Resources Contact Us Home
Browse by: INVENTOR PATENT HOLDER PATENT NUMBER DATE
 
 
Method and system for wavefront measurements of an optical system
7408651 Method and system for wavefront measurements of an optical system
Patent Drawings:Drawing: 7408651-10    Drawing: 7408651-11    Drawing: 7408651-3    Drawing: 7408651-4    Drawing: 7408651-5    Drawing: 7408651-6    Drawing: 7408651-7    Drawing: 7408651-8    Drawing: 7408651-9    
« 1 »

(9 images)

Inventor: Latypov, et al.
Date Issued: August 5, 2008
Application: 10/848,179
Filed: May 19, 2004
Inventors: Latypov; Azat M. (Danbury, CT)
Poultney; Sherman K. (Wilton, CT)
Vladimirsky; Yuli (Weston, CT)
Assignee: ASML Holding N.V. (Veldhoven, NL)
Primary Examiner: Turner; Samuel A
Assistant Examiner:
Attorney Or Agent: Sterne, Kessler, Goldstein & Fox, P.L.L.C.
U.S. Class: 356/515
Field Of Search: 356/488; 356/494; 356/499; 356/508; 356/509; 356/510; 356/512; 356/520; 356/521; 356/124; 356/515
International Class: G01B 9/02
U.S Patent Documents:
Foreign Patent Documents: 11-304653; 2003-309066; WO 98/33096; WO 98/38597; WO 01/63233; WO 03/076891
Other References: Robert Monteverde. "Spatial Light MOdulators Illuminate a Wide Variety of Application Spaces", Laser Focus World, Jan. 2004. cited by examiner.
Braat et al., "Improved Ronchi Test with Extended Source," J. Opt. Soc. Am. A, vol. 16, No. 1, pp. 131-140 (Jan. 1999). cited by other.
Naulleau et al., "Static Microfield Printing at the Advanced Light Source with the ETS Set-2 Optic," Proc. SPIE 4688-05, pp. 64-71 (2002). cited by other.
Durr, P. et al., "Characterization of Spatial Light Modulators for Micro Lithography," Proceedings of SPIE, vol. 4985, pp. 266-276 (2003). cited by other.
Durr, P. et al., "Test system for micro mirror arrays," Proceedings of SPIE, vol. 4178, pp. 358-364 (2000). cited by other.
Lakner, H. et al., "Design and Fabrication of Micromirror Arrays for UV-Lithography," Proceedings of SPIE, vol. 4561, pp. 255-264 (2001). cited by other.
Sandstrom, T. et al., "Pattern Generation with SLM Imaging," Proceedings of SPIE, vol. 4562, pp. 38-44 (2002). cited by other.
Sandstrom, T. and Eriksson, N., "Resolution extensions in the Sigma 7000 imaging pattern generator," Micronic Laser Systems AB, Strategic Development, Molndalsvagen 91, SE-435 54 Goteborg; Sweden, 11 pages. cited by other.
Search Report for Singapore Patent Application No. 200503134-9 dated Sep. 24, 2007, 7 pages. cited by other.
Office Action and Translation of Office Action for Japanese Patent Application No. 2005-147251 mailed May 16, 2008, 7 pgs. cited by other.
English Abstract for Japanese Publication No. JP2003-524175T published Aug. 12, 2003, 1 pg. cited by other.









Abstract: A wavefront measurement system includes a source of electromagnetic radiation. An illumination system delivers the electromagnetic radiation to an object plane. A source of a diffraction pattern is in the object plane. A projection optical system projects the diffraction pattern onto an image plane, which includes a mechanism (e.g., a shearing grating) to introduce the lateral shear. A detector is located optically conjugate with the pupil of the projection optical system, and receives an instant fringe pattern, resulting from the interference between sheared wavefronts, from the image plane. The diffraction pattern is dynamically scanned across a pupil of the projection optical system, and the resulting time-integrated interferogram obtained from the detector is used to measure the wavefront aberration across the entire pupil.
Claim: What is claimed is:

1. A wavefront measurement system, comprising: an illumination system configured to direct electromagnetic radiation onto an object plane; an object in the object planeconfigured to produce a diffraction pattern; a projection system configured to project the diffraction pattern onto an image plane; a shearing grating in the image plane configured to create a fringe pattern from the diffraction pattern; and adetector configured to receive the fringe pattern, wherein the object in the object plane is configured to move to project the diffraction pattern across a pupil of the projection optical system by moving the object in the object plane relative to theelectromagnetic radiation.

2. The system of claim 1, wherein the object includes a tiltable mirror for scanning the diffraction pattern across the pupil.

3. The system of claim 1, wherein the object includes a diffraction grating with variable pitch for scanning the diffraction pattern across the pupil.

4. The system of claim 1, wherein the object includes a refractive prism with a varying wedge angle for scanning the diffraction pattern across the pupil.

5. The system of claim 1, wherein the object includes a spatial light modulator for scanning the diffraction pattern across the pupil.

6. The system of claim 1, wherein the object generates a non-linear phase variation across to scan the diffraction pattern across the pupil.

7. The system of claim 1, wherein the object comprises a stress-birefringent material to scan the diffraction pattern across the pupil.

8. The system of claim 1, further comprising reflective optics for scanning the diffraction pattern across the pupil.

9. The system of claim 1, further comprising refractive optics for scanning the diffraction pattern across the pupil.

10. The system of claim 1, wherein the object includes a movable mirror with a variable surface slope for scanning the diffraction pattern across the pupil.

11. The system of claim 1, wherein the diffraction pattern is dynamically scanned across a pupil of said projection optical system.

12. The system of claim 1, wherein the detector is located in a plane that is optically conjugate with the pupil.

13. The system of claim 1, wherein: the shearing grating in the image plane generates the diffraction pattern that comprises first and second wavefronts; and the first and second wavefronts interfere with each other to produce the fringepattern.

14. A wavefront measurement system, comprising: an illumination system that delivers electromagnetic radiation to an object plane; an object in the object plane that generates a diffraction pattern from the electromagnetic radiation; aprojection optical system that projects the diffraction pattern onto an image plane; a shearing grating in the image plane that creates a fringe pattern from the diffraction pattern; and a detector that receives the fringe pattern, wherein as thediffraction pattern is projected across a pupil of the projection optical system, the diffraction pattern is scanned by moving the object in the object plane relative to the electromagnetic radiation.

15. The system of claim 14, wherein the object includes a tiltable mirror for scanning the diffraction pattern across the pupil.

16. The system of claim 14, wherein the object includes a diffraction grating with variable pitch for scanning the diffraction pattern across the pupil.

17. The system of claim 14, wherein the object includes a refractive prism with a varying wedge for scanning the diffraction pattern across the pupil.

18. The system of claim 14, wherein the object includes a spatial light modulator for scanning the diffraction pattern across the pupil.

19. The system of claim 14, wherein the source of the diffraction pattern generates a non-linear phase variation across to scan the diffraction pattern across the pupil.

20. The system of claim 14, wherein the source of the diffraction pattern includes a stress-birefringent material for scanning the diffraction pattern across the pupil.

21. The system of claim 14, further comprising reflective optics for scanning the diffraction pattern across the pupil.

22. The system of claim 14, wherein the object includes a movable mirror with a variable surface slope for scanning the diffraction pattern across the pupil.

23. The system of claim 14, wherein the detector is located in a plane that is optically conjugate with the pupil.

24. The system of claim 14, wherein: the shearing grating in the image plane generates the diffraction pattern that comprises first and second wavefronts; and the first and second wavefronts interfere with each other to produce the fringepattern.

25. A method of measuring a wavefront of an optical system, comprising: generating electromagnetic radiation at a source; delivering said electromagnetic radiation to an object plane of said optical system; generating a diffraction patternusing an object in said object plane; scanning the diffraction pattern across a pupil of said optical system by moving the object in the object plane relative to the electromagnetic radiation; receiving the diffraction pattern from said object plane; generating a fringe pattern associated with the diffraction pattern; determining wavefront parameters from said fringe pattern; and generating an interferogram graph from the wavefront parameters.

26. The method of claim 25, wherein the scanning step includes tilting a mirror to direct the diffraction pattern across the pupil.

27. The method of claim 25, wherein the scanning step includes moving a diffraction grating with variable pitch to direct the diffraction pattern across the pupil.

28. The method of claim 25, wherein the scanning step includes adjusting a refractive prism with a varying wedge angle to direct the diffraction pattern across the pupil.

29. The method of claim 25, wherein the scanning step includes adjusting a spatial light modulator to direct the diffraction pattern across the pupil.

30. The method of claim 25, wherein the scanning step uses reflective optics to scan the diffraction pattern across the pupil.

31. The method of claim 25, wherein the scanning step uses refractive optics to scan the diffraction pattern across the pupil.

32. A method of measuring a wavefront of a projection optical system, comprising: using an object in an object plane to generate a diffraction pattern, the diffraction pattern being directed at the projection optical system; receiving thediffraction pattern at an image plane; using a shearing grating to produce a fringe pattern on a detector based on the diffraction pattern; positioning the detector below an image plane of the projection optical system; receiving the fringe pattern ofthe diffraction pattern at the detector while simultaneously scanning the diffraction pattern across a pupil of the optical system by moving the object in the object plane relative to the electromagnetic radiation; determining wavefront aberrations fromthe fringe pattern; and generating an interferogram graph from the determined wavefront aberrations.

33. A wavefront measurement system, comprising: means for generating electromagnetic radiation; means for generating a diffraction pattern from the electromagnetic radiation in at an object plane of a projection optical system, the projectionoptical system being configured to project an image of said diffraction pattern onto an image plane; means for generating a fringe pattern in the image plane based on the diffraction pattern; means for detecting the fringe pattern at said image plane; and means for scanning the diffraction pattern across a pupil of said projection optical system by moving said means for generating a diffraction pattern relative to the electromagnetic radiation.

34. The system of claim 14, further comprising refractive optics for scanning the diffraction pattern across the pupil.

35. The wavefront measurement system of claim 1, wherein the object is configured to scan the projected diffraction pattern across the pupil.
Description: BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally related to photolithography systems, and more particularly, to measuring wavefront parameters in a photolithographic system.

2. Related Art

Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently usedsubstrate for such applications is a semiconductor wafer. One skilled in the relevant art would recognize that the description herein would also apply to other types of substrates.

During lithography, a wafer, which is disposed on a wafer stage (WS), is exposed to an image projected onto the surface of the wafer by an exposure system located within a lithography system. The exposure system includes a reticle (also called amask) for projecting the image onto the wafer.

The reticle is usually mounted on a reticle stage (RS) and generally located between the wafer and a light source. In photolithography, the reticle is used as a photo mask for printing a circuit on the wafer, for example. Lithography lightshines through the mask and then through a series of optical lenses that shrink the image. This small image is then projected onto the wafer. The process is similar to how a camera bends light to form an image on film. The light plays an integral rolein the lithographic process. For example, in the manufacture of microprocessors (also known as computer chips), the key to creating more powerful microprocessors is the size of the light's wavelength. The shorter the wavelength, the more transistorscan be formed on the wafer. A wafer with many transistors results in a more powerful, faster microprocessor.

As chip manufacturers have been able to use shorter wavelengths of light, they have encountered a problem of the shorter wavelength light becoming absorbed by the glass lenses that are intended to focus the light. Due to the absorption of theshorter wavelength light, the light fails to reach the silicon wafer. As a result, no circuit pattern is created on the silicon wafer. In an attempt to overcome this problem, chip manufacturers developed a lithography process known as ExtremeUltraviolet Lithography (EUVL). In this process, a glass lens can be replaced by a mirror.

The problem of measuring the undesirable perturbations of the wavefront (often referred to as wavefront aberrations) is a persistent one for the lithographic applications. These wavefront aberrations result from various physical causes, such aschanges in refractive or reflective properties of the optical elements (lenses or mirrors) occurring as a result of mechanical displacements or deformations, or changes in the optical properties of the optical elements caused by heating, or light-inducedcompaction. In particular, it is desirable to be able to measure wavefront quality in the photolithographic tool during wafer production and exposure, rather than having to take the tool offline in order to do so, which increases cost of ownership,reduces through-put or introduces some other type of inefficiency.

SUMMARY OF THE INVENTION

The present invention is directed to a scanning interferometer with dynamic pupil fill that substantially obviates one or more of the problems and disadvantages of the related art.

An embodiment of the present invention includes a wavefront measurement system including a source of electromagnetic radiation. An illumination system delivers the electromagnetic radiation to an object plane. An object generates a diffractionpattern and is located in the object plane. A projection optical system projects an image of the object onto an image plane. A detector receives a fringe pattern from the image plane. The diffraction pattern is scanned across a pupil of the projectionoptical system.

Another embodiment of the present invention includes a wavefront measurement system with an illumination system that delivers electromagnetic radiation at an object plane. A source of a beam of the electromagnetic radiation is in the objectplane. A projection optical system focuses the beam onto an image plane. A detector receives a fringe pattern of the beam from the image plane. The beam is scanned across a pupil of the projection optical system.

Another embodiment of the present invention includes a method of measuring a wavefront of an optical system including generating electromagnetic radiation at a source; delivering the electromagnetic radiation at an object plane of the opticalsystem; generating a diffraction pattern at the object plane; scanning the diffraction pattern across a pupil of the optical system; receiving an image of the source while scanning the diffraction pattern; and determining wavefront parameters from theimage.

Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the invention will berealized and attained by the structure and particularly pointed out in the written description and claims hereof as well as the appended drawings.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THEDRAWINGS

The accompanying drawings, which are included to illustrate exemplary embodiments of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the descriptionserve to explain the principles of the invention. In the drawings:

FIG. 1 shows a portion of an exemplary photolithographic system of the present invention.

FIGS. 2 and 3 illustrate the use of an interferometer to produce shear wavefronts.

FIG. 4 illustrates an example of interference fringes as they appear at the focal plane with the use of the present invention.

FIG. 5 illustrates the rationale for the present invention, showing the optical exposure system in a stylized, schematic form.

FIG. 6 shows how magnitudes of the diffraction orders and change in an interferogram with an extended object in the object plane.

FIG. 7 illustrates an effect of modulating the extended object by a Ronchi grating.

FIG. 8 is another illustration of the arrangement of the optical elements that may be used in the present invention.

FIG. 9 is an illustration of dynamic pupil fill using a tilting reflective Ronchi grating.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

It is convenient to characterize field-dependent aberrations of a projections optics (PO) by an aberration of a wavefront of a spherical wave emitted from a corresponding field point in the object plane. Various interferometry techniques can beused to measure aberration of this spherical wave. Shearing interferometry based on an extended incoherent source in the object plane superimposed with an object-plane grating matching the shearing grating is described in J. Braat and A. J. E. M.Janssen, Improved Ronchi test with Extended Source, J. Opt. Soc. Am. A, Vol. 16, No. 1, pp. 131-140, January 1999, incorporated by reference herein. Also, the paper by Naulleau et al., Static Microfield Printing at the ALS with the ETS-2 Set Optic,Proc. SPIE 4688, 64-71 (2002) (http://goldberg.lbl.gov/papers/Naulleau_SPIE.sub.--4688(2002).pdf- ), incorporated by reference herein describes the dynamic pupil fill illumination system for EUV implemented in order to control partial coherence duringprinting at a synchrotron light source where illumination is coherent.

FIG. 1 illustrates a photolithographic system 100 according to the present invention. The system 100 includes an illumination source 105, a condenser lens 102, an extended object 103 (located in the object plane), projection optics 104 with apupil 105, an image plane shearing grating 106, a detector lens 107 and a CCD detector 108, arranged as shown in the figure. These elements will be discussed further below.

The grating 106 includes both transmissive and opaque regions. The opaque regions can be formed of materials that absorb the radiation (for example, for 13.5 nm exposure wavelength in the case of EUV lithography, or optical radiation in the caseof lithographic systems using longer wavelengths), such as nickel, chromium or other metals.

It will also be appreciated that although the present invention is applicable to lithographic systems that use refractive optical elements (such as the projection optics 104, and the imaging optics), the invention is also applicable to systemsusing other wavelengths, with appropriate transmissive/refractive components used in place of reflective ones, as needed.

The grating 106 also can include reflective (or opaque) regions. These reflective regions can be formed of materials that absorb the radiation (for example, for 13.5 nm EUV exposure wavelength), such as nickel, chromium or other metals.

The pitch of the grating 106 is chosen to provide an appropriate shear ratio, where the CCD detector 108 is in the fringe plane (i.e., below the focal, or image, plane of the system), and "sees" a pattern of fringes (an interferogram) or a numberof overlapping circles, as will be discussed further below. The shear ratio is a measure of the overlap of two circles, where a shear ratio of zero represents perfect overlap. Note also that it is desirable for the CCD detector 108 to "see" only thezeroth order and the + and -1.sup.st order diffraction images, and to eliminate the + and -2.sup.nd order diffraction images. Furthermore, the extended object 103 is constructed to aid in eliminating unwanted orders. It is important, however, thatwhichever pattern of transmission and reflection areas is used, that it be a regular pattern.

The pitch of the source module grating 203 is also preferably chosen to match the pitch of the shearing grating so as to redistribute the light in the pupil to those locations that will mutually overlap as a result of shearing.

FIGS. 2 and 3 illustrate reference wavefronts and shear in a lateral shearing interferometer 210. The lateral shearing interferometer 210 interferes a wavefront with itself, or, phrased another way, it interferes a shifted copy of the wavefrontwith itself. As shown in FIGS. 2 and 3, the grating 106, positioned in the image plane, acts as a shearing interferometer, and generates a transmitted waves 204 with a wavefront 211A, and a diffracted reference wave 205 with a wavefront 211B. Thus, thelateral shearing interferometer 210 creates one or more apparent sources, whose wavefronts 211A, 311B interfere to produce fringes 212.

FIG. 4 illustrates the wavefront fringes (212A, 212B in FIG. 2) as seen by the CCD detector 108. As shown in FIG. 4, in the upper right-hand photograph, sheared fringes for a single object space slit are shown, where the slit is positioned infront of an incoherent, diffuse source that fills the maximum numerical aperture and smoothes any wavefront inhomogeneities. The bottom right-hand figure shows a fringe visibility function 401, with zeroth order pattern 502 and first order diffractionpatterns 403. The 50% duty cycle on the grating 106 makes all even orders of the diffraction pattern invisible. At the bottom left of FIG. 4, the image space sharing grating 106 is shown, with a shear ratio of 0.5.

FIGS. 2 and 3 illustrate reference wavefronts and shear in a lateral shearing interferometer 210. The lateral shearing interferometer 210 interferes a wavefront with itself, or, phrased another way, it interferes a shifted copy of the wavefrontwith itself As shown in FIGS. 2 and 3, the grating 106, positioned in the image plane, acts as a shearing interferometer, and generates a transmitted waves 204 with a wavefront 211A, and a diffracted reference wave 205 with a wavefront 211B. Thus, thelateral shearing interfero meter 210 creates one or more apparent sources, whose wavefronts 211A, 211B interfere to produce fringes 212.

Thus, the problem of measuring wavefront aberrations has to balance two competing interests: filling the entire pupil 105 (but at the cost of very low intensity), or having sufficient intensity, but only on a small portion of the pupil 105.

The following methods can be used to ensure the desired pupil fill in shearing interferometry:

(1) introducing a transmissive pattern inside the extended object 103 that matches the shearing grating (e.g., a Ronchi grating) and providing a fully incoherent illumination of the extended object 103 [see Baselmans, supra]; and

(2) placing an extended incoherent source into an object plane (by using a critical illumination or a diffuser in the object plane) and superimposing it with a Ronchi grating that matches the shearing grating [see Bratt et al., supra].

These methods have several problems:

(1) speckle disturbance of the measured interferograms occurs due to the remaining coherence of the effective source (this applies to methods 1 and 2) or due to a finite size of the object plane diffuser elements (this applies to method 2). Thespeckle disturbance adds high-frequency intensity fluctuations to the measured interferograms, resulting in wavefront measurement errors;

(2) the need to switch to a special illumination mode (this applies to methods 1 and 2) during the measurement complicates the wavefront measurement process; and

(3) a significant portion of light is diffracted from the extended object 103 away from the pupil 105 and does not participate in a formation of the sheared interferogram.

The present invention thus applies to the situation when the size of the extended object 103 needed to ensure the required optical throughput is such that the characteristic width of the diffraction pattern is much less than the object side NA ofthe PO 104, i.e., .lamda./extended object size <<NA object.

The pupil fill by the extended object 103 can be achieved dynamically. During the measurement of the interferogram, the extended object 103 can be dynamically modified, so that the diffraction pattern from this object scans across the wholeentrance pupil 105. The CCD detector 108 that measures the sheared interferogram integrates (or sums) the momentary interferograms occurring in the process of measurement.

The dynamic modification of an extended object 103 can be performed by using a reflective element, such as a tilting mirror, or by using a refractive object with varying slope and/or other characteristics (e.g., a parabolic or spherical lens)moved against an aperture.

The reflected and transmissive extended objects described above use dynamic phase variation of light induced by linearly varying complex reflectivity/transmittance within or across the extended object 103. However, arbitrary (non-linear) phasevariation effect can be also used to fill the pupil 105 dynamically. Many physical arrangements well known to those skilled in the art are possible for realizing such arbitrary non-linear phase variations. For instance, they can be achieved by using anextended movable object with its structure changing within or across it, and/or dynamically deformed, and/or otherwise dynamically modified (e.g., using spatial light modulators). Other possible realizations of dynamically introduced phase variation caninclude a diffusor pattern formed on a transmissive or reflective flexible substrate that can be physically deformed, including plastics, piezoelectric materials, and stress-birefringent materials whose stresses are induced by actuators, etc.

Unless very small shears are utilized, in either of the above methods, the extended object 103 must have a transmittance pattern (an object plane Ronchi grating matching the shearing grating) superimposed on it to provide additionalredistribution of light in the pupil 105, as described in Bratt et al. and Baselmans, cited above.

The dynamic modification is performed so that the transmittance function within the extended object has a time-dependent linear variation of the phase that ensures that the diffraction pattern from the extended object is shifted within the pupil105, dynamically sweeping the pupil 105 during the act of measurement.

The measurement of the interferogram is performed by the CCD detector 108 that records energy distribution across the CCD detector 108 plane. The CCD detector 108 is capable of integrating the time-varying intensity at every point in thedetector 108 plane to collect a sufficient number of photons during the act of measurement. The CCD arrays used in present-day wavefront sensors (like the CCD detector 108) satisfy this requirement.

As noted above, the dynamic modification of the extended object 103 can be achieved by any number of mechanisms. For instance, a reflective extended object 103 may be used. Examples of such reflective objects include:

(1) A tilting flat mirror can be used in a combination with an aperture, or having only a small flat portion of a large tilting object to be reflective. A relatively large tilting mirror is easier to control (e.g., to tilt and rotate) comparedto a micro-mirror. The extended object 103 in this case coincides with the tilting flat mirror, as shown in FIG. 9.

(2) A tilting micro-mirror, such as a mirror from a spatial light modulator (SLM) array, can be used as the entire extended object 103 (see FIG. 9). In order to sweep the pupil 105, the micro-mirror has to tilt in two axes in the object plane. If the micro-mirror can only tilt in one axis, it can be rotated around the axis perpendicular to the object plane, thus allowing a conical sweep of the 2D pupil 105. In practice, such a case is rare.

(3) A reflective object with a varying slope of a reflective surface, such as a parabolic or spherical mirror, which is moved linearly behind a small aperture, can be used as the entire extended object 103.

Note that in the case of reflective elements used to scan the diffraction pattern across the pupil 105, they need not always be located at the object plane. For example, a flat tilting mirror could be located between the object plane and thepupil 105 of the PO 104 (also acting to fold the optical axis of the system).

The extended object 103 can also be transmissive. In that case, the dynamic pupil fill can be achieved by moving a refractive element with a varying slope of one of its surfaces (e.g., a spherical or parabolic lens) against a small aperture, asshown in FIG. 8. A transmissive grating can also be used, such that various regions on the grating have different grating pitch, and the grating is moved linearly in its plane (i.e., perpendicular to the direction of the propagation of theelectromagnetic radiation) so as to vary the direction of the beam (i.e., to scan it across the pupil 105). It is also important to realize that, depending on the particular type of extended object 103 used, the size of the pupil 105 and the scanningapproach, maintaining proper focus in the image plane may become a problem, as the diffraction pattern is being scanned across the pupil 105. However, it is currently believed that although it is preferred to maintain focus, some de-focusing isacceptable.

Unless very small shears are utilized, in any of the above implementations, the extended object 103 must have a transmittance pattern (an object plane Ronchi grating matching the shearing grating) superimposed on it to provide additionalredistribution of light in the pupil 105, as described in Bratt et al. and Baselmans. In addition, any of the above objects are preferably translatable in two lateral dimensions to accomplish the phase shift readout of fringes preferred in the shearinginterferometer measurement.

The final sheared interferogram measured by the CCD detector 108 is a result of integration in time of the momentary sheared interferograms resulting from most of the light concentrated within a small portion of the pupil 105. The momentarysheared interferograms may have high contrast interference fringes only within a relatively small portion of the pupil image in the detector plane formed by the interfering diffraction orders. Their time integral measured by the CCD detector 108 haswell-defined interference fringes across the whole pupil 105 that can be used (typically in conjunction with phase-stepping) to compute the wavefront aberration.

This is due to the fact that dynamic pupil fill described above is equivalent to the use of a stationary source corresponding to an actual source convolved with the dynamic movement (source scanning). Thus, regardless of the degree of coherenceof illumination from the actual source, the effective source provides fully incoherent illumination.

FIG. 5 illustrates the rationale for the present invention, showing the optical exposure system in a somewhat stylized, schematic form. This figure relates to the use of a pinhole in the object plane in order to generate the spherical wave thatfills the pupil 105 and whose aberration is measured by the shearing interferometer. As shown in FIG. 5, going from top to bottom in the figure, light from a light source goes through a condenser lens 102, and then through an object plane with apinhole. The magnitude of the field at the pupil 105 of the projection optics 104 is shown by diagram A, where the pupil 105 coordinate is given as "f". Light then is focused onto an image plane 605, then passes through optional detector projectionoptics 107, and is detected by the detector 108 in the detector plane. Graph B shows magnitudes of the -1 and +1 diffraction orders formed by the shearing grating, which is located in the image plane 605. Graph C shows an interferogram resulting formthe diffraction orders from the shearing grating. Note the visible image variation that is due to the aberrations (phase variations) that are present in the resulting interferogram.

FIG. 6 shows how the magnitudes of the diffraction orders and change in the interferogram, when an extended object 103 is placed in the object plane that fills only a small portion of the pupil 105, resulting in interferogram fringes of verysmall (negligible) contrast observed in the detector plane within the non-overlapping peaks corresponding to the sheared diffraction pattern of the extended object 103. As in FIG. 5, Graph A in FIG. 6 shows the magnitude of the field in the pupil 105 ofthe projection optics 104, with the pupil coordinate "f". Graph B shows the magnitudes of the 0th, -1, and +1 diffraction orders formed by the shearing grating 106, when an extended object 103 is present in the object plane. Graph C shows theinterferograms resulting from the diffraction orders from the shearing grating. Because the diffraction orders do not overlap sufficiently, the resulting interferogram only weakly depends on the wavefront aberrations.

FIG. 7 illustrates the effect of modulating the extended object 103 by a Ronchi grating, that redistributes the light in the pupil 105 to those locations that will mutually overlap as a result of shearing. As with FIGS. 5 and 6, Graph A showsthe magnitude of the field in the pupil 105 of the projection optics 104. Graph B shows magnitudes of the 0th, +1 and -1 diffraction orders formed by the shearing grating, and Graph C shows the resulting interferogram from the diffraction orders fromthe shearing grating and the matching Ronchi grating. As a result of the overlap between the 0th, +1 and -1 diffraction orders, the interferogram in the overlap region (inside the peaks) strongly depends on the wavefront aberrations.

FIG. 8 is another illustration of the arrangement of the optical elements that may be used in the present invention, namely a dynamic pupil fill using a moving refractive object. The illustration of FIG. 8 is primarily applicable to atransmissive extended object 103. For example, as shown in FIG. 8, a transmissive Ronchi grating 801 can be used, with a refractive object having a varying slope that is moved against the object plane. The graphs on the lower right of FIG. 8 illustratethe interferograms resulting with this arrangement. Note that the refractive object, as noted above, may be, for instance, a spherical or a parabolic lens that is being moved against a small aperture.

FIG. 9 is another illustration of dynamic pupil fill using a tilting reflective Ronchi grating 901, with the beam patterns not shown. As shown in FIG. 9, a beam splitter 902 may also be necessary. The reflective extended object 103 (in thiscase, the Ronchi grating 901) is placed on a tilting mirror. The diagrams at bottom right illustrate the resulting interferogram patterns. The large tilting mirror can be used in combination with an aperture, or can have only a small, flat partportion, or can be a small flat portion of a large tilting object, which is made reflecting. A relatively large mirror, or a larger object, is easier to handle (in other words, to tilt and rotate) compared to a micromirror. The extended object 103 inthis case would coincide with the large tilting mirror, as shown in FIG. 9.

Also, a tilting micromirror (such as a mirror in a spatial light-modulator array) may be used as the entire extended object 103.

The present invention has a number of advantages over conventional systems. For example, the dynamic pupil fill eliminates the need for a diffuser in the object plane (e.g., in EUV wavefront sensors), thus resulting in elimination or reductionof speckle-induced wave front measurement errors.

The dynamic pupil fill also eliminates the need to switch to a special illumination mode during the wavefront measurement. The same illumination mode used during the exposure can be used to perform the wavefront measurement. (However, one stillhas to properly position a reticle stage with the tilting mirror on it.)

The dynamic pupil fill also allows to fill the PO pupil 105 "tightly," thus significantly reducing the loss of light that occurs with other methods. If necessary or desirable, the dynamic pupil fill allows sampling only the portions of the POpupil that are of interest.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. Thus, the breadth and scope of thepresent invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

* * * * *
 
 
  Recently Added Patents
Hoists
System and method for conditionally sending a request for data to a home node
Method for measuring and improving organization effectiveness
Cosmetic product including vegetable oil blend
Fuser member
Transmission apparatus and network protection method
Interferer region identification using image processing
  Randomly Featured Patents
Stable flexible pouch and method for making the pouch
Treatment of helminth infections with substituted phenyl-thiourea derivatives
System and method for computing rows since sequence function in a database system
Cutting tool holder retention system
Optical fibres and coatings therefor
Herbicidal compositions with increased crop safety
Electronic identification system employing a data bearing identification card
Resin sheet with copper foil, multilayer printed wiring board, method for manufacturing multilayer printed wiring board and semiconductor device
Heat-exchanger tube for condensing of vapor
Method for controlling the charging and discharging phases of a backup capacitor and a circuit configuration for carrying out the method