Uniform Tetragonal WSi 2 Layers Formed by RTA
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UNIFORM TETRAGONAL WSi2 LAYERS FORMED BY ETA M. SI•GAL , J.J. SANTIAGO , J. VAN DER SPIEGEL , W.R. GRAHAM , AND M. SETTON . Laboratory for the Research of Structure and Matter, *Department of Materials Science and Engineering, **Moore School of Electrical Engineering, Center for Sensor Technologies, University of Pennsylvania, Philadelphia,
Pa.
19104
ABSTRACT Thin films of tungsten silicide with resistivities of 30 - 35 •-cm have been formed by sputter depositing 71 nm of W metal onto (I00) oriented, 5 •-cm, p-type silicon wafers that were etched in BOE 500 solution. The samples were fast radiatively processed in an RTA system under high vacuum for time anneals ranging from 15 - 50 seconds at a temperature of - II00°C. The inevitable oxide barrier at the interface is shown to decrease with increasing RTA time.
INTRODUCTION Thin films of refractory metal silicides are being studied for use in VLSI circuit designs as interconnects and gate metallization schemes. As devices continue to get smaller, it has become imperative to develop new materials with low resistivities and high temperature stability.[l,2] We have previously reported the use of rapid thermal annealing (RTA) to form thin films of tetragonal WSi2 on a Si(100) substrate which had been etched in 5:1 HF solution before the W metal was sputter deposited.[3] In this paper, we discuss the formation of uniform WSi2 layers grown on Si (I00) substrates which had been chemically etched in BOE 500 solution (Buffered Oxide Etchant, 500 A SiOe etched per minute) prior to W deposition, resulting in a reduced carbon and oxygen concentration within the film. Table I summarizes recently reported resistivities for WSie films formed on Si-substrates.[3-6] Regardless of deposition or annealing technique, these values range consistently from 45 - 70 •-cm, with the exception of Goltz et. al., who report p = 30 •-cm for a co-sputter deposited WSi2 film annealed in a conventional furnace (CF) at 1000°C in 30 minutes.[6] To achieve such low resistivities, it is important to produce a film as free of impurities as possible. In particular, the influence of oxygen in the formation of tungsten silicides has been well-studied. Silversmith et. al.[7] noted that due to the gettering capabilities of W, deposition of a Wfilm in vacuum ambients where P • 2 x 10E-7 Torr, the incorporation of oxygen, probably in the form of WO3, is ensured. The usual high temperature anneal (a 1000°C) for WSi= formation results in the creation of an isolating SiOe diffusion barrier between the W film and the Si crystalline substrate, preventing silicon diffusion and hence WSie formation. This interfacial SiO2 layer is known to occur for nearly all metal disilicide films formed on a Si-substrate annealed in a residual oxygen environment at temperatures ranging from 1000°C - 1200°C.[8]
674
Table I: o(uQ-cm)
Previously Reported WSI2
Deposition
Resistivities
Annealing
Conditions
Ref.
55-60
sputtered W on Si
RTA,
1000°C
3
45-50
co-sputtered or CVD WSi
RTA,
1200°C
4
53-66
co-sputtered WS
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