MEMS COMPONENTS
[Original graphic courtesy of Khalil Najafi, University of Michigan]
Sensors, Transducers and Actuators • A sensor is a device that receives and responds to a signal. • Transducers change one form of energy into another form of energy.
A microphone converts sound into electrical impulses A loudspeaker converts electrical impulses into sound A solar cell converts light into electricity A thermocouple converts thermal energy into electrical energy An incandescent light bulb produces light by passing a current through a filament An electric motor is a transducer for conversion of electricity into mechanical energy or motion
• Actuators An actuator is something that actuates or moves something and is a device that converts energy into motion.
• There are many types of sensors, transducers, and actuators found in microelectromechanial systems (MEMS).
Out-of-Plane Accelerometer
SEM courtesy of Khalil Najafi, University of Michigan, Silicon Wet Etching Presentation]
Out-of-Plane Accelerometers • MEMS accelerometers convert motion to electrical energy. • An out-of-plane accelerometer consists of an inertial mass suspended by fabricated springs. • Forces affect this mass as a result in an acceleration or inclination. • The forces cause the mass to be deflected from its nominal position. • The deflection of the mass is sensed as a change in capacitance between the proof mass (moveable electrode) and a fixed electrode.
Springs (Tether)
[Original graphics courtesy of Khalil Najafi, University of Michigan, Capacitive Sensors Presentation]
Cs
Elect rode 1
Elect rode 2
Lateral In-Plane Accelerometer
Lateral In-Plane Accelerometer • •
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Lateral in-plane accelerometers sense the lateral movement of a proof mass (moveable electrode) as it moves parallel to the plane of the substrate. As the mass moves due to an acceleration, inclination, or deceleration, the capacitance between the mass fingers and the fingers of the stationary electrodes changes. The amount of change in capacitance represents the amount of movement. The figure illustrates how the space between the fingers of the mass and the electrodes is seen as a capacitance.
3-Axes Accelerometer Out-of-plane Device
In-plane Devices
[Graphic courtesy of Khalil Najafi, University of Michigan]
Polysilicon Connectors
3-Axes Accelerometers • An inertial sensor might have three accelerometers, one for x, y, and z directions. • The sensor in the diagram has two in-plane accelerometers for the x and y movements and an out-of-plane accelerometer for the z movement. • The MEMS sensor uses the accelerometers’ movements to monitor the acceleration and/or inclination of an object (e.g. cars, computers, cameras) • Movement is sensed by a change in capacitance between the proof mass (moveable electrode) and a fixed electrode.
MEMS Cantilevers [Cantilever array developed by and printed with permission of Seyet LLC]
MEMS Cantilevers • •
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The microcantilever is one of the most versatile transducers in MEMS sensors. It can convert pressures, chemical reactions, changes in mass, and temperatures to outputs such as changes in frequency, resistance, or angular deflection of light reflected off its surface. To detect a specific target molecule, the cantilever is fabricated with a probe coating on one or both surfaces as shown in the graphic. The probe coating is a chemically sensitive layer that provides specificity for molecular recognition. The left graphic shows viruses (red spheres) being adsorbed by a cantilever coating of a specific antibody (blue).
Surface Reactions on Cantilevers / Beams
Surface Reactions on Cantilevers • Surface reaction is when the analytes are confined to the surface of the probe coating. • The figure shows a coating as a monolayer of probe molecules on top of a gold layer. • The reaction is chemisorption of the analytes on the cantilever's surface. • Notice how the analytes are confined to the surface. The reaction at the surface causes thermal expansion of the probe coating. Because the gold layer is not experiencing the same thermal stress as the surface, it tends not to expand. This mismatch results in a bending of the cantilever. • This same reaction can be seen by making a cantilever out of layers of two different metals. When heated, the two metals will react differently, resulting in an upward or downward bend.
Strain Gauge (A piezoelectric transducer)
Strain R = ρ (L / (Wt), where R is resistance, L is length, W is width, and t is thickness Total Resistance = n * R, where n is the number of legs in the gauge
Strain Gauge (A piezoelectric transducer) So how do MEMS measure the amount of “bend” in a cantilever or beam? How about a strain gauge? • A strain gauge can be made by depositing a metal on a region of a non-conductive solid that experiences strain (stress). • The change in the resistance of the metal legs of the gauge provides a measure of the strain. • The gauge pattern can have one or more “gauge legs” that will stretch due to strain. • By connecting the gauge to an electrical circuit, the sensor can measure any change in the gauge’s resistance by a change in current. Each leg of the gauge has a resistance R, with a total resistance of n*R (n being the total number of legs in the gauge).
Examples of MEMS with Strain Gauges
Examples of MEMS Strain Gauges • Microcantilever sensors use strain gauges or piezoresistive layers fabricated into the microcantilever. (Top image) • The bottom image of the diaphragm and strain gauges is actually a Wheatstone bridge MEMS circuit with four resistors (two fixed and two variable). – The bottom right picture is the electrical drawing of a Wheatstone Bridge. – The variable resistors are the strain gauges. Notice that their placement on the diaphragm is such that they will experiences the greatest amount of stress when the diaphragm flexes. – The two resistors on the edge of the diaphragm are considered “fixed” since they will experience a negligible amount of stress. – The pads are the input and output nodes for the circuit.
Example of MEMS Strain Gauge (Pressure Sensor)
Example of a Diaphragm MicroPressure Sensor [University of New Mexico, MTTC]
Electrode Transducers
Electrode Transducers •
Just like cantilevers, electrodes can be used as transducers to sense analytes in a sample.
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This electrode transducer consists of a piezoelectric (PZT) layer and a probe coating.
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Just like with the cantilever, the probe coating detects and captures the target molecules.
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A chemical reaction causes the probe coating to expand (or contract).
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The PZT layer expands with the coating. This creates a change in the PZT resistance.
Surface Acoustic Wave (SAW) Sensors
Delay Line
Surface Acoustic Wave (SAW) Sensors Surface Acoustic Wave (SAW) sensors are currently used as electronic filters, delay lines, and resonators in communication systems. They are being tested for use as biosensors, torque and pressure sensors, and humidity and gas sensors. • A SAW uses two interdigited transducers (IDT) (an input and an output) to sense a shift in the frequency of an input wave after it travels across a “delay line.” • The delay line could be a sensing film or probe coating that identifies and captures target molecules (e.g. a specific gas or biomolecule such as a virus). • The input IDT generates the original wave. The output IDT measures a shift in frequency caused by the analytes on the sensing film. • When the wave hits the output IDT, it causes a mechanical shift in the transducer fingers which is sensed as a voltage change. [“Design and Fabrication of Novel SAW Bio/Chemical sensor in CMOS.” Tigli, Zaghloul. George Washington University. IEEE 2005]
Electrostatic Actuators
[Original graphic courtesy of Khalil Najafi, University of Michigan]
Electrostatic Actuators Many applications require the generation of forces vertical and parallel to the wafer’s surface. • The top actuator, the varying gap actuator, uses an electrostatic actuation to produce a vertical movement of the mass. An electrical signal is applied to the fixed electrode which creates movement in the moveable electrode. (Opposites attract, likes repel) • Comb drive actuators are used to generate forces parallel to the wafer surface. In the actuator, we have a lateral movement – the fingers of the moveable electrode move farther into or out of the fixed electrode. • Both electrostatic actuators can produce variable movement with the applications of a variable voltage allowing for precise positioning of devices such as pop-up micromirrors.
Electrostatic Comb Drives
Comb drives of a 3-layer polysilicon microengine are shown here. The springs in the center provide the restoring force, returning the electrostatic comb teeth to their original position. [Image courtesy of Sandia National Laboratories, www.mems.sandia.gov.]
Electrostatic Comb Drives • •
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A common MEMS actuator is the linear comb drive, also called an interdigitated comb drive. A comb drive consists of two combs that face each other such that their teeth interlock as shown in the Scanning Electron Microscope (SEM) image. One comb is fixed or anchored; the second comb is moveable and attached to a spring. During operation, each comb acts as an electrode. Comb drive fabrication ensures that the teeth of the two combs do not touch and are always mechanically and electrically separate.
Comb Drive Operation Fixed comb
Moveable comb
Travel stop
Spring
Comb drive showing Travel Stops and Springs of a Comb drive [Images courtesy of Sandia National Laboratories, www.mems.sandia.gov]
Comb Drive Operation •
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Comb drives move due to electrostatic interactions, those interactions that cause opposite charges to attract and like charges to repel. One comb is grounded. A positive voltage is applied to the other comb. Electrons move from ground to the tips of the grounded comb’s teeth. The opposing charges pull the moveable comb linearly into the fixed comb. When the voltage is removed, the tension of the spring returns the moveable comb back to its resting position.
MEMS Electrodes
This MEMS switch is actuated when a voltage is applied to the pull down electrode. Such switches are used in RF (radio frequency) applications. This technology is called digital light processing or DLPTM, a trademark owned by Texas Instruments, Inc. Electrodes are used to move the mirror into the ON (into the light) or OFF (away from the light) position. [Image courtesy of Texas Instruments]
MEMS Electrodes In MEMS, electrodes have many uses. Other slides show electrodes used as chemical sensors and as teeth or combs in accelerometers and comb drives, respectively. Other applications of MEMS electrodes include the following: • Detect the flow of electrons as in a Scanning Electron Microscope • Conduct current between points (e.g. switches) • Transmitter / Receiver (e.g. pacemakers, retinal implants, neural probes) • Actuators to create movement when voltage is applied and/or removed (e.g. digital mirrors, cantilevers, comb drives)
Micropumps
The diagram illustrate a nonmechanical pump used in bubblejet print heads. A heater is used to evaporate the ink, forming a bubble. The bubble pushes the ink below it out of the nozzle. When the heater is turned off, capillary action allows the ink to refill the reservoir.
The pump in the picture is less than one fourth the size of existing insulin-pump devices. Once tested and approved this pump will be encased in a nearly invisible patch placed on the skin. [Printed with permission from Debiotech SA]
Micro and Nanopumps MEMS pumps are used in • drug delivery systems, • ink jet print heads, • lab-on-a-chip (LOC), and • microfluidic research (just to name a few). Just like larger pumps, micropumps can be mechanical or nonmechanical. Actuation of micropumps include electrostatic, pneumatic, electromagnetic, piezoelectric, heaters, electroosmotic flow, and ultrasonics.
Diaphragm Micropumps
Diaphragm Micropumps •
Diaphragm micropumps have moving parts that create changes in pressure within the pump. These pressure changes cause the fluid to flow.
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A popular use of the diaphragm pump is the inkjet printer shown in the graphic. In this inkjet print head, piezoelectric crystals initiate the pumping action of the micropump by moving a diaphragm up and down. –
As the diaphragm moves up, ink flows from the reservoir into the ink jet head where the pressure is lower.
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When the diaphragm moves down, the pressure increases and the ink is pushed out the nozzle.
Batteries The left graphic illustrates a MEMS battery for use in vivo (internal) to power components of a retinal prosthesis. The image below shows the artificial retina (an electrode microarray placed on retina.) [Images courtesy of Sandia National Laboratories]
MEMS Batteries Sandia National Laboratories has designed a battery that can be used in vivo (internal) to power the components of a retinal prosthetics. (See Images) “A team of in the Georgia Tech School of Materials Science and Engineering has developed a nanogenerator that converts motion into electrical current. The nanogenerator is an array of tiny filaments-zinc oxide nanowires that produce continuous direct-current electricity from mechanical energy. Safe enough for use in biomedical applications, the nanoscale generator could generate energy from internal vibrations and even blood flow.” (“Science of the Small”. Courtney Howard. Military and Aerospace Electronics. June 2007.)
MEMS Valves
MEMS Valves • Microvalves can be used to control the flow of gases or liquids. • Microvalves are used in MEMS where fluid flow is required to be turned On or Off. • MEMS valves are normally electrostatic devices, but pressure and temperature actuated valves are being developed. • The graphic shows a piezoelectric unimorph actuator in the actuated state (top). This valve operates by applying a voltage across the piezoelectric (PZT) layer. This leads to an uneven temperature change in the two cantilever layers due to different materials with different temperature coefficients. The result is an upward bending of the cantilever creating an opening between the inlet and fluidic channel.
ACKNOWLEDGEMENTS Copyright 2009 - 2010 by the Southwest Center for Microsystems Education and The Regents of the University of New Mexico. Southwest Center for Microsystems Education (SCME) 800 Bradbury Drive SE, Suite 235 Albuquerque, NM 87106-4346 Phone: 505-272-7150 Website: www.scme-nm.org email contact:
[email protected] The work presented was funded in part by the National Science Foundation Advanced Technology Education program, Department of Undergraduate Education grants: 0830384.
