Spectrometer-on-a-chip Based On Etched Diffracton Grating

Lab-on-a-chip is currently a hot research topic among the scientific society. The research objective is to realize the sample detection and analysis on a small, low cost and effective chip to replace the traditional expensive and cumbersome instrument in biological and chemical laboratories. Spectral analysis is a major way to analyze the sample composition, therefore, to make spectrometer-on-a-chip is an important research issue in lab-on-a-chip. Many structures can be chosen for the on-chip spectral analysis. Among them, planar waveguide grating, including Etched diffraction grating (EDG) and arrayed waveguide grating (AWG), has the advantage of low insertion loss, small wavelength channel spacing, easiness to scale up to large counts of waveguides, low fabrication cost and so on. Planar waveguide grating was gradually matured along with the fast development of wavelength division multiplexing technology and it is a very potential one for making commercial-use spectrometer-on-a-chip. In this thesis, we design, fabricate and characterize a series of CMOS compatible spectrometer-on-a-chip based on silicon oxynitride (SiON) waveguide platform with Etched diffraction grating working below1100nm, which is the absorption edge of silicon material.The thesis starts from the theoretical background of planar integrated waveguide, including the ray theory and electromagnetic theory for the analysis of two-dimensional planar waveguide, and the mode coupling theory used for analyzing the mode coupling inside a waveguide or between waveguide. Besides, numerical simulation method, used for the calculation of mode field distribution and propagation in a waveguide, is introduced in the thesis, including finite differentiation method (FDM), finite element method (FEM), and beam propagation method (BPM).Also introduced are the one-stigmatic method, two-stigmatic mehod and three-stigmatic method used for the design of grating structure in Etched diffraction grating, along with the scalar diffraction theory for the simulation of light propagation in the grating, and the overlap integral method for calculating the mode coupling efficiency between waveguides. The relationship between the grating structure parameters and the spectrometer performance parameters are discussed in detail in the thesis, which provide an efficient instruction to design the grating structure with optimization. By pointing out the difficulty to obtain large spectral analysis range and high spectral resolution at the same time with the common design method, The dissertation introduces a cross-order high resolution spectrmeter on a chip with a separated wavelength band based on the frequency periodicity of the Etched diffraction grating. It can be used in the occassion that fluorescence spectra detection at multi wavelength bands is required, a practical EDG based spectrometer-on-a-chip is designed, with a separated wavelength band of680nm～690nm and735nm～745nm and a wavelength channel spacing of0.2nm.After that, an EDG based spectrometer-on-a-chp on SiON waveguides working below1100nm is demonstrated. The detailed design process was introduced. Some key restrictions of fabrication for high performance spectrometer-on-a-chip are throughly studied, including photo-lithography, SiON thin film growth and the deep etching of SiON and SiO2waveguides. The56-channel spectrometer-on-a-chip is characterized with our self-established test platform and measurement results are obtained and discussed.We further analyze the theoretical background of metal-semiconductor-metal (MSM) photodetector and the coupling structure between waveguide and photodetector, and integrate MSM photodetector array with EDG microspectrometer on SOI wafer by CMOS compatible fabrication process. The measurement results show that, the29wavelength channel operating from838nm to853nm has a channle spacing of0.494nm, a3dB channel bandwidth of0.4nm and a channel nonuniformity of1.5dB.In the end. the thesis is summarized and the future research work is planned.