The Woodruff School

SUMMARY

Photovoltaic manufacturing is material intensive; the cost of the silicon wafer, used as the substrate, can represent 40% to 60% of the solar cell fabrication cost. Consequently, there is a growing trend to reduce the silicon (Si) wafer thickness and to increase wafer size leading to new technical challenges related to manufacturing. Specifically, the breakage of thin Si wafers during handling and/or transfer is a significant issue. Therefore improve methods for breakage-free handling of thin and large Si wafers are needed to address this problem. An important prerequisite for realizing such methods is fundamental understanding and modeling of the effects of the handling device variables on the deformation, stresses, and fracture of mono- and multi- crystalline silicon wafers. This knowledge is lacking for wafer handling devices such as the Bernoulli gripper. In this thesis, a computational fluid dynamics (CFD) model of the flow generated by a Bernoulli gripper has been developed. This model predicts the air flow, pressure distribution and lifting force generated by the gripper. The model explicitly considers the non-steady characteristics of the air flow generated in the gripper and represents an enhancement over prior work in this area. The model is experimentally verified for a rigid substrate through measurement of the air pressure distribution. For thin wafers, the CFD model is combined with a finite element(FE) model of the wafer to analyze the effect of flexibility on the equilibrium pressure distribution and lifting force. Results show that the effect of wafer flexibility is significant. The model is used to analyze the handling stresses in cast and EFG wafers. Finally, a systematic approach for the analysis of the total stress state (handling plus residual) produced in crystalline Si wafers and the relationship to wafer breakage during handling is presented. Results confirm the capability of the approach to predict wafer breakage during handling given the crack size, location and fracture toughness. This methodology is general and can be applied to other thin wafer handling devices besides the Bernoulli gripper.

SUBJECT:	Ph.D. Dissertation Defense

BY:	Xavier Brun

TIME:	Wednesday, August 27, 2008, 3:00 p.m.

PLACE:	MARC Building, 114

TITLE:	Analysis of Handling Stresses and Breakage of Thin Crystalline Silicon Wafers

COMMITTEE:	Dr. Shreyes N. Melkote, Chair (ME) Dr. Steven Danyluk (ME) Dr. Suresh K. Sitaraman (ME) Dr. W. Steven Johnson (MSE) Dr. Paul M. Griffin (ISyE) Dr. Juris Kalejs (American Solar Technologies)

SUMMARY Photovoltaic manufacturing is material intensive; the cost of the silicon wafer, used as the substrate, can represent 40% to 60% of the solar cell fabrication cost. Consequently, there is a growing trend to reduce the silicon (Si) wafer thickness and to increase wafer size leading to new technical challenges related to manufacturing. Specifically, the breakage of thin Si wafers during handling and/or transfer is a significant issue. Therefore improve methods for breakage-free handling of thin and large Si wafers are needed to address this problem. An important prerequisite for realizing such methods is fundamental understanding and modeling of the effects of the handling device variables on the deformation, stresses, and fracture of mono- and multi- crystalline silicon wafers. This knowledge is lacking for wafer handling devices such as the Bernoulli gripper. In this thesis, a computational fluid dynamics (CFD) model of the flow generated by a Bernoulli gripper has been developed. This model predicts the air flow, pressure distribution and lifting force generated by the gripper. The model explicitly considers the non-steady characteristics of the air flow generated in the gripper and represents an enhancement over prior work in this area. The model is experimentally verified for a rigid substrate through measurement of the air pressure distribution. For thin wafers, the CFD model is combined with a finite element(FE) model of the wafer to analyze the effect of flexibility on the equilibrium pressure distribution and lifting force. Results show that the effect of wafer flexibility is significant. The model is used to analyze the handling stresses in cast and EFG wafers. Finally, a systematic approach for the analysis of the total stress state (handling plus residual) produced in crystalline Si wafers and the relationship to wafer breakage during handling is presented. Results confirm the capability of the approach to predict wafer breakage during handling given the crack size, location and fracture toughness. This methodology is general and can be applied to other thin wafer handling devices besides the Bernoulli gripper.