The Basic Principles of Plasma Diagnostics

Plasma are one of the most complex and useful industrial processes in the manufacture of advanced products. They are used to etch or deposit layers on a surface, or to modify the surface chemistry to increase the value of a material. They are non linear so that small changes in the input parameters can cause large fluctuations in the output of the plasma process parameters such as etch rate, film thickness or film quality. Often the cause and effect of changes in the process are difficult to understand and plasma diagnostics combined with modelling are becoming increasingly important in helping to understand what is happening and to try and better control the plasma process.

There are a wide variety of plasma diagnostics commonly used in industry and research. The most advanced form of plasma diagnostics occur in the area of fusion plasma. Traditional fusion plasma are scientific projects and are run like physics experiments rather than the normal industrial processes. This means that they are more expensive, but it has given time to scientist to use plasma diagnostics to monitor it and develop complex models of how the fusion plasma functions in more depth. This understanding has led to better control of the plasma. In fusion plasma it is routine to come across non-linear control strategies implemented based on a range of plasma diagnostics.

It is expected that as the complexities of industrial plasma increases there will be a move towards better control of the plasma based on the relevant plasma diagnostics. The most common plasma diagnostics in industrial applications are optical based systems. These optical systems are relatively inexpensive and sit outside the chamber and do not disturb the process. The most common optical system is OES (optical emission spectroscopy) and it can be used to monitor the chemical content of the plasma process. It is very useful in applications such as process end-point in etch. When the plasma etch is finished the gas composition of the plasma changes and this is detected by a change in the emission of certain lines from the plasma.

Electrical or electrostatic probes monitor the current as a function of voltage on a probe inserted in the plasma. These probes are very important in research to characterise it but have limited use in industrial plasma as the probes are easily coated and such coatings will affect the quality of the measurements. Finally mass probes based on quadrupole detectors can be used to monitor the species within a type. These systems require complex ion optics to guide the ions out of the plasma and measure the mass of ions to identify the important species. These systems have very limited use in industrial plasma as the systems are expensive and bulky.

However, advances in plasma diagnostics in all three areas, optical, mass and electrical are continually being made it is reasonable to expect that in the future more and more industrial systems will incorporate an increasing level of control and understanding.