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 Ion beam pre-cleaning

Ion beam pre-cleaning is an extremely effective and simple method of pre-cleaning substrates ‘in situ’ in the vacuum system. It is particularly effective when carried out immediately prior to deposition and has many advantages over other ‘in situ’ pre-cleaning techniques.

The ability to carry out effective substrate pre-cleaning prior to a deposition process is one of the most common problem areas. A majority of those involved in thin film deposition processes will have at some time encountered problems directly attributed to surface contamination, including loss of film adhesion, poor optical properties, and contact resistance issues.

Whilst various wet cleaning techniques and care in substrate handling procedures may be employed, having available an efficient method of performing additional cleaning in the vacuum chamber is an important option.

Problems with conventional DC/RF plasma pre-cleaning

DC and RF Plasma discharges for many years have been an accepted method of pre-cleaning substrates prior to a deposition process. The former, primarily being used with thermal evaporation processes, and the latter, with magnetron sputtering.

Both of these methods need to be engineered into the process chamber using specific electrode geometry in order to create a plasma discharge. In the case of RF, the engineering required is both complex and costly, and the plasma often of limited efficiency due to substrate holder geometry and chamber electrical capacitance effects.

However, both methods have two things in common, firstly, a relatively high pressure is required for their operation which is typically not compatible with that required for the deposition process, and secondly, a large degree of ‘plasma spread’ is always present and which is capable of releasing substantial levels of gases and contaminants from process chamber walls and fixturing where exposed to the plasma.

Advantages of ion beam pre-cleaning

An Ion Source is an extremely efficient plasma generator, not only able to operate at low pressures, but also over a range of pressures, which are compatible with thermal evaporation, e-beam and magnetron sputtering processes.

An ion source also produces a ‘confined and directed plasma’ in the form of an ion beam, with user selectable controlled ion energy and current density. This allows the pre-cleaning activity to be confined specifically to the substrate area and adjusted in intensity to suit process needs, whether this may be for removal of contaminants, water vapour and volatiles from the substrate surface, or physical removal of 30–50 angstroms of native oxide.

Unlike DC and RF ‘diffuse’ plasma cleaning processes mentioned previously, which can scatter material and cause recontamination, it is important to note that in the case of Ion Beam Pre-cleaning, any contaminants and sputtered material removed from the substrate, is ejected at normal incidence, and away from the substrate surface.

Thin film adhesion benefits

In addition to the Ion Beam Substrate Pre-cleaning mentioned previously, due to its flexibility of operation, an ion source can also be an extremely powerful process aid where adhesion of the thin film to the substrate is of importance.

Traditionally, substrate pre-cleaning followed by subsequent thin film deposition has been treated as two separate process steps. The introduction of an ion source offers the capability to ‘overlap’ these process steps by permitting deposition to commence while the ion beam pre-cleaning process is still operational. This has the advantage of enabling the initial seed layer of the thin film material, to be deposited on what is essentially, an atomically clean substrate surface.

However, if required, further adhesion improvements may be achieved by increasing the ion energy at this stage to encourage low-level implantation of the thin film seed layer into the substrate surface. Further adjustments may be made to deposition rate and ion beam parameters, allowing the ion beam irradiation to be phased out, or alternatively, continuing during the deposition process to perform that which is now well known as Ion Assist, or Ion Beam Assisted Deposition (IBAD).

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