The polished glass is spincoated with an adhesion promoter and then photoresist, followed by curing in an oven. The layer of photoresist has a thickness of 0.12 micron, with a tolerance of 0.003 micron. This thickness controls the depth of the pits, essential to efficient data retrieval; 0.12 micron is exactly one-fourth of the wavelength of the read laser (in the polycarbonate of the final compact disk). Note that the relative thicknesses shown are not to scale!
An extremely clean environment is required to avoid contamination of the photoresist. The entire mastering process is carried out in a class-100 clean room (fewer than 100 particles per cubic foot of air). For comparison, a "dirt density" of 100,000 is common for homes and offices, 10,000 for a hospital operating room.