Illumination system
There is provided an illumination system. The illumination system includes (a) a mirror, (b) a diaphragm in a light path downstream of the mirror, and (c) a field plane in the light path, downstream of the diaphragm.
Latest Carl Zeiss SMT AG Patents:
1. Field of the Invention
The invention concerns an illumination system for wavelengths ≦100 nm, wherein the illumination system has an object plane and a field plane. In illumination systems ≦100 nm, the problem exists that light sources of such illumination systems emit radiation that can lead to an undesired exposure of the light-sensitive object in the wafer plane. Furthermore optical components of the exposure system, such as, for example, the multilayer mirror, can be heated up in this way.
In order to filter out the undesired radiation, transmission filters are used in illumination systems for wavelengths ≦1 00 nm. Such filters have the disadvantage of high light losses. In addition, they can be disrupted very easily by heat stress.
The object of the invention is to provide an illumination system for wavelengths ≦100 nm, particularly in the EUV range, in which the above-named disadvantages can be avoided.
According to the invention, this object is solved by an illumination system that has at least one grating element and at least one physical diaphragm in a diaphragm plane. The physical diaphragm is situated in the beam path from the object plane to the field plane after the grating element.
2. Description of the Prior Art
Grating elements, for example, reflection gratings, particularly echelette gratings, which are also known as blazed gratings, have been known for a long time from monochromator construction for synchrotron radiation sources. For these elements good experiences, particularly with very high fluxes, were made.
With respect to the use of diffraction gratings in monochromators, reference is made to the following publications, whose disclosure content is incorporated to the full extent in the present Application:
-
- H. Petersen, C. Jung, C. Hellwig, W. B. Peatman, W. Gudat: “Review of plane grating focusing for soft x-ray monochromators”, Rev. Sci. Instrum. 66(1), January 1996.
- M. V. R. K. Murty: “Use of convergent and divergent illumination with plane gratings”, Journal of the Optical Society of America, Vol. 52, No. 7, July 1962, pp. 768-773.
- T. Oshio, E. Ishiguro, R. Iwanaga: “A theory of new astigmatism and coma-free spectrometer”, Nuclear Instruments and Methods 208 (1993) 297-301.
The inventors have now recognized that a grating element can be used in the beam path from the object plane to the image plane for spectral filtering in an illumination system for wavelengths ≦100 nm, if the individual diffraction orders and the wavelengths are clearly separated from one another.
This is most simple for a grating element within a convergent beam bundle. The convergent beam bundle has a focus with a limited diameter.
In order to obtain a stigmatic imaging of an object into the plane of the physical diaphragm with the aid of a grating element situated in a convergent beam path, in a first embodiment of the invention the optical element is curved concave in a meridional plane. The meridional plane of the optic element is defined as the plane which is perpendicular to the carrier surface of the grating element and to the grating lines.
Alternatively or additionally to this, the optical element can be curved convex in the sagittal plane, which is perpendicular to the carrier surface and the meridional plane, and contains the centre of the grating element.
If an internal diffraction order (k=1, 2, 3) is used, the refractive power in the meridional direction is greater than in the sagittal direction, i.e., the element is concave, e.g., in the meridional direction and planar in the sagittal direction, or it is planar in the meridional direction and convex in the sagittal direction, or it is formed concave in the meridional direction and convex in the sagittal direction.
If an external diffraction order (k=−1, −2, −3) is used, the refractive power in the sagittal direction is greater than in the meridional direction, i.e., the element is concave in the sagittal direction and planar in the meridional direction, or it is planar in the sagittal direction and convex in the meridional direction, or it is formed concave in the sagittal direction and convex in the meridional direction.
In the present application, the order which is diffracted to the surface normal line is denoted the internal order and assigned positive numbers, while the order which is diffracted away from the surface normal line is designated the external order and is assigned negative numbers.
In another embodiment of the invention, the stigmatic imaging is achieved by a variation of the distance between the grating lines.
The at-least one physical diaphragm according to the invention essentially serves for the purpose that light with wavelengths far above 100 nm does not enter into the illumination system. This can be achieved particularly by blocking the zeroth diffraction order. Due to the one physical diaphragm, all diffraction orders are preferably blocked except for a so called used order. The used order, for example, can be the 1st order.
It is particularly preferred if the rays have wavelengths in the range of 7 to 26 nm after the physical diaphragm, due to the combination of grating and physical diaphragm.
The grating element is preferably designed as a blazed grating, which is optimized to a maximal efficiency in a pregiven diffraction order. Blazed gratings are known, for example, from the Lexikon der Optik [Optics Lexicon], edited by Heinz Haferkorn, VEB Bibliographic Institute, Leipzig, 1990, pp. 48 to 49. They are characterized by an approximately triangular groove profile.
In order to avoid too high of a heat load on the physical diaphragm in the diaphragm plane, a part of the undesired radiation can be filtered out by additional diaphragms in the illumination system.
In addition to the illumination system, the invention also provides a projection exposure system with such an illumination system as well as a method for the production of microelectronic components.
BRIEF DESCRIPTION OF THE DRAWINGSAn example of the invention will be described below on the basis of the figures.
Here:
Due to the several partial diaphragms 7.1, 7.2, arranged in front of physical diaphragm 7.3, undesired radiation can be filtered out beforehand, in order to reduce the heat load on physical diaphragm 7.3. The physical diaphragm has a circular opening, which is situated in the focal plane of the desired diffraction order, here the −1. order 16. The diaphragms 7.1, 7.2 may also be cooled, but this is not shown. In addition, grating element 1 can be cooled, for example, by a cooling on the back side. The device 8 for back-side cooling of grating element 1 is preferably a liquid cooling device with inlet 10.1 and outlet 10.2. Due to grating element 1 and physical diaphragm 7.3, the 0th order that encompasses all wavelengths of the light source can be completely blocked out in the illumination system according to the invention. In addition, all of the other orders except for the −1st order are blocked.
If the grating in an illumination system with collector according to
How this is derived will be given in the following.
The starting point for subsequent considerations is the grating equation for a parallel beam bundle:
Nk·λ=sin α+sin β (1)
wherein N is the number of lines, k is the diffraction order, λ is the wavelength, α is the angle of incidence and β is the diffraction angle (relative to the surface normal of the carrier surface and referred to the chief ray CRbefore or CRafter). The nomenclature which is used in the following derivation is oriented to the “Lexikon der Optik [Optics Lexicon] in two volumes, edited by H. Paul, Heidelberg, Berlin, Spektrum Academic Publishers, 1999, Vol. 1, A-L, pp. 77-80.
Reference is made to
In the case shown in
If one now considers in the convergent beam path of an illumination system a reflection grating, which is placed as shown in
cff=cos (β)/cos (α) (2)
The following results for the bundle cross-section at the grating:
dafter=dbeforecff (3)
or for the numerical aperture NA
NAafter=NAbefore/cff (4)
wherein dafter denotes the bundle cross-section of the diffracted beam bundle 14 in the plane 108 and dbefore denotes the bundle cross-section of the incident beam bundle 100 in plane 106, NAafter denotes the numerical aperture of the diffracted beam bundle 14 and NAbefore denotes the numerical aperture of the incident beam bundle.
The following results for the image width s as previously defined, calculated starting at the grating:
safter=sbeforecff2 (5)
Care has to be taken that the grating acts only in the meridional or dispersive direction. In order to obtain a stigmatic imaging it is advantageous to introduce an additional optical effect, e.g., in the sagittal direction.
This can be achieved, for example, for the case when an internal diffraction order (k=1, 2, 3) is used, by a convex curvature in the sagittal direction.
For the case when an external diffraction order (k=−1, −2, −3) is used, it is advantageous that the grating is selected as sagittal concave.
Alternatively to a curved grating, the grating line distance may also be varied.
For the case of a sagittal convex curvature, the radius must be selected such that an image width of sbeforecff2 is obtained from the image width Sbefore in the 0th order. The sagittal focal distance fs can be calculated by means of the imaging equation:
fs=sbefore/(1/cff2−1) (6)
Finally, the sagittal radius results together with the angle of diffraction:
Rs=fs(cos α+cos β) (7)
It will be estimated in the following on an example of embodiment how a grating element 1 must be constructed that the following conditions are fulfilled:
-
- the beam bundles of the 0th and 1st order or −1st order are separated, i.e., at the focal point of one beam pencil of one diffraction order, there is no overlap of this beam bundle by a beam bundle of another diffraction order;
- the utilization wavelength used must be separate from the unwanted wavelengths;
- the distance to the intermediate focus must be small, so the grating does not become too large;
- the diffraction geometry must be optimized for best diffraction efficiency;
- the astigmatism, which produces a defocusing effect for the internal order and a focusing effect for the external order, should remain small.
In particular, the first condition is decisive for the effectiveness of the grating element. A formula for estimating the separation of the beam pencil of the different diffraction orders from one another can be derived as follows with reference to
Δx0=sbefore sin (α+β) (8)
and the distance Δx1 between the chief rays of the beam bundles of different diffraction orders at the focal point of the diffraction order e.g. the 1st or −1st diffraction order is:
Δx1=sbeforecff2 sin (α+β) (9)
Since the respective other beam bundle is not focused, which means that it has an extension, it is necessary for estimating whether the beam pencils do not overlap in the focal point, to estimate the extension of the other beam bundle. This can be estimated by the divergence or the numerical aperture. For the extension of the beam bundle of the 0th order at the focal point of the diffraction order, the following results:
Δd1=2NA cff|sbeforecff2−sbefore| (10)
and for the extension of the beam bundle of the 1st order or of the −1st order at the focal point of the 1st order or the −1st order:
Δd0=2NA|sbeforecff2−sbefore| (11)
The difference between, e.g., Δx0 and Δd0/2 yields, e.g., the distance of the edge rays of the diffracted beam bundle from the focal point of the beam bundle of the 0th order. In order to prevent an overlap of different beam bundles, this distance should correspond to at least half the diameter of the beam bundle in the focal point, which is denoted Δxf; a sufficient separation of the beam bundle of the 0th diffraction order from the beam bundles of other diffraction orders is then achieved.
The following is thus applied:
sbefore sin (α+β)−NA|sbeforecff2−sbefore|>Δxf (12)
or
sbeforecff2 sin (α+β)−NA cff|sbeforecff2−sbefore|cff>Δxf (13)
With the above-given considerations and formulas, the grating element with sagittal convex curvature, which is characterized by a grating efficiency of 56% can be constructed, which is characterized by the values given below in Table 1.
With the grating element 1 according to the embodiment in Table 1 in combination with a diaphragm, wavelengths above approximately 18 nm and below 8 nm can be almost completely filtered out. The heat load on the mirror of a projection system can be clearly reduced in this way.
In order to obtain a grating element 1 with optimal diffraction efficiency, the grating element is preferably configured as a blazed grating.
A blazed grating with approximately triangular groove profile is shown in
If one uses such a grating element, whose local blazed depth B changes with position on the grating, then a maximal efficiency is obtained according to
In
As can be seen from
An EUV projection exposure system with a grating element 1 according to the invention is shown in
Light source 3, which can be, for example, a laser plasma source or a plasma discharge source, is arranged in the object plane of the illumination system. The image of the primary light source, which is also designated as the secondary light source, comes to lie in the image plane 7.3 of the illumination system.
Additional diaphragms 7.1, 7.2 are arranged between grating element 1 and the physical diaphragm 7.3 in order to block the light of undesired wavelengths, particularly wavelengths longer than 30 mm. According to the invention, the focus of the 1st order comes to lie in the plane of diaphragm 7.3, i.e., light source 3 is imaged nearly stigmatic in the plane of diaphragm 7.3 by the collector and the grating spectral filter in the 1st diffraction order. The imaging in all other diffraction orders is not stigmatic.
In addition, the illumination system of the projection system comprises an optical system 20 for forming and illuminating field plane 22 with a ring-shaped field. The optical system comprises two faceted mirrors 29.1, 29.2 as well as two imaging mirrors 30.1, 30.2 and a field-forming grazing-incidence mirror 32 as the mixing unit for homogeneous illumination of the field. Additional diaphragms 7.4, 7.5, 7.6, 7.7 are arranged in optical system 20 for suppressing stray light.
The first faceted mirror 29.1, the so-called field-faceted mirror, produces a plurality of secondary light sources in or in the vicinity of the plane of the second faceted mirror 29.2, the so-called pupil-faceted mirror. The subsequent imaging optics image the pupil-faceted mirror 29.2 in the exit pupil of the illumination system, which comes to lie in the entrance pupil of the projection objective 26. The angle of inclination of the individual facets of the first and second faceted mirrors 29.1, 29.2 are designed in such a way that the images of the individual field facets of the first faceted mirror 29.1 overlap in the field plane 22 of the illumination system and thus an extensively homogenized illumination of a pattern-bearing mask, which comes to lie in the field plane 22, is possible. The segment of the ring field is formed by means of the field-forming grazing-incidence mirror 32 operated under grazing incidence.
A double-faceted illumination system is disclosed, for example, in U.S. Patent US-B-6,198,739, imaging and field-forming components in PCT/EP/00/07258. The disclosure contents of these documents is incorporated to the full extent in the present Application.
The pattern-bearing mask, which is also designated as the reticle, is arranged in field plane 22. The mask is imaged by means of a projection objective 26 in the image plane 28 of field plane 22. The projection objective 26 is a 6-mirror projection objective, such as disclosed, for example, in U.S. application No. 60/255214, filed on Dec. 13, 2000, in the U.S. Patent Office for the Applicant or DE-A-10037870, the disclosure content of which is fully incorporated into the present application. The object to be exposed, for example, a wafer, is arranged in image plane 28.
The replica technique is considered, for example, as a possible manufacturing method for a grating element according to the invention.
The invention gives for the first time an illumination system, with which undesired wavelengths can be selected directly after the light-source unit and which represents an alternative to filter foils, which are problematic, particularly with respect to the heat load.
REFERENCE LIST
- 1 grating element
- 3 light source
- 5 collector
- 7 .1, 7.2, 7.3
- 7.4, 7.5, 7.6
7.7 diaphragms
- 8 cooling device
- 10.1,10.2 inlet and outlet of the cooling device
- 11 incident radiation
- 12 0th order of the wavelength used
- 14 1st order of the wavelength used
- 16 −1st order of the wavelength used
- 20 optical system
- 22 field plane
- 26 projection objective
- 28 image plane of the field plane
- 29.1, 29.2 faceted mirrors
- 30.1, 30.2 imaging mirrors
- 32 field-forming mirror
- 34 exit pupil of the illumination system
- 100 convergent incident beam bundle
- 102 striking point of the chief ray CRbefore on grating 1
- 106 plane, which is perpendicular to the chief ray CRbefore
- 106 plane, which is perpendicular to the chief ray CRafter
- 112 focal point of the beam pencil diffracted in the 0th order
- 114 focal point of the beam pencil diffracted in the 1st order
- 200, 202
- 204, 206 diffraction efficiency η(1) for different materials
Claims
1-17. (canceled)
18. An illumination system comprising:
- a mirror;
- a diaphragm in a light path downstream of said mirror; and
- a field plane in said light path, downstream of said diaphragm.
19. The illumination system of claim 18, wherein said mirror is a facetted mirror having a plurality of facets.
20. The illumination system of claim 19, further comprising a normal incidence mirror in said light path, downstream of said facetted mirror and upstream of said field plane.
21. The illumination system of claim 20, wherein said normal incidence mirror is situated downstream of said diaphragm.
22. The illumination system of claim 20, further comprising a grazing incidence mirror situated in said light path, downstream of said normal incidence mirror and upstream of said field plane.
23. The illumination system of claim 22, wherein said diaphragm is situated in said light path, downstream of said normal incidence mirror and upstream of said grazing incidence mirror.
24. The illumination system of claim 19, further comprising a first normal incidence mirror and a second normal incidence mirror, both of which are in said light path, downstream of said facetted mirror and upstream of said field plane.
25. The illumination system of claim 24, wherein said diaphragm is situated downstream of said first normal incidence mirror and upstream of said second normal incidence mirror.
26. The illumination system of claim 19, further comprising a grazing incidence mirror situated in said light path, downstream of said facetted mirror and upstream of said field plane.
27. The illumination system of claim 19, wherein said plurality of facets includes a first facet and a second facet that are imaged into said field plane as a first image and a second image that at least partially overlap one another.
28. The illumination system of claim 19,
- wherein said facetted mirror is a first facetted mirror, and said plurality of facets is a first plurality of facets, and
- wherein said illumination system further comprises a second facetted mirror having a second plurality of facets, situated in said light path downstream of said first facetted mirror and upstream of said diaphragm.
29. The illumination system of claim 28, wherein said second plurality of facets includes a first facet and a second facet that are imaged into an exit pupil of said illumination system.
30. The illumination system of claim 19,
- wherein said facetted mirror is a first mirror,
- wherein said plurality of facets includes a first facet and a second facet, and
- wherein said illumination system further comprises a second mirror in said light path, downstream of said first mirror and upstream of said field plane, that images said first and second facets into said field plane.
31. The illumination system of claim 18,
- wherein said mirror is a first mirror, and
- wherein said illumination system further comprises a second mirror in said light path, downstream of said first mirror and upstream of said field plane.
32. The illumination system of claim 31, wherein said second mirror is a facetted mirror having a plurality of facets.
33. The illumination system of claim 31, wherein said diaphragm is situated downstream of said second mirror.
34. The illumination system of claim 18, further comprising a light source that emits light into said light path, upstream of said mirror, having a wavelength of less than or equal to about 100 nm.
35. The illumination system of claim 34,
- wherein said diaphragm is a first diaphragm, and
- wherein said illumination system further comprises a second diaphragm situated in said light path downstream of said light source and upstream of said mirror.
36. The illumination system of claim 18, wherein said diaphragm suppresses stray light.
37. The illumination system of claim 18, wherein said diaphragm partially surrounds said light path, and partially suppresses stray light.
38. A projection exposure system comprising:
- (a) an illumination system for illuminating a pattern-bearing mask, wherein said illumination system includes: a mirror; a diaphragm in a light path downstream of said mirror; and a field plane in said light path, downstream of said diaphragm, for accommodating the pattern-bearing mask;
- (b) a holder for holding a light-sensitive object; and
- (c) a projection objective for imaging the pattern-bearing mask onto the light-sensitive object.
39. A method, comprising:
- producing a microelectronic component, wherein said producing includes employing a projection exposure system having: (a) an illumination system for illuminating a pattern-bearing mask, wherein said illumination system includes: a mirror; a diaphragm in a light path downstream of said mirror; and a field plane in said light path, downstream of said diaphragm, for accommodating the pattern-bearing mask; (b) a holder for holding a light-sensitive object; and (c) a projection objective for imaging the pattern-bearing mask onto the light- sensitive object.
40. A method comprising:
- situating a diaphragm in a light path in an illumination system, downstream of a facetted optical element and upstream of a field plane,
- wherein said diaphragm suppresses stray light in said illumination system.
Type: Application
Filed: Jun 19, 2007
Publication Date: Oct 18, 2007
Applicant: Carl Zeiss SMT AG (Oberkochen)
Inventors: Markus Weiss (Aalen), Wolfgang Singer (Aalen), Bernd Kleemann (Aalen)
Application Number: 11/820,375
International Classification: G02B 5/08 (20060101);