Beam Blanker Moiré Imaging for Fast STEM Strain Mapping
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Solution Overview
Problem
Current scanning transmission electron microscopy (STEM) techniques face challenges in achieving high-contrast moiré patterns at fast scan speeds, leading to low moiré-fringe contrast and long acquisition times, which are inadequate for precise strain measurement in semiconductor devices.
Innovation Solution
Incorporating a fast pre-sample beam blanker in the electron-beam column to gate the electron beam during STEM imaging, allowing for high-contrast moiré pattern acquisition with dwell times smaller than 5 μs/pixel and enabling accurate optical alignment while reducing irradiation dose.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If fast scan speeds are used in STEM imaging, then productivity is improved, but moiré-fringe contrast deteriorates
Solution Approach 1:
The beam blanker is driven by a periodic drive signal that modulates the electron beam intensity at a specific frequency. This periodic gating of the beam during scanning creates the moiré pattern by selectively illuminating regions based on the interference between the beam modulation and the crystal lattice periodicity, enabling high-contrast imaging even at fast scan speeds
Solution Approach 2:
The invention changes the temporal parameter of the electron beam by modulating its intensity through the beam blanker. By adjusting the drive signal frequency and the beam gating timing, the system optimizes the contrast of moiré fringes while maintaining fast scanning, effectively decoupling scan speed from image quality
2Productivity
If dwell time is reduced to enable fast scanning, then productivity is improved, but measurement precision deteriorates
Solution Approach 1:
The periodic modulation of the electron beam intensity creates moiré patterns that encode strain information in the fringe spacing and orientation. This allows strain measurement precision to be maintained even with reduced dwell times, as the moiré effect amplifies the strain signal across multiple pixels rather than relying on long integration times at each pixel
Solution Approach 2:
The moiré pattern creates a magnified copy of the strain field information in the form of interference fringes. The fringe pattern serves as a visual and quantitative representation of the underlying lattice distortion, allowing precise strain measurement to be derived from the pattern geometry rather than direct atomic-resolution imaging
3Object-affected harmful factors
If beam blanking is applied to reduce irradiation dose, then harmful factors are reduced, but image quality may deteriorate
Solution Approach 1:
The beam blanker periodically gates the electron beam, creating intervals of beam presence and absence. This periodic action reduces the total irradiation dose by limiting beam exposure only to when moiré signal acquisition is needed, while the moiré pattern formation efficiency ensures that adequate signal is obtained within these reduced exposure windows
Solution Approach 2:
The beam blanking is synchronized with the scanning process to ensure continuous acquisition of useful moiré signal throughout the scan. By maintaining beam presence during the critical signal acquisition phases and eliminating it during non-productive periods, the system achieves both dose reduction and maintained image quality
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enables fast and accurate acquisition of high-contrast moiré patterns, improving strain mapping precision and reducing irradiation dose, thus meeting the demands of semiconductor industry requirements for rapid and precise strain measurement.
Implementation Method 1
a beam blanker configured to gate the electron beam in response to a drive signal
Implementation Method 2
an electron detector configured to measure a flux of transmitted or scattered electrons having interacted with the sample
Implementation Method 3
Moiré is a general phenomenon where two closely related patterns create a more slowly varying pattern. In scanning transmission electron microscopy (STEM), moiré patterns can be created when a sampling interval of the microscope is relatively close to a periodicity of the specimen's crystal lattice
Data Source
AI summary
In some embodiments, a scientific instrument includes an electron-beam column configured to scan an electron beam across a sample. The electron-beam column includes a beam blanker configured to gate the electron beam in response to a drive signal. The scientific instrument also includes an electron detector configured to measure a flux of transmitted or scattered electrons having interacted with the sample and an electronic controller configured to acquire an image of the sample using values of the flux measured with the electron detector for a plurality of electron-beam scan locations. The electronic controller is further configured to cause the drive signal to have a gating frequency at which the image has a moiré pattern therein.