Three-dimensional imaging in charged-particle microscopy

Abstract

A method of investigating a specimen using charged-particle microscopy, and a charged particle microscope configured for same. In one embodiment, the method includes: (a) selecting a virtual sampling grid on a surface of a specimen, the virtual sampling grid extending in an XY plane and comprising nodes to be impinged upon by a beam of charged particles; (b) selecting a landing energy for the beam, the landing energy associated with a penetration depth; (c) generating a scan image by irradiating the specimen at each of the nodes with the beam, and detecting output radiation emanating from the specimen in response thereto; (d) repeating steps (b) and (c) for a series of different landing energies corresponding to an associated series of penetration depths, (e) pre-selecting an energy increment by which the landing energy is to be altered after a first iteration of steps (b) and (c); (f) associating the energy increment with a corresponding depth increment; (g) selecting the virtual sampling grid to have a substantially equal node pitch p in X and Y, which pitch p is matched to the value of the depth increment so as to produce a substantially cubic sampling voxel; and (h) selecting subsequent energy values in the series of landing energies so as to maintain a substantially constant depth increment between consecutive members of the series of penetration depths.

Claims

1. A method of investigating a specimen using charged-particle microscopy, comprising: (a) on a surface of the specimen, selecting a virtual sampling grid extending in an XY plane and having grid nodes to be impinged upon by a charged-particle probing beam during a two-dimensional scan of said surface; (b) selecting a landing energy E.sub.i for said charged-particle probing beam, with an associated nominal Z penetration depth d.sub.i below said surface; (c) at each of said grid nodes, irradiating the specimen with said charged-particle robing beam and detecting output radiation emanating from the specimen in response thereto, thereby generating a scan image I.sub.i; (d) repeating steps (b) and (c) for a series {E.sub.i} of different landing energies, corresponding to an associated series {d.sub.i} of different penetration depths; (e) pre-selecting an initial energy increment E.sub.i by which E.sub.i is to be altered after a first iteration of steps (b) and (c); (f) associating energy increment E.sub.i with a corresponding depth increment d in the value of d.sub.i; (g) selecting said virtual sampling grid to have a substantially equal node pitch p in X and Y, which pitch p is matched to the value of d so as to produce a substantially cubic sampling voxel; and (h) selecting subsequent energy values in the series {E.sub.i} so as to maintain a substantially constant depth increment d between consecutive members of the series {d.sub.i}.

2. A method according to claim 1, wherein, after completion of step (h), a physical slicing process is used to remove from said surface a layer of material of nominal thickness L, thereby exposing a new surface.

3. A method according to claim 2, wherein: the series {E.sub.i} comprises a maximum landing energy E.sub.max; a maximum penetration depth d.sub.max is associated with E.sub.max; Ld.sub.max; and steps (a)-(h) are repeated on said new surface.

4. A method according to claim 1, wherein, in step (h), values of E.sub.i in the series {E.sub.i} are selected using at least one of: a functional relationship between E.sub.i and d.sub.i derived from a physical model; and at least one of extrapolation, interpolation and averaging, carried out with respect to an empirical relationship between E.sub.i and d.sub.i that is derived from a prior calibration.

5. A method according to claim 1, wherein, for each successive landing energy in said series {E.sub.i}, said output radiation emanating from the specimen is selectively detected in at least one of the following manners: by detecting only a given energy range .sup.i of the total energy spectrum of said output radiation, where .sup.i depends on E.sub.i; and by detecting only a given angular range .sup.i of the total angular spectrum of said output radiation, where .sup.i depends on E.sub.i.

6. A method according to claim 1, wherein the series {E.sub.i} comprises a maximum landing energy E.sub.max, and wherein: the sample comprises biological tissue and E.sub.max is in a range of from 5 keV to 8 keV; or the sample is substantially non-biological and E.sub.max is in a range of from 30 keV to 60 keV.

7. A method according to claim 1, wherein d is in a range of from 1 nm to 10 nm.

8. A method according to claim 1, wherein a series of scan images resulting from step (d) is not subjected to a mathematical deconvolution procedure.

9. A method according to claim 2, wherein, in step (h), values of E.sub.i in the series {E.sub.i} are selected using at least one of: a functional relationship between E.sub.i and d.sub.i derived from a physical model; and at least one of extrapolation, interpolation and averaging, carried out with respect to an empirical relationship between E.sub.i and d.sub.i that is derived from a prior calibration.

10. A method according to claim 3, wherein, in step (h), values of E.sub.i in the series {E.sub.i} are selected using at least one of: a functional relationship between E.sub.i and d.sub.i derived from a physical model; and at least one of extrapolation, interpolation and averaging, carried out with respect to an empirical relationship between E.sub.i and d.sub.i that is derived from a prior calibration.

11. A method according to claim 2, wherein, for each successive landing energy in said series {E.sub.i}, said output radiation emanating from the specimen is selectively detected in at least one of the following manners: by detecting only a given energy range .sup.i of the total energy spectrum of said output radiation, where .sup.i depends on E.sub.i; and by detecting only a given angular range .sup.i of the total angular spectrum of said output radiation, where .sup.i depends on E.sub.i.

12. A method according to claim 3, wherein, for each successive landing energy in said series {E.sub.i}, said output radiation emanating from the specimen is selectively detected in at least one of the following manners: by detecting only a given energy range .sup.i of the total energy spectrum of said output radiation, where .sup.i depends on E.sub.i; and by detecting only a given angular range .sup.i of the total angular spectrum of said output radiation, where .sup.i depends on E.sub.i.

13. A method according to claim 4, wherein, for each successive landing energy in said series {E.sub.i}, said output radiation emanating from the specimen is selectively detected in at least one of the following manners: by detecting only a given energy range .sup.i of the total energy spectrum of said output radiation, where .sup.i depends on E.sub.i; and by detecting only a given angular range .sup.i of the total angular spectrum of said output radiation, where .sup.i depends on E.sub.i.

14. A method according to claim 2, wherein the series {E.sub.i} comprises a maximum landing energy E.sub.max, and wherein: the sample comprises biological tissue and E.sub.max is in a range of from 5 keV to 8 keV; or the sample is substantially non-biological and E.sub.max is in a range of from 30 keV to 60 keV.

15. A method according to claim 3, wherein the sample comprises biological tissue and E.sub.max is in a range of from 5 keV to 8 keV; or the sample is substantially non-biological and E.sub.max is in a range of from 30 keV to 60 keV.

16. A method according to claim 2, wherein d is in a range of from 1 Mil to 1 nm.

17. A method according to claim 3, wherein d is in a range of from 1 nm to 10 nm.

18. A method according to claim 1, wherein a series of scan images resulting from step (d) is not subjected to a mathematical deconvolution procedure.

19. A Charged Particle Microscope, comprising: a specimen holder, for holding a specimen; a source, for producing a probing beam of charged particles; an illuminator, for directing said probing beam so as to irradiate the specimen; a detector, for detecting a flux of output radiation emanating from the specimen in response to said irradiation; and a processor that is configured to: (a) on a surface of the specimen, select a virtual sampling grid extending in an XY plane and having grid nodes to be impinged upon by said probing beam during a two-dimensional scan of said surface; (b) select a landing energy E.sub.i for said probing beam, with an associated nominal Z penetration depth d.sub.i below said surface; (c) at each of said grid nodes, irradiate the specimen with said probing beam and use said detector to detect output radiation emanating from the specimen in response thereto, thereby to generate a scan image I.sub.i of said surface, (d) repeat steps (b) and (c) for a series {E.sub.i} of different landing energies, corresponding to an associated series {d.sub.i} of different penetration depths; (e) pre-select an initial energy increment E.sub.i by which E.sub.i is to be altered after a first iteration of steps (b) and (c); (f) associate energy increment E.sub.i with a corresponding depth increment d in the value of d.sub.i; (g) select said virtual sampling grid to have a substantially equal node pitch p in X and Y, which pitch p is matched to the value of d so as to produce a substantially cubic sampling voxel; and (h) select subsequent energy values in the series{E.sub.i} so as to maintain a substantially constant depth increment d between consecutive members of the series {d.sub.i}.

20. A Charged Particle Microscope according to claim 19, wherein a series of scan images resulting from step (d) is not subjected to a mathematical deconvolution procedure.

Description

(1) The invention will now be elucidated in more detail on the basis of exemplary embodiments and the accompanying schematic drawings, in which:

(2) FIG. 1 renders a longitudinal cross-sectional view of a CPM in which the present invention is implemented.

(3) FIG. 2 renders an illustration of a principle underlying an embodiment of the present invention.

(4) FIG. 3 shows an example of a functional relationship between landing energy and penetration depth in an embodiment of the present invention.

(5) In the Figures, where pertinent, corresponding parts may be indicated using corresponding reference symbols.

EMBODIMENT 1

(6) FIG. 1 is a highly schematic depiction of an embodiment of a CPM in which the present invention is implemented; more specifically, it shows an embodiment of a microscope M, which, in this case, is a SEM (though, in the context of the current invention, it could just as validly be a (S)TEM, or an ion-based microscope, for example). The microscope M comprises an illuminator (particle-optical column) 1, which produces a probing beam 3 of input charged particles (in this case, an electron beam) that propagates along a particle-optical axis 3. The illuminator 1 is mounted on a vacuum chamber 5, which comprises a specimen holder 7 and associated stage/actuator 7 for holding/positioning a specimen S. The vacuum chamber 5 is evacuated using vacuum pumps (not depicted). With the aid of voltage supply 17, the specimen holder 7, or at least the specimen S, may, if desired, be biased (floated) to an electrical potential with respect to ground.

(7) The illuminator 1 (in the present case) comprises an electron source 9 (such as a Schottky gun, for example), lenses 11, 13 to focus the electron beam 3 onto the specimen S, and a deflection unit 15 (to perform beam steering/scanning of the beam 3). The apparatus M further comprises a controller/computer processing apparatus 25 for controlling inter alia the deflection unit 15, lenses 11, 13 and detectors 19, 21, and displaying information gathered from the detectors 19, 21 on a display unit 27.

(8) The detectors 19, 21 are chosen from a variety of possible detector types that can be used to examine different types of output radiation E emanating from the specimen S in response to irradiation by the input beam 3. In the apparatus depicted here, the following (non-limiting) detector choices have been made: Detector 19 is a solid state detector (such as a photodiode) that is used to detect photoluminescence emanating from the specimen S. It could alternatively be an X-ray detectorsuch as Silicon Drift Detector (SDD) or Silicon Lithium (Si(Li)) detectoror an electron sensor (e.g. a (silicon/evacuated) photomultiplier), for instance. It may, if desired, be movable (e.g. so as to allow it to capture specific angular ranges of the flux E), and/or provided with an energy filter (e.g. so as to allow it to examine specific energy ranges of the flux E). Detector 21 is a segmented silicon electron detector, comprising a plurality of independent detection segments (e.g. quadrants) disposed in annular configuration about a central aperture 23 (allowing passage of the primary beam 3). Such a detector can, for example, be used to investigate the angular dependence of a flux of output backscattered electrons emanating from the specimen S. It will typically be biased to a positive potential, so as to attract electrons emitted from the specimen S.
The skilled artisan will understand that many different types of detector can be chosen in a set-up such as that depicted.

(9) By scanning the input beam 3 over the specimen S, output radiationcomprising, for example, X-rays, infrared/visible/ultraviolet light, secondary electrons (SEs) and/or backscattered electrons (BSEs)emanates from the specimen S. Since such output radiation is position-sensitive (due to said scanning motion), the information obtained from the detectors 19, 21 will also be position-dependent. This fact allows (for instance) the signal from detector 21 to be used to produce a BSE image of (part of) the specimen S, which image is basically a map of said signal as a function of scan-path position on the specimen S.

(10) The signals from the detectors 19, 21 pass along control lines (buses) 25, are processed by the controller 25, and can be displayed on display unit 27. Such processing may include operations such as combining, integrating, subtracting, false colouring, edge enhancing, and other processing known to the skilled artisan. In addition, automated recognition processes (e.g. as used for particle analysis) may be included in such processing.

(11) It should be noted that many refinements and alternatives of such a set-up will be known to the skilled artisan, including, but not limited to: The use of dual beamsfor example an electron beam 3 for imaging and an ion beam for machining (or, in some cases, imaging) the specimen S; The use of a controlled environment at the specimen Sfor example, maintaining a pressure of several mbar (as used in a so-called Environmental SEM) or by admitting gases, such as etching or precursor gases, etc.

(12) In the specific context of the current invention, the illuminator 1/electron source 9 can be adjusted so as to alter the landing energy E.sub.i of the probing beam 3; more specifically, E.sub.i can be increased (or decreased) incrementally so as to cause the beam 3 to penetrate to successively greater (or lesser) depths d.sub.i in the specimen S. Making use of a known relationship d.sub.i=f(E.sub.i) between E.sub.i and d.sub.i (see Embodiment 3, for example), one can (pre-)select a series {E.sub.i} of incrementally altered energy values in such a way that the associated series {d.sub.i} of incrementally altered depth values has successive members that mutually differ by a substantially constant depth increment dthereby ensuring that successive sub-surface levels/bands probed by the beam 3 are substantially equally spaced in Z. Moreover, one can pre-match the X/Y dimensions of a scan grid on a presented surface of the specimen S such that a (repeating) cell of said grid is substantially square with a side length substantially equal to d; this scan grid is then used by controller 25 to execute an XY (e.g. serpentine, raster or spiral) scanning motion of beam 3 relative to specimen S, e.g. by sending appropriate setpoints to stage 7 or/and deflector 15.

(13) It should be noted that actions such as determining the relationship d.sub.i=f(E.sub.i) [or performing an equivalent calibration], determining {E.sub.i}, etc., can be performed fully automatically (e.g. with the aid of software/firmware executed by controller 25), or fully manually, or using a hybrid automatic/manual approach, as desired.

EMBODIMENT 2

(14) FIG. 2 renders a schematic illustration of a principle underlying the present invention. The Figure graphically depicts sub-surface intensity curves for probing beams of successively increasing (or decreasing) landing energy that impinge on a presented surface S.sub.i of specimen S, whereby: The curves in portion (A) correspond to a prior-art approach; The curves in portion (B) correspond to an embodiment of the current invention.
Note that individual curves are essentially bell-shaped, and are here depicted after subtraction of a background signal level S.sub.B. In particular, note that, as compared to the prior-art curves (A), the curves in portion (B) are sharper, with a more Z-confined peak and steeper flanks. As a result, the point of overlap of the shoulders of neighboring curves in portion (B) is further down from the peak than in case (A), resulting in crosstalk reduction as set forth above. An Airy disk associated with curves (B) is also more confined than for curves (A).

EMBODIMENT 3

(15) This embodiment presents a possible manner in which to determine a functional relationship d.sub.i=f(E.sub.i) using a calibration routine, whereby use is made of a combination of physical cutting and multi-energy (ME) radiative slicing to analyze BSE information depth. It should be noted that localized information corresponds mainly to the peak position in an emission layer, although the total BSE signal is spread across a wider range. A possible embodiment of this calibration involves alternating ME BSE imaging with serial physical cutting of the same volume. To obtain an optimally accurate calibration, both physical and ME radiative slicing are ideally performed with the highest resolution possible (smallest depth steps). After a sufficiently large dataset is obtained, subsurface ME images are matched to the most similar ones in the physical slicing datasetwhereby similarity can, for example, be assessed based on mathematical measures such as the Sum of Squared Differences (SSD), Sum of Absolute Differences (SAD), or Structural Similarity Index (SSI) metrics, for instance. Given that each layer in the physical slices stack is associated with a known depth value, this comparison will lead to a depth-of-information curve that interrelates landing energy and detected BSE depth. FIG. 3 shows an example of such a curve.