This paper reports on the new thermo-optic dispersion formula for AgGaSe2 that provides a good reproduction of the temperature-dependent phase-matching conditions for second-harmonic generation (SHG) and sum-frequency generation (SFG) of a CO2 laser and a Nd:YAG laser-pumped KTP optical parametric oscillator (OPO) in the 1.5558 - 10.5910 μm range as well as those for difference-frequency generation (DFG) between the signal and idler outputs of a Nd:YAG laserpumped 90° phase-matched CsTiOAsO4 OPO at λi = 7.5190μm when combined with the modified Sellmeier equations of the present authors [E. Takaoka and K. Kato, Appl. Opt., 37, 561-564 (1998)].
The wavelength region between 190 and 200 nm is especially relevant to semiconductor manufacturing. In
contrast to ArF excimer lasers, frequency up-converted solid-state lasers offer tuning, coherence and beam
quality characteristics that are essential to high performance semiconductor processing. This paper reviews
various methodologies for implementing pulsed non-linear optical interactions in this wavelength region
given a wide range of laser operating formats and describes the utilization of these sources for the specific
semiconductor applications of interference lithography and photoresist materials studies.
We have developed a solid-state 193-nm laser source operating at 5-kHz that generates a near-diffraction-limited TEM00
beam with 35 mW average power. The frequency spectrum is Gaussian, with a linewidth ~7-pm (FWHM),
corresponding to a coherence length of ~2-mm. The output beam also has a very high degree of spatial coherence. This
source was used in an interferometric liquid-immersion lithography test stand to produce 40- and 35-nm half-pitch
grating structures over a ~0.6-mm field of view with a commercially available chemically-amplified photoresist.
We describe an improved solid-state 193-nm laser source tailored specifically for high resolution photomask phase metrology. This source operates at a repetition rate of 5 kHz and produces 10-mW average power with a spectral bandwidth of 30 pm and near-TEM00 mode quality. The enhanced optical performance results from an optical parametric oscillator of improved design, and the use of the nonlinear crystal cesium-lithium-borate for fourth-harmonic generation. We also discuss the design evolution of our photomask phase metrology tool. The use of actinic illumination facilitates imaging of photomask phase structures, and ensures that optical path difference measurements and printing simulations are performed in-band and without off-wavelength accuracy errors.
We discuss the major challenges facing interferometric metrology and review several optical architectures that have evolved to meet the demands of the photolithography industry. Reliable image formation at extreme values of k1 requires the precise characterization of advanced photomasks, which may themselves contain near- and sub-illumination-wavelength feature sizes. The limitations of available photomask phase metrology tools have driven the development of a new actinic phase metrology architecture that facilitates optical path difference measurements of isolated or dense features on a sub-200-nm spatial scale. We describe the optical considerations that affect optical photomask metrology, and illustrate the new coherent-probe technique with preliminary results obtained using a leading-edge chromeless-phase lithography (CPL) reticle and a high-resolution actinic 193-nm microscope.
Attaining acceptable yields in the manufacture of advanced photomasks will require higher performance optical metrology tools. Key to improving these tools is the development of new ultraviolet light illumination sources that operate at the actinic wafer exposure wavelength, which is now projected to be 193 nm for the 65 and 45 nm device nodes. The use of an actinic light source for metrology facilitates imaging of photomask phase structures, and ensures that optical path difference measurements and printing simulations are performed in-band and are not subject to off-wavelength accuracy errors. We review the solid-state laser technologies that have been employed to generate 193 nm, and describe the development of a new solid-state 193 nm laser platform tailored specifically for high resolution photomask phase metrology. This source operates at a repetition rate of five-kilohertz, produces 2.5 mW average power with a spectral bandwidth of 10 pm and has excellent mode quality. Additionally, we present high-resolution, 200X-magnification photomask images obtained using this new illumination source.
The recent introduction of chromeless-phase lithography (CPL) has provided lithographers with a powerful wavefront engineering tool for patterning at k1 values below 0.3. Reliable image formation at such extreme k1 requires a well-characterized CPL photomask. However, the limitations of available optical inspection tools have made the test and measurement of CPL photomasks a difficult task. In this paper, we describe preliminary imaging and phase metrology results on a leading-edge CPL reticle using an high-resolution 193-nm microscope. This microscope features a solid-state 5-kHz repetition rate, 193.4 nm actinic light source in conjunction with high-numerical-aperture (0.75 NA) optics to provide 200X magnification, 150-nm Rayleigh resolution and 35-nm pixel size over a 30-micron image field. A recently-developed phase metrology architecture facilitates optical path difference (OPD) measurements of isolated or dense features on a sub-200-nm spatial scale. We discuss the phase measurement process and present images and corresponding OPD measurements of line and contact structures on an ArF CPL reticle that is designed mainly for the 65 nm technology node. We compare these OPD measurements with predictions based on surface nano-profilometer (SNP) step-height measurements of the same feature regions.
Aerial images formed by exposure tool simulation microscopes provide valuable information in determining the printability of photomask patterns prior to actual photoresist exposure. Exact simulation of production-grade step-and-scan exposure tools generally requires a complex optical system. This paper describes an accurate aerial imaging simulation technique using a relatively simple actinic, high-resolution inspection microscope incorporating image processing software. A high-resolution aerial image is captured by the inspection tool's CCD camera. The photomask spatial frequency information is multiplied by a digital low-pass apodization filter which preferentially attenuates the higher spatial frequencies. The exact shape of the filter depends upon the imaging and illumination configuration of the exposure tool under simulation, and is adjustable in software. The technique is well-suited to both manual and high-speed automated photomask inspection and defect detection and classification using a single low-cost platform.
We describe the application of a novel imaging technique to ultra-high spatial resolution measurements of photomask phase shift. An Actinix TMT-193 photomask transmission tool has been retrofitted with a breadboard Mach-Zehnder interferometer and ancillary signal detection electronics and software to demonstrate actinic phase shift measurement capability for ArF-generation photomasks. Using a simple proof-of-principle layout, we have successfully acquired high-contrast interference fringe data on ultra-high resolution (250 nm) photomask geometries. Minimum detectable feature sizes are limited only by the resolution of the imaging system. The measurement techniques are suited to both embedded-attenuator and alternating-aperture phase-shift masks, and may be directly applied to F2-generation mask tools. Engineering advances in the laser source will increase measurement precision to the 0.4° level as called for in the 2001 ITRS. We discuss the tool architecture and measurement method, and present recent results of phase scans performed on a customer-supplied MoSi EAPSM.
Acintic measurements of optical transmission are critical for determining the quality of repairs on advanced ArF- generation photomasks. We describe a new 193 nm tool designed to provide mask makers and uses the capability to resolve and measure photomask features with dimensions less than 0.20 micron. Measurements of transmission with sub-1 percent error are achieved in 30 seconds by using a laser probe beam imaged onto the top of the mask surface. We present high-resolution actinic images of embedded attenuator phase shift masks and of binary masks and discuss various methods and results of measuring mask transmission. A scheme to add phase retardation measurement capability to the tool is described, and preliminary results are discussed.