Patent Publication Number: US-10310155-B2

Title: Multiple-stack wire grid polarizer

Description:
CLAIM OF PRIORITY 
     This application is a divisional of U.S. Nonprovisional patent application Ser. No. 15/715,407, filed on Sep. 26, 2017, which claims priority to U.S. Provisional Patent Application Nos. 62/425,201, filed on Nov. 22, 2016, and 62/433,619, filed on Dec. 13, 2016, which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present application is related generally to wire grid polarizers. 
     BACKGROUND 
     A wire grid polarizer (WGP) can transmit one polarization (e.g. p-polarization) and reflect or absorb an opposite polarization (e.g. s-polarization). High reflectivity of the opposite polarization (e.g. high Rs) can be important because some applications use both polarized light beams (e.g. s &amp; p). High absorption/low reflectivity of the opposite polarization (e.g. low Rs) can be important in some applications because reflection of this polarization (Rs) can interfere with the optical system. For example, the reflected s-polarization can cause ghosting in an image projector. Some WGPs are designed for high reflection and others for high absorption of the s-polarization. 
     High transmission of one polarization (e.g. high Tp) can be an important feature of WGPs in order to minimize light-source power requirements. Low transmission of the opposite polarization (e.g. Ts) can be important for improved light image resolution. The quality or performance of WGPs can be shown by efficiency (Tp*Rs) and contrast (Tp/Ts). 
     Polarization can be improved by increased aspect ratio (wire thickness/wire width) of wires in a wire grid polarizer. Manufacture of wires with a sufficiently large aspect ratio is a difficult manufacturing challenge. 
     SUMMARY 
     It has been recognized that it would be advantageous to provide a wire grid polarizer (WGP) with a desired Rs (high or low), high Tp, and low Ts, and a high aspect ratio. The present invention is directed to various embodiments of WGPs that satisfy these needs. Each embodiment may satisfy one, some, or all of these needs. 
     The WGP can comprise a substrate having a first side and a second side opposite of the first side. A first array of wires, defining a first array, can be located over the first side of the substrate. A first thin film can be located over the first array and can be transparent. A second array of wires, defining a second array, can be located over the first thin film. A second thin film can be located over the second array and can be transparent. 
     In one embodiment, an aspect ratio can be greater than 5, where aspect ratio=T/W, T is a sum of a thickness of wires of the first array plus a thickness of wires of the second array, and W is a maximum width of wires of the first array and of the second array. 
     In another embodiment, each wire of the first array can comprise a first absorptive rib and a first reflective rib, the first absorptive rib being sandwiched between the first reflective rib and the substrate; and each wire of the second array can comprise a second absorptive rib and a second reflective rib, the second reflective rib being sandwiched between the second absorptive rib and the second thin film. 
     In another embodiment, each wire of the first array can comprise a stack of ribs in the following order extending outward from the substrate towards the first thin film: a first transparent rib, a second transparent rib, and a first reflective rib. A material composition of the first transparent rib can be different from a material composition of the second transparent rib. Each wire of the second array can comprise a stack of ribs in the following order extending outward from the first thin film towards the second thin film: a second reflective rib, a third transparent rib, and a fourth transparent rib. A material composition of the fourth transparent rib can be different from a material composition of the third transparent rib. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS (DRAWINGS MIGHT NOT BE DRAWN TO SCALE) 
         FIG. 1  is a schematic, cross-sectional side-view of a wire grid polarizer (WGP)  10  including a substrate  15 , a first array  11  over the substrate  15 , a first thin film  01  over the first array  11 , a second array  12  over the first thin film  01 , and a second thin film  02  over the second array  12 , in accordance with an embodiment of the present invention. 
         FIG. 2  is a schematic perspective-view of WGP  20 , similar to WGP  10  of  FIG. 1 , including the first array  11  over the substrate  15 , in accordance with an embodiment of the present invention. 
         FIG. 3  is a schematic, cross-sectional side-view of WGP  30 , similar to WGP  10  of  FIG. 1 , each wire of the first array  11  can comprise a first absorptive rib  21  and a first reflective rib  31 , and each wire of the second array  12  can comprise a second absorptive rib  22  and a second reflective rib  32 , in accordance with an embodiment of the present invention. 
         FIG. 4  is a schematic, cross-sectional side-view of WGP  40 , similar to WGP  10  of  FIG. 1 ; each wire of the first array  11  can comprise a first transparent rib  41 , a second transparent rib  42 , and a first reflective rib  31 ; and each wire of the second array  12  can comprise a second reflective rib  32 , a third transparent rib  43 , and a fourth transparent rib  44 , in accordance with an embodiment of the present invention. 
         FIG. 5  is a schematic, cross-sectional side-view of WGP  50 , similar to WGP  10  of  FIG. 1 , further comprising a first conformal coating  51  over the first array  11  between the first array  11  and the first thin film  01 , and a second conformal coating  52  over the second array  12  between the second array  12  and the second thin film  02 , in accordance with an embodiment of the present invention. 
     
    
    
     DEFINITIONS 
     As used herein, the term “conformal coating” means a thin film which conforms to the contours of feature topology. For example, “conformal” can mean that a minimum thickness of the coating is greater than 1 nm and a maximum thickness of the coating is less than 20 nm. As another example, “conformal” can mean that a maximum thickness divided by a minimum thickness of the coating is less than 20, less than 10, or less than 5. 
     As used herein, the term “elongated” means that a length L of the wires is substantially greater than wire width W 11  or W 12  or wire thickness Th 11  or Th 12  (e.g. L can be at least 10 times, at least 100 times, at least 1000 times, or at least 10,000 times larger than wire width W 11  or W 12  and/or wire thickness Th 11  or Th 12 ). 
     As used herein, the terms “on”, “located on”, “located at”, and “located over” mean located directly on or located over with some other material between. The terms “located directly on”, “adjoin”, “adjoins”, and “adjoining” mean direct and immediate contact with no other solid material between. 
     As used herein, the term “nm” means nanometer(s) and the term “μm” means micrometer(s). 
     As used herein, the term “parallel” means exactly parallel, parallel within normal manufacturing tolerances, or nearly parallel, such that any deviation from exactly parallel would have negligible effect for ordinary use of the device. 
     As used herein, the term “substrate” means a base material, such as for example a glass wafer. The term “substrate” also includes multiple materials, such as for example a glass wafer with thin film(s). 
     As used herein, the term “thin film” means a continuous layer that is not divided into a grid and having a thickness less than 10 μm, less than 1 μm, or less than 0.5 μm, depending on the light spectrum of interest. 
     Materials used in optical structures can absorb some light, reflect some light, and transmit some light. The following definitions distinguish between materials that are primarily absorptive, primarily reflective, or primarily transparent. Each material can be considered to be absorptive, reflective, or transparent in a specific wavelength range (e.g. ultraviolet, visible, or infrared spectrum) and can have a different property in a different wavelength range. Such materials are divided into absorptive, reflective, and transparent based on reflectance R, the real part of the refractive index n, and the imaginary part of the refractive index/extinction coefficient k. Equation 1 is used to determine the reflectance R of the interface between air and a uniform slab of the material at normal incidence: 
                   R   =           (     n   -   1     )     2     +     k   2             (     n   +   1     )     2     +     k   2                 Equation   ⁢           ⁢   1               
Unless explicitly specified otherwise herein, materials with k≤0.1 in the specified wavelength range are “transparent” materials, materials with k&gt;0.1 and R≤0.6 in the specified wavelength range are “absorptive” materials, and materials with k&gt;0.1 and R&gt;0.6 in the specified wavelength range are “reflective” materials.
 
     DETAILED DESCRIPTION 
     As illustrated in  FIG. 1 , a wire grid polarizer (WGP)  10  is shown comprising a substrate  15  having a first side  15   f  and a second side  15   s  opposite of the first side  15   f . The substrate  15  can be transparent. A first array of wires, defining a first array  11 , can be located over the first side  15   f  of the substrate  15  with a proximal end  11   p  closer to the substrate  15  and a distal end  11   d  farther from the substrate  15 . The first array  11  can be parallel and elongated, with channels  13  between adjacent wires. A first thin film  01  can be located at the distal end  11   d  of the first array  11  and can be transparent. A second array of wires, defining a second array  12 , can be located over the first thin film  01  with a proximal end  12   p  closer to the first thin film  01  and a distal end  12   d  farther from the first thin film  01 . The second array  12  can be parallel and elongated, with channels  13  between adjacent wires. A second thin film  02  can be located at the distal end  12   d  of the second array  12  and can be transparent. 
     The first array  11  can adjoin the substrate  15  and/or the first thin film  01 , or other material can be located therebetween. The second array  12  can adjoin the first thin film  01  and/or the second thin film  02 , or other material can be located therebetween. 
     In one embodiment, material of the first array  11  and material of the second array  12  can be reflective. In another embodiment, material(s) of the first array  11  and material(s) of the second array  12  can be a reflective, transparent, absorptive, or combinations thereof. 
     As shown in  FIGS. 3 and 4 , a plane  33  can pass through a center of a wire of the first array  11  and can be perpendicular to the first side  15   f  of the substrate  15 . As shown in  FIG. 3 , the wire of the first array  11  can be aligned with an associated wire of the second array  12  such that the plane  33  also passes through the associated wire of the second array  12 , and can even pass through a center of the associated wire of the second array  12 . Alternatively, as shown in  FIG. 4 , wires of the first array  11  and wires of the second array  12  may be misaligned or offset so that the plane does not pass through the associated wire of the second array  12 . A decision of whether or not to so align wires of the first array  11  and wires of the second array  12  can be made depending on the effect of such alignment/misalignment compared with manufacturing difficulties. 
     The first thin film  01  and the second thin film  02  can be formed by various methods, including sputter deposition. Sputtering can result in covering tops of the arrays of wires  11  and  12  without filling the channels  13 . The first thin film  01  can span the channels  13  while covering the distal ends  11   d  of the first array  11  and the second thin film  02  can span the channels  13  while covering the distal ends  12   d  of the second array  12 . Thus, the channels  13  can be adjacent to the wires and can be air filled, and degradation of WGP performance from solid-filled channels  13  can be avoided. See for example USA Patent Publication Number US 2012/0075699, which is incorporated herein by reference. Use of sputtering and the method described in US 2012/0075699 can result in continuous thin films unbroken by boundary layers. Thus, the first thin film  01  can extend across the first array  11  unbroken by any boundary layer extending parallel to the first array  11  and the second thin film  02  can extend across the second array  12  unbroken by any boundary layer extending parallel to the second array  12 . In contrast, shadow deposition can result in boundary layers running parallel with the wires. 
     An alternative to sputtering and the thin films  01  and  02  spanning the channels  13  is filling the channels  13 . Thus, the channels  13  can be filled with material of the first thin film  01  and the second thin film  02 , such as for example by atomic layer deposition. 
     An aspect ratio can be based on a combined thickness Th 11  and Th 12  of both arrays of wires  11  and  12  divided by wire width W 11  or W 12  (whichever is greater). With present manufacturing abilities, the combined thickness Th 11  and Th 12  of both arrays of wires  11  and  12  can be greater than a thickness of a single array. Therefore, a much larger effective aspect ratio may be achieved with stacked, multiple arrays. Although two arrays of wires  11  and  12  are shown in the drawings, an even higher effective aspect ratio may be achieved by a third, fourth, or even more arrays of wires stacked above the substrate with an intermediate thin film. For example, an aspect ratio can be greater than 3, greater than 5, greater than 10, greater than 15, greater than 20, or greater than 30, where aspect ratio=T/W. T is a sum of a thickness of wires of the first array  11  plus a thickness of wires of the second array  12  (i.e. T=Th 11 +Th 12 ). Unless specified otherwise, W is a maximum width of wires of the first array  11  and of the second array  12  (i.e. maximum of W 11  and W 12 ). Alternatively, if so specified, W is a maximum width of wires of the first array  11  or of the second array  12  (i.e. maximum of W 11  or W 12 ). 
     WGP  30 , shown in  FIG. 3 , is a variation of WGP  10 . Each wire of the first array  11  can comprise a first absorptive rib  21  and a first reflective rib  31 . The first absorptive rib  21  can be sandwiched between the first reflective rib  31  and the substrate  15 . Each wire of the second array  12  can comprise a second absorptive rib  22  and a second reflective rib  32 . The second reflective rib  32  can be sandwiched between the second absorptive rib  22  and the second thin film  02 . 
     This WGP  30  can absorb a polarization of light from each of two opposite sides of the WGP  30 , which can be advantageous in image projection systems. For example, in one embodiment, the WGP  30 , in a light wavelength range of 450 nm through 700 nm, can absorb at least 80% of one polarization of light from both sides of the WGP  30  and can transmit at least 80% of an opposite polarization of light. Advantages of the design of WGP  30  are also described in U.S. Pat. No. 9,684,203, which is incorporated herein by reference. 
     WGP  40 , shown in  FIG. 4 , is a variation of WGP  10 . Each wire of the first array  11  can comprise a stack of ribs in the following order extending outward from the substrate  15  towards the first thin film  01 : a first transparent rib  41 , a second transparent rib  42 , and a first reflective rib  31 . A material composition of the first transparent rib  41  can be different from a material composition of the second transparent rib  42 . Each wire of the second array  12  can comprise a stack of ribs in the following order extending outward from the first thin film  01  towards the second thin film  02 : a second reflective rib  32 , a third transparent rib  43 , and a fourth transparent rib  44 . A material composition of the fourth transparent rib  44  can be different from a material composition of the third transparent rib  43 . 
     Performance of WGP  40  can be improved if an index of refraction of the first transparent rib (n1) is greater than an index of refraction of the second transparent rib (n2) and an index of refraction of the fourth transparent rib (n4) is greater than an index of refraction of the third transparent rib (n3). This relationship between the indices of refraction n1-n4 can be across a wavelength range of intended use, such as for example across a wavelength range of at least 100 nm, 200 nm, 300 nm, or 400 nm in the ultraviolet, visible, or infrared spectrums of light. 
     Performance of WGP  40  can be improved, particularly in uniformity of efficiency and contrast from each of two opposite sides of the WGP  40 , if a material composition of the first transparent rib  41  is the same as a material composition of the fourth transparent rib  44  and if a material composition of the second transparent rib  42  is the same as a material composition of the third transparent rib  43 . 
     One example of possible performance of WGP  40  is that a percent reflection of one polarization of incident light on the first side  15   f  of the substrate  15  (Rs1) is greater than 93% and a percent reflection of one polarization of incident light on the second side  15   s  of the substrate  15  (Rs2) is greater than 93%. Such performance can be across a wavelength range of intended use, such as for example across a wavelength range of at least 100 nm, 200 nm, 300 nm, or 400 nm in the ultraviolet, visible, or infrared spectrums of light. 
     WGP  40  can have high efficiency (Rs*Tp) and high contrast (Tp/Ts) on each of two opposite sides of the WGP  40 . Advantages of the design of WGP  40  are also described in U.S. Provisional Patent Application No. 62/425,201, filed on Nov. 22, 2016, which is incorporated herein by reference. 
     WGP  50 , shown in  FIG. 5 , is a variation of WGP  10 . WGP  50  includes a first conformal coating  51  over the first array  11 , coating sides of wires of the first array  11 , a distal end  12   d  of wires of the first array  11  between the first array  11  and the first thin film  01 , and an exposed surface of the first side  15   f  of the substrate  15 . WGP  50  also includes a second conformal coating  52  over the second array  12 , coating sides of wires of the second array  12 , a distal end  12   d  of wires of the second array  12  between the second array  12  and the second thin film  02 , and an exposed surface of the first thin film  01 . The conformal coatings  51  and  52  can protect the wires  11  and  12 , as described in U.S. Pat. Nos. 6,785,050 and 9,703,028.