Marc D. Levenson

Improving resolution in photolithography with a phase-shifting mask

The phase-shifting mask consists of a normal transmission mask that has been coated with a transparent layer patterned to ensure that the optical phases of nearest apertures are opposite. Destructive interference between waves from adjacent apertures cancels some diffraction effects and increases the spatial resolution with which such patterns can be projected. A simple theory predicts a near doubling of resolution for illumination with partial incoherence σ < 0.3, and substantial improvements in resolution for σ < 0.7. Initial results obtained with a phase-shifting mask patterned with typical device structures by electron-beam lithography and exposed using a Mann 4800 10X tool reveals a 40-percent increase in usuable resolution with some structures printed at a resolution of 1000 lines/mm. Phase-shifting mask structures can be used to facilitate proximity printing with larger gaps between mask and wafer. Theory indicates that the increase in resolution is accompanied by a minimal decrease in depth of focus. Thus the phase-shifting mask may be the most desirable device for enhancing optical lithography resolution in the VLSI/VHSIC era.

Frequency modulation (FM) spectroscopy - Theory of lineshapes and signal-to-noise analysis

Frequency modulation (FM) spectroscopy is a new method of optical heterodyne spectroscopy capable of sensitive and rapid measurement of the absorption or dispersion associated with narrow spectral features. The absorption or dispersion is measured by detecting the heterodyne beat signal that occurs when the FM optical spectrum of the probe wave is distorted by the spectral feature of interest. A short historical perspective and survey of the FM spectroscopy work performed to date is presented. Expressions describing the nature of the beat signal are derived. Theoretical lineshapes for a variety of experimental conditions are given. A signal-to-noise analysis is carried out to determine the ultimate sensitivity limits.

The phase-shifting mask II: Imaging simulations and submicrometer resist exposures

Submicrometer optical lithography is possible with conventional projection cameras when the mask controls the phase of the light at the object plane. Two-dimensional imaging simulations for the Mann 4800 projection camera show that the maximum spatial frequency for 60-percent contrast increases from 640 1/mm to 896 1/mm. The geometrical quality of the images of typical microcircuit patterns was shown to be acceptable for feature sizes of 0.7, 0.6, and 0.5 µ, respectively, and various parameters of the irradiance patterns were calculated. Exposures were made using a high-performance two-layer photoresist system and a mask containing patterns similar to those in the simulation. The phase-shifting mask was shown to increase exposure latitude and to produce a 95-percent yield of 833 1/mm (0.6 µ line and gap) patterns, whereas the transmission mask gave a 7-percent yield. Half micrometer features were patterned with a 22-percent yield using 0.436-µ light.

Optically heterodyned coherent Raman spectroscopy

A convenient variation of Raman-induced Kerr effect spectroscopy (RIKES) which overcomes many of the previous limitations of coherent Raman spectroscopy is introduced. Quantum limited signal-to-noise ratios may be approached by the use of optical heterodyne detection (OHD). Detector current, linearly proportional and phase sensitive to the third-order nonlinear susceptibility, and linearly proportional to the Raman scattering cross section, is produced. Heterodyne detection in coherent Raman spectroscopy enables the detection of weak Raman signals that would otherwise be obscured by noise resulting from background sources.

The potential dependence of surface plasmon-enhanced second-harmonic generation at thin film silver electrodes

Abstract The intensity of second-harmonic light generated from plasmon surface polaritons at thin film silver electrodes in contact with an aqueous electrolyte exhibits a strong dependence on electrode potential for a variety of electrochemical systems. The surface plasmons enhance the electromagnetic field at the surface, thereby greatly increasing the second-harmonic intensity at the interface. We have measured the potential dependence of surface plasmon-enhanced second-harmonic generation at a silver electrode for aqueous solutions of sodium perchlorate, sodium thiocyanate, urea (a strong adsorbate), and for a buffered solution of lead acetate in the underpotential deposition region where a monolayer of lead is deposited onto the silver. The results can be interpreted by a simple theory relating the second-harmonic signal to the static electric field at the surface; to first order this field is proportional to the excess charge density on the metal.

Wavefront Engineering for Photolithography

Physicists have always been fascinated by the very small. These days, the inner structures of everyday items such as the personal computer fall into that size category and warrant our interest. The critical dimensions of individual features of state‐of‐the‐art memory chips are now as small as 500 nanometers and are getting smaller. The microwave transistors in some satellite dish receivers require gates smaller than a quarter of a micron.

Projection photolithography by wave-front conjugation

Conjugate wave-front generation by degenerate four-wave mixing has been employed to project images with submicrometer features onto photoresist-coated substrates. The developed patterns demonstrate a resolution of >800 line pairs per millimeter for 413-nm illumination, consistent with theoretical expectations. The patterns are not degraded by speckle or edge enhancement, and the magnification is within 0.1% of unity. Focusing is accomplished by a novel interferometric procedure.

High-resolution imaging by wave-front conjugation

Conjugate-wave-front generation by degenerate four-wave mixing is used to produce images with a resolution better than 500 lines/mm with 514.5-nm light.

Second harmonic generation at thin film silver electrodes via surface polaritons

The generation of second harmonic light from a silver–mica capacitor and from thin film silver electrodes provides information on the charge and composition of the metal–dielectric interface. The second harmonic light is created by the injection of surface polaritons onto the electrode surface via prism coupling techniques. A theory relating the second harmonic intensity to the square of the charge on the electrode successfully predicts the dependence of the SHG on applied voltage or potential and the time dependence of the SHG in response to a potential step. The time dependent second harmonic signal is also used to monitor the growth of a lead monolayer onto the silver electrode in the region of underpotential deposition.

Squeezing of classical noise by nondegenerate four-wave mixing in an optical fiber

Nondegenerate four-wave mixing in an optical fiber is shown to attenuate one quadrature of random sideband fluctuations created by external modulators. A theory of the nonlinear interaction that includes nonlinear dispersion fits the results. Analogous experiments on quantum noise inputs should prove successful.