Phyllis R. Nelson

Dynamics of polysilicon parallel-plate electrostatic actuators

Abstract The response of a polysilicon parallel-plate electrostatic actuator to a.c. signals at different bias voltages has been measured with a laser interferometer. Using microhinges, large plates (with areas from 100 μm2 to ≃ 0.1 mm2) with long thin support beams (such as 600 μm × 3 μm × 1.5 μm) are rotated off the surface of the substrate to form a parallel-plate capacitor. Fabricated structures having 100 μm gaps can be closed electrostatically with voltages as low as 50 V. This new actuator is estimated to output a force of up to 50 μN. With the exception of the resonant Q-value, the experimental results are in good agreement with simulations based on a simple nonlinear model for the actuator.

Luminescence of Cu+-β“-alumina

Abstract A series of monovalent copper containing β -alumina single crystals have been prepared by standard ion exchange techniques. The crystals exhibit strong luminescence which we attribute to the 3d 9 4s → 3d 10 interconfigurational transition. The peak wavelength of this transition can be varied by 135 nm by the presence of other cations in the β “-alumina conduction plane, allowing the luminescence to be varied throughout the visible spectrum. Single-pass gain measurements of several of these crystals show gains of the order of 0.1/cm, suggesting the potential use of these materials for solid-state tunable lasers.

Elimination of extra spring effect at the step-up anchor of surface-micromachined structure

This paper describes the concept, analysis, fabrication, and testing of a new anchor for surface-micromachined beams. The anchor is designed to eliminate the extra spring effect at the step-up anchor common in conventional surface-micromachined beams, so that the boundary condition follows the ideal anchoring condition more accurately. The idea is to form a reinforcement hump at the beam anchor through a minor modification in the sacrificial-layer mask. No modification in the fabrication process is necessary. Formation of the reinforcement hump is tested using the multiuser MEMS Process (MUMP) foundry service at the Microelectronics Center of North Carolina (MCNC). The effectiveness of the new anchor is analyzed by finite-element analysis based on the actual anchor geometry obtained from the fabrication directly. Experimental verification is provided by making overhanging microcantilever pairs, one with the new anchor and the other with conventional, through MUMPs and postprocessing and comparing their frequency responses. Small-signal frequency response measurements are made with a modified Michaelson interferometer. Resonant frequency of a 2-/spl mu/m-thick 300-/spl mu/m-long polysilicon cantilever with the new anchor differed by less than 0.1% from the ideal anchor case. In comparison, the resonant frequency of the same beam with a conventional anchor is off by over 1%.

Optical methods for characterization of MEMS device motion

Micro-electromechanical system (MEMS) devices are small compared to normal mechanical devices, but they are still large compared to the wavelength of visible light. Thus, simple low- cost optical measurement techniques can be adapted for precise characterization of the motions of these small objects. The results of such measurements are important for verification of simulations, especially for devices in which nonlinear effects such as squeeze film damping play a significant role. The advantages and challenges of optical metrology for MEMS are examined using an electrostatically-actuated microgripper structure as an example device. Interferometric measurements of static rotation and of small-signal sinusoidal and impulse responses are presented.

Optical properties of β-Ga2O3:Cr3+ single crystals for tunable laser applications

Abstract Single crystals of β-Ga2O3:Cr3+ with various doping levels have been grown by the floating zone technique. In this material, the room temperature fluorescence is dominated by the broad band 4T2→4A2 emission extending between 650 and 950 nm, with a 210 μs lifetime. At 77 K, the 4T2 level is depopulated so that fluorescence originates only from the 2E level with a 2.3 ms lifetime. The temperature dependence of the lifetime indicates that the 4T2 level lies approximately 600 cm-1 above the 2E level. These characteristics are similar to those of alexandrite: Cr3+. Therefore, β-Ga2O3:Cr3+ may be an interesting new material for tunable laser applications.

Modeling spaces for real-time embedded systems

No system in the real world can compute an appropriate response in reaction to every situation it encounters, or even most situations it is likely to encounter. Biological systems address this issue with four strategies: (1) a repertoire of already computed responses tied to a situation recognition process, (2) organized in a response-time hierarchy that allows a quick response to occur immediately, and one or more slower and more deliberate responses to begin at the same time, with (3) decision processes that allow one of them to take over after a little while, or that (4) merge several of them in a combined and possibly novel response. In this paper, we describe an approach to building self-adaptive computing systems that incorporates these strategies, to cope with their intended use in hazardous, remote, unknown, or otherwise difficult environments, in which it is known a priori that the system cannot keep up with all important events, and that “as fast as possible” is not appropriate for some interactions. The key to implementing these strategies is an abstraction/refinement hierarchy of behavioral models and processes at multiple levels of granularity and precision. The key to coordinating these different models is the collection of integrative mappings among them, which are developed along with the models, and used for managing system behavior. We also describe the system development process that we use to build such systems, which differs from conventional methods by taking the basic artifacts of development, considered as partial models of aspects of the system in its environment, and retains them all in a model hierarchy, which eventually becomes the definition of the run time system. We show how to implement such systems, explain why we think they are good candidates for real-time operational environments, and illustrate the method with an example implementation.

Systems Engineering for Organic Computing: The Challenge of Shared Design and Control between OC Systems and their Human Engineers

The term “emergence” is usually used to mean something surprising (and often unpleasant) in the behavior of a complex system, without further qualification. Designers of OC systems want to manage emergence in complex engineered systems so that it can contribute to, or even perhaps enable, accomplishing the system’s performance goals. That is, OC designers aim to construct systems that are more flexible and adaptable in complex environments, to gain some of the advantages in robustness and adaptability that biological systems seem to gain from these phenomena. In this chapter we suggest some principles that we believe underlie the enormous flexibility and opportunistic adaptability of biological systems. We show how these principles might map to systems engineering concepts when they do, and what to do instead when they don’t. We then describe five specific challenges for the engineering of OC systems, and how we think they might be addressed. We also discuss the key role played by language and representation in this view of designing and deploying an OC system. Finally, we describe our progress and prospects in addressing these challenges, and thus in implementing systems to demonstrate the capabilities that we have identified as essential for successful OC systems.

Sol-gel-derived lead and calcium lead titanate pyroelectric detectors on silicon MEMS structures

Pyroelectric detectors based on PbTiO3 (PTO) and Pb1-xCaxTiO3 (PCTx) were fabricated and evaluated on thin silicon micro electrical-mechanical structures (MEMS) utilizing a new sol-gel technique. The sol-gel approach allows arbitrary Pb:Ca ratio, and is fully compatible with existing CMOS processes. The novel precursor chemistry developed in our laboratory is not moisture sensitive, and has a shelf-life of many months. Layers of PTO precursor were spin-coated onto 4 micrometers thick silicon membrane structures. Heat treatment at 700 degree(s)C yielded crack-free films approximately 0.3 micrometers thick. Contacts to the top of the pyroelectric film and to the silicon membrane were made by thermal evaporation of aluminum. The reduced thermal capacitance and improved thermal isolation offered by the MEMS membrane structure have significantly improved the performance of our device over typical bulk single crystal or ceramic devices. Pyroelectric responsivity of 2.5 volt/watt was measured at (lambda) equals 10.6 micrometers for a first generation simple membrane structure without poling or electrical bias. SPICE analysis of a distributed thermal circuit model indicates the potential for two orders of magnitude improvement in device sensitivity for optimized membrane structures currently under construction. These results also indicate that a fully integrated high-performance room temperature imaging array for use in the far IR can be realized at low cost.

EFFICIENT LATERAL MINORITY CARRIER TRANSPORT IN PROTON-IMPLANTED P-TYPE SILICON

A highly efficient lateral transport mechanism has been observed in stable defect layers (SDL) in p‐type silicon. The SDLs were produced by proton implantation followed by rapid thermal anneal. Photogenerated carriers have been collected at a Schottky junction several millimeters away from the generation site. This transport distance is more than 30 times the diffusion length in comparable bulk material. A model is proposed in which bending of the energy bands near the SDL expels majority carriers, leaving no substantial recombination mechanism for minority carriers trapped in the layer.

Self-modeling and Self-awareness

The purpose of this chapter is to discuss why self-aware systems must pay special attention to self-modeling capabilities, clarify what is meant by both strong and weak self-modeling, and describe some of the defining characteristics of self-modeling. This chapter is also about self-management via run-time model creation by the operational system, explaining why systems need to build models at run time, what phenomena they need to model, and how they can build models effectively. A system that is expected to operate in a dynamic environment needs to be able to update and occasionally dramatically change its models to maintain synchrony with that environment. We describe several example systems, one rather extensively, to show how the notions apply in practice.