Aluminum nitride ceramic materials

Yoshimura HN, Molisani AL, Narita NE, Cesar PF, Goldenstein H. Porosity dependence of elastic constants in aluminum nitride ceramics. Materials Research 2007 10(2) 127-133. 

Materials used for substrates can be broadly classified into ceramics and metals. Gommonly used ceramics, ie, alumina, aluminum nitride, and beryUia, can be easily incorporated into a hermetic package, ie, a package permanently sealed by fusion or soldering to prevent the transmission of moisture, air, and other gases. 

Although beryllium oxide [1304-56-9] is in many ways superior to most commonly used alumina-based ceramics, the principal drawback of beryUia-based ceramics is their toxicity thus they should be handled with care. The thermal conductivity of beryUia is roughly about 10 times that of commonly used alumina-based materials (5). BeryUia [1304-56-9] has a lower dielectric constant, a lower coefficient of thermal expansion, and slightly less strength than alumina. Aluminum nitride materials have begun to appear as alternatives to beryUia. Aluminum nitride [24304-00-5] has a thermal conductivity comparable to that of beryUia, but deteriorates less with temperature the thermal conductivity of aluminum nitride can, theoreticaUy, be raised to over 300 W/(m-K) (6). The dielectric constant of aluminum nitride is comparable to that of alumina, but the coefficient of thermal expansion is lower. 

Most structural PMCs consist of a relatively soft matrix, such as a thermosetting plastic of polyester, phenolic, or epoxy, sometimes referred to as resin-matrix composites. Some typical polymers used as matrices in PMCs are listed in Table 1.28. The list of metals used in MMCs is much shorter. Aluminum, magnesium, titanium, and iron- and nickel-based alloys are the most common (see Table 1.29). These metals are typically utilized due to their combination of low density and good mechanical properties. Matrix materials for CMCs generally fall into fonr categories glass ceramics like lithium aluminosilicate oxide ceramics like aluminnm oxide (alnmina) and mullite nitride ceramics such as silicon nitride and carbide ceramics such as silicon carbide. 

A variety of other ceramics are prepared by pyrolysis of preceramic polymers.32,38 Some examples are silicon carbide, SC, tungsten carbide, WC, aluminum nitride, AIN, and titanium nitride, TiN. In some cases, these materials are obtained by simple pyrolysis in an inert atmosphere or under vacuum. In other cases a reactive atmosphere such as ammonia is needed to introduce some of the atoms required in the final product. Additional details are given in Chapter 9. 

In the following sections some examples are given of the ways in which these principles have been utilized. The first example is the use of these techniques for the low temperature preparation of oxide ceramics such as silica. This process can also be used to produce alumina, titanium oxide, or other metal oxides. The second example describes the conversion of organic polymers to carbon fiber, a process that was probably the inspiration for the later development of routes to a range of non-oxide ceramics. Following this are brief reviews of processes that lead to the formation of silicon carbide, silicon nitride, boron nitride, and aluminum nitride, plus an introduction to the synthesis of other ceramics such as phosphorus nitride, nitrogen-phosphorus-boron materials, and an example of a transition metal-containing ceramic material. 

Aluminum nitride is one of the few materials that is both a good thermal conductor and a good electrical insulator. It is also a high-temperature ceramic, that has a low thermal expansion coefficient, and a low dielectric constant. It is also stable to molten metals such as aluminum, has good wear resistance, and good thermal shock resistance. 

Metals and ceramics (claylike materials) are also used as matrices in advanced composites. In most cases, metal matrix composites consist of aluminum, magnesium, copper, or titanium alloys of these metals or intermetallic compounds, such as TiAl and NiAl. The reinforcement is usually a ceramic material such as boron carbide (B4C), silicon carbide (SiC), aluminum oxide (A1203), aluminum nitride (AlN), or boron nitride (BN). Metals have also been used as reinforcements in metal matrices. For example, the physical characteristics of some types of steel have been improved by the addition of aluminum fibers. The reinforcement is usually added in the form of particles, whiskers, plates, or fibers. 

Thermal Evaporation The easiest way of evaporating metal is by means of resistance evaporators known commonly as boats . Boats, made of sintered ceramics, are positioned side by side at a distance of approximately 10 cm across the web width (Fig. 8.1). Titanium boride TiB2 is used as an electrically conductive material with boron nitride BN (two-component evaporator) or BN and aluminum nitride AIN (three-component evaporator) as an insulating material [2]. By combination of conductive and insulating materials, the electrical properties of evaporators are adjusted. 

Aluminum nitride is most commonly used for its high thermal conductivity. Recently, a poreless composite material, TiAl-TiB2-AlN, was obtained by reacting a Ti-F(0.7-0.95)A1+(0.05-0.50)8 mixture at 30- to 100-atm nitrogen pressure (Yamada, 1994). The use of high-pressure nitrogen gas was found to be effective for simultaneous synthesis and consolidation of nitride ceramics with dispersed intermetallic compounds (e.g., TiAl). Dense, crack-free products with uniform grains (approximately 10 mm in size) were obtained. 

Included in the term nonoxide ceramics are all non-electrically conducting materials in the boron-carbon-silicon-aluminum system. The industrially most important representatives, apart from carbon (see Section 5.7.4), are silicon carbide (SiC), silicon nitride (Si3N4), boron carbide (B4C), boron nitride (BN) and aluminum nitride (AIN). 

Numerous ceramics are deposited via chemical vapor deposition. Oxide, carbide, nitride, and boride films can all be produced from gas phase precursors. This section gives details on the production-scale reactions for materials that are widely produced. In addition, a survey of the latest research including novel precursors and chemical reactions is provided. The discussion begins with the mature technologies of silicon dioxide, aluminum oxide, and silicon nitride CVD. Then the focus turns to the deposition of thin films having characteristics that are attractive for future applications in microelectronics, micromachinery, and hard coatings for tools and parts. These materials include aluminum nitride, boron nitride, titanium nitride, titanium dioxide, silicon carbide, and mixed-metal oxides such as those of the perovskite structure and those used as high To superconductors. 

Aluminum nitride has received attention as an alternative to Si02 dielectric layers in microelectronic circuits because of its high dielectric strength. Being a refractory ceramic with high thermal conductivity, AIN is useful for electronics packaging. There are also uses for AIN as a piezoelectric material because of its high surface acoustic wave velocity. 

The synthesis of nanophase ceramics is one of these concepts, it allows micro-porous ceramic materials with ceramic grains in the nanometer range to be obtained. Research in the field of nanophase materials is very active. A number of results on the control of microstructure and temperature stability of metal oxide ceramics can be applied to membrane preparation. Works carried out on non-oxide ceramics such as silicon carbide, silicon oxinitride or aluminum nitride should be regarded in order to extend the domain of available membrane materials. 

Multi-chip modules, in which chips are attached with TAB directly to an interconnection substrate, typically silicon or aluminum nitride or other ceramics, represent an emerging technology. The module interconnect circuits are generated with IC fabrication technology, but at a relatively modest level of resolution. Typically, as shown in Figure 6, the substrates can include power and ground planes plus one or more signal planes, and low dielectric constant interlayer materials such as polyimides. These modules, which have feature sizes ca. 5 to 50 pm, and which are mounted directly on the PC board or serve as the board itself,... 

A review of the literature showed that the nanoparticles used in the production of nanofluids were aluminum oxide (AI2O3), titanium dioxide (Ti02), nitride ceramics (AIN, SiN), carbide ceramics (SiC, TiC), copper (Cu), copper oxide (CuO), gold (Au), silver (Ag), silica (Si02) nanoparticles and carbon nanotubes (CNT). The base fluids used were water, oil, acetone, decene and ethylene glycol. Modem technology allows the fabrication of materials at the nanometer scale, they are usually available in the market under different particle sizes and purity conditions. They exhibit... 

Four examples were given in the patent, all using supercritical propane with polysilane and aluminum isopropoxide, so-called precursor ceramic materials. In one instance scanning electron microscopy was used to demonstfate that polysil me was deposited as a smooth surtace film on the alumina fibers and fine silicon nitride whiskers. A silicon carbide material with internal pore openings as small as 10 microns was penetrated by supercritical propane laden with aluminum isopropoxide. Weight gains of up to 43% can be obtained depending on the density of the initial host ceramic. 

Thermally conductive adhesives may be filled with metal, ceramic, or inorganic particles. Silver-filled epoxies have high thermal conductivities, but may not be used where there is a risk of electrical shorting. In such cases, epoxies or other polymers filled with electrically resistive, but thermally conductive materials such as aluminum nitride, boron nitride, alumina, or beryllia must be used. Some applications for thermally conductive adhesives include attachment of power devices, heat sinks, large components such as capacitors and transformers, large ceramic substrates, and edge connectors. 

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