Method for preparing a cross-section with a focused ion beam

Abstract

Method for preparing a cross-section in a substrate (1) with at least one focused ion beam (2), wherein a cut surface (3) is generated in the substrate (1) with the ion beam (2), wherein before and during the generation of the cut surface (3) a surface area of ​​the substrate (1) at the edge of the cut surface (3) is protected with a hard mask (4), which is provided as a single component and positioned at the edge of the cut surface (3) with at least one micromanipulator, wherein the hard mask (4) is a hard mask made of doped silicon and the hard mask (4) is not fixed to the substrate (1), but is held in place with the micromanipulator during the generation of the cut surface (3).

Description

Technical application area

[0001] The present invention relates to a method for preparing a cross section in a substrate, in which a cutting surface is produced in the substrate using at least one focused ion beam, wherein before and during the production of the cutting surface, a surface region of the substrate at the edge of the cutting surface is protected with a hard mask, which is provided as a single part and is positioned at the edge of the cutting surface using at least one micromanipulator.

[0002] Focused ion beams are suitable for producing cross-sections in substrates, which can then be examined more closely, for example, using electron microscopy. FIB (Focused Ion Beam) systems are used, among other things, in the semiconductor industry for process control or failure analysis. The focused ion beam can be used to prepare a cross-section locally at a desired location on the substrate without having to break or grind the substrate, such as a wafer, or the respective component. By creating a suitable cutting surface with the focused ion beam, only individual components or test structures on the substrate are destroyed.

[0003] In rare cases, single-beam systems are used, in which only one ion column is installed vertically, and dual-beam systems, in which a second, tilted ion column is arranged next to a vertically installed electron column. This allows non-destructive cross-sectional images to be taken using SEM (scanning electron microscopy) after the cross-section has been prepared. The substrate support or stage in these systems can be tilted accordingly.

[0004] When creating cross-sections for material analysis using FIB, a protective layer or mask must be applied to protect the surface before the cross-section is created. This serves to protect the uppermost material layers, as well as the upper region, to prevent edge rounding and thus measurement errors. State of the art

[0005] Two different techniques are currently known for protecting surfaces and preventing edge rounding. The first technique involves depositing a protective layer on the surface edge of the cut surface to be created, as described, for example, in T. Ishitani et al., "Cross-sectional sample preparation by focused ion beam: a review of ion-sample interaction", Microsc Res Tech. 1996, 35(4), pages 320-333. Deposition is carried out using a dual-beam system with the aid of an electron beam and an ion beam from the gas phase. Since deposition of the layer with an ion beam would damage the sample surface, a first thin layer (layer thickness approximately 200 nm) must first be deposited using the electron beam. A second layer with a greater thickness (layer thickness approximately 3 to 8 µm) is then deposited onto this layer using gas-induced deposition with the aid of the ion beam.The deeper the cross-sections to be created extend into the substrate or the harder the substrate material, the thicker this layer must be. The total deposition time varies from approximately 20 minutes for standard sections (approximately 15 µm wide) to 60 minutes for larger cross-sections (approximately 70 µm wide).

[0006] In the second known technique, instead of the protective layer, a cuboid-shaped individual part made of a suitably hard material is fixed to the surface area as a hard mask, i.e., glued or welded. This part is positioned at the desired location using a lift-out system of the FIB system, i.e., a micromanipulator, and fixed to the surface at the edges using gas-assisted deposition or electron beam-sensitive resist. The needle of the micromanipulator to which this part is attached is cut off and retracted after fixation. This technique reduces the long deposition times, since the fixation time is only approximately 15 minutes, regardless of the size of the cross-section to be created.

[0007] Both techniques utilize gas-induced deposition processes, which are also time-consuming. The deposition of material also creates contamination on the substrate surface, which leads to contamination problems, particularly in semiconductor manufacturing with regard to semiconductor materials and cleanroom processing. In the worst case, the wafer can no longer be used, resulting in significant losses, especially with expensive semiconductor materials.

[0008] In T. Vermeij et al., "Preventing damage and redeposition during focused ion beam milling: The "umbrella" method", Ultramicroscopy 186 (2018), pages 35-41, a method for surface protection during ion beam milling is described. A micromanipulator is used to press a hard mask made of PDMS onto the surface to be protected in order to prevent unwanted ion implantation or material deposition on the surface.

[0009] The object of the present invention is to provide a method for preparing a cross-section in a substrate using a focused ion beam, which requires less time and causes no or only reduced contamination of the substrate surface compared to the known techniques. Description of the invention

[0010] The object is achieved by the method according to patent claim 1. Advantageous embodiments of the method are the subject of the dependent patent claims or can be derived from the following description and the exemplary embodiment.

[0011] In the proposed method for preparing a cross-section in a substrate, a cutting surface is created in the substrate using at least one focused ion beam, which can then be examined, for example, using electron microscopy. Before the cutting surface is created with the ion beam, a surface region of the substrate at the edge of the cutting surface is protected with a hard mask, which is provided as a preferably cuboid-shaped individual part and positioned at the edge of the desired cutting surface using at least one micromanipulator. The proposed method is characterized in that a hard mask made of doped silicon is used and that the hard mask is not fixed to the surface of the substrate, i.e. not connected to it, but is held in the appropriate position with the micromanipulator during the creation of the cutting surface.After the cutting surface has been created, or after a subsequent examination of the cutting surface, this hard mask is then removed from the substrate using the micromanipulator.

[0012] The proposed method therefore eliminates the need for gas-induced deposition processes to create a protective layer or to fix the hard mask provided as a single part. This eliminates the processing times for these deposition processes. Most importantly, there is no risk of contamination of the substrate surface by materials used for deposition.

[0013] In a particularly advantageous embodiment, the hard mask is not placed on the surface of the substrate during the process, meaning it does not come into contact with it. Instead, the underside of the hard mask remains at the smallest possible distance from the substrate surface, preferably less than 1 µm. Distances in the range of 100 or several hundred nm are optimal. Even with such a distance from the surface, or with porous material or a structured surface, it is possible to create corresponding cross-sections in this way. The hard mask floats above the substrate surface and does not introduce any contamination onto it.

[0014] In an alternative embodiment, the hard mask can also be placed on the substrate surface and held in the appropriate position using the micromanipulator. The electrically conductive design of the hard mask makes it possible to avoid charging of the substrate by the ion beam. FIB processing applies a large amount of electrical charges to the substrate, whether during processing by the ions (positively charged) or during the optional subsequent irradiation by electrons (negatively charged) when creating an SEM image. In the latter case, the hard mask remains on the substrate until after this image has been taken. The electrical charges are then discharged via the hard mask and the micromanipulator. The charging of the substrate surface in cases of low or poor conductivity of the substrate surface (e.g.Otherwise, contact with the needle of the micromanipulator via the electrically conductive hard mask, which is grounded via the needle and the micromanipulator, allows the applied charges to flow away, significantly improving the homogeneity and quality of the cross-section compared to processing without contact between the hard mask and the substrate. In this embodiment, too, contact between the hard mask and the substrate is released by lifting it after the cross-section has been created or, if necessary, after performing an SEM image or similar. This means that no contamination remains on the surface of the substrate.

[0015] The hard mask material is compatible with semiconductor manufacturing processes, thus preventing foreign contamination in semiconductor cleanrooms. Silicon hard masks meet the requirements for ion beam hard masks and can be manufactured with reasonable effort. Silicon hard masks are preferably doped to achieve an electrical conductivity in the range between 1 S / m and 10 5 S / m is obtained. By using a material that does not represent foreign contamination in semiconductor cleanrooms, the material removed from the hard mask by the ion beam during sputtering does not represent a contamination of the substrate surface during semiconductor manufacturing.

[0016] The hard masks used in the proposed method as individual parts, in particular as blocks or plates, preferably have an edge length in the range of 10 to 1000 µm. The edge length here is understood to be the length of the edge of the preferably cuboid-shaped individual part that extends along the edge of the cutting surface. The edge length must be selected so that it corresponds at least to the width of the cutting surface at the substrate surface. When using Ga-FIB systems, hard masks with edge lengths of 20 to 30 µm are often used due to the areas of application of these systems; for plasma FIB systems, hard masks with edge lengths in the range of 400 to 500 µm are often used. The thickness of the hard masks in the direction perpendicular to the substrate surface can be on the order of magnitude of the thickness of corresponding protective layers, for example in the range between 3 and 25 µm.The width of the hard masks (perpendicular to the thickness and the cutting edge) can be chosen almost arbitrarily, as long as the hard masks are still manageable. The wider the hard masks are chosen, the more frequently they can be used for this type of process.

[0017] Typical micromanipulators in FIB systems have a needle, to the end of which the hard mask is then attached. Attachment can be achieved, for example, by gas-assisted deposition onto the contact point between the hard mask and the needle, or even without gas inlet using targeted redeposition sputtering with the ion beam. This connection then remains intact during cross-section creation until the hard mask is removed from the substrate by the micromanipulator. It is also possible to use a micromanipulator that picks up and positions the hard mask in another way, for example, using a clamping device.

[0018] The proposed method can be used in all technical fields where a cross-section needs to be created locally in a substrate, which then allows for examination, for example, using SEM. Examples include life sciences, materials science, and semiconductor technology. Suitable FIB systems and lift-out systems, i.e., corresponding micromanipulators, are used in these fields, from development in research institutes through production in factories to failure analysis. For example, every semiconductor component manufacturer's factory has several FIB systems available, which produce cross-sections in parallel in shift operation for process control. The method can be implemented in all existing FIB systems with lift systems.Since the parts serving as hard masks can be manufactured with different dimensions, the process can be used in both the widely used Ga-FIB systems and in plasma FIB systems such as those based on Xe. Short description of the drawings

[0019] The proposed method is briefly explained below using exemplary embodiments in conjunction with the drawings. Herein: Fig. 1A / B a schematic representation of a first embodiment of the proposed method; Fig. 2A / B a schematic representation of a second embodiment of the proposed method; and Fig. 3A / B a schematic representation of the generated cutting surface after carrying out the method according to Fig. 2. Ways to implement the invention

[0020] In the proposed method, a focused ion beam from an FIB system is used to locally create a cross-section in a substrate, for example a semiconductor substrate, which can then be analyzed, for example, using SEM. In the method, a hard mask is used as a single part, e.g. in the form of a block, to protect the surface area of ​​the substrate adjacent to the cutting surface and to prevent edge rounding. The hard mask is not fixed to the substrate surface or firmly bonded to the substrate surface as in previously known techniques, but is left on the manipulator, in particular the manipulator's needle, positioned with it, and held in this position during processing.

[0021] Fig. 1A / B shows a highly schematic first embodiment of the method, in which the substrate 1, the focused ion beam 2, and the cutting surface to be created in the substrate 1 are indicated. In this example, the hard mask 4 made of doped silicon, which is attached to the needle 5 of the micromanipulator, is positioned over the substrate before processing begins such that the lower right edge in this example lies exactly over the edge of the cutting surface to be created. In this example, the positioning is such that the underside of the hard mask 4 is kept at a small distance from the surface of the substrate 1, thus not touching it during processing. The hard mask 4 therefore floats above the substrate 1 during processing and does not introduce any contamination onto the substrate surface.The silicon removed from the mask 4 by sputtering during processing by the ion beam 2, which impinges on the substrate 1, does not represent foreign contamination in semiconductor cleanrooms. The hard mask 4 is held in this position throughout the entire processing, i.e., the creation of the desired cutting surface, and is only subsequently removed again with the micromanipulator. In this way, the hard mask 4 can be reused multiple times. Fig. Figure 1A shows a design with a single-beam FIB system in which only one ion column is installed vertically, Fig. 1B shows a configuration with a two-beam FIB system, which, in addition to a vertically installed electron column, has a tilted ion column. In both configurations, the hard mask 4 must be attached to the needle 5 of the micromanipulator at different angles, as can be seen from the Fig. 1A and Fig. 1B can be seen.

[0022] In a different embodiment of the proposed procedure, as described in Fig. As shown schematically in Figures 2A / B, the hard mask 4 is not held at a small distance above the substrate surface by the manipulator during processing, but is placed onto it and then held in this position by the micromanipulator. Due to the doping of the silicon hard mask 4, as is also used in this example, it has electrical conductivity, which prevents the substrate 1 from becoming charged during processing with the ion beam. The hard mask 4 is grounded accordingly via the needle 5 and the micromanipulator. Following the creation of the cutting surface, the hard mask 4 is again removed from the substrate 1 using the micromanipulator and can be reused several times. Fig. 2A again shows a design with a single-beam FIB system in which only one ion column is installed vertically, Fig. 1B shows a configuration with a two-beam FIB system, which, in addition to a vertically mounted electron column, has a tilted ion column. In both configurations, the hard mask 4 must be attached to the needle 5 of the micromanipulator at different angles, as can be seen from the Fig. 2A and Fig. 2B can be seen.

[0023] The hard mask can be used especially in the design of the Fig. 2 also remain on the substrate for the creation of an SEM image of the cut surface, in order to prevent charging of the substrate 1 by the electron beam. Two-beam FIB systems have both an ion beam column and an electron beam column tilted against it, in order to then analyze the cut surface prepared with the ion beam by creating an SEM image with the electron beam. Fig. 3A / B shows a highly schematic representation of the cutting surface 3 created with the ion beam and the electron beam 6 falling on this cutting surface to create a SEM image, each for the single-beam FIB system of the Fig. 2A ( Fig. 3A) and for the two-beam FIB system of the Fig. 2B ( Fig. 3B). The ion beam was used to create a correspondingly larger depression in the substrate 1, one side wall of which represents the desired cutting surface 3, as shown in Fig. 3A / B is indicated.

[0024] In the process design without contact between the hard mask and the substrate surface, the hard mask material prevents contamination of the substrate surface during cleanroom processes in semiconductor manufacturing. Compared to the previous techniques mentioned in the introduction to the description, the process saves time, ranging between 15 and 60 minutes depending on the technique. The hard masks used in the process can be reused multiple times, thus reducing preparation costs. The other process design, in which the hard mask is applied to the substrate surface, offers the same advantages. In addition, this design can reduce substrate charging by dissipating the electrical charges via the hard mask, which is grounded via the micromanipulator.The process enables the processing not only of semiconductor substrates, but also of substrates made of other materials such as ceramic, metal, polymer, or composite materials. The process can also be used to create lamellae in the substrate, which can then be separated and analyzed using, for example, a TEM. The production of lamellae requires the creation of two parallel cut surfaces at a short distance from each other, corresponding to the lamella thickness. List of reference symbols 1 substrate 2 focused ion beam 3 Cutting surface 4 Hard mask 5 Needle of the micromanipulator 6 Electron beam

Claims

[1] Method for preparing a cross section in a substrate (1) with at least one focused ion beam (2), in which a cutting surface (3) is produced in the substrate (1) with the ion beam (2), wherein before and during the production of the cutting surface (3) a surface region of the substrate (1) at the edge of the cutting surface (3) is protected with a hard mask (4), which is provided as a single part and is positioned at the edge of the cutting surface (3) with at least one micromanipulator, wherein a hard mask made of doped silicon is used as the hard mask (4) and the hard mask (4) is not fixed to the substrate (1), but is held with the micromanipulator during the production of the cutting surface (3). [2] Method according to claim 1, characterized by that a hard mask made of doped silicon is used as the hard mask (4), which has an electrical conductivity between 1 S / m and 10 5 S / m. [3] Method according to one of claims 1 or 2, characterized by that the hard mask (4) is firmly connected to a needle (5) of the micromanipulator before positioning. [4] Method according to one of claims 1 to 3, characterized by that the hard mask (4) is positioned at a distance above the surface area which is less than 1 µm. [5] Method according to one of claims 1 to 4, characterized by that the hard mask (4) has an edge length of an edge of the hard mask (4) positioned at the edge of the cutting surface (3) which is between 10-600µm. [6] Method according to one of claims 1 to 5 for preparing the cross section in a semiconductor substrate in semiconductor manufacturing.

Images