Positioning apparatus

A positioning apparatus includes a moving member, an actuator, and a controller. The moving member can move in at least a first direction. The actuator is provided along the first direction. The controller controls a current applied to the actuator in order to support the weight of the moving member. The bending rigidity of the moving member in the first direction is greater than the bending rigidity of the moving member in a second direction perpendicular to the first direction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a positioning apparatus, and more specifically, it relates to a positioning apparatus used for positioning a substrate in a photolithography machine.

2. Description of the Related Art

A semiconductor photolithography machine makes exposure light incident on an original pattern drawn on a reticle. The light transmitted or reflected by the reticle is reduced with an exposure optical system, and the reduced pattern is projected onto a semiconductor substrate (wafer). In this way, a semiconductor photolithography machine performs exposure operation. The reticle having the pattern to be transferred is mounted on a reticle stage and positioned at a predetermined position. The reticle is irradiated from above with exposure light by an illumination system. The exposure light then enters a reduced projection optical system. This optical system forms an image at a predetermined position. A wafer stage carries and positions a wafer such that a predetermined area on the wafer is positioned at the point where the image is formed. The positional information of the wafer relative to the wafer stage has been obtained in advance by measuring the position of an alignment mark on the wafer with an alignment optical system. When exposure is performed, the wafer is positioned at the predetermined position on the basis of this alignment information.

Throughput is one of the indicators of the performance of a photolithography machine. The throughput is expressed as the number of wafers that the photolithography machine can process per unit time. In order to increase the throughput, it is necessary to move the wafer stage in a short time. For this purpose, it is necessary to increase the moving velocity in addition to the acceleration and deceleration when the wafer stage is moved. In order to achieve high acceleration and deceleration, high moving velocity, and highly accurate positioning performance, conventional wafer stages generally have a coarse/fine-motion multistep configuration including a fine-motion stage and a coarse-motion stage. The fine-motion stage carries and positions a wafer with high accuracy. The coarse-motion stage moves the fine-motion stage in the horizontal direction at high acceleration and deceleration and high moving velocity. In this configuration, a coarse-motion actuator needs to accelerate and decelerate the combined mass of the coarse-motion stage and the fine-motion stage. The higher the acceleration, the greater the necessary thrust. Consequently, the coarse-motion actuator tends to be large, and the entire stage apparatus also tends to be large. This tendency is undesirable because it causes an increase in the production cost and an increase in the area for installing the apparatus.

In addition, recently, a twin-stage configuration has been proposed. In the twin-stage configuration, while a wafer on one stage is exposed, another wafer to be exposed next is mounted on the other stage and aligned. In the twin-stage configuration, two stages individually convey wafers, and each stage repeats a cycle of wafer mounting, alignment operation, exposure operation, and wafer pickup. Therefore, the two stages use a common alignment optical system, exposure optical system, and wafer exchanger at different times. In the case of the conventional coarse/fine-motion stage, a complex configuration is necessary to interchange positions of two stages.

To solve this problem, a surface-motor stage has been devised.FIGS. 15A and 15Bshow a surface-motor stage apparatus that can perform positioning in six directions by Lorentz force (see Japanese Patent Laid-Open No. 2004-254489, corresponding to US Patent Application No. 2004-126907).

The stage apparatus includes a stage (mover)110and a coil unit (stator)100. The stage110has a magnet unit114on the underside. The coil unit100faces the magnet unit114. The magnet unit114includes a plurality of permanent magnets. The plurality of permanent magnets are arranged in the XY direction in a so-called Halbach array. The coil unit100includes a plurality of coils. The coil unit100includes a layer116aof coils arranged in the X direction and a layer116bof coils arranged in the Y direction. Although not shown inFIG. 15A, the coil unit100further includes another layer of coils arranged in the X direction and another layer of coils arranged in the Y direction. By selectively applying a current to these coil layers, a Lorentz force is generated between the magnet unit114and the coil unit100, and consequently the stage110can be moved.

Using the coil layer116a, a thrust in the X direction is given to the stage110. Using the coil layer116b, a thrust in the Y direction is given to the stage110. Using the other coil layers, thrusts in the Z direction (vertical direction), θx direction (rotating direction around the X axis), θy direction (rotating direction around the Y axis), and θz direction (rotating direction around the Z axis) are given to the stage110. The weight of the stage110is supported by the coil layers that give the stage110the thrust in the Z direction.

The coils constituting each coil layer generate desired forces in pairs. Each pair of coils (a phase A coil and a phase B coil) is adjacent to each other. The magnet unit114has cyclic (for example, sine-wave) magnetic-flux-density distributions in the X direction and the Y direction. Therefore, when a certain current is applied to the phase A coils, the generated thrust is a sine wave whose argument is a position of the magnet unit114relative to the coil unit100. The magnet unit114and the coil unit100are arranged such that, when a certain current is applied to the phase B coils, the generated thrust is a sine wave that is out of phase with the thrust of the phase A coils by 90 degrees. Therefore, by obtaining a rectification value from the position of the magnet unit114relative to the coil unit100and applying a current multiplied by the rectification value to the phase A coils and the phase B coils, desired forces can be generated.

However, in the case where the weight of the stage is supported using coils arranged in the X direction (or the Y direction), the application points of the forces applied to the stage to support the weight of the stage change as the stage moves in the X direction (or the Y direction). That is to say, when the stage is at a position, only the phase A coils apply forces to the stage; when the stage is at another position, only the phase B coils apply forces to the stage; and when the stage is at yet another position, both phase A coils and phase B coils apply forces to the stage. Such change in the application points of forces can cause undesirable deformation of the stage.

In photolithography machines, in general, a laser interferometer is used for measuring the position of a stage. A reflecting surface (mirror) is provided in the stage. The laser interferometer measures the position of the reflecting surface (mirror) by irradiating the reflecting surface (mirror) with laser light. Therefore, the positional relationship between the reflecting surface (mirror) and the exposed area on the wafer must be fixed. If the above-described deformation occurs, the positional relationship between the reflecting surface (mirror) and the exposure area on the wafer changes and therefore the exposure accuracy deteriorates.

SUMMARY OF THE INVENTION

The present invention is directed to a positioning apparatus.

In an aspect of the present invention, a positioning apparatus includes a moving member, an actuator, and a controller. The moving member can move in at least a first direction. The actuator is provided along the first direction. The controller controls a current applied to the actuator in order to support the weight of the moving member. The bending rigidity of the moving member in the first direction is greater than the bending rigidity of the moving member in a second direction perpendicular to the first direction.

In another aspect of the present invention, a positioning apparatus includes a moving member, an actuator, and a controller. The moving member can move in at least a first direction. The actuator is provided along the first direction. The controller controls a current applied to the actuator in order to support the weight of the moving member. The controller controls the current so as to reduce the bending force exerted on the moving member as the moving member moves in the first direction.

In another aspect of the present invention, a positioning apparatus includes a moving member, an actuator, and a controller. The moving member can move in at least a first direction. The actuator is provided along the first direction. The controller controls a current applied to the actuator in order to support the weight of the moving member. The controller controls the current so as to reduce an amount of deformation of the moving member as the moving member moves in the first direction. The amount of deformation of the moving member is obtained in advance.

In another aspect of the present invention, a positioning apparatus includes a moving member, an actuator, a controller, an interferometer, and a mirror. The moving member can move in at least a first direction. The actuator is provided along the first direction. The controller controls a current applied to the actuator in order to support the weight of the moving member. The interferometer measures the position of the moving member in a direction of gravitational force. The mirror is provided in the moving member and reflects light from the interferometer. The mirror is provided along the first direction.

This configuration can reduce the effect of the deformation caused by the change in the application point of force when the stage moves.

The positioning apparatus of the present invention can be applied to not only the photolithography machines and device-manufacturing machines, which are described as the embodiments, but also various high-precision processing machines and various high-precision measuring machines.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1is a schematic view of a semiconductor photolithography machine. A lens barrel supporting member1is mounted on a mount3. The lens barrel supporting member1is insulated with a vibration absorber2so as to be insulated against vibrations from the floor. A projection optical system4is supported by the lens barrel supporting member1. A reticle stage (not shown) is provided above the projection optical system4. A wafer stage5is provided below the projection optical system4.

A wafer7is mounted on the wafer stage (mover)5with a wafer chuck6. The wafer stage5can be moved with a so-called surface motor. The surface motor includes a magnet unit8and a coil unit (stator)9. The magnet unit8is provided on the underside of the wafer stage5. The coil unit9is provided on the mount3. The surface motor will hereinafter be described in detail.

When the wafer stage5is driven, a reaction force is exerted on the coil unit9. In order to prevent the reaction force from being transmitted to the mount3, the coil unit9can move on the mount3in the XY direction. Such configuration is discussed in Japanese Patent Laid-Open No. 11-190786 (corresponding to U.S. Pat. No. 6,414,742).

In order to measure the position of the coil unit9relative to the mount3, a linear encoder that measures the position of the coil unit9in the X direction and the Y direction is provided. The coil unit9is driven by a linear motor10relative to the mount3in the X direction and the Y direction. Means for driving the coil unit9is not limited to a linear motor.

The position of the wafer stage5is measured with a laser interferometer12. A mirror (not shown) is provided in the wafer stage5. The laser interferometer12measures the position of the wafer stage5by making laser light13reflect from the surface of the mirror.

FIG. 2illustrates the surface motor.FIG. 2shows the magnet arrangement in the magnet unit8viewed from above through the wafer stage5inFIG. 1. Of coil layers in the coil unit9, only a coil layer in which coils are arranged in the X direction is shown. The coil unit9shown inFIG. 2is simplified. In the real coil unit9, the number of coils is larger and the length of coils is longer than one shown inFIG. 2as shown inFIGS. 15A and 15B.

Magnets21(shown in light gray) and magnets22(shown in dark gray) are main pole magnets polarized in the vertical direction (Z direction). The magnets21have the south pole on the −Z side (the side facing the coil unit9). The magnets22have the north pole on the −Z side. Magnets23(shown in white) are auxiliary pole magnets polarized in the horizontal direction. The ends of each auxiliary pole magnet23are in contact with the main pole magnets. The pole at an end of each auxiliary pole magnet corresponds to the pole on the −Z side of the main pole magnet that is in contact with the end of the auxiliary pole magnet. Such a magnet arrangement is called a Halbach array. The magnet unit8has a cyclic magnetic-flux-density distribution on the −Z side. In the magnet unit8, a plurality of magnets are arranged in the XY direction in the Halbach array. The plurality of magnets are arranged symmetrically in a substantially square pattern in the XY direction.

The coil unit9includes a plurality of coils. The coil unit9includes a layer of coils arranged in the X direction and a layer of coils arranged in the Y direction. In the coil unit9, coils are arranged in a grid pattern. By selectively applying a current to these coils, Lorentz force is generated between the magnet unit8and the coil unit9, and consequently the wafer stage5can be moved.

The current applied to the coils will be described. A coil pitch CP is the distance between two adjacent coils. A magnetic-pole pitch MP is the distance between magnets having the same pole (main pole magnets having the same pole on the −Z side). There is the following relationship between the coil pitch CP and the magnetic-pole pitch MP:
MP=4/3*CP  (1)

In the state ofFIG. 2, the positional coordinate x in the X direction of the stage is zero. Let us suppose that the positional coordinate x is greater than or equal to zero and less than CP. The coils used for driving the stage in the X direction are c6to c11. These coils c6to c11do not overlap with the magnetless portions on the upper-left and lower-right corners of the magnet unit8. The winding direction of the coils is clockwise inFIG. 2.

As described above, the magnet unit8forms a magnetic-flux-density distribution above the coils. If the average value of the magnetic-flux-density distribution in the Z direction can be approximated by a sine wave with respect to the X axis, the force fi (i=6 to 11) in the X direction generated when a current I [A] is applied to coils ci (i=6 to 11) is described by the following functions of positional coordinate x. Since the coil unit9moves as described above, variable x in the trigonometric functions in the following equations needs to be corrected using the measurement value obtained in the measurement of the position of the coil unit9. Here, to simplify the explanation, this correction is omitted.
f6,f10=−I*Kx*cos(2*π/MP*x)  (2)
f8=I*Kx*cos(2*π/MP*x)  (3)
f7,f11=I*Kx*sin(2*π/MP*x)  (4)
f9=−I*Kx*sin(2*π/MP*x)  (5)
Here, Kx is a constant.

When the phase is the same as the above functions and the current Ii (i=6 to 11) applied to the coil ci (i=6 to 11) is
I6,I10=−I*cos(2*π/MP*x)  (6)
I8=I*cos(2*π/MP*x)  (7)
I7,I11=I*sin(2*π/MP*x)  (8)
I9=−I*sin(2*π/MP*x)  (9)
each of the sums of forces (f6+f7), (f8+f9), and (f10+f11) is
I*Kx*cos ^2(2*π/MP*x)+I*Kx*sin ^2(2*π/MP*x)=1*Kx(10)
and therefore the total thrust F is 3*I*Kx. That is to say, the current I required for generating a force F is obtained from the following equation:
I=F/Kx/3  (11)
When the positional coordinate x is greater than or equal to −CP and less than zero, coils c5to c10are used. In this case, currents I6to I10according to equations (6) to (9) are applied to coils c6to c10, respectively. As for coil5, current I9according to equation (9) is applied. Each of the sums of forces (f5+f6), (f7+f8), and (f9+f10) is thus expressed by equation (10).

Since the coils are arranged at intervals of CP, in the case where the stage moves by the distance CP, the positional relationship between the magnet unit8and the coil unit9is the same except for that the coil numbers differ by one, and therefore the current I for generating force F is obtained from equation (11) as discussed above. That is to say, at whatever position above the coil unit9the stage is located, the currents applied to coils in order to generate a desired driving force F in the X direction are obtained by multiplying the current of equation (11) by the rectification values of equations (6) to (9).

Here, the rectification values for the currents applied to coils according to equations (6) and (9) have a negative sign. However, if the direction of the corresponding coils is reversed, the negative sign is unnecessary. Therefore, in the case of even-numbered coils, the current is multiplied by a rectification value according to the cosine function of equation (7), and in the case of odd-numbered coils, the current is multiplied by a rectification value according to the sine function of equation (8).

In this way, the stage5can be moved in the X direction. Although not shown inFIG. 2, the stage5can be moved in the Y direction using a layer of coils arranged in the Y direction.

In order to generate a moment force in the θz direction, the coils located under the magnetless portions of the magnet unit8are used. When the positional coordinate x is greater than or equal to zero and less than CP, a force is generated with (f2, f3) and (f14, f15). When the positional coordinate x is greater than or equal to −CP and less than zero, a force is generated with (f1, f2) and (f13, f14). In these cases, since the magnet unit8has the magnetless portions as shown inFIG. 2, the thrust position is displaced in the Y direction. By generating forces in different directions with the above groups, a moment force in the θz direction can be generated.

In the case where a driving force in the Z direction is generated, when the positional coordinate x is greater than or equal to zero and less than CP, groups (f5, f6), (f7, f8), (f9, f10), and (f11, f12) are used, and when the positional coordinate x is greater than or equal to −CP and less than zero, groups (f4, f5), (f6, f7), (f8, f9), and (f10, f11) are used. The magnet unit8forms a magnetic-flux-density distribution in the X direction above the coils. The average value of the magnetic-flux-density distribution can be approximated by a sine wave with respect to the X axis. Considering that the direction of the even-numbered coils c2, c6, c10. . . is opposite from the direction of the odd-numbered coils c1, c5, c9. . . , the force f generated in the Z direction when a current of I [A] is applied to each coils is obtained from the following equations:
fn=I*Kz*cos(2*π/MP*x)n: odd number  (12)
fm=−I*Kz*sin(2*π/MF*x)m: even number  (13)
When the phase is the same as the above functions and the currents applied to the coils are
In=I*cos(2*π/MP*x)n: odd number  (14)
Im=−I*sin(2*π/MP*x)m: even number  (15)
the sum of forces of the above four groups is obtained from the following equation:
Fz=4*I*Kz(16)
Therefore, the current I required for generating a desired magnitude of force in the Z direction (Fz) is obtained from the following equation:
I=Fz/Kz/4  (17)

To generate a moment force in the θy direction, the four groups of coils used for generating the force in the Z direction are used. Of the four groups, two groups located on the +X side and two groups located on the −X side generate a couple of forces. In the same way, a moment force in the θx direction can be generated.

In this way, any magnitude of force can be generated in six directions X, Y, Z, θx, θy, and θz. The force in the X direction and the forces in the Z and θy directions may be generated with the same layer of coils. Alternatively, the force in the X direction and the forces in the Z and θy directions may be generated with different layers of coils. The same applies to the force in the Y direction and the forces in the Z and θx directions. Therefore, it is necessary for the coil unit9to have at least two layers of coils, that is to say, a layer of coils arranged in the X direction and a layer of coils arranged in the Y direction.

As described above, the position of the stage is measured with the laser interferometer12. This measurement value is used as a feedback signal for position control of the stage. In addition, this measurement value is used for calculating the phase of the current. The position of the stage in the Z direction is also measured using another laser interferometer (not shown).

The measured positional information of the stage is input into a position controller (not shown). In the position controller, a driving command to be sent to the stage is generated from a position command and the stage-position measurement information. On the basis of the driving command, using the above-described current command method, predetermined currents are applied to the coils with a current driver (not shown). In this way, positioning control of the stage is performed.

The application points of forces for supporting the weight of the wafer stage5in the above-described positioning apparatus will be described. The weight of the wafer stage5is supported by the thrust in the Z direction generated by applying currents to coils arranged in the X direction. At this time, if the wafer stage5moves in the X direction, the application points of forces change depending on the positional relationship between the magnet unit8and the coil unit9.

FIGS. 3A to 3Gshow a group of coils (c4to c12) that generate forces supporting the weight of the wafer stage5, and the magnet unit8viewed from the Y direction. The positions (application points) and magnitudes of the forces in the Z direction generated by the group of coils (c4to c12) are shown by arrows.FIGS. 3A to 3Gshow the process of movement of the wafer stage5.

Of the magnets in the magnet unit8, only the main pole magnets21(shown in gray) and22(shown in white) that generate forces in the Z direction are shown so as to clarify the arrangement in the X direction. The group of coils (c4to c12) extends in the direction perpendicular to the drawings (Y direction) and are arranged in the X direction. The even-numbered coils are shown in gray, and the odd-numbered coils are shown in white. On the basis of the above equations (12) to (15), when the stage is located at a position x, the odd-numbered coils generate forces in proportion to square of the cosine of (2*π/MP*x), and the even-numbered coils generate forces in proportion to square of the sine of (2*π/MP*x).

FIG. 4shows the relationship between the stage position and the ratio between the forces generated by the odd-numbered coils and the forces generated by the even-numbered coils. The horizontal axis represents the stage position. The stage position is converted into magnetic-pole pitch. When the stage position x is zero, the magnetic-pole pitch is zero. When the stage position x is MP, the magnetic-pole pitch is one.

When the stage position x is zero (as shown inFIG. 3A), only the odd-numbered coils (c5, c7, c9, and c11) generate forces in the Z direction. In the case where the center of gravity G of the stage is in the center of the magnet unit8shown inFIGS. 3A to 3G, when x is zero, the application points of the forces in the Z direction are symmetrical with respect to the center of gravity G.

When the stage position x is ⅛*MP (as shown inFIG. 3B), the force ratio between the odd-numbered coils (c5, c7, c9, and c11) and the even-numbered coils (c6, c8, c10, and c12) is 1:1. However, in this state, the application points are not symmetrical with respect to the center of gravity G, and a moment is generated in the −θy direction. Therefore, in addition to the forces inFIG. 3B, coils c5to c12generate a correction moment to compensate for the −θy moment.

When the stage position x is 2/8*MP (as shown inFIG. 3C), only the even-numbered coils (c6, c8, c10, and c12) generate forces in the Z direction. The coils generate a −θy moment greater than that in the state ofFIG. 3B.

When the stage position x is ⅜*MP (as shown inFIG. 3D), the force ratio between the odd-numbered coils (c5, c7, c9, and c11) and the even-numbered coils (c6, c8, c10, and c12) is 1:1, and the θy moment is zero.

When the stage position x is 4/8*MP (as shown inFIG. 3E), only the odd-numbered coils generate forces, and the θy moment is in the positive direction.

When the stage position x is ⅝*MP (as shown inFIG. 3F), the force ratio between the odd-numbered coils and the even-numbered coils is 1:1. However, in this state, the application points are not symmetrical with respect to the center of gravity G, and a moment is generated in the +θy direction. Therefore, in addition to the forces inFIG. 3F, coils c5to c12generate a correction moment to compensate for the +θy moment.

When the stage position x is 6/8*MP (as shown inFIG. 3G), only the even-numbered coils generate forces, and no θy moment is generated. From the relationship between the magnetic-pole pitch MP and the coil pitch CP, 6/8*MP is equal to CP. That is to say, the state ofFIG. 3Gcan be considered as a state such that the odd-numbered coils inFIG. 3Aare replaced with the even-numbered coils. Therefore, when the stage position x moves beyond 6/8*MP, the states ofFIGS. 3A to 3Gare repeated.

When the center of gravity G shifts due to movement of the stage, and application points of the forces in the Z direction supporting the weight of the stage also change, the forces are unbalanced, and a moment in the θy direction is generated. Although shown separately for purposes of illustration inFIGS. 3A to 3G, of course, these moments really change continuously. In other words, due to movement of the stage, forces in the Z direction are applied to the stage with the positions and balance of the forces continuously changing. The change in the forces in the Z direction creates a resultant force that deforms the stage.

The method for measuring the position of the stage will be described with reference toFIG. 5. The position of the stage5in the X direction is measured by irradiating a reflecting mirror14provided in the stage5with laser light13from a laser interferometer (denoted by reference numeral12inFIG. 1) and making the laser light13reflect from the reflecting mirror14. Similarly, the position of the stage5in the Y direction is measured by irradiating another reflecting mirror provided in the stage5with laser light from another laser interferometer (not shown) and making the laser light reflect from the reflecting mirror.

The position of the stage5in the θz direction is calculated in the laser interferometer for the X direction or Y direction, using two measurement axes a certain distance apart in the horizontal direction, from the difference between the measurement values and the distance between the measurement axes.

The position of the stage5in the Z direction is measured by emitting laser light16in the Y direction from a laser interferometer and by making the laser light reflect from a reflecting mirror15provided in the stage5. The reflecting mirror15is formed by beveling an edge of the stage at an angle of 45 degrees and mirror-finishing the beveled surface. The reflecting mirror15is elongated in the X direction, and therefore the position where the laser light16is incident on the reflecting surface can be made the same as the position of the exposure light axis19in the X direction.

The laser light reflected by the reflecting mirror15is then reflected by a reflecting mirror (not shown) for measuring the position of the stage5in the Z direction provided in the lens barrel supporting member. The reflecting mirror for measuring the position of the stage5in the Z direction has a reflecting surface perpendicular to the Z direction and is elongated in the Y direction. When the position of the stage5changes in the Z direction, the optical path length between the laser interferometer and the reflecting mirror for measuring the position of the stage5in the Z direction changes. When the stage5moves in the Y direction, the optical path length also changes. Therefore, the measurement value in the Z direction is obtained by subtracting the measurement value in the Y direction from the measurement value of the laser interferometer for the Z direction.

The positions of the stage5in the θx and θy directions are calculated in the laser interferometers for the X direction and the Y direction respectively, using two measurement axes a certain distance apart in the vertical direction, in the same way as in the case of the θz axis.

Although the reflecting mirror15provided in the stage5is slanted in this example, alternatively, a reflecting surface perpendicular to the Z direction may be provided in the stage5. In this case, the laser interferometer irradiates the stage5with laser light from the Z direction. For this purpose, for example, the reflecting surface of the reflecting mirror for measuring the position of the stage5in the Z direction provided in the lens barrel supporting member is slanted. Providing a stage with a reflecting surface perpendicular to the Z direction is discussed in Japanese Patent Laid-Open No. 2002-319541 (corresponding to U.S. Pat. No. 6,819,433).

Next, deformation of the stage5will be described. The degree of curving deformation of the stage on which a wafer is mounted is shown by α. As described above, the position of the stage5is measured with the laser interferometer12, and therefore the displacement of the reflecting mirror14provided on the side surface of the stage5is measured. The rigidity of the wafer chuck (not shown) and the wafer7is lower than the rigidity of the stage5, and the wafer chuck and the wafer7are vacuum-attracted to the stage5. Therefore, the surface shape of the wafer7follows the surface shape of the stage5. If the degree α of curving deformation changes, the distance L between the reflecting mirror14and the exposure position changes. This change in the distance L causes deterioration of measurement accuracy, and consequently deteriorates exposure accuracy.

In this embodiment, the position in the X direction of the laser light16for measuring the position of the stage5in the Z direction is the same as that of the exposure light axis19. Therefore, if any curving deformation occurs, the measurement value of the stage position in the Z direction is not affected.

FIG. 6shows the rib configuration of the wafer stage5inFIG. 1. The stage should be lightweight and have high rigidity. If the mass of the stage is large, a great force is required to accelerate and decelerate the stage. Therefore, a large amount of energy is applied to the surface motor. In addition, a large amount of heat is generated in the stator coil. If this heat is transferred to the stage, thermal expansion causes a change in the positional relationship between the laser interferometer and the exposure position, and deteriorates the exposure accuracy.

In order to achieve positioning control in the high bandwidth, it is necessary to increase the elastic-mode natural frequency of the stage structure. The reason is that the elastic-mode vibration of the stage structure is transferred to the position measurement signal through the reflecting mirror for the laser interferometer, and if high feedback gain is used, the stage structure can oscillate. When the elastic-mode natural frequency is high, even if the frequency component of the elastic-mode natural frequency appears in the position measurement signal, the influence on the feedback control system can be reduced using a lowpass filter or notch filter. In order to realize a lightweight and highly rigid stage structure, a hollow rib structure using a ceramic material is used. In order to increase the natural frequency, as shown inFIG. 6, the square stage structure is provided with a rhombus-shaped rib31.

In addition, in the present invention, in order to increase the rigidity of the stage5in the direction in which the curving deformation occurs as shown inFIG. 5, ribs32parallel to the Y direction are provided. The ribs32parallel to the Y direction are, that is to say, ribs parallel to the direction in which the application points of the forces supporting the weight of the stage5shift when the stage is moved. By providing such ribs32, if the positions and balance of the forces in the Z direction change due to movement of the stage5in the X direction, the change in the degree α of curving deformation shown inFIG. 5can be reduced.

FIG. 7shows a modification of embodiment1. Recently, in order to further reduce the weight of a stage structure, FRP (fiber-reinforced plastic) materials have been used. In FRP materials, rigidity depends on the bending direction due to the orientation of the fibers. InFIG. 7, the rigidity is reinforced in the direction of the arrow.FIG. 7shows a stage5in which the change in the degree a of curving deformation is reduced by using a material having anisotropic rigidity and by increasing the bending rigidity in the X direction. In addition, since the position in the X direction of the laser light for measurement of the position of the stage5in the Z direction is the same as that of the exposure light axis, if any deformation of the stage structure occurs, the measurement value of the stage position in the Z direction is not affected.

Since high exposure accuracy is required, just reinforcing the rigidity in the bending direction of the stage structure cannot meet the requirement for photolithography machines. In such a case, as shown inFIG. 8, actuators33using a piezoelectric element are provided on the top surface of the stage structure. Since the wafer chuck is placed in the center of the stage structure, the piezoelectric element actuators are disposed around the center of the stage structure. Both ends in the X direction of each piezoelectric element actuator are attached to the stage structure. By adjusting the voltage applied to the piezoelectric element actuators, the piezoelectric element actuators can expand and contract in the X direction. Therefore, a bending force counteracting the curving deformation shown inFIG. 5can be generated. By adjusting the voltage applied to the piezoelectric element actuators depending on the stage position, the degree a of curving deformation can be reduced to such a degree that there is no problem with the exposure accuracy.

FIG. 9illustrates the curve-correcting forces in the state ofFIG. 3C. At this time, all forces in the horizontal direction are generated by the odd-numbered coils. In addition to the control forces generated for positioning, the coils c5and c11generate the curve-correcting forces f1and f2, respectively. In this surface motor, a force is generated in the center of each coil. By generating the forces f1and f2shown inFIG. 9with the coils c5and c11, a bending force to compensate for the curving deformation shown inFIG. 5can be applied to the stage structure through the movable magnets. Since the forces f1and f2have the same magnitudes and opposite directions, the resultant force in the X direction is zero. The correcting forces are adjusted depending on the stage position. The coils for generating correcting forces are not limited to the coils shown here. Any coils that generate a bending force to compensate for the curving deformation shown inFIG. 5can be used.

The methods for obtaining the command value of the voltage applied to the piezoelectric element actuators and the command value of correcting force will be described. The methods for obtaining the command values include a method using an apparatus different from the photolithography machine and a method using the photolithography machine. First, a method using an apparatus different from the photolithography machine will be described.

FIGS. 10 and 11show an apparatus for obtaining the command value to be sent to the piezoelectric element actuators. An apparatus200includes a stage structure, a mirror18for measuring deformation provided on the stage structure, a coil unit (stator)9, a laser interferometer212for measuring the position of the stage5in six directions, and a measuring plate201on the underside of which the laser interferometer212is attached. The measuring plate201is mounted on a mount203. The measuring plate201is insulated with a vibration absorber202to insulate against vibrations from the floor. Since the mechanism for positioning the stage is almost the same as that described inFIG. 1, only the difference will be described.

The coil unit9can move in the X direction along the guide211provided in the mount203. The coil unit9is moved by a ball screw215. The force of the ball screw215is transferred to the coil unit9by a drive shaft (drive mechanism)214. Incidentally, the drive mechanism is not limited to this configuration.

The position of the coil unit9in the X direction is measured with an instrument (not shown), for example, a linear encoder. For the positional servo of the stage5, positional relationship information between the stage5and the coil unit9is used. This positional relationship information is obtained from the measurement value of another laser interferometer216and the measurement value of the above-mentioned instrument.

In addition to the laser interferometer216used for positional servo, yet another laser interferometer217for measuring the deformation of the stage is provided in the measuring plate201. The measurement light213emitted from the laser interferometer217are reflected by the mirror18for measuring deformation provided on the stage5. The laser interferometer217can move in the XY direction. The mirror18for measuring deformation moves in response to the movement of the laser interferometer217.

The method for obtaining the command value to be sent to the piezoelectric element actuators33using the above-described apparatus will be described. First, the stage5is positioned at a predetermined target position using a servomechanism. At this time, the command value of the voltage applied to the piezoelectric element actuators33inFIG. 8or the correction command value ofFIG. 9is zero. Next, with the stage5positioned, the coil unit9is moved with the drive mechanism, and the output of the laser interferometer217is recorded.

Here, the coil unit9needs only be moved by one coil pitch. The reason is that the cycle of the change of the forces in the Z direction accompanying the movement of the stage5is one coil pitch. By moving not the stage5but the coil unit9, the position where the measurement light of the laser interferometer217is incident on the reflecting surface can be fixed. If the stage is moved, the irradiation position of the measurement light changes, and therefore measurement is affected by the surface accuracy and the installation error of the reflecting mirror.

Next, the average value of the recorded output of the laser interferometer217is calculated. The coil unit9is returned to the initial position and is then moved again. In this movement operation, a voltage command value is given to the piezoelectric element actuators33or a correction command value is given to the coil unit9so that the output of the laser interferometer217approaches the calculated average value. The voltage command value and the correction command value can be adjusted.

By repeating these adjustments and slight displacement of the coil unit9, a table of command values concerning the positional relationship between the coil unit9and the stage5can be obtained. In order to make the table more reliable, the values in the table can be modified by changing the position of the mirror18for measuring deformation and by performing the same measurement.

Next, the method for obtaining the command value to be given to the piezoelectric element actuators33using the photolithography machine will be described. Around the projection optical system4inFIG. 1, an alignment optical system (not shown) and a focus detecting system (not shown) are provided. The alignment optical system measures the position of an alignment mark provided on the wafer, thereby detecting displacement of the alignment mark with respect to the projection optical system4.

First, the stage5is positioned using a servomechanism at a position where the alignment mark can be measured, and the position of the alignment mark is measured with the alignment optical system. Next, with the stage5positioned, the coil unit9is moved with a drive mechanism and the position of the alignment mark is measured. A voltage command value is given to the piezoelectric element actuators33or a correction command value is given to the coil unit9so that the displacement of the alignment mark is reduced. The voltage command value and the correction command value can be adjusted.

By repeating these adjustments and slight displacement of the coil unit9, a table of command values concerning the positional relationship between the coil unit9and the stage5can be obtained.

The focus detecting system can detect the height in the Z direction of the wafer surface. Therefore, by referring to this value, a more accurate correction command value can be obtained. In this way, also in the photolithography machine, a table of command values concerning the positional relationship between the coil unit9and the stage5can be obtained.

By controlling the piezoelectric element actuators33or the coil unit9using the above-described table of correction command values, the bending force exerted on the stage5due to the change in the application points of forces can be reduced.

FIG. 12is a block diagram of the above-described correction system using piezoelectric element actuators33. The positional information of the stage5and the coil unit9is input into a control unit41, and the control unit41outputs a correction command value. The control unit41has the above-described correction-command-value table in the memory. The positional relationship between the stage5and the coil unit9is a continuous value. In the case where the correction-command-value table has discrete values, it is necessary to interpolate the values in order to obtain a correction command value from the correction-command-value table. As the method for interpolation, a commonly used method such as an approximation method may be used.

The correction command value is sent to the piezoelectric element actuators33or the coil unit9. The correction command value varies depending on the type of actuator used for correction. Therefore, the correction-command-value table is tailored to the actuator to be used.

In addition, by adjusting the command value to be sent to the stage using the above-described correction-command-value table, the bending force exerted on the stage can be reduced.

FIG. 13is a block diagram of a correction system using a stage position command. This correction command value can be obtained using the above-described method for making the correction-command-value table on the photolithography machine. When exposure is performed, a stage position command is corrected using a correction command obtained from the correction-command-value table, and the corrected position command is sent to the stage control system. That is to say, the amount of wafer displacement caused by the stage deformation due to the change in the positional relationship between the stator and mover is calculated, the stage is positioned in consideration of the amount of displacement, and consequently exposure can be performed at the correct position.

Next, a process of manufacturing semiconductor devices using this photolithography machine will be described.FIG. 14shows the flow of the whole manufacturing process of semiconductor devices. In step S1(circuit design), a semiconductor device circuit is designed. In step S2(mask making), a mask is made on the basis of the designed circuit pattern.

In step S3(wafer fabrication), wafers are fabricated using a material such as silicon. Step S4(wafer process) is called a front end process. In step S4, actual circuits are formed on the wafers by lithography using the mask and the photolithography machine. Step S5(assembly) is called a back end process. In step S5, semiconductor chips are made of the wafers processed in step S4. The back end process includes an assembly process (dicing and bonding) and a packaging process (chip encapsulation). In step S6(inspection), inspections such as an operation confirmation test and a durability test of the semiconductor devices made in step S5are conducted. Through this process, the semiconductor devices are completed and shipped in step S7.

The wafer process of step S4includes the following steps. An oxidation step in which the surface of a wafer is oxidized. A Chemical Vapor Deposition (CVD) step in which an insulating film is formed on the wafer surface. An electrode formation step in which electrodes are formed on the wafer by vapor deposition. An ion implantation step in which ions are implanted in the wafer. A resist process step in which a photosensitive material is applied to the wafer. An exposure step in which the circuit pattern is transferred to the wafer with the photolithography machine. A development step in which the exposed wafer is developed. An etching step in which the wafer is etched except for the developed resist image. A resist stripping step in which the resist is removed. These steps are repeated, and multilayer circuit patterns are formed on the wafer.

This application claims the benefit of Japanese Application No. 2005-033017 filed Feb. 9, 2005, which is hereby incorporated by reference herein in its entirety.