JP2003294420A - Adjustment apparatus for surface height of sample and automatic focusing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To hold the distance between an optical system and a photomask in a focus depth without bringing about the fluctuations of a focus position due to distrurbance-like return light and to enhance flaw inspection accuracy. <P>SOLUTION: An automatic focusing apparatus for aligning the surface of the photomask 11 has an LSI pattern formed thereon with the focal position of an inspection optical system 13 arranged in opposed relation to the surface of the photomask 11. The surface of the photomask 11 is irradiated with measuring light from a light source 21, the reflected light from the surface of the photomask 11 is detected by an optical sensor 22, and the detection signal of the optical sensor 22 is sampled at a constant cycle by an A/D converter 31. The signal subjected to A/D conversion is temporarily stored in a height measuring circuit 32, and the histogram distribution of a plurality of measuring signals for a definite period is calculated. The height position of the surface of a sample is measured on the basis of the histogram distribution and a Z-table 12 on which the photomask 11 is placed is driven on the basis of the measured height position by a Z-table drive circuit 33. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sample surface height adjusting device for adjusting the height of a sample surface to a target position.
The present invention also relates to an autofocus device for aligning the sample surface with the focus position of an inspection optical system such as a defect inspection device.

[0002]

2. Description of the Related Art One of the major causes of a decrease in yield in the manufacture of large scale integrated circuits (LSI) is a pattern defect occurring in a photomask used when manufacturing a device by a photolithography technique. Various defect inspection devices for inspecting such defects have been developed and put into practical use.

In this type of defect inspection apparatus, the pattern of the photomask to be inspected is formed by exposing and etching after depositing a metal film such as chromium on a glass substrate. The patterns of these photomasks are becoming finer year by year, and therefore, the inspection optical system for picking up a pattern image has been increased in magnification and NA.

Due to the higher magnification and higher NA of the optical system, the depth of focus, which is the permissible range of the distance between the optical system and the photomask capable of sharply forming an image, becomes narrower. Even a slight change in the distance between and causes blurring of the pattern image, which becomes a hindrance to the subsequent defect detection processing.

For this reason, an autofocus device is used to keep the distance between the optical system and the photomask constant. In this autofocus device, the photomask is irradiated with measurement light, the light reflected from the photomask is converted into an electric signal by a photodetector, and the height position of the photomask is measured from the change in the electric signal. And the optical system are controlled so that the distance between them is always constant. Specifically, the stage on which the photomask is placed is moved in the height direction (Z direction) according to the measurement position of the photomask.

However, this type of autofocus device has the following problems. That is, when the measurement light crosses the edge of the pattern drawn on the photomask, scattered light or diffracted light at the edge and uneven light amount due to the reflectance distribution cause the light intensity distribution in the photodetector element. Change. As a result, the measured value fluctuates, and the stage is controlled so as to match the changed measured value, which causes a problem that the observed image is blurred.

[0007]

As described above, due to the miniaturization of the pattern conventionally drawn on the photomask, the magnification and NA of the inspection optical system are increased, so that the observation image can be clearly formed. The depth of focus has become narrower, and fluctuations in the focus position due to disturbing return light due to diffraction of the measurement light generated in the pattern drawn on the photomask cannot be ignored.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a sample surface height adjusting device capable of accurately adjusting the height of the sample surface to a target position. To provide.

Another object of the present invention is to maintain the distance between the optical system and the sample surface within the depth of focus without causing fluctuations in the focus position due to disturbing return light, and to improve the defect inspection accuracy. An object is to provide an autofocus device that can contribute to improvement and the like.

[0010]

(Structure) In order to solve the above problems, the present invention adopts the following structure.

That is, the present invention is a sample surface height adjusting device for adjusting the height direction of a sample, comprising: a light source for irradiating the surface of the sample with measuring light; and a surface of the sample for irradiating with the measuring light. A light detecting element for detecting the reflected light of the light, an A / D converter for sampling the output of the light detecting element at a constant cycle, a memory for temporarily storing the A / D converted measurement signal, and a memory for storing in the memory. And a means for adjusting the height position of the sample on the basis of the obtained histogram distribution, and a means for adjusting the height position of the sample based on the obtained histogram distribution. .

The present invention also provides an autofocus device for aligning the surface of a sample on which a predetermined pattern is formed with the focal position of an optical system arranged opposite to the surface,
A light source that irradiates the surface of the sample with measurement light, a photodetector element that detects reflected light from the surface of the sample due to the irradiation of the measurement light, and an A / D that samples the output of the photodetector element at a constant cycle. A converter, a memory for temporarily storing the A / D converted measurement signal, a histogram distribution circuit for obtaining a histogram distribution of a plurality of measurement signals stored in the memory for a certain period, and the obtained histogram A height determination circuit that determines the height position of the sample surface based on the distribution, and a means that controls movement of at least one of the sample and the optical system in the height direction based on the determined height position. It is characterized by being done.

Here, the following are preferred embodiments of the present invention.

(1) The optical system is an inspection optical system for inspecting a pattern defect on the sample surface.

(2) The sample is placed on the stage,
The stage is moved in the height direction based on the height position determined by the height determination circuit.

(3) The optical system for measuring the height position of the sample surface irradiates the surface of the sample with light from an oblique direction,
A so-called grazing incidence illumination system in which the reflected light is received by a position sensor such as a PSD (Position Sensitive Device).

(4) The height is determined while continuously moving the sample in one direction in a plane orthogonal to the height direction, and the measurement light is on the surface of the sample in a direction not perpendicular to the moving direction of the sample. To scan.

(5) The height determination circuit determines the position with the highest frequency in the histogram distribution as the measured height, and the position where the offset amount to be specified is added to the measured height determination position is the final measured height. To do.

(6) If there are a plurality of points with a high frequency in the histogram distribution, the height determination circuit determines the average position of the measured heights of the points with a high frequency as the measured height, or Select a large number of positions as the measurement height judgment position, and make the position where the offset amount specified is added to the measurement height judgment position as the final measurement height.

(Operation) In the present invention, the measuring light reflected and returned from the surface of the sample is converted into an electric signal by the photodetector, and the height position of the sample is not measured by the obtained electric signal. A histogram distribution is obtained from the obtained electric signal at regular intervals, and the height position of the sample is measured from the obtained histogram distribution. By doing so, the influence of temporary disturbances such as scattered light, diffracted light, and light amount unevenness generated at the pattern edge becomes low in the histogram distribution, and the influence on the height measurement can be eliminated.

Therefore, according to the present invention, the height of the sample surface can be accurately adjusted to a target position. More specifically, the distance between the optical system and the sample surface can be kept within the depth of focus without causing fluctuations in the focus position due to disturbing return light, which contributes to improvement in defect inspection accuracy. It will be possible.

Further, with respect to the measured height obtained in this way, the stage for mounting the sample is controlled in consideration of the offset amount, so that the height information obtained by the photodetector element can be obtained. It is possible to effectively cope with a case where the height is to be slightly changed due to the influence of the process on the object.

[0023]

DETAILED DESCRIPTION OF THE INVENTION The details of the present invention will be described below with reference to the illustrated embodiments.

(First Embodiment) FIG. 1 shows a first embodiment of the present invention.
2 is a schematic configuration diagram showing an autofocus device according to the embodiment of FIG. This configuration is a general grazing incidence type displacement meter, and detects the mask height displacement from the optical axis displacement of the reflected light.

The photomask (sample) 11 is placed on a vertically movable Z table 12, and the Z table 12 is horizontally movable by an XY table (not shown). Above the photomask 11,
A defect inspection optical system 13 for detecting defects in the mask pattern is arranged. The inspection optical system 13 is a CCD
And the like. The photomask 11 is inspected by moving the photomask 11 in one direction, and a strip-shaped region having the above line width is inspected.
It is possible to inspect the whole of. Note that the autofocus in the present embodiment is to adjust the focus position of the inspection optical system 13 to the surface of the photomask 11.

The autofocus mechanism includes a light source 21 for irradiating the photomask 11 to be inspected with light for height measurement and an optical sensor 22 such as PSD for measuring the position of the light reflected by the photomask 11. It is equipped with a main optical measurement system. The optical measurement system further includes a condenser lens 23 for condensing light on the photomask 11.
An objective lens 24 for collecting light on the sensor 22 is arranged between the Z table 12 and the sensor 22 between the Z table 12 and the Z table 12. In addition, if the measurement light is fixed, the measurement light continues to be affected by the pattern edge when the measurement light is just applied to the edge of the pattern. Therefore, the movable mirror 25 is provided between the light source 21 and the photomask 11. Is arranged, and the measuring light is scanned on the photomask 11 by swinging the mirror 25 at a small angle at high speed so that the influence of the pattern edge does not continue. Further, the movable mirror 26 is arranged between the photomask 11 and the sensor 22, and the movable mirrors 25, 26 are shaken in synchronization with each other so that the reflected light does not shake on the sensor 22.

The electric signal obtained by the sensor 22 is A / D
It is converted into a digital signal by the converter 31 and supplied to the height measuring circuit 32. The height measuring circuit 32 outputs a differential signal with respect to the input offset value and the target height. By inputting this offset value, a desired offset amount is given to the measured height, and the differential signal with respect to the target height is changed. By doing so, it is possible to perform positioning by shifting the measurement height by a desired amount.

The difference signal from the target value output from the height measuring circuit 32 is supplied to a Z table driving circuit 33 for driving the Z table 12. Z table drive circuit 33
Then, the Z table 12 is driven according to the difference signal, and control is performed so that the distance between the optical system 13 and the photomask 11 is always constant. The offset value given to the height measuring circuit 32 may be given to the Z table driving circuit 33.

In order to keep the distance between the optical system 13 and the photomask 11 constant, the optical system 13 is driven in the Z direction instead of driving the photomask 11 in the Z direction. The configuration may be adopted.

A concrete structure of the height measuring circuit 32 is shown in FIG. The height measuring circuit 32 includes a memory circuit 41, a histogram distribution circuit 42, and a height determining circuit 43. A measurement signal obtained by A / D converting the electric signal output from the optical sensor 22 of FIG. 1 by the A / D converter 31 of FIG. 1 at a constant sampling period is input to the memory circuit 41. The memory circuit 41 is configured so that new data always remains like a FIFO, and stores a fixed amount of the A / D converted measurement signal. Histogram distribution circuit 42
Is connected to the memory circuit 41 and refers to a constant number of measurement signals among the measurement signals stored in the memory circuit 41 to obtain the histogram distribution thereof.

The height judging circuit 43 is a circuit for judging the measured height from the histogram distribution obtained by the histogram distribution circuit 42. An offset value can be designated from the outside in the height determination circuit 43, and the designated offset amount is taken into consideration with respect to the measured height determined from the histogram distribution obtained by the histogram distribution circuit 42. Then, a difference signal from the target height for driving the Z stage 12 is output. Here, the memory circuit 41, the histogram distribution circuit 42, and the height determination circuit 43 can be configured by programs as long as there is no problem in processing speed.

Next, a method of determining the height position of the sample surface from the histogram will be described with reference to FIG. The measurement signal obtained by A / D converting the output of the optical sensor 22 at a constant sampling period is displayed along the flow of time (a). An image obtained by extracting a measurement signal for a certain period from this is shown in (b). The result of converting the measurement signal of (b) into a histogram distribution is (c). In this example, the place where the measurement signal changes from the low level to the high level is extracted. That is, when considered with a photomask, it means a state in which a glass surface having no pattern is transferred to, for example, a chrome pattern.

Looking closely at this measurement signal, when the level changes from a low level to a high level, the measurement signal value is like a corner due to the influence of the edge of the pattern. In the conventional method, since the Z table 12 is directly controlled by this measurement signal, the observed image is blurred at this edge portion. On the other hand, in the present embodiment, on the histogram distribution, the jumping part of this signal is not affected because it has a low frequency, and the low-level signal has a high frequency, so the low-level position is the measurement height. Deciding.

The low-level position means the photomask 11
In the case of means a glass surface substantially. If the depth of focus is about the thickness of the chrome pattern, by holding the focus position on the glass surface, inspection can be performed on all parts without blurring. Even if the Z table 12 is driven to the second highest position (chrome surface) in the histogram, the glass surface as well as the chrome surface is within the depth of focus, so that inspection can be performed on all parts without blurring. .

However, when the Z table 12 is driven to a signal jumping portion such as an edge portion, the glass surface deviates from the depth of focus and blurring occurs. The jumping part of the signal may be larger than shown in the figure,
In this case, the chrome pattern surface will also be out of the depth of focus.

In order to obtain effective height information at the current measurement position from the histogram, not only the current measurement position but information before and after the current measurement position are necessary. In the present embodiment, as shown in FIG. 4, the measurement light is scanned in a direction inclined by, for example, 45 degrees with respect to the table moving direction. Therefore, the electric signal obtained by the optical sensor 22 includes not only the information of the original measurement position but also the information of the positions before and after the original measurement position, whereby an effective histogram can be obtained. Here, the scanning direction of the measurement light needs to have a component of the table movement direction in order to obtain an effective histogram, and thus may be a direction that is not perpendicular to the table movement direction.

As described above, according to the present embodiment, the height measurement signal is stored for a certain period, the histogram distribution is obtained from the data, and the place where the frequency of the obtained histogram distribution is high is set as the measurement height. It is possible to minimize height measurement signal error caused by scattered and diffracted light generated at the pattern edge of the photomask 11, uneven light amount due to reflectance distribution, and the like. Therefore, the effect of a temporary height measurement signal error due to the pattern edge of the photomask 11 can be eliminated, and accurate height adjustment can be performed.

Therefore, the pattern defect of the photomask 11 can be inspected with high accuracy. Further, it becomes possible to realize a more practical pattern defect inspection apparatus that meets various inspection requirements.

(Second Embodiment) FIG. 5 illustrates another example of the height determining method. In this example, as in FIG. 3, the image obtained by extracting the data for a certain period from the portion (a) in which the multilevel signal is displayed along the flow of time is shown in (b). What is different from FIG. 3 is that FIG. 4 (b), which is an image obtained by extracting data for a certain period, is slightly shifted in time from FIG. 3 (b).

In this case, the result of converting the multi-valued signal of FIG. 4 (b) into a histogram distribution is shown in FIG. 4 (c), where the high frequency is at both the low level (glass surface) and the high level (chrome surface). Exists in. This just means that the height measurement position is near the boundary between glass and chrome.

In this case, for the determination of the measurement height, the measurement height may be intermediate between the low level (glass surface) and the high level (chrome surface), or the high level (chrome surface) may be fixedly measured. Good as it is. Similarly, it is also possible to set a fixedly low level (glass surface) as the measurement height. That is,
Using this method, the measurement height can be set at a position convenient for the entire system. Then, the offset amount can be added to the measured height by the offset value given from the outside.

Considering a case where the depth of focus of the inspection optical system becomes extremely narrow due to the higher magnification and higher NA of the inspection optical system in the future, the glass surface and the chrome surface are faithfully observed in the height direction. It is desirable to perform alignment. At this time, if the height position of the photomask 11 is measured directly from the sensor output instead of the histogram as in the conventional case, the measurement signal jumps up due to the influence of the pattern edge at the boundary between the glass surface and the chrome surface. When the Z table is driven and controlled, the inspection of the chrome surface is in a defocused state near the edge. In the present embodiment, such inconvenience can be prevented by controlling based on the histogram.

Further, as shown in FIG. 6, when the obtained histogram distribution has a width, the average of the distribution may be obtained and its position may be set as the measurement height. Specifically, the average of a portion exceeding a certain threshold value or the average value of several right and left sides of the maximum value may be taken. Also in this case,
It is possible to minimize the height measurement signal error caused by scattered and diffracted light generated at the pattern edge of the photomask 11, uneven light amount due to reflectance distribution, and the like. Therefore,
The same effect as the first embodiment can be obtained.

(Modification) The present invention is not limited to the above embodiments. In the embodiment, the so-called oblique incidence method of irradiating light from the oblique direction and detecting the light reflected in the oblique direction is adopted, but a so-called confocal method may be adopted as shown in FIG. 7. In this method, the measurement light is applied to the photomask 11 from a vertical direction via a half mirror. Then, the measurement light reflected by the photomask 11 is detected at two points having different optical distances with respect to the photomask 11. Specifically, the light detecting elements 51 and 52 detect light that has passed through the pinholes PH1 and PH2 arranged before and after the focus position.

The detection output of the light passing through PH1 and PH2 is as shown in FIG. 8, and the difference signal between them is as shown by the bold line in the figure. The position where the difference signal becomes 0 is the focus position. Therefore, the table may be driven in the Z direction so that the difference signal becomes zero. In this case, the histogram distribution of the difference signal may be obtained and the height may be adjusted based on the histogram as in the previous embodiment.

Although the table on which the photomask is placed is moved in the embodiment, the inspection optical system may be moved in the Z direction instead. Further, the present invention is not necessarily limited to the defect inspection apparatus, but can be applied as long as it is necessary to accurately adjust the height position of the surface of the sample.

In addition, various modifications can be made without departing from the scope of the present invention.

[0048]

As described in detail above, according to the present invention, not the detection signal itself of the photo-detecting element for detecting the reflected light from the surface of the sample but the histogram distribution from the obtained electric signal at regular intervals. By measuring the height position of the sample from the obtained histogram distribution, the height of the sample surface can be accurately adjusted to a target position. In addition, when applied to defect inspection etc., the influence of temporary disturbance such as scattered light or diffracted light generated at the pattern edge, uneven light intensity, etc. becomes low on the histogram distribution, so the focus position of the disturbance return light Without causing fluctuations
The distance between the optical system and the sample surface can be kept within the depth of focus, which can contribute to the improvement of defect inspection accuracy.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an autofocus device according to a first embodiment.

FIG. 2 is a block diagram showing a specific configuration example of a height measuring circuit in FIG.

FIG. 3 is a schematic diagram for explaining a height determination method according to the first embodiment.

FIG. 4 is a diagram showing a scanning direction of measurement light on a sample surface.

FIG. 5 is a schematic diagram for explaining a height determination method according to the second embodiment.

FIG. 6 is a schematic diagram for explaining another example of the height determination method according to the second embodiment.

FIG. 7 is a view for explaining a modified example of the present invention, showing a configuration of a confocal measurement system.

8 is a diagram showing a measurement signal waveform obtained by the measurement system of FIG.

[Explanation of symbols]

11 ... Photomask (sample) 12 ... Z table 13 ... Optical system for inspection 21 ... Light source 22 ... Optical sensor 23, 24 ... Lens 25, 26 ... Mirror 31 ... A / D converter 32 ... Height measuring circuit 33 ... Z table drive circuit 41 ... Memory circuit 42 ... Histogram distribution circuit 43 ... Height determination circuit

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 2F065 AA24 BB02 CC18 FF01 HH04                       HH12 JJ03 JJ16 LL12 LL13                       PP12 QQ03 QQ43                 2H051 AA15 BB27 DA03                 2H095 BD02 BD11

Claims (5)

[Claims] 1. A light source for irradiating the surface of a sample with measuring light, a light detecting element for detecting reflected light from the surface of the sample due to irradiation of the measuring light, and sampling the output of the light detecting element at a constant cycle. Do A
A / D converter, a memory for temporarily storing the A / D converted measurement signal, a histogram distribution circuit for obtaining a histogram distribution of a plurality of measurement signals stored in the memory for a certain period, and the obtained histogram A means for adjusting the height position of the sample based on the distribution, and a device for adjusting the height of the sample surface. 2. An autofocus device for aligning a surface of a sample on which a predetermined pattern is formed with a focal position of an optical system arranged to face the surface, and irradiating the surface of the sample with measurement light. A light source, a photodetector for detecting light reflected from the surface of the sample due to irradiation of the measurement light, and an output of the photodetector for sampling at a constant cycle A
A / D converter, a memory for temporarily storing the A / D converted measurement signal, a histogram distribution circuit for obtaining a histogram distribution of a plurality of measurement signals stored in the memory for a certain period, and A height determination circuit for determining the height position of the sample surface based on the histogram distribution, a means for controlling movement of at least one of the sample and the optical system in the height direction based on the determined height position, An autofocus device comprising: 3. The height judgment is performed while continuously moving the sample in one direction in a plane orthogonal to the height direction, and the measuring light is perpendicular to the moving direction of the sample on the surface of the sample. The autofocus device according to claim 2, wherein scanning is performed in a non-direction. 4. The height determination circuit determines a position having the highest frequency in the histogram distribution as a measurement height, and determines a position obtained by adding an offset amount designated to this determination position as a final measurement height. The autofocus device according to claim 2, wherein: 5. The height determination circuit, when there are a plurality of places with a high frequency in the histogram distribution, determines the average position of the measurement heights of the respective places with a high frequency as the measurement height, or one of the frequencies. 3. The autofocus device according to claim 2, wherein a position having a large number of points is selectively used as a measurement height determination position, and a position obtained by adding an offset amount to the determination position is determined as a final measurement height.