Lasers Enable High-Efficiency Processing to Support Solar Roadmap

Tag : silicon strips
The use of DPSS lasers for edge isolation not only enables greeneredge isolation, but also increases yields and improves deviceefficiency. Specifically, current-generation systems are nowrelying more heavily on DPSS lasers operating at either visible(532 nm) or UV (355 nm) because of the significantly higherabsorption of c-Si at these shorter wavelengths. 4 Silicon absorption is some four to five orders of magnitudestronger at 355 nm compared with IR (1064 nm), allowing highlylocalized front-surface scribing when using Q-switched UV DPSS lasers (Fig. 3). In addition to shorter penetration depths, UVwavelengths allow narrower grooves to be scribed in a colderprocess with minimized peripheral thermal damage such asmicrocracks, which are potentially yield-killing. This enables thegrooves to be placed closer to the cell edges, reducing the"dead" area and thereby maximizing the efficiency of thecell.

Cost of ownership
Manufacturing costs for solar cells and panels are constantly underreview, as this critically impacts on the $/W passed through tofinal solar installers. Cost reduction is routinely sought eitherby reducing raw material costs or through the use of productionline equipment with the lowest capital expenditure and runningcost.
DPSS lasers provide an ideal solution here, due mainly to theirvery low operating costs. Further, the solar industry instantlybenefits from industrially qualified laser designs already involume use within semiconductor production lines. These legacyapplications have set target levels for uptime, spare-partsavailability and on-site service response. The solar industry nowstands to gain considerably from this. For example, a high-powerlaser operating in the UV spectral region with tens of watts outputoffers fully loaded running costs of typically $3–5/hr whenoperating at full power, and used 24/7 over five-year periods. Production yield
Although technically less demanding and involving fewer processsteps, solar cell production yields lag that of semiconductor yieldlevels by a considerable margin for a host of reasons. 5 These include problems resulting from the installation andoptimization of new precision equipment tools operating within ahigh-volume manufacturing environment and from the challenge inmaintaining trouble-free operation with reasonable uptime. Sohistorically, within solar, yield optimization has played secondfiddle to issues such as long equipment leadtimes and the resultingrush to have equipment installed and producing solar cells to meetmarket demand. The bottom line: Yield levels <90% are notuncommon within the solar industry, with the leading suppliers nowstarting to report levels of >95%. But clearly, photovoltaic(PV) manufacturing processes have not evolved and settled to theextent required to produce semiconductor yield levels.
There is also another development that is poised to impact yieldsand, hence, production equipment selection over the next 3–5years: the move to thinner and larger wafers. With thicknesses soonto go below 200 μm, wafers will be mechanically weaker than everbefore. Put simply, any contact technology used with these brittlewafers will carry the risk of further lowering yield levels inproduction. Therefore, the non-contact nature of laser processingoffers significant inherent advantages, both to reduce waferbreakages and minimize the effects of microcracking, currently oneof the main contributors to non-conforming product output. Throughput
Historically, manufacturing cost reductions in the PV industry,achieved by growing plant capacities and thereby decreasing coststhrough economies of scale, are similar to those observed in themicroelectronics industry. Over the past decade, a 20% reduction inmodule cost has been achieved with each doubling of worldwidemanufacturing output. To put this in perspective, astate-of-the-art c-Si cell production line today can have an hourlythroughput exceeding 3000 wph. Manufacturing sites generally have ahandful of parallel production lines with a total annual capacityup to several hundreds of megawatts, a number expected to soon riseto the landmark gigawatt-level plant size. Scaling cost reductionsare similarly projected as thin-film panel sizes increase from Gen5 to Gen 8 or bigger.
What does this mean for suppliers of laser-based process equipment?For many c-Si laser processes, production line throughput scalesalmost directly with average laser power level. Therefore, a keydriver here is increasing the average power levels from pulsed DPSSlasers while maintaining characteristics that determine processyields and operating costs. These include beam quality, productlifetime and pulse-to-pulse stability levels. Figure 4 shows how the power of Q-switched 355 and 532 nm DPSS lasers have evolved to keep pace with increased throughput demands.