Pulsed Charged Particle Beam Scanning Without Image Fringes
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Solution Overview
Problem
In charged particle beam apparatuses like electron microscopes, adjusting the illumination dose of the electron beam often requires changing the condenser lens excitation or aperture size, which complicates axis alignment and aberration correction, and can result in bright and dark fringes on the scan image due to beam switching.
Innovation Solution
A charged particle beam apparatus with a pulse mechanism and deflector that illuminates the specimen with pulses of the beam, controlled by a unit ensuring the dwell time (T) is a natural multiple (n × t) of the pulse cycle (t), preventing bright and dark fringes by maintaining constant illumination dose.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If the excitation of the condenser lens is changed to adjust illumination dose, then the illumination dose can be adjusted, but axis alignment and adjustment of the aberration correction apparatus are needed
Solution Approach 1:
The patent changes the control parameter from condenser lens excitation to pulse width modulation. By varying the pulse width (duration of beam emission) while keeping the condenser lens excitation constant, the illumination dose is adjusted without requiring realignment or readjustment of the aberration correction apparatus, thus resolving the technical contradiction.
Solution Approach 2:
The patent employs periodic pulsed emission of the charged particle beam instead of continuous emission. By controlling the duty cycle (ratio of pulse width to pulse period) of the periodic pulses, the illumination dose is precisely controlled while maintaining constant optical system parameters, eliminating the need for complex alignment adjustments.
2Illumination intensity
If the size of the aperture is changed to adjust illumination dose, then the illumination dose can be adjusted, but the aperture has to be adjusted and the convergence angle of the electron beam changes
Solution Approach 1:
The patent replaces aperture size adjustment with pulse width modulation as the control parameter. The aperture remains fixed at its optimal setting, and illumination dose is controlled by varying the duration of beam emission pulses, thereby avoiding the complexity of aperture adjustments and maintaining constant convergence angle.
3Illumination intensity
If the illumination dose is changed by conventional methods, then the illumination dose can be adjusted, but the conditions of the optical system change and the optical system has to be adjusted
Solution Approach 1:
The patent uses periodic pulsed beam emission with variable duty cycle to control illumination dose. This approach decouples dose control from optical system parameter changes, allowing dose adjustment without requiring any optical system adjustments, thereby significantly improving ease of operation.
4Object-affected harmful factors
If pulses of charged particle beam are used to illuminate the specimen, then specimen damage can be reduced by spreading beam emission, but bright and dark fringes may appear on the scan image if beam switching causes illumination dose variation
Solution Approach 1:
The patent employs periodic pulsed beam emission where the pulse width and pulse period are precisely controlled. By ensuring that the product of pulse width and scanning speed results in an integer number of pulses per pixel dwell time, uniform illumination is achieved across all pixels, eliminating bright and dark fringes while maintaining the specimen damage reduction benefits of pulsed operation.
Solution Approach 2:
The patent implements feedback control by synchronizing the pulse generation with the scanning deflector signals. The control unit monitors the scanning position and adjusts the pulse timing and width dynamically to ensure that each pixel receives an equal number of pulses, thereby preventing illumination non-uniformity and image artifacts.
Data Source
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AI summary
A charged particle beam apparatus (100) for scanning a specimen (S) with a charged particle beam and acquiring a scan image, the charged particle beam apparatus (100) including: an optical system (2) that includes a pulse mechanism (20) for illuminating the specimen (S) with pulses of the charged particle beam, and a deflector (40) that deflects the charged particle beam and scans the specimen (S) with the deflected charged particle beam; and a control unit (80) that controls the optical system (2), the control unit (80) controlling the optical system (2) so as to satisfy T = n × t (n is a natural number), where T represents a dwell time of the charged particle beam in each pixel of the scan image, and t represents a cycle of pulses of the charged particle beam.