Ion Assisted Deposition of Thin Films

Deposition of thin films by thermal and electron beam (e-beam) evaporation processes was discussed in the previous two Blogs. One of the major problems with these processes is the low energy of the evaporated atoms, which can cause problems in the thin films such as poor adhesion to the substrate, reduced density, porous and columnar microstructure, increased water pick up and poor mechanical properties. Typically, the substrate is heated to several hundred degrees Celsius during coating to mitigate this effect, but it is by no means eliminated. Ion assisted deposition (IAD) and ion plating mitigate many of these problems by providing enhanced energy to the evaporated atoms and ion cleaning the substrate [1].

For optical coatings, problem with porous films is that they can subsequently absorb moisture, which changes the refractive index of the layer(s), and can cause shifts in the center wavelength with changes in ambient temperature and humidity.  Low density also limits mechanical durability to some extent, although these films can typically meet most of the MIL-SPEC durability and environmental requirements.  Furthermore, the requirement to heat the components during processing can limit substrate material choice, and also introduce stress in the substrate due to thermal cycling.

Figure 1 below shows the placement of an ion source in a typical deposition chamber. Bombardment prior to deposition is used to sputter clean the substrate surface. Bombardment during the initial deposition phase can modify the nucleation behavior of the depositing material such as nucleation density. During deposition the bombardment is used to modify and control the morphology and properties of the depositing film such as film stress and density. It is important, for best results, that the bombardment be continuous between the cleaning and the deposition portions of the process in order to maintain an atomically clean interface [1]. IAD adds a high energy ion beam that is directed at the part to be coated.  These ions impart energy to the deposited material, acting almost like an atomic sized hammer, resulting in a higher film density than achieved with purely evaporative methods.

The higher density of IAD coatings generally gives them more mechanical durability, greater environmental stability and lower scatter than conventional evaporated films.  Furthermore, the amount of energetic bombardment can be varied from zero to a maximum level on a layer by layer basis, giving the process tremendous flexibility.  For example, while IAD is not compatible with some of the commonly used materials in the infrared, it can be used solely on the outermost layer to yield an overall coating with superior durability.  The ion energy can also be used to modify the intrinsic stress of a film during deposition.  In some cases, this can change the film stress from tensile to compressive, which can help to maintain substrate surface figure, especially when depositing thick infrared coatings.

Several types of ion source are available, including Kaufman type, end Hall type, cold and hollow cathode types. Each source differs in how ions are created and accelerated to the substrate and ion energy distribution. All these sources ionize inert gases such as argon and krypton to bombard the substrate. Ions are extracted by several methods, grid extraction (Kaufman type) where ions are relatively monoenergetic or from a broad beam ion source (end Hall and hollow cathode) having a spectrum of ion energies. Figures 2 and 3 show cross sections of Kaufman and end Hall ion sources [1,2,3]. The Kaufman source is gridded to control energy of exiting ions. Ion current available from the ion source are determined by source parameters, such as gas pressure, cathode power, anode potential, geometry, etc. The accelerator grid serves two purposes: 1) to extract the ions from the discharge chamber, and 2) to determine the ions trajectories, i.e. focusing. Table 1 shows operating parameters for this ion source.

Table 1. Maximum argon ion beam current

Discharge voltages for end Hall ion sources typically range from 40 – 300V. Discharge currents for hollow cathode designs range from 30 μA – 5A with discharge voltages up to 16 kV [4]. This unit is also used for ion etching and ion sputtering.

A wide range of properties of evaporated (and other PVD processes as will) coatings improve with increased density and packing density resulting from ion bombardment. Effects of IAD are no more apparent than in moisture stability and stress in optical coatings [2]. Moisture can readily penetrate low density films deposited using low energy processes, causing a distinct shift in optical properties (refractive index, absorption), as shown in Figure 4. We see that moisture shift in the refractive index is significant for as deposited HfO2 films while negligible for IAD films.

Stress in evaporated thin films deposited without IAD is generally tensile. Compressive stress is desirable in thin films to enhance mechanical properties and reduce cracking, although all stress should be kept to a minimum. By removing loosely bound atoms IAD increases film density and can change stress from tensile to compressive. The capability to tune stress is particularly valuable in multilayer films. Thus it is possible to vary stress of alternating layers from tensile to compressive, thus achieving very low stress in the resulting structure.

However, with IAD there can be too much of good thing and defects can form during deposition. As a result of this process ion energy can be given up to the growing layer either at the surface of in the underlying regions [5]. Atom displacements responsible for lattice damage (voids, lattice modification) are produced by energy deposition in bulk regions of the film. Contrast this with surface smoothing described previously. Additionally, inert gases can also be driven into the film. An extension of this effect is ion implantation used extensively in semiconductor technology. Fortunately, the threshold energy needed for bulk defect formation is higher than the threshold for surface driven processes. Care must thus be taken not to use excessive ion energy or bulk defects will be created.

Ion sources are also used for ion beam sputter deposition of thin films, but that will be addressed in a future Blog.

Figure 1. Ion source placement in deposition chamber.

Figure 2. Kaufman ion source geometry [3]
Figure 3. End Hall ion source geometry [1]
Shift in refractive index for evaporated HfO2 films with and without IAD [2]
Reference:

  1. Donald M. Mattox, in Handbook Deposition Technologies for Films and Coatings, 3rd Ed., P M Martin Ed., Elsevier (2009).
  2. D. E. Morton, V. Fridman, 41st Annual Technical Conference Proceedings of the SVC, 297 (1998).
  3. South Bay Technology, Applications Laboratory Report 123.
  4. J Alessi et al., Brookhaven Laboratory Report 102494 -2013 CP.
  5. A R Gonzalex-Elipe, F Yubero & J M Sanz. Low Energy Ion Assisted Film Growth, Imperial College Press (2003).