Fracture of silicon wafers and silicon beam transducers

McLaughlin, Judith Clare (1988) Fracture of silicon wafers and silicon beam transducers. University of Southampton, Doctoral Thesis.

Abstract

The main objectives of this work were to attempt to understand the materials factors limiting the fracture strength of modern silicon single crystals and the effect of eutectic bonding the silicon beam to the substrate on the fracture stress of the beam. The state of knowledge of the fracture characteristics of silicon and the assembly of silicon beam transducers are reviewed. Values of fracture toughness for silicon are reviewed and compared. The technique used to measure the fracture stress of silicon in the present work involved simply supporting a wafer on a ring concentric to the load axis. Typical values of the fracture stress obtained by this method, for different crystals, vary between 2 and 3.5 GPa. In the first part of the study, the role of the surface on the fracture behaviour has been investigated in detail. While the surface perfection of the tensile surface has a major effect on the fracture stress (as shown in previous studies), some of the results were found to be sensitive to the compressive surface as well. In the cases where the results are sensitive to the compressive surface finish the fracture stress rose from 3.7 to 8.8 GPa as the surface finish was improved while in the cases where they were not sensitive the fracture stress remained at about 3.5 - 4.6 GPa. Only in the float-zone (FZ) material were fracture stresses approaching 8.8 GPa observed. At his level of fracture stress, the behaviour is believed to be sensitive to surface defects less than 0.01 μm in size. These results can be analysed in terms of surface controlled defects under conditions where surface defects are dominant and bulk controlled defects where these defects are dominant. In this manner bulk defects can be isolated from surface ones. This gives the opportunity to study the effects of specific defects on the fracture stress and results are discussed in terms of the role of surface and internal defects on the fracture stress. Studies of the fracture stress of wafers after improvement of surface finish thus allowed a study of bulk defects on fracture behaviour. The fracture stress of Czochralski (CZ) silicon was reduced from 4.4 GPa to 3.0 GPa after a precipitation heat treatment. The effects of various surface treatments (thermal oxidation, ion implantation and rapid thermal annealing (RTA), nitride layers and annealing in hydrogen at 1250^oC) on the fracture stress were also studied. The breaking stresses of silicon beams bonded to Kovar baseplates, were measured in contraflexure and the bonds examined with optical and scanning electron microscopy. The eutectic bond/silicon interface was found to be non-planar exhibiting facets larger than the calculated critical crack length of the silicon beam and poor wetting was noted in places. One of the most effective ways of increasing the fracture stress of the silicon beam was found to be the elimination of the sharp corner at the inside edge of the eutectic bond/silicon interface using stepped beams.

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