ION SOURCE INCLUDING STRUCTURED SAMPLE FOR IONIZATION

Abstract

An ion source is provided that includes a structured sample and a method for the ionization and/or its enhancement is provided, which preferably relies on field emission and/or field ionization processes. These processes can be brought about by structures with appropriate geometries, which cause a high electric field gradient at or near the sample.

Claims

1. An ion source comprising a sample (1) to be ionized and extraction means for generating a first electric field gradient orthogonal to the surface of the sample (1) in order to extract and/or accelerate ions from said sample (1), characterized in that the ion source comprises a structure (2) comprising at least two galvanically separated substructures (3,4) and means for supplying one or more electric voltages to said substructures (3,4) to generate a second electric field gradient in close proximity to the surface of the sample (2) so as to improve ion production efficiency, wherein the substructures (3,4) of the structure (2) are separated at least in one region by a distance of less than 10.sup.−3 m and the means for supplying one or more electric voltages are configured to apply an electric voltage between said substructures to generate a second electric field gradient of at least 10.sup.6 V/m at a zone in close proximity to the surface of the sample (1).

2. The ion source according to claim 1, characterized in that the zone in which the second electric field gradient is generated encompasses an area of at least 10 μm.sup.2.

3. The ion source according to claim 1, characterized in that the means for supplying one or more electric voltages are configured to apply an electric voltage of at least 10 V between said substructures (3,4).

4. The ion source according to claim 1, characterized in that the means for supplying one or more electric voltages are configured to apply an electric voltage between said substructures (3,4) to generate a second electric field gradient of at least 10.sup.6 V/m.

5. The ion source according to claim 1, characterized in that the substructures (3,4) are separated at least in one region by a distance of less than 10.sup.−4.

6. The ion source according to claim 1, characterized in that the structure (2) is positioned within a distance of less than 10 μm of the surface of the sample (1).

7. The ion source according to claim 1, characterized in that the substructures (3,4) are separated at least in one region by a distance of less than 10.sup.−6 m and wherein an electric voltage of at least 10 V is applied between said substructures (3,4).

8. The ion source according to claim 1, characterized in that the substructures (3,4) exhibit the geometry of a comb comprising two or more teeth elements, wherein the substructures (3,4) are arranged such that the teeth elements interleave and the zone comprises areas enclosed by teeth of the interleaved substructures (3,4).

9. The ion source according to claim 1, characterized in that the ion source comprises a grounded extraction plate situated above the surface of the sample and the means for supplying one or more electric voltages to the substructures (3,4) are configured to simultaneously add to an extraction voltage to the at least two substructures (3,4), while providing a differential voltage between the substructures (3,4).

10. The ion source according to claim 1, characterized in that the one or more electric voltages applied to the substructures (3,4) are at least partially time-variable voltages.

11. The ion source according to claim 1, characterized in that the second electric field gradient is at least partially parallel to the surface of the sample (1).

12. The ion source according to claim 1, characterized in that the structure (2) is fabricated from conductive metal, conductive metal alloys and/or other electrically conductive substances.

13. The ion source according to claim 1, characterized in that the ion source comprises an analysis chamber designed to accommodate the sample (1) and the structure (2), wherein the analysis chamber comprises at least one feedthrough for the introduction of one or more cables supplying electric voltages to the substructures (3,4).

14. The ion source according to claim 1, characterized in that a sample holder suited to hold the sample (1) is configured to comprise means to provide one or more electric voltages to the substructures (3,4).

15. The ion source according to claim 1, characterized in that the sample holder comprises at least two galvanically separated elements, each in conductive contact with a carrier of the electric energy and each conductively connected to one or more of the at least two substructures (3,4).

16. Use of an ion source according to claim 1 in a mass spectrometer or for the ionization of a gas or gas mixture.

17. A method for fabrication and use of an ion source in according to claim 1, comprising: providing a sample (1) to be ionized and extraction means for generating a first electric field gradient orthogonal to the surface of the sample (1) in order to extract and/or accelerate ions from said sample (1), applying a structure (2) to a sample (1), wherein the structure (2) comprises at least two galvanically separated substructures (3,4) separated at least in one region by a distance of less than 10.sup.−3 m connecting the substructures (3,4) to electric signals appropriate to generate a second electric field gradient of at least 10.sup.6 V/m sufficient for inducing the ionization of neutral particles at a zone at or near the surface of the sample (1), supplying sufficient energy to the sample (1) in order to generate ions and/or neutral particles, wherein the production of ions is enhanced by the presence of the second electric field gradient and the first electric field gradient serves for extracting and/or acceleration the ions.

18. A method for the fabrication and use of a structure for the ionization of a substance (1), comprising: applying a structure (2) at an interface to an area deploying a substance (1) across the area, applying an electric field gradient of at least 10.sup.6 V/m within the area by use of the structure (2), inducing ionization of the substance (1), the structure (2) comprises at least two galvanically separated substructures separated at least in one region of the area by a distance of less than 10.sup.−3 m and wherein an electric voltage of at least 10 V is applied between said substructures (3,4) and whereas the area has a size of at least 10 μm.sup.2.

19. The method for according to claim 17, wherein the structure (2) is applied by means of optical, electron beam and/or ion lithography.

20. The method for according to claim 18, wherein the structure (2) is applied by means of optical, electron beam and/or ion lithography.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0113] FIG. 1 Schematic illustration a preferred embodiment of an ion source in which the structure on exhibits two interleaved comb-like elements (A: top view. B: side view)

[0114] FIGS. 2 and 3 Schematic Illustration of a preferred embodiment for using the structure described herein as an ion gas source.

[0115] FIG. 4 Photograph of a SIMS sample holder with test structures ranging from 10 μm to 1 μm spacing and having 24 comb structures in total.

[0116] FIG. 5 Experimental data on the enhancement of the 23Na+ signal using a differential voltage of ˜2.8 kV p-p between two substructures of a 1 μm comb structure and a 250 pA 16O-primary ion beam on a 50×50 μm raster. Teeth were 1 μm wide and the gap between teeth having differing voltages was also 1 μm.

DETAILED DESCRIPTION OF THE FIGURES

[0117] The preferred embodiment comprises two comb-like elements forming two galvanically separated substructures 3, 4 which are interleaved as shown. The interleaved teeth as well as the transverse component of each element are shown. The distance between two next-neighbour teeth as well as between the teeth-tips and the transverse component is on the order of a micrometer or less. For example, the teeth line width may be 1 μm and the distance between teeth of the respective substructures 3 and 4 may be also 1 μm. It is however not required that the width of the teeth and the spacing between the teeth is the same. The thickness of such a teeth, e.g. in form as a lithographic line (vertical direction above sample surface) can be for example between 50 nm and 500 nm, preferably on the order of 100 nm. The two elements form two electrodes which could be biased for example at +/−3 kV in order to generate the desired electrical field gradient. In order to prevent electron emission and arcing between the two structures it may be preferred to operate the voltages in an AC mode.

[0118] Preferably the ion source may comprise a grounded extraction plate 7 situated above the surface of the sample 1 and the means for supplying one or more electric voltages to the substructures 3, 4 are configured to simultaneously add an extraction voltage (e.g. 10 kV) to the at least two substructures, while providing a differential voltage between the substructures (e.g. +/−3 kV). Thereby a second electrical field gradient is generated in between the interleaving teeth of the substructures 3, 4 in order to facilitate the generation of ions as described herein, while at the same time a first electric field gradient is provided orthogonal to the surface of the sample 1 to extract and/or accelerate the produced ions, e.g. for a further use.

[0119] The preferred embodiment illustrated in FIG. 1 may be for instance used in a secondary ion mass spectrometer. To this end a primary ion source (not shown) may be used to generate a focussed ion bean onto the sample in order to generate secondary ions, which may be subsequently passed to a mass analyser and detector. For the primary ion source, the mass analyser and the detector different variants may be used as known in the art. Advantageously, in all cases the production of secondary ions can be reliably enhanced by the generating a second electric gradient using a structure on top of the sample as described herein.

[0120] FIGS. 2 and 3 illustrates an alternative embodiment of utilizing a structure 2 as described herein in order to produce ions from neutral gas atoms and molecules.

[0121] For this preferred embodiment an electrically non-conductive thin film 5 may be perforated as shown at regular intervals, providing numerous micron-sized holes 6 through which a gaseous sample 1 can pass. One side of the non-conductive thin film 5 would have the gas as a sample 1 at low vacuum or possibly up to a pressure similar to atmospheric pressure. The other side of the non-conductive thin film 5 may be set be at a high vacuum exhibiting pressures preferably of 10.sup.−4 Pascal or below. Thereby the differential gas pressure would cause gas atoms/molecules to move through the micron sized holes.

[0122] Two preferred geometries for the structured ion source under these conditions may be envisioned.

[0123] As illustrated in FIG. 2 closely juxtapositioned substructures 3 and 4 forming the electrodes for the generation of a second electric field gradient may be printed on the same side of the non-conductive thin film 5 in an alternating, parallel arrangement,

[0124] As illustrated in FIG. 3 substructures 3 and 4 forming the electrodes for the generation of a second electric field gradient may be printed on opposite sides of the non-conductive thin film 5.

[0125] Both arrangements advantageously allow a close spacing of the substructures to generate the desired magnitudes of electric field gradient. In both cases it is preferred to have a separate extraction electrode (not shown) biased by e.g. several thousand Volts relative to the mean voltage on the two substructures 3 and 4 acting as ionization electrodes on the vacuum side of the thin film 5.

[0126] The neutral gas atoms and molecules are ionized by the second electric field gradient, while the generated ions are accelerated into the high vacuum part of the apparatus by the first, orthogonal electric field gradient provided by the extraction electrode.

[0127] FIG. 4 shows a photograph of a SIMS sample holder with simple glass slide in the position where the structured sample would normally be located. Note the two copper contacts for providing electrical signals to the structured sample. A tip of a pen is placed for scale.

[0128] FIG. 5 shows the record of data acquired from measurement, with the x-axis showing time (in seconds) and the Y-axis showing ion intensity (in ions per second). The numerous spikes starting at time−70 s reflect the 2.8 kV p-p voltage being applied (sharp increase in ion count rate) and being removed (sharp decrease in the ion count rate) from the structured sample.

Example 1

[0129] To demonstrate the enhanced SIMS using an ion source as proposed herein a sample stage capable of accepting two separate voltages for the sample holder was installed on a Cameca 1280. An electric feedthrough was rated at a maximum voltage of 20 kV relative to the supporting con-flat flange. During the test the SIMS ion source was operated with a DC extraction potential of 10 kV over a distance of ˜4.5 mm. This is equivalent to an orthogonal first electric field gradient of ˜2.2e+6 V/m.

[0130] The electronics provided a differential voltage of 2.8 kV peak-to-peak. An exemplary SIMS sample holder with test comb structures ranging from 10 μm to 1 μm of teeth spacing is shown in FIG. 4. The sample disk had three different types of test structures: 10 μm, 3 μm and 1 μm tooth width and gap width. For each different type 8 test structures were present. The teeth were oriented in the horizontal, so that any deflection they gave to the ions would be parallel to the entrance slit of the mass spectrometer.

[0131] A test was run with a 10 picoampere (pA) 16O-primary focused to ˜2 μm diameter beam (primary ion beam) and rastered over a 50×50 μm area with 10 kV dc and ˜1.4 kV p-p on a 3 μm comb structure. Vacuum pressure in the sample chamber was in the high e-7 Pascal range. The settings are equivalent to a second field gradient parallel to the sample surface of ˜4.7 e+8 V/m.

[0132] Using the arrangement an enhancement in the 23Na+ signal of a couple of percent on the comb structure were seen. Boosting the SIMS instrument's primary beam current to 250 pA and shifting to the 1 μm comb structure an enhancement of ˜10% between the application of no differential voltage (10 kV DC extraction only) and AC mode (1.4e+9 V/m with 10±1.4 kV applied to the two sample feedthroughs) was observed.

[0133] Providing a differential voltage of ˜2.8 kV p-p between the two substructures and using a 250 pA, 50×50 μm raster on the 1 μm comb spacing test object an enhancement of a factor of ˜2× on the 23Na+ signal was observed (see FIG. 5). The setting corresponded to a field gradient of roughly 2.8e+9 V/m parallel to the sample's surface. These data were measured on the SIMS' discrete dynode ion counting system with an integration time of 1 second. The large spikes at e.g., +350 s are when the voltage supplied to the comb was turned on and at e.g. 450 s the voltage was turned off.

[0134] A further test using identical conditions except that a 16O-primary ion beam at 1 nA was used showed an enhancement of similar magnitude.

REFERENCE SIGNS

[0135] 1 Sample or substance [0136] 2 Structure [0137] 3, 4 Substructures [0138] 5 non-conductive thin film [0139] 6 Micron-size holes allowing gas to pass through the non-conductive thin film