WO2008110212A1

WO2008110212A1 - Optical imaging arrangement

Info

Publication number: WO2008110212A1
Application number: PCT/EP2007/052434
Authority: WO
Inventors: Albrecht Hof; Dietmar Neugebauer; Bernd JÄNKER; Steffen Lauer; Rasmus Debitsch; Matthias Manger
Original assignee: Carl Zeiss Smt Ag
Priority date: 2007-03-15
Filing date: 2007-03-15
Publication date: 2008-09-18
Also published as: TW200846843A

Abstract

There is provided an optical imaging arrangement (101) comprising an optical projection unit (102), at least one of a mask unit (103) and a substrate unit (104), a first imaging arrangement component, a second imaging arrangement component and a metrology arrangement (107). The mask unit (103) is adapted to receive a mask comprising a pattern. The substrate unit (104) is adapted to receive a substrate. The optical projection unit (102) comprises a group of optical element units holding an optical element group (106), the optical element group (106) being adapted to transfer an image of the pattern onto the substrate. The first imaging arrangement component is a component of the optical projection unit (102) while the second imaging arrangement component is a component of one of the mask unit (103) and the substrate unit (104). The metrology arrangement (107) captures a spatial relationship between the first imaging arrangement component and the second imaging arrangement component. The metrology arrangement (107) comprises a reference element (107.5), the reference element (107.5) being mechanically connected to the first imaging arrangement component.

Description

Optical Imaging Arrangement

BACKGROUND OF THE INVENTION

The invention relates to optical imaging arrangements used in exposure processes, in particular to optical imaging arrangements of microlithography systems. It further relates to a method of capturing a spatial relationship between components of an optical imaging arrangement. It also relates to method of transferring an image of a pattern onto a substrate. The invention may be used in the context of photolithography processes for fabricating microelectronic devices, in particular semiconductor devices, or in the context of fabricating devices, such as masks or reticles, used during such photolithography processes.

Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices comprise a plurality of optical element units comprising optical elements, such as lenses and mirrors etc., in the light path of the optical system. Those optical elements usually cooperate in an exposure process to transfer an image of a pattern formed on a mask, reticle or the like onto a substrate such as a wafer. Said optical elements are usually combined in one or more functionally distinct optical element groups. These distinct optical element groups may be held by distinct optical exposure units. In particular with mainly refractive systems, such optical exposure units are often built from a stack of optical element modules holding one or more optical elements. These optical element modules usually comprise an external generally ring shaped support device supporting one or more optical element holders each, in turn, holding an optical element.

Optical element groups comprising at least mainly refractive optical elements, such as lenses, mostly have a straight common axis of symmetry of the optical elements usually referred to as the optical axis. Moreover, the optical exposure units holding such optical element groups often have an elongated substantially tubular design due to which they are typically referred to as lens barrels.

Due to the ongoing miniaturization of semiconductor devices there is a permanent need for enhanced resolution of the optical systems used for fabricating those semiconductor devices. This need for enhanced resolution obviously pushes the need for an increased numerical aperture and increased imaging accuracy of the optical system.

One approach to achieve enhanced resolution is to reduce the wavelength of the light used in the exposure process. In the recent years, approaches have been made to use light in the extreme ultraviolet (EUV) range using wavelengths down to 13 nm and even below, typically between 20 to 5 nm. In this EUV range purely reflective optics have to be used since it is not possible to use conventional refractive optics any more due to absorption effects. Another approach to achieve higher resolution with more or less conventional optics is to increase its numerical aperture. To achieve this goal immersion techniques are applied, where a liquid is placed between the optical element located closest to the wafer and the waver itself.

However, not only the transition to the use of high numerical aperture (e.g. NA > 0.5), reflective systems in the EUV range or the use of immersion techniques but also the increasing accuracy demands on conventional systems, e.g. operating at a wavelength of 193 nm, lead to considerable challenges with respect to the design of the optical imaging arrangement.

Among others, there exist increasingly strict requirements with respect to the relative position between the components participating in the exposure process. Furthermore, to reliably obtain high-quality semiconductor devices it is not only necessary to provide an optical system showing a high degree of imaging accuracy. It is also necessary to maintain such a high degree of accuracy throughout the entire exposure process and over the lifetime of the system.

Currently, typical structures to be created on the wafer have a size of about 70 nm, for example, leading to the requirement that the wafer stage typically has to be positioned at an accuracy well below 10 nm. The accuracy requirements are further tightened when using multiple exposure processes where a specific structure on the wafer is generated in a plurality of exposure steps separated by intermediate chemical process steps.

As a consequence, the optical imaging arrangement components, i.e. the mask, the optical elements and the wafer, for example, cooperating in the exposure process must be supported in a defined manner in order to maintain a predetermined spatial relationship between said optical imaging arrangement components as well to provide a high quality exposure process. To maintain the predetermined spatial relationship between the optical imaging arrangement components throughout the entire exposure process, even under the influence of vibrations generated internally or introduced via the ground structure supporting the arrangement and under the influence of thermally induced position alterations, it is necessary to at least intermittently capture the spatial relationship between certain components of the optical imaging arrangement and to adjust the position of at least one of the components of the optical imaging arrangement as a function of the result of this capturing process.

To deal with these problems, in conventional mainly refractive systems, the optical elements and the metrology devices necessary to capture the spatial relationship mentioned above are substantially rigidly mounted to a so called metrology frame. Such a metrology frame, in general, is a heavy, generally plate shaped body. The metrology frame is supported on the ground structure via vibration isolating means to reduce the influences of vibrations of the ground structure usually lying in the range of about 30 Hz and below. Furthermore, considerable effort is necessary to avoid thermally induced deformations of the metrology frame. Either the metrology frame has to be made of a generally expensive material with a very low coefficient of thermal expansion or an expensive temperature stabilization system has to be provided. Furthermore, when using immersion techniques, considerable forces to be accounted for are introduced into the optical projection system and, thus, the metrology frame via the immersion medium mechanically contacting, both, the moving wafer stage and parts of the optical projection system. Thus, in any case, the metrology frame is a very complex and, thus, expensive part of the system.

In conventional refractive systems, the leveling of the wafer, i.e. the adjustment of the position of the wafer along the optical axis of the optical projection system (often vertical and, thus, often referred to as the z-axis), is often provided using the measurement results of a metrology arrangement mounted to the metrology frame and projecting a beam of light onto the wafer at an oblique angle from a location close to the outer periphery of the last refractive optical element of the projection system. The measurement beam is reflected from the surface of the wafer at an oblique angle as well and hits a receptor element of the metrology arrangement at a location also close to the outer periphery of the last refractive optical element. Depending on the location where the measurement beam hits the receptor element, the location of the wafer with respect to the metrology arrangement may be determined. EP 1 182 509 A2 (by Kwan), the disclosure of which is incorporated herein by reference, discloses an imaging system where the positioning of the mask relative to the optical projection system is provided using the measurement results of a mask metrology arrangement mounted in part to the housing of the optical projection system and in part to the mask table carrying the mask. Here, the mask table carries the reference elements of the metrology arrangement, i.e. reflectors for interferometry measurements or 2D-gratings for encoder measurements. While this solution eliminates the need for mounting components of the mask metrology arrangement to a metrology frame it still has the disadvantage that parts of the metrology arrangement are mounted to the housing of the optical projection system. Since the housing may be affected by thermally induced expansion effects altering the position of the optical elements received therein, it is necessary to account for these effects when positioning the mask table adding further complexity to the system thus rendered more expensive.

SUMMARY OF THE INVENTION

It is thus an object of the invention to, at least to some extent, overcome the above disadvantages and to provide good and long term reliable imaging properties of an optical imaging arrangement used in an exposure process.

It is a further object of the invention to reduce the effort necessary for an optical imaging arrangement while at least maintaining the imaging accuracy of the optical imaging arrangement used in an exposure process.

These objects are achieved according to the invention which is based on the teaching that a reduction of the effort necessary for an optical imaging arrangement while at least maintaining the imaging accuracy of the optical imaging arrangement may be achieved if, on the one hand, at least a part of the metrology system capturing a spatial relationship between components of the optical imaging arrangement, in particular, one or more reference elements of the metrology system, is mechanically connected to a component of the optical projection unit being adapted to transfer an image of the pattern onto the substrate, and if, on the other hand, certain components of the optical projection unit are arranged so as to receive least a part of the metrology system, in particular, one or more reference elements of the metrology system. Connection of at least parts of the metrology system to a component of the optical projection unit allows to reduce the effort for a metrology frame up to even entirely eliminating the metrology frame.

Thus, according to a first aspect of the invention there is provided an optical imaging arrangement comprising an optical projection unit, at least one of a mask unit and a substrate unit, a first imaging arrangement component, a second imaging arrangement component and a metrology arrangement. The mask unit is adapted to receive a mask comprising a pattern. The substrate unit is adapted to receive a substrate. The optical projection unit comprises a group of optical element units holding an optical element group, the optical element group being adapted to transfer an image of the pattern onto the substrate. The first imaging arrangement component is a component of the optical projection unit while the second imaging arrangement component is a component of one of the mask unit and the substrate unit. The metrology arrangement captures a spatial relationship between the first imaging arrangement component and the second imaging arrangement component. The metrology arrangement comprises a reference element, the reference element being mechanically connected to the first imaging arrangement component.

According to a second aspect of the invention there is provided a method of capturing a spatial relationship between a first component and a second component of an optical imaging arrangement. The method comprises providing an optical imaging arrangement comprising an optical projection unit, at least one of a mask unit and a substrate unit, a first imaging arrangement component, a second imaging arrangement component and a metrology arrangement; the mask unit being adapted to receive a mask comprising a pattern, the substrate unit being adapted to receive a substrate; the optical projection unit comprising a group of optical element units holding an optical element group, the optical element group being adapted to transfer an image of the pattern onto the substrate; the first imaging arrangement component being a component of the optical projection unit, the second imaging arrangement component being a component of one of the mask unit and the substrate unit. The method further comprises providing a reference element, the reference element being mechanically connected to the first imaging arrangement component. The method further comprises capturing a spatial relationship between the first imaging arrangement component and the second imaging arrangement component using the reference element. According to a third aspect of the invention there is provided a method of transferring an image of a pattern onto a substrate. The method comprises, in a transferring step, transferring the image of the pattern onto the substrate using an optical imaging arrangement, in a capturing step of the transferring step, capturing a spatial relationship between a first component and a second component of the optical imaging arrangement using the method according to the second aspect of the invention, and, in a controlling step of the transferring step, controlling the position of at least one component of the optical imaging arrangement as a function of the spatial relationship between a first component and a second component captured in the capturing step.

According to a fourth aspect of the invention there is provided an optical imaging arrangement comprising an optical projection unit, at least one of a mask unit and a substrate unit, a first imaging arrangement component, a second imaging arrangement component and a metrology arrangement. The mask unit is adapted to receive a mask comprising a pattern while the substrate unit is adapted to receive a substrate. The optical projection unit comprises an optical element group, the optical element group being adapted to transfer an image of the pattern onto the substrate. The first imaging arrangement component and the second imaging arrangement component are different components, each being a component of one of the optical projection unit, the mask unit and the substrate unit. The metrology arrangement captures a spatial relationship between the first imaging arrangement component and the second imaging arrangement component using a differential signal being obtained from a first signal and a second signal. The metrology arrangement comprises a first reference element mechanically connected the first imaging arrangement component and a second reference element mechanically connected the second imaging arrangement component. The metrology arrangement obtains the first signal using the first reference element and obtains the second signal using the second reference element.

According to a fifth aspect of the invention there is provided a method of capturing a spatial relationship between a first component and a second component of an optical imaging arrangement. The method comprises providing an optical imaging arrangement comprising an optical projection unit, at least one of a mask unit and a substrate unit, a first imaging arrangement component, a second imaging arrangement component and a metrology arrangement; the mask unit being adapted to receive a mask comprising a pattern, the substrate unit being adapted to receive a substrate; the optical projection unit comprising an optical element group, the optical element group being adapted to transfer an image of the pattern onto the substrate; the first imaging arrangement component and the second imaging arrangement component being different components and each being a component of one of the optical projection unit, the mask unit and the substrate unit. The method further comprises providing a first reference element being mechanically connected to the first imaging arrangement component and a second reference element being mechanically connected to the second imaging arrangement component. The method further comprises capturing a spatial relationship between the first imaging arrangement component and the second imaging arrangement component using a differential signal being obtained from a first signal and a second signal; the first signal being obtained using the first reference element and the second signal being obtained using the second reference element.

Further aspects and embodiments of the invention will become apparent from the dependent claims and the following description of preferred embodiments which refers to the appended figures. All combinations of the features disclosed, whether explicitly recited in the claims or not, are within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic representation of a preferred embodiment of an optical imaging arrangement according to the invention with which preferred embodiments of methods according to the invention may be executed;

Figure 2 is a schematic partially sectional view of the optical imaging arrangement of Figure 1 along line M-Il;

Figure 3A is a schematic partially sectional view of the optical imaging arrangement of Figure 1 along line Ill-Ill in a first operational situation;

Figure 3B is a schematic partially sectional view of the optical imaging arrangement of Figure 1 along line Ill-Ill in a second operational situation;

Figure 4 is a schematic partially sectional view of the optical imaging arrangement of Figure 1 along line IV-IV;

Figure 5 is a block diagram of a preferred embodiment of a method of transferring an image of a pattern onto a substrate according to the invention which may be executed with the optical imaging arrangement of Figure 1 ; Figure 6 is a schematic representation of a further preferred embodiment of an optical imaging arrangement according to the invention with which preferred embodiments of the methods according to the invention may be executed.

Figure 7 is a schematic representation of a detail of a further preferred embodiment of an optical imaging arrangement according to the invention with which preferred embodiments of the methods according to the invention may be executed.

DETAILED DESCRIPTION OF THE INVENTION

First embodiment

In the following, a first preferred embodiment of an optical imaging arrangement 101 according to the invention with which preferred embodiments of methods according to the invention may be executed will be described with reference to Figures 1 to 4.

Figure 1 is a schematic and not-to-scale representation of the optical imaging arrangement in the form of an optical exposure apparatus. The optical exposure apparatus 101 comprises an optical projection unit 102 adapted to transfer an image of a pattern formed on a mask 103.1 of a mask unit 103 onto a substrate 104.1 of a substrate unit 104. To this end, the optical exposure apparatus 101 comprises an illumination system 105 illuminating the mask 103. The optical exposure unit 103 receives the light transmitted through the mask 103.1 and projects the image of the pattern formed on the mask 103.1 onto the substrate 104.1 , e.g. a wafer or the like.

The optical projection unit 102 comprises an optical element group 106. This optical element group 106 is held within a housing 102.1 of the optical projection unit 102, often also referred to as the projection optics box (POB) 102.1. Typically, the housing 102.1 is formed by a stack of optical element holders holding the optical element group 106. The optical element group 106 comprises a number of optical elements in the form of lenses 106.1. These lenses 106.1 are at least in part actively positioned with respect to one another along an axis 102.2 of the optical projection unit 102, typically referred to as the optical axis 102.2 of the optical projection unit 102, in up to all six degrees of freedom.

The optical projection unit 102 receives the part of the light path between the mask 103.1 and the substrate 104.1. The optical elements 106 cooperate to transfer the image of the pattern formed on the mask 103.1 onto the substrate 104.1 located at the end of the light path.

The mask 103.1 is received on a mask table 103.2 of the mask unit 103, the mask table 103.2 being supported by a suitable support structure (not shown) on a ground structure (not shown). In a similar way, the substrate 104.1 is received on a substrate table 104.2 of the substrate unit 104, the substrate table 104.2 as well being supported by a suitable support structure - not shown - on the ground structure. (not shown).

The image of the pattern formed on the mask 103.1 is usually reduced in size and transferred to several target areas of the substrate 104.1. The image of the pattern formed on the mask 103.1 may be transferred to the respective target area on the substrate 104.1 in two different ways depending on the design of the optical exposure apparatus 101. If the optical exposure apparatus 101 is designed as a so called wafer stepper apparatus, the entire image of the pattern is transferred to the respective target area on the substrate 104.1 in one single step by irradiating the entire pattern formed on the mask 103.1. If the optical exposure apparatus 101 is designed as a so called step-and-scan apparatus, the image of the pattern is transferred to the respective target area on the substrate 104.1 by progressively scanning the mask table 103.2 and thus the pattern formed on the mask 103.1 under the projection beam while performing a corresponding scanning movement of the substrate table 104.2 and, thus, of the substrate 104.1 at the same time.

In both cases, the relative position of the optical elements 106.1 , i.e. the lenses 106.1 , of the optical element group 106 with respect to each other as well as with respect to the mask 103.1 and with respect to the substrate 104.1 has to be maintained within predetermined limits to obtain a high quality imaging result.

During operation of the optical exposure apparatus 101 , the relative position of the lenses 106.1 , with respect to each other as well as with respect to the mask 103.1 and the substrate 104.1 is subject to alterations resulting from, both, intrinsic and extrinsic, disturbances introduced into the system. Such disturbances may be mechanical disturbances, e.g. in the form of vibrations resulting from forces generated within the system itself but also introduced via the surroundings of the system, e.g. the ground structure. They may also be thermally induced disturbances, e.g. position alterations due to thermal expansion of the parts of the system. In order to keep the above predetermined limits of the relative position of the lenses 106.1 with respect to each other as well as with respect to the mask 103.1 and the substrate 104.1 , the lenses 106.1 may be actively positioned in space via actuator units (not shown). Similarly, the mask table 103.2 and the substrate table 104.2 may be actively positioned in space via the respective support structure (not shown).

The active positioning of these parts is performed on the basis of the measurement results of a plurality of metrology arrangements capturing the spatial relationship between certain components of the optical exposure apparatus 101.

A first metrology arrangement 107, in six degrees of freedom (DOF), captures the spatial relationship between a first imaging arrangement component of the optical exposure apparatus 101 and a second imaging arrangement component of the optical exposure apparatus 101 different from said first imaging arrangement component.

The first imaging arrangement component is a component of the optical projection unit 102, namely the housing 102.1 of the optical projection unit 102. The second imaging arrangement component is the substrate table 104.2, i.e. a component of the substrate unit 104. Since the spatial relationship between the substrate table 104.2 and the substrate 104.1 is known, e.g. due to a measurement operation immediately preceding the exposure process, the first metrology arrangement 107 also allows to capture the spatial relationship between the substrate 104.1 , as a component of the substrate unit 104, and the housing 102.1 .

The first metrology arrangement 107 and all further metrology arrangements described herein in the following operate in a contactless manner by using metrology light beams indicated in the appended Figures by dotted lines. However, it will be appreciated that, with other embodiments of the invention, other measurement techniques may be used instead of or in an arbitrary combination with these optical measurement techniques. For example, capacitive measurement techniques or even mechanical measurement techniques may be used.

As can be seen, for example, from Figures 2, 3A and 3B the first metrology arrangement 107 comprises four first emitter and receiver units 107.1 to 107.4 connected to the substrate table 104.2 . The metrology arrangement 107 further comprises a first reference element 107.5 being mechanically connected to the housing 102.1 of the optical projection 102, i.e. to the first imaging arrangement component, as will be explained in further detail below. The plate shaped reference element 107 is arranged such that its plane of main extension is substantially perpendicular to the optical axis 102.2 of the optical projection unit 102.

The first reference element 107.5 is held such that it is located in close proximity to the substrate table 104.2 in order to provide good measurement results. Depending on the working principle of the first metrology arrangement, the first reference element may be a reflecting element, e.g. a reflective surface or an element providing a plurality of reflecting surfaces such as a so called corner cube prism or the like, when an interferometry principle is used or a diffractive element, e.g. an optical grid or grating, when an encoder principle is used. In the embodiment shown in Figure 1 to 4, the first reference element 107.5 is formed by a generally plate shaped element.

Since the first metrology arrangement 107 shown in Figure 1 to 4 uses an encoder principle, the surface of the reference element 107.5 facing the substrate unit 104 is a reflective surface with a grating, e.g. a grating directly exposed onto the surface 107.6 facing the substrate unit 104. However, it will be appreciated that, with other embodiments of the invention, the first reference element may also be provided by one or more separate, generally plate shaped elements mechanically connected to a suitable carrier which in turn is mechanically connected to the housing 102.1. For example, it may be a reflective surface or a grating etc. on a separate part that is connected via a positive connection, a frictional connection, an adhesive connection or any combination thereof directly to the first mirror. For example it may be screwed, clamped, adhesively or otherwise fixedly connected to the first mirror.

In the embodiment shown in Figure 1 to 4, the first emitter and receiver units 107.1 to 107.4 and the first reference element 107.5 are part of an encoder arrangement of the first metrology arrangement 107. Thus, the first reference element 107.5 comprises four one- dimensional gratings 107.7 to 107.10 formed on the surface 107.6, e.g. directly exposed onto the surface 107.6.

As can be seen particularly well from Figures 3A and 3B (where the substrate unit 104 is shown as a semi-transparent component for the reasons of a better understanding), the first reference element 107 is a generally ring shaped plate with a central opening 107.1 1 forming a passageway for the light path on the exposure light used when projecting the image of the pattern formed on the mask 103.1 onto the wafer 104.1. The four gratings 107.7 to 107.10 divide the planar ring shaped reference element 107 into four segments, each extending over an angle of about 90°. The first grating 107.7 and the diametrically opposing third grating 107.9, both, form substantially parallel gratings used for obtaining position information in the x direction. The second grating 107.8 and the diametrically opposing fourth grating 107.9, both, form substantially parallel gratings used for obtaining position information in the y direction. Thus, the first grating 107.7 and the second grating 107.8 are substantially perpendicular to each other.

Each one of the emitter and receiver units 107.1 to 107.4 is associated to one of the gratings 107.7 to 107.10. Each one of the first emitter and receiver units 107.1 to 107.4 of the encoder arrangement emits at least one light beam 107.12 and 107.13, respectively, towards the associated grating 107.7 to 107.10 and receives at least a part of the light beam 107.12 and 107.13, respectively, reflected back by the associated grating 107.7 to 107.10. When there is a relative movement between the respective grating 107.7 to 107.10 and the respective associated emitter and receiver unit 107.1 to 107.4 in a direction transverse to the respective light beam, the intensity of the light received at respective the emitter and receiver unit 107.1 to 107.4 varies in a well known manner due to the structure of the grating 107.7 to 107.10 leading to a correspondingly pulsed signal by the respective emitter and receiver unit 107.1 to 107.4. A control unit 107.14 uses these signals in a well known manner to draw conclusions on the relative movement and to control active positioning of the substrate table 104.2 as well as of certain optical elements 106.1.

Hereby, the emitter and receiver unit 107.1 and its associated grating 107.7 as well as the emitter and receiver unit 107.3 and its associated grating 107.9 provide corresponding position change information in the x direction. Similarly, the emitter and receiver unit 107.2 and its associated grating 107.8 as well as the emitter and receiver unit 107.4 and its associated grating 107.10 provide corresponding position change information in the y direction upon relative movement between the substrate unit 104 and the first reference element 107.

The encoder arrangement, with the signals from the emitter and receiver unit 107.1 to 107.4, provides position information in at least three degrees of freedom, namely the two translational degrees of freedom (often referred to as translation along x-axis and y-axis) in a plane perpendicular to the optical axis 102.2 of the optical projection unit 102 and the rotational degree of freedom (often referred to as rotation about z-axis) about the optical axis 102.2. It will be appreciated that the measurement of the relative position between the first reference element 107.5 and the substrate table 104.2 in all six degrees of freedom may be made using one or more measurement beams provided by each one of the emitter and receiver units 107.1 to 107.4. For example, information on the sharpness of the image information received by the respective emitter and receiver units 107.1 to 107.4 may be used to gain position information in the remaining three degrees of freedom (translation along z-axis, rotation about x-axis and y-axis). As an alternative, at least one of the emitter and receiver units 107.1 to 107.4 may be arranged to emit its measurement beam under an oblique angle onto the reference element 107.5 in order to obtain this position information in the remaining degrees of freedom

Furthermore, for example, the encoder principle of the encoder arrangement of the first metrology arrangement 107 may be combined with interferometric and/or capacitive sensors to provide the position information in the remaining three degrees of freedom. In the embodiment shown in Figure 1 to 4, this may be achieved with three measurements along the optical axis 102.2 (z-axis). It will be appreciated that the gratings 107.7 to 107.10 already provide a convenient reference element for this purpose. For example, capacitive sensors or interferometric sensors using diffraction patterns reflected off the gratings 107.7 to 107.10 may be used.

However, it will be appreciated that, with other embodiments of the invention, further encoder elements located at other locations than the surface 107.6 of the reference element 107.5 may be used in the first metrology arrangement 107. Thus, for example, the entire metrology arrangement may operate according to the encoder principle.

The use of the encoder arrangement has several advantages. It is much easier to implement and operate than known interferometric systems using less parts and less space. Furthermore, the encoder element, e.g. the grating is easier to manufacture than the high quality mirror surfaces required for known interferometric systems.

A further advantage of the use of the encoder arrangement is that a homing function may be easily implemented in a well known manner by incorporating into the gratings 107.7 to 107.10, for example, multiple home index pulses in both translational degrees of freedom (x-axis and y-axis). Thus, the substrate table 104.2 may be repeatedly driven to a known home position relative to the optical projection unit 102 without a need for complex initialization procedures. It will be appreciated that, to position the substrate table 104.2 in the capture range of the home index pulses, the positioning accuracy of the positioning unit of the substrate table 104.2 may be more than sufficient.

The emitter and receiver units 107.1 to 107.4 are located evenly distributed at the outer circumference of the substrate 104.1. They have a mutual distance that equals at least half of the maximum diameter of the substrate 104.1 , thus reducing the maximum Abbe arm that may occur at the extreme positions of the substrate table 104.2.

As can be seen from Figures 3A and 3B (showing extreme positions of the substrate table 104.2) the size of the reference element 107.5 and the central opening 107.1 1 is chosen such that, at least two, namely three, of these emitter and receiver units 107.1 to 107.4 are functional to provide position information described above at any time in the maximum range of relative motion defined between the optical projection unit 102 and the substrate unit 104 during normal operation of the optical imaging arrangement 101. In other words, each of the emitter and receiver units 107.1 to 107.4 is functional to provide position information in a part of the range of relative motion between the optical projection unit 102 and the substrate unit 104, these parts of the range of relative motion having an overlap such that three of these emitter and receiver units 107.1 to 107.4 are functional to provide position information at any time during normal operation of the optical imaging arrangement 101.

It will be appreciated that, with other embodiments of the invention, another number of emitter and receiver units may be used. Even one single emitter and receiver unit may be used. However, preferably at least two, more preferably at least three emitter and receiver units, measuring at the same resolution in at least one common degree of freedom are used in order to avoid overly large encoder elements.

It will be appreciated that the four first emitter and receiver units 107.1 to 107.4 provide redundant position information that may be used, among others, by the control unit 107.14 for encoder calibration.

It will be appreciated that, with other embodiments of the invention, the respective first emitter and receiver unit does not necessarily have to be mounted to the substrate unit. For example, the emitter and receiver unit may be executed as a separate emitter unit and a separate receiver unit. Either of such a emitter and receiver unit or separate emitter unit and receiver unit may be mounted external to the substrate unit. In such a case a beam directing device, e.g. a mirror, would be mounted to the substrate unit in order to direct the respective light beam to and from the encoder element.

A crucial factor to the quality of the measurement results obtained from the first metrology arrangement 107 is the connection between the first reference element 107.5 and the housing 102.1 of the optical projection unit 102. This connection has to be a very rigid connection in order to provide as little relative movement between the first reference element 107.5 and the housing 102.1 as possible. In particular, the connection has to be made in such a way that as few mechanical disturbances as possible are introduced from the housing 102.1 into the first reference element 107.5 which might cause such relative movement.

In order to achieve this, the first reference element 107.5 is connected to the housing 102.1 via a connector element 108. As can be seen from Figure 1 and 4, the connector element 108 is formed by a continuous conical shell surrounding the lower part of the optical projection unit 102.

The conical shell 108 has the advantage that it provides a connector element that is a very rigid in itself. However, it will be appreciated that, with other embodiments of the invention, connector element may have any suitable cross-section other than the circular cross-section shown in Figure 4. For example, the connector element may be a continuous polyhedral shell having a polygonal cross section as it is illustrated, by way of example, in Figure 4 by the dashed contour 109. Furthermore, any other suitable cross-section with a combination of curved and/or straight sections may be selected. Finally, it is not necessary that there is provided one single continuous connector element; rather, it is also possible that a plurality of connector elements is used for connecting the first reference element to the housing of the optical projection unit.

The connector element 108 is connected to the housing 102.1 at a first connection location. This first connection location is located close to the location where the housing 102.1 is connected to a support structure 109 supporting the optical projection unit 102.

The optical projection unit 102, when mounted to the support structure 109 has a defined vibrational behavior transverse to the optical axis 102.2 with a natural mode of first order and, consequently, a node of said natural mode of first order, i.e. a location where minimum excursion (with respect to a rest position) transverse to the optical axis 102.2 occurs upon vibrational excitation of the optical projection unit 102. The first connection location is located as close as possible to this node of the natural mode of first order of the optical projection unit 102. Thus, upon vibrational excitation of the optical projection unit 102, at the first connection location as few excursion as possible occurs. Consequently, as few vibration energy as possible is introduced into the connector element 108 and, subsequently, into the first reference element 107.5. This contributes to a minimised disturbance in the relative position between the first reference element 107.5 and the housing 102.1.

The connector element 108 is connected to the first reference element 107.5 at a second connection location. This second connection location is selected such that the natural mode of first order of the plate shaped first reference element 107.5 in the direction of the optical axis, i.e. in the z direction, is maximised. This results in a beneficial vibrational behavior of the first reference element 107.5 itself further contributing to a minimised disturbance in the relative position between the first reference element 107.5 and the housing 102.1 upon vibrational excitation.

In order to achieve this, the diameter of the connector element 108, at the second connection location, is selected such that, in the x and y direction, the connector element is located substantially midway between the outer circumference 107.15 of the first reference element 107.5 and the inner circumference 107.16 of the first reference element 107.5.

The vibrational behavior of the first reference element 107.5 may be further optimised by connecting damping elements, such as e.g. oil dampers or other vibration reducing elements, to the first reference element as it is indicated in Figure 4 by the dashed contours 1 10. The damping elements 1 10 may be placed at any suitable location. In the embodiment shown in Figure 4 they are located at the corners of the first reference element 107.5 close to its outer circumference 107.15, i.e. in a region where the maximum excursion of the first reference element 107.5 is to be expected upon vibrational excitation.

In the embodiment shown in Figure 4 the first reference element 107.5 is of rectangular shape. However, it will be appreciated that, with other embodiments of the invention, the first reference element may have any other suitable shape. In particular, a circular shape with a circular outer circumference and/or a circular inner circumference may be selected as it is indicated by the dashed contours 1 11 and 1 12 in Figure 4. This may be particularly beneficial with respect to the vibrational behavior of the first reference element. A further beneficial influence on the vibrational behavior of the first reference element 107.5 may be achieved by connecting the first reference element to the 102 at a further, third connection location remote from the first connection location via one or more second connector elements as is indicated in Figure 1 by the dashed contour 1 13. Preferably, the respective connector element only provides restriction of movement in one degree of freedom (DOF), namely in the direction of main excursion of the first reference element 107.5 - i.e., here, in the z direction - upon vibrational excitation.

The direct mechanical connection of the first reference element 107.5 as a part of the first metrology arrangement 107 with the optical projection unit 102 as well as the direct mechanical connection, more precisely integration, of further parts of the first metrology arrangement 107, in particular the emitter and receiver units 107.1 to 107.4, directly to the substrate table 104.2 makes it possible to eliminate large and bulky structures such as a metrology frame or the like between the ground structure and the housing of the optical projection unit 102.

The elimination of the conventional metrology frame that may be achieved according to the invention provides a number of advantages over the optical projection systems currently known:

One advantage is that the elimination of the conventional metrology frame frees up a considerable amount of building space close to the substrate. This facilitates the design and dimensioning of the projection optics. A further advantage is that the housing 102.1 of the optical projection unit in itself has already to be highly stabilized in terms of thermal stability in order to reduce thermal drift effects between the optical elements received therein. Thus, the housing 102.1 is particularly suitable for mounting parts of the first metrology arrangement and the considerable effort usually necessary for thermal stabilization of a metrology frame has not to be taken. This considerably reduces the overall costs of the optical projection apparatus 101.

It will be appreciated that, with other embodiments of the invention, the encoder arrangement as described above may be used in combination with optical projection units comprising, partly or exclusively, other types of optical elements, such as reflective or diffractive optical elements. Furthermore, the encoder arrangement as described above may be used in combination with optical projection units working in other wavelength ranges. It will be further appreciated that the embodiment shown is particularly useful when immersion techniques are used, i.e. where a part of the optical element located closest to the substrate unit is immersed, together with a part of the substrate, in an immersion medium such as highly purified water.

A second metrology arrangement 1 14 is provided capturing the spatial relationship between the housing 102.1 and the mask table 103.2 in order to use the result in positional adjustment of the mask table 103.2 with respect to the optical projection unit 102.

The second metrology arrangement 1 14 comprises second emitter and receiver units 1 14.1 connected to the mask table 103.2 and a second reference element 114.2 mechanically connected to the housing 102.1. Here again, the second metrology arrangement 1 14 may capture, in six degrees of freedom (DOF), the spatial relationship between the housing 102.1 as a first imaging arrangement component of the optical exposure apparatus 101 and the mask table 103.2 as a second imaging arrangement component of the optical exposure apparatus 101.

The second reference element 1 14.2 is connected to the upper end of the housing 102.1. Depending on the working principle of the second metrology arrangement, the second reference element may be a reflecting element, e.g. a reflective surface or an element providing a plurality of reflecting surfaces such as a so called corner cube prism or the like, when an interferometry principle is used, or a diffractive element, e.g. a grating, when an encoder principle is used. In the embodiment shown in Figure 1 , the second reference element 1 14.2, is a reflective surface with a grating, e.g. a grating directly exposed onto the front side surface 108.4.

However, it will be appreciated that, with other embodiments of the invention, the second reference element may also be an element separate from the first mirror and mechanically connected directly thereto. For example, it may be a reflective surface or a grating etc. on a separate part that is connected via a positive connection, a frictional connection, an adhesive connection or any combination thereof directly to the housing of the optical projection unit. For example it may be screwed, clamped, adhesively or otherwise fixedly connected to the first mirror.

Furthermore, it will be appreciated that, with other embodiments of the invention, the second metrology arrangement 1 14 may be replaced by a conventional metrology arrangement as it is known from EP 1 182 509 A2. Finally it will be appreciated that the second metrology arrangement may also be provided in a configuration that this is similar to the one described in the context of the first metrology arrangement 107, i.e. with a second reference element connected to the housing of the optical projection unit by a connector element similar to the connector element 108.

Finally, a third metrology arrangement (not shown) is provided within the housing 102.1 capturing the spatial relationship between certain lenses 106.1 in order to use the result in the positional adjustment of the lenses 106.1 with respect to the housing 102.1. However, it will be appreciated that, with other embodiments of the invention, such a third metrology arrangement may be omitted.

With the optical exposure apparatus 101 of Figure 1 to 4 a preferred embodiment of a method of transferring an image of a pattern onto a substrate according to the invention may be executed as it will be described in the following with reference to Figure 5.

In a transferring step 1 15 of this method, an image of the pattern formed on the mask 103.1 is transferred onto the substrate 104.1 using the optical projection unit 102 of the optical imaging arrangement 101.

To this end, in a capturing step 1 15.1 of said transferring step 1 15, the spatial relationship between the housing 102.1 as a first component of the optical imaging arrangement 101 and the substrate table 104.2 as well as the mask table 103.2, each forming a second component of the optical imaging arrangement 101 is captured using a preferred embodiment of the method of capturing a spatial relationship between a first component and a second component of an optical imaging arrangement 101 according to the invention.

In a controlling step 1 15.2 of the transferring step, the position of the substrate table 104.2, and the mask table 103.2 as well as certain lenses 106.1 with respect to the housing 102.1 is controlled as a function of the spatial relationship previously captured in the capturing step 115.1. In an exposure step 1 15.3, immediately following or eventually overlapping the controlling step 1 15.2, the image of the pattern formed on the mask 103.1 is then exposed onto the substrate 104.1 using the optical imaging arrangement 101.

In a step 115.4 of the capturing step 1 15.1 , the mask unit 103 with the mask 103.1 and the substrate unit 104 with the substrate 104.1 is provided and positioned in space. It will be appreciated that the mask 103.1 and the substrate 104.1 may also be inserted into the mask unit 103 and the substrate unit 104, respectively, at a later point in time prior to the actual position capturing or at an even later point in time prior to the exposure step 115.3.

In a step 115.5 of the capturing step 1 15.1 , the components of the optical projection unit 102 are provided and supported according to a preferred embodiment of a method of supporting components of an optical projection unit according to the invention. To this end, in a step 1 15.6, the optical elements 106.1 of the optical projection unit 102 are provided and positioned within the housing 102.1 of the optical projection unit 102. In a step 1 15.7, the housing 102.1 with the optical elements 106.1 is supported in a vibration isolated manner on the ground structure via the structure 109 to provide a configuration as it has been described above in the context of Figure 1.

In a step 1 15.8 of the capturing step 1 15.1 the first to third metrology arrangements 107, 1 14 are provided to provide a configuration as it has been described above in the context of Figure 1. It will be appreciated that the first and secondthird reference elements 107.5, 1 14.2 have already been provided at an earlier point in time. However, with other embodiments of the invention, the first and second reference elements may be provided together with the other components of the first to third metrology arrangement at a later point in time prior to the actual position capturing.

In a step 1 15.9 of the capturing step 1 15.1 , the actual spatial relationship between the housing 102.1 as a first component of the optical imaging arrangement 101 and the substrate table 104.2, the mask table 103.2 is and the respective lenses 106.1 each forming a second component of the optical imaging arrangement 101 is then captured.

It will be appreciated that the actual spatial relationship between the housing 102.1 as a first component of the optical imaging arrangement 101 and the substrate table 104.2, the mask table 103.2 and the respective lenses 106.1 each forming a second component of the optical imaging arrangement 101 may be captured continuously throughout the entire exposure process. In the step 1 15.9, the most recent result of this continuous capturing process is then retrieved and used.

As described above, in the controlling step 1 15.2, the position of the substrate table 104.2, the mask table 103.2 and the respective lenses 106.1 with respect to the housing 102.1 is then controlled as a function of this spatial relationship previously captured before, in the exposure step 1 15.3, the image of the pattern formed on the mask 103.1 is exposed onto the substrate 104.1. Second embodiment

In the following, a second preferred embodiment of an optical imaging arrangement 201 according to the invention with which preferred embodiments of methods according to the invention may be executed will be described with reference to Figure 6.

Figure 6 is a schematic and not-to-scale representation of the optical imaging arrangement in the form of an optical exposure apparatus 201 operating in the EUV range at a wavelength of 13 nm. The optical exposure apparatus 201 comprises an optical projection unit 202 with an optical element group 206 comprising a plurality of mirrors 206.1 adapted to transfer an image of a pattern formed on a mask 203.1 of a mask unit 203 onto a substrate 204.1 of a substrate unit 204. To this end, the optical exposure apparatus 201 comprises an illumination system - not shown - illuminating the reflective mask 203.1. The optical projection unit 203 receives the light reflected from the mask 203.1 and projects the image of the pattern formed on the mask 203.1 onto the substrate 204.1 , e.g. a wafer or the like.

The embodiment of Figure 6, in its design and functionality, largely corresponds to the embodiment of Figure 1. In particular, in Figure 6, like or identical parts have been given the same reference numeral increased by 100. Thus, it is here mainly referred to the explanations given above and, primarily, only the differences will be discussed.

The main difference with respect to the first embodiment lies within the fact that the first imaging arrangement component to which the second reference element 214.2 of the second metrology arrangement 214 is connected to is formed by a support element 202.3 of the optical element unit 202 supported in a vibration isolated manner on the ground structure 209 and exclusively supporting the components of a third metrology arrangement 202.4 capturing the spatial relationship between the mirrors 206.1 of the optical element group 206.

It will be appreciated that, although not shown in further detail in Figure 6, the first reference element of a first metrology arrangement operating similar to the first metrology arrangement described above in the context of the first embodiment may also be connected to the support element 202.3. It will be appreciated that, with this embodiment as well, the methods according to the invention as they have been described above with reference to Figure 5 may be executed as well. Thus, in this context, it is here only referred to the above explanations.

Third embodiment

In the following, a third preferred embodiment of an optical imaging arrangement 301 according to the invention with which preferred embodiments of methods according to the invention may be executed will be described with reference to Figure 7.

The embodiment of Figure 7, apart from the second metrology arrangement corresponds to the embodiment of Figure 1 to 4. In particular, in Figure 7, identical parts have been given the same reference numeral as in Figure 1 to 4. Thus, it is here mainly referred to the explanations given above and, primarily, only the difference with respect to the second metrology arrangement 314 will be discussed.

Figure 7 is a schematic and not-to-scale representation of a part of the optical imaging arrangement in the form of an optical exposure apparatus. The optical exposure apparatus 301 comprises the optical projection unit 102 with the optical element group 106 which is adapted to transfer an image of the pattern formed on a mask 103.1 of the mask unit 103 onto the substrate 204.1 (not shown in Figure 7).

The spatial relationship between the housing 102.1 of the optical projection unit 102 the mask unit 103 in the direction of the optical axis 102.2, i.e. in the z direction, is captured using an interferometric metrology arrangement 315 of the second metrology arrangement 314. The interferometric metrology arrangement 315 comprises an emitter and receiver unit 315.1 mounted to the housing 102.1 and emitting a light beam 315.2 in the x direction towards a beam directing device in the form of a prism 315.3. The prism 315.3 is also mounted to the housing 102.1 and directs the light beam 315.2 via a beam directing surface 315.4 in the z direction towards the mask table 103.2.

The light beam 315.2 exits the prism 315.3 at an exit surface 315.5 which forms a first reference element in the sense of the present invention mounted to the housing 102.1 as a first imaging arrangement component in the sense of the present invention. When exiting the prism 315.3, a first fraction of the light beam 315.2 is folded back to the beam directing surface 315.4 of the prism 315.3 and from there to the emitter and receiver unit 315.1. At the emitter and receiver unit 315 this first fraction of the light beam 315.2 generates - in a well known manner which will not be explained here in further detail - a first interferometric signal which is used in capturing the spatial relationship between the housing 102.1 of the optical projection unit 102 the mask unit 103 as will be explained in further detail below.

On its further way the light beam 315.2 hits a second reference element 315.6 in the sense of the present invention mounted to the mask table 103.2 forming a second imaging arrangement component in the sense of the present invention. The second reference element is a reflecting surface 315.6 provided on the surface of the mask table 103.2 facing the optical projection unit 102. From this reflecting surface of 315.6 the light beam 315.2 is folded back to the prism 315.3 and is directed by the beam directing surface 315.4 back to the emitter and receiver unit 315.1. At the emitter and receiver unit 315 the returning fraction of the light beam 315.2 generates - in a well known manner - a second interferometric signal.

In order to determine the spatial relationship between the housing 102.1 of the optical projection unit 102 the mask unit 103 a differential signal is obtained using the first interferometric signal and the second interferometric signal. This differential signal provides information about the distance between the first reference element 315.5 and the second reference element 315.6 in the direction of the optical axis 102.2, i.e. it provides an information on the translation in the z direction.

This embodiment has the advantage that the differential signal is obtained using two relatively simple reference elements 315.5 and 315.6 which may be easily mounted and positioned to the optical projection unit 102 and the mask unit 103, respectively. In particular, the mechanical connection of the emitter and receiver unit 315.1 to the optical projection unit 102 is less critical to the measurement result. It may even be possible to connect to the emitter and receiver unit to a structure different and eventually mechanically isolated from the optical projection unit. Furthermore, the emitter and receiver unit 315.1 may be placed at a location where sufficient space is available while the prism 315.3 requires only few space allowing a relatively narrow gap between the optical projection unit

102 and the mask table 103.

It will be appreciated that, in order to gain information on the relative rotation about the x axis and the y axis between the mask unit 103 and the optical projection unit 102, three or more interferometric metrology arrangements 315 may be provided between the mask unit

103 and the optical projection unit 102, preferably in an evenly distributed manner. It will be further appreciated that position information in the remaining three degrees of freedom (translation in the x and y direction, rotation about the z direction) may be obtained as it has been described above. For example, encoder devices as they have been described above may be used as is indicated in Figure 7 by the dashed contour 316. These encoder devices, in a similar manner, may use such prisms for beam directing as well.

It will be further appreciated that, with other embodiments of the invention, the emitter and receiver unit as well as the beam directing prism may be mounted to the mask table while they reflecting surface forming the second reference element may be mounted to the optical projection unit. Furthermore, it will be appreciated that a similar interferometric metrology arrangement with a beam directing prism may also be used for capturing the spatial relationship between the optical projection unit 102 and the substrate unit 104 of Figure 1.

Although, in the foregoing, embodiments of the invention have been described where the optical elements are exclusively refractive and reflective elements, respectively, it will be appreciated that, with other embodiments of the invention, reflective, refractive or diffractive elements or any combinations thereof may be used for the optical elements of the optical element units.

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Claims

What is claimed is:

1. An optical imaging arrangement comprising: an optical projection unit, at least one of a mask unit and a substrate unit, a first imaging arrangement component, a second imaging arrangement component and a metrology arrangement; said mask unit being adapted to receive a mask comprising a pattern, said substrate unit being adapted to receive a substrate; said optical projection unit comprising an optical element group, said optical element group being adapted to transfer an image of said pattern onto said substrate; said first imaging arrangement component being a component of said optical projection unit, said second imaging arrangement component being a component of one of said mask unit and said substrate unit; - said metrology arrangement capturing a spatial relationship between said first imaging arrangement component and said second imaging arrangement component; said metrology arrangement comprising a reference element; said reference element being mechanically connected to said first imaging arrangement component.

2. The optical imaging arrangement according to claim 1 , wherein said reference element is located in close proximity to said second imaging arrangement component .

3. The optical imaging arrangement according to claim 1 , wherein said first imaging arrangement component is a holder holding an optical element of said optical element group.

4. The optical imaging arrangement according to claim 1 , wherein said metrology arrangement is a first metrology arrangement and said first imaging arrangement component is a support element supporting a second metrology arrangement; said second metrology arrangement capturing a spatial relationship between an optical element of said optical element group and said support element.

5. The optical imaging arrangement according to claim 1 , wherein said reference element comprises a reference surface, said reference surface being at least one of a reflective surface and a diffractive surface.

6. The optical imaging arrangement according to claim 1 , wherein - said metrology arrangement comprises an interferometry arrangement and said reference element comprises at least one reflecting surface forming part of said interferometry arrangement.

7. The optical imaging arrangement according to claim 7, wherein said reference element is a first reference element and - said interferometry arrangement comprises a second reference element; said second reference element being mechanically connected to said first imaging arrangement component and comprising at least one reflecting surface; said interferometry arrangement capturing a spatial relationship between said first imaging arrangement component and said second imaging arrangement component using a differential signal being obtained from a first signal and a second signal; said interferometry arrangement obtaining said first signal using said first reference element and obtaining said second signal using said second reference element.

8. The optical imaging arrangement according to claim 1 , wherein said metrology arrangement comprises an encoder arrangement and said reference element comprises at least one optical grid forming part of said encoder arrangement.

9. The optical imaging arrangement according to claim 8, wherein said first imaging arrangement component and said second imaging arrangement component define a maximum range of relative displacement during normal operation; said reference element being of sufficient size to capture said spatial relationship between said first imaging arrangement component and said second imaging arrangement component over said maximum range using said reference element.

10. The optical imaging arrangement according to claim 1 , wherein said first imaging arrangement component has a natural mode of first order and a node of said natural mode of first order; said reference element being mechanically connected to said first imaging arrangement component at least in proximity to said node.

1 1. The optical imaging arrangement according to claim 10, wherein said reference element is connected to said first imaging arrangement component via at least one connector element; said connector element forming at least a segment of one of a cylindrical shell, a conical shell and a polyhedral shell.

12. The optical imaging arrangement according to claim 1 1 , wherein said connector element forms one of a cylindrical shell, a conical shell and a polyhedral shell surrounding at least a part of said first imaging arrangement component.

13. The optical imaging arrangement according to claim 10, wherein said reference element has an outer circumference and an aperture allowing light to pass said reference element when said image of said pattern is transferred onto said substrate, said aperture defining an inner circumference of said reference element; said connector element being connected to said reference element at a connection location;

14. The optical imaging arrangement according to claim 13, wherein said reference element has a natural mode of first order; said connection location being selected such that said natural mode of first order of said reference element is substantially maximized.

15. The optical imaging arrangement according to claim 13, wherein said connection location is located substantially midway between said inner circumference and said outer circumference of said reference element.

16. The optical imaging arrangement according to claim 10, wherein said reference element is mechanically connected to said first imaging arrangement component at a further location remote from said node.

17. The optical imaging arrangement according to claim 16, wherein said reference element has a natural mode of first order and a main direction of excursion at said natural mode of first order; said connection between said reference element and said first imaging arrangement component at said further location remote from said node restricting movement of said reference element in said main direction of excursion.

18. The optical imaging arrangement according to claim 17, wherein said connection between said reference element and said first imaging arrangement component at said further location remote from said node restricts movement of said reference element in said main direction of excursion only.

19. The optical imaging arrangement according to claim 1 , wherein said first imaging arrangement component has a location of minimum vibration amplitude where the first imaging arrangement, upon a vibrational excitation, shows the smallest amplitude of excursion from a rest position; said reference element being mechanically connected to said first imaging arrangement component at least in proximity to said location of minimum vibration amplitude.

20. The optical imaging arrangement according to claim 1 , wherein said reference element is a substantially planar element.

21. The optical imaging arrangement according to claim 1 , wherein at least one vibration damping element is connected to said reference element.

22. The optical imaging arrangement according to claim 1 , wherein said metrology arrangement comprises an capacitive metrology arrangement and - said reference element comprises at least one capacitive element forming part of said capacitive metrology arrangement.

23. The optical imaging arrangement according to claim 1 , wherein said second imaging arrangement component is a component of said mask unit.

24. The optical imaging arrangement according to claim 1 , wherein said second imaging arrangement component is a component of said substrate unit.

25. The optical imaging arrangement according to claim 1 , wherein a control unit is provided; said control unit being connected to said metrology arrangement to receive said spatial relationship between said first imaging arrangement component and said second imaging arrangement component; said control unit controlling the position of at least one component of said optical imaging arrangement during the transfer of said image of said pattern onto said substrate as a function of said spatial relationship received from said metrology arrangement.

26. The optical imaging arrangement according to claim 1 , wherein said mask unit and said optical projection unit are adapted to use light in the UV range.

27. A method of capturing a spatial relationship between a first component and a second component of an optical imaging arrangement, said method comprising: providing an optical imaging arrangement comprising - an optical projection unit, at least one of a mask unit and a substrate unit, a first imaging arrangement component, a second imaging arrangement component and a metrology arrangement; said mask unit being adapted to receive a mask comprising a pattern, said substrate unit being adapted to receive a substrate; said optical projection unit comprising a group of optical element units holding an optical element group, said optical element group being adapted to transfer an image of said pattern onto said substrate; said first imaging arrangement component being a component of said optical projection unit, said second imaging arrangement component being a component of one of said mask unit and said substrate unit; - providing a reference element, said reference element being mechanically connected to said first imaging arrangement component; and capturing a spatial relationship between said first imaging arrangement component and said second imaging arrangement component using said reference element.

28. The method according to claim 27, wherein said providing said reference element comprises connecting said reference element to said first imaging arrangement component at least in close proximity to a location where the first imaging arrangement, upon a vibrational excitation, shows the smallest amplitude of excursion from a rest position..

29. The method according to claim 27, wherein said reference element is located in close proximity to said second imaging arrangement component when capturing said spatial relationship.

30. A method of transferring an image of a pattern onto a substrate, said method comprising - in a transferring step, transferring said image of said pattern onto said substrate using an optical imaging arrangement, in a capturing step of said transferring step, capturing a spatial relationship between a first component and a second component of said optical imaging arrangement using the method according to claim 27; in a controlling step of said transferring step, controlling the position of at least one component of said optical imaging arrangement as a function of said spatial relationship between a first component and a second component captured in said capturing step.

31. An optical imaging arrangement comprising: an optical projection unit, at least one of a mask unit and a substrate unit, a first imaging arrangement component, a second imaging arrangement component and a metrology arrangement; said mask unit being adapted to receive a mask comprising a pattern, - said substrate unit being adapted to receive a substrate; said optical projection unit comprising an optical element group, said optical element group being adapted to transfer an image of said pattern onto said substrate; said first imaging arrangement component and said second imaging arrangement component being different components and each being a component of one of said optical projection unit, said mask unit and said substrate unit; said metrology arrangement capturing a spatial relationship between said first imaging arrangement component and said second imaging arrangement component using a differential signal being obtained from a first signal and a second signal; said metrology arrangement comprising a first reference element mechanically connected said first imaging arrangement component and a second reference element mechanically connected said second imaging arrangement component. - said metrology arrangement obtaining said first signal using said first reference element and obtaining said second signal using said second reference element.

32. The optical imaging arrangement according to claim 31 , wherein said metrology arrangement comprises an interferometry arrangement; said first reference element and said second reference element each comprising at least one reflecting surface.

33. A method of capturing a spatial relationship between a first component and a second component of an optical imaging arrangement, said method comprising: providing an optical imaging arrangement comprising an optical projection unit, at least one of a mask unit and a substrate unit, a first imaging arrangement component, a second imaging arrangement component and a metrology arrangement; said mask unit being adapted to receive a mask comprising a pattern, said substrate unit being adapted to receive a substrate; said optical projection unit comprising an optical element group, said optical element group being adapted to transfer an image of said pattern onto said substrate; said first imaging arrangement component and said second imaging arrangement component being different components and each being a component of one of said optical projection unit, said mask unit and said substrate unit, providing a first reference element being mechanically connected to said first imaging arrangement component and a second reference element being mechanically connected to said second imaging arrangement component; and capturing a spatial relationship between said first imaging arrangement component and said second imaging arrangement component using a differential signal being obtained from a first signal and a second signal said first signal being obtained using said first reference element and said second signal being obtained using said second reference element.

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