Aluminum nitride ceramic metallization 5 methods, which is the most powerful?

Aluminum nitride ceramic metallization 5 methods, which is the most powerful?

2022-05-24 13:35:37 5

In recent years, with the development of large-scale integrated circuits and electronic devices to high-speed, multi-functional, miniaturization, low-power direction, the related applications for high-performance, high-density circuit demand is increasing. Among them, the chip manufacturing industry has also put forward requirements for package testing of chips, and there is a need to find more superior materials for package preparation.


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Aluminum Nitride Properties


Aluminum nitride (AlN) ceramics have many advantages that make them ideal for electronic packaging materials and can be used as substrates to make printed circuit (PCB) boards for high frequency circuit applications. In addition, AlN ceramics have superior heat transfer properties and are suitable for high-power circuits.


(1) High thermal conductivity.

(2) A coefficient of thermal expansion that matches that of semiconductor silicon wafers.

(3) High insulation resistance and dielectric strength.

(4) with low dielectric constant and dielectric loss.

(5) has high mechanical properties and good machinability.

(6) Very low secondary electron emission coefficient.

(7) Non-toxic.


However, the application of AlN ceramics in high-frequency and high-power applications usually encounters the problem of joining with metals or ceramics. Since AlN is a compound with strong covalent bonds, the general brazing material cannot wet the ceramic surface. Therefore, it is usually necessary to modify the surface of AlN to make it metallic (i.e. metallization), and then use conventional brazing process to realize the connection between AlN and metal.


The main methods of AlN ceramic metallization are: thin film metallization (e.g. Ti/Pd/Au), thick film metallization (low temperature metallization, high temperature metallization), chemical plating metallization (e.g. Ni), direct copper cladding method (DBC) and laser metallization.


Thin Film Metallization

Thin film metallization method uses vacuum coating method such as sputtering coating to combine film material and substrate together, usually in multi-layer structure substrate, the metal inside the substrate and the surface metal are not the same, the thin film metal in contact with the ceramic substrate should have good reactivity and strong bonding with the substrate, the surface metal layer is mostly chosen from the metal with high electrical conductivity and not easy to oxidize. Because it is vapor-phase deposition, in principle, any metal can form a film, any substrate can be metallized, and the deposited metal layer is uniform and has high bonding strength. However, thin film metallization requires a subsequent patterning process to realize the graphic preparation of metal leads, which is costly.


Thick film metallization method

The thick film metallization method is to form metal layers for sealing, conductors (circuit wiring) and resistors, etc. by screen printing on ceramic substrates, and to form brazed metal layers, circuits and lead joints, etc. by sintering. The steps of thick film metallization generally include: pattern design, preparation of original pattern and paste, screen printing, drying and sintering. The advantages of the thick film method are good electrical conductivity, simple process, suitable for automation and multi-species small batch production, but the bonding strength is not high, and is greatly affected by temperature, and the bonding strength is very low at high temperature.


Electroplating metallization method

The chemical plating metallization method uses a reducing agent to reduce the metal ions in solution on the catalytically active surface of the object without the passage of an external current, forming a metallic plating layer on the surface of the object. The bonding strength of chemical plating metallization depends largely on the roughness of the substrate surface, and within a certain range, the greater the roughness of the substrate surface, the higher the bonding strength; on the other hand, the adhesion of chemical plating metallization is poor, and the preparation of metallic patterns still requires a patterning process to achieve.


Direct copper cladding method

The direct copper cladding method uses a high-temperature melt diffusion process to clad the ceramic substrate with high-purity oxygen-free copper. The metal layer formed has the advantages of good thermal conductivity, high adhesion strength, excellent mechanical properties, easy etching, insulation and high thermal cycling capability, but the subsequent also requires a patterning process, while the oxide layer formed during the surface heat treatment of AlN will reduce the thermal conductivity of the AlN substrate.


Laser metallization method

The laser metallization method uses the thermal effect of the laser to cause thermal decomposition of the AlN surface, which directly generates a metallic conductive layer. After the laser irradiates to the AlN ceramic surface, the ceramic surface absorbs the energy of the laser and the surface temperature rises. When the temperature of the AlN surface reaches the thermal decomposition temperature, the AlN surface will undergo thermal decomposition and precipitate metallic aluminum. With the advantages of low cost, high efficiency and simple equipment maintenance, it has been widely used in production practice.


However, laser metallization also faces many problems, such as: the formation of agglomerates and porosity on the surface of the metallized layer, poor adhesion of the metallized layer and uneven metal thickness, etc.


Reference sources.

[1] Huang Ping Prize. Progress of laser metallization of AlN ceramics

[2] Gao Longqiao et al. Exploration of metallization technology for AlN ceramics

[3] Qin Diancheng. Research status and development trend of surface metallization of ceramic substrates 

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