X-Ray Ringfield lithography

Abstract: Ringfield projection apparatus using x-ray radiation e.g. of 130 Å wavelength is suitable for lithographic patterning in the fabrication of integrated circuits at design rules of 0.25 µm and below. The design permits reduction from an enlarged mask as well as substantial throughput, the latter due to unexpectedly large slit width. Incorporation of a folding mirror (29) as the third of four mirrors improves fabrication expediency by moving the device being fabricated to the other side of the system from the mask.

Description: Background of the Invention 1. Technical Field The invention relates to ringfield projection pattern delineation expediently used in the fabrication of devices, e.g. integrated circuits, using submicron design rules, as well as to apparatus of such design as to serve in such fabrication. Utilization of reflective optics facilitates operation within the x-ray spectrum - in particular, at "long" wavelengths at and below 300 Å down to tens of Å (1Å = 0.1 nm). A variety of design features are of particular consequence. Throughput capability in such fabrication results from lens design permitting substantial width as well as substantial straight line length of the scanning ringfield slit - factors resulting also in the ability to pattern delineate otherwise desired large dimensioned LSI chips within a single scan sequence. Fabrication of Very Large Scale Integrated devices - electronic as well as optical and hybrid - built to design rules of 0.25 µm and below is contemplated. 2. Description of the Prior Art The saga of Large Scale Integration from inception to the present time is well known. Evolution of the present 1-2 megabit chip built to design rules at or slightly below 1 µm does not represent the ultimate product. Lithographic definition has played, and will continue to play, a significant role. Fabrication of state-of-the-art devices depend on use of near-ultraviolet wavelength (e.g. of λ = 3650 Å). Intensive development effort is based on next generation devices which use shorter wavelength - or "deep UV" (e.g. of λ = 2480 Å). Still smaller design rules will likely dictate use of shorter wavelengths, and it is expected that electromagnetic radiation used for patterning will necessarily be in the x-ray spectrum. R&D activity concerned with x-ray fabrication of future generation devices has concentrated on x-ray wavelengths of λ = 10-15 Å. Considerations relating to characteristics of materials - poor reflectance, transmission/absorption - arc all considered to preclude useful optics. In consequence, this x-ray effort has been largely directed to "proximity printing" - effectively contact printing with small separation to protect fragile masks - an approach necessarily limited to 1:1 object-to-image size ratio. Despite the above problems, a number of considerations support a continuing parallel effort directed to projection X-ray lithography. The effort is largely due to hopes for object-to-image reduction. Fabricating of 1:1 masks particularly at or below 0.25 µm design rules presents problems while enlarged masks of high integrity, suitable for use in reduction systems, may be made using present technology. In addition, thick, robust reflective masks, precluded in proximity printing, may be used in projection. Early efforts on projection x-ray lithography used the two spherical mirror Schwarzschild system. (See, "Soft x-ray Reduction Lithography Using Multilayer Mirrors" by Kinoshita, et al, J. Vac. Sci. Technol. , vol. B6, p. 1648 (1989) and "Soft X-ray Projection Lithography Using an X-ray Reduction Camera" by Hawryluk and Seppala, J. Vac. Sci. Technol. , vol. B6, p. 2161 (1988).) There are three primary factors which restrict the use of a Schwarzschild design for practical lithography - field curvature, difficulty in achieving telecentricity, and degradation due to central obscuration. Promising work concerns use of ringfield scanning systems. All ringfield optical forms are based on radial dependence of aberration and use the technique of balancing low order aberrations (i.e. third order) with higher order aberrations to create narrow annular regions of correction away from the optical axis of the system (regions of constant radius, rotationally symmetric with respect to the axis). The shape of the corrected region is, in consequence, an arcuate strip rather than a straight strip. Strip width is a function of design rule, with increasing residual astigmatism at distances greater or smaller than the design radius being of greater consequence for greater resolution. A relevant apparatus based on this principle, but for use at longer wavelengths, is the Offner 1:1 ringfield system marketed by Perkin-Elmer Corporation. Useful description is included in the disclosure of U.S. Pat. 3,748,015, issued July 24, 1973. It is a zero distortion, telecentric system used at unit magnification. Because of the symmetry of this system (here we refer to the fact that the optical system and beam paths are effectively identical from the object to the stop as they are from the stop to the image the "stop" refers to the point of crossover of principal rays, or the position in the system at which the aperture is generally placed) coma and distortion are intrinsically corrected. Balancing of low and high order astigmatism provides a narrow circular region of correction. A significant advantage of the Offner 1:1 system is that the ringfield correction is achieved with spherical mirrors. Disadvantages of this system are that it has no reduction, and throughput is limited by the small ring width corresponding with the narrowness of the corrected zone. As noted, a primary motivation toward x-ray projection lithography is the difficulty in 1: 1 mask fabrication as required both in proximity printing and in the Offner ringfield system. Relevant disclosures describing systems using all-reflective three element objectives, are included in U.S. Pat. 4,240,707, issued December 23, 1980, and U.S. Pat. 4,733,955, issued March 29, 1988. Both these designs are based on configurations which comprise a negative (concave) mirror interposed between two positive (convex) ones, and both are designed for the visible/IR part of the spectrum. These systems are well suited for imaging distant objects (at infinity) over either two dimensional or high aspect ratio (straight slits) fields of view. However, neither of these systems is telecentric in the image space nor is corrected for image distortion. Another disadvantage of three-mirror designs for the purpose at hand is that both object and image are located on the same side of the system. The result is severe restriction in wafer motion to avoid obstruction of the optical system. Another relevant disclosure is included in U.S. Pat. 4,747,678 issued May 31, 1988. It describes a 4:1 reduction optical system for deep UV lithography, which uses a combination of a four mirror reflective system and three groups of refractive lens systems. This is a ringfield system; it is telecentric and has low distortion. The fundamental concept in this design is that the refractive lenses allow for use of spherical mirrors in reduction configuration. This system achieves 0.5 µm resolution at 2500 Å wavelength, but is not suitable for the soft x-ray wavelength region due to significant absorption by the refractive lenses, leading, in turn, to concentration on reflective lenses. Despite the inherent advantages of the 1: 1 ringfield approach as used in the scanning mode, there is little confidence that it will play a valuable role in LSI fabrication. Aside from the 1: 1 mask fabrication problem, the general belief is that throughput of such a scanning system is inherently small due to extreme demands placed on minimization of aberrations across the width of the ringfield. Consequent narrow ring-shape slit result in very low throughput. EP-A2-0 252 734 discloses a reflection reduction imaging system, disposed between the mask stage and the wafer stage, including reflecting mirror arrangement, containing at least three but not more than five reflecting mirrors (M1,M2,M3) coated with multi-layer films, for receiving X-rays from the mask and directing them to the wafer to expose the wafer to the pattern of the mask with the X-ray in a reduced scale. In a reduction form of a ringfield optical coma and distortion are no longer cancelled by the symmetry of the configuration. As a result, more design variables are required for the control of these aberrations in addition to spherical aberration, astigmatism and field curvature. In the design described in U.S. Patent 4,747,678, the variables take form of additional refractive lenses. For the x-ray designs the additional variables are in the form of aspheric departures of each mirror from a sphere. Aspheric coefficients for third order aberration correction (often called conic constants) and for higher order correction are very effective means for controlling aberrations and, generally, each coefficient can control or correct one order of one aberration term. The use of aspheric mirrors is currently thought to be inevitable for projection x-ray system. All-spherical mirror designs promising from the standpoint of aberration correction, have been developed, but these systems involve up to ten mirrors. Poor reflectivities of soft x-rays at near-normal incidence - ∼ 60% for available multi-layer mirrors - is well below the reflectivity of optical/UV wavelengths (typically 98% and higher) and this imposes a severe restriction on a number of mirrors which can be used in the projection system. Other difficulties are perhaps of still greater consequence. The 10-15 Å wavelength x-ray radiation used in proximity printing is outside the range of presently available x-ray optics as discussed above. This has led to consideration of longer wavelength radiation - in the range centering about 130 Å, e.g. in the range of 100-150 Å. While suitable materials of somewhat larger refractive index values at such longer wavelengths are available, limitation on throughput based on slit width, W - a limitation related, not to radiation wavelength, but to device design rules continues to be a problem. (See, Wood, et al, Proc. of 33rd Int. Symp. on Electron, Ion, and Photon Beams , "Short Wavelength Annular-field Optical Systems for Imaging Tenth Micron Features", (1989).) Available refractive indices at ∼ 100 Å, while still quite low, are sufficient to permit fabrication of multi-layered reflective optics ("Distributed Bragg Reflectors"). DBR reflective optics resulting in ∼ 45% reflectance for use with 140 Å radiation have been constructed and used to obtain one-tenth µm feature sizes. (See, Bjorkholm et al, J. Vac. Sci. Technol. B , "Reduction Printing at Better Than 0.1 Resolution Using 14 nm Radiation and Multilayer-Coated Optics", (Dec. 1990).) This approach, providing for full feature (non-scanning), reduction projection is severely limited by field curvature. While needed resolution is obtainable, field size is very small, e.g. 100 µm x 200 µm for feature size of 0. µm. While adaptable for use in ring-field scanning, the same field limit applies to result in a slit width W , of the order of a few microns (corresponding with a slit length, L, of the desired several mm). In short, the lure of ring-field projection printing e.g. for 0.25 µm and below due to considerations such as mask safety and possibility of object-to-image reduction, has not yielded to feasible processing. Very high camera/source costs, due to throughput limitations, are considered responsible. Summary of the Invention The invention provides a scanning ringfield reduction production apparatus as defined in claim 1. From the processing standpoint, the essence of the invention is the observation of ringfield projection properties to result in increased throughput of devices in fabrication while retaining the excellence associated with this approach. Embodiment of the invention attain slit dimensions which both accommodate image width during single scan while permitting large scan area. A particularly profound observation is in the fact that certain principle aberrations are of opposite slope in terms of dependence on radius from the optical center. This in turn permits design of optics to result in substantial cancellation of the aberrations, thereby permitting increased slit width, W , while permitting slit length L at least equal to a single pattern overall dimension. Activity directed toward x-ray ringfield fabrication has been impeded by expected costs. Extrapolation from considerations implicit in the work described above leads to very low device throughput - leads to ineffective use of large apparatus investment. Useful production rates for the Perkin-Elmer apparatus approach are in large part due to acceptable scan rates, which, in turn, are the consequence of large slit width (characteristically ∼ 2 mm). Required aberration limits across the slit width (to either side of the included arc of constant radial distance upon which the design is based) becomes limiting for smaller design rules. As a rough approximation reduction of design rules, from 1.0 µm to the presently contemplated 0.1 µm, results in a needed concomitant reduction in width (to ∼ 0.2 mm or 20 nm). Other work suggests need for still narrower slits. Practical difficulty in illumination of consequent high aspect ratio slits is of particular consequence for relatively expensive x-ray sources. Further, since slit width for given aberration must be decreased with increasing radius to maintain aberration control, practical considerations lead to limitation on the radius. Lessening radius, however, also lessens the available straight line dimension of the slit arc. Consequences are of concern - consequent reduction of the scan area increases overhead (time required to reposition the apparatus in between scans) - permitted scan lengths may be reduced to less than desired image pattern widths, so requiring an extremely high degree of precision at the slit butting edges. Synchrotron sources, effectively columniated, may be used to illuminate small scan width, W , with relative ease (perhaps as limited by fringing due to near-coherence - an effect sometimes suggesting deliberate modification to lessen columniation). In general, small source area, for Lambertian as well as columnar sources, is desirable. Larger area sources, e.g. plasma sources (both pinch plasmas and electron pumped plasmas) require relatively expensive, precisely designed condensers which increase both initial apparatus cost and processing cost (the latter to the extent to which condenser fabrication is imperfect). An important aspect of the invention is directed to apparatus of design particularly suited to high throughput. This aspect of the invention is directed to all-reflective, reduction, ringfield projection apparatus designed for use in the wavelength range of from about 300 Å to 40 Å (within the so-called "long" or "soft" x-ray spectrum). While processes herein may make use of a broader class of apparatus designs, apparatus claimed is exemplary. Designs embodying the invention provide for throughput believed feasible for commercial manufacture while retaining advantages in terms of resolution and freedom from consequential aberrations previously associated with low throughput prior art apparatus. In more specific terms, apparatus of the invention provides for the scanning arcuate slit-shape image field associated with earlier designs. Slit dimensions consistent with resolution/aberration needs may be of both length to accommodate pattern dimension without need for butting and width to make effective use of the x-ray source. Design options include substantial radial distance from the optical axis to permit large slit length (L) - length sufficient to permit single scan patterning of larger chips. For a preferred embodiment, this design flexibility translates into slit length (L) to match pattern width and substantial slit width (W) permitting effective use of a variety of x-ray sources, so resulting in high throughput. Large slit width also eases design requirements for providing uniform illumination over the entire slit. In an embodiment a reflecting triplet aspheric mirror system is configured for use as reduction projection system in an unobscured ringfield form. (Avoiding central obscuration permits maximum contrast across the entire spatial frequency range - i.e. across the entire range of line spacing of image consequence). The basic conic constants for the mirrors are selected for each mirror to correct or balance spherical aberration, coma, astigmatism, and distortion. The mirror radii are selected for each mirror to act in concert with conic constants to provide optimal correction and to maintain an unobscured ringfield form which is telecentric in the image space. Sixth order aspheric departures are also used to provide optimal diffraction limited performance. Preferred configuration in the design space have one of the three mirrors as a base sphere (i.e. conic constant of zero) with a small sixth order aspheric departure. Systems embodying the invention use a fourth mirror (in prototypical systems, a folding mirror - of zero or finite optical power) between the second and third mirrors of the reflecting triplet. This fourth mirror serves three important purposes: (1) to fold the system - to place the wafer on the other side of the optics thereby permitting motion flexibility without obstruction; (2) to offer additional design flexibility in the optics to overcome other limitations of the triplet; and (3) to increase clearance between optics and wafer. Increased slit width is ascribed, in significant part, to this design. A fourth order aspheric departure is added to this mirror, whether flat or curved, to provide complete correction of astigmatism as shown in FIG. 5, resulting in a system whose usable field may be limited only by distortion. The imaging performance in terms of resolution (neglecting distortion) for this design form is limited by the residual field curvature resulting from the balance of its third and fifth orders. In a preferred configuration, the packaging mirror is spherical (but full astigmatic correction is maintained). Description above is likely to be directly implemented in commercial apparatus, at least in initial designs. In fact, details set forth apply largely in historical sense in that most applicable prior work took the form of the triplet as modified by a planar folding mirror. While historically correct, it is apparent that certain of the functions of the fourth element may be assigned to member/s of the triplet. An optically equivalent system may be based on use of a folding mirror, perhaps spherical, containing no aspheric terms - a system in which the correction is made by appropriate aspheric correction to the other elements. In fact the second Example, that reported in tabular form in Tables V-VII, is essentially such a system. In that work, the value of aspheric correction as introduced via the folding mirror was indeed accomplished by assignment of the equivalent or near-equivalent aspheric term to the triplet. In that Example, the folding mirror was of appropriate convex curvature, and patterning-effective aspherical correction was accomplished by restricting asphericity to elements of the triplet. By the same token, although it is expected that a folding mirror will be used in most commercial apparatus, a simple triplet system may be designed to yield the increased W of the invention. It is conceivable that the advantage of reduced absorption loss could compensate for the convenience of the folding mirror. It is convenient to set forth the attributes of the inventive system in terms of allowable features, both from the design and operational standpoint. In listing such features, it should be kept in mind that there are circumstances under which economics or other conditions may suggest omission of one or more. Back clearance Beam clearance at mirrors Unobscured Coaxial (all mirror centers of curvature and centers of ring-shaped object and image fields have common axis, which simplifies the system assembly and alignment) Only conic plus 6th order departures (reduces aspheric mirror slope and thereby, fabrication complexity) Aspheric departures are low Complete astigmatic correction extends usable field (increases ring width, W) Very high level of distortion correction One and sometimes two of the four mirrors are base spheres Astigmatic correction over a broad range of system configurations allowing a packaging flexibility Brief Description of the Drawing FIG. 1 schematically depicts the main elements of exemplary x-ray lithography apparatus according to the invention; FIG. 2 and 3 schematically show, in side view and top view, respectively, the relevant parts of an exemplary inventive projection system from mask to image plane; off-axis portions of parent rotationally symmetric mirrors are shown. FIG. 4 shows the image field shape and size of an exemplary inventive projection system. FIG. 5 is a plot of astigmatism resulting from use of an exemplary projection system according to the invention (elimination of all orders of astigmatism is shown by coincidence of the x and y focus curves). FIG. 6 is a plot relating distortion to slit width for an example herein Detailed Description General Systems Characteristics While four-mirror systems are preferred as being useful in terms of convenience, both in design and operarion, optical objectives affording scan area/throughput are achievable in three-mirror (triplet) systems. Aspherics adequate for astigmatism nulling, consistent with other aberration lessening, are permitted in the triplet. From a generic standpoint, innovative advances may be discussed on this basis, Design choices prominently depend on determination as to whether there is an intermediate image (whether there is re-imaging) and whether the initial mirror (that closest to the object) will be positive (concave) or negative (convex), Generally there is a performance penalty associated with re-imaging in that attainment of high resolution (as contemplated for devices herein) is made difficult. Since re-imaging is not required for contemplated device fabrication, re-imaging was not provided for. Often for large format systems, a negative first mirror is used to reduce the effective field of view delivered to the following mirrors. This design option was pursued but it was found that the telecentric requirement could not be met. As a result, preferred systems of the invention depend upon a positive first mirror. Telecentricity, a desired property of systems/processes herein, is descriptive of the normal incidence of image-forming rays with respect to the focal plane (e.g. with respect to the wafer). It is of primary value in permitting slight movement of the wafer to a position outside of the focal plane without resulting in image deterioration due to image shifting (attendant upon non-normal angle of incidence) and/or to change in image magnification for image forming rays which are non-parallel. The above information was input to a computer using a lens design program to yield the design information described in the Examples - e.g. that of Tables I-IV. The particular program used was "Code V"™ offered by Optical Research Associates (ORA) although a variety of programs of similar capability are available and may be used. Exemplary Preferred Embodiments In a preferred embodiment the projection system comprises, from object plane to image plane, a first concave, a second convex, a third convex, and fourth concave mirrors. To obtain an extremely low distortion (∼ 1 nm) over the ring field, at least two of these mirrors have to be aspherics. The radii absolute values of the four mirrors are, again from object to image and as a fraction of the system focal length, 1.17, 0.69, 1.27, and 0.72, all to within ±5%. Equivalently, the radii of the four mirrors are, in the same order, in the ratio of 1.00, 0.59, 1.09 and 0.61, all to with ±5%. The axial separation of the mirrors, as a fraction of the system focal length, are 0.32 (first concave to second convex), 0.26 (second convex to third convex), 0.35 (third convex to fourth concave), and 0.62 (fourth concave mirror to the image), all within ±10%. The numerical aperture of the system at the image is 0.1 and can be reduced if high resolution of 0.08 µm is not needed. The numerical aperture can be increased to ∼0.14 without significant loss in the image quality (still diffraction limited performance), if the image distortion requirement is relaxed. In preferred embodiments the usable image field (based on imaging performance and neglecting distortion) has shape of the arc with two dimensions R and W, where R is the radius of the arc and W is the width of the arc as shown in FIG. 4 (R > W), and W > 1 mm. This system is compact with the back clearance (necessary for system implementation in vacuum environment) of at least 2 inches (50.8 mm). Since astigmatism may be completely corrected over any size field as shown in FIG. 5, the width of the ring is not limited by astigmatism. The usable field width for x-ray lithographic application for contemplated design rules of 0.25 µm and smaller is limited by distortion. The distortion in this system is a balance of third and higher order distortion and for usual design in accordance with the invention, distortion is the limiting aberration. The maximum value of W depends on distortion tolerance. FIG. 6 on coordinates of slit width on the abscissa and distortion on the ordinate shows distortion as a function of slit position for an illustrative slit - distortion rises to a maximum of - 4 nm at the two central positions for each side of a 2 mm slit. Examples The relationship between distortion and field width for 0.1 µm resolution for a particular apparatus in accordance with the invention is set forth in Table A. This table is based on field width as measured at the image plane (generally as measured at the surface plane of the wafer under fabrication). In prior art ringfield systems the ring widths typically would be limited by astigmatism before they were limited by distortion, although at these distortion levels (i.e. 1 nm) there has been very little work done. The resolution over this image field is 0.08 µm. The following table, Table B, describes performance data in terms of resolution across the field width of the slit. It shows the chief ray distortion, 84% encircled energy diffraction spot diameter, diffraction spot centroid shift, the RMS wavefront error at wavelength of 13 nm. Zone Height (mm) Chief Ray Distortion (nm) 84% Encircled Diffraction Spot Size [1] (microns) Diffraction Spot Centroid (nm) RMS Wavefront Error, [2] (waves at 13nm) 1.00 2.875 0.20 -8 0.040 0.90 -0.108 0.80 -2.258 0.21 -8 0.044 0.70 -3.658 0.60 -4.397 0.21 -8 0.048 0.50 -4.561 0.40 -4.243 0.21 -8 0.050 0.30 -3.533 0.20 -2.525 0.21 -8 0.051 0.10 -1.315 0.00 0.000 0.21 -8 0.050 -0.10 1.322 -0.20 2.549 0.21 -8 0.047 -0.30 3.579 -0.40 4.308 0.21 -8 0.042 -0.50 4.630 -0.60 4.435 0.20 -9 0.035 -0.70 3.615 -0.80 2.057 0.20 -9 0.029 -0.90 -0.353 -1.00 -3.731 0.20 -10 0.028 In particularly preferred embodiments of the invention the image reduction is at least 2:1, W is at least 1 mm, R is at least 25 mm, and the resolution over the ring-shaped image field is higher than 0.1 µm. (it can be reduced to 0.2 µm - 0.5 µm by simply reducing the aperture in the system). The aperture stop in preferred embodiment of the invention is near the tertiary mirror. Detailed Description of Some Exemplary Preferred Embodiments FIG. 1 schematically depicts exemplary inventive apparatus for semiconductor x-ray lithography. The apparatus comprises a radiation source 11 (exemplarily a synchrotron or a laser plasma source) that emits x-ray beam 12, typically a polychromatic beam. Optional filtering system 13 serves to narrow the spectral width of the beam 12 such that a substantially monochromatic beam 12′ results. Such beam is next introduced into condenser 18 which emits a beam 12″ of configuration to uniformly illuminate the requisite portion of mask 14. The emerging patterned beam is introduced into the optical system 10, which projects an image of mask 14, shown mounted on mask stage 15, onto wafer 16 which is in turn mounted on stage 17. Element 18, an x-y scanner, scans mask 14 and wafer 16 in such direction and at such relative speed as to accommodate the desired mask-to-image reduction. Since, as will be discussed below, the inventive projection system is an all-reflective system, use of a filtering system may not always be necessary. However, best resolution will generally be obtained if longer wavelengths are removed from the beam. Furthermore, by matching the spectral width of the incident radiation to the reflectivity bandwidth of the mirrors in the projection system, the stability of the system will typically be enhanced, since the mirrors will not have to carry the load of unwanted radiation. FIGS. 2 and 3 depict apparatus as used in development reported in the first example. The two figures are 90° views of the same apparatus. Such apparatus, depicted in operation consists of x-ray source 20 producing beam 21, in turn illuminating mask 22 (corresponding with mask 14 of FIG. 1) over slit width, W, 23. The figures are simplified schematic representations so that, for example, the x-ray beam shown as emanating from source 20 is, in the usual instance, the consequence of a condenser intermediate source 20 and mask 22 not shown. Reflected beam 24, shown as diverging, is reflected by concave mirror 25 so as to illuminate convex mirror 26 with now converting beam 27. Reflection from convex mirror 26 results in beam 28, illuminating folding mirror 29, which in the instance shown, is also convex to result in illumination of concave mirror 30 by now diverging beam 31. Curvature and location of mirror 30 is such as to result in-reflected radiation which is telecentric in image space. Illumination of wafer 32 by beam 33 is shown converging so as to represent such image reduction relative to mask 22 as provided for. For tutorial purposes, completed coaxial structures of which mirrors 25, 26 and 30 are a part, are shown with broken line portions 25a, 26a and 30, completing such structures - all shown as coaxial relative to axis 34. Folding mirror 29 is shown centrally disposed with reference to axis 34 - an arrangement as actually used in the first example. Ring-shaped object and image fields are coaxial with reference to axis 34 and their planes (i.e. the object/image planes containing the arcuate fields) are normal to axis 34. FIG. 3 depicts the apparatus of FIG. 2 as viewed at right angles to the representation of the earlier figure. Source 20 again illuminates mask 22, this time with imaging radiation 23a of major dimension sufficient for illumination of the slitto uniformly illuminate the slit in its major dimension, length, L. The remainder is as shown in FIG. 2: beam 24 is reflected by mirror 25 to illuminate mirror 26 with beam 27 resulting in illumination of folding mirror 29 by beam 28. This in turn results in beam 31 which illuminates mirror 30 to produce focused image on wafer 32 by means of beam 33. In operation, the light beam 24 emanating from various points on the mask 22 is reflected in converging pattern by the primary mirror 25, a concave aspheric, onto secondary mirror 26, a convex aspheric. The radiation 27 received by the secondary miser 26 is reflected as radiation 28 on the convex surface of tertiary mirror 29. In the currently preferred embodiment the tertiary miser is a quasi-flat weak aspheric, but optionally could be spherical as in another example. The tertiary mirror 29 is located at the aperture stop of the system - at the position at which principal rays cross, and serves to fold, i.e. to move the image to the other side of the optics. Like other of the optic elements it is centered with respect to optic axis 34. The power added to this mirror is required to provide the back clearance necessary for the projection lithography application. From the tertiary mirror the radiation is reflected in a diverging pattern upon quaternary mirror, a concave aspheric. The curvature and location of this mirror are chosen such that radiation reflected off it is telecentric in the image space. As shown, the inventive exposure system is rotationally symmetric in the aperture and the ring-shaped object and the image fields and both object and image planes are normal to optic axis 34. Furthermore, it is an all reflective, reduction, unobscured, finite conjugate system adapted to form a flat arc-shaped x-ray image of a flat mask. The image can be diffraction-limited, with high resolution over the image field. The image field has shape of the arc with dimensions W and L, where W is the arc width and L is the arc length. The arc length L depends on the ring radius R and may be defined as shown in FIG. 4 FIG. 4 is a schematic representation showing slit 40 of dimensions, W, 41 and length, L, 42, depicted as a portion of ringfield 43 defined by radial dimension, R, 44 spanning the distance from optic axis 45 and the center 46 of slit 40. Exemplarily, W is about 2 mm and R is about 25 mm, which makes L about 25 mm. However, rectangular, square or other off-axis field shapes are also possible and are contemplated. Both conic and general aspheric mirror geometries may be utilized in various embodiments which are made possible by the invention. The invention also provides an optical system which may attain various levels of image quality, typically in the range of 0.5 µm to 0.05 µm, and distortion correction from 1 µm to 0.01 µm. FIGS. 5 and 6 are included as representative of astigmatism and distortion characteristics of a representative system in accordance with the invention. FIG. 5 on coordinates of focus (focal distance) in mm on the abscissa, and angle relative to the optic axis in degrees, is a plot of focal distance for x (solid line) and y (broken line) directions. Information here plotted for the first apparatus example, representative for preferred apparatus herein, most importantly shows near-perfect nulling of astigmatism over the entire range plotted. While the focal distance varies with the angle of divergence relative to the optic axis, so defining a focal field of some curvature. There is a substantially flat field region (for the conditions plotted, from ∼ -5 ° to -5.61 ° - corresponding with a nominal radius of 25 mm (radial distance to the center of the arcuate slit as measured in the width direction) or, stated differently, with a slit width, W , of 2 mm. FIG. 6 on coordinates of slit width, W, on the abscissa and distortion in nanometers on the ordinate relates those qualities for a system herein - data presented is that of the apparatus of the second example. While the curve form is generally representative, data on which it is based is with reference to a 2 mm slit width, W, (the center of the slit in the direction W, corresponds with 0 on the abscissa and, therefore, extends from -1.00 to +1.00 mm. Ordinate units are those of distortion in units of manometers. The specific data plotted, therefore, indicates a peak distortion of 5.0 nm as occurring at the two extremities in the W direction. In projection system according to the invention the mirror parameters can readily be chosen to cooperate to give a high degree of correction of spherical aberration, coma and Petzval curvature. Through the use of 6th order aspheric terms the total elimination of all orders of astigmatism can be achieved, as shown in FIG. 5. For the projection lithography application, the elimination of all orders of astigmatism clearly extends the region of astigmatic correction and makes optimal use of the flat field region which exists where the 3rd order Petzval is balanced by 5th order Petzval. Unique, or more restrictive, features of the design include the low distortion (less than 1 nm over 1 mm arc width; less than 5 nm over 2 mm arc width, and less than 10 nm over 2.5 mm 2.5 width), telecentric image space (non-telecentricity errors is less than 1 nm per 1 µm defocus), and large back clearance (more than 2 inches) through the addition of a packaging fold and the use of power of the added mirror to control the packaging clearance. Typically, the resolution over the image field is better than 0.2 µm in the inventive systems. In an exemplary system according to the invention the resolution is 0.08 µm over a ring field of 2.5 mm in width and 25 mm of its radius. Table B shows calculated performance data for the currently preferred embodiment of the inventive projection system, for wavelength of 13 nm. In particular, Table B shows the chief ray distortion, 84% encircled energy diffraction spot diameter, diffraction spot centroid shift and RMS wavefront error for different field points across 2 mm ring width. Tables I-IV contain constructional data and other relevant information for the exemplary currently preferred projection system of FIGS. 2 and 3. This system is a 5:1, 0. 1 NA, 2.0 mm telecentric ring field system. The reference wavelength is 13 nm. Table I shows mirror radii and spacings, and taken together with Tables II-IV, describes the apparatus of the first Example. Quantities set forth are in terms well-known to those conversant with optical design as described, for example, in ORA Code V Reference Manual, Version 7.20 (Dec. 1988), Optical Research Associates, Pasadena, Calif. Terms of art include "INF" (descriptive of a flat, of a peripheral surface of a circle of infinite radius), "AIR" (indicative of an index of refraction, n, value of unity), "REFL" indicative of a reflector, therefore, of index which is the negative of that previous and showing change in direction of the reflected beam - e.g. "REFL" of C-1 defines an n of - 1 while "REFL" of C-2 suggests n = +1, "Thickness" is the art-approved term for surface-to-surface distance so that e.g. 700.0000 represents a distance of 700 mm from Object surface to that of C-1. - Fabrication Data Element Number Radius of Curvature Thickness Aperture Diameter Object INF 700.0000 AIR 1 A(1) -80.3450 C-1 REFL 2 A(2) 65.2250 C-2 REFL APERTURE STOP 18.2500 3 A(3) -90.0000 30.0000 REFL 4 A(4) 157.9470 C-3 REFL Image INF Asphericity values, A(1)-A(4) as well as aperture data, see C-1 thru C-3 are set forth in Table II and Table III, respectively. - Aspheric Constants Z = CURV Y 2 1 + (1 - 1 + K CURV Y 2 ) 1/2 + (A)Y 4 + (B)Y 6 + (C)Y 8 + (D)Y 10 ASPHERIC CURV K A B A(1) -0.00336791 -0.621100 0.00000E+00 -1.89000E-14 A(2) -0.00566733 0.000000 0.00000E+00 -2.66546E-12 A(3) 0.00308725 0.000000 1.80625E-08 -2.10800E-13 A(4) 0.00549315 0.199750 0.00000E+00 3.97949E-14 - Aperture Data DIAMETER DECENTER Aperture Shape X Y X Y C-1 Rectangle 110.000 60.000 0.000 57.500 C-2 Rectangle 50.000 35.000 0.000 17.500 C-3 Rectangle 75.000 55.000 0.000 -22.500 Table III gives data on size and decenter of used off-axis portions of rotationally symmetric mirrors for a 2 mm wide, 25 mm radius, and 25 mm long section of the ring at the image. Table II gives the values of the aspheric coefficients. The surface of the primary mirror has the shape of an ellipsoid with 6th order departure term added to it. The secondary mirror is spherical with 6th order aspheric departure. The tertiary mirror is quasi-flat with 4th and 6th orders aspheric departures. Finally, the surface of the quaternary mirror has the shape of an oblate spheroid with 6th order aspheric departure. Table IV gives first order data on the exemplary system. - System Characteristics Infinite Conjugates EFL = 254.2727 BFL = 106.7429 EFL = 563.5122 F/NO = 4.9986 At Used Conjugates Reduction = 0.2012 Finite F/NO = 5.0311 Object Dist = 700.0000 Total Track = 752.8270 Image Dist = 157.9470 Paraxial Image HT = 26.1616 Image Dist = 157.9134 Semi-field Angle = 5.6138 Entr Pupil Diameter = 50. 8692 Distance = 571.4761 Exit Pupil Diameter = 1624.1691 Distance = -8011.7604 Notes - FFL is measured from the first surface - BFL is measured from the last surface Another specific example of the invention is shown in Tables V, VI and VII. - Fabrication Data Element Number Radius of Curvature Thickness Aperture Diameter OBJECT INF 700.0000 AIR A(1) -8.9786 153.3938 REFL 152.8858 -67.2925 2 A(2) 99.1509 73.0320 REFL APERTURE STOP 19.7163 3 229.4010CX -98.3590 19.7163 REFL 4 A(3) 98.3580 89.0625 REFL 70.5707 IMAGE DISTANCE = 92.8839 Image INF - Aspheric Constants Z = (CURVY 2 1 + (1 - 1 + K CURV Y 2 ) 1/2 + (A)Y 4 + (B)Y 6+ (C)Y 8 + (D)Y 10 Aspheric Curv K A B A(1) -0.00286601 0.000000 0.00000E+00 -7.46578E-15 A(2) -0.00349335 3.702396 0.00000E+00 7.61721E-14 A(3) 0.0050387 0.134446 0.00000E+00 1.45433E-13 - Systems Characteristics INFINITE CONJUGATES EFL = 259.0835 BFL = 40.7516 FFL = 587.938 F/NO = 5.0050 AT USED CONJUGATES Reduction = 0.2012 Finite F/NO = 5.0290 Object Dist = 700.0000 Total Track = 815.7367 Image Dist = 92.8839 OAL = 22.8797 Paraxial Image Ht = 26.1510 Image Dist = 92.8692 Semi-Field Angle = 5.5455 Entr Pupil Diameter = 51.7650 Distance = 593.8654 Exit Pupil Diameter = 2262.7263 Distance = -11284.1847 Notes - FFL is measured from the first surface - BFL is measured from the last surface This system is also a 5:1, 0.1 NA, 2.0 mm telecentric ring field system, designed for the wavelength of 13 nm. The system is also rotationally symmetric with respect to system optical axis and has similar performance across a 2.00 mm slit width, with a 25 mm radius, of the ring at the image plane as described in FIG. 6. This system has only three aspherical mirrors with the tertiary mirror being a pure sphere. The distinctive features of this design include considerably more back clearance for packaging and nearly 2.5 times less aspheric departure from the best fit sphere for the steepest aspheric in this design compared to the previous exemplary design. The surface of secondary mirror in this design has the shape of sphere with 6th order aspheric departure, the secondary and quaternary mirrors are oblate spheroids with 6th order aspheric departure. Preceding description under "Examples", including Tables A, B and I-VII relate to actual working embodiments of the invention. While such matter is included for the sake of completeness, it does not set limits on the inventive teaching which is properly described in the Claims. Processing Considerations Detailed description thus far is largely in apparatus terms, although processing implications have not been ignored. Clearly, flexibility in slit dimensions is of paramount value. Increased slit width over which relevant aberration is reduced permits throughput advantage for submicron devices under fabrication. As noted, this, in turn, may translate into increased slit length to permit expedient fabrication of larger chips and to reduce overhead. Apparatus design presented has been somewhat restrictive. The now recognized capability of aberration reduction over greater width will give rise to other design approaches - approaches outside the category presented. Activity in this direction will be provoked by inherent processing advantages. X-ray lithographic patterning - to some extent soft x-ray in particular - presents inherent advantages for the contemplated design rule range of.25 µm and below. An exciting ongoing development relating to high resolution resist has taken the form: first of multi-level resists (in which only the top level is exposed and developed, subsequently to have its pattern transferred to underlying material) and; second of surface activation resist - itself initially in the form of "near-surface" activation and finally of true-surface activation. In near-surface activation conditions/materials are so chosen as to resist penetration of image forming radiation to the top region - a region of perhaps 2,000 or 3,000 Å in thickness. True surface activation, a recent emerging field, involves activation through a thickness of only one or a few monolayers on the free surface. Near-surface and true-surface activation have, to this time, required activity directed toward deliberate increase in opacity to restrict penetration of patterning radiation. The soft x-ray range primarily addressed in this disclosure is inherently suited to these approaches. For example, the 130 Å radiation treated in many considerations above, has an inherently limited penetration depth in most real materials. Organic resist materials otherwise suitable are penetrated only to depth to the order of a few thousand Å. Other resist materials, containing heavier elements further reduce depth of penetration. Advantages of surface and near-surface activation - advantages which have provoked deliberate effort toward increasing opacity in the UV wavelengths - advantages of particular significance for small design rules contemplated - all lead to reliable attainment of high resolution. Depth of but a few thousand Å allow more flexibility for x-ray processing in which depth of focus may be 1 µm or less. Transfer procedures by which the image involved in surface activation is transferred to underlying material, show promise of near-perpendicular patterning - of anisotropic transfer to minimize undercutting. While apparatus/processing of the invention contemplates shorter wavelength, e.g. down to ∼ 55 Å, most significant commercial usage may in fact entail use of the longer wavelengths contemplated. Initial usage, in permitting uniform composition single-material resist layers of the needed ∼ 1µm thickness is expected to command attention. Use of 55 Å radiation, which may penetrate through the entirety of a resist layer (likely in the 0.5-1.0 µm thickness range) permits resist/development in the usual commercial manner, with well-understood economy implications (reduction in required processing steps, higher expected yield, etc.). It is likely, however, that consideration set forth above will lead to emphasis on surface activation. For detailed discussion of the foregoing, see Proceedings of SPIEE , vol. 1343, (1990), "Resist Schemes For Soft X-ray Lithography" (presented at Symposium on XUV Lithography, San Diego, Calif., July 9-12, 1990). The inventive advance is of greatest significance in offering economically feasible fabrication throughput to compete or exceed that of alternative approaches. Processes of the invention, utilizing available x-ray sources, permit pattern exposure at a rate of 1 cm 2 per second and above. This is equivalent to writing a six inch (152.4 mm) wafer in three minutes. Assuming one minute overhead time (for wafer transport and alignment) this results in a throughput of 15 six inch (152.4 mm) diameter wafers per hour. Exposure at a rate of 0.5 cm 2 /sec with the same overhead time results in a throughput of 8 wafers per hour - still an acceptable throughput. Work to date suggests resist sensitivity in the range of from 5-10 mj/cm 2 (millijoules/square centimeter) as a compromise between throughput and resolution. Resists of greater sensitivity, e.g. at or below 1 mj/cm 2 may suffer from degraded resolution due to statistical fluctuations. If one square centimeter per second is to be exposed, it is necessary to illuminate such area at 10 mj/sec (i.e. equivalent to 10 mw of power incident on the wafer). Projection systems such as those described, containing four mirrors and a reflective mask result in source-to-image transmission efficiency of about 3% on the assumption that each surface has a reflectivity of about 50% at normal incidence. This translates into a power requirement of about 300 mw incident on the mask. Usable radiation bandwidth is largely a function of the bandwidth of reflectivity of condenser and mirrors. Bragg mirrors and condenser designs thus far studied have bandwidth Δlambda/lambda = 5% at a center wavelength of 130 Å (a suitable wavelength for use with molybdenum-silicon multi-layer coatings, i.e. DBR structures). Such radiation is obtainable from a bending magnet of a large synchrotron (The National Synchrotron Light Source Vacuum Ultraviolet Ring at the Brookhaven Laboratories produces 1.26 mw at a bandwidth of 1%/milliradian/100 ma). Experimental work has depended upon a synchrotron storage ring operating at 300 ma delivering x-ray radiation of a power of - 1700 mw (for the 5% bandwidth region centering about 130 Å) to conservatively yield in excess of the required 300 mw mask illumination as delivered by state-of-the-art condenser design. For further details, see Proceedings of the SPIE , vol. 1343 (1990), "Soft X-ray Projection Lithography: Experiments and Practical Printers" (to be published and delivered at 1990 SPIE International Symposium on Optical and Optoelectronic Applied Science and Engineering).

Claims: Scanning, ringfield reduction projection apparatus for lithographic patterning by scanning an image region (40) substantially contained within an arcuate slit (23) during device fabrication for yielding devices fabricated to design rule of a maximum of 0.4 µm, said apparatus for use with x-ray radiation within the wavelength range of from 30 nm to 4 nm, the optics of such apparatus being all-reflective and comprising distributed Bragg reflectors, said optics including a triplet of first, second, and third optical elements (25),(26),(30) which have the same optical axis (34) and which as viewed from object to image are in succession concave, convex, and concave, said slit (23) including a region being at such radius from the optical axis as to permit effective use of an x-ray radiation source (20) to yield total aberration sufficiently small to permit patterning at said design rule, CHARACTERIZED in that said apparatus optics includes a fourth optical element (29) having the same optical axis and located between said second and third optical elements (26, 30), so that an image on the opposite side of the optics from the object is obtained and in that said fourth optical element or said elements of said triplet are aspherical, the shape and position of said fourth optical element being such as to obtain working space sufficient to permit expedient fabrication of said device when positioned within a surface corresponding with that of the image. Apparatus of Claim 1 in which the said image region (40) is arcuate and corresponds with the said arcuate slit. Apparatus of Claim 2 in which the dimension of the said image region (40) in the scanning direction is at least 1 mm. Apparatus of Claim 2 in which the straight line dimension of the said arcuate slit (40) as measured normal to the scanning direction is at least 25 mm. Apparatus of Claim 1 in which the surface of the said fourth optical element (29) is convex and aspherical. Apparatus of Claim 2 in which the dimension of the said image region (40) in the radial direction decreases from a central maximum value to compensate for otherwise increased curvature-induced exposure. Apparatus of Claim 2 in which the said arcuate slit (40) is of length approximately corresponding with that of a 60 ° chord of the said ringfield. Apparatus of Claim 1 in which the surface of the said fourth optical element (29) is convex and aspherical and in which the absolute values of the radii of the said optical elements as viewed from object to image, expressed as a fraction of the system focal length are respectively 1.17, 0.69, 1.27, and 0.72 all to within ± 5%.

Inventors: JEWELL TANYA E, THOMPSON KEVIN, JEWELL, TANYA E., THOMPSON, KEVIN
Applicants: AT & T CORP, AT&T CORP.
Application year: 1991
Main classification: G02B17
IPC codes: G03F7, H01L21, G02B17