Fabrication and Characterization of Ultra-Thin Silicon Crystalline Wafers for Photovoltaic Applications using a Stress-Induced Lift-off Method

Name: Alex Masolin

• Prof. dr. ir. Robert Pierre Mertens (promotor)
• Prof. dr. ir. Jozef Poortmans (mede-promotor)

In order to reduce material-related costs, there is a need to develop new wafering techniques to produce thin (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 flow is as follows: a layer of a material with a coefficient of thermal expansion (metal or 
polymer) significantly 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 field 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 fixed 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 efficiency of the final 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 ◦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-efficiency solarcell processing.

https://lirias.kuleuven.be/handle/123456789/358121

•   Prof. dr. ir. Robert Pierre Mertens (promotor)
•   Prof. dr. ir. Jozef Poortmans (mede-promotor)
•   Prof. dr. ir. Paul Van Houtte (voorzitter/chairman)
•   Prof. dr. Marc Seefeldt (secretaris/secretary)
•   Prof. dr. ir. Robert Puers
•   Prof. dr. Andre Stesmans
•   Prof. dr. ir. Johan Driesen
•   Prof. dr. João Manuel de Almeida Serra , Universidade de Lisboa