Lithographic apparatus, distortion determining method, and patterning device

The invention relates to a lithographic apparatus arranged to transfer a pattern from a patterning device onto a substrate, wherein apparatus is operable to measure higher-order distortions and/or image plane deviations of the patterning device, apparatus comprising: a device for transmission image detection; and a processor configured and arranged to model higher-order distortions of the patterning device using signals received from the device for transmission image detection; wherein patterning device has a main imaging field, and a perimeter and apparatus is operable to model higher-order distortions using signals resultant from alignment structures comprised in perimeter and/or in the imaging field.

BACKGROUND

1. Field of the Invention

The present invention relates to a lithographic apparatus, a patterning device for use with such a lithographic apparatus, and a method for determining higher-order distortions of a patterning device of a lithographic apparatus.

2. Description of the Related Art

The substrate (e.g. a wafer) is usually supported by a wafer stage. By moving the wafer stage, the substrate can be positioned relative to the reticle. In present lithographic apparatus, a device for transmission image detection is used in order to align a reticle to a wafer stage. Said device comprises a structure (e.g. a grating) on a reticle and a complementary structure on a transmission image detector plate of a transmission image detector. By transmitting a radiation beam through the structure on the reticle and detecting an image of said structure by the transmission image detector on the wafer stage the position and focus of the image can be determined. In practice, the transmission image detector comprises several structures. Beneath each structure a photodiode is located to detect the transmitted radiation beam. The transmission image detector is conventionally used to measure first-order positioning terms like translation, magnification and rotation of the reticle with respect to the wafer stage, so that the transmission image detector is able to measure first-order reticle distortions.

However, higher-order reticle distortions remain unresolved and any reticle distortion, including these higher-order reticle distortions, may lead to image distortions of the pattern on the substrate resulting in overlay errors and focus errors.

Overlay is an important factor in the yield, i.e. the percentage of correctly manufactured devices. Overlay is the accuracy within which layers are printed in relation to layers that have previously been formed. The overlay error budget will often be 10 nm or less, and to achieve such accuracy, the substrate must be aligned to the pattern on the reticle with great accuracy.

Reticle distortions may have many different causes. To name a few, distortions may arise as a consequence of:reticle writing errors;reticle heating;holding the reticle in a reticle support (clamping effects and gravitational sag);reticle fabrication;assembling the reticle with other devices, such as a so-called pellicle.

A pellicle is a device that prevents dust particles from landing on the reticle as it is easier and less risky to replace a “dirty” pellicle than to clean a “dirty” reticle, and keeps dust particles at an out-of-focus distance from the reticle, so that their influence on the pattern to be imaged on a wafer is minimal. A pellicle usually comprises a pellicle membrane which is transparent to the radiation beam, a pellicle frame, and an adhesive to attach the pellicle to the reticle. The pellicle is attached to the reticle such that it covers the pattern on the reticle and shields that part of the reticle from the rest of the lithographic apparatus.

Because of the rigid nature of the pellicle frame, the reticle will experience mechanical distortions upon placement of the pellicle, resulting in higher-order image distortion. Furthermore, when the reticle is subsequently placed, i.e. chucked, on a reticle support to hold the reticle, this distortion becomes worse by gravitational sag and chucking stress.

During the fabrication of a reticle several distortions may be introduced, which will be elucidated here. When a reticle blank or substrate is fabricated, it is not perfectly flat, but it will have a concave shape, a convex shape, or an even more complex shape. This non-flat shape will introduce distortions to the image. The reticle blank is further coated with one or more (absorber) layers and a resist. The stress in these layers may have an impact on the shape of the reticle. Subsequently, an image of the pattern which has to be transferred to the reticle is transferred to the resist on the reticle by a reticle writing tool. This can be done by means of laser radiation or charged particle (e.g. electron) radiation. Irradiation of the reticle can result in local heating effects and distort the reticle. The use of the charged particle radiation (e.g. an e-beam tool) can result in charging effects and distort the image during writing. Subsequently, the resist is processed and this processing can introduce further distortions of the image in the resist. The image in the resist is then transferred into the underlying layer (and if required into the substrate). The underlying (absorber) layer or layers (and if required the substrate) are locally removed or etched. This can introduce distortion of the image in the underlying layer or layer(and if required the substrate). Removal of material in the layer or layers (and if required the substrate) can result in relaxation of stress that was build up earlier by the deposition of the (absorber) layer(s) and the resist. As a consequence, the reticle can experience distortion of the image.

When irradiating a reticle, some areas will absorb the radiation, and other parts will transmit the radiation, thereby forming a patterned radiation beam. However, absorbing the radiation will result in a local increase in temperature, thereby introducing distortions to the image field.

The pellicle induced distortions, reticle fabrication induced distortions and the chucking induced distortions have a static, or quasi-static nature, i.e. there magnitude is relatively constant over time. As a pellicle may be replaced by another pellicle, the pellicle induced distortion may change abruptly due to this replacement.

The distortions induced by heating have a dynamic nature and are dependent amongst others on the pattern on the reticle. The more radiation that is absorbed by the pattern, the more the reticle will deform due to heating.

It has been shown that especially for high-throughput lithographic apparatus, the higher-order distortions have a large contribution in the overlay budget.

SUMMARY

It is desirable to provide a lithographic apparatus with reduced overlay errors resulting from higher-order distortions of the patterning device.

According to an aspect of the invention, there is provided a lithographic apparatus arranged to transfer a pattern from a patterning device onto a substrate, wherein said apparatus is operable to measure higher-order distortions and/or image plane deviations of the patterning device, said apparatus comprising:a device for transmission image detection; anda processor configured and arranged to model higher-order distortions of the patterning device using signals received from the device for transmission image detection; wherein said patterning device has a main imaging field, and a perimeter and said apparatus is operable to model said higher-order distortions using signals resultant from alignment structures comprised in said perimeter and/or in the imaging field.

According to an embodiment of the invention, there is provided a lithographic apparatus arranged to transfer a pattern from a patterning device onto a substrate, wherein said apparatus is operable to measure higher-order distortions and/or image plane deviations of the patterning device, said apparatus comprising:a device for transmission image detection; anda processor configured and arranged to model higher-order distortions of the patterning device using signals received from the device for transmission image detection;
wherein said patterning device has a main imaging field, and a perimeter and said apparatus is operable to model said higher-order distortions using signals resultant from alignment structures comprised in at least three sides of said perimeter and/or in the imaging field.

According to another aspect of the invention, there is provided a method of determining higher-order distortions of a patterning device of a lithographic apparatus, said method comprising:imparting a radiation beam with a pattern in its cross-section to form a patterned radiation beam using a patterning device, said patterning device comprising a main imaging field, a perimeter and a plurality of alignment structures;detecting transmission of radiation transmitted through said alignment structures of said patterning device and into a device for transmission image detection,producing measurement signals from the detected radiation, anddetermining higher-order distortions and/or image plane deviations of said patterning device using measurement signals resultant from radiation transmitted through alignment structures comprised in said perimeter and/or in said imaging field.

According to an embodiment of the invention there is provided a method of determining higher-order distortions of a patterning device of a lithographic apparatus, said method comprising: creating a radiation beam; imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam using a patterning device, said patterning device comprising a main imaging field, a perimeter and a plurality of alignment structures; detecting transmission of radiation transmitted through said alignment structures of said patterning device and into a device for transmission image detection, producing measurement signals from the detected radiation, and determining higher-order distortions and/or image plane deviations of said patterning device using measurements signals resultant from radiation transmitted through alignment structures comprised in at least three sides of said perimeter and/or image field.

According to a further aspect of the invention, there is provided a patterning device for use in a lithographic apparatus, said patterning device having a main imaging field, and a perimeter wherein said patterning device is provided with additional alignment structures for the improved measurement of distortion and/or field plane deviations of said patterning device.

According to yet another aspect of the invention, there is provided an alignment apparatus comprising a patterning device for use in a lithographic apparatus, said patterning device having a main imaging field, and a perimeter wherein said patterning device is provided with additional alignment structures for the improved measurement of distortion and/or field plane deviations of said patterning device.

Other features of the invention are as described in the appended claims.

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.

In an ideal situation, the mask MA would be perfectly flat without showing any distortions to ensure that the pattern comprised in the mask MA is transferred correctly to the target portion on the substrate. However, in practice, the mask MA will show distortions which may result from:mask writing errors;mask heating due to absorption of radiation;the mask support MT holding the mask;mask fabrication; andassembling the mask MA with other devices, such as pellicles.The distortions of the mask MA result in a distorted pattern transfer from mask to the substrate, i.e. overlay errors and focus errors. The distortions of the mask can be measured using the alignment marks M1, M2and additional alignment marks M3-M10. InFIG. 1, the alignment marks M1-M10are provided in two columns along the Y-direction, thereby allowing the determination of higher-order (≧2) distortions of the mask MA in the Y direction and first-order distortions in the X direction using a device for transmission image detection (not shown). The information obtained about the distortions can subsequently be used to compensate for the distortions and improve the overlay and focus of the apparatus.

FIG. 2depicts schematically a device for transmission image detection, also referred to as a transmission image detector or transmission image sensor. A transmission image detector per se is known from the prior art. The radiation beam B is incident on a first object G0for example a grating in the mask MA. The first grating G0comprises a plurality of openings arranged for creating an image from the radiation beam B. The openings in the first grating G0each emit a radiation beam RB originating from the radiation beam B. The radiation beams RB emitted by the plurality of openings in G0, pass through a lens for example, the projection lens system PS. The optical properties of such projection lens system are such that an image of G0, G0′, is formed at a given plane below the projection lens system PS. The transmission image detector TD is positioned below the projection lens system. The transmission image detector TD comprises a slot pattern G1and a photo sensor PH device. The slot pattern G1is an opening over the photo sensor PH device which has the shape of a slit or a square. Advantageously, applying a pattern on the opening over the photo sensor PH device increases the number of edges which may increase the signal level and thus the signal/noise ratio of the photo sensor PH.

The transmission detector TD is arranged on the substrate table WT, seeFIG. 1. The transmission image detector TD allows accurate positioning of the wafer relative to the position of the projection lens system PS and the mask MA in three orthogonal directions X, Y, Z. By scanning along these three directions the intensity of the image G0′ can be mapped as a function of the XYZ position of the transmission image detector, for example in an image map (a 3D map), which comprises the coordinates of sampling locations and the intensity sampled at each location. From the 3D map, a computer or processor connected to the transmission image detector TD can derive the position of the image by using for example a parabolic fit of the top position using a least squares fitting method.

One issue with the above apparatus, in use, which should be taken into account, is that higher-order distortions of the mask MA cannot be measured with only two measurement positions in the field.

FIG. 3shows an example of a transmission image detector29according to the state of the art with 4 gratings30,31,32,33in a litho layer34. The litho layer23is manufactured on a quartz window35. Beneath each of the gratings30,31,32,33an associated photodiode36,37,38,39is provided. Using only4measurement positions in the corners of the image field as is standard, means that the transmission image detector29can only measure zero- and fist-order terms like translation, magnification, rotation, focus and focus tilt of the reticle with respect to the wafer stage and thus only first-order reticle distortions. Higher-order distortions, defined as some or all distortions of some or all polynomial orders above 1 (quadratic, cubic, etc.), generally remain undetected because these generally require more independent measurement positions in or around the image field. Such higher-order distortions may arise as a consequence of reticle heating and/or lens heating which is a dynamic distortion and/or (quasi-)static distortions such as mechanical induced distortions and may increase the overlay errors.

Instead of measuring just a small number of measurement positions in the corners of the image field the transmission image detector may be used to measure the actual reticle deformation locally to reduce the overlay errors. Dependent on the required accuracy of the measured overlay errors, the number of measurement positions to measure the actual reticle deformation locally, with a transmission image detector, may be adjusted. The locally measured values can be put in a matrix and by using a feed-forward model, the deformation behavior of a reticle, including higher-order distortions, can be predicted so that smaller overlay errors can be obtained. This feed-forward model is an extension to the known ‘basis’ Reticle Align (RA) model with additional XY inputs. By using more measured independent inputs, this model is less sensitive to model assumptions than the known ‘basis’ Reticle Align (RA) model.

The model may comprise a third order model with dX and dY terms as a function of X and Y, which means that 20 parameters are needed for a full decomposition, which in turn requires 20 independent measurements to avoid an undetermined system. In practice, some parameters (or terms) can be skipped or modeled from other parameters, reducing the required input measurements. However, it is still preferable to have dX, dY measurements both along the top/bottom (function of X) and left/right (function of Y) sides.

In practice a third-order model is most practicable due to the correction potential of the machine (and the number of available independent input measurements, and noise propagation considerations), but this is not fundamental and in some cases (e.g. when the load is highly asymmetric) or in the future, it may indeed be worthwhile to include some fourth-order terms or higher.

When the model is extended with additional XY inputs, the additional gratings to be measured at reticle level may be surrounded by very small chrome borders (e.g. as in scribe lanes) or even no chrome borders at all. In these cases spurious effects due to the limited or missing chrome borders for the intra-field gratings may occur and these effects should be suppressed. In these cases it can be advantageous to only use standard RA gratings for the actual first reticle align, while measuring and storing/calibrating the intra-field gratings for subsequent delta measurements/corrections. In such a scenario, a lower order correction model, using only the standard RA gratings, would be used for the first reticle align, while for subsequent wafers a relative higher-order correction model can be applied, using the standard RA gratings measurements in an absolute manner plus the additional XY grating measurements relative to the first reticle align.

FIG. 4shows a reticle that may be used in conjunction with a transmission image detector in an apparatus such as shown inFIG. 1. The standard reticle has an image field400and a perimeter410. The perimeter410is provided with four sets of x and y gratings420, one in each corner, with further x and y gratings420along the top and bottom and a number of single direction gratings430along the sides. Conventionally the corner gratings are used for basic alignment, while the other gratings are used in reticle shape correction for determining image plane deviation.

According to a first embodiment of the invention, the reticle ofFIG. 4can be used in this standard form with the apparatus depicted inFIGS. 1 and 2, and using the extended feed forward model described above, for the calculation of local reticle deformation. In particular, the use of gratings on all four sides (as opposed to just the top and bottom) of the reticle means that higher-order distortions, particularly in terms of y, can be calculated.

FIG. 5shows an enhanced reticle. One drawback with the reticle ofFIG. 4is that the gratings found along the left and right hand sides are “y-gratings” which are designed to perform measurements along the y-axis. While this is useful, measurement along the x-axis would be better in case of a distortion that tends to distort more in the middle of the sides as opposed to the corners (barrel distortion), such as heating of the mask MA. Consequently, another embodiment uses a reticle, as shown inFIG. 5, with additional “x-gratings”500along each side, as well as along the top and bottom. This allows for a more extensive modeling of higher-order terms in x.

FIG. 6shows a further enhanced reticle having a matrix of gratings600which is provided in the image field itself. So as not to interfere with the image, it is proposed that the best place to locate these image field gratings is in the scribe lanes610, as shown. This enhanced reticle allows for the production of a denser matrix to allow for more accurate local corrections. When using this reticle, not only a more detailed third-order model can be achieved, but also higher-order modeling of dx and dy, as a function of both x and y, with a polynomial order depending on the overlay issue and/or correction potential, including x-y cross terms, is achievable.

FIG. 7shows an enhanced reticle with a matrix of gratings600in the image field scribe lanes610, and further gratings700in the image field410outside of the scribe lanes. This is possible particularly where the image is known, in which case the gratings can be placed anywhere in the image field that does not interfere with the image itself.

FIG. 8shows an enhanced reticle with a matrix of grating800outside of the scribe lanes allowing third-order corrections in X-direction and Y-direction.

The scan should preferably be done during regular transmission image detector alignment to minimize the impact on throughput. In each case, the transmission image detector, as already described, comprises four gratings arranged on a litho layer (it is within the scope of the invention to use an improved transmission image detector, with an increased number of gratings, to increase throughput). The gratings are positioned so as to receive an image produced by the standard RA gratings on the reticle placed on the mask table MT of the lithographic apparatus, seeFIG. 1(alignment marks M1 to M10). The transmission image detector further comprises radiation sensitive sensors arranged to receive radiation coming through one of the gratings and to produce a measurement signal. The litho layer may for example be a chrome layer patterned with a plurality of gratings arranged in a row. The measurement signals are input to a processing device which is arranged to determine higher-order distortions of the projection system (i.e. the lens) and/or of the patterning device. These distortions can be used to adjust the components of the lithographic apparatus, but they can also be used to improve alignment of the substrate table relative to the patterning device.

While the above techniques have described the use of alignment gratings, the skilled person will appreciate that any alignment marks, patterns or structures may be used without departing from the scope of the invention. Furthermore the number and arrangement of the alignment marks and scribe lanes and the general arrangement of the reticles may differ significantly from the purely illustrative examples shown. Also by adding more chrome around the gratings to reduce stray light, accuracy can be improved.

It should be noted that any type of image sensor or image detector may be used, and is not necessarily be limited to the transmission image detector as described. For example a wavefront detector or interferometer may be used. Also, improvements can be made to the transmission image detector such as improving the fitting algorithms to better deal with a varying signal environment.

Note that the above description considers only the overlay terms. However, as the transmission image detector also yields focus results every scan, the disclosed concept can also be applied as an extension of reticle shape correction. This extension can be in the spatial domain by adding additional focus measurement points, or in the temporal domain by tracking changes in reticle shape correction over time (e.g. due to reticle heating).

Furthermore the above techniques can also be used to measure image plane deviations caused by the inadequate flatness of the wafer table.

It should be noted that the above techniques do not have to be performed for every wafer, as this would result in a throughput penalty that might be too great to be practical. Instead these measurements can be performed only once for a predetermined number of wafers, the number depending on the error range of the calculating/modeling software, which increases between recalibration. Alternatively, these measurements could be done off-line. In each case the results can be stored for further usage.

To summarize the invention, information of the distortions of a reticle can be obtained by measuring the relative position of alignment structures on the reticle, preferably using a device for transmission image detection. A processor can then be used to model the distortions based on the measurement signals of the alignment structures. Depending on the amount of alignment structures and the desired order of modeling the distortions, a full order model can be obtained or a partial order model can be obtained. As an example, for a third order model in both x and y, 20 independent measurement results have to be available from the alignment structures.

The model can be adjusted to match the compensation potential of the lithographic apparatus. In this way, the amount of alignment structures is optimized for the model that can be used to compensate for the distortions. In principle, the less amount of alignment structures, the less chance of throughput penalty.

The model can also be obtained by using less alignment marks than necessary for directly deriving the model from the alignment marks and combining the measurement results from the alignment marks with a physical model of the distortion or in case of a (quasi-)static distortion results from an external measurement.

The reticle can also be measured once externally to obtain the required model, and subsequently, the changes in the model can be derived from a limited set of alignment marks. This technique can be used for both dynamic and static distortions.

Another way of reducing throughput penalty is to measure the alignment marks parallel. This can be achieved by properly placing the alignment marks so that full use can be made of the device for transmission image detection and/or by increasing the number of devices for transmission detection.