- Email: [email protected]
A system for MBE growth and high-resolution RBS analysis
76
Nuclear
Instruments
A SYSTEM FOR MBE GROWTH AND HIGH-RESOLUTION P.M.J.
MAREE,
A.P.
DE JONGH,
J.W.
and J.F. VAN
DERKS
and Methods
RBS ANALYSIS DER
FOM - Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, Received
25 March
in Physics Research B28 (1987) 76-81 North-Holland, Amsterdam
VEEN The Netherlamb
1987
A UHV system is described, in which an MBE apparatus is combined with a setup for high-resolution RBS in conjunction with ion channeling and blocking and with facilities for RHEED, LEED and AES. This configuration makes it possible to grow atomic layers with excellent control over composition and thickness and to analyse the structure of these epitaxial films in situ with near-monolayer depth resolution.
1. Introduction
Molecular beam epitaxy (MBE) is a film growth technique with exceptional capabilities. The use of atomic or molecular beams, the slow growth rate (about one monolayer (ML) per second) and the low process temperature (400-800 o C) allow accurate control over thickness and composition and make this growth technique applicable to a large variety of semiconductor/metal/insulator systems. In the last decade it has been demonstrated that many conventional and novel electronic and optical devices can be fabricated by MBE [l]. However, for device production, in order to compete with established growth techniques it is necessary to reduce cost and to increase throughput [2]. This has led to the development of new production-oriented MBE systems with high deposition rate, capability of handling 6 in. or even 8 in. wafers, uniform film deposition (rotating substrate holder), parallel processing, easy operation and good reliability (3-61. The versatility, the excellent growth control and the compatibility with a variety of analytical methods in the ultrahigh vacuum (UHV) environment make MBE an outstanding technique for research applications. The requirements for a system designed for this purpose are different from those for a production facility. Throughput and ease of operation are only of secondary importance and may in part be sacrificed to improve the flexibility. Sample dimensions can be kept small in order to reduce size, complexity and cost of the growth chamber. On the other hand, a number of diagnostic methods should be available for in situ monitoring and characterisation of the epitaxial films during the different stages of growth. In this paper an MBE apparatus is described, which is part of a UHV system with various analytical possi0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
bilities. During growth the reflection high-energy electron diffraction (RHEED) pattern and the residual-gas mass spectrum can be monitored. In between preparation or growth cycles the samples can be transported in UHV to an analysis chamber. Besides the standard surface diagnostic tools of Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED), this analysis chamber contains equipment to perform highresolution Rutherford backscattering spectrometry (RBS) in conjunction with ion channeling and blocking. This technique, also known as medium-energy ion scattering (MEIS), allows for detailed investigations of the composition and atomic structure of thin films, surfaces and interfaces [7]. In section 2 the MBE apparatus is described, section 3 deals with the complete analysis system and in section 4 some applications are discussed.
2. MBE apparatus The UHV chamber for MBE growth consists of two compartments: a lower compartment which contains an electron-gun evaporation unit and an upper one in which Knudsen cell evaporators, sample manipulator and auxilary equipment are installed. A schematic of this arrangement is shown in fig. 1. The two compartments are separated by a manually operated shutter valve which can be opened to allow the beam generated by the electron-gun evaporation unit to reach the sample. The small-area valve acts as a shutter and at the same time prevents contamination of the electron-gun unit by material evaporated from the Knudsen cells. The upper compartment is pumped by a 190 l/s turbomolecular pump and a unit which contains a titanium sublimator surrounded by liquid-nitrogen (LN,)-cooled cryogenic panels. This unit is directly
P.M.J. Ma&e et al. /A system for MBE growth and high-resolution RBS analysis sample manipulator
RHEED
/nY
exchange chamber
phosphor scree” \
shutter valve thickness man
1
pumps Fig. 1. Schematic view of the apparatus for molecular beam epitaxy.
opposite to the Knudsen cell assembly, in order to condense the molecular beams after they have passed the sample. The lower compartment is evacuated by a 350 l/s turbomolecular pump, a 400 l/s ion pump and two Ti sublimation units. In addition, LN, cryoshields surround the effusion sources in both compartments. The LN, supply for the vessels is automatically regulated. Between the turbomolecular pumps and the rotary pumps are catalyser traps to prevent backstreaming of forepump oil into the UHV system. The chamber wall is made of stainless steel and the flanges are sealed with copper gaskets. After exposure to air the system is routinely baked at - 150 o C for at least 24 h. The base pressure in the apparatus is - 5 X 10F9 Pa. Due to differential pumping of the electron-gun unit in the
lower compartment upper compartment A quadrupole mass pressures of residual 2. I. Evaporation
the pressure during growth in the can be kept in the lo-* Pa region. spectrometer monitors the partial gases.
sources
The lower compartment of the MBE chamber is a modified version of the Si-MBE source chamber designed by De Jong et al. [8]. In houses a lo-kW electron-gun evaporation unit [9] with 270 o electron beam deflection and a single water-cooled copper crucible of 40 cm3 capacity, filled with a silicon slug. This Si source is shrouded by a copper shield, fastened to a LN, vessel. The Cu shield is roughened by sand-blasting, to in-
78
P.M.J. Mar&e et al. /A
system for MBE growth and high-resolution RBS analysis
crease adherence of the evaporated Si. This precaution reduces unwanted “flaking” [4]. The Si beam can reach the sample via a diaphragm in the Cu shield if the shutter valve between both chambers is opened. Two more holes in the shield allow supervision of the electron-beam scanning over the Si slug and monitoring of the beam flux by a quartz-crystal thickness monitor. The complete Si source, including the LN, cryoshield, is mounted on a single 12” Wheeler flange to facilitate refilling and servicing. The upper compartment contains four nearly horizontally mounted Knudsen cells for deposition of Ge, Ga, Sb, Sn, etc. The crucibles are made of pyrolitic BN and have a capacity of 3.5 cm3 [lo]. The flux distribution is influenced by changing the hole dimensions of a front plug on the crucible [3]. The power consumption per cell at the maximum operating temperature of 12OO’C is - 175 W. The cell temperatures are measured by thermocouples and are feedback controlled by PIDs [ll] to within + 0.5 o C. The cells are water-cooled and are surrounded by a LN, tank, to minimize mutual interference (“cross-talk”), both thermally and chemically. The Knudsen cell assembly with the LN, tank is mounted on a 100 in. Conflat flange. In front of each cell is a molybdenum shutter, which is operated manually via a rotational feedthrough. Beam fluxes are calibrated by RBS measurement of the amount of deposited material on a test sample. The long-term reproducibility of the flux for a fixed temperature setting of the cells is better than 10%. 2.2. Sample holder and manipulator The samples are mounted on special holders which contain provisions for heating. A holder with sample is introduced in the MBE system and plugged into the manipulator with the use of a transfer rod as shown in fig. 2. A bayonet fitting mechanism at the front of the rod allows for quick attachment of the holder to the rod and easy release once the holder is plugged into the manipulator. Attachment and release is simply achieved by clockwise and anticlockwise rotation of the rod, respectively. The aluminium oxide plate at the rear side of the
Fig. 3. The sample manipulator in the MBE chamber. The three different rotational degrees of freedom and their ranges are indicated.
sample holder is equipped with a S-pin receptacle which fits into a S-pin plug mounted in the sample manipulator. In this way the necessary electrical connections for sample heating and measurement of current and temperature are made automatically upon plugging-in of the sample. For heating of the sample several options are available: direct resistive heating or radiative heating with a sample clamped between Ta and Si plates, or mounting a sample with small Ta clamps or with In solder on a MO block. This block is heated by electron bombardment or radiant heating from a filament (fig. 2), or by a resistively heated thermocoax spiral [12]. Each sample has its optimal mounting and heating configuration, depending on substrate material and the
sample stage transfer
rod
MO block
flexible bavonet fitting mechanism
filament
S-pin receptacle
Fig. 2. Sample transfer system.
P.M.J. Marke et al. / A system for MBE growth and high-resolution RBS analysis temperature required for cleaning and growth. Temperatures are measured with a calibrated infrared pyrometer or with a Pt resistance embedded in the MO heater block. The sample size is - 1 cm*. Temperature differences over the sample are smaller than 10 ’ C and the uniformity in deposition is better than 5%. The manually operated manipulator (fig. 3) allows three orthogonal translations and the sample is rotatable about three different axes. Via two concentric rotational feedthroughs the sample can be rotated in the horizontal plane over more than 360° and tilted over 135O in the vertical plane along the arc (fig. 3). In the downward looking position the sample faces the (Si) electron beam evaporator and in the horizontal position it faces the Knudsen cell assembly. At intermediate positions, e.g. at a tilt angle of - 45 “, the sample surface is exposed to both electron beam evaporator and Knudsen cells. It is then possible to co-deposit Si and Ge or to dope Si layers with Sb or Ga during growth. For RHEED analysis it is necessary that the azimuthal angle of the sample is varied by rotation about the normal. In order to keep the construction of the manipulator as simple as possible, this rotation is performed with the aid of the transfer rod. Note, that in the downward looking position (Si MBE) the azimuth is simply adjusted by a rotation in the horizontal plane using the rotational feedthrough. 2.3. Preparation and analysis facilities For in-situ substrate cleaning use is made of the heater that is installed in the sample holder, eventually in combination with the molecular beams. In addition, a sputter ion gun is available, which generates a 0.5-0.8 keV Ar+ beam with a current density of - 1 pA/cm2. The sample can be positioned in any orientation with respect to this beam. Gas is introduced into the UHV chamber via an adjustable leak valve. During growth, the samples are analysed by RHEED. This technique, which has become a standard tool in most MBE systems, gives valuable information about surface morphology and crystal structure [13,14] and about the growth mechanism [15]. An electron gun [16] directs a beam of lo-15 keV electrons at grazing incidence onto the sample surface. The resulting diffraction pattern is made visible on a phosphor screen. In the configuration used in our setup (fig. 1) the RHEED pattern can be monitored in any of the growth positions of the sample. For the use of other surface analytical techniques (AES, LEED, RBS) the sample is transported to the analysis chamber, see below. 3. System description The MBE apparatus is part of a larger consisting of several UHV chambers separated
system, by gate
hipbresolution
analysis
79
RBS
chamber
I
IIICRr
Fig. 4. General layout of the system, showing MBE apparatus, analysis chamber, exchange chamber, storage/loading chamber and the available diagnostic techniques.
valves (fig. 4). The analysis chamber has a diameter of 1 m. It is coupled to a 200 kV ion accelerator via a differentially pumped beam line. This UHV scattering chamber is pumped with ion pumps, Ti sublimators, a LN, vessel and a turbomolecular pump which can be shut off by a pneumatic valve. Partial pressures of residual gases are measured by a quadrupole mass analyser. The base pressure in this chamber is - 7 X 10m9 Pa. Samples are introduced in the system via a vacuum interlock, which contains a carrousel with five storage positions for sample holders. After inserting the samples, the loading chamber is pumped down by a 270 l/s turbomolecular pump to a pressure of - 10m4 Pa within 15 min. In the centre of the system, connected with the MBE apparatus, the analysis chamber and the fast entry lock, is an exchange chamber. In this UHV chamber, which is pumped by a 190 l/s turbomolecular pump and a Ti sublimation unit in a LN,-cooled cryoshield, a base pressure in the lo-* Pa range is reached. Samples can be transported within the system without breaking the vacuum. Tbe sample holders are compatible with all manipulators in the system. With use of the bayonet fitting mechanism shown in fig. 2, holders are picked up from the manipulators or from the storage
80
P.M.J. Marie et al. /A
system for MBE growth and high-resolution RBS analysis
positions by magnetically coupled transfer rods. Samples are exchanged between the MBE apparatus and the scattering chamber via the intermediate exchange chamber. Introduction of new sample holders into the system takes place via the same chamber. This has the advantage that the exchange chamber acts as a buffer between the loading chamber and the UHV environment in the growth and scattering chambers, so that the pressure rise during introduction is minimal. Sample exchange between different chambers takes only a few minutes. Because the sample heaters are built into the holders they are transported along with the samples, which greatly improves the flexibility of the system. When a sample is plugged into any of the three manipulators in the system or in any of the five storage positions, five electrical leads connect themselves to the sample holder in the way described in section 2.2. These can be used for current and temperature measurement and for supplying the heater power. Additional tools for sample preparation in the system, apart from the facilities in the MBE chamber, are present in the analysis chamber: a regulated gas inlet, a differentially pumped sputter ion gun and a crystal-cleavage stage. The scattering chamber is equipped with an number of surface-sensitive analytical techniques. LEED optics and a cylindrical mirror analyser for AES are available. The sample manipulator in this chamber consists of a high-precision goniometer [17], by which samples can be accurately aligned. A medium-energy (50-200 kev) ion beam enters the analysis chamber and is scattered from the sample. A toroidal analyser detects backscattered ions simultaneously over an angular range of - 20 o and energy-selects them with a resolution of AE/E = 4 x 10e3 [18]. This corresponds to a depth resolution of 3-10 A, depending on primary ion energy, scattering geometry and sample material. These capabilities make the setup suitable for structural analysis of surfaces, interfaces and ultrathin films [7]. A modified version of the high-resolution RBS system is commercially available [19,20].
4. Applications The analytical capabilities of the MEIS technique are based on the high depth resolution, that can be obtained with the electrostatic analyser, and on the sensitivity for the atomic structure, which arises from the use of channeling and blocking techniques. An example of a MEIS spectrum, which has been collected in a single energy scan, is shown in fig. 5 in a three-dimensional plot. The plot displays the yield of backscattered ions from an epitaxial Ge/Si(lll) film, resolved in energy and scattering angle. The amopnt of Ge deposited is equivalent to a thickness of 13 A. The growth temperature was 500 o C. From the energy distri-
Fig. 5. Example of a ME’,S spectrum: The yield of backscattered ions from an 13 A Ge film grown at 500°C on a Si(ll1) substrate. The intensity is plotted as a function of energy and scattering angle. The energy distribution of backscattered He+ ions gives information about composition and morphology (islands), the angular distribution about structural aspects (epitaxy. strain, surface reconstruction).
bution of the backscattered He+ ions it can be inferred that the Ge has formed islands: the Ge signal is extended to lower energies (i.e. greater depth) and the leading edge of the Si signal is shifted to its surface position. Blocking minima in the Si substrate signal are continued in the yield from the Ge islands, indicating epitaxy. From a more detailed comparison of the angular distribution of ions backscattered from Ge and Si the mismatch-induced strain in the epitaxial Ge islands can be found [21,22]. The combination of MBE and high-resolution RBS in a single UHV system opens up new fields of research. On the one side the MBE chamber offers new possibilities to prepare clean surfaces and interfaces for crystallographic studies. On the other side, the MEIS technique can be used to address key-problems in epitaxial growth, such as analysis of nucleation and growth processes, dopant incorporation and defect formation mechanisms. The first experiments in this system, taking advantage of the new possibilities offered by the integration of MBE and MEIS, were performed on epitaxial films of the monoatomic semiconductors Si and Ge, grown on various semiconductor substrates. Several episilicide-silicon systems were studied also. Si and Ge were evaporated from the electron gun and Knudsen cell sources respectively, both with deposition rates of 0.1-2 ML/s. Metals (Ni, Co) were sublimated (at a rate
P.M.J. Mar&e et al. /A
system for h4BE growth and high-resolution RBS analysis
of 1O-3-1O-2 ML/s) from resistively heated wires, which were installed in both the scattering and the MBE chamber. Studied subjects were crystallinity and composition of the films [14,23], conditions for epitaxy [24], mismatch-induced strain [21], morphology and growth mechanisms [22], strain-induced reconstruction effects [25] and crystallography of surfaces and interfaces. In conclusion, the system described in this paper comprises both the means to grow epitaxial layers with the control over composition and thickness given by MBE and the means to perform structure analysis on them by MEIS, RHEED, LEED and AES. We gratefully acknowledge the craftsmanship of H. Neerings, W.H. Brouwer, W.J. Barsingerhom, C. van der Zweep, J.A. van Wel, A.F. Neuteboom, A. van ‘t Ent and S. Doom. Many thanks are due to B.A. Joyce and J.H. Neave from the Philips Research Laboratories in Redhill for valuable advice on the construction of the Knudsen cells, to G.P.J. Hermans, F.M. Mulders and K. Nakagawa for assembly and adjustment of the cells, and to P.C. Zalm from the Philips Research Laboratories in Eindhoven for supplying us with the phosphor screen. This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie (FOM) and is made possible by financial support from the Nederlandse Organisatie voor Zuiver Wetenschappelijk Onderzoek (ZWO) and the Stichting voor Technische Wetenschappen (S’I’W).
References
111A.Y. Cho, in: The Technology and Physics of Molecular Beam Epitaxy, ed., E.C.H. Parker (Plenum, New York, 1985) Ch. 1. 121D. Bellavance, in: Proc. First Int. Symp. on Si MBE, ed., J.C. Bean (Electrochemical Society, Pennington, 1985) p. 436. 131G.J. Davies and D. Williams, in: The Technology and Physics of Molecular Beam Epitaxy, ed., E.C.H. Parker (Plenum, New York, 1985) Ch. 2. [41 Y. Ota, Thin Solid Films 106 (1983) 1. [51 J.C. Bean and E.A. Sadowski, J. Vat. Sci. Technol. 20 (1982) 137.
81
[6] J.C. Bean and P. Butcher, in: Proceedings of the First International Symposium on Si MBE, ed., J.C. Bean (Electrochemical Society, Pennington, 1985) p. 427. [7] J.F. van der Veen, Surface Sci. Rep. 5 (1985) 199. [8] T. de Jong, L. Smit, V.V. Korablev R.M. Tromp and F.W. Saris, Appl. Surf. Sci. 10 (1982) 10; T. de J0ng;W.A.S. Douma, L. Smit, V.V. Korablev and F.W. Saris, J. Vat. Sci. Technol. Bl (1983) 888. Film Technology Division, 191Model 989-1105, Varian/Thin 611 Hansen Way, Palo Alto, CA 94303, USA. Ltd, 227 Berwick Avenue, Slough WI Fulmer Components Trading Estate, Slough, Berks SLl 4QT, UK. type 407, Control & Readout Ltd., PII Digital controller Woods Way Goring by Sea Worthing, West Sussex BN12 4TH, UK. et Cie, 10, rue de la Passerelle, 92150 WI Thermocoax Suresnes, France. ]I31 B.A. Joyce, J.H. Neave, P.J. Dobson and P.K. Larsen, Phys. Rev. B 29 (1984) 814. P41 A.E.M.J. Fischer, P.M.J. Mar&e and J.F. van der Veen, Appl. Surf. Sci. 27 (1986) 143. D51 P.J. Dobson, B.A. Joyce and J.H. Neave, J. Cryst. Growth 81 (1987) 1. Lane, East 1161 LET 110, VG Scientific Ltd., Imberhome Grinstead, West Sussex RH19 lUB, UK. P71 W.C. Turkenburg, E. de Haas, A.F. Neuteboom, J. Ladru and H.H. Kersten, Nucl. Instr. and Meth. 126 (1975) 241. R.M. Tromp, H.H. Kersten, A.J.H. WI R.G. Smeenk, Boerboom and F.W. Saris, Nucl. Instr. and Meth. 195 (1982) 581. F.W. Saris, 1191 R.M. Tromp, H.H. Kersten, E. Granneman, R. Koudijs and W.J. Kilsdonk, Nucl. Instr. and Meth. B4 (1983) 155. WI High Voltage Engineering Europe B.V., Amsterdamseweg 61, P.O. Box 99, 3800 AB Amersfoort, The Netherlands. WI P.M.J. Maree, R.I.J. Oltbof, J.W.M. Frenken, J.F. van der Veen, C.W.T. Bulle-Lieuwma, M.P.A. Viegers and P.C. Zalm, J. Appl. Phys. 58 (1985) 3097. WI P.M.J. Maree, K. Nakagawa, F.M. Mulders, J.F. van de Veen and K.L. Kavanagh, Surf. Sci. to be published. v31 P.C. Zalm, P.M.J. Marbe and R.I.J. Olthof, Appl. Phys. Lett. 46 (1985) 597. (241 P.M.J. Ma&e, K. Nakagawa, F.W. Saris and J. Haisma, Appl. Surf. Sci. 28 (1987) 128. (251 K. Nakagawa, P.M.J. Marie and J.F. van der Veen, in: Proc. 18th Int. Conf. on the Physics of Semiconductors, ed., 0. Engstrom (World Scientific, Singapore, 1987) p. 93.
Nuclear
Instruments
A SYSTEM FOR MBE GROWTH AND HIGH-RESOLUTION P.M.J.
MAREE,
A.P.
DE JONGH,
J.W.
and J.F. VAN
DERKS
and Methods
RBS ANALYSIS DER
FOM - Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, Received
25 March
in Physics Research B28 (1987) 76-81 North-Holland, Amsterdam
VEEN The Netherlamb
1987
A UHV system is described, in which an MBE apparatus is combined with a setup for high-resolution RBS in conjunction with ion channeling and blocking and with facilities for RHEED, LEED and AES. This configuration makes it possible to grow atomic layers with excellent control over composition and thickness and to analyse the structure of these epitaxial films in situ with near-monolayer depth resolution.
1. Introduction
Molecular beam epitaxy (MBE) is a film growth technique with exceptional capabilities. The use of atomic or molecular beams, the slow growth rate (about one monolayer (ML) per second) and the low process temperature (400-800 o C) allow accurate control over thickness and composition and make this growth technique applicable to a large variety of semiconductor/metal/insulator systems. In the last decade it has been demonstrated that many conventional and novel electronic and optical devices can be fabricated by MBE [l]. However, for device production, in order to compete with established growth techniques it is necessary to reduce cost and to increase throughput [2]. This has led to the development of new production-oriented MBE systems with high deposition rate, capability of handling 6 in. or even 8 in. wafers, uniform film deposition (rotating substrate holder), parallel processing, easy operation and good reliability (3-61. The versatility, the excellent growth control and the compatibility with a variety of analytical methods in the ultrahigh vacuum (UHV) environment make MBE an outstanding technique for research applications. The requirements for a system designed for this purpose are different from those for a production facility. Throughput and ease of operation are only of secondary importance and may in part be sacrificed to improve the flexibility. Sample dimensions can be kept small in order to reduce size, complexity and cost of the growth chamber. On the other hand, a number of diagnostic methods should be available for in situ monitoring and characterisation of the epitaxial films during the different stages of growth. In this paper an MBE apparatus is described, which is part of a UHV system with various analytical possi0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
bilities. During growth the reflection high-energy electron diffraction (RHEED) pattern and the residual-gas mass spectrum can be monitored. In between preparation or growth cycles the samples can be transported in UHV to an analysis chamber. Besides the standard surface diagnostic tools of Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED), this analysis chamber contains equipment to perform highresolution Rutherford backscattering spectrometry (RBS) in conjunction with ion channeling and blocking. This technique, also known as medium-energy ion scattering (MEIS), allows for detailed investigations of the composition and atomic structure of thin films, surfaces and interfaces [7]. In section 2 the MBE apparatus is described, section 3 deals with the complete analysis system and in section 4 some applications are discussed.
2. MBE apparatus The UHV chamber for MBE growth consists of two compartments: a lower compartment which contains an electron-gun evaporation unit and an upper one in which Knudsen cell evaporators, sample manipulator and auxilary equipment are installed. A schematic of this arrangement is shown in fig. 1. The two compartments are separated by a manually operated shutter valve which can be opened to allow the beam generated by the electron-gun evaporation unit to reach the sample. The small-area valve acts as a shutter and at the same time prevents contamination of the electron-gun unit by material evaporated from the Knudsen cells. The upper compartment is pumped by a 190 l/s turbomolecular pump and a unit which contains a titanium sublimator surrounded by liquid-nitrogen (LN,)-cooled cryogenic panels. This unit is directly
P.M.J. Ma&e et al. /A system for MBE growth and high-resolution RBS analysis sample manipulator
RHEED
/nY
exchange chamber
phosphor scree” \
shutter valve thickness man
1
pumps Fig. 1. Schematic view of the apparatus for molecular beam epitaxy.
opposite to the Knudsen cell assembly, in order to condense the molecular beams after they have passed the sample. The lower compartment is evacuated by a 350 l/s turbomolecular pump, a 400 l/s ion pump and two Ti sublimation units. In addition, LN, cryoshields surround the effusion sources in both compartments. The LN, supply for the vessels is automatically regulated. Between the turbomolecular pumps and the rotary pumps are catalyser traps to prevent backstreaming of forepump oil into the UHV system. The chamber wall is made of stainless steel and the flanges are sealed with copper gaskets. After exposure to air the system is routinely baked at - 150 o C for at least 24 h. The base pressure in the apparatus is - 5 X 10F9 Pa. Due to differential pumping of the electron-gun unit in the
lower compartment upper compartment A quadrupole mass pressures of residual 2. I. Evaporation
the pressure during growth in the can be kept in the lo-* Pa region. spectrometer monitors the partial gases.
sources
The lower compartment of the MBE chamber is a modified version of the Si-MBE source chamber designed by De Jong et al. [8]. In houses a lo-kW electron-gun evaporation unit [9] with 270 o electron beam deflection and a single water-cooled copper crucible of 40 cm3 capacity, filled with a silicon slug. This Si source is shrouded by a copper shield, fastened to a LN, vessel. The Cu shield is roughened by sand-blasting, to in-
78
P.M.J. Mar&e et al. /A
system for MBE growth and high-resolution RBS analysis
crease adherence of the evaporated Si. This precaution reduces unwanted “flaking” [4]. The Si beam can reach the sample via a diaphragm in the Cu shield if the shutter valve between both chambers is opened. Two more holes in the shield allow supervision of the electron-beam scanning over the Si slug and monitoring of the beam flux by a quartz-crystal thickness monitor. The complete Si source, including the LN, cryoshield, is mounted on a single 12” Wheeler flange to facilitate refilling and servicing. The upper compartment contains four nearly horizontally mounted Knudsen cells for deposition of Ge, Ga, Sb, Sn, etc. The crucibles are made of pyrolitic BN and have a capacity of 3.5 cm3 [lo]. The flux distribution is influenced by changing the hole dimensions of a front plug on the crucible [3]. The power consumption per cell at the maximum operating temperature of 12OO’C is - 175 W. The cell temperatures are measured by thermocouples and are feedback controlled by PIDs [ll] to within + 0.5 o C. The cells are water-cooled and are surrounded by a LN, tank, to minimize mutual interference (“cross-talk”), both thermally and chemically. The Knudsen cell assembly with the LN, tank is mounted on a 100 in. Conflat flange. In front of each cell is a molybdenum shutter, which is operated manually via a rotational feedthrough. Beam fluxes are calibrated by RBS measurement of the amount of deposited material on a test sample. The long-term reproducibility of the flux for a fixed temperature setting of the cells is better than 10%. 2.2. Sample holder and manipulator The samples are mounted on special holders which contain provisions for heating. A holder with sample is introduced in the MBE system and plugged into the manipulator with the use of a transfer rod as shown in fig. 2. A bayonet fitting mechanism at the front of the rod allows for quick attachment of the holder to the rod and easy release once the holder is plugged into the manipulator. Attachment and release is simply achieved by clockwise and anticlockwise rotation of the rod, respectively. The aluminium oxide plate at the rear side of the
Fig. 3. The sample manipulator in the MBE chamber. The three different rotational degrees of freedom and their ranges are indicated.
sample holder is equipped with a S-pin receptacle which fits into a S-pin plug mounted in the sample manipulator. In this way the necessary electrical connections for sample heating and measurement of current and temperature are made automatically upon plugging-in of the sample. For heating of the sample several options are available: direct resistive heating or radiative heating with a sample clamped between Ta and Si plates, or mounting a sample with small Ta clamps or with In solder on a MO block. This block is heated by electron bombardment or radiant heating from a filament (fig. 2), or by a resistively heated thermocoax spiral [12]. Each sample has its optimal mounting and heating configuration, depending on substrate material and the
sample stage transfer
rod
MO block
flexible bavonet fitting mechanism
filament
S-pin receptacle
Fig. 2. Sample transfer system.
P.M.J. Marke et al. / A system for MBE growth and high-resolution RBS analysis temperature required for cleaning and growth. Temperatures are measured with a calibrated infrared pyrometer or with a Pt resistance embedded in the MO heater block. The sample size is - 1 cm*. Temperature differences over the sample are smaller than 10 ’ C and the uniformity in deposition is better than 5%. The manually operated manipulator (fig. 3) allows three orthogonal translations and the sample is rotatable about three different axes. Via two concentric rotational feedthroughs the sample can be rotated in the horizontal plane over more than 360° and tilted over 135O in the vertical plane along the arc (fig. 3). In the downward looking position the sample faces the (Si) electron beam evaporator and in the horizontal position it faces the Knudsen cell assembly. At intermediate positions, e.g. at a tilt angle of - 45 “, the sample surface is exposed to both electron beam evaporator and Knudsen cells. It is then possible to co-deposit Si and Ge or to dope Si layers with Sb or Ga during growth. For RHEED analysis it is necessary that the azimuthal angle of the sample is varied by rotation about the normal. In order to keep the construction of the manipulator as simple as possible, this rotation is performed with the aid of the transfer rod. Note, that in the downward looking position (Si MBE) the azimuth is simply adjusted by a rotation in the horizontal plane using the rotational feedthrough. 2.3. Preparation and analysis facilities For in-situ substrate cleaning use is made of the heater that is installed in the sample holder, eventually in combination with the molecular beams. In addition, a sputter ion gun is available, which generates a 0.5-0.8 keV Ar+ beam with a current density of - 1 pA/cm2. The sample can be positioned in any orientation with respect to this beam. Gas is introduced into the UHV chamber via an adjustable leak valve. During growth, the samples are analysed by RHEED. This technique, which has become a standard tool in most MBE systems, gives valuable information about surface morphology and crystal structure [13,14] and about the growth mechanism [15]. An electron gun [16] directs a beam of lo-15 keV electrons at grazing incidence onto the sample surface. The resulting diffraction pattern is made visible on a phosphor screen. In the configuration used in our setup (fig. 1) the RHEED pattern can be monitored in any of the growth positions of the sample. For the use of other surface analytical techniques (AES, LEED, RBS) the sample is transported to the analysis chamber, see below. 3. System description The MBE apparatus is part of a larger consisting of several UHV chambers separated
system, by gate
hipbresolution
analysis
79
RBS
chamber
I
IIICRr
Fig. 4. General layout of the system, showing MBE apparatus, analysis chamber, exchange chamber, storage/loading chamber and the available diagnostic techniques.
valves (fig. 4). The analysis chamber has a diameter of 1 m. It is coupled to a 200 kV ion accelerator via a differentially pumped beam line. This UHV scattering chamber is pumped with ion pumps, Ti sublimators, a LN, vessel and a turbomolecular pump which can be shut off by a pneumatic valve. Partial pressures of residual gases are measured by a quadrupole mass analyser. The base pressure in this chamber is - 7 X 10m9 Pa. Samples are introduced in the system via a vacuum interlock, which contains a carrousel with five storage positions for sample holders. After inserting the samples, the loading chamber is pumped down by a 270 l/s turbomolecular pump to a pressure of - 10m4 Pa within 15 min. In the centre of the system, connected with the MBE apparatus, the analysis chamber and the fast entry lock, is an exchange chamber. In this UHV chamber, which is pumped by a 190 l/s turbomolecular pump and a Ti sublimation unit in a LN,-cooled cryoshield, a base pressure in the lo-* Pa range is reached. Samples can be transported within the system without breaking the vacuum. Tbe sample holders are compatible with all manipulators in the system. With use of the bayonet fitting mechanism shown in fig. 2, holders are picked up from the manipulators or from the storage
80
P.M.J. Marie et al. /A
system for MBE growth and high-resolution RBS analysis
positions by magnetically coupled transfer rods. Samples are exchanged between the MBE apparatus and the scattering chamber via the intermediate exchange chamber. Introduction of new sample holders into the system takes place via the same chamber. This has the advantage that the exchange chamber acts as a buffer between the loading chamber and the UHV environment in the growth and scattering chambers, so that the pressure rise during introduction is minimal. Sample exchange between different chambers takes only a few minutes. Because the sample heaters are built into the holders they are transported along with the samples, which greatly improves the flexibility of the system. When a sample is plugged into any of the three manipulators in the system or in any of the five storage positions, five electrical leads connect themselves to the sample holder in the way described in section 2.2. These can be used for current and temperature measurement and for supplying the heater power. Additional tools for sample preparation in the system, apart from the facilities in the MBE chamber, are present in the analysis chamber: a regulated gas inlet, a differentially pumped sputter ion gun and a crystal-cleavage stage. The scattering chamber is equipped with an number of surface-sensitive analytical techniques. LEED optics and a cylindrical mirror analyser for AES are available. The sample manipulator in this chamber consists of a high-precision goniometer [17], by which samples can be accurately aligned. A medium-energy (50-200 kev) ion beam enters the analysis chamber and is scattered from the sample. A toroidal analyser detects backscattered ions simultaneously over an angular range of - 20 o and energy-selects them with a resolution of AE/E = 4 x 10e3 [18]. This corresponds to a depth resolution of 3-10 A, depending on primary ion energy, scattering geometry and sample material. These capabilities make the setup suitable for structural analysis of surfaces, interfaces and ultrathin films [7]. A modified version of the high-resolution RBS system is commercially available [19,20].
4. Applications The analytical capabilities of the MEIS technique are based on the high depth resolution, that can be obtained with the electrostatic analyser, and on the sensitivity for the atomic structure, which arises from the use of channeling and blocking techniques. An example of a MEIS spectrum, which has been collected in a single energy scan, is shown in fig. 5 in a three-dimensional plot. The plot displays the yield of backscattered ions from an epitaxial Ge/Si(lll) film, resolved in energy and scattering angle. The amopnt of Ge deposited is equivalent to a thickness of 13 A. The growth temperature was 500 o C. From the energy distri-
Fig. 5. Example of a ME’,S spectrum: The yield of backscattered ions from an 13 A Ge film grown at 500°C on a Si(ll1) substrate. The intensity is plotted as a function of energy and scattering angle. The energy distribution of backscattered He+ ions gives information about composition and morphology (islands), the angular distribution about structural aspects (epitaxy. strain, surface reconstruction).
bution of the backscattered He+ ions it can be inferred that the Ge has formed islands: the Ge signal is extended to lower energies (i.e. greater depth) and the leading edge of the Si signal is shifted to its surface position. Blocking minima in the Si substrate signal are continued in the yield from the Ge islands, indicating epitaxy. From a more detailed comparison of the angular distribution of ions backscattered from Ge and Si the mismatch-induced strain in the epitaxial Ge islands can be found [21,22]. The combination of MBE and high-resolution RBS in a single UHV system opens up new fields of research. On the one side the MBE chamber offers new possibilities to prepare clean surfaces and interfaces for crystallographic studies. On the other side, the MEIS technique can be used to address key-problems in epitaxial growth, such as analysis of nucleation and growth processes, dopant incorporation and defect formation mechanisms. The first experiments in this system, taking advantage of the new possibilities offered by the integration of MBE and MEIS, were performed on epitaxial films of the monoatomic semiconductors Si and Ge, grown on various semiconductor substrates. Several episilicide-silicon systems were studied also. Si and Ge were evaporated from the electron gun and Knudsen cell sources respectively, both with deposition rates of 0.1-2 ML/s. Metals (Ni, Co) were sublimated (at a rate
P.M.J. Mar&e et al. /A
system for h4BE growth and high-resolution RBS analysis
of 1O-3-1O-2 ML/s) from resistively heated wires, which were installed in both the scattering and the MBE chamber. Studied subjects were crystallinity and composition of the films [14,23], conditions for epitaxy [24], mismatch-induced strain [21], morphology and growth mechanisms [22], strain-induced reconstruction effects [25] and crystallography of surfaces and interfaces. In conclusion, the system described in this paper comprises both the means to grow epitaxial layers with the control over composition and thickness given by MBE and the means to perform structure analysis on them by MEIS, RHEED, LEED and AES. We gratefully acknowledge the craftsmanship of H. Neerings, W.H. Brouwer, W.J. Barsingerhom, C. van der Zweep, J.A. van Wel, A.F. Neuteboom, A. van ‘t Ent and S. Doom. Many thanks are due to B.A. Joyce and J.H. Neave from the Philips Research Laboratories in Redhill for valuable advice on the construction of the Knudsen cells, to G.P.J. Hermans, F.M. Mulders and K. Nakagawa for assembly and adjustment of the cells, and to P.C. Zalm from the Philips Research Laboratories in Eindhoven for supplying us with the phosphor screen. This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie (FOM) and is made possible by financial support from the Nederlandse Organisatie voor Zuiver Wetenschappelijk Onderzoek (ZWO) and the Stichting voor Technische Wetenschappen (S’I’W).
References
111A.Y. Cho, in: The Technology and Physics of Molecular Beam Epitaxy, ed., E.C.H. Parker (Plenum, New York, 1985) Ch. 1. 121D. Bellavance, in: Proc. First Int. Symp. on Si MBE, ed., J.C. Bean (Electrochemical Society, Pennington, 1985) p. 436. 131G.J. Davies and D. Williams, in: The Technology and Physics of Molecular Beam Epitaxy, ed., E.C.H. Parker (Plenum, New York, 1985) Ch. 2. [41 Y. Ota, Thin Solid Films 106 (1983) 1. [51 J.C. Bean and E.A. Sadowski, J. Vat. Sci. Technol. 20 (1982) 137.
81
[6] J.C. Bean and P. Butcher, in: Proceedings of the First International Symposium on Si MBE, ed., J.C. Bean (Electrochemical Society, Pennington, 1985) p. 427. [7] J.F. van der Veen, Surface Sci. Rep. 5 (1985) 199. [8] T. de Jong, L. Smit, V.V. Korablev R.M. Tromp and F.W. Saris, Appl. Surf. Sci. 10 (1982) 10; T. de J0ng;W.A.S. Douma, L. Smit, V.V. Korablev and F.W. Saris, J. Vat. Sci. Technol. Bl (1983) 888. Film Technology Division, 191Model 989-1105, Varian/Thin 611 Hansen Way, Palo Alto, CA 94303, USA. Ltd, 227 Berwick Avenue, Slough WI Fulmer Components Trading Estate, Slough, Berks SLl 4QT, UK. type 407, Control & Readout Ltd., PII Digital controller Woods Way Goring by Sea Worthing, West Sussex BN12 4TH, UK. et Cie, 10, rue de la Passerelle, 92150 WI Thermocoax Suresnes, France. ]I31 B.A. Joyce, J.H. Neave, P.J. Dobson and P.K. Larsen, Phys. Rev. B 29 (1984) 814. P41 A.E.M.J. Fischer, P.M.J. Mar&e and J.F. van der Veen, Appl. Surf. Sci. 27 (1986) 143. D51 P.J. Dobson, B.A. Joyce and J.H. Neave, J. Cryst. Growth 81 (1987) 1. Lane, East 1161 LET 110, VG Scientific Ltd., Imberhome Grinstead, West Sussex RH19 lUB, UK. P71 W.C. Turkenburg, E. de Haas, A.F. Neuteboom, J. Ladru and H.H. Kersten, Nucl. Instr. and Meth. 126 (1975) 241. R.M. Tromp, H.H. Kersten, A.J.H. WI R.G. Smeenk, Boerboom and F.W. Saris, Nucl. Instr. and Meth. 195 (1982) 581. F.W. Saris, 1191 R.M. Tromp, H.H. Kersten, E. Granneman, R. Koudijs and W.J. Kilsdonk, Nucl. Instr. and Meth. B4 (1983) 155. WI High Voltage Engineering Europe B.V., Amsterdamseweg 61, P.O. Box 99, 3800 AB Amersfoort, The Netherlands. WI P.M.J. Maree, R.I.J. Oltbof, J.W.M. Frenken, J.F. van der Veen, C.W.T. Bulle-Lieuwma, M.P.A. Viegers and P.C. Zalm, J. Appl. Phys. 58 (1985) 3097. WI P.M.J. Maree, K. Nakagawa, F.M. Mulders, J.F. van de Veen and K.L. Kavanagh, Surf. Sci. to be published. v31 P.C. Zalm, P.M.J. Marbe and R.I.J. Olthof, Appl. Phys. Lett. 46 (1985) 597. (241 P.M.J. Ma&e, K. Nakagawa, F.W. Saris and J. Haisma, Appl. Surf. Sci. 28 (1987) 128. (251 K. Nakagawa, P.M.J. Marie and J.F. van der Veen, in: Proc. 18th Int. Conf. on the Physics of Semiconductors, ed., 0. Engstrom (World Scientific, Singapore, 1987) p. 93.