Chapter 5 Polymer Micromachining

Transcription

1 Chapter 5 Polymer Micromachining 5.1 Thick Resist Lithography Polymethylmethacrylate (PMMA) resist (1)PMMA was originally used as a resist material for the LIGA technique Substrate Metal Seed layer PMMA Structure polymer 2008/11/23 1

2 (2) Methods to apply the PMMA on a substrate Multiple spin coating induces high interfacial stresses Prefabricated sheets Casting Plasma polymerization (polymerized with plasma) (3) Light source: X-Ray X by synchrotron facilities, wavelength=0.2 nm-2 2 nm (4) Mask: beryllium ( 鈹 ), titanium, Si 3 N 4 (5) Absorber material: Gold, Tungsten, Tantalum ( 鉭 ) (6) Developer: 20 vol% tetrahydro-1, 4-oxazine4 5 vol% 2-aminoethanol2 aminoethanol-1 60 vol% 2-(22 (2-butoxy-ethoxy) ethanol 12 vol % water 2008/11/23 2

3 5.1.2 SU-8 Resist Negative photoresist Light source: near-uv (365 nm- 436 nm) Components: (i) Epoxy resin such as EPON SU-8 (ii) Solvent (gamma-butyrolactone butyrolactone,, GBL) (iii) Photoinitiator such as triarylium-sulfonium ( 三芳香族羥基 - 酸性硫酸基 )salts Standard SU-8 8 process (i) Spin coating (ii) Soft bake: to evaporate the solvent (iii) Exposure (iv) Post exposure bake (PEB) (v) Developing (vi) Hard bake (vii) Remove /11/23 3

4 Microfludic applications (i) As spacer (a) Single layer + Electroplating Substrate material: glass Seed layer: metal (deposition) SU-8 is spin-coated and structure Electroplating of gold Etching SU-8 as the microchannel: Ether: oxygen plasma; Mask: Al Bonding the top glass plate Glass Exposed SU-8 Metal /11/23 4

5 (b) Multiple layer 1st layer is coated and exposed on a silicon substrate 2nd layer is spin-coated and exposed Microchannel is formed by developing The channel is covered by a glass plate with a thin unexposed SU-8 layer. The bonded surface is cross-linked by a blanket exposure through the glass plate. PEB to form the microchannel The silicon substrate can be etched away to yield an optically transparent device. Mask Si SU-8 Exposed SU-8 Glass /11/23 5

6 (ii) Closed SU-8 channel with embedded mask Expose the first SU-8 layer to form the bottom of the channel Coat the 2nd SU-8 layer Sputter a thin metal layer to form the bottom of the channel. Pattern the metal layer by common lithography and etching. Coat the 3rd SU-8 layer then expose it to fabricate the top wall of the channel. Develop all three layers in a single step. Wash away the embedded mask. Mask Si SU-8 Exposed SU /11/23 6

7 (iii) Covered SU-8 channel with selective proton writing Spin-coat the SU-8 layer. Write (selective proton writing or proton beam micromachining) with low energy forms the top of the channel. A thin polymerized layer is formed. Write with higher energy to polymerize the sidewalls of the channel. Develop the exposure SU /11/23 7

8 (iv) Whole SU-8 channel with a sacrificial layer The first SU-8 layer is coated, exposed, and developed to form the bottom of the channel. A sacrificial structure is deposited and patterned. e. g. positive photoresist, thermal plastic, waxes, epoxies The sidewall and the channel ceiling are formed with a 2nd SU-8 layer. Develop the 2nd SU-8 layer. Remove the sacrificial material /11/23 8

9 5.1.3 Comparison of Different Thick Film Resist 2008/11/23 9

10 5.2 Polymeric Surface Micromachining Polymers are used either as structure material or as sacrificial material Polyimide A polymer of imide monomers. Imide ( 硫亞氨 ) Photoresist: Proimide 348 or 349, PI-2732 Thickness: up to 40 μm Fluorinated polyimide: In RIE processed, fluorite ( 氟石 ) radicals are released from the fluorinate polyimide and act as etchants. Substrate polyimide: Metal such as aluminum, titanium and platinum can be sputtered on polyimide using the lift-off techniques. 2008/11/23 10

11 5.2.2 Parylene (Polymer of p-xylene) Can be deposited with CVD at room temperature. Can be used as a structure material for microvalves and micropumps. Can improve the biocompatibility of microfludic device. p-xylene ( 二甲苯 ) Parylene N: High dielectric strength, low dissipation factor, a frequency-independent dielectric const. Parylene C: Useful combination of electrical and physical properties, low permeability to moisture and other corrosive gases. Parylene D: Properties of C pluses withstand higher temperature. 2008/11/23 11

12 5.2.3 Electrodeposition Photoresist Originally developed for printed circuit board. e.g. Eagle ED 2100 and PEPR 2400 Process: Cataphoretic electrodeposition (the movement of an electrically charged substance under the influence of an electric field) Thickness: μm Application: It is difficult to spin on a resist after patterning the substrate surface. 2008/11/23 12

13 5.2.4 Conductive Polymers (conjugated polymer) In the doped state (electrochemically, electrically, chemically, or physically with ion implantation, it is electrically conducting. Applications: Material for diodes, LED, transistor; e.g. polypyrrole (PPy) light-emitting diode (LED) based on PPV Deposition techniques: Spin coating of polymers Spin-coating of precursor and subsequent polymerization Chemical vapor deposition Electrochemical deposition /11/23 13

14 5.2.5 Fabrication of Microchannels With Polymeric Surface Micromaching Metal Si Sacrificial polymer Coating polymer Structure Photoresist SiO polymer 2 /Si 3 N /11/23 14

15 Spin-coat the sacrificial polymer. Sputter a metal layer on the polymer as a mask. Transfer the channel pattern to the mask. Use RIE to structure the sacrificial layer. Remove the resist and mask layers. Deposit the structure layer over the sacrificial structure. At elevated temperature the sacrificial polymer decomposes into volatile ( 易揮發的 )products. 5.3 Soft Lithography Soft lithography refers to a set of methods for fabricating or replicating structures using "elastomeric stamps, molds, and conformable photomasks. It is a nonoptical transfer techniques. Soft: Refers to an elastomeric stamp with patterned relief structure on its surface. Elastometric material: PDMS 2008/11/23 15 Rogers, J. A. & Nuzzo, R. G. (2005, February). Recent progress in soft lithography. In Materials today, 8,

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17 5.3.1 PDMS Courses/ce435/Polysiloxanes PDMS has a low interfacial free energy such that molecules of most polymers won t stick on or react with its surface. The interfacial free energy can be manipulated with plasma treatment. PDMS is stable against humidity temperature. PDMS is optically transparent and can be cured by UV light. PDMS can attach on nonplanar surfaces. PDMS is mechanically durable. Disadvantages: Volume change and elastic deformation. Recommended aspect ratio: /11/23 17

18 5.3.2 Fabrication of PDMS Device Microcontact Printing with a PDMS Stamps /11/23 18

19 SAM (self-assembled monolayers): Can be created by immersion of the substrate in a solution containing a liquid Y(CH 2 ) n X. X: The head group that determines the surface property of the monolayer. Y: The anchoring group Surface properties of patterned SAM can be used as templates for selective deposition of other material. e.g. hydrophilic SAM traps liquid prepolymer on its surfaces /11/23 19

20 Biotin sensing 2008/11/23 20

21 5.3.4 Micromolding with a PDMS Replica Master (i) Replica molding Replica master: PDMS Resolution: less then 10 nm (ii) Microtransfer molding Apply the liquid prepolymer Planarize the prepolymer Place the master on a planar substrate UV exposure or heating solidifies the prepolymer 2008/11/23 21

22 (iii) Micromolding in capillaries: Uses capillary forces to fill the gaps between the substrate and the PDMS master. The PDMS master is pressed tightly on a planar substrate. Elastic PDMS seals off walls and creates capillary channels. A drop of liquid prepolymer is placed at the ends of these channels and fills them automatically due to capillary force. Cure and peel off the PDMS master. (iv) Solvent-assisted micromolding: Uses a solvent to wet the PDMS stamp and soften the structure polymer. Dissipate 2008/11/23 and evaporate of the solvent. 22

23 Home Work 5-1: Carefully read the following papers and write your comments. 1. Xia, Y. and Whitesides, G. M., Soft Lithography, Annual Review of Material Science, Vol. 28, p , Rogers, J. A. & Nuzzo, R. G. (2005, February). Recent progress in soft lithography. In Materials today, 8, /11/23 23

24 5.4 Micromolding Injection Molding The mold is heated to the softening temperature of the polymer to prevent the injected polymer material from hardening too early. The reciprocating screw shears, melts, and pumps the polymer into the accumulation zones. After cooling, the melt solidifies, and can be taken out from the mold. Mold material: Metal Required 2008/11/23 pressure: bars 24

25 5.4.2 Hot Embossing people/faculty/oleschuk/ embossing.html Heat the mold and the thermoplastic to the glass transition temperature. Once the thermoplastic begins to soften it takes on the form and shape of the mold. The mold and thermoplastic are cooled below the glass transition temperature to harden the thermoplastic. 2008/11/23 25

26 Feature size: less than 100 nm Nanoimprinting: To supplement lithography techniques with hot embossing to fabricate large number of nanostructures in plastics. Reference: Chou, S., Krauss, P. R., and Renstrom, P. J., Applied Physics Letters, Vol. 67, pp , /11/

27 Parameters: embossing temperature, deembossing temperature, embossing pressure, hold time Parameter Material Embossing temperature Deembossing temperature Embossing pressure Hold time PMMA C 95 C bar sec PC Laser Micromaching Fiber optical control Controller Making head Laser beam Laser control Laser Sample Enclosure 2008/11/

28 A direct machining method Laser: Heat the irradiated material such that it can be decomposed, leaving a void in the polymer material. Making head: To steer the laser beam in two orthogonal directions. Relation between the smallest possible focal spot diameter d, the relative aperture of the optical system a, and the wave length λ. d = π λ 4 a a=d/f D: the aperture of the optical system f: focal length of the focusing lens For small feature size, shorter λ and higher a are required. UV is better than infrared radiation. 2008/11/23 28

29 Microvessel Scaffold Fabrication 2008/11/23 29

30 Schematic diagram of a microvessel scaffold with circular microchannels 2008/11/23 30

31 Microvessel Scaffold Fabrication 2008/11/23 31

32 Semi-cylindrical photoresist master fabrication Photolithography Mask Mask JSR THB-120N Glass Development Photoresist melting Spin-coating 60 μm thick JSR THB-120N on glass (350 rpm for 10 sec, 500 rpm for 25 sec); Exposure and development (developer solution: THB-D1 by JSR Inc.); Melting the photoresist (170 C for 30 minutes) 2008/11/23 32

33 Complete development vs. Incomplete development Incomplete development Complete development Incomplete development : Inaccurate scaffold dimension Fragmented demoding 2008/11/23 33

34 Semi-cylindrical microchannel structure after thermal reflow MO image 3D confocal microscope image 2008/11/23 34

35 Microstructure Fabrication Micromolding the bottom plate Casting PLGA solution Semi-cylindrical mold PLGA bottom plate Demolding PLGA solution preparation: Dissolve 85/15 POLY, IV: (dl/g), Mw: (Da) in acetone in a 1:4 w/w ratio. Stir the mixture with a magneto agitator at 60 C for 60 min. Shake the PLGA solution by an ultrasonic shaker for 15 min to remove bubbles created during mixing. 2008/11/23 35

36 PLGA bottom plate 3D confocal microscopy image Microchannel depth=60 μm 2008/11/23 36

37 Micromolding the top plate Connecting holes for the connection of injecting or circulating conduits : Drilling a hole at both the front and rear ends of the semi-cylindrical mold. Inserting φ= 1 mm silica-gel conduits into the holes. Casting. 2008/11/23 37

38 Alignment and bonding Alignment grooves Alignment marks The concave groove on the top plate and convex groove on the bottom plate allow a preliminary alignment to be executed. The alignment marks on both plates further reduce the mismatch probability. The plates are examined using a mask aligner (OAI/500) to make certain of the alignment. 2008/11/23 38

39 PLGA microvessel scaffold with circular microchannels OM image SEM image 2008/11/23 39

40 Circular Microchannels with Inner Nano Patterns Photolithography Mask JSR THB-120N Al Development Photoresist melting Ti and Al sputtering Anodization 2008/11/23 40

41 Semi-cylindrical microchannel structure after anodization OM image Etched by oxalic acid SEM image 2008/11/23 41

42 OM image Etched by phosphoric acid SEM image 2008/11/23 42

43 Semi-cylindrical PLGA microchannel structure after replicating OM image Rms:10.179nm Etched by oxalic acid AFM image 2008/11/23 43

44 OM image Rms:81.315nm Etched by phosphoric acid AFM image 2008/11/23 44