1. Litografia da fascio elettronico

2. SEM Imaging - System overview - Types of Electron Beam columns
- Magnification and deflection system
- Focussing (contamination dots)
- Imaging

3. Technical setup of EBL tools
Source: SPIE Handbook of Microlithography

4. Types of Electron Beam Columns
Typical Electron
Beam Column
Zeiss
Gemini™ column

5. Schottky field-emitter tip
Courtesy of Leica Lithography Systems Ltd.
(courtesy of Leo)

6. Electron Sources Yes ~ 1% / h 10 – 50 ~ 4000 2000 h (+) << 20nm
Suitable for EBL
Emission current drift
Maximum probe current / nA
Filament cost /€
excl labor
Filament life time
Spot diameter
@ Iprobe @ Uacc
Litho Resolution
in PMMA resist
Emitter
Yes
~ 1% / h
1000 50
1000 h
100 h
~ 4 nm
20pA/ 30keV
~ 5 nm
5 pA / 30keV
< 30 nm
~ 30 nm
LaB6
Tungsten
restricted
~20-30 % / h
< 1 ?
2000 h (+)
~ 2 nm
>20pA/20keV
<< 20 nm
Cold FE
Yes
~ 1% / h
10 – 50
~ 4000
2000 h (+)
~ 2 nm
>100pA/20keV
<< 20nm
ThermalFE
Current density (A/cm²) 25
160
> 650
> 3200
Focussing & Stigmation + ++
++++

7. Comparison of stage specifications

8. Beam deflection (E-static/magnetic)
Either magnetic or electrostatic fields can be used to focus electrons just as glass lenses are used to focus rays of light.
Electro-static
Electro-magnetic
F = q · (E + v  B)
EM-lenses
more simply
Fast deflection

9. Beam deflection (E-static/magnetic)
Electron lenses have very poor performance (spherical and chromatic aberrations) compared to light lenses; thus electrons must be kept very close to the axis - small 
Spherical aberration
Chromatic aberration

10. Beam deflection E-static or B?
Magnetic deflection
- lower aberrations (as with electrostatic lenses (= condensor in gun))
- but max. frequency is limited by the coil resonances to ~10 MHz.
- max. angle limited by off-axis aberrations to about degree
- Trade off between resolution and field size
Increasing working distance:
deflection angle and thus off-axis aberrations decrease
on-axis aberrations increase 
 Compromise field size is much smaller than most chips!

11. Beam-blanking Beam-blanker off Beam-blanker on Filament Anode
+250V
GND
Beam-blanker
Aperture

12. to be good in SEM imaging!
Motivation
To be good in EBL
means
to be good in SEM imaging!

13. Focus/Stigmator correction

14. Focussing - contamination dots
top view
side view
(Images taken during the acceptance test of Raith 200 at the University of
Neuchâtel (Switzerland) 4/98)

15. Focussing = WD and stigmation
(Image taken at KTH Stockholm (Sweden), see

16. Influence of acceleration voltage
Low (short penetration depth)
High (large penetration depth)
+ Clear surface structures
+ Less damage
+ Less charge up
+ Less edge effect
– Lower resolution
+ Higher resolution
– Unclear surface structures
– More edge effects
– More sample damage (heating)
(A guide to Scanning Microscope Observation, Jeol web page 1999)

17. Influence of beam current - aperture
Low (small aperture)
High (large aperture)
+ Higher resolution
+ Less damage (heating)
+ Larger depth of focus
– Grany image
+ Smooth image
+ Good Signal to noise
– Deteriorated resolution
– More damage (heating)
– Lower resolution
– Smaller depth of focus
(A guide to Scanning Microscope Observation, Jeol web page 1999)

18. Influence of working distance
Small
Large
+ Higher resolution
– Smaller depth of focus
+ Larger depth of focus
– Lower resolution
(A guide to Scanning Microscope Observation, Jeol web page 1999)

19. Important rule for SEM imaging
Take always the low magnification images first!!!

20. E-Beam Lithography Methods
E-beam strategies
Motivation
Stage movement strategies
EBL writing strategies

21. Motivation Applications of EBL
mask fabrication (e.g. chromium on glass)
direct write (rapid prototyping)
nano devices in R&D ….
different writing strategies required
Recommended Literature:
SPIE HANDBOOK OF MICROLITHOGRAPHY, MICROMACHINING AND MICROFABRICATION Volume 1: Microlithography, Chapter 2.1

22. Stage Movement Strategies
stationary stage
moving stage
versus
stripes
fields
"write-on-the-fly“

23. EBL Writing Strategies
round (Gaussian) beam
shaped beam
versus
vector scan
raster scan
versus

24. EBL Writing Strategies
strategy
beam
scan
mode
stage 1
(Raith)
gaussian
vector
fixed 2
(Etec)
raster
moving 3
(Leica)
shaped

25. gaussian beam, vector scan, fixed stage
1st Strategy (Raith)
gaussian beam, vector scan, fixed stage
chip
write field
wafer
shapes
beam
path

26. gaussian beam, vector scan, fixed stage
1st Strategy (Raith)
gaussian beam, vector scan, fixed stage
meander mode
line mode

27. gaussian beam, vector scan, fixed stage
1st Strategy (Raith)
gaussian beam, vector scan, fixed stage
+ fast writing of sparse patterns (unwritten areas are skipped)
+ easy dose variation from shape to shape
– settling time and hysteresis have to be calibrated
– overhead time caused by increased stage settling time
Applications: nano lithography, R&D, …

28. gaussian beam, raster scan, moving stage
2nd Strategy (Etec)
gaussian beam, raster scan, moving stage
stage motion
beam motion
(e.g. used by MEBES (Etec Systems Inc.))

29. gaussian beam, raster scan, moving stage
2nd Strategy (Etec)
gaussian beam, raster scan, moving stage
+ very simple
+ very repeatable calibration possible
– sparse patterns take as long as dense patterns
– difficult to adjust dose during writing
Applications: mask making, R&D, …

30. shaped beam, moving stage
3rd Strategy (Leica)
shaped beam, moving stage
electron beam
first shaping aperture
second shaping aperture
beam deflectors
two fields
wafer

31. 3rd Strategy (Leica) shaped beam, moving stage
+  10 x faster than equivalent gaussian beam machines
– extremely complex electron optical column
– complicated calibration routines
– resolution and focus varies with shape size
Applications: mask making,
advanced chip development
- Similar to Gaussian vector scan but exposes an entire rectangle (up to typically 2µm x 2µm) in a single "flash"
- Some machines combine vector scan with "write-on-the-fly“
Extension: Cell projection
(one of the square apertures is replaced by a
more complex shape such as a DRAM cell.

32. Summary: 3 strategies strategy beam scan mode stage 1 (Raith) gaussian
vector
fixed 2
(Etec)
raster
moving 3
(Leica)
shaped

33. Summary: Strategy used by Raith
vector scan
meander
mode
stationary stage
fields
vector scan
line mode
gaussian beam
Applications:
Nano device fabrication, R&D

34. Basic resist theory
Motivation transparency: Dose test in PMMA

35. Contents Electron scattering in resist and substrate Proximity effect
Resist interactions (positive /negative/chemically amplified resists, resist contrast)
Dose definition
Influence of beam energy (penetration depth)
Resolution limits

36. Forward scattering Forward scattering events Properties very often
scattering under small angles
small-angle hence very inelastic
(color symbolizes kinetic energy)
generation of secondary electrons
with a few eV kinetic Energy

37. Backscattering Backscattered electrons Properties occasionally
scattering under large angles
large angle hence mainly elastic (color symbolizes kinetic energy)
high kinetic energy, range of the primary electrons

38. What leads to an exposure?
Secondary electrons with few eV kinetic energy are responsible for most of the resist exposure
Hence forward scattering within the resist is responsible for exposure
And backscattering is responsible for exposure apart from incidence

39. Scattering range versus energy

40. Proximity effect
Test pattern in resist purposely overexposed to enhance the proximity effect

41. Proximity effect - simulation
Simulation of electron trajectories:
1.5 µm resist thickness on a silicon wafer
50 trajectories, at 25 keV beam energy.
(Mark A. McCord, Introduction to Electron-Beam Lithography, Short Course Notes Microlithography
1999, SPIE's International Symposium on Microlithography March, 1999; p. 22)

42. Proximity effect – energy dependence
(D. F. Kyser and N. S. Viswanathan, "Monte Carlo simulation of spatially distributed beams in
electron-beam lithography", J. Vac. Sci. Technol. 12(6), (1975))

43. Resist interactions
Positive resists usually work by polymer chain scission; exposed polymer becomes more soluble
Negative resists usually work by cross linking between polymer chains; exposed polymer becomes insoluble

44. Induced chain scission (PMMA)

45. Chemically amplified resist
Chemically amplified resists, are modified during exposure. However the actual exposure takes place during the post exposure bake, when the acids are activated.

46. Resist contrast = Slope in resist
Positive
Negative
remaining thickness
remaining thickness
log(Dose)
log(Dose) - D0 D1 D0 D1
Contrast  = [log10(D1)-log10(D0)] -1
(Mark A. McCord, Introduction to Electron-Beam Lithography, Short Course Notes Microlithography
1999, SPIE's International Symposium on Microlithography March, 1999; p. 22)

47. Resist contrast High contrast: Low contrast:
+ Steeper side walls
+ Greater process latitude
+ Better resolution (not always)
+ Less sensitivity to proximity effects
Low contrast:
+ 3d lithography

48. Calculation of dose Ibeam = beam current Tdwell = dwell time
s = step size
[µAs/cm²]

49. Dose table for PMMA (950k) 10 kV 20 kV 30 kV
Areas 100 µC/cm² 200 µC/cm² 300 µC/cm²
SPLs 300 pC/cm 600 pC/cm 900 pC/cm
Dots 0.1 pC 0.2 pC 0.3 pC
(developer: MIBK + IPA, 1:3)
Doses are values which resist manufactures will give you such that the resist is "cleared or fully developed" for a certain dose of current. With the measured beam current, pixel size, and dose value, the dwell time per pixel can be calculated.
If the doses are wrong, then the resist can be over or under exposed.
The sensitivity of the resist increases (dose value decreases) as you go down in kV. This means you can do faster exposes at lower kV - but may be with loss in terms of resolution.
The above values are good starting points. The best way to get optimum results is to perform a dose scaling: SPLs 0.5 – 5, Dots 0.1 – 10

50. Dose versus voltage Increase of dose with voltage for all resists
Graph gives general behavior

51. Design must be adapted to dose
Line width in design
Dependence of line width versus dose for different line width in design
Johannes Kretz, Infineon, Munich

52. Influence of beam energy
100 keV
+ Small scattering in resist
+ Small proximity effect
– High beam damage
– Strong sample heating
20 keV
+ Small beam damage
+ Small sample heating
+ Best electron-optical performance
(classical columns)
– Scattering in thick resist
– Strong proximity effect
2 keV
+ No beam damage
+ No proximity effect
+ High throughput (high resist sensitivity)
– High scattering in resist
– Needs very thin resists

53. Penetration depth versus energy
At small kVs you should keep an eye on the penetration depth
Y. Lee, W. Lee, and K. Chun 1998/9, A new 3 D simulator for low energy (~1keV) Electron-Beam Systems

54. Resolution limits Beam resolution Resist limits
Thick resists (forward scattering)
Thin resists (~0.5nm by diffraction, de Brogli wavelength)
Resist limits
Polymer size (~5-10nm)
Chemically amplified resists (acid diffusion ~50nm)
(Mark A. McCord, Introduction to Electron-Beam Lithography, Short Course Notes Microlithography
1999, SPIE's International Symposium on Microlithography March, 1999; p.63)

55. Risoluzione dg : riduzione della sorgente dg=dv/M-1
ds : aberrazioni sferiche ds=1/2 Cs a3
dc : aberrazioni cromatiche dc= Cc aDV/Vbeam
ddiff : diffrazione ddiff=1.2l/a
Il limite di diffrazione non è così importante (elettroni accelerati a 15 keV è l=0.1Å. se a è 10 mrad):
ddiff=1.2l/a = 1 nm
La somma in quadratura di questi contributi
dà una dimensione dello spot nei più moderni sistemi di ~2nm.

56. Resolution limits Secondary electron range (~5-10nm)
In practice, the best achievable resolution in polymer resists is about 20nm, with inorganic resists (currently impractical for most applications) 5nm.
(Mark A. McCord, Introduction to Electron-Beam Lithography, Short Course Notes Microlithography
1999, SPIE's International Symposium on Microlithography March, 1999; p.63)

57. Ultra high resolution in PMMA (45nm thickness):
What is possible ?
Ultra high resolution in PMMA (45nm thickness):
16nm line width in resist

58. Process Technology Pattern Transfer substrate resist Spin coating
Exposure
Developing
Wafer
Coating or stripping step
After x process steps
Remover
Lift-Off
Etching
metal
Pattern
Transfer

59. Process Technology Pattern Transfer substrate resist Spin coating
Exposure
Developing
Raith support
Special
Knowledge, e.g. EBL resist database
Remover
Lift-Off
Etching
metal
Pattern
Transfer
General
Knowledge

60. Raith Application Lab: Typical Instrumentation
substrate
Spin coating
Exposure
Developing
Cleaning
Instrumentation
Film thickness
measurement
Cleaning
Spin coating
Exposure
Developing
Inspection
wet bench
(eye-) shower for
accidents with acids
storage for chemicals
stove / hotplate
refrigerator
spin coater
Film Thickness Probe
Raith 150 / R50 FE
optical microscope
sputtering machine
Process step
Spin coating
Film thickness
measurement
Exposure
Developing
Inspection

61. Lift-Off Tips & Tricks obtain an undercut resist profile by
using a double layer resist
using low beam energy
overexposure or
overdeveloping
use an aspect ratio of resist:
metal as large as possible
if possible use evaporation,
not sputtering

62. Etching Tips & Tricks Resist Wafer Wafer obtain cross-sections without
undercut or overcut by
using high beam energy
avoiding overexposure and overdeveloping
Resist
apply postbake to improve resist stability during etching
for organic resists avoid etching processes with oxygen
Wafer

63. Process Steps substrate Spin coating Wafer resist substrate Exposure
inspection
e.g. resist thickness
measurement
e.g. electron
beam lithography
substrate
Developing
substrate
inspection
optical microscope
SEM
metrology

64. How to avoid charging effects
On isolating samples an additional metal layer between resist an substrate or on top of the resist is required, e.g.
use additional Al layer on top of the resist.
thickness should be 10 nm to 20 nm
remove with KOH
With an additional isolating interlayer, e.g. SiO layer
use a higher acceleration voltage, e.g kV for 1 µm SiO.

65. Resist characterization I:
Dependence of thickness on spin speed
open cover
closed cover

66. Resist characterization II:
Contrast curves

67. Positive EBL Resists application dose resolution commentsnm
High resolution:
PMMA ~200 <10 dry etch (--), low sensitivity or low contrast
ZEP xx ~ 50 <20 dry etch(3x better than PMMA), adhesion (-)
Mask Making:
EBR-9 ~20 >200 long shelf life, large process latitude
PBS ~2 >250 low contrast, difficult process (--)
AZ5206 ~20 ~ high contrast, good etch resistance
Double Layer with PMMA (e.g. Lift-Off):
P(MMA-MAA) ~ 70 <200
PMGI ~ 40 <100
more info at:

68. Negative EBL Resists Application dose resolution commentsnm
High resolution
Calixarene ~4000 <10 very low sensitivity, high resolution (+)
HSQ ~300 <20 dry etch(+)
ma-N2400 ~100 <70 DUV & E-beam sensitive
NEB xx ~ 20 <40 dry etch(+), adhesion (-)
SAL ~10 <100 high contrast, dry etch (+)
SU8 ~2 <100 3D EBL applications
COP ~1 >1000 difficult process (--)
Mask Making
more info at:

69. Which resist for which application?
decide about positive or negative resist with respect to the minimum area that will be exposed
check literature and resist suppliers for resist performance with respect to e.g.
resolution
sensitivity
etching stability
check suitability for your lab, e.g. required baking steps and chemicals

70. EBL Applications

71. PMMA 950K for High Resolution
resist thickness: 45 nm
closed cover
developing 15 s
without moving sample
linewidth < 20 nm
reproducibel!
HEMT
16 nm line width
100 nm gate
50 nm lines

72. Calixaren for High Resolution
18 nm linewidth

73. Resists - 3Dimensional Lithography
Gradation Curve
Contrast
Conventional lithography
Third
Dimension!
Positive resist
Negative resist
Dirk Brüggemann: MEMS application DOEs Microlenses Phase holograms Optical mirrors

74. SU8 and PMMA50K for 3D EBL SU8 PMMA50K Fresnel lens
Deutsches Museum München

75. Long Lines in PMMA line width = 20 nm pitch = 50 nm length = 10 mm
stitching error < 20 nm
50 nm PMMA 950K

76. Large Gratings → exposure time < 16 h line width = 40 nm
pitch = 100 nm
size = 5 x 5 mm2
write field = 100 µm
→ 2500 WF
Optimizing exposure time
small aperture (depth of focus!)
but: fast resist (e.g. ZEP)
Layout: SPLs
in addition: SPLs in
„area mode“ exposed
→ exposure time < 16 h

77. Exposure of Circles Circular mode (V. 3.0) Circle arrays
Fresnel lens / Zone plate

78. Proximity Effect Each pattern element needs a different dose
- here shown by colors -

79. Large Dot Arrays Application:
2 x 2 mm2 array with 200 nm wide circles and 250 nm pitch
Idea 1: Exposure of single dots
(1x settling time, 1x transmission time per circle)
Problem: Upper limit of diameter, because positive resists behave like
negative when overexposed by a factor of about 1000
Solution: Exposure with defocussed beam
resist residues
Defokussiert

80. Large Dot Arrays
Idea 2:
Problem Settling Time: 64,000,000 cirles x 5 ms  90 h
Solution:
0,5 ms settling time and layout in meander form
Set BeamParkPosition on first element
 Overall Settling Time < 9 h
Meander shaped sorting
→ constant distances for the beam movement