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MICROSCOPY RESEARCH AND TECHNIQUE 38:519–524 (1997)
Evaporated Germanium Films as Supports for Microanalysis
of Carbon and Silicon Containing Specimens
B.V. JOHANSEN,1,2* AND HEIDI ORMSTAD1
1Department
2EMLB,
of Environmental Medicine, National Institute of Public Health, Torshov, 0403 Oslo, Norway
University of Oslo, Blindern, 0316 Oslo, Norway
KEY WORDS
specimen support; transmission electron microscopy; germanium films; microanalysis; suspended particular matter (SPM)
ABSTRACT
A preparation procedure is described for producing specimen supports of evaporated germanium. The germanium film is used as a replacement for films of carbon and silicon when
microanalytical techniques like energy dispersive X-ray microanalysis (XRMA) or electron energy
loss spectroscopy (EELS) are focusing on particulates containing these elements. The supports can
be produced with high reproducibility within a thickness range of 15 to 30 nm and of a quality
suitable also for high resolution transmission electron microscopy. Microsc. Res. Tech. 38:519–524,
1997. r 1997 Wiley-Liss, Inc.
INTRODUCTION
In transmission electron microscopy (TEM) evaporated carbon films have been used for more than four
decades as reinforcement for plastic supports (Bradley,
1954), and for the last half of this period plain carbon
films have been the support of choice in high resolution
TEM of bio-molecules (e.g., Johansen, 1974). Carbon
evaporation has also been used to produce replicas from
a variety of specimens ranging from fractured steel
surfaces (Johansen, 1968) to freeze fractured biological
materials (Bullivant, 1969). Traditionally, carbon evaporation is carried out in high vacuum but recently carbon
coating procedures have been described also for low vacuum
equipment, like the sputter coater (e.g., Namork, 1985).
In environmental analytical transmission electron
microscopy, carbon extraction replicas are usually the
method of choice when air particulates are sampled
onto polycarbonate filters (e.g., Chatfield 1986). The
importance of thin film replicas is imperative when
analytical techniques like electron diffraction (ED),
X-ray microanalysis (XRMA), and electron energy loss
spectroscopy (EELS) are involved.
For the majority of applications in analytical electron
microscopy, the use of carbon as top-coating, replicating
material, or specimen support is usually uncomplicated. However, coating with carbon is not very successful in situations where the presence of carbonaceous
material is of interest.
For example, with immuno-electron microscopy we
have shown recently that allergens from cat (Fel d 1)
bind mostly onto soot particles in air samples, collected
from the indoor environment (Ormstad et al., 1995).
One way to investigate this further is to look at the
ratios between the amounts of soot (carbon) and other
small airborne house dust particulates (pure and complex silicates; organic, inorganic fibres, etc.). The inhaleable suspended particulate matter (SPM) of interest is
less than 10 and 2.5 µm in diameter (PM10 and PM2.5,
respectively). For obvious reasons we had to find a
substrate and a replicating (film) material other than
carbon or silicon for our further experiments.
r 1997 WILEY-LISS, INC.
Germanium is a group-IVA element in the periodic
table, which is known to produce amorphous films,
when deposited under certain conditions (Slope and
Tiller, 1963). The amorphism is of importance when
investigating crystalline particulates by selected area
electron diffraction (SAED), or when setting proper
imaging conditions based on defocus dependent phase
contrast granulation (e.g., Johansen, 1973).
This paper deals with the methodology of preparing
germanium films and it shows examples of how such
films can be applied in the study of airborne particulates collected in the environment.
MATERIALS AND METHODS
Evaporation Procedures
Germanium (grade 99.999%; granularity 0.7–3.5 mm
f from Balzer Union) was evaporated from a twothread, twisted tungsten basket (Balzers Union, Balzers, FL), (A in Fig. 1). Before loading the germanium
granules, the basket was heated for about 3 minutes to
glowing-red in high vacuum (5 3 1023 Pa), to remove
the oxide layer from the tungsten filament surface.
The evaporation was carried out in a JEOL (Skandinaviska AB) high vacuum coating unit (JEE 4X/5B),
using a liquid nitrogen LN2 trap in the baffle region to
prevent back-streaming of oil vapour to the chamber.
The specimens to be coated were placed above the
tungsten basket, facedown. The distance between the
basket and the specimen holder was 100 mm. To
evaporate accurate amounts of germanium and to
protect the specimens from photons during the filament
heat-up, a retractable shield was placed above the
tungsten filament (B in Fig. 1).
A multi-purpose holder, loadable with six specimens
(C in Fig. 1), was made to accommodate either 25 mm f
polycarbonate filters or 17 3 17 mm mica chips. For
*Correspondence to: Professor B.V. Johansen, Dept. of Environmental Medicine, National Institute of Public Health, P.O. Box 4404, Torshov, 0403 Oslo,
Norway. E-mail: bjorn.johansen@bio.uio.no
Received 27 August 1996; accepted in revised form 30 April 1997
520
B.V. JOHANSEN AND HEIDI ORMSTAD
Fig. 1. The setup for evaporating germanium onto either 25 mm f
polycarbonate filters or 17 3 17 mm cleaved mica sheets. The
evaporation source A is a two-thread, twisted tungsten basket located
at a distance of 100 mm below the specimen holder, C. The specimens
are protected by a retractable shield, B, during the heating-up
procedure. The detector crystal of the film thickness monitor is located
in position D of the specimen holder. Fig. 2. A diagram showing the
relationship between evaporated germanium mass and deposited film
thickness with a source-to-specimen distance of 100 mm.
accurate measurement of deposited film thickness, the
detector of a Polaron film thickness monitor (Model
E-5550) was always kept in one of the specimen positions (D in Fig. 1).
The coating unit power supply delivers 25 V AC. After
melting the germanium granules in the basket filament,
the current regulator was set to 30 A. With a steady
deposition rate (1–2 nm sec21) the radiation shield is
retracted from the evaporation path for as long as it takes
to produce the required thickness on the thickness monitor.
Without a monitor the film thickness can be approximated by relating the thickness to the mass of evaporated germanium by weighing the loaded tungsten
filament before and after deposition. The thickness-tomass relationship for the evaporating conditions and
setup used here is shown in Figure 2.
A Mettler MT5 (Greifensee, Switzerland) balance
was used, with a readability and reproducibility of 1
and 0.8 µg, respectively.
the surface of distilled water in a small beaker, as is
usually done for plain carbon films (e.g., Johansen,
1974). With a water suction pump, the floating film is
lowered gently onto uncoated copper grids. The grids
had previously been immersed and placed on a filter
paper on top of a stainless steel mesh. After the water is
drained well below the grid level, the steel mesh, with
the grids in position, is lifted out of the beaker and
placed in a petri dish to dry overnight.
Specimen Support Films From Coated Mica
Plain specimen supports are made by coating germanium onto freshly cleaved mica. The germanium film is
subsequently released from the mica by floating it onto
Coating of Polycarbonate Filters
One of our applications involved quantitative and
qualitative analysis of airborne suspended particulate
matter (SPM). SPM was collected on 25 mm f polycarbonate filters pre-coated with 10 to 20 nm layer of
germanium. The collected SPM mass per unit volume of
air (µg m23 ) was determined by weighing the coated
polycarbonate filter before and after air sampling. In
instances where the SPM mass exceeded around 30 µg
m23 an additional layer of approximately 20 nm germanium was evaporated onto the polycarbonate filter.
Discs, 3 mm in diameter, were punched out from
SPM-exposed filters and transferred to 3 mm, 200mesh copper grids. The grids were placed in a Jaffe-
EVAPORATED GERMANIUM FILMS AS SPECIMEN SUPPORTS
521
washer containing chloroform to remove the polycarbonate part of the specimen (Jaffe, 1948). The specimens
were kept in contact with chloroform for 24 hours.
Further details about how to handle polycarbonate
filter specimens for transmission electron microscopy
can be found in an International Organisation for
Standardisation catalogue (ISO 10312-1995). The treatment resulted in a germanium replica of the polycarbonate filter.
Electron Microscopy and Microanalysis
A JEOL 100CX transmission electron microscope was
used and operated at 80 KeV. The high resolution work
was carried out with a 200 and a 60 µm f aperture in
the condenser II and objective lenses, respectively.
Micrographs of focal series of plain germanium films
were Fourier transformed to a power spectrum to
determine objective lens focus and image quality using
either an optical diffractometer (Johansen, 1977) or the
crystallographic image processing programme CRISP
(Calidris; Sollentuna Sweden).
Identification of the element carbon (fractal aggregates of soot) was performed with a Gatan (Warrandale,
PA) electron energy loss spectrometer (EELS) coupled
to a JEOL 2000 FX transmission electron microscope
using a lanthanum-hexaboride (LaB6) electron emitter.
The EELS was run with a resolution of 1 eV per
channel.
RESULTS
Evaporated germanium films thicker than 30–40 nm
often obscured high resolution details in objects supported by the film. On the other hand, supports with a
thickness less than 10 nm were often experienced as
brittle and sometimes too fragile for practical work.
With the setup described in this paper, the thickness
range of 15–25 nm gave acceptable and reproducible
support films. According to the diagram in Figure 2 this
corresponds to the evaporation of approximately 20–35
mg of germanium.
The morphology of a 15-nm-thick germanium film
floated off mica is shown in Figure 3. The selected area
electron diffraction (SAED) diagram from this film,
presented as an insert, shows wide and diffuse diffraction rings, similar to what is seen in films of amorphous
carbon and vitreous ice.
The amorphousness of a germanium film, supporting
carbon black particles, is revealed by the defocus dependent granulation, at a much higher magnification, in a
five picture focal series (Fig. 4a–e), taken from over to
under focus. The corresponding optical Fourier transforms, (Fig. 4f–j) show the Thon-rings, which change
through focus, in the same way as a conventional
carbon support film. In the focal mini-series, Figure 4c
and h were recorded close to optimum (Scherzer) focus.
The fact that the phase contrast granularity of the
germanium film responds to objective lens, defocusing
in the same way as a conventional carbon specimen
support, allows precise focusing and correction of astigmatism at high magnifications.
A 10 1 15-nm-thick ‘‘sandwiched’’ germanium extraction replica of a polycarbonate filter, containing suspended particulate matter (PM10) from the indoor air
of a children’s day-care center, is shown in Figure 5a.
An area of the replica has been selected where the
Fig. 3. An electron micrograph of a 15-nm-thick germanium film.
Its corresponding selected area electron diffraction (SAED, inset)
pattern reveals broad and continuous diffraction rings similar to what
can be expected when electrons are scattered in amorphous materials
(e.g., evaporated carbon and vitreous ice). Scale bar 5 0.5 µm.
majority of the contaminating structures consist of soot
aggregates (arrows). The electron energy loss spectrum
(EELS) in Figure 5b, is recorded from such a soot
aggregate (black circle in Fig. 5a) and confirms the
presence of carbon. The spectrum in Figure 5c is
recorded from an area (white circle in Fig. 5a) with no
presence of carbon.
DISCUSSION
After carbon and silicon, germanium is the third
element in group IVA of the periodic table with a
physical density (5.32 g cm23 ) more than twice that of
carbon (2.26 g cm23 ), which is the most commonly used
material for specimen supports. Based on experiences
with ion-beam sputtered films of chromium (7.19 g
cm23 ) and niobium (8.4 g cm23 ) (Johansen, unpublished results), it was confirmed that the scattering
contrast from the germanium film itself did not obliterate structural details in particulates supported by the
germanium film, provided the film was not thicker than
30–40 nm. Going to higher accelerating voltages (.120
KeV), however, has shown that this thickness limit can
be exceeded accordingly.
522
B.V. JOHANSEN AND HEIDI ORMSTAD
Fig. 4. The focal series of carbon black particles supported on a
15-nm-thick germanium film imaged from over to under focus (a–e),
reveals typical Fresnel diffraction around the particles. Fourier analysis of the defocus dependent granulation of the germanium film, using
the PC assisted CRISP program, shows the characteristic Thon-rings
through focus from f to j. The micrograph closest to Scherzer-focus (d)
reveals a ‘‘nanometer crystallinity,’’ which is frequently seen in
germanium but never in evaporated carbon films. The gaps between
the ‘‘nanometric-islands’’ are detected in the Fourier transforms and
can easiest be seen in h and i, indicated by arrows. Scale bar 5 50 nm.
In attempts to establish if carbon is present in
specimens like those described in this study, it is crucial
that the germanium extraction replica itself does not
contain carbon, for example, left by the chloroform
digested polycarbonate filter. Electron energy loss analysis on sampled (Fig. 5c) and unsampled replicas, does
not reveal detectable amounts of carbon.
It is also our belief that germanium replicas of
polycarbonate filters appear much cleaner, with no or
very little filter residues after 12–24 hours chloroform
digestion, as compared to what we experience with
carbon-coated filters processed in a similar way. This
effect may be due to the fact that the energy transfer
between evaporation source and filter is significantly
lower when depositing germanium (approximately 70°C)
than coating filters with carbon (100–150°C).
The amorphous structure of any deposited film is
crucial if it is to be used as an extraction replica or a
specimen support in transmission electron microscopy.
Examining the texture of evaporated germanium films
by selected area electron diffraction revealed a diffuse
ring pattern close to what visually can be considered as
amorphous.
The deposition speed is known to affect the texture of
any evaporated film (Slope and Tiller, 1963). With
constant evaporating conditions (pressure, voltage, current) it was experienced that with reduced germanium
mass in the basket filament, the time required to reach
a preset thickness decreased, i.e., deposition speed
increases. The maximum evaporation speed was mea-
sured to 0.5 nm sec21, which is orders of magnitude below
the rate required to produce crystalline or even polycrystalline germanium films (Slope and Tiller, 1963).
The literature revealed some controversy about which
temperature germanium films transform from amorphous to poly-chrystalline state (e.g., Slope and Tiller,
1963; Wolsky et al., 1965).
When depositing 30-nm-thick films, the maximum
substrate temperature measured never exceeded 70°C,
which should be well below the transition temperature
of 300–350°C cited as critical (Slope and Tiller, 1965).
However, when inspecting the over- and underfocused micrographs in Figure 4 (a and e) a coarser
structure may be sensed. This ‘‘nano-crystallinity,’’ which
approaches amorphism, is best detected in the Fourier
transform of Figure 4h and i, where the reflections can
be seen as a narrow circle. The circular reflection
appears on the radius of the Fourier transform close to
where the frequency spectrum of the defocus dependent
granulation starts to expand outwards and should not
create a serious problem at low and intermediate
resolution and magnifications. The effect on high resolution structures has not yet been investigated.
Assuming that the nano-crystallinity of the germanium films is not caused by deposition rate or temperature, the explanation may be sought in the environment
within the bell jar during evaporation.
It is generally known that backstreaming of hydrocarbon oil vapours to the chamber of an untrapped
vacuum evaporator will affect the structure and quality
EVAPORATED GERMANIUM FILMS AS SPECIMEN SUPPORTS
523
Fig. 5. An indoor air specimen of suspended particulate matter
(SPM) collected in a children’s day-care center. An area of a ‘‘sandwiched’’ 10 1 15-nm-thick germanium extraction replica from the
polycarbonate filter shows large soot aggregates (carbon fractal structures) as indicated with arrows in a. A similar specimen processed as a
carbon extraction replica will not allow the soot particles to be
separated from the carbon film support by either X-ray microanalysis
(XRMA) or electron energy loss spectroscopy (EELS). The EELS
profile (b) is processed from the area within the black circle B in a and
reveals the K-shell excitation of carbon. The spectrum (c) is from the
soot-free area in circle C (a) and includes no carbon signal. Scale bar 5
1 µm.
of the evaporated films. In this coating unit the forevacuum and the diffusion pump were loaded with
fluorine-based (‘‘contamination free’’) oils. It was found
that germanium films evaporated with LN2 in the trap
located between the vacuum chamber and the diffusion
pump, seemed more stable and less fragile than those
prepared without LN2 in the trap. One can therefore
suggest that remaining gas molecules, within the chamber of the evaporator, are interstitially packed into the
condensed atomic structure of the germanium film and
hence produce the nano-crystalline morphology. The
conditions will most certainly be improved by evaporating in an ion-getter or turbomolecular pumped vacuum
system. This approach will benefit coating of polycarbonate filters but will most probably make separation of
the germanium film from mica substrates very difficult.
ACKNOWLEDGMENTS
The authors are indebted to Mr. Jan Zahlin for
skillful technical assistance during the evaporation
524
B.V. JOHANSEN AND HEIDI ORMSTAD
experiments. We also gratefully acknowledge Mr. Erik
Soerbroeden and Professor Johan Tafto, Material Science Center, University of Oslo, for performing the
EELS analysis.
REFERENCES
Bradley, D.E. (1954) A high resolution evaporation-carbon replica
technique for the electron microscope. J. Inst. Met., 83:35–42.
Bullivant, S. (1969) Freeze fracturing of biological materials. Micron,
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Chatfield, E.J. (1986) Asbestos measurements in workplaces and
ambient atmospheres. In: Electron Microscopy in Forensic, Occupational and Environmental Health Sciences. S. Basu and J.R. Millette, eds. Plenum Press, New York, pp. 149–186.
ISO (1995) Ambient air: Determination of asbestos fibres–Direct
transfer to transmission electron microscopy method. ISO 10312:
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