Patent Publication Number: US-2022216263-A1

Title: Biometric Sensor and Methods Thereof

Description:
PRIORITY DATA 
     This is a continuation application of U.S. patent application Ser. No. 17/007,455, filed Aug. 31, 2020, which is a divisional application of U.S. patent application Ser. No. 15/905,391, filed Feb. 26, 2018, now issued U.S. Pat. No. 10,763,296, which claims priority to U.S. Provisional Patent Application Ser. No. 62/590,055, filed Nov. 22, 2017, the entire disclosures of which are herein incorporated by reference. 
    
    
     BACKGROUND 
     The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of processing and manufacturing ICs. 
     For example, there is considerable interest in providing consumer and/or portable electronic devices (e.g., smart phones, electronic tablets, wearable devices, and so on) with biometric sensors (e.g., optical sensors for fingerprint recognition) inside limited device housing. Surface space is often a particularly limited resource in electronic devices. A need exists for biometric sensors to stack with other components (e.g., display panels) inside device housing to avoid assigning valuable surface space exclusively to biometric sensors that may only be used briefly during a user identification step. By stacking biometric sensors, a biometric object (e.g., a user&#39;s finger) outside the electronic device is further distanced away from the sensors. Interferences from stray light and ambient light may grow stronger, resulting in poorer sensitivity of the biometric sensors. For example, fingerprint images acquired by an optical biometric sensor may become blurred due to degradation of signal-to-noise ratio (SNR) of received light under the interferences. Therefore, although conventional means of integrating biometric sensors inside electronic device housing have been generally adequate for their intended purposes, they are not satisfactory in all respects. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  illustrates an electronic device with a biometric sensing region on surface space, according to various aspects of the present disclosure. 
         FIG. 2  is a cross-sectional view of an electronic device integrated with an optical sensor under a display panel, according to various aspects of the present disclosure. 
         FIGS. 3A and 3B  are top views of a biometric sensing region of an electronic device integrated with an optical sensor under a display panel, according to various aspects of the present disclosure. 
         FIG. 4  illustrates a flowchart of a method for capturing biometric images, according to various aspects of the present disclosure. 
         FIG. 5  illustrates a flowchart of a method for fabricating an electronic device integrated with an optical sensor under a display panel, according to various aspects of the present disclosure. 
         FIGS. 6, 7, 8, 9, 10, 11, 12, 13, and 14  are cross-sectional views of an electronic device at various stages of the method of  FIG. 5 , according to various aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the disclosure. 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 beyond the extent noted. 
     Moreover, the formation of a feature on, connected to, and/or coupled to another feature in the present disclosure that follows may include embodiments in which the features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the features, such that the features may not be in direct contact. In addition, spatially relative terms, for example, “lower,” “upper,” “horizontal,” “vertical,” “above,” “over,” “below,” “beneath,” “up,” “down,” “top,” “bottom,” etc. as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) are used for ease of the present disclosure of one features relationship to another feature. The spatially relative terms are intended to cover different orientations of the device including the features. 
     The present disclosure is generally related to electronic devices and fabrication. More particularly, some embodiments are related to integrating one or more optical sensors under a display panel of an electronic device for biometric detection. It is an objective of the present disclosure to provide methods for effectively enhancing signal-to-noise ratio (SNR) of incident light received by the optical sensors during a biometric detection. 
     Biometric sensing systems, such as fingerprint recognition systems, have been one approach drawing considerable interest to provide security features to electronic devices, and more particularly, consumer and/or portable electronic devices (e.g., smart phones, electronic tablets, wearable devices, and so on). Biometric sensing systems are based on unique features of a user and may not rely on memorization or the use of other input devices by the user, such as password input. Biometric sensing systems also provide the advantage of being difficult to hack for the same reason. 
     Among various biometric sensing techniques, fingerprint recognition is a reliable and widely used technique for personal identification or verification. A fingerprint recognition system generally includes fingerprint sensing and matching functionalities, such as collecting fingerprint images and comparing those images against known fingerprint information. In particular, one approach to fingerprint recognition involves scanning a reference fingerprint and storing the reference image acquired. The characteristics of a new fingerprint may be scanned and compared to the reference image already stored in a database to determine proper identification of a person, such as for verification purposes. A fingerprint recognition system may be particularly advantageous for authentication in consumer and/or portable electronic devices. For example, an optical sensor for acquiring fingerprint images may be carried inside the housing of an electronic device. 
     The effectiveness of biometric security systems may be affected by the accuracy with which the unique biometric data is able to be detected. In the case of fingerprint recognition systems, this means improving SNR of the incident light arriving optical sensors and thereby enhancing resolution of the images acquired. 
     Meanwhile, the availability of space within the device housing weights much during design efforts to integrate biometric security systems. Many electronic components contend for this space. Available surface space is often a particularly limited resource. In various embodiments, an electronic device includes a display panel and a separate fingerprint sensor located adjacent to the display panel. By placing the fingerprint sensor in adjacent areas, the fingerprint sensor avoids being obstructed by the display panel and will receive stronger incident lights. However, by assigning this valuable portion of the surface space exclusively to the fingerprint sensor, the display panel has to shrink to accommodate the fingerprint sensor, instead of expanding to substantially the whole surface space of the electronic device (e.g., for a better user viewing experience). 
     Referring initially to  FIG. 1 , an electronic device  100  is now described. The electronic device  100  is illustratively a mobile wireless communication device (e.g., a smart phone). The electronic device  100  may be any other suitable electronic device, such as a laptop computer, an electronic tablet, a portable gaming device, a navigation device, or a wearable device. The electronic device  100  includes a housing  102  together with other components, such as processor(s) and memories, inside the housing  102 . 
     A display panel  104  is also carried by the housing  102 . In the illustrated embodiment, the display panel  104  is an organic light-emitting diode (OLED) display panel. In various embodiments, the display panel  104  may be any other suitable type display panel, as will be appreciated by those skilled in the art, such as liquid-crystal display (LCD) panel, light-emitting diode (LED) display panel, or active-matrix organic light-emitting diode (AMOLED) display panel. 
     In the illustrated embodiment, the display panel  104  expands substantially to the whole surface space of the electronic device  100 . Some margins between the display panel  104  and edges of the housing  102  may be left for bezel panels  106 . The display panel  104  stacks above image sensing features for fingerprint detection, or other suitable biometric sensing features. The image sensing features will be described further in details later. The display panel  104  acts as both a display and an input device through which the image sensing features acquires fingerprint images. As such, the display panel  104  performs multiple device functions in response to user input. For example, the display panel  104  may first display a prompt (e.g., a finger icon or an instruction text) on screen when the electronic device  100  is in a lock status. The display panel  104  may further highlight a sensing region  108 . When a user&#39;s finger  110  or another suitable object is placed inside the sensing region  108 , in either near field or in direct contact with the display panel  104 , the image sensing features are activated and acquire biometric data (e.g., a fingerprint image) from the user&#39;s finger  110 . Such biometric data is sent to processor(s) for matching and/or spoof detection. If the biometric data matches a reference fingerprint image stored in memories, the electronic device  100  may thereafter transit into an unlock status, and the display panel  104  starts to show desktop icons or response to various other user inputs. The display panel  104  may further integrate with touch sensor arrays. In such case, the display panel  104  is also a touch display panel. 
     Referring to  FIG. 2 , a cross-sectional view of a portion of the electronic device  100  is illustrated. This portion of the electronic device  100  carries the fingerprint recognition function and can be regarded as a fingerprint recognition system  200 . The fingerprint recognition system  200  is in a stack-up configuration, including a display panel  202  on the top, a light conditioning layer  204  in the middle, and an image sensing layer  206  at the bottom. The display panel  202  illuminates the sensing region  108  above. When light emitted from the display panel  202  is reflected from the user&#39;s finger  110  or other suitable objects, the reflected light travels downwardly through the display panel  202  and the light conditioning layer  204  and eventually arrives at the image sensing layer  206 . The image sensing layer  206  includes one or more optical sensing elements  207 , such as complementary metal oxide semiconductor (CMOS) sensors and/or charged coupled device (CCD) sensors. The optical sensing elements  207  are capable of detecting intensities of the incident light. The image sensing layer  206  thereby convers the incident light into a pixel image, which includes biometric characteristics of the user&#39;s finger  110 . Each pixel of the pixel image may correspond to intensity of the incident light recorded at a corresponding location of an optical sensing element  207 . 
     In some embodiments, the display panel  202  includes a cover glass  214  (or cover lens) that protects inner components of the electronic device  100 . The sensing region  108  is defined above the cover glass  214 . A top surface  216  of the cover glass  214  forms a sensing surface, which provides a contact area for the user&#39;s finger  110  or other suitable objects. Inside the sensing region  108 , the user&#39;s finger  110  may directly touch the top surface  216  or keep a small distance away from the top surface  216  as during a near field sensing. The cover glass  214  may be made of glass, transparent polymeric materials, or other suitable materials. 
     The display panel  202  includes a display layer  220  under the cover glass  214 . The display layer  220  includes an array of light emitting pixels  222 . Different light emitting pixels  222  may be configured to emit different colors, such as the ones emitting red light (denoted as  222 R), the ones emitting green light (denoted as  222 G), or the ones emitting blue light (denoted as  222 B). Due to geometry relationships with the sensing region  108 , the light emitting pixels  222  can be categorized into two groups, one group directly under the sensing region  108  and another group outside of the sensing region  108 . The light emitting pixels  222  outside of the sensing region  108  perform regular display functions, while the light emitting pixels  222  directly under the sensing region  108  perform both regular display functions and illumination function during biometric sensing, depending on applications. In various embodiments, pixel distance D 1  between adjacent light emitting pixels  222  is in a range from about 5 um to about 30 um. In a specific example, the pixel distance D 1  may be in a range from about 10 um to about 20 um. 
     In some embodiments, the display panel  202  further includes a blocking layer  224 . The blocking layer  224  is a semitransparent or opaque layer that may be disposed below the display layer  220 . Outside of the sensing region  108 , the blocking layer  224  is continuous, obscuring components under the display layer  220  from the light emitted by the light emitting pixels  222  and from ambient light. Directly under the sensing region  108 , the blocking layer  224  has a plurality of openings  226 . Each opening  226  locates between two adjacent light emitting pixels  222 . The openings  226  allow the light reflected from the sensing region  108  to travel through. In the illustrated embodiment, there is one opening  226  located between two adjacent light emitting pixels  222 . The opening  226  may have a width (or diameter) D 2  in a ratio to the pixel distance D 1  from about 40% to about 90%. In some other embodiments, there are two or more openings  226  located between two adjacent light emitting pixels  222 . The opening  226  may thus have a width (or diameter) D 2  in a ratio to the pixel distance D 1  from about 20% to about 40%. 
     In various embodiments, the display layer  220  may be a LCD display (using backlight with color filters to form RGB pixels), a LED display (e.g., a microLED, in which the pixel material can be inorganic material used in LED), an OLED display, or any other suitable displays. In the illustrated embodiment, the light emitting pixels  222  are organic light emitting diodes (OLED) and the display layer  220  is an OLED display. Examples of an OLED display may include active-matrix OLED (AMOLED), passive-matrix OLED (PMOLED), white OLED (WOLED), and RBG-OLED, and/or other suitable types of OLED. An OLED display is usually thinner, lighter, and more flexible than other types of displays, such as LCD or LED displays. OLED display does not require a back light, since the light can be generated from the organic light emitting material in an OLED, which allows a pixel to be turned completely off. The organic light emitting material can be an organic polymer, such as polyphenylenevinylene and polyfluorene. Due to the organic light emitting material producing its own light, the OLED display can also have a wider viewing angle. This can be in comparison to a LCD display, which works by blocking light that can lead to obstruction of certain viewing angles. 
     The OLED diodes emit light using a process called electroluminescence. Electroluminescence is a phenomenon where the organic light emitting material can emit light in response to an electric current passing through. In some examples, the OLED diodes can include hole injection layers, hole transport layers, electron injection layers, emissive layers, and electron transport layers. The color of light emitted by an OLED diode depends on the type of organic light emitting material used in the emissive layer. Different colors can be obtained with a variety of chemical structures of the organic light emitting material. For example, the light emitting pixel  222 R can be formed with an organic light emitting material that emits red light; the light emitting pixel  222 G can be formed with an organic light emitting material that emits green light; and the light emitting pixel  222 B can be formed with an organic light emitting material that emits blue light. The intensity of light can depend on the number of emitted photons or the voltage applied on the OLED diodes. In some embodiments, each light emitting pixel  222 R,  222 G, or  222 B is formed with the same organic light emitting material that generates white light, but further includes a red, green, or blue color filter to filter out colors other than the target color, respectively. The color filter can be formed using a cholesteric filter material such as a multilayer dielectric stack that includes materials with different indices of refraction configured to form an optical filter. 
       FIGS. 3A and 3B  illustrate two exemplary arrangements of light emitting pixels  222  and openings  226  from a top view of the sensing region  108 . In  FIG. 3A , each light emitting pixel  222 R,  222 G, or  222 B has approximately the same shape and dimensions, for example, a rounded corner rectangular shape with aspect ratio (height L 1 /width W 1 ) ranging from about 2:1 to about 8:1. The light emitting pixels  222 R,  22 G, and  222 B form an N×M array. Each column of the array includes light emitting pixels with the same color. For example, as illustrated in the  FIG. 3A , the leftmost column includes only red light emitting pixels ( 222 R); the second column to the left includes only green light emitting pixels ( 222 G); the third column to the left includes only blue light emitting pixels ( 222 B); and so on. Such light emitting pixel arrangement is also referred to as an R-G-B stripe arrangement. Other stripe arrangements, such as R-B-G-G-B-R stripe arrangement or R-R-G-G-B-B stripe arrangement, may also be suitable for the display layer  220 . In some embodiments, the display layer  220  may have more than three primaries colors, such as a combination of red, green, blue, and yellow (RGBY) or a combination of red, green, blue, yellow, and cyan (RGBYC). The light emitting pixel arrangement may respectively be R-G-B-Y strip arrangement or R-G-B-Y-C strip arrangement, or any other suitable strip arrangement. In the display layer  220 , there may be substantially the same amount of light emitting pixels under each color. 
     As illustrated in the  FIG. 3A , one or more columns of the openings  226  locate between two adjacent columns of the light emitting pixels  222 . The width D 2  of the opening  226  may be smaller or larger than the width W 1  of the light emitting pixel  222 . In some embodiments, the width D 2  is substantially the same as the width W 1 . Similarly, the length L 2  of the opening  226  may be smaller or larger than the length L 1  of the light emitting pixel  222 . In some embodiments, the length L 2  is substantially the same as the length L 1 . In one example, the opening  226  has substantially the same shape and dimensions with the light emitting pixel  222 . In another example, the opening  226  is about 20% longer than the light emitting pixel  222 , while about 10% narrower. This dimension arrangement is merely illustrative, and is not intended as limiting. Numerous alternative exemplary dimension arrangements exist for the light emitting pixels  222  and the openings  226 . 
     In  FIG. 3B , the display layer  220  has as twice as many green light emitting pixels  222 G as there are blue light emitting pixels  222 B or red light emitting pixels  222 R. The green light emitting pixels  222 G are in round or oval shape and smaller, while the red and blue ones are in diamond shape and larger. In some embodiments, the blue light emitting pixel  222 B is even larger than the red emitting pixel  222 R. Such light emitting pixel arraignment takes advantages of the usually most efficient and longest lasting lifetime of the green light emitting pixels  222 G and balances the usually shortest lifetime of the blue emitting pixels  222 B. The openings  226  are not group in columns but interleaved among the light emitting pixels  222 . The opening  226  may be in a round shape with a diameter D 2 . In some embodiments, the opening  226  is smaller than the green light emitting pixel  222 G. In some embodiments, the opening  226  is larger than the green light emitting pixel  222 G but smaller than the blue light emitting pixel  222 B or the red light emitting pixel  222 R. 
     Referring back to  FIG. 2 , under the sensing region  108 , the light conditioning layer  204  is stacked under the display panel  202 . The light conditioning layer  204  includes a collimator  240  and an optical filtering film  242 . The collimator  240  includes an array of apertures  246 . Each aperture  246  is directly above one or more optical sensing elements  207  in the image sensing layer  206 . The array of apertures  246  is formed by any suitable techniques, such as plasma etching, laser drilling, or the like. The array of apertures  246  conditions incident light reflected from the sensing region  108 . With the image sensing layer  206  stacked at the bottom, the display panel  202 , especially the relative thick cover glass  214 , adds extra vertical distance between the user&#39;s finger  110  and the image sensing layer  206 , which causes stray light from nearby regions of the user&#39;s finger  110  also arrives an optical sensing element  207  together with the light from a small spot directly above. The stray light contributes to image blurring. The array of apertures  246  helps filtering out the stray light and substantially only allows the light from the small spot directly above to be detected, resulting in sharper images. 
     A metric of the collimator  240  is an aspect ratio of the aperture  246 , defined as the height H divided by the diameter d of the aperture  246 . The aspect ratio of the aperture  246  is sufficiently large to allow light rays at normal or near normal incidence to the collimator  240  to pass and reach the optical sensing element  207 . Examples of suitable aspect ratio of the aperture  246  are ranging from about 5:1 to about 50:1 and sometimes ranging from about 10:1 to about 15:1. In an embodiment, the height H of the aperture  246  is in a range from about 30 um to 300 um, such as about 150 um. In various embodiments, the collimator  240  may be an opaque layer with array of holes. In some embodiments, the collimator  240  is a monolithic semiconductor layer, such as a silicon layer. Other examples of the collimator  240  may include plastics such as polycarbonate, PET, polyimide, carbon black, inorganic insulating or metallic materials, or SU-8. 
     Still referring to  FIG. 2 , the light conditioning layer  204  also includes the optical filtering film  242 . The optical filtering film  242  selectively absorbs or reflects certain spectrums of incident light, especially components from the ambient light  250 , such as infrared light and/or a portion of other visible light (e.g., red light). The optical filtering film  242  helps reducing the optical sensing element  207 &#39;s sensitivity to ambient light  250  and increasing its sensitivity to the light emitted from the light emitting pixels  222 . In an example, the optical filtering film  242  may include a thin metal layer or a metal oxide layer that absorbs or reflects light in certain spectrums. In another example, the optical filtering film  242  may include dye(s) and/or pigment(s) that absorb or reflect certain light components. Alternatively, the optical filtering film  242  may include several sub-layers or nano-sized features designed to cause interference with certain wavelengths of incident light. The optical filtering film  242  may be deposited on a dielectric layer  252 . Optionally, there may be a capping layer  254  protecting the optical filtering film  242 . In the illustrated embodiment, the optical filtering film  242  is a continuous film under the collimator  240 . In some embodiments, the optical filtering film  242  is disposed between the display panel  202  and the collimator  240 , such as between the blocking layer  224  and the collimator  240 . In some embodiments, the optical filtering film  242  can be disposed between the display layer  220  and the blocking layer  224 . 
     The ambient light  250 , such as sun light, may include abundant infrared light components, which can penetrate the user&#39;s finger  110  or other objects in the sensing region  108  and arrive the optical sensing elements  207 . In contrast to human eyes, CMOS and CCD image sensors are usually also sensitive to infrared light (including near infrared light). The infrared light penetrating the user&#39;s finger  110  does not carry biometric information, which reduces contrasts of the useful reflected light emitted from the light emitting pixels  222 . Such infrared light can be considered as a source of noise in the pixel image generated by the image sensing layer  206 . Therefore, SNR of the incident light is reduced due to the unwanted infrared light. When ambient light  250  becomes stronger, the infrared light may even saturate the optical sensing elements  207  and SNR may be below a threshold for any meaningful biometric detection. For example, the biometric detection function may fail when the electronic device is under strong sunlight. In some embodiments, the optical filtering film  242  is an infrared light (IR) filter, also known as an infrared light cut-off filter (IRCF), such that infrared light can be substantially blocked, while visible light emitted from the light emitting pixels  222  can transmit through the IRCF filter. 
     The dielectric layer  252  may include a single or multiple material layers. In some embodiments, the dielectric layer  252  includes an anti-reflection (AR) film  256  deposited at the bottom. In some embodiments, the dielectric layer  252  further includes a red light reduction film  258  stacked above the AR film  256 . The red light reduction film  258  weakens intensity of red light components in the incident light. Compared to light in other colors, red light is easier to diffract from the optical sensing elements  207  and cause a secondary red image known as a ghost image. The red light reduction film  258  may help suppressing the ghost image. 
     The stack of the dielectric layer  252 , the optical filtering film  242 , and the capping layer  254  may further have a few openings  260 . The openings  260  allow bondwires  262  to interconnect the bondpads  264  on the top surface of the image sensing layer  206  to external circuits, such as a processor of the electronic device  100 . The bondpads  264  route to control signal lines and power/ground lines embedded in the image sensing layer  206 . The image sensing layer  206  may further include alignment marks  266  on the top surface for alignment control during fabrication and assembly. The sidewall of the capping layer  254  adjacent to the opening  260  may have a notched corner  269 . The reason a notched corner  269  exists will be explained in details later on. 
       FIG. 4  shows a flowchart of a method  400  for capturing a biometric image from an input object illuminated by a display panel integrated with a light conditioning layer, according to examples of the disclosure. The method  400  will be described below with references to the exemplary electronic device  100  illustrated in  FIG. 2 . 
     At block  402 , the method  400  begins with displaying a prompt on the screen. The screen of the electronic device  100  may be in a lock status. The prompt may be an icon, such as a fingerprint icon or an instruction text. The prompt highlights a sensing region  108  on the screen. The prompt is shown by light emitting pixels  222  under the sensing region  208 . The light emitting pixels  222  can be OLED diodes. The light emitting pixels  222  outside of the sensing region  108  may be turned off in the lock status or display preset screen saver images. 
     At block  404 , the method  400  detects an input object shown up in the sensing region  108 , such as the user&#39;s finger  110 . The detection may be implemented by sensing the incident light variation at the optical sensing elements  207 . Alternatively, the display panel  202  may be a touch screen and include touch sensor(s), and the detection may be implemented by the touch sensor(s). In some applications, the user&#39;s finger  110  is not necessary to physically touch the top surface  216  of the display panel  202 . Instead, a near-field imaging can be used for sensing touches detected through a user&#39;s glove or other barriers such as oils, gels, and moisture. When the user&#39;s finger  110  stays steady for more than a predetermined time, such as the user holding a finger steady for about one hundred milliseconds, the method  400  enters a biometric detection mode. Otherwise, the method  400  returns to block  402 , waiting for a new user input. 
     At block  406 , the prompt shown on the screen is turned off and the light emitting pixels  222  under the sensing region  208  start to illuminate the user&#39;s finger  110 . The light  270  emitted from the light emitting pixels  222  travels through the cover glass  214  and arrives at the user&#39;s finger  110 . The user&#39;s finger  110  can include ridges  272  and valleys  274 . The ridges  272  of the finger can reflect more light due to a closer distance to the top surface  216  than the valleys  274 , and the valleys  274  can reflect less light. The light  270  is in turn reflected back towards the light conditioning layer  204 . 
     At block  408 , method  400  filters stray light components in the light  270  at the collimator  240 . With high aspect ratio of the apertures  246 , the collimator  240  only allows light rays reflected from the sensing region  108  at normal or near normal incidence to the collimator  240  to pass and eventually reach the image sensing layer  206 . The optical sensing element  207  can be used to measure the intensity of light and convert the measured intensity into pixel image of the input object, such as the user&#39;s finger  110 . On the other hand, stray light with a larger angle from normal, strike the collimator  240 , either on its top surface or at surface within the apertures  246  (e.g., aperture sidewalls) and are blocked and prevented from reaching the image sensing layer  206  below. The aspect ratio of the apertures  246  is sufficiently large to prevent stray light from traveling through the collimator  240 , such as from about 5:1 to about 50:1. As an example, a light ray reflected from the valley  274  may travel in a large angel to norm direction and arrive at one sensor element directly under the ridge  272  in the absence of the collimator  240 . The image produced by the one sensor element is therefore blurred due to mixing the lights from regions of the ridge  272  and the valley  274 . Such a light ray is referred to as stray light. Larger aspect ratios of the apertures  246  restrict the light acceptance cone to smaller angles, improving the optical resolution of the system. In some embodiments, the apertures  246  are cylindrical or conical in shape. The sidewalls of the apertures  246  may further include grooves or other structures to prevent stray light from reflecting off the walls and reaching the image sensing layer  206  below. 
     At block  410 , the method  400  filters certain spectrums of light at the optical filtering film  242 . In some embodiments, the optical filtering film  242  is an infrared light cut-off filter, which filters (or reduces) infrared light component from the incident light, such as by absorbing or reflecting. The ambient light  250 , such as sunlight, is the major source of infrared light. The infrared light may easily penetrate the user&#39;s finger  110 . Thus the infrared light does not carry useful information of biometric characteristics of the finger and can be considered as part of the noise. Blending the infrared light component from the ambient light with the reflected light from the light emitting pixels reduces the sensitivity of the optical sensing elements  207 . By filtering the infrared light before sensing, SNR of the incident light will be increased. In some other embodiments, the optical filtering film  242  may target light in certain spectrums other than infrared light, for example, red light in the visible spectrum or ultra violet light. The light filtering profile of the optical filtering film  242  may be formulated to give a particular appearance of color, texture, or reflective quality thereby allowing for optimized filtering performance. In some embodiments, the optical filtering film  242  is an infrared light cut-off filter and there is a separate film stacked under or above for filtering red light to reduce ghost image. 
     At block  412 , the method  400  acquires a fingerprint image at the image sensing layer  206 . The optical sensing elements  207  inside the image sensing layer  206  convert the incident light into electrical outputs. Each optical sensing element  207 &#39;s output may correspond to one pixel in the fingerprint image. The optical sensing elements  207  may comprise color image sensors and/or monochromatic image sensors. In some embodiments, each of the optical sensing elements  207  may be configured to correspond with specific light wavelengths, such as a sensor element under a red light emitting pixel  222 R for sensing a red light wavelength, a sensor element under a green light emitting pixel  222 G for sensing a green light wavelength, and a sensor element under a blue light emitting pixel  222 B for sensing a blue light wavelength. 
     At block  414 , the method  400  compares the acquired fingerprint image with an authentic reference image previously stored in a memory. If the fingerprint images match, the method  400  proceeds to block  416  to unlock the screen. The light emitting pixels  222  under the sensing region  108  will stop illumination and join the other light emitting pixels  222  outside of the sensing region  108  to start display regular desktop icons as in an unlock status. If the fingerprint images do not match, the method  400  proceeds back to block  402  to wait for new biometric detection. 
       FIG. 5  is a flowchart of a method  500  of fabricating a workpiece  600  with a biometric sensing system. The workpiece  600  may be substantially similar to the electronic device  100  of  FIG. 2  in many regards. Additional steps can be provided before, during, and after the method  500 , and some of the steps described can be replaced or eliminated for other embodiments of the method  500 . The method  500  is described below in conjunction with  FIGS. 6-14 .  FIGS. 6-14  show cross-sectional views of the workpiece  600  at various stages of the method  500  according to various aspects of the present disclosure. 
     Referring first to block  502  of  FIG. 5  and to  FIG. 6 , a workpiece  600  is received that includes a semiconductor substrate  602 . In various examples, the semiconductor substrate  602  includes an elementary (single element) semiconductor, such as silicon or germanium in a crystalline structure; a compound semiconductor, such as silicon germanium, silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; a non-semiconductor material, such as soda-lime glass, fused silica, fused quartz, and/or calcium fluoride (CaF 2 ); and/or combinations thereof. 
     The workpiece  600  also includes one or more optical sensing elements  207 . In one embodiment, the optical sensing elements  207  may be disposed over the front surface and extended into the semiconductor substrate  602 . The optical sensing elements  207  each may comprise a light-sensing region (or photo-sensing region) which may be a doped region having n-type and/or p-type dopants formed in the semiconductor substrate  602  by a method such as diffusion or ion implantation. The optical sensing elements  207  may include photodiodes, pinned layer photodiodes, non-pinned layer photodiodes, reset transistors, source follower transistors, transfer transistors, select transistors, complimentary metal-oxide-semiconductor (CMOS) image sensors, charged coupling device (CCD) sensors, active pixel sensors, passive pixel sensors, and/or other sensors diffused or otherwise formed in the semiconductor substrate  602 . The optical sensing elements  207  may comprise a plurality of sensor pixels disposed in a sensor array or other proper configuration. The plurality of sensor pixels may be designed having various sensor types. For example, one group of sensor pixels may be CMOS image sensors and another group of sensor pixels may be passive sensors. Moreover, the optical sensing elements  207  may comprise color image sensors and/or monochromatic image sensors. 
     Additional circuitry and input/outputs are typically provided adjacent to the optical sensing elements  207  for providing an operation environment for the optical sensing elements  207  and for supporting external communications with processors. For example, the optical sensing elements  207  may further comprise or be coupled to components such as an electric circuit so that the optical sensing elements  207  are operable to provide a proper response to incident light. A plurality of dielectric layers and a plurality of conductive features including a plurality of metal structures coupled to a plurality of contact and/or via structures may be also formed over the front surface of the substrate. The plurality of metal structures and the plurality of contact/via structures may be formed in an integrated process, such as a damascene process or a dual damascene process, and further, vertical and horizontal features may be formed in various processes, such as photolithography and etching processes. The plurality of metal structures may be routed to one or more bondpads  264 . The bondpads  264  may be disposed on the front surface of the semiconductor substrate  602 . The bondpads  264  provide landing areas for bondwires, which provide external connections to the plurality of metal structures, such as connections of power supply, ground, controls, and data lines. The workpiece  600  may further include other features, such as alignment marks  266 . The alignment marks  266  may be disposed on the top surface of the semiconductor substrate  602 . Alignment marks and the alignment of subsequent lithographic patterns with respect to those marks are an important part of the semiconductor manufacturing process. Alignment of one pattern layer to previous layers is typically done with the assistance of special alignment patterns designed on a previous mask layer. When these special patterns are aligned, it is assumed that the remainder of the circuit patterns is also correctly aligned. 
     Referring to block  504  of  FIG. 5  and to  FIG. 7 , the method  500  deposits a dielectric layer  252  covering the workpiece  600 . The dielectric layer  252  may include material composition such as silicon oxide, silicon nitride, silicon oxynitride, and/or silicon carbide. In an exemplary embodiment, the dielectric layer  252  includes silicon oxide. The dielectric layer  252  may be formed to any suitable thickness and by any suitable process including thermal growth, chemical vapor deposition (CVD), high-density plasma CVD (HDP-CVD), physical vapor deposition (PVD), atomic-layer deposition (ALD), and/or other suitable deposition processes. The dielectric layer  252  may be uniform in composition or may include various layers, such as a red light reduction film and/or an anti-reflection (AR) film at the bottom. The thickness of the dielectric layer  252  may be in a range of about 1 um to about 10 um, such as about 2 um. 
     Referring to block  506  of  FIG. 5  and to  FIG. 8 , the method  500  deposits an optical filtering film  242  covering the workpiece  600 . The optical filtering film  242  selectively absorbs or reflects certain spectrums of incident light. In the illustrated embodiment, the optical filtering film  242  filters infrared light. The optical filtering film  242  may be uniform in composition or may include various layers. The optical filtering film  242  may include metal, metal oxide, dyes, and/or pigments. In one embodiment, the optical filtering film  242  includes Tatanlum dioxide. In another embodiment, the optical filtering film  242  includes silicon nitride. The optical filtering film  242  may be formed to any suitable thickness and by any suitable process including chemical vapor deposition (CVD), high-density plasma CVD (HDP-CVD), physical vapor deposition (PVD), atomic-layer deposition (ALD), and/or other suitable deposition processes. The thickness of the optical filtering film  242  may be in a range of about 2 um to about 8 um, such as about 4 um. 
     Referring to block  508  of  FIG. 5  and to  FIG. 9 , the method  500  may optionally deposits a capping layer  254  covering the workpiece  600 . The capping layer  254  protects the optical filtering film  242  underneath from subsequent fabrication processes. The capping layer  254  may include a dielectric such as a silicon oxide, a silicon nitride, a silicon oxynitride, and/or a silicon carbide. In various embodiments, the capping layer  254  and the dielectric layer  252  may include same or different material compositions. In an exemplary embodiment, the capping layer  254  includes silicon nitride and the dielectric layer  252  includes silicon oxide. The capping layer  254  may be formed to any suitable thickness and by any suitable process including thermal growth, chemical vapor deposition (CVD), high-density plasma CVD (HDP-CVD), physical vapor deposition (PVD), atomic-layer deposition (ALD), and/or other suitable deposition processes. The thickness of the capping layer  254  may be in a range of about 0.1 um to about 2 um, such as about 0.5 um. 
     Referring to block  512  of  FIG. 5  and to  FIG. 10 , the method  500  forms trenches  260  directly above the bondpads  264 . The bondpads  264  are not exposed in the trenches  260 , as a layer of the dielectric layer  252  at the bottom of the trenches  260  remains covering the bondpads  264 , which protects the bondpads  264  from subsequent fabrication processes. Block  512  may include a variety of processes such as photolithography and etching to form the trenches  260 . The photolithography process may include forming a photoresist (not shown) over the workpiece  600 . An exemplary photoresist includes a photosensitive material sensitive to radiation such as UV light, deep ultraviolet (DUV) radiation, and/or EUV radiation. A lithographic exposure is performed on the workpiece  600  that exposes selected regions of the photoresist to radiation. The exposure causes a chemical reaction to occur in the exposed regions of the photoresist. After exposure, a developer is applied to the photoresist. The developer dissolves or otherwise removes either the exposed regions in the case of a positive resist development process or the unexposed regions in the case of a negative resist development process. After the photoresist is developed, the exposed portions of the workpiece  600  may be removed by an etching process, such as wet etching, dry etching, Reactive Ion Etching (RIE), ashing, and/or other etching methods. After etching, the photoresist may be removed. In some embodiments, inside the trench  260 , the etching process removes the capping layer  254 , the optical filtering film  242 , and upper portions of the dielectric layer  252 . The depth H t  of the trench  260  may be in a ratio of the total thickness Hd of the capping layer  254 , the optical filtering film  242 , and the dielectric layer  252 , ranging from about 40% to about 90%, such as about 50% in one example. In some embodiments, the material compositions of the dielectric layer  252  are different from the capping layer  254  and the optical filtering film  242 . By selecting an etchant or etchants that target material compositions of the capping layer  254  and the optical filtering film  242  while resist etching of the dielectric layer  252 , the dielectric layer  252  functions as an etching stop layer and substantially remains covering the bondpads  264 . 
     Referring to block  514  of  FIG. 5  and to  FIG. 11 , the method  500  stacks a semiconductor substrate  1202  above the capping layer  254 . In some embodiments, the two semiconductor substrates  602  and  1202  are silicon substrates. However, the disclosed structure and the method are not limiting and are extendable to other suitable semiconductor substrates and other suitable crystal orientations. For examples, either of the semiconductor substrates  602  and  1202  may include an elementary semiconductor, such as germanium; a compound semiconductor, such as silicon germanium, silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; or combinations thereof, in the same or different crystalline structures. For example, the semiconductor substrate  602  is a silicon germanium substrate and the semiconductor substrate  1202  is a silicon substrate. 
     In the illustrated embodiment, the stacking of the semiconductor substrate  1202  is implemented by bonding a semiconductor wafer (or die) to the capping layer  254  through a proper bonding technology, such as direct bonding, eutectic bonding, fusion bonding, diffusion bonding, anodic bonding or other suitable bonding methods. In one embodiment, the material layers are bonded together by direct silicon bonding (DSB). For example, the direct silicon bonding process may include preprocessing, pre-bonding at a lower temperature and annealing at a higher temperature. A buried silicon oxide layer (BOX) may be implemented when the two substrates are bonded together. In some examples, the semiconductor substrate  1202  may be thinned down, such as by grinding or polishing, to proper thicknesses after the bonding, such as thinning from about 700 um down to about 150 um. 
     Referring to block  516  of  FIG. 5  and to  FIG. 12 , the method  500  patterns the bonded semiconductor substrate  1202  with a plurality of apertures  246 , thereby forming the collimator  240 . The top surface of the capping layer  254  is partially exposed in the apertures  246 . Examples of suitable aspect ratio of the apertures  246  are in a range from about 5:1 to about 50:1 and sometimes in a range from about 10:1 to about 15:1. The patterning process may also remove portions of the bonded semiconductor substrate  1202  that covers the trenches  260 . The patterning process may be an etching process that includes any suitable etching technique such as wet etching, dry etching, RIE, ashing, and/or other etching methods. For example, a dry etching process may implement an oxygen-containing gas, a fluorine-containing gas (e.g., CF 4 , SF 6 , CH 2 F 2 , CHF 3 , and/or C 2 F 6 ), a chlorine-containing gas (e.g., Cl 2 , CHCl 3 , CCl 4 , and/or BCl 3 ), a bromine-containing gas (e.g., HBr and/or CHBR 3 ), an iodine-containing gas, other suitable gases and/or plasmas, and/or combinations thereof. For example, a wet etching process may comprise etching in diluted hydrofluoric acid (DHF); potassium hydroxide (KOH) solution; ammonia; a solution containing hydrofluoric acid (HF), nitric acid (HNO 3 ), and/or acetic acid (CH 3 COOH); or other suitable wet etchant. In the illustrated embodiment, the apertures  246  are formed by a plasma etching process. The remaining dielectric layer  252  in the trench  260  protects the bondpads  264  and other electronic components underneath from damages during the plasma bombardment. 
     Referring to block  518  of  FIG. 5  and to  FIG. 13 , the method  500  removes the dielectric layer  252  from the bottom of the trenches  260  and exposes the bondpads  264 . The portions of the dielectric layer  252  may be removed by an etching process, such as wet etching, dry etching, Reactive Ion Etching (RIE), ashing, and/or other etching methods. Due to the etching selectivity of the selected etchant, the capping layer  254  may also have top surface etch loss during the etching process, resulting in the notched corner  269  adjacent to the collimator  240 . The portions of the capping layer  254  exposed in the apertures  246  may also have some but relatively less top surface etch loss and can be regarded as substantially remaining during the etching process, which is due to the high aspect ratio of the apertures  226  and corresponding loading effects of the etchant. 
     Referring to block  520  of  FIG. 5  and to  FIG. 14 , the method  500  may proceed to further processes in order to complete the fabrication of the workpiece  600 . For example, the method  500  may bond the bondpads  264  with conductive features, such as bondwires  262 . The bondwires  262  extend through the openings  260  and routes internal routings in the semiconductor substrate  602  to external control signals, data lines, and power lines. The method  500  may also assemble the display panel  202  above the collimator  240 . The display panel  202  may include the cover glass  214 , the display layer  220 , and/or the blocking layer  224 . 
     Although not intended to be limiting, one or more embodiments of the present disclosure provide many benefits to a biometric sensing system, such as a fingerprint recognition system in a consumer (or portable) electronic devices. For example, optical signals are enhanced with higher resolution and lower noise interferences from stray light and ambient light. Fingerprint image with enhanced signal-to-noise ratio (SNR) can be acquired at image sensors. Further, the disclosed methods can be easily integrated into existing semiconductor manufacturing processes. 
     In one exemplary aspect, the present disclosure is directed to a sensing apparatus. In an embodiment, the sensing apparatus includes an image sensor; a collimator above the image sensor, the collimator having an array of apertures; an optical filtering layer between the collimator and the image sensor, wherein the optical filtering layer is configured to filter a portion of light transmitted through the array of apertures; and an illumination layer above the collimator. In an embodiment, the portion of light is infrared light. In an embodiment, the optical filtering layer includes metal oxide. In an embodiment, the optical filtering layer extends continuously directly under the collimator and has an opening outside of the collimator. In an embodiment, the sensing apparatus further includes a conductive feature coupled to the image sensor, the conductive feature extending through the opening. In an embodiment, the collimator is formed by bonding a wafer substrate above the optical filtering layer. In an embodiment, a ratio of a height of an aperture of the array of apertures to a diameter of the aperture is within a range of 10:1 to 15:1. In an embodiment, the illumination layer includes a plurality of light emitting pixels, a portion of the plurality of light emitting pixels is configured to illuminate an object placed above the illumination layer. In an embodiment, the array of apertures has a first pitch; the plurality of light emitting pixels having a second pitch; and the first pitch is equal to or smaller than the second pitch. In an embodiment, the illumination layer is an organic light emitting diodes (OLED) display. In an embodiment, the sensing apparatus further includes a blocking layer between the illumination layer and the collimator, wherein the blocking layer has a plurality of openings under the portion of the plurality of light emitting pixels, the opening allowing light reflected from the object to pass through. 
     In another exemplary aspect, the present disclosure is directed to a device. In an embodiment, the device includes a touch display panel; a light conditioning layer under the touch display panel, the light conditioning layer includes a collimator and an infrared light filter; and an image sensing layer under the light conditioning layer, the image sensing layer is configured to sense light emitted from the touch display panel. In an embodiment, a portion of the touch display panel is configured as a fingerprint sensing region. In an embodiment, the touch display panel includes a plurality of organic light emitting diodes (OLED). In an embodiment, the infrared light filter is between the touch display panel and the collimator. In an embodiment, the infrared light filter is between the collimator and the image sensing layer. 
     In yet another exemplary aspect, the present disclosure is directed to a method of fabricating a sensing apparatus. In an embodiment, the method includes providing a substrate, the substrate including one or more image sensors; depositing an infrared light filtering film above the substrate; bonding a semiconductor layer above the infrared light filtering film; and etching the semiconductor layer to form a plurality of apertures. In an embodiment, the etching of the semiconductor layer includes plasma etching. In an embodiment, the method further includes thinning the semiconductor layer, prior to the etching of the semiconductor layer. In an embodiment, the method further includes forming an opening in the infrared light filtering film, wherein the opening is directly above a bondpad on a top surface of the substrate; and bonding a conductive feature to the bondpad, the conductive feature extending through the opening. 
     The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. 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.