Broad-beam Scanning Exposure Applied In The Fabrication Of Holographic Gratings

Diffraction gratings are widely used in diverse fields including spectral analysis,precision metrology,and chirped-pulse-amplification systems.In conventional interference lithography for grating fabrication,two collimated coherent beams generate interference fringes on the photoresist-coated substrate for exposure.This method has two drawbacks.First,the defects of the exposure interference field are recorded in the photoresist,resulting in curved grating lines,rough surface,and stray light,which deteriorate the detection limit of spectrometers.Second,the size of the fabricated gratings is limited by the aperture diameter of the collimating lenses,making it impractical to obtain meter-sized pulse-compression gratings needed in high-power chirped-pulseamplification systems.This dissertation focuses on a new type of broad-beam scanning exposure method that is free of these two drawbacks.The key technique in the scanning exposure is the use of the interference fringes generated by a reference grating or a latent grating to adjust in real time the phase and attitude of the exposure interference field relative to the substrate.The principle of scanning exposure and the principle of fringe locking that uses a reference grating or a latent grating are introduced.When a low-stray-light grating is being recorded,the substrate moves a distance of about one thousand grating periods in the direction parallel or perpendicular to the grating grooves in the exposure interference field.During scanning,the phase variation of the exposure interference field is detected and then adjusted by means of an acoustic optical modulator or a piezoelectric transducer,so that the phase of the exposure interference field is kept stationary relative to the substrate.Scanning exposure averages the highfrequency defects of the interference field,and therefore decreases the stray light of the grating.Several scanning-exposed gratings were successfully made,which had straighter grooves and smoother surface than gratings recorded still,and their stray light levels around the first and second diffraction orders were decreased by 30% and 80%,respectively.This is a simple and practical method to make low-stray-light gratings.When a large-size grating is being recorded,the substrate passes through the exposure interference field at a constant speed in the grating vector direction.We use the latent grating generated on the exposed area of the substrate to adjust in real time the phase and attitude of the exposure interference field relative to the substrate.A photomultiplier is used to lock the phase rapidly,while an electron-multiplying CCD is used to lock,at a lower speed,the attitude and correct the phase-locking error caused by the diffraction efficiency variation of the latent grating.The linear exposure technique realizes steady transition from stationary to scanning exposure and thus eliminates the exposure seams effectively.Since the latent grating is generated continuously on the substrate during scanning exposure,this method can achieve an expanded recording length along the scanning direction.Several scanning-exposed gratings of a size 200 mm × 100 mm were successfully made,whose duty cycle variation across the dimension parallel to the scanning direction was within 0.03,and the peak-valley value of the firstorder diffraction wavefront error was within 0.25 ?.This method may be applicable to fabricating larger-size gratings.