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Today, it's undoubtedly the era of information technology coming. The rapid development of the microelectronics industry has made Aluminum Nitride ceramics an instant success in the production of ultra-large-scale integrated circuits. And they are attracting attention as highly thermally conductive ceramic substrates and packaging materials.
The thermal conductivity of AlN is currently the most widely used Al2O3 ceramics as much as seven times, while low dielectric constant, with alumina rivals the excellent electrical properties, high strength, low density and non-toxic. But there are gains and lossed, the current aluminum nitride substrate is facing a larger dilemma, which is the cost to a certain extent affecting the promotion of its application.
So where do these costs come from? As mentioned, AlN has outstanding thermal conductivity, but due to impurities and defects in ceramics, the thermal conductivity of the product is far from the theoretical value.
Therefore, in order to get as close as possible to the theoretical thermal conductivity and highlight the advantages of AlN itself, manufacturers must be careful in the preparation to avoid obvious performance shortcomings. Each process of excellence, so that comes the high cost.
Preparation of AlN powder
As a raw material for the preparation of the final ceramic products, the purity, particle size, oxygen content and other impurities have an important impact on the thermal conductivity, sintering and forming process. And they are the cornerstone of excellent performance of the final products.
There are several methods of synthesis of AlN powders as follows:
① Direct nitridation method: In the high temperature nitrogen atmosphere, aluminum powdder is directly chemically combined with nitrogen to produce aluminum nitride powder, and the reaction temperature is generally in the range of 800℃~1200℃.
② Carbon thermal reduction method: the mixture of Al2O3 powder and carbon powder in the high temperature (1400℃~1800℃) in the flow of nitrogen gas reduction nitridation reaction to produce AlN powder.
③ Self-propagation high-temperature synthesis method: It is the direct nitridation of aluminum powder, making full use of the characteristics of the direct nitridation of aluminum powder as a strong exothermic reaction, the aluminum powder in the nitrogen point then, the use of high chemical reaction heat between aluminum and nitrogen to maintain the reaction on its own, the synthesis of AlN.
④Chemical vapor deposition method: the use of aluminum volatile compounds and nitrogen or ammonia gas reaction, precipitation from the gas phase of aluminum nitride powder. According to the choice of aluminum source, divided into inorganic and organic chemical vapor deposition method.
Method | Advantage | Disadvantage |
Direct Nitriding Method | Strong exothermic reaction, easy self-sintering, simple process, suitable for mass production | Controlled by gas-solid diffusion, requiring high temperature and long reaction time, poor quality stability and difficult to synthesize high-purity, fine-grained products. |
Carbon thermal reduction method | Synthetic powder with high purity, stable performance, fine and uniform powder size, prepare high-grade powder | Sensitive to process conditions, high reaction temperature, long synthesis time, need for secondary decarburization, high cost |
Chemical vapor deposition method | Reliable process, easy to realize the process continuity, obtain high purity nanoscale powder | Low production efficiency |
Self-propagating high-temperature synthesis method | Fast reaction speed, short synthesis time, simple process, low cost, high production efficiency and high activity of the powder produced | The reaction process is difficult to control, making it difficult to prepare high quality powder |
Organic salt cracking method | Continuous production, high purity of the prepared powder, ultra-fine | High cost of raw materials |
Plasma chemical synthesis method | Small particle size, high activity and good process performance of the prepared powder | Lower purity, complex equipment |
Obviously, the process required for high purity, fine particle size and narrow particle size distribution of AlN powder is costly, or the preparation process is complicated, the production efficiency is low, or the equipment requires high conditions, and the consequence of this series of difficulties is the high price of quality aluminum nitride powder.
Forming of AlN
There are various molding process for AlN powder, and traditional molding processes such as molding, hot pressing, and isostatic pressing are applicable. Among them, hot pressing ang isostatic pressing are suitable for the preparation of high performance bulk aluminum nitride ceramic materials, but the high cost and low production efficiency cannot meet the increasing demand for aluminum nitride ceramic substrate in the electronics industry. In order to solve this problem, recently people use the flow casting method to form aluminum nitride ceramic substrates. The casting method has also become the main molding process for aluminum nitride ceramics in the electronics industry.
In addition, due to the strong hydrophilicity of AlN powder, to reduce the oxidation, contact with water should be avoided as much as possible during the molding process, which means that aluminum nitride ceramic blanks need to be prepared using organic slurry. But since the organic solvent used has a strong volatility, it will cause adverse effects on the environment and human body. Or improve the surface resistance to hydrolysis of AlN powder, such as the formation of coating wraps on the surface of AlN with help of hydrophobic and hydrophilic organic substances, or heat treatment in a certain oxygen partical pressure atomosphere to form a dense aluminum oxide layer on its surface, etc.
Sintering of AlN
Aluminum nitride sintering process is relatively harsh, sintering or hot pressing sintering temperature is often up to 1800℃ or more, both to achieve dense sintering, reduce content of impurity and grain boundary phase, but also to simplify the process, reduce costs, in the sintering process of AlN ceramics key to do: First, choose the appropriate sintering process and atmosphere. Second, choose the appropriate sintering additives.
1.Sintering process
The sintering process of AlN substrates is very difficult beacuse of its small self-diffusion coefficient.
① Hot pressure sintering: Sintering ceramics under a certain pressure, can make heating sintering and pressurized forming at the same time, so that can get fine grains, high relative density and good mechanical properties of ceramics.
② No pressure sintering: Sintering process is simple, atmospheric pressure sintering aluminum nitride ceramics general temperature range of 1600-2000℃. The appropriate increase in sintering temperature and extend the holding time can improve the density of aluminum nitride ceramics, but the strength is relatively low.
③ Microwave sintering: Microwave sintering is also a fast sintering method, using the interaction between microwaves and the medium to produce dielectric loss and the overall heating of the blank sintering method.
④ Discharge plasma sintering: Incorporating plasma activation, hot pressing, resistance heating and other technologies. It has the characteristics of fast sintering speed and uniform grain size, but the equipment cost is high and the size of the processed workpiece is limited.
⑤Self-propagation sintering: The dense AlN ceramics are prepared directly by self-propagation high-temperature synthesis reaction under ultra-high pressure nitrogen gas. However, it is difficult to obtain dense AlN ceramics due to the high temperature combustion reaction in which the Al in the raw material melts easily and prevents the nitrogen from penetrating into the blank.
Among the above five sintering processes, hot pressing sintering is the main process for preparing high thermal conductivity densified AlN ceramics. However, it is a more complex process with high equipment requirements and low production efficiency, so the cost naturally goes higher.
2. Sintering atmosphere
Currently, there are three sintering atmosphere for AlN ceramics: neutral atmosphere, reducing atmosphere and weakly reducing atmosphere. The neutral atmosphere uses the commonly used N2, the reducing atmosphere uses CO, and the weak redcing atmosphere uses H2.
In the reducing atmosphere, the sintering time and holding time of AlN ceramics should not be too long, and the sintering temperature should not be too high to avoid the reduction of AlN. In neutral atmosphere, the above situation does not occur, so generally choose to sinter in nitrogen, so as to obtain a higher performance of AlN ceramics.
3. Sintering additives added
In the sintering process of aluminum nitride ceramic substrate, excepts the process and atmosphere affect the performance of the product, the choice of sintering additives is also particularly important.
AlN sintering additives are generally alkali metal oxides and alkaline earth metal oxides, sintering additives have two main roles: on the one hand, the formation of low melting point phase, liquid phase sintering, reduce the sintering temperature, and promote the densification of the blank. On the other hand, high thermal conductivity is an important performance of AlN substrates. Due to the existence of various defects such as oxygen impurities, thermal conductivity is lower than the theoretical value, the addition of sintering aid can react with oxygen to complete the lattice and thus improve thermal conductivity.
The sintering aids used for sintering AlN ceramics are mainly Y2O3、CaO、Yb2O3、Sm2O3、Li2O3、B2O3、CaF2、YF3、CaC2, etc. or their mixtures. The choice of multiple compound sintering aids can often obtain better sintering effect than single sintering aids, achieve AlN low-temperature sintering, reduce energy consumption, and facilitate continuous production. In order to find the right low-temperature sintering additives, manufacturers often need to invest a lot of time and effort in research and development, so this part will also be reflected in the price of AlN substrate.
This article is reprinted from 360powder.com
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