Real-Time Optical Monitoring of Tungsten Nucleation During Laser Induced Pyrolytic CVD
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XIAFANG ZHANG, AND SUSAN D. ALLEN Tulane University, Laser Microfabrication Lab, New Orleans, LA 70118
ABSTRACT The nucleation of tungsten on bare Si and SiO 2/Si surfaces by Ar ion laser induced CVD has been studied with high speed real-time specular reflection and scattering. Our experimental results show that the induction time of tungsten nucleation decreases with laser power and is longer on thicker SiO 2 surfaces than on thin native Si0 2 surfaces at the same laser power. The activation energy of tungsten nucleation on native SiO 2/Si obtained from an Arrhenius plot of induction time is about 33 kJ/mol at temperatures less than 1000 K and 16 kJ/mol at temperatures higher than 1000 K. The results on bare Si with and without H2 reveal that the initial reaction mechanism appears to be the same in both cases. An Arrhenius plot of induction time shows two different apparent activation energies at different temperature ranges, which suggest that W nucleation is controlled by H atom desorption from the Si surface at temperatures less than 714 K and is dominated by Si atom reduction of WF 6 molecules at surface temperatures greater than 800 K.
INTRODUCTION Because tungsten has relatively low resistivity, good electromigration resistance and excellent step coverage,1 laser chemical vapor deposition (LCVD) of W shows promise for ULSI applications. For controlling the microstructure and improving the properties of thin films, insitu process monitoring is of primary importance and interest in real-time characterization of thin film growth processes has increased steadily in the past few years. 2'3 For LCVD of tungsten on Si, 38 percent of the laser beam is reflected from the Si surface. Since the deposition area in LCVD is too small to use other in situ techniques, reflection and scattering from the reaction zone has been used to monitor the growth processes of tungsten in situ. The specular reflection and scattering intensity are determined by optical properties of the substrate and surface roughness of its overlayers. Therefore, a combination of the time dependence of these two reflected and scattered light signals from the growth surface allows us to study the tungsten nucleation reaction on various substrate surfaces and film growth formation during deposition processes, as well as kinetic processes in LCVD and conventional chemical vapor deposition (CVD). In this paper, we report in situ monitoring of the specular reflection and small angle scattering signals during laser induced chemical vapor deposition of tungsten to demonstrate the viability of reflectance methods for the real-time study of nucleation growth process during LCVD of the tungsten. The results provide insight into the reaction mechanism involved in the initial deposition as well as structure information.
619 Mat. Res. Soc. Symp. Proc. Vol. 397 ©1996 Materials Research Society
EXPERIMENTAL The experimental apparatus used in the optical monitoring of LCVD of tungsten
growth during deposition is shown
Detector
in Fig. 1. The laser beam source w
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