High Resolution 640 x 512 15um Pitch InSb Detector Chen-Sheng Huang, Bei-Rong Chang, Chien-Te Ku, Yau-Tang Gau, Ping-Kuo Weng* Materials & Electro-Optics Division National Chung Shang Institute of Science and Technology P.O.Box 90008-8-6, Lungtan, Taoyuan 325, Taiwan * Corresponding author: pk.weng@msa.hinet.net ABSTRACT Over the last two decades, NCSIST has developed and manufactured high quality InSb FPAs. Since 2007, we estibilished 320 x 256 with 30um pitch InSb FPA processes, and integrated these detectors with corresponding dewars and coolers into IDDCAs. The tendency of infrared detector technology is to develpoe higher resolution and smaller pitch. NCSIST had successfully developed 640 x 512 with 15um pitch FPAs in 2010. It shows high uniformity, low NETD (20-25mK) and high operability (typically better than 99.8%). The high resolution InSb FPA can be assembled in IDDCA to apply in many applications such as IR fire control system for armored vehicle, IR seeker for missile, and IR air defense system. Keywords: InSb, Focal Plane Array (FPA), Indium bump, Hybridization, Integrated Detector Dewar Cooler Assembly (IDDCA) 1. INTRODUCTION Integrated Detector Dewar Cooler Assembly (IDDCA) is the key component of infrared thermal image camera. An InSb IDDCA consists of InSb focal plane array (FPA) detector, dewar, and cooler. A FPA consists of Infrared detector and Readout Integrated Circuit (ROIC) (Fig. 1). Figure 1:IDDCA includes FPA, Dewar and Cooler.
Over the last two decades, NCSIST has developed and manufactured high quality InSb FPAs. The response wavelength of InSb is 3 to 5 um, which can not be absorbed by atmosphere. InSb detector utilizes the thermal radiation emitted from the object and background to get photo current, then ROIC can transform the current into thermal images to distinguish the target. Based on the physical characteristics, infrared thermal image camera can be used without external illumination. NCSIST had successfully developed high resolution 640 x 512 InSb FPA with 15um pitch (640-15um FPA) in 2010. The 640-15um FPA s size is the same as mid resolution 320 x 256 InSb FPA with 30um pitch (320-30um FPA). Due to the same size of 640-15um and 320-30um FPA, the image resolution can be further improved by replacing the original 320-30um FPA, any modifications of the dewar or optics components of the IDDCA are not needed, and furthermore the cooler is still suitable to use. NCSIST s 650-15um FPAs show high uniformity, low noise equivalent temperature difference (NETD) 20-25mK and high operability (typically better than 99.8%). The image in Fig. 2 is presented without any uniformity correction or bad pixels replacement. Figure 2:Raw signal of 640-15um FPA. 2. DETECTOR ARRAYS Fabrication techniques for plannar type InSb detector arrays use Be implantation to form a p + region on n-type InSb substrate. To approach the minimum surface leakage current, insulating layers grown by CVD were used as passivation. Then the metals were evaporated as ohmic contact and bonding pad. Each focal plane array consists of 640 x 512 element p + /n diodes. The pitch size is 15 um, and active areas are 10 x 10 um 2. IR radiation is absorbed by the thin substrate with the charge collected by the pn junction and injected into the input stage via the indium bumps which connect each element of the CMOS ROIC.
3. READOUT INTEGRATED CIRCUITS(ROIC) CMOS circuit has the advantages of low power and low noise while providing sophisticated multiplexing and control logic for an infrared FPA readout. A high performance CMOS switched FET readout multiplexer was developed using standard foundry compatible processing. Each unit cell in the readout has a direct injection input circuit, a source follower amplifier, a reset MOSFET, and a cell access MOSFET. The input circuit is a common gate amplifier that performs the functions of interfacing to the detectors, integrating the photocurrent and converting this current into a voltage. The gate bias labeled IG is adjustable externally to maintain the operating point of the detector. Internal timing generator generates most of the control clocks so that only three clocks are required. Two rows (512 inputs) are sampled simultaneously by peripheral sample and hold amplifiers. The readout also incorporates logic that can be programmed to select 4-output mode. Multiple outputs will enhance the readout data speed. Since the rate that each output can multiplex data is at least 2 MHz, the FPA can be operated up to 120 frames per second using the 4 outputs respectively. 4. HYBRIDIZATION Indium bumps are utilized to realize the electrical and mechanical interconnection between the InSb detector array and the silicon readout integrated circuit (ROIC). Identical indium bumps are grown on arrays and readout chips. Flip chip bonding technology is used to hybridize the 640 512 InSb detector array and silicon readout multiplexer. The bonding equipment provides simultaneous viewing of the detector and ROIC by a microscope. After alignment, these bumps are joined together. A built-in contact force indicator permits continuous monitoring of the bonding force during hybridization. The major concern in this process is that all the detector pixels should be connected electrically and mechanically to the readout silicon multiplexer with minimum degradation in the signal-to-noise ratio. Hybridization is a critical step for the whole process, both the shape and uniformity of the indium bumps can influence greatly to the hybridization yield. After hybridization, the detector backside was thinned by mechanically lapping to about 50 um thickness, followed by mechanically polishing to about 20 um thickness to optimize its quantum efficiency. Chemical etching technique was used in order to remove the defects caused by previous steps and therefore achieve better thickness uniformity of the InSb substrate. Finally, the detector backside was covered with anti-reflection coating for protection and optimization of the performance.
5. HIGH RESOLUTION AND SMALL PITCH ADVANTAGE High resolution and high sensitivity infrared FPA is getting more important in infrared applications. The number of pixels of a 640 x 512 infrared FPA is four times of a 320 x 256 infrared FPA, so its detail image will be distinguishable more easily even the targets are complicated. The ability of identifying object of high resolution infrared FPA is also better than low resolution infrared FPA, it is helpful to identify and lock targets earlier if high resolution infrared FPA is used in military weapons. Producing 640 x 512 InSb FPA with 30um pitch (640-30um FPA) is easier but the the FPA size will become larger than the 320-30um FPA. With the number of pixels increasing, the FPA size increases as well. Therefore, a better cooling system to lower the operating temperature down to 77 K is needed. For an IDDCA, the cost of high power cooler will be much higher. NCSIST did a lot of effort on 640-15um FPA, it can be used to replace 320-30um FPA in IDDCA without changing its dewar and cooler, and the resolution is improved. Besides, we also make low resolution 128 x 128 InSb FPA with 15um pitch (128-15um FPA) and mid resolution 320 x 256 InSb FPA with 15um pitch (320-15um FPA), these design can make small-size IDDCAs, which can be used on portable weapons as well. 6. DEVELOPMENT OF FPA IN NCSIST Since 1993, NCSIST started producing 128 x 128 InSb FPA with 50um pitch (128-50um FPA) and install them in IDDCA. Every three years, we develop a newer version of FPA with upgrated resolution and reduced pixel pitch. The FPA development progress is shown in Fig. 3. Figure 3:NCSIST s development in FPA Up to now, we have successfully developed and produced 640-15um FPA, and the resolution of 1280 x 1024 InSb FPA with 12um pitch (1280-12um FPA) is under development. The main product in NCSIST is 15um pitch IR FPA. By assembling our IDDCA with different cooler constructions, we have a total of 6 main products,
which is shown in the figure below. (Fig. 4) Figure 4:NCSIST has 6 IDDCA products. The following table (Table. 1) is the product specification of 15um pitch FPAs. We have 2 different formats, 320 x 256 and 640 x 512. The minimum analyzable NETD is about 20mK, and quantum efficiency can reach 85%, operability can exceed 99.5%, and the Cooling system can be used for more than 10 years. Table 1. 15um pitch InSb FPA Specifications Parameters Specifications Detector InSb 320 256 InSb 640 512 Pixel Size 15μm 15μm 15μm 15μm Die size 5mm 5mm 10mm 10mm Frame rate 50Hz(4 output) 50Hz(4 output) NETD 20mK 20mK Operability 99.5% 99.5% QEFF 60% 60% NCSIST has successfully produced 640-15um FPAs, the thermal image shows that it has high operability and good uniformity (Fig. 5). We installed the 640-15um FPA in IDDCA and had trial runs for it. The result is positive and had been demonstrated in 2010, it also achieved mass production in 2013. It has been successfully used on thermal imaging camera of weapons. NCSIST has produced various 15um FPAs with small sized where low power cooling systems can be used, making our products/systems light and price low.
Figure 5 Images of 640-15um FPA. Recently, NCSIST is developing mega-pixels 1280 x 1024 InSb FPA with 12um pitch FPA and plans to reduce the pitch size to 7.5um in the near future. The chip size between 1280-7.5um and 640-15um FPA will be the same, therefore, the IDDCA configrations can keep the same with 640-15um FPA. Besides InSb FPA for MWIR, NCSIST also has experences on developing GaN FPA for UV band, InGaAs FPA for SWIR, QWIP FPA and microbolometers for LWIR, respectively (Fig. 6). Figure 6 NCSIST has all kinds of FPAs.
7. APPLICATIONS In modernization war, it is important to win the night fightings. In Gulf War in 1990-91, US army used infrared thermal image technologies to wipe out Iraq army. From this example, we understood the importance of IDDCA on battlefields so NCSIST continues developing high resolution and small pitch FPAs. The current radars on aircrafts cannot detect armies fighter at a considerable distance so most aerial warfare will happen within a very short distance, thus the infrared detector will play the key role to win. The wavelength of infrared is shorter than microwaves so it can identify images clearer than radar. However, infrared is easier to decay in atmosphere because the range of infrared wavelength is easy to be absorbed by atmosphere. According to the different advantages of IR FPA and Radar, infrared/radar dual-seeker was getting more and more important during the past years. Radar can detect the missle at larger distances. When it becomes closer, we rely on high resolution infrared seeker to obtain thermal images and analyze the shape of enemies ballistic missiles while anti-ballistic missile system is trying to destroy targets. IDDCA has a lot of application such as thermal imaging observer on armored vehicles, seekers, air defense systems. The corresponding images are shown as Fig. 7. NCSIST s high resolution 640-15um FPAs have good quality for infrared thermal imaging. It has already been used on the weapons mentioned above. (a) IR fire control system (b) IR seeker (c) IR air defense system Figure 7:IDDCAs can use in differents weapon systems (a) IR fire control system, (b) IR seeker, and (c) IR air defense system 8. CONCLUSION The high performance 640-15um FPA has been successfully developed at NCSIST. Initial goals of the development program have been met or exceeded. The arrays have high operability and good uniformity; the quantum efficiency is close to theory; the readout noise and detector dark current are low. Larger formats of focal plane arrays will be developed in the future. We believe this system will be developed further by NCSIST and have more applications in the weapons.