Abstract:
Lithography using controlled polarization. One aspect generates an electromagnetic radiation suitable for production of microelectronic devices, linearly polarizes at least a portion of the radiation at a pupil plane of a projection system to form linearly polarized radiation, and exposing a substrate using the linearly polarized radiation at a high exposure angle.

Description:
BACKGROUND  
       [0001]     This disclosure relates to lithography systems and techniques that use controlled polarization.  
         [0002]     Lithography is a printing process in which a pattern is rendered on a substrate. The pattern can be rendered using lithography masks and other optical elements that transfer the pattern to the substrate. The resultant pattern can be used, e.g., to form integrated circuits on semiconductor wafers. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0003]      FIG. 1  shows a block diagram of a lithography system.  
         [0004]      FIG. 2  shows an example pattern that can be focused on a working surface.  
         [0005]      FIG. 3  illustrates a prior art technique for exposing a substrate.  
         [0006]      FIGS. 4 and 5  illustrate implementations of systems and techniques for exposing a substrate.  
         [0007]      FIGS. 6-9  illustrate implementations of how radiation can be polarized to improve the quality of printing.  
         [0008]      FIGS. 10-13  are graphs that show impacts of the use of linearly polarized radiation in printing.  
         [0009]      FIG. 14  shows a block diagram of a lithography system.  
         [0010]      FIG. 15  shows an implementation of a pupil plane polarizer.  
         [0011]      FIGS. 16-17  show another implementation of a pupil plane polarizer. 
     
    
       [0012]     Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0013]      FIG. 1  shows a photolithography system  100 . System  100  can be, for example, an air photolithography system or an immersion photolithography system. System  100  includes an illumination source  105 , an illumination system  110 , a mask  115 , a projection system  120 , and a substrate  125 .  
         [0014]     Illumination source  105  is a device capable of generating and emitting electromagnetic radiation  130 . Radiation  130  can be coherent in that the emitted optical waves maintain a fixed and predictable phase relationship with each other for a period of time. Radiation  130  can be selected for use in lithographic patterning of microelectronic devices. For example, radiation  130  can be sub-visible wavelength radiation such as 193-nm radiation.  
         [0015]     Illumination system  110  can include an aperture, a condenser, a filter, as well as additional devices for collecting, collimating, filtering, and focusing radiation  130  emitted from source  105 .  
         [0016]     Mask  115  is positioned in system  100  by a mask stage to influence the incidence of radiation  130  upon substrate  125 . Mask  115  can include different regions that transmit electromagnetic radiation with different transmissivities and phases. For example, mask  115  can be a strong or a weak phase shift mask, such as an alternating phase shift mask or an embedded phase shift mask.  
         [0017]     Projection system  120  can include an aperture, an objective, as well as additional devices, for collecting, filtering, and focusing the portion of radiation  130  that passes through mask  115 . Projection system  120  generally includes one or more pupil planes  135 . The spatial intensity distribution at pupil plane(s)  135  generally corresponds to an angular distribution of the radiation at substrate  125 .  
         [0018]     Substrate  125  is a workpiece to be patterned by system  100 . Substrate  125  includes a working surface  140 . Substrate  125  can be presented to system  100  by a vacuum chuck or other support (not shown) so that radiation  130  is focused on working surface  140  to form a pattern. Substrate  125  can include a resist material above a base material. Substrate  125  can be patterned to form all or a portion of a microelectronic device. The resist material can be a material that is sensitive to radiation  130 . For example, the resist material can be a positive or negative photoresist.  
         [0019]      FIG. 2  shows an example pattern  200  that can be imaged by system  100  onto working surface  140 . Pattern  200  includes an alternating series of lines  205  and spaces  210 . Lines  205  and spaces  210  are oriented with a main axis in the y-direction and a smallest pitch in the x-direction. Pitch is the spatial periodicity of features. For example, the pitch of lines  205  in the x-direction is the sum of the width of lines  205  and the width of spaces  210 .  
         [0020]     Lines  205  and spaces  210  can be formed using any of a number of different masks including, e.g., alternating phase shift masks and embedded phase shift masks. A point  215  is located in the midst of one of lines  205  and a point  220  is located in the midst of one of dark spaces  210 .  
         [0021]      FIG. 3 , taken along section A-A ( FIG. 2 ), illustrates a prior art technique for imaging pattern  200  onto working surface  140 . Although the entirety of pattern  200  can be exposed at one time using this technique, for the sake of simplicity,  FIG. 3  only schematically illustrates a portion of the incident radiation exposing point  215  on substrate  125 . Point  215  is exposed by a cone of radiation  130  that can subtend a maximum exposure angle  305 . Maximum exposure angle  305  is generally determined at least in part by the numerical aperture of the system  100 . The conical expanse spanned by exposure angle  305  includes rays  310 ,  315 ,  320 . Ray  310  is incident upon point  215  of substrate  125  at an angle that is substantially normal to working surface  140 . Rays  315 ,  320  are incident upon point  215  of substrate  125  at high exposure angles in the vicinity of the maximum exposure angle  305 . For example, rays  315 ,  320  can be incident upon point  215  at exposure angles of 45 degrees and above. In an alternate but less preferred implementation, rays  315 ,  320  can be incident upon point  215  at exposure angles of 55 degrees and above.  
         [0022]     Radiation  130  includes optical waves  325 ,  330 ,  335 . Optical wave  325  travels along ray  310 . Optical wave  330  travels along ray  315 . Optical wave  335  travels along ray  320 . Optical waves  325 ,  330 ,  335  are circularly polarized in that the direction of the electric fields of waves  325 ,  330 ,  335  circumscribe helices as waves  325 ,  330 ,  335  propagate.  
         [0023]     Given that the electric fields of waves  325 ,  330 ,  335  circumscribe helices, the electric fields of waves  325 ,  330 ,  335  are generally not oriented in the same direction when waves  325 ,  330 ,  335  are incident upon point  215 . Similar discrepancies in orientation of the electric fields also occur with other optical waves that are incident on other points on working surface  140  (not shown). The net effect of these discrepancies in orientation is a limitation of interference effects and reduced contrast on working surface  140 . These reductions in contrast increase as the exposure angles increase.  
         [0024]      FIG. 4 , also taken along section A-A ( FIG. 2 ), illustrates an implementation of a system and a technique for exposing point  215  on substrate  125 . Point  215  is exposed by optical waves  325 ,  330 ,  335  each traveling along the respective of rays  310 ,  315 ,  320 . Optical wave  325  is circularly polarized, whereas optical waves  330 ,  335  are linearly polarized with the electric fields of waves  330 ,  335  being directed into and out of the plane of the page (i.e., in the y-direction) as waves  330 ,  335  propagate. Waves  330 ,  335  are thus linearly polarized perpendicular to their propagation direction and parallel to the image plane on substrate  125 . Up to all of the optical waves in the vicinity of the outer bounds of maximum exposure angle  305  can be linearly polarized in this manner, as discussed further below.  
         [0025]     Since the electric fields of waves  330 ,  335  are oriented in the same direction when waves  330 ,  335  are incident upon point  215 , constructive interference between waves  330 ,  335  at point  215  increases the intensity of the exposure of point  215 . Similarly, destructive interference of waves  330 ,  335  at point  220  decreases the intensity of the exposure of point  220 . These interference effects increase the contrast of the pattern  200  imaged onto working surface  140 .  
         [0026]      FIG. 5 , also taken along section A-A ( FIG. 2 ), illustrates another implementation of a system and a technique for exposing point  215  on substrate  125 . Optical waves  330 ,  335  remain linearly polarized and are incident upon point  215  on substrate  125 . However, the radiation that travels along rays in the vicinity of ray  310  has been attenuated or eliminated. Selected portions of radiation  130  (such as optical wave  325 ) can thus be prevented from exposing point  215 .  
         [0027]     For the sake of clarity of illustration,  FIGS. 4 and 5  show optical waves  330 ,  335  perfectly linearly polarized. However, optical waves  330 ,  335  need not be perfectly linearly polarized. Rather, optical waves  330 ,  335  can be somewhat elliptically polarized with the proportion of radiation polarized in the illustrated direction being significant enough to achieve the described interference effects and increased contrast.  
         [0028]      FIGS. 6-9  illustrate implementations of how radiation  130  can be polarized in a pupil plane (e.g., pupil plane  135 ) in system  100  to improve the quality of printing. The illustrated polarizations can be obtained using a number of different techniques, including transmission polarizing, reflection polarizing, scattering techniques, double refraction techniques, and birefringence techniques.  
         [0029]      FIG. 6  shows an area  600  in pupil plane  135  that encompasses the angular distribution of the radiation that is to expose the pattern to be formed on substrate  125 . Area  600  can be entirely exposed by radiation  130  or area  600  can include dark portions where the intensity of radiation  130  is zero.  
         [0030]     Area  600  includes linearly polarized regions  605 ,  610 . Regions  605 ,  610  are located at high angles in the x-direction and can encompass optical waves in the vicinity of the outer bounds of exposure angle  305  ( FIGS. 4 and 5 ). Regions  605 ,  610  include radiation that is linearly polarized in the y-direction.  
         [0031]      FIG. 7  shows another implementation of area  600  that includes linearly polarized regions  705 ,  710 . Regions  705 ,  710  are located at high angles in the y-direction. Regions  705 ,  710  include radiation that is linearly polarized in the x-direction. Regions  705 ,  710  provide improved contrast of patterns having features with a small pitch in the y-direction in much the same way that regions  605 ,  610  provide improved contrast of patterns having features with a small pitch in the x-direction (such as pattern  200  ( FIG. 2 )).  
         [0032]      FIG. 8  shows another implementation of area  600  that includes linearly polarized regions  605 ,  610 ,  705 ,  710 . With the inclusion of regions  605 ,  610 ,  705 ,  710 , improved contrast of patterns having small pitches in both the y-direction and x-direction can be obtained.  
         [0033]      FIG. 9  shows another implementation of area  600  that includes an annular region  905 . Radiation at each point in annular region  905  is linearly polarized perpendicular to the propagation direction of the radiation and parallel to the substrate. With such a polarization, improved contrast of patterns having small pitches in any direction can be obtained.  
         [0034]      FIG. 10  is a graph  1000  that shows an impact of the use of linearly polarized radiation in the printing of y-line features (such as lines  205  in pattern  200 ). Graph  1000  shows the best focus intensity in air as a function of position along a line in the image plane perpendicular to the y-line features. The best focus intensity is normalized with respect to the clear field intensity. The positions in graph  1000  are given in micrometers. The results shown in graph  1000  were calculated using the Yeung vector method for a system that was assumed to include an embedded phase shift mask having y-line features with a  140  nm pitch (60 nm wide EPSM, 80 nm wide glass) in the x-direction. The radiation was assumed to have a wavelength of 193 nm and the photolithography system was assumed to have a numerical aperture of 0.92 and to provide cross quadrupole illumination with quadrupole centers at 0.75 and radii of 0.2 (e.g., one implementation of  FIG. 8 ).  
         [0035]     Graph  1000  includes a first trace  1005  and a second trace  1010 . Trace  1005  corresponds to the best focus intensity as a function of position when printing y-line features using radiation of a given intensity polarized in the y-direction. Such radiation can be obtained with an area  600  that includes polarized regions  605 ,  610  ( FIG. 6 ).  
         [0036]     Trace  1010  corresponds to the best focus intensity as a function of position when printing y-line features using radiation of the same intensity polarized in the x-direction. Such radiation can be obtained with an area  600  that includes polarized regions disposed like regions  605 ,  610  but including radiation polarized in the x-direction instead of the y-direction.  
         [0037]     The range of intensities spanned by trace  1005  is greater than the range of intensities spanned by trace  1010 . Thus, for the given intensity of radiation, the contrast between the y-line features and background is greater when imaged using radiation polarized in the y-direction than when imaged using radiation polarized in the x-direction.  
         [0038]      FIG. 11  is a graph  1100  that shows an impact of the use of linearly polarized radiation in the printing of y-line features. Graph  1100  shows the best focus intensity in air as a function of position along a line perpendicular to the y-line features in the image plane for a system under the same assumptions as  FIG. 10 . Graph  1100  was again calculated using the Yeung vector method and gives the positions in micrometers.  
         [0039]     Graph  1100  includes a first trace  1105  and a second trace  1110 . Trace  1105  corresponds to the best focus intensity as a function of position when printing y-line features using radiation of a given intensity polarized in both the y-direction and the x-direction. The polarization is set such that it is always nearly parallel to the substrate (x-polarized for large y exposure angles and y-polarized for large x exposure angles). Such radiation can be obtained with an area  600  that includes each of polarized regions  605 ,  610 ,  705 ,  710  ( FIG. 8 ) or by polarized region  905  ( FIG. 9 ).  
         [0040]     Trace  1110  corresponds to the best focus intensity as a function of position when printing y-line features using circularly polarized radiation of the same intensity.  
         [0041]     The range of intensities spanned by trace  1105  is greater than the range of intensities spanned by trace  1110 . Thus, for the given intensity of radiation, the contrast between the y-line features and background is greater when imaged using radiation polarized in both the x-direction and the y-direction than when imaged using circularly polarized radiation.  
         [0042]      FIG. 12  is a graph  1200  that shows an impact of the use of linearly polarized radiation in the printing of y-line features (such as lines  205  in pattern  200 ). Graph  1200  shows the intensity in air as a function of position along a line perpendicular to the y-line features in the image plane. The intensity is normalized with respect to the clear field intensity. The positions are given in graph  1200  in micrometers. The results shown in graph  1200  were calculated using the Yeung vector method for a system that was assumed to include an alternating phase shift mask having y-line features with 70 nm wide chrome and a 140 nm pitch in the x-direction. The illumination radiation was assumed to have a wavelength of 193 nm and the photolithography system was assumed to have a numerical aperture of 0.92 and conventional illumination with a partial coherence of sigma 0.2.  
         [0043]     Graph  1200  includes a first trace  1205 , a second trace  1210 , and a third trace  1215 . Trace  1205  corresponds to the intensity as a function of position when printing y-line features using radiation of a given intensity polarized in the y-direction. Such radiation can be obtained with an area  600  that includes polarized regions  605 ,  610  ( FIG. 6 ).  
         [0044]     Trace  1210  corresponds to the intensity as a function of position when printing y-line features using radiation of the same intensity polarized in the x-direction. Such radiation can be obtained with an area  600  that includes polarized regions disposed like regions  605 ,  610  but including radiation polarized in the x-direction instead of the y-direction.  
         [0045]     Trace  1215  corresponds to the intensity as a function of position when printing y-line features using circularly polarized radiation of the same intensity.  
         [0046]     The range of intensities spanned by trace  1205  is greater than the range of intensities spanned by traces  1210 ,  1215 . Thus, for the given intensity of radiation, the contrast between the y-line features and background is greater when imaged using radiation polarized in the y-direction than either when imaged using radiation polarized in the x-direction or when imaged using circularly polarized radiation.  
         [0047]      FIG. 13  is a graph  1300  that shows an impact of the use of linearly polarized radiation in the printing of y-line features (such as lines  205  in pattern  200 ). Graph  1300  shows the resist size of the y-line features as a function of defocus for a number of different radiation intensities. Resist size is the printed size of a feature and is given in graph  1300  in micrometers. Defocus is the distance between the focal plane and the working surface of a substrate and is given in graph  1300  in micrometers. The results shown in graph  1300  were calculated using the Yeung vector method for a system that was assumed to include an alternating phase shift mask having y-line features with 70 nm chrome and a 140 nm pitch in the x-direction. The illumination radiation was assumed to have a wavelength of 193 nm and the photolithography system was assumed to have a numerical aperture of 0.92 and conventional illumination with a partial coherence of sigma 0.2.  
         [0048]     Graph  1300  includes traces  1305 ,  1310 ,  1315 ,  1320 ,  1325 ,  1330 ,  1335 ,  1340 ,  1345 ,  1350 ,  1355 . Traces  1305 ,  1310 ,  1315 ,  1320 ,  1325  correspond to the resist size of y-line features as a function of defocus for circularly polarized radiation having intensities of 18 mJ, 19 mJ, 20 mJ, 21 mJ, and 22 mJ, respectively.  
         [0049]     Traces  1330 ,  1335 ,  1340 ,  1345 ,  1350 ,  1355  correspond to the resist size of y-line features as a function of defocus for radiation polarized in the y-direction and having intensities of 19 mJ, 20 mJ, 21 mJ, 22 mJ, 23 mJ, and 24 mJ respectively. Such radiation can be obtained with an area  600  that includes polarized regions  605 ,  610  ( FIG. 6 ).  
         [0050]     As shown in graph  1300 , the sensitivity of resist size to dose variation is decreased when imaging using radiation polarized in the y-direction as compared to imaging using circularly polarized radiation. The decreased sensitivity to dose variation improves the uniformity of patterning and reduces the probability of defects.  
         [0051]      FIG. 14  shows an implementation of photolithography system  1400 . System  1400  includes illumination source  105 , illumination system  110 , mask  115 , projection system  120 , and substrate  125 , as well as a pupil plane polarizer  1405 . Pupil plane polarizer  1405  is positioned at pupil plane  135  in system  1400 .  
         [0052]     Pupil plane polarizer  1405  is a device or material for suppressing or minimizing particular optical waves in radiation  130 . Pupil plane polarizer  1405  can be effective to polarize optical waves so that an increased proportion of the radiation in the vicinity of the outer bounds of an exposure angle is linearly polarized perpendicular to the direction angle and parallel to the image plane. In one implementation, pupil plane polarizer  1405  is effective to perfectly linearly polarize all of the radiation in the vicinity of the outer bounds of the exposure angle.  
         [0053]     Pupil plane polarizer  1405  can be used to achieve one or more of the described radiation polarizations in area  600  ( FIGS. 6-9 ). Pupil plane polarizer  1405  can be a transmission polarizer in that radiation  130  passes through pupil plane polarizer  1405 .  
         [0054]      FIG. 15  shows an implementation of pupil plane polarizer  1405  from below. Pupil plane polarizer  1405  includes a polarizer substrate  1505  and a collection of polarizing features  1510 . Polarizer substrate  1505  can be a made from a material that is at least partially transparent to radiation  130 . For example, when radiation  130  is EUV radiation, polarizer substrate  1505  can be a Si 3 N 4  film peripherally supported by an annular ring. Polarizing features  1415  can be features effective to polarize transmitted radiation  130  as described above. Polarizing features  1415  can be formed using any of a number of high resolution printing techniques, such as electron beam printing, imprint techniques, ion-beam printing, and EUV and x-ray lithography. Pupil plane polarizer  1405  can include a center region  1515  that subtends low exposure angles. Center region  1515  can either be opaque or at least partially transparent to radiation  130 .  
         [0055]      FIG. 16  shows another implementation of pupil plane polarizer  1405 . Pupil plane polarizer  1405  includes a polarization modulator  1610  and a stress modulator  1620 . Stress modulator  1620  is in communication with polarization modulator  1610 , and may be used to apply a stress to polarization modulator  1610 . An optional controller  1630  may be used to control the amount and/or direction of stress applied to polarization modulator  1610  by stress modulator  1620 . For example, in some implementations controller  1630  may be used to turn the applied stress on and off, or to vary the spatial and/or temporal distribution of stress applied to polarization modulator  1610 .  
         [0056]     Stress modulator  1620  may be implemented in a number of ways. For example, stress modulator  1620  may be implemented using a clamping mechanism, so that an inner diameter of stress modulator  1620  may be mechanically decreased to apply stress to polarization modulator  1610 . In another example, stress modulator  1620  may be implemented using material with a thermal expansion coefficient sufficiently large to apply a desired stress amount in response to a change in temperature. In another example, stress modulator  1620  may restrict the outer diameter of polarization modulator  1610 , and polarization modulator  1610  may be heated or cooled. In this example, the thermal expansion or contraction of the polarization modulator  1610  is used to produce the stress. Many other implementations are possible.  
         [0057]     In an implementation, polarization modulator  1610  is a glass cylinder or disc. As illustrated in  FIG. 17 , external stress  1705  applied to glass creates internal stress  1710  within the glass. Stresses  1705 ,  1710  make the glass optically birefringent, with a refractive index asymmetry that is a function of the angle around the cylinder. The resulting stress-induced birefringence causes light that passes through the element to change polarization. Light output from modulator  1610  thus has a polarization state P 2  different than polarization state P 1  of the input light.  
         [0058]     In this implementation of pupil plane polarizer  1405 , the principle stress directions and associated optical axis vary as a function of angle within modulator  1610 . For an implementation in which modulator  1610  is a glass cylinder, the top and bottoms of the glass cylinder rotate the polarization of incoming light in an equal but opposite direction than do the left and right sides of the cylinder. By providing a modulator  1610  having an appropriate geometry and stress profile, the polarization may be customized.  
         [0059]     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. For example, reflection polarizing or birefringence techniques can be used to polarize radiation in the manners described. The described systems and techniques can be used in immersion and air photolithography. Pupil plane polarizer  1405  can include a dark field at its middle. Areas  600  can be rotated to any extent about their center and the polarized regions can subtend different and varying extents of areas  600 . Accordingly, other implementations are within the scope of the following claims.