Mechanism behind dry etching of Si assisted by pulsed visible laser

Abstract

Listen

Translate article

Poly-Si films were etched using a 13.56 MHz capacitively coupled plasma source while simultaneously being exposed to a pulsed Nd:YAG laser using 266 and 532 nm lines, with Gaussian pulse durations of 100 Hz and 7 ns. For a fluorocarbon etch recipe of 50:8 sccm Ar:C4F8 with varying O2, a minimum laser intensity for the etch onset was necessary to overcome CFx polymer deposition. This etch onset occurred at 6 ± 1 mJ/cm2/pulse; beyond this onset, the etch rate increased linearly with laser intensity. Null results of laser etch enhancement using continuous wave diode sources demonstrated the necessity of the instantaneous application of the pulsed Nd:YAG source. To determine the mechanism of laser etch enhancement at 532 nm, highly doped Si samples were tested, with varying optical absorption depths while keeping the photon energy constant. It was shown that at phosphorus contents of 1019 cm−3 and 1021 cm−3, 532 nm etch enhancement trends were 1.7× and 3.7× higher than those on intrinsic Si, showing that instantaneous surface heating was key in desorbing involatile etch products. Further investigation of the surface fluorine content via X-ray photon spectroscopy showed that distinct desorption stages occurred for increasing pulse energy—trends which aligned very well with SiFx desorption promoted by steady-state wafer heating. Gas arrival/surface saturation experiments with varying pressures and pulse rates showed that, in straightforward etching discharges such as Ar/SF6, laser removal per pulse plateaus when the pulse rate is lower than the rate of surface saturation, while in fluorocarbon-rich etch chemistries such as Ar/C4F8/O2 mixtures, a minimum pulse rate must be maintained to overcome the CFx polymer layer being deposited.