1 Introduction
Ceramic materials are a class of inorganic non-metallic materials formed from natural or synthetic compounds through shaping and high-temperature sintering. They possess advantages such as high melting points, high hardness, and high wear resistance, and can be used as structural materials and functional materials, offering broad application prospects [1]. The sintering of ceramic materials is primarily a process in which one or more solid raw material powders undergo material transport under high-temperature heating, causing the powder to aggregate and densify. This process typically takes several hours or even tens of hours. Macroscopically, sintering manifests as increased density and strength; essentially, it is an energy decrease from particle surface energy to interface energy, driven by atomic diffusion under chemical potential gradients at different sites, and microscopically, it manifests as the elimination of pores between grains [2].
2 Metallization Processes
2.1 Direct Plated Copper Metallization
In the direct plated copper (DPC) metallization process, after the substrate is laser-drilled and thoroughly cleaned, a seed layer is deposited on the clean, dried ceramic substrate. A dry film is then applied, followed by development and exposure, and subsequently electroplating is performed to generate the desired metal circuitry. Thereafter, the excess dry film and seed layer are removed, and a non-active metal is coated on the copper surface to protect the copper layer for subsequent brazing processes.
Although DPC ceramic substrates offer advantages such as high thermal conductivity, high circuit precision, and reduced packaging volume through via-hole interconnections, the copper layer thickness is generally limited to no more than 150 μm due to the constraints of the electroplating process. Currently, DPC technology is primarily used in the packaging of high-power LEDs. In high-heat-generation applications such as high-brightness LEDs and deep-ultraviolet LEDs, not only is a high-thermal-conductivity substrate required on the backside for heat dissipation, but the front-side packaging materials must also consider thermal stability and reliability. Traditional resin packaging materials are prone to aging and failure under ultraviolet light and high temperatures. Therefore, current research also tends to use inorganic or metallic materials such as kaolin, nickel, and copper to form dam structures on DPC substrates, combined with transparent quartz encapsulation, to improve device reliability [3].
Currently, the direct plated copper metallization process has been widely applied, but it still faces issues such as low efficiency, poor via-filling, and poor bath versatility. Among these, poor via-filling can affect device performance, stability, and reliability. The reason is that during electroplating, copper tends to deposit more readily on the via surface, causing the via to close before the interior is fully filled, ultimately forming voids inside the via. The quality of electroplated via filling is influenced by the plating current and additive formulation; optimizing the plating solution composition and auxiliary process parameters can improve via-filling quality.
In addition, DPC ceramic substrates may encounter problems during electroplating, including excessively long plating times, non-uniform coating thickness, and macroscopic residual stresses within the coating. Among these, excessive residual stress may cause coating cracking or warping, and the accumulation of residual stress within the copper layer may affect the thermal stability of the ceramic substrate [4].
2.2 Direct Bonded Copper Method
The direct bonded copper (DBC) technology was first pioneered by Burgess et al. in 1973 [5]. Its basic principle is that the oxide layer on the copper foil surface forms a Cu–O eutectic melt at high temperature. This melt possesses excellent wetting properties and, at the eutectic temperature of 1065 °C, can effectively bond the ceramic substrate with the unreacted copper foil, ensuring a strong bond after cooling and solidification.
Near 1065 °C, the Cu–O eutectic phase forms. Since the melting point of pure copper is 1083 °C, the eutectic bonding must be carried out in the temperature range of 1065 °C to 1083 °C, with actual operations mostly concentrated between 1070 °C and 1075 °C. During cooling, the supersaturated oxygen in the Cu–O eutectic precipitates as Cu₂O; in Al₂O₃ or AlN ceramics, additional reaction products such as CuAlO₂ and CuAl₂O₄ may also appear [5]. The formation of the eutectic liquid and its oxygen content are crucial to bonding effectiveness. Given that the diffusion rate of oxygen in the copper melt is extremely low (10⁻⁵ cm²/s), it is difficult to introduce sufficient oxygen during the bonding process; therefore, pre-oxidation of the copper foil is generally performed to form Cu₂O on the copper foil surface to promote eutectic liquid formation. The oxygen content in copper also significantly affects the bonding interface strength, so precise control of this parameter is a core aspect of ensuring bonding performance [6].
The direct bonded copper process requires specific substrates for use. Pure copper melt has poor wettability on Al₂O₃, AlN, and Si₃N₄, with contact angles exceeding 130°. By increasing the oxygen partial pressure during bonding and the oxygen content of the copper melt, the contact angle on Al₂O₃ surfaces can be greatly reduced [7]. Although the wettability on AlN can also be improved by increasing the oxygen partial pressure during bonding, bonding in a vacuum environment, or extending the bonding time, the effect is very limited [8]. Therefore, AlN is generally pre-oxidized to form a surface layer of Al₂O₃, and then bonded using the above methods. However, none of these methods can easily improve the wettability of copper melt on Si₃N₄, so DBC is rarely applied to Si₃N₄.
2.3 Active Metal Brazing Metallization
In the new energy vehicle industry, SiC modules are highly valued. However, when the junction temperature of SiC power devices rises to 250 °C, the poor temperature-cycle reliability of DBC ceramic substrates under high-temperature conditions limits their application. To address this issue, researchers developed AMB (Active Metal Brazed) ceramic substrates.
First, a thin layer of solder is applied onto a clean ceramic substrate. Copper foil is then placed on the solder, and the assembly is heated in a vacuum environment at 800 °C to 950 °C to melt the solder. Upon cooling, a robust joint is formed. Next, wet etching is used to create metallic patterns to meet the electrical interconnection requirements of high-power devices.
Because conventional metals generally exhibit poor wettability on ceramic substrates, active metal solders are commonly used to improve wettability and enhance joint strength. Active metal solders contain at least one active metal element, with Ti and lanthanide elements currently being the main active elements. The commonly used active solders in AMB processes include Sn–Ag–Ti and Ag–Cu–Ti systems, where Ti serves as the active metal to improve wettability between the solder and the ceramic, while Sn and Ag act to lower the melting point and improve the thermal conductivity of the joint.
However, the AMB process must be carried out under high vacuum or a protective atmosphere, which limits its process applicability. To overcome this limitation, researchers have developed Reactive Air Brazing (RAB) technology, which can be performed in air. The brazing filler metals used in RAB are mainly composed of noble metals (such as Ag, Ag–Pd alloys) and metal oxides (such as CuO, V₂O₅ [9], Nb₂O₅, SiO₂, and Al₂TiO₅), thereby endowing the joint with good oxidation resistance. During RAB, metal oxides can adhere to and react with the ceramic substrate surface, enhancing the wettability of the ceramic substrate through the synergistic action of the molten filler metal and the interface. Meanwhile, the good ductility of noble metals helps relieve internal thermal stresses in the joint, and the addition of metal oxides helps reduce residual stresses arising from CTE mismatches between the joint and the ceramic matrix.
Active metal brazing metallization offers advantages such as simple equipment and process, high reliability, and no restrictions on ceramic substrate types, making it the most promising metallization process for high-power device applications. However, given the rapid development of the high-power electronic device industry, higher standards are being placed on the mechanical properties and long-term operational reliability of AMB processes, necessitating continuous optimization and improvement. At the same time, AMB ceramic substrates face the same technical bottleneck of insufficient circuit precision as DBC substrates. If new technologies can be developed to achieve circuit precision comparable to that of the DPC process, AMB ceramic substrates are expected to replace other similar substrates in the future, showing great application potential [4].
2.4 Laser-Induced Metallization
In the laser-induced metallization process, a laser beam is used to selectively irradiate an aluminum-containing ceramic substrate. The irradiated ceramic material is reduced to activated metal atoms. The substrate is then immersed in an electroless plating solution containing Cu²⁺, where the activated atoms promote the reduction of Cu²⁺ and its deposition on the irradiated areas, forming metallic circuit patterns [10].
Electroless plating is an autocatalytic redox process that does not require an external current; metal ions are reduced to solid metal by chemical reducing agents in the solution [11], and the energy driving this reduction comes from the chemical reducing agents in the solution. Typically, metal ions in the plating bath are not easily reduced spontaneously, and a catalyst is often needed as an intermediate medium to lower the activation energy for metal nucleation. Once catalyst particles are successfully deposited on the substrate surface, large-scale metal deposition can be triggered.
Laser-induced metallization is commonly performed on aluminum-containing substrates because laser irradiation can form activated Al atoms. However, the catalytic performance of Al atoms is not ideal, and other catalysts are needed to improve deposition efficiency.
The design of laser-induced metallization processes is highly sensitive to laser parameters, ceramic substrate characteristics, and electroplating process parameters. Although this technique combines the cost advantage of copper electroplating with the high circuit precision of LAM (laser-assisted metallization) processes, the high cost of laser equipment and the environmental pollution caused by electroless plating remain important factors limiting its further widespread adoption.
2.5 Thick-Film Metallization
Thick-film metallization involves screen-printing metal layers for sealing, conductors (circuit wiring), resistors, etc., onto ceramic substrates, followed by sintering to form brazing metal layers, circuits, and lead attachment points. Thick-film pastes are generally composed of metal powders with particle sizes around 1.5 μm, a few percent of permanent binder, and an organic vehicle (including organic solvents, thickeners, surfactants, etc.), prepared by ball milling and mixing. The steps of thick-film metallization generally include: pattern design and artwork preparation, paste preparation, screen printing, drying, and sintering [12].
Thick-film metallization utilizes the principle of screen printing. First, the required metal layers for packaging or electronic components such as resistors are attached to the aluminum nitride ceramic substrate. Next, the metal layers and electronic components are bonded to the ceramic substrate surface through high-temperature sintering, achieving firm connections between all parts. This process has wide applications in electronic device packaging and circuit wiring. The conductive paste is the key factor affecting thick-film metallization quality; its composition mainly consists of metal powders (1–5 μm), glass, binders, and an organic vehicle mixed by ball milling [13].
The thick-film metallization method is applicable to a wide variety of ceramic types and has a simple process [14]. However, limited by screen sizes and conductive pastes, it is difficult to fabricate wires with line widths below 60 μm. The electrical properties and adhesion strength of the metal layer are relatively poor, making it suitable only for electronic devices with lower power and size requirements. Conductive pastes specifically suitable for aluminum nitride thick-film metallization are still relatively scarce, and commercially available paste formulations are not directly applicable; otherwise, blistering at the interface may occur [15].
2.6 Thin-Film Metallization
Thin-film metallization is a process in which metal materials are vaporized and deposited onto ceramic surfaces by physical vapor deposition methods such as vacuum evaporation or sputtering to form a thin metal film, followed by masking, etching, and other steps to create metallized circuit patterns.
In theory, this process can form micron-scale uniform metal films on various substrate materials via evaporation or sputtering. However, due to the significant difference in thermal expansion coefficients between ceramics and metallic copper, direct copper deposition on aluminum nitride ceramics introduces large stresses between the metal and ceramic layers, affecting the adhesion strength of the coating to the ceramic and the thermal cycling stability of the substrate. Therefore, in recent years, multilayer deposition approaches have become popular. The first layer is typically a Ti layer, and the second layer is chosen from metals such as Cu, Ag, or Au. When dislocation slip and interactions in a single layer are transferred to another layer due to large internal stresses, the internal stresses within the metal layer are also relieved.
Thin-film metallization offers high quality, strong adhesion, uniform coatings, and fine pattern resolution. However, it can only produce very thin metal layers, and the preparation process is relatively complex, involving multiple steps including surface treatment, metal deposition, and post-treatment, requiring strict control of process parameters. This results in high manufacturing costs, severely constraining its development.