Patent Publication Number: US-2022230011-A1

Title: Thin, multi-lens, optical fingerprint sensor adapted to image through cell phone displays

Description:
BACKGROUND 
     Many modern cell phone operating systems, including Apple iOS and Android, are configurable to use biometrics, such as fingerprints, as an alternative to user entry of unlock codes to validate user identity. A prior optical sensor for reading fingerprints used an electronic camera equipped with a single lens and an image sensor with a single array of photosensors to image a fingerprint surface of a finger through an OLED cell-phone display. To image a reasonable area of the finger, the lens and array of photosensors were large and required considerable space between lens and the array of photosensors—posing issues in the limited space available in a cell phone. 
     SUMMARY 
     In an embodiment, a multiple-lens optical fingerprint reader adaptable to read fingerprints through a display has an image sensor integrated circuit comprising at least one photosensor array; a spacer; and a plurality of lenses organized in a microlens array, each lens of the plurality of lenses being configured to focus light arriving at that lens from a portion of a fingerprint region of a finger adjacent a surface of the display through the spacer to form an image on a plurality of photosensors associated with that lens, the photosensors being of a photosensor array of the at least one photosensor array, the image being formed independently of other lenses of the plurality of lenses. 
     In another embodiment, a method of verifying identity of a user includes illuminating a fingerprint region of a finger of the user with an organic light emitting diode display; focusing light from the fingerprint region through an array of microlenses onto at least one photosensor array of an integrated circuit, each microlens focusing light from a portion of the fingerprint region onto multiple photosensors of the at least one photosensor arrays; reading the at least one photosensor array to form overlapping electronic fingerprint images; extracting features by a method selected from extracting features from the overlapping electronic fingerprint images and extracting features from a stitched image formed from the overlapping electronic fingerprint images; and comparing the features to features of at least one user in a library of features associated with one or more fingers of one or more authorized users. 
     In an embodiment, the fingerprint s reader is made by forming an infrared filter on a bottom side of a thin glass substrate, the glass substrate being from 0.1 mm and 0.15 mm in thickness; depositing a light-absorbing coating on the infrared filter; masking and etching the light absorbing coating to form openings; forming an array of microlenses by reflowing reflowable optical material onto a top side of the glass substrate and shaping the optical material with a preformed wafer-sized stamp; aligning, and bonding the substrate to a wafer of integrated circuits, each of the integrated circuits having at least one array of photosensors; dicing the wafer of integrated circuits; and bonding the integrated circuits to a flexible printed circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a top view of an optical fingerprint sensor module configured for placement beneath an OLED cell phone display and having a 4×6 array of microlenses and a spacer atop an image sensor, and a circuit board. 
         FIG. 2  is a cross sectional diagram of a finger, OLED display, the optical fingerprint sensor module of  FIG. 1  taken along line A-A in  FIG. 1 , and a battery; the optical fingerprint sensor module having a microlens array, spacer, image sensor, and a flexible circuit board. 
         FIG. 3  is an enlarged copy of a portion of  FIG. 2 , showing overlapping fields of view of image sensor photodiode arrays with traced light paths. 
         FIG. 4  is a flowchart illustrating a method for fabrication of the optical fingerprint sensor. 
         FIG. 5  is a flowchart illustrating a method for how the optical fingerprint sensor is used. 
         FIG. 6  is a block diagram illustrating a cellular telephone device in which the optical fingerprint sensor may be used. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     A fingerprint sensor module  100  ( FIG. 1 ) has a microlens array  104  of microlenses  102 , in this example a 2×3 array. In other examples, it is anticipated that the microlens array may have other numbers of lenses, such a 3×3, 3×6, 4×4, 4×8, 6×6, 6×8, 6×10, or larger lens array. The microlenses  102  of the microlens array  104  are surrounded by a black mask  106 . The lens array  104  and black mask  106  are mounted atop a transparent spacer ( 208  in  FIG. 2 ) mounted atop an image sensor integrated circuit  108  that may in some embodiments also include other functions such as processor and memory functions. The image sensor integrated circuit  108  may in some embodiments be mounted directly to a processor printed circuit board of a cell phone or other fingerprint-activated unit, or in other embodiment be mounted to a flexible printed circuit  110  that extends beyond integrated circuit  108  so it may be coupled to a connector, such as connector  202  ( FIG. 2 ) attached to a processor printed circuit board  204  of a cell phone or other fingerprint-activated or fingerprint-detecting unit. Fingerprint sensor module  100 ,  206  has the flexible printed circuit  110  that may couple through connector  202  to other components of the phone. 
     Under the spacer  208 , in infrared-sensing embodiments, there may be an infrared filter  210 , which is omitted in other embodiments that image fingerprints with visible light. There is also an opaque, black, mask  212  with openings  214  that align with photosensor arrays  216  of integrated circuit  108   
     In a typical application, the optical fingerprint sensor module  100  is positioned under an organic light-emitting diode (OLED) display panel  220  of the cell phone, the OLED display panel  220  being of a known thickness and at least semitransparent to light at infrared wavelengths if infrared filter  210  is present, or semitransparent to some visible light wavelengths if infrared filter  210  is absent. 
     The optical fingerprint sensor module  100  is also typically positioned in front of a battery  222  that is positioned in front of a back plate  224  of the cell phone, the distance from a back side of back plate  224  to a front side of the OLED display panel  220  defining thickness of the cell phone. 
     When a finger  226  of a user is positioned in contact with the front of the OLED display panel  220 , some light reflected from a fingerprint region  228  of the finger  226  passes through OLED display panel  220  and is focused by microlenses  102  onto photosensor arrays  216 . 
     In an embodiment, each microlens  102  of the lens array as an aspheric single-element lens with distance from a front surface of the lens between 1.5 mm and 2.1 mm, Fstop of 1.0, a field of view FOV=23°, and an effective focal length EFFL=0.113 mm. Each lens is 0.09935 mm in diameter and 0.0526 mm tall. 
     As illustrated in  FIG. 3 , each microlens  102  of the microlens array  104  images a portion  302 ,  304 ,  306  of the fingerprint region  228  of finger  226  and produces an image on a separate photosensor array  216  of integrated circuit  108  of that portion of the fingerprint region. In an embodiment, the portion  302 ,  304 ,  306  of the fingerprint region  228  of finger  226  that each lens images onto the photosensor array  216  is centered directly above, but is larger than, the photosensor array. In an embodiment, each photosensor array typically is at least a 100×100 array of photosensors. In an alternative embodiment, all the lenses project images onto a single array of at least 400×400 photosensors, where the lenses of the lens array each project its image onto a separate area of the single array of photosensors. 
     The fingerprint sensor module  100  is produced by a process  400  according to  FIG. 4 . The infrared filter  210  is deposited 402 on a bottom side of a thin glass substrate that will become spacer  208  of between 100 um and 150 um thickness (inclusive). Black light-absorbing coatings, or masks,  212  are then deposited 404 on the bottom side of the glass substrate  208 , if the infrared filter  210  is present the light-absorbing coating  212  is deposited over the infrared filter  210 . In some embodiments black light-absorbing coating  106  is also deposited on a top side of the glass substrate or spacer  208 . The bottom black light absorbing coating  212 , and top light absorbing coating  106  if used, are then masked and etched to form openings  214 ,  215  and alignment marks (not shown), these black coatings form baffles that improve image quality when lenses are formed with small pitch and large image overlap areas. 
     The microlens array  104  is formed  406  as a wafer level lens array by reflowing reflowable optical material onto a top side of the glass substrate or spacer  208  and the reflowable optical material is shaped with a preformed wafer-sized stamp. The alignment marks are used to align the stamp and optical material with the previously formed openings  214 ,  215  in the light absorbing coating. The bottom side of the glass substrate or spacer  208  with light absorbing coating  212  is then aligned, and bonded  408 , to a wafer of integrated circuits  108 . The assembled wafer with microlenses  102 , glass substrate serving a spacer  208 , and integrated circuits  108  may be tested and defective circuits inked. The assembled wafer is then diced, typically by sawing, and individual microlens array  104 , substrate or spacer  208 , light absorbing coatings  106 ,  212 , and integrated circuit  108  assemblies bonded  410  using a ball-bond reflow technique to flexible printed circuit  110 . 
     The fingerprint sensor module  100 ,  206  is used in a cellular telephone  600  ( FIG. 6 ); the cellular telephone  600  incorporates OLED display panel  220 , typically having touch sensing capability, operable under control by one or more processors  606  coupled to receive raw images or extracted features from fingerprint sensor  206 . On or more processors  606  operate under control of firmware and an operating system  608  in a memory system  610 , and are also coupled to one or more digital radios  612  configured for two-way communications with at least digital cellular towers. The processors  606  are also coupled to a global positioning system receiver and other sensors  614  such as accelerometers, a microphone and speaker  616 , and in many embodiments a serial port  618  coupled to a universal serial bus (USB) interface  620 . Cellular telephone  600  is powered by the battery  222 , through a power supply circuit and recharged by a charger  622 . 
     The fingerprint sensor is operated by a method  500  ( FIG. 5 ) including illuminating  502  the fingerprint region  118  of the finger  226  using the OLED display panel  220 ; light from the fingerprint region  228  is focused by microlenses  102  onto the photosensor arrays  216  of integrated circuit  108 , each microlens  102  focuses light onto multiple photosensors of the photosensor arrays. The photosensor arrays are then read  506  to form overlapping electronic fingerprint images. The overlapping electronic fingerprint images may in some embodiments then be stitched  508  to form a single electronic fingerprint image. Features are then extracted  512  from the single electronic fingerprint image or from the overlapping electronic fingerprint images, these features are then compared  514  to features associated with one or more users in a feature library  630  of features comprising features associated with one or more fingers of one or more authorized users in memory system  610 , a successful comparison verifies identity of a user to whom finger  226  belongs. 
     Changes may be made in the above methods and systems without departing from the scope hereof. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall therebetween.