A novel scatterometer for the dimensional characterisation of nanostructured surfaces has been developed and set up at PTB. It is used in particular in the deep ultraviolet wavelength range (DUV).
Novel DUV scatterometer ready for operation
PTB News 1/2007 (432 kB) PTB News 1/2007, English edition, Issue April 2007
By means of a scatterometer, the diffracted or scattered radiation is analysed on a sample without employing imaging optics (objectives). The measurements are in principle not limited by the resolution limit of imaging methods which, in optical microscopy, lies in the order of magnitude of half a wavelength. Thus, scatterometry is particularly suited to the quantitative determination of the cross sectional profiles of periodically nanostructured surfaces.
The new scatterometer was so designed that as many characteristics as possible of the scattered radiation are detected. It enables the highly accurate measurement of the radiant power and the polarisation condition as well as the direction of propaga-tion (diffraction angle). All scatterometric quantities of reflectometry, ellipsometry and diffractometry which are dependent on the angle of incidence can be measured with the new instrument. The complex evaluation allows the quantitative characterisation of the nanostructured surfaces also far below the resolution limit of optically imaging methods.
As radiation source, a titanium-sapphire laser system with frequency multiplication is used. In addition to the tunable fundamental wavelength of between 772 nm and 840 nm it is possible to access the 2nd, 3rd and 4th harmonic. In the DUV, there is thus a usable variable wavelength range of 193 nm to 210 nm available.
For the important lithography wavelength of 193 nm, the PTB can thus offer additionally so-called “at-wavelength-metrology”. This comprises, among other things, polarisation-/depolarisation- and transmission-measurements (Müller-Matrix-measurements) on structured and unstructured samples. As a result of the very good agreement of first polarisation measurements with simulation calculations, the design parameters of a nanostructure sample could be confirmed.
Thanks to this highly automatic system, the PTB has now available a measuring device, unique worldwide, for the metrology of photolithographic masks of current and future technology generations.