JPH05297262A - Automatic focusing device - Google Patents

Abstract

PURPOSE:To make accurate focusing using a simple configuration even with no pattern formed, even in case interface having different reflection factor exists. CONSTITUTION:Light from a light source 30 is cast on an object 33 by an irradiation optical system 31, and the reflected beam having passed a beam splitter 36 is taken out from an objective lens 43. The taken-out beam is diverged in two directions by a branching optical system 46, and one of the divergent beams is passed through the first optical system 47 and the first pinhole plate 48, installed for example ahead of the focal point of this system 47, to be received by the first photo-receiving element 49, while time other divergent beam is forwarded via the second optical system 50 and the second pinhole plate 51, arranged for example behind the focal point of the system 50, to be received by the second photo-receiving element 52. An adjusting means 60 adjusts the position of the objective lens 43 relative to the object 33 so that the photo- reception amounts of these first and second receiving elements 51, 52 are identical.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an autofocus device for an object such as a glass mask used for semiconductor inspection.

[0002]

2. Description of the Related Art There are various methods for an autofocus device depending on the object and the device to which it is applied. The phase difference method shown in FIG. 5 is a method applied to an autofocus camera, in which an image from the object 1 is collected by the imaging lens 2 and guided to the two lenses 4 and 5 through the lens 3. Then, the images focused by these lenses 4 and 5 are detected by the linear image sensors 6 and 7, respectively. These linear image sensors 6 and 7 detect images 8 and 9 as shown in FIG.
When these images 8 and 9 coincide with each other, they are focused.

However, in this phase difference method, since the images of the linear image sensors 6 and 7 are matched,
The object 1 needs to have a pattern formed,
For example, in a semiconductor, it is impossible to focus on a place where no pattern is formed or before pattern formation.

The astigmatism method shown in FIG. 7 is a method applied to an optical disc or a compact disc. The optical disc or the compact disc is irradiated with light, and the reflected light is passed through lenses 10 and 11 to a four-division sensor 12. Irradiate. In the four-divided sensor 12, the beam shape of the reflected light changes as shown in FIG. 8 according to the deviation from the focal point, so that when the circular shape shown in FIG.

The optical lever method shown in FIG. 9 is a method applied to a semiconductor inspection device or the like. After passing light through a slit plate 13, it is reflected by a mirror 14 and guided to an objective lens 15 to an object 16. Irradiate. Then, the reflected light from the object 16 is narrowed down again through the objective lens 15 and applied to the position detection sensor 17. In this case, when the position of the object 16 changes, the focal position of the reflected light by the objective lens 15 changes. Therefore, when the reflected light is focused at the center position of the position detection sensor 17, it becomes a focal point.

However, in the astigmatism method and the optical lever method, the focus position is obtained from the distribution of received light intensity on the sensor surfaces of the four-division sensor 12 and the position detection sensor 17,
If the object is a glass mask, the reflectance is 7 to 8 times different between the Cr surface and the glass surface, and an error occurs at the Cr-glass interface. Further, if the object has a tilt, an error occurs.

Vibration slit (pinhole) shown in FIG.
According to the method, the light emitted from the light source 18 is reflected by the mirror 19 and condensed by the condenser lens 20 to irradiate the object 21. The reflected light from the object 21 is condensed by the condenser lens 20, and the slit 22 is formed near the focus of the condenser lens 20.
Is vibrated at a predetermined frequency in the direction of arrow (a). In this state, the reflected light that has passed through the slit 22 is received, and when the amount of the received light is twice the vibration frequency and the phases are synchronized, the focal point is obtained. In addition, each position 22 of the slit 22
a and 22b are focal positions corresponding to the positions 21a and 21b of the object.

However, in this vibrating slit method, even if there is no pattern on the object and no error occurs even at the boundary surface of Cr-glass or the like, the mechanism for vibrating the slit 22 and the vibration frequency of the received light amount are doubled. Must be equipped with a complicated signal processing device that determines that the frequencies and the phases are synchronized.

[0009]

As described above, in the phase difference method, the focus cannot be obtained unless the pattern is formed, and in the astigmatism method and the optical lever method, the Cr surface, the glass surface and the reflectance are Errors occur at different interfaces. Further, in the vibration slit method, a mechanism for vibrating and a complicated signal processing device must be provided.

Therefore, the present invention provides an autofocus device having a simple structure and capable of accurately focusing even if a pattern is not formed or a boundary surface having different reflectance exists. The purpose is to

[0011]

SUMMARY OF THE INVENTION According to the present invention, light emitted from a light source is transmitted through a beam splitter and then condensed by an objective lens to illuminate an object, and reflected light from the object is reflected by the objective lens. An irradiation optical system that narrows the light by using a beam sputter and splits it by a beam sputter, a branching optical system that splits the branched reflected light into two directions, a condensing optical system that condenses the reflected light by this branching optical system, and this condensing optical system. The first pinhole plate arranged on the front side or the rear side of the focal point of the
A first light receiving element that receives the reflected light that has passed through the pinhole plate, a second pinhole plate that is arranged behind or in front of the focal point of the other reflected light that is split by the split optical system, and the second A second light receiving element that receives the reflected light that has passed through the pinhole plate, and an adjusting means that adjusts the position of the objective lens with respect to the object so that the light receiving amounts of the first and second light receiving elements are the same. It is an autofocus device that aims to achieve the above object.

[0012]

By providing such means, the light from the light source passes through the beam splitter of the irradiation optical system, is condensed by the objective lens and is irradiated onto the object, and the reflected light is reflected by the objective lens. The beam is narrowed down and branched by the beam sputter. The branched reflected light is branched and condensed in two directions by the branching and converging optical system, passes through the first pinhole plate arranged on the front side or the rear side of the focal point of the optical system, and is then transmitted by the first light receiving element. Received light. At the same time, the other reflected light passes through the second pinhole plate arranged on the rear side or the front side of the focal point and is received by the second light receiving element. Then, the adjusting means adjusts the position of the objective lens with respect to the object so that the light receiving amounts of the first and second light receiving elements are the same.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of an autofocus device. The laser light output from the semiconductor laser 30 is transmitted from the irradiation optical system 31 to the mask 3 placed on the XY table 32.
Irradiate to 3.

Explaining the configuration of the irradiation optical system 31, a collimator lens 34, a beam expander 35, and a polarization beam splitter 36 are arranged on the optical path of the laser light output from the semiconductor laser 30. The beam expander 35 has lenses 35a and 35b arranged therein, and a pinhole plate 35c is arranged between the lenses 35a and 35b. The diameter of the pinhole formed in the pinhole plate 35c is limited to the diffraction limit, and a point light source is obtained here.

A quarter wavelength plate 37 and two rotatable wedge prisms 38a and 38b are arranged on one side of each branching direction of the polarization beam splitter 36, and further, lenses 39 and 40 of the relay optical system and Each dichroic mirror 41, 42 is arranged. An objective lens 43 is arranged on the reflected light path of the mirror 42.

The wedge prisms 38a and 38b have their optical axes slightly tilted, so that the irradiation position of the laser light on the mask 33 is as shown in FIG.
It becomes the Q point deviated from the center position of. This is because the mask 33 is displaced from the inspection region W of the sensor that performs the defect inspection.

Here, the lenses 39 and 40 are arranged at positions satisfying the relationships of s1 = f1, s2 + s3 = f2, and s4 = f0. Note that s1 is the distance between the wedge prisms 38a and 38b and the lens 39,
s2 is the distance between the lens 40 and the dichroic mirror 42, and s3 is the dichroic mirror 42 and the objective lens 43.
To the entrance pupil, s4 is the distance from the principal point of the objective lens 43 to the entrance pupil, f1 is the focal length of the lens 39, and f1 = a + b. Further, f0 is the focal length of the objective lens 43.
On the other hand, on the other side of the polarization beam splitter 36, the lenses 44 and 45 of the relay optical system and the beam splitter 46 are arranged.

A condenser lens 47, a pinhole plate 48, an interference filter 70, and a light receiving sensor 49 are arranged in one branching direction of the beam splitter 46. this house,
The pinhole plate 48 is arranged in front of the focal point of the first condenser lens 47 by a distance s5. A condenser lens 50, a pinhole plate 51, an interference filter 71, and a light receiving sensor 52 are arranged in the other branching direction of the beam splitter 46. Of these, the pinhole plate 51 is disposed behind the focal point of the second condenser lens 50 by a distance s6. The interference filters 70 and 71 have a characteristic of transmitting only wavelengths near the wavelength of the semiconductor radar light.

These optical systems are arranged so as to satisfy the following conditions. f3 = f4 s5 = s6 Note that f3 and f4 are focal lengths of the condenser lenses 47 and 50, respectively.

The pinhole diameters d1 and d2 of the pinhole plates 48 and 51 are d1 = d2, and

S7 + s8 = f5 s9 = s10 s11 + s9 = f6 + f3 (s11 + s10 = f6 + f4) Note that f5 is the focal length of the lens 44, s7 is the distance between the wedge prisms 38a and 38b and the polarization beam splitter 36, s8 is the distance between the polarization beam splitter 36 and the lens 44, and s11 is the distance between the lens 45 and the beam splitter 46. , S9, s10 are respective distances between the beam splitter 46 and the condenser lenses 47, 50.

On the other hand, the adjusting means 60 includes the respective light receiving sensors 49,
It has a function of adjusting the position of the objective lens 43 with respect to the mask 33 so that the respective light receiving amounts of 52 are the same. Each light receiving sensor 4 is connected to the input terminal of each amplifier 61, 62.
9, 52 are connected, and a multiplexer 65 is connected to the output terminals of these amplifiers 61, 62 via sample-hold circuits 63, 64. The output terminal of the multiplexer 65 is CP via the A / D converter 66.
U67 is connected.

The CPU 67 is provided with respective light receiving sensors 49, 52.
Then, the levels I1 and I2 of the respective received light signals are obtained and the change value .DELTA.I.DELTA.I = (I2-I1) / (I2 + I1) is obtained from these signal levels I1 and I2. Then, the CPU 67 has a function of creating a position adjustment signal that sets the change value ΔI to “0”. This CPU
A driver 68 is connected to 67, and an actuator 69 for displacing the objective lens 43 is connected to the driver 68.

The defect inspection optical system of the mask 33 is XY.
The inspection light is applied to the mask 33 from below the table 32, and the light transmitted through the mask 33 passes through the objective lens 43 and is received by the defect detection sensor arranged above the XY table 32. .. Next, the operation of the device configured as described above will be described.

The laser light output from the semiconductor laser 30 is converted into parallel light by the collimator lens 34, and its diameter is expanded by the next beam expander 35. Next, this laser light is emitted from the polarization beam splitter 36 to 1/4.
The wedge prisms 38a, 38 are transmitted through the wave plate 37.
The optical axis is slightly tilted by b. Then, this laser light passes through the lens 39, is reflected by the dichroic mirror 41, further passes through the lens 40, is reflected by the next dichroic mirror 42, and enters the objective lens 43. Thus,
The laser light is condensed by the objective lens 43, and the mask 33
Irradiated on.

On the other hand, the reflected laser light from the mask 33 passes through an optical path opposite to the irradiation on the mask 33 and is 1/4.
It enters the wave plate 37. Here, since the polarization direction of the reflected laser light is rotated 90 ° by reciprocating the quarter-wave plate 37, the reflected laser light is reflected by the polarization beam splitter 36 and sent to the lens 44.

The reflected laser light that has passed through this lens 44 passes through the lens 45 and is split into two directions by the beam splitter 46. One of the reflected laser beams split by the beam splitter 46 passes from the condenser lens 47 to the pinhole plate 4
The light passes through 8 and enters the light receiving sensor 49. Then, the light receiving sensor 49 outputs a light receiving signal P1 having a level corresponding to the amount of received light.

At the same time, the other reflected laser beam split by the beam splitter 46 enters the light receiving sensor 52 from the condenser lens 50 through the pinhole plate 51. And
The light receiving sensor 52 outputs a light receiving signal P2 having a level according to the amount of received light. Here, the mask 33 and each lens 47,
There is a conjugate relationship (corresponding to an image) with each focus of 50.

The received light signals P1 and P2 are both sent to the adjusting means 60, and the amplifiers 61 and 62 of the adjusting means 60.
Is amplified by each of the sampling and holding circuits 63 and 64. The received light signals sampled and held are passed through the multiplexer 65 and the A / D converter 66.
Is converted into a digital signal and sent to the CPU 67.

This CPU 67 controls each light receiving signal level I1,
A change value ΔI ΔI = (I2 −I1) / (I2 + I1) is calculated from I2, and a position adjustment signal for making this change value ΔI “0” is created.

By the way, when the mask 33 and the focal points of the lenses 47 and 50 satisfy the conjugate relation as described above, when the distance between the mask 33 and the objective lens 43 changes, the level I1 of each received light signal, I2 and the change value .DELTA.I change as shown in FIG. However, if the above optical system does not satisfy the conjugate relation, for example, the level I1 of each received light signal is
, I2 change as shown by I1a and I2a, respectively, and the change value ΔI changes as shown by ΔIa, and the sensitivity becomes asymmetric.

The position adjustment signal issued from the CPU 67 is sent to the driver 68, and the driver 68 operates the actuator 69. As a result, the objective lens 43 is displaced in the vertical direction with respect to the mask 33 and adjusted to the in-focus position.

As described above, according to the above-described embodiment, the reflected laser light from the mask 33 is passed through the pinhole plates 48 and 51 and the interference filters 70 and 71, so that external light, for example, for mask inspection is used. It can cut illumination light and indoor light,
Further, even if the pattern is not formed on the mask 33,
Further, for example, even if there is a boundary surface having a different reflectance between the Cr surface and the glass surface, it is possible to focus accurately.

Further, since the mask 33 and the respective focal points of the lenses 47 and 50 are in a conjugate relationship, it is possible to accurately focus even if the mask 33 is inclined, and further the mask 33 is inclined and defocused. In this case, both light receiving sensors 49, 52
Since the respective light receiving signal levels of 1 are uniformly reduced, they are not affected by the inclination of the mask 33.

On the other hand, by changing the pinhole diameter of each pinhole plate 48, 51 and its position, each sensitivity can be set as shown in FIG. For maximum sensitivity,
The pinhole diameter may be formed below the diffraction limit. The pinhole may be located near the focal position of each lens 47, 50. The reason for this is that the peaks of the changes in the levels I1 and I2 of the respective received light signals are sharp.

The deviation ΔZ that determines the displacement of the objective lens 43 is ΔZ = Z (I1max) -Z0 = Z (I2max) -Z0 is the deviation ΔL from the focus position of each pinhole and the total longitudinal magnification M ^{2 of the} detection system. (Total magnification of detection system: M) ΔZ = ΔL / 2M ² Have a relationship.

When an offset OFT occurs between the autofocus optical system and the defect detection optical system, the control is performed so that ΔI-OFT = 0. At this time, the offset value may change due to lens replacement for switching the magnification of the defect detection optical system. In this case, since the CPU 67 controls the actuator 69, the offset O
Can easily respond to changes in FT. Further, the offset OFT can be changed by finely adjusting the relay lens 39 or 40 of the irradiation optical system.

The present invention is not limited to the above-mentioned one embodiment, and may be modified within the scope of the invention. For example, the present invention can be applied not only to autofocusing on the mask 33 but also to autofocusing on a light-transmissive object in which a pattern is not formed.

[0040]

As described above in detail, according to the present invention, the focus is accurately adjusted with a simple structure even if a pattern is not formed and a boundary surface having different reflectance exists. It is possible to provide an autofocus device that can do this.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of an autofocus device according to the present invention.

FIG. 2 is a diagram showing an irradiation position of laser light for autofocus in the same apparatus.

FIG. 3 is a diagram showing a level change of each light receiving sensor output in the same apparatus.

FIG. 4 is a diagram showing a change in sensitivity of the apparatus.

FIG. 5 is a configuration diagram of an autofocus device using a phase difference method.

FIG. 6 is a diagram showing an autofocus function of the device.

FIG. 7 is a configuration diagram of an autofocus device based on an astigmatism method.

FIG. 8 is a diagram showing an autofocus function of the apparatus.

FIG. 9 is a configuration diagram of an autofocus device using an optical lever method.

FIG. 10 is a configuration diagram of an autofocus device using a vibration slit method.

[Explanation of symbols]

30 ... Semiconductor laser, 31 ... Light irradiation system, 33 ... Mask,
36 ... Polarizing beam splitter, 43 ... Objective lens, 46
... Beam splitter, 47, 50 ... Condensing lens, 48,
51 ... Pinhole plate, 49, 52 ... Light receiving sensor, 60 ...
Adjusting means, 69 ... Actuator.

Claims (1)

[Claims] 1. A light emitted from a light source is transmitted through a beam splitter and then condensed by an objective lens to illuminate an object, and reflected light from the object is narrowed down by the objective lens and taken out by the beam sputter. An irradiation optical system, a branching optical system for branching the reflected light extracted by the beam splitter into two directions, a converging optical system for condensing the reflected light, and a front side of the focal point of the converging optical system. And a first pinhole plate, a first light receiving element for receiving the reflected light that has passed through the first pinhole plate, and a rear side of the focal point of the other reflected light branched by the branch optical system. A second pinhole plate, a second light receiving element for receiving the reflected light that has passed through the second pinhole plate, and the first light receiving element
And an adjusting means for adjusting the position of the objective lens with respect to the object so that the respective light receiving amounts of the second light receiving element are the same.