Microfabrication for fluidics basics and silicon sami franssilaaalto

Microfabrication for fluidics, basics and silicon sami. franssila@aalto. fi

Channels, top view

Channels, cross section view SU-8 epoxy polymer Glass and quartz Santeri Tuomikoski, Aalto Univ.

Channels and chambers Basically we want to make miniature piping. Foremost strategy is this: Make a groove first, and then bond a roof.

Photomask = original pattern

Lithography = copying the pattern

Lithography and etching Exposed polymer is developed away Known as ”positive resist”

Microfabrication linewidths Office laser printer enables ~ 100 µm wide lines 0. 1 € Industrial laser printer ~ 10 µm wide lines 10 € Dedicated microfabrication laser ~ 1 µm wide lines 1000 € Electron beam system ~ 0. 1 µm wide lines 10 000 € Most microfluidic structures are in 10 -100 µm range

Photoresist as an etch mask – Most simple to use – Allows etching of 10’s of micrometers deep – Will be removed after etching photomask photoresist Lithography: Photoresist spinning Lithography: UVexposure Photoresist development Etching with resist mask

Plasma etched silicon

Cell growth forced into arbitrary shapes formed by plasma etching

Anisotropic wet etching silicon (111) 54. 7 o (100) • KOH, e. g. 20% at 80 o. C. • Anisotropic wet etched profiles in <100> wafer. The sloped sidewalls are the slow etching (111) planes; the horizontal planes are (100). Etching will terminate if the slow etching (111) planes meet.

Anisotropic etching of silicon (100) and (110) crystal planes etch fast: 1 µm/min typical (111) plane etches slow, 10 nm/min typical Uses Si. O 2 as etch mask (photoresist and oxide etching is used to make patterns in oxide, and this is immersed in KOH)

Cell growth stimulator: wet and plasma etching

Isotropic etching • • Proceeds as a spherical wave Undercuts structures (proceeds under mask) Most wet etching processes are isotropic HF etching of Si. O 2 and glass, H 3 PO 4 etching of Al

After lithography a) Ion implantation /doping b) Wet etching a b c) moulding f) lift-off f c d) Plasma etching e e) electroplating d

Bonding Ensure flatness and smoothness Clean the surfaces 1) Particle removal 2) Surface chemistry Join the wafers (at room temperature) Apply force (pressure, heat, voltage) to strengthen the bond

Etched and bonded channel Silicon etching (using oxide hard mask)

Bonded sieve Glass wafer Silicon

Nanofluidics: molecular size equals channel size Side view Top view

Bond alignment One wafer holds channel; other is planar Both wafers hold structures; need alignment Misalignment ! Is channel cross section important ?

Thin films are layers 1 -1000 nm thick They serve several functions: -heater electrodes (W, Pt, Ti. N, Al, . . . ) -temperature sensors (Pt) -catalysts (Pt, Pd, . . . ) -mirrors (many metals; λ/4 dielectric stacks)) -electrodes for electrical sensing (Pt, Pd, Au, . . . ) -electrical isolation (Si. O 2, Si 3 N 4) -optical coatings (filters, windows, . . . ) -antisticking coatings (Teflon) -protective layers (Si. O 2, Si 3 N 4, Cr, Al, etch masks)

Metal evaporation -source metal is heated in crucicble -high enough vapor pressure atoms released wafer In high vacuum these atoms are transported to wafer. electron beam gun crucible Condensation of metal vapor into solid = film formation

Metal sputtering target Electric field inonizes argon gas Electric field accelerates argon ions into target metal Metal atoms shot apart from target wafer Metal atoms fly in vacuum to wafer and condense to form film.

CVD: Chemical Vapor Deposition Source gases introduced into gas flow diffusion through boundary layer Adsorption and chemical reaction film formation; byproduct desorption

Common CVD processes • Si. H 4 (g) ==> Si (s) + 2 H 2 (g) • Si. Cl 4 (g) + 2 H 2 (g) + O 2 (g) ==> Si. O 2 (s) + 4 HCl (g) • 3 Si. H 2 Cl 2 (g) + 4 NH 3 (g) ==> Si 3 N 4 (s) + 6 H 2 (g) + 6 HCl (g)

Step coverage in deposition H A B Ratio of film thickness on sidewall to horizontal surfaces (100% = conformal coverage) Cote, D. R. et al: Low-temperature CVD processes and dielectrics, IBM J. Res. Dev. 39 (1995), p. 437

In-plane microneedles

Thin film heater processing 1. Metal sputtering 2. Photoresist spinning & baking 3. Lithography with resistor mask 4. Resist image development 5. Metal etching 6. Photoresist stripping Can be done on any wafer ! Glass wafers, polymer, . . .

Simple linear microreactor Heater electrode Nitride membrane Catalyst underneath Flow channel Bonded to glass wafer Microreactor dimensions Shin & Besser,

Linear microreactor R. M. Tiggelaar et al. / Sensors and Actuators A 119 (2005) 196– 205

Microreactors Besser: J. Vac. Sci. Technol. B 21. 2. , Mar/Apr 2003 Shin & Besser:

Etching of glass with hard mask DFR = dry film resist =laminate resist = resist which is used to make large, non-critical structures

Channel considerations Material • silicon (semi)conducting, opaque • glass (insulator), transparent • polymer (insulator), transparent or opaque Channel walls • vertical/round/slanted • smooth/polished/textured/porous • surface charging/electric double layer • surface-volume ratio Fluid dynamics • wetting/hydrophilic/hydrophobic • self-filling/capillary forces • flow dynamics/Reynolds number • size effects/diffusion • thermal effects/Fourier number