Semiconductor Materials and Processes

The silicon carbide production process includes the preparation of material end substrates and epitaxy, the design and manufacture of subsequent chips, and then the packaging of devices, and finally flows to the downstream application market. Among them, the substrate material is the most challenging link in the silicon carbide industry. The silicon carbide substrate is hard and brittle, and it is very difficult to cut, grind, and polish, which makes it easier to produce waste products and reduce the yield rate during processing. At present, most international silicon carbide manufacturers are planning to change silicon carbide wafers from 6 inches to 8 inches.

Silicon carbide substrate The core material of the industrial chain, the preparation is difficult

The preparation of silicon carbide substrates mainly includes raw material synthesis, silicon carbide crystal growth, ingot processing, ingot cutting, wafer grinding, wafer polishing, and polishing pad cleaning. The link is the focus and difficulty in the entire substrate production link, and has become a bottleneck that limits the yield and production capacity of silicon carbide.

Crystal growth

Crystal growth difficulties:

● Crystal growth is slow. The growth rate of silicon carbide is only 0.3-0.5mm/h, and the maximum crystal length can only reach 2-5cm, which is quite different from silicon-based substrates. And as the size of silicon carbide crystals expands, the difficulty of its growth process increases geometrically.

● The yield rate of black box operation is low. The core parameters of silicon carbide substrates include micropipe density, dislocation density, resistivity, warpage, surface roughness, etc. The production process is completely completed in a high-temperature sealed graphite cavity. It is necessary to arrange the atoms in the closed high-temperature cavity in an orderly manner to complete the crystal growth and control the parameter indicators at the same time. It is very dependent on the manufacturer's process experience and is prone to various defects and other problems. The crystal growth process is difficult, the yield rate is low, and the output is small.

● There are many types of silicon carbide crystal structures, but only a few of them are required materials. It is difficult to control impurities, and polymorphic inclusions are likely to occur, which reduces product yield.

Cutting, Grinding and Polishing

After the silicon carbide crystal is prepared, it needs to be cut into thin slices with a thickness of no more than 1mm along a certain direction, and ground with diamond abrasive fluids of different particle sizes to remove knife marks and metamorphic layers and control the thickness before CMP After polishing to achieve global planarization, it enters the final cleaning process.

Difficulties in cutting, grinding and polishing: Since silicon carbide is a brittle material with high hardness, it has serious problems such as warping and cracking during processing, and the loss is huge. Under the traditional reciprocating diamond-bonded abrasive multi-wire cutting method, the overall material The utilization rate is only 50%. After polishing and grinding, the cutting loss ratio is as high as 75%, and the usable part ratio is relatively low.

Slicing of SiC single crystal

As the first process in the silicon carbide single crystal processing process, the performance of the slice determines the processing level of subsequent thinning and polishing. Slicing is easy to produce cracks on the surface and sub-surface of the wafer, which increases the chip fragmentation rate and manufacturing cost. Therefore, controlling the crack damage on the surface of the wafer is of great significance to promote the development of silicon carbide device manufacturing technology.

Main Influencing Factors and Optimizing Measures of Slicing Quality

Surface crack damage is closely related to slice quality. The epitaxial growth on the silicon carbide substrate material, the device manufacturing process and the device performance are all related to the crystal orientation. In order to avoid brittle cracks in wafers caused by orientation sensitivity during slicing, crystal orientation detection is required before slicing silicon carbide ingots. Silicon carbide ingots are generally grown on the SiC { 0001} plane, and cutting along the SiC crystal plane parallel to the growth direction of the ingot can effectively reduce the through-thread dislocation density on the slice surface and improve the slice quality.

Thinning of silicon carbide wafers

2.1 The thinning of silicon carbide slices is mainly achieved by grinding and grinding.

Grinding

The most representative form of wafer grinding is self-rotation grinding. While the wafer is self-rotating, the spindle mechanism drives the grinding wheel to rotate, and at the same time the grinding wheel is fed downwards, thereby realizing the thinning process. Although self-rotating grinding can effectively improve processing efficiency, the grinding wheel is prone to passivation with the increase of processing time, the service life is short, and the wafer is prone to surface and sub-surface damage. The existence of processing defects seriously restricts the processing accuracy and efficiency. In order to solve these problems, researchers have developed different auxiliary technologies, such as online dressing of grinding wheels, or the development of new soft abrasive grinding wheels. At present, the main technologies include ultrasonic vibration assisted grinding and online grinding. Electrolytic dressing assisted grinding.

Ultrasonic assisted grinding is a method of reducing grinding force and grinding wheel wear through ultrasonic vibration, and improving processing quality. Many studies have shown that under certain process conditions, ultrasonic-assisted grinding is more suitable for thinning of hard and brittle materials than ordinary grinding.

Under the action of electrolysis during online electrolytic dressing assisted grinding, an insulating oxide film is formed on the surface of the grinding wheel, which can slow down the loss of the grinding wheel, and at the same time support a large number of electrolytically shed abrasive particles, which is similar to the grinding effect of free abrasive particles, which is conducive to improving the quality of the grinding surface.

Lapping

The lapping process can be divided into single-side and double-side grinding, and single-side and double-side grinding technologies for small-sized silicon carbide wafers have been developed one after another. When grinding the surface of silicon carbide slices, the abrasive used is usually boron carbide or diamond, which can be divided into rough grinding and fine grinding. Coarse grinding is mainly to remove the knife marks caused by slicing and the metamorphic layer caused by slicing, and the abrasive grains with larger particle sizes are used. The purpose of fine grinding is to remove the surface damage layer left by coarse grinding, improve the surface roughness, and use finer abrasive grains.

2.2 The main factors affecting the thinning effect

The study found that the wafer material removal rate in the thinning process is closely related to abrasive particle size, density, grinding disc rotation speed, grinding pressure and other factors. The higher the hardness of the abrasive grains in the slurry, the larger the grain size, and the greater the surface roughness of the processed wafer. Too hard grinding discs will damage and pollute the surface of the workpiece, soft grinding discs can allow more sliding movement of the abrasive, and the surface finish after processing is high, but the flatness is low.

Polishing of silicon carbide wafers

3.1 Research status of polishing technology

The polishing process of silicon carbide wafers can be divided into rough polishing and fine polishing. Mainly include electrochemistry, magnetorheology, plasma, photocatalysis, etc., and mechanical synergistic methods mainly include ultrasonic assistance, mixed abrasive grain and consolidated abrasive grain polishing, etc.

3.2 Key factors and development trends affecting CMP

Polishing results are optimized when the mechanical and chemical effects of CMP are balanced. The polishing effect of CMP is mainly affected by three parameters: process parameters, polishing liquid, and polishing pad. Polishing fluid and polishing pads are the main consumables in CMP, and controlling and optimizing their properties to ensure repeatable polishing efficiency is critical for process stability. Improving the polishing solution and developing a polishing pad with self-catalysis is the research direction of CMP consumables in the future.