Silicon Dioxide finds widespread application as a low index coating when used in combination with a high index coating in the near UV to IR wavelength range. Common applications include AR coatings, beam splitters, band-pass filters etc.
Unlike quartz, which is the bulk form of silicon dioxide, thin films of silicon dioxide are rarely crystalline and optimum bulk properties such as refractive index, density, hardness and water impermeability are not normally achieved in thin films. However, with the application of ion-beam assisted deposition and enhanced substrate temperatures, atom packing densities can be increased such that moisture permeability can be effectively minimized. In practical terms, this is seen as an increase in refractive index relative to the normal electron-beam evaporated coatings, and IR absorption spectroscopy clearly demonstrates a reduction in the water molecule assigned absorption bands near 3µm and 6µm. In addition, the increased packing density reduces the sensitivity of these thin-films to water vapor as seen experimentally by a stabilization of characteristic properties such as small refractive index drift.
Silicon dioxide thin films have a range of transparency from 0.2 µm to 9µm. The refractive index at 550nm decreases asymptotically from 1.53 to 1.45 over the wavelength range 200nm to +2000nm. This is typically referred to as the dispersion.
Transparency is very dependent upon the quality of the silicon dioxide and influences of the deposition method. Lorad silicon dioxide granules are of the highest chemical purity; better than 99.99% which means that the Lorad material contains less than 100 parts of absorbing material per one million parts of silicon dioxide. A simple examination of Beers Law of Absorption shows that in a 100nm coating, a 100ppm impurity level would only cause a 0.001% absorption. Absorption losses are more usually the result of processing issues. Notably, loss of stoichiometry and microscopic inhomogeneities, such as micro-porosity in the deposited thin film.
Oxygen deficient thin films will normally show increased absorption. The introduction of a controlled, low background level of oxygen during the evaporation is sufficient to maintain good stoichiometry and high transmission.
| Typical Evaporation Conditions | |
|---|---|
| Source Temperature: | approx. 1200oC |
| Deposition Rate: | 1-2 Angstroms/second |
| Oxygen Partial Pressure: | 10-5torr or 1.33x10-3 Pa |
| Substrate Temperature: | 200-300o C |
The most common form of Silicon Dioxide requested by our customers is 1-3mm and 1-4mm high purity fused quartz granules. Lorad typically holds hundreds of kilograms of these granules in stock and offers special rates for bulk purchases.