2005 Conference on Lasers and Electro-Optics Europe

Laser micromachining of silicon in sulfur hexafluoride atmosphere:experiment and numerical simulation

Yuri M. Yashkir*, Seongkuk Lee, Maher Harb, Yuriy Yu. Yashkir

University of Toronto, 60 St.George Street, Suite 331, Toronto, ON, CANADA L6H 5T6email: yyashkir@optics. utoronto ca

Micromachining conditions for silicon using a CW lasers and different gas ambients werereported in [1 - 5]. Most of data are related to chlorine used as a laser- stimulated etching agent.Comparatively, data on etch rates and quality of etching using other ambients such as sulfurhexafluoride are rare. For many practical application the process in noncorrosive environment iscritical. We report results of experimental investigation of the etch rate of silicon using a CW 532nm laser in sulfur hexafluoride atmosphere, and three- dimensional temporal computer modelingof the process.

Experimental details

Silicon wafer was placed in a chamber filled with sulfur hexafluoride at 100 Torr pressure.The CW laser beam (532 nm; power 3 to 4 W) was focused with a lens (focal length 66 mm) throughthe chamber window onto the wafer surface. The laser power was just slightly above the meltingthreshold of silicon. The Aerotech micropositioning stage was programmed to scan a set of parallellines with at various feed rates and laser powers. Spatial profiles of etched grooves were analyzedusing interferometric profilometer WYCO, and rates of the material removal were calculated.

Numerical simulation

Modeling of the interaction of the laser focused beam with the material requiredsimultaneous modeling of the electromagnetic wave propagation, the heat diffusion processes, andchemical etching of the material. A time- dependent, three- dimensional heat- flow model wasimplemented numerically through the finite- difference method as further development of resultspresented in [6]. This model has been developed to predict and understand etch rates, heat flowand general light- matter phenomena during laser- material interactions. The main equation used inthe model was the diffusion of heat density with a source of heat due to the absorbed laser light.Material is heated up to the melting point and is being removed by chemical etching. Laser beam ismoving at a given feed rate along an arbitrary trajectory. A special adaptive step procedure wasdeveloped and implemented to reduce calculation time (smaller spatial cells are used in the volumearound the moving laser beam focus where the temperature gradient is higher, etc.).

Results

It was experimentally found that below melting point no noticeable etching takes place.There is no material removal in the melting phase without sulfur hexafluoride (only surfacedisturbance was observed in this case). At melting conditions in sulfur hexafluoride atmospherethe experimental etch rate was identified and its dependence on the laser power was studied.

Numerical simulation based on our model and available material parameters of siliconprovided very good fit to experimentally etched grooves. The only calibration micro- parameterwas the rate of chemical reaction. It was determined to be - 200p3/S

1. T.J.Chuang, The Journal of Chemical Physics Vol 74(2) pp. 1453- 1460. January 15, 1981; D.J.Ehrlich, R.M. Osgood, and T.F. Deutsch, Appl. Phys. Lett. 38, 1018 (1981).

2. M.Nantel, Y.Yashkir, Seong- Kuk Lee, B.Hockley. SPIE TDOI, 187 - 189 (2002); Y.Yashkir, M.Nantel, B. Hockley. International Journal of Applied Electromagnetics and Mechanics, v.19, 373 -377 (2004)

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