Mirror unit and exposure apparatus

A mirror unit includes: a mirror; a plurality of non-contact type actuators including movable elements and stators, and configured to change a shape of the mirror; a supporting plate to which the stators are fixed; and a structure configured to hold the mirror and the supporting plate. The movable elements are attached to a surface of the mirror that is opposite to an optical surface, and the structure holds the supporting plate via a kinematic mount.

BACKGROUND OF TEE INVENTION

Field of the Invention

The present invention relates to a deformable mirror unit capable of correcting the wavefront error and distortion of an optical system in an exposure apparatus, astronomical telescope, or the like, and an exposure apparatus.

Description of the Related Art

Recently, as a demand on the resolution of an exposure apparatus is becoming stricter, a demand for correcting aberration induced by exposure is also becoming stricter. To correct aberration induced by exposure, there has been proposed an arrangement using a deformable mirror.

Japanese Patent No. 4361269 discloses a mirror support structure in which an outer ring supports a reaction assembly to which a pneumatic actuator for deforming a mirror is attached, and an inner ring supporting the mirror. Japanese Patent Laid-Open No. 2004-64076 discloses a servo control mechanism including many actuators which support a deformable mirror at least three rigid positions, and displacement sensors located near them.

In Japanese Patent No. 4361269, the pneumatic actuator is mechanically coupled to the rear surface of the mirror via a rod, and thus excessively constrained with respect to the mirror. The mirror shape is readily affected by an assembly error. In the mirror support structure disclosed in Japanese Patent No. 4361269, the reaction force of the pneumatic actuator is transferred to the mirror via the reaction assembly, inner ring, and outer ring, and may affect the mirror shape.

Japanese Patent Laid-Open No. 2004-64076 has proposed a mirror unit with a displacement feedback driving control system including many actuators and displacement sensors located near them. The mirror unit with the displacement feedback driving control system measures the displacement of the mirror at or near each driving point, and controls the actuators so that the displacement coincides with a target displacement value in a given accuracy range. Hence, the mirror unit is free from the influence of a driving reaction force. However, this mirror unit has a complicated arrangement, requires many displacement sensors, and is expensive.

SUMMARY OF THE INVENTION

The present invention provides a mirror unit which reduces the influence of a driving reaction force from an actuator on the mirror shape.

The present invention in its first aspect provides a mirror unit comprising: a mirror; a plurality of non-contact type actuators including movable elements and stators, and configured to change a shape of the mirror; a supporting plate to which the stators are fixed; and a structure configured to hold the mirror and the supporting plate, wherein the movable elements are attached to a surface of the mirror that is opposite to an optical surface, and the structure holds the supporting plate via a kinematic mount.

The present invention in its second aspect provides an exposure apparatus for projecting, onto a substrate via a projection optical system, a pattern image of a reticle illuminated with light from an illumination optical system, thereby exposing the substrate, wherein at least one of the illumination optical system and the projection optical system includes a mirror unit defined in the first aspect.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will now be described with reference to the accompanying drawings. A non-contact type actuator described in the present invention is of a type in which a movable element and stator are not mechanically coupled to each other in the actuator. The non-contact type actuator is, for example, a linear motor in which a movable element and stator are coupled by an electromagnetic force, or an electromagnet. Although the following embodiments set forth will exemplify a linear motor, the non-contact type actuator is not limited to only the linear motor.

FIGS. 1A and 1Bare views showing the arrangement of a mirror unit according to the first embodiment of the present invention. InFIG. 1B, the Z direction is the upward vertical direction. A mirror1having an optical surface1ais held on its side surface by holding members2. The holding members2are fixed to a structure3. Movable elements (magnets)4aof linear motors4are bonded and attached to a rear surface1bof the mirror1that is a surface opposite to the optical surface. Supporting members5support stators (coils)4bof the linear motors4. The plurality of supporting members5are fixed to a supporting plate6. It is also possible that the stators (coils)4bare fixed on the supporting plate6directly. The supporting plate6receives a reaction force along with driving of the linear motors4. The supporting plate6is kinematically held with respect to the structure3by bipod mounts7(kinematic mounts) located at three portions. The bipod mounts7are located at an interval of 120° in the rotational direction along the periphery of the mirror1. In the first embodiment, the structure3constitutes a lens barrel which surrounds the mirror1, the plurality of linear motors4, and the supporting plate.

In the first embodiment, the movable element4ais directly bonded to the rear surface1bof the mirror1. However, the movable element4amay be bonded to the rear surface is of the mirror1via a member which relaxes a heat stress. When the difference between the linear expansion coefficient of the mirror1and that of the movable element4ais large, the use of the member which relaxes a heat stress is effective in relaxing a heat stress. In general, the linear expansion coefficient of the mirror1is smaller than that of the movable element4a. As the member which relaxes a heat stress, the same material as that of the mirror1, a material having physical property values close to those of the material of the mirror1, or a material smaller in linear expansion coefficient than at least the movable element4ais used.

FIG. 2is a schematic view showing the bipod mount7inFIG. 1B.

FIG. 2is a schematic view showing the bipod mount7. The supporting plate6and the structure3are coupled by flexures111which operate similarly to ball joints, a rigid bar112, and a fixing member113. Three bipod mounts7of this type are used and can fix the supporting plate6to the structure3kinematically in the range of a small displacement.

Since the supporting plate6is kinematically fixed to the structure3, it can be prevented to deform the structure3upon transferring, to the structure3, the deformation of the supporting plate6caused by a driving reaction force upon driving the linear motor4to deform the mirror1. Hence, the shape of the mirror1can be deformed at high accuracy without deforming the mirror1by the driving reaction force of the linear motor4via the structure3and holding member2.

This is effective particularly in a mirror unit which deforms the mirror1by only high-accuracy force control using a non-contact type actuator such as the linear motor4. In the mirror unit which controls the shape of the mirror1by only the non-contact type actuator, the accuracy of the shape of the mirror1depends not only on the force accuracy but also on each factor affecting the mirror shape. For example, if the driving reaction force for deforming the mirror1affects the mirror shape via the structure3, this directly leads to poor accuracy of the shape of the mirror1.

As the holding members2of the mirror1, inFIG. 1B, the holding members2may be held kinematically using the aforementioned bipod mounts7. However, the holding members2of the mirror may be holding members other than the kinematic mounts. For example, the mirror1may be fixed to the structure3by bonding or clamped mechanically.

FIG. 3shows the second embodiment of the present invention. A mirror1having an optical surface1ais fixed to a plate-shaped second structure3bserving as part of a structure3by a holding member2′ which holds the center portion of a rear surface1bof the mirror1. In the second embodiment, the structure3is constituted by a first structure3awhich constitutes a lens barrel, and the plate-shaped second structure3b.

The second structure3bis fixed to the first structure3a. Movable elements (magnets)4aof linear motors4are bonded to a rear surface1bof the mirror1except for the center portion. Stators (coils)4bare fixed to a supporting plate6via supporting members5. The supporting members5extend through through holes3cformed in the second structure3b, and are fixed to the supporting plate6located below the second structure3b. The supporting plate6is coupled to the first structure3aat three portions via bipod mounts7. Similar to the first embodiment, even in the second embodiment, the influence of the driving reaction force of the linear motor4on the shape of the mirror1is blocked.

FIG. 4shows a modification of the second embodiment. The arrangement inFIG. 4is different from that inFIG. 3in that at least three holding members109are used to hold the rear surface1bof the mirror1.FIG. 5shows the arrangement of the holding members109which couple the rear surface1bof the mirror1and the second structure3b.

FIG. 6shows the third embodiment of the present invention. A mirror1having an optical surface1ais held by a holding member2′ at the center portion of a rear surface1b. The holding member2′ extends through a through hole6aformed in a supporting plate6and is fixed to a second structure3blocated below the holding member2. Movable elements (magnets)4aof linear motors4are bonded to the rear surface1bof the mirror1except for the center portion. Stators (coils)4bare fixed to the supporting plate6via supporting members5. The supporting plate6is coupled to a first structure3avia bipod mounts7.

Similar to the first embodiment, even in the third embodiment, the influence of the driving reaction force of the linear motor4on the shape of the mirror1is blocked.

FIG. 7shows a modification of the third embodiment. The arrangement inFIG. 7is different from that inFIG. 6in that the first structure3aand second structure3bare integrally formed. The structure3described in each of the second and third embodiments is divided into two structures, but the present invention is not limited to this. In practice, the structure3may be constituted by one or more members.

FIG. 8is a view showing the arrangement of a mirror unit according to the fourth embodiment of the present invention.FIG. 9is a schematic view showing three V-groove mounts100constituted by three V-grooves104and three balls102. The bails102fixed to a supporting plate6are coupled to the V-grooves104formed in a structure3to couple the supporting plate6and structure3without excessively constraining them. The supporting plate6is thus held kinematically with respect to the structure3. When this type of mount is used, the deformation of the structure3is not transferred to the supporting plate6in a frictionless ideal state. In practice, the accuracy of the kinematic mount of this type is decided by the influence of the shape error of the ball102, the shape error and position error of the V-groove104, the friction between the ball102and the V-groove104, and the like.

In the example OfFIG. 9, the coupling between the V-groove104and the ball102constraints two degrees of freedom at two points at which the groove104and ball102point-contact each other. Since the three V-groove mounts100are used, the supporting plate6is constrained at six degrees of freedom with respect to the structure3. The V-groove mount100has high positioning reproducibility because, even if work of separating the supporting plate6and structure3and then recoupling them is repeated, the V-groove104and ball102are always coupled at the same position. Since the V-groove104and ball102point-contact each other, the force is transferred from the V-groove104to the ball102or reversely from the ball102to the V-groove104. However, a moment greatly concerning deformation is not transferred.

In a mount using a leaf spring or hinge, the rigidity is not zero at a low-rigidity portion of the leaf spring, the hinge portion, or the like serving as the point-contact portion between the V-groove104and the ball102, and not only the force but also the moment is transferred. According to the fourth embodiment, the transfer of deformation can be blocked much more than in the case in which the leaf spring or hinge is used.

Although the optical axis of the mirror1is oriented in the vertical direction in the first to fourth embodiments, the orientation of the mirror1is not limited to this. Even when the optical axis of the mirror1is oriented in an arbitrary direction, the mirror unit according to the present invention is applicable. For example, when the optical axis of the mirror1is oriented in the horizontal direction, the mirror unit is rotated by 90°. In this case, the three V-groove mounts100inFIG. 9may he formed vertically, or V-groove mounts100as shown inFIG. 10are also available. The vertical V-groove mounts100inFIG. 10are constituted by V-grooves104formed in the structure3, and balls102which are coupled to the V-grooves104. The balls102are fixed to the supporting plate6(not shown). Although the bipod mounts7as shown inFIG. 2can he employed, the arrangement can be changed in accordance with a concrete design. The present invention has exemplified well-known two types of mounts, but is not limited to them. If necessary, the supporting plate6, structure3, or the like may have a cooling function.

An exemplary exposure apparatus to which the mirror unit according to the present invention is applied will be explained. As shown inFIG. 11, the exposure apparatus includes an illumination optical system501, a reticle stage502on which a reticle is mounted, a projection optical system503, and a substrate stage504on which a substrate is mounted. The exposure apparatus projects, onto a substrate via the projection optical system503, the pattern image of a reticle illuminated with light from the illumination optical system501, thereby exposing the substrate. The exposure apparatus may comply with a step & repeat projection exposure method or a step & scan projection exposure method. The mirror unit according to the present invention can be used as a mirror unit for correcting the wavefront error and distortion of at least one of the illumination optical system501and projection optical system503.

The mirror unit according to the present invention can also be used as a mirror unit for correcting the wavefront error and distortion of an optical system in an astronomical telescope other than the exposure apparatus.

[Method of Manufacturing Device]

Next, a method of manufacturing a device (for example, a semiconductor device or liquid crystal display device) will be explained. The semiconductor device is manufactured through a pre-process of forming an integrated circuit on a wafer, and a post-process of completing, as a product, an integrated circuit chip formed on the wafer by the pre-process. The pre-process includes a step of exposing a wafer coated with a photosensitive agent by using the above-mentioned exposure apparatus, and a step of developing the wafer. The post-process includes an assembly step (dicing and bonding) and a packaging step (encapsulation). The liquid crystal display device is manufactured through a process of forming a transparent electrode. The process of forming a transparent electrode includes a step of applying a photosensitive agent to a glass substrate on which a transparent conductive film is deposited, a step of exposing the glass substrate coated with the photosensitive agent by using the above-described exposure apparatus, and a step of developing the glass substrate. According to the method of manufacturing a device in the embodiment, a higher-quality device than a conventional one can he manufactured.

This application claims the benefits of Japanese Patent Application Nos. 2013-086060, filed Apr. 16, 2013 and 2014-037323, filed Feb. 27, 2014, which are hereby incorporated by reference herein in their entirety.