1. Micro/Nanolithography
Dale E. Ewbank
unL052010_microe.ppt
Micro/Nanolithography

2. Micro/Nanolithography
OUTLINE
Micro/Nanolithography
Masks
Optical Lithography
Photoresist
Sensitivity
Processing
Exposure Tools
Advanced Processes
Micro/Nanolithography

3. Micro/Nanolithography
MICROLITHOGRAPHY
The production of ultra-small three dimensional relief images based on
exposure and subsequent development of a photon sensitive polymer
called photoresist.
DRAM LITHO
FEATURE EXPOSURE TOOL
YEAR SIZE WAVELENGTH COST
nm 436 nm $0.8 MILLION
$1.0
$3.0
$5.5
$7.0
$10
$20-30
or 193i $30-50
Micro/Nanolithography

4. Micro/Nanolithography
MASKS
Reduction reticle with pellicle: ASML, Canon, Nikon, others
1X projection with pellicle:
Perkin Elmer Micralign
Micro/Nanolithography

5. TRANSMISSION PROPERTIES OF OPTICAL GLASS
Micro/Nanolithography

6. Ultra Violet Radiation
248 nm KrF laser
193 nm ArF laser
365 nm i-line
436 nm g-line
Modified from
Micro/Nanolithography

7. Micro/Nanolithography
Optical Lithography
Illumination
System
Source
Condenser Lens
Mask
Objective Lens
Substrate
Illumination
System
Source
Condenser Lens
Mask
Substrate
Gap (z) q
Proximity/Contact
Projection
NA = n sinq
Micro/Nanolithography

8. Micro/Nanolithography
DIFFRACTION
Micro/Nanolithography

9. Proximity/Contact System Resolution Micro/Nanolithography
FRESNEL DIFFRACTION
Proximity/Contact System Resolution
Wmin ~ 0.7 (Z)1/2
W = space opening width on the mask
 = exposing wavelength
Z = gap (distance) from the mask to the wafer
Note that Z cannot go be zero because of resist thickness
Micro/Nanolithography

10. Micro/Nanolithography
RAYLEIGH CRITERIA
Projection System Resolution
P/2 = k1 / NA NA = n sinq
DOF = +/- k2 / (n sin2q) k2= 0.5
g-line i-line KrF KrF ArF ArF k
 [nm] i NA
P/2 [nm]
DOF[+/- nm] (66) 95
Micro/Nanolithography

11. NEGATIVE AND POSITIVE PHOTORESISTS
Photomask
Photoresist
Film to be Patterned
Substrate
Light
Negative Resist
Positive Resist
Rendered Insoluble
Rendered Soluble
After
Development
After Etch
and Strip
Micro/Nanolithography

12. ONE AND TWO COMPONENT RESIST HIERARCHY
Positive Resist
Components
Chemically Amplified Resist
Components
ONE
TWO
TWO
Acrylates
(PMMA)
Diazo naphtho quinone
Novolac
(Shipley 1400)
Acid Generator -- Onium salt
Polymer backbone -- Polystyrene
Micro/Nanolithography

13. Micro/Nanolithography
NOVALAC RESIN OH
CH2 OH
CH3
CH2 n
CH3
Novalac resins are soluble in organic solvents,
exhibit good film forming characteristics, are
capable of combining with orthoquinone diazide
sensitizers
Micro/Nanolithography

14. PHOTOACTIVE COMPOUND (PAC)
Stable ketene
molecule hu R O R
add H2O
Naphthoquinone
diazide (PAC) + N2 H O C OH R
Keto-carbene
and Nitrogen
Base soluble
carboxylic acid R
Micro/Nanolithography

15. POSITIVE NOVOLAC PHOTORESIST
Novolac Base Matrix Resin %
Sensitizer or Photoactive Compound (PAC) %
Casting Solvent %
Additives %
PAC - such as naphthoquinone diazide
Solvents - such as EL (Ethyl Lactate) or PGMEA
(Propylene Glycol Monomethyl Ether Acetate)
Additives - such as adhesion promoters, surfactants, dyes,
antioxidants, polymerization inhibitors
Unexposed resists dissolves in an alkali developer
at a rate of 2 nm/sec
Partially exposed resists dissolves in an alkali developer
at a rate of 200 nm/sec
Micro/Nanolithography

16. Micro/Nanolithography
SENSITIVITY
unexposed resist
0.6
Resist absorbs light
in the base resin and in
the PAC. The difference
is the more important
parameter
0.4
difference
Absorbance
0.2
exposed resist
300
340
380
420
500
460
Wavelength (nm)
Micro/Nanolithography

17. Deep ULTRA VIOLET RADIATION
Micro/Nanolithography

18. CHEMICALLY AMPLIFIED RESIST
Poly(t-butoxycarbonyloxystyrene)
with onium salt cationic photoinitiator for acid generation
Micro/Nanolithography

19. Micro/Nanolithography
THICKNESS LOG DOSE g
Dose to clear
The higher the slope, gamma, then the smaller the difference needs to be between exposure in areas to be cleared and areas to leave resist. That is the required aerial image modulation is smaller.
Micro/Nanolithography

20. Micro/Nanolithography
MODULATION
Mask m(x)
Modulation = Imax - Imin
Imax + Imin
Ideal 1
Imax
Aerial image
I(x)
Actual
Imin
Wafer
Micro/Nanolithography

21. Micro/Nanolithography
PROCESSING
Substrate Cleaning
Coat track
Priming if needed
BARC coat and bake
Spin Coating Photoresist
Soft-Bake (PAB)
Top Coat and bake if needed
Stepper Exposure
Develop track
Post Exposure Bake (PEB)
Develop
Rinse
Hard-Bake (PDB)
Etching
Stripping
Micro/Nanolithography

22. SUBSTRATE CLEAN AND PRIME
Cleaning is done with a high pressure (2000 psi) water scrub
A dehydration bake is typically done on a hot plate at 250 °C
for 1 min. (Wafers are clean and dry just after removing
from oxide growth furnace)
HMDS (hexa-methyl-di-silizane), - Adhesion
promoter or primer: Are commonly applied as a liquid
or vapor. HMDS attaches to remaining OH molecules
releasing ammonia gas and creating an organic-like
surface improving adhesion
Too much HMDS is detrimental to sensitivity and adhesion.
Micro/Nanolithography

23. Micro/Nanolithography
HMDS PRIME
Micro/Nanolithography

24. Micro/Nanolithography
SPIN COATING
Most spin coating is performed at spin speeds from 3000 to 6000 RPM
for 20 to 60 seconds, producing coating uniformities to +/ Å
Micro/Nanolithography

25. Micro/Nanolithography
SOFT-BAKE
The main purpose is to reduce the solvents from a level
of % down to 4 - 7%. Baking in a convection oven
about 20 minutes is equivalent to hot plate baking for about 1 minute.
Forced Air Oven
Exhaust
Photoresist
wafer
Hot Plate
90 TO 100 °C
Fan
Micro/Nanolithography

26. Micro/Nanolithography
EXPOSURE
E = I t
where E is exposure dose in mJ/cm2
I is irradiance in mW/cm2
t is exposure time in seconds
Humidity should be 40-44% because exposed PAC
requires water to convert to carboxylic acid
Micro/Nanolithography

27. Micro/Nanolithography
POST EXPOSURE BAKE
Post exposure bake increases speed of resist
Post exposure bake reduces standing wave effects
Post exposure bake is require for
chemically amplified and image reversal resists
(100 to 115 °C for 1 min.)
Micro/Nanolithography

28. Micro/Nanolithography
DEVELOP AND RINSE
Develop is done in an alkali solution such as NaOH or KOH (Metal Containing Developers) Trace quantities of these metals can cause transistor threshold voltage shifts.
Metal Ion Free Developers are available (TMAH)
Developer Concentration and Temperature of Developer are the most important parameters to control.
Micro/Nanolithography

29. Micro/Nanolithography
HARD BAKE
Hard Bake is done at or slightly above the glass transition temperature. The resist is crosslinked (and is toughened prior to plasma etch). The resist flows some as shown below. Pinholes are filled. Improves adhesion also. No flow should occur at the substrate. Photo stabilization involves applying UV radiation and heat at 110 °C for dose of 1000 mJ/cm2 then ramping up the temperature to °C to complete the photo stabilization process.
After Develop
After Hard Bake
125 to 140 °C for 1 min.
Micro/Nanolithography

30. Micro/Nanolithography
ETCHING
Isotropic Etching - etches at equal rate in all directions
Wet Chemical Etching - is isotropic
Anisotropic Etching - etches faster vertically than horizontally
Plasma Etching (Dry Etch or Reactive Ion Etching, RIE) – is
either isotropic or anisotropic depending on ion energy and chemistry of etch.
Photoresist
Oxide wet etched
Micro/Nanolithography

31. Micro/Nanolithography
STRIPPING
Oxygen Plasma Ashing
Plasma Damage is Possible to sensitive gate oxide layers
Hot Sulfuric Acid and Hydrogen Peroxide
If underlying layers are not etched by these chemicals
Organic Solvents are available
Micro/Nanolithography

32. Micro/Nanolithography
GCA 6700 G-LINE Stepper
Micro/Nanolithography

33. Micro/Nanolithography
CANON FPA-2000 i1 STEPPER
i-Line Stepper l = 365 nm
NA = 0.52, s = 0.6
Resolution = 0.7 l / NA = ~0.5 µm
20 x 20 mm Field Size
Micro/Nanolithography

34. Micro/Nanolithography
STEPPER X Y R
UV-Light Source
Diffuser
Condenser Lens, NAC
Filter
Reticle Alignment Motors
Thru Lens Alignment
Detector
Illumination
Reticle
5X Reduction Lens, NA O
Auto Focus
Aperture Blades
Baseline
Stage Motors
Auto Leveling X-Y Stage
Reference Mirror
Alignment Microscope
and TV
Zeeman Two Frequency
Laser Interferometer
Fiducial Marks
Micro/Nanolithography

35. Micro/Nanolithography
STEP AND SCAN
Micro/Nanolithography

36. Micro/Nanolithography
ADVANCED PROCESSES
Tri-layer Process
Bi-layer Process
Lift-off Process
Reversal Process
Dyed Resists
Anti-reflective Coatings
Contrast Enhancement Materials
Chemically Amplified Resists
Silylation Process
Micro/Nanolithography

37. Micro/Nanolithography
TRI-LAYER
Film to be Etched
Substrate with Topology
Coat with Planarizing Layer
Coat with Barrier Layer
Coat with Photoresist
Image Photoresist
Etch Barrier Layer
Reactive Ion Etch Planarizing Layer
Etch Film
Micro/Nanolithography

38. Coat with Planarizing Layer
SILYLATION
Film to be Etched
Coat with Planarizing Layer
of Photoresist
Polymerized Areas
Expose Desired Pattern
causing exposed areas
to become polymerized
SiO2, 10% in Photoresist
Reactive Ion Etch in
Oxygen coverts silicon
into SiO2, Only
polymerized areas are
etched. Silicon
containing areas form
and in-situ mask
After Reactive
Ion Etch
Soak in HMDS Vapor
Note: HMDS is incorporated
into the non polymerized
areas only
Etch Film
Micro/Nanolithography

39. Micro/Nanolithography
LIFT-OFF
1. Create a reverse slope or
undercut resist edge profile
2. Deposit film by evaporation
3. Chemically strip photoresist and lift off film, leaving film in desired pattern
Film
Substrate
Photoresist
Micro/Nanolithography

40. Post Exposure Reversal Bake Micro/Nanolithography
REVERSAL PROCESS
No Extra
Processing
After Development
Normal Process
Photoresist
Substrate
Reversal Process
Coat
Expose
Post Exposure Reversal Bake
Flood Expose
After Development
Micro/Nanolithography

41. Micro/Nanolithography
REFERENCES
Micro/Nanolithography Science and Technology, Second Edition, edited by Kazuaki Suzuki and Bruce W. Smith, CRC Press, 2007.
Introduction to Micro/Nanolithography, Second Edition, Edited by Larry F. Thompson, C.Grant Willson and Murrae J. Bowden, ACS Professional Reference Book, American Chemical Society, Washington, DC 1994.
Micro/Nanolithography, David Elliott, McGraw Hill Book Company, 1986.
accessed 03/13/2009.
accessed 06/10/2008.
Micro/Nanolithography