Metals silicon nitrides

Other useful refractory nitrides for corrosion protection are silicon nitride (Si3N4) and boron nitride (BN). Silicon nitride has good corrosion resistance and is not attacked by most molten metals as shown in Table 17.6 (see Ch. 10). 

In a further improvement, the NTD Ge spider-web bolometer developed by the Caltech/JPL group [69], and shown in Fig. 15.10, the absorber is a metallized silicon nitride spider-web structure, and the thermometer is a very small NTD germanium crystal [14], The spacing of the web pattern is much smaller than the wavelength, so it acts... 

Insulator sputtering is similar to the process described for metal sputtering. The only difference is that the source target is a dielectric film. There is less control of the chemical nature and quality of the film as compared to a CVD deposited film. Common sputtered films include silicon nitride and silicon oxide. 

Considerable recent activity in the area of ceramic processing is aimed toward the formulation of materials with high strengths, comparable to the room temperature strength of metal alloys, at high temperatures (of the order of 2000 K). The impetus comes from the significant gains made in the last 20 years with materials formed from submicron powders of silicon nitride and silicon carbide and the promise of similar improvements in the near future. 

The carbides and nitrides are well known for their hardness and strength, and this section will briefly compare a number of these properties with those of the pure metals. Concentration will be placed here on the first row compounds, since these constitute a complete series, and Mo and W, since these are the most commonly studied metals. As will be shown, the physical and mechanical properties of carbides and nitrides resemble those of ceramics not those of metals. Comparisons will be made with boron carbide (B4C), silicon carbide (SiC), aluminium nitride (AIN), silicon nitride (Si3N4), aluminium oxide (A1203), and diamond, as representative ceramic materials. 

Table 4.6. A selection of chemical shifts for silicon nitride, silicon oxynitride and SiAION compounds and related metal silicon nitrides and oxynitrides, with respect to TMS.

II. Representation of Silicon Nitride-Metal Oxide Systems. 128... 

This chapter is not intended to be a literature survey, since an excellent review paper on phase diagrams of silicon nitride-metal oxide systems has been written by Sorrell [5], This chapter, is, however, intended to illustrate the use of phase diagrams to study silicon nitride ceramics. 

Silicon nitride ceramics are one of the most promising materials for structural applications. The properties of the silicon nitride ceramics depend on the phases present and their microstructures. This paper describes how to approach microstructural design of these materials by using the phase equilibrium diagrams of silicon nitride-metal oxide systems. 

The formation of silicon nitride whiskers was observed in several different reactions, including vapor deposition, CVD, and growth from a melt. However, only the following techniques are considered to have commercial significance nitriding of metallic silicon or silicon-silica mixture, carbothermal reduction of silica with simultaneous nitridation, and thermal decomposition of silicon halides. 

Some ceramic materials are not found widely or at all in nature, and thus are synthesized for use. To prepare more complex ceramic compositions such as perovskites of general structural formula ABO3, and ferrites, of formula MFc204, the individual oxides or salts of the cations A, B, and M are often combined as powders and then reacted at high temperature by a solid-state diffusion mechanism. Silicon nitride (Si3N4) can be manufactured from either the nitridation of silicon metal or from the reaction of silicon tetrachloride with ammonia. Silicon carbide (SiC) is obtained from the reduction of silica with a carbon containing source. 

The only useful materials for heat removal are metal or ceramics. The ceramics obviously can be used as a substrate material in their own right, and beryllia is the standard for this kind of application. Now, other materials such as Aluminium Nitride and Silicon Carbide are being developed to replace beryllia because of its reputation as a poisonous product. 

Plasma etchants are available for most of the films commonly used in micromachining. Etching of oxide, nitride, and silicon is usually done with fluorine-containing compounds such as SiF4, CF4, and SF6. Aluminum and other metals can be etched in chlorine plasmas such as SiC l4 and BC13. In certain conditions, better results can be obtained by combining several gases. 

Fig. 1. Cross section of an integrated circuit with three levels of Al/Si/Cu metallization separated by spin-on-glass (SOG) and plasma enhanced oxide (PEOX). The top passivation layer is typically silicon nitride. The silicon substrate is not shown. After [2].

Stoichiometric silicon nitride (Si3N4) often is used for chemical passivation and encapsulation of silicon bipolar and MOS devices, because it serves as an extremely good barrier to diffusion of water and sodium ions. Water causes device metallization to corrode and sodium causes devices to become electrically unstable. Silicon nitride is also used as a mask for the selective oxidation of silicon, and as a strong dielectric in MNOS (metal-nitride-oxide-silicon) structures. 

Many metal nitrides exhibit interesting properties such as extreme hardness, high melting points and resistance towards organic solvents and inorganic acids (Table 7-1). Several non-metallic nitrides also are of great interest. Among them, boron nitride and silicon nitride are of particular interest. The properties and preparation of these two compounds by CVD are described elsewhere (see Chapter 6). Most nitrides can be prepared by solid state reactions [142]. However, for the preparation of thin films, CVD is the method of choice. This section focuses on nitrides which have not been described in previous chapters. 

Nitrides are the binary salts of metals and nitrogen, containing the anion. While the nitrides of alkali metals and some alkaline earth metals are ionic, most other metal nitrides are covalent. Nitrides of silicon and selenium are covalent and polymeric. Many complex and polynuclear metal nitrides are known. The formulas and CAS numbers for some nitrides are presented in Table 57.1. 

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