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Hollow Cathode Plasma Sources – a wider operating pressure range than you might think

We 20190422_143455want to correct some recent disinformation in a journal article, the review paper J. Vac. Sci. Technol. A 37 (2019) 030902, on plasma ALD, appears to have mistakenly reported a very narrow operating range for our hollow cathode plasma sources of only 0.3 to 2 Torr. It is unclear what this range was based on as none of the references cited actually provide a pressure range. Apparently it is a simple misconception of the authors, presumably due to their inexperience with hollow cathode plasma sources. However, all our designs have had larger pressure ranges than those given in the review, and in fact there have been some exciting recent advances in our designs that have extended operation down to tens of milliTorr and perhaps even lower.

For clarification, the operating pressure range of our earliest hollow cathode design is provided in Japanese Journal of Applied Physics 51 (2012) 01AF02. The optimum pressure for that cathode – used for CVD work – was 1 Torr, however measurements presented there clearly show an operating range of at least 0.23 to 6.6 Torr for that very early design, though in fact it was probably much broader.

When designing for ALD systems, lower optimum pressure ranges have been required, so from about 2014, our designs have been optimised for operation between 0.3 and 1 Torr, though the full operating range of our barrel design plasma sources – such as our series 50 source, commonly used as an ICP replacement – have  a lower  limit of approximately 100 mTorr and an upper limit of > 5 Torrs. The picture below shows one of these sources at 120 mTorr with nitrogen at 100 watts of RF power.

Our newer designs, based on our large area hollow cathode plasma sources, go to even lower pressures, which we’ve now run to the base pressure of our test systems, to as low as 35 mTorr. A measurement system upgrade will be required to see just how low these sources can go, an example at ~50 mTorr is shown top left.

Check out the Meaglow website at or contact us at or +1 807 252 4391, for more information on our plasma products.


120 mTorr nitrogen 100 watts 0.0005 sec exposure

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