ABOUT THE THIM FILM PROCESS
Ion Assisted Deposition (IAD)
Our Telescope Mirror Coating process utilizes the latest methods in Optical Coating technology, perfectly suited for the Telescope Mirror industry. State of the art Ion Assisted Deposition - IAD - enhances the quality of the deposited material in the ways listed below.
Density and Hardness:
Films produced with (IAD) exhibit mechanical properties very close to there bulk material counterparts. Conventional vapor deposition has always had the drawback of producing films that are loosely packed, soft, and porous, due in part to the very low kinetic energy of the arriving evaporant (< 1ev). With our "Troth Style®" Linear Ion Source that we have developed in house, we can greatly enhance the film growth at the substrate surface by bombarding the arriving evaporant with a relatively low energy (~60ev), high current oxygen ion beam. The term "low energy" in this case is in contrast to most ion sources which operate at several kilovolts (~2kv or greater). Through years of research, coating companies have learned that high voltage ion sources do not enhance but degrade the optical properties of thin films needed for low absorbing, high transmitting films. Special low energy (~60ev), high current ion sources had to be developed and Spectrum Coatings has developed one of the finest in the industry.
Reduced Absorption of Visible Light:
Sub oxides are evaporated, (mainly Titanium Oxide (TiO) and Silicon Monoxide SiO) and are then transformed into higher oxides (Titanium Dioxide (TiO2) and Silicon Dioxide (SiO2 or Quartz) at the substrate surface when bombarded with the highly reactive ionized oxygen ion beam. Not only does the oxygen ion beam have high energy (60ev, now compared to < 1ev of conventional deposition) but the oxygen is in a highly reactive state in the form of O2 ions, monatomic oxygen (very unstable and wanting to bond), and ionized monatomic oxygen. Thus, the high current ion beam enhances the growing films by allowing the film to fully oxidize while "beating" the film down causing arriving atoms and molecules to have greater surface mobility which produces a fully oxidized, absorption free thin film. Most front surface mirrors (using conventional means) are over-coated with Silicon Monoxide (SiO). SiO exhibits optical absorption in the blue end of the visible spectrum. Also, unless very high substrate temperature are used the film is porous and soft and will eventually degrade and start to peel. At Spectrum Coatings we produce absorption free Quartz over-coated front surface mirrors.
Hard Quartz with No Substrate Heating:
Because of the issues raised above our overcoats are done using NO substrate heating. Once again, it is known that one cannot produce durable, absorption free optical coatings without ample surface mobility of the growing film. The options in this case are very limited. Two methods of obtaining high surface mobility of the growing film are 1. Very high substrate temperatures (~300°C or 572°F) or 2. Ion Assisted Deposition (No substrate temperature needed). Due to time and cost of resources most optical coating companies DO NOT HEAT telescope mirrors when applying the protective overcoat resulting in what we have already talked about, mainly, poor quality overcoats. Spectrum Coatings produces hard oxide films without any heating of the substrate. A true state of the art method of manufacturing coatings today.
Coating Process Overview:
A brief description of our front surface mirror coating process will be discussed below.
Old coating is stripped by soaking in Ferric Chloride (very mild - does not react with glass).
Glass blank is now cleaned with acetone and lastly methanol.
Blank is loaded into coating fixture (which rotates during coating process for a very uniform deposition) the coating fixture is then loaded into the vacuum system facing down.
System is rough pumped, then the high vacuum valve is opened and the chamber is in a "high vacuum" state (better than 2x10-6 torr).
Quartz lamps are switched on breifly to bake water off of the chamber walls.
System is left to pump until deposition pressure is reached.
The Substrate (Mirror) is prepared for deposition (proprietary process - NO ADHESION LAYERS ARE USED). This is very important. Most coating companies use some form of adhesion layer. This is unnecessary if the correct process has been developed. Adhesion layers are, more times than not, very difficult to remove, and, they must be removed before re coating the mirror.
Aluminum is flash evaporated at very high deposition rates (~500-700Å/sec.). Flash evaporation is the best method for making highly reflective, no scattering aluminum and silver films.
Ion source is switched on just as SiO (silicon monoxide) deposition starts. As the SiO film grows it is fully oxidized by the reactive oxygen ion beam which transforms the SiO into a hard durable layer of Quartz (SiO2). No optical interference of the quartz thin film through the visible to the UV. This method is the only method proven to produce pure quartz. Methods without an efficient ion source, even using very high substrate temperature (higher than 270°C) produce what is known as SiOx, which has varying index of refraction and hardness problems. TiO2 films are produced in the same manner for the EAL and MaxR coatings.