Patent Publication Number: US-2020292796-A1

Title: Optical fingerprint sensing module and display device with optical fingerprint detection

Description:
RELATED APPLICATIONS 
     The present application is a continuation in part of U.S. patent application Ser. No. 16/742,932, filed Jan. 15, 2020, which is a continuation application of U.S. patent application Ser. No. 15/293,295, filed Oct. 14, 2016, U.S. Pat. No. 10,539,765, which claims priority to U.S. Provisional Patent Application No. 62/241,156, filed Oct. 14, 2015, all of which are herein incorporated by reference. The present application claims priority to U.S. Provisional Patent Application No. 62/898,557, filed Sep. 11, 2019, which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to an optical fingerprint sensing module and a display device with optical fingerprint detection. 
     Description of Related Art 
     Accompanying with the development of portable devices such as smart phones and tablet computers, techniques of individual identification are highly demanded. Among individual identification techniques, the fingerprint identification technique is thought to be promising from the standpoints of cost, size and identification accuracy. In conventional smart phones, the fingerprint identification modules are equipped in the cell phones at a region out of the display panel. Recently, some smart phones equipped with OLED panels integrate fingerprint identification modules into the OLED panels in order to minimalize the frame size of the smart phones. However, this technique is not suitable for other types of display panels because the optical mechanism and the structures of the display panels are quite different. Therefore, there is a need for a fingerprint identification technique which is suitable for various display panels. 
     SUMMARY 
     On aspect of the present disclosure is to provide an optical fingerprint sensing module. The optical fingerprint sensing module includes an image sensing device, at least one light source and a light shielding structure. The image sensing device is configured to sense light transmitted from a fingerprint of a finger on a display panel. The image sensing device includes a light sensing plane having a first geometric center. The at least one light source is adjacent to the image sensing device. The light source includes a light emitting plane having a second geometric center, and the first geometric center is separated from the second geometric center by a distance ranged from 2 mm to 20 mm. The light shielding structure is disposed between the image sensing device and the light source. 
     According to some embodiments of the present disclosure, the optical fingerprint sensing module further includes a field angle controller cooperated with the light source such that light emitted from the light source or passing through the field angle controller has a field angle ranged from 5 degrees to 60 degrees. 
     According to some embodiments of the present disclosure, the field angle ranges from 15 degrees to 35 degrees. 
     According to some embodiments of the present disclosure, the field angle ranges from 20 degrees to 30 degrees. 
     According to some embodiments of the present disclosure, the field angle controller includes a top plate extending laterally from a top of the light shielding structure, and the top plate has an aperture aligned with the light emitting plane. 
     According to some embodiments of the present disclosure, the field angle controller further includes a wall extending downwards from the top plate, and the light source is positioned between the wall and the light shielding structure. 
     According to some embodiments of the present disclosure, the wall surrounds the light shielding structure and the light source. 
     According to some embodiments of the present disclosure, the field angle controller includes a lens disposed over the light emitting plane. 
     According to some embodiments of the present disclosure, the field angle has an axis that is substantially perpendicular to the light sensing plane. 
     According to some embodiments of the present disclosure, the distance ranges from 4 mm to 10 mm. 
     According to some embodiments of the present disclosure, the image sensing device has a bottom that is coplanar with a bottom of the light source. 
     According to some embodiments of the present disclosure, the light shielding structure surrounds the image sensing device. 
     According to some embodiments of the present disclosure, the light shielding structure has an opening exposing the image sensing device. 
     An optical fingerprint sensing module includes an image sensing device, a light shielding structure, at least one light source, and a field angle controller, according to yet various embodiments. The light shielding structure surrounds the image sensing device, and the light shielding structure has an opening exposing the image sensing device. The at least one light source is positioned out of the light shielding structure such that the light shielding structure is located between the light source and the image sensing device. The field angle controller is adjacent to the light source and cooperated with the light source such that light emitted from the light source or passing through the field angle controller has a field angle ranged from 5 degrees to 60 degrees. 
     According to some embodiments of the present disclosure, the image sensing device includes a light sensing plane having a first geometric center, and the light source includes a light emitting plane having a second geometric center. The first geometric center is separated from the second geometric center by a distance ranged from 2 mm to 20 mm. 
     According to some embodiments of the present disclosure, the field angle controller includes a top plate extending laterally from a top of the light shielding structure, and the top plate has an aperture aligned with the light emitting plane in a direction perpendicular to the light emitting plane. 
     According to some embodiments of the present disclosure, the field angle controller further includes a wall extending downwards from the top plate, in which the light source is positioned between the wall and the light shielding structure. 
     According to some embodiments of the present disclosure, the field angle ranges from 15 degrees to 35 degrees. 
     On aspect of the present disclosure is to provide a display device with optical fingerprint detection. The display device includes a display panel and the optical fingerprint sensing module according to any one of embodiments or examples of the present disclosure. The display panel has a display side and a backside opposite to the display side. The optical fingerprint sensing module is disposed at the backside of the display panel. 
     According to some embodiments of the present disclosure, the light emitting plane and the light sensing plane face the backside of the display panel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  is an exploded perspective view schematically illustrating an optical fingerprint sensing module according to some embodiments of the present disclosure. 
         FIG. 2  is a diagram showing the distance between the first and second geometric centers according to some embodiments of the present disclosure. 
         FIGS. 3-5  are cross-sectional views schematically illustrating display devices with optical fingerprint detection, according to some embodiments of the present disclosure. 
         FIG. 6A  is a cross-sectional view schematically illustrating a display device with optical fingerprint detection, according to yet some embodiments of the present disclosure. 
         FIG. 6B  is a plane view schematically showing the image sensing device, the light shielding structure, the light source and the field angle controller of  FIG. 6A , according to some embodiments of the present disclosure. 
         FIG. 7A  is a cross-sectional view schematically illustrating a display device with optical fingerprint detection, according to yet some embodiments of the present disclosure. 
         FIG. 7B  is a plane view schematically showing the image sensing device, the light shielding structure, the light source and the field angle controller of  FIG. 7A , according to some embodiments of the present disclosure. 
         FIG. 8  is diagram showing the relationship between the SNR of the detected fingerprint and the field angle θ according to some examples of the present disclosure. 
         FIG. 9  is cross-sectional view schematically showing possible light paths in the display device of  FIG. 7A . 
         FIG. 10  is an enlarged cross-sectional view to illustrate certain possible mechanism. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     Still further, when a number or a range of numbers is described with “about,” “approximate,” and the like, the term is intended to encompass numbers that are within a reasonable range including the number described, such as within +1-10% of the number described or other values as understood by person skilled in the art. For example, the term “about 5 nm” encompasses the dimension range from 4.5 nm to 5.5 nm. 
     According to one aspect of the present disclosure, an optical fingerprint sensing module is provided. The optical fingerprint sensing module is capable of detecting or sensing a fingerprint of a user&#39;s finger. In various embodiments, the optical fingerprint sensing module detects the light transmitted from the fingerprint of a finger on a display panel of an electronic device. 
       FIG. 1  is an exploded perspective view schematically illustrating an optical fingerprint sensing module  100  according to some embodiments of the present disclosure. As shown in  FIG. 1 , the optical fingerprint sensing module  100  includes an image sensing device  110 , one or more light sources  120 , and a light shielding structure  130 . The optical fingerprint sensing module  100  may optionally include other elements or features, which are described in detail hereinafter, according yet some embodiments of the present disclosure. 
     The image sensing device  110  includes a light sensing plane  112  which is capable of detecting or sensing incident light. For instance, the image sensing device  110  may include a number of light sensitive elements, called pixels, arranged on the light sensing plane  112 . Photons incident onto the light sensing plane  112  generate charges that can be read and converted into digital signal. In some examples, the image sensing device  110  may include a charge coupled device (CCD) detecting infrared light or visible light, or a complementary metal-oxide-semiconductor (CMOS) device detecting infrared light or visible light. The light sensing plane  112  has a first geometric center  112   a . The term “geometric center” herein refers to the general meaning in mathematics and physics, and particularly the “first geometric center” refers to the arithmetic mean position of all the points on the light sensing plane. 
     The one or more light sources  120  are arranged adjacent to the image sensing device  110 . Although  FIG. 1  depicts a plurality of light sources  120 , the present disclosure is not limited thereto. In some embodiments, the image sensing device  110  may include a single light source  120 . The light source(s)  120  is capable of emitting infrared light or visible light, depending on the types of the image sensing device  110 . In some examples, the light source(s)  120  may be a visible light emitting diode when the image sensing device  110  includes a CCD or CMOS device detecting visible light. In yet some examples, the light source(s)  120  may be an infrared light emitting diode when the image sensing device  110  includes a CCD or CMOS device detecting infrared light. 
     The light source  120  may include a light emitting plane  122  having a second geometric center  122   a . The term “light emitting plane” herein refers to the plane which emits light when observed in a top view of the light source  120 . Furthermore, the term “geometric center” herein refers to the general meaning in mathematics and physics, and particularly the “second geometric center” refers to the arithmetic mean position of all the points on the light emitting plane. In some embodiments, the light source  120  includes an LED chip, and the second geometric center  122   a  of light emitting plane  122  is approximately the same as the geometric center of the top surface of LED chip. 
     The second geometric center  122   a  is separated or spaced apart from the first geometric center  112   a  by a distance D ranged from 2 mm to 20 mm. According to various embodiments, the distance D between the first and second geometric centers  112   a ,  122   a  is critical and that provides certain technical effects. In particular, when the distance D is less than a certain level such as 2 mm, the signal-to-noise ratio (SNR) of the detected fingerprint decreases to an unaccepted level, according to some embodiments. On the other hand, when the distance D is greater than another certain value such as 20 mm, the signal-to-noise ratio of the detected fingerprint also decreases to an unaccepted level, according to yet some embodiments. Therefore, a relatively better range of the distance D is present, and the range of the distance D is from 2 mm to 20 mm, specifically 4 mm to 10 mm. For instance, the distance D may be 2 mm, 4 mm, 6 mm, 8 mm, 10 mm, 12 mm, 14 mm, 16 mm, 18 mm, or 20 mm. The possible mechanism of the criticality of the distance D will be discussed in detail hereinafter. 
     In some embodiments, the light sensing plane  112  may be substantially parallel to the light emitting plane  122 . The light emitting plane  122  may be higher or lower than the light sensing plane  112 . Furthermore, the light sensing plane  112  has a first axis  112   c  perpendicular to the light sensing plane  112  and passing through the first geometric center  112   a . The light emitting plane  122  has a second axis  122   c  perpendicular to the light emitting plane  122  and passing through the second geometric center  122   a . The distance D between the first and second geometric centers  112   a ,  122   a  is defined as the distance between the first axis  112   c  and the second axis  122   c.    
     In yet some embodiments, the light sensing plane  112  is not parallel to the light emitting plane  122 , as shown in  FIG. 2 . The light emitting plane  122  may be higher or lower than the light sensing plane  112 . The upper portion of  FIG. 2  depicts that the light emitting plane  122  is higher than the light sensing plane  112 . The lower portion of  FIG. 2  depicts that the light emitting plane  122  is lower than the light sensing plane  112 .  FIG. 2  further depicts a virtual plane  160  that is parallel to the light sensing plane  112 . In these embodiments, the distance D between the first and second geometric centers  112   a ,  122   a  is defined as the distance between the normal projection  112   a ″ of the first geometric centers  112   a  and normal projection  122   a ″ of the second geometric centers  122   a  onto the virtual plane  160 . 
     Referring back to  FIG. 1 , according to some embodiments, the image sensing device  110  has a bottom  114  that is coplanar with a bottom  124  of the light source  120 . For instance, the bottom  114  of the image sensing device  110  and the bottom  124  of the light source  120  may be attached or bonded onto a plane  152  of a substrate  150 , such as for example a circuit board. However, in yet some embodiments, the bottom  114  of the image sensing device  110  and the bottom  124  of the light source  120  are not coplanar. 
     In yet some embodiments, the optical fingerprint sensing module  100  includes a plurality of the light sources  120 , and the light sources  120  are symmetrically arranged with respect to the first geometric center  112   a  in a plane view. For example, one or more pairs of the light sources  120  may be disposed, in which the paired light sources  120  are positioned at opposite sides of the image sensing device  110 . The paired light sources  12  are equidistant from the image sensing device  110 . 
     The light shielding structure  130  is disposed between the image sensing device  110  and the light source  120 . The light shielding structure  130  extends from a position lower than the light emitting plane  122  to a position higher than both of the light emitting plane  122  and the light sensing plane  112 . In some embodiments, the light shielding structure  130  surrounds the image sensing device  110 . Specifically, the light source(s)  120  may be positioned out of the light shielding structure  130  whereas the image sensing device  110  is positioned inside the light shielding structure  130 . In this manner, the light shielding structure  130  prevents the light sensing plane  112  from the interference of the light directly emitted from the light source  120  as well as other noise light that is irrelevant to the target fingerprint. According to yet some embodiments, the light shielding structure  130  may have an opening  132  exposing the image sensing device  110 . In examples, the opening  132  is overlapped and/or substantially aligned with the image sensing device  110 . Accordingly, light may be transmitted through the opening  132  to the light sensing plane  112  of the image sensing device  110 . 
     Although  FIG. 1  depicts that the light shielding structure  130  surrounds the image sensing device  110 , the present disclosure is not limited thereto. For example, the light shielding structure  130  may be a single “partition” or a plurality of discrete “partitions” stood in between the light source(s)  120  and the image sensing device  110 . The partition(s) vertically extends upwards to a position higher than both of the light sensing plane  112  and the light emitting plane  122 . 
     A lens set  170  may optionally be included in the optical fingerprint sensing module  100 , according to some embodiments of the present disclosure. The lens set  170  is disposed over the image sensing device  110 , and configured to generate an image of the fingerprint for the image sensing device  110 . In some embodiments, the lens set  170  may include a biconvex lens, a convex-concave (positive meniscus) lens, a plano-convex lens, a plano-concave lens, or a combination thereof. 
     The optical fingerprint sensing module  100  may further include a filter (not shown in  FIG. 1 ) which filters unnecessary light having interferential wavelengths. For example, when the image sensing device  110  serves to detect infrared light (and the light source  120  serves to emit infrared light), the filter functions to filter visible light and/or ultraviolet light. The filter may be disposed over the image sensing device  110  and cover the light sensing plane  112 . In some example, the filter may be a coating layer formed on one or more lenses of the lens set  170 . 
       FIG. 3  is a cross-sectional view schematically illustrating a display device  200   a  with optical fingerprint detection, according to some embodiments of the present disclosure. The display device  200   a  may be a part of an electronic device such as for example a smart phone, a tablet computer, a laptop, a communication device, an electronic security guarding device, or any device with requirement of fingerprint identification. As illustrated in  FIG. 3 , the display device  200   a  includes an optical fingerprint sensing module  100   a  and a display panel  210  over the optical fingerprint sensing module  100   a . As to the optical fingerprint sensing module  100   a , reference numerals are repeated herein to show the same or similar features shown in  FIG. 1 , and the description above applies equally to the embodiments described below, and the details thereof are not repeatedly described. The display panel  210  has a display side  211  and a backside  212  opposite to the display side  211 . The optical fingerprint sensing module  100   a  can be disposed near the backside  212  of the display panel  210 . The optical fingerprint sensing module  100   a  is configured to sense or detect light transmitted from fingerprint of the finger  300  on the display panel  210 . In various embodiments, the light emitting plane  122  and the light sensing plane  112  face the backside  212  of the display panel  210 . 
     The optical fingerprint sensing module  100   a  further includes a field angle controller  140 , as compared to the embodiments shown in  FIG. 1 . The field angle controller  140  cooperates with the light source  120  such that light emitted from the light source  120  or the light passing through the field angle controller  140  has a field angle θ in a predetermined range, which can be preferably ranged from 5 degrees to 60 degrees. The field angle θ is critical in terms of the signal-to-noise ratio of the detected fingerprint. In particular, when the field angle θ is less than a certain level such as 5 degrees, the SNR of the detected fingerprint decreases to an unaccepted level, according to some embodiments. On other hand, when the field angle θ is greater than a certain value such as 60 degrees, the SNR of the detected fingerprint also decreases to an unaccepted level, according to yet some embodiments. Therefore, a relatively better range of the field angle θ is present between 5 to 60 degrees, specifically ranged from 15 degrees to 35 degrees, more specifically from 20 degrees to 30 degrees. For instance, the field angle θ may be 5 degrees, 10 degrees, 15 degrees, 20 degrees, 25 degrees, 30 degrees, 35 degrees, 40 degrees, 45 degrees, 50 degrees, 55 degrees or 60 degrees. The possible mechanism of the criticality of the field angle will be discussed in detail hereinafter. It is noted that when the field angle controller  140  is used and controls the field angle θ of the light in the range from 5 degrees to 60 degrees, the distance D between the first and second geometric center  112   a ,  122   a  is possible to be out of the range of 2 mm to 20 mm. 
     The field angle controller  140  may be disposed at any suitable position adjacent to the light source  120  as long as it can cooperate with the light source  120  to control the field angle of the light in a predetermined range, for example, the range from 5 degrees to 60 degrees. In some embodiments, the field angle controller  140  may include a lens  144  disposed over the light emitting plane  122 . For example, the lens may be a plano-convex lens or positive meniscus lens, or the like. The plano-convex lens or positive meniscus lens focuses (or concentrate) the light emitted from the light source  120  to control the field angle of the light emitted from the light source  120 . In yet some embodiments, the field angle controller  140  further includes a shielding dome  142  mounted on the lens  144 . The shielding dome  142  functions to shield the light with a large angle with respect to the normal axis of the light emitting plane  122 . In addition, the shielding dome  142  has an aperture that exposes the second geometric center  122   a  of the light source  120 . 
     The light emitted from the light source  120  illuminates a finger  300 , and the image sensing device  110  detects the light transmitted from the finger  300 . Therefore, the fingerprint of the finger  300  may be detected. 
     In some embodiments, the display device  200   a  may optionally include a light guide plate  220 , a diffuser  230 , and a reflector  240 . The light guide plate  220  is disposed between the display panel  210  and the optical fingerprint sensing module  100   a . The diffusor  230  is disposed between the light guide plate  220  and the display panel  210 . The reflector  240  is disposed between the light guide plate  220  and the optical fingerprint sensing module  100   a.    
     In some embodiments, display panel  210  is a liquid crystal display (LCD) panel. In examples, the LCD panel may be an IPS LCD panel, a MVA LCD panel, a TN mode LCD panel, a semi-reflective LCD panel, or the like. As shown in  FIG. 3 , a plurality of optical elements is disposed between the finger  300  and the optical fingerprint sensing module  100   a . The detection of the image of the fingerprint is more difficult than other kinds of display panels. Therefore, the aforementioned SNR is important for the purposes of precise detection of the fingerprint, and the aforementioned distance D and/or field angle θ are critical, according to some embodiments of the present disclosure. 
       FIG. 4  is a cross-sectional view schematically illustrating a display device  200   b  with optical fingerprint detection, according to yet some embodiments of the present disclosure. The display device  200   b  is similar to the display device  200   a  shown in  FIG. 3 , except that the light shielding structure  130  includes a wall  134 . In some embodiments, an eaves portion  136  can be further included. The eaves portion  136  can laterally extend from a top of the wall  134  towards the second axis  122   c  of the light source  120 . In some embodiments, the eaves portion  136  laterally extends to a position not beyond the second axis  122   c  of the light emitting plane  122 . The eaves portion  136  also assists to enhance the aforementioned SNR of the detected fingerprint. 
       FIG. 5  is a cross-sectional view schematically illustrating a display device  200   c  with optical fingerprint detection, according to yet some embodiments of the present disclosure. The display device  200   c  is similar to the display device  200   b  shown in  FIG. 4 , except that the structure of the field angle controller  140  is different. In  FIG. 5 , the field angle controller  140  includes a collimator  143  and a lens  144 . The collimator  143  and lens  144  collectively serves to manage the directions of the light so that the field angle of the light through the collimator  143  may be limited in a desired range. In some examples, the lens  144  is directly mounted on the light source  120 , and the collimator  143  is fixed to the eaves portion  136 . The collimator  143  is above the lens  144  and substantially aligned with the lens  144 . In yet some examples, the collimator  143  may be directly placed on the lens  144 , and is put around the lens  144 . 
       FIG. 6A  is a cross-sectional view schematically illustrating a display device  200   d  with optical fingerprint detection, according to yet some embodiments of the present disclosure. The display device  200   d  is similar to the display device  200   a  shown in  FIG. 3 , except that the structure of the field angle controller  140  is different. In  FIG. 6A , the field angle controller  140  includes a top plate  145  extending laterally from a top of the light shielding structure  130 . The top plate  145  has an aperture  145   a  aligned with the light emitting plane  122  in a direction perpendicular to the light emitting plane  122 . In some examples, the aperture  145   a  has a width less than a width of the light emitting plane  122 . Therefore, the light transmitted through the aperture  145   a  of the top plate  145  may be constrained within a certain field angle. However, in some examples, the width of the aperture  145   a  may be greater than the width of the light emitting plane  122 . For example, another optical element (e.g., lens  144  shown in  FIG. 5 ) may be disposed over the light emitting plane  122  of the light source  120  to facilitate constraining the field angle θ. The field angle θ of the light through the aperture  145   a  can be constrained in the range from 5 degrees to 60 degrees, and then the constrained light projects to the finger  300 . The field angle θ has an axis C that directs to the finger  300  so that the image of the fingerprint may be detected by the image sensing device  110 . In examples, the axis C is substantially perpendicular to the light emitting plane  122  and the light sensing plane  112 . 
       FIG. 6B  is a plane view schematically showing the image sensing device  110 , the light shielding structure  130 , the light source  120  and the field angle controller  140  of  FIG. 6A , according to some embodiments. As shown in  FIG. 6A  and  FIG. 6B , the top plate  145  of the field angle controller  140  is physically connected to the light shielding structure  130 . For instance, the top plate  145  laterally extends from the top of the light shielding structure  130  to a position past the light source  120 . The top plate  145  may be extended in a direction substantially perpendicular to the light shielding structure  130  and/or the axis C of the field angle θ. The top plate  145  may have a plurality of apertures  145   a , and each of the apertures  145   a  exposes partial or entire light emitting plane  122  of the light source  120 . The top plate  145  shields the light with a large angle with respect to the axis C, and therefore the SNR of the detected fingerprint may be enhanced. 
       FIG. 7A  is a cross-sectional view schematically illustrating a display device  200   e  with optical fingerprint detection, according to yet some embodiments of the present disclosure. FIG.  7 B is a plane view schematically showing the image sensing device  110 , the light shielding structure  130 , the light source  120  and the field angle controller  140  of  FIG. 7A . The display device  200   e  is similar to the display device  200   d  shown in  FIG. 6A , except that the field angle controller  140  further includes a wall  146  extending downwards from the top plate  145 . In some embodiments, the wall  146  extends downwards to a position below the light emitting plane  122 . In some examples, the wall  146  may extend from the top plate  145  to the substrate  150 , and therefore the wall  146  has a height substantially equal to a height of the light shielding structure  130 . Moreover, the bottoms of both the wall  146  and the light shielding structure  130  may be firmly fixed onto the substrate  150 , and therefore the stability and accuracy of the construction of the light shielding structure  130  and the field angle controller  140  are secured. In other words, the accuracy and reliability of the entire optical fingerprint sensing module are improved. For example, the light shielding structure  130 , and the top plate  145  and the wall  146  of the field angle controller  140  may be integrated into a single piece. Specifically, a molding process may be used to form an integrated piece comprised of the light shielding structure  130  and field angle controller  140 . 
     In yet some embodiments, the wall  146  surrounds the light shielding structure  130  which surrounds the image sensing device  110 . Particularly, the light source  120  is positioned between the wall  146  and the light shielding structure  130 . More particularly, the light source  120  is enveloped in the space defined by the light shielding structure  130 , the substrate  150 , and the top plate  145  and the wall  146  of the field angle controller  140 . Only the aperture  145   a  provides a light path for the light emitted from the light source  120 . As a result, the SNR of the detected fingerprint and the reliability of the optical fingerprint sensing module are improved. 
       FIG. 8  is diagram showing the relationship between the SNR of the detected fingerprint and the field angle θ of light projected to the finger of a user according to some examples of the present disclosure. It is unexpected that the SNR of the detected fingerprint is considerably relevant to the field angle θ. For example, when the field angle θ is less than about 15 degrees, the SNR is less than or approximate to 10 (dB). On the other hand, when the field angle θ is greater than about 35 degrees, the SNR is less than or approximate to 10 (dB) as well. There exists a certain range of field angle θ that is capable of obtaining a relatively higher SNR of the detected fingerprint. The results associated with  FIG. 8  are evidence showing the criticality of the field angle θ. 
       FIG. 9  is cross-sectional view schematically showing possible light paths in the display device  200   e  of  FIG. 7A , in which the fingerprint of the finger  300  is enlarged for better illustration. As shown, the fingerprint has a number of protrusive portions A and a number of recessed portions B. The light emitted from the light source  120  transmits through the aperture  145   a  of the field angle controller  140 , and projects to the finger  300 . The light further transmits into the finger  300  and is scattered (and/or reflected) by the tissue in the finger  300 . The scattered light transmits along the light path  310  to the light sensing plane  112  through the fingerprint of the finger  300 , and therefore the fingerprint is detected by the light sensing plane  112 . If the fingerprint is detected in the optical mechanism described above, the protrusive portions A of the finger  300  are detected as “dark” patterns whereas the recessed portions B are detected as “bright” patterns. That is because the light passing through the protrusive portions A has a longer light path in the tissue of the finger  300 , and the tissue absorbs parts of the light and decreases the light intensity. 
     However, there is another light path present in the display device  200   e , i.e., light reflection. As the light with a large incident angle transmits along the light path  320  to the surface of the fingerprint, the light is reflected and directed to the light sensing plane  112 . While this light path occurs, the protrusive portions A are detected as “bright” patterns whereas the recessed portions B are detected as “dark” patterns, and these patterns are just in contrast to that of light path  310  described in the previous paragraph.  FIG. 10  is an enlarged cross-sectional view to illustrate the possible mechanism. As the light with a large incident angle illuminates the protrusive portions A along the light path  320 , shadows A″ of the protrusive portions A is generated covering the recessed portions B. Therefore, the protrusive portions A of the finger  300  are detected as “bright” patterns, but the recessed portions B are detected as “dark” patterns. The detected pattern associated with the light path  320  is contrary to that associated with the light path  310 . Accordingly, the light path  320  derived from light with large incident angles may interfere with the detected patterns associated with the light path  310 . When the field angle θ of the light projected to the finger is increased, the amount of the light with large incident angles increases, leading to an increase in the reflection of the light path  320 . Accordingly, when the field angle θ is greater than a certain level, the SNR of the detected fingerprint is decreased. 
     Moreover, the field angle θ also affects the light path  310 . Referring back to  FIG. 9 , when the field angle θ is narrowed, the amount of the scattered light transmitted along the light path  310  possibly decreases. A wider field angle θ possibly enhances the multi-directionality of the light scattering because a wider field angle θ provides more incident directions. Therefore, a narrowed field angle θ possibly suppresses the multi-directionality of the light scattering, leading to the decrease in the amount of scattered light transmitted along the light path  310 . Accordingly, when the field angle θ is less than a certain level, the SNR of the detected fingerprint is also decreased. 
     Referring back to  FIG. 3 , the distance D also affects the light path  310  and the light path  320 , and therefore a relative better range of the distance D is also present in terms of the SNR of the detected fingerprint. 
     Nevertheless, the present application is not intended to be bound to any theory or optical mechanism. The relevant theory and mechanism described in the present disclosure is only for the purpose of better understanding of the criticality of the field angle θ and the distance D. 
     The foregoing has outlined features of several embodiments so that those skilled in the art may better understand the detailed description above. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure.