Abstract:
Computerized acquisition and frequency-domain analysis of dynamic reflection high-energy electron diffraction (RHEED) intensity data is obtained during growth by molecular-beam epitaxy (MBE). Rapid, accurate determination of the frequency of RHEED oscillations can be obtained not only when these oscillations are well resolved, but also when the growth conditions yield oscillations that are too poorly resolved to permit frequency analysis by conventional procedures. The method has been used to study transients in the growth of AlGaAs on GaAs substrates and also to investigate the hetero-epitaxial growth of GaAs on Si.

Description:
GOVERNMENT SUPPORT 
     The Government has rights in this invention pursuant to Contract Number F19628-85-C-0002 awarded by the Department of the Air Force. 
    
    
     COPYRIGHT 
     Appendix A of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     BACKGROUND ART 
     Measurement of film thickness during growth is required in a number of arts and especially in the growth of thin film semiconductors. Such films may comprise amorphous or polycrystalline or epitaxial films. Such films may be grown or deposited in a variety of reactors. One such reactor is a molecular beam epitaxy (MBE) reactor. 
     Recently, the examination of images formed by in situ reflection high-energy electron diffraction (RHEED) intensity data has become one of the most useful tools for the analysis of growth by MBE. Static RHEED images obtained when growth is interrupted, yield detailed information on surface reconstruction. Analysis of dynamic RHEED images (&#34;RHEED oscillations&#34;), plotted as a function of intensity versus time, and obtained as growth is taking place, can be employed to determine epitaxial growth rates and therefore alloy compositions. (See U.S. Pat. No. 4,855,013 issued Aug. 8, 1989.) This method is applicable because epitaxial growth causes a variation in surface roughness on the atomic scale that under favorable conditions produces well-resolved oscillations in the RHEED intensity whose period is the time required for the growth of one complete monolayer. Frequently, however, the conditions used for epitaxial growth yield oscillations that are not sufficiently obvious to permit the period to be obtained directly from plots of intensity versus time. 
     A need exists, therefore, for a method and apparatus which would permit analysis of RHEED or other oscillations under such unresolved growth conditions. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, digital signal processing and frequency domain analysis is used to enable the a growth oscillation frequency to be extracted from data obtained under unresolved growth conditions, as well as from data exhibiting well-resolved oscillations. This system has the further advantage of allowing the frequency to be determined from even a single oscillation, so that it can be used for the investigation of growth transients. The system is described in connection with RHEED oscillations generated during MBE growth, but may find similar application in any growth system which exhibits detectable growth oscillations over time. 
     In accordance with the invention a layer of material being grown in a reactor is subjected to an energy beam. Oscillatory variations in the beam resulting from beam incidence on the growing material, are detected. The detected time varying pattern is converted to a frequency varying pattern to disclose pattern characteristics not readily observable in the time domain. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a measurement system in accordance with the invention. 
     FIG. 2 is a plot of RHEED intensity versus time for a GaAs on GaAs MBE growth experiment using prior art techniques. 
     FIG. 3 is a plot of the power spectrum, i.e., power versus frequency of the RHEED data for the same experiment using the method and and apparatus of the invention. 
     FIG. 4 is a plot of RHEED intensity versus time for a GaAs-on-GaAs growth experiment which did not yield well-resolved oscillations. 
     FIG. 5 is a plot of the power spectrum for the experiment of FIG. 4. 
     FIG. 6 is a plot of RHEED intensity versus time for a GaAs-on-GaAs growth experiment showing quickly damped oscillations. 
     FIG. 7 is a plot of the power spectrum for the experiment of FIG. 6. 
     FIG. 8 is a plot of RHEED intensity versus time for the last five seconds of the experiment of FIG. 6. 
     FIG. 9 is a plot of the power spectrum for the data of FIG. 8. 
     FIG. 10 is a plot of the power spectrum for a GaAlAs.-on-GaAs growth experiment. 
     FIG. 11 is a cycle-by-cycle plot of RHEED frequency for the experiment of FIG. 10. 
     FIG. 12 is a plot of RHEED intensity versus time for a GaAs-on-Si experiment. 
     FIG. 13 is a power spectrum for the experiment of FIG. 12. 
     FIG. 14 is a power spectrum for a GaAs-on-GaAs growth experiment. 
     FIG. 15a-d are method diagrams of the steps of converting collected time domain data to frequency domain data. 
     FIG. 16a-d are a series of plots of the wave shape of the collected data resulting from the steps of FIG. 15. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described in detail in connection with the drawings. FIG. 1 is a schematic drawing of a preferred embodiment of the system of the invention, as illustrated by a RHEED measurement system, in accordance with the invention. 
     A Varian GEN II modular 3 in (MBE) reactor system 10 was used for the growth of GaAs, AlAs, and AlGaAs on GaAs substrates and the growth of GaAs on Si substrates. The GaAs substrates were prepared by chemical cleaning and etching followed by in situ oxide desorption at ˜600° C. The Si substrates were prepared by chemical cleaning followed by an HF dip and in situ oxide desorption at 750°-850° C. The Ga source fluxes were adjusted to give GaAs growth rates between 0.8-1.1 μm/h, the Al fluxes were adjusted to give AlAs growth rates of 0.2-0.5 μm/h. and the V:III ratios were adjusted to give beam-equivalent pressure ratios of 5-10:1. The substrate temperatures during growth, which were monitored by thermocouple and optical pyrometer measurements (not shown), ranged from 500° to 620° C. 
     A RHEED image 11 was generated with a standard Varian RHEED electron gun 2 operated at 8 kV with an emission current of 2-3 A. The resultant image obtained on the phosphor screen 17 was captured with a CCD TV camera 16 equipped with a macro lens 14 that permits the magnification of any part of the image. The [110] azimuth of the As-stabilized 2×4 reconstructed GaAs (001) surface of wafer 12 in MBE chamber 10 for analysis and the lens 14 focused on the specularly reflected region of the diffracted image 11. The video signal from the CCD chamber 16 was connected to a Colorado Video Model 321 video analyzer 18. Analyzer 18 permits the selection of any pixel on the TV image 20 by the use of manually adjustable cross-hair controls. The voltage associated with the intensity of this pixel was read by a digital voltmeter and an X-Y recorder 22 with a time base. The digital voltmeter converted the measured voltage into digitized data at rates of 30-70 sample/s. The data were sent over an IEEE-488 bus 23 to an 80386-based desktop computer 24 equipped with a math co-processor and a commercial software package (National Instruments Lab Windows®) for data collection and analysis. The software package provided a tool for developing a digital signal processing program (see Appendix A) to accomplish such tasks as base line correction to compensate for drift digital filtering to remove undesirable noise components, fast Fourier and fast Hartley transform analysis, and power spectrum measurements. 
     The following procedure was generally employed in obtaining the RHEED oscillation data for growth on GaAs substrates. A GaAs layer about 0.5- μm thick was first grown under conventional conditions. Growth was then interrupted momentarily by manually closing the Ga source shutter while keeping the As shutter open to maintain an As stabilized surface. 
     After about 30 s, a pixel was selected on the specularly reflected streak, and collection of intensity data was initiated. Growth was resumed by reopening the Ga shutter, continued for a predetermined interval, and again interrupted by closing the Ga shutter while keeping the As shutter open. Data collection was automatically terminated at some later time. Usually, 2048 data samples were collected over an interval of about 30 s. The RHEED oscillation frequency was then found by the computer analysis program. The computation utilizes an accurate value for the voltmeter sampling rate, which is determined from the number of samples and the total data collection time measured by using the internal clock of the computer. For GaAs growth on Si substrates the same procedure was followed after about 1 μm of GaAs had been deposited. 
     To a first order the RHEED oscillation signal consists of periodic (sinusoidal) and nonperiodic (damped) components. Therefore, one can estimate the limiting precision of determining the exact oscillation frequency. First, since the periodic signals of interest have a frequency of ˜1 Hz and the sampling rate is ˜70 Hz, the system is &#34;over-sampling&#34; and not limited by Nyquist considerations. Second, since data is taken over 30-s intervals repeated (albeit damped) oscillations of the periodic signal are sampled. In the frequency domain, the spacing between points is therefore given by Δf(Hz)=1/NT, where N-number of samples and T=sampling interval in the time domain. 
     In this work, N=2048 and T=14.3 ms yielding Δf=0.0342 Hz. Without further analysis, this value yields a maximum frequency uncertainty of ±Δf/2=0.0171 Hz. By using a simple algorithm (described later) for determining the centroid of the peak of the power spectrum, a minimum frequency uncertainty of less than 0.003 Hz is attained which has been confirmed by repeated measurements during growth on the same high-quality GaAs substrate. The frequency uncertainty can be reduced still further by taking more samples, increasing the sampling rate, or using more sophisticated algorithms for centroid location. 
     FIG. 2 is a recorder trace of intensity versus time that was obtained for a run in which GaAs was grown on a high-quality GaAs substate at ˜500° C. under conditions that were optimized for obtaining well-resolved RHEED oscillations. (The signal plotted is not linearly related to the pixel intensity, since the video analyzer output was inverted and offset by a dc bias supplied by the recorder.) Opening the Ga shutter produces an abrupt change in intensity, which is followed by a large number of distinct oscillations that decay in amplitude with time. When the Ga shutter is closed, the intensity again changes rapidly to approximately the initial value, and no further oscillations are observed. The average frequency determined from the time required for the first 17 oscillations is 0.872 Hz, which corresponds to a growth rate of 0.887 μm/h, since the height of a (100) GaAs monolayer is half the lattice constant (a o  =0.565 33 nm). FIG. 3 shows the power spectrum obtained for the same experiment by using the new measurement method with contains a clearly resolved peak at 0.879 Hz, corresponding to a growth rate of 0.894 μm/h. which agrees with the value obtained from the recorder trace. 
     FIG. 4 is a recorder trace obtained on another day for GaAs growth under conditions similar to those for the experiments of FIGS. 2 and 3, but on a GaAs substrate of inferior quality. In this case, the oscillations are so weak and indistinct that the frequency cannot be accurately determined by inspection of the trace. However, by applying the new analysis method which performed a base line correction to compensate for the long-term nonperiodic variation in the signal, the RHEED oscillation frequency can be obtained. The power spectrum obtained, which is shown in FIG. 5, contains a dominant peak at 0,860 Hz, corresponding to a growth rate of 0.875 μm/h. The excellent agreement between this value and the growth rate determined from FIG. 3 is further evidence for the validity of the new method, since the growth rate of GaAs under the conditions employed in the two experiments is primarily determined by the Ga flux, which was nominally the same. 
     To further demonstrate the power of the new frequency measurement system, a growth experiment was performed with the GaAs substrate temperature raised to ˜580° C. As shown by the plot of FIG. 6, this procedure resulted in a series of rapidly damped RHEED oscillations. Computer analysis of all the intensity data yielded the power spectrum shown in FIG. 7, which has a peak at 1.036 Hz. FIG. 8 is a plot of intensity vs time on an expanded scale for the last 5 s of the experiment. The data are so noisy that no oscillations can be observed. However, the computer analysis yielded the power spectrum shown in FIG. 9, which has a predominant peak at 1.034 Hz, almost exactly the value obtained for the experiment as a whole. 
     In some power spectra the frequency peak is strongly asymmetric, as shown in FIG. 10, for an experiment on the growth of GaAlAs. The asymmetry appears to be due to flux transients in the Ga and Al sources. Since the computer analysis can be performed on a single oscillation (although the accuracy is significantly reduced because the number of samples taken per cycle is relatively small), a cycle-by-cycle analysis was carried out for the experiment of FIG. 10. As shown in FIG. 11, the frequency is initially about 1.70 Hz, but quickly stabilizes at about 1.56 Hz. 
     The frequency-domain analysis has also been applied in experiments on GaAs-on-Si growth. FIG. 12 is an intensity vs time plot obtained in one such experiment. Although the effect of opening and closing the Ga shutter is apparent, the diffraction is weak, and oscillations cannot be clearly resolved. However, computer analysis yields the power spectrum shown in FIG. 13, which has a predominant peak at 0.828 Hz, in good agreement with the growth rate determined by sample thickness measurements. We are presently using the frequency analysis technique to investigate the initial stages of GaAs-on-Si heteroepitaxy, where the growth is more three-dimensional in nature. 
     Finally, an interesting feature that is present in a number of power spectra is illustrated in FIG. 14. In addition to the main peak at frequency f, there is also a resolvable peak at 2f, although a second harmonic is not apparent in the intensity vs time data. Initially, we thought that such additional peaks at 2f might be the result of some nonlinearity in the response of our system. However, even the intentional introduction of nonlinear response by clipping the data at a fixed amplitude did not produce 2f peaks of the magnitude shown in FIG. 14. Others have directly observed harmonic components in intensity-time RHEED data taken at different azimuthal angles. Perhaps similar harmonics are being resolved by our measurement technique even though they are too weak to be seen in the time-domain data. 
     Referring now to Appendix A and FIGS. 15 and 16, the details of the signal processing method to convert the time domain data, obtained from the digital voltmeter of unit 22 (FIG. 1), the frequency domain data will now be described. A primary function of the computer program of Appendix A is to obtain RHEED oscillation frequency data from a digitized signal stored or read directly from the voltmeter of unit 22 as the growth process is proceeding in reactor 10. This data can then be utilized by computer 24 to feedback control signals to reactor controls 28 to change growth patterns to optimize the desired growth in reactor 10. 
     In the first main routine of the process, the computer is programmed to collect and display the input voltage versus time RHEED oscillation data (FIG. 15, Step A). This routine generally extends from item of Appendix A, to item II page 6. In the Display Mode (item III), the collected data and/or processed data is displayed on the screen 26. Furthermore, only certain data may be selected for display using the window sub-routine &#34;zero data&#34; described at Appendix A at VI, and shown at FIG. 16A. 
     The windowed data is then processed to determine the mean of the voltage versus time data curve using the sub-routine &#34;meandata&#34; which begins at item IV. Next, the baseline trend of the curve is calculated by fitting a fifth order polynomial to the data and the baseline trend is subtracted from the data (see item V continuing through item VIII, &#34;Poly Fit&#34; sub-routine and FIGS. 15C and 16C). The data is now ready for signal processing to determine the power spectrum, i.e., the power versus frequency curve of the pixel under inspection (see FIGS. 15D and 16D). The sub routines for power spectrum extraction are the &#34;fht data&#34; routine and &#34;ps data&#34;. The &#34;fht data&#34; routine is described (item VII) wherein the fast Hartley transform of the data is displayed and optional data filtering is performed. The sub routine &#34;ps data&#34; starts at item IX and comprises calculation of the power spectrum of the data and finding the peak frequency using a centroid finding algorithm. 
     In summary, a computerized system for the frequency-domain analysis of time-dependent RHEED intensity data has been described. In addition to the rapid, accurate analysis of well-resolved RHEED oscillations, this method permits the extraction of oscillation frequencies from data, like those obtained for GaAs-on-Si growth, that cannot be analyzed by conventional time domain methods. The additional information that can be provided by the new technique on the more subtle features of RHEED oscillations may contribute to improved understanding of the nature of epitaxial growth. 
     Equivalents 
     Those skilled in the art will know, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. 
     While the invention has been described in connection with RHEED oscillations, other applications are envisioned for this system. For example, any in situ method for generating an incident beam on a specimen during growth and detecting the resultant beam variations with time will benefit from the invention. Thus, light scattering, rather than diffraction phenomena, may be detected and used as the time domain data for determining growth oscillation. Photo-emissions or reflected beams may also generate detectable oscillations The beam may comprise any suitable energy beam, such as an electron beam, photon beam, ion beam or X-ray beam. 
     All manner of growth chambers, including (PVD) or the like, are contemplated, as well as a wide variety of growth materials, such as superconductors, silicides and metal organic films of semiconductors material, without limitation. 
     These and all other equivalents are intended to be encompassed by the following claims. ##SPC1##