J. Eur. Opt. Society-Rapid Publ. 21, 7( 2025) 57
denoted as optical critical dimension( OCD) metrology, have proven astonishing capabilities in dimensional nanometrology down to the few-Nanometre feature size regime. However, they are limited to periodic structures, require relatively large target fields and provide only mean integral measurement values for structure parameters [ 3, 4 ]. For several applications like field effect transistors( FET), local defect characterisation or to characterise local parameter variations, imaging metrology is indispensable([ 1, 2 ], and references therein).
Optical microscopy is a metrology technique to provide physical information about samples with sufficient contrast. It is the method of choice for spatial measurements of individual structures. However, conventional optical microscopy suffers from a fundamental resolution limit due to diffraction [ 5 ]. Therefore, several methods have been developed and applied to increase the sensitivity to specific structural features and the measurement capabilities of optical microscopy for nanoscale structures and features: Different types of interference microscopy is adding phase information to the microscope image, which can enhance the sensitivity and signal-to-noise ratio to specific nanoscale features [ 6 ]. Phase information without the need of reference fields can also be obtained by detection of different afocal images in Through Focus Microscopy( TFM). Besides phase information, Tomographic Diffractive Microscopy( TDM) is adding additional spatial frequency information by artificial aperture extension. This is achieved by the application of different combinations of illumination and detection schemes, which can provide even higher contrast and resolution images [ 7 ]. However, TFM and, to a significantly larger degree, TDM require numerical calculations to provide( purely computational) high quality images of the nanostructures under test. Another interesting approach is Alternating Grazing Incidence Darkfield Microscopy( AGID), which can extend the measurement capability of the widths of individual structures down to 1 / 10 of the wavelength used [ 8, 9 ].
Microscopy methods such as scatterfield microscopy [ 10 ] or through-focus microscopy [ 11 ] have been developed and tested for quantitative dimensional metrology on( deep) sub-wavelength structures. These do not belong to the classical super resolution techniques but show astonishing sensitivity regarding sub-resolution features, allowing for CD measurements down to a few 10 nm.
In Section 2, we present a through-focus microscopy method and a corresponding metrology system developed at the Danish National Metrology Institute( DFM), which can be seen as a sophisticated reference system of what can be achieved with“ classical”( i. e. non-super-resolution) microscopic measurement systems.
In recent decades, many super-resolution microscopy( SRM) techniques that exceed the lateral and / or axial resolution limit have been developed [ 12, 13 ]. Focusing on far-field methods, SRM methods can roughly be classified in four different approaches:
1. Far-field techniques derived from objective modifications or metamaterial based super-lenses which in principle can detect high spatial frequency information from label-free samples [ 13 ].
2. Non-uniform illumination / collection, including linear and non-linear structured illumination microscopy( SIM) [ 14 – 17 ].
3. Deterministic functional pump / probe techniques such as stimulated emission depletion( STED) microscopy [ 18, 19 ] based on switch-able detection channels( mainly via molecular state population modifications) and non-linear interactions.
4. Stochastic functional opto-numerical SRM methods such as photo-activated localization microscopy( PALM) [ 20 ] or stochastic optical reconstruction microscopy( STORM) [ 21 ].
Many of these methods offer an excellent optical resolution down to a few nm. However, while the objective modification or super-lens methods are often very limited in practical applications and in some cases convincing experiment verifications are missing the other SRM methods are mostly based on fluorescence microscopy. They typically require specific sample preparations with fluorescence markers( labels) and are therefore mainly suitable for biological samples, but usually not accessible for inorganic applications such as semiconductor metrology.
In recent years, in fact also many very interesting approaches have been discussed and partly also demonstrated experimentally to provide label-free SRM. An excellent overview is given by Astratov [ 13 ]. However, most of this research has focussed on qualitative imaging, often still on biological samples, and fluorescence microscopy and many methods are limited to very specific applications or materials.
What still is missing for applications in quantitative nanoscale optical metrology is a universal label-free SRM suitable for( dimensional) nanometrology in technical applications and inorganic materials, which has the potential to complement for example OCD metrology in semiconductor manufacturing.
Therefore, in the context of a larger European research project( European Metrology Programme for Innovation and Research) [ 22 ], several partners have been cooperating to investigate potential options and develop promising methods to reach this goal [ 23 ]. This article is giving an overview about different methods which have been investigated and developed within our consortium to achieve such label free SRM options or at least SRM methods with alternative labelling schemes acceptable and suitable for dimensional metrology and technical applications.
Different routes and approaches for label-free or alternative-label SRM( Sect. 3) have been analysed theoretically.
In Section 4, two promising methods are described in more detail and with first experimental results: Wide-field Imaging with Super-resolution Enabled signal( WISER, Sect. 4.2) and a STED-like pump-probe method for NV centres in artificial diamond as an example for STED microscopy of inorganic materials with suitable four-level systems( Sect. 4.3).
In Section 5 we shortly discuss which, either from our research or beyond, are from our perspective the most promising routes to a universal microscopic-optical