Surface Interaction Between Wf 6 and GaAs Under UV Laser Illumination
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SURFACE INTERACTION BETWEEN WF 6 AND GaAs UNDER UV LASER ILLUMINATION
M.TABBAL, R.IZQUIERDO, M.MEUNIER and A.YELON Ddpartement de Gdnie Physique et Groupes des Couches Minces, Ecole Polytechnique de Montreal, Montreal, Quebec, Canada, H3C 3A7. ABSTRACT
We have studied laser-CVD of W on GaAs by X-ray Photoelectron Spectroscopy (XPS). Deposition of W is obtained by irradiating GaAs samples with a KrF excimer laser at normal incidence to the substrate, in a cell containing WF 6 mixed with H2 and Ar. We have previously shown that WF 6 and GaAs react at room temperature even without laser illumination. GaF 3 formation and a loss of As were detected at the surface of the samples by XPS. At laser fluences of 50mJ/cm 2 , this reaction appears to be enhanced by laser heating of the substrate, but no metallic W is formed. At laser fluences of 67 mJ/cm 2, metallic W begins to be deposited, through a pyrolytic dissociation reaction with the substrate. INTRODUCTION
There has recently been considerable interest in depositing W on GaAs using a CVD process with WF 6 as the metal precursor gas [1-4]. Tungsten [5] and its silicides [6] form thermally stable Schottky contacts on GaAs, and it is therefore used in the high temperature self-aligned gate process. Chemical vapor deposition (CVD) processes have some important advantages over the physical deposition techniques [2,3]. Laser-CVD offers the possibility of deposition in a selective manner, so that resistless laser projection patterning can be achieved, in appropriate circumstances. We have therefore investigated laser CVD of W from WF 6 on GaAs using a KrF excimer laser, both with and without H2 as a reducing gas, and have observed a surface reaction between WF 6 and GaAs under laser illumination. We have used insitu XPS to study the mecanisms of this reaction. GaF 3 formation and a loss of As were detected at the surface of the samples. Metallic tungsten begins to be deposited at laser fluences of 67 mJ/cm 2 , even when no reducing gas, such as H2 , was used. The presence of a threshold for metallic W formation suggests that the deposition process is pyrolitic. EXPERIMENTAL
The W deposition cell is a stainless steel high vacuum five-way cross, evacuated by a rotary pump and a turbomolecular pump, to obtain a base pressure of the order of 10-6 Torr. WF6, H2 , Ar and SiH 4 can be introduced into the cell. The KrF excimer laser (X = 248 nm) beam is weakly focused and is incident perpendicular to the sample. The maximum repetition rate of our laser (Lumonics TE-261-2) is 2 Hz and most of the experiments were carried at a repetition rate of 1 Hz. A high vacuum transfer rod is connected to the cell and allows vacuum transfer of the samples from and to an XPS spectrometer (VG ESCALAB3 MklI) which also has an Ar+-sputtering gun for sample in-situ cleaning. The Al Kac radiation (1486.6 eV) from a nonmonochromatized source was used to obtain the XPS spectra. Semi-insulating LEC GaAs (100) substrates were degreased in boiling trichloroethane, acetone and propanol, then dipped for 5 m
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