| Interference lithography is the key procedure in the holographic gratingfabrication, which includes static interference lithography and scanning beaminterference lithography (SBIL). The static interference lithography and SBIL bothrequire keeping the fixed phase relationship between the interference fringes and thegrating substrate, and the relative phase stability determines the grating’sperformance. In the process of static interference lithography, the interference fringedrifts caused by the environmental changes can be reduced by the interference fringestatic phase-locking technology. Then, the lithographic quality and the grating’sperformance can be improved. There are advantages in making the large-areagratings using SBIL.In the process of SBIL, the fringe’s phase must be regulated inreal time to compensate the adjacent scanning phase mismatches caused by the stagemovement error. This is the key to SBIL’s success. In order to further improve thelevel of the grating lithography and satisfy the urgent needs for high performanceand large-area gratings in the fields of science, the research is mostly on theinterference fringe static and dynamic phase-locking technology in the lithographysystem of the holographic grating. First, the interference fringe’s phase changes in the static interferencelithography system have impacts on the exposure contrast and the grating maskprofile. These impacts are analyzed. The fringe’s phase changes are categorized intothree categories, linear drifts, sine drifts and high frequency vibrations. By means oftheoretical analysis and numerical calculation, it is found the linear drifts and sinedrifts decrease the exposure contrast and it’s become difficult to control the maskprofile through the exposure dose. The low amplitude high frequency phasevibrations have little effect on exposure contrast and grating mask profile. The lowfrequency phase drift can be suppressed by static phase locker.Second, a frequency-shift static phase-locking method is proposed. According tothe method, a static phase-locking system is designed. Acousto-optic modulators(AOM) regulate the beam frequency to correct the fringe’s phase errors which aremeasured by the moirépatterns. The phase adjusting system and controller designmethod are given, and the influences AOM on the exposure parameter are analyzed.The experimental results illustrate the low-frequency drifts are restrained effectively.Fringe’s Phase fluctuations are within±0.02interference fringe period (3σ) whichmeets the needs of lithography.Third, the key methods and technologies in dynamic phase-locking are studied.Dual-frequency laser interferometer measures stage displacement with nanometeraccuracy. Heterodyne interferometry is introduced to measure the interferencefringe’s phase changes in high-precision. Moreover, the measuring optical path isdesigned. For extending system bandwidth, the stage displacement and phaseinformation are acquired at high speed by the hardware interface of the highperformance heterodyne calculation cards. The dynamic phase-locking controlsystem based on the PXI and FPGA card is built which is good at real-time dataprocessing and multifunction coordinating, and the software is developed byLabVIEW.Fourth, the mathematical model of the dynamic phase-locking reference isestablished. The compensating algorithm for the following control system is devised. The error factors leading to the interference fringe’s phase mismatch are analyzed.The generalized reference model with universal meaning is built. Based on thegeneralized model, the evolutionary model I and evolutionary model II on twodistinct occasions are derived. Aiming at the situation that the stage’s accuracy isrestricted, the evolutionary model II is simplified and modified. The system transferfunction is identified by the step response and frequency response method. After thesystem bandwidth is determined, the compensating algorithm is designed andverified by simulation.Fifth, the accuracy of the dynamic phase-locking is analyzed and calculated,then the engineering implementation of the dynamic phase-locking system in SBILis completed. An80mm×70mm holographic grating is firstly fabricated in China bySBIL. Stage displacement and interference fringe’s phase measurement accuraciesare calculated carefully. Besides, the fringe period is an important parameter indynamic phase-locking, so the influence of the period measurement error isdiscussed. The phase-locking experiments show the fixed phase-locking accuracyreach6.43nm (1800l/mm,3σ), and the low frequency components reach2.75nm(below500Hz，1800l/mm,3σ). The fringe phase shifting has nano-scale steady stateprecision. In the exposure process, dynamic follow accuracy reaches18.07nm(1800l/mm,3σ), and the low frequency components which are the main factoraffecting lithography are limited within2nm (below300Hz，1800l/mm,3σ). |
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