Beam Current Spatial Distribution Optimization of High Voltage Power Supply for Ion Beam Assisted Deposition of Optical Films
Ion beam assisted deposition combines physical vapor deposition with concurrent ion bombardment to produce thin films with enhanced density, improved adhesion, and tailored microstructure. The technique is particularly valuable for optical coatings where film properties such as refractive index, absorption, and stress must be precisely controlled. The ion beam parameters including energy, current density, and spatial distribution critically affect the film properties and the uniformity across the substrate. The high voltage power supply that powers the ion source must enable optimization of these parameters.
The ion beam assisted deposition process typically uses a separate ion source that directs a beam of energetic ions toward the substrate while a conventional evaporation or sputter source deposits the film material. The arriving ions provide energy and momentum to the growing film, influencing the adatom mobility, the nucleation density, and the film structure. The ion to atom arrival ratio, the ratio of ion flux to depositing atom flux, is a key parameter that determines the degree of ion assistance. Typical ion energies range from tens to hundreds of electron volts, sufficient to affect film growth without causing significant sputtering or implantation.
The ion source for ion beam assisted deposition is typically a Kaufman type broad beam source or a radio frequency ion source. In a Kaufman source, a thermionic cathode emits electrons that ionize the process gas in a discharge chamber. A multi aperture grid system extracts and accelerates the ions, forming a broad beam with controlled divergence. The high voltage power supply provides the discharge power for ionization and the acceleration voltage for the extraction grids. The beam current and energy are controlled independently through the discharge power and the acceleration voltage respectively.
Beam current spatial distribution refers to the variation of ion current density across the beam cross section. A uniform distribution provides consistent ion assistance across the substrate, essential for uniform film properties. Nonuniform distributions cause spatial variations in ion assistance that translate to variations in film density, stress, and optical properties. The beam distribution is affected by the ion source design, the extraction grid geometry, the discharge conditions, and the beam neutralization.
Grid system design strongly influences the beam current distribution. The extraction grids consist of multiple plates with arrays of apertures that extract, accelerate, and focus the ion beam. The aperture pattern, the grid spacing, and the grid voltages determine the beam extraction per aperture and the overall beam shape. Curved grids can produce focused or divergent beams for specific applications. The grid alignment and cleanliness affect the uniformity of extraction across the grid area.
Discharge conditions in the ion source affect the plasma density distribution and thus the available ion current for extraction. The magnetic field configuration in the discharge chamber influences the plasma confinement and the density profile. The cathode emission uniformity affects the electron availability for ionization across the chamber. The gas flow distribution affects the local gas density and ionization rate. The high voltage power supply for the discharge must provide stable power that maintains consistent plasma conditions.
Beam neutralization is required to prevent space charge blowup of the ion beam after extraction from the source. Without neutralization, the positive charge of the ion beam creates electric fields that diverge the beam and distort the current distribution. Neutralization is typically provided by electrons from a thermionic neutralizer or a plasma bridge neutralizer. The neutralizer current must match the beam current to achieve complete neutralization. Incomplete neutralization causes beam expansion and nonuniform current distribution at the substrate.
Substrate motion during deposition can improve the uniformity of both the deposited material and the ion assistance. Planetary rotation systems move the substrate through different positions relative to the evaporation source and the ion beam, averaging out spatial variations. The motion pattern and speed must be coordinated with the deposition rate and the ion current to achieve the desired averaging. Static depositions require more uniform beam distributions to achieve acceptable film uniformity.
Characterization of the beam current distribution uses Faraday cup measurements at various positions in the beam. A Faraday cup is a biased collector that measures the ion current entering a defined aperture. By scanning the Faraday cup across the beam, the current density profile can be mapped. Beam profile monitors using multiple collectors or scanning probes provide real time distribution information for process control. The measurements inform adjustments to the ion source parameters to optimize the distribution for specific substrate sizes and geometries.
