Redistribution of Dopant and Impurity Concentrations During the Formation of Uniform Wsi 2 Films by RTP
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REDISTRIBUTION OF DOPANT AND IMPURITY CONCENTRATIONS DURING THE FORMATION OF UNIFORM WSi 2 FILMS BY RTP Michael
P.
Siegal
and Jorge J. Santiago*. Laboratory for the Research of
*Department of Materials Science and Engineering, Structure and Matter, **Moore School of Electrical Engineering, Center for Sensors Technology, University of Pennsylvania, Philadelphia, Pennsylvania 19104.
ABSTRACT Secondary ion mass spectroscopy has been used to study the effects of rapid thermal processing on the formation of tetragonal tungsten disilicide thin films on Si(100), p-type 5 0,cm wafers. The substrates were chemically etched, followed by an RF sputter depostion of 710A W metal. The samples were then fast radiatively processed in an RTP system for time intervals ranging from 15 to 45 seconds at high temperature (-II00°C) under high vacuum. The redistribution of the boron dopant concentration profile is studied
and shows that boron moves from the Si-substrate into the growing WSi
2
film,
eventually escaping into the vacuum. Oxygen is the major impurity in these samples and its removal from the interface has been shown to improve the quality of the silicide film. Trace quantities of F, Cl, Na, K, C and Cr have also been detected.
INTRODUCTION Refractory metal silicides have been widely studied as a replacement for polycrystalline silicon and aluminum for use in VLSI (very large scale integration) metallization schemes. In particular, tungsten disilicide has been recognized as a prime candidate due to its low resistivity and high temperature stability.[l] Nevertheless, the high processing temperatures needed to form WSi 2 (> 1000°C) can greatly alter the concentration profiles of the dopant [2-6], incorporate oxygen from the annealing ambient [7-19] and redistribute impurities inherent to the deposition processes.[20-22] Each
of
these
phenomena
can
change
the
characteristics
therefore they must become better understood. The redistribution of dopant atoms during
temperature
such
as
etching
threshold voltage, contact barrier and resistivity.[2]
The one
affects
properties, property
a device;
high
processing
that
appears
important to
be
device
WSi
of 2
characteristics
universal for all dopants is
the speed with
which they diffuse into silicides.[3] This was first noted by Tsai et. al. [4] when they looked at the redistribution of P atoms in WSi 2 . Films that were passivated with a Si0 2 layer showed a fast and complex diffusion of P atoms from the doped poly-Si into WSi 2 . Absence of the passivating layer led to a rapid out-diffusion of P into the atmosphere presumably by evaporation through the silicide surface. The special affinity WSi 2 has for B has been evidenced not only by the resulting high concentration of B reached in WSi 2 from an adjacent doped poly-Si layer at 1000°C [5], but also by a gradual buildup of B in WSi 2 in lower temperatures up to 800°C. The presence of a maximum in the B concentration at the free surface of a silicide layer and even of an asdeposited W metal layer ha
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