Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE

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Introduction / project goals
Silicon wafering for the PV industry is made by multi-wire slurry saw (MWSS).The cost of solar cells is
mainly driven by the cost of silicon raw materials. One way to decrease these costs is to cut thinner
wafers. The current wafer thickness in production is about 220 – 200 µm. This thickness is limited by
the wafers strength due to the presence of sub-surface defects (SSD) introduced by the sawing process.
The production of stronger wafers will not only allow to cut thinner wafers, but also to increase
the wafers yield during subsequent production steps.
The goal of this project is to understand the principle of MWSS and the influence of sawing parameters
on wafers strength to finally be able to cut 120 µm thick wafers. In order to do this, a dual approach
was chosen: in the one hand a more fundamental direction aiming at understanding the deformation
process of silicon during contact loading and on the other hand a more applied direction aiming at
understanding the influence of the sawing parameters on the SSD and on the wafers strength. Thus,
the goals of the projects were:
1. To understand the deformation process of silicon during scratching and indentation
2. In-situ observation of deformation process, crack initiation and propagation during scratching
and indentation.
3. To control the roughness of surfaces, and the initiation and propagation of microcracks during
sawing.
4. To identify and understand the interaction of slurry flow, wire motion and rolling-indenting particles.
5. To understand and model the crack propagation with respect to the different length scale
6. Classification of the Sub-Surface Defects (SSD) with respect to MWSS parameters
7. Set-up a test bench to measure the mechanical fracture strength of sawn wafers
Technical Summary
In essence, a Mutli-Wire Slurry Saw (MWSS) consists of a wire being managed to constitute a multiwire
web, allowing an abrasive liquid to penetrate an ingot driven through the web. More specifically, it
consists of one 120-160 µm-diameter steel wire moving, either uni-directionally or bi-directionally, on
the surface of the workpiece (e.g., a silicon ingot). The single wire is wound on wire-guides carefully
grooved with constant pitch forming a horizontal net of parallel wires or web (Figure 1a). The wireguides
are rotated by drives, causing the entire wire-web to move at a relatively high speed (10-
20m/s). A couple of high flow-rate nozzles are feeding the moving wires with an abrasive suspension
(“slurry”) into the cutting zone. The workpiece (or the wire-web) is moved vertically. The wire tension is
maintained constant (20-30 N) during the cutting process with state-of-the-art feedback control. A wire
feed reel provides the length of a new wire and a wire take-up reel stores the used wire.
In MWSS, material is removed by third-party free abrasive grains transported in a liquid media (slurry).
The primary functions of the slurry are as follows: (i) carrying the abrasive particles to the cutting zone;
(ii) flush away workpiece chips and residues (kerf); (iii) heat removal by the slurry liquid carrier. Most
commonly used slurries are oil-based, water-soluble and water-based coolant, with generally SiC as
abrasive.
Upper Table
Slurry Manifolds
Inlet spool
Take-up spool
Ingots
Wire
web
Lower Table
Before Cut After Cut
Figure 1: a) Schematic of a mutli-wire slurry saw, b) silicon wafer sawn, c) silicon wafer containing no defects
SIWIS: Ultra Thin Silicon Wafer Sawing by Multi-Wire Sawing, A. Bidiville, Empa
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