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As a new type of electronic device packaging material, AlN ceramic has high thermal conductivity and strength, low coefficient of thermal expansion and dielectric loss, high temperature and chemical corrosion resistance, good insulation, and non-toxic environmental protection. So it is one of the most promissing ceramic materials by domestic and international cnsensus.
Aluminum nitride ceramic substrate, a material ideal for higyh-power, high-lead and large-size chip packaging, the thermal conductivity has been the industry's focus on. The current thermal conductivity of commercial AlN substrate from its theoretical thermal conductivity is still a big gap. Therefore, the improvement of higher thermal conductivity of AlN ceramic substrate while reducing the sintering temperature of AlN ceramic is of great significance for the rapid development of electronic devices.
In order to prepare aluminum nitride substrates with higher thermal conductivity, it is necessary to investigate what factors affect the thermal conductivity.
Heat Conduction Mechanism
Thermal conductivity, is one of the most important properties of thermalling conductive materials to measure heating conductive ability. It is a covalent compund and has no freely movable electrons inside, so the transfer of heat is achieved in the form of lattice vibrations, which is called phonon heat transfer. The high temperature part of the crystal has high energy and the low temperature part has low. The energy is transferred from high to low through the interaction between phonos, and the migration of energy leads to the conduction of heat.
Phonon Heat Transfer
The atoms inside the lattice are seen as small balls, which are connected to each other by springs (covalent bonds), so that the vibration of each atom has to pull the surrounding atoms and make it through the crystal in the form of elastic waves. This lattice vibration generates quanta of energy, called phonons, which interact to transmit the vibrations, thus allowing energy migration and heat transfer.
The relation for thermal conductivity K in phonon heat transfer is given by:
The above c is the heat capacity of the ceramic body itseld, v is the average velocity of phonon motion, and λ is the average free range of the phonon. The heat capacity of the material itself (c) is close to constant, and the large heat capacity of aluminum nitride is one of the reasons for the high thermal conductivity of aluminum nitride. The phonon velocity (v) is only related to the crystal density and the elastic mechanical properties, which can also be considered as a constant, so the propagation distance of the phonon, is the key that affects the thermal conductivity performance of the final macroscopic aluminum nitride ceramic.
Therefore, it is clear from the heat conduction mechanism of phonons inside aluminum nitride that for high thermal conductivity, it is necessary to make the phonons propagate farther to reduce the resistance to propagation, which is generally comes from various scattering during the phonon diffusion. The sintered ceramics usually have various crystal defects, impurities, porosity and introduced second phases inside, which act to scatter the phonons and thus affect the final thermal conductivity.
Key Factors Affecting Thermal Conductivity
It has been confirmed through continuous reseach that among the many factors affecting the thermal conductivity of AlN ceramics, the microstructure and oxygen impurity content of AlN ceramics are particularly prominent.
(1) Influence of microstructure of AlN ceramics on thermal conductivity.
In practical applications, various sintering aids are often added to AlN to lower the sintering temperature. And at the same time, a second phase is introduced into the AlN lattice, resulting in a decrease in thermal conductivity due to scattering of phonons during the heat condution process.
The second phase introduced by the addition of sintering aids can occur in several ways: in terms of distribution, it can be divided into islands and continuous distribution at grain boundaries; in terms of distribution location, it can be divided into distribution at grain boundary trianles and other locations at grain boundaries. Continuouslu distributed grains can provide more direct access to phonons, and direct contact with AlN grains has higher thermal conductivity than isolated AlN grains, so it is better for the second phase to be continuously distributed; AlN ceramics distributed at the grain boundary triangle produce less interference scattering during heat conduction and can keep contact between AlN grains, so it is better for the second phase to be continuously distributed; AlN ceramics ditributed at the grain boundary triangle produce less interference scattering during heat conduction and can keep contact between AlN grains, so it is better for the second phase to be distributed at the grain boundary triangle.
Schematic diagram of the distribution of the second phase within the aln crystal
In addition, the non-uniform distribution of similar grain boundaries leads to thr presence of a large number of pores, which hinders the scattering of phonons and leads to a decrease in the thermal conductivity of AlN. The grain boundary content, grain boundary size, and porosity also have an effect on the performance of thermal conductivity.
Therefore, during the sintering of AlN ceramics, the thermal conductivity of AlN ceramics can be improved by improving the sintering process by such means as increassing the sintering temperature, extending the holding time, and heat treatment to improve the internal defects of the crystal and to make the second phase distributed continuously as well as located at the trigonal grain boundaries as much as possible.
(2) The effect of oxygen impurities on thermal conductivity.
AlN is highly susceptible to hydrolysis and oxidation, resulting in oxidation of the aluminum nitride surface, leading to the formation of aluminum vacancy defects in the oxygen solid solution into the AlN lattice. And it leads to an incease in phonon scattering, a decrease in the mean free range, and a consequent decrease in thermal conductivity.
Oxygen Content(wt%) | Thermal Conductivity(W/m·K) |
0.31 | 130 |
0.24 | 146 |
0.19 | 165 |
0.13 | 171 |
0.12 | 185 |
Oxygen content and thermal conductivity in the AlN lattice
Therefore, in order to improve the thermal conductivity, adding a suitable sintering aid to remove the oxygen impurities in the lattice is an effective approach.
Key Control Elements for Sintering
AlN is a covalent compound with small self-diffusion coefficient of atoms and strong bonding energy, which makes it difficult to sinter densely. Its melting point is up to 3000℃ or more, and the sintering temperature is even higher than 1900℃. Such a high sintering temperature seriously restricts the pratical application of AlN in industry.
In addition, the oxygen impurities in the surface layer of AlN start to diffuse to the interior of its lattice only at high temperatures, so low-temperature sintering has another function, namely, to delay the diffusion of oxygen impurities in the surfce layer to the interior of the AlN lattice during sintering and to reduce the oxygen impurities in the lattice, so the research of low-temperature sintering technology is imperative for the preparation of AlN ceramic materials with high thermal conductivity.
At present, there are various ways of sintering AlN ceramics in the industry, and different sintering methods can be adopted to obtain dense ceramics bodies according to actual needs. Regardless of the sintering method, refining the original AlN powder and adding suitable low temperatures sintering additives can effectively reduce the sintering temperature of aluminum nitride ceramics.
(1) Using small particle size aluminum nitride powder
The driving force of aluminum nitride sintering process is surface energy, and the fine-grained AlN powder can enhance the sintering process is surface energy, and the fine-grained AlN powder can enhance the sintering activity and increase the sintering driving force to accelerate the sintering process. Studies have confirmed that when the starting particle size of the original AlN powder is 20 times smaller, the sintering rate of ceramics will increase 147 times.
Sintering raw materials should be selected from aluminum nitride powder with small particle size and uniform distribution, which can prevent secondary recrystallization, and the large internal particles are prone to abnormal grain growth which is not conducive to densification sintering; if the particles are not uniformly distributed, individual crystals are prone to abnormal growth during the sintering process and affect sintering.
Aluminum nitride grain growth
Sometimes the sintering mechanism of aluminum nitride ceramics is influenced by the size of the original powder. Micron-sized aluminu nitride powders are sintered according to the bulk diffusion mechanism, while nanometer-sized powders are sintered according to the grain boundary diffusion or surface diffusion mechanism.
However, for now, the preparation of fine and uniform AlN powders is very difficult, and most of them are prepared by wet chemical method combined with carbon thermal reduction method, which is not only complex in sintering process but also energy-consuming, and there are still some limitations for large-scale promotion and application. Domestic supply of small particle size high performance aluminum nitride powder is still very scarce.
(2) Selection of low-temperature sintering additives for aluminum nitride ceramics
By adding some low melting point sintering additives in the sintering process, it can produce liquid phase to promote the dense sintering. In addition, some sintering additives can not only generate liquid phasem, but also react with the oxygen impurities in the lattice, which can play the role of removing oxygen impurities to purify the lattice, thus improving the thermal conductivity of AlN ceramics.
Schematic diagram of the action process of sintering additives
However, sintering additives should not be added blindly, and the amount added should be appropriate, otherwise it may have a detrimental effect. Sintering additives introduce a second phase, and the distribution control of the second phase has a large impact on thermal conductivity.
After research, in the selection of low-temperature sintering additives for AlN ceramic should refer to the following points:
1) The additive has a low melting point and is able to form a liquid phase at a lower sintering temperature and promote sintering through the liquid phase;
2) Additives can react with Al2O3 to remove oxygen impurities and purify the AlN lattice, thus improving thermal conductivity;
3) Additives do not react with AlN to avoid the generation of defects;
4) Additives do not induce decomposition and oxidation of AlN to produce Al2O3 and AlON, avoiding a sharp decrease in the thermal conductivity of aluminum nitride ceramics.
The materials found to be suitable as sintering additives are Y2O3, CaO, Li2O, BaO, MgO, SrO2, La2O3, HfO2 and CeO2, which do not react with AlN, as well as some fluorides of rare earth metals and alkaline earth metals and a small number of compounds with reducing properties (CaC2, YC2, TiO2, ZrO2, TiN, etc.).
Using a single sintering additive alone, sintering at atmospheric pressure usually requires a temperature higher than 1800°C. Using compound additives and designing reasonable additives and ratios can further reduce the sintering temperature effectively, and it is also a commonly used method for low-temperature sintering of aluminum nitride at present.
Summary
Aluminum nitride ceramic substrate electronic packaging field of application is becoming more and more widespread, there are also some domestic enterprises in this field has been built, however, compared to the long-near red sea of overseas markets, China's aluminum nitride ceramic substrate development is still in its infancy, in the preparation and production of high-performance powder and high thermal conductivity substrate still has a certain gap. In-depth understanding of the mechanism of the material, from the root of the right medicine, in order to make China's ceramic substrate industry to a higher level.
Reference:
Preparation of AIN Ceramics Substrate with High Thermal Conductivity and Packaging for High-power LED, Li Hongwei, China Jiliang University.
This article is reprinted from 360powder.com.
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