A Hybrid Integrated Silicon Diode Array for Visible Earth-Horizon Sensing

An earth-horizon sensing device which operates principally in the visible portion of the spectrum has been designed as a hybrid integrated circuit. The circuit was fabricated and tested for use in the LES-8/9 communications satellites as part of the system which maintains the satellites´ orientation with respect to earth. The hybrid circuit consists of four silicon chips mounted on a high-density alumina ceramic substrate. Each silicon chip contains eight photodiodes which are shallow junction (approximately 1.5 micrometers)n on p type, and each diode is surrounded by a one-micrometer deep p+ channel stop. The individual diodes in the array have as typical parameters dark currents at five volts of less than 5 nanoamps, breakdown voltages measured at 10 microamps in excess of 70 volts, and a uniformity of photoresponse across an array of + 5%. The choice of the techniques used in the hybridization of this circuit was dictated by the requirements that the four silicon chips be aligned to one another within + 0.001 in., and that the entire assembly survive unpackaged and unencapsulated in a potentially corrosive prelaunch environment. For precise silicon die alignment, the dies were mounted with a gold-filled epoxy. To provide maximum corrosion resistance, a Ti/Pd/Au metallization system was used on both the silicon chips and the alumina substrate, and interconnections from the chips to the substrate were made with thermocompression bonded gold wires. Environmental tests indicate that the complete assembly will survive a Mil Std 202D immersion test with no degradation. The diodes will survive extended aging at 200°C, but the epoxy deteriorates badly after a few hundred hours. A completed sensor aged at 125°C for 720 hours has shown no significant changes.