D. Allen

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.

Crack propagation and fracture in silicon wafers under thermal stress

The microcrack propagation and cleavage behaviour in silicon wafers during thermal annealing has been studied by in situ X-ray diffraction imaging (topography).

Prediction of the propagation probability of individual cracks in brittle single crystal materials

We show that x-ray diffraction imaging (topography) and finite-element modelling can determine accurately the probability of propagation of individual cracks in brittle single crystal materials. The x-ray image of the crack provides a critical parameter for crack propagation which informs a predictive model, enabling us to identify critical defects that lead to catastrophic shattering of silicon wafers during high temperature thermal processing. Wafers fracture on cooling and finite element modelling shows that, during cooling, the tangential stress at the wafer edge is tensile and results in crack propagation. The predicted fracture geometry agrees extremely well with that observed experimentally.

Dislocation sources and slip band nucleation from indents on silicon wafers

The nucleation of dislocations at controlled indents in silicon during rapid thermal annealing has been studied by in situ X-ray diffraction imaging (topography). Concentric loops extending over pairs of inclined {111} planes were formed, the velocities of the inclined and parallel segments being almost equal. Following loss of the screw segment from the wafer, the velocity of the inclined segments almost doubled, owing to removal of the line tension of the screw segments. The loops acted as obstacles to slip band propagation.

X-RAY DIFFRACTION IMAGING OF DISLOCATION GENERATION RELATED TO MICROCRACKS IN Si-WAFERS

The nucleation of dislocations at indents in silicon following rapid thermal annealing (RTA) has been examined by X-ray diffraction imaging (topography). For indentation loads below 200mN, no slip bands were generated from the indent sites following RTA at 1000C under spike conditions. Upon plateau annealing at 1000C, slip dislocations were propagated from some indents but not all. Slip was also observed from edge defects not associated with indentation. For 500mN indentation load, large scale dislocation sources were generated from the indent sites, propagating on two of the four {111} slip planes. These dislocations multiplied into macroscopic-scale slip bands. A significant change in morphology was observed in the 60 dislocation segments after the screw segment reached the rear surface of the wafer. Dislocations changed line direction and in some cases appeared to leave the Peierls trough during glide.

Three-dimensional X-ray diffraction imaging of process-induced dislocation loops in silicon

In the semiconductor industry, wafer handling introduces micro-cracks at the wafer edge and the causal relationship of these cracks to wafer breakage is a difficult task. By way of understanding the wafer breakage process, a series of nano-indents were introduced both into 20 × 20 mm (100) wafer pieces and into whole wafers as a means of introducing controlled strain. Visualization of the three-dimensional structure of crystal defects has been demonstrated. The silicon samples were then treated by various thermal anneal processes to initiate the formation of dislocation loops around the indents. This article reports the three-dimensional X-ray diffraction imaging and visualization of the structure of these dislocations. A series of X-ray section topographs of both the indents and the dislocation loops were taken at the ANKA Synchrotron, Karlsruhe, Germany. The topographs were recorded on a CCD system combined with a high-resolution scintillator crystal and were measured by repeated cycles of exposure and sample translation along a direction perpendicular to the beam. The resulting images were then rendered into three dimensions utilizing open-source three-dimensional medical tomography algorithms that show the dislocation loops formed. Furthermore this technique allows for the production of a video (avi) file showing the rotation of the rendered topographs around any defined axis. The software also has the capability of splitting the image along a segmentation line and viewing the internal structure of the strain fields.

A novel X-ray diffraction technique for analysis of die stress inside fully encapsulated packaged chips

Manufacturing-induced thermal stress created during the fabrication of packaged integrated circuits can potentially lead to device failure. Therefore, the need to develop metrologies that can be used to effectively measure stress/strain in systems-on-chip or systems-in-package is identified by the International Technology Roadmap for Semiconductors (ITRS). In this study, a novel technique for non-destructive analysis of strain/warpage inside completely encapsulated packaged chips, at room temperature and processed at elevated temperatures up to 115°C, is developed using a laboratory-based X-ray diffraction tool. Maps are produced of the entire silicon die, which reveal warpage via mapping of rocking curve full-widths-at-half-maximum (FWHM) as a function of position across encapsulated packages, using a technique known as 3-dimensional surface modelling. We develop complete Si die maps of the large thermal stresses that are developed during the die attach process due to the coefficient of thermal expansion mismatch between different materials. These are confirmed by in situ X-ray diffraction annealing experiments, as well as finite element analysis (FEA).

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 〈110〉 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 simulation.

Local strain and defects in silicon wafers due to nanoindentation revealed by full-field X-ray microdiffraction imaging

Quantitative characterization of local strain in silicon wafers is critical in view of issues such as wafer handling during manufacturing and strain engineering. In this work, full-field X-ray microdiffraction imaging using synchrotron radiation is employed to investigate the long-range distribution of strain fields in silicon wafers induced by indents under different conditions in order to simulate wafer fabrication damage. The technique provides a detailed quantitative mapping of strain and defect characterization at the micrometer spatial resolution and holds some advantages over conventional methods.