A microstrip-line microwave interferometer for monitoring of plasma density of processing plasmas

Abstract

Summary form only given. We developed a planar transmission-line microwave interferometer for monitoring of electron density for applications in process real-time feedback control of plasma based semiconductor fabrication tools, such plasma etchers or PECVDs. The principle of this technique is the same as the conventional microwave interferometers except that the sensing microwave propagates along a transmission line structure. In this study, the sensor was a microstrip transmission-line where microwave propagates at a phase velocity determined by the structure and the electron density of the surrounding plasma. Thus the variation of plasma density can be estimated from the phase shift of the transmitted microwave from one to the other end of the transmission-line. Compared to the conventional microwave interferometers where line-averaged plasma density is measured, the transmission-line type microwave sensor will be less susceptive to the interference caused by multi-passes reflection/refraction effect resulting from nonuniformity of the plasma density profiles. Therefore, it provides a measurement of higher sensitivity and wider dynamic range. In this work, an U-shaped transmission line with 3 mm in width and 86 mm in length, the quartz based substrate thickness is 3.5 mm, and the distance between input and output ports is 23 mm. The dispersion characteristics of this microstrip line immersed in a plasma of different electron densities was simulated by using a commercial tool, High Frequency Structure Simulation code (Ansoft HFSS), a full- wave electromagnetic simulator using the finite element method. The simulation results show that the phase shift increases gradually with the plasma electron density with a sensitivity 12 degree / 1010 cm3. Experimental demonstration has been performed with an inductively coupled plasma. The sensor was mounted on the inner wall of plasma chamber. Measurement results show that the dependence of electron density of plasma source RF power predicted by the sensor agrees with the Hairpin probe measurements.