Lalitha Parameswaran

A merged MEMS-CMOS process using silicon wafer bonding

A process for fabricating integrated silicon micromachined sensors is demonstrated. The process uses silicon wafer bonding to create a substrate that can be inserted into an existing IC fabrication line without perturbation of the line. After circuits are completed, micromachining steps are performed to release the silicon membranes and form the sensors. A variety of test structures including MOSFETs, piezoresistive pressure sensors and cantilever beams were successfully fabricated, and all were functional, indicating that the additional micromachining steps and CMOS thermal cycles caused no adverse effects to the devices.

Silicon Pressure Sensors Using A Wafer-bonded Sealed Cavity Process

A sealed cavity microstructure can be used as the base for a variety of sensors. The central features of this technology are the seamless manner in which the micromechanics process interfaces with IC processes, and the diversity of sensors which can be fabricated. This paper discusses new results on the design, fabrication and testing of both capacitive and piezoresistive pressure sensors using this technology. n-substrate I I I

Reversible Electrowetting on Dual-Scale-Patterned Corrugated Microstructured Surfaces

The ability to reversibly switch between a hydrophobic Cassie state and a hydrophilic Wenzel state is often not possible on textured surfaces because of energy barriers which result from the geometry of the microstructure. In this paper, we report on a simple microstructure geometry that allows an aqueous droplet to be reversibly switched between these states by the application of electrowetting. We demonstrate reversible electrowetting in air on microstructured surfaces consisting of parallel corrugations and show that this geometry can be engineered to produce a Cassie state and can be electrically controlled to switch to a Wenzel wetting state having high adhesion. When the electric field was removed, we observed spontaneous dewetting along the corrugations as the droplet transitioned from the Wenzel state back to a Cassie state.

IC process compatibility of sealed cavity sensors

Sealed cavities formed by wafer bonding represent one technological means of integration of MEMS with electronics. In this work, the survivability of sealed cavity plates subjected to typical CMOS high temperature steps is evaluated. Defect generation in such plates is predicted with modelling of stresses and examined through experiments. Additionally, it is shown that a full CMOS process flow has no detectable impact on the mechanical properties of the bonded layer if the sealed cavity plate is properly sized according to the stress model.

An adaptive calibration technique for micromachined pressure sensors

An adaptive calibration technique which continuously calibrates the linearity of capacitive microsensors is described. The sensing system utilizes two sense capacitors of different size and two sigma-delta (/spl Sigma//spl Delta/) analog-to-digital converters (ADCs). A small, linear sensor is used to calibrate the linearity of a larger, higher-resolution sensor. The integrated circuit containing the ADCs is built in a 0.6 /spl mu/m, double-poly CMOS technology and uses 150 mW from a 5 V supply.

Electrically switchable diffractive waveplates with metasurface aligned liquid crystals

Diffractive waveplates and equivalent metasurfaces provide a promising path for applications in thin film beam steering, tunable lenses, and polarization filters. However, fixed metasurfaces alone are unable to be tuned electronically. By combining metasurfaces with tunable liquid crystals, we experimentally demonstrate a single layer device capable of electrically switching a diffractive waveplate design at a measured peak diffraction efficiency of 35%, and a minimum switching voltage of 10V. Furthermore, the nano-scale metasurface aligned liquid crystals are largely independent of variations in wavelength and temperature. We also present a computational analysis of the efficiency limits of liquid crystal based diffractive waveplates, and compare this analysis to experimental measurements.

Nonreciprocal acoustoelectric interaction of surface waves and fluorine plasma-treated AlGan/GaN 2DEG

This paper demonstrates acoustoelectric (AE) effects for surface acoustic waves (SAWs) propagating in an AlGaN/GaN 2DEG, where a fluorine based plasma has been used to tune the carrier concentration. Incorporation of fluorine ions in the AlGaN barrier is shown to reduce sheet carrier density in the 2D layer, which is required to prevent screening of piezoelectric fields. Tuning of the 2DEG channel is also observed with a progressive shift of the threshold voltage for co-fabricated HEMT structures. The monolithic gain devices exhibit nonreciprocal insertion losses under applied DC bias for higher-order Rayleigh modes in GaN on sapphire at 728 MHz and 1.48 GHz. This constitutes a first step in implementing the carrier control required for AE gain in 2D semiconductors with intrinsically high sheet density.

Enhanced coupling of broadband light into amorphous silicon via periodic nanoplasmonic arrays

Achieving enhanced coupling of solar radiation over the full range of the silicon absorption spectrum up to the bandgap is essential for increased efficiency of solar cells, especially thin film versions. While many designs for enhancing trapping of radiation have been explored, detailed measurements of light scattering inside silicon cells is still lacking. Here, we demonstrate experimentally and computationally that plasmonic-assisted localized and traveling modes can efficiently couple red and infrared radiation into ultrathin amorphous silicon (a-Si) layers. Utilizing patterned periodic arrays of aluminum nanostructures on thin a-Si, we perform specular and diffuse reflectivity and transmission measurements over a broad spectrum. Based on these results, we are able to separate parasitic absorption in aluminum plasmonic arrays from enhanced light absorption in the 200 nm thick amorphous silicon layer, as compared to a blank silicon layer. We discover a very efficient near-infrared a-Si absorption mechanism that occurs at the transition from the radiative to evanescent diffractive coupling, analogous to earlier surface-enhanced infrared studies. These results represent a direct demonstration of enhanced radiation coupling into silicon due to large angle scattering and show a path forward to improved ultrathin solar cell efficiency.

Solar metadevice with enhanced absorption, scattering, and spectral control

Lightweight, portable solar blankets, constructed from thin film photovoltaics, are of great interest to hikers, the military, first responders, and third-world countries lacking infrastructure for transporting heavy, brittle solar cells. These solar blankets, as large as two square meters in area, come close to satisfying specifications for commercial and military use, but they still have limited absorption due to insufficient material efficiency, and therefore are large and too heavy in many cases. Metasurfaces, consisting of monolayers of periodic and semi-random plasmonic particles patterned in a scalable manner, are explored to enhance scattering into thin photovoltaic films (currently of significant commercial and military value), in order to enhance absorption and efficiency of solar blankets. Without nano-enhancement, absorption is limited by the thickness of the thin photovoltaic active layer in the long-wavelength region. In this study, lithographically patterned, periodic Al nanostructure arrays demonstrate experimentally a large absorption enhancement, resulting in a predicted increase in short-circuit current density of at least 35% and as much as 70% for optimized arrays atop 200-nm amorphous silicon thin films. Optimized arrays extend thin-film absorption to the near infrared region. This impressive absorption enhancement and predicted increase in short-circuit current density may significantly increase the efficiency and reduce the weight of solar blankets, enabling their use for commercial and military applications.

Fast, electrically tunable filters for hyperspectral imaging

Tunable, narrow-wavelength spectral filters with a ms response in the mid-wave/long-wave infrared (MW/LWIR) are an enabling technology for hyperspectral imaging systems. Few commercial off-the-shelf (COTS) components for this application exist, including filter wheels, movable gratings, and Fabry-Perot (FP) etalon-based devices. These devices can be bulky, fragile and often do not have the required response speed. Here, we present a fundamentally different approach for tunable reflective IR filters, based on coupling subwavelength plasmonic antenna arrays with liquid crystals (LCs). Our device operates in reflective mode and derives its narrow bandwidth from diffractive coupling of individual antenna elements. The wavelength tunability of the device arises from electrically-induced re-orientation of the LC material in intimate contact with antenna array. This re-orientation, in turn, induces a change in the local dielectric environment of the antenna array, leading to a wavelength shift. We will first present results of full-field optimization of micron-size antenna geometries to account for complex 3D LC anisotropy. We have fabricated these antenna arrays on IR-transparent CaF2 substrates utilizing electron beam lithography, and have demonstrated tunability using 5CB, a commercially available LC. However, the design can be extended to high-birefringence liquid crystals for an increased tuning range. Our initial results demonstrate <60% peak reflectance in the 4- 6 μm wavelength range with a tunability of 0.2 μm with re-orientation of the surface alignment layers. Preliminary electrical switching has been demonstrated and is being optimized.