Doctoral research project

Person in charge of the project:
MERTENS ROBERT PIERRE, member of research team Associated Section of ESAT - INSYS, Integrated Systems
Fabrication and Characterization of Ultra-Thin Silicon Crystalline Wafers for Photovoltaic Applications using a Stress-Induced Lift-off Method
Project summary:
In order to reduce material-related costs, there is a need to develop new wafering techniques to produce thin (< 100 µm) crystalline silicon wafers for photovoltaic applications.This work presents a new kerf-free wafering process for single crystal silicon which relies only on thermo-mechanical treatments. The process is named SLIM-Cut (Stress-Induced LIft-off Method).The process &#64258;ow is as follows: a layer of a material with a coef&#64257;cient of thermal expansion (metal or polymer) signi&#64257;cantly different from silicon is deposited on top of a bare silicon substrate that could be a few centimeters thick. The system formed by the silicon substrate and the stress-inducinglayer later undergoes a thermal process. Upon cooling, the stress-inducing layer will shrink more than the silicon creating a stress &#64257;eld inside the silicon substrate, provided that the bonding is strong enough to withstand this stress. When the stress reaches a threshold value, the system tends to relax the constraints by propagating acrack either in the stress-inducing layer itself, along the interface, or completely through the substrate (ingot). If the mechanical parameters are chosen carefully, there is a third option for the trajectory of the crack: propagating inside the silicon at a &#64257;xed distance from the interface, parallel to the surface, i.e. substrate spalling.The quality of the resulting material must be assessed to ensure that this innovative silicon foil approach does not jeopardize the potential ef&#64257;ciency of the &#64257;nal solar cell. Microwave-detected photoconductance decay, deep-level transient spectroscopy, electron spin resonance and optical inspections after defect etching of the foils surface were performed to asses wafer quality in terms of electronic activity, defect density and location.A metal-based approach for the stress inducing layer involves high-temperatures, above the transition temperature for silicon from brittle to ductile. This leads to poor foil quality due to plastic deformations of the material and possible contamination of the foil. Currently, a metal-based approach involving high temperatures is not suitable for the fabrication of PV material.Conversely, a polymer-based approach involves only low-temperature steps (max 150 &#9702;C). The obtained foils show important roughness and thickness variations which could be reduced avoiding manual processing. Analysesof the silicon foils fabricated in this way indicate that the material quality is preserved, e.g. high bulk lifetimes and low defect densities demonstrate the suitability of the foils for high-ef&#64257;ciency solarcell processing.
ph.D student :
Faculty of Engineering Science
Doctoral Programme in Engineering Science (Leuven)

ph.D defence : 06.11.2012
Full text ph.D