Vacuum processing for solar cells

by Katherine Derbyshire, Contributing Editor, Solid State Technology

Oct. 23, 2008 – Equipment for making wafer-based silicon solar cells reports throughput not in hundreds, but in thousands of wafers/hour. Equipment for deposition of amorphous silicon must accommodate substrates as large as 5.7m2. Though the processes used in solar cell manufacturing appear familiar at first glance, the actual equipment requirements are substantially different.

Like integrated circuit and flat-panel display manufacturing, solar cell manufacturing depends on a variety of vacuum-based processes, from PECVD silicon deposition to lamination of the finished module. High pumping speeds and rapid chamber cycling are essential if these processes are to meet the throughput requirements of the solar cell industry. Yet vacuum-based processes in solar cell manufacturing pose special challenges for pump designers. For example, the amorphous silicon cells that achieve the best conversion efficiency incorporate substantial amounts of hydrogen. In fact, researchers at ULVAC found that a-Si performance actually improves at lower deposition rates, probably because slower deposition allows more complete hydrogen incorporation and reduces the number of dangling bonds. While a slower deposition means that chamber load and unload time is a smaller fraction of the total process time, it also means that the chamber pump time is one of only a few levers available to improve the process speed.

The hydrogen passivates dangling bonds and densifies the film, improving carrier lifetime and reducing recombination. As Clive Tunna, technical and commercialization director for Oerlikon Leybold, explained in a recent interview, hydrogen is bad news for vacuum pump designers. The lightest of all gases, it is notoriously difficult to pump. The cold traps in standard cryopumps operate at liquid nitrogen temperature (77K) where hydrogen is still a gas. Hydrogen molecules are small enough to leak through seals that would contain other gases, and high concentrations of hydrogen anywhere in the system pose an explosion risk.

As if large hydrogen flows weren’t challenging enough by themselves, a-Si deposition chambers also require frequent cleaning, usually by means of NF3 etching. The etch gas tends to corrode seals and pump components, a problem that Oerlikon Leybold addresses by flooding these components with purge gas. At the same time, the cleaning process generates a large volume of dust. Handling both dust and light gases in the same system requires careful optimization, Tunna said.

Problems with dust also appear in wafer-based solar cell manufacturing, as the crystal growth process produces large amounts of metallic silicon dust. This dust is especially hazardous because it is pyrophoric (spontaneously ignites below room temperature, and/or reactive with water). To eliminate explosion risks, traditional system designs place a complex metal dust filter before the pump assemblies. Instead, Tunna explained that Oerlikon’s pump design mixes air or oxygen with the silicon dust, forming non-reactive SiO2 which can be captured after the pump by a less expensive standard dust filter.

Vacuum processing appears in a different form toward the end of the solar cell assembly process, as wafer-based cells are laminated into the module frame and thin film panels are encapsulated to protect them from the environment. These processes take place under vacuum in order to make sure that air and water vapor aren’t trapped against the cell surfaces. However, curing EVA outgasses volatile monomers which can attack pump seals and react with pump oil. Standard pumps require oil replacement after as little as 200 hours in this environment, and dry pumps can be damaged if process gases infiltrate the gear box. In contrast, Tunna said, Oerlikon’s Screwline SP 630 pump uses oil-cooled rotors to prevent polymerization of EVA byproducts, while shaft seal purging helps isolate process gases from the pump oil. Oerlikon claims that their design requires maintenance only 1×/year.

Because of their different pressure regimes, vacuum deposition processes manage dust and hazardous gases differently than crystal pulling and encapsulation systems do. Yet all three offer unique challenges for pumping systems as they strive to achieve the performance solar cell manufacturing requires. — K.D.


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