Tungsten diffusion

The formation of a rhenium thermionic emission layer and a tungsten diffusion prevention layer... 

Based on these results, a (0001) oriented rhenium thermionic emission layer ( 1(X)// m ), and a tungsten diffusion prevention layer (5(X) /im) were formed on a molybdenum substrate, and emitters with a composition gradient at each interface were produced. 

V. Hoffman, Titanium tungsten diffusion barrier metallization. Solid State Technol. 

The only metals having good or excellent resistance to corrosion by amalgamation with mercury are vanadium, iron, niobium, molybdenum, cesium, tantalum, and tungsten (8). The diffusion rates of some metals in mercury are given in Table 5. 

Another important function of metallic coatings is to provide wear resistance. Hard chromium, electroless nickel, composites of nickel and diamond, or diffusion or vapor-phase deposits of sUicon carbide [409-21-2], SiC , SiC tungsten carbide [56780-56-4], WC and boron carbide [12069-32-8], B4C, are examples. Chemical resistance at high temperatures is provided by aUoys of aluminum and platinum [7440-06-4] or other precious metals (10—14). 

Fig. 47. Arrhenius plot of diffusion coefficient for (a) H and (b) D atoms on the (110) face of a tungsten crystal at coverage degree 0.1-0.9 as indicated. The cusps on the curves correspond to the phase transition.

Difoggio and Corner [1982] and Wang and Comer [1985] have discovered tunneling diffusion of H and D atoms on the (110) face of tungsten. They saw that the Arrhenius dependence of the diffusion coefficient D sharpy levels-off to the low-temperature limit (D = D ) at 130-140 K (fig. 47) the values of depend but slightly on the mass of the tunneling particle for the D and... 

Repeatedly in this book, the important functions of dopants , intentional additives made in small amounts to materials, have been highlighted the use of minor additives to the tungsten used to make lamp filaments is one major example. The role of impurities, both intentional and unintentional, in matters such as phase transformations, mechanical properties and diffusion, was critically reviewed in one of the early seminar volumes published by the American Society for Metals (Marzke 1955). But extreme purity was not considered that came a little later. 

Interconnect. Three-dimensional structures require interconnections between the various levels. This is achieved by small, high aspect-ratio holes that provide electrical contact. These holes include the contact fills which connect the semiconductor silicon area of the device to the first-level metal, and the via holes which connect the first level metal to the second and subsequent metal levels (see Fig. 13.1). The interconnect presents a major fabrication challenge since these high-aspect holes, which may be as small as 0.25 im across, must be completely filled with a diffusion barrier material (such as CVD titanium nitride) and a conductor metal such as CVD tungsten. The ability to fill the interconnects is a major factor in selecting a thin-film deposition process. 

The interconnecting holes are narrow and deep (at times less than 0.25 im wide and up to 2 im or more in depth) and, after a diffusion-barrier layer is applied, it must be filled completely with a high-conductivity metal (usually aluminum or tungsten) to provide the low-resi stance plug for inter-layer connections. Typically, CVD provides better step coverage and conformity than sputtering and other physical-vapor deposition processes. 

A typical example of the complexity of interconnection assembly consists of a MOCVD-TiN diffusion barrier combined with a low-temperature (260°C) CVD aluminum with low resistivity (<3 lQ-cm). A copper doping forms an overlying sputtered Al-Cu film. This aluminum plug provides a large reduction in resistance compared to the classical tungsten process. k" ... 

Copper is intrinsically a better metal than aluminum for the metallization of IC s. Latest developments in MOCVD show that it can be readily deposited without major changes in existing processing equipment. Diffusion problems are minimized and it appears that present barrier materials, such as titanium nitride or titanium-tungsten alloys, should provide adequate diffusion barriers for the copper-silicon couple, certainly up to the highest temperatures presently used in IC s processing (see Ch. 6). The development of CVD copper for semiconductor metallization is on a considerable scale at this time.Clt ]... 

Titanium-tungsten is an intermetal lie, composed mostly of tungsten. It is an excellent barrier if it is stuffed, that is, with nitrogen added into the crystalline defects. It prevents diffusion of silicon into aluminum up to 500°C. It is deposited mostly by sputtering. 

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