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Light microscope measures nanostructures

PTB has developed two procedures that enable detecting structures as small as one hundred nanometres with a light microscope, thus well below the classical resolution limit.

Many fields of natural sciences and technology are using ever smaller structures with dimensions in the micro- to nanometre range. Therefore, there is a need to determine the geometric lengths of these structures, for instance, to assess the functionality of microelectronic components. Light microscopic measurements are simple, fast and inexpensive. However, if the dimensions of the structures to be measured are of the same size as the optical wavelength applied, then “proximity effects” pose a problem. In that case diffraction patterns superimpose and interfere with the structural features.

At PTB it has been demonstrated that these effects can be significantly reduced by illuminating structures under grazing incidence and by changing the direction of the incident light. By illuminating with a laser of appropriate polarisation in one direction light is primarily scattered by structural edges facing the direction of the impinging light. By changing the direction of the light one can selectively switch between diffraction patterns of the rising and falling edges and one can measure them separately in a time-resolved mode.

Based on this principle two (dark field-) microscopic methods have been developed. Both procedures allow highly precise measurements of structures well below the classic resolution limit for light microscopy. In due course the measurement uncertainties could be reduced from 50 nm to 20 nm.

In the first method, AGID-microscopy (alternating grazing incidence dark field microscopy), the structure to be investigated is illuminated by incident light (reflection mode), whereas in the second method, FIRM (frustrated total internal reflection microscopy), the illumination occurs in transmission. To achieve this special optics had to be realized to couple the light into the substrate of the sample.

Both methods have already been picked up by industry and are to be applied in the field of inspection systems for photo-masks in the semiconductor industry as well as to characterise structure sizes in nano- and microparticles.

At the time being the technology is under further development within a joint project with an industrial partner. Goal of the project is to provide users improved measuring systems that will allow linear measurements of structural widths down to 100 nm using light of a wavelength of 375 nm. Compared with conventional bright-field microscopes with the same wavelength this constitutes an improvement by a factor of two.

Contact at PTB:

Phone: +49-531-592-0