Design and Fabrication of Novel Photodetector Arrays Mary J. Li"2, Christine A. Allen2 , Shahid Aslam"2, Tina C. Chen3'4, Fred M. Finkbeiner5'6, Scott A. Gordon7, Jonathan L. Kuhn7, David B. Mott2, Carl M. Stahle2, Caroline K. Stahle5, Nilesh Tralshawala"2, Liqin Wang8'9 'Raytheon ITSS, Lanham, MD 20706, USA 2NASA, Goddard Space Flight Center, Code 553, Greenbelt, MD 20771, USA 3Global Science & Technology Inc., Greenbelt, MD 20770, USA 4NASA, Goddard Space Flight Center, Code 685, Greenbelt, MD 20771, USA 5NASA, Goddard Space Flight Center, Code 662, Greenbelt, MD 2077 1 , USA 6Department of Astronomy, University of Maryland, College Park, MD 20742, USA 7NASA, Goddard Space Flight Center, Code 542, Greenbelt, MD 2077 1 , USA 8Unisys Corp., Lanham, MD 20706, USA NASA, Goddard Space Flight Center, Code 54 1 , Greenbelt, MD 2077 1 , USA Abstract We are developing novel photodetector arrays based on superconducting transition-edge sensor (TES) and pop-up detector (PUD) technologies. The TES has the potential for a new generation of high sensitivity photodetectors from the infrared to the x-ray. This is directly due to the sharpness of the resistance change with temperature at the superconducting transition. The TESs are deposited on the PUD arrays and serve as the sensing elements. The PUDs are close-packed, folded membrane arrays that provide the TES substrate and the thermal isolation required by the bolometers and microcalorimeters. This paper presents the processing-related characterization results of preliminary TES and PUD designs. The goal of this work is to fabricate a new generation of x-ray calorimeters and infrared bolometers for space flight projects. Al-Ag and Mo-Au TES bilayers were fabricated by electron-beam deposition in an ultra-high vacuum system. To achieve the desired transition temperatures and stability, materials-related properties such as adhesion, surface morphology, film stress, and electrochemical reaction ofthe bilayers were studied. Mo-Au bilayers showed better chemical and electrical stability than Al-Ag bilayers. The PUD arrays were fabricated with a 0.2mm pitch on silicon wafers. MEMS techniques were used in the fabrication of PUD arrays. Mechanical folding tests were conducted to inspect the mechanical stress distribution for the array structures. The test results showed yields for fourteen designs. Mechanical modeling was conducted using non-linear static and dynamic simulations. The simulations agreed with the trends of mechanical folding tests. Scanning electron microscopy was utilized to examine the flatness of detectors and the alignment of detectors in arrays. Key Words: Superconducting transition edge sensor, Pop-up detector, Mo-Au bilayer, Al-Ag bilayer, bolometer, microcalorimeter, Anisotropic etching, FEM simulation. 1. Introduction While the thermal detector using a semiconductor thermistor is approaching its practical limit of sensitivity, the superconducting transition edge thermometer has the advantage to increase the sensitivity by l2 orders of magnitude. The sensitivity, a, is defined as d(logR)/d(logT). The sensitivity increase is directly due to the sharpness of resistance change with temperature at the superconducting transition of the materials. There is a long history for applying superconducting materials in thermal detectors. Known as transition edge sensors, TES detectors have been used in bolometers since 1942 [1]. Researchers adjust the superconducting transition temperature, i.e. the critical temperature, T, utilizing the proximity effect [2]. When a superconductor (5) and a normal metal (N) are put together forming an S-N bilayer structure, the proximity effect may take place. Researchers adjust the relative thickness of N and S to obtain desired T of the bilayers. Different metallic bilayer films have been studied including normal metals like gold, silver, copper, and palladium, and superconductors like aluminum, niobium, tantalum, and titanium. Very promising results were reported by Irwin et al. at NIST on Al-Ag bilayers in thermal detector application [3J. The NIST bilayers exhibited a values in excess of 1000. For Part of the Conference on Device and Process Technologies for MEMS and Microelectronics • Royal Pines Resort, Queensland, Australia 192 October 1999 SPIE Vol. 3892 • 0277-786X/99/$1O.OO