JPS60217322A - Focus detecting device - Google Patents

Abstract

PURPOSE:To detect a focal shift with high sensitivity by providing two photoelectric detecting means each having a non-photosensitive body part of the almost same shape as a spot optical image, on a part of the photodetecting surface by shifting them from the image forming surface separated by a half mirror. CONSTITUTION:A luminous flux from a light source 1 is made parallel by a lens 2, reflected by a half mirror 3 and made to form an image on a data 5 by a lens 4. Its reflected light is divided into two by a half mirror 8 through the lenses 4, 7, provided onto glass plates 9a, 10a having circular light shielding parts 9b, 10b, and photoelectrically converted 9, 10. Glass plates 9, 10, for instance, are shifted by (d) forward and backward from surfaces FP1, FP2 on which the reflected light is to be focused. Output of the detectors 9, 10 are amplified by a circuit which is not shown in the figure and denoted as A and B, A+B and A-B are derived and a division (A-B)/(A+B) is executed, and the lens 4 is moved forward and backward by driving a motor 12 so that said value becomes minimum. In such a way, the focusing which is precise and has high sensitivity is executed. It is suitable for observing a pattern of a mask or a wafer.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a focus detection device for an optical system, and particularly to a focus detection device suitable for a microscope optical system used in an observation device or an inspection device for minute objects. .

(Background of the Invention) When observing a pattern on a mask or a pattern on a wafer using an S** objective lens, it is necessary to accurately focus the objective lens on the surface of a sample such as a mask or wafer. In order to automatically perform this focusing, a focus detection device is required to accurately detect the imaging state of the optical image formed by the objective lens. Conventionally, a spot of laser light is irradiated onto the sample surface through an objective lens, the reflected light is focused through the objective lens toward a predetermined imaging plane, and the spot is focused at a position away from the imaging plane (device). A device that performs focus detection can be considered by arranging a 2-split or 4-split light-receiving element at a focus position) and measuring the position and center of the defocused image of the spot light formed on the light-receiving surface of the light-receiving element. ing. Using a two- or four-split photodetector like this is good when the sample is stationary, but when the sample is placed on a moving stage and moved at high speed for inspection or observation, When the contrast on the sample surface is not constant and changes, there is a problem that a difference occurs in the amount of light received by each of the two or four divided light receiving elements, making accurate focus detection difficult.

(Object of the Invention) The present invention solves the above-mentioned drawbacks and provides a focus detection device that easily and accurately detects defocus without being affected by high-speed movement of a sample or changes in surface contrast. With the goal.

(Summary of the Invention) The present invention irradiates a focused spot light onto the surface of a sample (object) through an imaging optical system, and reflects light of the spot light on the sample surface through an imaging optical system. In a device that detects a defocus from the imaging state of a spot light image that is converged towards a predetermined imaging plane and formed on the imaging plane, the reflected light that has passed through the imaging optical system is In the optical path, a light-receiving surface is placed at two positions separated by a predetermined distance in the front and rear directions of the optical axis of the imaging optical system, with a light-insensitive area partially having a shape similar to that of the spot light image. outputs a photoelectric signal according to the amount of reflected light that reaches the part of the light-receiving surface other than the light-insensitive area.
The key technical point is to provide two photoelectric detection means and a detection circuit that detects defocus based on the photoelectric signals of the two photoelectric detection means.

(Embodiment) FIG. 1 is a schematic optical layout diagram of a focus detection device according to an embodiment of the present invention. The light source 1 includes a light emitting element, such as a laser diode, an LE, for forming a spot light on the sample.
It has an incandescent light bulb or the like, and a light shielding plate having a pinhole that is illuminated by the light emitting element. The light flux from the pinhole is made into a parallel light flux by lens 2, and
- It is reflected by the mirror 3 and enters the objective lens 4.

This parallel light beam is converged by the objective lens 4, and the sample 5
A pinhole image, or spot light, is formed on the surface of the The stage 6 places the sample 5 on it and moves two-dimensionally so that a spot light is formed at an arbitrary position on the surface of the sample 5. The light reflected on the surface of the sample 5 is again transmitted to the objective 1 and lens 4.
The light passes through the half mirror 3 and reaches the condensing 1/lens 7. The reflected light converged by the condenser lens 7 is split into two directions by the half prism 8 so that the light intensity is approximately 1:1.

The reflected light that has passed through the Nof prism 8 is converged toward a predetermined image plane FPI determined by the objective lens 4 and the condensing lens 7. This imaging plane FP is the position where the image of the pinhole of the light source 1 is accurately formed when the focal point of the objective 1/lens 4 exactly coincides with the surface of the sample 5. Then, the '1st photoelectric detector 9 is placed at a position (front focus position) a distance d away from the imaging plane F'PI toward the objective lens 4. The light receiving surface of the photoelectric detector 9 is circular with a predetermined diameter,
A circular glass plate 9a is provided on the light receiving surface. In the center of the glass plate 9a, there is a half prism 8 which is placed in a state where the image of the pinhole of the light source 1 is accurately formed on the imaging plane FPI, that is, when the focusing of the objective 1/lens 4 is achieved. A circular light-shielding portion 9b having a diameter such that almost all of the reflected light passing through the circular light-shielding portion 9b is blocked is provided with low-reflection chrome or the like.

On the other hand, the reflected light from the sample 5 reflected by the half prism 8 converges on an image plane F'P2 that is conjugate with the image plane FPI.

Then, a second photoelectric detector 1 having the same shape as the photoelectric detector 9 is provided.
0, it is arranged at a position away from the imaging plane FP2 by a distance d on the opposite side from the objective lens 4 (rear focus position). The photoelectric detector 10 also includes a circular glass plate 10a and a circular light shielding part 10b, and outputs a photoelectric signal according to the amount of reflected light passing around the circular light shielding part 10b. Also, during automatic focusing,
In order to move the objective lens 4 in the optical axis direction, a nut portion 4a provided integrally with the objective lens 4, a feed screw 11 screwed into this nut portion 4a, and a feed screw 11 that is rotated forward and backward by a predetermined amount. A rotating servo motor 12 is provided. Although the objective lens 4 is assumed to be moved here, the stage 6 may be moved in the optical axis direction.

Here, the circular light shielding part 9 provided on the glass plate QallO&
The shape of b and 10b will be explained using FIG. 2. FIG. 2(a) is a plan view of the glass plate 9a and the circular shield portion 9b, and FIG. 2(b) is a sectional view thereof. The distance between the surface of the glass plate 9a and the imaging plane FPI is d so that the optical axis l of the objective lens 4 passes through the center of the circular glass plate 9a.
Further, when the reflected light 1d from the sample 5 is accurately focused on the imaging plane FP1, the diameter C of the circular light shielding part 9b is the distance d from the imaging plane F'PI in this embodiment. It is assumed that the intensity distribution is determined to be approximately the same as the intensity distribution of the spot light image formed by the differential focus at the position. However, the magnitude of the intensity distribution at diameter C and distance d does not necessarily have to be the same, and the state shown in Fig. 2(b) (i.e., the focusing of objective 1/lens 4 is achieved) (state), the diameter C may be made smaller than the size of the intensity distribution of the defocused image at the distance d. Conversely, if the diameter C is made larger than the size of its intensity distribution,
During focus detection, a dead zone occurs near the in-focus point, which is undesirable. This will be discussed later.

FIG. 3 is a circuit block diagram of a detection circuit that performs focus detection based on the photoelectric signals of the photoelectric detectors 9 and 10 of FIG. 1. The photoelectric signal from the photoelectric detector 9 is amplified by a predetermined amount by a preamplifier 20, and the photoelectric signal from the photoelectric detector 10 is amplified by a predetermined amount by a preamplifier 21. The subtracter 22 outputs a signal (AB) obtained by subtracting the analog value of the photoelectric signal B amplified by the preamplifier 21 from the analog value of the photoelectric signal A amplified by the preamplifier 20 . The signal (h, l B ) is obtained as an analog value proportional to the difference in the amount of light received by the photoelectric detectors 9 and 10, and its polarity (positive/negative) represents the direction of the defocus (front focus or rear focus). The absolute value corresponds to the amount of focus shift. In principle, focus detection is possible just by obtaining this signal (A-B), but if the reflectance of the surface of the sample 5 is not uniform and changes, the amount of light incident on the photoelectric detectors 9 and 10 will be It changes depending on its reflectance. Therefore, although the direction of defocus can be detected based on the polarity of the signal (A-B), the exact amount of defocus cannot be determined.

Therefore, in this embodiment, the analog value of the photoelectric signal A and the analog value of the photoelectric signal B are further added to produce a signal (A+B
), the signal (A-B) is input as the numerator, the signal (A + B) is input as the denominator, and (A-
A divider 24 is provided which performs division of B)/(A + B) and outputs a signal E representing the divided value. Signal (A+B
) represents an analog value according to the total amount of light incident on the two photoelectric detectors 9 and 10, so the signal E is the signal (A-B)
represents an analog value normalized so as not to depend on the reflectance of sample 5. This signal E is applied to the focus adjustment motor 12 via the servo amplifier 25.

Next, the operation of this embodiment will be explained with reference to FIGS. 4, 5, and 6. FIG. 4 schematically represents the imaging state of the spot light image. When the focus position of the objective 1/lens 4 is in the focused state exactly matching the surface of the sample 5, the reflected light 1d passing through the objective lens 4 and the condenser lens 7 is reflected as shown in FIG. 4(a). The convergence position coincides with the imaging planes FPI and FP2. Therefore, both the light intensity distributions on the circular light shielding parts 9b and 10b of the two photoelectric detectors 9 and 10 are
). In FIG. 5, the vertical axis is the circular light shielding part 9b,
represents the light intensity of the reflected light ld at the position 10b,
The horizontal axis represents the position in the diametrical direction of the cross section of the reflected light ld.

When in focus, the light intensity distribution P1 as shown in Fig. 5(a)
is almost equal to the diameter C and most of the light is blocked, so
Of the reflected light ld, only extremely small base portions (hatched portions) at both ends of the light intensity distribution P1 are received by the photoelectric detectors 9 and 10.

Therefore, the analog values of the photoelectric signals A and B both become equal, and the analog value of the signal E becomes zero.

This signal E is applied to the motor 12 to rotate the motor 12, but in this case, since the analog value of the signal E is zero, the motor 12 does not rotate and the focus state is left blank.

Next, for example, when moving the stage 6 to observe another position on the surface of the sample 5, for some reason the position of the sample 5 in the optical axis direction is lower than the position in the focused state, that is, the objective lens 4 position further away from (rear pin state)
Assume that it has changed to . At this time, as shown in FIG. 4(b), the convergence position of the reflected light ld is on the imaging plane FP1. It changes in a position after FP2, that is, in a direction closer to photoelectric detector 10.

Therefore, the light intensity distribution on the circular light shielding part 10b is as shown in FIG.
As shown in b), the light intensity distribution P2 changes to a narrower and sharper light intensity distribution than the light intensity distribution P1 in FIG. 5(a). The light intensity distribution P2 is completely blocked by the diameter C, and the photoelectric signal B of the photoelectric detector 10 is smaller than the analog value in the focused state and becomes almost zero.

On the other hand, in the pinned state after this, the convergence position of the reflected light 1d has changed in the direction away from the photoelectric detector 9. Therefore, the light intensity distribution on the circular light shielding portion 9b changes to a light intensity distribution P3 that is wider and broader than the light intensity distribution P1 in FIG. 5(a), as shown in FIG. 5(C). Since the base portion of the light intensity distribution P3 that is not blocked by the diameter C rapidly increases, the photoelectric signal A of the photoelectric detector 9 becomes larger than the analog value in the focused state. As described above, in the rear pin state, the signal (A-B) has a positive polarity, and the signal E changes to an analog value according to the amount of displacement of the rear pin. The analog value of the signal E at that time is 10E1, the relative distance between the objective lens 4 and the sample 5 in the optical axis direction (two directions) in the in-focus state is zO, and the relative distance in the optical axis direction (assumed to be two directions) is When the interval is set to 21, the change characteristics of the signal E have a substantially linear relationship as shown in FIG. Therefore, the rotation L2 is performed until the analog value of the motor 121d' signal E becomes zero from 10E1, and the position of the objective lens 4 is pulled back to the in-focus state.

In this way, the movement of the objective lens 4 in two directions is servo-controlled so that the signal E always remains at zero.

Now, from the position ZO when the sample 5 is in focus, the objective 1/
It is assumed that the position has changed to position Z2 (front focus state), which is closer to the lens 4. At this time, as shown in FIG. 4(C), the convergence position of the reflected light ld is at the imaging plane FPI.

Change from FP2 to the previous position. Therefore, in the rear pin state, the negative polarity of the signal EI-i is the analog value -E2.
Changes to Therefore, the motor 12 rotates in the opposite direction to that in the rear focus state, and returns the position of the objective 1/lens 4 to the position zO where the analog value of the signal E becomes zero.

The present embodiment has been described above, but here, the circular light shielding part 9b,
A case will be described in which the diameter C of the beam 10b is larger than the diameter of the intensity distribution of the reflected light ld. In this case, in the focused state, both photoelectric signals A and B are approximately zero, and signal E is also zero. However, it is possible that both the photoelectric signals A and B do not change from zero even when the in-focus state becomes slightly out of focus. Only when the amount of out-of-focus reaches a certain level does a difference occur between photoelectric signals A and B, and signal E changes from zero. If the diameters of the circular light-shielding parts 9b and 10b are too large as described above, the range in which the in-focus state is detected will apparently expand, which is not preferable for precise focusing. However, since there is an advantage that the focusing operation becomes faster, this method can be implemented satisfactorily in practice when the depth of focus of the objective lens 4 is large.

Next, another embodiment of the present invention will be explained with reference to FIG. 7th
The figure is a block diagram of the detection circuit, and has the same effect as in Figure 3.
Components that operate are given the same reference numerals. In this embodiment, intensity modulation is applied to the light beam emitted from the light source 1 at a specific frequency.

When a laser diode or an LED is used as the light source 1, the drive current may be modulated. In the case of incandescent light bulbs, a mechanical chopper (e.g. rotating sulin, top plate, etc.)
The light beam can be turned on and off using the . For this reason,
The photoelectric signals A and B become alternating current signals whose analog values corresponding to the defocus are modulated at the modulation frequency of the light source 1.

Now, the photoelectric signals A and B are each passed through a bandpass filter (hereinafter referred to as B) that passes a frequency component that matches the modulation frequency of the light source 1.
PF) 30.32. BPF30,32
Each output signal of
3 and is converted into a DC voltage corresponding to the amplitude component of the AC signal.

As an example of the AC/DC converters 31, 3.9, an RMS/DC converter that detects the effective value of a sinusoidal AC signal is suitable. This AC/DC converter 31.
The output signals i and j of 33 correspond to the photoelectric signals A and H when the light source 1 is not modulated.

These signals i and j are input to a subtracter 22 and an adder 23,
A divider 24 generates a signal E depending on the defocus.

In this embodiment, the light intensity of the light source 1 is modulated and only the modulated frequency component is extracted from the photoelectric signal, so it is not affected by ambient light and a high rejection rate is obtained for noise generated on the circuit. be able to. This has the effect of improving the S/N ratio of the signal E in response to defocus, and achieving good focusing accuracy.

Each embodiment of the present invention has been described above, and modifications common to each embodiment will be described below.

As shown in FIGS. 1 and 2, two circular light shielding parts 9b,
10b both have the same diameter C, and the imaging planes FPI, F'P
2, the circular light shielding parts 9b and 10b may be different by the distance d.

In that case, the signal E does not become zero in the focused state, but exhibits a change characteristic in which a certain offset voltage is superimposed. That is, the characteristic shown in FIG. 6 is changed to a characteristic in which the signal E is shifted by a certain amount in the positive or negative direction. Also, the circular light shielding part 9
b and 10b are made to have different diameters, and the imaging planes F'PI and FP are respectively set so that the signal E becomes zero when in focus.
On the other hand, the shape of the spot light image does not have to be circular, and may be rectangular, triangular, etc. In that case, the light source 1 is provided with a field diaphragm, and a light shielding plate having a rectangular, triangular, or other minute aperture instead of a pinhole is provided at the position of the field diaphragm or a position conjugate thereto, and the surface of the sample 5 is A rectangular or triangular spot light image is formed. and photoelectric detectors 9, 10
A similar effect can be obtained by providing rectangular or triangular light shielding portions 9b and 10b similar to the shape of the spot light image.

Further, the photoelectric detectors 9, 10 and the glass plate 9a19b do not need to be in close contact with each other, and the photoelectric detectors 9, 10 are provided separately from the glass plates 9a + 9b, and a condensing lens etc. may be provided.

Furthermore, a reflective member or the like is inserted between the half prism 8 and the photoelectric detector 10 shown in FIG.
0 may be placed on the same plane.

In this case, the photoelectric detectors 9 and 10 are made into a single light-receiving element with a large light-receiving surface, and in front of the light-receiving surface, two circular light-shielding parts 9b and 10b are formed spaced apart on a glass plate. A chopper plate is provided in front of the two circular light shielding parts 9b and 10b, and the time during which the reflected light ld is directed only to the circular light shielding part 9b and the time during which the reflected light ld is directed only towards the circular light shielding part 10b are alternated at extremely short intervals. The chopper plate is vibrated or rotated repeatedly. The detection circuit is provided with a multiplexer or the like that separates (demodulates) a time-series photoelectric signal from a single light-receiving element into two photoelectric signals A and H using a repeated cycle of a chopper plate. Signal E can be similarly generated from these two divided photoelectric signals A and B. In this way, since two photoelectric signals each having front pin information and rear pin information are obtained from a single light receiving element, it is expected that the detection accuracy will be improved more than using two photoelectric detectors. This is because variations in the light receiving sensitivity of the photoelectric detector are eliminated.

Furthermore, when a single light receiving element is used, an embodiment as shown in FIG. 8 is also conceivable. The basic optical system is the first
It is the same as the one in the figure, except that a filter 40 with a circular light shielding part formed in the center is provided in the optical path of the reflected light ld converged by the lens 7, and this filter 40 is vibrated with a constant amplitude in the optical axis direction. an oscillator 42 that outputs an oscillation signal for vibration, and a filter 40.
The point is that a detection circuit 44 is provided which synchronously detects a photoelectric signal from a light receiving element 43 placed after the light receiving element 43 using an oscillation signal. In this way, the detected signal (so-called S curve signal)
corresponds to the amount and direction of defocus, and similarly focus detection is possible.

Here, it is desirable that the center of vibration of the filter 40 coincides with a predetermined imaging plane FP of the reflected light ld, but this is not always necessary. If the circular light-shielding portion of the filter 40 vibrates between two positions separated in the optical axis direction in the optical path of the reflected light ld, that is, a position where there is a difference in sight per unit area, focus detection can be performed in the same way. In addition, a liquid crystal plate is installed at each of the two positions, and the liquid crystal displays the circular light-shielding area and makes it transparent, which is repeated alternately for the two liquid crystal plates, and synchronous detection is performed using a single light-receiving element. It's okay.

In addition, in the optical system shown in Fig. 1, in order to provide an eyepiece optical system for visual observation, etc., a 1/- optical system is inserted between the condensing lens 7 and the lens 80, and the objective 1/lens A similar effect can be obtained by arranging photoelectric detectors 9 and 10 before and after a surface that is conjugate with the image forming surface formed by the condenser lens 4 and the condenser lens 7. Furthermore, as a photoelectric detector (light-receiving element), a light-insensitive part is provided in the center of the light-receiving surface to form a donut (ring zone)-shaped light-receiving surface, which is similar to the light shielding parts 9b and 10b. You can use it as is.

(Effects of the Invention) According to the present invention, two photoelectric detection means each having a light-insensitive portion having a shape substantially similar to the shape of a spot light image on a part of the light-receiving surface are shifted from a predetermined image-forming plane. Since it is provided at a certain position, it is possible to photoelectrically detect only the spread of the intensity distribution of the spot light image due to deviation from the focused state, and there is an effect that defocus can be detected with high sensitivity.

Furthermore, since defocus is detected based on the difference between the photoelectric signals from the two photoelectric detection means, there is also the effect that the detection sensitivity is further increased. Furthermore, even when the sample (object) is moving at high speed in the direction of the optical axis of the imaging optical system, the amount and direction of defocus can always be detected, making it possible to observe specimens using the imaging optical system. It can be done quickly without any complication, and it may also shorten the processing time when testing samples.

[Brief explanation of the drawing]

FIG. 1 is a schematic optical layout diagram of a focus detection device according to an embodiment of the present invention, FIGS. Figure 3 is a circuit block diagram of the detection circuit, Figure 4 is a diagram schematically showing the state of focus, Figure 5 is a diagram showing changes in the intensity distribution of reflected light produced at the circular light shielding part of the photoelectric detector, 6 is a characteristic diagram of the signal E according to the focus shift, FIG. 7 is a block diagram of a detection circuit according to another embodiment of the present invention, and FIG. 8 is a configuration diagram of a focus detection device according to yet another embodiment. be. [Explanation of symbols of main parts] 1...Light source, 4...Objective 1/lens,
5...Sample, 9,10...Photoelectric detector, 9a, 10k...Glass plate, 9b, 10b...
...Circular light-shielding part, 12...Motor,
22...subtractor, 23...adder,
24...Divider. Applicant Nippon Kogaku Kogyo Co., Ltd. Agent Takao Watanabe Figure 1 Figure 2 (θ) (O) Figure 3 ((1) (C)

Claims (1)

[Claims] irradiating a focused spot light onto the surface of the object via an imaging optical system, and converging the reflected light of the spot light on the surface toward a predetermined imaging plane via the imaging optical system; In the device for detecting a focal shift of the imaging optical system with respect to the surface from the imaging state of a spot light image formed on the imaging surface, in the optical path of the reflected light converged through the imaging system, At two positions separated by a predetermined distance in the optical axis direction of the imaging optical system, light-receiving surfaces each having a light-insensitive portion having a shape substantially similar to the shape of the spot light image are arranged. , two photoelectric detection means for outputting a photoelectric signal according to the amount of the reflected light reaching a portion of the light-receiving surface other than the light-insensitive portion; A focus detection device comprising: a detection circuit for detecting.