Babar Minhas

Ellipsometric scatterometry for the metrology of sub-0.1-μm-linewidth structures

We describe a modification to our existing scatterometry technique for extracting the relative phase and amplitude of the electric field diffracted from a grating. This modification represents a novel combination of aspects of ellipsometry and scatterometry to provide improved sensitivity to small variations in the linewidth of subwavelength gratings compared with conventional scatterometer measurements. We present preliminary theoretical and experimental results that illustrate the possibility of the ellipsometric scatterometry technique providing a metrology tool for characterizing sub-0.1-mum-linewidth.

The effects of an electromagnetic crystal substrate on a microstrip patch antenna

The effects of a two-dimensional (2D) electromagnetic bandgap substrate on the performance of a microstrip patch antenna are investigated. The microstrip patch antenna is placed on a defect in the electromagnetic bandgap substrate that localizes the energy under the antenna. Finite-difference time-domain calculations are employed to determine the effects of the substrate. The excitation frequency of the antenna near the resonance frequency of the defect mode can be used to control the coupling between antennas that are placed in an array.

Metallic inductive and capacitive grids: theory and experiment.

We present theoretical modeling and experimental validation of both capacitive (dot) and inductive (hole) metallic crossed gratings in the mid-infrared (2-5 microm). The gratings are fabricated by use of interferometric lithography and modeled by use of rigorous coupled-wave analysis. Our experimental and numerical investigations of the transmittance spectra of these gratings suggest that, as in inductive grids, the behavior of capacitive grids is described by the coupling of the incident light into surface plasma waves.

Scatterometry measurement of sub-0.1 μm linewidth gratings

The effort discussed here addresses the use of shorter incident wavelengths for characterizing sub-0.1 μm linewidths and the corresponding influence on scatterometry measurement sensitivity to linewidth variations. A sensitivity metric, based on the variance statistic, was developed using well-characterized, large-pitch (0.80 μm) photoresist grating structures on Si illuminated at 633 and 442 nm. The same metric was applied to short-pitch (0.20 μm), etched gratings on InP, with the result that appreciable scatterometry sensitivity was measured, even at the 633 nm incident wavelength. Modeling was used to estimate scatterometry sensitivity at three wavelengths for photoresist critical dimensions of 100 and 70 nm on Si. A significant increase in sensitivity was not found until the incident wavelength was reduced to 325 nm. We are presently investigating techniques to improve measurement sensitivity for short-pitch structures using the 633 nm incident wavelength.

Deeply etched grating structures for enhanced absorption in thin c-Si solar cells

Sub-wavelength periodic structures in crystalline-silicon (c-Si) for solar cell applications can be designed for maximizing optical absorption in thin films. We have investigated optical response of deeply etched c-Si grating structures using rigorous modeling, hemispherical reflectance, one-sun LIV, and internal quantum efficiency measurements. Model calculations predict that almost /spl sim/ 100 % optical absorption can be achieved in subwavelength 2D structures etched to a depth of /spl sim/ 15 /spl mu/m. Using advanced reactive ion etching techniques, subwavelength deeply etched grating structures have been fabricated and integrated into solar cells. Preliminary one-sun solar cell measurements from /spl sim/ 10-/spl mu/m 2D period structures have demonstrated short-circuit current enhancement of /spl sim/ 10 mA. The cell efficiencies were poor due to the lack of surface passivation and emitter optimization. Subwavelength grating solar cells failed to provide any performance boost probably due to the lack of surface passivation. Optimization of emitter formation on these types of deeply etched grating surfaces is expected to lead to high-efficiency, thin-film c-Si solar cells.

Fabrication of 1D and 2D vertical nanomagnetic resonators

We have successfully fabricated the first resonant magnetic nanostructures exhibiting a negative permeability in the midinfrared. These metal-dielectric structures exhibit local resonances based on metallic inductive-capacitive effects that are controlled by the dimensions of the individual nanostructures and are independent of the periodicity, which is much smaller than the resonance wavelengths. Compared to other commonly used structures for obtaining negative permeability in the microwave and THz regions which are based on a planar fabrication paradigm, this structure is oriented vertically so that the smallest features are controlled by deposition rather than by lithography.

Over 80% subwavelength transmission in annular coaxial metallic arrays

Arrays of coaxial metallic structures (~ 210-nm gap) were fabricated. An enhanced transmission resonance (> 80% at 2 mum) is observed compared with comparable-diameter hole arrays as a result of electromagnetic modes supported by the complex unit cell

Massively parallel solution of the inverse scattering problem for integrated circuit quality control

The authors developed and implemented a highly parallel computational algorithm for solution of the inverse scattering problem generated when an integrated circuit is illuminated by laser. The method was used as part of a system to measure diffraction grating line widths on specially fabricated test wafers and the results of the computational analysis were compared with more traditional line-width measurement techniques. The authors found they were able to measure the line width of singly periodic and doubly periodic diffraction gratings (i.e. 2D and 3D gratings respectively) with accuracy comparable to the best available experimental techniques. They demonstrated that their parallel code is highly scalable, achieving a scaled parallel efficiency of 90% or more on typical problems running on 1024 processors. They also made substantial improvements to the algorithmics and their original implementation of Rigorous Coupled Waveform Analysis, the underlying computational technique. These resulted in computational speed-ups of two orders of magnitude in some test problems. By combining these algorithmic improvements with parallelism the authors achieve speedups of between a few thousand and hundreds of thousands over the original engineering code. This made the laser diffraction measurement technique practical.