ELEMENTAL ANALYZING APPARATUS
United States Patent 3665182
A compact instrument for solid surface analysis comprising a compact ion generator, a movable target support, an energy analyzer and an ion detector mounted together on a flange adapted to form the end wall of a vacuum chamber.
US Patent References:
LOW-ENERGY ION SCATTERING APPARATUS AND METHOD FOR ANALYZING THE SURFACE OF A SOLID
Smith - November 1969 - 3480774
LOW-ENERGY ION SCATTERING APPARATUS AND METHOD FOR ANALYZING THE SURFACE OF A SOLID
Smith - November 1969 - 3480774
Inventors:
Goff, Robert F. (White Bear Lake, MN)
Smith, David P. (North Hudson, WI)
Smith, David P. (North Hudson, WI)
Application Number:
04/850810
Publication Date:
05/23/1972
Filing Date:
08/18/1969
View Patent Images:
Export Citation:
Assignee:
Minnesota Mining and Manufacturing Company (St. Paul, MN)
Primary Class:
Other Classes:
250/397, 313/362.100, 250/289, 250/309, 250/305
International Classes:
H01J49/14; H01J49/28; H01J49/48; H01J49/00; H01J49/10; H01J49/26; H01J37/26
Field of Search:
250/49.5PI,41.9SA,41.9SB,41.9SR,41.9SE,41.9D 313/63
Primary Examiner:
Lawrence, James W.
Assistant Examiner:
Nelms D. C.
Claims:
What is claimed is
1. A compact elemental analyzing apparatus adapted to fit within an evacuable chamber containing a noble gas and having particular utility in determining the elemental composition of a solid surface by measuring the energy and the quantity of primary ions scattered from elements forming said solid surface and received by an energy analyzer positioned adjacent to said surface to thereby selectively identify said elements and determine the quantity of each said element respectively, said apparatus including in combination a target support for supporting in a predetermined location a sample to be elementally analyzed, an ion generating means for producing a beam of primary ions having known mass and substantially the same known kinetic energy and for directing said primary ions along a beam axis toward said sample, an energy analyzer for transmitting scattered primary ions which have a second known kinetic energy value less than the original kinetic energy of said primary ions to measure the energy lost as a result of collision with said surface to identify the atomic mass of the element scattering said primary ions, and ion detector means to receive the transmitted primary ions for converting the received ions into an electrical signal, wherein said ion generating means comprises:
2. A compact elemental analyzing apparatus of claim 1 wherein said energy analyzer comprises,
3. A compact elemental analyzing apparatus of claim 2 wherein the ion generator means further comprises,
4. A compact elemental analyzing apparatus of claim 3 wherein said target support comprises,
5. A compact elemental analyzing apparatus of claim 4 wherein said compact ion generator further comprises,
6. A compact elemental analyzing apparatus adapted to fit within an evacuable chamber containing a noble gas and having particular utility in determining the elemental composition of a solid surface by measuring the energy and the quantity of primary ions scattered from elements forming said solid surface and received by an energy analyzer positioned adjacent to said surface to thereby selectively identify said elements and determine the quantity of each said element respectively, said apparatus including in combination a target support for supporting in a predetermined location a sample to be elementally analyzed, an ion generating means for producing a beam of primary ions having known mass and substantially the same known kinetic energy and for directing said primary ions along a beam axis toward said sample, an energy analyzer for transmitting scattered primary ions which have a second known kinetic energy value less than the original kinetic energy of said primary ions to measure the energy lost as a result of collision with said surface to identify the atomic mass of the element scattering said primary ions, and ion detector means to receive the transmitted primary ions for converting the received ions into an electrical signal, wherein said energy analyzer comprises,
7. A compact elemental analyzing apparatus of claim 6 wherein the ion generator means further comprises,
8. A compact elemental analyzing apparatus of claim 7 wherein said target support comprises,
9. A compact elemental analyzing apparatus of claim 8 wherein said ion generating means comprises,
10. A compact elemental analyzing apparatus of claim 9 wherein said compact ion generator further comprises,
11. A compact elemental analyzing apparatus adapted to fit within an evacuable chamber containing a noble gas and having particular utility in determining the elemental composition of a solid surface by measuring the energy and the quantity of primary ions scattered from elements forming said solid surface and received by an energy analyzer positioned adjacent to said surface to thereby selectively identify said elements and determine the quantity of each said element respectively, said apparatus including in combination a target support for supporting in a predetermined location a sample to be elementally analyzed, an ion generating means for producing a beam of primary ions having known mass and substantially the same known kinetic energy and for directing said primary ions along a beam axis toward said sample, an energy analyzer for transmitting scattered primary ions which have a second known kinetic energy value less than the original kinetic energy of said primary ions to measure the energy lost as a result of collision with said surface to identify the atomic mass of the element scattering said primary ions, and ion detector means to receive the transmitted primary ions for converting the received ions into an electrical signal, wherein the improvement comprises,
12. a flange adapted to form an end wall of the evacuable chamber,
13. a support frame secured to said flange,
14. a tubular housing supported by said frame and adapted to support a compact ion generator,
15. a multi-positionable target support secured to said frame and located adjacent to said tubular housing to position successive samples within a predetermined target location, including
16. an energy analyzer supported by said frame and having an entrance positioned adjacent to said target support and extending adjacently along said tubular housing for transmitting ions therethrough which have a predetermined energy, and
17. ion detector means supported by said frame and positioned adjacent to said tubular housing and said energy analyzer for converting the transmitted ions into an electrical signal, wherein the primary ions from said compact ion generator are directed to bombard a first sample from which the scattered primary ions are received and transmitted to said ion detector means for indicating the surface elemental composition of the sample by said electrical signal.
18. The method of determining the elemental composition of a solid surface by measuring the energy and quantity of primary ions developed by an ion generator and scattered from a surface to be analyzed, the improvement comprising the steps of
19. The method of claim 12 further including the step of gettering the interior of a said chamber after it has been evacuated to said predetermined active gas pressure to chemically adsorb substantially all active gases in said chamber.
20. The method of claim 12 wherein the evacuating of said chamber comprises the step of cryogenically pumping said chamber.
21. The method of claim 12 wherein said energy analyzer inserted within said chamber has conductive entrance and exit diaphragms formed with slits and wherein said method further comprises the step of positively biasing said diaphragms to restrict the noise level afforded by sputtered and extraneous scattered ions.
1. A compact elemental analyzing apparatus adapted to fit within an evacuable chamber containing a noble gas and having particular utility in determining the elemental composition of a solid surface by measuring the energy and the quantity of primary ions scattered from elements forming said solid surface and received by an energy analyzer positioned adjacent to said surface to thereby selectively identify said elements and determine the quantity of each said element respectively, said apparatus including in combination a target support for supporting in a predetermined location a sample to be elementally analyzed, an ion generating means for producing a beam of primary ions having known mass and substantially the same known kinetic energy and for directing said primary ions along a beam axis toward said sample, an energy analyzer for transmitting scattered primary ions which have a second known kinetic energy value less than the original kinetic energy of said primary ions to measure the energy lost as a result of collision with said surface to identify the atomic mass of the element scattering said primary ions, and ion detector means to receive the transmitted primary ions for converting the received ions into an electrical signal, wherein said ion generating means comprises:
2. A compact elemental analyzing apparatus of claim 1 wherein said energy analyzer comprises,
3. A compact elemental analyzing apparatus of claim 2 wherein the ion generator means further comprises,
4. A compact elemental analyzing apparatus of claim 3 wherein said target support comprises,
5. A compact elemental analyzing apparatus of claim 4 wherein said compact ion generator further comprises,
6. A compact elemental analyzing apparatus adapted to fit within an evacuable chamber containing a noble gas and having particular utility in determining the elemental composition of a solid surface by measuring the energy and the quantity of primary ions scattered from elements forming said solid surface and received by an energy analyzer positioned adjacent to said surface to thereby selectively identify said elements and determine the quantity of each said element respectively, said apparatus including in combination a target support for supporting in a predetermined location a sample to be elementally analyzed, an ion generating means for producing a beam of primary ions having known mass and substantially the same known kinetic energy and for directing said primary ions along a beam axis toward said sample, an energy analyzer for transmitting scattered primary ions which have a second known kinetic energy value less than the original kinetic energy of said primary ions to measure the energy lost as a result of collision with said surface to identify the atomic mass of the element scattering said primary ions, and ion detector means to receive the transmitted primary ions for converting the received ions into an electrical signal, wherein said energy analyzer comprises,
7. A compact elemental analyzing apparatus of claim 6 wherein the ion generator means further comprises,
8. A compact elemental analyzing apparatus of claim 7 wherein said target support comprises,
9. A compact elemental analyzing apparatus of claim 8 wherein said ion generating means comprises,
10. A compact elemental analyzing apparatus of claim 9 wherein said compact ion generator further comprises,
11. A compact elemental analyzing apparatus adapted to fit within an evacuable chamber containing a noble gas and having particular utility in determining the elemental composition of a solid surface by measuring the energy and the quantity of primary ions scattered from elements forming said solid surface and received by an energy analyzer positioned adjacent to said surface to thereby selectively identify said elements and determine the quantity of each said element respectively, said apparatus including in combination a target support for supporting in a predetermined location a sample to be elementally analyzed, an ion generating means for producing a beam of primary ions having known mass and substantially the same known kinetic energy and for directing said primary ions along a beam axis toward said sample, an energy analyzer for transmitting scattered primary ions which have a second known kinetic energy value less than the original kinetic energy of said primary ions to measure the energy lost as a result of collision with said surface to identify the atomic mass of the element scattering said primary ions, and ion detector means to receive the transmitted primary ions for converting the received ions into an electrical signal, wherein the improvement comprises,
12. a flange adapted to form an end wall of the evacuable chamber,
13. a support frame secured to said flange,
14. a tubular housing supported by said frame and adapted to support a compact ion generator,
15. a multi-positionable target support secured to said frame and located adjacent to said tubular housing to position successive samples within a predetermined target location, including
16. an energy analyzer supported by said frame and having an entrance positioned adjacent to said target support and extending adjacently along said tubular housing for transmitting ions therethrough which have a predetermined energy, and
17. ion detector means supported by said frame and positioned adjacent to said tubular housing and said energy analyzer for converting the transmitted ions into an electrical signal, wherein the primary ions from said compact ion generator are directed to bombard a first sample from which the scattered primary ions are received and transmitted to said ion detector means for indicating the surface elemental composition of the sample by said electrical signal.
18. The method of determining the elemental composition of a solid surface by measuring the energy and quantity of primary ions developed by an ion generator and scattered from a surface to be analyzed, the improvement comprising the steps of
19. The method of claim 12 further including the step of gettering the interior of a said chamber after it has been evacuated to said predetermined active gas pressure to chemically adsorb substantially all active gases in said chamber.
20. The method of claim 12 wherein the evacuating of said chamber comprises the step of cryogenically pumping said chamber.
21. The method of claim 12 wherein said energy analyzer inserted within said chamber has conductive entrance and exit diaphragms formed with slits and wherein said method further comprises the step of positively biasing said diaphragms to restrict the noise level afforded by sputtered and extraneous scattered ions.
Description:
BACKGROUND OF THE INVENTION
The science of analyzing the elemental composition of a solid surface for elemental analysis, impurity detection or to determine the cleanliness of the surface has generated the need for a concept and apparatus to non-destructively analyze such a surface.
It is known to scatter high-energy ions, derived from a noble gas, from a target surface and at a relatively large angle therefrom to determine the charge state and scattered energy of a primary ion resulting from such a collision. The energy of the scattered primary ion was interpreted in terms of a binary-elastic collision model. This high-energy ion scattering technique, wherein the ion energy is in the order of 40 keV to 80 keV, is disclosed by Sheldon Datz and Cornelis Snoek, in an article entitled "Large-Angle, Single-Collision Scattering of Argon Ions (40-80 keV) from Metals," Physical Review, Volume 134, Nos. 2A, Apr. 20, 1964.
Analyzing the atoms of a solid by elastically scattering ions off the target surface described in the article by Datz and Snoek has serious disadvantages. The primary ion beam directed at the target is required to have a kinetic energy value in the order of 40-80 keV. Such a relatively high-energy ion beam exhibits "multiple charge stripping" during scattering, has a relatively deep penetration capability and causes considerable removal of surface atoms. A large percentage of the primary ions exhibit "multiple charge stripping" whereby such ions have a plurality of electrons stripped therefrom. Therefore, an energy spectrum derived from scattered high-energy ions from a single element exhibits several distinct peaks. The many peaks for a single target atom due to "multiple charge stripping" makes analysis of the energy spectrum ambiguous and inaccurate. Also, when it is desired to examine isotopes of a specific element, besides the necessary high resolution of the examining apparatus, the concept and apparatus of Datz and Snoek also requires that the single crystals be precisely oriented with respect to the incident ion beam to avoid multiple collisions within a crystal.
Another apparatus and method for analyzing the composition of elements forming a solid surface has been disclosed and taught by the D. P. Smith patent application Ser. No. 641,582, now U.S. Pat. No. 3,480,774. The low-energy ion beam analyzing technique utilized in the Smith invention results in substantial improvement in accuracy of surface analysis relative to the technique or method described in the article by Datz and Snoek. The ion beam utilized by Smith has such a low energy level that "multiple charge stripping" effects are negligible whereby extra peaks in the energy spectrum due to "multiple charge stripping" are eliminated. Further, the use of a low-energy ion beam has the advantage of minimizing the damage to the surface being analyzed.
The relatively non-destructive method of surface analysis taught by the Smith U.S. Pat. No. 3,480,774 includes, inter alia, the steps of producing a beam of primary ions having known mass and energy values, directing the beam of primary ions toward a surface to be analyzed, analyzing the energy of the primary ions scattered from elements forming the solid surface wherein the energy lost by the scattered primary ions as a result of the collision with the elements is measured to identify the atomic mass of the various elements. Analysis of a solid surface, as taught by the Smith U.S. Pat. No. 3,480,774 is based on a binary-elastic collision theory and formulation based on the following assumptions. First, it is assumed that the target atoms are essentially unbound from their neighboring atoms. Thus, during collision the target atoms behave as independently as though they were gas atoms. Second, it is assumed that the target atoms are substantially stationary before collision with the primary ions. Third, the energy losses of the primary ions during collision with the target atoms are assumed to be completely kinetic. Based on these three assumptions and the principles of conservation of energy and momentum, a prediction of the postcollision energies of the primary ions can be determined in terms of scattering angles, mass ratio and precollision energies. Most importantly, this is accomplished without recourse to adjustable or empirical factors. The following equation for the kinetic energy ratio of the primary ion after collision to the primary ion prior to collision, and simplified by selection of the scattering angle θ = 90°, becomes:
and when solved for M 2 yields:
where
M 1 = mass of primary ion
M 2 = mass of target atom
E 0 = kinetic energy of primary ion prior to collision
E 1 = kinetic energy of primary ion after collision.
Some of the main components preferred for performing ion scattering surface analysis are: a monoenergetic ion source with good focusing properties at low energies, an energy analyzer capable of defining a percise scattering angle together with good energy resolution, an ion detector, and an advancement means for advancing and positioning successive samples in a predetermined position without necessitating the loss of vacuum and inert gas atmosphere. To increase the resolution at a given electrical signal level with a low energy scattering instrument it becomes critical to
1. minimize the ion energy spread within the ion beam,
2. to optimize the collimation and focusing of the ion beam prior to bombardment of the sample, and
3. to optimize the energy analyzer resolving power at a known scattering angle.
In obtaining the compromise between the desired high instrument resolution and an adequate electrical signal level it becomes advantageous to locate the ion generating means and the energy analyzer in close proximity to the sample being analyzed.
SUMMARY OF THE INVENTION
The present invention relates to a low energy elemental analyzing apparatus for achieving greater resolution while maintaining a sufficient signal, with a compact apparatus that permits closer positioning of the components to improve the resolution and includes
a compact ion generating means for producing a beam of substantially monoenergetic ions wherein the energy variation is 0.5 percent or less of the primary ion beam value at full width half maximum and wherein the ions are well collimated and focused prior to bombardment of the sample, with the generating means close to the sample to reduce ion-ambient gas collisions, and
a compact energy analyzer closely positioned adjacent to the sample for maximizing the number of scattered ions received by said energy analyzer to optimize the number of ions received by the ion detector and utilizing an entrance diaphragm having a narrow slit width therein to minimize the divergence of the ions passing through the entrance slit, resulting in improved resolution.
Thus, a primary advantage of the apparatus of the present invention is that the resolution of the system, which definitively and unequivocally determines the atomic mass of the target element, has been improved with a compact apparatus.
A further advantage of the apparatus of the present invention is that greater ion beam stability throughout the ion beam operating range has been obtained with a relatively compact and simplified apparatus.
Yet another advantage of the present invention is that the ion beam generating means has a stable operability range below 100 eV.
Still another advantage of the present invention is that the apparatus can be operated under a static gas condition, to enhance stability and simplicity of operation, and eliminates the need for dynamic pumping of the noble gases during operation. The present invention also provides a structure where the several parts are disposed in a single chamber and are in open communication with each other, thus avoiding the need for multiple chambers, connecting valves and pumps.
These and other advantages of the present invention become apparent and can be best understood from reading the following detailed description which refers to the accompanying drawing wherein:
FIG. 1 is a side elevational view illustrating an apparatus constructed in accordance with the present invention;
FIG. 2 is a block diagram, a partial pictorial and schematic diagram illustrating the electrical components of the apparatus of FIG. 1;
FIG. 3 is a graph illustrating analysis of a solid surface and the improved resolution and definition resulting from biasing of the entry and exit diaphragms; and
FIG. 4 is a graph illustrating the analysis of a copper sample showing scattering from the individual isotopes Cu 63 and Cu 65 .
In the accompanying drawing and description of the present invention, and referring in particular to FIG. 1, there is shown a compact elemental analyzing apparatus comprising a flange 12 adapted to fit onto a standard size vacuum chamber (not shown) as is commercially available, a support frame 16 secured to the flange, a multipositionable target support 60 secured to the frame, an ion generating means 26, an energy analyzer 45, an ion detector 70, and indicating apparatus 80.
The flange 12 is a standard 8-inch ultra-high vacuum flange including a plurality of apertures about is periphery utilized to secure the flange to a housing shown in phantom in FIG. 1 which in combination defines the ultra-high vacuum chamber. After the flange is secured to the housing, wherein the frame, target support, ion generating means and other apparatus shown in FIG. 1 is located within the same vacuum chamber, a vacuum pump (not shown) evacuates the chamber to a pressure of less than about 10 - 8 Torr. A getter (not shown) is positioned within the chamber to further purify the active elements remaining in the chamber. The pumping is discontinued by shutting off the vacuum pump. A noble gas is released by suitable valves into the chamber, until the static pressure is increased to approximately 5 × 10 - 5 Torr, as measured on Bayard-Alpert type pressure gauges, and all openings to the chamber are closed. Thus, no additional noble gas is directed into the chamber. The noble gas atmosphere within the chamber is utilized to analyze the elements forming the solid surface of the sample. The noble gas used herein may be any noble gas, however, Helium (He) and Argon (Ar) are commonly used. Insulated electrical feed throughs or connectors 14 project through the flange to provide the necessary electrical connections between the components within the chamber and the electrical apparatus located outside of the chamber.
The support frame 16, which is secured to the flange 12, includes a base 18, a base support 20 to provide clearance between the electrical connectors 14 and the base, and a vertical column 22 secured to the base to support the target support 60.
The multipositionable target support 60 includes a rotatable octagonal target wheel 61, side arms 62 for rotatably mounting the wheel relative to the vertical column 22, and for insulating the wheel from the column 22, and wheel advancement means including tooth ratchet wheel 63 to sequentially advance the target wheel one-sixteenth of a revolution per actuation of the solenoid. On each planar spaced peripheral surface or face 66 of the octagonal wheel may be placed a sample which is to be elementally analyzed. The sample is held on each face by any suitable temporary fastening such as screw or spring fasteners. It should be apparent that the target wheel may be constructed with a different number of faces, e.g., hexagonal, and the ratchet wheel may have a different number of teeth numerically corresponding to the number of faces on the target wheel. The target support includes a sliding contact arm insulated from said frame 16 and engageable with indents to electrically connect the wheel 61 and the sample being bombarded with a current measuring device 81 (see FIG. 2) for monitoring the level of ion beam current. The solenoid 64, which is a standard vacuum solenoid, is electrically connected to a target selector power supply 82 which is independently actuated for advancing and positioning successive samples into the predetermined target location (as shown in FIG. 1).
The ion generating means comprises a grounded tubular housing 25 essentially 2 × 3 × 4 inches adapted to support the operative components of the compact ion generator. The compact ion generator structure, essentially 1 × 1 × 3 inches includes a heated filament 27 for producing electrons, a highly transparent grid 28, having greater than 80 percent open area and defining within extractor plate 31 an ionization region 29, a repeller 30 encircling the filament 27, a first 33, second 35, third 37, and fourth 39, anode plates, and a feed-back stabilization loop 41.
A filament power supply 84 powers the filament to produce electrons and a grid power supply 83 biases the grid with respect to the filament. The produced electrons from the filament are accelerated, by the relatively transparent grid 28, to a potential sufficient to ionize the noble gas atoms. For example, the electrons would have from 100 to 125 electron volts of energy which is sufficient to ionize helium which has an ionization potential of about 24 electron volts. The repeller 30 is at filament potential and repels or deflects any approaching electrons to result in a long electron path which increases the probability of the electrons striking an atom of the gas to ionize the gas atom.
If the static pressure of the noble gas within the evacuatable chamber is increased then the ion beam current is increased. Therefore, by regulating the electron current at a constant gas pressure the ion beam current is regulated. In the plasma source of the prior art, many adjustments could be made to regulate ion beam current but none of the adjustments could be easily related to the ion beam current. With such a plasma source it is extremely difficult to build a beam current regulating feed-back circuit. The feed-back stabilization loop 41, of the present embodiment, has been established between the filament power supply and the grid power supply. This stabilization loop maintains a stable electron grid current which controls the ion beam current throughout pressure changes within the evacuatable chamber.
An ion gun voltage divider network 85 biases the extractor plate 31 to a potential to extract positive ions from the ionization region 29. The network 85 includes nine R 1 fixed resistors each having a resistance of 470 kohms, three R 2 10 turn linear potentiometers of 500 kohms arranged in a series and parallel circuit to selectively bias the extractor plate 31 and the anode plates 33, 35, and 37, except the fourth anode plate 39 which is grounded.
The extractor plate 31 includes an extractor aperture 32 of about one-quarter inch (0.6 centimeters), located about the beam axis 42, to extract the positive ions and the ions are then directed into a beam by the anode plates. Each anode plate has a potential applied thereto from the network 85. The first anode plate 33 is primarily used to control, modulate and initially focus the extracted ions into a collimated beam. The second anode plate 35, which is spaced from the first plate 33 a distance greater than the spacing between the other plates, is the primary beam collimating and focusing anode. The third anode plate 37 is run at a substantially fixed potential from the voltage divider network 85 and the fourth plate 39 is at ground potential or could be connected to one side of a high voltage power supply 86 and biased with respect to ground. The anode plates are each formed with a small aperture and of very thin conductive material to control the ion flow and to maintain a monoenergetic beam. The plates are, for example, 0.010 inch thick to minimize the wall surface defining the apertures for minimizing of the interaction of the passed ions with the wall surface and loss of energy in the ions passing therethrough.
The beam passing out of the tubular housing 25 is now directed through the noble gas atmosphere a distance of less than 10 cm toward a face supporting a sample to be analyzed. Beam perturbing collisions are estimated to limit the mean free path of ions at this pressure range to about 30 cm. A pair of deflector plates 57, positioned near the end of the housing 25 and on opposite sides of the beam, serve to deflect the beam slightly to scan the sample. The plates 57 are charged by an ion deflector power supply 87 controlled either manually or programmed. The deflector plates 57 also help in optimizing the the scattering angle or aligning the beam on the surface of the sample so the axis of the scattered ions is aligned with the entrance diaphragm of the energy analyzer for electrical signal optimization.
The ion beam strikes or bombards the sample on the preselected target face and the impinging primary ions are scattered therefrom. The current to the sample by the impinging beam is measured by the current measuring device 81 and such measured current is used to determine the current density striking the surface of the sample.
The energy analyzer 45 comprises an entrance diaphragm 46, having a rectangular entrance slit 47 (long and narrow), an exit diaphragm 49, having a rectangular exit slit 50, and two curved electrostatic analyzer plates 48. The entrance diaphragm 46 and exit diaphragm 49 are charged by a diaphragm biasing power supply 88. The diaphragms may be separately or simultaneously grounded or biased to similar or different positive potentials. The slits in the diaphragms have a preferred width of .005 inch and the entrance diaphragm is spaced about one centimeter from the surface of the sample being analyzed.
By charging the entrance diaphragm 46 and exit diaphragm 49 some scattered ions are repelled from entering the energy analyzer to effectively reduce the detected electrical background signal. As shown in FIG. 3 the effect of positive biasing of the entrance and exit diaphragms is shown. In the graph the energy ratio (E 1 /E 0 ) of the positive scattered ions collected is plotted against the ion current of 1,500 eV Helium ions scattered from an aluminum surface having a light aluminum oxide layer thereon. The first plotting 92 shows the typical spectrum of the sample with the diaphragms at 0 potential and the plotting 93 indicates the spectrum with the diaphragms with a positive bias. The peaks in the plots 92 and 93 indicate the presence of aluminum and oxygen at the energy ratios of about 0.55 and 0.7 respectively. Thus the graph clearly represents the increased resolution and elimination of background signals with the bias placed on the diaphragms to suppress the background ion spectrum occurring from the sputtered ions.
The analyzer plates 48 are charged by the output from an analyzer plate sweeping power supply 90 receiving power from a dual power supply 89. The analyzer plate sweeping power supply 90 permits a suitable charge to be applied to the plates to direct ions having a predetermined mass and energy through the slit in the exit diaphragm. The analyzer plates 48 have a mean radius of 2 inches. The illustrated analyzer 45 is a standard 127° analyzer.
The scattered ions are thus received from the sample by the energy analyzer and the ions having a predetermined energy value are passed therethrough. The number of ions being passed are detected and converted into electrons by the ion detector 70, to be received by the electron collector 68. The electron collector 68 converts the collected electrons into an electronic signal received by a pulse counting circuit 100 of an indicating apparatus 80. The data from the circuit 100 may then be permanently recorded as a graph on an X--Y recorder 102, as shown by the graph of FIG. 3, and/or visually indicated on an oscilliscope 101.
The ion detector 70, within enclosure 69, is a continuous channel electron multiplier 71, powered by a high voltage power supply 99, having an 8 millimeter diameter cone entrance which encompasses the entire exit slit in the exit diaphragm of the 127° energy analyzer. The electron multiplier may be a commercially available device such as Model No. CEM--4028 manufactured by Bendix Corporation, Ann Arbor, Michigan.
The science of analyzing the elemental composition of a solid surface for elemental analysis, impurity detection or to determine the cleanliness of the surface has generated the need for a concept and apparatus to non-destructively analyze such a surface.
It is known to scatter high-energy ions, derived from a noble gas, from a target surface and at a relatively large angle therefrom to determine the charge state and scattered energy of a primary ion resulting from such a collision. The energy of the scattered primary ion was interpreted in terms of a binary-elastic collision model. This high-energy ion scattering technique, wherein the ion energy is in the order of 40 keV to 80 keV, is disclosed by Sheldon Datz and Cornelis Snoek, in an article entitled "Large-Angle, Single-Collision Scattering of Argon Ions (40-80 keV) from Metals," Physical Review, Volume 134, Nos. 2A, Apr. 20, 1964.
Analyzing the atoms of a solid by elastically scattering ions off the target surface described in the article by Datz and Snoek has serious disadvantages. The primary ion beam directed at the target is required to have a kinetic energy value in the order of 40-80 keV. Such a relatively high-energy ion beam exhibits "multiple charge stripping" during scattering, has a relatively deep penetration capability and causes considerable removal of surface atoms. A large percentage of the primary ions exhibit "multiple charge stripping" whereby such ions have a plurality of electrons stripped therefrom. Therefore, an energy spectrum derived from scattered high-energy ions from a single element exhibits several distinct peaks. The many peaks for a single target atom due to "multiple charge stripping" makes analysis of the energy spectrum ambiguous and inaccurate. Also, when it is desired to examine isotopes of a specific element, besides the necessary high resolution of the examining apparatus, the concept and apparatus of Datz and Snoek also requires that the single crystals be precisely oriented with respect to the incident ion beam to avoid multiple collisions within a crystal.
Another apparatus and method for analyzing the composition of elements forming a solid surface has been disclosed and taught by the D. P. Smith patent application Ser. No. 641,582, now U.S. Pat. No. 3,480,774. The low-energy ion beam analyzing technique utilized in the Smith invention results in substantial improvement in accuracy of surface analysis relative to the technique or method described in the article by Datz and Snoek. The ion beam utilized by Smith has such a low energy level that "multiple charge stripping" effects are negligible whereby extra peaks in the energy spectrum due to "multiple charge stripping" are eliminated. Further, the use of a low-energy ion beam has the advantage of minimizing the damage to the surface being analyzed.
The relatively non-destructive method of surface analysis taught by the Smith U.S. Pat. No. 3,480,774 includes, inter alia, the steps of producing a beam of primary ions having known mass and energy values, directing the beam of primary ions toward a surface to be analyzed, analyzing the energy of the primary ions scattered from elements forming the solid surface wherein the energy lost by the scattered primary ions as a result of the collision with the elements is measured to identify the atomic mass of the various elements. Analysis of a solid surface, as taught by the Smith U.S. Pat. No. 3,480,774 is based on a binary-elastic collision theory and formulation based on the following assumptions. First, it is assumed that the target atoms are essentially unbound from their neighboring atoms. Thus, during collision the target atoms behave as independently as though they were gas atoms. Second, it is assumed that the target atoms are substantially stationary before collision with the primary ions. Third, the energy losses of the primary ions during collision with the target atoms are assumed to be completely kinetic. Based on these three assumptions and the principles of conservation of energy and momentum, a prediction of the postcollision energies of the primary ions can be determined in terms of scattering angles, mass ratio and precollision energies. Most importantly, this is accomplished without recourse to adjustable or empirical factors. The following equation for the kinetic energy ratio of the primary ion after collision to the primary ion prior to collision, and simplified by selection of the scattering angle θ = 90°, becomes:
and when solved for M 2 yields:
where
M 1 = mass of primary ion
M 2 = mass of target atom
E 0 = kinetic energy of primary ion prior to collision
E 1 = kinetic energy of primary ion after collision.
Some of the main components preferred for performing ion scattering surface analysis are: a monoenergetic ion source with good focusing properties at low energies, an energy analyzer capable of defining a percise scattering angle together with good energy resolution, an ion detector, and an advancement means for advancing and positioning successive samples in a predetermined position without necessitating the loss of vacuum and inert gas atmosphere. To increase the resolution at a given electrical signal level with a low energy scattering instrument it becomes critical to
1. minimize the ion energy spread within the ion beam,
2. to optimize the collimation and focusing of the ion beam prior to bombardment of the sample, and
3. to optimize the energy analyzer resolving power at a known scattering angle.
In obtaining the compromise between the desired high instrument resolution and an adequate electrical signal level it becomes advantageous to locate the ion generating means and the energy analyzer in close proximity to the sample being analyzed.
SUMMARY OF THE INVENTION
The present invention relates to a low energy elemental analyzing apparatus for achieving greater resolution while maintaining a sufficient signal, with a compact apparatus that permits closer positioning of the components to improve the resolution and includes
a compact ion generating means for producing a beam of substantially monoenergetic ions wherein the energy variation is 0.5 percent or less of the primary ion beam value at full width half maximum and wherein the ions are well collimated and focused prior to bombardment of the sample, with the generating means close to the sample to reduce ion-ambient gas collisions, and
a compact energy analyzer closely positioned adjacent to the sample for maximizing the number of scattered ions received by said energy analyzer to optimize the number of ions received by the ion detector and utilizing an entrance diaphragm having a narrow slit width therein to minimize the divergence of the ions passing through the entrance slit, resulting in improved resolution.
Thus, a primary advantage of the apparatus of the present invention is that the resolution of the system, which definitively and unequivocally determines the atomic mass of the target element, has been improved with a compact apparatus.
A further advantage of the apparatus of the present invention is that greater ion beam stability throughout the ion beam operating range has been obtained with a relatively compact and simplified apparatus.
Yet another advantage of the present invention is that the ion beam generating means has a stable operability range below 100 eV.
Still another advantage of the present invention is that the apparatus can be operated under a static gas condition, to enhance stability and simplicity of operation, and eliminates the need for dynamic pumping of the noble gases during operation. The present invention also provides a structure where the several parts are disposed in a single chamber and are in open communication with each other, thus avoiding the need for multiple chambers, connecting valves and pumps.
These and other advantages of the present invention become apparent and can be best understood from reading the following detailed description which refers to the accompanying drawing wherein:
FIG. 1 is a side elevational view illustrating an apparatus constructed in accordance with the present invention;
FIG. 2 is a block diagram, a partial pictorial and schematic diagram illustrating the electrical components of the apparatus of FIG. 1;
FIG. 3 is a graph illustrating analysis of a solid surface and the improved resolution and definition resulting from biasing of the entry and exit diaphragms; and
FIG. 4 is a graph illustrating the analysis of a copper sample showing scattering from the individual isotopes Cu 63 and Cu 65 .
In the accompanying drawing and description of the present invention, and referring in particular to FIG. 1, there is shown a compact elemental analyzing apparatus comprising a flange 12 adapted to fit onto a standard size vacuum chamber (not shown) as is commercially available, a support frame 16 secured to the flange, a multipositionable target support 60 secured to the frame, an ion generating means 26, an energy analyzer 45, an ion detector 70, and indicating apparatus 80.
The flange 12 is a standard 8-inch ultra-high vacuum flange including a plurality of apertures about is periphery utilized to secure the flange to a housing shown in phantom in FIG. 1 which in combination defines the ultra-high vacuum chamber. After the flange is secured to the housing, wherein the frame, target support, ion generating means and other apparatus shown in FIG. 1 is located within the same vacuum chamber, a vacuum pump (not shown) evacuates the chamber to a pressure of less than about 10 - 8 Torr. A getter (not shown) is positioned within the chamber to further purify the active elements remaining in the chamber. The pumping is discontinued by shutting off the vacuum pump. A noble gas is released by suitable valves into the chamber, until the static pressure is increased to approximately 5 × 10 - 5 Torr, as measured on Bayard-Alpert type pressure gauges, and all openings to the chamber are closed. Thus, no additional noble gas is directed into the chamber. The noble gas atmosphere within the chamber is utilized to analyze the elements forming the solid surface of the sample. The noble gas used herein may be any noble gas, however, Helium (He) and Argon (Ar) are commonly used. Insulated electrical feed throughs or connectors 14 project through the flange to provide the necessary electrical connections between the components within the chamber and the electrical apparatus located outside of the chamber.
The support frame 16, which is secured to the flange 12, includes a base 18, a base support 20 to provide clearance between the electrical connectors 14 and the base, and a vertical column 22 secured to the base to support the target support 60.
The multipositionable target support 60 includes a rotatable octagonal target wheel 61, side arms 62 for rotatably mounting the wheel relative to the vertical column 22, and for insulating the wheel from the column 22, and wheel advancement means including tooth ratchet wheel 63 to sequentially advance the target wheel one-sixteenth of a revolution per actuation of the solenoid. On each planar spaced peripheral surface or face 66 of the octagonal wheel may be placed a sample which is to be elementally analyzed. The sample is held on each face by any suitable temporary fastening such as screw or spring fasteners. It should be apparent that the target wheel may be constructed with a different number of faces, e.g., hexagonal, and the ratchet wheel may have a different number of teeth numerically corresponding to the number of faces on the target wheel. The target support includes a sliding contact arm insulated from said frame 16 and engageable with indents to electrically connect the wheel 61 and the sample being bombarded with a current measuring device 81 (see FIG. 2) for monitoring the level of ion beam current. The solenoid 64, which is a standard vacuum solenoid, is electrically connected to a target selector power supply 82 which is independently actuated for advancing and positioning successive samples into the predetermined target location (as shown in FIG. 1).
The ion generating means comprises a grounded tubular housing 25 essentially 2 × 3 × 4 inches adapted to support the operative components of the compact ion generator. The compact ion generator structure, essentially 1 × 1 × 3 inches includes a heated filament 27 for producing electrons, a highly transparent grid 28, having greater than 80 percent open area and defining within extractor plate 31 an ionization region 29, a repeller 30 encircling the filament 27, a first 33, second 35, third 37, and fourth 39, anode plates, and a feed-back stabilization loop 41.
A filament power supply 84 powers the filament to produce electrons and a grid power supply 83 biases the grid with respect to the filament. The produced electrons from the filament are accelerated, by the relatively transparent grid 28, to a potential sufficient to ionize the noble gas atoms. For example, the electrons would have from 100 to 125 electron volts of energy which is sufficient to ionize helium which has an ionization potential of about 24 electron volts. The repeller 30 is at filament potential and repels or deflects any approaching electrons to result in a long electron path which increases the probability of the electrons striking an atom of the gas to ionize the gas atom.
If the static pressure of the noble gas within the evacuatable chamber is increased then the ion beam current is increased. Therefore, by regulating the electron current at a constant gas pressure the ion beam current is regulated. In the plasma source of the prior art, many adjustments could be made to regulate ion beam current but none of the adjustments could be easily related to the ion beam current. With such a plasma source it is extremely difficult to build a beam current regulating feed-back circuit. The feed-back stabilization loop 41, of the present embodiment, has been established between the filament power supply and the grid power supply. This stabilization loop maintains a stable electron grid current which controls the ion beam current throughout pressure changes within the evacuatable chamber.
An ion gun voltage divider network 85 biases the extractor plate 31 to a potential to extract positive ions from the ionization region 29. The network 85 includes nine R 1 fixed resistors each having a resistance of 470 kohms, three R 2 10 turn linear potentiometers of 500 kohms arranged in a series and parallel circuit to selectively bias the extractor plate 31 and the anode plates 33, 35, and 37, except the fourth anode plate 39 which is grounded.
The extractor plate 31 includes an extractor aperture 32 of about one-quarter inch (0.6 centimeters), located about the beam axis 42, to extract the positive ions and the ions are then directed into a beam by the anode plates. Each anode plate has a potential applied thereto from the network 85. The first anode plate 33 is primarily used to control, modulate and initially focus the extracted ions into a collimated beam. The second anode plate 35, which is spaced from the first plate 33 a distance greater than the spacing between the other plates, is the primary beam collimating and focusing anode. The third anode plate 37 is run at a substantially fixed potential from the voltage divider network 85 and the fourth plate 39 is at ground potential or could be connected to one side of a high voltage power supply 86 and biased with respect to ground. The anode plates are each formed with a small aperture and of very thin conductive material to control the ion flow and to maintain a monoenergetic beam. The plates are, for example, 0.010 inch thick to minimize the wall surface defining the apertures for minimizing of the interaction of the passed ions with the wall surface and loss of energy in the ions passing therethrough.
The beam passing out of the tubular housing 25 is now directed through the noble gas atmosphere a distance of less than 10 cm toward a face supporting a sample to be analyzed. Beam perturbing collisions are estimated to limit the mean free path of ions at this pressure range to about 30 cm. A pair of deflector plates 57, positioned near the end of the housing 25 and on opposite sides of the beam, serve to deflect the beam slightly to scan the sample. The plates 57 are charged by an ion deflector power supply 87 controlled either manually or programmed. The deflector plates 57 also help in optimizing the the scattering angle or aligning the beam on the surface of the sample so the axis of the scattered ions is aligned with the entrance diaphragm of the energy analyzer for electrical signal optimization.
The ion beam strikes or bombards the sample on the preselected target face and the impinging primary ions are scattered therefrom. The current to the sample by the impinging beam is measured by the current measuring device 81 and such measured current is used to determine the current density striking the surface of the sample.
The energy analyzer 45 comprises an entrance diaphragm 46, having a rectangular entrance slit 47 (long and narrow), an exit diaphragm 49, having a rectangular exit slit 50, and two curved electrostatic analyzer plates 48. The entrance diaphragm 46 and exit diaphragm 49 are charged by a diaphragm biasing power supply 88. The diaphragms may be separately or simultaneously grounded or biased to similar or different positive potentials. The slits in the diaphragms have a preferred width of .005 inch and the entrance diaphragm is spaced about one centimeter from the surface of the sample being analyzed.
By charging the entrance diaphragm 46 and exit diaphragm 49 some scattered ions are repelled from entering the energy analyzer to effectively reduce the detected electrical background signal. As shown in FIG. 3 the effect of positive biasing of the entrance and exit diaphragms is shown. In the graph the energy ratio (E 1 /E 0 ) of the positive scattered ions collected is plotted against the ion current of 1,500 eV Helium ions scattered from an aluminum surface having a light aluminum oxide layer thereon. The first plotting 92 shows the typical spectrum of the sample with the diaphragms at 0 potential and the plotting 93 indicates the spectrum with the diaphragms with a positive bias. The peaks in the plots 92 and 93 indicate the presence of aluminum and oxygen at the energy ratios of about 0.55 and 0.7 respectively. Thus the graph clearly represents the increased resolution and elimination of background signals with the bias placed on the diaphragms to suppress the background ion spectrum occurring from the sputtered ions.
The analyzer plates 48 are charged by the output from an analyzer plate sweeping power supply 90 receiving power from a dual power supply 89. The analyzer plate sweeping power supply 90 permits a suitable charge to be applied to the plates to direct ions having a predetermined mass and energy through the slit in the exit diaphragm. The analyzer plates 48 have a mean radius of 2 inches. The illustrated analyzer 45 is a standard 127° analyzer.
The scattered ions are thus received from the sample by the energy analyzer and the ions having a predetermined energy value are passed therethrough. The number of ions being passed are detected and converted into electrons by the ion detector 70, to be received by the electron collector 68. The electron collector 68 converts the collected electrons into an electronic signal received by a pulse counting circuit 100 of an indicating apparatus 80. The data from the circuit 100 may then be permanently recorded as a graph on an X--Y recorder 102, as shown by the graph of FIG. 3, and/or visually indicated on an oscilliscope 101.
The ion detector 70, within enclosure 69, is a continuous channel electron multiplier 71, powered by a high voltage power supply 99, having an 8 millimeter diameter cone entrance which encompasses the entire exit slit in the exit diaphragm of the 127° energy analyzer. The electron multiplier may be a commercially available device such as Model No. CEM--4028 manufactured by Bendix Corporation, Ann Arbor, Michigan.