Study on damage layer of lapping lithium niobate crystal

Source: Wanfang Data
It is a common concern to improve the efficiency of lapping and polishing. Because the processing time of lapping and polishing is very long and the removal amount per unit time is very small, the surface roughness of the previous process has a great influence on the processing allowance of the later process. In this case, the efficiency of a single process does not guarantee the overall efficiency of the whole process. This paper presents a new idea to improve the efficiency of lapping and polishing. Process combination optimization was carried out by establishing process database to shorten the total processing time. This paper presents the strategy and implementation method of process combination optimization, and analyzes it with a typical example. The results show that this process combination optimization method is effective.
With the development of materials science and technology, new materials have been more and more widely used in various components, and people have higher and higher requirements for machining accuracy. Ultra-precision lapping and polishing technology using the "evolutionary" machining principle, can obtain high precision and close to the geometric shape of the ultra-smooth perfect surface, has a wide range of applications in modern materials processing. At present, the final processing of silicon wafer, crystal oscillator substrate, lithium niobate substrate, lithium tantalate substrate and other crystal substrates of large-scale integrated circuit adopts ultra-precision plane lapping and polishing technology. With the popularization of the application of components and the increasing production volume, the need for high quality and high efficiency processing is becoming more and more urgent.
In this field, many scholars and experts have made a lot of research from the way of lapping and polishing, the improvement of abrasive and polishing liquid, the optimization of single process parameters and so on, so that the efficiency of lapping and polishing is constantly improved. The method of lapping and polishing is used for processing, usually including rough lapping, fine lapping, rough polishing and fine polishing four processes. Of course, sometimes one or two processes will be omitted according to the needs, but generally speaking, it is a multi-process processing. Because the processing time of lapping and polishing is very long, the removal amount per unit time is very small, and the more to the later process, the less the removal amount per unit time, the surface roughness of the previous process has a great impact on the processing allowance of the later process. In this case, the high efficiency of a single process cannot guarantee the overall high efficiency of the whole process, which leads to the problem of process combination optimization. Although computer aided process design has been widely used in the machinery manufacturing industry and has produced good results, for a long time, in terms of precision lapping and polishing processing, how to choose the process route, as well as the processing parameters of the specific process are judged by the experience of workers, which results in too much dependence on experienced workers. Moreover, the process route selected according to experience and the process parameters of the specific process also have their randomness, which may not be optimized and reasonable. Therefore, this paper takes the shortest total processing time as the optimization goal, and uses computer technology and database technology to conduct a preliminary study on the optimization problem of lapping and polishing process combination.
Process database is the basis of process combination optimization. Up to now, there is no special database or process manual about ultra-precision grinding and polishing in China. Only through a large number of experiments to obtain data, and then the application of mathematical statistics to the experimental data comprehensive analysis, induction and sorting, the ultra-precision grinding and polishing process parameters and surface roughness and the relationship between the removal rate, as the basis for processing. In order to facilitate the storage and management of data and the application in the process combination optimization, the experimental data were divided into four groups according to rough grinding, fine grinding, rough polishing and fine polishing, which were described by classification number. For one kind of processing material, each group stores 4 ~ 6 kinds of process parameters. In terms of the design of the base table of the database, a main process table and a process list are built. In addition, there are grinding liquid list, grinding disk list, material list and optimization history record table. The type, particle size, concentration and composition of abrasive liquid are stored in the list of abrasive liquid. In the process optimization, you can first look for the same record in the optimization history table, if there is, you can directly use. Table 1 and Table 2 show the design of the header of the process master table and process detail table. In the process combination optimization, mainly according to the main process table. After the optimization results are obtained, the relevant parameters are obtained from the procedure list for machining control. In order to improve the speed of retrieval, the corresponding indexes were built based on surface roughness and removal rate respectively. From the point of view of economy and easy maintenance, the background database uses SQL Sever 2000.
According to the workpiece to achieve the roughness, can determine the final processing process. According to the surface roughness value to be achieved by machining, the final machining process is searched in the database, and the conditions of the final machining process are determined as: Rae(attainable value)<Rao(required value), and the removal rate is the highest. Once the final process is determined, the possible process combinations are determined accordingly. The details are shown in Table 3. For example, if the final finish is finishing, there are eight possible combinations of processes.
The total processing time is taken as the optimization objective function, that is, the sum of the processing time of each process. The conversion time between processes is ignored here. Firstly, to reduce the complexity of system establishment, and secondly, considering the actual situation of ultra-precision grinding and polishing, the processing time is much longer than that of other auxiliary processing. Set the total processing time as T and the processing time of each process as ti; That is, the minimum value of the entire processing time is equal to the minimum sum of the processing time of each process, among which, there are at most four processes and at least one process, so n is an integer value between 1 and 4. It should be emphasized that the minimum value of the entire processing time is not equal to the sum of the minimum processing time of each process. Because of the particularity of lapping and polishing, the machining allowance of each process is closely related to the surface roughness achieved in the previous process, so it cannot be simplified to the sum of the minimum processing time of each process. That is, the constraint condition of optimization is that the sum of the machining allowance of each process should be equal to the machining allowance required by the workpiece, and the machining allowance of the initial machining procedure is determined accordingly. Suppose the required machining allowance of the workpiece is S, the machining allowance of each procedure is Si, and the machining allowance of the initial machining procedure is S1, then if n=1, then S1=S. Assuming that the removal rate of the initial process is q1, the processing time t1 of the initial processing process can be expressed as
For specific processing, even if the initial processing and final processing procedures are determined, there may still be four situations, that is, the workpiece through one, two, three, four processing procedures to complete the whole processing. In addition to the initial machining process, the machining allowance of each subsequent process is determined as follows: k is the coefficient ratio between the machining allowance and the initial surface roughness. It is generally believed that the subsequent process should remove the metamorphic layer of the previous process. Considering that Rt value is generally 7 ~ 8 times of Ra value in the measurement index of surface roughness, and considering the influence of processing process on the sub-surface, k=15 ~ 25 can be taken according to experience. When the surface quality requirement is high, the value of k is larger, and vice versa. If the removal rate of a certain process is qi, the processing time of the process is ti, which can be expressed as, therefore, the total processing time (D) can be expressed as if the total processing time T of a certain process combination is the minimum, then the process combination is the optimal process combination. The process of process combination optimization is shown in Figure 1.
The processing experimental data of a component are shown in Table 4. The machining allowance is 0.1mm and the required surface roughness is 0.02μm. For process combination optimization, take k=20. The optimization results show that when the process combination is 1-4-6-7, the total processing time is the shortest, which is 490min. The process combination with the highest removal rate at each stage is 1-3-5-7, and the total processing time is 561min; the process combination with the lowest surface roughness is 2-4-6-8 at each stage, and the total processing time is 607min. Therefore, the overall processing efficiency can be effectively improved through process combination optimization.
Because the processing time of lapping and polishing is very long and the removal amount per unit time is very small, the surface roughness of the previous process has a great influence on the processing allowance of the later process. In this case, the efficiency of a single process does not guarantee the overall efficiency of the whole process. In this paper, the process combination optimization is realized by establishing the process database and using the computer aided method. The results show that the process combination optimization can effectively shorten the overall processing time and improve the processing efficiency.