Frit bonding: a way to larger and more complex silicon components

Abstract

The value of glass frit bonding to assemble silicon parts was demonstrated by the successful evaluation of cryostability of a small multi-piece silicon mirror. This bonding technique has been extended to the assembly of a 44kg block of silicon, 113 x 400 x 400mm. Such an assembly was considered to be a cost competitive alternative to the purchase of a custom sized silicon boule. Various types of evaluation provided the foundation upon which this accomplishment is based. Included were cryocycling of frit bonded plates, comparison of the strength of bonded silicon bend bars with that of silicon bend bars, and the fabrication and cryotest of a small concave frit bonded silicon mirror. Of the 16 bonded bars in two groups only 2 had failures in the bondline, 11 failed in the silicon, and origins could not be determined for 3 bars. The two groups of bonded silicon bars had average strengths that were 84% and 91% of the average strength of the plain silicon bars. In view of the relatively small number of bars in each group this is not surprising. The cryostability of the concave bonded silicon mirror was demonstrated by a figure error of less than 0.06 wave rms at 633nm, cold to warm, compared to a specification of 0.1 wave rms, and 0.014 wave rms, warm to cold to warm, over an 80% clear aperture. These results are reviewed before interesting features of the large block are discussed. Finally, projections are made regarding possible future applications for this bonding process.