Research on cutting technology of silicon carbide single crystal

Release date：2019-07-01   Views：2457

Silicon carbide single crystal material is the representative of the third generation of wide band gap semiconductor materials, which plays a strong support for the development of electronic industry. Wafer processing technology is an important foundation and basic guarantee for the production of devices. Any material with excellent characteristics can only play a practical role under the successful and effective processing technology. This paper introduces the cutting process, and determines the cutting process parameters through the process experiment and the analysis of the experimental results.

Silicon carbide single crystal material with its excellent properties, can achieve semiconductor lighting, improve optical storage density, improve equipment performance, has a wide range of applications and broad application prospects. According to the use of devices, high surface quality is required for silicon carbide wafers: smooth surface, low surface roughness, no defects, no damage, and excellent surface parameters such as TTV and warp. Wafer processing technology is an important foundation and basic guarantee for device production. Only with the support of successful and effective wafer processing technology can any material with excellent characteristics play the greatest value. Even if there is high-quality silicon carbide single crystal, without matching processing technology and means, it can not be practical. The machining quality and precision of silicon carbide directly affect the performance of the device. For example, when there are micro defects on the wafer surface, they will be inherited to the epitaxial growth film and become the fatal defect of the device. According to the needs of application, the machining technology of silicon carbide single crystal polished wafer has become an important problem that we must solve.

It is very difficult to process this material, which is mainly reflected in the following aspects: (1) the hardness of SiC single crystal material is between 9.2 and 9.6, which is only about 0.5 lower than that of diamond; (2) it has high chemical stability, hardly reacts with any strong acid or alkali, and can resist any known acid corrosive agent at room temperature; (3) Silicon carbide is a new generation of semiconductor materials, a large number of problems have not been found, the mechanism research is very lack; (4) Processing equipment is relatively backward. Processing equipment is the basis of ultra precision processing of materials. Most of the key processing equipment with high degree of automation are imported from abroad. The research on processing equipment in China is relatively backward, especially in the aspects of equipment design, manufacturing concept, processing accuracy, equipment material and so on, there is a great gap with foreign countries.

Compared with other semiconductor materials, silicon carbide has incomparable advantages. From the comparison in Table 1, it can be seen that the technical parameters of silicon carbide such as working temperature, breakdown electric field strength and thermal conductivity are significantly higher than those of silicon, GaAs and other semiconductor materials.

For silicon carbide material processing technology, crystal cutting is a key link, which plays a decisive role in wafer quality. At present, China mainly relies on its own technical strength to solve the demand problem of silicon carbide single crystal polishing chip, improve the geometric parameters and surface quality of silicon carbide crystal polishing chip as soon as possible, and realize the practicality of silicon carbide crystal polishing chip, which is of great significance to the development of new-type electronic components based on silicon carbide in China.

       1 Experiment

Crystal cutting technology is one of the key technologies in the process of silicon carbide single crystal material processing. The quality of crystal cutting directly affects the processing quality and efficiency of the following process. For example, TTV, warp and other problems are mainly caused by the slicing process. If the TTV and warp of the chip are large, it will cause great difficulties to the processing of the later process, and even make the wafer scrapped; if there are deep scratches in the crystal cutting process, it will take a lot of time and energy to eliminate them in the subsequent grinding and polishing process. Silicon carbide single crystal cutting experimental equipment: struersaccu-tom-50 cylindrical cutting machine (as shown in Figure 1); main raw materials: silicon carbide single crystal, diamond blade. Through the process experiment, the influence of each technological condition on the cutting quality is further analyzed, and the best cutting process condition is determined.

2 results and discussion

2.1 selection of crystal propulsion mode

In the cutting process, the advancing way of crystal is mainly divided into three ways: horizontal pushing, swinging and rotating. For silicon carbide crystal with high hardness and high wear resistance, it is very important to choose a suitable and reasonable crystal pushing mode, because it will directly affect the success or failure and quality of single crystal cutting.

2.1.1 horizontal pushing mode

Traditional semiconductor materials, such as silicon, germanium, etc., adopt the flat pushing method when cutting the outer or inner circle. This method is relatively simple and the transmission system is relatively stable. Compared with silicon carbide, these materials are hard and brittle, but their hardness is very different, not in the same order of magnitude. According to the inherent characteristics of silicon carbide single crystal, there are some disadvantages in using flat pushing method to cut silicon carbide

(1) The cutting blade is circular. At the beginning of cutting, the contact between the blade and the crystal is a point. With the deepening of cutting, the cutting surface gradually becomes an arc, which leads to the increase of cutting resistance and the load of cutting blade. In addition, with the increase of cutting process, the cooling and lubrication of the blade are very difficult, which makes the temperature of the blade increase, which may lead to the failure of cutting;

(2) The cutting diameter is directly related to the diameter of the blade. When the flat pushing method is adopted, the cutting diameter is smaller than the radius of the blade. Especially in the later stage of cutting, almost all the blades are submerged in the crystal body. The phenomenon of diameter jump and end jump of the blade will be relatively "enlarged", which is easy to cause chip damage and directly affect the processing quality.

       

2.1.2 swing mode

This method is to rotate back and forth under the set angle, and the rotation angle is generally between 5 ° and 20 °. Compared with the flat pushing method, this method increases the rotation and reduces the cutting resistance. However, in the later stage of cutting, it is very difficult to control the radial jump and end jump of the blade, as well as cooling and lubrication.

2.1.3 rotation mode

In this way, the crystal and the blade are kept in reverse rotation, so that the cutting is always carried out at a "point", which can reduce the cutting resistance; In addition, the rotary cutting can increase the cutting diameter by one time, because the cutting point is gradually close to the crystal center, and finally ends in the crystal center part. That is to say, when the feed rate keeps the crystal radius length, a chip can be cut. Using this method can not only improve the processing efficiency, but also prolong the tool life to ensure the smooth cutting. Through the comparison of the above three ways, it is the most scientific and reasonable way to cut silicon carbide, which is a special crystal with high hardness.

2.2 blade speed control

In the cutting process, the control of the cutting blade speed is very important. Through the analysis of the theoretical and technological test results, we can see that when the speed is too low, the cutting efficiency is low, which is very easy to cause damage to the cutting surface, making the cutting surface quality poor; when the speed is too high, the cutting efficiency is high, and the cutting surface quality is good at the initial stage of cutting, but the surface quality gradually becomes worse after cutting. Through the analysis, when the blade speed is too high, the blade quality requirements are also very high, because the blade damage speed is too fast when cutting, when the blade quality and cutting speed do not match, resulting in the decline of chip surface quality and geometric parameters, so choosing the appropriate cutting speed is very important to the cutting quality and cutting success or failure. The test data is shown in Figure 2. Through the test, we choose the cutting speed in the range of 2400 ~ 2800r / min, in which the chip cutting quality is better.

2.3 effect of cutting speed on wafer geometric parameters

The cutting speed directly affects the quality of TTV and bow. Theoretically, the faster the cutting speed, the higher the efficiency. However, if the cutting speed is too fast, the blade will bend, which will affect the geometric parameters of the cutting blade. In addition, if the cutting speed is too fast, it will cause excessive wear of the blade and eventually affect the quality of the cutting blade. Therefore, the cutting feed rate should be kept at a relatively low level for SiC crystal with high hardness, and the cutting quality will be improved. The relationship between the cutting rate and TTV and the service life of the blade is obtained through the process test, as shown in Fig. 3. The results show that when the cutting speed is below 8 μ M / s, the TTV of the chip will drop to less than 20 μ m, and the life of the blade will be extended to about 4 h.

3 conclusion

The hardness of SiC crystal is very large, and the traditional cutting method, cutting equipment, cutting tools and cutting process parameters can not adapt to it. The best process is determined through process research. In the process of cutting silicon carbide crystal, the blade speed and cutting speed are the most important process parameters, which will directly affect the surface quality of the cutting wafer. The cutting speed is 2400 ~ 2800r / min, and the cutting speed is less than 8 μ M / s.

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