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2001. 11.1 KIGAM Ion Beam Laboratory
MNC 2001, Japan
Ion Beam Lithography Ion Beam Lithography
Using Membrane Masks Using Membrane Masks
Y.S. Kim*, W. Hong, H.J.Woo,
H.W.Choi, G.D. Kim, J.H. LeeIon Beam Laboratory
Korea Institute of Geoscience and Mineral ResourcesGajeongdong 30, Yuseonggu, Daejeon 305-350, Korea
S. LeeDepartment of Chemistry Daejeon University
Daejeon 300-716, Korea
2001. 11.1 KIGAM Ion Beam Laboratory
MNC 2001, Japan
MotivationMotivation■ 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 doneWhat has been done■ Feasibility of the IBL using membrane masks has been studied both by simulat
ion and experiment
■ A full procedure of membrane mask fabrication has been developed
■ IBL was performed using a 2 m Si3N4 mask and a 4.5 m Si channeling with 400 - 500 keV proton beam
2001. 11.1 KIGAM Ion Beam Laboratory
MNC 2001, Japan
Advantage and Disadvantage of IBLAdvantage and Disadvantage of IBL
Advantage■ Good sensitivity for 0.1 m pattern
● X-ray : 375 mJ/cm2
● e-beam : 100 C/cm2
● IBL : 4.5 C/cm2 (720mJ)
■ Good intrinsic resolution ● 10 nm : limitation not from the wavelen
gth but from PR
Disadvantage■ In vacuum treatment■ 1:1 mask ■ lateral straggling■ non familiar method - no extensive
study
Comparison of limiting resolutions
Line Width [m]
0.01 0.1 1
cont
rast
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
multilayer resistsingle layer resist
ION
X-RAY
OPTICS
E-BEAM
Ref. : P.H. Rose, NIM B37/38, p26
2001. 11.1 KIGAM Ion Beam Laboratory
MNC 2001, Japan
Position distribution of protons entering resist surface through a 1m width slit membrane maskMembrane : 2m Si3N4
PR : 200nm PMMA
Distance from Slit Center [nm]
-2000 -1500 -1000 -500 0 500 1000 1500 2000
Num
ber
of E
vent
s [a
rbitr
ary]
0
2000
4000
6000
8000
10000
12000
350 keV
400 keV
450 keV
500 keV
14 to 86 % width
440 nm
260 nm
190 nm
160 nm
Effect of Effect of angular spreadangular spread at the membrane at the membrane on on lateral resolutionlateral resolution - - TRIM simulationTRIM simulation
Change of position distribution of protons passing through a 200nm PMMA resistafter 1m width slit membrane maskmembrane : 2m Si3N4Proton Energy : 450 keV
Distance from Slit Center [nm]
-2000 -1500 -1000 -500 0 500 1000 1500 2000
Num
ber
of E
vent
s [a
rbitr
ary]
0
1000
2000
3000
4000
5000
6000
7000
before PR
after PR
50% dose position = 505 nm14 to 86 % width = 220 nm
50 % dose position = 507 nm14 to 86 % width = 195 nm
Gaussian fit to the differentiated edge
• 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 patternEffect of Angular Spread at the Membrane to the PR pattern
Distance from slit center [m]
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
nu
mb
er
of
eve
nts
at
wa
fer
(=p
roto
n d
ose
)
0
5000
10000
15000
20000
25000
slit width = 1m
0.80.6
0.40.2
0.1
0.05
0.02
slit patternmembrane
wafer
400keV proton
10m
develop untilthis dose region
■ Effect of Angular Spread : Contradiction of replicated pattern for small patterns← Can be solved by the pattern size control at the mask
TRIM simulation
2001. 11.1 KIGAM Ion Beam Laboratory
MNC 2001, Japan
Angular Spread MeasurementAngular Spread Measurement
Experimental SetupAngular Distribution of protons passing through a 4.5 m [100] Si membrane
Scattering Angle [degree]
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
Re
lativ
e Y
ield
0
100
200
300
400
500
600
0.41
0.42
0.46
0.45
0.38
0.46
0.40
450keV
500keV
600keV
800keV
1000keV
1000keV Au 300A
600keV Au 300A
Width(FWHM
800keV Au 300A
0.36
• Angular spread is insensitive to the incident energy
→ Other words, insensitive to the membrane thickness
GoniometerCollimator
P
SSB detector
[100] Simembrane
SSB detector
Au spoton Be
20cm
X-Y translator
2001. 11.1 KIGAM Ion Beam Laboratory
MNC 2001, Japan
Incident Proton Energy [keV]
400 500 600 700 800 900 1000 1100
FW
HM
of
angl
uar
dist
ribut
ion
(per
uni
t so
lid a
ngle
)
0.1
1
10
100
Random direction (TRIM sim.)
Channeling direction (measured)
Incident proton energy [keV]
400 500 600 700 800 900 1000 1100
Pro
ton
Ene
rgy
afte
r 4.
5m
mem
bran
e [k
eV]
0
200
400
600
800
1000
channeling direction (measured)
random direction (calculation)
Angular Spread and Residual Energy Angular Spread and Residual Energy of channeled and non channeled protonsof channeled and non channeled protons
Width of angular distribution Residual energy
● For protons passing through 4.5 m Si
2001. 11.1 KIGAM Ion Beam Laboratory
MNC 2001, Japan
Preparation of Membrane MasksPreparation of Membrane Masks
■Two kinds of masks fabricated
●non-channeling mask :
▶m low stress silicon nitride
▶Fabrication procedure very similar to the X-ray mask
●Channeling mask :
▶4.5 m Si membrane
▶Fabrication procedure as shown
[100] Si wafer
Thermal diffusion of B- 9h at 1100deg for 4.5m Si
E-beam deposition- 300A Au/ 20A Ti
Spin coating- PMMA 4000A
E-beam writing and develop
Au electroplating and lift off- Fwd-Rev pulsing method
Backside opening and etching- EDP for 10h at 105 deg
2001. 11.1 KIGAM Ion Beam Laboratory
MNC 2001, Japan
Scattering Angle [degree]
0 5 10 15 20 25
Yie
ld
0
100
200
300
400
500
600
700
1000A 2000A 3000A 4000A 5000A
crit of [100] Si :
1.05 for 450keV proton
Percent channeled protons :3.9% after 2000A
non channeledchanneled
Optimization of Pattern and membrane thicknessOptimization of Pattern and membrane thickness- - for channeling maskfor channeling mask
Membrane thickness■ As thick as possible provided the residual energy is enough for penetrating through the object PR (about 100keV)
→ minimization of pattern distortion during irradiation
Pattern thickness
■ For 450 keV protons, 200nm thick pattern is enough for scattering 96% of protons incident on the pattern
→ easy fabrication sub 100nm patterns
2001. 11.1 KIGAM Ion Beam Laboratory
MNC 2001, Japan
Proton Dose [x 1013/cm2]
0 1 2 3
Dev
elop
Spe
ed [
nm/s
]
0
200
400
600
800
standardmordmeaea10ea20ea50ea80m1i3m1i5e1i3e1i5pgpe1m2padgmeegpeegbe1m2p
at 35oC
PMMA 950k
Optimization of Resist DevelopmentOptimization of Resist Development
Choice of Developer
■ Choose a developer which shows the best contrast
Contrast = slop in the dose vs. develop speed curve
▶Best so far :20% morpholine5% etanolamine,60% diethylenglycol - monobutylether15% distilled water
■ Choose a temperature at which the contrast becomes best
Optimum develop condition
2001. 11.1 KIGAM Ion Beam Laboratory
MNC 2001, Japan
SEM Images of Mask and Replicated PatternSEM Images of Mask and Replicated Pattern
Electroplated mask pattern
Replicated Pattern on PMMA by non-channeling mask
Mask to wafer distance : 10 m, Angular spread : 5o to 10o
Energy too large
Energy too small
Energy normal
2001. 11.1 KIGAM Ion Beam Laboratory
MNC 2001, Japan
ConclusionConclusion
■ 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.