Optical lithography has been the dominant patterning process for semiconductor fabrication for over 40 years. The patterning process evolved initially from methods used in the printing industry, but as integrated circuits became more complex, and as device geometries shrank, sophisticated new imaging methods evolved. Today’s optical lithography systems represent the highest resolution, most accurate optical imaging systems ever produced. This remarkable evolutionary process continues to this day, paced by "Moore's Law". The evolutionary development of lithography systems over the last 40 years is reviewed along with a brief discussion of options for the future.
With the development of 157nm laser lithography, calcium fluoride (CaF2) has become a very important material for excimer laser components. The optical performance of CaF2 at 157nm is strongly related to its surface quality. By analyzing the phase change of reflected p-polarization near the Brewster angle at wavelengths between 150nm and 1000nm, a quasi-Brewster angle technique (qBAT) was developed to characterize the surface quality of CaF2. This technique suggests that the slope of the quasi-Brewster angle is dominated by top surface roughness, while the displacement of the quasi-Brewster angle is determined by subsurface damage. A simple model developed to describe the surface quality of CaF2 includes both top surface and subsurface effects. The relationship between subsurface damage and quasi-Brewster angle shift was determined. Using an ultraviolet ozone (UVO) cleaning process, four surface characteristics, e.g., top surface roughness, top surface contamination, subsurface damage, and subsurface contamination could be distinguished.
The discovery of a significant spatial-dispersion-induced birefringence (intrinsic birefringence) in CaF2 at ultraviolet wavelengths has had a major impact on the design of 157 nm lithography systems, requiring complete redesign of the optics to take account of the imaging aberrations resulting from the birefringence and the accompanying index anisotropy. This intrinsic birefringence phenomena results from a symmetry-breaking effect of the finite wave vector of the photon on the symmetry of the light-matter interaction in fluorite-structure cubic crystals. As a follow-up to our original concise report of measurements and theory of the effect in CaF2 and BaF2, we present here a more detailed analysis of the theory, focusing on the symmetry and its consequences. We also provide the full directional dependence of the effect in useful closed forms. We analyze the implications for precision optical design with CaF2 optical elements, and discuss qualitatively the approaches being considered to compensate for it.
The evolution of optical lithography is traced back more than 30 years to its beginnings with contact printing. As the complexity of integrated circuits increased, the intolerance for defects drove the industry to projection printing. Projection printing was introduced in the early 1970s by imaging the full wafer at 1:1 magnification. The rapid increase in wafer sizes was accommodated by annular field scanning using 1:1 imaging mirror systems. Decreased linewidths and tighter overlay budgets combined with larger wafers created huge difficulties for the mask maker which weren't relieved until the introduction of reduction step-and- repeat printing of small blocks of chips in the late 1970s. Further demands for smaller linewidths and larger chips have driven optical lithography to shorter wavelengths and to scanning the chip in a step-and-scan printing mode. Future advancements in lithography will likely combine novel scanning techniques with further reductions in wavelength.
The evolution of optical lithography is traced back more than 30 years to its beginning with contact printing. As the complexity of integrated circuits increased, the intolerance for defects drove the industry to projection printing. Projection printing was introduced in the early 1970s by imaging the full wafer at 1:1 magnification. The rapid increase in wafer sizes was accommodated by annular field scanning using 1:1 imaging mirror systems. Decreased linewidths and tighter overlay budgets combined with larger wafers created huge difficulties for the mask maker which weren't relieved until the introduction of reduction step-and- repeat printing of small blocks of chips in the late 1970s. Further demands for smaller linewidths and larger chips have driven optical lithography to shorter wavelengths and to scanning the chip in a step-and-scan printing mode. Future advancements in lithography will likely combine novel scanning techniques with further reductions in wavelength.
The evolution of optical lithography is traced back more than 30 years to its beginnings with contact printing. As the complexity of integrated circuits increased, the intolerance for defects drove the industry to projection printing. Projection printing was introduced in the early 1970's by imaging the full wafer at 1:1 magnification. The rapid increase in wafer sizes was accommodated by annular field scanning using 1:1 imaging mirror systems. Decreased linewidths and tighter overlay budgets combined with larger wafers created huge difficulties for the mask maker which weren't relived until the introduction of reduction step- and-repeat printing of small blocks of chips in the late 1970's. Further demands for smaller linewidths and larger chips have driven optical lithography to shorter wavelengths and to scanning the chip in a step-and-scan printing mode. Future advancements in lithography will likely combine novel scanning techniques with further reductions in wavelength.
The evolution of optical lithography is traced back more than 30 years to its beginnings with contact printing. As the complexity of integrated circuits increased, the intolerance for defects drove the industry to projection printing. Projection printing was introduced in the early 1970's by imaging the full wafer at 1:1 magnification. The rapid increase in wafer sizes was accommodated by annular field scanning using 1:1 imaging mirror systems. Decreased linewidths and tighter overlay budgets combined with larger wafers created huge difficulties for the mask maker which weren't relieved until the introduction of reduction step- and-repeat printing of small blocks of chips in the late 1970's. Further demands for smaller linewidths and larger chips have driven optical lithography to shorter wavelengths and to scanning the chip in a step-and-scan printing mode. Future advancements in lithography will likely combine novel scanning techniques with further reductions in wavelength.
Tanya Jewell, M. Becker, John Bjorkholm, Jeffrey Bokor, Ludwig Eichner, Richard Freeman, William Mansfield, Alastair MacDowell, M. O'Malley, Eric Raab, William Silfvast, L. Szeto, Donald Tennant, Warren Waskiewicz, Donald White, David Windt, Obert Wood, John Bruning
We demonstrate nearly diffraction limited printing using soft x-ray radiation of approximately 36
and 14 nm wavelength. As an imaging system we used a-20X-reduction Schwarzschild-type objective
with iridium coated mirrors for use at 36 nm and Mo/Si multilayer coated mirrors for use at 14 nm.
An off-axis aperture and illumination were used to eliminate the central obscuration in the imaging
system caused by the primary mirror. Two types of masks were used for exposures: an open-stencil
one for 36 nm and a silicon membrane with a Ge absorber for 14 nm. The high absorption of carbonbased
polymers at these wavelengths requires that imaging resist be very thin. Thin resist layers are
not robust and, by themselves, not very useful for processing. By incorporating them into a tn-level
resist, however, we have exposed, developed, and transferred features as small as 0.1 jm into silicon.
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