During IC fabrication, the whole wafer is completely covered with a layer or multi-
layers of silicon dioxide, silicon nitride, or metal. Etching is the process of 
removing unwanted parts of these layers in order to form holes for diffusion, 
electrical interconnections, to name a few. Etching is a chemical process. Different 
kinds of etching techniques included wet chemical, electrochemical, pure plasma, 
reactive ion etching (RIE), and high-temperature vapor etching. Wet chemical 
etching is the oldest form available to IC manufacturing and the least expensive. 
It involves a beaker of etching solution and few beakers of rinsing water. The 
wafer is literally soaked in etching solutions then rinsed clean with water. The 
etching bath is temperature controlled and the solution is continuously being 
filtered and circulated. Hot deionized water is used in the rinsing cycle. No 
etching reactant should remain on the wafer after it is cleaned. Otherwise, this 
will result in defective chips. The time required for rinsing can be determined 
by monitoring the resisitivity of the deionized water bath.  The wafer is 
considered clean once the resisitivity of rinsing water returns to its previous 
value. A common etching solution is diluted HF. From monitoring the resistivity 
of rinsing water, it is one of the easiest to remove from the wafer. Nitric acid 
is next when it comes to ease of removal and sulfuric acid being the most 
difficult to remove. An advanced automated etching process involves multiple 
rinsing tanks. After the etchant is introduced, a robotic arm will transfer the 
wafer from tank to tank for rinsing. Cascading the tanks to allow water flow 
from one to the other minimizes the use of deionized water. Another approach to 
the etching-rinsing process is to simply dump the etchant rapidly and fill the 
tank with water. The step is repeated until the wafer is clean of etchant.