In the etching and deposition steps in the production of semiconductor chips, plasma
processing is required for three main reasons. First, electrons are used to dissociate
the input gas into atoms. Second, the etch rate is greatly enhanced by ion bombardment,
which breaks the bonds in the first few monolayers of the surface, allowing the
etchant atoms, usually Cl or F, to combine with substrate atoms to form volatile
molecules. And third, most importantly, the electric field of the plasma sheath
straightens the orbits of the bombarding ions so that the etching is anisotropic,
allowing the creation of features approaching nanometer dimensions.
The plasma sources used in the semiconductor industry were originally developed
by trial and error, with little basic understanding of how they work. To achieve this
understanding, many challenging physics problems had to be solved. This chapter is
an introduction to the science of radiofrequency (rf ) plasma sources, which are by far
the most common. Sources operating at zero or other frequencies, such as 2.45 GHz
microwaves, lie outside our scope. Most rf sources use the 13.56 MHz industrial
standard frequency. Among these, there are three main types: (1) capacitively coupled
plasmas or CCPs, also called reactive ion etchers (RIEs); (2) inductively coupled
plasmas (ICPs), also called transformer coupled plasmas (TCPs); and (3) helicon
wave sources, which are new and can be called HWSs