1、 Aluminum nitride ceramics

Aluminum nitride (AlN) is hexagonal crystal, and pure AlN is blue and white, usually gray or gray white. As a new ceramic material with excellent comprehensive properties, aluminum nitride ceramics have a series of excellent characteristics, such as excellent thermal conductivity, reliable electrical insulation, low dielectric constant and dielectric loss, non toxicity and thermal expansion coefficient matching with silicon. It is considered to be an ideal material for the packaging of a new generation of highly integrated semiconductor substrates and electronic devices.

2、 Preparation method of aluminum nitride ceramics

The preparation process is basically similar to that of 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 preparing the final ceramic products. Its purity, particle size, oxygen content and the content of other impurities have an important impact on the thermal conductivity, subsequent sintering and molding process of the subsequent products. It is the cornerstone of whether the performance of the final products is excellent or not. The synthesis methods of aluminum nitride powder are as follows:

① Direct nitriding method: in high temperature nitrogen atmosphere, aluminum powder is directly combined with nitrogen to produce aluminum nitride powder, and the reaction temperature is generally 800 ℃ ~ 1200 ℃.

② Carbothermal reduction method: the mixed powder of alumina powder and carbon powder is reduced and nitrided in flowing nitrogen at high temperature (1400 ℃ ~ 1800 ℃) to form AlN powder.

③ Self propagating high temperature synthesis method: this method is the direct nitriding of aluminum powder, which makes full use of the characteristics of direct nitriding of aluminum powder as a strong exothermic reaction. The aluminum powder is placed at the midpoint of nitrogen, and then the reaction is maintained by the high chemical reaction heat between aluminum and nitrogen to synthesize AlN.

④ Chemical vapor deposition method: the volatile compounds of aluminum react with nitrogen or ammonia to precipitate aluminum nitride powder from the gas phase; According to different aluminum sources, it can be divided into inorganic (aluminum halide) and organic (alkyl aluminum) chemical vapor deposition.

2. AlN forming process

There are many molding processes of aluminum nitride powder, and the traditional molding processes such as molding, hot pressing and isostatic pressing are applicable. Due to the strong hydrophilicity of aluminum nitride powder, in order to reduce the oxidation of aluminum nitride, contact with water should be avoided during the forming process. In addition, hot pressing and isostatic pressing are suitable for preparing high-performance bulk aluminum nitride ceramic materials, but they have high cost and low production efficiency, and can not meet the increasing demand of electronic industry for aluminum nitride ceramic substrate. In order to solve this problem, aluminum nitride ceramic substrate has been formed by tape casting in recent years. Tape casting has also become the main basic forming process of aluminum nitride ceramics for electronic industry.