The STF proprietary CIGS manufacturing process is utilizing all-vacuum process that ensures even, defect-free coating and impurity control, as well as scalable deposition methods developed for the glass substrate. Glass was chosen as the flattest, lowest cost substrate available offering structural support and high temperature capability. Another specific feature of the STF process is that for safety reasons, solid elemental selenium is used, and not the hydrogen selenide.

STF offers a process for the formation of poly-crystalline thin-films of CIGS for photovoltaic applications that is inherently compatible with large-scale manufacturing. This all-vacuum process has several advantages:

• adaptability to high throughput
• absence of major manufacturing hazards
• efficient use of materials
• increased reliability and ease of maintenance
• excellent control over process parameters

Using this process, PV device efficiencies in excess of 12% have been achieved. The Mo layer is deposited on the glass by DC magnetron sputtering. This process is carried out in a multi-chamber, in-line sputtering system, where properties of the Mo are sensitive to the sputtering conditions. After laser patterning of the Mo, the glass substrate is transferred to another in-line vacuum system where extensive use is made of sources capable of downward evaporation. Three custom designed sources are employed to supply the Cu, In, Ga and Se needed to form the Cu(In,Ga)Se₂ compound. During this process the glass is heated. After deposition of the CdS (or other junction-forming material) and scribing of the CIGS, a second in-line sputtering system is used to deposit highly conducting zinc oxide as the top transparent electrode. This electrode is then patterned by scribing, and the plate is ready for testing. There are nine distinct steps worked into the flow line process:

• Step 1: Glass preparation
• Step 2: Sputter deposition of molybdenum
• Step 3: Patterning of the molybdenum conductor
• Step 4: Compound formation to create CIGS
• Step 5: Patterning to open up the CIGS material
• Step 6: Sputter deposition of the zinc oxide transparent conductor
• Step 7: Patterning of the zinc oxide
• Step 8: Encapsulation
• Step 9: PV module testing

The heart of the technology proprietary Step # 4, the CIGS compound formation. STF and its R&D partners have designed this key patented piece of equipment, so that multiple different compound formation can be accomplished. The following processes were successfully demonstrated:

• Compound precursors - CuSe, InSe2, GaSe2, InGaSe₂
• Metallic precursors - Cu, In, Ga
• Coevaporation - Cu, In, Ga, Se
• Combination compound and metallic precursors

All the above steps require a selenization and anneal step. STF has demonstrated device efficiencies of greater than 12% for all the above steps. In the manufacturing optimization, besides the high efficiency, film adhesion is a most important parameter. In the recommended STF compound formulation, metallic and compound precursors are combined with both thermal evaporation and sputter deposition.