Development of a ridged microstrip microwave interferometer for plasma electron density measurements

Abstract

Here we report the development of a microwave interferometer based on a ridged microstrip transmission line structure for the monitoring of plasma density in plasma processing tools. A special ridged shaped microstrip structure with a quartz dielectric is adopted for yielding a large phase shift of the microwave, and thus a higher sensitivity of the interferometer. During operation, the plasma density sensor is installed on the chamber wall where the microstrip transmission line is immersed in plasma and a microwave is launched from one end of the line and exits through the other end. As in conventional microwave interferometers, the plasma density is determined by the phase shift of the microwave propagating through the transmission line. 3D electromagnetic numerical simulations, where plasma is treated as a dielectric medium having a plasma permittivity determined by plasma density and microwave frequency, were employed to determine the phase shift/plasma density relation of this sensor. The sensor is designed to operate at 2.4 GHz microwave frequency, with a compact size and materials that are compatible with most plasma processing tools. Measurement results show that plasma density measured by the sensor, although placed at the chamber wall, does reflect the variations of the plasma density near the chamber center. In the real-time plasma etch process, the dependence of plasma densities on source powers and pressures measured by the sensor is also consistent with the results of ion current on the wafer electrode obtained from an impedance meter.

Local pressure calibration method of inductively coupled plasma generator based on laser Thomson scattering measurement

“…However, this comparison depends on accurate and reliable measurements of electron density. At present, the commonly used measurement methods for ICP electron density include Langmuir probe method 14 , emission spectroscopy 15 , microwave interferometry 16 , laser interferometry 17 and laser Thomson scattering (TS) measurement 18 . Among these techniques for measuring plasma electron density, the TS can measure the electron density in a given region without disturbing the plasma, and has very high spatial and temporal resolution 19 .…”

Electron density in amplitude modulated microwave atmospheric plasma jet as determined from microwave interferometry and emission spectroscopy

“…to objects with dimensions comparable to the wavelength. Recent research employs all sorts of advanced approaches, including heterodyne detection [33][34][35][36], dual lasers [37], holographic interferometer [38], transmission line design [39], one-port microwave interferometer [36,40], combined techniques [41], radio frequencies [42], harmonics [43], dielectric waveguides [44][45][46], and micro-strip line [47].…”