Large area backside illuminated CCD arrays using bonded and etched back silicon on epoxy

Details Large area backside illuminated CCD arrays using bonded and etched back silicon on epoxy Large area backside illuminated CCD arrays using bonded and etched back silicon on epoxy

Oct 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt........15b&link_type=abstract

Thesis (PhD). UNIVERSITY OF CALIFORNIA, DAVIS, Source DAI-B 61/04, p. 2098, Oct 2000, 144 pages.

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Charge Coupled Devices (CCD) are presently used for many astronomical instrumentation applications. While CCDs exhibit excellent spectral response, they show a reduced sensitivity at wavelengths below 400 nm. This results from the absorption of photons in the CCD's polysilicon controlling gates. The spectral response can be improved by thinning the bulk silicon from the CCD and illuminating it from the back side. Since a thinned CCD will be only 15 to 20 micrometers thick, it must be supported by another structure. To achieve this, a thinning procedure had to be developed that bonds the CCD to a handle for mechanical support prior to thinning. Aluminum metalization and charge sensitive circuitry prevents the use of thermal or anodic bonding, respectively. Therefore, adhesion bonding was done with an epoxy. This research started with a commercially produced CCD array, designed for the astronomical community. The CCD, acquired as a wafer, was bonded to a handle wafer. Four separate bonding issues were investigated: adhesive bonding, mechanical thinning, chemical polishing, and contact via etching. First, the bonding required an investigation into potential adhesives. Various properties of an ideal adhesive were defined, and a number of experiments devised to test a number of adhesives. Then a procedure was developed that would produce a void free bonded wafer set. This involved methods to mix, and apply the adhesive to the wafers. Second, mechanical thinning techniques were studied to thin the device wafer. Initial selection of commercial grinders proved unsatisfactory, so equipment, and techniques were acquired to perform a custom lapping procedure. Third, a chemical thinning method was developed for two types of CCD wafers used. One CCD was constructed on a lightly doped epitaxial layer above a highly doped bulk wafer. The other wafer was constructed with a Si-Ge-B etch stop grown prior to the lightly doped CCD epitaxial layer. Last, the device wafer had to be selectively etched to expose the metal bonding pads that were buried at the interface. A contact via etching procedure needed to be developed that would expose the aluminum, without damaging the CCD electronics.

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