同步辐射扫描透射X射线显微成像系统振动的闭环控制方法研究

Research on synchronous radiation scanning transmission X-ray microscopy system closed-loop control method

上海同步辐射光源(Shanghai Synchrotron Radiation Facility,SSRF)扫描透射X射线显微成像(Scanning Transmission X-ray Microscopy,STXM)实验站是国内首个自主研制的STXM实验系统,目前该系统波带片聚焦直接成像分辨率达到15 nm,扫描相干衍射成像方法成像的分辨率达到7.32 nm。系统振动是影响其成像质量的主要因素。本文针对波带片和样品之间系统振动进行详情分析,并提出相应的抑制策略,旨在提升STXM系统的成像质量。采用激光干涉仪和现场可编程门阵列(Field Programmable GateArray,FPGA)技术,从软件层面开发了一种针对STXM中波带片和样品位置稳定性的闭环控制系统。该控制系统在最佳条件下可将STXM系统振动的均方根值(Root Mean Square,RMS)有效控制到4.833 nm,减少至原振动水平的约1/3,为STXM系统的成像质量进一步提升奠定了技术基础。

[Background]The Scanning Transmission X-ray Microscopy(STXM)endstation,located at the Shanghai Synchrotron Radiation Facility(SSRF),stands as China's sole STXM device.It boasts the capability for high spatial resolution imaging down to a 15 nm scale.Utilizing the scanning coherent diffraction imaging method(i.e.ptychography),this station achieves an optimal resolution of 7.32 nm,necessitating utmost system stability.However,the STXM system operates within a complex environment where the impact of external vibrations on imaging quality is increasingly problematic.[Purpose]This study aims to develop a closed loop control method for the vibration of STXM system to improve imaging quality.[Methods]First of all,system vibration between the Fresnel zone plate(FZP)and the sample was analyzed in detail,so did the vibration suppression methods.Then,a software-driven closed-loop control system was developed on Field Programmable Gate Array(FPGA)platform by leveraging these insights,and a Fast Fourier Transform(FFT)based approach was implemented to process precise(picometer-scale)and rapid(1 kHz)positional data captured by a laser interferometer.Finally,the internal sensor of motor controller was replaced by laser interferometer to realize the closed-loop control to suppress vertical vibration,and vibration data were applied to a piezo motor for effectively replacing its internal sensors to suppress system vibrations.[Results]The optimal performance achieved by the closed-loop control scheme shows that the root mean square(RMS)value of the vibration is reduced to 4.833 nm,which is around one third of the value before.[Conclusions]The successful implementation of this closed-loop control scheme has furnished robust technical foundations for further enhancements in the imaging resolution of the STXM system.