Surface Plasmon Enhanced Contact Lithography SPECL

Another interesting effect was observed during the study of PLL. It was noted that the quality of the image above the planar lens could be enhanced by surface plasmons on the underlying metallic film, and this effect has been termed Surface Plasmon Enhanced Contact Lithography (SPECL). This is illustrated in Fig. 17.23, which compares the near field intensity profiles for two 140 nm period Cr gratings illuminated at 341 nm, embedded in a medium with refractive index of 1.6.

Fig. 17.23(a) shows the usual ENFOL case, where the material beneath the mask has a uniform refractive index (or is an index-matched stack). In this case the image has poor contrast at the exit plane of the mask, has a limited depth of focus, and that at depths greater than 80 nm beneath the mask an image reversal occurs. These effects reduce the process latitude for faithful patterning in this case.

Fig. 17.23. Near field intensity profiles for a 140 nm pitch grating embedded in a medium with refractive index of 1.6 and illuminated at 341 nm: (a) with the entire underlying material index matched with n = 1.6; and (b) with a 40 nm thick silver layer 50nm beneath the mask to show the surface plasmon enhanced contact lithography (SPECL) effect. The normalised intensity is plotted from 0 (black) to 2 (white) in linear steps of 0.2

Fig. 17.23. Near field intensity profiles for a 140 nm pitch grating embedded in a medium with refractive index of 1.6 and illuminated at 341 nm: (a) with the entire underlying material index matched with n = 1.6; and (b) with a 40 nm thick silver layer 50nm beneath the mask to show the surface plasmon enhanced contact lithography (SPECL) effect. The normalised intensity is plotted from 0 (black) to 2 (white) in linear steps of 0.2

The addition of a SPECL layer within the resist stack can significantly improve the depth of field and process latitude, as is shown in Fig. 17.23(b). In this case a 40 nm thick silver layer is included 50 nm beneath the mask. The contrast at the exit plane of the mask is improved, and high contrast without image reversal is preserved throughout the entire resist layer. This improvement is attributed to the generation of surface plasmons on the underlying silver layer, which illuminate from beneath in addition to the incident illumination from above. This goes against conventional wisdom that dictates that the best resolution in a multilayer resist is obtained when all the layers are index matched.

The improvement in process latitude is shown further in Fig. 17.24, which plots expected pattern linewidth versus depth for the two intensity distributions shown in Fig. 17.23. These are extracted from iso-intensity contours at I = 0.4I0, 0.5I0 and 0.6I0, a ±20% variation about the middle value. In the case of the index-matched substrate (Fig. 17.24(a)) the linewidth for the I = 0.5I0 exposure increases by more than a factor of three between depths of 15 nm and 35 nm beneath the mask. This compares to the SPECL situation in which the linewidth increase is less than 20%. Variations in exposure intensity will also be less critical for SPECL. A 20% increase in exposure intensity will increase the linewidth 30 nm beneath the mask by more than 40% for the index matched substrate exposure, whereas the equivalent increase for SPECL is less than 20%. This is encouraging for the development of a stable and repeatable process.

= 0.4Io

t

/

- = 0.5I

/ -

--- 0.6I

/

J

/ r v*'

-

**

-

15 20 25 30 35 Depth (nm)

Fig. 17.24. Linewidth as a function of depth beneath the exit plane of the mask for the two intensity distributions shown in Fig. 17.23(a) index matched substrate; (b) a 40 nm thick silver layer 50 nm beneath the mask. Linewidths are shown for the iso-intensity contours I = 0.4I0 (dotted), 0.5I0 (solid) and 0.6I0 (dashed)

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