Characterization of 100 MHz inductively coupled plasma (ICP) by comparison with 13.56 MHz ICP

Abstract

The effect of the excitation frequency on the dissociative process of the C4F8 gas was investigated by comparing a 100 MHz [very high-frequency (VHF)] inductively coupled plasma (ICP) with a 13.56 MHz [radio frequency (rf)] ICP. The same apparatus except for the wave generator and matching network was used for both ICPs in order to investigate the frequency effect as precisely as possible. The electron density and electron temperature in an Ar plasma were measured by using a Langmuir probe. From the dependence of the radial distribution of the Ne on the ICP source power, it was found that the rf ICP was produced in a cylindrical space under the coil area, while the VHF ICP was generated throughout the reactor. In C4F8/Ar plasma, the CFx (x=1, 2, 3) radical densities near the reactor wall were measured by using appearance mass spectrometry, and the F radical density was measured by using actinometry through the optical emission spectroscopy of Ar (750.4 nm) and F (703.7 nm). The degree of dissociation of the C4F8 in the VHF ICP was higher than that in the rf ICP, but the CF2/F density ratio in the VHF ICP was 3–5 times that in the rf ICP. This result indicates that the dissociation of high-molecule gases, including C4F8, proceeds more easily in the VHF ICP than in rf ICP and that there is much less dissociation of high-order radicals (e.g., CF2+e→CF+F+e) in VHF ICP than there is in rf ICP. Hence, VHF ICP suppresses F radical generation better than rf ICP.