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 Ion beam systems FAQ

Ion Beam Technology and the associated systems in which it is employed is a rather complex science. There are many variables to consider before you choose a suitable product. The process, both now and future anticipated, must be the primary concern.

Supposedly an RF Ion Source will allow greater process flexibility?

Not necessarily, unless specifically ‘filament-less’ operation is required and as such, would be used to avoid the need for filament changing during long runs or when use is required with reactive gas species.

Explain how substrates are cooled and temperature limitations?

The substrate stage is normally fed with cooling water at 20 deg.C. This will keep substrates below 50 deg.C during processing. There are no water to vacuum seals utilised as the design incorporates high quality ‘ferrofluidic’ type feedthroughs. If lower temperatures are required, ie. < 50 deg.C a closed circuit refrigeration system is available as an option. This also provides the capability of heating substrates to approx. 80 deg.C if required.

Is a temperature change required for venting/loading?

No – after processing, the temperature of the substrates will be above the dew point and not hot enough to cause oxidation. It is our normal suggestion that the system should be vented to atmosphere with dry N2 gas. If the system is being operated in excessively humid conditions, then a hot/cold water changeover facility for the chamber can be provided as an option.

‘Mean time for maintenance between DC and RF Ion Sources?’

This depends on what is considered to be ‘maintenance’. Neither DC or RF ion sources, can be promoted as being maintenance free. As shown in the table above, a DC source would be expected to require a change of filament after approximately 50 hours. The most prone filament is the neutraliser which is immersed in the ion beam and which may be simply and rapidly changed. The KDC Source is equipped with two switchable cathode filaments, so there is a spare available. Although simple to change, these are more difficult to rapidly access. The DC source is generally very tolerant to milling conducting, semi-conducting and dielectric materials and insulators are well shielded by design. Nevertheless, periodic cleaning of the key parts will be required – this is wholly process dependent.

The RF source in comparison, is based on a ‘filament-less’ discharge, but still requires a neutraliser. Contamination of the source plasma chamber walls by back sputtered deposits from the process, typically form a conducting or semi-conducting ‘RFshield’ preventing plasma discharge, this then will require cleaning. There are a number of different designs of neutraliser which may be used with an RF ion source – all of these require maintenance, and some are reliant on being fed with a supply of very high purity argon and associated gas handling to achieve long lifetime. Although operating under a shroud of inert gas which contributes to long operational times before failure, all are subject to ‘poisoning’ or oxidation reducing electron emission which eventually result in failure and process interruption while repairing the defective unit.

The LFN2000 neutraliser provided with the RFICP Source has a lifetime of up to 200 hours but is also provided with the advantage of a unique ‘plug-in’ design. In the event of failure, this may be instantly replaced with a spare unit body (optional) while the defective unit may be bench serviced for rapid re-use. The RFICP Source, unlike most RF ion sources currently commercially available which operate at 13.56 MHz, utilises the more recently developed trend of using an RF frequency of 2.0 MHz.

The major significance of this, is that an ion source operating at a frequency of 2.0 MHz, provides a great deal of immunity to the thickness of back sputtered deposits on the plasma chamber wall blocking the RF signal.

This occurs due to an effect known as ‘RF Skin Depth Attenuation’. The period between maintenance being required (cleaning), is therefore very substantially increased. However, In order to simply quantify the benefits of this, in a given process, when repeatedly milling the same amount of the identical material, the same number of substrates and same number of process runs, the RFICP Source will operate for an approximately 3.5 – 4 times longer period than 13.56 MHz RF powered ion sources before cleaning of the discharge chamber is required. As an option, a spare discharge chamber is normally offered to enable a rapid changeover and so minimise system down time whilst the contaminated unit is being cleaned. In addition, the impedance matching of the RFICP Source is achieved electronically which is a much faster and more consistent method than that used by typical mechanically actuated matching networks.

What is the best size ion source for obtaining +/- 3% uniformity or better, on a batch of 3x3” dia. wafers?

The simple answer to this question, generally, would be to recommend the use of the largest ion source possible within budget. However, there are also many variables apart from this:

Ion Source design and history – Can manufacturers claims be verified? Are supporting beam profiles available showing a range of operational parameters matching those to be used in the process? How many sources are in the field?

Ion Source to substrate distance – Generally, the further away the better – but at the cost of reducing milling rates. Grid design is also an important factor to consider with this.

Grid Optic design – Self aligning grids are without doubt, always the most desirable, they offer consistent uniformity and repeatability run to run, and also less grid wear, (which tend to be high cost replacement items). Thermal stability is also vital as ion source grids get extremely hot during operation. Grids which suffer ‘movement’ during operation will destroy any hopes of process repeatability and uniformity. Collimated, defocused, and focused grid designs should be available and interchangeable to suit future process requirements. However, it should be noted, that with the correct grid design coupled with the corresponding source to substrate distance, can often be a viable and more economical alternative to using a larger ion source. Ion Beam process parameters used – Depending on both ion source and grid optic design, the beam parameters used, (ie. beam current, voltage and accelerator voltage), can all greatly affect uniformity. It is pointless being able to provide uniform milling when the resultant low milling rates will produce an unacceptably long process time – or conversely, causing damage to sensitive substrates if too high a beam voltage has to be used when trying to achieve a higher milling rate to increase throughput.

Substrate Stage design – Rotation, precise and repeatable angular adjustment are essential, as also is the ability to accurately position the substrates centred or ‘off-axis’ of the ion source for uniformity and control of substrate vertical wall angle features. An effective substrate cooling interface is essential to provide uniform cooling over the substrate area.

System Geometry – It is essential that the system is designed from the outset to meet the required process requirements. To try to force the process to ‘fit into’ a pre-packaged system design rarely will give the process results required and usually there will need to be some process compromise.

Uniformity of the material or thin films to be removed - When specifying the required etch uniformity for an ion source or milling system, it is not unusual for very tight limits to be specified by the intended user. However, in practice it has often been observed that the uniformity of deposited thin films is far worse than that being expected from an Ion Beam Milling System. It is worth noting that very tight uniformity limits carry cost implications in terms of Ion Source size and system design.

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