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HiTUS deposition explaned


Dr Peter Hockley
March 2017


A sputter deposition system using the PQL ‘HiTUS’ technology essentially differs from a conventional sputter deposition system in that the target is not itself required to generate the gas plasma required for the sputtering process. Instead an attached remote Plasma Source (the PLS) separately generates a magnetised plasma which is guided to the target by a DC electromagnetic field.


The PLS is able to generate a vacuum gas plasma over a wide process pressure range independently of the target condition or type and so a much wider range of process options and target materials are usable than is the case with conventional sputtering processes. The PLS is intrinsically compatible with UHV and reactive process environments; in common with conventional sputter processes, argon is the usual primary gas used for sputter coating, though oxygen, nitrogen, hydrogen, helium, carbon tetrafluoride and silane have all been shown to work effectively with the PLS, either separately or in combination.


Visually, the plasma source appears to produce a ‘tube’ of brightly glowing gas plasma of colour and intensity dependent on the process gases used and the PLS power – typically bright purple-blue for high power operation with argon gas. This is directed using DC electromagnets to terminate over the selected sputter target area as shown below.




Interior View of the Vacuum Chamber of a Typical S500 during Sputter Coating Process


The sputter targets are usually circular planar of 5cm to 20cm diameter (depending on chosen system configuration) and 6mm thickness. Sputter targets can be metallic, semiconducting or insulating, and require the use of DC, pulsed DC or AC electrical bias as appropriate, the latter most usually at RF frequency.


As the plasma itself does not induce sputtering, multiple targets may be mounted on a single, rotatable circular planar holder and individually indexed into an aperture in a dark shield that otherwise blocks the plasma and thereby prevents sputtering from the non-selected targets. Multiple target units generally use 5cm to 10cm diameter circular targets, depending on system size and required target numbers (typically 4 to 6).


As in a conventional sputter system, targets are required to be negatively biased in order for the sputter coating process to occur. However, as the target is not required to strike or maintain the plasma in any way, usable sputtering rates can generally be obtained for negative bias voltages as low as 50 to 80V. Above about 100V negative bias, the target current is essentially independent of bias voltage, being set by the remote plasma RF power, as shown below, and the system operating pressure. This allows an unusually wide range of process options to be selected for a given target power (hence sputter rate), providing new opportunities for fine tuning coating properties.



















Example Dependence of Sputter Target Current on Target Voltage and Remote Plasma Source RF Power.



A HiTUS system may be run at very low RF power and magnetic field strength to produce a low plasma density (e.g. less than 1012 cm-3), up to maximum RF power and magnetic field settings to produce a very high plasma density (e.g. 5x1013 cm-3). Independently (subject to target power supply limitations) the target may be voltage biased to just above the sputtering threshold (for very slow deposition) to maximum bias without significantly changing the plasma density. Using these techniques, sputter deposition rates for a set target / system configuration can be controllably varied over more than 3 orders of magnitude within a deposition cycle, allowing very accurate, optimally fast layer deposition to be achieved.


The magnetised plasma itself also interacts with the substrate coating process in an unusual and beneficial manner compared to conventional sputter processes. Essentially an optimal balance of plasma density and coating rate may be set to deliver a low energy ‘plasma assist’ to the deposition process without the need for substrate bias, beneficially impacting the thin film coating properties or enhancing reactive deposition processes. Additionally, the process is highly efficient in achieving improved conformal coating and/or coating modification if substrate bias is used.


In summary, by freeing the sputter target of the need to generate the required gas plasma, a HiTUS system provides new process capability for sputter coating process technology, thereby enabling the realisation of new thin film based structures and products.  Very high deposition rates and highly densified, high performance thin film coatings may be readily achieved with the PQL plasma source through appropriate adjustment of the enhanced range of process parameters allowed by the remote magnetised plasma generation scheme.


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