This paper presents a new image-processing approach to estimating a pointing error of the electron probe of a scanning electron microscope. An environmental disturbance causes a pointing error of the probe that is reflected upon a specimen image in a microscope. The new approach uses the deteriorated specimen image to estimate the pointing errors of the probe. The microscale is used as a specimen in this experiment, and a simple mathematical model is used to simulate a microscale image. The mathematical model is obtained by using the surfcace tilt and shadowing contrast of the microscale and by approximating the delta and step functions to differentiable functions. Simulated microscale images are identified by a least-squares procedure with measured images to estimate the pointing error of the probe. The estimated pointing errors are used to design a controller for vibration isolation of a scanning electron microscope. The designed controller is based on a transfer function from acceleration sensor outputs to the pointing errors. An acceleration sensor is situated close to the specimen stage in the microscope chamber to detect the stage motion. Sinusoidal excitation tests are performed to determine the transfer function. The sensor outputs are passed through the designed controller to compute the inputs into the image-shifting coils, and the coils move the electron probe to cancel the pointing errors. The performance of the designed controller is verified by comparing specimen images with and without control when the microscope vibrates. The comparison shows the pointing errors are significantly reduced in a region of lower frequencies.
We propose a new approach to estimating a pointing error of the electron probe of a scanning electron microscope. The approach is formulated to estimate the pointing errors using a specimen image. Specimen images are numerically simulated by a mathematical model and identified with measured images of the specimen using a least-squares procedure to determine the pointing errors. The pointing errors are estimated by the proposed approach and used to design a controller for vibration isolation of a scanning electron microscope. Acceleration sensors are located at the root of the specimen chamber and are used to detect any environmental disturbance into the microscope. The designed controller is based on a transfer function from the sensor outputs to the pointing errors, and the transfer function is determined by sinusoidal excitation tests for the microscope. The controller is implemented as a digital filter on a PC and is used to move the electron probe to cancel the pointing errors using image-shifting coils. The pointing errors are successfully reduced by the controller in a lower frequency region that contains the first four natural frequencies.
KEYWORDS: Microscopes, Sensors, Video, Digital signal processing, Signal processing, Electron microscopes, Digital filtering, Vibration isolation, Scanning electron microscopy, Mathematical modeling
This paper proposes a new approach to control of image-shifting coils for vibration isolation of a scanning electron microscope. Image-shifting coils move the electron probe of the microscope out of phase with undesirable motion of a specimen due to any disturbance source. Two acceleration sensors are located at the root of the specimen chamber of the microscope to detect the disturbance. The outputs of the acceleration sensors are fed forward into a controller to move the probe by the image-shifting coils. The feed-forward controller is based on a transfer function from the sensor outputs to the relative displacement of a specimen to the electron probe that is assumed to be at rest. The microscope is put on a table attached to a shaker. Sinusoidal excitation tests are made by the shaker to measure the transfer function data using a video signal and the sensor outputs. The relative displacement of the specimen is estimated by identifying the measured video signal with a simulated one in a least-squares sense. The controller is implemented as a digital filter running on a digital signal processor. The amplitude of the vibrating images is significantly reduced by the controller at the natural frequency of the system.
KEYWORDS: Sensors, Digital signal processing, Signal processing, Microscopes, Scanning electron microscopy, Electron microscopes, Digital filtering, CRTs, Video, Image sensors
This paper proposes a new approach to reducing an effect of floor vibration on an image of a scanning electron microscope. An image-shifting coil is used to move the electron probe in order to cancel undesirable motion of a specimen due to the floor vibration. The floor vibration is structurally transmitted through the microscope and detected by two acceleration sensors at the root of the specimen chamber of the microscope. The outputs of the acceleration sensors are fed forward into a controller to move the electron probe by the image-shifting coil. The feed-forward controllers are designed in two ways. The first one is based on a transfer function from the sensor outputs to the relative displacement of a specimen to the electron probe being at rest. The microscope is put on a table attached to a shaker. Sinusoidal excitation tests are done many times to estimate the transfer functions from vibrating images of a micro scale. Moreover, the second controller is designed by manually amplifying and delaying the sensor outputs so as to minimize amplitude of the vibrating images on a CRT. Those two controllers are implemented as a digital filter running on a digital signal processor.
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