If the evidence is paper or an item that is relatively flat, it can be held to the rack with magnets. All other forms of evidence are suspended from the rack. Special jigs can be fashioned for specific types of evidence i.e. bullet casings.
The heat sources are then loaded with the metals to be evaporated. The metals can be evaporated from two types of sources, foil boats or crucible heaters.
Metal foil boats are made from thin refractory metal stampings, usually tungsten or molybdenum. These boats have dimples which hold the evaporation material.
Crucible heaters are an open, circular wound filament which allows crucibles to be inserted inside the windings. The crucibles are commonly manufactured from alumina, carbon, quartz and boron nitride. Crucibles have insulating properties which allow a uniform melt temperature. Crucible evaporation is very stable because of its uniform heating.
The chamber door is then closed and the system pump down is initiated.
The mechanical pump, ‘roughing pump’, starts the process. This pump sweeps the air from the chamber. The mechanical pump, in a diffusion pumped system, has two functions. The first is to remove 99% of the air from the chamber. The second is to back the diffusion pump. The mechanical pump acts as the exhaust for the diffusion pump. A diffusion pump cannot operate at atmospheric pressure. The mechanical pump reduces the pressure in the chamber to the correct operating range of the diffusion pump.
When the mechanical pump has reduced the pressure in the chamber to the operating range of the diffusion pump, then an operating valve is opened and the diffusion pump will lower the pressure in the chamber to where the deposition process can begin. The complete pump down process can take anywhere from 30 to 45 minutes, depending on the size of the chamber.
Once the chamber is at desired pressure then the metals can be evaporated. This is where the equipment operator’s skill and experience takes over. The amount of evaporation is controlled by the VMD operator. It is easy for an inexperienced operator to under or over develop an item. Evidence that is under developed can be reprocessed. Reprocessing takes time and wastes resources. Over development can be a serious error. The development process cannot be reversed.
All GoEvidence VMD technicians are highly skilled in the process. Not only do we provide the service, but we can also help with any technical questions, evaporation boats, evaporation metals, vacuum chamber door gasket sealants, and diffusion pump oil.
Vacuum Metal Deposition (VMD) machine