Selection of ceramic substrate materials and Process Flow

With the continuous development of technology, semiconductor power devices will continue to develop in new directions. Now semiconductor power devices will pay more attention to intelligence, integration, reliability analysis and energy consumption management, and will also pay more attention to applications in special environments such as high temperature, high pressure and strong electric field. With the iteration and update of semiconductor power devices, its device integration and power density continue to increase, and the heat dissipation requirements for electronic packaging systems are also increasing.

In recent years, the ceramic substrates that have been widely used in large-scale production mainly include: Al₂O₃, BeO, SiC, AlN, Si₃N₄, etc. These are high thermal conductivity substrate materials to deal with the heat dissipation problem of devices.

Common Ceramic Substrate Materials and Applications

Selection and advantages and disadvantages of ceramic substrate materials

So far, Al₂O₃ ceramic substrate is the substrate material with the largest number of applications and the widest application field in the electronics industry. It has the advantages of mature technology, low price, excellent performance and abundant raw material sources. It can meet a variety of needs and can be made into different shapes. Al₂O₃ ceramic substrates mainly include 95Al₂O₃, ceramic substrates and 99Al₂O₃, ceramic substrates. 95Al₂O₃ ceramic substrates are mainly used as thick film hybrid integrated circuit substrates, and 99Al₂O₃, ceramic substrates are mainly used as thin film hybrid integrated circuit substrates. Its processing technology often adopts the casting method, which is suitable for mass production.

Although SiC ceramics have high thermal conductivity, they have high dielectric loss and low breakdown voltage, which is not conducive to application in high-frequency and high-voltage working environments.

The most outstanding performance of BeO ceramic substrate material is its large thermal conductivity, which is 6 to 10 times that of alumina. Unfortunately, BeO ceramic powder is highly toxic.

AlN ceramic substrate has the characteristics of high thermal conductivity and good insulation, and is currently the most commonly used ceramic substrate in Si-based semiconductor materials. However, the low mechanical strength, easy deliquesce and high manufacturing cost of AlN ceramics limit the development of AlN substrates.

Si₃N₄ ceramics are the ceramic substrate materials with the best comprehensive performance, with a thermal conductivity of 90-120W/(m·k), a thermal expansion coefficient of 3.2×10-6/℃, and excellent mechanical strength, good chemical stability and thermal shock resistance.

At the same time, the thermal expansion coefficient of Si₃N₄ ceramic substrate is close to that of the third-generation semiconductor substrate SiC crystal, which makes it more stable in matching with SiC crystal materials, making Si₃N₄ the first choice for high thermal conductivity substrate materials for third-generation SiC semiconductor power devices.

Factors affecting thermal conductivity of Si₃N₄ ceramics

In terms of raw material selection, the β-phase content in high thermal conductivity silicon nitride ceramics should be greater than 40%. As the β-phase ratio in the final fired silicon nitride ceramic products gradually increases, the thermal conductivity of silicon nitride ceramics also gradually increases.

However, in terms of the selection of raw material crystal form, α-Si₃N₄ powder is used as the raw material. During the sintering process, due to the high activity of the powder and the higher sintering driving force, it is easier to realize the dissolution precipitation mechanism, promote the α-β phase transition of silicon nitride, and finally obtain silicon nitride ceramics with high β phase content and high thermal conductivity; while β-Si₃N₄ powder is used as the material, although the final silicon nitride ceramic has a high β phase content, its sintering driving force is small, and the ceramic is difficult to sinter densely, resulting in a large number of pores inside the ceramic, which reduces the thermal conductivity of the ceramic.

Studies have shown that adding a certain amount of β-Si₃N₄ seed crystals to the Si₃N₄ raw material can promote the rapid dissolution and precipitation of fine particles on the β-Si₃N₄ crystal during the sintering process, promote grain growth, discharge impurities and defects in the grain boundary, purify the lattice, and thus improve thermal conductivity.

In the selection of ceramic sintering aids, the commonly used metal oxides and rare earth oxides are Al₂O₃, MgO, ZrO₂, SiO₂, RE₂O₃ (RE=La, Nd, Gd, Y, Yb, Sc), etc. In addition, the research on sintering aids has developed from a single sintering aid to two or more composite sintering aids.

Manufacturing process of ceramic substrate

In addition to the casting method mentioned above, the manufacturing method of ceramic substrates sometimes also uses gel injection molding, dry pressing molding, 3D printing molding and other processes.

The preparation process of ceramic substrates is a key link in the production process of a product, because it will affect the quality and cost of the ceramic substrate. The overall price of the equipment is affected. It can be said that the manufacturing process of ceramic substrates is a crucial step. The flatness, surface roughness, and dimensional stability of ceramic substrates are key factors affecting the subsequent preparation of copper cladding and etching circuits on the substrate. This puts high demands on the forming process of ceramic substrates. At the same time, mass production also requires the forming method to have high production efficiency and low cost.

1. Casting process

The casting method is currently the most commonly used method, also known as th e scraper molding method, belt casting method, etc.

The basic process of ceramic substrate casting is: batching, vacuum degassing, molding, debinding, sintering, etc. Among them, obtaining a casting slurry with a high solid content and suitable viscosity is the key to casting. Tape casting has the dual advantages of high production efficiency and ultra-thinness, but due to the low density of the green body, it is easy to deform during sintering, and the quality rate of large-sized substrates is low. Therefore, improving thermal conductivity and controlling the yield rate are the main problems it faces. Large-sized ceramic substrates are often prone to deformation. The powder molding process combining tape casting and isostatic pressing is used to increase the density of the ceramic substrate tape casting green body, further reduce the shrinkage of the green body during sintering, and thus help to obtain large-sized high thermal conductivity ceramic substrates.

The slurry of tape casting is the key factor that determines the performance of the green sheet. The slurry includes powder, solvent, dispersant, binder, plasticizer and other additives. Although tape casting has unique advantages compared to other molding processes, in actual operation, due to the different stress release mechanisms, it is easy for the tape-cast sheet to bend, crack, wrinkle, and have uneven thickness when drying. In order to prepare uniform and stable tape casting slurry and smooth and flat tape-cast sheet after drying, it is necessary to pay attention to factors such as the wettability and stability of the slurry and the thickness of the green sheet while keeping the formula unchanged.

Ceramic substrate molding process

2. Gel injection molding

 Gel casting is a term in materials science and technology published in 2011 (from Baidu Encyclopedia）Simply put, it is a molding process that adds organic monomers, crosslinking agents, initiators, and catalysts to ceramic slurry, injects the slurry into a non-porous membrane, and uses temperature to induce the polymerization reaction of the organic monomers to solidify and form a green body. First, the ceramic powder is dispersed in an aqueous or non-aqueous solution containing organic monomers and crosslinking agents to obtain a concentrated suspension with high viscosity and high solid volume fraction (>50vol%), and then the initiator and catalyst are added to the ceramic slurry, and then the suspension is injected into a non-porous mold. Under specific temperature conditions, the polymerization reaction of the organic monomer is induced to solidify and form a green body.

Gel injection molding is currently difficult to achieve large-scale production. Although gel injection molding can produce parts with complex shapes and the produced green bodies are high in strength and relatively low in cost, its process requirements have high requirements on slurry fluidity, flow pattern of injection into the mold, gel time control, green body drying uniformity, etc., which also determines the difficulty of quantitative production control. However, it is difficult for companies to invest in mass production.

3. Dry pressing

Dry pressing is to put the powder that has been granulated, has good fluidity and appropriate particle size into the mold, and apply pressure through the press to press the powder into a green body of a certain shape. The dry pressing process is simple, easy to operate, suitable for mass production, short cycle, high work efficiency, and easy to realize mechanized and automated production. The green body has high density, relatively precise size, small firing shrinkage, high mechanical strength of the porcelain parts, and good electrical properties. Dry pressing is suitable for pressing round and thin sheet products. The product density is high and the flatness of the substrate is easy to ensure, but the production efficiency is low and it is difficult to prepare ultra-thin substrates.

4.3D printing technology

  3D printing technology was first produced in the late 1970s and early 1980s, and is one of the technologies that has attracted much attention. The processing idea of 3D printing "additive manufacturing" gets rid of the limitations of molds on traditional molding. In today's increasingly competitive market, 3D printing can achieve frequent product trials and modifications, which has incomparable advantages over traditional processing methods. At present, some domestic and foreign scholars have tried to use 3D printing technology to prepare sheet ceramic materials. In addition, 3D printing technology can greatly simplify the process steps of multi-layer co-fired ceramic substrates, realize rapid prototyping, and provide a new method for the integrated prototyping of multi-layer co-fired ceramic substrates.

In addition to the above methods, there are many types of manufacturing technologies for ceramic products, and it is said that there are more than 30 types, which are not listed here. Generally, factories use dry pressing molding, which has a relatively simple manufacturing process and low cost. Since the shape of the ceramic substrate is not complicated, the dry pressing molding method is more suitable and can produce large quantities of ceramic PCBs.

Manufacturing process of ceramic substrates

1. Molding

Use high-purity alumina (Al₂O₃ content ≥95%) powder and additives (mainly binders, dispersants, etc.) to form "slurry" or processing materials. Then dry-press the embryo after mixing evenly. The embryo after dry pressing can be processed before sintering, such as external dimensions and drilling, but attention should be paid to the compensation of dimensional shrinkage caused by sintering (enlarging the shrinkage size). Green pieces can also be made by tape casting. Glue liquid (alumina powder + solvent + dispersant + binder + plasticizer, etc. are mixed evenly + sieved) manufacturing + tape casting (the glue liquid is evenly applied to the metal or heat-resistant polyester belt on the tape casting machine) + drying + trimming (holes can also be processed) + degreasing + sintering and other processes. It can be produced automatically and on a large scale.

2. Sintering

After the green body is formed, there will still be impurities, voids, air and organic matter inside. At this time, we need to remove the impurities through "sintering". In this process, volatilization, combustion and extrusion are used to remove impurities, and the alumina particles will also achieve close contact or bonding. However, after sintering, the ceramic green body (cooked body) will experience changes such as weight loss, size shrinkage, shape deformation, increased compressive strength and reduced porosity.

There are several methods for sintering ceramic green bodies:

(1) Normal pressure sintering method

(2) Pressurized (hot pressing) sintering method

(3) Hot isostatic pressing sintering method

Normal pressure sintering is mainly carried out under normal pressure. This method will cause greater deformation to the green body and is not commonly used. Hot pressing is currently a more commonly used method. It is carried out under pressure, so the sintered product has better flatness. Static pressure sintering method, general products will not use this method, this method is characterized by the use of high pressure and high temperature gas for sintering, so that the performance of the sintered product is relatively balanced, the quality is better, but the cost is relatively high, usually aerospace, defense and military products are mostly used in this sintering method, such as military reflectors, nuclear fuel, barrels and other products.

3. Surface finishing

The surface of the ceramic blank just fired is very rough, this is due to the imbalance of particle distribution, voids, impurities, organic matter, etc. in the green body, which will cause deformation and unevenness or excessive roughness and difference. These problems need to be solved to obtain a smooth surface. After surface polishing (mainly using polishing materials such as SiC, B₄C) or diamond paste, the surface is polished step by step from coarse to fine abrasives. Generally speaking, most of them use AlO powder or diamond paste ≤1μm, or use laser or ultrasonic treatment to achieve. ) In this way, a mirror-like reflection and a very high smoothness can be obtained.

4. Strengthening treatment

After surface polishing, in order to improve the mechanical strength (such as bending strength, etc.), a layer of silicon compound film can be coated by electron ray vacuum coating, sputtering vacuum coating, chemical vapor deposition and other methods, and heat treatment at 1200℃ ~ 1600℃ can significantly improve the mechanical strength of the ceramic blank!

5. Wiring (circuit formation)

To process and form a conductive pattern (circuit) on a ceramic substrate, first, a copper-clad ceramic substrate must be manufactured, and then a ceramic printed circuit board is manufactured according to the printed circuit board process technology. Commonly used conductive pattern formation methods on ceramic PCBs are selected according to application requirements, such as power requirements, circuit density, conductivity and cost. There are many different options.

①　Film process

Thick film process is one of the most widely used methods for making conductive patterns on circuit boards, suitable for materials such as (Al2O₃) and ceramic aluminum (AlN). By screen printing on the ceramic body, a thick film paste containing conductive components (such as silver paste, gold paste, copper paste, palladium paste, etc.) is printed on the ceramic surface according to the circuit pattern. Then the printed ceramic substrate is sintered at high temperature (800℃ to 1000℃), and the conductive slurry will be combined with the substrate to form a conductive circuit.

②　 Thin film process

Mainly, a layer of metal film (such as copper, gold, molybdenum, etc.) is deposited on the ceramic substrate by physical vapor deposition (PVD) or chemical vapor deposition (CVD). This is the basis before the conductive pattern is formed. After the f

ilm is formed, photolithography and polishing are required, photoresist is coated on the metal film, and the circuit pattern is formed by exposure and etching, and finally the unnecessary metal area is removed to obtain the conductive pattern. The conductive layer made by this method is very thin, which is suitable for tiny circuits and lead circuits. However, the process is complicated, the equipment cost is high, and the production speed is relatively slow, so it is applied to the manufacture of ordinary electronic products, usually used for the manufacture of high-frequency circuits and precision electronic components.

③　Direct metallization

First, the ceramic surface needs to be treated, laser marked or roughened to make the surface suitable for metal attachment, and then a metal film (copper, gold, etc.) is plated on the ceramic substrate by chemical methods. After the film is formed, it is necessary to perform photolithography and polishing, and then etch to form a circuit diagram.

④　Laser direct writing

This mainly uses high-precision lasers to directly etch conductive materials on ceramic substrates to form conductive patterns.

⑤　 Nanomaterial spraying

Use conductive slurry containing nano-metal particles (such as silver and copper) to form circuit patterns through spraying technology.

Summary:

There are many types of ceramic materials that can be used as substrates, and each material has its ideal properties and applications. Alumina, zirconium oxide, silicon carbide, silicon nitride, and glass ceramics are the most commonly used ceramic substrates in various industries. Selecting the right ceramic material for a specific application requires careful consideration of its characteristics and performance requirements. With continuous research and development, the potential applications of ceramic substrates are endless.