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Optical characterization of sub-wavelength structures

Non-imaging procedure allows the size of grating structures to be measured far below the optical resolution limit

PTB-News 3.2015
Especially interesting for

the semiconductor industry

the optical industry

fundamental research

A measurement method developed at PTB offers the possibility of measuring the size of grating structures that are clearly smaller than the optical wavelength used. Such structures play an important role, for example, in developing novel, more efficient lens systems and in manufacturing modern nanoelectronic components lithographically; however, they cannot be measured by means of conventional optical procedures. A fast procedure to measure the dimensions of sub-wavelength structures is now available.

Cross section profile (red curve) of a silicon grating structure, determined optically. For com-parison, in the background: cross section profile of an identically manufactured specimen, measured by means of electron microscopy; the grating had to be cut perpen-dicular to the grating lines to take this picture.

Comparison of the measured reflection curves (dots) with the best fit model computation (lines) at varying angle of incidence. The graphics illustrate the measurement configurations used (plane of incidence relative to the grating orientation and to the polarization of the light).

In numerous fields of natural sciences and technology, ever smaller structures are being used whose dimensions are clearly below 100 nm. Hereby, it is important, for example, for the operability of a nanoelectronic component to be able to measure the geometric dimensions of these structures very accurately and efficiently, without influencing, contaminating – or even destroying – these. Microscopy and classical scatterometry have, to date, been used successfully for this purpose.

In the case of scatterometry, the grating geometries are reconstructed based on the measurement of the intensity distribution of the different diffraction orders. In the case of dimensions far below the optical (Abbe's) resolution limit, these methods can, however, no longer be used, since in the case of grating structures with too short a periodicity, no higher diffraction orders occur. This is the reason why alternative optical measurement procedures are urgently needed – especially in the semiconductor industry – to develop and manufacture improved electronic components.

PTB has shown that even grating-like structures with periods down to a few 10 nm can be reliably characterized by means of modified optical scatterometry procedures at an illumination wavelength of 266 nm. Hereby, the remaining “diffraction order” – the specular reflection – is measured in various configurations as a function of the angle of incidence. From the measured data, it is possible to reconstruct the structure geometries very exactly. One can even detect small details of the edge profiles (such as rounded edges) which are only a few nanometers in size.

Thus, a highly efficient optical procedure is now available to measure and calibrate the dimensions of sub-wavelength structures as well as linewidth standards. This will be used, for example, in the semiconductor industry in the field of wafer metrology where it may be applied, especially for process control during manufacturing.


Bernd Bodermann
Department 4.2 Imaging and Wave Optics
Phone: +49 (0)531 592-4222
E-mail:  bernd.bodermann(at)ptb.de


Scientific publication

J. Endres, M. Wurm, J. Probst, M. Schoengen, A. Diener, B. Bodermann: Determination of sub-wavelength grating structure geometry by optical scatterometry (submitted to Optics Letters)