DE102018218162A1 - Optical arrangement, in particular for an EUV lithography system - Google Patents

Abstract

The invention relates to an optical arrangement (1), in particular for an EUV lithography system, comprising: at least one optical element (2), in particular for reflection of EUV radiation, a measuring element attached to the optical element (2) via at least one splice Target (3), a measuring device (8) for determining an actual position (P _{T, I} ) of the at least one measuring target (3), at least one moisture sensor (9) for determining a measure (P (RF)) for a Relative humidity (RF) in an environment of at least one splice (6), and a calculation device (14) for calculating an actual position (P _{M, I} ) of the optical element (2) in dependence on the actual position (P _{T , I} ) of the measuring target (3), wherein the optical arrangement (1) is formed, in the calculation of the actual position (P _{T, I} ) of the optical element (2) the measure (P (RF)) for the relative humidity (RF) in the environment of at least one splice (6) to be considered.

Description

Background of the invention

The invention relates to an optical arrangement, in particular for an EUV lithography system, comprising: at least one optical element, in particular for reflection of EUV radiation, a measuring target attached to the optical element via at least one splice, and a measuring device for determining an actual Position of the at least one measuring target.

The EUV lithography system may be an EUV lithography system for exposing a wafer or other optical arrangement using EUV radiation, for example, an EUV inspection system, e.g. an arrangement for the measurement or inspection of masks, wafers or the like used in EUV lithography. The optical elements of an EUV lithography system are typically operated in a vacuum environment.

During maintenance work on such an EUV lithography system, the projection lens is usually vented and subsequently vented again, i. the vacuum is restored. In this process, the glue or adhesive between the optical elements and the components attached thereto via at least one splice, e.g. in the form of measuring targets or the like, absorb humidity and release it again. The relatively long (possibly several days) change in the humidity in the vicinity of the splice (s) during maintenance can lead to a drift of the measuring target, i. lead to a deviation of the measuring target from its actually fixed position relative to the optical element. Such a deviation or drift can lead to a mispositioning of the optical element and, as a result, in the exposure of a wafer, to a significant overlay error.

Object of the invention

The object of the invention is to provide an optical arrangement in which a malpositioning of the optical element, which is due to a change in the humidity in an environment of the splice, as completely as possible can be avoided.

Subject of the invention

This object is achieved by an optical arrangement of the aforementioned type, further comprising: at least one moisture sensor for determining a measure of a relative humidity in an environment of at least one splice, and a calculation device for calculating an actual position of the optical element as a function of the actual position of the measuring target, wherein the optical arrangement is designed to take into account in the calculation of the actual position of the optical element, the measure of the relative humidity in the vicinity of the at least one splice.

According to the invention, it is proposed to determine a measure of the relative humidity in the vicinity of the bond between the optical element and the measuring target and to deduce the change in volume of the adhesive at the bond using this measure. For this purpose, the measure of the relative humidity of the calculation device can be provided as an input variable which takes into account this measure or the relative humidity in the calculation of the actual position of the optical element. In this case, the calculation device can, for example, resort to tables, measuring curves, etc., which enable a correlation between the measure of the relative humidity and correction data for the position correction in the calculation of the actual position of the optical element on the basis of the measured relative humidity.

Alternatively, the measure of the relative humidity of the humidity sensor may be determined such that this measure is directly referred to as positional information, e.g. in the form of a deviation from a nominal value for the distance between the optical element and the measuring target, which can be superimposed on the actual position of the measuring target determined by the measuring device, so that the calculation device already has the input by the adhesive drift (as a deviation provided by a desired value or a desired distance between the reflective optical element and the measuring target) corrected actual position of the measuring target is provided. In both cases, the relative humidity information in the vicinity of the splice is used for feed forward for position control of the optical element.

In one embodiment, the moisture sensor has a distance measuring device for measuring a distance or a deviation from a desired distance between at least two components connected in each case via a further splice, wherein the distance or the deviation serve as a measure of the relative humidity. The moisture sensor may, for example, have two plate-shaped components, between which a further splice is formed. The distance measuring device can be designed to measure the distance between the two plate-shaped components, which corresponds to the thickness of the respective further splice. The measurement of the distance By means of the distance measuring device, for example, optically.

The distance measuring device or the moisture sensor can also be used to determine a deviation of the distance from a desired distance between the two components, which is due to a deviation of the relative humidity of a desired value. Since the change in relative humidity results in a relatively small relative volume change or change in length of the adhesive of the splice in the longitudinal direction, i. perpendicular to the two components, leads, it is advantageous if the splice has the greatest possible thickness in the longitudinal direction. Since the thickness of the splice can not be arbitrarily increased, two or more splices can be connected in series, i. a plurality of generally plate-shaped components of the moisture sensor can be connected to each other in a sandwich construction via a respective splice.

In a further embodiment, the adhesive at the at least one further splice coincides with the adhesive at the splice between the optical element and the measuring target. In this case, an identical adhesive is used at the other splice and at the splice. For example, the (identical) adhesive may be an epoxy resin based adhesive. Due to the identical adhesive, a change in the relative humidity on the splice and on the further splice affects in the same way, i. the volume and thus the change in length with a change in relative humidity are identical. Therefore, in the case that the humidity sensor is configured to measure a deviation of a distance from a target distance, the deviation of the distance from the target distance is typically proportional to the change in length at the joint between the optical element and the measurement target. In this case, the deviation of the distance from the desired distance between the two or more components of the humidity sensor can be converted into a corresponding change in the distance between the optical element and the measuring target using a suitable proportionality factor. The moisture sensor can thus itself generate an output signal in the form of a position signal which compensates for the change in relative humidity caused by the volume change at the splice to the determined by the measuring device actual position of the measuring target. In this case, the output signal of the humidity sensor can be superimposed directly on the output signal of the measuring device, so that the calculation device is provided with the already corrected by the adhesive drift actual position of the optical element as an input signal.

In a further development, the moisture sensor has a lever mechanism for increasing the measuring sensitivity in determining the distance. In this case, one of the plate-shaped components form a lever, wherein at a first end or at a first lever arm of the plate-shaped component, the splice or the splices are formed, while at a second end or a second lever arm of the plate-shaped component, which laterally beyond the splice (s) also protrudes, the distance measuring device determines the distance to the plate-shaped component. The length of the first lever arm is in this case smaller than the length of the second lever arm, so that the accuracy in the determination of the distance by means of the distance measuring device in the ratio of the lever arms is increased.

In a further embodiment, the optical arrangement additionally comprises a control device for regulating the position of the optical element to a desired position on the basis of the actual position of the optical element. The corrected for the adhesive drift actual position of the measuring target forms a feedback signal for the control device, which is designed to minimize the deviation or the error between the actual position of the optical element and the desired position of the optical element.

In a further embodiment, the optical element has a reflective coating for reflection of EUV radiation. In particular, when the reflective optical element is used in a projection objective of an EUV lithography system, to avoid aberrations and an overlay error in the (multiple) exposure of a wafer, it is necessary that the position of the optical element be exactly (with an accuracy in the Order of magnitude of nanometers) coincides with the desired position in the EUV lithography system.

For the purposes of this application, a position is understood to mean not only the position of the optical element in three-dimensional space, but also the orientation of the optical element in three-dimensional space. It is understood that typically several measuring targets are needed to control or regulate the position of the optical element with all six degrees of freedom. It may suffice if only a single moisture sensor is used to determine a measure of relative humidity in the vicinity of the splices of all measurement targets, since the relative humidity in the vicinity of one and the same optical element is approximately constant.

In one embodiment, the measuring target has a reflective coating for reflection of measuring radiation of the measuring device. The reflective coating is usually arranged in a measuring arm of the measuring device. The measuring radiation typically impinges perpendicularly on the reflective coating of the measuring target and is reflected back to the measuring device.

A change in volume of the adhesive of the splice only affects the determination of the actual position of the measuring target if the change in volume or in length is parallel to the measuring direction of the measuring radiation. One measure for reducing the effect of the adhesive drift on the determination of the actual position of the measuring target is therefore to attach the splice so that the change in length or volume of the adhesive runs as perpendicular as possible to the measuring direction. Even an angle of 45 ° between the measuring direction, i. The direction of the measuring radiation, and the direction of the adhesive drift or the volume change significantly reduces the error in determining the actual position of the measuring target.

In a further embodiment, the splice is arranged in a gap formed between the optical element and the measuring target, wherein the gap or splitting direction is preferably aligned substantially parallel to the measuring radiation (i.e., the measuring direction) emitted by the measuring device. The gap has a gap width across the gap direction, which typically corresponds to the (nominal) thickness of the splice. As described above, it is favorable if the direction along which the gap width changes does not coincide with the measuring direction. The adhesive gap is ideally oriented so that the drift direction of the adhesive, which leads to a change in the gap width, runs as orthogonal as possible to the measuring direction.

It is understood that as an alternative or in addition further measures can be taken to reduce the adhesive drift. For example, attempts can be made to slow down or reduce the ingress and egress of moisture into the adhesive of the splice. This can be achieved, for example, by keeping the period of time during maintenance, in which the EUV lithography system is open, so that there is no vacuum in it, as low as possible. When the EUV lithography system is open, active rinsing of the EUV lithography system can also be carried out, for example. done with dry air.

For the reduction of the adhesive drift, it is also favorable if the adhesive of the splice has a low stretch stiffness, in particular in relation to the stretch stiffness of the optical element or of the measuring target. The stretch stiffness is defined herein as the modulus of elasticity of the adhesive multiplied by the area of the bond transverse to the thickness direction of the bond. The measuring target and the optical element are usually made of glass. In the event that the measurement target and the optical element outside the bond have glass-to-glass contact, the glass-on-glass contact and adhesive at the bond behave like two springs connected in parallel. If the glass-to-glass contact is very stiff compared to the adhesive "spring", the elongation at the bond due to the change in humidity only has a minor effect on the total elongation of the parallel-connected springs. It is therefore advantageous if the adhesive has a lower elongation rigidity than the (glass) material of the optical element or of the measuring target by at least one order of magnitude, for example by a factor of 20. The aim of the design is therefore a glass-to-glass contact that is as stiff as possible (since it does not react sensitively to changes in humidity) and the softest possible adhesive bond.

Further features and advantages of the invention will become apparent from the following description of embodiments of the invention, with reference to the figures of the drawing, which show details essential to the invention, and from the claims. The individual features can be realized individually for themselves or for several in any combination in a variant of the invention.

list of figures

• 1a eine schematische Schnittdarstellung einer optischen Anordnung, welche einen EUV-Spiegel mit zwei Mess-Targets aufweist,
• 1b eine Detaildarstellung eines Mess-Targets von 1a mit einer Klebestelle zur Befestigung an dem optischen Element,
• 2 eine schematische Darstellung eines Feuchtigkeitssensors zur Bestimmung eines Maßes für die relative Feuchte in der Umgebung der Klebestelle von 1b, sowie
• 3 ein Signalflussplan einer Positions-Regelung für das optische Element von 1a,b.

Embodiments are illustrated in the schematic drawing and will be explained in the following description. It shows

• 1a 1 is a schematic sectional view of an optical arrangement which has an EUV mirror with two measuring targets,
• 1b a detailed representation of a measuring target of 1a with a splice for attachment to the optical element,
• 2 a schematic representation of a humidity sensor for determining a measure of the relative humidity in the vicinity of the splice of 1b , such as
• 3 a signal flow chart of a position control for the optical element of 1a, b ,

In the following description of the drawings, identical reference numerals are used for identical or functionally identical components.

In 1a is schematically an optical arrangement 1 for an EUV lithography system which is a reflective optical element 2 as well as two measuring targets 3 having. The reflective optical element 2 has a surface with a coating 4 for reflection of EUV Radiation is formed. As in 1b is to be recognized, is a respective measurement target 3 over a splice 6 on the optical element 2 attached. The splice 6 runs annular and is in a gap 7 between a substantially spherical shell-shaped surface 3a of the measuring target 3 and a substantially conical surface 2a of the optical element 2 arranged.

Each of the two measuring targets 3 has a reflective coating 5 auf, which is designed for the reflection of measuring radiation, which of an in 1b shown measuring device 8th is sent out and reflected back to this. The measuring device 8th allows an actual position P _{T, I} (see. 3 ) of the measuring target 3 More precisely, the reflective coating 5 of the measuring target 3 to determine with high accuracy.
The optical arrangement 1 also has an in 2 illustrated humidity sensor 9 to determine a measure of relative humidity RF (see. 3 ) in the vicinity of the splice 6 is trained. The moisture sensor 9 is stationary, for example, attached to a support frame or the like and in the vicinity of the splice 6 arranged.

The moisture sensor 9 has three plate-shaped components 11a-c on, between the first and second component 11a . 11b a first further splice 10a and between the second component 11b and the third component 11c a second additional splice 10b is formed. The glue on the two other splices 10a . 10b agrees with the adhesive at the splice 6 between the optical element 2 and the measuring target 3 match. In the example shown, each is the same adhesive based on epoxy resin. The moisture sensor 9 from 2 is designed as a measure of the relative humidity RF in the vicinity of the splice 6 the distance A between each two adjacent plate-shaped components 11a-c to determine. The distance A agrees with the thickness of the respective splice 10a , b match.

For the determination of the distance A more precisely for the determination of the sum of the distances A1 . A2 between each two adjacent plate-shaped components 11a-c points the humidity sensor 9 a distance measuring device 13 on. The distance measuring device 13 For this purpose, measure a distance from a fixed reference to the top of the uppermost plate-shaped component 11a , This has a section twelve on, extending laterally over the splices 10a , b and the other two plate-shaped components 11b , c extends beyond. The first plate-shaped component 11a with the protruding section twelve forms a lever which is rotatable about a plane perpendicular to the plane of rotation D rotation. At a first, shorter lever arm are the two splices 10a B. The distance measuring device 13 is used to measure the distance to the end of the second, longer lever arm of the first plate-shaped component 11a ,

A change in the distance A between each two of the plate-shaped components 11a-c is with a leverage, which corresponds to the ratio of the length of the lever arms, from the distance measuring device 13 measured. Due to the presence of two splices 10a , b, which are connected in series, that of the distance measuring device changes 13 measured distance with a change in the distance ΔA between two adjacent plate-shaped components 11a-c by a factor of two, multiplied by the leverage ratio. Through the lever mechanism and through the series connection of several other splices 10a , b can thus the measuring sensitivity of the humidity sensor 9 be increased.

3 shows a signal flow plan of a position control for the optical element 2 from 1 , A control device 15 the position control becomes a difference between a target position P _{M, S} of the optical element 2 and an actual position P _{M, I} of the optical element 2 specified as input. The control device 15 acts on a not shown adjusting device, for example in the form of a Lorentz actuator, on the optical element 2 to bring this to the desired position P _{M, S} to move or to the difference between the desired position P _{M, S} and the actual position P _{M, I} to minimize. The control device 15 the input position is the actual position P _{M, I} of the optical element 2 supplied by a computing device 14 depending on the actual position P _{T, I} of the measuring target 3 is calculated. For this purpose, the calculation device 14 that of the measuring device 8th certain actual position P _{T, I of} the measuring target 3 provided as an input.

At the in 3 The example shown is that of the calculation device 14 Actual position supplied as input variable P _{T, I} corrected by a volume change of the adhesive (glue drift) at the splice 6 is taken into account. For this purpose, the humidity sensor generates 9 as a measure of the relative humidity RF in the vicinity of the splice 6 a position signal P (RF), the deviation of the relative humidity RF from a desired humidity or the deviation of the means of the distance measuring device 13 measured distance A from a desired distance corresponds, in which the splice 6 a target thickness and thus the measurement target 3 a desired distance from the optical element 2 having. The position signal P (RF) becomes that of the measuring device 8th certain actual position P _{T, I} of the measuring target 3 superimposed, ie the calculation device 14 becomes the actual position already corrected by the glue drift P _{T, I} of the measuring target 2 transmitted as input.

Alternatively, the humidity sensor 9 be configured to determine another measure of the relative humidity RF, which is the calculation device 14 is fed directly as input. In this case, in the calculation device 14 the correction of the actual position dependent on the relative humidity RF P _{M, I} made the control device 15 is supplied as input. For this purpose, the calculation device 14 rely on a database of tables and / or on empirical traces, which in previous measurements at different values of relative humidity in the vicinity of the splice 6 were determined.

In the manner described above, the influence of the adhesive drift, ie the change in volume of the adhesive at the splice 6 , be corrected, so that the control device 15 the reflective optical element 2 with high precision to the specified target position P _{M, S} emotional. In this way, when using the optical arrangement 1 In a projection objective of an EUV lithography system, an overlay error attributable to the adhesive drift during the exposure of a wafer can be avoided or significantly reduced. The overlay error represents the absolute error of the position of the image field (exposed field on the wafer) with respect to the so-called alignment marks, with which the position of the wafer in the lithography system is measured.

Will the volume change of the adhesive at the splice 3 not compensated in the manner described above in the control, this leads to an unwanted movement (drift) of the reflective optical element 2 , For example, to an unwanted rotation of the reflective optical element 2 and thus to an overlay error. For a typical epoxy based adhesive, the maximum drift that can occur is estimated. At a volume change (shrinkage) of the adhesive at a respective splice 6 by 2% results in a measurement target drift, which depends on the thickness of the glue gap 7 and the contact stiffness of the splice 6 depends. The overlay error is all the optical elements present in the projection lens 2 as involved as all on a respective optical element 2 via respective splices 6 attached measuring targets 3 , An acceptable value for the overlay error is approximately 40 pm / min. The feedforward compensation described above can ensure that the above-specified value for the overlay error is generally not exceeded.

Claims (8)

Optical arrangement (1), in particular for an EUV lithography system, comprising: at least one optical element (2), in particular for reflection of EUV radiation, and at least one measuring element attached to the optical element (2) via at least one splice (6) Target (3), a measuring device (8) for determining an actual position (P _{T, I} ) of the at least one measuring target (3), characterized by at least one moisture sensor (9) for determining a measure (P (RF) ) for a relative humidity (RF) in an environment of the at least one splice (6), and a calculation device (14) for calculating an actual position (P _{M, I} ) of the optical element (2) as a function of the actual position (P _{T, I} ) of the measuring target (3), wherein the optical arrangement (1) is formed, in the calculation of the actual position (P _{T, I} ) of the optical element (2) the dimension (P (RF) ) for the relative humidity (RF) in the vicinity of the at least one splice (6). Optical arrangement according to Claim 1 in which the moisture sensor (9) has a distance measuring device (13) for measuring a distance (A1, A2) or a deviation from a desired distance between at least two components (11a-c) respectively connected via a further splice (10a, 10b) as a measure (P (RF)) of the relative humidity (RF). Optical arrangement according to Claim 2 in which the adhesive at the further splice (10a, 10b) coincides with the adhesive at the splice (6). Optical arrangement according to Claim 2 or 3 in which the humidity sensor (9) has a lever (12) for increasing the measuring sensitivity in determining the distance (A1, A2) or the deviation from the desired distance. Optical arrangement according to one of the preceding claims, further comprising: a control device (15) for controlling the position of the optical element (2) to a desired position (P _{M, S} ) based on the actual position (P _{M, I} ) of the optical Elements (3). An optical arrangement according to any one of the preceding claims, wherein the optical element (2) comprises a reflective coating (4) for reflection of EUV radiation. Optical arrangement according to one of the preceding claims, wherein the measuring target (3) has a reflective coating (5) for the reflection of measuring radiation of the measuring device (8). Optical arrangement according to one of the preceding claims, in which the splice (6) is arranged in a gap (7) formed between the optical element (2) and the measuring target (3), the gap (7) preferably being substantially parallel is aligned with the measuring radiation emitted by the measuring device (8).

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