Mass spectrometry systems

Abstract: A mass spectrometry system includes a double-focusing magnetic sector mass spectrometer (7) and a time-of-flight spectrometer (9) arranged in parallel. The spectrometers share a common means (3) for exciting ions from a sample, and a common transfer optics system (5). An interleaved control system (11) for the two spectrometers (7,9), is arranged also to control a sampling handling arrangement (2) dependent on the output of the time-of-flight spectrometer (9), so as to enable the double-focusing magnetic sector mass spectrometer (7) to analyse a region of interest on a sample.

Description: This invention relates to mass spectrometry systems. In recent years there has arisen a need to analyse small sample volumes. This need arises, for example, where there is only a limited quantity of a unique sample, the sample being distributed in a particulate or other form in a matrix. Thus modern materials analysis problems require a spatial resolution over the sample region of 1 micron or less, together with the ability to find the specific locations of the sample in the matrix prior to analysis. Most known magnetic sector or quadrupole mass spectrometry systems do not include means for lateral imaging of the sample surface combined with precise means of sample manipulation controlled through the image. Those that do, consume significant amounts of sample material in the process thus destroying the areas from which detailed information is sought. GB2,217,907A discloses an instrument comprising a direct imaging type SIMS instrument having a TOF spectrometer mode which is used with certain special substances. It is an object of the present invention to provide a mass spectrometry system which is capable of locating a sample within a matrix but which also makes an efficient use of the ions emitted by the sample. According to the invention there is provided a mass spectrometry system for analysis of a sample, comprising a source for deriving ions from the sample, a magnetic sector mass spectrometer, a time-of-flight mass spectrometer, a transfer optics system and a control system for controlling the magnetic sector mass spectrometer, the time-of-flight mass spectrometer and the transfer optics system, the transfer optics system being effective to collect ions from the sample and to direct and match collected ions selectively either into the magnetic sector mass spectrometer or into the time-of-flight mass spectrometer under the control of the control system characterised in that the control system enables the time-of-flight mass spectrometer to analyse ions directed thereto by the transfer optics system to identify a mass range of interest and enables the magnetic sector mass spectrometer to perform a more detailed analysis of ions having masses in said mass range of interest identified by the time-of-flight mass spectrometer. One embodiment of a mass spectrometry system in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawing which is a schematic diagram of a mass spectrometry system in accordance with the embodiment of the invention. Referring to the Figure, the mass spectrometry system in accordance with the present invention, to be described comprises a sample holder 1, whose position may be adjusted by use of a sample handling arrangement 2. A source 3 is arranged so as to be capable of ablating and ionising atoms or molecules from a sample mounted on the sample holder. A transfer optics system 5 is arranged to focus ions from the sample, either into a double-focusing magnetic sector mass spectrometer 7, hereinafter referred to as a DFMS, or into a time-of-flight mass spectrometer 9, hereinafter referred to as a TOF. The DFMS 7 and TOF 9 are both operated under control of an interleaved control system 11, which is also arranged to control the sample handling system 2 and the operation of the transfer optics 5 so as to direct ions either towards the DFMS 7 or the TOF 9. The source 3 may take any suitable form, and in particular may comprise a combination of different primary probes dependent on the likely applications of the system. Thus the system may be configured as a secondary ionization mass spectrometer (SIMS) system, the source 3 being arranged to produce pulsed and/or continuous primary ion beams to stimulate ion emission from the sample carried by the sample holder 1. The source 3 may alternatively or additionally include a fine laser probe or a micro-focus pulsed primary ion probe. The transfer optics system 5 is capable of matching a two or three dimensional source having a large energy spread into the TOF 9 or DFMS 7. The transfer optics system 5 is arranged to combine a high efficiency screened extraction field with optical gating and dynamic emittance matching. This will ensure that the DFMS 7 can be operated so as to have high mass resolution from a small sample area, typically less than 10 microns, without significant transmission losses of the ions through the system. The transfer optics system 5 ensures that the field of view of the DFMS can be scanned across a larger sample area in synchronisation with the scanning action of the source 3 over the sample area. The lens design of the transfer optics system 5 will take account of the need to prevent excessive temporal pulse spreading so as to maintain high resolution in the TOF 9. In practice this will necessitate, at least temporarily, operating the ion optical elements in what is known as the "accelerating mode". In some circumstances, however, the ion optical elements will be operated in a retarding mode. It will be appreciated that the screened extraction field of the transfer optics system 5 will enable the fields on insulating samples to be reduced, thus improving the performance possible for the analysis of insulating samples. By floating a primary ion probe 3 at the screen or sample potentials, it will also be possible to cause low energy primary ions to impinge within the spectrometers' fields of view. It will be seen that by having the TOF 9 and the DFMS 7 in parallel, the TOF can be used to locate the areas of interest on a sample before using the DFMS 7 for a variety of localised analysis functions. Thus the sample may be initially scanned with a micro-focused pulsed primary ion probe, such imaging requiring the consumption of extremely small quantities of the sample but yielding spatial information across a wide mass range. As the TOF 9 possesses nearly absolute mass calibration it can also be used to determine the mass range of interest present in the sample. The source 3 may then be switched to a continuous excitation mode, the control system 11 causing the transfer optics system 5 to switch the ion beam emitted from the system 5 from the TOF 9 to the DFMS 7. The DFMS 7 is then used to perform a more detailed analysis of the sample with a high mass resolution over a limited mass range as determined by the use of the TOF 9. The control system 11 may be arranged to maintain a record of the variation in intensity of the signals measured by the spectrometers 7 and/or 9 as a function of the position on the sample from which the ions have originated, together with an indication of the accumulated flux. An alternative mode of operation for the parallel combination of the TOF 9 and the DFMS 7 is to switch rapidly between the TOF and the DFMS. This mode of operation will, for example, enable low dose imaging of buried features to be alternated with high sensitivity depth profiling. Although the DFMS is shown located on the axis of the transfer optics system 5 in the spectrometry system shown in Figure 1 , it will be appreciated that many other configurations are possible. If however, both spectrometers are located off axis, further deflection means will be necessary to direct the ions from the transfer optics system 5 to the two spectrometers 7,9. The output of the TOF 9 will be used by the control system 11 to control the sampling handling system 2 to bring the desired sample area into the field of view of the transfer optics system 5. By linking the sample handling arrangement 2 to the control system 11, regions of interest in the sample may be selected in real time for detailed analysis. Typically, the sampling handling arrangement must be capable of orthogonal movements of up to 25 mm amplitude, with a precision of up to 1 micron. It will be appreciated that the parallel system described herebefore finds particular application in the semiconductor industry, for example in the analysis of fully and part processed semiconductor wafers and semiconductor materials. It will be appreciated that in some applications it may be advantageous to combine a series arrangement comprising a magnetic sector mass spectrometer and a time-of-flight spectrometer, together with an appropriate transfer optics system and control system, with a parallel arrangement of spectrometers in accordance with the invention in a single spectrometry system. It will also be appreciated that whilst the magnetic sector mass spectrometer described by way of example is an energy focussing spectrometer, in particular a double-focussing magnetic sector mass spectrometer, other forms of magnetic sector mass spectrometers may be used in a system in accordance with the invention. It will be appreciated that the means for generating ions from the sample may take any suitable form. These include ion beams, fast atom beams, and electron beams all operated in either a pulsed or continuous mode, and a pulsed laser. It will also be appreciated that the transfer optics system may take any suitable form. Whilst a quadrupole system may be appropriate in some circumstances, other electrode structures which lack cylindrical symmetry may also be used. It will also be appreciated that a mass spectrometry system in accordance with the invention may also incorporate other components if necessary. In particular, such a parallel spectrometer system may include a buncher which uses electrodynamic bunching, or other means, to produce short pulses of ions suitable for the time-of-flight spectrometer, the magnetic sector mass spectrometer using a continuous ion beam. Furthermore, an electron beam may be used for the purpose of charge neutralisation at the sample surface.

Claims: A mass spectrometry system for analysis of a sample, comprising a source (3) for deriving ions from the sample, a magnetic sector mass spectrometer (7), a time-of-flight mass spectrometer (9), a transfer optics system (5) and a control system (11) for controlling the magnetic sector mass spectrometer (7), the time-of-flight mass spectrometer (9) and the transfer optics system (5), the transfer optics system (5) being effective to collect ions from the sample and to direct and match collected ions selectively either into the magnetic sector mass spectrometer (7) or into the time-of-flight mass spectrometer (9) under the control of the control system (11) characterised in that the control system (11) enables the time-of-flight mass spectrometer (9) to analyse ions directed thereto by the transfer optics system (5) to identify a mass range of interest and enables the magnetic sector mass spectrometer (7) to perform a more detailed analysis of ions having masses in said mass range of interest identified by the time-of-flight mass spectrometer (9). A mass spectrometry system according to claim 1 in which the control system (11) is capable of measuring the signal from either spectrometer (7,9) in such a way as to maintain a record of the variation in intensity of the signals as a function of the position on the sample from which the ions have originated, and the accumulated flux. A mass spectrometry system according to either one of the preceding claims in which the control system (11) is capable of selecting the area of the sample to be analysed. A mass spectrometry system according to any one of the preceding claims in which the magnetic sector mass spectrometer (7) is an energy focussing magnetic sector mass spectrometer. A mass spectrometry system according to any one of the preceding claims in which said source (3) comprises a combination of different probes. A mass spectrometry system according to claim 5 in which said source (3) comprises a pulsed source for use with the time-of-flight mass spectrometer (9) and a continuous source for use with the magnetic sector mass spectrometer (7). A mass spectrometry system according to any one of the preceding claims in which said source (3) is effective to scan a beam of particles across the sample to excite ions from different regions of the sample, and the transfer optics system is arranged to scan the field of view of the spectrometers (7,9) in synchronisation with the scanning action of the source (3). A mass spectrometry system according to any one of the preceding claims wherein the time-of-flight spectrometer (9) is used to locate areas of interest on the sample prior to using the magnetic sector mass spectrometer (7) for localised analysis functions. A mass spectrometry system according to any one of the preceding claims in which the fields of view of the spectrometers are limited by the transfer optics system (5)

Inventors: DERRICK PETER JOHN, DOWSETT MARK G, WALDRON JOHN DESMOND, DERRICK, PETER JOHN, DOWSETT, MARK G, WALDRON, JOHN DESMOND
Applicants: KRATOS ANALYTICAL LTD, KRATOS ANALYTICAL LIMITED
Application year: 1991
Main classification: H01J49
IPC codes: H01J49, G01R33, G01Q20