Resolution enhancement methods in optical microscopy for dimensional optical metrology
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-01-01 |
| Journal | Journal of the European Optical Society Rapid Publications |
| Authors | Bernd Bodermann, Mohammad Nouri, P. Olivero, Stefanie Kroker, Ivano Ruo Berchera |
| Citations | 1 |
| Analysis | Full AI Review Included |
Resolution Enhancement Methods in Optical Microscopy for Dimensional Optical Metrology
Section titled âResolution Enhancement Methods in Optical Microscopy for Dimensional Optical MetrologyâExecutive Summary
Section titled âExecutive Summaryâ- Core Objective: The research investigates and develops universal, label-free Super-Resolution Microscopy (SRM) techniques suitable for accurate and traceable dimensional nanometrology, particularly for inorganic and semiconductor structures.
- Classical Enhancement: Through-Focus Confocal Microscopy (TFCM) simulations demonstrated enhanced nanoscale sensitivity and improved axial resolution by extracting phase information from afocal images and applying deconvolution.
- Raman-Based SRM (WISERS): Wide-field Imaging with Super-resolution Enabled by Raman Scattering (WISERS) successfully combined structured illumination with hyperspectral imaging, achieving a 1.6x resolution improvement over the diffraction limit on implanted diamond samples.
- Alternative Labeling (NV Centers): Nitrogen-Vacancy (NV) centers embedded in artificial diamond were validated as a suitable, non-organic, four-level system for STED-like pump/probe SRM methods.
- STED Performance: STED imaging on NV center substrates provided significantly better contrast and clear delineation of feature borders (e.g., cross-shaped patterns) compared to conventional confocal microscopy.
- Traceability Focus: The work emphasizes the potential of NV centers in nanodiamonds to serve as stable, traceable reference standards for calibrating and quantifying the performance of future SRM metrology tools.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| TFCM Wavelength | 405 | nm | Simulation light source |
| TFCM Objective NA | 0.95 | - | Numerical Aperture used in simulation |
| Si Grating Pitch (TFCM) | 250, 300, 400 | nm | Test structure dimensions |
| NV Nanodiamond Size | 125, 180 | nm | Median size of raw material |
| Proton Irradiation Energy | 2 | MeV | H+ ion beam for vacancy creation |
| Irradiation Fluence | 2 x 1016 | cm-2 | Optimal fluence for NV concentration |
| High-Temp Annealing | 800 | °C | 2 h in N2 flow (NV center formation) |
| Surface Purification | 500 | °C | 8 h or 12 h in air (thermal oxidation) |
| WISERS Resolution Gain | 1.6 | x | Improvement over diffraction limit achieved |
| WISERS Structured Illumination | 1200 | nm | Line spacing projected onto sample plane |
| STED Excitation Wavelength | 561 | nm | Pulsed laser line |
| STED Depletion Wavelength | 775 | nm | Pulsed laser line (donut beam) |
| STED Objective NA | 1.4 | - | Oil immersion objective |
| Maximum STED Power | 3 | W | Maximum pulsed STED laser power |
| Optimized STED Power | 5 | % | Percentage of max power used (to prevent damage) |
| Optimized Excitation Power | 2 | % | Percentage of max power used (to prevent damage) |
| STED Cross Pattern Width | 2.1 | ”m | Size of fabricated test structure |
| NV Center ZPL Peaks | 575, 637 | nm | Zero-Phonon Lines for NV0 and NV- states |
Key Methodologies
Section titled âKey Methodologiesâ- Through-Focus Confocal Microscopy (TFCM): A digital twin approach was used, involving rigorous modeling of light-matter interaction. Simulated 3D image stacks were generated by calculating far-field angular scattering using the Fourier modal method, followed by deconvolution with the objectiveâs Point Spread Function (PSF) to enhance z-resolution.
- NV Center Sample Preparation: Diamond nanocrystals (125 nm and 180 nm) were drop-casted onto a silicon wafer substrate, forming a layer approximately 30 ”m thick.
- Vacancy Creation: Samples were irradiated using a 2 MeV H+ ion beam at a fluence of 2 x 1016 cm-2 to introduce vacancies into the diamond lattice.
- NV Center Formation: Thermal annealing was performed at 800 °C for 2 hours in N2 flow, promoting the coupling of generated vacancies with nitrogen impurities to form NV centers.
- Surface Treatment: Surface purification was achieved via thermal oxidation at 500 °C (8 hours or 12 hours in air) to remove disordered carbon phases (sp2/sp3) and stabilize photoluminescence.
- Wide-field Imaging with Super-resolution Enabled by Raman Scattering (WISERS): This technique utilized structured illumination (1200 nm line spacing) projected onto the sample at three phases and three orientations, combined with hyperspectral imaging to extract high spatial frequency information from the Raman signal (~1410 cm-1).
- STED Microscopy: A commercial STED system employed 561 nm (excitation) and 775 nm (depletion) pulsed lasers, with a vortex phase plate generating a donut beam profile. Imaging parameters were optimized (e.g., 5% STED power, 2% excitation power) to maximize resolution while preventing sample damage.
Commercial Applications
Section titled âCommercial Applicationsâ- Semiconductor Manufacturing Metrology: Provides high-speed, non-destructive dimensional metrology for critical dimensions (CD), line widths, and overlay measurements, serving as an imaging complement to non-imaging Optical Critical Dimension (OCD) techniques.
- Nanophotonics and Optical Device Fabrication: Essential for accurate characterization and quality control of complex micro- and nano-scale structures (e.g., gratings, waveguides) where device functionality is highly dependent on precise geometry.
- Traceable Optical Standards: Utilization of NV centers in artificial nanodiamonds to create stable, high-photostability reference standards for the calibration and validation of advanced SRM systems, ensuring measurement traceability to SI units.
- Label-Free Industrial Inspection: Implementation of label-free SRM methods (like WISERS or SAX/SUSI variants) suitable for inorganic materials, enabling super-resolution imaging in environments (e.g., wafer manufacturing) where organic fluorescence markers are prohibited.
- Local Defect Analysis: Capability to characterize individual structures and local defects, which is crucial for R&D and process control, particularly for analyzing local parameter variations in advanced electronic devices.
View Original Abstract
In this paper, we discuss several enhancement approaches to increase the resolution and sensitivity of optical microscopy as a tool for dimensional nanometrology. Firstly, we discuss a newly developed through-focus microscopy technique providing additional phase information from the afocal images to increase the nanoscale sensitivity of classical microscopy. We also explore different routes to label-free or semiconductor compatible labelling super-resolution microscopy suitable for a broad range of technical applications. We present initial results from, a new wide-field super-resolution imaging technique enabled by Raman scattering. In addition, we discuss super-resolution imaging using NV centres in nano-diamonds as labels and their application in future reference standards.