Plasma parameters in very high frequency helium and argon plasmas at atmospheric pressure

Abstract

A simple procedure has been proposed to estimate atmospheric-pressure (AP) plasma parameters based on the electric field simulation of a very-high-frequency plasma system including the impedance matching unit. The capacitively coupled plasma is generated between metal electrodes with a narrow gap, where the standard Langmuir probe method cannot be applied. The amplitudes of current density (J0) and voltage (V0) between the electrodes are determined by the three-dimensional computer simulation of the system in the impedance matched state using two experimental parameters (i.e., the separations between two parallel-plate capacitors) in the matching unit. Approximate analytical solutions relating the central electron density (n0) and the sheath thickness to J0 and V0 are derived based on a simplified inhomogeneous plasma model with collisional sheaths. The average electron temperature Te is estimated using a power balance relation between the total powers absorbed and lost in the ionizing plasma. The results on power dependences of n0 and Te show reasonable agreement with the predictions by the particle and power balance relations. In the case of AP plasma of Ar or He mixed with impurity or process gas molecules, it is impossible to determine Te. However, n0 can be obtained and the collisional energy loss per electron-ion pair creation (ϵc) can be estimated, which brings certain information on the change of plasma chemistry. Since the proposed procedure is simple and non-intrusive, it might become a useful tool for discussing AP plasma properties in process developments.