Diagnostic Studies of Silicon and Silicon Dioxide Etching in Fluorine and Chlorine-Containing Inductively Coupled Plasmas

Plasma etching processes are widely used to produce patterns in the fabrication of microelectronic devices. The recent development of micro-/nano- technologies for micro-electro-mechanical systems, as well as the reduction of critical dimensions in transistors towards 7 nm and below, has brought out the need to control plasma etching processes. The very severe requirements in terms of etch rate, selectivity, profile control and surface damage caused by ion bombardment have been at the origin of the development of mechanistic studies by means of plasma diagnostics and surface analysis. In the present study, chemical reaction probabilities, defined as the number of silicon atoms removed per incident fluorine atom, have been investigated in mixtures of NF3 and SF6 plasmas in an inductively-coupled plasma reactor. Fluorine atom densities were measured by optical emission actinometry, and isotropic etching rates were measured by the degree of undercutting of SiO2-masked silicon, using cross-sectional scanning electron microscopy (SEM). The F atom reaction probabilities derived from isotropic etching rates indicate a ~30-fold higher reaction probability in SF6 plasmas compared with values in NF3 plasmas. This surprising enhancement of reaction probabilities for F with Si in SF6 plasmas is further investigated based on the mechanism of adsorbed sulfur acting as a catalyst to greatly enhance the etching rate of Si. Further, we discussed the use of glow discharge optical emission spectroscopy (GD-OES) for in-situ, real-time characterization of surfaces exposed to plasmas. A small coupon piece was mounted on an rf-biased electrode and inserted into an opening in the reactor wall. Silicon or SiO2 substrates on a separately rf-biased electrode were etched in an inductively-coupled plasma (ICP) of Cl2/Ar/O2 or C4F8/O2, respectively. Pulsed bias was applied to sputter the surface of the coupon piece. Optical emission from the region above the coupon surface was collected and spectrally resolved. The difference in intensity between the coupon bias on and off condition was used to determine what species were present on the surface. A quantification method for converting emission intensities into atomic composition depth profiles is presented.