Until the advent of electron bombardment vaporization, only materials that melted at moderate temperatures (2000°C) could be incorporated into thin-film coatings. Low-temperature materials produce softer, less durable coatings and consequently require specialized handling or protection.
Electron bombardment is capable of vaporizing even difficult-to-vaporize materials such as titanium oxide and zirconium oxide. A high-flux electron gun (1 A at 10 kV) is aimed at the film material contained in a large, water-cooled copper crucible. Intense local heating melts and vaporizes some of the coating material in the center of the crucible without causing under heating of the crucible itself.
Plasma Ion-Assisted Bombardment
Plasma ion-assisted deposition (PIAD) is a coating technique, often applied at low temperatures, which offers unique benefits including an increased durability to heat and humidity. Also, ion-assisted bombardment during the coating process leads to a higher atomic or molecular packing density in the thin-film layers (increasing index of refraction), minimizes wavelength shift, and achieves the high adhesion and low absorption.
Finally, the ion-assisted coating can also be used for cold or low-temperature processing. Eliminating the need to heat parts during coating allows cemented parts, such as cemented achromats, to be safely coated.
Ion Beam Sputtering (IBS)
Ion beam sputtering is a deposition method using a very high-kinetic energy ion beam. The target is external to the ion source, which allows for independent or automated control of the ion energy and flux. The energy and flux of ions is composed of neutral atoms which allow either insulating or conducting targets to be sputtered directly onto the substrate; this allows for a wide range of coating options.
The high energy flux impacts the target source and ejects atoms directly towards the intended substrate. Direct sputtering provides a high level of accuracy and repeatability over numerous coating runs. IBS deposition produces dense coating layers with almost no scatter or absorption, which minimizes or eliminates spectral shift due to moisture absorption.
Magnetron sputtering is a thin film deposition process that utilizes a magnet behind a cathode to trap free electrons in a circuitous magnetic field close to the target surface. A metered gaseous plasma of ions or neutral particles is introduced and the accelerated electrons collide with the neutral gas atoms in their path. These interactions cause ionizing collisions and drive electrons off the gas atoms. The gas atom becomes unbalanced and will have more positively charged protons than negatively charged electrons.
The positively charged ions are accelerated towards the negatively charged electrode and impact the target material. The energy transfer is greater than the binding energy of the target material, causing the release of free electrons, erosion of the target material, and ultimately the sputtering process. The ejected source material particles are neutrally charged and therefore unaffected by the negative magnetic field. The ejected atoms are transferred to a substrate into densely packed coating layers, resulting in little or no spectral shift caused by moisture absorption. The release of free electrons feeds the formation of ions and the propagation of the plasma. Due to close proximity, the percentage of confined electrons that cause ionizing collisions dramatically increases. This allows for very high deposition rates at which the target material is eroded and subsequently deposited onto the substrate.