Developing standards tailored specifically to the needs of solar cell production, less complex and costly than those used in the semiconductor industry, is one weapon in the battle to reduce manufacturing costs. John Baxter and Al Bousetta from Swagelok discuss a new PV component processing specification that matches testing, cleaning, and packaging steps for stainless steel components.
by John Baxter, Al Bousetta, Swagelok Co.
January 13, 2009 – A global push to reduce dependency on non-renewable energy sources is driving increased demand for photovoltaic (PV) technologies. However, the promise of solar power becoming a leading source of renewable energy is hampered by the high cost of solar cell production. To prosper, this market must become competitive with the cost of more traditional electric power. That means reducing the cost of solar power to achieve grid parity between solar power and traditional electricity.
One means of reducing costs is to match standards for component cleanliness and purity with the true process requirements for the production of solar cells. Thus far, the PV industry has looked to the semiconductor industry for standards but, in reality, ultrahigh purity (UHP) semiconductor standards are typically well beyond what is required for solar cell manufacturing.
Solar cell manufacturers are learning that utilizing components rated for UHP semiconductor processing are, in many cases, overspecifying their components. UHP components demand a price premium due to the highly controlled manufacturing and cleaning protocols used in their production, which may exceed those required for PV components. To date, the alternative to UHP components is “non-qualified” components, which in the context of this article are defined as products that are subject to non-UHP manufacturing and cleaning protocols. But, such components introduce risks, such as downtime due to contamination and potential system integrity issues.
A solution to this dilemma is a component processing standard that is specifically designed for the PV industry. Swagelok Company has taken a first step towards reaching this type of standard by issuing a new “Photovoltaic Process Specification” . The specification matches testing, cleaning, and packaging steps for stainless steel components to the needs of the PV industry. At the same time, the new specification retains a set of baseline requirements ensuring that the components will meet the highest purity requirements of the PV industry.
Cleanliness in PV manufacturing
The new specification supports the industry’s effort to establish specific guidelines to help facilitate market growth. The SEMI Photovoltaic Committee is also currently investigating and addressing specifications related directly to the PV fabrication market. Based on the company’s work with several PV manufacturers, the Swagelok specification shares policies and procedures for the manufacturing of fluid system components at a level of cleanliness that meets — but does not unduly exceed — the requirements of today’s PV manufacturers. The specification helps decrease total PV system costs compared to UHP semiconductor systems by reducing component manufacturing steps.
Degrees of component cleanliness are directly related to the amount of processing involved to improve surface finishes, remove impurities from metals, and minimize corrosion and particle generation in service. To achieve higher levels of cleanliness, components must go through additional manufacturing steps. For example, stainless steel may be reprocessed to reduce surface defects and sulfur content, which, in turn, reduces the metal’s potential for contamination and corrosion.
Figure 1. As part of the manufacturing cleanliness chain, an ultrasonic bath (a) is used to rinse stainless steel components (b), removing any impurities prior to final assembly and/or packaging. The SC-06 process specification calls for the appropriate level of cleaning to match needs for PV processing applications.
A high level of processing is required for fluid system components used in UHP semiconductor wafer manufacturing operations to minimize corrosion and particle generation. Small line widths and high device density drive the need for UHP gas and chemical delivery as any particles carried downstream may contaminate the wafer, resulting in increased scrap and operating expenses. Substrates used in solar cell production are more tolerant of particles, which is, in part, why UHP processing standards are too stringent for PV manufacturing.
Still, it is important for PV components to be held to a suitable cleanliness standard. Unqualified components may corrode and generate particles when used in corrosive PV manufacturing gas streams. Corrosion sometimes leads to cross contamination of gases, causing reactions to occur in gas lines. Particles and gas reactions may slow down or even clog lines, resulting in lower deposition rates, reduced system efficiency, and downtime.
A clean processing environment is also critical to ensure proper adhesion between layers in thin-film solar cells (TFSCs), particularly during the wiring process. Properly rated PV components provide the right level of contamination control for reliable TFSC production.
Safety is a further consideration related to corrosion and cleanliness. Certain gases used in PV manufacturing are highly reactive. If component connections corrode or otherwise deteriorate, these gases may escape to the atmosphere as fugitive emissions, creating a potentially dangerous work environment.
PV manufacturing protocols
The new specification outlines protocols for stainless steel component design, material selection, and manufacturing steps that are similar to, but in some cases less stringent than, those used for UHP semiconductor components (see table). For example, surface finish requirements are relaxed in the PV specification. Minimizing surface flaws and inclusions within a component reduces the total wetted surface area, which, in turn, improves purging and moisture removal. While these characteristics are important to maintain cleanliness in both semiconductor and PV processes, PV production systems are much more tolerant of small amounts of possible contamination that may result from less polished surfaces.
|Characteristic||Common UHP semiconductor specifications||Proposed photovoltaic process specifications|
|Manufacturing and surface finish||
|Electropolishing and passivation||
|Surface chemistry analysis and critical pitting temperature (CPT) testing||
|Work area classification||
Under the specification, design promotes clean operation. Components must be able to be cleaned and purged quickly and easily, generate few particles in service, and contain minimal areas of entrapment. Sound component designs are of particular importance in TFSC production, which utilizes a continuous manufacturing process. In such an environment, equipment must be very reliable to avoid shutting down the entire line. Component cleanliness specifications must therefore be commensurate with high reliability.
Material guidelines cover chemical composition, material properties, inclusions, and other material characteristics to help ensure components will be clean and leak-tight.
In accordance with the specification, manufacturing techniques utilize advanced tooling, machining, finishing, and fabricating methods to produce consistent products with smooth surface finishes for reliable performance.
The areas in which UHP semiconductor and PV component manufacturing protocols differ most notably are during the end stages of the product manufacturing cycle, including the cleaning, verification and testing, and assembly and packaging steps. In these stages, the practices outlined in the specification enable suppliers to reduce costs compared to UHP component manufacturing. The resulting savings may be passed on down the supply chain, helping to reduce the total cost of solar power generation. Further, during installation, end users may realize efficiencies based on a less stringent packaging requirement.
Each step in the manufacturing process introduces contamination. After each step, therefore, UHP semiconductor components must be cleaned thoroughly using organic solvents or alkaline- or acid-based cleaners.
Similarly, PV components must be also cleaned during manufacturing. However, the PV market collectively acknowledges that UHP semiconductor grade cleaning is not required for crystalline silicon (c-Si) or thin-film PV technologies, the two most common manufacturing methods for producing solar cells. Compared to UHP component cleaning standards, the specification reduces requirements for bath controls, including resistivity and bacteria levels.
The more lenient cleaning specifications suggested for PV components are directly related to the technologies used in solar cell production. PV materials and line widths are typically much less demanding in terms of contamination sensitivity when compared to semiconductor production. Rather than following today’s stricter UHP purity standards, the new specification aligns PV component cleaning methods to the purity needs of the market.
Verification and testing
UHP and PV components undergo a variety of tests during manufacturing to confirm product cleanliness and quality. Component producers are particularly concerned with verifying the corrosion-resistant properties of the chromium-enriched oxide surface layer of stainless steel components. These surfaces are enhanced during manufacturing through electropolishing and passivation processes that remove surface iron and smooth surfaces.
UHP standards typically call for advanced surface chemistry analysis techniques to confirm corrosion resistance. The techniques analyze a series of discrete points on a sample. Common testing methods include auger electron spectroscopy (AES), electron spectroscopy for chemical analysis (ESCA), and secondary ion mass spectroscopy (SIMS). These tests are complex and are often performed by third-party labs, two factors that add cost to UHP component manufacturing operations.
The specification drives out those added costs by specifying an alternative testing method, known as the critical pitting temperature (CPT) test. CPT testing is based on ASTM G150 . It evaluates the entire passivated surface of the sample by stressing a sample’s chromium oxide surface layer to the point of failure to determine its resistance to localized pitting corrosion. The temperature at which the surface layer fails is known as the CPT. Because CPT testing analyzes the full wetted surface area of a sample, it provides a better indication of how well a component will stand up to a harsh environment compared to surface chemistry analysis techniques. Further, CPT testing provides more consistent results, in less time, and at a lower cost, all of which make it a preferred choice for testing PV components.
Assembly and packaging
Components must be protected from airborne contamination during assembly and packaging to help maintain their cleanliness. UHP specifications for semiconductor components require final assembly and packaging to be performed in a Class 100 cleanroom. This level of contamination protection is generally unnecessary for PV components. Instead, the specification calls for assembly and packaging in a controlled environment in which basic precautions are taken to preclude equipment-generated particles, airborne fibers, and common forms of contamination. Without the added time and expense related to operating in a cleanroom, suppliers realize savings that can be passed on to end users.
|Figure 2. By working with industry leaders and analyzing its own production processes, Swagelok was able to determine industry-appropriate specifications for components used in PV manufacturing.|
Further savings are generated by reducing the amount of packaging. To meet semiconductor UHP specifications each component must be double bagged. The inner bag, an abrasion-resistant nylon packaging material, is purged and filled with dry, filtered nitrogen. This sealed nylon bag is then heat sealed in an outer polyethylene bag. Per the new specification, a single abrasion-resistant bag is sufficient to keep products free from outside contaminants during shipping. The single PV component bag is still purged, but fewer packaging steps translate into cost savings for suppliers and end users.
While assembly and packaging requirements differ between UHP semiconductor and PV components, suppliers must follow similar contamination protection protocols for both types of products. No lubricants are used on wetted areas of the components. End connections are covered to help prevent contamination during shipment. Sealed packages are packed individually for shipment. In addition, identification and traceability information is visible without opening the product package.
Because installation represents the final link between component manufacturing and an operable production system, system assemblers need to retain the original cleanliness of fluid system components. Single-bagged PV components should be opened in a controlled environment with care taken to not generate particles during opening. Using a sharp razor, rather than scissors, helps prevent particle generation. Connections should be properly aligned and assembled. Components of differing purity and sulfur content should not be mixed. Authorized training from the component supplier is a valuable resource in an efficient and effective installation.
As the PV industry seeks to reduce costs associated with solar cell production, attention is being directed towards the development of industry-specific standards for manufacturing equipment and materials. Such standards will be tailored to the needs of the PV industry, thereby reducing complexity and costs associated with the commonly followed standards for UHP semiconductor manufacturing. The Photovoltaic Process Specification is a valid first step towards establishing a standard for stainless steel fluid system components. It provides multiple opportunities to reduce component costs, which should ultimately lead to less expensive energy production.
John Baxter received his BS in mechanical engineering from Cleveland State U. and is manager, products and technology, at Swagelok Co., 31500 Aurora Rd., Solon, OH 44139; ph.: 440-349-5934; e-mail [email protected].
Al Bousetta received his MS in solid state physics from the U. of Montpellier, France, and a PhD in electrical engineering from the U. of Manchester Institute of Technology, UK, and is a marketing manager of gas products at Swagelok Co.
. Swagelok Photovoltaic Process Specification (SCS-00006) for Stainless Steel Components.
. ASTM G150: “Standard Test Method for Electrochemical Critical Pitting Temperature of Stainless Steels.”