PATENT DOCUMENT

Publication Number: US-10133418-B2
Application Number: US-201615258810-A
Country: US
Kind Code: B2

Title: Force sensing in an electronic device using a single layer of strain-sensitive structures

Abstract:
A strain-sensitive structure includes two resistive structures connected in series and formed on one surface of a substrate. One resistive structure is formed with a first trace arranged in first trace pattern. The other resistive structure is formed with a second trace arranged in a second trace pattern. The first resistive structure is configured to experience strain in response to an applied stress on the substrate. The second resistive structure is configured to experience less strain in response to the applied stress on the substrate compared to the first resistive structure. Together the strain-sensitive structure and the substrate form a force sensing layer that can be included in an electronic device.

Claims:
What is claimed is: 
     
       1. An electronic device, comprising:
 a display layer positioned below a cover layer; 
 a force sensing layer positioned adjacent the display layer, the force sensing layer comprising:
 a substrate; and 
 a plurality of strain-sensitive structures formed on a surface of the substrate; 
 
 a processing device operably connected to the plurality of strain-sensitive structures and configured to:
 receive a strain signal from each strain-sensitive structure; and 
 correlate at least one strain signal into an amount of force applied to the cover layer, 
 
 wherein each strain-sensitive structure in the plurality of strain-sensitive structures comprises:
 a first trace arranged in a first trace pattern on the surface of the substrate and forming a first resistive structure; 
 a second trace arranged in a second trace pattern on the surface of the substrate and forming a second resistive structure, the second resistive structure connected in series with the first resistive structure, wherein 
 the first trace pattern of the first resistive structure is sensitive to strain in one or more directions in response to an applied stress on the substrate, 
 the second trace pattern of the second resistive structure is less sensitive to strain than the first trace pattern of the first resistive structure in response to the applied stress on the substrate; and 
 in response to the applied stress on the substrate, a first resistance value of the first resistive structure changes more compared to a second resistance value of the second resistive structure. 
 
 
     
     
       2. The electronic device of  claim 1 , wherein:
 a drive signal line is received at a first node of one strain-sensitive structure; and 
 the drive signal line is shared with an adjacent strain-sensitive structure and is received at a different second node of the adjacent strain-sensitive structure. 
 
     
     
       3. The electronic device of  claim 1 , wherein the substrate is attached to a bottom surface of the display layer. 
     
     
       4. The electronic device of  claim 1 , wherein:
 a first terminal of the first resistive structure is connected to a first drive signal line; 
 a second terminal of the second resistive structure is connected to a second drive signal line; and 
 a common terminal of the first and the second resistive structures is connected to a strain signal line. 
 
     
     
       5. The electronic device of  claim 1 , wherein the first trace comprises a first section that includes a first set of legs arranged in a serpentine pattern, each leg in the first set of legs having a first length. 
     
     
       6. The electronic device of  claim 5 , wherein the second trace comprises a second section that includes a second set of legs arranged in the serpentine pattern, each leg in the second set of legs having a second length that is shorter than the first length. 
     
     
       7. The electronic device of  claim 6 , wherein:
 one end of the first resistive structure is connected to a first drive signal line and another end of the first resistive structure is connected to a strain signal line; and 
 one end of the second resistive structure is connected to a second drive signal line and another end of the second resistive structure is connected to the strain signal line. 
 
     
     
       8. The electronic device of  claim 1 , wherein the first and the second traces comprise a metal or metal alloy formed in or on a plastic substrate. 
     
     
       9. The electronic device of  claim 1 , wherein the first trace pattern and the second trace pattern are arranged on the surface of the substrate to reduce magnetic interference. 
     
     
       10. The electronic device of  claim 1 , further comprising at least one cavity formed in the substrate adjacent at least a portion of the second trace. 
     
     
       11. The electronic device of  claim 1 , wherein:
 the first trace comprises:
 a first section comprising a first set of legs arranged in a serpentine pattern, the first set of legs having a first length extending along a first axis; 
 a second section comprising a second set of legs arranged in the serpentine pattern, the second set of legs having the first length extending along the first axis; and 
 a third section including a leg extending along a second axis and connecting the first section to the second section; and 
 
 the second trace comprises:
 a fourth section positioned adjacent three sides of the first section, the fourth section comprising a third set of legs arranged in the serpentine pattern, the third set of legs having a second length that is less than the first length; and 
 a fifth section connected to the fourth section, the fifth section positioned adjacent three sides of the second section, the fifth section comprising a fourth set of legs arranged in the serpentine pattern, the fourth set of legs having the second length. 
 
 
     
     
       12. The electronic device of  claim 11 , wherein the second trace further comprises a sixth section positioned adjacent, and connected to, the fifth section, the sixth section comprising a fifth set of legs arranged in the serpentine pattern, the fifth set of legs having the second length extending along the second axis. 
     
     
       13. The electronic device of  claim 11 , wherein the first trace pattern and the second trace pattern are duplicated and rotated at least once on the substrate. 
     
     
       14. The electronic device of  claim 13 , wherein the first and the third sections of the first trace are arranged to detect strain along the first axis and a rotated first section and a rotated third section of the first trace are arranged to detect strain along the second axis. 
     
     
       15. The electronic device of  claim 11 , wherein the second axis is normal to the first axis. 
     
     
       16. The electronic device of  claim 11 , wherein the fourth and the fifth sections each experience less strain in response to the applied stress on the substrate compared to the first and the second sections. 
     
     
       17. The electronic device of  claim 11 , further comprising at least one cavity formed in the substrate adjacent at least a portion of the second trace. 
     
     
       18. The electronic device of  claim 11 , further comprising trenches formed in the substrate between and around each leg in the first, the second, and the third sections of the first trace and between and around each leg in the fourth and the fifth sections of the second trace. 
     
     
       19. The electronic device of  claim 18 , further comprising an insulating layer disposed between the substrate and the first and second traces.

Description:
FIELD 
     The described embodiments relate generally to strain sensing. More particularly, the present embodiments relate to force sensing in an electronic device using strain-sensitive structures formed in or on one surface of a substrate. 
     BACKGROUND 
     Many electronic and input devices include a touch-sensitive surface for receiving user inputs. Devices such as smart telephones, tablet computing devices, laptop computers, track pads, wearable communication and health devices, navigation devices, and kiosks can include a touch-sensitive surface. In some cases, the touch-sensitive surface is integrated with a display to form a touch-screen or touch-sensitive display. 
     The touch-sensitive surface may detect and relay the location of one or more user touches, which may be interpreted by the electronic device as a command or a gesture. In one example, the touch input may be used to interact with a graphical user interface presented on the display of the device. In another example, the touch input may be relayed to an application program operating on a computer system to effect changes to the application program. 
     Touch-sensitive surfaces, however, are limited to providing only the location of one or more touch events. Moreover, touch, like many present inputs for computing devices, is binary. The touch is either present or it is not. Binary inputs are inherently limited insofar as they can only occupy two states (present or absent, on or off, and so on). In many examples, it may be advantageous to also detect and measure the force of a touch that is applied to a surface. In addition, when force is measured across a continuum of values, it can function as a non-binary input. 
     SUMMARY 
     Certain embodiments described herein reference a strain or force sensing layer that includes at least one strain-sensitive structure. A strain-sensitive structure includes a first trace formed on only one surface of a substrate and a second trace formed on the same surface of the substrate. The first trace forms a first resistive structure and the second trace forms a second resistive structure. The second trace is connected in series with the first trace. The first trace includes: a first section comprising a first set of legs arranged in a serpentine pattern, the first set of legs having a first length extending along a first axis; a second section comprising a second set of legs arranged in the serpentine pattern, the second set of legs having the first length extending along the first axis; and a third section including a leg extending along a second axis and connecting the first section to the second section. The second trace includes: a fourth section positioned adjacent three sides of the first section, the fourth section comprising a third set of legs arranged in a serpentine pattern, the third set of legs having a second length that is less than the first length; and a fifth section connected to the fourth section, the fifth section positioned adjacent three sides of the second section, the fifth section comprising a fourth set of legs arranged in a serpentine pattern, the fourth set of legs having the second length. In some embodiments, the first and second trace patterns can be arranged to reduce, minimize, or substantially cancel magnetic interference. 
     In some embodiments, a strain-sensitive structure includes a first trace arranged in a first trace pattern on a surface of a substrate and a second trace arranged in a second trace pattern on the surface of the substrate. The first trace forms a first resistive structure and the second trace forms a second resistive structure that is connected in series with the first resistive structure. The first resistive structure experiences strain in response to an applied stress on the substrate. The second resistive structure experiences less strain than the first resistive structure in response to the applied stress on the substrate. Additionally or alternatively, a resistance of the first resistive structure changes more compared to a resistance of the second resistive structure in response to one or more forces that are applied to the surface of the substrate. 
     Further embodiments described herein may relate to, include, or take the form of an electronic device that includes a display layer positioned below a cover layer, and a force sensing layer positioned over or below the display layer. The force sensing layer includes a substrate and a plurality of strain-sensitive structures formed on a surface of the substrate. A processing device is operably (e.g., electrically) connected to the plurality of strain-sensitive structures. The processing device is configured to receive a strain signal from each strain-sensitive structure and correlate at least one strain signal into an amount of force applied to the cover layer. At least one strain-sensitive structure in the plurality of strain-sensitive structures includes a first trace arranged in a first trace pattern on the surface of the substrate and forming a first resistive structure, and a second trace arranged in a second trace pattern on the surface of the substrate and forming a second resistive structure, the second resistive structure connected in series with the first resistive structure. The first trace pattern of the first resistive structure is sensitive to strain in one or more directions in response to an applied stress on the substrate. The second trace pattern of the second resistive structure is less sensitive to strain in any direction in response to the applied stress on the substrate. 
     In some embodiments, a method of fabricating a strain-sensitive structure on a substrate includes forming a conductive material over the substrate and forming the strain-sensitive structure by etching a first trace and a second trace in the conductive material in one etching operation. The first trace is etched into a first trace pattern and the second trace is etched into a second trace pattern and the first trace and the second trace are connected in series. The substrate is then etched to produce trenches around and between the first trace and the second trace. When the substrate is etched, the first trace and the second trace act as a mask such that the trenches are self-aligned with the first trace and the second trace. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
         FIG. 1  shows one example of an electronic device that can include one or more strain-sensitive structures; 
         FIGS. 2A-2E  show a first example of a strain-sensitive structure that is suitable for use in a force sensing layer; 
         FIG. 2F  depicts a second example strain-sensitive structure that is suitable for use in a force sensing layer; 
         FIG. 2G  shows a second example strain-sensitive structure that is suitable for use in a force sensing layer; 
         FIG. 3  depicts a schematic diagram of the strain-sensitive structure connected to a reference element; 
         FIG. 4  shows a simplified view of a portion of a force sensing layer that includes an array of strain-sensitive structures; 
         FIG. 5  depicts one example of the second trace on a substrate; 
         FIG. 6  shows an example of the first trace and the second trace on a substrate; 
         FIG. 7  depicts a flowchart of an example method of fabricating the first and second traces shown in  FIG. 6 ; and 
         FIG. 8  shows a cross-sectional view of a portion of the display taken along line  8 - 8  in  FIG. 1 . 
     
    
    
     The use of cross-hatching or shading in the accompanying figures is generally provided to clarify the boundaries between adjacent elements and also to facilitate legibility of the figures. Accordingly, neither the presence nor the absence of cross-hatching or shading conveys or indicates any preference or requirement for particular materials, material properties, element proportions, element dimensions, commonalities of similarly illustrated elements, or any other characteristic, attribute, or property for any element illustrated in the accompanying figures. 
     Additionally, it should be understood that the proportions and dimensions (either relative or absolute) of the various features and elements (and collections and groupings thereof) and the boundaries, separations, and positional relationships presented therebetween, are provided in the accompanying figures merely to facilitate an understanding of the various embodiments described herein and, accordingly, may not necessarily be presented or illustrated to scale, and are not intended to indicate any preference or requirement for an illustrated embodiment to the exclusion of embodiments described with reference thereto. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following descriptions are not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims. 
     Embodiments described herein reference an electronic device that includes an input device configured to determine an amount of force applied to an input surface of the input device. The input device includes a force sensing layer having one or more strain-sensitive or strain-responsive structures formed on one surface of a substrate. The force sensing layer can be incorporated into any suitable type of input device. Example input devices include, but are not limited to, a touch display, a button, a section of an enclosure of the electronic device, and a track pad. 
     Each strain-sensitive structure includes two resistive structures connected in series. One resistive structure is formed with a first trace arranged in a first trace pattern. Sections of the first trace have legs that are arranged in a serpentine pattern. 
     The other resistive structure is formed with a second trace arranged in a second trace pattern. The second trace includes legs that are also arranged in a serpentine pattern. The number of legs in the second trace is greater than the number of legs in the first trace, and the legs in the second trace are shorter than the legs in the first trace. 
     In particular, the resistance of the resistive structure formed by the first trace changes in response to strain while the resistance of the resistive structure formed by the second trace changes less in response to the strain. This is at least partly due to the legs in the second trace contacting the substrate over shorter distances (e.g., the length of the legs in the second trace is less than the length of the legs in the first trace). When the second trace is formed with a material having a high Young&#39;s modulus (e.g., metal), and is supported by a substrate formed with a material having a lower Young&#39;s modulus (e.g., plastic), strain is reduced in the second trace and experienced by the substrate. Thus, the strain experienced by the first trace only can be measured and correlated to an amount or magnitude of applied force. 
     In some embodiments, the strain-sensitive or responsive structure(s) can be operably connected to two reference resistive structures in a balancing network configuration, such as a Wheatstone bridge configuration. Because the strain-sensitive structures are formed on only one surface of the substrate and operate in a balancing network configuration, the strain-sensitive structures operate relatively independent of changes in temperature and environment (e.g., humidity). Additionally, matching the resistances of the first and second resistive structures may be more accurate because the first trace and the second trace can be formed at the same time, with the same material, and with the same fabrication process. 
     Moreover, the strain-sensitive structures can be easier to fabricate because the first and second traces are formed on only one surface of a substrate at the same time, with the same material, and with the same fabrication process. For example, in one embodiment, the first trace and the second trace are formed with a copper-nickel alloy (e.g., Constantan). The copper-nickel alloy is formed on the substrate (e.g., deposited or grown) and/or patterned (e.g., with photolithography) at the same time and on the same side of the substrate. In other embodiments, the first trace and the second trace can be formed with a different material, such as isoelastic, nichrome, tantalum nitride, and chromium nitride. Additionally or alternatively, the first trace and/or the second trace may be formed with a stack of conductive layers. For example, the conductive material for the first trace and the second trace can include a first layer of a first conductive material (e.g., a nichrome material) and a second layer of a second conductive material (e.g., Constantan) formed over the first layer. 
     These and other embodiments are discussed below with reference to  FIGS. 1-8 . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. 
       FIG. 1  shows one example of an electronic device that can include one or more strain-sensitive structures. In the illustrated embodiment, the electronic device  100  is implemented as a smart telephone. Other embodiments can implement the electronic device differently. For example, an electronic device can be a laptop computer, a tablet computing device, a wearable computing device, a digital music player, a display input device, a kiosk, a remote control device, a television, and other types of electronic devices that include one or more strain-sensitive structures. 
     The electronic device  100  includes an enclosure  102  at least partially surrounding a display  104  and one or more input/output (I/O) devices  106 . The enclosure  102  can form an outer surface or partial outer surface for the internal components of the electronic device  100 . The enclosure  102  can be formed of one or more components operably connected together, such as a front piece and a back piece. Alternatively, the enclosure  102  can be formed of a single piece operably connected to the display  104 . 
     The display  104  can provide a visual output to the user. The display  104  can be implemented with any suitable technology, including, but not limited to, a multi-touch sensing touchscreen that uses a liquid crystal display (LCD) element, a light emitting diode (LED) technology, an organic light-emitting display (OLED) element, an organic electroluminescence (OEL) element, or another type of display element. In some embodiments, the display  104  can function as an input device that allows the user to interact with the electronic device  100 . For example, the display  104  can be a multi-touch touchscreen LED display. 
     In some embodiments, the I/O device  106  can take the form of a home button, which may be a mechanical button, a soft button (e.g., a button that does not physically move but still accepts inputs), an icon or image on a display, and so on. Further, in some embodiments, the I/O device  106  can be integrated as part of a cover layer  108  and/or the enclosure  102  of the electronic device  100 . Although not shown in  FIG. 1 , the electronic device  100  can include other types of I/O devices, such as a microphone, a speaker, a camera, and one or more ports, such as a network communication port and/or a power cord port. 
     The cover layer  108  may be positioned over the front surface of the electronic device  100 . At least a portion of the cover layer  108  can receive touch and/or force inputs. The cover layer  108  can be formed with any suitable transparent material, such as glass, plastic, sapphire, or combinations thereof. In one embodiment, the cover layer  108  covers the display  104  and the I/O device  106 . Touch and force inputs can be received by the portion of the cover layer  108  that covers the display  104  and/or by the portion of the cover layer  108  that covers the I/O device  106 . In another embodiment, the cover layer  108  covers the display  104  but not the I/O device  106 . In such embodiments, the I/O device  106  can be positioned in an opening or aperture formed in the cover layer  108  and/or in the enclosure  102 . The I/O device  106  can receive touch and/or force inputs as well as the portion of the cover layer  108  that covers the display  104 . 
     A strain-sensitive structure or structures  110  can be included in one or more locations of the electronic device  100 . In the illustrated embodiment, the strain-sensitive structures  110  are formed as an array of rectilinear strain-sensitive structures, although other shapes and array patterns can also be used. In many examples, each individual strain-sensitive structure  110  may have a selected shape and/or pattern. For example, in certain embodiments, a strain-sensitive structure  110  may be formed, patterned, or deposited with one trace formed in a first trace pattern and a second trace formed in a second trace pattern, as is described in more detail in conjunction with  FIGS. 2A-2E . 
     In one embodiment, the strain-sensitive structures  110  are formed in a patterned metal layer. Any suitable material or materials that exhibit a change in resistance can be used to form the strain-sensitive structures  110 . Such materials include, but are not limited to, metal or a metal alloy, such as a copper-nickel alloy (e.g., Constantan), isoelastic, nichrome, tantalum nitride, chromium nitride, polyethyleneioxythiophene, carbon nanotubes, graphene, silver nanowire, other metallic nanowires, and the like. 
     The type of material(s) that is used to form a strain-sensitive structure  110  can be based at least in part on the location of the strain-sensitive structure  110  within the electronic device  100 . For example, in one embodiment, the strain-sensitive structure(s)  110  may be included in a display stack of the display  104 . The strain-sensitive structures  110  can be used to measure an amount of force and/or a change in force that is applied to the display  104  or to a portion of the display  104 . In such embodiments, the one or more strain-sensitive structures  110  can be formed with an optically transparent material. 
     In another embodiment, one or more strain-sensitive structures  110  may be included in the I/O device  106 . The strain-sensitive structure(s)  110  can be used to measure an amount of force and/or a change in force that is applied to the I/O device  106 . Additionally or alternatively, one or more strain-sensitive structures  110  can be positioned under at least a portion of the enclosure  102  to detect a force and/or a change in force that is applied to the enclosure  102 . In such embodiments, the strain-sensitive structures  110  can be formed with an optically transparent material or with an opaque material. 
     Embodiments are described herein in conjunction with a display stack of a display (e.g., display  104 ). In one non-limiting example, the entire top surface of the display  104  (or the cover layer  108  disposed over the top surface of the display  104 ) may be an input region that is configured to receive touch and/or force inputs from a user. As will be described in more detail later, the strain-sensitive structures  110  are formed on a single surface of a substrate to produce a force sensing layer. 
       FIGS. 2A-2E  show a first example of a strain-sensitive structure that is suitable for use in a force sensing layer. With respect to  FIG. 2A , the strain-sensitive structure  200  includes a continuous first trace  202  and a continuous second trace  204  that is connected in series with the first trace  202 . The first trace  202  forms a first resistive structure in the strain-sensitive structure  200 . The second trace  204  forms a second resistive structure in the strain-sensitive structure  200  that is connected in series with the first resistive structure. The first and the second resistive structures can have any suitable resistance. In one embodiment, the resistance of the first and the second resistive structures is approximately twenty-five (25) to one hundred (100) kilohms. 
     Three terminals or signal lines  310 ,  312 ,  316  are connected to the strain-sensitive structure  200 . A first terminal  310  connects to one end of the first trace  202  and a second terminal  312  connects to a first end of the second trace  204 . A common terminal  316  connects to the other ends of the first and second traces  202 ,  204 . The three terminals or signal lines  310 ,  312 ,  316  are discussed in more detail in conjunction with  FIG. 3 . 
     As described earlier, the first and the second traces  202 ,  204  can be formed with any suitable material or materials that exhibit a change in resistance. Such materials include, but are not limited to, metal or a metal alloy, such as a copper-nickel alloy (e.g., Constantan), isoelastic, nichrome, tantalum nitride, chromium nitride, polyethyleneioxythiophene, carbon nanotubes, graphene, silver nanowire, other metallic nanowires, and the like. 
     For ease of understanding only, the pattern that forms the strain-sensitive structure  200  is divided into four sections  206 ,  208 ,  210 ,  212  using dashed lines  214 ,  216 . In one embodiment, the pattern is a single pattern with the first and the second traces  202 ,  204  running continuously through the pattern. In other embodiments, one or more sections  206 ,  208 ,  210 ,  212  may be formed on the surface of the substrate and used to detect strain and correlate the detected strain to an amount of force. For example, in one embodiment, a strain-sensitive structure may be formed with only two sections (e.g., sections  206  and  212 ). Alternatively, a strain-sensitive structure may be formed with only one section (e.g.,  206  or  208 ). 
       FIG. 2B  depicts the first trace  202  in the first section  206  of the strain-sensitive structure (identified as  206   a ). The first trace  202  is formed into a first trace pattern  218 . The first trace pattern  218  includes sections  220  having legs that are arranged in a serpentine pattern. The legs in the sections  220  have a first length that extends along a first axis  234  (see  FIG. 2D ). As used herein, the term “length” refers generally to a segment or leg of the first trace or of the second trace that does not experience a change in direction or trajectory. For example, in  FIG. 2B , the first length refers to each segment that extends along a respective axis (e.g., the first axis  234 ). Alternatively, in  FIG. 2F , the first length of the legs in the first trace  202  refers to the segments that have a given radius of curvature. 
       FIG. 2C  depicts the second trace  204  in the first section  206  of the strain-sensitive structure (identified as  206   b ). The second trace  204  is arranged in a second trace pattern  222 . The second trace pattern  222  includes legs  224  that are arranged in a serpentine pattern. The legs have a shorter second length compared to the length of the legs in sections  220 . Accordingly, the second trace  204  includes more conductive material than the first trace  202  because the serpentine pattern of the second trace  204  includes a greater number of legs. The legs in the second trace  204  are denser or more compact compared to the legs in the first trace  202 . In  FIG. 2C , the second length refers to each segment or leg that extends along a respective axis (e.g., the second axis  242 ). 
     As depicted in  FIGS. 2B and 2C , the first and second trace patterns  218 ,  222  complement each other in that each section  220  of the first trace pattern  218  is positioned in a U-shaped arrangement  226  of the second trace pattern  222 . In other words, sections  220  of the first trace  202  are bordered on three sides by the second trace  204 . 
     Other embodiments can arrange the first and second trace patterns  218 ,  222  differently. For example, two sections  220  of the first trace  202  can be included within a U-shaped arrangement  226  of the second trace  204 . In another example, the first section can include only one section  220 , and that one section  220  may be bordered on three sides by the second trace  204 . Alternatively, the first and/or the second trace  202 ,  204  can be arranged in a pattern that includes diagonal sections. In some embodiments, the open end of one U-shaped arrangement  226  can be adjacent a first side of the section  206  while the open end of another U-shaped arrangement  226  can be adjacent a different section side of the section  206  (e.g., an opposing second side of the section  206 ). 
     As shown in  FIG. 2D , the first section  206  is formed by combining the first and second trace patterns  218 ,  222 . The combination of the first trace pattern  218  and the second trace pattern  222  is referred to herein as a pattern template. The pattern template  228  is used to produce the pattern of the strain-sensitive structure  200  shown in  FIG. 2A . 
     As shown in  FIGS. 2B and 2D , the first trace  202  includes a first section  230  that includes a first set of legs  232  arranged in a serpentine pattern, the first set of legs  232  having a first length (L 1 ) extending along a first axis  234 . The first trace  202  further includes a second section  236  that includes a second set of legs  238  arranged in a serpentine pattern. The second set of legs  238  extends along the first axis  234  and has the first length L 1 . 
     A third section  240  includes a leg that connects the first section  230  to the second section  236 . The third section  240  extends along a second axis  242 . 
     With respect to  FIGS. 2C and 2D , the second trace  204  includes a fourth section  244  positioned adjacent a first side of the first section  230 . The fourth section  244  extends along the first axis  234  and includes a third set of legs  224  that are arranged in a serpentine pattern. The third set of legs  224  in the serpentine pattern extends along the second axis  242  and has a second length (L 2 ) that is different from the first length. In the illustrated embodiment, the second length is less than the first length. 
     The second trace  204  also includes a fifth section  246  positioned adjacent a second side of the first section  230 . The fifth section  246  extends along the first axis  234  and includes a fourth set of legs  224  that is arranged in a serpentine pattern. The fourth set of legs  224  in the serpentine pattern extends along the second axis  242  and has the second length L 2 . 
     A sixth section  248  of the second trace  204  connects the fourth section  244  to the fifth section  246 . In the illustrated embodiment, the sixth section  248  connects a first end of the fourth section  244  to a first end of the fifth section  246 . The sixth section  248  extends along the second axis  242 . The sixth section  248  includes a fifth set of legs  224  that is arranged in the serpentine pattern. The fifth set of legs  224  in the serpentine pattern extends along the first axis  234  and has the second length L 2 . 
     A seventh section  250  of the second trace  204  connects to the fifth section  246 . In the illustrated embodiment, a first end of the seventh section  250  connects to a second end of the fifth section  246 . The seventh section  250  is positioned adjacent a first side of the second section  236 . The seventh section  250  extends along the first axis  234  and includes a sixth set of legs  224  that is arranged in the serpentine pattern. The sixth set of legs  224  in the serpentine pattern extends along the second axis  242  and has the second length L 2 . 
     An eighth section  252  of the second trace  204  is positioned adjacent a second side of the second section  236 . The eighth section  252  extends along the first axis  234  and includes a seventh set of legs  224  that is arranged in the serpentine pattern. The seventh set of legs  224  in the serpentine pattern extends along the second axis  242  and has the second length L 2 . 
     A ninth section  254  of the second trace  204  connects the seventh section  250  to the eighth section  252 . In the illustrated embodiment, the ninth section  254  connects the second end of the seventh section  250  to a first end of the eighth section  252 . The ninth section  254  extends along the second axis  242  and includes an eighth set of legs  224  that is arranged in the serpentine pattern. The eighth set of legs  224  in the serpentine pattern extends along the first axis  234  and has the second length L 2 . 
     A tenth section  256  of the second trace  204  connects to the eighth section  252  (e.g., to a second end of the eighth section  252 ). The tenth section  256  extends along the first axis  234  and includes a ninth set of legs  224  that is arranged in the serpentine pattern. The ninth set of legs  224  in the serpentine pattern extends along the second axis  242  and has the second length L 2 . 
     Returning to  FIGS. 2B and 2D , the first trace  202  can include an eleventh section  258  that is positioned adjacent to the tenth section  256  of the second trace  204  (the side of the tenth section  256  that is opposite the second section  236  of the first trace  202 ). The eleventh section  258  includes a leg that extends along the first axis  234 . A twelfth section  260  of the first trace  202  connects the eleventh section  258  to the second section  236  of the first trace  202 . 
     In  FIG. 2D , the second axis  242  is normal to the first axis  234 , although this is not required. Alternatively, in some embodiments, the first and second axes  234 ,  242  can be normal to each other and both axes  234 ,  242  rotated a given angle (e.g., rotated at a forty-five degree angle). 
       FIG. 2E  shows an exploded view of the four sections  206 ,  208 ,  210 ,  212  of the strain-sensitive structure  200 . To produce the pattern shown in  FIG. 2A , the pattern template  228  in section  206  is rotated ninety degrees to produce the second section  208 . The pattern template  228  in section  206  is rotated one hundred and eighty degrees to produce the third section  210 . And the pattern template  228  in section  206  is rotated two hundred and seventy degrees to produce the fourth section  212 . 
     In other words, each section is rotated a positive ninety (+90) degrees and a negative ninety (−90) degrees to produce its neighboring (contiguous) sections. The first section  206  is rotated a positive ninety degrees to produce the second section  208  and a negative ninety degrees to produce the fourth section  212 . The second section  208  is rotated a positive ninety degrees to produce the third section  210  and a negative ninety degrees to produce the first section  206 . The third section  210  is rotated a positive ninety degrees to produce the fourth section  212  and a negative ninety degrees to produce the second section  208 . Finally, the fourth section  212  is rotated a positive ninety degrees to produce the first section  206  and a negative ninety degrees to produce the third section  210 . 
     In the strain-sensitive structure  200 , the first trace  202  is sensitive to strain because the legs in the sections  230 ,  236 ,  240 ,  258 , and  260  are in contact with the substrate over relatively long distances. Consequently, the first trace  202  does not have an opportunity to be strained differently from the substrate. The first trace  202  compresses or elongates in response to an applied stress. Additionally, based on the rotated first trace patterns  218  in the strain-sensitive structure  200 , the first trace  202  detects strain in a first direction and in a second direction normal to the first direction (e.g., along axes  234  and  242 ). 
     Unlike the first trace  202 , the second trace  204  can be less sensitive to strain. The legs  224  in the serpentine pattern are in contact with the substrate over shorter distances. Consequently, the legs  224  experience less tension or compression in response to strain compared to the first trace  202 . The strain is instead applied to the substrate. In other words, the second trace  204  acts as a strain relief structure, allowing the strain experienced by the strain-sensitive structure  200  to be determined based on the net strain on the surface of the substrate. The second resistive structure formed by the second trace  204  may act as a reference for the first resistive structure formed by the first trace  202 . 
     Additionally, the strain-sensitive structure  200  may operate relatively independent of changes in temperature and environment because the first and second traces  202 ,  204  are formed on only one surface of the substrate using the same material or materials. Moreover, the strain-sensitive structure  200  may be easier to fabricate because the first and second traces  202 ,  204  can be formed at the same time using the same fabrication process. Because the first and second traces  202 ,  204  are formed at the same time and with the same material, the resistances of the first and second traces  202 ,  204  may be matched more accurately. 
     As described earlier, the first resistive structure formed by the first trace  202  is typically the only resistive structure that detects strain. When a user exerts a force on an input surface (e.g., cover layer  108  in  FIG. 1 ), the input surface may flex in response. When the force is sufficient, the flex in the input surface transfers to the force sensing layer, which causes the surfaces of the substrate to compress or elongate. This compression or elongation causes the strain-sensitive structures to experience compression or tension. Based on the first and second trace patterns  218 ,  222  of the strain-sensitive structure  200 , an electrical property (e.g., electrical resistance) of the strain-sensitive structure  200  changes as a result of the compression or tension. The change in resistance is represented in a signal level of a strain signal (e.g., a voltage level). The strain signal can be correlated to the amount of force applied to the input surface. 
     In particular, with the strain-sensitive structure  200 , the resistance of the first resistive structure formed by the first trace  202  changes based on stress. As discussed earlier, the second resistive structure formed by the second trace  204  experiences less strain compared to the first trace  202 , so the resistance of the second resistive structure changes less (e.g., has insignificant changes) in response to the stress. Accordingly, the strain signal received from the strain-sensitive structure  200  represents the strain experienced substantially by the first resistive structure in the strain-sensitive structure  200 . 
     Thus, the resistances of the first and second traces  202 ,  204  are substantially the same when a force is not applied to the substrate. When one or more forces are applied to the substrate, the resistance of the first trace  202  changes more compared to the resistance of the second trace  204 . 
       FIG. 2F  depicts a second example strain-sensitive structure that is suitable for use in a force sensing layer. The strain-sensitive structure  262  includes a continuous first trace  202  and a continuous second trace  204  that is connected in series with the first trace  202 . The first trace  202  forms a first resistive structure in the strain-sensitive structure  262 . The second trace  204  forms a second resistive structure in the strain-sensitive structure  262  that is connected in series with the first resistive structure. 
     Three terminals or signal lines  310 ,  312 ,  316  are connected to the strain-sensitive structure  262 . A signal line  310  connects to one end of the first trace  202  and a second signal line  312  connects to a first end of the second trace  204 . A common signal line  316  connects to the other ends of the first and second traces  202 ,  204 . The three terminals or signal lines  310 ,  312 ,  316  are discussed in more detail in conjunction with  FIG. 3 . 
     The first trace  202  is arranged in a first trace pattern and the second trace  204  is arranged in a second trace pattern. In the illustrated embodiment, the first and second traces  202 ,  204  are arranged in partial concentric arcs or curves with the second trace  204  positioned inside the first trace  202 . The partial concentric curves of the first trace  202  are configured in a first serpentine pattern and the partial concentric curves of the second trace  204  are arranged in the first serpentine pattern (e.g., each trace double backs one or more times). In other embodiments, the first trace  202  can be positioned inside of the second trace  204 . 
     The first trace  202  includes one or more legs  264  that are curved and have a first length. In the illustrated embodiment, the first length refers to the segments or legs that have a given radius of curvature (e.g., the longer segments or legs that extend from one side of the strain-sensitive structure  262  to the other side of the strain-sensitive structure  262 ). 
     The second trace  204  is arranged in a second trace pattern that includes legs  266  that are arranged in a second serpentine pattern. The number of legs  266  in the second trace  204  is greater than the number of legs  264  in the first trace  202 . The legs  266  have a shorter second length (e.g., L 2  in  FIG. 2C ) compared to the length of the legs  264  in the first trace  202 . 
     The arrangement of the first trace  202  causes the first resistive structure to be sensitive to strain in one or more directions while the arrangement of the second trace  204  causes the second resistive structure to be less sensitive to strain in any direction. The first trace  202  stretches or compresses based on an applied stress while the second traces  204  does not substantially elongate or compress based on the applied stress. Accordingly, the second resistive structure experiences less strain that the first resistive structure in response to an applied stress. In this manner, the second resistive structure may act as a reference for the first resistive structure. 
     In the absence of an applied stress, the resistances of the first and second traces  202 ,  204  can be substantially the same. This is due at least in part to the first and second traces  202 ,  204  being fabricated at the same time and with the same material. 
       FIG. 2G  depicts a third example strain-sensitive structure that is suitable for use in a force sensing layer. The strain-sensitive structure  268  includes a continuous first trace  202  and a continuous second trace  204  that is connected in series with the first trace  202 . The first trace  202  forms a first resistive structure in the strain-sensitive structure  268 . The second trace  204  forms a second resistive structure in the strain-sensitive structure  268  that is connected in series with the first resistive structure. 
     Three terminals or signal lines  310 ,  312 ,  316  are connected to the strain-sensitive structure  268 . A first terminal  310  connects to one end of the first trace  202  and a second terminal  312  connects to a first end of the second trace  204 . A common terminal  316  connects to the other ends of the first and second traces  202 ,  204 . The three terminals or signal lines  310 ,  312 ,  316  are discussed in more detail in conjunction with  FIG. 3 . 
     The first trace  202  is arranged in a first trace pattern that includes one or more legs  270  that have a first length. In the illustrated embodiment, the first length refers to each longer segment that extends along an axis. The second trace  204  is arranged in a second trace pattern that includes legs  272  that are arranged in a serpentine pattern. The number of legs  272  in the second trace  204  is greater than the number of legs  270  in the first trace  202 . The legs  272  have a shorter second length (e.g., L 2  in  FIG. 2C ) compared to the length of the legs  270  in the first trace  202 . 
     As shown in  FIG. 2G , the first trace  202  is arranged in a “Y” shape with the legs  270  of the first trace  202  forming a serpentine pattern within each leg of the Y (e.g., doubles back one or more times within each leg of the Y). The second trace  204  is positioned outside of the first trace  202 . The second trace  204  substantially conforms to the Y shape formed by the first trace  202 . The second trace  204  is arranged in a serpentine pattern along each side of the Y shape (e.g., doubles back one or more times on each side of the Y shape). In other embodiments, the second trace  204  may form the Y shape and the first trace  202  may be positioned outside of the second trace  204 . 
     In the absence of an applied stress, the resistances of the first and second traces  202 ,  204  can be substantially the same. This is due at least in part to the first and second traces  202 ,  204  being fabricated at the same time and with the same material. 
     The arrangement of the first trace  202  causes the first resistive structure to be sensitive to strain in one or more directions while the arrangement of the second trace  204  causes the second resistive structure to be less sensitive to strain in any direction. The first trace  202  stretches or compresses based on an applied stress while the second trace  204  does not substantially elongate or compress based on the applied stress. Accordingly, the second resistive structure experiences less strain that the first resistive structure in response to an applied stress. Thus, in some embodiments, the second resistive structure formed by the second trace  204  can act as a reference for the first resistive structure formed by the first trace  202 . 
       FIG. 3  shows a schematic diagram of the strain-sensitive structure connected to a reference element. The variable first resistor  202  represents the first resistive structure formed by the first trace  202 . Similarly, the second resistor  204  represents the second resistive structure formed by the second trace  204 . Thus, the first and second resistors  202 ,  204  represent the strain-sensitive structure  200  shown in  FIG. 2A . The first and second resistors  202 ,  204  are connected in series and form a first voltage divider. 
     Similarly, a first reference resistor or resistive structure  300  is connected in series with a second reference resistor or resistive structure  302  to form a second voltage divider. The first and second reference resistors  300 ,  302  have a known resistance and are coupled to the first and second resistors  202 ,  204  in a balancing network configuration, such as a Wheatstone bridge configuration. In particular, the first reference resistor  300  is connected to the variable first resistor  202  at a top node  304  and the second reference resistor  302  is connected to the second resistor  204  at a bottom node  306 . The terms “top” and “bottom” are for descriptive purposes only and are not intended to limit the arrangement of the resistors, nor limit which signals are received by the nodes. 
     The first and second resistors  202 ,  204  and the first and second reference resistors  300 ,  302  are operably connected to drive circuitry  308  at nodes  304 ,  306 . The drive circuitry  308  can include one or more circuits that are configured to produce the drive signals for the first and second resistors  202 ,  204  and the first and second reference resistors  300 ,  302 . For example, the drive circuitry  308  may include one or more analog circuit components, digital circuit components, passive circuit components, and/or active circuit components (e.g., signal generators, amplifiers, digital-to-analog converters, and multiplexers). In some embodiments, one or more functions or outputs of the drive circuitry  308  can be partially or wholly implemented in software. A drive signal can be any suitable signal such as a voltage bias, a current signal, a voltage signal, and so on. 
     Drive circuitry  308  applies a drive signal (V T ) to the top node  304  on first drive signal line  310  (see  310  in  FIGS. 2A, 2F, 2G ), and a drive signal (V B ) to the bottom node  306  on second drive signal line  312  (see  312  in  FIGS. 2A, 2F, 2G ). Node  304  represents a first end of the first resistor  202  while the node  306  represents a first end of the second resistor  204 . 
     Readout circuitry  314  receives a strain signal from the strain-sensitive structure  200  on strain signal line  316  (see  316  in  FIGS. 2A, 2F, 2G ). Strain signal line  316  is connected to a second end of the first resistor  202  and to a second end of the second resistor  204  (e.g., a common terminal). A reference signal is also detected on output signal line  318 . The strain and reference signals can be any suitable signal, such as a voltage signal. 
     When the strain-sensitive structure  200  experiences strain, the strain-sensitive structure  200  and the reference resistors  300 ,  302  produce a measurable voltage differential that is detected by the readout circuitry  314 . The voltage differential can be correlated to an amount of force applied to the input surface (e.g., cover layer  108 ). 
     The readout circuitry  314  can include one or more circuits that are configured to receive the strain signals on strain signal line  316  and the reference signals on output signal line  318 . For example, the readout circuitry  314  may include one or more analog circuit components, digital circuit components, passive circuit components, and/or active circuit components (e.g., amplifiers, digital-to-analog converters, and multiplexers). In some embodiments, one or more functions or outputs of the readout circuitry  314  can be partially or wholly implemented in software. 
     The first and second reference resistors  300 ,  302  can be formed or positioned on the same substrate as the strain-sensitive structure  200 , although this is not required. In some embodiments, the first and second reference resistors  300 ,  302  are formed as a strain-sensitive structure similar to the strain-sensitive structure  200  shown in  FIG. 2A . The first and second reference resistors  300 ,  302  can be formed with the same material that forms the first and second traces  202 ,  204 . Alternatively, the first and second reference resistors  300 ,  302  may be independent high-precision resistors, or may be formed as an array or network of independent resistors. In some cases, the two reference resistors  300 ,  302  may be variable; the resistance of the two reference resistors  300 ,  302  may be changed and/or adjusted dynamically. 
       FIG. 4  shows a simplified view of a portion of a force sensing layer that includes an array of strain-sensitive structures. In the illustrated embodiment, the first and second traces in each strain-sensitive structure are arranged as shown in  FIG. 2A . However, other embodiments are not limited to this arrangement. The first and second traces can be arranged in any suitable arrangement that connects the second trace in series with the first trace and results in the first trace experiencing more strain than the second trace in response to an applied stress. For example, the first and second traces may be arranged as illustrated in  FIGS. 2F and 2G . 
     The force sensing layer  400  includes the strain-sensitive structures  402 ,  404 ,  406 ,  408  formed on only one surface of the substrate  410 . The density of the strain-sensitive structures  402 ,  404 ,  406 ,  408  on the substrate  410  can be any suitable density. In one embodiment, the density of the strain-sensitive structures  402 ,  404 ,  406 ,  408  is approximately one per square centimeter. 
     Each strain-sensitive structure  402 ,  404 ,  406 ,  408  is electrically connected to three signal lines. In the illustrated embodiment, a first drive signal line  412 ,  418 ,  422 ,  426  (e.g., similar to signal line  310  in  FIGS. 2 and 3 ) is connected to respective strain-sensitive structures  402 ,  404 ,  406 ,  408  (e.g., at top node  304  shown in  FIG. 3 ). A second drive signal line  412 ,  414 ,  418 ,  422  (e.g., similar to signal line  312  in  FIGS. 2 and 3 ) is connected to respective strain-sensitive structures  402 ,  404 ,  406 ,  408  (e.g., at bottom node  306  shown in  FIG. 3 ). A strain signal is received from each strain-sensitive structure  402 ,  404 ,  406 ,  408  on a respective strain signal line  416 ,  420 ,  424   428  (e.g., similar to signal line  316  in  FIG. 3 ). 
     The drive signal lines  412 ,  414 ,  418 ,  422 , and  426  are each connected to drive circuitry (see  FIGS. 3 and 8 ). As described earlier, the drive circuitry can include one or more circuits that are configured to produce the drive signals for the strain-sensitive structures  402 ,  404 ,  406 ,  408  and for the reference resistors (e.g., reference resistors  300 ,  302 ). 
     The strain signal lines  416 ,  420 ,  424 , and  428  are each connected to readout circuitry (see  FIGS. 3 and 8 ). As described earlier, the readout circuitry may include one or more circuits that each receives a strain signal from the strain-sensitive structures  402 ,  404 ,  406 ,  408  on a respective strain signal line  416 ,  420 ,  424 , and  428 . The readout circuitry may also receive an output signal from the reference resistors on an output signal line (e.g., reference resistors  300 ,  302  on output signal line  318 ). In some embodiments, the readout circuitry is configured to detect changes in an electrical property (e.g., resistance) of each strain-sensitive structure  402 ,  404 ,  406 ,  408  based on the received reference and strain signals. In other embodiments, a processing device can receive the strain and output signals and detect changes in an electrical property (e.g., resistance) of each strain-sensitive structure  402 ,  404 ,  406 ,  408 . The changes in the electrical property can be correlated to a magnitude or amount of force or stress that is applied to the substrate  410 . 
     In the illustrated embodiment, two adjacent strain-sensitive structures  402 ,  404 ,  406 ,  408  share the drive signal lines  412 ,  418 , and  422 . Sharing drive signal lines reduces the total number of drive signal lines needed for the strain-sensitive structures  402 ,  404 ,  406 ,  408 . For example, when signal lines are not shared, two adjacent strain-sensitive structures need a total of six signal lines (four drive signal lines and two strain signal lines). By sharing one drive signal line, the total number of signal lines needed for two adjacent strain-sensitive structures is reduced to five signal lines (two strain signal lines and three drive signal lines). Accordingly, in the illustrated embodiment, five drive signal lines  412 ,  414 ,  418 ,  422 ,  426  are used instead of six drive signal lines. 
     Additionally, in the illustrated embodiment, the shared drive signal lines are connected to different nodes of the adjacent strain-sensitive structures. For example, for strain-sensitive structures  406  and  408 , the drive signal transmitted on the drive signal line  412  can be the drive signal V B  (e.g., received at the bottom node  306  in  FIG. 3 ) for the strain-sensitive structure  406  and V T  (e.g., received at the top node  304  in  FIG. 3 ) for the strain-sensitive structure  408 . Similarly, the drive signal line  418  can be connected to the strain-sensitive structures  404  and  406 . The drive signal transmitted on the drive signal line  418  can be received by the strain-sensitive structure  404  as the drive signal V B  (received at the bottom node) and received by the strain-sensitive structure  406  as the drive signal V T  (received at the top node). Finally, the drive signal line  422  may be connected to the strain-sensitive structures  402  and  404 . The drive signal transmitted on the drive signal line  422  can be received by the strain-sensitive structure  402  as the drive signal V B  (received at the bottom node) and may be received by the strain-sensitive structure  404  as the drive signal V T  (received at the top node). 
     In  FIG. 4 , the signal lines  426  and  414  that are connected to the two outer strain-sensitive structures  402  and  408 , respectively, are not shared because there is not a second strain-sensitive structure adjacent the strain-sensitive structures  402 ,  408  (e.g., there is only one strain-sensitive structure adjacent each strain-sensitive structure  402 ,  408 ). The drive signal transmitted on the signal line  426  may be received by the strain-sensitive structure  402  as the drive signal V T  (received at the top node) while the drive signal transmitted on the signal line  414  may be received by the strain-sensitive structure  408  as the drive signal V B  (received at the bottom node). 
     Magnetic fields are created when the drive signals pass through the strain-sensitive structures. In many cases, changing magnetic fields can induce a measurable electromotive force within the strain-sensitive structures. The patterns or arrangements of the first and second traces  202 ,  204  in each strain-sensitive structure are designed to reduce, minimize, or cancel the effects due to magnetic fields. The arrangement (e.g., rotations) of the first and second traces  202 ,  204  on the substrate is the same (or substantially the same), which reduces, minimizes, or cancels the effects due to changing magnetic fields. 
     In some embodiments, the drive signal lines  412 ,  414 ,  418 ,  422 , and  426  and the strain signal lines  416 ,  420 ,  424 , and  428  can be formed in a second layer in the substrate or in a different substrate (not shown). This allows the drive signal lines  412 ,  414 ,  418 ,  422 , and  426  and the strain signal lines  416 ,  420 ,  424 , and  428  to be formed with a material that is different from the material in the first and second traces (e.g., traces  202  and  204 ). For example, the strain and drive signal lines can be formed with a material that has a higher conductivity than the material in the first and second traces. 
       FIG. 5  shows one example of the second trace on a substrate. The second trace  204  is formed on a surface of a substrate  500 . The substrate  500  can be configured to flex in a free, controlled, or limited manner. For example, the substrate  500  may be supported along its perimeter by a chassis or frame. The substrate  500  may be single layer or multi-layer including materials such as, but not limited to, a plastic (e.g., polyamide), metal, ceramic, polyethylene terephthalate, or any combination thereof. 
     When the second trace  204  is formed with a material having a high Young&#39;s modulus (e.g., a metal), and is supported by a substrate formed with a material having a lower Young&#39;s modulus (e.g., plastic), strain is reduced in the second trace  204  and experienced by the substrate  500 . As discussed earlier, this is because the second trace  204  contacts the substrate over shorter distances. Consequently, the second trace  204  experiences less tension or compression in response to strain compared to the first trace  202 . 
     In some embodiments, the substrate  500  can be etched or cut to form one or more trenches or cavities  502  along and adjacent the second trace  204  (or along and adjacent one or more portions of the second trace  204 ). The one or more cavities  502  may further reduce the amount of strain experienced by the second trace  204 . When the substrate  500  is stressed (e.g., bent), the strain around the second trace  204  may be redirected to (or concentrated in) the one or more cavities  502  next to the second trace  204  rather than in the second trace  204 . This can increase the strain relief function of the second trace  204 . 
       FIG. 6  depicts an example of the first and second traces on a substrate. Only a portion of the first and second traces  202 ,  204  are shown for simplicity. An insulating layer  602  is formed over the substrate  600 , and the first and second traces  202 ,  204  are formed over the insulating layer  602 . In one embodiment, the insulating layer  602  is formed with an insulating material having a higher Young&#39;s modulus (e.g., an oxide or nitride material), the first and second traces  202 ,  204  are formed with a material having a higher Young&#39;s modulus (e.g., a metal), and the substrate  600  is formed with a material having a lower Young&#39;s modulus (e.g., plastic). 
     In the illustrated embodiment, the sections  604  of the insulating layer  602  that are around and between the first and second traces  202 ,  204 , around and between the legs  232 ,  238  in the first trace  202  (see  FIGS. 2B and 2D ), and around and between the legs  224  in the second trace  204  (see  FIGS. 2C and 2D ) are removed. The insulating layer  602  that is directly below the first and second traces  202 ,  204  remains and forms support bases  606 . Each support base  606  supports a respective leg  224 ,  232 ,  238  in the first and second traces  202 ,  204 . 
     Accordingly, as shown in the Detail A-A, channels or trenches  608  are formed between adjacent legs in the first and second sections  230 ,  236  of first trace  202  (e.g., between legs  232 ). Trenches  610  are also formed between adjacent legs  224  in the second trace  204  (see Detail B-B). Removing the sections of the insulating layer  602  around the tight turns of the shorter legs  224  in the second trace  204  allows the strain in the shorter legs  224  to be lower than the strain in the longer legs of the first trace  202 . 
     Any suitable method can be used to remove the portions of the insulating layer  602 . For example, a wet (e.g., chemical) etching process or a dry etching process (chemical or physical) can be used to remove the sections of the insulating layer  602  around and between the first and second traces  202 ,  204 , around and between the legs  232 ,  238  in the first trace  202 , and around and between the legs  224  in the second trace  204 . An example method of forming the first and second traces is described in more detail in conjunction with  FIG. 7 . 
     In some embodiments, the sections of the substrate  600  corresponding to the sections  604  of the insulating layer  602  may be removed along with the sections  604  of the insulating layer  602 . In such embodiments, the same removal process may be used to remove the sections  604  of the insulating layer  602  and the corresponding sections of the substrate  600 . Alternatively, one removal process (e.g., a first etching process) can be used to remove the sections  604  of the insulating layer  602  and a different removal process (e.g., a second etching process) may be used to remove the corresponding sections of the substrate  600 . 
     Alternatively, the insulating layer  602  may be omitted in some embodiments. In such embodiments, the sections of the substrate  600  around and between the first and second traces  202 ,  204 , around and between the legs  232 ,  238  in the first trace  202 , and around and between the legs  224  in the second trace  204  are removed. The substrate  600  that is directly below the first and second traces  202 ,  204  remains and forms support bases, with each support base supporting a respective leg  224 ,  232 ,  238  in the first and second traces  202 ,  204 . 
     Alternatively, in some embodiments, only the sections of the insulating layer  602  and/or the substrate  600  around and between the legs  224  in the second trace  204  are removed. The sections of the insulating layer  602  and/or the substrate  600  around and between the legs  232 ,  238  in the first trace  202  are not removed. In such embodiments, the substrate  600  and/or the insulating layer  602  that is directly below the second trace  204  form(s) support bases, with each support base supporting a respective leg  224  in the second trace  204 . 
       FIG. 7  shows a flowchart of an example method of fabricating the first and second traces shown in  FIG. 6 . Initially, an insulating material is formed over a substrate (block  700 ). For example, the insulating material can be deposited or laminated over the substrate. In some embodiments, the insulating material can include one or more layers of insulating material. As described earlier, the insulating material is optional and may be omitted in some embodiments. 
     Next, as shown in block  702 , the conductive material for the first and second traces is formed over the insulating layer. Like the insulating material(s), the conductive material for the first and second traces may be deposited (e.g., physical vapor deposition) over the insulating material. In some embodiments, the material for the first and second traces includes a stack of conductive layers. For example, the conductive material for the first and second traces can include a first layer of a first conductive material (e.g., nichrome material) and a second layer of a second conductive material (e.g., Constantan) formed over the first layer. 
     The first and second trace patterns (e.g.,  218  in  FIG. 2B and 222  in  FIG. 2C ) are then formed in the conductive material to produce the first and second traces, as shown in block  704 . The first trace and the second trace can each have any suitable dimensions. The dimensions of a respective trace is based, at least in part, on the number of sections in each trace pattern (e.g.,  230 ,  236 ,  240 ,  244 ,  246 ,  248 ,  250 ,  252 ,  254 ,  256 ,  258 ,  260  in  FIGS. 2B-2D ), the number of legs in respective sections (e.g., legs  224 ,  232 ,  238  in  FIGS. 2B-2D ), the density of the strain-sensitive structures on a substrate, and the like. In a non-limiting embodiment, the total length of the first trace is approximately one thousand millimeters, the total length of the second trace is approximately one thousand (1000) millimeters, the thickness of each trace  202 ,  204  is approximately two hundred (200) to two hundred and fifty (250) nanometers, and the width of each trace  202 ,  204  is approximately ten (10) to fifteen (15) microns. The spacing between the first and second traces is approximately ten (10) to fifteen (15) microns. 
     Since the first and second traces are formed of the same material or materials, the first and second traces can be formed in the same patterning operation. Any suitable technique can be used to pattern the conductive material. For example, in one embodiment, a mask that defines the pattern of one or more strain-sensitive structures can be positioned over the conductive material. The conductive material may then be etched to remove the conductive material that is not used to form the strain-sensitive structure(s). Thus, the first and second traces are formed during one etching operation. 
     Next, as shown in block  706 , the sections of the insulating layer and/or the substrate that are around and between the first and the second traces, around and between the legs in the first trace, and between the legs in the second trace are removed. The substrate can be etched to any suitable depth. For example, in one embodiment, the depth of etching of the substrate around the first and the second traces is approximately three (3) to five (5) microns. 
     In some embodiments, the first and the second traces are used as a mask when the insulating layer and/or the substrate are etched. After the substrate is etched, the trenches slightly undercut the first and second traces. In such embodiments, the trenches are self-aligned with the first and second traces. In other embodiments, the mask that is used to pattern the conductive material in block  704  can be used to pattern the insulating layer and/or the substrate. 
     In some embodiments, a protective material may then be formed over the force sensing layer at block  708  (e.g., over the first and second traces, the insulating material, and/or the substrate). The protective material may also encapsulate the force sensing layer. In a non-limiting example, the protective material can be a material that has a lower Young&#39;s modulus. Block  708  is optional and may be omitted in other embodiments. 
     Other embodiments can produce the first and second traces differently. For example, the insulating material can be formed over the substrate and patterned to form the supporting bases. The conductive material for the first and second traces can then be formed over the insulating layer (including the supporting bases) and the conductive material that is not used to form the first and second traces may be removed. Alternatively, the conductive material for the first and second traces may be formed only on the top surfaces of the supporting bases. 
     As described earlier, the force sensing layer can be included in a display stack of a display.  FIG. 8  depicts a cross-sectional view of a portion of the display taken along line  8 - 8  in  FIG. 1 . The cross-sectional view illustrates one example of a display stack  800  that is suitable for use for the display  104 . At least a portion of the cover layer  108  may be positioned over the display stack  800 . The cover layer  108  can have any suitable dimensions. For example, in one embodiment, a thickness of the cover layer  108  is approximately eight hundred (800) microns. 
     In the illustrated embodiment, the cover layer  108  can be disposed over a front polarizer  802 . The cover layer  108  can be a flexible touchable surface that is made of any suitable material, such as, for example, a glass, a plastic sapphire, or combinations thereof. The cover layer  108  can act as an input region for a touch sensor and a force sensor by receiving touch and force inputs from a user. The user can touch the cover layer  108  with one or more fingers or with another element such as a stylus. 
     An adhesive layer  804  can be disposed between the cover layer  108  and the front polarizer  802 . Any suitable adhesive can be used in adhesive layer  804 , such as, for example, a liquid optically clear adhesive. A display layer  806  can be positioned below the front polarizer  802 . As described earlier, the display layer  806  may take a variety of forms, including a liquid crystal display (LCD) element, a light-emitting diode (LED) display element, and an organic light-emitting diode (OLED) display element. In some embodiments, the display layer  806  can be formed from glass or have a glass substrate. Embodiments described herein include a multi-touch touchscreen LCD display layer. 
     Additionally, the display layer  806  can include one or more layers that is/are not shown in  FIG. 8 . For example, a display layer  806  can include a VCOM buffer layer, an LCD display element, and one or more conductive layers disposed over and/or under the display element. In one embodiment, the conductive layer(s) may each comprise an ITO layer. 
     A rear polarizer  808  may be positioned below the display layer  806 , and a force sensing layer  810  below the rear polarizer  808 . The force sensing layer  810  includes a substrate  812  having a first set of independent strain-sensitive structures  814  formed on a first surface  816  of the substrate  812 . The substrate  812  may have any suitable dimensions. In one non-limiting embodiment, the thickness of the substrate  812  ranges from one hundred (100) to one hundred and fifty (150) microns. 
     The first set of strain-sensitive structures  814  can include one or more strain-sensitive structures  814 . An adhesive layer  818  may attach the substrate  812  to the rear polarizer  808 . 
     The combination of the cover glass  108 , the front polarizer  802 , the adhesive layers  804 ,  818 , the display layer  806 , the rear polarizer  808 , and the force sensing layer  810  can have any suitable thickness. In one illustrative embodiment, the thickness is less than two thousand (2000) microns. 
     A back light unit  820  can be disposed below the force sensing layer  810 . The back light unit  820  may be configured to support one or more portions of the substrate  812  that do not include strain-sensitive structures. For example, as shown in  FIG. 8 , the back light unit  820  can support the edges of the substrate  812 . Other embodiments may configure a back light unit differently. 
     Each strain-sensitive structure  814  is connected to drive circuitry  822  through signal line  824 . The signal line  824  represents the drive signal lines connected to each strain-sensitive structure  814  (e.g., drive signal lines  412 ,  414 ,  418 ,  422 ,  426  in  FIG. 4 ). Each strain-sensitive structure  814  is connected to readout circuitry  826  through signal line  828 . The signal line  828  represents the strain signal lines connected to each strain-sensitive structure (e.g., strain signal lines  416 ,  420 ,  424 ,  428  in  FIG. 4 ). The readout circuitry  826  is configured to receive a strain signal from each strain-sensitive structure  814 . In some embodiments, the readout circuitry  826  is configured to detect changes in an electrical property (e.g., resistance) of each strain-sensitive structure  814  based on the strain signals received from the strain-sensitive structures  814 . 
     The strain signals output from the readout circuitry  826  can be received by a processing device  830 . The processing device  830  is configured to correlate the strain signals to an amount of force applied to the cover layer  108 . In some embodiments, the readout circuitry  826  may also be configured to provide information about the location of a touch based on the relative difference in the change of resistance of the strain-sensitive structures  814 . 
     In the illustrated embodiment, a gap  832  exists between the force sensing layer  810  and the back light unit  820 . The gap  832  can have any suitable dimension between the force sensing layer  810  and the back light unit  820 . For example, in one embodiment, the gap  832  is approximately one hundred microns. 
     Strain measurements intrinsically measure the force at a point on the first surface  816  of the substrate  812  plus the force from the bottom at that point on the back surface of the substrate  812 . When the gap  832  is included in the display stack  800 , there are no forces on the back surface of the substrate  812 . Thus, the forces on the front surface  816  can be measured independently of the forces on the back surface. In some embodiments, the gap  832  may be filled with an open cell or closed cell foam. 
       FIG. 8  depicts the force sensing layer  810  positioned below the display layer  806 . In other embodiments, the force sensing layer  810  may be disposed over the display layer  806 . In such embodiments, the substrate  812  and the first set of independent strain-sensitive structures  814  can each be formed with a transparent material or materials. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not targeted to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings.

Metadata:
Filing Date: 20160907
Publication Date: 20181120
Grant Date: 20181120
Priority Date: 20160907
Inventors: SMITH, JOHN STEPHEN
Assignee: APPLE INC
CPC Classifications: [{"code": "G06F3/0412", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/045", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0418", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F2203/04103", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/0414", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04105", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/04164", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/04144", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/04144", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/04164", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/045", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01L1/2293", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04105", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F2203/04103", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/0412", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04105", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F2203/04103", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/045", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0412", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 61280447