Method for improving TTV of large-size ultrathin lithium niobate grinding substrate

Abstract

The invention discloses a method for improving TTV of a large-size ultrathin lithium niobate grinding substrate. The method is a method for providing a wafer with more excellent flatness for a subsequent polishing procedure. The method comprises the following steps: firstly, calculating revolution and rotation speeds of a central gear, an outer gear ring, a lower disc surface, an upper disc surface and a wandering star wheel; wherein the rotation direction of the upper disc surface is opposite to the rotation direction of the wandering star wheel, so that the moving trajectory of the wafer can be calculated only by calculating the relative speed of a certain point on the wafer relative to the upper disc surface or the lower disc surface. For any point of the wafer in the test period, the moving trajectory is calculated, and the distance between the wafer and the test period can be obtained by integrating the displacement. The grinding machine sets corresponding program parameters, calculates and measures the comprehensive distance range in the selected five-point measurement period, and optimizes the program to balance the difference of TTV by taking the two as the basis after comprehensively considering the sharpness of the moving trajectory. The method has the beneficial effects that the TTV of the substrate after large-size ultrathin lithium niobate is ground can be reduced to be within 2 microns, the surface flatness of the ground wafer is greatly improved, high-quality supplied materials are provided for the subsequent polishing procedure, and a series of problems such as wafer running and tray collapse possibly caused by poor TTV of the grinding procedure are reduced. And the TTV level of the grinding procedure is improved, the stability of the grinding procedure is guaranteed, and the grinding yield is greatly increased. And directions are pointed out for scratches and other abnormities caused by follow-up TTV and grinding tracks.

Description

Technical Field

[0001] The invention relates to the field of semiconductor materials, and in particular to a method for improving TTV of a large-size ultra-thin lithium niobate grinding substrate, which is a semiconductor substrate material. Background Art

[0002] Lithium niobate (LiNbO3, hereinafter referred to as LN) is a multifunctional material that integrates piezoelectric, ferroelectric, pyroelectric, nonlinear, optoelectronic, photoelastic, photorefractive and other functions. LN has received more and more attention due to its excellent physical properties and has been widely used in aviation, aerospace, civilian optoelectronic products and other fields. Lithium niobate crystals have the characteristics of multiple photoelectric effects, strong controllable performance, stable physical and chemical properties, and wide light transmission range. This is caused by the lattice structure and rich defect structure of lithium niobate crystals. Many properties of lithium niobate crystals can be greatly controlled by crystal composition, element doping, valence control, etc. The physical and chemical properties of lithium niobate crystals are quite stable and easy to process; the light transmission range is wide, it has large birefringence, and it is easy to prepare high-quality optical waveguides. Therefore, optoelectronic devices such as surface acoustic wave filters, optical modulators, phase modulators, optical isolators, etc. based on lithium niobate crystals have been widely studied and applied in the fields of electronic technology, optical communication technology, laser technology, etc.

[0003] The Mohs hardness of lithium niobate crystal is 5, which is between the first-generation hard and brittle semiconductors and the third-generation soft and brittle semiconductors. It has unique mechanical properties, and its processing mechanism is fundamentally different from the above two. Before lithium niobate crystals are made into high-performance devices, they need to go through the following steps: first, prepare a single crystal ingot, then cut it into wafers and grind it. During this process, sub-surface damage will occur, and the ultra-precision CMP method needs to be used to remove the damaged layer to obtain a high-quality, damage-free and smooth surface. Before polishing, large-sized lithium niobate substrates need to be ground to ensure that the polished material meets the TTV requirements and reduce the impact on the polishing process. Summary of the invention

[0004] The purpose of the present invention is to provide a method for improving the TTV of large-sized ultra-thin lithium niobate grinding substrates, so as to provide wafers with better surface flatness for subsequent polishing processes.

[0005] The present invention provides a method for improving the TTV of a large-sized ultra-thin lithium niobate polishing substrate, which specifically comprises the following steps:

[0006] a) Set They are the central gear, outer gear ring, lower disk, upper disk, planetary gear revolution, and planetary gear rotation speed, in rpm, α is the ratio of the outer gear ring to the central gear pitch radius (α>1), are the outer ring gear, center gear, and planetary gear pitch circle radius respectively. The speed CW is positive and CCW is negative. , after adjustment, the characteristic equation of the planetary gear train is , from which we can get ,Other .set up They are the central gear, outer gear ring, lower disk, upper disk, planetary gear revolution, and planetary gear rotation angular velocity, respectively, multiplied by 2π / 60 Conversion income.

[0007] b) Set The orbital angular velocity and rotational angular velocity of the planetary wheel. When calculating the orbital angular velocity, the relative velocity needs to be calculated according to the different rotation speeds of the upper and lower disks. The relative running trajectories of the planetary wheel with respect to the upper and lower disks are calculated respectively. The orbital direction of the planetary wheel is CCW, the rotation direction of the upper disk is CW, and the rotation direction of the lower disk is CCW. The relative orbital velocity of the upper disk is the orbital velocity of the planetary wheel plus the rotation speed of the disk, that is, the The relative revolution speed of the lower disk is the revolution speed of the planetary wheel minus the rotation speed of the lower disk, that is, .

[0008] c) Set The distance from the center of the disk to any point on the planetary wheel and the distance from the center of the disk to the center of the planetary wheel are respectively. Then the vector from the center of the disk to any point on the chip is Can be decomposed into vectors and vector Assuming that when time t=0s, the starting point coordinate y=0, the displacement of any point on the chip in the time period t1 to t2 is D=\sqrt {\left [ {x(t2)-x(t1)} \right ]^{2}+\left [ {(y(t2)-y(t1)} \right ]^{2}} ; Take t=0s as the starting point, ti=0.1~0.5s as the time interval, T uses the cycle of the maximum processing load, according to the vector By plotting the obtained coordinates, we can get the running track of grinding at any point on the wafer. By integrating the displacement of any point on the wafer during the test cycle, we can get the distance of the wafer during the test cycle.

[0009] d) The grinding machine sets the corresponding program parameters, mainly including the lower platen, upper platen, outer ring gear speed, center gear speed, operation buffer, unit pressure, processing load, processing time, and supply liquid. The fixed prerequisite of the present invention is to apply a full plate, that is, the number of single plate processing is 15 pieces, and the main consideration is the speed of the relevant lower platen, upper platen, outer ring gear, and center gear when the processing load reaches the maximum weight, wherein the lower platen speed is 8-20rpm, the upper platen speed is 2-8rpm, the outer ring gear speed is 2-10rpm, and the center gear speed is 2-10rpm.

[0010] e) Select 5 points on the large-size lithium niobate wafer to measure TV5, and calculate its running trajectory and distance. The 5 points selected for measuring TV5 are the center of the substrate and the four points 10 mm away from the circumference of the substrate, and there are two points where the line connecting them is perpendicular to the main reference edge. Due to the different distribution of sand and liquid on the surface of the wafer during grinding, the center of the wafer is usually thicker because the fluidity of the sand and liquid is not as good as that of the edge of the wafer. In fact, the center point of the wafer can be used as the thickest reference point of the wafer. After obtaining the running distance of the 5 points, calculate the difference between the maximum and minimum values ​​of the comprehensive distance, that is, calculate the arithmetic average of the running distance of any point on the wafer relative to the upper disk surface and the running distance relative to the lower disk surface as the comprehensive distance of the point.

[0011] f) Taking the comprehensive distance range and the sharpness of the running track as the standard, set the parameters such as the speed of the grinding machine center gear and the outer ring gear, gradually optimize the program, and finally comprehensively evaluate the optimized program, and measure and compare the TTV after grinding.

[0012] For the above step d), as a preference, different grinding programs can be set for different grinding disc shapes, that is, different rotation speed differences between the center gear and the outer gear ring are matched. When the height difference between the inner and outer diameters of the disc surface meets the requirements within ±10μm, the corresponding program is used. The positive and negative rotation of the planetary wheel has different emphases. Generally, the planetary wheel is used for positive rotation of the concave disc and the planetary wheel is used for negative rotation of the convex disc. When the disc shape does not meet the requirements, a correction wheel needs to be used to repair the disc until the disc shape meets the standards.

[0013] Regarding the above step e), as a preferred embodiment, when selecting the running trajectory, a curve with a larger trajectory turning radius should be selected to reduce the generation of abnormalities such as fragments.

[0014] With respect to the above step e), as a preferred embodiment, the present invention is applicable to the case where the sand-liquid ratio, the use cycle and the supply amount, the rising trend of the processing load and the maximum value are relatively fixed, that is, the grinding process is stable and the production line operates continuously.

[0015] For the above step f), as a preferred embodiment, the range of the comprehensive distance length within 20 minutes of grinding calculation is within 30m.

[0016] The beneficial effect of the present invention is that the TTV of a large-sized ultra-thin lithium niobate substrate after grinding can be reduced to less than 2μm, which greatly improves the surface flatness of the wafer after grinding, provides high-quality materials for the subsequent polishing process, and reduces a series of abnormal problems such as wafer runaway and collapse that may exist in polishing due to poor TTV in the grinding process. The present invention improves the TTV level of the grinding process and ensures its stability through a unified procedure, greatly increasing the grinding yield. It also points out the direction for improving the subsequent abnormalities such as scratches caused by TTV and grinding tracks. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of vector decomposition of any point on the chip of the present invention.

[0018] Figure 2 Schematic diagram of wafer point selection according to the present invention.

[0019] Figure 3 This is a schematic diagram of process parameters of Procedure-1 of Example 1 of the present invention.

[0020] Figure 4 Schematic diagram of machine parameters of process-1 of embodiment of the present invention.

[0021] Figure 5 This is a schematic diagram of process parameters of Procedure-2 after optimization according to an embodiment of the present invention.

[0022] Figure 6 Schematic diagram of machine parameters of process-2 after optimization according to an embodiment of the present invention.

[0023] Figure 7 Schematic diagram of the upper disc surface grinding trajectory using Program-1 when Eh=158mm in an embodiment of the present invention.

[0024] Figure 8 Schematic diagram of the grinding trajectory of the lower plate surface when Eh=158mm using Program-1 in an embodiment of the present invention.

[0025] Fig. 9 Schematic diagram of the upper disc surface grinding trajectory using Program-1 when Eh=28mm in an embodiment of the present invention.

[0026] Fig.10 Schematic diagram of the grinding trajectory of the lower plate surface when Eh=28mm using Program-1 in an embodiment of the present invention.

[0027] Fig.11 Schematic diagram of the upper disc surface grinding trajectory using Program-2 when Eh=158mm in an embodiment of the present invention.

[0028] Fig.12 This is a schematic diagram of the grinding trajectory of the lower plate surface using Program-2 when Eh=158mm in an embodiment of the present invention.

[0029] Fig.13 Schematic diagram of the upper disc surface grinding trajectory using Program-2 when Eh=28mm in an embodiment of the present invention.

[0030] Fig.14 Schematic diagram of the grinding trajectory of the lower plate surface when Eh=28mm using Program-2 in an embodiment of the present invention.

[0031] Fig.15 This is a schematic diagram of TTV at 300 points after grinding in Procedure-1 of Example 1 of the present invention.

[0032] Fig.16 This is a schematic diagram of TTV at 300 points after polishing in Procedure-2 after optimization according to an embodiment of the present invention.

[0033] in, Figure 2 Among them, 21, 22, 23, 24, and 25 are the positions of the five measuring points when measuring TV5. DETAILED DESCRIPTION

[0034] The following specific examples are listed to further illustrate the present invention. It should be understood that the examples are not intended to limit the scope of protection of the present invention.

[0035] a) For 6-inch large-size lithium niobate wafers, the grinding machine sets the corresponding program parameters. The prerequisite is to use a full plate, that is, a single plate processing quantity of 15 wafers. Another major consideration is the speed of the lower plate, upper plate, outer ring gear, and center gear when the processing load reaches the maximum weight. Among them, the speed of the plate under the forward program is 10rpm, the speed of the upper plate surface is 3.3rpm, the speed of the outer ring gear is 4.7rpm, and the speed of the center gear is 2.1rpm. The corresponding data is calculated as follows Figure 3 shown.

[0036] b) Taking Eh=158mm as an example, the calculation results are as follows Figure 4 .

[0037] c) The two points with the largest distance difference on the wafer are 21 and 25, where point 21 corresponds to Eh=158mm and point 25 corresponds to Eh=28mm. Since the grinding removal is 50μm and the maximum processing load time is 20min, the distances traveled by the two points in 20min are 175.2m for point 21 relative to the upper disk surface and 150.4m for point 21 relative to the lower disk surface, and 163.3m for point 25 relative to the upper disk surface and 126.4m for point 25 relative to the lower disk surface. The grinding paths are as follows with an interval of ti=0.1s and a running time of T1=60s: Figure 7 to Figure 10As shown. The comprehensive distance of point 21 is 307.65m, and the comprehensive distance of point 25 is 144.85m. The distance difference of program 1 is about 162.8m, and the running track of point 21 relative to the lower disk surface is too sharp, which is easy to cause scratches and edge collapse. After grinding, the wafer TTV is measured to be about 4μm.

[0038] d) According to the distance difference, the program is optimized and compared with each other. After the optimization, the speed of the lower plate is 15rpm, the speed of the upper plate is 5rpm, the speed of the outer ring gear is 3rpm, and the speed of the central gear is 4.6rpm. Figure 5 shown.

[0039] e) Taking Eh=158mm as an example, the calculation results are as follows Figure 6 .

[0040] f) The two points with the largest distance difference on the wafer are 21 and 25, where point 21 corresponds to Eh=158mm and point 25 corresponds to Eh=28mm. Since the grinding removal is 50μm and the maximum machining load time is 20min, the distances traveled by the two points in 20min are 235.6m for point 21 relative to the upper disk and 206.6m for point 21 relative to the lower disk, and 234.8m for point 25 relative to the upper disk and 238.6m for point 25 relative to the lower disk. The grinding paths are as follows with an interval of ti=0.1s and a running time of T1=60s: Figure 11 to Figure 14 As shown: The comprehensive distance of point 21 is 221.1m, and the comprehensive distance of point 25 is 236.7m. The distance difference of program 2 is about 15.6m. The grinding tracks of points 21 and 25 are smooth without obvious edges and corners. With the higher upper and lower disc speeds, the grinding efficiency is improved by about 15%. Fig.15 and Fig.16 The following are the TTV diagrams of 300 points before and after the optimization process. The TTV of the wafer has been significantly improved.

[0041] Although embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that various changes, modifications and substitutions may be made to the embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for improving the TTV of a large-size ultra-thin lithium niobate grinding substrate, characterized in that: The specific steps include: a) Calculate the angular velocity of the grinding machine center gear, outer gear ring, lower disk surface, upper disk surface, planetary wheel revolution, and planetary wheel rotation respectively; b) Calculate the relative angular velocity of the upper disk surface and the lower disk surface corresponding to any point on the substrate; c) drawing the running track of grinding at any point on the substrate, and integrating the displacement of any point on the substrate during the test cycle to obtain the distance of the substrate during the test cycle; d) Under the premise of ensuring the flatness of the plate surface, set the corresponding program parameters on the grinding machine, mainly including the lower platen speed, upper platen speed, outer ring gear speed, center gear speed, operation buffer, unit pressure, processing load, processing time, and supply fluid; e) Calculate the comprehensive distance range of any point among the five points selected on the substrate as the evaluation criterion; f) According to the evaluation criteria in e), set the center gear and outer ring gear of the grinder, the speed of the upper and lower disks, and optimize the program to the maximum extent with the minimum range and the sharpness of the running trajectory, and use the corresponding program to grind the substrate and measure the TTV after grinding.

2. A method for improving TTV of a large-size ultra-thin lithium niobate polishing substrate according to claim 1, characterized in that: In the step a), the default chip size for the corresponding scenario is Φ=6 inches, and the number of full-plate chips is 15.

3. A method for improving TTV of a large-sized ultra-thin lithium niobate polishing substrate according to claim 1, characterized in that: In the step c), the interval time ti of the points used to calculate the motion trajectory is 0.1-0.5s.

4. The method for improving TTV of a large-sized ultra-thin lithium niobate polishing substrate according to claim 1, characterized in that: In the step d), the height difference between the inner and outer diameters of the disk surface must be within ±10 μm, and the grinding machine program parameters are set to have a fixed disk speed of 8-20 rpm, an upper disk speed of 2-8 rpm, an outer ring gear speed of 2-10 rpm, and a center gear speed of 2-10 rpm.

5. The method for improving TTV of a large-sized ultra-thin lithium niobate polishing substrate according to claim 1, characterized in that: In the step e), five points are selected to measure TV5, which are the center of the substrate and four points 10 mm away from the circumference of the substrate, and the line connecting two of the points is perpendicular to the main reference edge.

6. The method for improving TTV of a large-sized ultra-thin lithium niobate polishing substrate according to claim 1, characterized in that: In the step f), the range of the comprehensive distance length within 20 minutes of grinding calculation is within 30 m.

Images