EP1658632B1 - Mass spectrometer and liquid-metal ion source for a mass spectrometer of this type - Google Patents

Abstract

The invention relates to a mass spectrometer comprising an ion source for producing a primary ion beam, which has a heatable ion emitter coated by a liquid metal layer essentially comprised of pure metallic Bismuth or of a low-melting-point alloy containing, in essence, Bismuth. A Bismuth ion mixed beam can be emitted by the ion emitter under the influence of an electric field. From the Bismuth ion mixed beam, one of a number of Bismuth ion types whose mass is a multiple of monatomic singly or multiply charged Bismuth ions Bi1p+, is to be filtered out in the form of a mass-pure ion beam that is solely comprised of ions of a type Binp+, in which n≧2 and p≧1, and n and p are each a natural number.

Description

The invention relates to a mass spectrometer for analyzing secondary ions and deionized neutral secondary particles with an ion source for generating a primary ion beam for irradiating a sample and generating secondary particles, which source has a heatable ion emitter, which is coated in the field-exposed area with a liquid metal layer, the contains ionizable metal that is emitted and ionized as a primary ion beam, the primary ion beam containing metal ions with different ionization levels and cluster states, as well as a spectrometer unit for mass analysis of the secondary particles. The invention also relates to the ion source for such a mass spectrometer.

In the description, the notation "bismuth" instead of "bismuth" according to IUPAC recommendation is selected (see RÖMPP, CHEMIELEXIKON, 9th ed., Keywords "bismuth" and "bismuth"). Furthermore, the common name for ions in clusters in terms of their mass and their charge is used as follows:

Bi _n ^{p +}

where n is the number of atoms in a cluster and p + is the charge.

It is known to use liquid metal ion sources in secondary ion mass spectrometry, which is especially operated as Time of Flight Secondary Ion Mass Spectroscopy (TOF-SIMS). Applicant proposes a liquid metal gold cluster ion source for a spectrometer (see Prospectus "Liquid Metal Gold Cluster Ion Gun for Improved Molecular Spectroscopy and Imaging", no date, published 2002), which represents the state of the art according to the cited generic term.

The efficiency of the TOF-SIMS measurements compared to primary ion beams of monatomic gallium ions could be significantly increased with gold primary cluster ions, for example of the Au ₃ ⁺ type. A disadvantage of using gold as a material for the primary ion beam, however, is that in the production of gold ions of the type Au ₁ ⁺ predominate, while cluster formats such as Au ₂ ⁺ , Au ₃ ⁺ , only moderately small proportions in the total ionic current turn off.

In the intensive search for further cluster-forming, only one natural isotope substances for secondary ion mass spectroscopy was successfully tested bismuth. Bismuth is an anisotopic element with a melting point of 271.3 ° C. Besides, bismuth alloys such as Bi + Pb, Bi + Sn and Bi + Zn are known which have a lower melting point (46 ° C - 140 ° C) than pure bismuth. For a liquid metal ion source, however, pure bismuth is preferred.

The document US 6,002,128 describes a secondary ion mass spectrometer in which the primary beam is generated by a gallium liquid metal ion source. Other possible metal ions for the primary beam include cesium, indium, bismuth and gold.

Also, in the JP 03-084435 a calibration alloy for a secondary ion mass spectrometer specified, can be obtained with the mass spectra with high resolution. The elements V, Ge, Cd, Os and Bi are mentioned as elements with high negative secondary ionization. The isotope curves (patterns) with the aforementioned elements give characteristic, repeatable spectra. However, this document does not speak of clustering or of a liquid metal ion source. In addition, it is not stated that bismuth is particularly well suited for cluster production.

The invention thus has the task of developing an ion source with improved yield of cluster ions for the operation of secondary ion mass spectrometers in order to achieve a high efficiency of secondary ion formation with simultaneously high data rates and thus short analysis times. The proposed improvement combines a high efficiency E of secondary ion formation from unchanged sample surfaces with high cluster currents and leads to a corresponding shortening of the analysis times.

This object is achieved by a secondary ion mass spectrometer according to claims 1 or 6 and 9 in which the liquid metal layer consists of pure metallic bismuth or a low melting bismuth alloy, wherein with the ion emitter under the influence of an electric field, a bismuth ion mixing beam is emitted one of several Bismutionenarten, the mass of which is a multiple of the monatomic, singly or multiply charged bismuth Bi ₁ ^{p +,} is filtered out using a filter device as a mass pure ion beam consisting exclusively of ions of a type Bi _n ^pk , where n≥2 and p≥1 is and n and p are each a natural number.

Since secondary ion mass spectrometry is based on the sputtering of the analyzed solid surface, part of the surface is destroyed. From a given solid surface, therefore, only a limited number of molecular secondary particles can be generated and detected. In particular, the molecular components of the solid surface disintegrate due to the primary ion bombardment and are thus no longer available for analysis. A broader use of the TOF-SIMS for the analysis of molecular surfaces requires an increase of the previously achievable detection sensitivity for organic materials. Such sensitivity enhancement requires more efficient formation of secondary particles, especially secondary ions, from thicker organic layers. The proposed improvement increases the efficiency E of secondary ion formation from unchanged sample surfaces.

The value of efficiency E corresponds to the number of secondary particles detected by the spectrometer, which can be detected per unit surface area of a fully consumed monolayer. From the efficiency it can be calculated, therefore, how many secondary ions are to be detected in a small-area chemical analysis under the selected bombardment conditions.

It is particularly advantageous when the filtered out for a mass-pure ion beam ions belong to one of the following type: Bi ₂ ^+, Bi ₃ ^+, Bi ₃ ^2+, Bi ₄ ^+, Bi ₅ ^+, Bi ₆ ^+, Bi ₅ ²⁺ or Bi ₇ ²⁺ . It should preferably be worked with an ionic species, which makes up a relatively high proportion of the total number of ions.

Preferably, the mass spectrometer is operated as a time-of-flight secondary ion mass spectrometer (TOF-SIMS), since there is much experience for this type and the experimental operation has shown that the greatest user potential lies here.

In terms of wettability, stability and processability, for bismuth coatings, an ion emitter equipped with a nickel-chromium tip is, according to current knowledge, a favorable solution.

As the average current in the operation of the secondary ion mass spectrometer is chosen for the emission current between 10 ^-8 and 5x10 ^-5 A.

In the event that a bismuth metallic alloy is chosen instead of pure bismuth, it is preferable to determine one having a low melting point with a high bismuth content. Here, for example, bismuth alloys with one or more of the following metals as liquid metal coating in question: Ni, Ag, Pb, Hg, Cu, Sn, Zn, wherein preferably an alloy is selected whose melting point is below the melting point of pure bismuth ,

Fig. 1

ein Schema des Aufbaues eines Erzeugungssystems einer Flüssigmetall-Ionenquelle;

Fig. 2

Vergleich der Emissionsstromanteile, normiert auf die atomaren, einfach geladenen Spezies Bi₁ ⁺ bzw. Au₁ ⁺ für entsprechende Emitter bei 1µA Emissionsstrom;

Fig. 3

verschiedene Aufnahmen einer lateralen Farbstoffverteilung (413u und 640u) eines Farbfilter-Arrays mit verschiedenen Primärionenspezies, wobei als Analysebedingungen 25 keV Primärionenenergie bei einem Gesichtsfeld von 50 x 50 µm² gewählt wurden.

Essential properties, advantages and construction principles are explained with reference to a drawing, whose figures show:

Fig. 1

a diagram of the structure of a generating system of a liquid metal ion source;

Fig. 2

Comparison of the emission current components normalized to the atomic, singly charged species Bi ₁ ⁺ and Au ₁ ⁺ for respective emitters at 1μA emission current;

Fig. 3

various images of a lateral dye distribution (413u and 640u) of a color filter array with different primary ion species, wherein as analysis conditions 25 keV primary ion energy were selected at a field of view of 50 x 50 microns ² .

The general structure of a TOF-SIMS is well known, so here only on the FIG. 1 and the associated description of DE 44 16 413 A1 the applicant is referenced.

A liquid metal ion source suitable for a TOF-SIMS is disclosed in U.S. Pat FIG. 1 shown. Liquid metal ion sources are widely used for material processing and surface analysis. These ion sources have a very small virtual source size of about 10 nm and a high angle intensity. Due to these properties, liquid metal ion sources can focus very well, whereby up to 7 nm beam diameter can be achieved with relatively high beam currents.

In FIG. 1 schematically the generation system for ions from a liquid metal ion source with an emitter unit 1 is shown. The support unit 7 carries at its two ends depending on a rigid lead wire 6, wherein via the lead wires 6 a variable in its thickness heating current is supplied; Both supply wires 6 are connected to a reservoir 5 in which a supply of molten bismuth is present during operation of the emitter unit 1. An emitter needle 1 protrudes centrally from the reservoir 5. The emitter needle 1 can thus be held at a temperature at which the bismuth remains molten and wets the needle.

The emitter needle 1 is made of a nickel-chromium alloy and is wetted to its tip with liquid bismuth 4. The emitter needle has a wire diameter of about 200 microns and a radius of curvature at the top of 2 to 4 microns. The emitter needle 1 is positioned centrally in front of an extraction panel 2 and surrounded by a suppression unit 3.

If a high voltage is applied between the extraction aperture 2 and the wetted emitter needle 4, a sharp cone formed of liquid bismuth, the so-called Taylor cone, is formed at the tip of the needle at a certain voltage. The consequent rejuvenation of the tip leads to a significant increase in field strength. If the field strength is sufficient for field desorption, the emission of metal ions starts at the tip of the Taylor cone. The emission current of the liquid metal ion source of the type shown is approximately between 0.2 and 5 μA.

FIG. 2 shows the emission current components for bismuth and gold, normalized to the atomic, singly charged ions for AuGe and Bi emitters at an emission current of 1 μA.

It can be seen that the standardized, relative emission proportions are much better for bismuth than for gold. Another advantage over gold, which requires alloying components to achieve low melting points, is that bismuth can be used as the pure metal. The melting point is relatively low at 271.3 ° C. In addition, with bismuth the vapor pressure prevailing at the melting point is lower than with gold. Another advantage is the fact that in gold, the emitted ionic current is mixed with alloying constituents, such as germanium, so that there is a higher requirement for mass filtering.

The absolute emission currents of Au ₁ ⁺ and Bi ₁ ⁺ are approximately equal. While the atomic, singly charged beam components Au ₁ ⁺ and Bi ₁ ^{+ are} comparable in size, the cluster yield shows a clear difference. For the singly charged ions, the advantage of Bi _n ⁺ over Au _n ⁺ increases steadily with the size of the clusters. Double-charged cluster ions are emitted only with bismuth with appreciable intensity.

In the FIG. 2 The cluster components shown refer to a total emission current of 1 μA. Since the cluster components are emission current-dependent, the cluster current can still be increased depending on further parameters for bismuth.

To compare the invention with the prior art, the same liquid metal ion mass spectrometer was used to analyze the same organic surfaces with different kinds of primary ions (cf. Fig. 3 ). The sample is a color filter array, which, for example, is switched in digital cameras in front of a photosensitive CCD surface to provide the color information. This sample is very well suited as a comparison standard, since it is made very homogeneous and reproducible. In addition, the differences achieved between the primary ion species are quite typical and can be qualitatively transferred to other molecular solid surfaces.

In the FIG. 3 Series of images shown show the lateral distribution of two dyes used with the masses 413u and 641u. Due to the increasing destruction of the surface as a result of the Primärionenbeschusses the signal intensity decreases continuously. Shown for all primary ion species of the designated type is the summed signal intensity with the same degree of surface damage (1 / e decrease in signal intensity). The signal intensity achieved is thus a measure of the efficiency of the analysis.

The very low Au ₃ ⁺ cluster currents lead to relatively long measurement times. By using Bi ₃ ⁺ clusters, the primary ion currents over Au ^₃₊ can be increased by a factor of 4 to fifth Due to the slightly increased yield, the increase in data rates is even higher. The 1 / e decrease of the signal intensity is observed with Au ₃ ⁺ primary ions after 750 s and with Bi ₃ ⁺ primary ions already after a significantly reduced analysis time of 180s reached. The reduction in the measuring time is essentially due to the increased Bi ₃ ⁺ cluster currents. The choice of Bi ₃ ⁺⁺ leads to similarly short measurement times. An increase in efficiency can be achieved by using larger clusters, such as Bi ₇ ⁺⁺ , but these cluster currents are relatively small, so the analysis times are generally longer.

Since the measurement time makes up the major part of the analysis time, increasing the data rates by using Bi ₃ ⁺ or Bi ₃ ⁺⁺ leads to a correspondingly higher sample throughput.

In addition to the described advantages in terms of measurement time, bismuth emitters have advantages over gold emitters in terms of emission stability at low emission currents and mass separation of the emitted ion species. The advantages described therefore show that bismuth emitters have significant economic and analytical advantages which were not to be expected without further ado.

Claims (10)

1. A mass spectrometer for the analysis of secondary ions and post-ionized neutral secondary particles with an ion source to produce a primary-ion beam for irradiating a sample and to produce secondary particles, which ion source comprises a heatable ion emitter which is coated in the region exposed to a field with a liquid-metal layer which contains a metal which is capable of being ionized and which is emitted as a primary-ion beam and is ionized, wherein the primary-ion beam contains bismuth metallic ions with different stages of ionization and cluster states, and with a spectrometer unit for the mass analysis of the secondary ions and secondary particles, wherein the liquid-metal layer consists of pure metallic bismuth or a low-melting alloy containing bismuth, wherein a bismuth-ion mixed beam is capable of being emitted by the ion emitter under the influence of an electrical field, characterized in that the said mass spectrometer contains a filter apparatus by which one of a multiplicity of types of bismuth ions - the mass of which amounts to a multiple of the monatomic, singly or multiply charged bismuth ion Bi₁ ^p+ as a mass-pure ion beam which consists exclusively of ions of a type Bi_n ^p+ in which n ≥ 2 and p ≥ 1 and n and p are in each case a natural number - is capable of being filtered out from the bismuth-ion mixed beam.
2. A mass spectrometer according to Claim 1, characterized in that the ions filtered out for a mass-pure ion beam belong to one of the following type: Bi₂ ⁺, Bi₃ ⁺, Bi₃ ²⁺, Bi₄ ⁺, Bi₅ ⁺, Bi₆ ⁺, Bi₅ ²⁺ or Bi₇ ²⁺.
3. A mass spectrometer according to Claim 1 or 2, characterized in that the secondary-ion mass spectrometer is capable of being operated as a flight-time secondary-ion mass spectrometer.
4. A mass spectrometer according to any one of the preceding Claims, characterized in that the emission current of the primary-ion beam during operation amounts to between 10^-8 and 5 × 10^-5 A.
5. A mass spectrometer according to any one of the preceding Claims, characterized in that a metallic alloy of bismuth with one or more of the following metals is selected as a liquid-metal covering: Ni, Ag, Hg, Cu, Sn, Zn, wherein it is preferable to select an alloy of which the melting point is below the melting point of pure bismuth.
6. A method of analysing samples or surfaces, characterized in that the sample or surface is irradiated with a mass-pure ion beam of bismuth ions of a type Bi_n ^p+ in which n ≥ 2 and p ≥ 1 and n and p are in each case a natural number, so that secondary particles and/or secondary ions are formed, and wherein the mass of the secondary particles and/or secondary ions is analysed.
7. A method according to the preceding Claim, characterized in that the mass-pure ion beam is generated by the bismuth ions of one type being filtered out from an ion beam generated in a liquid-metal ion source.
8. A method according to Claims 6 and 7, characterized in that the sample or surface has organic materials.
9. Use of a mass spectrometer according to any one of Claims 1 to 5 for the analysis of samples and/or surfaces.
10. Use according to the preceding Claim, characterized in that the samples or surfaces have organic materials.

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