Crack propagation and fracture in silicon wafers under thermal stress

The behaviour of microcracks in silicon during thermal annealing has been studied using in situ X-ray diffraction imaging. Initial cracks are produced with an indenter at the edge of a conventional Si wafer, which was heated under temperature gradients to produce thermal stress. At temperatures where Si is still in the brittle regime, the strain may accumulate if a microcrack is pinned. If a critical value is exceeded either a new or a longer crack will be formed, which results with high probability in wafer breakage. The strain reduces most efficiently by forming (hhl) or (hkl) crack planes of high energy instead of the expected low-energy cleavage planes like {111}. Dangerous cracks, which become active during heat treatment and may shatter the whole wafer, can be identified from diffraction images simply by measuring the geometrical dimensions of the strain-related contrast around the crack tip. Once the plastic regime at higher temperature is reached, strain is reduced by generating dislocation loops and slip bands and no wafer breakage occurs. There is only a small temperature window within which crack propagation is possible during rapid annealing

Observation of nano-indent induced strain fields and dislocation generation in silicon wafers using micro-raman spectroscopy and white beam x-ray topography

In the semiconductor manufacturing industry, wafer handling introduces micro-cracks at the wafer edge. During heat treatment these can produce larger, long-range cracks in the wafer which can cause wafer breakage during manufacture. Two complimentary techniques, micro-Raman spectroscopy (μRS) and White Beam Synchrotron X-ray Topography (WBSXRT) were employed to study both the micro-cracks and the associated strain fields produced by nano-indentations in Si wafers, which were used as a means of introducing controlled strain in the wafers. It is shown that both the spatial lateral and depth distribution of these long range strain fields are relatively isotropic in nature. The Raman spectra suggest the presence of a region under tensile strain beneath the indents, which can indicate a crack beneath the indent and the data strongly suggests that there exists a minimum critical applied load below which cracking will not initiate

X-ray asterism and the structure of cracks from indentations in silicon

The asterism observed in white radiation X-ray diffraction images (topographs) of extended cracks in silicon is investigated and found to be associated with material that is close to breakout and surrounded by extensive cracking. It is a measure of the mechanical damage occurring when the fracture planes do not follow the low-index cleavage planes associated with the crystal structure. It is not related to a propensity for some cracked wafers to shatter during subsequent high-temperature processing. There is no correlation between crack morphology and alignment of an indenter with respect to the orientation of a silicon wafer, the cracks being generated from the apices of the indenter and having threefold symmetry for Berkovich indents and fourfold symmetry for Vickers indents. X-ray diffraction imaging (XRDI) of indents does not reveal this underlying symmetry and the images exhibit a very substantial degree of variation in their extent. This arises because the XRDI contrast is sensitive to the long-range strain field around the indent and breakout reduces the extent of this long-range strain field. Breakout is also detected in the loss of symmetry in the short-range strain field imaged by scanning micro-Raman spectroscopy. Weak fourfold symmetric features at the extremes of the images, and lying along directions, are discussed in the context of slip generated below the room-temperature indents. Scanning electron microscopy imaging of the region around an indent during focused ion beam milling has permitted the three-dimensional reconstruction of the crack morphology. The surface-breaking Palmqvist cracks are found to be directly connected to the median subsurface cracks, and the presence of extensive lateral cracks is a prerequisite for material breakout at indenter loads above 200 mN. The overall crack shape agrees with that predicted from simulatio

Thermal slip sources at the extremity and bevel edge of silicon wafers

High-resolution X-ray diffraction imaging of 200 mm silicon wafers following rapid thermal annealing at a temperature of 1270 K has revealed the presence of many early stage sources of thermal slip associated with the wafer edge. Dislocation sources are primarily at the wafer extremity, though many are generated by damage at the edge of the bevel incline on the wafer surface. A smaller fraction of sources is associated with other regions of localized damage, probably relating to protrusions on the wafer support. The geometry of the latter is similar to that of dislocation sources generated by controlled indentation on the wafer surface. It is concluded that rapid spike annealing at high temperature does not suppress the nucleation of slip, but rather the rapidity of the process prevents the propagation of the dislocations in the slip band into the wafer