The low voltage (usually below 1kV) imaging of scanning electron microscopy has become a key paradigm for observing sensitive materials such as biological samples, organic semiconductors, two-dimensional materials, lithium containing battery materials, etc. The core challenge lies in how to maintain sufficient resolution and signal strength while reducing electron beam damage? Modern SEM achieves a delicate balance through multi-dimensional collaborative innovation.
1、 The inherent contradiction and solution path between resolution and damage
Traditional high-voltage (such as 10-30kV) electron beams have deep penetration, small beam spot, and high resolution, but they can cause serious charging, thermal damage, and radiation chemical damage. Although low voltage can limit the interaction volume within a few nanometers of the sample surface, greatly reducing deep damage and charging effects, it also faces two major challenges:
Increased color difference: The electron wavelength becomes longer, the lens color difference significantly deteriorates, and the beam spot is difficult to converge.
Signal attenuation: There is a peak in the secondary electron yield in the low voltage region, but the overall signal strength decreases and the signal-to-noise ratio decreases.
2、 The four technological pillars for achieving balance
1. Innovation in electronic optical systems
Monochromator/Deceleration Field Technology: Using a monochromator or sample stage to apply negative bias voltage (deceleration mode) inside the tube, electrons are transmitted at high voltage to obtain small beam spots, and then decelerated to low landing voltage when approaching the sample, balancing small beam spot size and low landing energy.
Composite objective design: Adopting immersion or hybrid objectives to enhance the convergence ability of electron beams under low voltage, pushing the low voltage resolution to below 1nm.
2. Detector and signal extraction optimization
In situ detection system: Placing secondary electron detectors (such as Through the Lens Detector, TLD) within the optical axis greatly improves the collection efficiency of low-energy secondary electrons.
Multi signal collaboration and spectrum analysis: synchronously collect signals such as secondary electrons, backscattered electrons, and cathodoluminescence, and utilize complementary responses of different signals to sample characteristics to obtain maximum information at low doses.
3. Intelligent imaging and dose control
Conditional optimization algorithm: The software automatically recommends the optimal voltage and beam current combination based on the sample material.
Adaptive scanning and region segmentation: Ultra low dose rapid scanning is used for sensitive areas, and high-resolution imaging is performed for regions of interest to achieve "on-demand supply" of dose allocation.
3、 The Art of Balance: Resolution Damage Balance Curve
Modern low-voltage SEM has successfully shifted the optimal equilibrium point to the lower left through the aforementioned techniques, achieving better resolution than traditional modes at lower voltages and doses. The core concept has shifted from "resolution" to "optimizing information acquisition efficiency within acceptable damage thresholds".
Conclusion
Low voltage SEM imaging is no longer a compromise on the performance degradation of high-voltage modes, but a systematic reconstruction through electron optics, detectors, sample preparation, and intelligent algorithms, forming an active and precise control paradigm for electron sample interactions. It makes it possible to observe and understand the true state of materials that are extremely sensitive to electron beams at the atomic/molecular scale, promoting cutting-edge research in materials science, life science, and semiconductor fields.