Amplifying sound-wave induced reflection and diffraction signals

Published on July 21, 2023
Category Light-Matter Interaction

ARCNL researchers Thomas van den Hooven and Paul Planken have found a way to enhance acoustic-wave-induced diffraction changes from a sample similar to those used for wafer alignment in nanolithography. With a gold-covered segmented grating they can induce a so-called plasmonic resonance. This amplifies reflection and diffraction changes caused by laser-induced acoustic waves in the sample. The relative enhancement of the diffraction signal is even two to three times larger than the enhancement of the reflection signal. The researchers have published their findings in Photoacoustics on May 9th 2023.

Nanolithography machines print several layers of nano-sized structures on a wafer to produce state-of-the art computer chips and components. To make sure consecutive layers are aligned accurately, wafers contain grating lines that act as markers. Detecting these alignment gratings is not always straightforward, because they get buried under many layers of material, some of which can be opaque to light. The Light Matter Interaction group at ARCNL develops techniques to overcome this problem.

In 2020 group leader Paul Planken and his colleagues showed that they could detect a hidden nanostructure using very high frequency sound waves induced by light. “The sound waves coming from the surface, reflect off the hidden grating and copy their shape in the process. A second light pulse can diffract off the acoustic-wave copy of the grating, when the wave reaches the surface”, PhD student Thomas van den Hooven explains. “Unfortunately these signals are very weak. That is why our group sought for ways to enhance them and thus improve our previous work.”

Plasmonic resonance
Van den Hooven and his colleagues use a layer of gold to induce a phenomenon called plasmonic resonance. When light hits a metal-semiconductor interface, it can transfer part of its energy to electrons that start to oscillate in the metal, forming a surface plasmon resonance, but only under very specific conditions. The wavelength of the light and its angle of incidence are important, but so are the material properties and shape of the metal.
Van den Hooven: “A former PhD student in our group, Guido de Haan, showed that a grating on the metal surface induces a plasmonic resonance that is very sensitive to changes in the reflection signal caused by laser-induced acoustic waves from a sample.” (see news item)

Schematic view of the segmented grating, the pump and probe pulses. A) A pump pulse generates acoustic waves (in red, arrows indicate direction of propagation) at the surface of the grating. B) The reflected acoustic wave returns to the surface, where a second pulse is used to probe both reflection (R) and diffraction (D). C) The surface plasmon polaritons (SPPs) are generated on the shorter-period grating.

Segmented grating
To investigate whether plasmonic resonance could also enhance changes in diffraction from a hidden grating, Van den Hooven designed a so-called segmented grating, in which the peaks consist of a series of smaller peaks themselves. “This type of grating is typically used for alignment in nanolithography. Also diffraction measurements are relevant for applications”, he says. “We use a segmented grating etched in glass and on top of that, we deposit a layer of gold which takes the same form. So, our grating is not hidden, in contrast to the first experiments.”
As his colleagues did before, Van den Hooven induced acoustic waves in his sample with a pump laser pulse and he subsequently measured the diffraction and reflection of light at different wavelengths. “We found that, for some type of acoustic waves, the measured reflection and diffraction changes are opposite to when we measure with a probe wavelength longer or shorter than the plasmonic resonance wavelength”, he says. “While the total amount of diffracted light is less than the reflected, we see a larger relative change in the diffraction signal due to the acoustic waves. This shows that plasmonic resonance can be used to enhance these very subtle signals.”

Van den Hooven’s experiments have shown that plasmonic resonance can enhance both diffraction and reflection measurements, but there are still some hurdles to take before this principle can be applied to nanolithography. “First of all, our segmented grating is not hidden, contrary to the alignment gratings we are ultimately trying to measure”, he says. “Further research will have to show whether the light-induced acoustic waves can cause enough disturbances at the wafer surface to induce plasmonic resonance, and whether this will work with other metals too. But first we will focus on increasing the measurement speed. We, therefore, changed our experiment so that we can probe a sample with many colors of light at once, instead of probing it with each wavelength separately.”

Thomas van den Hooven, Paul Planken, Surface-plasmon-enhanced strain-wave-induced optical diffraction changes from a segmented grating, Photoacoustics, 31, June (2023).