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Interconnected UHV facilities for materials preparation and analysis
Nuclear Instruments and Methods in Physics Research A 435 (1999) 514 } 522
Interconnected UHV facilities for materials preparation and analysis S. Kubsky *, L. Borucki , F. Gorris , H.W. Becker , C. Rolfs , W.H. Schulte , I.J.R. Baumvol, F.C. Stedile Institut fu( r Experimentalphysik III, Ruhr-Universita( t Bochum, D-44780 Bochum, Germany Instituto de Fn& sica, Instituto de Qun& mica, UFRGS, Porto Alegre, Brazil Received 14 June 1999; accepted 21 June 1999
Abstract A UHV system for in-situ preparation and analysis of ultra thin "lms has been built. The system includes a rapid thermal processing furnace which allows production of samples over a wide range of temperatures and pressures using isotopically enriched gases. XPS, AES, and LEED analyses provide information on the surface structure and composition. With a transportable UHV chamber, the samples can be transferred to a 4p c-ray spectrometer facility (in UHV), where analytical ion beam methods can be used to determine isotopic depth pro"les and total amounts of isotopes in the "lms. Furthermore, an ion beam deposition facility (in UHV) can produce isotopically enriched silicon "lms on Si substrates for in situ isotopic tracing. 1999 Elsevier Science B.V. All rights reserved. Keywords: RTP; UHV; XPS; NRA
1. Introduction The fabrication of reliable ultra-thin gate dielectrics represents a key issue in very-large-scale-integrated (VLSI) technologies [1,2]. Presently, the dielectrics are fabricated in the form of thermally grown oxide or oxynitride "lms on Si, with a thickness of a few nm [3}11]. However, their growth properties could not be understood within the standard Deal}Grove model [12]. Several attempts have ascribed [1,3,4] the observed deviations from the Deal}Grove model to the initial stages of growth, assuming either the existence of an extended reactive layer near the interface or the
* Corresponding author.
in#uence of special surface phenomena on the oxide growth. In both cases the initial stages may depend on the hydrogen termination and roughness of the surface as well as on the wafer cleaning [1,13]. As a consequence, the modi"ed models could not describe consistently all available data. To improve the situation, new data are needed. For example, all models assume that Si atoms are immobile during the growth process; however, an experimental proof of this assumption is missing. Experimental investigations of the initial stages of thermally grown oxide or oxynitride "lms on Si are hampered by the fact that an Si surface is not stable under the presence of oxygen. A cleaned Si surface oxidizes at room temperature and atmospheric pressure within a short time to an SiO of thickness of about 1 nm [14]; for example, at an
0168-9002/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 6 4 8 - 8
S. Kubsky et al. / Nuclear Instruments and Methods in Physics Research A 435 (1999) 514 } 522
O partial pressure of 10\ mbar one monolayer of SiO grows already within 1 h [3]. Thus, improved studies must ful"ll the following requirements. (i) The pressure of the reactive gases must be below 10\ mbar during all processing steps, including the transfer from the growth facility to the analysis facility; the exception is the growth step itself, where the furnace is pressurized with the reactive gas. (ii) The cleanliness and crystallinity of the sample surface must be monitored before and after the processing steps. (iii) The atomic transport during thermal growth must be investigated, i.e. the atomic composition of the grown layer must be analyzed with high depth resolution (of the order of 1 nm). The present work describes experimental e!orts to implement these requirements [15,16]. Brie#y, the "rst requirement is ful"lled using an interconnected setup in which thermal growth of dielectric "lms as well as their analyses are carried out under ultra-high vacuum (UHV) conditions. The growth facility consists of a rapid thermal processing (RTP) furnace and the analysis facility includes lowenergy electron di!raction (LEED), Auger-electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). In addition, ion beam analysis as well as ion beam deposition can be performed, for which the samples are transferred to these UHV facilities using a transportable UHV chamber. The third requirement can be ful"lled by combining "lm growth and isotopic tracing: isotopically enriched layers are produced either by thermal growth in enriched gases (such as O ) or by ion beam de position (such as Si) and the produced "lms are analyzed using nuclear resonance reaction analysis (NRRA) and deuteron induced gamma emission (DIGME) as well as XPS and AES.
2. Experimental equipment An extended UHV system including several subsystems has been built (Fig. 1), where the subsystems are connected by magnetically coupled transfer rods to move a sample from one subsystem to another. The system includes an RTP furnace, a loading chamber (A), a storage chamber (C), and
515
Fig. 1. Schematic diagram of the interconnected UHV system (for details, see text).
an analysis chamber (D) for XPS, AES, LEED, and SEM. A transportable UHV chamber (B) can be used to access a 4p c-ray spectrometer facility [17] at the Bochum accelerators, a remote UHV subsystem for NRRA and DIGME. The chamber can also be used to transfer samples to an ion beam deposition facility [18,19], another remote UHV subsystem. The total system is designed for processing and analyzing samples of 1 in diameter. The analysis chamber and its equipment were supplied by FISONS and the pumps and pressure meters for all chambers by LEYBOLD. The sample transport to the analysis chamber is based on a sample holder from FISONS, the POD system. The RTP furnace [20] has been built as a compact design (Fig. 2), where the small volume (850 cm) allows to use isotopically enriched process gases at a pressure of several 100 mbar, at moderate costs. The furnace consists of a quartz tube with a diameter of 55 mm and a wall thickness of 3 mm, surrounded by 12 halogene lamps. The quartz tube is connected to a stainless-steel #ange using a combination of seven rings of di!erent adapting glasses. The process gas is introduced into
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Fig. 2. Schematic diagram of the rapid thermal processing furnace (for details, see text).
the chamber through a needle valve and its pressure is monitored using a Baratron manometer. The furnace can be operated at a static gas pressure or in a #ow mode using a turbo-molecular pump and a membrane pump to circulate the gas. These pumps evacuate the furnace to a pressure of about 10\ mbar. A pressure of 2;10\ mbar is achieved in the furnace with an ion getter pump installed at the loading chamber (A). A quadrupole mass spectrometer (QMS, Fig. 1) is connected with the furnace and monitors the residual gas composition as well as the gas composition during processing. After processing, the gas can be restored using a zeolite trap. The halogen lamps provide a heating power of 1000 W each. The quartz piston of the halogen lamps has nearly identical absorption characteristics as the quartz tube of the furnace: the radiation which would be absorbed by the quartz tube is already absorbed by the lamp piston and thus leads to a negligible heating of the furnace walls. The power of the lamps is controlled by six channel thyristor circuits allowing a six zone control of the furnace and thus for an optimization of the radiation power distribution. The lamps are surrounded by a water-cooled re#ector, which has a thin gold layer evaporated on its polished inner surface. A fan provides air cooling of the quartz tube. The samples are centered in the quartz furnace by a quartz support, which is designed such that thermal contacts are minimized. The temperature of the sample has been measured as a function of lamp power using thermocouples. In praxis,
a pyrometric temperature control (PbSe detector, j"2.7}4.3 lm) has been implemented that allows for a closed-loop operation. Temperature gradients of more than 200 K/s can be reached, and the maximum temperature is about 1450 K. The power of the halogen lamps is monitored by a microcontroller, thus allowing #exible heating cycles. Individual RTP recipes are downloaded from a PC via a serial RS232 interface. A spherical chamber (D, Fig. 1) with an inner diameter of 300 mm houses the equipment for XPS, AES, LEED, and SEM: the analysis chamber. The chamber is made from l-metal to screen stray magnetic "elds. The 1503 hemispheric electron energy analyzer CLAM2 has a central de#ection radius of 100 mm; the electrons are detected with a channeltron. The width of the entrance and exit apertures of the analyzer can be varied between 2 and 4 mm. A maximum counting rate of 10 events/s can be tolerated at an energy resolution of *E/E"0.5%. Both manual and computer controls of the spectrometer are available. The X-ray source XR3E2 with a twin anode (Mg and Al) has been installed at an angle of 553 with respect to the energy analyzer and is used for XPS. For AES, an electron beam with an energy up to 5 keV and a current up to 2 mA is produced by the electron gun LEG62. The minimum electron beam diameter is about 3 lm at an electron current of 100 nA. Furthermore, a scintillation crystal can detect secondary electrons correlated with the lateral displacement of the electron beam and provides thus an image of the sample surface with a lateral resolution of about 20 lm (SEM). The LEED system RVL/5, installed on a CF150 #ange, allows to evaluate the crystalline properties of sample surfaces via visual inspection and analysis of the LEED patterns. Finally, an Ar sputter ion gun AG5000 has been installed for in situ cleaning of the samples. For a precise and reproducible position of the sample at the center of the chamber, the sample is mounted on the POD system. The chamber is pumped by a 270 l/s ion getter pump. The baking cycle of the chamber as well of the UHV chambers described below is controlled by a microcontroller. A pressure of 2;10\ mbar (measured with ionization gauges) is achieved after baking the chambers at 1203C for about 24 h.
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A cylindrical chamber of 150 mm diameter (C, Fig. 1) is used for storage and transfer of up to six samples mounted on the sample holder POD. The chamber is evacuated by a 800 l/s cryogenic pump. To process a wafer in the RTP furnace, it has to be removed from the metallic POD system and transferred into the furnace. A similar transfer procedure is needed for ion beam analyses and deposition. The transfer is performed in the loading chamber (A) involving two manipulators and a wobble stick [16]. The cylindrical chamber (A) of 200 mm diameter is pumped by a 120 l/s ion getter pump. A transportable UHV chamber (B, Fig. 1) has been built for the transfer of up to nine samples between the UHV facilities. The cylindrical chamber of 150 mm diameter is equipped with a magnetically coupled transfer arm and a manipulator and is evacuated by a 220 l/s ion getter pump (operated on a battery during the transport to remote facilities). Details of the ion beam deposition system have been described elsewhere [18,19]. Brie#y, the UHV system has been designed as an integrated part of other facilities, i.e. the transportable UHV chamber can be used to transfer samples from and to this deposition system. The deposition system allows to produce thin isotopically enriched "lms for a wide range of elements using a decelerated ion beam with an energy as low as 30 eV, the so-called soft landing. For the production of Si "lms, the problem of mass interference between Si isotopes and molecular ion beams after the analyzing magnet (e.g. Si and SiH) limits the isotopic purity of the grown "lms and introduces hydrogen contaminants to the "lms. In this work, the isotopic purity could be improved by orders of magnitude with the use of negative ion beams produced in a sputter ion source. The c-rays emitted in NRRA and DIGME are observed with a 12 ;12 NaI(Tl) crystal, which has a central bore hole of 35 mm diameter and a 0.5 mm Al wall thickness. The ion beam enters a cylindrical target chamber (32 mm diameter, 27 cm length) and is stopped at the target. With the target at the center of the crystal, the total e$ciency of this 4p spectrometer was observed to be nearly
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100% for c-rays up to 10 MeV. Details of the spectrometer and its UHV system are given in Ref. [17]. The sample handling system of the spectrometer has been extended [21] to allow a sample transfer from and to the spectrometer using the transportable UHV chamber. With the spectrometer connected to the 400 kV single-ended and the 4 MV tandem accelerators at Bochum, the measurement of depth pro"les of special isotopes using NRRA and of total amounts of isotopes using DIGME can be performed.
3. Some performance tests In the following some "rst performance tests are described to illustrate the features of the system and its components. In one test, Si(1 1 1) wafers were placed in the ion beam deposition system and cleaned here by thermal heating. Subsequently, thin "lms of Si have been deposited on the substrates at an energy of 30 eV. The samples were then transferred to the 4p c-ray spectrometer using the UHV transport chamber. The NRRA yield curve obtained with the spectrometer at the E "324 keV resonance in 0 Si(p, c)P for one of the samples is shown in Fig. 3.
Fig. 3. Yield curve of the E "324 keV resonance in 0 Si(p, c)Si obtained with the 4p c-ray spectrometer and a thin Si "lm produced by ion beam deposition on an Si substrate. The curves through the data points and the inset represent the results of a simulation of the Si depth pro"le.
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Fig. 4. XPS spectrum of an Si sample with SiC islands on its surface. An extended spectrum obtained near the C1s line is shown in the lower part, where the dashed curves represent the contribution of individual lines and the solid curve represents their sum. The line due to the SiC bond is separated from the C line by 2.0 eV.
The curves through the data points and the inset represent a simulation of the observed Si depth pro"le: the deposited "lm with a 90% Si abundance has a thickness of about 7 nm (with a relatively sharp interface of about 2 nm), while at deeper layers the natural abundance of Si (4.7%) is visible. The results demonstrate an acceptable quality of the deposited thin "lms needed for isotopic tracing of Si in the growth of siliconoxid "lms (see below). Analyses of the growth of SiC on Si(1 1 1) using scanning tunnel microscopy (STM) showed the
formation of islands [22]. However, the chemical composition of the islands could not be obtained from STM. Such a sample was placed in the analysis chamber. The resulting XPS spectrum is shown in the upper part of Fig. 4: lines from C, O, and Si are visible. A spectrum accumulated near the C line (lower part of Fig. 4) reveals a high-energy shoulder on the C line due to the presence of SiC bonds (energy di!erence between the bonds C and SiC"2.0 eV). The results lead to an energy resolution of the CLAM2 analyzer of 0.7%, consistent with expectation. As discussed in Section 1, the cleanliness of Si wafers represents an important aspect for the production of ultrathin "lms on Si. For this reason, an Si(1 1 1) wafer after HF etching (4%, 20 s) and rinsing in ultra-pure water was introduced into the UHV system (Fig. 1). An XPS spectrum is shown in Fig. 5a: lines from the contaminant elements C (E "283 eV) and O (E "531 eV) are visible. To reduce or remove these contaminants, the sample was transferred from the analysis chamber to the RTP furnace and thermally treated under vacuum at 9003C for 20 s. The sample was then moved back to the analysis chamber: the XPS spectrum (Fig. 5b) exhibits a signi"cant reduction of C and O but the asymmetric shape of the Si2p line (E "99.4 eV) indicates still the formation of SiO bonds. Subsequently, the sample was treated at 3003C for 600 s in 10 mbar O (#ow-mode) fol lowed by short #ashes to temperatures of 9003C under vacuum. The XPS spectrum (Fig. 5c) shows now that this cleaning step removed nearly all C and O from the sample surface. An LEED picture of this sample for an electron energy of 32 eV is shown in Fig. 6: the visibility of the 7;7 reconstruction of the Si(1 1 1) surface demonstrates a good crystallinity of the surface. Another test was devoted to the study of the monolayer-hydrogenation of an Si(1 0 0) surface using the vibrational Doppler spectroscopy in combination with the E "6.40 MeV narrow reson0 ance in H(N, ac)C (for details, see Ref. [21] and references therein). For this purpose, the RTP furnace was used to clean and hydrogenate the wafer (H pressure"10 mbar, temperature" 6003C, time"10 s). The sample was then transferred via the transportable UHV chamber to the 4p
S. Kubsky et al. / Nuclear Instruments and Methods in Physics Research A 435 (1999) 514 } 522
c-ray spectrometer and a H depth pro"le was obtained using the above resonance. The resulting Doppler width of 10.03$0.24 keV lies between the expected values for ordered (11.5 keV) and random
519
(8.6 keV) surface layers. However, the absolute hydrogen density corresponded to about 3.6 monolayers requiring an improved sample preparation in future e!orts.
Fig. 5. XPS spectra obtained from an Si(1 1 1) sample: (a) as introduced to the UHV system after HF-cleaning, (b) after thermal cleaning in the RTP furnace (9003C, 20 s), (c) after oxygen etching (3003C, 600 s, 10 mbar O ) and thermal cleaning (#ashing at 9003C).
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Fig. 5. (Continued).
Fig. 6. LEED picture at an electron energy of 32 eV of an Si(1 1 1) sample after oxygen etching and thermal cleaning (c): a reconstructed 7;7 surface is visible.
Finally, a Si(1 1 1) wafer was cleaned in the RTP furnace, then transferred (here and later: always using the transportable UHV chamber) to the ion
beam deposition system, where a thin "lm of Si was deposited. The sample was then transferred to the 4p c-ray spectrometer, where an Si depth pro"le was obtained using the E "324 keV reson0 ance in Si(p, c)P. A pro"le similar to that shown in Fig. 3 was found. The sample was then transferred to the RTP furnace, where the sample was oxidized (O pressure"180 mbar, temper ature"10703C, time"300 s). The sample was then moved back to the spectrometer and another Si depth pro"le was obtained. The data (Fig. 7) reveal a reduction of Si near the surface (from 70% to 15%) accompanied with a distribution to deeper layers (about a factor 2). This observation could indicate a signi"cant mobility of Si during the oxidation process. However, a severe ion-beam induced damage of the sample was observed subsequently using an electron microscope. This damage could have in#uenced the above observations. With the availability of a new 500 kV accelerator at Bochum, the E "417 keV resonance in 0 Si(p, c)P will be accessible, with which ionbeam-induced damage will be reduced by about a factor 20 due to a correspondingly higher resonance strength.
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Fig. 7. Yield curve of the E "324 keV resonance in Si(p, c)Si obtained from a thin Si "lm on a Si substrate after oxidation in the 0 RTP furnace. The c-ray yield has been converted to an Si concentration reaching its natural level (4.7%) at deeper layers. The dashed curve through the data points is to guide the eye only.
4. Summary The performance tests just discussed illustrate the potential of the interconnected system, where in-situ sample production and analysis can be carried out under UHV conditions. Thus, improved information on the thermal growth of ultra-thin dielectric "lms as well as other issues of materials science may be obtained in the future. Acknowledgements The authors appreciate the assistance of S. Baier, M. Berheide, T. Gutt, and N. Piel in the initial phase of this work. References [1] H.Z. Massoud, E.H. Poindexter, C.R. Helms, Electrochem. Soc. 96 (1996) 97.
[2] E. Garfunkel, E. Gusev, A. Vul, Fundamental Aspects of Ultrathin Dielectrics on Si-based Devices, Kluwer Academic Publishers, The Netherlands, 1988. [3] C.J. So"eld, A.M. Stoneham, Semicond and Sci. Technol. 10 (1995) 215. [4] M. Morita, T. Ohmi, E. Hasegawa, M. Kawakami, M. Ohwada, J. Appl. Phys. 68 (1990) 1272. [5] F.J. Himpsel, F.R. McFeely, A. Taleb-Ibrahimi, J.A. Yarmo!, G. Hollinger, Phys. Rev. B 38 (1988) 6084. [6] T. Ohmi, T. Shibata, J. Electrochem. Soc. 92 (1992) 32. [7] K.F. Schuegraf, C. Hu, Semicond. Sci. Technol. 9 (1994) 989. [8] H. Hwang, W. Ting, B. Maiti, D.L. Kwong, J. Lee, Appl. Phys. Lett. 57 (1990) 1010. [9] R. Wrixon, A. Twomey, P. O'Sullivan, A. Mathewson, J. Electrochem. Soc. 142 (1995) 2738. [10] I.J.R. Baumvol, F.C. Stedile, J.J. Ganem, I. Trimaille, S. Rigo, Appl. Phys. Lett. 69 (1996) 2385. [11] I.J.R. Baumvol, J.J. Ganem, L.G. Gosset, I. Trimaille, S. Rigo, Appl. Phys. Lett. 72 (1998) 2999. [12] B.E. Deal, A.S. Grove, J. Appl. Phys. 36 (1965) 3770. [13] F.C. Stedile, I.J.R. Baumvol, I.F. Oppenheim, I. Trimaille, J.J. Ganem, S. Rigo, Nucl. Instr. and Meth. B (1999), in press.
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[14] J.J. Ganem, S. Rigo, I. Trimaille, G.N. Lu, Nucl. Instr. and Meth. B 64 (1992) 784. [15] S. Kubsky, L. Borucki, M. Berheide, S. Baier, H.W. Becker, F. Gorris, C. Grunwald, T. Gutt, G. KruK ger, M. Mehrho!, N. Piel, C. Rolfs, W.H. Schulte, Nucl. Instr. and Meth. B 113 (1996) 63. [16] S. Kubsky, Thesis, Ruhr-UniversitaK t Bochum, 1999. [17] M. Mehrho!, M. Aliotta, I.J.R. Baumvol, H.W. Becker, M. Berheide, L. Borucki, J. Domke, F. Gorris, S. Kubsky, N. Piel, G. Roters, C. Rolfs, W.H. Schulte, Nucl.Instr. and Meth. B 132 (1997) 671.
[18] F. Gorris, C. Krug, S. Kubsky, I.J.R. Baumvol, W.H. Schulte, C. Rolfs, in preparation. [19] F. Gorris, Thesis, Ruhr-UniversitaK t Bochum, 1999. [20] F. Roozeboom, History and perspectives of rapid thermal processing, Advances in Rapid Thermal Processing, NATO Conference Proceedings, Maratea, Italy, 1995. [21] L. Borucki, H.W. Becker, S. Kubsky, W.H. Schulte, C. Rolfs, Eur. Phys. J. A5 (1999) 327. [22] U. KoK hler, Ruhr-UniversitaK t Bochum, private communication, 1999.
Interconnected UHV facilities for materials preparation and analysis S. Kubsky *, L. Borucki , F. Gorris , H.W. Becker , C. Rolfs , W.H. Schulte , I.J.R. Baumvol, F.C. Stedile Institut fu( r Experimentalphysik III, Ruhr-Universita( t Bochum, D-44780 Bochum, Germany Instituto de Fn& sica, Instituto de Qun& mica, UFRGS, Porto Alegre, Brazil Received 14 June 1999; accepted 21 June 1999
Abstract A UHV system for in-situ preparation and analysis of ultra thin "lms has been built. The system includes a rapid thermal processing furnace which allows production of samples over a wide range of temperatures and pressures using isotopically enriched gases. XPS, AES, and LEED analyses provide information on the surface structure and composition. With a transportable UHV chamber, the samples can be transferred to a 4p c-ray spectrometer facility (in UHV), where analytical ion beam methods can be used to determine isotopic depth pro"les and total amounts of isotopes in the "lms. Furthermore, an ion beam deposition facility (in UHV) can produce isotopically enriched silicon "lms on Si substrates for in situ isotopic tracing. 1999 Elsevier Science B.V. All rights reserved. Keywords: RTP; UHV; XPS; NRA
1. Introduction The fabrication of reliable ultra-thin gate dielectrics represents a key issue in very-large-scale-integrated (VLSI) technologies [1,2]. Presently, the dielectrics are fabricated in the form of thermally grown oxide or oxynitride "lms on Si, with a thickness of a few nm [3}11]. However, their growth properties could not be understood within the standard Deal}Grove model [12]. Several attempts have ascribed [1,3,4] the observed deviations from the Deal}Grove model to the initial stages of growth, assuming either the existence of an extended reactive layer near the interface or the
* Corresponding author.
in#uence of special surface phenomena on the oxide growth. In both cases the initial stages may depend on the hydrogen termination and roughness of the surface as well as on the wafer cleaning [1,13]. As a consequence, the modi"ed models could not describe consistently all available data. To improve the situation, new data are needed. For example, all models assume that Si atoms are immobile during the growth process; however, an experimental proof of this assumption is missing. Experimental investigations of the initial stages of thermally grown oxide or oxynitride "lms on Si are hampered by the fact that an Si surface is not stable under the presence of oxygen. A cleaned Si surface oxidizes at room temperature and atmospheric pressure within a short time to an SiO of thickness of about 1 nm [14]; for example, at an
0168-9002/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 6 4 8 - 8
S. Kubsky et al. / Nuclear Instruments and Methods in Physics Research A 435 (1999) 514 } 522
O partial pressure of 10\ mbar one monolayer of SiO grows already within 1 h [3]. Thus, improved studies must ful"ll the following requirements. (i) The pressure of the reactive gases must be below 10\ mbar during all processing steps, including the transfer from the growth facility to the analysis facility; the exception is the growth step itself, where the furnace is pressurized with the reactive gas. (ii) The cleanliness and crystallinity of the sample surface must be monitored before and after the processing steps. (iii) The atomic transport during thermal growth must be investigated, i.e. the atomic composition of the grown layer must be analyzed with high depth resolution (of the order of 1 nm). The present work describes experimental e!orts to implement these requirements [15,16]. Brie#y, the "rst requirement is ful"lled using an interconnected setup in which thermal growth of dielectric "lms as well as their analyses are carried out under ultra-high vacuum (UHV) conditions. The growth facility consists of a rapid thermal processing (RTP) furnace and the analysis facility includes lowenergy electron di!raction (LEED), Auger-electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). In addition, ion beam analysis as well as ion beam deposition can be performed, for which the samples are transferred to these UHV facilities using a transportable UHV chamber. The third requirement can be ful"lled by combining "lm growth and isotopic tracing: isotopically enriched layers are produced either by thermal growth in enriched gases (such as O ) or by ion beam de position (such as Si) and the produced "lms are analyzed using nuclear resonance reaction analysis (NRRA) and deuteron induced gamma emission (DIGME) as well as XPS and AES.
2. Experimental equipment An extended UHV system including several subsystems has been built (Fig. 1), where the subsystems are connected by magnetically coupled transfer rods to move a sample from one subsystem to another. The system includes an RTP furnace, a loading chamber (A), a storage chamber (C), and
515
Fig. 1. Schematic diagram of the interconnected UHV system (for details, see text).
an analysis chamber (D) for XPS, AES, LEED, and SEM. A transportable UHV chamber (B) can be used to access a 4p c-ray spectrometer facility [17] at the Bochum accelerators, a remote UHV subsystem for NRRA and DIGME. The chamber can also be used to transfer samples to an ion beam deposition facility [18,19], another remote UHV subsystem. The total system is designed for processing and analyzing samples of 1 in diameter. The analysis chamber and its equipment were supplied by FISONS and the pumps and pressure meters for all chambers by LEYBOLD. The sample transport to the analysis chamber is based on a sample holder from FISONS, the POD system. The RTP furnace [20] has been built as a compact design (Fig. 2), where the small volume (850 cm) allows to use isotopically enriched process gases at a pressure of several 100 mbar, at moderate costs. The furnace consists of a quartz tube with a diameter of 55 mm and a wall thickness of 3 mm, surrounded by 12 halogene lamps. The quartz tube is connected to a stainless-steel #ange using a combination of seven rings of di!erent adapting glasses. The process gas is introduced into
516
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Fig. 2. Schematic diagram of the rapid thermal processing furnace (for details, see text).
the chamber through a needle valve and its pressure is monitored using a Baratron manometer. The furnace can be operated at a static gas pressure or in a #ow mode using a turbo-molecular pump and a membrane pump to circulate the gas. These pumps evacuate the furnace to a pressure of about 10\ mbar. A pressure of 2;10\ mbar is achieved in the furnace with an ion getter pump installed at the loading chamber (A). A quadrupole mass spectrometer (QMS, Fig. 1) is connected with the furnace and monitors the residual gas composition as well as the gas composition during processing. After processing, the gas can be restored using a zeolite trap. The halogen lamps provide a heating power of 1000 W each. The quartz piston of the halogen lamps has nearly identical absorption characteristics as the quartz tube of the furnace: the radiation which would be absorbed by the quartz tube is already absorbed by the lamp piston and thus leads to a negligible heating of the furnace walls. The power of the lamps is controlled by six channel thyristor circuits allowing a six zone control of the furnace and thus for an optimization of the radiation power distribution. The lamps are surrounded by a water-cooled re#ector, which has a thin gold layer evaporated on its polished inner surface. A fan provides air cooling of the quartz tube. The samples are centered in the quartz furnace by a quartz support, which is designed such that thermal contacts are minimized. The temperature of the sample has been measured as a function of lamp power using thermocouples. In praxis,
a pyrometric temperature control (PbSe detector, j"2.7}4.3 lm) has been implemented that allows for a closed-loop operation. Temperature gradients of more than 200 K/s can be reached, and the maximum temperature is about 1450 K. The power of the halogen lamps is monitored by a microcontroller, thus allowing #exible heating cycles. Individual RTP recipes are downloaded from a PC via a serial RS232 interface. A spherical chamber (D, Fig. 1) with an inner diameter of 300 mm houses the equipment for XPS, AES, LEED, and SEM: the analysis chamber. The chamber is made from l-metal to screen stray magnetic "elds. The 1503 hemispheric electron energy analyzer CLAM2 has a central de#ection radius of 100 mm; the electrons are detected with a channeltron. The width of the entrance and exit apertures of the analyzer can be varied between 2 and 4 mm. A maximum counting rate of 10 events/s can be tolerated at an energy resolution of *E/E"0.5%. Both manual and computer controls of the spectrometer are available. The X-ray source XR3E2 with a twin anode (Mg and Al) has been installed at an angle of 553 with respect to the energy analyzer and is used for XPS. For AES, an electron beam with an energy up to 5 keV and a current up to 2 mA is produced by the electron gun LEG62. The minimum electron beam diameter is about 3 lm at an electron current of 100 nA. Furthermore, a scintillation crystal can detect secondary electrons correlated with the lateral displacement of the electron beam and provides thus an image of the sample surface with a lateral resolution of about 20 lm (SEM). The LEED system RVL/5, installed on a CF150 #ange, allows to evaluate the crystalline properties of sample surfaces via visual inspection and analysis of the LEED patterns. Finally, an Ar sputter ion gun AG5000 has been installed for in situ cleaning of the samples. For a precise and reproducible position of the sample at the center of the chamber, the sample is mounted on the POD system. The chamber is pumped by a 270 l/s ion getter pump. The baking cycle of the chamber as well of the UHV chambers described below is controlled by a microcontroller. A pressure of 2;10\ mbar (measured with ionization gauges) is achieved after baking the chambers at 1203C for about 24 h.
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A cylindrical chamber of 150 mm diameter (C, Fig. 1) is used for storage and transfer of up to six samples mounted on the sample holder POD. The chamber is evacuated by a 800 l/s cryogenic pump. To process a wafer in the RTP furnace, it has to be removed from the metallic POD system and transferred into the furnace. A similar transfer procedure is needed for ion beam analyses and deposition. The transfer is performed in the loading chamber (A) involving two manipulators and a wobble stick [16]. The cylindrical chamber (A) of 200 mm diameter is pumped by a 120 l/s ion getter pump. A transportable UHV chamber (B, Fig. 1) has been built for the transfer of up to nine samples between the UHV facilities. The cylindrical chamber of 150 mm diameter is equipped with a magnetically coupled transfer arm and a manipulator and is evacuated by a 220 l/s ion getter pump (operated on a battery during the transport to remote facilities). Details of the ion beam deposition system have been described elsewhere [18,19]. Brie#y, the UHV system has been designed as an integrated part of other facilities, i.e. the transportable UHV chamber can be used to transfer samples from and to this deposition system. The deposition system allows to produce thin isotopically enriched "lms for a wide range of elements using a decelerated ion beam with an energy as low as 30 eV, the so-called soft landing. For the production of Si "lms, the problem of mass interference between Si isotopes and molecular ion beams after the analyzing magnet (e.g. Si and SiH) limits the isotopic purity of the grown "lms and introduces hydrogen contaminants to the "lms. In this work, the isotopic purity could be improved by orders of magnitude with the use of negative ion beams produced in a sputter ion source. The c-rays emitted in NRRA and DIGME are observed with a 12 ;12 NaI(Tl) crystal, which has a central bore hole of 35 mm diameter and a 0.5 mm Al wall thickness. The ion beam enters a cylindrical target chamber (32 mm diameter, 27 cm length) and is stopped at the target. With the target at the center of the crystal, the total e$ciency of this 4p spectrometer was observed to be nearly
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100% for c-rays up to 10 MeV. Details of the spectrometer and its UHV system are given in Ref. [17]. The sample handling system of the spectrometer has been extended [21] to allow a sample transfer from and to the spectrometer using the transportable UHV chamber. With the spectrometer connected to the 400 kV single-ended and the 4 MV tandem accelerators at Bochum, the measurement of depth pro"les of special isotopes using NRRA and of total amounts of isotopes using DIGME can be performed.
3. Some performance tests In the following some "rst performance tests are described to illustrate the features of the system and its components. In one test, Si(1 1 1) wafers were placed in the ion beam deposition system and cleaned here by thermal heating. Subsequently, thin "lms of Si have been deposited on the substrates at an energy of 30 eV. The samples were then transferred to the 4p c-ray spectrometer using the UHV transport chamber. The NRRA yield curve obtained with the spectrometer at the E "324 keV resonance in 0 Si(p, c)P for one of the samples is shown in Fig. 3.
Fig. 3. Yield curve of the E "324 keV resonance in 0 Si(p, c)Si obtained with the 4p c-ray spectrometer and a thin Si "lm produced by ion beam deposition on an Si substrate. The curves through the data points and the inset represent the results of a simulation of the Si depth pro"le.
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Fig. 4. XPS spectrum of an Si sample with SiC islands on its surface. An extended spectrum obtained near the C1s line is shown in the lower part, where the dashed curves represent the contribution of individual lines and the solid curve represents their sum. The line due to the SiC bond is separated from the C line by 2.0 eV.
The curves through the data points and the inset represent a simulation of the observed Si depth pro"le: the deposited "lm with a 90% Si abundance has a thickness of about 7 nm (with a relatively sharp interface of about 2 nm), while at deeper layers the natural abundance of Si (4.7%) is visible. The results demonstrate an acceptable quality of the deposited thin "lms needed for isotopic tracing of Si in the growth of siliconoxid "lms (see below). Analyses of the growth of SiC on Si(1 1 1) using scanning tunnel microscopy (STM) showed the
formation of islands [22]. However, the chemical composition of the islands could not be obtained from STM. Such a sample was placed in the analysis chamber. The resulting XPS spectrum is shown in the upper part of Fig. 4: lines from C, O, and Si are visible. A spectrum accumulated near the C line (lower part of Fig. 4) reveals a high-energy shoulder on the C line due to the presence of SiC bonds (energy di!erence between the bonds C and SiC"2.0 eV). The results lead to an energy resolution of the CLAM2 analyzer of 0.7%, consistent with expectation. As discussed in Section 1, the cleanliness of Si wafers represents an important aspect for the production of ultrathin "lms on Si. For this reason, an Si(1 1 1) wafer after HF etching (4%, 20 s) and rinsing in ultra-pure water was introduced into the UHV system (Fig. 1). An XPS spectrum is shown in Fig. 5a: lines from the contaminant elements C (E "283 eV) and O (E "531 eV) are visible. To reduce or remove these contaminants, the sample was transferred from the analysis chamber to the RTP furnace and thermally treated under vacuum at 9003C for 20 s. The sample was then moved back to the analysis chamber: the XPS spectrum (Fig. 5b) exhibits a signi"cant reduction of C and O but the asymmetric shape of the Si2p line (E "99.4 eV) indicates still the formation of SiO bonds. Subsequently, the sample was treated at 3003C for 600 s in 10 mbar O (#ow-mode) fol lowed by short #ashes to temperatures of 9003C under vacuum. The XPS spectrum (Fig. 5c) shows now that this cleaning step removed nearly all C and O from the sample surface. An LEED picture of this sample for an electron energy of 32 eV is shown in Fig. 6: the visibility of the 7;7 reconstruction of the Si(1 1 1) surface demonstrates a good crystallinity of the surface. Another test was devoted to the study of the monolayer-hydrogenation of an Si(1 0 0) surface using the vibrational Doppler spectroscopy in combination with the E "6.40 MeV narrow reson0 ance in H(N, ac)C (for details, see Ref. [21] and references therein). For this purpose, the RTP furnace was used to clean and hydrogenate the wafer (H pressure"10 mbar, temperature" 6003C, time"10 s). The sample was then transferred via the transportable UHV chamber to the 4p
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c-ray spectrometer and a H depth pro"le was obtained using the above resonance. The resulting Doppler width of 10.03$0.24 keV lies between the expected values for ordered (11.5 keV) and random
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(8.6 keV) surface layers. However, the absolute hydrogen density corresponded to about 3.6 monolayers requiring an improved sample preparation in future e!orts.
Fig. 5. XPS spectra obtained from an Si(1 1 1) sample: (a) as introduced to the UHV system after HF-cleaning, (b) after thermal cleaning in the RTP furnace (9003C, 20 s), (c) after oxygen etching (3003C, 600 s, 10 mbar O ) and thermal cleaning (#ashing at 9003C).
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Fig. 5. (Continued).
Fig. 6. LEED picture at an electron energy of 32 eV of an Si(1 1 1) sample after oxygen etching and thermal cleaning (c): a reconstructed 7;7 surface is visible.
Finally, a Si(1 1 1) wafer was cleaned in the RTP furnace, then transferred (here and later: always using the transportable UHV chamber) to the ion
beam deposition system, where a thin "lm of Si was deposited. The sample was then transferred to the 4p c-ray spectrometer, where an Si depth pro"le was obtained using the E "324 keV reson0 ance in Si(p, c)P. A pro"le similar to that shown in Fig. 3 was found. The sample was then transferred to the RTP furnace, where the sample was oxidized (O pressure"180 mbar, temper ature"10703C, time"300 s). The sample was then moved back to the spectrometer and another Si depth pro"le was obtained. The data (Fig. 7) reveal a reduction of Si near the surface (from 70% to 15%) accompanied with a distribution to deeper layers (about a factor 2). This observation could indicate a signi"cant mobility of Si during the oxidation process. However, a severe ion-beam induced damage of the sample was observed subsequently using an electron microscope. This damage could have in#uenced the above observations. With the availability of a new 500 kV accelerator at Bochum, the E "417 keV resonance in 0 Si(p, c)P will be accessible, with which ionbeam-induced damage will be reduced by about a factor 20 due to a correspondingly higher resonance strength.
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Fig. 7. Yield curve of the E "324 keV resonance in Si(p, c)Si obtained from a thin Si "lm on a Si substrate after oxidation in the 0 RTP furnace. The c-ray yield has been converted to an Si concentration reaching its natural level (4.7%) at deeper layers. The dashed curve through the data points is to guide the eye only.
4. Summary The performance tests just discussed illustrate the potential of the interconnected system, where in-situ sample production and analysis can be carried out under UHV conditions. Thus, improved information on the thermal growth of ultra-thin dielectric "lms as well as other issues of materials science may be obtained in the future. Acknowledgements The authors appreciate the assistance of S. Baier, M. Berheide, T. Gutt, and N. Piel in the initial phase of this work. References [1] H.Z. Massoud, E.H. Poindexter, C.R. Helms, Electrochem. Soc. 96 (1996) 97.
[2] E. Garfunkel, E. Gusev, A. Vul, Fundamental Aspects of Ultrathin Dielectrics on Si-based Devices, Kluwer Academic Publishers, The Netherlands, 1988. [3] C.J. So"eld, A.M. Stoneham, Semicond and Sci. Technol. 10 (1995) 215. [4] M. Morita, T. Ohmi, E. Hasegawa, M. Kawakami, M. Ohwada, J. Appl. Phys. 68 (1990) 1272. [5] F.J. Himpsel, F.R. McFeely, A. Taleb-Ibrahimi, J.A. Yarmo!, G. Hollinger, Phys. Rev. B 38 (1988) 6084. [6] T. Ohmi, T. Shibata, J. Electrochem. Soc. 92 (1992) 32. [7] K.F. Schuegraf, C. Hu, Semicond. Sci. Technol. 9 (1994) 989. [8] H. Hwang, W. Ting, B. Maiti, D.L. Kwong, J. Lee, Appl. Phys. Lett. 57 (1990) 1010. [9] R. Wrixon, A. Twomey, P. O'Sullivan, A. Mathewson, J. Electrochem. Soc. 142 (1995) 2738. [10] I.J.R. Baumvol, F.C. Stedile, J.J. Ganem, I. Trimaille, S. Rigo, Appl. Phys. Lett. 69 (1996) 2385. [11] I.J.R. Baumvol, J.J. Ganem, L.G. Gosset, I. Trimaille, S. Rigo, Appl. Phys. Lett. 72 (1998) 2999. [12] B.E. Deal, A.S. Grove, J. Appl. Phys. 36 (1965) 3770. [13] F.C. Stedile, I.J.R. Baumvol, I.F. Oppenheim, I. Trimaille, J.J. Ganem, S. Rigo, Nucl. Instr. and Meth. B (1999), in press.
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[14] J.J. Ganem, S. Rigo, I. Trimaille, G.N. Lu, Nucl. Instr. and Meth. B 64 (1992) 784. [15] S. Kubsky, L. Borucki, M. Berheide, S. Baier, H.W. Becker, F. Gorris, C. Grunwald, T. Gutt, G. KruK ger, M. Mehrho!, N. Piel, C. Rolfs, W.H. Schulte, Nucl. Instr. and Meth. B 113 (1996) 63. [16] S. Kubsky, Thesis, Ruhr-UniversitaK t Bochum, 1999. [17] M. Mehrho!, M. Aliotta, I.J.R. Baumvol, H.W. Becker, M. Berheide, L. Borucki, J. Domke, F. Gorris, S. Kubsky, N. Piel, G. Roters, C. Rolfs, W.H. Schulte, Nucl.Instr. and Meth. B 132 (1997) 671.
[18] F. Gorris, C. Krug, S. Kubsky, I.J.R. Baumvol, W.H. Schulte, C. Rolfs, in preparation. [19] F. Gorris, Thesis, Ruhr-UniversitaK t Bochum, 1999. [20] F. Roozeboom, History and perspectives of rapid thermal processing, Advances in Rapid Thermal Processing, NATO Conference Proceedings, Maratea, Italy, 1995. [21] L. Borucki, H.W. Becker, S. Kubsky, W.H. Schulte, C. Rolfs, Eur. Phys. J. A5 (1999) 327. [22] U. KoK hler, Ruhr-UniversitaK t Bochum, private communication, 1999.