WO2017186198A1

WO2017186198A1 - Method for characterization of a sample surface by using scanning electron microscope and scanning probe microscope

Info

Publication number: WO2017186198A1
Application number: PCT/CZ2017/000031
Authority: WO
Inventors: Jan NEUMAN; Zdenek NOVACEK; Michal PAVERA
Original assignee: Nenovision S.R.O.
Priority date: 2016-04-27
Filing date: 2017-04-27
Publication date: 2017-11-02

Abstract

A method of a sample surface characterization by a scanning electron microscope (SEM) and a scanning probe microscope (SPM), characterized in that the scanning of the sample surface by the scanning electron microscope and the scanning probe microscope is carried out simultaneously, wherein the primary electron beam is stationary while the sample table is moved in the XY plane during the scanning and the SPM probe is stationary in this XY plane. The table, which is typically a piezo scanner, therefore moves in the XY plane and in the direction of Z axis, or only in the XY plane, where the movement in the direction of Z axis is carried out by the SPM probe.

Description

SAMPLE SURFACE CHARACTERIZATION METHOD BY SCANNING ELECTRON MICROSCOPE AND SCANNING PROBE MICROSCOPE

Field of the Invention

The invention relates to a method of surface characterization of an examined sample by means of a scanning electron microscope and a scanning probe microscope.

Background of the Invention

Background of the invention comprises usage of various microscopy methods of the sample surface examination. The utilized methods are usually used in the individual devices and it is necessary to transfer the samples between the individual devices which can negatively affect the entire measuring, because the sample is exposed to the environment and this can lead for example to the sample surface oxidation etc. If more measuring methods are implemented into a single device, it is usually for the purposes of control of one of the measurements or for finding the suitable sample area. Techniques of the scanning electron microscopy (SEM) and methods summarized under the term scanning probe microscopy (SPM) are often used for characterization and visualization of the surfaces. Both approaches have their strengths and limiting factors.

The scanning electron microscope, further also only SEM, is an instrument designed for observation of microscopic and nanoscopic objects. A primary electron beam is concentrated on the examined sample by a system of electromagnetic lenses. There are various phenomena occurring during the interaction of the primary electrons with the sample, many of which can be used for creation of the resulting image. The most commonly utilized phenomenon is detection of secondary electrons, the electrons ejected from atoms of the sample by the primary electrons, wherein the penetration depth can be up to hundreds of nanometers. Secondary electron detectors, most commonly of the Everhart-Thornley type, are for the secondary electron detection located in the area above the sample outside the optical axis of the microscope.

The second most common type of detected electrons are backscattered electrons, which are the primary beam electrons that have been deflected by scattering processes so that they fly out of the material and can be detected. The most commonly used detectors for detection of the backscattered electrons are scintillation type detectors or semiconductor detectors utilizing phenomenon of the electron-hole pair recombination. A diffraction pattern generated by the backscattered electrons can be also detected in some applications, in such cases an EBSD (electron backscatter diffraction) detector is used.

Various types of radiation can be generated during the dispersion and precipitation processes within the sample, out of which X-ray and cathodoluminescent radiation are used for the detection. X-ray radiation is generated either as a result of the deceleration of the primary electrons, or due to the characteristic atomic transitions due to the ejection of the electrons from the inner electron shells. Characteristic X-ray radiation can be used for qualification of the elements contained in the sample. Qualification of the X-ray radiation can be performed through its energy (EDX - energy dispersive X-ray) or wavelength (WDX - wavelength dispersive X-ray). Scintillation detectors or a recently widespread multi-channel proportional detector are used for X- ray radiation detection.

Cathodoluminescent radiation occurs in semiconductors and on the borders of mineral grains. It is a result of ejection or excitation of electrons from outer atom shells. The consequence of this is the creation of the radiation amount in the visible to ultraviolet portion of the spectrum. These amounts can either be conducted by the optical fiber out of the microscope chamber, where they are further specified by monochromators, or they can be detected by a CCD chip.

Another method of elemental analysis in the scanning electron microscopes is the use of Auger electrons. These electrons are created as a result of the electron ejection from the inner atom shells, wherein the energy released by the subsequent re-filling of the shell is not radiated in the form of radiation amount but it is carried by another electron of the inner shell in the form of its kinetic energy.

The image in the scanning electron microscope is generated by deflection of the primary beam in the defined area by moving the primary beam in lines (screening/scanning) over the static sample, wherein the resulting image is generated by combining the individual lines together. The size of the sample surface scanning area is typically in micrometers with lateral resolution often up to nanometers. Although it is possible to observe a certain three-dimensional aspect in the images taken by the scanning electron microscopes, it is not appropriate to use them as source of information about surface topography.

A focus ion beam (FIB) is often used for the purposes of depth profiling, optionally for the formation of micro and nanostructures. Accelerated ions (mostly of heavy metals) are, as in the case of the primary electron beam, centered on the area where they sputter original atoms, thereby creating craters or defects. The resulting defects, optionally structures, can be further immediately visualized by the electron beam. The problem in this case remains in the inability to measure the depth of craters or defects, and to measure the entire profile of the examined area.

Scanning probe microscopy, further also only SPM, is a common name for dozens of different methods functioning on the principle of detection of changes in the interaction extent between the probe and the sample surface. The SPM probe most commonly consists of a cone-shaped tip measuring several tens of nanometers, attached to an arm measuring several hundreds of micrometers, which is attached to a rail controlled by piezo-crystalline manipulators.

When moving the probe over the sample surface, there is an interaction between the tip and the sample surface (nature of the interaction varies in the individual methods), on which basis the tip is brought closer or further away from the surface by means of feedback. In this way, a mapping of the surface relief is achieved. Similarly to the electron microscopy, the probe is guided so as to scan the specified area line by line, wherein the resulting image is generated by merging individual measurements into one map. Lateral resolution of the SPM is strongly dependent on the scanning speed and tip dimensions, and it may vary up to the atomic level depending on the used method.

The scanning of the sample surface is achieved by probe movement or sample movement, depending on the used microscopy method. Possibilities of moving the probe in the directions of X, Y and Z axes over the static sample, as well as moving the sample in the directions of X, Y and Z axes under the static probe are known from the state of the art. Combination is also possible, in which the sample can be moved in the X and Y plane and the probe is moved only in the direction of the Z axis.

Because of the temporal instability of the scanned samples, the disadvantage of separate dedicated measuring devices is represented mostly by the amount of time needed for the successive measuring by the first method and then by the second one. If the sample is scanned by both methods in separate devices or chambers, the susceptibility of the sample to damage is greater during its handling between chambers or devices. When moving the sample from one device to another, the sample is exposed to the environment and, for example, oxidation of its surface may occur. Measurement of the sample after the transfer between devices does not take place under the same atmospheric conditions, which is a major disadvantage.

Contemporary scanning electron microscopes and scanning probe microscopes are built and utilized mostly as separate devices (methods). Only in isolated cases it is possible to encounter systems with different technologies integrated in a single device, wherein either scanning probe microscope scans moving sample or the sample is fixed and scanned by moving primary electron beam. Cases in which the electron microscopy and the scanning probe microscopy systems were implemented in one device are listed below. Patent JP4223971 describes a device for sample observation by methods of scanning electron microscopy and scanning probe microscopy. The subject-matter of the invention are two holders for holding the sample, one of which is rotatably movable. Elimination of distracting vibrations is achieved by a special attachment of the sample. The sample is in this device placed on a table movable in the directions of X and Y axes. The potential movement in the direction of Z axis is performed during the scanning in the scanning probe microscope mode. The table is fixed during the scanning in the scanning electron microscope mode. However, the document does not in any way mention the possibility of simultaneous scanning of the sample, the described procedures essentially exclude this option, or the possibility of merging the resulting images together.

Patent KR101382111 describes a device for measuring electrical and mechanical properties of the sample. It is a combination of the scanning electron microscope and the scanning probe microscope. Local measuring of electrical and mechanical properties of the sample is performed by attaching a SPM probe with a tip capable of moving in the directions of X, Y and Z axes to the fixed table for sample attachment, wherein this SPM probe is connected to the sample by electrical circuit. Further, the sample surface is scanned and the conductivity is mapped, or more precisely, there is a mechanical contact with the sample and a deformation mapping. However, these measurements are not used to complete the image taken by the electron microscope.

Patent EP0899561 describes the possibility of simultaneous imaging of the resulting images obtained by the electron beam of the scanning electron microscope and by the probe of the scanning probe microscope. However, the scanning device described in the document contains several vacuum chambers, each containing one part of the system, i.e., the first chamber contains a holder with SPM probe, the second one contains a sample holder, and the third one contains a table for SPM probe and sample. Both the sample and the SPM probe have to be transported to the table by manipulators, resulting in system expansion and in time lag between setup of the sample and the measurement itself. US4874945 describes a combination of transmission electron microscope

(TEM) and scanning probe microscope (SPM). The beam of the transmission electron microscope impinges the surface with normal line perpendicular to the axis of the optical microscope, thereby reflecting the electrons from the surface. The SPM probe is introduced perpendicularly to the surface. The resulting image therefore displays both the surface and the probe. Another image shows scanning result of the scanning probe microscope. However, due to the different rotation of the sample in both measurements, the images cannot be merged and therefore there the result does not contain any extra information.

When measuring by the method of scanning tunneling microscopy (STM), only a small area of the sample is scanned. In practice, it may be very difficult to find the required area for scanning by the scanning tunneling microscope. Therefore, the document US5081353 provides a possibility of combining methods of the scanning electron microscopy and scanning tunneling microscopy in such a way that the electron microscope simultaneously images a larger area of the sample surface together with the tip of the STM probe which can be placed on the selected area of the sample surface on the basis of the image from the scanning electron microscope. In combination with the markers which have to be applied on the sample, the user can relatively easily navigate and find the desired area to be measured. The disadvantage of this arrangement lies in the necessity of application of the markers on the sample. Moreover, imaging by the scanning electron microscope does not provide any information about the sample structure, it is used only for orientation.

Similarly to the previous case, document US5229607 protects a combination of the scanning electron microscope and the scanning probe microscope in one chamber. The scanning electron microscope serves for orientation on the surface of the examined sample and for finding a suitable area for scanning of the sample surface with the scanning probe microscope, thereby simultaneously scanning both the sample surface and the probe tip of the scanning probe microscope. Although the table on which the sample is placed has the possibility of moving in the directions of X, Y and Z axes, this movement is not employed when scanning the sample surface with the scanning electron microscope and the electron beam is deflected by deflection coils in the microscope tube, thus the table with sample remains at rest. The table with sample moves when measuring by AFM method (Atomic Force Microscopy) utilizing Van der Waals forces for the detection of, for example, sample surface structure or its irregularities. The movement for the measurement by the scanning tunneling microscopy is enabled by a mounting for the STM probe. Document US525686 protects a possibility of imaging the image from the scanning tunneling microscope into the image of the scanning electron microscope, but the individual images are scanned separately. The chamber of the scanning electron microscope contains a table movable in the directions of X and Y axes. A stand with the sample is placed on the table in a way that the normal line of the sample surface forms a nonzero angle with the electron beam axis. Simultaneously, the scanning tunneling microscope is installed in the chamber at such angle that the tip of STM is perpendicular to the sample surface. When scanning by the scanning electron microscope, the beam is deflected in the microscope tube. Subsequently, measuring by the scanning tunneling microscope is performed in the same area as the measuring by the scanning electron microscope, wherein the image generated by the scanning tunneling microscope is imaged into the image taken by the scanning electron microscope.

In the prior art solutions, the scanning electron microscopes are used for informative imaging of larger areas of the sample surface (in the order of up to cm²) and for defining the area of interest on the sample (typically 1-50 urn²). The area of interest is then analyzed in detail by the scanning probe microscope, where the position of the SPM probe tip is controlled by the scanning electron microscope.

In the field of the sample surface analysis, methods of the so-called correlative imaging are often used, which enable direct alignment of images obtained by two different techniques. However, the present methods of sample characterization utilizing solution which integrates the scanning electron microscope and the scanning probe microscope are in correlative imaging accompanied by a number of problems caused by significant nonlinearities, different sampling, resolution, etc. The significant source of these problems is an independent scanning in SEM (by electron beam) and in SPM (by probe or sample) which runs sequentially. It is necessary to significantly reduce or completely eliminate these defects to enable utilizing techniques of correlative imaging for SPM and SEM methods.

Summary of the Invention

The above-mentioned drawbacks are eliminated by means of sample surface characterization method by the scanning electron microscope (SEM) and the scanning probe microscope (SPM), characterized in that scanning of the sample surface by the scanning electron microscope and the scanning probe microscope is carried out simultaneously, wherein the primary electron beam is stationary, while the table with the sample is moved in the XY plane during the scanning and the SPM probe is in this XY plane stationary. In a preferred embodiment, the table is also movable in the direction of Z axis parallel to the XY plane, wherein this Z axis is preferably perpendicular to the XY plane. The table, which is typically a piezo scanner, therefore moves in the XY plane and in the direction of the Z axis, or only in the XY plane while scanning the sample surface, where the movement in the direction of the Z axis is carried out by the SPM probe.

This method of surface sample characterization requires a change in the approach to the sample surface imaging by the scanning electron microscope where the primary electron beam does not scan over the sample surface, on the contrary, it is stationary, and the scanning of the sample surface by the scanning electron microscope is provided by moving the table with the sample. This embodiment is not known in the scanning electron microscopes and it is not used for separate imaging because deflection of the primary electron beam by means of electromagnetic lenses is considerably simpler than a very accurate mechanical movement of the sample. In the case of integration of the scanning electron microscope and the scanning probe microscope, however, it was found out during the development of the present solution that it was possible and very convenient to use the SPM table, typically a piezo scanner, for scanning, and to unify the scanning of the sample surface of both methods by moving the table with the sample. Both the SPM probe and the primary electron beam are static during the measurement at least in the XY plane, which is often parallel to the plane of the table. Scanning the sample surface with the scanning electron microscope by moving the SPM table brings along limitations in a small scanning range, typically 5-50 μιη², in comparison to the conventional scanning of the sample surface by deflecting the primary electron beam.

Similarly to the prior art solutions, the method of the sample surface characterization can be preceded by conventional scanning by the scanning electron microscope utilizing deflection of the primary electron beam to determine the area which is further characterized by the method of the present invention. The present solution therefore allows, after focusing, scanning of the sample surface in the selected area with the scanning electron microscope and the scanning probe microscope simultaneously, thereby detecting two types of signals at the same time. Thus, the advantage over the prior art solutions lies in the possibility of simultaneously obtaining information about the sample surface from the same selected area and in the same time by methods of scanning electron microscopy and scanning probe microscopy.

The main aspect of this invention is the same coordination system and the same resolution of the images obtained by the scanning electron microscope and the scanning probe microscope. The resulting imaging of these two images of the sample surface contains the same manifestation of non-linearity and table (piezo scanner) hysteresis, eventual defects of the sample surface are therefore reflected in the two images equally. The invention has a wide range of uses. It is advantageous to merge the resulting images of the two measurements into one map, the so-called correlative imaging, and thus obtain detailed information about examined sample surface. In this way, the user will get not only information about the topography of the sample surface, but also other information which is normally provided by the SPM and SEM measurement methods

Furthermore, it is very advantageous to apply this method to the creation of 3D nanostructures directly in the scanning electron microscope in which the focused ion beam (FIB) is installed. For example, in semiconductor integrated circuits, a Ga beam is used for testing and quality control. Focused ion beam technology also further allows, for example, removal of material from the sample by individual layers, or removal of undesired electrical contacts. It is possible to obtain a 2D image of the sample surface by means of the scanning electron microscope, but not to find out the information about depth and other sample surface parameters, which is in turn allowed by method of the scanning probe microscopy. Another possible use of this method is quality control and testing in the semiconductor industry (Intel Corporation, Samsung Industry, Qualcom, Toshiba Semiconductor, Texas Instruments, Broadcom, AMD, NVIDIA). Device with combination of the scanning electron microscope and the scanning probe microscope scanning simultaneously the sample surface has a sufficient resolution and complex capacity characterization for the purposes of this field.

Researchers can use the device with combined scanning electron microscope and scanning probe microscope for surface characterization in basic and applied laboratory research. For example, nanostructures used in various research areas (NanoDevices, solar cells, LabOnChip, nanosensors, and storage media) require complex surface characterization. Further development can hardly be achieved without a suitable technological device which would allow complete analysis in sub-nanometer resolution.

In a preferred embodiment, the trace of the primary electron beam of the scanning electron microscope and the probe tip of the scanning probe microscope are, during the simultaneous scanning, spaced apart from each other, in the so-called offset, the size of which is typically in the order of nanometers to microns. Offset is used when the measuring SPM probe can interfere with (shield) detection of signal electrons. The so-called offset is selected according to the probe type used and the size of the electron beam trace.

Brief Description of Figures

The present invention will be illustrated by figures in which the fig. 1 shows scheme of the chamber of the device with scanning electron microscope and scanning probe microscope, fig. 2 shows areas of sample surface scanning by the scanning electron microscope and the scanning probe microscope in a case, in which the trace of the primary electron beam of the scanning electron microscope and the probe tip of the scanning probe microscope are spaced apart from each other by a predefined offset. Description of the Preferred Embodiments

The invention will be described, not by way of limitation, in the following description by means of exemplary embodiments with reference to the accompanying figures. Fig. 1 shows a schematic arrangement of the chamber 9 of the device for performing the method according to the invention comprising both the scanning electron microscope 5 and the scanning probe microscope 1 The primary electrons 6 which impinge the sample 4_surface to be examined protrude from the scanning electron microscope 5. Signal electrons 8, which are flying out of the sample 4 surface are detected by a signal electron detector 7. Sample 4 is positioned on the table 3 for moving the sample, typically in the form of piezo scanner, which is attached to the manipulator 10. The device further comprises scanning probe microscope 1 which also scans the sample 4 surface by means of the SPM probe 2.

This device in the first exemplary embodiment characterizes the sample 4 surface with the scanning electron microscope 5 and the scanning probe microscope i so that the scanning of the sample 4_surface by the scanning electron microscope 5 and the scanning probe microscope Λ takes place simultaneously. The primary electron beam 6 is stationary during the characterization of the sample 4 surface, i.e., it is not deflected by deflection coils. While scanning, i.e. while moving between the individual scanning points of the sample surface, the table 3 with sample 4 moves in the plane XY, which is in this case identical to the table 3 plane. The table 3 is arranged horizontally, as shown in Fig. 1. The SPM probe 2 is stationary in the XY plane when scanning the sample 4 surface. The table 3 is stationary in the direction of the Z axis which is perpendicular to the XY plane, while on the contrary, the SPM probe 2 moves in this direction and moves the tip of the SPM probe 2 closer and further away from the sample 4.

In another exemplary embodiment, the characterization of the sample 4 surface, its scanning by the scanning electron microscope 5 and the scanning probe microscope 1 is again carried out simultaneously, and the primary electron 6 beam is stationary during the sample 4 surface characterization. Table 3, with its plane diverted from the horizontal plane by 30 degrees, moves again during the scanning with the sample 4 in the XY plane, which is in this case again identical with the table 3 plane. However, the table 3 also moves in the direction of the Z axis, which is perpendicular to the XY plane, while the SPM probe 2 is stationary not only in the XY plane but also in the direction of the Z axis during the characterization of the sample 4 surface. The tip of the SPM probe 2 is therefore brought closer and further away from the sample 4 surface by movement of the table 3 and not by movement of the SPM probe 2.

Fig. 2 shows the scanning areas of the sample 4 surface by the scanning electron microscope 5 and the scanning probe microscope when the trace of the primary electron beam of the scanning electron microscope and the probe tip of the scanning probe microscope are spaced apart from each other in each moment at a predetermined offset 14. The overlap of SPM image 11_, and a SEM image 13 is displayed, wherein the coordination system of the SPM image 1 and the SEM image 13 is displaced by the given offset 14. The offset 14 ensures that shielding of the primary electron 6 beam by the SPM probe 2 is avoided. The size of the offset 14 can vary in the ones of nanometers to tens of micrometers according to the range of scanned fields. The offset 14 size is equal to the distance of the trace of the primary electron 6 beam of the scanning electron microscope 5 and the SPM probe 2 tip. It is advantageous to precisely determine the offset 14 size before scanning the sample surface, thereby making it easier to combine the images of the scanned areas. However, the information about the offset 14 is not a requirement because the respective software can combine these images 11. , 13 automatically.

A typical example may be a 1 x 1 cm sample 4_surface characterization by means of signal electrons 8 and SPM probe 2. First, the sample 4 surface is scanned by the scanning electron microscope 5 to determine further examined area, wherein the scanning is carried out by deflection of the primary electron 6 beam. After that, the characterization method according to the present invention is applied, in which the primary electron 6 beam is fixed and the selected surface area of the sample 4 is simultaneously scanned by the scanning electron microscope 5 and the scanning probe microscope Λ . During such measurement, the primary electron 6 beam is accelerated by 15 kV voltage, wherein the primary electron 6 beam is not deflected (it is stationary). The working distance is 9 mm. The primary electron 6 beam is concentrated into the close proximity of the SPM probe 2 tip, the distance of which, the so-called offset 14, is for example 100 to 200 nm. The scanning area of the sample 4 surface is adjusted by piezo scanner 3, in this case its size being 10 x 10 μηι. To scan the deflecting positions of the SPM probe 2 tip, a measuring Tunning fork probe with a tip or Akyiama probe is used. "Tapping mode" measuring mode with oscillation amplitude of 20 - 80 nm has a resonance frequency of 30-60 kHz and a sample surface scanning frequency of 1 Hz/line. After setting these parameters, the scanning of the sample 4 surface is initiated which is scanned by the piezo scanner 3 in the matrix of e.g. 500 x 500 dots. The signal of the SPM probe 2 is detected by its movement in the direction of the Z axis. Signals from the SPM probe 2 and from the secondary electron 8 detector 7 are brought into the scanning probe microscope controller where they are simultaneously recorded point-for-point (basic principle of the scanning probe microscope) by means of the same software. Each signal represents one measuring channel. The image of each channel is formed by data about the position of the scanner of X, Y axes and about the signal from the secondary electron 8 detector 7_or the Z axis of the piezo scanner 3. A preferred resultant three-dimensional correlative imaging is achieved in the common scanning 12 area by moving the SPM image H by a given offset 14 (size and direction). The impact of the SPM probe 2 shape is suppressed/eliminated. The typical time of such a measurement is in the order of minutes.

By selecting the appropriate SPM probes 2/detector 7 of the signal electrons 8, images with a combination of e.g. chemical properties, electrical properties, mechanical properties, and the like can be obtained.

Technical Applicability

The combination of the scanning electron microscope and the scanning probe microscope has a wide application both among industrial users with specific needs and among scientists in research. The resulting images of the two measurements can be merged into one map to obtain a correlative imaging. Additionally, this method of sample surface characterization can be applied in the creation of 3D nanostructures directly in the scanning electron microscope in which a focused ion beam (FIB) is installed. This solution can be also used for testing and quality control in the semiconductor industry.

List of Reference Signs

1- Scanning probe microscope

2- SPM probe

3- Table

4- Sample

5- Scanning electron microscope

6- Primary electrons

7- Signal electron detector

8- Signal electrons

9- Chamber

10- Manipulator

11 - SPM image

12- Common scanning area

13- SEM image

4- Offset

Claims

1. A method of the sample (4) surface characterization by a scanning electron microscope (5) and a scanning probe microscope (1), characterized in that scanning of a sample (4) surface by the scanning electron microscope (5) and the scanning probe microscope (1) is carried out simultaneously, wherein a primary electron beam (6) is stationary while a table (3) with the sample (4) moves in the XY plane during the scanning, and a probe (2) of the scanning probe microscope is stationary in this XY plane.

2. A method according to claim 1 , characterized in that the table (3) is movable also in the direction of the Z axis parallel to the XY plane (3).

3. A method according to claim 1 or 2, characterized in that the trace of the primary electron (6) beam and the probe (2) tip of the scanning probe microscope are spaced apart from each other during simultaneous scanning.