What is E-Beam Evaporation?
E-Beam Evaporation also has very high material utilization efficiency compared to other PVD processes reducing costs. The E-Beam system only heats the target source material, not the entire crucible, resulting in a lower degree of contamination from the crucible. By concentrating the energy on the target rather than the entire vacuum chamber, it helps reduce the possibility of heat damage to the substrate.
Several different layers of coating from different target materials can be applied with a multiple crucible E-Beam evaporator without breaking the vacuum making it adapt easily to a variety of lift-off masking techniques.
Because of the high deposition rate and high material utilization efficiency, E-Beam Evaporation is used in a wide variety of applications ranging from high performance aerospace and automotive industries where high temperatures and wear resistance are key, to durable hard coatings for cutting tools, and for chemical barriers and coatings that protect surfaces in corrosive environments like marine fittings.
Electron Beam Evaporation is used for optical thin films ranging from laser optics, solar panels, eye glasses and architectural glass to give them the desired conductive, reflective and transmissive qualities.
There are basically three types of E-Beam configurations for heating the target material. Electromagnetic focusing, electromagnetic alignment and pendant drop configuration.
With the electromagnetic focusing and electromagnetic alignment configuration which magnetically curve the E-Beam, the target material is heated in a crucible with the magnetic fields focusing and diffusing the E-Beam. The pendant drop configuration utilizes the target material as a rod. Ingots are heated in a copper crucible, whereas rods are mounted in a socket.
The basic E-Beam deposition process is that the target material to be evaporated is placed in the vacuum chamber in a crucible or socket. A tungsten filament is placed below it as the anode or negative charge applying a large voltage of electrons focused by magnets to the target source material to be evaporated. The substrate to be coated is placed above it in the vacuum chamber as the diode with a positive charge.
E-Beam Evaporation systems configure the crucibles that hold the source material to be evaporated in three different configurations: Single Pocket, Rotary Pocket and Linear Pocket. A single pocket E-Beam Evaporation system has a single inverted truncated cone “Pocket” used to hold the target material, or hold the copper crucible the material is melted in. Rotating Pockets hold the source material in rotating pockets that can be dialed into position for different target materials to be applied to the substrate. Linear Pocket systems place the multiple pockets in a linear configuration.
Two magnetic coils surround each pocket or crucible. One magnetic coil focuses the high energy E-Beam to the center of the pocket. The other magnetic field known as “XY Sweeping” diffuses the energy to heat the target material more evenly.
Target materials with lower melting points do not require XY Sweeping because they melt more quickly and fill the crucible. Coating materials with higher melting points require XY Sweeping to keep the E-Beam from boiling a hole in the target and “Spitting” an uneven discharge of the coating material onto the thin film being deposited.
The E-Beam’s power is swept throughout the target material to be heated and evaporates it to a gaseous phase. Whether the target material is an ingot heated in a crucible or a rod in a socket, the high energies involved requires that they must be cooled which is accomplished with circulating water. This cooling is crucial to reduce impurities from the crucible, an issue requiring close attention at each step of the E-Beam Evaporation process.
Depending upon the pressures, E-Beam Coating is a line of site deposition technique from the crucible to the substrate – with average working distances being 300 mm to 1 meter with the substrate to be coated placed above the crucibles. Atoms or molecules evaporating off the target material in a high vacuum chamber travel in a straight line of site known as the mean free path until they collide with another atom, diffusing their trajectory into a vapor cloud.
The Electron Beam’s power needs to be controlled to achieve and maintain the desired speed of evaporation for optimal vapor tension, which is unstable and can vary quickly. This is done with quartz crystal control which measures as a feedback loop the increase in weight with a growing film’s thickness upon the oscillation frequency of a quartz crystal – making almost instantaneous adjustments regulating the speed of evaporation and the vapor tension in real time. An optical monitor is commonly used when coating precision optics which provides an optical
signal associated with the growing film and automatically providing a cut based on a pre-programmed optical thickness for that film.
With many Electron Beam Evaporation applications the addition of an ion sources maybe employed for in-situ cleaning, ion beam assisted deposition (IBAD), and Surface modification and activation (SM)
Although E-Beam Evaporation is used in a wide variety of applications, it is particularly efficient in transferring pure and precise metal coatings that require high melting temperatures to substrates on the atomic and molecular level.
