JP2003186181A - Mask board placing table and mask board discriminating device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a mask board placing table capable of exactly detecting the size of a mask board of a prescribed kind and suitable for judging that the mask board is placed while being inclined or that a mask board other than the prescribed kind is placed. <P>SOLUTION: Supports 36a, 36b, 36c and 36d each in the form of a quadrangular prism are erected at four corners on a base 35 and a plurality of steps are provided in the upper part of each of the supports. The step sides of the supports 36a and 36b face the step sides of the supports 36c and 36d respectively, and the upper surfaces of the first, second and third stages from the top of each of the supports respectively form a placing surface for various kinds of dedicated mask boards. Between the supports 36a and 36c and between the supports 36b and 36d, a light emitting body 37 with a large number of light emitting elements arrayed in the form of a line vertically to the base 35 and a light receiving body 38 with a large number of similarly arrayed light receiving elements are provided. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mask plate mounting table capable of detecting the thickness of a mounted mask plate and a mask plate discriminating apparatus provided with such a mask plate mounting table.

[0002]

2. Description of the Related Art In the manufacture of semiconductors, a pattern is drawn by an energy beam such as an electron beam (referred to as mask drawing) on a mask plate obtained by coating a resist on a substrate such as glass, and then etching or the like is performed. Then, a mask original plate is produced, the mask original plate is set in a transfer device, and an IC pattern is produced on a semiconductor substrate such as a silicon wafer by an energy beam such as light.

FIG. 1 shows a schematic example of an electron beam drawing apparatus for drawing a mask.

In the figure, reference numeral 1 denotes a mask plate storage in which a large number of mask plates are stored, which can be moved up and down by an elevating mechanism 2.

Reference numeral 3 denotes a mask plate transport mechanism for mounting the mask plate stored in the mask plate storage 1 on the arm 4 and setting it on the mask plate mounting table 5. The mask plate transport mechanism includes an arm drive mechanism 6 that drives the movement of the arm 4 and a lifting / rotating mechanism 7 that drives the lifting / rotation of the arm 4.

The mask plate mounting table 5 detects the thickness of the mask plate and the like.

Reference numeral 8 denotes a mask plate transfer mechanism for mounting the mask plate mounted on the mask plate mounting table on the arm 9 and setting it on the alignment table 11 installed in the preliminary chamber 10. The mask plate transport mechanism includes an arm drive mechanism 12 that drives the movement of the arm 9 and a lifting / rotating mechanism 13 that drives the lifting / rotating of the arm 9.

The alignment table 11 is for aligning the mask plate, and an alignment mechanism 14 is provided for that purpose.

The mask plate aligned by the alignment table 11 is transferred to the drawing chamber by a mask plate transfer mechanism (not shown) having the same structure as the mask plate transfer mechanisms 3 and 8 provided in the preliminary chamber 10. It is set on the stage 16 provided in 15.

An electron optical system lens barrel 23 having an electron gun, a focusing lens, a deflector and the like is provided on the stage 16 and an electron beam generated from the electron gun is set on the stage. It is possible to focus on the mask plate and scan an arbitrary region on the mask plate with the focused electron beam based on the pattern data to be drawn.

In the figure, 17 and 18 are gate valves, and 19 and 2.
Reference numeral 0 is an exhaust valve, and 21 and 22 are vacuum pumps.

In such an apparatus, the mask plate of the mask plate storage 1 is placed on the arm 4 of the arm driving mechanism 6 and set on the mask plate placing table 5. The mask plate mounting table 5 detects the thickness of the set mask plate.

The mask plate is an arm 9 of an arm drive mechanism 8.
Placed on the alignment table 11 in the auxiliary chamber 10 in which the gate valve 17 is opened and the gate valve 17 is opened. At this time, the gate valve 18 is closed.

The arm 9 of the arm drive mechanism 8 is replaced by the spare chamber 10.
After returning to the original position outside the spare chamber, the gate valve 17 is closed, and the preliminary exhaust chamber is evacuated to a high vacuum by the vacuum pump 21. Then, the mask plate set on the alignment table 11 is aligned by the alignment mechanism 14.

After that, the gate valve 18 is opened, and the aligned mask plate is placed on the stage 16 in the drawing chamber 15 by an arm (not shown) of an arm driving mechanism (not shown) provided in the preliminary chamber. Can be posted.

Then, the gate valve 18 is closed, and a pattern is drawn at a predetermined position on the mask plate placed on the stage by electron beam irradiation.

The mask plate mounting table 5 detects the thickness of the mask plate, and the size of the mask plate is calculated by a computer (not shown) based on this thickness. The size of the mask plate placed on the screen is displayed on a monitor (not shown).
The detection of the thickness of the mask plate on the mask plate mounting table 5 is an important process for the next alignment process.

That is, when the mask plate is placed on the alignment table 11, adjustment for supporting the mask plate is performed based on the size of the mask plate. Therefore, it is necessary to know the size of the mask plate actually sent before placing the mask plate on the alignment table 11.

Further, when the mask plate is placed on the mask plate mounting table 5 in a tilted manner or when a mask plate having a size which does not meet the specifications of the apparatus is placed, the alignment operation is hindered. Before the mask plate is placed on the alignment table 11, it is necessary to know the thickness of the mask plate actually sent.

Such a mask plate mounting table has the following structure.

FIG. 2 shows an outline of the mask plate mounting table, which is viewed obliquely. FIG. 3 shows an outline of a mask plate discriminating apparatus provided with such a mask plate mounting table.
Regarding the mask plate mounting table, the AA cross section of the mask plate mounting table shown in FIG. 2 is shown. In the figure, reference numeral 25 is a base, and square pillar-shaped support columns 26a, 26b, 26 are provided on the four corners above this
c and 26d are set up.

These support columns 26a, 26b, 26c,
The upper portion of 26d has a structure having one step, and the step surfaces of 26a and 26b and the step surfaces of 26c and 26d face each other. Further, these support columns are movable along a line connecting the facing support columns. In addition, the upper surface of the lower stage of each support column forms a support surface for the mask plate.

The base 2 is provided between the supporting columns 26a and 26c.
5 is provided with a light emitting body 27 in which a large number of light emitting elements are arranged in a line in a direction perpendicular to each other, and between the supporting columns 26b and 26d, a large number of light receiving elements are arranged in a direction perpendicular to the base 25. A light receiving body 28 arranged in a line is provided so as to face the element.

In the mask plate mounting table having such a structure,
The arm 4 of the mask plate transfer mechanism 3 on which the mask plate is placed is moved by the arm drive mechanism 6 to the support columns 26a, 26b, 26.
Move directly above the center of c and 26d. Next, the mask plate M ₀ is placed on the supporting surface (upper surface of the lower stage) of each supporting column by moving downward by the elevating and rotating mechanism 7. After this, the operator visually checks the gap between each edge of the mask plate M ₀ and the step surface (backrest surface) of the support surface of each of the support columns 26a, 26b, 26c, 26d so that the gap is almost eliminated. In addition, each support pillar 26a, 26b, 26c, 26
By moving d, the instability of the support of the mask plate M ₀ is eliminated.

In such a stable supporting state, as shown in FIG.
As shown in FIG. 5, a part of the light from each light emitting element of the light emitting body 27, which is detected by each light receiving element of the light receiving body 28, is blocked by the mask plate M ₀ between the light receiving body 28 and the light emitting body 27. To be done. Due to this interruption, the length (for example, L _b ) corresponding to the number of light receiving elements that have stopped receiving light corresponds to the thickness of the mask plate M ₀ .

The signal from the photoreceptor 28 (the signal corresponding to the length L _b ) is sent to the computer 29.

The computer calculates a signal corresponding to the thickness (eg, t _b ) of the mask plate M ₀ from the signal from the light receiving body 28 (the signal corresponding to the length L _b of the non-light receiving light receiving element). In the computer, data corresponding to the thickness of the mask plate and data corresponding to the size of the mask plate corresponding to the data corresponding to the thickness are tabulated (eg, FIG. 5). Causes the monitor 30 to display the size (for example, Sb × Sb) of the mask plate M ₀ actually mounted on the mask plate mounting table 5 with reference to the data in this table.

At this time, the alignment table 11 is adjusted for supporting the mask plate based on the size of the mask plate.

The mask plate is, as described above,
It is placed on the alignment table 11 and aligned there.

[0030]

However, there are the following problems.

For some reason, the mask plate M_oIs a mask plate
For example, tilt it on the mounting table as shown in FIG.
The length corresponding to the number of light receiving elements that do not receive light,
L_b′ And the actual thickness of the mask plate (eg t_b)
Length L corresponding to_FromDoes not mean In this case,
The pewter 29 uses tabled thickness data for length
L _bSince there is no data corresponding to ', the monitor 30
The sent mask plate is displayed to indicate that it is abnormal.
The operator can use this display to make this mask plate a mask.
Alignment table 1 by arm 9 of plate drive mechanism 8
1 so that the mask plate drive mechanism 8 is temporarily
Stop the operation.

However, the thickness calculated by the computer,
That is, when the thickness corresponding to the length L _{b 'corresponding} to the number of non-light-receiving light-receiving element, which matches the one of the thickness data being tabulated (t _a or t _c) to the computer 29,
A size different from the actual size of the mask plate (Sb × Sb) is recognized, and the alignment table 11 makes adjustments for supporting the mask plate based on the recognized size of the mask plate. Therefore, the alignment operation on the alignment table 11 is hindered.

Further, even if the mask plate placed on the mask plate mounting table 5 has a thickness that matches one of the thickness data tabulated in the computer 29, the actual size is the specification of the device. If the size is not suitable for
Similarly, the alignment operation on the alignment table 11 is hindered.

The present invention has been made to solve such a problem, and an object thereof is to provide a novel mask plate support and a mask plate discriminating apparatus.

[0035]

Means for Solving the Problems A mask plate supporting base according to the present invention has at least a part of a peripheral portion of a mask plate, each mounting surface of which has a different size, on a base of a mask plate mounting base. A step body having a plurality of inward steps is provided between the mounting surfaces so that the mask plate can be mounted by supporting the mask plate, and between the uppermost mounting surface and the lowermost mounting surface. An optical sensor capable of detecting the thickness of the mask plate is provided. A mask plate discriminating apparatus according to the present invention mounts a mask plate on a base of a mask plate mounting table by supporting at least a part of a peripheral portion of a mask plate having different mounting surfaces. A step body with multiple inward steps is provided between each mounting surface so that the thickness of the mask plate can be detected between the uppermost mounting surface and the lowermost mounting surface. An optical sensor is provided, and the optical sensor is a mask plate mounting table including a light-emitting body that emits linear light in a direction perpendicular to the mounting surface and a light-receiving body that can receive linear light. Each of the mounting surfaces is a dedicated mounting surface for each of various mask plates whose size is determined with respect to the thickness, and when a mask plate of a predetermined type is placed on each mounting surface in a predetermined state, the mask plate The data corresponding to the light receiving area of the photoreceptor that senses the thickness of the When the thickness of the placed mask plate corresponds to the light receiving area of the photoreceptor but the stored data corresponding to the light receiving area of the photoreceptor does not exist, the actually placed mask board is a problematic mask board. And a control device configured to determine that

[0036]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 7 shows an outline of the mask plate mounting table shown as an example of the present invention, and is a view seen obliquely.
FIG. 8 shows an outline of a mask plate discriminating apparatus provided with such a mask plate mounting table.
8 is a sectional view taken along line BB of the mask plate mounting table shown in FIG. 7.
In the figure, reference numeral 35 denotes a base, and square pillar-shaped support columns 36a, 36b, 36c, 36d are erected at four corners on the base.

These support columns 36a, 36b, 36c,
The upper part of 36d has a plurality of steps (three steps in this example).
The structure has a difference), and each of 36a and 36b
The step surface side and the step surface sides of 36c and 36d are respectively
Facing each other. In addition, the upper surface of each step of each support pillar, that is,
The top surface (36a₁, 36b₁，
36c₁, 36d₁) Upper surface of second stage (36a_Two, 36
b_Two, 36c_Two, 36d _Two) Upper surface of the third stage (36
a_Three, 36b_Three, 36c_Three, 36d_Three) Are various
Dedicated mask plate (predetermined thickness and size according to the equipment specifications
The mask plate is a set) and forms the support surface.

The base 3 is provided between the supporting columns 36a and 36c.
5, a light emitting body 37 in which a large number of light emitting elements are arranged in a line is provided, and a large number of light receiving elements in a direction perpendicular to the base 35 are provided between the support columns 36b and 36d. A light receiving body 38 arranged in a line is provided so as to face the element.

The light-emitting body 37 and the light-receiving body 38 are the base 35.
May be attached to the base 35 or may be provided separately from the base 35.

The signal from the light receiver 38 is sent to the computer 39. The computer recognizes the size of the mask plate mounted on the mask plate mounting base on the basis of the signal from the light receiving body 38, or the mounted mask plate is an abnormal mask plate (a mask plate that is mounted at an inclination or an apparatus). It is determined whether the mask plate has a size that does not meet the specifications of (1), and the recognition value or the determination content is displayed on the monitor 40.

The recognition of the size of the mask plate or the determination as to whether the mask plate is abnormal or not is made based on the reference of the table stored in advance in the computer.

FIG. 9 shows an example of the tabulated contents, in which three types of non-light receiving light receiving element ranges are associated with the sizes of the mask plate corresponding to the respective ranges. That is, the mask plate Ma having a thickness of Ta and a size of Sa × Sa is mounted on a dedicated mounting surface (the mounting surfaces 36a ₁ , 36b ₁ , 3 of the first step from the top).
6c ₁ , 36d ₁ ) and the Hm of the light receiving element
_{1 to} Hm ₂ does not receive light, and has thickness Tb and size Sb × Sb
Of the mask plate Mb is a dedicated mounting surface (the second mounting surface 3 from the top).
6a _{2, when 36b 2,} 36c 2, mounted on the 36d _{2), Hn} 1 ~Hn ₂ of the light receiving element is not received, the thickness T
c magnitude Sc × Sc mask plate Mc private placement surface (mounting from the top of the 3-stage surface _{36a 3, 36b 3, 36c 3} , 3
6d ₃ ), the light receiving elements Ho ₁ to Ho ₂
It is confirmed in advance by experiments that the light will not be received by, and a table is created as shown in FIG. 9 based on these data and stored in the computer 39. The non-light receiving element range (length) of Hm _{1 to} Hm ₂ is Hn ₁ to Hn _{2 in the} thickness Ta.
The non-light receiving element range (length) of the thickness Tb is Ho ₁ to Ho
_{The two} non-light-receiving element ranges (lengths) correspond to the thickness Tc, respectively.

The light emitting body 37 and the light receiving body 38 are the base 35.
It may be attached to or may be provided independently.

When the mask plate mounting table having such a structure is used in an electron beam drawing apparatus for mask drawing as shown in FIG. 1, the arm 4 of the mask plate transfer mechanism 3 on which the mask plate is placed.
Is the support pillars 36a, 36 by the arm drive mechanism 6.
Move right above the center of b, 36c, 36d.

Next, by moving downward by the lifting rotary mechanism 7, the mask plate M _Q rests either mounting surface of each support post.

There is a mounting place on the mask plate mounting surface of the arm 4 according to the size of the mask plate to be mounted. Normally, when mounting the mask plate from the mask plate storage 1, the size of the mask plate to be mounted is set. The mask plate can be placed on the placement place according to the above. Further, if the arm 4 is lowered as it is, the mask plate is mounted on a dedicated mounting surface according to the size of the mask plate, so that the arm 4 is supported by the arm drive mechanism 6 so that the support columns 36a, 36b, 36c. , 36d can be moved right above the center.

[0048] For example, at this time, the mask plate M _Q that is placed on the arm 4 thickness t _b in size is to only there by S _b × S _b, the arm 4 is lowered, as shown in FIG. 8 , A dedicated mounting surface of Mb (the mounting surface 36 of the second stage from the top).
a ₂ , 36b ₂ , 36c ₂ , 36d ₂ ).

In this state, as shown in FIG.
Each of the light emitting elements 37 detected by each light receiving element of the light receiving element 38
A part of the light from the light emitting element is generated by the light receiver 38 and the light emitter 37.
Mask plate M between_QShut off by. To block this
From the light receiving element Hn₁~ Hn _TwoIs no longer receiving light,
The pewter 39 is masked by the signal from the photoreceptor 38.
Board M_QThickness is T_bIs calculated. At the same time,
Table, the non-light receiving light receiving element is Hn₁~ Hn_TwoAnd
Therefore, the placed mask plate M_QIs the thickness T_bIn size
Is S_b× S_bIt is judged as the mask plate of the
Board size (type M_bHowever, if you display
Alignment table adjustment mechanism (not shown)
The mask plate size data is sent to the imagement mechanism 14.

Therefore, in the alignment table 11, adjustment for supporting the mask plate based on the mask plate is performed. Then, this mask plate is placed on the alignment table 11 and aligned as described above.

Meanwhile, when the arm 4 is lowered, for some reason, the mask plate M _Q that has been placed on the arm mounting table mask plate, only the mounting surface (the second stage from the top of the M _b not stray exactly the placement surface _{36a 2,} 36b _2, 36c 2, 36d _2), for example, as shown in (a) of FIG. 11, one on one side the mounting of the second-stage surface _36b 2, 36d ₂ When the other side is placed on the placing surfaces 36a ₁ and 36c ₁ of the first stage from the top (that is, when it is placed at an angle), the non-light-receiving element is shown in FIG. 11 (b). As shown, Hn ₂ ~ H
Suppose that it is n ₃ .

The computer 39, which has received the signal from the light receiving body 38 (the signal indicating that the non-light receiving light receiving elements are Hn ₂ to Hn ₃ ), refers to the inside of the table, and Hn ₂ to Hn ₃ indicate the non light receiving light receiving elements. Make sure it is not in range.

[0053] As a result, the computer 39 is a mask plate M _Q placed on the mounting table mask plate, it is determined that abnormal mask plate, with displays that an abnormal mask plate monitor 40, a mask plate carrying mechanism 8 Send a pause signal to.

At this time, the non-light receiving light receiving elements Hn _{2 to} Hn
_Even if the thickness of the mask plate calculated from ₃ coincides with either Ta or T _C , the computer 39 is looking at the non-light receiving light receiving element range, that is, the detected non-light receiving light receiving element range is the data of the table. Therefore, when the mask plate is tilted and placed, the mask plate transport mechanism 8 always stops temporarily.

Even if the mask plate is placed on any one of the placement surfaces of the mask plate placement table without tilting, if the combination of the thickness and the size is not the specified one, the abnormal mask plate is placed. Is judged as.

Because the size of the mask plate is the table
Even if there is data inside (for example, Sa × Sa), the thickness
Data must be paired with its size (Ta)
Then, the mask is placed on the mounting surface of the first stage,
Light receiving element range is Hm₁~ Hm _Two  do not become.

Further, even if the thickness of the mask plate is the data in the table (for example, T _C ), if the size is not a data paired with the thickness (Sc × Sc), three levels are set. It cannot be placed on the eye-holding surface, and the non-light receiving element range is Ho ₁
~ It doesn't become Ho ₂ .

The light-emitting body 37 is composed of a large number of light-emitting elements arranged in a line, but it may be composed of one line-shaped light-emitting element.

Further, although the light emitting body 37 and the light receiving body 38 are arranged so as to face each other with the mask plate interposed therebetween, the light emitting body 37 and the light receiving body 38 are placed on the same side with respect to the mask plate, and the light emitting body 37 emits light. The light reflected from the side surface of the mask plate may be received by the light receiver, and the thickness of the mask plate may be detected based on the number of light receiving elements received.

In the above example, the mask plate mounting table used in the electron beam writing apparatus has been described, but the mask plate mounting table used in the ion beam writing apparatus or the laser writing apparatus or a large sample is observed. Alternatively, the present invention can be applied to a mask plate mounting table or the like used in a scanning electron microscope or the like for inspection.

[Brief description of drawings]

FIG. 1 shows one schematic example of an electron beam drawing apparatus for drawing a mask.

FIG. 2 shows an outline of a mask plate mounting table.

3 is a cross-sectional view taken along the line AA ′ of FIG.

FIG. 4 is a diagram used for explanation of detecting the thickness of a mask plate.

FIG. 5 shows an example of contents of a table stored in a computer.

FIG. 6 shows a state in which a mask plate is tilted and mounted on a mask mounting table.

FIG. 7 is a diagram showing an outline of a mask plate mounting table shown as an example of the present invention, as viewed obliquely.

8 is a schematic view of a mask plate discriminating apparatus including a mask plate mounting table as shown in FIG.

FIG. 9 shows an example of contents of a table stored in a computer which is a part of the mask plate discriminating apparatus of the present invention. .

FIG. 10 is a diagram showing a range of a non-light-receiving light receiving element when a dedicated max plate is placed on each dedicated placing surface of the max placing table of the present invention.

FIG. 11 shows a state in which a mask plate is tilted and mounted on the mask mounting table of the present invention.

[Explanation of numbers]

1 ... Max plate storage 2 ... Elevating mechanism 3 ... Drawing chamber 4 ... Arm 5 ... Mask plate placing table 6 ... Arm drive mechanism 7 ... Elevating / rotating mechanism 8 ... Mask plate transport mechanism 9 ... Arm 10 ... Preliminary chamber 11 ... Alignment table 12 ... Arm drive mechanism 13 ... Elevating / rotating mechanism 14 ... Fry noodles and mechanism 15 ... Drawing chamber 16 ... Stage 17, 18 ... Gate valves 19, 20 ... Exhaust valves 21, 22 ... Vacuum pump 23 ... Electron optical system lens barrel 26a , 26b, 26c, 26d ... support posts 27 ... light-emitting body 28 ... photoreceptor 29 ... Konpiyuta 30 ... monitor _{M O, M} Q _... mask plate 36a, 36b, 36c, 36d ... support posts _{36a 1, 36b 1, 36c 1} , 36d 1 _... first placement surface _{36a 2, 36b 2, 36c 2} , 36d 2 ... second placement surface _{36a 3, 36b 3, 36c 3} , 36d 3 ... third mounting face 37 ... Light emitter 38 ... Light receiver 39 ... Computer 40 ... Monitor

Claims (13)

[Claims] 1. A mask plate mounting table is mounted on a base so that each mounting surface supports at least a part of a peripheral portion of a mask plate having a different size so that each mask plate can be mounted. An optical sensor capable of detecting the thickness of the mask plate between the uppermost mounting surface and the lowermost mounting surface is provided by providing a step body having a plurality of inward steps between the mounting surfaces. The mask plate mounting table provided. 2. The mask plate mounting table according to claim 1, wherein the step body is divided into four parts that are separated from each other, and two step surfaces face each other. 3. The mask plate according to claim 1, wherein the optical sensor includes a light-emitting body that emits light in a line in a direction perpendicular to the mounting surface and a light-receiver that can receive the light in a line. Mounting table. 4. The mask plate mounting table according to claim 3, wherein the light emitter and the light receiver are arranged so as to face each other with the mask plate interposed therebetween. 5. The mask plate mounting table according to claim 3, wherein the light emitter and the light receiver are arranged so that the light emitted from the light emitter and reflected by the side surface of the mask plate can be received by the light receiver. 6. The mask plate mounting table according to claim 3, wherein the light emitter and the light receiver are arranged so as to face each other along the direction in which the step surfaces face each other. 7. The mask plate mounting table according to claim 3, wherein the light emitter and the light receiver are attached to a base. 8. A mask plate mounting base is mounted on a base so that each mounting surface supports at least a part of a peripheral portion of a mask plate having a different size so that each mask plate can be mounted. An optical sensor capable of detecting the thickness of the mask plate between the uppermost mounting surface and the lowermost mounting surface is provided by providing a step body having a plurality of inward steps between the mounting surfaces. The optical sensor is provided with a mask plate mounting table including a light-emitting body that emits linear light in a direction perpendicular to the mounting surface and a light-receiving body that can receive linear light, and each of the mounting surfaces. Is a dedicated mounting surface for each of the various mask plates whose size is determined with respect to the thickness, and the thickness of the mask plate is sensed when a predetermined type of mask plate is placed on each mounting surface in a predetermined state. The data corresponding to the light receiving area of the photoreceptor to be stored is stored, and the mask plate of the actually placed mask plate is stored. When the thickness corresponds to the light receiving area of the photoreceptor but the stored data corresponding to the light receiving area of the photoreceptor does not exist, the mask plate actually placed is determined to be a problem mask plate. A mask plate discriminating device including a control device. 9. A table is preliminarily stored as a table for specifying the type of mask plate in association with the data corresponding to the light receiving area of a photodetector that senses the thickness of various mask plates. 9. The mask plate discriminating apparatus according to claim 8, wherein the type of the corresponding mask plate is specified by collating the data corresponding to the light receiving area of the photodetector, which has sensed the thickness of the mask plate, with the data in the table. 10. The mask plate discriminating apparatus according to claim 9, wherein the mask plate is specified by the size of the mask plate. 11. The mask plate discriminating apparatus according to claim 8, wherein the light emitter and the light receiver are attached to a base. 12. The mask plate discriminating apparatus according to claim 8, wherein the light emitter and the light receiver are arranged so as to face each other with a mask plate interposed therebetween. 13. The mask plate discriminating apparatus according to claim 8, wherein the light emitter and the light receiver are arranged so that the light emitted from the light emitter and reflected by the side surface of the mask plate can be received by the light receiver.