(57) Spherically convex microgrids may be prepared from a flat microgrid structure by
using a process depending on the differential thermal expansion between the material
of the flat structure and a layer of other material applied thereto. A flat process
blank 2 is etched with grid apertures in a microgrid area to which a curvature is
to be added during fabrication. The blank is then subject to the deposition of a thin
layer 4 of aluminium nitride (or other differential thermal expansion material having
a positive thermal expansion mismatch with the material of the microgrid substrate)
over at least the microgrid area (Step A). The differential thermal expansion layer
is coextensively with the grid area, so as to confine the warping substantially to
the grid area where curvature is desired. Curvature (Step B) is accomplished by a
controlled heat warping step in an inert atmosphere (for a silicon microgrid structure
of 250 micrometers thickness, heating at 1200C to soften the silicon). Permanent deformation,
occuring as a result of the differential thermal expansion, is controlled by selection
of temperature and time of heat treatment to achieve the desired curvature. The aluminium
nitride layer (Step C) is removed prior to cooling.
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