Projection exposure system for microlithography and method of monitoring a lateral imaging stability

Abstract

A projection exposure system (10) for microlithography. The system includes projection optics (12) configured to image mask structures into a substrate plane (16), an input diffraction element (28) which is configured to convert irradiated measurement radiation (21) into at least two test waves (30) directed onto the projection optics (12) with differing propagation directions, a detection diffraction element (34; 28) which is disposed in the optical path of the test waves (30) after the latter have passed through the projection optics (12) and is configured to produce a detection beam (36) from the test waves (30) which has a mixture of radiation portions of both test waves (30), a photo detector (38) disposed in the optical path of the detection beam (36) which is configured to record the radiation intensity of the detection beam (36), time resolved, and an evaluation unit which is configured to determine the lateral imaging stability of the projection optics (12) from the radiation intensity recorded.

Claims

1. A projection exposure system for microlithography comprising: projection optics arranged to image mask structures into a substrate plane, and a measurement device configured to determine a lateral imaging stability of the projection optics with a resolution of greater than 0.5 nm with a measuring speed of at least 10 Hz, wherein the lateral imaging stability comprises an ability of the projection optics to image mask structures stably into the substrate plane with regard to a lateral shift of the image of the mask structure.

2. The projection exposure system according to claim 1, wherein the resolution of the measurement device is greater than 0.1 nm.

3. The projection exposure system according to claim 1, wherein the resolution of the measurement device is greater than 30 pm.

4. The projection exposure system according to claim 1, wherein the measuring speed is at least 50 Hz.

5. The projection exposure system according to claim 1, wherein the measuring speed is at least 500 Hz.

6. The projection exposure system according to claim 1, wherein the measuring speed is at least 2 kHz.

7. The projection exposure system according to claim 1, wherein the projection optics are configured to image the mask structures with light in at least the extreme-ultraviolet frequency wavelength range into the substrate plane.

8. The projection exposure system according to claim 1, comprising: an input diffraction element configured to convert irradiated measurement radiation into at least two test waves directed onto the projection optics with differing propagation directions, such that a system pupil of the projection optics is illuminated by the at least two waves in areas which are separated locally from each other.

9. The projection exposure system according to claim 8, further comprising an illumination diffraction element disposed in the optical path of the measurement radiation upstream of the input diffraction element and is configured to convert the measurement radiation into at least two measurement radiation partial beams with differing propagation directions.

10. The projection exposure system according to claim 1, further comprising a detection diffraction element disposed in the optical path of test waves after the test waves have passed through the projection optics and configured to produce a detection beam from the test waves which has a mixture of radiation portions of the at least two test waves.

11. The projection exposure system according to claim 10, wherein the detection diffraction element is configured to produce, in addition to the first detection beam, at least a second detection beam and a third detection beam from the test waves, and wherein the second detection beam has at least a radiation portion of a first of the two test waves and the third detection beam has at least one radiation portion of the second of the two test waves.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following exemplary embodiments of the projection exposure system for microlithography according to the invention and of the method according to the invention are described in greater detail with reference to the attached diagrammatic drawings. These show as follows:

(2) FIG. 1 a diagrammatic sectional view of a first embodiment of a projection exposure system according to the invention with projection optics and a measurement device for determining a lateral imaging stability of the projection optics,

(3) FIG. 2 an illustration of a measurement optical path in the projection exposure system according to FIG. 1,

(4) FIG. 3 the section on the mask side of the projection exposure system according to the invention in a further embodiment according to the invention,

(5) FIG. 4 the section on the mask side of the projection exposure system according to the invention in a further embodiment according to the invention,

(6) FIG. 5 the section on the substrate side of the projection exposure system according to the invention in a further embodiment according to the invention,

(7) FIG. 6 the section on the substrate side of the projection exposure system according to the invention in a further embodiment according to the invention,

(8) FIG. 7 a diagrammatic side view of a second embodiment of the projection exposure system according to the invention with a retro-reflector,

(9) FIG. 8 the projection exposure system according to FIG. 7 in a further embodiment according to the invention,

(10) FIGS. 9A and 9B an illustration of the mode of operation of the retro-reflector according to FIGS. 7 and 8, and

(11) FIGS. 10A and 10B the projection exposure system according to FIG. 7 in a further embodiment according to the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS ACCORDING TO THE INVENTION

(12) In the exemplary embodiments described below, elements which are similar to one another functionally or structurally are provided as far as possible with the same or similar reference figures. Therefore, in order to understand the features of the individual elements of a specific exemplary embodiment, one should refer to the description of other exemplary embodiments or to the general description of the invention.

(13) FIG. 1 illustrates a first embodiment of a projection exposure system 10 according to the invention for microlithography, e.g. in the form of an EUV projection exposure system designed as a so-called “scanner”. The projection exposure system 10 is configured to image mask structures on a mask, not illustrated by the drawing of FIG. 1, using projection optics 12 onto a substrate in the form of a so-called “wafer” disposed in a substrate plane 16. For this purpose the projection exposure system 10 comprises a mask shifting table in the form of a so-called “reticle stage” and a substrate shifting table in the form of a so-called “wafer stage”.

(14) For this purpose the projection exposure system 10 comprises an exposure radiation source 46 shown e.g. in FIG. 3, for example in the form of an EUV radiation source for producing extreme ultraviolet exposure radiation 48 with a wavelength of 13.5 nm, and illumination optics 52 for illuminating the mask with the exposure radiation 48.

(15) The projection optics 12 comprise a number of optical elements. In the case where the projection exposure system 10 is operated with exposure radiation 48 in the EUV wavelength range, the projection optics 12 are designed catoptrically and only comprise reflective optical elements in the form of mirrors.

(16) The projection exposure system 10 further comprises a measurement device 18 for determining the lateral imaging stability of the projection optics 12. In this context the lateral imaging stability of the projection optics 12 specifies, as already explained above, to what extent the lateral position of the image of structures imaged with the projection optics 12 from the mask plane 14 into the substrate plane 16 during the exposure operation of the projection exposure system 12 remains stable over time. Therefore, the lateral imaging stability defines the capability of the projection optics 12 to image mask structures into the substrate plane 16 stablely with regard to the lateral shift of the image of the mask structures in the substrate plane 16.

(17) In other words, the measurement device 18 determines a lateral shift of the image of the projection exposure system 10 which takes place due to the aberrations in the projection optics 12. In terms of a wavefront error a tilt of the wavefront is detected. For this purpose, a measurement optical path is provided which either replaces the exposure or imaging optical path temporarily, is coupled to or uncoupled from the imaging optical path or samples a part of the projection optics 12 not used by the exposure optical path, the properties of which are representative of the whole projection optics 12.

(18) The measurement device 18 comprises a measurement radiation source 20 for producing measurement radiation 21. The measurement radiation 21 can comprise electromagnetic radiation in the infrared, visible or ultraviolet wavelength range, e.g. with a wavelength of 1064 nm, 780 nm, 632 nm, 532 nm, 365 nm, 248 nm or 193 nm. In the case where the projection exposure system 10 is configured as an EUV exposure system, in one embodiment according to the invention the individual mirrors of the projection optics 12 are provided with a coating which reflects well both with the EUV wavelength and with the wavelength of the measurement radiation 21.

(19) In one embodiment according to the invention the measurement radiation 21 can have the same wavelength as the exposure radiation for imaging the mask structures. In this case, the measurement radiation source 20 can correspond to the exposure radiation source 46.

(20) The measurement device 18 according to FIG. 1 further comprises a collimator 22, an optional illumination diffraction element in the form of an illumination diffraction grating 24, an optional imaging optical element 26 and an input diffraction element in the form of an input diffraction grating 28. The collimator 22 focuses the measurement radiation 21 onto the illumination diffraction grating 24, from which the measurement radiation 21 is converted by diffraction into three measurement radiation partial beams 25 with differing propagation directions. The individual measurement radiation partial beams 25 are formed by the radiation diffracted into the 0th, −1st and +1st diffraction order. The measurement radiation partial beams 25 then strike the input diffraction grating 28 disposed in the mask plane 14. From the latter the radiation of the individual measurement radiation partial beams 25 is diffracted once again, by which five so-called test waves 30, also referred to as test partial beams, are formed.

(21) As illustrated in FIG. 2, the test waves 30 are respectively formed from a number of diffraction individual beams 31. For example, the test wave 30 identified by “0” comprises the following three diffraction individual beams 31: (+1,−1), (−1,+1) and (0,0). Here the first figure in the bracket characterising one respective test wave 30 indicates the diffraction order of the associated measurement radiation partial beam 25 and the second figure the diffraction order when diffracting this measurement radiation partial beam 25 on the input diffraction grating 28 in order to produce the respective diffraction individual beam 31. The test wave 30 identified by “+1” comprises the following diffraction individual beams 31: (+1, 0) and (0,+1). Furthermore, the diffraction individual beam (−1,+2), for example, can also contribute to the formation of this test wave 30.

(22) For the basic function of the method according to the invention it is essential that at least two test waves 30 with differing propagation directions pass through the projection optics 12. After passing through the projection optics 12 the test waves 30 strike a detection diffraction element in the form of a detection diffraction grating 34 on which the test waves 30 are converted by diffraction into the detection partial beams 36 identified by “−3” to “+3”.

(23) Analogously to the test waves 30, the detection partial beams 36 are formed from diffraction individual beams 37. Therefore, for example, the detection partial beam 36 identified by “−2” comprises the following three diffraction individual beams 37: (−1, −1, 0), (0, −1, −1) and (−1, 0, −1). Here the first figure in the brackets characterising one respective detection partial beam 36 indicates the diffraction order of the associated measurement radiation partial beam 25, the second figure indicates the diffraction order of the associated test wave 30, and the third figure indicates the diffraction order of the diffraction individual beam 37 produced by diffracting this test wave 30 on the detection diffraction grating 34. Furthermore, the diffraction individual beams (−1, −2, +1), (−1, +1, −2), etc. for example can also contribute to the formation of the test partial beam 30 identified by “−2”.

(24) The measurement device 18 further comprises for each of the detection partial beams 36 a photo detector in the form of a photodiode 38. The photodiodes 38 record the development over time of the respective intensity of the individual detection partial beams 36. The intensity signals recorded by the photodiodes 38 correlate to the lateral shift of the illumination diffraction grating 24, to the lateral shift of the input diffraction grating 28, to the lateral shift of the detection diffraction grating 34 and to the lateral image shift in the substrate plane 16 which is brought about by the wavefront tilt 32 of the projection optics 12.

(25) If one holds the illumination diffraction grating 24, the input diffraction grating 28 and the detection diffraction grating 34 sufficiently securely, the lateral image shift going back to the projection optics 12 and so the lateral imaging stability of the projection optics 12 can thus be measured from the intensity signals recorded.

(26) This is implemented by reading out the intensity signals from the photodiodes 38 via a read-out unit 40 and a corresponding evaluation of the intensity signals with an evaluation unit 42. In order to be able to determine the lateral imaging stability, the intensity of at least a first detection partial beam 36 must be read out which has a mixture of radiation portions of at least two test waves 30. This is the case, for example, for the detection partial beam 36 characterised by “−1” which has diffraction individual beams 37 going back to at least two test waves 30, for example: (−1, −1, +1) and (−1, +1, −1).

(27) Furthermore, in one advantageous embodiment the respective intensity of at least two further detection partial beams 36 is recorded, one of these detection partial beams 36 comprising at least one radiation portion of a first of the aforementioned two test waves 30, and the other detection partial beam 30 comprising at least one radiation portion of the second of the aforementioned test waves 30. Therefore, standardisation of the measured intensity of the first detection partial beam 36 can take place. As already explained above, the signals of the photo detectors 38 are only definite as far as the grating period. The interpolation within the grating period is facilitated by the further detection beams. The signals of the individual photo detectors 38 are still periodic in relation to translation of the gratings and the lateral position of the test waves 30 on the detection diffraction grating 34, but respectively offset in relation to one another by a fraction of the period of the detection diffraction grating 34.

(28) The design of the gratings 24, 28 and 34 with regard to the grating period, grating shape, blaze angle, phase range and position in the optical path is such that photodiode signals are produced which can be further processed electronically. With an electronic evaluation of the photodiode signals measurement accuracy is achieved which is better than the grating period of the detection diffraction grating 34 by at least three to four orders of magnitude. The measurement signal is always available while the measurement radiation source 20 is in operation. It is not linked to a movement in the imaging system such as e.g. a scan movement of the wafer stage or to the provision of exposure radiation 48 by the exposure radiation source 46 of the projection exposure system 10.

(29) The input diffraction grating 28 and the detection diffraction grating 34 do not have to, as shown in FIG. 1, be disposed exactly in the mask plane 14 or the substrate plane 16. The gratings 28 and 34 also do not have to be conjugated exactly to one another. A defocus can be useful for optimisation of the signals of the photodiodes 38. In one embodiment according to the invention not shown in the drawings, the measurement optical path is inverted, i.e. the input diffraction grating 28 is disposed on the substrate side and the detection diffraction grating 34 is disposed together with the photodiodes 38 on the substrate side.

(30) The evaluation unit 42 converts the determined lateral imaging stability of the projection optics 42 into a control signal which is relayed to the control electronics of the projection exposure system 10. Upon the basis of this control signal the control electronics correct the lateral position of the image during the exposure process in real time so that the lateral position of the mask structures imaged onto the substrate remain stable to a large extent.

(31) With the measurement device 18 according to the invention it is possible, in particular, to determine the lateral imaging stability of the projection optics 12 with a resolution of better than 30 pm with a measuring speed of at least 2 kHz. Therefore, the lateral position of the image of the mask structures can be corrected with a very high repetition rate and accuracy during the exposure.

(32) FIG. 3 shows the section on the mask side of the projection exposure system 10 according to FIG. 1 in a first embodiment according to the invention. According to this embodiment the measurement radiation 21 is coupled in the form of the test waves 30 using a coupling mirror 44 into an exposure optical path 50 of the projection exposure system 10. The exposure optical path 50 guides the exposure radiation 48 produced by the exposure radiation source 46. During the exposure operation of the projection exposure system 10 the exposure radiation 48 is irradiated via illumination optics 52 onto a product mask disposed in the mask plane 14 in order to image mask structures disposed on the latter into the substrate plane 16. The coupling of the measurement radiation 21 into the exposure optical path 50 using the coupling mirror 44 takes place in the embodiment illustrated in FIG. 3 at a point between the mask plane 14 and the projection optics 12 such that the exposure optical path 50 is not interfered with. The gratings 24 and 26 are disposed, like the measurement radiation source 20, outside of the exposure optical path 50.

(33) FIG. 4 shows a further embodiment of the section on the mask side of the projection exposure system 10. Here, after passing through the collimator 22, the measurement radiation 21 is coupled into the exposure optical path 50 at a point disposed in the optical path of the exposure radiation 48 in front of the mask plane 14. Here too the coupling of the measurement radiation 12 into the exposure optical path 50 is implemented such that the exposure optical path 50 is not interfered with. The gratings 24 and 28 are disposed here in the exposure optical path 50, the imaging optical element 126 disposed between being part of the illumination optics of the projection exposure system 10.

(34) FIG. 5 shows the section on the substrate side of the projection exposure system 10 in a further embodiment according to the invention. In this embodiment an additional grating 54 is disposed downstream of the detection diffraction grating 34 for the coherent superposition of the test waves 30. On the mask side the illumination diffraction grating 24 can then be omitted. In this case only three test waves 30 pass through the projection optics 12. On the detection diffraction grating 34, in the same way as in the methodology illustrated with reference to FIG. 2, splitting of the three test waves 30 into five detection partial beams 36 and on the additional grating into seven prepared detection partial beams 56 takes place.

(35) FIG. 6 shows a further embodiment of the configuration of the section on the substrate side of the projection exposure system 10. According to this embodiment the measurement radiation 21 in the form of the test waves 30 is uncoupled using an uncoupling mirror 58 from the exposure optical path 50 of the projection exposure system 10 in front of the substrate plane 16. This makes it possible to integrate the measurement device 18 into the projection exposure system 10 without any adverse effect upon the structure in the region of the substrate table or of the so-called “wafer stage”.

(36) FIG. 7 shows a further embodiment of the projection exposure system 10 according to the invention wherein the test waves 30 pass through the projection optics 12 with a double passage. For this purpose a retro-reflector 60 is disposed in the substrate plane 16 with which after having passed through the projection optics 12, the test waves 30, referred to in the following as incoming test waves 30a, are reflected back on themselves. Therefore, the test waves 30 pass through the projection optics 12 as backwards running test waves 30b along the same path as the incoming test waves 30a. In a further version of the embodiment according to FIG. 7, in the same way as the version according to FIG. 6, an uncoupling and coupling mirror is disposed on the substrate side in order to uncouple the incoming test waves 30a from the exposure optical path of the projection exposure system and for coupling the backwards running test waves 30b into the exposure optical path. Similarly to the detection diffraction grating 34 according to FIG. 6, in this embodiment the retro-reflector 60 is disposed to the side of the exposure optical path.

(37) The measurement radiation 21 is coupled by a partially transmissive coupling mirror 144 into the exposure optical path. The detection partial beams 36 produced from the backwards running test waves 30b are partially transmitted by the coupling mirror 144 so that the latter can be recorded by the photodiodes 38. The input diffraction grating 28 is also used in the reverse passage as a detection diffraction grating.

(38) FIG. 10a illustrates the optical path of the measurement radiation 21 passing to the retro-reflector 60 extending from the measurement radiation source 20 to the retro-reflector 60 and FIG. 10b illustrates the optical path of the backwards running measurement radiation 21 extending from the retro-reflector 60 to the detectors 38. Here an embodiment is shown which manages with the minimum number of test waves 30, i.e. two test waves.

(39) The retro-reflector 60 is designed in the form of a Littrow grating which, scale corrected, has half the period of the diffraction grating 28 on the object side. Moreover, only three detectors 38 are provided for recording the intensities of the detection partial beams 36. This embodiment can be used if there is only very little space available for the optical path of the measurement device 18. This is the case in particular with EUV systems.

(40) The reflectivities of multiple layers and so-called “multilayer” layers applied for the reflection of EUV radiation onto the mirrors of the projection optics 12 are so small for the wavelength of the measurement radiation 21 with approximately 0.6 that after the double passage of the measurement radiation 21 through a 4 mirror system, only very slight intensity comes back. This problem is resolved according to the invention by working with the minimum number of test waves 30. The cross-sections, or so-called “footprints” of the partial beams on the mirrors are then relatively small and are provided with a reflective coating 70 optimised to the wavelength of the measurement radiation 21.

(41) The reflection in the substrate plane 16 is not implemented with a flat mirror because then the sensor would have no effect. The retro-reflector 60 ensures that the wavefront tilt effects the image offset two times and does not cancel out as with simple mirroring. As already stated above and illustrated e.g. in FIG. 7, the retro-reflector 60 can be in the form of a Littrow diffraction grating. Alternatively, a spherical mirror in the cat's eye position or a retro-reflector based on a prism in the form of so-called “corner cubes” can be used.

(42) A Littrow diffraction grating can be written directly onto the wafer, as illustrated in the embodiment according to FIG. 8. This embodiment differs from the embodiment according to FIG. 7 in that illumination optics 52 for the exposure radiation are disposed between the coupling mirror 144 and the diffraction grating 28. The diffraction grating 28 is disposed on a peripheral region of a product mask 62.

(43) FIGS. 9a and 9b illustrate the mode of operation of the retro-reflector 60 according to FIG. 7 in the form of a Littrow grating. For this purpose, in FIG. 9a a transmission grating 60a and in FIG. 9b a reflection grating 60b is shown. With a Littrow grating the grating period, wavelength and diffraction order are correlated. In accordance with the diffraction formula with a non-perpendicular incidence

(44) $\begin{array}{cc}\sin \alpha +\sin \beta =\frac{m.Math.\lambda }{p_B}& \left(1\right)\end{array}$
the correlation emerges between angle α of the incoming beam 66 to the grating normal and angle β of the outgoing beam 68 diffracted in the first order to the grating normal with wavelength λ, grating period p.sub.B and diffraction order m. With a reflection grating shown in FIG. 9b the outgoing beams 68 are folded upwards. Therefore, for all of the incoming test waves 30a it is possible for the respective diffracted beam 30b to pass back precisely into the corresponding incident beam 30a.

(45) For this purpose the following must be fulfilled: α=β, i.e.
sin α=sin β,  (2)
where

(46) $\begin{array}{cc}\sin \alpha =\frac{m.Math.\lambda }{2p_B}& \left(3\right)\end{array}$

(47) If the angle spectrum corresponds to the diffraction pattern on a grating 28 on the object or mask side with a grating period p.sub.O with a perpendicular incident:

(48) $\begin{array}{cc}\sin \alpha =\frac{m.Math.\lambda }{p_o}& \left(4\right)\end{array}$
it follows that the reflective Littrow grating 60 must have half the period of the equivalent grating on the object side:

(49) $\begin{array}{cc}{p}_B=\frac{p_o}{2}& \left(5\right)\end{array}$

(50) The retroreflection of the optical path is then automatically guaranteed for the whole order.

(51) In the embodiment according to FIG. 7 the grating 28 on the object side is passed through twice with the measurement optical path, which increases the measuring accuracy. It is furthermore possible for incident and outgoing beams to be slightly offset in relation to one another in the mask plane 14. This can take place by defocus and tilting of the Littrow grating 16 or use of a prism or mirror retro-reflector. The grating 28 can then be written differently for the incoming measurement radiation 21 and the backwards running test waves 30b.

(52) The diffraction elements 24, 28, 24 and 54 shown in FIGS. 1 to 10 can also be in the form of hologram structures instead of gratings.

(53) The above description of the exemplary embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the structures and methods disclosed. The applicant seeks, therefore, to cover all such changes and modifications as fall within the spirit and scope of the invention, as defined by the appended claims, and equivalents thereof.