The Research Of Fiber Spectrum Sensor Film Thickness Meter

As the development of the science and technology, thin film materials are usedmore and more broadly in many fields. The thickness which is an important indexof the films definitively affects the optical property, mechanical property,electromagnetic property and other properties of the films. For example, most ofthe semiconductor devices and integrated circuits are composed by different filmsin the semiconductor production which makes the measurement of the filmthickness very important.There are many methods to measure the thickness of the films. Such asmechanical methods, optical methods, weight methods, electrical methods, atomicmethods and so on. Optical methods have extensive application prospect as theyhave high accuracy, no touching and other advantages. Polychromatic lightinterference spectrum methods which belong to optical methods can not onlymonitor the thickness of the films but also can measure the thickness of the filmswhich have already been done. Use fiber sensor in polychromatic light interferencespectrum methods to receive light signal release the complexity of the system.The data processing of optical film thickness measure systems are verycomplex that it is always done by PC. Such systems have many strongpoints suchas fast processing, friendly interface, etc. But they also have some disadvantageswhich include bulkiness, high power consumption, etc. As the development ofembedded processors especially the Digital Signal Processor (DSP) makes thecomplex data operation can be finished on DSP but not the PC to design smalleroptical equipment. Thickness measure systems based on this structure are small,low power consumption, fast operation speed and many other features. The dataoperation for processing the light spectrum signal of the system which is discussedin this thesis is based on DSP.The research that have done include: Analyze the basic principle of the double beam interference. Give thefundamental algorithm of the polychromatic light interference spectrum to measurethe thickness.Discuss the denoising and study the denoising method using wavelet. Choosea method adapt the denoising of the spectrum signal which called adaptivedenoising method. Bai Ying Kui use this method to process the spectrum of theether and received very good conclusion. Use the method to process the sine signalwith noise to simulate the denoising and got a perfect result. This simulateexperiment verify that this denoising method is fit to the thickness data processing.Establish a thickness measurement system which uses LED as its light source.LED is a self emission source which has characteristics such as low powerconsumption, small bulk and high stability, etc. White LED which is an importantimprovement of LED is the last published. Choose proper fiber spectrum sensor toconstitute fiber propagating unit with other components. The fiber spectrum sensortests the interference light which is modulated in the film but not in the fiber sensor.The fiber sensor only wave guide of the light and the light propagating in the fiberis not affected by the thickness. There is an optical processing unit whichcomposed by the grating and other optical components. This unit uses the gratingdispersion capability to separate the spectrum data. ACCD camera is used to inceptand convert the data. CCD camera is a kind of image collection device which usethe CCD sensor technology. It is made up of the CCD sensor and the electroniccircuit. CCD sensors have line scan sensors and area scan sensors. The system usePiranha2 camera produced by DALSA which contain IL P1 4096 line scan CCD, 8bit or 10 bit output interface and arithmetic operation unit. The system useTMS320VC5402 that belongs to 5000 series, produced by TI as the data processingunit. The software for data collection, wave let denoising, geting the extremal pointand calculating the thickness is programmed in the CCS which is the IDE forTMS320.Analyze the error of the measurement and discuss the error from the light source, the fiber, the optical path difference and the unevenness of the film. Presentsome method to solve the error problem. Especially discuss the error caused by theunevenness of the film and find a way to evaluate the film to give a reference forthickness measurement which improves the precision and reliability of themeasurement.The system is used in the thickness measurement of SiO 2film and comparedwith conclusion using elliptical polarization equipment. Validate that the systemhas high precision and the evaluating the film gauge uniformity is help to theaccurate.Such structural system also has some disadvantages. The major one is that thesystem can't measure the"very thin"film. The extreme value method needs thespectrum extreme value so that the film must have enough thickness. It canmeasure thinner film by changing the light source to one that have shorterwavelength. But it is nearly impossible to find the source when film thickness isthinner and thinner to 0nm . At last, an assumption that can measure the gapbetween two reflect boards by the spectrum is presented. The assumption canmeasure the thickness of the film that between the gap and the thinnest film canreach to 0nm theoretically.