Our work

Super-resolution Lenses

The technology roadmaps for the semiconductor industry aim  at constantly shrinking device dimensions to achieve higher  level of performance through miniaturisation and integration. Furthermore, device geometries become increasingly complex,  and 3D scaling is anticipated to introduce new functionalities and to make optimum use of the available space. The trends are such that conventional technologies currently used for production and control  are reaching their physical limits.  In order to sustain such a development, technological breakthroughs and new disruptive technological platforms are required.
Experimental set-up for the characterisation of the lens beam spot.

The objective of the Beyond Conventional Optics (BCO) project is to develop super resolutions optical sensors for novel metrology instruments. Such instruments aim at providing the level of resolution required by the current trends of the semiconductor industry, but without the typical problems of current systems based on Scanning Electron Microscopes (SEMs), or Atomic Force Microscopes (AFMs).

In the last decades, metamaterials and metasurfaces have received an enormous attention, proving their applicability in many application fields and in an extremely wide range of frequencies (from acoustic waves up to optical frequencies). By artificially engineering the electromagnetic properties of these materials, it is possible to achieve unprecedented electromagnetic characteristics. These unconventional material properties (e.g. negative refractive indexes, ad-hoc anisotropy profiles) can lead to major breakthroughs in sensing, imaging and miniaturisation. In particular, optical metamaterials emerged in the last years as a disruptive technology platform for the development of innovative optical components and systems. These structures, in fact offer not only the possibility to realise optical systems able  to go well beyond the Abbe-Rayleigh diffraction limit of conventional optics,  but offer also full simultaneous polarisation and phase control, allowing not only the replacement of existing bulk optical components with ultra-thin surfaces, but also the realisation of novel components which do not actually have a bulk conventional counterpart.

Competitive advantage

Current state-of-the-art metrology systems are based on SEMs or AFMs. These systems can indeed provide very high resolutions, better than conventional optical systems, but they have also limitations and disadvantages. SEMs have the well-known disadvantages of large measurement times and potential radiation damages of the photo-resist layer. The main issues for AFMs are the frequent damages of the cantilever tips during the scanning, which require an expensive maintenance, but also the possible damages of the surface under inspection, due to undesired contacts between the  tip and the surface.

An optical system would allow faster measurements with a less intrusive approach, avoiding the above mentioned damages to the structure under test. Nevertheless, a system based on conventional optics would be limited in resolution by the above mentioned diffraction limit. The development of novel metamaterial based lenses, capable to go beyond such a limit, would allow a major step forward towards the realisation of this new generation of metrology instruments.

Figure 1. a. Interferometric image of the SOL demonstrator; b. image of the SOL obtained by retro illuminating the lens with withe light; Experimental set-up for the characterisation of the lens beam spot.

Application areas

For super-resolution lenses the main application areas are metrology and microscopy. In addition, optical meta-surfaces in general can be used to realise very thin flat surfaces for any kind of light wave front manipulation e.g. flat lenses, polarisers, polarisation scramblers, Computer Generated Holograms, filters. These components can find applications  in space instruments, in miniaturised spectrometer and imaging systems, and more in general, for coupling and controlling light in integrated optics.

Proven specifications

A SOL (Super Oscillatory Lens) demonstrator has been manufactured and tested (see Fig. 1). The experimental results have demonstrated the capability of the SOL to focus the light in spots with beamwidths smaller than the diffraction limit. In particular, the developed demonstrator has achieved a spot of 0.33 λ beamwidth (λ = 634 nm), at a focal distance of about 5.4 µm.

A flat lens demonstrator, based on dielectric resonators (see Fig. 2), has been manufactured and tested at λ = 634 nm. The designed flat lens is characterised by  a numerical aperture of NA = 0.56.

Target specifications

The ultimate goals is to develop this type of lenses at Deep Ultra Violet (DUV) wave-lengths, achieving a resolution in the range from 40 nm down to better than 10 nm.

Figure 2. Metasurface flat lens based on Silicon dielectric resonators embedded in glass. a. SEM image of the array of dielectric resonators; b. detail of the dielectric resonators.
Figure 3. Experimental characterisation of a wafer containing several flat lenses.
Contact

Dr Stefan Bäumer

  • optics
  • System architecture
  • Optical design
  • Optical Metrology
  • Micro- optics
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