Improvements of the PLD (Pulsed Laser Deposition) Method for Fabricating Photocathodes in ICMOS (Intensified CMOS) Sensors

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1 , pp Improvements of the PLD (Pulsed Laser Deposition) Method for Fabricating Photocathodes in ICMOS (Intensified CMOS) Sensors Dae-Hee Lee 1,2*, Youngsik Park 1, Bongkon Moon 1, Yong-Woo Kang 1, Won-Kee Park 1, Seoung-Chul Bang 1, HyeunSeok Choi 3, So-Jeong Na 3, Sung-Bok Kang 3, Nikolay Vedenkin 4, Alexander Malyshev 4 and Yang-Soo Kim 4 1 Korea Astronomy and Space Science Institute, Daejeon, South Korea 2 University of Science and Technology, Daejeon, South Korea 3 Korea Institute of Industrial Technology, Cheonan-si, South Korea 4 Satbyul Corporation, Ltd., Suwon, South Korea * dhlee@kasi.re.kr Abstract. We introduce new technologies improving the PLD (Pulsed Laser Deposition) method to fabricate visible (370 ~ 600 nm) and NUV (Near Ultraviolet, 185 ~ 320 nm) photocathodes for ICMOS (Intensified CMOS) sensors. First, we have improved the PLD VC (Vacuum Chamber) by utilizing optical window viewports and a couple of internal carousels, so that we can do cleaning, deposition various alkalis, measurement of the QE (Quantum Efficiency) in-situ, for multiple photocathode targets in a single process. Second, we have designed a Load/Degassing/Assembly (LDA) VC to prepare, load, degas, and assemble the alkali targets and photocathode substrates. Finally, we have connected the PLD VC with the LDA VC through a vacuum gate to prevent the photocathodes from oxidation and water contamination during the process. In this paper, we describe detail procedures of our new technologies and discuss about the design results of ICMOS products. Keywords: Pulsed Laser Deposition, Photocathode, ICMOS 1 Introduction An image intensifier is a device which amplifies photons so that low light level signals can be detected. The major components of an intensifier are a photocathode, phosphor screen and microchannel plate (MCP). The image intensifier works by converting incident photons into electrons at the photocathode via the photoelectric effect. Therefore, the photocathode is essential in the intensifier because it determines the photon wavebands and the quantum efficiency of the intensifier [1], [2]. In this paper, we introduce new technologies to improve the PLD (Pulsed Laser Deposition) method for fabricating photocathodes in ICMOS (Intensified CMOS) sensors. We describe the detail design and procedure of our new technologies in section 2 and discuss about the design results of ICMOS sensor products in section 3. Finally, the summary is presented in section 4. ISSN: ASTL Copyright 2018 SERSC

2 2 Photocathode Fabrication & Assembly Various photocathodes are currently used to improve the sensitivity of photon counting or imaging detectors. The choice of photocathode material is determined by the spectral range where the device sensitivity is crucial. Alkali metal compounds have been shown to be very efficient photo converters in the near ultraviolet (NUV) wavelength ranges. CsTe is known to be one of the most efficient among them, and therefore it is widely used in many detecting devices [3], [4]. It is also relatively stable under short exposure to atmosphere, which substantially simplifies production and handling of intensifiers with CsTe photocathodes. In the visible spectra one of the most efficient photocathodes is multi-alkali S20 [3], [5]. 2.1 Pulsed Laser Deposition (PLD) Method Pulsed laser deposition method has a lot of advantages for ICMOS photocathode production: Process of deposition is easiest for control Quality and thickness of photocathode can be controlled during the process Very good and uniform coating quality can be achieved Relatively low cost of equipment (small and clean vacuum camera and impulse laser) Final cleaning of target, photocathode deposition and testing can be done without opening the camera Manufacturing process can be easily scaled and is well-reproducible and modifiable The PLD VC consists of a main vacuum chamber with necessary openings including viewports, electrical inputs and flanges for connecting pumps, gauges, effectors, and an internal structure includes two carousels independently rotated by stepping motors for placing substrates and targets. There are ancillary instruments such as a laser system and measurement and control tools. The rotating carousels allow to do following operations with no need of VC opening and repeated vacuuming: To produce in series several photocathodes with technological variances to explore the most optimal process parameters. To accomplish different operations on input window (multiple layers ablation, photocathode measuring etc.). To control main photocathode parameters during process. The Load/Degas/Assembly (LDA) VC is for loading substrates and transport in vacuum to deposition chamber as well as for assembling the image intensifier in the Copyright 2018 SERSC 47

3 final stage. When the loading targets, including preparation of liquid alkali metal targets from glass ampoules in vacuum, are ready then targets and substrates are degassed and transported to deposition chamber in vacuum conditions. 2.2 Connecting Vacuum Chambers to Assemble the Image Intensifier When the PLD VC and LDA VC are evacuated to necessary pressure ( Torr), then the gate valve to the PDA chamber opens. The input window with photocathode deposited is transported by the transporter fork and placed in the assembling jig in the LDA VC as seen in Fig. 1. Fig. 1. Connecting the vacuum chambers via the vacuum gate. The transporter fork can access the LDA chamber to the PLD chamber through the gate. 3 ICMOS (Intensified CMOS) Sensor Image intensifiers were initially developed for military applications as night vision goggles. Recently, high-speed fluorescence, medical, and astronomical markets of image intensifiers have been grown rapidly. 48 Copyright 2018 SERSC

4 Table 1. Visual and NUV ICMOS sensors design results. Parameter Visual NUV Detector type Intensified CMOS Photocathode material S20 CsTe Photocathode active area diameter 40 mm Spatial resolution on Photocathode 25 um Wavelength 370 ~ 600 nm 185 ~ 320 nm Photocathode Quantum Efficiency (QE) > 20% > 15 % Luminance gain > 5000 > 2000 Readout rate 100kHz ~ 12 MHz (per pixel) Lifetime (count/pixel) > 10E6 Global count rate (cps) > 1E5 Data format in accumulation mode 1024 x 1024 pixel Operation temperature -40 C ~ +40 C With pessimistic evaluation QE PC = 15% for S20 photocathode and QE PC = 8% for CsTe photocathode; K MCP = 1000; QE SCR = 0.05 for P43 phosphor [3] and V = 4000V calculate that the photon gain for visible spectrum intensifier (S20 photocathode) G PH = and for NUV spectrum intensifier (CsTe photocathode) G PH = 16000, which are acceptable to our design goal of the ICMOS sensors in Table 1. 4 Summary New technologies to improve the PLD (Pulsed Laser Deposition) method for fabricating photocathodes in ICMOS (Intensified CMOS) sensors are introduced in this research. We adapt two vacuum chambers (VCs): the PLD VC and the LDA (Load/Degas/Assembly) VC. The PLD VC consists of a main vacuum chamber with necessary openings including viewports, electrical inputs and flanges for connecting pumps, gauges, effectors, and an internal structure includes two carousels independently rotated by stepping motors for placing substrates and targets. The LDA VC is for loading substrates and transport in vacuum to the PLD VC as well as for assembling the image intensifier in the final stage. When the loading targets, including preparation of liquid alkali metal targets from glass ampoules in vacuum, are ready then targets and substrates are degassed and transported to deposition chamber in vacuum conditions. The two VCs are connected by a vacuum gate that the input window with photocathode deposited in the PLD VC is transported by the transporter fork through the vacuum gate and placed in the assembling jig in the LDA VC We have designed a visible and NUV ICMOS for both military and astronomical applications, according to our photocathodes QE data. The design results show that the photon gain for visible spectrum intensifier (S20 photocathode) and NUV spectrum intensifier (CsTe photocathode) is G PH = and G PH = 16000, respectively. Copyright 2018 SERSC 49

5 Acknowledgments. This research is funded by Korea s National Research Council of Science and Technology (CMP KASI). References 1. Stanford Computer Optics, 2. Photek, 3. Hamamatsu, 4. Johnson, Stuart, M. Jr.: Ultraviolet angular response of cesium-telluride photocathodes, APPLIED OPTICS, Vol. 31, No. 13 (1992) 5. Townsend, P. D., Valberg, L., Momchilov, N., Harmer, S. W., Downey, R. and Cormack, A. J.: Optimization of photomultiplier spectral sensitivity with a graded thickness photocathode, J. Phys. D: Appl. Phys. 41 (2008) 6. Eppeldauer, G. P., Podobedov, V. B.: NIST traceable measurements of radiance and luminance levels of night-vision-goggle test-instruments, Proc. 9071, SPIE DSS (2014) 50 Copyright 2018 SERSC