Patent Publication Number: US-2016231487-A1

Title: High Contrast Inverse Polarizer

Description:
CLAIM OF PRIORITY 
     This claims priority to US Provisional Patent Application No. 62/113,101, filed on Feb. 6, 2015, which is hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present application is related generally to wire-grid polarizers. 
     BACKGROUND 
     Wire-grid polarizers (WGPs or WGP for singular) can be used to divide light into two different polarization states. One polarization state can mostly pass through the WGP and the other can be mostly absorbed or reflected. The effectiveness or performance of WGPs is based on a high percent transmission of one polarization and minimal transmission of an opposite polarization. A percent transmission of the primarily-transmitted polarization divided by a percent transmission of the opposite polarization is called contrast. It can be difficult to manufacture WGPs that provide sufficiently-high contrast. High contrast can sometimes be obtained by reducing the pitch of the wires/ribs, but doing so can be a difficult manufacturing challenge, especially for smaller wavelengths. It would be beneficial to find a way to improve WGP performance by some way other than a reduction in pitch. 
     SUMMARY 
     It has been recognized that it would be advantageous to improve wire-grid polarizer (WGP or WGPs for plural) performance by some way other than a reduction in pitch. The present invention is directed to various embodiments of embedded, inverse WGPs, methods of polarizing light, and methods of designing embedded, inverse WGPs, that satisfy these needs. Each embodiment may satisfy one, some, or all of these needs. For the following WGPs and methods, E ∥  is a polarization of light with an electric field oscillation parallel to a length L of the ribs and E 195   is a polarization of light with an electric field oscillation perpendicular to a length L of the ribs. 
     The embedded, inverse WGP can comprise ribs located over a surface of a transparent substrate, gaps between the ribs, and a fill-layer substantially filling the gaps. The ribs can be elongated and can be formed into an array. At a wavelength of light incident upon the WGP, E ∥  transmission can be greater than E 195   transmission. The fill-layer can have an index of refraction greater than 1.4 at the wavelength of the light. 
     The method of polarizing light can comprise providing an inverse, embedded WGP and transmitting more E ∥  through the WGP than E ⊥ . The method of designing an embedded, inverse WGP can comprise calculating a pitch of an array of ribs of the WGP for E ∥  transmission&gt;E 195   transmission at a desired wavelength and calculating an index of refraction of a fill-layer, located over the array of ribs and substantially filling gaps between the ribs, for E ∥  transmission&gt;E ⊥  transmission at the desired wavelength. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS (DRAWINGS MIGHT NOT BE DRAWN TO SCALE) 
         FIG. 1 a    is a schematic cross-sectional side view of an embedded, inverse wire-grid polarizer (WGP)  10  comprising ribs  13  located over a surface of a transparent substrate  11 , gaps  16  between the ribs  13 , and a fill-layer  15  substantially filling the gaps  16 , in accordance with an embodiment of the present invention. 
         FIG. 1 b    is a schematic perspective-view of an embedded, inverse wire-grid polarizer (WGP)  10  comprising ribs  13  located over a surface of a transparent substrate  11 , gaps  16  between the ribs  13 , and a fill-layer  15  substantially filling the gaps  16 , in accordance with an embodiment of the present invention. 
         FIG. 2  is a schematic cross-sectional side view of WGP  20 , similar to WGP  10 , except that the fill-layer  15  of WGP  20  extends from the gaps  16  over the ribs  13 , in accordance with an embodiment of the present invention. 
         FIG. 3  is a schematic cross-sectional side view of WGP  30 , similar to WGPs  10  &amp;  20 , except that the ribs  13  of WGP  30  include a substantial difference between a lower-rib-width W L  and an upper-rib-width W H , in accordance with an embodiment of the present invention. 
         FIG. 4  is a schematic perspective view of an integrated circuit (IC) inspection tool  40 , using at least one WGP  44  to polarize light  45 , in accordance with an embodiment of the present invention. 
         FIG. 5  is a schematic perspective view of a flat panel display (FPD) manufacturing tool  50 , using at least one WGP  54  to polarize light  55 , in accordance with an embodiment of the present invention. 
     
    
    
     DEFINITIONS 
     As used herein, the term “elongated” means that a length L (see  FIG. 1 b   ) of the ribs  13  is substantially greater than rib width W or rib thickness Th 13  (see  FIGS. 1 a   ,  2 , and  3 ). For example, WGPs for ultraviolet or visible light can have a rib width W between 20 and 100 nanometers and rib thickness between 50 and 500 nanometers; and rib length of about 1 millimeter to 20 centimeters or more, depending on the application. Thus, elongated ribs  13  can have a length L that is many times (e.g. at least 10 times in one aspect, at least 100 times in another aspect, at least 1000 times in another aspect, or at least 10,000 times in another aspect) larger than rib width W or rib thickness Th 13 . 
     As used herein, the term “light” can mean light or electromagnetic radiation in the x-ray, ultraviolet, visible, and/or infrared, or other regions of the electromagnetic spectrum. 
     As used herein, the term “thin-film layer” means a continuous layer that is not divided into a grid. 
     As used herein, the term “width” of the rib means the maximum width of the rib, unless specified otherwise. 
     Many materials used in optical structures absorb some light, reflect some light, and transmit some light. The following definitions are intended to distinguish between materials or structures that are primarily absorptive, primarily reflective, or primarily transparent. Each material can be primarily absorptive, primarily reflective, or primarily transparent in a specific wavelength of interest (e.g. all or a portion of the ultraviolet, visible, or infrared spectrums of light) and can have a different property in a different wavelength of interest. 
     1. As used herein, the term “absorptive” means substantially absorptive of light in the wavelength of interest.
         a. Whether a material is “absorptive” is relative to other materials used in the polarizer. Thus, an absorptive structure will absorb substantially more than a reflective or a transparent structure.   b. Whether a material is “absorptive” is dependent on the wavelength of interest. A material can be absorptive in one wavelength range but not in another.   c. In one aspect, an absorptive structure can absorb greater than 40% and reflect less than 60% of light in the wavelength of interest (assuming the absorptive structure is an optically thick film—i.e. greater than the skin depth thickness).   d. In another aspect, an absorptive material can have a high extinction coefficient (k), relative to a transparent material, such as for example greater than 0.01 in one aspect or greater than 1.0 in another aspect.   e. Absorptive ribs can be used for selectively absorbing one polarization of light.       2. As used herein, the term “reflective” means substantially reflective of light in the wavelength of interest.
       a. Whether a material is “reflective” is relative to other materials used in the polarizer. Thus, a reflective structure will reflect substantially more than an absorptive or a transparent structure.   b. Whether a material is “reflective” is dependent on the wavelength of interest. A material can be reflective in one wavelength range but not in another. Some wavelength ranges can effectively utilize highly reflective materials. At other wavelength ranges, especially lower wavelengths where material degradation is more likely to occur, the choice of materials may be more limited and an optical designer may need to accept materials with a lower reflectance than desired.   c. In one aspect, a reflective structure can reflect greater than 80% and absorb less than 20% of light in the wavelength of interest (assuming the reflective structure is an optically thick film—i.e. greater than the skin depth thickness).   d. Metals are often used as reflective materials.   e. Reflective wires can be used for separating one polarization of light from an opposite polarization of light.   
       3. As used herein, the term “transparent” means substantially transparent to light in the wavelength of interest.
       a. Whether a material is “transparent” is relative to other materials used in the polarizer. Thus, a transparent structure will transmit substantially more than an absorptive or a reflective structure.   b. Whether a material is “transparent” is dependent on the wavelength of interest. A material can be transparent in one wavelength range but not in another.   c. In one aspect, a transparent structure can transmit greater than 90% and absorb less than 10% of light at the wavelength of interest or wavelength range of use, ignoring Fresnel reflection losses.   d. In another aspect, a transparent structure can have an extinction coefficient (k) of less than 0.01, less than 0.001, or less than 0.0001 in another aspect, at the wavelength of interest or wavelength range of use.   
       4. As used in these definitions, the term “material” refers to the overall material of a particular structure. Thus, a structure that is “absorptive” is made of a material that as a whole is substantially absorptive, even though the material may include some reflective or transparent components. Thus for example, a rib made of a sufficient amount of absorptive material so that it substantially absorbs light is an absorptive rib even though the rib may include some reflective or transparent material embedded therein.   

     DETAILED DESCRIPTION 
     As illustrated in  FIGS. 1 a , 1 b   ,  2 , and  3 , embedded, inverse wire-grid polarizers (WGPs or WGP for singular)  10 ,  20 , and  30  are shown comprising ribs  13  located over a surface of a transparent substrate  11 . The ribs  13  can be elongated and can be formed into an array. The ribs  13  can be reflective, or can include a reflective portion. The ribs  13  can include an absorptive portion. The ribs  13  can be a metal or a dielectric or can include different regions, at least one of which is a metal and at least one of which is a dielectric. 
     For the following discussion, E ∥  is a polarization of light with an electric field oscillation parallel to a length L of the ribs and E 195   is a polarization of light with an electric field oscillation perpendicular to a length L of the ribs. In typical WGP use, E 195   is primarily transmitted and E ∥  is primarily reflected or absorbed. A WGP can be used as an inverse WGP in a wavelength range of light where E ∥  is primarily transmitted and E ⊥  is primarily reflected or absorbed (E ∥  transmission&gt;E 195   transmission). Merely having E ∥  transmission&gt;E ⊥  transmission is insufficient for many applications, and it can be important to optimize performance of the inverse WGP, meaning a high E ∥  transmission and/or high contrast (E ∥  transmission/E 195   transmission). The WGP structure can be optimized for improved inverse WGP performance. 
     The WGPs  10 ,  20 , and  30  can have gaps  16  between the ribs  13 . The term “gap” means a space, opening, or divide, separating one rib from another rib. A fill-layer  15 , substantially filling the gaps  16 , and especially a fill-layer  15  with a relatively large index of refraction, can improve inverse WGP performance. For example, an index of refraction of the fill-layer  15  can be greater than 1.4 in one aspect, greater than 1.5 in another aspect, greater than 1.6 in another aspect, or greater than 1.8 in another aspect. The aforementioned index of refraction values are those at the light wavelength of intended use, where E ∥  transmission&gt;E 195   transmission. The fill-layer  15  can be a solid material or liquid. The fill-layer  15  can be transparent. Examples of fill-layer materials, for polarization of ultraviolet light, include Al 2 O 3  (n=1.8 at λ=300 nm), ZrO 2  (n=2.25 at λ=361 nm), and HfO 2  (n=2.18 at λ=365 nm). 
     Use of a fill-layer  15  to improve WGP performance, and especially use of a fill-layer with a relatively large index of refraction, is contrary to conventional WGP design theory. For example, see U.S. Pat. No. 6,288,840, column 6, line 59 through column 7, line 15. A conventional WGP (E 1  transmission&gt;E ∥  transmission) may include a fill-layer for protection of the ribs, accepting a reduction in WGP performance. For example, see U.S. Pat. No. 6,288,840, column 1, lines 18-54. 
     The fill-layer  15  of WGPs  20  and  30  in  FIGS. 2-3  substantially fills the gaps  16  and extends from the gaps  16  over the ribs  13  such that the fill-layer  15  in each gap  16  extends continuously over adjacent ribs  13  to the fill-layer  15  in each adjacent gap  16 . Extending the fill-layer  15  over the ribs  13 , and using certain thicknesses Th 15  of the fill-layer  15  over the ribs  13 , can improve inverse WGP performance. The fill-layer  15  can extend over the ribs for a thickness Th 15  optimized for the desired wavelength range of use. For example, the fill-layer  15  can extend over the ribs for a thickness Th 15  of at least 25 nanometers in one aspect, at least 50 nanometers in another aspect, or at least 60 nanometers in another aspect, and less than 90 nanometers in one aspect, less than 100 nanometers in another aspect, or less than 150 nanometers in another aspect. 
     Use of a substrate  11  and/or a thin-film layer  31  (see  FIG. 3 ) between the ribs  13  and the substrate  11 , with a relatively large index of refraction, can improve inverse WGP performance and can shift the wavelength range at which E ∥  transmission&gt;E ⊥  transmission. For example, an index of refraction of the substrate  11  and/or the thin-film layer  31  can be greater than 1.4 in one aspect, greater than 1.5 in another aspect, greater than 1.6 in another aspect, or greater than 1.8 in another aspect. The aforementioned index of refraction values are those at the light wavelength of intended use, where E ∥  transmission&gt;E 195   transmission. 
     Rib  13  pitch P can be selected to improve inverse WGP performance and to shift the wavelength range at which E ∥  transmission&gt;E 195   transmission. In conventional WGPs, the pitch needed for high-performance polarization can be less than one-half of the smallest wavelength in the desired polarization wavelength range. Consequently, a pitch of less than 150 nanometers is typically used for polarization of visible light (λ/P≈150/400=2.67), and around 100 nanometers or less for polarization of ultraviolet light. Manufacture of such polarizers can be difficult and costly due to this small pitch. Fortunately, optimal pitch P, for inverse WGPs described herein, can be larger than a pitch needed for conventional polarizers, thus improving the manufacturability of these inverse WGPs. 
     For example, the wavelength of the light of desired inverse polarization divided by a pitch P of the ribs  13  can be less than 2.5 in one aspect, less than 2.0 in another aspect, less than 1.9 in another aspect, less than 1.8 in another aspect, or less than 1.7 in another aspect. As another example, for inverse polarization of light with a wavelength of less than 400 nanometers (e.g. ultraviolet light), a pitch P of the ribs  13  can be greater than 140 nanometers. Pitch P of the ribs  13  and an index of refraction n of the fill-material  15  can be selected by the following equation: P*(n−0.2)&lt;λ&lt;P*(n+0.2), where λ is the wavelength of the light of desired inverse polarization. 
     Although pitch P for inverse polarization may be relatively large, for polarization of small wavelengths of light, such as less than 260 nanometer light in one aspect or less than 200 nanometer light in another aspect, small pitches P may be needed, such as for example less than 100 nanometers in one aspect, less than 80 nanometers in another aspect, or even less than 60 nanometers in another aspect. 
     A duty-cycle (W/P) of the ribs  13  can be selected to improve inverse WGP performance and to shift the wavelength range at which E ∥  transmission&gt;E 195   transmission. For example, the following duty-cycles can improve contrast: greater than 0.45 in one aspect or greater than 0.55 in another aspect, and less than 0.60 in one aspect, less than 0.65 in another aspect, less than 0.70 in another aspect, or less than 0.80 in another aspect. 
     A lower duty-cycle can be selected to improve transmission of E ∥ , and can broaden the wavelength range of high E ∥  transmission, but possibly by sacrificing contrast. Thus, a duty-cycle can be selected for improved transmission of E ∥ , such as for example less than 0.7 in one aspect, less than 0.6 in another aspect, less than 0.5 in another aspect, or less than 0.4 in another aspect. For example, for a wavelength range of light of at least 30 nanometers, E ∥  transmission&gt;E 195   transmission and E ∥  transmission can be greater than 80%. This wavelength range of light can be in a region of the electromagnetic spectrum of less than 400 nanometers, e.g. ultraviolet spectrum. 
     A smaller rib thickness Th 13  can improve contrast. For example, rib thickness Th 3  can be less than 70 nanometers in one aspect, less than 55 nanometers in another aspect, or less than 45 nanometers in another aspect. 
     Rib  13  shape can be selected to improve inverse WGP performance and to shift the wavelength range at which E ∥  transmission&gt;E ⊥  transmission. Edges E (i.e. corners) of the ribs  13  can be approximately 90 degrees, thus forming rectangular-shaped ribs  13 , as shown in  FIGS. 1 a  and 1 b   . Alternatively, the edges E of the ribs  13  can be rounded, and thus a cross-sectional-profile of the ribs  13  can include a rounded shape, as shown in  FIGS. 2-3 . One, two, three, or more than three of the edges E of each rib  13  can be rounded. An end of the ribs  13  farther from the substrate (i.e. top of the ribs  13 ) can have a rounded-shape and/or an end of the ribs  13  closest to the substrate (i.e. bottom of the ribs  13 ) can be rounded. The ribs  13  can be formed with different shapes by adjusting the anisotropic/isotropic character of the etch, and other etch parameters, throughout the etch process. 
     Ribs  13  with multiple widths W L  and W H  in each rib  13 , as shown on WGP  30  in  FIG. 3 , can broaden the wavelength range of high contrast. For example, a difference between a lower-rib-width W L  and an upper-rib-width W H  can be greater than 10 nanometers in one aspect, greater than 20 nanometers in another aspect, or greater than 30 nanometers in another aspect. Lower-rib-width W L  means a maximum width of the ribs  13  in a lower-half of the rib  13  closer to the substrate  11 . Upper-rib-width W H  means a maximum width of the ribs  13  in an upper-half of the rib  13  farther from the substrate. The inventors found that, by selecting a difference between a lower-rib-width W L  and an upper-rib-width W H  of greater than 20 nanometers, for a wavelength range of light of at least 20 nanometers in the ultraviolet spectrum, the E ∥  transmission divided by the E 195   transmission can be at least 300. The ribs  13  can be formed with a different lower-rib-width W L  and an upper-rib-width W H  by adjusting the anisotropic/isotropic character of the etch, and other etch parameters, throughout the etch process. 
     WGPs described herein can be made with E ∥  transmission&gt;E 195   transmission, with high contrast (E ∥  transmission/E ⊥  transmission), and with high E ∥  transmission, even in the difficult to polarize regions of the electromagnetic spectrum. For example, the WGPs described herein can have E ∥  transmission&gt;E ⊥  transmission and contrast of at least 10 in one aspect, at least 100 in another aspect, at least 300 in another aspect, at least 400 in another aspect, at least 1000 in another aspect, at least 5000 in another aspect, or at least 10,000 in another aspect, at a certain wavelength or wavelength range. As another example, the WGPs described herein can have E ∥  transmission of at least 70 &amp;, at least 80%, or at least 90%, at a certain wavelength or wavelength range. These WGP performance numbers can even be achieved at a wavelength or a wavelength range of light in the electromagnetic spectrum of less than 400 nanometers in one aspect, less than 300 nanometers in another aspect, less than 270 nanometers in another aspect, or a wavelength in or across the ultraviolet spectrum in another aspect. 
     A method of polarizing light can comprise one or more of the following:
     1. providing an inverse, embedded WGP as described herein; and   2. transmitting more E ∥  through the WGP than E ⊥  with contrast (E ∥  transmission/E ⊥  transmission) as described above and at a wavelength or wavelength range as described herein.   

     A method of designing an embedded, inverse WGP can comprise one or more of the following for matching or tuning the inverse WGP performance (E ∥  transmission&gt;E 195   transmission) to a desired wavelength or wavelength range and/or for improving WGP performance (contrast and/or %E ∥  transmission) at that wavelength or wavelength range:
     1. calculating a pitch of an array of ribs  13 ;   2. calculating an index of refraction of a fill-layer  15 , located over the array of ribs  13  and substantially filling gaps  16  between the ribs  13 ;   3. selecting rib  13  material;   4. selecting rib thickness Th 13 ;   5. selecting duty cycle (W/P);   6. selecting rib  13  shape;   7. selecting thickness Th 15  of the fill-layer  15  over the array of ribs  13 ; and   8. selecting substrate material.   

     Integrated circuits (ICs or IC) can be made of semiconductor material and can include nanometer-sized features. ICs can be used in various electronic devices (e.g. computer, motion sensor, etc.). Defects in the IC can cause the electronic device to fail. Thus, inspection of the IC can be important for avoiding failure of the electronic device, while in use by the consumer. Such inspection can be difficult due to the small feature-size of IC components. Light, with small wavelengths (e.g. ultraviolet), can be used to inspect small feature-size components. It can be difficult to have sufficient contrast between these small feature-size components and defects or their surroundings. Use of polarized light can improve integrated circuit (IC) inspection contrast. It can be difficult to polarize the small wavelengths of light (e.g. ultraviolet/UV) used for IC inspection. Polarizers that can polarize such small wavelengths, and that can withstand exposure to high-energy wavelengths of light, may be needed. 
     The WGPs described herein can polarize small wavelengths of light (e.g. UV) and can be made of materials sufficiently durable to withstand exposure to such light. The fill-material  15  can protect the ribs  13  from UV light damage. An IC inspection tool  40  is shown in  FIG. 4 , comprising a light source  41  and a stage  42  for holding an IC wafer  43 . The light source  41  can be located to emit an incident light-beam  45  (e.g. visible, ultraviolet, or x-ray) onto the IC wafer  43 . The incident light-beam  45  can be directed to the wafer  43  by optics (e.g. mirrors). The incident light-beam  45  can have an acute angle of incidence  49  with a face of the wafer  43 . To improve inspection contrast, a WGP  44  (according to an embodiment described herein) can be located in, and can polarize, the incident light-beam  45 . 
     A detector  47  (e.g. CCD) can be located to receive an output light-beam  46  from the IC wafer  43 . An electronic circuit  48  can be configured to receive and analyze a signal from the detector  47  (the signal based on the output light-beam  46  received by the detector  47 ). To improve inspection contrast, a WGP  44  (according to an embodiment described herein) can be located in, and can polarize, the output light-beam  46 . 
     The WGPs described herein can be used in the manufacture of flat panel displays (FPDs for plural or FPD for singular). FPDs can include an aligned polymer film and liquid crystal. An FPD manufacturing tool  50  is shown in  FIG. 5 , comprising a light source  51 , a stage  52  for holding an FPD  53 , and a WGP  54  (according to an embodiment described herein). The light source  51  can emit ultraviolet light  55 . The WGP  54  can be located between the light source  51  and the stage  52  and can polarize the ultraviolet light  55 . Exposing the FPD  53  to polarized ultraviolet light  55  can align the polymer film. See U.S. Pat. No. 8,797,643 and 8,654,289, both incorporated herein by reference. Exposing the FPD  53  to polarized ultraviolet light  55  can help repair the FPD  53 . See U.S. Pat. No. 7,697,108, which is incorporated herein by reference.