5.01.00 Solar (Photovoltaic) Industry
Overview of Process Control
Ref. 5.00.00 Semiconductor Wet Chemical Processes (pdf)
Typically, the chemical suppliers are responsible for the in-coming chemical quality. This leaves the semiconductor fabricators with a very limited capability to detect problems with the process chemicals that they receive from their suppliers. Poor chemical quality is revealed through production line test data or poor e-sort performance. Changes, which can occur during on site handling are disregarded and not monitored. Furthermore, factors such as human error in container handling and equipment failure at the distribution point are not taken into account. Implementation of the K-Patents Semicon Refractometer is economically feasible (costing less than the labour requirement for correcting a major mishap). It should also be noted, that expertise in the chemical distribution area should not be overlooked or underestimated. Instead this should be treated as a critical tool set, capable of making a significant impact on the fab very quickly.
Ref. 5.00.01 Bulk Chemical Delivery System (pdf)
Wet etching is used to chemically remove layers (metal, silicon, photoresist) from the surface of a wafer during manufacturing. Etchants that erode the substrate equally in all directions are called isotropic. Modern processes prefer anisotropic etches because they produce sharp, well-controlled features. Several anisotropic wet etchants are available for silicon. For instance, Potassium hydroxide (KOH) is commonly used for this purpose, especially in the MEMS processing.
Ref. 5.00.02 KOH Etch of Silicon (pdf)
Spray solvent tools are used in the post etch polymer residual removal process to clean the surface of wafers. EKC is a commonly used chemical in the process that employs two tanks. The process operating temperature of EKC is kept between 65°C (149°F) and 75°C (167°F). The actual residue removal takes place in the spray solvent tool where EKC is fed to, according to a pre-determined cycle of chemical re-circulation and wafer run.
Ref. 5.00.03 Post-Etch Residue Removal Using Spray Tool (pdf)
Chemical mechanical planarization (CMP) is the process of smoothing and polishing the wafer surface. This is done with the aid of an oxidising agent, e.g. Hydrogen peroxide (H2O2), which contains abrasive particles suspended in the carrier fluid. H2O2 oxidizes the silicon wafer surface to silicon dioxide. The polishing pad is then capable of polishing the wafer surface more efficiently because the newly oxidized layer is much softer than the silica. The CMP slurries require mixing or dilution before use. Oxide polishing slurries are commonly purchased in concentrated form and diluted with water on-site to minimize shipping and labor costs. Some multi-component polishing slurries may only be blended just prior to their use because of their short post mix lifetime. In both cases, it is essential to measure the H2O2 concentration of the mixed slurry, because altering the concentration of the slurry constituents will affect the chemical reaction rates and wafer polishing rate. Too much H2O2 can result in the contamination of the wafer. Process tools that drive the semiconductor manufacturing processes, like CMP, are referred to as critical process systems. They are typically operated by a fab’s facility management team. Automated chemical and slurry handling systems have tremendous implications to the safety, purity and up-time of the fabrication processes.
Ref. 5.00.04 Peroxide Blending and Dispense at CMP (pdf)
Silicon crystal growing and casting plants are either an integrated unit of the photovoltaic manufacturer or independent sub-contract producers. They need not be sited in the proximity of solar cell production plants because wafer transportation is inexpensive. Most are though, because the Photovoltaic manufacturers have invested to ensure a secure wafer supply to their cell plants. The process at these plants, is to first cut the ingots into bars, then slice them into wafers (slightly bigger than a compact disc) using a wire saw and finally to clean the product in preparation for cell manufacturing. The cleaning phase consists of an acetic or lactic acid bath, where the wafers are immersed in order to remove glue and other residues such as Si particles from the wafer surfaces. Solar cell plants then take the cleaned wafers through a high technology processing sequence to create working solar cells.
Ref. 5.01.01 Removal of Residual Sawing Material from Solar Wafers (pdf)