Selective CVD

One of the most important advantages of CVD for some applications is the possibility to carry out the deposition selectively. In this case the substrate is presumed to include a surface to be coated with deposited fihn (which will be referred to here as simply the substrate ) and a masked area that is intended to be devoid of film ( the mask ). Successful selective CVD can eliminate subsequent patterning steps that would be needed if a more conventional deposition blanketed the entire substrate, including the mask, with fihn. Selective deposition is very helpful for maintaining tight tolerances in current microelectronic devices. 

The concentration of reactant on the surface is a dynamic balance between arrival rate, adsorption and desorption. The net surface coverage may be expressed as in Equation 10.8 or as a ratio of the adsorption rate ka to the desorption rate kj as  

Differential adsorption probability Substrate adsorption mask adsorption  

Differentiai desorption probability Mask desorption substrate desorption 

Differential nucleation rate Substrate nucleation mask nucleation 

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CVD processing can be used to provide selective deposition on certain areas of a surface. Selective tungsten CVD is used to fill vias or holes selectively through siUcon oxide layers in siUcon-device technology. In this case, the siUcon from the substrate catalyzes the reduction of tungsten hexafluoride, whereas the siUcon oxide does not. Selective CVD deposition can also be accompHshed using lasers or focused electron beams for local heating. 

HampdensmithMJjKodas TT (1995) Chemical-Vapor-Deposition of Metals. 2. Overview of Selective Cvd of Metals. Chemical Vapor Deposition 1(2), 39-48... 

There are two aspects of tungsten CVD for integrated circuits that have taken on commercial importance. One is the blanket deposition and subsequent patterning, so it can be used as a conductor to replace high-resistivity doped poly. The second area of interest is the "selective" CVD of tungsten, where deposition occurs on silicon but not on silicon dioxide. Here one can selectively fill via holes to either provide a thin barrier metal or to deposit a thicker layer to help planarize the circuit. Both applications involve only one processing step, and are attractive for this reason. 

The formation of CVD-W plug was studied by a nonselective or blanket CVD-W and a selective CVD-W. Blanket CVD-W is deposited by the hydrogen or monosilane reduction of tungsten hexafluoride as shown in Eqns. (27), (35) and (36). At present, monosilane is mainly used as a reducing agent because of its high rates even at a low temperature. 

On the other hand, a selective CVD-W makes good use of the selective nature of the deposition chemistry, i.e. tungsten can be selectively deposited on silicon, but not on Si02. 

Many of the first papers which discussed the use of (selective) CVD of tungsten for IC applications used conventional hot wall tube CVD reactors [Broadbent et al.44, Pauleau et al.45, Cheung47]. This type of reactor was and still is the workhorse in IC fabs. Excellent films such as TEOS based oxides, thermal silicon-nitride and poly-silicon can be grown in such equipment. Hot wall tube reactors are suitable for these films because such materials stick very well to quartz tubes and are quite transparent to IR radiation of the heating elements. Thus neither particle nor temperature control is a problem. One other major advantage is that high throughputs are typically obtained. 

Extensive literature has been published on blanket and selective CVD-W, in which a vast amount of (sometimes conflicting) information can be found. What is clearly needed is a book where all relevant and pertinent material is gathered in a condensed format. It is the intention of this book to provide such a compilation of the literature with emphasis on the material which has appeared in the last 10 years. In addition, unpublished material obtained in the laboratory of the author is included. After reading this work, the reader will have all the necessary background to bring up, fine tune and maintain successfully a CVD-W process in a production line. Others seeking a quick overview of the current status of CVD-W will also benefit from this book. 

Selective Selective CVD, selective Possible to fill only the required... 

Simon AH, Bolom T, Niu C, Baumann FH, Hu C, Parks C, et al. Electromigration comparison of selective CVD cobalt capping with PVD Ta(N) and CVD cobalt liners on 22nm-groundrule dual-damascene Cu interconnects. In Reliability physics symposium (IRPS), 2013 IEEE international, 14—18 April 2013 2013. pp. 3F.4.1—3F.4.6. 

A typical example of selective CVD is the deposition of W on Si using WFg gas. [10] This reaction is carried out in two stages, nucleation and growth. The selective W process is shown schematically in Figure 12.23. The nucleation step reaction is... 

Figure 12.23 The selective CVD process based on WFg decomposition catalyzed during the nucleation step by Si for the initial reaction in the absence of H2 and the growth reaction catalyzed by W in the presence of H2.

Selective CVD deposits material selectively on exposed substrate rather than on a surrounding masked area. 

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