Patent Number: 062263463
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT The following is a detailed description of the presently preferred embodiments of the present invention. However, the present invention is in no way intended to be limited to the embodiments discussed below or shown in the drawings. Rather, the description and the drawings are merely illustrative of the presently preferred embodiments of the invention. The present invention is a photolithography optical system designed for use with extreme ultraviolet (EUV) radiation. FIG. 4 schematically depicts the exemplary inventive apparatus for semiconductor EUV lithography. The apparatus comprises a radiation source 401 that emits EUV radiation 403. The EUV radiation 403 is processed by a condenser 405 which produces a EUV beam 407 to uniformly illuminate a portion of mask 409. The radiation reflected from the mask 409 produces a patterned EUV beam 411, which is introduced into optical system 413. The optical system 413 projects a reduced image 415 of the mask 409 onto a wafer 417. EUV radiation has a wavelength .lambda. between about 4 to 20 nm and may be produced by any suitable means, including laser produced plasma, synchrotron radiation, electric discharge sources, high-harmonic generation with femto-second laser pulses, discharge-pumped x-ray lasers and electron-beam driven radiation devices. Laser-produced plasma (LPP) sources focus an intense pulsed laser beam onto a target. Suitable targets are metals and noble gases. Targets of noble gas molecule clusters in a supersonic jet produce a bright "spark" with a broad emission spectrum from the visible to the EUV radiation. High-repetition-rate (3,000-6,000 Hz) pulsed laser drivers deliver 1,500 W of focused power to the target regions. The LPP gas source then converts the incident laser power into EUV light in the required spectral bandwidth. The condenser collects EUV power from the LPP source and conditions the radiation to uniformly illuminate the mask. The condenser provides the EUV radiation in a narrow ring field with at least 1% uniformity at the mask in the cross scan dimension. The condenser further directs the EUV beam into the entrance pupil of the inventive optical system with a partial coherence of approximately 0.7. Separate collection channels each act in parallel, directing radiation across the entire ring field and the optical system entrance pupil. Since EUV radiation is absorbed by all materials, only reflective elements are suitable for EUV optical systems. The inventive optical system comprises four reflective optical elements (mirrors) listed in order from mask to substrate: M1, M2, M3 and M4. The optical system is placed in a vacuum or other suitable atmosphere. During lithography, EUV rays are collimated and directed at a mask, producing patterned radiation. The object end of the inventive optical system departs enough from telecentricity so that a reflective mask can be used without any vignetting of the light rays by mirror edges. Referring to FIG. 5, an exemplary optical system is shown for EUV semiconductor lithography. Because this is a ring field optical configuration, the lower section of the first optical element M1 505, the lower section of the second optical element M2 509, all of the third optical element M3 513, and the upper section of the fourth optical element M4 517 are exposed to EUV radiation. The optical elements are all arranged in a coaxial configuration. The portions of the mirrors actually used are designated with solid lines, and the sections of the parent mirror that are not used are designated with dotted lines. An EUV Beam 1 501 diverges from a reflective mask 503 onto convex aspheric mirror M1 505. Beam 2 507 is reflected from mirror M1 505 in a divergent cone to a concave aspheric mirror M2 509. Beam 3 511 is reflected from mirror M2 509 in a convergent cone to a convex spherical mirror M3 513, which also functions as an aperture stop. Beam 4 515 is reflected from mirror M3 513 in a divergent cone to a concave aspheric mirror M4 517. Beam 5 519 is reflected from mirror M4 517 in a convergent cone, projecting a reduced image of the mask 503 pattern onto a wafer 521. The chemical reaction of a photoresist layer on the wafer 521 to the patterned EUV exposure enables subsequent semiconductor processing by well-known means. Concave mirrors have positive optical power and convex mirrors have negative optical power. Using this convention, the optical power configuration of the first embodiment of the inventive system from object to image can be described as: negative, positive, negative and positive, corresponding to mirrors M1 505, M2 509, M3 513, and M4 517, respectively. This inventive placement of optical power allows the projection system to produce a reduction ratio of 4.times. and a telecentric imaging bundle at the wafer that is normal to the wafer (substrate plane), while simultaneously providing the necessary opto-mechanical clearances and achieving a near-zero Petzval sum (flat field condition). The numerical aperture of the system is about 0.1. The absolute radii of the mirrors M1 505, M2 509, M3 513, and M4 517, relative to the system focal length, are listed in Table 1. TABLE 1 Mirror radii from object plane to image plane as a Mirror fraction of the system focal length .+-.5% M1 5.471 M2 1.984 M3 0.711 M4 0.924 The axial separations of the mirrors M1 505, M2 509, M3 513 and M4 517, relative to the system focal length, are listed in Table 2. For a 4-to-1 reduction, the distance of the mask to M1 305 is 1005.654 mm. TABLE 2 Axial separations of the mirrors as a Surface fraction of the system focal length .+-.10% M1 to M2 1.119 M2 to M3 0.897 M3 to M4 0.459 M4 to image 0.826 The inventive optical system projects a mask image onto a wafer through the step and scan method. Referring to FIG. 2, the usable field projected by the inventive optical system is in the form of an arcuate slit 201. The inventive optical system can be configured with length 207 of 26 mm at an angle 209 of approximately 30.degree., a ring field width 205 of 1.5 mm and a ring field radius 203 of 52.75 mm. The inventive system achieves a resolution of 0.1 .mu.m or better with a depth of focus greater than 1 .mu.m across the projected image field. The inventive optical system arcuate slit dimensions are an improvement over the prior art. The prior art typically produced an arcuate slit with a length of approximately 16 mm at an angle of approximately 60.degree., a ring field width of approximately 0.5 mm to 1.0 mm, and a ring field radius of approximately 31.5 mm. The ring field width of the present invention (1.5-3.0 mm) is significantly wider than that of the prior art. Because the ring field radius of the present invention is larger than that of the prior art, the present invention improves the unit area coverage within a single field on the wafer, thereby improving wafer throughput per hour. Tables 3-6 contain constructional data and other relevant information for the inventive optical system shown in FIG. 5. The inventive optical system is a telecentric ring field system with a slit width of 1.5-3.0 mm, a 4:1 reduction ratio, and a numerical aperture (NA) of at least 0.1. The reference wavelength is 13.4 mm. Table 3 shows the mirror radii and spacings, and taken with Table 4 and Table 5, completely describes the apparatus of the example. Table 4 lists the aspheric constants. Table 6 shows the performance of the system as described by the root mean square (RMS) wavefront error and corresponding Strehl ratio. TABLE 3 Element number Radius of Curvature Thickness Object Infinite 1005.654000 1 2985.14000 -610.424000 2 1082.16000 489.255000 3 388.09000 -250.491500 4 504.16000 450.481604 Image Infinite Dimensions are given in mm. Positive radius indicates center of curvature to the right. Thickness is axial distance to next surface. TABLE 4 Mirror CURV K A B C D M1 0.00033499 -30.1000 0.0 -3.10082E-16 -3.97717E-21 0.0 M2 0.00092408 -0.80000 0.0 -9.57030E-17 -6.27533E-22 0.0 M3 0.00257672 0.0 0.0 0.0 0.0 0.0 M4 0.00198350 0.11828 0.0 1.64273E-16 -2.25401E-21 0.0 TABLE 5 Center of ring field (mask) -211.0 mm Effective focal length 545.592 mm Paraxial reduction ratio 0.25 Finite F/N.sub.O 5.0 Total track 1084.475 mm TABLE 6 rms wavefront error Ringfield Radius (l = 13.4 nm) Strehl ratio 52.00 mm 0.020.lambda. 0.984 52.25 mm 0.013.lambda. 0.993 52.50 mm 0.008.lambda. 0.998 52.75 mm 0.005.lambda. 0.999 53.00 mm 0.009.lambda. 0.997 53.25 mm 0.015.lambda. 0.991 53.50 mm 0.021.lambda. 0.982 The aspheric profile is uniquely determined by its K, A, B, C, and D values, such as given in Table 4. The sag of the aspheric surface (through 10th order) parallel to the z-axis (z) is given by Equation 1: ##EQU1## Wherein, h is the radial coordinate; c is the curvature of the surface (1/R); and A, B, C, and D are the 4th, 6th, 8th, and 10th order deformation coefficients, respectively. The equation coefficients A and D are zero for the mirrors M1, M2, and M4, thus they use only 6th and 8th order polynomial deformations. The optical elements M1, M2, M3, and M4 can be described via the base conic as a hyperboloid, prolate ellipse, sphere, and oblate spheroid, respectively. Another advantage of this first embodiment of the present invention is that the centroid distortion magnitude is balanced across the ring field width. This balance distortion curve results in a minimization of dynamic (scanning) distortion. Referring to FIG. 6, the centroid distortion curve of the inventive optical system is shown. The centroid location is the geometric center of an arcuate radiation ray imaged from a point on the mask to the wafer (substrate). The distortion curve is balanced when the shape of the centroid distortion curve is quadratic across the ring field width, and the point of inflection of the curve is located substantially at the center of the ring field width. The shape of the curve may be either "U" or ".solthalfcircle.", depending on how the input field was defined at the mask, and with the magnitude of distortion substantially equal at the edges of the ring field width. In an optical system where static distortion is balanced, the path of the projected image (on the moving wafer) folds back on itself, causing the image blur to be minimized. When static distortion is not balanced, then the blurring becomes elongated. This could, in principle, be corrected by scanning the wafer at a slightly different velocity. However, if this is done, the location of the printed image will be incorrect and a straight line will be printed as curved. By balancing the centroid distortion across the ring field width, dynamic distortion and image smearing are reduced and the ring field width can be increased. The balanced static centroid distortion curve of the present invention is obtained by adjusting the asphericity of optical elements M1, M2, and M4. The distortion in an optical system can be expanded in a power series of odd terms, with the 3rd and 5th order terms being the lowest ones. The 3rd and the 5th order static distortions can be adjusted so that the centroid distortion is symmetrically balanced across the ring field width. These aspheric surfaces on M1, M2, and M4 also help correct astigmatism across the ring. The aspheric surfaces can also be expanded as a departure from a base sphere as a function of aperture radius in a power series of even terms. The expansion of the distortion and the aspheric surfaces are interrelated in that the 4th and 6th order aspheric terms influence the 3rd and 5th order distortion terms. By adjusting the aspheric terms, the magnitude and sign (+or -) of the 3rd and 5th order distortion terms can be controlled. The prior art systems adjusted the aspherics terms in order to minimize the overall distortion across the ring field slit width, but not to shape the distortion curve, thus the dynamic (scanning) distortions were never optimized. Table 7 below shows the deviation (distortion) of the image centroid at the wafer from its ideal location. TABLE 7 Ideal Image Location (mm) Centroid Distortion (nm) 52.000 10.50 52.075 7.60 52.150 4.97 52.225 2.62 52.300 0.55 52.375 -1.24 52.450 -2.74 52.525 -3.95 52.600 -4.87 52.675 -5.50 52.750 -5.83 52.825 -5.85 52.900 -5.57 52.975 -4.98 53.050 -4.07 53.125 -2.85 53.200 -1.32 53.275 0.55 53.350 2.73 53.425 5.25 53.500 8.10 In scanning lithography, the mask and wafer are synchronously scanned so that the projected ring field at the mask will cover the entire wafer field. The scanning process has a substantial effect on the image aberrations, particularly distortion. The image distortion due to the relative movement of the image and the substrate during radiation exposure is dynamic distortion which can smear an image out along a field-dependent trajectory as the image crosses the ring field width. Although some of the radiation incident to the optical elements is reflected, a large percentage of the incident radiation is absorbed. The total reflectivity of a four mirror optical system is described by the formula: R.sub.total =R.sub.1.times.R.sub.2.times.R.sub.3.times.R.sub.4, where R.sub.x represents the reflectivity of Mirror.sub.x. Different reflective coatings have different reflectivities. Reflective coatings, which have been found to have acceptable EUV reflectivity, include multilayer coatings of molybdenum/silicon (Mo/Si) and molybdenum/beryllium (Mo/Be). The maximum theoretical reflectivity of a multilayer mirror made of Mo/Si is approximately 72%. The Mo/Si multilayer structure includes an alternating layer stack of Mo and Si. The Mo and Si layers are deposited by a dc-magnetron sputtering system or an ion beam sputtering system. The thickness of each layer is determined by simultaneously maximizing the constructive interference of the beams reflected at each interface and minimizing the overall absorption to enable more interfaces to contribute to the reflectance. In addition to being highly reflective, the optical elements must have extremely accurate surface figures and surface roughness. State-of-the-art techniques are used to fabricate mirror surface figures with an accuracy of about 0.25 nm rms or better. Interferometers are used to measure the dimensional accuracy of the figure of the aspheric mirrors and the wavefront of assembled projection. Commercially available tools are capable of measuring surface roughness. Although suitable reflective surfaces exist for EUV radiation, the wavelength must be kept within a tight tolerance to maintain acceptable reflectivity. The described reflectivity varies with the wavelength of the radiation. Referring to FIG. 7, a plot of the theoretical reflectivity versus wavelength at normal incidence is shown for a 40 bilayer Mo/Si multilayer. The maximum theoretical reflectivity of approximately 72% is achieved when the radiation wavelength is 13.4 nm; however, a deviation of 0.4 nm in the radiation wavelength results in reflectivity of only 12%. Shifts in radiation wavelength are equivalent to changes in the incidence angle, i.e., both result in reduced reflectivity. The inventive optical systems maintain a high overall system reflectivity by utilizing shallow incidence angles that are within the highly reflective region of the multilayer surface. The thickness of the layer coatings can be adjusted to maximize the reflectivity for a specific range of incidence angles. The maximum theoretical reflectivity of over 70% for a suitably constructed Mo/Si multilayer optical element is obtained when the radiation incidence angle is 5.degree..+-.5. At incidence angles of 12.5.degree. and 15.degree., the reflectivity decreases to about 40% and 12%, respectively. Keeping the incidence angles low at M1, M2, and M4, maximizes the system reflectivity by ensuring that the multilayer is being used at its highest reflectivity at all times. The maximum theoretical reflectivity of over 70% can be shifted to higher incidence angles at the expense of the angular bandwidth of the coating. The design of the Mo/Si multilayer is adjusted so the reflectance peak is shifted to about 12.degree..+-.1, where the reflectivity of this multilayer coating decreases more rapidly as the incidence angle deviates from 12.degree.. At incidence angles of 0.degree. and 17.degree., the reflectivities decrease to approximately 55% and 40%, respectively. The reflectivity M3 is maximized by using reflective optics specifically designed for incidence angles between approximately 11.degree. and 13.degree.. In Table 8, the mean angle of incidence, the angle of incidence range and the corresponding Mo/Si reflectivity are listed for the optical elements of the present invention. TABLE 8 Optical Mean Angle Angle of Element of Incidence Incidence Range Reflectivity Range M1 3.48.degree. 3.8.degree. 70-71% M2 6.56.degree. 0.8.degree. 71% M3 12.0.degree. 1.6.degree. 70% M4 6.0.degree. 1.4.degree. 70.5-71% A benefit of the inventive optical system configuration is that the intensity of the illumination in the imaging bundle is uniform without the use of complex graded multilayer coatings. Prior art optical systems may require graded multilayer coatings to achieve this same level of illumination uniformity. In addition, in the present optical system the high radiation intensity and heat generation, which can degrade the reflective coating and cause thermal distortion of the optical element, is minimized. Because the inventive system has a high total reflectivity, the absorbed energy is minimized. The inventive system is also able to dissipate the absorbed energy more readily because mirror Ml has a large surface area. By spreading the radiation energy across a broader surface area, the radiation intensity and resulting heat generation are minimized. The inventive optical system requires only low aspheric mirrors, which are significantly easier to fabricate and test than highly aspheric mirrors. Mirrors with asphericities less than approximately 12 micrometers (low asphericity) can be tested at the center of curvature without the need for complex auxiliary test optics. In addition, low asphericity mirrors are more easily polished to an excellent surface finish of less than 1 .ANG.. Table 9 below shows the maximum aspheric departure from a best-fit spherical surface centered on the off-axis section of the parent asphere for each mirror. While the listed asphericities are for the preferred embodiment, these asphericities may vary by .+-.3.0 .mu.m for M1, .+-.4.0 .mu.m for M2, and .+-.3.0 .mu.m for M4 for different projection systems. In addition, mirror M3 may also be aspheric, with a maximum departure of .+-.2.0 .mu.m from the best fit sphere. TABLE 9 Mirror Maximum Aspheric Departure M1 6.2 .mu.m M2 9.6 .mu.m M3 0 M4 2.7 .mu.m Yet another advantage of the inventive optical system is that the design has an accessible, real aperture stop on mirror M3. The accessible aperture stop makes the projected imagery stationary. The imagery is independent of the position in the ring field. More specifically, the physically accessible aperture stop ensures that imaging bundles from each field point are not clipped or vignetted across the ring field width. M3 is configured with at least 5 mm of clearance around its diameter from other radiation beams making it accessible for adjustment. FIG. 8 illustrates a second embodiment of the present invention. Modifications to the first embodiment of the present invention depicted in FIG. 5 enable larger numerical apertures, which allows increased resolution. By changing the distribution of optical power within the projection system, numerical apertures in excess of 0.10, and typically between 0.1 and 0.14, can be achieved. Increasing the numerical aperture decreases the ray clearance around mirrors M1 and M3. Ray clearance can be improved substantially by increasing the field bias. However, the field dependent aberrations become more difficult to correct as the field bias is increased. In addition, incident angles at each mirror increases with increasing field bias. This necessarily complicates the multilayer coating task. A better approach to solving the ray clearance problem is to decrease the physical distance from the mask plane to mirror M1, while reducing the radius of the ring field at the mask to keep the field bias constant. In this manner, the incident angles at each of the mirrors can be constrained to acceptable limits. The reduction ratio of the system decreases as the distance from the mask to M1 is decreased. To increase the reduction back to 4.times., the negative power of mirror M1 is increased relative to the system focal length. The Petzval sum must be substantially zero to achieve high resolution imagery. This increase in the power of mirror M1 must be balanced against a decrease in the power of mirror M3 to maintain a well-corrected Petzval sum. For this second embodiment shown in FIG. 8, the absolute radii of the mirrors M1 801, M2 803, M3 805, and M4 807, relative to the system focal length, are listed in Table 10. TABLE 10 Mirror radii from object plane to image plane Mirror as a fraction of the system focal length .+-.5% M1 4.297 M2 2.411 M3 0.867 M4 1.036 The axial separations of the mirrors M1 801, M2 803, M3 805, and M4 807, relative to the system focal length, are listed in Table 11. For a 4-to-1 reduction, the distance of the mask to M1 801 is 726.688 mm. TABLE 11 Axial separations of the mirrors as a Surface fraction of the system focal length .+-.10% M1 to M2 1.488 M2 to M3 1.251 M3 to M4 0.514 M4 to image 0.902 Tables 12-15 contain constructional data and other relevant information for the inventive optical system shown in FIG. 8. The inventive optical system is a telecentric ring field system with a slit width of about 0.5-3.0 mm, a 4:1 reduction ratio, and a numerical aperture (NA) in the range of about 0.1 to 0.14. The reference wavelength is 13.4 mm. Table 12 shows the mirror radii and spacings, and taken with Table 13 and Table 14, completely describes the apparatus of the example. Table 13 lists the aspheric constants. Table 15 shows the performance of the system as described by the root mean square (RMS) wavefront error and corresponding Strehl ratio. The aspheric profile is uniquely determined by its K, A, B, C, and D values, such as given in Table 13. The sag of the aspheric surface (through 10th order) as a function of radial coordinate (h) given by Equation 1. Mirrors M1, M2, and M4, are base conics with 6th, 8th, and 10th order polynomial deformations. Mirror M3 has only a 6th order polynomial deformation. Identifying the mirror type via the base conic, the optical elements M1 and M2 are a hyperboloid and prolate ellipsoid, respectively. Both optical elements M3 and M4 are oblate spheroids. TABLE 12 Element number Radius of Curvature Thickness Object INFINITY 726.688000 1 1824.00000 -631.705000 2 1023.60000 530.859078 3 368.17500 -218.256865 4 439.83400 382.841399 Image INFINITY 0.000000 Dimensions are given in mm. Positive radius indicates center of curvature to the right. Thickness is axial distance to next surface. TABLE 13 Mirror CURV K A B C D M1 0.00054825 -13.470700 0.0 9.015830E-16 -5.054450E-20 3.951610E-25 M2 0.00097694 -0.517400 0.0 -2.821200E-17 -1.395030E-21 7.556670E-27 M3 0.00271610 0.407900 0.0 4.389380E-15 0.000000E+00 0.000000E+00 M4 0.00227359 0.155600 0.0 3.890150E-16 -1.108150E-20 4.030120E-25 TABLE 14 Center of ring field (mask) -200.0 mm Effective focal length 424.519 mm Paraxial reduction ratio 0.25 FiniteF/N.sub.o 4.15 Total track 790.427 mm TABLE 15 Ringfield rms wavefront error Strehl Radius (l = 13.4 nm) ratio 52.00 mm 0.028.lambda. 0.970 52.25 mm 0.015.lambda. 0.991 52.50 mm 0.009.lambda. 0.997 52.75 mm 0.020.lambda. 0.985 53.00 mm 0.0036.lambda. 0.951 This second embodiment also has an accessible, real aperture stop on mirror M3. More specifically, the accessible aperture stop ensures that imaging bundles from each field point within the ring field are not clipped or vignetted. This helps to ensure that the projected imagery, setting aside the effects of the field dependent aberrations and input illumination, is independent of position in the ring field. Such imagery is termed stationary. Another advantage of this second embodiment of the present invention is that the distortion is balanced across the width of the ring field. This balanced distortion curve results in a minimization of dynamic (scanning) distortion as explained previously. Referring to FIG. 9, the centroid distortion curve of this inventive optical system is shown. Table 16 below provides a tabular listing of the deviation of the image centroid its ideal location at the wafer plane. This deviation is the centroid distortion. TABLE 16 Ideal Image Location Centroid Distortion (mm) (nm) 49.500 11.69 49.550 9.39 49.600 7.32 49.650 5.48 49.700 3.87 49.750 2.49 49.800 1.35 49.850 0.45 49.900 -0.22 49.950 -0.64 50.000 -0.82 50.050 -0.75 50.100 -0.43 50.150 0.13 50.200 0.95 50.250 2.02 50.300 3.35 50.350 4.94 50.400 6.79 50.450 8.91 50.500 11.29 This second embodiment of the present invention also requires only low aspheric mirrors. Table 17 below shows the maximum aspheric departure from a best-fit spherical surface centered on the off-axis section of the parent asphere for each mirror. While the listed asphericities are for the preferred embodiment, these asphericities may vary by .+-.2.0 .mu.m for M1, .+-.2.0 .mu.m for M2, .+-.1.0 .mu.m for M3, and .+-.2.0 .mu.m for M4 for different projection systems. TABLE 17 Mirror Maximum Aspheric Departure M1 8.80 .mu.m M2 11.70 .mu.m M3 0.16 .mu.m M4 4.70 .mu.m While the present invention has been described in terms of the preferred embodiments above, those skilled in the art will readily appreciate that numerous modifications, substitutions and additions may be made to the disclosed embodiment without departing from the spirit and scope of the present invention. For example, although optical systems shown in FIGS. 5 and 8 have been described for use with a semiconductor photolithography system, those skilled in the art will readily appreciate that the inventive optical systems may be utilized in any similar lithography device and that the present invention is in no way limited to the mechanisms described above. It is intended that all such modifications, substitutions and additions fall within the scope of the present invention, which is best defined by the claims below.