Optical characterization of sub-wavelength structures
Non-imaging procedure allows the size of grating structures to be measured far below the optical resolution limit
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.
Contact
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)