Two improvements have been made to solve two problems that arise in conjunction with a fabrication process in which an HgCdTe photodetector is mounted on an alumina substrate with epoxy. Although most of the HgCdTe-photodetector industry has abandoned this process in favor of epitaxial deposition, epoxy mounting is still used in some special applications.
The first problem is to reduce large variations in detector response as a function of wavelength; that is, to obtain more nearly even spectral response. The sharp variations in the spectral response are associated with resonances caused by interference between reflections from the alumina substrate and the front surface of the detector. The first improvement is to load the epoxy with 0.05-μm alumina grit; this solves the spectral-response problem by suppressing reflections from the substrate.
The second problem arises as follows: During the mounting procedure, the HgCdTe/epoxy/alumina structure is compressed, causing some of the epoxy to flow out and form a ridge around the perimeter of the HgCdTe wafer. The epoxy ridge prevents the use of contact photolithography in subsequent fabrication steps, making it necessary to resort to more expensive and difficult noncontact optical photolithography. The problem is to prevent the formation of the epoxy ridge so that contact photolithography can still be used Accordingly, the second improvement is to change the mounting procedure to prevent the formation of the epoxy ridge. In the improved mounting procedure, weight is applied to the HgCdTe/epoxy/alumina structure while the structure is heated. Any excess epoxy squeezed out is removed by use of solvent spray. The epoxy is then cured.
This work was done by Bob Martineau, Andre Burgess, Sridhar Manthripragada, Frank Peters, Brent Mott, and Peter Shu of Goddard Space Flight Center; Kelley Hu and Jack Shi of HSTX; and Sachidananda Babu of Ball Aerospace. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Components and Systems category. GSC-13811