Study On Specular Surface Quality Inspection Based On Optical Three Dimensional Metrology

With the accelerating development of the science and technologies,the requirement of measuring specular surface increases rapidly in the fields ranging from optical mirror fabrication to the semiconductor industry,mold manufacture,and cell phone industry.Among all the optical three-dimensional(3D)surface metrologies,the phase measuring deflectometry(PMD),which is based on structured illumination,raises extensive attentions to researchers as it has the advantages of being a full-filed measurement,incoherent,high sensitivity,stable and cost-efficient.Its system is simply composed by a camera and a liquid crystal display(LCD)screen,while millions points can be measured in a single measurement with the lateral and height resolution 1?m and 1nm,respectively.Accuracy is always the key concern for any metrology including PMD.In order to enhance the accuracy of PMD,this dissertation studies the system calibration,phase error reduction,dynamic measurement method and system development of PMD.Several innovation ideas are carried out in the research of high precision calibration method,phase error reduction method,and high performance dynamic measurement method.The details are listed below.1.A new calibration method based on phase-slope model is proposed to enhance the system calibration flexibility and accuracy.The calibration error has been one of the main errors in PMD.The LCD screen calibration procedure during the existing geometry calibration method is inflexible,and a range of factors will affect its accuracy.The proposed method avoids the LCD screen calibration and can achieve 2.5 times higher measurement accuracy than the existing calibration method in our experiment.2.In order to increase the phase accuracy,the following three problems are studied.Firstly,the random phase error suppressing method when measuring low reflectivity specular surface is researched.This dissertation deduces the analytical expression of random phase error.The relations between random phase error and system parameters are proven by the simulation and experiment.This work could be the guide for acquiring high quality fringe pattern.Secondly,an effective nonlinear carrier phase removal method is studied.The carrier phase frequency of the captured fringe pattern is no longer a constant when the imaging system is non-telecentric.Based on the analytical expression of the nonlinear carrier phase,this dissertation presents a novel carrier removal method.Compared to the existing series-expansion method,the proposed method is 6 times faster,but with similar accuracy.Lastly,the phase ambiguous problem is studied.This dissertation deduces the unambiguous phase-slope relation analytical expression,and a low phase ambiguous system setup is proposed with the aid of a beam splitter consequently.The novel system setup has 10 times less phase ambiguous error than normal system when the angle of the test surface is smaller than 5°.3.The discontinuity-induced systematic error model is carried out and a corresponding error reduction method is proposed.When the measurement comes to the area around the discontinuity edge,the discontinuity-induced systematic error will make one side of the edge raise above the surface and the other side drop below the surface.The error mathematical model is researched in this dissertation,and an algorithm is proposed to compensate discontinuity-induced systematic error.In our experiment of measuring a hole work piece,the discontinuity-induced systematic error is 4 times smaller with error compensation,and the hole radius estimation error is 4-10 times smaller.4.To solve the frequency spectrum overlapping problem in existing dynamic measurement method,an orthogonal color encoding method is proposed.The proposed fringe encoding way suffers much less in spectrum overlapping problem,and is proven to have 4 times less phase error by our experiment.5.A PMD system with 44?m lateral resolution,108mm×90mm view field,4.5 slope resolution and 1nm height resolution achievable is designed and built.With the assistance of a home-made synchronization software,this machine can accomplish the entire measurement by pressing a single button.The measurement result of a wafer is compared with an interferometer,and the results of the two machines are similar both in shape,peak-valley value,and root mean square(RMS)height value.The measurement result of a sphere mirror by this PMD system has 182 nm RMS error comparing to its ideal shape.