MSN 551 Class Notes LITHOGRAPHY

Transcription

1 MSN 551 Class Notes LITHOGRAPHY

2 Why do we need lithography?

3 Why do we need lithography?

4 LITHOGRAPHY It is a general name given to processes used to transfer patterns on to a substrate to define structures that make up devices Optical lithography: Uses light Electron Beam lithography: Uses electrons Ion beam lithography: Uses energetic ions to bombard and pattern surfaces Soft lithography: Uses mechanical contact indentation to transfer patterns

5 Optical Lithography: Summary A photoresist is exposed with light Photoresist undergoes chemical modification A developer is used to etch away modified (or unmodified) parts of the photoresist Pattern transfer is achieved. coat expose develop

6 Main performance paremeters for lithography Resolution: How small features can you write Registration: How tight can you align different layers (about 1/3 resolution for optical lithography) Throughput: How fast (cm^2/minutes) can you expose?

7 Coating resists: Spin coating Easiest method of coating a flat substrate The wafer (substrate) is placed on a rotating table (chuck). The chuck is rotated at a fixed rotational speed (rpm, rotations per minute) for a fixed amount of time. Depending on viscosity of the resist, a certain thickness of film remains on the substrate. We want uniform (constant thickness) films that conform the surface (coat sharp edges and deep groves)

8 Coating resists: Spin coating

9 Coating: Alternatives Spray coating Dip coating Substrate is dipped and then the resists is let to flow with gravity Meniscus coating Resist is sprayed onto the substrate Substrate is inverted and pulled over a resist bath Plasma coating Thin photoresist films can be deposited by certain plasmas Success of each method depends on wettability and viscosity

10 Spin coating Issues You can't spin coat everything on everything Wetting issues Fluid dynamical instabilities may cause nonuniform or noncontinous films Surface treatment is a solution. Conformity Works best when surface topographic variations are less than the film thickness If deep feature is what you have you may want to try other coating techniques.

11 Spray coated substrate with deep features

12 Spin coating nonplanar topography

13 Spin coating: Cleanliness

14 Spin coating: Thickness Inverse Square Root law

15 Spin coating recipe

16 Surface treatment: HMDS Problem: nanometer thick water film on surfaces degrades photoresist sticktion

17 Surface treatment: HMDS

18 Light sources Arc Lamp

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25 Mask materials

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27 Alignment marks

28 Alignment marks

29 Photoresist Chemistry Polymer with photosensitive crosslinkers Bisazide crosslinking

30 Photoresist Chemistry Polymer with photosensitive crosslinkers Poly(vinyl cinnamate)

31 Photoresist Chemistry

32 Exposure process: Thin film interference

33 Exposure process: Thin film interference

34 Exposure Process

35 We can write a differential equation to estimate the dose received by each point

36 Exposure process: Thin film interference

37 Edge beads Near the edges, photoresist piles up A mask in contact can be tens of microns away from the substrate plane

38 Edge beads: Removal develop

39 Diffraction effects

40 Diffraction effects

41 Diffraction effects

42 Phased Masks Standard binary mask Phase shifted masks

43 Resist polarity

44 Resist contrast

45 Resist Profiles Depending on polarity of resist, inverted trapezoid can be printed.

46 Simulations can predict development

47 Resist Profiles Negative resist, exposed with lines

48 Resist Profiles Negative resist, exposed with lines

49 Thin film interference effects Post exposure bake (P.E.B.) reflows resist and smoothens out walls

50 Post exposure Bake

51 Why is it important to control the profiles Lift-off A name given to process where an evaporated material is patterned by the shadowing effect of the photoresist barrier. Conformal coatings

52 Lift-off process

53 Lift-off process: Problems

54 Lift-off process

55 Multiple layer resist coatings

56 Bi-layer coating: T shaped profiles

57 Bi-layer coating: T shaped profiles

58 Chlorobenzene treatment Soak sample in chlorobenzene for minutes after exposure. Chlorobenzene is toxic, and is not being widely used anymore!

59 Other Exposure types Direct laser writing

60 Interference lithography

61 Interference lithography

62 Interference lithography

63 Gray scale lithography Location dependent exposure dose may be used to fabricate 3D structures with arbitrary profiles.

64 Photoresist cleaning Photoresist removers Sulfiric acid and Hydrogen peroxide Oxygen plasma