In the electronic devices area, courses include basic and advanced electronics classes as well as classes in solid state and device physics. Additionally, device fabrication techniques are taught both in theory and in practice.
In the photonics area, undergraduate classes are offered in lasers, fiber optics, optical sensors, and photonic devices. Graduate classes include optical devices and quantum electronics. An expanding effort in optical sensors has resulted in a new undergraduate course offering and a new graduate course offering.
These undergraduate and graduate classes in the photonics and sensors area are currently supported by a dedicated Photonics Teaching Laboratory. This laboratory (first initiated in 1991 by a National Science Foundation grant and currently supported by Fluke, Tektronix, and Intel) offers a broad range of support for classroom and independent projects in optics and photonics.
The research pursued by members of the group includes chemical and biological microsensors and optical sensors (S. Yee and Afromowitz); electronic and photonic materials synthesis (Pearsall); nonlinear and quantum optics (Babbitt); Plasma heating for nuclear fusion (Nelson); semiconductor device modeling, photodetectors, and transport phenomena in solids (Darling); power device modeling, and novel power device structures (Lauritzen and H.P. Yee); semi conductor super-lattices, quantum wells and optical properties of semiconductors (Pearsall and Kuhn); waveguide and array optical modulators (S. Yee and Kuhn); industrial display technology (Kuhn and Pearsall); and optical measurement of water quality (Kuhn and Yee).
Research highlights of the past few years include:
New: Surface plasmon resonance technology was developed for determining the charge of refractive index due to specific chemicals, proteins, etc. in aqueous solutions as well as in gas phase (S. Yee)
Table of Contents
Sponsor: Washington Technology Center, Concis, Senmed Medical Systems, Defense Advanced Research Projects Agency
Abstract: In collaboration with Dr. Alan Blanchard of the Dept. of Molecular Biotechnology, we have designed and built a system that can machine 3-D structures of arbitrary geometry into silicon wafers with micron resolution and unlimited depth. Initially, this system, installed in the Washington Technology Center (WTC), will support the research efforts of a WTC-related project concerned with research into micromachined pumps and valves (Fred Forster, P.I., supported in part by Concis, LLC of Vancouver, WA). Use of this facility by a second major project, concerned with the development of clinical laboratory modules on a chip (Paul Yager, P.I., supported in part by Senmed Medical Ventures, Inc. of Cincinnati, OH), is planned for the future.
This project builds on technologies that have been demonstrated by others, primarily by Bloomstein and Ehrlich at Lincoln Laboratory (MIT). The technique is a unique research and development tool for machining microsensors, microactuators, micro-optics, and other chip-level structures. This project will enhance the ability of faculty at the University of Washington and Washington State companies associated with the WTC to compete and excel in the silicon micromachining arena.
At present, our etching facilities include wet chemical etching for silicon and certain dry etch processes, such as ion beam etching, sputter etching and reactive ion etching (RIE). However, all these techniques require the use of photolithographic masks and the transfer of patterns to the wafer using etch resists. Thus, the etching is binary, that is, one either removes material in an exposed area to some predetermined depth or one doesn't. The material may be removed isotropically, leaving behind rounded features with some mask undercutting, or anisotropically, where crystal planes or the directionality of collimated ion beams define etched wall slopes. One has no ability to etch shapes that do not conform to these constraints. Although micromachinists have been very clever in utilizing these etching techniques and overcoming their limitations in novel ways, it remains that a less restrictive etching technique would permit the machining of structures heretofore impossible to fabricate.
On the other hand, LACE does not use etch resists. The system is shown schematically in Figure 1. The silicon wafer to be etched sits in a stainless steel cell surrounded by low pressure chlorine vapor. By directing a moderately powerful (approximately 5 watt) focused laser beam at the silicon wafer surface through a quartz window in the cell, the laser energy heats the illuminated spot to the silicon melting point, and the chlorine gas reacts with the hot silicon and removes it. By allowing the laser beam to dwell on a spot longer, proportionally more silicon is etched away. By moving the wafer around on an x-y stage, intricate patterns may be etched, all under computer control. Thus, a pattern need only be established in the computer (a CAD/CAM program is ideal for this), and the pattern can be etched. This is very attractive for prototyping work. Walls of arbitrary slope can be obtained by etching away layer after layer and offsetting the etching position as a function of depth. The only restriction is that the etched structure cannot have hidden cavities.
A recent project to fabricate and test microvalves and pumps revealed the difficulties of using only the wet chemical etching techniques to define the flow channel geometries. Sharp corners at the required angles could not be fabricated by either isotropic or anisotropic etching. Dry etching techniques, such as RIE, could not be used either because they would not etch deep enough.
The LACE equipment can be used with modification for etching GaAs and metals, selective deposition of plastics by photo-polymerization, and selective deposition of many materials, including silicon, aluminum, gold, copper, cobalt and platinum. Polymerization of plastics on substrates at locations defined by the laser beam can be used to fabricate micro-optic structures, including waveguides and couplers.
Figure 1. Schematic of Laser-Assisted Chemical Etching System
The LACE facility will be operational in limited form in March 1996. Initial objectives are to design for etch features no smaller than approximately 4 microns. This will permit us to use a moderately long focal length lens (~f/8), and achieve almost vertical etch walls, if required. By changing a final lens in the laser beam path, higher resolution can be easily achieved. f/1 optics will permit the removal of less than one cubic micron of silicon. Tests will be performed to establish the resolution limitations of this technique. Higher resolution ultimately requires more accuracy in the positioning of the focused laser beam on the substrate. Further extensions of the system for photopolymerization or material deposition work will await the establishment of specific materials requirements.
Sponsor: Washington Technology Center and Concis, LLC
Abstract: We have been pursuing the analysis, design, fabrication and testing of micro-fluidic pumping systems. Whereas the most effective pumping mechanism in low-Reynolds number flow relies on positive displacement, such as a moving membrane, without valves the fluid will be forced equally in both the input and output directions. Valves for such micro-pumps have already been demonstrated by others which utilize techniques ranging from passive flapping membranes to complex thermally-controlled active devices. Our research is directed specifically at the development of valves that have no moving parts. These valves rely upon an intrinsic fluidic diodicity created by their non-reciprocal forward vs. backward flow geometry. That is, a larger flow resistance is encountered in the backward flow direction than in the forward flow direction.
Figure 1. A scanning electron microscope photograph of one of our designs, which curiously is an adaptation of a design patented by Nicholas Tesla, the famous electrical engineer and inventor, in 1920.
The advantages of such a valve are obvious. First, no-moving-parts valves are simple and do not require special control electronics. Secondly, since all the former valve designs include valve seats, where two mating surfaces meet to close off a channel, they suffer from variable leakage and outright failure as the valve seat becomes fouled or dirty. No-moving-parts valves do not have a valve seat. And thirdly, any valve with a valve seat cannot be used successfully in fluids that contain particulates (such as inks), or especially cells (such as blood). These fluids represent a large fraction of the applications proposed for these devices.
The areas of emphasis of our work on these devices includes improved methods of fabrication, characterization of the valves, and design of integrated sensors to permit the measurement and control of pressures and flow rates.
Sponsor: Center for Process Analytical Chemistry
Abstract: Optical fibers have been developed for many interesting sensor applications. The most sensitive detection methods involve interferometric devices, such as the Mach-Zehnder Interferometer (MZI), in which slight changes in the effective pathlength of light propagating in one fiber can cause enormous changes in the amplitude of two such beams interfering with each other. The main difficulty, in fact, is that these methods are so sensitive that slight changes in the temperature in the laboratory can produce enormous "noise" signals.
We are attempting to fabricate an integrated optic MZI chemical sensor device, incorporating waveguide channels created on a glass-covered silicon substrate, that will have the required sensitivity, but not be susceptible to environmental temperature effects. This integrated optical device is expected to outperform fiber-based MZI designs for several compelling reasons. In general, the entire length of each optical arm, from the first beam splitter to the detector, is sensitive to environmental effects. In fiber-optic MZIs, only a short length of one arm is treated to be sensitive to the chemistry of interest. The rest of the MZI arm picks up environmental noise. In the proposed integrated optical device, almost the entire length of the detector arm will be made chemically sensitive. Since the overall MZI length is reduced from typically a meter down to a centimeter, environmental noise will constitute a much smaller fraction of the output signal. The arms of the MZI cannot move with respect to one another in the integrated optical device. Thus, differential strains from fiber movement, microbending, stretching, etc., that plague fiber-based MZI structures will be eliminated. The waveguides of the integrated optic MZI are embedded below the surface of a glass-covered silicon chip. Therefore, they are rendered immune to humidity effects that can swell the buffer layer of optical fiber MZI arms, causing phase shifts from the induced strain. The chip also will tend to reduce rapid environmentally-induced temperature fluctuations between the arms.
A unique three-arm configuration is proposed for this design, and will force environmentally-induced thermal gradients to produce a different output pattern than the chemically-induced calorimetric or refractive-index response of the sensor. To our knowledge, this feature has never been investigated before. Finally, the potential for developing this device into an integrated array of chemical detectors is very attractive. As a generic micro-chemical sensor, any chemically-sensitive patch that either produces heat or changes the index of refraction can be incorporated onto the MZI. The exquisite sensitivity of the MZI to thermal or refractive index imbalance between the arms of the interferometer suggests that once we control the effects of environmental noise, as proposed herein, a wide variety of chemistries can be sensed with excellent quantitative results.
Sponsor: Senmed Medical Systems, Defense Advanced Research Projects Agency
Abstract: In collaboration with faculty in the Center for Bioengineering and the Departments of Mechanical Engineering and Laboratory Medicine, we are working on the design of a series of optic-based devices that will be used for the clinical analysis of microliter samples of blood. This project, known as "The Portable Stat Lab," seeks to miniaturize the apparatus used for chemical and physical analysis of blood by orders of magnitude, thus permitting critical analyses to be done in med-evac helicopters, in ambulances, and/or at accident sites if necessary, instead of requiring the patient to be brought to the hospital first.
Two paths are being pursued:
1) Designing devices that will permit the currently accepted methods to be used, but in miniature form, and
2) Inventing entirely new methods that are made possible because of the miniaturization.
Our part of this large interdisciplinary effort involves the design and development of micro-fluidic, micro-optical modules in which whole blood (possibly with a variety of added reagents) flows through micro-channels on a chip, and optical absorption, fluorescence or scattering measurements indicate the important clinical parameters, such as pH, oxygen saturation, ionic concentrations, hematocrit, white blood cell count, etc.
The advantages of working in micro-channels can be shown easily for the case of quantitative measurement of the concentrations of the various forms of hemoglobin (Hb) found in blood. In the clinical laboratory, this measurement is make using blood whose cells have been lysed, that is, broken up either by ultrasonic agitation or addition of strong detergents. The hemoglobin in the red blood cells then colors the plasma, and absorption spectra on this largely homogeneous sample reveal the concentrations of oxygenated and de-oxygenated Hb. The cells are lysed so that optical scattering from the cells will not interfere with the absorption measurement. We have shown that accurate measurements of Hb can be made in analysed whole blood, as long as the transmission cell is on the order of 50 um in thickness. Such transmission cells can be easily fabricated using silicon micromachining techniques.
Sponsor: Washington Technology Center and Combustion Specialists, Inc.
Abstract: We have developed improved techniques for measuring the temperature of the gases inside large utility boilers (steam plants used to generate 600 MW of electrical power). The temperature of the boiler gases ranges upwards of 1700deg. C, and is an important parameter in the control of the entire system. The measurement technique relies upon the fact that the speed of sound in an ideal gas is proportional to the square root of the absolute temperature of the gas. Thus, whereas at room temperature and standard atmospheric pressure, sound travels at 346 m/sec in air, the velocity increases to 887 m/sec at 1700deg. C. By sending a sharp pulse of sound across the 40 foot expanse of the boiler and measuring the time of flight, one can infer the average temperature along the sound path.
A further enhancement that we are planning to investigate involves the use of many such acoustic paths, intersecting each other in a grid pattern. By analyzing the sound propagation along all the paths, one can infer the temperature distribution over the whole space probed by the different paths. This acoustic tomography approach is very useful for finding hot and cold spots in the boiler and for adjusting the fuel delivery for optimum efficiency of heat transfer to the steam.
Sponsor: Air Force Office of Scientific Research
Abstract: Real-time, wideband information storage and signal processing devices are critical to many military and commercial systems in order to perform complex functions such as secure communications, electronic surveillance and tracking, pattern recognition, database management, and tactical air reconnaissance. Optical coherent transient technology has the potential to perform real-time storage and signal processing at data rates in excess of 10 GHz, with storage/pattern densities on the order of 100 gigabits per centimeter squared, and with data block sizes/time-bandwidth products in excess of 10,000. Despite extensive research on the physics of coherent transient memories and signal processors and several implementation techniques that have been proposed and conceptually demonstrated, analysis of the performance of coherent transient systems has only recently been addressed. Until an experimentally validated model of coherent transient memory and signal processing systems is developed that can verify the technology's true potential, development of coherent transients and associated enabling technologies cannot proceed productively.
The research program covers six areas:
1) Continued development of the analytical tools needed to evaluate coherent transient systems,
2) Evaluation of coherent transient memory and signal processing systems,
3) Experimental validation of the analytical predictions,
4) Development of coherent transient materials,
5) Experimental demonstrations of the feasibility and performance potential of coherent transient systems, and
6) Fostering interaction by supporting workshops in this area and operating an electronic bulletin board.
Sponsor Center for Process Analytical Chemisty
Abstract: The long-term goal of this work is to develop technology and prototype systems for applying microelectrode arrays to the task of multicomponent chemical analysis using voltammetric techniques. Ultimately, this work is planned to lead to automatic, portable electrochemical analysis instrumentation that can operate within realistic field conditions. The use of microelectrodes for voltammetric analysis is one of the most sensitive and versatile electroanalytical techniques. Microelectrodes afford many advantages, including predictable diffusion-limited transport, rapid equilibration and response time, detection sensitivities into the low ppb range, and small sample volumes.
Our system consists of: (1) A sampling chamber, (2) A microelectrode probe containing an array of working, auxiliary, and reference electrodes, (3) An electrode multiplexer (MUX) which allows any electrode of the array to be connected to any one of several measurement bus lines, (4) A source-monitor unit (SMU) which contains a digitally controlled test waveform generator, a wide dynamic range picoammeter and floating point analog-to-digital converter, and electrometer opamps for feedback control of the auxiliary electrodes, (5) A personal computer interface which allows the PC hardware to directly control the SMU and MUX and to collect the digitized data, and (6) A system of chemometric analysis software and in-situ techniques which process and characterize the data, sequences the measurements, and formats the results into a clear presentation of the analysis. To address the issues of microelectrode fragility and fouling, both polishable microelectrode arrays and disposable microelectrode arrays are being developed. These are being designed for fabrication by commercial integrated circuit vendors, and are being developed in collaboration with Prof. Greg Kovacs' group at Stanford University.
Our present application of this technology is the determination of heavy metals in aqueous solutions. The proposed system is envisioned for water and wastewater testing, monitoring of industrial effluents, hazardous spill assessment and control, environmental ground water analysis, and quality control analysis of industrial plating baths. Instrument prototypes are being field tested in various locations around the U.S. Instrumentation for the quantitative analysis of solutions usually falls into one of two categories: highly sensitive laboratory equipment which is large, cumbersome, slow, and expensive or inexpensive but inaccurate hand-held meters which are usually based upon a single ion-sensitive electrode. The present work offers the possibility of creating a family of instrumentation which is midway between these two extremes, and which should find widespread use in field work and industrial process monitoring.
Sponsor: National Science Foundation, Tektronix, Fluke Corp., Hewlett-Packard
Abstract: The Photonics Teaching Laboratory provides laboratory support for senior elective classes in photonics and sensors (EE 465, 466, 468, 484, and 488). The intent is to provide an environment where general equipment is made available to support a wide variety of optic and photonic experiments. The laboratory contains a number of permanent optical systems (such as a SPEX Im spectrometer, an Acton Gm spectrometer, and a Tektronix OTDR) as well as a wide selection of optical parts (mirror, lens, optical mounts, fiber optic chucks, laser diodes, HeNe lasers) for constructing small optical systems for short term experiments.
Sponsor: NSF
Abstract: Gallium Nitride and Aluminum Nitride have numerous potential applications in electronics and optoelectronics. These materials are drawing renewed attention because of the role that they might play in high-temperature integrated circuit technology. At present, electronic grade materials can only be grown above 1000deg.C. Our research seeks to replace the thermal energy required for growth by chemical energy. We use metal-organic chemical vapor deposition to grow thin films of GaN and AlN. By the use of novel pre-cursor chemistry, we can make the substrate behave as a catalyst to increase the growth rate of these compounds at lower temperatures.
Representative project: Epitaxial deposition of AlN on sapphire and silicon using dimethylethylamine-alane and ammonia.
Figure 1. The X-ray diffraction spectrum of crystalline Aluminum Nitride grown at low temperature on Silicon by MO-CVD.
Significant Publications and Presentations Include:
J.N.Kidder, Jr., J. S. Kuo, A. Ludviksson, T.P. Pearsall, J.W. Rogers, J.M. Grant, L.R. Allen, and S.T. Hsu, Jr., "Deposition of AlN at lower temperatures by Atomospheric Metalorganic Chemical Vapor Depositon using Dimethylethylamine-alane and Ammonia", J. Vac Sci. Tech A13, 711-715 (1995).
Figure 2. Electroluminescence of Gallium Nitride-based LEDs is a key to full-color visual displays.
Sponsor: Texas Instruments
Abstract: Gallium Arsenide and Gallium Nitride are well-known semiconductors. GaAsN is an alloy of these two materials. Our investigations indicate that GaAsN alloys should show semi-metallic behavior if they exist. Thermodynamic calculations indicate that these two materials are incompatible and will not form an alloy phase. We are exploring novel epitaxial growth techniques which may lead to the first synthesis of this unusual material.
Figure 1. Calculations of the dependence of the bandgap on the alloy composition.
Sponsors: Thermionics, NW; Washington Technology Center; NSF
Abstract: We are developing process monitor and control instrumentation using optical spectroscopy. Data aquisition and analysis are performed in real time during processing. Our measurements include temperature, gas composition and deposited composition of thin films. This work has attracted both attention and funding from the semiconductor industry. Representative project: Real-time optical temperature monitor with an absolute precision of +/-1deg.C over the range 0 < T < 1000 K.
Applications:
* Processes involving corrosive or hazardous substances. (e.g., medium to low pressure processing of various substrates).
* Processes involving very low concentrates of reactants. (e.g., deposition of very pure thin films in high to ultra high vacuum).
* Processes requiring fast response real-time control. (e.g., deposition of etching of very thin films, or processes with a specified narrow composition range).
* Real-time analysis or monitoring of combustion or process products (e.g., jet engine exhaust, environment monitoring).
Significant Publications and Presentations Include:
T.P. Pearsall, S.R. Saban, J. Booth, B.T. Beard, Jr. and S. Johnson, "Precision of Non-invasive Tempperature Measurement by Diffuse Reflectance Spectroscopy,", Rev. Sci. Instruments 66, 1995, pp. 4977-4980
Sponsor: National Science Foundation
Abstract: We are investigating the roles of chemistry and symmetry in determining the electronic structure of crystalline materials. Silicon and germanium have the same +4 valence chemistry. Ge-Si superlattices with a short period, similar to that of the unit cells of elemental Si or Ge (4 atomic monolayers) have measureable electronic and optical properties that are distinct from those of the Ge-Si random alloy of the same average composition. The difference comes from the symmetry of the Ge-Si superlattice which is defined by the number of atomic monolayers of Ge and the number of atomic monolayers of Si per superlattice period. Our samples are grown in Germany through a collaboration with Daimler-Benz.
Representative project: Measurement of the optical absorption constant ofGe-Si superlattices and alloys
Figure 1. This graph shows the measured optical absorption coefficient of a Ge-Si short-period superlattice at room temperature. It is measured as a function of the photon energy above the bandgap of the Ge-Si superlattice.
Significant Publications and Presentations Include:
T.P. Pearsall, "Electronic Properties of Ge-Si Superlattices," Progress in Quantum Electronics 18, 1994, pp. 97-152 .
Sponsors: Royalty Research Foundation, National Science Foundation
Abstract:: Project Goal:We are creating nanostructures on silicon by harnessing localized chemical reactions that are mediated by a scanning tunneling microscope (STM). We have demonstrated the use of the microscope to draw a line of oxide with a width of 20 nanometers. The oxide may then be used as a lithography mask to define other features in the silicon. The goal of our research is to determine and describe the physics and the chemistry of the modifications. Therefore, there are two objectives to the proposed work. One is to identify the processes involved in STM- stimulated deposition. The second is to show the capability of the method by using it to define novel controlled structures with dimensions on the order of 100 atoms or less.
Figure 1. This pattern was written using the STM located inside the SEM. The ambient is vacuum (10-5 Torr). This pattern was written at positive bias: +5V and 15nA. A variety of scanning speeds was used as explained in the text.
We are performing AFM lithography experiments at the Stanford Nanofabrication Facility. The AFM tip is coated with 300Å of evaporated Ti. The AFM image in Fig. 2 is a cross section of the oxide layer written at -5V tip bias and .8 um/s in air. The lines are approximately 20 Å high, 450Å wide. These oxide lines are clearly distinct from the Si substrate. Figure 3 is the same pattern after the KOH and HF etches. The lines are now approximately 120Å high, 430Å wide.
Figure 2. This is a cross section of oxide lines written with an AFM at -5V tip, 0.8 um/s.
Figure 3. This an AFM cross section of Si features transferred via KOH and HF etches using the Fig. 8 oxide patterns as a mask. Note that the etch did not significantly undercut the structures.
These STM/AFM results are significant for two reasons. First, we have demonstrated scanning nanolithography with state-of-the art resolution. Second, we can show that the field threshold for tunnel-induced deposition of material is much less than the field required for sequential depassivation (at positive tip bias) and subsequent oxidation decoration of the depassivated region.
Significant Publications and Presentations Include:
Steven Konsek, and T.P. Pearsall, "Scanning Probe Nanolithography", 38th Electronic Materials Conference, Santa Barbara, June 28, 1996, paper Y3.
Sponsor: National Science Foundation
Abstract: This project is a program of research and education in the development of advanced chemical microsensors and optical sensors for industrial process monitoring and control. The objective of this project is to enhance the competitveness of American chemical and high technology industries by providing a means to monitor their processes in real time. In order to perform this task, advanced sensor technology is being developed to be employed on-line in industrial processes. The important aspects of advanced sensors are the abilities to be rugged enough to withstand harsh environments, to be robust enough to handle complex samples, to have self-contained control, calibration and data interpretation, and and to be compact for remote installation.
Specific research projects supported by this grant appear on the next several pages.
Graduate Student: Kyle Johnston (Ph.D.)
Sponsor: National Science Foundation
Abstract: Optical sensors based on surface plasmon resonance (SPR) are highly sensitive to the optical characteristics of the medium surrounding the sensor interface. SPR sensor systems are commonly utilized to monitor the dynamics of thin films on the sensor surface, such as antigen-antibody and enzyme reactions. Several commercial SPR sensor systems with the ability to distinguish 1 Å changes in film thickness are marketed specifically to support research into bio-chemical reactions. Investigations of SPR sensor performance and system design issues for this type of research can be found in many publications.
A potential application of SPR sensors that has been identified but has not been adequately investigated is refractive index (RI) measurements of liquid samples. With potential sensitivity better than 1 x10-5 Index of Refraction Units (IRU), SPR sensors are capable of resolving analyte concentration levels of less than 10 ppm in aqueous solutions over a range of 0% up to 50% molecular analyte concentration. One of the potential reasons that the range and sensitivity of SPR sensors have not been exploited for RI sensing is the issue of sensor calibration, which can be illustrated by examining an experimental calibration of fructose. If a stepwise dilution of fructose is carried out in the sample cell of the SPR sensing system shown in Figure 1, the reflection spectra shown in Figure 2a can be obtained.
Figure 1. Experimental setup for fructose concentration calibration with a white light planar
SPR sensor system.
The incident beam of collimated white light excites SPR on the surface of the sensor and the reflected signal light has an attenuated dip centered around the wavelength that best coupled to the surface plasmon wave (lspr). As the concentration (and hence the RI) of the sample is changed, the wavelength location of the attenuated dip shifts. Figure 2b shows a calibration plot of concentration vs. lspr for the data shown in Figure 2a.
Figure 2. Experimental SPR reflection spectra (a) and concentration vs. minimum wavelength (lspr) calibration (b) of a stepwise dilution of a saturated fructose solution into water.
Calibration of the sensor is difficult because the response of the SPR sensor is so nonlinear that it is almost asymptotic to both the concentration and wavelength axis. A continuous function can not be adequately fit to the curve in Figure 2b, prompting a piece-wise calibration to be used. The calibration will not be accurate if the sensor is moved to another sensor system, requiring a method to be developed so that both sensors and systems be calibrated separately. Calibration is further complicated by the fact that the curve in Figure 2b is application specific, an identical sucrose dilution experiment would yield a substantially different calibration curve. Finally, the apparatus in Figure 1 is not ideal for industrial applications where there is a need to somehow bring the sensor to the process being monitored while providing for the stability of the measurement.
The goals of this research program are to address the difficulties listed above. A new patent pending on SPR sensor design utilizing state of the art optical design techniques has been developed to facilitate industrial sensing needs. Additionally, new calibration algorithms and techniques have been developed and are being investigated to find ways to address the issues of interchanging sensors between systems and applications.
Graduate Student: Mimi Mar
Abstract: The use of optical sensors employing surface plasmon resonance (SPR) to characterize biomolecular interaction is a promising development in biosensor technology. SPR sensors are highly sensitive to changes in the optical characteristics of the sample near the sensor interface. In a typical SPR biosensing experiment, a biomolecule is first immobilized onto a sensor surface, followed by the introduction of its compliment molecule. By monitoring the changes in the interfacial optical characteristics, the SPR sensor can determine the binding affinity and kinetics of a biospecific pair.
Since SPR is highly sensitive to changes at the sensor surface, fouling of the surface is a major concern. Non-specific adsorption of undesired solutes to the metal sensor surface will invalidate binding experiments. Some proteins can be denatured and unable to bind their biospecific partners upon exposure to the metal surface.
Ideally, biomolecules should be covalently immobilized onto the SPR sensor surface. Covalent bonds resulting the strongest molecular attraction between atoms. In addition, the molecules will have increased stability when bound.
A new film that potentially combines specific protein binding with a non-fouling background has been developed. This thin (approximately 200 Å) film is comprised primarily of a polyethylene oxide) (PEO)-like layer which provides a passivation layer that resists non-specific molecule uptake. Amine groups are distributed throughout this layer, providing sites to which specific proteins can be immobilized using established chemistries. The two film components are deposited simultaneously by a radio frequency (RF) glow discharge plasma deposition (GDPD) reaction.
The proposed film provides practical advantages. Current methods of functionalizing SPR sensor surfaces involve time-consuming, costly application of self-assembled monolayers followed with a modified dextran matrix. The film under development is deposited simultaneously in minutes and is extremely inexpensive. The films also require no special storage or handling.
Significant Publications and Presentations Include:
Mimi N. Mar, "Development of a Surface Plasmon Resonance Biosensor Using a Plasma Deposited Functionalization Film", MSEE Thesis, 1995.
M.N. Mar, B.D. Ratner, K.S. Johnston and S.S. Yee, "Enhanced Protein Binding on a Surface Plasmon Resonance Sensor Using a Plasma Deposited Functionalization Film", SPIE /BiOS 2388 B, Proceedings of An International Symposium on Biomedical Optics, San Jose, CA , Feb. 1995, pp. 585-592.
M.N. Mar, R.C. Jorgenson, S. Letellier and S.S. Yee, "In-Situ Characterization of Multilayered Langmuir-Blodgett Films Using a Surface Plasmon Resonance Fiber Optic Sensor", Proceedings of the 15th annual conference of the IEEE/EMBS, San Diego, CA, 1993, pp. 1551-1552 .
Graduate Student : Shuai Shen, Scott Karlsen
Sponsor: Partially supported by Center for Analytical Chemistry
Abstract: Scientific papers on surface plasmon resonance (SPR) based chemical sensors have been prolific in recent years, yet there has been no published research on the development of the First Order SPR based sensor. SPR based sensing applications have ranged from biochemical sensing, industry sensing and environmental sensing. However, all these sensors which have been developed are either the Zeroth order sensors which measure the variable at one time or multiplexed sensors, which use arrays of the Zeroth order sensors with different chemical specificity. The First Order SPR sensors, which measure multiple variables at a time, like an absorbance spectrometer measures the absorbance at many wavelengths of the light, have distinct advantages over the Zeroth Order SPR sensors. A Zeroth Order sensor suffers from the need to be perfectly chemically selective. That is, the sensor can not respond to a chemical sample other than the one for which it was designed. For most practical purpose, it is chemically impossible to have a perfectly selective Zeroth Order sensor. On the other hand, the First Order SPR sensor achieves the chemical selectivity by measuring a characteristic spectrum. Chemical samples can be identified and their concentration are determined from their characteristic spectrum using multivariate calibration techniques. The major objectives of this project are: 1) to prove the principle of operation of the First Order SPR sensor system with bulk optics and 2) to explore a planar lightpipe sensor configuration which is applicable to the process line monitoring.
Publications and Presentations:
Scott Karlsen, "Surface Plasmon Resonance Based Optical Dispersion Sensor" , MS thesis, University of Washington
S.R.Karlsen, K.S.Johnston, R.C. Jorgenson, and S.S. Yee, "Simultaneous Determination of Refractive Index and Absorbance Spectra of Chemical Samples Using Surface Plasmon Resonance" Sensors and Actuators, B 24-25, 1995, pp. 747-749.
S.R.Karlsen, K.S.Johnston, S.S. Yee and C.C. Jung, "First Order Surface Plasmon Resonance Sensor System Based on the a Planar Light pipe", Accepted , Sensors and Actuators, 1996.
Jiri Homola and S.S. Yee, "Surface Plasmon Resonance Sensor Based on Planar Lightpipe : Theoretical Optimization Analysis," Submitted to Sensors and Actuators, 1996.
Graduate Student: Spencer Nelson
Sponsor: Sandia National Labs
Abstract: The Surface Plasmon Resonance (SPR) transduction principle is widely used as an analytical tool for measuring small changes in the optical refractive index (RI) of biological fluids at the sensor interface. The change of RI is related to the on-going biochemical reactions occurring on the sensor surface. Thus, this enables researchers to study the bio-reaction dynamics in real time without the need for labeling chemistry. For such applications, it is extremely desirable to have the greatest sensitivity possible from the sensing system in order to differentiate certain reactions.
In the literature, almost all the SPR sensing systems are operating on the basis of measuring the shift of the minimum of the reflection spectrum amplitude. This minimum corresponds to the resonance condition at the sensor that is detected by (a) a single wavelength for multiple incident angles, known as angle modulation, or (b) a single angle for multiple wavelengths, known as wavelength modulation. The minimum angle or wavelength is then referred to as the coupling angle or wavelength. A common sensor configuration consists of a silver film 50 nm thick on a BK7 glass substrate. At 0.628 mm wavelength illumination, typical sensitivity for angle modulation is 0.02 change in the coupling angle for a 1X10-4 RI change in the sample medium. Similarly, using the wavelength modulation configuration at a 76 incident angle, a 0.24 nm change in the coupling wavelength has been observed. However, our calculations show that the phase of reflected light shifts 4.0 for a single wavelength/single angle configuration. This shift in phase should result in higher sensitivity if an appropriate phase detection scheme is employed.
Using a standard Kretschmann configuration, monochromatic laser light is reflected off the sensor at a single angle of incidence. The relative phase change is measured using an optical heterodyning technique. Light from a He-Ne laser is passed into an acoustic optic modulator to create a second frequency-shifted beam to be used as a reference. The primary beam is TM polarized and reflected off the sensing surface to pick up a phase change, while the reference beam is TE polarized and passes through the sensor unaffected. Both beams are then combined using an analyzing polarizer and the beat frequency, plus any phase change, is detected with a photo detector. Finally, a lock-in amplifier is used to resolve the phase difference as the conditions on the sensor surface change.
Graduate Student: Tim Chinowsky
Abstract: We seek to extend the capabilities of metal ion sensing by combining electrochemical analysis with surface plasmon resonance (SPR) spectroscopy at the electrode surface. These techniques are complementary. While SPR is very sensitive to electrode reactions and, as an optical technique, is resistant to interference, it provides limited information about the nature of the reaction. Electrochemical analysis, on the other hand, can provide specific information about the reaction occurring, but is vulnerable to electrical noise. This project will investigate sensors which combine the best features of both of these techniques while optimizing sensor sensitivity, stability, and practicality.
Initial research will focus on applications of SPR to anodic stripping voltammetry (ASV). In ASV, very sensitive detection of metals is obtained by a pre-concentration step in which metal ions are drawn from solution and are plated on to the electrode surface. This high concentration provides raw material for the stripping step in which the reaction is reversed by ramping of the electrode voltage. The voltage at which the reaction occurs indicates the species of metal ion, and the integrated current indicates the amount of material participating in the reaction. We will apply SPR to the stripping step, using SPR instead of current to monitor the progress of the reaction. By substituting an optical measurement for an electrical one, and by sensing the total material deposited instead of a rate of deposition, we expect to see a great gain in sensitivity.
In addition to sensor structure, geometry, and fabrication, we will be addressing the question of how best to incorporate the sensor in a sensor system--that is, how best to create a sensing instrument which is quickly and easily applicable to a wide variety of problems. Towards this end we are developing DSP-based, low-noise, low-power instrumentation (below), designed for portability and ease of use while also providing the real-time processing capabilities needed for exploring the ultimate capabilities of an advanced sensor.
Publication:
C. C. Jung, S. B. Saban, S. S. Yee, R. B Darling, "Chemical Electrode Surface Plasmon Resonance Sensor", Accepted, Sensors and Actuators, 1996.
Figure 3. Block Diagram of Instrument Controller
Engineering) and Sinclair Yee
Graduate Students: Linda S. Jung (Ph.D)
Sponsor: The Center of Process Analytical Chemistry (CPAC), Sandia National Labs, Department of Energy (DOE) and Texas Instruments (TI)
Abstract: The goal of this research is to create reproducible and stable coverages of organic adlayers on metal or oxide surfaces for the purposes of functionalizing the sensing surface of a surface plasmon resonance (SPR) instrument. This would result in the fabrication of highly specific sensors based on SPR technology that can be used for chemical analysis, enzyme/protein detection, etc.
SPR is sensitive to the refractive index local to the surface/analyte interface to within ~250 nm. This is measured as a change in the resonant wavelength at which the reflectance is minimized due to the energy being pumped into the evanescent plasmon field. By functionalizing the probe's surface with the proper receptor group, different analytes can be selectively bound and detected.
Organo-functionalized thiols are currently being used to functionalize the Au surface of the SPR sensor. The use of thiols to functionalize metals has already been studied extensively since its ability to form stable, well-ordered monolayers makes it attractive for model systems. Silane coupling agents which are used to functionalize oxide surfaces will also be studied. A high surface area (porous) SiO2 is first applied to the gold surface and then functionalized with silane coupling agents. The higher surface area significantly increases the amount of receptor groups that can be bound to the surface resulting in increased sensitivity. The quality of these resulting films is characterized with various surface analytical techniques that include time of flight-secondary ion mass spectrometry (TOF-SIMS), scanning tunneling microscopy (STM), and XPS.
Much of this work has utilized a SPR probe developed by the Yee group, which has been shown to have a sensitivity of ~2.8 nm per 10-3 index of refraction units, and a detection limit of ~1x10-5 index of refraction units. A more stable planar geometry, which uses glass slides instead of optical fibers, will also be used as it is expected to be even more sensitive. Results so far have shown that functionalization with
1-decanethiol (S-(CH2)9-CH3) did not adversely affect the instrument's sensitivity and that the SPR probe, after being functionalized with mercaptoundecanoic acid (COOH-(CH2)10-SH), was able to bind and detect trace amounts of triethylamine ((CH3)3NH) in solution.
Publications and Presentations:
Linda S. Jung, Lara Gamble and C.T. Campbell, "Sensor Development: Surface Receptors," CPAC Meeting, November 1994
Lara Gamble, Linda S. Jung and C.T. Campbell, "Sensor Development: Surface Receptors," CPAC Meeting, May 1995
Linda S. Jung, Lara Gamble and C.T. Campbell, "Surface Functionalization: Silane Coupling Agents on TiO2(110) and Thiols on Gold," American Vacuum Society Pacific Northwest Regional Conference , September 1995
Lara Gamble, Linda S. Jung and C.T. Campbell, "Sensor Development: Surface Receptors," CPAC Meeting, November 1995
Graduate Student: Wei Chih Wang
Sponsor: Ford Research Fellowship (1995), and TRW Development Fellowship
Abstract: The aim of this project is to develop a compact fiber optic viscosity and mass flow sensor capable of monitoring manufacturing processes in real time. The work is primarily focused on two efforts: the first aims at comprehensive understanding of the mechanics of the fluid measurement and the second focus on developing different sensor configurations for specific measuring conditions. Four fiber optic based sensors are currently under development. The first technique utilizes forward light scattering from a vibrating optical fiber for viscosity and flow rate measurement. The second technique measures viscosity and flow rate based on light intensity modulation in a fiber optic Fabry-Perot cavity. The third technique uses light modulation due to vibration induced microbending in a fiberoptic cavity. The fourth technique involves a gas viscosity sensor using optically excited and interrogating silicon based microresonator.
The concept of the fluid viscosity measurement is derived from the fluid's frictional damping on the surface of the immersed vibrating fiber optic probe. This frictional damping, which becomes the dominant factor in the fluid damping under a small fiber's vibration in a still fluid, is primarily a function of viscosity. Based on this viscous fluid damping model, a viscous imposed frequency response of the fiber's vibration is generated. As a result, the fluid viscosity can be deduced based on an equivalent damping coefficient or the maximum displacement derived from the frequency response. The concept of flow rare and mass density measurment is similar to that of viscosity measurement. By increasing the velocity of the flow or the vibration of the fiber, the viscous fluid damping is gradually replaced by a rising pressure drag. As a result, the displacement response of the vibrating fiber becomes inversely proportional to the fluid's mass density and the fluid's velocity. We have shown that the mass flow rate can be deduced based on the measured maximum vibration amplitude or on the half-width frequency. Unlike the viscosity case, the changes in the magnitude of the peak displacement and the half-width frequency will be seen as a change in the velocity or mass density of the flow.
Publications and presentations:
W.C. Wang, M. Afromowitz, B. Hannaford, "Technique for Mechanical Measurement Using Optical Scattering from a Micro-Pipette," IEEE Trans. Biomedical Eng., March, 1994.
W.C. Wang, S.P. Yee, P. Reinhall, "Optical Viscosity Sensor Using forward Light Scattering," Sensor & Actuator Proc. for the Fifth Int'l Meeting on Chemical Sensors Rome, Italy 1994.
W.C. Wang, S.P. Yee, P. Reinhall, "Optical viscosity sensor using forward light scattering," Sensors and Actuators, B 24-25, 1995, pp. 753-755.
W.C. Wang, S.P. Yee., P. Reinhall, "Fluid Viscosity and Mass flow Sensor Using Forward Light Scattering," SPIE Proc. for the Pacific Northwest Fiber Optic Sensor Workshop, May, 1995.
W.C. Wang, P. Reinhall, S.P. Yee., "Fluid Viscosity Sensor using Forward light Scattering," Journal of Dynamic Systems, Measurement and Control, submitted, 1996.
W.C. Wang, P. Reinhall, S.P. Yee. "Viscosity Measurement Using Nonlinear Vibration Effect (Superharmonic Response)," Journal of Applied Physics, in preparation, 1996.
W.C. Wang, P. Reinhall, S.P. Yee "Viscosity Measurement Using Nonlinear Vibration Effect (Subharmonic Response)," Journal of Applied Physics, in preparation, 1996.
[8] Wang W-C, Reinhall P., Yee, S. "Viscosity Measurment using embedded fiber optic microbending sensor," Review of Scientific Instrumentation, in preparation, 1996.
Patent Disclosure:
Measure fluid viscosity, mass density, and mass flow rate using forward light scattering from an optical fiber and Fiber-Optic Fabry-Perot Interferometer. (Patent Pending, Disclose to Office of Technology Transfer, Univ. of Washington, OTT # 05-94-78)
Graduate Student: Timothy Chinowsky
Sponsor: Washington Sea Grant
Abstract:: The objectives of the proposed research are to develop protein-based microbiosensors for application in the marine environment. The primary thrust of the research will be to develop two fundamental signal transduction technologies that can be used to develop a broad range of sensors for detecting specific organic compounds, bacteria, viruses, and metal ions in the marine environment. One sensor system will employ chemiluminescence as a signal generation system and the other will make use of surface plasmon resonance fiber optic technology.
Graduate Student: Lawrence Lam, (Ph.D)
Abstract: Process maturity of silicon and high demand of optical modulators raise the research interest of researchers. Free carrier dispersion effect is the main optical modulation means in silicon so far. However, an effective optical modulation does not occur until a high injection carrier takes place. This presents high current and high power issues. The goal of this research is to investigate a novel optical modulation scheme in silicon to suppress the drawbacks. One method is to use geometrical means to enhance the free-carrier injection modulation effect. This is accomplished by introducing a refractive step discontinuity in the wave guiding region. A radiation loss occurs as light is guided and scatter in the refractive step discontinuity. A FDTD simulation work is done and showing an order of magnitude improvement in the modulation length required to achieve the similar effect. Actual devices are designed, fabricated and tested. Experimental result shows a modulation depth up to 30% can be accomplished. We are in the process of data modeling. Future research work including high speed modulation using majority carrier, cascading modulation effect, discrete component packaging and on-chip electronic integration.
Publications and presentations:
L. Lam, H.C. Huang, S.P. Yee, "An Application of Radiation Loss Mechanism in a Silicon Guided-Wave Optical Modulator using Free-Carrier Dispersion Effect," PIERS 24 - 28 July, 1995.
H.C. Huang, Lo, L. Lam, "Integration of the Finite-Difference-Time Domain and Mode-Propagation-by-Fourier-Expansion Methods for Guided-Wave Device Simulation," SPIE OE/LASE +94, January 1994, pp. 22-29.