1. The development background of ceramic circuit boards.
The first-generation semiconductor technology represented by silicon (Si) and germanium (Ge) materials is mainly used in the field of data computing and lays the foundation for the microelectronics industry. Second-generation semiconductors, represented by gallium arsenide (GaAs) and indium phosphide (InP), are mainly used in the communications field to produce high-performance microwave, millimeter wave and light-emitting devices, laying the foundation for the information industry. With the continuous expansion of technological development and application needs, the limitations of the two have gradually emerged, making it difficult to meet the use requirements of high frequency, high temperature, high power, high energy efficiency, resistance to harsh environments, and lightweight and miniaturization. The third-generation semiconductor materials represented by silicon carbide (SiC) and gallium nitride (GaN) have the characteristics of large band gap, high critical breakdown voltage, high thermal conductivity, and high carrier saturation drift speed. Their production Electronic devices can work stably at temperatures of 300°C or higher, and have broad application prospects in semiconductor lighting, automotive electronics, new generation mobile communications (5G), new energy and new energy vehicles, high-speed rail transit, consumer electronics and other fields. The application prospects are expected to break through the bottleneck of traditional semiconductor technology, complement the first and second generation semiconductor technologies, and have important application value in optoelectronic devices, power electronics, automotive electronics, aerospace and other fields. With the rise and application of third-generation semiconductors, semiconductor devices are gradually developing in the direction of high power, miniaturization, integration, and multi-function, which also puts forward higher requirements for the performance of packaging substrates. Ceramic circuit boards have the characteristics of high thermal conductivity, good heat resistance, low thermal expansion coefficient, high mechanical strength, good insulation, corrosion resistance, radiation resistance, etc., and are widely used in electronic device packaging.
2. Technical Classification of Ceramic Circuit Boards Ceramic circuit boards include ceramic substrates and metal circuit layers.
For electronic packaging, the packaging substrate plays a key role in connecting the previous and the next, connecting internal and external heat dissipation channels, and has functions such as electrical interconnection and mechanical support. Ceramics have the advantages of high thermal conductivity, good heat resistance, high mechanical strength, and low thermal expansion coefficient. It is a commonly used substrate material for power semiconductor device packaging. According to different preparation principles and processes, currently commonly used ceramic substrates can be divided into Thin Film Ceramic Substrate (TFC), Thick Printing Ceramic Substrate (TPC), and Direct Bonded Copper Ceramic Substrate (DBC), Direct Plated Copper Ceramic Substrate (DPC), etc. This article analyzes the physical properties of commonly used ceramic substrate materials (including Al2O3, AlN, Si3N4, BeO, SiC and BN, etc.), focusing on introducing the preparation principles, process flows and technical characteristics of various ceramic substrates.
2.1Thin-film ceramic circuit board
Thin film ceramic circuit board (TFC), also known as thin film circuit, generally uses a sputtering process to directly deposit a metal layer on the surface of the ceramic substrate, and uses photolithography, development, etching and other processes to pattern the metal layer into circuits ... Because TFC uses high-precision photoresist as the photoresist material, combined with photolithography and etching technology, TFC's distinctive feature is high pattern accuracy, such as line width/slit width less than 10 μm. TFC deposits thin film capacitors, thin film inductors, thin film resistors and distributed parameter circuit components on a ceramic substrate. It has a wide range of component parameters, high precision, and good temperature and frequency characteristics. It can work in the millimeter wave band and has a high level of integration. Due to its small size, the product is mainly used in small current devices in the field of communications. Due to the high operating frequency and the great impact of parasitic parameters on circuit performance, the TFC itself is small in size and has high component density. Therefore, there are very high precision and consistency requirements for circuit design, substrate and film patterning.
2.2 Thick film ceramic circuit board
The TPC substrate can be prepared by coating the metal slurry on the ceramic substrate through screen printing, drying and sintering at high temperature. Depending on the viscosity of the metal slurry and the size of the screen mesh, the thickness of the prepared metal circuit layer is generally 10 μm ~ 20 μm. Due to limitations of the screen printing process, TPC substrates cannot obtain high-precision lines (the minimum line width/line spacing is generally greater than 100 μm). In addition, in order to lower the sintering temperature and improve the bonding strength between the metal layer and the ceramic substrate, a small amount of glass phase is usually added to the metal slurry, which will reduce the electrical and thermal conductivity of the metal layer. Therefore, TPC substrates are only used in the packaging of electronic devices (such as automotive electronics) that do not have high requirements for circuit accuracy.
2.3 Direct bonding to ceramic substrate
To prepare the DBC ceramic substrate, oxygen element is first introduced between the copper foil (Cu) and the ceramic substrate (Al2O3 or AN), and then the CuO eutectic phase is formed at about 1065°C (the melting point of metallic copper is 1083°C), which is then combined with the ceramic substrate. The film and copper foil react to generate CuAlO2 or Cu(AO2)2, achieving eutectic bonding between copper foil and ceramics. Because ceramics and copper have good thermal conductivity, and the eutectic bonding strength between copper foil and ceramics is high, the DBC substrate has high thermal stability and has been widely used in insulated gate bipolar diodes (GBT), lasers (LD ) and focused photovoltaics (CPV) and other devices are being packaged for heat dissipation. DBC substrate copper foil has a large thickness (generally 100μm-600μm), which can meet the needs of device packaging applications in extreme environments such as high temperature and high current. Although DBC substrates have many advantages in practical applications, the eutectic temperature and oxygen content must be strictly controlled during the preparation process, which requires high equipment and process control, and the production cost is also high. In addition, due to the limitation of thick copper etching, it is impossible to prepare a high-precision circuit layer. In the DBC substrate preparation process, oxidation time and oxidation temperature are the two most important parameters. After the copper foil is pre-oxidized, the bonding interface can form enough CuxOy phase to wet the Al2O3 ceramic and copper foil, and has a high bonding strength; if the copper foil is not pre-oxidized, the CuxOy wettability is poor, and the bonding interface will be A large number of voids and defects remain, reducing bonding strength and thermal conductivity. For the preparation of DBC substrates using AlN ceramics, the ceramic substrate needs to be pre-oxidized to first form an Al2O3 film, and then react with the copper foil to produce a eutectic reaction. Xie Jianjun and others used DBC technology to prepare Cu/Al2O3 and Cu/AlN ceramic substrates. The bonding strength between copper foil and AlN ceramics exceeded 8N/mm. There was a transition layer with a thickness of 2 μm between the copper foil and AlN. Its components were mainly Al2O3 and CuAlO2. And Cu2O.
2.4 Direct electroplating of ceramic substrates
The DPC ceramic substrate preparation process is as follows: first, a laser is used to prepare through holes on the ceramic substrate (apertures are generally 60 μm ~ 120 μm), and then ultrasonic waves are used to clean the ceramic substrate; magnetron sputtering technology is used to deposit a metal seed layer on the surface of the ceramic substrate ( Ti/ Cu), then complete the circuit layer production through photolithography and development; use electroplating to fill holes and thicken the metal circuit layer, and improve the solderability and oxidation resistance of the substrate through surface treatment, and finally remove the dry film and etch the seed layer to complete the substrate preparation. The front end of DPC ceramic substrate preparation uses semiconductor micro-processing technology (sputtering coating, photolithography, development, etc.), and the back end uses printed circuit board (PCB) preparation technology (graphic plating, hole filling, surface grinding, etching, surface processing, etc.), the technical advantages are obvious. Specific features include: (1) Using semiconductor micro-machining technology, the metal circuits on the ceramic substrate are finer (the line width/line spacing can be as low as 30μm~50μm, related to the thickness of the circuit layer), so the DPC substrate is very suitable for applications with higher alignment accuracy requirements. High-quality microelectronic device packaging; (2) Using laser drilling and electroplating hole filling technology to achieve vertical interconnection on the upper/lower surface of the ceramic substrate, three-dimensional packaging and integration of electronic devices can be achieved, and the device volume can be reduced; (3) Electroplating growth is used to control the thickness of the circuit layer (generally 10μm~100μm), and the surface roughness of the circuit layer is reduced through grinding to meet the packaging requirements of high-temperature and high-current devices; (4) The low-temperature preparation process (below 300°C) avoids high-temperature damage to Substrate materials and metal circuit layers are adversely affected, while also reducing production costs. To sum up, the DPC substrate has the characteristics of high pattern accuracy and vertical interconnection, and is a true ceramic circuit board. The bonding strength between the metal circuit layer and the ceramic substrate is the key to affecting the reliability of the DPC ceramic substrate. Due to the large difference in thermal expansion coefficient between metal and ceramic, in order to reduce interface stress, it is necessary to add a transition layer between the copper layer and ceramic, thereby improving the interface bonding strength. Since the bonding force between the transition layer and ceramics is mainly based on diffusion adhesion and chemical bonding, metals with higher activity and good diffusivity such as Ti, Cr and Ni are often selected as the transition layer.
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