Partial agglomeration during Co silicide film formation
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C. A. Canovai and C. M. Osburna> Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina 27695-7911 (Received 18 March 1991; accepted 4 October 1991)
The agglomeration of Co silicide films formed on Si substrates processed with evaporated Co film thicknesses from 9 to 28 nm was investigated by TEM and four-point-probe resistivity measurements. It was found that the upper portion of a reacting Co or Co silicide film can agglomerate independently from the main body of the silicide layer. This phenomenon is designated partial agglomeration in contrast to whole film agglomeration which generally occurs at higher temperatures. Partial agglomeration appears to develop more extensively for thinner films and poses a serious limitation for the application of thin silicide contact layers for advanced VLSI devices. The formation mechanism of partial agglomeration and the reason for its variation with film thickness are explained on the basis of a previously presented [MRS Proc. Vol. 202 , p. 101 (1991)] theoretical model of grain boundary grooving and the onset of islanding in silicide films. Kirkendall voids and phase transformation induced volume changes play an important role in the process.
Cobalt silicide is under consideration for use in ultra large scale integration (ULSI) as a contact and interconnection material because of its low resistivity.1 However, during heat treatments above about 1000 °C CoSi2 films degrade, resulting in a dramatic increase in sheet resistance. The physical process of the degradation is agglomeration of the silicide film into discrete islands.2-3 As there is often some high temperature processing subsequent to film formation, e.g., reflow or diffusion from silicide, thermal stability becomes a critical requirement for silicides. Recently, during an investigation of the agglomeration of silicide films formed from Co layers initially 9 to 28 nm thick, a new phenomenon (partial agglomeration) was observed. That is, the top portion of the reacting Co or Co silicide film was found to partially agglomerate into discrete globules during the silicide film formation, while the bottom layer of the silicide film either formed islands or remained continuous, depending on the initial Co film thickness.3 In this report more detailed experimental results are presented and the partial agglomeration is analyzed using the recently published theoretical model for grain boundary grooving and islanding.3 Implications for Co silicide reliability in ULSI processing are discussed. Phosphorus doped, 10 f2-cm, (100) oriented 4 inch diameter Si wafers were used in this study. After standard RCA cleaning and an (NH 4 F: NH 4 H 2 PO 4 : HF = 2 7 : 2 : 2 ) substrate etch, Co was evaporated at room tema)
MCNC, Center for Microelectronics, Research Triangle Park, North Carolina 27709. J. Mater. Res., Vol. 7, No. 2, Feb 1992 http://journals.cambridge.org
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perature in a cryo-pumped e-beam system with a base pressure of 1 x 10"5 Pa. The Co thicknes
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