JPH0896738A - Automatic focusing device - Google Patents

Abstract

PURPOSE: To provide high accuracy by performing a process of allowing a current, saturating a magnetic field of an objective lens, to flow in positive/ negative sides, before setting a proper current, so as to improve repeatability of the magnetic field by the same current value. CONSTITUTION: In accordance with a wafer information file, after a stage 8 is started to move to a measuring point, a positive exciting current, saturating a magnetic field of an objective lens 3, is set. After the excited magnetic field is stabilized, a negative current, saturating a magnetic field, is set. After control in the above, whether leading or not to in the vicinity of a stage target point is judged, to start a Z sensor when in the vicinity of the target point, and measuring height is performed, to obtain, from this result, an exciting current value set. A point of starting the Z sensor is set previously to the point where a time required for moving to the target point from the point of starting this Z sensor and a time required for stabilizing an obtained image by setting the current value from starting the Z sensor are generated in almost the same length.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to automatic focusing in a scanning electron microscope.

[0002]

2. Description of the Related Art An automatic focusing device using an electron beam length measuring device and a height detecting device (hereinafter referred to as a Z sensor) for obtaining a sample height from the position of reflected light and a flow of operations of a conventional electron beam length measuring device are described. explain.

In FIG. 1, an observation sample 4 is irradiated with an electron beam 1 generated from an electron gun (not shown) which is finely focused by an objective lens 3. Reference numerals 2X and 2Y are deflection coils for scanning the electron beam 1 on the sample. The secondary electrons 7 generated from the sample 4 are detected by the detector 5, and the output signal from the amplifier 6 is brightness-modulated in synchronization with the deflection coils 2X and 2Y to obtain an image of a scanning electron microscope. However, the above coils and lenses are electromagnetic lenses. The stage 8 is controlled by the stage controller 15. The controller stores the coordinate data of the measurement point from the wafer information file stored in the external storage device 9 in which the coordinates of the point to be measured are registered. Can be received and the wafer can be moved to the appropriate measurement point. This wafer information file is used for each wafer process such as gate creation or contact hole creation process
A memory or a 4M memory wafer or the like is registered for each type of wafer, and a file corresponding to a wafer to be measured is called for each wafer from an external storage device at the time of measurement.

Next, Z in the apparatus having the above configuration
An automatic focusing mechanism using a sensor is described.

The height detector collects the probe light emitted from the light source 11 such as a light emitting diode or a laser on the sample 4 at an angle θ, and determines the position of the center of gravity of this reflected light by PSD.
It is detected by an element or an optical position detector 12 such as a CCD camera. In the case of a PSD element, the barycentric position of the light intensity of incident light is detected. As an example of the optical system, an example in which the optical lenses 13 and 14 are arranged in front of and behind the sample is shown. In this height detector, when the sample height changes by δZ _WAF with respect to the position w0 of the probe light incident on the optical position detector when the wafer is at the preset reference height W0, the optical position detector changes The position of the incident light changes by δZ _OPT . Therefore, the height change amount δZ _WAF of the sample detects δZ _opt ,

[0006]

## EQU1 ## δZ _WAF = δZ _OPT / (M × 2 sin θ) (Equation 1) However, M shows the magnification of the optical system.

On the other hand, in focusing of the scanning electron microscope, the objective lens controller 10 controls the amount of exciting current flowing through the objective lens 3 to control the focus height of the electron beam by controlling the objective lens 3 and the height of the observation sample surface. According to the situation. Wafer is δ
When the image of the electron microscope is focused on this at the height of Z _WAF , the focus position of the electron beam is also δZ _WAF , but the focus height of this electron beam δZ _WAF and the corresponding excitation of the objective lens. When the function with the current value δI is calculated,

[0008]

## EQU00002 ## .delta.I = f (.delta.Z _WAF ) (Equation 2) If the composite function of Equations 1 and 2 is g, if the output .delta.Z _OPT from the height detector is known, the excitation of the objective lens from Equation 3 The current value δI is obtained.

[0009]

Equation 3] _{δI = g (δZ OPT) ...} ( Equation 3) Thus, determined the number 3 at the time of device adjustment, if saved, in the automatic focusing run, seeking .delta.Z _opt, the objective current .delta.I
The process ends by calculating and setting. The objective lens control unit 10 also controls the automatic focusing device by the Z sensor. Next, the flow of the conventional processing of one sample wafer in the apparatus having the above configuration is shown in FIG. 2 and below.

1. The sample wafer is loaded into the sample chamber.

2. Load the wafer information file corresponding to the wafer.

3. 3. Perform origin correction and θ correction for stage coordinates according to the wafer information file. It moves to the coordinate point to be measured according to the wafer information.

5. Performs automatic focusing by the Z sensor.

(Height measurement by Z sensor, calculation and setting of objective current value by equation 3) 6. Find the pattern to be measured and set it to the measurement magnification (example: 50,000 times or more).

7. Measure and observe the measurement points.

8. If there are other measurement points, 4. ~ 8. I do.

9. The wafer is unloaded from the sample chamber.

[0018]

The problems to be solved by the present invention are to improve the magnetic field reproducibility for the same objective current value and to start the Z sensor operation after the stage is stopped until the image becomes stable after the operation. It saves time.

In the prior art, the automatic focusing accuracy depends on the accuracy of wafer height measurement and the accuracy of magnetic field reproducibility. Here, the following points were raised as problems regarding magnetic field reproducibility.

1. Even if the wafer height is measured accurately, if the magnetic field reproducibility is not good, the relationship between the wafer height obtained when adjusting the device and the objective current at the positive focus (Equation 3) does not hold and good performance is obtained. I can't.

The reason why the magnetic field reproducibility is deteriorated is that a magnetic hysteresis is generated in order to control a magnetic field by using an electromagnetic lens applying an electromagnet as an objective lens and operating an exciting current amount flowing in the electromagnetic lens at the time of focusing. is there.

2. In the prior art, after the wafer stopped at the measurement point, the height was measured by the Z sensor, and the exciting current value to be set was calculated and set from this result using Equation 3.
In this method, it takes about several seconds until the image becomes stable after the automatic focusing processing time and the setting of the objective current value.

That was a problem. Although this is a short time for autofocusing alone, it is a function to perform only the number of measurement points when considered as a system, and when trying to shorten the processing time, it becomes a big factor and shortening is strongly desired. is there.

[0024]

SUMMARY OF THE INVENTION With respect to the above-mentioned first problem, the present invention adopted as a method for removing magnetic hysteresis is as follows. Before the objective current calculated from the detection height of the Z sensor is set, an exciting current that saturates the magnetic field of the objective lens is passed in the positive and negative directions to create a fixed magnetic history.

2. Before passing the objective current value obtained from the height detection value of the Z sensor, be sure to perform the process of passing the maximum value of the movable range of the objective current used for focusing on the objective lens to the next minimum value (or It is characterized by creating a fixed magnetic history with the minimum value first and then the maximum value.

In the second aspect of the present invention, which is taken to shorten the execution time of the automatic focusing apparatus using the Z sensor, the above-mentioned items 1 and 2 for forming a fixed magnetic history during the movement of the stage. This is characterized in that the processing is performed, the height detection processing and the objective current setting are completed immediately before the stage is stopped, and the magnetic field is in a stable state when the stage is stopped, and a stable image is obtained.

[0027]

Operation: To obtain the height of the sample and perform a process for improving the reproducibility of the magnetic field through a certain history in the automatic focusing device that sets the exciting current in the electromagnetic lens so that the height is focused. Therefore, the total processing time is shortened by improving the accuracy and performing this processing while the stage is moving. Further, the height detection and the setting of the exciting current of the electromagnetic lens are completed immediately before the stage is stopped, thereby shortening the processing time.

[0028]

EXAMPLE FIG. 3 shows an example of a procedure for carrying out the present invention.

After moving the stage to the measurement point according to the wafer information file, a positive exciting current is set to saturate the magnetic field of the objective lens. After stabilizing the excited magnetic field, it is set to a negative current that saturates the magnetic field. This state is shown in FIG.

However, in the case of this method, (1) it takes too much time until a stable magnetic field is obtained after setting the exciting current.

(2) There is a limitation that the device must have a capacity capable of passing an exciting current to saturate the magnet.

Therefore, this control may be performed as follows. That is, it is a method of setting the exciting current of the objective lens to the maximum value of the control range of the lens after starting the movement of the stage to the measurement point, and setting the minimum value of the control range of the lens after stabilizing the excited magnetic field. In this case, all the limiting items when setting the current that can obtain the saturation magnetic field are good. This state is shown in FIG.

In any case, the order of setting the currents may be reversed. That is, a positive current may be set after setting a negative value, or a maximum value may be set after setting a minimum value. However, this control must always be performed in the same order.

After the above control, it is judged whether or not the stage has reached the vicinity of the target point, and if it is in the vicinity of the target point, the Z sensor is activated and the height is measured. Obtain and set the value. The point at which the Z sensor is activated is the time required to move from that point to the target point, and Z
The current value is set after the sensor is activated, and is set in advance to a point where the time required for the obtained image to stabilize becomes approximately the same.

[0035]

As described above, according to the present invention, in the automatic focusing apparatus for the particle beam apparatus, the accuracy of the automatic focusing apparatus is improved and the processing time is shortened by improving the reproducibility of the magnetic field obtained by applying the same excitation current to the objective lens. Can be realized.

By adopting the present invention, since the same history can be maintained during execution of automatic focusing, there is a one-to-one relationship between the objective current and the magnetic field, and high accuracy can be achieved.

Further, according to the conventional method, the history is made the same during movement of the stage of the present invention, and the process from height detection to automatic focusing is completed before the stage is stopped. The execution time of these can be shortened. Assuming this execution time is 3 seconds,
3 to measure 5 points in one sample with conventional equipment
If it takes minutes, 3 seconds × 5 points = 15 seconds, 15 seconds / 3 minutes × 100 = 8.3 according to the present invention.
(%), And the processing time can be shortened by about 8%.

[Brief description of drawings]

FIG. 1 is a block diagram of an automatic focusing device using a height detection device and a scanning electron microscope.

FIG. 2 is a flowchart of conventional processing of one sample wafer in a conventional apparatus.

FIG. 3 is a flowchart of processing one sample wafer according to the present invention.

FIG. 4 is a magnetic hysteresis characteristic diagram when an objective current is passed until the magnetic field is saturated.

FIG. 5 is a magnetic hysteresis characteristic diagram when the maximum and minimum currents in the variable range of the objective current are passed.

[Explanation of symbols]

1 ... Electron beam, 2X, 2Y ... X and Y deflection coils, 3 ...
Objective lens, 4 ... Sample, 5 ... Detector, 6 ... Amplifier, 7 ...
Secondary electron, 8 ... Stage, 9 ... External storage device, 10 ... Objective lens control device, 11 ... Light source, 12 ... Optical position detector,
13, 14 ... Optical lens, 15 ... Stage control device.

Claims (4)

[Claims] 1. A surface of a sample is irradiated with probe light and the height of the sample is determined from the position of the reflected light. In an automatic focusing device that performs focusing by obtaining an appropriate exciting current from the table or an approximate expression, be sure to set the current at which the magnetic field of the objective lens is saturated to the positive side and the negative side before setting the appropriate current. An automatic focusing device characterized by improving the reproducibility of a magnetic field at the same current value by performing a flowing process. 2. The maximum value of the movable range of the objective current used for focusing with respect to the objective lens, before flowing the objective current value obtained from the height detection value of the detection device according to claim 1. An automatic focusing device that improves the reproducibility of the magnetic field with the same excitation current by performing the process of flowing the minimum value. 3. The automatic focusing device according to claim 1, wherein the operation is completed while the sample is moving to the measurement point. 4. The height detection operation according to claim 1, which is started and ended when the sample moves to the vicinity of the measurement point, and when the measurement point is reached, an appropriate excitation current value to be supplied to the objective current. An auto-focusing device that completes the calculations and settings of.