An apparatus, for photo-luminescent analysis of the surface of crystalline silicon, is disclosed, in which the photons emitted from the sample are passed through a two-beam (or two-arm) interferometer, having the usual beamsplitter, fixed mirror, and movable mirror. The interferometer output is directed to a detector which is a germanium photo-diode, cooled in a Dewar, which also cools the initial electronic circuitry to which the detector output is input. Using the disclosed apparatus, methods are available for readily eliminating the negative effect of the electron-hole-dropler phenomenon, and for utilizing the no-photon no-phonon region of the spectrum to identify otherwise unidentified impurity (or dopant) materials.
The questions raised in reexamination request No. 90/001,324, filed Sep. 9, 1987, have been considered and the results thereof are reflected in this reissue patent which constitutes the reexamination certificate required by 35 U.S.C. 307 as provided in 37 CFR 1.570(e).
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1. An apparatus, for photo-luminescent analysis of impurities in a sample, comprising:
means for applying laser radiation to the surface of the sample to cause emission of photons by the sample; the sample having significant photon emissivity within the radiation wavelength range of 0.8 to 1.4 microns; means for collecting the sample-emitted photons and providing a photon beam for interferometer input; an interferometer which receives the input photon beam and outputs a spectrally encoded photon beam; and a detector which receives the interferometer output beam and converts it into electronic signals.
36. A method of calibrating spectrographic analysis of samples based on photoluminescence data which comprises:
directing a laser beam having a certain power at the surface of a sample; collecting the photons from the sample caused by the laser excitation; passing the collected photons through a fourier Transform spectrometer to provide a spectrograph; determining from the spectrograph whether there is substantial electron-hole-droplet formation in the sample at the selected laser beam power; and repeating the foregoing steps until that laser beam power has been essentially determined at which electronhole-droplet formation in the sample is substantially stabilized.
39. A method of obtaining new information from spectrographic analysis of samples based on photoluminescence data which comprises:
directing a laser beam having a certain power at the surface of a sample; collecting the photons from the sample caused by the laser excitation; passing the collected photons through a fourier Transform spectrometer to provide a spectrograph; the spectral coverage extending into sufficiently high frequencies to include the no-phonon region; the resolution of the spectrographic data being sufficiently fine to identify one or more impurity materials in the no-phonon region that are not identifiable in the lower frequency portions of the spectrum.
8. An apparatus, for photo-luminescent analysis of impurities at the surface of a sample, comprising:
means for supplying laser excitation radiation to the surface of the sample, for the purpose of causing the sample to emit photons; the sample having significant photon emissivity within the radiation wavelength range of 0.8 to 1.4 microns; cooling means for the sample containing liquid helium in which the sample is immersed; means for collecting sample-emitted photons and providing an output photon radiation beam; an interferometer beamsplitter to which said photon radiation beam is directed, and which partially reflects and partially transmits the photon radiation beam; two reflecting means, one fixed and one movable for scanning purposes, which reflect back to the beamsplitter its partially reflected and partially transmitted photon beams; and a detector which receives a recombined photon beam from the beamsplitter, and which converts the intensity of that beam into electronic output signals.
2. The apparatus of
3. The apparatus of claim 2 1 wherein: the interferometer output beam includes radiation wavelengths within the range of 0.8 to 1.4 microns; and the detector's quantum efficiency in response to radiation wavelengths in the range of 0.8 to 1.4 microns is substantially flat.
5. The apparatus of
a cooling container enclosing the germanium photodetector and maintaining it at a temperature no greater than approximately that of liquid nitrogen; and a pre-amplifier also enclosed in the container and having as its input the electronic signal output of the detector.
6. The apparatus of
means for preventing radiation other than sample-emitted photons from reaching the detector.
7. The apparatus of
means for preventing radiation other than sample-emitted photons from reaching the detector.
9. The apparatus of
a monochromatic radiation subsystem, including a source of such radiation, which subsystem is caused by operation of the interferometer to provide clocking signals which determine the frequency of sampling of detector output signals; and means for preventing the monochromatic radiation from affecting the photon radiation beam, including means for substantially enclosing the source of the monochromatic radiation and filtering means which limits the radiation from the source to the wavelength required for the clocking function.
10. The apparatus of
a broad band radiation subsystem, including a source of such radiation, which subsystem is caused by operation of the interferometer to provide a scan starting signal for the analysis of the photon radiation beam entering the interferometer; and means for preventing the wide band radiation from affecting the photon radiation beam, including means for turning off the wide band source during each scan of the photon radiation beam, and means for turning on the wide beam source between successive scans of the photo radiation beam.
11. The apparatus of
means associated with the detector for substantially preventing it from receiving radiation outside a certain desired range; and means for covering the entire photo-luminescent, interferometer, and detector, apparatus, in order to isolate it from ambient atmospheric and radiation conditions.
12. The apparatus of
means associated with the detector for substantially preventing it from receiving radiation outside a certain desired range; and means for covering the entire photo-luminescent, interferometer, and detector apparatus, in order to isolate it from ambient atmospheric and radiation conditions.
13. The apparatus of
a broad band radiation subsystem, including a source of such radiation, which subsystem is caused by operation of the interferometer to provide a scan starting signal for the analysis of the photon radiation beam entering the interferometer; and means for preventing the wide band radiation from affecting the photon radiation beam, including means for turning off the wide band source during each scan of the photon radiation beam, and means for turning on the wide band source between successive scans of the photon radiation beam.
14. The apparatus of
means associated with the detector for substantially preventing it from receiving radiation outside a certain desired range; and means for covering the entire photo-luminescent, interferometer, and detector, apparatus, in order to isolate it from ambient atmospheric and radiation conditions.
15. The apparatus of
a cooling container enclosing the detector and maintaining it at a temperature substantially that of liquified nitrogen; and electronic circuitry located in the cooling container which receives the electronic output signal from the detector and provides a first stage of pre-amplification of those signals.
16. The apparatus of
17. The apparatus of
18. The apparatus of
means for supplying excitation radiation to the surface of the sample, for the purpose of causing the sample to emit photons; means for collecting sample-emitted photons and providing an output photon radiation beam; an interferometer beamsplitter to which said photon radiation beam is directed, and which partially reflects and partially transmits the photon radiation beam; two reflecting means, one fixed and one movable for scanning purposes, which reflect back to the beamsplitter its partially reflected and partially transmitted photon beams; and a detector which receives a recombined photon beam from the beamsplitter, and which converts the intensity of that beam into electronic output signals; the sample is being crystalline silicon and the radiation wavelengths to be analyzed are being in the neighborhood of 1.1 microns.
19. The apparatus of
the means for supplying concentrated radiation to the surface of the sample is an argon ion laser beam perpendicular to the surface of the sample; and the means for collecting the sample-emitted photons is a lens (or lens system) having an aperture through which the argon ion laser beam passes on its way to the sample.
20. The apparatus of
a mirror which directs the photon radiation beam from the sample toward the beamsplitter, and which has an aperture through which (a) the argon ion laser beam passes on its way to the sample and (b) specular reflection from the sample exits the system.
21. The apparatus of
means for supplying excitation radiation to the surface of the sample, for the purpose of causing the sample to emit photons; means for collecting sample-emitted photons and providing an output photon radiation beam; an interferometer beamsplitter to which said photon radiation beam is directed, and which partially reflects and partially transmits the photon radiation beam; two reflecting means, one fixed and one movable for scanning purposes, which reflect back to the beamsplitter its partially reflected and partially transmitted photon beams; a detector which receives a recombined photon beam from the beamsplitter, and which converts the intensity of that beam into electronic output signals; and means for deriving from the detector's electronic output signals a spectrograph having a resolution no coarser than two wave numbers.
22. The apparatus of
23. The apparatus of
24. The apparatus of
the spectrograph requires accumulated data from successive scans obtained during a period no longer than six minutes; and the spectrograph has a signal-to-noise sensitivity ratio of at least 25.
25. The apparatus of
26. The apparatus of
27. The apparatus of
means for supplying excitation radiation to the surface of the sample, for the purpose of causing the sample to emit photons; means for collecting sample-emitted photons and providing an output photon radiation beam; an interferometer beamsplitter to which said photon radiation beam is directed, and which partially reflects and partially transmits the photon radiation beam: two reflecting means, one fixed and one movable for scanning purposes, which reflect back to the beamsplitter its partially reflected and partially transmitted photon beams; a detector which receives a recombined photon beam from the beamsplitter, and which converts the intensity of that beam into electronic output signals; and means for deriving from the detector's electronic output signals a spectrograph having a scan width extending at least from 8600 to 9300 wave numbers.
28. The apparatus of
29. The apparatus of
30. The apparatus of
31. The apparatus of
means for supplying excitation radiation to the surface of the sample, for the purpose of causing the sample to emit photons; means for collecting sample-emitted photons and providing an output photon radiation beam; an interferometer beamsplitter to which said photon radiation beam is directed, and which partially reflects and partially transmits the photon radiation beam; two reflecting means, one fixed and one movable for scanning purposes, which reflect back to the beamsplitter its partially reflected and partially transmitted photon beams; a detector which receives a recombined photon beam from the beamsplitter, and which converts the intensity of that beam into electronic output signals; and means for deriving from the detector's electronic output signals a spectrograph requiring accumulated data from successive scans obtained during a period no longer than six minutes.
32. The apparatus of
33. The apparatus of
34. The apparatus of
means for supplying excitation radiation to the surface of the sample, for the purpose of causing the sample to emit photons; means for collecting sample-emitted photons and providing an output photon radiation beam; an interferometer beamsplitter to which said photon radiation beam is directed, and which partially reflects and partially transmits the photon radiation beam; two reflecting means, one fixed and one movable for scanning purposes, which reflect back to the beamsplitter its partially reflected and partially transmitted photon beams; a detector which receives a recombined photon beam from the beamsplitter, and which converts the intensity of that beam into electronic output signals; and means for deriving from the detector's electronic output signals a spectrograph having a signal-to-noise sensitivity ratio of at least 25.
35. The apparatus of
37. The method of
38. The method of
40. The method of
41. The method of using photoluminescence to analyze impurities at the surface of a sample, comprising the following steps:
supplying laser excitation radiation to the surface of the sample, thereby causing the sample to emit photons within the radiation wavelength range of 0.8 to 1.4 microns: collecting the sample-emitted photons; using such photons to provide an output photon radiation beam; directing that beam to an interferometer beamsplitter, which partially reflects and partially transmits the photon radiation beam, thereby creating so as to create two photon beams; reflecting one of such photon beams back to the beamsplitter from a fixed reflecting means; reflecting the other of such photon beams back to the beamsplitter from a moving reflecting means; recombining the photon beams at the beamsplitter to provide a spectrally scanned output beam; and directing the scanned photon output beam to a detector which converts it into electronic output signals.
42. The method of
43. The method of
44. The method of
45. The method of
46. The method of
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This application is a continuation-in-part of Auth U.S. application Ser. No. 555,607, filed Nov. 28, 1983, abandoned.
This invention relates to the field of photoluminescence (PL) analysis, in which a light source is used to excite a sample, and the photons emitted by the sample are passed through a spectrometer which provides desired spectral information.
Although its potential uses ae much broader, the primary concern of the present invention relates to the determination of impurity concentrations in single crystal silicon (Si), whether such impurities are intentionally or unintentionally present in the silicon. These impurity determinations are an important means of evaluating the characteristics of electronic devices in integrated circuit chips. The use of PL analysis is essentially directed at determining surface characteristics, whereas other types of analysis , throughout throughput mode of operation the beam aperture need not be varied for the higher resolution scans; it is only necessary to move the mirror farther, thus increasing the amount of the interferogram information obtained.
Adequate resolution is important to resolve the narrowly spaced "no-phonon replica" features which appear at the high frequency end of the spectrum. This is readily accomplished with the FTPL of the present invention.
Among the accomplishments of the FTPL are:
(a) Discovery of several new luminescence line groups associated with different types of defects or impurities in silicon samples, heretofore unmentioned in the literature.
(b) Enhanced understanding of the extent of the excitation intensity dependence of the luminescence lines of interest in silicon.
(c) Learning much about the competitive nature of the different impurities in silicon, important for the advancement of quantitative analysis of impurity concentrations in silicon.
(d) Improving dramatically the lower concentration detection limits for the dopant impurities with the FTPL's enhanced sensitivity.
(e) Improving dramatically the ability to unequivocally identify (particularly in the no-phonon region) the closely-spaced shallow impurity luminescence lines, with the FTPL's enhanced resolution capabilities.
The remaining figures further demonstrate the validity of the claims made for the present invention. FIG. 11 shows a single spectrograph which illustrates both the resolution power and the breadth of the spectrum scanned by the FTPL. The resolution in FIG. 11 is 56 units, having a per unit value of 0.01 cm-1 ; and the laser power is 8 mW. In other words, the resolution is approximately one-half wave number, whereas the available resolution in monochromator PL systems is usually five wave numbers.
A particularly impressive result of this high resolution spectrograph is the data in the no-phonon region, which is at the far right of the figure. Three clearly distinct peaks are seen, representing (as marked) arsenic, aluminum, and phosphorous, respectively. The value of the data from the no-phonon region lies in the fact that it makes possible identification of one or more features which cannot be separated in the center region of the spectrograph (which shows broader features, and which fails to disclose the aluminum in this sample). One reason for the sharper available lines in the no-phonon region is that the recombination of the hole-electron pair does not involve a phonon.
FIGS. 12A-12D use the same raw data as FIG. 11, but they show progressively coarser resolution values; and they show only the no-phonon region. FIG. 12A has a resolution of 0.56 cm-1 (the same as FIG. 11); FIG. 12B has a resolution of 1.0 cm-1 ; FIG. 12C has a resolution of 1.5 cm-1 ; and FIG. 12D has a resolution of 2.0 cm-1. As the fineness of resolution decreases, from that shown in FIG. 12A to that shown in FIG. 12D, the aluminum peak is gradually merged into the arsenic peak, and thus disappears. The general practice, using the FTPL system, is to obtain spectra both at 2 wave numbers (cm-1) and at one-half wave number (cm-1).
The same sample as that analyzed in FIGS. 11 and 12A-12D was also analyzed on a monochromator PL system, as shown in FIGS. 13A-13C, which are three separate spectrographs, each of which only shows a small portion of the spectrum covered by FIG. 11. FIG. 13A started at 8630 cm-1, and had a step size of 5 Angstrom (distance between data points). FIG. 13B started at 8795 cm-1, and had a smaller step size of 1 Angstrom. FIG. 13C started at 9273 cm-1, and had a smaller step size of 0.5 Angstrom. FIG. 13C is in the no-phonon region. Even with its greatly reduced step size, and very small spectral width, it is unable to distinguish the three impurities which are clearly visible in FIG. 11.
Comparing FIGS. 13A-13C with FIG. 11 provides additional evidence of the magnitude of the advantages provided by the present invention. Not only is the difference in sensitivity dramatically illustrated; but also the single spectrograph of FIG. 11 provides extensive spectral data between and beyond the three spectrographs shown in FIGS. 13A-13C.
FIGS. 14A and 14B are of interest because they show that the present invention has recorded new photoluminescence features which were heretofore unreported in the literature, and were, therefore, presumably unobserved. FIG. 14A, showing data first observed on Jan. 17, 1984, indicates with arrows four new (previously unreported) photoluminescence features. FIG. 14B, showing data first observed on Apr. 6, 1984, indicates with arrows three new (previously unreported) photoluminescence features.
FIGS. 15A-15C illustrate the performance of the FTPL system in a particularly complex and subtle aspect of PL analysis. This aspect is the "competitive" nature of the different impurities in silicon in PL analysis. FIGS. 15A-15C show FTPL-produced spectrographs based on three different silicon samples, each having essentially the same concentration of boron. (This information is derived both from the boron quantity predicted from the sample manufacturing process, and from bulk analysis of the silicon samples using standard transmission-type spectroscopy). In all three figures, the laser power on the sample was 16 mW; and the resolution was 0.56 cm-1.
In FIGS. 15A and 15B, the boron concentration signals identified as BTO (BE) are of substantially the same height as the intrinsic signals identified as ITO (FE). However, in FIG. 15C, the boron signal is significantly lower than the intrinsic signal. It appears that this boron signal difference in FIG. 15C is caused by the presence of arsenic in this sample, as indicated by the signal identified as ASNP (BE) in the no-phonon region.
FIG. 16 is included as an example of a spectrograph produced by the FTPL system having extremely wide spectral coverage, extending from 6000 wave numbers to 13000 wave numbers. The material analyzed in FIG. 16 is gallium arsenide doped with tellurium. The germanium diode detector tends to drop off at the lower end of the spectrum in FIG. 16, particularly below 6500 wave numbers.
From the foregoing description, it will be apparent that the apparatus and method disclosed in this application will provide the significant functional benefits summarized in the introductory portion of the specification.
The following claims are intended not only to cover the specific embodiments disclosed, but also to cover the inventive concepts explained herein with the maximum breadth and comprehensiveness permitted by the prior art.
In some of the claims, quantified minimum values are stated of one or more of the four primary characteristics--sensitivity, resolution, spectral coverage, and speed of data acquisition. The purpose of such claims is to enhance the differentiation from prior art photoluminescence analysis systems. The figures used have necessarily been chosen arbitrarily, because the present invention represents differences in degree of performance of such magnitude that they become, in effect, differences in kind; but there are no dramatic change-over points. For example, referring to resolution capability, a broader resolution claim value "no coarser than two wave numbers", and a narrower claim value "approximately one-half wave number", have been specified. Referring to the width of a spectral scan, a broader claim value of "at least from 8600 to 9300 wave numbers", and a narrower claim value of "from 6500 to 13,500 wave numbers", have been specified. Referring to speed of data acquisitions, a broader value of "requiring accumulated data during a period no longer than six minutes", and a narrower value "only requires accumulated data obtained during a period of three minutes", have been specified. And referring to sensitivity, a broader value of "a signal-to-noise sensitivity ratio of at least 25", and a narrowr value of "a signal-to-noise sensitivity ratio of at least 100", have been specified.
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