Mechanical Property Of Diamond Wire Sawn Multi-crystalline Silicon Wafers And Its Improvement

Diamond wire saw technology has outstanding advantages, including higherproduction efficiency, less environmental impact and so on. It has taken the place ofslurry wire saw technology in the production of mono-crystalline silicon wafersgradually. However, the poor fracture resistance of diamond wire sawnmulti-crystalline silicon wafers is a potential barrier to commercial development ofdiamond wire saw technology in PV industry. In the present thesis, a systematicinvestigation of the mechanical property of diamond wire sawn multi-crystallinesilicon wafers was carried out, including elastic moduli and fracture resistance, in thedirection parallel to the cutting marks and in the direction vertical to the cutting marks.And then the methods to improve the fracture resistance are explored. For a betterunderstanding, mono-crystalline wafers and slurry wire sawn silicon wafers were alsoinvestigated as references.3-point bending tests were carried out to measure the apparent elastic moduli ofwafers in bending. The measuring results were analyzed by the impulse excitation ofvibration tests and the anisotropic calculation of elastic moduli. The results show that,elastic moduli of multi-crystalline silicon wafers are larger than that ofmono-crystalline silicon wafers. The apparent elastic moduli in bending are smallerthan their theoretical values. It is caused by the surface roughness, which leads tolower effective thicknesses. Both the sawing technology and the relationship betweenthe direction in bending and the cutting marks may have appreciable effect on theapparent elastic moduli in bending.Critical strain is chosen to evaluate the fracture resistance of wafers. In thispaper, it is measured by3-point bending test. The results show that, compared toslurry wire sawn silicon wafers, although diamond wire sawn wafers are stronger inthe direction parallel to the cutting marks, they are much weaker in the directionvertical to the cutting marks. In direction vertical to the cutting marks, the criticalstrains of diamond wire sawn multi-crystalline silicon wafers are only57%of thoseof slurry wire sawn multi-crystalline silicon wafers. It may leads to an unacceptable high breakage rate in production. In order to investigate the effect of the surface stateon the critical strains of wafers, chemical polishing of diamond wire sawnmulti-crystalline silicon wafers were carried out. The results show that, Chemicalpolishing can increase the critical strains of wafers greatly, and as the polishing depthincreases, the critical strains increase, too. The dispersion of critical strains alsobecomes larger after chemical polishing. In addition, the improvement in the directionparallel to the cutting marks is greater than that in the direction vertical to the cuttingmarks.In this study, it has been found that, the acidic texturization in themulti-crystalline silicon solar cell manufacturing has significant effect on the fracturebehavior of diamond wire sawn multi-crystalline silicon wafers. In general, etchingby the rich HNO3solution raises the critical strains, while etching by the rich HFsolution reduces the critical strains. In addition, reducing the proportion of H2O in thesolution and raising the etching temperature can obtain larger critical strains.Annealing treatment is a very effective method, and the temperature doesn’t need toohigh. Annealing at as low as400℃can raise the critical strains of diamond wire sawnmulti-crystalline silicon wafers to a level similar to those of slurry wire sawnmulti-crystalline silicon wafers. In this thesis, the mechanisms of fracture and twoimproving method were analyzed and discussed.