Imaging interferometric lithography: A wavelength division multiplex approach to extending optical lithography

The critical dimension (CD) limits of conventional optical lithography foll ow directly from the low-pass filter characteristics of the imaging optical system (\k\less than or equal to 2 pi NA/lambda where lambda is the optica l wavelength and NA the numerical aperture). In contrast, the linear system s limits of optics extend to spatial frequencies of 4 pi/lambda (interferen ce between counterpropagating beams at grazing incidence). Imaging interfer ometric lithography is introduced as a technique to approach this linear sy stems limit while retaining the arbitrary pattern capability of an imaging optical system. Multiple, wavelength-division-multiplexed exposures are use d, each exposure recording a different portion of frequency space. A conven tional, coherent illumination exposure provides the low frequency informati on, within the lens passband. Offset exposures provide the high spatial fre quency information. Off-axis illumination shifts a portion of the high spat ial frequency diffraction from the mask into the lens passband and interfer ence with a reference beam resets the frequencies once they are transmitted through the optical system. For a typical x-y geometry pattern, offset exp osures in the x and y directions provide a sufficient coverage of frequency space. Model calculations illustrate that the imaging capabilities of imag ing interferometric lithography (ILL) for dense features extend to -lambda/ 3 (130 nm at I line; 65 nm at an ArF exposure wavelength). Initial experime nts are reported at I line with a modest (NA=0.04) optical system. The resu lts are in good agreement with the model calculations. A resolution enhance ment of similar to 3 X from dense 6 mu m CDs for a conventional, coherent i llumination exposure to similar to dense 2 mu m CDs for an IIL exposure seq uence is demonstrated. (C) 1998 American Vacuum Society.