Abstract
A system for creating a substantially planar face in a substrate, the system including directing one or more beams at a first surface of a substrate to remove material from a first location, the beam being offset from a normal to the first surface by a curtaining angle; sweeping the one or more beams in a plane that is perpendicular to the first surface to mill one or more initial cuts, the initial cuts exposing a second surface that is substantially perpendicular to the first surface; rotating the substrate about an axis other than an axis normal to the first beam or parallel to the first beam; directing the first beam at the second surface to remove additional material from the substrate without changing the curtaining angle; and scanning the one or more beams in across the second surface to mill one or more finishing cuts.
Claims
1. A method of creating a sample lamella for observation by a transmission electron microscope, the method comprising forming a first substantially planar face of the sample lamella by: directing a first beam at a first surface of a substrate positioned on a sample stage to remove material from a first location in the substrate, the first beam being offset from a normal to the first surface by a first nonzero curtaining angle; sweeping the first beam in a plane that is perpendicular to the first surface to mill one or more first initial cuts in the substrate, the first initial cuts exposing a second surface that is tapered but substantially perpendicular to the first surface; determining a nonzero rotation angle according to the equation: where θ.sub.r is the nonzero rotation angle, θ.sub.m is a desired milling angle, and θ.sub.c is the nonzero curtaining angle; rotating the substrate without tilting the sample stage with respect to the first beam and through the nonzero rotation angle about an axis other than an axis that is normal to the first beam or parallel to the first beam; directing the first beam at the second surface to remove additional material from the substrate without changing the first nonzero curtaining angle; and scanning the first beam in a pattern across the second surface to mill one or more first finishing cuts in the substrate, the one or more first finishing cuts substantially removing the taper of the second surface, wherein rotating the substrate occurs after milling the one or more first initial cuts and before milling the one or more first finishing cuts.
2. The method of claim 1, further comprising forming a second substantially planar face of the sample lamella by: directing a second beam at the first surface of the substrate to remove material from a second location in the substrate, the second beam being offset from a normal to the first surface by a second nonzero curtaining angle, the second location in the substrate disposed sufficiently proximal to the first location in the substrate to form a lamella of a desired thickness; sweeping the second beam in a plane that is perpendicular to the first surface to mill one or more second initial cuts in the substrate, the second initial cuts exposing a third surface that is tapered but substantially perpendicular to the first surface and substantially parallel to the second surface; rotating the substrate without tilting the sample stage with respect to the second beam and through a nonzero rotation angle about an axis other than an axis that is normal to the second beam or parallel to the second beam; directing the second beam at the third surface to remove additional material from the substrate without changing the second nonzero curtaining angle; and scanning the second beam in a pattern across the third surface to mill one or more second finishing cuts in the substrate, the one or more second finishing cuts substantially removing the taper of the third surface, wherein rotating the substrate without tilting the sample stage with respect to the second beam and through the nonzero rotation angle about the axis other than the axis that is normal to the second beam or parallel to the second beam occurs after milling the one or more second initial cuts and before the one or more second finishing cuts.
3. The method of claim 1, further comprising rotating the substrate through a nonzero rotation angle about an axis perpendicular to the first surface.
4. The method of claim 1, in which the first beam is selected from a group comprising: a focused ion beam, an electron beam, a laser beam, and a water jet from a water jet cutter.
5. The method of claim 2, in which the second beam is selected from a group comprising: a focused ion beam, an electron beam, a laser beam, and a water jet from a water jet cutter.
6. The method of claim 1, further comprising imaging the substrate with a scanning electron microscope during milling.
7. The method of claim 1, in which the first nonzero curtaining angle is the angle between the beam and a normal to the first surface of the substrate.
8. The method of claim 1, in which the nonzero rotation angle is the angle between a plane including the beam and the axis about which the substrate is rotated and a plane including the second surface of the substrate.
9. The method of claim 2, in which the directing of said first beam and the directing of said second beam occurs time-sequentially, and wherein said first beam and said second beam are the same beam.
10. The method of claim 2, in which the directing of said first beam and the directing of said second beam occurs generally concurrently, and wherein said first beam and said second beam are different beams.
11. The method of claim 1, in which the one or more first finishing cuts removes material that is redeposited on and/or flows onto the second surface during and/or after milling the one or more first initial cuts.
12. The method of claim 1, in which the one or more first finishing cuts remove material on the second surface that has been made amorphous during milling of the one or more first initial cuts.
13. The method of claim 2, in which the one or more first finishing cuts and the one or more second finishing cuts substantially remove variations in thickness between the top and the bottom of the lamella.
14. The method of claim 2, in which the one or more first finishing cuts and the one or more second finishing cuts separate the lamella from the substrate.
15. The method of claim 2, further comprising directing a first beam and second beam at the substrate to increase throughput, the first beam removing material from the first location in the substrate, and the second beam removing material from the second location in the substrate.
16. The method of claim 2, in which the method is performed automatically without manual intervention from an operator.
17. The method of claim 15, further comprising directing the first beam at the second surface to remove additional material from the substrate, and directing the second beam at the third surface to remove additional material from the substrate.
18. A method for creating a substantially planar face in a substrate comprising: directing a first beam at a first surface of a substrate to remove material from a first location in the substrate, the first beam being offset from a normal to the first surface by a first nonzero curtaining angle, the substrate being positioned on a sample stage that can rotate about no more than one axis, the one axis being an axis other than an axis that is normal to the first beam or parallel to the first beam; sweeping the first beam in a plane that is perpendicular to the first surface to mill one or more initial cuts in the substrate, the initial cuts exposing a second surface that is substantially perpendicular to the first surface; determining a nonzero rotation angle according to the equation: where θ.sub.r is the nonzero rotation angle, θ.sub.m is a desired milling angle, and θ.sub.c is the nonzero curtaining angle; rotating the substrate through the nonzero rotation angle about the one axis; directing the first beam at the second surface to remove additional material from the substrate without changing the first nonzero curtaining angle; and scanning the first beam in a pattern across the second surface to mill one or more finishing cuts in the substrate.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) For a more thorough understanding of the present invention, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
(2) FIG. 1A shows a prior art FIB system in an initial orientation for preparing a sample lamella for TEM analysis from a substrate;
(3) FIG. 1B shows a prior art FIB system in a tilted orientation for post-processing a sample lamella using overfilling, polishing, and/or undercutting;
(4) FIG. 2 shows a sample 200 exhibiting the curtaining effect;
(5) FIG. 3 shows a particle beam system used to implement one or more embodiments of the present invention;
(6) FIG. 4 shows an idealized three dimensional view of sample lamella 400 and ion beam 18 to demonstrate the various angles that are used to determine the orientation of ion beam 18 relative to lamella 400 in accordance with one or more embodiments of the present invention;
(7) FIG. 5 shows a graph 500 illustrating milling angle θ.sub.m as a function of rotation angle θ.sub.r for various curtaining angles θ.sub.c;
(8) FIG. 6 shows a flowchart 600 showing a method of operating focused ion beam system 8 in accordance with one or more embodiments of the present invention;
(9) FIG. 7 shows a sample 700 exhibiting no curtaining effect;
(10) FIG. 8 shows a top schematic view of a three-column assembly 800 containing two FIB columns 802a and 802b, one on each side of a central SEM column 804;
(11) FIG. 9 shows a side schematic view of the three-column assembly 800 of FIG. 8 containing two FIB columns 802a and 802b, one on each side of a central SEM column 804;
(12) FIG. 10 is a side view illustrating an electron-beam imaging mode of a flat sample surface 22 prior to FIB milling;
(13) FIG. 11 is a side view illustrating an electron-beam imaging mode of the side walls of lamella 400;
(14) FIG. 12 shows focused ion beam columns 802a and 802b using ion beams 18a and 18b to simultaneously mill two cutouts 1202a and 1202b, one cutout on each side of lamella 400 to expose the vertical faces of lamella 400;
(15) FIG. 13 is a close-up view of focused ion beam columns 802a and 802b and focused ion beams 18a and 18b from FIG. 12, showing how the two FIB columns may operate independently of each other;
(16) FIG. 14 illustrates a top view of the milling of finishing cuts in the lamella preparation process with a dual FIB system in accordance with an embodiment of the present invention;
(17) FIG. 15 illustrates a top view of dual-FIB columns simultaneously polishing the same face 402 of lamella 400 to reduce curtaining;
(18) FIG. 16 illustrates a close-up side view from FIG. 15 of dual-FIB columns simultaneously polishing the same face 402 of lamella 400 to reduce curtaining;
(19) FIG. 17 illustrates the milling of finishing cuts in the lamella preparation process with a dual FIB system using stage rotation in accordance with a preferred embodiment of the present invention;
(20) FIG. 18 shows an alternative embodiment of the present invention that includes two electron beam columns combined with two FIB columns; and
(21) FIG. 19 shows another alternative embodiment of the present invention that includes two electron beam columns combined with two FIB columns.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
(22) Embodiments of the present invention are directed to devices and methods for creating flat-sided cuts in a sample substrate to create sample lamellae, preferably for use in a transmission electron microscope (TEM). The embodiments include positioning an ion beam in such a way that the beam is not normal to the top surface of the sample substrate. The beam is scanned in the plane defined by the ion beam column and a normal to the top surface of substrate, and the substrate is rotated about the normal to its top surface in order to vary the apparent milling angle.
(23) FIG. 3 shows a focused ion beam system 8 that can be used to implement one or more embodiments of the present invention. Focused ion beam system 8 includes evacuated envelope 12 in which ion source 14 is located to provide ions for ion beam focusing column 16. Ion beam 18 passes from source 14 through column optics 16 and between electrostatic deflection mechanism 20 toward substrate 22, which comprises, for example, a semiconductor device positioned on sample stage 24 within lower chamber 26. In at least one embodiment, sample stage 24 has no more than four degrees of freedom, obviating the need for tilting stage. In alternative embodiments, sample stage 24 may include a tilting stage having more than four degrees of freedom. Preferably, sample stage 24 can translate in the x, y, and z directions, and sample stage 24 can rotate about single axis that is perpendicular to the top surface of sample stage 24.
(24) High voltage power supply 34 is connected to ion source 14 as well as to appropriate electrodes in focusing column 16 for forming ion beam 18 and directing ion beam 18 toward sample 22. Deflection controller and amplifier 36, operated in accordance with a prescribed pattern provided by pattern generator 38, is coupled to deflection plates 20 whereby beam 18 may be controlled to trace out a corresponding pattern on a surface of sample 22 (also termed “substrate”). In some systems, the deflection plates 20 are placed before the final lens (i.e., within focusing column 16), as is well known in the art.
(25) The ion beam source 14 is brought to a focus at substrate 22 for either modifying a surface of substrate 22 by ion milling, material deposition, or for the purpose of imaging the surface. A charged particle multiplier 40 used for detecting secondary ion or electron emission for imaging can be connected to video circuit and amplifier 42. Other image detectors known in the art can also be used, such as backscattered electron or x-ray detectors. Video circuit and amplifier 42 supplying a video signal for video monitor 44. Video monitor 44 also receives deflection signals from controller 36. The location of charged particle multiplier 40 within chamber 26 can vary in different embodiments. For example, in one embodiment, charged particle multiplier 40 can be coaxial with the ion beam and include a hole for allowing the ion beam to pass. A scanning electron microscope 41 (SEM), along with its power supply and controls 45, are preferably provided with the FIB system 8. SEM 41 can be used to image the substrate with electron beam 48 after milling with the FIB 18 or concurrently with FIB milling to monitor the progress of the milling process.
(26) Signals applied to deflection controller and amplifier 36, cause the beam 18 to move within a target area to be imaged or milled on substrate 22 according to a pattern controlled by pattern generator 38. Emissions from each sample point are collected by charged particle multiplier 40 to create an image that is displayed on video monitor 44 by way of video circuit 42. An operator viewing the image may adjust the voltages applied to various optical elements in column 16 to focus beam 18 and adjust beam 18 for various aberrations. Focusing optics in column 16 may comprise mechanisms known in the art for focusing or methods to be developed in the future.
(27) FIG. 4 shows an idealized three dimensional isometric view of sample lamella 400 and ion beam 18 to demonstrate the various angles that are used to determine the orientation of ion beam 18 relative to lamella 400 in accordance with one or more embodiments of the present invention. FIB column 16 directs ion beam 18 at a surface of lamella 400. The curtaining angle θ.sub.c is the angle of ion beam 18 with respect to the z-axis. That is, curtaining angle θ.sub.c is the angle between ion beam 18 and a normal to the top surface 401 of substrate 22. Put another way, curtaining angle θ.sub.c is the angle of ion beam 18 from the z-axis, measured in the xz plane, when θ.sub.r is equal to zero. In one embodiment of focused ion beam system 8 of FIG. 3, FIB column 16 is disposed in a fixed position within focused ion beam system 8 and curtaining angle θ.sub.c cannot be varied. In an alternative embodiment, the tilt angle of FIB column 16 can be adjustable prior to milling lamella 400 from substrate 22, enabling focused ion beam system to mill different substrates at different curtaining angles. However, in this alternative embodiment, the position of FIB column 16 with respect to the top surface 401 of substrate 22 is not changed while milling lamella 400 from substrate 22 so that curtaining angle θ.sub.c is constant throughout the milling process.
(28) The rotation angle θ.sub.r is the angle between of ion beam 18 and the x-axis projected into the xy plane.
(29) The milling angle θ.sub.m is the angle between ion beam 18 and the z-axis projected into the yz-plane. Milling angle θ.sub.m can be determined from rotation angle θ.sub.r and curtaining angle θ.sub.c according to the following equation:
θ.sub.m=tan.sup.−1[sin(θ.sub.r)tan(θ.sub.c)],
(30) where:
(31) θ.sub.m=milling angle; component of FIB in plane of z-axis and lamella face normal
(32) θ.sub.c=curtaining angle; mechanical angle between FIB and z-axis
(33) θ.sub.r=rotation angle between FIB-z-axis plane and lamella plane
(34) Alternatively, the necessary sample stage rotation about the z-axis (i.e., rotation angle θ.sub.r) necessary to achieve the desired milling angle θ.sub.m can be determined according to the following equation:
(35)
(36) For example, if focused ion beam system 8 has FIB column 16 tilted 45 degrees with respect to the z-axis (i.e., curtaining angle θ.sub.c=45°), the relationship between necessary rotation angle θ.sub.r and desired milling angle θ.sub.m ends up being approximately linear for small milling angles θ.sub.m:
(37)
(38) for small θ.sub.m
θ.sub.r≈sin.sup.−1[sin(θ.sub.m)]
θ.sub.r≈θ.sub.m
(39) To achieve a 6 degree milling angle θ.sub.m in the example above, sample stage 24 is rotated 6 degrees, and the scanning pattern of ion beam 18 is rotated 6 degrees. Post-processing milling can then be performed without having to tilt sample stage 24 or FIB column 16 with respect to the other. If FIB column 16 is mounted with a curtaining angle θ.sub.c that is much smaller than 45 degrees, the linear approximation no longer holds. FIG. 5 shows a graph 500 illustrating milling angle θ.sub.m as a function of rotation angle θ.sub.r for various curtaining angles θ.sub.c (θ.sub.c=10, 20, 30, 45, and 80 degrees from the z-axis).
(40) FIG. 6 is a flowchart 600 showing a method of operating focused ion beam system 8 in accordance with one or more embodiments of the present invention. The method begins at initiator 601. At step 602, ion beam 18 is directed toward a horizontal top surface 401 of substrate 22 at a nonzero curtaining angle θ.sub.c. At step 604, ion beam 18 is swept in a plane to mill one or more initial cuts in substrate 22, exposing a substantially planar surface that is perpendicular to the top surface 401 of substrate 22. The initial cuts are “rough cuts” to remove bulk sample material from the substrate to expose the planar surface. By directing ion beam 18 at the substrate with a nonzero curtaining angle θ.sub.c, the shadows cast by materials in substrate 22 having higher mill rates are reduced compared to a vertically-oriented ion beam (i.e. an ion beam directed perpendicular to the top surface 401 of substrate 22). Reducing the shadows cast by materials in substrate 22 having higher mill rates reduces the undesirable surface variations due to the curtaining effect. FIG. 7 shows a sample 700 exhibiting no curtaining effect.
(41) The planar surface may be tapered from top to bottom, due to the shape and energy distribution of ion beam 18. Also, during the initial cuts, material may be redeposited on or flow onto the planar surface. A first reason for finishing cuts is to remove any unwanted taper of the planar surface. A second purpose for finishing cuts is to remove a thin layer from a beam affected zone of the planar surface—this thin layer may comprise an amorphous layer, a redeposited layer, or a layer that flowed onto the planar surface. A third purpose for finishing cuts is to undercut the lamella in preparation for removal—this undercutting procedure comprises FIB milling at both the bottom and two side edges of the lamella. To make the finishing cuts, substrate 22 is rotated through a nonzero rotation angle θ.sub.r about an axis that is neither perpendicular nor parallel to beam 18 (step 606). The rotation angle θ.sub.r is determined according to the desired milling angle to be used for post-processing substrate 22. That is, instead of tilting sample stage 24 or FIB column 16 with respect to the other, sample stage 24 is rotated about an axis perpendicular to the top surface 401 of substrate 22 to achieve the desired milling angle. Milling angle θ.sub.m can be determined from rotation angle θ.sub.r and curtaining angle θ.sub.c according to the following equation:
θ.sub.m=tan.sup.−1[sin(θ.sub.r)tan(θ.sub.c)],
(42) where:
(43) θ.sub.m=milling angle; component of FIB in plane of z-axis and lamella face normal
(44) θ.sub.c=curtaining angle; mechanical angle between FIB and z-axis
(45) θ.sub.r=rotation angle between FIB-z-axis plane and lamella plane
(46) After rotating substrate 22 through a nonzero rotation angle θ.sub.r, ion beam 18 is directed at the exposed planar surface in substrate 22 without tilting sample stage 24 with respect to ion beam 18 (step 608). Ion beam 18 is scanned in a pattern across the exposed planar surface to mill one or more finishing cuts in substrate 22 (step 610). After completion of the first cut in substrate 22, step 612 exits along the “No” branch, leading back to step 604 to initiate a second cut adjacent to the first cut. Completion of this second cut creates two cavities separated by a wall bounded by the two exposed planar surfaces. The wall separating the two cavities can be used as a lamella. After completion of the second cut, step 612 exits along the “Yes” branch to step 620, signifying completion of the lamella. One or more of the finishing cuts can include undercutting the wall at the base so that the wall can be removed.
(47) To improve throughput and reduce processing time, embodiments of the present invention may include more than one FIB column and more than one electron beam column. FIG. 8 shows a top schematic view and FIG. 9 shows a side schematic view of a three-column assembly 800 containing two FIB columns 802a and 802b, one on each side of a central SEM column 804. For illustration, FIB columns 802a-802b are shown at 45 degree angles with respect to SEM column 804, but the exact angles may vary according to the size of the columns, the distance of the beams from substrate 22, and/or other factors. In the example shown here, the optical axes of the three columns 802a, 802b, and 804, converge to a single point on the surface of substrate 22 directly under the electron beam column. In other embodiments, it may be preferred to configure the three columns 802a, 802b and 804, so that the optical axes do not converge to a single point. In some embodiments, all the beams from columns 802a, 802b, and 804, may simultaneously be directed to the substrate 22. In other embodiments, only one or two of the beams from columns 802a, 802b, and 804, may be directed to the substrate 22 simultaneously. The above calculations for the determination of the curtaining angle θ.sub.c apply for the case of multiple FIB columns as well as for the single FIB column illustrated in FIGS. 3 and 4. The arrangement of detectors for the columns may be one of the following:
(48) 1) Through-the-lens detectors within each column, all operating independently of each other. For SEM column 804, secondary and/or backscattered electrons could be collected to generate an imaging signal. For FIB columns 802a and 802b, secondary electrons or secondary ions could be collected to generate a supplemental imaging signal during milling and polishing. This signal could be used for endpoint detection.
(49) 2) One or more detectors located to the sides of the three columns (not shown), and capable of detecting secondary electrons, backscattered electrons and/or secondary ions in order to generate one or more imaging signals.
(50) FIG. 10 and FIG. 11 are side views illustrating two differ ent electron-beam imaging modes employing SEM column 804. In FIG. 10, electron beam 48 is normally-incident to the surface of substrate 22—this mode would be useful during initial navigation around on substrate 22 to locate the specific features which are to be included in preparation of lamella 400. Electron beam 48 is scanned telecentrically to avoid positional errors induced by parallax which can occur with tilted beams. In FIG. 11, the double-deflection system within SEM column 804 is used to tilt electron beam 48 with respect to the two sides of lamella 400, enabling the sides to be imaged for endpoint detection (i.e., when the feature of interest is exposed, milling stops before the feature is milled away). In order to achieve adequate tilt angles (e.g., ±7 degrees to the substrate normal), a double-deflection system is employed which is optimized for large deflection angles and minimal deflection-induced aberrations.
(51) FIG. 12 shows focused ion beam columns 802a and 802b using ion beams 18a and 18b to simultaneously mill two cutouts 1202a and 1202b, one cutout on each side of lamella 400 to expose the vertical faces of lamella 400. SEM column 804 is not shown so that cutouts 1202a and 1202b can be seen. The FIB columns may have the same type of ion source (e.g., Ga) or may be configured with one column having a heavy ion source for rapid milling while the other column is configured with a lighter ion source for fine milling and polishing. It is also possible that each FIB column may have the same type of ion source, but different beam-defining apertures to enable one column to produce high ion fluxes for rapid (bulk) milling, while the other FIB column has smaller apertures to produce lower ion fluxes suitable for fine milling and polishing. In cases where the FIB columns differ, it may be necessary to compensate for the differential milling rates when curtaining is being removed, as illustrated in FIG. 16 and FIG. 17. Another alternative is to configure the FIB columns for multiple operating modes, wherein one mode produces a large beam with high currents, and another mode produces small beams with low currents—these alternative modes can be selectable by changes in lens voltages and/or by mechanical aperture motions.
(52) FIG. 13 is a close-up view of focused ion beam columns 802a and 802b and focused ion beams 18a and 18b from FIG. 12, showing how the two FIBs may operate independently of each other, either milling on opposite sides of lamella 400 (shown) or both FIBs milling on the same side of lamella 400 (not shown). To mill cutouts 1202a and 1202b, the FIBs are scanned in modified raster patterns in which there are more scan lines near lamella 400 and fewer scan lines farther away from lamella 400. Scanning the FIBs in this modified raster pattern produces two triangular-shaped cutouts where the deepest side of each cutout creates one of the two faces of lamella 400. In this mode, the two FIBs will strike the top surface 401 of substrate 22 at roughly 45 degrees to the normal, which may increase the milling rates compared with normally-incident beams (this is a function of the particular ion species employed in the FIB columns). The substrate 22 rotation methodology described in FIGS. 3-5 for a single FIB column embodiment is equally applicable here for a dual FIB column embodiment, since both column tilt angles θ.sub.c are fixed, while angles θ.sub.m, and θ.sub.r will change simultaneously for columns 802a and 802b as the substrate 22 is rotated around an axis perpendicular to the surface of substrate 22. During this raster scanning process, the electron beam column may be periodically activated to produce an image of the cutouts in progress as an aid to end-pointing. The detectors within the FIB columns may also be used for real-time endpoint detection.
(53) FIG. 14 and FIG. 15 illustrate the milling of finishing cuts in the lamella preparation process with a dual FIB system in accordance with an embodiment of the present invention. Cutouts 1202a and 1202b have been completed (see FIGS. 12-13) and now focused ion beams 18a and 18b are deflected against the substantially vertical faces of lamella 400 as shown. FIG. 14 illustrates the case where focused ion beams 18a and 18b are simultaneously milling on opposite sides of lamella 400. FIG. 15 illustrates the case where focused ion beams 18a and 18b are simultaneously milling on the same side of lamella 400. The double-deflectors within each FIB column are used to tilt the focused ion beams outwards so that they strike the lamella sides at a near grazing angle. For FIG. 14, substrate 22 may be rotated to direct the ion beams closer to the normal to the two lamella sides.
(54) FIG. 16 is a close-up view of dual-FIBs 18a and 18b simultaneously polishing the same face 402 of lamella 400 to reduce curtaining. Protrusion 1602 is shown (e.g., a tungsten contact or via within the metal interconnect stack of the device—due to differential milling rates, these features may end up protruding due to slower milling than the other elements of the stack). For purposes of illustration, the two FIBs 18a and 18b have an enclosed angle of approximately 90 degrees in this example, although other enclosed angles may be used depending up on the application. The enclosed angle will generally be twice the angle between each of the FIB columns 802a and 802b and the SEM column 804 between them. If the SEM column 804 is mounted perpendicular to the substrate surface, a 90° enclosed angle corresponds to curtaining angles θ.sub.c=45° for both beams 18a and 18b. As can be seen in the figure, because the beams cross at a large angle, very little shadowing is possible beneath protrusion 1602, thereby reducing the curtaining under protrusion 1602.
(55) FIG. 17 illustrates the milling of finishing cuts 1202a and 1202b in the lamella preparation process with a dual FIB system using stage rotation in accordance with a preferred embodiment of the present invention. Sample stage 24 and/or three column assembly 800 is rotated through nonzero rotation angle θ.sub.r so that lamella 400 is rotated about an axis that is neither perpendicular nor parallel to focused ion beams 18a and 18b. After rotating sample stage 24 and/or three column assembly 800 through a nonzero rotation angle θ.sub.r, focused ion beams 18a and 18b are directed at the exposed planar surfaces 402a and 402b in lamella 400 without tilting sample stage 24 with respect to the ion beams. Ion beams 18a and 18b are then scanned in a pattern across the exposed planar surfaces 402a and 402b to mill one or more finishing cuts on lamella 400.
(56) FIGS. 8-17 have been described in relation to a three-column configuration comprising two focused ion beam columns 802a and 802b, one on each side of a single central electron beam column 804. Alternative column configurations are also possible within the scope of the present invention, including, but not limited to, the following:
(57) 1) Two Electron beam Columns and Two-FIB Columns—in this configuration, instead of a single electron beam column with its axis normal to the substrate surface, two electron beam columns 804a and 804b are combined with two FIB columns 802a and 802b, as illustrated in FIG. 18. The advantage of this configuration is that the two electron beams have a much better angle with respect to the sides of the lamella for imaging during milling, fine-milling and polishing. An obvious disadvantage is the additional cost for the second electron beam column and its supporting electronics. Some of this cost disadvantage may be mitigated through the use of mini-SEM and mini-FIB columns. In particular, an all-electrostatic SEM column, or mini-SEM column, may utilize less expensive electrostatic optics (lenses and voltage supplies) instead of the more expensive magnetic optics (magnetic coils and current supplies) employed in prior art SEM columns.
(58) 2) Multiple Groups of Columns—in this configuration, two or more groups of columns are employed to further increase lamella-preparation throughput. For example, a system could comprise two groups of three columns, such as those shown in FIG. 8 and FIG. 9, which could be spaced apart along an axis perpendicular to the plane containing the three columns—this would enable the closest-packing of the columns, allowing for the smallest vacuum enclosure. Another embodiment could be two or more groups of four column assemblies (two electron beams and two FIBs, as shown in FIGS. 18 and 19).
(59) 3) One tilted Electron beam Column and Two FIB Columns—in this configuration (equivalent to FIG. 18 with one of the two Electron beam columns 804a and 804b removed), a single Electron beam (SEM) column would be mounted as shown in FIG. 18, accompanied by two FIB columns having an approximately 90 degrees enclosed angle between them as shown by the two FIB columns in FIG. 8. These two FIB columns produce focused ion beams with approximately a 90 degrees angle between them as illustrated in FIG. 16. In this concept, substrate 22 would be mounted on sample stage 24 with a rotation axis enabling the SEM column to image each side of the lamella during milling (the stage rotation axis would be perpendicular to the substrate). The two FIB columns can be identical, or can be configured such that one FIB column is optimized for higher current operation, probably at higher beam voltages, while the other FIB column is optimized for lower current operation, possibly at lower beam voltages.
(60) Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.
