Design Strategies Of Eddy Current Sensors With Sub-nanometer Accurancy

Requirements for high performance displacement sensors increase rapidly with the development of ultra-precision manufacturing, semiconductor industry, micro-electromechanical systems, and nanotechnology, and the measurement accurancy increases from micrometer to nanometer scale. Capacitive sensors can achieve extremely high resolution and stability, while they are very sensitive to enivronmental containmetns, which make them only can be used in hermetic cavities and clean room. Eddy current sensors (ECSs) are widely used in industrial and laboratory applications, as one of the most important non-contact displacement sensors, because of their low cost, high resolution, wide bandwidth, and high tollerance to environmental contaminants. Compared to capacitive sensors, eddy current sensors have poorer resolution and higher thermal drift. The biggest obstacle of ECS to be used for ultra-precision measurement applications is the poorer resolution and high temperature drift coefficient. Even though kinds of ECS products are available in the market, the relationship between the design parameters and the ECS performance is still lack of data. Most commercial products are made by the technological achievements in a few decades ago. So far, the design of most ECS products still strongly depends on experience and experimentation.A clear, simple, effective and comprehensive analysis of designing and implementing a high resolution, low thermal drift eddy current displacement sensor system is urgently needed. The design strategies of ECS with sub-nanometer accurancy are studied in this dissertation. Based on a full survey and comprehensive analysis of ECS through theoretical model and FEA, an ultra-stable ECS prototype with sub-nanometer resolution was developed. And the target selectivity of ECS has been analyzed and solved. A novel noncontact thickness measurement method of metal films based on ECS was also developed, which is independent of the distance variation. The main research contents and innovations of this dissertation are as follows:1) A simple and effective analysis and discussion on how to design a high performance eddy current displacement sensor were presented. A transformer based equivalent model was presented to explain the influence of conductivity and working frequency to the sensitivity and thermal stability of an ECS. The theoretical formula of ECS’s response were obtained also through this model. The simulation and optimization technologies for ECSs were also developed to analyze the relationship between parameters and their performance by using COMSOL Multiphysics. Both the FEA and theoretical results show that the higher the conductivity and the frequency, the higher the ECS’s performance.2) Through a comprehensive analysis of the relationship between the coil’s diameter and its magnetic field distribution, it can be concluded that the sensitivity of ECS with small coil diameter is higher, while its inductance is lower, resulting in limited resolution. For a given diameter D, the best inner diameter is0.2D, and the thickness of the coil should be less than0.05D. The selection of wire diameter and working frequency for ECS coil is also analyzed and discussed. A summarization of the traditional and advanced manufacturing process for ECS is presented. Finally, a compact eddy current sensor without magnetic shielding based on meander-spiral coils is designed and tested.3) A novel AC bridge with specific parameters is designed, which can precisely measure any tiny variation of the inductance and resistance, separately. Based on this bridge and a lock-in amplifier, a signal conditioning board for ECS has been designed and tested. Based on a disk shape wire-wound coil, the whole ECS system, including the calibration and test experimental setup are developed. Test results show that the ECS prototype has extremely high resolution of0.07nm, and the noise power spectral density is less than0.01nm/√Hz.4) A Comprehensive analysis of the thermal drift sources of ECS is presented. By using the inductance and resistance signals simultaneously, the temperature drift of the ECS can be completely canceled without any additional hardware and temperature sensor, which reduce the temperature drift of the ECS by two orders of magnitude. A comprehensive temperature compensation method base on signal processing is also developed, which makes the thermal drift of ECS less than4nm/℃, even in a harsh environment with a very high temperature variation speed of10℃/h.5) The quantitative error of an ECS caused by target conductivity was analyzed using a complex image method. The response curves (L-x) of the ECS with different targets were similar and could be overlapped by shifting the curves on x direction with0.7074δ. Both finite element analysis and experiments match well with the theoretical analysis, which indicates that the measured error of high precision ECSs caused by target conductivity can be completely eliminated, and the ECSs can measure different materials precisely without calibration. 6) The slope of the lift-off curve (LOC) in the R-L impedance plane is found to be a good feature for characterizing target thickness independent of lift-off distance variation. The essential relationship between the slope of LOC (SLOC) and target properties was obtained by the equivalent model. A full FEA was also conducted to analyze the relationship between SLOC feature and target thickness, and the results matched very well with the model results. A sensor coil probe was then manufactured and used to measure the thickness of copper films with high performance, and the capability of this technique for online noncontact thickness measurement was verified. The basic characteristics and performances of this thickness measurement technique were tested and discussed.The design strategies discussed in this dissertation will promot the development of eddy current displacement sensors. The physical behavior and response characteristics of ECSs can be explained and predicted using the equivalent model developed in this thesis. It is believed that the FEA using a computer will greatly enhance the design efficiency of ECS system, and eliminated the imperfections before manufacture. This new ECS system with sub-nanometer resolution and ultrahigh stability could be widely used in all kinds of advanced applications, even in harsh environments. The above research achievements will bring ECSs into a variety of ultra-precision displacement measurement applications with their extraordinary performance and environment tolerance.