E. P. Donovan

Fundamentals of ion‐beam‐assisted deposition. I. Model of process and reproducibility of film composition

An ion‐beam‐assisted‐deposition (IBAD) system is under development to fabricate Si1−x Nx films for optical devices. Reproducible film composition requires characterization of the relationship between the incorporated nitrogen atom fraction x and the real time experimental measurable quantities. In this paper a simple model is presented which relates the film composition x to the measured beam current density JF , the vapor impingement rate Q, and the chamber pressure p. Effects included in the model are reflection of energetic particles, sputtering from the film surface, and charge exchange neutralization of the ions. Each term in the model is examined as a potential source of both systematic and random deviations of the data from the model. Data on film composition as a function of the nitrogen ion current to deposition rate ratio are presented for several sets of ion source voltages and chamber pressures. It is shown that by modifying the deposition system so as to minimize the identified sources of err...

Near infrared rugate filter fabrication by ion beam assisted deposition of Si (1− X ) NX films

The rugate filter employs a sinusoidal refractive index depth profile to produce high reflection in a narrow band of wavelengths. Fabrication relies on a continuously variable index of refraction in the wavelength regime of interest. The near IR refractive index of amorphous silicon-nitrogen films decreases continuously as the composition varies from pure silicon to stoichiometric silicon nitride (Si(3)N(4)). Ion implantation was found unsuitable as a fabrication method for rugate filters. Homogeneous and inhomogeneous films up to 5 microm in thickness have been produced by simultaneous deposition of electron beam evaporated silicon and of energetic nitrogen particles arising from an ion beam. The relative fluxes of beam and evaporant are found to determine the ratio of nitrogen to silicon in the films and therefore to determine the index. Single-band reflection filters of the rugate design of high peak optical density were fabricated under computer control using a quartz crystal oscillator shielded from the beam to monitor the silicon evaporation and three suppressed Faraday cups to monitor the ion beam current.

Fundamentals of ion‐beam‐assisted deposition. II. Absolute calibration of ion and evaporant fluxes

A method is given to obtain an absolute calibration of the ion and evaporant fluxes in an ion‐beam‐assisted deposition system based upon a Kaufman ion source and an electron beam vapor source. The nitrogen‐ion silicon‐vapor material system is used for the calibration; Rutherford backscattering is used for measurement of composition and thickness of Si1−x Nx films deposited on C and Si substrates. It is shown that quantitative predictions of the ion‐to‐atom impingement ratio, film composition, and film thickness can be obtained when sputtering, reflection, charge exchange neutralization of the ions, and species content of the nitrogen beam are considered.

Characterization of a 3 cm Kaufman ion source with nitrogen feed gas

Abstract A commercial high current dual grid optics 3 cm ion source (normally used in argon etching systems) was purchased for use in a nitrogen ion beam assisted, electron beam evaporation (IAD) system. Control of the concentration of the reactive ions in the growing film required characterization of the source for nitrogen. The inter-relationships of beam current, beam spatial profile measurements, chamber pressure, accelerator voltage, beam voltage (500, 1000 and 1500 V), discharge voltage, and filament current are reported. The ratio of atomic to molecular ion content ( N + N + 2 ) was also investigated under some of these conditions.

Ion-beam-assisted deposition of molybdenum nitride films

A series of molybdenum nitride films was synthesized by electron beam evaporative deposition of molybdenum, with simultaneous bombardment by nitrogen ions from a Kaufman ion source. The nitrogen ions were accelerated to 500 or 1000eV. The film compositions and structures were determined using Rutherford backscattering spectrometry and X-ray diffraction respectively. Effective reflection and sputtering coefficients for nitrogen incident on the molybdenum nitride surfaces were extracted from the data. These coefficients were used to calibrate the deposition system and allowed the deposition of molybdenum nitride films with control of the nitrogen atom concentration to ±2.3 at.%. In general, the films were polycrystalline with a high degree of texturing. The phases found in order of increasing measured nitrogen content were as follows: ‘1-Mo2N(f.c.c.), /3-Mo16N7(b.c.t.), Bi-MoN (f.c.c.) then 8-MoN (h.c.p.).

Ion beam assisted deposition of substoichiometric silicon nitride

Thin films of substoichiometric silicon nitride of uniform composition have been produced by ion beam assisted deposition (IAD) of electron beam evaporated silicon on amorphized Si substrates. IAD films are found to have improved adhesion, increased density, and markedly lower oxygen contamination than non-IAD films produced simultaneously. The Reststrahlen features of the IR spectrum (due to local vibrational modes) are largely unaffected by crystallization at 900°C, indicating stable Si-N bond formation during IAD. The nitrogen content of the films was measured by RBS and in several cases by AES. Infrared reflection spectra were taken and analyzed to derive the index of refraction. The index decreases monotonically with increasing nitrogen fraction in agreement with the Lorentz-Lorentz relations for both the as deposited (amorphous) and annealed (crystalline) samples. The relationship between the incorporated nitrogen fraction and the ratio of the arriving nitrogen atom flux to the silicon atom flux is reported and modelled in a simple manner.

Physical aspects of ion beam assisted deposition

Abstract Progress toward obtaining an understanding of the physical processes that are active during ion beam assisted deposition (IBAD) is reviewed. A model is presented that includes the effects of sputtering, reflection of ions, multiple species beam. charge exchange neutralization, and incorporation of ambient gas atoms. Good agreement is found with data for the composition of silicon nitride. boron nitride and titanium nitride films as a function of the arrival ratio of nitrogen ions to evaporant atoms. Calculations based upon collision cascade simulation are reviewed which predict the energy dependence of the critical arrival ratio to achieve low intrinsic stress in deposited films. Factors which influence the choice of IBAD system geometry, the choice of ion beam energy, and applications of the films are also discussed.

Effect of annealing on the optical properties of ion‐implanted Ge

Infrared reflection and transmission measurements are used to study (111)‐ and (100)‐oriented Ge samples which were implanted with sufficient fluences to produce a continuous amorphous layer. Two optical states of amorphous Ge are identified: (i) as‐implanted, amorphous state which has an infrared refractive index about 8% larger than that for single crystal Ge, nc=0.92 nI; (ii) thermally stabilized amorphous state with an intermediate refractive index, nII=(0.963±0.002)nI. A shift of the fundamental absorption edge to higher energy occurs with the transition from the as‐implanted to the thermally stabilized, amorphous Ge state, but no change in the density is observed for the transition. Annealing at about 300 °C for 2 h produces the thermally stabilized state. Annealing for longer time or at higher temperatures causes measurable epitaxial regrowth. The regrowth rates and activation energies for both orientations are also determined and compared with values measured previously by another method. With the...

The pitting behavior of silicon nitride ion beam assisted deposited coatings on aluminum

Abstract The pitting potential of silicon nitride ion beam assisted deposited (IBAD) coatings on aluminum in deaerated 0.1 M NaCl solutions increased with coating thickness for coatings ranging from 0.01 to 2.0 μm. Rutherford backscattering spectroscopy and optical techniques showed the films to be nearly stoichiometric Si 3 N 4 . X-ray photoelectron spectroscopy showed that the surface of the IBAD samples is composed of Si 3 N 4 , SiO 2 , and a silicon oxynitride species. Pit propagation beneath the coatings proceeds by blister formation and rupture.

Thermal stability of silicon nitride coatings produced by ion assisted deposition

Abstract Amorphous Si-N alloy films containing from about 20 to 60 at.% N were deposited by combined e-beam evaporation of Si and ion bombardment of N. A Kaufman-type ion gun produced the 500-eV nitrogen ion beam. Films up to 1-μm thick were deposited on single-crystals of silicon and sapphire in a vacuum of about 2 × 10 −4 Torr. The as-deposited films were characterized by Rutherford backscattering spectroscopy for composition, visible and near-infrared spectrophotometry to measure index of refraction and absorption bands from Si-N bond vibrations, and X-ray diffraction for crystal structure. Subsequently, samples were annealed in a thermogravimetric analyzer at temperatures up to 1350°C to ascertain their thermal stability against crystallization, oxidation, and reaction with the substrate. Postanneal examination by Rutherford backscattering, spectrophotometry. X-ray diffraction, and optical and scanning electron microscopy provided detailed information on the thermally induced changes in the films. Crystallization of Si occurred in N-poor samples, while α Si 3 N 4 crystallized in N-rich samples after 1200°C anneals. Blisters sometimes also appeared following 1200°C anneals of N-rich samples. For anneals up to 1200°C, no reactions with sapphire or Si substrates were observed and minimal oxidation was found.