Aluminum nitride ceramics have the characteristics of high thermal conductivity, good mechanical properties, corrosion resistance, excellent electrical properties, and weldability. They are ideal large-scale integrated circuit heat dissipation substrates and packaging materials.
In electronic packaging applications, the lightweight and ultra-smooth surface of aluminum nitride ceramic substrates can reduce the volume, reduce internal resistance, and facilitate the heat dissipation of chips. It is usually required that the surface is ultra-smooth, the surface roughness Ra ≤ 8 nm, and the damage depth reaches the nanometer level; in integrated circuit chip applications, the surface accuracy of aluminum nitride ceramic substrates after polishing needs to meet RMS < 2 nm. The high hardness, high brittleness and low fracture toughness of aluminum nitride ceramics make it easy to produce surface defects and subsurface damage during processing. How to obtain high-quality flattened processing surfaces, improve processing efficiency, and reduce defects and damage during processing has always been a research hotspot in the field of ultra-precision processing.
At present, in order to obtain aluminum nitride ceramic substrates with high surface quality, ultra-precision processing methods such as chemical mechanical polishing, magnetorheological polishing, ELID grinding, laser processing, plasma-assisted polishing and composite polishing are mainly used.
1. Chemical Mechanical Polishing Process of Aluminum nitride ceramics
Chemical Mechanical Polishing (CMP) is the most widely used global flattening technology in the semiconductor industry. Its process device mainly consists of three parts: a rotating polishing disc, a specimen holder, and a polishing liquid delivery device. The polishing disc is attached to a polishing pad and rotates by itself. The external carrier applies positive pressure to the wafer so that there is a suitable positive pressure between the wafer and the polishing pad, which can produce relative movement. At present, a series of progress has been made in the CMP research of aluminum nitride ceramics.
2. Magnetorheological Polishing Process of Aluminum Nitride Ceramic
Magnetorheological polishing technology is a polishing method between contact polishing and non-contact polishing. Compared with traditional polishing methods, it has the advantages of high polishing accuracy, no tool wear, no clogging, high removal rate, and no introduction of subsurface damage.
3. ELID grinding technology of aluminum nitride ceramics
ELID grinding technology is a composite mirror processing technology that combines traditional grinding, lapping and polishing. It has the characteristics of high efficiency, simple process and high grinding quality. The grinding fluid used is a weak electrolyte aqueous solution, which has no corrosive effect on machine tools and workpieces. The device is simple and suitable for promotion. However, during the grinding process, the change of the correction current can easily lead to discontinuity of the oxide layer, uneven surface of the workpiece, and burns, residual stress, cracks and other defects when grinding the workpiece.
4. Laser processing of aluminum nitride ceramics
Laser processing is an advanced processing technology with non-contact processing, no tool wear, high precision, and strong flexibility. It is a processing method suitable for brittle and hard ceramic materials. Its working principle is that light energy reaches an extremely high energy density after being focused by a lens, causing the material to decompose at high temperature. The laser processing method is low-cost and high-efficiency, but it is difficult to control the precision and surface quality of the product.
5. Plasma-assisted polishing process of aluminum nitride ceramics
Plasma-assisted polishing (PAP) is a dry polishing technology. Because it combines plasma irradiation to modify the surface, the modified layer can be removed by ultra-low pressure or using soft abrasives, so it is often used to process difficult-to-process materials. At present, due to the influence of the grinding stone, the material removal rate of plasma-assisted polishing is lower than that of other processing technologies, and the processing equipment of PAP is expensive and not suitable for large-scale processing.
6. Composite polishing process of aluminum nitride ceramics
For typical hard and brittle materials, non-contact processing methods, such as chemical etching and laser polishing, often have problems such as environmental pollution, high processing costs, and low processing efficiency. In contrast, contact abrasive processing methods include diamond grinding and free abrasive polishing. Although they have high processing efficiency and good workpiece shape accuracy, they will introduce serious surface and sub-surface damage and are only suitable for rough processing. They must be combined with etching or polishing processes to achieve damage layer removal and stress release.
From the above analysis, it can be seen that a single processing method cannot have various advantages at the same time. To improve the surface quality and processing efficiency of aluminum nitride ceramic substrates, domestic and foreign scholars have also used a variety of processing methods to study composite polishing technology. Common composite polishing processes include ultrasonic vibration-assisted grinding, ultrasonic abrasive water jet polishing, and ultrasonic-assisted consolidated abrasive chemical mechanical polishing.