Observations of Effects Produced by ionBeam Assisted Deposition

Densification of films - Deposition of thin films at low substrate temperatures (Zone I and T in the Movchan diagram) is particularly troublesome to the optical coatings industry because the pick-up of water vapor by pipes and pores between columnar grains causes a change in the refractive index of the film and makes it difficult to reproduce the properties of the films. Martin et. al. (9) have demonstrated that the bombardment of Zr02 films deposited on glass substrates will produce a dense film above a specific ion current density. They used ions of both Ar+ and 02+ at energies of 600-1200 ev to achieve the densification. Nuclear reaction analysis of the hydrogen content of the films showed the films prepared without ion bombardment had a H concentration of 13 x 1022 atoms/cm3 at the surface dropping to 0 at 140 nm depth while films prepared by bombardment above the critical current density had H contents of only 1 x 1022 gr/cm3 at the surface, tapering to 0 at 140 nm depth. Similar observations of densification for a variety of other optical materials have been reported (10).

Miilier has examined the interaction of the ion beam with the film growth processes from a number of perspectives. Attempts to reproduce the densification process using thermal stimulation of a 2-dimensional molecular dynamics model of film growth were inadequate to explain the observations (11). An alternative approach using forward recoils in the collision cascade as the dominant process has provided a better fit to the observations (12) although the model is still rather crude.

Grain Size - An example of the effect of IAD on grain size is provided in the work of Huang et al (13). Using a dual ion beam system, they deposited Ag on a silica film while bombarding the film with Ar+ ions. X-ray measurements were used to measure grain size, texture and stress in the films. The energy of the beam was varied and expressed in terms of an average energy per atom (E/No. of atoms deposited). The grain size was found to decrease from 42 nm grain diameter at 0 ev/atom to 20 nm at 20 ev/atom and further to 15 nm at 190 ev/atom. The stress in the film changed from slightly tensile ( + 0.6 x 108 N/m2) to compressive (-4.5 x 108 N/m2) at 40 ev/atom and stayed at that level to 190 ev/atom. The film texture was also observed to shift away from < 111 > with increasing energy/atom.

Film Stress - The stress in thin films deposited under normal vapor deposition conditions is often so high that the films spontaneously spall or peel. The bombardment of the growing film with an ion beam is effective in reducing the stress in the film and its propensity to spall. Hirsch and Varga (14) conducted a series of experiments in which a Kaufman gun with a known beam profile was directed on a growing Ge film. The film adhered at a certain beam diameter which defined a critical current density for adhesion. The critical current density was found to be energy dependent and measurements of the critical current density were made at 6 energies ranging from 65 to 3000 ev. The critical current density was found to vary approximately as E"3'2. Hirsch and Varga analyzed the data using a thermal spike model and found an e_5/3 dependence. Brighton and Hubler (15) reanalyzed the data using the Monte Carlo computer code MARLOWE which simulates collision cascades using a binary collision model. They found the data fit an Et3/2 dependence for the critical current density and further that the critical current density corresponded to the condition where every atom in the film had been displaced once in the collision cascade. This result indicates that the stress in the film is relaxed by some process in the collision cascade.

Adhesion of Films - One of the major effects produced by bombardment of a surface with energetic ions is the improvement of adhesion. Many of these observations have been qualitative in nature with comments such as "passes the scotch tape test." Baglin (16) has conducted one of the most comprehensive investigations of the factors which influence adhesion of metal films on ceramic substrates using a peel test. While not an IAD experiment since the films were deposited prior to bombardment, most of the factors identified will also apply to IAD conditions. Copper films were deposited on Al203 at room temperature in a configuration that permitted a peel test of the film adhesion. High energy (200, 250 keV) ion beams of He+ and Ne+ ions were used to examine the contribution of elastic collisions and electronic excitation to adhesion. Energies of the beams were adjusted to give approximately the same value of (dE/dX)Bec at the interface while the nuclear stopping power was much larger for the Ne ions. The increase in peel strength was found to be 10x larger for the Ne+ ions thus indicating that nuclear collisions were more important than electronic excitation. However, further experiments indicated several other factors were important such as sputter cleaning of the surface prior to deposition of the film, the presence of specific contaminants on the surface and local atomic rearrangements at the filmsubstrate interface (16)..

NRL Experiments - High energy ion beams

Kant and Sartwell (6) have conducted a wide variety of experiments on thin films grown under reactive IAD conditions.. An electron beam evaporator was used to deposit Ti films on a variety of substrates with ion bombardment by Ti+ or N+ ions in a reactive atmosphere of N2 gas at a pressure of 10"5 Torr. Ion energies have been varied from 40 to 200 keV, ion to atom arrival rate ratios from .001 to 0.2 and substrate temperatures from 23-525°C, although a complete coverage of this matrix space is not yet complete. X-ray and electron diffraction results show that TiN is formed under these conditions with no bombardment and with either N+ or Ti+ ion bombardment. TEM studies of films deposited at room temperature with and without ion bombardment show both films are fine grained, crystalline TiN but the film grown under IAD conditions exhibits a bimodal grain size indicative of grain growth of preferentially oriented grains. Auger analysis of a variety of films deposited on 52100 steel shows several effects of arrival rate ratio as shown in Fig! 5. Both films show flat Ti and N concentration profiles across the film thickness indicating uniform deposition conditions. The film.grown with an R = 0.2 shows a broader film-substrate interface than one with R = 0.002 thus indicating a greater degree of mixing at the interface.



Fig. 5 Composition-depth profiles of IAD films of TiN produced at R values of 0.002 and 0.2 using Auger electron spectroscopy and depth profiling by sputtering.


Fig. 5 Composition-depth profiles of IAD films of TiN produced at R values of 0.002 and 0.2 using Auger electron spectroscopy and depth profiling by sputtering.

Another effect of arrival rate ratio is the change in texture of the growing film (17). X-ray diffraction measurements of films deposited at four different R values show a pronounced change from the < 111 > texture prominent in vapor deposited films (R = 0) to a strong <200> texture in films deposited with an R value of 0.1. This change in texture is believed to be produced by channeling of ions. Deep channeling of ions distributes the energy over a larger volume of crystal and these grains are observed to be most likely to survive under film growth conditions (18).

Film densification has also been observed in these experiments on TiN as shown in Fig. 6. A TiN film was deposited under conditions such that part of the film was grown under IAD conditions, part was grown under PVD conditions and

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