AVS 56th International Symposium & Exhibition
    Plasma Science and Technology Thursday Sessions
       Session PS-ThP

Paper PS-ThP10
Influence of Sheath on Measurement of Electron Density in Frequency Shift Probe and its Application to Measurement of Electron Temperature

Thursday, November 12, 2009, 6:00 pm, Room Hall 3

Session: Plasma Science Poster Session
Presenter: K. Nakamura, Chubu University, Japan
Authors: K. Nakamura, Chubu University, Japan
Q. Zhang, Chubu University, Japan
H. Sugai, Chubu University, Japan
Correspondent: Click to Email
In advanced materials processing for manufacturing LSI devices, improvement of accuracy and repeatability has been required to achieve high performance plasma processes. In general, temporal variation of plasma components is believed to be one of reasons for the problems, so it is important to develop technologies for accurate plasma control. We have developed a frequency shift probe as a novel in-situ plasma monitoring technology. The probe enables us to measure an electron density from variation of resonance frequency of the probe head, and the density measurement is possible under minimum disturbance to the processing plasma because of its plane structure. Furthermore, the probe is applicable to a reactive plasma such as fluorocarbon plasmas since the deposited polymer has no significant effects on the resonance frequency. When the resonance frequency of the probe varies from f0 (GHz) to fr (GHz) by producing the plasma with the electron density of ne, (1010 cm-3), the value of ne is given by ne=(fr2-f02)/0.81. However influences of a sheath formed around the probe have not been considered in the formula. In this work, sheath effects on the frequency shift probe were investigated based on finite difference time domain (FDTD) simulation, and it was examined how much influences on the measured density the sheath has. Furthermore, the frequency shift probe was tried to be applied to measurements of electron temperature using the sheath effects. As the sheath thickness increased, the resonance frequency decreased since effective permittivity of the media around the probe head decreased. 　 Such a decrease of the resonance frequency was observed regardless of the slit width, however its dependence on the sheath width was affected by the slit width, and it became significant as the slit width decreased. The sheath width is proportional to Debye length, so the resonance frequency of the frequency shift probe is a function of electron density ne and electron temperature Te. This means that resonance frequencies obtained in two frequency shift probes having different sheath dependence gives an unique solution of ne and Te. Actually, in a experiment using a plasma produced with 13.56 MHz RF power up to 400 W at an argon pressure of 3 mTorr, the present method derived 7.5x1010 cm-3 in ne and 4.8 eV in Te which comparatively showed good agreements with values measured by a Langmuir probe.