Research On 3-D Displacement Measurement Technology Based On Double Grating Interference

With the continuous progress of modern scientific technology and industrial production, the requirements for the dimension and precision of the displacement measurement technology are getting higher and higher in the microelectronics manufacturing industry and semiconductor manufacturing industry. Ultra-precision displacement measurement technology has become a hot-spot in the measuring filed and an important development direction of the measurement technology in the modern industry. Among them, the grating displacement measurement technology is widely used in various ultra-precision measurement fields with its high stability, high resolution and low cost. So far, the research on the grating displacement measurement technology is extensive and a lot of achievements have been acquired. However, there are still insufficient in measuring dimensions and measuring range. Therefore, the study of a grating displacement measuring system with multi-dimension measurement capability and high precision is of great significance.In this paper, a 3D displacement measurement system based on double grating interference has been studied. By combining the double grating interference principle, the incident light has been multiple divided and merged with a combined transmission scanning grating and a two-dimensional scale grating in order to realize the displacement measurement of the grating plane in X and Y direction, and combining the classical Michelson interferometer principle to achieve the displacement measurement in Z direction which is perpendicular to the grating plane.First of all, based on the coupled wave vector diffraction theory, the vector diffraction characteristic models of the one-dimensional rectangular grating and the twodimensional rectangular grating have been established, and the process of calculating the diffraction efficiency of the grating has been derived, and the simulation and analysis have been carried out for the influence of structure parameters on the diffraction characteristics of grating. The work are laying the theoretical foundation for subsequent design of the grating structure parameters which meets the demands of the measurement system.Secondly, the optical structure of the measurement system is introduced. Based on the double grating interference principle, the optical structure of the X and Y direction measurement has been designed and the specific working principle has been also analyzed; based on the Michelson interferometer principle, the optical structure of Z-axis measurement has been also designed and specific measurement principle has been analyzed. The overall design of the optical structure of the measurement system has been completed and the requirements for the diffraction grating properties of the measurement system have been proposed. The interference signals of the X, Y and Z direction of the measurement system has been analyzed, and the simulations between the intensity of the interference signals and displacement have been done; and it is demonstrated that the displacement measurement of each direction has achieved a 2 optical subdivision, and displacement information of each direction is independent, not coupled.Again, the hardware structure of the measurement system is designed. Based on the vector diffraction theory, the structure parameters of the scanning grating and the scale grating according to the system requirements have been designed and the machining size of the scanning grating and the scale grating according to the special form of the scanning grating have been also designed to ensure the system work normally. In order to achieve the interference signal, the photoelectric detection circuit and the signal conditioning circuit have been designed which includes the low-pass filter circuit and eliminate DC biasing circuit to eliminate the influence of interference signal.Finally, the diffraction properties of the scanning grating and the scale grating have been tested and the results have been compared with the theoretical design results, and the factors causes the errors between the test and the theory have been also analyzed. The experimental platform is built and the experiments to verify the validity of the measurement principle in the X, Y and Z direction have been completed. The results show that the system can realize the displacement measurement of three dimensions and the displacement information in three directions is not coupled. Without the electronic subdivision, the resolution of the system in X and Y direction is 2√2μm and the resolution of the system in Z direction is λ/2, which consists with the design in theory.