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

2. Channels, top view

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

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

5. Photomask = original pattern

6. Lithography = copying the pattern

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

8. Microfabrication linewidths Office laser printer enables ~ 100 µm wide lines Industrial laser printer ~ 10 µm wide lines Dedicated microfabrication laser ~ 1 µm wide lines Electron beam system ~ 0.1 µm wide lines 0.1 € 10 € 1000 € 10 000 € Most microfluidic structures are in 10-100 µm range

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

10. Plasma etched silicon

11. Cell growth forced into arbitrary shapes formed by plasma etching

12. Anisotropic wet etching silicon KOH, e.g. 20% at 80 o C. Anisotropic wet etched profiles in 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. 54.7 o (100) (111)

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

14. Cell growth stimulator: wet and plasma etching

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

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

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

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

19. Bonded sieve Silicon Glass wafer

20. Nanofluidics: molecular size equals channel size Side view Top view

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

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

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

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

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

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

27. Step coverage in deposition A H 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

28. In-plane microneedles

29. 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,...

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

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

32. Microreactors Besser: J. Vac. Sci. Technol. B 21.2., Mar/Apr 2003 Shin & Besser:

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

34. 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

35. Integration: bonding 3 wafers

36. Bonding 6 wafers for GC

37. Microwell with platinum heaters and glass cover Transparent glass for detection; silicon spreads heat well for temperature uniformity

38. Cell interrogation chip Greve et al: MicroTAS 2003

39. The end