COMET Capacitively Coupled Plasma(CCP) Diagnostic

March 29, 2024

• CCPs are simple, low impurity plasmas commonly used for processing. EEDF influences the rate of reactions and collisions, impacting species generation and processing parameters
• EEDF varies over the course of one RF cycle (74 ns period in 13.56 MHz supply)
• ns resolution time resolved Langmuir probe used to measure electron density and EEDF throughout the RF cycle.

This work uses a time-resolved Langmuir probe to measure the electron energy distribution function (EEDF) in a capacitively-coupled parallel-plate (CCP) plasma reactor. The EEDF completely determines the plasma chemistry in a low-temperature plasma, and that is why it is so important to obtain. By seeing how the EEDF changes throughout an RF cycle, both as a function of time and position, one then knows the extent by which altering the RF waveform can affect the energy of the electrons. Often industry mixes RF frequencies to alter the plasma — particularly the ion energy distribution at the substrate. Here we add a second frequency in a systematic manner and examine the changes in the instantaneous EEDF. We also examine the turn on and turn off times of the RF generator itself. Specialized circuits were designed for this work to ensure high frequency fidelity so digitization at 1.5 GHz is possible and accurate. A set of experiments were conducted to show how only altering circuit parameters affect the results, and steps were taken to eliminate those effects. Spatial variations of the resulting EEDFs were investigated, especially near the edge of the CCP reactor, to see which aspects change the most with radius.