IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 4, AUGUST 2009

1Development of 4-Sides Buttable CdTe-ASIC Hybrid
Module for X-ray Flat Panel Detector
Mitsuru Tamaki, Yoshio Mito, Yasuhiro Shuto, Tatsuya Kiyuna, Masaya Yamamoto, Kenichi Sagae, Tooru Kina,
Tatsuhiro Koizumi, and Ryoichi Ohno

Abstract—A 4-sides buttable CdTe-ASIC hybrid module suitable for use in an X-ray flat panel detector (FPD) has been developed by applying through silicon via (TSV) technology to the256 channels of charge inreadout ASIC. The ASIC has 12825.7tegration type readout circuitry and an area of 12.9 mm
mm. The CdTe sensor of 1 mm thickness, having the same areaand pixel of 100
pitch, was fabricated from the Cl-doped CdTe
single crystal grown by traveling heater method (THM). Then theCdTe pixel sensor was hybridized with the ASIC using the bumpbonding technology. The basic performance of this 4-sides buttable
module was evaluated by taking X-ray images, and it was compared with that of a commercially available indirect type CsI(Tl)
FPD. A prototype CdTe FPD was made by assembling 9 pieces ofthe 4-sides buttable modules into 3 3 arrays in which the neighboring modules were mounted on the interface board. The FPD
covers an active area of 77 mm 39 mm. The results showed thegreat potential of this 4-sides buttable module for the new real time
X-ray FPD with high spatial resolution.

mIE
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oofFig. 1. Structure of (a) the conventional CdTe-ASIC hybrid module and (b) the4-sides buttable CdTe-ASIC hybrid module.

Index Terms—CdTe, four-sides buttable, through-silicon-via,
X-ray imaging.

these modules which can be assembled in all four directions. In
this work, we describe the development of the 4-sides buttable
CdTe-ASIC hybrid module for a Flat Panel Detector (FPD).
II. MATERIAL AND METHOD

I. INTRODUCTION

A. Through Silicon Via (TSV)ADMIUM TELLURIDE (CdTe) is a promising material
for X-ray imaging detectors in the field of the medical,dental and nondestructive testing applications because of thehigh detective quantum efficiency and high energy resolution
for X-ray. Various types of CdTe-ASIC hybrid modules hadbeen already developed for X-ray imaging [1], [2]. Fig. 1(a)shows a schematic view of the structure for conventional CdTeASIC hybrid module. However, as long as they use the ASIC
with conventional structure where the I/O pads are arranged
along one edge of the ASIC to be wire-bonded to an interface
board, the I/O pads area of the ASIC causes an inevitable deadspace for plural hybrid modules to be assembled adjoining eachother. After all, this structure of the so-called 3-sides buttable
CdTe-ASIC hybrid modules has restricted the arrangement ofthe modules for a two-dimensional X-ray imager.
By eliminating the wire-bonding and the dead space fromthe conventional structure, an active area of the CdTe sensor
can be extended up to the edge of the ASIC shown in Fig. 1(b).
The so-called 4-sides buttable structure is realized by using

CManuscript received November 13, 2008; revised March 04, 2009 and May
19, 2009.
The authors are with ACRORAD Co., Ltd., Okinawa 904-2234, Japan(e-mail: tamaki@acrorad.jp).
Digital Object Identifier 10.1109/TNS.2009.2024417

Through Silicon Via (TSV) technology, which has been recently developed for the advanced Silicon devices having threedimensional stacking structure, was applied to the readout pixel
ASIC to eliminate the wire-bonding and the dead space from3-sides buttable module. Fig. 2 shows (a) a schematic view ofTSV near the edge of the ASIC and (b) a cross sectional imageof TSV near the front-side I/O pad. Via holes were etched fromthe back-side of ASIC to reach the front-side I/O pad. The diand 250, reameter and the depth of via hole are 80
spectively. The oxide insulation layer and the conductive layer
were formed on the sidewall of via and they were extended to beconnected to the new back-side I/O pads of the ASIC. The surface of the ASIC was protected by the passivation layer. Total
number of I/O pads and via holes is 26 which are formed along
one edge of the ASIC chip and they are divided into analog anddigital lines which have the function of GND, analog output,ASIC configuration, power supply, clock, and tuning. The fabrication of TSV was carried out in a Silicon MEMS foundry.
B. Hybridization
The CdTe sensor was fabricated from the Cl-doped CdTe
single crystal grown by Traveling Heater Method (THM) [3],[4]. CdTe single crystal wafers of 1 mm thickness were sliced
from CdTe ingot with (111) orientation and polished mechanically and chemically. The Schottky contact was formed on CdTe
wafers using indium (In) as a common electrode and platinum(Pt) as pixel electrodes. The thickness of the indium layer is0018-9499/$26.00 © 2009 IEEE

2IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 4, AUGUST 2009

Fig. 2. (a) Schematic view of TSV near the edge of the ASIC and (b) a crosssectional image of TSV near the front-side I/O pad.

Fig. 4. Photograph of the 4-sides buttable hybrid module.

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TABLE ITHE SPECIFICATIONS OF THE 4-SIDES BUTTABLE HYBRID MODULE

Fig. 3. Photographs of the surface of (a) the pixelized CdTe sensor and (b) theASIC. (c) A schematic view of the 4-sides buttable hybrid module.

300 nm. Fig. 3(a) shows a photograph of the pixel electrode ofsquare and pixel pitch isthe CdTe sensor. Pixel size is 70. The diameter of contact hole for bump bonding is 30
100. The CdTe sensor has. A 15-wide guard ring is formed surrounding most outer pixels,
which is well known to block the leakage current injection fromthe edge of the CdTe sensor to pixel electrodes [5]. The distance
between the most outer pixel and the guard ring electrode is 15, and the distance from the edge of the CdTe sensor to the.
guard ring is 25
The ASIC was designed for integration and read out of charge
pulses from a CdTe sensor working in hole collection mode. The
CMOS technology.
ASIC was fabricated with AMIS 0.35with a pitch of 100
The ASIC has. Each pixel contains a bump bonding pad, an input transistor, an integration capacitor, a sampling capacitor, a buffer
amplifier and switches to control the sampling, reset and readoutof the integrated charge. The total power consumption is lessthan 15 mW. The frame rate can be set up to 50 fps for a fast
moving object. Fig. 3(b) shows a photograph of the pixel electrode of the ASIC. Bumps were formed on pixel electrodes ofthe ASIC in wafer level process. The soldering bumps with thelow melting point were used to avoid the damage for the CdTesensor. The CdTe sensor was hybridized with the ASIC using
the flip-chip-bonding technology shown in Fig. 3(c). These hybrid bare modules were mounted on the interface board andheated during the re-flow process. Back side I/O pads of theASIC were connected to the interface board by outer bumps.

Outer bumps with the diameter of 750, which are larger than, were used.
inner bumps with the diameter of 60
The photograph and the specification of the 4-sides buttablehybrid bare module are shown in Fig. 4 and Table I, respectively.

III. RESULTS AND DISCUSSTION

A. Spatial Resolution

The spatial resolution characteristic was evaluated by taking
an X-ray image of a resolution chart pattern. A micro focus
X-ray source (HAMAMATSU L9181S) was used for the mea. Thesurement. The X-ray source has a focal spot size of 40
distance between the X-ray source and the surface of the CdTe
hybrid module was 60 cm. The measured dose rate without thefiltration was approximately 800 mR/min at tube voltage of 80. A commercially available inkV and tube current of 100
direct X-ray conversion type CsI(Tl) FPD with the same pixel
pitch was used for the comparison [6]. The CsI(Tl) FPD utilize-thick CsI (Tl) scintillator plate with a needle struca 200ture. Fig. 5 shows the X-ray images taken by the CsI(Tl) and theCdTe at 80 kV. For CsI(Tl), it was difficult to discriminate line
pairs at 5 lp/mm which corresponded to the theoretical Nyquist
frequency. On the other hand, for the CdTe it was clearly visible.
The CdTe shows excellent spatial resolution characteristics.
The pre-sampling Modulation Transfer Function (MTF) wasmeasured by the edge method which was the standardized
method to evaluate the spatial resolution characteristics for
medical digital X-ray imaging devices [7]. A tungsten plate

TAMAKI et al.: DEVELOPMENT OF 4-SIDES BUTTABLE CdTe-ASIC HYBRID MODULE

Fig. 7. Photograph of a prototype CdTe FPD by assembling 9 pieces of the4-sides buttable modules into 3 3 arrays.

233 arrays with the gap of 50
among neighboring hybrids shown in Fig. 7. The dead space corresponding to oneline-width was induced among neighboring hybrids where thecenter-to-center distance of most outer pixels of neighboring hy. The FPD covers the active area of 39 mm
brids was 230
77 mm.
The bias voltage of 500 V was applied to the top electrodesof all modules. A periodic bias reset technique was used tosuppress the polarization effect [8]. The polarization effect iswell known to degrade the energy resolution and shift the photopeaks under a continuous bias voltage applying. By removing
the bias voltage and re-applying, the detector can recover to theinitial state. In this study, the similar phenomenon was observed
as the degradation of the spatial resolution in several tens ofminutes at a continuous bias voltage of 500 V. However, it was
found that momentary removal of the bias returns the spatial
resolution to its initial state. We applied the periodic bias reset
operation to prevent the polarization effect, where the bias-ontime and the off-time were set to be 5 min and 0.5 sec, respectively. These values were consistent with those of the bias resetparameters reported in [8].
The interface board was connected to a control circuit boardequipped with ADCs, a FPGA, memory ICs, regulators andCamera Link PCI interface. The ASICs were controlled by theFPGA. Output analog signals from the modules were converted
to 12 bit digital signals with ADCs and they were stored temporarily. Then the data were transferred to the PC. The electronic devices on the control circuit board were shielded from
X-rays irradiation by 1-mm-thick lead sheets. The ASICs wereprotected by the 1-mm-thick CdTe sensor which could be agood X-ray beam attenuator. Actually, we tested the radiation
hardness of the hybrid using other hybrids. The measured dose
rate was approximately 0.1 MR/hr with the distance from X-ray.
source of 2 cm at tube voltage of 120 kV and current of 300
The dark signal and the gain linearity in the irradiated region
were measured up to 10 hours in the operation of the periodic
bias reset mode. The results showed that the dark signal slightly
began to increase beyond 6 hours (the total dose was 0.6 MR).
The dark signal increased by 1% of the full dynamic range after
10 hours (the total dose was 1 MR). No change of the gain linearity was observed.

IEEPr E
oofFig. 5. Comparison of X-ray images of a resolution chart pattern taken by theCsI(Tl) FPD and the CdTe module at 80 kV.

3Fig. 6. Comparison of the pre-sampling MTF for CsI(Tl) FPD and CdTe
module as a function of spatial frequency. Open and filled circles show theMTF for CsI FPD and the CdTe module, respectively. The theoretical sincfunction curve was also plotted (dashed line).

with 1 mm thickness was placed in front of the hybrid modulesurface with the distance of 5 mm. The edge of the plate wasaligned to the center axis of the X-ray beam and tilted byan angle less than 3 relative to the axis of the pixel rows.
An X-ray image of the edge of the plate taken at 80 kV wasanalyzed. The MTF was calculated from a Fourier transformof the line spread function (LSF) of the edge at three differentpositions. Fig. 6 shows a comparison of the MTF curves for theCsI(Tl) FPD and the CdTe hybrid module. The horizontal andthe vertical axes represent the spatial frequency and the MTF,
respectively. Filled and open circles show the MTF curves for
the CdTe module and CsI(Tl) FPD, respectively. The MTF
value of the CdTe module was found to be significantly higher
than that of the CsI(Tl) FPD in all frequency range investigatedhere. The theoretical sinc function with the pixel pitch of 100
was also plotted in Fig. 6 (dashed line). The MTF curveof the CdTe was lower than the sinc function. This difference
could be explained by the inter-pixel signal leakage accordingto [2].
B. Performance of Prototype CdTe FPD
A prototype CdTe FPD was made by assembling 9 piecesof the 4-sides buttable bare modules on an interface board into

4IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 4, AUGUST 2009

Fig. 8. X-ray image of a USB flash drive taken at 80 kV.

Fig. 9. (a) One frame of a real time X-ray image series of a watch taken at 100
kV at 50 fps, (b) an enlarged image of central area of the X-ray image.

These results showed the great potential of this 4-sides buttable CdTe module for a new real time X-ray FPD with highspatial resolution.

IEEPr Eoof

The uniformity of gain for the CdTe FPD was evaluated bytaking a flat- field X-ray image. The proportion of dead pixels
including bump failure pixels, high leakage current pixels and irregular gain linearity pixels was approximately 0.3% in one hybrid module. Both of standard deviations of gain among pixels
and hybrids were less than 5%.
Fig. 8 shows an X-ray image of an USB flash drive taken by
the prototype CdTe FPD at 80 kV, where the image correction
was performed by adjustment of differences in the gain linearity
among pixels and replacement of missing data due to the gaps
with average of the neighboring pixels. The detail of the drive isclearly visible. A high resolution X-ray image was successfullyobtained.
Fig. 9 shows one frame of a real time X-ray image series ofa watch taken at 100 kV at 50 fps. It is the raw image without
any image correction. Details of gear pairs and the fast rotatingbalance wheel are visible.
IV. CONCLUSION

The 4-sides buttable CdTe-ASIC hybrid module for X-rayFlat Panel Detector was successfully developed by applying
TSV technology. The 4-sides buttable hybrid module has the excellent spatial resolution characteristic compared with the commercially available indirect X-ray conversion type CsI(Tl) FPD.
Especially, the MTF value of the CdTe module was found tobe significantly higher than that of the CsI(Tl) FPD in all frequency range. The prototype CdTe FPD was made by assembling 9 pieces of the 4-sides buttable hybrid modules into 3
3 arrays with the 50-gap among hybrids. A high resolution
real time X-ray image was obtained at high frame rate of 50 fps.

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