Patent Description:
Transmission electron microscopy is used extensively in the semiconductor industry for looking at the finest details of transistors and memory structures down to the atomic level. 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 in the order of tens of nanometres is lifted out from the sample under investigation. The sample slice needs to be thin enough to exhibit electron transparency. In order to protect the structure itself from being milled away while preparing the thin slice, a mask and a protective layer is needed. This protective layer can be applied by a variety of methods : spin-on, 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, for example comprising a patterned polymer photoresist layer, are prone to damage by any of the methods mentioned above or they do not exhibit enough contrast with the protective layer to be distinguished during transmission electron microscopy observations. Depositing an additional contrasting layer by sputtering techniques is not an option in the case of vulnerable structures such as polymer resist lines or porous silicon structures, because sputtering techniques will damage the structures.

The publication 'Properties and applications of cobalt-based material produced by electron- beam-induced deposition', <NPL> discloses the use of electron beam induced deposition (EBID) for depositing cobalt-containing microcolumns on a substrate. The deposition is performed in the specimen chamber of an environmental scanning microscope. A cobalt-containing precursor is vaporized and introduced via a tube into a microchamber placed in the microscope's specimen chamber. A <NUM>-mm diameter hole allows transmission of the primary electron beam. Further growth (in height and width) of an existing column can be induced by irradiating a neighbouring point within the backscattering radius of the electron beam directed at the neighbouring point.

The invention aims to provide a solution to the problems set out above. This aim is achieved by the method disclosed in the appended claims. A substrate is provided comprising on its surface a patterned area defined by a given topography of nano-sized features, for example a set of parallel polymer resist lines. The substrate is to be processed for obtaining a TEM sample in the form of a slice of the substrate cut out transversally to the substrate surface, with the aim of visualizing the topography by TEM. According to the method of the invention, a thin conformal layer (i.e. a layer that follows said topography) of contrasting material is deposited on the topography, by depositing a thicker layer of the contrasting material on a local target area of the substrate, spaced apart from the patterned area, i.e. located at a non-zero distance from the patterned area. The material deposited on the target area is deposited by Electron Beam Induced Deposition (EBID), i.e. without using a mask to cover the substrate surface outside the local target area. By a judicial selection of the thickness of the layer deposited in the target area, and the distance of said target area to the patterned area, a conformal layer of the contrasting material is formed on the topography of the patterned area. This is followed by the deposition of the protective layer, which does not damage the topography in the patterned area, as it is protected by the conformal layer. The TEM sample is prepared in a manner known in the prior art, for example by FIB. The conformal contrasting layer provides a good contrast with the protective layer, thereby allowing a high quality TEM analysis.

The invention is in particular related to a method for preparing a sample for transmission electron microscopy, hereafter abbreviated as TEM, comprising the steps of :.

characterized in that the method further comprises, before the step of depositing the protective layer, a step of producing a contrast layer on the topology by depositing a layer of contrasting material locally in at least one target area spaced apart from the patterned area, wherein the local deposition is performed by Electron Beam Induced Deposition applied only to the at least one target area, in such a manner that a portion of the contrasting material is deposited also around the target area, thereby forming a conformal layer of the contrasting material on at least some of the features in the patterned area.

According to an embodiment, the features of the patterned area are formed of polymer, and the contrasting material is a heavy metal, for example Pt.

According to an embodiment, the contrasting material is deposited in a single target area, and the thickness of the conformal layer decreases as a function of the distance to the target area.

According to an embodiment, the contrasting material is deposited in two or more target areas, and the conformal layer is at least partially formed by the addition of conformal layers formed as a consequence of the deposition of the contrasting material in the two or more target areas.

A preferred embodiment of the invention will be described for the case of a set of parallel polymer resist lines. Cited materials and processes which are known as such are mentioned only as examples and are not intended to limit the scope of the present invention. <FIG> shows a substrate <NUM> which may be a glass substrate, with a layer of silicon <NUM> on its surface. On the Si layer is a patterned area <NUM> comprising an array of parallel polymer resist lines <NUM>, produced by a lithographic patterning technique known as such in the art. The width (measured in the plane of the drawing) and height of the lines <NUM> are in the order of nanometres, for example between <NUM> and <NUM> nanometres. The pitch of the array of lines is of the same order of magnitude. The aim is obtaining a TEM sample that allows the verification of these dimensions. In order to do this, a protective spin-on carbon (SoC) layer is to be deposited on the resist lines <NUM> and a TEM sample of the substrate is to be produced in a focused ion beam (FIB) tool by milling away material on either side of a thin slice, oriented in the direction perpendicular to the lines <NUM>. An outline of the sample <NUM> is indicated in the top view of <FIG>. According to the invention however, an additional step is performed prior to depositing the SoC layer.

As illustrated in <FIG>, a layer <NUM> of platinum having a thickness T is deposited locally in a rectangular target area <NUM> to one side of the array of resist lines <NUM>, spaced apart from the array by a distance D, the distance D extending in a transversal direction relative to the lines <NUM>, in this case perpendicular to said lines. The local deposition is done by Electron Beam Induced Deposition (EBID), preferably in the FIB tool that is to be used for producing the TEM sample. The EBID technique is known as such and details of this technique are not described here. When the EBID deposition is limited to a given target area <NUM> that is located at a distance D from the patterned area <NUM> comprising the lines <NUM>, a thin layer <NUM> of the deposited material is produced also in a region surrounding the target area <NUM>. The thin layer is a result of the generation of secondary and backscattered electrons in the polymer material of the lines <NUM> and in the deposited material itself. By judiciously choosing the distance D, the thickness T of the Pt in the target area <NUM>, and the deposition parameters applied in the EBID process, the thin layer <NUM> is formed conformally on the resist lines <NUM>, i.e. the layer follows the topography defined by the lines <NUM> and does not fill the spaces between two adjacent lines <NUM>.

When the material of the layer <NUM>/<NUM> is not reactive with respect to the polymer, as is the case for Pt, the conformal layer <NUM> does not damage the polymer lines <NUM>, given the fact that the conformal layer <NUM> is formed outside the area <NUM> that is directly affected by the EBID process. As seen in <FIG> and in more detail in <FIG>, the conformal layer <NUM> has a thickness of a few nanometres, which decreases gradually as a function of the distance from the target area <NUM>. Preferably, the distance D and the thickness T are chosen as a function of the dimensions of the array of lines <NUM> (height and width of the lines and pitch of the array), so that all the lines <NUM> of the array receive a contrast layer that is detectable by TEM. The parameters D and T and other deposition parameters may therefore depend on the exact dimensions of the patterned area <NUM> and of the features within said area. A limited number of trials is however sufficient for finding a suitable set of deposition parameters.

As seen in <FIG>, a layer <NUM> of spin-on carbon is then deposited on top of the Pt layer <NUM> to serve as the protective layer required during the TEM sample processing. The protective layer could be another suitable material known in the art, applied by any technique known for this purpose. The substrate is then moved back to the FIB tool for producing the TEM sample <NUM>. The TEM image obtainable from the sample <NUM> corresponds to the section view shown in <FIG>. Even though the conformal layer <NUM> does not have a constant thickness, it provides a clear contrast between the lines <NUM> and the SoC layer <NUM>, and thereby permits the lines <NUM> to be clearly visualised in the TEM image so that the dimensions of the lines can be measured and/or verified. Furthermore, the Pt layer <NUM> protects the polymer lines <NUM> from any damage during the deposition of the SoC layer <NUM>. The deposition by EBID is applied only to the target area <NUM>, i.e. not directly to the area of interest <NUM>, thereby avoiding possible damage to the polymer lines <NUM> caused by the high electron currents applied in the EBID process.

<FIG> shows an embodiment wherein local Pt layers 5a and 5b of equal thickness T are deposited on both sides of the patterned area <NUM> comprising the array of polymer resist lines <NUM>, in two equal-sized rectangular target areas 6a and 6b, placed at equal distance D from the array. The layers 5a and 5b are applied sequentially, i.e. through deposition by EBID in area 6a followed by area 6b or vice versa. The decreasing thicknesses of the conformal Pt layers 7a and 7b resulting from the two Pt depositions now add up, and form a contrast layer with a substantially constant thickness, as shown in the detail images in <FIG>. The image obtained from the TEM sample <NUM> now resembles the view shown in <FIG>. The contrast layer 7a+7b has a substantially constant thickness across the array of resist lines <NUM>.

However, by depositing two Pt layers 5a and 5b of lower thickness than in the example shown, or further away from the array of resist lines <NUM>, the combined conformal layer 7a+7b could have a higher thickness on the outer lines than in the middle of the array, this lower thickness however being sufficient to provide the required contrast. It is also possible to deposit layer 5a at a different distance from the array <NUM> than the layer 5b, for example if the available space for the target areas is not the same on both sides of the array. In that case the thickness T of the layers 5a and 5b could be different, in order to ensure that a conformal layer of suitable thickness is eventually formed on the lines <NUM>. More than two layers 5a,5b,5c,. could be deposited sequentially in more than two respective target areas 6a,6b,6c,. if the size and other characteristics of the patterned area would require this. The in-plane shape of the target <NUM> or areas 6a,6b,. could be other than the rectangular shape illustrated in the drawings. If the contrast layer <NUM> (or 7a+7b+. ) is needed only on some of the features in a sub-area of the patterned area <NUM>, the thickness T and/or the distance D and possibly other parameters could be adapted so that the contrast layer <NUM> is at least deposited on said sub-area of the patterned area <NUM>. The method of the invention thus allows a degree of flexibility, as a function of the characteristics of the structure of which a TEM sample is required.

The following EBID parameters are suitable for obtaining a contrasting layer of Pt on an array of polymer resist lines like the array illustrated in the drawings, the width of the lines, measured perpendicular to the longitudinal direction of the lines, being about <NUM>, the height about <NUM>, the pitch about <NUM>.

In plane-dimensions of the Pt target areas <NUM> : typically <NUM> x <NUM> but may be chosen depending on local structure features.

The invention is not limited to any of the materials cited above. The invention is primarily useful for producing TEM samples comprising features of a vulnerable material such as polymer or porous silicon, and/or a material that shows little or no contrast with the protective layer required for the TEM sample preparation. The contrast layer may be formed of any material that is not reactive with the material of the features that are to be imaged by TEM. For the imaging of polymer structures, other heavy metals besides Pt are suitable as materials for the contrast layer, e.g. W, Hf, Mo, Au, Ir,. which can be deposited in a FIB instrument using an appropriate chemical precursor and the EBID mode.

The structure that is to be imaged may be any patterned structure defined by a given topography. The invention is applicable for example to all scaled structures and stacks used in patterning where the top is a resist, or a structure with complex layers of resist as used in DSA (Directed Self-Assembly), SADP/SAQP (Self-Aligned Double and Quadruple Patterning) methods, or a structure where analysis of a polymeric activation layer for selective deposition is required.

Claim 1:
A method for preparing a sample for transmission electron microscopy, hereafter abbreviated as TEM, comprising the steps of :
- providing a substrate (<NUM>,<NUM>) comprising on its surface a patterned area (<NUM>) comprising pattern features (<NUM>) which define a topology,
- depositing a protective layer (<NUM>) on the patterned area (<NUM>),
- producing the sample in the form of a thin slice (<NUM>) of the substrate by removing material on either side of the slice, the slice being oriented transversally to at least a number of the features (<NUM>), so as to visualize said features by TEM,
characterized in that the method further comprises, before the step of depositing the protective layer (<NUM>), a step of producing a contrast layer (<NUM>) on the topology by depositing a layer (<NUM>) of contrasting material locally in at least one target area (<NUM>) spaced apart from the patterned area (<NUM>), wherein the local deposition is performed by Electron Beam Induced Deposition applied only to the at least one target area (<NUM>), in such a manner that a portion of the contrasting material is deposited also around the target area (<NUM>), thereby forming a conformal layer (<NUM>), of the contrasting material, i.e. a layer that follows said topography, on at least some of the features in the patterned area (<NUM>).