Chemistry in long residence time fluorocarbon plasmas

Abstract

The densities of radicals and neutrals in fluorocarbon (FC) plasmas have been investigated in an inductively coupled plasma system to understand the predominant gain and loss mechanisms of dissociative products and their interaction with chamber surfaces. The input parameters varied in this experiment are the source to chuck gap and the F: C ratio of the feed gas. The densities of F, CF2, CF3, CF4, C2F4, SiF4, COF2, CO, and CO2 are measured and analyzed. In addition, two different forms of C4F8, the standard cyclic c and a radical linear l structures are observed in C4F8 containing plasmas. l-C4F8 is shown to be the primary dissociation product of c-C4F8 and, thus, cannot be neglected from calculations of the loss rate of c-C4F8 to electron collisions. This implies that the typically cited dissociative products of c-C4F8 (primarily C2F4) can have dual production channels: one from l-C4F8 and the other directly from c-C4F8. Furthermore, the measured density of CF4 shows strong correlation to the loss of F to the surfaces, indicating its primary production mechanism is from surface recombination. Similarly, CF3 is mainly produced from deposited FC film on chamber surfaces and lost to electron impact reactions. The etch products resulting from etch of the quartz window are shown, and an overall neutral density model is developed.