Microtechnology -- Material Etching
This page intends to present the different major techniques used in microfabrication to etch a material so that the layer you've deposited will let the shape you wanted appear. Etching is a large enough subject to write a whole encyclopedia about it. I just want to show the main differences between techniques, as well dry etching as wet etching.
Etching a material: What must we think about?
As I've said, surface engineering consists in depositing material layers that cover the whole substrate, and then remove it so that we keep only the pattern we want.
This is definitely not magic, so, to etch only the material we want to remove, we must use a mask!
I won't discuss too much about the making of masks, because here, it would be a never ending explanation. A mask is a material layer processed so that the etching of your target will give you the patterns you want.
At the beginning, the first pattern you get on a substrate is given by lithography. This gives you a polymer mask. For some etching processes, polymers don't make good masks. So you first process another material that will become your real mask, and then you etch your target.
Complicated? OK! Let's see an exemple:
I need to make a TMAH etching. TMAH is a chemical product. I want to transfer my pattern to the silicon, that is the target material. A lithography could give me a polymer layer with the desired shape. The problem is that TMAH is able to remove completely the polymer!! I can't use it as a mask. So, I use silicon dioxide as a mask. I first make an oxide layer above silicon, on which I make my lithography. Then I etch the silicon dioxide so that it gets the pattern I want to transfer. Finally, I remove polymer and put the substrate in TMAH. TMAH etches very very slowly silicon dioxide, so my mask is efficient. Once the etching of silicon is over, I can remove oxide with... another etching step!
This shows a very important point in etching: selectivity!!
The term «etching» is used because the processes really etch materials. This involves physical and/or chemical reactions between an etchant, and the etched material. Of course, chemistry helps us guessing which kind of etchant will be useful to etch a certain kind of material. One of the main problem with etching, is that except for the very first layer, we would like to avoid the etchant to etch the patterns already processed. This is where we look for the selectiviy of the etching between the materials.
Now, considering there are a lot of different materials involved in microtechnology, there are a lof of techniques to etch them, you can guess people could write books just with etching ratios, to find the best possible selectivity for one etching!
Still thinking about masks? Now let's think about a very simple physics principle:
If molecules in a liquid react with the material, there is no reason for them to react more with the bottom of the substrate rather than the side of the hole you've etched...
This is called underetching. You etch under the sides of your mask. This is often a limit to the technology. If you cannot control the underetching, you cannot use the smallest pattern you've made with your lithography, but a slightly larger one. There are other reasons to like it and other phenomenas to consider in underetching study. We will see some of them in appropriate time in thoses pages.
There are two process classes to etch a material:
- Wet etching: in which we use molecules in water as an etchant. The susbtrate is immersed into the solution during the necessary time.
- Dry etching: we use other ways of getting a chemical and/or physical reaction. This is the domain of plasmas etching
Each of them brings a lot of possibilities and counterparts. Each of them is a very large subject of discussion!! So don't compare them directly, just read through their pages!