Chemistry of selective tungsten

In an effort to quantify the dark field counts, Chow et al.67 compared these counts with two other techniques, RBS and Total Reflectance X-Ray Fluorescence (TRXRF). Some discrepancies between these techniques were found. See section 3.5 for more details. To the authors knowledge no relationship between yield and actual selectivity loss has yet been reported. 

The displacement reaction Although it is believed that the key reason for selectivity is the activated adsorption of hydrogen on tungsten there is obviously no tungsten available in the beginning of the deposition (in a contact to silicon). The first reaction which will occur is that between silicon and WF6, the so called displacement reaction [Broadbent et al.44]  

This was confirmed in another study using RBS spectroscopy by Kuiper et al.72. We see that in this case the silicon consumption is twice that of reaction 3.1. The presence of H2 has only a negligible effect on the course of the reaction since the reaction between Si and WF6 is so much faster than that between H2 and WF6 [Broadbent et al.44]. In the remaining part of this section we will summarize some explanations offered in the literature for the observed thickness range. 

Another explanation is that the presence of the native oxide (in which there are pinholes) can cause thicker films of tungsten [Green et 

In a detailed analysis Hitchman et al.74, came to an interesting conclusion and pointed out that trace amounts of water can influence the result tremendously. For instance, WF6 will not react with Si02 in a dry ambient possibly due to the formation of a protecting W03 film on the oxide. However, if water is present there will be a reaction between WF6 and H20 according to  

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