Tungsten etch-back

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.

Figure 2.20. Etch back scheme using a sacrificial layer, in this case polyimide. Situation after polyimide spin (a) and after polyimide/tungsten etch back (b).

Degree of Planarization (DOP) Whereas the result of the blanket tungsten etch back will be very sensitive for the DOP, selective tungsten is virtually... 

In addition, several issues arise with the use of either RIE etch back or CMP to remove the metal from on top of ILD. Dual layw inlaid metal schemes have been demonstrated for tungsten, aluminum, copper,and gold metallization systems. 

As with oxide CMP (Chapter 5), metal CMP may enhance yields by virtue of reduced defect densities. In addition to a reduction in nonplanarity induced defects (Section 5.24), CMP is a cleaner process than the relatively dirty RIE etch back processes. Figure 6.5 shows a 3X reduction in particles using CMP vs. RIE. The result is a decrease in metal-to-metal shorts on the subsequent interconnection level (Figure 6.6). As with oxide CMP, increased die yields is one of the major driving forces for acceptance of metal CMP processes for tungsten stud formation. ... 

Unlike W plasma etch back process, the typical W CMP process usually removes the adhesion layer such as Ti/TiN or TiN during the primary polish. As a result, during the over polish step there is some oxide loss. Since the oxide deposition, planarization CMP (oxide CMP), and tungsten CMP steps are subsequent to each other, the oxide thickness profile could become worse further into the process flow. Therefore, the across-wafer non-uniformity of the oxide loss during W CMP process is one of the very important process parameters needs to be optimized. To determine the effect of the process and hardware parameters on the polish rate and the across-wafer uniformity, designed experiments were run and trends were determined using analysis of variance techniques. Table speed, wafer carrier speed, down force, back pressure, blocked hole pattern, and carrier types were examined for their effects on polish rate and across-wafer uniformity. The variable ranges encompassed by the experiments used in this study are summarized in Table I. 

WSi would be a very acceptable candidate with regard to the in situ deposition possibility in the CVD-W reactor. WSix can relatively easily be deposited prior to tungsten deposition in the same (cold wall) reactor by using the SiH4/WF6 chemistry [see also chapter IX, Chiu et al.10]. Unfortunately, WS exhibits only a moderate adhesion towards oxide which results in missing plugs after the etch back step [Ellwanger et al.7]. 

At least two distinct approaches can be followed for etching back blanket tungsten 1) etch back without the use of a sacrificial layer and 2] etch back with the use of a sacrificial layer. 

Another solution to reduce the loading effect was investigated by van Laarhoven et. al.61, (see figure 2.19). In their approach there was a 0.3 urn PECVD silicon nitride layer deposited atop the oxide prior to the contact opening. The normal procedure of adhesion layer (TiW), tungsten deposition and etch back was followed. Since the nitride etches with about the same rate as the tungsten (selectivity W SiN=0.8) both the loading is... 

Degree of planarization (DOP) The blanket tungsten process is in fact a planarization method for contacts and vias, Therefore, if a topography exists before the tungsten deposition is done there will be a risk of tungsten residues or "stringers" after the etch back step (see figure 2.22). This cannot be tolerated since shorts between aluminum lines will occur. 

Figure 2.22. Formation or tungsten residues due to insufficient DOP. Situation after tungsten deposition (top) and after etch back (bottom).

Kaanta et al.142 implemented tungsten for contact fill, via studs, and the first interconnect metal. The via plug was produced by the blanket etch back method. The apparent disadvantage of the higher resistivity of tungsten as the interconnect was compensated by ... 

Applied Materials method In this approach backside deposition does occur, however, the tungsten at the backside is stripped back using a plasma etch. The etch is done such that the tungsten at the frontside remains untouched. 

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