METHOD TO PREPARE A SAMPLE FOR ATOM PROBE TOMOGRAPHY (APT), PREPARATION DEVICE TO PERFORM SUCH METHOD AND METHOD TO INVESTIGATE A REGION OF INTEREST OF A SAMPLE INCLUDING SUCH PERFORMING METHOD

Abstract

To prepare a sample for atom probe tomography, a raw sample body having a surface and a region of interest (ROI) to be inspected by APT is provided. Pillars containing the ROI are formed into the surface of the raw sample body via ablation of material of the raw sample body from the surface with an ultra-short pulsed laser. Redeposited ablated material is removed in the region of the formed pillars. The surface of the formed pillars is polished. A preparation device to perform such a preparation method includes a sample handling unit, a pillar forming unit including an ultra-short pulsed laser, a removal unit to remove redeposited ablated material, and a polishing unit. The result is an efficient preparation of robust samples for atom probe tomography. To investigate a region of interest of a sample, the preparation method is performed and then atom probe tomography of the region of interest is performed.

Claims

1. A method of preparing a sample for atom probe tomography, the method comprising: providing a raw sample body having a surface and a region of interest (ROI) to be inspected via APT; using an ultra-short laser to ablate material from the raw sample body to form pillars containing the ROI; removing redeposited ablated material in a region of the pillars; and polishing the surface of the pillars.

2. The method of claim 1, further comprising prior to forming the pillars, forming a sacrificial layer (SL) on a portion of the surface of the raw sample body.

3. The method of claim 2, further comprising, after forming the pillars, removing a portion of the SL.

4. The method of claim 3, comprising using a member selected from the group consisting of laser ablation, ion milling, wet chemistry, plasma, a mechanical mechanism to remove the portion of the SL.

5. The method of claim 4, comprising using charged particle deposition to form the SL.

6. The method of claim 2, comprising using charged particle deposition to form the SL.

7. The method of claim 2, wherein the ultra-short laser comprises a femtosecond laser.

8. The method of claim 2, further comprising, during formation of the pillars, clearing a region of the surface around the pillars.

9. The method of claim 2, further comprising, after removing the redeposited ablated material, applying a protection layer.

10. The method of claim 2, wherein the method is performed automatically.

11. The method of claim 2, further comprising performing atom probe tomography of the ROI.

12. The method of claim 1, wherein the ultra-short laser comprises a femtosecond laser.

13. The method of claim 1, further comprising, during formation of the pillars, clearing a region of the surface around the pillars.

14. The method of claim 13, wherein a radius of the region of the surface around the pillars is more than five times a height of the pillars.

15. The method of claim 1, further comprising, after removing the redeposited ablated material, applying a protection layer.

16. The method of claim 1, wherein the method is performed automatically.

17. The method of claim 16, further comprising performing atom probe tomography of the ROI.

18. The method of claim 1, further comprising performing atom probe tomography of the ROI.

19. A preparation device, comprising: a sample handling unit; a pillar forming unit comprising an ultra-short pulsed laser configured to form pillars in a surface of a sample; a removal unit configured to remove redeposited ablated material in a region of the pillars; and a polishing unit configured to polish the sample surface in the region of the pillars.

20. The preparation device of claim 19, further comprising: a deposition unit configured to deposit a sacrificial layer (SL) on a portion of the sample surface prior to forming the pillars; and an SL removal unit configured to remove a portion of the SL from the sample surface.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Illustrative embodiments of the disclosure are now described with reference to the figures, in which.

[0048] FIG. 1 shows an upper view of a sample prepared for atom probe tomo-graphy (APT);

[0049] FIG. 2 shows a sectional view according to section line II-II in FIG. 1;

[0050] FIG. 3 shows schematically a preparation device to perform a method to prepare the sample according to FIGS. 1 and 2;

[0051] FIG. 4 a schematical flow chart showing an embodiment of a sample preparation method; and

[0052] FIG. 5 another schematical flow chart showing another embodiment of the sample preparation method.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0053] A sample 1 is prepared for inspection with atom probe tomography (APT). The sample 1 can be cut from a silicon wafer. The sample 1 can include a handling section 2 and a sample pillar section 3. The handling section 2 can be used for handling the whole sample 1. At the handling section 2, for example, a tweezer of a sample handling unit 4 of a preparation device 5 as shown schematically in FIG. 3 can grip the sample 1.

[0054] A total thickness T of the sample 1 may be equal to the initial wafer thickness is approximately 0.8 mm. A typical width dimension W of the sample handling unit 4 is 3 to 5 mm.

[0055] The sample pillar section 3 can have a base body 6 with an elongated contour (FIG. 1) with a length L of about 2 mm and a width WS of about 0.5 mm.

[0056] In the sample pillar section 3, a plurality of pillars 7 extends from the base body 6.

[0057] To facilitate the description of orientations and dimensions, in the following a Cartesian coordinate system is used. In FIG. 1, the x-axis is directed to the right and the y-axis is directed upwards. The z-axis extends perpendicular to the drawing plane of FIG. 1 in the direction to the viewer.

[0058] The pillars 7 can have an extension of approximately 50 μm in the z-direction. The pillars 7 generally have the shape of a needle. They can have an approximately square cross section with an extension of 10 μm in the x- and in the y-direction. Opposing side walls 8 of the pillars 7 can be approximately parallel to each other. Alternatively, a slide conical angle in the range between 1 deg and 10 deg is possible. Alternative to a square xy-cross section, the pillars 7 may have a rounded and/or circular cross section with a typical diameter of 10 μm.

[0059] The pillars 7 can be arranged in a row which extends in the x-direction. Alternatively, the pillars may be arranged in a xy-array.

[0060] In the FIG. 1/FIG. 2 embodiment, the sample 1 has a total of four pillars 7. Depending on the desired APT parameters, alternative embodiments of the sample 1 may have a number of pillars 7 in the range between 1 and 1000, for example between 1 and 100, such as between 1 and 50.

[0061] A distance d between adjacent pillars 7 can be 300 μm, i.e. is more than five times the z height of the respective pillar 7. The distance d is not shown in scale as compared to the z height of the pillars shown in FIG. 2. The distance d may, depending on the respective embodiment, vary between 50 μm and 1 mm.

[0062] In the whole area surrounding the neighbouring pillars 7, the material of the base body 6 is removed. The result of such clearance is that between adjacent pillars 7 there is free space and the respective pillars 7 are the structures with by far the largest height as compared to the surrounding base body 6. A typical xy-dimension of such clearance is com-parable to the distance d.

[0063] Highlighted via dashed circles in FIG. 2 are regions of interest ROI 9 for APT. Further, these regions of interest 9 also are highlighted in the upper view of FIG. 1. The region of interest 9 may include a stack of material layers.

[0064] In a method to prepare the sample 1, initially a raw sample body can be provided having a surface 10 (cf. FIGS. 1 and 2) with the region of interest 9 to be inspected by APT. Such raw sample body provision may be done by cutting a wafer in a base body shape as shown in the upper view of FIG. 1 including the handling section 2 and a raw sample section having the contour of sample pillar section 3 in FIG. 1. In some embodiments, the sample can be adapted to dimensions similar to that of a standard TEM grid, for example, with a 3 mm diameter in length, less than 3 mm in height, and a thickness no greater than that compatible with fitting into a TEM sample holder (typically, no more than 100 um).

[0065] After the raw sample body provision, the pillars 7 are formed into the surface 10 of the raw sample body via ablation of material of the raw sample body from the surface 10 with an ultra-short pulsed laser. A typical xy-cross section or diameter value of the pillars 7 may be smaller than 20 The ultra-short pulsed laser to be used to form the pillars 7 may be an fs- (femtosecond) laser or a ps- (picosecond) laser.

[0066] During the forming of the pillars 7, a region of the surface 10 around the pillars 7 in a radius, which can be more than 5 times the height of the pillars, is cleared.

[0067] Prior to forming the pillars 7, a sacrificial layer SL 11 may be deposited at least on a part of the surface 10 of the raw sample body. After the forming of the pillar 7, at least part of such sacrificial layer SL may be removed. The sacrificial layer SL may be deposited by charged particle deposition. Such deposition may be done by use of a FIB-SEM referred to in https://www.zeiss.com/microscopy/int/cmp/mat/20/nanomaterials/fslaser/laserfib.html. Such an instrument is also is referred to as a Crossbeam laser or referred to as a laser FIB.

[0068] The source of unwanted redeposition comes from the sample itself, so the redeposited material is sample dependent. Examples for the protective, cap and sacrificial layers are given in C. Kang, C. Chandler, and M. Weschler, Chap. 3, Gas Assisted Ion Beam Etching and Deposition, Focused Ion Beam Systems, N. Yao, Ed., Cambridge University Press, 2007.

[0069] Beam-induced deposition may be done with any charged particle beam (electrons or ions). Chapter 3 of the Yao book above describes deposition. FIB redeposition and FIB deposition further are mentioned elsewhere in the Yao book. More details on deposition are in section 3.3 of Yao's book.

[0070] The sacrificial layer 11 may be a resist capping layer and/or a charged-particle beam-induced protective layer. It can also be a Si mask as described in: Subramaniam S, Smath L, Brown A, Johnson K. Use of Single Crystal Masks for Improved Mill Characteristics in High Current Xenon Plasma FIB instrumentation. Microscopy and Microanalysis. 2016; 22(53):152-3.

[0071] After the forming of the pillars 7, redeposited ablated material in the region of the formed pillars 7, and in particular, in the cleared region of the surface 10, is removed. Part of such removal step may be the removal of at least part of the sacrificial layer 11 from the structure remaining after the ablation process step. Such sacrificial layer removal serves at least partly to remove the redeposited ablated material.

[0072] The removal of the sacrificial layer 11 may be done via laser ablation or via ion milling or via wet chemistry or plasma, or via a combination of at least two of these removal techniques. A Si mask may be lifted off or removed in a sonication bath.

[0073] After the removal step of the redeposited ablated material, and prior to any polishing steps, the surface of the sample is polished in the region of interest 9. The polishing may be done with a focused ion beam (FIB).

[0074] After the removal of the redeposited ablated material, a protection layer may be applied to the region of interest 9. Such application of the protection layer is done before polishing of the region of interest 9.

[0075] Such preparation method may be done in a vacuum environment. During the preparation method, all preparation steps may be performed automatically. For navigation of the sample during the preparation method, correlative microscopy might be used including SEM and TEM or STEM.

[0076] In a further embodiment, a protection layer may be deposited on the surface 10 with the region of interest 9 prior to form the pillars 7.

[0077] The preparation device 5 includes besides the sample handling unit 4 a pillar forming unit 12 including the ultra-short pulsed laser to form the pillars 7 in the surface 10 of the sample 1. A removal unit 13 of the preparation device 5 serves to remove the redeposited ablated material in the region of the formed pillars 7. A polishing unit 14 serves to polish the sample surface 10 in the region of the formed pillars 7. The polishing unit 14 may include an ion beam source and further a focusing optics to focus the generated ion beam.

[0078] Further, the preparation device 5 may include a deposition unit 15 to deposit the sacrificial layer 11 at least on a part of the sample surface 10 prior to the formation of the pillars 7. Further, the preparation device 5 may include a sacrificial layer removal unit 16 to remove at least part of the sacrificial layer 11 from the sample surface 10.

[0079] In a method to investigate the region of interest 9 of the sample 1, initially the above described method to prepare the sample 1 is performed. After that, atom probe tomography (APT) of the region of interest 9 is performed.

[0080] With APT, 3D tomography is done at atomic resolution.

[0081] To prepare the region of interest 9, fiducials or other positioning markers may be placed on the sample 1. This may be done by use of a laser or a charged particle beam.

[0082] A preparation of the sample 1 may be done from the front side, but alternatively may be done from the back side of the sample 1. This in particular is done in case of a wafer having a thickness T which is less than 100 μm.

[0083] The APT inspection may be combined with SEM/TEM (scanning electron microscopy/transmission electron microscopy), in particular with scanning transmission electron microscopy (STEM). The cleared material around the pillars can enable the TEM to have a clear line of site, i.e. a clear, unobstructed path, for transmitting electrons from the source, through the ROI, to the detector.

[0084] FIGS. 4 and 5 show schematical flow charts of embodiments of the method to prepare a respective sample 1 to form pillars 7 for APT inspection.

[0085] In an initial step, the raw sample body 20 having the surface 10 and having the at least one region of interest (ROI) 9 to be inspected is provided. FIG. 4 shows two different examples for such raw sample bodies 20. The raw sample body 20 as shown on the left hand side in the head area of FIG. 4 has an extended region of interest 9 directly at the surface 10. The other raw sample body 20 shown right next to it has a smaller ROI 9 being located at a distance to the surface 10. A distance between the ROI 9 and the surface 10 may be a few μm, e.g. maybe 10 μm at most.

[0086] After the provision of the raw sample body 20, the sacrificial layer 11 is deposited on the surface 10 of the raw sample body 20. A respective deposition step is indicated at 20a in FIG. 4.

[0087] After that, the pillars 7 in an initial coarse shape which may be cylindrical are formed into the surface 10 of the raw sample body 20 via ablation of material of the raw sample body 20. This forming of the coarse shaped pillars 7 is done with an ultra-short pulse (USP) laser. An intermediate product showing the base body 6 and three of such coarsely shaped pillars 7 is shown in FIG. 4 in a second line. At the top of such coarsely shaped pillars 7 there is a layer showing the ROI 9 capped by the sacrificial layer 11. Further, FIG. 4 shows a detail enlargement of one of these coarsely shaped pillars 7 showing in addition laser recast/redeposit material 21, which is recast/redeposited on the surface of the coarse pillar 7.

[0088] In a next preparation step, at least a portion or a part 22 of the sacrificial layer 11 is removed as shown at removal step 23 in FIG. 4. Such removal may be done via laser ablation, via ion milling, via plasma ash or another suitable removal process as described above. With such removal step 23 also at least a portion of the laser recast/redeposit material 21 is removed.

[0089] After the removal step 23, in a polishing step 24 the coarsely shaped pillar 7 is polished to prepare the final needle shaped pillar 7, which is shown at the bottom of FIG. 4. Such polishing is done with a focused ion beam (FIB). FIG. 4 further shows a detail enlargement of a needle tip 25 of the needle pillar 7 showing the layer structure of a needle body 26, the ROI 9 and the sacrificial layer 11 at the very end of the needle tip 25. Such final needle has a high quality, smooth surface with no significant topography or projections.

[0090] Removal of the part 22 of the sacrificial layer 11 avoids that during the subsequent polishing an undesired, not smooth surface results in the top portion of the resulting needle pillar.

[0091] FIG. 5 shows a variant of the preparation method in case a raw sample body 27 is present having the ROI 9 more deeply buried into its body volume. In this case, a distance between the surface 10 and the ROI 9 is more than a few μm, e.g. more than 10 μm or more than 25 μm.

[0092] Components, functions and steps of such preparation method variant which have been described with reference to the other fig. and in particular with reference to FIG. 4 show the same reference numerals and are not discussed in detail again.

[0093] In that case, after forming the coarse pillars 7 on the base body 6, which is done similar to the FIG. 4 case (compare FIG. 5 top right and the detail enlargement below), laser re-cast/redeposit material 21, which may include a cap layer 28, is removed from the coarse pillar 7 in a removal step 29. This also is done via laser ablation or ion milling. As indicated in the part of FIG. 5 showing the removal step 29, such removal may be done perpendicular to a cylinder axis of the coarse pillar 7 or may be done at an oblique angle as indicated at 30.

[0094] After the removal step 29, the polishing step 24 takes place as discussed above, in particular with respect to FIG. 4. The result of such polishing step 24 in FIG. 5 is the final needle shaped pillar 7 having the ROI 9 in the region of its needle tip.

[0095] In the FIG. 5 preparation method no sacrificial layer deposit takes place. Instead of the removal of the portion or the part 22 of the sacrificial layer 11, in the FIG. 5 preparation method the cap layer 28 is removed from the top of the coarse pillar 7. This again avoids a low quality top surface region of the resulting needle shaped pillar 7.