Lithographic apparatus, projection apparatus and device manufacturing method

A control system to control a position of a substrate table for a lithographic apparatus, including: a first detection device to generate a position signal representative of the position of the projection system, a second detection device to generate a projection system feed-forward signal, a comparative unit to generate a servo error signal by subtracting a signal representative of an actual substrate table position from a substrate table position reference signal and adding the projection system position signal, a control unit to generate a first control signal on the basis of the servo error signal, an addition unit to generate a second control signal by adding the feed-forward signal and the first control signal, and an actuator unit to actuate the substrate table to a desired substrate table position on the basis of the second control signal. The control system further includes a projection system position signal filter unit.

BACKGROUND

1. Field of the Invention

The present invention relates to a lithographic apparatus, a projection apparatus and a method for manufacturing a device.

2. Description of the Related Art

The known lithographic apparatus comprises a control system to control the position of the substrate table. This control system is not only configured to place the substrate table in a desired position on the basis of a reference signal, but also to follow the lens in a horizontal direction in order to minimize image errors caused by movements of the lens, which movement are for instance caused by vibrations in the lithographic apparatus. For this reason a projection system position signal is added to the substrate table position reference signal, so that the controller error is adapted for movements of the lens. Usually the projection system position signal is obtained by a interferometer measuring system. To further increase the accuracy in following the movements of the lens, the control system comprises a feed-forward branch which adds a feed-forward signal representative for the acceleration of the lens to a output signal of the controller unit of the control system. The feed-forward signal is obtained by accelerometers arranged on the lens. The feed-forward signal is conditioned by a feed-forward filter unit. This feed-forward filter unit may comprise analogue filtering of the feed-forward signal and may also comprise digital filters to shape the feed-forward correctly.

A disadvantage of the known control system is that the filtering of the feed-forward signal introduces phase lag/delay in the feed-forward branch, which results in that the actual position of the substrate table lags behind the actual position of the lens, and hence creating a servo error/lens tracking error. This servo error/lens tracking error may be countered by adding lead-lag filters in the feed-forward branch, which introduce a phase advantage at the dominant lens resonance frequencies. However, this is only possible for a limited number of frequencies and as a consequence the response becomes worse at other frequencies. For this reason, the performance that can be gained with this strategy reaches its limits when the number of lens resonance frequencies increases and/or the position accuracy requirements become higher. Furthermore, the lead-lag filters are highly lens dependent, and therefore often have to be tuned by hand, which increases the risk on a poor response on the movements of the lens.

SUMMARY

It is desirable to provide a control system which is configured to further reduce the effect of movements of the projection system on the servo error and/or the projection system tracking error.

According to an embodiment of the invention there is provided a lithographic apparatus comprising a projection system configured to project a patterned radiation beam onto a target portion of the substrate, the substrate being supported on a substrate table, wherein the lithographic apparatus comprises a control system configured to control a position of the substrate table, the control system comprising a first detection device configured to generate a projection system position signal representative for the position of the projection system, a second detection device configured to generate a projection system feed-forward signal, a comparative unit configured to generate a servo error signal by subtracting a signal representative for an actual substrate table position from a substrate table position reference signal and adding the projection system position signal, a control unit configured to generate a first control signal on the basis of the servo error signal, an addition unit configured to generate a second control signal by adding the projection system feed-forward signal and the first control signal, and an actuator unit configured to actuate the substrate table to a desired substrate table position on the basis of the second control signal, wherein the control system further comprises a projection system position signal filter unit configured to filter the projection system position signal before adding the projection system position signal to the reference substrate table position signal.

According to an embodiment of the invention there is provided a lithographic apparatus comprising:

a projection system configured to project a patterned radiation beam, the patterned radiation beam being imparted by a patterning device being supported on a patterning device support, wherein the lithographic apparatus comprises a control system configured to control a position of the patterning device support, the control system comprising a first detection device configured to generate a projection system position signal representative for the position of the projection system, a second detection device configured to generate a projection system feed-forward signal, a comparative unit configured to generate a servo error signal by subtracting a signal representative for an actual patterning device support position from a patterning device support position reference signal and adding the projection system position signal, a control unit configured to generate a first control signal on the basis of said servo error signal, an addition unit configured to generate a second control signal by adding the projection system feed-forward signal and the first control signal, and an actuator unit configured to actuate the patterning device support to a desired patterning device support position on the basis of the second control signal, wherein the control system further comprises a projection system position signal filter unit configured to filter the projection system position signal before adding the projection system position signal to the reference patterning device support position signal.

According to an embodiment of the invention there is provided a projection apparatus comprising a control system configured to control a position of a first component, on the basis of a first component reference signal and a position of a second component, the control system comprising:

a first detection device configured to generate a second component position signal representative for the position of the second component, a second detection device configured to generate a second component feed-forward signal, a comparative unit configured to generate a servo error signal by subtracting a signal representative for an actual first component position from a first component position reference signal and adding the second component position signal, a control unit configured to generate a first control signal on the basis of the servo error signal, an addition unit configured to generate a second control signal by adding the second component feed-forward signal and the first control signal, and an actuator unit configured to actuate the first component to a desired first component position on the basis of the second control signal, wherein the control system further comprises a second component position signal filter unit configured to filter the second component position signal before adding the second component position signal to the reference first component position signal.

According to an embodiment of the invention there is provided a device manufacturing method comprising using a projection system for projecting a patterned beam of radiation onto a substrate, the substrate being supported on a substrate table, wherein a control system configured to control the position of the substrate table is used, wherein a projection system position signal representative for a position of the projection system is added to the difference between a substrate table position reference signal and an actual substrate table position resulting in a servo error signal which is used as an input for a control unit, wherein in a feed-forward branch a projection system feed-forward signal is added to a first control signal of the control unit resulting in a second control signal, wherein the second control signal is used to actuate the substrate table to a desired substrate table position, and wherein the projection system position signal is filtered by a projection system position signal filter unit before being added to the difference between said substrate table position reference signal and said actual substrate table position.

According to an embodiment of the invention there is provided a device manufacturing method comprising using a projection system for projecting a patterned beam of radiation, the pattern being imparted by a patterning device, the patterning device being supported on a patterning device support, wherein a control system configured to control the position of the patterning device support is used, wherein a projection system position signal representative for a position of the projection system is added to the difference between a patterning device support position reference signal and an actual patterning device support position resulting in a servo error signal which is used as an input for a control unit, wherein in a feed-forward branch a projection system feed-forward signal is added to a first control signal of the control unit resulting in a second control signal, wherein the second control signal is used to actuate the patterning device support to a desired patterning device support position, and wherein the projection system position signal is filtered by a projection system position signal filter unit before being added to the difference between the patterning device support position reference signal and the actual patterning device support position.

According to an embodiment of the invention there is provided a device manufacturing method comprising using a projection apparatus, wherein a control system is used, the control system being configured to control a position of a first component on the basis of a first component reference signal and a position of a second component, wherein a second component position signal representative for a position of the second component is added to the difference between a first component position reference signal and an actual first component position resulting in a servo error signal which is used as an input for a control unit, wherein in a feed-forward branch a projection system feed-forward signal is added to a first control signal of the control unit resulting in a second control signal, wherein the second control signal is used to actuate the first component to a desired first component position, and wherein the second component position signal is filtered by a second component position signal filter unit before being added to the difference between said first component position reference signal and the actual first component position.

DETAILED DESCRIPTION

The lithographic apparatus may be of a type having two (dual stage) or more substrate tables or “substrate supports” (and/or two or more mask tables or “mask supports”). In such “multiple stage” machines the additional tables or supports may be used in parallel, or preparatory steps may be carried out on one or more tables or supports while one or more other tables or supports are being used for exposure.

InFIG. 2a control scheme for controlling the position of the substrate table in a horizontal direction according to the prior art is shown. The control scheme comprises a control unit C, a filter unit F and the substrate table having a transfer function P. The actual position y of the substrate table is in a comparative unit CU subtracted from a substrate table position reference signal r, resulting in a servo error erwhich is used as an input for the control unit C. When the substrate table position reference signal is changed the control unit C will actuate the substrate table by an actuator unit AU to move to the desired position, wherein the actual substrate table position equals the substrate table position reference signal resulting in a zero servo error. It is remarked that the control scheme may further comprise several elements which are not directly related to the present invention, such as for instance reference signal acceleration feed-forward. Such elements are not shown in the control scheme ofFIG. 2.

The filter F may be used to condition the control signal and may for instance comprise a notch filter which dampens the response of the substrate table at the resonance frequencies of the substrate table. Any other suitable filter-type for filtering the control signal may be applied.

Due to vibrations or other movements in the lithographic apparatus, the projection system, or a part thereof may move, which movement may have a negative influence on the image that is projected by the lithographic apparatus on the substrate, thus resulting in imaging errors. Therefore, it is desirable that the substrate table reacts to movements of the projection system, for instance a lens (lens column), in order to avoid or at least reduce these imaging errors caused by the movements of the lens.

In order to make the substrate table follow movements of the, a signal u representative for the actual position of the lens is in the comparative unit added to the difference between the substrate table position reference signal and the actual substrate table position. The signal u may be obtained by a first detection device D1, such as an interferometer measurement system.

To further improve the reaction of the substrate table on movements of the lens, a feed-forward branch is introduced into the control scheme. In this feed-forward branch a feed-forward signal a representative for the acceleration of the lens is multiplied by m, m representing the mass of the substrate table by a multiplier M and consequently added to the output signal of the control unit C by an addition unit A. Before multiplying with the mass m of the substrate table, the feed-forward signal is filtered by feed-forward filter Q. In an alternative embodiment the order of the feed-forward filter Q and the mass multiplier m may be swapped.

In this application a signal representative for the acceleration of the lens may also be a acceleration signal already multiplied with m, being approximately the mass of the substrate table including a substrate supported thereon, since that signal is still. For instance the signal produced by the accelerometer may already be a signal in which the mass of the substrate table has been accounted for. The signal may also later in the feed-forward branch be multiplied with m, as shown inFIG. 2.

The acceleration signal a may be obtained by a second detection device D2, such as an accelerometer, but this second detection device D2may also be the same as the first detection device D1, whereby the latter case the acceleration of the projection system is obtained by differentiating the position signal u. This second embodiment is shown in the control scheme ofFIG. 2, wherein u is multiplied by s2to obtain the accelerationsignal a. In practice, it is preferred to use separate accelerometers to obtain the acceleration signal a.

The feed-forward filter Q may comprise the analogue filtering of the accelerometer signal and may also comprise possible digital filters configured to shape the feed-forward signal. The analogue filters of the feed-forward filter Q and the F filter may introduce phase lag into the feed-forward branch (from a to y). Also in a digital system some delay may be present due to e.g., the DAC's that perform a zero order hold function, a calculation delay and such. Therefore, y will lag behind u which will create a servo error er.

The digital filters in feed-forward filter unit Q may comprise lead-lag filters which introduce a phase advantage at the dominant lens resonance frequencies, to compensate for phase lag/delay in the feed-forward branch. However, this is only possible for a limited number of frequencies and as a consequence the response becomes worse at other frequencies. For this reason, the performance that can be gained with this strategy reaches its limits when the number of lens resonance frequencies increases and/or the position accuracy requirements become higher. Furthermore, the lead-lag filters are highly lens dependent, and therefore often have to be tuned by hand, which increases the risk on a poor response on the movements of the lens. There is a need for other solutions to decrease the servo error and/or the lens tracking error, i.e., the difference between the actual position of the projection system and the actual position of the substrate table.

InFIG. 3a control scheme of a control system according to an embodiment of the invention is shown. The control scheme includes a filter L, which is configured to filter the projection system position signal before it is added at the comparative unit. Further, in the control scheme the lens tracking error (projection system tracking error) euis shown, i.e., the difference between the (change in) position of the lens u and the (change in the) actual position of the substrate table y. This lens tracking error eumay be used to provide an indication of the imaging error that may be obtained when projecting an image on the substrate by a misalignment between substrate table and the projection system. The comparison between u and y in the control scheme is in practice not actually a physical part of the control scheme, but is here shown to indicate the lens tracking error eu.

By adding a filter L to filter the projection system position signal u before feeding it to the comparative unit it is possible to further improve the response of the substrate table to a change in the position of the lens as will be explained hereinafter.

Two possible selection criteria for the selection of the transfer function of the filter L are here proposed. Other selection criteria may also be possible and are deemed to fall within the scope of the present invention.

In a first embodiment the filter L is selected to minimize the servo error er. To minimize this servo error erL is chosen to be equal to the feed-forward path from u to y in the control scheme, i.e.,
L=QF

Hereby it is assumed that the process transfer function P does not contain any dynamics that make it deviate from 1/ms2. If P would contain such dynamics, these dynamics should also be included in the transfer function of filter L.

The filter L=QF can easily be implemented because the filter consists of two filters which are “proper,” i.e., have a pole excess. Filter L selected according the first embodiment will hereinafter be indicated as L1.

In a second embodiment the filter L is selected to minimize the lens tracking error eu, i.e., the difference between the (change in) position of the lens and the actual position of the substrate table y is miminized. To minimize eu, first the overall transfer function from u to y is calculated.

If this transfer function equals 1, euwill be zero. Now, by setting y/u=1, L2can be calculated to be:

Also here it is assumed that the process transfer function P does not contain any dynamics that make it deviate from 1/ms2. If P would contain such dynamics, these dynamics should also be included in the transfer function of filter L.

This choice for L is less easy to implement as the filter L now has a zero excess of 3, mainly due to the term s2in the numerator and the product CFP in denominator. To make this choice of L implementable a low-pass filter of the order3is included in L. In the example the low-pass cut-off frequency is set at 10000 Hz. A lower value may probably also be applied in practice. Filter L selected according to the second embodiment of the invention will hereinafter be indicated as L2.

The effects of the application of the filter units L1and L2according to the first and second embodiment of the invention will now be compared with the control scheme according to the prior art without a filter L in an example.

In this example the filter F is a notch filter at 150 Hz, the control unit C is a PID plus low-pass controller at a bandwidth of 100 Hz. Q is a low-pass filter with a cut-off frequency of 500 Hz. The transfer function of the process P is 1/ms2as was indicated above. The mass m equals 10 kg.

FIG. 4shows the effect of the usage of L1and L2on the transfer function y/u (i.e., reaction of the substrate table on lens movements), respectively. The vertical axis shows y/u in [dB]. It can be seen that without filtering (L=1), y/u can reach a gain of +8 dB, which means that the substrate table over-reacts with a factor of 2.5. On the other hand, when using a filter L1, y/u never shows a response larger than 0 dB. For higher frequencies, both curves follow the low-pass filtering characteristics in Q, because for these frequencies the open-loop gain is small. When using L2, y/u is substantially equal to 0 dB, indicating that y is almost equal to u for all frequencies.

FIG. 5shows the transfer function of eu/u (also in [dB]), i.e., how well the substrate table actually follows the lens for a specific frequency. A smaller value in this case is better, because it indicates a smaller difference between the lens position and the substrate table position. It can be seen that for low frequencies, the use of L1deteriorates performance compared to when no filter unit L is used, as the magnitude of the transfer function is larger. From around 65 Hz, use of L1improves the behavior compared to no filter unit L. Lens frequencies typically start around 90 Hz, so the net effect will still be positive. When using L2, the overall transfer function is much smaller, indicating a generally smaller eu.

FIGS. 6,7and8show the time response of the system with no filter unit L, with filter L1and with filter L2, respectively, for a lens movement of 100 and 190 Hz with respective amplitudes of 10 and 2 nm. In each ofFIGS. 6,7and8, the top window shows the servo error er, and the bottom window shows the lens tracking error eu.

FIG. 6shows that without a filter unit L, the substrate table error with respect to the lens (i.e., eu, bottom window) is about 12 nm, which matches the gain of the dotted line inFIG. 5at 100 Hz. It is also clear that the servo error erand the lens tracking error euare the same.

As can be seen inFIG. 7the servo error erreduces to zero, as expected, when using a filter unit with filter L1. The lens tracking error reduces to about 6 nm, which matches the dashed line inFIG. 5at 100 Hz, as a gain of −6 dB corresponds to a decrease by a factor ½.

FIG. 8shows the result when a filter unit L2is used. Now, the lens tracking error euis close to zero, while the servo error erhas become approximately 6 nm.

From the results of the example it can be concluded that using a filter L for filtering the projection system position signal, may reduce both the servo error and the lens tracking error substantially. The choice of L determines which error is reduced the most. When selecting a filter L, L may for example be chosen to minimize the servo error, to minimize the lens tracking error, to minimize both servo error and lens tracking, for example the (weighted) sum of servo error lens tracking error, or any other suitable optimization criterium.

In the example two possible choices for the selection of L, namely L1and L2have been shown. L1and L2were selected to minimize servo error erand lens tracking error eu, respectively. The use of L1and L2also had a positive reducing effect on euand er, respectively. Other design methods for selecting a transfer function of the filter unit L may be used depending on the system requirements. In this respect it is remarked that the transfer er/u and/or eu/u only needs to approach zero for those frequencies that are actually present in the lens movements u. In the example presented here, this lens frequency spectrum was not yet taken into account. However, this lens frequency spectrum may be taken into account when an actual filter L is selected.

The use of a filter L according to embodiments of the invention is in particular suitable for providing a horizontal reaction movement of the substrate table in response to a horizontal movement of the projection system, such as the lens, in the same direction. However, the control scheme may also be used to make a substrate table response to a vertical movement of the projection system.

The control scheme comprising the filter L may also be used for optimizing the feed-forward of any other signal which is used in a lithographic apparatus. In particular the control scheme may be useful to make the response of a patterning device support on a change in a position of the projection system more accurately. Furthermore, the control scheme may particular successfully be applied in reducing the servo error and/or tracking error of a system, wherein it is desirable that a first component of a lithographic apparatus or more general a projection apparatus substantially follows, or corrects for, the movements (change of position) of a second component of the lithographic apparatus. For instance, the control scheme may also be used to control the position of the substrate table, wherein the control system is configured to react on movements of the patterning device support.

In view of the above, the features of the invention described in this application with respect to the control system for controlling the position of the substrate table may also be applied for controlling the position of a patterning device support, or in general the position of the first component.