US3798447A

US3798447A - Apparatus for directing an energizing beam on a sample to cause secondary ion emission

Info

Publication number: US3798447A
Application number: US00144078A
Authority: US
Inventors: P Lanusse; R Hernandez; G Slodzian
Original assignee: Office National dEtudes et de Recherches Aerospatiales ONERA
Current assignee: Office National dEtudes et de Recherches Aerospatiales ONERA
Priority date: 1970-05-27
Filing date: 1971-05-17
Publication date: 1974-03-19
Anticipated expiration: 1991-03-19
Also published as: FR2087652A5

Abstract

Two enclosures are separated by a diaphragm; one of them contains means for producing a beam of primary ions and the other contains means for focusing the beam of primary ions and the secondary beam emitted by the sample. Means for scanning the sample with the primary beam are provided. The device enables high resolution in spectral analyses.

Description

United States Patent 9 Lanusse et al.

[ Mar. 19, 1974 APPARATUS FOR DIRECTING AN ENERGIZING BEAM ON A SAMPLE TO CAUSE SECONDARY ION EMISSION [73] Assignee: Office National DEtudes Et De Recherches Aerospatiales, Chatillon-sous-Bagneux, France [22] Filed: May 17, 1971 [21] Appl. No.: 144,078

3,418,465 12/1968 Hahn et a1. 250/49.5 3,517,191 6/1970 Liebl.. 250/495 3,585,383 6/1971 Castaing et al. 250/495 FOREIGN PATENTS OR APPLICATIONS 464,571 12/1968 Switzerland 250/495 OTHER PUBLICATIONS Mass-spectrometric Micro-Surface Analysis published by the Geophysics Corporation of America, Bedford, Mass.

A Theoretical Assessment of the Possibility of Selected-Area Mass-spectrometric Analysis Using a Focused Ion Beam by J. V. P. Long from the British Journal of Applied Physics, Vol. 16, Sept., 1965,

- pages 1277-1284.

Primary Examiner--William F. Lindquist Attorney, Agent, or Firm-Larson, Taylor & Hinds 57 ABSTRACT Two enclosures are separated by a diaphragm; one of them contains means for producing a beam of primary ions and the other contains means for focusing the beam of primary ions and the secondary beam emitted by the sample. Means for scanning the sample with the primary beam are provided. The device enables high resolution in spectral analyses.

14 Claims, 1 Drawing Figure [30] Foreign Application Priority Data May 27, 1970 France 70.19276 [52] U.S. Cl. 250/495 P, 250/41.9 SE [51] Int. Cl. G01n 23/22 [58] Field of Search... 250/419 SE, 49.5 P, 49.5 B,

[56] References Cited UNITED STATES PATENTS 3.005.099 10/1961 Fournier et a1 250/515 3,124,680 3/1964 Van Dorsten et' al.. 3,415,985 12/1968 Castaing et a1. 250/495 BEAM ON A SAMPLE TO CAUSE SECONDARY ION EMISSION -The invention relates to improvements to chambers for energising by an ion emission for mass spectrometric analysis of solid samples in which the energising is obtained by an ion beam, called primary ion flow, the flow of ions emitted by the sample being then called secondary ion flow.

It is known that, for mass spectrometric analysis of solid samples, whether said analysis is effected by spectrography or by spectrometry, there are arranged two principal methods of energising, namely energising by spark and ionic energising, the latter method being better adapted than the previous one to analyses of high definition, especially in depth, due to the fact that the area of the emission of the ion beam to be analysed is more stable, better localised and that the thickness of the material consumed is less and more regular.

The ion energisation should therefore enable the exploitation fully of the performances of mass analysers with high separating power called double focusing, and in particular mass spectrographs of the Mattauch and Herzog'type in which there is produced by an ion beam of small aperture emerging from an image projected on an input slot, a double focusing by the conjunction of an electrostatic analyser and of a magnetic analyser imposing respectively on the ion beam to be analysed deviations of opposite sense, the resulting focusing being thus of the first order for the whole of the ion species to-be separated.

However ion energising chambers of known type are badly adapted to mass analysers with double focusing since they do not enable, either the obtaining ofvsecondary ion beams of small aperture, the focusing an ion image in the plane of the input slit, or the obtaining of a uniform ionic illumination of the region of the sample to be analysed.

It is an object of the invention to provide a chamber for ionic emission with ion energisation which fulfills the aforesaid conditions and which enables the exploitation to the best advantage of the performances of a mass spectral. analyser and in particular ofa doublefocusing, mass spectrograph; v to provide such a chamber which, better than the chambers of the known type, gives ion beams from strictly defined areas of a solid sample, and this without any pollution of neighboring areas, and which enables in addition the layers of materials consumed in the course of an analysis operation to be of regular thickness by uncovering facets of subjacent planes and well defined adapted for further analysis;

to provide such a chamber capable of maintaining the energising and the emission under high vacuum, without notable polution by traces of residual gas; a, condition particularly important for mass spectrography, in particular for the analysis of superficial layers;

to provide such a chamber which enables the bringing rapidly to high temperature of the samples even in the course of examination.

The chamber of ion emission by ionic excitation according to the invention comprises two vacuum enclosures of which the first contains emitting and focusing means for a beam of primary ions extracted from a flow of gas or of vapor and of which the second comprises means for deflection and projection of the said primary beam on to a solid sample to be excited, the two enclosures being provided with independent vacuum means and communicating through an orifice of such dimensions that it gives passage to the useful portion of the said primary beam whilst limiting the flow rate of the passage of gas from the first enclosure towards the second enclosure. This arrangement enables the obtaining of an excellent vacuum in the second enclosure by means of a small delivery pump but without risking polluting the residual atmosphere, the said pump being advantageously an ion, pump whose action is possibly completed by a sublimator.

The second enclosure comprises an ion optic device forming an ion image carried by the beam of ions emit ted by the excited sample, or secondary ion beam, in the plane of an orifice formed in the said second enclosure and substantially coincidant with the input orifice of the analyser, the two enclosures being relatively arranged in such a way that the axis of the primary beam before passage into the deflection means of the second enclosure and the axis of the secondary beam are substantially at right-angles and the said deflecton means bend the primary beam on to the sample to be energised This arrangement enables the ion optic device-to be housed in the second enclosure without impeding the passage of the primary beam.

Deflection means for the primary beam into the second enclosure comprise two pairs of electrodes acting respectively on the orientation of the-said beam in perpendicular directions and borne respectively to adjustable continuous potentials and/or adjustable alternating potentials of which the frequencies aredifferent,

' these electrodes enabling a defined zone of the sample to be swept by the primary beam. The face of the sample subjected to the impact of th primary ions is covered by a metal screen advantageously monoisotopic borne to the same potential as the sample and pierced by an orifice limiting the area of particle fall-back. As will be seen in the example given below, the combination of these features enables the obtaining of an excellent resulution of the analysis in area and in depth without pollution of the limitrophic zones. Lastly the second enclosure contains a samplecarrier device which frees the face of the example nonexposed to the impact of the primary beam, and a retractable electronic gun projecting an electron beam on to the said non-exposed face, thus enabling, if necessary, rapid heating and without polluton of the said sample, in the course of examination if such is necessary.

There will now be described, with reference to the single FIGURE of the drawings, one embodiment of the energising chamber according to the invention; By way of simplification, there has been omitted in the F16 URE the elements or devices of which the representation is not necessary to understand the invention such as the-means of vacuum, shuttering, assembly, sealing, mechanical or electrical connections etc. all features whose construction is known to those skilled in the art.

The enclosure 1, vacuum-tight, of the ion emission chamber according to the invention communicates a. by a flange 2, with a sump and a valve not shown enabling the introduction of the borne object 3 without breaking the vacuum, I

b. by means of the orifice of a diaphragm 4 with an enclosure 5 treating the ion beam emitted by the extraction electrode 6 of the primary ion gun 7,

c. with an enclosure 8 of the double beam spectral analysis apparatus, not shown, of which however is seen the input slit 9,

d. by an orifice 10 with vacuum means which will be considered below.

All the enclosures, flanges, valves, mechanical linkages with devices situated outside the enclosures etc.. are obviously vacuum-tight.

The gas to be ionised by the gun 6 penetrates through the passage 11 under a pressure of 10' bars. The gun, of known type, delivers through the central orifice, of a diameter of 0.5 mm of the extraction electrode 6, brought to a potential of 29 kV, ions whose average energy is of the order of l 000 to l 500 electron-volts.

The chamber 5, called space-gun, comprises means 12, shown by a rectangle in discontinuous lines, enabling acceleration of the ions to 30 kilovolts and focusing of them onto the central aperture, of diameter 0.5 mm, of the diaphragm 4 of monoisotopic metal for example of tantalum, which separates the space-gun from the space-object. At the output of the diaphragm 4, the characteristics of the ion beam are for example the following intensity 10.10 A; aperture 2.4.10 radians; brilliance 6.6 A/cm2/steradian.

An oil diffusion pump, provided with a liquid nitrogen trap, and acting in the enclosure 5 through the orifice 13, ensures. a pumping flow rate of 100 litres per second for a pressure of 2.5.10 bars.

Under these conditions the pressure in the gun 6 supplied with gas through the orifice 11 is 10' bars.

The vacuum necessary in the enclosure 1 is ensured firstly by an absorption pump then by anion pump of the triode type, the said pumps acting through the orifice 10. The triode pump, of which tthe total delivery rate is of the order of 100 litres per second, enables the obtaining in the enclosure 1 of a total pressure of the order of 10 bars.

The combination of the pumping means described with the diaphragm 4 separating the space-gun from the space-object is extremely advantageous. It enables the use, for the pumping of the space-gun of a high delivery pump and; for the pumping of the space-object of a triode pump with much lower delivery rate, but ensuring a high vacuum without risk of contaminating the space-object by hydrocarbon vapors, of flexible and positive usefulness and preserving a notable pumping delivery rate even for the rare gases (helium, argon etc. introduced into the gun to produce the primary ion beam.

There will now be given the description of the devices contained in the enclosure 1 and ensuring, among other functions, the bombardment of the target and focusing of the secondary ion beam emitted by the said target.

The object-carrier 3 supports a metallic tube 14, open at both ends on which is fixed the object or target 15, for example by means of a metallic collar not shown supporting the target 15, by its periphery, against the tube 14.

The primary ion beam emerging from the central orifice of the diaphragm 4 is firstly guided by two pairs of deflection plates, the first pair 16 being shown in section and the second pair 17 in superposition, then passes through a tantalum diaphragm 18 having a central orifice of diameter 1.5 mm, and whose role is to avoid electrostatic charges of the insulating supports (not shown) of the plates 17 from disturbing the beam, finally it is directed to the center of the target by a spherical deflector 19 of known type.

Several types of spherical deflectors can be used, either for example, a deflector for bombarding the object 15 with positive ions, or again a 45 deflector followed by a neutralising chamber 20, such as a metallic tube filled with gas or vapor to bombard the object with neutral atoms of variable energy, obtained by exchange of charges.

By way of example, if the object is assumed to be at the potential of 2.10 V. with respect to the ground, there are used primary ions of argon of which the potential at the outlet of the diaphragm 4 is 3.10 volts. If, by means of the deflector 19, these ions are directed into the electrostatic field of the object with an incidence of 30 with respect to the normal, they strike the object with an energy of 10 electron-volts at an incidence of 60.

The guide and deflection device described above comprises also a diaphragm 21 of metal advantageously monoisotopic, such as tantalum or gold, placed at a slight distance from the surface of the sample 15, held at the same potential as the said sample, for example by electrical contact and comprising a central orifice of diameter about 3mm. The said diaphragm 21 protects the limitrophic zones of the object from the examination zone against the pollution by neutral particles torn of by the impact of the primary beam and by the secondary ions reflected by the adaptation optics which will be considered below. The object-carrier 3 being provided with mechanical means (not shown) actuated from outside the enclosure 1 for translation of the object in the plane of the face examined whilst the diaphragm 20 is rigidly fixed to the enclosure 1, it is thus possible to proceed to the analysis of superficial layers of an extended region of the said examined face without risking closer and closer contamination.

As for the pairs of electrodes 16 and 17, they are on one hand respectively polarised by continuous voltages of adjustable value enabling the obtaining of a precise guidance of the primary beam, on the other hand respectively polarised by alternating voltages of adjustable amplitude, the frequency supplying one pair being much greater than the frequency supplying the other pair. The amplitudes of the alternating voltages are adjusted so that the primary beam sweeps through the orifice of the diaphragm l5 and without contact with the edges of the said orifice a zone of predetermined location and dimensions of the surface under examination, the said zone being thus uniformly illuminated by the primary beam and the eroded layers being of regular thickness. If the surface under examination is flat, the facet uncovered by erosion is also flat and lends itself to further examination. The invention hence enables mass spectral analysis effected layer by layer with an excellent definition in depth.

The FIGURE shows also an electron gun 22 enabling in the course of operation, the local heating of the object 15 by its rear face. The said gun can be retracted by means of its support 23 actuated from the outside to enable translation of the object-carrier 3.

The FIGURE shows also a cryogenic pumping device constituted by a toric reservoir 24 of metal, a good conductor of heat which can be filled from the outside, by

means of a tube 25, with a liquid gas such as nitrogen so as to perfect the vacuum in the region of the enclosure 1 where the object is placed.

The enclosure 1 contains lastly an ionic optics device enabling the projection of the ionic image of the examined zone of the object 15 into the plane of the input slit 9 of the mass spectral analyser 8. This device comprises an accelerating electrode 26 connected to the earth or to ground and followed by an electrostatic lens constituted by three

electrodes

27, 28, 29, the

electrodes

27 and 29 being grounded and the electrode 28 being brought to a high DC voltage.

In the embodiment described, the ion optics are calculated to give the observed zone of the object 15 an image with an amplification of two-thirds in the plane of the slit 9.

It is important to note that the field of the invention is not limited by the description above and that the embodiment described can be varied in numerous ways.

Thus g the deflection plates 16 and 17 can be arranged on both sides of the spherical deflector 19 or again upstream or downstream of the said deflector with respect to the direction of flow of the primary ions;

other means of heating the sample than the electron gun 22 can be adopted;

there can be adopted for withdrawal movement of the said gun, either translation in the direction of the axis 23, or rotation of the said axis around an axis situated in the plane of the FlGURE, etc.

The invention is in no way limited to the embodiment given by way of example nor to the several variations indicated. In addition, the excitation chamber according to the invention is usable both with a mass spectrometer and with a massspectrograph or with a mass spectral analyser effecting both functions.

What we claim is:

1. Mass spectrometric analysis apparatus comprising 7 a chamber for ionic emission energization containing at least one solid sample for spectral mass analysis thereof, said chamber including means for directing a primary, energizing beam upon said sample so that said sample emits a secondary ion beam and a metallic diaphragm positioned parallel to and closely adjacent the face of the sample upon which said primary beam is directed, said metallic diaphragm having an orifice therein defining a passage for said primary beam substantially the size of the surface of said sample impinged by said beam and for said secondary ion beam emitted by said sample, said apparatus further comprising means for maintaining said metallic diaphragm at the same potential as the sample.

2. Analysis apparatus in accordance with claim 1, wherein the metallic diaphragm is formed of a monoisotopic metal.

3. Analysis apparatus in accordance with claim 2, wherein said metallic diaphragm is formed of tantalum.

4. Analysis apparatus in accordance with claim 2, wherein said metallic diaphragm is formed of gold.

5. Mass spectrometric analysis apparatus according to claim 1, comprising means for deflecting said primary beam, said deflecting means comprising first and second pairs of electrodes located so as to act on said primary beam in mutually perpendicular planes, means for applying a potential of variable amplitude and a first frequency to said first pair of electrodes, means for applying a potential of variable amplitude and a second, different frequency to said second pair of electrodes, and means for controlling the amplitude of said potentials so that the primary beam sweeps an area on the surface of the sample substantially the size of said orifice in said diaphragm.

6. Mass spectrometric analysis apparatus according to claim 1, comprising a cryogenic container located adjacent the sample around said primary beam and said secondary ion beam emitted by said sample, said cyrogenic container being filled with liquified gas.

7. Mass spectrometric analysis apparatus comprising a chamber for ionic emission energization containing at least one solid sample for spectral mass analysis thereof, said chamber including means for directing a primary, energizing beam upon the sample so that said sample omits a secondary ion beam and a sample carrier for supporting the sample so that one face thereof is exposed to the primary beam and another surface of said sample opposed to said exposed surface is free, said chamber further containing an electron gun so located as to direct an electron beam against said another surface of the sample for providing localized heating of the sample.

8. Mass spectrometric analysis apparatus according to claim 7, comprising a cryogenic container located adjacent the sample around said primary beam and said secondary ion beam emitted by said sample, said cryogenic container being filled with a liquified gas.

9. Analysis apparatus in accordance with claim 8 wherein said cryogenic container is of annular shape.

10. Analysis apparatus in accordance with claim 8 wherein said cryogenic container is filled with liquid nitrogen.

11. Analysis apparatus in accordance with claim 8 wherein said cryogenic container is filled with liquid helium.

12. Mass spectrometric analysis apparatus according to claim 7, comprising deflecting means for deflecting said primary, energizing beam and a neutralizing chamber positioned in the path of said primary beam between said deflecting means and said sample for producing an energizing beam containing neutral particles.

13. Analysis apparatus in accordance with claim 12 wherein said neutralizing chamber comprises a metal tube filled with a fluid for bombarding the sample with neutral atoms of variable energy obtained by charge exchange.

14. Analysis apparatus in accordance with claim 7 further comprising a retractable support for mounting said electron gun.

Claims

1. Mass spectrometric analysis apparatus comprising a chamber for ionic emission energization containing at least one solid sample for spectral mass analysis thereof, said chamber including means for directing a primary, energizing beam upon said sample so that said sample emits a secondary ion beam and a metallic diaphragm positioned parallel to and closely adjacent the face of the sample upon which said primary beam is directed, said metallic diaphragm having an orifice therein defining a passage for said primary beam substantially the size of the surface of said sample impinged by said beam and for said secondary ion beam emitted by said sample, said apparatus further comprising means for maintaining said metallic diaphragm at the same potential as the sample.

2. Analysis apparatus in accordance with claim 1, wherein the metallic diaphragm is formed of a mono-isotopic metal.