A method of time-of-flight mass spectrometry adapted for the analysis of ions up to a required mass limit comprises the following sequences of events: PA1 (a) producing, during a first time interval, a pulse of charged particles, PA1 (b) directing said charged particles towards the entrance of a mass analyzer; PA1 (c) recording the times-of-flight of said charged particles after they pass through said mass analyzer; PA1 (d) closing a gating means, which is disposed in the path of said charged particles between said source and said mass analyzer, after a second time interval which, measured from the start of said first time interval, is sufficient for substantially all of said charged particles having mass less than or substantially equal to said mass limit to travel from said source to and through said gating means; PA1 (e) keeping said gating means closed until the end of a third time interval which, measured from the start of said first time interval, is at least as long as the time taken for substantially the most massive of said charged particles to travel from said source to said gating means, and opening said gating means at substantially the end of said third time interval; PA1 (f) repeating the procedure above, by producing another pulse after a fourth time interval measured from the start of said first time interval.

This invention relates to a method and apparatus for time-of-flight mass 
spectrometry, particularly though not exclusively adapted for use in 
secondary ion mass spectrometry to analyze the composition of surfaces. 
In a time-of-flight mass spectrometer a mass spectrum is obtained by 
arranging that the time taken for each ion to travel a flight path depends 
upon its mass. Ions of equal kinetic energy travelling through a 
field-free region naturally disperse according to the square-root of their 
masses, though in practice it is desirable to compensate for an initial 
variation in kinetic energy. This variation may be overcome to an extent 
by applying a linear electric field which accelerates the ions according 
to their ratio of mass to charge, then the time of flight of each species 
of ion is a function of not only the the initial kinetic energy but also 
that imparted by the accelerating force. Time-of-flight mass spectrometers 
employing this technique have been described, for example by W. C. Wiley 
and I. H. McLaren in The Review of Scientific Instruments, volume 15(12), 
pp 1150-1157, 1955, and by B. T. Chait and K. G. Standing in The 
International Journal of Mass Spectromery and Ion Physics, volume 40, pp 
185-193, 1981. 
An improved design of time-of-flight mass spectrometer was described by W. 
P. Poschenreider in The International Journal of Mass Spectrometry and Ion 
Physics, volume 9, pp 357-373, 1972. This type of analyzer is known as 
`energy-focussing` because, by the application of a toroidal electrostatic 
field, ions of equal mass to charge ratio travel equal flight times, those 
of higher energy travelling longer distances in the electrostatic field 
than those of lower energy. An alternative form of mass analyzer achieving 
`momentum-focussing`, by the application of a magnetic sector field, has 
also been described by W. P. Poschenrieder in The International Journal of 
Mass Spectrometry and Ion Physics, volume 6, pp 413-426, 1971. 
A further design of energy-focussing, time-of-flight, mass spectrometer has 
been described by B. A. Mamyrin V. A. Karataev and D. M. Shmikk in British 
Patent Specification No. 1474149 and in U.S. Pat. No. 4,072,862, and by B. 
A. Mamyrin and D. M. Shmikk in Soviet Physics, JETP, volume 49(5), 1979, 
pages 762 to 765. In that instrument, which is known as the linear mass 
reflectron, the ions traverse a linear region and compensation for 
differing energies is achieved by reflecting the ions through 180.degree. 
in a system of electrostatic fields. 
In general, in time-of-flight mass spectrometry, regardless of the design 
of analyzer, the ions are provided for analysis in the form of a pulsed 
beam, each pulse containing the range of ion masses. The time of flight of 
each type of ion in a pulse is measured by electronic timing circuits from 
the time of creation of the pulse to the time of detection of the ion. 
Several methods of creating a pulsed beam of ions have been described, for 
example J. M. B. Bakker, in The Journal of Physics E, volume 7, 1974, pp 
364-368 and J. D. Pinkston et al, in The Review of Scientific Instruments, 
volume 57(4), 1986, pp 583-592, describe systems which chop a continuous 
beam by deflecting the beam across a slit at the entrance to the flight 
region. Alternatively the ion beam may be created in pulses by a pulsed 
ionization process, e.g. by the impact of a pulsed primary ion beam. 
One important application of time-of-flight analysis is in Secondary Ion 
Mass Spectrometry (SIMS), a technique developed for the analysis of the 
atomic and molecular composition of surfaces, in which a surface is 
bombarded by a beam of primary ions causing it to release characteristic 
secondary ions. The secondary ions are then collected and analysed using a 
time-of-flight or other form of mass analyzer, for example a 
magnetic-sector mass spectrometer. More generally, ions may be released 
from a surface by some other means, for example laser ionisation or 
electron impact and again a time-of-flight mass spectrometer may be used 
to identify the released ions and so analyse the composition of the 
surface. A review of analytical techniques using time-of-flight mass 
spectrometry has been published by Price et al in The International 
Journal of Mass Spectrometry and Ion Processes, volume 60, pp 61-81, 1984. 
Time-of-flight apparatus designed for SIMS has been described by A. R. 
Waugh et al in Microbeam Analysis, San Francisco Press Inc., pp 82-84, 
1986 and also by P. Steffens et al, in The Journal of Vacuum Science and 
Technology, volume 3(3), pp 1322-1325, 1985. Both these instruments 
comprise an energy-focussing analyzer of the type described by 
Poschenrieder in 1972. The pulsed beam of secondary ions is generated by 
applying a pulsed primary ion beam to the surface under analysis. However, 
a problem with time-of-flight SIMS instruments arises because whereas it 
would be advantageous to arrange that the pulse repetition rate 
corresponds to the flight-time of the most-massive ion of interest, ions 
of greater mass in each pulse must be allowed to clear the flight tube 
before the next pulse is admitted, otherwise consecutive pulses interfere. 
One solution to this problem would be to reject as many pulses as 
neccessary, after admitting one pulse, to allow the admitted pulse to 
fully pass through the analyzer. Methods of rejecting alternate pulses are 
described by Bakker and by Pinkston et al in the context of overcoming 
problems in shaping a chopped beam. But rejecting alternate pulses is not 
neccessary for pulse-shaping when the ions are created by pulsed 
ionization, and furthermore it is not a satisfactory solution for a SIMS 
instrument because rejecting half, or more, of the emitted secondary ions 
reduces the sensitivity of the instrument. 
It is the object, therefore, of this invention to provide a method of 
time-of-flight, mass spectrometry in which interference with the analysis 
by ions of mass greater than the highest mass of interest is substantially 
eliminated, without adversely affecting the sensitivity of the analysis. 
It is a further object of the invention to provide a time-of-flight, mass 
spectrometer in which interference with the analysis by ions of mass 
greater than the highest mass of interest is substantially eliminated, 
without adversely affecting the sensitivity of the spectrometer. 
Thus according to one aspect of the invention there is provided a method of 
time-of-flight mass spectrometry adapted for the analysis of ions up to a 
required mass limit comprising the following sequence of events: 
(a) producing from a source, during a first time interval, a pulse 
comprising charged particles which are distributed over a range of masses; 
(b) extracting said charged particles from said source and directing them 
towards the entrance of a mass analyzer; 
(c) recording the times-of-flight for those of said charged particles which 
reach a detector disposed in their path after they pass through said mass 
analyzer; 
(d) closing a gating means, which is disposed in the path of said charged 
particles between said source and said mass analyzer, after a second time 
interval which, measured from the start of said first time interval, is 
sufficient for substantially all of said charged particles, produced 
during said first time interval and having mass less than or substantially 
equal to said mass limit, to travel from said source to and through said 
gating means; 
(e) keeping said gating means closed until the end of a third time interval 
which, measured from the start of said first time interval, is at least as 
long as the time taken for substantially the most massive of said charged 
particles to travel from said source to and through said gating means, and 
opening said gating means at substantially the end of said third time 
interval; 
(f) repeating the procedure described in (a) to (e) above, by first 
producing another pulse after a fourth time interval measured from the 
start of said first time interval. 
In this way there is produced a sequence of pulses of charged particles, 
each created with pulse width equal to said first time interval, and the 
period of the sequence being equal to said fourth time interval. 
According to another aspect of the invention there is provided a 
time-of-flight mass spectrometer adapted for the analysis of charged 
particles up to a required mass limit comprising: 
(a) means for producing from a source, during a first time interval, a 
pulse comprising charged particles distributed over a range of masses; 
(b) a preliminary mass separating means, having a first entrance and an 
exit, said charged particles travelling between said first entrance and 
exit in a time, which for each of said charged particles, is dependent 
upon the mass of that charged particle; 
(c) a time-of-flight mass analyzer having a second entrance; 
(d) extraction means, disposed between said source and said preliminary 
mass separating means, which accelerates said charged particles from said 
source towards said first entrance of said preliminary mass separating 
means; 
(e) a gating means, disposed between said exit of said preliminary mass 
separating means and said second entrance of said time-of-flight mass 
analyzer; 
(f) means for controlling said gating means adapted to 
(i) close said gating means after a second time interval which, measured 
from the start of said first time interval, is sufficient for 
substantially all of said charged particles, produced during said first 
time interval and having mass less than or substantially equal to said 
mass limit, to travel from said source, through said preliminary mass 
separating means, to and through said gating means; and 
(ii) keep said gating means closed until the end of a third time interval, 
which measured from the start of said first time interval is at least as 
long as the time taken for substantially the most massive of said charged 
particles to travel from said source to said gating means, and to open 
said gating means at substantially the end of said third time interval; 
and 
(g) means for producing a plurality of said pulses successively, the time 
between the start of one pulse and the start of the next pulse being equal 
to a fourth time interval. 
In a preferred embodiment of the invention the preliminary mass separating 
means comprises a drift region, substantially free of electrostatic 
fields. In a further preferred embodiment the preliminary mass separating 
means comprises a region in which there is at least one electrostatic 
field. The preliminary mass separating means may comprise a toroidal 
electrostatic field having energy-focussing properties, or an 
electrostatic mirror having energy-focussing properties. The essential 
feature of the preliminary mass separating means is that it should 
separate the charged particles, by flight-times, according to their 
masses. 
Preferably the gating means comprises deflector plates and is opened by 
applying voltages to the deflector plates which allow or deflect the 
charged particles into the entrance of the mass analyzer, and is closed by 
applying voltages to the plates which deflect charged particles away from 
the entrance of the mass analyzer. Conveniently, the gating means may be 
opened by earthing the deflector plates. Such deflector plates may be 
provided to give deflections in X and Y directions, orthogonal to the 
direction of travel of the charged particles before deflection, as 
commonly understood, and deflection voltages may be applied in one or both 
X and Y directions as convenient. 
In a further preferred embodiment the gating means comprises a repeller 
grid, and may be closed by applying a repelling voltage to that grid, 
thereby repelling the charged particles away from the entrance of the mass 
analyzer; for example, a grid may be disposed across the entrance of the 
mass analyzer and a voltage applied to reflect the charged particles 
through substantially 180.degree.. Alternatively the gating means may 
comprise at least one accelerating electrode, conveniently in the form of 
an accelerating grid, and may be closed by applying an accelerating 
voltage to accelerate the charged particles, still allowing them to 
proceed substantially towards the entrance of the mass analyzer, but 
giving them a kinetic energy outside pass energy band of the mass 
analyzer, thereby preventing the analysis of those charged particles 
having mass greater than the mass limit. 
In a preferred embodiment of the invention the means for producing pulses 
of charged particles from a source comprises means for irradiating the 
surface of a sample with primary radiation, in which case the source 
comprises said surface and the charged particles are produced as a result 
of the interaction of the primary radiation with the surface. 
Also in a preferred embodiment the primary radiation comprises a pulsed 
beam of primary ions, in which case the charged particles are secondary 
ions and the time-of-flight mass spectrometer of the invention is known as 
a time-of-flight, secondary ion mass spectrometer. Alternatively the 
primary radiation may comprise a pulsed beam of neutral atoms, electrons 
or laser radiation. The invention may also comprise means for ionising 
neutral particles released from the source, or more specifically from the 
surface, thereby producing during said first time interval a pulse of 
charged particles comprising ionised neutral particles. 
The extraction means may conveniently comprise an extractor plate having an 
aperture through which the charged particles may pass. An electric 
extraction field is applied to accelerate the charged particles from the 
surface of the sample towards the extractor plate. The invention may be 
adapted to analyse particles of either positive or negative electric 
charge by the appropriate choice of the direction of the extraction field. 
In the embodiments of the invention described above, in which the primary 
radiation comprises a pulsed beam of ions, neutral atoms, electrons or 
laser radiation, the extraction field is maintained with substantially 
constant magnitude and direction, the charged particles are then produced 
in pulses because the primary radiation beam is pulsed. Alternatively, the 
invention may comprise means for producing a substantially continuous beam 
of primary radiation, comprising ions, neutral atoms, electrons or laser 
radiation, and then the charged particles are produced in pulses by 
applying a pulsed electric extraction field. 
In any embodiment in which a primary radiation beam, whether pulsed or 
continuous, is provided, means may also be provided to scan the primary 
radiation beam across the surface of the sample to perform a 
two-dimensional analysis. 
In a further embodiment of the invention the means for producing pulses of 
charged particles comprises means for applying a pulsed electric field to 
a sample, causing the release of charged particles from its surface, a 
technique known as pulsed field desorption. 
The time-of-flight mass analyzer of the invention may comprise at least one 
region substantially free of electric fields, or at least one region in 
which an electric field is maintained. Preferably the time-of-flight mass 
analyzer comprises an electrostatic, energy-focussing, time-of-flight 
analyzer. In a preferred embodiment of the invention the time-of-flight 
mass analyzer comprises an energy-focussing, toroidal electrostatic field. 
Alternatively the time-of-flight mass analyzer may comprise at least one 
energy-focussing, linear electrostatic field. In a further preferred 
embodiment the invention comprises a magnetic-sector, momentum-focussing 
time-of-flight analyzer. 
The time at which the gating means is to be closed, the end of the second 
time interval, can be calculated from particle dynamics, because it 
corresponds to the flight time of the most massive charged particle of 
interest through the preliminary mass separating means. The time at which 
the gating means is re-opened, at the end of the third time interval, can 
similarly be calculated if the mass of the most massive charged particle 
is known. In practice, however, the most massive charged particle may not 
be known and the time intervals may have to be adjusted to eliminate the 
most massive charged particles from the mass spectrum. In the preferred 
embodiment of the invention, described in detail below, it is convenient 
to set the end of the third time interval at the time when the most 
massive charged particle of interest has been detected after passing 
through the mass analyzer; it is found that this ensures the elimination 
of the most massive charged particle which is not of interest, for most 
samples. 
Also, it is preferable to allow a delay between the end of the third time 
interval and the start of the next pulse, at the end of the fourth time 
interval, to allow the voltages on the gating means to stabilise after 
opening the gating means.

Referring first to FIG. 1, there is shown in schematic form a 
time-of-flight secondary ion mass spectrometer comprising: 
(i) means for producing pulses of charged particles from a source, which 
comprises a primary ion gun 1, and a sample 2, in which sample 2 is the 
said source and the charged particles are secondary ions emitted from the 
surface of sample 2 under the action of primary ions from ion gun 1; 
(ii) extraction means 3, comprising extractor plate 4, with aperture 5; 
(iii) preliminary mass separating means 6, which is a drift region 
substantially free of electrostatic fields, having a first entrance 7 and 
an exit 8; 
(iv) gating means 9 comprising X-deflector plate pair 10, and Y-deflector 
plate pair 11; 
(v) time-of-flight mass analyzer 12, having second entrance 13; and 
(vi) detector 14. 
Ion gun 1 typically comprises a liquid metal ion source with means to focus 
and scan pulses of primary ions 15 across the surface of sample 2 to 
perform a two-dimensional analysis, if required, as known in the art. 
Sample 2 is maintained at an electric potential of approximately +5kV or 
-5kV with respect to earthed extractor plate 4, thereby establishing an 
electrostatic field in extraction region 16. That electrostatic field 
accelerates the secondary ions in pulse 17, produced from the surface of 
sample 2, substantially in the direction of the entrance 13 of mass 
analyzer 12. The distance between sample 2 and extractor plate 4 is 
approximately 5 mm. The distance between extractor plate 4 and Y-deflector 
plate pair 11 is approximately 300 mm. 
Time-of-flight mass analyzer 12 is an energy-focussing analyzer having a 
toroidal electrostatic field. 
Also shown in FIG. 1 are deflector plate voltage supply 18 and the means to 
produce a plurality of pulses, timing unit 19. It will be appreciated that 
items 1 to 14 are enclosed within a conventional vacuum chamber and that 
there are power supplies and control units for items 1,3,12 and 14 not 
shown on FIG. 1. 
Referring now to FIG. 2, there is shown a timing sequence for events in the 
operation of the spectrometer (the time intervals are not drawn to scale). 
T.sub.1 is the time during which a pulse of secondary ions 17 (FIG. 1) is 
emitted from sample 2, i.e. T.sub.1 is the initial width of pulse 17 
before dispersion. T.sub.4 is the period of the cycle of pulses. T.sub.2 
is the time taken by the slowest ion of interest in pulse 17 to travel 
from sample 2 to gating means 9. T.sub.5 is the time taken by the slowest 
ion in pulse 17 to reach gating means 9. T.sub.3 follows T.sub.5 and is 
the time after the start of T.sub.1 when the gating means is reopened. 
The method of operating the invention is as follows: 
A cycle in the operation of the mass spectrometer is started when timing 
unit 19 sends a signal to ion gun 1 causing it to emit a primary ion pulse 
15, directed towards the surface of sample 2. 
When primary ion pulse 15 strikes the surface of sample 2, a pulse of 
secondary ions 17 is emitted and is attracted towards extractor plate 4, 
passes through aperture 5, entrance 7, preliminary mass separating means 
6, exit 8 and continues towards gating means 9. Until the end of time 
period T.sub.2, ions within pulse 17 are allowed through gating means 9 to 
continue towards entrance 13, and to pass through mass analyzer 12 to 
reach detector 14. The time-of-flight between sample 2 and detector 14 can 
then be recorded for each detected ion, and a mass spectrum derived by 
conventional means. At the end of time T.sub.2, in response to a signal 
from unit 19, voltage supply 18 changes the voltages on either or both of 
deflector plate pairs 10 and 11 to deflect any further ions away from 
entrance 13, thereby closing gating means 9. Gating means 9 is kept closed 
until the end of time interval T.sub.3, and re-opened at the end of time 
interval T.sub.3, the most massive of the ions in the pulse having reached 
the gating means, and been deflected, by the earlier time T.sub.5. In the 
preferred embodiment it is convenient to reopen gating means 9, i.e. to 
set the end of time interval T.sub.3, when the most massive ion of 
interest has been detected at detector 14, because it is found that this 
ensures that T.sub.3 is longer than T.sub.5, for most samples of interest. 
There is then a further delay between the end of time T.sub.3 and the 
start of the next pulse from ion gun 1, this delay is approximately 10 
.mu.s and is sufficient to allow the voltages on the deflector plates to 
stabilise. The cycle is then repeated as necessary to collect sufficient 
data as required by the analysis. 
In a typical analysis in which, for example, secondary ions up to 300 amu 
are of interest, the period of the cycles (T.sub.4) is approximately 50 
.mu.s, i.e. a frequency of 20 kHz. Typically, the width of primary ion 
pulse 15 is in the range from 1 ns to 50 ns, and the initial width 
(T.sub.1) of secondary ion pulse 17 is approximately equal to this. 
By the method and apparatus described above a mass spectrum is obtained in 
which interference between consecutive pulses is substantially eliminated.