Silicon is easily the most common substrate material used in any cleanroom. The most common growth method is to melt the polycrystalline silicon in a large slowly rotating chamber made of graphite or quartz, and allow it to condense over a seed crystal. Crystalline silicon is extremely hard and difficult to slice. Diamond-coated blades are required to slice the ingot into wafers. Silicon wafers are mass produced for the electronics and solar industry, and as a result, their cost is relatively low even for ultra high-purity wafers with extremely low defect densities. Substrates serve multiple functions during a fabrication process: they provide mechanical rigidity, flatness, and temperature stability during processes such as photolithography, deposition and etching; they provide a semiconducting electrical material whose conductivity can be modified by selectively introducing impurities to create diodes and transistors; and they provide a defined crystal orientation on which subsequent layers can be grown as lattice-matched crystalline thin films.