Tungsten gates

Mobile ion contamination Whereas tungsten gates fabricated using sputter deposition need ultra pure targets [Yamamoto et al.199], CVD-W (using 99.5% purity WF6) showed very low (0.01 ppm) mobile ion contamination [Kobayashi et al.200]. 

Tungsten silicide gates interconnections of cold wall 1 Torr... 

Hot- and cold-mirror CVD coatings are used in projectors to maintain the film gate at low temperature and avoid damaging the film. They are also used increasingly in tungsten-halogen lamps. 

Chemical Vapor Deposition. Deposition of tungsten, molybdenum, and their silicides by chemical yapor deposition (CVD) is of relatively recent interest in the microelectronics industry. These materials arc useful for gates and interconnects in metal oxide semiconductors (MOS) devices. Aluminum, the widely used interconnect material, has a comparatively low melting point (600°C) and a markedly different coefficient of thermal expansion (compared to silicon), so that over a period of years researchers have been seeking an alternative for aluminum,... 

Tungsten silicide has been widely used as a gate electrode for the following reasons ... 

At present, tungsten silicide is usually used on the top of gate polysilicon as polycide structure which is an abbreviated name for polysilicon-silicide as shown in Fig. 6. 

Improvements in the performance of integrated circuits and the trend towards VLSI-technology require the replacement of polycrystalline silicon by materials with a lower resistivity for use as gate electrodes. Transition metal silicides appear to be valuable possibilities for these applications. Timgsten-silicon compounds could be suitable precursors for the precipitation of tungsten-silicide thin films. Moreover tungsten-silicon compounds are nearly unknown and of scientific interest. 

In the following section we will show that tungsten as the metal gate can possibly solve these issues. A very attractive property of refractory metals, such as Mo, W, Ti and Ta, is that they have mid gap workfunctions which is very beneficial in CMOS technology. 

In the discussion below we limit ourselves to pure metal gates although alternatives have been proposed such as a tungsten/poly-silicon stack [Wong and Saraswat201]. It is helpful to keep the process flow as depicted in figure 8.1 in mind. 

A consistent model of the action of fluorine on gate oxide was proposed by Wright and Saraswat243. In this study the fluorine was introduced in the poly-Si by means of ion implantation. This approach allows various fluorine levels to be studied and eliminates complications from other elements like tungsten or hydrogen or stress induced effects. To... 

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