4.16

SOI pixel sensor opens a new era ofparticle and nuclear physics

A pixel detector plays an important role in particle(KEKDTP) [2] was founded. With the KEKDTP sup-physics to study decay of short lived particles involv-

port, an intensive R&D program for pixel sensor wasing heavy quarks or the o lepton with excellent time

initiated using the SOI CMOS technology of Lapis

and spatial resolution under harsh radiation environ-Semiconductor Co., Ltd., a leading company thatment in a high luminosity particle accelerator.

provides foundry services for SOI devices. Devel-In silicon-on-insulator (SOI) technology, CMOS

opment of a large area X-ray 2D image sensor was(complementary metal oxide semiconductor) circuits

the first goal of this project, which has since been

are fabricated on a thin SiO2 insulating layer (buriedachieved and the device widely used in experimentsoxide, BOX) on the silicon substrate. As each circuitof material science.

element like a transistor is isolated on BOX, parasitic

Recent improvement of the SOIPIX structure withcapacitance and leakage current are minimized inan additional silicon layer on BOX enables to com-an SOI circuit realizing an ideal characteristic beyond

pensate the effect of accumulated charge due tothat of a normal CMOS. A pixel sensor using SOI

radiation and makes the sensor radiation-tolerant,

technology (SOIPIX) would, therefore, serve as aneven in a harsh environment as high as 1 MGy at aexcellent particle detector, where charges induced

collision point of high luminosity collider experiments

by radiation in the silicon substrate (bottom) are col-like SuperKEKB [3].

lected and processed in the CMOS circuit (top) on

In January 2017, a group in the SOIPIX collabo-the BOX (Fig. 1). The small feature size of CMOS

ration working for particle physics (*) performed ahelps to realize high performance, monolithic pixel

beam test with 120 GeV protons at the Fermi Na-sensors.

tional Accelerator Laboratory (Fermilab), USA [4] (**),

The SOIPIX collaboration was formed in 2005
to develop generic SOI pixel sensor technology[1] when the KEK Detector Technology Project

Fig. 2. SOIPIX sensor stack involving four FPIX2 and two SOFIST1
sensors used for the beam test in Fermilab.

Fig. 1. Concept of the SOI pixel sensor.

682016 a t K E K A n n u a l R e p o r t(*) KEK, Tohoku University, University of Tsukuba and Osaka University. Work supported by JSPS (Japan Society for the Promotion ofScience) KAKENHI Grant Number JP25109006.
(**) Supported by Japan-US cooperative program in High Energy
Physics.

to demonstrate the capability of SOIPIX sensors forsuch a residual distribution demonstrating the posi-particle detection. Two types of sensors, FPIX2 [3]tioning accuracy of better than 0.89 +m and 1.4 +m

and SOFIST1 [5] were tested. FPIX2 is characterizedin chip FPIX2 and SOFIST1, respectively. A Monte-

by an ultra small pixel size (8 +m × 8 +m). SOFIST1

Carlo simulation confirmed that the above distribu-

is designed with a pixel size of 20 +m × 20 +m fortions could be reproduced with an intrinsic spatial

good measurements both in charge and timing forresolution of 0.7 +m in FPIX2.

the International Linear Collider (ILC) [5]. Four FPIX2

The sub-micron (<1 +m) spatial resolution

and two SOFIST1 sensors were integrated into theachieved by the SOIPIX group in KEKDTP signifies atest setup (Fig. 2). A total of 5.6M proton tracks

first time achievement in the history of particle detec-

were recorded during the data acquisition period oftors. Realization of ultra small pixel size and extreme

three weeks. After careful and fine alignment of thelevel of accuracy in spatial measurement is likely tosensors, the proton trajectories were reconstructed

open a new era of particle and nuclear physics re-using FPIX2 sensors; here, the impact points weresearch.

precisely calculated in a charge centroid algorithm
among several pixels. Accuracy of spatial measurement is evaluated in terms of residual distribution,References

which is defined as the shift of the proton impact[1] Y. Arai, I. Kurachi, Synthesis Lectures on Emerging En-

point from the measured value by the target sensor

gineering Technologies, Morgan & Claypool Publishers

to that traced by other sensors. Figures 3 shows(2017)

Eventshttp://www.morganclaypool.com/doi/10.2200/
S00750ED1V01Y201701EET009.

FPIX2800[2] KEK Detector Technology Project http://rd.kek.jp/project/700soi/.

600
500

[3] K. Hara et al., The Technology and Instrumentation in Par-

400

ticle Physics 2017, TIPP2017, (Beijing, China, 2017).

300

http://indico.ihep.ac.cn/event/6387/session/53/contribu-

200tion/25/material/slides/0.pdf.
[4] http://ftbf.fnal.gov/.

1000-10[5] S. Ono et al., The Technology and Instrumentation in Par-8-6-4-2

02

468

10

Eventsresidual [+m]ticle Physics 2017, TIPP2017 (Beijing, China, 2017).
http://indico.ihep.ac.cn/event/6387/session/53/contribu-

220

SOFIST1

200

tion/109/material/slides/0.pdf.

180160140120100806040
200-15-10-5051015

residual [+m]Fig. 3. Residual distribution for FPIX2 (top) and for SOFIST1 (bottom). Residual is defined as a measure of spatial resolution
by a shift from the proton incident position measured by thetarget sensor to that traced with the other sensors. FPIX2
clearly demonstrates the world’s best resolution significantly
better than 1 +m.

4 Research Highlights

69