Papers

AVS1997 Session AS-ThM: New Approaches of Established Techniques

Thursday, October 23, 1997 8:20 AM in Room J2

Thursday Morning

Start	Invited?	Item
8:20 AM		AS-ThM-1 A Study of the Epitaxial Growth of SrTiO₃ on SrTiO₃(001) using a Novel Oblique-Incidence Reflectance Difference Technique. X.D. Zhu (University of California, Davis); H.B. Lu, G.Z. Yang, D.Z. Zhang (Chinese Academy of Science, P.R. China) In a measurement of pulsed laser deposition of SrTiO₃ on SrTiO₃(001), we demonstrate that the difference in the relative change between the reflectivities for s- and p-polarized light can be effectively used in monitoring the growth mode and quality of thin films at atomic level. This reflectance-difference technique has the same sensitivity (0.01 monolayer), the real-time monitoring capability, and the spectral resolution as the conventional reflectance difference spectroscopy (RDS) developed by Aspnes and coworkers. Complementary to the conventional RDS, the present technique does not rely on the presence of optical anisotropy within the surface plane and therefore is applicable to investigation and control of thin film growth on all surfaces. Compared to the surface photo-absorption technique developed by Kobayashi and Horikoshi, our technique improves the signal-to-noise ratio by at least another order of magnitude by reducing the background to below R/R = 10^-5.
8:40 AM		AS-ThM-2 Application of Ellipsometry in Studying the Reactions of Li with Small Molecules in UHV F. Kong, G.R. Zhuang, K. Wang, P.N. Ross, F. McLarnon (Lawrence Berkeley National Laboratory and University of California, Berkeley) Ellipsometry in combination with Auger electron spectroscopy has been used to understand the surface chemical reactions of clean Li and small molecules, such as O₂ and H₂O, in UHV. We demonstrated that spectroscopic ellipsometry can be a valuable tool to follow the formation and interconversion of possible reaction products, especially the evolution of the overlayer structure produced by these chemical processes. We found that the clean Li surface is very reactive toward oxygen, with essentially a sticking coefficient of unity at the initial stage of oxidation at 5 x 10^-8 torr O₂. Oxidation of the entire film(75 Å) maximum) to lithium oxide proceeded with an unit reaction probability. A model of oxidation with in-plane contraction of the oxide film (continuously exposing fresh Li) is proposed to explain both the high rate of reaction and observed variation of ellipsometric parameters with time of exposure to O₂. Based on the knowledge of optical properties of Li and lithium oxide, we extended the same methodology to investigate the Li and O₂. Our results revealed that LiOH is formed after the Li surface is exposed to H₂O at low temperature , but LiOH is in a metastable state and is slowly converted into lithium oxide. The conversion pathway of LiOH and its implication for Li batteries be will be discussed.
9:00 AM		AS-ThM-3 Ellipsomicrosopy for Surface Imaging - A Novel Tool to Investigate Surface Dynamics G. Haas, T.D. Pletcher, G. Bonilla (Purdue University); H.H. Rotermund (Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany); J. Lauterbach (Purdue University) Ellipsometry measures relative shifts in the polarization state upon oblique reflection and it has shown to be sensitive to sub-monolayer coverages of adsorbates on metal surfaces. Ellipsomicroscopy for surface imaging (EMSI) is a novel powerful tool for in situ microscopy, which spatially resolves changes in the ellipsometric parameters of surfaces by measuring changes of the degree of polarization of laser light reflected from a surface. The lateral resolution is around 5 microns, the temporal resolution is simply restricted by the speed of the CCD camera recording the images. The high surface sensitivity, in conjunction with the applicability to a wide range of ambient media, makes EMSI particularly attractive for study of surface phenomena. Unlike the majority of surface-sensitive, electron spectroscopy based techniques, light based techniques can be used to investigate material surfaces in situ, e.g., during surface etching processes, deposition of materials, or during catalytic reactions. All optical parts are installed outside the process chamber allowing easy access for tuning and maintenance. We will present a variety of examples of EMSI measurements performed on the spatio-temporal pattern formation on platinum catalysts during CO oxidation at reactant pressures between 1 and 150 mTorr.
9:20 AM		AS-ThM-4 Optical Determination of Free Carrier Profiles in Silicon using IR Ellipsometry T.E. Tiwald, D. Thompson, J.A. Woollam (University of Nebraska, Lincoln) The distribution of free carriers in semiconductors were determined from IR-variable angle spectroscopic ellipsometric data by fitting the parameters of various optical models to the free carrier effects in the spectra. Different carrier distribution models were applied to ellipsometric data from ion-implant, doped epilayer and semiconductor junction samples -- starting with single, uniformly-doped layers; and progressing to the Gaussian, Pearson type IV, and efrc(x) distributions. All models were based on the classical Drude response, and the continuous distributions were represented as graded-multilayers. The effects of dopants on the effective mass and mean scattering time are analyzed. For each model, the results are discussed in terms of correlation, goodness of fit (mean squared error), and compatibility with physical measurements such as spreading resistance probe and secondary ion mass spectrometry.
9:40 AM		AS-ThM-5 Auger Spectroscopy in Semiconductor Defect Review: Applications and Artifacts P.L. King (Advanced Micro Devices) Defects which are much larger than about a third of the critical dimension of an integrated circuit are considered as having a real potential to be killer defects. With critical dimensions shirinking towards quarter micron geometries and defects of interest trending toward 0.1 micron, it becomes increasingly challenging to perform defect review using conventional SEM/EDS. Due to its superior surface sensitivity and analytical lateral resolution, Auger electron spectroscopy (AES) has always offered an alternative technique for the review of small particles. With the recent introduction of Auger hardware supporting full 200 mm wafer review, Auger spectroscopy has become a more practical and increasingly attractive technique for semiconductor defect review. This talk reviews the principles of Auger spectrscopy and describes the state of the art in Auger hardware geared toward the semiconductor industry. Examples of AES successfully applied to semiconductor defect review are described. Artifacts and difficulties in the application of AES are recognized and equal attention is devoted to the description of problems commonly encountered during AES analysis. The impact of sample charging, electron beam damage, electron beam scattering and chemical effects on quantification are discussed.
10:20 AM		AS-ThM-7 Applications of a 200mm Wafer-Capable AES Defect Review Tool H.A. Krasinski, D.K. Fillmore (Micron Technology, Inc.) Although 200mm wafer-capable scanning electron microscopes and defect review tools (DRTs) have been available for some time, commercially available 200mm wafer-capable Auger electron spectroscopy (AES) analytical systems have only become available in the last 24 months. The ability to perform AES analysis of large samples with an automated stage offers unique opportunities for detailed elemental characterization of the typical films, particles, and defects found in semiconductor manufacturing. In this paper, applications of a commercially available 200mm wafer-capable DRT using AES analytical techniques in a cleanroom installation are presented. These include but are not limited to particle characterization on 200mm wafers, automated multiple sample analysis, in-line film monitoring, and methods of charge neutralization.
10:40 AM		AS-ThM-8 Imaging of Doped Silicon Using High Energy Resolution Auger Electron Spectroscopy G.W. Jones, K.S. Robinson, J. Wolstenholme (VG Scientific, United Kingdom) A small energy shift is observed in the high energy resolution Si KLL Auger peak depending upon the type of dopant (n-type or p-type) in silicon. Such shifts (typically 0.7eV) are easily measured using an Auger electron instrument equipped with a high energy resolution (0.02%) Spherical Sector Analyser. In this paper we show how areas containing such dopants may also be spatially resolved using high energy resolution Scanning Auger Mapping (SAM). Silicon(111) single crystal was doped with phosphorus during growth with a bulk concentration of 1x1016 atom cm-3. The dopant was thermally diffused to the surface to produce a diffusion layer of approximately 1 m. The sample was then fractured in vacuum to reveal a fresh cross section surface. High energy resolution Si KLL spectra show a difference in peak energy due to the dopant. Non-Linear Least Squares Fitting (NLLSF) of Si KLL spectra taken as a spectrum linescan across the differently doped regions enables an energy shift profile to be obtained. High energy resolution SAM images acquired at energies corresponding to the two peak positions clearly differentiate the differently doped region from the substrate.
11:00 AM		AS-ThM-9 2D Imaging of Surface Morphology by Energy-analyzed Secondary Electrons and Auger Electron Spectroscopy for Stepped Si(111) Surfaces M. Suzuki, K. Mogi (NTT Advanced Technology Corp., Japan); Y. Homma (NTT Science and Core Technology Labs, Japan) Secondary electron microscope (SEM) has conventionally been used to observe surface morphology. Recently, it has been reported that a monoatomic step structure can be imaged and different crystallographic surface structures on an atomically smooth surface distinguished by SEM¹. There have been few reports on the detailed relationship between the imaging mechanism and the energy distribution of secondary electrons, though, so we have experimentally investigated two-dimensional imaging of stepped Si surfaces by energy-analyzed electrons. A stepped Si(111) surface was prepared by direct-current heating to around 1250oC in UHV. The stepped surface consisted of atomically flat terraces and step bands where atomic steps bunched. The specimen was transferred to the manipulator stage for Auger analysis from a special stage mounted in a scanning Auger microscope (SAM) instrument for high-temperature annealing. An electron-emission spectrum, generated with a field-emission-type electron gun, was observed with the SAM instrument. The electron-emission spectrum obtained in the direct mode included only characteristic Auger peaks of silicon and a secondary-electron (SE) peak. It was impossible to image the stepped morphology from the electrons energy-analyzed at the peak positions of Si KLL, Si LVV, and SE when the peak intensities were simply defined as the peak height. We found, however, that the stepped structure could be imaged by using only the energy-analyzed SEs when the intensity was defined as the difference between the peak height and the background intensity. The image from the energy-analyzed SEs was in good agreement with the conventional SEM image obtained with non-energy-analyzed SEs. Also, the electron-emission spectrum obtained from the atomically flat Si(111) terrace between the step bands when using the primary electron beam will be compared with the spectra from sputtered clean silicon surfaces in the conference presentation. ¹e.g. Y. Homma, M. Suzuki, and M. Tomita, Appl. Phys. Lett., 62, 3276 (1993).
11:20 AM		AS-ThM-10 SIMS Technique for SiGe/Si and SiGeC/Si Heterojunctions and Superlattices A. Li-Fatou, M. Sardela (Charles Evans & Associates) This paper investigates new SIMS analytical approaches applied to the characterization of SiGe/Si(Ge) heterojunctions. A comparative study of three different SIMS analysis methods is presented. Use of Cs primary ion beam in the analysis of CsX+ ions was investigated at different primary ion beam energies (3 and 5.5 keV, respectively). The results showed a linear dependence of CsGe+ to CsSi+ ion yield ratio with the alloy composition but the technique had the drawback of poor sensitivity to major alloy dopants (such as boron in heterobipolar devices). The low-energy oxygen (LEOx) primary bombardment technique provided superior depth resolution and excellent sensitivity to dopants. In this case, reduced matrix effects allowed for accurate stoichiometry analysis with excellent results in the most important range of Ge concentration (up to 30 at%). Improvements in depth resolution were achieved by reducing the angle of incidence and the impact energy maintaining enhanced ion yield associated with oxygen bombardment. Depth profiles of SiGe/Si multilayer junctions showed in this case an improvement in depth resolution by a factor of 2 over conventional approaches with dramatic consequences in the determination of interfacial abruptness in this system. The method was also employed in the SIMS characterization of short-period SiGe/Ge superlattices. In these particular systems, ultimate depth resolution and minimum knock-on effects are crucial requirements in order to identify Ge/Si intermixing in the nanometric scale. Depth profiles from 50-periods 3-nm-thick Si0.24Ge0.76/Ge superlattices acquired with the LEOx SIMS technique exhibited clear layer separation with excellent depth resolution maintained up to the 50-th alloy layer in the structure.
11:40 AM		AS-ThM-11 A Plasma-Polymerized Protective Film for TEM Specimen Preparation by FIB Etching N. Kato, N. Miura, N. Tsutsui (ITES, IBM Yasu, Japan) Focused ion beam (FIB) milling has been intensively used for cross-sectional transmission electron microscopy (X-TEM) specimen preparation. In FIB fabrication, a protective layer is normally formed on the specimen surface by ion-beam-assisted metal deposition to avoid the damage caused by highly accelerated ion beams. However, the metal deposition itself induces damages in the specimen. It causes problems when the region near the surface of a specimen is of interest. We have investigated the subsurface damages through X-TEM observations. It is found that the induced defects extend over several hundred angstroms from the surface. In the case of a single crystal of silicon, a several-hundred- angstrom layer just below the surface is converted into an amorphous layer. We propose the use of a plasma- polymerized organic film instead of the deposited metal. This film is formed by plasma-polymerizing of methane and ethylene. We optimized the condition of the film formation so that the damage to the specimen surface was minimized. We used the organic film as a protective layer and fabricated TEM specimens without damaging the fine subsurface structures. As for a single crystal of silicon, no amorphous layer was observed, and even the several-angstrom-thick native oxide layer on the crystalline silicon could be observed. Application of the plasma-polymerized film allows closer observation of the specimen subsurface by FIB-TEM methods.