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

The critical dimension (CD) limits of conventional optical lithography follow directly from the low-pass filter characteristics of the imaging optical system (|k|≲2πNA/λ where λ is the optical wavelength and NA the numerical aperture). In contrast, the linear systems limits of optics extend to spatial frequencies of 4π/λ (interference between counterpropagating beams at grazing incidence). Imaging interferometric lithography is introduced as a technique to approach this linear systems limit while retaining the arbitrary pattern capability of an imaging optical system. Multiple, wavelength-division-multiplexed exposures are used, each exposure recording a different portion of frequency space. A conventional, coherent illumination exposure provides the low frequency information, within the lens passband. Offset exposures provide the high spatial frequency information. Off-axis illumination shifts a portion of the high spatial frequency diffraction from the mask into the lens passband and interference with a reference beam resets the frequencies once they are transmitted through the optical system. For a typical x-y geometry pattern, offset exposures in the x and y directions provide a sufficient coverage of frequency space. Model calculations illustrate that the imaging capabilities of imaging interferometric lithography (IIL) for dense features extend to ∼λ/3 (130 nm at I line; 65 nm at an ArF exposure wavelength). Initial experiments are reported at I line with a modest (NA=0.04) optical system. The results are in good agreement with the model calculations. A resolution enhancement of ∼3× from dense 6 μm CDs for a conventional, coherent illumination exposure to ∼dense 2 μm CDs for an IIL exposure sequence is demonstrated.