Pellet Radical Probes

Radicals are very important for many of the processes that occur during the production of computer chips and other processes involving plasmas. However, the direct measurement of radical density is difficult due to their reactivity and tendency to recombine with themselves. Instead, by catalyzing the recombination process and measuring the heat released, one can infer the density of radicals present in a system at equilibrium. For probes not at equilibrium, the temperature time derivatives become important, treating the probes as 0D objects with heating and cooling terms. F-radical measurement is especially difficult due to catalyst fouling and low F-F recombination coefficients. Etching-based probes use the high etch rates of fluorine to improve measurement signal. Since thin coatings etch too quickly, pellets containing thermocouples that measure dT/dt can be used instead, allowing for the calculation of density. Al and W pellet probes were placed in SF₆ and NF₃ plasmas, comparing measurements of F radical density to spectroscopy. Comparing probes made of al, the aluminum probe is only heated by the plasma, whereas the tungsten turns red hot due to the creation of WF_6, showing material sensitivity.