Abstract
A method is provided of reducing the thickness of a region of a target sample. Reference data is obtained that is representative of x-rays generated by a particle beam being directed upon part of a reference sample under a first set of beam conditions. Under a second set of beam conditions the particle beam is directed upon the region of the target sample. The resultant x-rays are monitored as monitored data. Output data are then calculated based upon the reference and the monitored data. Material is then removed from the region, so as to reduce its thickness, in accordance with the output data.
Claims
1. A method of reducing the thickness of a region of a target sample, comprising: a) obtaining reference data that is representative of x-rays generated by the interaction of a particle beam with part of a reference sample under a first set of beam conditions, wherein the reference sample has a predetermined composition; b) causing the particle beam, under a second set of beam conditions, to impinge upon the region of the target sample; c) monitoring x-rays generated by interaction between the particle beam and the target sample so as to produce monitored data; d) calculating output data based upon the monitored and reference data; and e) removing material from the region of the target sample so as to reduce the thickness of the region in accordance with thickness that is determined from the output data.
2. A method according to claim 1, wherein the reference sample has dimensions that are known or sufficient to act as a bulk material in response to the particle beam.
3. A method according to claim 1, wherein step (a) is performed in accordance with a theoretical representation in which the reference data is calculated based upon a collection solid angle for the generated x-rays and in accordance with the first set of beam conditions.
4. A method according to claim 1, wherein the reference data in step (a) is obtained by causing the particle beam to impinge upon part of a physical reference sample under the first set of beam conditions and monitoring x-rays generated by the interaction between the particle beam and the reference sample so as to produce the reference data.
5. A method according to claim 1, wherein each of steps (a) to (e) is performed in situ within the same apparatus.
6. A method according to claim 1, wherein each of steps (a) to (e) is performed within a focused ion beam scanning electron microscope.
7. A method according to claim 1, wherein the x-rays are monitored with an x-ray detector and wherein the solid angle between the reference sample and the x-ray detector is substantially the same as that between the target sample and the x-ray detector.
8. A method according to claim 1, wherein the first set of beam conditions is different from the second set of beam conditions.
9. A method according to claim 8, wherein the first set and second set of beam conditions include the beam current and wherein the method further comprises monitoring the beam current for one or each of the first and second sets.
10. A method according to claim 9, wherein the calculation of the output data is based upon the beam currents in accordance with the said first and second sets of beam conditions.
11. A method according to claim 8, wherein the beam current is monitored using at least one of: a Faraday cup, an electron absorbent material with an associated electrical current monitor, a known reference material monitored with an electron back scatter detector apparatus.
12. A method according, to claim 1, wherein the thickness of the region of the target sample is reduced to a predetermined thickness.
13. A method according to claim 1, further comprising obtaining a thickness reduction value, and wherein step (e) reduces the thickness of the region in accordance with both the output data and the obtained thickness reduction value.
14. A method according to claim 12, further comprising repeating steps (b) to (e) until the region has the predetermined thickness or the thickness of the region has been reduced by the obtained thickness reduction value.
15. A method according to claim 1, wherein the removing of material from the region of the target sample is performed by a focused ion beam.
16. A method according to claim 1, wherein the particle beam is an electron beam or an x-ray beam.
17. A method according to claim 1, wherein said beam conditions include one or each of the beam energy, current or duration.
18. A method according to claim 1, wherein the output data is calculated in accordance with a model of the region of the target sample.
19. A method according to claim 18, wherein the model includes predicting the x-ray spectrum produced by a specimen of known structure and composition being impinged upon by an electron beam of known characteristics.
20. A method according to claim 18, wherein the model includes predicting a k-ratio for one or more elements in the region of the target sample based upon the reference data.
21. A method according to claim 20, wherein the output data are calculated by comparing the said predicted k-ratios with the k-ratios generated from the monitored data.
22. A method according to claim 18, wherein the model is adjusted iteratively.
23. A method according to claim 18, wherein the model includes layers and wherein the method further comprises identifying the presence of one or more damaged layers in the region of the target sample using the monitored data.
24. A method according to claim 23, further comprising removing material from the damaged layer in the region of the target sample in accordance with the output data.
25. A method according to claim 23, wherein the damaged layer contains a contaminant material.
26. A method according to wherein the reference sample is an elementally pure sample.
27. A method according to claim 1, wherein the target sample has the form of a lamella.
28. A method according to claim 1, wherein the target sample is a protruding portion of an original sample, which remains attached to the original sample and is formed by removing material from parts of the original sample adjacent the target sample so as to form trenches around the protruding portion.
29. A method according to claim 1, wherein the reference sample comprises a region of the original sample.
30. A method according to claim 1, wherein the reference sample is separate from and is mounted near the target sample.
31. A method according to claim 1, wherein the reference sample is mounted on a probe.
32. A method according to claim 1, wherein the target sample is mounted on a probe so that the target sample may be moved and reoriented relative to the particle beam.
33. A method according to claim 1, wherein the target sample is mounted on a probe so that the target sample may be moved and reoriented relative to the particle beam and wherein the reference sample comprises a portion of the probe on which the target sample is mounted.
34. A method according to claim 1, wherein the target sample is mounted on an electron-transparent substrate, the electron-transparent substrate being mounted on an electron-opaque structure and wherein the reference sample comprises an area of the electron-opaque structure.
35. A method according to claim 34, wherein the electron-opaque structure is a TEM grid and wherein the reference sample comprises a region of a grid bar.
36. A method according to claim 1, wherein the target sample is mounted on a post of a sample carrier and wherein the reference sample comprises a region of the post of the sample carrier.
37. A method according to claim 1, wherein when the particle beam causes the generation of spurious x-rays in material other than the target sample, the method further comprises absorbing the particle beam when it has exited the region of the target sample.
38. A method according to claim 37, further comprising absorbing the particle beam using either, part of a probe formed from a material different from the target sample, or, a shield material held by a probe, the shield material being formed from material different from that of the target sample.
39. A method according to claim 28, further comprising applying a shielding layer of material to a trench on an exit side of the target sample, the shielding layer being formed from a material different from the target sample.
40. A method according to claim 1, wherein the target sample is formed from a part of an original sample, wherein at least the said part of the original sample is coated with a second material, said second material being different from the material of the original sample, such that part of the target sample is formed so as to comprise the second material, the part of the target sample formed from the second material being of substantially the same thickness as another part of the target sample; and wherein the said region of the target sample is formed from the second material.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Some examples of methods and apparatus according to the invention are now described, with reference to the accompanying drawings, in which:
(2) FIG. 1 is a schematic illustration of a focussed ion beam scanning electron microscope apparatus for performing the claimed method;
(3) FIG. 2 is a flow diagram illustrating a first example method according to the invention;
(4) FIG. 3 is a flow diagram illustrating the analysis process of the claimed method;
(5) FIG. 4 is a flow diagram illustrating the thickness calculation process of the example method;
(6) FIG. 5 shows the x-ray measurements of the reference and target samples in the first example method;
(7) FIG. 6 shows the x-ray measurement of the reference and target sample in a second example method;
(8) FIG. 7 shows the x-ray measurement of a target sample that is mounted on a probe, in accordance with a third example method;
(9) FIG. 8 shows the x-ray measurement of the target and reference sample in the third example method;
(10) FIG. 9 shows an alternative arrangement for the x-ray measurement of the reference sample;
(11) FIG. 10 shows the arrangement of the reference sample, original sample and target sample in accordance with a fourth example method;
(12) FIGS. 11, 12 and 13 show the mounting of the target sample upon a carrier grid in accordance with a fifth example method;
(13) FIG. 14 shows the mounting of the target sample upon a carrier post in accordance with a sixth example method;
(14) FIG. 15 is a flow diagram illustrating an eighth example method;
(15) FIG. 16 is a graph displaying the acquired x-ray spectra from the target sample both before and after the thinning of the sample in accordance with the eight example method;
(16) FIG. 17 is a table containing the results of the thickness calculation performed in the eighth example method;
(17) FIG. 18 is a flow diagram illustrating a ninth example method;
(18) FIG. 19 shows two arrangements in which unwanted x-ray radiation is generated by transmitted and backscattered electrons and is collected by the detector;
(19) FIG. 20a shows the blocking by way of a shield of transmitted electrons in accordance with a tenth example method;
(20) FIG. 20b shows the blocking of transmitted electrons by way of a shielding layer applied to the trench sidewall in accordance with an eleventh example method;
(21) FIG. 21a shows the blocking of transmitted electrons using a probe in accordance with a ninth example method;
(22) FIG. 21b shows the x-ray measurement of an additional layer of material applied to the top surface of the target sample in accordance with a twelfth example method;
(23) FIG. 22 shows the original sample of the twelfth example method, having had an additional layer of material applied to its surface and subsequently, having been milled so as to form a lamella, and a side on cross section of this subsequent form.
DESCRIPTION OF EMBODIMENTS
(24) Apparatus suitable for the implementation of the method is described first. A schematic diagram of the apparatus is shown in FIG. 1. The apparatus comprises a focused ion beam scanning electron microscope (FIB-SEM) 1. As is known in the art the FIB-SEM comprises a vacuum chamber 2 containing an electron beam 5 and an ion beam 7, and the required electron optics 9 and ion optics 11 for focussing and scanning with the beams, respectively. The vacuum chamber also contains a sample stage 19, and side ports to hold a manipulator 15, a gas injection system 17 and an x-ray detector 13. The apparatus is controlled by a computer having a processor 27, this processor also receiving signals from the x-ray detector 13. Upon the sample stage 19 is positioned an original sample 21 which contains an area of interest. This original sample 21 comprises a target sample in the form of a lamella 25, trenches 23, formed by the removal of material from the region of the original sample surrounding the target sample and a reference sample 24 which in this embodiment comprises a part of the original sample 21 having dimensions sufficient to act as a bulk material in response to the electron beam 5. As will be understood the target sample may be formed from the original sample 21 using ion beam milling in the FIB-SEM according to known methods.
(25) The target sample 25 may be moved, tilted and rotated with respect to the vacuum chamber 2 such that electron beam 5 may be caused to impinge upon the target sample 25 without being obscured or blocked by other parts of original sample 21. X-rays are produced within the lamella 25 by incident electron beam 5. X-ray detector 13 is directed towards the lamella, and is positioned near to the lamella such that the solid angle over which the produced x-rays radiating from the lamella are detected by the detector 13 is maximised.
(26) FIG. 5 further illustrates the arrangement of apparatus within vacuum chamber 2 in accordance with the present embodiment. The schematic depicts target sample lamella 25 being still attached to original sample 21, and being tilted with respect to electron beam 5. The figure depicts the electron beam 5 first being focused onto the calibration sample 24, which is a selected suitable surface of the original sample 21, and subsequently focussed onto the lamella 25. While the solid angle over which excited x-rays emitted from the reference sample 24 and the target sample 25 respectively (28, 29) appears to be different, this is for illustrative purposes only and in reality the solid angle between the target area of each sample and x-ray detector 13 would be the same.
(27) An example method of the invention is now described for an in situ method. It will be understood that this method may be modified readily to effect the separation of the monitoring of the reference sample from that of the target and thereby include a consideration of different beam conditions in the calculations for guiding the thinning process. Indeed, if the necessary effort is expended to determine reference data representing the effective collection solid angle and efficiency for the x-ray detector that correspond to an available measure of beam current, then the physical monitoring of the reference sample can be avoided.
(28) Referring now to FIG. 2, the method begins at step 101 in which a reference sample 24 having a predetermined composition and dimensions sufficient to act as a bulk material in response to an electron beam, is provided. In the embodiment depicted by FIG. 1, a region of the original sample 21 is provided as the reference sample 24. Notably at this stage the original sample is in an unmilled state. In this case the region chosen is of known composition and presents to the electron beam 5 a region of material having sufficiently large dimensions so as to be able to contain the entire volume within which x-rays are produced due to interactions between the sample material and the incident electron beam.
(29) At step 103 the electron beam 5 is then focussed upon part of the reference sample 24 by first positioning the original sample such that the electron beam 5 coincides with the chosen part of the reference sample. The incident beam causes x-rays to be produced by the excitation of electrons within the reference sample.
(30) X-ray detector 13, which is positioned so as to maximise the proportion of the produced x-rays that are detected, collects the x-rays generated by the interaction between the electron beam and the reference sample. The spectrum of the produced x-rays is recorded at step 105, to produce reference data according to the spectrum of x-rays generated by the interaction between the electron beam and the reference sample.
(31) At step 107 the target sample 25 is milled from the original sample 21. A focussed ion beam 7 is used to mill trenches around an area of interest in the original sample 21. Material is removed from the original sample 21 in this way so as to form a trench 23 from the bottom of which a thin lamella 25 protrudes, the lamella containing the area of interest. The shape of the trenches 23 is configured to enable a line of sight to be established between the electron beam 5 and the lamella 25 from a range of angles, unobscured by the sidewalls of the trenches so that electrons of electron beam 5 are incident only upon the lamella 25 and not on any other part of the original sample 21.
(32) Electron beam 5 is then focussed upon part of the lamella 25 at step 109. Electron beam 5 is configured to impinge upon each of the reference sample and target sample with the same beam energy and such that an identical beam current is incident upon each of the reference sample and the target sample. In the embodiment illustrated at FIG. 1 the lamella 25 is tilted with respect to the electron beam. A direct line of sight from the area of the lamella within which x-rays are generated and the x-ray detector is possible due to the size and width of the trench 23. At step 111 x-ray detector 13 collects the x-rays generated by the interaction between the electron beam and the target sample in the form of lamella 25. The spectrum of these x-rays is recorded to produce monitored data.
(33) At this stage two data sets of x-rays have been recorded, namely, monitored data that contains the recorded spectrum of x-rays generated within the target sample, and, reference data containing the recorded spectrum of x-rays generated within the reference sample. These sets of data are compared at step 113 in order to calculate output data, which contains the determined thickness of the lamella 25.
(34) In the current embodiment, the thickness of the region of the target sample is to be reduced to a predetermined thickness by the method of the invention so as to enable further materials analysis or some other desired use. This is achieved by way of an iterative process wherein the thickness of the lamella is reduced and then measured, repeatedly until the lamella has been thinned to the predetermined thickness. An example thickness is 50 nanometers. This is illustrated at step 115 in FIG. 2, wherein the subsequent step of the method is determined by whether the thickness has been reduced by, or to, a predetermined amount. If the thickness of the lamella, as determined in accordance with the output data, is the desired thickness, then the process is complete (step 120). If the determined lamella thickness remains greater than the predetermined desired thickness, then material is removed from the lamella 25 so as to reduce the thickness of the lamella further, at step 117. In the current embodiment, the lamella 25 is milled using ion beam 7 so as to remove material from the lamella 25 and reduce its thickness. The amount of milling carried out at this step is guided by the difference between the predetermined objective lamella thickness and the thickness that is determined from the output data.
(35) When the milling of stage 117 is complete, the process then returns to step 109 wherein the electron beam 5 is again caused to impinge upon the target sample 25.
(36) FIG. 3 provides further details of the x-ray analysis steps previously shown at 103, 105, 109, 111 and 113. Referring now to the flow chart at FIG. 3, at step 301 a spectrum of x-rays is acquired by focussing a 20 kV electron beam on the reference sample and recording the x-rays produced over a known period of time. The recorded spectrum is used at step 302 to determine the k line intensity for the reference sample material in units of photons per second.
(37) At step 303 the x-ray intensities that would have been obtained from pure bulk samples of the elements of interest within the target sample are derived. This deduction is made either from previous measurements on pure standard materials comprising the elements of the reference and target samples using the same detector, or from theoretical predictions.
(38) The electron beam is then focussed upon the lamella at step 304, while keeping the energy and current of the electron beam unchanged. At 305 x-ray intensities for the elements of the target sample are obtained in units of photons per second from the spectrum acquired at step 304. These intensities are then converted, using the bulk element intensities derived at step 303, to experimental k-ratios.
(39) At step 306 an iterative calculation (see FIG. 4) is performed by computer software to calculate best estimates for the thicknesses of the elements of interest within the target sample layers that would generate k-ratios consistent with those experimental k-ratios obtained at step 305. Thereafter at step 306, the target sample can be thinned further, in accordance with the calculated thickness, as per step 117.
(40) The flow diagram of FIG. 4 illustrates the iterative procedure performed at step 306 by which the thickness of each layer of the target sample is calculated. Starting at 401, an initial thickness value T.sub.0(m) is assigned to each layer m of the lamella. For each element i within the lamella that produces k level x-ray emission lines, the experimental k-ratio k.sub.exp(i) as obtained at 305 and theoretical k-ratios, modelled in accordance with theory, k.sub.theory(i) (see for example J. L. Pouchou. X-ray microanalysis of stratified specimens, Analytica Chimica Acta, 283 (1993), 81-97)) are also taken as starting parameters. The iterative process then begins for the initial value of iteration index j. The cycle begins at step 402 with the calculation of the sum squared deviation D.sub.j. The procedure at step 403 is then performed for each layer m. The thickness value T.sub.j for the current iteration step j is used to pursue a minimum sum squared deviation. Taking T.sub.j(m)/20 as a convenient value for delta, this value is added to, and subtracted from, T.sub.j(m) to give T.sub.j.sup.+(m) and T.sub.j.sup.(m), respectively. The thickness value that corresponds to the minimum in the parameter defined by the three points (T.sub.j.sup.(m), D.sup.), (T.sub.j(m), D.sub.j), (T.sub.j.sup.+(m), D.sup.+), where D.sup.+ and D.sup. are the sum squared deviations calculated using T.sub.j.sup.+(m) and T.sub.j.sup.(m), respectively is assigned as the starting thickness value T.sub.j+1(m) for each given layer m for the next iteration cycle j+1.
(41) The estimation of the thickness value is deemed to have converged when T.sub.j is stationary within a certain threshold sigma. The check for convergence is made at step 405, wherein if the difference between the thickness T.sub.j for the current iteration step j and the determined thickness T.sub.j+1 for the next iteration step j+1 is less than sigma for each of the layers m then the cycle is complete. In the case that T.sub.j+1(m)T.sub.j(m) is greater than or equal to sigma for any of the layers m, then the cycle is repeated and iteration continues with step j+1, returning to step 402.
(42) Within each iteration step, a method of oscillation damping is applied at step 404 in order to prevent the estimated thickness from oscillating about the convergence value. This is achieved by assessing the deviation D.sub.j+1 for the new thickness estimate for each layer T.sub.j+1(m), and in the case that D.sub.j+1 is greater than the deviation D.sub.j for the current iteration step j, setting the thickness value T.sub.j+1(m) for the next step to be the mean of the current thickness T.sub.j and the thickness T.sub.j+1 of the next step. This damping factor is applied up to four times in repetition, by way of assigning an initial value of zero to index p, incrementing p by 1 for each application of oscillation damping to the estimated thickness and repeating 404 for the case when p is less than 4. Note that k-ratios are used to avoid the need to determine x-ray detector collection solid angle and efficiency and the need to measure beam current explicitly. If beam current measurement is available and collection solid angle and efficiency have been pre-determined, then either the intensities corresponding to pure bulk elements can be predicted so that sample intensities can be converted into k-ratios or the iteration scheme can be modified to use the measured x-ray intensities in place of measured k-ratios and a theoretical prediction of x-ray intensity, rather than a prediction of k-ratio within the scheme.
(43) FIG. 6 depicts a second embodiment of the invention wherein rather than selecting part of original sample 21 as the reference sample 24, the calibration is performed upon a physically separate reference sample 24 that is mounted on a probe of a manipulator 15 which may be inserted and retracted from the FIB-SEM or SEM chamber. For this embodiment and the other embodiments now described the method is the same as for the first embodiment unless indicated to the contrary.
(44) FIGS. 7 and 8 present another alternative arrangement of both target sample 25 and reference sample 24, in accordance with a third embodiment of the invention. In this embodiment, after being prepared using one the methods known to those skilled in the art (for instance by milling trenches using an FIB), the lamella 25 is attached to a manipulator 15, and lifted out of the trench. The probe enables free movement of the target sample within the chamber and allows reorientation of the lamella with respect to the electron beam. Additionally, a suitable area on the manipulator probe itself may be used, illustrated at FIG. 8, to calibrate the measurement. A beam calibration measurement is performed by acquiring an x-ray spectrum from the selected area of the probe 24 which is of known composition. The manipulator is then moved and rotated such that the lamella is under the electron beam 5 and the electron beam is positioned on the side of interest upon the prepared lamella. Alternatively, the reference sample 24 may be inserted on a second probe 16 as per the second embodiment, and as illustrated in FIG. 9, rather than using an area of the (first) probe of manipulator 15 for calibration. As explained earlier, in the event that the probe material is different from that of the lamella 25, the reference data may be converted to be equivalent to that of the material of the lamella 25.
(45) A fourth embodiment comprises a separate reference sample 24 being mounted to the sample stage 19 near the target sample 25 within the FIB-SEM chamber. An example of such an arrangement is illustrated at FIG. 10. Positioning the reference sample 24 in close proximity to the lamella 25 allows the calibration measurement and target measurement to be carried out in quick succession, thereby facilitating the beam current to be kept constant between calibration and measurement of the lamella and ensuring that the calibration is effective.
(46) In a fifth embodiment the method of the invention is performed with the lamella 25 having been transferred onto an electron-transparent substrate in the form of a film 30, as illustrated in FIGS. 11 and 12. The electron transparent film 30 upon which target sample 25 sits may be formed for instance of thin amorphous carbon. With reference to the figures, film 30 is formed on a carrier, known within the art as a grid, the carrier having large regions which are electron transparent and other regions which are electron opaque 31. An example of the abovementioned grid structure is depicted in FIG. 13. In this example, the grid contains a multiplicity of fields of electron transparent film 30. Calibration of electron beam 5 may be performed, in the current embodiment, using a non-electron transparent region 31 of the grid as the reference sample 24. With particular reference to FIG. 15, wherein the carrier takes the form of a grid with multiple, orthogonally intersecting, grid bars surrounding regions 30 of electron transparent film, a grid bar close to the field 30 that contains the lamella 25 may be used for calibration. Measurement performed as per the current embodiment requires the software performing the measurement to calculate the emission for layers corresponding to the lamella 25 on top of a layer corresponding to the support film 30. The grid is usually formed from copper.
(47) A sixth embodiment represents a further alternative for mounting the target sample after it has been lifted out from original sample 21. With reference to FIG. 14, the lamella 25 is welded to a post 33 instead of being supported by a film or substrate. In the current embodiment the material of carrier 32 is suitable for the calibration of electron beam 5, therefore a region of the post 33 to which lamella 25 is welded is selected for use as reference sample 24.
(48) A seventh embodiment comprises performing the method in accordance with any of the previous embodiments, except that an x-ray beam is used instead of electron beam 5 to excite x-rays within the samples 24 and 25 and thereby allow the measurement of the thickness of the target sample.
(49) In an eighth embodiment, the method of the invention includes the identification, thickness measurement and removal of one or more layers of damaged material on the target sample. This may be particularly applicable to any embodiment where ion beam milling is used to form or thin the lamella. In this case, the focussed ion beam that is used to mill the trenches 23 around target sample 25 implants some of its constituent gallium ions into the surface of the lamella 25. The surface layers of the lamella, the atomic structure of which has been amorphised by the gallium ion implantation, are removed as part of the material removal stage of the present embodiment of the invention method. Practically this is may be achieved by modifying the parameters that control the ion beam milling process.
(50) The flow diagram of FIG. 15 illustrates this further. First, at step 501 similarly to the fifth embodiment, lamella 25 is prepared by milling trenches using the FIB on either side of the lamella from an original sample 21 (which may be composed of silicon for example), and the lamella is lifted out the trench and placed on a suitable TEM grid 31 at step 502. A beam calibration measurement is then carried out at step 503 by acquiring an x-ray spectrum from a reference sample 24 having a known composition. At step 504 the lamella 25 is positioned under the electron beam 5 and the electron beam is focused on the site of interest on the prepared lamella. At step 505 an x-ray spectrum is acquired from the sight of interest on the prepared lamella 25. The recorded x-ray spectrum is then processed at step 506 to measure the intensity of the silicon and gallium k line emissions from the spectrum and obtain k-ratios using the deduced intensities for pure bulk silicon and gallium, using these to calculate the thickness of the lamella and the effective thickness of the gallium layer. The computational process to determine the thickness of the layers is performed by applying to the lamella a theoretical model wherein the layers comprise a beryllium substrate, a silicon layer and a layer containing gallium and silicon with a composition of 10% atomic gallium. In this example the damaged (amorphised) layer is assumed to be, and is modelled as being, on one side of the lamella only and is approximated as a SiGa.sub.x compound. This modification of the model to include a beryllium substrate, as opposed to one that corresponds to self-supporting layers with a vacuum beneath them is chosen so that the multilayer theoretical model mimics the real situation with a non-scattering vacuum on the beam exit side of the lamella, given that the light element beryllium would only weakly reflect electrons.
(51) The determined thicknesses of the silicon and damaged layers then guide the process at step 507. In the case that the desired lamella and damaged layer thickness have been achieved, the process proceeds to step 509 and the lamella may be lifted out and transferred to a TEM grid. Should the lamella thickness be too great the process instead continues to step 508, wherein the sample is processed further to reduce the thickness of the lamella, similarly to the first embodiment. Additionally, should the lamella thickness be determined at step 507 to be close to the desired value, albeit with the thickness of the gallium damage layer being too great, the indication is that there is significant amorphisation of the lamella surface and this surface damage is removed for instance by low kV milling at step 508.
(52) The process through steps 504 to 507 is then repeated until the desired lamella and damage layer thicknesses have been achieved. FIG. 16 shows a first x-ray spectrum acquired from a target sample prior to the thickness reduction at step 508, overlaid with a second spectrum obtained from the same sample with the same electron dose after having removed material from the sample at step 508. The reduction in signal intensity from the silicon peak is indicative of the reduction in thickness of the lamella, while the reduction in the height of the gallium peak shows that the gallium damaged layer has also been reduced in thickness.
(53) The quantitative results of the thickness calculation procedure as applied to the current example are shown at FIG. 17. In this case, the result of the analysis of the first and second x-ray spectra acquired from the target sample indicates that the thickness of the damaged layer is 20 nanometers and the thickness of the undamaged layer is 47 nanometers prior to reaching step 507. The second x-ray spectrum, recorded for 30 seconds subsequent to the second thinning step at step 508, is processed to show that the gallium silicon layer is thinner by approximately 10 nanometers whereas the undamaged silicon layer is unaffected and remains at 47 nanometers.
(54) A ninth embodiment of the invention method is illustrated by the flow diagram at FIG. 18. This example differs from the previous embodiment firstly in that the measurement and thinning process is carried out with the lamella remaining attached to the original sample, in accordance with the first embodiment. In this example, the problem of unwanted x-ray radiation being produced by electrons that have scattered out of the lamella and have interacted with the material of the trench sidewalls is remedied by way of a technique for blocking or absorbing the electrons from the particle beam. This problem is visualised by the two geometries depicted at FIG. 19, wherein electrons 6 that are transmitted through the lamella interact with the sidewalls of trench 23 to produce x-rays 35 and therefore detector 13 receives x-rays originating at multiple points. The blocking means is provided at step 511 in the method of FIG. 18, prior to the impingement of the electron beam upon the target sample at step 504. This step is illustrated at FIG. 20a, wherein a manipulator probe 15 is used to hold a shield material 36 which may for instance be welded to the probe tip or held by a gripper probe. Shield 36 is positioned on the side of lamella 25 out of which transmitted electrons 6 exit the lamella. The shield material is different from the material present in the trench 23 and is different from the materials present in the lamella so as to obviate the production of artefacts in the x-ray data. The shield 36 is provided to be sufficiently thick to absorb the electron beam 6, so that the transmitted electrons 6 will be prevented from striking the surface of the trench 23.
(55) If the shield 36 is positioned immediately beneath the lamella 25 so that the electrons that are backscattered from the shield strike the lamella, then the theoretical model used for FIG. 3 may be modified so that the material of the shield 36 is modelled as the substrate rather than beryllium or a vacuum.
(56) A tenth embodiment, wherein the detection of unwanted x-rays 35 generated by transmitted electrons 6 is prevented by an alternative method is illustrated at FIG. 21a. In this embodiment, prior to acquiring x-ray data from the lamella, the manipulator probe 15 is positioned on the electron beam exit side of the lamella 25. The probe material is different from the material present in the trench and in the lamella, and the probe is sufficiently thick that electron beam 6 is fully absorbed.
(57) An eleventh embodiment includes a further alternative means of preventing the detection of spurious x-rays 35 and is illustrated at FIG. 20b. In this example an additional layer 37 is deposited on the sidewalls of trench 23 in order to block electrons from entering the sidewall and stopping the excitation of x-rays from therein. The additional layer may be deposited for instance using gas injection system 17. The gas is injected through a needle and a localised deposition of material on the trench sidewalls can be achieved by rastering the focussed ion beam 7, configured to be incident parallel to the target surface of the lamella, on the trench walls. The layer 37 is of sufficient thickness to absorb the electron beam and stop x-rays being generated within the specimen material in the trench, for example a 150 nanometer thick layer of platinum is sufficient to absorb a 10 kV incident electron beam 5.
(58) A twelfth embodiment of the invention comprises an alternative means of mitigating the effect of unwanted x-rays 35 on the determination of the target sample thickness, and is illustrated at FIG. 21b. In contrast to the previous three embodiments, electrons 6 are not physically blocked from entering and interacting with the material of the sidewall in trench 23. Instead, the lamella thickness is determined indirectly by measuring the spectrum of x-rays acquired from a top layer 38 of carbon or platinum. Prior to the preparation of the lamella from original sample 21, a layer 38 of platinum, carbon or tungsten is deposited on the surface of original sample 21 from which the lamella is to be milled.
(59) FIG. 22 shows an example of a target sample being produced in this way. The deposition of additional layer 38 is carried out by injecting a gas close to the surface of original sample 21 and decomposing it using an electron beam or an ion beam. As shown in FIG. 22 material 38 is commonly deposited along a strip corresponding to the width and thickness of the lamella that is to be formed. This is done in order to protect the target sample 25 during the subsequent processing when the material surrounding the lamella is removed using the focussed ion beam 7. The thickness of the layer deposited is considerably larger than the final width of the lamella 25 and the layer remains present on top of the lamella after the material surrounding the lamella has been removed to form trenches 23. The thickness of layer 38 being considerably greater than that of lamella 25 means that the electron beam 5 may be positioned so that the x-rays acquired are produced within the protective layer material 38, without exciting x-rays in the lamella 25. Since the thickness of the protrusion formed by layer 38 is the same as that of the lamella, the x-ray signal acquired from the platinum, tungsten or carbon, for example, will be indicative of the lamella thickness, providing that the material used in layer 38 is not present anywhere proximal to the electron beam 5.
(60) The thickness of the top layer may therefore be determined by the same approach as the flow chart of FIG. 3, modified in that in place of layers of SiGa.sub.x and silicon, a single layer corresponding to the top layer, for example platinum and carbon, is used and emissions of x-rays at energies corresponding to these elements are measured. In this way, x-rays that are characteristic to silicon may be excluded from the thickness determination calculation and therefore the spurious x-rays, if produced, may be ignored.
