Introduction and motivation
The microfabrication process of wet etching is the most straightforward way to remove material from a wafer. A wafer is immersed in an acid and the acid etches films with which it comes into contact. Acids have the ability to selectively etch films. For instance, hydrofluoric acid can etch silicon dioxide without etching silicon. Because the acid is fluidic, layers are etched not only vertically, but also horizontally. This is best demonstrated with diagrams. Consider the following structure:
Poly | | Polysilicon
Now suppose this wafer is immersed in hydrofluoric acid (HF). Initially, the acid contacts only the silicon dioxide directly beneath the window in the polysilicon. However, as the oxide is etched, the HF etches oxide underneath the polysilicon. The lateral etching is known as undercutting. When the HF finally etches all the way to the silicon wafer, it will look something like this:*
* Actually the undercutting will produce circular sidewalls.
Poly | | Polysilicon
Oxide \ / Silicon dioxide
It is clear that wet-etched holes will always be larger than the holes in the etch mask. For integrated circuit fabrication, "small is better" and the undercutting is a disadvantage of wet etching. However, for MEMS (microelectromechanical systems) fabrication, wet etching can be very useful. Consider the diagram below that shows what happens when our wafer is immersed in HF for even longer.
Poly | | Polysilicon
The polysilicon layers have been released and have formed cantilevered beams. Wet etching has created free-moving structures on the wafer. While this example is simplistic, it is very similar to the method used to make acceleration sensors in airbags. In the example, motion of the polysilicon beams could be detected by the change in capacitance between the beams and the wafer. If you dream a little, you can imagine more than simple beams. Imagine miniature silicon motors (already reality). Imagine microrobots (already reality). The fabrication of movable structures using microfabrication techniques is called MEMS. MEMS and integration of MEMS with integrated circuits are very exciting areas of engineering research.
Specific wet etches
Because of the excitement about MEMS, researchers have amassed a huge amount of knowledge about the etch rates of various materials in different etchants. I will discuss only the etches most pertinent to integrated circuit fabrication.
Hydrofluoric acid is by far the best etchant for silicon dioxide since it does not etch silicon. Ammonium fluoride (NH4F) is often added to create a buffered hydrofluoric acid solution (BHF). There is a useful way to determine when HF has etched through oxide and reached silicon. Silicon dioxide is hydrophilic, so water adheres to it. Crystalline silicon is hydrophobic, and water is "repelled" by it. When the HF has reached silicon, "dewetted" spots can be observed on the wafer.
There are several ways to etch silicon. One way is to oxidize the silicon and then etch it in HF. The most common wet oxidizer is HNO3. A typical solution to etch polysilicon is 10HNO3:1HF:10H2O. This etch is isotropic, meaning that silicon is etched at an equal rate in all directions. Crystalline silicon can also be etched anisotropically by KOH, NaOH, and other echants. These etchants etch silicon far faster in some crystalline directions than in others. Anisotropic etching can make sharp peaks and grooves that are interesting for MEMS applications or for making AFM tips, but undesirable for integrated circuit purposes.
Silicon nitride is typically wet-etched in phosphoric acid (H3PO4). Phosphoric acid also etches photoresist, so it's by no means perfect. Phosphoric acid can also be used to etch aluminum.
Piranha--a mixture of 7 parts sulfuric acid to 3 parts hydrogen peroxide-- is a very common cleanup etchant. Piranha attacks organic material (i.e. photoresist), preparing a wafer for high-temperature steps such as thermal growth or chemical vapor deposition. The standard piranha dip is for 10 minutes at 120 °C.
For information about other microfabrication etching techniques, try the nodes integrated circuit, reactive ion etching, and sputtering. For a huge amount of information about etchants and etch rates, see http://www-bsac.eecs.berkeley.edu/db/etch.pdf.