WO1996037905A1 - High-frequency-operated magnetron glow discharge ionisation process and ion source - Google Patents

Abstract

The invention relates to a process for ionising sample material in which a plasma is generated on a target containing the sample material by means of a high-frequency-operated magnetron glow discharge and sample material is released from the target with the aid of a sputter gas for transmission to a mass spectrometer and ionised. Here a magnetic field is generated on the surface of the target in such a way that the magnetic field lines in the plasma region are oriented towards the target surface. It is of advantage to arrange a magnet on the side of the target away from the plasma in such a way that the magnetic field lines in the plasma region are oriented towards the target surface.

Description

Description

Process for high-frequency-operated magnetron glow discharge ionization, and ion source

The invention relates to methods according to the preamble of claim 1 for high-frequency operated magnetron glow discharge ionization, in particular in a low-pressure argon plasma for mass spectrometry, in particular non-conductive solid samples. Furthermore, the invention relates to an ion source according to the preamble of claim 3.

Various ionization methods are used as prior art in mass spectrometry for direct evaporation (atomization) and ionization of the sample material to be examined, and ion sources are used which allow simultaneous evaporation (atomization) of the solid samples and ionization of the vaporized sample atoms.

Evaporation and ionization in a glow discharge plasma with direct current excitation (dc-GDMS, Direct Current Glow Discharge Mass Spectrometry). However, this method is only possible with conductive sample materials (such as metals, alloys or semiconductors) or with non-conductive samples by using special sample preparations such as a mixture with pure powdered metals or high-purity graphite with non-conductive powder samples, and by means of a secondary cathode applicable for non-conductive, compact samples.

However, such methods can only be used to a limited extent because they have disadvantages such as e.g. have low sputtering rates of non-conductive sample materials and consequently relatively high detection limits of the elements. They are also not applicable to all isolating sample materials.

For this reason, high-frequency-operated Climadladungsmassspectrometrie (rf-GDMS, radio frequency Glow Discharge Mass Spectrometry) by JW COBURN (Thin Solid Film 1989, 171.65), WW HARRISON (in Inorganic Mass Spectrometry eds. Adams, F., Gijbels, Van Grieken, R .; _1993. 8, 935) and RK MARCUS (J. Anal. At. Spectrom., 1993, j3, 935) for the direct analysis of conductive materials without matrix modification nicht¬ entwik- Celtic. These mass spectrometric methods can be applied to the elemental analysis of non-conductive sample materials. However, the detection limits for trace element analysis are only in the ppm concentration range or above. However, an ultra-sensitive trace analysis of non-conductive solid samples and thick layers is not possible.

It is therefore an object of the invention to provide a method which makes it possible to simultaneously vaporize (atomize) all the elements for mass spectrometry by means of high-frequency-operated magneton glow discharge ionization, molecular or atomic constituents of solid samples of different conductivity, with high sensitivity and efficiency. and ionize. Furthermore, it is an object of the invention to provide an ion source with improved ionization effectiveness and lower detection limit than known ion sources.

The object is achieved by a method according to the entirety of the features according to claim 1. A further expedient variant is found in the dependent sub-claim 2. Furthermore, the object is achieved by an ion source according to claim 3.

The invention according to claim 1 provides a method for highly sensitive evaporation (atomization) and ionization in a high frequency operated magnetron Glow discharge ion source at low argon pressure.

In particular, this method can be used for mass spectrometric analysis of conductive, semiconducting and non-conductive solids, liquids and gases. The method according to the invention is used both for the determination of the main, minor or trace element concentrations and for isotope or depth profile analysis.

The method can be used universally for solving analytical problems in solid state physics such as in surface physics, materials research, geology, nuclear physics and solid state chemistry. On the one hand it is for the quantitative determination of trace element concentrations, on the other hand it is also. suitable for deep profile analysis of bulk materials and thick layers.

Finally, the method is used in the isotope analysis of all chemical elements, especially in solid samples with different conductivity, such as metals, alloys, semiconductors or insulators.

The invention further relates to a system with an ion source for sample evaporation and ionization (ion source), for example in combination with one high-resolution double-focusing mass spectrometer for ultra-sensitive trace element analysis and depth profile analysis of various sample materials - especially non-conductive ceramics and thick insulating layers.

The glow discharge ion source, which contains a chamber in which a target as cathode, a sputtering gas for removing particles from the target, and an opening to a mass spectrometer unit, has a magnet on the back of the target, which is arranged so that the magnetic field lines in the plasma area are oriented towards the target surface.

With the magnetic field thus applied, an improved ratio of the number of target ions to the number of plasma ions is achieved. The ion source can thus be operated with a lower sputtering gas pressure, in particular argon pressure, and the ion source as a result has a higher sensitivity.

The method according to the invention allows sample material to be effectively vaporized (atomized) and ionized in a high-frequency magnetron glow discharge of a low-pressure argon plasma. The invention is explained in more detail with reference to figures and embodiment. Show it:

Figure 1: Planar magnetron glow discharge ion source

with the following reference numbers:

1 - sample and magnetic holder; 2 - copper back;

3 - flat sample (cathode);

4 - gas (argon) inlet capillary;

5 - interface to mass spectrometer;

6 - ion source housing (anode); 7 - magnetic ring;

8 - Viton O-ring;

9 - Macor isolator; MS mass spectrometer; HFG radio frequency generator; GD glow discharge.

Figure 2: Schematic diagram of the interface between the high-frequency generator and the high-frequency glow discharge ion source

with the following reference numbers: 1 - high frequency generator;

2 - power meter;

3 - matching box;

4 - high frequency filter; 5 - low frequency filter;

HS high-voltage power supply (1 - 15 KV); MS mass spectrometer.

Figure 3: Planar magnetron configuration profilogram of a sputter crater from a glass sample (power - 30 W, argon pressure - 5 Pa, t = 120 min)

with the following reference numbers:

Magnetic poles (N, S); B - magnetic field lines;

VB, VE - magnetic and electrical field gradient vector; V - electron drift vector.

Figure 4: Coupling of a high-frequency magnetron glow discharge ion source with a mass spectrometer

with the following reference numbers: 1 - ion source;

2 - interface;

3 - ion optics;

4 - ion acceleration;

Uß - acceleration voltage; MS mass spectrometer.

Table 1: Influence of the magnetic field on the ion intensities of the gas discharge species and the matrix components using a planar high-frequency GDMS for a glass sample.

The experimental arrangement of a planar magnetron glow discharge ion source for flat sample geometries is shown in FIG. 1.

The ion source consists of a magnet and sample holder 1 made of copper for flat and pin-shaped sample geometries, the ion source housing 6, preferably made of VA steel anode with a back 2 made of copper and a flat sample 3, basically supplied big size [cathode (3)]. In addition, the arrangement also has an argon inlet system 4, an interface 5 to the mass spectrometer MS, a magnetic ring 7, and a Viton O-ring 8 and an insulating intermediate piece 9 [Macor (9)].

The glow discharge ion source (GD ion source) is directly coupled to a mass spectrometer (e.g. double-focusing system). The GD ion source is located in an ion source chamber which is used for evacuation, e.g. with a turbomolecular pump is provided.

Highly pure argon (for example with a purity of 99.9995%) is preferably introduced as the discharge gas into the GD ion source via the gas inlet system 4. The argon pressure in the GD ion source (PGD) is - depending on the selected sample material - 0.1 to 10 Pa, for example. Optimal discharge conditions with maximum ion current intensities are measured in such a system with an output of 20-30 W and a plasma gas pressure of 1-3 Pa.

The back cover made of solid copper of the GD ion source is connected to a cooling system (vapor of liquid nitrogen). In this way, a temperature of the ion source of up to 50 ° C can be maintained during the sputtering process.

From FIG. 2, in which the electrical circuit diagram for the voltage supply on the sample is shown, A power meter 2, a high-frequency filter 4, a low-frequency filter 5, a DC voltmeter and a high-voltage power supply HS can be removed. The high-frequency power of, for example, 13.6 MHz, which is generated in a high-frequency generator 1, is coupled in via a matching box 3 and the sample holder.

By attaching a magnet to the back of the sample holder - that is, by the effect of an additional magnetic field on the sample surface and in the gas discharge space, as can be seen in FIG. 3 - the sputtering rate, in particular of compact, non-conductive sample materials, and thus the sensitivity of the analysis method (high-frequency-operated magneton GDMS) can be increased by at least one order of magnitude. Due to the lower working pressure compared to the high-frequency GDMS (PQD 0.1 -lhPa), the molecular ion or cluster ion formation rate (e.g. the argide) drops significantly. As a result, the detection limits for element trace analysis are significantly reduced down to the ppb concentration range.

By means of the high-frequency-operated magnetron GDMS, it is possible to use solids with extreme physical and chemical properties (eg high.) Without additional devices (eg mask technology - secondary cathode) Temperature resistance, chemical resistance, high hardness, high specific resistance and complicated matrix composition - as is the case with ceramics, for example) with regard to trace composition and element distribution (depth profile analysis).

For a depth profile analysis of non-conductive materials (e.g. thick ceramic layers), the dimensions of the magnet must be selected so that a smooth crater floor is created during the sputtering process.

The technical implementation of the coupling of a high-frequency glow discharge ion source with a commercial mass spectrometer is shown in FIG.

Because of the molecular and cluster ions formed in plasma ion sources (often with a large number and frequency), it can be advantageous to couple the glow discharge ion source with a high-resolution mass spectrometer. A coupling e.g. be provided with an HR-ICP-MS (high resolution inductively coupled plasma mass spectrometer), the inductively coupled plasma ion source being substituted by the high-frequency magnetron glow discharge ion source.

An advantage of such a device configuration is that the ion source is at ground potential and 05

12 consequently, the structure of the high-frequency magnetron GDMS is easy to implement.

Table 1 shows the analytical results from mass spectra of the high-frequency GDMS without and with the influence of a magnetic field on the sample surface in comparison to one another.

The experimental results show an increase in the total ion current by more than an order of magnitude. Furthermore, an increase in the intensity of the sample ions which are relevant for the analysis and on the other hand a reduction in the plasma ions which disrupt the analysis were achieved in this way.

Table 1 :

Influence of the magnetic field on the ion intensities of the

Gas discharge species and the matrix components below

Using a planar high frequency GDMS for a

Glass sample

Experimental gas discharge species atomic ions of the sample

Parameters Ar ⁺ Ar ²⁺ Ar ₂ ⁺ Arϊ co ₂ ⁺ o ₂ ⁺ Si ⁺ Na ⁺ Al ⁺ Mg ⁺

without magnet: RF power - 30 W; 3 0, 10 0.47 0.21 argon pressure - 30 Pa; 100 12 3.5 16 0.3 0.01 0.024 ion current - 1'10 ^"10 A with magnet: HF power - 30 W; argon pressure - 6 Pa; 100 2.1 0.42 1.5 0.10 0.025 2.0 0.83 0.041 0.10 ion current - 1 10 ^"9 A.

Intensity ratio with magnet / without magnet 1 0.18 0.12 0.10 0.3 0.25 4.2 4.0 4.1 4.2

Claims

claims 1. A process for the ionization of sample material, in which a plasma is generated on a target containing the sample material by means of a high-frequency magnetron glow discharge and sample material is released from the target and ionized with the aid of a sputter gas for transmission to a mass spectrometer, characterized in that A magnetic field is formed on the surface of the target in such a way that the magnetic field lines in the plasma region have an orientation toward the target surface. 2. The method according to claim 1, characterized in that a magnet is provided on the target side facing away from the plasma, which is arranged so that the magnetic field lines in the plasma region have an orientation towards the target surface. 3. Glow discharge ion source with a chamber in which a target as cathode, a sputter gas for extracting particles from the target, an opening to a mass spectrometer unit are provided, characterized by a magnet on the back of the target, which is arranged in this way that the magnetic field lines in the plasma region have an orientation towards the target surface. 4. Rf-GDMS with a glow discharge ion source according to claim 3.