Apparatus and method for bearing a lithography mask

An apparatus for bearing a lithography mask with a reticle stage includes a resting support holder for the lithography mask. The resting support holder has bearing points which bear the lithography mask. Optionally, the resting support holder optionally has exactly four bearing points. An associated method adjusts the height of the fourth bearing point until all bearing points bear the same supporting force.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 U.S.C. § 119 of German Patent Application Number 10 2015 015 423.0, filed Nov. 27, 2015, the entire contents of which are incorporated by reference herein.

FIELD

The disclosure relates to an apparatus for bearing a lithography mask, including a reticle stage with a resting support holder for the lithography mask, wherein the resting support holder has bearing points which bear the lithography mask, as well as a related method.

BACKGROUND

So-called microlithographic projection exposure apparatuses are usually used for producing microstructured components such as e.g. integrated circuits or LCDs. In these apparatuses, the image of a lithography mask (=reticle) is projected onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) via a projection lens in order to transfer the mask structures onto the light-sensitive coating of the substrate. In the process, the lithography mask is displaced laterally with the aid of a so-called reticle stage in order also to successively image relatively large structures. By way of subsequent developing, etching and doping process, cost-effective large quantities of microstructured components may be achieved in this manner.

Here, the quality of the lithography mask plays a decisive role since an error in the mask structure propagates into the components emerging therefrom.

Therefore, complicated measurement apparatuses were developed, by which the mask structures may be measured after and during the production process of the lithography masks.

SUMMARY

Bearing the lithography masks both in the measurement apparatuses and in the projection exposure apparatuses becomes ever more important on account of the advancing miniaturization of the structure dimensions in the field of microlithography. This is because the bearing of the lithography mask determines, for example, the flexing thereof due to its inherent weight or the behavior thereof in view of disturbing vibrations.

Because three points always span a plane, lithography masks were previously borne by way of a three-point bearing. This helped prevent wobbling of the lithography mask. However, such a three-point bearing can have, in part, an inadequate behavior in relation to disturbing vibrations.

The disclosure seeks to provide an improved mount, in particular with a higher rigidity, for the lithography mask.

In one general aspect of the disclosure, an apparatus includes a resting support holder has four, in particular exactly four, bearing points which are arranged so that all bearing points bear the same supporting force.

The inventors have recognized that the behavior of the lithography mask in relation to disturbing vibrations is improved if the lithography mask is borne in such a way that the frequency of the first eigenmode of the lithography mask lies above the frequency of the first structure mode of the reticle stage. In general, in currently conventional reticle stages, the frequency of the first structure mode of the reticle stage lies at approximately 800 Hz.

The inventors have further recognized that a frequency of above approximately 800 Hz for the first eigenmode of the lithography mask is not reached in the case of the customary lithography masks with the known three-point bearing.

Therefore, the inventors developed an apparatus in which four bearing points bear the lithography mask such that the frequency of the first eigenmode of the lithography mask lies above the frequency of the first structure mode of the reticle stage.

Lithography masks typically are manufactured plane with an uncertainty of <1 μm. Flexing on account of gravity is also of this order of magnitude. Therefore, the bearing points should be matched in relation to the lithography mask in such a way that the supporting force, which may primarily be traced back to the gravitational force acting on the lithography mask, is approximately the same on the four bearing points within the scope of these uncertainties. Here, the same supporting force should be understood to mean supporting forces which differ from one another by less than 5%.

To this end, at least one of the bearing points may be height adjustable by way of an actuator.

Because three points always describe a plane, a single height-adjustable bearing point already facilitates matching of the bearing points in order to distribute the supporting force equally.

Even though very different actuators are conceivable, the actuator preferably has a piezo element with an adjustment range of up to 5 μm, in particular up to 20 μm, in particular up to 50 μm. Such piezo actuators are known and offer a sufficient accuracy. Thus, the resolutions of piezo actuators are a few nanometers, e.g. 10 nm, with adjustment dynamics of 1:2000.

Here, the apparatus may have an actuator measurement system, by which the movement of the actuator is captured in order to linearize the actuation thereof. In this manner, it is possible to compensate nonlinear properties, in particular of piezo actuators.

The apparatus may further include a proximity measuring device including a vibration device, by which a vibration may be superimposed onto the movement of the bearing point in the case of a height adjustment, and a capturing device, by which a power consumption of the actuator may be captured. In the case of such an apparatus, it is possible to superimpose a relatively small vibration movement onto the movement when driving up the height-adjustable bearing point from a position without a supporting force. There is a change in the vibration movement if the bearing point then approaches the lithography mask on account of the coupling to the lithography mask. This has an effect on the power consumption or the driver voltage of the actuator, which is captured with the aid of the capturing device, in order to determine the height with an equally distributed supporting force.

The apparatus may include a frequency response measuring device which may capture a transfer function of the reticle stage including the borne lithography mask as a response to a short stroke excitation.

In the three states—support on three fixed bearing points; support on four bearing points; support on two fixed bearing points and one height-adjustable bearing point—the lithography mask exhibits different eigenmodes which have an effect up into the transfer function of the mirror block of the reticle stage.

Therefore, it is possible to identify an occurrence or a breakdown of these eigenmodes when the reticle stage with the lithography mask borne therein is excited by way of a short stroke excitation and the transfer function is determined. In particular, this allows identification as to whether the frequency of the first eigenmode of the lithography mask lies above the frequency of the first structure mode of the mirror block.

Preferably, the frequency response measuring device includes at least one interferometer which cooperates with a mirror of a mirror block of the reticle stage. Because the reticle stage involves a mirror block, the mirrors of which cooperate with corresponding interferometers, for positioning the lithography mask, the components of a frequency response measuring device are already present.

The bearing points preferably have ruby spheres which are in direct contact with the lithography mask. As a result, a high rigidity of the bearing is achieved.

A method according to the disclosure for bearing a lithography mask in a reticle stage includes the following steps:a) providing four, in particular exactly four, bearing points;b) placing the lithography mask onto three of the four bearing points;c) adjusting the height of the fourth bearing point until all bearing points bear the same supporting force.

Preferably, step b) includes the following steps in this case:a) adjusting the height of the fourth bearing point with an actuator, in particular with a piezo actuator, into a position in which this bearing point bears no supporting force;b) bringing the fourth bearing point closer to the lithography mask, with a vibration movement, in particular in and counter to the direction of approach, being superimposed onto the actuator;c) capturing a power consumption of the actuator during the approach;d) determining the height of the fourth bearing point at which the vibration movement couples to the lithography mask, from the power consumption of the actuator.

In this way, it is possible to ascertain the height of the height-adjustable bearing point at which the supporting forces are equally distributed among the four bearing points. Step b) may also include the following steps for the same purpose:a) adjusting the height of the fourth bearing point with an actuator, in particular with a piezo actuator, into a position in which this bearing point bears no supporting force;b) bringing the fourth bearing point closer to the lithography mask step-by-step, with a transfer function of the reticle stage being measured between two steps as a response to a short stroke excitation;c) determining the height of the fourth bearing point at which the transfer function exhibits an eigenmode of a four-point-bearing with an equally distributed supporting force.

Because, with the mirror block of the reticle stage and the interferometers cooperating therewith, components of a frequency response measurement system, by which the transfer function may be determined, are already present, there is no need for the provision of any additional measurement systems. Moreover, the frequency behavior which is also decisive for the behavior in relation to disturbing vibrations, is used directly in this procedure for determining the ideal height of the fourth bearing point.

DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS

FIG. 1shows a mask examination apparatus, denoted by10overall, in which a microlithographic lithography mask12is arranged on a holding apparatus14for checking purposes.

The mask examination apparatus10includes an EUV light source16which generates an EUV light beam18used to examine the lithography mask12.

Various beamforming elements, such as a stop20and concave mirrors22,24and26are used to form the beam of the EUV light beam18and to direct the EUV light beam18to the lithography mask12, which is held upside down inFIG. 1.

The holding apparatus14includes a reticle stage28to displace the lithography mask12along two scanning directions (from left to right and perpendicular to the plane of the drawing inFIG. 1) in order to laterally displace the lithography mask12in respect of the point of incidence of the EUV light beam18. Moreover, the reticle stage28facilitates a height adjustment perpendicular to the two scanning directions.

A mirror block30is arranged on the movable part of the reticle stage28, the mirror block in this case having obliquely arranged mirrors32,34on the sides, the mirrors cooperating with of laser interferometers36,38in order to determine the position of the movable part of the reticle stage28. In a real system, the mirror block30has further mirrors (not shown here), which cooperate with further laser interferometers in order to measure all movement directions.

The laser interferometers36,38are connected to a frequency response measurement system39such that it is possible to determine the transfer function of the movable part of the reticle stage28, including the lithography mask12, from the movements of the mirror block30after a short stroke excitation, i.e. after a short impulse excitation of the movable part of the reticle stage28, for example with the aid of the reticle stage actuators.

In order to hold the lithography mask12, the reticle stage28includes a resting support holder40, elements of which are shown inFIG. 2and explained in more detail below. The EUV light42reflected downward by the lithography mask12is directed to an EUV CCD camera46via a mirror arrangement44in the further course of the beam path. The EUV CCD camera is used to capture the state of the lithography mask12.

Because the structures to be measured on the lithography mask12are very small, it is desirable to ensure that possible disturbing vibrations influence the measurement as little as possible. In particular, the modal behavior of the lithography mask12is set by way of the configuration of the resting support holder40in such a way that the first eigenmode lies at a frequency that is as high as possible. In particular, this frequency should lie above the first structure mode of the reticle stage28.

FIG. 2shows elements of the resting support holder40.

The resting support holder40has four carriers50a-dwhich protrude inwardly from a frame (not shown here) of the reticle stage28. Supporting tips52a-dare respectively provided on the carriers50a-das bearing points for the lithography mask12, wherein the supporting tips should have a rigidity that is as high as possible and are depicted as cones in this case. At the upper end thereof, the supporting tips52a-deach carry a ruby sphere54a-d, on which the lithography mask12then is borne.

The supporting tip52dincludes a piezo actuator56, known per se, which may be used to adjust the height of the supporting tip52d. Here, the piezo actuator56is connected in parallel with a known measurement system (not depicted in any more detail here) for linearizing the movement control.

For actuation purposes, the piezo actuator56is connected to a vibration device57, which can be used to superimpose a vibration onto the movement of the piezo actuator56during a height adjustment. For the purposes of capturing the power consumption of the piezo actuator56, a capturing device59is still interposed between the piezo actuator and the vibration device57.

An overall rigidity emerges for the height-adjustable supporting tip52dfrom the rigidity of the piezo material, the rigidity of the transition to the ruby sphere54a-dand the rigidity of the ruby sphere54a-d. Therefore, a rigidity of the transition which is as high as possible should be sought after by appropriate forming (Hertzian contact).

As may be gathered fromFIG. 3, the four supporting tips52a-dare distributed laterally with the ruby spheres54a-dthereof in such a way that the lithography mask12is borne at four supporting points60a-dwhich are arranged at the center of the sides of the lithography mask12, which is a square in this case. Here, the support is such that all four bearing points support the same supporting force.

Alternatively, the supporting points60a-60cmay be arranged in a ratio of 2:1 in accordance withFIG. 4. The fourth supporting point60d, on which the height-adjustable supporting tip52drests, is arranged slightly outside of the center line of the lithography mask12here in order to avoid symmetry with the three-point bearing.

In order to set the height of the supporting tip52dsuch that all four supporting tips52a-dsupport the same supporting force, it is possible, for example, to use the two procedures explained below in an alternative or complementary manner.

Determining the Height of the Same Supporting Force Via a Proximity Measuring Device

First, the height-adjustable supporting tip52dis withdrawn with the aid of the piezo actuator56to such an extent that the supporting tip certainly does not abut against the lithography mask12. Then, the piezo actuator56is made to vibrate with the aid of the vibration device57while being driven upward. In the process, the power consumption or the driver voltage of the piezo actuator56is monitored with the aid of the capturing device59. If the supporting tip52dapproaches the lithography mask12, the vibration couples with the lithography mask12. This is identified by the capturing device59, and so it is thereby possible to ascertain the ideal height for an equal distribution of the supporting force.

Determining the Height of the Same Supporting Force Via a Frequency Response Measuring Device:

Here too, the height-adjustable supporting tip52dis withdrawn with the aid of the piezo actuator56to such an extent that the supporting tip certainly does not abut against the lithography mask12. This state corresponds to a three-point support.

The transfer function depicted in the Bode diagram ofFIG. 5is determined with the aid of the frequency response measuring device39, the interferometers36,38and the mirrors32,34of the mirror block30.

As may be identified therein, the frequency of the first eigenmode72of the lithography mask12lies below the frequency of the first structure mode70of the mirror block30in this case.

Now, the supporting tip52dis made to approach the lithography mask12step-by-step and the transfer function is determined in each case.

When the height is reached at which the supporting force at all four supporting tips52a-dis approximately the same (four-point support), the transfer function shown in the Bode diagram ofFIG. 6is obtained.

As may be identified therein, the frequency of the first eigenmode74of the lithography mask12lies above the frequency of the first structure mode70of the mirror block30in this case.

A lithography mask12borne thus has a higher rigidity, and so the behavior in relation to disturbances is improved because only relatively high disturbing vibrations, which usually have a smaller amplitude, lead to resonances in the system.