Research Progress on Sintering Technology of Nitride Ceramic Substrates

1

Performance Characteristics of Nitride Ceramics

Nitride ceramics are ceramics primarily composed of refractory compounds in which nitrogen is combined with metallic or non-metallic elements through covalent bonds. They represent a class of ceramic materials featuring high melting points, high hardness, high strength, high-temperature resistance, and excellent thermal and electrical properties. Currently, they are increasingly being applied in engineering fields such as metallurgy, chemical industry, electronics, and machinery.

Nitride ceramics are an important class of structural and functional materials. Their main characteristics include [1]:

(1) Most nitrides have relatively high melting points. Some nitrides, such as Si₃N₄, BN, and AlN, do not melt at high temperatures but sublime and decompose directly, with their decomposition temperatures or melting points approaching or exceeding 2000°C;

(2) High hardness and high strength. Si₃N₄, TiN, and cubic boron nitride (c-BN) all exhibit high hardness, among which c-BN is a superhard material with hardness comparable to diamond. Meanwhile, Si₃N₄, Sialon, AlN, and TiN possess relatively high strength;

(3) For most nitrides, the temperature corresponding to a vapor pressure of 10⁻⁶ Pa is approximately 2000°C. Compared with oxides, nitrides have relatively poor oxidation resistance, which somewhat limits their use under air conditions. Overall, nitride structural ceramics demonstrate favorable mechanical, chemical, electrical, thermal, and high-temperature physical properties, and can serve as high-strength mechanical components, heat-resistant parts, and corrosion- and wear-resistant components, finding extensive applications in industries such as metallurgy, aerospace, chemical engineering, automotive engines, electronics, machinery, and semiconductors.

Table 1 Crystal Structures and Properties of Nitride Structural Ceramics

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Currently, the most widely applied nitride ceramics include silicon nitride (Si₃N₄), aluminum nitride (AlN), and boron nitride (BN) ceramics. Among these, due to their excellent hardness, mechanical strength, and heat dissipation properties, silicon nitride ceramics and aluminum nitride ceramics can be fabricated into ceramic substrates for electronic packaging, showing promising development prospects. The greatest advantage of aluminum nitride ceramic substrates lies in their high thermal conductivity and thermal expansion coefficients matching those of semiconductor materials such as Si, SiC, and GaAs, making them indeed highly effective in solving heat dissipation issues for high-power devices. Silicon nitride ceramics, on the other hand, excel in overall performance. Among existing ceramic materials usable as substrate materials, Si₃N₄ ceramics exhibit high flexural strength (greater than 800 MPa) and good wear resistance, and are recognized as ceramic materials with the best comprehensive mechanical properties, outperforming other materials in high-strength heat dissipation environments. BN materials possess relatively good comprehensive properties, but as substrate materials, they lack outstanding advantages, are expensive, and have mismatched thermal expansion coefficients with semiconductor materials; they currently remain under research.

2

Sintering Technologies for Nitride Ceramic Substrates

Currently, the mainstream materials for nitride ceramic substrates are silicon nitride (Si₃N₄) and aluminum nitride (AlN). The commonly used sintering technologies are as follows:

Hot Pressing Sintering (HPS)

Hot pressing sintering (HPS) is a process that applies axial mechanical pressure, typically 30–50 MPa, to the sintering body in the mold during the heating stage of sintering. This pressure application provides substantial sintering driving force for the powder sintering process, thereby increasing the ratio of densification rate to high-temperature grain growth rate, and reducing the temperature and time required for ceramic densification. This method provides additional sintering driving force through pressure application, shortens sintering time, lowers sintering temperature, and reduces the amount of sintering additives required, thereby decreasing the grain boundary glass phase in the ceramic sintered body and improving its high-temperature resistance.

However, simple hot pressing sintering can no longer keep pace with the rapid development of microwave devices. Therefore, many researchers have attempted to introduce new technologies on the basis of hot pressing. Liu Haihua from Fuzhou University [2] varied the addition amount of yttrium oxide, particle size distribution, holding time, and heat treatment time, but the optimal thermal conductivity achieved was only 160 W/m·K. Deeley et al. [3] first introduced MgO as a sintering additive in their research, then employed a hot pressing process to prepare fully densified silicon nitride materials. Such silicon nitride products were quickly applied, such as Norton Company's NC-132 grade silicon nitride.

Spark Plasma Sintering (SPS)

Spark plasma sintering (SPS), also known as plasma-activated sintering, involves directly introducing pulsed current between powder particles for heating and sintering. Compared with other sintering processes, the advantages of SPS include fast heating rates (reaching 1600°C in 30 minutes) and short sintering times. The disadvantage is that the short sintering time often results in relatively low ceramic thermal conductivity.

Researchers including Kobayashi from the University of Tokyo [4] added Y₂O₃-CaO-B (LaB₆) during SPS sintering of AlN, reducing the temperature to 1450°C, but the thermal conductivity ranged between 30–80 W/m·K. The generally lower thermal conductivity of samples prepared by this method compared with pressureless sintering may be due to fine grains limiting the thermal conductivity of the sintered body. Yang et al. [5] prepared Si₃N₄ ceramics via SPS with flexural strength of 857.6 MPa, hardness of 14.9 GPa, and fracture toughness of 7.7 MPa·m¹/²; however, the maximum thermal conductivity was only 76 W/(m·K).

Gas Pressure Sintering (GPS)

Gas pressure sintering (GPS) is a sintering method in which a certain gas pressure is introduced and maintained during the heating and holding stages of the sintering process. Typically, GPS is conducted in a closed furnace chamber with nitrogen gas at a pressure of 1–10 MPa to assist sintering. This method ensures high densification while offering simpler sintering processes and more convenient operation compared with hot pressing or hot isostatic pressing processes.

Mitomo [6] was the first to discover through research that the densification degree of gas pressure sintered silicon nitride was significantly higher than that of pressureless silicon nitride. The introduction of high-pressure nitrogen gas can effectively promote the densification of silicon nitride and inhibit its high-temperature decomposition. Considering comprehensive sintered product performance, production cycle, and production costs, GPS is currently the most suitable sintering process for silicon nitride ceramic substrates.

Pressureless Sintering (PS)

Pressureless sintering (PS), also known as atmospheric pressure sintering, refers to a process in which the nitrogen pressure in the furnace during sintering is at standard atmospheric pressure. Pressureless sintering is generally divided into solid-phase sintering and liquid-phase sintering. Pure solid-phase sintering of AlN ceramics is difficult to achieve full densification, so liquid-phase sintering is generally selected. Zhou Heping et al. obtained aluminum nitride ceramics with a density as high as 3.26 g/cm³ and thermal conductivity reaching 189 W·m⁻¹·K⁻¹ using relatively simple equipment at sintering temperatures above 1800°C. However, this method requires high sintering temperatures, long sintering times, and high energy consumption. Moreover, the prepared sintered bodies exhibit lower density, non-uniform grain sizes, and more blocky second phases can be observed at grain boundaries.

Typically, pressureless sintering of high-performance silicon nitride requires higher sintering temperatures or longer holding times, as well as appropriate sintering additives such as yttrium oxide (Y₂O₃) and aluminum oxide (Al₂O₃) to lower sintering temperature and improve densification. Although this method is simple and easy to implement, the mechanical properties of the resulting silicon nitride ceramics may be somewhat inferior compared with other methods.

Hot Isostatic Pressing Sintering (HIP) [7]

Hot isostatic pressing sintering is a densification method conducted at high temperatures using gas to transmit pressure, typically above 1000°C. High-pressure protective gas in a sealed environment transmits pressure to the ceramic body. During operation, the internal pressure of the equipment reaches up to 200 MPa. Under the combined action of temperature and force fields, the ceramic body is subjected to balanced pressure from all directions.

In the sintering of silicon nitride ceramics, two sintering methods have emerged during the development of HIP sintering. One is direct HIP sintering, namely the glass encapsulation process. In this process, the formed silicon nitride body is placed in a glass encapsulant that easily deforms at high temperatures for HIP sintering. After sintering, the encapsulant on the silicon nitride surface is removed mechanically. This sintering method can produce high-density, high-reliability, high-strength silicon nitride ceramics in a single sintering step, and has been successfully applied in certain special fields, such as high-temperature silicon nitride heat engine components prepared in the United States, Norton's NT-164, and GTE's PY-6.

Microwave Sintering

Microwave sintering is a technology that achieves sintering by heating the material overall to the sintering temperature through dielectric loss of the material in a microwave electromagnetic field. Microwaves simultaneously increase the activity of powder particles, facilitating mass transfer. It enables overall heating, greatly shortening sintering time and inhibiting grain growth, resulting in ceramics with fine and uniform crystals. Using Nd₂O₃-CaF₂-B₂O₃ as sintering additives, AlN ceramics with thermal conductivity of 66.4 W/(m·K) can be obtained by microwave sintering at the low temperature of 1250°C.

During the sintering process of silicon nitride, an α→β-Si₃N₄ phase transformation occurs. Research has found that microwave sintering promotes this phase transformation in silicon nitride. Compared with traditional sintering processes, microwave sintering of silicon nitride ceramics offers advantages such as promoting phase transformation, lowering sintering temperature, promoting densification, improving microstructure, and enhancing material properties.

3

Sintering Process Optimization

Selection and Ratio of Sintering Additives

The selection and ratio of sintering additives have significant effects on the sintering performance of nitride ceramics. For example, adding appropriate sintering additives helps densify nitride ceramics, yielding ceramics with fine and uniform grains. Furthermore, by regulating the types and contents of sintering additives, the properties of nitride ceramics can be further optimized.

Li et al. [8] studied the effects of the Y₂O₃/MgO sintering additive ratio on the densification, phase transformation, microstructural evolution, and thermal conductivity of Si₃N₄ ceramics. At a Y₂O₃/MgO ratio of 3:4, they prepared Si₃N₄ ceramics with thermal conductivity of 98.04 W/m·K, flexural strength of 875 MPa, and fracture toughness of 8.25 MPa·m¹/². Jin Ye [9] doped CeO₂ and Y₂O₃ binary sintering additives into AlN powder through a hot pressing sintering process to improve the thermal conductivity of AlN ceramics. When the doping contents of Y₂O₃ and CeO₂ were 5 wt% and 1 wt%, respectively, the AlN powder after hot pressing sintering achieved a thermal conductivity of 207.8 W/m·K and a relative density of 96.15%.

Sintering Temperature and Time [9,10]

Increasing sintering temperature facilitates mass transfer processes such as dissolution and diffusion, reducing system viscosity and increasing fluidity, thereby promoting densification. However, excessively high temperatures not only waste energy but also lead to excessive liquid phase, excessively low viscosity, causing product deformation, property deterioration, and decreased densification. Therefore, controlling appropriate sintering temperatures and holding times is a consideration that must be addressed in most research.

Luo Jie et al. studied the effect of sintering temperature on the densification of Si₃N₄ ceramics. Using MgSi₂ as a sintering additive and controlling the temperature between 1300–1500°C for plasma-activated sintering, they found that when the temperature was below 1350°C, the relative density of samples was below 70%; when the temperature reached 1400°C, the relative density was 99.6%; when the temperature exceeded 1400°C, the sample density almost no longer changed. The study indicated that after reaching 1400°C, the rapid dissolution of α-Si₃N₄ in the liquid phase was promoted, and through precipitation of β-Si₃N₄, further shrinkage of Si₃N₄ ceramics was achieved, thereby greatly improving the densification degree.

Wang Liying et al. sintered in the range of 1500–1800°C and found that increasing temperature favored the increase in thermal conductivity of AlN ceramic materials, with the obtained thermal conductivity of AlN ceramics increasing from 76.9 W/(m·K) to 113.9 W/(m·K). In the sintering furnace, the uniformity of sintering temperature profoundly affects AlN ceramics. Research on sintering temperature uniformity also provides assurance for mass production and reduced production costs, facilitating the commercial production of AlN ceramic substrate products.

Sintering Atmosphere and Equipment

Regarding sintering atmosphere, silicon nitride ceramic sintering adopts high-pressure nitrogen sintering. The nitrogen atmosphere can effectively inhibit the high-temperature decomposition of Si₃N₄ ceramics, allowing Si₃N₄ ceramics to be sintered at higher temperatures, promoting the dissolution-precipitation process of Si₃N₄ ceramics, improving the α-β phase transformation of silicon nitride, and enhancing the thermal conductivity of silicon nitride ceramics.

Additionally, to prevent oxidation of AlN ceramics during sintering, non-oxidizing protective atmospheres are typically selected, such as strongly reducing atmospheres (e.g., CO), reducing atmospheres (e.g., H₂), or neutral atmospheres (e.g., N₂). Industrially, AlN ceramics are generally sintered in highly flowing N₂ atmospheres.

4

Development Trends in Nitride Ceramic Sintering Technologies [11]

Development of Novel Sintering Additives

Adding effective sintering additives can not only improve the microstructure and properties of nitride ceramic matrix composites but also reduce the manufacturing costs of high-performance nitride ceramics. In current research, determining the optimal particle size of sintering additives and their uniform dispersion in the matrix are issues requiring focused attention. Meanwhile, given the current situation where research on non-oxides as sintering additives is relatively scarce, the mechanisms by which non-oxides affect sintering processes and densification effects remain unclear, and research on high-temperature properties of materials is lacking, future research on nitride ceramic sintering additives should focus on strengthening these aspects.

Exploration of Low-Temperature Sintering Technologies

As electronic devices develop toward higher power and miniaturization, higher requirements are being placed on the thermal conductivity of ceramic materials. However, traditional high-temperature sintering technologies not only consume high energy but may also cause thermal stress damage to devices. Therefore, developing low-temperature sintering technologies has become an important direction. Lower sintering temperatures result in very little liquid phase generated during the densification stage by additive systems with high eutectic points, and the liquid phase has high viscosity. Diffusion of solute atoms is difficult, and particle rearrangement and dissolution-precipitation are affected, making it difficult for silicon nitride ceramics to achieve densification. Phase transformation is also inhibited, thereby affecting the properties of silicon nitride ceramics.

Recently, the team led by Wang Hong at Southern University of Science and Technology successfully developed dense oriented boron nitride (BN) matrix ceramic composites sintered at extremely low temperatures (such as 150°C), with thermal conductivity as high as 42 W/(m·K), far exceeding existing low-temperature ceramics, providing new ideas for low-temperature sintering technologies.

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