Selective deposition typically relies upon the reaction of a gas, such as tungsten hexafluoride, with substrates, such as silicon, to leave tungsten on the silicon surface. Selective deposition is also possible on metals and silicides.

 

With blanket deposition, tungsten is deposited over the entire surface and then etched back so that tungsten ideally remains only in the vias or as, for example, interconnects. While this process is conceptually simple, tungsten does not adhere well to silicon dioxide and practical problems arise. For example, after the deposition has been completed, the tungsten film may simply peel off the silicon dioxide which is also an obviously undesirable result.

 

The use of a glue layer has been proposed to overcome the adhesion problem just discussed. A glue layer is a layer of material deposited prior to the tungsten and which has good adhesion both to the underlying dielectric layer and to the tungsten. Several substances have been proposed for glue layers. For example, the use of both elemental metals, such as Ti, and metallic silicides, such as WSi2, has been proposed. See, for example, Comparison of Two Contact Plug Techniques for Use with Planarized Oxide and A Contact Filling Process with CVD-Tungsten for Multilevel Metallization Systems, Proceedings of the V-MIC Conference, pp. 403-410, and pp. 443-449, June 9-10, 1986, respectively. A common feature of the approaches adopted by these references in the use of relatively thick glue layers. Thicknesses of approximately 100 nm or more were used.

 

 

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