E-Beam or Electron Beam Evaporation is a form of Physical Vapor Deposition in which the target material to be used as a coating is bombarded with an electron beam from a charged tungsten filament to evaporate and convert it to a gaseous state for deposition on the material to be coated. Taking place in a high vacuum chamber, these atoms or molecules in a vapor phase then precipitate and form a thin film coating on the substrate.
E-Beam evaporation and sputtering are the two most common types of PVD. While sputtering can be very useful, E-Beam deposition has several unique advantages. It permits the direct transfer of energy with the electron beam to the target material to be evaporated, making it ideal for metals with high melting points. E-Beam evaporation can yield high deposition rates – from 0.1 nm per minute up to 100 nm per minute – resulting in high density film coatings with exceptional adhesion to the substrate. It also has remarkably high material utilization efficiency compared to other PVD processes, creating the potential for reduced costs. An 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 and chamber. This allows for several different layers of coating from different target materials to be applied by use of a multiple pocket crucible within the E-Beam evaporator, without breaking the vacuum. This helps it to adapt easily to a variety of lift-off masking techniques.
Evaporation Process
Because of the high deposition rate and material utilization efficiency, E-Beam evaporation is used in a wide variety of applications in high performance aerospace and automotive industries where high temperatures and wear resistance are key, as well as for 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 also used for optical thin-films including laser optics, solar panels, eyeglasses and architectural glass, to give them the desired conductive, reflective, or transmissive qualities.
There are three primary types of E-Beam configurations for heating the target material: electromagnetic focusing, electromagnetic alignment, and pendant drop configuration.
Through the electromagnetic focusing and electromagnetic alignment configurations, the E-Beam can be magnetically curved and the target material can be 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 involves the target material to be evaporated being placed in the vacuum chamber in a crucible or socket. A tungsten filament is placed below it as the anode to apply 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 crucibles can be arranged 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 the copper crucible the material is melted in. A rotating pocket system will hold the source material in rotating pockets that can be dialed into position for different target materials to be applied to the substrate. A linear pocket system places 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. A target material with a lower melting point does 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 boring a hole into 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 then evaporated 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 require cooling by water circulation. This cooling is crucial to reduce impurities from the crucible, an issue requiring close attention at each step of the E-Beam Evaporation process.
Typically, E-Beam evaporation is a line-of-sight technique, with average working distances ranging from 300 mm to 1 meter. The substrate to be coated is placed above a set of crucibles inside the chamber. 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.
In the use of an electron beam, control is paramount to achieving and maintaining the desired speed of evaporation. This allows for better control of the optimal vapor tension, which is unstable and can vary quickly. This is typically done with a quartz crystal monitor which measures, through the use of a feedback loop, the increase in weight of a quartz crystal and the resulting change in the quartz crystal’s oscillation frequency. This allows for almost instantaneous adjustments to regulate the speed of evaporation and the vapor tension in real time.
Other valuable tools in E-Beam deposition are optical monitors and ion sources. An optical monitor is commonly used when coating precision optics to provide an optical signal associated with the growing film and automatically provides a cut based on a pre-programmed optical thickness for that film. Ion sources can be useful for process requirements such as in-situ cleaning, ion beam assisted deposition, and surface modification and activation.
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.
Matt Hughes is President of Semicore Equipment Inc, a leading worldwide supplier of sputtering equipment for the electronics, solar energy, optical, medical, military, automotive, and related high tech industries. Please let our helpful support staff answer any questions you have regarding “What is E-Beam Evaporation?” and how to implement the best techniques and equipment for your specific Thin Film Deposition by Evaporation needs by contacting us at sales@semicore.com or by calling 925-373-8201.
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