Planar Lens Lithography PLL

Following Pendry's prediction that a planar silver layer excited near its plasma frequency could produce a sub-diffraction-limited resolution image in the near field simulations and experiments have been performed to test this for near field lithography (Blaikie 2002; Melville 2004). The system that has been studied is shown in Fig. 17.18. Within a free-space background medium a metal grating of period p and thickness t1 is illuminated from above with transverse magnetic (TM) polarised light of wavelength X. At a distance f1 beneath the exit plane of the grating there is a planar layer (PL) of thickness t2. In a simplistic negative refraction picture the presence of this layer is predicted to cause an image of the mask pattern to be formed a distance f «f below its bottom surface (see ray diagram). Silver (Ag) is chosen for the metallic material as its complex permittivity, s = s' + js", has a negative real part and small complex part for blue/UV wavelengths used for photolithography. For example, at X = 341 nm its complex permittivity is e = -1 +0.4, (Johnson and Christy 1972) which is closely index-matched to the surrounding free-space medium. This is the illumination wavelength chosen for this work - tuning the illumination wavelength so that the layer is index-matched to some other dielectric medium will produce similar results.

Simulation results comparing the near field intensity distribution with and without a silver NFPL are shown in Fig. 17.19. Exposures are for a mask with period p = 140 nm, which is sub-diffraction limited. Fig. 17.19(a) shows the case with f = 20 nm and t2 = 40 nm. Two sub-diffraction-limited images of the mask are observed, the first in the center of the PL, and the second at a distance of f = 23 nm below it. This second image could be used for nanolithography by placing a photoresist in this plane. This second image is superimposed on a background intensity that decays with distance below the PL, making the image difficult to discern. This arises because the mask period is sub-diffraction-limited, and the image is constructed from evanescent fields beneath the NFPL. Nonetheless, the peak intensity in the second image is 74% of the incident intensity, so exposure times for this new technique should be similar to those for conventional contact lithography with the same source and resist. The intensity distribution for the non-PL case shown in Fig. 17.19(b) shows no similar lensing effects. There is a region directly beneath the mask in which a shadow image forms, but the intensity and visibility of this image decays smoothly with distance. This simple near field intensity distribution has been used to obtain sub-diffraction-limited resolution, but there is a requirement of intimate mask-resist contact for higher resolution.

Image Plane

Fig. 17.18. Geometry & illumination condition for the near field planar lensing simulations

Image Plane w y

Fig. 17.18. Geometry & illumination condition for the near field planar lensing simulations

Fig. 17.19. Near field intensity profiles for a 140 nm period grating illuminated at 341 nm, (a) with a near field planar lens (NFPL) and (b) without a NFPL. The normalised intensity is plotted from 0 (black) to 2 (white) in linear steps of 0.1

Experimental verification of PLL has been achieved (Melville 2004), although not yet with sub-diffraction-limited resolution. Fig. 17.20 shows comparative AFM images of 1 pm-period gratings exposed with and without the PL layer. The exposure time was 120 s and the development time was 8 s in both cases. These AFM images clearly show that the silver PL is effective in forming a near field imaging of the the mask, whereas in the proximity exposure all resolution for the 1 |im-period grating is lost. Whilst there is some degree of overexposure for the proximity exposure of Fig. 17.20(a), faithful reproduction of the mask object is not achieved even for shorter exposures (Melville 2004). This is the first experimental demonstration of near field imaging through a silver layer, and efforts are now underway to determine the resolution of the technique. By reducing the thickness of the silver and spacers layers improved resolution can be obtained, and sub-wavelength resolution has been achieved by using a 50 nm thick silver layer, as shown in Fig. 17.21. The fidelity of the micron-scale feature is much improved in this case, and resolution down to a period of 250 nm is evident.

Fig. 17.20. Atomic force microscope (AFM) images of 1-micron period gratings exposed into 50 nm thick photoresist: (a) exposure performed with a 120 nm thick PMMA spacer; (b) exposure performed through a PMMA/silver/PMMA stack with 60nm/120nm/60nm thicknesses respectively

Simulations for the exposure geometries of Fig. 17.20 have been performed, and the results are shown in Fig. 17.22. In this case the FDTD method was used to perform these simulations. The simulation results for the proximity exposure are shown in Fig. 17.22(a), and it is clear that the pattern in the resist layer is of low contrast. There are some near field interference features apparent in this simulation, which may have been expected to give rise to discernable patterns in the developed resist. However the simulation is only for a single wavelength, and for the broadband source used for the exposure many such interference pattern will superimpose to produce a featureless final exposure pattern. For the PL exposure shown in Fig. 17.22(b) there is a much clearer image present in the resist layer, as observed in the experimental results. In this case the broadband nature of the exposure does not change the simulation results significantly, as the self-filtering nature of the silver PL only allows significant transmission at this wavelength.

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