Transcript Ion Beam Lithography Using Membrane Masks

MNC 2001, Japan
Ion Beam Lithography
Using Membrane Masks
Y.S. Kim*, W. Hong, H.J.Woo,
H.W.Choi, G.D. Kim, J.H. Lee
Ion Beam Laboratory
Korea Institute of Geoscience and Mineral Resources
Gajeongdong 30, Yuseonggu, Daejeon 305-350, Korea
S. Lee
Department of Chemistry Daejeon University
Daejeon 300-716, Korea
2001. 11.1
KIGAM Ion Beam Laboratory
MNC 2001, Japan
Motivation
■ Ion Beam Lithography (IBL) using membrane masks has been forgotten for
more than 10 years.
■ The reason seems to be that the angular spread of the incident ion beam in the
membrane is difficult to overcome even when channeling masks are used.
■ Membrane mask has, however, many advantages such as rigidity, easy
fabrication, durability, etc and deserves to be studied further.
■ The angular beam spread of channeling masks (about 0.5o) is enough for
obtaining sub 100nm pattern as will be shown
What has been done
■ Feasibility of the IBL using membrane masks has been studied both by
simulation and experiment
■ A full procedure of membrane mask fabrication has been developed
■ IBL was performed using a 2 mm Si3N4 mask and a 4.5 mm Si channeling with
400 - 500 keV proton beam
2001. 11.1
KIGAM Ion Beam Laboratory
MNC 2001, Japan
Advantage and Disadvantage of IBL
Advantage
Comparison of limiting resolutions
Good sensitivity for 0.1 mm pattern
1.0
X-ray : 375 mJ/cm2
0.9
e-beam : 100 mC/cm2
0.8
Good intrinsic resolution
10 nm : limitation not from the wavelength
but from PR
contrast
IBL : 4.5 mC/cm2 (720mJ)
0.7
X-RAY
OPTICS
ION
0.6
0.5
E-BEAM
0.4
0.3
0.2
multilayer resist
single layer resist
0.1
Disadvantage
In vacuum treatment
1:1 mask
lateral straggling
non familiar method - no extensive
study
2001. 11.1
0.0
0.01
0.1
1
Line Width [mm]
Ref. : P.H. Rose, NIM B37/38, p26
KIGAM Ion Beam Laboratory
MNC 2001, Japan
Effect of angular spread at the membrane
on lateral resolution - TRIM simulation
Position distribution of protons entering resist surface
through a 1mm width slit membrane mask
Change of position distribution of protons
passing through a 200nm PMMA resist
after 1mm width slit membrane mask
Membrane : 2mm Si3N4
PR : 200nm PMMA
membrane : 2mm Si3N4
Proton Energy : 450 keV
7000
12000
Number of Events [arbitrary]
Number of Events [arbitrary]
6000
14 to 86 % width
10000
440 nm
8000
350 keV
6000
260 nm
400 keV
4000
190 nm
450 keV
2000
160 nm
500 keV
0
-2000
Gaussian fit
to the differentiated edge
50% dose position = 505 nm
14 to 86 % width = 220 nm
5000
4000
3000
after PR
2000
50 % dose position = 507 nm
14 to 86 % width = 195 nm
1000
before PR
0
-1500
-1000
-500
0
500
1000
Distance from Slit Center [nm]
1500
2000
-2000
-1500
-1000
-500
0
500
1000
1500
2000
Distance from Slit Center [nm]
• Meaning : Resolution depends rather on the resist contrast
2001. 11.1
KIGAM Ion Beam Laboratory
MNC 2001, Japan
Effect of Angular Spread at the Membrane to the PR pattern
25000
number of events at wafer (=proton dose)
TRIM simulation
400keV proton
slit pattern
membrane
20000
15000
10000
slit width = 1mm
0.8
0.6
0.4
0.2
develop until
this dose region
0.1
10mm
wafer
0.05
5000
0.02
0
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Distance from slit center [mm]
■ Effect of Angular Spread : Contradiction of replicated pattern for small patterns
← Can be solved by the pattern size control at the mask
2001. 11.1
KIGAM Ion Beam Laboratory
MNC 2001, Japan
Angular Spread Measurement
Angular Distribution of protons
passing through a 4.5 mm [100]
Si membrane
Experimental Setup
SSB detector
[100] Si
membrane
Au spot
on Be
600
Width(FWHM
P
500
1000keV Au 300A
Goniometer
400
SSB detector
X-Y translator
20cm
Relative Yield
Collimator
0.36
800keV Au 300A
0.40
600keV Au 300A
300
200
0.38
1000keV
0.46
800keV
0.45
600keV
0.46
500keV
100
• Angular spread is insensitive to
the incident energy
0
-1.0
0.42
450keV
-0.8
-0.6
0.41
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Scattering Angle [degree]
→ Other words, insensitive to the
membrane thickness
2001. 11.1
KIGAM Ion Beam Laboratory
MNC 2001, Japan
Angular Spread and Residual Energy
of channeled and non channeled protons
● For protons passing through 4.5 mm Si
Width of angular distribution
Residual energy
1000
Proton Energy after 4.5mm membrane [keV]
FWHM of angluar distribution
(per unit solid angle)
100
Random direction (TRIM sim.)
10
1
Channeling direction (measured)
0.1
800
channeling direction (measured)
600
400
200
random direction (calculation)
0
400
400
500
600
700
800
900
Incident Proton Energy [keV]
2001. 11.1
1000
1100
500
600
700
800
900
1000
1100
Incident proton energy [keV]
KIGAM Ion Beam Laboratory
MNC 2001, Japan
Preparation of Membrane Masks
[100] Si wafer
■Two kinds of masks fabricated
●non-channeling mask :
Thermal diffusion of B
- 9h at 1100deg for 4.5mm Si
▶2mm low stress silicon
nitride
E-beam deposition
- 300A Au/ 20A Ti
▶Fabrication procedure very
similar to the X-ray mask
Spin coating
- PMMA 4000A
E-beam writing and develop
●Channeling mask :
▶4.5 mm Si membrane
▶Fabrication procedure as
shown
2001. 11.1
Au electroplating and lift off
- Fwd-Rev pulsing method
Backside opening and etching
- EDP for 10h at 105 deg
KIGAM Ion Beam Laboratory
MNC 2001, Japan
Optimization of Pattern and membrane thickness
- for channeling mask
Pattern thickness
■ For 450 keV protons, 200nm thick
pattern is enough for scattering 96% of
protons incident on the pattern
700
600
channeled
1000A
2000A
3000A
4000A
5000A
non channeled
500
Yield
400
→ easy fabrication sub 100nm patterns
crit of [100] Si :
1.05 for 450keV proton
300
Membrane thickness
Percent channeled protons :
3.9% after 2000A
200
100
0
0
5
10
15
Scattering Angle [degree]
2001. 11.1
20
25
■ As thick as possible provided the
residual energy is enough for
penetrating through the object PR
(about 100keV)
→ minimization of pattern distortion
during irradiation
KIGAM Ion Beam Laboratory
MNC 2001, Japan
Optimization of Resist Development
Choice of Developer
Optimum develop condition
■ Choose a developer which
shows the best contrast
▶Best so far :
20% morpholine
5% etanolamine,
60% diethylenglycol monobutylether
15% distilled water
■ Choose a temperature at
which the contrast becomes best
2001. 11.1
standard
mor
dm
ea
ea10
ea20
ea50
ea80
m1i3
m1i5
e1i3
e1i5
pgpe
1m2pa
dgme
egpe
egbe
1m2p
PMMA 950k
at 35oC
600
Develop Speed [nm/s]
Contrast = slop in the dose vs.
develop speed curve
800
400
200
0
0
1
2
13
2
Proton Dose [x 10 /cm ]
KIGAM Ion Beam Laboratory
3
MNC 2001, Japan
SEM Images of Mask and Replicated Pattern
Electroplated mask pattern
Energy too
large
Energy
normal
Energy too
small
Replicated Pattern on PMMA by non-channeling mask
Mask to wafer distance : 10 mm, Angular spread : 5o to 10o
2001. 11.1
KIGAM Ion Beam Laboratory
MNC 2001, Japan
Conclusion
■ The IBL using channeling mask was studied already about 20
years ago, but was forgotten for many years afterwards.
■ We want to emphasize, however, the method deserves to get
attention, mainly because the problem with angular spread
cannot be an fatal restriction.
■ Simulation and some preliminary experiment on the angular
spread shows the promising characteristics of the method.
■ Provided a good channeling membrane mask is fabricated, sub
0.1 um patterning can be done rather simply.
2001. 11.1
KIGAM Ion Beam Laboratory