Metrology runs side by side with scientific and technologic progress. Manipulating matter at the nanoscale, in a scientifically reliable and predictable way, urges the metrology community to provide the appropriate metrology solutions. Such novel or improved metrology solutions then, in turn, stimulate again the next technological and scientific developments.
In this context, optical measurement methods play an instrumental role as the development in four out of the six Key Enabling Technologies (Nanotechnology, Micro-nanoelectronics, Photonics and Advanced materials) identified by the EU is strongly underpinned by optics-based measurement methods. The importance of holding a dominant scientific and technologic position in these sectors is clear if one considers that altogether they are worth more than € 800 billion on the global market and Photonics and Micro-nanoelectronics already employs more than 400,000 people in Europe.
Despite the many advantages of optical systems (speed, non-invasiveness, high-precision, moderate investments involved, integrability) the operational spatial resolution attainable in classical optical metrology is still essentially limited by the wavelength used for the optical probe. Therefore, novel and robust metrology solutions are needed that maintain all the recognised benefits of optical methods while substantially overcoming the current limitations.
Challenges and Approaches:
Rayleigh's criterion for assessing the resolution limit of optical systems, no matter how complex and which wavelength λ they employ, states that the smallest detail that can be resolved in an object is of the order of 0.6 λ/NA, where NA is the numerical aperture of the system. For systems operating in air, NA is a positive number smaller than 1, which results in a resolution of about half of the wavelength. While different criteria can be adopted to quantify such limit, they all agree on the fact that the wavelength is the limiting factor.
This is a severe limitation for optical methods, especially in view of the growing impact of nanostructured devices and materials in current and expected technological developments in our society. For specific applications, smart alternatives have been found e.g. for the imaging of biological samples, they can be stained by fluorophores, and hence time-resolved fluorescent methods (such as Stimulated Emission Depletion (STED), Stochastic Optical Reconstruction Microscopy (STORM), Photo Activated Localisation Microscopy (PALM) etc.) have flourished in the last years, with proven resolutions at few tens of nanometres level. Unfortunately, such techniques are of no use for inorganic materials in the semiconductor sector, where no natural fluorescent response exists nor is a contamination with external markers allowed. Additionally, the quantification of the measurements results is complex and currently rarely addressed.
One way to approach this issue is by progressively moving to smaller wavelengths. Techniques such as Extreme Ultra Violet (EUV) and X-ray microscopy are examples and these new spectral windows can help resolve smaller details, although with the disadvantage of large instrumental complexity and investments levels. Other disadvantages also include the increased invasiveness, shorter penetration depths (about 100 nm, which does not allow investigation of deep objects), the need to use vacuum conditions (because of the oxygen absorption bands in that spectral range) and a reduction in the amount of information on other parameters, such as refractive index, absorption, electronic and molecular transitions.
Another approach to beat the resolution limit without scaling down the wavelength of the source is to make use of all the prior information available on the target. In this approach, the target is expressed in terms of a set of physical and geometrical parameters. Then the actual measurements are combined with rigorous simulations performed on the idealised target model. The most representative example of non-imaging techniques based on this approach is Optical Scatterometry, which has shown deep subwavelength metrology capabilities on special targets of industrial relevance, such as diffraction gratings or contact holes, and it has become the current metrology standard de facto within the nanometrology lines of the semiconductor industry.
- To develop stable and reliable methods to achieve deep sub-wavelength spatial resolution by exploiting higher-order (beyond Born’s regime) probe-target interactions. To design metamaterials-based structures that can enhance such interaction and bring it to detectable levels for a large class of targets, not only strong scatterers. The goal is to reach, for a well-defined class of samples (e.g. diffraction gratings on silicon substrates, isolated nanoparticles on both opaque and transparent substrates), a traceable spatial resolution at the λ/10 level and sub-nanometre uncertainty, with λ being the wavelength of the light probe
- To exploit invariant topological structures in electromagnetic fields, in their polarisation, amplitude and phase distributions, and map how such topological information transforms after interacting with matter, especially in the case of nanostructured materials endowed with specific geometric symmetries (e.g. diffraction gratings, spiral geometries, bio-inspired circularly-symmetric objects). The ultimate goal is to implement spectroscopy-like measurement concepts, leading to robust and high-precision dimensional and physical measurement results
- To realise and demonstrate near-field techniques to measure deep sub-wavelength gratings down to the regime << l/10 which allow accurate and traceable optical procedures to characterise nanostructured optical components and to measure effective optical material parameters. In addition to link such near-fields methods to far-field optical methods of specific applied interest
- To apply sub-shot noise quantum technologies to optical systems, addressing both low and high Numerical Aperture (NA) systems. To realise input fields with spatially-entangled optical channels and to map their coupling with the geometry of nano-targets. The potential of quantum metrology in optical systems will be explored through spatial modes entanglement and its integration into existing optical systems. The aim is to find a natural link with the exploitation of topologic information in classical fields, as discussed hereinabove
- To facilitate the take up of the technology developed in the project by the end users.