Standard Operating Procedure
Bendix High Vacuum Thermal Evaporator
This page serves as a tutorial for the Bendix high-vacuum thermal deposition system and an introduction to the control and vacuum system. At the bottom of this page, a PDF document may be found with the condensed SOP for the unit.
This basic thermal deposition unit is configured for four sources and is primarily intended for optical thin-film coatings. The filaments would normally be loaded with both metals (such as aluminum) and dielectric materials such as magnesium fluoride (in a crucible) and sodium hexafluoroaluminate (cryolite) allowing fabrication of dielectric mirrors and filters. Unlike most of our other systems this is a completely manual system optimally suited for laboratory and prototyping use.
The active filament is selected via the rotary switch on the control stack just below the ion gauge. Filament current is controlled via a variable autotransformer on top of the unit. Short, straight filaments require control settings in the range 20-35 while larger helical filaments require higher settings. Crucibles heated with helical filaments can require control settings up to 120 for high melting point, low vapour pressure materials.
Pressures in the system are measured using a Varian 843 gauge which includes two TC gauges and an ion gauge. The TC gauges are used to monitor the chamber pressure during roughing as well as the foreline pressure. The roughing pump will pull the chamber below 1*10-3 torr - so low that the chamber TC gauge (TC 1) will 'bottom out' at which point is is safe to open the high vacuum valve as well and turn the ion gauge on. DO NOT turn the ion gauge filament ON until the chamber TC gauge 'bottoms out'. As well as monitoring the chamber pressure the second gauge (TC2) montitors the foreline pressure of the diffusion pump which must be kept below the maximum tolerable pressure for this pump - preferably below 75 mtorr, absolute maximumof 100 mtorr. When opening the high vacuum valve it is important to watch this gauge and open the valve as slowly as required to keep TC 2 below this maximum pressure. This will minimize the amount of possible backstreaming during the critical changeover point between rough pumping and high-vacuum.
Other uses of the TC gauge include monitoring the foreline pressure during rough pumping of the chamber during which the foreline pressure must be checked periodically and the foreline repumped as required to keep the foreline pressure below the maximum tolerable value. Finally, the gauge allows detection of failure of the high-vacuum valve seal. If the high-vacuum valve did not completely seal before the chamber is vented the foreline pressure will increase rapidly and the failure will be immediately evident - the solution is to simply tighten the high-vacuum valve to ensure a more complete seal.
The ion gauge (tube seen to the left, located behind the unit) allows pressures in the chamber to be monitored when in the high-vacuum domain. While the TC gauge 'bottoms-out' at 1*10-3 torr the ion gauge works down to 1*10-10 torr. The ion gauge must never be used until the chamber is under high vacuum though since air in the gauge will burn out the filament.
To set the density of the material, press the PROGRAM key (if not already in program mode) and then press the DENSITY button. Enter the density and press the ENTER key to store it. There is no decimal point key so the density of aluminum would be entered as '2', '7', '0', then enter. The decimal place is implied at two places. Tooling factors ranging from 10 to 200 percent are entered in a similar manner by pressing the TOOLING key beside the DENSITY key. This factor defaults to 100% at power-up, begin with a value of 152.0% with the crystal monitor located at the center of the substrate deck.
When a deposition is started (i.e. just before the shutter is opened), press the ZERO button to reset the display. It is also important to ensure the crystal is oscillating before the chamber is pumped. The XTAL lamp will blink if there is a problem with the crystal or the oscillator package.
PDF Document outlining the basic procedures required to operate the Bendix System.
|Filament #||Material||Index of Refraction||Deposition Rate (Angstroms/sec)|
|1||Cryolite||1.33||25 - 50|
|2||Aluminum||10 - 50|
|3||Magnesium Fluoride||1.38||1 - 10|
|4||Cryolite||1.33||25 - 50|