Double-duty tool cuts costs for c-Si solar-cell wafering
The most expensive component of crystalline silicon-based PV manufacturing flow is the wafer — reducing wafer fabrication cost is considered key to the goal of making solar energy competitive with grid power. To this end is the goal of a new platform from Applied Materials that slices ingots into ultra-thin wafers, and the company claims it will drive down the cost of manufacturing photovoltaic (PV) cells by up to $0.18/Watt (assuming a polysilicon price of $55/kg).
Figure 1. Double-wire solution to single-wire problem. (Source: Applied Materials)
A key technical advancement of the HCT MaxEdge wire saw is a dual-wire management system that enables the system to deliver higher throughput and load capacity with a smaller footprint and fewer operators for equivalent megawatt output. To produce thinner wafers, traditional wire saws reduce the ingot length (load) and the cutting speed, explained Ian Latchford, GPM and director of Applied’s c-Si solar new products group, to SST. By enabling larger loads and using thinner wires at higher cutting speeds, the new tool is able to deliver ultrathin wafers without sacrificing throughput (Figs. 1 and 2).
Figure 2. Dual-wire management. (Source: Applied Materials)
The dual-wire management system employs four independently-controlled direct drive motors (20m/sec cutting speed) and advanced process control to lower wire tension, reducing wire wear and avoiding ingot scrap and unplanned downtime. Reduced tension also allows use of smaller-diameter cutting wires, resulting in significantly less silicon loss. A central slurry nozzle fixed to the web is an enhancement to the cutting head in Applied’s previous wiresawing solution, the HCT B5.
Typical production performance of the new wire saw cited by the company is 5861 wafers/cut (156mm ?? 156mm), a wafering yield of 95.4%, with a typical PM and downtime percentage of 7%. That translates to typical throughput of >13MW/year, or 3.5 million wafers/year/machine, said Farhan Ahmad, global product manager, planarization, in Applied’s thin films group.
That level of throughput has a direct savings in a real-world manufacturing environment. A 50MW wafering plant using four HCT MaxEdge tools with two operators would need as many as 18 tools and nine operators for one competitor’s solution, and eight tools/four operators for another competitor’s product, according to Latchford. And fabs using the older HCT B5 would need six tools and three operators to achieve the same output as the new tool. —D.V.