Characterization of argon electron-cyclotron-resonance-heated mirror plasmas for materials processing applications.

Abstract

Electron cyclotron resonance (ECR) plasma sources have been used in both fusion and ion source applications. More recently, ECR sources have been developed for applications in plasma etching and deposition processes relevant to semiconductor manufacturing. The most commonly studied ECR reactors contain a magnetic field profile which can be varied from a condition in which it is monotonically decreasing from the plasma source region to the substrate, to one in which it is in a weak mirror configuration. The profile of the magnetic field and the shape of the resonance region have been found to have a significant effect on the plasmas produced in these reactors. The availability of several magnetic field configurations available on the Michigan Mirror Machine (MIMI) make it an ideal source to investigate these effects. For this experiment three configurations were studied: symmetric mirror, minimum-B (magnetic well), and an asymmetric mirror. A gridded energy analyzer has been implemented on MIMI to measure the energy distribution of Argon ions in the endloss plasma flowing from the mirror. Langmuir probes, microwave interferometry, and optical spectroscopy have also been used to monitor the core plasma parameters (electron density, temperature, and ambipolar potential) and their dependencies upon microwave power, pressure, and magnetic field configuration have been determined. An additional comparison between two microwave heating frequencies (2.45 GHz and 7.43 GHz) has also been made. At a microwave heating frequency of 7.43 GHz, the plasma density was found to be generally insensitive to the microwave power and pressure, remaining roughly 2 $\times$ 10$\sp{11}$ cm$\sp{-3}$. Better microwave coupling was obtained for the 2.45 GHz heating frequency by utilizing an alumina window at the end of the microwave waveguide. This resulted in plasma densities up to 2 $\times$ 10$\sp{12}$ cm$\sp{-3}$ at the midplane and 5 $\times$ 10$\sp{12}$ cm$\sp{-3}$ at the ECR zone for the symmetric mirror configuration. Operation of the minimum-B magnetic field configuration was found to increase the plasma stability and provide densities up to 1 $\times$ 10$\sp{12}$ cm$\sp{-3}$. The asymmetric mirror configuration yielded the lowest plasma densities ranging from 2 $\times$ 10$\sp{11}$ cm$\sp{-3}$ to 1 $\times$ 10$\sp{12}$ cm$\sp{-3}$ as the mirror strength increased.