Nanolithography simulation of surface plasmon resonant interference lithography

Optical lithography that exploits surface plasmon resonances in the optical near field is being developed as a possible technique for the fabrication of nanoscale features beyond the diffraction limit. By using a photoresist that is sensitive at the surface plasmon resonant frequency, the exposure of a thin layer of photoresist directly below a contact mask can create an aerial image on nanometer length scales. This surface plasmon resonant nanolithography technique is not diffraction limited, and can produce subwavelength features using broad beam illumination with visible light.

Surface plasmons are optical waves trapped near a metal-dielectric interface due to the resonant interaction between the associated electromagnetic field and electrons collectively oscillating near the surface of the metal. Surface plasmons are bound to the interface with fields decaying exponentially away in both directions. With nanoscale lithographic patterning, such metal-dielectric surfaces can lead to a strongly enhanced nanoscale spatial distribution of optical energy.

Step 1. Create the FDTD Solutions model of the surface plasmon resonant nanolithography contact mask

Surface plasmon resonant lithography representation of silver-quartz contact mask in FDTD Solutions.

A 2D cross-section through the quartz substrate (blue), silver contact mask (grey), photoresist (pink) and silicon wafer (red) is shown here in the FDTD Solutions layout editor, along with the sources and monitors used in the simulation. • The simulation region is bounded by the orange outline. • The source polarization of the incident plane wave is shown by the blue arrow, its propagation direction by the purple arrow. • Monitors, shown in yellow, are used to determine the desired near-field profiles and create a movie of the simulation dynamics. • At the bottom is the script window, which allows customized commands and additional analysis to be performed.

Step 2. Watch the movie of the surface plasmon resonance lithography exposure process

If your browser does not support mpeg movies, you can download a copy of the resulting FDTD Solutions movie here.

Step 3. Analyze the surface plasmon resonance lithography near field data

Near-field aerial image resulting from exposure of quartz/silver mask with surface plasmon resonance waves. The rapidly-varying spatial features lends the technique to nanolithography applications.

With the detailed results produced by FDTD Solutions, all the complex optical wave interactions at the many material interfaces, including the reflection from the silicon substrate, are accurately treated. • A plot of the near field intensity in cross-section through the silver mask layer (y = 0 to 60 nm) and the photoresist layer (y = -50 to 0 nm) is shown on a logarithmic scale. Surface plasmon modes are clearly seen at the silver mask/photoresist interface. • The periodic structure allows the normal incident beam to couple to counter-propagating surface plasmon waves, which gives rise to subwavelength variation in the nearfield intensity inside the photoresist layer.

Step 4. Verify the subwavelength features resulting from surface plasmon resonance nanolithography as simulated by FDTD Solutions

Near-field intensity at the aerial image plan consists of 80 nm feature sizes resulting from the surface plasmon resonant lithography process.

FDTD Solutions simulation data may be easily analyzed and plotted either in the graphical user interface or using the sophisticated built-in scripting language. • The nearfield optical intensity in the middle of the photoresist layer, 30 nm below the contact mask, is plotted as a function of position. The high contrast ratio shown allows for patterns with minimum size about 80 nm to be transferred to the photoresist. • This sub-wavelength result achieved with surface plasmons is well below the diffraction limit for a 436 nm source and is therefore amenable to nanolithography applications.

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