Technical field

The invention relates to the use of sample movement compensation, where the observed sample place moves within the field of view of a charged particle beam device, wherein the deviation of the observed sample place from the original position is caused by rotation or tilt of the sample.

Background of the invention

When working with a device that uses a charged particle beam for its operation, in particular scanning electron microscope (SEM), transmission electron microscope (TEM), scanning transmission electron microscope (STEM), or focused ion beam (FIB) device, the studied sample is positioned on a sample holder where it is irradiated by a charged particle beam. The charged particle beam is used to observe the sample and obtain information about its internal structure, or to process the sample and create surface structures.

The work activity then requires frequent manipulation of the sample, which consists of rotating, tilting, and shifting the sample. For this reason, the sample holding stages are adapted to be displaced in three mutually perpendicular axes (x, y, z), to rotate about a vertical axis (z), and to tilt about at least one horizontal axis (x and/or y). Other possible sample stage designs allow for rotation or tilt about at least one axis different from the sample displacement axes.

There are two approaches in terms of sample tilt. In case of an eucentric tilt, the tilt axis passes directly through the place observed by the charged particle beam. Thus, when tilted about this axis, the sample still has the same position relative to the charged particle beam axis. In case of a compucentric tilt, the tilt axis passes through another place, and the observed place changes its position relative to the charged particle beam axis, and the observed sample escapes the field of view over time. To compensate for this movement, it is then necessary to move the sample back into the field of view of the charged particle beam by shifting it in the direction of the horizontal x and y axes. A similar shifting of the sample occurs also in case of rotation about the vertical axis, especially when the rotation axis does not directly intersect the sample region observed.

The solution to the sample movement correction problem is described, for example, in the document US 4,627,009 titled Microscope stage assembly and control system. The microscope according to an embodiment described therein comprises a processing unit adapted to compensate for sample movement, where sample deflection is caused by rotation and tilt of the sample, in particular tilt about a compucentric axis. The operator enters the desired sample tilt and/or rotation values, the processing unit calculates the theoretical displacement value from these values, and after rotating and/or tilting the sample, it ensures that the sample is displaced such that it returns to its original position relative to the charged particle beam axis.

A similar approach is presented in the document EP 1 071 1 12 B1 titled Scanning charged-particle beam instrument. The size of the sample displacement is calculated from the position of the rotation axis, the rotation angle, and the position of the observed point. After calculating the total displacement and after transferring the sample to the second position by rotation, the observed point is returned to the field of view by means of reverse displacement.

Even though the precision of the mechanical component processing is increasing and thus accuracy of the partial displacements is improving, this accuracy is limited. Another source of inaccuracy lies in the calculation of the compensating displacement itself. The input parameters do not have to be entered precisely and are already in principle burdened with a certain error in the measurement of their values. Similarly, the calculation, or rather its result, has a certain error value. Therefore, not all measures lead to a perfectly accurate adjustment of the resulting position, and certain correction procedures must always be implemented. It would therefore be useful to come up with a solution that allows accurate and continuous compensation for the movement of the observed place on the sample out of the field of view when the sample is rotated or tilted. Summary of the Invention

The disadvantages mentioned above are addressed or to some extent counteracted by the method of operation of a charged particle beam device. The charged particle beam device comprises at least a charged particle source, at least one lens adapted to shape the charged particle beam or to focus the beam onto a selected area, where the lens is embodied as an electromagnetic lens. Further, the device comprises a sample manipulation stage for positioning the sample, where the stage is adapted to move the sample translationally and rotate it or tilt it about at least one axis, and the sample positioned on the manipulation stage. The translational shifting then takes place in three non-parallel axes, which can be perpendicular to each other. The charged particle beam device further comprises a signal particle detector, a control unit for receiving instructions about the operation of the device and sending instructions to the device, a display unit for generating and displaying an image of a sample of the charged particles detected by a signal particle detector communicatively connected to the signal particle detector, a correction unit including at least a memory adapted to store the images, and a calculation unit for calculating the displacement of the sample. Signal particles are mainly charged particles scattered from the sample, or scattered electrons, or other particles emitted by the sample due to interaction with the incident charged particle beam, e.g., secondary electrons, protons, ions, or uncharged particles, e.g., atoms, molecules, or photons. The method of operation of the charged particle beam device comprises a sequence of steps of

- moving the sample to a first sample position;

- generating at least an initial image of the sample in a first position and storing this image in the memory, wherein the image is generated by the display unit;

- moving the sample to a second sample position, wherein this position is different from the first sample position, where the moving of the sample to the second sample position is due to rotation or tilt of the sample, wherein the sample remains within the field of view of the charged particle beam device throughout the position change, and generating at least a first auxiliary sample image in a position different from the first sample position, and storing the image in the memory, wherein the image is generated by the display unit from electrons scattered from the sample, secondary electrons; - determining a sample displacement value by comparing at least a pair of the initial and first auxiliary sample images, wherein the determination of the value is made by the calculation unit of the correction unit;

- moving the sample by the determined sample displacement value to a third sample position different from the first and second sample positions, wherein this moving consists only of translation by the determined sample displacement value.

The present method of operation of the charged particle beam device solves the problem of movement of the observed sample region in the field of view of the charged particle beam device, which occurs by rotation or tilt of the sample. This movement can cause the image to be defocused as the charged particle beam is not permanently focused on the same place on the sample. By means of the present method, a sample displacement value is quickly and accurately obtained, by which the sample is subsequently displaced such that the selected place on the sample is still observed.

In a preferred embodiment, the first auxiliary sample image is generated in the second sample position. This embodiment has the advantage of directly obtaining a determined sample displacement value by which the sample must be displaced to keep the observed sample place in the same position relative to the charged particle beam device as in the first position.

In a preferred embodiment, the first auxiliary sample image is generated during the sample position change, subsequently, at least a second auxiliary sample image is generated during the sample position change, and subsequently, a final sample image is generated in the second sample position, wherein the images are generated by the display unit and stored in the memory. Further, a first intermediate sample displacement value is determined by comparing the initial and the first auxiliary sample image by the calculation unit of the correction unit. Further, a second intermediate sample displacement value is determined by comparing the first and second auxiliary sample images by the calculation unit of the correction unit. Further, a third intermediate sample displacement value is determined by comparing the second auxiliary sample image and the final sample image by the calculation unit of the correction unit. The determined sample displacement value is then given by the sum of the first, second, and third intermediate sample displacement values. An advantage of this embodiment lies in the fact that a more accurate value of the determined sample displacement value is obtained due to its continuous calculation already during the sample position change.

Preferably, the calculation unit may calculate a calculation sample displacement value from the values specifying the sample position change. The determined sample displacement value is then specified as the average of the above specified displacement value and the calculation displacement value. The dual methodology of specifying the determined sample displacement value increases the accuracy of the present method.

Preferably, a mark may be created on the sample, for example by a focused ion beam. The creation of a mark creates a prominent point on the sample that facilitates the comparison of a pair of images to obtain the sample displacement value, especially for samples with smooth relief without prominent structures.

Preferably, a cross-correlation function is then used to compare the images, which allows for a fast and efficient calculation of the displacement of two signals, where a signal in context of this application means a sample image.

Description of Drawings

The summary of the invention is further clarified using exemplary embodiments thereof, which are described with reference to the accompanying drawings, in which:

Fig. 1 shows schematically the charged particle beam device including the correction unit for calculating displacement of observed region on the sample after sample tilt;

Fig. 2 is a block diagram of basic procedure for determining the sample position change value;

Fig. 3 is a block diagram of procedure for determining the sample position change value by comparing the initial image in the first sample position and the auxiliary image in the second sample position;

Fig. 4 is a block diagram of procedure for determining the sample position change value by comparing the sample images in the first sample position, auxiliary images generated during the sample position change, and the final sample image in the second sample position; Fig. 5a is a block diagram of procedure for determining the sample position change value by comparing the initial image in the first sample position and the auxiliary image in the second sample position, together with calculation of the sample displacement from the entered sample position change values;

Fig. 5b is a block diagram of procedure for determining the sample position change value by comparing the sample images in the first sample position, auxiliary images generated during the sample position change, and the final sample image in the second sample position, together with calculation of the sample displacement from the entered values of the sample position change;

Fig. 6a shows schematically the sample in the first sample position;

Fig. 6b shows schematically the sample in the second sample position;

Fig. 6c shows schematically the sample in the third sample position ; Embodiments of the Invention

The invention will be further clarified using example embodiments with reference to the respective drawings. In Fig. 1 a charged particle beam device 1 is shown, including a charged particle source 2, at least one lens 3, 4 for shaping the charged particle beam or focusing the beam on a selected area, and a manipulation stage 5 for positioning a sample 6 adapted to be displaced in three mutually perpendicular axes and to at least rotate or tilt the sample 6 about two mutually different axes. For example, the charged particle beam device 1 has at least one condenser lens 3 for shaping the charged particle beam and at least one objective lens 4 for focusing the beam on the selected area. The device 1 according to the exemplary embodiment of the invention further comprises a signal particle detector 7. Signal particles are mainly charged particles scattered from the sample 6, or scattered electrons, or other particles emitted by the sample 6 due to interaction with the incident charged particle beam, e.g., secondary electrons, protons, ions, or uncharged particles, e.g., atoms, molecules, or photons. On the basis of the detected signal particles, the device 1 is adapted to generate an image of the imaged sample 6 region via a display unit 9 communicatively connected to the signal particle detector 7. A communication connection means a connection enabling transfer of information between the communicatively connected elements, as such it can be implemented, e.g., by interconnecting via network cables or wirelessly in the form of WiFi, Bluetooth, etc. The device 1 further comprises a control unit 8 for receiving instructions given by the user and adapted to send these instructions further to the device 1, where they are executed by the respective components. In an exemplary embodiment of the invention, these instructions mean, e.g., entering the desired magnification, focusing, selecting the desired region to be imaged, storing the processed image, further processing the image, etc. The invention is not limited to these functions mediated by the control unit 8. An exemplary embodiment of the invention further comprises a correction unit 10 communicatively connected to the control unit 8 and comprising an intrinsic memory 1 1 in which images of the sample 6 are stored, and a calculation unit 12 adapted to calculate the sample 6 displacement. The control unit 8 gives an instruction to move the sample 6 to a new position to which the sample 6 is moved by the manipulation stage 5. The calculation unit 12 receives information from the control unit 8 about the sample 6 position change, or data about a first sample 6 position and a second sample 6 position, and calculates a calculation sample 6 displacement value based on these data. The calculation of this calculation sample 6 displacement value is carried out according to the following formulas:

AZ = WD - Z,

Z _new = Z - AZ • (1/ cos(a) - 1) and

^new = T + AZ • tan(a), where AZ represents the calculated value by which the sample 6 has to be displaced in the Z-axis direction after tilting the sample 6 by an angle a, WD represents the Working Distance parameter, or the distance of the observed point on the sample 6 from the objective lens 4, Y represents the current position in the /-axis, and Z _new and K _new are the calculated sample 6 displacement values in the Z-axis or /-axis, see Figs. 6a, 6b and 6c.

The movement of the sample 6 consists of translation, tilt and rotation. The translation is usually performed along at least two axes which are, for example, mutually perpendicular. For correct implementation of the present method of operation of the charged particle beam device 1, the possibility of translation along only one axis is sufficient. Alternatively, it is then possible to select manipulation stage 5 allowing movement in three mutually perpendicular axes. The rotational movement of the sample 6 then comprises rotation about at least one axis, wherein this axis may be different from the axes of the translational movement of the sample 6, the rotation of the sample is generally possible in the full range of the rotational movement, i.e., 360°. The tilt of the sample 6 is then determined by its tilt about an axis other than the rotation axis. In an exemplary embodiment of the manipulation stage, it is possible to perform two independent rotational movements of the sample 6 about two mutually different axes. The rotational and translational movement of the sample 6 is realized by the manipulation stage 5. The function of the stage can also be performed by another manipulator, e.g., a needle manipulator.

The tilt of the sample 6 can be realized by two methods. The first method is a compucentric tilt method, where the tilt axis 5a does not pass through the observed place on the sample 6, and when the sample 6 is tilted or rotated, the observed sample 6 region within the field of view of the charged particle beam device 1 moves significantly, and the observed sample place 13 may thus be defocused, but more importantly, it may leave the field of view of the charged particle beam device T The second method is the eucentric tilt, where the tilt axis passes through the observed sample 6 place.

In the context of this invention application, charged particle beam device 1 means an electron microscope, in particular a scanning electron microscope (SEM), a transmission electron microscope (TEM), a scanning transmission electron microscope (STEM), a focused ion beam (FIB) device, or combined electron beam and focused ion beam devices.

In an exemplary embodiment of the operation method of the charged particle beam device 1, see Fig. 2, the sample 6 to be processed, analyzed, or observed is positioned in the corresponding place on the manipulation stage 5. A working chamber 15 of the charged particle beam device 1 is closed and pumped to the desired pressure, below atmospheric pressure. The sample 6 is adjusted by the manipulation stage 5 to the first sample 6 position. In the first sample 6 position, the sample 6 is subsequently irradiated by a charged particle beam that is focused and shaped by the condenser lens 3 and the objective lens 4. The interaction of the charged particle beam with the sample 6 results in creation of signal particles, e.g., back-scattered electrons, secondary electrons, or photons. These particles are then detected by the signal particle detector 7. The signal of these particles captured by the signal particle detector 7 is subsequently processed by the display unit 9. The signal processing by the display unit 9 results in at least an initial sample 6 image in the first sample 6 position, which is subsequently stored in the memory 1 1 . In the next step, the control unit 8 gives instruction to move the sample 6 to the second sample 6 position different from the first sample 6 position. The control unit 8 passes this instruction on to the charged particle beam device 1, and the sample 6 is moved to the second sample 6 position by the manipulation stage 5. At the same time, in a position different from the first position, the sample 6 is irradiated by the charged particle beam. As a result of the interaction of the charged particle beam with the sample 6, the particles are scattered from the sample 6 or signal particles are released from the sample 6. The signal particles are subsequently captured by the signal particle detector 7, and at least an auxiliary sample 6 image in a position different from the first sample 6 position is generated from this signal by the display unit 9 and subsequently stored in the memory 1 1 . In the following step, a comparison of the pair of the initial sample 6 image and the auxiliary sample 6 image is performed. The comparison of the pair of these images is performed using the calculation unit 12 of the correction unit 10. The output of this comparison is then a determined sample 6 displacement value, or a value of the movement of the observed place 13 of the sample 6 relative to the field of view of the charged particle beam device 1, which is caused by the rotation and/or tilt of the sample 6, and in which the observed place 13 on the sample 6 may partially leave the field of view of the charged particle beam device 1, in addition, its height and position relative thereto may change, thereby defocusing or shift of the image. In an exemplary embodiment of the invention, the comparison of the pair of the initial sample 6 image and the auxiliary sample 6 image is performed by method of correlation or cross-correlation. The sample 6 is then subsequently shifted to the third sample 6 position different from the first and second sample positions by the determined sample 6 displacement value. The exemplary embodiment of the operation method of the charged particle beam device 1 according to Fig. 3 is identical to the exemplary embodiment of the operation method of the charged particle beam device 1 according to Fig. 2, except that the auxiliary sample 6 image is generated in the second sample 6 position.

In signal processing, correlation is a function that describes the similarity of the shape of signals. In case of a pair of signals which are similar in waveform but may be displaced by a certain value p of the phase displacement, the cross-correlation of these signals results in the value of the mutual displacement of these signals. The cross- correlation of signals can be used also in case of two-dimensional signal, which corresponds, for example, to a pair of images. In an exemplary embodiment of the invention, a software module is implemented in the calculation unit 12 of the correction unit 10 allowing for application of the cross-correlation function of signals to a pair of sample 6 images at different sample 6 positions. The result of this process is a determined sample 6 displacement value.

Based on the above obtained sample 6 displacement value calculated by the correction unit 10 using the method described above, the sample 6 is then moved on the manipulation stage 5 to a third sample 6 position different from the first position and the second sample 6 position. The moving of the sample 6 to the third position consists only of a translational movement, no rotation or tilt of the sample 6 is required.

In another exemplary embodiment of the operation method of the charged particle beam device 1, see Fig. 4, the sample 6 to be processed, analyzed, or observed is positioned in the corresponding place on the manipulation stage 5. The working chamber 15 of the charged particle beam device 1 is closed and pumped to the desired pressure, below atmospheric pressure. The sample 6 is adjusted by the manipulation stage 5_to the first sample 6 position. In the first sample 6 position, the sample 6 is subsequently irradiated by a charged particle beam that is focused and shaped by the condenser lens 3 and the objective lens 4. The interaction of the charged particle beam with the sample 6 results in the creation of signal particles, e.g., back-scattered electrons, secondary electrons, or photons. These particles are then detected by the signal particle detector 7. The signal of these particles captured by the signal particle detector 7 is subsequently processed by the display unit 9. The signal processing by the display unit 9 results in at least an initial sample 6 image in the first sample 6 position, which is subsequently stored in the memory 1 1 . In the next step, the control unit 8 gives instruction to move the sample 6 to the second sample 6 position different from the first sample 6 position. The control unit 8 passes this instruction on to the charged particle beam device 1, and the sample 6 is moved to the second sample 6 position by the manipulation stage 5. This exemplary embodiment further comprises step of generating at least a first auxiliary sample 6 image, which is recorded during the sample 6 position change from the first sample 6 position to the second sample 6 position, and at least a second auxiliary sample 6 image, which is recorded during the sample 6 position change from the first sample 6 position to the second sample position after the first auxiliary image. Further, at least a final sample 6 image is generated in the second sample 6 position. These images are then stored in the memory 1J.. Subsequently, a first intermediate sample 6 displacement value is determined by comparing the initial and the first auxiliary sample 6 image by the calculation unit 12 of the correction unit 10. Subsequently, a second intermediate sample 6 displacement value is determined by comparing the first and the second auxiliary sample 6 images by the calculation unit 12 of the correction unit 10. Subsequently, a third intermediate sample 6 displacement value is determined by comparing the second auxiliary and the final sample 6 image by the calculation unit 12 of the correction unit 10. The determined sample displacement value is then given by the sum of the first, second, and third intermediate sample 6 displacement values. In this exemplary embodiment of the invention, the comparison of the pairs of the sample 6 images is performed by the method of correlation or cross-correlation. The sample 6 is subsequently moved from the second sample 6 position to the third sample 6 position different from the first and second sample positions by the determined sample displacement value.

Further exemplary embodiments of the operation method of the charged particle beam device 1 according to Figs. 5a and 5b are identical to the exemplary embodiments of the operation method of the charged particle beam device according to Figs. 3 and 4, except that the sample 6 is moved from the second sample 6 position to the third sample position by a determined sample 6 displacement value, which is given as the average of the calculation sample 6 displacement value and the determined sample 6 displacement value.

All exemplary embodiments of the operation method of the charged particle beam device 1 may further comprise a step of creating at least one mark on the sample 6. In this exemplary embodiment, the mark creation is realized by a focused ion beam (FIB). The ion beam is focused on a specific place on the sample 6, wherein the impact of the charged particles (ions) on the sample 6 results in the so-called sputtering of the sample 6, where the atoms and molecules of the studied sample 6 are ejected due to the impact of the ions. As a result of this, it is possible to create a mark on the sample 6. The creation of this mark facilitates orientation on the sample 6, especially when the surface structure of the sample 6 is homogeneous and without significant reliefs. The creation of the mark on the sample 6 also results in increasing the accuracy of the calculation of the sample 6 displacement by the cross-correlation method, especially if, e.g., the structure of the analyzed sample 6 is smooth and without significant reliefs. The mark on the sample 6 then serves as an auxiliary point for the cross-correlation algorithm.

In another exemplary embodiment of the operation method of the charged particle beam device 1 according to this invention, the operation of the device 1 may be realized as follows and according to Figs. 3 or 5a. The sample 6 is positioned on the manipulation stage 5 and moved to the first sample 6 position, see Fig. 6a. In the first sample 6 position, the sample 6 is positioned such that at least one manipulation stage 5 tilt axis 5a passes directly through the sample 6. Subsequently, the observed place 13 of the sample 6 is selected for further observation or processing. The charged particle beam is focused and directed to the observed place 13 of the sample 6 by means of condenser lenses 3 and objective lenses 4. Further, the initial sample 6 image in the first position is recorded and stored in the memory 1 1 . Subsequently, the sample 6 is moved to the second sample 6 position, see Fig. 6b, wherein throughout the position change, the sample 6 remains in the field of view of the device 1, but the position of the observed place 13 may change. After the sample 6 is positioned in the second sample 6 position, the auxiliary sample 6 image in the second position is generated and stored in the memory 1 1 . The field of view of the charged particle beam device 1 is the region from which the image is generated after the region has been scanned by the charged particle beam or by a camera or other device capable of generating an image of the region. In the next phase, a pair of images is selected, where the first image of the pair is the initial sample 6 image in the first sample 6 position and the second image of the pair is the auxiliary sample 6 image in the second sample 6 position. This pair of images is then processed by the calculation unit 12 of the correction unit 10, wherein the comparison of this pair of images is performed by the method of cross-correlation of this pair of images. The processing of this pair of images results in a determined sample 6 displacement value specifying the deflection of the observed place 13 of the sample 6 in the second position compared to the first position. The calculation unit 12 may further specify the calculation sample 6 displacement value, which is specified from the data about the entered sample 6 position change value by the calculation according to the relationship described above. Data means e.g., coordinates of the sample 6 position and values describing its angular orientation, e.g., relative to the axis of the charged particle beam device 1_. The determined sample 6 displacement value may then be specified as the average of the calculation value and the determined value obtained by comparing the initial sample 6 image and the auxiliary sample 6 image. Thereafter, the sample 6 is displaced by the sample 6 displacement value to the third sample 6 position different from the first position and the second sample 6 position, wherein the moving of the sample 6 to the third sample 6 position consists only of translational movement, see Fig. 6c. In addition, the sample 6 image can also be obtained using another recording medium, e.g., a camera, infrared camera, or ICCD camera.

Industrial applicability

The method and device described above may be used in the field of electron microscopy or in other devices using a charged particle beam for sample treatment and/or observation.

List of reference numerals

1 - charged particle beam device

2 - charged particle source

3 - condenser lens

4 - objective lens

5 - manipulation stage

5a - manipulation stage tilt axis

6 - sample

7 - signal particle detector

8 - control unit

9 - display unit

10 - correction unit

11 - memory

12 - calculation unit

13 - observed sample place

14 - charged particle beam axis

15 - working chamber