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A lithium liquid metal ion source with a narrow angle emission for writing beam lithography
Microelectronic Elsevier
Engineering
23 (1994) 11 l-l 14
111
A lithium liquid metal ion source with a narrow angle emission for writing beam lithography E. Hesse*, F. K. Naehringb
and J. Teichert’
Research Center Rossendorf Inc. ’ Institute for Ion Beam Physics and Material Research b ABV Rossendorf P.O.Box 51 01 19, D-013 14 Dresden, Germany
Parameters of a lithium liquid metal ion source have been determined. The angular intensity is the highest yet reported for liquid metal ion sources. This high angular intensity and the large range of light ions suggest the liquid metal ion source to be applied for writing beam lithography. PMMA resist layers were exposed by a focused lithium ion beam.
1. INTRODUCTION In the lithographic process the ion beam exposure of resists has the opportunity of very high resolution structuring. The ions deposit their energy in a very narrow range, producing secondary electrons of very low energy and therefore short range. There is no electron backscattering. The proximity effect is suspected to be very small as compared to electron irradiation [ 11. Light ions should produce small and deeply exposed regions because of the prevailing electronic stopping. The higher energy density and smaller collision cascade of ions with respect to electrons is the basis for the interest in the development of ion beam lithography. Siegel [l] calculated, that writing 50 nm-lines, ion beam lithography would have a limiting rate two magnitudes faster than electron beam lithography (100 MHz versus 1 MHz). Both gaseous field ionization and liquid metal ion sources provide high-resolution, high-density focused ion probes that could be used to write patterns in resists in the same manner as electron beams. Using liquid metal ion sources chromatic aberration limits the size of the focused beam [2,3]. The diameter of the probe increases linearly with the energy broadening AE. In the expression of Knauer [4] AE is proportional to the cubic root of the ion mass. The first metal in the table of elements, lithium, should have the smallest energy 0167-9317/94/$07.00
broadening. Mair and Mulvey calculated about 2 eV near the onset current [2]. Furthermore, a high angular intensity of the lithium ion source is expected. This results from the calculated atomic mass dependence of the angular intensity [ 51. Hence the chromatic aberration of the ion optical system will be low and a large numerical aperture may be allowed for the needed lateral resolution (i.e. beam diameter). This results in a high current density and in fast lithographic processing. The values of melting temperature and vapour pressure at the melting point are 186” C and 2* 10“ Pa respectively. Because of these low values it is possible to use the pure lithium element as source material contrary to other low mass metals like boron or beryllium. This results in an additional gain in ion current density.
2. THE ION SOURCE To restrict evaporation loss and ionization of thermally evaporated atoms a closed reservoir is used. The emitter consists of a capillary reservoir with direct current heating and a moveable tungsten needle (Fig. 1). This needle of 0.3 mm diameter was etched in NaOH to form the emitter tip. The intensive reaction of lithium alkali metal with air requires preparation in an inert environment, e.g.
0 1994 - Elsevier Science B.V. All rights reserved.
E. Hesse et al. I A lithium liquid metal ion source
flange
feed
mechanical liquid
lithium
through
:I’
-----_.___
.I
,/
manipulator
needle
emitter
needle
-~--
capillary 12 mm
reservoir * 0.5 mm
-A
-p-‘;p~ .~ ~_~ _+_._-__i ~~~*< ‘I
I.D.
__~ -~,-,wfl A-
_/~m--emitter tip ,extraction electrode __u-’ Figure I. Scheme emitters region. benzine.
To
of the lithium
fill
the
liquid
capillary
with
metal
ion
lithium
source.
it is
The
right
side
- visual
/
-
ly---
-mIr-
shows
;1 ten
observation
times
cnlargcmcnt
of the fluorescence
punched into the lithium block immersed in benzine. When the capillary is inserted in the ion source and the ion source is put into the ion optical
target electrode - irradiation of a silicon target and the implanted lateral distribution
column
- angular
a thin
capillary
surface
film
of benzine
and protects
remains
the lithium
on the from air.
distribution
Faraday
1 . -1
Ld--
of the
spot at the
Jetcrmlnation by SIMS
measurements
01‘
by a rotating
cup.
During the ion emission a red fluorescence spot is visible at the target electrode This collector llphr 3. RESULTS
ON THE
LITHIUM
was
ION SOURCE
presumed
emitter 3.1. Current-voltage
curve
to
arise
previously and resputtert
collector
surface
Fig. 2 shows
a current-voltage
curve of the lithium
intensity
was estimated
liquid
ion
secondary
observed
homogenous
metal
source.
The
electron
current is suppressed by a target electrode bias of +30 V with respect to the extraction electrode. The onset
voltage
about
0.74
is 5.0 keV. The curve
has a slope
of
PA/V.
The stability of the lithium liquid metal ion source strongly depends on the residual pressure. At 2* 10m4Pa it was necessary to use the electronic current control mode to avoid a current decrease Fig 3 shows the stability of the ion current At ion current values of 2.4 and 23.5 pA the noise 1s below
1 %. Maximal
3.3. Angular The following angular
intensity methods
intensity
fluctuations
are below
were used to determmc
of the lithium
ion source:
3 %.
the
from
the
escltatlon
deposited
of
on
the
[6,7]. The angular the diamctcr
tluorescencc
spot.
of the the 1011
current and the emitter-target-distance [X 1. At a line crossing the center of‘ the irrndiatlon spot on a silicon target scvcral depth profiles of lithium SIMS.
3.2. Stability
from
material,
concentration The maximum
have
been
of the
measured
profiles
has
by been
evaluated for the angular distribution of the ion source. Post- accclcration of the ions IS IICWSS;I~ to get ion ranges being sufficlcntly deep ~OI- SIMS. The resulting angular intensity distrihution has been corrected for the focusing cl‘l‘cct 01‘ ~hls post accclcration l’he Faraday cup 1s moved 011 u clrclc around the emitter tip which intersects the emission bundle at the optical axis. This results in a set of angular distribution curves that are Jcp~~tcd 111I:ig 5 The results of these three methods arc compatible.
In I:lg. 5 the lithium
beam
profiles
E. Hesse et al. I A lithium liquid metal ion source I
’
113
I
-800 -600 -400 -200
0
200
400
600
800
EMISSION ANGLE (mr)
Figure 4. Plot emission angle
of angular at different
intensity (dI/dR) current levels.
vs
Figure 2. Current-voltage curve of the lithium ion source. I
Li’ 2.7/.LA (visual observation1
Li+
,Ga+
--;
2.0pA ww
Figure
3. Stability
of the lithium
ion emission
determined by visual observation and SIMS respectively are compared with that measured at gallium and bismuth sources. The axial angular intensities of a lithium source and a gallium source in Fig. 6. The axial angular [9] are shown intensity of the lithium liquid metal ion source is the highest one reported for liquid metal ion sources. The results are consistent with experiments and calculations on the atomic mass dependence of angular intensity [5] (Pig. 7). The enhancement of
; I
5.5 /LA i
;
[Swamon : et al.] ’
I
1’
; 4.0/LA wanson et Iis .t-.>
I \\ I ‘I, ’ I/ 1 / 1 , I /
1 I
EMLWON
al
ANGLE
(mr)
Figure 5. Comparison of the lithium angular intensity with experimental results from gallium and bismuth liquid metal ion sources [9].
E. Hesse et al. / A lithium liquid metal ion source
114
4. LITHOGRAPHIC To
further
EXPERIMENTS
test
the
lithium
liquid
metal
ion
source it was inserted into a focusing ion beam system with a focusing capacity of 5 pm PMMA resist layers were exposed with the focused lithium ion beam.
TOTAL
CURRENT
@A)
Figure 6. Comparison of the axial lithium angular intensity with experimental results from gallium[9].
Figure lithium
8. Lines
in PMMA
ion beam.
2.3 nC/cm;
resist written
Ion energy:
development:
by focused
25 kcV;
lint
dose
5 s m ethyl
hcnzcne
“VLSI
l’lectronics
at
22” c
ATOMIC
Figure
7. Angular
elements lithium.
at 10 PA
the lithium
angular
REFERENCES
MASS
intensities [5] and
intensity
on axis for different experimental
with respect
metal
ion sources
of other
elements
high
in
current
range
the
low
result for
to liquid
is especially up
Probably the kink of the axial I,(I)-curve current range observed at other liquid
to
20 PA.
in the low metal ion
sources, e.g. of gallium and aluminium [9,10], exists also at the lithium source. On the assumption of a nearly constant value of the energy spread at low Ion currents a maximum value of the chromatic angular intensity
one appears
on certain
conditions
[8].
1. B.
M.
Siegel
in
Microstructure Science” (N G. Einspruch, cd.). Vol. 16, chapter 5, Academic I’rcss. Orlando 1987 2. G. L. R. Mair Engineering, 3. L. Bischoff
and T. Mulvey, 3 (1985)
Microclcctromc
133-146
et al. , Microelectronic
13 (1991) 367 4. W. Knauer, Optik 5. D. R. Kingham
Engineering.
49 (1981)335-354
and L. W. Swanson
Appl.
PLys
A34 (1984) 123 6. G. L. R. Mair and A.von Engel. .I Appl. l’hys 50 (1979) 5592 A. J. Dixon and A. von Engel 1080 (Inst. l’hys Conf. Ser. 54) ch. 7, p. 292 E. Hesse and F K. Nachrlng 1903 .I I’hys I1 Appl. Phys. 26 (1993) 7 17-7 18 L. W. Swanson et al., 1979 J. Vat. SCI ‘l‘echnol 16 (1979) 1864 IO. Y. Torii and Ii. Yamada,
Proc.
Int
Ion Eng.
Congress CISIAT and IPAT (‘83), Kyoto (Japan). ed: T. Tagaki (Inst. of electrical Engineers 01 Japan,
Tokyo,
1983) p. 363
Engineering
23 (1994) 11 l-l 14
111
A lithium liquid metal ion source with a narrow angle emission for writing beam lithography E. Hesse*, F. K. Naehringb
and J. Teichert’
Research Center Rossendorf Inc. ’ Institute for Ion Beam Physics and Material Research b ABV Rossendorf P.O.Box 51 01 19, D-013 14 Dresden, Germany
Parameters of a lithium liquid metal ion source have been determined. The angular intensity is the highest yet reported for liquid metal ion sources. This high angular intensity and the large range of light ions suggest the liquid metal ion source to be applied for writing beam lithography. PMMA resist layers were exposed by a focused lithium ion beam.
1. INTRODUCTION In the lithographic process the ion beam exposure of resists has the opportunity of very high resolution structuring. The ions deposit their energy in a very narrow range, producing secondary electrons of very low energy and therefore short range. There is no electron backscattering. The proximity effect is suspected to be very small as compared to electron irradiation [ 11. Light ions should produce small and deeply exposed regions because of the prevailing electronic stopping. The higher energy density and smaller collision cascade of ions with respect to electrons is the basis for the interest in the development of ion beam lithography. Siegel [l] calculated, that writing 50 nm-lines, ion beam lithography would have a limiting rate two magnitudes faster than electron beam lithography (100 MHz versus 1 MHz). Both gaseous field ionization and liquid metal ion sources provide high-resolution, high-density focused ion probes that could be used to write patterns in resists in the same manner as electron beams. Using liquid metal ion sources chromatic aberration limits the size of the focused beam [2,3]. The diameter of the probe increases linearly with the energy broadening AE. In the expression of Knauer [4] AE is proportional to the cubic root of the ion mass. The first metal in the table of elements, lithium, should have the smallest energy 0167-9317/94/$07.00
broadening. Mair and Mulvey calculated about 2 eV near the onset current [2]. Furthermore, a high angular intensity of the lithium ion source is expected. This results from the calculated atomic mass dependence of the angular intensity [ 51. Hence the chromatic aberration of the ion optical system will be low and a large numerical aperture may be allowed for the needed lateral resolution (i.e. beam diameter). This results in a high current density and in fast lithographic processing. The values of melting temperature and vapour pressure at the melting point are 186” C and 2* 10“ Pa respectively. Because of these low values it is possible to use the pure lithium element as source material contrary to other low mass metals like boron or beryllium. This results in an additional gain in ion current density.
2. THE ION SOURCE To restrict evaporation loss and ionization of thermally evaporated atoms a closed reservoir is used. The emitter consists of a capillary reservoir with direct current heating and a moveable tungsten needle (Fig. 1). This needle of 0.3 mm diameter was etched in NaOH to form the emitter tip. The intensive reaction of lithium alkali metal with air requires preparation in an inert environment, e.g.
0 1994 - Elsevier Science B.V. All rights reserved.
E. Hesse et al. I A lithium liquid metal ion source
flange
feed
mechanical liquid
lithium
through
:I’
-----_.___
.I
,/
manipulator
needle
emitter
needle
-~--
capillary 12 mm
reservoir * 0.5 mm
-A
-p-‘;p~ .~ ~_~ _+_._-__i ~~~*< ‘I
I.D.
__~ -~,-,wfl A-
_/~m--emitter tip ,extraction electrode __u-’ Figure I. Scheme emitters region. benzine.
To
of the lithium
fill
the
liquid
capillary
with
metal
ion
lithium
source.
it is
The
right
side
- visual
/
-
ly---
-mIr-
shows
;1 ten
observation
times
cnlargcmcnt
of the fluorescence
punched into the lithium block immersed in benzine. When the capillary is inserted in the ion source and the ion source is put into the ion optical
target electrode - irradiation of a silicon target and the implanted lateral distribution
column
- angular
a thin
capillary
surface
film
of benzine
and protects
remains
the lithium
on the from air.
distribution
Faraday
1 . -1
Ld--
of the
spot at the
Jetcrmlnation by SIMS
measurements
01‘
by a rotating
cup.
During the ion emission a red fluorescence spot is visible at the target electrode This collector llphr 3. RESULTS
ON THE
LITHIUM
was
ION SOURCE
presumed
emitter 3.1. Current-voltage
curve
to
arise
previously and resputtert
collector
surface
Fig. 2 shows
a current-voltage
curve of the lithium
intensity
was estimated
liquid
ion
secondary
observed
homogenous
metal
source.
The
electron
current is suppressed by a target electrode bias of +30 V with respect to the extraction electrode. The onset
voltage
about
0.74
is 5.0 keV. The curve
has a slope
of
PA/V.
The stability of the lithium liquid metal ion source strongly depends on the residual pressure. At 2* 10m4Pa it was necessary to use the electronic current control mode to avoid a current decrease Fig 3 shows the stability of the ion current At ion current values of 2.4 and 23.5 pA the noise 1s below
1 %. Maximal
3.3. Angular The following angular
intensity methods
intensity
fluctuations
are below
were used to determmc
of the lithium
ion source:
3 %.
the
from
the
escltatlon
deposited
of
on
the
[6,7]. The angular the diamctcr
tluorescencc
spot.
of the the 1011
current and the emitter-target-distance [X 1. At a line crossing the center of‘ the irrndiatlon spot on a silicon target scvcral depth profiles of lithium SIMS.
3.2. Stability
from
material,
concentration The maximum
have
been
of the
measured
profiles
has
by been
evaluated for the angular distribution of the ion source. Post- accclcration of the ions IS IICWSS;I~ to get ion ranges being sufficlcntly deep ~OI- SIMS. The resulting angular intensity distrihution has been corrected for the focusing cl‘l‘cct 01‘ ~hls post accclcration l’he Faraday cup 1s moved 011 u clrclc around the emitter tip which intersects the emission bundle at the optical axis. This results in a set of angular distribution curves that are Jcp~~tcd 111I:ig 5 The results of these three methods arc compatible.
In I:lg. 5 the lithium
beam
profiles
E. Hesse et al. I A lithium liquid metal ion source I
’
113
I
-800 -600 -400 -200
0
200
400
600
800
EMISSION ANGLE (mr)
Figure 4. Plot emission angle
of angular at different
intensity (dI/dR) current levels.
vs
Figure 2. Current-voltage curve of the lithium ion source. I
Li’ 2.7/.LA (visual observation1
Li+
,Ga+
--;
2.0pA ww
Figure
3. Stability
of the lithium
ion emission
determined by visual observation and SIMS respectively are compared with that measured at gallium and bismuth sources. The axial angular intensities of a lithium source and a gallium source in Fig. 6. The axial angular [9] are shown intensity of the lithium liquid metal ion source is the highest one reported for liquid metal ion sources. The results are consistent with experiments and calculations on the atomic mass dependence of angular intensity [5] (Pig. 7). The enhancement of
; I
5.5 /LA i
;
[Swamon : et al.] ’
I
1’
; 4.0/LA wanson et Iis .t-.>
I \\ I ‘I, ’ I/ 1 / 1 , I /
1 I
EMLWON
al
ANGLE
(mr)
Figure 5. Comparison of the lithium angular intensity with experimental results from gallium and bismuth liquid metal ion sources [9].
E. Hesse et al. / A lithium liquid metal ion source
114
4. LITHOGRAPHIC To
further
EXPERIMENTS
test
the
lithium
liquid
metal
ion
source it was inserted into a focusing ion beam system with a focusing capacity of 5 pm PMMA resist layers were exposed with the focused lithium ion beam.
TOTAL
CURRENT
@A)
Figure 6. Comparison of the axial lithium angular intensity with experimental results from gallium[9].
Figure lithium
8. Lines
in PMMA
ion beam.
2.3 nC/cm;
resist written
Ion energy:
development:
by focused
25 kcV;
lint
dose
5 s m ethyl
hcnzcne
“VLSI
l’lectronics
at
22” c
ATOMIC
Figure
7. Angular
elements lithium.
at 10 PA
the lithium
angular
REFERENCES
MASS
intensities [5] and
intensity
on axis for different experimental
with respect
metal
ion sources
of other
elements
high
in
current
range
the
low
result for
to liquid
is especially up
Probably the kink of the axial I,(I)-curve current range observed at other liquid
to
20 PA.
in the low metal ion
sources, e.g. of gallium and aluminium [9,10], exists also at the lithium source. On the assumption of a nearly constant value of the energy spread at low Ion currents a maximum value of the chromatic angular intensity
one appears
on certain
conditions
[8].
1. B.
M.
Siegel
in
Microstructure Science” (N G. Einspruch, cd.). Vol. 16, chapter 5, Academic I’rcss. Orlando 1987 2. G. L. R. Mair Engineering, 3. L. Bischoff
and T. Mulvey, 3 (1985)
Microclcctromc
133-146
et al. , Microelectronic
13 (1991) 367 4. W. Knauer, Optik 5. D. R. Kingham
Engineering.
49 (1981)335-354
and L. W. Swanson
Appl.
PLys
A34 (1984) 123 6. G. L. R. Mair and A.von Engel. .I Appl. l’hys 50 (1979) 5592 A. J. Dixon and A. von Engel 1080 (Inst. l’hys Conf. Ser. 54) ch. 7, p. 292 E. Hesse and F K. Nachrlng 1903 .I I’hys I1 Appl. Phys. 26 (1993) 7 17-7 18 L. W. Swanson et al., 1979 J. Vat. SCI ‘l‘echnol 16 (1979) 1864 IO. Y. Torii and Ii. Yamada,
Proc.
Int
Ion Eng.
Congress CISIAT and IPAT (‘83), Kyoto (Japan). ed: T. Tagaki (Inst. of electrical Engineers 01 Japan,
Tokyo,
1983) p. 363