6.5 Etching Techniques

6.5.1 General Remarks

After we have produced all kinds of layer, we must now proceed to the structure module of our basic process cycle. First we discuss etching techniques.
Lets see what it means to produce a structure by etching. Lets make, e.g., a contact hole in a somewhat advanced process (and do some of the follow-up processes for clarity).
What the stucture contains may look like this:
Etching a contact hole
Obviously, before you deposit the Ti/TiN diffusion barrier layer (and then the W, and so on), you must etch a hole through 3 oxide layers and an Si3N4 - and here we don't care why we have all those layers. (The right hand side of the picture shows a few things that can go wrong in the contact making process, but that shall not concern us at present).
There are some obvious requirements for the etching of this contact hole that also come up for most other etching processes.
1. You only want to etch straight down - not in the lateral direction. In other words, you want strongly anisotropic etching that only affects the bottom of the contact hole to be formed, but not the sidewalls (which are, after all, of the same material).
2. You want to stop as soon as you reach the Si substrate. Ideally, whatever you do for etching will not affect Si (or whatever material you want not to be affected). In other words, you want a large selectivity (= ratio of etch rates).
3. You also want reasonable etching rates (time is money), the ability to etch through several different layers in one process (as above), no damage of any kind (including rough surfaces) to the layer where you stop, and sometimes extreme geometries (e.g. when you etch a trench for a capacitor: 0,8 µm in diameter and 8 µm deep) - and you want perfect homogeneity and reproducibility all the time (e.g. all the about trenches on one 300 mm wafer containg 256 Mbit DRAMs must be identical to the ones on the other 500 - 1000 wafers you etch today, and to the thousands you etched before, or are going to etch in the future).
Lets face it: This is tough! There is no single technique that meets all the requirements for all situations.
Structure etching thus is almost always a search for the best compromise, and new etching techniques are introduced all the time.
Here we can only scratch at the surface and look at the two basic technologies in use: Chemical or wet etching and plasma or dry etching.

6.5.2 Chemical Etching

Chemical etching is simple: Find some (liquid) chemical that dissolves the layer to be etched, but that does not react with everything else.
Sometimes this works, sometimes it doesn`t. Hydrofluoric acid (HF), for example will dissolve SiO2, but not Si - so there is an etching solution for etching SiO2 with extreme selectivity to Si.
The other way around does not work: Whatever dissolves Si, will always dissolve SiO2, too. At best you may come up with an etchant that shows somewhat different etching rates, i.e. some (poor) selectivity.
Anyway, the thing to remember is: Chemical etchants, if existing, can provide extremely good selectivity and thus meet our second request from above.
How about the first request, anisotropy? Well, as you guessed: It is rotten, practically non-existent. A chemical etchant always dissolves the material it is in contact with, the forming of a contact hole would look like this:
Chemical structure etching
There is a simple and painful consequence: As soon as your feature size is about 2 µm or smaller, forget chemical structure etching.
Really? How about making the opening in the mask smaller, accounting for the increase in lateral dimensions?
You could do that - it would work. But it would be foolish: If you can make the opening smaller, you also want your features smaller. In real life, you put up a tremendous effort to make the contact hole opening as small as you can, and you sure like hell don't want to increase it by the structure etching!
Does that mean that there is no chemical etching in Si microelectronics? Far from it. There just is no chemical structure etching any more. But there are plenty of opportunities to use chemical etches (cf. the statistics to the 16 Mbit DRAM process). Lets list a few:
Etching off whole layers. Be it some sacrificial layer after it fulfilled its purpose, the photo resist, or simply all the CVD layers or thermal oxides which are automatically deposited on the wafer backside, too - they all must come off eventually and this is best done by wet chemistry.
Etching coarse structures, e.g. the opening in some protective layers to the large Al pads which are necessary for attaching a wire to the outside world.
Etching off unwanted native oxide on all Si or poly-Si layers that were exposed to air for more than about 10 min.
All cleaning steps may be considered to be an extreme form of chemical etching. Etching off about 1,8 nm of native oxide might be considered cleaning, and a cleaning step where nothing is changed at the surface, simply has no effect.
While these are not the exciting process modules, experienced process engineers know that this is where trouble lurks. Many factories have suffered large losses because the yield was down - due to some problem with wet chemistry.
A totally new field, just making it into production for some special applications, is electrochemical etching. A few amazing (and not yet well understood) things can be done that way; the link provides some samples.

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© H. Föll (Electronic Materials - Script)