AVS2001 Session EL-ThA: In-Situ Semiconductor Characterization

Thursday, November 1, 2001 2:00 PM in Room 124

Thursday Afternoon

Start	Invited?	Item
2:00 PM	Invited	EL-ThA-1 In-situ Analysis, Monitoring and Control of III-V-Semiconductor Epitaxial Growth W. Richter (TU Berlin, Germany) Epitaxial growth of semiconductor structures requires control of the growth process on the atomic scale. This, however, is difficult to achieve by just controlling macroscopic growth parameters like partial pressures (fluxes) and temperatures. It is thus desirable to have in real time microscopic information from the growing surface for a direct analysis and conseqently control on the ongoing growth process. Optical probes offer in this respect important advantages: they are in general non invasive, they can be fast in obtaining the needed information and finally they can be applied in all growth environments (vacuum or gasphase based growth methods). The "optical" disadvantage of having a low spatial resolution can be overcome quite often by extra "calibration" measurements with high resolution spatial probes like STM, AFM, LEED. I will discuss first the analysis of reconstructed surfaces by reflectance anisotropy (RAS). Simultaneous measurements in MBE and MOVPE together with theoretical calculations allow to correlate optical spectra with surface reconstructions. Based on these basic data it is possible to describe adsorption and desorption processes of the basic constituents (Group III and V elements). Moreover, growth in dependence of partial pressure and temperature can be described via different modes in a flux - temperature phase diagram, allowing thus to define the surface growth status nearly independent of the special epitaxial equipment. Monitoring and control of device growth seem to be now possible.
2:40 PM		EL-ThA-3 Direct Numerical Inversion of Real-time Ellipsometric Data for Monitoring and Control of Optical Filter Deposition D. Kouznetsov, A. Hofrichter, B. Drevillon (Ecole Polytechnique CNRS, France) In situ ellipsometry is well known to be one of the most sensitive, non-disturbing tools for controlling and monitoring the growth of thin films. However the rapid advances in ellipsometric instrumentation, especially the increasing real time spectroscopic capabilities of state of art ellipsometers necessitate the development of new algorithms for an optimal assessment of the available information. In this work we apply a new direct numerical inversion algorithm for the real-time reconstruction of homogeneous and inhomogeneous refractive index profiles for the monitoring and the control of optical thin film depositions. The algorithm is based on a second order Taylor decomposition of the coefficients of the Abeles matrices of the newly grown layer. The variation of the real-time spectroscopic ellipsometry data are expressed as polynomial functions depending on the dielectric constant and the thickness of the newly grown layer. This allows a direct inversion of the ellipsometric signal and assures the high speed of the algorithm. Typical inversion times are 150 ms for 16 wavelength with a typical precision of 0.02 for the refractive index and less than 2% error in the reconstructed thickness. The algorithm is successfully applied for the real-time material characterization of transparent and weakly absorbant silicon oxynitrides deposited by plasma enhanced chemical vapor deposition. The complete process space can thus be explored in one single run. Combined with traditional ellipsometric control algorithm this method allows to grow multilayer and gradient optical coatings with high accuracy in respect to initial design.
3:00 PM		EL-ThA-4 Integrated Multiwavelength Parallel-processing Rotating-compensator Ellipsometer/Reflectance-difference Spectrometer for Real-time Measurement and Control of OMCVD Growth K. Flock, M. Ebert, K.A. Bell, D.E. Aspnes (North Carolina State University) Previous research has underlined the importance of real-time diagnostics for epitaxial growth of semiconductor materials and structures. We describe an optical system integrated with a modified rotating-sample commercial organometallic chemical vapor deposition (OMCVD) reactor that combines the functions of spectroscopic ellipsometry (SE) and reflectance-difference spectroscopy (RDS) in a single optical path to return information about growth rates, compositions, and film thicknesses (SE) and surface chemistry (RDS). The system is essentially a rotating-compensator ellipsometer for increased diagnostic power relative to rotating-polarizer or rotating-analyzer designs, with the rotation rate of the compensator synchronized to that of the sample. Synchronization reduces noise in the SE data by eliminating random optical-anisotropy contributions and also allows us to measure the non-normal-incidence RD spectrum directly. The original sample spindle was replaced with a design that allows sample runout to be adjusted dynamically to provide additional improvements in signal-to-noise ratios. A 1024-element photodiode array detector with a 0.2 m spectrograph provides a spectral range of 240 to 840 nm. Software designed to optimize both real-time response and repetition rate permits complete spectra to be obtained at a 6 Hz rate with a 400 MHz computer. Using a prototype rotating-polarizer system we demonstrated sample-driven closed-loop feedback control of a graded-composition InGaP structure grown on GaAs, where the composition was varied parabolically with thickness. During the growth of this structure it was necessary to reduce the flow through the trimethylindium by about half, which the RD data indicate is due to a change of incorporation efficiency due to an observed change of surface reconstruction.
3:20 PM		EL-ThA-5 In-situ Monitoring of Ag Film Growth on Si(111)7x7 Surface by Optical Second Harmonic Generation H. Hirayama, T. Kawata, K. Takayanagi (Tokyo Institute of Technology, Japan) A growth of Ag films on the Si(111)7x7 surface was monitored by optical second harmonic generation (SHG). Ag-coverage dependent intensity oscillation was observed in p-polarized SHG signals with 1.20,1.30 and 1.40 eV pump photon energy. As has been reported recently by Pedersen et al, the SHG intensity oscillated with the Ag coverage. However, on the contrary to the previous report, the first peak in the oscillations was observed at 3ML for all the pump energy. The peak position was independent on the pump energy. Meanwhile, the subsequent peaks shifted toward lower coverage with the increase in the pump photon energy. A detailed comparison of the SHG intensity and AFM images of the Ag film grown on the Si(111) substrate showed that many three-dimensional Ag islands nucleated at the coverage for the first peak. The surface morphology changed to be two-dimensional smooth one at the coverage for the subsequent peaks. The AFM images and the SHG spectrum taken at the coverages of the peaks showed that the first peak in the SHG intensity oscillation corresponded to the local plasmon resonance with the 3D Ag island formation. The subsequent peaks were caused by the transitions with the quantized electronic states confined in the thin, flat Ag films. The characteristic transitions between the electronic states localized at the Ag/Si(111)7x7, 1x1 and √3x√3 interfaces were also detected as resonant peaks in the s-polarized SHG spectrum with the excitation photon energy ranged from 1.05 to 1.70eV.
4:00 PM		EL-ThA-7 RHEED Intensity Oscillation during Thermal Oxidation on Si(001) Surface with O₂ Y. Takakuwa, F. Ishida (Tohoku University, Japan) Auger electron spectroscopy combined with reflection high energy electron diffraction (RHEED-AES) was applied to investigate the surface reaction dynamics during thermal oxidation on Si(001)2x1 surface with O₂. In the RHEED-AES measurement, O KLL Auger electrons excited by a grazing-incident electron beam for RHEED observation were detected, enabling to observe simultaneously the SiO₂ coverage and surface morphology. In the temperature region of two-dimensional growth of SiO₂ islands at 630~800°C as confirmed by the time evolution of O KLL Auger electron intensity, an oscillatory behavior in RHEED half-order spot intensity was observed, indicating that etching of the surface occured between SiO₂ islands. The etching rate obtained by the oscillation period was 0.039 ML/s at an O₂ pressure of 2x10^-7 Torr independently of the temperature and SiO₂ coverage, and increased in proportion to the O₂ pressure, suggesting that the etching reaction was rate-limited by O₂ supply. Since part of adsorbed oxygen atoms is consumed for nucleation and two-dimensional growth of SiO₂ islands in this temperature region, the assigned rate-limiting reaction means that the etching reaction takes place not only through SiO desorption but also through SiO₂ growth, that is, incorporation of Si atoms into SiO₂ islands. The fraction of the amount of incorporated Si atoms into SiO₂ islands to that of etched Si atoms will be discussed against SiO₂ coverage.
5:00 PM		EL-ThA-10 SR-TXRF for the Investigation of the Deposition Mechanism of Trace Cu Impurities on Si Wafer Surfaces K. Baur (Stanford Synchrotron Radiation Laboratory); T. Homma, J. Tsukano (Waseda University, Japan); M. Watanabe (Komatsu Electronic Metals, Japan); A. Singh, S. Brennan, P. Pianetta (Stanford Synchrotron Radiation Laboratory) Total Reflection X-ray Fluorescence (TXRF) spectroscopy using synchrotron radiation is one of the most powerful techniques for trace impurity analysis on Si wafer surfaces. In addition, among the more sensitive techniques, it is the only one that is non-destructive. We present the status of the transition metal analysis activity at the Stanford Synchrotron Radiation Labororatory (SSRL) which has matured to a point where a facility exists at which semiconductor companies are able to perform industrially relevant measurements at state of the art detection limits. This facility features clean wafer handling and automated data acquisition making routine analytical measurements possible. The best sensitivity demonstrated to date is 3.4 E7 atoms/cm2 for a 5000 second count time corresponding to 7.6 E7 atoms/cm2 for a standard 1000 second count time. This is more than a factor of 100 better than what can be achieved with conventional TXRF systems. A new development at SSRL is the investigation of the deposition mechanisms of trace metal impurities during a wet cleaning process on Si wafer surfaces. This is considered to be influenced by the oxidation conditions at the silicon wafer surface. This study requires high surface sensitivity and renders Synchrotron Radiation TXRF the technique of choice. We will present our results on the deposition mechanism of Cu trace impurities on Si wafers immersed into ultra pure water, focussing on the correlation between the deposited Cu concentration and the amount of dissolved oxygen present in the ultra pure water.