IMPROVEMENTS IN ATOM PROBES

Abstract: An atom probe with an ion detector (7) capable of giving direct electrical indication of the position of incidence of ions field-evaporated from a specimen (1). Position information is obtained serially for successive ions and correlatd with time of flight signals related to the same ions. In this way data is made available for subsequent computer-assisted three-dimensional reconstruction of the atomic-scale chemistry of the specimen.

Description: IMPROVEMENTS IN ATOM PROBES In the field-ion microscope (FIM) an image of atom positions on the surface of a " sharply pointed specimen can be produced by the field lonisation of gas atoms close to the surface. Later developments have permitted analysis of the atoms visualised in the FIM by two broad means: 1) In the atom probe (AP) , the specimen is moved so that the image of the atom or region of interest lies over the aperture into a mass spectrometer (either time-of-flight or magnetic sector). Typically this aperture would be a hole In the phosphorescent screen of the FIM and would cover the image of 10 - 100 atoms on the specimen surface. Ions field evaporated from the specimen surface, either by a voltage pulse, a laser pulse or a sufficiently high d.c. voltage, and passing through the aperture are analysed in the spectrometer. 2) The Imaging atom probe (IAP) uses a combined detector and imaging screen placed close to the specimen. This detector can thus collect ions field evaporated from the whole of the imaged area. By using pulsed field evaporation, the resultant charge from the detector can be used as a time-of-flight analysis of the field evaporated layer. Alternatively, a time gating technique can be used to selectively image one kind of field evaporated species, that is ions having a given time-of-flight. It is the purpose of the present invention to extend the capabilities of the presently available techniques by incorporating the high spatial resolution of the AP with the extended field of view of the IAP. This involves a new detector and electronics system, which may include on-line » computing facilities, allowing the simultaneous measurement of the positions of ion impact and the ion times-of-flight. Since position of ion impact can be related directly to the original location of the atom on the tip surface, both the chemistry and distribution of the surface species can be obtained. The resulting instrument is thus able to carry out analyses in either of the modes of previous instruments, and also produce selected images for any or all of the field evaporated species simultaneously. It also allows three-dimensional reconstruction of the microchemical variations within the specimen. According to the invention there is provided an atom probe comprising a specimen holder for holding the specimen to be examined; means for field evaporating ions from the surface of the specimen; an ion detector arrangement for receiving the ions and comprising a position detector effective to produce electrical signals representative of the position of incidence of the ions on the detector arrangement; timing means for determining the times of flight of the ions from the specimen to the detector arrangement and thereby allowing the ion species to be identified; and means for correlating, in terms of time, the outputs of the timing means and the ion detector arrangement so as to enable the species of each ion to be identified with * the ion position. This ar angement allows direct electrical signals to be obtained which, represent the species and corresponding position of each ion, and thus atom. In contrast to the use of optical information derived from a phosphorescent screen, direct electrical signals are more efficient and are capable of electronic processing and correlation at speeds which allow a three- dimensional pattern of the specimen to be built up from ions which success¬ ively impinge upon the ion detector arrangement. Preferably, the detector arrangement comprises a charged particle multiplier for producing a quantity of charged particles corresponding in amount and position to the number of ions incident and the respective positions of incidence of each ion, the position detector being situated to respond to the position of the quantity of charged particles from the multiplier produced by each incident ion. The position detector may comprise, for example, a wire array, a resistive anode or a photodiode array which responds directly to the quantity of charged particles. Generally, the quantity of charged particles will be elec-trons and a preferred form of charged particle multiplier is a microchannel plate arrangement. However, preferably the position detector comprises a plate bearing electrodes which are shaped differentially in X and Y coordinate directions and electrically connected to give outputs on coordinate leads representative of the coordinate position of incidence on the electrodes of the quantity of charged particles. Essentially, the invention relies upon the processing of ion information serially as the ions reach the detector arrangement in turn. Serial processing allows timing signals to be used to correlate the individual ion positions with their species. Such serial processing differs in concept from the parallel presentation on the screen of a field ion microscope or an imaging atom p-robe. Also, it does not require the provision of a very large number of parallel processing channels. However, there are restrictions imposed by serial processing. Firstly, processing must be fast enough to allow successiv ions to be dealt with. While, under some circums ances, it is possible to envisage the use of a single processing channel, inherent electrical delays in the processing equiopment will generally make it advisable to cycle the processing of information from successive ions through a set of processing channels which are gated in turn. However, the number of channels need not be large. Secondly, it is possible that more than one ion of a particular species may be liberated from the specimen at the same time. Consequently, such ions will arrive at the detector simultaneously at different positions. Under these circumstances, the system cannot identify the different positions. With correct operation of the system, this is found to be of little significan but it is necessary to know that the situation has arisen so that the infor- ation can be automatically disregarded. Preferably, means are provided responsive to the total number of ions received in a given time interval to identify simultaneous arrival of ions and to effect automatic disregard of the associated positional Information. If such simultaneous arrivals are found to occur more often than is desirable, it is possible to reduce automatically the number of ions evaporated at each pulse by reducing the magnitude of the pulse, be it voltage or laser. The invention will further be described with reference to the accompanying drawings, of which:- Figure 1 is a schematic diagram of an atom probe in accordance with the invention. Figure.2 is a diagram showing at (a) and (b) two possible configurations of ion detector arrangement for use in the atom probe of Figure 1. Figure 3 is a diagram of the position detector of the atom probe of figure 1. Figure 4 is a schematic block diagram of the atom probe and associated circuitry. Referring to Figure 1, there is shown a specimen 1 made from the material to be analysed, and shaped by mechanical, chemical or other means into a fine needle point _2_, held in a support means 3. The said support means _3 may also include some means for linear translation of the specimen in any direction and rotation of the specimen about a point approximately coincident with the pointed end of the specimen. The instrument includes: a) means (not shown) to cool the specimen to cryogenic temperatures (in the range 20 - 100K) either by liquefied gases or by a closed-cycle refrigerator or other means, and coupled thermally to the specimen by a rigid member _4 and a flexible thermal connection 5, b) means to supply to the specimen a high d.c. voltage, in the range 0 - 30kV, via a terminal 6, c) means to supply to the specimen a high voltage pulse of amplitude in the range approximately 0 - 5kV, risetime about 0.5 nanoseconds, and duration about 10 nanoseconds, also via terminal 6_, d) means (not shown in Figure 1) to allow irradiation of the specimen with a laser pulse of duration about 5 nanoseconds and energy density of 2 approximatelylOmJ/cm , and e) an ion detector arrangement 1_ to detect ions field evaporated from the specimen surface by either the said voltage pulse means or by the said laser pulse means, in combination with the said d.c. voltage means, the said detector means generating electronic pulses which may then be analysed by electronic means to determine both the time-of-flight of the ion and its position of impact on the detector means. ~~ The s ~ pec " ιmen and detector, and any necessary parts of a - e are held within an air-tight chamber 8 having conducting walls, typically of stain¬ less steel, and having regions of transparent walling where it is necessary to view the inside of the chamber. Means are provided to evacuate the chamber -10 of atmosphere, down to a pressure of about 10 mbar. There are also means to exchange the specimen held in the support means, and means to admit a gas (e.g. neen, argon), at the discretion of the operator, to a pressure of approximately 10 mbar. The arrangement _7 consists of a charged particle multiplier comprising of a plurality of microchannel plates 9 and a position detector 10 in the form of a shaped electrode assembly, on an insulating substrate, placed behind the said multiplier, in order to collect the charge produced as a result of the impingement of an ion upon the frontsurface of the detector arrangement. As shown in Figure 2, each microchannel plate 11,12 is constructed from a res st ve g assy mater a so as o cons s o many sma usua y c rcu ar channels, the length of each being much greater than its width, distributed across its area so as to have their entrances and exits on opposite faces of the plate. The two faces of the plate are coated with a conductive material to allow a voltage to be applied via the. terminals 11a, lib, 12a and 12b. The material of the plate is such that a charged particle striking the wall of a channel will produce a number of secondary electrons. Thus impingement of one ion upon the detector surface will lead to the deposition of about 10 electrons upon the position detector 10. The voltages applied across the assembly may vary according to the configuration and conditions, but each plate will typically have about IkV across it. Figure 2a shows the use of two double thickness channel plates. Figure 2b shows three single thickness channel plates. Here single thickness applies to plates which have a ratio of channel length to channel width of about 40. Double thick¬ ness plates have a channel length-to-width ratio of at least 60- The arrangement of the electrodes of the position detector are such that the charge received is divided between at least two electrodes such that the proportion of charge collected by the electrodes is dependent on the point of impact of the ion on the front surface of the position detector. One possible arrangement, shown schematically in Figure 3, divides sthe charge between three electrodes J , _15_ and _1 such that the charge on the X electrode increases with the x ordinate of the ion, up to a maximum of half of the total charge, the charge on the Y electrode increases with the y ordinate of the position, up to half of the total charge, and the Z electrode collects the remaining charge. A charge signal can also be derived from the output of the channel plate closest to the position sensing means, amounting to the total charge.deposited on the X, Y and Z electrodes. In Figure 3 the effective diameter , of the electron "beam" from the multiplier, is shown at 17. It is to be understood that for the sake of illustration the electrode pattern of Figure 3 is not to scale. In practice, it will be very much finer. The instrument uses electronic means to measure the time-of-flight of each ion and the quantity of charge collected on each electrode as a result of the impact of each ion on the detector surface. Referring to Figure 4, the timing means consists of a multi-event time-to-digital converter _18_ which will measure the intervals between a start pulse and several stop pulses applied to it. A start pulse Is obtained either by picking off a small fraction of the pulse voltage from the pulse generator 19 or by using the signal from a photodiode _2_0_ irradiated with a portion of the laser pulse from the laser 21, depending on which pulse means is being used. The bias voltage generator is shown at _22_. Stop pulses may be derived either from the sum of the pulses from the X, Y and Z electrodes, or from the output of the channel plate, as above. These pulses are amplified by electronic means, placed close >to the detecting means, before being routed to the time and charge measuring means. - The charge measuring means consists of three channels _2_3, 24, 25, each containing three charge sensitive analogue-to-digital converters, e.g. 26, 27, 28, which may be gated by a separate signal from unit 18 to determine the period during which each converter is active. During this period, charge arriving at the input to each converter in the gated channel is integrated, such that the total charge input during the period is measured. The charge output from the electrodes X, Y and Z, amplified by suitable electronic means as above, is divided equally between the charge measuring channels by a suitable resistor divider. On the arrival of an ion at the detector means, one of the channels is gated to measure the charge on each of the electrodes X, Y and Z due to this ion. On the arrival of a subsequent ion, a second channel is gated to measure the charges due to this second ion. In this way, the charges on the electrodes due to several ions can be measured up to a maximum number of ions equal to the maximum number of charge measuring channels, and given that no two ions arrive within a single gating time of the charge measuring means. Alternatively, a single channel, working sufficiently quickly, may be used to make several measurements, removing the need for other channels. The minimum possible gating time (or integrating time) of the charge measuring channels will depend on the exact details of the electronic configuration, such as the time constant of the amplifying means. The gating time must therefore be sufficiently long, to ensure a measurement of the collected charge to the accuracy required for the determination of ion positions. Timing correlation between the time-of-flight output from unit _18_ and the position output from channels 23 to 25 is effected in a correlation unit 30. In the event that two ions arrive within a single gating time, the position sensing means described above cannot be used to determine the position of the two ions. This case is therefore detected to..prevent the incorrect determination of ion positions. Such a coincident arrival of two ions can be detected either by the timing means, if the ions are separated by greater than the pulse pair resolution of the timing means, or by the total charge arising from the two ions striking the detector means. For detector means using suitable multiplication means, as in Figure 2, the total charge produced by a single ion is approximately constant. The coincident impact of two ions on the detecting means w s ill thus result in twice the charge generated by a single ion. In Figure 4, a coincidence signal is given under these circumstances from channel _23_ to _2_5 over a line to an inhibit unit 31. Unit 31 applies an output to unit 30 to inhibit processing of the positional information. The pulse intensity may be reduced via line 32. Once the times and charges have been measured, the ion positions and mass-to-charge ratios can be calculated. This may be effected by transferring the measurements of time and charge to an on-line computer, which may then carry out the necessary calculations. As an intermediate measure, the data may be recorded on a magnetic or other medium, as indicated at 33. For charge. Q , Q and Q collected on the X, Y and Z electrodes respectively, x y z the ion position is given by: 2Q x = x y - 2Q. The total charge may also be obtained by measuring the -charge from the channel plate output, T, in place of the charge on the Z electrode, in which case the ion position is calculated using: x = 2Q χ / Q t ■ y - 2Q y / Q t For specimen voltage V, and pulse amplitude V ,, the ion mass-to- dc r r pul charge ratio is given by: 1.93xlθ "4 m/n ( V. + bV ) t 2 dc pul d 2 where t is the flight time in nanoseconds, d is the flight length (mm) and b is an experimental factor (approximately 1) to allow for attenuation of the voltage pulse due, for example, to impedance mis-match at the specimen. When two or more ions have been detected coincidentally, as described above, only the mass-to-charge ratios can be calculated. Once position and mass-to- charge data have been obtained, these may be stored and used to calculate the chemical analysis from all or part of the sampled area, and from all or part of the analysed depth. The spatial distribution of a given species or group of --species may also be calculated, inclusively in three dimensions, since the order of detection of the ions gives a measure of the depth from the original surface. The position sensing means may be made from a transparent electrode material on a transparent substrate, and coated with a phosphorescent material to allow simultaneous imaging of the detected ions. This will also allow field-ion images of the specimen surface to be observed, by introducing a small quantity of gas into the instrument, as previously described. Means to carry out time gating of field evaporated species may also be incorporated, either by applying a pulse of approximately IkV to the output of the channel plate assembly, or by applying a pulse of about 100V between two of the channel plates in the assembly. Multiple pulses may also be applied, to image species of more than one time-of- flight,e.g carbon species at m/n = 6 (G ), 12 (C ), 24 (C„ ) etc. In the absence of the image facility, field ion images may be reconstructed by determining the positions of the field-ionised gas atom ' s incident OΠL the detector means, as above. A second imaging means, such as a single channel plate and phosphor screen, may also- be included in the instrument. The invention is not restricted to the details of the embodiment described with reference to the accompanying drawings. For example, correlation of positional and time-of-flight signals has been described as being effected by electronic circuit units. Provided timing signals are associated, however, these time-of-flight and positional signals can be recorded for subsequent processing and correlation, perhaps by computer. The number of charge measuring and other channels is also not restricted to the numbers which have been quoted for purposes of illustration.

Claims: CLAIMS 1. An atom probe comprising a specimen holder for holding a specimen to be examined; means for field evaporating ions from the surface of the specimen; an ion detector arrangement for receiving the ions and comprising a position detector effective to produce electrical signals representative of the position of incidence of the ions on the detector arrangement; timing means for determining the times-of-flight of the ions from the specimen to the detector arrangement and thereby allowing the ion species to be identified; and means for correlating, in terms of time, the outputs of the timing means and the ion detector arrangement so as -to enable the species of each Ion to be identified with the ion position. 2. An atom probe as claimed in claim 1 wherein the detector arrangement comprises a charged particle multiplier for producing a quantity of charged particles corresponding in amount-and position to the number of ions incident and the respective positions of incidence of each ion, the position detector being situated to respond to the position of the quantity of charged particles from the multiplier produced by each incident ion. 3. An atom probe as claimed in claim 2 wherein the position detector comprise a plate bearing electrodes which are shaped differentially in X and Y coordinate directions and electrically connected to give outputs on coordinate leads representative of the coordinate position of incidence on the electrodes of the quantity of charged particles 4. An atom probe as claimed in any of the preceding claims wherein the ion detector arrangement gives an output representative of the total ion chargs received in a given time interval, and there are provided means responsive to said total ion charge to identify simultaneous arrival of ions and to effect automat c sregar o t e assoc ate pos t ona n ormat on. 5. An atom probe as claimed in any of the preceding claims wherein means are provided for regulation of the means for field evaporating ions in response to rate of arrival of ions at the detector, as indicated by the total ion charge collected. 6. An atom probe as claimed in any of the preceding claims including means for storing the outputs from the timing means and the ion detector arrangement, or signals derived therefrom, with time correlation information, for subsequent computer processing. 7. An atom probe as claimed in any of the preceding claims wherein a plurality of measuring channels is provided, connected to measure 'the output from the position detector, and gating means is effective to gate the output from the ' position detector to the different channels ' in turn, whereby the positions of successive ions are measured in different channels.

Inventors: CEREZO ALFRED, SMITH GEORGE DAVID WILLIAM, CEREZO, ALFRED, SMITH, GEORGE, DAVID, WILLIAM
Applicants: CEREZO ALFRED, SMITH GEORGE DAVID WILLIAM, CEREZO, ALFRED, SMITH, GEORGE, DAVID, WILLIAM
Application year: 1986
Main classification: H01J37
IPC codes: H01J37, H01J49