Broadband Optical Detection of Ultrafast Strain Waves in Metals

As semiconductor devices continue to shrink and grow in complexity, some fabrication processes, such as those used to manufacture 3D NAND, involve depositing thick, opaque layers onto the wafer. These layers also cover alignment markers, that are used to vertically align the different layers that make up the device. While ultrafast, laser-induced strain waves can be used to detect gratings buried underneath such layers, they currently cannot be optically detected with the required efficiency.
This thesis presents research on improving the optical detection of ultrafast strain waves, by investigating the wavelength dependence of the detection process. First, the wavelength-dependent strain-wave-induced changes in reflection and diffraction of a gold-covered, segmented grating are investigated. This grating is able to sustain three SPP resonances, and we find enhanced strain-wave-induced reflectance and diffraction changes for probe wavelengths close to the SPP resonance wavelengths. Second, we use an ultrafast reflection spectrometer to simultaneously measure the reflectance changes of the grating between 470 and 760 nm, which includes both the interband transition (IBT) of gold and the three SPP resonances. Surprisingly, the reflectance changes near the IBT are almost as large as those near the SPP resonances. Third, we investigate the strain-wave-induced reflectance changes of a 30 nm thick ruthenium film for wavelengths between 475 and 1000 nm. We find that the highest detectable strain-wave frequency is inversely proportional to the optical penetration depth of the probe wavelength.
In conclusion, the optical response of these metals depends strongly on the optical detection wavelength and that experimental measurements are essential to fully characterize the optical response.