Following my previous blog post I hope you have created a good etch mask…do not get me started again on masks…so what else do we need to consider? Well, when I was conducting research for my PhD, my supervisor said the first thing to do every day is to make a small sacrifice to the plasma gods….I think we can do better than that.
Plasma chemistry is next, correctly choosing this can be the difference between an etch that lifts your device to great heights or one that consigns it to the bin.
An etch process always consists of three elements, each with its own advantages and disadvantages:
- Physical component where the surface is bombarded by high energy ions and material is smashed off:Very directional, smooth surface
Slow, low selectivity to mask, high energy ions can introduce damage
- Purely chemical component where elements in the plasma combine with those on the surface to form compounds which are intrinsically volatile and evaporate from the surface:Low damage, highly selective, fast
Will etch in all directions at the same time, possible rough surface
- Ion assisted component where the compounds formed on the surface are removed by the bombarding ions but at typically lower energies than the pure physical etch:Low damage, selective, fast, directional
The gas you choose to create your plasma will dictate which of these components dominate in your process. Choose an inert gas such as Argon (Ar) and you will get physical etching only. A gas such as Sulfur hexafluoride (SF6) will etch Silicon (Si) very nicely in a chemical way but unless controlled will give an undercut profile. In order to choose your gas take a look at the compounds it forms with your sample and what temperatures these become volatile, as this will tell you how they will interact with your wafer. Then, using combinations of gases can give more of one style of etching than another allowing control over the selectivity, rate and profile.
Well to some degree anyway! Clearly this is not the whole story, the balance between the components is also affected by parameters such as power, pressure and temperature….back to those sacrifices for the plasma gods? Maybe for now, until my next blog post where I reveal more tips on plasma etch.