In the simplest implementation of interferometric lithography (IL), two coherent light beams are incident on a resist-coated substrate at their point of intersection (Figure 1). Interference between the two beams produces a sinusoidal intensity distribution that can be used to form grating patterns in a photoresist film. The period of the grating is defined by the vacuum wavelength of the exposing light, the angle of intersection of the beams, and by the refractive index n of the medium in which the interference takes place.

We have pioneered the use of IL as a tool for the evaluation and characterization of advanced photoresist materials. IL offers several advantages for this application: it is maskless and lensless, so scattering, diffraction and optical aberrations do not degrade the aerial image. Similarly its large depth of field eliminates ambiguities due to focus errors. The pitch is continuously variable over a wide range, and allows imaging at spatial dimensions smaller than those achievable with conventional lithographic exposure tools of the same wavelength. Finally, the sinusoidal aerial image has essentially perfect contrast, and that can be readily manipulated.

IL has been applied by us to validate predictions of a PEB model for CA resists, to study the effects of imaging CA resists at high numerical aperture¹, and to test the capability of CA resists for exposure using liquid immersion lithography².

One example serves to demonstrate the utility of IL for resist characterization. Line-edge roughness (LER) is a random fluctuation in the width of a resist feature after development, The amplitude of LER can be a significant fraction of the overall resist feature width at small feature dimensions, and as in such cases poses a difficulty in dimensional control. LER is a key factor hindering the advancement of lithography to nanoscale dimensions. Possible sources of LER include the molecular weight and molecular weight distribution of the resist polymer and the molecular structure of resist components, inhomogeneity in the distribution of resist components within the film, statistical effects influencing film dissolution, and intrinsic properties of the imaging chemistry. One key issue is whether LER is influenced by the contrast of the exposing aerial image. We have used IL to address this question.

With balanced beams, the IL light image is intrinsically 100% modulated, but this modulation can be varied in a controlled manner to any desired value. One convenient way to vary the effective image contrast as seen by the resist is to carry out a exposure sequence where each site is exposed twice in succession - once with a two beam imaging exposure and once with a flood exposure using a single beam (Figure 2)³. By appropriate selection of the two doses, any desired image contrast and integrated dose can be attained. The photoresist effectively sums the two exposures. The scanning electron micrographs shown in Figure 2 result from a series of IL exposures of a CA resist film where the overall exposure dose is constant but where the contrast or modulation of the sinusoidal light image is varied over a wide range. Although the width of the resist image remains constant, the LER clearly increases dramatically as the light image contrast is degraded. This result shows that aerial image contrast is a significant factor controlling LER for single-layer resists.

 References

1. T. A. Brunner, J.A. Hoffnagle, W. D. Hinsberg, F. A. Houle, M. I. Sanchez, J. Microlith. Microfab. Microsys., 1, 188-196 (2002).
2. J. Hoffnagle, W. Hinsberg, M. Sanchez and F. Houle, J. Vac. Sci Techn. B, 17, 3306-3309 (1999).
3. W. Hinsberg, F. Houle, J. Hoffnagle, M. Sanchez, G. Wallraff, M. Morrison and S. Frank, J. Vac. Sci Techn B, 16, 3689-3694 (1998).