Study On Novel Fast Full Spectrum Reflectance Difference Spectrometer

Reflectance difference spectroscopy (RDS) is a linear optical instrument which measures the difference in the normal incidence reflectivity for two mutually perpendicular orientations of the polarization vector as a function of photon energy. The technique is extremely sensitive to any kind of the in-plane optical anisotropy of matters and has been successfully applied in the studies of semiconductor surfaces, metal surfaces, and polymer surfaces. Particularly, it is widely used as a powerful tool for the real-time inline measurement in industry. According to the continuous extending of its application in different fields, the instruments can do fast full spectrum acquisition are required. This has stimulated lot of research and development efforts carried out in both industry and universities over the world. Focusing on the realization of this challenging task, in the PhD work, two systems based on different phase modulation techniques have been development. For the first system, we have set up a prototype multi-channel RD spectrometer by modifying a commercial single-channel RD spectrometer which is based on photoelastic modulator (PEM) techniques. Then, with a new concept of RDS configuration based on rotating compensation techniques established in this work, a prototype machine is developed. The testing results show that the time consumed for one full spectrum is one order of magnitude less than that for commercial one without loosing measurement precision. Therefore, the function of fast data acquisition is successfully realized. The main achievements of this PhD work are listed by time sequence in the following:1. A new scheme for multi-channel PEM based RDS was introduced based on a general high speed data acquisition board and virtual instrumentation technique on PC. And a two-channel PEM based RDS was built up as a prototype of the new kind of multi-channel instrument. The influences of acquisition board performances and two frequency domain analysis methods on measurement precision, such as FFT and Lock-in amplifier, were discussed. Two more general PEM retardation corrections for full spectrum range were promoted for calculating the exact phase retardation of PEM at every wavelength.2. The mathematic model of rotating-compensator based RDS was established by Stokes matrix. All kinds of measurement errors including systematic and statistical ones and their origins were studied in detail. Perticular attention has been given to the imperfections of optical elements, azimuth misalignments and error signals from detector. The mathematic relationships between measurement errors and these error sources were derived. Two new calibration / correction methods for system were promoted. One was based on polarizer's azimuth and another was an on-line method based on polarizer rotation. A combination of the real time angular positions of the compensator and a least-square method was applied to data analysis.3. A prototype of rotating-compensator based RDS was built up, which composes 3 parts namely, optical probe system, electronic control system, and operation software. A method of reading simultaneously both the real time angular position of the compensator and the integrated intensity on the detector was used for data acquisition.4. The test and analysis for each component's imperfection was conducted, especially for the light source, the compensator, and the CCD detector. Then, several mathematical methods in data analysis for improving signal to noise ratio were discussed. At last, the comparison of measurement precision between the commercial single-channel PEM based RDS and the new spectrometer was performed.5. In order to evaluate the performance of the new instrument thoroughly, some samples in addition to Si(110) were measured by both the new instrument and the commercial RDS. The results show that in the full range of RDS signal (between -1 and +1), the new spectrometer can measure the signal exactly and the measurement speed is 10 times faster than that of the commercial RDS. It also indicates that RDS is very sensitive to anisotropy of surface electronic structure, whatever it comes from subnano-scale atom structure, nano-scale cluster, and submicro-scale micro-maching structure.