Comparison of the operational performance of a compact electron cyclotron resonance plasma source at excitation frequencies of 2.45 GHz or 915 MHz

Abstract

This article presents the results of a study in which the performance of a compact coaxial electron cyclotron resonance plasma source for molecular-beam epitaxy has been experimentally evaluated and compared at the two microwave excitation frequencies of 2.45 GHz and 915 MHz. Diagnostic measurements, using Langmuir probes and a multigrid energy analyzer, are made at absorbed power levels varying from 80 to 250 W, at flow rates ranging from 2 to 30 sccm and the corresponding pressures of ∼10−5–10−4 Torr. It is seen that the plasma source is capable of producing high density discharges, about high 1010 to low 1011 cm−3, with excitation at either 915 MHz or 2.45 GHz. Also, at either frequency, ion energy distribution functions are relatively narrow, with full width at half-maxima of less than 20 eV for a typical operating condition. However, the average ion energy for a 915 MHz discharge (30–50 eV) is in general higher than the average ion energy of a 2.45 GHz discharge (15–25 eV), when operated at the same power and pressure conditions. As measured by the double Langmuir probe, the electron temperatures of the high energy tail are higher for 915 MHz excitation than similar electron temperatures of a 2.45 GHz excited discharge.