High Thermal Conductivity Silicon Nitride Ceramics

Journal of the Korean Ceramic Society Vol. 49, No. 4, pp. 380~384, 2012. http://dx.doi.org/10.4191/kcers.2012.49.4.380 Review High Thermal Conductivity Silicon Nitride Ceramics Kiyoshi Hirao, You Zhou, Hideki Hyuga, Tatsuki Ohji, and Dai Kusano* National Institute of Advanced Industrial Science and Technology (AIST), Nagoya 463-8560, Japan *Japan Fine Ceramics Co. Ltd., Sendai 981-3203, Japan (Received May 22, 2012; Accepted June 21, 2012) ABSTRACT This paper deals with the recent developments of high thermal conductivity silicon nitride ceramics. First, the factors that reduce the thermal conductivity of silicon nitride are clarified and the potential approaches to realize high thermal conductivity are described. Then, the recent achievements on the silicon nitride fabricated through the reaction bonding and post sintering technique are presented. Because of a smaller amount of impurity oxygen, the obtained thermal conductivity is substantially higher, compared to that of the conventional gas-pressure sintered silicon nitride, while the microstructures and bending strengths are similar to each other between these two samples. Moreover, further improvement of the thermal conductivity is possible by increasing β/α phase ratio of the nitrided sample, resulting in a very high thermal conductivity of 177 W/(m K) as well as a high fracture toughness of 11.2 MPa m 1/2. Key words : Silicon nitride, Thermal conductivity, Strength, Fracture toughness, Lattice oxygen, Reaction bonding I 1. Introduction n recent years, power electronic devices capable of efficient control and conversion of electric power have been widely used for a variety of applications including industrial robots, hybrid motor vehicles, and advanced electric trains. 1) Furthermore the replacement of Si semiconductor by the wide-bandgap materials such as SiC and GaN is expected to allow higher voltage, larger current, higher power density, and smaller size of the power devices. 2) Since large electric power of several 10 to several 100 kw is controlled and converted in the power devices, high electric insulation, high heat dissipation, and high heat resistance are required to their circuit substrates. Aluminum nitrides, which have high thermal conductivity (estimated value of pure crystal: 320 W/(m K)), have been used for circuit substrates of power devices with high power density such as in-vehicle inverters. However, the higher power density is strongly demanded nowadays in many applications as above stated. In addition, the devices loaded in motor vehicles are subject to large temperature changes, leading to cracks forming in ceramic substrates due to high thermal internal stresses arising at joints with conductive circuits as shown in Fig. 1. 3) Therefore, excellent mechanical reliability as well as high thermal conductivity is required to these substrates. Fig. 2 shows relationship between strengths and thermal conductivities of alumina, aluminum nitride, and silicon nitride that are commercially available for the substrates Corresponding author : Kiyoshi Hirao E-mail : k-hirao@aist.go.jp Tel : +81-52-736-7099 Fax : +81-52-736-7405 Fig. 1. Structure of ceramic substrate for power device. Fig. 2. Relationship between strengths and thermal conductivities of commercially available ceramic substrates. currently. 4) While aluminum nitride has a high thermal conductivity, its strength is low compared to silicon nitride. On the other hand, silicon nitride with excellent mechanical 380

July 2012 High Thermal Conductivity Silicon Nitride Ceramics 381 properties has a thermal conductivity below a half of that of aluminum nitride. Generally sintered body of aluminum nitride consists of equiaxed grains and has low fracture toughness; therefore it is difficult to improve its strength. However, silicon nitride sintered body, which is composed of well developed fibrous grains, has high fracture toughness and strength, and furthermore, the theoretical thermal conductivity of β-si 3 crystals is estimated at 200 W/(m K) or higher. Thus, silicon nitride is a promising material for the next-generation substrates due to excellent mechanical properties and potentially high thermal conductivity, and significant improvement of its thermal conductivity is strongly desired for this goal. 2. Factors Reducing Thermal Conductivity Silicon nitride crystals are divided into two phases of α and β that are low and high temperature phases, respectively. When the raw powder is α- Si 3, it transforms into β phase in liquid phase sintering, resulting in microstructure of well developed fibrous grains. Although the estimated theoretical thermal conductivity of β-si 3 crystals is higher than 200 W/(m K) as above stated, the real values of commercially available high thermal conductivity silicon nitrides are substantially low, 60~90 W/(m K), for the following reasons. Fig. 3 shows an example of microstructure of silicon nitride sintered body. As indicated, the factors reducing the thermal conductivity include grain boundary phases of low thermal conductivity and lattice defects in Si 3 crystals. Since silicon nitride has strong covalency and low diffusivity, it is generally sintered in liquid phase using sintering additives such as rare-earth oxides. During sintering, these additives react with small amount of impurity silica contained in silicon nitride raw powder to form liquid phase, and densification proceeds thereby. After sintering, the liquid phase remains as glassy phase of low thermal conductivity (about 1 W/(m K)) at grain boundaries. These grain boundary phases typically exist as isolated triple point junctions surrounded by three grains, or as continuous thin film (1 nm thickness) boundaries between two grains. The effect of the latter on the thermal conductivity is substantial due to its continuity, but becomes relatively small when the grain size exceeds several microns. 5,6) Thus, coarsening Fig. 3. Typical microstructure of silicon nitride sintered body. grains via e.g. long heat-treatment is one of the effective approaches to obtain a high thermal conductivity. Heat transfer in silicon nitride that is insulator occurs due to lattice vibration (phonon). Lattice defects in β-si 3 crystals induce phonon scattering and reduce the thermal conductivity. Therefore, it is essentially important to remove these defects for increasing the thermal conductivity. It has been reported that solution of oxygen into Si 3 crystals generates defects (vacancies) at sites of Si in crystal lattice as expressed in the following relation, and degrades the thermal conductivity of the crystals substantially. 7,8) 2SiO 2 2Si Si + 4O N + V Si (1) The effective approaches to decrease the lattice oxygen content in Si 3 crystals include using sintering additives with high oxygen affinity (e.g. rare earth oxides), 9) and increasing nitrogen/oxygen ratio in the liquid phase. 7) For example, while Yb 2 O 3 -MgO sintering additive system resulted in a thermal conductivity of 120 W/(m K) after 1900 o C-48 h sintering, it increased to 140 W/(m K) by using MgSiN 2 instead of MgO for raising the nitrogen/oxygen ratio. 10) However, there is a limit to controlling nitrogen/oxygen ratio only through selection of sintering additives; the combination of reaction bonding and post sintering that is described in the following section will be another effective approach. 3. High Thermal Conductivity Through Reaction Bonding and Post Sintering The reaction bonding and post sintering has been known as a fabrication process of dense silicon nitride sintered body where Si powder compact is heat-treated in nitrogen atmosphere for transformation into Si 3 and post-sintered for densification. 11) In this approach, the nitridation and post-sintering are carried out without exposing the raw powder to air, leading to substantial reduction in the impurity oxygen content. Furthermore, even coarse Si powder is decomposed into fine Si 3 powder in nitridation as shown in Fig. 4, facilitating full densification during the post sintering. 12,13) Therefore, the reaction bonding and post sintering is a promising method to reduce the impurity oxygen content and to improve the thermal conductivity. 14) Zhou, et al. 13) tried to fabricate high thermal conductivity silicon nitride using Si starting powder containing small oxygen content through the reaction bonding and post sintering. The used raw powder was a high purity Si powder with an oxygen content of 0.28 wt%, a total metallic impurity content of <0.01 wt%, and a mean particle size (d 50 ) of 8.5 µm. High purity 2 mol%y 2 O 3-5 mol%mgo were added to the Si powder as sintering additives during post-sintering, and were mixed in methanol using a planetary mill. Generation of new surfaces as well as mechano-chemical reactions during the planetary-milling process leads to oxidation of the Si particles. Therefore, the oxygen content of Si

382 Journal of the Korean Ceramic Society - Kiyoshi Hirao et al. Vol. 49, No. 4 Table 1. Characteristics of the Raw Si and Si3N4 Powders12,14) Si powder (for SRBSN) Commercial Si3N4 powder (for GPSSN) Particle size 8.5 µm 0.2 µm Purity (except oxygen) >99.99% >99.9% Oxygen content (as received) 0.28 mass% - Oxygen content (after milling) 0.51 mass% - Oxygen content in Si3N4 (after nitridation)* 0.31 mass% 1.2 mass% *Estimated oxygen content in a fully nitrided material Fig. 5. SEM images of microstructures for SRBSN (a) and GPSSN (b). Fig. 4. SEM images of Si raw powder, nitrided compact, and sintered body. powders after planetary milling increased to 0.51 wt% (from 0.28 wt%), which was still relatively low compared to those of the commercial high-purity Si3N4 powders (typically 1 wt% or higher), as shown in Table 1. Fig. 5(a) shows a scanning electron microscopy (SEM) image of microstructure for sintered reaction-bonded silicon nitride (SRBSN) which was obtained through reaction-bonding (nitridation) at 1400oC for 8 h under 0.1 MPa nitrogen pressure and post-sintering at 1900oC for 12 h under 0.9 MPa nitrogen pressure. It can be known that large fibrous grains are embedded in fine grain matrix without pores. For comparison, microstructure of gas-pressure sintered silicon nitride (GPSSN) obtained under the same sintering conditions using a commercial high-purity Si3N4 powder (mean particle size: 0.2 µm, oxygen content: 1.2 wt%) is shown in Fig. 5(b). Because of the similar microstructures, the four-point bending strength was 620 MPa and 700 MPa for the SRBSN and GPSSN, respectively, indicating no substantial difference between them. However, the thermal conductivity of the SRBSN is 120 W/(m K), which was about 20% higher than that of the GPSSN, 98 W/(m K), due to a smaller amount of impurity oxygen contained in the starting powder. Fig. 6 shows relationship between strengths and thermal conductivities of the GPSSN, SRBSN and modified SRBSN. With increasing the sintering time, the thermal conductivity increases for all the samples due to grain coarsening and Fig. 6. Thermal conductivities of GPSSN, SRBSN and modified SRBSN with increasing sintering time. diffusion of lattice oxygen into grain boundaries. It can be known that the SRBSN shows both high strength and high thermal conductivity compared to the GPSSN. Moreover, Zhou, et al. 15) investigated in detail the effects of nitridation conditions on the properties of the nitrided and post-sintered samples, and revealed that further improvement of the thermal conductivity is possible by increasing β/α phase ratio of the nitrided sample from conventional 60:40 to 83:17 via controlling the nitrogen atmosphere in the nitridation (the modified SRBSN in Fig. 6). This is plausibly due to further reduced content of impurity oxygen in the post-sintered sample derived from the nitrided sample of high β

July 2012 High Thermal Conductivity Silicon Nitride Ceramics Fig. 7. Relationship between thermal conductivities and lattice oxygen contents for GPSSN, SRBSN and modified SRBSN. 383 thermal conductivity and lattice defects (lattice oxygen) in Si3N4 crystals, and discussed the potential approaches to realize high thermal conductivity, such as coarsening grains via e.g. long heat-treatment, using sintering additives with high oxygen affinity, increasing nitrogen/oxygen ratio in liquid phase during sintering, and combining reaction bonding and post sintering. Then, we described the recent achievements on the silicon nitride obtained through the reaction bonding and post sintering technique from Si powder. Because of a smaller amount of impurity oxygen, the developed material showed higher thermal conductivity compared to that of the conventional gas-pressure sintered silicon nitride fabricated through the same sintering condition from Si3N4 powder, while the microstructures and bending strengths are similar to each other between these two samples. Moreover, it has been found that further improvement of the thermal conductivity was possible by increasing β/α phase ratio of the nitrided sample, resulting in a very high thermal conductivity of 177 W/(m K) as well as a high fracture toughness of 11.2 MPa m1/2. High thermal conductivity ceramic substrates are indispensable components to a variety of next generation devices including, not only power modules described in this paper, but also semiconductor laser and high-brightness LED. Silicon nitrides that have high thermal conductivity as well as high mechanical properties are promising substrate materials for applications where, e.g., high reliability should be ensured under extreme environments. For this purpose, the development of low-cost fabrication process is also critically required, in addition to the improvement of material performance16). REFERENCES Fig. 8. SEM image of fracture surface for modified SRBSN. phase ratio, since it is known that an amount of oxygen solved into β-si3n4 is smaller than that into α-si3n4. Fig. 7 shows relationship between thermal conductivities and lattice oxygen contents for the GPSSN, SRBSN and modified SRBSN.14) Despite of the different process conditions, close correlation is observed between them, clearly indicating that the thermal conductivity increases with decreasing the lattice oxygen content. The modified SRBSN obtained through sintering at 1900oC for 60 h and slow cooling with a rate of 0.2oC/min demonstrated a very high thermal conductivity of 177 W/(m K). Fig. 8 shows an SEM image of fracture surface for this modified SRBSN, which has microstructure consisting of large fibrous grains and a high fracture toughness of 11.2 MPa m1/2.15) 4. Summary This paper gave an overview on the recent developments of high thermal conductivity silicon nitride ceramics. First, we clarified the factors that reduce the thermal conductivity of silicon nitride, including grain boundary phases of low 1. The Institute of Electrical Engineers of Japan, Power Semiconductor That Runs the World (in Japanese), Ohmsha, Ltd., Tokyo, 2009. 2. C. R. Eddy Jr. and D. K. Gaskill, Silicon Carbide as a Platform for Power Electronics, Science, 324 [5933] 1398-400 (2009). 3. M. Yamagiwa, Packaging Technologies of Power Modules for Hybrid Electric Vehicles and Electric Vehicles (in Japanese), Bull. Ceram. Soc. Japan, 45 [6] 432-37 (2010). 4. K. Hirao, Development of Ceramic Substrates with High Thermal Conductivity (in Japanese), Bull. Ceram. Soc. Jpn, 45 [6] 444-47 (2010). 5. M. Kitayama, K. Hirao, M. Toriyama, and S. Kanzaki, Thermal Conductivity of Beta-Si3N4: I, Effects of Various Microstructural Factors, J. Am. Ceram. Soc., 82 [11] 310512 (1999). 6. A. Okada and K. Hirao, Conduction Mechanism and Development of High Thermal Conductive Silicon Nitride (in Japanese), Bull. Ceram. Soc. Jpn, 39 [3] 172-76 (2004). 7. M. Kitayama, K. Hirao, A. Tsuge, K. Watari, M. Toriyama, and S. Kanzaki, Thermal Conductivity of Beta-Si3N4: II, Effect of Lattice Oxygen, J. Am. Ceram. Soc., 83 [8] 198592 (2000).

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