KR101156214B1

KR101156214B1 - Calibration method for free magnification in scanning electron microscope

Info

Publication number: KR101156214B1
Application number: KR1020100127699A
Authority: KR
Inventors: 이준희; 전정범
Original assignee: (주)코셈
Priority date: 2010-12-14
Filing date: 2010-12-14
Publication date: 2012-07-03

Abstract

PURPOSE: A method for adjusting self-magnification of a scanning electron microscope is provided to measure the accurate size of an object by automatically adjusting the scanning electron microscope according to arbitrary magnification without feedback control. CONSTITUTION: A focused control coefficient is calculated based on a correlation between an electronic objective lens and working distance(S101). A shape correction coefficient is calculated by revising the shape of an image according to the working distance after controlling the focus. An actual size coefficient is calculated after shape correction of the image. The coefficient of the functions becomes the database by determining equipment set values according to arbitrary magnification(S113). A zero point coefficient according to the working distance is calculated based on a distance correlation between a sample holding stage and the electronic objective lens.

Description

Calibration Method for Free Magnification in Scanning Electron Microscope

The present invention relates to a scanning electron microscope (SEM), and more specifically, when observing a sample at an arbitrary magnification, the adjustment of the scanning electron microscope to an arbitrary magnification is automatically performed without feedback control. The present invention relates to a method of adjusting the free magnification of a scanning electron microscope in order to quickly observe an image and measure an accurate size.

In general, a scanning electron microscope detects secondary electrons or back scattered electrons having the highest probability of generating various signals generated from a sample when an electron beam is scanned on a sample surface. It is the equipment to observe a target sample.

Scanning electron microscopes have the advantage of being able to obtain information mainly on the surface of the sample and are not limited to the thickness, size and preparation of the sample.

These scanning electron microscopes are very complicated and difficult in a series of processes until accelerated projection of electrons to observe an image. In particular, the amount of current of the objective lens to focus the image according to the working distance (WD) from the objective lens to the sample and the accelerating voltage to project the electrons (Object Lens Current) OLC), the magnification of the phase and the amount of current in the scan coil to correct the distortion, such as different equipment settings.

For example, there are equipment setting values for observing a normal phase in the two-dimensional plane variable formed by the working distance and the acceleration voltage. Not only is it too complicated to represent these relationships as a function, it is unknown. However, knowing this correlation, you can automatically perform focusing, distortion correction, and size calculation to observe normal images without feedback control.

Commercially available scanning electron microscopes automatically perform focusing, distortion correction, and size calculation without knowing such correlations. The method is to perform feedback control or zeroing or a combination thereof.

In other words, the scanning electron microscope sets about 30 kinds of device setting values for the experts in semiconductor, materials, biotechnology, etc. to obtain the microscopic images required for their specialty. The result of this setting value is represented as an image, and knowledge of physics, optics and electro-optics, mechanical engineering, electronic engineering, software, etc. is required to know the correlation. Typically, it is extremely impossible for an equipment operator to have all this expertise, and most equipment operators operate their equipment through experience.

In this case, if only the feedback control is performed alone, since it covers the entire area, it takes a long time to achieve the goal and it is difficult to format the image, and thus it is difficult to have satisfactory performance.

For this reason, the operator who is familiar with using the equipment performs the automatic function by observing and setting the equipment setting values which achieves the optimum for the 2D plane variables in advance (during manufacturing). This is called calibration, and when used in conjunction with feedback control, more accurate automatic functions can be performed quickly in operation time. In other words, in order to operate the equipment smoothly without the help of an operator, the equipment manufacturer developed a related algorithm through image processing such as image processing, so that accurate images could be observed without expertise.

As described above, the method of performing the automatic function through the zero adjustment has the advantage of obtaining an optimal image as described above, but it is difficult to form an image, and the dimension space of the variable space for determining about 30 device setting values is increased. Too high and takes considerable time. To solve this problem, the zero point adjustment is used in the manufacturing process of the equipment, and based on this value, the optimization method is achieved in the vicinity of the variable space.

However, the method of performing the automatic function through the zero adjustment also has a disadvantage that the two-dimensional plane variable can not have a continuous value. More easily expressed, there is a disadvantage that the image cannot be observed at any magnification (or free magnification), and the sample can be observed only at a specific magnification set through the zero adjustment during the manufacturing process of the equipment.

In addition, the conventional methods require considerable time for zero adjustment by selecting a specific magnification and performing zero adjustment for a considerable amount of magnification to express continuity. This is a tedious task that the operator repeatedly performs over a long period of time, which is very inconvenient.

Accordingly, the present invention has been proposed to solve various problems occurring in the conventional commercialized scanning electron microscope as described above.

The problem to be solved by the present invention, when observing the sample at any magnification (free magnification) automatically performs the adjustment of the scanning electron microscope to any magnification without feedback control (Feedback control), to quickly observe the image It is to provide a free magnification adjustment method of a scanning electron microscope to measure the exact size.

Another problem to be solved by the present invention is to provide a free magnification adjustment method of a scanning electron microscope that enables the observation of an image at a continuous magnification so that the subject can be clearly identified.

"Free magnification adjustment method" according to the present invention for solving the above problems,

Calculating a focus adjustment coefficient based on a correlation between the working distance, which is the distance between the sample stage and the electronic objective lens, and the electronic objective lens;

A second step of calculating a shape correction coefficient for correcting a shape of each work distance after focus adjustment;

A third step of calculating an actual size coefficient of the image observed for each working distance after the shape correction;

And a fourth process of databaseting coefficients calculated through each process into coefficients of a function for determining equipment setting values according to an arbitrary magnification.

The first process,

Calculating a zero coefficient for each working distance based on the distance correlation between the sample object and the electronic objective lens;

Calculating an electron objective lens current coefficient according to the working distance by obtaining a correlation between the working distance and an electron objective lens current (OLC) for moving the electron objective lens;

And calculating a correlation between the electron objective lens current and the acceleration voltage, and calculating the electron objective lens current coefficient for each acceleration voltage.

The second process,

Calculating a rotational movement value of the image for each working distance based on the focal length and the rotational relationship of the image;

Computing the distortion correction value for correcting the image distortion caused by the magnetic field effect for each working distance.

The third process,

The actual size factor of the phase is calculated based on a correction value for the length calculated using a standard sample for each working distance.

"Free magnification adjustment method" according to the present invention for solving the above problems,
Calculating a focus adjustment coefficient based on a correlation between the working distance, which is the distance between the sample stage and the electronic objective lens, and the electronic objective lens;
Calculating a shape correction coefficient for correcting the shape of the image for each working distance after the focus adjustment;
Calculating an actual size coefficient of the image observed for each working distance after the shape correction;
Databaseting coefficients calculated through the respective processes into coefficients of a function for determining equipment setting values according to an arbitrary magnification;

Receiving an arbitrary magnification to be manipulated through the graphical user interface;

Drawing a function for determining equipment setting values;

Extracting a coefficient of a function from which a coefficient of a function for determining equipment setting values according to an arbitrary magnification is extracted from a database having a database;

Substituting the extracted coefficient into the function to extract equipment setting values for the arbitrary magnification;

And controlling the magnification by automatically controlling each device according to the extracted device setting values.

The process of receiving the arbitrary magnification,

Characterized in that the magnification is input through the mouse or the joystick.

The process of adjusting the magnification,

The magnification is continuously adjusted according to an arbitrary magnification inputted through the mouse or the joystick.

According to the present invention, the scanning electron microscope can be automatically adjusted to any magnification without feedback control when observing the sample at any magnification (free magnification). It has the advantage of being able to measure the size.

In addition, according to the present invention, it is also possible to observe the image at a continuous magnification, it is also possible to clearly identify the object.

1 is a schematic configuration diagram of a free magnification control apparatus of a scanning electron microscope to which the present invention is applied.
Figure 2 is a flow chart of a first embodiment of a method for adjusting the free magnification of a scanning electron microscope according to the present invention.
Figure 3 is a flow chart of a second embodiment of the free magnification adjustment method of the scanning electron microscope according to the present invention.
4A and 4B are diagrams showing the relationship between the magnetic flux intensity and the electron objective lens current.
5 is a relationship between the electron objective lens current and the working distance.
6 is a rotation shift value correction relationship diagram in the present invention.
7 is a shape distortion correction relationship diagram in the present invention.
8 is an explanatory diagram for explaining an open loop and a feedback control loop in the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

In the scanning electron microscope, the equipment control values can be calculated quickly by substituting the appropriate conditions (acceleration voltage, working distance, etc.) to obtain an accurate image in a function for determining the equipment setting values. Each instrument in the microscope can be automatically controlled at any magnification.

To this end, in the present invention, a function for equipment control is obtained through the process as shown in FIG. 2, and the coefficients for substituting the function are pre-built into a database.

FIG. 2 is a flowchart illustrating a first embodiment of a method of adjusting a free magnification of a scanning electron microscope, and calculates a focus adjustment coefficient based on a correlation between a working distance, which is a distance between a sample stage and an electron objective lens, and an electron objective lens. The first process (S101 ~ S105) to perform; A second process (S107 to S109) for calculating a shape correction coefficient for correcting the shape of the image for each working distance after the focus adjustment; A third step of calculating an actual size coefficient of the image observed for each working distance after the shape correction (S111); A fourth process S113 is performed to database the coefficients calculated through the above processes into coefficients of a function for determining equipment setting values according to an arbitrary magnification.

In the first embodiment of the present invention, the free magnification adjustment method of the scanning electron microscope according to the present invention calculates the zero coefficient for each working distance based on the physical distance correlation between the sample object and the electron objective lens (S101). Focusing is essential for observing the correct image of the sample. The focal point can be expressed by the correlation between the working distance and the current of the electronic objective lens. For this purpose, it is necessary to physically measure the distance between the sample stage and the electronic objective lens (working distance).

In other words, the actual position of the stage in the chamber must be known from the stage board, the GUI, and be able to be manipulated to reach the desired position. This requires knowing the correlation between the current position and the control signal and the actual distance traveled.

Since the working distance is the distance from the electronic objective lens, the distance between the home position and the electronic objective lens is measured. The distance from the groove is the correlation between the known control signal and the actual distance traveled (scanning electron microscopes are designed to move 1 mm per wheel, and you can set the number of pulses required to turn a motor). Able to know. The working distance here has a 1: 1 correspondence with the electron objective current.

Through this process, the working distance is determined, and then, the correlation between the working distance and the electronic objective lens current OLC for moving the electronic objective lens is obtained, thereby calculating the electronic objective lens current coefficient according to the working distance.

The working distance and the focal length must be clearly coincident, and as shown in FIG. 4A, the electron objective lens current and the focal length are inversely related. Therefore, find the relationship between them. To this end, there may be a method of finding the current value of the electronic objective lens while fixing the high voltage (HV) and changing the working distance. However, the magnetic flux intensity (B-I curve), which determines the current of the electronic objective lens and the focus of the lens, has hysteresis as shown in FIG. 4B, so the relationship between the two does not have a 1: 1 relationship. So, in order to have a functional relationship, a method of removing hysteresis must be devised.

To this end, in the present invention, as shown in Fig. 4B, the graph is made one by starting from the "0" point in order to eliminate hysteresis. Then, the electronic objective lens current that is focused by the working distance is measured. After that, the measured value of the electron objective lens current is stored as the electron objective lens current coefficient.

Here, the reason for normalizing is to draw an OLC-WD curve for each of the high voltages. Therefore, if you get a certain distance, you can get the working distance = F (OLC, HV).

Next, the correlation between the electron objective lens current and the acceleration voltage is obtained, and the electron objective lens current coefficient is calculated for each acceleration voltage ACC (S105).

Since there is a difference in the intensity of the electron objective lens current for forming the same focal length for each acceleration voltage, these correlations are obtained. That is, the relationship between the WD-OLC according to the high voltage HV is as shown in FIG. 5, and there is an overlapping relationship when normalized, so that the WD-OLC relationship calculated in step S103 is known and a proportionality constant for obtaining a prototype ( Once you know Pivot, you can get it easily. Therefore, when WD is 5mm (min), the OLC value (Max) is calculated as the electron objective lens current coefficient for each acceleration voltage. Here, the proportional constant means a value that is multiplied by the normalized WD-specific OLC value to return to the original value. That is, the OLC value is the proportional constant value when WD is minimum.

Through this process, the process for focus is terminated, and second processes S107 to S109 for calculating a shape correction coefficient for correcting the shape of the image are performed.

First, a rotation movement value of each image is calculated based on the focal length and the rotational relationship of the image (S107). The electrons that pass through the electron lens move while rotating in the screw direction under the influence of Lorentz force. Due to this effect, the image is rotated to appear as the focal length increases, and a rotation shift value that can correct this is found as shown in FIG. 6.

For example, Lorentz forces cause the beam to descend while rotating counterclockwise as it passes through the lens (magnetic field). This creates a phenomenon in which the observation rotates clockwise as the WD moves away and must be corrected.

In general, when the X-axis and the Y-axis are moved, the X-axis and the Y-axis should be moved on the screen. Otherwise, correction is necessary. After calibration, the screen moves only in the axial direction.

To do this, position WD at 5 mm (minimum) and zero R to move in the axial direction when moving the X or Y axis (R-Calibration). After that, the angle that needs to be corrected is measured as the WD changes, and this is calculated as a rotation movement value coefficient.

The correction value is transformed into an axial rotation matrix and calculated and multiplied by a reference voltage when performing linear transformation on a scan block value.

Next, the distortion correction value for correcting the image distortion caused by the influence of the magnetic field for each working distance is calculated (S109). The electrons that pass through the electron lens move while rotating in the screw direction under the influence of Lorentz force. Typically, the magnetic field of the electron objective lens is formed in an extremely narrow area, and most of the distance to reach the sample is outside the range of influence of the magnetic field. Therefore, the Lorentz force acting cannot act the same for the scanning area, resulting in image distortion. A value must be calculated to correct this.

Unlike the CL and OL, the scan coil does not generate a circular magnetic field, so a regular sawtooth wave does not generate a scan region in a square shape. To correct this is shearing distortion. According to the experiment, it was observed that the rotation correction after the distortion correction is normally formed when the rotation and the distortion are corrected for each WD. Measure and store the correction value of the shape to be modified as the WD changes.

Next, a third process (S111) of calculating the actual size factor of the image observed for each working distance after the shape correction is performed. Here, when the first and second processes are performed, the image may be observed without distortion in the focused state. However, the actual size and the size of the observed image may not match, so obtain a correlation to correct this function. Here, the actual size is stored by calculating a correction value for the length calculated using a standard sample (the length of which is already known) for each WD.

A fourth process S113 is performed to convert the coefficients calculated through this process into a database DB as a coefficient of a function for determining equipment setting values according to an arbitrary magnification.

The method of adjusting the free magnification in the actual scanning electron microscope based on the database information is as follows.

1 is a schematic configuration diagram of an apparatus for free magnification adjustment of a scanning electron microscope to which the present invention is applied, the input unit 10 for a user to input an arbitrary magnification, and the set values according to an arbitrary input magnification coefficient database 60. And a controller 20 for automatically controlling each device of the scanning electron microscope 30 by extracting from the scanning electron microscope 30 and controlling the display of the image extracted from the scanning electron microscope 30.

In addition, the free magnification adjustment device of the scanning electron microscope according to the present invention, in conjunction with the controller 20, the movement of the electronic objective lens (OL) according to each control value (OLC, ACC, BIAS, FILA, etc.) And a scanning electron microscope (30) for detecting secondary electrons or reflected electrons generated from a sample, a signal processor (40) for signal processing an image of a sample detected by the scanning electron microscope (30), and the signal processor ( And an indicator 50 for displaying the image of the sample processed in 40 on the screen.

3 is a flowchart of a second embodiment showing a free magnification adjustment method of a scanning electron microscope according to the present invention.

As shown therein, step S201 of receiving an arbitrary magnification manipulated through a graphic user interface; Extracting a function for determining equipment setting values (S203); Extracting a coefficient of the extracted function from a database in which a coefficient of a function for determining equipment setting values is stored according to an arbitrary magnification (S205); Extracting equipment setting values for the arbitrary magnification by substituting the extracted coefficients into the function (S207); According to the extracted device setting values, the control is automatically performed to adjust the magnification (S209).
Although not shown in the drawings, the focus adjustment coefficient is determined based on the correlation between the working distance and the electronic objective lens, which is the distance between the sample stage and the electronic objective lens, as in FIG. Calculating process; Calculating a shape correction coefficient for correcting the shape of the image for each working distance after the focus adjustment; Calculating an actual size coefficient of the image observed for each working distance after the shape correction; Databases of coefficients calculated through the above processes are determined as a coefficient of a function for determining equipment setting values according to an arbitrary magnification. A second embodiment of the present invention is shown in FIG. As such, the process of databaseting the coefficients of a function is omitted.

The second embodiment according to the present invention made as described above starts with the database stored as a coefficient of a function and stored in the database through the process as shown in FIG. 2.
That is, an arbitrary magnification is received from the user through the input unit 10 which is a graphic user interface (S201). In this case, the user sets the desired magnification through the input unit 10. For convenience of input and continuous expression of the desired magnification, it is preferable to input an arbitrary magnification through the mouse or the joystick. Therefore, the user can input the desired magnification.

Next, when an arbitrary magnification is determined from the input unit 10, the controller 20 draws a function for determining equipment setting values (S203). Such a function is a function for determining equipment setting values, and it is preferable to calculate and store the result in advance.

After that, the coefficient for substituting the extracted function is extracted from the coefficient database 60 (S205), and the apparatus setting values are extracted by substituting the function (S207).

Then, the extracted device setting values are transferred to the scanning electron microscope 30 as a control value, and thus, the scanning electron microscope 30 performs an operation according to the transmitted control value to measure and output an image of the sample as an image (S209). ).

The extracted image is subjected to signal processing on the image through the signal processor 40 and displayed on the display unit 50 under the control of the controller 20, so that the image can be observed at a magnification determined by the user.

On the other hand, the above method is a method without feedback control, as shown in Figure 8 (hereinafter referred to as "Open Loop"), in the present invention to perform the image shaping to the open loop as described above By adding feedback loop II, you can observe the image more optimally.

A specific method for this is to obtain an image of an arbitrary magnification from the scanning electron microscope through step S209 of FIG. 3, and to perform signal processing on the image, instead of simply performing signal processing on the image in the signal processor 40. This is accomplished by shaping the image once again using a known image shaping algorithm. Since the image shaping algorithm is one of the feedback control methods used in the conventional scanning electron microscope, detailed description thereof will be omitted.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims and their equivalents. Of course, such modifications are within the scope of the claims.

10... Input
20... Controller
30... Scanning electron microscope
40 ... Signal processor
50... Indicator
60 ... Coefficient database

Claims

In the method of adjusting the magnification in a scanning electron microscope,
Calculating a focus adjustment coefficient based on a correlation between the working distance, which is the distance between the sample stage and the electronic objective lens, and the electronic objective lens;
A second step of calculating a shape correction coefficient for correcting a shape of each work distance after focus adjustment;
A third step of calculating an actual size coefficient of the image observed for each working distance after the shape correction;
And a fourth process of databaseting the coefficients calculated through each process into coefficients of a function for determining equipment setting values according to arbitrary magnifications.

The method of claim 1, wherein the first process,
Calculating a zero coefficient for each working distance based on the distance correlation between the sample object and the electronic objective lens;
Calculating an electron objective lens current coefficient according to the working distance by obtaining a correlation between the working distance and an electron objective lens current (OLC) for moving the electron objective lens;
And calculating a correlation between the electron objective lens current and the acceleration voltage, and calculating the electron objective lens current coefficient for each acceleration voltage.

The method of claim 1, wherein the second process,
Calculating a rotational movement value of the image for each working distance based on the focal length and the rotational relationship of the image;
And calculating a distortion correction value for correcting an image distortion caused by a magnetic field effect for each working distance.

The method of claim 1, wherein the third process,
A free magnification adjustment method of a scanning electron microscope, characterized in that the actual size factor of the image is calculated based on a correction value for the length calculated using a standard sample for each working distance.

In the method of adjusting the magnification in a scanning electron microscope,
Calculating a focus adjustment coefficient based on a correlation between the working distance, which is the distance between the sample stage and the electronic objective lens, and the electronic objective lens;
Calculating a shape correction coefficient for correcting the shape of the image for each working distance after the focus adjustment;
Calculating an actual size coefficient of the image observed for each working distance after the shape correction;
Databaseting coefficients calculated through the respective processes into coefficients of a function for determining equipment setting values according to an arbitrary magnification;
Receiving an arbitrary magnification to be manipulated through the graphical user interface;
Drawing a function for determining equipment setting values;
Extracting a coefficient of a function from which a coefficient of a function for determining equipment setting values according to an arbitrary magnification is extracted from a database having a database;
Substituting the extracted coefficient into the function to extract equipment setting values for the arbitrary magnification;
And adjusting the magnification by automatically controlling each device according to the extracted device setting values.

The method of claim 5, wherein the receiving of the arbitrary magnification is performed.
A free magnification adjustment method of a scanning electron microscope, characterized in that an arbitrary magnification is input through a mouse or a joystick.

The method of claim 6, wherein the adjusting of the magnification is performed.
Free magnification adjustment method of the scanning electron microscope, characterized in that to display by continuously adjusting the magnification according to any magnification input through the mouse or joystick.

In the method of adjusting the magnification in a scanning electron microscope,
Calculating a focus adjustment coefficient based on a correlation between the working distance, which is the distance between the sample stage and the electronic objective lens, and the electronic objective lens;
Calculating a shape correction coefficient for correcting the shape of the image for each working distance after the focus adjustment;
Calculating an actual size coefficient of the image observed for each working distance after the shape correction;
Databaseting coefficients calculated through the respective processes into coefficients of a function for determining equipment setting values according to an arbitrary magnification;
Receiving an arbitrary magnification to be manipulated through the graphical user interface;
Drawing a function for determining equipment setting values;
Extracting a coefficient of a function from which a coefficient of a function for determining equipment setting values according to an arbitrary magnification is extracted from a database having a database;
Substituting the extracted coefficient into the function to extract equipment setting values for the arbitrary magnification;
Adjusting the magnification by automatically controlling each device according to the extracted device setting values;
And adjusting the magnification of the image obtained from the scanning electron microscope and displaying the image on a display.