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DIALOG(R)File 652:US Patents Fulltext
(c) format only 1998 Knight-Ridder Info. All rts. reserv.
00509151
Utility
METHOD AND APPARATUS FOR ION BOMBARDMENT USING NEGATIVE IONS
PATENT NO.: 3 ,573,454
ISSUED: April 06, 1971 (19710406)
INVENTOR(s): Andersen, Cristian A., Solvang, CA (California), US (United
States of America)
Roden, Henry J., Santa Barbara, CA (California), US (United
States of America)
ASSIGNEE(s): Applied Research Laboratories, Inc , Sunland, CA (California),
US (United States of America)
[Assignee Code(s): 4861]
EXTRA INFO: Assignment transaction [Reassigned], recorded November 13,
1984 (19841113)
POST-ISSUANCE ASSIGNMENTS
ASSIGNEE(s): E.I. DU PONT DE NEMOURS AND COMPANY, WILMINGTON, DE., A DE
CORP.
Assignor(s): BUCKFELDER, JOHN J. -- signed: 09/25/1984;
SCHLEINITZ, HENRY M. -- signed: 09/25/1984
Recorded: November 13, 1984 (19841113)
Reel/Frame: 4326/0513
Brief: ASSIGNMENT OF ASSIGNOR'S INTEREST
Rep.: FRANK C. HILBERG, JR. LEGAL DEPARTMENT E. I. DU
PONT DE NEMOURS & CO. WILMINGTON, DE 19898
ASSIGNEE(s): LLOYDS BANK INTERNATIONAL LIMITED, ONE SEAPORT PLAZA 199 WATER
STREET NEW YORK, NY 10038 A BRITISH BANKING CORP.; JAMES
TALCOTT, INC., 1633 BROAWAY NEW YORK, NY 10019 A NY CORP.
Assignor(s): APPLIED RESEARCH LABORATORIES, INC., A MA CORP.
-- signed: 08/28/1984; ARL APPLIED RESEARCH LABORATORIES, S.A.
A SWITZERLAND CORP. -- signed: 08/28/1984
Recorded: November 13, 1984 (19841113)
Reel/Frame: 4326/0899
Brief: SECURITY INTEREST
Rep.: FINNEGAN, HENDERSON FARABOW ET AL 1775 K ST., NW
WASHINGTON, DC 20006
APPL. NO.: 4-723,026
FILED: April 22, 1968 (19680422)
U.S. CLASS: 437-20 cross ref: 250-282; 250-309; 250-492.2; 437-930;
437-931
INTL CLASS: [ ] H01j 39-34; H01l 7-00
FIELD OF SEARCH: 148-1.5; 250-041.9ISB; 250-041.9ISE; 250-049.59
References Cited
U.S. PATENT DOCUMENTS
2,947,868 8/1960 Herzog 250-49.59
3,328,210 6/1967 McCaldin et al. 148-1.5
3,336,475 8/1967 Kilpatrick 250-43
3,341,754 9/1967 Kellett et al. 148-1.5
OTHER REFERENCES
zhurnal Tekhnicheskoi Fiziki, Vol. 30, No. 1, Fogel et al., pgs. 63--73,
January, 1960, 250--49.5(9)
PRIMARY EXAMINER: Lawrence, James W.
ASST. EXAMINER: Birch, A. L.
ATTORNEY, AGENT, OR FIRM: Stone, Hoffman
CLAIMS: 3
DRAWING PAGES: 1
DRAWING FIGURES: 3
ART UNIT: 256
FULL TEXT: 175 lines
ABSTRACT
Improved stability and control in ion bombardment is achieved in the case
of many materials under bombardment by the use of negative ions. The method
is especially advantageous for the analysis of insulating materials by
secondary ion emission and also enables improved results in ion
implantation processes.
DETAILED DESCRIPTION
The invention will now be described in greater detail in connection with
the accompanying drawing, wherein:
FIG. 1 is a chart illustrating the secondary yield of Al sup + ions from
an aluminum specimen in response to bombardment with positive oxygen ions;
FIG. 2 is a chart on a comparable scale of the yield of Al sup + ions
from an alumina specimen in response to bombardment by negatively charged
oxygen ions; and
FIG. 3 is a schematic block diagram of apparatus according to the
invention.
BRIEF SUMMARY
This invention relates to a novel method of ion bombardment enabling the
achievement of improved and heretofore unachievable results in processes
that include the step of bombarding a material with charged particles.
Heretofore in ion bombardment work, either for spectro-chemical analysis
by secondary ion emission, or for other purposes such as, for example, the
making of solid state electronic devices by ion implantation, the
bombarding ions have been selected from among those having a positive
charge. Extremely useful results have been achieved, especially in mass
spectro-chemical analysis. However, it has not heretofore been possible to
analyze insulating materials by this method with a reasonable degree of
reliability. The surfaces of insulating and poorly conducting materials
appear to become electrically charged, not only by reason of the charges
delivered by the impinging ions, but also by the departure of secondary
electrons driven from the surfaces by the ions. After a relatively brief
bombardment, the surfaces of many insulators become charged to the point
where the bombarding beam is diffused, or repelled, or in some other way
adversely affected so that it is no longer possible even to detect the
emission of secondary ions.
It has previously been suggested that this effect could be overcome by
evaporating a grid of conductive material on the surface to be bombarded so
that the charges delivered to the surface by the bombarding ions and
created thereon by the departure of the secondary electrons could leak off,
having to travel only a short distance along the insulating surface.
Another previous suggestion was to place a source of electrons close to the
surface of the specimen under bombardment and biased negatively relative to
the specimen so that electrons from the source would be drawn to the
specimen to neutralize the positive charge. These expedients have been
found to be of very little practical use in most instances.
Briefly, in accordance with the present invention, it has been found that
the hereinabove described problems may be substantially completely
overcome, and excellent results achieved if the specimen is bombarded with
negative ions in place of the previously used positive ions.
The theory on which the invention is based is not understood, but is
believed probably to be related to the known phenomenon that all materials
release secondary electrons when subjected to ion bombardment. In the case
of materials that are electrically insulating, the statistical yield of
secondary electrons is greater than the number of impinging ions when the
bombarding ion energies are in the range ordinarily used in microanalysis
work. The electrons also greatly outnumber the yield of sputtered ions, and
the charging effect on the surface of the material being bombarded seems to
be primarily determined by charges delivered by the impinging ions and
charges taken away by the secondary electrons. The secondary electrons seem
to be usually of relatively low energy so that, as the surface under
bombardment starts to charge in the positive direction due to the departure
of secondary electrons in greater numbers than the arriving negative ions,
enough of the secondary electrons are electrostatically attracted back to
the surface to establish an equilibrium condition. The small positive
potential at the point of impact of the bombarding ions is insignificant
compared to the acceleration imparted to the bombarding ions, which is
usually in the range of about 1,000 to 20,000 electron volts.
Thus, by the use of negative ions, the heretofore disastrous effects
produced by electrostatic charge accumulation on the surface of the
specimen are avoided. The technique has been found to be extremely useful
in the operation of an ion microprobe of the type described and claimed in
the copending application of Helmut J. Liebl, Ser. No. 494,388, filed Oct.
11, 1965, entitled "Ion Microprobe" and assigned to the present assignee.
It is also thought that it will be found highly beneficial for sputtering
in general, and in other applications also, especially in connection with
ion implantation. For example, a growing degree of interest is currently
evident in industry in the use of ion implantation methods for making solid
state electronic devices. The use of negatively charged ions in accordance
with the invention is expected to facilitate the achievement of higher
concentrations of implanted ions into more precisely defined regions,
relative to implantation by bombardment with positive ions.
Insofar as is presently known, an impinging ion produces the same effect
in a material regardless of its charge in respect of sputtering of material
from the specimen and the release of secondary electrons. The impinging ion
is simply a submicroscopic bullet delivering energy in accordance with its
velocity and mass. It appears to become electrically discharged as it
approaches or meets the surface. The polarity of the charge carried by the
impinging ions, therefore, appears to be immaterial in the bombardment of
electrically conductive specimens. Whether the ions are electrically
negative or positive, the sputtering results are substantially identical
for similar beam currents and particle energies. Either positive or
negative ions may be chosen on the basis of collateral considerations such
as the characteristics of the ion source.
When electrically insulating specimens are to be bombarded, however, the
charges carried by the bombarding ions and deposited upon the surfaces of
the specimens are trapped there, and the selection of ions of the proper
charge becomes a matter of vital importance.
FIG. 1 illustrates the output of secondary positive aluminum ions of
atomic weight 27 sputtered from an aluminum specimen in response to
bombardment by a beam of O sup + ions (nascent oxygen, atomic weight 16) in
an ion microprobe of the type described in the hereinabove identified
copending application of Helmut J. Liebl. It is seen that within a few
seconds of the start of the bombardment, the output of sputtered ions
reaches a relatively high and stable value.
The use of oxygen ions in place of the more commonly used chemically
inert ions such as argon ions is described and claimed in the copending
patent application of Christian A. Andersen and Helmut J. Liebl, Ser. No.
678,840, filed Oct. 30, 1967, entitled "Analysis by Bombardment with Oxygen
Ions," now abandoned, and the continuation-in-part thereof, Ser. No.
753,822, filed Jul. 12, 1968, entitled, "Analysis by Bombardment with
Chemically Reactive Ions."
FIG. 2 is a chart on the same scale as the chart of FIG. 1 showing the
emission of secondary Al sup + ions from a specimen of relatively pure
alumina (Al sub 2O sub 3) under bombardment in the same instrument by a
beam of 0 sup - ions. In both cases, the current in the ion beam was about
3 times 10 sup sup -9 amperes, the accelerating potential was about 8
kilovolts, and the ion beam was defocused to cover an area of between about
20 and 50 mu sup 2.
It is seen that the output of aluminum ions from the alumina shows a
characteristic very similar to that obtained in the case of metallic
aluminum. The output of sputtered ions rises rapidly to about the same
relatively high and stable value. The selection of negative ions to
constitute the primary beam successfully alleviates the problems heretofore
encountered due to building of an electrical charge on the surface of the
insulator.
The negative ions may be produced in any desired way. It is known, for
example, that by properly positioning the outlet orifice of an ion source
of the type known as a duoplasmatron, negative ions can be drawn from it in
relatively large numbers. No claim is made in this application relative to
the negative ions per se, or to the means for producing them. The invention
is directed chiefly to the concept of using negative ions for bombarding
materials, especially for bombarding electrically insulating materials, for
any desired purpose. Also, since the nature of the matter sputtered from
the bombarded specimen appears to be independent of the electrical polarity
of the bombarding ions, the practice of the invention in spectrochemical
work is not is any way limited in respect of the selection of sputtered
ions for analysis, or of the methods used for analyzing the sputtered ions
or other materials.
FIG. 3 schematically represents the ion microprobe described in the
hereinabove-identified box labeled FOCUSING indicates a system of lenses
for producing an ionic image of the source upon the surface of the
specimen.
The secondary electrons are ejected from the bombarded surface at
relatively low velocities, and it is desirable to provide a collector
electrode 10 (FIG. 3) fairly close to the surface to ensure against the
accumulation of an excessively large negative space charge adjacent to the
surface. Electrodes suitable for this purpose are normally present in all
ion bombardment instruments equipped for mass spectrometric analysis of
secondary ions, and instruments of this type ordinarily need no special
modification for collecting the secondary electrons. In other types of ion
bombardment apparatus, it may be desirable to add a simple, positively
biased electrode spaced within an inch or so of the specimen to be
bombarded. The spacing and bias are not critical.
1. Method of implanting particles of atomic dimension in a specimen of an
electrically insulating material comprising the step of bombarding the
specimen with negatively charged ions, imparting energy to the ions at a
value selected to cause them to drive secondary electrons out of the
specimen to remove negative charges at a rate to compensate fully for the
negative charges carried to the surface of the specimen by the ions thereby
to avoid the accumulation of an excessive negative electrical
2. Method of sputtering an electrically insulating material comprising
bombarding the material with negatively charged ions to cause particles of
the material to be ejected from it, imparting energy to the ions at a value
selected to cause them to drive secondary electrons out of the specimen to
remove negative charges carried to the surface of the specimen by the ions
thereby to avoid the accumulation of an excessive negative
3. Method of analyzing an electrically insulating material comprising the
steps of bombarding a specimen of the material with negatively charged ions
with enough energy to sputter secondary ions from the material, imparting
energy to the ions at a value selected to cause them to drive secondary
electrons out of the specimen to remove negative charges at a rate to
compensate fully for the negative charges carried to the surface of the
specimen by the ions thereby to avoid the accumulation of an excessive
negative electrical charge on the surface of the specimen, and mass
spectrometrically analyzing the sputtered ions.
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