1. Tutorial 2 Derek Wright Wednesday, January 26 th, 2005

2. Some Important Units 10 Å = 1 nm –Ex) Si-Si bonds are 2.33 Å, or 0.233 nm 1 micron = 1  m 1 atm = Standard Atmospheric Pressure = 101.3 kPa = 760 torr = ~ 1 bar (1 bar = 100 kPa)

3. Why are we learning about thin film process? It is easier to grow nanometer-scale films vertically than to mask nanometer-scale patterns horizontally. Combining thin films with very good lithography leads to nanometer-scale devices

4. Photoresist Application Exposure/ Developing Deposition/ Growth or Etching Photoresist Etching Building Devices

5. Overview Deposition (Growth) –Good vs. Bad Films –Physical Methods Evaporation Pulsed Laser Deposition Sputtering –Chemical Methods CVD PECVD Optical CVD Chemical Solution Deposition LB Films

6. So What’s “Good Quality Film”? A bad film has defects Defects are different for crystalline and amorphous films Crystal Defects: –Vacancies (voids), Interstitials (stuff jammed into the lattice), and Dislocations (fractures in the lattice) Amorphous Defects: –Coordination Defects (dangling bonds)

7. Deposition: Physical Methods Physical deposition means that nm sized chunks of material fly at the substrate and stick onto it The hotter the substrate, the more easily these pieces of material can move around (surface mobility) –They find their point of lowest energy resulting in a better film

8. Surface Mobility and Sticking

9. Evaporation Material to be deposited is heated until it becomes vapor phase The heated material flies into the substrate The hotter the substrate, the better the film quality Can deposit very fast relative to other methods, but not always good quality film (up to 200 nm/s film growth)

10. Evaporation

11. Pulsed Laser Deposition Similar to Evaporation method, except uses a laser to heat the material to be deposited Different because the intense energy creates a plasma Plasmas not only contain inert material, but also ions and radicals which could chemically react with the surface –Depends on chemistry of reactants

12. Pulsed Laser Deposition

13. Sputtering The target (material to be deposited) and substrate are placed facing each other A plasma is ignited between them under vacuum A voltage bias between them causes ions from the plasma to ram into the target The ions eject pieces of the target that “sputter” onto the substrate

14. Sputtering

15. Magnetron Sputtering A big magnet is used to force the electrons into spiral paths so that they spend more time ionizing neutral gas particles This increases the number of ions More ions increases the chances of knocking out some of the material to be sputtered Increases efficiency

16. Deposition: Chemical Methods In chemical deposition, the material being deposited on the substrate reacts with the surface –Form bonds with the surface –Chemical reaction with the surface The substrate as well as reactant temperature play a role in the rate of reaction

17. Chemical Vapor Deposition Precursor gas (a gas phase version of what will be deposited on the surface) is pumped into the reaction chamber It’s heated until reactive species form –Ex) SiH 4  SiH 2 + H 2 The reactive species chemically interact with the surface to stick to (or react with) it Surface properties and temperature can determine how well something sticks

18. Chemical Vapor Deposition

19. Plasma Enhanced CVD Uses an RF or microwave E-field to strip electrons off the precursor gasses Since e - are so much lighter than the rest of the molecule (ion), they accelerate in the E-field faster than the molecules

20. Plasma Enhanced CVD By the time the E-field changes direction (at RF or microwave frequencies) the electron has gained a lot of momentum and the remaining molecule (ion) has barely started to move Thus, the e - have a high temperature and the molecules (ions) have a low temperature This means that the substrate can have a lower temperature, too –Enables new substrates like glass and plastic –This is how TFT-LCD displays can be made

21. Plasma Enhanced CVD Electrode Substrate RF Source Plasma Gas

22. Optical CVD Not always applicable Uses different wavelengths of light to break precursor gas bond to form reactive species –Ex) Cl 2 + h (photon)  2Cl  (radicals) Also enables low temperature deposition

23. Chemical Solution Deposition Material is deposited on the substrate in the liquid state –Spin Coating: Some liquid is placed on the substrate and it’s spun really fast until only a thin coating is left –Dip Coating: Dunk the substrate in solution –Spray Coating: Like spray painting the substrate –Screen Printing: Put a stencil on the substrate and use a squeegee to pull solution across –Ink-jet Printing: Same as in an ink-jet printer for a PC

24. Langmuir-Blodgett (LB) Films A form of dip coating You have a solution with a layer of special molecules on the surface One side of the molecule is water-soluble, and the other is not (like soap) Thus all the molecules are aligned on top of the solution

25. Langmuir-Blodgett (LB) Films When you dunk the substrate in, you get a monolayer (one layer) of aligned molecules on the substrate If you keep dunking it you’ll get a new layer each time –The water soluble side of one layer aligns with the water soluble side of the next (alternating alignment)

26. Langmuir-Blodgett (LB) Films

27. Photoresist Application Exposure/ Developing Deposition/ Growth or Etching Photoresist Etching Building Devices

28. Lithography When a pattern is applied to the substrate The most common is optical lithography where a mask is used to expose a pattern onto a substrate –Like how a transparency on an overhead projector works The better the lithography, the smaller the feature size Small feature size  nanoelectronics

29. Technology Nodes

30. Optical Lithography “Resist” is spin-coated onto the substrate A “mask” is placed in front of the substrate –A mask is a clear plate with a pattern on it, like an overhead transparency A light shines through the uncovered parts of the mask and chemically changes the resist (exposure) The exposed resist is etched away with a solvent (developing)

31. Optical Lithography Smaller features need smaller wavelengths of light –UV: 365nm - 436nm –Deep UV (DUV): 157nm - 250nm –Extreme UV (EUV): 11nm - 14nm –X-ray: < 10nm

32. Optical Lithography Three types: –Contact: The mask is directly against the substrate – good minimum feature size, bad for the mask and substrate to touch –Proximity: The mask is a few  m away from the substrate – degrades minimum feature size but good for reliability because mask doesn’t touch substrate –Projection: Lenses are used to focus the mask’s image onto the substrate – good minimum feature size, good for reliability

33. Extreme UV Lithography Pretty soon UV lithography will hit the limit in terms of minimum feature size EUV is the next step Few materials allow EUV light to pass through, so reflective (instead of transmissive) optics must be used Mask pattern must be really absorbent to EUV light, so heavy metals are used

34. X-ray Lithography After EUV comes X-ray lithography Enables super-high resolution pattern transfer There are technical hurdles to overcome before x-ray lithography systems are in place –Synchrotrons need to be further developed as a source for x-rays

35. E-beam Lithography Uses a focused beam of electrons to directly write to the substrate Works much like a CRT TV – an electron gun fires electrons and the beam is directed with magnetic fields There is a limit to how many electrons can be in the beam because they will start to repel each other and blur the beam

36. E-beam Lithography Very precise, but very slow method Can be accomplished in two ways: –Use a narrow beam and turn it on and off to write or not write a pixel –Use a wide beam and a mask to block the parts that shouldn’t be written Typically very slow and costly – good for making optical lithographic masks for use in UV, EUV, and X-ray

37. Nano-imprint Technology Much like forging steel, except at a very small scale A stamp is fabricated at the nm-scale using traditional process methods A substrate is coated with some kind of polymer The polymer is stamped with the nm-scale stamp –The polymer is either cured with heat or light

38. Thank You! This presentation will be available on the web.