Study on sintering properties of aluminum oxide nano-powder for electronics packaging

The microstructures of two kinds of nano powders (ɑ-Al2O3, γ-Al2O3) are studied using a scanning electron microscope (SEM), X-ray diffraction (XRD) under the customized low-temperature sintering process (under the maximum sintering temperature 1600 °C) for the frame material of the customized ceramic for high power electronics packaging application. The experimental results show that the structure of ɑ-Al2O3 is stable while the structure of γ-Al2O3 is unstable. The crystalline form of γ-Al2O3 is changed to ɑ-Al2O3 under the sintering process. The two kinds of powders are all bonded together with a laminate structure. The sintered samples of the γ-Al2O3 are the most compacted.


Introduction
General aluminum oxide ceramics have attracted much research due to their non-oxidizability, corrosion-resistant, antiwear, and high-temperature resistance.However, the wide application of aluminum oxide ceramics has been limited due to their sintering temperature (above 1700 ℃), low toughness, and high brittleness, making it difficult for industrial production [1][2][3] .The quality of ceramic products of the micro powder depends on the grain fineness directly.The smaller the grain fineness is, the better the sintering property is.Meanwhile, the sintering process needs less energy.The maximum sintering temperature can be reduced to 70-100 ℃ when 10%~15 wt% pure nano aluminum oxide powder is added into generic micro powder.But the industrial application of the process is limited because of the cost of the nano aluminum oxide powder production and the difficulty of meeting the requirement of fine particle size using the current manufacturing process [4][5][6] .
The nano powder of aluminum oxide is a homogeneous polycrystalline oxide with α-Al 2 O 3 , γ-Al 2 O 3, and β-Al 2 O 3 .Much research on composite materials based on nano aluminum oxide powder applied in the automobile industry, communications engineering, and household electronic products has recently been reported because of their small size and nano surface effect.The nano aluminum oxide powder can enhance the strength and toughness of rubber, ceramic, plastic, and fire-resistive composite materials.Obviously, the smoothness, density, thermal fatigue, fracture toughness, and creep resistance of the ceramic products are improved.The increasing demand for improving the performances of ceramic substrates for electronics packaging is attracting attention in various industry applications [7][8][9] .
Temperature and process are the two key elements of preparing nano-alumina ceramics.The high activity of the powder is governed by sintering, and the growth of grains is governed by the sintering process [10][11][12] .Therefore, studying the sintering properties of nano aluminum oxide powder and its application is of great meaning, thereby promoting social benefits and economic value.
Here, the sintering properties of customized nano-alumina powder under the given procedure are studied by reducing the particle size of alumina powder, increasing the surface activation energy, and reducing the sintering temperature.The effect of the change fromγ-Al 2 O 3 to ɑ-Al 2 O 3 is studied qualitatively during the sintering.These works provide some references for solving the low-temperature sintered alumina ceramic substrate used in electronics packaging.

Experimental details
SEM has characterized two kinds of aluminum oxide powders (SU5000 Hitachi Enterprise Co. Ltd., Japan).All kinds of aluminum oxide powders are comprised of loose near-spherical particles (Figure 1 and Figure 2)     and an average diameter is 9.37468 m  .The determination of the sintering temperature is related to the compositions, particle size, surface state, and the property required for the product [13][14][15] .The ideal sintering temperature is 0.5 T m , and a long period is needed.Considering the strength, hardness, toughness, ductility, porosity, and particularly mechanical strength, the sintering temperature shall be above 0.6-0.8T m .The finer the powder is, the more active the powder surface is, and the lower the sintering temperature.The sintering temperature and holding time can vary with the product's different physical and mechanical requirements.The holding time during the sintering process depends on the temperature, the required porosity, and the pore shape.Because of the required porosity, the holding time is shorter if the sintering occurs at a higher temperature and is longer at a lower temperature.In practice, the sintering temperature and holding time can be determined by experimentation.Generally, low sintering temperature and short holding time can be preferred for sintering to reduce the requirements and increase the productivity of the sintering facility.High reliability is fundamental for electronic packaging materials [16][17][18][19][20] .This paper's sintering process (Figure 5) is based on its matched packaging chips.The sintered samples (Figure 6) are sintered using the box-type muffle furnace (SG-XS1700, Shanghai SAGER Industrial CO., Limited, China) under the condition of aerobic atmospheric pressure.The samples are bonded together after the sinter, though there are a small number of gaps and voids in the middle of the samples from the appearance of the sintered samples.About 1/4 of the part is not sintered into a block in the ɑ-Al 2 O 3 sintered sample, although the surface activation energy of ɑ-Al 2 O 3 is higher than that of γ-Al 2 O 3 in theory.

Results and discussion
The analysis of XRD patterns and diffraction peaks characterize the crystalline phase of the sintered samples.This is particularly important in the case of synthesized nano aluminum oxide powders.The results of XRD analysis (Figure 7 and Figure 8) show that the structure of ɑ-Al 2 O 3 is unchanged under the sintering procedure with the same intensity and shape of the peak compared with that of the powder before sintering.Another interesting thing is that the structure of sinteredγ-Al 2 O 3 is quite different from that of the powder before sintering but is the same as that of ɑ-Al 2 O 3 .It also shows that the crystal structure of nanoγ-Al 2 O 3 powder after sintering under the given sintering procedure is changed into nano-ɑ-Al 2 O 3 .
Two kinds of aluminum oxide sintered samples have been characterized by SEM using the same way as previously mentioned, as shown in Figure 9 and Figure 10.From the scanning electron microscopy comparison, the nano-ɑ-Al 2 O 3 powder and the nano-γ-Al 2 O 3 powder particles are aggregated, and the spherical powders are bonded to form a lamellar structure.Still, the flake aggregation phenomenon is not dense enough, and there is delamination.The nano γ-Al 2 O 3 powder is much more compact than nano ɑ-Al 2 O 3 powder.But they all can be sintered at the low temperature.It is found that the generated grain size is greater than 1000 nm (micro-level) under the given sintering procedure after analyzing the grain size data using matched software Jade.
Aluminum oxide ceramic materials are difficult to sinter because the material is a high hardness compound with a high melting point (2054 ℃) and high boiling point (2980 ℃).Alumina oxide has a small self-diffusion coefficient and a low sintering activity.The sintering temperature of alumina oxide usually refers to 1750-1900 ℃, which is a high sintering temperature.The low sintering temperature is 1600~1700 ℃.Low-temperature sintering has become a new research direction for alumina oxide ceramics to reduce production costs.The two kinds of nano aluminum oxide powders here can be sintered at 1600 ℃ due to their high specific surface area and short diffusion distance, which increase the driving force of sintering and accelerate the formation of liquid phase substances and flow.In addition, the heat released to the system during the conversion of γ-Al 2 O 3 to ɑ-Al 2 O 3 also contributes to the powder sintering.Therefore, the sintering compactness of nano γ-Al 2 O 3 is better than that of nano-ɑ-Al 2 O 3 .However, the higher the purity of nano-alumina powder is, the more difficult it is to achieve complete density by sintering.There are pores in the ceramic grain or grain boundary, which greatly limits some practical applications of alumina ceramic materials, but also provides the possibility for the study of new direct bonding metal substrates for high-power electronics packaging.The results of the experiments can be used to further investigate the cover metal substrate for high-power electronics packaging.

Conclusions
The nanoɑ-Al 2 O 3 powder is a stable nano material, while the nano γ-Al 2 O 3 powder is unstable, which is changed to ɑ-Al 2 O 3 .The compactness of nanoγ-Al 2 O 3 powder is much better than the nano ɑ-Al 2 O 3 powder under the given sintering procedure.
Meanwhile, the two kinds of nano aluminum oxide powder can be sintered, and the particles are bonded together.They all grow into micro grains with laminate structures, which can support the follow-up research on direct bonding metal ceramic substrates for high-power electronics packaging.

Figure 2 .
Figure 2. SEM image ofγ-Al 2 O 3 powder (×3000).A laser diffraction analyzer measures the particle size distribution of the two powders (LA-960, Seishin Enterprise Co. Ltd, Japan).A mixed solution containing ɑ-Al 2 O 3 and γ-Al 2 O 3 being the solutes separately and an appropriate amount of 95 wt% ethanol being the solvent are formed for testing before sintering.

Figure 4 .
Figure 4.The particle-size distribution ofγ-Al 2 O 3 powder.As shown in Figure3and Figure4, the γ-Al 2 O 3 powder has a narrower size distribution, and the particle size is smaller than the ɑ-Al 2 O 3 powder.The ɑ-Al 2 O 3 powder exhibits a particle size of D 50 = 3.5048 m  and an average diameter is 3.96351 m  , while the γ-Al 2 O 3 powder possesses a particle size of D 50 = 8.58033 m  and an average diameter is 9.37468 m  .The determination of the sintering temperature is related to the compositions, particle size, surface state, and the property required for the product[13][14][15] .The ideal sintering temperature is 0.5 T m , and a long period is needed.Considering the strength, hardness, toughness, ductility, porosity, and particularly mechanical strength, the sintering temperature shall be above 0.6-0.8T m .The finer the powder is, the more active the powder surface is, and the lower the sintering temperature.The sintering temperature and holding time can vary with the product's different physical and mechanical requirements.The holding time during the sintering process depends on the temperature, the required porosity, and the pore shape.Because of the required porosity, the holding time is shorter if the sintering occurs at a higher temperature and is longer at a lower temperature.In practice, the sintering temperature and holding time can be determined by experimentation.Generally, low sintering temperature and short holding time can be preferred for sintering to reduce the requirements and increase the productivity of the sintering facility.High reliability is fundamental for electronic packaging materials[16][17][18][19][20] .This