Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
1999-07-09
2002-06-04
Anderson, Bruce (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S305000, C250S307000
Reexamination Certificate
active
06399944
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the measurement of film thicknesses. The invention relates particularly to such measurement using an electron analyzer.
BACKGROUND ART
Many technologically advanced devices rely upon composite structures having a very thin, substantially planar film covering a substrate of another material. An example of such a device is a magnetic recording or read head which has an active surface layer of a ferromagnetic material. High-performance ferromagnetic materials based on, for example, heavier elements such as cobalt, are often brittle and subject to oxidation so that it is common practice to cover the ferromagnetic layer with a very thin protective layer, often of a carbon-based material such as diamond. However, the performance and durability of such devices depend on the manufacturing process to produce a thickness of the covering layer within a relatively narrow range. If the structure is an electromagnetic sensor, the coating thickness must be closely controlled so as to not degrade the sensor performance.
Many thickness measurement techniques are available to measure such film thicknesses, which often are in the range of 1 to 150 nm. Optical techniques are simple, but their dependence on optical properties of both the film and substrate preclude their use for some combination. Auger electron spectroscopy, to be described below, is commonly used for compositional control and in principle can be used for measuring film thickness. However, it is entirely too slow for use in a production environment if the film thickness exceeds 3 nm and is practically useless at thicknesses above 5 nm. Scanning electron microscopy is straightforward, but it is a very slow process and requires the sectioning of the sample being tested. Frequent sampling in a production environment requires a fast, non-destructive technique.
Electron spectroscopy is a well known technique for characterizing the atomic constituents in a solid. Briggs et al. have edited a complete reference in
Practical Surface Analysis, vol.
1
, Auger and X-ray Photoelectron Spectroscopy,
2
nd
ed., (Wiley, 1990). In the typical practice of Auger spectroscopy, the solid is probed with an electron beam in the low keV range of energies and produces a secondary electron through an Auger transition process having a well defined Auger energy E
AUGER
. In Auger spectroscopy, the probing radiation ejects an inner-shell electron from an atom. Then in the Auger transition, a first outer-shell electron falls into the inner-shell vacancy and a second outer-shell electron is ejected carrying the difference in energy. The spectrometer analyzes the energy of the ejected electron as the Auger energy E
AUGER
. The Auger energy E
AUGER
is for the most part unique for each atom, primarily dependent upon the atomic number Z. Thus, the measured electron energy can be used to determine the composition of the material, at least near its surface. These energies are generally in the range of a few hundred eV to a few keV for the typical practice of Auger electron spectroscopy. Usually to enhance the Auger signal, the primary energy Ep is made twice or more the Auger energy E
AUGER
. Auger electron spectroscopy allows the very quick and highly accurate measurement of film thicknesses up to about 30 nm. Other types of electron spectroscopy are possible with similar equipment, and the technology is close to electron microscopy.
A generic electron spectrometer is schematically illustrated in FIG.
1
. Other geometrical relationships may be used. An electron gun
10
emits a primary radiation beam
12
of energy E
p
towards a sample
14
under test, which is supported on a holder
15
. An electron energy analyzer
16
receives a beam
18
of secondary electrons emitted from the sample
14
and characterized by energy E
s
. The low electron energies require that the entire analyzer be operated at very high vacuum levels. The secondary beam
18
tends to be spatially very broad. The electron energy analyzer
16
typically has a spatially fixed entrance slit
20
to fix the angle between the analyzer
16
and the sample
14
, and it internally analyzes the secondary energy E
s
by means of a electrostatic retarder or a magnetic analyzer or other means. Although in some automated applications, the electron analyzer
16
outputs a small number of experimentally determined parameters, the typical analyzer at some level outputs an energy spectrum from which the energy location of one or more peaks is extracted. Such electron spectrometers are well known, very often as Auger or ESCA spectrometers, and are commercially available from several sources, including Physical Electronics (PHI), a division of Perkin-Elmer of Eden Prairie, Minn., Vacuum Generators of the United Kingdom, and Omicron of Delaware.
Although electrons are often used as the probing radiation, other types of radiation can By produce similar effects, for example, X-ray or positrons.
One of the major experimental effects in electron spectroscopy is background noise introduced by inelastic scattering of the primary electrons (if an electron source is used) and of secondary electrons as they pass through the material between its surface and their points of interaction with the constituent atoms of the material. All electrons experience both elastic and inelastic collisions. Inelastically scattered electrons have a wide distribution of energies beginning at the energy E
p
of the probing beam and extending downwardly.
Primary electrons used for Auger spectroscopy typically have energies of a few keV while the Auger transitions are typically below 1 keV. A 1 keV electron has a mean free path in a solid of about 3 nm; a 3 keV electron, 15 nm. X-rays exhibit significantly deeper penetration. Furthermore, secondary Auger electrons are subject to the same type of inelastic scattering. Many technical articles have attempted to explain and quantify the effects of inelastic scattering in order to extract the Auger spectrum. Other technical articles have used the inelastic spreading as a way of measuring the scattering cross-sections between electrons. Elastic scattering depends upon the average atomic number Z of the material and is stronger in materials with higher Z.
SUMMARY OF THE INVENTION
A method and apparatus for quickly and non-destructively determining the thickness of an overlayer of one material formed over an underlayer of another material having a different effective atomic number. A source of primary radiation, for example, keV electrons or X-rays, creates a wide spectrum of inelastically scattered secondary electrons. The spectrum of secondary electrons emitted through the overlayer of a test sample is measured and compared to similar spectra for reference samples having known thicknesses of the overlayer to thereby determine the overlayer thickness in the test sample. The apparatus may be derived, in the case of electrons being the primary radiation, from conventional electron spectrometers.
In one embodiment of the invention, ratios of the intensity of a portion of the spectrum of inelastically scattered electrons to that of elastically scattered electrons are measured both for the reference and test samples.
REFERENCES:
patent: 4766034 (1988-08-01), Sato et al.
patent: 5280176 (1994-01-01), Jach et al.
patent: 5594245 (1997-01-01), Todokoro et al.
patent: 6067154 (2000-05-01), Hossain et al.
patent: 26 11 411 (1977-09-01), None
patent: 859406 (1998-08-01), None
patent: 2 054 136 (1981-02-01), None
Briggs et al.; “Auger and X-Ray Photoelectron Spectroscopy”; Practical Surface Analysis; vol. 1; 2ndedition; Wiley, 1990. pp. 19-83.
Goulet et al.; “A Procedure for Determining Low-Energy (<10EV) Electron Mean Free Paths in Molecular Solids: Benzene”; Journal of Electron Spectroscopy and Related Phenomena; 41 (1986) pp. 157-166.
Michaud et al.; “Total cross sections for slow-electron (1-20 eV) scattering in solid H2O”; The American Physical Society; vol. 36; No. 10; Nov. 15, 1987; pp. 4672-4683.
Bronshtein, et al.; “Inelastic Scattering
Borodyansky Sergey
Bryson, III Charles E.
Klyachko Dmitri
Linder Robert
Vasilyev Leonid A.
Anderson Bruce
FEI Company
Scheinberg Michael O.
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