Lecture 10 Part II: Micro f b a i ca i tion for Microfluidics Microfluidics and aand Microfluidics Devices

Transcription

1 Lecture 10 Part II: Microfabrication i for Microfluidics and Microfluidics Devices Silicon Etching Polymer-based Micromachining Assembly and Packaging Biocompartibility

2 Techniques involved: Wet etching of channels in Si and glass (isotropic, anysotropic) Dry etching Resist lithography h PDMS soft lithography Hot embossing Other machining techniques in plastics, glass etc. Bonding

3 Wet etching of (100) Silicon

4 Wet etching of other orientation of Silicon

5 Isotropic etching of Silicon Etchant: 66% HNO3 and 34% HF Etching rate: 5um/min On importance of stirring.

6 Chemical dry etching Deep trenches with high aspect ratios can be made in Si, glass or plastic Gases used: Fluorine chemistry (CHF 3, SF 6, CF 4 ) Chlorine chemistry (HCl, Cl 2 ) Oxygen

7 Bulk micromachined channels

8 Silicon surface micromachining PECVD oxide oxide PECVD oxide wet etching

9 Polymer based micromachining Thick resist lithography Polymeric based micromachining Soft lithography Microstereo lithography Micromolding

10 SU-8 resist Negative photoresist for NUV exposure Really thick layers in one spin! L/S 10/30 20/60 30/90

11 Example of SU-8 structures Fabrication of open channels Fabrication of covered channels embedded d selective sacrif.layer mask proton writing

12 Soft lithography Uses elastomeric stamp, usually PDMS (Polydimethylsiloxane) to transfer the pattern. Microcontact printing Micromolding

13 Fabrication of microchannels using soft lithography 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 oc for couple of hours Peel off treat with ozon or Oxygen plasma and attach to clean glass, silicon or another PDMS

14 Microstereo lithography Single photon adsorbtion Single photon adsorbtion Two photon absorbtion Layer-by-Layer photolithography

15 Micromolding Injection molding: high pressure injection of molten PMMA, PC (polycarbonate), PSU (polysulfone) etc. Compression molding (hot embossing)

16 Other micromachining techniques Laser micromachinig (usually using an excimer lasers, Nd:YAG or CO 2 lasers) Focused ion beam Microelectro discharge Ultrasonic micromachining

17 Assembly and packaging Anodic bonding (T=400 o C, V=1kV) Silicon direct bonding reaction of OH groups on Si surfaces at T= o C Glass direct bonding (T=600 o C for 6-8h) Polymer direct bonding Adhesive bonding (low melting glass ( o C, photoresists, UV curable epoxies, epoxies etc.) Eutectic bonding (e.g. gold/silicon eutectic at 363 o C)

18 Other microfabrication issues: Biocompartibility Material responce to biological environments (swelling, corrosion etc.) Tissue and cellular responce to the material

19 Microfluidics: Devices for Flow Control Valves Pumps Micromixers

20 Microvalve consist of closure element driven by an actuator Normally open Active Microvalves Normally closed Bistable Active Microvalves Analog Digital Analog Digital Pulse-width modulation Fluidic DAC

21 Pneumatic Thermopneumatic Active Microvalves Thermomechanical Piezoelectric Electrostatic Electromagnetic Ac ctuation princip le Electrochemical Capillary forcel

22 Valve specification: leakage valve capacity L valve = Q Q closed open C valve = Δp Q max max /( 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

23 Design considerations Specification required Materials to be used Cost Suitable type of actuators Optimal valve spring and valve seat

24 Design consideration: Actuators Moving function (enough force, displacement and controllability) Holding function (should keep valve in a set position) Dynamic function (required responce time) Energy density: E = ' a Fa s V a a

25 Design consideration: Valve spring 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

26 Design consideration: Valve seat Main requirements: Zero leakage Resistance against particles trapped

27 Design consideration: Pressure compensation Aim: Maintain closing force when the inlet pressure vary Passive flow controller

28 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:

29 Thermopneumatic valves Relies on the change in volume of sealed liquid id or solid under thermal loading. Usually utilize solid/liquid and liquid/gas phase transition for maximum performance

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31 Example Thermopneumatic valve with air. Height of the expansion cylinder 500um. Assuming the volume constant: T T p p T = T p p = = 329K = 2 1 = C

32 Example

33 Design examples Heater is placed on SiN membrane, 50um Silicon rubber used for membrane 9um paraffin used as actuation material 2um deflection with 50mW heating

34 Thermomechanical valves Solid expansion Bimetallic Shape-memory alloys

35 Solid expansion valves Generated force: F γδt where γ is the thermal expansion coefficient

36 Bimetallic valves Uses difference in thermal expansion coefficient of two metals F γ 2 γ ) ΔT ( 1

37 Bimetallic valves: Design examples

38 Shape memory Alloy valves 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)

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40 Piezolelectric valves generate small strain (0.1%) and high stresses (MPa), therefore suitable for applications with high force and low displacement ϕ = ϕ = d l v t ϕ = d 33 E el 31 E el

41 Example:

42 Design examples

43 Electrostatic valve Based on attractive force between two oppositely charged plates V F 1 2 = ε r ε A( ), 8.854* ε 0 = F / m 2 d Advantages: fast responce Disadvantages: high h voltage and small discplacement

44 Design examples

45 Electromagnetic valves Uses solenoid actuator with a magnetic core and a coil Advantage: large deflection Disadvantage: low efficiency

46 Design examples

47 Electrochemical l valves Actuated by a bubble created by water electrolysis Consumes 4.3uW (10000 smaller that thermopneumatic!!!) 2.5 V operational voltage

48 Capillary force valves Capacity of double layer. Electrocapillary Maximum value of surface tension at V 0.

49 Thermocapillary effect Caused by temperature dependence of surface tension: viscosity and surface tension both drop with the temperature increase

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

51 Micropumps micropumps mechanical Non-mechanical

52 Mechanical pumps p Require an electromechanical actuator, either external or integrated External actuators: drawback: large size, advantages: large force and displacement. Integrated actuators: fast responce and reliability

53 Parameters of micropumps p Maximum flow rate (determined at 0 back pressure) Maximum back pressure (at which h flow becomes 0), also described as pump head Pump efficiency

54 Check-valve pump Function under conditions of small compression ration and high pump p pressure Compression ratio High pump pressure Design rules Minimize i i critical pressure Maximize stroke volume Minimize the dead volume Maximize the pump pressure using large force actuators

55 Design examples Typical micro check valves

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57 Peristalstic pumps p 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.

58 Example A peristaltic pump has 3 chambers and 3 circular unimorph piezodiscs, membrane diameter 4mm, frequency 100Hz, maximum deflection 40um.

59 Design examples

60 Valvless rectification pumps Diffusers/nozzles used instead of check valves for flow rectification 2 pu Δ p = ξ 2 ξ - pressure e loss coefficientc e Fluidic diodicity: ξnegative η F = ξ positive Flow rate: Q = 2ΔVf η η F F χ - rectification efficiency

61 Design examples Tesla pump: χ=0.045, η=1.2

62 Micromixers i mixing in microscale relies mainly on diffusion i due to laminar flow at low Reynolds numbers 2 d τ = 2D

63 Lamination in mixer parallel l lamination sequential lamination

64 Focusing in mixer geometric and hydrodynamic focusing

65 Example: design a mixing for EtOH and water, if flow rates are 10ul/min and channel crossection is 100x100um D=0 0.84x10-5 cm 2 /s how many laminates would be required to fit the design into 1mm length?