19 - Fundamentals of Microfabrication: 
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Unformatted text preview: Fundamentals of Microfabrication: 
 Guest Lecture, Winter 2010! Micro Actuator Systems for Compressible Gas Flow Control Professor Hanseup Kim " Electrical and Computer Engineering " University of Utah- Salt Lake City " 50 S. Central Campus Drive " Salt Lake City, UT 84112 " (801) 587-9497 " Email: hanseup@ece.utah.edu " Example of Microfluidic Application ! Courtesy of Prof. Carlos Mastrangelo, University of Utah Microfluidics is: •  A high precision fluidic manipulation technique; •  A method to achieve chemical reactions fast and at small amount; •  Difficult to achieve under limited force and deflection available. Need for Microfluidic Integration ! Courtesy of Prof. Carlos Mastrangelo, University of Utah •  •  Microfluidic Transducers ! A fluid is a material (gas or liquid) that deforms continually under shear stress, i.e., the material can flow and has no rigid three-dimensional structure." Micromachining applications in fluidics have become more important as people strive to create complete fluidic systems in miniaturized formats. Many of the key building blocks (flow channels, flow restrictors, mixers, pumps, valves, sensors, and etc) are either fairly mature or are already under development. A broad variety of materials are available for fabricating the systems or their components, including glass, plastics/polymers, metals, ceramics, and semiconductors." Applications of micromachined fluidic systems include chemical analysis, biological and chemical sensing, drug delivery, molecular separation, amplification, sequencing or synthesis of nucleic acids, environmental monitoring; and many others." •  Issues in Microfluidics ! •  To take full advantage of micromachining technologies for microfluidics, one must deal with significant additional issues, such as packaging, interfaces between components (often made from different materials), and testing." In the early development of electronic systems, similar issues were present and were dealt with by standardized packaging, interconnects like printed circuit broad and with modern electronic test components." Unfortunately, the analogy between electronic and fluidic systems is not perfect, since electronic components do not generally require consideration of flowing materials, chemical compatibilities, high pressures, packaging with openings for fluidics access, etc. It is likely, however, that as high-volume markets are identified, mass production issues will drive the development of solutions to these problems." •  •  Difference Between Macroscopic and Micromachined ! Issues! Unwanted turbulent flow?" Very small dead volume?" Problems purging bubbles?" Efficient liquid pumps available?" Efficient liquid valves available?" Efficient gas pumps available?" Efficient gas valves available?" Simple interconnect scheme?" Chemical resistant materials available?" Low power?" Sub-cm2 volume?" High surface-area-to-volume ratio?" Batch fabricated" Macroscopic! Y" Varies" N" Y" Y" Y" Y" Y" Y" N" N" N" N" Micromachined! N" Y" Y" Not yet" Not yet" N" Y" N" Varies" Varies" Y" Y" Y (not packaging)" •  •  General comparison of performance of macroscopic vs. micromachined fluidic devices from 1998. Courtesy of Prof. Greg Kovacs, Stanford University, Micromachined Transducers Sourcebook, WCB/McGraw-Hill, 1998." Progress in the past >10 years: Some efficient liquid pumps for selected applications; beginning to see gas pumps with some efficiency, but not able to achieve high vacuum." Microfluidic Components and Systems ! Introduction: Nature ! •  Squid:
 http://seawifs.gsfc.nasa.gov/OCEAN_PLANET/HTML/squid_move.html! History of an Acoustic Resonator ! Chladni (1787)! Mode shapes of vibrating plates.! Faraday (1831)! Circulatory air currents on Chladni plates.! Helmholtz (1862)! Frequency selection from a complex sound
 in resonators.! Rayleigh (1877, 1884)! Jet produced by Acoustic resonator.! Eckart (1948)! Performance parameters.! Scaling Arguments ! •  •  Rayleigh [1877]:! Thrust:! fo: A V ~ LLLS: ~ LL2LS: Cavity resonant frequency Throat cross sectional area Resonator volume Resonator surface area Change in resonator volume AR ~ LL2: •  Reynolds Number:! ΔV ~ hoLL2: •  For L ~ 10-5, ho ~ 10-6: –  Thrust per unit area ~ 102 N/m2 = 100 µN/mm2. –  Silicon density ~ 2.3g/cc = 23 µN/mm3. Reynolds number ~ 102. Motivation ! Dilemma of Electrostatic Actuation ! To increase thrust (Th)" Need to " increase volume change (ΔV)" Increase membrane deflection (Δd)" Increase the gap distance (Δd) 
 (between the membrane and the electrode)" But, the increased gap distance increases 
 operating voltages. (Δvolt)" Curved Electrode ! Reduced gap around the edges all the time" Reduced voltage operation while achieving the large deflection" Original position Movable membrane Strong electric field around the edges Curved fixed electrode Operation Principle of the Helmholtz Resonator with a Curved Electrode ! Fabrication Challenge ! How to form an out-of-plane curved profile on a wafer surface?" Analog lithography or RIE Lag?" ? Hard to control a depth where a curved surface forms" Mechanical etching or Molding?" Hard to form a micro scale surface" Dirty / not IC process compatible " Buckling for a Curvature ! Compressive stress applied" Simple process
 one mask process" Stressed films have stronger structural strength
 better electrode: minimize its own movement as the ʻfixedʼ electrode" Close distance to a membrane around the edges " Buckling generates a curvature" Then, a directionality is added by tensile nitride thin film on top of composite films (Fan et al., 1990)" H.Kim et al, Transducers 2003 Buckled Electrode Results ! Buckled electrode after TMAH release Cross-section view of a buckled electrode and an undercut cavity Smooth curved electrode with a center depth of 8.8~18.7 µm in different versions." High uniformity across a wafer (> 97%)" Excellent structural strength (at least three times stronger than the thickest (~ 10 µm) boron-doped electrode available" Microfabrication ! Specifications of a microthruster: 1.2 × 1.2 mm2, oxide/polySi/nitride layers of a moving membrane, buckled electrode center depth of 8.8 µm, buckled electrode thickness of 4.4 µm: oxide 0.5, polySi 3.8, nitride 0.1 µm, 8 throats." Measurement Results ! Thrust air veloctiy measurements Under a 120 V sinusoidal signal 1.1 •  Op. frequency" 50-70 kHz" Throat Throat Air velocity (m/s) 0.9 0.7 0.5 0.3 0.1 -0.1 0 0.5 •  Op. voltage V" 100-140 •  Hot-wire measurement" 70 kHz 62 kHz 50 kHz 1 Positions (x-direction) (mm) 1.5 2 •  Hot-wire distance" 1-5 mm" •  Max. velocity was measured on top of throats." •  Max. velocity" •  Avg. velocity" •  Dimension (mm3)" 1.2 m/s " 1.0 m/s" 1.2 × 1.2 × 1 " Jetting Visualization ! Generated ethanol column: 12 cm " Thrust Measurement: Pendulum ! Tanθ= Δd L = Thrust mg Measured thrust = 55.6 µN θ L Δd THRUST FLOW Ruler Δd Pendulum length: ~1.9 m" Device weight: 0.56 g (2.1 g including a mount)" Device displacement: ~5 mm " Curved vs. Flat electrode: Modulated Pendulum ! THRUST FLOW Ruler Curved Amplitude modulation of input signals at a frequency of 0.43 Hz." Max. Displacement: 8.4 cm, (previous flat electrd.: 2.7 cm)" A Gas Micropump for MicroGC Systems ! Micro pump " Micro column" Micro sensor array" WIMS µGC in the University of Michigan! -  Small size: < a few cc" -  Small power: < 1 W" -  Fully-integrated system including even a micro pump! Electrical control circuits" Previous µGCʼs
 - No gas micro pumps included" •  Goal: Develop a micro pump for the functioning µGC! –  Flow rate: 2-50 sccm, Pressure: 0.2 to 0.5 atm, Multi-Mode" –  Power: <100 mW, Size: < 10 × 10 × 2 mm3" Integrated Multi-stage Gas Micropump ! Goal: Develop a gas micropump for the µGC! Flow rate: 2-50 sccm, for Pressures: 20 to 50 kPa" Power: <100mW" Size: <1cm × 1czzm × 2mm" Approach: ! Inlet valve Peristaltic multi-stage:! High pressure" Electrostatic actuation:! Fast, low-power" Double-sidzzzzed curved electrodes:! Large displacement" High flow" Polymer membrane:! Large displacement" Low power" Resonant operation:! High flow" Low power" Pump Outlet valve ΔP ΔP ΔP ΔP ΔP ΔP Peris taltic multi -stag e pum p Reference: US Patent # 7,008,193 K. Najafi, H.Kim, et.al., “Micropump Assembly For A Micro gas Chromatograph And The Like”, March 7, 2006 Design of the 2-Stage Pump ! Electrostatic Actuation " Active Micro Valves" Multi-Stage Design" Polymer Membrane " Dual-Electrode Actuation " Dual-Chamber Layout" Curved Electrode" Bottom Pump! Chamber! Pump! Membrane! Top Silicon Wafer! Inlet µValve! (Checker-Board)! Top Pump! Electrode! Top Pump ! Chamber! Valve! Membrane! Paryle ne Wafers -Bonded! ! Bottom Si Wafer! Bottom Pump! Electrode! Outlet µValve! (Checker-Board)! Operation Principle of the Peristaltic Pump ! Peristaltic gas progression in synchronized motions of pump, inlet valve, and outlet valve membranes, respectively." Dual-chamber structure reduces overall pump dimensions." Dual-electrode operation increases membrane stroke volume." Multi-Stage Layout and the Microvalves ! The multi-stage pump can be layed out to generate any number of stages needed. " Layout of 18-stage pump shown below. Two-, four-, and 18-stage pumps have been designed and fabricated." Gas flow is controlled by the integrated 19 checkerboard 
 microvalves shown on the right." Hole on top electrode and valve membrane Hole on bottom electrode Microvalve Timing and Multi-Mode Pumping ! HFT: minimized gas compression min. pressure rise, max. flow rate" HPT: extended gas compression higher pressure rise, lower flow rate" Technology Developments ! Parylene Wafer Bonding! Low-Temp, <230°C" Thin Layers (<0.5µm)" Reliable, No Voids" Alignment" Curved Electrodes! Efficient Electrostatic Drive" High Force, Low-Voltage" No Need for Special Techn." P-bonded wafer interface Over 50-µm distance at the bonded wafer surface Silicon Cavity Area 13.2um squares Parylene Membrane Transfer! Wafer-Level" High-yield (>90%)" Bonded Thin films, over deep Area cavities" Microfabrication and Technologies ! Microfabrication Process! Tech. Developments! 1. Out-of-plane curvature 
 1 Development of a buckled electrode 
 (Kim et al., Transducers ʼ03)" 2 2. Freestanding membrane
 Development of a Parylene Membrane Transfer technique
 (Kim and Najafi, Transducers ʼ03, submitted to JMEMS) " 3. Bonding wafers 
 3 Development of a wafer bonding using Parylene 
 (Kim and Najafi, JMEMS 2005)" The Microfabricated Micropump ! A complete fluidic path through two wafers" Dual-electrodes" Polymer membrane" Families of Fabricated Gas Micropumps ! •  Several different generations of micropumps have been fabricated & tested •  2-stage, 4-stage, and 18-stage pumps have been fabricated and tested •  Nanoports are used to provide fluidic connection to the pump Reference: US Patent # 7,008,193 K. Najafi, H.Kim, et.al., “Micropump Assembly For A Microgas Chromatograph And The Like”, March 7, 2006 Summary of Pressure-Flow Testing of 18-, 4-, and 2-stage Pumps ! Max. measured pressure differences @~15kHz: 17.5kPa (18-stage), 7.0 kPa (4-stage), and 2.5 kPa (2-stage), respectively. Practical range for app." The highest pressure generated by any electrostatic MEMS micro gas pump." The first realization of gas pressure build-up through a multi-stage pump.! Maximum flow rates were measured at ~17kHz as 4.0 sccm in the 18-stage pump, & at ~14kHz as 3.0 & 2.1 sccm in the 4- & 2-stage pumps." Measured power consumption: 57mW/18-stage, 15.1mW/4-stage, 9.1mW/2-stage" Higher maximum pressures & flow rates were measured at voltages >100V.! Efficiency and Mode Testing of 18-, 4-, and 2-stage Pumps ! Higher flow rate was generated by using a small (2×2 mm2) membrane at a high frequency Fluidic resonance, appropriate control of microvalves. " The highest efficiency, defined by the flow rate to membrane stroke volume, by any membrane-based MEMS micro pump.! Two operation modes (HFT and HPT) were obtained reliably by actively adjusting the opening duration of microvalves. " The first multiple-modes pumping operation using integrated microvalves by any MEMS micro pumps.! Visualization: Bubble Test ! Visualization: Droplet Transportation ! Visualization: Manometer Test ! Toward Bio-Microsystems in Moving Fluids! 16 control ports for 256 µ-valves
 limited space, leakage" 16 pressure controller" Highly pressurized gas tank (high volume)" Thorsen, et.al., “Microfluidic large scale integration”, Science, v298, n5593, pp. 580-584, 2002. Motivation: to eliminate conventional pneumatic control components, large number of fluidic interconnections, gas leaks, complexity in control and structure, and external components (gas tanks or syringe pumps). ! Hydraulic Actuator! Deflection amplification" Force amplification" Electrostatic Hydraulic Actuator! Electrodes DI water Fabricated device H.Kim, S. Lee, and K. Najafi, “High-force liquid-gap electrostatic hydraulic micro actuators”, microTAS ‘07, Paris, France. Hydraulic Piston Actuator Array! H. Kim and K. Najafi, “An electrically-driven, large-deflection, high-force micro piston hydraulic actuator array for large-scale micro fluidic systems”, MEMS ‘09, Sorrento, Italy. Preliminary Performance! ...
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