# Imaging Pulsed Laser Deposition oxide growth by in-situ Atomic Force
Microscopy

W.A. Wessels  T.R.J. Bollmann<EMAIL_ADDRESS>D. Post  G. Koster
G. Rijnders  University of Twente, Inorganic Materials Science, MESA+
Institute for Nanotechnology, P.O. Box 217, NL-7500AE Enschede, The
Netherlands

###### Abstract

To visualize the topography of thin oxide films during growth, thereby
enabling to study its growth behavior quasi real-time, we have designed and
integrated an atomic force microscope (AFM) in a pulsed laser deposition (PLD)
vacuum setup. The AFM scanner and PLD target are integrated in a single
support frame, combined with a fast sample transfer method, such that in-situ
microscopy can be utilized after subsequent deposition pulses. The in-situ
microscope can be operated from room temperature (RT) up to 700∘C and at
(process) pressures ranging from the vacuum base pressure of 10-6 mbar up to 1
mbar, typical PLD conditions for the growth of oxide films. The performance of
this instrument is demonstrated by resolving unit cell height surface steps
and surface topography under typical oxide PLD growth conditions.

##  I Introduction

A fascinating material class are the perovskite oxides due to their wealth in
physical properties such as superconductivity, ferromagnetism, ferro- and
dielectricity Eason (2009). Induced by the discovery of high-$T_{C}$
superconductors, pulsed laser deposition (PLD) has become a popular thin film
growth technique to fabricate high quality oxide materials Bednorz and Muller
(1986); Dijkkamp  _et al._ (1987). The strength of depositing complex oxides
with PLD comes from the fact that relative high oxygen pressures can be used
while still having high kinetic energy ablated species, which strongly
influences the films properties. Development of the current $in-situ$
diagnostic tools such as high-pressure reflection high-energy electron
diffraction (RHEED) Rijnders  _et al._ (1997), surface x-ray diffraction
(SXRD) Eres  _et al._ (2002) and the more rarely used optical spectroscopic
ellipsometry (SE), enabled to observe $in~{}operando$ the oxide thin film
growth. Using RHEED e.g., the growth mode and in some conditions the number of
grown unit cells can be deduced by measuring the step density over time Neave
_et al._ (1983); Shitara  _et al._ (1992); Blank  _et al._ (1999). The
specular rod in SXRD descibes layer filling when the diffuse scattering
contains information about the spatial distribution of islands, whereas with
SE the evolution of the electronic structure can be monitor during film growth
Blank and Rogalla (1997); Bijlsma  _et al._ (1998). Although these scattering
techniques are well established tools for monitoring the growth of oxides
$in~{}operando$, the reciprocal information contained can be hard to interpret
and they do not allow to probe growth properties of individual thin film
islands as the surface reflectivity signal typically probes and averages over
a surface area of millimeters in size. Besides this, diffraction techniques
typically require crystalline surface planes to enable observation at all.

For microscopic real-space observations a popular diagnotic tool available are
the scanning probe microscopes (SPM), enabling monitoring the surface topology
at the (sub)nanometer spatial resolution Lippmaa  _et al._ (1998, 2000); Iwaya
_et al._ (2010); Shimizu  _et al._ (2014). As most perovskite oxides are
insulators or have a large band gap, application of scanning tunneling
microscopy (STM) is rather limited as it is based on a tunneling current
flowing between sample and tip Binnig  _et al._ (1982); Sudheendra  _et al._
(2007). Within the field of (PLD) oxide growth, microscopy analysis on
surfaces is therefore typically done by post-deposition $ex-situ$ atomic force
microscopy (AFM) or by $in-situ$ ultra high vacuum (UHV) AFM. However,
microscopically probing the surface during deposition is however a
prerequisite in order to broaden our understanding of thin oxide film growth.
Real space microscopy during film growth would give complementary information
besides reciprocal techniques about microscopic events in thin film growth
such as diffusion processes, ripening, defect formation etc. by measuring the
nucleation density and individual thin film island evolution over time in
between subsequent deposition pulses Rost  _et al._ (2009), as in PLD
deposition and growth are separated in time.

A first design and demonstration of a conventional AFM operating at metal-
oxide PLD conditions has been reported Broekmaat  _et al._ (2008); Broekmaat
(2008), however the main drawback of typical AFM is the low sample throughput,
as $in-operando$ monitoring requires high-speed AFM instrumentation.
Conventional AFMs are slow due to the low bandwidth of the cantilever, AFM
scanner and electronics and optical detection signal. Recently, a lot of
progress has been made to increase the bandwidth of these components in
different enviroments, which shows the potential of high speed AFM in PLD
conditions Rost  _et al._ (2005); Kodera, Sakashita, and Ando (2006); Ando,
Uchihashi, and Fukuma (2008); Schitter and Rost (2008); Ando (2012).

Here, we present the concept, specifications, design and performance of an
atomic force microscope (AFM) in a pulsed laser deposition (PLD) vacuum setup.
The setup consists of an in-situ AFM joined in an aluminum frame with an in
geometrically position separated PLD position. Combined with a fast sample
transfer system, this enables in-situ microscopy after subsequent deposition
pulses. The AFM has been developed such that tapping mode (TM) and frequency
modulated (FM) AFM can be applied at typical PLD conditions ranging from room
temperature (RT) up to 700∘C and at (process) pressures ranging from the
vacuum base pressure of 10-6 mbar up to 1 mbar. The performance of this
instrument is demonstrated by resolving unit cell height steps of a
SrTiO3(001) surface at PLD conditions as well as the evolution of the surface
topography of a grown BiFeO3 film, a prototypical perovskite film. We conclude
this paper with an outlook towards future applications as well as limitations
of the current design.

##  II Concept

Figure 1: Conceptual drawing of the PLD setup combined with $in-situ$ AFM.
Imaging and deposition are geometrically in position seperated to prevent
hindering of deposition and tip contamination. A fast transfer stage is moving
the sample back and forth between imaging and deposition position. 1) support
frame, 2) AFM scanner, 3) plasma screen, 4) PLD target and 5) sample/heater
transfer stage.

Fig. 1 shows a schematic illustration of a concept of AFM during PLD. In a
support frame, an AFM scanner and PLD target are installed and a sample
transfer stage is included. The AFM scanner is positioned geometrically
separated from the PLD position for a main reason: It prevents blocking of
deposition on the as-grown surface and unwanted AFM tip contamination. As
deposition and growth in PLD are separated in time, growth can be studied in
between subsequent pulses using this concept. A plasma screen between AFM and
PLD should prevent any PLD plasma contamination on the AFM scanner.

In this concept, a sample is transferred back and forth between AFM and PLD,
where the sample transfer is based on linear motion, see Fig. 1. A sample
heater is mounted on top of a linear motion slider. The concept is such that
the side approach Wessels  _et al._ (2013) can be applied to reduce the time
delay between PLD and AFM monitoring of the sample surface. The proposed
configuration is straightforward to combine PLD with other diagnostics tools,
such as scattering techniques as RHEED and plasma diagnostic tools for an
increased understanding of thin film growth in PLD.

##  III Specifications

To image the surface topography at PLD conditions, the AFM has to be operated
at pressures ranging from 10-6 mbar up to process pressures of 10-1 mbar. The
applied gases typically involve (a mixture of) oxygen, argon and/or nitrogen.

The temperature window of typical growth conditions runs from room temperature
(RT) up to about 700∘C, where the exact substrate temperature depends on the
substrate material and ablated species.

AFM imaging, resolving the growth properties of individual thin film islands
requires a stable AFM imaging at an image size of about 1$\times$1$\mu$m2 A
short mechanical loop between the to be imaged substrate and AFM cantilever is
required such that the noise levels, both, electronic and vibrational, should
not exceed the substrate stepheight, typical $<$0.4 nm as typical oxide
substrate steps are around 0.4 nm in height. This resolution is required both
out-of-plane and in-plane.

To suppress thermal drift, an effective temperature stabilization is required.
A thermal drift out-of-plane and in-plane, which enables measurement during 1
h would be required, as it is a typically duration involved in a PLD
experiment. As we want to make use of image sizes of about 1$\times$1 $\mu$m2,
this requires any in-plane drift to stay below 50 nm/min, thereby requiring
minimal manual correction for imaging. The perpendicular thermal drift should
stay within the piezo range to circumvent the requirement of a slow re-
approach procedure.

To visualize the rapid growth processes involved, one needs to scan at high-
speed, typically an image within seconds would be desirable, possibly even
faster.

In order to reduce the dwell time between deposition and imaging, a fast
sample transfer is required to minimize the time between geometrically
seperated PLD position and AFM imaging. The repositioning error for subsequent
back and forth motions has to stay below 100 nm.

The requirements can be summarized as follows:

  * •

Pressures during imaging: Ranging from 10-6 mbar (vacuum) up to 10-1 mbar in
background gasses of (a mixture of) oxygen, argon and/or nitrogen.

  * •

Temperature range for imaging: RT up to 700∘C.

  * •

Imaging resolution: step resolution at oxide substrates ($<$0.4 nm) at all
mentioned pressure and temperature conditions.

  * •

Imaging rate: on the order of (tens of) seconds per image of 256$\times$256
pixels with images of about 1$\times$1$\mu$m2.

  * •

Thermal drift: below 700 nm/h in perpendicular direction, below 50 nm/min in-
plane.

  * •

Transfer time before and after imaging: transfer time back and forth within
0.5 s with a repositioning error below 100 nm.

##  IV Design

In this section, the general architecture of the vacuum setup, the designed
_in-situ_ AFM within a PLD vacuum chamber and the design of the AFM scanhead
is described in detail.

###  IV.1 Vacuum chamber and peripheral equipment

Figure 2: Schematic drawing of the vacuum setup and peripheral equipment. 1)
PLD vacuum chamber, 2) AFM-PLD frame, 3) door of chamber, 4) Halcyonics MOD2-S
active damping control element, 5) air damped pods, 6) Aluminum profile
system, 7) passive damping stage, 8) active damping stage, 9) incoming
beampath for the excimer laser, 10) DN100CF five-way cross, 11) fiber
feedtrough, 12) electrical feedtrough, 13) mounting of the passive damping
stage, 14) and 15) DN40CF flanges for pressure sensors, 16) gasinlet, 17)
DN63CF flange with quartz glass, 18) adaptive pressure controlled valve, 19)
valve for laser energy measurement, 20) closing unit door, 21) Aluminum tube
for feedtrough and connecting the active damping stage with the supporting
frame, 22) DN200CF flange towards turbopump.

Fig. 2 shows a schematic representation of the designed PLD chamber and
peripheral equipment. The PLD chamber is a single vacuum system, which differs
in shape compared to traditional cylindrical chambers. In the designed vacuum
system, the base pressure of 10-7 mbar is limited by O-rings of the
vacuumdoor, valve on the flange on the laser beam path and the pressure
controled valve. Typical PLD background pressures of 10-6 \- 1 mbar are
measured by almost closing this valve and varying the incoming gas flow
between zero and 100 ml/min. The vacuumchamber is a square-like box design
such that a vacuum chamber door is used to open up the system for
accessibility to install the supporting frame for AFM and deposition. The
vacuumdoor has a DN100CF flange viewport window, revolves by a shaft and can
be closed by two closing knobs. Four airpod damping units together with matted
corks placed underneath provide passive damping of system resonances validated
by measuring the transfer function in x- and y-direction Segerink, Korterik,
and Offerhaus (2011). The vacuum chamber houses an aluminum tube connecting
the active damped system towards the supporting frame for AFM and deposition.
The active damping stage is mounted as a lid on this PLD chamber ensuring good
vibration isolation.

On top of the active damping stage, a five-way cross has been assembled
providing feedtroughs for optical and electrical signals from and to the AFM
scanner, coarse approach steppermotor, slider piezomotor drive and heater. The
DN40CF flange ports are used for vacuum pressure gauges to measure both,
process pressures as well as (high) vacuum pressures.

A KrF excimer laser beam aligned to an optical rail is integrated such that a
248 nm pulsed laser beam, having a typical pulse duration of 25 ns, is focused
on the PLD target. As an entrance flange for the PLD excimer laser light, a
flange is designed to enable the alignment of the laser onto the PLD target in
an angle of 15∘ parallel to the targets surface. The flange is integrated with
a valve to support laser intensity measurements behind the quartz window as
well as a gasinlet for the applied background gasses. A flow meter controlled
gas manifold has been designed to separately let (a mixture) of nitrogen,
oxygen and/or argon into the vacuum chamber through this gas-inlet. At the
bottom of the vacuum chamber, a DN200CF flange is connected to an adjustable
valve typically used for automated pressure control followed by a
turbomolecular pump.

###  IV.2 Support frame for microscopy and deposition

Fig. 3, schematically illustrates the supporting frame which contains the AFM
and deposition stage. This aluminum frame, exhibiting a robust and high
stiffness design, is attached by an aluminum tube to an active damping stage,
see also Fig. 2. This damping stage has the functionality to lower the
vibrational level in the mechanical loop of the AFM. The chosen shape and size
of the support frame is based on required sizes and shape of the AFM scanner,
PLD deposition and highest mechanical stiffness such that the required
vibrational level is not exceeded. The aluminum frame consist of two assembled
frames, where the top frame is used for integration of both the AFM scanner
and PLD target. The $z$-approach stage, containing a stepper motor having a
step resolution of 100 nm/step, is positioned in the top frame to provide
coarse approach towards the sample. A homebuilt mounting bracket connects the
coarse approach stage to the AFM scanhead. The PLD target can be mounted onto
a holder, adjustable in both height and lateral position along the sample
transfer direction. In this design, the distance between the AFM tip and
center of the PLD target is typically 50-60 mm, where the PLD target is
shielded by a plasmascreen from the AFM scanhead.

The bottom frame has the functionality to integrate a sample transfer stage
enabling back and forth sample transfer between the geometrically seperated
AFM and deposition stage. A vacuum piezomotor with low vibrational level, a
repositioning resolution within 20 nm and a maximum speed of 244 mm/s is
installed in the bottom frame in order to propel the sample transfer stage
which is designed to have minimal mass.

Figure 3: Schematic drawing of support frame including AFM and PLD. 1) Al
frame, 2) AFM flexure scanner, 3) mounting bracket, 4) PLD target 5) coarse
approach stage, 6) Vexta 5 phase stepper motor PK545-B, 7) HR4 nanomotion
ultrasonic piezomotor, 8) macor heater stage, 9) sample transfer stage, 10)
plasma screen, 11) two Kamet HDA pt200 RTD’s connected in serie, 12)
5$\times$5 mm2 sample on top of an 5$\times$5 mm2 Pt plate and 13) location of
screwed cube for integration of acceleration level sensors.

###  IV.3 AFM flexure scanner

Figure 4: Schematic drawing of the high resonance frequency AFM flexure
scanner, with 1) the Ti6Al4V flexure, 2) stacked XY piezo actuators, 3) AFM
chip holder, 4) screw holes, The insets shows the: 5) connection plate between
AFM chip base plate and the flexure, 6) cantilever base plate 7) electrical
contacts of the dither piezo, 8) ferrule, 9) cantilever chip, 10) chip holder,
11) cantilever clamping spring, 12) dither piezo stack, 13) screws, 14) AFM
cantilever, 15) optical fiber end.

The AFM scanner is based on a flexure, see Fig. 4, which is often used in the
field of high speed AFM Ando (2012). Flexure scanners are used as an
alternative to commonly used low resonance frequency piezotubes to extend the
mechanical scanner bandwidth several orders of magnitude into the kHz range
Yong  _et al._ (2012). In addition, stacked piezo’s integrated in a flexure
results in less cross-coupling, hysteresis and creep as compared to generally
used piezotubes. The AFM flexure in this work has been made out of Ti6Al4V to
achieve high stiffness, a resulting high resonance frequency and low thermal
expansion in order to minimize thermal drift. A simulation using finite
element analysis resulted in a lowest resonance of 19 kHz for the flexure in
the XY-plane. The maximum achievable linerate, estimated as 1/100th - 1/10th
of the lowest resonance frequency Yong  _et al._ (2012), is therefore between
0.19 - 1.9 kHz for this scanner design.

In order to achieve the desired scan range, we make use of piezo actuators
with an expansion up to 4 $\mu$m, which strongly reduces up to 1.4 $\mu$m due
to the high preload. The Z-flexure, a piezoring with an effective scan range
of 1.4 $\mu$m is aligned nearly in the center of the XY-flexure and can be
positioned with high precision within the XY-plane by the double stacked XY-
actuators. A counterbalance piezo, integrated in the Z-flexure, counters the
momentum generated by center of mass movement of the Z-piezo. The unit,
holding the AFM chip, is displaced by this Z-piezo such that the actual
cantilever is excited, see the inset in Fig. 4.

The bottom part of this unit is screwed on top of the Z-flexure and is milled
under an angle of 7∘. On top of this a flat plate with dither piezo stack is
mounted. To detect the cantilevers deflection we make use of optical
interferometry through the ZrO2 ferrule shown in Fig. 4 instead of the more
common optical beam deflection method. Beam deflection requires use and
adjustment of optical instrumentation, such as mirrors and is unpractical
under PLD conditions, where space is lacking. Therefore, usage of
interferometric detection results in a more compact design. A laser beam is
reflected from the cantilever top side and fiber-air interface, where the
cantilever and fiber are separated by 40-50 nm. A reference wave is reflected
by the planar end of the fiber (glass-air interface). The detected wave is
reflected by the top face of the cantilever. Both light waves interfere on a
photodiode for detection. The optimal interference working point distance of
the cavity can be adjusted by a DC voltage over the dither piezo. On top of
this $V_{\mathrm{DC}}$, an AC voltage (typically $V_{\mathrm{AC}}$ = 1 mV - 1
V) is applied to oscillate the cantilever at its resonance frequency $f_{0}$
in dynamic mode AFM.

##  V Performance

In this section we demonstrate the performance of the crucial components of
the experimental setup. We focus here on the vibrational level and
characteristics of the setup under PLD conditions. We then turn to the imaging
performance by demonstrating the imaging of a typical oxide surface,
SrTiO3(001), under some typical conditions experienced in a PLD experiment as
well as the imaging of the growth of a BiFeO3 film under its PLD growth
conditions.

###  V.1 Vibrational level and stage translation

Fig. 5 shows the X,Y,Z acceleration levels of the AFM-PLD setup operating at
0.1 mbar oxygen background pressure in comparison to that of the acceleration
levels of a commercial AFM setup (Bruker Dimension Icon) operating in ambient
conditions. Both systems are placed on the same VC-G classified floor. The
vibrational level sensors are positioned in the mechanical loop from tip to
the sample, see Fig. 3. The passive damping system suppresses the higher
frequencies $\geq$ 100 Hz, whereas the active damping stage significantly
decreases acceleration levels from 10 - 100 Hz. Damping of the system by the
airpods is mainly done in the Z-direction. There is no significant influence
in acceleration levels observed for the peripheral equipment. Acceleration
levels of the designed setup are similar to the commercial AFM setup, see Fig.
5. Only resonance frequencies of the aluminum support frame, see Fig. 2, are
found at 140, 190 and several peaks between 300 - 400 Hz, marked by arrows in
Fig. 5.

Crucial for the designed setup, is the performance (directly) after stage
transfer. We therefore recorded the time domain signal together with the
acceleration levels during such transfer. The vibrational decay time of the
mechanical AFM tip-sample loop is determined from the time domain signal from
the moment that the transfer stage velocity is zero at the AFM position after
sample transfer from PLD position to the AFM position, for more details we
refer to our earlier report Wessels  _et al._ (2013). In the parallel
acceleration level measurements, resonance peaks are observed at 140 and 280
Hz directly after stage deceleration and a corresponding delay time of 0.4 s.
The back and forth sample transfer between AFM and PLD is possible in 0.5 s
driving at maximum acceleration and velocity.

Figure 5: Vibrational spectra of the, (a) designed setup at a typical oxygen
PLD pressure of 10-1 mbar O2 in comparison to a commercial (b) AFM setup. The
peaks marked by arrow correspond to resonance frequencies of the aluminum
support frame.

###  V.2 The role of PLD conditions

Typical PLD conditions involve substrates at elevated temperatures and process
pressures up to several mbar, a pressure regime where cantilever response is
known to alter dramatically Lübbe  _et al._ (2011). The cantilever resonance
frequency $f_{0}$ and the cantilever quality $Q$ factor are nearly constant at
pressures ranging from 10-6 \- 0.1 mbar Mertens  _et al._ (2003). However, $Q$
increases from a few hundred to tens of thousands in the pressure regime 1 bar
- 0.1 mbar Lübbe  _et al._ (2011), corresponding to a broadening in its
frequency response.

Also the elevated substrate temperatures involved in PLD growth, can result in
temperature variations of the cantilever during stage transfer. These
temperature variations are known to result in unwanted drift of the eigen
frequency $f_{0}$ of the cantilever as the cantilevers dimensions alter and
the Young’s modulus of silicon is known to vary reasonably upon temperature.
Measuring $f_{0}$ at RT and within close vicinity of a sample heated up to
600∘C at a process pressure of 10-1 mbar resulted in a frequency shift $\Delta
f$ of 200 Hz, corresponding to a temperature increase of the cantilever of
$\approx$ 70 ∘C. By use of cantilevers with reasonable but rather low $Q$
factors, the used side-approach Broekmaat  _et al._ (2008) upon stage transfer
can still be made succesfull, having the compromise of lower resolution. By
the use of a radiation shield we prevented thermal radiation to influence the
working point distance of the interferometric setup

###  V.3 AFM imaging

In order to demonstrate the scan speed performance we performed tapping mode
(TM)-AFM measurements in air, shown in Fig. 6. The scan speed is increased for
figures Fig. 6 (a-c) measuring the SrTiO3(001) substrate steps of 0.4 nm in
height. At conventional AFM scan speeds, having a 2 Hz line rate
(corresponding to an acquisition time of 256 s/frame), the subnanometer SrTiO3
substrate steps are clearly resolved, see also the height profiles in Fig. 6.
From this line-profile, a peak-to-peak RMS of $<$0.1 nm was found. Upon
reducing the acquisition time, see Fig. 6(b) and (c), towards respectively 39
s/frame and 20 s/frame, this peak-to-peak RMS increases towards $\approx$0.3
nm. In addition, a resonance frequency of 1.6 kHz is appearing, a vibrational
mode of the mounting bracket connection with the coarse approach stage.

The increase in peak-to-peak z-noise can be attributed to two sources; firstly
the electronic noise which increases proportional to $\sqrt{f_{bw}}$, note
that the used electronics are known to deliver sufficient resolution at video
rate and beyond Rost  _et al._ (2005). Secondly, the currently used
photodetector which is limited to 400 kHz, thereby practically limiting to
cantilevers of $\sim$ 300 kHz. A reduced pixel dwell time results in less
oscillations used to determine a RMS value for every single pixel.

Figure 6: (Color online) TM-AFM 1$\times$1 $\mu$m2 512$\times$512 pixels2
images of a SrTiO3(001) substrate in air at RT with in the bottom panels the
corresponding height profiles. The scan speed is (a) 256 s/frame, (b) 39
s/frame and (c) 20 s/frame. The resolution decreases significantly at a scan
speed of 20 s/frame due to the limitation of the used cantilever resonance
frequency $f_{0}$.

Typical PLD conditions involve process pressures ranging from high vacuum
conditions up to 1 mbar. To demonstrate the use of the microscope at these
conditions, in Fig. 7 we show AFM images recorded under typical PLD
conditions. In Fig. 7(a), a frequency-modulated (FM) AFM image is depicted of
a SrTiO3(001) substrate containing unit cell vacancy islands of 0.4 nm deep
recorded in a background pressure of 10-6 mbar at RT Koster  _et al._ (1998).
In this pressure regime, the FM-AFM imaging mode is typically used due to the
high $Q$ for image stability reasons and lower transient time compared to TM-
AFM Garcia and Perez (2002); Albrecht  _et al._ (1991). In this FM-AFM image,
the vertical resolution is similar to the z-noise level ($<$0.1 nm) and unit
cell (0.4 nm) deep vacancy islands are resolved having lateral sizes of
$\approx$20 nm.

In Fig. 7(b), a TM-AFM image is depicted of a SrTiO3(001) substrate at 10-1
mbar oxygen background pressure , which is a typical PLD pressure to deposit
perovskite oxide films ensuring good crystal quality, properties,
stoichiometric transfer and 2D growth Groenen  _et al._ (2015). At 10-1 mbar
oxygen background pressure, the free cantilever amplitude was set to $\sim$
120 nm and is found to be the most critical parameter for AFM imaging. The
lateral AFM resolution on terraces is lowered as a SrTiO3(001) step broadening
of $\approx$20 nm is found at this pressure. An average in-plane drift of 2
nm/min is measured over a period of 3.5 hours under these conditions.

The same AFM settings have been used for a sample measured at T=600∘C. At
pressures of 10-1 mbar oxygen and sample temperatures of T=600∘C vacancy
islands can easily be resolved. The AFM noise increase due to temperature is
negligible based on the fact that z-noise level and SrTiO3(001) step
broadening remained at T=600∘C Broekmaat (2008). Under these conditions, a
drift of 15 nm/min is measured as depicted in Fig. 7. After hours of stable
AFM imaging under oxide PLD conditions, the z-piezo stayed within range
($\pm$700 nm).

Figure 7: (Color online) In situ AFM 1.2$\times$1.2 $\mu$m2 (512*512 pixels2)
images of SrTiO3(001) substrates under different conditions (and different
miscut angles) recorded with an acquisition time of 256 s/frame. a) FM-AFM
image, $\Delta$f = -28 Hz, $A_{osc}$ = 10 nm, at RT and P = 10-6 mbar, b) TM-
AFM image, Aosc = 44 nm, at RT and PO2 = 10-1 mbar, c) TM-AFM image, Aosc = 44
nm, at T = 600∘C and PO2 = 10-1 mbar O2, d) TM-AFM image, Aosc = 44 nm, at T =
600∘C and PO2 = 10-1 mbar O2 after 30 min AFM imaging. Circle markers surround
unit cell vacancy islands with a depth of 0.4 nm and square markers surround
etch pits of several unit cells deep.

###  V.4 AFM imaging during and after PLD

BiFeO3 has been deposited as a prototypical ABO3 film using the described PLD
system. This material has multiferroic properties and its growth and property
relation is intensively studied Lim  _et al._ (2008); Béa  _et al._ (2011);
Kanashima  _et al._ (2014); Deepak  _et al._ (2015); Solmaz  _et al._ (2016).
It is reported that BiFeO3 tends to grow in several types of domains by
modifying the SrTiO3(001) termination Solmaz  _et al._ (2016). Dependent on
the substrate termination, BiFeO3 grows either 1D/2D or 3D. Here, BiFeO3 films
have been grown on TiO2 terminated SrTiO3(001) substrates anticipating 3D
growth. BiFeO3 was deposited with a laser fluence of 2.0 J/cm2 , 0.3 mbar
oxygen background pressure, a sample temperature of 600-670∘C during
deposition and a repetition rate $f_{rep}$=0.5 Hz, all settings similar to
previous work where only a target-substrate distance of 45 mm is used instead
of 55 mm Solmaz  _et al._ (2016). Fig. 8 shows AFM images obtained after and
during BiFeO3 deposition. In Fig. 8(a), an ex-situ AFM image is depicted of a
BiFeO3 film after deposition of 1000 pulses at 670∘C with a film thickness of
$\approx$3-4 nm. This BiFeO3 film was deposited as a reference to reported
literature Solmaz  _et al._ (2016). It is reported that BiFeO3 grows 3D on
TiO2 terminated SrTiO3(001), which is similar to the results obtained here.
Terraces with on top 3D islands are visible after 1000 PLD pulses of BiFeO3
deposited on TiO2 terminated SrTiO3(001). The 3D BiFeO3 islands on top of the
terraces suggest that next layer nucleation starts before a previous layer is
completely covered. Island step heights are found of 0.2 nm, 0.4 nm and its
multiples up to a maximum peak-to-peak height of 3 nm. Islands of 25-30 nm in
lateral size are obtained on top of random shaped islands. From these results,
it is expected that BiFeO3 growth will continue 3D, similar to what has been
reported Solmaz  _et al._ (2016).

In Fig. 8(b), an in-situ AFM image is presented, which was recorded at
T=600∘C, as this is the maximum stable operating temperature for AFM
measurements. The process pressure used was 0.3 mbar oxygen background
pressure depositing again 1000 pulses of BiFeO3. The AFM image was taken after
BiFeO3 deposition, sample transfer from PLD to AFM and an AFM stabilization
time of 2h at the AFM position. After thermal stabilization, AFM was started
with similar settings used for imaging of a SrTiO3(001) substrate. It became
clear that due to an increase in surface roughness after deposition, the
integral gain of AFM electronic feedback had to be increased significantly to
visualize the smallest BiFeO3 islands. Stable AFM imaging was continued for
several hours on BiFeO3 without thermally drifting out of the z-range. Thermal
drift causes small distortions at AFM image edges. Smallest lateral BiFeO3
island sizes of 20-30 nm have been measured, similar to the results obtained
with an ex-situ AFM on BiFeO3 after deposition at T=670∘C Solmaz  _et al._
(2016). Some of these small islands are surrounded by a white circle marker in
Fig. 8. In Fig. 8(b), the square white marker represents a zoom-in of a BiFeO3
taken from an 0.8$\times$0.8 $\mu$m2 AFM image.

Afterwards, the BiFeO3 film grown on SrTiO3(001) at T=600∘C had a cooldown of
$\approx$15∘C/min in its deposition pressure of 0.3 mbar oxygen background
pressure. Once the sample reached RT, it was exposed to a maximum oxygen flow
up to atmospheric pressure. In Fig. 8(c), an ex-situ AFM image is depicted of
BiFeO3 after oxygen exposure up to atmospheric pressure. The AFM image in Fig.
8(c) is slightly different compared to Fig. 8(b). One difference is that
BiFeO3 islands of $\approx$20-30 nm are hardly visible in the ex-situ AFM
image, see Fig. 8(c). Both AFM images have in common that BiFeO3 islands
within polygon markers are similar in size. Note that, $Q$ decreases more than
an order of magnitude from 10-1 mbar background pressure up to atmospheric
pressure.

This study reveals that BiFeO3 can be imaged using a Si AFM tip at T=600∘C and
0.3 mbar process pressure after deposition on a TiO2 terminated SrTiO3(001)
substrate. Neck formation has not been observed using this tip-sample
combination under these conditions. One of the ideas is to deposit BiFeO3 on
mixed terminated SrTiO3(001) in order to study growth (front evolution)
differences on both SrO and TiO2 termination. However, AFM imaging (Si tip) on
mixed terminated SrTiO3 substrates results in neck formation at T=600∘C and
0.1 mbar process pressure, while stable AFM imaging was achieved on a TiO2
deposited film on TiO2 terminated SrTiO3(001) under the mentioned conditions.
From the described measurements and the neck formation of a Si AFM tip with a
SrRuO3 film (SrO termination) at T=600∘C and 0.1 mbar oxygen background
pressure, it seems plausible to argue that a Si AFM tip forms a neck if the
surface contains a SrO top layer Rijnders  _et al._ (2004); Broekmaat (2008).
For this tip-sample combination, another tip material/tip coating needs to be
selected.

Figure 8: (Color online) Fig. 3.13 TM-AFM 1.2$\times$1.2 $\mu$m2
(512$\times$512 pixels2) images, acquisition time=256 s/frame, after 1000
pulses of BiFeO3 deposited on SrTiO3(001) (a) ex-situ at RT and in air after
deposition at T=670∘C, (b) in-situ at T=600∘C and PO2=0.3 mbar O2 after 1000
pulses of BiFeO3 deposited on SrTiO3(001), (c) ex-situ at RT and in air after
deposition at T=600∘C. Square markers point to 3D BiFeO3 islands, circle
markers point to small BiFeO3 islands and polygon markers point to larger
islands.

##  VI Conclusions and outlook

To visualize the topography of thin oxide films during growth, we have
designed and integrated an atomic force microscope (AFM) in a pulsed laser
deposition (PLD) vacuum setup. The in-situ microscope is demonstrated to
operate at typical PLD conditions, thereby resolving unit cell height surface
steps and surface topography.

We end this paper by discussing some aspects that might improve the
performance of this microscope. The setup described here is a major step
towards real-time AFM during PLD conditions. To enable quasi real-time AFM
monitoring of island growth during PLD, the bandwidth of both the cantilever
and (optical) detection system need to be improved, as the acquisition rate is
currently limited by them. As high resonance cantilevers dictate smaller
physical dimensions on the cantilever itself, the optical detection system
might also need reconsideration. We are in the process of the development of
self-sensing piezo-electric cantilevers in order to increase the bandwidth by
(an) order(s) of magnitude.

Besides this, the thermal drift of the cantilever resulting from varying
temperature gradients during sample transfer also needs reconsideration as
slight temperature variations of the cantilever might result in a considerable
eigenfrequency shift of it. Moreover, increasing both the AFM feedback
bandwidth and transfer stage speed will result in a faster tip-sample approach
time using the discussed side approach. We therefore currently develop a
modification of the geometry such that the temperature of both sample and
cantilever is better controlled together with a faster approach.

##  VII Acknowlegdement

This work is part of the research programme of NanoNext NL project 9A
nanoinspectation and characterization, project 07 real-time atomic force
microscopy growth monitoring during pulsed laser deposition of oxides.

##  VIII References

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