Mechanism of ArF Laser induced Photolytic Deposition of W From WF 6 on Etched Si and Unetched Si
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ABSTRACT ArF excimer laser-induced photolytic chemical vapor deposition of W on etched Si substrates using tungsten hexafluoride has been studied. Our experimental results show that tungsten film thickness is proportional to the laser irradiation time and fluence, and that the deposition rate initially increases, then decreases with increasing WF 6 pressure. The activation energy obtained from an Arrhenius plot is much less than that for conventional CVD. A deposition mechanism has been proposed which yields results in good agreement with the experimental data. The absorption cross section of WF 6 is determined to be 2.75x10-18 cm2/molecule. INTRODUCTION The selective deposition of W by silicon reduction of WF 6 has been used for filling submicrometer contacts in microelectronics manufacturing processes.' The most obvious advantage of using an excimer laser is its high output power in the ultraviolet region which allows for photolytic deposition of W films by photo-dissociation of WF6 .2 Photolytic deposition can be performed with a laser beam aligned parallel or perpendicular to the substrate. In the parallel configuration, absorption occurs exclusively by molecules in the gas phase. In the perpendicular configuration, absorption may occur in the gas phase followed by transport of products to the surface or may occur in an adsorbed layer on the surface. The film deposition only takes place in the laser irradiated area via photochemistry and local heating. The perpendicular configuration was used in the experiments presented in this paper. ArF excimer laser induced CVD of W by H2 reduction of WF 6 has been investigated extensively. 2 For non-oxide coated substrates the Si reduction reaction determines the properties of the W/Si interface such as adherence, contact resistance and interdiffusivity. The important reaction mechanisms in the ArF laser photolytic CVD of W by Si reduction of WF 6 are not known at present. The purpose of this research is to study a mechanism of ArF laser induced photolytic deposition of W on Si substrates from WF 6 by correlating the W film thickness and growth conditions with laser fluence, irradiation time, WF 6 pressure and surface temperature. EXPERIMENTAL An ArF laser (LAMBDA PHYSIK model LPX 200), which emits a beam of ultraviolet light of wavelength 193 nm passing through an aperture to produce a relatively uniform beam intensity, was focused through a fused quartz window into a chemical vapor deposition cell containing a silicon substrate. One biconvex fused silica lens combined with a cylindrical lens focused the laser beam to an area 2 mm long by 0.4 mm wide. The power of the excimer laser was computer-controlled. We used n-type (111) Si wafers covered with a native oxide (SiO 2) as 139 Mat. Res. Soc. Symp. Proc. Vol. 388 0 1995 Materials Research Society
the substrate. The Si wafers were ultrasonically cleaned 3 times with tetrahydrofuran (THF), isopropyl alcohol (IPA) and acetone to remove all organics from the sample surface, and then rinsed in deionized water to remove the
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