GB2345378A

GB2345378A - Ion source for elemental analysis

Info

Publication number: GB2345378A
Application number: GB9930663A
Authority: GB
Inventors: Volker Hoffmann; Guenther Pietzsch; Klaus Wetzig; Klaus Moehler; Mihaly Kunstar; Detlef Schiel; Reinhard Jaehrling
Original assignee: Institut fuer Festkoerper und Werkstofforschung Dresden eV
Current assignee: Institut fuer Festkoerper und Werkstofforschung Dresden eV
Priority date: 1998-12-30
Filing date: 1999-12-24
Publication date: 2000-07-05
Anticipated expiration: 2019-12-24
Also published as: GB2345378B; GB9930663D0

Abstract

An ion source for elemental analysis in respect of solid material samples by means of analysis of the plasma of a low-pressure gas discharge comprises a chamber having an inlet channel (8) and a suction-extraction channel (9) for a working gas. The chamber has a cathode (2) and a cylindrical hollow anode (1) with which a plasma (10) is generated over a material sample (5). The chamber exhibits an aperture (4) for emission into a mass spectrometer of the ions to be analysed. The chamber also has an additional channel (11) for the extraction of working gas by suction which leads into the chamber to the side of the aperture (4) for emission of the ions to be analysed. This allows an increase in discharge output even though the pressure of the working gas is lowered. In a modification (fig 3) a window allows radiation from the plasma (10) to be used for optical spectroscopy.

Description

Ion Source for Elemental Analysis Description The invention relates to an ion source for the elemental analysis of solid material samples by means of analysis of the plasma of a low-pressure gas discharge.

Known processes for investigating solid material samples by elemental analysis are, inter alia, optical glow-discharge spectroscopy (GD-OES) and glow-discharge mass spectroscopy (GD-MS).

In the case of the known ion sources for glow-discharge mass spectroscopy a partial vacuum of a working gas is generated within the source, a plasma is ignited between two electrodes, matter pertaining to the material sample which is at cathode potential is introduced into the plasma by sputtering, and said plasma is then analysed by mass spectroscopy (N. Jakubowski, D. Stuewer, G. Toelg, Int. Mass Spectrom. Ion Proc. 71 (1986), 183 [1] ; DE 1 589 389 [2]; Y. Shao and G. Horlick, Spectrochim. Acta, Vol.

46B, No. 2,165-174 (1991) [31).

The ions get into the mass spectrometer, via a usually differential pumping system, through an aperture with a size of approximately 1 mm which is located opposite the material sample. The size of this aperture and the pressure prevailing in the source determine the pressure that arises in the mass spectrometer. Due to the necessary high vacuum in the mass spectrometer, at high pressures use therefore has to be made of small apertures. The pressure of the working gas within the source essentially determines, along with the material sample that is used, the discharge current that arises at a particular voltage. Within a wide working range the discharge output and hence the rate of erosion increases with rising pressure.

A high rate of erosion is required, in order to advance as rapidly as possible into the interior of the sample and therefore to be able to carry out effectively depth-resolved analyses or to eliminate superficial impurities rapidly by so-called preliminary sputtering. As a result of varying the pressure, in addition the shape of the crater profile can be optimised in the direction of high depth resolution.

In case the spacing between the electrodes is smaller than the mean free path-length of the atoms of the discharge gas, the discharge is limited there. Spacings smaller than approximately 0.1 mm cannot be used, since the redeposition of the eroded sample matter on the surface of the sample rapidly results in short-circuits.

In commercial plants and in all other plants that have become known from other literature in the field of GD-MS the spacing between the electrodes of the discharge source, i. e. between the material sample and the anode, is kept small, typically < 0.5 mm. Almost exclusively, both electrodes have hitherto been separated by insulating material in the vicinity of the sample. When the output is increased as a result of raising the pressure, instabilities and hence also contamination of the plasma occur in this region as a result of flash-overs. This is true both for the known rod sources or pin sources and for the flat sources.

In [1] and [3] cylindrical anodes were employed in which the material boundary between anode and cathode does not occur in the vicinity of the material sample but rather in the external region of the source. However, effective evacuation of the space between the anode and the cathode plate in accordance with [2] was dispensed with in both cases.

Instead, for example in [3], the working gas is even admitted in the source through this gap, resulting in a local increase in pressure at this point. The suction-extraction lines that are present with this source serve exclusively for evacuation of the source prior to the analysis and are closed during the analysis.

The determination of extremely low elemental concentrations in the region of 1O 9 ig/g requires the measurement [of] very high ion intensities of the principal element in the region of 10' ions/s. Experiments have shown that the evacuation of the interspace between the electrodes has a negative effect on the ion current that is transported into the mass spectrometer. At constant voltage between the electrodes, higher gas pressures and the associated higher outputs do not necessarily result in an increase in the particles that are transported into the mass spectrometer and consequently in an increase in the analytical signal, despite an increased rate of sputtering of the material sample. Although not described hitherto in the literature, it can be concluded from this that the stream of gas between the electrodes that is not directed towards the mass spectrometer impedes the transport of the ions of the material sample to the mass spectrometer.

Summing up, it may be noted that all the ion sources known hitherto pertaining to GD-MS operate within a comparatively low pressure range and output range in comparison with GD-OES. Typical for GD-OES are, for 0 8 mm and 1,000 V, 100 mA, and 10 mA in GD-MS. Since the measurement of the pressure in the discharge source is usually not exact and the discharge parameters in the case of a defined material sample are defined by voltage and current, the precise specification of the pressure can be dispensed with.

Experience has shown that it lies between 1 and 10 Torr in GD-OES, and an order of magnitude lower in GD-MS.

The object underlying the invention is to design an ion source for elemental analysis in respect of solid material samples by means of analysis of the plasma of a lowpressure gas discharge in such a way that the pressure in the space between the electrodes is lowered and at the same time an increase in the discharge output and a higher measured intensity of the particles that are removed by sputtering is brought about.

In accordance with the invention, in the case of an ion source consisting of a chamber which exhibits an inlet channel and a suction-extraction channel for a working gas as well as an aperture for emission into the mass spectrometer of the ions to be analysed and which is equipped with a cathode and a cylindrical hollow anode for the generation of plasma over the material sample, this object is achieved in that, in addition, a channel for the extraction of working gas by suction leads in the chamber to the side of the aperture for emission of the ions to be analysed.

In advantageous manner the free cross-section of the additional channel is designed to be larger than the cross-section of the aperture for emission of the ions to be analysed.

According to another configuration of the invention the cylindrical hollow anode [is] subdivided in its interior in the axial direction into two channels, one channel being sealed with a light-transmitting window at its end facing away from the material sample and being intended for optical spectroscopy of the plasma. The other channel exhibits at its end facing away from the material sample an aperture for emission ; mass spectrometer of the ions to be analysed. A light guide may expediently be coupled onto the light-transmitting window outside the source.

In this configuration the ion source can be employed for optical glow-discharge spectroscopy which is operated with d. c. voltage (DC-GD-OES), for glow-discharge mass spectrometry which is operated with d. c. voltage (DC-GD-MS), high-frequency-assisted optical glow-discharge spectroscopy (RF-GD OES) and high-frequency-assisted glow-discharge mass spectroscopy (RF-GD-MS).

With the ion source according to the invention it is also possible, in addition to the high stability of the discharge at high output densities, to increase the ion current by approximately an order of magnitude in comparison with the known sources.

The invention is elucidated in greater detail below on the basis of embodiment examples. Shown in the associated drawings are: Fig. 1: the schematic sectional representation of an ion source for GD-MS, Fig. 2: the GD-MS spectrum of a Cu standard sample with Pb, Fig. 3: the schematic sectional representation of an ion source for GD-OES and GD MS.

Example 1 The ion source that is represented in Fig. 1 for a mass spectrometer is constructed with an anode 1, a cathode 2 and an insulator 3. The anode 1 exhibits an aperture 4 which leads into the mass spectrometer for emission of the ions to be analysed. Opposite this aperture a material sample 5 to be analysed is arranged on the cathode 2. The interior of the ion source is tightly sealed with the material sample 5 and the sealing rings 6 and 7. Within the ion source a working gas which is supplied through an inlet channel 8 and withdrawn through a suction-extraction channel 9 is conducted over the surface of the material sample 5. The working gas is ignited between the cathode 2 and the anode 1. As a result a plasma 10 is formed, into which the matter of the material sample 5 which is at cathode potential is introduced. Upstream of the exit aperture 1 towards the mass spectrometer an evacuation channel 11 leads in the anode 2, through which evacuation channel the plasma 10 is aspirated in the direction of the exit aperture 4 during implementation of the analysis. The cross-section of the orifice of the evacuation channel 11 within the ion source is designed to be larger than the cross-section of the exit aperture 4. In this way, although the pressure of the working gas between the electrodes is lowered, in advantageous manner an increase in the discharge output and an increase in the intensity measured by the mass spectrometer are achieved.

Fig. 2 shows the result of the analysis in respect of a material sample 5 consisting of Cu with 996 ig/g Pb, which was analysed at 650 V and 150 mA. The matrix ion current of the Cu was in the region of 101 ions/s. From the intensity of the Pb signals it is evident that ultratrace analyses are possible with approximately 30 ions/s for each ng/g. The comparison of the measured isotope ratios with the known natural isotope ratios (values in brackets) proves the high stability of the discharge.

Example 2 The ion source that is represented in Fig. 3 can be used both for optical glow-discharge spectroscopy and for glow-discharge mass spectroscopy. It is constructed with an anode 15, a cathode 16 and an insulator 17. The interior of the ion source is tightly sealed with the material sample 18 which is arranged on the cathode 16 and with the sealing rings 19 and 20. In the ion source a working gas which is supplied through an inlet channel 21 and withdrawn through a suction-extraction channel 22 is conducted over the surface of the material sample 18. The working gas is ignited between the anode 15 and the cathode 16. As a result a plasma 23 is formed, into which the matter of the material sample 18 which is at cathode potential is introduced. The space above the material sample 18 is subdivided into two channels 24; 25. The channel 24 exhibits an aperture 26 at its end facing away from the material sample 18, through which aperture the ions that are formed in the ion source can emerge for an investigation by mass spectrometry. Upstream of the aperture 26 towards the mass spectrometer a channel 28 leads in the anode 15, through which channel the ions are aspirated in the direction of the aperture 26 during implementation of the analysis. The cross-section of the orifice of the channel 28 is designed to be larger than the cross-section of the aperture 26. In this way, although the pressure of the working gas between the electrodes 15 ; 16 is lowered, in advantageous manner an increase in the discharge output and an increase in the intensity measured by the mass spectrometer are achieved.

The channel 25 is sealed with a light-transmitting window 27. The radiation emanating from the plasma 23 emerges through said lighttransmitting window and so can be utilised for the purpose of optical glow-discharge spectroscopy.

Claims

Claims 1. Ion source for elemental analysis in respect of solid material samples by means of analysis of the plasma of a low-pressure gas discharge, consisting of a chamber for receiving the material sample, whereby the chamber exhibits an inlet channel and a suction-extraction channel for a working gas and is equipped with a cathode and a cylindrical hollow anode, with which a plasma is generated over the material sample, and whereby the chamber exhibits an aperture for emission into the mass spectrometer of the ions to be analysed, characterised by an additional channel (11; 28) for the extraction of working gas by suction which leads into the chamber to the side of the aperture (4 ; 26) for emission of the ions to be analysed.
2. Ion source according to claim 1, wherein the free cross-section of the additional channel (11; 28) is larger than the cross-section of the aperture (4 ; 26) for emission of the ions.
3. Ion source according to claim 1, wherein the cylindrical hollow anode (15) is subdivided in its interior in the axial direction into two channels (24,25), one channel (25) being sealed with a light-transmitting window (27) at its end facing away from the material sample (1) and being intended for optical spectroscopy of the plasma, and the other channel (24) exhibiting at its end facing away from the material sample (18) an aperture (26) for emission into a mass spectrometer of the ions to be analysed.
4. Ion source according to claim 1, wherein a light guide is coupled onto the lighttransmitting window (27) outside the source.