Paper PS2-TuM2
In-situ Measurement of High-Frequency Current and Voltage in Etching Chambers

Tuesday, October 19, 2010, 8:20 am, Room Galisteo

A set of electrical probes have been developed to measure high-frequency current and voltage, close to electrode surfaces in inductive and capacitive etching chambers. Attenuating radio-frequency (RF) voltage probes are often used for voltage measurements in plasma chambers. However, at high frequencies over 50 MHz, the reading of RF voltage by these probes strongly depends on how the grounding wire is connected to a posited earth point. On the other hand, a voltage probe of the capacitive divider-type provides the voltage reading defined by the direction of dominant electric field, resulting in physically meaningful data even over 100 MHz. The latter approach is therefore taken. Since this voltage probe is designed with a high input impedance, the 50 ohm cable of an arbitrary length can be connected so as to place its sensing part in a small space of a chamber. Our current probe design is based on the pick-up coil approach. However, this current probe is designed to remove contamination of electric field from current reading. After precise calibration, both the probes can be mounted at any surface of the plasma etching chamber.

The probes are firstly used to monitor the etching uniformity in a capacitive plasma discharger operated at 162 MHz. The experimental concept is based on the electrical control methodology proposed by Sobolewski et al. (1999). Due to the 162 MHz standing wave built up in the transmission part of the chamber, the interpretation of current and voltage measurements is not so straight-forward. However, the voltage probe, mounted close to the wafer position shows better correlation with etching uniformity compared with another commercialized voltage probe mounted at the exit of the RF match box.

In the second experiment, these electrical probes are used to monitor the phase difference between two floating electrode in a capacitive discharging etching chamber. This experiment is aimed at testing the approach suggested by Bera et al. (2008). While experiments on uniformity-control via RF-phase was reported in the literature of Sung et al. (2009), one problem with these experiments is that the phase shift was monitored in the pre-match position. Unless special care is taken to characterize the match state, the phase change controlled by the phase-shifter would not have provided precise information on phase between the two electrodes. In our experimental set-up, this issue will be overcome by monitoring voltages close to the electrode surfaces.

These preliminary experiments indicate that these high-frequency voltage/current probes have a broad range of in-situ applications in the future design of etching chambers.