THE LIFEBLOOD OF WAFER MANUFACTURING
HOW EFFECTIVE SLURRY REPROCESSING CAN IMPROVE QUALITY AND CONTROL COSTS.
Source: InterPV, November, 2009.
THE MARKET EROSION
During the last six months, the PV industry has had to endure an exceptional amount of volatility. In the context of the larger global recession, the impact of a depressed market is everywhere. Governments, under the strain of the battered economy, have slashed subsidies for solar industry expansion. The collapse of Spain’s photovoltaic sector, in a country that accounted for over 40% of the world’s solar energy in 2008(1) has been well documented. Bloomberg reported in June 2009 that Germany’s feed-in tariffs, which had built the solar industry up to US$8.8 billion in sales, were being reduced(2).
The effects of such tumult are many. We’ve seen a significant decrease in PV module demand along with oversupply in the entire PV supply chain. The effect of this oversupply has already been seen in the price of finished silicon wafers, which have dropped by more 50% in 2009. All of these factors have led to significant market erosion that may ultimately lead to substantial fallout. Industry-wide, margins are getting tighter and are not expected to increase dramatically anytime soon.
COST REDUCTION IS REQUIRED
Analysts indicate that manufacturers expecting to endure this crisis will have to make meaningful and significant cost reductions. In any business, cost efficiencies are important. In the current environment, operating efficiently and reducing costs wherever possible may be the difference between survival and closed doors.
One area that solar wafer manufacturers can look to help contain costs is their slurry reprocessing operations. Slurry is at the heart of wafer manufacturing, where it is used to cut the wafers out of silicon ingots. Slurry is one of the factors that help ensure precise cuts that conform to extremely tight tolerances, and is composed of Silicon Carbide (SiC) and a carrier such as Polyethylene Glycol (PEG) or oil. Fed along a guide wire, the abrasive silicon carbide in the slurry (the “grit”) cuts through the ingots to produce the wafers. To keep the number and quality of the wafers high while maintaining acceptable unit cost, slurry must be reprocessed.
SLURRY REPROCESSING COST
Not only is slurry important to the process, it’s also a significant production cost. Often it’s second only to the raw material silicon, itself a big number. Prices fluctuate, but at a market price of US$3.50 per kilogram, a customer with 10 wire saws could spend up to US$16 million a year for the silicon carbide slurry alone, not including disposal fees or labor. For the vast majority of manufacturers, slurry reprocessing ranks in the top five operating costs. Optimizing this aspect of the business can have dramatic, and, more important, lasting effects on wafer production costs and overall quality.
For wafer manufacturers that are not currently reprocessing, instituting a slurry reprocessing program or starting day-one in a greenfield operation are among the most significant margin-enhancing improvements that can be implemented. For a wafering operation running 10 wire saws with a volume of 385 metric tons per month, slurry reprocessing can result in a savings of US$8-10 million per year, net of recycling costs, utilities, and infrastructure investment.
THE IMPORTANCE OF RECOVERY RATE
There are several different ways to reprocess slurry to produce a good cutting medium and leave behind the unnecessary materials such as kerf particles. A fairly robust and straightforward process that doesn’t involve chemical components is to run the fluid through several centrifuging steps followed by filtration or distillation. This allows for the separation of large particles that are still viable cutting materials from small particles that are no longer effective. The recovered material is then recombined with a minimal portion of virgin material to replace the portion that has been eliminated.