Nitrides silicon

Silicon nitride, Si3N4, has wide applications as a ceramic and refractory material and in the form of whiskers (see Section 28.6). It is a white, chemically inert amorphous powder, which can be formed by reaction 14.90, or by combining Si and N2 above 1650 K. 

The two most important polymorphs, a- and (3-Si3N4, possess 3-dimensional structures containing distorted tetrahedral Si atoms, and 3-coordinate, near-planar N atoms. The detailed crystal stractures and the presence of lattice defects have been the subject of debate for many years. A denser, harder polymorph, y-Si3N4, has been obtained by high- 

Silicon nitride can be sintered in five diOererrt methods  

Apart from reaction sintering, all the other processes reqture sintering additives. This corrsists of nitriding powdery sihcon compact at about 1,300-1,350°C and partially fihing the porosity (final residrral porosity = 15%) at the time of the reaction  

Sihcon nitride exists imder two crystalline stmctures of hexagonal symmetry the low temperature a variety, whose crystals are equiaxed and the high temperature P variety, generaUy in the form of adcirlar crystals. The transformation a P is not reversible. 

Sintering and the various mecharrisrrrs which take place during densification are weU-known [BRO 77]. It is possible to modirlate the microstracture (size and shape of the crystals) in a broad domain according to the desired mechanical properties or apphcahorts in question. 

Solid particles in a hquid exhibit a solubility that varies with their size. This difference in solubility forms the driving force of the crystalline growth and corresponds to the phenomena of Ostwald ripening. For two soluble grains with radius rj and r2 (rj r2), the rate of growth of the coarsest grain is expressed in the 

The wide applications of silicon nitride, Si3N4, as a ceramic and refractory material and in the form of whiskers (see 

Silicon dioxide has selectivity to many silicon etchants and therefore is a good mask material for self-aligned etching process. In combination with silicon nitride, multistep etching process of three-dimensional structure is possible. Silicon dioxide is also used to seal microchannels. The insulating property of sihcon dioxide makes it a good coating layer of channels in microfluidics. 

Silicon nitride is created by CVD process of silane (SiH4) and dichlorosilane (SiH2Cl2) in ammonia atmosphere at high temperature. The reactions for LPCVD is 

Silicon nitride is a good insulator and acts as barrier against all kinds of diffusion to water and ions. Due to thermal insulation properties, heater structures are suspended on silicon nitride membrane or fixtures. Its ultrastrong resistance to oxidation and many etchants makes it a superior material for masks in deep etching. It is also used as high-strength electrical insulator. 

The Shockley equations describe a linear dependence between drain current and permittivity of the gate dielectric. In order to lower the supply voltage of OFETs, silicon nitride was examined because its permittivity Cnitride 8 is almost twice as large as that of silicon dioxide. Moreover, with a roughness of 1-2 nm rms it has the smoothest surface of the chemical vapour deposited dielectrics, which is one essential requirement for the growth of a well-ordered pentacene film. 

In the following, a silicon nitride layer was deposited as the gate dielectric on a thermally oxidised silicon wafer. The nitride layer was re-oxidised to enhance the electrical stability. The silicon dioxide below the nitride film adopted the function of a buffer layer to reduce mechanical stress between the silicon and silicon nitride due to different thermal coefficients of expansion. To deposit the dielectric film, ammonia gas and triethylsilane were put into the process tube in a ratio of 1 5, at 800 °C and at a process pressure of 0.3 mbar. The thickness of the deposited dielectric film was about 75 nm in total. 

One reason for the high off-current of the transistor at E g = 0 V could be interface charges at unsaturated interface bonds of the gate dielectric. Another cause for the off-current could be found in carbon bonds, for example ethyl groups, that were separated from the chemical product triethylsilane during the 

Off-currents have also been measured at some organic field effect transistors using silicon dioxide as the gate dielectric. During the investigations it was not possible to detect the real reason for this sporadically occurring behaviour, which was only detectable after the pentacene deposition. One explanation could be the generation of interface traps as a consequence of the pentacene deposition. 

The high deposition temperature excluded the manufacturing of simple circuits using silicon nitride as gate dielectric for transistors with a metal gate electrode. Therefore, another gate dielectric, deposited at lower process temperatures, is necessary. 

There did not appear to be any gross evidence of hydrogen in the films, but none of the analytical techniques used by these authors to evaluate the film composition were sensitive to this impurity. 

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As noted, the oxidation resistance of silicon nitride ceramics depends on the type and concentration of the sintering aids. In materials designed for high temperature appHcations the specific weight gain resulting from oxidation upon a 500-h air exposure at 1200°C and 1350°C is about 1—2 g/m and 2—4 g/m, respectively. The kinetics of the oxidation process have been iavestigated (63,64) as has the corrosion resistance (65). Corrosion resistance is also dependent on material formulation and density. 

Silicon Nitride. SiUcon nitride is manufactured either as a powder as a precursor for the production of hot-pressed parts or as self-bonded, reaction-sintered, siUcon nitride parts. a-SiUcon nitride, used in the manufacture of Si N intended for hot pressing, can be obtained by nitriding Si powder in an atmosphere of H2, N2, and NH. Reaction conditions, eg, temperature, time, and atmosphere, have to be controlled closely. Special additions, such as Fe202 to the precursor material, act as catalysts for the formation of predorninately a-Si N. SiUcon nitride is ball-milled to a very fine powder and is purified by acid leaching. SiUcon nitride can be hot pressed to full density by adding 1—5% MgO. 

Fig. 12. (a) A cross section of multiple coatings of TiN on TiC on a silicon nitride-based tool material (b) multicoatings on a SiAlON-based tool material. 

Creep Resistsince. Studies on creep resistance of particulate reinforced composites seem to indicate that such composites are less creep resistant than are monolithic matrices. Silicon nitride reinforced with 40 vol % TiN has been found to have a higher creep rate and a reduced creep strength compared to that of unreinforced silicon nitride. Further reduction in properties have been observed with an increase in the volume fraction of particles and a decrease in the particle size (20). Similar results have been found for SiC particulate reinforced silicon nitride (64). Poor creep behavior has been attributed to the presence of glassy phases in the composite, and removal of these from the microstmcture may improve the high temperature mechanical properties (64). 

This mixture of gases has been used to prepare silicon nitride particles photochemically, the overall reaction being represented by... 

The covalently-bonded silicon carbide, silicon nitride, and sialons (alloys of Si3N4 and AI2O3) seem to be the best bet for high-temperature structural use. Their creep resistance... 

Silicon nitride SiaN (3Si02 + 2N2) = -629 Gold (AU2O3) +80... 

Ceramics themselves are sometimes protected in this way. Silicon carbide, SiC, and silicon nitride, Si3N4 both have large negative energies of oxidation (meaning that they oxidise easily). But when they do, the silicon in them turns to Si02 which quickly forms a protective skin and prevents further attack. 

High -performance engineering ceramics Diamond Dense alumina Silicon carbide Silicon nitride Zirconia Sialons... 

When you pour boiling water into a cold bottle and discover that the bottom drops out with a smart pop, you have re-invented the standard test for thermal shock resistance. Fracture caused by sudden changes in temperature is a problem with ceramics. But while some (like ordinary glass) will only take a temperature "shock" of 80°C before they break, others (like silicon nitride) will stand a sudden change of 500°C, and this is enough to fit them for use in environments as violent as an internal combustion engine. 

Zirconia, ZrOj, is made from the natural hydrated mineral, or from zircon, a silicate. Silicon carbide and silicon nitride are made by reacting silicon with carbon or nitrogen. Although the basic chemistry is very simple, the processes are complicated by the need for careful quality control, and the goal of producing fine (<1 jiva) powders which, almost always, lead to a better final product. 

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