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Connection of a Scanning Tunneling Microscope with a Molecular Beam Epitaxy Chamber and Analysis of the
Vibration Isolation System
P. Krapf, J.P. Lainé, Y. Robach, L. Porte
To cite this version:
P. Krapf, J.P. Lainé, Y. Robach, L. Porte. Connection of a Scanning Tunneling Microscope with a Molecular Beam Epitaxy Chamber and Analysis of the Vibration Isolation System. Journal de Physique III, EDP Sciences, 1995, 5 (11), pp.1871-1885. �10.1051/jp3:1995231�. �jpa-00249422�
Classification Physics Abstracts
07.79Cz 68.55Bd 82.42+m
Connection of a Scanning Tunneling Microscope with a Molecular Beam Epitaxy Chamber and Analysis of the Vibration Isolation
System
P- Krapf, J-P- LainA(*). Y- Robach and L- Porte
DApartement de Physicochimie des MatAriaux (UA CNRS 848), and (")DApartement de MA- canique des Solides~ Ecole Centrale de Lyon~ B-P- 163, 69131 Ecully Cedex, France
(Received 22 March 1995, revised 12 July1995, accepted 19 July1995)
Abstract. A scanning tunneling microscope has been connected to a high vibrating com- mercial Molecular Beam Epitaxy (MBE) system, in order to study InGaAs layers gro,vn on
InP- An original and well efficient vibration isolation system has been designed and built: the
microscope support, stiffly linked to a vibration free inert
mass has been connected through a highly flexible bellow to the MBE system. The main characteristic of this design is
a low trans- mission factor at low frequencies. A detailed analysis of this system has been made to determine the main parameters of the transfer function and optimize the performances. Crucial problems
concern the bellow which for efficient damping of the eigen mode vibrations has to be adapted~
and the sample weight which has to be lowered. The efficiency of this vibration isolation system has been demonstrated by high resolution imaging of the graphite surface and of the surface of
an InGaAs buffer layer epitaxially grown on an InP (loo) substrate.
1. Introduction
The realization of high quality heterostructures by Molecular Beam Epitaxy (MBE) requires a good understanding of the surface morphology and growth modes of epitaxial layers. Because
of its real space imaging capability, the Scanning Tunneling Microscope (STM) has become a
powerful complementary tool to the more widely used m-situ Reflection High-Energy Electron Diffraction (RHEED) technique. Recent studies on the epitaxial growth of metals on metal
substrates [1-3] or semiconductors on semiconductor substrates [4-8] have clearly illustrated the unique advantage of STM for investigating the surface structure or any specific surface
reconstruction, as well as the nucleation and growth of 2-dimensional or 3-dimensional islands.
Such experiments require a transfer of the sample from the MBE chamber to the STM chamber without breaking the vacuum, in order to prevent contamination or oxidation of the surface.
In comparison to the use of an Ultra High Vacuum (UHV) transfer chamber [9,10], which requires fine procedures to maintain the sample in a UHV environment, a direct connection of
a STM chamber to a pre- existing MBE system appears more suitable for a subsequent control
@ Les Editions de Phvsique 1995
and epitaxial regrowth on STM characterized surfaces. Nevertheless this last solution may be
a rather complex issue due to the high sensitivity of STM to environmental vibrations.
An extensive work has been already done in our laboratory ill,12], concerning the growth
modes and relaxation mechanisms of strained InGaAs layers epitaxially grown on InP (100).
Experiments were carried out in a commercially available MBE system equipped with RHEED.
To obtain a better understanding of the epitaxial growth mechanism, we have connected a UHV-STM chamber to the pre-existing MBE system. In this paper, we describe the overall
design of the STM system that we have studied and built. For this design, we had to take into
account some constraints: a high vibrating environment, a compactness of the added system,
an adaptability to the sample holder of the MBE system, and simply an adaptability to the ex-
isting plant of a new high vacuum chamber supporting the STM and the vacuum manipulators
necessary to transfer the sample from the MBE to the STM. In the unfavourable conditions of high vibration level commonly encountered with commercial MBE systems, the capability of high resolution imaging requires an efficient isolation towards the vibration sources: conse-
quently, a special attention was paid to the analysis and modelling of the developed isolation system. Finally, the efficiency of this system was tested by imaging a graphite surface at atomic resolution and one of the first images of an InGaAs buffer layer is presented. Images of InGaAs epitaxial layers grown on InP (100) substrates are now currently recorded.
2. Description of the UHV System and Characterization of the Vibration Envi- ronment
A compact ultra high vacuum STM chamber was rigidly connected by a gate valve to a pre-
existing commercial MBE system (RIBER 2300). This chamber was divided into two compart- ments, a first one for the sample transfer and the second for the installation of the scanning tunneling microscope. As it will be described in more detail in the next Section, the microscope mounted on a UHV flange was linked to the STM chamber through a welded bellow. The MBE chamber is part of a multi- chamber UHV equipment which additionally includes a Plasma En- hanced Chemical Vapor Deposition (PECVD) system and a surface characterization chamber
equipped with X-ray Photoelectron Spectroscopy (XPS) and Low-Energy Electron Diffraction
(LEED) techniques. A central transfer unit allows the sample transfer from one chamber to another. An overview of our global UHV equipment is given in Figure 1. The base pressure
of this vacuum system is r-
10~~ Torr in the transfer unit and
r-
10~~° Torr in the MBE and STM chambers. The main sources of vibrations and acoustic noises are constitued by 4
turbomolecular pumps which are continuously in operation, and by an air conditioning system for temperature regulation. Contribution of other apparatus and cooling systems to vibrations and noise appeared negligible. The samples consist typically of 0-3-0.4 mm thick InP wafers,
on which epitaxial layers are grown, bonded with indium to a standard 2-inch molybdenum
RIBER sample holder; it allows compatibility with all manipulators for the UHV transfer in the whole system.
Two kinds of mechanical solicitations on the UHV system can be distinguished: contin-
uous vibrations due to pumps or other electric apparatus, and transient excitations due for example to shocks or valves manipulations. Continuous vibrations were measured on the STM chamber using a piezoelectric accelerometer which gives a reliable measurement above 1 Hz. The acceleration was recorded for a few seconds and frequency spectra were obtained
by Fourier transform using a Hanning's windowing in order to minimize the secondary peaks
due to the windowing: before the Fourier transform, the signal was multiplied by the function
g(t) = 1+ cos 2nj
,
where t is the time coordinate and [-T/2, +T/2] the period of 2
TOP VIEW
P.E.C.V.D.
M.B.E.
R,H.E,E,D, © X-P-S.
I
U-P-S-
3 4 ,,/--,,_ L.E.E.D.
S.T,M.
6
5 ~
50 cm
Load lock chamber
Fig. 1. Overview of the experimental UHV system. The sample is at 1 for MBE growth. To
translate the sample from 1 to STM, it has first to be taken successively by rotary-linear manipulators
2 and 3. A special vertical rotary-linear manipulator at 4 (not shown) allows to disconnect the molybloc sample holder on 3 and put the wafer on translator 5. Finally it is taken from 5 by vertical translator 6 (not shown) which put the wafer horizontal on the 3 STM legs (see Fig. 3).
measurement. In the following, a direct Cartesian coordinate system is used: the x and y
axes are in the horizontal plane which corresponds to the plane of the STM sample and the vertical z-axis direction corresponds to the tunneling tip axis. Acceleration versus frequency spectra exhibiting the same frequency dependence, but with different amplitudes were obtained
along the three directions. A typical acceleration spectrum for the z-axis direction is given in
Figure 2. Maximum acceleration amplitudes were obtained at 24 Hz, 50 Hz and 100 Hz. As
displacements are more relevant than accelerations for this study about STM where the tip to
sample displacement is of first concern, acceleration amplitudes were converted to displacement amplitudes Az (or Ax, Ay) by the relation [13],
az =
~~ ~~~
ai 11)
where f is the frequency. The vibration amplitude reached a maximum value of 0.6 pm at the
frequency of 50 Hz in the z-axis direction. Comparable displacements were obtained at lower
frequencies despite of their very low acceleration amplitudes. Relation (1) shows that for low
frequencies, low accelerations can correspond to large displacements. That is the main reason to look with great caution at low frequencies excitation sources before installation of a STM.
The inset of Figure 2 shows the vibration spectrum up to 5 Hz obtained on the underground.
8
~
3
m
E 2
cy
~o
_i
~f 0 2
~ 4
2
0 50 100 150 200
Frequency/ Flz
Fig. 2. Acceleration magnitude in the z-axis direction as a function of mechanical oscillation frequency, on the UHV chamber and on the underground (inset).
For higher frequencies, no significant signal at all was measured. The peak near 0 Hz is an artefact due to the accelerometer. Clearly, this underground should be a very good place to
put on the STM and we chose to build the basement of our STM implantation on it.
3. The Vibration Isolation System
3.1. DESCRIPTION. To obtain atomic resolution by STM imaging, one has to be able to
measure corrugations as small as 100 pm (surface atomic corrugation on metals). Therefore,
the major criterion of the microscope design must be a minimization of the vibrations [13-16].
An efficient vibration isolation system should then lead to a residual mechanical amplitude of the tip to sample separation situated well below 100 pm [17]. This goal was looked for by
combining a rigid and compact scanning tunneling microscope with an efficient isolation of this unit from the high vibrating vacuum chamber. The basic idea of our vibration isolation system
was to link the STM unit as weakly as possible to the vibrating vacuum chamber through a
UHV bellow and as stiffly as possible to a vibration free ground through a rigid support. The STM had to be installed on the first floor, but taking in account the spectrum of Figure 2
(inset), its basement was built on the underground. The overall design of our isolation system is shown in Figure 3. The microscope mounted on a 3.5-inch UHV flange is linked through
the bellow to the UHV-STM chamber. On the other hand, the microscope supporting flange
is fixed on a rigid support which consists in an iron tripod scelled on a concrete block. A 0.4 meter thick, 3 meters long and 2.80 meters large concrete block, which constitutes the inert basis, was built on the underground and disconnected from the foundations of the building.
A wall was built on this basis, forming a rectangular cavity which was filled with earth and covered with a new concrete layer. This intends to make the whole structure as stiff as possible
since too long iron beams for the tripod would have been detrimental for the stiffness. A metallic tripod structure made of 5 mm thick iron beams of square and U section was fixed on this construction. The beams go through the floor without contacts and support the STM base
flange. Though the ideal shape would have been a symmetrical tripod made with 3 convergent
sioEviEw
TRANSLATORS
VACUUM VERTICAL
CHAMBER TRANSLATOR 6
TLNNEUNG WAFER
MICROSCOPE
~~~~~
l m
wALL
UNDERGROUND
Fig. 3. Overall design of the vibration isolation system. The STM unit is rigidly fixed on the
basement and linked by a soft bellow to the UHV-MBE system.
beams of square section, we had to deal with the existing installation and to keep place for an additional ion pump. Therefore some lateral beams were added to improve the stiffness.
The highly flexible bellow which links the microscope supporting flange to the UHV-STM chamber is intended to absorb the vibrations transmitted from the MBE system. The bellow consists in 50 welded AM 350 stainless steel membranes of 0.2 mm thickness, 132 mm outer diameter and 102 mm inner diameter. The length of the bellow in its present configuration is 167 mm. An additional damping of the bellow turned out to be necessary to reduce excessive tip to sample displacements associated with the excitations of the bellow resonance frequencies.
This damping was obtained by surrounding the bellow with an elastomer material.
The Scanning Tunneling Microscope (a commercial Besocke "beetle type" STM) is rigid,
small and temperature compensated, yielding reduced sensitivity to mechanical and acousti- cal vibrations and temperature variations. Viton dampers ensure a second stage of isolation
between the microscope support and the tunneling assembly.
The two subsystems I-e- the isolation system and the STM unit, will be modelled separately
in the next paragraphs. These models will allow the determination of the global transfer function and a discussion of the attainable efficiency.
3.2. MODEL OF THE ISOLATION SYSTEM. The previously described isolation system can
be modelled by a system of springs and masses, as shown in Figure 4. Only translations had to be considered since in the present configuration of the vacuum chamber, displacements due to rotations are extremely small compared with translation amplitudes. The same model is valid for the three directions. The bellow is modelled by a spring of stiffness l~b,z
" 1000 N/m along
the z axis direction, and I(b,~ " Kb,y " 918 N/m along the x and y directions [18]. These values are in good agreement with calculated values obtained by numerical simulation using a
ANSYS software [19]. ANSi~S is a finite element calculation software which allows to simulate
complex mechanical structures. The resonance frequencies of the bellow were also determined by numerical simulation: the first resonance frequency was found equal to 6.6 Hz along x or y (flexion mode) and 13.4 Hz along z (traction-compression mode).
z
--~- ~fl
Vibrating VaCUUm Chamber ~
Az
o
K~
M Az~
~
K
Fig. 4. Simplified model of the vibration isolation system. Kb and Ka are respectively the stiffness of the bellow and of the rigid support, and Ma the moving mass of the support.
Consider now the spring and mass model for the rigid support of the microscope: one end of the spring, linked to the concrete block, does not move while the other end is linked to a mass denoted M~. The evaluation of this mass is not straightforward since it models the mass of the microscope and part of the mass of the tripod (the "moving mass" ). On one hand,
an exact value of the mass Ml of the microscope and of the elements on which it is directly
fixed (UHV flange, ...) can be measured. On the other hand, the beams of the tripod do not move uniformly as they are fixed on one side and allowed to move on the other. Thus the metallic tripod was modelled by numerical simulation (ANSYS software) in order to evaluate the "moving mass" lI2,z of the beams for a solicitation in the considered direction (I = x,y
or z). One gets then M~,~ = Ml + M2,~ with different values of M2,z and M~,~ for the three directions. In order to have a model which fits the real system as well as possible, the resonance frequencies were determined experimentally: the eigen modes of the structure were excited by
shocks and the fundamental resonance frequencies fo,~, relative to the three directions (with
= x, y or z) were obtained by accelerometer measurements. The spring stiffness K~,~ used to model the tripod structure were calculated using the relation (2~fo,~)~ = K~,~/M~,~.
On the basis of this model, we can now calculate the transfer function F of the isolation system. If zhzo represents the vibration amplitude, in the z direction, on the vacuum chamber linked to the MBE system. and Azfl_ the vibration amplitude applied on the microscope base
flange, the transfer function is given by:
~ ~~~'~~~°
1- (iffo)~ ~~~~ ~ b,~~~ji~,z ~~~ ~ /~
~
A same expression applies for x and y directions with Kb,~ (or Kb,y), K~,~ (or K~,~) and M~,~ (or lI~,~) instead of h'b,z, K~,z and lI~,z. Addition of some little damping of the bellow
-4o
x
(ta
~o Z
3
lo loo loco
Frequwqy/liz
Fig. 5. Calculated transfer functions of the vibration isolation system, for the three directions, using the model of Figure 4. Note the low value of the gain at low frequencies.
Table I. Caicttlated valttes of the eigen freqttency lo of the vibration isolation system and of the transfer fttnction D at low freqttencies: ttsing the model of Figttre $, lo and D are given by
~°
/~~
~'~~ ~ b,~~'k~,~'
"~~ ~'~ °~ ~'
direction fi£ (kg) Ks ~N/m) fo ~lb) D
x 49 7.7 x 20 1.2 x
y 54 2.9 x 37 3.I x
z 65 6 x 79 7.7 x
does not induce important changes in the transfer function since the quality factor I§ very
high due to the high stiffness of the support. The Bode's diagrams (20 log(F)) relative to the three directions, are given in Figure 5. Our vibration isolation system is characterized by
a low value of the transfer function D below the resonance frequency. Numerical results are given in Table I. Using the most important amplitude of vibrations Azo acting on the UHV
chamber, I.e. 0.6 pm at 50 Hz, we obtained a displacement of the microscope base flange along the z direction of Azfl_ = 8 x 10~~~
m. We found similarly, in the x and y directions, Axfl_ =1.3 x 10-~°
m and Ayfl
= 2.2 x 10~~°
m.
Our vibration isolation system represents an original alternative to the use of two-stage spring
isolation system or of frame suspended from pneumatic dampers that have been adopted by
m
~ Az~
K
m~ Az~
~~ "d
Az~
Fig. 6. Simplified model of the microscope unit; ms and mb are respectively the mass of the sample
holder and of the base plate, Kp the stiffness of a piezo-tube, I~d and ad respectively the stiffness and damping coefficient of a Viton damper.
other authors ii,13, 20,21]. In those cases, the isolation is verv efficient for vibration frequencies well over the first resonance frequency of the system, but frequencies below this resonance
are completely transmitted. The only way to isolate low frequencies with such a system is
to lower as much as possible the first eigen frequency. If it can be obtained by using long spring suspensions, it is not an easy way in a compact vacuum chamber. The suspension from pneumatic dampers can be appropriate with a quite small, specially designed system but cannot be considered in our case where a STM chamber is connected to a pre-existing big MBE system. In the case of a suspension ~vith a first proper frequency of 5 Hz [17], the transfer
function has a gain value of 0 dB below 5 Hz and -40 dB at 50 Hz. In our case, we got a
calculated gain value of -80 dB at 50 Hz and below, with the advantage of low gain at low
frequencies.
3.3. MODEL OF THE STM UNIT. The transfer function of interest for the global system
gives the variation of the tip to sample distance along with the amplitude of excitation on the MBE plant. Before obtaining this global transfer function, one has to introduce the microscope
structure. The tunneling assembly of the microscope consists in 4 piezo-tubes mounted on a
base plate. The central piezo-tube carries the tip and is used for the tip-to-sample distance
regulation during the scanning process. The 3 peripheral piezo-tubes carry an helicoidal ring
on which the sample holder is deposited; they are used in combination with the helicoidal ring
for the coarse tip adjustment (with a maximum coarse displacement amplitude of 0.5 mm),
for the scanning process and for the lateral displacement of the sample [22]. The base plate is isolated from the support linked to the flange by 3 Viton dampers. The small size and high rigidity of the microscope should result in relatively high meclianical resonance frequencies.
The microscope is modelled by the springs and masses system shown in Figure 6. Let m~ be the mass of the sample holder (ms
= 28 g) and mb the mass of the base plate of the microscope
unit (mb " 66 g). In a first step, we calculate the different parameters of our model. The
stiffness I[p for a piezo-tube is obtained from standard formulas [23] Kp = ~d e Eli for the
z direction and K[ = 3E J/l~, with J
=
~ [d~ (d 2e)~], for the x and y directions; is the length of the piezo-tube, d its outer diz~n~eter and e the thickness of the ceramic, E and J
respectively the Young modulus and inertia moment of the ceramic. Using E
= 66 GPa [24],
