Surface Studies of Laser Processing of W on GaAs from WF 6
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SURFACE STUDIES OF LASER PROCESSING OF W ON GaAs FROM WF 6
M.TABBAL, A.LECOURS, R.IZQUIERDO, M.MEUNIER and A.YELON. Groupe des Couches Minces and D6partement de Gdnie Physique, Ecole Polytechnique, Montr6al, Canada H3C 3A7.
ABSTRACT
Laser Chemical Vapor Deposition of tungsten on GaAs from WF 6 using a focused cw scanning argon-ion laser beam has been investigated. Lines have been produced using different mixtures of WF6 :H2 and WF6:SiH 4 and in some cases, without any reducing gas. Depositions are found to occur within a narrow process window, and are difficult to reproduce. In order to understand this process, we have performed surface analysis on GaAs samples exposed to WF6 . X-Ray Photoelectron Spectroscopy studies on the interaction between WF 6 and GaAs in the absence of laser illumination show that fluorinated tungsten compounds are present on the GaAs surface. Furthermore, the existence of a chemical reaction leading to the formation of GaF 3 at the surface and to a loss of the stoichiometry of the substrate surface is detected. Possible mechanisms, and the effects of these reactions on the deposition process are discussed. INTRODUCTION
It has recently been reported (1,2) that W is a good material for forming Schottky contacts on GaAs, since W is a low resistivity refractive metal and does not react with Ga or As below 1000°C (2). As laser chemical vapor deposition (LCVD) is a selective and low temperature process, it is an attractive technique for depositing W on temperature sensitive substrates such as GaAs. We have previously presented preliminary results on LCVD of W from WF6 on GaAs (3). We present here the results of the LCVD of W on GaAs using a cw Ar+ laser. Lines are deposited in a narrow process window but are difficult to reproduce. In some cases, deposits are obtained without any reducing gas, suggesting that GaAs plays a role in the deposition. We then report on the interaction between WF6 and GaAs under the conditions of laser deposition using X-ray Photoelectron Spectroscopy (XPS). We found that the surface oxides (especially As20 3) react with the WF6 atmosphere, leading to a highly surface localized reaction. Tungsten subfluorides and oxyfluorides are chemisorbed onto the surface, where a loss in As and formation of GaF 3 are detected. LASER PROCESSING OF W ON GaAs The substrates are (100) semi-insulating liquid-encapsulated Czochralski
GaAs. Samples are degreased in hot trichloroethane, acetone and propanol, rinsed in deionized water then etched in H3P0 4:H20 2:H20 (1:1:10) for 5 seconds. Samples are then rinsed in hot HCI:H 20 (1:1) for 5 minutes and then in deionized water and dried in flowing nitrogen. Mat. Res. Soc. Symp. Proc. Vol. 236. ©1992 Materials Research Society
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The laser direct writing system has been described elsewhere (3). It consists of a 514 nm cw Ar+ laser focused on the substrate which is placed in a stainless steel chamber with a quartz window. This chamber is mounted onto computer controlled X-Y translation stages having a spatial resolution of 0.1gm and a
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