Efforts are under way to develop back-surface-illuminated, thinned silicon charge-coupled devices (CCDs) with delta doping and integral optical filters to be used as image detectors in the ultraviolet wavelength range. The concept of delta doping of back-surface-illuminated, thinned silicon CCDs as part of an overall design to make CCDs sensitive to ultraviolet light is not new in itself. Delta-doped CCDs were invented at NASA's Jet Propulsion Laboratory in 1992, and it is well established that this process produces ultraviolet-sensitive CCDs with stable and uniform 100-percent internal quantum efficiency. The novelty lies in the proposed fabrication of such CCDs in which both delta doping and optical filter layers would be deposited as integral parts of unitary device structures.
Because silicon CCDs are sensitive to visible light, one of the major challenges in the development of ultraviolet imaging CCDs is to satisfy the need for filters that will reject visible light but pass ultraviolet light. Another major challenge is posed by the fact that the naturally-formed SiO2 on the air-exposed Si surfaces absorbs light strongly at wavelengths < 140 nm. Hence, it would be desirable to eliminate the SiO2 layers as well as to deposit visible-light-rejecting filters and antireflection layers on the back surfaces of the CCDs.
The use of integral filters (as distinguished from external filters that are fabricated on separate substrates) would (1) increase the robustness of image detectors by eliminating the external filters, which are delicate; (2) eliminate the need for structural supports for the filters; (3) eliminate the need for the substrates on which external filters are constructed and which introduce optical losses that degrade detector responses at short wavelengths; and (4) reduce the number of optical surfaces, thereby reducing overall optical losses by eliminating the loss (typically at least 2 to 3 percent) associated with each such surface eliminated.
Because the delta-doped layer lies permanently ~5-10 Å beneath the back surface of a CCD, the delta doping process does not pose an impediment to the subsequent deposition of optical filters and antireflection layers. The problem then becomes one of depositing these optical layers directly on the silicon surface, without the formation of an intervening SiO2layer. The approach taken in the present development effort is to perform delta doping in one ultrahigh-vacuum molecular-beam epitaxy (MBE) chamber and then, without breaking vacuum, transfer the CCD to a connected metal/insulator MBE chamber wherein the filter layers are deposited. By refraining from breaking vacuum until after the deposition of the filter layers, one can prevent the formation of the SiO2 layer (see figure). At the time of reporting the information for this article, MgF2 antireflection layers optimized for the wavelength range of 200 to 300 nm had been deposited on delta-doped CCDs and were found to result in a modest increase in the quantum efficiency of the CCDs at a wavelength of 180 nm.
This work was done by Shouleh Nikzad, Peter Deelman, Paula Grunthaner, Frank Grunthaner, Michael Hoenk, and R.W. Terhune of Caltech forNASA's Jet Propulsion Laboratory.
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