Opportunity
In electron microscopy and related imaging apparatuses such as scanning electron microscopes (SEMs) and semiconductor inspection tools, a critical challenge lies in achieving and maintaining precise orthogonality between the two scanning directions of the electron beam. The deflector, typically comprising electromagnetic coils, moves the electron beam in a first (e.g., X) and a second (e.g., Y) scanning direction within the same plane to raster-scan the specimen. The quality of the resulting image is heavily dependent on these two scanning directions being perfectly orthogonal (i.e., at a 90-degree angle). Any deviation from orthogonality introduces image distortion, compromising measurement accuracy and visual fidelity. This non-orthogonality primarily stems from inherent manufacturing tolerances and assembly errors in the deflector coils. These imperfections cause the actual scanning axes to be skewed relative to the intended orthogonal reference axes. Existing systems often struggle to compensate for these errors effectively in real-time, potentially requiring complex calibration procedures with specific samples or offline corrections, which reduces operational efficiency and can limit imaging precision, especially in high-throughput inspection or critical research applications.
Technology
This patent addresses the orthogonality problem by introducing a specialized control unit and method that dynamically corrects the scanning directions of the deflector. The core innovation is a real-time feedback and adjustment system. The control unit first determines the current, actual first and second scanning directions. This determination can be achieved by analyzing a produced image of the specimen to assess distortion or by directly measuring the extent of orthogonality between the scanning directions. The unit then processes these determined directions using a set of predetermined mathematical equations. These equations calculate the required adjustments to the drive signals controlling the deflector coils. A key set of equations is: A = A' - B'·sin φ and B = B'·cos φ, where A' and B' are the detected amplitudes of the drive signals in the skewed scanning directions, A and B are the required amplitudes for orthogonal reference directions, and φ is the measured deviation angle of the second scanning direction from its ideal reference. For small deviation angles (e.g., less than 10 degrees), a simplified equation set (A = A' - B·sin φ; B = B') can be used, enabling even more efficient computation. Based on the results of this processing, the control unit generates and sends a control signal (e.g., adjusting currents) to the deflector. This signal adjusts one or both scanning directions so that they fall within a required orthogonality limit—for instance, bringing the angle between them to between 89 and 91 degrees, effectively making them substantially orthogonal. The implementation can be via a dedicated analog circuit, a field-programmable gate array (FPGA), or a processor-based digital system, allowing for fast, real-time correction without the need for specimen-specific calibration.
Advantages
- Enables real-time, dynamic correction of scanning orthogonality, minimizing image distortion during operation.
- Improves image quality and measurement accuracy in electron microscopes and inspection apparatuses.
- Compensates for inherent manufacturing and assembly tolerances in deflector systems.
- Eliminates or reduces the need for complex, sample-specific calibration routines.
- Offers computational efficiency, especially when using simplified equations for small deviation angles.
- Can be implemented through various hardware (analog circuit, FPGA, processor), providing flexibility in system design.
- Enhances the reliability and consistency of imaging outputs.
Applications
- Scanning Electron Microscopes (SEMs) for materials science, life sciences, and failure analysis.
- Semiconductor inspection and metrology tools for critical dimension measurement and defect review.
- Electron beam-based lithography systems where precise beam positioning is crucial.
- Any imaging or inspection apparatus utilizing a scanned electron beam and a deflector system.
