1999 MRS Spring Meeting & Exhibit

Orlando Auciello 
Materials Science Division 
Argonne Natl Laboratory 
Bldg 212 
Argonne, IL 60439-4838 
630-252-1685

 

Eugene A. Irene
Dept of Chemistry
Univ of North Carolina
Chapel Hill, NC 27599
919-966-1652

 

Alan R. Krauss 
Dept of Matls Sci & Chemistry 
Argonne Natl Laboratory 
MSD/CHM-200 
Argonne, IL 60439
630-252-3520

 

J. Albert Schultz
Ionwerks
Houston, TX 77005
13-522-9880

* Invited paper

8:30 AM *U1.1 
IN-SITU, REAL-TIME, STUDIES OF FILM GROWTH PROCESSES USING ION SCATTERING AND DIRECT RECOIL SPECTROSCOPY TECHNIQUES. Vincent S. Smentkowski , General Electric Corporate Research and Development Center, Microstructure and Microanalysis Program, Schenectady, NY; O. Auciello, Materials Science Div, Argonne, IL; J. Im, A.R. Krauss, D.M. Gruen, Argonne National Laboratory, Materials Science and Chemistry Div, Argonne, IL; J.C. Holecek, K. Waters, J.A. Schultz, Ionwerks, Houston, TX.

Time-of-flight ion scattering and recoil spectroscopy (TOF-ISARS) enables the characterization of the composition and structure of surfaces with 1-2 monolayer specificity. It will be shown that surface analysis is possible at ambient pressures up to  10 mTorr using TOF-ISARS techniques; allowing for real-time, in-situ, studies of film growth processes. TOF-ISARS comprises three analytical techniques: ion scattering spectroscopy (ISS), which detects the backscattered primary ion beam; direct recoil spectroscopy (DRS), which detects the surface species recoiled into the forward scattering direction; and mass spectroscopy of recoiled ions (MSRI), which is a variant of DRS capable of isotopic resolution for all surface species ­ including H and He. The advantages and limitations of each of these techniques will be discussed. The use of the three TOF-ISARS methods for real-time, in-situ, film growth studies at high ambient pressures will be illustrated. It will be shown that MSRI analysis is possible during sputter deposition. It will be also be demonstrated that the analyzer used for MSRI can also be used for time of flight secondary ion mass spectroscopy (TOF-SIMS) under high vacuum conditions. The use of a single analyzer to perform the complimentary surface analytical techniques of MSRI and SIMS is unique. It will be demonstrated that the dual functionality of the MSRI analyzer provides surface information not obtained when either MSRI or SIMS is used independently.

 

9:00 AM *U1.2 
IN-SITU MONITORING AND CONTROL OF III-NITRIDES THIN FILMS SURFACE COMPOSITION BY TIME OF FLIGHT LOW ENERGY ION SCATTERING. A. Bensaoula , E. Kim, I. Berishev, Nitride Materials and Devices Laboratory, SVEC, The University of Houston, Houston, TX; J.A. Schultz, Ionwerks, Houston, TX.

Time of flight mass spectroscopy of recoiled ions (TOF-MSRI) and reflection high energy electron diffraction (RHEED) were used in-situ for the determination of the surface composition, crystalline structure, and growth modes of GaN thin films deposited by three different molecular beam epitaxy (MBE) methods: Nitrogen-plasma MBE, ammonia MBE, and chemical beam epitaxy (triethyl gallium (TEG) and ammonia). For all techniques, the substrate nitridation process is clearly determined by TOF-MSRI, which is shown to be more sensitive to onset of nitridation than RHEED. In the case of N2- plasma, the optimum temperature and process time for a maximum O to N exchange on a sapphire substrate was demonstrated. N2-plasma nitridation is shown to be more effective than ammonia gas. N2-plasma-MBE resulted in continuous crystalline buffer layers while non-crystalline discontinuous buffer layers were grown by ammonia-MBE. TOF-MSRI identified that nucleation layers could not be deposited directly on the sapphire substrate using all chemical precursors. The use of TEG and N2-plasma species allowed the growth of crystalline buffers. The growth temperature and N2 flow strongly affected the crystallinity of these buffer layers. Amorphous and polycrystalline buffer layers became island-like upon annealing to 900 C in vacuum while the crystalline buffer layers remained unchanged. Similarly the epilayers' crystal quality depended critically on the growth temperature and TEG flows. TOF-MSRI studies found that the residual carbon level is highly temperature dependent and influenced greatly the GaN surface morphology and crystallinity (from RHEED). It is only within a narrow temperature window, 800-825 C that the carbon level is at a minimum. Within that range, GaN epi-layers with strong band edge PL signals were obtained. Finally, TOF-MSRI was also utilized for the determination of the surface termination and polarity of GaN surfaces. Our results show that GaN thin films with a [000-1 ] surface polarity are obtained with plasma-MBE, those deposited with an all chemical process have [0001] surface polarity.

 

10:00 AM U1.3 
STUDIES OF INTIAL PHYSICAL VAPOR GROWTH STAGES OF STRONTIUM BISMUTH TANTALATE FILMS VIA IN-SITU TIME-OF-FLIGHT ION BEAM ANALYSIS. J. Im , A.R. Krauss, Materials Science and Chemistry Divisions, Argonne National Laboratory, Argonne, IL; O. Auciello, Materials Science Division, Argonne National Laboratory, Argonne, IL.

We have developed and are using a time-of-flight ion scattering and direct recoil spectroscopic (ToF-ISARS) technique that does not interfere with the deposition process and permits compositional and structural analysis of the growing films during physical vapor-deposition. Currently, we are studying the growth mechanism in relation to ion beam sputter-deposition (IBSD) of ferroelectric thin films. In this paper, we will report initial studies of the growth mechanism of SrBi₂Ta₂O₉(SBT) thin films produced by IBSD. The thin film growth mechanisms have been the subject of extensive research, because of their crucial role in the fabrication of a variety of thin film based devices. We will present results that demonstrate that we can use the ToF-ISARS technique for in situ characterization of the film growth mode. The three main film growth modes [Frank-van der Merwe (layer-by-layer), Stranski-Krastanov, and Volmer-Weber (three dimensional)], generally observed in vapor-deposition of thin films, can be identified by differences in the ToF-ISARS signals during growth by comparing the time evolution of the signal corresponding to the film and substrate. ToF-ISARS analysis suggests that perovskite SBT films deposited on Pt/MgO substrates at 700C in oxygen follows the Stranski-Krastanov growth model. We also investigated the metallic elements and oxygen incorporation during SBT film growth on a Pt/MgO substrate at 700C in oxygen (P(O₂) = 5x10^-4 Torr). The result indicates that the initial SBT layers (thickness <150) grown using the IBSD method on Pt/MgO at 700C in oxygen are Bi deficient, and Bi incorporation in the film becomes possible only when the film thickness is larger than 150. Studies of growth processes at different temperatures will be presented to show the growth mode vs. temperature dependence. Work supported by the U.S. Department of Energy, BES-Material Sciences, under Contract W-31-109-ENG-38.

 

10:15 AM U1.4 
COMBINED SPECTROSCOPIC ELLIPSOMETRY AND ION BEAM SURFACE ANALYSIS FOR IN-SITU REAL TIME CHARACTERIZATION OF COMPLEX OXIDE FILM GROWTH PROCESSES. A.H. Mueller , Y. Gao, E.A. Irene, University of North Carolina at Chapel Hill, Dept of Chemistry, Chapel Hill, NC; O. Auciello, A.R. Krauss, Argonne National laboratory, Materials Science and Chemistry Divisions, Argonne, IL; J.A. Schultz, Ionwerks, Houston, TX.

Composition, structure and growth dynamics determine the final properties of complex oxide thin films. In order to characterize these films in-situ, an integrated ion beam deposition system with spectroscopic ellipsometry (SE) and time of flight ion scattering and recoil spectrometry (ToF-ISARS) has been developed and built. ToF-ISARS incorporates mass spectrometry of recoiled ions (MSRI), direct recoil spectrometry (DRS) and ion scattering spectrometry (ISS). The use of differentially pumped ion beam and detector lines permit surface composition and structure analysis of thin films during processing while simultaneously SE provides the information on film thickness and the evolution of the optical properties. 
The sensitivity and resolution of the above techniques will be demonstrated by selected studies, including the investigation of complex structure barrier stability by MSRI, and the oxygen incorporation mechanism in c-axis oriented yttrium barium copper oxide (YBCO) films by angle resolved ion scattering spectrometry, each complemented by SE. The application of these unique techniques in the study of high dielectric constant materials such as barium strontium titanate (BST) and superconducting YBCO thin films will be presented, showing the advantages of in-situ thin film characterization provided by these complimentary techniques.

 

10:30 AM U1.5 
IN SITU REAL-TIME DEPTH PROFILING BY ELASTIC RECOIL DETECTION ANALYSIS AND ITS APPLICATIONS TO ION NITRIDING OF STAINLESS STEEL. Olaf Kruse , Stefano Parascandola, Wolfhard Moeller, Forschungszentrum Rossendorf, Dresden, GERMANY.

A dual beam experiment has been set up combining real-time depth profiling and low energy ion implantation in order to study the kinetics of ion nitriding. The choice of low energy ion implantation allows precise and independent control of the important ion nitriding parameters ion energy, ion current density, temperature and residual gas composition. Real-time depth profiling is achieved by Elastic Recoil Detection Analysis (ERDA) using an angle resolving ionisation chamber. A large solid angle (5 msr) allows a fast data acquisition and hence real-time depth profiling with a time resolution of about 30 s combined with a depth resolution of about 10 nm. The capabilities of the set-up and its limitations will be discussed. Measurements revealing the role of mechanisms relevant for the nitrogen transport during ion nitriding of stainless steel i.e. diffusion, sputtering, adsorption of oxygen from the residual gas will be presented.

 

10:45 AM U1.6 
IN-SITU CHARACTERIZATION OF THIN-FILMS BY THE FOCUSED ION BEAM. Sung H. Choi , Mike K. Pak, Shujun Cai, Kang L. Wang, UCLA, Electrical Engineering Dept, Los Angeles, CA; Martin S. Leung, Gary W. Stupian, Nathan Presser, The Aerospace Corp, Electronics Technology Center, El Segundo, CA.

The wide spread usage of Focused Ion Beam (FIB) has been recognized by many people working in the device characterization for long time. The need for fast and precise cross-sectioning and inspection of thin film structures in multi-layered integrated circuit devices continues to grow and may become a major force in the development of focused ion beam milling techniques. In our work, silicon-based multi-layered nanometer-size wires for Vertical Metal Oxide Semiconductor Field Effect Transistor (VMOSFET) was milled to produce a trench with a micron-scale rectangular area for a subsequent inspection by the Field Emission Scanning Electron Microscope (FESEM). The trench had a shape of an inverse Gaussian as expected from the beam profile. However, we found the shape of an inverse Gaussian became sharp, narrow, V-shaped, when the beam current was decreased to less than 10pA. The milling process was being monitored by the real-time Scanning Ion Microscope (SIM) equipped in the FIB. This in situ monitoring enabled the precise control of milling process. The small volume of trench was also prepared by the FIB milling process on the Ohmic contact of GaN Field Effect Transistor (FET) for a depth profiling of elements by a subsequent Auger analysis. The cross-sectioning view by the FESEM enabled the investigation of individual film thickness and morphology. We may also avoid the calibration step for defining the depth of sputtering during Auger analysis by preparing a small volume of trench by the FIB. This in situ FIB cross-sectioning process can be a convenient technique for a subsequent investigation by the FESEM and the Auger analysis, which provides a rapid and precise characterization of multi-layered thin film structures.

IN SITU ETCH RATE MEASUREMENTS BY -PARTICLE ENERGY LOSS. Y. Levy , W.J. Mandeville, V. Pacradouni, A. Schmalz, T. Tiedje, J.F. Young, Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, CANADA; M. Davies, Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, ON, CANADA; I. Kelson, School of Physics and Astronomy, Tel Aviv University, Tel Aviv, ISRAEL.

When -particles pass through thin films they lose energy by an amount proportional to the film thickness with the proportionality constant depending on the film composition. Thus, from measuring the energy loss one can determine the film thickness. We have applied this technique to measurements of the etch rate of various III-V semiconductor layers grown by molecular beam epitaxy. Prior to film growth, GaAs substrates were recoil implanted with the -emitting ²²⁴Ra isotope by exposure to a 10Ci source of ²²⁸Th. The implanted isotope decays with a half-life of 3.7 days which leaves a practical time window of about two weeks after implantation. Following growth the samples were etched in an electron cyclotron resonant etcher using a BCl₃/Cl₂/Ar gas mixture. As the film is etched the energy of the -particles emitted from the surface increases. By introducing a high resolution Si detector into the etcher we are able to measure changes in the -emission spectrum without removing the sample from the etcher. Thickness changes with an uncertainty of 5-10nm are obtained in 5 minute measurements at the end of each etch step. Some of the samples were also measured by SEM yielding results in good agreement with values obtained by the -particle measurements. As an example of an application of the technique we will describe measurements of the temperature dependence of the etch rate of GaAs in the 20-150C temperature range using optical bandgap thermometry to determine the substrate temperature. In a second example, we explore the application of the technique to etch rate of short pitch (250-500nm) gratings. In this case the shape of the -spectrum is sensitive to the profile of the etched trenches.

 

11:15 AM U1.8 
IN SITU GROWTH STUDIES OF ARTIFICIAL LAYERED (Ba,Sr,Ca)CuO_x ON QUASI-IDEAL SrTiO₃ SUBSTRATES BY HIGH PRESSURE RHEED. Gertjan Koster , Guus Rijnders, Horst Rogalla and Dave H.A. Blank, University of Twente, Applied Physics, Low Temperature Division, NETHERLANDS.

The layered structure of oxides, like the high-Tc cuprates, has been topic of research for some years now. The possibility to control thin film deposition on an atomic level has made fabrication of artificial structures and junctions accessible by depositing atomic layers or molecular blocks sequentially. Perfectly smooth substrate surfaces are hereby a prerequisite. In this presentation, latest results using our method to obtain single terminated, quasi-ideal SrTiO₃ surfaces will be presented. Using Pulsed Laser Deposition (PLD), different perovskite oxide materials have been deposited on these SrTiO₃ substrates. With in situ high pressure Reflection High Energy Electron Diffraction the growth at different temperatures and oxygen pressures has been studied. Ex situ AFM is used to study the morphology after deposition. Since most of the oxides under investigation are not stable in a wide oxygen pressure and temperature range, to one usually cannot observe all possible growth modes like step-flow, layer-by-layer (2D) or island growth (3D) by just varying these parameters. Usually, due to a limited diffusivity, 2D growth modes are inhibited. An exception is the homo-epitaxy on SrTiO₃, where it possible to choose in a wide range of temperatures and pressures and, both, step flow as well as layer-by-layer growth have been realized. Here we present a new approach in obtaining 2D growth, regardless of the deposition conditions implied by the way how we deposit material. Alternating intervals of high super-saturation depositing of one unit cell layer with intervals of lower super-saturation we are able to force a layer-by-layer growth mode. This approach is in principle unique for PLD. We applied this technique to fabricate the layered infinite structure (Ba,Sr,Ca)CuO_x.

 

11:30 AM U1.9 
QUANTITATIVE RHEED ANALYSIS OF POLYCRYSTALLINE FILM TEXTURE AND ORIENTATION. J.W. Hartman and H.A. Atwater, Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA.

Reflection high energy diffraction (RHEED) is one of the most widely used in-situ diagnostic techniques for single crystal growth but has not been used widely for analysis of polycrystalline films. We have developed kinematical RHEED simulation algorithms for polycrystalline films that are able to simulate the effects of grain size, surface roughness, and distribution of crystallographic texture and in-plane orientation on RHEED patterns. In our simulations of kinematic-RHEED from mosaic crystals of MgO, the shapes of the high order diffraction spots reveal the mosaic spread in the grain texture and in-plane orientation. By studying higher order diffraction spots, we find that we can discern the widths of these texture distributions to within 1 degree. Comparisons are made with experimental RHEED data for biaxially textured MgO deposited by ion beam assisted deposition (IBAD). The validity of the kinematical scattering approximation and the role of dynamical scattering effects will be discussed. 
 

1:30 PM *U2.1 
REAL TIME CHARACTERIZATION OF SURFACES AND FILM GROWTH PROCESSES BY SPECTROSCOPIC ELLIPSOMETRY AND POLARIMETRY. R.W. Collins , Joohyun Koh, A.S. Ferlauto, P.I. Rovira, Center for Thin Film Devices, Materials Research Laboratory and Dept. of Physics, The Pennsylvania State University, University Park, PA; Ilsin An, Dept. of Physics, Hanyang University, Ansan, KOREA.

Over the past ten years, we have developed and applied rotating-polarizer multichannel ellipsometry as a real time probe of surfaces and thin films. Full spectra consisting of  50 pairs of (, ) values from 1.5 to 4.5 eV can be collected in times as short as 16 ms during surface processing or film growth. At this speed, the precision in (, ) is sufficient for monolayer sensitivity. For spectra consisting of  100 pairs of (, ),  0.1 monolayer sensitivity ( 0.01) can be achieved with acquisition times as short as 100 ms. Although very powerful, the rotating-polarizer instrument has some disadvantages. In particular, it is prone to errors that arise in measuring non-ideal samples when  is near 0 or 180 (i.e., for nearly linear polarization). Such non-idealities can result from macroscopic non-uniformities such as thickness gradients, polycrystalline surface texture, and macroparticles. In addition, the rotating polarizer instrument loses sensitivity for bulk dielectric surfaces and film growth on such surfaces. As a result, we have developed a rotating compensator multichannel polarimeter that provides spectra in the four components of the (unnormalized) Stokes vector that is generated upon reflection of polarized light from the sample surface. Thus, for randomly polarized light superimposed onto a pure polarization state, (, ), p, R can be determined, where p is the degree of polarization and R is the sample reflectance. This approach provides much greater immunity to sample non-idealities, and, in fact, features such as film thickness gradients and macroscopic structure can be quantified from analyses of p and R, in addition to the optical properties, thickness, and microscopic structure from analyses of (, ). Examples of this approach will be taken from our studies of diamond thin film growth on seeded substrates and amorphous semiconductor solar cell fabrication on glass coated with textured SnO₂. Future directions in real time optical spectroscopy will also be highlighted.

 

2:00 PM *U2.2 
MODELING AND REAL-TIME PROCESS MONITORING OF ORGANOMETALLIC CHEMICAL VAPOR DEPOSITION OF III-V PHOSPHIDES AND NITRIDES AT LOW AND HIGH PRESSURES. Klaus J. Bachmann ^a, Beatriz Cardelino^b, Carlos Cardelino^c, Sonya McCall^a and Craig Moore^d, ^aDepartment of Materials Science and Engineering, North Carolina State University, Raleigh, NC; ^bSpelman College, Atlanta, GA; ^cSchool of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA and ^dNASA George C. Marshall Space Flight Center, Huntsville, AL.

In this presentation we compare the flow dynamics in two reactors for high pressure organo-metallic chemical vapor deposition, labeled for the purpose of discussion A and B, respectively, that have been built in the authors laboratory. Reactor A employs a fused silica inner shell that is surrounded by a pressure bearing stainless steel enclosure with differential pressure regulation between inside and outside of the silica reactor. It is rated for operation in the pressure range 0.06  p  6 bar. Real-time process monitoring in this reactor is possible by p-polarized reflectance spectroscopy (PRS) or near normal incidence reflectance spectroscopy (NIRS). In both cases this requires passage of the probe beam through the dense vapor atmosphere, which, in turn, mandates provision of purge flows of pure carrier gas to avoid clouding of windows. Unfortunately mixing of the purge flows with the process gas stream at the location of the substrate cannot be avoided in this case. In order to counter adverse effects of this flow on process uniformity the substrate is rotated. However, this causes problems with substrate positioning and wobble, both contributing to tripping of eddies at low Reynolds number flow and interferring with real-time process monitoring. Also, reactor A - representing a glass blown structure - suffers from tripping of eddies on wall irregularities for low Reynolds number flow. 
Reactor B employs an inner machined and polished alumina shell that is far less likely of tripping eddies, and an outer stainless steel shell with internal pressure equalization. It is rated for 1  p  100 bar, and avoids the use of purge flows at the site of the substrate by replacing PRS by attenuated total reflection (ATR) spectroscopy. Using two single pass substrate plates that are incorporated into the top and bottom channel walls provides access from the back thus eliminating the need for the laser probe beam to cross the flow channel. Reactor B also provides for gas phase analysis by absorption spectroscopy and for the characterization of the flow by recording/line shape analysis of scattered light intensity. Using results of PRS studies under low- pressure conditions for the identification of critical stages in process evolution. The present state of process modeling of OMCVD of III-V compounds and necessary corrections to accommodate to high-pressure conditions are discussed. This includes a critical assessment of input parameters for simulations of OMCVD growth of selected group III phosphides and nitrides.

 

2:30 PM U2.3 
EXTENSION OF MULTICHANNEL SPECTROSCOPIC ELLIPSOMETRY INTO THE ULTRAVIOLET FOR REAL TIME CHARACTERIZATION OF THE GROWTH OF WIDE BANDGAP MATERIALS FROM 1.5 TO 6.5 eV. J.A. Zapien , R.W. Collins and R. Messier, Department of Engineering Science and Mechanics and the Materials Research Laboratory, The Pennsylvania State University, University Park, PA.

Multichannel spectroscopic ellipsometry is a powerful tool for the characterization of surfaces and thin films. Current state-of-the-art instruments can collect ellipsometric spectra (,) from 1.5 to 4.5 eV, providing monolayer sensitivity with acquisition times as short as  20 ms. The spectral range limitation of the current instruments result from the strong Xe source output in the visible, and its sharply falling output and poor overall system response in the ultraviolet (uv). This leads to a problem characteristic of broadband multichannel systems, namely, detection of weak signals in the uv in the presence of significant stray light from the visible range. Here, we report the results of the first successful effort to extend multichannel spectroscopic ellipsometry into the ultraviolet region above 5 eV. Key modifications in system design include (i) incorporation of a see-through deuterium (D₂) lamp that permits a tandem Xe/D₂ lamp design configuration for flat spectral output from 1.5 to 6.5 eV, (ii) MgF₂ polarizers, and (iii) a grating spectrograph blazed at 250 nm with internally mounted order-sorting filters. With this new design, we can collect 138-point ellipsometric spectra over the range from 1.5 to 6.5 eV with a minimum acquisition time of 25 ms (one optical cycle), while retaining sub-monolayer sensitivity. In this first report, we describe instrumentation issues including stray light avoidance, image persistence correction, and order sorting.