JPH09251208A - Method and device for exposure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To give adequate exposure on a photosensitive substrate even though the luminance of a light source is so high that the exposure can not be adjusted to an adequate value only by the control of the scanning speed of the photosensitive substrate to an illumination optical system. SOLUTION: The combination of adequate control condition is set on plural luminous flux control means 12 based on a stored result S6 which is obtained by correspondingly storing the control condition of a luminous flux control means 12 controlling respective primary luminous fluxes L6 to L8 outgoing from plural light sources 10 and the illuminance of secondary luminous fluxes L1 to LS obtained by condensing and dividing the primary luminous fluxes L6 to L8 by a light transmission path 14 in a storing means 23.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exposure apparatus and an exposure method, and can be applied to, for example, exposing a large-area pattern on a photosensitive substrate for manufacturing a liquid crystal display device.

[0002]

2. Description of the Related Art Conventionally, when a large area pattern is exposed on a photosensitive substrate, partial patterns are repeatedly exposed until a desired area is reached. On the other hand, a photosensitive substrate for manufacturing a liquid crystal display device has recently been required to have a large area. For this reason, it has been desired for the exposure apparatus to expand the exposure area per unit time. In order to expand the exposure area per unit time, a scanning exposure apparatus provided with a plurality of projection optical systems has been proposed.

This scanning type exposure apparatus is provided with a plurality of illumination optical systems, and illuminates different small areas (illumination areas) on a mask with light beams emitted from the respective illumination optical systems. By the way, the illumination optical system of this scanning type exposure apparatus uses a field diaphragm to shape a light beam emitted from a light source and having a uniform amount of light through an optical system including a fly-eye lens, into a desired shape and mask pattern. Illuminate the surface. Subsequently, the scanning exposure apparatus projects the image of the pattern on the illuminated mask onto different projection regions on the photosensitive substrate through each of the plurality of projection optical systems to form an image. In this scanning type exposure apparatus, the mask and the photosensitive substrate are synchronously scanned with respect to the illumination optical system and the projection optical system to transfer the entire pattern area on the mask onto the photosensitive substrate.

[0004]

By the way, there is an appropriate value for the exposure amount of the resist when the pattern on the mask is transferred onto the photosensitive substrate. The exposure amount is represented by the product of the illuminance of the light flux emitted from the light source and the exposure time, that is, the exposure amount = exposure illuminance on the substrate surface × exposure time. The illuminance of the light flux emitted from the light source increases or decreases in proportion to the brightness of the light source. Further, the brightness of the light source is maximum at the beginning of use and decreases according to the use time.
Therefore, when the brightness of the light source is high, the scanning exposure apparatus described above needs to increase the scanning speed of the mask and the substrate in order to shorten the exposure time. On the other hand, when the brightness of the light source decreases, the scanning type exposure apparatus described above needs to reduce the scanning speed so as to obtain a constant exposure amount for the resist on the photosensitive substrate.

However, the brightness of the light source is generally non-uniform due to manufacturing errors. The scanning speed of the mask and the substrate with respect to the illumination optical system and the projection optical system generally has an upper limit due to the characteristics of the control system. That is, there is a limit in shortening the exposure time by controlling only the scanning speed. For this reason, if the brightness of the light source is too large to be handled by the upper limit scanning speed, the difference between the exposure time at the upper limit scanning speed and the exposure time at the scanning speed required for proper exposure becomes the overexposure time. However, there is a problem that the mask pattern cannot be accurately transferred to the photosensitive substrate.

The present invention has been made in consideration of the above points, and even if the brightness of the light source is too large to adjust the exposure amount to an appropriate value only by controlling the scanning speed of the photosensitive substrate with respect to the illumination optical system. , An exposure apparatus and an exposure method capable of giving a proper exposure amount to a photosensitive substrate.

[0007]

In order to solve such a problem, an explanation will be given in association with FIG. 2 showing an embodiment, in the exposure apparatus according to claim 1, from the plurality of ultra high pressure mercury lamps 10, respectively. A pattern is formed by the plurality of light fluxes L _{1 to} L ₅ obtained by collecting the plurality of emitted light fluxes L _{6 to} L ₈ by the light guide 14 and then dividing the light fluxes by the light guide 14. A plurality of different illumination areas M on the mask
1 to M5, respectively, and a plurality of projection optical systems 4A to 4E arranged corresponding to the plurality of illumination areas M1 to M5, and each of the plurality of illumination areas M1 to M5. In the exposure apparatus that projects an image on the photosensitive substrate 5 via each of the plurality of projection optical systems 4A to 4E, the respective light fluxes L ₆ emitted by the plurality of ultra-high pressure mercury lamps 10
Based on the illuminance signals S1 to S5, the aperture ratios of the plurality of shutters 12 that control the L to L ₈ with the aperture ratio, the photodetector 21 that detects the illuminance of the light fluxes L _{1 to} L ₅ And a memory 23 that associates the illuminances of the plurality of light fluxes L _{1 to} L ₅ and stores them as illuminance data S6,
A control unit 22 for controlling the shutter 12 is provided based on the illuminance data S6 stored in the memory 23, and the luminous fluxes L _{1 to} L
_When the illuminance of ₅ exceeds a predetermined value, the luminous fluxes L _{6 to} L ₈ emitted from the extra-high pressure mercury lamp 10 which emits arbitrary luminous flux components of the luminous fluxes L _{1 to} L ₅ are regulated by the aperture ratio, L ₁ ~
The illuminance of L ₅ is regulated below a predetermined value.

In the exposure apparatus according to the second aspect, the light guide 14 is formed by bundling a plurality of optical fibers. In the exposure apparatus according to claim 3, the shutter 12 has a light flux L.
The aperture ratios of the apertures that allow _{6 to} L ₈ to pass through are controlled to regulate the luminous fluxes L _{6 to} L ₈ . In the exposure apparatus according to claim 4, parts 4A, 4C and 4E of the plurality of projection optical systems 4A to 4E are arranged in a line along the Y direction of the optical axis,
Other parts 4B and 4D of the plurality of projection optical systems 4A to 4E
Is a part of a projection optical system having a plurality of optical axes 4A, 4C and 4E.
Are arranged in a row in parallel with each other and at a predetermined interval, are substantially orthogonal to the Y direction, and scan the mask 3 and the photosensitive substrate 5 in the in-plane direction of the photosensitive substrate 5 in synchronization.

In the exposure apparatus according to the fifth aspect, the light beams L ₆ and L ₇ emitted from the first and second ultra-high pressure mercury lamps 10, respectively, are condensed by the light guide 14 and then are converged on the light guide 14. First and second luminous fluxes L obtained by splitting
₁ and L ₂ illuminate different first and second illumination regions M1 and M2 on the patterned mask 5, respectively, and the first and second illumination regions M1 and M2 are illuminated.
The respective images of the first and second projection optical systems 4A and 4A
The exposure method for projecting B a on the photosensitive substrate 5 via respectively, each opening with respect to light beam L ₆ of the first shutter 12 to regulate the light beam L ₆ emitted from the first ultra-high pressure mercury lamp 10 by an aperture First processing for detecting the illuminance of the first and second luminous fluxes L ₁ and L ₂ and second shutter for regulating the luminous flux L ₇ emitted from the second ultra-high pressure mercury lamp 10 by the aperture ratio. Second processing for detecting the illuminances of the first and second luminous fluxes L ₁ and L ₂ for each aperture ratio of the luminous flux L ₇ by 12 and the illuminance signal S1 by the first and second processings.
And S2, the respective aperture ratios of the first and second shutters 12 and the first and second light fluxes L ₁ and L
_A third process in which the illuminance of ₂ is associated and stored as illuminance data S6, and the illuminance of the first and second light fluxes L ₁ and L ₂ is made uniform when the pattern is projected onto the photosensitive substrate 5. , Based on the illuminance data S6 by the third processing,
A fourth process for controlling the first and second shutters 12 is provided.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows a scanning type exposure apparatus 1 as a whole, which exposes a large area pattern by only one-dimensional scanning.
Scanning exposure apparatus 1, the five light beams L ₁ ~L ₅ that uniform illumination from the illumination optical system 2 is injected, the light beam L ₁ ~L ₅
Therefore, different small illumination areas M on the mask 3
Illuminate 1 to M5. The scanning exposure apparatus 1 includes a plurality of light beams transmitted through the mask 3 and different projection optical systems 4A to 4A.
It projects on the photosensitive substrate 5 for manufacturing a liquid crystal display device through 4E, and the pattern image of illumination area | region M1-M5 is imaged in five different projection areas P1-P5.

Incidentally, projection areas P1 to P5 on the photosensitive substrate 5 are provided.
Are adjacent projection regions (for example, P1 and P2, P2 and P
3) are separated from each other by a predetermined distance in the X direction,
The ends of adjacent projection regions are arranged so as to overlap each other in the Y direction orthogonal to the X direction. Therefore, the projection optical systems 4A to 4E are also separated from each other in the X direction by a predetermined distance in correspondence with the arrangement of the respective projection areas P1 to P5, and are also arranged to overlap in the Y direction.

The projection optical systems 4A to 4E are all equal-magnification erecting systems, and their arrangement is such that the illumination areas M1 to M5 on the mask 3 are arranged.
It will be the same arrangement as. Therefore, the arrangement of the projection areas P1 to P5 on which the pattern images of the illumination areas M1 to M5 are formed is the same as that of the illumination areas M1 to M5. Luminous flux L ₁ of illumination optical system 2
The plane arrangement of the optical axis of L ₅ is as follows:
It is arranged similarly to M5.

The mask 3 and the photosensitive substrate 5 are integrally held by the stage holding base 6 in a state of facing each other. As a result, the mask 3 and the photosensitive substrate 5 are mechanically coupled so that their mutual positions are constant. The stage holding table 6 is provided with a driving device that drives the stage holding table 6 with a long stroke in the scanning direction (X direction). During scanning, the scanning type exposure apparatus 1 uses the drive unit to move the stage holder 6
Are driven in the X direction to scan the mask 3 and the photosensitive substrate 5 one-dimensionally with respect to the illumination optical system 2 and the projection optical systems 4A to 4E. By this synchronous scanning, the image of the entire surface of the pattern area 3A on the mask 3 can be transferred onto the photosensitive surface 5A of the photosensitive substrate 5.

The mask 3 is supported by a mask stage 7 arranged on a stage holder 6, and is positioned at an arbitrary position in the in-plane direction of the mask 3 by a plurality of fine movement actuators (not shown). . As a result, the scanning exposure apparatus 1 can be positioned by this fine movement actuator and correct the positional deviation of the pattern image due to the inclination of the stage holding table 6 with respect to the projection optical systems 4A to 4E.

As shown in FIG. 2, the illumination optical system 2 is provided with, for example, three super high pressure mercury lamps 10, and the luminous fluxes L ₆ emitted from the super high pressure mercury lamps 10 respectively.
~ L ₈ is an optical guide 1 formed by bundling a plurality of optical fibers through an elliptic mirror 11, a shutter 12, and a lens system 13.
It is incident on 4 and is condensed. The illumination optical system 2 uses the light guide 14 to convert the condensed light fluxes L _{6 to} L ₈ into five light fluxes L _{1 to} L.
₅ to divide the respective light beams L ₁ ~L ₅ obtained by dividing equalized by connexion illuminance fly-eye lens in the exit portion (not shown) is emitted. Incidentally, the shutter 12 is also used in the configuration of a normal illumination optical system, and does not increase the number of parts.

Subsequently, the illumination optical system 2, for example, the light flux L ₁
Is irradiated onto the field stop 17 via the half mirror 15 and the condenser lens 16, and the light beam L is emitted by this field stop 17.
Shape ₁ into each desired shape. The illumination optical system 2 is
The shaped light beam L ₁ is irradiated onto the pattern surface of the mask 3 via the relay lenses 18 and 19 to form an image of the field stop 17.

The illumination optical system 2 is provided with a half mirror 15 between the exit end of the light guide 14 and the condenser lens 16, and makes a part of the light beam L ₁ enter the photodetector 21. The illumination optical system 2 processes the light fluxes L _{2 to} L _{5 in the} same manner as the light flux L ₁ and irradiates the pattern surface of the mask 3 with the light.
Part of each of the light fluxes L _{2 to} L ₅ is incident on the photodetector 21.

The illumination optical system 2 causes the photodetection element 21 to generate illuminance signals S1 to S5 according to the illuminance of a part of the luminous flux given to the photodetection element 21 via the half mirror 15 and the lens system 20. , The illuminance signals S1 to S5
Give to 2. The illumination optical system 2 controls the illuminance of each of the light fluxes L _{1 to} L ₅ based on the illuminance signals S1 to S5.
Then, the illuminance data S6 generated in step S1 and associated with the aperture ratios of the respective shutters 12 are stored in the memory 23.

The illumination optical system 2 reads the illuminance data S6 stored in the memory 23 to the control unit 22, and the control signals S7 to S generated by the control unit 22 based on the illuminance data S6.
9 to the shutter control units 24 to 26, respectively. As a result, when the illuminance exceeds the appropriate value, the illumination optical system 2
The control unit 22 adjusts the aperture ratio of each of the three shutters 12 to reduce the illuminance of the light fluxes L _{1 to} L ₅ to an appropriate value and equalize the illuminance between the light fluxes L _{2 to} L ₅ . Let

Here, when obtaining the illuminance data S6, the illumination optical system 2 operates in accordance with the illuminance data acquisition procedure shown in FIG. 3, for example. That is, the illumination optical system 2 has a step SP0.
Then, the parameter M is cleared in step SP1 and the process proceeds to step SP2. In step SP2, the illumination optical system 2 fully opens all the N (here, 3) light sources, that is, all the shutters 12 of the ultra-high pressure mercury lamp 10, sets the aperture ratio to 100%, and proceeds to step SP3.

In step SP3, the illumination optical system 2
When the illuminance of the light fluxes L _{1 to} L ₅ in this setting is detected,
Proceeds to step SP4, in association with the opening ratio of the light beam L ₁ ~L 3 single shutter 12 each illumination of ₅ at this time. Then, the illumination optical system 2 is switched to step SP5.
Then, the parameter M is incremented to M + 1 and the process proceeds to step SP6. In step SP6, the illumination optical system 2 uses the M (here, 1) th super high pressure mercury lamp 1
The shutter 12 of 0 is closed, the aperture ratio is set to 0%, and the process proceeds to step SP7.

In step SP7, the illumination optical system 2
Detects the illuminance of the light fluxes L _{1 to} L ₅ , the step SP
8, the illuminance of each of the light fluxes L _{1 to} L ₅ at this time is stored in association with the aperture ratios of the three shutters 12. Subsequently, the illumination optical system 2 moves to step SP9 and determines whether or not the parameter M satisfies M = N. Step SP
When a negative result is obtained in 9, the illumination optical system 2 determines that the illuminance corresponding to the aperture ratios of the shutters 12 of all the ultra-high pressure mercury lamps 10 is not detected, and proceeds to step SP10. In step SP10, the illumination optical system 2 is M
The aperture ratio of the shutter 12 of the (here, 1st) super high pressure mercury lamp 10 is set to 100% again, the process returns to step SP5, and the above-described procedure is repeated.

When a positive result is obtained in step SP9, the illumination optical system 2 turns all the ultra high pressure mercury lamps 1 into one.
When it is determined that the illuminance corresponding to the aperture ratio of the shutter 12 of 0 is detected, the process proceeds to step SP11 to end the illuminance data acquisition procedure. The illuminance of the light fluxes L _{1 to} L _{5 at} the aperture ratio of 100% thus obtained and the light flux L at the aperture ratio of 0%
By integrating the difference with the illuminance of _{1 to} L ₅ , the illumination optical system 2 determines that each of the luminous fluxes L _{6 to} L ₈ emitted from the extra-high pressure mercury lamp 10 corresponding to the shutter 12 whose aperture ratio is controlled. The illuminance can be detected.

When the illuminance of each of the luminous fluxes L _{6 to} L ₈ is known, the illumination optical system 2 changes the luminous fluxes L _{6 to} L ₈ into luminous fluxes L _{1 to} L ₅ which are secondary luminous fluxes. Luminous flux of L ₆
Each division ratio of ~L ₈ can be detected based on the difference described above. It is clear that this division ratio is constant unless the configuration of the light guide 14 is changed.

In the above configuration, the illumination optical system 2 obtains the illuminance data S6 corresponding to each of the three ultra high pressure mercury lamps 10 before exposing the photosensitive substrate 5 of one lot and immediately after the lamp replacement, and stores it in the memory. It is stored in 23. The illumination optical system 2 obtains the illuminance data S6 by using the luminous fluxes L ₁ to
When it is detected that the illuminance of any one of L ₅ exceeds the predetermined value, it is determined that the brightness of any of the ultra-high pressure mercury lamps 10 is excessive, and the illuminance of the light fluxes L _{1 to} L ₅ is calibrated.

That is, the illumination optical system 2 reads out the illuminance data S6 obtained immediately before the exposure from the memory 23 and the illuminances of the luminous fluxes L _{6 to} L ₈ emitted by the three ultra high pressure mercury lamps 10 and the three shutters 12. Aperture ratio and luminous fluxes L _{1 to} L ₅
Based on the respective division ratios of the luminous fluxes L _{6 to} L ₈ to
The optimum aperture ratio combination for the three shutters 12 is set.

As a result, even if the brightness of any of the ultra-high pressure mercury lamps 10 is excessive and it is difficult to adjust the exposure amount to an appropriate value only by adjusting the scanning speed,
It is possible to illuminate the entire surface of the photosensitive substrate 5 with a luminous flux having a uniform illuminance and to expose the photosensitive substrate 5 with an appropriate exposure amount.

According to the above construction, the aperture ratio of the shutter 12 which regulates the luminous fluxes L _{6 to} L ₈ emitted from the plurality of ultra-high pressure mercury lamps 10 and the luminous fluxes L _{6 to} L ₈ are guided to the light guide 14. Light beams L _{1 to} L ₅ obtained by condensing and dividing
Of the scanning speed of the photosensitive substrate with respect to the illumination optical system 2 by setting the optimum combination of the aperture ratios for the three shutters 12 before exposure based on the illuminance data S6 stored in the memory 23 in association with the illuminance. Even if the brightness of the extra-high pressure mercury lamp 10 is too large to adjust the exposure amount to an appropriate value only by control, the photosensitive substrate 5 can be provided with an appropriate exposure amount.

In the above embodiment, the case of exposing the photosensitive substrate 5 for manufacturing the liquid crystal display device is described, but the present invention is not limited to this, and any photosensitive substrate, for example, a semiconductor element is manufactured. It can also be applied to the case of exposing a photosensitive substrate for.

Further, in the above-mentioned embodiment, the present invention is applied.
The case where the present invention is applied to the scanning type exposure apparatus having the two projection optical systems 4A to 4E has been described, but the present invention is not limited to this.
It can also be applied to an exposure apparatus in which an arbitrary number of projection optical systems other than five are arranged.

Further, in the above-mentioned embodiment, the case where the present invention is applied to the exposure apparatus 1 which scans and exposes only one dimension is described. However, the present invention is not limited to this, and two-dimensional scanning and exposure is performed. It can also be applied to an exposure apparatus.

Further, in the above-mentioned embodiment, the case where the illuminance of the luminous fluxes L _{1 to} L _{5 as} the secondary luminous flux is calibrated before the exposure is described, but the present invention is not limited to this, and the primary luminous flux during the exposure. Can be controlled to control the illuminance of the secondary light flux.

Further, in the above embodiment, the luminous flux L ₆
Although the case where the aperture ratio of the shutter 12 is controlled in order to regulate ~ L ₈ has been described, the present invention is not limited to this, and the luminous flux regulating means for regulating the luminous flux may have any configuration.

Furthermore, in the above-mentioned embodiment, the case where the light beam is condensed and split by the light guide 14 has been described, but the present invention is not limited to this, and the light beam is collected by an optical system of any configuration. It can also be applied to light and splitting.

[0036]

As described above, according to the present invention, the regulation state of the luminous flux regulating means for regulating each primary luminous flux emitted from the plurality of light sources and the primary luminous flux are collected by the optical transmission line. Based on the storage result stored in the storage means in association with the light and the illuminance of the secondary light flux obtained by splitting, by setting the optimum combination of the regulation states for the plurality of light flux regulation means, It is possible to realize an exposure apparatus and an exposure method capable of giving an appropriate exposure amount to a photosensitive substrate even if the light source has an excessive brightness such that the exposure amount cannot be adjusted to an appropriate value only by controlling the scanning speed of the photosensitive substrate. .

[Brief description of drawings]

FIG. 1 is a perspective view showing a scanning type exposure apparatus according to an embodiment of an exposure apparatus and an exposure method according to the present invention.

FIG. 2 is a schematic diagram for explaining a configuration of an illumination optical system according to an example.

FIG. 3 is a flowchart for explaining an illuminance data acquisition procedure according to an embodiment.

[Explanation of symbols]

1 ... Scanning exposure apparatus, 2 ... Illumination optical system, 3 ... Mask, 3A ... Pattern area, 4A-4E ... Projection optical system, 5 ... Photosensitive substrate, 5A ... Photosensitive surface, 6 ... Stage holder, 7 ... Mask stage, 10 ... Super high pressure mercury lamp, 11 ... Elliptical mirror, 12 ... Shatter, 13 ... Lens system, 14 ... Optical guide, 15 ... Half mirror, 1
6 ... Condenser lens, 17 ... Field stop, 18, 1
9 ... Relay lens, 20 ... Lens system, 21 ... Photodetector, 22 ... Control unit, 23 ... Memory, 24-26
...... Shutter control section, L _{1 to} L ₈ ...... Light flux, P1 to P5
...... Projection area, M1 to M5 ...... Illumination area.

Claims (5)

[Claims] 1. A plurality of secondary light fluxes obtained by concentrating a plurality of primary light fluxes respectively emitted from a plurality of light sources by an optical transmission path and then dividing the plurality of primary light fluxes by the optical transmission path, An illumination optical system that illuminates a plurality of different areas on a mask on which a pattern is formed, and a plurality of projection optical systems that are arranged corresponding to the plurality of areas, and each of the plurality of areas is provided. In an exposure apparatus for projecting an image on a photosensitive substrate via each of the plurality of projection optical systems, a plurality of light flux restricting means for restricting the respective primary light fluxes emitted by the plurality of light sources, and the secondary light flux. An illuminance detecting unit that detects the illuminance, and a storage unit that stores, based on the detection result, each regulation state of the plurality of light flux regulating units and the illuminances of the plurality of secondary light fluxes in association with each other. Based on memory result And a control means for controlling the light flux regulating means, wherein when the illuminance of the secondary light flux exceeds a predetermined value, the light source emitting any light flux component in the secondary light flux emits the light. An exposure apparatus which regulates a secondary light flux and regulates the illuminance of the secondary light flux to be equal to or less than the predetermined value. 2. The exposure apparatus according to claim 1, wherein the optical transmission line is configured by bundling a plurality of optical fibers. 3. The exposure apparatus according to claim 1, wherein the light flux regulating means regulates the primary light flux by controlling an aperture ratio of an opening through which the primary light flux passes. 4. A part of the plurality of projection optical systems are arranged in a line with their optical axes aligned in a first direction, and the other part of the plurality of projection optical systems have their optical axes aligned with each other. The mask and the photosensitive substrate are arranged in a line in parallel with a part of the plurality of projection optical systems and at a predetermined interval, substantially orthogonal to the first direction, and in the in-plane direction of the photosensitive substrate. The exposure apparatus according to claim 1, wherein scanning is performed in synchronization with and. 5. A first and a second light beam obtained by dividing the primary light fluxes emitted from the first and second light sources by an optical transmission line and then dividing the light beam by the optical transmission line. By the next light flux, the first different from each other on the mask on which the pattern is formed
And a second area, respectively, and an image of each of the first and second areas is projected onto a photosensitive substrate via a first and a second projection optical system, respectively. A first process for detecting the illuminance of the first and second secondary luminous fluxes for each regulation state for the primary luminous flux by the first luminous flux regulation means for regulating the primary luminous flux emitted from the light source; Second detecting the illuminance of the first and second secondary luminous fluxes for each regulation state for the primary luminous flux by the second luminous flux regulating means for regulating the primary luminous flux emitted from the second light source And the regulation states of the first and second light flux regulating means and the illuminances of the first and second secondary light fluxes based on the detection results of the first and second processes. And a third process for storing In order to make the illuminances of the first and second secondary light fluxes uniform when projecting onto the photosensitive substrate, the first and second light fluxes are stored based on the storage result of the third processing.
And a fourth process for controlling the light flux restricting means.