160 GHz Gaussian beam microwave interferometry in low-density rf plasmas

Abstract

160 GHz Gaussian beam microwave interferometry is realized for electron density analysis in low pressure rf plasmas. Measurement of electron densities lower than 10 16 m −3 with corresponding phase shift less than 0.3 • demands high stability of the interferometer frequency and minimum disturbance due to external interfering voltages and mechanical vibrations of the optical components. The interferometer consists of a frequency stabilized (phase lock loop) heterodyne system operating at a frequency of f MWI = 160.28 GHz and wavelength of λ MWI = 1.87 mm, respectively. A quasi-optical setup is used, considering specially designed horn antennas and elliptical mirrors as well as components which have to comply with the aperture limit in relation to the Gaussian microwave beam and its optimal coupling and focusing into the plasma center. A spatial and temporal resolution of about 10 mm (beam waist 5 mm) and 0.2 µs is achieved, respectively. In cc-rf plasma the lowest measurable phase shift is in the order of 0.01 • , which corresponds to a line-integrated electron density of about 5 × 10 13 m −2 or an electron density of 5 × 10 14 m −3 averaged over the electrode diameter. Results are presented and discussed concerning line-integrated electron density in an asymmetric argon cc-rf plasma in dependence on rf power and total pressure.

E‐H Transition in Argon/Oxygen Inductively Coupled RF Plasmas

“…The vacuum chamber is already described by Dittmann et al [44]. The complete setup with the used diagnostics is shown in figure 1.…”

“…The experimental setup is similar to the gaseous electronics conference (GEC) reference cell [43]. The vacuum chamber is already described by Dittmann et al [44]. The complete setup with the used diagnostics is shown in figure 1.…”

“…The local measurements are done in the center of the discharge (r = 0) in the optical axis of the microwave at a = 30 mm, see below. The used 160.28 GHz frequency stabilized (PLL) heterodyne Gaussian beam microwave interferometer is described in detail by Dittmann et al [44]. It measures the phase shift ΔΦ between the plasma on and off phase.…”

Dynamics and Electronegativity of Oxygen RF Plasmas

“…The rf electrode was powered via a matching network in cw or pulsed mode operation by a 13.56 MHz generator at forward power from 5 to 150 W. Due to the larger grounded electrode (rf electrode shielding and chamber wall) an asymmetric rf discharge with self-bias voltage from -100 to -600 V is generated, [2,14]. The vacuum pumping and gas supply system allowed a base pressure of 5 × 10 −5 Pa, whereas the plasma process is driven by a pressure of between 5 and 100 Pa at a constant gas flow rate of 5 sccm.…”

“…The vacuum pumping and gas supply system allowed a base pressure of 5 × 10 −5 Pa, whereas the plasma process is driven by a pressure of between 5 and 100 Pa at a constant gas flow rate of 5 sccm. The rf electrode was powered via a matching network in cw or pulsed mode operation by a 13.56 MHz generator at forward power from 5 to 150 W. Due to the larger grounded electrode (rf electrode shielding and chamber wall) an asymmetric rf discharge with self-bias voltage from -100 to -600 V is generated, [2,14]. The MW interferometer for electron density analysis consists of a frequency stabilized (PLL) heterodyne system operating at a frequency of 160.28 GHz with output power of 1.3 mW, [14].…”