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F. Babarada, M. Profirescu, C. Dunare
University Politehnica Bucharest, RO

Keywords: Modellig, MEMS

PARALLEL PLATE PLASMA ETCHING FOR MEMS PROCESSING- REACTOR MODELLING Florin Babarada, Marcel Profirescu-Department of Electronics and Telecommunications, University Politehnica of Bucharest, Romania, tel.: +4021 4113193, email: Camelia Dunare- Department of Electronics and Electrical Engineering, University of Edinburg, UK 1. Introduction The model presented analyses the uniformity of dry etching polyslicon films which are used in resonator cavity for MEMS fabrication/1,2/. The increase of the uniformity is very important for nanotechnology and very strong required of MEMS because the dimensional configuration precision of mechanical structures has a direct influence on the mechanical properties. It was found that the uniformity can be improved by decreasing the power and pressure or by increasing the flow rate. 2. Model formulation The model was developed for a radial symmetric single-wafer parallel plate plasma reactor shown schematically in fig.1. Feed gas enters uniformly through the porous upper electrode. Etching products and unreacted feed gas are pumped radially outwards. The wafer is in good thermal and electrical contact with the lower grounded electrode and the etching surface temperature is assumed to be constant. Gas temperature variations are neglected. Due to the consumption on the wafer surface large concentration gradients may develop at the boundary between the wafer and the surrounding electrode. Because the etching rate is usually a function of the local etch agent concentration, the etching is nonunifom. Assuming constant gas physical properties and negligible volume change during reaction, the momentum equation can be decoupled from the mass and heat transfer equations. Considering the continuum approximation which is valid for pressure above about 0.2 torr, we used the Navier-Stokes equations and the continuity equations. The differential equation can be solved only by numerical techniques /3/. When we consider the wall Reynolds number Rw<1 and by using regular perturbation techniques /4/ we can obtain approximate solutions which are plotted in fig.2. 3. Experimental The parallel single-wafer etcher has a 4" diameter, hard-anodized aluminum powered showered upper electrode held at a distance of 3 cm from the lower electrode. The temperature for the lower electrode is controlled by a thermostat. Gases are pumped by a two stage rotary vane pump and the base pressure is 10-4 torr. Chamber pressure and gas flow rate were independently controlled by a pressure transducer, an exhaust throttle valve and a controller. Power is applied to the upper electrode by a 13,56 MHz, 200W RF generator. 4. Results and discussion The radial velocity (Fig.2a) is zero in the reactor center and increases linearly towards the exit. For Rw<<1 this velocity has a parabolic profile. The axial velocity (Fig.2b) has a maximum value at the upper electrode, where gas enters and decreases monotonically with the axial position. We can observe that this velocity is independent of radial position. The experimental data in Fig.3 and Fig.4 show the dependence of uniformity with power respectively with pressure and is in good quantitative agreement with the model prediction. 5. Conclusions The results show the possibility to use the parallel plate single wafer plasma, usually used in CMOS-IC fabrication for MEMS processing /5/. Topic: Equipment Modelling, Application area: MEMS 6. References 1. Florin Babarada et al, Vacuum encapsulated polysilicon resonators with electrostatic excitation and capacitive detection, MME 1996, Barcelona Spain, pg. 300, 1996. 2. Florin Babarada et al, Pressure and vapor resonant transducers in CMOS technology, Kongressmesse fur industrielle Messtechnik, Germany, pg.14, 1995. 3. M.D. Profirescu et al, O noua metoda de macromodelare, The 6th Semiconductors Conference, Timisu de Sus, pg.139, 1983. 4. A.H. Nayfed, Perturbation Methods, 1972. 5. Florin Babarada et al, CMOS - Compatible integrated microresonator design, International Semiconductor Conference, Sinaia, Romania, pg. 405, 1994. Fig.1-Parallel plate plasma single-wafer etcher Fig.2-Radial (a) and axial (b) gas velocities as a function of the axial and radial coordinate Fig. 3-Polysilicon etch rate Fig. 4-Polysilicon etch rate versus power density versus pressure

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