summary
In order to explore the material removal mechanism and subsurface damage evolution law of gallium oxide single crystal in the grinding process, the material removal mechanism in the grinding process was explored by simulating the process of removing material from a single abrasive grain by means of variable depth nano scratch test, and the grinding test of gallium oxide single crystal was carried out by using diamond grinding wheels with particle sizes of SD600, SD1500 and SD5000 respectively, and the morphology of the grinding surface and the damage evolution law of the subsurface were analyzed. The results show that the staggered slip zone that expands along different crystal directions during the removal of gallium oxide single crystals may lead to irregular fracture pits, and the orientation cracks are seriously affected by the (-3-10) slip surface. With the decrease of the particle size of the grinding wheel, the grinding surface morphology gradually evolves into a completely plastic surface dominated by crushing pits and oriented cracks.
Semiconductor materials are the cornerstone of supporting the modern information society and the forerunner of promoting the development of high technology, and are also the guarantee for the development of science and technology and information industry for national defense Bandgap/ultra-wide bandgap (Eg>2.3 eV) semiconductors are called third-generation semiconductors. Among them, Ga2O3 is a new type of semiconductor material with ultra-wide bandgap, with a bandgap width of 4.8 eV, and because of its superior physical and chemical properties, it is an ideal material for devices working in extreme environments such as high temperature, high pressure, high power, and high corrosion. In addition, its unique "solar blindness" characteristics (wavelength less than 285 nm) make it the preferred material for UV detectors, and has long-term development prospects in the fields of military detection, regional communications, and optoelectronic equipment.
β-Ga2O3 is the most thermodynamic state of gallium oxide single crystals, and it belongs to the monoclinic C2/m space group. In addition, due to the large interplane distance between the crystal planes of β-Ga2O3 (100) and (001), the inter-atomic binding force is weak, so the interplane cleavage phenomenon is very easy to occur, which is a typical difficult-to-process material. β Gong Kai et al. used grinding pads made of silk, wool, sandpaper, tin discs and W1.5 free abrasives to grind gallium oxide wafers, which proved that the hardness of the grinding pads will affect the cleavage of the surface material, among which the cleavage phenomenon caused by the tin disc is the most serious, and the soft grinding pad can inhibit the cleavage phenomenon in the processing process, but the removal rate is small. Gao et al. studied the subsurface damage characteristics of gallium oxide wafers ground by grinding wheels with different grain sizes, analyzed the variation of surface/subsurface damage with the cutting depth of abrasive grains, and established a grinding damage model of gallium oxide wafers. Huang et al. explored the mechanism of the grinding process on the crystal plane of β-Ga2O3 crystal (100) by using water-based and oil-based slurries, and the results showed that there were a large number of cleavage steps on the crystal surface with water-based slurries, which belonged to brittle removal. Hoshikawa et al. studied the free abrasive grinding process of the (001) and (100) planes of the gallium oxide wafer, and compared the processing performance and yield of the two crystal planes, and the results showed that when grinding the (001) and (100) planes of the gallium oxide wafer, the surface quality and yield of the (001) crystal plane were better than those of the (100) crystal plane under the same processing conditions, indicating that the (100) crystal plane had stronger cleavage. Blevins et al. optimized the grinding/polishing slurry and grinding/polishing pad in the grinding and polishing process of gallium oxide wafers, and obtained the processing route of rough grinding using 20 μm corundum abrasive slurry and cast iron grinding disc, 1 μm corundum abrasive slurry and lead grinding disc fine grinding, alkaline sol polishing slurry and felt polishing pad polishing. Lee et al. studied the effect of groove density (groove width/groove spacing) on the surface quality and material removal rate of the grinding disc, and the results showed that the groove density of the grinding disc has a direct effect on the liquid film thickness and fluid state during the grinding process, and the groove width of the grinding disc needs to be increased in order to improve the surface quality and material removal rate of the grinding wafer.
Summarizing the above research status, it is not difficult to find that the research of domestic and foreign scholars on the processing technology of gallium oxide single crystal mainly focuses on the grinding process, but the grinding process is very easy to cause the cleavage of gallium oxide single crystal, although the above-mentioned scholars have achieved some results by using different grinding pads and slurries, but they cannot solve the problem of gallium oxide single crystal cleavage caused by grinding process. In recent years, the ultra-precision grinding process based on the workpiece rotation method has been widely used in the processing of semiconductor wafers, which can stably remove materials and obtain high surface integrity. Through preliminary research, it is found that the workpiece rotation grinding process can effectively inhibit the cleavage phenomenon of gallium oxide single crystal, which is a high-quality and efficient processing method, but the grinding characteristics of gallium oxide single crystal are not clear at present. Therefore, in order to meet the requirements of ultra-precision machining of high-precision and high-quality gallium oxide wafers in the manufacture of high-performance semiconductor devices, it is necessary to systematically study the removal mechanism and damage evolution law of the grinding surface material of the new generation of semiconductor materials gallium oxide single crystals. Therefore, in this paper, the microscopic material removal mechanism and damage evolution law of gallium oxide single crystals are systematically studied by means of variable depth nano scratch test and grinding test.
1 Research protocol
In order to explore the removal mechanism and damage evolution law of gallium oxide single crystal grinding surface material, the surface of gallium oxide single crystal (-201) was selected as the test crystal plane, the size of the test sample was 10 mm×10 mm×0.68 mm, and the surface roughness Ra was less than 1 nm after chemical mechanical polishing. The nano-scratch method is a test method that can simulate the interaction between diamond abrasive particles and workpieces, and has been widely used to study the removal and deformation characteristics of various semiconductor materials. The parameters of the nano-scratch test are shown in Table 1.The grinding test is carried out on the Okamoto VG401 MKII. ultra-precision grinding machine equipped with an aerostatic spindle for high-precision rotation and feeding. VG401 MKII. ultra-precision grinding machine and grinding principle schematic diagram is shown in Figure 1. The grinding wheel adopts diamond grinding wheels from Asahi Diamond Industrial, with grain sizes of SD600, SD1500 and SD5000 respectively, and the cooling method is deionized water cooling, in order to reflect the grinding characteristics of gallium oxide single crystal under different grain size grinding wheels, no light grinding is carried out in this paper, and the grinding test parameters are shown in Table 2.
The surface and subsurface of gallium oxide single crystal specimens were characterized by various methods. Ground and scratch surface topography was observed using JEOL JSM-7610 Plus scanning electron microscope (SEM), ground surface roughness and 3D topography were measured by Zygo NewView 9000 3D surface profiler, transmission electron microscope samples were prepared using FEI Helios G4 UX dual-beam focused ion beam (FIB)-SEM and TEM observation of the sample cross-section by FEI Tecnai F20 transmission electron microscope. In order to protect the grinding surface from damage, a layer of Pt.
2 Results and Discussion
2.1 Microscopic removal mechanism of materials
In order to analyze the microscopic removal mechanism of gallium oxide single crystal in the grinding process, the overall morphology of the scratch surface is obtained by the variable depth nano-scratch test, as shown in Figure 2. The scratch depth selects 4 areas from shallow to deep for specific analysis, as shown in Figure 3 ~ Figure 6 respectively.In the initial stage of scratch, it can be found that there is no crack in the scratch, damage such as breakage, and the scratch groove is relatively smooth, which is manifested as a typical plastic removal, as shown in Figure 3 (b). The presence of micro-cracks, crushing pits and micro-cracks indicates that fractures have begun to occur inside the scratch, and with the increase of the indentation load, the material removal characteristics of the scratch in the middle section 1 gradually change (Fig. 4(a)), and the banded chips gradually change to massive chipping chips, which are abundantly distributed on both sides of the scratch, and this phenomenon can be clearly observed in Fig. 4(b), Fig. 4(c), and Fig. 4(d), where the underside of the scratch is found to have obvious orientation The degree of cracking is greater than that of the crushing pit in Fig. 3(c). As can be seen from Fig. 5(a)(b), slip zones along the [132] and [1-12] directions are found on the outside of the scratch, and a plastic flow line in the scratch groove is found in this area. When the load continues to increase, it can be seen from Figure 5(c) that a large number of slip zones along the [132] direction begin to appear, and the density of the slip zone increases significantly. In addition, orientation cracks were found on the underside of the scratch, as shown in Fig. 5(d), which is consistent with the crack found in Fig. 4(c), and a large number of chipping chips were observed on the outside of the scratch, indicating that the material removal pattern at this time was brittle. As shown in Figure 6, more slip zones can be found to be activated, i.e., slip zones in the [132], [1-12] and [112] directions can be observed; due to the increase of the indentation load, the extended length of the slip zone at the end of the scratch is also significantly increased, and the density of the plastic flow line in the groove is also significantly increased, and a long crack exists at the cone angle of the indenter at the end of the scratch, and the crack orientation is consistent with the slip zone in the [132] direction, according to our previous research results In the case of gallium oxide single crystals, the sharp slip will cause the crack to propagate along the slip zone, where the crack propagates because the stress concentration is strongest at the edges and corners of the indenter.
Based on the test results of the above-mentioned nano-scratches, it can be found that with the increasing indentation load, cracks with obvious orientation appear in the scratches, and then slip zones and plastic flow lines in multiple directions are found at the edge and inside of the scratches, respectively, and finally long cracks propagating along the slip zone are found at the end of the scratches.
By observing the surface morphology of gallium oxide single crystal nano-scratches, it is not difficult to find that the cracks with obvious orientation are distributed from the front to the end of the scratch, while the slip zone is distributed in the middle and rear sections of the scratches. For single gallium oxide crystals in the monoclinic crystal system, Yamaguchi et al.'s study shows that the crystal planes (-201), (101), (-310) and (-3-10) are close-packed planes, and the crystal structure is prone to slip along these planes, where the (-201) plane and the (101) plane are perpendicular to the (010) plane and coplanar to the [010] direction, respectively, while the (-310) plane and the (-3-10) plane are both perpendicular to the (001) plane and intersect the (-201) plane, as shown in Figure 7. Combined with the test conditions in this paper, the (-310) and (-3-10) planes are the crystal planes that are most likely to produce slip cracks on the (-201) plane, and through further analysis, it is found that the angle between the crack propagation direction in the scratch and the [010] direction is about 57°~62°, which is close to the theoretical value of 58°, indicating that the crack is found to be propagated by the (-3-10) surface in the (-201) surface nanoscratch, but It should be noted that the orientation crack only occurs on one side of the scratch, which is mainly related to the deflection of the indenter during the scratch process and the stress concentration of the indenter bevel, which will be discussed in detail below. In addition, slip zones in multiple directions are found at the edge of the scratch, which is due to the special atomic arrangement of the single crystal gallium oxide (-201) surface, which is discussed in detail in our previous work and will not be repeated in this article.
Figure 8 shows the position of the Glass indenter and the plastic flow line at the end of the nano scratch. As can be seen from Fig. 8(a), the scratching process does not strictly follow the edge forward, and there is an angle of 7.5° between the front edge of the indenter and the scratch direction [010]. It should be noted that the shape of the tip of the Glass indenter used in the test is a regular triangular pyramid, so the morphology reflected on the scratch should be a regular triangle, and the scratch topography of the end section of the scratch is further completed by connecting the line by combining the plastic flow line and the front edge mark in the scratch groove. As can be seen from Fig. 8(b), the composition is about a regular triangle, which indicates that the formation of the plastic flow line in the scratch groove is affected by the posture of the indenter during the scratch process. In the process of rubbing the material by the indenter, the normal force applied to the indenter will produce tangential force on the contact surface and accumulate at the front end of the indenter, and increase with the increase of normal load and cutting depth, which aggravates the plastic deformation of the material at the front end of the indenter. As the indenter continues to go deeper, the potential energy is continuously stored and released in the form of kinetic energy, which will lead to the periodic sliding phenomenon of the indenter, which leads to the plastic flow line within the scratch. In addition, in the nano-scratching process, because the front edge of the indenter cannot achieve the ideal full forward, there will also be a certain angle between the oblique edge on both sides and the scratch direction, which will also be reflected in the scratching process, as shown in the plastic flow line in the groove in Fig. 8(c), which are 52.6° and 67.8° respectively in this paper, which is consistent with the position of the indenter in Fig. 8(b), which further proves that the formation mechanism of the plastic flow line in the scratch groove is related to the Glass indenter.
Combined with the above analysis, the material removal characteristics on both sides of the scratch are different due to the deflection of the front edge of the indenter at a certain angle. In order to further analyze the scratching process, two finite element models, the deflection and the non-deflection of the front edge of the indenter during the scratching process, were established by the finite element method, and the stress distribution in the scratching process was simulated. The stress distribution of the indenter when it is not deflected in the nano-scratch process is shown in Figure 9, and it can be seen from Figure 9 that the stress on both sides of the indenter is symmetrically distributed on both sides of the scratch surface and on the scratch subsurface. The stress distribution of the indenter during deflection in the process of nano scratching is shown in Fig. 10, and it can be seen from Fig. 10 that when the front edge of the indenter is deflected at 7.5°, although both sides of the indenter are in contact with the material, the surface and subsurface stress distribution of the deflection side of the indenter is significantly greater than that of the other side, and the stress distribution on the contact surface of the indenter is also significantly different, and the indenter contact surface on the deflection side bears more material removal, so it shows a greater stress value. For gallium oxide single crystals, due to its monoclinic structure with poor symmetry, the difference in stress distribution leads to different material deformation characteristics on both sides of the scratch, which is manifested as the crack propagating along the (-3-10) plane is first induced to be generated on the deflection side of the indenter, and with the continuous accumulation and release of energy and the stress concentration on the deflection side of the indenter, the crack begins to appear repeatedly, as shown in Fig. 3(c), Fig. 4(c) and Fig. 5(d). The other side of the scratch is the first to be activated by the multi-directional slip zone after continuous accumulation due to the small stress, as shown in Fig. 5 (c) and Fig. 6 (c), until the end of the scratch produces a crack along the [132] direction under the action of the stress concentration in front of the indenter, as shown in Fig. 6 (d).
In summary, with the increasing indentation load, the crushing pits, cracks, plastic flow lines and multi-directional slip zones are found in the process of variable depth nano-scratches, and the material fragmentation occurs in the front section of the scratches, indicating that gallium oxide single crystals are very prone to damage during the scratching process, and the damage behavior is significantly different from that of monocrystalline silicon, silicon carbide and gallium nitride.
2.2 Grinding surface material removal features
In order to explore the material removal characteristics of gallium oxide single crystal under different grinding conditions, diamond grinding wheels with different grain sizes were used for grinding tests. The surface morphology of SD600, SD1500 and SD5000 grinding wheels grinding gallium oxide single crystal is shown in Fig. 11, Fig. 12 and Fig. 13 respectively. When grinding with the SD600 grinding wheel, a large number of cracks and irregular crushing pits were found to be distributed on the grinding surface, and there was almost no continuous surface, as shown in Fig. 11(a) and Fig. 11(b), showing obvious brittle removal characteristics, and brittle fracture dominated the material removal at this time. As can be seen from Fig. 11(c), there are a large number of cracks on the grinding surface that propagate in the same direction and have a certain angle with the grinding direction, indicating that there is obvious orientation in the propagation of these cracks, and the roughness of the grinding surface Ra is 241.37 nm. The morphology of the grinding surface of the SD1500 grinding wheel is shown in Fig. 12(a), at this time, the irregular crushing phenomenon of the grinding surface is significantly reduced, although the surface damage such as deep scratches and cracks can still be observed, but the grinding surface is no longer completely dominated by brittle fracture [29], as shown in Fig. 12(b), it can be clearly observed that the plastic flow traces of the material have begun to appear, indicating that the grinding surface at this time has gradually changed from complete brittle removal to partial plastic removal. Under the premise that the grinding process parameters are consistent, due to the decrease of abrasive particle size, the cutting depth of abrasive grains decreases accordingly, so the surface grinding depth also gradually decreases and the grinding density increases, and the grinding surface roughness Ra decreases significantly compared with the grinding surface of SD600 grinding wheel, which is 58.13 nm. However, similar to the grinding surface of SD600 grinding wheel, the cracks existing on the surface also show obvious orientation, as shown in Figure 12(b). Select a local area for magnified observation, as shown in Fig. 13(b), it can be observed that there is only a uniform distribution of shallow grinding lines on this surface, the quality of the grinding surface is the best, and its three-dimensional surface morphology is shown in Fig. 13(c), it can also be found that the material removal of the whole grinding surface is relatively uniform, and no orientation crack similar to that of the grinding surface of SD600 and SD1500 grinding wheels is observed, and the surface roughness Ra is 2.06 nm, which is much smaller than the first two.
In summary, with the decrease of abrasive particle size, the grinding surface of gallium oxide single crystal gradually changes from a fully brittle surface to a partially plastic surface, and finally the full plastic removal surface is realized, and the surface morphology of the grinding surface is that the surface damage such as a large number of crushing pits and orientation cracks gradually transitions to a uniform distribution of shallow grinding grains, and the surface roughness is significantly reduced, indicating that the grinding of gallium oxide single crystal by a fine-grained grinding wheel can effectively inhibit the surface damage and achieve high-quality grinding processing.
When there are brittle fractures on the surface of the gallium oxide single crystal during the grinding process, there are a large number of crushing pits and oriented cracks on the grinding surface, which still exists even when grinding with the SD1500 grinding wheel, which is significantly different from the grinding characteristics of traditional semiconductor materials such as single crystal silicon, which indicates that the gallium oxide single crystal is very prone to grinding damage during the processing process, and the results of the variable depth nano scratch test are consistent. Combined with the test results of nano scratches, it is not difficult to analyze that when the gallium oxide single crystal is in a brittle grinding state, the cutting action of the abrasive particles and the interlacing of the multi-directional slip zone will remove the dominant material, in this process, due to the violent interaction between the slip zones, a large amount of material will be removed in the form of chipping chips, and leave a large number of irregular crushing pits, at the same time, affected by the complex stress in the grinding process, the crack that extends along the (-3-10) plane may also extend to the grinding surface to form an orientation crack, as shown in Figure 11 Shown in . With the reduction of the grinding wheel particle size and the depth of cut of the abrasive grain, the staggered effect of the slip zone is gradually weakened, and the irregular crushing pits on the grinding surface are significantly reduced at this time, but at this time it is still affected by the propagation cracks along the (-3-10) surface, and the cracks on the grinding surface still show obvious orientation, as shown in Figure 12, and the grinding surface belongs to partial plastic removal at this time. With the further reduction of abrasive particle size and abrasive depth of cut, the grinding surface is completely plastically removed, and no obvious damage is observed on the grinding surface at this time, as shown in Figure 13.
2.3 Evolution of subsurface damage in grinding
The subsurface of the SD600, SD1500 and SD5000 grinding wheels was observed by TEM, and the crystal quality of the GaN single crystal matrix was judged by Selected Area Electron Electron Fractional (SAED). The orderly arrangement of diffraction spots indicates that the quality of gallium oxide single crystals is good, so the observed subsurface damage is caused by the grinding process. As can be seen from Fig. 14, the subsurface damage is more severe, and there are nanocrystals of uneven thickness near the grinding surface, and cracks are found on both the surface and the subsurface, and as the grinding process continues, the cracks on the subsurface spread to the grinding surface, resulting in brittle fracture of gallium oxide single crystals. As shown in Figure 15, it can be found that the damage behavior of the subsurface is significantly reduced compared with that of SD600 grinding subsurface, and no obvious cracks are observed in the selection area The nanocrystals can still be observed on the grinding surface, and a large number of parallel slip bands can be observed from the bottom of the nanocrystal layer to the matrix.
Fig. 16 shows the subsurface damage of GaN single crystal grinding on SD5000 grinding wheel. As can be seen from Fig. 16, the analysis of the subsurface damage depth is lower than that of the first two parts of the high-resolution transmission electron microscopy (low, near the grinding surface area, and the high resolution transmission electron microscopy [HRTEM] of the dotted wire frame) shows that there is a delamination fault phenomenon in the area away from the grinding surface. In summary, with the decrease of abrasive particle size, the depth of damage to the grinding subsurface of gallium oxide single crystal is also gradually decreasing. The damage evolution in the grinding process is as follows: cracks and nanocrystals in the grinding stage of the brittle domain, slip zones and nanocrystals in the brittle-plastic transition stage, and lamination faults and nanocrystals in the grinding stage of the plastic domain.
3 Conclusion
In order to explore the removal mechanism and subsurface damage evolution law of gallium oxide single crystal grinding surface materials, a systematic study was carried out by variable depth nano-scratch test and grinding test, and the deformation characteristics and damage evolution of grinding subsurface were analyzed by transmission electron microscopy, and the conclusions are as follows:
1) Through the nano-scratch test, it is found that gallium oxide single crystals are very prone to damage during the removal process, which is obviously different from semiconductor materials such as monocrystalline silicon, silicon carbide and gallium nitride, and the staggered effect of the extended slip zone along the crystal direction [112], [1-12] and [132] leads to irregular crushing pits on the grinding surface, and the cracks with obvious orientation during the scratch process are seriously affected by the slip surface of single crystal gallium oxide (-3-10).
2) Through the grinding test, it is found that with the decrease of the particle size of the grinding wheel, the grinding surface shows different morphological characteristics, and when the SD600 grinding wheel is used for grinding, there are a large number of crushing pits and orientation cracks on the grinding surface, and the overall expression is brittleness, and the surface roughness is Ra=241.37 nm. The crushing pits on the grinding surface of the SD1500 grinding wheel were significantly reduced, but the orientation cracks still existed, which was manifested by partial plastic removal and the surface roughness was Ra= 58.13 nm. The grinding surface obtained with the SD5000 grinding wheel is completely plastically removed without significant surface damage and has a surface roughness of Ra= 2.06 nm.
3) In the grinding process of gallium oxide single crystals, with the decrease of abrasive particle size, the damage depth of gallium oxide single crystals gradually decreases, and the damage evolution law of the grinding process is manifested as nanocrystals and cracks in the grinding stage of brittle domain, nanocrystals and slip zones in the grinding stage of brittle to plastic, and nanocrystals and lamination faults in the grinding stage of complete plastic domain.
Source: GaN World
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