JPH0157885B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focus control device for automatically controlling the focal position of an objective lens in an optical observation device, measurement device, etc. to always obtain a clear imaging state. is focused by the target lens,
The present invention relates to a device that irradiates an object with minute spot light and controls the focal position based on the reflected light.

Traditionally, in cameras, etc., photoelectric elements are placed in two places, the front focusing plane and the rear focusing plane, before and after the optimal focusing plane (planned focal plane), and based on the photoelectric output, the imaging lens ( A device is known that controls the focal position so that the image plane (or its conjugate plane) of object light transmitted through an objective lens coincides with the optimal focusing plane. However, when such automatic focus control devices are applied to devices that precisely measure pattern line widths on wafers or masks, devices that inspect IC pattern defects, or reduction projection exposure devices, even higher accuracy is required. and a wider working area is required.

Generally, each type of device is equipped with an objective lens or a projection lens with extremely high resolution. In a high-resolution lens, the blurred image between the front focal plane and the rear focal plane becomes asymmetrical due to inherent lens aberrations. For this reason, simply controlling the focal position based on photoelectric information from the front focal plane and the rear focal plane cannot achieve the required precision, for example, precisely within the focal depth of the objective lens. There were some shortcomings.

SUMMARY OF THE INVENTION An object of the present invention is to solve these drawbacks and provide a focus control device with high precision and a wide operating range.

Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view of a line width measuring device that scans a semiconductor wafer or mask with a spot of laser light to measure the line width and edge spacing of a pattern. This device measures the line width by photoelectrically detecting the diffracted light and scattered light generated from the edge when a spot light is irradiated onto the edge of the pattern. The details of this device are disclosed in Japanese Patent Publication No. 56-25964, ``Pattern Line Width Measuring Apparatus,'' so a description of the line width measurement, which is not directly related to the present invention, will be omitted. In FIG. 1, a laser beam from a laser light source 1 is expanded by a beam collimator 2 to become a parallel beam of light, which is reflected by a reflection mirror 4 to an objective lens 5 via a half mirror 3. The parallel laser light incident on the objective lens 5 is a minute spot light (1μ in diameter).
m), and irradiates the sample 6 as an object to be observed or measured. The reflected light from the sample surface of the sample 6 enters the objective lens 5 in the reverse direction, becomes a parallel beam again, is reflected by the reflection mirror 4, and then is reflected by the half mirror 3, and the defocus of the objective lens 5 is detected. incident on the detection optical system. This detection optical system will be further explained with reference to FIG. The beam from the half mirror 3 is split into two beams by the beam splitter 7, one beam enters a short focal length lens 9 (first imaging means) via the mirror 8, and the other beam is split into two beams. is a long focal length lens 10
(second and third imaging means). The light beam passing through the lens 9 enters the beam splitter 13 and is further split into two light beams. The light beam transmitted through the splitting surface 13b of the beam splitter 13 passes through the optical path length correction glass 14, and then passes through a one-dimensional photo array, such as a charge-coupled device (hereinafter referred to as
It reaches the center of the 15 light-receiving surfaces (CCD) divided into three in the element arrangement direction. On the other hand, the luminous flux reflected by the splitting surface 13b of the beam splitter 13 is reflected by the reflecting surface 13c, and the beam is reflected by the CCD 1.
5 (lower part in Figure 2). Furthermore, the light beam passing through the lens 10 is transmitted through the optical path length correction glass 1
1, is reflected by mirror 12, is reflected by reflection surface 13a of beam splitter 13, and is reflected by CCD
15 (upper part in FIG. 2).
Here, on the light receiving surface of the CCD 15, the area that receives the light beam that has passed through the lens 10 is defined as the focus detection area 15a, and the area that receives the light beam that has passed through the beam splitter 13 is defined as the focus detection area 15a.
The areas that receive the light flux are respectively designated as rear focus detection areas 15.
b. This will be referred to as the front focus detection area 15c. The light receiving surface of the focus detection area 15a is the objective lens 5.
When the focal position of the spot light coincides with the sample surface, the spot light entering the objective lens 5 in the opposite direction is located at the position where the image is formed, that is, on the optimal focal plane (planned focal plane). At this time, the rear focus detection area 15b is determined to be located behind the optimal focusing plane along the optical axis direction of the objective lens 5, and the front focus detection area 15c
is determined to be located in front of the optimal focusing plane, and regions 15b and 15c are determined to have equal optical path lengths from region 15a.

Furthermore, the lenses 9 and 10 have focal lengths of approximately 300 mm and 900 mm, for example.

Next, a control circuit for controlling the focus based on the photoelectric output of the CCD 15 will be explained with reference to FIG. CCD
The amount of accumulated charge as photometric information corresponding to the light intensity of each light-receiving element 15 is determined by the CCD drive circuit 16.
The data are sequentially extracted in chronological order in response to timing pulses from. The amount of electric charge swept out from each light-receiving element is amplified by an amplifier 17 and then input to a sample-and-hold circuit 18 . The sample and hold circuit 18 samples and holds the peak value of the output voltage of the amplifier 17 at every timing pulse of the CCD drive circuit 16. The divider 19 optimizes the output voltage of the sample and hold circuit 18 for the next input level range of the A/D converter 20. The output voltage optimized by the divider 19 is A/
is converted into a digital value by the D converter 20 in response to the timing pulse of the CCD drive circuit 16,
The digital values are sequentially stored in memory 21 in a random access format by high speed I/O transfer (eg, direct memory access (DMA) method). At this time, the memory 21 contains the CCD
Digital values of accumulated charge amounts assigned corresponding to each of the 15 light receiving elements are sequentially stored.

Now, the digital values stored in the memory 21 are processed by a microprocessor 22 and a high-speed processor 23 dedicated to multiplication and division according to a specific algorithm. At this time, the microprocessor 22 determines the amount of deviation between the focal position of the objective lens 5 and the sample surface at the time when the amount of charge on the CCD 15 is swept out, and adjusts the amount of the objective lens 5 corresponding to the amount of deviation.
The output signal is output to the drive device D/A converter 24. A drive device 25 using a motor etc. is a D/A converter 24.
In response to the output signal, the objective lens 5 is moved by a predetermined amount in the optical axis direction.

Note that the divider 19 uses the output data of the microprocessor 22 converted into analog data by the D/A converter 26 as the denominator, and divides the data into the sample and hold circuit 18.
Divide the output voltage in an analog way. This divider 19 functions via a microprocessor 22 as a so-called automatic gain control (AGC). In addition, the microprocessor 22
It includes a memory section that stores programs for performing predetermined operations, an arithmetic register section that performs various calculations and comparisons, and an I/O port section that communicates with external circuits. .

Next, the operation of this device will be explained based on the flowchart shown in FIG. This flowchart shows the operation sequence of the microprocessor 22. Sample 6 whose line width is to be measured
is placed under the objective lens 5. At this time, the objective lens 5 and the sample 6 are set at a predetermined interval in consideration of the thickness of the sample 6. When the focus control operation starts, “CCDRD1” in step 100
All the photometric information from the CCD 15 is read out for each light receiving element and sequentially stored in the memory 21 via the A/D converter 20. Then, the microprocessor 22 reads the photometric information corresponding to the front focus detection area 15c and the rear focus detection area 15b of the CCD 15 from among the photometry information stored in the memory 21, and performs the "area detection" process in step 101. Execute. This "area detection" is performed using the objective lens 5.
This is to detect whether the amount of deviation between the focal point position and the sample surface can be immediately determined based on the read photometric information. If the amount of deviation is extremely large
As shown in FIG. 5a, the light intensity distribution on the light receiving surface of the CCD 15 becomes substantially flat over each of the detection areas 15a to 15b. In FIG. 5, Ds corresponds to the maximum value that can be digitally converted by the A/D converter 20. This "area detection" detects whether there is a peak (or dip) of a predetermined size in the light intensity distribution of the front focus and rear focus detection areas 15c and 15b. If the light intensity distribution is flat as shown in FIG. 5a, the microprocessor 22 determines that it is out of the area in the next step 102, and moves on to ``lens drive A'' in step 103.
The "lens drive A" process is to drive the objective lens 5 by a certain amount in the optical axis direction. Specifically, the microprocessor 22 outputs a predetermined constant value to the D/A converter 24, for example, a value corresponding to 20 μm as the amount of movement of the objective lens 5. According to this value, the driving device 25 controls the objective lens 5.
move. Once this movement is complete, the micro
Processor 22 repeats the above-described operations from step 100 again.

Now, if it is determined in step 102 that the area is within the area, the microprocessor 22 returns to step 1.
Step 104 "CCDRD2" is executed in the same way as in step 00. However, "CCDRD2" in step 104 also performs a process of determining a constant that becomes the denominator of the divider 19 and optimizing the input voltage of the A/D converter 20.

Therefore, this optimization will be explained using FIG. 5b. Now, if the light intensity distribution of the CCD 15 read first is generally lower than the maximum value Ds, as shown in distribution i as the output voltage of the divider 19, then
The photometric value (charge amount) for each light-receiving element stored in the memory 21 is searched to find the peak value in the distribution i. And this peak value and maximum value Ds
and outputs a constant corresponding to the ratio to the D/A converter 26. As a result, the denominator of the divider 19 is updated to a small value, and then at the same focal position,
When the photometric information of the CCD 15 is read, the output voltage of the divider 19 has a peak value close to the maximum value Ds as shown in distribution j. In this way, by detecting and optimizing the peak component of photometric information, digital errors during A/D conversion are reduced and processing can be performed with a sufficient S/N ratio, resulting in extremely improved reproducibility. do.

Now, in step 105 "Focus calculation A",
Among the photometric information optimized and stored in the memory 21, the front focus and rear focus detection areas 15 of the CCD 15
The amount of shift in the focal position of the objective lens 5 is calculated based only on the photometric information of c and 15b (hereinafter referred to as front focus information and rear focus information). This calculation basically involves determining the difference between the front focus information and the rear focus information and converting it into a deviation amount. In "focus calculation A", the microprocessor 22 uses the memory 21
Read the front pin information inside and set the front pin detection area 1.
The sum of the photometric intensities of each light-receiving element 5c is determined. This corresponds to finding the area S ₁ on the distribution j of the front focus information as shown by diagonal lines in FIG. 5c. Next, find the peak value P ₁ in the previous pin information,
Divide by the sum (take S ₁ ). At this time, a real number calculation is performed on P ₁ /S ₁ using the calculation-dedicated processor 23,
Save the results. Next, the microprocessor 22 reads the rear pin information from the memory 21 and, in the same way as above, calculates the summation value shown by diagonal lines in FIG.
S ₂ and peak value P ₂ are determined, and the processor 23 calculates
Perform real number calculation on P ₂ /S ₂ . Then, the previously obtained calculation result P ₁ /S ₂ and this P ₂ /S ₂ are subtracted.
This subtraction value corresponds to the amount of shift in the focus position including the front focus and back focus, and ideally, if the subtraction value becomes zero, the shift amount also becomes zero, which means that the object is almost in focus. Furthermore, the sign of the subtraction value is determined by the fact that the conjugate image formed by the objective lens 5 is relative to a predetermined focal plane.
Indicates whether it is located at the front or rear. In this way, when processing the front focus and rear focus information, it is normalized (divided) by the sum of the photometric intensities.
P ₁ /S ₂ , P ₂ /S ₂ ) has the advantage that the accuracy does not deteriorate due to differences in reflectance from sample to sample.

Now, the subtraction value calculated in this way does not directly represent the amount of deviation of the focal position. Therefore, in order to convert the subtraction value into a deviation amount, the microprocessor 22 searches a conversion table prepared in advance in a storage circuit (ROM, etc.) to find the corresponding deviation amount. This conversion table is for objective lens 5
It is best to create one for each device, including its optical characteristics. The microprocessor 22 performs the above processing in step 105 "focus calculation A", and executes the next step 106. In step 106, it is determined whether the calculated amount of deviation is less than or equal to a predetermined value. The predetermined value corresponds to, for example, a vertical movement of the objective lens 5 in the optical axis direction, and a deviation amount of ±1 μm from the optimal focal position. If it is determined in step 106 that the value is greater than or equal to the predetermined value, the "lens drive B" step is performed in step 107.
is executed. Here, the microprocessor 22
In step 105, the amount of movement of the objective lens 5 corresponding to the amount of deviation determined in step 105 is determined. The correspondence between the amount of shift and the amount of movement is created in advance as a reference table in a storage circuit such as ROM. Therefore, the microprocessor 22 can immediately retrieve the corresponding movement amount simply by searching this reference table. This amount of movement is output to the D/A converter 24, which drives the objective lens 5 in the optical axis direction by the amount of movement via the drive device 25. "Lens drive B"
When the process is completed, the microprocessor 22 repeats the same operation from step 104 "CCDRD2".

Above, steps 104, 105, 106, 10
At step 7, the focal position of the objective lens 5 is roughly adjusted.
Completion of the coarse adjustment is determined in step 106, but
The criterion for this determination is determined by whether the subtraction value (differential information) obtained from the front focus and rear focus information is the minimum limit at which the objective lens 5 can be driven. This is because even if the focal positions match accurately, the blurred image inherent to the objective lens 5 will be asymmetrical between the front and rear focal planes.

Now, when it is determined in step 106 that the coarse adjustment (for example, within ±1 .mu.m from the in-focus position) has been completed, "CCDRD3" in step 108 is executed. "CCDRD3" is similar to "CCDRD2",
The photometric information of the CCD 15 is read into the memory 21.
In addition, in "CCDRD3", optimization by the divider 19 may be performed, but since the processing time becomes correspondingly longer, it is not necessary to perform optimization. At this time, the light intensity distribution of the photometric information stored in the memory 21 is
The result is as shown in Figure d. That is, by rough adjustment, the front focus detection area 15c, the rear focus detection area 15c of the CCD 15,
The blurred image of the laser spot light on 15b has a substantially equal Gaussian distribution shape. In addition, focus detection area 1
The light intensity of the spot light on 5a has an extremely sharp peak.
It becomes a Gaussian distribution with Pm. Furthermore, the area 15a
The size of the spot light image above is 1 of CCD15.
sufficiently larger than two light receiving elements. The imaging magnification for the area 15a is the same as that for the other two areas 15a.
Since the imaging magnification is set to be larger than the imaging magnification for pixels b and 15c, the magnitude of the peak Pm changes extremely sensitively to slight fluctuations in the focal position.

Therefore, the next step 109 "Focus calculation B"
Then, the microprocessor 22 reads the photometric information (hereinafter referred to as focus information) of the focus detection area 15a in the memory 21, and calculates the value of the peak Pm in the focus information (the shaded area in FIG. 5). demand. this peak
The value of Pm is stored in microprocessor 22. In step 110, it is determined whether or not the obtained peak Pm has reached a maximum within a slight vertical movement range of the objective lens 5. If it is determined in step 110 that it is not the maximum, the microprocessor 22 executes "lens drive C" in step 111. "Lens drive C" moves the objective lens 5 by a minute amount, for example, 0.1 μm, in the optical axis direction. Then, the microprocessor 22 repeats execution from "CCDRD3" again. Step 1
When it is determined in step 10 that the peak Pm has become the largest, the process proceeds to the next step 112, ``position storage''. In this "position storage", the microprocessor 22 reads and stores the position of the objective lens 5 with respect to the base on which the sample 6 is placed using a potentiometer (not shown) or the like.

Note that the above steps 108, 109, 110, 1
11 performs fine adjustment of the focal position. In the above embodiment, this fine adjustment is performed at the peak of the focusing information.
This was a so-called servo control operation in which the objective lens 5 was moved minute by minute so that Pm was maximized. However, as another method, the objective lens 5 is moved by a minute amount, the peak Pm value is sampled, and the changes in the peak Pm are sequentially stored within the fine adjustment range of the objective lens 5 (for example, within ±1 μm). . Then, through software processing, the objective lens 5 corresponding to the maximum value of peak Pm is adjusted.
, and immediately place the objective lens 5 at that position.
You can also move it.

Now, the position information stored in "position storage" in step 112 will be used when the next sample is placed.
It is used to immediately control the focus of the objective lens 5. Generally, there is almost no difference in thickness between wafers in the same process and in the same lot. Therefore, when inspecting a large number of such wafers, extremely high-speed focus control becomes possible by moving the objective lens 5 based on the stored position information. At this time, if the movement of the objective lens 5 is coarsely adjusted based on the position information and then finely adjusted as described above, accurate focal position control can be achieved even for slight curvature or warpage of the wafer. In addition, by using this position information, it is possible to implement an automatic return function, which allows the camera to quickly return to its original state even if the focus shifts significantly due to some circumstances.

Furthermore, it goes without saying that the sample 6 may be moved up and down instead of the objective lens 5. Further, even if the objective lens 5 includes a focus adjustment mechanism (focus ring, etc.), if this mechanism is driven by the drive device 25 in the same manner as in the above embodiment, the same effect can be obtained.

Further, in the embodiment, the light beam from the objective lens 5 is divided into two parts and enters the lenses 9 and 10, respectively. However, a beam splitter or the like is provided after the lens 10 to split the light beam into two, and the light beam that passes through one of the optical paths enters the area 1 of the CCD 15.
5a, and the light beam passing through the other optical path passes through a newly provided lens to areas 15b and 15.
It may be configured to reach c. Of course, also in this case, the imaging magnification of the lens 10 alone is set to be larger than the imaging magnification determined by the combination of the lens 10 and the newly provided lens.

Of course, the two areas 15b and 1 of the CCD 15
It goes without saying that a lens with a short focal length may be provided independently for each of the lenses 5c.

As described above, in the embodiment of the present invention, the image formed on the CCD 15 is a spot of laser light, but it does not necessarily have to be laser light, so-called coherent light. Further, when inspecting a wafer in the manufacturing process as sample 6, the aluminum vapor-deposited surface has extremely high reflectance, and the amount of accumulated charge in the CCD 15 may become saturated. Therefore, depending on the surface condition of the wafer (photoresist, silicon oxide, polysilicon, or aluminum), the CCD drive circuit 16 may be controlled to make the read cycle of the CCD 15 variable or to change the storage time.

In addition, in the optimization by the divider 19, A/
When the input voltage of the D converter 20 exceeds the maximum value Ds,
The digital value to be converted is limited to the largest digital value determined by the number of bits. Therefore, when the microprocessor 22 determines the denominator of the divider 19 based on the photometric information in the memory 21, if there is one maximum digital value in the memory 21, the denominator is updated to a larger value. Then, A/D
Digital saturation of the converter 20 may be prevented. In this way, the photometric information read into the memory 21 is free from saturation and has a good S/N ratio, improving the accuracy and reproducibility of focus control.

As explained above, in the present invention, reflected light from an object to be focused is separated into two beams, one of which is guided to a photoelectric detector for detecting front focus and rear focus, and the other is guided to a photoelectric detector for detecting alignment. At the same time, the imaging magnification for the photoelectric detector for focus detection was made larger than the imaging magnification for other photoelectric detectors. This has the effect of widening the control range of the focus position and realizing extremely high-precision automatic focus control.

[Brief explanation of drawings]

FIG. 1 is a perspective view of a line width measuring device incorporating a focus control device according to an embodiment of the present invention, FIG. 2 is a layout diagram showing a focus detection optical system, and FIG. 3 is a diagram of a control circuit for performing focus control. The block diagram, Figure 4, is a flowchart diagram explaining the focus control operation, Figure 5.
The figure shows CCD15 changing with focus control operation.
FIG. 3 is a diagram for explaining the light intensity distribution of FIG. [Explanation of symbols of main parts], 1... Laser light source,
5...Objective lens, 9...Short focal length lens,
10... Long focal length lens, 15... One-dimensional charge-coupled device (photo array).

Claims (1)

[Scope of Claims] 1. An objective lens into which light from an object enters; a first photoelectric detector that detects an image formation state at a predetermined focal plane of the objective lens; 2 front and rear sensors that sandwich the predetermined focal plane second and third photoelectric detectors each detecting an imaging state at two positions; first, second, and third photoelectric detectors that guide light from the objective lens to the first, second, and third photodetectors, respectively; a control circuit for controlling an image formed by the objective lens to be formed on a predetermined focal plane based on output signals of the three photoelectric detectors; A focus control device characterized in that the imaging magnification of is set to be larger than the imaging magnification of other imaging means. 2. A patent characterized in that the second and third imaging means are combined by one lens means 9 so as to guide the light from the objective lens to both the second and third photoelectric detectors. A focus control device according to claim 1. 3. In the device according to claim 1,
The three photoelectric detectors are composed of one photo array 15 in which a plurality of light receiving elements are arranged in one dimension, and the light receiving surface on the photo array is divided into three regions in the element arrangement direction, and each region is divided into three A focus control device characterized in that it is used as a first, second, and third photoelectric detector. 4. In the device according to claim 1,
The control circuit roughly adjusts the distance between the objective lens and the object so that the output signals of the second and third photoelectric detectors are approximately equal, and then adjusts the output signal of the first photoelectric detector to a maximum value. A focus control device comprising a microprocessor 22 that finely adjusts the interval so that