Selective Tungsten

Although there are a number of reactions from which the selective deposition of tungsten could be implemented, the only ones that have been investigated are the reduction of WF6 by either Si or H2 [see Equations (5) and (6)]. The first of these will obviously be selective. The second can be selective when the H2 reduction is favored on W surfaces, as compared to oxide surfaces. 

Since the mechanism whereby silicon reduces WF6 involves the consumption of silicon (i.e., production of SiF4, SiF2, etc.), there is concern that there should not be too much encroachment of the tungsten into the silicon. Assuming the silicon is consumed uniformly, then two atoms of silicon will be released for each tungsten atom deposited. As tungsten is a denser material than Si, this translates into a thickness of Si consumed which is twice the thickness of tungsten deposited. 

Finally, the type of dielectric also plays a role in selectivity. The percentage monolayer coverage achieved in a WF6 deposition at 300°C for 40 minutes is shown in Table 2 for several different films.24 

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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. 

The second reaction. Equation (3), is the basis for the selective tungsten process we will discuss later. It also plays some role in the blanket process. 

Table 2 Monolayer Coverage of Selective Tungsten on Different Dielectrics27...

Figure 18 Selective tungsten deposition in a contact hole.

A second approach to a cold wall system is the single-wafer CVD reactor developed by Varian-Torrex. A schematic of the reaction chamber is shown in Figure 25. Again, tungsten silicide is deposited in this cold-wall reactor. Other conducting films such as blanket and selective tungsten can also be deposited. 

Keywords alkanes-isomerization iron-promoter n-pentane-isomerization platinum-promoter selectivity tungsten oxide zirconia-tungstated-acidity zirconia-tungstated-redox properties... 

The attractive features of a selective tungsten process in IC s were mentioned more than 10 years ago. Two problems have prevented a timely implementation ... 

In the near future we will see that the use of aluminum will diminish and other interconnect materials will come into the picture. Nevertheless most studies of selective tungsten in vias are presently focused on aluminum. 

Figure 3.2. Severe wormhole formation in the source area of a MOS transistor due to selective tungsten deposition. [TEM courtesy J. Planner, Signetics Corporation],...

Although the reaction between silicon and WF6 is necessary to start the tungsten deposition, at the same time it causes many problems and in fact has prevented the successful implementation of selective tungsten based on the H2/WF6 chemistry. TEM studies of the tungsten-silicon interface... 

Figure 3.5. Top view or contacts filled with selective tungsten (H WF6 chemistry).

The resistivity of the a-W phase in this study is more than twice the values obtained at higher deposition temperatures (>400°C) [Fuhs et al.80]. This is probably due to the relatively high silicon content (however, see section 5.3.5). The increased resistivity for selective tungsten is in most applications not a problem. 

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