Re: How does a dry plasma etcher work?

Dear Cristina,
A dry plasma etcher works on the principle of taking a gas which contains 
reactive gaseous elements like Chlorine(Cl) or Fluorine(F) within its 
stable molecular structure, and making a gas plasma of this gas.  The 
plasma is created by putting the gas in a vacuum chamber at about 1 torr  
pressure, and making an electrical discharge within the gas.  The 
electrical discharge breaks apart the gas molecules releasing free 
Fluorine, or Chlorine, or other reactive gases which are in the original 
gas molecule.  These very reactive free atoms chemically react with the 
surface material being etched.

For example if the material to be etched is silicon, then free fluorine 
atoms can react with the silicon to make SiF4, which is a gas; and then the 
silicon that is etched is pumped away as a gas.  There are no liquids 
present.  That is why this is called dry etching.  In the same way aluminum 
can be etched with BCl3 gas plasma, where the free chlorine reacts to make 
AlCl3, which is a very volatile material and can be p[umped away as a gas 
if the surface is kept warm.   Similarly carbon can be dry etched by free 
oxygen atoms which are made in a plasma of O2, oxygen gas, and form CO2 
which is as gaseous product.

The design of the chamber to do dry plasma etching has wide variations 
depending upon what is being etched.  For semiconductor microchips, where 
this equipment is very widely used to etch the fine lines in chips, the 
necessity is to etch vertically down into the surface so that the resists 
mask pattern is very precisely reproduced in the etched surface.  Etching 
beneath the patterned resist mask is not permissible.  There are many 
variations of chamber design to accomplish this.

If you wish more information please inquire again and I will try to answer 
 your questions.  My email address is: rlbersin@ieee.org

R. Bersin