Crack Propagation in Large Diameter PV Silicon

Abstract

We report on crack propagation in CZ silicon wafers grown for photovoltaic (PV) applications at faster growth rates than those typical for 200 mm diameter ingots. Enhanced thermal gradients and point defect/impurity distributions associated with rapid growth produces large localized stresses in the wafer core which impacts the direction of propagating cracks. The stress modified crack behavior deviates considerably from the energetically favorable Si[110]/(111) systems enabling scenarios that behave like a ductile fracture following a radial path. Minority carrier lifetime mapping and microscopic etch pit measurements confirmed the presence of a series of radial oxygen precipitate (OP) rings, which impact the energy transfer process at the expanding crack-tip, thereby modifying the crack stress path. Raman spectroscopy was used to quantify the local stresses due to radial oxygen precipitate variations, whereas TEM studies confirmed the presence of OP's, their influence on defect generation and probable role on crack path deviation.

Oxygen Precipitation Related Stress-Modified Crack Propagation in High Growth Rate Czochralski Silicon Wafers

“…Radially symmetric trace of crack propagation near the wafer core highlights the deviation from energetically favorable 110 (111) crack systems in (100) oriented CZ wafers. In reference to our prior work 14 and further study presented here, we deduce that the crack must initiate at the edge of the wafer, probably from a micro-crack/flaw. Thereafter, it propagates along a slip system until perturbed by stress gradient (tensile/compressive) at the wafer core.…”

“…40 However, beyond 90 • change in the crack path, it propagated along 110 crystallographic direction, which was one of the cleave direction for (100) oriented wafer and parallel to initial crack path. 14 These fracture features clearly demonstrate the effect of compressive stress induced crack-front instabilities associated with crack velocity changes.…”

“…Our X-ray topographic analysis presented elsewhere had previously revealed that the crack was initiated at the edge of the wafer and propagated along the favorable slip lines before interacting with differentially stressed region in the wafer core. 14 Investigating the crack path further to Location 3, where the crack abruptly deviated to 90 • , Raman data have shown that the crackfront was evidently interacting with an intensive compressive stress field (see Fig. 3).…”