JPH08271220A - Traveling device and method for manufacturing reference scale used to the device - Google Patents

Abstract

PURPOSE: To provide a traveling device for accurately directing straight advance over a wide stroke range, and a method for manufacturing a reference scale for directing straight advance used for the device. CONSTITUTION: A first traveling stand 2 driven in X-axis direction is mounted to a support stand 1, and a second traveling stand 5 is mounted to the first traveling stand 2 via a piezoelectric actuator 6 for giving a small displacement in Y-axis direction. A reference scale 10 where a pair of reflection patterns 13a and 13b symmetrically including a drawing error for a virtual reference line 12 are formed is provided at the support stand 1. An interference fringe projector 22 for reading the pattern of the reference scale 10 and photo detectors 23a and 23b are provided at the tip of arms 21a and 21b mounted to the traveling stand 6 and an interference length-measuring circuit 31 and y-yaw circuit 32 for performing the feedback control of the piezoelectric actuator 6 according to the differential output between the detectors 23a and 23b are provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a moving device which is applied to an electron beam drawing apparatus, a reduction projection exposure apparatus, a linear scale and the like and has excellent straightness, and more particularly, to a deviation of straightness in a moving direction of a moving table. The present invention relates to a moving device having a reference straightness indicating reference scale that guarantees the straightness of movement by detection compensation, or corrects the movement perpendicular to the moving direction of a moving base in the case of two-dimensional movement, and a method of manufacturing the reference scale.

[0002]

2. Description of the Related Art Conventionally, as a moving device that enables parallel movement of a moving table with an accuracy of 0.1 μm to 0.01 μm,
The optical drawing device shown in FIG. 7 is known. A first moving table 602 is mounted on a support table 601 by a cross roller guide 603, and can be moved in the X axis direction by a ball screw 604. A second moving table 606 is further mounted on the first moving table 602 via a piezoelectric actuator 605 for giving a minute displacement in the Y-axis direction. A drawing arm 608 having an optical drawing head 607 at its tip is fixed to the second moving table 606, and the optical drawing head 607 draws on a substrate 609 placed on the support table 601.

In order to enable the second movable table 606 to move precisely in the X-axis direction, an optical straight-movement indicating mechanism is provided for the purpose of measuring the straightness. The reference reflecting mirror 610 fixed to the second movable table 606 serves as a reference for directing the vehicle straight. In order to configure two laser interferometers with this reference reflecting mirror 610, a He-Ne laser 611, a beam splitter 612 that splits the output laser light beam into two light beams, a prism mirror 613, and a reflected light from the reference reflecting mirror 610 are separated. Beam splitter 614,615
Etc. optical systems are arranged. The two beam splitters 614, 615 are separated by a yaw measurement span.

The reflected light of two systems from the reference reflecting mirror 610 is detected by the photodetectors 616 and 617, and the detection output thereof enters the interference measuring circuit 618. The interference measurement circuit 618 measures an error from the design distance between the beam splitter 614 and the reference reflecting mirror 610, and also an error from the design distance between the beam splitter 615 and the reference reflecting mirror 610. These errors are amplified by the y-yaw correction circuit 619 and fed back to the piezoelectric actuator 605. Accordingly, the distance between the reference reflecting mirror 610 and the beam splitters 614, 615 is controlled so as to always maintain a constant value, and the straightness of the movement of the second moving table 606 is within the accuracy of the flatness of the reference reflecting mirror 610. Guaranteed by.

[0005]

However, in the conventional apparatus shown in FIG. 7, first, the stroke of the movable table is set to the length of the reference reflecting mirror 610 in the X direction minus the span for yaw measurement. However, there is a problem that it is difficult to realize a sufficient stroke while securing the flatness of the reference reflecting mirror 610. Secondly, there is a drawback that the price of the laser interferometer forming the optical linear indicator is extremely expensive as compared with the moving table. Among them, the He-Ne laser 611 is expensive.

For example, a recent manufacturing apparatus for a liquid crystal display device is required to have an accuracy of straight line error of 0.1 μm or less with a stroke of 1 m or more, and it is difficult for the prior art to meet this requirement. It is desired to realize such a linear indicator of a manufacturing apparatus for a liquid crystal display device so as to be used at a size and a price of a linear scale or in a state of being incorporated in the linear scale.

The present invention has been made in view of the above points, and provides a moving device capable of highly accurate linear instruction over a large stroke range, and a method of manufacturing a reference scale for linearity instruction used in the moving device. It is intended to be provided.

[0008]

A moving device according to the present invention includes a support base, a first movable base attached to the support base and driven to move straight in a first axial direction within a plane of the support base. A second moving table attached to the first moving table via an actuator for giving a minute displacement in a second axial direction orthogonal to the first axis of the supporting table surface; and a predetermined axial direction on the supporting table. A reference scale formed with a light reflection pattern serving as a reference for instructing the straightness of the movement of the first movable table in the first axis direction over the range, and the reference scale attached to the second movable table, respectively. Light irradiation means for irradiating a light beam for reading the light reflection pattern and light detection means for detecting the reflected light from the reference scale, and the output of the light detection means is processed to eliminate the straight line error of the first moving table. Before to compensate It is characterized by having a feedback control means for driving the actuator.

In the present invention, preferably, the reference scale is formed in a direction of the second axis, which is formed symmetrically with respect to an imaginary reference line parallel to the first axis and includes a symmetrical drawing error. A pair of light reflection patterns in which the reflection portions are repeatedly arranged at a cycle is provided, and the light irradiation means has a cycle equal to the cycle of the light reflection patterns in a range covering the pair of light reflection patterns with respect to the reference scale. Interference fringe projector for producing interference fringes, the photodetection means is constituted by a pair of photodetectors for detecting reflected light from the pair of light reflection patterns, respectively, and the feedback control means is provided for the pair of photodetectors. It is composed of a differential circuit that takes a difference between the outputs of the photodetectors and a drive circuit that drives the actuator so that the output of the differential circuit becomes constant. It is.

The present invention is also a method of manufacturing a straightness indicating reference scale used in the above-described moving device, comprising a second transparent substrate which is long in the first axis direction and orthogonal to the first axis.
A second light-shielding substrate is formed by forming a first light-shielding pattern consisting of a plurality of stripes arranged at a predetermined cycle in the axial direction and using the first transparent substrate on which the first light-shielding pattern is formed as a mask for close contact or proximity transfer. A second light-shielding pattern, which is a reverse pattern of the first light-shielding pattern, is formed on the first transparent substrate and the second transparent substrate are arranged so that their light-shielding pattern surfaces are on the same plane. By closely contacting or closely transferring the light-shielding pattern on the reference scale substrate, a pair of light reflection patterns that are symmetrical with respect to the virtual reference line parallel to the first axis and that have a symmetrical drawing error are formed on the reference scale substrate. It is characterized by forming.

[0011]

According to the present invention, the second moving table is formed by using the reference scale having the light reflecting pattern as a reference for indicating the linearity of the movement of the first moving table in the first axial direction on the support table. The light reflection pattern of the reference scale is read by the light irradiation means and the light detection means provided in the above, and the straight line error of the first moving table is compensated by the feedback control. As a result, it is possible to instruct the straightness with high accuracy within the stroke range determined by the reference scale.

Particularly in the present invention, the reference scale is
Assuming that a pair of light reflection patterns in which reflecting portions are repeatedly arranged at a predetermined cycle in the second axis direction are formed and drawn with a drawing error symmetrical with respect to a virtual reference line parallel to the first axis, As an irradiation means, an interference fringe projector that creates an interference fringe having a cycle equal to the cycle of the light reflection pattern in a range covering the pair of light reflection patterns with respect to the reference scale is used, and as a light detection means, from the pair of reflection patterns. By using a pair of photodetectors that detect reflected light respectively, and by performing feedback control of the moving table so that the difference between the outputs of the pair of photodetectors is constant, it is affected by drawing errors on the reference scale. Without, it is possible to guarantee the straightness over a long range.

The pair of light reflection patterns having a drawing error symmetrical with respect to the virtual reference line as described above is mounted on a support table and is attached to a movable table which moves in the first axis direction of the support table surface. Is used to form a first light-shielding pattern composed of a plurality of stripes arranged in a first axis direction on a first transparent substrate in a second axis direction that is long in the first axis direction and orthogonal to the first axis at a predetermined period. A second transparent substrate, which is an inversion pattern of the first light-shielding pattern, is brought into close contact or proximity transfer using the first transparent substrate on which the light-shielding pattern is formed as a mask.
A light-shielding pattern is formed, and the first transparent substrate and the second transparent substrate are arranged so that the light-shielding patterns are on the same plane, and these patterns are used as a mask to closely or closely transfer to the reference scale substrate. To be

In such a method, starting from a pattern obtained by using a drawing device of ordinary accuracy or a drawing device of a large area, a pair having drawing errors symmetrical with respect to the virtual reference line is obtained. The light reflection pattern can be obtained, and by using the reference scale having such a light reflection pattern, it becomes possible to cancel the influence of the drawing error on the straight movement control of the carriage.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. 1A and 1B are a plan view and a front view showing the configuration of a moving device according to an embodiment of the present invention. A first moving table 2 is mounted on a fixed supporting table 1 by a cross roller guide 3. The first moving table 2 is driven by the ball screw 3 in the in-plane first axial direction (X-axis direction) on the support table 1. The error in linearity of the first moving table 2 in the X-axis direction and yaw are determined by the machining accuracy and are about 10 μm.

On the first moving table 2, precision moving tables 5 which are second moving tables for compensating for the linear error are attached at four corners via piezoelectric actuators 6. That is, the precision moving table 5 gives a small displacement in the second axis direction (Y-axis direction) orthogonal to the X-axis in the plane of the support table 1 by driving the piezoelectric actuator 6 to perform displacement correction in the Y-axis direction. The yaw component can be corrected.

A reference scale 10 for indicating straightness which is long in the X-axis direction is attached to a region of the support base 1 adjacent to the movable bases 2 and 5. The reference scale 10 has a virtual reference line 12 parallel to the X-axis indicated by the alternate long and short dash line on the scale substrate 11, and forms a pair of light reflection patterns 13a and 13b in mirror image symmetry with respect to the virtual reference line 12. It was done. Each of the light reflection patterns 13a and 13b is long in the X-axis direction, and stripe-shaped light reflection parts and absorption parts (or transmission parts) are alternately arranged in the Y-axis direction at a constant period p. Although a specific method of manufacturing the reference scale 10 will be described later, the pair of light reflection patterns 13a,
13b is arranged not only symmetrically with respect to the virtual reference line 12, but also with a drawing error symmetrical with respect to the virtual reference line 12.

Light reflection pattern 13 of the reference scale 10
As a device for optically reading a and 13b, two reading arms 21a and 21b are attached on the precision moving table 5 at a predetermined distance in the X-axis direction. Specifically, in this embodiment, the distance between the two arms 21a and 21b is the yaw measurement span. Reading arms 21a, 2
At the tip of 1b, a light irradiation means for irradiating the reference scale 10 with a reading light beam, and the reference scale 1
Light detecting means for detecting the reflected light beam modulated by 0 is provided.

The light irradiating means provided on the reading arms 21a and 21b specifically changes the light quantity in a sinusoidal manner in the Y-axis direction with respect to the reference scale 10 at the same period p as the period of the light reflection patterns 13a and 13b. The interference fringe projector 22 is configured to project the interference fringes. In addition, the light detection means,
It is composed of a pair of light reflection patterns 13a, 13b and a pair of photodetectors 23a, 23b for detecting the amount of reflected light obtained by overlapping the interference fringes projected on these.

The movable table 5, and hence the reading arm 21a,
When 21b is displaced in the Y-axis direction, the pair of photodetectors 23
The outputs of a and 23b are sinusoidal outputs having a period of the displacement amount p. Further, when the pair of light reflection patterns 13a and 13b of the reference scale 10 is composed of the reflection portion and the absorption portion having the width p / 2, and the distance between them is an integral multiple of p, the pair of light The outputs of the detectors 23a and 23b are mutually phase-inverted. Therefore, the pair of detectors 23a, 23
The displacement in the Y-axis direction can be known by taking the difference between the outputs of b. For example, the pitch p is set to the light reflection pattern 13
If the maximum straightness error of a and 13b is doubled, the displacement reading accuracy in the Y-axis direction can be p / 200 or more.

For the two reading arms 21a and 21b, a reading device comprising a similar interference fringe projector and a pair of photodetectors is constructed, and the output of each photodetector pair of these reading devices is a kind of differential circuit. Interferometry circuit 3
The differential output that is input to 1 and detected is input to the y-yaw circuit 32 that controls and drives the piezoelectric actuator 6.
In the y-yaw circuit 32, the displacement of the movable table 5 in the Y-axis direction,
That is, the error from the straightness in the X-axis direction and the yaw component are compensated by the feedback control. That is, the arm 21
Y by the output of either the a or 21b photodetector pair
The directional displacement is detected, the outputs of the two pairs of photodetectors are arithmetically processed, and the yaw component is detected.

2 (a) and 2 (b) show a specific configuration of the optical system of the above-mentioned reading device. Interference fringe projector 22
To the detectors 23a and 23b, the entire optical system is housed in the light shielding container 200. The light source of the interference fringe projector 22 is a semiconductor laser 201, and its output laser light is collimated by a collimator lens 202, reflected by a 45 ° reflection mirror 203, and incident on a diffraction grating 204. The incident light is divided into zero-order and ± first-order diffracted lights by the diffraction grating 204, and is once condensed by the projection lens 205 for each diffracted light component, and only the zero-order component is removed by the zero-order diffracted light trap 206.

Then, the ± 1st-order diffracted light is projected onto the projection lens 207.
The light is condensed by each of the perforated prism reflectors 20.
The interference fringes are projected on the reference scale 10 through the holes of No. 8. At this time, the pitch of the interference fringes is 10
It is set so as to match the pitch p of the light reflection patterns 13a and 13b. Part of the interference fringes is the light reflection pattern 13
a and 13b, as shown by the broken line, and further reflected by the prism 208, and the detection lenses 209a and 209.
b is focused on the photodetectors 23a and 23b, respectively.

The amount of light detected by the pair of photodetectors 23a and 23b changes sinusoidally with a period of pitch p with respect to the movement of the reading device in the diffraction direction, and the phase of the sinusoidal wave is the reflection pattern. It changes according to the amount of drawing error and the sum of the displacement of the reading device with respect to the reflection pattern. The details will be described later.

Next, a method of manufacturing the reference scale 10 will be described with reference to FIGS. FIG. 3 shows a drawing apparatus for manufacturing a reference scale. A moving base 303 is attached to the support base 301 by a cross roller guide 302, and the moving base 303 is driven in the X-axis direction by a ball screw 304. Drawing arm 3 on the moving stand 303
05, and an optical drawing head 306 is attached to the tip of the drawing arm 305. Optical drawing head 3
Reference numeral 06 designates, for example, an interference fringe projector similar to that described in FIG.

First, using the drawing apparatus as described above, the support base 3
On the first transparent substrate 401 placed on 01, a first light shielding pattern 402 is formed which is composed of a plurality of stripes which are long in the X-axis direction and are arranged at a period p in the Y-axis direction orthogonal to the X-axis. Specifically, a chromium film is previously formed on the first transparent substrate 401, and a resist is applied on the chromium film. An interference fringe pattern having a period p in the Y-axis direction is formed on the chromium film by the optical drawing head 306. Projection is performed while moving the optical drawing head 306 in the X-axis direction, and development is performed. As a result, as shown in FIG.
The light shielding pattern 402 is formed.

Next, with respect to the second transparent substrate 403 on which the chrome film is formed and the resist is applied, as shown in FIG.
As shown in (b), the first transparent substrate 401 is turned upside down, and the first transparent substrate 401 is placed in close contact or in close proximity as a mask, exposed and developed. As a result, on the second transparent substrate 403,
A second light shielding pattern 404, which is an inverted pattern of the first light shielding pattern 402, is formed.

The two transparent substrates 401 thus obtained,
As shown in FIG. 4C, 403 is arranged so that the light-shielding patterns 402 and 404 are on the same plane, and is used as a mask so as to be in close contact with or close to the scale substrate 11 on which the chrome film and the resist are formed. It is exposed and developed. By repeating such pattern transfer, a pair of light reflection patterns 13a and 13b including an error amount of the first pattern drawing with respect to the virtual reference line 12 in a mirror image symmetry relationship are formed on the scale substrate 11. be able to.

Next, with reference to FIG. 5, the operation of detecting the displacement in the Y-axis direction and canceling the drawing error by the pattern reading device in the moving device shown in FIG. 1 will be specifically described. FIG. 5 shows a pair of light reflection patterns 13 of the reference scale 10.
a and 13b are enlarged and shown. The shaded area is the reflection area A,
Between them are absorption parts B, and in the figure, four reflection parts A are illustrated on both sides of the virtual reference line 12, but in reality, about 100 reflection parts A are arranged. Further, in the figure, there is shown a case where there is a drawing error such that the reflection patterns 13a and 13b become wider as they go upward in the X-axis direction positions x1, x2 and x3.

The column of rectangles 501 indicated by broken lines for x1 and x3 and the solid line for x2 is an illumination unit that is brightly illuminated by the interference fringe projector 22. Further, regarding the position x2, regions 502a and 502b surrounded by a solid line indicate detection regions by the pair of photodetectors 23a and 23b.

At the position x2, the light reflection pattern 13
The interval between a and 13b is set to an integral multiple (5 × p in the figure) of the period p of the reflecting section A and the absorbing section B as designed, and in both cases half of the light from the irradiation section 501 is reflected. It is detected in the opposite phase. When the irradiation light pattern shifts to the right along the Y-axis, the output of one photodetector 13a decreases and the output of the other photodetector 13b increases. Therefore, the change of the outputs of the photodetectors 13a and 13b at the position x2 with respect to the displacement in the Y direction is as shown in FIG. 6, and the difference output gives the amount of displacement of the projector irradiation unit 501 in the Y axis direction.

At the position x1, the light reflection patterns 13a, 1
3b shows the virtual reference line 12 from the position x2 due to drawing error.
And the interval becomes 5p-Δ. This position x
2, the outputs of the photodetectors 13a and 13b are shifted to the left side and the right side, respectively, as shown in FIG. 6, and the output values are smaller than x2. However, since the drawing error of the light reflection patterns 13a and 13b is symmetrical with respect to the virtual reference line 12, the difference output between them is smaller than the position x2 in the inclination, but the zero point position does not change, and the drawing error occurs. The canceled displacement is obtained. At the position x3, the light reflection pattern 13
The distances a and 13b are farther from the virtual reference line 12 than the position x2, and the distance is 5p + Δ. Also in this case, similarly, by taking the difference output of the photodetectors 13a and 13b, the displacement output in which the drawing error is compensated is obtained as shown in FIG.

As described above, according to the moving device of FIG. 1, when the moving table is moved in the X-axis direction, it is displaced in the Y-axis direction at two points without being affected by the drawing error of the light reflection pattern of the reference scale 10. Can be detected and the Y-axis direction displacement and the yaw component can be compensated. According to this embodiment, the straightness error and the yaw are set to about 10 μm due to the machining accuracy, and even if the light reflection pattern on the reference scale 10 is formed with the accuracy of about 10 μm, the optical pattern reading is performed with the light reflection pattern. This is possible with an accuracy of 1/200 of the pitch p, and since the drawing error of the light reflection pattern is automatically canceled, the straightness can be secured with an accuracy of 0.1 to 0.01 μm.

Also, in the conventional reference reflecting mirror and the laser interferometer using a He-Ne laser or the like, the stroke is limited by the size of the reference reflecting mirror,
In order to realize a large stroke, it is difficult to secure the flatness of the reference reflecting mirror, and the device is also extremely expensive, whereas in this embodiment, a moving device that maintains a highly accurate straightness over a large stroke is used. It can be realized at low cost.

Specifically, the straightness indicator according to this embodiment is effective when it is incorporated in an electron beam drawing apparatus, an exposure apparatus, or a linear scale to control the straightness of the movement of the stage or scale. Furthermore, the present invention can be applied to various steppers that perform two-dimensional movement.

[0036]

As described above, according to the present invention, a light reflection pattern serving as a reference for indicating straightness is formed over a large stroke range by using a reference scale formed so as to cancel a drawing error. It is possible to provide a moving device that enables highly accurate straight ahead instruction, and a method of manufacturing a straightness indicating reference scale used in this moving device.

[Brief description of drawings]

FIG. 1 shows a configuration of a moving device according to an embodiment of the present invention.

FIG. 2 shows the configuration of the optical reading apparatus according to the embodiment.

FIG. 3 shows a drawing apparatus for manufacturing a reference scale of the embodiment.

FIG. 4 shows a manufacturing process of a reference scale of the same example.

FIG. 5 is an explanatory diagram of a reflective pattern reading operation of the embodiment.

FIG. 6 is an explanatory diagram of a reflective pattern reading operation of the embodiment.

FIG. 7 shows a configuration of a conventional moving device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Support stand, 2 ... 1st moving stand, 5 ... Precision moving stand (2nd moving stand), 6 ... Piezoelectric actuator, 10 ... Reference scale for linearity instruction | indication, 11 ... Reference scale board | substrate, 12 ... Virtual reference line, 13a, 13b ... Light reflection patterns, 21a, 21
b ... Reading arm, 22 ... Interference fringe projector, 23a, 2
3b ... Photodetector, 31 ... Interferometric length measuring circuit, 32 ... y-ya
w circuit.

Claims (4)

[Claims] 1. A support base, a first movable base mounted on the support base and driven to move straight in a first axial direction within a plane of the support base, and a first shaft of the support base surface on the first movable base. Second orthogonal to
A second movable base mounted via an actuator for giving a small displacement in the axial direction, and a linearity of movement of the first movable base in the first axial direction over a predetermined range in the first axial direction on the support base. And a reference scale on which a light reflection pattern serving as a reference is formed, and a light irradiating means for irradiating a light beam for reading the light reflection pattern of the reference scale, which is attached to the second moving table, respectively. A light detecting means for detecting the reflected light from the reference scale; and a feedback control means for processing the output of the light detecting means to drive the actuator so as to compensate the straight-line error of the first moving table. Characterized moving device. 2. The reflector is formed with a predetermined cycle in the second axis direction, which is drawn and formed symmetrically with respect to an imaginary reference line parallel to the first axis and including a symmetrical drawing error. Has a pair of light reflection patterns that are repeatedly arranged, the light irradiation means, in the range that covers the pair of light reflection patterns with respect to the reference scale, interference fringes of a cycle equal to the cycle of the light reflection pattern. An interference fringe projector for producing, the photodetector means is constituted by a pair of photodetectors for detecting reflected light from the pair of reflection patterns, and the feedback control means is an output of the pair of photodetectors. 2. The moving device according to claim 1, wherein the moving device comprises a differential circuit that takes a difference between the differential circuit and a driving circuit that drives the actuator so that the output of the differential circuit becomes constant. . 3. The second moving table is supported by a plurality of piezoelectric actuators capable of displacement and yaw component correction in the second direction, and the reference scale is a virtual reference line parallel to the first axis. A pair of light reflections that are formed symmetrically with respect to each other and that include a drawing error and that have a predetermined cycle in the second axis direction and that are repeatedly arranged with a spacing of an integral multiple of the cycle. A pattern, wherein the light irradiating means is arranged in the first axis direction for producing interference fringes having a cycle equal to the cycle of the light reflection pattern in a range covering the pair of light reflection patterns with respect to the reference scale. It is configured by two interference fringe projectors arranged at a predetermined distance, and the light detecting means detects the reflected light from the pair of reflection patterns irradiated by the two interference fringe projectors, respectively. The feedback control means includes a differential circuit that takes the difference between the outputs of the two pairs of photodetectors and the output of the differential circuit. 2. The moving device according to claim 1, further comprising a drive circuit that drives the plurality of actuators so as to be constant and performs a second axial displacement and a yaw component correction. 4. The first transparent substrate is provided with a first elongated substrate in the first axial direction.
A first light-shielding pattern formed of a plurality of stripes arranged at a predetermined cycle in a second axis direction orthogonal to the axis is formed, and the first transparent substrate on which the first light-shielding pattern is formed is used as a mask for close contact or proximity transfer. To form a second light-shielding pattern, which is a reverse pattern of the first light-shielding pattern, on the second transparent substrate, and the light-shielding pattern surfaces of the first transparent substrate and the second transparent substrate are on the same plane. By arranging the light-shielding patterns as masks and closely or closely transferring them to the reference scale substrate, there is a drawing error symmetrical and symmetrical with respect to the virtual reference line parallel to the first axis on the reference scale substrate. A method of manufacturing a reference scale for indicating straightness, which comprises forming a pair of light reflection patterns.