Standard for Raman microscopy

In Raman spectroscopy, characteristic vibrations in molecules are excited by means of laser irradiation. The stray light contains redshifted wavelengths (Stokes lines) which are measured by means of a spectrometer and are used to identify the molecules. Confocal Raman spectrometry, as a spatially resolved surface analysis method, has meanwhile become widely used in chemical analysis as well as in quality and process checks. It is a non-invasive imaging procedure in which the sample surface is scanned point by point with a spatial resolution better than 1 µm. Hereby, each measurement point is attributed a full Raman spectrum, which results in a 2D representation of the surface composition (“Raman mapping”) when individual, substance-specific bands are analyzed. If the increment and the number of measurement points enter into the analysis, then parameters such as the domain size or the degree of coverage of the detected components can be determined quantitatively.

A fundamental aspect of spatially resolved Raman microscopy is the combination of chemical and dimensional metrology. To ensure the traceability of the measurement results to the International System of Units (SI), both a chemical standard and a dimensional standard are therefore required. Whereas the reference spectra of numerous substances can either be retrieved from databases or can be generated by means of reference materials, there had, to date, been no dimensional standard adapted to the requirements of Raman microscopy.

Such a standard has now been developed at PTB for which a patent application has been filed. The standard consists of a silicon chip (a material with high Raman activity) on whose surface individual areas have been covered with a thin Au/Pd layer, so that the areas generating an intensive Raman signal are clearly delimited from areas with attenuated Raman intensity. Thus, one-dimensional and two-dimensional structures (lattices, checkerboard pattern) of different periodicity (from 4 µm to 0.8 µm) as well as point scattering centers (circles) of different sizes (from 10 µm to 0.1 µm), which are arranged separately and in pairs, have thus been generated in separate areas. This allows both the two lateral main axes (x,y) of the positioning unit to be calibrated – simultaneously and under the same conditions as those prevailing during the subsequent sample measurement – and the optical resolution of the Raman microscope to be determined for diverse configurations of the device. The dimensional quantities were traced to the SI unit “meter” by means of a calibrated scanning force microscope.

We are currently looking for a license partner for the new standard.