US3916191A

US3916191A - Imaging apparatus and method for use with ion scattering spectrometer

Info

Publication number: US3916191A
Application number: US447339A
Authority: US
Inventors: John A Leys; Robert F Goff
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: Kratos Analytical Ltd; Kratos Ltd
Priority date: 1974-03-01
Filing date: 1974-03-01
Publication date: 1975-10-28
Anticipated expiration: 1992-10-28
Also published as: CA1026875A

Abstract

Improved apparatus and method for displaying an image representing the position and concentration of a given element on a sample surface. A signal identifying the concentration of the selected element is produced according to ion scattering spectroscopy in which the primary ion beam is scanned in two dimensions across the sample surface, and the energy analyzer is tuned to transmit scattered ions indicative of a given element. The signal thereupon produced is used to modulate a display device synchronized with the scanning of the primary ion beam.

Description

United States Patent Leys et al. Oct. 28, 1975 [54] IMAGING APPARATUS AND METHOD FOR 3,665,182 5/1972 Goff et a1 250/305 USE SCATTERING 1332??? 1111333 3 323533? ugan SPECTROMETER 3,767,926 10/1973 Coates et al.... 250/310 [75] Inventors: John A. Leys, Stillwater; Robert F. 3, 3,2 l/1974 Cre e 250/307 Goff, White Bear Lak b th of 3,829,691 8/1974 Hufnagel 250/307 Minn. P E J W. L [73] Assignee: Minnesota Mining & Manufacturing 2:22: g:$ g z Company Paul Attorney, Agent, or FirmA1exander, Sell, Steldt & [22] Filed: Mar. 1, 1974 Hu [21] Appl. No.: 447,339 ABSTRACT Improved apparatus and method for displaying an [52] 250/306; 250/307; 250/309 image representing the position and concentration of a [51] Ilrt. Cl. COIN gi element on a Sample surface. A ig identify [58] new of Search 250/3O5 ing the concentration of the selected element is pro- 250/310 duced according to ion scattering spectroscopy in which the primary ion beam is scanned in two dimen- [56] References Clted sions across the sample surface, and the energy analy- UNITED STATES PATENTS zer is tuned to transmit scattered ions indicative of a 3,229,087 1/1966 Shapiro 250/306 g en lement. The signal thereupon produced is used 3,517,191 6/1970 Liebl 250/309 to modulate a display device synchronized with the 3,614,311 10 1971 Fujiyasu et a1.... 250/310 scanning of the primary ion beam. 3,628,014 12/1971 Grubic 250/310 3,660,655 5 1972 Wardell 250/309 11 Claims, 1 Drawmg Flgllre POWER SUPPLY r- 1 I 'TARGETSELECTm .11.

r DUAL POWER SUPPLY .DlAPHRAGM 1 amsms POWER 1 SUPPLY IMAGING APPARATUS AND METHOD FOR USE WITH ION SCATTERING SPECTROMETER BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to improved apparatus and methods utilizing ion bombardment of a surface to obtain information concerning surface composition, by detecting ions scattered off the surface after bombardment.

2. Description of the Prior Art Various techniques and apparatus for analyzing solid surfaces by scattering ions from the surfaces are disclosed in US. Pat. Nos. 3,480,774, issued to Smith of Nov. 25, 1969, 3,665,182, issued to Goff and Smith on May 23, 1972 and 3,665,185, issued to Goff on May 23, 1972. Such techniques include the provision of moving a primary ion beam with respect to the sample to control the point of impact of the beam on the sample to optimize the scattered ion signal. The energy of the scattered ions is thereafter measured and the intensity of a signal associated with the measured scattered ions is plotted as a function of the relative energy of the scattered ions to at least semiquantitatively identify the elemental composition of the bombarded surface. In

that technique, there is no provision for scanning the sample surface to repetitively analyze the composition of an area of the sample surface.

A related technique, also utilizing ionic bombardment to analyze a solid surface is disclosed in US. Pat. No. 3,479,505, issued to Liebl on Nov. 18, 1969, which technique includes the direct mass analysis of ions sputtered from the surface as a result of the bombardment. In one embodiment of that technique, an ion beam is scanned in a rastered pattern over a limited portion of the same surface and the sputtered ions' are mass analyzed to produce an electrical signal indicative of the instantaneous emission of ions having a selected mass. The signal is applied in amplified form to modulate the intensity of the electron beam in an oscilloscope scanned in synchronization with the raster pattern on the sample, thereby producing an optical image indicating variations in the concentration of a particular element. Because of the limitations inherent in ion microprobe analysis, namely the destruction of the surface due to the sputtering action, the variation in sensitivity for the various elements in the periodic table and the severe influence of the matrix (i.e., sample composition and crystalline structure), the resultant display raises complex interpretational problems and prevents repetitive analysis of the same surface.

SUMMARY OF THE INVENTION In contrast to the above referenced secondary ion mass spectrometric display technique, the present invention provides an ion scattering apparatus for surface analysis in which a display is produced corresponding to the concentration of atoms of a given element by measuring scattered ions having a predetermined relative energy. The apparatus includes a target support for supporting a sample in a predetermined location, the surface of which is to be analyzed. An ion generator produces a beam of primary ions having known mass respect to the sample to enable the beam to scan a predetermined area of the surface, thereby forming a scanning pattern of the primary ions within the predetermined area. An energy analyzer transmits scattered primary ions having a second known kinetic energy value less than the original kinetic energy of the primary ions. An ion detector subsequently receives the transmitted primary ions and converts the received ions into an electrical signal. An electronic display apparatus is provided and is synchronized to the movement of the beam of primary ions for displaying a raster modulated by the electrical signal.

In a preferred embodiment, the apparatus includes at least two pair of electrostatic deflection plates positioned along the primary beam axis to repetitively deflect the beam along two substantially orthogonal axes when suitably energized.

BRIEF DESCRIPTION OF THE DRAWING The apparatus of the present invention will be more fully understood upon reading the following detailed description which refers to the accompanying drawing wherein the FIGURE is a schematic diagram illustrating the apparatus constructed in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The FIGURE is substantially that set forth as FIG. 2 in US. Pat. No. 3,665,182, the disclosure of which is incorporated herein by reference.

In the drawing there is shown a compact elemental analyzing apparatus comprising multipositionable target support 60, an ion generating means 26, beam deflection members 110, analyzer 45, an ion detector and display device 112.

In operation, the apparatus described above with the exception of the display device 112, is located within a vacuum chamber (not shown), a vacuum pump evacuates the chamber to a pressure of less than about 10 Torr. A getter and a cryopanel are positioned within the chamber to further purify the active elements remaining in the chamber. The pumping is discontinued and noble gas is released 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), Neon (Ne) and Argon (Ar) are commonly used. Insulated electrical feed throughs or connectors provide the necessary electrical connections between the components within the chamber and the electrical apparatus located outside of the chamber.

The multipositionable target support 60 includes a rotatably octagonal target wheel 61 and wheel advancement means including tooth ratchet wheel 63 to sequentially advance the target wheel through an increment each time the solenoid is activated. On each planar space 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 suitable supporting members and engageable with indents to electrically connect the wheel 61 and the sample being bombarded with a current measuring device 81 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. Any variety of similar multiple sample supports may likewise be provided.

The ion generating means preferentially comprises a grounded tubular housing 25, essentially 2 X 3 X 4 inches, (5.1 X 7.6 X cm) adapted to support the operative components of the ion generator. The ion generator structure, essentially 1 X l X 3 inches (2.5 X 2.5 X 7.6 cm) 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 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 evacuable 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. The feed-back stabilization loop 41 maintains a stable electron grid current which controls the ion beam current throughout pressure changes within the evacuable 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 a number of resistors 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. The ions are then focused and apertured by the anode plates, forming a primary ion beam. 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 are constructed of very thin conductive material to control the ion flow and to maintain a monoenergetic beam. The plates are, for example, 0.010 inch (0.25 mm) 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 toward the sample to be analyzed. Under normal operating conditions beam perturbing collisions do not cause serious deviations in analysis.

Two pair of

deflector plates

57 and 114, positioned near the end of the housing 25 and on opposite sides of the beam axis serve to deflect the beam to scan the beam about a predetermined area of the sample. The plates 57 are charged by an ion deflector power supply 87, while the plates 114 are charged by a similar ion deflector power supply 116.

The power supplies 87 and 116 include time base sweep generators 118 and 120 respectively, such as Tektronix, Inc. Model 2867, which may be used with the Tektroniz, Inc. RM561A scope, and

amplifiers

122 and 124. Such supplies are capable of delivering i 140 V sawtooth waveforms, and when used to charge onehalf inch (1.27 cm) long by one-eighth inch (0.32 cm) wide deflection plates positioned at the exit apertures of the housing 25 are sufficient to deflect a 3,500 e\/ Ne ion beam approximately 3 mm in the horizontal direction and approximately 4.5 mm in the vertical di rection at the specimen surface. Other deflection circuits providing sawtooth, triangular or other preferred waveshapes may similarly be provided. It is preferred that the outputs of the supplies 87 and 116 be ungrounded, hence providing equal positive and negative outputs to drive each plate of a given pair of deflection plates such that the beam axis is maintained at substantially ground potential. It is further preferred to provide a DC bias via bias supplies 126 and 128 on the output of each supply to facilitate positioning of the scanned beam on the sample surface. In one test, the primary beam diameter was about 1 mm, and the beam was scanned over an area approximately 3 X 4.5 mm to produce a scanning raster. It is, of course, readily apparent that the number of lines and the size of the raster are readily controlled by varying the deflection voltage and repetition rates, the size of the deflection plates and the energy of the primary ion beam.

Signals from the sweep generators 118 and 120 are also coupled via leads 130 and 132 to a display device 134, such as an oscilloscope, in order to synchronize the deflection of a displayed raster on the oscilloscope.

The deflected ion beam strikes or bombards the sample on the sample surface about the predetermined area, 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 approximate current density striking the surface of the sample.

The energy analyzer 45 comprises an entrance diaphragm 46, having a rectangular entrance slit 47, an exit diaphragm 49, having a rectangular exit slit 50, and two curved electrostatic analyzer plates 48. The entrance diaphragm 46 and exit diaphragm 47 may be 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 0.005 inches (0.125 mm) and the entrance diaphragm is spaced about one centimeter from the surface of the sample being analyzed.

The analy zer 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 (5.1 cm). The illustrated analyzer 45 is a standard 127 energy 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.

As disclosed in US. Pat. No. 3,665,172, the mass of the atoms in the sample surface from which the ions of the primary beam have scattered is readily determined when the scattering angle is restricted to 90. Accordingly, in the present invention it is preferred to maintain the scattering angle at substantially 90, in spite of the further requirement of the present invention that the beam be moved with respect to the sample. This may be accomplished in several ways. Thus, it is within the scope of the present invention that the motion of the primary beam of ions with respect to the sample be effected by mechanically translating the sample in a plane substantially parallel to the bombarded surface at the point of beam incidence. In such an embodiment, the angle of the primary ion beam with respect to the analyzer input would, of course, not be varied.

In another embodiment, the sample is fixed in position during bombardment and the primary ion beam is deflected within limits across the predetermined area of the surface of the sample. In such an embodiment, the deflection angle is maintained sufficiently close to 90 such that a 90 scattering angle may be assumed.

In a further embodiment, larger ion beam deflections may be utilized, however, a determination of the mass of the target atoms as set forth in US. Pat. No. 3,480,774 must include a more complex equation involving a consideration of the sine and cosine of the scattering angle (where the scattering is 90 as discussed hereinabove, it is easily seen that such a determination is considerably simplified).

Since the scattering angle may be varied by scanning the primary ion beam, the measured scattered ion energy from a given element will also vary. Thus, the change in energy must be correctly interpreted to determine the mass of the surface atoms. This may be accomplished by a system which determines the analog correction factor and which supplies a signal to the display device 112 such that the corrected elemental determinations are made in order to singularly identify ions scattered from a surface atom of a given atomic mass over a given range of values. A further method for accomplishing the same correction may involve adjustments such as varying the potential applied to the energy analyzer 45 or varying the energy of the primary ion beam.

The ion detector 70, within the 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. CHM-4028 manufactured by Galileo Electro-Optics Corporation, Galileo Park, Sturbridge, Mass. 01,581.

In the present invention, the electronic signal is preferably coupled through a signal processor circuit 136 to extend the duration of pulses associated with discrete scattering events. The processed signal is thereafter coupled to the display device 134 to modulate the synchronized raster, thereby forming an image in real time representative of the scattered ions having a preselected energy relative to the energy of the primary ions, which pictorially illustrates the distribution of a given element within the scanned area of the surface.

In an exemplary procedure, a test specimen was prepared by embedding a 0.025 inch (0.052 cm) gold wire in a copper cylinder, which was polished on one end to form a flat surface. The specimen was then positioned on the target support 60 such that the portion containing the gold insert was facing and approximately centered along the primary beam axis. In this procedure, the entrance slit 47 of the energy analyzer 45 was also approximately 0.005 inches (0.125 mm) wide, and the primary ion beam diameter was approximately one millimeter. Such conditions severely limit the resolution of a resultant image on the oscilloscope, yet when a 3,500 eV Ne ion beam was scanned over an area of the specimen, centered on the gold wire, and the energy analyzer was tuned to transmit only such scattered ions as had a relative energy associated with scattering from a gold atom, an image on the oscilloscope was produced showing a generally dark background (indicative of the absence of gold atoms in most portions of the scanned portion of the specimen surface) and a light center area, representative of the gold wire. When the energy analyzer was readjusted to transmit only such scattered ions as had a relative energy associated with scattering from copper, the light and dark regions of the oscilloscope image were inverted. In this procedure, the primary beam was scanned in the horizontal direction at a repetition rate of approximately 20 Hz, and in the vertical direction at a repetition rate of approximately 0.1 Hz. Because it thus took ten seconds to sweep through a single field, the display was ob-' served on a storage oscilloscope such as Tektronix, Inc., Model RM564. Alternatively, a conventional oscilloscope and camera was used to form a permanent image, in which case the camera shutter was simply opened for at least the duration of a single field.

The pulses received from the ion detector as a result of a given scattering event are generally extremely short, such as less than about one microsecond, and may be insufficient to adequately excite conventional display apparatus. Accordingly, it is preferred to couple the electrical signal through the signal processor circuit 136 to extend the duration of the pulses between 15-60 microseconds, thereby ensuring adequate excitation durations.

In another embodiment, it is preferred to display the scanned portion of the sample surface on conventional video display equipment, thus enabling remote viewing of a displayed image, such as is commonly done in closed circuit video systems. In such an event, the power supplies 87 and 116 are synchronized with conventional video deflection circuits, and the processor circuit 136 further amplifies and shapes the electrical signal to ensure compatibility with conventional video monitor devices.

With the improvement it should be realized that the specific determination of localized elements can now be readily identified. Once this identification is made, the raster image can be further adapted to localize the primary ion on a selected location to enable in depth analysis at that location. Such analyses are critical to the solution of many metallurgical and impurity problem areas such as those involved in air pollution, particulate defects and the like.

Having described the present invention with reference to a preferred embodiment, it is appreciated that changes may be made without departing from the spirit or scope of the invention as defined in the appended claims.

What is claimed is:

1. An ion scattering apparatus for surface analysis, said apparatus comprising:

a. a target support for supporting in a predetermined location a sample, the surface of which is to be analyzed;

b. an ion generator means for producing a beam of primary ions having known mass and substantially the same known kinetic energy;

c. means for directing said primary ions along a beam axis towards a surface of said sample;

d. energy analysis means for transmitting scattered primary ions having a second known kinetic energy value less than the original kinetic energy of said primary ions; and

e. ion detector means for receiving the transmitted primary ions and for converting the received ions into an electronic signal; wherein the improvement comprises f. means for moving said beam of primary ions with respect to said sample in at least two directions to cause said beam to impinge on a predetermined area of said surface; and

g. display means synchronized to said moving means for displaying an image formed by a raster modulated by said electronic signal.

2. An apparatus according to claim 1, wherein said energy analysis means is positioned to receive only such primary ions as are scattered from said sample to substantially 90 with respect to the direction of the beam axis towards the sample, and wherein said moving means is constrained to substantially maintain said 90 scattering conditions.

3. An apparatus according to claim 2, wherein said moving means comprises means for repetitively translating said sample in the plane parallel to the sample surface at the point of beam incidence.

4. An apparatus according to claim 1, wherein said apparatus further comprises means for compensating for changes in the energy of scattered ions depending on the scattering angle to ensure that said ions scattered from a surface atom from a given atomic mass are singularly identified over a given range of angles.

5. An apparatus according to claim 1, wherein said moving means is adapted to repetitively deflect said primary ions in two substantially orthogonal directions across said predetermined area.

6. An apparatus according to claim 5, wherein said moving means includes means for repetitively deflecting said beam at a rate determined by conventional video display standards.

7. An apparatus according to claim 1, further comprising means for processing said electronic signal to extend the duration of pulses constituting portions of said signal such as correspond to said transmitted primary ions to facilitate said raster modulation and display.

8. An apparatus according to claim 1, wherein said moving means further comprises at least two pairs of electrostatic deflection plates, said pairs being orthogonally disposed with respect to each other and positioned along said beam axis to deflect said primary ion beam when suitably energized.

9. An apparatus according to claim 8, further comprising deflection power supply means coupled to said pairs of deflection plates for energizing said plates, said supply means being adapted to alternatively energize the plates of a pair of given plates with a positive and negative voltage with respect to a given fixed potential.

10. An apparatus according to claim 9, wherein said power supply means are further adapted to vary said given fixed potential to control the position of said predetermined area on said sample.

11. A method of surface analysis comprising:

a. providing a target support for supporting in a predetermined location a sample, the surface of which is to be analyzed;

b. producing a beam of primary ions having known mass and substantially the same known kinetic enc. directing said primary ions along a beam axis towards a surface of said sample;

d. transmitting scattered primary ions having a second known kinetic energy value less than the original kinetic energy of said primary ions; and

e. receiving the transmitted primary ions and converting the received ions into an electrical signal; wherein the improvement comprises f. moving said beam of primary ions with respect to said sample in at least two directions to cause said beam to impinge on a predetermined area of said surface; and

g. displaying a raster synchronized to said moving beam and modulated by said electronic signal.

Claims

1. An ion scattering apparatus for surface analysis, said apparatus comprising: a. a target support for supporting in a predetermined location a sample, the surface of which is to be analyzed; b. an ion generator means for producing a beam of primary ions having known mass and substantially the same known kinetic energy; c. means for directing said primary ions along a beam axis towards a surface of said sample; d. energy analysis means for transmitting scattered primary ions having a second known kinetic energy value less than the original kinetic energy of said primary ions; and e. ion detectOr means for receiving the transmitted primary ions and for converting the received ions into an electronic signal; wherein the improvement comprises f. means for moving said beam of primary ions with respect to said sample in at least two directions to cause said beam to impinge on a predetermined area of said surface; and g. display means synchronized to said moving means for displaying an image formed by a raster modulated by said electronic signal.

2. An apparatus according to claim 1, wherein said energy analysis means is positioned to receive only such primary ions as are scattered from said sample to substantially 90* with respect to the direction of the beam axis towards the sample, and wherein said moving means is constrained to substantially maintain said 90* scattering conditions.

11. A method of surface analysis comprising: a. providing a target support for supporting in a predetermined location a sample, the surface of which is to be analyzed; b. producing a beam of primary ions having known mass and substantially the same known kinetic energy; c. directing said primary ions along a beam axis towards a surface of said sample; d. transmitting scattered primary ions having a second known kinetic energy value less than the original kinetic energy of said primary ions; and e. receiving the transmitted primary ions and converting the received ions into an electrical signal; wherein the improvement comprises f. moving said beam of primary ions with respect to said sample in at least two directions to cause said beam to impinge on a predetermined area of said surface; and g. displaying a raster synchronized to said moving beam and modulated by said electronic signal.