JP4550863B2 - Automatic focusing device - Google Patents

Description

  The present invention relates to an automatic focusing apparatus used in a semiconductor lithography apparatus, a semiconductor inspection apparatus, and the like, and more particularly to an automatic focusing apparatus for focusing an objective lens on the surface of a test object such as a semiconductor substrate or a glass substrate.

  In an optical apparatus such as a lithography apparatus that transfers a circuit pattern to a semiconductor substrate and a defect inspection apparatus that inspects a pattern of a semiconductor integrated circuit, a high resolution equal to or smaller than the circuit pattern size is required. For this reason, an automatic focusing device that maintains a constant distance between an object such as a semiconductor substrate and the objective lens is essential, and high accuracy of the automatic focusing device is required.

  As a conventional automatic focusing apparatus, there is an optical lever system (for example, see Patent Document 2). In this apparatus, the surface of the test object is irradiated with light from an oblique direction, the reflected light from the test object is detected by the light receiving sensor, and the detection output of the light receiving sensor is converted into position displacement information in the height direction. Thus, the height position is measured. Then, by feeding back the measurement signal to the stage control circuit, the relative distance between the test object and the main optical system can be maintained with high accuracy.

  As another method, there is an operating pinhole type (see, for example, Patent Document 2). In this apparatus, the light reflected by the surface of the test object is collected by the light receiving lens and branched into two optical paths by the beam splitter. The first optical path has a pinhole and light receiving before the focal point of the light receiving lens. A sensor is installed, a pinhole and a light receiving sensor are installed in the second optical path after the focal point of the light receiving lens, and an electrical difference / sum of signals obtained by the respective light receiving sensors is calculated, whereby a test object is obtained. Can be measured.

  However, when this type of automatic focusing apparatus is used, the following problems occur. That is, when the height position of the surface of the test object is detected while moving the stage on which the test object is placed in the horizontal direction, a steep change exceeding the change in the surface shape of the original test object occurs. The problem that ends.

As a result of examination by the present inventors on this problem, when the incident angle θ between the pattern surface and the incident light exceeds a certain value for a fine pattern having a line width comparable to the light source wavelength, the reflected light It has been found that the phenomenon of the reflection spot position shifting occurs. This is due to a resonance phenomenon that occurs when the spatial period, incident angle, and wavelength of the pattern satisfy a specific relationship. Since this reflection spot shift is regarded as a movement of the focal position on the light receiving sensor surface, the output of the position detection circuit changes in proportion to the amount of movement of the spot position, and the height of the test object is detected. It becomes a big error.
JP-A-6-102011 JP-A-5-297262

  As described above, in the conventional automatic focusing apparatus, when a light beam is incident on a fine pattern having a line width comparable to the light source wavelength on the test object, the spot position change of the reflected light changes. Along with this, there was a problem that a measurement error occurred.

  The present invention has been made in consideration of the above circumstances, and the object of the present invention is a reflection that occurs when a light beam is incident on a fine pattern having a line width comparable to the wavelength of the light source on the test object. An object of the present invention is to provide an automatic focusing apparatus that can suppress measurement errors caused by changes in the spot position of light and can improve the reliability of focusing.

  In order to solve the above problems, the present invention adopts the following configuration.

That is, the present invention is an automatic focusing apparatus for focusing an objective lens on the surface of an object held on a stage movable in a horizontal direction and a height direction, and is oblique to the surface of the object. An irradiation optical system for irradiating spot-like light from a direction, a beam splitter for branching reflected light from the surface of the test object, a first photodetector for detecting one of the branched reflected light , detecting the other of the branched reflected light, a second photodetector for detecting the predetermined direction of the polarization component of the reflected light, the test object from an electrical signal obtained by the first light detecting element The position detection circuit for detecting the position in the height direction of the stage and the position detection signal obtained by the position detection circuit are monitored in real time, and the amount of change per unit time of the position detection signal accompanying the horizontal movement of the stage is predetermined. Beyond the level and A correction circuit for correcting an excess amount exceeding a value corresponding to the surface shape of the test object from the position detection signal when the intensity of the electrical signal obtained by the second photodetection element exceeds a predetermined normal range. And a stage control circuit for controlling the position in the height direction of the stage based on a position detection signal obtained via the correction circuit.

  The present invention also provides an automatic focusing device for focusing an objective lens on the surface of a test object held on a stage movable in a horizontal direction and a height direction, wherein the objective lens is focused on the surface of the test object. An irradiation optical system for irradiating spot-like light from the direction, a first wave plate for making light incident on the surface of the test object from an oblique direction into a substantially circularly polarized light, and from the surface of the test object A second wave plate for making the reflected light substantially linearly polarized light, a reflected light obtained through the second wave plate, a component along the polarization direction of the second wave plate, and a component orthogonal to the polarization direction A polarization beam splitter that branches into the first wavelength detector, a first light detection element that detects light of a component along the polarization direction of the second wave plate obtained through the polarization beam splitter, and the polarization beam splitter. Of the second wave plate obtained by A second light detection element for detecting light of a component orthogonal to the direction, a position detection circuit for detecting a height direction position of the test object from an electrical signal obtained by the first light detection element, The position detection signal obtained by the position detection circuit is monitored in real time, the amount of change per unit time of the position detection signal accompanying the horizontal movement of the stage exceeds a predetermined level, and is obtained by the second light detection element. A correction circuit for correcting an excess exceeding a value corresponding to the surface shape of the test object from the position detection signal when the electrical signal exceeds a predetermined value, and a position detection signal obtained via the correction circuit And a stage control circuit for controlling the position of the stage in the height direction.

  The present invention also provides an automatic focusing device for focusing an objective lens on the surface of a test object held on a stage movable in a horizontal direction and a height direction, wherein the objective lens is focused on the surface of the test object. An irradiation optical system for irradiating spot-like light from a direction, a wave plate for making light incident on the surface of the test object from an oblique direction into substantially circularly polarized light, and reflected light from the surface of the test object A beam splitter that branches in two directions, a first polarization beam splitter that splits one reflected light branched by the beam splitter into first and second lights whose polarization directions are orthogonal to each other, and the beam splitter A second polarization beam splitter for branching the other reflected light into a third and a fourth light whose polarization directions are orthogonal to each other and whose polarization direction is 45 degrees different from that of the first and second lights; , Detecting the first light A first light detecting element, a second light detecting element for detecting the second light, a third light detecting element for detecting the third light, and a second light detecting element for detecting the fourth light. 4 and a plurality of electrical signals obtained by the first to fourth photodetectors are input, and an electrical signal with a minimum amount of signal change is selectively processed to increase the height of the test object. A position detection circuit for detecting a vertical position and a stage control circuit for controlling a height direction position of the stage based on an output of the position detection circuit are provided.

  According to the present invention, a correction circuit that detects an abrupt change in the position detection signal and corrects an excess exceeding a value corresponding to the surface shape of the object to be detected is provided, or a plurality of light detection elements are provided to provide reflected light. This is due to the change in the reflected light spot position that occurs when a light beam is incident on a fine pattern with the same line width as the light source wavelength on the test object by selecting the light detection element with the smallest spot position change. Measurement error can be suppressed. Thereby, it is possible to improve the reliability of measurement, and it is possible to improve the reliability of the semiconductor lithography apparatus and the semiconductor inspection apparatus.

  The details of the present invention will be described below with reference to the illustrated embodiments.

(Reference example)
In general, the frequency of the displacement signal change is low because the height change of the surface of the test object is a physical displacement when the change in the height of the test object or the oscillation cycle of the sample stage where the test object is placed is gentle. . On the other hand, the frequency of the signal change of the signal error caused by the spot position displacement of the reflected light is higher than the height fluctuation of the test object, and the change is steep. Therefore, by detecting a high frequency component or a steep signal change, it is possible to detect the occurrence of a signal error due to the reflected light spot position displacement. Then, by removing the signal error component, it is possible to improve the focus position stabilization accuracy.

  FIG. 1 is a schematic configuration diagram showing an automatic focusing apparatus according to a reference example of the present invention based on such an idea. This apparatus is applied to a pattern defect inspection apparatus for inspecting a pattern on a test object. A position detection mechanism that measures the relative distance of the test object surface, and a test object is installed to detect the pattern of the test object. A stage control mechanism for controlling the position is provided.

  The stage control mechanism includes a stage 9 on which an object 8 is placed and a stage control circuit 7. The stage control circuit 7 forms a closed loop based on the position signal obtained by the position detection circuit 6 described later, and controls the stage 9 so as to keep the distance between the surface of the test object 8 and the objective lens 2 constant. . The stage 9 displaces the position of the test object 8 in the height direction (Z direction) by a piezoelectric element or the like according to the control of the stage control circuit 7. In order to inspect the entire surface of the test object 8, the stage 9 is movable in the horizontal direction (XY direction).

  The position detection mechanism includes a light source 1, an objective lens 2, a light projecting lens 3, a mirror 25, a light receiving lens 4, a light receiving sensor (light detecting element) 5, a position detecting circuit 6, a signal correcting circuit 20, and the like. The light beam emitted from the light source 1 is incident on the pattern surface on the surface side of the test object 8 through the light projecting lens 3 and the objective lens 2 and forms a spot-like light source image. As the light source 1, for example, a laser diode is used. The reflected light from the test object 8 is imaged on the light receiving sensor 5 through the objective lens 2, the mirror 25, and the light receiving lens 4. An output signal from the light receiving sensor 5 is input to the position detection circuit 6, and the position signal output from the circuit 6 is corrected in the signal correction circuit 20 and input to the stage control circuit 7.

  The position detection circuit 6 detects the position of the test object pattern surface by calculating the position of the center of gravity of the secondary light source or the light quantity distribution based on the output signal from the light receiving sensor 5. In this reference example, a line sensor is used as the light receiving sensor 5, but a quadrant sensor or the like may be used. The position detection circuit 6 and the signal correction circuit 20 may be integrated.

  The light receiving sensor 5 outputs two signals Ia and Ib depending on the light amount distribution on the light receiving surface. In the position detection circuit 20, after filtering the signal noise, the calculation of (Ia−Ib) / (Ia + Ib) is performed by the addition circuit, the subtraction circuit, and the division circuit. The difference (Ia−Ib) is a value proportional to the position signal, but depends on the amount of light flux. Therefore, the division is performed by the sum (Ia + Ib) for normalization. This is a position detection signal indicating the height position of the surface of the test object 8.

  The signal correction circuit 20 corrects the position detection signal obtained by the position detection circuit 6. The average settling time from when the displacement of the test object 8 is input to the stage control circuit 7 until the stage 9 responds is longer than the signal change period associated with the abnormal displacement of the spot position. Therefore, since the stage 9 does not respond immediately for a certain period after the abnormal displacement of the spot position of the reflected light occurs, a steep change appears in the position detection signal. This steep signal change is larger than the change in the position detection signal corresponding to the height displacement of the test object 8 and the surface shape of the test object 8. Therefore, by observing a steep change in the position detection signal (detecting that the amount of change per unit time exceeds a predetermined level), an abnormal displacement of the reflected light spot position can be detected.

  The signal correction circuit 20 monitors the difference between the position detection signal and the feedback target value, and detects an abnormality when the difference exceeds a predetermined threshold assuming the surface shape of the test object 8. When an abnormality is detected, the signal correction circuit 20 outputs a signal obtained by correcting the abnormal displacement of the signal based on a predetermined algorithm from the position detection signal.

  More specifically, this reference example uses the following algorithm. A feedback target value is output for a certain time after abnormality detection, and the difference between the position detection signal and the feedback target value when the rate of change of the position detection signal falls within a predetermined value is used as an offset. Thereafter, by subtracting the calculated offset value, only the abnormal displacement component can be removed. As other algorithms, various means such as a method of differentiating the position detection signal and outputting a feedback target value when a predetermined threshold value is exceeded can be used.

  As described above, according to the present reference example, by providing the signal correction circuit 20 that detects an abrupt change in the position detection signal and corrects an excess amount exceeding the value corresponding to the surface shape of the test object 8, A measurement error caused by a change in the spot position of reflected light, which occurs when a light beam is incident on a fine pattern having a line width comparable to the light source wavelength on the object 8, can be suppressed. Therefore, the influence of the abnormal displacement of the reflected light spot generated with a pattern size smaller than the light source wavelength can be suppressed, and focusing can be performed with high reliability.

(First embodiment)
The present inventors have found that there is a correlation between the spot position abnormal displacement of the reflected light and the polarization state of the reflected light. FIG. 2 shows the correlation between the signal intensity of the polarization sensor that measures the polarization state of the reflected light and the spot position displacement. As shown in this figure, the polarization state of the reflected light when the spot position is abnormally displaced is broken compared to the normal state. Therefore, if the polarization state of reflected light is measured and the detection of the change in polarization state and the detection of a steep signal change are satisfied at the same time, the possibility of the correction process malfunctioning is reduced by making a determination to perform the correction process. can do. Thus, by measuring the polarization state of the reflected light, the accuracy of correction can be increased.

  FIG. 3 is a schematic configuration diagram showing an automatic focusing apparatus according to the first embodiment of the present invention based on such an idea. In addition, the same code | symbol is attached | subjected to FIG. 1 and an identical part, and the detailed description is abbreviate | omitted.

  The stage control mechanism includes a stage control circuit 7 and a stage 9 as in the reference example. The position detection mechanism is provided with a beam splitter 11 and a polarization detection mechanism 26 in addition to the configuration of the reference example.

  Similar to the reference example, the light beam emitted from the light source 1 is incident obliquely on the pattern surface of the test object 8 through the light projection lens 3 and the objective lens 2 to form a spot-like light source image. The reflected light from the test object 8 is imaged on the light receiving sensor 5 through the objective lens 2, the mirror 25, the light receiving lens 4, and the beam splitter 11.

  Here, the reflected light beam is branched into two by the beam splitter 11 before entering the light receiving sensor 5, and one of the branched beams is incident on the polarization detection mechanism 26. The polarization detection mechanism 26 may be any mechanism that can detect a polarization component in a predetermined direction. For example, the polarization detection mechanism 26 may include a polarizing plate and an optical sensor. An output signal from the light receiving sensor 5 is changed to a position detection signal by the position detection circuit 6, an error component is removed by the signal correction circuit 20, and is input to the stage control circuit 7.

  The signal correction circuit 20 corrects the position detection signal obtained by the position detection circuit 6. The signal correction circuit 20 monitors the difference between the position detection signal and the feedback target value, and the difference exceeds a predetermined threshold assuming the surface shape of the test object 8, and the polarization information signal input from the polarization detection mechanism 26. Is detected as abnormal when the value is outside the allowable range. When the abnormality is detected, the signal correction circuit 20 outputs a signal obtained by correcting the abnormal displacement of the signal from the position detection signal based on a predetermined algorithm. A specific correction algorithm may be the same as in the reference example. In addition, when abnormality can be detected reliably only with the polarization information signal obtained by the polarization detection mechanism 26, it is possible to eliminate the need to monitor the difference between the position detection signal and the feedback target value.

  As described above, according to the present embodiment, in addition to the signal correction circuit 20 that detects an abrupt change in the position detection signal and corrects an excess amount exceeding the value corresponding to the surface shape of the test object, the spot position abnormal displacement is detected. By providing the polarization detection mechanism 26 that changes the detection output when the light is generated, the same effect as in the previous reference example can be obtained, and the abnormality can be detected more reliably and more reliably. Highly accurate focusing can be performed.

(Second Embodiment)
The present inventors have devised a method of installing a back surface sensor that measures the height displacement of the back surface of the test object separately from the surface sensor that measures the height of the surface of the test object. When the height of the test object is displaced, the same displacement signal is observed on both the front surface sensor and the back surface sensor. When an abnormal displacement occurs in the reflected light position measured by the front surface sensor, the front surface sensor observes the displacement, but the back surface sensor does not observe the displacement. Therefore, by constantly comparing the measured values of the front and back sensors, and correcting the front sensor when the back sensor does not detect the signal change obtained by the front sensor, the possibility of malfunction is suppressed and the accuracy of the correction Can be raised.

  FIG. 4 is a schematic configuration diagram showing an automatic focusing apparatus according to the third embodiment of the present invention based on such an idea. In addition, the same code | symbol is attached | subjected to FIG. 1 and an identical part, and the detailed description is abbreviate | omitted.

  The stage control mechanism is resisted by the stage control circuit 7 and the stage 9 as in the reference example. The position detection mechanism includes two systems for measuring the height of the pattern surface (front surface) and the back surface of the test object 8. In the present embodiment, the two detection mechanisms have the same method on the pattern surface and the back surface, but different detection methods can be used. The position detection mechanism on the front side is the same as that in the reference example, and the position detection mechanism on the back side includes a light source 27, an objective lens 28, a light projection lens 29, a mirror 30, a light receiving lens 31, and a light receiving sensor 15. Signals from the respective light receiving sensors 5 and 15 are simultaneously input to the position detection circuit 6 to obtain respective position detection signals.

  The signal correction circuit 20 corrects the position detection signal obtained by the position detection circuit 6. The signal correction circuit 20 monitors the difference between the position detection signal from the surface light receiving sensor 5 and the feedback target value, and if the difference exceeds a predetermined threshold assuming the surface shape of the test object, the surface sensor signal is abnormal. Is detected. Then, the position measurement values of the front side light receiving sensor 5 and the rear side light receiving sensor 15 are always compared, and the signal change obtained by the front side light receiving sensor 5 is regarded as abnormal only when the rear side light receiving sensor 15 does not detect it. . When the sensor signal is abnormal, the signal correction circuit 20 outputs a signal obtained by correcting the abnormal displacement of the signal from the position signal based on a predetermined algorithm. A specific correction algorithm may be the same as in the reference example. If an abnormality can be reliably detected only by the difference between the position detection signals from the light receiving sensors 5 and 15, monitoring of the difference between the position detection signal and the feedback target value can be made unnecessary.

  As described above, according to the present embodiment, the signal correction circuit 20 that detects an abrupt change in the position detection signal and corrects an excess exceeding the value corresponding to the surface shape of the test object is provided, and the test object 8 By measuring and comparing the two surfaces, it is possible to increase the detection accuracy of the abnormal displacement of the reflected light spot, and as a result, the correction reliability can be increased.

(Third embodiment)
As a means for measuring the polarization state of the reflected light from the test object described in the second embodiment, a method using a wavelength plate and a polarizing beam splitter was considered. The wavelength plate is adjusted so that the polarization state of the reflected light from the test object 8 incident on the light receiving sensor 5 is substantially linearly polarized in a state where the test object has no pattern. Therefore, normally, almost no light is incident on the polarization sensor.

  If the polarization state of the reflected light is disrupted in this state, the amount of light incident on the polarization sensor increases. Accordingly, by measuring the intensity of light incident on the polarization sensor, the polarization state of the reflected light can be measured in real time. Therefore, the signal of the light receiving sensor is input to the position detection circuit, and when the signal of the polarization sensor exceeds the threshold value in the signal correction circuit, the steep change exceeding the signal change due to the unevenness of the test object is corrected. As a result, the possibility of excessive correction can be greatly suppressed, and abnormal displacement of the reflected light spot position can be corrected with high reliability.

  FIG. 5 is a schematic configuration diagram showing an automatic focusing apparatus according to the fourth embodiment of the present invention based on such an idea. In addition, the same code | symbol is attached | subjected to FIG. 1 and an identical part, and the detailed description is abbreviate | omitted.

  The stage control mechanism includes a stage control circuit 7 and a stage 9 as in the reference example. The position detection mechanism includes wave plates 41 and 42, wave plates 16 and 17, a polarization beam splitter 18, and a polarization sensor 19 in addition to the configuration of the second embodiment.

  The light beam emitted from the light source 1 is incident on the pattern surface of the test object obliquely via the light projection lens 3, the wave plates 41 and 42, and the objective lens 2 to form a spot-like light source image. The wave plates 41 and 42 are λ / 2 plates and λ / 4 plates, respectively, and are adjusted so that the polarization state when entering the test object 8 is almost circularly polarized light. The reflected light from the test object 8 is imaged on the light receiving sensor 5 and the polarization sensor 19 via the objective lens 2, the mirror 25, the wave plates 16 and 17, the light receiving lens 4, and the polarization beam splitter 18. The wave plates 16 and 17 are λ / 2 plates and λ / 4 plates, respectively, and are adjusted so that the polarization state when there is no pattern on the surface of the test object 8 is substantially linearly polarized light. The polarization beam splitter 18 is installed so that the light quantity of the light receiving sensor 5 is maximized when there is no pattern on the surface of the test object 8.

  With such a configuration, the light beam branched by the polarization beam splitter 18 enters the polarization sensor 19 only when the polarization state of the reflected light is normal or changed. An output signal from the light receiving sensor 5 is input to the position detection circuit 6, and this circuit 6 outputs a position detection signal to the signal correction circuit 20.

  The signal correction circuit 20 corrects the position detection signal obtained by the position detection circuit 6. The signal correction circuit 20 monitors the difference between the position detection signal and the feedback target value, the difference exceeds a predetermined threshold assuming the surface shape of the test object, and the signal obtained from the polarization sensor 19 is out of the allowable range. If an error occurs, it is detected as abnormal. When an abnormality is detected, the signal correction circuit 20 corrects an error component of the signal based on a predetermined algorithm from the position signal, and outputs a position detection signal to the stage control circuit 7. A specific correction algorithm may be the same as in the reference example. In addition, when abnormality can be detected reliably only with the signal obtained by the polarization sensor 19, it is possible to make it unnecessary to monitor the difference between the position detection signal and the feedback target value.

  As described above, according to the present embodiment, in addition to the signal correction circuit 20 that detects an abrupt change in the position detection signal and corrects an excess amount exceeding the value corresponding to the surface shape of the test object, the spot position abnormal displacement is detected. By providing the polarization sensor 19 whose detection output changes when the phenomenon occurs, the same effect as in the second embodiment can be obtained.

(Fourth embodiment)
The abnormal displacement of the spot position of the reflected light from the test object also depends on the incident angle of the incident light. Therefore, as shown in FIG. 6, the light beam is incident on the test object 8 from a plurality of angles, and the optical path and the light receiving sensor are provided corresponding to each light beam, so that the influence of the pattern on the test object 8 is minimized. It is possible to obtain a signal that is limited. Since the height displacement of the test object 8 is equally reflected in the signal of the light receiving sensor corresponding to each light beam, an abnormal displacement of the reflected light spot position is selected by selecting a signal with the least signal change among the light receiving sensors. The influence of can be suppressed.

  FIG. 7 is a schematic configuration diagram showing an automatic focusing apparatus according to the fifth embodiment of the present invention based on such an idea. In addition, the same code | symbol is attached | subjected to FIG. 1 and an identical part, and the detailed description is abbreviate | omitted.

  The stage control mechanism includes a stage control circuit 7 and a stage 9 as in the reference example. As the position detection mechanism, two systems having different incident angles are provided. That is, the position detection mechanism includes the light source 1, the beam splitter 11, the mirrors 32 and 33, the light projection lenses 3 and 34, the objective lens 2, the mirrors 25, 35 and 36, the light reception lenses 4 and 37, the light reception sensors 5 and 15, the position. It comprises a detection circuit 6.

  The light beam emitted from the light source 1 is branched by the beam splitter 11, and one branched light is obliquely incident on the pattern surface of the test object 8 through the light projecting lens 34 and the objective lens 2, and the other branched light is mirror 32, The light is incident obliquely on the pattern surface of the test object 8 through the light projection lens 3 and the mirror 33, and forms a spot-like light source image. The incident angles of the respective light beams on the test object 8 are set to be different from each other as shown in FIG.

  In the present embodiment, one of the branched light beams enters the test object from the outside of the objective lens 2, but an optical path passing through the objective lens 2 can also be taken. The reflected light from the test object 8 is imaged on the light receiving sensor 5 via the objective lens 2, the mirror 25 and the light receiving lens 4, or on the light receiving sensor 15 via the mirror 35, the mirror 36 and the light receiving lens 37. The Output signals from the light receiving sensors 5 and 15 are simultaneously input to the position detection circuit 6, which outputs a position signal to the stage control circuit 7.

  The position detection circuit 6 has a selection function for selecting one of the position detection signals as well as position detection based on the detection signals of the sensors 5 and 15. More specifically, position detection is performed based on each signal obtained by the light receiving sensors 5 and 15, and the position detection signal having a smaller temporal change is selected, or each of the light receiving sensors 5 and 15 is obtained. The position detection is performed by selecting the signal with less signal change.

  Next, the position detection circuit 6 selects position signals obtained from the respective light receiving sensors 5 and 15. In the position detection circuit 6, the signal having the smallest amount of change is selected as the input signal from the light receiving sensors 5 and 15 and is output to the stage control circuit 7. As a result, the signal change due to the abnormal displacement of the reflected light beam spot position can be removed, and the autofocus operation can be performed normally.

  As described above, according to the present embodiment, by providing a plurality of position detection mechanisms having different incident angles and selecting the detection signal of the sensor with the smallest spot position change of the reflected light, the light source wavelength on the test object 8 can be determined. It is possible to suppress a measurement error caused by a change in the spot position of reflected light, which occurs when a light beam is incident on a fine pattern having the same line width. Therefore, the same effect as the reference example can be obtained.

  As described above, for a fine pattern having a line width comparable to the light source wavelength, when the incident angle θ between the pattern surface and the incident light exceeds a certain value as shown in FIG. A phenomenon that the reflected spot position of the reflected light shifts as shown in FIG. 15 is observed. This is due to a resonance phenomenon that occurs when the spatial period, incident angle, and wavelength of the pattern satisfy a specific relationship. Therefore, it is easily understood that the influence on the sensor is different by changing the incident angle as in the present embodiment. That is, selecting the detection signal of the sensor with the least change in the spot position of the reflected light selects the sensor output in which the above-described resonance phenomenon does not occur, thereby enabling highly accurate focusing.

(Fifth embodiment)
The abnormal displacement of the spot position of the reflected light from the surface of the test object depends on the incident direction of the incident light with respect to the pattern direction on the test object. Therefore, as shown in FIG. 8, the influence of the pattern on the test object 8 is minimized by providing light beams to the test object 8 from a plurality of directions and providing light receiving sensors corresponding to the respective light beams. A signal can be obtained. Since the height displacement of the test object 8 is equally reflected in the signal of the light receiving sensor corresponding to each light beam, by selecting a signal with the least signal change among the light receiving sensors, the reflected light spot position is abnormal. The influence of displacement can be suppressed.

  FIG. 9 is a schematic configuration diagram showing an automatic focusing apparatus according to the sixth embodiment of the present invention based on such an idea. The same parts as those in FIG. 7 are denoted by the same reference numerals, and detailed description thereof is omitted.

  The basic configuration is the same as that of the fifth embodiment shown in FIG. 7, and the two position detection mechanisms are different in incident direction instead of incident angle.

  Even in such a configuration, by selecting the detection signal of the sensor with the least change in the spot position of the reflected light, the light beam is incident on a fine pattern having a line width comparable to the light source wavelength on the object 8 In this case, it is possible to suppress a measurement error caused by a change in the spot position of reflected light. Therefore, the same effect as the fifth embodiment can be obtained.

(Sixth embodiment)
FIG. 10 is a diagram showing a basic configuration of an automatic focusing apparatus according to the seventh embodiment of the present invention. In addition, the same code | symbol is attached | subjected to FIG. 1 and an identical part, and the detailed description is abbreviate | omitted.

  The abnormal displacement of the spot position of the reflected light from the surface of the test object 8 takes different values depending on the polarization component of the reflected light. As shown in FIG. 10, the reflected light is separated by the beam splitter 12, and the respective light beams are further separated by the polarization beam splitters 18, 21, and the light receiving sensors 5, 22, 23, 24 corresponding to the separated light beams are provided. Use. The two polarizing beam splitters 18 and 21 are installed so that the polarization directions of the finally obtained four light beams are different from each other by 90 ° or 45 °. The height displacement of the test object 8 is equally reflected in the signals of the light receiving sensors 5, 21, 22, and 23 corresponding to the light beams. Accordingly, when the signal with the smallest signal change is selected from the light receiving sensors 5, 21, 22, and 23, a signal having the minimum influence of the abnormal displacement of the spot position of the reflected light is obtained. In this way, the influence of abnormal displacement of the reflected light spot position can be suppressed.

  FIG. 11 is a schematic configuration diagram showing the above-described automatic focusing apparatus more specifically. In addition, the same code | symbol is attached | subjected to FIG. 1 and an identical part, and the detailed description is abbreviate | omitted.

  The stage control mechanism includes a stage control circuit 7 and a stage 9 as in the reference example. The position detection mechanism includes a light source 1, an objective lens 2, a light projecting lens 3, wave plates 41 and 42, a mirror 25, a light receiving lens 4, a beam splitter 38, polarizing beam splitters 18 and 21, and light receiving sensors 5, 22, 23 and 24. The position detection circuit 6 is provided.

  The light beam emitted from the light source 1 is incident on the pattern surface of the test object 8 through the light projecting lens 3, the wave plates 41 and 42, and the objective lens 2 to form a spot-like light source image. The wave plates 41 and 42 are λ / 2 plates and λ / 4 plates, respectively, and are adjusted so that the polarization state when entering the test object 8 is almost circularly polarized light. The reflected light from the test object 8 is imaged on the light receiving sensors 5, 22, 23, and 24 via the objective lens 2, the mirror 25, the light receiving lens 4, the beam splitter 38, and the polarization beam splitters 18 and 21. The reflected light from the test object 8 is set by the beam splitter 38 and the polarization beam splitters 18 and 21 so as to be separated into four light beams whose polarization directions are different from each other by 90 ° or 45 °. More specifically, for example, polarized light of 0 ° is detected by the light receiving sensor 5, 90 ° by the light receiving sensor 22, 45 ° by the light receiving sensor 24, and 135 ° by the light receiving sensor 23. In this embodiment, a line sensor is used as the light receiving sensor, but a quadrant sensor or the like may be used.

  The position detection circuit 6 selects one from position signals for the light receiving sensors 5, 22, 23, and 24. The input signals from the light receiving sensors 5, 22, 23, 24 are compared, and the signal with the smallest change amount is selected and output to the stage control circuit 7. The light receiving sensors 5, 22, 23 and 24 receive light beams whose polarization directions are different from each other by 90 ° or 45 °. In other words, since the abnormal shift of the reflected light beam spot depends on the polarization direction of the reflected light, a signal with less error can be extracted by selecting the smallest signal change. With such a configuration, a signal change due to an abnormal displacement of the reflected light beam spot position can be removed, and the autofocus operation can be performed normally.

  As described above, according to the present embodiment, the four light beams having different polarization directions are detected by the independent light receiving sensors, and the detection signal of the sensor having the smallest change in the spot position of the reflected light beam is selected. It is possible to suppress a measurement error caused by a change in the spot position of reflected light, which occurs when a light beam is incident on a fine pattern having a line width comparable to the upper light source wavelength. Therefore, the same effect as the reference example can be obtained.

(Seventh embodiment)
The phenomenon of spot movement of reflected light in a fine pattern is closely related to the angle φ formed by the incident direction of incident light and the pattern direction. When φ = 0, since there is no repetitive structure in the incident direction, spot movement of reflected light does not occur. Further, when the incident direction is φ and φ + π, the amount of movement of the spot position is the same and the moving direction is reversed. For this reason, in the above-described condensing optical system, it is considered that astigmatism occurs because the focus position does not change with the component of φ = 0, and the focus position changes with the component of φ = π / 2. Can do. By measuring the direction and degree of astigmatism, measurement of error generation and error reduction can be realized. In this embodiment, it is possible to confirm the beam shape by using a surface sensor as the light receiving sensor, and it is possible to measure astigmatism and reduce errors.

  FIG. 12 is a schematic configuration diagram showing an automatic focusing apparatus according to the eighth embodiment of the present invention based on such an idea. In addition, the same code | symbol is attached | subjected to FIG. 1 and an identical part, and the detailed description is abbreviate | omitted.

  As the light source 1, for example, a laser diode having a wavelength of 600 to 700 nm is used. The light emitted from the light source 1 is collimated by the collimator lens 10, passes through the beam splitter 11, and is collected on the surface of the test object 8 by the objective lens 2. The light reflected by the test object 8 is transmitted again through the objective lens 2, reflected by the beam splitter 11, and then collected by the light receiving lens 4. A beam splitter 12 is disposed immediately after the light receiving lens 4, and the light is branched into two optical paths. A surface light receiving sensor (a two-dimensional area sensor composed of a CCD or the like) 39 is installed in the first optical path before the focal point of the light receiving lens 4, and the surface light receiving sensor 40 is disposed in the second optical path after the focal point of the light receiving lens 4. Is installed. The surface light receiving sensor is a two-dimensional area sensor composed of a CCD or the like. Sensor signals obtained by the surface light receiving sensors 39 and 40 are input to the position detection circuit 6. The position detection circuit 6 processes the input sensor signal, calculates it with an algorithm described later, and then outputs a height signal of the test object 8. The surface light receiving sensor here is, for example, a CCD and may be any sensor that can output the intensity distribution of the beam as an electric signal.

  The position detection algorithm is based on the following principle, for example. As shown in FIG. 13A, when there is no astigmatism and the focal position is located between the surface light receiving sensors 39 and 40, the beam shapes obtained from both sensors are the same. On the other hand, as shown in FIG. 13B, when the beam spot position fluctuation occurs in a fine pattern, the focus shift occurs in the direction orthogonal to the L / S direction, so that it can be obtained by the surface light receiving sensors 39 and 40. The beam shape is an ellipse. If it is assumed that the L / S direction corresponds to y in the figure and the focus shift is in the + direction in the figure, the beam shape obtained by the surface light receiving sensor 39 becomes an ellipse with the x direction as the short axis, The beam shape obtained by the sensor 40 is an ellipse whose minor axis is the y direction. Further, since it is assumed that the focus shift is in the + direction, the ellipticity of the beam shape obtained by the surface light receiving sensor 39 is larger.

  From these facts, the ellipticity of the beam shape obtained by the surface light receiving sensors 39 and 40 is compared, and it can be determined that the beam diameter in the major axis direction with the larger ellipticity does not include an error. Since the minor axis direction of the beam shape obtained by the surface light receiving sensor 39 is the y direction, the beam diameter in the y direction of the beam shape obtained by the surface light receiving sensors 39 and 40 does not include an error, and the y direction beam diameter of both sensors. By calculating the ratio, it is possible to obtain the exact height of the test object. That is, the recognition of the direction with no error is realized by comparing the ellipticity of the beam shape obtained by the surface light receiving sensors 39 and 40 in the above method. This presupposes that the focal point is located in the center of the sensors 39 and 40 in advance. Therefore, it can be easily realized when a mechanism for raising and lowering the height of the test object 8 is provided and a servo operation based on the height signal is performed. That is, by setting the servo target value so that the focal position is located at the center between the sensors, the state can be maintained.

  Based on such a principle, a specific processing flow is as follows, for example. First, the position detection circuit 6 captures signals obtained by the two surface light receiving sensors 39 and 40 by irradiating light on a position where it is known that there is no fine pattern on the test object 8. The position detection circuit 6 calculates the beam diameter in an arbitrary direction, calculates the difference / sum of the beam diameters of the sensors 39 and 40, and starts the servo operation based on this value. Thereafter, the position detection circuit 6 constantly monitors the ellipticity of the beam shape obtained by the surface light receiving sensors 39 and 40. A threshold value is set in advance for the ellipticity, and when the ellipticity of one sensor image exceeds the threshold value, the position detection circuit 6 obtains the major axis direction φ of the ellipse. Thereafter, the beam diameters in the φ direction of the surface light receiving sensors 39 and 40 are calculated, their difference / sum is calculated, and this value is output as a servo signal. Thereafter, the position detection circuit 6 constantly monitors the ellipticity, compares the ellipticity of the surface light receiving sensor 39.40, and determines the beam diameter in the major axis direction φ with the larger ellipticity to both the surface light receiving sensors 39, 40. The operation of outputting the difference / sum as a servo signal is continued.

  Thus, according to the present embodiment, the reflected light from the test object 8 is branched into two, one light is received in front of the focal point, and the other light is received behind the focal point. When the position detection is performed based on the beam diameter in the major axis direction with the larger ellipticity when the ellipticity of the sensor image exceeds the threshold, the position detection is performed on the object 8 It is possible to suppress a measurement error caused by a change in the spot position of reflected light, which occurs when a light beam is incident on a fine pattern having a line width comparable to the light source wavelength. Therefore, the same effect as the reference example can be obtained.

  In addition, this invention is not limited to each embodiment mentioned above. In the embodiment, the example in which the pattern defect inspection apparatus that inspects the pattern of the surface of the test object is described has been described. However, the embodiment can be applied to a lithography apparatus that transfers the pattern to the surface of the test object. Furthermore, the present invention can be applied to various devices that require focusing with an objective lens. Moreover, the optical component of each part can be suitably changed according to a specification. In addition, various modifications can be made without departing from the scope of the present invention.

The schematic block diagram which shows the automatic focusing apparatus concerning a reference example. The figure which shows the correlation with the signal strength of the polarization sensor which detects reflected light, and spot position displacement. 1 is a schematic configuration diagram showing an automatic focusing apparatus according to a first embodiment. The schematic block diagram which shows the automatic focusing apparatus concerning 2nd Embodiment. The schematic block diagram which shows the automatic focusing apparatus concerning 3rd Embodiment. The schematic diagram which shows a mode that light is irradiated with a different incident angle. The schematic block diagram which shows the automatic focusing apparatus concerning 4th Embodiment. The schematic diagram which shows a mode that light is irradiated in a different incident direction. The schematic block diagram which shows the automatic focusing apparatus concerning 5th Embodiment. The figure which shows the basic composition of the automatic focusing apparatus concerning 6th Embodiment. The schematic block diagram which shows the automatic focusing apparatus concerning 6th Embodiment. The schematic block diagram which shows the automatic focusing apparatus concerning 7th Embodiment. The schematic diagram showing a position detection algorithm. The schematic diagram which shows the relationship between a pattern surface, an incident direction, and an incident angle. The schematic diagram which shows the shift phenomenon of a reflected light spot position.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,27 ... Light source 2,28 ... Objective lens 3,29,34 ... Light projection lens 4,31,37 ... Light reception lens 5,15,22,23,24 ... Light reception sensor 6 ... Position detection circuit 7 ... Stage control circuit DESCRIPTION OF SYMBOLS 8 ... Test object 9 ... Stage 10 ... Collimator lens 11, 12 ... Beam splitter 16, 17, 41, 42 ... Wave plate 18, 21 ... Polarizing beam splitter 19 ... Polarization sensor 20 ... Signal correction circuit 25, 30, 32, 33, 35, 36, 38 ... mirror 26 ... polarization detection mechanism 39, 40 ... surface sensor

Claims (3)

An automatic focusing device for focusing an objective lens on a surface of an object held on a stage movable in a horizontal direction and a height direction,
An irradiation optical system for irradiating the surface of the test object with spot-like light from an oblique direction;
A beam splitter that branches reflected light from the surface of the test object;
A first photodetector for detecting one of the branched reflected light ;
Detecting the other of the branched reflected light, a second photodetector for detecting the predetermined direction of the polarization component of the reflected light,
A position detection circuit for detecting a height direction position of the test object from an electrical signal obtained by the first light detection element;
The position detection signal obtained by the position detection circuit is monitored in real time, the amount of change per unit time of the position detection signal accompanying the horizontal movement of the stage exceeds a predetermined level, and the second light detection element A correction circuit that corrects an excess exceeding a value corresponding to the surface shape of the test object from the position detection signal when the intensity of the obtained electrical signal exceeds a predetermined normal range;
A stage control circuit for controlling the height direction position of the stage based on a position detection signal obtained via the correction circuit;
An automatic focusing apparatus comprising: An automatic focusing device for focusing an objective lens on a surface of an object held on a stage movable in a horizontal direction and a height direction,
An irradiation optical system for irradiating the surface of the test object with spot-like light from an oblique direction;
A first wave plate for making light incident on the surface of the test object from an oblique direction into substantially circularly polarized light;
A second wave plate for making reflected light from the surface of the test object substantially linearly polarized light;
A polarization beam splitter that branches reflected light obtained through the second wave plate into a component along a polarization direction of the second wave plate and a component orthogonal to the polarization direction;
A first light detecting element for detecting light of a component along the polarization direction of the second wave plate obtained through the polarizing beam splitter;
A second photodetector for detecting light of a component orthogonal to the polarization direction of the second wave plate obtained via the polarizing beam splitter;
A position detection circuit for detecting a height direction position of the test object from an electrical signal obtained by the first light detection element;
The position detection signal obtained by the position detection circuit is monitored in real time, the amount of change per unit time of the position detection signal accompanying the horizontal movement of the stage exceeds a predetermined level, and the second light detection element When the obtained electrical signal exceeds a predetermined value, a correction circuit that corrects an excess exceeding a value corresponding to the surface shape of the test object from the position detection signal;
A stage control circuit for controlling the height direction position of the stage based on a position detection signal obtained via the correction circuit;
An automatic focusing apparatus comprising: An automatic focusing device for focusing an objective lens on a surface of an object held on a stage movable in a horizontal direction and a height direction,
An irradiation optical system for irradiating the surface of the test object with spot-like light from an oblique direction;
A wave plate for making light incident on the surface of the specimen from an oblique direction into substantially circularly polarized light,
A beam splitter that branches reflected light from the surface of the test object in two directions;
A first polarization beam splitter for branching one reflected light branched by the beam splitter into first and second lights whose polarization directions are orthogonal to each other;
The second reflected light branched by the beam splitter is branched into third and fourth lights whose polarization directions are orthogonal to each other and whose polarization directions are 45 degrees different from the first and second lights. A polarizing beam splitter;
A first light detecting element for detecting the first light;
A second light detecting element for detecting the second light;
A third photodetector for detecting the third light;
A fourth photodetector for detecting the fourth light;
A position where a plurality of electrical signals obtained by the first to fourth light detection elements are inputted, an electrical signal having a minimum signal change amount is selectively processed, and a height direction position of the test object is detected. A detection circuit;
A stage control circuit that controls the position in the height direction of the stage based on the output of the position detection circuit;
An automatic focusing apparatus comprising:

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