Nuclear Instruments and Methods in Physics Research A 636 (2011) S31–S36

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Nuclear Instruments and Methods inPhysics Research A
journal homepage: www.elsevier.com/locate/nima

Development of SOI pixel process technology
Y. Arai a,n, T. Miyoshi a, Y. Unno a, T. Tsuboyama a, S. Terada a, Y. Ikegami a, R. Ichimiya a, T. Kohriki a,
K. Tauchi a, Y. Ikemoto a, Y. Fujita a, T. Uchida a, K. Hara b, H. Miyake b, M. Kochiyama b, T. Sega b,
K. Hanagaki c, M. Hirose c, J. Uchida c, Y. Onuki d, Y. Horii d, H. Yamamoto d, T. Tsuru e, H. Matsumoto e,
S.G. Ryu e, R. Takashima f, A. Takeda f, H. Ikeda g, D. Kobayashi g, T. Wada g, H. Nagata g, T. Hatsui h,T. Kudo h, A. Taketani h, T. Kameshima h, T. Hirono h, M. Yabashi h, Y. Furukawa h, M. Battaglia i, P. Denes i,
C. Vu i, D. Contarato i, P. Giubilato i, T.S. Kim i, M. Ohno j, K. Fukuda k, I. Kurachi k, M. Okihara l,
N. Kuriyama l, M. Motoyoshi maInstitute of Particle and Nuclear Studies, High Energy Accelerator Research Org., KEK, Tsukuba 305-0801, Japan
Institute of Science, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japanc
Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japand
Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578, Japane
Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japanf
Kyoto University of Education, 1 Fujinomori-cho, Fukakusa, Fushimi-ku, Kyoto 612-8522, Japang
Institute of Space and Astronautical Science, JAXA, Sagamihara, Kanagawa 229-8510, Japanh
Riken, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japani
Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USAj
National Institute of Advanced Industrial Science and Technology(AIST), Tsukuba 305-8568, JapankOki Semiconductor Co. Ltd., Higashiasakawa, Hachioji, Tokyo 193-8550, Japanl
Oki Semiconductor Miyagi Co. Ltd., Ohira-mura, Kurokawa-gun, Miyagi 981-3693, Japanm
ZyCube Co. Ltd., 4259-3 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8510, Japanba r t i c l e in fo

abstract

Available online 4 May 2010

A silicon-on-insulator (SOI) process for pixelated radiation detectors is developed. It is based on a
0.2 mm CMOS fully depleted (FD-)SOI technology. The SOI wafer is composed of a thick, high-resistivity
substrate for the sensing part and a thin Si layer for CMOS circuits. Two types of pixel detectors, one
integration-type and the other counting-type, are developed and tested. We conﬁrmed good sensitivity
for light, charged particles and X-rays for these detectors.
For further improvement on the performance of the pixel detector, we have introduced a newprocess technique called buried p-well (BPW) to suppress back gate effect. We are also developing
vertical (3D) integration technology to achieve much higher density.
& 2010 Elsevier B.V. All rights reserved.

Keywords:
SOI
Pixel
X-ray imaging
Particle tracking

1. Introduction
A silicon-on-insulator (SOI) technology is a very attractive
method to get high-speed and low-power LSI circuit. With its
unique structure of two different Si layers in a single wafer,
researchers developing imaging devices have long expected torealize high-performance monolithic pixel detectors via the SOI
technology. However, thus far, development of SOI radiation
image sensor has been mostly limited to laboratory scale [1].
We have been developing an SOI pixel process based on OKI
Semiconductor Co. Ltd. 0.2 mm CMOS fully depleted (FD-)SOI
process [2]. The SOI wafer is composed of a thick, high-resistivitynCorresponding author.
E-mail address: yasuo.arai@kek.jp (Y. Arai).

0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2010.04.081

substrate (sensor part) and a thin low-resistivity Si layer (CMOS
circuitry) sandwiching a buried oxide (BOX) layer. After removing
the top Si and the BOX layer in the region of the sensing nodes,
p or n dopant is implanted to the substrate. Then contact vias andmetal connections from the p–n junction to the transistors are
created (see Fig. 1) [3–5]. This detector has many good features forapplications in high-energy experiments, astrophysics, material
analysis, medical imaging and so on.

 There is no mechanical bump bonding, so obstacles whichcause multiple scattering are eliminated and smaller pixel size
is possible.
 Parasitic capacitances of sensing nodes are very small(  10 fF), so large conversion gain and low noise are possible.
 Full CMOS circuitry can be implemented in the pixel.

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 The cross-section of single event effects caused by radiation is Emerging vertical (3D) integration techniques are a naturalvery small. A latch-up mechanism, which destroys conventional bulk CMOS LSI, is absent.
 Unlike conventional CMOS process, there is no leakage path tobulk. Thus SOI transistors are shown to work over a very large
temperature range from 4 to 600 K.
 The technology is based on industry standards, so further
progress and lower cost are foreseeable.

extension of the SOI technology, so a much higher integration
density is possible.
In Section 2, we describe the details of the process. The R&D effort
on vertical integration is shown in Section 3. In Section 4, some of thetest results of the SOI pixel detectors we have developed are shown.

2. SOI pixel process
In Fig. 2, a simpliﬁed procedure for the fabrication of the SOI
pixel is shown. First, conventional SOI processes are performed toFig. 1. Cross-sectional view of the SOI pixel detector.

Fig. 3. A photograph of a wafer processed in 2008. The mask size used is20.8  20.8 mm. In addition to Japanese laboratories and universities, US
institutions also contributed designs.

Fig. 2. Simpliﬁed SOI pixel process ﬂow.

Table 1
SOI pixel process speciﬁcations.
Process

0.2 mm Low-leakage fully depleted SOI CMOS, 1 Poly, 4 metal
layers, MIM cap., DMOS option core (I/O) voltage ¼ 1.8 (3.3) V

SOI wafer

Diameter: 200 mmf,
Top Si: Cz,  18 O-cm, p-type,  40 nm thickBuried oxide: 200 nm thickHandle wafer: Cz n-type 700 O-cm, 725 mm thickBackside

Thinned to 260 mm and sputtered with Al (200 nm).

Transistors

Normal and low threshold transistors are available for both coreand IO transistors.
Three types of structures (body-ﬂoating, source-tie andbody-tie) are available.

Optional
process

Buried p-well formation
Vertical integration with m-bumps.

Fig. 4. Simulation results of NMOS transistor threshold voltage shift by the backgate voltage. By creating guard ring around the transistor at the distance shown,
the shift can be reduced.

Fig. 5. (a) Normal implantation method to create p–n junction in the substrateand (b) buried p-well implantation method. By ﬁxing the BPW potential underperipheral circuit, the back gate effect is completely suppressed. In the pixel area,
BPW may be used to extend sensor area.

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form transistors. Then the BOX layer is opened to implant p+ andn+ dopant to the handle wafer. After the implantation, annealing
and contact via formation through the BOX are done. Then,conventional backend processes are followed.

Fig. 6. Backside voltage dependence of an NMOS Id–Vgs curve (a) without BPW
and (b) with BPW connected to 0 V.

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The main speciﬁcations of the process are summarized inTable 1. The process has two kinds of transistors: core and I/Otransistors. There are three types of transistor structures:body-ﬂoating, body-tied and source-tied. Metal–insulator–metal(MIM) capacitors, depletion MOS (DMOS) transistors, lateral
diodes and several kinds of resistors are also available.
To reduce the development cost, several chip designs are put
on a mask. We submitted two runs in year 2009. In each run, we

Fig. 8. Photograph of the m-bumps created on the SOI pixel.

Fig. 7. Process ﬂow of the m-bump bonding.

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included about 16 designs from our collaborators [6–8]. A
photograph of a wafer from the recent run is shown in Fig. 3.

2.1. Back gate effect
One of the major difﬁculties in creating a sensor from the SOI
wafer substrate is the back gate effect. Since the area under thetransistor acts as a back gate, its potential affects the thresholdvoltage and the leakage current of the transistor.
The back gate effect is small compared to the front gate since
the front gate oxide (  4 nm) is much thinner compared with thatof the buried oxide (  200 nm). However the required backside
bias voltage is very high (4100 V) since sensor construction
usually requires a thick depletion layer. So the back gate effectcannot be neglected.
One method to reduce the back gate effect is the creation of aguard ring around the transistors. By keeping the distance
between the transistors and the guard ring less than 10 mm orso, the threshold voltage shift can be reduced to an acceptablelevel (Fig. 4). However this method needs additional area anddecreases the integration density, so this is not a desirable
solution in most of the cases.
To avoid the area penalty, we developed Buried p-well (BPW)
process (Fig. 5). We implant p-type dopant through the top
Si layer and create a buried p-well (BPW) region under the BOX.
This BPW region will help to stabilize the underside potential of
the transistors. The doping level of the BPW is about 3 orders ofmagnitude lower than that of the p + sensor node and drain/source region, so it does not affect the transistor characteristics. In
addition, the implantation energy is controlled so that the peakdensity will be located under the BOX region.
Fig. 6 shows Ids–Vgs curve of an NMOS transistor when the backgate voltage is applied. The Ids–Vgs characteristics, especially on
NMOS, change dramatically when the back gate voltage is applied.
However, by introducing the BPW and connecting it to a ﬁxed voltage,
this effect is completely suppressed.
The suppression of the back gate effect is also conﬁrmed inexisting pixel chips. Signal charge increases by increasing thedetector voltage, but in the absence of a BPW layer the output
signal will decrease if the detector voltage exceeds 15 V or so.
With the addition of the BPW layer, the same kind of pixel chipworks for more than 100 V detector voltage without problem.
The BPW process also introduces many attractive features forthe pixel application: it reduces electric ﬁeld gradients at a criticalpoint, so that break down voltage increases. Furthermore, we cancreate large sensing nodes without removing the top Si layer.

3. Vertical integration
Since the performance improvements by shrinking the process
technology are approaching the limit, vertical (3D) integration
technologies are emerging in the semiconductor industry. In
addition to higher circuit density, in 3D technology, signal
propagation time can be shortened, so that higher performanceand lower power can be realized.
Vertical integration is especially desirable in pixel applications,since it can enhance pixel functions without increasing pixel size.
The SOI technology is well suited for vertical integration.
We have designed a test chip applying vertical integration. We
used m-bump technology of ZyCube Co. Ltd. [9]. Minimum pitch of
the bump is 5 mm. The bonding process is shown in Fig. 7. The test
chip, which includes 13 mm pitch pixels and several test elements,
is fabricated in OKI Semiconductor and then bonded at ZyCube.
Photograph of the m-bumps and the cross-section of the bondingis shown in Figs. 8 and 9. Detailed tests are under preparation.

4. Results of pixel detectors
We are mainly developing two kinds of pixel detectors. One is
an integration-type pixel called INTPIX, and the other is acounting-type pixel called CNTPIX. There are also many otherdesigns by MPW users.
4.1. Integration-type pixel
The basic schematic of the integration-type pixel (INTPIX) is
shown in Fig. 10. INTPIX2 and 3 chips are 5  5 mm in size having

Fig. 9. Cross-sectional view of the m-bump bonding after stacking.

2.2. Radiation hardness
Since the active Si layer is very thin ( 40 nm), radiation
generated charge is also small. Thus the Single Event Effect (SEE)cross-section of the SOI chip is normally small compared withthat of the conventional CMOS circuit.
As for the Total Ionizing Damage (TID), the SOI chip is notnecessarily radiation hard since it has relatively thick buried oxide(BOX) of  200 nm, so radiation generated holes will tend to be
trapped. The transistor threshold voltage shifts a few hundreds ofmV after a dose of one hundred krad (Si). This becomes worse
when we apply the backside voltage.
In this aspect, the reduction of BOX electric ﬁeld by using the
BPW layer helps to increase radiation hardness, since it increases
recombination probability of generated electron–hole pairs in theBOX. Further studies to reduce the effect of hole trap in the BOX
are also ongoing.

Fig. 10. Schematic of the integration type pixel (INTPIX).

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128  128 pixels each 20 mm square. Reverse bias voltage isapplied from the bottom surface or top n+HV ring. Leakage
current and break down voltage of the INTPIX3 are shown in

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Fig. 11. The leakage current is about 10 nA at 100 V bias voltage,
and the break down voltage is about 160 V.
Fig. 12 shows a test chart image taken by the INTPIX2 with
 8 keV X-rays. It indicates 20 lines/mm are resolved well. The
present INTPIX has a window area, in which there are no metal ortransistors, allowing illumination from the topside to ease testing.
It may be possible to shrink the pixel size to less than 10 mm
squares.
4.2. Counting-type pixel
A pixel circuit of the counting-type pixel (CNTPIX) is shown in
Fig. 13. The preampliﬁer circuit is based on that proposed byKrummenacher
[10]
which
contains
leakagecurrent
compensation circuitry. After the low and high thresholddiscriminator, the input signal is processed in a 16-bit counter.

Fig. 11. Sensor leak current and break down voltage (INTPIX3, 5  5 mm2 chip,measured at 5 1C).

Fig. 12. X-ray image taken with the integration type pixel (INTPIX2).

Fig. 14. Layout of a pixel in the CNTPIX2 chip. The size of a pixel is 60  60 mm,
and each pixel contains about 600 transistors.

Fig. 13. Circuit of the CNTPIX2 pixel.

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control signals and the other handles TCP/IP protocol to communicate with PC via Ethernet. Each pixel chip is mounted on a subboard and connected to the SEABAS board through four 64-pinsconnectors. The block diagram and photograph of the SEABAS
main/sub board is shown in Fig. 16.
The SEABAS board also has 65 MHz 12 bit ADC, a four channel
12 bit DAC, and NIM I/O ports for easy testing.

5. SummaryFig. 15. X-ray image taken with the CNTPIX2. X-ray generator with Cu target wasused.

We have developed a SOI pixel process based on a commercial
0.2 mm FD-SOI process. We have demonstrated basic performanceof the SOI integration-type and counting-type pixel detectors.
A new implantation process which creates buried p-well(BPW) under the BOX is introduced. We conﬁrmed the BPW
suppress the back gate effect very efﬁciently. The BPW process
also gives us the possibility to increase break down voltage andradiation hardness.
We are also developing 3D integration technology by using the
m-bump bonding technique. The m-bump pitch is only 5 mm, so
we can increase the functionality of a pixel without increasing the
pixel size.
We also developed an easy-to-use test board.

Acknowledgments
The authors wish to thank F. Takasaki and J. Haba for theircontinuing support to this project. The authors also thank manyusers who are participating in the SOI MPW runs.
This work is supported by KEK Detector Technology Projectand JSPS KAKENHI (21244040).
This work is also supported by VLSI Design and Education
Center (VDEC), The University of Tokyo with the collaboration
with Cadence Corporation, Synopsys Corporation, and Mentor
Graphics Corporation.
References
Fig. 16. (top) Block diagram of the SEABAS board. (bottom) Photograph of the
SEABAS board and its sub-board.

The layout of the CNTPIX2 is shown in Fig. 14. The size of a pixel isabout 60 mm square, and there are 128  128 pixels in a 10 mm
square chip.
A metal plate image taken with 8 keV X-ray is shown in Fig. 15.
4.3. Test system
We have developed a test board called SEABAS (SOI Evaluation
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