Research On Ultra-precision Diamond Cutting And Metrology Techniques For Microstructure Arrays

Microstructure array is typically composed of microstructure cells arranged in a deterministic pattern. The functions of a microstructure array are dominated by not only the geometries of the individual microstructure cells but also the spacing between the cells. With incomparable superiorities, microstructure arrays serve as key components in various of fileds, such as photonics, microoptics, metrology artifacts, communication, precision engineering, etc. For example, optical films with microlens array for flat panel displays, optical retroreflectors with micro pyramid arrays for space exporation, micro groove arrays for the solar cells, and high-aspect-ratio deep trench arrays for advanced dynamic random access memory. As high-tech products are now develoing towads the direction of high performance, high precision and high integration, microstructured surfaces are playing increasingly important roles in the the field of aerospace, semiconductor manufactoring, biomedical and so on. Correspondingly, they also drive the development of ultra-precision machining techniques of microstructured surfaces.Compared with electronic and optical methods, fast tool servo (FTS) based diamond cutting, with the advantage of generating complicated surfaces with high form accuracy and good surface finish, is a very effective way for the fabricaiton of microstructure arrays. Although great advances have been made in ultra-precision cutting, with the increasing requirement on manufacturing scale, precision and efficiency, the technique of FTS based ultra-precision diamond cutting faces a number of new challenges. Firstly, in face of the growing size of the manufacturing products, one of the highest priorities is to reduce the influence of tool wear and tool damage occuring in the time-consuming cutting process with an extremly long length of cutting path. Secondly, when cutting hard or even brittle materials, microstructure defect is easy to occure due to the complicated interaction between the cutting tool and the material surface. Thirdly, with the complexity of the cutting process, it is necessary to carry out precision measurement of machine tools, diamond tools and ultra-precision fabricated parts to guarantee the manufacturing accuracy. With the aim of giving effective solutions to the above issues, this thesis proposed several methods and key techniques on ultra-precision cutting as well as on-machine measurement. A force sensor integrated fast tool servo which is referred as FS-FTS, a device for electropolishing of high aspect ratio nano probes, and a new prototype of large-area scanning tunneling microscopy are developed. Based on these instruments, research are conducted in four related areas that are stitching fabrication of a large-area microstructure array, in-process detection and repair of microstructure defects, on-machine measurement of the cutting edge contours of diamond tools, and large-area profile measurement of microstructure surfaces, respectively. A large number of experiments, including those experiments on the diamond turning machine, have been carried out. The feasibility of the proposed methods have been confirmed by the experimental results.The contents of this thesis including six chapters are summerized as below:In Chapter1, the backgroud of the technique of FTS based ultra-precision diamond cutting is presented. The new difficulties and challenges which the technique faces are summerized based on an intensive literature survey. After an introduction of the current research status and progress, the research contents and significance of this thesis are demostrated, respectively.In Chapter2, stitching fabrication of a microstructure array on the outer surface of a roller mould is presented. The measurement function of the FS-FTS is demostrated, the basic performances of the FS-FTS operated for surface measurement are investigated. In addition, a new method of tool positioning with respect to the fabricated microstructures is proposed. Without using any other measuring instrument, the replacement cutting tool is brought to scan across the fabricated microstructures with a constant contact force by servo controlling the tool displacement. The tool position with respet to the fabricated microstructures can be thus obtained based on the measured tool scanning traces. Stitching fabrication, including line stitching, area stitching and filling-in stitching, are achieved respectively based on the accurate positioning of the replacement tools.In Chapter3, an in-process measurement method for repair of defective microstructures by using the FS-FTS is presented. A real-time cutting force map is captured in the process of cutting the microstructures by the FS-FTS to indicate the cutting status with respect to the cutting tool position in the coordinate system of the machine. Based on the cutting force map, the positions of the defects are indentified in real time from the locations of the abnormal variations in the map associated with the occurrences of the defects. Characterization of the defects is then conducted by employing the force-controlled cutting tool as a measureing probe. The indentified positions and the characterized profiles of the defects are then fed back to carry out the repair fabrication of the defective microstructures. Experiments of in-process detection and repair of the micro-defects in the microstructures over the outer surface of a roll mould are carried out to confirm the feasibility of the proposed method.In Chapter4, on-machine measurement of the cutting edge contours of diamond tools is presented. Without using any accurate reference artifacts and additional surface form measuring instruments, the diamond cutting tool, serving as a measuring stylus, is brought to scan across a sharp-edge structure based on the force feedback control loop of the FS-FTS. During the scanning, the contact force between the micro tool and the sharp-edge structure is maintained constant by servo controlling the displacement of the cutting tool so that the cutting edge contour can be obtained from the tool scan trace profile which is provied by the linear encoder of the machine slide and the displacement sensor of the FS-FTS. Measurement experiments of several diamond cutting tools including worn tools are carried out to demostrate the feasibility of the proposed method.In Chapter5, large area profile measurement of microstructurd surfaces by using a new prototype scanning tunneling microscopy is presented. Firstly, based on systematic study of electropolishing process, an improved tungsten probe fabrication technique is presented. A control strategy for determining the probe shaping instant and the optimal setting of the parameters during the electropolishing procedure are developed which facilitate the fabrication of probe with controllable tip profile. Then, by improving the traditional technique, a new prototype scannning tunneling microscopy, with the capability of conducting large area and high aspect ratio surface profile measurement, is developed. A self-calibration method of sample tilt is proposed in order to achive large-area scanning without tip-crashing or losing of tip-sample interaction. Profile measurement experiments of sinusoidal freeform surfaces are carried out.In Chapter6, the research contents of this thesis is summarized, the Innovation points are highlighted, and some future works are discussed.