WO2007025961A1

WO2007025961A1 - Method for preparation of a planar sample body and preparation

Info

Publication number: WO2007025961A1
Application number: PCT/EP2006/065766
Authority: WO
Inventors: Mathias Senoner; Martin Schmidt; Janko Theisen
Original assignee: BAM Bundesanstalt für Materialforschung und -prüfung
Priority date: 2005-08-31
Filing date: 2006-08-29
Publication date: 2007-03-08
Also published as: DE102005042075B3

Abstract

The invention relates to a method for preparation of a planar sample body (10) for a subsequent analysis and/or measurement of a cross-sectional area (24) of the sample body (10) and a preparation produced by said method. At least one sample surface (12) of the sample body (10) and at least one contact surface (16) of a metal body (14) have a planar connection by means of a metallic connection (22) arranged between the sample surface (12) and the contact surface (16).

Description

Method for the preparation of a flat specimen and preparation

The invention relates to a method for the preparation of a flat sample body, in particular of wafers or sheets, for a subsequent analysis and / or measurement of a cross-sectional area of the sample body. The invention further relates to a corresponding preparation, which can be produced in particular by the method.

The analysis of layers, depth profiles or the internal structure of flat, in particular plane-parallel samples in the micrometer or millimeter range, for example of sheet metal or semiconductor wafers, in many cases requires the preparation of cross sections of these samples. These cross-sections can then be examined by means of imaging methods and / or methods of chemical surface analysis. Most methods of surface analysis analyze with

Electron, ion or photon beams the material surface. Examples are Scanning Electron Microscopy (SEM), Auger Electron Spectroscopy (AES), Secondary Ion Mass Spectrometry (SIMS) and Electron Spectroscopy for Chemical Analysis (ESCA, XPS).

In the analysis of surface structures or near-surface structures with charged particles (electrons or ions), it is necessary to embed the cross-section of the specimen such that there are no three-dimensional edges in the vicinity of the cross-sectional area to be examined, that is, the cross-sectional area substantially planar embedded in an embedding material. Otherwise, such edges lead to inhomogeneous electric fields and thus image distortions. In addition, it is important that no charge of the sample occurs. In order to achieve a discharge of the charged particles, a conductive preparation of the sample to be examined with a ground fault is an essential prerequisite for an accurate imaging and analysis. Since some methods of chemical surface analysis require ultrahigh-vacuum conditions with pressures below ICT ⁹ mbar in the measuring chamber, the materials used for the preparation must be highly vacuum-compatible, that is, they must not outgas under these conditions.

In most cases, the examination of a material cross-section requires the preparation of a cross-section of the sample to be examined. Small samples, such as parts of sheets or semiconductor wafers, have to be fundamentally embedded for grinding and polishing because of their low material thickness. For this purpose, it is known to use epoxy resins as embedding. However, because epoxy resins are nonconductive, epoxies with conductive fillers such as silver flake or graphite powder are known for the conductive preparation of cross sections. However, it has been found that this measure does not completely solve the problem of electrical charging. Since the conductive fillers are present as isolated particles in the epoxy resin, there are still certain charges in the microscopic areas between the fillers, which make disturbing measurements. In addition, in some analyzers it is not possible to achieve the required ultrahigh vacuum with samples embedded in epoxy resin because the epoxy outgasses. Another known method of reducing charge is coating the epoxy-embedded sample with a thin gold film. The conductive gold layer leads to a distribution of electric charges on the entire surface of the cross section, thus reducing the effect of charging. However, prior to analysis, the gold layer must be removed again from the area to be analyzed. In addition, the applied gold layer is very sensitive to mechanical stress and does not always guarantee a short to ground. Since epoxy resins are also used in this type of preparation, this method is also suitable only for ultra-high vacuum analyzes.

From documents JP 2000-105 180 A, JP 03-243 844 A, JP 03-274 437 A, JP 2003-322 599 A and JP 09-166 526 A, methods for the preparation of specimens are known, in which a sample surface a specimen is combined with a contact surface of another body by means of a compound of adhesives, mostly epoxy resins. According to JP 2000-199 735 A, two similar materials are bonded by means of an adhesive, while in JP 09-210 885 A two dissimilar materials are bonded to each other by means of epoxy resin, wherein the substrate material is then etched away and the sample material is exposed. The disadvantages of epoxy resin compounds, in particular their lack of electrical conductivity, have already been mentioned above.

From DE 698 20 361 T2, a method for the preparation of specimens for the subsequent analysis is known, in which each one sample surface of two identical specimens are joined together via a compound of hardenable resin arranged between them. Furthermore, this document discloses a method for examining a laminate body, consisting of a silicon wafer with sputtered layers of titanium (Ti) and titanium nitride (TiN). This object to be examined is thinned by ion irradiation in places so far that a transmission electron microscopic examination is made possible by this point. A connection of the sample body with other bodies is not provided. Due to the high material thickness of the specimen no embedding is necessary here.

In US 6,361,626 Bl a method for the investigation of

Solder alloys, in particular Sn-Al-Zn and Sn-Bi-Al alloys, described. Two copper bodies are soldered together using these alloys and the solder joint is tested for various mechanical parameters. Here, therefore, the specimen itself forms the metallic connection of two bodies.

JP 2000-146783 A relates to a method for the investigation of epitaxially grown semiconductor wafers of gallium arsenide GaAs. In this case, a compound is produced by magnetic force to an iron body via an applied and magnetized nickel layer. Finally, the rear side of the GaAs layer remote from the nickel layer and the iron body is examined by mass spectroscopy. An examination of a cross-sectional area, however, does not take place. An electrostatic force-based compound is known from JP 2004-061 204 A.

The present invention is therefore based on the technical object of providing a preparation method of a flat sample body for a subsequent analysis and / or measurement of a cross-sectional area of the sample body which produces a preparation whose electric charge during a subsequent irradiation with charged particles is largely prevented and that is fully suitable ultra high vacuum. It is also intended to provide a corresponding preparation of a flat specimen.

This object is achieved by a method and by a preparation having the features of claims 1 and 15, respectively.

The method according to the invention provides that at least one sample surface of the sample body and at least one contact surface of a metal body are joined together in a planar manner via a metallic connection arranged between the sample surface and the contact surface. In this case, a flat specimen is understood as meaning a structure consisting of a metal or semiconductor with at least one substantially flat surface whose thickness (material thickness) is typically less than the extents of the at least one planar major surface of the specimen referred to herein as the specimen surface. Preferably, the sample body is a structure having at least two mutually opposite substantially plane-parallel sample surfaces, in particular with exactly two substantially plane-parallel sample surfaces, which are spaced from each other by the thickness d. The compound can be prepared by material or physical connection of the sample and metal bodies (see below). It is crucial that a flat, electrically conductive contact between sample and contact surface is formed.

The at least one metal body joined to the specimen forms an embedding necessary for subsequent processing, in particular grinding and / or polishing. On the other hand, by the electrically conductive metallic compound of the specimen with the metal body ensures the necessary ground fault, which prevents charging of the preparation during irradiation with charged particles. Another advantage of the method results from the virtually exclusive use of metallic components see. In fact, since the process does not require the use of polymeric materials, such as epoxy resin or adhesives, the conductive cross-sectional preparation according to the invention is of unlimited high vacuum capability.

A particularly preferred embodiment of the method provides to assemble two opposing sample surfaces of the sample body and each a contact surface of at least one metal body via a metallic compound. In this connection, in particular, each of the sample surfaces is joined together with one metal body each.

According to an alternative embodiment, a single metal body is used with a recess, which preferably has at least two contact surfaces. In this case, the sample body is inserted into the precisely fitting recess, so that in each case a metallic connection between the two opposite sample surfaces of the sample body and the two contact surfaces of the recess of the metal body is generated. In the case of a plane-parallel sample body, the (groove-shaped) recess of the metal body may have a tapering or widening cross section, so that a clamping voltage is applied to the inserted sample body.

A further advantageous embodiment of the method provides to assemble at least one of the sample surfaces of a first sample body with a sample surface of a second sample body via a metallic compound. Such a structure of two or more specimens can then on a further metallic compound in the manner described above with one or more metal bodies are joined together. In this context, it is particularly preferred to use specimens made of a same material or a material with similar properties, in particular similar hardnesses. In this way, the subsequent material removal is facilitated.

In all the variants listed above, the metallic compound is preferably produced by first producing a metallic coating on the at least one sample surface of the sample body and / or on the at least one contact surface of the metal body, preferably both on the sample surface and on the contact surface to be applied the metal body is applied in each case a metallic coating. Subsequently, the surfaces coated in this way are joined together and joined together. If the metallic coatings on specimens and metal body of the same material, the joining can be done in particular by welding, with diffusion welding is preferred due to its good material conservation. If different materials for the metallic coating on the sample surface on the one hand and the contact surface on the other hand used, the joining can be done by soldering. In addition to the material connection by welding or soldering, the metallic compound can alternatively be realized solely by physical contact, for example by adhesion, by the flat sample and contact surfaces (of which at least one is metallically coated) are held under pressure with a pressing force. The production of a metallic connection between two specimens takes place in an analogous manner. The metallic compound or the metallic coating on the sample surface and / or the contact surface consists essentially of a metal or a metal alloy or a eutectic mixture, with materials with low tendency to oxidation and high electrical conductivity, in particular a noble metal or a noble metal-containing alloy are preferred , For example, gold or eutectic alloys, for example gold / tin (Au / Sn), have proven to be very suitable.

According to a preferred embodiment, the cross-sectional area of the specimen to be analyzed is produced by subsequent material removal of the structure, consisting of at least one specimen, at least one metal body and the corresponding metallic connections between them. In particular, this structure is ground and / or lapped and / or polished, so that a flat and smooth cross-sectional area of the sample body is formed.

According to an alternative embodiment, the sample body has a smooth fracture surface, so that the cross-sectional area of the sample body to be analyzed is generated by being flush-mounted with the at least one contact surface of the at least one metal body in the manner described.

The specimen is typically a wafer, in particular a semiconductor wafer, for example of silicon (Si) or gallium arsenide (GaAs). In principle, however, any planar structure, for example a metal sheet, can be prepared with the method. Particularly good results are achieved with material thicknesses of at most 10 mm, in particular of at most 1 mm. Typical thicknesses of Semiconductor wafers are 100 to 1000 microns, especially 300 to 600 microns. There is no lower limit on the thickness of the specimens, so that specimens with a thickness in the micrometer range or less can be used.

According to a particularly advantageous embodiment of the invention, the sample body has a pattern as a focusing aid and / or calibration aid for a measuring device, in particular a spectrometric or microscopic device on. In particular, the cross-sectional area of the sample body may be a

Have striped pattern of at least two distinguishable materials with defined intervals, which allows a determination of the lateral resolution of the measuring device used. A preferred arrangement of stripe patterns in the nanometer range is described in the earlier application DE 10 2005 009 514.3, the contents of which are fully incorporated in the present application.

The invention further relates to a preparation which can be produced according to the method described above and which comprises at least one flat specimen with at least one specimen surface and at least one metal body with at least one contact surface, wherein the at least one specimen surface of the specimen and the at least one contact surface of the specimen via a metallic compound layer are joined together flat.

Further preferred embodiments of the invention are the subject of the remaining dependent claims.

The invention will be explained in more detail in embodiments with reference to the accompanying drawings. Show it: Figure la-d process steps for producing a cross-sectional preparation according to a first embodiment of the invention;

2 shows a finished cross-sectional preparation after a second

Embodiment of the invention;

Figure 3 process steps for the preparation of a cross-sectional preparation according to a third embodiment of the invention and

4 shows a finished cross-sectional preparation according to a fourth embodiment of the invention.

According to the process variant shown in Figure 1 is a

Sample body 10, each metallically connected to a metal body 14. The specimen 10 is a semiconductor wafer or a portion thereof, for example of silicon or gallium arsenide, with two substantially plane-parallel, with a thickness d spaced sample surfaces 12 (Figure Ia). The thickness d of the specimen in the present example is about 400 μm. Each of the sample surfaces 12 is connected to a respective contact surface 16 of a metal body 14. The material of the metal body 14 is preferably selected so that its mechanical properties, in particular its hardness, are as similar as possible to the material of the specimen. The material of the metal body 14 should also be electrically conductive and non-magnetic. In the case of Si or GaAs wafers, for example, stainless steel has proven itself. In this case, the steel composition can be matched to the mentioned material properties of the sample. In the illustrated example, both the sample surfaces 12 of the sample body 10 and also the contact surfaces 16 of the metal bodies 14 are each provided with a metallic coating 18 or 20 (FIG. 1b). The metallic coatings 18, 20 consist for example of gold or of a eutectic mixture, wherein the coatings 18 of the sample body 10 may have a different material composition than the coatings 20 of the metal bodies 14.

In accordance with the arrow directions shown in FIG. 1b, the components are joined to one another on their coated side surfaces 12 and 16. Subsequently, the connection of the metallic coatings 18, 20 by welding (in the case of similar metallic coatings 18, 20), in the present example by diffusion welding. For this, the entire structure is subjected to a contact pressure and stored over, for example, one day at a temperature below the melting temperature of the coating material, here at 400 ^° C. The result is shown in Figure Ic, wherein the generated metallic connections between the sample body 10 and the metal bodies 14 are designated 22.

To produce a cross-sectional area of the specimen 10, the entire structure shown in Figure Ic, consisting of the specimen 10, the two metal bodies 14 and the interposed metallic compounds 22 is first ground and / or lapped and finally polished. The resulting cross-sectional preparation 100, which has the cross-sectional area 24 to be analyzed at least on one side, is shown in FIG.

Also, according to the embodiment of the present invention shown in Figure 2, both sample surfaces 12 of the Sample body 10, each with a metal body 14, 14 a via metallic connections 22 joined together. In contrast to the example shown in FIG. 1, however, one of the two metal bodies (14a) has an L-shaped longitudinal section, so that a further metallic connection 26 is present between the lower leg of the metal body 14a and the sample body 10 and the second metal body 14 , The additional metallic connection 26 between the two metal bodies 14, 14 a leads to an increase in the mechanical stability of the preparation 100.

According to the exemplary embodiment illustrated in FIG. 3, the specimen body 10 is connected to a single metal body 14 which has a recess 28 which has two opposing contact surfaces 16. FIG. 3 a shows the corresponding process stage in which both the sample surfaces 12 of the sample body 10 with a metallic coating 18 and the contact surfaces 16 of the metal body 14 with a metallic coating 20 are already provided. The groove 28 preferably has an increasing thickness in the direction of the metal body bottom, that is to say in the present illustration. In order to insert the coated sample body 10 in the recess 28, the metal body 14 is slightly spread apart. The wedge-shaped configuration of the recess 28 has the advantage that a loading of the structure with an external contact pressure for the subsequent welding is eliminated. Alternatively, this design also makes it possible to dispense with a material connection by welding or soldering and instead to generate the connection exclusively physically by frictional connection, by holding the specimen body 10 under clamping stress of the metal body 14 in its groove 28. FIG. 3b shows the microstructure after the welding (or the frictional connection), wherein the metallic see connections are again denoted by 22. This is followed by grinding and polishing of the surface (result not shown).

FIG. 4 shows a finished cross-sectional preparation 100 in which a first flat specimen 10 and a second specimen 10 a of the same material are joined together. For the preparation of the sample body 10 and 10a were first coated with a metal layer, joined together and welded. This package was then connected to the two metal bodies 14 according to the procedure described with reference to FIG. Of course, the embodiments shown in FIGS. 2 and 3 can also be carried out with two or more sample bodies 10.

LIST OF REFERENCE NUMBERS

10 specimen 10a second specimen

12 sample area

14 metal bodies

14a metal body

16 contact surface 18 metallic coating of the specimen

20 metallic coating of the metal body

22 metallic compound

24 cross-sectional area

26 metallic connection 28 recess

100 preparation

Claims

claims

1. A method for the preparation of a flat specimen

(10) for a subsequent analysis and / or measurement of a cross-sectional area (24) of the sample body (10), characterized in that at least one sample surface (12) of the sample body (10) and at least one contact surface (16) of a metal body (14) surface via a between the sample surface (12) and the contact surface (16) arranged metallic compound (22) are joined together.

2. The method according to claim 1, characterized in that the sample body (10) has at least two substantially plane-parallel sample surfaces (12), in particular exactly two.

3. The method according to any one of the preceding claims, characterized in that two opposing sample surfaces (12) of the sample body (10) and each a contact surface (16) at least one metal body (14) via a respective metallic compound (22) are joined together , in particular each having a metal body (14) per sample surface (12).

4. The method according to claim 3, characterized in that the sample body (10) is joined together with at least two metal bodies (14) and between the at least two metal bodies (14). Body (14) at least one interface, a metallic compound (26).

5. The method according to any one of the preceding claims, characterized in that the sample body (10) in a at least two contact surfaces (16) having recess (28) of a metal body (14) is inserted and in each case a metallic compound (22) between two sample surfaces ( 12) and the two contact surfaces (16).

6. The method according to any one of the preceding claims, characterized in that the sample body (10) via at least one sample surface (12) with a second sample body (10a), in particular of the same material or a material having similar properties, via a metallic compound (22 ) is connected.

7. The method according to any one of the preceding claims, characterized in that the metallic compound (22) by generating a metallic coating (18) on the at least one sample surface (12) of the sample body (10) and / or a metallic see coating (20 ) is produced on the at least one contact surface (16) of the metal body (14) and subsequent joining.

8. The method according to any one of the preceding claims, characterized in that the joining by material connection, in particular by welding or soldering, or by physical connection, in particular by adhesion, takes place.

9. The method according to any one of the preceding claims, characterized in that the metallic compound (22) consists essentially of a metal or a metal alloy, in particular a noble metal or a noble metal-containing alloy.

10. The method according to any one of claims 1 to 9, characterized in that the to be analyzed cross-sectional area (24) of the sample body (10) by subsequent machining material removal, in particular by grinding and / or lapping and / or polishing, is generated.

11. The method according to any one of claims 1 to 9, characterized in that the to be analyzed cross-sectional area (24) of the sample body (10) is a smooth fracture surface and the at least one sample surface (12) of the sample body (10) and the at least one contact surface ( 16) of the metal body (14) are joined flush.

12. The method according to any one of the preceding claims, characterized in that the sample body (10) has a thickness (d) of at most 10 mm, in particular of at most 1 mm.

13. The method according to any one of the preceding claims, characterized in that the sample body (10) is a semiconductor wafer or a portion of such.

14. The method according to any one of the preceding claims, characterized in that the sample body (10) has a pattern as a focusing aid and / or calibration aid for a measuring device, in particular a spectrometric or microscopic device.

15. A preparation (100), which can be produced by a method according to one of claims 1 to 14, comprising at least one flat sample body (10) with at least one sample surface (12), at least one metal body (14) with at least one contact surface (16) the at least one sample surface (12) and the at least one contact surface (16) are joined together flat over a metallic connecting layer (22).