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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 Ink Jet Printer Head • well contains a small volume of ink • surface tension • droplet propelled using thin-film resistor made of tantalumaluminum 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 Ink Jet Printer Head • 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 Ink Jet Printer Head Sample Fabrication • 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. Ink Jet Printer Head 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 Ink Jet Printer Head 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 Ink Jet Printer Head Sample Fabrication • spin on polyimide and partially cure • patterned to leave a channel through which ink flows to the resistor • fabricate nickel orifice plate separately using electroforming or laser ablation • aligned and bonded to silicon structure by the polyimide 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 Micromachined Valve from Redwood Microsystems • Membrane is heated to either open or close the valve Micromachined Valve from Redwood Microsystems • 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 Micromachined Valve from Redwood Microsystems • 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 Micromachined Valve from Redwood Microsystems • 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 Micromachined Valve from Redwood Microsystems • fluid flow through an ideal orifice depends on the differential pressure across it • volume flow rate CD A0 2P / ΔP is the difference in pressure ρ is the density of the fluid A0 is the orifice area CD is the discharge coefficient 0.65 for a wide range of orifice geometries Micromachined Valve from Redwood Microsystems Fabrication • 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 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 Micromachined Valve from TiNi Alloy Company • 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 Micromachined Valve from TiNi Alloy Company • 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 Micromachined Valve from TiNi Alloy Company • 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 Micromachined Valve from TiNi Alloy Company Fabrication • thin-film deposition and anisotropic etching • form the silicon elements of the valve • orifice and the spacer wafers is simple Micromachined Valve from TiNi Alloy Company 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 Micromachined Valve from TiNi Alloy Company 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 Micromachined Valve from TiNi Alloy Company 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 Micromachined Valve from TiNi Alloy Company 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 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 Sliding Plate Microvalve • • • • • • 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 Sliding Plate Microvalve • 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 Sliding Plate Microvalve • 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 Sliding Plate Microvalve • • • • 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 Sliding Plate Microvalve • 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. Sliding Plate Microvalve • 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. Sliding Plate Microvalve • 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). Sliding Plate Microvalve Physical Desc • 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 Sliding Plate Microvalve 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 Sliding Plate Microvalve 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 heatflow rate • reduces power consumption • slows the response when cooling Sliding Plate Microvalve Fabrication • 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 Sliding Plate Microvalve 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 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