Lecture 10 Part II:

MicrofabricationMicrofabrication forfor MicrofluidicsMicrofluidics andand MicrofluidicsMicrofluidics DevicesDevices

SiliconSilicon EtchingEtching PolymerPolymer--basedbased MicromachiningMicromachining AssemblyAssembly andand PackagingPackaging BiocompartibilityBiocompartibility TechniquesTechniques involved:involved:

„ WetWet etchingetching ofof channelschannels inin SiSi andand glassglass (isotropic,(isotropic, anysotropicanysotropic)) „ DryDry etchingetching „ ResistResist lithographylithography „ PDMSPDMS softsoft lithographylithography „ HotHot embossingembossing „ OtherOther machiningmachining techniquestechniques inin plastics,plastics, glassglass etc.etc. „ BondingBonding WetWet etchingetching ofof (100)(100) SiliconSilicon WetWet etchingetching ofof otherother orientationorientation ofof SiliconSilicon IsotropicIsotropic etchingetching ofof SiliconSilicon andand GlassGlass

Silicon: Etchant: 66% HNO3 and 34% HF Etching rate: 5um/min

Glass: Etchant: HF (or BHF) ChemicalChemical drydry etchingetching

„ Deep trenches with high aspect ratios can be made in Si, glass or plastic

„ Gases used:

– Fluorine chemistry (CHF3, SF6, CF4) – Chlorine chemistry (HCl, Cl2) – Oxygen BulkBulk micromachinedmicromachined channelschannels SiliconSilicon surfacesurface micromachiningmicromachining

PECVD oxide

oxide

PECVD oxide

wet etching PolymerPolymer basedbased micromachiningmicromachining

„ ThickThick resistresist lithographylithography „ PolymericPolymeric basedbased micromachiningmicromachining „ SoftSoft lithographylithography „ MicrostereoMicrostereo lithographylithography „ MicromoldingMicromolding SUSU--88 resistresist

„ NegativeNegative photoresistphotoresist forfor NUVNUV exposureexposure

ReallyReally thick thick layers layers in in one one spin! spin!

L/S 10/30 20/60 30/90 ExampleExample ofof SUSU--88 structuresstructures Fabrication of open channels Fabrication of covered channels

embedded selective sacrif.layer mask proton writing SoftSoft lithographylithography

„ Uses elastomeric stamp, usually PDMS (Polydimethylsiloxane) to transfer the pattern. – Microcontact printing

– Micromolding FabricationFabrication ofof microchannelsmicrochannels usingusing softsoft lithographylithography

„ Advantages of PDMS: – Low cost – Transparency in VIS and NUV – Chemically inert „ Technology: – Mix prepolymer and curing agent 10:1 – 5:1 – Pour into solid master made in SU-8 with inlets defined by glass posts – Cure at 60 – 80 oC for couple of hours – Peel off – treat with ozon or Oxygen plasma and attach to clean glass, silicon or another PDMS MicrostereoMicrostereo lithographylithography

Single photon adsorbtion Two photon absorbtion Layer-by-Layer photolithography MicromoldingMicromolding

„ Injection molding: high pressure injection of molten PMMA, PC (polycarbonate), PSU (polysulfone) etc.

„ Compression molding (hot embossing) OtherOther micromachiningmicromachining techniquestechniques

„ Laser micromachinig (usually using an excimer lasers, Nd:YAG or CO2 lasers)

„ Focused ion beam

„ Microelectro discharge

„ Ultrasonic micromachining AssemblyAssembly andand packagingpackaging

„ Anodic bonding (T=400 oC, V=1kV)

„ Silicon direct bonding reaction of OH groups on Si surfaces at T=300 – 1000 oC „ Glass direct bonding (T=600 oC for 6-8h) „ Polymer direct bonding „ Adhesive bonding (low melting glass (400-600oC, photoresists, UV curable epoxies, epoxies etc.) „ Eutectic bonding (e.g. gold/silicon eutectic at 363oC) OtherOther microfabricationmicrofabrication issues:issues: BiocompartibilityBiocompartibility

„ MaterialMaterial responceresponce toto biologicalbiological environmentsenvironments ((swellingswelling,, corrosioncorrosion etc.)etc.) „ TissueTissue andand cellularcellular responceresponce toto thethe materialmaterial MicrofluidicsMicrofluidics:: DevicesDevices forfor FlowFlow ControlControl

•Valves •Pumps •Micromixers Microvalve consist of closure element driven by an actuator Normally open

Active Microvalves Normally closed

Bistable

Analog Active Microvalves Digital

Analog Digital

Pulse-width modulation Fluidic DAC Pneumatic

Thermopneumatic

Thermomechanical

Piezoelectric Active Microvalves Electrostatic

Electromagnetic Actuation principle

Electrochemical

Capillary forcel Valve specification: Q L = closed valve  • leakage Qopen  Qmax Cvalve = • valve capacity ∆pmax /(ρg) • power consumption – total power consumption in active state • closing force (pressure range) – pressure generated by the valve • temperature range • responce time • reliability • biocompartibility • chemical compartibility DesignDesign considerationsconsiderations

„ SpecificationSpecification requiredrequired „ MaterialsMaterials toto bebe usedused „ CostCost „ SuitableSuitable typetype ofof actuatorsactuators „ OptimalOptimal valvevalve springspring andand valvevalve seatseat DesignDesign considerationconsideration:: ActuatorsActuators

„ Moving function (enough force, displacement and controllability) „ Holding function (should keep valve in a set position) „ Dynamic function (required responce time)

F s E ' = a a Energy density: a Va DesignDesign considerationconsideration:: ValveValve springspring

„ For normally closed valves (NC) – large spring constant to resist the pressure „ For normally open valves (NO) – soft spring constant, optimized for actuator closing force DesignDesign considerationconsideration:: ValveValve seatseat Main requirements: „ Zero leakage „ Resistance against particles trapped DesignDesign considerationconsideration:: PressurePressure compensationcompensation

Aim: Maintain closing force when the inlet pressure vary

Passive flow controller Stack of 3 directly bonded Si wafers, Si membrane 25 um thick

Silicon rubber membrane 25 um thick prepared by spin coating, hole drilled with laser

Silicon rubber membrane 30 um thick prepared by surface micromachinig, photoresist used as a sacrificial layer. Boron implantation

Typical parameters of pneumatic valves: ThermopneumaticThermopneumatic valvesvalves

„ ReliesRelies onon thethe changechange inin volumevolume ofof sealedsealed liquidliquid oror solidsolid underunder thermalthermal loadingloading.. UsuallyUsually utilizeutilize solid/solid/liquidliquid andand liquidliquid/gas/gas phasephase transitiontransition forfor maximummaximum performanceperformance

ExampleExample

„ ThermopneumaticThermopneumatic valvevalve withwith air.air. HeightHeight ofof thethe expansionexpansion cylindercylinder 500um.500um. „ AssumingAssuming thethe volumevolume constantconstant::

T1 p1 p2 109.67 = → T2 = T1 = 300 = 329K = 56C T2 p2 p1 100 ExampleExample DesignDesign examplesexamples

Heater is placed on SiN membrane, 50um Silicon rubber used for membrane

9um paraffin used as actuation material 2um deflection with 50mW heating ThermomechanicalThermomechanical valvesvalves

„ SolidSolid expansionexpansion „ BimetallicBimetallic „ ShapeShape--memorymemory alloysalloys SolidSolid expansionexpansion valvesvalves

„ Generated force: F ∝ γ∆T where γ is the thermal expansion coefficient BimetallicBimetallic valvesvalves

„ Uses difference in thermal expansion coefficient of two metals

F ∝ (γ 2 −γ 1)∆T BimetallicBimetallic valvesvalves:: DesignDesign examplesexamples ShapeShape memorymemory AlloyAlloy valvesvalves

„ Shape memory alloys (SMA) are materials that have property to return to their original undeformed shape upon a change of temperature „ Advantages: high force and large stroke „ Disadvantages: low efficiency, low frequency (bandwidth)

PiezolelectricPiezolelectric valvesvalves

„ generate small strain (0.1%) and high stresses (MPa), therefore suitable for applications with high force and low displacement

ϕl = ϕt = d31Eel

ϕv = d33Eel ExampleExample:: DesignDesign examplesexamples ElectrostaticElectrostatic valvevalve

„ Based on attractive force between two oppositely charged ε ε plates 1 V F = A( )2 ,ε = 8.854*10−12 F / m 2 r 0 d 0

„ Advantages: fast responce „ Disadvantages: high voltage and small discplacement DesignDesign examplesexamples ElectromagneticElectromagnetic valvesvalves

„ Uses solenoid actuator with a magnetic core and a coil

„ Advantage: large deflection „ Disadvantage: low efficiency DesignDesign examplesexamples ElectrochemicalElectrochemical valvesvalves

„ Actuated by a bubble created by water electrolysis

Consumes 4.3uW (10000 smaller that thermopneumatic !!!) 2.5 V operational voltage CapillaryCapillary forceforce valvesvalves

„ ElectrocapillaryElectrocapillary

Capacity of double layer.

Maximum value of surface tension at V0. ThermocapillaryThermocapillary effecteffect

„ Caused by temperature dependence of surface tension: viscosity and surface tension both drop with the temperature increase PassivePassive capillarycapillary effecteffect

Example: a bubble valve is designed with a vapour bubble between channel sections 50um and 200um sections. What pressure can it withstand? -3 σw=72*10 N/m MicropumpsMicropumps

mechanical „ micropumps Non-mechanical MechanicalMechanical pumpspumps

„ Require an electromechanical actuator, either external or integrated „ External actuators: drawback: large size, advantages: large force and displacement. „ Integrated actuators: fast responce and reliability ParametersParameters ofof micropumpsmicropumps

„ Maximum flow rate (determined at 0 back pressure) „ Maximum back pressure (at which flow becomes 0), also described as pump head „ Pump efficiency CheckCheck--valvevalve pumppump

„ Function under conditions of small compression ration and high pump pressure – Compression ratio

– High pump pressure

Design rules „ Minimize critical pressure „ Maximize stroke volume „ Minimize the dead volume „ Maximize the pump pressure using large force actuators DesignDesign examplesexamples

„ Typical micro check valves

PeristalsticPeristalstic pumpspumps

„ Do not require passive valves to set flow direction „ Require 3 or more chambers (actually just valves connected in seria) „ Drawbacks: leakage and small pressure difference, require a one check valve to prevent back flow „ Design rules: large stroke and large compression ratio. ExampleExample

„ A peristaltic pump has 3 chambers and 3 circular unimorph piezodiscs, membrane diameter 4mm, frequency 100Hz, maximum deflection 40um. DesignDesign examplesexamples ValvlessValvless rectificationrectification pumpspumps

„ Diffusers/nozzles used instead of check valves for flow rectification

pu 2 ∆p = ξ ξ - pressure loss coefficient 2

ξnegative Fluidic diodicity: ηF = ξ positive

η F −1 Flow rate: Q = 2∆Vf ηF +1 χ - rectification efficiency DesignDesign examplesexamples

Tesla pump: χ=0.045, η=1.2 MicromixersMicromixers

„ mixingmixing inin microscalemicroscale reliesrelies mainlymainly onon diffusiondiffusion duedue toto laminarlaminar flowflow atat lowlow ReynoldsReynolds numbersnumbers

d 2 τ = 2D LaminationLamination inin mixermixer

„ parallelparallel laminationlamination

„ sequentialsequential laminationlamination FocusingFocusing inin mixermixer

„ geometricgeometric andand hydrodynamichydrodynamic focusingfocusing MixingMixing byby twistingtwisting thethe flowflow

StroockA. D., DertingerS. K. W., AjdariA., MezicI., Stone H. A., Whitesides G. M. “Chaotic Mixer for Microchannels” Science 295, 647-651 (2002) ElectrohydrodynamicElectrohydrodynamic mixingmixing

„ Difference in liquids conductivities and permittivities created transversal flows across the interface and destabilized it, promoting mixing. MagnetoelectrodynamicMagnetoelectrodynamic mixingmixing

„ Alternating electric fields were applied to the chamber with two independent center electrodes in the presence of a magnetic field MechanicalMechanical mixingmixing

„ StirringStirring „ OscillatingOscillating wallswalls „ MagneticMagnetic particlesparticles etc.etc.