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. In turn, via nanoscale lithographic patterning, metal-dielectric surfaces can lead to a strongly enhanced nanoscale spatial distribution of optical energy.  This surface plasmon resonant nanolithography technique is not diffraction limited, and can produce subwavelength features using broad beam illumination with visible light.

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

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.

Two-dimensional model of surface-plasmon enhanced nano- lithography setup to realize sub wavelength features.

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

Movies showing the simulation dynamics are easily created, and provide information that facilitates understanding of device behavior. Here the incident source light tunnels through the thin silver contact mask and couples to surface plasmon modes at the silver/photoresist interface. The rapid lateral field variation facilitates the lithographic exposure of the underlying photoresist with nanoscale features. As demonstrated in the movie, surface plasmon resonance lithography is capable of creating aerial images with spatial features much smaller than the illumination wavelength.

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

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 incidence beam to couple to counter-propagating surface plasmon waves, which gives rise to subwavelength variation in the nearfield intensity inside the photoresist layer.

Near-field electric field intensity showing enhanced field intensities underneath the mask openings are an order of magnitude larger than elsewhere.

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

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.

Near-field electric field intensity showing significant, sub-wavelength variations below the silver contact lithography mask.