GB2414594A

GB2414594A - A time of flight secondary ion mass spectrometer

Info

Publication number: GB2414594A
Application number: GB0411961A
Authority: GB
Inventors: Andrew Hoffman
Original assignee: Ionoptika Ltd
Current assignee: Ionoptika Ltd
Priority date: 2004-05-28
Filing date: 2004-05-28
Publication date: 2005-11-30
Also published as: GB0411961D0

Abstract

A time of flight secondary ion mass spectrometer (TOF-SIMS) 200 comprises an ion gun 210 which fires a continuous beam of primary ions 211 at a sample 220 to generate secondary ions 221. The secondary ions 221 are then collected by a collection electrode arrangement (collection optics) 230. The secondary ions 211 pass from the collection optics 230 into a cooling electrode arrangement 240. The secondary ions 221 then enter an acceleration means 260 adapted to periodically apply an impulse to ions 221 in a direction orthogonal to the ions direction of travel. The ions 221 are thus directed towards a detector 280 via a reflectron 270. By calculating the time interval between an impulse generated by the acceleration means 260 and the arrival of ions at the detector, the charge to mass ratio and relative abundance of various ions can be determined, in turn enabling the composition of the sample 220 to be determined. Application to purity testing of semiconductor materials and devices is disclosed. Other applications include the analysis of polymers, metals, chemical samples or biological materials, and pharmaceutical absorption testing.

Description

24 1 4594

MASS SPECTROMETRY

The present invention relates to mass spectrometry and in particular to secondary ion mass spectrometry (SIMS).

Time of flight (TOF) mass spectroscopy involves the ionisation of a sample and the acceleration of the ions towards a detector. The flight time of the ions to the detector after acceleration is indicative of the charge to mass ratio of the ions and by analysing the relative abundances of ions with different charge to mass ratios, an indication of the composition or purity of the sample can be obtained. In different types of mass spectrometer the paths along which ions travel to the detector differ. In linear mass spectrometer arrangements, the ion flight path is substantially linear, sometimes having an ion mirror or reflectron or similar device to extend the flight path and provide some compensation for energy spread. In orthogonal TOF mass spectrometers the ions are cooled and then travel along in a first direction before being accelerated in a substantially perpendicular direction. In such orthogonal TOF mass spectrometers the ions are also typically reflected from an ion mirror or reflection between acceleration and the detector.

Secondary ion mass spectrometry involves ionising a sample by firing a beam of primary ions at the sample and ana]ysing the resulting secondary ions by acceleration towards a suitable detector as above. The secondary ions travel along a linear path to the detector, SIMS instruments are arranged in this linear manner because it is a tried and tested configuration for this application which is relatively easy to manufacture with ultra high vacuum capability.

One particular use of SIMS is in assessing the purity of semiconductor materials and devices, although other there are other applications such as in polymer chemistry, metallurgy, biochemistry and intra-cellular analysis. In semiconductor purity analysis, a pulsed and bunched primary ion beam is focussed upon different areas of the sample in turn. An instantaneous pulse of secondary ions is generated upon the incidence of each bunched pulse of the primary ion beam. A pulsed and bunched primary ion beam is used to ensure all the secondary ions produced by each pulse are produced at substantially the same time improving the accuracy of the charge to mass ratio of the ions deduced from the time of flight.

To obtain an ideal assessment of purity, the ion beam should be focussed on as small an area of the sample as possible. The minimum size of this area is however constrained by bunching the ion pulses, which tends to spread the beam laterally.

Accordingly, the number of ions in each bunched pulse is limited to improve the focussing and hence the number of secondary ions produced by each bunched pulse is also is limited. Additionally, the pulse rate is limited by the need to ensure that ions produced by consecutive pulses do not overlap in the detector. As a result of these limitations, only around 100-1000 analysis cycles can be completed each second if a high mass range is required. The relatively slow pulse rate means that the sample must be held in a very good vacuum to ensure that each pulse generates secondary ions from the sample itself and not from vapour material deposited on the surface.

Consequently, it takes a relatively long time to detect sufficient ions to adequately assess the purity of the sample at a given area of the surface and thus it also takes along time to obtain overall or comparative assessments based upon analysis of the purity of a variety of different areas of the sample.

It is therefore an object of the present invention to provide a method and - 3 apparatus for conducting purity analysis of a surface by mass spectrometry that overcomes or alleviates some or all of the above problems.

According to a first aspect of the present invention there is provided a method of mass spectrometry comprising the steps of: directing a primary ion beam at a sample, said ion beam generating secondary ions from the sample material, wherein a proportion of the secondary ions enter an acceleration means; periodically accelerating said secondary ions which enter the acceleration means towards a detector; and measuring the time of flight between the acceleration and the detection of said secondary ions to determine the charge to mass ratio of said secondary ions characterized in that said ions are accelerated by said acceleration means in a direction substantially orthogonal to that in which the ions have travelled between said sample and said acceleration means.

In this manner a secondary ion mass spectrometer (SIMS) is provided wherein analysis of the sample ions is earned out in an orthogonal time of flight (TOP) mass spectrometer arrangement. This allows improved resolution of charge to mass ratio of individual secondary ions and a much faster analysis cycle than conventional linear TOF arrangement.

Preferably, the primary ion beam is run continuously, or with very long pulses if some beam off time is required for other reasons. This allows the beam to be focussed more narrowly than the pulsed and bunched primary ion beam used in the prior art. Additionally, the use of a continuous, or almost continuous, primary beam means that a greater flux of ions can be focussed on a particular area at a particular time, this increase in the number of primary ions increasing the number of secondary ions generated. An additional benefit of running the primary ion beam continuously or almost continuously is that the sample need not be held in such a high vacuum as samples in the prior art. This is because a continuous stream of ions is being produced from the surface, preventing the deposition of gaseous material on the surface in between pulses, thus giving a more accurate assessment of the purity or S composition of the sample.

In order to accurately measure the time of flight of individual secondary ions or groups of secondary ions, when a continuous stream of such ions is entering the acceleration means, the acceleration means is preferably operated in a pulsed manner to periodically accelerate ions towards the detector. Each pulse of the acceleration means accelerates ions towards the detector and given that there is a continuous stream of secondary ions and that more secondary ions are produced by this method than by the pulsed beam of the prior art, the acceleration means may be pulsed more rapidly than the primary beam was in the prior art whilst still producing high quality results. This allows the apparatus to operate at up to 10, 000 cycles per second, considerably faster than the prior art. Furthermore the mass resolution of the individual cycles is improved.

Preferably, this method is adapted for surface analysis of a sample and involves repeating the above cycle on different areas of the sample surface to obtain an overall and or a comparative composition or purity assessment. Preferably, the method is adapted to be applied to purity testing of semiconductor components or devices. It is of course possible for the method to be used in other applications such as analysis of polymers, metals, chemical samples or biological materials. In addition to acquiring spectra, the primary beam can be scanned to generate images of chemical distributions in the surface and plots of chemical concentrations against depth can be - 5 acquired by etching into a sample. Preferably, the acceleration means is a suitable electrode arrangement. Many such suitable arrangements will be known to those skilled in the art. In some embodiments the acceleration means may direct the ions towards the detector via an ion mirror or reflection.

The primary ion beam may be provided by any suitable ion gun, and may consist of any suitable ions. In one alternative embodiment, the ion gun may be an ion gun adapted to fire C60 or 'Buckyballs' at a sample. Such an ion gun would be of particular use for analysing organic samples including pharmaceutical absorption testing of the type wherein a pharmaceutical substance is administered to a subject, a cell sample is subsequently obtained from the subject, the cell sample being frozen and sliced open before being analysed by SIMS.

Preferably, the secondary ions are collected, cooled and focussed into a beam before being inserted into the acceleration means. Preferably, the collection, cooling and focussing of the ions is achieved by use of a suitable electrode arrangement having a collection electrode arrangement, a cooling electrode arrangement and a focussing electrode arrangement.

The cooling electrode arrangement is preferably an electrode arrangement of the quadrupole type. Most preferably, the cooling arrangement is optimised to provide a fast transit time for ions from the sample to the acceleration means, rather optimum cooling of the ions. Preferably, the transit time is less than 30ps and most preferably is ofthe order of 15ps.

The collection and focussing electrode arrangements may comprise any suitable collection and focussing arrangements. Many suitable arrangements are known to those skilled in the art. - 6

According to a second aspect of the present invention there is provided a mass spectrometer apparatus adapted to carry out the method of the first aspect of the present invention.

The apparatus may incorporate any features or alternatives described in relation to the method of the first aspect of the invention.

In order that the invention is more clearly understood, it will now be described further herein, by way of example only and with reference to the following drawings in which: Figure 1 shows a linear time of flight mass spectrometer according to

the prior art; and

Figure 2 shows an orthogonal time of flight mass spectrometer according to the present invention.

Referring now to figure 1, a conventional time of flight (TOF) secondary ion mass spectrometer (SIMS) 100 comprises an ion gun 110 firing a pulsed bunched primary ion beam 1 1 1 at a sample 120. The incidence of the primary ion beam 11 1 ionises the sample 120 to produce secondary ions. A proportion of the secondary ions are collected and focused by collection electrodes 130 and accelerated linearly by further electrodes (not shown) to detection chamber 140, wherein a detector (not shown) is provided. The detector detects the number of ions arriving at particular time intervals after each bunched pulse of the primary ion beam l l l and thus the relative abundances of ions of different charge to mass ratios can be determined.

Use of a pulsed ion beam l l l is necessary in order that the time of flight of the secondary ions can be accurately calculated. However in order to generate sufficient secondary ions at one time rather than a range of times the beam must be bunched, which reduces the accuracy of the focussing of the beam and therefore requires that a less intense beam is used.

The above problems mean that less secondary ions are generated than is desirable from each pulse and that the ions are not generated from as restricted an area of the sample as is desired. Furthermore use of a pulsed beam limits the number of analysis cycles that may be completed every second to between 100 and 1000 cycles.

This in combination with the lower than desired number of secondary ions produced by each pulse increases the length of time it takes to obtain an accurate analysis of the composition of the sample 120.

10The TOF-SIMS 100 and the sample 120 are placed in a high vacuum (10-6 Torr or better) during the analysis. However even at these pressures, in the interval between pulses gaseous material can be deposited on the surface of the sample reducing the accuracy of the analysis.

Referring now to figure 2, a TOF-SIMS 200 according to the present invention 15is shown. As in the conventional TOF-SIMS 100, an ion gun 210 fires a beam of primary ions 211 at a sample 220 to generate secondary ions 221. The secondary ions 221 are then collected by a collection electrode arrangement (collection optics) 230.

In the TOF-SIMS 200 however the primary ion beam 211 runs continuously.

The secondary ions 211 pass from the collection optics 230 into a cooling electrode arrangement 240. There is a relatively high pressure within the cooling arrangement 240 which aids the cooling or slowing of the secondary ions. The cooling arrangement 240 comprises a set of quadrupole electrodes 241. The quadrupole arrangement is optimised to allow a fast transit time between the sample and the detector 280 to enable the maximum number of analysis cycles to be run per - 8 second. Typically the transit time is of the order of 15-301ls.

Upon exit from the cooling arrangement 240, secondary ions may optionally pass through a focussing electrode arrangement 250, which is not shown in figure 2.

The secondary ions 221 then enter an acceleration means 260 comprising a suitable electrode arrangement. The acceleration means is adapted to periodically apply an impulse to ions 221 entering the acceleration means. The impulse is in a direction orthogonal to the ions direction of travel. The ions 221 are thus directed towards a detector 280 via a reflection 270. By calculating the time interval between an impulse generated by the acceleration means 260 and the arrival of ions at the detector, the charge to mass ratio and relative abundance of various ions can be determined, in turn enabling the composition of the sample 220 to be determined.

As the acceleration means 260 generated impulses periodically which direct ions to the detector, there is no need for the primary ion beam 211 to be pulsed and bunched. Accordingly, the primary ion beam may be run continuously which enables an accurate focus to be achieved and for a reasonable amount of secondary ions 221 to be generated. The greater number of secondary ions generated and the use of the acceleration means to divert ions to the detector 280 allows 10,000 or more analysis cycles to be completed per second. The cooling of the ions 221 before acceleration reduces the variation in initial conditions and thus increases the achievable resolution.

As the primary ion beam is run continuously, problems caused by the deposition of gaseous material on the surface of the sample 220 are minimised and thus the sample may be held in a lower vacuum than that used in the prior art without adversely affecting the analysis. - 9 -

It is of course to be understood that the invention is not to be limited to the details of the above embodiment which is described by way of example only. - 10

Claims

Claims 1. A method of mass spectrometry comprising the steps of:

directing a primary ion beam at a sample, said ion beam generating secondary ions from the sample material, wherein a proportion of the secondary ions enter an acceleration means; periodically accelerating said secondary ions which enter the acceleration means towards a detector; and measuring the time of flight between the acceleration and the detection of said secondary ions to determine the charge to mass ratio of said secondary ions characterized in that said ions are accelerated by said acceleration means in a direction substantially orthogonal to that in which the ions have travelled between said sample and said acceleration means.
2. A method as claimed in claim 1 wherein the primary ion beam is run continuously.
3. A method as claimed in claim 1 wherein the primary ion beam is run with very long pulses.
4. A method as claimed in any preceding claim wherein the acceleration means is operated in a pulsed manner.
5. A method as claimed in claim 4 wherein the acceleration means is pulsed at up to 10,000 cycles per second.
6. A method as claimed in any preceding claim wherein the method is adapted for surface analysis of a sample. - 11
7. A method as claimed in any preceding claim wherein the method is repeated on different areas of the sample surface to obtain an overall and/or a comparative composition or purity assessment.
8. A method as claimed in any preceding claim wherein the primary beam is scanned to generate images of chemical distributions in the surface of the sample.
9. A method as claimed in any preceding claim wherein by etching into a sample, plots of chemical concentrations against depth are acquired.
10. A method as claimed in any preceding claim wherein the acceleration means is an electrode arrangement.
11. A method as claimed in any preceding claim wherein the acceleration means directs the ions towards the detector via an ion mirror or reflection.
12. A method as claimed in any preceding claim wherein the secondary ions are collected, cooled and focussed into a beam before being inserted into the acceleration means.
13. A method as claimed in claim 12 wherein the collection, cooling and focussing of the ions is achieved by use of an electrode arrangement having a collection electrode arrangement, a cooling electrode arrangement and a focussing electrode arrangement.
14. A method as claimed in claim 13 wherein the cooling electrode arrangement is an electrode arrangement of the quadrupole type. - 12
15. A method as claimed in claim 13 or claim 14 wherein the cooling arrangement is optimised to provide a fast transit time for ions from the sample to the acceleration means.
16. A method as claimed in claim 15 wherein the transit time is less than 30ps.
17. A method as claimed in claim 15 or claim 16 wherein the transit time is of the order of 151ls.
18. A method as claimed in any preceding claim wherein the primary ion beam comprises C60 ions.
19. A method of purity testing of semiconductor components or devices using the method of any one of claims 1 to 18.
20. A method of analysis of polymers, metals, chemical samples or biological materials using the mass spectrometry method of any one of claims 1 to 18.
21. A method of pharmaceutical absorption testing wherein a pharmaceutical substance is administered to a subject, a cell sample is subsequently obtained from the subject, the cell sample being frozen and sliced open before being analysed by using the mass spectrometry method of any one of claims 1 to 18.
22. A mass spectrometer apparatus comprising: an ion gun for directing a primary ion beam at a sample, said primary ion beam generating secondary ions from the sample; acceleration means for periodically accelerating a proportion of said secondary ions; and a detector for detecting the accelerated secondary ions characterized in that said ions are accelerated by said acceleration means in a direction substantially orthogonal to that in which the ions have travelled between said sample and said acceleration means. - 13
23. An apparatus as claimed in claim 22 wherein the acceleration means is an electrode arrangement.
24. An apparatus as claimed in claim 22 or claim 23 wherein the acceleration means directs the ions towards the detector via an ion mirror or reflection.
25. An apparatus as claimed in any one of claims 22 to 24 wherein the secondary ions are collected, cooled and focussed into a beam before being inserted into the acceleration means.
26. An apparatus as claimed in claim 25 wherein the cooling electrode arrangement is an electrode arrangement of the quadrupole type.
27. An apparatus as claimed in claim 25 or claim 26 wherein the cooling arrangement is optimised to provide a fast transit time for ions from the sample to the acceleration means.
28. An apparatus as claimed in any one of claims 22 to 27 wherein the ion gun is an ion gun adapted to fire C60 at a sample.
29. An apparatus as claimed in any one of claims 22 to 28 used to perform the method of any one of claims 1 to 21.
30. A method of mass spectrometry substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.
31. A mass spectrometer apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.