Applications: Actuated Systems CSE 495/595: Intro to Micro- and Nano- Embedded Systems Prof. Darrin...

Applications:Actuated Systems

CSE 495/595: Intro to Micro- and Nano- Embedded Systems

Prof. Darrin Hanna

Ink Jet Printer Head

• Hewlett-Packard, Inc., Palo Alto, California• Early inkjet heads used electroformed nickel nozzles• More recent use nozzle plates drilled by laser ablation

• silicon micromachining more expensive• High resolution printing – micromachined nozzles

• 1,200 dots per inch (dpi)• spacing between adjacent nozzles is 21 µm• cheaper using micromachining


• well contains a small volume of ink• surface tension

• droplet propelled using thin-film resistor made of tantalum-aluminum alloy

• locally heats water-based ink to over 250ºC• within 5 µs, a bubble forms

• peak pressures reach 1.4 MPa (200 psi)

• expels ink out of the hole• after 15 µs, the ink droplet is ejected from the nozzle

• volume on the order of 10-10 liters


• within 24 µs of the firing pulse, the tail of the ink droplet separates

• bubble collapses inside the nozzle• results in high cavitation pressure

• within less than 50 µs, the chamber refills• ink meniscus at the hole settles


• oxidize silicon wafer for thermal and electrical isolation• sputter 0.1 µm of tantalum-aluminum alloy

• TaAl is resistive, near-zero thermal coefficient of expansion

• sputter aluminum containing a small amount of copper• aluminum and TaAl are patterned leaving an Al/TaAl “sandwich” to form conductive traces.

Sample Fabrication


• remove aluminum from the resistor location leaving TaAl resistors• resistors and conductive traces are protected by layers of PECVD silicon nitride and silicon carbide

• SiN -- electrical insulator• SiC -- electrically conductive at elevated temperatures but more chemically inert than SiN

Sample Fabrication


• bilayer passivation with appropriate thermal properties and needed chemical protection reduces pinholes• SiC/SiN layers are patterned to make openings over the bond pads• tantalum sputtering is followed by gold sputtering • Ta acts as an adhesion layer for the Au• Au and Ta remain only on the contact pads and resistor• Au etched off of the resistor

Sample Fabrication


• spin on polyimide and partially cure• patterned to leave a channel through which ink flowsto the resistor• fabricate nickel orifice plate separately using electroforming or laser ablation• aligned and bonded to silicon structure by the polyimide

Sample Fabrication

Valves

• Applications• difficult to compete with traditional valves (price and performance) – more of a niche product

Micromachined Valve from Redwood Microsystems

• Membrane is heated to either open or close the valve• Fluorinert perfluorocarbon from 3M


• Membrane is heated to either open or close the valve


• boiling point ranges from 56° to 250ºC• large temperature coefficients of expansion (~ 0.13% per degree Celsius)• electrically insulating• control liquid choice determines:

• actuation temperature• power consumption • switching times


• NO-1500 Fluistor normally open gas valve• control of the flow rate for noncorrosive gases• flow rate ranges from 0.1 sccm up to 1,500 sccm• maximum inlet supply pressure is 690 kPa (100 psig)• switching time is typically 0.5s• average power consumption is 500 mW


• The NC-1500 Fluistor normally closed gas valve • similar pressure and flow ratings as NO-1500• switching response is 1s and it consumes 1.5W• measures approximately 6 mm × 6 mm × 2 mm

• Fluistor relies on the absolute temperature• valve cannot operate at elevated ambient temperature

• rated for operation from 0° to 55ºC


• fluid flow through an ideal orifice depends on the differential pressure across it• volume flow rate

ΔP is the difference in pressureρ is the density of the fluidA0 is the orifice areaCD is the discharge coefficient

0.65 for a wide range of orifice geometries

0 2 /DC A P


• intermediate silicon layer etched using KOH• both sides of the wafer

• front-side etch forms the cavity to be filled with liquid• bottom side forms the fulcrum as well as the valve plug• timed etch rate of both etches form thin diaphragm

Fabrication

Micromachined Valve from TiNi Alloy Company

• very different• actuation mechanism is titanium-nickel (TiNi)

• a shape-memory alloy• very efficient actuators • can produce a large volumetric energy density

• approximately five to 10 times higher than other methods• TiNi processing is not easily integrated in regular MEMS processing


• three silicon wafers• one berylliumcopper spring

• maintain a closing force on the valve poppet (plug)• one wafer incorporates an orifice• second wafer is a spacer • third wafer contains the poppet suspended from a spring structure made of a thin-film titaniumnickel alloy


• sapphire ball • between a beryllium-copper spring and third wafer• pushes the poppet out of the plane of the third wafer through the spacer of the second wafer to close the orifice in the first wafer

• normally closed


• current flow through the titanium-nickel alloy heats the spring above its transition temperature (~ 100ºC)

• contracts and recover its original undeflected position• pulls the poppet back from the orifice - opens


• thin-film deposition and anisotropic etching • form the silicon elements of the valve

• orifice and the spacer wafers is simple

Fabrication


• third wafer containing the poppet and the titanium-nickel spring• SiO2 is deposited on both sides of the wafer• back side -- timed anisotropic etch using the SiO2 as a mask defines a silicon membrane.

• TMAH because of its extreme selectivity to SiO2

Fabrication


• sputter titanium-nickel film, a few micrometers thickness on front• pattern

• this film determines the transition temperature• double-sided lithography ensures that the TiNi pattern aligns with the cavities on the back side

Fabrication


• evaporation and pattern Au • defines the bond pads and the metal contacts to the TiNi actuator

• wet or plasma etch from the back side to remove thin Si membrane• frees the poppet

Fabrication


• bond the three wafers together using glass thermo-compression• Si fusion bonding not practical since TiNi rapidly oxidizes at temperatures above 300ºC (that would be a bad thing)

• assembling valve elements is manual• list price for one valve is about $200

Fabrication

Sliding Plate Microvalve

• many micromachined valves use a vertically movable diaphragm or plug over an orifice

• diaphragm or plug sustains a pressure difference across it• pressure difference x area = force that must be overcome for the diaphragm to move• high pressures and flow rates large forces for a tiny device


• low power consumption• fast switching speeds• consumes less than 200 mW• switches on in about 10 ms and off in about 15 ms• maximum gas flow rate & inlet pressure 1,000 sccm and 690 kPa • valve measures 8 mm × 5 mm × 2 mm


• intended for use in such automotive applications• braking and air conditioning• require ability to control liquids or gases at high pressures

• ~2,000 psi (14 MPa)• wide temperature range

• –40°C to +125°C


• a plate, or slider, moves horizontally across the vertical flow from an orifice• forces due to pressure can be balanced to minimize the force that must be supplied to the slider


• once again, three layers of Si• inlet and outlets ports formed in the top and bottom layers• normally open valve• One of the two paths of fluid flow

• past the top orifice between the slider and the top wafer• through the second layer of Si• down out of the outlet port formed in the bottom wafer


• A second path of the two paths of fluid flow• through the slot in the slider• under the slider• through the lower controlling orifice• out of the outlet port.


• reduce or turn off the flow• actuator moves the slider to the right

• reduces the area of the two controlling orifices• pressure inside the slot = the inlet pressure pin

• horizontal pressure forces on internal surfaces of the slot are equal and opposite (balanced)• horizontal pressure forces on external surfaces of the slot balance each other because the pressure outside the slot is equal to the outlet pressure pout.


• pressure forces also balanced vertically• pressures on the top and bottom surfaces of the slider are equal to the inlet pressure • not perfect, but good

• operation is few MPa (hundreds of psi).


• actuator is entirely in the middle Si layer• a small gap above and below all moving parts to allow motion

• approximately .5 to 1 µm• thermal actuator - mechanically flexible “ribs” suspended in middle and anchored at edges

• electrically resistive

Physical Desc


• current flow through ribs heats them • expand

• centers of ribs push movable pushrod to the left

• torque about the fixed hinge• moves slider tip in the opposite direction.

• after current stops ribs cool down • mechanical restoring force of the hinges and ribs returns the slider to its initial position

Physical Desc


• depending on the geometry of the actuator ribs the actuation response time can vary

• few to hundreds of ms• depth of recesses above and below ribs can be increased to lower the heat-flow rate

• reduces power consumption• slows the response when cooling

Physical Desc


• shallow recess cavities are etched in top and bottom • KOH etch creates the ports, deep recess, and through hole for electrical contacts• actuator in the middle wafer is etched using DRIE• Si fusion bonding to stack wafers• metal for electrical contacts in middle wafer• ports are protected with dicing tape to keep them clean

Fabrication


• typical design includes ten or more rib pairs• each rib is approximately 100 µm wide, 2,000 µm long, and 400 µm thick, and is inclined at an angle of a few degrees• water at pressures reaching 1.3 MPa (190 psig) and flows of 300 ml/min• does not match automotive requirements yet

Fabrication

Micropumps

• must compete with traditional small pumps• Lee Company of Westbrook, Connecticut, manufactures a family of pumps

• 51 mm × 12.7 mm × 19 mm (2 in × 0.5 in × 0.75 in) • weigh only 50g (1.8 oz)• dispense up to 6 ml/min with a power consumption of 2W from a 12-V dc supply

• micromachined pumps can be readily integrated along with other fluidic components

• automated miniature system

Micropumps

• four wafers!• bottom two wafers - two check valves at inlet and outlet• top two wafers - the electrostatic actuation unit

• voltage applied between the top two wafers actuates the pump diaphragm

• expands the volume of the inner chamber• draws liquid through the inlet check valve to fill the additional chamber volume

Micropumps

• when applied ac voltage goes through 0• diaphragm relaxes• pushes the liquid out through the outlet check valve

• flap can each move only in a single direction• inlet valve flap moves only as liquid enters to fill the pump inner chamber• outlet valve is opposite

Micropumps

• So, is this bidirectional or will this only pump fluid in one direction?

Micropumps

• So, is this bidirectional or will this only pump fluid in one direction?

!

Micropumps

• as long as pump diaphragm displaces liquid at a frequency lower than the natural frequencies of the two valve flaps• at higher actuation frequencies—above the natural frequencies of the flap—the response of the two flaps lags the actuation drive

Micropumps

• when pump diaphragm draws liquid into the chamber• inlet flap can’t respond instantaneously

• remains closed for a moment longer• outlet flap is still open from previous cycle and does not respond quickly to closing

• the outlet flap is open and the inlet flap is closed• draws liquid into the chamber through the outlet

• phase difference between the flaps and the actuation must exceed 180º

Micropumps

• pump rate rises with frequency• peak flow rate of 800 µl/min at 1 kHz

• at exactly the natural frequency of the flaps (1.6 kHz)• pump rate rapidly drops to zero• phase difference is precisely 180º

• both valves are simultaneously open— no flow• after natural frequency the pump reverses direction • further increase in frequency reaches a peak backwards flow rate of –200 µl/min at 2.5 kHz

Micropumps

• at ~10 kHz actuation is much faster than the flaps’ response • flow rate is zero

• peak actuation voltage is 200V• power dissipation is less than 1 mW

Micropumps

Fabrication

Microfluidics

• rectangular trenches in a substrate with cap covers on top, capillaries, and slabs of gel• cross-sectional dimensions on the order of 10 to 100 µm • lengths of tens of micrometers to several centimeters• fluid drive or pumping methods

• applied pressure drop (common)• capillary pressure (common)• electrophoresis (common)• electroosmosis (common)• electrohydrodynamic force• magnetohydrodynamic force

Microfluidics

• pressure drive • apply positive pressure to one end of a flow channel• negative pressure (vacuum) can be applied to the other end

Microfluidics

• Electrophoretic flow can be induced only in liquids or gels with ionized particles

• apply voltage across the ends of the channel• produces an electric field along the channel that drives positive ions through the liquid toward the negative terminal and the negative ions to the positive terminal

• velocity of the ions is proportional to the electric field and charge and inversely related to their size• in liquids

• velocity is also inversely related to the viscosity• in gels

• velocity depends on porosity.

Microfluidics

• Electroosmotic flow occurs because channels in glasses and plastics tend to have a fixed charge on their surfaces• in glasses silanol (SiOH) groups at walls lose the hydrogen as a positive ion, leaving the surface with a negative charge

• negative ions attract a layer of + ions forming a double layer• layer of positive ions not tightly bound

• can move under an applied electric field• moving ions drag the rest of the channel volume along creating electroosmotic flow

• velocity at the center of the channel is about the same or slightly less, giving the fluid a flat velocity profile

Applications: Actuated Systems CSE 495/595: Intro to Micro- and Nano- Embedded Systems Prof. Darrin...

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