Hello,
I am trying to structure p-GaN-layers by RIE-etching. Therefor I use a 200 nm Al/ 200 nm Ni etching mask. However, after the etching process (ICP plasma, I use Cl and N2 suring the etching process) I see a very strong "balling-up" effect. There is almost no adhesion of the aluminium to the GaN after the process. Normally I would think that this happens under high temperature treatment. The etching process should not heat up the sample very much (only 2 minute process @ 35°C). Has anyone seen something similar? Could it be due to surface heating due to the plasma?
Thank you
Hello,
I never experienced such a problem but indeed it looks to be connected with the sample heating. Do you use a small piece or a whole wafer? In case this is a small piece indeed - is there any 'bonding' between it and the wafer table, something like vacuum oil? In case this is a wafer - is this cooled from the backside by He flow?
Finally, what are plasma discharge powers you use?..
Also: what is the need for a bilayer mask? 200 nm of Ni alone would be excellent for any reasonable etch depths. I can suppose that Al underlayer serves for further removing of Ni top layer, through an Al wet etching.
Hi Victor,
Thank you for your support. I use small samples from a 4" wafer (about ~1-2cm^2). There is no wafer bonding. Just the plain wafer on a Si/SiO2 wafer underneath as a carrier. It is cooled by He flow though. So the temperature is regulated to be 35°C. That is true for the Si/SiO2 wafer. The actual temperature on the surface of the GaN is unknown of course.
I use 300W RF and 150W ICP as plasma sources.
I want to use Al as a mirror contact to GaN. So in the end I need a direct GaN/Al junction. The Ni is the actual etching mask to protect the Al contact (e.g. from oxidation). And Ni should be easier to wire bond with Au wire.
Hi Jan,
Thanks for the details. It became clear to me that the actual temperature of your small piece at these etch conditions is incredibly high. The sample bonding with oil is absolutely necessary for you, otherwise the piece will heat up to a few hundreds degrees above the carrier wafer temperature during the two minutes etch at 300 Watts of RF power!
We typically use a small drop of Santovac 5P to make a thermal contact, this is then easily wiped out by IPA from both the sample and the carrier wafer.
In case the use of any oil is totally prohibited for your plasma tool, you could either try to reduce the RF power and/or use short 'etch pulses' repeated, e.g. 5-10 seconds of etch and then 30-60 seconds of no-discharge waiting time to enable the sample cooling. This will greatly extend the total process time of course, also the tool should be able to start the plasma reliably and repeatably, without noticeable transient periods.
Now I understand the need for Al in your mask, thanks. Good luck!
Thank you very much. Very good feedback there. I will check if vacuumoil is an option. The pulsing seems to make a lot of sense. I will definately try this. process time shouldn't be a limiting factor so pulsing and/or reducing plasma power seems to be the most reasonable approach to deal with the adhesion problem. Thank you very much!
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