The pillar approach

As will be clear from the following explanation, the pillar process has the same advantages, in terms of Si real estate, as has the tungsten plug process (see chapter I). The process steps have been described by Welch et al.184(see also Yeh et al.185 for pillar variations). In figure 8.10 we see a sketch of the complicated process flow. The starting metal layer consists of two AlCu films sandwiched between three TiW layers. On the stack a photoresist is spin coated in which the inverse via mask is printed (figure 

The TiW layer is etched in a fluorine based plasma with good selectivity towards the AlCu layer after which the resist is stripped (figure 

A disadvantage of this approach is that it obviously will not work for the fill of contacts to silicon (however, see Yeh et al.185). This implies that two plug techniques are necessary to overcome the reliability issues, one (blanket) for the contacts and one (pillars) for the vias. 

The previous part of this book covers the chemical vapor deposition of tungsten. However, there are several reasons to include a discussion about the chemical vapor deposition of tungsten silicide (CVD-WSix). These are  

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