Method for preparing a sample for transmission electron microscopy

Abstract

A substrate is provided comprising a patterned region defined by a given topography on its surface. The substrate will be processed to obtain a TEM sample in the form of a slice of the substrate. According to the method of the invention, a conformal layer of contrast material is deposited on the topography by depositing a layer of contrast material on a local target region of the substrate separated from the patterned region. The material is deposited by Electron Beam Induced Deposition (EBID). The deposition parameters, the thickness of the layer deposited in the target region and the distance of said target region to the patterned region are such that a conformal layer of contrast material is formed on the topography of the patterned region. This is followed by the deposition of a protective layer which does not destroy the topography in the patterned region as the topography is protected by the conformal layer. The TEM sample is prepared in a manner known in the art, for example by FIB. The conformal contrast layer provides a good contrast with the protective layer, thereby allowing a high quality TEM analysis.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of transmission electron microscopy (TEM), and in particular to a method for preparing a TEM sample for visualizing nano-scale structures as produced in semiconductor processing. BACKGROUND

[0002] Transmission electron microscopy is widely used in the semiconductor industry for observing the finest details of transistor and memory structures, down to the atomic scale. One of the difficult steps is the transmission electron microscopy sample preparation. This is done in a focused ion beam milling (FIB) tool, whereby a thin slice of the order of tens of nanometers is taken out of the sample under study. The sample slice needs to be thin enough to show electron transparency. In order to protect the structure itself from being milled when preparing the slice, a mask and a protective layer are needed. This protective layer can be applied by various methods: spin coating, physical vapor deposition, chemical vapor deposition, evaporation or a combination of two or more of these methods applied sequentially. However, samples with a polymeric top surface, such as including a patterned polymeric resist layer, are easily damaged by any of the above methods, or these samples do not exhibit sufficient contrast with the protective layer to be distinguished during the transmission electron microscopy observation. In the case of fragile structures such as polymeric resist lines or porous silicon structures, it is not possible to choose to deposit an additional contrast layer by sputtering techniques, as sputtering techniques would damage the structure. SUMMARY

[0003] It is an object of the present invention to provide a solution to the above problems. This object is achieved by the method disclosed in the appended claims. A substrate is provided, comprising on its surface a patterned region defined by a given topography of nano-sized features, such as a set of parallel polymeric resist lines. This structure is to be processed to obtain a TEM sample in the form of a substrate slice cut transversely to the substrate surface, aiming at visualizing the topography by TEM. According to the method of the present invention, a thin conformal layer of contrast material is deposited on the topography by depositing a thicker layer of contrast material on a local target region of the substrate, separated from the patterned region, i.e. located at a non-zero distance from the patterned region. The material deposited on the target region is deposited by electron beam induced deposition (EBID), i.e. without using a mask to cover the substrate surface outside the local target region. By a reasonable choice of the thickness of the layer deposited in the target region and of the distance of said target region to the patterned region, a conformal layer of contrast material is formed on the topography of the patterned region, i.e. a layer following the topography but not filling the spaces between adjacent features of the topography. This is followed by the deposition of a protective layer, which does not destroy the topography in the patterned region, as this topography is protected by the conformal layer. The TEM sample is prepared in a manner known in the art, e.g. by FIB. The conformal contrast layer provides a good contrast with the protective layer, thereby allowing a high quality TEM analysis.

[0004] The invention particularly relates to a method for preparing a sample for transmission electron microscopy (hereafter abbreviated as TEM), comprising the steps of:

[0005] - providing a substrate comprising a patterned region on its surface, the patterned region comprising pattern features defining a topography,

[0006] - depositing a protective layer on the patterned region,

[0007] - producing a sample in the form of a thin section by removing material on either side of the thin section of the substrate, the section being transversely oriented with respect to at least a number of features so as to visualize said features by TEM.

[0008] characterized in that the method further comprises, prior to the step of depositing a protective layer, a step of producing a contrast layer on the topography by locally depositing a layer of contrast material in at least one target region spaced apart from the patterned region, wherein the local deposition is performed by electron beam induced deposition applied only to the at least one target region, so that a portion of the contrast material is also deposited around the target region, thereby forming a conformal layer of contrast material on at least some of the features in the patterned region.

[0009] According to an embodiment, the features of the patterned region are formed by a polymer, and the contrast material is a heavy metal, such as Pt.

[0010] According to an embodiment:

[0011] - the features are parallel lines defined by a given width, height and pitch,

[0012] - the patterned region is an array of such lines, and

[0013] - the at least one target region is located on one side of the array, spaced apart from the array in a direction transverse to the lines.

[0014] According to an embodiment, the contrast material is deposited in a single target region, and the thickness of the conformal layer decreases as a function of the distance from the target region.

[0015] According to an embodiment, the contrast material is deposited in two or more target regions, and the conformal layer is at least partially formed by adding the conformal layer formed as a result of depositing the contrast material in the two or more target regions.

[0016] The present invention also relates to the use of electron beam induced deposition to deposit a contrast material layer on a patterned area comprising pattern features of defined topography, by locally depositing a contrast material layer in at least one target area spaced apart from the patterned area, such that a portion of the contrast material is also deposited around the target area, thereby forming a conformal layer of contrast material on at least some of the features in the patterned area. This conformal layer is suitable as a contrast layer when producing a TEM sample of the patterned area. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 A front view and a top view of an array of polymer resist lines on a substrate are shown.

[0018] Figure 2 A top view of a substrate is shown, according to an embodiment of the present invention. Figure 1 A top view of a substrate is shown, according to an embodiment of the present invention.

[0019] Figure 3a A top view of a substrate is shown, according to an embodiment of the present invention. 3b Details of the conformal layer on the array of resist lines before and after depositing a layer of spin-on carbon are shown, for the case of a single Pt deposition to one side of the resist lines.

[0020] Figure 4 An embodiment is shown in which two local depositions are performed, once on one side of the array of resist lines.

[0021] Figure 5a A top view of a substrate is shown, according to an embodiment of the present invention. 5b Details of the conformal layer on the array of resist lines before and after depositing a layer of spin-on carbon are shown, for the case of two Pt depositions. DETAILED DESCRIPTION

[0022] Preferred embodiments of the present invention will be described for the case of a set of parallel polymer resist lines. The materials and processes mentioned are known per se only as examples and are not intended to limit the scope of the present invention. Figure 1 A substrate 1, which can be a glass substrate, is shown having a silicon layer 2 on its surface. On this Si layer is a patterned area 8 comprising an array of parallel polymer resist lines 3, which patterned area is produced by a lithographic patterning technique known per se in the art. The pitch of the array of lines has the same order of magnitude. The goal is to obtain a TEM sample that allows verifying these dimensions. To this end, a protective layer of spin-on carbon (SoC) is to be deposited on the resist lines 3, and a TEM sample of the substrate is to be produced in a focused ion beam (FIB) tool by milling away the material on either side of a thin slice oriented perpendicular to the lines 3. The profile of the sample 4 is indicated in a top view in Figure 1 However, according to the present invention, an additional step is performed before depositing the SoC layer.

[0023] As shown in Figure 2 , a platinum layer 5 having a thickness T is locally deposited into a rectangular target area 6 on one side of an array of resist lines 3, spaced apart by a distance D, which extends in a transverse direction with respect to the lines 3, in this case perpendicular to said lines. The local deposition is preferably done by Electron Beam Induced Deposition (EBID) in a FIB tool used to produce the TEM sample. The EBID technique is known per se and the details of the technique are not described here. When the EBID deposition is limited to a given target area 6 located at a distance D from a patterned area 8 comprising the lines 3, a thin layer 7 of the deposited material is also produced in the area surrounding the target area 6. This thin layer is a result of the generation of secondary and backscattered electrons in the polymer material of the lines 3 as well as in the deposited material itself. By reasonably choosing the distance D, the thickness T of Pt in the target area 6 and the deposition parameters applied in the EBID process, the thin layer 7 is formed conformally on the resist lines 3, i.e. the layer follows the topography defined by the lines 3 and does not fill the space between two adjacent lines 3.

[0024] When the material of the layer 5 / 7 does not react with the polymer (as is the case for Pt), the conformal layer 7 does not damage the polymer lines 3 given the fact that it is formed outside the area 6 directly affected by the EBID process. As seen in Figure 2 and in more detail in Figure 3a , the conformal layer 7 has a thickness of a few nanometers which decreases gradually as a function of the distance from the target area 6. Preferably, the distance D and the thickness T are chosen as a function of the dimensions of the array of lines 3 (height and width of the lines and pitch of the array) so that all lines 3 in the array receive a contrast layer which can be detected by the TEM. The parameters D and T as well as other deposition parameters can thus depend on the exact dimensions of the patterned area and of the features within said area. However, a limited number of trials is sufficient to find a suitable set of deposition parameters.

[0025] As seen in Figure 3b , a spin-on carbon layer 10 is then deposited on top of the Pt layer 7 to serve as a protective layer needed during the TEM sample processing. This protective layer can be another suitable material known in the art, applied by any known technique for this protective purpose. The substrate is then moved back into the FIB tool to produce the TEM sample 4. The TEM images that can be taken from the sample 4 correspond to Figure 3bThe cross-sectional view is shown in Fig. 2. Even though the conformal layer 7 does not have a constant thickness, it provides a clear contrast between the lines 3 and the SoC layer 10 and thereby permits the lines 3 to be clearly visualized in the TEM image such that their dimensions can be measured and / or verified. Furthermore, the Pt layer 7 protects the polymer lines 3 from any damage during deposition of the SoC layer 10. Deposition by EBID is only performed on the target areas 6, i.e. not directly on the area of interest 8, thereby avoiding possible damage to the polymer lines 3 caused by the high electron current applied in the EBID process.

[0026] Figure 4 An embodiment is shown, in which local Pt layers 5a and 5b with equal thickness T are deposited in two equally sized rectangular target areas 6a and 6b on both sides of the patterned area 8 comprising the array of polymer resist lines 3, the two target areas being placed at equal distance D from the array. The layers 5a and 5b are applied sequentially, i.e. by EBID deposition in area 6a and subsequently in area 6b (or vice versa). The reduced thickness of the conformal Pt layer 7a and 7b resulting from the two Pt depositions now adds up and forms a contrast layer with a substantially constant thickness, as shown in the detail image in Fig. 3. Figure 5a and 5b The image taken from the TEM sample 4 now resembles the view shown in Fig. 4. The contrast layer 7a+7b has a substantially constant thickness across the array of resist lines 3. Figure 5b

[0027] However, by depositing two Pt layers 5a and 5b with a lower thickness than in the shown example or further away from the array of resist lines 3, the combined conformal layer 7a+7b can have a higher thickness on the outer lines than in the middle of the array, however the lower thickness is sufficient to provide the required contrast. Layer 5a can also be deposited at a different distance from the array 8 than layer 5b, e.g. in case the available space for the target areas is different on both sides of the array. In this case, the thicknesses of layers 5a and 5b can be different to ensure that a conformal layer with the appropriate thickness is eventually formed on the lines 3. More than two layers 5a, 5b, 5c,... can be sequentially deposited in more than two corresponding target areas 6a, 6b, 6c,... if required by the pitch and other characteristics of the patterned area. The in-plane shape of the target 6 or areas 6a, 6b,... can differ from the rectangular shape shown in the respective figures. If the contrast layer 7 (or 7a+7b+...) is only required on certain features in a sub-area of the patterned area 8, the thickness T and / or the distance D and possibly other parameters can be adapted such that the contrast layer 7 is at least deposited on said sub-area of the patterned area 8. The method of the present invention thereby allows a certain degree of flexibility depending on the characteristics of the structure for which the TEM sample is required.

[0028] Numerical example:​

[0029] The following EBID parameters apply to obtain a Pt contrast layer on a polymer resist line array as the one shown in the figures, with a line width of about 14 nm, a height of about 15 nm, a pitch of about 30 nm, measured perpendicular to the longitudinal direction of the lines.

[0030] Primary beam energy: 5 keV

[0031] Current through aperture: 1.6 nA

[0032] Deposition time: 60 s

[0033] Distance D( Figure 2 ) : 2 to 4 meters

[0034] Temperature: room temperature

[0035] Thickness T( Figure 2 ) : 2 to 20 nm

[0036] In-plane dimensions of the Pt target area 6: typically 0.3 m x 0.2 m, but can be chosen depending on local structural features.

[0037] The present invention is not limited to any of the materials described above. The present invention is mainly used to produce TEM samples comprising features of a delicate material such as a polymer or porous silicon and / or a material showing little or no contrast with a protective layer required for TEM sample preparation. The contrast layer can be formed of any material that does not react with the material of the features to be imaged by the TEM. For imaging of polymer structures, other heavy metals than Pt are suitable as material for the contrast layer, for example W, Hf, Mo, Au, Ir,... that can be deposited in a FIB tool using suitable chemical precursors and EBID modes.

[0038] The structure to be imaged can be any patterned structure defined by a given topography. The present invention is applicable to all scaled structures and stacks used in e.g. patterning where the top is a resist, or structures with a composite resist layer as used in DSA (directed self-assembly), SADP / SAQP (self-aligned double and quadruple patterning) methods, or structures where a polymer activation layer needs to be analyzed for selective deposition.

[0039] Although the application has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. Other variations of the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed application, from an study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A method for preparing a sample for transmission electron microscopy, TEM, comprising the steps of: providing a substrate (1, 2) comprising a patterned region (8) on its surface, the patterned region (8) comprising pattern features (3) defining a topography, depositing a protective layer (10) on the patterned region (8), producing the sample in the form of a thin slice (4) of the substrate by removing material on either side of the slice, the slice being oriented transversely to at least some of the features (3) so as to visualize the features by TEM, characterized in that the method further comprises, prior to the step of depositing the protective layer (10), the step of producing a conformal layer (7) on the topography by locally depositing a contrast material layer (5) in at least one target region (6) spaced apart from the patterned region (8), wherein the local deposition is performed by electron beam-induced deposition applied only to the at least one target region (6), so that a portion of the contrast material is also deposited around the target region (6), thereby forming a conformal layer (7) of the contrast material on at least some of the features in the patterned region (8).

2. The method of claim 1, wherein the features (3) of the patterned region are formed by a polymer, and wherein the contrast material is a heavy metal.

3. The method of claim 2, wherein The contrast material is Pt.

4. The method of claim 1, characterized in that: the features are parallel lines (3) defined by a given width, height and pitch, the patterned region (8) is an array of such lines, and the at least one target region (6) is located at one side of the array, spaced apart from the array in a direction transverse to the lines (3).

5. The method of claim 1, wherein the contrast material is deposited in a single target region (6), and wherein the thickness of the conformal layer (7) decreases as a function of the distance from the target region (6).

6. The method of claim 1, wherein the contrast material is deposited in two or more target regions (6a, 6b), and the conformal layer is at least partially formed by adding the conformal layer (7a, 7b) formed as a result of depositing the contrast material in the two or more target regions (6a, 6b).

7. Use of electron beam-induced deposition in preparing a sample for transmission electron microscopy, TEM, prior to depositing a protective layer (10), to deposit a contrast material layer (5) on a patterned region (8) comprising pattern features (3) defining a topography, by locally depositing the contrast material layer (5) in at least one target region (6) spaced apart from the patterned region (8), so that a portion of the contrast material is also deposited around the target region (6), thereby forming a conformal layer (7) of the contrast material on at least some of the features in the patterned region (8).

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