Overcoming the Diffraction Limit

Scanning electron microscope (SEM) images of high-resolution ENFOL-defined patterns are shown in Fig. 17.6. The patterns have been dry etched approximately 100 nm deep into silicon to facilitate imaging. Fig. 17.6(a) shows a 280 nm period grating formed by exposure through a mask with 70 nm isolated lines. After exposure, development and dry etching, the line width has been reduced to less than 50 nm, and there is some line edge roughness (LER) evident. It should be noted that the width of these lines is less than Xmin/7, where Xmin is the minimum exposing wavelength (365 nm). The period is approximately 0.77 hmin.

Fig. 17.6(b) shows a 280 nm period grating formed by exposure through a mask with 70 nm apertures, the negative of the mask pattern used for Fig. 17.6(a). These 70 nm apertures are less than Xmin/5 wide, however, the exposure time for this pattern is the same as for patterns with large features. This indicates that there is no significant attenuation of the incident light through these sub-wavelength apertures. High transmission through sub wavelength sized circular aperture arrays has been previously reported (Ebbesen et al. 1998), and was attributed to plasmon resonances. In the ENFOL case, we have rectangular aperture that are long with respect to the wavelength, and high transmission of one polarisation will always be possible. Fig. 17.6(c) shows a 140 nm period grating formed in resist after exposure and development. This was exposed though a mask with 70 nm lines and 70 nm apertures. The lines in this pattern are discontinuous in places due to LER, however continuous lines are obtained for all patterns with periods greater than 200 nm. The grating period for the pattern shown in Fig. 17.6(c) is well below the diffraction limit for an equivalent projection lithography system. For normal incidence coherent illumination at wavelength X the minimum resolvable period pmin in a projection optical lithography system is (Rai-Choudhury 1997),

P min

Where NA is the numerical aperture of the system. The maximum possible NA is equal to the refractive index of the imaging medium, n, which is 1.6 for the photoresist we are using.

Therefore, the best result that projection lithography into this photoresist could achieve would be pmin = 228 nm for the shortest available exposure wavelength in our source (2 = 365 nm). The structure shown in Fig. 17.6(c) has a period 40% smaller than this diffraction limit. Index-matched projection lenses are not generally used so in practice a projection optical lithography system is limited to NA < 1. If, together with this, we consider the peak exposing wavelength transmitted through the masks (X = 436 nm) then pmin = 436 nm and the structure shown in Fig. 17.6(c) has a period which is approximately 1/3 of the diffraction limit for the equivalent projection system.

Fig. 17.6. Scanning electron microscope images of patterns produced using ENFOL and subsequent reactive ion etching: (a) 280 nm period grating exposed through a mask with 70 nm lines, (b) 280 nm period grating exposed through a mask with 70 nm apertures, and (c) 140 nm period grating exposed through a mask with 70 nm lines and 70 nm apertures

100 nm

Fig. 17.6. Scanning electron microscope images of patterns produced using ENFOL and subsequent reactive ion etching: (a) 280 nm period grating exposed through a mask with 70 nm lines, (b) 280 nm period grating exposed through a mask with 70 nm apertures, and (c) 140 nm period grating exposed through a mask with 70 nm lines and 70 nm apertures

ENFOL's ability to pattern nanometre-scale and micron-scale features at the same time using the same exposure conditions is illustrated in Fig. 17.7. This figure shows a SEM micrograph for a grating structure of 2 |im wide lines with 1 |im spaces exposed and processed simultaneously with gratings that have periods down to 140 nm. These patterns show no pinhole defects and are uniform over the 40 x 40 |im2 area of each grating on the substrate, and are uniform between fields distributed over the 5 x 5 mm2 patterned area.

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Fig. 17.7. Scanning electron microscope image of large feature patterns with 2 )j,m lines and 1 )j,m spaces produced using ENFOL. Patterns with feature sizes down to 70 nm have also been exposed and produced simultaneously from the same mask

Fig. 17.7. Scanning electron microscope image of large feature patterns with 2 )j,m lines and 1 )j,m spaces produced using ENFOL. Patterns with feature sizes down to 70 nm have also been exposed and produced simultaneously from the same mask

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