氮化铝陶瓷的金属化

An information-analytical study of metallization technology for aluminum nitride ceramic is performed. Development work on thin- and thick-film technologies for metallization of AIN ceramic is being widely con.ducted in domestic and foreign enterprises. Various articles for electronic technology have been developedusing metalized AlN articles.

An important problem of modern technology in connection with the development of highly reliable, fast, highpower devices is removing heat, since heat degrades reliabil-ity and shortens the service life of electronic devices. Heat-conducting materials can help to solve this problem [1].Possible heat-conducting materials for this are berylliumoxide BeO silicon carbide SiC, aluminum nitride AIN andboron nitride BN.

However, beryllium oxide is toxic, silicon carbide is asemiconductor with poor dielectric properties and boronnitride requires complicated technological methods to manu-facture articles. Aluminum nitride has the optimal set ofproperties and is nontoxic and articles can be manufacturedfrom it by simple and efficient methods (2].Aluminum nitride ceramic used in radio and electronicengineering at the last step of production of an article undergoes metallization followed by bonding soldering with metalstructures.

Ceramic is metalized in order to create a current conducting layer on a part of the article's surface serving as an electrode as well as to obtain a strong, and in some cases vacuum-tight, ceramic - metal connection, i.e., to create ametal-ceramic structure. Metallization is also done to depositthin-layer of metallic components of an electronic circuit onceramic bases, substrates and other articles .

The choice of metal and technology of metallization depends on the purpose, shape and dimensions of the metalcoating as well as the operating conditions. For the most part.precious metals  Ag, Au and Pt  are used to deposit electrically conduction layers. Aluminum and its alloys, refractory metals — W, Mo, Ta, Cr and iron-group alloys — are used for the electron components [4].

The thickness of metallic coatings varies from fractionsto several tenths of a micron and depends on the purpose ofthe coating. Metallization is said to be thin-film for coatingthickness to several microns and thick-film for thicker coat-ings ([3].

In the thick-film technology, a layer of finely dispersemetal on organic binder, secured by burn-in, is deposited onthe surface of a ceramic. In the thin-film technology, a metalfilm is sputtered on a ceramic surface by condensation ofatoms or ions (4].

The metal - ceramic interaction is due to chemical andphysical adsorption processes, mechanical bonding and diffusion. When a ceramic is metalized a multilayer transitionalzone consisting of the products of chemical interaction arisesat the metal - ceramic interface (4].To increase the mechanical bonding between the filmand the substrate with thick-film metallization technologylow-melting additives in the form of glass solders and otherfluxes are introduced into the pastes (4].

The thin-film technology is more complicated. Plasmaand arc spaying on pre-sintered ceramic in special facilitiesare used. Thin-film metallization requires the surface of thearticle to have a very high degree of purity with an adsorbedwater vapor and other substances removed completely [3].In (5] thin-film metallization was accomplished by depositing different substances from the vapor phase by rf sputtering in vacuum. For better adhesion, copper or aluminumto 5 um thick was deposited on a 0.03 - 0.05 um thick sub-layer of chromium or other metals, such as nichrome, tungsten, titanium and molybdenum. A layer of gold or nickelwas deposited to prevent a chemical reaction of the film.

In the manufacture of high-power modules, the authorsof [6] used aluminum nitride AlN and beryllium oxide BeOsubstrates to secure reliably operation for modules and high-voltage insulation as well as the lowest possible heat resis-tance. In the opinion of the authors, the range of applicationof AlN in engineering is wider than that of BeO because ofthe latter's toxicity. In addition, the thermal expansion coeffi.cient of AlN ceramic is closer to that of silicon than in thecase of BeO. This makes AlN substrates better form the engineering standpoint (5].

The authors of [5] propose a combined technology forobtaining on ceramic plates a thick copper layer suitable forefficient operation with currents >50 A and voltages1 - 4 kV. The required thick layer of a conductor is attainedby galvanic metallization of thinfilm conductors preparedbeforehand. This technology gives good metal ceramic adhesion.