JP2000098219A - Focusing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a focusing device wherein a focusing retrieval range required for each sample to be inspected is set and a time required for focusing retrieval operation is remarkably shortened. SOLUTION: Measuring light irradiates on the surface of a semiconductor wafer 6' through an objective lens 5, and the objective lens 5 is moved in an optical axis direction based on reflected light, so that focusing for the surface of the semiconductor wafer 6' is retrieved by the focusing device. In this case, focusing operation is executed while making the optical axis of the objective lens 5 move along a measuring line set on the semiconductor wafer 6', and the coordinate position of the objective lens 5 in the optical axis direction at each measuring point is detected by a counter 17. The focusing retrieval range is set from the maximum value and the minimum value of the coordinate position at that time by a focusing retrieval range setting means 18, so that the focusing operation for the surface of the semiconductor wafer 6' is executed based on the focusing retrieval range.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focusing device used for a microscope or an optical measuring device.

[0002]

2. Description of the Related Art Conventionally, as a focusing device, for example, as disclosed in Japanese Patent Application Laid-Open No. Sho 59-177510, a subject surface is irradiated with measurement light through an objective lens, and based on the reflected light, 2. Description of the Related Art There is known an apparatus in which focusing is performed on a surface.

FIG. 6 shows a schematic configuration of such a focusing device. A laser beam emitted from a semiconductor laser 1 is reflected by a deflecting beam splitter 2 and converted into a parallel light beam by an imaging lens 3 to obtain 1/1/1. After passing through the four-wavelength plate 4, the light is focused on the surface of the subject 6 on the stage 601 via the objective lens 5. Then, the light reflected on the surface of the subject 6 is again transmitted to the objective lens 5, the quarter-wave plate 4, and the imaging lens 3.
, And is incident on the deflection beam splitter 2, and then transmitted through the deflection beam splitter 2,
, And one of the light beams is divided into a first stop 8 located a distance L ahead of the focal point P of the imaging lens 3.
And the other light beam is received by the second light-receiving element 10 via the second stop 10 disposed behind the focal point P of the imaging lens 3 by the distance L. 11 to receive light. Further, an electric signal corresponding to the amount of reflected light from the first light receiving element 9 and the second light receiving element 11 on the surface of the subject 6 is input to the signal processing system 12, and each of the input electric signals is Thus, a predetermined calculation is performed, and an error signal corresponding to the displacement of the surface of the subject 6 is output.

In this case, the signal processing system 12 is supplied with electric signals A and B having characteristics as shown in FIG. 7A as inputs from the first light receiving element 9 and the second light receiving element 11. In this case, the calculation of (AB) / (A + B) is performed as a signal for detecting the displacement of the surface of the subject 6, and an error signal which becomes 0 at the focal point F as shown in FIG. The object 6 is located at a position where the error signal becomes 0.
The driver 13 (13 ') relatively moves between the objective lens 5 and the subject 6 in the optical axis direction (Z direction) so that the surface is located, so that a focus position is obtained.

The relative movement between the objective lens 5 and the subject 6 is limited by, for example, providing a limit detector 14 to prevent mechanical collision and collision between the objective lens 5 and the subject 6. The position is monitored and the focus search range is limited.

Accordingly, for example, as shown in FIG.
The limit position where the distance between the objective lens 5 and the subject 6 is the maximum is the focus search upper limit position (upper limit U), and the objective lens 5 and the subject 6
Is the focus search lower limit position (lower limit D) at which the distance is minimum, the search for the focus position is performed in the range between the upper limit U and the lower limit D. In the drawings, 15 is an illumination power supply for observation, and 16 is an observation optical system.

[0007]

The subject 6
Is a semiconductor wafer, for example, the stage 60
The height of the wafer surface is different between the search position A and the search position B as shown in FIG. May be different. In such a case, the focus search range required for the search position A is A ', whereas the focus search range required for the search position B is B', and these required focus search ranges A 'And B' are different.

For this reason, if the search position A is focused in the focus search range A ′, the search position is changed to B.
In the focus search range A ′ with respect to the search position B, no focus is obtained, and the apparatus temporarily moves the distance between the objective lens 5 and the subject 6 to the focus search upper limit position (upper limit U). The focus search is performed for the entire focus search range while reducing the distance from this position in the direction of the focus search lower limit position (lower limit D). There is a problem that work efficiency such as observation is greatly reduced.

The present invention has been made in view of the above circumstances, and can set a required focus search range for each subject.
It is an object of the present invention to provide a focusing device that can significantly reduce the time required for a focus search operation.

[0010]

According to the first aspect of the present invention,
The object surface is irradiated with measurement light through an objective lens, and the distance between the object and the objective lens is relatively moved in the optical axis direction based on the reflected light to search for the focus on the object surface. In the focusing device, while performing a focusing operation along a measurement line set on the subject, position detecting means for detecting the position of the subject or objective lens at each measurement point in the optical axis direction, A focus search range setting means for setting a focus search range from a maximum value and a minimum value of the position of the object or the objective lens in the optical axis direction detected by the position detection means; It is characterized in that a focusing operation on the subject surface is performed based on the focus search range set by the range setting means.

According to a second aspect of the present invention, in the first aspect, the measurement line set on the subject is:
It is characterized by comprising a straight or curved line passing at least through the center of the subject.

According to a third aspect of the present invention, in the first aspect of the present invention, further comprising a focus correction table storing a focus correction value for each objective lens, the focus correction table corresponding to the objective lens. The focus search range set by the focus search range setting means is corrected based on the focus correction value.

As a result, according to the first aspect of the present invention,
Since the focus search range required for the subject can be narrowed down when searching for the focus position, the time required for the focus search operation can be greatly reduced.

According to the second aspect of the present invention, the focus search range can be set based on the measurement values at a wide range of measurement points including the central part of the subject, so that the focus search range with high accuracy can be set. it can. According to the third aspect of the invention, it is possible to set an appropriate focus search range according to the subject in consideration of the amount of parallax correction even if the amount of parallax correction exists between the objective lenses.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 1 shows a schematic configuration of a focusing apparatus to which the present invention is applied, and the same parts as those in FIG. 6 are denoted by the same reference numerals.

In this case, it is assumed that a semiconductor wafer 6 ′ is mounted on the stage 601 as a subject. The center of the semiconductor wafer 6 ′ is suction-held by the air chuck 601 a on the stage 601.

A counter 17 is connected to the driver 13. The counter 17 detects the distance between the objective lens 5 and the semiconductor wafer 6 ′. Here, information on the optical axis direction (Z direction) between the objective lens 5 and the semiconductor wafer 6 ′ is transmitted from the driver 13. Receiving, Z of objective lens 5
This is for detecting the coordinate position in the direction.

The counter 17 is connected to a focus search range setting means 18. This focus search range setting means 18
Has a memory 181 and a comparator 182. Here, the memory 181 stores the contents of the counter 17. In this case, there are two data stored in the memory 181, and one is the upper limit data (MAX) of the focus search range.
Value), and the other is the lower limit data (MIN
Value). Further, the comparator 182 has a counter 1
7 and the contents of the memory 181. The MAX value and M in the memory 181 are compared according to the comparison result.
The IN value is updated.

A focusing controller 19, an XY driver 20 and an offset unit 21 are connected to the focusing search range setting means 18. The focus controller 19 is inserted before the driver 13, and controls the focus search range by the outputs of the memory 181 and the comparator 182. The XY driver 20 is for driving the stage 601 in the XY directions so as to move the semiconductor wafer 6 ′ in a direction orthogonal to the optical axis direction (Z direction). Also, the offset unit 2
Numeral 1 is for correcting the focus search range set by the focus search range setting means 18.

Next, the operation of the embodiment configured as described above will be described. First, the semiconductor wafer 6 'arbitrarily extracted as a basic sample is set on the stage 601, and a focus search range is set.

Also in this case, the center of the semiconductor wafer 6 'is suction-held by the air chuck 601a on the stage 601. The objective lens 5 has the lowest magnification. This is because low-magnification lenses have better focus tracking ability, and are not necessarily limited to this.

Next, as shown in FIG.
The measurement line SP for setting the focus search range is determined above. In this case, the measurement line SP designates a peripheral portion on the semiconductor wafer 6 ′ as the measurement start point S, and extends straight from the measurement start point S through the center of the semiconductor wafer 6 ′ toward the diagonal peripheral portion. Consists of lines.

In this state, first, the stage 601 is driven by the XY driver 20, and the focusing operation is performed by aligning the optical axis of the objective lens 5 with the measurement start point S.
Thus, the coordinate position of the objective lens 5 in the Z direction is detected from the information in the optical axis direction (Z direction) between the objective lens 5 and the semiconductor wafer 6 ′. The focus coordinate position of the measurement start point S is
The initial value is written to the MAX value and the MIN value in the memory 181.

Next, the stage 60 is controlled by the XY driver 20.
1 to move the optical axis of the objective lens 5 along the measurement line SP to perform the focusing operation.
At this time, the coordinate position of the objective lens 5 in the Z direction is detected.
Hereinafter, similarly, the optical axis of the objective lens 5 is set to the measurement line SP.
The focusing operation is performed for each measurement point while moving along the axis, and the coordinate position of the objective lens 5 in the Z direction is detected.

In this case, the coordinate position of the objective lens 5 in the Z direction is such that the center of the semiconductor wafer 6 ′ is supported on the stage 601 and the center is curved in a bulging direction as described above. It is obtained as a locus as shown in FIG. 3 along the curvature.

In this case, each object coordinate position is compared with a value written in the memory 181 by the comparator 182. If the coordinate position is larger than the value written in the MAX value in this comparison, Updates the MAX value with the value, and if the value is smaller than the value written in the MIN value, updates the MIN value with the value. Accordingly, the MAX value and the MIN value of the memory 181 at the time when the focusing operation along the measurement line SP is completed are the maximum values of the coordinate position in the Z direction of the objective lens 5 at the time of focusing on the measurement line SP. AF_U ′ and the minimum value AF_D ′ are respectively written.

In this case, if the focus search range measured in this way is set as it is as a focus search range for the subsequent focus operation, the focus search is performed when the focus operation is performed other than on the measurement line SP. Beyond the search range, as described in the conventional example, the objective lens 5 is raised to the focus search upper limit position,
The focus search may be performed for the entire focus search range while reducing the distance from this position in the direction of the focus search lower limit position.

Therefore, here, after the measurement is completed, the focus search range obtained as shown in FIG. 3 is modified in the following manner. In this case, the offset amount F is added from the offset unit 21 to the MAX value of the memory 181 and subtracted from the MIN value, and the focus search upper limit position AF_U and the focus search lower limit position AF_D of these new values are set to MAX. Values and MIN values are written to the memory 181 and these M
The range from the AX value to the MIN value is set as the focus search range. Here, the offset amount F of the offset unit 21
Is, for example, as shown in FIG. 3, the distance from the maximum value AF_U 'of the focus search range to the focus search upper limit position (upper limit U) is R, and the minimum value AF_D' is the focus search lower limit position (lower limit D). Assuming that the distance to R ′ is the maximum value, (R + R ′) / 4 is determined from this range, and a new value is focused on the focus search upper limit position AF_U by the determined offset amount F. The search lower limit position AF_D is set.

Thereafter, a focusing operation is performed at an arbitrary observation point on the semiconductor wafer 6 'based on the focusing search range set as described above. The focusing operation in this case is the focusing search upper limit position A written in the MAX value of the memory 181.
F_U and focus search lower limit position A written in the MIN value
By moving the objective lens 5 in the range of F_D, a search for a focal point is performed. That is, to perform the focus position search operation, the focus controller 19 moves the objective lens 5 to the position indicated by the focus search upper limit position AF_U, and then moves the objective lens 5 to the focus search lower limit position AF_D. While moving the objective lens 5 toward the position,
The focus search is performed.

Therefore, according to such a configuration, in order to search for the focus position, conventionally, the objective lens 5 is once extended to the focus search upper limit position (upper limit U), and the focus is adjusted from this position. The focus search is performed for the entire focus search range while reducing the distance in the direction of the search lower limit position (lower limit D), but the focus search range required for the semiconductor wafer 6 ′, that is, the focus search upper limit position AF_U is changed. Focus search lower limit position AF_
D can be narrowed down to perform the focus search operation, so that the time required for the focus search operation can be greatly reduced. In particular, many defect inspections of the same type of semiconductor wafer 6 'are performed. In such a case, time loss associated with the focus search operation can be minimized, and work efficiency can be dramatically improved.

In the first embodiment, the center portion of the semiconductor wafer 6 'serving as a basic sample is sucked and held by the air chuck 601a on the stage 601, but the periphery of the semiconductor wafer 6' The present invention can also be applied to a device that holds a part by suction. In this case, the focus search range is set in a state where the central portion of the semiconductor wafer 6 'is curved in a concave direction.

The counter 17 determines the Z of the objective lens 5.
The case where the coordinate position in the direction is detected has been described.
A coordinate position of 01 in the Z direction may be detected.
Further, the determination of the measurement line SP on the semiconductor wafer 6 'serving as a basic sample at the time of setting the focus search range is performed by taking the maximum value and the minimum value of the Z-axis coordinate of the objective lens 5 at the time of subsequent focus. Various applications are possible. For example, if a plurality of measurement lines SP intersecting at the center of the semiconductor wafer 6 ′ are used, the focus search range can be set more accurately, and a spiral that covers the sample surface to some extent can be obtained. Various settings can be made in combination with the offset amount, such as a curved line. Further, the measurement line SP is effective not only in the horizontal direction but also in combination with the rotational movement of the semiconductor wafer 6 ′ or in consideration of the inclination and the bending.

Further, the offset amount set by the offset unit 21 is not limited to a constant value, and may be variable based on, for example, the magnification of the objective lens 5 or the depth of focus.
Further, the value may be a value in consideration of the characteristics of the objective lens 5 depending on the temperature.

In addition, setting of the semiconductor wafer 6 ′ on the stage 601, replacement of the objective lens 5, capture of the maximum value and minimum value of the objective position on the measurement line SP, and capture value A series of flows, such as setting of a focus search range from, may be automatically performed in combination with an electric stage, an electric revolver, or the like, so that time efficiency may be further improved. Of course, some operations such as stage movement can be replaced with manual operation.
(Second Embodiment) FIG. 4 shows a schematic configuration of a second embodiment of the present invention, and the same parts as those in FIG. 1 are denoted by the same reference numerals.

In this case, reference numeral 5 'denotes another objective lens in place of the objective lens 5. The focus search range setting unit 18 further includes a focus correction table 183 and the adder 1.
84. The parallax correction table 183 stores the parallax correction values for each of the objective lenses 5 and 5 ′. Here, based on the objective information, the current objective position with respect to the reference objective in the Z-axis direction is stored. The correction amount is output. Thereby, when the objective lens 5 is changed to the objective lens 5 ′, the correction amount shown in the confocal correction table 183 is added to the value from the memory 181 via the adder 184 for the changed objective lens 5 ′ Then, by moving the position of the objective lens 5 'in the Z direction, the parfocal correction at the time of objective exchange is performed.

Next, the operation of the embodiment configured as described above will be described. In this case, it is assumed that the objective lens 5 has been changed to another objective lens 5 '. Also in this case, the focus search range is set, but during the setting of the focus search range,
The correction amount is not output from the parallax correction table 183. Therefore, the adder 184 outputs the value from the memory 181 as it is, and the same setting operation as described in the first embodiment is performed.

Here, the focus search range setting operation sets the focus search range a as the upper limit AF as shown in FIG.
_U and the lower limit AF_D are respectively set. When the focusing operation is performed in this state, the focus correction table 183 outputs a correction amount in the Z-axis direction from the changed objective information of the objective lens 5 ′. Assuming that the changed focus correction amount of the objective lens 5 'is Z_OFF_1, which is a brass value, the upper limit AF_U and the lower limit AF_D of the focus search range a shown in FIG. The search range a is corrected to the focus search range b shifted by the correction amount Z_OFF_1.

Similarly, even when the post-change focus correction amount of the objective lens 5 'is a negative value Z_OFF_2, the upper limit AF_U and the lower limit AF_D of the focus search range a shown in FIG. By 184, AF_U + Z_OFF_2 becomes AF_D + Z_OFF_2, and the focus search range a is corrected to the focus search range c shifted by the correction amount Z_OFF_2.

In this case, it goes without saying that which of the focus search ranges b and c gives priority to the range of the upper limit U and the lower limit D, which are the range of the limit value. Therefore, according to such a configuration, even if a parallax correction amount exists between the objective lenses 5 and 5 ', an appropriate focus search corresponding to the semiconductor wafer 6' in consideration of the parallax correction amount is performed. Since the range can be set, a more stable focusing operation can be performed quickly.

In the first and second embodiments described above, the case where the number of light receiving elements for performing photoelectric conversion is two has been described, but the number is not limited to this. In the above description, the objective lens 5 is moved in the optical axis direction (Z
Direction), the stage 601 may be driven in the optical axis direction (Z direction) using the driver 13 ′. Further, the focus detection means is not limited to the above-described method, but may employ another known method such as a pupil division method.

[0041]

As described above, according to the present invention, the required focus search range can be set for each subject, so that the time required for focus search can be reduced, and the minimum time Can be used for focusing.

Even if there is a parallax correction amount between the objective lenses, an appropriate focus search range can be set in accordance with the subject in consideration of the parallax correction amount, and a more stable focusing can be achieved. The operation can be performed quickly.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of a first embodiment of the present invention.

FIG. 2 is a diagram for explaining the operation of the first embodiment.

FIG. 3 is a diagram for explaining the operation of the first embodiment.

FIG. 4 is a diagram showing a schematic configuration of a second embodiment of the present invention.

FIG. 5 is a diagram for explaining the operation of the second embodiment.

FIG. 6 is a diagram showing a schematic configuration of a conventional focusing device.

FIG. 7 is a diagram for explaining a focusing operation of a conventional focusing device.

FIG. 8 is a diagram for explaining a focusing operation of a conventional focusing device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser 2 ... Deflection beam splitter 3 ... Imaging lens 4 ... Wave plate 5.5 '... Objective lens 6 ... Subject 6' ... Semiconductor wafer 601 ... Stage 601a ... Air chuck 7 ... Beam splitter 8.10 ... Aperture 9 First light receiving element 11 Second light receiving element 12 Signal processing system 13 Driver 14 Limit detector 15 Illumination light source 16 Observation optical system 17 Counter 18 Focusing search range setting means 181 Memory 182 Comparator 183 Parallax correction table 184 Adder 19 Focus controller 20 XY driver 21 Offset unit

Claims (3)

[Claims] An object surface is irradiated with measurement light through an objective lens, and a distance between the object and the objective lens is relatively moved in an optical axis direction based on the reflected light, so as to adjust the distance to the object surface. In a focusing apparatus for searching for a focus, a focusing operation is performed along a measurement line set on the subject, and a position of the subject or the objective lens in each of the measurement points in the optical axis direction is detected. A position detection unit; and a focus search range setting unit that sets a focus search range from a maximum value and a minimum value of the position of the subject or the objective lens in the optical axis direction detected by the position detection unit. A focusing device for performing a focusing operation on the subject surface based on the focus search range set by the focus search range setting means. 2. The focusing apparatus according to claim 1, wherein the measurement line set on the subject is at least a straight or curved line passing through the center of the subject. 3. The apparatus according to claim 1, further comprising a focus correction table storing a focus correction value for each objective lens, wherein the focus search range is based on the focus correction value of the focus correction table corresponding to the objective lens. 2. The focusing apparatus according to claim 1, wherein the focusing search range set by the setting unit is corrected.