Position measurement system and lithographic apparatus

A measurement system includes a sensor arranged to co-operate with a first pattern arranged on a structure of the measurement system to determine a first position quantity of the sensor relative to the structure, and arranged to co-operate with a second pattern arranged on the structure to determine a second position quantity of the sensor relative to the structure, wherein the first and second patterns are arranged on different surfaces of the structure.

FIELD

The present invention relates to a position measurement system and a lithographic apparatus.

BACKGROUND

In order to project the pattern of the patterning device on the appropriate position on the substrate, accurate knowledge of the instantaneous position of the patterning device and the substrate is desired. In order to obtain this information, the lithographic apparatus is provided with one or more position measurement systems. The most commonly used position measurement systems include interferometer based measurement systems and encoder based measurement systems. The latter generally includes a sensor arranged to co-operate with a grating or pattern enabling the relative position of the sensor and the grating to be determined. In general, such an encoder based measurement system may be used for determining the position of a stage apparatus in a lithographic apparatus. In principle, such an encoder based measurement system provides information regarding a relative displacement of the sensor relative to the pattern when the sensor is moved from one position to an other. As an example, the pattern may be a periodic pattern of lines arranged adjacent to each other along the displacement direction, during the displacement, the sensor counts the number of lines that have passed (note that the individual lines of the pattern, in general, will extend in a direction substantially perpendicular to the displacement direction). Such encoder based measurement systems have been expanded to provide additional information regarding the position of the sensor relative to the grating. Examples are the use of a two-dimensional grating (e.g. including a checkerboard pattern) to provide positional information in two dimensions. Other configurations of the basic encoder measurement system as described above may include a second pattern in order to enable an absolute position measurement. As an example, the pattern may include a reference mark at some position, detection of this mark by the sensor enables a reference position to be set by the sensor.

In case two positional quantities are measured, the grating design tends to become complex, e.g. a checkerboard pattern rather than a pattern of lines, or the inclusion of a reference mark at a specific position within a pattern of lines. In the latter case, the accuracy of the linear grating may be affected by the introduction of e.g. a reference mark. As such, the manufacturing of the grating or pattern may be difficult.

SUMMARY

It is desirable to provide a measurement system of which the grating or pattern is easier to manufacture.

According to an aspect of the present invention, there is provided a measurement system for measuring a position of an object including a sensor arranged to co-operate with a first pattern to determine a first position quantity of the object and with a second pattern to determine a second position quantity of the object, wherein the first pattern and the second pattern are mounted on different surfaces of a structure.

According to an other aspect of the invention, there is provided a lithographic apparatus including an illumination system configured to condition a radiation beam; a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate, the apparatus further includes a measurement system for measuring a position of the support and/or the substrate table, the measurement system including a sensor arranged to co-operate with a first pattern arranged on a structure of the measurement system for determining a first position quantity of the support and/or the substrate table and with a second pattern arranged on the structure for determining a second position quantity of the support and/or the substrate table, wherein the first pattern and the second pattern are mounted on different surfaces of the structure.

In embodiments of the present invention, a position quantity may represent, for example, an absolute position reference, a distance traveled, a displacement between two positions, . . . etc.

DETAILED DESCRIPTION

The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the patterning device support (e.g., mask table) MT, and is patterned by the patterning device. Having traversed the patterning device (e.g. mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor MS (e.g. an interferometric device, linear or planar (2D) encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted inFIG. 1) can be used to accurately position the patterning device (e.g. mask) MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan.

In an embodiment of the present invention, either the position sensor MS and/or the position sensor arranged to measure the position of the patterning device (e.g. mask) MA includes a measurement system configured to measure a position of an object (i.e. the mask MA or the substrate W) including a sensor arranged to co-operate with a first pattern to determine a first position quantity of the object and with a second pattern to determine a second position quantity of the object, wherein the first pattern and the second pattern are mounted on different surfaces of a structure. Such a measurement system provides the benefit that the structure including the patterns is easier to manufacture compared to pattern structures (or gratings) of conventional measurement systems. In general, movement of the patterning device support (e.g. mask table) MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the patterning device support (e.g. mask table) MT may be connected to a short-stroke actuator only, or may be fixed. Patterning device (e.g. mask) MA and substrate W may be aligned using mask alignment marks M1, M2and substrate alignment marks P1, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device (e.g. mask) MA, the mask alignment marks may be located between the dies.

FIG. 2schematically depicts a XZ-view of a position measurement system as known in the art together with an XY-view of the grating of the measurement system. The system includes a grating100on a structure110and a sensor120arranged the to assess or determine a position quantity of the sensor relative to the grating, in the arrangement as shown, an X-position of the sensor relative to the grating. In the arrangement as shown, the sensor is arranged to project a radiation beam130onto the grating100and includes, in the example as shown, two detectors140,150for receiving the reflected beams160,170of the grating. Reflected beams160,170may e.g. be the +1 and −1 order (or wave-form) of the reflected beam. In the arrangement as shown, the measurement system constitutes an incremental measurement system, i.e. it may not provide information on the absolute position of either the sensor or the grating.

FIG. 3schematically depicts a second position measurement system as known in the art. The figure shows a 2D pattern (or grating)200provided on a structure (not shown), comparable to a checker board pattern. The arrangement as shown enables the position measurement of the sensor210relative to the grating200in both X-direction and Y-direction. This may be achieved by detecting a reflected beam (not shown) originating from the sensor210towards the detector elements220of the sensor210. As will be appreciated by the skilled person, the manufacturing of the checker board pattern may be more complicated than the manufacturing of a linear grating as e.g. shown inFIG. 2(XY-view of grating100). In order to benefit from both the possibility of obtaining two different position quantities, while maintaining the manufacturability of the linear grating, it is proposed to provide two patterns co-operating with a sensor on different surfaces of a structure.

FIG. 4schematically depicts an example of such a structure in accordance with an embodiment of the invention. The structure300as shown includes a first pattern310on a first surface320of the structure and a second pattern330on a second, different surface340of the structure. The first pattern310may e.g. be a grating including of a plurality of lines, the grating (pattern) extending in a first direction (X-direction), the second pattern330may e.g. be a similar grating but extending in a second direction (Y-direction). Note that the individual lines of the gratings actually extend in a direction perpendicular to the direction of the grating, i.e. the individual lines of the grating310actually extend in the Y-direction, whereas the grating as a whole extends in the X-direction. It should however be noted that in general, that the first pattern may not extend in a direction that is perpendicular to the direction in which the second pattern extends. In addition, it may be noted that it is not required that the surfaces on which the patterns are mounted are parallel to each other.

The structure as shown inFIG. 4may be applied in an embodiment of the present invention as shown inFIG. 5.FIG. 5schematically depicts a structure400similar to the structure as shown inFIG. 4, the structure including a first pattern (or grating)410(the pattern extending in the Y-direction) mounted to a top surface of the structure, and a second pattern (or grating)420(the pattern extending in the X-direction) mounted to a bottom surface of the structure.FIG. 5further shows a radiation beam440originating from a sensor (not shown) directed towards the bottom surface of the structure. Upon incidence, the beam reflects on the grating420, the reflection being depicted by the reflection beams450and460which e.g. may be the +1 and −1 order of the radiation beam440. As the lines of the grating420extend in the Y-direction, the +1 and −1 order reflect in a plane perpendicular to the Y-direction, i.e. the XZ-plane. The reflected beams450and460may further be captured by detector elements (not shown) of the sensor co-operating with the grating. The non-reflected part of the radiation beam440propagates through the structure and incidents the grating410. Similar to the incident with the grating420, the radiation beam is reflected and a +1 and a −1 order of the reflected beam (470,480) are captured by detector elements. As the lines of the grating410extend in the X-direction, the reflected beams may be found in a plane perpendicular to the X-direction, the YZ-plane. Such an arrangement may e.g. be applied in order to determine a first and second position quantity of an object. As an example, such an arrangement may e.g. be applied in a lithographic apparatus for determining the X- and Y-position of a substrate table provided with a substrate. As such, the structure including the gratings may e.g. be mounted to a reference frame or reference part (e.g. a projection lens) of the lithographic apparatus, whereas the co-operating sensor is mounted to the substrate table or to a positioning device that controls and positions the substrate table.

Embodiments of the present invention are not limited to embodiments wherein the first and second position quantities relate to information with respect to different directions. To illustrate this,FIG. 6schematically depicts an XZ-view of an embodiment of the present invention including a sensor500arranged to co-operate with a first grating510mounted to a structure520and with a second grating530mounted to a different surface of the structure. Both gratings extend in the X-directions and include a plurality of lines extending in the Y-direction (perpendicular to the XZ-plane). As can be seen, the two gratings510and530differ in that they have a different pitch (indicated by ‘d’ and ‘D’ inFIG. 6.FIG. 6further shows the incident beam550origination from the sensor500and the reflected beams from the surface including the first grating and the surface including the second grating, towards detector elements560and570of the sensor. It can be shown that the arrangement as depicted allows for an absolute position measurement of the sensor relative to the structure, over a period (in the X-direction) equal to D/(D-d). Within this period, each position is characterized by a unique combination of signals originating from both gratings.FIG. 7schematically depicts a 3 dimensional (3D) view of such an arrangement. The figure schematically shows a structure600including a first grating610extending in the X-direction and a second grating620, also extending in the X-direction, the second grating having a different pitch (indicated by the larger distance between the different lines of the grating). A radiation beam that is incident on the structure may partly reflect on the first grating, and partly propagate through the structure to the second grating. It will be appreciated that in this case, the reflected beams from both gratings may be arranged in a plane perpendicular to the Y-direction, i.e. the direction parallel to the lines of the first and second grating.

In an embodiment of the present invention, the first pattern includes a linear grating enabling an incremental position measurement, whereas the second pattern includes a reference mark. Such a reference mark enables the determination of the absolute position at a certain position; when the reference mark is noticed by the sensor, absolute position information is obtained for the sensor relative to the structure. Once this reference is established, displacement along the direction in which the first grating extends, may provide absolute positional information on the sensor relative to the structure. The presence of such a reference mark (or reference marks) may be applied during initialization of the measurement system e.g. during start-up or after a power surge. This procedure may also be referred to as homing or zeroing.

FIG. 8schematically depicts an XZ-cross-section view on such a system. The measurement system as shown includes a sensor700arranged to co-operate with a first grating710mounted to a structure720and a reference marker730mounted to a different surface of the structure. As in the system described inFIG. 6, the reflected beams of an incident radiation beam may be detected by the sensor700and enables two position quantities to be determined; in the example as shown, the two position quantities consist of a incremental position measurement and an absolute position detection at a certain position (determined by the position of the reference marker730)

In an embodiment of the present invention, a pattern mounted to the structure is embedded in the structure, or, phrased different, is mounted to an inner surface of the structure. In order to manufacture a structure as applied in the present invention, one may apply the first and second pattern on two different outer surface of the structure. Alternatively, the structure including the two patterns may be manufactured by joining two structure parts, each including a grating. Joining the two parts may be done in such manner that one of the gratings is actually embedded in the structure (i.e. mounted to an inner surface of the structure) rather than being on an outer surface. Such an embedded pattern may be preferred because it may be less susceptible to contamination and may enable the cleaning the structure more easily (without the risk of damaging the pattern). Such a structure is illustrated inFIG. 9.

FIG. 9schematically shows a structure800in its non-assembled state (left side of the figure) including a first structure part810including a first pattern820and a second structure part830including a second pattern840. When joined together, the structure as shown on the right of the figure (850) may be obtained, resulting in the second pattern being embedded in the structure.

The gratings or patterns as have been described in the embodiments above, may be either be phase-gratings or a reflective gratings.

It will be appreciated that the radiation beam as referred to in the various embodiments may be, for example, a beam of visible light. More generally however, it will be appreciated that any form of radiation may be applied. As such, the patterns as described herein may have any form or shape. As long as the pattern enables the radiation beam to be affected (i.e. modified in some way), the modified beam may be detected and enables the determination of a position quantity of the sensor relative to the structure.