AVS 59th Annual International Symposium and Exhibition
    Plasma Science and Technology Tuesday Sessions
       Session PS1-TuM

Invited Paper PS1-TuM1
Monitoring Plasma Etch Processes with Wave Cut-Off, Langmuir, and Radio-Frequency Probes

Tuesday, October 30, 2012, 8:00 am, Room 24

Session: Plasma Diagnostics, Sensors and Control 1
Presenter: M.A. Sobolewski, National Institute of Standards and Technology (NIST)
Correspondent: Click to Email
In industrial plasma etching processes, etch rates, profiles, and the critical dimensions of etched features may depend very sensitively on ion current density and other plasma parameters. The reproducibility of plasma conditions is thus an important concern, but the many mechanisms that can cause plasma properties to vary during processing have not been much studied. Here, such mechanisms were investigated using a wave cut-off probe and a Langmuir probe, in tetrafluoromethane/argon plasmas in an rf-biased, inductively coupled plasma reactor. The wave cut-off probe provided measurements of plasma electron density every few seconds. Because it is based on a frequency measurement, it can precisely measure small changes in electron density, often as small as 1 %. Internal resonances in the cut-off probe, however, can degrade this precision. The Langmuir probe provided measurements of the ion current density, electron and ion densities, and electron energy distribution function, with a time resolution of tens of seconds.

Several different phenomena were investigated. First, we investigated the changes in plasma electron density that occur when the rf bias is turned on or off. Analysis shows that such changes can be explained by the stochastic heating of electrons by rf bias power, the interaction of the rf bias and the inductive source, and perturbations in gas composition caused by etch products. Second, we investigated the changes in all plasma parameters that occur when etches of silicon dioxide films on silicon substrates reach endpoint. Analysis shows that the changes observed at etch endpoint were primarily due to changes in gas composition, rather than changes in electron emission from the wafer surface. Third, we also observed nonidealities in the inductive source that affect its ability to maintain constant plasma parameters.

Simultaneous with these measurements, the rf current and voltage were measured outside the reactor, in the rf bias and inductive source circuitry. These external measurements were strongly correlated to — and explained by — the wave cut-off and Langmuir probe data. These results strongly suggest that nearly all of the phenomena observed here can be monitored equally well by external rf measurements, instead of invasive probes.