2X Reduction Imaging Interferometric Lithography
Ruth DeJule, Associate Editor -- Semiconductor International, 4/1/1999
CD's as small as 65 nm may be achievable with a 193-nm lithography tool.
Imaging interferometric lithography (IIL) combines the arbitrary pattern capabilities of optical lithography with the proven high resolution of interferometric techniques. Developed at the University of New Mexico in Albuquerque, 2X reduction imaging has been demonstrated with a 3X improvement in resolution over traditional optical lithography. Using standard binary chrome-on-quartz masks with no resolution enhancements, CDs as small as 65 nm may be achievable with a 193-nm lithography tool.
A useful means of describing the resolution limits of an optical system is with spatial frequency analysis, corresponding to the 2D Fourier transform of the aerial image intensity at the wafer. The highest spatial frequency accessible to optics (in air) is 2/l, corresponding to counter-propagating plane waves at grazing incidence to the wafer. For conventional optical systems, the lens imposes a smaller bandpass limit of approximately NA/l where the NA refers to the angular acceptance cone of the lens. These bandpass limits are similar to the effects of passing an electrical pulse through a low-pass filter where low frequencies go through and high frequencies are blocked, smearing out the edges of the pulse. For any lithography technique, higher resolutions require higher spatial frequencies.
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| Fig. 1. Compared to conventional coherent illumination (bottom row), IIL (top row) has demonstrated resolution of 12 to 2 µm patterns (k1 = 0.22) using a 364 nm laser source and 0.04 NA system. |
Traditional quadrupole off-axis illumination extends the frequency space coverage of an optical system. Each pole corresponds to a separate partially coherent exposure, with the final image an incoherent sum of four simultaneous exposures. For each pole, off-axis illumination at sin(Qoff-axis) changes the frequency coverage from a single circle of radius NA/l centered at zero frequency to a pair of circles of the same radius centered at radius shifted away from the origin so that the highest available spatial frequency is increased. The off-axis angle is chosen to be less than the numerical aperture to ensure that the zero-frequency reference beam is incident on the wafer. In any optical imaging scheme, interference between the zero-frequency reference beam and the diffracted beams from the mask produces the image.
IIL is essentially off-axis illumination taken to the extreme where the zero-order beam does not pass through the imaging pupil, but is brought around the lens to allow larger off-axis angles and consequently higher spatial frequencies. Unlike plane wave interference, which is restricted to periodic structures with no image information, modeling studies indicate that IIL can image arbitrary patterns with dense CDs as small as l/3. IIL extends the resolution beyond optical lithography limits to (1 + NA)/l, Brueck said.
The potential afforded by IIL for CDs as small as l/3 means imaging 130 nm dense lines with i-line exposures and 65 nm dense lines with 193 nm light. In an initial experiment, using a modest 0.04 NA, 2X reduction optical system and a 364 nm exposure source (Ar-ion laser), Brueck's group demonstrated resolutions to 2 µm, approximately a factor of three improvement over the resolution of the same optical system with conventional coherent illumination (see Figure). Work is underway to extend these results to sub-wavelength resolutions with a higher NA optical system.
