With the rapid development of nanotechnology and ultrafine material science, the demand for high-purity and ultrafine powder is increasing day by day. Silicon nitride ball, with its excellent hardness, very low wear rate and good chemical stability, has gradually become the key grinding medium in the field of ultrafine grinding. The purpose of this paper is to systematically describe the performance characteristics of silicon nitride ceramic balls as grinding media, and deeply discuss its application advantages and specific uses in the process of ultrafine grinding, so as to provide theoretical basis and practical guidance for the research and application in related fields.

 

Ultrafine grinding technology is an important branch of modern material preparation and processing, which is of great significance for improving material properties and expanding material application fields. Traditional grinding media such as steel ball, alumina ball, etc., often have problems in the grinding process, such as large wear, easy to pollute the material, it is difficult to meet the requirements of ultra-fine grinding for high purity and low pollution. Silicon nitride (Si₃N₄) balls, as a new high performance grinding medium, due to their unique physical and chemical properties, show great application potential in the field of ultrafine grinding.

 

Performance Characteristics of Silicon nitride ball

 

High hardness and wear resistance: the hardness of silicon nitride ceramic ball is second only to diamond and cubic boron nitride, which makes it stable in the process of high load and high-speed grinding, reducing its own wear, thereby extending the service life and reducing the grinding cost.

 

 

Low wear rate: Experimental data show that the wear rate of silicon nitride grinding ball after 24 hours of continuous work is only one millionth, which greatly reduces the loss of grinding media during the grinding process, reduces the impurities introduced due to media wear, and is conducive to obtaining higher purity ultra-fine powder.

 

 

Good chemical stability: silicon nitride ceramic ball in high temperature, strong acid, alkali and other harsh environment can still maintain stability, not easy to chemical reaction with grinding materials, to ensure the purity of the grinding process and product quality.

 

 

Moderate density and good dispersion: The moderate density of silicon nitride ceramic balls can provide good dispersion effect during the grinding process, promote the improvement of grinding efficiency, and reduce the caking phenomenon of materials.

 

 

Application Advantages and Specific Uses

 

Ultrafine powder preparation: In the fields of pharmaceutical, electronic materials, advanced ceramics and other fields, silicon nitride ceramic balls as grinding media can efficiently prepare ultrafine powders with uniform particle size distribution and high purity to meet the needs of the industry for high-quality raw materials.

 

 

High hardness material grinding: For the fine grinding of diamond, silicon carbide and other high hardness materials, silicon nitride ceramic balls show excellent performance, effectively avoiding the rapid wear and pollution problems caused by the lack of hardness of traditional grinding media.

 

 

Special environment grinding: Where grinding is required in high temperature or highly corrosive environments, such as wet grinding of semiconductor materials, silicon nitride ceramic balls are ideal for their excellent chemical stability.

 

 

Environmental protection and sustainable development: The low wear rate of silicon nitride ceramic ball reduces the production of grinding waste, in line with the concept of green manufacturing and sustainable development, which is of great significance for reducing production costs and reducing environmental pollution.

 

Conclusion

In summary, silicon nitride ceramic balls as grinding media in the field of ultrafine grinding show unique performance advantages and wide application prospects. Its high hardness, low wear rate, good chemical stability and moderate density provide a strong guarantee for the preparation of high purity and ultra-fine particle size powder. With the continuous progress of materials science and the in-depth development of nanotechnology, the application of silicon nitride ceramic balls in the field of ultra-fine grinding will be more extensive and become an important force to promote technological progress and industrial upgrading in related industries. In the future, it is necessary to further study the preparation process, performance optimization and application strategy in specific fields of silicon nitride ceramic balls in order to give full play to its potential and meet the growing demand for ultrafine grinding.

After entering the intelligent information age, semiconductor devices have quickly occupied our lives. Because the heat generated by the workpiece is a key factor causing the failure of semiconductor devices, in order to avoid many troubles caused by device failure and ensure its long-term effective and safe operation, it is necessary to be equipped with an efficient heat dissipation system.

 

At present, in the work of the industry for "heat dissipation", the replacement of the new power ceramic substrate is a very important part. Excellent high temperature resistance, corrosion resistance, high thermal conductivity, high mechanical strength, thermal expansion coefficient matched with the chip and not easy to deteriorate characteristics make the ceramic substrate more advantageous than metal, plastic and other materials, suitable for products with high heat and harsh outdoor environment, so it is more and more widely accepted by the public.

 

The ceramic substrate plays the following roles in semiconductor integrated circuits: providing mechanical support and environmental protection for chips and electronic components; Provides heat dissipation channels to avoid excessive local temperature, which improves device reliability. At present, the ceramic substrate materials that can meet these requirements are mainly alumina, aluminum nitride, beryllium oxide, silicon nitride and so on.

 

 

Among them, the advantage of silicon nitride ceramics is that it is a special ceramic with excellent comprehensive performance, with high strength, high hardness, high resistivity, good thermal shock resistance, low dielectric loss and low expansion coefficient, etc., all aspects of the performance is more balanced, is the best comprehensive performance of structural ceramic materials. And power electronic devices in high-speed rail, electric vehicles and other fields often face bumps, vibrations and other complex mechanical environment, so the excellent mechanical properties of silicon nitride ceramics precisely make it in the above fields have a strong competitiveness.

 

As for thermal properties, Lightfoot and Haggerty have proposed that the theoretical thermal conductivity of silicon nitride based on the structure of Si3N4 is between 200 and 300W/(m·K), so it is reasonable to say that silicon nitride has great potential in heat dissipation. However, the ideal is full, the reality is bone feeling, silicon nitride compared with other ceramic materials, the entry threshold is high, not only the technical difficulty is large, but also the processing cost is high, the current commercial silicon nitride substrate thermal conductivity is basically between 85-95W/m·K, higher thermal conductivity substrate (> 150W/m·K) is still in the laboratory stage.

 

In summary, with the in-depth development of the intelligent information age, the wide application of semiconductor devices has put forward higher requirements for heat dissipation technology. With its excellent mechanical support, environmental protection, heat dissipation performance and comprehensive performance, silicon nitride ceramics show great potential and competitiveness in the field of semiconductor devices, especially in complex mechanical environments such as high-speed rail and electric vehicles, and its advantages are more significant. However, the commercialization of silicon nitride ceramics is not a smooth road, and the technical difficulty and high processing cost have become the bottleneck restricting its large-scale application. Nevertheless, researchers are making unremitting efforts to break through technical difficulties and improve the thermal conductivity of silicon nitride ceramics, in order to achieve higher performance heat dissipation solutions in the future. We have reason to believe that with the continuous progress of technology and the gradual reduction of cost, silicon nitride ceramics will certainly shine in the field of semiconductor device heat dissipation and contribute an important force to the development of the intelligent information age.

Aluminum nitride (AlN) as an advanced ceramic material, because of its unique physical and chemical properties, has shown great application potential in electronic packaging, power electronics, high frequency communication and other fields. As a core component in these fields, aluminum nitride substrates not only have to meet the basic physical performance requirements, but also need to adapt to the complex needs of specific application scenarios. This paper will start from the physical characteristics of aluminum nitride ceramic substrate, discuss its specific requirements in different application fields in detail, and combined with cutting-edge research, analyze how to improve the comprehensive performance of aluminum nitride ceramic substrate by means of material design and preparation technology.

 

Physical Properties of Aluminum Nitride Substrate

Aluminum nitride ceramics are known for their high thermal conductivity, high electrical insulation, good mechanical strength, excellent thermal stability and chemical inertness. These characteristics provide a solid foundation for the application of aluminum nitride ceramic substrates in high-performance electronic packaging.

 

1. High thermal conductivity

The thermal conductivity of aluminum nitride is much higher than that of traditional ceramic materials such as alumina, which makes it a significant advantage in the heat dissipation of high-power density electronic components. The high thermal conductivity helps to quickly transfer heat from the heating element to the cooling system, thus maintaining the stable operation of the electronic components.

 

 

2. High electrical insulation

Aluminum nitride ceramics have excellent electrical insulation properties, which can meet the high insulation requirements of high frequency circuits and microwave circuits. This feature is essential to ensure the stability and reliability of the circuit.

 

 

3. Good mechanical strength

Aluminum nitride ceramics have high bending strength and hardness, can withstand large mechanical stress, suitable for a variety of complex environment applications.

 

 

4. Excellent thermal stability and chemical inertness

Aluminum nitride ceramics can still maintain good physical and chemical stability at high temperatures, and is not easy to react with the surrounding environment, thus extending the service life of electronic components.

 

 

Application Requirements of Aluminum Nitride Substrate

1. Power electronics

In the field of power electronics, aluminum nitride ceramic substrates are mainly used for the packaging of high power density electronic components such as IGBT and MOSFET. These components generate a lot of heat during operation, so the substrate is required to have a high thermal conductivity for rapid heat dissipation. At the same time, because the power electronic components usually work in high voltage, high frequency environment, the electrical insulation and mechanical strength of the substrate also have high requirements. In addition, in order to meet the needs of miniaturization and integration, aluminum nitride ceramic substrates also need to have excellent processing properties and dimensional stability.

 

2. High-frequency communications

In the field of high frequency communication, aluminum nitride ceramic substrate is favored because of its low dielectric constant, low loss Angle tangent and excellent thermal conductivity. These characteristics make aluminum nitride ceramic substrates an ideal carrier for high frequency and microwave circuits. In the high frequency communication system, the transmission speed and stability of the signal are very important, so the substrate is required to have low loss, low noise and good electromagnetic compatibility. In addition, with the continuous development of new generation communication technologies such as 5G and 6G, the performance requirements for aluminum nitride ceramic substrates will also become higher and higher.

 

3. Automotive electronics

In the field of automotive electronics, aluminum nitride ceramic substrates are mainly used in the power electronic modules of new energy vehicles such as electric vehicles and hybrid electric vehicles. These modules need to withstand high voltages, high currents and complex working environments, so the substrate is required to have high thermal conductivity, high electrical insulation and good mechanical strength. In addition, due to the high reliability and safety requirements of automotive electronic systems, it is also necessary to carry out rigorous reliability testing and certification of aluminum nitride ceramic substrates.

 

Cutting-edge Research and Performance Enhancement

In order to meet the specific requirements of aluminum nitride ceramic substrates in different application fields, researchers are constantly exploring new material design, preparation processes and performance testing methods.

 

1. Material design

By adjusting the chemical composition and microstructure of aluminum nitride ceramics, the thermal conductivity, electrical insulation and mechanical strength can be further improved. For example, by doping an appropriate amount of rare earth elements or transition metal elements, the lattice structure of aluminum nitride ceramics can be optimized, thereby improving its thermal conductivity and mechanical properties. In addition, the densification and microstructure optimization of Al nitride ceramics can be achieved by controlling the parameters of atmosphere, temperature and time during sintering.

2. Preparation process

With the continuous development of preparation technology, researchers have developed a variety of efficient and environmentally friendly preparation processes for aluminum nitride ceramics. For example, advanced sintering technologies such as reaction sintering and discharge plasma sintering can significantly improve the density and properties of aluminum nitride ceramics. In addition, by introducing advanced processing technology and equipment, such as laser cutting, ultrasonic processing, etc., precision processing and efficient production of aluminum nitride ceramic substrates can be achieved.

3. Performance testing and evaluation

In order to accurately evaluate the performance of aluminum nitride ceramic substrates, researchers are constantly developing and improving various performance test methods and evaluation standards. For example, by using advanced equipment such as thermal conductivity tester and electrical insulation tester, key performance indicators such as thermal conductivity and electrical insulation of aluminum nitride ceramic substrate can be accurately measured. At the same time, the reliability and durability of aluminum nitride ceramic substrate in complex working environment can be evaluated by simulation and experimental verification.

 

Conclusion

In summary, aluminum nitride ceramic substrate has a wide range of application prospects in power electronics, high frequency communication, automotive electronics and other fields. In order to meet the specific requirements of different application fields, researchers are constantly exploring new material design, preparation processes and performance testing methods. In the future, with the continuous development of material science and preparation technology, the performance of aluminum nitride ceramic substrate will be further improved, the cost will be further reduced, and make greater contributions to the development of the electronics industry. At the same time, we also need to pay close attention to the development trend of new technologies and new materials, and continue to expand the application field and market space of aluminum nitride ceramic substrates.

 

Si3N4 powder is the main raw material for the preparation of silicon nitride balls. Selecting the appropriate treatment method to obtain the powder with regular shape and uniform particle size distribution is the basis for the stable implementation of Si3N4 ceramic ball forming, sintering, processing and other processes.

According to the different atomization methods, the spray granulation methods of Si3N4 powder mainly include centrifugal spray granulation, pressure spray granulation and two-fluid spray granulation. Pressure spray granulation The slurry with uniform Si3N4 powder is sprayed into the granulation tower under high pressure for atomization, and the droplets are quickly dried into spherical powder by hot air flow, which can prevent the agglomeration and sedimentation of various components in the slurry. By controlling the volatilization rate of the solvent on the surface of the particles, the regular particle morphology can be obtained, and the spray granulation powder with uniform particle size distribution, good fluidity and suitable loose density can be packed. Thus, the performance of the powder filling mold is improved, and the density and uniformity of the blank are increased. Therefore, the pressure spray granulation method was selected to study the effect of the loose density of spray granulation powder on the properties of Si3N4 ceramic balls.

 

Test Material

Si3N4 powder (when the cumulative volume fraction in the particle size distribution is 50%, the corresponding particle size D50=1.5μm, α-Si3N4 content is 93%, the purity is 99.9%), Y2O3 powder (D50=1.8μm, the purity is 99.9%), Al2O3 powder (D50=2.2μm, the purity is 99.95%), etc.

 

Sample preparation

According to Si3N4∶Y2O3∶Al2O3= 92%∶4%∶4%(mass ratio), the mixture was added to the ball mill, anhydrous ethanol was used as the solvent, Si3N4 ball was used as the grinding medium for mixing and dispersing, mixing time was 24h, the mass ratio of Si3N4 ball and mixed powder was 3:1. After uniform mixing, the mass fraction of solid phase of the slurry is 55%, and the viscosity is 4000MPa·s. By controlling the inlet temperature of the spray drying tower and the diameter of the spray plate, the granulated powder with different loose density was obtained. The Si3N4 spray granulation powder was pressed into a ceramic pellet with a diameter of 8.731mm by a dry press, and then the atmosphere pressure sintering was carried out at 1850℃, the heating rate was 3℃ /min, the holding time was 1.5h, and the nitrogen pressure was 9MPa. The properties were tested after preparation.

 

Result

The screening fraction data and loose density of the granulated powder prepared by different spray granulation processes with the same batch slurry are shown in Table 1. The density of ceramic pellet blank, sintering density, bending strength, crushing load, fracture toughness and Vickers hardness of the ceramic pellet pressed by spray granulation powder with different loose density are shown in Table 2.

 

The mechanical properties of Si3N4 ceramic ball pressed by 5# granulated powder are the best. Too high or too low loose density will affect the pressing performance of the powder and the density of Si3N4 ceramic ball blanks, thus affecting the mechanical properties of Si3N4 ceramic balls. The mechanism is that the loose density directly affects the porosity of the spray granulation powder after pressing, and the gas is difficult to discharge, resulting in the long migration distance of particles and substances during the sintering process, which is not conducive to sintering densification.

The particle morphology of the 5# Si3N4 spray granulation powder is solid and spherical (Figure 1).

The microstructure of Si3N4 ceramic ball GPS by 1# ~ 9# spray granulation powder is shown in Figure 2. With the increase of loose density of granulation powder, the number of pores inside Si3N4 ceramic ball after GPS first increases and then decreases, and the density of Si3N4 ceramic ball first increases and then decreases.

SEM was used to observe the microstructure and grain fracture morphology of the crushed samples of Si3N4 ceramic balls with the best mechanical properties and poor mechanical properties. As shown in Figure 3, the density of Si3N4 ceramic balls first increased and then decreased with the increase of loose density of granulated powder. Too high or too low loose density would lead to uneven grain growth and internal pores.

 

Conclusion

With Si3N4 powder as raw material and Y2O3 and Al2O3 as sintering additives, the influence of loose density of spray granulation powder on densification and mechanical properties of ceramic ball sintering was analyzed. The following conclusions were drawn:

1) The density of Si3N4 ceramic ball blank increases first and then decreases with the increase of the bulk density of granulated powder. When the loose packing density is 0.89g /cm3, the Si3N4 ceramic ball has the highest densification degree and the best mechanical properties.

2) When the bulk density of spray granulation powder is 0.89g /cm3, the formed Si3N4 ceramic ball has the smallest pores, uniform grain size, and mainly adopts the mode of transgranular fracture.

With the continuous progress of microelectronics packaging technology, the power and integration of electronic components have significantly increased, which has led to a significant increase in the heat generation per unit volume, which has put forward more stringent requirements for the heat dissipation efficiency (that is, its heat conduction performance) of the new generation of circuit boards. At present, researchers are working to develop a variety of ceramic substrate materials with high thermal conductivity, including aluminum nitride (AlN), silicon carbide (SiC) and beryllium oxide (BeO). However, BeO is environmentally limited due to its toxicity; SiC is not suitable for use as substrate material because of its high dielectric constant properties. In contrast, AlN is the preferred substrate material choice due to its similar thermal expansion coefficient and moderate dielectric constant to silicon (Si) materials.

 

Traditionally, thick film slurps are mainly designed for alumina (Al2O3) substrates, but the composition of these slurps is prone to chemical reactions when in contact with AlN substrates, producing gases, which poses a serious threat to the stability and performance of thick film circuits. In addition, since the coefficient of thermal expansion of the AlN substrate is lower than that of the Al2O3 substrate, directly applying the slurry and sintering process suitable for the Al2O3 substrate to the AlN substrate will lead to the problem of thermal expansion mismatch, which will affect the performance of the circuit. Therefore, it is not advisable to simply copy the material system and production process of the Al2O3 substrate to the AlN substrate. This paper describes in detail the fabrication process of the resistance designed for AlN substrate, and studies and analyzes the performance of the resistance.

 

resistance temperature coefficient measurement

The resistance temperature coefficient (TCR) represents the relative change of the DC resistance value of the resistor at the test temperature to the DC resistance value at the reference temperature, that is, the relative change of the resistance value ΔTCR for every 1 ° C temperature between the test temperature and the reference temperature:

 

Where: R1 is the resistance value at the reference temperature; R2 is the resistance value at the test temperature. T1 is the reference temperature; T2 is the test temperature.

 

The thick film resistance on the AlN substrate was measured by TCR. The high temperature temperature coefficient (HTCR) test data were shown in Table 1, and the low temperature temperature coefficient (CTCR) test data were shown in Table 2. From the test data, it can be seen that the design size has a certain effect on the temperature coefficient of the resistance. All the resistance models have a positive temperature coefficient on this AlN substrate, and the TCR of FK9931M is less than 150×10-6/℃, and the remaining models are less than 100×10-6/℃.

 

resistance stability assessment

Resistance can be regarded as a three-dimensional network structure composed of many conductive chains. When the resistance layer is subjected to tension, the more fragile conductive chain will break or locally elongate, so that the overall conductive capacity will be reduced and the resistance value will be increased. Conversely, when the coefficient of thermal expansion of the resistance layer is obviously smaller than that of the substrate, the stress inside the resistance layer is pressure. When the resistance layer is subjected to pressure, the contact between the particles will be tighter, and even a new conductive chain will be added, thus enhancing the conductive ability of the entire thick film resistor, and the resistance value will be reduced on the macro level. Because the thick film resistor is firmly bound to the substrate and the stress release is slow, the resistance value will change when the thick film resistor is stored at a certain temperature. The greater the difference between the thermal expansion coefficient of the thick film resistance and the substrate, the greater the stress inside the thick film resistance, and the greater the change rate of the thick film resistance when stored at high temperature.

 

According to different design sizes, four kinds of square resistance resistors were printed on the AlN substrate, and the resistors were adjusted by laser. After temperature storage at 150℃ and 1000h, the change of resistance values before and after temperature storage was compared. The resistance of each square resistance measures the resistance value of five resistors. As can be seen from Table 4 to Table 6, the resistance value change rate is less than 1.5% after being stored at high temperature.

 

In summary, with the rapid development of microelectronics packaging technology, the power and integration of electronic components have achieved a qualitative leap, but also put forward unprecedented challenges to the heat dissipation efficiency of the circuit board. Researchers have actively responded to this challenge by exploring and developing a series of ceramic substrate materials with high thermal conductivity, among which aluminum nitride (AlN) stands out among many candidate materials with its superior thermal expansion matching and moderate dielectric constant, and has become the focus of current research.

 

In this paper, the limitations of the traditional thick film slurry in the application of AlN substrate are analyzed in depth, and the resistance manufacturing process designed for the characteristics of AlN substrate is described in detail. The experimental results show that the thick film resistance on AlN substrate has stable performance, its temperature coefficient is within the acceptable range, and the resistance change rate is very small after high temperature storage, which verifies the feasibility and effectiveness of the production process.

 

In the future, with the further research and optimization of the AlN substrate and its supporting production process, we have reason to believe that the AlN substrate will play a more important role in the packaging of high-power density electronic components, and promote the development of the microelectronics industry to higher performance and higher integration.

Although the processing of silicon carbide substrate is difficult, in order to make the application of single crystal silicon carbide in electronic components become the future direction of development, so that silicon carbide devices are large-scale application and promotion, it is necessary to find a way to solve the problem of difficult silicon carbide processing.

 

 

At present, the SiC material processing technology mainly has the following processes: directional cutting, chip rough grinding, fine grinding, mechanical polishing and chemical mechanical polishing (fine polishing). Among them, chemical-mechanical polishing is the final process, and its process method selection, process route arrangement and process parameter optimization directly affect the polishing efficiency and processing cost.

 

However, due to the high hardness and chemical stability of SiC materials, the material removal rate in the traditional CMP polishing process is low. Therefore, the industry began to study the auxiliary efficiency technology supporting the flattening ultra-precision machining technology, including plasma assisted, catalyst assisted, ultraviolet assisted and electric field assisted, as follows:

 

01 Plasma assisted technology

YAMAMURA Kazuy et al. first proposed the plasma-assisted polishing (PAP) process, which is an auxiliary chemical-mechanical polishing that oxidizes surface materials to a softer oxide layer through plasma, while still removing materials by abrasive friction and wear.

 

The basic principle is: through the RF generator reaction gas (such as water vapor, O, etc.) to produce a plasma containing free groups (such as OH free groups, O free radicals), with strong oxidation capacity of free groups on the surface of the SiC material oxidation modification. A soft oxide layer is obtained, and then the oxide layer is removed by polishing with soft abrasives (such as CeO2, Al2O3, etc.), so that the surface of SiC material reaches the atomic level smooth surface. However, due to the high price of PAP process test equipment and processing costs, the promotion of PAP process processing SiC chips is also limited.

 

02 Catalyst assisted process

In the industrial field, in order to explore the high-efficiency ultra-precision machining technology of SiC crystal materials, researchers use reagents for catalytic assisted chemical-mechanical polishing. The basic mechanism of material removal is that the soft oxide layer is formed on the SiC surface under the catalysis of reagents, and the oxide layer is removed by the mechanical removal of abrasive. For a high quality surface. In the literature, Fe3O4 catalyst and H2O2 oxidizer were used to assist the enhancement of chemical mechanical polishing technology with diamond W0.5 as abrasive. After optimization, the surface roughness Ra=2.0 ~ 2.5 nm was obtained at the polishing rate of 12.0 mg/h.

 

03 UV-assisted technology

In order to improve the SiC surface flattening processing technology. Some researchers have used ultraviolet radiation to assist catalysis in chemical-mechanical polishing process. Uv photocatalytic reaction is one of the strong oxidation reactions. Its basic principle is to produce active free radicals (·OH) by photocatalytic reaction between photocatalyst and electron catcher under the action of UV light.

 

Due to the strong oxidation of OH free groups. The oxidation reaction occurs on the SiC surface layer to generate a softer SiO2 oxide layer (MOE hardness is 7), and the softened SiO2 oxide layer is easier to be removed by abrasive polishing; On the other hand, the bonding strength between the oxide layer and the surface of the wafer is lower than the internal bonding strength of the SiC wafer, which reduces the cutting force of the abrasive in the polishing process, reduces the scratch depth left on the surface of the wafer, and improves the surface processing quality.

 

04 Electric field assisted technology

In order to improve the removal rate of SiC materials, some researchers have proposed electrochemical mechanical polishing (ECMP) technology. The basic principle is: by applying direct current electric field to the polishing liquid in the traditional chemical mechanical polishing treatment, the oxidation layer is formed on the SiC polishing surface under electrochemical oxidation, and the hardness of the oxide layer is significantly reduced. Abrasive is used to remove the softened oxide layer to achieve efficient ultra-precision machining. However, it should be noted that if the anode current is weak, the machining surface quality is better, but the material removal rate does not change much; If the anode current is strong, the material removal rate is significantly increased. However, too strong anode current will lead to lower surface accuracy and porosity.

 

In short, chemical-mechanical polishing is still the most potential flattening ultra-precision machining method for SiC materials, but in order to obtain high-quality SiC wafers more efficiently, the above mentioned auxiliary processes are potential options. However, due to the lack of relevant studies, the impact on SiC materials is still lack of predictability. Therefore, if we can deeply study the influence of related auxiliary processes on chemical-mechanical polishing technology, and further reveal the processing mechanism of chemical-mechanical polishing auxiliary efficiency enhancement technology by quantitative and qualitative research means, it will be of great significance for realizing the industrialization application and promotion of SiC materials.

With the rapid development of modern microwave communication technology, the design of microwave circuit components with high performance and high precision has become increasingly important. In microwave radio frequency (RF) modules, thin-film circuit technology has become a key design method with its unique advantages. In this paper, several thin film circuit components based on alumina substrate are introduced in detail, including thin film microstrip circuit, thin film filter, thin film load, thin film equalizer and thin film power divider. These components play an irreplaceable role in microwave circuits, and their design accuracy and performance directly affect the performance of the whole microwave system.

 

 

Applications of alumina substrate in circuit

 

1 Thin film microstrip circuit

Aluminum oxide ceramic substrate is used to design thin film microstrip circuit, the thickness of the gold layer can be up to 3.5um, and the metal wire bonding can be used to connect with the external circuit. The common plate thickness is 0.127mm, 0.254mm, 0.381mm, 0.508mm, and the transmission frequency can reach more than 40GHz. It meets the frequency band requirements of most microwave RF module modules, and the thin film circuit line accuracy of the thin film process is ±5um. The microwave RF field often uses ceramic substrates for microstrip transmission lines or circuit design with high precision.

 

2 Thin film filter

The frequency of the thin film filter made of alumina ceramic substrate can be as high as 40GHz, which is often used as a functional unit of frequency selection in various microwave module modules and systems. Thin film filter is processed by thin film technology, through sputtering, lithography, wet or dry etching, cleaning, slicing to obtain thin film filter substrate, common interfinger type, hairpin type, comb type, parallel coupling type, C-type and other structural filters can be designed based on alumina ceramic substrate processing, can be designed low pass high pass band pass band resistance and other different types of filters. Because the dielectric constant of the alumina ceramic sheet is greater than that of the general PCB substrate, the volume of the film filter made by the alumina ceramic sheet is smaller than that of the general microstrip filter, and the electrical parameters of the film filter made by the alumina ceramic sheet are good. It is usually fixed with conductive adhesive or gold tin eutectic.

 

3 Film load

The aluminum oxide ceramic substrate is used to design the thin film load, which is often used to match the terminal of the module assembly of the microwave circuit to absorb the excess reflected power. For the load, the machining accuracy of the resistance value is very important, and the greater the deviation, the worse the final load performance. Due to the controllable square resistance of the tantalum nitride film layer in the film process, the film load with high precision can be produced, and the film load volume is very small, which is a good choice for the miniaturization of component modules. It is usually fixed to the end of the circuit using conductive adhesive or gold tin eutectic.

 

4 Film equalizer

The thin-film equalizer is designed with alumina ceramic substrate, which is often used to adjust the broadband power flatness of microwave circuits. The output waveform of the device is adjusted by changing the square resistance of the integrated tantalum nitride film layer and the different resistance values of the graphic design, so as to balance the power signal at the front end to achieve the power flatness adjustment effect.

 

5 Film power divider

The thin-film power divider designed with alumina ceramic substrate is often used in the multi-channel communication network system. It has the function of power distribution according to a certain proportion, with one input and multiple output. Since the isolation resistor can be integrated into the thin film circuit using the tantalum nitride film layer to design a suitable resistance value, this can avoid the deterioration of the circuit performance due to the welding of the patch resistance of the microstrip power divider and the instability of the resistance value of the patch resistor, and the thin film power divider is easier to achieve the multi-stage ultra-wideband design, and the designed object is small in size, easy to integrate and good in performance.

 

In summary, alumina ceramic substrate plays a crucial role in the design of thin-film circuit components. From thin film microstrip circuit to thin film power distributor, these components not only meet the requirements of high precision and high frequency of microwave RF modules, but also realize the miniaturization and high performance of the components. Through the precise control of thin film technology, we can obtain thin film circuit components with excellent electrical parameters, which further improves the overall performance of microwave systems. In the future, with the continuous progress of microwave communication technology, thin film circuit components based on alumina ceramic substrates will continue to play a greater role in the microwave field, contributing to the development of wireless communication, radar detection and other fields.

In exploring silicon nitride (Si3N4) substrate materials as core to a high-performance thermal management solution, our understanding of their heat transfer mechanisms is critical. The main heat transfer mechanism of silicon nitride is known to rely on lattice vibration, a process that transfers heat through quantized hot charge carriers called phonons.

 

The propagation of phonons in the lattice is not a simple linear motion, but is affected by the complex coupling between the lattice, resulting in frequent collisions between phonons, which significantly reduces the mean free path of phonons, that is, the mean distance that phonons can travel freely between two collisions. This mechanism directly affects the thermal conductivity of silicon nitride materials.

 

Furthermore, various defects, impurities and grain interfaces in Si3N4 crystals become the main sources of phonon scattering. These scattering events also lead to a decrease in the mean free path of phonons, which in turn reduces the overall thermal conductivity of the material. In particular, lattice oxygen, as one of the main defects affecting the thermal conductivity of silicon nitride ceramics, significantly hinders the smooth propagation of phonons and reduces the thermal conductivity efficiency of the material.

 

To overcome this challenge and improve the thermal conductivity of the silicon nitride substrate, we started at the source and focused on reducing the oxygen content in the lattice. Specific strategies include:

 

 

Optimize raw material powder

Choosing Si powder with low oxygen content as starting material is the key. The oxygen impurity content in the initial raw material is reduced through a rigorous raw material screening and pretreatment process. Subsequently, a two-step nitrided sintering process is used, in which Si powder is first heated in a nitrogen atmosphere to close to its melting point (1414℃), so that it reacts with nitrogen to form porous Si3N4 sintered body. This process ensures adequate nitriding of Si while controlling the oxygen content in the newly generated silicon nitride. Then, porous Si3N4 was further sintered at high temperature to promote grain growth and pore closure, and finally the Si3N4 ceramic substrate with high density, low oxygen content and high thermal conductivity was formed.

 

 

Direct sintering of high purity α-Si3N4 powder

Another way is to use high purity α-Si3N4 powder with very low oxygen content for sintering. This method avoids the conversion process from Si to Si3N4 and directly uses α-Si3N4 powders with high purity and specific crystal structure for sintering, reducing the possibility of oxygen impurity introduction. By precisely controlling sintering parameters such as temperature, atmosphere and pressure, silicon nitride substrates with high density, few defects and excellent thermal conductivity can be obtained.

 

 

Sintering application of β-Si3N4

Although β-Si3N4 may differ from α-Si3N4 in some physical properties, its low oxygen content and high purity are also suitable for the preparation of high-performance silicon nitride substrates. The use of β-Si3N4 powder for sintering can also prepare high thermal conductivity silicon nitride materials, especially in specific application scenarios, some characteristics of β-Si3N4 may be more advantageous.

 

In summary, the Silicon nitride (Si3N4) substrate material is a key component of a high-performance thermal management solution, and the optimization of its thermal conductivity is crucial to improve the overall thermal management efficiency. By deeply understanding the heat transfer mechanism of silicon nitride, namely lattice vibration and phonon conduction process, we realize that phonon scattering is one of the key factors affecting the thermal conductivity. In particular, oxygen defects in the lattice, acting as the main scattering source, significantly reduce the mean free path of phonons, thereby hindering the effective conduction of heat.

To overcome this challenge, we propose a variety of strategies to reduce the oxygen content in the silicon nitride substrate, thereby improving its thermal conductivity. From the optimal selection of raw material powder, to the direct sintering of high-purity α-Si3N4 powder, to the sintering application of β-Si3N4, each method aims to reduce the introduction of oxygen impurities at the source and achieve high density and low defect status of the material through fine process control.

Future research will further focus on exploring more efficient silicon nitride preparation processes and further understanding the mechanism by which different crystal structures and microstructure affect the thermal conductivity of silicon nitride. Through these efforts, we are expected to develop silicon nitride substrate materials with higher thermal conductivity and lower thermal resistance, providing strong support for high-performance thermal management in electronic packaging, aerospace, energy conversion and other fields.

In practical applications, in addition to high thermal conductivity and high electrical insulation properties, aluminum nitride substrates are also required to have high bending strength in many fields. At present, the three-point bending strength of aluminum nitride in circulation on the market is usually 400~500MPa, which seriously limits the promotion and application of aluminum nitride ceramic substrates, especially in the field of IGBT power devices with high reliability requirements. Due to the complex production process and high production cost of AlN materials, most of the domestic AlN materials are still unable to meet the application requirements of high thermal conductivity and high strength.

 

In the preparation of aluminum nitride ceramic substrate, the selection of sintering methods and sintering additives is often twice the result with half the effort, and the introduction of sintering additives is a common method for sintering aluminum nitride ceramics at present. On the one hand, the formation of low temperature eutectic phase, the realization of liquid phase sintering, promote the compact body; On the other hand, the oxygen impurity in aluminum nitride is removed, the lattice is improved, and the thermal conductivity is increased. At present, the sintering additives used in sintering AlN ceramics mainly include Y2O3, CaO, Yb2O3, Sm2O3, Li2O3, B2O3, CaF2, YF3, CaC2, etc., or their mixtures.

  

In the sintered aluminum nitride ceramic formula system, when Y2O3 is higher than 3.5wt%, the content of Y-Al-O increases significantly and aggregates in the sintering process. Due to the low thermal conductivity of Y3Al5O12 (about 9 W/ (m·K)), the thermal conductivity of aluminum nitride ceramic products after sintering is seriously affected. When the content of CaF2 and Li2O is higher than 1.33wt%, due to the volatilization of fluoride and Li-containing compounds, the porosity of the sintered aluminum nitride ceramic body is increased during the sintering process, and the density of the ceramic is reduced, resulting in a sharp decline in the bending strength of aluminum nitride ceramic products after sintering. When each additive is less than the minimum value, the effect of enhancing mechanical properties cannot be played or the effect is very small.

 

In summary, aluminum nitride ceramic substrate in practical applications face the comprehensive requirements of high thermal conductivity, high electrical insulation properties and high bending strength, but the flexural strength of the products in circulation on the market is generally low, limiting its wide application in the field of high reliability such as IGBT power devices. At the same time, the domestic AlN material is difficult to meet the application needs of high thermal conductivity and high strength due to the complex production process and high production cost. Therefore, in the preparation of aluminum nitride ceramic substrate, it is very important to select the appropriate sintering method and sintering additives, not only to form a low temperature eutectic phase to promote the compact body, but also to remove oxygen impurities to improve the thermal conductivity. However, the selection and dosage of sintering additives need to be strictly controlled to avoid negative effects on thermal conductivity and bending strength. In the future, in order to improve the performance of aluminum nitride ceramic substrates, it is still necessary to further optimize the sintering process and formulation system to meet the needs of higher levels of application.

In the field of high-tech materials, aluminum nitride (AlN) ceramics, with its excellent thermal conductivity, excellent electrical insulation properties and excellent mechanical strength, have become the core material in key fields such as electronic packaging, power electronics and microwave communication equipment. However, the preparation of aluminum nitride substrate is a complex process, in which the sintering process and the selection of sintering additives have important effects on the properties of the final product. In this paper, starting from the preparation process of aln ceramic substrate, the selection of sintering additives and their influence on the performance of the substrate will be discussed in detail, and combined with frontier research, how to improve the comprehensive performance of aluminum nitride substrate by optimizing the sintering additives and sintering process will be analyzed.

 

Preparation Technology of Aluminum Nitride Substrate

The preparation of aluminum nitride substrate mainly includes raw material preparation, mixing, molding, sintering and other key steps.

 

1. Raw material preparation

Raw material preparation is the first step in the preparation of aluminum nitride ceramic substrate, which mainly includes the selection and ratio of aluminum nitride powder, alumina powder and additives. As the main material, aluminum nitride powder is required to have the characteristics of high purity, small particle size, large specific surface area, low carbon content and low oxygen content. Alumina powder is usually used as part of the sintering aid to form a low melting point composite oxide during the sintering process to promote the densification of aluminum nitride ceramics. Additives are used to regulate the molding properties, sintering properties and mechanical properties of the material.

 

2. Mix and form

In the mixing process, it is necessary to mix aluminum nitride powder, alumina powder and additives according to a certain ratio to ensure the uniformity and stability of subsequent molding. Mixing can be done by dry or wet method. Molding is usually by pressing molding, injection molding or die molding, etc., the mixed raw materials are made into billets with a certain shape and size.

 

3. Sintering

Sintering is the last process and the most critical step in the preparation of aluminum nitride ceramic substrate. Under the condition of high temperature, the particles in the billet are combined by sintering to form a dense ceramic substrate. In the sintering process, temperature, atmosphere and time need to be controlled to ensure the forming quality and performance of the ceramic substrate. The commonly used sintering processes include hot pressing sintering, non-pressing sintering, microwave sintering, discharge plasma sintering and self-propagating sintering.

 

The Selection of Sintering Additives and Its Influence on The Properties of Substrate

Sintering additives play an important role in the preparation of aluminum nitride ceramic substrates. They react with the alumina composition on the surface of the aluminum nitride particle to form a composite oxide with a low melting point, resulting in a liquid phase in the sintered body. These liquid phases surround the aluminum nitride particles, and the particle rearrangement and internal pore discharge occur under the action of capillary force, and finally the compact sintering of aluminum nitride ceramics is realized.

 

1. Commonly used sintering additives

The commonly used sintering auxiliaries for aluminum nitride ceramic substrates include CaO, Li2O, B2O3, Y2O3, CaF2, CaC2 and CeO2. These materials play a dual role in the sintering process: first, they combine with aluminum oxide on the surface of aluminum nitride particles to form liquid aluminate, accelerating mass transfer and promoting sintering; Secondly, they can react with oxygen to reduce the lattice oxygen content and increase the thermal conductivity of aluminum nitride ceramics.

 

2. Principle of selection of sintering additives

When selecting sintering additives, it is necessary to consider their influence on the properties of aluminum nitride ceramic substrate. On the one hand, the sintering additives should promote the densification of aluminum nitride ceramics, improve the thermal conductivity and mechanical strength; On the other hand, sintering additives should avoid the introduction of excessive impurities, so as not to affect the electrical insulation properties and chemical stability of aluminum nitride ceramics. Therefore, when selecting sintering additives, a lot of experiments and optimization are needed to determine the best sintering additives formula.

 

3. Influence of sintering additives on substrate performance

The selection and content of sintering additives have significant influence on the performance of aluminum nitride ceramic substrate. For example, Y2O3, as one of the commonly used sintering additives, can significantly improve the thermal conductivity and bending strength of aluminum nitride ceramics when the content is moderate. However, when the content of Y2O3 is too high, Y-Al-O phase will be formed, resulting in a significant decrease in thermal conductivity. Therefore, when preparing aluminum nitride ceramic substrate, it is necessary to precisely control the content of sintering additives to obtain the best performance.

 

Improved Performance of AlN Substrate

In order to meet the high requirements for the performance of aluminum nitride ceramic substrates in different application fields, researchers are constantly exploring new sintering additives, optimizing sintering processes and developing new preparation technologies.

 

1. Development of new sintering additives

In order to further improve the performance of aluminum nitride ceramic substrates, researchers are actively developing new sintering additives. For example, by introducing rare earth elements or transition metal elements, the lattice structure of aluminum nitride ceramics can be optimized to improve thermal conductivity and mechanical properties. In addition, a variety of properties can be achieved through the use of composite sintering additives.

 

2. Optimization of sintering process

The optimization of sintering process is the key to improve the performance of aluminum nitride ceramic substrate. By adjusting sintering temperature, holding time and atmosphere, the microstructure of aluminum nitride ceramics can be accurately controlled. For example, rapid sintering methods such as microwave sintering or discharge plasma sintering can significantly shorten the sintering time and improve the production efficiency. At the same time, the microstructure and properties of Al nitride ceramics can be further optimized by precisely controlling the oxygen content and temperature gradient in the sintering atmosphere.

 

3. Innovation of preparation technology

With the continuous development of preparation technology, researchers are constantly exploring new preparation methods to improve the performance of aluminum nitride ceramic substrates. For example, the precise preparation of aluminum nitride ceramic substrate can be achieved by using advanced molding technologies such as casting molding and injection molding. In addition, the welding performance, stability and corrosion resistance of aluminum nitride ceramic substrate can be further improved by electroless copper plating, ceramic substrate encapsulation and other post-treatment processes.

 

Conclusion

In summary, the preparation of aluminum nitride ceramic substrate and the selection of sintering additives have important effects on its properties. The comprehensive properties of aluminum nitride ceramic substrate can be significantly improved by optimizing the sintering auxiliary formula, improving the sintering process and developing new preparation technology. In the future, with the continuous development of material science and preparation technology, the application field of aluminum nitride ceramic substrate will be further expanded, providing strong support for the development of electronic packaging, power electronic devices and microwave communication equipment. At the same time, we also need to pay close attention to the development trend of new technologies and new materials, constantly expand the research horizon, and promote the continuous progress and innovation of aluminum nitride ceramic substrate technology.