Study On Scatterometer For Planar Diffraction Grating Stray Light Measurement

The grating is a key component of spectral analysis technology.Therefore,the fabrication of high quality grating has become one of the core of the development of high precision spectral analysis instrument.The stray light is an important index of the quality of the grating,especially the high resolution grating.Stray light is mainly caused by the defects of gratings and it decreases the signal-to-noise ratio of spectral instrument greatly.Stray light directly affects the measuring range and measuring precision of high precision spectrometer.Only through measuring stray light's strength accurately can we calculate,judge and deal with the harm brought to spectral data by stray light.At present,the precise measurement of stray light is still difficult,and there is no uniform testing method at home and abroad.There are two main methods based on the principle of measurement: One is based on the electromagnetic theory,using convergent beam combined the vector diffraction theory with Kirchhoff diffraction equation,measuring the bidirectional reflectance distribution function(BRDF).However,using convergent light to measure the grating used in the parallel light will have a bigger error which will affect the final measurement accuracy.Moreover,BRDF theory which meets the measurement conditions of Rayleigh scattering,is mainly used for measuring stray light of rough surface.However,for the low density grating,the groove of the grating is generally larger than a ?,which does not meet the Rayleigh scattering conditions.For the high density grating,we want to learn about the small angle of stray light distribution,which does not meet the theoretical boundary conditions.The theoretical basis of measurement is not perfect and it is difficult to inverse the defect of gratings from the measured data.The other is using the classical scalar diffraction theory,based on the Fresnel-Kirchhoff diffraction equation,under a parallel beam to measure the grating distribution function(GDF).This method has a more strict derivation on grating stray light.But these instruments are use the Littrow structure to measure the stray light of the grating under the condition of parallel light.However,since the Littrow structure is relatively compact,it is difficult to eliminate the stray light inside the instrument.And the spherical mirror in the back-end that acts as both the collimating mirror and the focusing mirror causes secondary stray light,which makes the requirements of the spherical mirror surface's accuracy higher.In the view of this,the diffraction grating stray light measurement system is in-depth studied in this thesis.The main contents include the following aspects:First,using the classical scalar diffraction theory,based on the Fresnel-Kirchhoff diffraction equation,under the condition of parallel light incidence,build the expression of stray light intensity in the instruments and the measurement model of GDF,which provided the theoretical basis for the simulation of the stray light and the inversion of the surface parameters of the grating.Second,study on the correlation between the electromagnetic theory and the classical scalar diffraction theory,According to the BRDF expression based on the electromagnetic theory,the GDF is associated with the power spectral density(PSD),which provided guidance for mirror optical processing and the simulation of the stray light.Third,the modular design and integrated optimization design are combined to complete the system's optical design.At last,under the condition of parallel light incidence and use the optical attenuation method,the GDF of different grating and full aperture(15 * 15mm2 ~200 * 200mm2)at the wavelength ranging from 380 nm to 900 nm can be measured by high image quality and low instrument stray light.Fourth,with the study of stray light modeling and analysis,optimize the system in the aperture stops,light traps and baffles.In accordance with the result to complete the instrument itself stray light suppression design,and provided the basis for the slits,extinction painting selection and the optical processing.Finally,the requirement of the low instrument stray light and the accuracy of measurement can be 10-9 is realized.Fifth,align the system to determine the test capabilities and repeatability.The correction and processing algorithms of the measurement data are studied to obtain the GDF.And the grating surface parameter inversion model is built by combining the stray light theory of planar diffraction grating,so as to provide reference for the grating quality inspection and fabrication.