Development of Back-Supportless CCD and
Focal Plane Assembly for Soft X-ray Imager
onboard the NeXT Satellite
Shin-ichiro Takagi
Department of Physics, Faculty of Science
Graduate School of Kyoto University
January, 5, 2006

Abstract
NeXT (New X-ray Telescope) is the next Japanese X-ray astronomy satellite, whichis scheduled to be launched around 2010. Thanks to the included multilayer super
mirror, NeXT has a large eﬀective area in the 0.1–80 keV band. To detect all X-rays
collected by the super mirror, the focal plane detector needs to be sensitive over a
very wide energy band.
As the focal plane detector for NeXT, we have been developing a Wideband
hybrid X-ray Imager (WXI) consisting of X-ray CCDs and a pixelized CdTe detector.
The X-ray CCD of the WXI is required to maintain a high quantum eﬃciency up to
the high energy band and pass hard X-rays undetected in the depletion (sensitive)
layer.
In order to meet these requirements, we have invented, and have been developinga Back-Supportless CCD (BS-CCD) which has a thick depletion layer, a thinned Si
wafer and a back supportless structure. As a ﬁrst step, we manufactured a test
model of the BS-CCD using an Nchannel CCD (Nch CCD). The Nch CCD was
fabricated on a p-type silicon semiconductor, which is a standard type of CCD. The
purpose of the trial production of the BS-CCD was to (1) investigate the handlingand thinning process involved, and (2) conﬁrm the absence of any change in theperformance. From the view-point of mechanical strength and safe handling of thewafer, we decided to thin the wafer to ∼ 200 µm thickness for the test model. We
veriﬁed the thickness of the depletion layer and the wafer of the BS-CCD to be about
70 and 190 µm, respectively. The energy resolution at 5.9 keV of 144 eV and theread-out noise of 7 e− are equal to those of the unthinned CCD, thus conﬁrming that
our thinning process has no eﬀect on the performance. Based on these successful
results, we constructed the “CCD-NeXT1” as an evaluation model of the BS-CCD.
This device is a medium-sized CCD which can be loaded on to the satellite. The
performance of this device is as excellent as those of test devices.
Through the development of the Nch BS-CCD, we found that (1) the thicknessof the depletion layer is limited to 80–100 µm and (2) the thickness of the ﬁeld-freelayer is left 50–70 µm, as long as we adopt the Nch CCD. In order to remove theﬁeld-free layer completely, it is essential to increase the thickness of the depletion
layer to > 150 µm. A promising device to realize such a thick depletion layer is the
Pchannel CCD (Pch CCD), which is fabricated on an n-type silicon semiconductor.
1

2
We participated in the development of the Pch CCD, which was proceeded by agroup of National Astronomical Observatory of Japan (NAOJ). We evaluated theperformance of the Pch CCD, and were able to conﬁrm that the thickness of thedepletion layer reached over 300 µm. Furthermore, through our experiments toimprove the performance, we were able to show that Pch CCD achieved excellent
performance comparable to that of established X-ray CCDs .
In parallel to the development of a test device for the SXI, we also considered
and designed a focal plane assembly (FPA). The FPA consists of a CCD chip, cold
electronics of a thermal electric cooler (TEC), and associated electronics such as
ampliﬁers and cables.
Since the CCD for the SXI will have a > 4 times larger size than the X-ray CCD
onboard the Suzaku, reducing the heat input to a CCD chip is inevitable in order
to cool the CCD with low power consumption . We designed a new-type of TEC
to match the unique structure of the SXI, and we also invented various schemes toreduce the heat input. Consequently, we were able to successfully reduce the heat
input drastically and restrain the power consumption to cool the CCD.

Contents
1 Introduction

10

2 The N eXT mission
2.1 Exploring the Nonthermal Universe - Key Science of N eXT . . . . .
2.1.1 Non-thermal emission from supernova remnants (SNR) . . . .
2.1.2 Non-thermal emission from the Galactic center and Galactic
ridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.3 Non-thermal emission from a cluster of galaxies . . . . . . . .
2.2 Overview of the Observation System . . . . . . . . . . . . . . . . . .
2.2.1 Hard X-ray Imaging System . . . . . . . . . . . . . . . . . . .
2.2.2 Soft Gamma-ray Detector . . . . . . . . . . . . . . . . . . . .
2.2.3 Soft X-ray Imaging System . . . . . . . . . . . . . . . . . . . .
2.2.4 Comparison of goal performance of each detector onboard
N eXT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

12
12
13

3 Charge-Coupled Device
3.1 Physical Structure of the CCD . . . . . . . . . . . . . . . . . . . . . .
3.1.1 MOS capacitor . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.2 Surface-Channel and Buried-Channel CCD . . . . . . . . . . .
3.1.3 Front illuminated and Back illuminated CCD . . . . . . . . .
3.2 Charge Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1 Principle of Charge Transfer . . . . . . . . . . . . . . . . . . .
3.2.2 Scheme of Charge Transfer . . . . . . . . . . . . . . . . . . . .
3.3 Charge Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1 Structure of Output Ampliﬁer . . . . . . . . . . . . . . . . . .
3.3.2 Mechanism of Charge Measurement . . . . . . . . . . . . . . .
3.4 Noise Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.1 Readout Noise . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.2 Charge Transfer Ineﬃciency . . . . . . . . . . . . . . . . . . .
3.4.3 Spurious Charge . . . . . . . . . . . . . . . . . . . . . . . . .
3.5 X-ray Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.6 Data analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

22
23
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26
26
28
28
30
32
32
34
35
35
38
38
38
40

3

14
15
16
17
18
19
20

CONTENTS

4

3.6.1
3.6.2
3.6.3

Output image of CCD . . . . . . . . . . . . . . . . . . . . . . 41
X-ray event analysis method . . . . . . . . . . . . . . . . . . . 41
Estimation of Noise Level . . . . . . . . . . . . . . . . . . . . 44

4 Soft X-ray Imager
4.1 Role of Soft X-ray Imager . . . . . . . . . . . . . . . . . . . . . . . .
4.2 CCD for Soft X-ray Imager - Back Supportless CCD - . . . . . . . . .
4.2.1 Baseline Plan . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.2 Goal Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

48
48
49
56
57

5 Experimental Setup
5.1 Data acquisition system . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Overview of the CCD evaluation system . . . . . . . . . . . . . . . .
5.2.1 Multipurpose CCD camera (Mini chamber) system . . . . . .
5.2.2 Large chamber system . . . . . . . . . . . . . . . . . . . . . .

62
62
66
66
68

6 Development I -Back Supportless CCD71
6.1 Baseline Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.1.1 Development Plan . . . . . . . . . . . . . . . . . . . . . . . . 71
6.1.2 Fabrication of BS-CCD . . . . . . . . . . . . . . . . . . . . . . 71
6.1.3 Performance of BS-CCD . . . . . . . . . . . . . . . . . . . . . 73
6.1.4 Summary of baseline plan . . . . . . . . . . . . . . . . . . . . 89
6.2 Goal Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
6.2.1 Development Plan . . . . . . . . . . . . . . . . . . . . . . . . 90
6.2.2 Improvement in detection eﬃciency for hard X-rays . . . . . . 91
6.2.3 Improvement of detection eﬃciency for soft X-rays . . . . . . 99
6.2.4 Amp gain and CTI . . . . . . . . . . . . . . . . . . . . . . . . 104
6.2.5 Final test device . . . . . . . . . . . . . . . . . . . . . . . . . 106
6.2.6 Summary of the goal plan . . . . . . . . . . . . . . . . . . . . 108
7 Development II -Focal Plane Assembly110
7.1 Design Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
7.1.1 Peltier device . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
7.1.2 Heat budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
7.2 Trial production of TEC module . . . . . . . . . . . . . . . . . . . . . 117
7.3 Thermo-mechanical model of FPA for SXI . . . . . . . . . . . . . . . 121
8 Summary124

List of Figures
2.1 The X-ray image and spectra of SN1006. . . . . . . . . . . . . . . . .
2.2 The BeppoSAX spectrum of the galaxy cluster A2256. . . . . . . . .
2.3 The overall view of the N eXT satellite and its observation systems. .
2.4 Eﬀective areas and Limiting Sensitivities of the NeXT hard X-ray
telescope compared with various X-ray telescopes. . . . . . . . . . .
2.5 A schematic view of the Wideband X-ray Imager. . . . . . . . . . . .
2.6 A schematic view and eﬀective area of the SGD. . . . . . . . . . . . .

13
15
16

3.1 A schematic view of a MOS CCD . . . . . . . . . . . . . . . . . . . .
3.2 A schematic view of the surface/buried channel potential well. . . . .
3.3 Transmission of the surface electrode (poly-Si) and insulator (SiO2 ) .
3.4 Comparison of the sensitivities of XIS-FI and XIS-BI . . . . . . . . .
3.5 Bucket analogy used to describe CCD operation. . . . . . . . . . . . .
3.6 Schematic view of the 2-phase transfer method. . . . . . . . . . . . .
3.7 Schematic views of the virtual phase and 3-phase transfer methods. .
3.8 The scheme of charge transfer in FT, FFT, and IT CCDs. . . . . . .
3.9 Schematic view of the ﬂoating diﬀusion ampliﬁer. . . . . . . . . . . .
3.10 Schematic view of the output signal produced by the FDA . . . . . .
3.11 Schematic view of the delay method. . . . . . . . . . . . . . . . . . .
3.12 Schematic view of the correlated double sampling method. . . . . . .
3.13 Schematic view of the dual slope method. . . . . . . . . . . . . . . . .
3.14 Relative importance of the three major types of X-ray and γ-ray
interactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.15 Pattern diagram of the typical output image of a CCD . . . . . . . .
3.16 Criteria for grading into 8 grades . . . . . . . . . . . . . . . . . . . .
3.17 Schematic view of the ﬁtting method. . . . . . . . . . . . . . . . . . .
3.18 Division of the active region to measure CTI. . . . . . . . . . . . . .

23
24
27
27
28
29
30
31
32
33
34
35
35

4.1

17
18
19

39
41
43
45
46

Comparison of the sensitivity between a CCD (300-µm thickness) andCdTe (0.5- and 1-mm thickness). . . . . . . . . . . . . . . . . . . . . 49
4.2 Relationship between the detection eﬃciency and the depletion layer
thickness of the CCD . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5

LIST OF FIGURES
4.3
4.4
4.5
4.6
4.7
4.8
4.9

Schematic structures of various types of CCDs. . . . . . . . . . . . .
The vignetting function of HXT. . . . . . . . . . . . . . . . . . . . .
Examples of a mosaic CCD camera. . . . . . . . . . . . . . . . . . .
Schematic structures of various types of CCDs. . . . . . . . . . . . .
Electron and hole mobility as a function of impurity concentration . .
A mosaic camera with 8 CCD-CRESTs. . . . . . . . . . . . . . . . .
Structure of the LBNL Pch CCD[14]. The gate structure, on top of
the insulating oxide and protective nitride layers, is conventional, as
is the buried channel. A bias voltage on the back window/ electrodedepletes the entire substrate. . . . . . . . . . . . . . . . . . . . . . .
4.10 Schematic view of the CCD-NeXT2, 3, and 4 . . . . . . . . . . . . . .
5.1
5.2
5.3
5.4
5.5

The data acquisition system of the CCD evaluation system. . . . . . .
Picture of the DAQ system of the CCD evaluation system. . . . . . .
Circuit diagram of a board of the clock generator. . . . . . . . . . . .
Circuit diagram of a board of the integration ﬁlter . . . . . . . . . . .
A circuit diagram of a readout circuit which is used only for the
CCD-NeXT1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6 Mini chamber used for evaluation of the CCD . . . . . . . . . . . . .
5.7 Picture of the performance evaluation system . . . . . . . . . . . . .
5.8 Collimator of 109 Cd for measurement of the depletion depth . . . . .
5.9 The acquired image of the collimated X-rays of 109 Cd . . . . . . . . .
5.10 Picture of the CCD-NeXT1 mounted in the large chamber system . .
6.1

6
50
52
54
55
57
58

59
60
63
64
64
65
66
67
68
69
69
70

Schematic view of the chemical etching method and the grinding +polishing method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
6.2 Picture and schematic view of the test model of BS-CCD . . . . . . . 73
6.3 Spectrum of 55 Fe and 241 Am detected by BS-CCD of P15 6-5B1P-2(Deep1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
6.4 Spectrum of 55 Fe detected by BS-CCD of P15 14-5B1P-4 (Deep2K2). 76
6.5 Acquired image of 109 Cd detected by BS-CCD of P15 14-5B1P-4(Deep2K2). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
6.6 Spectrum of 109 Cd detected by BS-CCD of P15 14-5B1P-4 (Deep2K2). 77
6.7 Picture of the test model of the CCD fabricated on a 6-inch wafer . . 79
6.8 Spectrum of 109 Cd detected by BS-CCD of KF201 17-5B1KF-5 (Deep5) 79
6.9 Spectrum of 109 Cd detected by the BS-CCD of KF201 21-5B1KF-5(Deep5) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
6.10 Photograph of the test model of the WXI. . . . . . . . . . . . . . . . 82
6.11 Transmissivity and Q. E. of the BS-CCD. . . . . . . . . . . . . . . . . 82
6.12 Picture of the CCD-NeXT1 . . . . . . . . . . . . . . . . . . . . . . . 84
6.13 Tiny trails found on the grinding surface of CCD-NeXT1. . . . . . . . 84

LIST OF FIGURES

7

6.14 Surface condition of the ground wafer. . . . . . . . . . . . . . . . . . 85
6.15 Experimental setup for measurement of the thickness of the wafer of
CCD-NeXT1 (KG103 9-20A0KG-2) . . . . . . . . . . . . . . . . . . . 86
6.16 (left) Spectra of 241 Am detected by the CdTe detector. (b) The countratio at each emission line between the absorbed spectrum by the IAof the CCD-NeXT1 and the unabsorbed one. . . . . . . . . . . . . . . 86
6.17 Spectrum of 55 Fe and 109 Cd detected by CCD-NeXT1 of S/N KG103
22-20A0KG-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6.18 The cross-section of a CCD chip. . . . . . . . . . . . . . . . . . . . . 88
6.19 A photograph of the front-illuminated Pch CCD. Pixel size is 24µm
× 24µm and the format is 1024 × 256. . . . . . . . . . . . . . . . . 91
6.20 Acquired dark image with Pch3. . . . . . . . . . . . . . . . . . . . . 92
6.21 Relationship between the width of a generated charge cloud and absorbed depth in Pch CCD. . . . . . . . . . . . . . . . . . . . . . . . 94
6.22 Acquired X-ray image of 109 Cd with the Pch 5-9. . . . . . . . . . . . 94
6.23 Acquired image with Pch5(left) and Pch3(right). Incident X-rays arefrom radio isotope 109 Cd (mainly 22.2keV). . . . . . . . . . . . . . . 95
6.24 Schematic view of the potential well of an ordinary and the back bias
structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
6.25 Acquired image of 109 Cd with Pch 8A-7. . . . . . . . . . . . . . . . . 96
6.26 Acquired image of X-rays with the Pch14B-05. We irradiated collimated X-rays (109 Cd) to the white dotted circle on Pch 14B. The
white dotted line shows the border between active and VOC regions. 98
6.27 Acquired image of X-rays with Pch 15-14 (back bias voltage = 0) andits close up view. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.28 A photograph of the front-illuminated Pch CCD. Pixel size is 24µm
× 24µm and format is 512 × 512. . . . . . . . . . . . . . . . . . . . 100
6.29 Acquired image of 241 Am with Pch 9-17 with the back bias voltage of
5V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
6.30 Spectrum of 241 Am detected by Pch 9-17 with a back bias voltage of
5V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
6.31 Comparison of images acquired by Pch 9-17 with various back bias
voltages of 5V, 13V, and 15V. . . . . . . . . . . . . . . . . . . . . . . 102
6.32 Schematic view of the interaction of β-rays in a CCD. . . . . . . . . 103
6.33 Image of β-rays from 90 Sr detected by Pch15-22 with a back bias
voltage of 5V and its close-up view. . . . . . . . . . . . . . . . . . . 103
6.34 Image acquired by Pch 8A-7 with 6-sec irradiation of LED light. . . . 105
6.35 Relationship between luminous time of the LED and CTI of Pch 8A-7 106
6.36 Image of X-rays acquired by Pch 15-22 (back bias voltage = 10V)
and its close-up view. . . . . . . . . . . . . . . . . . . . . . . . . . . 107
6.37 Comparison of images acquired by Pch 15-22 and Pch 15-14. . . . . 109

LIST OF FIGURES

8

7.1 Schematic view of an ordinary TEC module. . . . . . . . . . . . . . . 111
7.2 Conceptual diagram of a Peltier device. . . . . . . . . . . . . . . . . . 112
7.3 Schematic view of FPAs of Suzaku XIS and MAXI-CCD. . . . . . . . 114
7.4 Conceptual diagram of heat transfer due to thermal conduction. . . . 115
7.5 Conceptual diagram of the heat transfer due to thermal radiation. . . 116
7.6 Simulation model for the heat performance of TEC . . . . . . . . . . 118
7.7 Result of simulation for cooling performance of TEC. . . . . . . . . . 119
7.8 Result of simulation for cooling performance of small-size TEC. . . . 120
7.9 Photo of the trial production of the 2-stage TEC module. . . . . . . . 121
7.10 Design drawing of the thermo-mechanical model of FPA for SXI . . . 122
7.11 A photo of the trial production of the thermo-mechanical model of
FPA for SXI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

List of Tables
2.1

Goal performance of each detector onboard N eXT . . . . . . . . . . . 21

3.1

Comparison of Various X-ray CCDs . . . . . . . . . . . . . . . . . . . 23

4.1 Comparison of Suzaku XIS, CCD-CREST, and MAXI-CCD . . . . . 51
4.2 Comparison of the performance of the baseline and goal plans . . . . 56
4.3 Comparison of the speciﬁcations of the evaluation/ﬂight model of thebaseline and goal plans . . . . . . . . . . . . . . . . . . . . . . . . . . 61
6.1
6.2

Speciﬁcation and performance of test models of BS-CCD . . . . . . . 75
Intensities of the lines of 241 Am, transmissivity and estimated Q. E.
of the BS-CCD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
6.3 Speciﬁcation and performance of CCD-NeXT . . . . . . . . . . . . . 87
6.4 Comparison of the detection eﬃciency for 109 Cd detected with Pch
3-10 at various gate voltages . . . . . . . . . . . . . . . . . . . . . . . 92
6.5 Speciﬁcation and typical performance of test models of Pch CCD(Front Illuminated) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
6.6 Comparison of the eﬃciency and grade ratio of 109 Cd detected by Pch
5-9 at various back bias voltages . . . . . . . . . . . . . . . . . . . . . 96
6.7 Speciﬁcations and typical performance of test models of Pch CCD(Back Illumination) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
6.8 Relationship between luminous time of the LED and CTI of Pch 8A-7 105
6.9 Comparison of the characteristics of Pch 15-22 for various back bias
voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
6.10 Comparison of grade ratio of 109 Cd detected by Pch 15-22 for various
back bias voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
7.1
7.2

Comparison with FPAs of existing missions and NeXT-SXI . . . . . . 117
Contests of the screening test performed in AISIN . . . . . . . . . . . 120

9Chapter 1
Introduction
Observations in the hard X-ray band are very important in astronomy in order toexplore the ”non-thermal universe” characterized by accelerated high energy particles. The big breakthrough in X-ray astronomy was the discovery of non-thermal
emission. Koyama et al., in 1995, discovered non-thermal X-ray emission from a supernova remnant SN 1006 using ASCA observations. This emission was considered
to be synchrotron X-rays caused by high energy electrons on the order of 100 TeV.
The distribution of such high energy electrons clearly departs from thermal equilibrium. This suggests that a portion of the particles is selectively accelerated andthere is an unknown physics process which actively produces the nonequilibrium
state.
After the discovery of non-thermal emission by Koyama et al., several supernova remnants (SNRs) emitting synchrotron X-rays were found. Furthermore, nonthermal X-ray emission has also been found in larger scale objects such as Galactic
center, Galactic ridge, and clusters of galaxies in succession. These facts indicatethat non-thermal X-ray emission occurs universally in celestial objects at various
scales. Furthermore, the total amount of energy of non-thermal particles is estimated to be equal to that of thermal particles.
It is thus clear that non-thermal X-ray emission is a very important physicalprocess for a comprehensive understanding of the universe. However, we still do nothave a clear answer about its origin. One of the key reasons for this is that thethermal X-ray emission overwhelms the non-thermal emission in the energy band
below 10 keV, where X-ray focusing optics so far have provided a high sensitivity
in previous and current X-ray astronomy satellites. In order to reveal the nature of
non-thermal X-ray emission, it is necessary to perform imaging and spectroscopic
observations, and to investigate the spot from where the non-thermal X-rays are
emitted. Because non-thermal emission becomes dominant in the energy regime
above 10 keV, imaging and spectroscopic observations in this band are essential tosolve the origins of the non-thermal emission.
The New X-ray Telescope (N eXT ) satellite will be the next Japanese X-ray
10

CHAPTER 1. INTRODUCTION

11

astronomy satellite following the Suzaku, and it is designed to carry out imaging
and spectroscopic observations in the energy band above 10 keV. Thanks to new
innovations including a multilayer super mirror, N eXT will be able to focus the
X-rays in the energy range from 0.1 keV to 80 keV. In particular, this super mirror
maintains a high reﬂectivity even in the high-energy (> 10 keV) band. For the
focal plane detector for N eXT , we have been developing a Wideband X-ray Imager(WXI). To match the wide energy range provided by HXT, we have combined (1)X-ray CCDs, which have excellent spatial and energy resolution, with (2) CdTe
pixelized detectors, which have a high detection eﬃciency for X-rays and moderatespatial resolution. The former and the latter are respectively named the Soft Xray Imager (SXI) and Hard X-ray Imager (HXI), based on the role played by eachdetector. X-ray CCDs are stacked above the CdTe detectors. The X-ray CCD
performs precise imaging and spectroscopic observations in the energy band of 0.1–
20 keV. On the other hand, since it is diﬃcult to detect hard X-rays of above 20 keVby the CCD, we make them “penetrate” the CCD and enter the CdTe pixelized
detector, which has suﬃcient sensitivity in the energy range up to 100 keV. The
X-ray CCD is required to maintain a high detection eﬃciency to the high-energy
band, while allowing hard X-rays to pass undetected through the depletion sensitive
layer.
In order to realize the WXI and SXI, which are key to reveal the origins of nonthermal X-ray emission, we have been designing and developing a “back-supportless
CCD” with a thick depletion layer, a thinned Si wafer and a back-supportless structure. In parallel, we have designed a focal plane assembly (a unit of a CCD and
its cooling system) for the SXI, in order to cool the CCD chip with low power
consumption, to the maximum extent possible.
In this thesis, in chapter 2, we ﬁrst give a brief overview of the N eXT mission
and the observation systems involved. In chapter 3, the principles and fundamental
knowledge of the CCD are described. In addition, we give an outline of the data
analysis method used which is peculiar to X-ray astronomy. In chapter 4, we describe
the positioning of the SXI in the detectors onboard N eXT and development plans
for the SXI. In chapter 5, we brieﬂy introduce the experimental setup, which is usedin the evaluation of the BS-CCD test device. In chapter 6, we state the results ofthe performance tests of the test devices for SXI and summarize the present statusof the development of the BS-CCD. In chapter 7, we describe the design of the FPA
for the SXI, especially focusing on the cooling system to cool a CCD chip eﬃciently
on board the satellite.

Chapter 2
The N eXT mission
The N eXT (New X-ray Telescope) satellite is the next Japanese X-ray astronomy
satellite mission after the Suzaku satellite and is slated to be launched around 2010.
With the use of the multilayer super mirror (see [62] and references therein), N eXT
has a large eﬀective area for X-rays in the energy range from 0.5 to 80 keV. In
particular, this super mirror can maintain a high reﬂectivity even in the high energy(> 10 keV) band. In order to meet the energy range covered by the super mirror, wehave developed a wideband camera which has high sensitivity in the energy bandfrom sub-keV to ∼ 100 keV. Satellite missions with X-ray focusing optics in theenergy band above 10 keV have never been realized thus far, and therefore, N eXT
is expected to carry out imaging and spectroscopic observations in the energy band
above 10 keV for the ﬁrst time in human history.

2.1

Exploring the Nonthermal Universe - Key Science of N eXT

The ASCA, Chandra, XM M -N ewton and Suzaku satellites have carried out precise imaging and spectroscopic observations mainly in the soft X-ray band (< 10 keV)and have successfully discoverd many important phenomena. Though these observatories have detected non-thermal X-ray emission from many X-ray objects, thelack of sensitivity for X-rays over 10 keV have prevented us from conﬁrming thenature of these emissions.
Since non-thermal emission, mainly due to accelerated high energy particles,
dominates the thermal X-ray emission in the energy regime above 10 keV, observations of this band are essential to study the non-thermal X-ray emission. Therefore,
the N eXT satellite is expected to open new windows to the “non-thermal universe”.

12CHAPTER 2. THE N EXT MISSION

2.1.1

13

Non-thermal emission from supernova remnants (SNR)

The energy of a supernova explosion is mainly transformed into the energy of theejected matter into interstellar space. Shortly after explosion, the ejected matter
carries most of the explosion energy. The ejected matter thermalized by the interstellar medium and form plasma. In previous results, the thermal X-ray emission
from high temperature thermal plasmas has been observed in the X-ray band. The
picture that an SNR is a high temperature plasma sphere has been established as a
general understanding for SNRs. However, ASCA, which is the 5th Japanese X-ray
Astronomy satellite, has discovered non-thermal emission separate from the thermal
emission in some young SNRs.

Figure 2.1: (left) The ASCA X-ray image of SN1006. X-ray spectra were obtained
from two elliptical regions[33]. (right) The X-ray spectra from the rim and interior
regions; the former is a non-thermal spectrum, while the latter is that of a thinthermal plasma typical of shell-like SNRs.

Figure 2.1 shows an X-ray image and spectra of the SN1006 SNR observed by
the ASCA satellite. The X-ray spectrum from the interior region reveals a spectrumwith many emission lines typically seen in a thermal plasma. However the spectrum
from the rim of the SNR yields a featureless power-law spectrum. This represents
non-thermal emission from the shell-type SNR. After this signiﬁcant discovery, SNRs
which show non-thermal emission have been discovered consistently in the hard Xray band (see [70] and reference therein). The radiation mechanism is considered
to be synchrotron emission by acceleration of high energy electrons. The electrons
emitting synchrotron radiation at these X-ray energies must have energies on theorder of 100 TeV [33].
Subsequently, the TeV gamma-ray telescope Cangaroo and H.E.S.S detected
TeV gamma-rays in the SNR RXJ 1713.7−3946 and RXJ 0852−4622, which shownonthermal X-ray emission [1, 2, 3, 10, 28]. Although the origin of the TeV gammarays is still debatable, they found that the emission mechanism of the observed TeV
gamma-rays is consistent with the inverse Compton scattering between relativistic

CHAPTER 2. THE N EXT MISSION

14electrons and photons of the cosmic microwave background (CMB). This may support the idea that the electrons are accelerated in an SNR, and then ions, including
protons, are also accelerated. They are the main contributors to cosmic rays, andtherefore the acceleration of charged particles in an SNR is a possible origin of cosmic
rays.
However we have not fully understood the relationship between cosmic rays andthe acceleration of charged particles in an SNR. Since it is diﬃcult to obtain X-ray
images in the energy band over 10 keV, we cannot observe the bending point of the
non-thermal X-ray spectrum. X-ray observations so far have not determined withcertainty how high energy an electron is being accelerated. Therefore we have notcome to a deﬁnitive conclusion as to whether the origin of the highest energy cosmic
rays is an SNR.
In addition, since nonthermal emission associated with SNRs has not been abundantly found so far, the question of what fraction of the (galactic) cosmic rays canbe explained by SNRs has not yet been resolved.
To solve these problems, it is essential to perform imaging and spectroscopic
observations in the hard X-ray band above 10 keV. Since N eXT will have unprecedented high sensitivity for hard X-rays over 10 keV, we should be able to detectand study the bending point of the nonthermal spectrum. Furthermore, we cansuccessfully separate nonthermal emission from thermal emission which dominatessigniﬁcantly and hides the nonthermal origin in the energy band below 10 keV.
These results will be invaluable to solve the origin of cosmic rays.
We will also obtain the precise position-velocity map of expanding gas by thehigh-dispersion spectroscopic and imaging observations using the Soft X-ray Spectrometer onboard the N eXT satellite. Based on this observation, we should be able
to identify the place and the physical process associated with the energy conversionof the bulk motion to the thermal and nonthermal components.

2.1.2

Non-thermal emission from the Galactic center andGalactic ridge

T enma discovered strong X-ray emission from the hot plasma in the Galactic center(GC) and Galactic ridge (GR) regions. These X-ray spectra cannot be explainedby one-temperature plasma and they have the nonthermal component extended
to the gamma-ray band. The origins of the X-ray emission from the GC and GR
remain unresolved. Since active star formation and supernova explosions occur in the
Galactic plane, these activities may promote the acceleration of charged particles.
By the N eXT observations in the hard (> 10 keV) X-ray band, we will
be successfully able to separate nonthermal emission from the thermal
emission diﬀused over the entire GC and GR, and obtain the spatial distribution
and spectra of non thermal emission. Based on these results, we aim to resolve theCHAPTER 2. THE N EXT MISSION

15

spot and origin of the accelerated charged particles and the heating of the plasma.

2.1.3

Non-thermal emission from a cluster of galaxies

The Einstein satellite revealed large scale X-ray diﬀuse emission in the inter-galactic
space of galaxy clusters [49]. The temperature of the diﬀuse plasma reaches ∼107−8 K and the total mass of the plasma is more than several times that of the opticalgalaxies. In order to bind such large-scale diﬀuse plasma with high temperature, theexistence of dark matter is necessitated, and its mass should be more than several
times that of the plasma.
The ASCA satellite ﬁrst performed imaging and spectroscopic observations ofclusters of galaxies in the wide energy band of 1–10 keV, and found a complex
structure of the temperature. This suggests that the clusters have experienced
collisions and mergers, and are not in an equilibrium condition. Since the clusters
have an enormous kinetic energy of 1064 erg, a part of this energy may be used toaccelerate charged particles via shock waves. In fact, spectra from several clusters
contained a hard X-ray component in addition to diﬀuse thermal emission.

Figure 2.2: The BeppoSAX spectrum of the cluster of galaxies A2256 [13].

Figure 2.2 shows the spectrum of the A2256 cluster of galaxies [13] observed by
BeppoSAX. In the energy band below 10 keV, thermal emission is observed by
the CCD. On the other hand, the hard X-ray component, which shows a powerlaw spectrum, is dominated in the energy band of 10–100 keV. Though a plausible

CHAPTER 2. THE N EXT MISSION

16interpretation of the hard X-ray emission is inverse Compton emission by the nonthermal electrons and CMB photons[13], the origin has still not been deﬁnitively
established.
Because this hard X-ray component will be distinguished clearly from the thermal
emission in the energy band over 10 keV, the evidence of large-scale nonthermal
emission will be obtained by further observations in the hard X-ray band. The hard
X-ray (>10 keV) image, which will be obtained by N eXT for the ﬁrst time, willallow us to identify the location of the hard X-ray emission. In particular, if theinverse Compton emission is really the origin of this emission, the distribution andthe spectrum of the hard X-ray emission will reveal the strength and distribution of
the magnetic ﬁeld in the clusters.

2.2

Overview of the Observation System

The observation system of N eXT consists of the Hard X-ray Imaging System, Soft
X-ray Imaging System, and a Soft Gamma-ray Detector. We show an overall viewof the N eXT satellite and its observation systems in Figure 2.3
HXT
Wideband X-ray Imager x3
Soft X-ray Spectrometer

Extended Optical Bench
Soft X-ray
Telescope

Focal length 12m

Soft Gamma-ray Detector

Hard X-ray Telescope x3

Figure 2.3: The overall view of the N eXT satellite and its observation systems.

In this section, we brieﬂy provide an overview of and explain the characteristics
of each of the observation systems.

CHAPTER 2. THE N EXT MISSION

2.2.1

17

Hard X-ray Imaging System

Hard X-ray Telescope (HXT) In the X-ray band over 10 keV, satellites with
X-ray focusing optics have never been available previously. Figure 2.4 shows theeﬀective areas of various X-ray telescopes. In the case of present X-ray telescopes
as Chandra, XM M − N ewton, and Suzaku, the eﬀective area declines rapidly inthe energy band over 10 keV.

Figure 2.4: (left) Eﬀective areas of the N eXT hard X-ray telescope, compared with
the XMM-Newton, Chandra, InFOCµS and XEUS hard X-ray telescope.
(right) Limiting Sensitivities of various missions (right) with 5σ detection for apoint-like sources. The NeXT X-ray telescope will achieve a sensitivity of 1.7 ×
1013 erg cm2 s−1 in the 10–80 keV region with a 100-ksec observation [50].

However, the Hard X-ray Telescope (HXT) onboard N eXT maintains a higheﬀective area up to the X-ray energy of ∼80 keV due to the “depth-graded multilayer” on its reﬂecting interface. This multilayer is a periodic structure which reﬂects
X-rays by interference from the interfaces, similar to Bragg reﬂections from crystal
lattice planes. Since its bandwidth is narrow and conﬁned to close to the Bragg
wavelength, the supermirror is a multilayer stack with diﬀerent sets (blocks) of periodic lengths and number of layer pairs to cover diﬀerent energy bands [74, 62, 50].
The multilayer is built up using platinum-carbon (Pt/C). From simulations performed by Okajima et al. (2004)[50], the eﬀective area of NeXT is expected to be
250cm2 per one HXT is 436, 210, and 73 cm2 , at 20, 40, and 60 keV, respectively.
The rapid decline of the eﬀective area at ∼ 80 keV is caused by the platinum K
absorption line.
Wideband X-ray Imager(WXI) The focal plane detector for HXT is required
to cover a very wide energy band from 0.1 to 80 keV. The standard focal plane
detector is a CCD with excellent spatial and energy resolution. However, since theCHAPTER 2. THE N EXT MISSION

18%%&(2#TGCFQWV%%&+OCIKPICTGC %%&#NEQCVKPI(TCOGUVQTGFCTGC$CEM
5WRRQTVNGUU

5:+

OOǾ
#WEQCVKPI2GNVKGEQQNGT%%&(2#%F6G*:+%QNF2NCVG*GCV2KRG$)1UJKGNF$CUG2NCVG

Figure 2.5: A schematic view of the Wideband X-ray Imager.
material of the CCD is silicon (low-atomic-number matter; low Z), the sensitivity
for high energy X-rays is low. For example, the detection eﬃciency of the Suzaku
XIS, which is one of the X-ray CCDs with the highest sensitivity, is limited to only
∼2% at 40 keV. On the contrary, the high Z solid detector has a high sensitivity
at the high energy band with a moderate positional resolution. On the other hand,the spectroscopic and imaging performance of high Z solid detectors such as CdTe
is poor below 10 keV compared to those of X-ray CCDs.
To compensate for the weak points of each other and to take advantage of both
detectors, we have been developing the Wideband X-ray Imager (WXI) consistingof X-ray CCDs and CdTe pixel detectors[56, 57, 58, 67] placed under the X-ray
CCDs. When the X-rays collected by the supermirror enter the WXI, the soft
X-rays are detected by the CCDs and the hard X-rays, undetected by the CCD,
penetrate the CCD and are detected by the CdTe detector. Therefore WXI enables
us to carry out wideband imaging and spectroscopic observations together and issuitable for the focal plane detector of the HXT.

2.2.2

Soft Gamma-ray Detector

The Soft Gamma-ray Detector has high sensitivity toward “soft Gamma-rays”
which have an energy of a few × 10 keV – 1MeV. As shown in Figure 2.6(left),the SGD consists of a stack of 24 double-sided silicon strip detectors (DSSDs) andCdTe pixel detectors[58]. This structure allows the incident gamma-ray events tointeracts twice in the detector. First, Compton scattering occurs in DSSDs, and sec-

CHAPTER 2. THE N EXT MISSION

19

ondoly, photo absorption of the scattered photon occurs in CdTe detectors. Basedon the kinematics of the Compton scattering, the SGD can determine the arrival
directions of detected gamma-rays within a cone in the sky (Compton ring). Thanks
to the structure of a narrow ﬁeld of view (FOV) using a well-type (phoswitch) active
shield, the background events are eﬃciently rejected if the reconstructed Compton
ring does not intercept the FOV[58].

Figure 2.6: (left) A schematic view of the SGD[58]. A stack of 24 Si DSSDs and CdTe
pixel detectors are assembled to form a semiconductor Compton Telescope. (right)
Eﬀective area of the SGD. The solid line shows the area calculated for the photoabsorption mode. The dashed line shows the area when the detector is operated asa Compton camera (Compton mode) [59].

The right side in ﬁgure 2.6 shows the eﬀective area of the SGD. The solid line
shows the area when the detector is operated in the photo-absorption mode, whichcalculated only the energy of the incident gamma-rays. The eﬀective area of thephoto-absorption mode is greater than that of the Compton mode. However, theCompton mode improves the S/N ratio, especially for observations of faint objects,
because this mode can determine the arrival direction of gamma-rays, as describedabove.

2.2.3

Soft X-ray Imaging System

Soft X-ray Spectrometer The Soft X-ray Spectrometer (SXS) adopts a TES(transitionedge sensor)-type micro-calorimeter. An X-ray micro-calorimeter absorbs X-rays
and produces signals proportional to the temperature rise induced by X-rays. The
micro-calorimeter is usually operated at a low temperature (typically 0.1K), The
TES is a very sensitive thermometer utilized by the rapid change of its resistancefrom the super conducting and normal states. The TES is thermally coupled toa cold bath whose temperature is below the transition temperature. When X-ray
photon enters into the TES, the heat is generated and makes its state and resistanceCHAPTER 2. THE N EXT MISSION

20

change. The heat rapidly moves the cold bath via a thermal link. Utilizing thesecharacteristics, the aim for the SXS is to realize performance of (1) an energy resolution of 5 eV (at 5.9 keV), (2) a detector size of 12 mm × 12 mm, and (3) a formatof 16 × 16pixels. Furthermore, a ﬁnal goal is to achieve an energy resolution of 3 eV
(at 5.9 keV) at a format of 32 × 32pixels.

2.2.4

Comparison of goal performance of each detector onboard N eXT

We summarize the performance goals of each detector onboard N eXT in Table 2.1.

CHAPTER 2. THE N EXT MISSION

21

Table 2.1: Goal performance of each detector onboard N eXT [47].
Hard X-ray Imaging
SystemSoft Gamma-ray Detector

HXT(Hard X-ray Telescope)Eﬀective area
500–1000 cm2 (at 30 keV, HXT×3)
Energy band
0.5–60 keV
Spatial resolution 30–40 arcmin
WXI(Wideband X-ray Imager)
HXI(Hard X-ray Imager)
Energy band
0.5–60 keVEnergy resolution 0.5–1 keV (FWHM)
Pixel size
200–500 µm
Detector size
20 mm × (20-30) mm
SXI(Soft X-ray Imager)
Energy band
0.5–20 keVEnergy resolution 130 eV (FWHM, 5.9 keV)
Pixel size
27 µm
Detector size
42mm × 43 mm (only imaging area)
SGD(Soft Gamma-ray Detector)
Energy band
Energy resolution
Eﬀective area

Soft X-ray Spectroscopic System

10keV–1 MeV
2 keV (FWHM, 40 keV)
525 cm2 (at 100 keV)
110 cm2 (at 500 keV)
100 cm2 (at 100 keV, Compton mode)
40 cm2 (at 500 keV, Compton mode)
Spatial resolution 1.5 deg (RMS, Compton mode, 500 keV)SXT (Soft X-ray Telescope)Eﬀective area
700 cm2 (at 7 keV)
Energy band
0.5–10 keV
Spatial resolution 30–40 arcmin
SXS (Soft X-ray Spectrometer)
Energy b and
0.5–10 keVEnergy resolution 2–5 eV (FWHM, 7 keV)Eﬀective area
525 cm2 (at 100 keV)
Pixel size
>12 mmFormat
12 × 12 – 32 × 32 pixels

Chapter 3
Charge-Coupled Device
In 1969, a charge coupled device (CCD) was invented by W. S. Boyle and G. E. Smithat Bell Telephone Laboratories[7]. Though they originally developed it as a memory device, M. F. Tompsett at Bell Telephone Laboratories suggested the idea
in 1971 of using the CCD as an imaging device. W. J. Bertram et al. (1972)
and C. H. Séquin et al. (1973) successfully developed CCDs of 98 pixels [6] and106 × 128 = 13568 pixels[53], respectively. After the CCD gained recognition as a
new 2-dimensional imaging area sensor, its development progressed rapidly. Basedon the progress of semiconductor processing, the number of pixels was eventually
increased to over 106 pixels, and the CCD became one of the most representativeand popular detectors among imaging devices.
Today, CCDs are widely used in various applications including commercial, industrial, and scientiﬁc. In astronomy, CCDs are also widely employed, especially in
optical, infrared, and X-ray astronomy. Since the sensitivity of a CCD is signiﬁcantly
higher than that of a ﬁlm[24] for photons in the 300–1000 nm-wavelength range, anda CCD has the moderate time resolution, most optical astronomical observatories
employ a CCD as a focal plane detector instead of ﬁlms.
The CCD has also become a standard detector for X-ray astronomy, with anenergy resolution of ∼ 130 eV at 5.9 keV (FWHM), position resolution reaching∼ 20µm, time resolution of several seconds, and suﬃcient detection eﬃciency in the
0.1–10 keV band. An X-ray CCD as the focal plane detector for X-ray astronomy
mission was ﬁrst used in the ASCA satellite launched in 1993. Since then, X-ray
CCDs have become standard focal plane detectors. The new generation of X-ray observatories, including Chandra(1999-), XM M -N ewton(1999-) and Suzaku(2005-),
are all equipped with CCDs as the focal plane detectors. Table 3.1 shows the performance of representative X-ray CCDs onboard various X-ray astronomy satellites[8, 61, 54, 69, 71].

22

CHAPTER 3. CHARGE-COUPLED DEVICE

23

Table 3.1: Comparison of Various X-ray CCDs
Observatory
Detector
Year
Country
Pixel size [µm]Energy resolution [eV]†
Depletion depth [µm]
Type
†

ASCA
SIS
1993–2000
Japan
27
∼130
35
MOS

Chandra
ACIS
1999–
USA
24
∼130
70
MOS

XM M − N ewton
EPIC-MOS EPIC-PN
1999–
ESA
40
150
∼130
∼130
35
280
MOS
PN

Suzaku
XIS
2005–
Japan
24
∼130
70(FI)/45(BI)
MOS: Energy resolution of the 5.9 keV line (FWHM).

3.1

Physical Structure of the CCD

3.1.1

MOS capacitor

The fundamental building block of a CCD is the MOS capacitor. This element isthe backbone for charge collection and charge transfer. We show a cross-sectionalview of a CCD in Figure 3.1. In the following, we assume that a MOS capacitor
consists of a p-type silicon semiconductor. A silicon semiconductor is oxidized toform an insulator (SiO2 ), and a poly-Si electrode is deposited as a gate structure toform a MOS capacitor.

1pixelGate(Poly Si)

Insulator(SiO2)
Depletion layer

Field Free layer

Figure 3.1: A schematic view of a MOS CCD

A positive voltage applied to the gate forms a depletion layer, wherein majority
carriers (holes in the case of a p-type semiconductor) are driven out from the surface.

CHAPTER 3. CHARGE-COUPLED DEVICE24

Uncompensated, negatively charged acceptors were left in the depletion layer. Then,the biased distribution of the charge prevents further drift of carriers and makes thebias condition in equilibrium, as shown in Figure 3.2(left). The potential is plotted
as a function of depth in the silicon. Below the depletion layer, there is a region
called the “ﬁeld-free region”, where forms little electric ﬁeld condition.
potential(V)depletion layer

depletion layer

field-freeelectronelectron

p-Si

n-Si

<Surface Channel CCD>elec
in trod
su e
la
to
r

el
ec
in trod
su e
la
to
r

field-freep-Si<Buried Channel CCD>

Figure 3.2: A schematic view of a potential well, in the case of surface channel CCD(left), and in the case of buried channel CCD (right).

When an X-ray photon is injected onto a CCD, it is absorbed by a Si atom
via photoelectric absorption. A hot electron is then emitted which collides withanother atom, multiplying the number of electrons. These are pulled up to the
CCD surface by the electric ﬁeld in the depletion layer. It is basically diﬃcult forthe charges generated in the ﬁeld-free region to reach the CCD surface, since there
is little electric ﬁeld in this region. Therefore, we usually call the depletion layer andthe ﬁeld-free layer as the “sensitive” and “dead” layers, respectively. The sensitivityof the X-rays depends on the thickness of the depletion layer. We can derive theequation of the thickness of the depletion layer, as shown in the following step [24];In the whole of the depletion layer, the number of holes driven out from thesurface equals the number of negative charges on the surface of a CCD (in otherwords, the electrode),Qs = −qNA xd ,(3.1)

where Qs is the ionized acceptor charge concentration beneath the depleted MOS
electrode, NA is the acceptor doping concentration, xd is the thickness of the depletion layer, and q is the elementary electric charge.
Then, a potential voltage ϕ(x) is generated from the surface to the depletion
layer. ϕ(x) is found by solving the following Poisson’s diﬀerential equation,∂ 2 (ϵSi ϵ0 ϕ(x))= qNA ,∂x2

(3.2)

CHAPTER 3. CHARGE-COUPLED DEVICE

25

where x is the depth measured from the surface, ϵSi is the relative permittivity ofsilicon, and ϵ0 is the permissivity of free space.
Assuming the following boundary conditions;• the electric ﬁeld is equal to zero at x = xd , or∂(ϵSi ϵ0 ϕ(xd ))
= 0,
∂x

(3.3)

• the potential is zero (reference potential) at x = xd , orϕ(xd ) = 0,

(3.4)

then we can solve Eq. (3.2) as follows;−qND (x − xd )2.
(3.5)
2ϵSi ϵ0
Assuming that the potential voltage of the surface of the CCD is ϕs , we can solve
the thickness of the depletion layer as:
ϕ(x) =√

xd =2ϵSi ϵ0 ϕs
qNA

(3.6)

It can be seen that the thickness of the depletion layer increases by the squareroot of the surface potential voltage ϕs and decreases with the square root of thedoping concentration NA . We can simplify the above equation to:
√

ϕs
.
(3.7)
NA
In the case of the p-type semiconductor, the resistivity of the semiconductor
ρp−type and impurity concentration NA is related as:xd ∝ρp−type =

1
,
µp NA

(3.8)

where µp is the mobility of holes, which is the major carrier in a p-type semiconductor.
By combining Eq. (3.7) and (3.8), the relationship for the depletion depth (xd,Nch )for a p-type semiconductor is given by:√

xd,Nch = C µp ρp−type ϕs .

(3.9)

In the same way, the thickness of the depletion layer (xd,Pch ) for an n-type semiconductor equals:√

xd,Pch = C µn ρn−type ϕs ,(3.10)

CHAPTER 3. CHARGE-COUPLED DEVICE

26

where ρn−type and µn are the resistivity of an n-type semiconductor and the mobility of electrons, which is the major carrier of an n-type semiconductor, respectively.
C is the common constant in the above two equations. It can be clearly seen thatthe thickness of the depletion layer signiﬁcantly depends on the resistivity of a semiconductor and the gate voltage. In general, the relationship for the thickness of thedepletion layer is given by:xd ∝√µρϕs ,(3.11)

where µ, ρ represent the mobility of the major carrier and the resistivity of asemiconductor, respectively.

3.1.2

Surface-Channel and Buried-Channel CCD

As the model considered in the above calculation, a CCD consisting of a silicon
semiconductor (p-type Si), an insulator (SiO2 ), and a gate electrode (poly-Si) has
the potential well at the interface of the Si-SiO2 (see also Figure 3.2 (left)). Therefore
the generated charge is stored and transferred along the surface of the semiconductor.
We call the CCD with such a potential structure as a surface-channel CCD. Since thestructure of the surface-channel CCD is very simple, earlier CCDs were formed withthis structure. However, there is a major problem associated with surface-channel
CCDs. Since there are many trap levels at the Si-SiO2 interface, signal charges
transferred near the interface are often trapped and this makes the performance of
charge transfer ineﬃciency (CTI) severely worse.
To avoid this surface-state problem, the “buried-channel CCD” was invented.
As shown in Figure 3.2 (right), a thin doped n-type Si region is formed between theinsulator and p-type Si. Then, the potential well is formed in the n-type Si layer.
Since the generated charges are stored and transferred in the potential well whichlies beneath the surface, they are free from the trap levels present at the surface. In
contrast to the surface-channel CCD, the CTI performance for the buried-channel
CCD is remarkably high. Today, most CCDs are equipped with a buried channel,
including the CCDs described in this thesis.

3.1.3

Front illuminated and Back illuminated CCD

CCDs are classiﬁed into two types, “front-illuminated CCD” (FI-CCD) and ”backilluminated CCD” (BI-CCD). In the FI-CCD, the photons enter the CCD from theelectrode side. Though the surface electrode and insulator are very thin, the absorption of soft X-rays below 1 keV is not negligible. Figure 3.3 shows the properties of
the transmission of the surface electrode (poly-Si) and insulator (SiO2 ). The thickness is assumed to be 0.45 µm for the poly-Si and 0.35 µm for the SiO2 , which are

CHAPTER 3. CHARGE-COUPLED DEVICE27

the nominal value for the FI-CCD onboard the Suzaku satellite (XIS-FI). We canclearly see soft X-rays below 1 keV are signiﬁcantly absorbed.

Figure 3.3: (left) Transmission of the surface electrode (poly-Si) and insulator(SiO2 ). The former and latter are shown by the dotted and solid curves, respectively.
(right) Total transmission of the surface of XIS-FI.

In the case of the BI-CCD, the ﬁeld-free region, which is virtually insensitive forphotons, is removed. Since photons directly enter the depletion layer, the sensitivity
for soft X-ray photons is improved compared with the FI-CCD. Figure 3.4 shows
a comparison of the sensitivity for X-rays of XIS-FI and XIS-BI (back-illuminated
CCD onboard the Suzaku satellite). It can be seen that the sensitivity of the XIS-BI
for soft X-rays below 1 keV is higher than that of the XIS-FI [5].

Figure 3.4: Comparison of the sensitivities of XIS-FI (dotted curve) and XIS-BI(solid curve) [5].

CHAPTER 3. CHARGE-COUPLED DEVICE

3.2

28

Charge Transfer

A CCD is a monolithic array of closely spaced MOS capacitors that transfers ananalog signal charge from one pixel to another, working as an analog shift register.
In this section, we describe the principle and mechanism of charge transfer in a
CCD.

3.2.1

Principle of Charge Transfer

The charge stored within a pixel is transferred from pixel to pixel, and is measured
via a readout node. Combining the horizontal transfer and vertical transfer, we
can obtain information on all pixels arranged in a 2-dimensional array with only
one readout node. To help in our understanding, we show the simplest and anunderstandable analogy for the operation of the CCD in Figure 3.5 [23, 24];

Figure 3.5: Bucket analogy used to describe CCD operation [23, 24].
In a CCD, the transfer of charge is carried out by manipulating the voltage
applied to the gate electrode, instead of the belt conveyor in the bucket analogy.
We change the voltage periodically and form potential slopes in order to transfer
the charges. There are two methods for forming the potential slopes;• Multiple transfer electrodes
A few transfer electrodes are deposited on each pixel. Applying voltage to eachelectrode independently, we can form a potential slope as needed. Though thepattern of the voltage is complex, there is little restriction in the way of chargetransfer.

CHAPTER 3. CHARGE-COUPLED DEVICE29• Additional dopant
We dope impurities non-uniformly on each pixel to form a natural potential
slope. The number of electrodes in this case is reduced, and it simpliﬁes thepattern of the voltage. However, the charge transfer is limited to one direction,
which is determined by the spatial distribution of the additional dopant.
All the CCDs developed in this work and described in this thesis employ the “twophase transfer method”, which is a combination of the above two methods. We
show the scheme of the two phase transfer method in Figure 3.6.
1pixel

+ + +
+ +

clock

+ + +
+ +

+ + +
+ +

+ + +
+ +

+ + +
+ +

dopantelectron

Figure 3.6: Schematic view of the 2-phase transfer method.

Two electrodes are deposited on each pixel, and applied arbitrary voltages canbe applied independently to each of the two. Furthermore, an additional impurity
is doped on the half side of each pixel. By applying the pattern of voltages (clock
pattern) shown in Figure 3.6 to each pixel, the periodic change in the potential wellin a CCD transfers the signal charges.
For comparison, we also describe the other methods of charge transfer;• Virtual phase transfer
As shown in Figure 3.7(left), a single electrode, which is half the size of a
pixel, is deposited on each pixel [22, 20]. The applied voltage is strikingly
limited because the voltage potential of the other half side of the pixel is ﬁxed.
However, by leaving half the pixel open, this method allows photons to enter
this open area without signiﬁcant absorption by the electrode. The sensitivity
to photons which cannot penetrate the electrode signiﬁcantly is thus improved.

CHAPTER 3. CHARGE-COUPLED DEVICE

30

1pixel1pixel

+ + +
+ +

clock

clock

+ + +
+ +

+ + +
+ +

+ + +
+ +

+ + +
+ +

dopantelectronelectron

Figure 3.7: Schematic views of the virtual phase (left) and 3-phase (right) transfer
methods.

• Three-phase transfer
As shown in Figure 3.7(right), in this method, three electrodes are depositedon each pixel. Any additional impurity doping is unnecessary, allowing us
to freely select the direction of transfer by applying suitable clock patterns.
Thus, this method allows for bi-directional charge transfer.

3.2.2

Scheme of Charge Transfer

CCDs can be classiﬁed into 3 types based on the diﬀerence in the scheme of thecharge transfer; frame transfer CCD (FT CCD), full frame transfer CCD (FFTCCD), and interline transfer CCD (IT-CCD). A schematic view of each type of
CCD is shown in Figure 3.8. In this subsection, we describe the scheme of chargetransfer and characteristics of each type of CCD.
1. Frame Transfer CCD (FT CCD)
The FT CCD consists of an imaging area (IA) exposed to incoming light anda frame stored region (FS) shielded from light. At the end of the exposure,the charges acquired in the IA are quickly transferred to the FS. During thereadout of the charges transferred to the FS, new exposure can proceed on the
IA. Therefore the dead time due to the readout time is signiﬁcantly reduced
by adopting this transfer scheme. Furthermore, because the charges in the IA
are transferred in a short time, a mechanical shutter to shield the incoming
light in the transfer phase is unnecessary, in general. However, approximately
half of the CCD is shielded in the FS region, and therefore the eﬀective area
is smaller than that of other CCD types.

CHAPTER 3. CHARGE-COUPLED DEVICE

31[Frame Transfer][Full Frame Transfer]

1
Imaging Region

1

Serial Shift Region
Imaging Region
Frame Stored Region
Amp[Inter lineTransfer]

Serial Shift Region2
1

2
Imaging Region
Paralell Shift Region

Amp

Amp

2

3

Figure 3.8: The scheme of charge transfer in (upper left) full frame transfer CCD,
(right) frame transfer CCD, and (lower left) interline transfer CCD.

2. Interline Transfer CCD (IT CCD)
The IT CCD is a spatially constructed device in which each column of theimaging (active) pixels is paralleled by a light-shielded (inactive) column ofstorage [19]. At the end of the exposure, charges stored in each active column
are quickly shifted to its neighboring light shielded column. The shift occurs ina very short time (only 1 time transfer!), and therefore, a mechanical shutter
is unnecessary. The shifted charges can be transferred to a readout node while
the active columns are newly exposed.
However, the IT CCD is rarely employed for X-ray use. This is because the
X-ray photons easily penetrate the aluminum coating which is usually too thinto absorb the X-ray photons.
3. Full Frame Transfer CCD (FFT CCD)
In this case, the whole area of a CCD is an IA. Compared to the FT CCD
and IT CCD, there is no dead area. However, during the transfer phase, the
FFT-CCD needs to be shielded against incoming photons with the use of amechanical shutter.
Most of the CCDs developed in this study and described in this thesis employ
the FFT-CCD method except as noted. Only the “CCD-NeXT1” device is classiﬁed
as an FT-CCD (see chapter 6).

CHAPTER 3. CHARGE-COUPLED DEVICE

3.3

Charge Measurement

3.3.1

Structure of Output Ampliﬁer

32

The charges are transferred to a readout node using one of the above methods andthen ﬁnally converted to a voltage which corresponds to the amount of charges storedin each pixel. A typical example of a readout node of a CCD sensor is an output
with a ﬂoating diﬀusion layer, termed the ”ﬂoating diﬀusion ampliﬁer (FDA)” We
show a schematic view of FDA in Figure 3.9.

Figure 3.9: Schematic view of the ﬂoating diﬀusion ampliﬁer.

The scheme to measure the amount of charge using the FDA can be explained
as follows;
1. SG–Low, RG–High (Reset phase)
When the reset gate (RG) is clocked high, the drain voltage of MOSFET1(reset drain: RD) is equal to the source voltage (ﬂoating gate: FG). Then the
charge stored in the FG is transferred to the RD and is reset.
2. SG–High, RG–Low (Floating phase)
When RG is clocked low, FG is isolated to RD. The voltage of the FG at thisphase is called the “ﬂoating level”.

CHAPTER 3. CHARGE-COUPLED DEVICE

33

3. SG–Low, RG–Low (Signal phase)
When the summing gate (SG) is clocked low, the charge stored in the SG istransferred to the FG. The voltage of the FG is then decreased, and is called
the “signal level” at this phase. The measurement of the amount of charge
stored in a pixel is then completed.
The scheme backs in the reset phase [1], and the charge is measured in thenext pixel.
Figure 3.10shows the typical shape of the output signal (OS) produced by the
FDA. The amount of charges stored in each pixel reﬂects the diﬀerence in voltage
between those of the ﬂoating level and the signal level.

Reset

Floating Signal
Level
Level

Figure 3.10: Schematic view of the output signal produced by the FDA

The sensitivity of the output ampliﬁer of a CCD is deﬁned by:
SV =

q
,
Cs

(3.12)

where SV is the node sensitivity (V/e− ), q is the elementary charge (1.6×10−19 C),
and Cs is the sense capacitance (F) given by:
Cs = CM OS + CF D ,(3.13)

where CM OS is the gate capacitance associated with the output MOSFET and
CF D is the capacitance related to the ﬂoating diﬀusion. These capacitances are

CHAPTER 3. CHARGE-COUPLED DEVICE

34

essentially determined by the layout design of the FDA. The node sensitivity is theconversion coeﬃcient from the amount of charges to the voltage. Therefore, a highnode sensitivity yields a high tolerance for noise from the external system other thanthe CCD itself.

3.3.2

Mechanism of Charge Measurement

As described in the previous subsection, the stored charge in each pixel is reﬂectedin the gap between the voltage of the signal phase and the ﬂoating phase. Next, we
brieﬂy explain the major methods to measure the voltage gap.
1. Delay method
Figure 3.11 shows the scheme of the delay method. We divide the OS signal
into two, and delay one signal with a delay circuit so that the signal level of thedelayed OS signal overlaps with the ﬂoating level of the original one. Then,
we can detect the voltage diﬀerence using a diﬀerence circuit. This method ischaracterized by the combination of the fundamental electrical circuits.
TCY
UKIPCNUCORNKPI

FGNC[GF
UKIPCN

Figure 3.11: Schematic view of the delay method.
2. Correlated double sampling method (CDS method)
The scheme of the correlated double sampling method is shown in Figure 3.12.
We sampled the voltage of the ﬂoating and the signal phase by ADC in advance, and subtracted each level. By multiple sampling of each phase andaveraging, we can decrease the eﬀects of random noise. However, a high-speed
ADC is necessary to increase the number of samplings.
3. Double slope method
We show the scheme of the dual slope method in Figure 3.12. Using the
integrator circuit, we integrated the ﬂoating voltage level onto the integrating
capacitor during the timing 1 is high. After integrating the ﬂoating level, thesignal voltage level is “invertedly” integrated onto the capacitor during thetiming 2 is high. After that, the integrated signal shows the voltage level whichcorresponds to the gap of the voltage level between the signal and ﬂoatingphase.

CHAPTER 3. CHARGE-COUPLED DEVICE35

UCORNKPIUCORNKPI
Figure 3.12: Schematic view of the correlated double sampling method.
This method is very eﬀective for removing random noise and does not requirea fast ADC. However, the integration ﬁlter usually requires a very complicatedcircuit system, and there is a possibility that we are aﬀected by the noise of
the ﬁlter itself.

OStiming1
timing2
Integrated

sampling

sampling

Figure 3.13: Schematic view of the dual slope method.

3.4

Noise Sources

In this section, we describe the factor of noise sources which degenerate the spectroscopic ability of a CCD.

3.4.1

Readout NoiseThe readout noise is mainly generated in the MOS transistor which composes the
FDA (see Figure 3.9). The noise of the MOS transistor consists of a random thermalCHAPTER 3. CHARGE-COUPLED DEVICE

36

noise, 1/f (ﬂickering) noise, and kT C (reset) noise.
Thermal noise is generated by the thermal agitation of electrons in channel resistance in the MOS transistor. Therefore, this noise is observed at random and its
power spectrum density is constant independent of frequency. The square mean voltage of this noise (Vn2 ) converted to its gate voltage is approximated by the following
equation:
Vn2 =

8 kT
,
3 gm ∆f(3.14)

where k is the Boltzmann constant, T is the temperature, f is the frequency, andgm is mutual conductance.
The 1/f (ﬂickering) noise is caused by the capture and emission of electrons at
the interface trap level in a channel of the MOSFET. The square mean voltage ofthis noise (Vn2 ) converted to its gate voltage is approximated by this equation [29],Vn2 =q 2 d2ox nT e,ϵ2ox W Lf(3.15)

where nT e is the trap concentration in a channel, ϵox is the permittivity of a gate oxide, W andF are the width and length of a gate respectively, and q is the elementaryelectric charge. This equation contains a frequency f in the denominator, therefore,
the power spectrum density is proportional to 1/f . W and F are also present in the
denominator, meaning that the 1/f noise is governed by that caused in the smallest
MOSFET.
Since the 1/f noise increases as the frequency is lower, the CDS circuit signiﬁcantly attenuates the 1/f noise, because the frequency response of the CDS falls oﬀ
with decreasing frequency [72]. Reducing the 1/f noise through signal processing
via the CDS allows us to reduce the size of the on-chip ampliﬁer. This in turnincreases the node sensitivity (V/e− ) because we can reduce the gate capacitanceassociated with the MOSFET ampliﬁer.
Reset noise is generated by the periodic resetting of the readout node by the
MOSFET. Since the reset noise is generated every time we reset the readout node,
the ﬂoating voltage level is diﬀerent from pixel to pixel. The noise induced on thereadout node is caused by the channel resistance of the MOSFET used to reset the
FG. The voltage of this noise is shown as:√

kT,(3.16)
C
where k, T, and C are the Boltzmann constant, temperature, and capacitance of theFG.
This equation can also be expressed in units of electrons as,
Vn =CHAPTER 3. CHARGE-COUPLED DEVICE
√

Nn =

kT C
,
q

37

(3.17)

where q is the elementary charge (1.6 × 10−19 C). The reset noise expressed this way
is called the kT C noise.
However, the reset noise voltage will not signiﬁcantly change over a pixel period
after the reset switch is turned oﬀ (in other words, after RG is low). Therefore,
by diﬀerencing a sample of ﬂoating level and signal level, we cancel out the resetnoise because the noise amplitude does not change over the sampling period. In
other words, we can completely remove the reset noise by using the delayed method,correlated double sampling, and dual slope methods as described in the previous
section.

Dark Current
Dark current is intrinsic to semiconductors and occurs naturally through the thermal generation of minor carriers. If there are imperfections or impurities within
the semiconductor or the Si-SiO2 interface, the energy level occurs in the forbidden
bandgap. Then, some electrons are excited to the conduction band from the valenceband via the generated energy levels in the forbidden bandgap, mainly by the thermal motion, thereby promoting the dark current. We call this source “dark current”
because it is produced even when the CCD is in complete darkness.
There are three principle factors for the generation of the dark current;
1. thermal excitation and diﬀusion of charges in the ﬁeld-free layer below thepotential well,
2. thermal excitation of charges in the depletion layer,
3. thermal excitation of charges at the Si-SiO2 interface.
The contribution from surface states is the dominant source of the dark current.
However we can reduce this current with the MPP mode [24]. Since dark current ismainly caused by thermal excitation, we can signiﬁcantly reduce it by cooling the
CCD. The relationship between the dark current Idark and the operating temperature
T is given by the following equation [35]:Eg),(3.18)
2kT
where Eg is the bandgap energy of silicon and k is the Boltzmann constant.
Note that the bandgap energy of silicon Eg varies with operating temperature
following the empirical formula [52],Idark ∝ T 1.5 exp(−

CHAPTER 3. CHARGE-COUPLED DEVICE

Eg = 1.1557 −

3.4.2

7.021 × 10−4 T 2[eV].
1108 + T38(3.19)

Charge Transfer Ineﬃciency

Charge transfer eﬃciency (CTE) is a measure of how eﬀectively the CCD transfers
charge from one pixel to the next. The CTE is deﬁned as the ratio of chargetransferred from the target pixel to the initial charge stored in the target pixel.
In X-ray astronomy, CTE is speciﬁed in terms of charge transfer ineﬃciency (CTI).
CTI , on the other hand, is the fraction of charge left behind in a single pixel transfer
and is simply deﬁned as CT I = 1 − CT E.
CTI is mainly limited by traps located in the signal channel. The traps ina pixel capture a ﬁxed quantity of charge when the charges are transferred to the
pixel. Though the trapped charges are slowly released, they are added to the trailing
pixels. The time to release the trapped charge is relatively short, the events which
trailing in the direction of transfer are observed. On the other hand, when the time
is longer, it becomes diﬃcult to see such a tail. Therefore charges seem to be lost
gradually with increasing number of transfers.
Traps can be created during the design and manufacture of the chips, and tosome degree, they are also found in the starting silicon material (bulk), even before
a CCD is made.

3.4.3

Spurious Charge

Spurious charge is generated when a CCD is clocked into inversion[25]. When thegate voltage is clocked low (a negative voltage), a CCD is in an inverted phaseand some holes become trapped at the Si-SiO2 interface. When the gate voltage
is clocked high (a positive voltage), the CCD is switched to a non-inverted phaseand trapped holes are accelerated from the Si-SiO2 interface with suﬃcient energy
to create electron-hole pairs by colliding with silicon atoms. These “spurious ”
electrons are then collected in the nearest pixel and contaminate the original signal
charge.
To reduce the Spurious charge, we reduce the change in the electric ﬁeld which
gives the trapped hole large kinetic energy. We usually avoid the spurious charge by(1) reducing the voltage gap between the high phase and low phase, and (2) slowing
down the rise time from the low to the high phase.

3.5

X-ray Detection

In the X-ray band (<50 keV), the interaction between X-ray photons and silicon isdominated by the photoelectric eﬀect, as shown in Figure 3.14.

CHAPTER 3. CHARGE-COUPLED DEVICE

39

Atomic Number Z

Pb
Photo-electric effect

Pair production

Compton scatteringSi
H 2O
Photon energy hν [MeV]

Figure 3.14: Relative importance of the three major types of X-ray and γ-ray interactions. The line shows the value of Z and hν for which two neighboring eﬀects are
just equal [11].

When an X-ray photon is injected onto a CCD, it is absorbed by a Si atom
via the photoelectric eﬀect; the X-ray photon undergoes interaction with a Si atom
wherein the photon completely disappears. Then, an energetic photoelectron (aprimary electron) is ejected by the atom from one of its bound shells. The kinetic
energy of the primary electron is given by,
Ee− = Ex − Eb

(3.20), where Ex is the energy of the absorbed X-ray and Eb is the binding energy forK-shell electrons (1.74 keV for Si).
The primary electron produces an ionizing trail of electron and hole pairs (e-h)
through inelastic collisions with orbital electrons of other Si atoms. The primaryelectron produces one electron and hole pair per energy of Ee−h , which for siliconis 3.65 eV at room temperature [9]. The binding energy used to ﬁll the K-shellagain is either converted into another X-ray, or creates additional electron/holepairs through a mechanism referred to as the Auger process. In the latter case,the atom returns to the ground state when electrons from the outer shells drop to
inner ones as free electrons are ejected from the atom. The energy therefore appears
largely as kinematic energy of the ejected Auger electrons rather than energy in theform of an X-ray photon. The Auger process occurs with much higher probability
than the X-ray emission process. The energy of the absorbed X-ray photon (Ex )is completely transferred to a number of electron and hole pairs. The generatedx. If a Si X-ray is produced, it will likely ”escape” from the original
charge is EEe−hpoint of interaction. The net signal generated under these circumstances is only
Ex −Eb, creating an escape peak. The Si X-ray that is produced may be absorbed
Ee−hby the CCD at some other pixel location resulting in an X-ray (Si-K line) chargebor it may escape from the sensor entirely.
packet of EEe−hCHAPTER 3. CHARGE-COUPLED DEVICE40

The energy of the incident X-ray photons is estimated by measuring the numberof generated electron and hole pairs. As described above, one electron and hole pairis generated per 3.65 eV. Therefore, ∼ 103 electron and hole pairs are generated for
an X-ray photon of a few keV.
If all the energy of an X-ray photon was used to produce electron-hole pairs
directly, there would be no statistical variation in their amount of charge generated
by the X-ray. However, a ﬁnite amount of energy is transferred to the Si lattice
by non-electron-hole processes (e.g., thermal), giving rise to a small statistical difference in the number of electron-hole pairs actually generated. This uncertainty
is characterized by the Fano factor, originally formulated by U. Fano in 1947 todescribe the uncertainty of the number of ion pairs produced in a volume of gas
following the absorption of ionizing radiation.
Then, the statistical variation of the number of electron-hole pairs are correctedwith the Fano factor F as follows,√

Ex,(3.21)
Ee−h
where Ex is the photon energy. Experimentally, the Fano factor has been determined
to be 0.12 for silicon.
In general, the energy resolution is determined by the root mean square of thestatistical variation of electron-hole pairs (σx ) derived in Eq. 3.21 and the system
noise in electrons (σnoise ) as follows,
σx =

√

F×

√

Ex
+ σnoise 2 [eV].
(3.22)
Ee−h
Examination of Eq. 3.22 reveals that the system noise must be minimized in order
to exploit the potential for X-ray spectroscopy oﬀered by the silicon properties.
Reducing system noise to ∼0 electron results in Fano noise-limited performance
down to the energy resolution (FWHM) of 120 eV for an X-ray of 5.9 keV.
∆ E(F W HM ) =3.6

8 log 2 × Ee−h ×

F×

Data analysis

The output signal (OS) of a CCD includes information not only of the signal charge
generated by the incident X-rays but also of the oﬀset level caused by the darkcurrent and Spurious charge, as well as variations caused by the readout noise, CTI,
and shot noise of the oﬀset. Since the OS is converted to digital data by an ADC,
we need to (1) properly subtract the oﬀset level from the digital data to estimate
the energy of incident X-rays, and (2) evaluate the noise level to conﬁrm whether
we are driving the CCD properly.
In this section, we describe the methods for (1) extraction of X-ray events, and(2) estimation of the performance of a CCD.

CHAPTER 3. CHARGE-COUPLED DEVICE

3.6.1

41

Output image of CCD

Figure 3.15 shows the pattern diagram of a typical output image of a CCD. The
pixels in the “active region“ are really exposed during the exposure time and they
contain the digital value corresponding to the charge stored in the exposure andtransfer time. In later, we refer to the digital value in each pixel as the “raw pulse
height (RPH)”. On the other hand, when we repeat the transfer over a number of
CCD pixels, the vertical overclocked region (VOC) and the horizontal overclocked
region (HOC) are available. These overclocked regions are useful to evaluate the performance of a CCD. The RPH of pixels in the HOC corresponds to the charge whichis generated only in the horizontal transfer. The RPH of pixels in the VOC corresponds to the charge which is generated only in the horizontal and vertical transfer.
We describe the evaluation of a CCD in detail in the following subsection 3.6.3.

44

VOC

256

Active

HOC

1040

60

Figure 3.15: Pattern diagram of the typical output image of a CCD. We repeat the
charge transfer 1200 × 300 times for a CCD of 1040 × 256 pixels.

3.6.2

X-ray event analysis method

The RPH in each pixel has some oﬀset level, even if no X-ray photon enters, whichis caused by the dark current, spurious charges and the electrical oﬀset by the FDA.
First, we subtract the oﬀset level from the RPH in each pixel. The oﬀset levelin each pixel is determined by averaging the RPH using multiple images obtained
under the same condition. By subtracting the oﬀset level from the RPH, we obtainthe “pulse height” (PH) in each pixel. The PH represents the amount of charges
attributed to be X-ray events.
Secondly, we recognize X-ray events from the images containing the PH in eachpixel and estimate the incident X-ray energy. As described above, an X-ray photon
absorbed in a CCD generates a number of electrons. While these electrons are pulledCHAPTER 3. CHARGE-COUPLED DEVICE42

up to the CCD surface by the electric ﬁeld in the depletion layer, they are spread bythermal diﬀusion, and in some case they are distributed over multiple pixels. Since
the number of charges is proportional to the incident X-ray energy and corresponds
to PH, we have to sum up all the PH spread over multiple pixels. We use two
methods to carry out this reconstruction, one is the “grade method” and the otheris the “ﬁtting method”. In the following, we describe the basic concepts of thesemethods.
3.6.2.1

Grade Method

The grade method is the standard X-ray events analysis method applied to CCD
detectors used in the ASCA, Chandra, XM M , and Suzaku satellites. In thismethod , we sum up all the charges spread over multiple pixels and distinguish
X-ray events from others using the following scheme;
1. We pick up pixels which have PH beyond the “Event threshold” and we callthese “event pixels”
2. For the charge distribution of 3 × 3 pixels surrounding the event pixel, we
pick up pixels which have the PH beyond the “Split threshold”. We call these“split pixels”. Hereafter, we ignore the pixels which are judged to be neither
event pixels nor split pixels.
3. Based on the position pattern of the event pixels and the split threshold, we
classify each X-ray event into eight grades (Figure 3.16).
4. We sum up the PH of the event pixels and split pixels. We note that we do
not add the PH of some split pixels in some grades.

3.6.2.2

Fitting Method

The ﬁtting method is employed as an alternate solution. For example, this method
is usually employed to rectify some X-ray events which are too widely spread toreconstruct using the grade method. We ﬁt the elliptical distribution of the PH
on the CCD surface with a two-dimensional non-spherical Gaussian function (seeFigure 3.17) [43].
(X − x) cos θ + (Y − y) sin θ2 (X − x) sin θ + (Y − y) cos θ2N orm.
exp −Z(X, Y ) = √2πσx σy2σx2
2σy2(3.23), whereCHAPTER 3. CHARGE-COUPLED DEVICE

43

Figure 3.16: criteria of grading pixels into 8 grades. The events that meet none of
the criteria (in other words, the more spread ones) are labeled grade 7.

CHAPTER 3. CHARGE-COUPLED DEVICE

44

1. N orm. : the incident X-ray energy as the total volume of the best ﬁt function
corresponds to it.
2. x, y : the position of the incident X-ray events.
3. σx , σy : ellipticities of the PH distribution.
4. θ : rotation angle between the horizontal axis of the CCD chip and the longer
axis of the ellipse.
The total pulse heights (Z) of each 5 × 5 pixels around the center pixel arecalculated based on the position of each pixel (X, Y ) using Eq. 3.23, and are ﬁt to
the real data. The ﬁtting method has several advantages over the grade method.
(1) This method can be applied to CCDs of any pixel size.
(2) By using this method, we can make use of more spread events, often caused
when X-ray photons of higher energy are injected and absorbed in the ﬁeld-free
region, by compensating for the lost charges. These spread events are labeled as
grade 7 and discarded when the grading method is used. Thus, we can get higher
detection eﬃciencies, especially in the higher energy band, without signiﬁcantly
degrading the energy resolution.
(3) We can analyze polarized X-ray events using this method. Tsujimoto et al.
(2000) [64] showed that the ﬁtting method can determine the degree of polarizationand the rotation angle of the polarized X-rays detected by the X-ray CCD.
(4) We do not need a split threshold anymore.
Kohno et al. (2000) [30] successfully measured the relationship between theextent of an event and the X-ray absorption depth. Using this, they showed thatthe thickness of the CCD depletion layer can be measured without calibrating theabsolute ﬂux of the monochromatic X-rays.

3.6.3

Estimation of Noise Level

In this subsection, we describe the method to determine the noise level of the readout noise, the dark current, the CTI and the spurious charge. As described in
section 3.6.2, the PH distribution gives us diﬀerent information in each region of
Active, VOC, and HOC.
We summarize the noise sources aﬀecting the PH in each region as follows;• Active region · · · readout noise + spurious charge + CTIv +CTIh + darkcurrent• VOC region · · · readout noise + spurious charge + CTIv +CTIh• HOC region · · · readout noise

CHAPTER 3. CHARGE-COUPLED DEVICE

45

Figure 3.17: Schematic view of the ﬁtting method. The charge distribution of 5 × 5
pixels are ﬁt with a two-dimensional Gaussian function.

In the following, we describe the method used to estimate the noise level ina concrete form. Except as noted, our derived noise levels of test devices (seechapter 6) are calculated in the following way.
Readout noise The readout noise is derived from the PH distribution in the HOC
region since PH in the HOC region suﬀers less from noise sources other than thereadout noise. We deﬁne the readout noise as the standard deviation (converted to
the number of electrons) of the PH distribution in HOC;
σr.o.n [e− ] = (PH standard deviation of in HOC[ADU])
gain[eV/ADU]×[electron]
3.65 eV/e−(3.24)

Dark current The dark current is derived from a comparison of the PH distribution between VOC and the active region. The essential diﬀerence between these tworegions is whether they are exposed or not. Therefore the gap of the peak channelsof the PH distribution reﬂect the dark current.

Idark =

peakchanne(VOC) − peakchannel(Active)[ch](Exposure time[sec])gain[eV/ADU][electron/sec/pixel]
×3.65 eV/e−(3.25)

CHAPTER 3. CHARGE-COUPLED DEVICE

46

CTI The CTI is derived from a comparison of the X-ray spectra in diﬀerent areasin the Active region as shown in Figure 3.18. We divided the active region into 2 × 2
areas. We obtained the spectra of the X-ray photons in each area and compared thepeak channels of the spectra.

1

2

3

4

read out
Figure 3.18: Division of the active region to measure CTI.

[

CTIvertical1(peakchannel(3) − peakchannel(1))
1=
2 No. of pix. in vertical(peakchannel(3))](peakchannel(4) − peakchannel(2))+[pixel−1(3.26)](peakchannel(4))[

CTIhorizontal

1
1(peakchannel(2) − peakchannel(1))=
2 No. of pix. in vertical(peakchannel(2))](peakchannel(4) − peakchannel(3))+[pixel−1(3.27)](peakchannel(4))

Spurious charge The Spurious charge is derived from a comparison of the PH
distribution between VOC and HOC. The essential diﬀerence between these twoareas is whether they are transferred vertically or not. Therefore the gap of thecenter channels of the PH distribution reﬂects the Spurious charge mainly generated
by the vertical transfer.

CHAPTER 3. CHARGE-COUPLED DEVICE

spurious charge = [(peakchannel(VOC)) − (peakchannel(HOC))]gain[eV/ADU]×[electron]
3.65 [eV/e− ]47(3.28)

Chapter 4Soft X-ray Imager
4.1

Role of Soft X-ray Imager

As shown in Table 2.1, the SXI covers the energy band of 0.5–20 keV. As describedin the previous section, since non-thermal emission begins to dominate in the energy
range over 10 keV, the imaging and spectroscopic observations of SXI in the energy
band of 10–20 keV is invaluable to explore and investigate the nature of non-thermal
emission precisely. Therefore, the detector for SXI is required to have excellent
imaging and spectroscopic abilities in this energy regime.
From the view-point of the X-ray CCD being equipped with high spatial andenergy resolutions, and moderate time resolution simultaneously, it is diﬃcult toadopt other X-ray detectors with the exception of the X-ray CCD for SXI. An
X-ray CCD, as the focal plane detector for X-ray astronomy missions, was ﬁrst
used in the ASCA satellite which was launched in 1993. Since then, the X-ray
CCD has been established to be one of the standard focal plane detectors and has
been adopted by most X-ray observatories, including Chandra, XM M -N ewton,
and Suzaku. Therefore, the X-ray CCD has yielded actual results as a detector
carried on satellites. The major weak point of this CCD however is its relatively
low sensitivity to hard X-rays. This is because the CCD has silicon which is a low
atomic number (Z) element. To compensate for this weak point, a CdTe detector
is added behind the CCD in the WXI. Since the CdTe detector consists of high-Z
matter, its sensitivity to hard X-ray photons is much higher compared to a CCD.
Figure 4.1 shows a comparison of the sensitivity for a CCD (300-µm thickness) andCdTe (0.5- and 1-mm thickness) . Even a CdTe detector with a thickness of 0.5 mm
provides good detection eﬃciency for the hard X-ray band up to 80 keV.
However, the position resolution of the CdTe pixel detector is > 200 µm[12, 59],
which is worse than that of an X-ray CCD, and in addition, the energy resolution forthe soft X-ray (< 10 keV) band is also worse than that for an X-ray CCD. Therefore,
a CCD is still suitable for imaging and as a spectroscopic detector for X-rays below∼ 20 keV.
48

CHAPTER 4. SOFT X-RAY IMAGER

49

Figure 4.1: Comparison of the sensitivity between a CCD (300-µm thickness) andCdTe (0.5- and 1-mm thickness).

4.2

CCD for Soft X-ray Imager - Back Supportless CCD -In this section, we ﬁrst discuss the requirements for a CCD adopted as the SXI.
Based on these requirements, we examine the development plan for the SXI.
To detect non-thermal X-ray emission with high sensitivity, we need to improvethe detection eﬃciency in the energy band above 10 keV as much as possible. The
detection eﬃciency of a CCD for X-rays with energies > 10 keV strongly dependon the thickness of its depletion layer (see also section 3.1.1). Figure 4.2 shows therelationship between the thickness of the depletion layer and the detection eﬃciency
of a CCD. It can be clearly seen that the detection eﬃciency for hard X-rays of
10 keV–20 keV improves signiﬁcantly with increasing thickness of the depletion
layer. Therefore, the thickness of the depletion layer is a key parameter for thedevelopment of the SXI.
Another key parameter is the thickness of the ﬁeld-free region. The CdTe pixel
detector (HXI) is present below the SXI to detect hard X-rays which cannot be
detected by SXI. However, a standard CCD has a ﬁeld-free layer other than thedepletion layer. Though the depletion layer is sensitive to X-rays, the ﬁeld-free layer
is insensitive to them. Therefore, a fraction of the hard X-rays which cannot be
detected by the CCD is absorbed and lost in the ﬁeld-free region, in other words,
this fraction cannot reach the HXI. Since a standard CCD has a thick ﬁeld-free layer

CHAPTER 4. SOFT X-RAY IMAGER

50
80um-depl
200um-depl
300um-depl

Efficiency

1

0.1

5

10

15
20
Energy[keV]

25

30

Figure 4.2: Relation between the detection eﬃciency and the depletion layer thickness of the CCD. The red, green and blue lines shows the detection eﬃciency in thecase of the 80 µm, 200 µm, and 300 µm-thick depletion layers, respectively.

of > 600 µm (cf. the thickness of the depletion layer is <80 µm), the loss of hard
X-rays is not negligible. The ﬁeld-free region of the CCD for SXI is required to bethinned down as much as possible. Furthermore, the CCD is usually sustained bya package which consists of plastic, ceramic plate, or so on. This package is usually
too thick to allow X-ray photons to pass through, and therefore, we need to get ridof this completely as well.
Front Illuminated CCD

Back Illuminated CCD

Back Supportless CCD

depletion layer

depletion layer

gatedepletion layer
field-free layer
packagegate

packagegate

package

Figure 4.3: Schematic structures of various types of CCDs. (left): normal front
illuminated CCD. (center): back illuminated CCD. (right): (back-illuminated) BSCCD.

Therefore, in order to realize the WXI, two characteristics are primarily required
for the SXI which is equipped at the upperstep;CHAPTER 4. SOFT X-RAY IMAGER

51

1. to detect soft (< 20 keV) X-rays as much as possible, in other words, toincrease the thickness of the depletion layer, and
2. to allow hard (> 20 keV) X-rays to pass through the CCD wafer, in otherwords, to decrease the thickness of the ﬁeld-free region and to remove thepackage.
To satisfy these requirements, we decided to develop a new-type of X-ray CCD“Back supportless CCD (BS-CCD)” for the SXI. Our aim was to realize a
BS-CCD that has a thick depletion layer and no ﬁeld-free layer, and package whichwastefully absorb X-rays. We anticipated that we needed to develop the BS-CCD
gradually, in order to conﬁrm (1) whether such a back supportless structure could
be realized, and (2) whether we could produce a BS-CCD with a stable and reliable
performance.
In cooperation with Hamamatsu Photonics K.K. (HPK), we have been researching and developing X-ray CCDs for over 10 years. Thus far, we have successfully
developed two types of X-ray CCDs at the top international level, the “CCDCREST”[4] and the “MAXI-CCD”[63, 39]. Table 4.1 shows a comparison of theperformance of the CCDs manufactured by HPK with that of the XIS onboard theSuzaku satellite, which has the world’s highest performance among X-ray CCDs.
Table 4.1: Comparison of Suzaku XIS, CCD-CREST, and MAXI-CCD

Readout noise (r.m.s.)Energy resolution1
Size of imaging area
Size of pixel
Format2
No. of readout node
Depletion layer
Illumination typeTransfer method
Surface coating
1

[e-]
[eV]
[mm2 ][µm2 ][pix2 ][µm]

Suzaku XIS
2–3
130–140
24.5 × 24.5
24 × 241024 × 1024 × 2
445(BI), 70(FI)BI, FI
FT(none)CCD-CREST
3–5
135
24.5 × 24.5
24 × 241024 × 1024 × 2
2
70
FI
FT
Al

CCD-CREST2
3–5
135
24 × 21.6
12 × 122000 × 1800 × 2
2
70
FI
FT
Al

MAXI-CCD<5
135
24.5 × 24.5
24 × 241024 × 1024
1
80
FI
FFT
Al

F.W.H.M. of Mn K-α (5.9-keV) line. 2 Sum of imaging area and stored area (except for MAXI-CCD)

We decided to develop the BS-CCD, continuing our cooperation with HPK, forthe following reason: It is expected that a number of trial and error steps are needed
for the development of the BS-CCD. Therefore, it is rational to select a domestic
manufacturer, considering the need for close communication and conferences. In
other words, to conﬁrm (1) and (2), HPK was considered to be the best partner
manufacturer for developing the BS-CCD. Furthermore, there was no doubt that
HPK can produce a CCD with world class performance. In fact, the MAXI-CCD

CHAPTER 4. SOFT X-RAY IMAGER

52

has been carried on the Hayabusa satellite [66] and is being driven normally inthe space environment. Therefore, it has already been conﬁrmed that the technicalcapabilities of HPK are suﬃciently high in order to manufacture CCDs to be carriedon satellites.
Prior to development, it is necessary to decide the goal speciﬁcations of the BSCCD. First, we examine the size of a CCD chip and the back supportless area toallow hard X-rays to pass through.
Since SXI is the best detector in terms of imaging ability on the N eXT satellite,
we need to enlarge the size of the imaging area of the CCD as much as possible
in order to cover as wide a ﬁeld of view (FOV) as possible. Figure 4.4 shows thevignetting function of HXT [47]. The vignetting function shows the relationshipbetween the eﬀective area and the oﬀ-axis angle. In focusing optics, since the reﬂectivity of the mirror assembly decreases with increasing oﬀ-axis angle, the vignetting
function shows a hat-like shape, as indicated in Figure 4.4. We note that we normalized the eﬀective area by that at the on-axis in Figure 4.4.

OOZOO+#OOZOO+#Figure 4.4: The vignetting function of HXT [47].

Based on this vignetting function of HXT, we determined the size of a CCD for
SXI. We ensured a wide FOV at least up to the oﬀ-axis angle where the normalized
eﬀective area at 8 keV is 50 %. Since the focal length between the SXI and HXT
is 12 m, 1 arcmin corresponds to 3.4 mm-length at the focal plane. Assuming
the IA size is 25 × 25 mm2 which is equal in size to that of the XIS, the FOV is∼ 7 × 7 arcmin2 . As seen in Figure 4.4, since the normalized eﬀective area at 8 keV

CHAPTER 4. SOFT X-RAY IMAGER

53

is still maintained high (60 %), we need to enlarge the size of IA in order to makeeﬃcient use of HXT performance. The IA size of 50 × 50 mm2 , which is 2 times
wider than that of XIS, corresponds to the FOV of ∼ 15 × 15 arcmin2 . This FOV
can be achieved (1) by enlarging the IA size of one CCD chip, or (2) by arranging
2 × 2 CCDs of 25 × 25 mm2 size in a mosaic-shaped pattern. Then, the normalized
eﬀective area at 8 keV is decreased to ∼30 %, therefore we concluded that the IA
size of 50 × 50 mm2 is a suitable size for the HXT.
The width of the vignetting function is narrower as the X-ray energy is higher,
therefore, hard X-ray photons rarely come on the rim of an SXI. Furthermore, thesize of the HXI is planned to be 20 × 30 mm2 . Therefore, we decided to remove thepackage only at the center of the imaging area. From the view point of mechanicalstrength and handling, this is clearly a safe approach compared to take the backsupportless structure at the whole area of imaging area. Based on the size of the
CdTe detector, we made a hole with 30 mm-diameter in the package at the centerof the IA.
It is hard to obtain a CCD chip with a large size such as 50 × 50 mm2 . Since
the size of most established X-ray CCD is limited to 25 × 25 mm2 , it is easily
expected that we will face the problem of low process yield when we try to produce
a CCD chip which has 4 times larger size than established ones. To cover such alarge area, we have a method wherein we arrange CCD chips in a mosaic-shaped
pattern (a mosaic CCD camera). This is a reliable method adopted for the Chandra
and XM M -N ewton satellites. Figure 4.5 shows pictures of the focal plane of the
XM M -N ewton (left) and Chandra (right). Though the IA size per CCD chip inboth cameras is ∼ 25 × 25mm2 , a mosaic of several CCDs can assure a wide FOV
which is over 2 times wider than that of a single CCD chip. However, as seen in
Figure 4.5, we cannot avoid the gap between each CCD chip.
Our objective is to fabricate a large-sized CCD with an IA of ∼ 50 × 50 mm2 .
However, in order to reduce the risk of failure in its development, we also planned tofabricate a medium-sized CCD with a ∼ 25 × 25 mm2 IA. If the development of thelarge-size CCD is not found to be promising, we will adopt a 2 × 2 mosaic with themedium-sized CCDs. We note that, in the case of the fabrication of the large-size
CCD, we need to process four of the same CCDs, electrically separated from eachother, on the same wafer. This is because in this way, we diversify the risk that thewhole of the CCD could break down at once, by imparting electrical redundancy tothe system.
Next, we examine the performance required for the BS-CCD. As described above,
the key parameters are the thickness of the depletion layer and the ﬁeld-free layer.
Based on these for developing the CCDs, we established a “Baseline plan” and a”Goal plan” to develop the BS-CCD. The former is a conservative plan in which
the number of new development items is comparatively small. On the other hand,
the latter is an innovative plan aimed at progressively high performance for the BS-

CHAPTER 4. SOFT X-RAY IMAGER

54

Figure 4.5: Examples of a mosaic CCD camera. (Left) A picture of the focal planeof XM M -N ewton EPIC-MOS. There are seven FI-CCDs. The size of IA is ∼25 × 25mm2 , so that a mosaic of seven covers a focal plane 62 mm in diameter.
(Right) A picture of the focal plane of the Chandra ACIS. A central mosaic of four
FI-CCDs is the imaging array. The size of IA is ∼ 25 × 25mm2 , so that a mosaic ofseven covers the focal plane of ∼ 50 × 50 mm2 -square.

CCD. We show schematic views of the BS-CCDs produced with the baseline andthe goal plan in Figure 4.6
In the baseline plan, we develop the BS-CCD based on CCDs already developedby HPK as well as by us. We selected an FI-CCD, since CCDs with a thick depletion
layer (70 − 80 µm) were fabricated as FI-CCDs (see Table 4.1). For reasons related
to manufacturing, it is diﬃcult to fabricate BI-CCDs with a thick depletion layerusing the conventional method. Additional details are included in the following
section 6.1.2.
In the goal plan, however, we have the challenge to develop a BI-CCD with athick depletion layer of 200 µm. To achieve such a device, it is necessary to change
the type of wafer from a p-type semiconductor, used conventionally, to an n-typesemiconductor. Though this change is drastic and has the risk of failure in development, the n-type semiconductor has the potential to realize an unprecedentedly
thick depletion layer. We will describe the details of these plans in the next section.
The goal for the readout noise and energy resolution is < 5 e− and 135 eV at
5.9 keV, respectively. Judging from the technical achievements of CCDs fabricatedby HPK (see Table 4.1), these goals are considered to be achievable.
Furthermore, we aim to coat the surface of a CCD chip with aluminum. Since
the CCD is also sensitive to optical photons, the aluminum evaporation plays therole of a ﬁlter for optical blocking. This is also useful to simplify the structure of
the SXI because we no longer need to assemble additional optical blocking ﬁlters.
In the case of the Suzaku XIS, since it has no surface Al coating layer, it needs
an optical blocking ﬁlter, making the structure of the XIS to become complicated.
As described in chapter 7, aluminum evaporation has the advantage of signiﬁcantly

CHAPTER 4. SOFT X-RAY IMAGER

55

C (TQPVKNNWOKPCVGF%%&D $CEM5WRRQTVNGUU%%&
=$CUGNKPG?

2CEMCIG

:TC[

:TC[

VJKPPKPI)CVG(KGNF(TGG4GIKQP&GRNGVKQP.C[GTE $CEMKNNWOKPCVGF%%&:TC[

F $CEM5WRRQTVNGUU%%&
=)QCN?

:TC[

Figure 4.6: The schematic structures of various types of CCDs. (a) Front-illuminated
CCD. (b) Front illuminated BS-CCD (the baseline plan). (c) Back illuminated CCD(d): Back-illuminated BS-CCD (the goal plan).

CHAPTER 4. SOFT X-RAY IMAGER

56

reducing the heat input to a CCD chip by thermal radiation. HPK has alreadysuccessfully developed a CCD with a surface Al coating (see Table 4.1), therefore
this goal is also considered to be very promising.
The goal performance of the SXI is summarized in Table 4.2. For comparison,
we also show the performance of the Suzaku XIS.
Table 4.2: Comparison of the performance of the baseline and goal plans.
Item
Readout noise
Energy resolution†
Wafer type
Imaging area
Back supportless area
Pixel size♭
Depletion layer
Field-free layer
Illumination typeSurface coating

Baseline plan Goal plan
< 5e−
135 eVN channel
P channel
50 mm × 50 mm
30mmϕ
24 × 24 µm2
70–100 µm
200 µm50–80 µm
0 µm
FI
BI
Al
Al

Suzaku XIS< 3e−
130 eVN channel
24.5 mm × 24.5 mm(none)
24 × 24 µm2
70/45 µm (FI/BI)> 500/0 µm (FI/BI)
FI/BI(none)†: Energy resolution for X-ray of 5.9 keV (FWHM).
: Size of Imaging area per a CCD chip.
♭: Values in active region.
‡

4.2.1

Baseline Plan

Most X-ray CCDs onboard X-ray observatories as focal plane detectors are “Nchannel CCDs” (Nch CCDs). Nch CCDs are fabricated on a p-type silicon semiconductor. Since the major carrier in a p-type semiconductor is a hole, the electron is
the transferred carrier. The mobility of electrons is higher than that of holes (seeFigure 4.7), so p-type semiconductors have an advantage in terms of high speedtransfer. Therefore most consumer products and CCDs for scientiﬁc use employ
Nch CCDs.
Our aim is to achieve the baseline plan based on the established method of
fabrication of the Nch CCD as the CCD-CREST and MAXI-CCD in cooperation
with HPK. First, in order to conﬁrm the realization of the BS-CCD to investigate any problems in the production process, we process and evaluate a small-sized(12 × 12 mm2 ) test CCD model with the same wafer used in the MAXI-CCD and
the CCD-CREST/CREST2. After conﬁrming that there is no performance degradation due to the wafer thinning process, we construct an evaluation model “CCDNeXT1”. This device is a medium-sized (24 × 24 mm2 ) CCD which can be loaded
on a satellite. After the development of the CCD-NeXT1, we fabricate a ﬂight

CHAPTER 4. SOFT X-RAY IMAGER

57

Figure 4.7: Electron and hole mobility as a function of impurity concentration.
The electron mobility is about 3 times larger than that of holes independent of theconcentration of the impurity.
model “CCD-NeXT2”, which will be carried on the satellite. As per the plan, the
CCD-NeXT2 is also medium-sized (24 × 24 mm2 ). In order to arrange them next
to each other with as little a gap as possible, the CCD-NeXT2 will need to have abuttable structure with a dead region less than 250 µm wide on three sides. Then,the gap between the adjacent two CCDs is restrained to ∼ 500 µm. We note thatsince the CCD-CREST/CREST2 and the MAXI-CCD have a buttable structure,
we consider that there should be no problem in the fabrication of a buttable CCD.
Figure 4.8 shows the picture of a mosaic camera with 8 CCD-CRESTs. Thanks
to a 3-side-buttable structure, this camera achieved a large-size imaging region of
50 × 100 µm2 .

4.2.2

Goal Plan

The other candidate for the SXI is the P-channel CCD (Pch CCD). This device isfabricated with a MOS-type CCD on an n-type silicon semiconductor. Thanks to thehigh-resistivity of the n-type silicon, the Pch CCD has the potential to have a thicker
depletion layer than the Nch CCD. The Pch CCD was ﬁrst fabricated by a groupat the Lawrence Berkeley National Laboratory (LBNL)[14][18]. They fabricateda back-illuminated and fully depleted CCD on high resistivity (10k – 12kΩ · cm)
n-type wafer. They reported the thickness of the depletion layer in their CCD tobe ∼ 300µm, which is ∼ 4 times thicker than that of established MOS CCDs suchas in Chandra ACIS, XM M -N ewton EPIC-MOS and Suzaku XIS. Furthermore,
the wafer is fully depleted, in other words, no ﬁeld-free layer exists. Therefore,
it is very easy to have the back-illuminated structure. This device acquires highsensitivity for low-energy X-rays (soft X-rays) by the back-illuminated structure

CHAPTER 4. SOFT X-RAY IMAGER

58

CCD-CREST

FS

50mm

IA

100mm

A mosaic camera of 8 CCD-CRESTs
Figure 4.8: A mosaic camera with 8 CCD-CRESTs. Thanks to the 3-side-buttablestructure, a large-sized imaging region of 50 × 100 mm2 is achieved.
and high sensitivity for high energy X-rays (hard X-rays) by the thick depletion
layer, simultaneously.
Judging from the device fabricated by LBNL[14], the Pch CCD is ideal for the
BS-CCD, if it can be realized. However, there are no actual results for the Pch
CCD as a sensor for astronomy observations. In Japan, a group of the National
Astronomical Observatory of Japan (NAOJ) is developing the Pch CCD as a fullydepleted CCD for an infrared and optical sensor [27] in cooperation with HPK. They
are developing the Pch CCD for use in the HyperSuprime[31], which is one of the
focal plane detectors of the Subaru telescope. The Pch CCD is also an attractive
image sensor in the optical band because the thick depletion layer depth greatly
improves the sensitivity for longer wavelengths (> 700 nm). In fact, Kamata etal. (2004) reported a test device of the Pch CCD which achieved a high quantum
eﬃciency of > 60% at 1 µm, which is roughly 5∼6 times higher than that of ordinary
CCDs [27]. We have adopted the Pch CCD for the goal plan of the SXI and toparticipate in the development which is proceeded by NAOJ.
First, we evaluate the performance of test models of Pch CCD. In particular, weneed to conﬁrm whether (1) the depletion layer is thicker than that in the Nch CCD,
and (2) the wafer is fully depleted. After conﬁrming that there are no problems inthe small-sized test devices (12 × 12 mm2 and 24 × 6 mm2 ) and that the achievementof excellent performance, which meets the demands for the goal plan, is reliable, weCHAPTER 4. SOFT X-RAY IMAGER

59

Figure 4.9: Structure of the LBNL Pch CCD[14]. The gate structure, on top of
the insulating oxide and protective nitride layers, is conventional, as is the buried
channel. A bias voltage on the back window/ electrode depletes the entire substrate.

CHAPTER 4. SOFT X-RAY IMAGER/GFKWOUK\G%%& %%&0G:6

(5

+#

60

.CTIGUK\G%%& %%&0G:6

(5

(5

+#

(5

(5

Figure 4.10: Schematic view of the medium-size CCD (left) and large size CCD(right). The large imaging area of 50 × 50 mm2 is realized by (1) the mosaic of 2 × 2
medium-size CCDs or (2) single large-size CCD.
will proceed with the development of the Pch CCD with the following steps; (1)We fabricate a medium-sized (25 × 25 mm2 ) Pch CCD as the “CCD-NeXT3”. This
device will be fully depleted and the thickness of the depletion layer will be 200 µm.
We fabricate this using the process established for the test device, and conﬁrm
that there is no problem in performance even if the size of the CCD chip becomeslarge. (2) After the successful development of the CCD-NeXT3, we will fabricate alarge-sized (50 × 50 mm2 ) Pch CCD as the “CCD-NeXT4”. As in the case of the
CCD-NeXT3, this device will have a fully-depleted wafer of 200 µm-thickness. As
described in the above section, we will fabricate four of the same CCDs, which areelectrically isolated from each other, on a single wafer. There is the possibility that
we will fail in the development of the CCD-NeXT4 because it is an unprecedented
large-sized CCD. In this event, we will adopt the CCD-NeXT3 for the SXI. As shownin Figure 4.10, the mosaic of the 2 × 2 medium-size CCDs covers the required large
imaging area of 50 × 50 mm2 . In Table 4.3, we summarize the comparison of theevaluation and ﬂight models of the baseline and goal plans.

CHAPTER 4. SOFT X-RAY IMAGER

61

Table 4.3: Comparison of the speciﬁcations of the evaluation/ﬂight models of thebaseline and goal plans
Code
Plan
PurposeReadout noise
Energy resolution†
Imaging area ‡Format♭Pixel size♭
No. of nodes
Depletion layer
Field-free layer
Illumination type
Buttable
Transfer method
Surface coating†

CCD-NeXT1 CCD-NeXT2Baselineevaluation
ﬂight

24 × 24 mm22000 × 2000
12 × 12 µm2
2
80 µm
70 µm
FI
N/A
FT
Al

CCD-NeXT4

Goal
evaluation/ﬂight ﬂight< 5e−
135 eV2
25 × 25 mm
25 × 25 mm2
50 × 50 mm21024 × 1024
1024 × 1024
2048 × 2048
2
2
24 × 24 µm
24 × 24 µm
24 × 24 µm2
2
48
70–100 µm
200 µm200 µm50–80 µm
0 µm
0 µm
FI
BI
BI
3-side3-side
N/A
FT
FT
FT
Al
Al
Al

: Energy resolution for X-ray of 5.9 keV (FWHM).
: Size of imaging area per a CCD chip.
♭: Values in active region.
‡

CCD-NeXT3

Chapter 5
Experimental Setup
In this chapter, we give an overview of the experimental setup and data acquisition(DAQ) system used for the evaluation of CCDs.

5.1

Data acquisition system

The DAQ system of CCDs consists mainly of a “ CCD driver system” and a “readout system”. The CCD driver system generates the clock and bias voltages, andsupplies them to the CCD. The read-out circuit (1) ampliﬁes the output signal fromthe CCDs, (2) shapes its waveform, and (3) changes it to a digital signal. We give
brief explanations for these systems in the following paragraph.
CCD driver system As shown in block diagram in Figure 5.1, the CCD driver
system consists of two parts; the digital generator (Logic Analyzer TLA 714, Tektronix) and the clock generator. The digital generator gives the high/low phase
information of each clock. The time resolution of the TLA 714 is 50 nsec, which isenough fast to activate the CCD clock (usually up to 100 kHz transfer). The clock
generator converts the digital clocking pattern provided to the digital generator toanalog signals. We use one circuit board of the clock generator to generate one
clock. The block diagram of this circuit board is shown in Figure 5.3. The circuitboards are arranged in a special case as shown in Figure 5.2 (lower right). We canfreely adjust the voltage levels of the high/low phases of the clocks within the rangeof -20V to +20V and change the rise and decay time of the clocks by adjusting thevariable resistance mounted on the circuit board of the clock generator. Since thedigital signal is supplied to the clock generator via a photocoupler, the HCPL-2730(see Figure 5.3), the analog ground is separated from the digital ground. Clocks
generated in the above process are supplied through the CCD head board and drive
the CCD.

62

CHAPTER 5. EXPERIMENTAL SETUP(A)CCD Driver system

63

55Fe

digitalclock

shutter
Be Window

1A clock generator
x 10
2A

X-ray

clock inanalogclock

digitalclock
digitalclock

signalCCD Head

CCD

signal out

Amp
T1,T2,T3

impedance
converter

T4

ADC

Integralfilter(B)standard
Read-out systemWSvacuumchamber

Digitalgenerator

Differential
Amplifier

Low-Pass
filterfits file(C)Read-out system
only for CCD-NeXT1

Figure 5.1: Block diagram of the DAQ system. The CCD driver system is shown by
the dotted square (A). To process the CCD output signal, we usually use a standard
read-out system shown by the dotted square (B). Only in the case of the evaluationof CCD-NeXT1 do we use the read-out system shown by the dotted square (C).

CHAPTER 5. EXPERIMENTAL SETUP

64

Vacuum Chamber

Clock Generator

Amplifier and Integration Circuit

Figure 5.2: Picture of the DAQ system of the CCD evaluation system.

7404
1A
1B
2A
2B

V01

A

V02

optocoupler
HCPL-2630

74125

Y0

+T1

B

Y1

+T2

C

Y2

+T3

Y3

+T4

74138

3 to 8demultiplexer

2.2nF
1A

1Y

-T1

Hex 3state 2Y
Bus Inverters

-T2

2A

+2.5V

-T4

+5V
1k Ω

C11
1µF

R30
R31

-T1
+T2
R16
1k Ω

R32

5.1k Ω

C23
0.22µ FR41
6.2k Ω

IN++

LM7171Z+-

MAX435
Iout+

R45
51Ω

+

LM7171
Z-

+

IN-

Rset

-T2
+T3

VR3
10k Ω
R17
1k Ω

VR5
50k Ω

-

-2.5V
+2.5V

R40
1k Ω

R38
10 Ω

R36
100 Ω

C12
1µF

C24
1pF
R33

R29

+

VR2
10k Ω

-2.5V
+2.5V

-T3

4Y

+T1
R15
1k Ω

-2.5V
+2.5V

3Y

4A

74366

LM6144

VR1
10k Ω

3A

C13
1µF

R37
7.5k Ω

C23
500pF

100pF

R35
10 Ω

+

-5V
-T3
+T4

VR4
10k Ω
R18
1k Ω
-

-2.5V

C14
1µF

+

-T4

Figure 5.3: Circuit diagram of a board of the clock generator.

OUT

CHAPTER 5. EXPERIMENTAL SETUP

65

Read-out system An output signal is processed with an ampliﬁer and integration
circuit, named the “integral ﬁlter” in Figure 5.2 (lower left). We show a diagram of
the circuit board of the integration ﬁlter in Figure 5.4. In this ﬁlter, the CCD signal isﬁrst ampliﬁed by two ampliﬁers. The output is then fed into an integrating ampliﬁer
used a MAX WTA (wide-band transconductance ampliﬁer), the M AX 4351 . Then,
we obtain the integrated charge that is proportional to the voltage gap between thesignal and ﬂoat level in a condenser. In the following circuit, we convert the charge
to the voltage and shift within ±10V, which is the input voltage range of the ADC.
This processed signal is ﬁnally sampled by a 12-bit ADC according to the timing
trigger generated by the digital generator.
We note that only in the case of the performance test of CCD-NeXT1 (seechapter 6), we use another read-out circuit consisting of a pre-ampliﬁer, diﬀerential
ampliﬁer, and low-pass ﬁlter (see part (c) in Figure 5.1 and Figure 5.5). The CCD
driver circuit is the same as that explained above.
F
+12V

+12V

D

R28
10kΩ

R16
10kΩ
VR6
5kΩ

B
C1
100µ F
+

R0
2k Ω

VR3
5kΩ

IC1
LM7171

-

+

R29
10kΩ

-

R2

C2
1nF

TP1
R17
10kΩ

-12V

R1
2k Ω

A

U2
2N7000R95.1k Ω

R10
2kΩ

R19

VR4

2kΩ

5kΩ

VR1
5kΩ
-

R11
20k Ω

+

+

-12V

TP2

R15
1kΩ

IC5
LM7171TP3
T1

R22
1kΩ R242kΩ-

IC4LM7171+C10
1.5 µF

R26
10Ω

R18
2kΩ

VR2
100k Ω

R9
10Ω

+12V

R30
2kΩ

T2

Vin

VR5
5kΩ

T3
U3
2N7000C

IC6 MAX435
IN+
Z+
Iout+
ZIN- Rset

VR7
50k Ω
R31
2kΩ

TP5

TP6

+

C19
5.6nF

-

IC7
LM7171

R23
1kΩ

TP4
U1
2N7000C19
0.047µ F-12V

C130.047µ F

20k Ω

R25
7.5kΩ

C16
0.022µ F T1

R35
1.1k Ω

TP7

-12V
-

E
+

IC8
LM7171 VoutG

R34
1.1k Ω

Figure 5.4: Circuit diagram of a board of the integration ﬁlter.

1

Datasheet is available at the following web site; http://pdfserv.maxim-ic.com/en/ds/MAX435-MAX436.pdf

CHAPTER 5. EXPERIMENTAL SETUP

66

Figure 5.5: Circuit diagram of a readout circuit which is used only for the CCDNeXT1.

5.2

Overview of the CCD evaluation system

We built two evaluation systems, (1) a multipurpose CCD camera (mini chamber)system and (2) a large chamber system. The former is mainly used for the evaluationof small-size test devices of Nch and Pch CCDs (see chapter 6). The latter is used
for the evaluation for the CCD-NeXT1 because the CCD-NeXT1 is too large for theformer system (see section 6.1.3).

5.2.1

Multipurpose CCD camera (Mini chamber) system

The X-ray CCD is generally driven in a cool temperature environment to reducethe dark current. We cooled it down in a vacuum chamber to prevent the formationof any water condensation. To make the evaluation of the CCD smooth, we builta “Multipurpose CCD camera (Mini chamber) System” consisting of a mini-size
vacuum chamber and a Stirling cooler, SRS-2110 (SHI cryogenics group, Japan).
The small size of this chamber allows for its evacuation in a short time. The Stirlingcooler has beneﬁts in the following respects; (1) it is easy to install a camera systembecause of its compact size, and (2) we can easily cool the CCD device without any
cooling medium such as liquid nitrogen.
Figure 5.6 shows a photograph (left) and a schematic view (right) of the vacuum

CHAPTER 5. EXPERIMENTAL SETUP(3)Cold Block

4GCT8KGY 5KFGQH%QQNGT(4) CCD Head67
5KFG8KGY 2GTURGEVKXG&TCYKPIԘ /GEJCPKECN%QQNGTCCD Clock inԝ 'XCEWCVKQP2KRG
ԙ %QNF*GCF

ԛ%%&*GCF

Ԛ%QNF$NQEM

Ԟ .GCM8CNXG

:TC[

%%&

Ԝ &UWD%QPPGEVQTCCD Signal out(2)Cold Head of Mechanical Cooler:TC[+PEKFGPV9KPFQY $G
1XGTXKGYQHOWNVKRWTRQUG%%&ECOGTCXGT56CMCIK

Figure 5.6: (left) Picture of the vacuum chamber. The volume of this chamber is
22 × 17 × 8(depth)cm. (right) Schematic view of the vacuum chamber.

chamber. The Stirling cooler is attached to the under-body of the chamber. Only
the cold head is placed in the chamber, so that we need to air-cool the metal cylinder
sealed in the Stirling-cycle engine. By using a ﬂexible copper wire braid, we joined
the cold head and the cold block contacting the CCD package directly. The CCD
clock and the CCD output signal are transferred via the hermetically-sealed D-sub
37 connector attached to the underbody of the chamber. The CCD head circuit canbe quickly changed according to the package conﬁguration and pin assignment of
the CCD.
Setup for performance evaluation
We irradiate X-rays with the conﬁguration shown in Figure 5.7. X-rays from sealed
radio isotopes enter the CCD in the chamber through the thin Be window with
< 1 mm-thickness and 3 cm diameter. To block X-rays in the transfer phase, themechanical shutter is placed in front of the Be window.
Setup for measurement of the depletion layer thickness
We estimate the thickness of the depletion layer (dDL ) from the detection eﬃciency(feﬀ )using the following formula for the photo-electric absorption:feﬀ = 1 − exp(−µρdDL ),(5.1)

where µ and ρ are the mass-absorption coeﬃcient and the density of silicon, respectively.
To obtain feﬀ , we need to know the absolute ﬂux of the X-rays entering the
CCD. The X-rays from the sealed radio isotopes are collimated using the collimatoras shown in Figure 5.8.
Since X-rays are collimated into a small size, all the X-rays from the collimator
enter the IA of CCD. Figure 5.9 shows the image of the X-rays acquired from theCHAPTER 5. EXPERIMENTAL SETUP

68%NQEM)GPGTCVQTGT

8CEWWOEJCOD:TC[$GYKPFQY/GEJCPKECN5JWVVGTFigure 5.7: Picture of the experimental setup for the performance evaluation test.
X-rays from the sealed radio isotope enter the CCD through the mechanical shutterand Be window.

collimator. We can clearly see that the spot size of the X-rays is suﬃciently small
compared to the size of the imaging area of a CCD. We obtain the ﬂux of the detected
X-ray events, divide it by the absolute ﬂux calibrated with the X-ray detector whose
detection eﬃciency is well-known, and then ﬁnally obtain the detection eﬃciency.
Since the ﬂux of the collimated X-rays is weak, the exposure time is usually much
longer than the transfer time. The number of events detected in the transfer phaseis expected to be negligible and can be ignored, compared to that of the exposure
phase, therefore we do not use the mechanical shutter in this setup.

5.2.2

Large chamber system

The large chamber system is used only for the purpose of evaluating the CCDNeXT1. As described in the following section (section 6.1.3.3), the CCD-NeXT1 is alarge-size device, and has a ﬂex printed cable and special connectors in anticipationof it being loaded on the satellite. Therefore, the vacuum chamber of the multipurpose camera system described above is too small to contain it. Thus, we use thelarge-size (80 × 50 × 50 cm) chamber. An overview of the system used can be seenin Figure 5.10.
We use the Pulse Tube Cryocooler (Iwatani Industrial Gases Corp.) to cool down
the CCD. The stainless (SUS) ﬂexible wire braid joins its cold head and the CCDNeXT1. The CCD-NeXT1 can be cooled down to ∼ −80 ◦ C. Since the transfer
method of the CCD-NeXT1 is frame transfer, the mechanical shutter is basically

CHAPTER 5. EXPERIMENTAL SETUPC 5EJGOCVKE8KGY

69

D 2JQVQITCRJ KPUVCNNGFOKPKEJCODGT

Figure 5.8: Collimator of 109 Cd for measurement of the depletion depth. (a) A
schematic view of the collimator. X-rays from 109 Cd are collimated by two thickbrass collimators. (b) Picture of the collimator installed on the mini chamber system.

81%

*1%

#EVKXG:TC['XGPVU EQNNKOCVGF
81%

*1%

Figure 5.9: The acquired image of the collimated X-rays of 109 Cd. The spot size ofthe collimated X-rays (dotted circle) is small enough comparing the size of activearea.

unnecessary. The frame stored area is covered by a thick Al plate so that X-rays
cannot enter the CCD in the transfer phase.
As described in the previous section, the readout system is diﬀerent from themultipurpose CCD camera system. The output signal is ﬁrst ampliﬁed by thepreampliﬁer in the vacuum chamber, second by the diﬀerential ampliﬁer, and ﬁnally ﬁltered by the low-pass ﬁlter (see Figure 5.5). The diﬀerential ampliﬁer andthe low-pass ampliﬁer are located outside the vacuum chamber. The CCD drivercircuit is the same as that of the multipurpose CCD camera system.

CHAPTER 5. EXPERIMENTAL SETUP

8CEWWOEJCODGT

70

575EQNFYKTGDTCKF%%&0G:6

EQXGTHQT
(5TGIKQP

Figure 5.10: Picture of CCD-NeXT1 mounted in the large chamber system. The
CCD-NeXT1 is joined to the cold head of Pulse Tube Cryocooler via a SUS ﬂex wire
braid. X-rays enter the CCD-NeXT1 in a direction perpendicular to the paper. In
order to block the entrance of X-rays during transfer, the frame stored area (FS) of
the CCD-NeXT1 is covered with a thick Al plate.

Chapter 6
Development I -Back Supportless
CCDWe next describe the development of a new-type CCD device for the SXI onboard
the N eXT satellite. Since the SXI will be one of the main focal plane detectors
for the HXT, focusing hard X-rays up to 60∼80 keV with an unprecedented large
eﬀective area, the SXI is required to achieve (1) high sensitivity for X-rays and (2)the back-supportless structure for combining a hard X-ray detector. In order toproduce such an unprecedented CCD, we programmed two plans, a “Baseline plan”
and a “Goal plan”, and advanced the development of the new CCDs in parallel.
Details of these plans are described in chapter 4.

6.1

Baseline Plan

6.1.1

Development Plan

As described in chapter 4, we have already developed the Nch CCDs as “CCDCREST” and “MAXI-CCD”, which have shown excellent performance ([4, 37, 44, 63]
and references there in). By removing the package and thinning down the CCD chip,
we fabricated the small-size (12×12 mm2 ) test device of BS-CCD. The main purposeof this fabrication is to examine (1) whether the thinning process and fabrication
could be performed as we expected, and (2) whether the thinning process yields any
performance degradation.

6.1.2

Fabrication of BS-CCD

For the fabrication of the BS-CCD, two characteristics are required; (1) In orderto satisfy the requirement of high sensitivity for hard X-rays, we need to enlarge
the depletion layer as much as possible. (2) In order to satisfy the requirement totransmit undetected X-rays as much as possible, we need to remove the ﬁeld-free
region and package which are not sensitive to X-rays.
71

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

72

To create the test model of the BS-CCD, we employed the “Deep 2K2” wafer,
which is used for the MAXI-CCD [42]. Among practical X-ray CCDs, the MAXICCD is an excellent CCD, achieving good energy resolution, high detection eﬃciency,
and low dark current. If there is no deterioration in the performance due to thethinning of the wafer, the test model of the BS-CCD is expected to achieve good
performance equivalent to that of the MAXI-CCD.

(a) Chemical Etching Method

Thick outer rim(b) Grinding + Polishing Method

Figure 6.1: Schematic view of (a) the chemical etching method and (b) the grinding+ polishing method.
We examined the method for the thinning of the wafer. To thin down the wafer
to ∼ 80 µm, equivalent to the thickness of the depletion layer, the thick rim area isrequired to maintain the strength of the chip and to handle the chip safely. In otherwords, we are required to process the chip in a concave shape (see Figure 6.1a).
In this case, we need to use chemical etching, however, the resistivity of this wafer
is too high to control the thickness as required. Therefore, we cannot apply thismethod for the Deep 2K2 wafer.
For the above reason, we decided to thin the wafer by grinding and polishing.
This mechanical method is available independent of the resistivity of the wafer. The
typical precision of thickness by the grinding and polishing processes are ±15 µm
and ±5 µm, respectively. The typical roughness is 5 µm or less after both polishingprocesses. However, this method cannot process wafers with complicate shapes, suchas the concave shape (see Figure 6.1b). We decided to thin the wafer to 200 µm
from the viewpoint of the safe handling, though it could be processed more thinly.
Thus, we thinned the Deep 2K2 wafer to 200 µm by this method. The ﬂatness was
about 17 µm within the size of 10 mm × 10 mm, which is the test model of theCHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

73

BS-CCD (see below). Using this thinned wafer and the special package removed thebackside, we assembled the test model of BS-CCD.

6.1.3

Performance of BS-CCD

6.1.3.1

Test model(b) Cross section[Baseline]

X-ray(thinning)

Back side

Front side(a) Picture
Front side

Back sideP15 6-5B1P-2 (Deep1) First, we fabricated the BS-CCD using the “Deep1”
wafer. Deep1 is one of the most well-informed wafers and is employed in many
CCDs fabricated in HPK. Though Deep1 has the disadvantage in that the depletion
layer needs to be enlarged because of its relatively lower resistivity compared to
Deep2K2 used in CCD-CREST and MAXI-CCD, Deep1 was employed by the test
device only to evaluate the eﬀect of the thinning of the backside. We show thespeciﬁcation of this device in Table 6.1.

Depletion Layer
Field-Free Region

Figure 6.2: (a) Picture of the test model of BS-CCD. Due to the back supportless
structure, the gate structure and the surface of the ﬁeld-free region are exposed. (b)Schematic view of the test model of BS-CCD.

Figure 6.2 shows a photograph and a schematic view of the test model of BSCCD. Due to the back-supportless structure, the gate structure and the surface ofﬁeld-free layer are exposed. We show the results of the performance evaluation test in
Table 6.1. We conﬁrmed that this device drove normally and successfully detected
incident X-rays. Figure 6.3 shows the spectrum of 55 Fe and 241 Am accumulating
grade 0, 2, 3, 4, 6 events1 . Though the readout noise of ∼ 35 e− seems to be bad,
it is mainly attributed to the noise of the clock generator and the readout circuit
1

The grade 0, 2, 3, 4, and 6 events are conventionally judged to be valid X-ray events in X-ray
astronomy [46].

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

74

at Kyoto University (see also section 5.1), and not to that of the device itself. As
described in the following paragraph, we have already conﬁrmed that a CCD whose
readout noise is equivalent to this device shows good readout noise (∼ 7 e− ) whenevaluated by the low-noise readout system at Osaka University[38].
 MG8 MG8 MG8 MG8 MG8

Figure 6.3: Spectrum of 55 Fe and 241 Am detected by BS-CCD of P15 6-5B1P-2(Deep1). Grade 0, 2, 3, 4, and 6 events are accumulated.
We also found that the dark current and the CTI were < 0.1 e− /sec/pixel, and
1.0 × 10−6 (horizontal) and 7.0 × 10−6 (vertical). These are essentially consistent
with those of the unthinned CCDs, for example CCD-CREST and MAXI-CCD.
From these results, we conﬁrmed that there is no problem in the process of thinningof the backside.

Table 6.1: Speciﬁcation and performance of test models of BS-CCD
Type No.
P15 6-5B1KF
P15 14-5B1P
KF201 17-5B1KF KF201 21-5B1KF
Format
512 × 512
Pixel size
24 µm × 24 µm
Clock phase2 phaseTransfer method
Full Frame Transfer
Illuminated method
Front IlluminatedWafer TypeDeep1(4inch)
Deep2K2(4inch)
Deep5(6inch)
Readout noise[e− (rms)]
7⋆
35
36
42
30
Energy resolution [eV(FWHM)]
140⋆⋆ (@5.9keV) 570 (@5.9keV)
670 (@22.2keV)
850 (@22.2keV)
610 (@22.2keV)−◦◦
Dark current[e /pixel/sec] <0.1 (-110degC) <0.1 (-110 C)<0.1 (-73 C)
0.1 (-73 ◦ C)−†
Node sensitivity[ µV/e ]
2.3
1.8
2.6
−6
CTI (vertical)[10 /transfer] 7.0
2.1
9.3
36
CTI (horizontal) [10−6 /transfer] 1.0
1.9
2.3
6.3
Depletion layer[µm]
10(ratio)
65
76
73
‡
Wafer
[µm]
200
190
-(not thinned)-(not thinned)⋆
read-out noise measured by Osaka University group.
⋆⋆
energy resolution measured by Osaka University group.
†(not measured).
‡nominal value reported by HPK (not measured).

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD75

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

76

Counts

102

10

1
1200

1400

1600
1800
Pulse height [ADU]

2000

Figure 6.4: Spectrum of 55 Fe detected by BS-CCD of P15 14-5B1P-4 (Deep2K2).
This data is obtained with E ∼ N A system of Osaka University. Single pixel events(grade 0) are accumulated.
P15 14-5B1P-4 (Deep2K2) Based on the successful results described in theprevious paragraph, we fabricated BS-CCD on the Deep2K2 wafer, which is expectedto achieve a thick depletion layer of ∼ 80 µm. We show the speciﬁcation of thisdevice in Table 6.1. The appearance of this device is the same as P15 6-5B1P-2 (seeFigure 6.2).
Figure 6.4 shows the spectrum of 55 Fe obtained by this BS-CCD, accumulating
single pixel (grade 0) events. This data is obtained with the E ∼ N A system of Osaka University [38]. The readout noise of this system is signiﬁcantly lower than thatof the readout system of Kyoto University. Mn Kα (5.9 keV) and Mn Kβ (6.5 keV)lines are clearly separated. The readout noise and energy resolution are 7 e− and 144
eV at 5.9 keV (Mn Kα ), respectively. These are excellent and essentially equivalent
to those of MAXI-CCD.
We also evaluated this device using our readout system. Though the readout
noise of our system is higher than that of the E ∼ N A system of Osaka University,
it is useful to compare the characteristics of P15 6-5B1P-2 (Deep1) and this device
evaluated in the same experimental setup. Figure 6.5 and 6.6 show the X-ray image
and spectrum of 109 Cd detected by this device, respectively. The readout noise is36e− , which is same as that of P15 6-5B1P-2. Therefore, the readout noise of P15
6-5B1P-2 (Deep1) itself might also be the same as this device. The dark current
and CTI are also equivalent to those of P15 6-5B1P-2 (Deep1).
Using the well-calibrated X-ray source of 109 Cd (see section 5.2.1), we measured
the detection eﬃciency of the BS-CCD and estimated the thickness of its depletion

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

VOC

77

HOC

ActiveFigure 6.5: Acquired image of 109 Cd detected by BS-CCD of P15 14-5B1P-4(Deep2K2).

Figure 6.6: Spectrum of 109 Cd detected by BS-CCD of P15 14-5B1P-4 (Deep2K2).
Single pixel events (grade 0) are accumulated.

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

78

layer. We accumulated X-ray events of grades 0, 2, 3, 4, and 6 and found thedetection eﬃciency to be 4.1% at 22.2 keV (Ag Kα ). This detection eﬃciency
is equivalent to that of the 65 µm-thickness depletion layer, which is essentiallyconsistent with the thickness of the depletion layer expected from the resistivityof the Deep 2K2 wafer. We also measured the transmissivity of the BS-CCD with109
Cd. The transmissivity of 87 % (at 22.2 keV) is equivalent to that of the 190µm-thickness Si wafer. This thickness is consistent with our designed value.
Based on these results, we conﬁrmed that there was no degradation in the performance of the BS-CCD compared to that of the unthinned CCD as MAXI-CCD;
in addition, no problems were found in the process of thinning of the backside.
KF201 17-5B1KF-5 (Deep5) As shown in the previous paragraph, we successfully developed the test model of BS-CCD. We judged that we have already learned
the process of manufacturing BS-CCD with a p-type semiconductor. Considering
the future demand for the production of large-size CCD as the ﬂight model onboard
N eXT , we changed the size of the Si wafer from a 4-inch radius to a 6-inch radius.
We could obtain 6-inch radius wafers (6-inch wafers) with quality as good as thatof the 4-inch radius wafers (4-inch wafers). However, the manufacturing line had tobe changed with the change in the wafer size, and so it was necessary to conﬁrm
whether the CCDs fabricated on the 6-inch wafer show equivalent characteristics
as those on the 4-inch wafers. We experimentally manufactured front-illuminated
CCDs with a 6-inch high-resistivity wafer, named “Deep5”, which is equivalent to
the Deep2K2 wafer. As shown in Table 6.1, the format is the same for the test
model of BS-CCD as P15 6-5B1P-2 (Deep1) and P15 14-5B1P-4 (Deep2K2). However, as shown in Figure 6.7, the structure of this device was not a back-supportless
structure, meaning that we did not thin the backside of this wafer and the backsideof the package was not removed.
In the inspection of this device before shipment, performed by HPK, the characteristics of this device were found to be essentially equivalent to those of the
CCDs fabricated on 4-inch wafers, however, the dark current was reported to be
565 e− /sec/pixel at −20 ◦ C, which is ∼ 10 times higher than that of the CCDs
fabricated on 4-inch wafers (Deep2K2). We evaluated this device paying special attention to the behavior of the dark current at low temperatures. The readout noise,
CTI, and the thickness of the depletion layer are equivalent to those of Deep2K2
CCD, as reported earlier. The dark current is also equal to that of the Deep2K2
CCD at a suﬃciently low temperature. The results are shown in Table 6.1. The
reason for the high dark current at high temperatures is unknown. However, we
judged the Deep5 wafer and the manufacturing process for the Deep5 CCD had no
problems because the dark current was suﬃciently low (< 0.1 e− /sec/pixel) at thestandard temperature (< −70 ◦ C) for driving X-ray CCDs.

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

79

Figure 6.7: Picture of the test model of the CCD fabricated on a 6-inch wafer.

Figure 6.8: Spectrum of 109 Cd detected by BS-CCD of KF201 17-5B1KF-5 (Deep5).
Single pixel events (grade 0) have accumulated.

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

80

Figure 6.9: Spectrum of 109 Cd detected by the BS-CCD of KF201 21-5B1KF-5(Deep5). Single pixel events (grade 0) have accumulated.

KF201 21-5B1KF-11 (Deep5) This device is also fabricated on the Deep5 wafer
in the same manufacturing line as the KF201 17-5B1KF-5. The format and appearance are entirely the same as those of KF201 17-5B1KF-5. Though this device
also showed a high dark current of 1487e− /sec/pixel at −20 ◦ C, we conﬁrmed that
this decreased to < 0.1e− /sec/pixel at a temperature of −76 ◦ C. As shown in Table 6.1, the characteristics are essentially equivalent to those of KF201 17-5B1KF-5.
We conﬁrmed that the fabrication process with the new manufacturing line had no
problem.
6.1.3.2

Prototype of hybrid detector

To demonstrate the characteristics of the BS-CCD, we combined the BS-CCD and
the CdTe detector (commercially available). The CdTe detector used in these experiments is the XR-100T-CdTe produced by Amptek Inc. As shown in Figure 6.10a,
the arrangement of the detectors are the same as that in the WXI. We irradiatedwith X-rays of 241 Am from above these devices.
First, we detached the BS-CCD and obtained the spectrum of 241 Am using only
the CdTe detector (see Figure 6.10b). Thus, the X-rays from the 241 Am entered
the CdTe detector directly. The exposure time was 300 sec. We show the observedof fintensity of each line in the raw CCdT
e in Table 6.2. From 10 keV to 60 keV, theof feﬃciency of this detector is almost 100%, therefore the ratio of CCdT
e represents the241
intensity ratio of each line from Am.
Secondly, as shown in Figure 6.10a, we set the BS-CCD above the CdTe detector.
Figure 6.10c and 6.10d show the spectra obtained with the BS-CCD and the CdTe

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

81

detector placed under the BS-CCD. The total exposure time of the BS-CCD and
the CdTe detector were 600 sec and 300 sec, respectively. The spectrum shownin Figure 6.10c accumulated events of grades 0, 2, 3, 4, and 6. We show the line
intensity of each line seen in Figure 6.10c (by the BS-CCD) and 6.10d (by the CdTe
ondetector) in the rows CCCD and CCdT
e in Table 6.2, respectively.
To verify the above results of the thickness of the wafer (dW F ) ∼ 190 µm, weof fon
calculated the transmissivity (CCdT
e /CCdT e , see Table 6.2) at the energy of eachline and compared it with the expected transmissivity of the Si wafer of 190 µm
thickness. As shown in Figure 6.11, these are approximately consistent with eachother. This also demonstrates that almost all the hard X-rays pass through the
BS-CCD wafer and are detected by the CdTe detector.
We also veriﬁed the results of the thickness of the depletion layer (dDL ) ∼ 65 µm.
Since the size of the imaging area of the BS-CCD is diﬀerent from that of the CdTeof fdetector, CCCD /CCdT
e does not indicate the detection eﬃciency of the BS-CCD.
of f
However, the ratio of CCCD /CCdT
e of each line is equal to that of the detection
eﬃciency at the energy of each line. Therefore, we estimated the detection eﬃciencyof fof the BS-CCD (QCCD ) at the energy of each line by normalizing CCCD /CCdT
e at
13.9 keV to 0.17 equal to the detection eﬃciency of the 65 µm-thick depletion layer,of fi.e., QCCD = CCCD /CCdT
e × 0.17/1.09 (see also Table 6.2). As shown in Figure 6.11,
we plotted the estimated detection eﬃciencies (QCCD ) and conﬁrmed they wereconsistent with the expected detection eﬃciencies of the 65 µm-thickness depletion
layer.
Table 6.2: Intensities of the lines of 241 Am, transmissivity and estimated detection
eﬃciency of BS-CCD

Detector
CdTe(without CCD)
CdTe(with CCD)
CCD

of f †
CCdT
e

13.9
9655

16.9
12071

17.7
5856

Line Energy [keV]
20.8
26.2
32.1
4871 2459 993

on
†
CCdT
5475
9275
4288 4164 2315 1047
e
of f
on
CCdT e /CCdT
0.57
0.77
0.73 0.85
0.94
1.05
e
†
CCCD
10552
9897
1559 392
of f
CCCD /CCdT e 1.09
0.55
0.32
0.16
QCCD ††
0.17
0.086
0.052 0.025 of f
† : Total counts of each line [counts]. †† : Q
CCD = CCCD /CCdT e × 0.17/1.09.

6.1.3.3

33.3
684

36.4
1876

59.6
30493

760
1.11
-

1856
0.99
-

30739
1.01
-

Evaluation model -CCD-NeXT1-

Based on these successful results, we started to fabricate the developed BS-CCD
device as “CCD-NeXT1”. This device is characterized as an evaluation model ofCHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

E
D














82







:TC[

$5%%&
F MG8 MG8 MG8 MG8 MG8 MG8 MG8











%F6G&GVGEVQTC

Figure 6.10: (a) Photograph of the test model of the WXI. (b)241 Am spectrum
acquired by the CdTe detector. There is no barrier between 241 Am and the detector.
(c) Spectrum of 241 Am acquired by the BS-CCD. Grades 0, 2, 3, 4, and 6 events
have accumulated. (d) Spectrum of 241 Am acquired by the CdTe detector placed
under the BS-CCD. X-rays penetrating the BS-CCD are detected.

Transmissivity, Efficiency

1

Transmissivity (d WF =190um)
0.1

on

off

CCdTe /C CdTe
Q.E (dDL =65um)
QCCD
0.01

10

20

30

40
50
Energy[keV]

60

70

80

Figure 6.11: Transmissivity and detection eﬃciency (Q .E.) of the BS-CCD. The
solid and dotted lines show the expected transmissivity of 190 µm-thick Si wafer
and the expected detection eﬃciency of the CCD with a 65 µm-thick depletion
layer, respectively. The solid circles and triangles show the observed transmissivof fonity (CCdT
e /CCdT e ) and the estimated detection eﬃciency (QCCD ), respectively (seeTable 6.2).

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-83

the baseline-plan SXI (see Table 4.3 in chapter 4).
Making use of the experience of developing domestic CCDs of CCD-CREST,MAXI-CCD, and the test model of BS-CCD, as described previously, we attempted
to produce the CCD-NeXT1. The format and pixel size were 2000 pixels and 12µm×12µm, which are the same as those of the CCD-CREST2. The charge transfer
method is the frame transfer. We show a picture of the CCD-NeXT1 in Figure 6.12.
The wafer was thinned down to 150 µm. Though we adopted the grinding andpolishing method for thinning the wafer in the case of the test model, this wafer
was thinned by only grinding. The reason for omitting the polishing process is thatphysical destruction frequently occurs in this process. Many microscopic trails areapparently present on the ground surface only due to the grinding method. We show
a close-up view of the ground surface of the CCD-NeXT1 in Figure 6.13. We can see
many tiny trails along the direction of the grinding. We observed these trails closelyusing a color laser three-dimensional proﬁle microscope (VK-9500; Keyence, Osaka,
Japan). Figure 6.14 shows the surface condition on the grinding surface. We foundthe depth and the width to be ∼ 0.3µm peak-to-peak and sub-µm, respectively.
Compared to the thickness of the wafer (∼ 150µm) and the size of the pixels (∼12µm), they are negligibly small. Therefore, we made a judgment that it is not a
problem to omit the polishing process. In the following, the thinning of the wafer isperformed only by the grinding method.
KG103 9-20A0KG-2 KG103 9-20A0KG-2 is fabricated on the Deep1 wafer thinned
down to 150 µm by the grinding method. We measured the thickness of the wafer inthis device. As shown in Figure 6.15, we set the IA of the CCD-NeXT1 between thesealed radio isotope of 241 Am and the CdTe detector, and acquired an “absorbed”
241
Am spectrum shown in the blue curve in Figure 6.16. Subsequently, we detached
CCD-NeXT1 and acquired an “un-absorbed” spectrum (the red curve in Figure 6.16)
under the same experimental arrangement. The intensity ratio of each characteristic
X-ray line between the absorbed and unabsorbed spectra shows the transmissivity(TCCD ) of this wafer. We calculated the absorptivity ACCD (1 − TCCD ) at eachcharacteristic X-ray line and compared the expected ACCD for X-rays absorbed bysilicon with 150-µm thickness. As shown in Figure 6.16, they are approximately
consistent.
KG103 22-20A0KG-2 We evaluated the performance of KG103 22-20A0KG-2.
This device was fabricated on the Deep5 wafer. The performance test for this device
was performed using the setup described in section 5.2.2. We irradiated X-rays from55
Fe over the whole area of CCD-NeXT1 uniformly. As described in section 5.2.2,
the frame stored area is covered by a thick Al plate. The CCD-NeXT1 was driven
by the frame transfer method in this evaluation. The results are shown in Table 6.3.
Figure 6.17 (left) shows the acquired spectrum of 55 Fe with accumulated grade 0

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

(1) Front Side

84

Thermometer
Micro-Dsub

Imaging Area

Frame Stored Area

FET
X-ray Incidence Plane(Depletion Layer)

Flex Cable(2) Back Side

Grinded Surface(Dead Layer)

Figure 6.12: Picture of the CCD-NeXT1. (1) Front side. (2) Back side. Due tothe presence of the Al coating, we can clearly see that the reﬂection forms on thesurface of both sides.

Figure 6.13: Tiny trails found on the grinding surface of the CCD-NeXT1. We
adjusted the color of the picture in order to clearly show these trails.

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

(1) raw image

85

B
B

A

cross section

cross section
AA
0.60 um(2) cross-section viewB(3) 3-D image

12 um (1pix)

Figure 6.14: Surface condition of the ground wafer measured by a color laser three
dimensional proﬁle microscope (VK-9500; Keyence, Osaka, Japan). (1) Raw imageof a part of the ground surface. (2) Cross-sectional view acquired by laser ranging.
(3) 3-D view.

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

CCD-NeXT1

241-Am86

CdTe detector

CdTe detector

Am-241

CCD-NeXT1

Figure 6.15: Experimental setup for measurement of the thickness of the waferof CCD-NeXT1 (KG103 9-20A0KG-2). We set the imaging area of CCD-NeXT1
between the sealed radio isotope of 241 Am and the CdTe detector.

red: w/o CCD
blue: with CCD

Absorption Ratio (data)

16.9&17.7keV
13.9keV20.8keV26.2keV

32.1&33.3keV
Expected absorption curve for 150-um Si

Figure 6.16: (left) Spectra of 241 Am detected by the CdTe detector. The red line
shows the spectrum obtained without CCD-NeXT1 between the radio isotope 241 Am
and the CdTe detector. The blue line shows the absorbed spectrum by the IA of
CCD-NeXT1 (KG103 9-20A0KG-2). (right) The absorptivity at each characteristic
X-ray line between the absorbed spectrum and the unabsorbed one. A solid curve
represents the expected absorptivity in the case where the thickness of the IA of
CCD-NeXT1 is 150 µm.

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

87

Figure 6.17: Spectrum of 55 Fe (left) and 109 Cd (right) detected by the CCD-NeXT1of S/N KG103 22-20A0KG-2

events. The readout noise and energy resolution were 21e− and 299 eV at the 5.9 keVline. Though the read-out noise of 21 e− appears to be bad, it is mainly attributed
to the noise of the clock generator and the readout circuit of Kyoto University (seealso section 5.1). The node sensitivity of 3.2 µV/e− is higher than those of thetest device, therefore the read-out noise of CCD-NeXT1 itself is expected to be< 7 e− , which is the same as that of P15 14-5B1P-4 measured by Osaka University(see Table 6.1). The CTI of 1.5×10−6 (vertical) and 1.2×10−6 /transfer (horizontal)
is also as good as that of the test device.
Table 6.3: Speciﬁcation and performance of CCD-NeXT1Type No.
KG103 22-20A0KG-2 (Front Illumination)
Number of pixels 2000 × 2000 ×2†
Pixel size
12 µm × 12 µm
Clock phase2 phaseTransfer method Frame Transfer
Readout noise (rms)21 e−‡
Energy resolution (FWHM)
300 eV (at 5.9keV)
Dark current
12e− /pixel/hour (at −77 ◦ C)
Thickness of the depletion layer 84 µm
Thickness of the wafer
625 µm(not thinned)
Node sensitivity
3.2 µV/e−CTI (vertical)
1.5 × 10−6 /transfer
CTI (horizontal)
1.2 × 10−6 /transfer†‡

Sum of imaging area and frame stored region.
Most of this value is thought to originate in the system noise.

We measured the thickness of the depletion layer with the well-calibrated X-ray
source of 109 Cd. Details of the experimental setup are described in section 5.2 In this

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

88

measurement, 75 % of the 109 Cd events are grade 7 events, which are conventionallyclassiﬁed as background events. This value is very large compared to the grade 7
ratio (8%) of the test device KF201 21-5B1KF-11 fabricated on the same Deep5
wafer. This is caused by the small pixel size (12µm × 12µm) of CCD-NeXT1 whichis half the size of that of the test devices. To help in our understanding, we show
the cross-section of a CCD chip and the appearance of charge distribution on thesurface of a CCD in Figure 6.18. When the pixel size becomes half, the charge cloud
spreads out over two times more number of pixels in appearance.

1pix(24um)

1pix(12um)

X-ray

X-ray

Gate

Depletion
layer

Figure 6.18: The cross-section of a CCD chip and the appearance of the charge
distribution on the surface of the CCD

Therefore, we consider that most grade 7 events are also valid X-ray events inthis evaluation test. To conﬁrm this speculation, we made image ﬁles binned 2 × 2
pixels in one pixel (2 × 2 binned image). We analyzed these binned image ﬁlesagain using the grading method, and found that the ratio of grade 7 decreased from
75% to 15%. This results shows that the conventional grade method should beapplicable to CCDs which have a pixel size of 24µm × 24µm, but it is diﬃcult toapply to those with a pixel size of 12µm × 12µm. We then reconstructed the 109 Cd
events by the ﬁtting method. As described in section 3.6.2, the ﬁtting method isapplicable regardless of the size of the events. We show the obtained 109 Cd spectrum
in Figure 6.17 (right).
The detection rate of 109 Cd is 0.14 count s−1 , which corresponds the detection
eﬃciency of 5.2 %. This detection eﬃciency is equivalent to that of the 84-µm
thickness of the depletion layer as we expected.

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

6.1.4

89

Summary of baseline plan

We summarize the development of the Nch CCD for the BS-CCD following thebaseline plan;• We developed the BS-CCD for the Wideband X-ray Imager onboard the NeXT
satellite.
• We built the test model of the BS-CCD using Deep 2K2 , the same wafer as
MAXI-CCD. We conﬁrmed that the thickness of the wafer and the depletion
layer are 190 µm and 65 µm, respectively.
• The performance of the test model of the BS-CCD was comparable to thatof MAXI-CCD. The energy resolution at 5.9 keV is 144 eV and the read-out
noise is 7 e− (r.m.s.).
• Based on these successful results, we built the evaluation model of the BS-CCD
as the “CCD-NeXT1”. This device also shows good performance, essentiallyequivalent to that of the test model of the BS-CCD.

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

6.2

90

Goal PlanAs shown in the previous section, we successfully developed the Nch BS-CCD which
has a thicker depletion layer (∼ 80 µm) and a thinner ﬁeld-free region (∼ 70 µm)
compared with established X-ray CCDs. However, in the process of developing
the Nch BS-CCD, we found some inevitable problems: (1) the thickness of thedepletion layer was limited to within ∼ 80 µm primarily because a wafer with above
5k Ω · cm-resistivity was not available, and (2) the thickness of the silicon wafer
cannot be thinned to 150 µm-thickness from the view-point of safe handling andmechanical strength. In the goal plan, we aim to signiﬁcantly increase the thicknessof the depletion layer by employing a Pch CCD.

6.2.1

Development Plan

The advantages of the Pch CCD are that (1) high sensitivity for hard X-rays canbe essentially acquired by thickening the depletion layer, and (2) high sensitivity forsoft X-rays can be acquired by using the back-illuminated structure. Furthermore,
we note that the characteristics (1) and (2) can be compatible, and we aim to realize
the “perfect” back-supportless structure using the fully-depleted wafer. However,
because the experience of developing the Pch CCD is very poor, we need to advance its development gradually. As described in chapter 4, we participated in thedevelopment of the Pch CCD conducted by NAOJ and shared in the performance
evaluation examinations of the test device with a group of NAOJ. We performed
the evaluation of test devices from the viewpoint of the X-rays detector and fed theresults back to the development of the device in the following process;(1) We developed devices which have a high sensitivity to hard X-ray photons.
To achieve this goal, it is essential to increase the thickness of the depletion
layer. We measured the sensitivity for hard X-rays of test devices fabricated onn-type semiconductor with high resistivity, and cleared up their performance
problems.
(2) We developed devices which have high sensitivity to soft X-ray photons. To
achieve this goal, it is essential to make the back-illuminated CCD. We veriﬁedthat the thick depletion layer, already achieved in step (1), could be maintained
and the performance does not degrade.
(3) Parallel to (1) and (2), we examined the basic characteristics of test devices.
In particular, we checked problems with the gain of the ampliﬁer and CTI in
detail to realize good energy resolution for which to be one of the strong pointsof X-ray CCD.

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

6.2.2

91

Improvement in detection eﬃciency for hard X-rays

From Eq. (3.11), the thickness of the depletion layer (dDL ) essentially depends onthe mobility of the major carrier (µ) and the resistivity of the Si wafer (ρ) for agiven bias voltage(V ), as indicated by the following equation,
dDL ∝

√

µρV .

(6.1)

In the Pch CCD, the major carrier is an electron whose mobility is a factor ofthree higher than that of a hole, which is the major carrier of the Nch CCD (seeFigure 4.7). N-type silicon, used for the fabrication of the Pch CCD with very highresistivity of over 10kΩ · cm, is available. On the other hand, the resistivity of thep-type silicon (ρNch ) used for the fabrication of the Nch CCD is limited to within
5 kΩ · cm. The above two features allow for the operation of the Pch CCD with athickness of the depletion layer of 300µm without applying a high bias voltage.
6.2.2.1

test device

Figure 6.19: Overview of the front-illuminated Pch CCD (Pch3, 5, 8A, 14B). The
pixel size is 24µm × 24µm and the format is 1024 × 256 pixels.

Pch3 This device is a prototype of the Pch CCD fabricated on the existing siliconwafer to conﬁrm the assembly method of the Pch CCD. Two kinds of prototypes
were made in every material that a resistivity was diﬀerent; the wafer resistivityof Pch 3-4 and Pch 3-10 is 1kΩ · cm and 6kΩ · cm, respectively. We successfully
drove these devices and detected X-ray photons. However, as shown in Figure 6.20,
there are many apparent vertical stripes in the active area. These stripes reﬂect theimperfections in the charge transfer, and the lattice imperfections may be causedby the processing conditions and the wafer material. We plan to solve this problem

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

92

by changing the wafer material and the optimization of the process conditions. We
measured the detection eﬃciency of Pch 3-10 by using the collimated X-ray source of109
Cd, which is described in section 5.2.1. The measurements of the X-ray detection
eﬃciency of test devices described in the following are performed under essentiallythe same experimental setup unless otherwise noted.
81%

*1%

#EVKXG

Figure 6.20: Acquired dark image with Pch 3-10. We see apparent vertical stripes
which may be caused by a lattice imperfection.

As shown in Table 6.4, we conﬁrmed that the detection eﬃciency of Pch 3-10
increases as we apply a higher gate voltage. We recorded a maximum detection
eﬃciency of 7.7 % at the gate voltage of −12 V, which is equivalent to that of thedepletion depth of 124 µm. These values are the same as expected, and the depletion
depth of ∼ 100 µm is thicker than any established Nch CCD fabricated on a p-typesemiconductor.
Table 6.4: Comparison of the detection eﬃciency for 109 Cd detected with Pch 3-10
at various gate voltages
Gate voltage Q. E.⋆ Depletion depth⋆⋆
[V]
[%]
[µm]
-2
5.6
89
-4
4.7
74
-6
6.4
103
-8
6.8
109
-12
7.7
124
⋆

: Detection eﬃciency at 22.2 keV (109 Cd Kα ). We
reconstructed the spectrum by accumulating grade 0,2, 3, 4, and 6.
⋆⋆: Thickness of depletion layer corresponds to detection eﬃciency.

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

93

Table 6.5: Speciﬁcation and typical performance of test models of Pch CCD (Front
Illumination)
Type No.
Pch 3-4
Pch 3-10 Pch 5-9
Pch 8A-7
Format1024 × 256
Pixel size
24 µm × 24 µm
Clock phase2 phaseTransfer method
Full Frame Transfer
Illuminated method
Front IlluminatedWafer Resistivity
1kΩ · cm 6kΩ · cm >10kΩ · cm >10kΩ · cm
Detection eﬃciency⋆ [%]
3.1
7.7
16.3
20.2
Depletion layer[µm] 49124
276
349⋆ Detection eﬃciency at 22.2 keV (109 Cd K ).
α

Pch 14B-5>10kΩ · cm
22.4
372

Pch5 We found that there was no big problem in the production process of the Pch
CCD judging from the results of the performance evaluation of Pch 3-10. As the next
step, we evaluated the test device of Pch 5-9 fabricated on a super high-resistivity(over 10kΩ · cm) wafer which was newly obtained. We were able to successfully
detect X-rays from 109 Cd. Figure 6.23 (left) shows the image of the detected eventsof 109 Cd. Most of them were classiﬁed as grade 7, spread out over 2 × 2 pixels.
Comparing with the 109 Cd events detected by Pch 3-10 (see Figure 6.23 (right)),
we can clearly see that the grade 7 events signiﬁcantly increase. We think that theincrease in the ratio of grade 7 is due to the very thick depletion layer of Pch 5-9.
In general, the width of the charge cloud is wider as the depth where X-ray photons
are absorbed is deeper. The extent (1-sigma width) of the X-ray event absorbed in
the depletion layer is described by the following equation [30];
√

σ=

2Dϵsi
dDL
ln(
) [µm],µqND
dDL − x(6.2)

where D, ϵsi , ND , dDL , and x are the diﬀusion constant, the dielectric constant ofsilicon, the densities of the donor, the thickness of the depletion layer, and theabsorption depth. Figure 6.21 shows the relationship between the 1-sigma widthof a generated charge cloud (σ) and the absorption depth(x) in Pch CCD. Judging
from the resistivity, the thickness of the depletion layer of Pch 5-9 is expected tobe ∼ 300µm. Therefore, based on the relationship shown in Figure 6.21, it is not asurprise even if we detected X-ray events spread 3–4 times wider than those detectedby Pch 3-10 (depletion layer 100µm). In the case of this device, we decided to include
the grade 7 events in the valid X-ray events (which usually include only grades 0,2, 3, 4, and 6). We accumulated grade 0, 2, 3, 4, 6, and 7 events and compiled
the spectrum of 109 Cd. We estimated the count rate of the detected X-rays of 109 Cd(mainly 22.2 keV) from this spectrum and found the detection eﬃciency to be 15.5%

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

94

(at 22.2 keV), which exceeds that of Pch 3-10 (6–8%) as expected. This detection
eﬃciency corresponds to that of the ∼260 µm-thickness depletion layer, which iscomparable to the thickness of the depletion layer of the Pch CCD developed by
LBNL (∼ 300µm)[14].
18

300um
200um
100um

1-sigma width [um]1614300umCCD(ND=3e11/cc)

12200umCCD(ND=5e11/cc)

10
8
6
4

100umCCD(ND=7e11/cc)

2
0

0

50

100

150
depth [um]

200

250

300

Figure 6.21: Relationship between the width of a generated charge cloud and absorbed depth in Pch CCD.

81%

*1%

#EVKXG

Figure 6.22: Acquired X-ray image of 109 Cd with the Pch 5-9. The white dottedline shows the border between the active and VOC regions.

As mentioned previously, when the depletion layer becomes thick, the distance,
which the signal carrier (hole) generated in the depletion layer moves to the gateelectrode, increases. Therefore, the drift time increases and the hole clouds spreadout widely by thermal diﬀusion before reaching the gate electrode. Because it isdiﬃcult to reconstruct the charge of widely spread events, we need to restrain thesize of the hole cloud to be as small as possible.
We solve this problem by installing a back bias electrode on the backside surfaceof the Pch CCD. In the case of the ordinary MOS-type CCD, the potential of
the backside surface is equal to that of the substrate. By forming a back bias

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD2EJ WOVJKEMPGUU 

95

2EJ WOVJKEMPGUU ITCFG
ITCFG

Figure 6.23: Acquired X-ray image with Pch 5-9(left) and Pch 3-10(right). Incident
X-rays are from the radio isotope 109 Cd (mainly 22.2 keV). The red circle shows
grade 7 events spread over 2 × 2 pixels and the green circle shows grade 0, 2, 3, 4,
6 events, spread within 2 × 2 pixels.

electrode and applying a reverse voltage (positive for Pch CCD) to it, we increased
the potential diﬀerence between the front and back side of a wafer. We show theschematic view of the back bias structure in Figure 6.24.
<Ordinary structure>< Back Bias structure>

+V
0

0
-V

p-Si

n-Si

field-freedepletion layerelec
in trod
su e
la
to
r

el
ec
in trod
su e
la
to
r

depletion layer

p-Si

n-Si

n
el + -Se(b ctr i
ac od
kb e
ia
s)

-V

field-free

hole

hole

Figure 6.24: Schematic view of the potential well of an ordinary and the back bias
structure.
Then, we can reduce the drift time of the holes and restrain the size of thehole cloud to be small as a result, because the electric ﬁeld in the depletion layer
is intensiﬁed. Furthermore, there is also another advantage in that we can safely
apply a large potential diﬀerence which is needed to make the depletion layer wider.
Pch 5-9 is the ﬁrst test device which is formed a back bias electrode. By applying
various voltages of +5, +10, and +15 V to the back bias electrode, we examined(1) whether the size of the charge distribution is really restrained, and (2) whether
the detection eﬃciency improves as we apply the higher voltage to the back bias
electrode. This examination was performed using the collimated X-ray source ofCHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

96

109

Cd (see section 5.2.1). We show the detection eﬃciency and the grade branchingratio of 109 Cd for various back bias voltages in Table 6.6. In this experiment, we
accumulated grade 0, 2, 3, 4, 6, and 7 events and compiled the spectrum of 109 Cd.
The detection eﬃciency improves as the back bias voltage becomes higher. The
detection eﬃciency of 16.3% (at a back bias voltage of =15V) corresponds to a
276 µm-thick depletion layer. We can also see that, at a high back bias voltage, theratio of grade 0 and grade 7 events tends to increase and decrease, respectively.
Table 6.6: Comparison of the eﬃciency and grade ratio of 109 Cd events detected by
Pch 5-9 at various back bias voltages
VBB † Peﬀ ‡ g0 g1 g2 g3 g4 g5 g6 g7
[V]
%
%
5
10.1 4.4 0.9 3.5 3.5 12 5.3 16 53
10
14.2 4.3 0.7 5.0 4.3 14 5.4 19 47
15
16.3 6.5 0.5 5.6 6.4 15 4.8 18 43†: Voltage applied to back bias electrode.
: Detection eﬃciency of 109 Cd. We reconstructed the spectrumby accumulating grades 0, 2, 3, 4, 6, and 7.
‡

Pch8A Pch 8A-7 is the test device fabricated on the same wafer as Pch 5-9 withnew processing conditions optimized based on the experience of the development of
Pch 3 and Pch 5.
81%#EVKXG*1%:TC['XGPVU EQNNKOCVGF

Figure 6.25: Acquired image of 109 Cd with Pch 8A-7. The white dotted line shows
the border between the active and VOC regions. We irradiated collimated X-rays(109 Cd) within the white dotted circle on Pch 8A-7.

Since, as in the case of Pch 5-9, most X-ray events of 109 Cd were classiﬁed into
grade 7, we reconstructed the X-ray events by the ﬁtting method. We found that thedetection eﬃciency of Pch 8A-7 reached 20.2%, which corresponds to a 349µm-thickCHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

97

depletion layer. This detection eﬃciency seems to be diﬀerent from that of Pch 5-9.
Since the amp gain of Pch 5-9 is smaller than that of Pch 8A-7, the readout noise
is high and this may prevent the reconstruction of a part of the 109 Cd events. We
describe the improvement of the amp gain in section 6.2.4.
Pch14B Pch 14B-5 is the test device formed on the new wafer with the highresistivity (>10 kΩ · cm). This wafer has a diameter of 6 inches, therefore, theprocess line is diﬀerent from than that of test devices described below (Pch3, 4, 5,
and 8A). The reason for changing the size of the wafer is discussed in section 6.1.3.
This device is also equipped with a guard ring structure so that we can safely apply
a high back bias voltage. The guard ring electrode is installed in order to restrain
the leakage current. Most leakage current is observed at the edge of the detector as
the surface leakage current, therefore, we installed the guard ring electrode aroundthe outside of the CCD. The leakage current is then absorbed in this electrode andwe can safely apply a high voltage to the detector.
We applied +12 V and +24 V to the back bias electrode and irradiated collimated X-rays of 109 Cd. We found that the detection eﬃciency is 21.8% and 22.4%
respectively for the two back bias voltages, which corresponds to that of the 360 and
372µm-thick depletion layer, respectively. We note that the X-ray events detectedby this device at the back bias voltage of +24 V show a horizontal tail which may becaused by a transfer failure. This suggests that we need to adjust the gate transfer
voltage with respect to each back bias voltage.
Pch15 Pch 15-14 is one of the ﬁnal devices among the test devices of Pch CCD.
This CCD is fabricated on the high resistivity (10 kΩ · cm) wafer and thinned to
200 µm. The illumination method for this device is back-illumination. Judging fromthe high resistivity of the wafer, since the thickness of the depletion layer of thisdevice is expected to be over 200 µm, we can expect that the device is fully depleted
by the thinning.
We irradiated collimated X-rays of 109 Cd and successfully detected the events asshown in Figure 6.27. We reconstructed the X-ray events by the ﬁtting method andfound the detection eﬃciency for X-rays from 109 Cd (22.2 keV) to be ∼ 193 µm.
This is essentially equal to the thickness of the wafer (200µm) and suggests that
this device is probably fully depleted. We describe the characteristics of this device
in detail as well as the back illuminated structure in the following section.

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-$$8
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#EVKXG$$881%:TC['XGPVU EQNNKOCVGF*1%#EVKXG$$8$$8%NQUGWR8KGY%NQUGWR8KGY

Figure 6.26: Acquired image of X-rays with Pch14B-05. (Upper) The back bias
voltage is 12 V. (Middle) The back bias voltage is 24 V. (Bottom) Close-up view ofdetected X-ray events. Due to its thick depletion depth and large energy of X-rays
from 109 Cd, most of the X-ray events are spread over 2 × 2 pixels.

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Figure 6.27: Acquired image of X-rays with Pch 15-14. The back bias voltage is 0 V.
(left) The white dotted line shows the border between active and VOC, and HOC
regions. We irradiated collimated X-rays (109 Cd) within the white dotted circle areaon this device. (right) Close-up view of detected X-ray events. Due to its thick
depletion depth, back-illuminated structure, and large energy of X-rays from 109 Cd,
many X-ray events are spread over 2 × 2 pixels and are classiﬁed into grade 7.

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

6.2.3

99

Improvement of detection eﬃciency for soft X-rays

As described in the previous section, we conﬁrmed that we can obtain a CCD which
has a high sensitivity for hard X-ray photons, by adopting a Pch CCD. We also
successfully achieved the thickness of the depletion layer over 300 µm. This makes
it easy to remove the ﬁeld-free region and adopt “a back illuminated structure” inorder to increase the quantum eﬃciency for soft X-rays. As we discovered in the
fabrication of Nch BS-CCD, we cannot apply the chemical etching method to thehigh resistivity wafer. Therefore, we are required to remove the ﬁeld free layer using
the mechanical grinding and polishing methods. From the experience of Nch CCD,
we have already conﬁrmed that we can mechanically thin the Si wafer to 150 µm
without any mechanical damage or performance degradation, which is independentof the resistivity of the wafer. In the case of Nch CCD, since the depletion layer
thickness increases to 80 µm, a ﬁeld free layer was left ∼ 70µm. On the other hand,since the depletion layer thickness of Pch CCD increases to over 300 µm, we can
expect that we can get rid of the ﬁeld-free layer completely by thinning the wafer
down to 300 µm.
In this section, we present the results from the performance evaluation for Pch
BI-CCD fabricated on the n-type semiconductor.
6.2.3.1

test device

Table 6.7: Speciﬁcations and typical performance of test models of Pch CCD (BackIllumination)
Type No.
Pch 9-17
Pch 15-14 Pch 15-22
Format
512 × 512
Pixel size
24 µm × 24 µm
Clock phase2 phaseTransfer method
Full Frame Transfer
Illuminated method
Back IlluminatedWafer Resistivity
10kΩ · cm 10kΩ · cm 10kΩ · cm
Depletion layer-†
193 µm-†Wafer⋆
200 µm200 µm200 µmRemarks
Al coating† : (not measured.)⋆ : Nominal value reported by HPK (not measured).

Pch9 Pch 9-17 is the ﬁrst test device of the BI-CCD fabricated on the n-typesemiconductor. The resistivity of the wafer is 10k Ω · cm. The format and packageof these devices are diﬀerent from those of the FI-CCDs described in the previous
section. We show an overview of the Pch 9-17 in Figure 6.28 (left).

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

100

Figure 6.28: A photograph of the back-illuminated Pch CCD. Pixel size is 24µm
× 24µm and format is 512 × 512 pixels. (Left) Pch 9-17 and Pch 15-14. (Right)
Pch 15-22. Due to the presence of the Al coating, the hand of the photographer isreﬂected on the surface.

The wafer is thinned to 200 µm, judging from the wafer resistivity, and the thedepletion layer is expected to increase in thickness to ∼ 300µm without thinning.
We can thus expect that the wafer is fully depleted after the thinning process.
As shown in Figure 6.29, we conﬁrmed that this device successfully detected the
X-rays of 241 Am. Figure 6.30 (left) shows the spectrum accumulated grade 0, 2,
3, 4, and 6 events. We could not ﬁnd the two characteristic X-ray lines of 16.9
and 17.8 keV separately, due to the high readout noise of ∼ 82 e− , most of which
can probably be attributed to the noise of the system. As compared to the 241 Am
spectrum detected by Nch CCD (P15 14-5B1P-4) which has a depletion layer 65-µm
thick (see Figure 6.30 right), we see that the magnitude relationship between the
13.9 keV and 16.9 keV lines is reversed. This means that the hard X-ray sensitivityof Pch 9-17 is higher than that of Nch CCD. We note, however, that it is diﬃcult
to evaluate this issue quantitatively due to the very high readout noise which may
be caused by the bad condition of the drive circuit.
We also found the phenomenon wherein we could not drive this device normallywhen we applied a voltage of 13 V or more to the back bias electrode. Figure 6.31
shows the acquired images using Pch 9-17 for various back bias voltages of 5 V,
13 V, and 15 V. In the case of back bias voltages of over 13 V, the OS signal
also clearly has an improper shape. We could not ﬁnd the cause of this unusual
behavior. However, the test device of Pch 14B-05, which has a guard ring electrodedrives normally even if we apply a voltage of 24 V to the back bias electrode (seeprevious section). Furthermore, Pch 15-22 which has the same structure as thisdevice also drives normally for the back bias voltage of = 25 V (see section 6.2.5).
Therefore, it is highly likely that this phenomenon can be solved by adopting theguard ring electrode and optimizing the manufacturing process at HPK.

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

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Figure 6.29: Acquired image of 241 Am by Pch 9-17 with the back bias voltage of 5V.
The white dotted line shows the border between active and VOC, and HOC regions.
We irradiated X-rays (241 Am) over the whole area of the CCD.







&GRNWO PQOKPCN&GRNWO OGCUWTGF

Figure 6.30: (left) Spectrum of 241 Am detected by Pch 9-17 with a back bias voltage
of 5V. Grade 0, 2, 3, 4, and 6 events have accumulated. (1), (2), (3), and (4) shows
the characteristic X-ray lines of 13.9 keV, 16.9&17.8 keV, 20.8 keV. and 26.2 keV,
respectively. We successfully detected X-ray events by the back-illuminated andfully-depleted X-ray CCD. However, we could not ﬁnd the two emission lines of
16.9 and 17.8 keV separately, due to the very high readout noise of ∼ 82 e− , mostof which can be attributed to the noise of the system. (right) Spectrum of 241 Am
detected by Nch CCD which has a 65-µm-thick depletion layer. (P15 14-5B1P-4).

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD$$8$$8102$$8Figure 6.31: Comparison of images acquired by Pch9-17 with various back bias
voltage of 5V (left), 13V(center), and 15V(right panel). The white dotted line
shows the border between active and VOC, and HOC regions. The exposure time is
1 sec. The dark current is apparently very high. Though the thermometer mounted
near the CCD indicated the temperature to be ∼ −90◦ C, the CCD device itself may
not be refrigerated fully due to problem of the refrigeration setup. When we apply
a voltage of 13V or more on the back bias electrode, Pch 9-17 stopped outputting aproper OS signal and we could not acquire normal image from the CCD.

Pch15 As already mentioned in the previous subsection, we conﬁrmed that the
BI-CCD of Pch 15-14 successfully detected X-rays and achieved a high detection
eﬃciency for hard X-rays. Figure 6.33 shows the acquired image of β-ray with
Pch 15-22. Pch 15-22 is essentially the same as Pch 15-14, except that the surfaceof Pch 15-22 is coated with Al with a 100 nm-thickness for optical blocking. We
irradiated the β-rays (90 Sr) over the whole area of the CCD. The energy of the βrays was 546 keV. A β-ray interacts with silicon with ionization loss and generates
electron-hole pairs along its track. The generated charges drift along the electricﬁeld in the depletion layer. They create a unique charge distribution whether the
charges are generated in the depletion layer or the ﬁeld-free region. We show aschematic view in Figure 6.32; when a β-ray interacts in the depletion layer, thecreated holes drift rapidly to the gate electrode, thereby forming a narrow charge
distribution of the track as a “tail”. On the other hand, when a β-ray interacts in theﬁeld-free layer, the created holes expand slowly by diﬀusion to reach the depletion
layer, thereby forming a wider charge distribution of the track in the form of a“head”. Therefore, if the device is fully depleted, there are no β-ray events whichhave the “head” structure. As shown in Figure 6.33, we cannot see β-ray eventswith a “head”, therefore, we can expect that almost the entire ﬁeld-free region isremoved.

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

103

0EJ%%&%NQUGWR8KGY

G
2EJ%%& (WNN[FGRNGVGF%NQUGWR8KGYG

Figure 6.32: Schematic view of the interaction of β-rays in a CCD and the exampleof the image of the β-ray track obtained by a CCD. (Upper) The case of the typical
Nch CCD. The image of β-ray track is obtained by CCD-CREST [64].(lower) The
case of the fully-depleted CCD.

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Figure 6.33: Image of β-rays from 90 Sr detected by Pch15-22 with a back bias voltage
of 5V. The white dotted line shows the border between active and VOC, and HOC
regions. We irradiated the β-rays (90 Sr) over the whole area of the CCD.

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

6.2.4

104

Amp gain and CTI

At the stage of trial production, we found that the test devices of Pch CCD suﬀer(1) low ampliﬁer gain, and (2) bad CTI as compared with those of Nch CCDs.
These imperfections brought about a degradation of the spectroscopic ability,
which is one of the features of a CCD. We solved these problems using the following
process.
6.2.4.1

Amp gain

The node sensitivity of Pch 3-10 and Pch 5-9 is 0.8 and 0.9 µV/e− , repectively,
which is much smaller than the values for Nch CCDs (about 2 − 3 µV/e− ,see also
Table 6.1). This is because in the layout design of the FDA of these devices, wediverted the existing mask design, which is optimized not for Pch CCD but for
Nch CCD. Therefore this small ampliﬁer gain is just as we expected. This problem
has been solved by optimization of the FDA design by HPK. The node sensitivity of
Pch 8A-7 was improved to 1.6 µV/e− , which is ∼ 2 times higher than that for Pch 310 and Pch 5-9. Kamata et al. (2004) [27] also reported that the node sensitivityof the test devices of FI8A-I (Pch 8A-2) and FI8A-IV(Pch 8A-7) were improved to
2.2 and 2.0 µV/e− , and their read out noise was 9 and 11 e− , respectively. Thisvalue is essentially equivalent to those of the established Nch CCD. As shown in thefollowing subsection 6.2.5, Pch 15-22, which is the ﬁnal test model of the Pch CCDs
also achieved a high node sensitivity of ∼ 1.8 µV/e− .
6.2.4.2

CTI

We also had the problem of poor CTI of the Pch CCD. In order to investigate thecause of the poor CTI, we performed the “Fat Zero” experiment. Fat Zero is thetechnique employed to improve CTI which is attributed to many trap levels. We
injected some charges (sacriﬁcial charges) to the whole area of the CCD in advance.
Then, trap levels were ﬁlled, which allows the signal charge to be transferred without
signiﬁcant loss due to the trap levels. Therefore, if the CTI is signiﬁcantly improved
by Fat Zero, then the main cause of the poor CTI is the existence of many traplevels. Note that this method is, however, rarely adopted for scientiﬁc observations,because it has the disadvantage of increasing the shot noise of the sacriﬁced charge
in addition to the readout noise.
We performed this examination using the test device Pch 8A-7. Without Fat
Zero, we found the CTI to be 5.2×10−4 /transfer (horizontal) and 6.4×10−4 /transfer(vertical) which are signiﬁcantly poorer than those of the Nch CCD (see also Table 6.1).
We installed a red LED in the vacuum chamber in order to irradiate visible light.
We evaluated improvements of the CTI with various amounts of optical photons

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

105

incident on the CCD. We adjusted the amount of optical photons by the length ofthe luminous time of the LED illumination. We acquired data at luminous times of0, 2, 4, and 6 sec. The exposure time of the CCD is 10 sec in all cases. Figure 6.34
shows an image acquired by the Pch 8A-7 with 6-sec irradiation of LED light. We
show the results in Table 6.8 and in Figure 6.35. It can be clearly seen that the CTI
is signiﬁcantly improved by the Fat Zero method, therefore, we conﬁrmed that apoor CTI mainly causes many trap levels in the Si wafer. Based on these results, we
decided to build a test device using wafers supplied by other wafer vendors. Pch 14B05 is one of the test devices fabricated on the new wafers (see also section 6.2.2). We
found that the CTI was signiﬁcantly improved to be 2.4 × 10−6 /transfer (horizontal)
and < 0/transfer (vertical), and reconﬁrmed that a poor CTI is attributable to thetrap levels.
81%

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Figure 6.34: Image acquired by Pch 8A-7 with 6-sec irradiation of LED light. We
also irradiated X-rays of 55 Fe on the whole area of the CCD. The exposure time is
10 sec. The temperature of the CCD is -89 ◦ C

Table 6.8: Relationship between luminous time of LED and CTI of Pch 8A-7
Luminous time [s]
0
2
4
6
−
Generated Charge[e ] 45 331 594 874
CTIH [10−4 /transfer]
5.2 1.1 0.6 0.9−4CTIV [10 /transfer]
6.4 4.2 3.4 2.5

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

106

CTI Vertical
CTI Horizontal

Generated Charge by LED[electron/pix]

Figure 6.35: Relationship between luminous time of the LED and CTI of Pch 8A-7

6.2.5

Final test device

As shown previously, we identiﬁed and solved problems in each development plan
through the performance evaluation of test devices of Pch CCD. Based on the resultsof these developments, the Pch 15-22 was built as a ﬁnal test device of Pch CCD.
We have already mentioned in section 6.2.3 that this device has a wafer with 200-µm
thickness and could be fully depleted. Using this device, we performed performance
evaluations in order to conﬁrm whether this device maintains the characteristics
which we have already achieved in other test devices.
Figure 6.36 shows an image of the X-rays from 109 Cd acquired by Pch 15-22 witha back bias voltage of 10V. We irradiated X-rays over the whole area of the CCD
and successfully detected them. Table 6.9 shows the characteristics of Pch 15-22
with various back bias voltages of 5, 10, 15, and 20V. It can be clearly seen that
there are not few changes in the performance dependent on the back bias voltage.
It should be noted that the readout noise of ∼ 50 e− is very high, most of which
could be attributed to the noise of the system. Because the node sensitivity of thisdevice (1.7–1.8µV/e− ) is equivalent to that of Pch 8A-7 (1.6µV/e− ), the readout
noise of the CCD itself is estimated to be ∼ 10 e− from the results showing that thePch 8A-7 recorded a low readout noise of ∼ 10 e− in NAOJ[27].
However, the CTI is rather bad compared to that of Pch14B and Nch CCD.
Though we do not yet know the factors for bad CTI, there is the possibility that
the CTI could be improved by the optimization of the transfer voltages and thecondition. The dark current was not detected signiﬁcantly in the 20-sec exposure
time, which means it is suﬃciently low to be used as an X-ray CCD.
Table 6.10 shows the grade branching ratio of 109 Cd detected by Pch 15-22 forCHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

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Figure 6.36: Image of X-rays acquired by Pch 15-22 with a back bias voltage of 5V.
(left) The white dotted line shows the border between active and VOC, and HOC
regions. The exposure time is 18 sec. The temperature of the CCD is -89◦ C We
irradiated X-rays (109 Cd) over the whole area of the CCD. (right) A close-up viewof the detected X-ray events. Due to its thick depletion depth, the back-illuminatedstructure, and the large energy of X-rays from 109 Cd, many X-ray events are spreadover 2 × 2 pixels.

Table 6.9: Comparison of the characteristics of Pch 15-22 for various back bias
voltages
†
VBB
[V]
5
10
15
20

r.o.n‡
[e− ]
56
54
51
54

∆E⋆
[keV]
0.95(0.94-0.96)
0.93(0.92-0.94)
0.83(0.83-0.84)
1.0(0.98-1.0)S⋆⋆v[µV/e− ]
1.7
1.7
1.8
1.8

♯
CTI♭x
CTI♭♭
Idark
y
[10−5 /transfer] [10−5 /transfer] [e− /sec/pix]
5.0(2.0-8.0)
0.95(<3.8)
0(<0.1)3.9(0.9-6.9)0(<1.7)0(<0.1)2.5(0.44-4.8)
1.3(<3.3)0(<0.1)2.0(<4.8)
2.3(< 4.8)0(<0.1)

Parentheses indicate the 1-sigma conﬁdence limit.
†: Voltage applied to back bias electrode.
‡: Readout noise (r.m.s.).
⋆: Energy resolution (F.W.H.M.) of 109 Cd Kα (22.2 keV).
⋆⋆: Node sensitivity.
♭,♭♭: Charge transfer ineﬃciency of horizontal(x) and vertical(y) transfer.
♯: Dark current at −90◦ C.

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

108

various back bias voltages. It can be seen that, for a high back bias voltage, the ratioof grade 0 events and grade 7 events tends to increase and decrease, respectively.
Table 6.10: Comparison of grade ratio of 109 Cd detected by Pch 15-22 for various
back bias voltagesa
VBB
g0 g1 g2
g3 g4 g5 g6 g7
[V]
[%]
5
7.5 0.2 6.8 2.9 2.9 0.5 38 41
10
7.6 0.2 7.5 3.1 3.1 0.8 44 33
15
8.0 0.3 8.3 3.1 3.0 0.8 46 30
20
11 0.1 10.4 5.0 4.3 1.0 51 17a: Voltage applied to the back bias electrode.

We coated the surface of Pch 15-22 with Al to block the optical photons, and weevaluated the eﬃciency of the optical blocking by the surface Al coating layer. We installed a red LED in the vacuum chamber. We determined the optical transmissivityof Pch 15-22 and compared it with the Pch 15-14 which has no Al layer. Figure 6.37
shows a comparison of images acquired by Pch 15-22 and Pch 15-14. We irradiatedlight from the LED on to Pch 15-22 and Pch 15-14 at luminous times of 3 sec and180 msec, respectively. The LED luminance is common in both cases. Neverthelessthe luminous time is much longer than that for Pch 15-14. We can clearly see thata small amount of charge is generated in the active area of Pch 15-22. This suggests that the surface Al coating layer successfully blocks optical light, as expected.
The amount of generated charge per unit time in Pch 15-22 and Pch 15-14 were
384 e− /s/pixel and 1.2 × 105 e− /s/pixel, respectively. Therefore, the transmissivityof the surface Al layer of Pch 15-22 was estimated to be 0.32 %. We note that,
however, the transmissivity is possibly overestimated. Since we irradiated LED to
the whole area of CCD chip, CCD probably detected LED light incident from theside face of CCD chip which is not coated with Aluminum

6.2.6

Summary of the goal plan

We summarize the development of Pch CCD for SXI following the goal plan as
follows.
• Using the high resistivity n-type silicon wafer, we successfully developed the
Pch CCDs which have unprecedented high sensitivity for hard X-rays. For example, the test device fabricated on a super high resistivity (> 10kΩ·cm) wafer
recorded a detection eﬃciency of over 20% for 22.2 keV, which is signiﬁcantly
higher than that of Nch CCD (∼ 5%).

CHAPTER 6. DEVELOPMENT I -BACK SUPPORTLESS CCD-

Pch 15-22 (w Al coat)109

Pch 15-14 (w/o Al coat)
VOC HOC

VOC HOC
ActiveActive3-sec exposure180-msec exposure

Figure 6.37: Comparison of images acquired by Pch 15-22 (with Al coating) and
Pch 15-14 (without Al coating).

• We fabricated the back-illuminated CCD on high resistivity n-type wafer.
From the experience of Nch CCD, we safely thinned the wafer to 200µm.
We conﬁrmed that this device successfully detected X-rays and was almost
fully depleted by the measurement of the detection eﬃciency for 109 Cd andthe shape of the β-ray track.

Chapter 7
Development II -Focal Plane
AssemblyIn order to reduce the dark current in the exposure time, we need to cool the X-ray
CCDs to a low temperature. Furthermore, by cooling the CCD, we can reduce thedegree of radiation damage to the CCD in the environment of orbit[41]. We will coola CCD chip to a temperature of −90 ◦ C using the thermo electric cooler module(TEC) which is assembled with many Peltier devices.
A TEC has already been applied as the main cooling method for X-ray CCDs
onboard ASCA and Suzaku. A TEC consists of multiple semiconductor-based
electronic components. When DC power is applied to a TEC, heat moves throughthe module from one side to the other in proportion to the applied voltage. A TEC
can be used either as a cooler or a heater, epending on the direction of current ﬂow
Because a TEC uses no moving parts, and employs no ﬂuids, thereby eliminating
the need for piping and mechanical compressors used in vapor-cycle cooling systems,
it has the advantage of compact and durable. Thus, a TEC is often adopted as thecooler for a CCD carried on a satellite. For eﬃcient use of the limited power resources
available onboard a satellite, we need to reduce the power consumption of the TEC
to be as low as possible.
A CCD carried on a satellite is usually assembled with the cooling system, including a TEC and associated electronics, in a single unit. We usually call thisunit the “Focal Plane Assembly” (FPA). Since they are closely connected to eachother, we cannot perform any estimations of the heat budget separately. In thischapter, we describe the design and development of the cooling system and focal
plane assembly (FPA) of the SXI.

7.1

Design Concept

In the design of FPA for SXI, we mostly paid attention to reducing the power
consumption of cooling. In order to cool down a CCD device eﬃciently, we need110

CHAPTER 7. DEVELOPMENT II -FOCAL PLANE ASSEMBLY-

111

Peltier device

Figure 7.1: Schematic view of an ordinary single-stage TEC module.

to (1) develop a TEC with high performance, and (2) reduce the heat input into a
CCD chip. In the following subsection, we describe (1) the development of the TEC
for SXI, (2) factors of heat input, and (3) several solutions to reduce the heat input.

7.1.1

Peltier device

A TEC module is assembled with many Peltier devices. We show a schematic viewof an ordinary TEC module in Figure 7.1.
In this subsection, we brieﬂy explain the principle of the Peltier device, as basic
knowledge for the design of the TEC module.
In 1834, Peltier discovered the inverse of the Seebeck eﬀect, now known as the”Peltier eﬀect”. By applying a voltage to thermocouple, a temperature diﬀerence iscaused between the junctions. This results in a small heat pump, which subsequently
came to be known as a “Peltier device”.
We show the schematic view of a typical Peltier device in Figure 7.2. The
structure of the Peltier device includes π-shaped p-type and n-type semiconductors
connected in series. A positive voltage applied to the n-type material drives electrons
from the p-type to the n-type material, and heat then moves from the cold side to
the hot side (downward in Figure 7.2) by the Peltier eﬀect. The temperature on thecold side decreases as heat is absorbed and that on the hot side increases. The heatbudget is given by the following relation;
Qc + pI 2 r + k∆T = αTc I(7.1)(1 − p)I 2 r + αTh I = Qh + k∆T.

(7.2)

Qc and Qh are the net heat absorbed at the cold side and the net heat removed
at the hot side, respectively. r and I represent the resistance and the current of theCHAPTER 7. DEVELOPMENT II -FOCAL PLANE ASSEMBLY-

electrical insurator

Qc

Tc

Heat Absorbed(cold side)
2

αTcI

h+

k∆T
αThI

112

e-

pI r
2

P-typeTh
electrical insurator

Qh(1-p)I r
N-typeHeat Removed(hot side)

Qc: net heat absorbed at the cold side
p: branching ratio of Joule heat
Qh: net heat removed at the hot sider: resistance of peltier device
Tc: temperature at the cold side
α: Seebeck constant of peltier device
Th: temperature at the cold side
k: thermal conductivity of peltier device

Figure 7.2: Conceptual diagram of a Peltier device.

Peltier device; the Joule heat (I 2 r) generated in the Peltier device is conducted tothe cold side and the hot side. p is the branching ratio of the Joule heat, which isordinarily 0.5. k is the thermal conduction coeﬃcient, and the heat of k∆T moves
upward (from the hot side to the cold side) by thermal conduction. α refers to the
Seebeck coeﬃcient of the semiconductor. By the Peltier eﬀect, the heat of αTc I isabsorbed at the cold side and the heat of αTh I is reduced at the hot side.
From these relations, we found the main factors to determine the characteristics
of a Peltier device.
We changed equation 7.1 as follows;1
αTc 2
α2 Tc2S
Qc = − (I −) +(− κ∆T )( ),
2
r
2ρ
l

(7.3)

where ρ, κ, S, and l are the resistivity, thermal conductivity, cross section, andlength of the semiconductor, respectively.
In order to absorb the heat at the cold side, Qc must be positive under someconditions, therefore:
α2 Tc2− κ∆T > 0,2ρ
α2 Tc2
∆T <
.
2ρκ

(7.4)
(7.5)

CHAPTER 7. DEVELOPMENT II -FOCAL PLANE ASSEMBLY-

113

Equation 7.5 shows the maximum diﬀerence (∆Tmax ) in temperature between
the cold side and hot side, or∆Tmax =

α2 Tc2
.
2ρκ

(7.6)

In the case of SXI, we planned that (1) the temperature of CCD be −90 ◦ C (coldside) and (2) the temperature of the heat sink be −45 ◦ C (hot side). Assuming theparameters of typical Bi-Te semiconductors of the Peltier device are equal to thoseof Peltier device adopted by the TEC of Suzaku XIS[48], orρ = 0.98 × 10−3 Ω cm,
κ = 1.65 × W cm

−1

(7.7)
−1

K ,

and

(7.8)
(7.9)

α = 1.98 × 10−4 V K−1 ,(7.10)∆Tmax = 40K.

(7.11)and Tc = −90 ◦ C, then,Therefore, a one stage TEC cannot achieve the required diﬀerence in temperature of 45 K at Tc = −90 ◦ C. By increasing the number of stages of a TEC, theeﬃciency of cooling of the TEC is improved because the temperature gap per one
stage becomes lower. On the other hand, the Peltier device is fragile and a very
thin, multi-stage TEC usually requires mechanical support using posts to support
the weight. For example, a three-stage TEC used as the cooler in the Suzaku XIS
needs three posts to support the CCD device (see Figure 7.3 left).
Due to these posts, there is a large heat input to the CCD chip by thermal
conduction. As described later (see Table 7.1), the fraction of the heat input attributable to the posts is over 25% of the total heat input. Furthermore, we havethe problem that the bottom of the CCD device is separate from the top side of the
TEC by thermal cycles, because of the diﬀerence of the thermal expansion between
the TEC and posts. Though this problem has already been solved by matching
the thermal expansion between the TEC and posts by improvements of the TEC
and by changing the material of the posts, this problem has the potential to cause
degradation of thermal contact and, at worst, destruction of the TEC by bending
and/or shear stress.
On the other hand, in the case of MAXI-CCD, the CCD chip is directly glued
to the top of the TEC and is supported only by the TEC itself, i.e. not by posts(see Figure 7.3 right), so that the heat input caused by thermal conduction can be
minimized. In order to reduce the weight of the CCD, (1) the package structure inwhich a CCD chip is usually installed is removed, and (2) the CCD chip is connectedCHAPTER 7. DEVELOPMENT II -FOCAL PLANE ASSEMBLY-(a) Overhead View

CCD chipPeltier device

114

Bonding wire
Side View

Top View(b) Side View

Figure 7.3: (left) Schematic view of FPA of Suzaku XIS. The CCD device is supported by a 3-stage TEC and three standoﬀs (posts). (right) Picture of MAXI-CCD.
To reduce the heat input to the CCD as much as possible, a unique structure is used.

via bonding wires to the hot side instead of a ﬂexible printed circuit. Since thebonding wire is aluminum wire which is thin (20-µm diameter) and short (3 mm),this is signiﬁcantly lighter than a ﬂexible printed circuit. Because the temperatureof a CCD chip is -60 ◦ C, which is a relatively high temperature, and the diﬀerence intemperature between the CCD chip and heat sink is 40 K which is relatively small,such thermal conditions can be realized by a single-stage TEC.
In the case of SXI, however, the temperature diﬀerence of 45 K between the
CCD chip and the heat sink is too large to be achieved using a single-stage TEC,
as shown in equation 7.11. We therefore designed the FPA consisting of a 2-stage
TEC without posts. We have examined the possibility of realizing such a uniquestructure in cooperation with HPK and Aisin Seiki Co. Ltd. (AISIN).

7.1.2

Heat budget

In this subsection, we estimate the heat input to a CCD chip and introduce ourdesign plan to reduce the heat input as much as possible.
Wire bonding In order to sustain a CCD device with only the TEC module, weneed to reduce the mass of the CCD chip as much as possible. In the case of XIS,
as shown in Figure 7.3 (left), the CCD is wired by a ﬂex print circuit. However,
because the ﬂex print circuit is very heavy, it is expected to be diﬃcult to support
only with the TEC. Based on the experience with MAXI-CCD, we decided to adopt
wire bonding for the electrical connection between a CCD chip and the hot side.

CHAPTER 7. DEVELOPMENT II -FOCAL PLANE ASSEMBLY-

115

Q1->2: net heat absorbed at the cold side
S: cross section, l:length
T1: temperature at the hot side
T2: temperature at the cold side

Figure 7.4: Conceptual diagram of heat transfer due to thermal conduction.

On the other hand, since the wires, which distribute the clock voltage for driving
the CCD, connect the hot side to the CCD (cold side), this is one of the main pathsof heat input. We need to study the heat input through these wires. Assuming thethermal condition shown in Figure 7.4, the heat transfer Qcond through the wires isthe following equation of heat conduction;
S
Qcond = ρ (T1 − T2 ),(7.12)lwhere ρ, S, l, T1 , and T2 are the coeﬃcient of thermal conductivity, a cross section,
length, temperature at the side of 1, and temperature at the side of 2.
Therefore, to reduce the heat input by the heat conduction, we need to design
the bonding wire, paying attention to the following points;• the material with low thermal conductivity coeﬃcient,• thin and long wire, as much as possible.
Though Au and Al wires are usually used for the bonding wire, the thermal
conductivity coeﬃcient of Al (238 Wm−1 K−1 ) is signiﬁcantly lower than that of Au(320 Wm−1 K−1 ). Therefore, we decided to use the Al wire. From the view pointof mechanical strength and tight bonding, the diameter of the Al wire is limited
to 20 mm and the length is up to 3 mm. We note that the speciﬁcation of thiswire is the same as that of the MAXI-CCD. Therefore, we can expect that bonding
processing using this wire has already been established. The heat input is 1.0 mW
per wire, which is calculated on the assumption that the diﬀerence in temperature
between a CCD cold side and hot side is 45 K.
Al coating The thermal radiation of the housing of SXI is also the main path of
the heat input to a CCD. Since the temperature of the housing is planned to be∼ −45 ◦ C, which is much higher than that of a CCD (−90 ◦ C), the heat input due toCHAPTER 7. DEVELOPMENT II -FOCAL PLANE ASSEMBLY-

116

T1, ε1T2, ε2

Q1->2: net heat transfer from 1 to 2
ε1,2: emissivity and absorption coefficientof the surface of the plate 1, and 2.
T1.2: temperature of the plate 1, and 2

Figure 7.5: Conceptual diagram of the heat transfer due to thermal radiation.

thermal radiation is not negligible. To reduce this heat input, it is eﬀective to coatthe surface of a CCD and the housing with the material which has low emissivity
and absorption coeﬃcient. This is also evident from the following equation for theheat transfer by radiation (this equation assumes the thermal condition shown in
Figure 7.5);
T4 − T4
Qrad = Sσ 1 1 1 2 ,+ ϵ2 − 1
ϵ1(7.13)

where S, σ, T1 , T2 , ϵ1 , and ϵ2 is the surface area of a CCD, Stefan-Boltzmanncoeﬃcient, temperature of plate 1 (assuming the housing), temperature of plate 2(assuming the CCD), emissivity and absorption coeﬃcient of plate 1, and emissivity
and absorption coeﬃcient of plate 2.
The emissivity of the CCD surface is usually ∼ 1. On the other hand, aluminum
evaporation on the surface of the CCD improves ϵ to 0.1, therefore we can expectthat the heat input to the CCD by radiation signiﬁcantly decreases. As shown in
Table 7.1, the heat input by the radiation of MAXI-CCD (15 mW) is smaller thanthat of Suzaku XIS (136 mW). This is because the aluminum is evaporated noton the surface of Suzaku XIS but on that of MAXI-CCD. We note that aluminum
evaporation plays the role of a ﬁlter for optical blocking and is useful to simplify the
structure of the housing of a CCD because we need no longer assemble an additional
optical blocking ﬁlter.
Assuming that we coat the surface of the CCD chip and the housing with thealuminum, we can calculate the heat input to the CCD chip by radiation. We assume
that (1) the size of the CCD chip is 75 × 50mm2 and (2) the CCD chip is enclosed in
an isothermal box of -45 ◦ C temperature which corresponds to the housing. Then,the heat input is 27 mW, which is also quite small compared to that of the Suzaku

CHAPTER 7. DEVELOPMENT II -FOCAL PLANE ASSEMBLY-

117

XIS (136 mW)
We summarize the estimated heat inputs to a CCD chip for SXI, comparing
the values with those of Suzaku XIS and MAXI-CCD, in Table 7.1. Assuming
the number of readout nodes and wires of SXI to be 8 and 110, amp heating andconduction heating through the wires is estimated to be 40 mW and 110 mW,
respectively. Then, the total heat input is 177 mW. Despite the larger size comparedto the Suzaku XIS, the total heat input is successfully restrained below that of the
Suzaku XIS.
Table 7.1: Comparison with FPAs of existing missions and NeXT-SXI
Suzaku XIS MAXI-CCD NeXT-SXIBasic Characteristics
Size of IA
25 × 25mm2 25 × 25mm2 50 × 50mm2
No. of nodes
4
1
8
No. of cables
57
26
110
CCD surface coating none
Al
Al
Temp. of CCD-90◦ C-60◦ C-90◦ C
∆T
50K
40K
45K
Heat Input
Radiation heating
136 mW
15 mW
27 mW
Amp heating
18 mW
2 mW
40 mW
Conduction heatingfrom cable 28.5 mW
26 mW
110 mW
from support posts 67.5 mW
0 mW
0 mW
Total heat input
250 mW
43 mW
177 mW
Characteristics of TEC
No. of stage
3
1
2
Power consumption
4.0 W
1.0 W
7.0 W

7.2

Trial production of TEC module

Based on the calculation results, we designed a TEC with a 2-stage Peltier module,in cooperation with HPK and AISIN. In the design, we established the following
goals;• to sustain a CCD chip of 75mm × 50mm × 300µm without additional posts,• to absorb heat of 180 mW at the cold side,• to realize a temperature diﬀerence of 45 K between the cold side (Tc ) and the
hot side (Th ) at Th is -45◦ C,

CHAPTER 7. DEVELOPMENT II -FOCAL PLANE ASSEMBLY-<simulation model>

118

Ag-paste joint
Conductivity (5W/m/K)

CCD(Si wafer)
Peltiermodule(Temp. of CCD)
<Large-Size TEC>
HeightSize

Stage(plate)
Upper
MiddleBottomremarks
φ35 hole(total)φ36 hole<Small-Size TEC>
Stage(plate)
Upper
MiddleBottomHeightSize
 O O  OO O O O O  OO  OO

remarks(total)Figure 7.6: Simulation model for the heat performance of TEC.

and• to make a hole of 35 mm-diameter to pass through the hard X-ray penetrated
a CCD (back supportless structure).
Large Size TEC First, we planned to build a large-size TEC module (large-size
TEC), with a suﬃciently large (75mm × 50mm) stage to load a CCD chip without
protruding. The material of the plates for the stage is aluminum nitride (AlN) which
has a similar coeﬃcient of thermal expansion as that of silicon. We adopted an AlN
plate to reduce the risk of destruction due to the diﬀerence in thermal expansion
between the CCD and plate caused by the rapid temperature change in cooling.
Under the simulation model shown in Figure 7.6, we found that (1) we needed 18
and 81 Peltier devices in the upper and lower stages in order to meet the goals of
the TEC, and then (2) we can reach a temperature diﬀerence of 45 K with a power
consumption of ∼ 5 W as a result of the simulation of the heat performance of TEC
performed by AISIN. The results are shown in Figure 7.7.
Though the heat input to the SXI is restrained to be lower than that of XIS,
the TEC power consumption of 5 W is slightly higher than that of XIS (4.0 W).
This is due to the diﬀerence in the number of stages. As described previously, withincreasing number of stages of the TEC, the eﬃciency of cooling of the TEC improves because the temperature gap of each Peltier device becomes lower. We foundthat this TEC has good anti-shock strength, equivalent to that of the MAXI-CCD,
as a result of analysis for the mechanical strength by the ﬁnite element method

CHAPTER 7. DEVELOPMENT II -FOCAL PLANE ASSEMBLY-

119

Power Consumption[W]Temp. of CCD

Temp. of CCD [degC]

Power Consumption[W]simulation(2)simulation(1)Heat input [mW]

Figure 7.7: Result of simulation for cooling performance of the large-size TEC. (left)Relationship between the temperature of the CCD and the total power consumptionof the TEC. The absorbed heat is ﬁxed to be 180 mW. (right) Relationship between
the absorbed heat and the total power consumption of the TEC. The temperatureof the cold side (CCD) is ﬁxed to be −90◦ C.

(FEM). Since the MAXI-CCD has already been conﬁrmed to have suﬃcient mechanical strength to pass the vibration and shock test to simulate a rocket launch,
the mechanical strength of this TEC is promising without any additional mechanicalsupport.
On the other hand we had a problem in building up the TEC module. Since
we attached the Peltier devices to an AlN plate by soldering, the rapid heating andcooling by soldering makes them deform. Due to the large size of the AlN plate,serious distortion of the AlN plate causes the destruction of the TEC modules. We
could not solve this problem though we tried to adopt a low temperature solder andto change the assembling process. We thought that the large size of the TEC itself isthe main cause of this trouble, and we decided to divide the TEC into two modules.
Small size TEC We designed a small size TEC, which has a 49mm×20mm stage.
We planned to support the CCD chip using 2 modules. We also adopted a thick(0.635 mm) AlN plate for the stage in order to reduce the distortion by thermal
stress to the stage. The simulation of the thermoelectric characteristics of this TEC
was performed under the simulation model shown in Figure 7.6. We found 19 and 41
Peltier devices are necessary, to be attached to the upper and lower stages, in order
to meet the goals of thte TEC. Figure 7.8 (left) shows the relationship between a
CCD temperature and the total power consumption of the TEC as a result of thesimulation. Each TEC module needs to absorb half of the heat input to a CCD
chip, therefore, we ﬁxed the heat input of this TEC to 100 mW. We determined a
power consumption of ∼ 3.5 W per one TEC in order to realize the temperature
gap of 45 K. Figure 7.8 (right) shows the relationship between the heat input andthe total power consumption of the TEC. Therefore the total power consumption

CHAPTER 7. DEVELOPMENT II -FOCAL PLANE ASSEMBLY-

120

Table 7.2: Contests of the screening test performed in AISIN
item
exam. condition
acceptance criterion◦◦
Thermal cycle-40 C/-85 C, 15min×20 cycle (1)internal resistancechange is < 2%
Annealing100◦ C, 1 hour(2) internal resistanceis 1.01–1.55Ω (upperstep) and 1.28–1.80Ω(lower step)
Visual inspection Microscope ×20–

is 7.0 W, which is slightly increased compared to the large size TEC. The power
consumption of the TEC is, however, still restrained to be only 2 times higher thanthat of XIS, even though the area of the imaging region of the SXI is 4 times larger
than that of the XIS.
simulation(1)simulation(2)Heat Input =100 mW

Power Consumption[W]Power Consumption[W]Temp. of CCD = -90 degC

Temp. of CCD [degC]

Heat input [W]Figure 7.8: Result of simulation for cooling performance of small-size TEC. (left)Relationship between the temperature of the CCD and the total power consumptionof TEC. The absorbed heat is ﬁxed to be 100 mW. (right) Relationship between theabsorbed heat and the total power consumption of TEC. The temperature of thecold side (CCD) is ﬁxed to be −90◦ C.

In this design of a small size TEC, the TEC module was successfully assembled
without any failures. Figure 7.9 shows the appearance of the small size TEC module.
By the screening test performed at AISIN before shipping, this module has alreadyconﬁrmed that the expected electrical characteristics are achieved for the criteria
shown in Table 7.2.

CHAPTER 7. DEVELOPMENT II -FOCAL PLANE ASSEMBLY-121

AlN PlatePeltier device
Figure 7.9: Photo of the trial production of the 2-stage TEC module.

7.3

Thermo-mechanical model of FPA for SXI

In parallel to the trial production of the TEC module, we also proceeded with thedesign of the thermo-mechanical model of FPA for SXI. The purpose of the trial
production of the thermo-mechanical model is to examine whether (1) the cooling
performance and (2) anti-vibration shock strength were achieved as expected.
We assembled the thermo-mechanical model in a form as similar as possible
to the FPA, which will actually be assembled as the device onboard the NeXT
satellite. Figure 7.10 shows a drawing of the thermo-mechanical model of SXI-FPA.
This model mainly consists of the following parts;• the trial production of the TEC module (small-size TEC),• dummy CCD fabricated from AlN,
• Tungsten-copper (CuW) base plate to attach the TEC module,• Aluminum bonding wires of 20 mm-diameter and 3 mm-length.
The reason why we adopted AlN to a dummy CCD is (1) the thermal characteristics are very similar to that of silicon, and (2) processing of the AlN is easierthan silicon. The tungsten copper alloy is suitable for the base plate because of
its high thermal conductivity (193W m−1 K−1 ) and low thermal expansion coefﬁcient (6 × 10−6 K−1 ). We note that the thermal expansion coeﬃcient of AlN of∼ 5 × 10−6 K−1 is almost equal to that of CuW. Though Aluminum and Stainlesssteel (SUS) show good thermal conductivity and are easier to procure and process,their thermal expansion coeﬃcients are 23×10−6 K−1 and 15×10−6 K−1 , respectively.
To avoid the risk of destruction due to the diﬀerence in the thermal expansion, as

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CHAPTER 7. DEVELOPMENT II -FOCAL PLANE ASSEMBLY122

Figure 7.10: Design drawing of the mechanical model of FPA for SXI.
CHAPTER 7. DEVELOPMENT II -FOCAL PLANE ASSEMBLY-

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much as possible, we decided to adopt the CuW alloy as the base plate. To simulate
the heat input caused by thermal conduction, we bonded the 140 wires between adummy CCD and ﬂexible printed cable (hot side). We also attached thermometersof Pt 1kΩ and resistances on the dummy CCD, in order to measure the temperatureof the surface and to simulate the heating of the load resistance of the readout FET
on a CCD, respectively. They are electrically interfaced through the bonding wires
and ﬂexible print cable with the outside.
This model has already been assembled at HPK. A picture of the trial productionof this model is shown in Figure 7.11. However, in the inspection performed at
HPK before shipping, we found a problem in that the internal resistance of the
TEC suddenly increased by the thermal cycle test. Since we have already conﬁrmedthat the TEC module itself is driven normally, we considered that the structureof the thermo-mechanical model caused this problem. At present, checking andmodiﬁcation of the structure and assembling process of this model are in progress
to solve this problem.

Chapter 8
Summary
Soft X-ray Imager The Soft X-ray Imager (SXI) is the key detector to realize a
Wideband X-ray Imager (WXI), which is one of the focal plane detectors of N eXT .
SXI is required to perform precise imaging and spectroscopic observations in theenergy band of 0.5–20keV. From the view point that the X-ray CCD is equippedwith high spatial and energy resolutions, the X-ray CCD is the most suitable detector
for SXI. On the other hand, the X-ray CCD is required to maintain a high detection
eﬃciency for X-rays in the high energy band, while allowing hard X-rays undetectedby the CCD to pass through in order to enter the HXI. To satisfy these requirements,
we decided to design and develop the “Back Supportless CCD (BS-CCD)“, which
has a thick depletion layer, a thin ﬁeld-free layer, and no package at the rear side. In
order to develop the BS-CCD gradually and safely, we developed a “Baseline plan”
and a ”Goal plan”. The former is a rather conservative plan based on the establishedmethod to fabricate Nch CCDs, and the latter is an innovative plan which aims tofabricate a Pch CCD.
Back Supportless CCD We fabricated a test model of the BS-CCD using Nch
CCD (Nch BS-CCD). The test model of the BS-CCD of P15 14-5B1P achieved
excellent performance of readout noise of 7 e− and energy resolution of 144 eV, which
are essentially equivalent to those of the established X-ray CCDs. The thickness ofthe depletion layer is ∼ 70 µm, meaning that this device has one of the most sensitive
X-ray detectors for hard X-rays among past and present established X-ray CCDs.
We successfully thinned the ﬁeld-free layer of this device without any performancedegradation and conﬁrmed that the thickness of wafer was ∼ 190 µm, in accordancewith our design. Through the subsequent development of the BS-CCD, we could
safely thin down the wafer to 150 µm. Based on these successful results of thetest model, we built a medium-size BS-CCD (CCD-NeXT1) in order to conﬁrm thespeciﬁcations and equipment with the aim of loading it onboard the satellite. CCDNeXT1 was also successfully driven and achieved excellent performance including (1)low dark current of 12 e− s−1 pix−1 , (2) thick depletion layer of 84 µm, (3) high node124

CHAPTER 8. SUMMARY

125

sensitivity of 3.2 µV/e− , and (4) low CTI of ∼ 1 × 10−6 /transfer. The readout noiseof 21 e− seems to be worse than that of the test device (7 e− ), which is attributed
to the diﬀerence in the CCD driver system. The node sensitivity of CCD-NeXT1(3.2 µV/e− ) is somewhat higher than that of the test devices (∼ 2 µV/e− ), therefore
the read out noise of the CCD-NeXT1 itself is expected to be equal to or better thanthat of the test devices.
In parallel with the development of the Nch BS-CCD, we participated in thedevelopment of the Pch CCD which is being undertaken by a group of at the National
Observatory of Japan (NAOJ). We evaluated the performance of the Pch CCD
and conﬁrmed that the thickness of the depletion layer reached over 300 µm, whichis ∼ 4 times thicker than that of established CCDs (∼ 80 µm). This thick depletion
layer suggests that the detection eﬃciency of hard X-rays is signiﬁcantly increased.
For example, the detection eﬃciency for 22.2-keV X-rays increased from ∼ 5 %to ∼ 20 % by increasing the thickness of the depletion layer from ∼ 80 µm to ∼300µm. Such a thick depletion layer allows for the fabrication of the back-illuminated
CCD (BI-CCD). We fabricated the test model of the BI-CCD using a wafer thinned
down to 200µm. We conﬁrmed that this device was driven normally and obtained
results which suggest that a ﬁeld-free region is no longer left by the measurementof the depletion layer thickness and irradiation of β-rays. Furthermore, through ourexperiments to improve the performance, Pch CCD achieved excellent performance.
(1) We found the problem of low amp gain of < 1µ/V− in the earlier test device of
Pch CCD . This problem was solved by the optimization of the amp structure by
HPK, and we conﬁrmed that the amp improved to ∼ 2µ/V− , which is comparable tothat of Nch CCDs. (2) We also found the problem of poor CTI (∼ 5×10−4 /transfer)in the test device of the Pch CCD. Through the Fat Zero experiment, we found the
major cause of the poor CTI to be many trap levels in the Si wafer. The new
test device was fabricated on a new wafer which was obtained from a diﬀerent
wafer vendor, and we found that this device shows good CTI performance of <5 × 10−6 /transfer. Finally, we successfully developed the ﬁnal test device of Pch
CCD, which is a BI-CCD and has a ∼ 200 µm-thick depletion layer. The readout
noise expected from the node sensitivity is ∼ 10 e− , which is comparable to thatof Nch CCD. The CTI is rather worse (∼ 2 × 10−5 /transfer) than that recordedby Pch 14B. Though there is the possibility that soptimization of the transferring
voltage could improve the CTI, we need to continue to investigate this problem .
Excluding this CTI problem, Pch CCD is a very promising device for BS-CCD.
Focal Plane Assembly We designed and developed the focal plane assembly(FPA) for SXI. To eﬃciently cool a CCD chip equipped in the SXI, we are required
to reduce the heat input as much as possible. We examined the paths of the heat
input and contrived various schemes to reduce the heat input; We designed a newtype of thermo electrical cooler (TEC) which consists of a 2-stage Peltier module andCHAPTER 8. SUMMARY

126

no additional posts, to generate a wide temperature gap of 45 K and to eliminate
heat input by thermal conduction through the posts. The thermal radiation of the
housing of SXI is also the principle path of the heat input. We found that the heat
input by thermal radiation was signiﬁcantly reduced by the metal coating on thesurface of the CCD chip and the housing. In the results, although the size of theimaging area of a CCD chip for SXI is 4 times larger than that of the Suzaku XIS,
we successfully restrained the heat input to 177 mW, which is much smaller thanthat of the XIS (250 mW). The power consumption is expected to be 7.0 W, which
is only 2 times larger than that of XIS.

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Acknowledgements
I am deeply grateful to Prof. K. Koyama for his continuous guidance and support
throughout the ﬁve years of my graduate school period. I am also deeply gratefulto Prof. T. Tsuru for his guidance and encouragement in this work. This thesis
would not be completed without his support. I sincerely thank Dr. H. Matsumotofor his helpful advice and heartfelt encouragement for my work. I am grateful to mycollaborators, Dr. A. Bamba, Dr. H. Katagiri, Mr. H. Nakajima, Mr. T. Inui, Mr.
H. Yamaguchi, Mr. Y. Hyodo, Mr. K. Ono, Ms. M. Ozawa and Mr. H. Uchiyama
for their cooperation in my work and constructive discussions. I also thank Dr.
A. Asahara and Mr. D. Nishida for helpful and constructive support for building
the DAQ system. I am grateful to all members of the cosmic-ray laboratories at
Kyoto University for their continuous discussions and encouragement. I would liketo express my thanks to Prof. T. Tanimori, Prof. K. Imai, and Prof. N. Saito for
their constructive refereeing and suggestive comments.
I would like to thank Prof. H. Tsunemi, Prof. K. Hayashida, Dr. E. Miyata, andmembers of their laboratory (Osaka University) for their continuous cooperation
with my work and many constructive discussions. I sincerely thank Dr. S. Miyazaki
and Ms. Y. Kamata (National Astronomical Observatory of Japan) who led thedevelopment of the Pch CCD and gave me much helpful advice.
I would like to thank Mr. K. Yamamoto, Mr. K. Miyaguchi, Mr. H. Muramatsu,
Mr. H. Suzuki, Mr. K. Maeda, Mr. H. Kobayashi, Mr. H. Kohno, and the staﬀ ofHamamatsu Photonics K.K., who manufactured the test devices of Nch and Pch BSCCD and the thermo mechanical models of FPA and gave me technical informationand helpful comments. I also thank Mr. Y. Ando and Mr. H. Sugiura and the staﬀ of
AISIN Seiki Co. Ltd., who manufactured the TEC device for the thermo mechanicalmodel of FPA despite our diﬃcult requests and gave me useful information regarding
the TEC module. I would like to thank Mr. K. Mori and Mr. S. Kubo for theirtechnical advice and helpful information regarding the CCD driver and readout
systems.
This work was supported by a Grant-in-Aid for the 21st Century COE Center
for Diversity and Universality in Physics , and the Japan Society for Promotionof Science for Young Scientists. I was also ﬁnancially supported by the Hayakawa
Satio Foundation, the Japan Student Services Organization, Gifu prefecture, and131

BIBLIOGRAPHY

132

Nakatsugawa city.
I also thank Mr. R. Yokoe, Ms. M. Yokoe and Mr. K. Yokoe for their spiritualsupport. They invited me to dinner many times and encouraged my research work.
Finally, I wish to thank my family for their supporting my graduate school lifeand continued encouragement.