JPH07245075A - Automatic focusing device - Google Patents

Abstract

PURPOSE:To speedily and easily achieve focusing of a charged particle beam device at high precision by measuring capacity between a sensor disposed close to an optical axis of the charged particle beam and a sample, detecting height information, and performing automatic focusing. CONSTITUTION:A sensor 11 is a doughnut-shaped plate having a hole for a beam passage 12 of an electron beam 5, and it is fixed to face a sample 4 to be an electrode for measuring capacity to the sample 4. The capacity to the sample 4 is measured by a lead 13 connected to the sensor 11, a current at an objective lens 2 is changed in accordance with the measured capacity, and the beam 5 is controlled to be focused on the sample 4. Automatic focusing in a device such as a short focus and high resolution scan type electron microscope can be performed at high precision speedily.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic focusing device for focusing a charged particle beam on a sample.

Microscopes using charged particles, such as scanning electron microscopes, are nowadays used in many fields of research and industry. In particular, in the semiconductor industry, with the miniaturization of processing, a dedicated scanning electron microscope has been used instead of a conventional optical microscope in various inspection processes. for that reason,
It is important that no special training is required for the operation, and it is necessary to automate it as much as possible, and in particular, there is a demand for automatically focusing the sample.

[0003]

2. Description of the Related Art Automatic focusing methods conventionally used in scanning electron microscopes are roughly classified into those using an electron beam itself and those other than that.

In the former method, the focus information is obtained from the secondary electron image on the sample surface, or the focus information is obtained not from the image but from the secondary electron signal obtained by line scanning. When the electron beam is narrowed down and irradiated on the sample surface and scanned, the change in the secondary electron image and the secondary electron signal on the sample surface increases, so this is detected and the current flowing through the lens is controlled. I try to focus automatically.

In the latter, as shown in FIG. 5, for example, a light beam other than an electron beam is obtained to obtain height information of a position where the sample is irradiated and reflected, and a current flowing through the objective lens 32 is controlled to focus the sample. I am trying. The configuration and operation of FIG. 5 will be briefly described below.

FIG. 5 shows an explanatory view of the prior art. In FIG. 5, the lens barrel 31 has an electron gun or the like for generating an electron beam provided therein.

The objective lens 32 focuses the generated electron beam on the sample 34. The sample chamber 33 is
This is a room for holding and moving the sample 34 in a vacuum.

The sample 34 is to be observed by irradiating it with an electron beam, and is, for example, a semiconductor wafer, which is placed on a sample moving mechanism (not shown). The electron beam 35 is an electron beam that irradiates the sample 34.

The laser beam 36 is generated by a laser generator 37.
Is a laser beam emitted toward the sample 34 from.
The laser beam 36 ′ is the laser beam reflected from the sample 34.

The detector 38 detects the laser beam 36 'reflected by the sample 34. Amplifier 3
9 is for amplifying the change in the signal reflected on the sample 34 and detected by the detector 38, here the change in the signal corresponding to the change in the height of the sample 34.

The objective lens power supply 40 is based on the signal from the amplifier 39, and when the surface of the sample 34 is raised, the objective lens 32 is detected and amplified by the detector 38 and the amplifier 39. To increase the current supplied to, or conversely decrease the current when the surface of the sample 34 becomes low.

Next, the operation will be briefly described. The surface of the sample 34 is irradiated with the laser beam 36 from the laser generator 37 on the sample 34 placed on the sample moving mechanism (not shown), and the laser beam 36 ′ reflected from the sample 34 is detected by the detector 38. Then, the detected signal is amplified by the amplifier 39,
The objective lens power supply 40 is controlled based on this amplified control signal, and the current supplied to the objective lens 32 is controlled. For example, when the surface of the sample 34 is raised by moving the sample 34, for example, when the surface of the semiconductor wear, which is the sample 34, is raised, the laser beam 36 ′ reflected from the wafer.
Since the position in the height direction of the object changes, the change in the height direction is detected by the detector 38, the detected signal is amplified by the amplifier 39, and the control signal is supplied to the objective lens power supply 40 and supplied to the objective lens 32. To increase the current. On the other hand, when the surface of the semiconductor wafer becomes low, the current supplied to the objective lens 32 is reduced. As a result, it becomes possible to always scan the surface of the sample 34 with the electron beam 35 in the imaged state, and observe the secondary electron image in the always focused state.

[0013]

The focus information is obtained from the secondary electron image obtained by irradiating the sample 34 with the above-mentioned electron beam or the signal obtained by line scanning, and the current of the objective lens 32 is controlled. And when adjusting automatically so that the focus is always on the surface of the sample 34, the electron beam is used, so the resolution is excellent,
In principle, highly accurate focusing can be performed.

However, in order to increase the S / N ratio of the secondary electron signal obtained by irradiating the sample 34 with the electron beam, a certain amount of information storage time is required, so that high-speed focusing is required. There was the problem of becoming an obstacle. That is, the image obtained by the scanning electron microscope is inferior in S / N ratio to the image obtained by the light source microscope,
If the intensity of the electron beam is increased, it will be damaged when the sample is irradiated, so that it cannot be unnecessarily strong, and it is absolutely necessary to accumulate signals to improve the S / N ratio.

On the other hand, according to the latter configuration of FIG. 5 described above, the deviation of the position of the laser beam 36 'which irradiates the sample 34 with the laser beam 36 and is reflected in response to the change in the height of the sample surface is detected. The height change is obtained by increasing the current of the objective lens 32 when the sample 34 moves in the high direction, and the objective lens 3 increases when the sample 34 moves in the low direction.
Two currents can be reduced for autofocus.

However, as the resolution of the scanning electron microscope becomes higher, the distance (working distance) between the objective lens and the sample surface becomes smaller, and the space for taking the optical path of the laser beam is lost. Further, as the resolution becomes higher, the depth of focus of the electron microscope becomes shallower, and the method shown in FIG. 5 cannot detect a sufficient change in height, resulting in defocusing.

The present invention solves these problems.
The height between the sensor and the sample (or the flat plate corresponding to the sample) placed near the optical axis of the charged particle beam is measured to detect the height information, and the charged particle beam is automatically focused. Focus The objective is to realize automatic focusing of a device such as a high-resolution electron microscope with high precision, speed and simple structure.

[0018]

[Means for Solving the Problems] Means for solving the problems will be described with reference to FIG. In FIG. 1, the sensor 1
Reference numeral 1 is for facing the flat plate fixed to the sample 4 or a portion interlocked with the movement of the sample 4 to measure the capacity corresponding to the height of the sample 4.

The sample 4 is a sample to be automatically focused on the charged particle beam.

[0020]

According to the present invention, as shown in FIG. 1, the sensor 1 fixed and arranged around the optical axis of the charged particle beam so as to face the sample 4.
1, the capacitance between the sample 4 and the sensor 11 is measured,
The control circuit controls the current or voltage measured in advance corresponding to the measured capacitance for automatic focusing.

At this time, as the sensor 11, one donut-shaped sensor 11 having a hole around the optical axis is fixedly arranged or divided into a plurality of holes having a hole at the optical axis. The sensors 11 are fixed, the capacitance between them and the sample is measured, and automatic focusing is performed.

Further, the capacitance between a flat plate and a center 11 which are arranged at a flat portion of a moving mechanism for moving the sample 4 or near the sample 4 and connected to substantially the same height,
I try to focus automatically.

An electron beam is used as the charged particle beam, and the electron beam is automatically focused on the sample 4. Therefore, the height information is detected by measuring the capacitance between the sensor 11 arranged near the optical axis of the charged particle beam and the sample 4 (or the flat plate corresponding to the sample 4) to automatically focus the charged particle beam. By performing the adjustment, it becomes possible to realize automatic focusing of a charged particle beam device such as a scanning electron microscope having a short focus and high resolution with high accuracy, speed and simple structure.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the construction and operation of an embodiment of the present invention will be described in detail with reference to FIGS.

FIG. 1 shows a block diagram of an embodiment of the present invention.
In FIG. 1, an electron beam 5 is emitted from an electron gun (not shown), and a current is passed through an objective lens 2 from a control circuit (not shown) to focus on the sample 4. A so-called secondary electron image is displayed by surface-scanning the focused electron beam, collecting secondary electrons generated at that time, scanning in synchronization with the display, and performing brightness modulation.

The objective lens 2 focuses the electron beam 5 on the sample 4, and a current is supplied from a control circuit (not shown) to adjust the strength of the magnetic lens to adjust the electron beam 5 to the sample 4. Focus on above.

The beam passage 12 is a passage through which the electron beam 5 passes. The passage 12 and the periphery of the sample 4 in the sample chamber are evacuated to a vacuum. Sample 4
Is a sample to be surface-scanned while the electron beam 5 is focused, and collects, for example, secondary electrons generated at that time to display a secondary electron image and observes the sample, which is, for example, a wafer.

The sensor 11 is a donut-shaped plate having a hole in the beam passage 12 of the electron beam 5 and is fixed so as to face the sample 4 and is used to measure the capacity between the sensor 4 and the sample 4. It becomes an electrode. Since the measured capacitance changes depending on the distance between the sensor 11 and the sample 4, the current of the objective lens 2 is changed corresponding to this capacitance so that the electron beam 5 is always focused on the sample 4. In addition, a control circuit (not shown) controls (described later with reference to FIGS. 2 and 3).

The lead wire 13 is for drawing out a conducting wire from the sensor 11 to the outside and measuring the capacitance between the lead wire 13 and the sample 4. Next, the operation when the capacitance between the sensor 11 and the sample 4 is measured and the electron beam 5 is automatically focused on the sample 4 in the configuration of FIG. 1 will be described in detail with reference to FIGS. 2 and 3. To do.

FIG. 2 is an explanatory view of the relationship between distance and capacity according to the present invention. This represents the relationship between the distance between the sensor 11, which is a donut-shaped flat plate having a hole through which the electron beam 5 passes in the center of FIG. 1, and the sample 4, for example, the wafer, and the capacitance. The horizontal axis represents the distance between the sensor 11 and the sample 4,
The vertical axis represents the measured capacitance between the sensor 11 and the sample 4.

Here, the distance between the sensor 11 and the sample 4 is h
When it is (mm), the measured capacity is Ch (farad). It is preliminarily measured that the capacitance Ch changes by ΔC when the distance h changes by Δh back and forth as shown in the figure. From this relationship, when it is measured that the capacitance Ch has changed by ΔC, it can be detected that the height of the sample 4 has changed by Δh. Then, the electron beam 5 can always be focused on the sample 4 by adjusting the current flowing through the objective lens 2 (current calculated using FIG. 3) by an amount corresponding to the height change Δh. Becomes

FIG. 3 is a diagram for explaining the relationship between the capacity and the focus current of the present invention. This is a sensor 11 which is a donut-shaped flat plate having a hole through which the electron beam 5 passes in the center of FIG.
The relationship between the capacitance between the sample 4, for example, a wafer, and the current i flowing through the objective lens 2 when the electron beam 5 is focused on the sample 4 is shown. The horizontal axis represents the focus current i when the electron beam 5 flowing through the objective lens 2 is focused on the sample 4, and the vertical axis represents the capacitance between the sensor 11 and the sample 4.

Here, the capacitance between the sensor 11 and the sample 4 is C
When h, the focus current for focusing the electron beam 5 on the sample 4 is measured as i0 in advance, and when the capacitance Ch changes by ΔC, if the focus current is changed by Δi corresponding to the ΔC, The electron beam 5 will be focused on the sample 4. Therefore, the relationship between the measured capacitance C between the sensor 11 and the sample 4 and the focus current i when the electron beam 5 at that time is focused on the sample 4 is obtained in advance,
The curve shown in the figure is stored as a function or set in a table. Then, when the capacitance C between the sensor 11 and the sample 4 is measured, the focus current i is calculated by substituting it into a stored function, or the focus current i is read out by referring to a table. Then, the control circuit supplies the focus current i to the objective lens 2, and even if the height of the sample 4 changes, it is detected as a change in capacitance, and the electron beam 5 is automatically focused on the sample 4. Is possible. For example, in the capacitance measurement, the accuracy between the sensor 11 and the sample 4 (for example, a wafer) can be detected with a resolution (accuracy) of about 0.1%. Therefore, the reference value h of the distance between the sensor 11 and the sample 4 is set. Two
If it is mm, the accuracy in the height direction can be detected as 2 mm × 0.1% = 2 μm 2. Detection accuracy (resolution) in this height direction
2 μm is a value that is sufficiently practical for automatic focusing for a scanning electron microscope. Further, in order to improve the detection accuracy in the height direction, if the measurement accuracy of the capacitance is improved to more than 0.1%, the accuracy of the automatic focusing in the height direction is further improved accordingly.

FIG. 4 shows an example of the shape of the sensor of the present invention.
1 to 3, the sensor 11 is a donut-shaped flat plate having a circular hole in the center, but this is extremely convenient for a flat sample 4 such as a wafer, and is practically used. The electron beam 5 can always be focused on the sample 4 with good accuracy, but if the surface of the sample 4 has irregularities, or if there are locally concave or convex portions,
The following optimum sensor 11 corresponding to the shape of the surface of 1 may be selected.

FIG. 4A shows an example of the doughnut-shaped flat plate sensor 11 in which the hole which becomes the passage of the electron beam 5 is provided at the center described above. The shaded portion is the sensor 11, and the capacitance between the shaded portion and the sample 4 is measured, and the focus current corresponding to the measured capacitance is passed through the objective lens 2 so that the electron beam 5 is always on the sample 4. It becomes possible to focus on. The sensor 11 having this shape is particularly suitable for the sample 4 having a flat surface such as a wafer.

FIG. 4B shows a structure in which three circular sensors 11 are provided in the center of the sample 4 while avoiding a part which is a passage for the electron beam 5 in the center. And the focus current is passed through the objective lens 2 so that the electron beam 5 is always focused on the sample 4. The three-divided sensor 11 measures the capacitances at different positions of the sample 4, adopts the average capacitance of the three measured capacitances, and determines the focus current corresponding to the average height position. The electron beam is made to flow through the objective lens 2 to focus the electron beam on an average position when there is unevenness on the sample 4, and the maximum (or minimum) of the three measured capacities.
By adopting the above capacity, the focus current corresponding to the position of maximum (or minimum) height is made to flow through the objective lens 2 and the electron beam is most convex (or most concave) when there is unevenness on the sample 4. ), And further, it is possible to focus on any position between and.

FIG. 4 (c) shows an example in which the donut-shaped flat plate of FIG. 4 (a) is divided into four parts, avoiding the part which becomes the passage of the electron beam 5 in the center, and the parts of the diagonal lines. , Sample 4
The capacitance between and is measured, and the focusing current is caused to flow through the objective lens 2 based on the measured capacitance.
Is always focused on the sample 4. The four-divided sensor 11 measures the capacitances at different positions of the sample 4, and is the most convex position, the average position, the most concave position, as in (b) of FIG. Alternatively, it becomes possible to pass a focus current corresponding to an arbitrary position of these and focus the electron beam 5 at a position corresponding to each. Furthermore, because it is an axis object, 4
Due to the difference in the measured capacitances of the respective divided sensor 11 portions, the inclination on the sample 4 is found, and the focus current is dynamically changed in the direction corresponding to this inclination, even if the sample 4 is inclined. It is also possible to automatically focus on the tilt.

FIG. 4D shows an example of wiring. this is,
When each sensor 11 is divided into a plurality of parts, as shown in the figure, the lead wires are shielded from each of the divided sensors 11 and drawn out to the outside (in the figure, they are collectively shown as one, but the number is actually equal to the number of divisions). The number of signal lines is drawn), and the capacitance is measured by the control circuit, and the focus current is determined based on the function or table of the capacitance-focus current obtained in advance, and the focus current is passed to the objective lens 2 for automatic focusing. It is for.

[0039]

As described above, according to the present invention,
Sensor 11 and sample 4 arranged near the optical axis of the charged particle beam
(Or a flat plate corresponding to the sample 4) The height information is detected by measuring the capacitance and the automatic focusing of the charged particle beam is adopted. Automatic focusing of a charged particle beam device such as an electron microscope can be realized with high precision, speed and simple structure. In particular, according to the present invention in which the height information is detected by measuring the capacitance between the sample 4 and the fixed sensor 11 and automatic focusing is performed, the signal of the conventional secondary electron image is differentiated to obtain the maximum size. It is possible to detect the capacitance and perform automatic focusing at an extremely high speed, and to simplify the configuration and control, as compared with a device that adjusts the focus current to achieve automatic focusing.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of the present invention.

FIG. 2 is an explanatory diagram of the relationship between distance and capacity according to the present invention.

FIG. 3 is a diagram illustrating the relationship between the capacitance and the focus current according to the present invention.

FIG. 4 is an example of the shape of the sensor of the present invention.

FIG. 5 is an explanatory diagram of a conventional technique.

[Explanation of symbols]

 2: Objective lens 4: Sample 5: Electron beam 11: Sensor 12: Beam passage 13: Lead wire

Claims (4)

[Claims] 1. An automatic focusing device for focusing a charged particle beam on a sample, comprising: a sensor (11) fixed and arranged to face the sample (4) around an optical axis of the charged particle beam; Control circuit for measuring the capacitance between the sensor (11) and the sensor (11), controlling the current or voltage measured in advance corresponding to the measured capacitance, and focusing the charged particle beam on the sample (4). An automatic focusing device comprising: 2. As the sensor (11), one donut-shaped sensor (11) having a hole around the optical axis is fixedly arranged, or a hole is provided around the optical axis. 2. The automatic focusing device according to claim 1, wherein the plurality of divided sensors (11) are fixedly arranged. 3. A flat part of a moving mechanism for moving the sample (4) or the vicinity of the sample (4) instead of measuring the capacitance between the sample (4) and the sensor (11). The automatic focusing device according to claim 1 or 2, wherein the capacitance between the flat plates connected to each other at substantially the same height is measured. 4. The automatic focusing device according to claim 1, wherein the charged particle beam is an electron beam.