The electrode of a crystal is critical for thickness monitoring and rate control. Currently, three standards materials for electrodes are available on the market: gold, silver and alloy.
Gold is the most widely used conventional material; it has low contact resistance, high chemical stability and is readily deposited. Gold is best suitable to monitor the thickness of low-stress materials e.g. gold, silver and copper. When a gold-coated crystal chip is used to monitor the above-mentioned products, even if the frequency drifts by 1 MHz, there is no negative effect. However, a gold electrode is not flexible so that it will transfer the stress from the film to the quartz substrate. The stress transferred will lead to frequency jitter of a crystal and seriously affect its quality and stability.
Silver is the nearly perfect material for electrode; it has very low contract resistance and excellent plasticity.
However, silver is readily sulfurized; sulfurized silver has high contact resistance, reducing the fastness of the film on a crystal.
Silver-aluminum alloy crystals are suitable to monitor the films of high-stress materials e.g. SiO2, MgF2, TiO2. Due to high tension or accumulative attraction, these high-stress films often lead to unstable crystals; high stress will deform the substrate, resulting in frequency jitter. Silver-aluminum alloy distributes the stress through plastic deformation or rheology; before the tension or stress deforms the substrate, the silver-aluminum electrode has released these stresses. It allows the silver-aluminum alloy crystals to have more stable vibration for longer time. Experiments show that the SiO2 coating with the silver-aluminum alloy crystal has a life 400% longer than the gold coating.
As the coating technology is advancing rapidly, it is difficult for a coating engineer to choose the best crystal depending on different coating processes. The following suggestions are provided for your information:
- For coating with low-stress materials, choose gold-coated crystals; the most commonly used coating materials are Al, Au, Ag, Cu; these films have nearly no stress, and can be applied at room temperature. The films are soft and prone to scratching; but they will not crack or produce negative effect on the substrate. It is recommended to use gold-coated crystals for above-mentioned coating; as experiments demonstrate, you can apply gold of thickness 60,000 A and silver of 50,000 A with gold-coated crystals.
- Use silver-coated or silver-aluminum-alloy-coated crystals to apply high-stress films; the materials Ni, Cr, Mo, Zr, Ni-Cr, Ti, Stainless Steel are prone to produce high stress; the films are prone to peel off from the substrate or crack, resulting in sudden jump of the rate or a series of sudden irregular positive/negative variations of the rate. Sometimes, these conditions are tolerable; but in some cases, they have adverse effect in controlling the power of evaporation sources.
- Use silver-aluminum-alloy crystals to apply dielectric optical films; thanks to their good optical transparency range or refractivity feature, the materials MgF2, SiO2, Al2O3, TiO2 are widely used in optical coating; but it is also most difficult to monitor these materials; only when the substrate is at temperature higher than 200 degrees, can these films have very good adhesion to the substrate; so when these materials are applied on a crystal with water-cooled substrate, it will produce huge stress in the condensation process, likely causing the crystal to fail within 1,000 A. In this case, it is preferably to choose alloy crystals, which will greatly reduce the chance of frequency adjustment. Laboratories show that when alloy crystals are used to monitor magnesium oxide, the effective life is 100% longer than the gold-coated ones. If the temperature of cooling water is increased from 20 degrees to 50 degrees, the life of crystals can be prolonged by one fold.