Photon Factory Activity Report 2016 #34 (2017)

BL-11D/2016G672

Measurement of quantum efficiency of back-illuminated imaging device
with pixel size 1.4 × 1.4 μm2
Takeo EJIMA* and Tadashi HATANOIMRAM, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, Japan
Using a commercially available PC board and the camera module attached, a soft X-ray imaging device was fabricatedof which the pixel size is 1.4 × 1.4 μm2 and about 1/10 of the conventional one. The quantum efficiency of the imaging
device was increased as the increase of photon energy from 250 eV to 1000 eV depending on the surface materials.
1 Introduction
Over the past 10 years, typical pixel size ofcommercially available soft X-ray image sensors has been
limited to 13 × 13 μm2 because of the process rule of themanufacturer [1, 2]. On the other hand, back-illuminated(BI) type CMOS sensors in visible (VI) wavelength
region have been supplied to the market and mounted on
popular cameras. The VI BI-CMOS sensors are called thesame "back-illuminated type sensor" as in SX wavelength
region, but a color filter is deposited on the surface
photodiode layer of the VI BI-CMOS sensor for color
separation [3, 4]. When a VI BI-CMOS sensor isirradiated by the SX light, the SX light is absorbed by thiscolor filter because of the strong interaction of the SX
light with the color filter. If the color filter layer can beremoved from the VI BI-CMOS sensor before theirradiation, the VI BI-CMOS sensor will have asensitivity in the SX wavelength region.
Raspberry Pi is an educational single board computer
which is equipped with a quad-core ARM chip, and Unix
derived OS, Raspbian, works on it [5]. Pi Camera V1.3 isa camera module of Raspberry Pi, which is equipped witha visible BI-CMOS sensor (OV5647, OmniVision) thathas an image area of 2592×1944 pixels with a pixel sizeof 1.4 × 1.4 μm2 [6]. This camera module can beseparated from the computer board, and images can beobtained by a standard program which works on Raspbian.
The pixel size of the image sensor OV5647 is smallenough comparing with the ones of the conventional SX
imaging devices, moreover, the cost of the image sensor
with peripheral equipment are very low. In this study,
operation of Pi Camera V1.3 is checked in SX
wavelength region after removing the color filter from thesurface of the image sensor [7, 8].
2 Experimental Details
The color filter layer of the image sensor was removed
at first. Color filters of image sensors are often made bymixing optical dyes in resist materials [3]. Peeling off
resist layers on the surface of semiconductors is astandard technique in semiconductor processes. In thepresent case, the color filter is made of acrylic resin andwas peeled off by a solution of a resist stripper. Then thesensor was rinsed with isopropyl alcohol, with water

solution of an optical neutral detergent, and with distilledwater. The OV5647 chip before and after removal of thecolor filter is shown in Figure 1 (a) and (b), respectively.
Fabricated image unit is shown in Figure 2. The front
side of the OV5647 chip was covered with a protection
case, and then a Peltier device (TES1-3102LT125, Laser
Create Corp., Japan) was bonded to the backside of thechip via a Cu plate which is 2 mm thick. A Pt resistancetemperature sensor (M222-100-A, Heraeus) was attachedto the Cu plate, and its temperature was measured tocontrol the Peltier device. The Cu block was adhered tothe hot side of the Peltier device, and the temperature of
the hot side decreases by the air through a vacuum flange
adhered tightly to the Cu block. The temperature wascooled down to 5 C or less under the ready mode of thePeltier device and was increased to 10 C under theoperation mode.

Figure 1. OV5647 chip before, (a), and after, (b),removal of the color filter.

Figure 2. Fabricated image sensor unit.

Photon Factory Activity Report 2016 #34 (2017)

The measurement was made using the beamline, BL11D, at Photon Factory, KEK, Japan [9]. The photon
energy region of the beamline is divided by two
diffraction gratings, G1 (2400 grooves/mm, R=55.2 m,250 – 1300 eV) and G3 (2400 grooves/mm, R=22.9 m, 60– 280 eV). A photodiode (SXUV100, Optodiode Corp.)
was also installed in front of the CMOS sensor tomeasure the absolute photon intensity.
3 Results and Discussions
Figure 3 shows the quantum efficiency curve of theCMOS sensor. The value of the quantum efficiency wasincreased as the increase of the photon energy. Some dipstructures can be observed at 150 eV, 300 eV, and 450 eV.
These structures are originated from the Si, C, and N
atoms of the sensor surface, which correspond to theresults of XPS measurements. The result of the quantum
efficiency suggest that the sensor will be useful in SX
measurements.
Acknowledgements
This work was supported by Grants-in-Aid for
Scientific Research (JSPS Kakenhi, Grant Number:
16H03902, and 17K190210). Mr. Y. Nunokawa and K.
Yamaki are thanked for design and construction of thesensor unit. Prof. K. Mase and Mr. T. Kikuchi areappreciated for assistance of the measurements.
References[1] CCD47-10 AIMO Back Illuminated Compact Pack
High Performance CCD Sensor, A1A-100029(5),e2v technologies (UK) limited, (2006).
[2] T. Ejima, et al., AIP CP 1234, (2010) pp. 811-814.
[3] K. Yonemoto, “Principles and Applications of
CCD/CMOS image sensors”, (CQ publishers Co.
Ltd., Japan, 2003) Chap. 3 (in Japanese).
[4] V. Venezia, et al., US 2011/0089311 (Apr. 21, 2011).
[5] http://ftp.jaist.ac.jp/pub/raspberrypi/raspbian/images/raspbian-2015-05-07/2015-05-05-raspbianwheezy.zip [Accessed 22, Jun. 2017].
[6] “OV5647, 5-megapixel product brief, ver. 1.2”,
OmniVision,(2010).
Availableat:
http://www.ovt.com/uploads/parts/OV5647.pdf[Accessed 8 Jun. 2015].
[7] Products of Raspberry Pi Foundation, Available athttps://www.raspberrypi.org/ [Accessed 22, Jun.
2017].
[8] R. J. Kinch, "Raspberry Pi: Information andProducts for Custom Applications and Engineering",
Available at: http://www.truetex.com/raspberrypi[Accessed 19, Oct. 2015].
[9] T. Hatano and T. Harada, J Electron Spectrosc. Relat.
Phenom. 196, (2014), pp. 156-158.
*e-mail address: takeo.ejima.e7@tohoku.ac.jp

Figure 3. Quantum efficiency of OV5647 after
removal of the colour filter deposited on the sensor
surface.