Wave-cutoff probe: overview and achievements

Through the dispersion relation of electromagnetic waves propagating into un-magnetized plasma or ordinary wave in magnetized plasma: ω² = ω_pe² + c²k²  where ω², ω_pe², c, and k are the wave frequency, the plasma frequency, the speed of light, and the wave number, the wave cutoff frequency becomes the plasma frequency as ω_cutoff = ω_pe = (n_ee²/ε₀m)^0.5 when k = zero. Thus, we can obtain the absolute electron density from the measurement of the ω_cutoff. This method is called the wave-cutoff probe [1]. In this talk, we present research overview and achievements on the wave-cutoff probe, as follows. First, theory and apparatus of the wave-cutoff probe method are introduced. Second, various technical methods, such as conventional wave-cutoff, phase resolved wave-cutoff, and Fourier wave-cutoff are presented [2]. Third, a method measuring electron temperature, as well as the electron density is shown [3]. Forth, flat cutoff probe for industrial plasma processing monitoring was developed [4]. Fifth, interesting and pioneering works were done with bar-type flat cutoff probe [5], such as electron density measurement at the sheath-plasma boundary and real-time electron density measurement even though the wafer. Finally, we will show the ion density measurement using the wave-cutoff probe [6]. Thanks to this accurate plasma metrology method that can measure the absolute ion and electron densities at the same time, we demonstrate the verified quasi-neutrality of the plasmas for the first time [6].