High performance temperature controlled UHV sample holder

High performance temperature controlled UHV sample holder
Hugo P. Marques,a兲 David C. Alves, Ana R. Canário,
Augusto M. C. Moutinho, and Orlando M. N. D. Teodoro
CEFITEC, Physics Department, New University of Lisbon, 2829-516 Caparica, Portugal
共Received 10 October 2006; accepted 29 January 2007; published online 9 March 2007兲
A requirement of many surface science studies is the capability to alter a sample temperature in a
controlled mode. Sample preparation procedures such as heating or cooling ramps, high temperature
spikes, fast annealing, or simply maintaining a sample at a very high, or very low, temperature are
common. To address these issues, we describe the design and the construction of a multipurpose
sample holder. Key points of this design are operation in an extended temperature range from liquid
nitrogen 共LN2兲 temperature to ⬃1300 K, temperature control during heating and cooling, low
thermal inertia with rates up to 50 K s−1 共heating兲 and −20 K s−1 共cooling兲, and small heated volume
to minimize background problems in thermal desorption spectroscopy 共TDS兲 spectra. With this
design the sample can be flash heated from LN2 temperature to 1300 K and cooled down again in
less than 100 s. This sample holder was mounted and tested in a multitechnique apparatus and adds
a large number of sample preparation procedures as well as TDS to the list of already available
surface analysis techniques. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2712892兴
Control of the sample temperature is essential in many
surface science techniques. For example, thermal induced
desorption is used to study the binding energies of adsorbates
in surfaces and induced decomposition products 共samples for
heterogeneous catalysis experiments also require controlled
surface temperature兲1,2 and induced structural changes in thin
films by rapid thermal annealing.3–5 Furthermore, since many
cleaning procedures are based in inert gas sputtering, annealing at high temperatures is required to release the resulting
stress in the crystalline structure and to promote surface reconstruction.
These processes require that the sample be heated and
cooled in a controlled mode, and often, in very short times.
Rapid heating or cooling will depend on the thermal inertia
of the holder; to achieve high performances the heated volume should be as small as possible.
Small heating volumes are of great importance for thermal desorption spectroscopy 共TDS兲, since when the holder is
at its lowest temperature 共⬃100 K兲 all the surfaces may adsorb residual or other introduced gases. If the heated volume
is larger than the sample, then adsorbed gases will be released from the surrounding surfaces, thus masking the TDS
signal or even leading to sample contamination.
In this work we describe the design details and performance of a multipurpose sample holder, which addresses the
following requirements: operation in an extended temperature range, temperature control while heating and cooling,
short heating and cooling times, and a small heated volume.
This holder is currently being used in a multitechnique surface analysis system, also developed in this research center.6
The prototype is an evolution of a previously reported
holder.7 Its design goals were to minimize the duration of the
heating and cooling processes and to keep the desorption
area as close to the sample size as possible. To reduce desorption from unwanted surfaces when the sample is being
heated, the rest of sample holder is kept at the cold source
temperature. This improved design introduces a smaller
sample mounting part, a new thermal connection, and an
evolved heating assembly.
Liquid nitrogen cooling was selected for the cold source
since it is inexpensive, easily available in the laboratory, and
most gases will adsorb at such temperatures. The upper temperature limit was set to ⬃1300 K as this is the temperature
required to anneal silicon—one of the highest annealing temperatures. Heating by electron bombardment was selected
over resistive heating so that the heating power may be concentrated in the smallest volume.
Figure 1 shows a cross section of the sample holder. The
sample has the shape of a short cylinder of 10 mm in diameter and 5 mm in height. Adapters may be used if other
sample shapes or sizes need to be used. The sample is
mounted in a molybdenum ringlike part. This part is grooved
in order to be mounted using sapphire balls in the space
between the ring and copper support part. The sapphires are
mounted as in ball bearings. Sapphire provides good electrical insulation and good thermal conductivity at low temperature but poor thermal conduction at high temperatures.
The sample is heated from the back side by electron
bombardment. Therefore, the sample is placed on the top of
a molybdenum disk to avoid sample damage and allow indirect heating. Temperature is measured using a type K thermocouple, in contact with the sample, mounted through a
hole drilled in the side of the copper part to the groove. The
space of a missing sapphire ball is used to connect the ther-
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78, 035103-1
© 2007 American Institute of Physics
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Rev. Sci. Instrum. 78, 035103 共2007兲
Marques et al.
FIG. 1. 共Color online兲 Cross section of the sample assembly. The sample is
held by compressing the sapphire spheres against the molybdenum ring.
mocouple to the molybdenum ring. An extra wire is used to
measure the sample current or to bias the sample in case of
As the molybdenum ring, the external copper part has a
side cut 共see Fig. 3兲. When an external bolt is fastened the
copper part is pushed against the sapphire balls, tightening
the molybdenum ring all around the sample. For improved
thermal contact a thin gold foil 共⬇50 ␮m兲 is pressed between the sample and the molybdenum parts.
The copper part is brazed to a long and thin stainless
tube—the liquid nitrogen 共LN2兲 reservoir. The heater assembly is mounted below the copper part thermally and electrically insulated by a Macor™ ceramics piece. Electrons are
produced through Ohmic heating of a thoriated tungsten filament. Being a design option to maintain all the parts exposed
to the analysis side at a well defined ground potential, the
accelerating voltage is provided by biasing the filament.
Thus, during the heating operation, the filament is biased to
−1000 V and the sample kept at ground potential. A special
electrode was designed with the help of a SIMION simulation
to confine the electrical potential in the filament side. The
back of the molybdenum holder was shaped to hide the sapphire balls from the electron beam and to focus the electrons.
This heater can provide up to 500 W of heating power. The
emission current is controlled by the filament current. This
current or the accelerating voltage may be used in a
proportional-integral-derivative 共PID兲 loop to control the
Care was taken to prevent electrons coming from the
heater assembly to reach the upper surface, thus producing
interference in the analytical techniques. The cut in the molybdenum ring has a V shape so that there is no straight line
for electrons, or any other particles, to cross the holder from
the heater side to the analysis side 共see Fig. 2兲.
A copper washer is fastened over the copper plate to
protect the sapphires from charging up and to avoid direct
exposure during depositions. Additionally, since the washer
remains cold even during sample heating, it will act as a trap
for gases released from the sapphire balls, preventing these
from reaching the sample surface. The sample surface is po-
FIG. 2. Detailed view of the molybdenum ring. The V shaped cut, as shown
in the side view, allows closing and opening for sample exchange. Additional partial cuts were made to assist continuous bending.
sitioned slightly above the washer plane 共⬃0.5 mm兲, allowing irradiation at any incident angle, even at grazing angles.
All materials were chosen to meet the requirements for
UHV operation and to withstand the temperature range
共100– 1300 K兲. The holder is mounted in a manipulator,
which allows linear movement in x, y, and z directions and
rotational movement around the z axis. Analysis can be performed even at grazing angles. The complete holder is shown
in Figs. 3共a兲 and 3共b兲.
All tests were performed using a graphite sample. Initially, it takes about 10 min to cool the sample down from
room temperature to 100 K. The minimum recorded temperature was 89 K after 2 h.
The sample was submitted to several heating/cooling
cycles using 100 W of heating power for gradually longer
periods. Results are shown in Fig. 4. Heating rates in excess
of 100 K s−1 were obtained at the beginning of the cycle,
although lowering at higher temperatures. These results show
FIG. 3. 共Color online兲 共a兲 Exploded view of the complete setup 共without
electrical connections兲. 共b兲 Fully assembled sample holder including the
LN2 reservoir and heating assembly.
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Rev. Sci. Instrum. 78, 035103 共2007兲
UHV sample holder
FIG. 4. 共Color online兲 Heating and cooling cycles of a graphite sample with
peak temperatures ranging from 375 to 1273 K. The heating power was
kept constant at 100 W.
that a constant heating ramp of 50 K s−1 is easily achieved as
well as a cooling ramp of about 20 K s−1, considering the full
temperature range.
Temperature was measured on the copper piece with a
second thermocouple. It remained low 共⬍150 K兲, confirming that the release of gas from surfaces other than the
sample should be negligible.
These performance details seem to meet or even exceed
most of the requirements for thermal treatments and TDS
studies. Furthermore, when comparing with some key aspects of other sample holders8–10 the achieved temperature
range and cooling speed are excellent. Other holder designs
do not address all of the requirements imposed here, but key
aspects such as temperature range and cooling 共or heating兲
speed can be individually comparable.
With this prototype the cool down times were greatly
improved when compared with the previous version.6 The
cool down time from 1300 to 100 K was reduced from
10 min to less than 100 s. A complete cycle of flash heating
up to 1000 K now requires less than 60 s.
This sample holder meets challenging requirements
in surface science: extended temperature range from
100 to 1300 K, very fast heating rates of ⬃50 K s−1, fast
cooling times, small heated volume for reduced thermal inertia, and a well defined region for gas desorption. Its performance is achieved by a novel solution to fit the sample
through the sapphire spheres in a ball-bearing-like arrangement.
The main drawback of this holder is the difficulty to
exchange the sample. Therefore, this holder is best suited
when a sample can remain in place for extended periods of
time. This is the case of many fundamental studies on adsorption and thin film growth. An improved version of this
holder is being planned to allow exchanging of the subassembly formed by the copper plate, the molybdenum ring,
and the sample. Then the sample could be exchanged ex situ
and remounted in the holder using a fast entry air lock.
This holder is being used in our current experiments
concerning the properties of noble metal growth on
TiO2共110兲 surfaces.11 These studies are being performed at
controlled temperatures using techniques such as Auger electron spectroscopy 共AES兲, x-ray photoelectron spectroscopy
共XPS兲, low energy ion spectroscopy 共LEIS兲, and work function 共WF兲. Due to the catalytic relevance of these nanostructured surfaces, gas adsorption studies as a function of the
surface temperature are planned in the near future.
The financial support of the Portuguese Foundation for
Science and Technology, POCTI, and FEDER is gratefully
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