EP0919067A2

EP0919067A2 - Charged particle velocity analyser

Info

Publication number: EP0919067A2
Application number: EP97936783A
Authority: EP
Inventors: Timothy Andrew 227 Avenue Road Extension STEELE; Ronald Charles Unwin; Adrian John Eccles
Original assignee: Millbrook Instruments Ltd
Current assignee: Millbrook Instruments Ltd
Priority date: 1996-08-17
Filing date: 1997-08-18
Publication date: 1999-06-02
Anticipated expiration: 2017-08-18
Also published as: GB9617312D0; DE69703624T2; WO1998008244A2; EP0919067B1; DE69703624D1; WO1998008244A3

Abstract

An ion velocity analyser 10 comprises a source (12) of ions, which source is in a vacuum. The ions are focused into a beam by a series of electrostatic lenses in the focus section (14) of the ion velocity analyser (10). The beam is modulated by a deflector (16) to give a pseudo-random binary signal having pulses of variable length. The modulator beam travels down in an evacuated drift tube (18) having a chosen electrostatic potential to a detector (20). The signal from the detector (20) is amplified and the signal intensity feature of an array of time channels is measured and recorded by a multichannel analyser (22); the resultant signal is a convolution of the pseudo-random binary signal and the time of flight spectrum of the ion sample. The signal from the detector is cross-correlated with the modulated signal and deconvoluted to give a time of flight spectrum for the ion sample. The time of flight spectrum is then inverted to give a mass spectrum.

Description

CHARGED PARTICLE VELOCITY ANALYSER

This invention relates to a charged particle velocity analyser and to a method of analysing the velocity of charged particles.

The velocity of a charged particle is determined by its mass and its energy. For a mono-energetic beam of particles the results of the velocity analysis can be inverted to give a mass spectrum. Mass spectroscopy is a widely used analysis technique. For a beam of particles of one mass eg. electrons, the results of the velocity analysis can be used to determine the energy spectrum. Electron energy spectroscopy is a widely used technique.

One type of mass analysis is known as time-of-flight (ToF) analysis. Other methods of analysis are quadrupole analysis and magnetic sector analysis.

Quadrupole analysis has the disadvantage that very precise mechanical and electronic components are required. A particular disadvantage is that particle transmission in the apparatus decreases with increasing mass.

Magnetic sector mass analysis apparatus requires magnets which are often large and heavy. The size and weight of the magnets detracts from the utility of this type of apparatus.

Quadrupole and magnetic sector instruments have the further disadvantage of processing information on a particular mass sequentially, rather than all masses simultaneously, which is very time consuming. Previous time-of-flight instruments have the benefit of being mechanically simple but have the disadvantage of having a low duty cycle. The duty cycle is the proportion of time of operation of the instrument during which data is recorded. A typical duty cycle for previous time-of-flight instruments is less than 0.1 % . The undesirable result is that long overall collection times are necessary and a large proportion of particles from a continuous source are wasted.

Two previous types of time-of-flight instruments have used a frequency modulation of the ion beam. In a first of these instruments many frequencies were encoded in one sequence. The data from the detector were deconvoluted by gating the detector. Gating selects a particular time-of-flight from the many times-of-flight present in the detected signal. Consequently, much of the signal is not used and is wasted. Also, the procedure must be repeated for each time of flight, which may take an undesirably long time.

A second of the previous time-of-flight instruments modulated the signal with a sine wave. The frequency was changed in a stepwise fashion and the detector was gated for a particular frequency, as with the last mentioned instrument. This second time-of-flight instrument also had the disadvantage of processing a single time-of-flight at a time.

It is an object of the present invention to address die disadvantages mentioned above.

According to one aspect of the present invention a time-of-flight velocity analyser comprises modulation means for imposing a substantially random binary sequence on a sample of charged particles, a detector for detecting the arrival of said charged particles thereat and producing an output signal, and transformation means for fransforming the signal from the detector with the aid of the substantially random binary sequence to provide time-of- flight values for a spectrum of velocities.

The substantially random binary sequence may be a pseudo random binary sequence or a random binary sequence.

The transformation means may be deconvoluting means for deconvoluting the detector signal or may be cross-correlation means.

The cross-correlation means may be arranged to provide time-of-flight values for a spectrum of velocities substantially simultaneously.

The charged particles may be ions. The charged particles may be electrons.

The velocity analyser may be a mass spectrometer. The mass spectrometer may be a secondary ion mass spectrometer.

The velocity analyser may be an electron energy analyser.

In the case where the charged particles are ions, the mass spectrometer may include a drift tube having a drift potential in the range 5eV to 800eV. Preferably, the drift potential in the drift tube is in the range 20eV to 200eV. The mass spectrometer may comprise a drift tube for which the time of flight values are in the range O. lμs to lOOOμs. Preferably, the time of flight values are in the range lμs to 500/xs.

The velocity analyser may comprise a drift tube for which the velocity of the particles is in the range 100 m/s to 1,000,000 m/s. Preferably the velocity of the ions is in the range 1,000 m/s to 500,000 m/s.

The sample of charged particles may comprise a beam of charged particles, which may be produced by collimation and focusing of the charged particles. The charged particles may be focused on the detector. The charged particles may be focused by one or more lenses. The lenses may comprise at least one reflecting lens. The lenses may comprise at least one electrostatic lens.

The modulation means may comprise means for deflecting said sample of charged particles from the detector. The sample beam direction may be changed by electro-static means, which electro-static means preferably comprise a parallel plate deflector, but may comprise a single plate deflector or an electro-static lens or a grid of fine wires. The sample may alternatively be deflected by magnetic means.

The modulation means may comprise an array of charge carrying members, which may be wires. The charge carrying members may be substantially parallel to each other. The modulation means may be short in the direction of travel of the sample so as not to broaden the pulse length imparted to die beam. The charge carrying members may, in use, have a polarity of charge with respect to the charged particles which is arranged to result in substantially no electric field from the charge carrying members outside the modulation means. The array may comprise a plurality of layers of charge carrying members, which may be transversely offset relative to one another, to thereby form a lens.

The sample may comprise a pulsed beam of charged particles. The length of each pulse may be determined by the substantially random binary sequence. The length of each pulse may be a multiple of a time unit of the binary sequence. The length of each pulse may be determined by a trigger from a timing means.

The timing means may provide a trigger to a sequence generator, which sequence generator may provide the substantially random binary sequence.

The timing means may be arranged to pulse with a period of between O.Olμs and 30μs. Preferably the timing means is arranged to pulse with a period of between 0.1 μs and 3μs.

The sequence generator may be arranged to generate, in use, a sequence having M digits, where M is an integer greater than 3. Preferably, the sequence will be a binary sequence having 2^N digits, where N may be an integer. Preferably, the sequence generator is arranged to generate, in use, a binary sequence of one or more of 128 digits, 256 digits, 512 digits, 1,024 digits, 2,048 digits and/or 4,096 digits. Most preferably, the sequence generator is arranged, in use, to generate a sequence of 1 ,024 digits.

The velocity analyser may include a transient recorder for recording the signal produced by die detector. The detector may be arranged to pass a signal to the transient recorder on direct detection of the sample. Preferably the detector may comprise a Faraday cup.

Alternatively the detector may be arranged to pass a signal to d e transient recorder on detecting a secondary cascade of charged particles caused by the sample. Suitably, the detector may comprise a channel electron multiplier or an electron multiplier array /multi-channel plate.

The transient recorder may record the signal from the detector in a plurality of time channels. The size of the time channels may be determined by the timing means. Each time channel may be defined by die time between successive digits of the substantially random binary sequence. Alternatively, the time channel may be defined by odier means. The transient recorder may be arranged to add signals from the detector resulting from a repeat of the substantially random binary sequence to the values recorded in previous cycles of the substantially random binary sequence. The transient recorder may be arranged to integrate the signals recorded in each channel for a plurality of repeats of the substantially random binary sequence. The transient recorder may comprise a digital signal processing (DSP) chip. Said DSP chip may be operatively linked to a computer. The transient recorder may comprise a peripheral component interconnect (PCI) card, which may be operatively linked to a computer.

The cross-correlation means may be arranged to use a mathematical function to separate the substantially random binary sequence from a time-of- flight spectrum of the sample of charged particles. Said mathematical function may be performed by computing means, which computing means may be a personal computer. The mathematical function may comprise involving a transformation of a signal from the frequency domain to time domain. Said transformation may be a Fourier transformation. Said transformation may be a fast Fourier transformation. Said transformation may be a Hadamard transformation.

According to another aspect of the present invention a memod of analysing the velocity of a sample of charged particles comprises modulating me sample with a substantially random binary sequence, detecting the arrival of the modulated sample of charged particles at a detector and cross- correlating a signal produced by the detector widi the substantially random binary sequence to provide time-of-flight values for a spectrum of velocities.

The method may include cross-correlating the signal produced by the detector with the substantially random binary sequence to provide time-of- flight values for a spectrum of velocities substantially simultaneously.

The method may include the charged particles beam being accelerated along a drift tube, which acceleration may be caused by a drift potential in the range 5eV to 800eV. Preferably the charged particle sample is accelerated with a potential of 20eV to 200eV.

The sample of charged particles may be produced by secondary ionisations.

All of the above aspects may be combined with any of the features disclosed herein in any combination. Specific embodiments of the present mvention will now be described widi reference to the accompanying drawing, in which:

Figure 1 is a functional diagram of a charged particle velocity analyser;

Figure 2 is a schematic side view of a first embodiment of a drift tube of the charged particle velocity analyser;

Figure 3 is a schematic side view of a second embodiment of a drift tube of the charged particle velocity analyser;

Figure 4a is a partial schematic side view of an embodiment of modulation means;

Figure 4b is a partial schematic front view of the embodiment of modulation means; and

Figure 4c is a partial schematic top view of the embodiment of modulation means.

An ion velocity analyser 10 comprises a source 12 of ions, which source is in a vacuum. The ions are focused into a beam by a series of electro-static lenses in the focus section 14 of die ion velocity analyser 10. The beam is modulated by a deflector 16 to give a pseudo-random binary signal having pulses of variable length. The modulated beam travels down an evacuated drift tube 18 having a chosen electro-static potential to a detector 20. The signal from the detector 20 is amplified and the signal intensity for each of an array of time channels is measured and recorded by a multi- channel analyser 22. The resultant signal is a convolution of the pseudorandom binary signal and me time-of-flight spectrum of the ion sample. The signal from the detector 20 is cross-correlated with the modulated signal and de-convoluted to give a time-of-flight spectrum for the ion sample. The time- of-flight spectrum is then inverted to give a mass spectrum.

The operation and construction of the ion velocity analyser 10 will now be described in greater detail.

The ions in the source 12 are created by a high energy ion beam directed onto the surface of a sample for analysis. The high energy ion beam ionises the surface of the sample. The resulting ions are used in the analysis. Alternative methods may be used to create a source of ions, including electron impact ionisation for gases (in which the velocity analyser would be a residual gas analyser giving mass spectra of die vapour phase particles); plasma ionisation for gases, liquids and solutions; high energy neutral beam ionisation for solids; glow discharge ionisation for solids; laser irradiation for solids; radioactive ionisation; secondary ionisation from ion beams; and other related methods. The ions from the sample are then collimated and focused to form a beam by a series of electro-static lenses in the focus section 14. In this example the energy spread of die ions in the beam is controlled to be a small fraction of the drift tube potential by using an energy filter.

Next, the beam of ions passes to the deflector 16 which can take a number of various forms, such as a parallel plate deflector, a single plate deflector, an electro-static lens, a series of electro-static fields formed by a grid of fine wires or a deflector using magnetic fields. The ions are deflected from being focused towards the detector 20. The deflector 16 is controlled to create a beam which is given a pseudo-random binary modulation. A clock 24 is used to set the width of the time channels to between 0. lμs and 3μs, the clock pulses with tins period. A pseudo-random digital sequence is provided by a sequence generator 26 on being triggered by the clock 24. The digital sequence is combined with the pulsed signal provided by the clock 24 to give a signal having a known sequence of ON/OFF pulses of known duration. This signal is amplified by an amplifier 28 and is used to control the deflector 16 by varying the voltage thereof according to die signal. The beam is deflected so diat it misses the detector 20 or is left un-deflected so that the beam hits the detector 20. The beam of ions dierefore comprises pulses of ions of varying lengths. The lengths of die pulses will be multiples of me channel width.

The number of ON/OFF digits is chosen to be 2^N, where N is an integer, so that die de-convolution can be performed efficiently. The cross- correlation function used in diis example is a fast Fourier transform which can be performed considerably more quickly if the number of data points is equal to 2^N, where N is an integer. In this example the number of cycles is 1 ,024 but others could be used such as 128, 256, 512, 2,048 or 4,096.

Furώer focusing of the beam is then carried out after it has passed dirough the deflector 16, however this may not be necessary for different sources of ions. The further focusing may be carried out in a number of ways but two examples are given in figures 2 and 3.

In figure 2 three spaced lenses 36 are located in die drift tube 18. The lenses may be electrostatic and may be controlled separately to focus the beam of ions. Figure 3 shows a second example of further focusing. In this example the drift tube 18 comprises first and second sections 38, 40 which form an angle. A lens 42 is located at die entrance to the flight tube 18. At me apex of die angle between the first and second sections 38, 40 of the flight tube 18 diere is a reflecting lens 44. Ions travelling down die first section 38 enter the reflecting lens 44 and are focused and reflected down die second section 40 to a further lens 46 and on to die detector 20. This example has the benefit of reducing flight time broadening due to energy spread.

The pulse beam then passes into the drift tube 18, which has a drift potential in die range 20 to 200eV. The drift potential determines the flight time of the ion when combined with the tube lengm and die ion mass. The mass is proportional to the square of flight time. In this embodiment die drift tube 18 is approximately 50 cm long.

Un-deflected ions will arrive on the detector 20 and dius form an electric current flow in the detector 20. The detector output signal is passed to an amplifier 30 which, typically, provides an output voltage of IV for a detected drift tube current of InA. The amplification may be varied by a factor of 1,000 though. Various detectors can be used wid this ion velocity analyser, in this example a Faraday cup is a used. Alternatively, an electron multiplier or electron multiplier array could be used to pre-amplify the ion current before the amplifier 30 by secondary cascade in the vacuum from lpA to lμA or InA respectively. This current is then passed to the amplifier 30 which provides an output voltage of lOmV for a detected cascade current of InA. The signal is then passed to an analogue to digital converter 32 so mat it can be analysed in me multi-channel analyser 22, which requires a digital input. The multi-channel analyser 22 in this example is a digital signal processing chip but this could be replaced by various equivalents such as a PCI card.

The multi-channel analyser 22 receives the pulses from the clock 24, which pulses are also received by the sequence generator 26, as mentioned above. For each pulse, which comprises one channel of die 1 ,024 channels, the intensity of the signal from the amplifier 30 is recorded by die multichannel analyser 22. Consequently, after one cycle of the pseudo-random signal 1,024 values of the detector current will be stored.

The cycle of 1 ,024 pulses is repeated and die intensity of the signal is added to die previously stored signal for a paπicular channel. The cycle of

1 ,024 pulses is repeated typically 256 times. A cumulative signal is thus built up as the 1 ,024 pulse cycle is repeated. The cycle is repeated until die signal to noise ratio is sufficiently high for the system.

The cumulative data for die 1 ,024 channels are then cross-correlated widi die output signal from the sequence generator 26 by a computer 34. The correlation function used in is example is a fast Fourier transform. A Hadamard transform may provide an alternative correlation function. The result is me intensity of the signal corresponding to each flight time divided into the clock timing units. This signal is then processed in the computer 34 to convert clock pulse units into time and convert the time-of-flight into its corresponding mass. As mentioned above, a time-of-flight corresponds to a particular mass for a given drift potential and length of drift tube 18. If a particular mass range is of interest then me correlation may be carried out on only a subset of the 1,024 channels. This will reduce die amount of time taken to perform the cross-correlation.

Several transformed results, up to 100 or more, may be added together to produce a spectrum which shows low intensity peaks more easily than a single transformed result.

The modulation of the beam may be a frequency modulation or an amplitude modulation.

An alternative modulation means 50 is shown in figures 4a, b and c. The alternative modulation means 50 comprises a diree layer array of wires 52. The wires are arranged transverse to the direction of particle travel, in diis example in a vertical orientation. All of die wires are substantially parallel giving an increase in transmission over a grid of wires. Each layer 54 of the array 52 comprises wires with a spacing (x) of 0.2- 1.5mm. The second layer 54b is offset transversely relative to the first layer 54a by a distance of approximately half the wire spacing (x). The distance between adjacent layers is 0.15-lmm. The wires of die third layer 54c are substantially aligned in me direction of particle travel with the wires of the first layer 54a. The wires of me array have the drift tube potential, to cause deflection die second layer 54b of die array has a potential difference to die drift potential.

A further alternative modulation means is to have a single layer of parallel wires in which the adjacent wires have opposite polarity but equal potential difference from drift tube potential to cause deflection. The alternative modulation 50 means functions in the same manner as described above. Additional benefits, however, include the feature diat the array of wires 52 functions as a micro lens and is uniform in effect over several mm² area of the beam. Also, the use of an array means that overall lengtii of die modulation means can be shortened giving shortened transit times and pulse periods. The required voltage of such an array is also lower from that of a corresponding square grid, making it easier to manufacture and operate.

An alternative to the detector described above is to use multiple synchronous detectors. This is achieved by using a sequence generator to modulate me signal, which signal then arrives at the detector at various times depending on die velocity of the ions. A delay line is used to delay die modulation for a chosen flight time to be measured. The signal at die detector is multiplied by the modulated and delayed output of the delay line. The multiplication results in the signal being multiplied by 1 when the modulation is 1 and dien by -1 when the modulation is zero. The average value of d e output from the multiplication is me intensity of the signal at die delay time (time of flight of choice) determined by die delay line.

The delay line is arranged to have several taps each defining a different flight time. Each tap has its own multiplier and averager. Any number of taps can be used so diat, in essence, the number of "tap, multiplier, averager" sets is me number of detector channels. The delay line can be a shift register and die multipliers can be simple +/- devices, die averager can be as simple as a resistor/capacitor pair (low pass filter). The multiple synchronous detector diat works on all arrival time channels is simply carrying out a transformation by hardware ra ier than in a computer. The ion velocity analyser disclosed herein provides significant advantages over prior art devices. The function of analysing a complete time- of-flight signal simultaneously, rather than one particular time-of-flight at a time, provides significant benefits in terms of the speed of analysis of a particular sample. Using a relatively low drift potential of 20 to 200eV compared to me more usual 3,000 to 10,000eV enables a variety of components to be used which, although decreasing the accuracy of the ion velocity analyser, result in considerable cost savings in the manufacture and maintenance of die device, as well as a more robust device, over prior art devices. For instance, a Faraday cup detector is cheaper than and has a considerably longer life span than die usual multiplier or multi-channel plate. The use of longer channel times and me resulting reduction of me amount of information enables the use of a PC to process die signal and provide a mass spectrum in a matter of seconds.

The ion velocity described herein offers a higher duty cycle dian a previous time-of-flight analyser. Additionally, inexpensive components can be used.

Previous instruments have deconvoluted die signal by gating before die detector. The advantage of deconvoluting me signal after the detector, using a computer (as is the case here), is that a much higher duty cycle can be used togedier with parallel collection of flight times.

The above advantages may be exploited either by shorter collection time for a particular spectrum, collection of very weak spectra or collection of repetitive spectra with a high repetition rate. The reader's attention is directed to all papers and documents which are filed concurrently widi or previous to diis specification in connection with this application and which are open to public inspection with this specification, and die contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any mediod or process so disclosed, may be combined in any combination, except combinations where at least some of such feamres and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving die same, equivalent or similar purpose, unless expressly stated odierwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to die details of die foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of me features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of me steps of any mediod or process so disclosed.

Claims

1. A time-of-flight velocity analyser comprises modulation means for imposing a substantially random binary sequence on a sample of charged particles, a detector for detecting the arrival of said charged particles diereat and producing an output signal, and transformation means for transforming the signal from die detector widi the aid of die substantially random binary sequence to provide time-of-flight values for a spectrum of velocities.

2. A time-of-flight velocity analyser as claimed in claim 1, in which the substantially random binary sequence is a pseudo random binary sequence.

3. A time-of-flight velocity analyser according to eidier claim 1 or claim

2, in which the transformation means are cross-correlation means.

4. A time-of-flight velocity analyser as claimed in claim 3, in which the cross-correlation means are arranged to provide time-of-flight values for a spectrum of velocities substantially simultaneously.

5. A time-of-flight velocity analyser as claimed in any preceding claim, which is a mass spectrometer.

6. A time-of-flight velocity analyser as claimed in claim 5, in which the mass spectrometer is a secondary ion mass spectrometer.

7. A time-of-flight velocity analyser according to any one of claims 1 to 4, which is an electron energy analyser.

8. A time-of-flight velocity analyser as claimed in either claim 5 or claim 6, which includes a drift tube having a potential in the range 5eV to 800eV.

9. A time-of-flight velocity analyser according to eidier claims 5 or claim 6, which includes a drift mbe having a drift potential in die range 20eV to

200eV.

10. A time-of-flight velocity analyser as claimed in any preceding claim, in which the charged particles are focused by one or more lenses.

11. A time-of-flight velocity analyser as claimed in any preceding claim, in which the modulation means comprises means for deflecting fhe sample of charged particles from the detector.

12. A time-of-flight velocity analyser according to claim 11 , in which die modulation means comprises an array of charge carrying members.

13. A time-of-flight velocity analyser according to claim 12, in which the charge carrying members are substantially parallel to each odier.

14. A time-of-flight velocity analyser according to eitiher claim 12 or claim 13, in which die charge carrying members, in use, have a polarity of charge with respect to die charged particles which is arranged to result in substantially no electric field from the charge carrying members outside die modulation means.

15. A time-of-flight velocity analyser according to one of claims 12 to 14, which comprises a plurality of layers of charge carrying members, which are transversely offset relative to one anomer.

16. A time-of-flight velocity analyser according to any one of die preceding claims, in which the sample comprises a pulsed beam of charged particles.

17. A time-of-flight velocity analyser according to claim 16, in which me lengm of each pulse is determined by die substantially random binary sequence.

18. A time-of-flight velocity analyser according to any one of die preceding claims, which includes a transient recorder for recording die signal produced from the detector.

19. A time-of-flight velocity analyser according to claim 18, in which the detector is arranged to pass a signal to die transient recorder on direct detection of die sample.

20. A time-of flight velocity analyser according to claim 18 or claim 19, in which die detector comprises a Faraday cup.

21. A time-of-flight velocity analyser according to claim 18, in which me detector is arranged to pass a signal to d e transient recorder on detecting a secondary cascade of charged particles caused by die sample.

22. A time-of-flight velocity analyser according to claim 21 , in which e detector comprises a channel electron multiplier or an electron multiplier array.

23. A time-of-flight velocity analyser according to any one of claims 18 to 22, in which the transient recorder records a signal from the detector in a plurality of time channels.

24. A time-of-flight velocity analyser according to any one of claims 18 to 23, in which the transient recorder is arranged to add signals from the detector resulting from a repeat of die substantially random binary sequence to die values recorded in previous cycles of die substantially random binary sequence.

25. A time-of-flight velocity analyser according to any one of claims 3 to 24, in which the cross correlation means are arranged to use a mathematical function to separate die substantially random binary sequence from a time-of- flight spectrum of the sample of charged particles.

26. A method of analysing me velocity of a sample of charged particles comprises modulating me sample with a substantially random binary sequence, detecting die arrival of die modulated sample of charged particles at a detector and cross-correlating a signal produced by die detector widi die substantially random binary sequence to provide time-of-flight values for a spectrum of velocities.

27. A mediod of analysing die velocity of a sample of charged particles according to claim 26, which includes cross-correlating the signal produced by the detector widi the substantially binary sequence to provide time-of-flight values for a spectrum of velocities substantially simultaneously.

28. A mediod of analysing me velocity of a sample of charged particles according to claim 26 or claim 27, which includes die charged particle beam being accelerated along a drift mbe, by a drift potential in the range 5eV to 800eV.

29. A time-of-flight velocity analyser substantially as described herein widi reference to die accompanying drawings.

30. A mediod of analysing the velocity of a sample of charge particles substantially as described herein with reference to the accompanying drawings.