How to get a "perfect" silicon carbide wafer?

At present, more than 95% of the world's semiconductor components are based on the first-generation semiconductor material silicon as the basic functional material. However, with the rise of new applications such as electric vehicles and 5G, silicon-based semiconductors are limited by the physical properties of silicon materials. The bottleneck is not easy to break through, so the third-generation semiconductor materials represented by gallium nitride (GaN) and silicon carbide (SiC) have begun to receive attention.
Among the third-generation semiconductor materials, SiC has the characteristics of large band gap, high breakdown electric field, high saturation electron drift speed, and high thermal conductivity. obvious advantage.
At the same time, SiC crystal is an ideal substrate material for GaN-based devices, such as LEDs and LDs, because of its highly matched lattice constant and thermal expansion coefficient and good thermal conductivity with the epitaxial layer material GaN. Therefore, SiC crystal material has become an indispensable substrate material in the field of semiconductor lighting technology.
Among them, SiC substrate processing technology is an important basis for device fabrication. The quality and precision of its surface processing directly affect the quality of epitaxial films and the performance of devices. Therefore, in its application, the wafer surface is required to be ultra-smooth, free of Defects, no damage, surface roughness value below nanometer level.
However, due to the high hardness, high brittleness, good wear resistance and extremely stable chemical properties of SiC crystals, the processing of SiC wafers becomes very difficult.
The ultra-precision machining process of SiC single wafer, according to its processing sequence, mainly goes through the following processes: directional cutting, grinding (rough grinding, fine grinding), polishing (mechanical polishing) and ultra-precision polishing (chemical mechanical polishing).
01
cut
Cutting is the process of cutting SiC crystal rods into crystal flakes along a certain direction. Cutting SiC ingots into wafers with small warpage, uniform thickness and low cutting loss is crucial for subsequent grinding and polishing. Compared with the traditional inner circle and outer circle cutting, multi-wire cutting has the advantages of high cutting speed, high machining accuracy, high efficiency and long life, and has been widely used in the efficient cutting of wafers.
The principle of multi-wire cutting process: The multi-wire cutting process is to cut the crystal ingot into a slice with a flat surface and a uniform thickness according to a certain crystal orientation, so as to facilitate the subsequent grinding.
The basic principle is that the high-quality steel wire moves back and forth at high speed on the surface of the crystal ingot, and the diamond particles in the cutting fluid attached to the steel wire cause severe friction to the crystal ingot, causing the material to shatter and fall off the surface of the parent body to achieve the effect of cutting.
02
grinding
The purpose of grinding is to remove the knife marks and surface damage layer on the surface of the SiC slices caused by the cutting process. Due to the high hardness of SiC, high-hardness abrasives (such as boron carbide or diamond powder) must be used to grind the crystal surface of SiC slices during the grinding process. Grinding can be divided into coarse grinding and fine grinding according to different processes.
Coarse grinding is mainly to remove the knife marks caused by cutting and the metamorphic layer caused by cutting, and use abrasive grains with larger particle size to improve processing efficiency. Fine grinding is mainly to remove the surface damage layer left by the rough grinding, improve the surface finish, and control the surface shape and the thickness of the wafer, which is beneficial to the subsequent polishing, so use abrasive grains with finer particle size to grind the wafer.
Due to the low fracture toughness of SiC, it is easy to crack during the grinding process, making the grinding of SiC wafers very difficult. Effective grinding requires the selection of appropriate grinding parameters for maximum material removal and control of surface integrity.
03
rough throw
Rough polishing mainly adopts mechanical polishing method, and uses hard abrasives with smaller particle size, such as B4C, diamond, etc., to trim the surface of the wafer.
It is used to remove the residual stress layer and mechanical damage layer in the grinding process, improve the surface flatness and surface quality, complete the material removal efficiently, and lay the foundation for the subsequent ultra-precision polishing.
04
Ultra-precision polishing
After the traditional rough polishing process, using fine-grained diamond or B4C polishing liquid to mechanically polish the SiC wafer, the flatness of the wafer surface is greatly improved, but there are many scratches on the machined surface and a deep residual stress layer. and mechanical damage layer.
In order to further improve the surface quality of the wafer, improve the surface roughness and flatness, and make the surface quality characteristic parameters meet the precision requirements in the subsequent processing, ultra-precision polishing is a very critical link in the SiC surface processing process.
With the development of ultra-precision polishing technology, at present, the ultra-precision polishing processing methods suitable for SiC single wafer mainly include mechanical grinding, magnetorheological polishing, ion beam polishing, chemical mechanical polishing, etc. Among them, chemical mechanical polishing (CMP) technology is the current The most efficient method to achieve global planarization of SiC wafers.
Schematic diagram of GMP process
Chemical mechanical polishing is the process of removing and flattening the surface material of the workpiece through the synergy of chemical corrosion and mechanical wear. The wafer undergoes chemical oxidation under the action of the polishing liquid, and a chemical reaction layer is formed on the surface, and then the reaction softening layer is removed under the mechanical action of abrasive grains.
Because chemical mechanical polishing technology involves multidisciplinary knowledge, such as chemistry, physics, friction, mechanics and materials science, etc., there are many factors affecting its polishing effect, mainly polishing fluid (abrasive particles, oxidants, pH value, additives, etc.), polishing Pad (hardness, elasticity, surface topography, etc.) and polishing parameters (polishing pressure, head/disk speed, slurry flow, etc.).
Among them, polishing liquid is the core of chemical mechanical polishing technology, so its influence is decisive. Generally speaking, CMP polishing liquid is composed of abrasive grains, oxidants, deionized water and additives, so the polishing effect can be regulated from these factors - for example, the simple abrasive grains include abrasive types, abrasive grains diameter, abrasive particle concentration, etc.
At present, the oxidants commonly used in CMP polishing liquids are mainly H2O2 and KMnO4, and the abrasive particles are mainly SiO2, CeO2 and Al2O3. Feng et al. used 30-135nm modified silicon solution to fine-polish SiC wafers, and found that the residual damage layer after the rough polishing process could be completely removed, thereby obtaining a high-quality polished surface.