1. ECE/ChE 4752: Microelectronics Processing Laboratory
Photolithography
ECE/ChE 4752: Microelectronics Processing Laboratory
Gary S. May
January 22, 2004

2. Outline Introduction Clean Rooms Exposure Masks Photoresist
Pattern Transfer
E-Beam Lithography

3. Definition
Photo-imaging method by which geometric patterns are transferred from a mask to the substrate (wafer).
Uses photosensitive polymer (called “photoresist”).
Features transferred to substrate surface by shining light through glass plates (called “masks”).

4. Basic Process Flow

5. Process Sequence 1) Clean wafer surface bake (get rid of H2O)
RCA clean
apply adhesion promoter (HMDS = “hexi-methyl-di-silizane”)
2) Deposit photoresist (usually by spin-coating)
3) Soft bake (or “pre-bake”) - removes solvents from liquid photoresist
4) Exposure (pattern transfer)
5) Development - remove soluble photoresist
6) Post bake (or “hard bake”) - desensitizes remaining photoresist to light
7) Resist removal (“stripping”)

6. Outline Introduction Clean Rooms Exposure Masks Photoresist
Pattern Transfer
E-Beam Lithography

7. The Need
Electronics fabrication requires a clean processing environment for lithography.
Goal: minimize dust particles that can settle on substrates or masks and cause DEFECTS.
Dust on a mask looks like an opaque feature; will get transferred to underlying layers; can lead to short circuits or open circuits.

8. Graphic Illustration
Particle 1 may result in formation of a pinhole in underlying layer.
Particle 2 may cause a constriction of current flow in a metal runner.
Particle 3 can lead to a short between the two conducting regions.

9. Class
Numerical designation taken from maximum allowable number of particles 0.5 mm and larger per ft3 (English system).
For IC fabrication, a class 100 clean room is required (about four orders of magnitude lower than ordinary room air).
For photolithography, class 10 or better is required.

10. Particle Size Distribution Curve

11. Sample Problem
A 300 x 300 mm square substrate is exposed for 1 minute under laminar flow at 30 m/min. How many dust particles will land on this substrate in a Class 1000 clean room?
SOLUTION:
Class 1000 => 35,000 particles/m3 (from graph)
Air flow volume over wafer/min = 30 m/min (0.3m x 0.3m) = 2.7 m3
# of particles = 35,000 x 2.7 = 94,500!!!
If each of these causes a defect, we are in serious trouble!

12. Outline Introduction Clean Rooms Exposure Masks Photoresist
Pattern Transfer
E-Beam Lithography

13. Performance Metrics
Resolution: minimum feature dimension that can be transferred with high fidelity to a resist film.
Registration: how accurately patterns on successive masks can be aligned (or overlaid) with respect to previously defined patterns.
Throughput: number of wafers that can be exposed/unit time for a given mask level.

14. Shadow Printing
Mask and wafer in direct contact (contact printing); or
Mask and wafer in close proximity (proximity printing).

15. Contact Printing*
Contact between the resist and mask provides a resolution of ~1 mm.
Drawback: dust particles on the wafer can be imbedded into mask where mask makes contact with the wafer.
Imbedded particles cause permanent damage to mask and result in defects with each succeeding exposure.
* We use this in lab.

16. when l = wavelength and g = gap
Proximity Printing
Small gap (10 – 50 mm) between the wafer and the mask.
Minimizes mask damage, but …
Gap results in optical diffraction at feature edges that degrades resolution to 2–5 mm.
Minimum linewidth (or critical dimension):
when l = wavelength and g = gap

17. Projection Printing Wafer many centimeters from mask
To increase resolution, only small portion of the mask is exposed at a time.
Small image area is scanned or stepped over the wafer to cover the entire wafer surface.
After exposure of one site, wafer is moved to next site and the process is repeated.
Called step-and-repeat projection, with a demagnification ratio M:1

18. Step and Repeat Projection
After exposuring one site, wafer moved to next site and the process repeats.
Demagnification ratio M:1

19. Resolution Given by: where k1 is a process dependent factor and
NA = numerical aperture, which is
where is the index of refraction

20. where k2 is another process-dependent factor
Depth of Focus
Expressed as:
where k2 is another process-dependent factor

21. Outline Introduction Clean Rooms Exposure Masks Photoresist
Pattern Transfer
E-Beam Lithography

22. Making Masks
CAD system used to describe the circuit patterns electrically.
Digital data produced by CAD system drives a pattern generator that transfers the patterns directly to electron-sensitized mask.
Mask consists of a fused silica substrate covered with chrominum.
Circuit pattern is first transferred to the electron-sensitized layer (electron resist), which is transferred into the underlying chrominum layer for the finished mask.

23. Use of Masks Patterns on a mask represent one level of an IC design.
Composite layout is broken into mask levels that correspond to the manufacturing process sequence.
15 – 20 different mask levels are typically required for a complete IC process.

24. Mask Composition Fused silica plate 15  15 cm, 0.6 cm thick
Accommodates lens field sizes for 4:1 or 5:1 optical exposure tools

25. Outline Introduction Clean Rooms Exposure Masks Photoresist
Pattern Transfer
E-Beam Lithography

26. Definition
Photosensitive polymer compound that either gets more or less soluble when exposed to light.
Photolithography labs have yellow light because photoresist is sensitive to wavelenghts > 500 nm.

27. Types Positive: gets more soluble after exposure
Negative: gets less soluble after exposure.

28. Development
More exposure energy vs. Higher resolution

29. Contrast Ratio
where: ET = sensitivity or “threshold” energy (where resist becomes completely soluble)
E1 = energy to reach 100% resist thickness (50% for negative resist)
Larger g => higher solubility of resist and sharper images
ET and E1 interchanged for negative resists

30. Outline Introduction Clean Rooms Exposure Masks Photoresist
Pattern Transfer
E-Beam Lithography

31. Steps Apply adhesion promoter (HMDS)
Spin coat photoresist at 1000 – 10,000 rpm
“Soft bake” (90 – 120°C for 60 –120 sec) to remove solvent
Alignment
Exposure
Development
“Post bake” (100 – 180°C) to increase adhesion
Etch exposed regions
Strip resist

32. Illustration

33. Alignment
Mask for each layer must be aligned to previous layer patterns
For a minimum feature size ~ 1 mm => alignment tolerance should be +/- 0.2 mm
To align, wafer is held on vacuum chuck and moved around using an xyz stage
Alignment marks: special patterns on mask used to facilitate accurate alignment.

34. Outline Introduction Clean Rooms Exposure Masks Photoresist
Pattern Transfer
E-Beam Lithography

35. Limitations of Optical Lithography
Resolution becoming a challenge for deep-submicron IC process requirements
Complexity of mask production and mask inspection
High cost of masks

36. Electron Beam Lithography
Involves direct exposure of the resist by a focused electron beam without a mask
Currently used to primarily produce photomasks
Resolution as low as 10 – 25 nm

37. Schematic Electron gun generates beam of electrons
Condenser lenses focus the e-beam
Beam-blanking plates turn beam on and off

38. Advantages Generation of submicron resist geometries
Highly automated and precisely controlled operation
Greater depth of focus than that available from optical lithography
Direct patterning on wafer without using a mask

39. Scanning
Raster: beam scans sequentially over every possible location on the mask and turned off where no exposure is required
Vector: beam directed only to requested features, jumps from feature to feature

40. Disadvantages Low throughput Expensive resists
Proximity effect: backscattering of electrons irradiates adjacent regions and limits minimum spacing between features