MINERVA: A facility to study Microstructure and INterface Evolution in Realtime under VAcuum

Abstract

A sample environment to enable real-time X-ray scattering measurements to be recorded during the growth of materials by thermal evaporation in vacuum is presented. The in situ capabilities include studying microstructure development with time or during exposure to different environmental conditions, such as temperature and gas pressure. The chamber provides internal slits and a beam stop, to reduce the background scattering from the X-rays passing through the entrance and exit windows, together with highly controllable flux rates of the evaporants. Initial experiments demonstrate some of the possibilities by monitoring the growth of bathophenanthroline (BPhen), a common molecule used in organic solar cells and organic light emitting diodes, including the development of the microstructure with time and depth within the film. The results show how BPhen nanocrystal structures coarsen at room temperature under vacuum, highlighting the importance of using real time measurements to understand the as-deposited pristine film structure and its development with time. More generally, this sample environment is versatile and can be used for investigation of structure-property relationships in a wide range of vacuum deposited materials and their applications in, for example, optoelectronic devices and energy storage.

Full text

MINERVA: A facility to study Microstructure and INterface Evolution in Realtime
under VAcuum
Chris Nicklin, Josue Martinez-Hardigree, Adam Warne, Stephen Green, Martin Burt, John Naylor, Adam
Dorman, Dean Wicks, Salahud Din, and Moritz Riede
Citation: Review of Scientific Instruments 88, 103901 (2017); doi: 10.1063/1.4989761
View online: http://dx.doi.org/10.1063/1.4989761
View Table of Contents: http://aip.scitation.org/toc/rsi/88/10
Published by the American Institute of Physics

REVIEW OF SCIENTIFIC INSTRUMENTS 88, 103901 (2017)
MINERVA: A facility to study Microstructure and INterface Evolution
in Realtime under VAcuum
Chris Nicklin,1,a) Josue Martinez-Hardigree,2,b) Adam Warne,1Stephen Green,1
Martin Burt,1John Naylor,3Adam Dorman,3Dean Wicks,3Salahud Din,3and Moritz Riede2
1Diamond Light Source, Ltd., Harwell Science and Innovation Campus, Chilton OX11 0DE, United Kingdom
2Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, United Kingdom
3Kurt J. Lesker Company, Hastings TN35 4NR, United Kingdom
(Received 12 June 2017; accepted 11 September 2017; published online 2 October 2017)
A sample environment to enable real-time X-ray scattering measurements to be recorded during the
growth of materials by thermal evaporation in vacuum is presented. The in situ capabilities include
studying microstructure development with time or during exposure to different environmental con-
ditions, such as temperature and gas pressure. The chamber provides internal slits and a beam stop,
to reduce the background scattering from the X-rays passing through the entrance and exit windows,
together with highly controllable flux rates of the evaporants. Initial experiments demonstrate some of
the possibilities by monitoring the growth of bathophenanthroline (BPhen), a common molecule used
in organic solar cells and organic light emitting diodes, including the development of the microstructure
with time and depth within the film. The results show how BPhen nanocrystal structures coarsen at room
temperature under vacuum, highlighting the importance of using real time measurements to understand
the as-deposited pristine film structure and its development with time. More generally, this sample
environment is versatile and can be used for investigation of structure-property relationships in a wide
range of vacuum deposited materials and their applications in, for example, optoelectronic devices
and energy storage. 2017 Author(s). All article content, except where otherwise noted, is licensed
under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
https://doi.org/10.1063/1.4989761
I. SCIENTIFIC MOTIVATION AND BACKGROUND
A key focus of materials science is the study of
structure-property relationships, in which macroscopic proper-
ties often arise hierarchically from atomic interactions through
mesoscale ordering. Common examples include the depen-
dence of mechanical strength of a polycrystalline metal on its
constituent grain size1or the modification of catalytic activ-
ity with film thickness or surface area.2Similar relationships
frequently play a role in photovoltaic devices based on emerg-
ing energy materials, such as charge-transfer dyes, organic
semiconductors, and organometal halide perovskites. Struc-
turally dependent parameters such as charge carrier mobility,
electronic band structure, or molecular orientation have been
shown to have a direct influence on power conversion effi-
ciency.3,4Several fabrication techniques have been used to
explore the effect of processing on structure-property rela-
tionships, including solution-based slot-die coating and ink-
jet printing,5as well as vacuum deposition,6the latter an
increasingly popular tool as a method for fabricating large
area electronic devices with fine control over nanoscale thick-
ness, deposition rate, and composition. There are two main
drawbacks of investigating post-deposition films: (a) it is chal-
lenging to identify interface effects during the growth and
resolve variations in microstructure at different depths inside
the film, though both can be critical for the performance of
a)chris.nicklin@diamond.ac.uk
b)josue.martinezhardigree@physics.ox.ac.uk; Joint first author.
devices made from such films; (b) that many emerging materi-
als are susceptible to degradation or morphological evolution
from ambient moisture or oxygen7–9or even illumination.10
As a consequence, ex situ investigations in which samples must
be transferred through air, as is the case with many systems
for atomic force microscopy (AFM), scanning and transmis-
sion electron microscopy (SEM, TEM), or laboratory based
X-ray scattering, may create the additional challenge of relat-
ing measurements taken hours or days after fabrication to the
structures captured in encapsulated, device-processed materi-
als. In situ techniques are needed to capture film growth as it
proceeds during real-world device processing.
The high flux of X-rays available at a synchrotron source
means that high quality, low noise structural data can be col-
lected very quickly. This has been recognised in the grow-
ing application of synchrotron radiation to real-time mea-
surements of vacuum-deposited thin films fabricated with a
wide range of deposition techniques including sputtering,11–14
chemical vapour deposition,15 molecular beam epitaxy,16,17
and organic vapor-jet deposition.18 Such scattering measure-
ments are complementary to local structure measurements
with AFM, as the elongated footprint of the X-ray beam at
grazing angles probes the statistical average structure over a
large area. Information such as the degree of crystallinity of
the film, surface roughness, and domain size in polycrystalline
regions and evidence of lattice strain or preferred orientation
can be probed with high spatial and time resolution. Moreover,
these parameters can be probed as a function of growth con-
ditions such as substrate temperature, deposition rate, or the
0034-6748/2017/88(10)/103901/10 88, 103901-1 ©Author(s) 2017

103901-2 Nicklin et al. Rev. Sci. Instrum. 88, 103901 (2017)
pre-deposition of a buffer or templating layer. It is also possi-
ble to monitor the development of the film structure with time,
with exposure to a specific gas or in an atmospheric pressure
of air (due to the penetrating nature of X-rays).
Research into emerging energy materials is progress-
ing across several materials and device architectures, from
organic bulk heterojunctions to planar perovskite absorbers,
and with many solution-based fabrication techniques spanning
spin-casting,19 blade-coating,20–22 and ink-jet printing.5How-
ever, vacuum deposition is increasingly popular as a scalable
method for fabricating large area electronic devices with fine
control over nanoscale thickness, deposition rate, and ambi-
ent composition.6Here we present an environmental chamber
specifically designed and optimised for the grazing incidence
wide angle X-ray scattering (GIWAXS) or grazing incidence
small angle X-ray scattering (GISAXS) study of in situ vac-
uum deposited layers on the synchrotron beamline I07,23 at
Diamond Light Source.
II. DESIGN CONSIDERATIONS AND CONSTRAINTS
GIWAXS and GISAXS are techniques where an X-ray
beam hits a sample at grazing angles, close to the critical angle
for total external reflection (∼0.1◦for most organic materials
at typical X-ray energies of ∼12.5 keV). The scattered X-rays
are collected using a large area two-dimensional detector to
image the full scattering pattern in both the in-plane and out-
of-plane directions (perpendicular and parallel to the sample
normal, respectively). The implementation available at beam-
line I07 uses a Pilatus P2M detector mounted at a distance
of ∼300 mm for GIWAXS (or up to 3 m for GISAXS) from
the sample to give a potential angular collection range of up
to 40◦. The use of a flat detector introduces some geometri-
cal distortions at such small distances that must be corrected
to remap the pixels to the true momentum transfer values (qxy
and qz). Many sample environments exist that have been devel-
oped to study pre-grown samples and optimise the signal to
noise for weakly scattering samples such as for organic photo-
voltaic (OPV) bulk heterojunction (BHJ) structures. They can
generally incorporate a flowing gas to reduce air scattering
or prevent oxidation/degradation of the interface during the
X-ray measurements. We utilise similar design principles for
our vacuum system, by incorporating elements such as inter-
nal pre-sample slits and a post-sample beam stop, to reduce
the background caused by the incoming or exiting X-ray beam
hitting the window material of the sample environment.
An additional constraint in the design is the requirement
for the evaporation sources (to vacuum deposit the materi-
als) to point upwards. Two sources need to be incorporated,
to enable studies of application relevant interfaces as well as
the growth of binary mixtures. In order to provide a versa-
tile and accurate system, the design also includes two Quartz
Crystal Microbalances (QCMs) for independent monitoring of
the evaporating fluxes, allowing co-evaporation of materials.
Other requirements are a sample shutter and a sample transfer
system to limit the changeover time between experiments as
well as a heatable substrate stage. To undertake the measure-
ments, it is necessary to have accurate control over the sample
motion, both the translations and rotations about the sample
position. Beamline I0723 is a beamline for grazing incidence
and surface diffraction studies, which uses a high precision
hexapod for sample alignment and a large diffractometer for
complex multicircle positioning. In the GIWAXS studies, the
P2M detector is fixed to the diffractometer detector circles,
and the sample does not need the full range of motions pro-
vided by the diffractometer. Therefore in order to produce a
stable platform for the GIWAXS studies, a frame that fits onto
the diffractometer base has been implemented with all sample
movement then provided via the hexapod. During GISAXS
measurements, the same sample manipulation is used, and
the P2M detector sits on an independent positioning platform
downstream of the diffractometer.
III. IMPLEMENTATION
A. General arrangement
The general arrangement of the chamber is shown in
Fig. 1, whilst a cut-through view of the chamber is shown in
Fig. 2. The MINERVA (Microstructure and INterface Evolu-
tion in Realtime under VAcuum) system is designed to meet
the design criteria and consists of four different sections:
(1) The lower deposition chamber contains the evaporation
sources and QCMs. MINERVA’s overall arrangement is
dictated by the requirement for the evaporation sources
to be upward pointing, to enable a constant evaporation
rate through material covering the base of the crucible.
They also operate at an optimum throw distance of
230 mm between the source and the sample position,
ensuring an even flux over the sample area. This cham-
ber also houses two QCMs to independently measure
the flux rates of the two materials and a baffle between
the deposition sources to ensure no cross talk in the
FIG. 1. Overview of the design of the MINERVA chamber. It consists of
four modules: the deposition chamber houses the low temperature evapora-
tion (LTE) sources and quartz crystal microbalances (QCMs); the scattering
chamber with beryllium windows and slits; the sample manipulator using an
external hexapod to allow accurate positioning of the sample; the vacuum
component chamber with all pumps, gauges, and valves.

103901-3 Nicklin et al. Rev. Sci. Instrum. 88, 103901 (2017)
FIG. 2. Cut-through of the MINERVAchamber looking from the front (access
ports), showing key internal components and the path of the X-ray beam.
flux calibrations. A separate ring baffle at the top of
the base chamber protects all components above from
being coated with an evaporating material, except for
the substrate.
(2) The central scattering chamber houses the X-ray trans-
parent beryllium entrance and exit windows. It also
contains a set of pre-sample slits to define the X-ray
beam and a post-sample beam stop, so that any unscat-
tered beam does not add to the background levels (see
Sec. III C). This chamber also provides an auto-
mated sample shutter which prevents deposition onto
the sample during initial evaporator configuration and
warmup.
(3) The upper sample manipulator consists of a hexapod
external to the vacuum, the motions of which are fed
into the vacuum chamber via a flexible vacuum bellows
and feedthrough. The stage, incorporating the sample
heater assembly, reaches down into the centre of the
scattering chamber.
(4) The vacuum component chamber contains the pump-
ing and ancillary services (venting, gauges), mounted
on a multiport vessel that is at the rear of the system
when it is mounted on the diffractometer available at
beamline I07.23 As GIWAXS/GISAXS measurements
do not require substantial movement of the diffractome-
ter, this allows short, simple cable and pipework man-
agement to common electrical controller and backing
pump locations.
MINERVA is built in a very modular way, to enable flex-
ibility in the studies for which it can be used. It allows, for
example, replacement of the bottom chamber with another
design that may contain different types of vacuum deposi-
tion sources. In addition, it would be possible to combine
the X-ray scattering measurements with other related tech-
niques by modifying the upper scattering chamber. The current
implementation includes four flanged ports with the two
smaller ports optically centred on the substrate position. These
are currently used for video monitoring of the sample during
alignment and deposition but could be used for introducing var-
ious kinds of probes to the sample. Although the system in its
current configuration does not have optical viewports set at 74◦
for in situ spectroscopic ellipsometry, a technique which has
been successfully combined with GIXD measurements,24 the
available ports could accommodate fibre-optic feedthroughs
for complementary optical techniques such as in situ differ-
ential reflectance.25,26 In addition, connections for enabling
electrical measurements of films in organic field-effect tran-
sistors (OFETs) during or after deposition27,28 can be easily
integrated using the current flange ports.
B. Deposition chamber
The lower deposition chamber contains two low tempera-
ture evaporator (LTE) sources (Kurt J. Lesker Company) with
individual flip shutters and power supplied through a common
electrical feedthrough flange. Their temperature range of up to
600 ◦C is sufficient for depositing the organic materials used
in OPVs. The sources are mounted directly on the base flange
with a baffle placed between them to remove any temperature
cross talk or cross contamination of the sources and to ensure
that the flux rates correspond only to the specific source. This
design allows very accurate studies of two component material
systems where the relative composition of the two components
can affect both the structure and the photovoltaic efficiency of
the sample. Separate QCMs record the flux rates of the evapo-
rants, enabling complex heterojunctions to be produced. This
may include growing a single component wetting layer before
a two-component material is deposited as required, for exam-
ple, in many BHJ structures of OPVs. The flux rates can be
calibrated before the deposition, as the sample is protected by
a pneumatically operated shutter. This acts off one of the ports
in the central scattering chamber, and the pneumatic actuator
rotates to either place a steel plate in the path of the evap-
orating materials (but not blocking the X-ray path to enable
alignment of a clean sample) or completely removing it out
of the way. This shutter can be actuated remotely allowing the
initial stage dynamics (e.g., nucleation, wetting behaviour, or
strain development) to be monitored during the deposition.
C. Scattering chamber: X-ray background reduction
The quality of the GIWAXS/GISAXS measurement is
critically dependent on the signal to noise ratio, as there are
very many weak signals that are important to fully understand
the structure of the developing interface. The measurements
typically use an X-ray incidence angle very close to the criti-
cal angle for total external reflection (typically 0.1◦for most
organic materials at X-ray energies of ∼12.5 keV). A well
defined X-ray beam is therefore required and I07 has beam
parameters of ∼80 µm vertically by ∼120 µm horizontally. The
vertical size will flood the sample at 0.1◦when the sample has a
diameter of ∼10 mm, but even then a strong reflected beam can
lead to background scattering. Much of the background scat-
tering originates from the intense X-ray beam hitting either
the entrance or exit window of the sample environment. In

103901-4 Nicklin et al. Rev. Sci. Instrum. 88, 103901 (2017)
a vacuum chamber, these windows are generally made from
polycrystalline beryllium that leads to some powder diffraction
but also has a grain structure leading to small angle scattering
close to the direct beam. In order to reduce the amount of
this scattering reaching the detector, we borrow ideas from
non-vacuum GIWAXS sample environments where the beam
definition slits are after the entrance window and an internal
beam stop is located to prevent the direct beam or reflected
beam from hitting the exit window. This combination blocks
the majority of the unwanted scattering that would reach the
detector, cleaning up the signal significantly. It is important
that the slits are high precision to define an accurate beam
size and the beam stop can be reliably moved and relocated in
order to enable initial alignment that is usually performed on
an attenuated direct X-ray beam. This implementation makes
use of in-vacuum stages incorporating SmarACT-precision lin-
ear drives (travel range of 10 mm and nm precision) in both
the slits and the beam stops. Figure 3shows the slit arrange-
ment that uses four separate linear drives, each attached to
one blade of the slits. These are software coupled to allow
the gap and position of the slit to be set directly. The slits
are mounted within a custom designed housing that splits to
allow easy maintenance of the individual slit pairs. They join
together using an O-ring mechanism that is compressed by
a series of bolts through the external part of the housing. The
slits mount directly onto the flange of the chamber, and a small
Be window is mounted on the upstream side as indicated in
Fig. 2. This combination serves two purposes: to define a small
beam onto the sample and to block the front window X-ray
scattering.
The beam stop assembly is constructed from two Smar-
ACT stages mounted perpendicular to each other to provide an
XY motion with a travel range of ±10 mm. The beam stop (see
Fig. 4) mounts directly onto these stages and is designed so
that it nominally blocks the beam at the centre of travel in both
directions. The beam stop is made from 3 mm tungsten in order
to ensure blocking of the intense direct beam when required
FIG. 3. Cut-through of the internal slit assembly. The slits are highlighted in
yellow and allow the vertical and horizontal widths of the beam to be adjusted.
FIG. 4. The internal tungsten beam stop assembly is mounted after the sample
with respect to the incoming X-ray beam. It allows cutting out the intense
direct beam or the direct and reflected beam without blocking too much of the
scattered signal.
and is arrow shaped to allow selective blocking of the direct
beam or the direct and reflected beam without shadowing too
much of the GIWAXS scattering.
D. Sample manipulator: Sample motion and heating
There are limited requirements for sample motion in
the GIWAXS/GISAXS measurements; there must be vertical
motion to position the sample accurately in the X-ray beam
and angular motion of a few degrees to flatten the sample in
two orthogonal directions and to set the beam incidence angle.
We implement all of these motions using a hexapod directly
attached to the sample through a vacuum feedthrough and a
flexible edge welded bellows. An internal tube mounts off the
flange and passes down to the sample heater assembly with
the cabling routed axially along the hexapod to prevent cable
snagging. In these experiments, there is no requirement for
azimuthal rotation although this could be added using either
an internal rotation stage or a differentially pumped rotary
seal attached to the bellows. A hexapod provides a number of
advantages for the sample positioning, the most significant of
which is the ability to define the rotational pivot point, allow-
ing optimal alignment even if the beam height entering the
chamber is varied. In addition, the hexapod allows translation
across the sample surface during the measurements which can
be important when determining the role of beam damage on
structural degradation. It should be noted that the hexapod local
Cartesian axes must be correctly aligned parallel (and perpen-
dicular) to the beam direction to ensure accurate calibration of
the incidence angle.
The design of the sample heater is shown in Fig. 5. It uses
resistive heating from a Nichrome wire threaded through a
machinable ceramic assembly, allowing sample temperatures
of up to ∼600 ◦C. The Nichrome wire is used to provide com-
patibility with heating in a non-vacuum environment that may
be required for certain sample processing or to study the effects
of gas exposure on sample structure. The heater is controlled
via a Eurotherm 2504 controller using feedback from the
K-type thermocouple mounted as close to the sample position
as possible. Samples are mounted on Omicron flag type sample

103901-5 Nicklin et al. Rev. Sci. Instrum. 88, 103901 (2017)
FIG. 5. Cut-through of the sample heater assembly. The upper section (green)
shows the substrate coupling that attaches to the alignment hexapod, whilst
the pink section and purple fixtures indicate the spring loaded sample clips
that hold the transferable sample holders rigidly during measurement. The
blue section is the machinable ceramic heater, housing a Nichrome heating
wire to enable heating in oxygen or ambient atmospheres. The transferable
sample plate and sample are shown in red.
plates that can be inserted either individually through the upper
access door or by replacing the upper door (see Fig. 1) with a
customised cluster flange that contains a sample transfer wob-
ble stick, a sample carousel for storage of up to twelve samples,
and viewing windows to enable simple sample transfers. This
reduces the time between experiments significantly, and the
only extended changeovers occur when evaporant materials
need to be exchanged.
E. Vacuum component chamber: Vacuum control
The base vacuum required during deposition, in a produc-
tion relevant process, is <1×10 6mbar so it is not necessary
to bakeout the chamber. This also enables a number of differ-
ent flange sealing techniques to be used. Where compatibility
with other ultrahigh vacuum (UHV) beamline equipment is
required, conflat flanges are used where a knife edge cuts into
a copper gasket, but in other places O-rings are used with com-
pression fittings. The chamber is pumped via a combination
of a backing line scroll pump and a 330 l/s turbomolecular
pump (TMP), whilst pressure measurement is provided via a
combination of a Pirani gauge for low vacuum and an Inverted
Magnetron Gauge (IMG) for the higher vacuum. Initial pump-
down is achieved by closing a pneumatic valve between the
backing pump and the turbomolecular pump and opening a
bypass valve to pump the main chamber. At a pressure of
10 2mbar, the bypass line valve is automatically closed and
the valve to the TMP backing line is opened. The main valve
between the TMP and the chamber is then opened, allowing
the vacuum to rapidly reach the required level. The ultimate
vacuum achieved is ∼5×10 8mbar after 2 h of pumping,
which is more than adequate for deposition of these molecular
species.
Control of the vacuum system is undertaken using a pro-
grammable logic controller (PLC) that allows the sequence
above to be performed automatically at the touch of a button
on a pressure sensitive screen, whilst monitoring the vac-
uum interlocks for safety and equipment protection. The PLC
controls electronically actuated valves to ensure that a consis-
tent and well defined sequence of events is used for different
operations. These include pumpdown as described above and
venting, opening up control of the vacuum system to the
non-expert.
IV. INITIAL TESTS
Initial functional tests included test pumpdown and leak
test to ensure that a useable vacuum could be reached as
quickly as possible. A pressure of ∼1×10 6mbar is achieved
after 20 min reaching ∼1×10 7mbar within an hour. Tests of
the sample heater showed rapid heat up and cooldown possi-
bilities, requiring approximately 20 min to stabilise once the
correct control parameters had been established. Test deposi-
tions were made with the sample held at room temperature,
where the evaporation rates measured using the QCM’s were
calibrated for samples deposited for a known amount of time
and subsequently measured ex situ using laboratory based
X-ray diffraction (see S1 in the supplementary material). The
film thickness for the sample grown at room temperature is in
excellent agreement with the expected thickness, confirming
that the flux rates from the QCMs are accurate. The alignment
procedure was simulated prior to first experiments, and the
range of motion of the samples was checked to establish the
parameters such as the pivot point and angular ranges for depth
profiling measurements. It is clear that the design goal of ±5◦
was easily achieved and more than enough to enable sample
alignment and setting of the incidence X-ray angle.
After installation on the beamline, initial X-ray alignment
tests were carried out. The sample was moved out of the beam
path by retracting the hexapod to its lowest position, and checks
were made that the direct beam passed through the chamber
and hit the detector at an appropriate position. The in-vacuum
pre-sample slits were aligned around the direct beam, and they
were calibrated to ensure that when fully closed there was not
any transmission of the X-rays. The beam size was then set to
be 100 µm×100 µm. The beam stop was aligned by driving it
to its lowest level vertically and then scanning in the horizontal
direction to find the position at which the direct beam was
blocked. The vertical motion was then scanned to find the
point of the beam stop so that it could be aligned appropriately
and block just the direct beam. Tests were then made on the
reproducibility of the positioning and the ability of the beam
stop to block the full X-ray beam when all attenuation had
been removed. The slit and beam stop assembly were shown
to behave very well, limiting the background scatter on the
detector.
V. FIRST RESULTS
The capabilities of the MINERVA chamber extend beyond
probing growth dynamics during in situ deposition, enabling
also the observation of freshly deposited films exposed to
external stimuli. Such investigations provide potentially criti-
cal information otherwise inaccessible in laboratory settings,
where thin films must be transferred to an inert ambient-
pressure glove box or to air. These experimental limitations
preclude the observation of thin film evolution from its pris-
tine, as-deposited state when exposed to air or analyte-carrying
gases. Such studies have become increasingly important

103901-6 Nicklin et al. Rev. Sci. Instrum. 88, 103901 (2017)
in the context of solvent annealing microstructural trans-
formations,29 organic semiconductor (OSC)-based sensors,30
and OPV stability.31 One frequently used molecule is
bathophenanthroline (BPhen), which is typically used as a
buffer or doped electron transport layer between the active
layer organic absorber/emitter and the evaporated metal cath-
ode, composed of materials such as aluminium. Although
ubiquitous in a number of organic light emitting diode (OLED)
and OPV architectures, few if any investigations of the micro-
structure of vacuum-deposited BPhen exist. We have inves-
tigated thin films of BPhen on silica, which is known to
transform from a glass-like organic layer into a more opaque,
translucent scattering film over the course of an hour.
Sublimation-purified BPhen (Luminescence Technology
Corp., Taiwan) was evaporated onto glass substrates (Thin
Film Devices, Inc., USA) that were cleaned following a stan-
dard procedure of sonication successively in 2.5% Hellmanex
(Sigma Aldrich) in deionised water, deionised water, acetone,
and 2-propanol and subsequently dried in a light stream of N2,
followed by surface cleaning under ultraviolet (UV) ozone for
10 min prior to loading in the chamber. The X-ray energy
was tuned to 12.5 keV (wavelength λ= 0.992 Å), and the
substrate was aligned using optical methods to accurately ori-
ent the sample along the direction of the beam. The reflected
spot position was recorded as the sample was translated across
the X-ray beam to level it in the orthogonal direction. To
monitor the surface of the film while limiting the scattering
from the bulk, the substrate was rotated such that the X-ray
incidence angle was Θ=0.071◦, well below both the glass
critical angle Θc,glass =0.152◦and the BPhen critical angle
Θc,Bphen =0.127◦. Although the intensity at this shallow graz-
ing angle is relatively low, we can achieve significant depth
sensitivity, with a minimum effective penetration depth32 given
by z0=1
2√reπ ρ, where reis the Thomson scattering length and
ρis the electron density of the compound. Assuming vacuum
deposited Bphen is crystalline, with an orthorhombic crys-
tal structure33 (a=7.253 Å, b=10.810 Å, c=21.14 Å), the
penetration depth z0=116 Å which is slightly larger than that
reported for crystalline P3HT of 85 Å.32 As a result, changes
in the depth dependence of the crystallographic features with
increasing film thickness can be monitored closely in films
relevant for optoelectronic devices.
The aim of the study was to investigate BPhen using the
parameter space accessible with MINERVA, initially by mon-
itoring the growth of a 40 nm thin film of BPhen and observing
its evolution while under vacuum. Figure 6(a) shows the real-
time deposition rate and thickness measured during growth
with 2D GIWAXS images recorded at key points shown in
Figs. 6(b) and 6(c). The appearance of powder rings in films
of thickness greater than 10 nm indicates an isotropically grow-
ing film with negligible preferential vertical ordering. Using
these images as a guide, radial regions of interest were defined
and applied to all images acquired during in situ deposition,
resulting in the plots shown in Fig. 7. These profiles show that
in-plane ordering occurs in films of thickness as low as 2 nm,
as evidenced by the appearance of two closely spaced peaks
at qxy = 0.61 Å 1and 0.62 Å 1. These features correspond to
spacings of 10.13–10.30 Å, which are relatively close to the
previously reported values for the [010] spacing of 10.4 Åin
solution-growth BPhen crystals.34 Notably, both these peaks
shift to slightly higher qvalues with increasing thickness, sug-
gesting that BPhen may exhibit closer in-plane packing when
grown on itself as compared to on bare glass. As the film grows
to nearly 40 nm thickness, new scattering features are observed
at qxy = 1.11 Å 1and 1.19 Å 1, respectively, indicating the
concomitant development of in-plane scattering features.
After deposition, images were recorded while the X-ray
incidence angle was varied from 0.073◦to 0.153◦(0.01◦incre-
ment) to evaluate any depth-dependence in the crystallinity of
the sample. Radial regions of interest identical to those shown
in Fig. 7were applied to the images, with results plotted in
Fig. 8. The large change in penetration depth between the ini-
tial and final incidence angles provides information about the
internal microstructure as a function of depth from the surface.
FIG. 6. In situ monitoring of BPhen grown on glass.
(a) Recorded quartz crystal monitor (QCM) readout
during BPhen deposition (blue) indicating a rate of
0.25−0.3 Ås−1. Green step function denotes 10 s detec-
tor exposure time and shaded regions with arrows indicate
the upper 11.6 nm probed in a given image based on the
penetration depth into the film at 12.5 keV. Lower panels
show GIWAXS images monitoring the 001 features after
total growth time and film thickness of (b) 700 s, 17.5 nm
and (c) 1470 s, 39 nm.

103901-7 Nicklin et al. Rev. Sci. Instrum. 88, 103901 (2017)
FIG. 7. Monitoring growth of BPhen
grown on glass at rates of 0.25−0.3
Ås−1. (a) Radial regions of interest
(i-v) and (b) profiles from 2D detec-
tor images. Linear colorbar denotes
the total film thickness at which each
image was acquired during deposition
[cf. Fig. 6(a)].
Several of the higher order qpeaks, corresponding to the 020
and 002 features at 1.21 Å 1, are clearly resolved at the highest
penetration depth. A striking contrast with Fig. 7is that the in-
plane low-q features no longer appear as two separate peaks but
are convoluted, with only a small shoulder near 0.62 Å 1. It is
the superposition of the q-shifted peaks of the various 11.6 nm
slices of the film observed in Fig. 7which results in this smear-
ing of a single peak with only a shoulder at 0.62 Å 1. By
comparison, the same range along qzreveals a more complex
depth-dependence of the features; upper layers show a single
FIG. 8. Angle of incidence dependent region of interest profiles for a 40 nm BPhen film on glass. The variation in penetration depth ranges from 11.6 nm at
Θ=0.071◦to the full 40 nm film thickness at Θ=0.15◦, with the full film probed at Θ=0.13◦.

103901-8 Nicklin et al. Rev. Sci. Instrum. 88, 103901 (2017)
broad peak with larger lattice spacing, while the layers in the
lower half of the film exhibit two separate peaks, with lattice
spacings larger and smaller, respectively, than at the surface.
The larger spacing at shallower angles suggests a degree of
surface relaxation in BPhen, a phenomenon observed in other
crystalline organic semiconductors.35 Although the evapora-
tion rate varied only 10% (see Fig. 6), we cannot entirely
rule out that the slightly higher evaporation rate (0.28 Ås 1vs
0.25 Ås 1) during the first few nm of the deposition could lead
to a kinetically constrained lower layer at the glass interface
(with a different microstructure), as could the OSC “wetting
layer” polymorphism observed in systems such as pentacene.36
The temperature- and rate-dependence of such trapping, as
well the influence of surface energy on BPhen arrangement,
will be the subject of future studies with MINERVA.
The observed surface relaxation in the post-deposition
scan of the BPhen film, coupled with the material’s relatively
low sublimation temperature of 140 ◦C, suggests that some
of the rapid crystallisation seen in air may already occur to
a limited degree under vacuum. To probe this behaviour, the
thin film was maintained at 24 ◦C under vacuum in the depo-
sition chamber (P=6×10−7mbar) and monitored at various
time steps. Comparison of the radial profiles (Fig. S2 in the
supplementary material) demonstrates a clear increase in scat-
tering intensity for all of the scattering features, consistent with
increased crystallisation of the film. Notably, comparison of
the qzfeatures near the 001 peak indicates a slight relaxation
within the upper part of the film, with the lattice spacing in
this layer (corresponding to dark lines) shifting by +0.05 Å,
while the layers near the glass substrate remain unchanged.
Additionally, we can more clearly resolve the peak splitting in
the 002 features for the film near the glass interface, as well as
the emergence of a higher order powder ring at 1.69 Å 1.
Comparison of the radial profiles for samples kept under
vacuum reveals that the film undergoes greater out-of-plane
reorganisation than in-plane, with the upper surface showing
an increase of nearly 0.08 Å between initial deposition and
the scan 2 h later. In addition to the changes with time, all
samples show a small but systematic decrease in the lattice
constant as the greater depth of the film is probed, as shown in
Figs. 9(a) and 9(b). This change becomes significantly larger
at an incidence angle of 0.13◦, where the X-ray penetration
depth is equal to the film thickness. At these higher angles, the
out-of-plane layer spacing (derived from the [001] peak posi-
tion) diverges into two separate peaks. The larger dimension
corresponds to a lattice spacing of 10.39 Å, very close to the
10.40 Å value found by Mazumdar et al.,34 but smaller than
10.81 Å identified by Ceolin et al.33 The smaller of the two lat-
tice constants is 9.97 Å, considerably smaller than previously
reported values. Given that this large difference in qclose to
the specular beam arises when we reach the critical angle of
the substrate, one possibility is that this distortion is due to
refraction at the vacuum/film interface.37,38 However, in that
case, ∆qzwould be expected to be of order ≤0.005 Å 1and so
does not account for the large separation of the two qzvalues.
Alternatively, the presence of more than one peak when the
sample is fully illuminated is most likely due to the presence
of several close families of the BPhen crystal structure with
different out-of-plane strain in the layers close to the interface,
a hypothesis supported by the additional peaks near the [002]
position (Figs. S2 and S3 in the supplementary material) which
do not exhibit any peak splitting above the critical angle of the
glass. Together with the absence of any change in qxy with
increasing time as seen in Fig. 9(a), these data suggest that the
structure of nanocrystalline BPhen may accommodate greater
reordering along the out-of-plane [00l] direction than along
other planes, likely arising from the steric hindrance imposed
by its two phenyl rings.
One key aspect of the MINERVA chamber is the abil-
ity to track the evolution of crystalline features with time,
either during growth or under vacuum. Figure 10 shows such
an evolution for the [001] peak, with initial time points corre-
sponding to monitoring the upper surface during in situ growth
and the final three-time points when kept under vacuum, post-
deposition. In the case of the [001] peak, the relaxation of
the upper surface with time can be compared against the
steady-state growth of the film. Of note is the appearance
of the [001] peak during the last few minutes of the film
growth. This increased scatter from the [001] peak might arise
from preferential out-of-plane growth, but it is also likely to
arise from an increase in film roughness which results in a
lower film density and hence a greater effective X-ray penetra-
tion depth probing out-of-plane scatter. Closer examination of
Fig. 7reveals that the film scatters highly in the in-plane
FIG. 9. Comparison of [010] and [001] evolution under vacuum at room temperature. Different symbols represent the time elapsed following the final deposition
as indicated in the legend. (a) Radial RoI along qxy and (b) along qz, showing the presence of a layer with different out-of-plane ordering near the substrate
interface. In-plane alignment does not change significantly while holding the sample under vacuum, while vertical ordering exhibits a greater change near the
free surface. The shaded region indicates the angle at which the penetration depth of the X-rays exceeds the sample thickness.

103901-9 Nicklin et al. Rev. Sci. Instrum. 88, 103901 (2017)
FIG. 10. Time series comparison of the [001] peak evolution. Earlier, closely
spaced time points correspond to in situ measurements of the growing upper
surface. The final three time points relate to the post-growth measurements
illustrated in Fig. 9(b). In all cases, the angle of incidence was Θ=0.07◦.
direction at low thickness, while it scatters more in the out-
of-plane direction at higher thickness, suggesting that accu-
mulation of orientational disorder during film growth is partly
responsible for the observed coarsening of the film surface
along the [001] direction.
VI. SUMMARY
A new facility (MINERVA, a facility to study Microstruc-
ture and INterface Evolution in Realtime under VAcuum) for
use on the synchrotron beamline I07 at Diamond Light Source
has been built for in situ monitoring of GIWAXS/GISAXS
measurements during growth of vacuum deposited materials
or modification of the environment in which the thin films are
studied. The chamber allows co-evaporation of two materials
and is configured to reduce the X-ray background, essen-
tial to monitor the scattering from, e.g., weakly scattering
organic molecules. We have successfully demonstrated the
parameter space accessible by MINERVA to observe the in
situ nucleation and growth of a nanocrystalline BPhen thin
film and observed how the nanocrystals coarsen with time at
room temperature under vacuum. Our measurements reveal
that BPhen, a common electrode interface material, under-
goes minor but continuous surface relaxation relative to the
bulk of the film when not covered by metal even in vacuum.
Although a simple model system, these findings underscore
the need to employ in situ methods to probe the effect of
thin film fabrication conditions on the OSC microstructure,
not only for the fabrication of high performance and stable
devices but also to understand the degree to which measure-
ments carried out even under vacuum (e.g., XPS/UPS39) on
samples at long times after deposition can be related to the
rapidly processed, device-relevant thin films used in large-
scale production facilities. Furthermore, MINERVA opens the
door for a wide variety of investigations of structure-property
relationships involving vacuum deposited materials and their
applications in optoelectronic and energy-related devices.
SUPPLEMENTARY MATERIAL
See supplementary material for calibration data used to
determine the accuracy of the flux rates. Also included are
data showing how the BPhen crystal structure develops as a
function of film thickness and with time in vacuum.
ACKNOWLEDGMENTS
C.N., M.R., and J.M.H. were responsible for the initial
design concept and overall management of the project. M.B.,
S.G., J.N., A.D., D.W., S.D., and A.W. detailed the design
of the components and were responsible for assembly. All
authors contributed to writing the manuscript under the coordi-
nation of C.N. We gratefully acknowledge funding to develop
the chamber through the STFC Challenge Led Applied Sys-
tems Programme (CLASP) under Grant Nos. ST/L006294/1,
ST/L006219/1 and access to Diamond beamtime at I07 under
experiments Nos. SI14220-1 and SI15207-1. M.R. gratefully
acknowledges funding through an EU FP7 Marie Curie Career
Integration Grant (No. PCIG14-GA-2013-630864). We thank
Simon Lay, Hugo Shiers, and Matthew Barnes (Diamond Light
Source) for their work implementing the vacuum control and
logic.
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