Planarization for the Integration of CMOS and Micro Mirror Arrays

摘要

A large format (2Kx2K) individually addressable Micro-Mirror-Array (MMA) has been developed at NASA, GSFC for possible application in the Next Generation Space Telescope (NGST). The 100micronx100micron aluminum micro-mirrors are built on top of CMOS driven address and driver circuit for individual addressing. The high voltage CMOS fabrication process produces about 2.8microns surface roughness on the silicon wafer. The wafer surface is planarized before integration of the MMA. Three different planarization materials were evaluated; polyimide, spin-on glass and BCB. BCB showed the best results for our application. A single layer of BCB coating reduced the surface topology from 2.8micron to less than 1,700Angstroms and two layers of BCB coating reduced the surface topology to about 600Angstroms. Since the MMA has to operate at 30K for the NGST application, a wafer coated with cured BCB was dunk tested in liquid nitrogen at 77K and no cracks were found after thermal cycling. For specific application in NGST, the optical reflectance of BCB was measured at 40K over l-5micron wavelength range and the results showed that BCB could absorb 30-40% of infrared light over this range. Details of coating, curing and etching properties of BCB are discussed along with its low temperature optical properties.