Optical Diagnostics of SF₆ Low Pressure Plasma Jet Applied to Silicon Etching

Abstract

In this work the applicability of a low pressure sulphur hexafluoride (SF 6 ) plasma jet for silicon etching was investigated by optical emission spectroscopy (OES) technique. Through the use of actinometry method the density of atomic fluorine was obtained as a function of process parameters namely radiofrequency (rf) power, axial magnetic field, SF 6 gas pressure and flow rate, and O 2 concentration in the SF 6 +O 2 mixture. The results indicate large fluorine concentrations (>10% in overall plasma volume) for the conditions studied. To confirm the applicability of these results for microelectronic material processing, we performed the etching of masked silicon (Si) substrates under some optimum process conditions. Etch rates of up to 1.2 µm/min were obtained for rf power of about 150W and operating pressures about 3.1 mTorr. These values of etching rates are comparable with those obtained in inductively coupled plasma (ICP) systems operating at similar process conditions.

Morphological and Chemical Analysis of Silicon Etched by SF6+O2 and CF4+O2 Low Pressure Constricted Plasma Jet

“…The experiments were performed in a constricted plasma jet reactor whose details are presented in references (10,14). In this reactor, the vacuum system is composed by a turbomolecular pump (EXT250 -Edwards) backed by a rotary pump (E2M18 -Edwards) allowing to attain residual pressures at about 10 -6 Torr.…”

“…In this study, the total gas flow was maintained constant at 10 sccm, resulting in a constant pressure of 3.2 mTorr in process chamber. The high eletronegativity of SF 6 , CF 4 and O 2 gases and low operation pressures of plasma jet are factors favorable to high loss rates of electrons by formation of negative ions and by recombination to the walls, making difficult the plasma sustainment (10,14). For this, an external axial B-field of 7.2 mT was applied with the use of a Helmholtz coil in order to reduce radial diffusion thus improving the confinement and sustainability of the plasma jet in this region.…”

“…However, the complexity of its construction involves high production costs, what limiting its use for specific applications. For this reason, alternative reactors with hollow cathode geometry are being developed and proposed (10,11). A system that is being increasingly studied in plasma processes is the constrictive reactor to generate plasma jet(s) at low pressures (11)(12)(13).…”