Study On Polymer Micro-Opto-Mechanical Vibration Sensors Integrated On Silicon

Being juxtaposed with communication and computer technologies, sensortechnology has been deemed as one of three mainstays of communicationtechnology in modern time. With the development of modern science, sensortechnology, as a new subject related closely with modern science, has beendeveloping rapidly. Sensor technology is being applied wider and wider inindustry automatization, measurement and detection technology, spaceflighttechnology, military engineering, and diagnosis of medical treatment. In the sametime, sensor technology has the function to accelerate developments of everysubject.In communication time, the quantity of communication needed to obtain ismore and more with the increase of the complexity and the grade ofautomatizatiom of systems, and the systems not only need the higher and higherprecision, dependability and response, but also require that the sensors have thestandard output forms in order to connect with systems easily. By all appearances,the traditional sensors hardly satisfy the requirement and will be washed outgradually because of their simple function and big bulk. Developing highperformance, advanced sensors made mainly with silicon has been inevitable,such as resonance sensors, capacitance sensors, optic-electric sensors and fieldeffect chemistry sensors. Although their sensitivity mechanisms are different,their common characteristics are development toward micromation andintelligentization. This is the mainstream of sensor development in the world nowand long future.One of characteristics of the micro sensors is their small bulk. The size oftheir sensing element commonly is the micrometer grade, and they made by thetechnique of micro-mechanical process, which include Photolithography, Etching,Deposition, Bonging and Encapsulation. Making use of the Anisotropy Etching,Sacrificial Layer Technology, LIGA (Lithographie, Galvanoformung andAbformung) Process, we can fabricate three dimension micro structures with thelarge difference between the layers, which include movable membranes,cantilever beams, bridges, grooves, holes, and cones. Combining with the specialfilms and high performance integrated circuits, these micro structures have beenapplied to develop various sensors even sensing element arrays withmulti-performance (such as optic-electro detectors), and realized pressure, force,acceleration, angle velocity, stress-strain, temperature, flux, imaging, magneticfield, humidity, pH value, gas composition, ion or molecule concentrationsensors, and biosensors.Application and realization of micro sensors will bring reformations withdeep affection in the many fields of technology, especially be more important forthe aviation and spaceflight, remote sensing, medical treatment and health care,and industry automatization.A micro-opto-mechanical vibration sensor with the polymer structureintegrated on silicon was presented, desdigned and developed firstly in thisdissertation.The sensor structure composes of micro a cantilever beam and an integratedcircuit of optical waveguides. The polymer micro cantilever beam is used assensing element. Fixed the sensor on the measured object, vibration oracceleration of the object can make the free end of the cantilever beam tovibration in the same as frequency of the object or displacement to realize theconversation from vibration/acceleration to displacement. The multimodeinterference (MMI) coupler integrated with input and output waveguides is usedas the transduction element from displacement to output light intensity difference.Because the input waveguide integrated in the cantilever beam moves with thecantilever beam, the position of the input light changes at the input end of MMIwaveguides. Based on the self-image effect of MMI waveguides, the motion ofinput light position brings about the change of the light power difference betweentwo output waveguides.By using optic-electro detectors, the two light power signals are receivedand transformed electric signals. Then the electric signals have been amplifiedwith the differential amplified circuit, so that the displayable mechanicalparameters of the measured object in the state of vibration or accelerated motion.The polymer constructed by Polymethyl Methacrylate (PMMA) wasselected as the material of the optical waveguides and the cantilever beam. It isknown that Young's modulus of polymer material is one twentieth of that of SiO2,and just one fifty-sixth of that of silicon. Indicated by theoretical analysis, thestatic sensitivity is inversely proportional to Young's modulus. That is, for thesame mass and the same structure parameters of micro cantilever beam, thesmaller the Young's modulus of the material is, the higher of the sensitivity.Therefore, the static sensitivity of polymer PMMA cantilever beam can beincreased to twenty times of that of SiO2 cantilever beam with the same structureparameters. Besides, the whole structure of the polymer sensor is made of thesame material, so that it does not occurred the mechanical structure distortion andthe measurement error because of the difference of internal stress and heatexpansion coefficient among the different waveguide materials.For the polymer material, waveguide films can be fabricated by usingspin-coating. By adjusting the concentration of polymer solution and combiningwith the proper spin-coating conditions, the thick film for making the cantileverbeam is easily fabricated by using spin-coating. The fabrication process is simple,cost is low, and does not need costly process equipments. Therefore, it isconvenient for the device practical application.In the aspect of device performance, the possible selected range of organicoptical material for fabricating optical thick films is much wider than that ofinorganic material, and the mechanical property of the organic optical materialcan be adjusted by designing and optimizing the molecular structure of theorganic optical material to satisfy the need of the optimization design of thedevices. The Young's modulus of polymer optical material used to fabricatecantilever beams could be adjusted in a wide range. Therefore, it is possible toincrease obviously the device sensitivity and dynamic range.The innovative and significative works accomplished in this dissertation aresummarized as follows:1.A micro-opto-mechanical vibration sensing structure of polymer integratedon silicon was presented, desdigned and developed firstly, and its feasibility andadvantage have been testified by the experiment;2.Based on the essential principle of material mechanics and vibrationmechanics, the structure and mechanical parameters of the micro cantilever beamin the sensor have been analyzed and designed, and the optimum structureparameters have been calculated.3.As a part of the transduction element of the sensor, the input and outputwaveguides of the waveguide integrated circuit have been optimized anddesigned. The mode property of the waveguides has been analyzed by using themethod of effective refractive indices. For realizing the waveguides work in thesingle mode, the mode cut-off equations have been derived based on the methodof effective refractive indices, and the figure of the single mode region has beenpresented. Combining the conditions of the waveguide fabrication process andconsidering the mode coupling between the fiber and waveguide and matchingthe cantilever beam, the width and thickness of the waveguides are designed as5μm and 3μm, respectively, and the waveguides work in the single mode.4.By using the method of effective refractive indices, the two-dimensionalself-image effect was simplified as one-dimensional self-image effect. Theapproximate errors of the length and effective refractive indices of the multimodewaveguide brought by different approximate conditions have discussed detailedly.The structure of the 1×2 MMI coupler with a dynamic input used in themechanical-opto signal transduction of the vibration sensor was designed. In theMMI waveguide, one input image formed of three self-images at the output endof the MMI waveguide. The double output waveguides were set at the positionsof two minimum value points of the distribution curve of the light field in thestatic state at output end of the MMI waveguide, respectively. Although thisstructure not is the standard 1×2 beam splitter structure, the differential outputlight signals with perfect linearity can be obtained with this structure. Indicatedby the theoretical calculation, the linearity is better than that reported in thereference. The simulated result with the guided-mode propagation analysismethod was compared with the simulated result with the BPM (BeamPropagation Method), and both optical field distributions are nearly identicalunder the condition of the effective width with the first approximation.5.Under the conditions of the practical device process and considering therequirement of waveguide-fiber coupling, I presented the detailed parameters ofthe device masks design. In the process of the device fabrication, many keytechniques problems have been solved successfully, and the perfect process flowfor polymer vibration sensors has been formed. All of above established theexperiment foundation for the applied research and industrialization in the future,and supported the tried experiment data.6.Based on the detailed theoretical analysis and optimized design, the modeldevice of the polymer micro-opto-mechanical vibration sensor in the laboratoryhas been developed successfully. The measuring result showed that the differenceof two output light powers was linearly varied with the shift of the free end of thecantilever beam in the measuring range. The ultimate stress was designed muchsmaller than the permissible stress, so that assured the applicability of Hooke law,which is the accelerator of the sensing mass in vibration strictly was in directproportion to the shift of the free end of the beam. Therefore, the linearity of theoutput quantity was assured effectively in the process of twice transductions ofthe measurand. Besides, it was indicated by the measurement of bendedcantilever beam that the practical ultimate target of the device was signally higherthan the design target. Consequently, it could be expected to upgrade thetechnical targets of polymer vibration sensing devices, and the advantages of thepolymer micro-opto-mechanical vibration sensing structure were shown.7.A novel and efficient method for measuring parameters of anisotropic slabwaveguides by using prism coupling technique was presented. This method canbe used directly to measure parameters of the anisotropic slab waveguides withsingle-mode, and needed not to measure firstly the multimode waveguide madeof the same material and then reversed the parameters of single-mode waveguide.The method broke through the bottle-neck of that the prism coupling technique isonly applicable for measuring multimode waveguides. By using the measurementsystem, guided mode refractive indices and leaky mode refractive indices ofanisotropic single mode waveguides can be measured in the same time, and thewaveguide parameters can be solved from the system of transcend equationsformed by guided mode equations and leaky mode equations. For testifying thevalidity of this method, an electric-optic polymer DR1/PMMA single modewaveguide was measured practically, and the accurate measuring result wasobtained. Although the measuring error brought by the calculation with theguided modes equations and the leaky modes equations is greater than that ofonly guided modes equations, the measuring precision satisfies the requirementof device design enough. Certainly, this method is fit to measure multimodewaveguides and isotropic waveguides. By using the same measurement systemand the data treatment software programmed by myself, the polymer PMMAmaterials of guided wave layers and cover layers compounded in differentbatches, both the refractive indices was determined as 1.51 and 1.49, respectively.The measuring result presented tried data for the device design. Thus it can beseen, the applied range of this method is abroad, and its measurement process issimple. The data treatment software programmed by myself is fit for differentwaveguide films. The software can distinguish guided mode or leaky modeautomatically, and adopts the relevant data treatment manner, so that its universalproperty is outstanding.To sum up, a micro-opto-mechanical vibration sensor with the structure ofpolymer integrated on silicon was invented in this dissertation. Based on thedetailed theoretical analysis, the design of the device structure has beenoptimized, and the detailed structure parameters of the device are supported.After a lot of exploratory experiments, a perfect fabrication process flow for thedevice has been formed. Finally, the model device of the polymermicro-opto-mechanical vibration sensor in the laboratory has been developedsuccessfully. The success of the device fabrication established the experimentfoundation for the applied research and industrialization in the future, andsupported the tried experiment data.For this vibration sensor, its sensing principle is advanced, the fabricationprocess is simple, and its cost is low, so that it is convenient for the practicalapplication. Composed with the sensors based on the piezoelectricity orcapacitance principle, micro-opto-mechanical vibration sensors have a uniqueadvantage because of its insensitivity to electromagnetic fields. Therefore, thisdevice has an important significance and a broad application foreground in bothmilitary and civilian fields.