Plasma etching can be a difficult, complex procedure but by following a few simple rules you can greatly increase the chance of achieving the results you need.
I would challenge anyone who has ever performed plasma etching to say with all honestly, hand on heart, that they have never ever had any problems with their process.
So what can you do to ensure that you get that smooth surface and low damage that your device needs? Clearly, buying an etch tool from Oxford Instruments Plasma Technology is step one…and with our lifetime process support, one step is all that’s needed!
However, for people who want to have the fun of developing their own process here are a few tips to help you out.
Mask, Mask, Mask
The mask is key and here are some rules to follow…
Do not come to my etch tool with:
- A mask that is wavy and expect a straight line in your sample. Every little kink in your mask will be replicated in your feature. Check your feature using a SEM to make sure it is smooth to the degree that you need your feature to be.
- A mask that is sloped and expect a feature that is vertical. The mask will also etch in the process and if it is thin at one edge this will etch away to reveal the material underneath. Once this is exposed this starts to etch and you will get a slope in your feature. This can be steep or shallow, only at the top or a portion of the way down but it can spoil your results.
- A mask that is too thin. The mask is used up in the process and if the mask is too thin it may not be able to reach the etch depth you require. A good rule of thumb is use twice the minimum mask that your selectivity (Selectivity is Sample Etch Rate/Mask Etch Rate) indicates to reach the target depth. Even a vertical mask develops a slope as it is etched so by having twice the amount of mask you can reduce the chance of having a slope in your feature.
- A mask that has not been fully removed in the areas you need to etch. These areas of mask residue will stop etching and if they do that, even for a brief moment, they will cause the surface to become ugly and roughened. There are many ways to help improve a dirty or partially patterned surface but knowing you have one to start with is essential.
- A temperature sensitive mask such as Photoresist (PR) and expect to be able to use high etch powers without cooling the sample. Plasma etching can increase the temperature of a sample to above 2000C in less than a minute, if high plasma densities (needed for high rates) are used. If no cooling is used, PR can burn and a burnt mask gives an awful profile which is difficult to remove. If you want high rates or have a tough sample and still want to use PR mask then clamping of the sample and active cooling is generally required. This is standard operating procedure with many etch tools but knowing you have to and running the tool in an appropriate way stops a lot of heartache.
These are some of the tips relating to creating a good mask. Next time I will talk about choosing the correct plasma chemistry. Watch this space!