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 Ion beam assisted deposition

An ion source can be operated during a deposition process to provide extremely beneficial effects on thin film nucleation and packing density. However, the ion source used for this operation needs to have specific design and performance qualities in order to obtain optimum process results.

When choosing an ion source for IBAD, there are a number of important considerations, which are easily overlooked. Firstly, and as previously mentioned, the ion source must be physically compact enough to permit unobtrusive installation into the thin film deposition system without interference to other process accessories, mechanically, electrically or magnetically. The ion source also needs to have a beam of sufficient diameter to adequately cover the required area at the substrate.

The importance of a high current ion beam at low energy

The ion source must be able to deliver the required level of ion beam current density at the substrate, which roughly equates to an arrival rate of one ion per atom of deposited material. Hence, the higher the deposition rate, the greater the ion current density required.

This high ion current needs to be delivered at lower ion energies. As the ion energy approaches and exceeds 100eV many materials will start to sputter (including chamber fixturing). This in turn, causes a potential contamination risk to the deposited film due to back sputtering. Use of too high an ion energy can also cause film growth problems and be deleterious to optical thin film properties.

Thin film contamination issues

It is also important that the ion source itself, does not contribute to thin film contamination. Ion sources can present a potential contamination risk to a sensitive thin film deposition process due to the intense plasmas present within the source. Sputtering of poorly shielded internal components. Use of incorrect materials, erosion of tungsten filaments and grid optics can also be a problem. For this reason, recent ion source designs have included the capability to rapidly change the material of key exposed source components, in modular form, to those, which match the native materials used in the deposition process, eg. titanium, and tantalum.

Ion source reliability during operation

Another important consideration is the ability of the ion source to operate reliably for extended periods with reactive gases. The use of oxygen can cause the formation of oxide layers on anode surfaces creating instability of operation and failure to initiate a plasma discharge within the source. Graphite components also need to be avoided due to their decomposition in the presence of oxygen. If the ion source is fitted with grid optics, these must be changed to a material such as molybdenum. However, this has a higher sputter rate, and grid erosion in the path of the ion beam can create a potential thin film contamination risk.

Thermionic filament vs. hollow cathode electron source

Whilst dealing with the issues of reliable ion source operation, the discussion often arises as to which is the most suitable – filament or hollow cathode. Whilst there are no fixed rules on this, it is better that this question is dealt with from a process point of view.

Thin film deposition cycles of relatively short duration, ie. Ophthalmic coating and similar applications are likely to be adequately served by use of a thermionic filament. The filament is inexpensive and simply replaced. Depending on usage and process, lifetimes up to 10 hours may be normally expected when used with an EH source as the filament design is particularly robust and not exposed to a high-energy ion beam. However, this is not the case with a gridded ion source with an immersed filament neutraliser.

Longer deposition cycles, as may be used in technical optical coating and which can last many hours, would normally benefit from use of a hollow cathode, which offers a high purity process and an operational lifetime of up to 1000 hours before maintenance is required.

However, it should be noted that the hollow cathode electron source does require a specific operational procedure in order to ensure that the internal electron emitter surface does not become contaminated. This normally involves initiating an argon gas purge before venting the deposition chamber to atmosphere, and also prior to pumpdown, which adds a little time to the overall deposition cycle. Although this is not of significance during a long process, it may be, if high throughput production, with minimum deposition cycle time is of critical importance.