A stroboscopic scanning electron microscope (SEM) has been proposed as a means of generating still or slow-motion pictures of moving structures in microelectromechanical systems (MEMS). Such imaging is used in characterizing the dynamics of MEMS; characterization of the dynamics is a critical component of the MEMS development cycle.
Conventional strobed-illumination microscopy with visible or infrared light provides adequate temporal resolution but insufficient spatial resolution for measuring subwavelength motions in the main plane of a typical MEMS. A conventional SEM provides adequate spatial resolution, but is inadequate for resolving motions at frequencies greater than several tens of hertz because the illuminating electron beam is continuous. The proposed stroboscopic SEM would offer both the spatial resolution of a conventional SEM and the temporal resolution of conventional optical stroboscopy, making it possible to form crisp images of moving (e.g., vibrating) MEMS structures.
According to the proposal, a conventional SEM would be augmented with an electronic beam blanker that would be operated in coordination with a signal generator. The output of the signal generator would control the vibrational excitation of a MEMS device mounted in the SEM (see figure).
In one mode of operation, the blanking-pulse-repetition frequency would be set equal to the frequency of vibration, so that the resulting SEM image would "freeze" the motion at some phase in the vibration cycle. The phase could be varied by adjusting the phase offset between the vibration-waveform and blanking-pulse generators. In another mode of operation, the blanking-pulse-repetition frequency would be made to differ slightly (no more than a few hertz) from the vibration frequency, yielding a sequence of images at slightly different phases (in other words, a slow-motion picture). Freeze-motion images taken at different phases could be used to quantify the shape of a vibrational mode at the frequency of excitation, while slow-motion pictures could be used to obtain qualitative understanding of the motion.
This work was done by Kirill Shcheglov and Russell Lawton of Caltech for NASA's Jet Propulsion Laboratory.
NPO-21056
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Snapshot SEM Imaging of Moving MEMS Structures
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Overview
The document presents a novel approach to imaging micro-electromechanical systems (MEMS) using an advanced scanning electron microscope (SEM) equipped with an electronic beam blanker. Developed by Kirill Shcheglov and Russell Lawton at NASA's Jet Propulsion Laboratory, this technology addresses the limitations of traditional SEMs and optical stroboscopic techniques in capturing dynamic motion at high frequencies.
Conventional SEMs utilize a continuous electron beam for illumination, which results in blurred images of moving structures. On the other hand, existing optical stroboscopic methods, while capable of capturing fast motion, suffer from low resolution due to the limitations of visible light, particularly when imaging features smaller than the wavelength of light. The proposed solution combines the high spatial resolution of SEM with the stroboscopic technique, allowing for dynamic characterization and visualization of MEMS structures.
The system operates by synchronizing the blanking-pulse repetition frequency of the electron beam with the vibrational frequency of the MEMS device. In one mode, the blanking pulse is set to match the vibration frequency, effectively "freezing" the motion of the MEMS at a specific phase in its vibration cycle. This allows for precise measurement of the structure's mode shapes. In another mode, the blanking pulse frequency is slightly offset from the vibration frequency, enabling the capture of a sequence of images at different phases, akin to a slow-motion video. This dual capability facilitates both quantitative analysis of vibrational modes and qualitative visualization of device operation.
The document outlines the technical disclosure of this invention, emphasizing its significance in the MEMS development cycle, where dynamic characterization is critical. By overcoming the limitations of previous imaging techniques, this stroboscopic SEM offers a powerful tool for researchers and engineers working with MEMS, enhancing their ability to analyze and understand the behavior of these intricate devices.
Overall, this innovative imaging technology represents a significant advancement in the field of MEMS characterization, promising to improve the development and optimization of these systems for various applications in technology and engineering.
