Silicon Growth Rate Enhancement Using Trisilane in a Laser Direct-Writing Technique
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EXPERIMENT The experimental set-up is that previously described in detail [6, 7, 20]. A stainless steel reaction chamber is placed on XYZO moving stages and connected to gas distribution and pumping facilities. A substrate is set on a holder inside the chamber. The laser used is a 5 W cw argon-ion laser (Spectra Physics, 168-OEM), operating in the TEM 00 multiline mode. Its beam, after passing through an optical assembly, is tightly focused onto the substrate surface using a microscope objective of X20 magnification. The full-width-half-maximum laser spot diameter at the focal plane of the objective has been measured and found to be 2 gm [17]. The substrates used were 0.1 g~m thick polysilicon/1 gtm thick silicon dioxide/ monosilicon multilayered structures. The silicon dioxide (SiO 2 ) film was thermally grown while the polysilicon overlayer was deposited using conventional CVD of SiH4 at 620 *C. The polysilicon film ensures the laser beam absorption to locally increase the surface temperature. The SiO2 layer thermally insulates the polysilicon top-coating from the monosilicon substrate which plays the role of a heat sink and remains at room temperature. The reaction chamber was first pumped to a base pressure of 10-3 mbar and then filled with trisilane (L'Air Liquide Co., 99.99%) at a pressure ranging from 0.1 to 30 mbar. To deposit a silicon line, the laser beam was turned on, the substrate was laterally moved using the XY translation stages in the focal plane of the objective, and the mechanical shutter was opened. The laser power must be set at a sufficiently high value to locally heat the substrate surface to a temperature ranging from 1000 to 1410 *C. All the experiments were performed in a static atmosphere, at a constant Si3 H 8 pressure. To determine the laser-induced surface temperature in order to perform relevant investigations of the growth kinetics, a rigorous methodology was developed and previously thoroughly described [6, 7, 18]. We used as a reference point the laser output power Pm required for melting the polysilicon overlayer when the reaction chamber was under vacuum, i.e., at a base pressure of 10-3 mbar. Since it has been verified, for our experimental configuration, that the laserinduced surface temperature is proportional to the laser output power [6, 7, 18], the measure of Pm at the beginning of each experiment allowed us to derive the deposition temperature. The silicon growth kinetics was investigated by measuring the thickness of the silicon lines for various scanning speeds of the laser spot (1-100 gm/s), Si3 H 8 pressures (0.1-30 mbar), and laser-induced surface temperatures (1000-1410 'C). A mechanical stylus (Tencor Instruments, Alpha-step 200) was used. RESULTS Figure 1 shows the dependence of the silicon thickness on the writing speed for a deposition temperature of 1200 *C and two Si 3 H 8 pressures, 5 and 10 mbar. For both pressures, the thickness of the silicon lines is inversely proportional to the scanning speed. Therefore, the deposition proceeds at a constant temperature
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