Through-Focus Scanning Optical Microscopy Applications

Through-Focus Scanning Optical Microscopy (TSOM) is a non-destructive optical metrology technique that harnesses multiple through-focus images to resolve three-dimensional features of micro- and nanoscale structures. By collecting a series of defocused images across a defined axial range, TSOM captures four-dimensional optical data—two spatial dimensions, focal position and intensity—which can be analysed to determine critical dimensions, depths and shapes with nanometre-scale sensitivity. Operating on a conventional optical microscope platform, TSOM delivers high throughput and cost-effectiveness without the need for complex hardware modifications. Recent innovations have extended its capabilities through adaptive optics to eliminate lateral instabilities, machine-learning algorithms to enhance accuracy and model-less approaches to simplify data interpretation. TSOM is increasingly employed in semiconductor process control, MEMS fabrication and defect inspection, offering rapid, in situ assessment of high-aspect-ratio trenches, nanoparticles and surface anomalies. Its ability to monitor three-dimensional profiles in real time positions TSOM as a versatile tool for both research and industrial quality assurance.

Research from Nature Portfolio

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Research from all publishers

Recent advancements in TSOM have emphasised integration with machine learning and adaptive optics. A 2022 study introduced a convolutional neural network-based TSOM method to measure high-aspect-ratio trenches in MEMS devices, successfully characterising individual silicon trenches up to 30 µm in width and 440 µm in depth with standard deviations on the order of 100 nm. This approach leverages deep learning to extract detailed intensity features from TSOM image stacks, enhancing dimensional accuracy for large-scale structures.

A 2021 investigation proposed a model-less TSOM protocol for defect height estimation in organic light-emitting diode (OLED) panels. Dispensing with a pre-built reference library, this method defined new TSOM parameters—height, area and volume—and demonstrated a linear correlation with atomic force microscopy measurements across defect heights from 140 to 950 nm, achieving an uncertainty of approximately 20 nm.

In 2020, a motion-free TSOM system employing a deformable mirror was reported to overcome lateral drift during through-focus scanning. By deforming the mirror surface rather than moving mechanical stages, the instrument achieved sub-4 nm accuracy for critical dimensions in the 60–120 nm range, a scanning update rate of 4 kHz and a ±25 µm focus range. This motion-free design significantly improves stability and repeatability for high-resolution metrology.

Through-Focus Scanning Optical Microscopy Applications publication trend

The graph below shows the total number of articles in through-focus scanning optical microscopy applications across all publications each year (not limited to Nature Index journals).

Technical terms

Through-Focus Scanning Optical Microscopy (TSOM): An optical metrology technique that acquires sequences of defocused images to reconstruct three-dimensional features of micro- and nanoscale structures.

High-Aspect-Ratio Structures: Features whose height significantly exceeds their width, common in MEMS and advanced semiconductor devices.

Critical Dimension (CD): The smallest feature size of a device, whose precise measurement is vital for process control and yield.

Model-Less TSOM: A TSOM variant that eliminates the need for a reference image database by directly correlating optical parameters with structural dimensions.

Convolutional Neural Network (CNN): A machine-learning architecture designed to analyse and extract spatial features from image data, improving TSOM measurement accuracy.

References

1. MEMS High Aspect Ratio Trench Three-Dimensional Measurement Using Through-Focus Scanning Optical Microscopy and Deep Learning Method. Applied Sciences (2022).
2. Machine-learning models for analyzing TSOM images of nanostructures.. Optics Express (2019).
3. Motion-free TSOM using a deformable mirror.. Optics Express (2020).
4. Resolving three-dimensional shape of sub-50 nm wide lines with nanometer-scale sensitivity using conventional optical microscopes. Applied Physics Letters (2014).
5. Feasibility study on 3-D shape analysis of high-aspect-ratio features using through-focus scanning optical microscopy.. Optics Express (2016).
6. Method for optical inspection of nanoscale objects based upon analysis of their defocused images and features of its practical implementation.. Optics Express (2013).
7. Defect height estimation via model-less TSOM under optical resolution.. Optics Express (2021).