SUBJECT: Ph.D. Proposal Presentation
BY: Kevin Skenes
TIME: Wednesday, May 15, 2013, 2:00 p.m.
PLACE: MARC Building, 431
TITLE: Characterization of Residual Stresses in Birefringent Materials applied to Multicrystalline Silicon Wafers
COMMITTEE: Dr. Steven Danyluk, Co-Chair (ME)
Dr. William Wepfer, Co-Chair (ME)
Dr. Shreyes Melkote (ME)
Dr. Peter Hesketh (ME)
Dr. Laurence Jacobs (CEE)
Dr. Ajeet Rohatgi (ECE)


Birefringence has been used to study transparent materials since 1815, and is based on the decomposition of a polarized ray of light into two distinct rays when passing through an optically anisotropic material. Cases involving applied stresses have been thoroughly investigated, but samples in a residual stress state with no applied load remain less understood. This research uses a near-infrared polariscope and the principles of birefringence to study residual stresses in multicrystalline silicon wafers. The numerical methods used for photoelastic analysis will be carefully examined and further developed. Noise in the isoclinic fringe measurement has been identified as a key source of error when separating the maximum shear stress into its principal normal components. One critical objective is to determine ways to reduce or eliminate this noise in an effort to obtain useful normal stress data. Smoothing techniques have been applied to the data, and new quality-guided integration schemes are being developed and implemented. Residual stress measurements will be tied to physical phenomena on a wafer-wide and localized scale. Properties on the wafer level which will be investigated include the effects of inclusion concentration, grain and grain boundary concentration, and the difference between n- and p-type wafers. On a local level, Vickers indentations will be used to create crack networks, and the residual stresses measured with the polariscope will be compared to a finite element model. The stresses accompanying these crack fields will be investigated, as will the effects of proximity to other cracks, inclusions, or grain boundaries.