1. Aluminum nitride ceramics
Aluminum nitride (AlN) is hexagonal, pure AlN is blue-white, usually gray or off-white. As a new type of ceramic material with excellent comprehensive performance, aluminum nitride ceramics have excellent thermal conductivity, reliable electrical insulation, low dielectric constant and dielectric loss, non-toxic and thermal expansion coefficient matching with silicon, etc. Excellent characteristics, is considered to be an ideal material for a new generation of highly integrated semiconductor substrates and electronic device packaging. This article briefly introduces the preparation and application of AlN materials.
Second, the preparation method of aluminum nitride ceramics
The preparation process is basically similar to other ceramic materials. There are three preparation processes: powder synthesis, molding and sintering.
1. Preparation of AlN powder
Aluminum nitride powder is used as the raw material for the preparation of the final ceramic product. Its purity, particle size, oxygen content, and other impurities all have an important impact on the thermal conductivity, subsequent sintering, and forming process of the subsequent product. Is it the performance of the final product? cornerstone. The synthesis methods of aluminum nitride powder are as follows:
①Direct nitriding method: In a high-temperature nitrogen atmosphere, aluminum powder is directly combined with nitrogen to produce aluminum nitride powder. The reaction temperature is generally 800℃~1200℃.
②Carbothermal reduction method: the mixed powder of alumina powder and carbon powder undergoes reduction and nitridation reaction under flowing nitrogen at high temperature (1400℃~1800℃) to generate AlN powder.
③Self-propagating high-temperature synthesis method: This method is the direct nitridation of aluminum powder, making full use of the characteristics of direct nitridation of aluminum powder as a strong exothermic reaction. Place the aluminum powder at the midpoint of nitrogen, then use the high temperature between aluminum and nitrogen The heat of chemical reaction keeps the reaction on its own to synthesize AlN.
④Chemical vapor deposition method: using aluminum volatile compounds to react with nitrogen or ammonia to precipitate aluminum nitride powder from the gas phase; according to the choice of aluminum source, it is divided into inorganic substances (aluminum halide) and organic substances (alkyl aluminum ) Chemical vapor deposition.
2. AlN molding process
There are many molding processes for aluminum nitride powder. Traditional molding processes such as molding, hot pressing, and isostatic pressing are all applicable. Due to the high hydrophilicity of aluminum nitride powder, in order to reduce the oxidation of aluminum nitride, contact with water should be avoided as much as possible during the molding process. In addition, although hot pressing and isostatic pressing are suitable for the preparation of high-performance bulk aluminum nitride ceramic materials, they have high costs and low production efficiency, and cannot meet the increasing demand for aluminum nitride ceramic substrates in the electronics industry. In order to solve this problem, in recent years, people have cast aluminum nitride ceramic substrate casting method. The casting method has also become the basic main forming process of aluminum nitride ceramics for the electronics industry.
3. Application of aluminum nitride ceramics
1. AlN as substrate material
Most ceramics are extremely strong ionic bonds or covalent bonds, have excellent comprehensive performance, are commonly used as substrate materials in electronic packaging, have high insulation properties and excellent high-frequency characteristics, and the linear expansion coefficient and electronic The components are very similar, the chemical properties are very stable and the thermal conductivity is high. From 1985 to 1988, the application of aluminum nitride in microelectronics encapsulation materials emerged. It turns out that the substrate material of the vast majority of high-power hybrid integrated circuits has been using alumina and beryllium oxide ceramics for a long time, but alumina has low basic thermal conductivity and a thermal expansion coefficient that is not well matched with silicon; although beryllium oxide has excellent comprehensive performance, but Its high production cost and the shortcomings of high toxicity limit its application and promotion. Therefore, considering the performance, cost and environmental protection factors, the two can no longer fully meet the development needs of modern electronic power devices.
2. AlN as an electronic film material
Electronic thin-film materials are the foundation of microelectronics and optoelectronic technology, so the research of various new electronic thin-film materials has become a hot spot for many researchers. Aluminum nitride was discovered as an electronic thin film material in the 1960s and has a wide range of applications. In recent years, wide band gap semiconductor materials and electronic devices represented by Group IIIA nitrides have developed rapidly, and are called the third generation after the first generation semiconductor represented by Si and the second generation semiconductor represented by GaAs semiconductor. As a typical Group IIIA nitride, AlN has received more and more attention from domestic and foreign researchers.
3. AlN as a coating for crucibles or refractory materials
The corrosion resistance of aluminum nitride can be infiltrated by molten aluminum but cannot react with it. Compound alloys including copper, lithium, uranium, iron and some super heat-resistant alloys; Many molten salts such as eutectic mixtures, chlorides and cryolite are stable. It can therefore be made into a coating of crucibles or refractory materials.
Aluminum nitride can be used as a container for vacuum evaporation and metal smelting, especially for crucibles for vacuum evaporation of Al. Although AlN is heated in a vacuum, the vapor pressure is low, but even if decomposed, it will not contaminate aluminum. AlN can also be used as a thermocouple protective sleeve. There is no erosion and damage if it is continuously immersed in an aluminum bath at 800~1000℃ in air for more than 3000h. In the semiconductor industry, using AlN crucibles instead of quartz crucibles to synthesize gallium arsenide can completely eliminate the pollution of Si to gallium arsenide and obtain high-purity products.
A variety of excellent properties of aluminum nitride determine its many applications. As a piezoelectric film, it has been widely used; as an electronic device and integrated circuit packaging, dielectric isolation and round material, it has important application prospects; as blue light, UV luminescent materials are also current research hotspots; as polymer materials, they can be used to fix molds, make adhesives, thermal grease, and heat dissipation pads… After further expansion of the market, the application range of aluminum nitride ceramic materials will be Getting wider and wider.