Blanket tungsten

We will review recent work in the blanket tungsten process first. 

Tungsten can be deposited by CVD by a number of different processes. Several that have received considerable study are  

The first is the hydrogen reduction process which can proceed on any surface raised to a suitable temperature. The second is the silicon reduction process where silicon reduces WF6. The third process is similar to the first, but substitutes chlorine for fluorine. The final process is related to the WSi2 deposition studied earlier. It has been shown17 that depending on the deposition conditions, one can deposit either W, WSi2 or W5Si3 from these two reactants. 

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. 

The major problem, then and now, in attempting to deposit blanket tungsten is the adhesion to silicon dioxide. Unless some steps are taken beforehand, the H2 reduction of WF6 on Si02 will not produce an adherent film. The solution to this problem was to undertake the selective process initially.18 It is then pointed out that WF6 will decompose on Si, but it will slowly etch Si02. Once the Si02 has been etched by WF6, then the W deposited from the H2 reduction process adheres very well. 

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Chemical Vapor Deposition- Deposition of silicon oxide films is accomplished by CVD equipment. Either plasma CVD or ozone oxidation is used. Blanket tungsten films are also deposited by CVD equipment to create contact and via plugs. Polysilicon and silicon nitride films are deposited in hot-wall furnaces. TiN diffusion barrier films are deposited by either sputtering or CVD, the latter giving superior step coverage. 

Finally, blanket tungsten can be deposited from tungsten carbonyl, W C0)6. Some data has been published and such depositions appear to result in films with appreciable carbon and oxygen.23... 

The Si consumption can be beneficial in that a more intimate contact between W and Si is promoted.25 It can, however, lead to encroachment problems that will be discussed later. As in the blanket tungsten deposition case, film resistivities are higher than bulk. For selective deposition, thinner films are useful, so it is important to recognize that resistivity varies with film thickness, as shown in Figure 16.26... 

FIGURE 5.29 Correlation of relative blanket tungsten removal rate (W-RR) and PECVD-TEOS coating time [86]. 

Inlaid metal patterning schemes were first used in the formation of tungsten studs to fill contact and via holes. In this application, a blanket tungsten film is deposited by CVD thick... 

In this chapter we will focus on contact and via fill using the blanket tungsten approach. In chapter 5 we will discuss another application of blanket tungsten, namely, that of tungsten as the interconnect material. 

Figure 2.1. Three key steps in the blanket tungsten process a) the deposition of the adhesion layer, b) after the blanket tungsten deposition and c) after tungsten etch back.

The blanket tungsten approach diffusion coefficient is now given by ... 

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