U.S. patent number 3,868,507 [Application Number 05/422,048] was granted by the patent office on 1975-02-25 for field desorption spectrometer.
This patent grant is currently assigned to The United States of America as represented by the United States Atomic. Invention is credited to John A. Panitz.
United States Patent |
3,868,507 |
Panitz |
February 25, 1975 |
FIELD DESORPTION SPECTROMETER
Abstract
A field desorption spectrometer which is capable of detecting
and identifying one or more atoms of a specimen and/or all the
atoms in an outer layer of the specimen or throughout the bulk of
the specimen may comprise an apertured electrode for applying an
electric field to field evaporate or field desorb ions from the
specimen through the aperture, an apertured wall for blocking
electric fields from the apertured electrode and specimen from a
field free drift region and for transmission of the desorbed ions
into the drift region, channel electron multiplier array means
positioned in the drift region for intercepting of substantially
all of said ions and for providing amplification of ion impacts by
electron multiplication at locations corresponding with the
locations where ions strike the array means, and means for sensing
the multiplied electrons corresponding to these locations.
Inventors: |
Panitz; John A. (Edgewood,
NM) |
Assignee: |
The United States of America as
represented by the United States Atomic (Washington,
DC)
|
Family
ID: |
23673183 |
Appl.
No.: |
05/422,048 |
Filed: |
December 5, 1973 |
Current U.S.
Class: |
250/287;
850/5 |
Current CPC
Class: |
H01J
49/16 (20130101); H01J 37/285 (20130101); H01J
49/025 (20130101) |
Current International
Class: |
H01J
49/02 (20060101); H01J 37/26 (20060101); H01J
49/10 (20060101); H01J 49/16 (20060101); H01J
37/285 (20060101); H01j 039/34 () |
Field of
Search: |
;250/286,287,306,307,309 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
The Atom Probe Field Ion Microscope by E. W. Muller from
Naturwissenschaften, Vol. 57, No. 5, May, 1970, pp. 222-230.
250-309. .
"Construction and Performance of an FIM-Atom Probe" by S. S.
Brenner et al. from "Surface Science," North-Holland Publishing
Co., Vol. 23, No. 1, 1970, pages 88-111..
|
Primary Examiner: Lawrence; James W.
Assistant Examiner: Church; C. E.
Attorney, Agent or Firm: Horan; John A. King; Dudley W.
Constant; Richard E.
Claims
What is claimed is:
1. A spectrometer for detecting and indentifying ions in a
diverging generally conical ion beam field desorbed from a specimen
surface, comprising a vacuum chamber having electrically conductive
walls surrounding an ion source section and a detector section;
said ion source section including means for supporting said
specimen, means for coupling a constant voltage to said specimen
through said supporting means, an electrode adjacent said specimen
having an aperture aligned therewith for emission and conical
definition of ions from said specimen, and means for coupling an
ion desorption field to said specimen through said electrode
opposite in polarity from said constant voltage for thus providing
a beam of ions from said specimen surface diverging in a cone-shape
through said aperture; an electrically conductive wall portion
intermediate said ion source section and said detector section and
spaced from said electrode, said conductive wall portion having an
aperture aligned with said specimen and said first mentioned
aperture of size no less than that of said first mentioned aperture
and no greater than outer margins of said ion beam as defined by
said first mentioned aperture; said detector section including a
channel electron multiplier array means disposed in registry with
each of said apertures and said ion beam as defined thereby and
impervious to direct passage of ions in said ion beam for producing
and multiplying electrons in response to said ions at locations
where said ions strike said array, said array means being spaced
equidistant from said specimen for all paths of travel of ions in
said ion beam, phosphor means for sensing means multiplied
electrons adjacent said locations, said array means having
sufficient gain to detect ions from said specimen surface, and
means responsive to said sensing means for identifying ion species
in said ion beam; means for coupling said chamber to a vacuum pump;
and means for connecting said chamber walls and said electrically
conductive wall portion to ground potential to provide shielding
for said ions and an ion drift region between said electrically
conductive wall portion and said channel electron multiplier array
means.
2. The spectrometer of claim 1 wherein said identifying means
includes variable delay means for gating said channel electron
multiplier array means on at a time commensurate with the arrival
of a particular ion species from said specimen surface for sensing
of said particular ion species by said sensing means.
3. The spectrometer of claim 1 wherein said identifying means
includes an oscilloscope and means for initiating the sweep of said
oscilloscope by said ion desorption field.
4. The spectrometer of claim 1 wherein said identifying means
includes a photosensitive device external to said vacuum
chamber.
5. The spectrometer of claim 1 wherein said channel electron
multiplier array means includes a plurality of adjacent, generally
coextensive channel plates facing and aligned with said apertures
and said ion beam, each having a generally spherical radius of
curvature centered at said specimen for providing substantially
equal travel distances for each ion from said specimen surface to
said plates.
6. The spectrometer of claim 1 wherein said channel electron
multiplier array means includes a plurality of adjacent, generally
coextensive planar channel plates facing and aligned with said
apertures and said ion beam, and a generally spherical grid
electrode disposed between said channel plates and said specimen
with a radius of curvature centered at said specimen for providing
substantially equal travel distances for each ion from said
specimen surface to said array means.
7. The spectrometer of claim 1 wherein said sensing means includes
an array of optical fibers on which is settled a phosphorescent
screen to provide optical coupling between said channel electron
multiplier array means and the exterior of said vacuum chamber.
8. The spectrometer of claim 7 wherein said optical fiber array
includes a generally spherical surface adjacent said channel
electron multiplier array with a radius of curvature centered at
said specimen.
9. The spectrometer of claim 7 including means for sensing the
locations of ions at said channel electron multiplier array means
through said fiber optic array.
10. The spectrometer of claim 7 including means for detecting the
location of an ion at said channel electron multiplier array means
through said fiber optic bundle.
11. The spectrometer of claim 1 wherein said apertures are circular
and form a diverging conical path from said specimen with a conical
half angle of from about 15.degree. to 45.degree..
12. The spectrometer of claim 11 wherein said electrode is spaced
from said specimen a distance of about 1 millimeter and from said
apertured wall a distance greater than the voltage breakdown
distance therebetween and less than about 2 millimeters.
Description
BACKGROUND OF INVENTION
Attempts have been made to provide atom-by-atom identification of
specimen surfaces using an apparatus sometimes referred to as an
atom-probe field ion microscope. With this microscope, a "map" of
the surface atoms is generally made using an ionized imaging gas to
identify the position of atoms or an atom of interest. The specimen
is then positioned so that the image of the surface atom of
interest is aligned with an aperture or probe hole spaced some
distance from the specimen, generally about 10 centimeters, and
located on a phosphorescent screen, the aperture being equal in
size to only one or at most a few atom images. The outer layer of
atoms from the specimen would then be field evaporated or field
desorbed from the specimen and the ion or ions resulting from the
desired atom or atoms passed through the aperture or probe hole
into either a drift-type mass spectrometer or a magnetic sector
spectrometer which would then identify the atom species. The
phosphorescent screen would prevent all other atoms from reaching
the detector.
The passage of a preselected ion from the specimen through the
"probe hole" may be difficult to achieve due to a difference in
trajectory between the imaging gas atoms and the corresponding atom
field desorbed from the surface. In addition, because of the small
size of the specimen and the hole in the screen and the distance
between them and between the screen and spectrometer detector,
alignment between specimen and detector will be difficult. In
addition, the specimen must be moved within a vacuum chamber to
shift the image of its surface with respect to the probe hole so as
to position the selected surface atom image over the probe hole,
and to accomplish this while providing specimen cooling to
cryogenic temperatures and high voltage and pulse voltage
connection to the specimen.
These microscopes were generally of fairly large volume which
required differential pumping of the specimen chamber or portion of
the microscope and the spectrometer section together with the
associated gauging and pumping apparatus.
In these previous instruments or microscopes, the ion kinetic
energy was generally determined by the sum of a DC bias voltage and
the amplitude and shape of a high voltage pulse. Since it is
difficult or impossible to measure or terminate the pulse
transmission line with its characteristic impedance at the
specimen, the pulse shape and amplitude may be indeterminate and
consequently the ion kinetic energy was uncertain. In order to
provide reasonably reproducible results, a complicated calibration
procedure was required. As the specimen shape is altered by the
evaporation or desorption of atom layers, the DC bias or
evaporation pulse or both had to be increased to maintain a
constant evaporation field. This meant that the ion kinetic energy
would change and a new calculation, and calibration, for each
unknown species would have to be performed at each new value of
applied voltage.
SUMMARY OF INVENTION
In view of the problems associated with the various atom-probe
field ion microscopes and their inability to provide certain
information, it is an object of this invention to provide a field
desorption spectrometer which is capable of identifying one or more
atoms at one or more locations on the surface of the specimen and
simultaneously providing a measurement of the number of atoms of
each species over the entire surface or a portion of the
specimen.
It is a still further object of this invention to provide a field
desorption spectrometer which is also capable of providing a "map"
of the location of every atom of a single species in each layer or
layer edges of the specimen.
It is another object of this invention to provide a field
desorption spectrometer which is capable of providing these
measurements without movement of the specimen or alignment thereof
with a particular location of the spectrometer detector.
It is a still further object of this invention to provide a field
desorption spectrometer which provides ions of substantially
constant kinetic energy at the detector portion of the spectrometer
and thereby to eliminate the complicated and containing calibration
procedures of previous atom probes.
It is a still further object of this invention to provide a field
desorption spectrometer which is capable of providing these
measurements in an instrument having a relatively small volume.
Various other objects and advantages will appear from the following
description of the invention, and the most novel features will be
particularly pointed out hereinafter in connection with the
appended claims. It will be understood that various changes in the
details, materials and arrangements of the parts, which are herein
described and illustrated in order to explain the nature of the
invention, may be made by those skilled in the art.
The invention comprises an ion source utilizing a triode-type
electrode configuration which includes the specimen biased at a
constant positive voltage, an electrode having an aperture
positioned adjacent the specimen for passage of ions desorbed from
the specimen by a negative voltage applied to the electrode, and a
grounded wall having an aperture aligned with the specimen and
electrode aperture for passage of the ions into a field free drift
region beginning at said aperture and continuing to a channel
electron multiplier array which is disposed in direct line of
flight with ions from said specimen through said apertures.
DESCRIPTION OF DRAWING
The invention is illustrated in the accompanying drawings
wherein:
FIG. 1 is a diagrammatic view of the field desorption spectrometer
of this invention showing the relative locations and sizes of the
various portions of the apparatus and the associated biasing and
control circuits to provide the various modes of operation;
FIG. 2a is an enlarged partially cutaway, perspective view of the
specimen and its associated electrode configuration;
FIG. 2b is an enlarged and somewhat diagrammatic, not to scale,
cross section of a portion of the channel electron multiplier array
and fiber optic bundle of FIG. 1;
FIG. 2c is a diagrammatic view of an alternate channel electron
multiplier array which may be utilized in the spectrometer of FIG.
1;
FIG. 3 is a representation of a typical helium gas image of a
tungsten specimem produced with the spectrometer of FIG. 1;
FIG. 4 is a representation of a typical image of the
tungsten.sup.+.sup.3 ion species from the same tungsten specimen
utilized in FIG. 3;
FIG. 5 is a representation of the hydrogen desorbed from the
tungsten specimen of FIG. 3; and
FIG. 6 is a graph of the ion species and their quantities detected
by the spectrometer of FIG. 1 during a typical evaporation event
from a tungsten specimen.
DETAILED DESCRIPTION
The field desorption spectrometer of this invention is illustrated
in FIG. 1 in somewhat diagrammatic and simplified form bringing out
the relative positions, proportions and sizes of the respective
elements of the spectrometer and typical electrical control and
biasing apparatus to provide the desired modes of operation. The
size, shape, and material of the specimen 10 (shown in greater
detail in FIG. 2a) and the various power supplies are selected to
provide an electric field of the order of magnitude of a few
hundred million volts per centimeter at the surface of tip 12 of
the specimen 10 resulting in field evaporation or field desorption
of atoms from the surface of tip 12 and also to provide electric
fields of the order of magnitude of about several hundred million
volts per centimeter resulting in gas imaging of the surface atoms
of tip 12. Generally, the specimen 10 is formed in the shape of a
rod of about 0.1 millimeter in diameter comprised of a conical
needle ending in an extremely sharp point and is made of a material
to be studied and analyzed. The tip 12 may be formed with a
generally hemispherical shape which is produced and otherwise
finished by mechanical or electrochemical etching and polishing to
dimensions well beyond the range of an optical microscope and the
tip finally finished by field evaporation to an atomically smooth
surface. In this condition, tip 12 provides atoms from its surface
which are desorbed and magnified and whose relative positions are
magnified to several million diameters by the spectrometer. From a
so-formed specimen, the primary atoms of interest will be emitted
in a generally conically diverging beam centered on the
longitudinal axis of the specimen 10 and having a conical half
angle of from about 40.degree. to 45.degree.. It will be
appreciated that as the radius of curvature of the tip 12
increases, the electric field density will decrease and may reduce
the field strength below the level at which desorption may take
place. The ions leaving the tip 12 may follow very closely along
lines of radius from the tip 12, if maintained in a field free
environment, and may provide a resolution of about 1 to 4 Angstroms
and several million magnification.
Because of the geometry of specimen 10 comprising tip 12, the
practical limit of the ion beam dimensions is near or below the
recited conical half angle above. In order to maintain the
evaporation voltage at as low a level as practical and to eliminate
electrical breakdown and other considerations, the specimen radius
of curvature at the areas where evaporation is to occur should be
maintained at below about 1,000 Angstroms. It has also been found
that it is desirable that the specimen 10 be maintained at
cryogenic temperatures, such as below about 78.degree. to
21.degree. Kelvin, by use of liquid gases like nitrogen or neon, to
enhance the image formed with the desorbed and ionized atoms and
from gas imaging ions. In order to prevent interference from other
materials and gases, it is preferred that the field desorption and
ion magnification be carried out in an evacuated atmosphere of from
about 10.sup.-.sup.6 to 10.sup.-.sup.10 Torr.
The specimen 10 to be examined may be mounted at the base of a
glass or ceramic "cold finger" 14 at the end of an electrically
conductive support element 16. The support element 16 may include
one or more conductors carried by and within the cold finger 14 and
sealed therein in an appropriate manner which may terminate with a
generally U-shaped support 16' and the specimen 10. The cold finger
14 may be partially filled with a liquid gas to maintain the
specimen 10 through the support member 16 at cryogenic
temperatures. A first electrode 18 having an aperture or passageway
20 aligned with the specimen 10 and the ion beam evolved therefrom
is positioned at a location adjacent specimen 10 and tip 12 which
will provide the desired field desorption electric field to tip 12.
The aperture 20 dimension is selected to be sufficiently large so
as not to impede or block the diverging ion beam from tip 12. The
spacing between tip 12 and electrode 18 may be typically about 1
millimeter while the aperture 20 may be about 2 millimeters in
diameter. The electrode 18 may be in the hollow tubular shape shown
or in a planar or spherical shape depending upon the desired means
of imaging and positioning of the electrode. An additional
electrode or wall 22 may then be positioned adjacent electrode 18
with electrode 18 intermediate tip 12 and electrode 22. The
electrode 22 is provided with an aperture or opening 24 which is
aligned and symmetrical with the longitudinal axis of specimen 10
and of sufficient diameter so as to minimize blocking or impeding
of the ion beam diverging from tip 12. Even though the aperture 24
should generally be made of sufficient diameter to be outside the
desired diverging ion beam, its diameter should be small enough to
act to contain the electric field produced by electrode 18 and
should thus preferably be just slightly greater than the diameter
of the desired ion beam. Additional shielding may be achieved by
placing a grid across aperture 24, however, with some consequent
degradation of the ion beam. The apertures 20 and 24 may form a
diverging conical path from specimen 10 with a conical half angle
of from about 15.degree. to about 45.degree..
The specimen 10, and electrodes 18 and 22 form a triode-type ion
source section of the field desorption spectrometer and may be
supported within an appropriate vacuum chamber 26 which may be
sealed and evacuated to a desired low pressure. Vacuum chamber 26,
in the area adjacent to and surrounding the ion source section,
should be formed from or include electrically conductive wall
portions which may be electrically grounded to act as a shield for
the ion source section. The various elements and conductive
feedthroughs to the ion source section may be suitably mounted and
sealed to maintain the desired vacuum levels through a suitable
vacuum pump 28. By way of example, the vacuum chamber 26 may be
formed from intersecting metal tubular sections 26a and 26b, as
shown, with the cold finger 14 and ion source section supported in
tubular section 26a and the vacuum pump 28 and ion detector
section, to be described, supported in tubular section 26b.
The ion detector section of the field desorption spectrometer may
include an ion drift region 30 which is maintained in a
substantially electric and magnetic field free condition by the
non-magnetic conductive wall 32, a part of which forms electrode
22. The wall 32 may expand conically or in any other appropriate
manner from the aperture 24 of electrode 22 to a diameter larger
than the desired magnified diameter of the ion beam. A channel
electron multiplier array 34, as shown for purpose of illustration
in greatly expanded cross section in FIG. 2b, is positioned at the
end of the drift region 30 within walls 32 at a location where the
ion beam reaches the desired magnification level. The diameter of
the channel electron multiplier array may typically be about 8
centimeters and be spaced about 10 centimeters from tip 12 of
specimen 10. The channel electron multiplier array 34 may be
provided with a curvature having its center of radius at tip 12 to
provide equal or substantially equal travel distances for ions
evolved from tip 12 to the array 34.
The channel electron multiplier array 34 may be made up of one or
more channel plates, for example channel plates 36 and 38 in FIG.
2b, and a luminescent or phosphorescent screen, such as phosphor
screen 40, disposed adjacent the channel plates on the side
opposite to the ion source section of the spectrometer. The channel
plates are formed of a multiplicity of parallel circular
passageways arranged in a honeycomb-like array and are often formed
from bundles of fused optical fibers which have been partially
etched away. Each of the passageways, for example passageways 42 in
channel plate 36 of FIG. 2b, are formed with a length equal to
about 40 times their diameters and may typically be about 37
micrometers in diameter with a distance between passageway centers
of about 50 micrometers. The inside surface of each passageway is
coated with a semi-insulating layer having a resistance typically
of about 10.sup.8 ohms, such as layer 44, while the outer surfaces
of the plates are coated with a conductive layer, such as layers 46
and 48 for channel plates 36, to effectively connect all the
passageways in a channel plate in electrical parallel, each
passageway functioning as a distributive dynode multiplier when an
appropriate bias is applied between the conductive layers 46 and
48. The passageways are preferably positioned at an angle, such as
about 15.degree., with respect to a particle or ion which is
traveling from tip 12 of specimen 10 so as to make the channel
plates effectively opaque to ions striking the plates at normal
incidence. The second channel plate 38 has its passageways
positioned at about normal incidence or an appropriate angle so
that it is effectively opaque to the electrons produced in the
first plate 36. When an ion enters a passageway of the first
channel plate 36, secondary electrons are emitted from the
semi-insulating layer 44 surface which in turn strike the inner
wall producing further secondary electrons. This process may be
repeated many times along the passageway with many electrons
emerging from the far end of the passageway. These electrons may be
accelerated to the next channel plate by the bias applied to its
surface layer and enter an adjacent passageway thereof and repeat
this secondary electron multiplication. With appropriate potentials
applied to the respective outer surface layers of the channel
plates beginning at ground potential at layer 46, gains of 10.sup.6
and more may be achieved. The electrons exiting from the passageway
of the second channel plate 38 may be accelerated against the
phosphor screen 40 by a bias applied thereto produce light images
at these locations which thus correspond with the magnified ion
image evolved from tip 12 of specimen 10.
The respective conductive layers on the outer surfaces of channel
plates 36 and 38 may be formed from appropriate deposition of such
as gold while the resistance layers in the passageways may be
formed of lead or the like by heat treatment of the channel
plates.
A fiber optic bundle 50, formed from a multiplicity of optical
fibers (shown as fibers 50') fused together to form an array having
outer dimensions similar to that of the channel electron multiplier
array 34, or a glass plate or other transparent window, may be
positioned adjacent to the array 34 through which phosphor screen
40, which may be deposited or settled on its surface facing array
34, may be viewed. The fiber optic bundle 50 may form the outer
wall of the vacuum chamber 26 at the terminal end of the drift
region 30. The optical fibers may be individually stacked to form
the bundle 50 with the interstices filled with appropriate bonding
material and the entire bundle then heated to a temperature to fuse
this bonding material to the fibers to form a solid, gas impervious
structure which leaves the integrity of the individual fibers
intact. The surface of the fiber bundle 50 adjacent to the channel
electron multiplier array 34 may be spherically curved to
correspond with the spherical curvature of array 34 to minimize
distortion of the image on the phosphor screen 40 from the channel
plates. It should be noted that in most applications, the fibers
may typically have a cross section of from about 20 to 40
micrometers so that more than one fiber of the fiber bundles 50 may
view the light emissions caused or produced through one or several
of the passageways of the channel plate and may provide collimation
and relatively distortion free viewing of the image on phosphor
screen 40.
As mentioned previously, the channel electron multiplier array 34
may be shaped with a spherical curvature having a radius centered
at tip 12 of specimen 10 to insure equal or approximately equal
travel distances for all ions evolved from tip 12. The channel
plates may also be formed with a planar configuration, such as
shown by channel plates 36' and 38' and phosphor screen 40' settled
on glass or fiber plate 50' of array 34' in FIG. 2c, and a high
transmission, mesh or grid 52 having a spherical radius of
curvature centered at tip 12 placed in front of the array 34'. With
appropriate bias applied to the grid 52 and the array 34', such as
by grounding the grid 52 and applying a negative voltage to the
first conductive layer of channel plates 36', the difference in
travel time of an ion located towards the center of the array 34'
compared to that of an identical ion towards the periphery of the
array 34' may be minimized.
The phosphor screen 40 or 40' may be formed in a conventional
manner with one or more types of phosphorescent or fluorescent
material deposited or settled on a conductive layer or forming a
part thereof. For example, a high speed phosphorescent material
having a low or short residual luminescence may be utilized where
differentiation between species having very similar or close travel
times is desired provided the apparatus utilized to sense the
phosphoresence light image of screen 40 has sufficient speed to
measure or detect this image. A slower, longer residual luminescent
material may be utilized where the detecting or sensing apparatus
requires longer periods of time to record the image. In some
applications, a mixture of two or more luminescent materials may
provide a compromise for combining of these functions.
The light image on screen 40 or 40' may be sensed through the fiber
optic bundle 50 by use of a light sensing or detecting apparatus
such as a camera to obtain an image of the entire phosphor screen
40 or 40' simultaneously on a photographic plate, by use of a
photosensitive device such as a photomultipler like photomultiplier
54 having a sensitivity at the particular luminescence wavelength
of phosphor screen 40 or 40' or by use of a photomultiplier having
similar spectral response. The photomultipler 54 may be provided
with a fiber optic 55 or similar light collimating structure for
isolating those fibers of bundle 50 producing a single luminescence
spot at a particular location of screen 40. It will be apparent
that with the latter arrangement two or more photomultipliers with
or without light collimators, such as photomultiplier 54', may be
utilized to measure the luminescence simultaneously at more than
one location of screen 40. Apparatus may be provided to move either
the photomultiplier or any fiber optics associated therewith from
location to location about the outer surface of the fiber optic
bundle 50 for these measurements. The signal generated by
photomultiplier 54 and 54' may be coupled through an appropriate
differentiating circuit 56 to an oscilloscope 58 or other recording
means. It is also apparent that by use of appropriate optical
mechanisms, such as beam splitters and the like and electrically
controllable shutters, that more than one optical sensing means may
be utilized simultaneously or in sequence to sense the information
and images produced by the channel electron multiplier array 34,
including image storage tubes or image dissecting tubes, vidicon
tubes and photodiode or the like arrays or combinations of one or
more cameras with one or more photomultiplier devices viewing the
fiber optic bundle 50 simultaneously or in controlled
sequences.
As mentioned previously, the specimen, through support element 16,
may be biased to a constant positive voltage to give the ions
evolved from specimen 10 a predetermined kinetic energy. The
voltage bias should be sufficiently large to assure that external
magnetic fields do not affect the trajectories of the ions. For the
size drift region of the present spectrometer, this may be at
voltages somewhat greater than 50 volts. The voltage bias should be
at a level which insures detectable and reliable travel times for
each ion species, and may be generally from about 2 to 4 kilovolts
and preferably less than one kilovolt. This bias voltage may be
applied via element 16 by the bias supply 60. The travel time of a
given ion species may be determined with sufficient accuracy by the
equation:
T = d[(m/n)/(0.193 V.sub.A)].sup.1/2,
where T is in microseconds, d is the distance between tip 12 and
the channel electron multiplier array 34 in meters, m/n is in amu
and is the charge to mass ratio of the ion species, and V.sub.A is
in kilovolts. The electrode 18 may be held at a negative potential
sufficient to establish the required imaging field by means of a
suitable feed-through conductor 61 and bias supply 62. The
electrode 18 voltage bias may typically be from about 0 to -20
kilovolts. If a high voltage pulse of sufficient amplitude to field
desorb an atomic layer from tip 12 of specimen, such as from about
0 to -20 kilovolts, is applied to electrode 18 from pulser supply
63 through an appropriate pulse forming network, an atomic layer
may be desorbed from the tip 12 surface and ionized. The ions may
travel from tip 12 through apertures 20 and 24 into the drift
region 30 and strike channel electron multiplier array 34. Between
the tip 12 and electrode 18, the ions are accelerated and between
electrode 18 and wall 22 they are decelerated so that the ion
kinetic energy within the field-free drift region 30 is determined
solely by the magnitude of the positive bias voltage from bias
supply 60. With the aperture 24 at the size indicated, the ions
will reach this kinetic energy level determined by the bias supply
60 directly adjacent to the aperture 24 regardless of the amplitude
of the bias or pulse at electrode 18 so that identical ions will
travel for the same time at each evaporation pulse. If it is
desired, temperature dependence or other effects may be measured by
supplementally heating specimen 10 during the desorption pulse by
use of a laser, electrical resistance heating by additional leads
coupled to specimen 10, by heated fluid inserted in cold finger 14,
or otherwise.
The oscilloscope 58 sweep may be initiated by sampling the pulser
supply 63 via the voltage divider 68 or other suitable means. The
phosphor screen 40 and the channel electron multiplier array 34 may
be suitably biased through bias supply 64. Screen 40 may be coupled
through network 70 and voltage divider 72 to the oscilloscope 58 so
that the travel times of all ions striking the channel electron
multiplier array 30 may be measured from the time of the desorption
pulse from pulser 63. All of the ions produced by the desorption
event may be monitored at one time in this manner to provide a
rapid evaluation of species concentration and identity over the
entire surface as well as the identification of these species at
various separate locations and their quantity. It will be apparent
that these measurements can be made at the same time using separate
oscilloscopes or multiple traces on a single oscilloscope.
If it is desired to make an atom-by-atom analysis of one or more
locations on tip 12 using the photomultiplier 54 and a fiber optic
coupler 55 between the fiber optic bundle 50 and photomultipler 54,
a suitable imaging gas may be introduced through imaging gas source
73 into the vacuum chamber 26 to a pressure of about 10.sup.-.sup.6
Torr to produce a conventional ion image of the specimen surface on
phosphor screen 40 of the channel electron multiplier array 34. The
suitably apertured photomultiplier 54 may then be placed over a
selected image spot through the fiber optic bundle 50. The imaging
gas may then be pumped from the chamber 26 and an atomic layer
desorbed from specimen tip 12 by the pulse from pulser 63. By
differentiating the output of the photomultiplier 54 and feeding
its signal to the oscilloscope 58, which was triggered by the
desorption pulse from pulser 63, the travel time of the ion
desorbed from the tip 12 at this location may produce luminescence
on the phosphor screen 40 at a time after desorption determined by
the ion species and opposite the apertured photomultiplier so that
the atom at that single image spot may be analyzed and identified.
As noted above, this analysis may be modified by the use of several
apertured photomultipliers to provide atom-by-atom analysis at
several different locations simultaneously.
The pulser supply 63 may be a pulsed generator capable of providing
a pulse amplitude of from about 0 to -20 kilovolts with a pulse
width of about 20 nanoseconds or less and a rise time of less than
about 1 nanosecond. Pulser supply 63 may be manually or
automatically triggered to supply single pulses, as required. Since
changes in pulse amplitude or duration only affect the evaporation
field and not the ion energy in the drift region 30, carefully
terminated pulse lines may no longer be required for successful
operation. The bias supply 60 may be an ultra-stable power supply
with about 0.001 percent regulation and a ripple of less than about
200 milovolts. As the desorption from the tip 12 of specimen 10
occurs, the radius of curvature of the tip may change from
evaporation pulse to evaporation pulse and require some change in
the amplitude of the evaporation pulse from pulser supply 63 or
bias supply 62 or a combination thereof, however with no effect on
the ion kinetic energy of the ions entering the drift region 30.
Also, as the radius of curvature of tip 12 changes from evaporation
pulse to evaporation pulse, any shifting of the image at the
channel electron multiplier array 34 may be compensated for merely
by movement of the photomultiplier 54 and its fiber optics, or
other light detector, external to the vacuum system.
The respective elements of the channel electron multiplier array 34
may be biased by suitable bias supplies similar to bias supply 64
which is shown in simplified form for purpose of illustration
without illustrating the different connections to each element of
array 34 through appropriate voltage dividers or separate bias
supplies to achieve the desired electron multiplication and ion
detection. For example, the surface layer 46 of channel plate 36
will normally be grounded while the back layer 48 is biased at
about 1 kilovolt with the front layer of channel plate 38 biased at
about 1 kilovolt to 1.4 kilovolts and the back layer at about 2
kilovolts with screen 40 at about 7 kilovolts.
If it is desired to record all the ions of a single species, e.g.,
provide an image at phosphor screen 40 of this ion species only,
the back layer of channel plate 38 may be biased at a direct
current potential of about 1,200 volts by bias supply 64 and the
layer pulsed by a positive pulse of about 800 volts from mass gate
pulser 74 at the desired time period after the evaporation pulse is
generated by pulser supply 63, as determined by a variable delay
circuit 76. Thus, the channel electron multiplier array 34 may be
gated on only during the arrival time of the desired ion species
and the resulting electrons produced therefrom by the array 34
reach the back surface layer of channel plates 38 to produce an
image on phosphor screen 40 of only these ions. If the mass gate
pulser 74 voltage pulse is terminated before any other ion species
may arrive, the image will be of only the desired ion species. In
order to optimize viewing and photographing of the gated image, a
long decay time phosphor may be desirable, but since the arrival of
the ions at the channel electron multiplier array 34 must be
accurately determined, a short rise time is also required. These
two diverse conditions may be satisfied by utilizing a mixture of
phosphors on phosphor screen 40.
FIG. 3 illustrates a typical helium ion image of a tungsten
specimen which may be utilized to obtain the crystallographic
orientation of the specimen 10 tip 12. FIG. 4 illustrates the image
of the tungsten.sup.+.sup.3 ion species from a tungsten specimen
using the mass gate pulser 74 and time delay circuit 76. FIG. 5
illustrates an image formed from hydrogen ion species using mass
gate pulser 74 and a different time delay by delay circuit 76. FIG.
6 illustrates a typical spectrometer record of the different ion
species evolved from a tungsten specimen illustrating by peak 78
the quantity of tungsten.sup.+.sup.3 ion species, by peak 80 the
quantity of tungsten.sup.+.sup.4 ion species, by peak 82 the
quantity of oxygen ion species and by peak 84 the quantity of
helium ion species in a single evaporation pulse as detected from
the signals produced on phosphor screen 40 and coupled to scope 58
by network 70 and voltage divider 72. For example, the peak 78 and
the peak 80 indicates that there is about 10 times as many
tungsten.sup.+.sup.3 ion species as tungsten.sup.+.sup.4 ion
species produced during the evaporation pulse at 78.degree.K. It
will be apparent that the spectrometer record illustrated in FIG. 6
may be provided by any electron multiplier arrangement other than
and including the channel electron multiplier array described.
Since the ion energy is determined solely by the direct current
bias supply to specimen 10 and is not affected by the amplitude or
duration of the desorption pulse, the field desorption spectrometer
described does not require extremely precise pulse characteristics.
With this field desorption spectrometer, various measurements and
determinations may be made of the specimen and its makeup as
described previously without affecting or entering the vacuum
chamber 26. The combined volume of both the ion source section and
detector section of this spectrometer may be about 1 liter.
* * * * *