JPS63239412A - Automatic focusing system - Google Patents

Abstract

PURPOSE:To remove the influence of variation in the thickness of a member and to improve defocusing detection accuracy by detecting defocusing optically in the form of the contrast ratio of two light and shade pattern projected on the surface of the member. CONSTITUTION:A wafer 12 on an XYZ base 10 is irradiated with an S-polarized laser beam 14 and vertical scattered light is received by a photodetection part 20 to extract a P and an S polarized light component and also move and scan the base 10. Parallel white light is projected on pattern plates 22 and 24, a lens 30 is adjusted to project a pattern of light and shade stripes on the wafer surface, and the intermediate point between both plates at the time of focusing is image-formed on the wafer. Shade parts 22a and 24a of the pattern are arranged alternately. A control circuit 34 controls and drives the base 10 with a signal from the output of a two-dimensional CCD 32 to cancel deviation in focusing. The visual field 32a of the CCD 32 is handled while image pickup areas A and B of the pattern stripes 24a and 22a are divided alternately. The intermediate point between two successive light and shade patterns is image- formed on the wafer surface, so any pattern becomes light images and the mean contrast is nearly 1. Consequently, the focusing is performed stably with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an autofocus method applied to focusing control between a member surface and an observation optical system in an apparatus for optically observing the surface of a member.

[Prior Art] As a mainstay device, there is a foreign matter inspection device that inspects minute foreign matter adhering to the surface of an integrated circuit wafer.

In this foreign matter inspection apparatus, generally, the viewing W11 optical system for detecting foreign matter on the wafer surface is fixed, and foreign matter detection is performed while moving the wafer. In devices with high detection accuracy, an autofocus method is applied to focus the observation optical system on the wafer surface during inspection.

Conventionally, this autofocus method detects the amount of defocus between the observation optical system and the wafer surface using a capacitance displacement meter fixed to the observation optical system, and then moves the wafer to [
By slightly moving the IJ optical measurement system in the optical axis direction,
The objective was to focus the observation optical system on the wafer surface.

[Problems to be Solved] However, since the detection accuracy of the amount of defocus is relatively low and it is easily affected by changes in the thickness of the wafer, the accuracy and stability of focusing have been inadequate.

This invention was made in view of this problem, and it is used in a device that optically observes the surface of a component such as a wafer foreign object inspection device, and allows for high-precision focusing between the observation optical system and the surface of the component such as a wafer. It is an object of the present invention to provide an autofocus method that can be performed stably and that allows easy adjustment of focus accuracy.

[Means for Solving the Problems] In order to achieve this L1 objective, the present invention provides a projection optical system fixed to the E optical measurement system for observing the surface of a member, and a projection optical system that a mechanism for stacking two pattern plates back and forth on an axis and fixing them so that their positions can be adjusted at least in the direction of the optical axis, the two pattern plates facing each other and having shading patterns; a two-dimensional image sensor for imaging the shading pattern of each of the pattern plates projected onto the surface of the member by a projection optical system, and an output signal of the two-dimensional image sensor as input;
Controlling a driving means for relatively moving the component and the observation optical system so that an average contrast ratio of the light and shade patterns of each of the pattern plates projected onto the surface of the M strip becomes approximately 1. Equipped with 1 stage focus control,
The projection optical system is configured such that when the surface of the member and the focal point of the observation optical system are aligned, an image is formed approximately at the midpoint between the two pattern plates on the surface of the member. be.

[Function] As described above, this invention is configured to optically detect defocus in the form of the contrast ratio of two shading patterns projected on the surface of a member, so it is not possible to use the conventional method using a capacitance displacement meter. The influence of variations in the thickness of the members can be essentially eliminated, and the accuracy of detecting defocus can be greatly improved.

Further, this contrast ratio is an average contrast ratio determined from an output signal of a two-dimensional image sensor having a wide field of view, and is hardly affected by minute foreign matter on the surface of the member.

Therefore, the surface of a member such as a wafer observed by the rm optical system and the observation optical system can be focused with high accuracy and stability.

In addition, since the front and rear pattern plates are provided with light and shade patterns where they face each other, and the position of the pattern plates at least in the optical axis direction of the projection optical system can be adjusted, as will be described in detail in connection with the embodiments, defocusing can be avoided. The width, lower limit, and upper limit of the allowable range can be arbitrarily set, and the autofocus system can be adjusted easily and accurately.

[Embodiment] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic diagram of an autofocus system of the present invention applied to a wafer foreign matter inspection apparatus.

In the figure, lO is an XYZ moving in each direction of x and ylz.
This is a stage, on which the wafer 12 to be inspected is fixed by negative pressure suction or the like.

In the foreign matter inspection apparatus of this embodiment, the surface of the wafer 12 is irradiated with an S-polarized laser beam, and the scattered light in the vertical direction is received by the IIJ optical measurement system, and the intensity of the scattered light is
Foreign matter on the surface of the wafer 12 is detected based on the intensity of the P-polarized component of the scattered light or the ratio of the intensities of the P-polarized component and the S-polarized component of the scattered light.

However, the autofocus method can be similarly applied to a foreign material Je4 scanning apparatus configured to irradiate a wafer surface with a light beam such as a non-polarized laser beam.

14 is a laser oscillator for irradiating the S-polarized laser beam, and 1B is an objective lens of the observation optical system. Scattered light in the vertical direction from the laser irradiation spot on the surface of the wafer 12 is transmitted through the objective lens 16 and the half mirror 1.
8 and enters the light receiving section 20 of the observation optical system. The light receiving section 20 includes means for extracting the P-polarized light component and the S-polarized light component, photoelectric conversion, etc. as described above. During the inspection operation, the wafer 12 is moved by the XYZ stage 10.
The wafer surface is scanned by moving in the direction and the Y direction.

Since the configuration related to such an inspection operation is not directly related to the gist of the present invention, further detailed explanation will be omitted.

The configuration of an autofocus method applied to such a foreign matter inspection device will be described. First, a description will be given of a portion in which a light and shade striped pattern for autofocus is projected onto the wafer surface. Note that this deep portion is provided in a fixed relationship with respect to the observation optical system.

Reference numerals 22 and 24 designate pattern plates, each of which has a dark and light striped pattern for autofocus provided on opposing surfaces of a glass plate by metal vapor deposition or the like. The dark and light striped pattern and the fixing mechanism of the pattern plates 22 and 24 will be described later.

26 is a light source for illuminating the pattern plates 22 and 24. The light (white light) from this light source 24 is converted into a parallel beam by a lens 26 and illuminates the pattern plates 22 and 24. The light and shade striped pattern on the pattern plates 22 and 24 is projected onto the surface of the wafer 12 via the lens 30, the half mirror 18, and the objective lens 16.

The half mirror 18 and the objective lens 1 are arranged so that when the wafer surface coincides with the focus of the observation optical system, the midpoint between the front and rear pattern plates 22 and 24 is imaged on the wafer surface.
The focal length of the lens 30, which together with 6 constitutes the projection optical system of the light and shade striped pattern, and the lens 30 and the pattern plate 22.
．． 24 is adjusted (details will be described later).

Next, a description will be given of an imaging system for an autofocus gray stripe pattern projected onto the wafer surface. This part is provided in a fixed relationship with respect to the observation optical system.

32 is a two-dimensional image sensor for capturing an image of the portion of the wafer surface onto which the shading stripe pattern is projected. In this embodiment, the two-dimensional image sensor 32 includes:
A 41×14 pixel COD image sensor is used. Reference numerals 34 and 36 denote a mirror and a cylindrical lens for forming an image on the imaging surface of the two-dimensional image sensor 32 of a portion of the wafer surface onto which the light and shade striped pattern is projected. 38 is an infrared cut filter.

Note that the projected portion of the shading stripe pattern on the wafer surface and the field of view of the two-dimensional image sensor 32 are shifted from the irradiation spot of the foreign object detection laser beam.

34 is a focus control circuit. This circuit receives a focus error signal S from the output signal of the two-dimensional image sensor 32.
7 is generated, and the focus adjustment motor 37 of the XYZ stage 10 is driven in accordance with this focus error signal so as to cancel out the focus shift. This motor 37 is, for example, a piezo motor, and moves the wafer 12 in a Z direction for focus adjustment.
This is a high-speed fine movement in the direction (1) downward. A motor for largely moving the wafer 12 in the Z direction is separately provided on the XYz stage 10.

Reference numeral 39 denotes a device control section that controls the movement of the XYZ stage 10 and other controls of the entire device.

FIG. 2 is an explanatory diagram of the autofocus shading stripe pattern provided on the pattern plates 22 and 24. In the figure, 22a is a part of the light and shade striped pattern provided on the pattern plate 22, and 24a is the edge of the light and shade striped pattern provided on the pattern plate 24. As is clear from this figure, when viewed from the optical axis direction, the dark and light striped patterns of the pattern plates 22 and 24 are located at each edge 22 a * 24
The relationship is such that a's are arranged alternately.

In addition, the shading stripe pattern for autofocus and wafer 1-
In order to avoid confusion with the circuit pattern, when this shading stripe pattern is projected onto the wafer surface, the shading stripe pattern intersects the X1Y direction, which is the basic lattice direction of the wafer 12, at an angle of approximately 45 degrees. has been done. In order to clarify this relationship, a reduced outline of the wafer 12 is shown by a chain line 12a. The circuit patterns on wafer 12 mostly run in the X or Y direction.

Furthermore, in order to avoid confusion between the circuit pattern and minute foreign matter on the wafer L and the dark and light striped pattern, the width and length of the deep and shallow parts of the dark and light striped pattern are determined to be larger than those of the circuit pattern and the striped pattern. .

FIG. 3 is an explanatory diagram of the field of view division of the two-dimensional image sensor 32. As shown in this figure, the visual field ffF32a of the two-dimensional image sensor 32 is an imaging area A of the edge 24a of the dark and light striped pattern of the +FI side pattern plate 24, and an image of the edge 22a of the dark and light striped pattern of the rear pattern plate 22. It is divided into 1i areas intersecting with area B and handled. Then, pattern plates 22 and 24 are arranged so that each imaging area has a corresponding dark and light striped pattern.
and the position of the two-dimensional image sensor 32 is adjusted.

FIG. 4 is a schematic block diagram of the focus control circuit 34. In this figure, 40 is a two-dimensional image sensor 32
42 is a circuit for calculating the average value (or total value) of the output signals of pixels corresponding to the imaging area A (FIG. 3) of the two-dimensional image sensor 32; This is a circuit for finding a value (or total value).

44 is a subtraction circuit that calculates the difference between the output signal value of the circuit 40 and the output signal value of the circuit 42 and outputs a focus error signal ERR. This focus error signal ERR is proportional to the average contrast ratio of the front and rear dark and light striped patterns projected onto the wafer surface, its absolute value corresponds to the amount of defocus, and its polarity corresponds to the amount of defocus. Corresponds to the direction.

A driver 46 drives the focus adjustment motor 37 in accordance with the focus error signal ERR.

Reference numeral 48 denotes a peak passage detection circuit provided to detect that the wafer surface has entered the autofocus pull-in range, and detects peak passage when the focus error signal ERR passes a peak equal to or higher than a predetermined threshold level. 4
Output No. 11 PT.

This peak passage detection signal PT is given to the device control section 39. When the peak passage detection signal PT is generated, the device control unit 39 stops the two-direction movement (ascending or descending) of the XYZ stage 10, turns off the inhibition signal DE to the driver 46, and puts the driver 46 in the operating position. Start autofocus operation.

FIG. 5 shows the relationship between the out-of-focus between the observation optical system and the wafer surface and the focus error signal ERR.

Wafer 12 from the just focus point with zero defocus
When the surface of
The light and shade striped pattern is clearly projected onto the wafer surface because its image plane coincides with the wafer surface. therefore,
The average value (or total value) of the imaging signals of this light and dark striped pattern, that is, the output signal shift value of the circuit 40 becomes the maximum. At this time, since the imaging plane of the rear shading stripe pattern is far away from the wafer surface, it is projected onto the wafer only as a faint image. The output signal value becomes almost zero.

Conversely, when the wafer surface is clear from the just focus point, the imaging plane of the light and shade striped pattern on the rear pattern plate 22 reaches the wafer surface, the output signal value of the circuit 42 becomes maximum, and the front side Average value of the imaging signal of the light and shade striped pattern (
or the total value) will be almost zero.

At the just-focus point, the midpoint between the front and rear shading stripes is imaged on the wafer surface, so both the front and rear shading stripes are projected onto the wafer surface as pale images, and the average contrast ratio of each \IL is It becomes almost 1.

The focus error signal ERR is output from the circuit 40.4.
This is a difference signal between the two output signals υ values, that is, a proportional signal 'I) of the average contrast ratio of the front and rear dark and light striped patterns projected onto the wafer surface. Therefore, when the wafer surface moves around the just focus point, the absolute value and polarity of the focus error 4K change as shown.

The operation of this embodiment configured as described above will be explained. When the wafer 12 is fixed on the XYZ stage 10,
Under the control of the device control section 39, the XY2 stage 10 is driven to the reference position in the X and Y directions. Note that at this point, the XYZ stage 10 has been lowered to the lowest position.

Next, under the control of the device control unit 39, the xYz stage 10
is driven upward, and the wafer 12 gradually rises. Then, the focus error signal ERR approaching the just focus point increases to a peak on the plus side, and immediately after that, the peak passage detection signal PT is output from the peak passage detection circuit 48. In other words, the wafer surface has entered the autofocus pull-in range.

In response to this peak passage detection signal PT, the device control unit 3
9 stops the upward drive of the XYZ stage io, and activates the driver 46 by turning off the inhibit signal 1)E (which had been on until now), and after a certain period of time has elapsed, the XYZ stage 10 The foreign object inspection operation is performed while moving in the X or Y direction.

Now, by turning off the inhibition signal DE, the focus control circuit 3
4 starts operating. When the focus error signal ERR has positive polarity, the focus adjustment motor 37 is driven by the driver 46 in a direction that slightly raises the wafer 12. Conversely, when the focus error signal ERR has negative polarity, the motor 3 moves in the direction of lowering the wafer 12.
7 is driven by a driver 46. In this way,
The height of the wafer 12 is finely adjusted so as to keep the focus error signal ERR at approximately zero.

The focus error in such a state where the focus servo is applied depends on the gain of the focus control circuit 34 and the distance between the front and rear pattern plates 22 and 24.

The thickness of the wafer 12 is fixed and is not affected by variations in the thickness of the wafer 12. In addition, the shading stripe pattern for focus is imaged not by a one-dimensional image sensor (2D image sensor 32), and from the output signal, a focus error signal corresponding to the average contrast ratio of the front and rear shading stripe patterns is detected. E
Since focus control is performed by creating an RR, there is almost no influence from minute foreign matter or circuit patterns on the wafer surface.

In other words, since images are captured using a two-dimensional image sensor with a wide field of view and the average contrast ratio is determined, by determining the direction and size of the dark and light striped pattern as described above, it is possible to detect minute foreign objects on the wafer surface and circuit patterns. This means that the effects can be more reliably eliminated.

As described above, according to the autofocus method of this embodiment, focus control between the observation optical system and the wafer surface can be performed with much higher precision and stability than in the past.

FIG. 6 is a schematic perspective view of the position adjustment and fixing mechanism of the pattern plates 22,24. In the figure, 50 is the pattern board 2
This is a 2-fine movement table for integrally moving 2 and 24 in two directions. A fixed portion of this Z fine movement table 50 is fixed to a fixed member 52.

54 is an X fine movement table for finely moving the pattern plates 22 and 24 in the X direction (optical axis direction), and its fixed part is
It is fixed to the movable part of the fine movement table 60. 58 is an X fine movement table for finely moving the pattern plate 24 in the X direction, and its movable part is fixed to the movable part of the X fine movement table 54 via a Z-shaped metal fitting 58. 60 is a Y fine movement table for finely moving the pattern plate 24 in the Y direction, and its fixed portion is fixed to the fixed portion of the X fine movement table 5B. 62 is a bracket to which the pattern plate 22 is attached, and this is fixed to the Z-shaped metal fitting 58.

64 is a bracket to which the pattern plate 24 is attached, and this is fixed to the movable part of the X fine movement table 60.

With such a structure, by operating the rotary successor 50a of the Z fine movement table 50, the X fine movement table 54
, lk movement table 56 and Y fine movement table 60 as a whole can be moved slightly in the Z direction to finely adjust the position of the pattern plates 22, 24 in the Z direction.

Further, by operating the rotary heir 54a of the X fine movement table 54, the portion fixed to the Z engraving tool 58 can be integrally moved finely, thereby making it possible to finely adjust the position of the pattern plates 22, 24 in the X direction.

Further, by operating the rotating heir 58a of the X fine movement table 56, the Y fine movement table 60 can be moved as a whole, and the position of the pattern plate 24 in the X direction can be finely adjusted.

Furthermore, by operating the rotary heir 80a of the Y fine movement table 60, the position of the pattern plate 24 in the Y direction can be finely adjusted.

As is clear from the above description, according to the present invention, the two light and shade striped patterns for autofocus are provided on two independent pattern plates 22.24, and the optical axis direction of the pattern plates 22.24 is Since the position and interval can be easily adjusted, the width of the allowable range of focus deviation (focusing accuracy) when the focus servo is applied, and its l-limit and lower limit positions can be set arbitrarily using functions 1-4. Also, it is easy to adjust.

Incidentally, when shading stripe patterns are provided on both sides of a single glass plate, the intervals between the shading stripes are fixed depending on the thickness of the glass plate, making it impossible to adjust the focusing accuracy. Further, the position of the glass plate determines the limit and upper limit of the allowable defocus range, and it is not possible to set each of them arbitrarily.

Moreover, since the pattern plates 22, 24 are provided with light and shade striped patterns on opposing surfaces, the pattern plates 22, 24
Regardless of the thickness of the (glass plate), the interval between the front and rear shading striped patterns can be made extremely small. In other words,
Relatively thick glass plates can be used as pattern plates 22,24.

Incidentally, when a pattern of light and dark stripes is provided on both sides of a single glass plate, the minimum thickness of the glass plate is automatically determined by factors such as strength, and focusing accuracy is determined by that thickness. , such restrictions disappear.

Furthermore, if a pattern of light and dark stripes is provided on both sides of a single glass plate, when the wafer is at the just focus point,
The distance between the projection optical system and the glass plate must be adjusted so that the midpoint of the glass plate, that is, the part without a pattern, is imaged onto the wafer, and there is no U-1 marker for adjustment. Adjustment is troublesome and tends to be inaccurate.

In contrast, in the present invention, since the light and shade striped patterns are provided on opposing surfaces of the pattern plates 22 and 24, the position of each light and shade striped pattern can be easily and accurately adjusted. In order to clarify this point, an example of the adjustment procedure will be described below.

First, the pattern plates 22 and 24 are brought into close contact with each other so that the front and rear shading striped patterns are almost the same. Then, the wafer is adjusted to the height position f4p.

In this state, the rotating heir 50
a * 60 Pattern board 22.24 by operation of a
Adjust the Z-direction position and Y-direction position of. Furthermore, the position of the pattern plates 22, 24 in the X direction (optical axis direction) is adjusted by operating the rotating heir 54a so that the output of the two-dimensional image sensor 32 is maximized.

When a pattern of light and dark stripes is provided on both sides of a single glass plate, this adjustment step needs to be performed at the midpoint of the glass plate, making the adjustment extremely troublesome because there is no target for the adjustment. However, in the present invention, the front and rear shading stripes patterns are brought into close contact with each other, and the shading stripe pattern can be targeted, so that the adjustment is easy and accurate.

Next, move the wafer to the upper limit of the allowable defocus range.
t Wr-. Then, the rotating heir 54a is operated so that the output of the two-dimensional image sensor 32 corresponding to the light and shade striped pattern on the rear pattern plate 22 is maximized.

Next, the wafer is lowered to the lower limit of the allowable defocus range. Then, by operating the rotating heir 58a, the position of the pattern plate 24 in the X direction is adjusted so that the output of the two-dimensional image sensor 32 corresponding to the light and shade striped pattern of the front pattern plate 24 is maximized.

This completes the adjustment of the position and interval of the pattern plates 22 and 24, that is, the 1-limit and lower limit positions and width (focusing accuracy) of the allowable defocus range.

Note that part of the functions of the focus control circuit 34 in this embodiment may be realized by software.

Further, the pattern form of the autofocus gray pattern may be modified as appropriate.

Furthermore, a two-dimensional image sensor 32 for autofocus
may use a sensor other than the COD image sensor.

Furthermore, although the above embodiment was applied to a wafer foreign matter inspection device, the autofocus method of the present invention is generally applicable to devices that optically observe the surface of a member, such as a mask substrate surface inspection device. It is possible.

[Effects of the Invention] As is clear from the description below, according to the present invention, the effects of variations in the thickness of the member and the effects of minute foreign matter on the surface of the member can be eliminated, and the surface of any member, such as a wafer, can be eliminated. The observation optical system can be focused with high accuracy and stability, and the width, lower limit, and upper limit of the allowable range of defocus can be arbitrarily set, and the adjustments can be made easily and accurately. can be achieved.

[Brief explanation of drawings]

FIG. 1 shows an embodiment of the autofocus system of the present invention applied to a wafer foreign matter inspection device. 2 is an explanatory diagram of a gray stripe pattern for autofocus, FIG. 3 is an explanatory diagram of field division of a two-dimensional image sensor for imaging a gray stripe pattern, and FIG. 4 is a block diagram of a focus control circuit.
FIG. 5 is a characteristic diagram showing the relationship between out-of-focus and focus error signals, and FIG. 6 is a schematic perspective view of a pattern plate position adjustment and fixing mechanism. 10...XYZ stage, 12...wafer, 16.
...Objective lens of 11J optical measuring system, 18...Half mirror 122, 24...Pattern plate, 30...Lens of projection optical system, 32...Two-dimensional image sensor, 34
--Mirror, 3B--Cylindrical lens, 34
...Focus control circuit, 37...Focus adjustment motor, 50...2 fine movement table, 54.56.-
X fine movement table, 60...Y fine movement table.

Claims (2)

[Claims] (1) A projection optical system that is fixed to the observation optical system that observes the surface of the component, and two pattern plates stacked back and forth on the optical axis of this projection optical system so that the position can be adjusted at least in the optical axis direction. and a mechanism for fixing the two pattern plates to each other, each of the two pattern plates having a shading pattern on opposing surfaces, and imaging the shading pattern of each of the pattern plates projected onto the surface of the member by the projection optical system. A two-dimensional image sensor is used to perform the image processing, and an output signal of the two-dimensional image sensor is input, and the image sensor is configured such that the average contrast ratio of the light and shade patterns of each of the pattern plates projected onto the surface of the member becomes approximately 1. focus control means for controlling a drive means for relatively moving the component and the observation optical system; An autofocus system characterized in that an image is formed on the surface of the member at approximately a midpoint between two pattern plates. (2) The focus control means calculates the average value or total value of the output signals of the two-dimensional image sensor corresponding to the shading pattern of one pattern plate projected onto the surface of the member, and the average value or total value of the output signal of the two-dimensional image sensor corresponding to the light and shade pattern of one pattern plate projected onto the surface of the member. 2. The autofocus method according to claim 1, wherein the difference between the average value or the total value of the output signals of the two-dimensional image sensor corresponding to the grayscale pattern is determined as the average contrast ratio.