Andrew R. Neureuther

Ultrabroadband mirror using low-index cladded subwavelength grating

We report a novel subwavelength grating that has a very broad reflection spectrum and very high reflectivity. The design is scalable for different wavelengths. It facilitates monolithic integration of optoelectronic devices at a wide range of wavelengths from visible to far infrared.

A general simulator for VLSI lithography and etching processes: Part I—Application to projection lithography

A simulator is described which produces line-edge profiles at various key stages in integrated circuit processing. Optical models are included for contact and projection lithography. The effects of multiple wavelengths, defocus, and partially coherent sources may be simulated in projection lithography. The positive resist model of Dill is used with the string development model of Jewett to obtain resist line-edge profiles. The string model is generalized to surface reaction rate limited etching of any layer. The application of the simulator to projection lithography is illustrated with a number of examples including monochromatic and multiwavelength exposure, the effect of a post-exposure anneal, plasma descum, and defocus.

Self-Organized Silver Nanoparticles for Three-Dimensional Plasmonic Crystals

Metal nanostructures that support surface plasmons are compelling as plasmonic circuit elements and as the building blocks for metamaterials. We demonstrate here the spontaneous self-assembly of shaped silver nanoparticles into three-dimensional plasmonic crystals that display a frequency-selective response in the visible wavelengths. Extensive long-range order mediated by exceptional colloid monodispersity gives rise to optical passbands that can be tuned by particle volume fraction. These metallic supercrystals present a new paradigm for the fabrication of plasmonic materials, delivering a functional, tunable, completely bottom-up optical element that can be constructed on a massively parallel scale without lithography.

A general simulator for VLSI lithography and etching processes: Part II—Application to deposition and etching

The extension of the general process simulator SAMPLE to plasma etching and metallization is described. The etching algorithm is divided into isotropic, anisotropic, and direct milling components and is suitable for modeling wet etching, plasma etching, reactive ion etching, and ion milling. Separate deposition algorithms are used for CVD, sputtering, and planetary deposition. With the extension, it is possible to use a simple keyword repertoire to simulate a sequence of photolithography, etching, and deposition steps to obtain device cross sections at each stage of fabrication.

Modeling phase-shifting masks

An algebraic image perturbation model is introduced which used the point spread function of the lens and the mutual coherence of the illumination to give insight into the interactions of auxiliary patterns with features for the phase shifting mask technology. The model is based on adding electric field contributions and the cross term is shown to characterize the dominant interaction as a function of the number of auxiliary features, the relative coherence of the illumination, and the spreading of the image of the auxiliary pattern toward the feature. Data on the point/line spread functions and the mutual coherence are given and used to verify the accuracy of quantitative predictions of the change in peak intensity for lines and contacts when nonprinting phase shifting auxiliary patterns are added.

Electron-beam resist edge profile simulation

Resist edge profiles of lines produced by electron-beam exposure are explored through the use of numerical simulation. The modeling approach uses Monte Carlo simulation of electron scattering and energy dissipation, a simple etch rate versus dose model for the resist, and a string development algorithm. The simulation was made on an IBM 370/158 and primarily considers a multiple-spot Gaussian beam exposure of PMMA resist. An absolute quantitative evaluation of the simulation accuracy is made based on resist exposure-development measurements and comparisons with SEM micrographs of experimental profiles. The comparisons show good quantitative agreement and indicate that modeling can be used as a quantitative processing aid. Simulation results illustrate the importance of resist, beam, and substrate parameters in the context of optical mask writing (0.5-µm resist on 0.08-µm Cr on SiO2) and direct wafer writing (1.0-µm resist on Si, Gd3Ga5O12, and Au).

Polarization effects in mask transmission

Polarization and interface reflection effects are examined for technology issues in mask making of chrome edge shape, overcoating, double phase-shifting removal of defects, and reflective masks. This study is based on the massively parallel rigorous electromagnetic simulator TEMPEST1, with extensions to include the TM polarization and boundary conditions for outgoing waves in optical materials. The analysis was carried out on a CM-2 connection machine with remote electronic access. A local workstation was used to write and preview the geometry as well as process images from files of diffraction efficiencies downloaded to the workstation from the network. Through this remote analysis procedure, the four proto-typical mask case studies described by Doi et al.2 were examined for polarization and boundary condition effects. For chrome masks of one wavelength wide opening in a IX system at 0.248 |im, moderate differences between the TE and the TM polarizations were observed in the peak intensities (10%). The TM polarization in general showed higher transmission and lower sidelobes. The two polarizations showed a similar increase in linewidth and peak intensity with decreasing chrome edge slope. The difference was not as pronounced for a 5X system at 0.365 |Lim. Overcoating chrome masks with anti-reflection layers improved resolution for both polarizations. For 5X chromeless phase-shifting masks at 0.365 pm, neither polarization effect nor phase-shifter edge slope was important. The peak intensity at the phase-shifting section changed by only 1% and the linewidth varied by less than 2% when the phase-shifter edge slope changed from 90° to 45°, keeping the mid-points of the phase-shifter edge fixed. To remove defects on such masks by double phase-shifting may drop the intensity level to 70% of the clear field value for a 0.1 X/NA sized defect. For IX reflective masks at one wavelength, both polarizations displayed similar behaviors. These reflective masks with built-in material-based phase-shifting improved the image slope, and the TM polarization was found to have slightly less ringing than the TE polarization.

Investigating phase-shifting mask layout issues using a CAD toolkit

Phase-shifting mask layout issues for nonregular designs are investigated using a new CAD (computer-aided design) toolkit. Using this toolkit, the periphery cells of a nonregular 16-Mb DRAM scaled for 64-Mb design rules are analyzed for their phase-shifting potential. On average, a 0.5 shrink factor applied to a layer of the DRAM periphery mask results in an effective shrink factor of 0.53, since some areas of the mask have phase conflicts. In this case, 94% of the cell area is easily shiftable using two phases. Possible solutions are recommended for the remaining difficult areas. The toolkit contains components for the global shrinking, phase-assignment, design-rule checking, region extraction, and region replacement of a mask layer. Using the initial shrink factor and violation data generated by the toolkit, an effective shrink factor based on phase-conflicts can be calculated for a particular design. The toolkit can also be used to assist in translating a traditional design into a phase-shifted one interactively from within a mask layout editor.<<ETX>>

Mask topography effects in projection printing of phase-shifting masks

Topography effects of glass edges in phase-shifting masks (PSMs) on image quality are assessed using the rigorous electromagnetic simulation program TEMPEST on three different optical systems for four PSM technologies including alternating, rim, attenuated, and chromeless. The scalar and thin mask approximations used in simulation programs such as SPLAT can be in error by as much as 20% for certain classes of shifter edges. A feature size independent bias of 0.021 /spl lambda//NA per edge is recommended for alternating masks with vertical edges because light is lost near the etched glass edges. No direct electromagnetic interaction between chromium edges and shifter edges was found for rim phase-shifting masks. The rim dimension can thus be designed solely on the basis of the sidelobe level and peak intensity. For attenuated PSM, edge effects are less severe but sidelobe problems occur. For a center to sidelobe contrast of 0.6 over a DOF of 3 RU, a lower transmission of 4% is recommended. For chromeless PSM, the imbalance in image peaks is shown to be affected by the optical stepper parameters. In any PSM technology, it appears that a 360/spl deg/ glass protrusion may produce a drastic drop in intensity due to resonant effects. >

Photoinduced diffusion of Ag in GexSe1−x glass

Diffusion of Ag in GexSe1−x (x∼0.1) under UV light irradiation is studied using Rutherford backscattering spectrometry and microlithography techniques. The diffusion coefficient at 21 °C and 2 mW/cm2 is determined to be 2.7 nm2/s. The temperature dependence of the diffusivity follows an Arrhenius‐type equation with an activation energy of 5.32 kcal/mole (0.23 eV) and a pre‐exponent factor of 2.5×104 nm2/s. The reciprocity of irradiation intensity and exposure time with respect to the diffusion distance is confirmed. Specifically, the diffusivity is found to be directly proportional to the irradiation intensity.