Semiconductor Industry Helps to Green Solar

Photovoltaics (PV) manufacturing faces two important challenges as it emerges as a significant new industry. First, it must reach parity in cost per watt with conventional sources of electricity, and second, it must ensure that it maintains a "green" manufacturing profile so that the environmental benefits of solar power are not negated by pollution resulting from the solar cell manufacturing process itself (Fig. 1).

1. Reducing the power consumption from vacuum pumps is one way to drive down the cost per watt for PV cells. Active utility control (AUC) can reduce power an additional 20%.

Historically, solar cell manufacturing has been based largely on semiconductor technology, but is now also embracing flat panel display (FPD) manufacturing techniques to produce large solar panels. This offers PV manufacturers an excellent opportunity to benefit from lessons learned over the past 20 years in the semiconductor and FPD industries to reduce both manufacturing costs and environmental impact.

Throughout its development, the semiconductor industry has been driven by cost and environmental considerations similar to those now faced by PV manufacturers. From its earliest days, consumer demand and competitive pressure fueled a relentless drive for more computing power and lower prices. Also, though initially considered to be relatively clean, the industry was eventually forced to recognize and solve major problems with water pollution and hazardous waste gases, as well as high consumption of water and electricity. As a result, the industry developed considerable expertise in the ways and means of reducing manufacturing costs and environmental impact through improvements in tool productivity, energy efficiency, system integration and waste management. This article will discuss some of the approaches and technologies that the solar industry can easily adopt in its efforts to address similar challenges.

The need to be perceived as green

The solar industry has several reasons to take a green manufacturing approach from the beginning. First, as an alternative energy source, one of its major appeals is that electricity generated via solar technology is pollution-free. If it turns out, however, that manufacturing solar panels results in as much pollution as gas- or coal-generated electricity, the industry risks the loss of much of its appeal — a weakness that alternative energy competitors would be quick to take advantage of. Issues have already arisen with gas abatement and the safe handling of PV fabrication byproducts in some emerging solar markets, and it behooves the industry as a whole to address these concerns promptly.

Solar manufacturers in markets such as Europe — a rapidly growing market for alternative energies — already face strict particulate, gas and water discharge regulations. Adoption of the German TA Luft standard is rapidly increasing throughout the industry, and is likely to become a de facto solar manufacturing standard worldwide (see "Abatement Choice and Environmental Impact"). Manufacturers in territories where environmental regulations are not as strict can still avoid long-term costs and reduce risk to liabilities by adhering to such prescriptive requirements. Solar cell manufacturers that anticipate environmental regulation can reduce their costs in the long run by avoiding the need to upgrade installed tools to meet emerging regulations. In doing so, they stand to gain an advantage over those competitors that fail to implement environmental controls until they are mandated.

To gain widespread adoption, though, the solar industry must address more than environmental concerns; it must gain grid parity, which will require further reductions in the cost of solar manufacturing. In many cases, clean manufacturing can play a significant role in lowering operating costs — for example, by reducing energy and water consumption. Reducing energy consumption can be particularly critical in emerging countries with relatively weak energy infrastructure in place, and reducing water consumption is important in arid regions, such as the Middle East, which offer great potential as sites for solar farms. Investing in green manufacturing now also helps solar manufacturers avoid potential pollution and cleanup costs — both monetary and human — that can have significant negative impacts on profitability and brand equity over the longer term.

The amount of energy consumed in solar manufacturing will become increasingly important as the solar market grows. In June, market researcher iSuppli Corp. predicted that there would by as many as 400 solar production lines able to produce at least 1 MW of PV cells per year by 2010, a fourfold increase over 2007.¹ As the solar industry grows, it will compete with other industries for increasingly expensive energy resources. It must minimize the amount of energy used in manufacturing solar panels if it is to reach grid parity with conventional energy sources and become an overall energy contributor rather than a net energy consumer.

The semiconductor industry has already grappled with many of the pollution issues now facing the solar industry. In addition, semiconductor equipment manufacturers, in the transition from 200 to 300 mm wafers, overcame cost-of-ownership issues comparable to those faced by the solar industry as it transitions to its next manufacturing generation. Both industries face very similar challenges to reduce manufacturing costs while producing a more powerful and effective product. The considerable similarities in the processes and materials used by the semiconductor and solar industries present solar manufacturers with an excellent opportunity to integrate subsystems developed for semiconductor manufacturing into their manufacturing flow.

Easily adopted processes and technologies

Some of those systems that can be easily transitioned from semiconductor to solar manufacturing include the vacuum pumps used to move the various gases used in the manufacturing process, pollution abatement systems and chamber cleaning systems.

In addition, systems integration lessons learned by the semiconductor industry can help increase tool productivity, a key factor in reducing manufacturing costs.

Wet vs. dry pumps

The semiconductor industry transitioned some time ago from "wet" vacuum pumps, which require oil to form the vacuum seal, to "dry" pumps. Eliminating the need for oil in the vacuum space significantly reduces costly and potentially dangerous maintenance requirements, while also helping to reduce the risks and costs of hazardous waste disposal. At the same time, dry pumps generally consume less electricity than wet pumps. In particular, a dry pump generally has a much improved service interval, allowing for more production output between services, a significant requirement for reducing total running costs.

Abatement systems

The waste abatement systems developed for semiconductor manufacturing present another excellent technology transfer opportunity for the solar industry. Since abatement technology is generally viewed as a cost rather than an added value, manufacturers of abatement systems for the semiconductor industry have competed fiercely to reduce the operating cost and environmental burden of their abatement systems — benefits that can be directly transferred to solar manufacturing.

The advanced abatement systems in use in the semiconductor industry offer significantly lower cost of ownership (CoO) than previous generations. For example, plasma-based abatement systems are very energy-efficient and use only electrical power, which can be generated relatively cleanly, thus simultaneously reducing both energy costs and environmental footprint. They have reduced consumable and maintenance requirements, which translate into lower costs and increased system uptime. When applicable, plasma-based abatement systems can be designed to maximize the destruction rate efficiency (DRE) for gases such as nitrogen fluoride (NF₃) and prevent the formation of carbon tetrafluoride (CF₄), both of which are strictly regulated greenhouse gases.

Other abatement technologies all but eliminate fuel costs by burning component gases of the exhaust stream to generate the heat required to destroy harmful waste compounds.

Where minimal water consumption is a priority, systems may use all-dry combustion reactor technology to break down harmful compounds and high-temperature particle filtration systems (instead of scrubbers) to remove solid byproducts from the gas stream. While this has obvious and immediate value in arid regions, limited availability of water is a growing concern around the globe and demand-driven price increases will likely give water conservation an increasingly important role in reducing manufacturing costs.

Wet vs. dry process chamber cleaning

As demonstrated in the FPD and semiconductor industries, dry process chamber cleaning using fluorinated gases (e.g., NF₃ or F₂) has become very common to increase tool throughput and reduce the amount of manual intervention required in maintaining the tool. Although most flat-panel solar manufacturing has already transitioned to dry (plasma) cleaning, wet cleaning is still common in crystalline silicon (c-Si) solar cell manufacturing. Dry cleaning processes do create highly corrosive gases, but specially designed turbomolecular pumps and dry pumps are available (once again developed for semiconductor processes) that resist chemical attack.

Systems integration

Solar manufacturing can also benefit from the systems integration expertise that has developed in the semiconductor industry. An effective systems integration approach provides the capacity and redundancy necessary to maximize system uptime without building in costly overcapacity. A fab-wide abatement capacity optimization scheme developed in the semiconductor industry provides an excellent example of the cost-saving benefits of systems integration.

Many semiconductor fabs add redundant abatement systems to avoid production downtime resulting from planned or unplanned maintenance on abatement systems. Typically, a single abatement system supports a single process tool. A traditional approach to ensure redundancy would then add a second, backup abatement system for each process tool. Although such an approach does maximize process uptime, it is costly and inefficient because the 100% additional capacity far exceeds the 5% downtime typical of abatement systems.

A more integrated approach, easily transferrable to solar applications, is an area-wide abatement solution (Fig. 2). In this model, incompatible waste gases are segregated as they leave the process tools and are directed to banks of abatement systems. These banks provide a centralized abatement capacity, with a modest redundancy provision that requires less than half the additional systems needed in the earlier scenario. Designing in just the right amount of redundancy ensures high uptime, increased productivity and lower manufacturing costs, all of which can make important contributions toward achieving grid parity.

2. An area-wide abatement solution is easily transferrable to solar applications. In the top scenario, six abatement units are required to maintain tool uptime if a single unit fails, while only four are needed in the systemized backup scenario (center). Systemizing abatement (bottom) minimizes the number of units required and minimizes waste generation.

Conclusion

Leveraging the experience gained from semiconductor manufacturing can help the rapidly growing solar industry maintain its green image, while also accelerating its progress toward cost parity with conventional energy sources. The key to reaching parity, of course, is reducing manufacturing costs.

A variety of systems, such as dry vacuum pumps, turbomolecular pumps, plasma-based abatement systems and plasma-based cleaning systems, can be adapted easily from semiconductor to solar manufacturing. These systems address key concerns for solar manufacturing, including reduced utility costs, lower water consumption and the effective management of greenhouse gases and other harmful waste products. In addition, the system integration expertise developed in the semiconductor industry can be used to ensure that these systems enhance overall manufacturing productivity and minimize costs.