U.S. patent number 3,876,305 [Application Number 05/389,088] was granted by the patent office on 1975-04-08 for demountable sputtering cathode for atomic absorption spectroscopy.
This patent grant is currently assigned to Commonwealth Scientific and Industrial Research Organization. Invention is credited to David Samuel Gough, Peter Hannaford, Alan Walsh.
United States Patent |
3,876,305 |
Gough , et al. |
April 8, 1975 |
Demountable sputtering cathode for atomic absorption
spectroscopy
Abstract
Techniques and apparatus for measuring the concentration of
elements in a solid metal sample by atomic absorption and
fluorescence are described. A silica disc with an annular
discharge-suppressing gap surrounding the sample area and an O-ring
seal are used to locate a surface of the sample for sputtering, and
gas passages in the disc allow sputtered atoms to be swept into the
body of a vacuum chamber for convenient analysis.
Inventors: |
Gough; David Samuel (Hawthorn,
Victoria, AU), Hannaford; Peter (Mount Waverley,
Victoria, AU), Walsh; Alan (Brighton, Victoria,
AU) |
Assignee: |
Commonwealth Scientific and
Industrial Research Organization (Campbell, AU)
|
Family
ID: |
3765268 |
Appl.
No.: |
05/389,088 |
Filed: |
August 17, 1973 |
Foreign Application Priority Data
Current U.S.
Class: |
356/314 |
Current CPC
Class: |
G01N
21/67 (20130101); H05H 3/02 (20130101); G01N
21/62 (20130101); G01N 21/74 (20130101) |
Current International
Class: |
G01N
21/71 (20060101); G01N 21/62 (20060101); G01N
21/74 (20060101); G01N 21/67 (20060101); H05H
3/00 (20060101); H05H 3/02 (20060101); G01j
003/02 () |
Field of
Search: |
;356/85,96,97
;204/192,298 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Russell et al.: Spectrochimica Acta, Vol. 10, 1959, pages 883-885.
.
Gatehouse et al.: Spectrochimica Acta, Vol. 16, 1960, pages
602-604. .
Goleb et al.: Analytica Chimica Acta, Vol. 28, May 1963, pages
457-466. .
Goleb et al.: Analytica Chimica Acta, Vol. 30, March 1964, pages
213-222..
|
Primary Examiner: Wibert; Ronald L.
Assistant Examiner: Evans; F. L.
Attorney, Agent or Firm: Sughrue, Rothwell, Mion, Zinn &
Macpeak
Claims
The claims defining the invention are as follows:
1. Apparatus for providing an atomic vapour for spectrochemical
analysis from a sample area on the surface of a sample body, the
sample area in use forming the cathode of a sputtering discharge,
comprising a vacuum chamber having an apertured wall, an anode
lying inside the chamber and below the wall aperture, and apertured
sample locating means lying outside the chamber around the wall
aperture and comprising an outer annular portion adapted in use to
abut the surface of the sample body and an inner annular portion
electrically insulated from the outer portion and the anode,
adapted in use to lie opposite the surface of the sample body but
spaced therefrom a short distance to form an annular gap
surrounding the sample area, the gapwidth being large enough to
electrically isolate the opposed surfaces thereof but smaller than
the width of the cathode dark space for the sputtering
discharge.
2. Apparatus as claimed in claim 1 in which the gapwidth is about
0.2 millimetre.
3. Apparatus as claimed in claim 1 in which the vacuum chamber
includes means whereby a stream of gas may be introduced into the
vacuum chamber while vacuum is maintained by a vacuum pump, such
that impurity atoms and molecules may be swept out of the chamber
while the sputtering discharge is operating.
4. Apparatus as claimed in claim 3 in which the sample locating
means is provided with at least one internal gas passage for
directing at least part of the stream of gas into the region of the
sputtering discharge so that sputtered atoms are thereby carried
away from the discharge region towards the body of the vacuum
chamber to facilitate spectrochemical analysis.
5. Apparatus as claimed in claim 4 in which the sample locating
means has a gas circulation passage into which at least part of the
stream of gas may be directed, and at least one gas outlet duct
leading into the aperture in the sample locating means.
6. Apparatus as claimed in claim 4 in which the sample locating
means has a gas circulation passage into which at least part of the
stream of gas may be directed, and at least one outlet duct leading
into the annular gap between the sample locating means and the
surface of the sample body.
7. Apparatus for spectrochemical analysis of a sample by means of a
sputtering discharge comprising in combination;
a vacuum chamber having an apertured wall;
an anode lying inside the chamber and below the wall aperture;
a disc-shaped centrally apertured sample locating means disposed
outside and around the wall aperture having a surface facing the
sample with an outer annular portion abutting the surface of the
sample and an inner annular portion electrically insulated from the
outer annular portion and the anode, stepped away from the sample
surface a distance large enough to electrically isolate the opposed
surfaces of the gap so formed but smaller than the width of the
cathode dark space for the sputtering discharge;
a sealing member in the form of an O-ring disposed around the
periphery of the sample locating means and between the sample and
the vacuum chamber wall so as to effect a vacuum seal
therebetween;
means to evacuate the vacuum chamber;
means to initiate and maintain the sputtering discharge between the
anode and the sample through the aligned apertures in the wall and
the sample locating means;
a spectral lamp providing light containing at least one spectral
line characteristic of an element under analysis; and
photodetector means for providing an indication of the degree to
which the characteristic spectral line is absorbed by atoms
sputtered from the sample by the discharge.
8. Apparatus for spectrochemical analysis of a sample as claimed in
claim 7 in which the degree of absorbance is determined by
measuring the fluorescent light re-emitted by the sputtered
atoms.
9. Apparatus for spectrochemical analysis as claimed in claim 7
including means whereby a stream of gas may be introduced into the
vacuum chamber while vacuum is maintained by a vacuum pump such
that impurity atoms and molecules may be swept out of the vacuum
chamber while the sputtering discharge is operating.
10. Apparatus as claimed in claim 9 in which the sample locating
means is provided with at least one internal gas passage for
directing at least part of the stream of gas into the region of the
sputtering discharge so that sputtered atoms are thereby carried
away from the discharge region towards the body of the vacuum
chamber to facilitate spectrochemical analysis.
Description
BACKGROUND OF THE INVENTION
This invention relates to spectrochemical analysis, and seeks to
provide a system which permits a rapid change-over of samples
submitted for analysis. Spectrochemical analysis, for the purposes
of this specification, should be taken to include atomic absorption
analysis, atomic fluorescence analysis, and spectral emission
analysis.
In spectrochemical analysis, it is necessary to create an atomic
vapour from the sample so that the spectral characteristics
determined are those of the atoms alone, and not those of atoms in
combination with other atoms -- whether of the same element or
different. In atomic absorption and fluorescence spectroscopy,
high-temperature flames have been used almost exclusively as the
means for converting samples into atomic vapour. A disadvantage of
this method is that, with solid samples, a preliminary
time-consuming step is necessary before the technique can be used,
as the sample for flame photometry must be in the form of a
solution. Moreover, the results of the tests on the solution must
be related back through the dissolution step to give the desired
result in the form of an analysis of the original sample.
The flame method also suffers to a greater or lesser extent from
other disadvantages, including pressure broadening of absorption
lines, quenching of fluorescence radiation, compound formation in
the flame, and opacity of the flame gases to vacuum-ultraviolet
light.
An alternative to the flame method, cathodic sputtering of the
sample atoms to form an atomic vapour, has been known for some time
(see, for example, our copending Australian patent application
37,184/68).
When a metal specimen is made the cathode of a d.c. glow discharge,
particles of the cathode material are sputtered from the surface by
energetic ions accelerated in the high field of the cathode fall.
These particles, which consist largely of single ground-state
neutral atoms are ejected from the cathode with large initial
energies, of the order of 10eV for the ion energies available in a
glow discharge. The sputtered atoms subsequently lose this energy
by elastic collisions with gas atoms as they diffuse out from the
cathode to sinks in various parts of the vessel. During transit,
the sputtered atoms pass through the negative-glow region of the
discharge where they may become excited or ionized by electron
impact or by collisions with metastable atoms.
At pressures in the range 1-10 torr, the sputtered atoms rapidly
come to thermal equilibrium with the gas, and sputtering and
diffusion rates are such that relatively large steady-state
concentrations accumulate in the region around the cathode. At the
same time, the negative glow is relatively well confined, and the
anode glow and positive column can be virtually eliminated by
positioning the anode at the edge of the negative glow. Thus large
concentrations of relatively emission-free sputtered vapours, in
thermal equilibrium with the gas, are available in the region just
beyond the negative glow.
When an alloy is vapourized thermally, the depletion of the higher
vapour-pressure elements at the surface is rapidly compensated for
by diffusion of atoms from the bulk, and net fractional
distillation of these elements occurs. For cathodic sputtering,
however, the temperature of the alloy is usually too low to permit
diffusion of atoms from the bulk, and a thin layer depleted in the
faster-sputtering elements rapidly forms near the surface. Once
equilibrium is established, this depleted layer apparently
compensates for the faster sputtering rates of the depleted
elements, and the composition of the sputtered material is then
approximately the same as that of the original alloy.
While this sputtering technique offers many advantages in certain
situations over flame photometry, it has not as yet gained wide
acceptance for routine analysis. One of the prime difficulties has
been the time necessary to change from one sample to the next,
mainly because of the contamination introduced into the chamber
during the change-over, and the relatively large amount of
contamination which must be removed from the sample body if it is
placed wholly inside the vacuum chamber. The time consumed in the
actual change-over of sample bodies has also given rise to
significant delays.
BRIEF SUMMARY OF THE INVENTION
The invention provides a method and apparatus whereby samples for
spectrochemical analysis by sputtering may be interchanged rapidly.
The invention avoids the contamination difficulties associated with
the introduction of the whole sample into the vacuum chamber by
allowing the bulk of the sample, if it is a solid block, to remain
outside the chamber. The sample may be in the form of a solid piece
of metal, for example a disc of about 4 cm diameter, or may be
formed on the surface of a similar solid block of holding material.
In either case, the block will be referred to herein as a "sample
body," and the area on the surface from which sample atoms are
sputtered will be referred to as the "sample area." The sample
area, and a small part of the surrounding surface, are the only
portions of the sample body subjected to the vacuum; the remainder
being left in contact with the surrounding air.
The term vacuum as used in this specification is intended to cover
a partial vacuum, and pressures inside the vacuum chamber are
typically of the order of 1-10 torr.
To allow the sample area to be included within the vacuum system,
the vacuum chamber is provided with an aperture in one of its
walls. The sample area can then form the cathode of a sputtering
discharge between itself and an anode located inside the vacuum
chamber.
However, it is not practicable for routine spectrochemical analysis
merely to place each sample in vacuum tight relation outside an
aperture in a vacuum chamber and establish a sputtering discharge
to form sample atomic vapours. Material sputtered from the samples
will accumulate on all surfaces close to the discharge and
interfere with operations by forming additional cathode areas on
the walls. These areas will be in electrically conductive relation
with the cathode, being at some point continuous therewith, so that
the discharge may switch to them as cathodes. Interference may be
produced because the material then sputtered may be left over from
previous samples, and even if the samples are the same, the
relative distribution of atoms in the atomic vapour may be quite
different to that in the samples because there is no compensation
for different sputtering rates; and furthermore, different current
densities characteristic of a new discharge path may result in
different discharge conditions. Even more extreme interference can
result with a heavy build-up of sputtered material on the walls
because eventually a conductive path can form between the anode and
cathode, short-circuiting the discharge.
Accordingly, in one aspect the invention comprises apparatus for
providing an atomic vapour for spectrochemical analysis from a
sample area on the surface of a sample body, the sample area in use
forming the cathode of a sputtering discharge, comprising a vacuum
chamber having an apertured wall, an anode lying inside the
chamber, and apertured sample locating means around the wall
aperture comprising an outer annular portion adapted in use to abut
the surface of the sample body and an inner annular portion
electrically insulated from said outer portion and said anode
adapted in use to lie opposite the surface of the sample body but
spaced therefrom a short distance to form an annular gap
surrounding the sample area, the gapwidth being large enough to
electrically isolate the opposed surfaces thereof but smaller than
the width of the cathode dark space for the sputtering
discharge.
For the purposes of this specification the inner and outer annular
portions should be understood as being electrically insulated from
each other if one or both are made from electrically insulating
material, including the case where they are formed integrally from
electrically insulating material.
Preferably also, the vacuum chamber is purged with clean inert gas
while maintaining the vacuum level appropriate to the glow
discharge so that impurity atoms are swept from the volume.
In a particularly advantageous form, the invention provides gas
passages within the disc so that purge gas is directed into the
area of the discharge in such a way that atoms sputtered from the
sample area are carried away from the discharge region towards the
body of the vacuum chamber to facilitate spectrochemical
analysis.
In a preferred form, in which a resilient vacuum sealing member is
located between the surface of the sample body and a wall of the
chamber, the external sides of the outer annular portion also
retain the sealing member in place and prevent it from being drawn
towards the aperture under the influence of the vacuum. In a
particularly preferred embodiment, the sample locating means is in
the form of a separate flat disc with a central aperture and an
inwardly stepped inner annular portion on one face .
Preferred features of the method and apparatus according to the
invention will become apparent from the following description of a
particular embodiment of the invention, in which:
FIG. 1 is a diagrammatic representation of apparatus for
spectrochemical analysis according to the invention;
FIG. 2 shows a cross-section of a sample holding arrangement of the
invention;
FIG. 3 shows in plan view a disc used in the sample holding
arrangement; and
FIGS. 4A and 4B show exploded plan views of a disc according to a
further embodiment of the invention;
FIGS. 5A and 5B show section views through the disc shown in FIGS.
4A and 4B;
FIG. 6 shows a section view of a further sample holding arrangement
according to the invention;
FIG. 7 shows a typical warm-up trace of fluorescence radiation
intensity obtained from a surface with no prior sputtering
treatment.
Turning briefly to FIG. 1, the sample body 1 is located against an
aperture (not shown) in vacuum chamber 2. The sample body is
electrically conductive and is connected to a negative supply
voltage line 3, and a discharge is struck between it and an anode
inside the chamber connected to positive voltage line 4. A spectral
lamp 5, operated to emit a pulsed high-intensity spectral line
characteristic of the element being estimated, is located so that
it illuminates the atomic vapour created inside chamber 2. If
fluorescence radiation is being examined, a photodetector 6 is
arranged to receive light emitted by the sample vapour in a
direction at right angles to the incident radiation. If atomic
absorption is being examined, a photodetector of monochromator 7 is
arranged in line with the incident radiation. In either case,
lenses may be arranged to focus incoming and outgoing light
relative to the vacuum chamber, light baffles may be used to
minimize the interference due to stray light reflections, and
filters may be interposed in the light paths to select appropriate
regions of the spectrum.
The detectors 6 and/or 7 are preferably arranged in synchromous
demodulation circuits, so that light pulses originating from lamp 5
are selectively detected. A fuller description of circuit and
optical arrangements may be found in our Australian patent
specification No. 163,586 in relation to atomic absorption and in
the specification of the aforementioned Australian patent
application No. 37,184/68 in relation to fluorescence.
In FIG. 2, a preferred sample holding means of the invention is
shown in more detail. Portions of the wall of the chamber 2 of FIG.
1 are shown at 8,8', with an aperture at 9. Anode 10 is located
inside the chamber at a point about 1 cm below the cathode formed
by sample area 11, the distance of 1 cm corresponding roughly to
the edge of the negative glow in order to eliminate as far as
possible background interference from an anode glow and positive
column. The discharge causes sputtering of sample atoms from the
cathode to form an atomic vapour, and the preferred location at
which this vapour is examined spectroscopically is about 2 cm below
the cathode, thus minimizing the contribution of the negative glow
to interfering background radiation.
The sample area 11 is located on the surface 12 of the sample body
13. Surface 12 is preferably ground flat and is pressed against the
wall 8,8' by a retaining member 14 assisted by outside air pressure
when the chamber 2 is evacuated. Interposed between the sample body
and chamber wall is a sample locating means 15 and a resilient
sealing member 16.
Sample locating means 15, shown in plan view in FIG. 3, in one
embodiment, takes the form of a solid disc of insulating,
heat-resisting material such as quartz, with a central aperture 17.
In use, this aperture is aligned with aperture 9 in wall 8,8' of
the vacuum chamber. The disc is relatively flat on the side which
is placed against wall 8,8' of the chamber, and on its other side
has two annular portions, the inner portion 18 being stepped
inwardly relative to the outer portion 19.
The ratio of the thickness of the disc to that of sealing member
16, preferably an O-ring, is set so that when the sample body is
forced against the disc, the O-ring is sufficiently compressed to
form a vacuum-tight seal between the surface 12 and the wall
8,8'.
The annular gap 20, formed between the surface 12 and the inner
annular portion 18, is approximately 0.2 mm wide, and extends
approximately 5 mm parallel to surface 12.
The sample body 13 may be cooled to predetermined temperature by
means of a water jacket 21, and gas, preferably argon may be passed
through chamber 2 via gas inlet and outlet connections (not shown).
The gas connections are preferably arranged on opposite sides of
the chamber relative to the sample area so that a flow of pure
argon across the cathode surface may be maintained to sweep
impurity molecules away from the viewing area.
In a further embodiment, disc 15 instead of being solid may be
provided with internal gas flow passages to direct gas into the
area of the discharge in such a way that sputtered atoms are swept
into the body of the vacuum chamber 2 to facilitate spectrochemical
analysis. FIGS. 4 - 6 of the accompanying drawings depict such an
arrangement.
In FIGS. 4A and 4B a disc 15 having internal gas passages is shown
in two halves prior to assembly.
FIG. 5A represents a cross-section through the halves of disc 15
shown in FIGS. 4A and 4B along the lines AA, with the halves
arranged in opposing relation. FIG. 5B is a similar view along
lines BB in FIGS. 4A and 4B. In FIG. 4A, a circular groove of
rectangular cross-section is shown on the upper face of the lower
half of the disc, and in FIG. 4B a corresponding groove 21' can be
seen on the lower face of the upper half of the disc. As shown in
FIG. 5B in particular, grooves 21 and 21' when aligned opposite
each other form a gas circulation passage within the composite disc
15. A gas inlet port 22 can be seen in FIGS. 4A and 5B leading into
gas circulation passages 21,21'. Returning to FIG. 4B, four
additional grooves 23 may be seen leading from the gas circulation
passage 21,21' to the central aperture 17. In use, the disc 15 is
arranged as shown in FIG. 6 with a gas inlet tube 25 inserted
partially into hole 22 so that argon, at a suitably low flow rate
is directed into the region of the sputtering discharge through gas
outlet ducts 24 (shown in FIG. 5A). It has been found unnecessary
in practice for a leak free connection to be made between gas inlet
tube 25 and inlet port 22, as adequate sweeping of atoms, and
molecules, out of the discharge region has been attained with quite
loose connections. The argon, preferably in a purified form, passes
from the discharge region to a vacuum pump, shown schematically in
FIG. 1, which is kept running while the sputtering discharge is
operating.
In an alternative construction, the gas circulation passage 21,21'
may be connected to a series of ducts leading into the region of
the annular gap 20 shown in FIG. 2, so that gas then passes out
from this region, through aperture 17 in sample locating means 15
and into the body of the vacuum chamber 2, and thence to the vacuum
pump.
In operation, the chamber is brought up to atmospheric pressure
with dry argon before being opened, and a fresh sample is placed on
the disc 15 over the aperture 9. The chamber is pumped down to a
vacuum of approximately 5 microns over about 1 minute to clear as
much contamination from the sample and chamber as possible. The
pressure of argon is then let up to about 5 torr, and a steady flow
of argon of about 0.2 litre per minute is maintained below the
sample. The sputtering discharge is then started and allowed a
certain time, usually a minute or so, to further remove surface
contaminants and settle to a steady condition before reliable
readings may be taken. FIG. 4 shows a typical warm-up trace of the
intensity of fluorescence radiation emitted from an atomic vapour
derived from the surface which had received no prior sputtering
treatment. As can be seen, the signal reaches a peak value after
only about 5 seconds, and then approaches a final equilibrium value
after 1 minute or so.
Repeated use of the system provided by the invention without
discernible interference caused by sputtered layers has been found
possible. The annular gap 20 has been found to contain the
discharge to the aperture 17 without any fringing into the gap, and
material sputtered on the disc and wall surfaces outside the
annular gap has not become electrically connected to the
cathode.
It will be apparent that many modifications can be made to the
embodiment of the invention described herein, and it is to be
understood that the invention is not limited to the details of the
construction illustrated, but includes all variations falling
within its spirit and scope.
* * * * *