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Microelectronic Engineering 46 (1999) 105-108
Sub-100 nm Structures by Neutral Atom Lithography
Th. Schulze, B. Brezger, P. O. Schmidt, R. Mertens, A. S. Bell, T. Pfau, and J. Mlynek.
Fakult/it fiir Physik, Universit/it Konstanz, 78457 Konstanz, Germany
Instead of using a solid mask to pattern a light beam (optical lithography) we used a mask made of light to
pattern a beam of neutral atoms (atom lithography). By making use of two special features of the atom-light
interaction we wrote structures with periods below A/2. In the first approach we inverted the focussing potentials
by switching the detuning of the light field during the deposition. The second method uses the fact that atoms
with a magnetic substructure in the electronic ground state are strongly sensitive to the polarization of the light
field. Both techniques produce sub-100 nm chromium structures in one and two dimensions on silicon substrates.
1. I n t r o d u c t i o n
I transversel laser cooling ]
Beams of neutral atoms and immaterial masks
of laser light have been employed for nanostructuring since 1992. Up to now, the generation of lines with period ,V2, where ~ is the
laser wavelength near an atomic resonance, has
been reported for sodium [1], chromium [2,3], alurninium [4], and cesium [5]. For chromium, also
various two-dimensional structures of the same or
larger period [6,7] have been achieved.
In these experiments the light mask contains intensity gradients which induce a so-called dipole
force [8] on the atoms. The intensity pattern has
only fourier components belonging to difference
vectors k j - k t and therefore with maximum modulus 2k which limits the generated structures to
periods of ,V2 or more. In this paper we present
two new techniques to go below this limit in two
dimensions. First by a detuning switch during the
deposition and second by the use of polarization
gradients in two dimensional light masks.
2. E x p e r i m e n t a l s e t u p
In all experiments described here, the experimental setup corresponds to fig. 1 (described
in more detail in [7]): immediately after the
chromium source, the transversel atomic velocities are reduced by polarization gradient laser
cooling [9] using laser light detuned by - F with
respect to the 4s 7S3 --+ 4p 7P4 transition in 52Cr
(,k = 425.6 nm, F = 5 MHz). This collimates the
atomic beam to below 0.4 mrad (FWHM) in two
0167-9317/99/$ - see front matter
PII: S0167-9317(99)00026-X
I light mask I
mirrors
BOcm
chrorn
sour
chromium beam ~
T > 1(
--~
substrate
t 425nm
I
Cr-
feedback
1. /
monomode~
.~,,~
[ ..... r r
o
J 850nm
fiber~
I
I 425nm
v
Figure 1. Experimental setup. The different experiments differ in the light mask configuration.
dimensions while simultaneously enhancing the
atomic flux in the beam center.
The laser light at ,k = 4 2 5 n m is produced
via second harmonic generation (SHG) of a
Ti:Sapphire laser and frequency-shifted using an
acousto-optical modulator (AOM).
3. D o u b l e e x p o s u r e w i t h d e t u n i n g s w i t c h
The technique of this experiment is analogous to a double exposure in photography: two
subsequent patterning steps generate two interlaced chromium patterns which add in height on
the substrate. An important issue is the ex-
© 1999 Elsevier Science B.V. All rights reserved.
106
Th. Schulze et al. I Microelectronic Engineering 46 (1999) 105-108
act overlay, which is achieved by making use of
a special property of a dipole force light mask:
the potential of an intensity gradient light mask
changes sign when switching from negative to
positive detuning between laser and atomic transition frequencies. The corresponding change in
laser wavelength and therefore in structure period ( A t / A ~ 6 . 10 . 7 in the experiment) is neglectable. In effect, structure minima and maxima exchange places and the density of peaks per
area adds up.
The employed light mask is the same as used
in [7]: three laser beams intersect under an an-
(a)
13 nrn
gle of 120 °. For this experiment we rotated the
polarization of the incoming beam by 90 o so the
beams have a linear polarization in the propagation plane. Although there are polarization gradients present in this light mask, our theoretical
analysis [10] and experimental results show that
in this case the effect of the light mask is mainly
due to the intensity pattern which is inverted with
respect to the case when all polarizations are orthogonal to the propagation plane, as employed
in [7].
The results in fig. 2 and fig. 3 were obtained
with a light power of 24 m W and a gaussian beam
of waist 100 #m. During the deposition the detuning was chosen to - 2 0 0 MHz for 20 min and subsequently to +200 MHz for another 10 rain without
changing anything else.
The AFM picture in fig. 2a shows chromium
lines on silicon at a position where two laser
beams overlap.
Due to the different expo-
ii
0.00 nm
0.96 nm
0 nm
1150 nm
(b)
~/.~= 245.7 nm
12
Y=
'~
t-
9
2300 nm
.......
i
i
::.........
! .........
','-
!
=l
i .........
i
0.00 nm
::......
6
0 nm
255
510
765
1020 1275
distance [nm]
Figure 2. (a) AFM picture showing chromium
lines on silicon created in the double exposure experiment with detuning switch. (b) Cross-section
perpendicular to the lines.
450 nm
900 nm
Figure 3. AFM picture showing chromium dots
on silicon created in the double exposure experiment with detuning switch. The sketched
hexagons of edge length ~A ~ 284nm illustrate
the two interlaced sets of peaks which correspond
to the intensity minima for red and maxima for
blue detuning.
107
Th. Schulze et al. I Microelectronic Engineering 46 (1999) 105-108
sure times the two interlaced line systems differ in height (see cross-section in fig. 2b). The
chromium lines deposited in 20 min with red detuning have an average height of 8 n m and an
average FWHM of 71 nm (after background subtraction). The lines deposited with blue detuning
are 6 nm high and have a FWHM of 66 nm.
The AFM picture in fig. 3 shows two interlaced
hexagonal chromium patterns at the central part
of the light mask. Compared to the results in [7]
we have doubled the density of the chromium dots
to 2.8,109 dots per square centimeter.
the maximally symmetric arrangement of three
linearly polarized laser beams with polarizations
orthogonal to each other (therefore constant intensity) and wavevectors in a plane. This configuration which we call " l i n ± lin ± lin" was mentioned also in theoretical work on VSCPT laser
cooling [12].
The light mask was generated in a mirror setup
out of one linearly polarized beam (power 36 roW,
detuning +200 MHz) as shown in fig. 4b. The
length of the equilateral mirror triangle was chosen to only 350 #m because .of experimental constraints. To prevent non adiabatic transitions [10]
4. P o l a r i z a t i o n g r a d i e n t l i g h t m a s k s
Polarization gradients in light masks induce
multiphoton Raman processes which open the
way to generate smaller spatial periods. In one
dimension, it has been demonstrated experimentally that using a lin _L lin light mask leads to a
chromium line pattern with A/8 spatial frequency
components [11].
To extend this technique to two dimensional
patterning we have developed the light mask
sketched in fig. 4a. This configuration represents
(a)
(a)
m
t atoms
=
=
X/3 = 142 nm
(b)
~ / 4 " ~ = 61 nm
36
E
27
..c
(angle 8 to plane)
107
Figure 4.
(a) Light field configuration " l i n ±
the wavevectors (drawn bigger) of
three beams lie in one plane under 1200 whereas
the corresponding linear polarizations lie under
an angle of 0 = 35.30 (sin(0) = 1/x/~) to this
plane so that they are perpendicular to each
other. (b) Experimental setup to generate this
light mask in the shaded central region. (do =
100 #m)
214
321
428
535
distance [nm]
lin ± l i E ' :
Figure 5. (a) AFM analysis of a chromium structure generated in the central region of the light
mask in fig. 4. (b) Cross section perpendicular
to chromium lines deposited outside the central
region where only two perpendicularly polarized
beams overlap.
108
Th. Schulze et al. / Microelectronic Engineering 46 (1999) 105-108
we applied a magnetic field (B=0.3 mT) parallel
to the atomic beam.
Fig. 5a shows an AFM analysis of the central
part of the light mask in fig. 4. The open circles
mark the hexagonal dot lattice which corresponds
to the lattice deposited in the red detuned intensity gradient light mask in [7], the lattice period
is 3 A ~ 284nm. The polarization gradients in
the light field lead to additional chromium dots
marked as black points in fig. 5. With this polarization gradient mask we were able to halve the
lattice period to ½A ~ 142nm. By this, the density of the chromium dots has been quadrupled
to 5.7 * 109 dots per square centimeter compared
to the results in [7].
The cross section in fig. 5b shows lines at a position of the mask where two beams overlap under
an angle of 1200 forming a "lin _k lin" configuration with a period of ~ ~ 61nm. The equidistant chromium lines have an average height of
13nm.
5. C o n c l u s i o n
By making use of two special features of the
atom-light interaction we wrote chromium structures with periods below A/2 in one and two dimensions on silicon substrates. In the double exposure experiment with detuning switch we wrote
chromium lines with a period of ~
~ 123nm
and two interlaced sets of chromium dots having
a density of 2.8 * 109 dots per square centimeter.
In the second experiment we used polarization
gradients in two dimensions to write chromium
lines with a period of ~ ~ 61 nm and a hexagonal dot pattern with lattice period ½~ ~ 142nm
which is half the lattice period of the intensity
gradient light mask in [7]. The density of the
chromium dots has been quadrupled to 5.7 * 109
dots per square centimeter.
Based on these results we will systematically
study in future experiments the pattern generation in polarization gradient light masks including the dependence on magnetic fields. In a second experiment we want to make use of the fact
that light masks are material selective. This special feature of light masks allows to make a ho-
mogeneous and a structured deposition at the
same time resulting in an artificial material with
a structured doping on a sub-100 nm lengthscale.
Such a material is a promising candidate for a
photonic crystal [13] or a quantum computer [14].
We appreciate financial support by the Deutsche
Forschungsgemeinschaft (SFB 513) and the Optik
Zentrum Konstanz.
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