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RETENTION OF PHOSPHORUS IN SILICON BENEATH TUNGSTEN SILICIDE CONTACTS DURING HIGH TEMPERATURE CYCLES

IP.com Disclosure Number: IPCOM000005621D
Original Publication Date: 1986-Oct-01
Included in the Prior Art Database: 2001-Oct-19
Document File: 5 page(s) / 176K

Publishing Venue

Motorola

Related People

I.A. Lesk: AUTHOR [+3]

Abstract

After metal contacts and short-range interconnects are deposited on silicon, there is often the need to carry out processing at high temperatures. Compound formation, grain size enhancement, contact improve- ment, and glass reflow for tapering of via holes in thick doped glass layers all can require exposure to temperatures in the 900.1000°C range for best results. To withstand such high temperatures, refractory metals or compounds are necessary. Of these, tungsten silicide (WSi>) with a melting temperature of 2164°C is a promising candidate! WSh can easily withstand temperatures above 1100°C if sufficient excess Si is available to satisfy the needs of any oxidation cycle, which produces asurface layerof SiOz, without lowering theSi content below that need- ed to maintain WSiz stoichiometry. To accomplish this, and to promote adhesion to SiOz, WSh is often deposited silicon-rich and onto a layer of poly SP; this is referred to as a polycide. To obtain a deposited WSh layer in its most conductive (<40pgcm) large grain form, a high temperature cycle, e.g., lo-120 minutes at lOOo"C, is needed.

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MOlOROLA Technical Developments Volume 6 October 1966

RETENTION OF PHOSPHORUS IN SILICON BENEATH TUNGSTEN SILICIDE CONTACTS DURING HIGH TEMPERATURE CYCLES

by LA. Lesk, Michael L. Kottke and Wayne Paulson

   After metal contacts and short-range interconnects are deposited on silicon, there is often the need to carry out processing at high temperatures. Compound formation, grain size enhancement, contact improve- ment, and glass reflow for tapering of via holes in thick doped glass layers all can require exposure to temperatures in the 900.1000°C range for best results. To withstand such high temperatures, refractory metals or compounds are necessary. Of these, tungsten silicide (WSi>) with a melting temperature of 2164°C is a promising candidate! WSh can easily withstand temperatures above 1100°C if sufficient excess Si is available to satisfy the needs of any oxidation cycle, which produces asurface layerof SiOz, without lowering theSi content below that need- ed to maintain WSiz stoichiometry. To accomplish this, and to promote adhesion to SiOz, WSh is often deposited silicon-rich and onto a layer of poly SP; this is referred to as a polycide. To obtain a deposited WSh layer in its most conductive (<40pgcm) large grain form, a high temperature cycle, e.g., lo-120 minutes at lOOo"C, is needed' '.

   Phosphorus (P) diffuses very rapidly in WSh. In a non-oxidizing (e.g., Nz, Ar, vacuum) environment, P will diffuse out of the substrate, through the WSh, and evaporate from the surface, leaving a greatly depressed P concentration in the interfacial substrate region. Loss of substrate P can result in a more efficiently-rectifying Schottky barrier with a concommitant large increase in contact resistance. An appreciable loss of P will occur even if there is a layer of poly Si between the N + substrate and the WSiz because P diffuses quite rapidly along grain boundaries in Si. If the WSh is used with an intervening poly Si layer as the gate metal for an MOS tran- sistor, loss of P during high temperature treatment may be sufficiently extensive to change the threshold voltage, raise the effective Si-SiOz barrier to electrons, reduce poly Si grain growth, and add an undesirable gate resistance.

   Reduced out-diffusion of P from a WSis-poly Si structure has been demonstrated with a 1000-5000 A SiO> or SizNd capping layer" 5 for passivation. Use of a thinner Si,Nd layer could augment fine geometry patterning while maintaining effective control of P content during high temperature treatment. The thin ShN, layer could be left in place to act as a barrier to subsequent incoming rapidly-diffusion impurities.

   The effectiveness of a thin SiaNA layer in preventing loss of P from poly Si in a WSh-poly Si-SiOa-Si layered structure is illustrated in Figs. 1-3. In each case, thicknesses of WSh and poly Si are 2500 and 2750 A respec- tively. Substrates were 20 Q -cm P-type < 100 > wafers with 400 A of thermal oxide. The 200 A SisNd layer was deposited by LPCV...