- The growth pressure used during the creation of the LBMO thin films significantly impacts the results of AFM nanolithography on the films. Specifically, higher growth pressures produce rougher film surfaces while lower pressures produce smoother surfaces. - The growth pressure affects the oxygen concentration and surface roughness of the LBMO thin films. As a result, the type of nanopatterns produced via AFM nanolithography and how those patterns relax over time depends on the film growth pressure. - Positive and negative tip voltages in AFM nanolithography produce different results depending on the growth pressure and corresponding oxygen concentration of the LBMO thin film. On oxygen deficient films, positive voltages produce lines while negative voltages produce large growths

1. AFM Nanolithography of La1-XBaXMnO3-d (LBMO) Thin Films:
The Effect of Oxygen Pressure Variations During Film Growth
Christopher Stumpf, David Schaefer, Rajeswari Kolagani, Grace Yong, Zoey Warecki, and Kevin Tanyi
Department of Physics, Astronomy and Geosciences, Towson University, Towson, MD 21252
Growth Pressure and Roughness
- The growth pressure dramatically affects the roughness of
the LBMO thin films. High growth pressures produce very
rough sample surfaces.
Figure 3: Example scan of
sample LBMO-STO-34
Figure 4: Example scan of
sample LBMO-STO-37
Sample 34
(50 mTorr)
Ra = 0.398 nm
Sample 37
(400 mTorr)
Ra = 1.60 nm
From Nanoscope,
Abstract
In AFM nanolithography, a bias voltage applied between the tip of an
atomic force microscope (AFM) and a sample is used to produce nanoscale
modifications of material surfaces. AFM nanolithography has been studied
extensively on a variety of materials, but limited studies have been performed
on perovskite manganites such as Lanthanum Barium Manganese Oxide
(LBMO). Studying such materials is important because of their potential
applications for room-temperature nanoscale spintronic devices.
Previous research on LBMO by our group has focused on how
parameters such as applied tip voltage, temperature, and humidity affect the
creation of nanopatterns. This paper reports on the influence of growth pressure
of the LBMO films grown by pulsed laser deposition. Films grown on (100)
SrTiO3 were studied for growth pressures ranging between 100 mTorr to 400
mTorr. Our studies indicate that the type of nanopatterns induced by AFM and
the relaxation dynamics of these patterns are sensitive to the film growth
pressure. The growth pressure is mainly known to affect the oxygen
concentration and the surface roughness, but possible variations in cationic
stoichiometry could also contribute to these results.
Line Patterns on LBMO-STO-39 (225 mTorr)
- Sample 39’s surface is smoother than sample 37 (400 mTorr)
but it is not as clean as sample 34 (50 mTorr).
- The surface has many small dots and growths on it. There are
clean and smooth areas between the dots.
- Sample 39 was created at a growth pressure of 225 mTorr
which is exactly between the pressures of samples 34 and 37.
Figure 9: A 10 V to 20 V
pattern on sample 39
Figure 10: A -20 V to -10 V
pattern on sample 39
- These two images
are examples of
patterns on sample
39
- The patterns were
created at 73.6°F
and 71% humidity. 10 v 20 v
-20 v
Line Patterns on LBMO-STO-37 (400 mTorr)
- Although the sample is rough, we were still able to create line
patterns on sample 37. However the lines are blurrier than the
line patterns on sample 34.
Figure 7: A 15 V to 25 V
pattern on sample 37
Figure 8: A -25 V to -15 V
pattern on sample 37- To the right are
examples of
line patterns on
sample 37.
- The lines were
created at 75°F
and 74%
humidity.
15 V
25 V
-25 V
-15 V
- Unlike the 50 mTorr film, both positive and negative tip
voltages produce line patterns.
Positive Tip Voltages on Sample 39 (225 mTorr)
- Like on sample 37, we performed many tests on sample 39
and averaged the results together to determine the average
line height that is produced by each positive tip voltage.
- Some of the lines were written overtop of the dots on sample
39’s surface. The dots affected the height of the voltage lines.
- Positive voltages
on sample 39
have a threshold
voltage of 10 V
and a saturation
height of 6.43 nm
at 31 V.
Nanolithography: Experimental Procedure
- After writing, we use Nanoscope to measure the height and
width of each voltage line. We can then plot the line height
or width versus the applied tip voltage.
- When applying a voltage to the LBMO sample, there is a
threshold voltage below which a line will not appear.
- When creating line patterns on the LBMO films, we keep
track of the temperature and humidity as each line is written.
- We use positive and negative tip voltages to write on the
LBMO film. The two polarities can produce different results.
- Sometimes the line patterns will not become any wider or
taller even if a larger voltage is applied. This is called a
saturation point.
Negative Tip Voltage: LBMO-STO-34 (50 mTorr)
- Unlike positive tip voltages, negative tip voltages produce
huge growths on the 50 mTorr sample 34.
Figure 6: A negative voltage
pattern on sample 34
- The huge growths are uncontrollable and irreproducible.
- The growth in Figure 6 occurred
when we applied a -15 V tip
voltage to sample 34 at 70%
humidity and a temperature of
72.4°F
- The growth covers a 40 µm area
and has a height of over 1 µm.
LBMO Thin Film Deposition
- The LBMO films were grown on SrTiO3 (100) substrates by
Pulsed Laser Deposition using a Krypton Fluoride laser of
wavelength 248 nm and operating at 10 Hz.
- The substrate temperature during film growth was
800°C with a laser pulse energy of 450mJ and an
energy density of 1J/cm2 on the substrate. The
films are grown under pressure.
- Figure 1: The Pulsed Laser
Deposition Chamber
- For our experiment we grew
three LBMO samples under
different growth pressures.
Thin Film Deposition (Continued)
Pulsed Laser Deposition
Kr-F (248 nm)
Target
Heater
Substrate
Film
Sample Growth Pressures:
LBMO-STO-34 at 50 mTorr
LBMO-STO-37 at 400 mTorr
LBMO-STO-39 at 225 mTorr
- The growth pressure affects the oxygen concentration and
surface roughness of the sample. Growth pressure is very
important to our research.
Figure 2: The laser plume deposits
material on the substrate.
- We created three samples with different growth pressures so we
could see how the growth pressure affects nanolithography.
- The 50 mTorr sample has
the lowest oxygen
concentration while the
400 mTorr sample has the
highest concentration.
Positive Tip Voltages on Sample 37 (400 mTorr)
- Just like sample 34, we wrote on sample 37 many times and
averaged the results to determine the average line height
produced by each voltage.
- Positive line
patterns on
sample 37 have
a threshold
voltage of 10 V.
- Unlike sample 34, sample 37 has a saturation height of 4.5
nm at 39 V. The 400 mTorr growth pressure appears to affect
how high the line patterns can grow.
Negative Tip Voltages on Sample 39 (225 mTorr)
- Some lines were not created when writing with negative tip
voltages on sample 39. Figure 11 is an example of this problem.
Figure 11: Partial writing of
a -30 V to -20 V pattern
- For the lines that did appear, we
averaged the results of many tests to get
the average line heights for each voltage.
- The threshold voltage for negative tip
voltages on sample 39 is – 16 V.
- The saturation height
is 9.6 nm at - 34 V.0
2
4
6
8
10
12
-35 -30 -25 -20 -15 -10
LineHeight(nm)
Appplied Tip Voltage (V)
LBMO-STO-39 Negative Tip Voltages vs.
Average Line Height
Nanolithography with an Atomic Force Microscope
STO substrate
LBMO thin film
- We performed our experiments with a Veeco Multimode
VII AFM in contact mode.
- We used Asylum Research AC160TS silicon coated tips to
scan the LBMO film.
- To create growths on the LBMO we put the AFM in a
humidity chamber and applied a voltage to the tip
with our custom LabView program.
Figure 3: AFM with
LBMO film in a
humidity chamber
Humidity
chamber
Applied Voltage
Positive Tip Voltage: Sample 34 (50 mTorr)
- We wrote dozens of line patterns on sample 34. We measured
the height and width of each line in the pattern. We then
averaged the results together to determine the average line
height and width that each voltage produces.
- Sample 34 has a threshold voltage of 5 V. Lines are not formed
below this voltage.
- The line widths saturate at a width of 1.13 µm at around 23 V.
The line heights do not saturate on sample 34.
0
5
10
15
20
0 5 10 15 20 25 30
LineHeight(nm)
Voltage (V)
LBMO-STO-34 Positive Tip
Voltage vs. Average Line Height
0.8
0.85
0.9
0.95
1
1.05
1.1
1.15
1.2
5 10 15 20 25 30
LineWidth(µm)
Voltage (V)
LBMO-STO-34 Average Line
Width (µm) vs. Tip Voltage (V)
Patterns on LBMO-STO-34 (50 mTorr)
Figure 5: A positive voltage
growth pattern on sample 34
- In AFM nanolithography, a bias voltage applied between the tip
of an Atomic Force Microscope (AFM) and a sample is used to
produce nanoscale modifications of material surfaces.
- The pattern in Figure 5 was made
at 76.5°F with 74% humidity and a
tip velocity of 1.6 µm/s.
- The properties of the growth
pattern are affected by the applied
tip voltage, temperature,
humidity, and tip writing speed.
- In Figure 5, we started at 10 V
and increased the applied voltage
by 1 V for each line, up to 30 V.
10 V
20 V
30 V
Negative Tip Voltages on Sample 37 (400 mTorr)
- Unlike the 50 mTorr sample 34, negative tip voltages applied
to sample 37 produce line patterns and not massive growths.
- The lines are usually very rough and blurry. They often have
large growths near them. Controlling the growth of negative
voltage lines is very difficult.
- Like the positive voltages, we averaged together multiple
negative voltage tests to determine the average line height that
each voltage produces.
- Negative lines on
sample 37 have a – 10
V threshold voltage
and a saturation height
of 5.4 nm at – 29 V. 0
1
2
3
4
5
6
7
-30 -25 -20 -15 -10 -5
LineHeight(nm)
Applied Tip Voltage (V)
LBMO-STO-37 Negative Tip Voltages
vs. Average Line Height
References and Acknowledgements
Conclusions
- RK and GY acknowledge support from the National Science Foundation Grant
ECCS 1128586
- Li. Run-Wei. “AFM lithography and fabrication of multifunctional nanostructures
with perovskite oxides.” Int. J. Nanotechnology,. Vol. 6, No. 12, 2009.
- The growth pressure used during the creation of the LBMO
sample dramatically affects the characteristics of the line
patterns produced by AFM nanolithography.
- The growth pressure affects oxygen stoichiometry and surface
roughness.
- Patterns on oxygen deficient films are sensitive to voltage
polarity. Positive tip voltages produce lines while negative
tip voltages produces massive growths.
- Both voltage polarities produce lines on oxygen rich films.