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Changing the Economics of Solar Cell Manufacturing

Brent Ames

Applied Vericell Solar Wafer Inspection System accurately predicts solar cell efficiency to deliver higher yields and lower costs.

Production yield and cell efficiency are critical in today’s c-Si solar cell manufacturing market. Average selling prices are declining, so both wafer and cell manufacturers need to keep costs low while maintaining significant R&D investments. Wafer manufacturers must deliver high quality wafers at a competitive price; cell manufacturers must mass-produce high-efficiency cells and maintain high production yields.

Applied Materials Cell Manufacturing Center of Excellence in Treviso, Italy.

Solar cell manufacturing is consumable-intensive and operates on razor-thin margins. About 70% of the cost of manufacturing solar cells comes from raw materials such as polysilicon and silver. This makes it very important for manufacturers to avoid, or minimize the effect of, subpar materials.

Average cell efficiency, or power conversion rate, for a multicrystalline solar cell is about 17.5%. As manufacturers know, there is only so much space on a roof or so many acres of land that can be dedicated to solar power installations. This means that tiny percentage differences in efficiency across arrays of hundreds of solar cells can add up to major competitive differentiation.

The need for better cell efficiency drives the need for better quality wafers. Factories face a host of challenges in the quest for the best wafers. Only about half of all wafers are inspected automatically. Manual inspection is a game of chance, with only spot inspections instead of batch inspections. The cost of manual inspections is rising along with the cost of labor. Some automated mechanical inspection units have been known to generate false positive rates of up to 4%, negating return on investment for the end user. New processes and cell architectures present new types of defects—which typically cannot be detected during manual inspections.

Introducing Applied Vericell Solar Wafer Inspection System

The Applied Vericell Solar Wafer Inspection System uses unique, proprietary image analysis and prediction algorithms to perform bare wafer inspection before cell processing begins. This fully automated system is the first in the industry to integrate full wafer inspection, multiple inspection modules and a sorter—plus photoluminescence that can predict cell efficiency—into a single tool. The ability to classify wafers by grade before production begins improves final production yield and lowers manufacturing costs by eliminating bad or low-efficiency wafers at the outset, before money is wasted manufacturing a cell that can’t be sold.

The Applied Vericell Solar Wafer Inspection System (see figure 1) uses multiple metrology modules to collect information about wafer thickness, thickness variation (TTV), resistivity, shape, edge chips, stains, saw marks, microcracks, and crystal fraction. The system uses photoluminescence technology combined with advanced algorithms and proprietary image analysis to determine final cell efficiency before the wafer is processed. Photoluminescence is also used to identify wafers with impurities and dislocation defects.

This data allows c-Si solar wafer and cell manufacturers to optimize their manufacturing lines so they can produce the highest grade cells possible. Applied’s breakthrough technology has been proven to increase average factory cell yield by 0.2% and factory production yield by 1%, resulting in potential annual savings of $750,000 to $2 million (USD).

The ability to predict cell efficiency has long been a goal for the solar cell market. The Applied Vericell Solar Wafer Inspection System uses its algorithms and analysis to achieve high prediction accuracy. The algorithms use a broad set of inspection data, along with customized reporting, that allows manufacturers to identify a wide range of potential improvements. Combined with photoluminescence technology and the industry’s lowest false alarm rates for microcracks and saw marks, this tool is a unique offering in the solar manufacturing marketplace.

The Applied Vericell Solar Wafer Inspection System loads bare wafers from coin-stack magazines or cassettes, then sends each wafer through an inspection line at the rate of one wafer per second. The tool first sorts the wafers based on customer-defined mechanical

Figure 1. The Applied Vericell Solar Wafer Inspection System helps improve cell- and factory-production yields by using metrology modules to analyze eight variables on each wafer, including photoluminescence, before the wafer is processed.

characteristics either after the wafering process is complete (wafer manufacturer) or before the cell manufacturing process starts (cell manufacturer). The system’s ability to predict efficiency lets manufacturers sort starting material, bin materials with similar efficiency ratings together, and potentially scrap wafers that have a predicted cell efficiency that is below their minimum specification.

All data collected by the system is easily transmitted into databases and connected to manufacturing execution systems through standard interfaces. Automated notification helps factory managers and engineers identify yield issues quickly.

An important benefit of the Applied Vericell Solar Wafer Inspection System is that it has extremely low false alarm rates—less than .25%, compared to up to 4% for other tools—for saw mark and microcrack inspection modules. False alarms directly affect factory yield, causing good material to be scrapped. If a tool runs 80,000 wafers in a day, scrapping just 4% of the wafers can potentially erase the return on investment from operating the inspection tool in the first place.

Value for Cell Manufacturers

Cell manufacturers can use the Applied Vericell Solar Wafer Inspection System to spot-check the quality of incoming wafers from manufacturers, and to perform inspections themselves so they can grade incoming material by potential cell efficiency. They can reject lowquality wafers that can’t be sold early in the production cycle, thus eliminating wasted consumable and labor expense on unsellable material. Knowing the capabilities of each wafer lets manufacturers improve planning and control of cell product yield, and helps them optimize equipment and production lines to improve cell efficiency yields.

Value for Wafer Manufacturers

The Applied Vericell Solar Wafer Inspection System offers significant benefits for wafer manufacturers as well. By tracking batches or individual wafers, they can further use cell efficiency measurements to pinpoint underperforming tools and ingots. Tool performance can then be modified to optimize ingot and wafer yield. Successful processes on the highest performing tools can be replicated across other tools, improving yield. Wafer manufacturers can use cell efficiency metrics to charge a premium price for high-quality wafers that have a known high potential. The ability to differentiate with verified wafer quality before cells are manufactured improves planning and control of wafer product yield.

Joint Study Demonstrates Prediction Accuracy

A study by Applied Materials and solar manufacturers Sun Edison (formerly MEMC Electronic Materials) and Gintech Energy Corporation showed that bare wafer prediction accuracy using multiple sensors is exceptional. Wafers from 12 bricks, 2,000 wafers in all, were tested. The wafers were indexed with laser marking. The Applied Vericell Solar Wafer Inspection System and predictive algorithms were used to forecast the final cell efficiency of each wafer after the manufacturing process. Once the wafers were manufactured into cells, end-of-line testing measured their cell efficiency. The result was a strong correlation between predicted wafer value and final measured efficiency. This correlation is expressed as mean average efficiency (MAE). Ideally, MAE will be .15% or less. MAE in these tests ranged between 0.08 and .1% (see figure 2).

Figure 2. Study results showed strong correlation between cell efficiency that was predicted using the Applied Vericell Solar Wafer Inspection System,and cell efficiency measured after cell production was complete.

Generally, a batch of wafers will have a small number of very high efficiency wafers, with the majority falling somewhere in the middle, and then a “tail” of very low efficiency wafers. Identifying this wafer tail allows manufacturers to reject very low efficiency cells. As shown in figure 3, this tail consisted of wafers with efficiency below 16.5%, totaling 1.75% of the wafer batch. In this example, a replacement or refund for these low-efficiency cells would be $.85 per wafer. Savings on cell processing would be $.55 per wafer. If a manufacturer scans about 25 million wafers per year, 1.75% of 25 million is 437,500 wafers, providing a total cost savings of $612,000—just by rejecting the extreme tail of the wafers (see figure 3).

In this study, the cell efficiency average was 18.45% without sorting materials by quality. Predicting cell efficiency improved cell efficiency by 0.2%, from 18.45 to 18.64%. Figure 4 shows that this was achieved by sorting starting materials and binning low-quality materials together so they could be identified and rejected.

Figure 3. The Applied Vericell Solar Wafer Inspection System can identify the lowest performing “tail” of a given batch of wafers. Rejecting these wafers before they are processed into cells can provide significant cost savings.

Figure 4. Wafers that do not meet cell efficiency standards can be binned together and rejected as a batch. In this study, eliminating three bins of the lowest performing wafers reduced costs and improved average factory cell efficiency from 18.45 to 18.64%.
 

Case Study: Global Wafer Manufacturer Achieves Productivity Gains and Cost Savings

In 2013, Applied Materials teamed with a leading global solar wafer manufacturer to evaluate the Applied Vericell Solar Wafer Inspection System. During the 4-month program, more than 6 million wafers were processed using the new tool.

The Applied Vericell Solar Wafer Inspection System was installed in a fab that was experiencing production problems leading to yield loss. Using the system to measure the total percentage of yield loss by type of mechanical defect, the manufacturer determined that wafer thickness variations were affecting 8% of yield. Saw marks were affecting another 10%.

The Applied Vericell Solar Wafer Inspection System uses advanced analysis software to identify root causes of yield loss. By comparing yields of identical equipment within a single process, managers were able to identify a single wire saw that was causing 10–20% more defects than other saws (see figure 5). They also identified two saws that were causing yield loss due to thickness variations. Photoluminescence inspection, unique to the system, identified two poorly performing ovens that were causing 0.24% cell efficiency loss. The system also analyzed and compared wafer performance by the wafer’s original location in a cast ingot, determining that certain locations in the ingot had higher levels of impurities.

The end results of this evaluation identified key areas for factory-wide yield improvement, using cell efficiency prediction and an upgrade from manual inspection to automated inspection. The Applied Vericell Solar Wafer Inspection System used proprietary yield management software to identify equipment that was performing poorly. The ability to accurately predict cell efficiency allows the manufacturer to sort and re-melt low-efficiency wafers, reducing waste and costs and improving overall shipped wafer quality. Minimized manual handling lowered labor costs and helped reduce breakage, improving yield. The manufacturer estimates a gain of 12.5% in productivity versus other automated inspection systems and has improved profits by about $156,000 per machine. For geometry measurements, the manufacturer estimates a savings of $31,000 per year by using automated instead of manual inspections.

Figure 5. The Applied Vericell Solar Wafer Inspection System identifies underperforming tools by comparing yields of identical equipment within a single process. In an evaluation with a global wafer manufacturer, the system discovered that a single wire saw caused 10–20% more defects than other saws.

New Technology Delivers a Competitive Edge

The solar cell market is at an inflection point for costs and technology. When the market was young, manufacturers focused on expansion and quantity. Eventually, as has happened in many other technology markets, producers found themselves facing dropping demand and overcapacity. Some companies found that it was cheaper to shutter factories rather than run them at a loss. As the solar industry matures, factories now are focusing on yield and process improvements. Instead of mixing good and bad materials to achieve a working average efficiency, solar wafer and cell manufacturers are at a point where they must drive yield improvements within the factory and place greater emphasis on quality. Tools such as the Applied Vericell Solar Wafer Inspection System are playing a critical role in delivering the efficiency, cost and quality improvements solar manufacturers need to compete and thrive.

For additional information, contact brent_ames@amat.com