JPS5919913A - Focus controller - Google Patents

Abstract

PURPOSE:To widen the control range of a focus position and to improve precision by splitting reflected light into two pieces of luminous flux, guiding one to a photoelectric detector for front and rear out-of-focus detection and the other to a photoelectric detector for focusing detection, and increasing the image formation magnifiction of the focusing detection. CONSTITUTION:Luminous flux from a half-mirror 3 is aplit into two pieces of luminous flux; one piece of luminous flux is incident to a short-focus lens 9 through a mirror 8, and the other piece of luminous flux is incident to a long- focus lens 10. The luminous flux passed through the lens 9 reaches the center part of a charge coupled device CCD15 through a split surface 13a. Luminous flux reflected by a split surface 13a reaches the leftside part of the CCD15 and luminous flux passed through a lens 10 reaches the right-side part of the CCD15. The photodetection surface of a focusing detection area 15a is arranged at an expected focal plane when the focal point of an objective 5 coincides with a sample surface. Then, the image formation magnification of a photoelectric detector for a focusing surface is increased. Consequently, the control range of the focus position is widened and the precision is improved.

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, and in particular to The present invention relates to a device that condenses light with a target lens, irradiates the target object as a minute spot light, and controls the focal position based on the reflected light.

Conventionally, in cameras, 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 photographing 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 an automatic focus control device is applied to a device that precisely measures pattern line widths on wafers or masks, a device that inspects defects in IC patterns, a reduction projection exposure device, etc.
Even higher precision and a wider working area are required.

Generally, each type of device is equipped with an objective lens or a projection lens with extremely high resolution. And with high resolution lenses,
Due to inherent lens aberrations, the blurred images between the front and rear focal planes become asymmetrical. For this reason, simply controlling the focal position based on photoelectric information from the front and rear focal planes cannot achieve the required accuracy, for example, precisely controlling the focal position to within the focal depth of the objective lens. ,
There were some drawbacks.

Therefore, the 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 d 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. For more information about this device, please refer to the following.
No. 4, "Pattern Line Width Measuring Apparatus" K is disclosed, so a description of line width measurement that is not directly related to the present invention will be omitted.

Referring to 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 reflecting mirror 4 to an objective lens 5 via a half mirror 3. The parallel l/-the light incident on the objective lens 5 is converged into a minute spot light (about 1 μm in diameter),
A sample 6 as an object to be observed or measured is irradiated. Sample 6
The reflected light from the sample surface enters the objective lens 5 in the opposite direction, becomes a parallel beam of light again, is reflected by the reflection mirror 4, and then becomes a parallel beam of light.
The light is reflected by the film mirror 3 and enters a detection optical system that detects the focal shift of the objective lens 5. This detection optical system will be further explained with reference to FIG. The beam from the no-f mirror 3 is divided into two beams by a beam collimator, one beam enters a short focal length lens 9 (first imaging means) via a mirror 8, and the other beam enters a short focal length lens 9 (first imaging means). The light enters a long focal length lens 10 (second and third imaging means). The light beam that has passed through the lens 9 is incident on the beam splitter 13, where it is further split into two light beams. The light flux transmitted through the split surface 13k) of the beam splitter 13 is
Through the optical path length correction glass 14, a one-dimensional photo array, such as a charge-coupled device (hereinafter referred to as a CCD)
The luminous flux reflected by the CCD is reflected by the reflecting surface 13c.
15 (lower side in FIG. 2). Furthermore, the light flux that has passed through the lens 10 passes through the optical path length correction glass 11 and is reflected by the mirror 12, and then is reflected by the reflective surface 13a of the beam splitter 13 and reaches the right side (upper part in FIG. 2) of the CCI 15. . Here, on the light-receiving surface of the CCD 15, the area that receives the light beam that has passed through the lens lO is defined as a focus detection area 15a, and the area that receives the two light beams that have passed through the beam splitter 13 is defined as a ridge-bin detection area 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 position where the spot light entering the objective lens 5 in the opposite direction is imaged when the focal position of the objective lens 5 matches the sample surface, that is, the optimal focusing plane (planned focal point). surface). At this time, the rear focus detection area 15b is determined to be located behind the optimal focus plane along the optical axis direction of the objective lens 5, and the front focus detection area 150 is determined to be located in front of the optimal focus plane. The regions 15b and 15c are defined to have the same optical path length from the region 15a.

Lens 9.10 also has a focal length of approximately 30
0m and 900tm.

Next, a control circuit for controlling focus based on the photoelectric output of the CCD 15 will be explained with reference to FIG. The amount of accumulated charge as photometric information corresponding to the light intensity of each light-receiving element of the CCD 15 is sequentially extracted in time series in response to a timing pulse from the CCD drive circuit 16. 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 is a sample hold circuit 18
The output voltage of the A/D converter 20 is optimized for the next input level range of the A/D converter 20. The output voltage optimized by the divider 19 is converted into a digital value by the A/D converter 20 in response to the timing pulse of the CCD drive circuit 16, and the digital value is transferred to a high-speed I10 transfer (for example, die reflex, tomemory, etc.). access (DMA) method),
The data is sequentially stored in a random access format memo I721. At this time, the memory 21 sequentially stores digital values of accumulated charge amounts assigned addresses corresponding to each light receiving element of the CCD 15.

Now, the digital values stored in the memo+721 are processed by the microprocessor 22 and the high-speed processor 23 dedicated to multiplication and division according to a specific algorithm. At this time, the microprocessor 221ri, C
Find 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 CD 15 is swept out, and calculate the amount of drive *B of the objective lens 5 corresponding to the deviation by 1) / A f converter 2
4FC output. And a driving device rt 2 using a motor etc.
5 moves the objective lens 5 by a predetermined amount in the optical axis direction in response to the output signal of the D/A converter 24.

Incidentally, the divider 19 divides the output voltage of the sample-and-hold circuit 18 in an analog manner using the output data of the microprocessor 22 converted into analog data by the /A converter 26 as a denominator. This JII calculator 19 functions via a microprocessor 22 as a so-called automatic gain control (AGC). That's naive.The microprocessor 22 has a memory section that stores programs to perform predetermined operations, an arithmetic register section that performs various calculations and comparisons, and a port section that communicates with external circuits. etc. are included.

Next, the operation of this apparatus will be explained based on the flowchart shown in FIG. This flowchart diagram is
It shows the operation sequence of the processor 22. A sample 6 whose line width is to be measured is placed under the objective lens 5. At this time, the distance between the objective lens 5 and the sample 6 is set at a predetermined interval in consideration of the thickness of the sample 6. When the focus control operation is started, rccDR in step 100
In the DI J, all photometric information from the CCD 15 is read out for each light receiving element, and sent to the memo 1 via the A/D converter 20.
The data are sequentially stored in J21. Then, the microprocessor 22 selects the CCD from among the photometric information stored in the memory 21.
The photometric information corresponding to the front bin detection area 15c and the rear bin detection area 15b of No. 15 is read out, and step 101 is performed.
Execute the "Area Detection" process. This "area detection" is to detect whether the amount of deviation between the focal position of the objective lens 5 and the sample surface can be immediately determined based on the read photometric information. If the amount of deviation is extremely large, CCD1
As shown in FIG. 5(8), the light intensity distribution on the light-receiving surface of No. 5 becomes substantially flat across each detection area 15a-15b. Note that in FIG. 5, Ds corresponds to the maximum value that can be digitally converted by the A/D converter 20. This "area detection"
is the front focus/back focus detection area 15c.

It is detected whether there is a peak (or dip) of a predetermined size in the light intensity distribution of 15b. If the light intensity distribution is flat as shown in FIG. 5(a), the microprocessor 22 determines that it is outside the area in the next step 102, and moves on to "lens drive A" in step 103. "
The process of ``lens drive A'' is to drive the objective lens 5 by a certain amount in the optical axis direction. Specifically, the microprocessor 22 supplies the D/A converter 24 with a predetermined constant value, for example, 20 as the amount of movement of the objective lens 5.
Outputs the value corresponding to the touch. Drive device 2 depending on this value
5 moves the objective lens 5. Once this movement is complete,
Microprocessor 22 again repeats the above operations starting at step 100.

Now, if it is determined in step 102 that the area is within the area, the microprocessor 22 again executes rccDRD2J in step 104 in the same manner as in step 100. however,
Step 104 (7) rccDRD2J is the divider 19
A process of optimizing the input voltage of the A/D converter 20 by determining a constant that becomes the denominator of is also performed.

Therefore, this optimization will be explained using FIG. 5(b). If the light intensity distribution of the CCD 15 read first is lower than the maximum value as shown in distribution 1 as the output voltage of the divider support 9, each light receiving element stored in the memo 1721 The peak value in the distribution i is found by searching for each photometric value (charge amount). Then, the ratio between this peak value and the maximum value Ds is determined, and a constant corresponding to the ratio is output to the D/A converter 26. From this K, the denominator of the divider 19 is updated to a smaller value, and when the next time the photometric information of the CCD 15 is read at the same focal position, the divider 19
The output voltage has a peak value close to the maximum value D8 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, processing can be performed with a sufficient S/N ratio, and reproducibility is greatly improved. .

Now, in the "focus calculation A" of the Stesoge 105, the CCD
Front pin and rear pin detection area 15c of I5.

The deviation of the focal position of the objective lens 5 is calculated based only on the both-side optical information (hereinafter referred to as front focus information and rear bin information) of 15b. This calculation basically involves determining the difference between the front pin information and the rear bin information and converting it into a deviation amount. In "focus calculation A", the microprocessor 22 reads the front focus information in the memory 21 and calculates the sum of the photometric intensities of each light receiving element in the front bin detection area 15c. This corresponds to finding the area S on the distribution j of the front focus information as shown by the diagonal line K in FIG. 5(c). Next, the peak value P in the previous bin information is calculated, and the sum (sl
). At this time, the calculation-dedicated processor 23
Using P,/s, perform a real number operation and save the result. Next, the microprocessor 22 reads the rear bin information from the memory 21, and similarly as above, the microprocessor 22 reads the rear bin information from the memory 21, and as shown in FIG.
The total sum $ and the peak value P2 indicated by diagonal lines are obtained, and the processor 23 calculates P2/82 with a real number.

Then, the previously calculated calculation result PI /82 and this P2
/S2. This subtraction value corresponds to the amount of deviation of the focus position including the front focus and the back focus, and ideally, if the subtraction value becomes zero, the deviation amount also becomes zero, which means that the object is in focus. The sign of the subtraction value indicates whether the image (or conjugate image) formed by the objective lens 5 is located in front or behind a predetermined focal plane.

In this way, when processing the front pin and rear bin information,
Normalized by the total photometric intensity (divided PI /82, Pt
/82) has the advantage that the accuracy does not deteriorate with respect 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 obtain the corresponding deviation amount. This conversion table is preferably created for each apparatus, including the optical characteristics of the objective lens 5. The microprocessor 22 performs the above processing using the "focus calculation A" of the Stella 7105, and executes the next step 106. In step 106, it is determined whether the calculated deviation amount 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 optimum focal position. If it is determined in step 106 that the value is equal to or greater than the predetermined value, "lens drive B" in step 107 is executed.

Here, the microprocessor 22 determines the amount of movement of the objective lens 5 corresponding to the amount of shift determined in step 105. 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 a ROM. Therefore, the microprocessor 22 can immediately retrieve the corresponding displacement amount simply by searching this lookup 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. When "lens drive B" is completed, the microfist processor 22 again performs the rCCDRD2 in step 104.
Repeat the same operation from J.

Above, proceed to step 404.10 with steps 106 and 107.
The focal position of the objective lens 5 is roughly adjusted. Completion of the rough adjustment is determined in step 106, but the criterion for this determination is only the subtraction value (differential information) obtained from the front focus and rear focus information, and whether it is the minimum limit that can drive the objective lens 5. It is determined. 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 focus planes.

Now, when it is determined in step 106 that the coarse adjustment (for example, within ±1 μm with respect to the in-focus position) has been completed, rccDRD3J in step 108 is executed. rccDR
The D3J reads the photometric information of the CCD 15 into the memo+721 in the same way as the rCCDRD2J. In addition, r CCI) I
For ID3J, optimization may be performed by the divider 19, but since the processing time increases accordingly, it is not necessary to perform optimization. At this time, the light intensity distribution of the photometric information stored in the ML in the memo IJ 21 is as shown in FIG. 5(d). That is, by rough adjustment, the front pin of CCD 15,
The blurred images of the laser spot light on the rear bin detection area 150 and tsb have approximately the same Gaussian distribution shape. Also,
The light intensity of the spot light on the focus detection area 15a has a Gaussian distribution with an extremely sharp peak Pm. Furthermore, area 15
The size of the spot light image on a is sufficiently larger than one light receiving element of the CCD 15. The imaging magnification for the area 15a is the other two areas 15b and 15cK.
Since the imaging magnification is set to be larger than the corresponding imaging magnification, the size of the beak Pm changes extremely sensitively to slight fluctuations in the focal position.

Therefore, in the next step 109 "focus detection XBJ, the microprocessor 22 selects the focus detection area 1 in the memory 21.
Load the photometry information (hereinafter referred to as focus information) of 5a,
The value of the peak Pm in the focus information (shaded area in FIG. 5) is determined. This value of peak Pm is stored in microprocessor 22. In step 110, the determined peak Py
It is determined whether yc 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 movement, in the optical axis direction. and microbook processor 22
- repeats the execution from rCCDRD3J again. If it is determined in step 110 that the peak Pyu 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.

In addition, the above steps 108, 109, 110, 11
1, the focal position IH is finely adjusted. In the above embodiment, this fine adjustment was a so-called servo control-like operation in which the objective lens 5 was moved minute by minute so that the peak Pm of the focusing information was maximized.

However, as another method, the value of the peak Pm is sampled by moving the objective lens 5 by a minute amount, 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, by software processing, the position of the objective lens 5 corresponding to the maximum value of the peak P□ can be determined, and the objective lens 5 can be immediately moved to that position.

Now, the position information stored in "position storage" in step 112 is used to immediately control the focus of the objective lens 5 when the next sample is placed. 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, the objective lens 5 is adjusted based on the stored position information.
If the lens is moved, extremely high-speed focus control becomes possible. At this time, the movement of the objective lens 5 based on the position information is coarsely adjusted,
Next, by making fine adjustments as described above, accurate focal position control can be achieved even for slight curvature or warping of the wafer. However, by using this location information, it is also possible to incorporate a function that allows the camera to quickly return to its original state even if the focus shifts significantly due to some situation, in other words, an automatic return function.

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 has a focus adjustment mechanism (such as a focus ring) incorporated therein, the same effect can be obtained if this mechanism is driven by the drive device 25 in the same manner as in the above embodiment.

Further, in the embodiment, the light beam from the objective lens 5 is divided into two parts, and each part enters the lens 9 and the IO. However, a beam splitter or the like is installed after the lens 1o to split the light beam into two, and the light beam that has passed through one optical path reaches the area 15a of the CCD 15, and the light beam that has passed through the other optical path is sent to a newly installed lens. through area 15b
, 15c. Of course, also in this case, the imaging magnification of the lens IO 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, it goes without saying that a lens with a short independent button focal length may be provided for each of the two areas 15b and 15c of the CCD 15.

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. When inspecting a wafer in the manufacturing process as the 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
The continuous output cycle of the CCD 15 may be made variable or the accumulation time may be changed by controlling.

Still, in the optimization using the divider 19, the k-transformer 2
If the zero input voltage exceeds the maximum value D8, the digital value to be converted will be limited to the largest digital value determined by the number of bits. So the microprocessor 22
When determining 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 and A/
It is sufficient to prevent digital saturation of the D converter 20.

In this way, the photometric information read into the memory 21 is free from saturation and has a good S/N ratio, improving the precision and reproducibility of focus control.

As explained above, in the present invention, the reflected light from the object to be focused is separated into two beams, one of which is guided to the photoelectric detector for detecting the front focus and the rear focus, and the other is guided to the photoelectric detector for detecting the focus. 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 the drawing]

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 control circuit for performing focus control. FIG. 4 is a flowchart for explaining the focus control operation, and FIG. 5 is a diagram for explaining the light intensity distribution of the CCD 15 that changes with the focus control operation. [Explanation of symbols of main parts] ■...Laser light source, 5...Objective lens, 9...Short focal length lens, IO--long focal length lens, 15
...-dimensional charge-coupled device (7 Otoare (a) Applicant: Takashi Watanabe, agent of Nippon Kogaku Kogyo Co., Ltd.: A71 Sen/172 Senko 3 prisoners - A74 figure, +-5 figure 15 (115615C)

Claims (1)

[Scope of Claims] +11 An objective lens into which light from an object is incident; a first photoelectric detector that detects an image formation state at a predetermined focal plane of the objective lens; second and third photoelectric detectors each detecting an imaging state at two positions;
first, second and third imaging means for guiding each of the photoelectric detectors;
a control circuit that controls the image formed by the objective lens to be formed on a predetermined focal plane based on each output signal of the three photoelectric detectors, and controls the imaging magnification of the first imaging means to another value. A focus control device characterized in that the imaging magnification is set to be larger than the imaging magnification of the imaging means. (2) 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 J 7-photo arrays 05 in which a plurality of light receiving elements are arranged in one dimension, and the light receiving surface on the photo array is A focus control device characterized in that it is divided into three regions in the element arrangement direction, and each region is used as the eleventh, second and third photoelectric detectors, respectively. (4) In the apparatus according to claim 1, the control circuit roughly adjusts the distance between the objective lens and the object so that the power signals of the second and third photoelectric detectors are approximately equal. A focus control device comprising: a microprocessor (support) for finely adjusting the interval so that the output signal of the first photoelectric detector is maximized.