Y. Kawai, Y. Ueda, S. Hiejima, M. Shindo, H. Imaizumi*, S. Kawakami* and N. Ishii**

Interdisciplinary Graduate School of Engineering Sciences
Kyushu University, Kasuga, Fukuoka 816, Japan
Kawaiigh@mbox.nc.kyushu-u.ac.jp

* Tokyo Electron Tohoku Limited
** Tokyo Electron Limited

Fluorocarbon gases such as C4F8 have been widely used for SiO2 etching in RF and ECR plasmas, and some radicals, CF and CF2, have been measured. However, the plasma parameters such as the density and temperature of electrons which play an important role in understanding the production of both radicals and ions have not been measured. Here we attempted to measure ECR C4F8/Ar plasma parameters with a heated Langmuir probe and an 8 -mm microwave interferometer [1].

An ECR C4F8/Ar plasma was produced with a conventional TE11 mode microwave (2.45 GHz, 5 kW) [2]. The vacuum chamber was made of stainless steel with an inner diameter of 290 mm and a length of 1200 mm. The gas pressure was kept at 1.5 mTorr. The Langmuir probe made of a tungsten wire whose diameter of 0.1 mm was heated by passing a current to prevent deposition of carbon like films. Although the analysis of probe characterization in magnetized plasmas is generally limited, we adopted a thin probe theory since the ion Larmor radius was larger than the probe radius in this experiment. Thus, the ion density , ni, in the magnetized plasma was derived from the ion saturation current of the Langmuir probe curve. An 8 mm-microwave interferometer with convex lenses was also used to measure the electron density, ne, using homodyne detection.

The ion density, ni, and electron temperature, Te, were measured as a function of microwave power and gas ratio C4F8/Ar. In this case, dominant ions were assumed to be CF+ or C2F4+. The electron temperature was around 3-4 eV. It was found that when C4F8 is increased, the difference (ni-ne) increases, that is, there are much negative ions in the plasma. In fact, the negative ions amounted to about 50%. Furthermore, we attempted to estimate what dominant ion species is by using the Langmuir probe characteristics [3]. It was found that a dominant ion species is C2F4+ for the microwave power of 2 kW while both CF+ and C2F4+ become dominant for 2.7 kW.

References

[1] M. Morimoto et al, Jpn. J. Appl. Phys. 36 (1997) 4659.
[2] Y. Ueda and Y. Kawai, Appl. Phys. Lett., 71(1997) 2100.
[3] S. Hiejima et al, Proc. of the 15th Symposium on Plasma Processing (Hamamatsu, Japan, Jan.1998) 322.