Patent Publication Number: US-2022238733-A1

Title: Sensor packages with wavelength-specific light filters

Description:
BACKGROUND 
     Electrical circuits are formed on semiconductor dies and subsequently packaged inside mold compounds to protect the circuits from damage due to elements external to the package, such as moisture, heat, and blunt force. To facilitate communication with electronics external to the package, an electrical circuit within the package is electrically coupled to conductive terminals. These conductive terminals are positioned inside the package but are exposed to one or more external surfaces of the package. By coupling the conductive terminals to electronics external to the package, a pathway is formed to exchange electrical signals between the electrical circuit within the package and the electronics external to the package via the conductive terminals. 
     SUMMARY 
     In examples, a sensor package comprises a die pad and a semiconductor die on the die pad. The semiconductor die has an active surface. The sensor package includes a light sensor on the active surface of the semiconductor die. The sensor package includes a mold compound covering the die pad, the semiconductor die, the conductive terminal, and a portion of the active surface. The sensor package includes a light filter covering the light sensor and abutting the mold compound. The light filter includes a combination of silicone, metal particles, and an organic dye. The combination is configured to reject light having a wavelength in a target wavelength range. The light filter has a minimum thickness of 0.5 millimeters. 
     In examples, a method comprises coupling a semiconductor die to a die pad, the semiconductor die having a light sensor. The method includes positioning the die pad and the semiconductor die in a mold chase. The method includes lowering a mold chase member having a dam toward the semiconductor die such that the dam covers the light sensor. The method includes injecting a mold compound into the mold chase to cover the die pad and the semiconductor die with the mold compound, the dam configured to prevent the mold compound from covering the light sensor. The method includes depositing a light filter solution including a combination of silicone, an epoxy including metal particles, and an organic dye onto the light sensor. The combination is configured to reject light having a wavelength in a target wavelength range. The method includes curing the light filter solution to form a light filter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A, 1B and 1C  are profile cross-sectional, top-down, and perspective views, respectively, of a sensor package having a wavelength-specific light filter, in accordance with various examples. 
         FIGS. 2A, 2B and 2C  are profile cross-sectional, top-down, and perspective views, respectively, of a sensor package having a wavelength-specific light filter, in accordance with various examples. 
         FIGS. 3A, 3B and 3C  are profile cross-sectional, top-down, and perspective views, respectively, of a sensor package having a wavelength-specific light filter, in accordance with various examples. 
         FIGS. 4A-4F  are a process flow for manufacturing a sensor package having a wavelength-specific light filter, in accordance with various examples. 
         FIG. 5  is a flow diagram of a method for manufacturing a sensor package having a wavelength-specific light filter, in accordance with various examples. 
         FIG. 6  is a block diagram of an electronic device including a sensor package having a wavelength-specific light filter, in accordance with various examples. 
     
    
    
     DETAILED DESCRIPTION 
     Some packages include sensors that are positioned on or in the semiconductor die. These sensors are configured to sense various properties in the environment, such as light, humidity, temperature, sound, potential of hydrogen (pH), and others. To sense many such properties, the sensor may be exposed to the environment, with the remainder of the package covered by an opaque mold compound. For example, to sense the pH of a liquid, the sensor may come into direct contact with the liquid. However, light sensors are not directly exposed to the environment. Instead, a transparent or translucent material may cover light sensors. This transparent or translucent material permits light to reach the sensor but simultaneously protects the sensor from damage. 
     In some light sensor applications, it is useful to filter (reject) specific wavelengths of light so such light does not reach the light sensor. For example, in some cases, a transparent mold compound containing a wavelength-specific light rejection dye is used in lieu of an opaque mold compound to cover the semiconductor die, the light sensor, and other package components. While somewhat effective in filtering specific wavelengths of light, such transparent mold compounds are expensive because they entail covering all package components, rather than just the light sensor, with the transparent mold compound. In other cases, a light filter film is used to cover the light sensor. Like the transparent mold compound, this light filter film is configured to reject specific wavelengths of light. Unlike the transparent mold compound, the light filter film does not cover all package components. However, the light filter film is still expensive because of the manner in which it is applied. Specifically, the film is applied in liquid form to a wafer at the wafer stage of the package manufacturing process. The wafer is then spun to produce an even film coating. Portions of the film not covering the light sensor are then removed. This waste results in an inefficient and expensive application process. 
     This description presents various examples of a sensor package with a wavelength-specific light filter. The sensor package includes an opaque mold compound that covers various components of the sensor package, such as the semiconductor die. The mold compound includes a cavity, and a light sensor formed on or in the semiconductor die is exposed to this cavity. The cavity includes a transparent or translucent light filter covering the light sensor, abutting the opaque mold compound, and having a thickness of at least 0.5 millimeters. The light filter includes a combination of silicone, epoxy, metal particles, and an organic dye. This combination of materials is specifically configured to reject light having a wavelength in a target wavelength range, such as the infrared light wavelength range. The specific wavelengths filtered may be controlled by adjusting the thickness of the light filter and/or the material composition of the light filter. The light filter is deposited as a solution (e.g., using a syringe) into the cavity after the mold compound has been injected and cured, and the light filter solution is then cured. Because the light filter is applied in the area of the light sensor rather than the entire semiconductor die or the entire package, costs are mitigated relative to other solutions. 
       FIGS. 1A, 1B and 1C  are profile cross-sectional, top-down, and perspective views, respectively, of a sensor package having a wavelength-specific light filter, in accordance with various examples. Specifically,  FIG. 1A  is a profile cross-sectional view of an example sensor package  100 .  FIG. 1A  shows the sensor package  100  as a wire bonded quad flat no-lead (QFN) style package, but other types of packages, such as dual in-line packages (DIP), packages with gullwing-style leads, etc., also fall within the scope of this description. The sensor package  100  includes a die pad  102  and conductive terminals  104 . The sensor package  100  includes a semiconductor die  106  (e.g., a silicon die) coupled to the die pad  102 . The semiconductor die  106  includes an active surface  107  on and/or in which various circuits and/or circuit components may be formed. Bond wires  108  couple to the active surface  107  and to the conductive terminals  104 . 
     The sensor package  100  includes a light sensor  110  on and/or in the active surface  107 . The light sensor  110  may be any suitable type of light sensor for a given application. For example, a smartphone containing the sensor package  100  may include a light sensor  110  that is suitable for capturing ambient light that is useful to other circuitry in the smartphone to adjust output settings on a display. An opaque mold compound  112  partially or fully covers the die pad  102 , the conductive terminals  104 , the semiconductor die  106 , the active surface  107 , and the bond wires  108 . The mold compound  112  includes a cavity  114  that may be partially hollow and partially filled or that may be fully filled, as described below. The dimensions of the cavity  114  may be chosen for a specific application, with wider and/or shallower cavities permitting light from a wider angle of view to reach the light sensor  110  and narrower and/or deeper cavities permitting light from a narrower angle of view to reach the light sensor  110 . The depth of the cavity  114  may be measured from a top surface  116  of the mold compound  112  to the active surface  107 , and the width of the cavity  114  may be measured as numeral  118  indicates. The light sensor  110  may be exposed to the cavity  114 . In examples, the term exposed to means that, other than the light filter  120  (described below), no other components or materials are positioned between the light sensor  110  and the cavity  114 . 
     The sensor package  100  includes a light filter  120 . The light filter  120  includes a combination of silicone, an epoxy base material with metal particles, and an organic dye. The specific types, quantities, and/or proportions of silicone, epoxy base, metal particles, and organic dyes used are collectively referred to herein as the material composition of the light filter  120 . This material composition may vary and may be selected to achieve specific properties of the light filter  120 , such as the physical properties described below, as well as the ability to filter specific target wavelength ranges. Examples of the silicone material include phenyl resin silicone and methyl silicone. Examples of the epoxy base material include biphenyl and bisphenol epoxy. Examples of the metal particles include aluminum oxide and titanium oxide. Examples of the organic dyes include squalilium and phthalocyanine. In examples, the light filter  120  is translucent or transparent. 
     In some examples, the light filter  120  fills the entire cavity  114 , and in other examples, the light filter  120  fills part, but not all, of the cavity  114 . In some examples, the light filter  120  has a minimum thickness of 0.5 millimeters. A thinner light filter  120  may not adequately filter light in the target wavelength range, although it is possible to adjust the material composition of a light filter  120  thinner than 0.5 millimeters as described below to increase filtering efficacy. A thickness of 0.5 millimeters is not possible in other solutions using light filter films because the light filter film is applied to a wafer and the wafer is subsequently spun, resulting in a light filter film that is consistently below 0.5 millimeters in thickness. Although various thicknesses may be possible in transparent mold compound solutions, these solutions are costly, as described above. In contrast to these other solutions, the light filter  120  of the sensor package  100  covers only an area in the vicinity of the light sensor  110  (e.g., sufficient to permit adequate light from a target angle of view to reach the light sensor  110 ) and is deposited into the cavity  114  after the remainder of the sensor package  100  has been assembled, resulting in an inexpensive solution that can reach thicknesses of 2.0 millimeters or more. In some examples, a thickness exceeding 5.0 millimeters may preclude adequate light from reaching the light sensor  110 . 
     In some examples, the material composition of the light filter  120  may be adjusted to produce a light filter  120  having specific properties, some of which may be realized prior to curing, and others which may be realized post-curing. Some of these properties are now described. For example, the light filter  120  may have a viscosity ranging from 1.0 Pascal-seconds to 1.5 Pascal-seconds at 23 degrees Celsius prior to curing. A more viscous solution may produce a light filter  120  with flaws, such as uneven thicknesses in the light filter  120 . A less viscous solution may cause patterning and contamination due to the spread of ink. In some examples, the light filter  120  may have a density ranging from 1.1 grams/cm 3  to 1.3 grams/cm 3 . In some examples, the light filter  120  may have a hardness ranging between 80 and 90 on the type D Shore hardness scale, with a hardness less than 80 possibly resulting in cosmetic scratches on the package and possibly causing internal damage to the silicon die, and with a hardness greater than 90 being difficult to realize due to the combination of materials used to form the light filter  120 . In some examples, the light filter  120  may have a flexural strength ranging from 50 to 60 Mega Pascals, with a flexural strength below this range possibly resulting in fractures in the package, and a flexural strength above this range being difficult to realize due to the combination of materials used to form the light filter  120 . In some examples, the light filter  120  may have a flexural modulus ranging from 1400 to 1500 Mega Pascals, with a flexural modulus below this range possibly resulting in internal damage to the silicon die, and a flexural modulus above this range possibly resulting in fractures to the package responsive to the package being under mechanical stress. 
     In some examples, the light filter  120  may have a glass transition temperature ranging from 160 to 180 degrees Celsius, with a glass transition temperature lower than this range possibly resulting in deformation under high temperatures, and a glass transition temperature higher than this range being difficult to realize due to the combination of materials used to form the light filter  120 . In some examples, if a mismatch in coefficients of thermal expansion exists between the silicon die and other package materials such as the mold compound, cracks may result in high or low temperature conditions. In some examples, the light filter  120  may have a volume resistivity ranging from 80 to 90 tera Ohm-meters, with a volume resistivity below this range possibly resulting in breakdown due to electrostatic discharge, and a volume resistivity above this range being difficult to realize due to the combination of materials used to form the light filter  120 . In some examples, the light filter  120  has a moisture permeation ranging from 3.0 to 5.0 grams/m 2 -days, with a moisture permeation below this range being difficult to realize due to the combination of materials used to form the light filter  120 , and a moisture permeation above this range possibly resulting in oxidation and damage to metals inside the package, such as to bond wires. 
     The material composition of the light filter  120  may be specifically configured to filter light having specific target wavelength ranges. For example, the light filter  120  may be configured to reject some or all infrared light. In some examples, the light filter  120  is configured to reject light in a specific target wavelength range that is a contiguous portion of the light spectrum, and in other examples, the light filter  120  is configured to reject light in a specific target wavelength range that is divided into two or more non-contiguous portions of the light spectrum. 
       FIG. 1B  is a top-down view of the sensor package  100 . In examples, the light sensor  110  has a rectangular shape in the horizontal plane. In examples, the cavity  114  has a rectangular shape in the horizontal plane. As  FIG. 1B  shows, the light filter  120  is transparent, making the light sensor  110  and the active surface  107  visible in this top-down view.  FIG. 1C  is a perspective view of the sensor package  100 . 
       FIGS. 2A, 2B and 2C  are profile cross-sectional, top-down, and perspective views, respectively, of a sensor package having a wavelength-specific light filter, in accordance with various examples. Specifically,  FIG. 2A  is a profile cross-sectional view of an example sensor package  200 . The sensor package  200  may be a QFN package, although other package types are included in the scope of this description. The sensor package  200  includes multiple conductive terminals  202 . The sensor package  200  includes a semiconductor die  204  having an active surface  206  on and/or in which various circuits and/or circuit components are formed. Conductive components  208  (e.g., solder balls) couple to the active surface  206  and to the conductive terminals  202 . The semiconductor die  204  includes an inactive surface  209 . The active surface  206  faces the conductive terminals  202 , and the inactive surface  209  faces away from the conductive terminals  202 . The inactive surface  209  includes a light sensor  210 . The semiconductor die  204  includes a conductive via  212  that extends through the thickness (e.g., the body) of the semiconductor die  204  and that is coupled to the light sensor  210  and to the active surface  206  (e.g., to a processor, microcontroller, or other suitable circuitry on the active surface  206  that can route, receive, and/or process signals from the light sensor  210 ). An opaque mold compound  214  covers some or all of the conductive terminals  202 , the semiconductor die  204 , and the conductive components  208 . A cavity  216  is formed in the mold compound  214 . The properties of the cavity  216  may be similar to those of the cavity  114  described above, and thus they are not repeated here. A light filter  218  fills some or all of the cavity  216 . The properties of the light filter  218  may be similar to those of the light filter  120  described above, and thus they are not repeated here.  FIG. 2B  is a top-down view of the sensor package  200 , and  FIG. 2C  is a perspective view of the sensor package  200 . 
       FIGS. 3A, 3B and 3C  are profile cross-sectional, top-down, and perspective views, respectively, of a sensor package having a wavelength-specific light filter, in accordance with various examples. Specifically,  FIG. 3A  is a profile cross-sectional view of an example sensor package  300 . The sensor package  300  may be a QFN package, a DIP, or any other suitable type of package. The sensor package  300  includes a die pad  302  and conductive terminals  304 . The sensor package  300  includes a semiconductor die  306  coupled to the die pad  302  and having an active surface  308 . The active surface  308  includes circuits and/or circuit components formed in and/or on the active surface  308 . Bond wires  310  are coupled to the active surface  308  and to the conductive terminals  304 . The active surface  308  includes multiple light sensors  312 ,  314 ,  316 , and  318 . In some examples, each of the light sensors  312 ,  314 ,  316 , and  318  may be similar to the light sensors  110 ,  210  described above. An opaque mold compound  320  covers some or all of the die pad  302 , the conductive terminals  304 , the semiconductor die  306 , and the bond wires  310 . The mold compound  320  includes cavities  322 ,  324 ,  326 , and  328 . Each of the light sensors  312 ,  314 ,  316 , and  318  is exposed to a different one of the cavities  322 ,  324 ,  326 , and  328 , respectively. Each of the light sensors  312 ,  314 ,  316 , and  318  is partially or fully filled with a different light filter  330 ,  332 ,  334 , and  336 , respectively. 
     Each of the light filters  330 ,  332 ,  334 , and  336  has a different material composition and/or thickness that affects the wavelengths of light that are filtered. Specifically, each of the light filters  330 ,  332 ,  334 , and  336  includes a different combination of silicone, epoxy base with metal particles, and organic dye and/or has a different thickness, such that each of the light filters  330 ,  332 ,  334 , and  336  rejects light having a different target wavelength range (such different ranges may overlap with each other). For example, the material composition and/or thickness of the light filter  330  may be set so the light filter  330  is configured to reject all light (including visible and invisible light, such as infrared light) except red light. For example, the material composition and/or thickness of the light filter  332  may be set so the light filter  332  is configured to reject all light (including visible and invisible light, such as infrared light) except green light. For example, the material composition and/or thickness of the light filter  334  may be set so the light filter  334  is configured to reject all light (including visible and invisible light, such as infrared light) except blue light. For example, the material composition and/or thickness of the light filter  336  may be set so the light filter  336  is configured to reject all light (including visible and invisible light, such as infrared light) except indigo light. By permitting only light with differing wavelengths to enter into each of the cavities  322 ,  324 ,  326 , and  328  and to reach the corresponding light sensors  312 ,  314 ,  316 , and  318 , the information obtained by each of the light sensors  312 ,  314 ,  316 , and  318  is more color-specific. This increased granularity in color-specific information may enable an electronic device containing the sensor package  300  to perform operations that it would otherwise be unable to perform. 
     In examples, the thickness of each of the light filters  330 ,  332 ,  334 , and  336  is at least 0.5 millimeters, as described above with reference to the light filters  120 ,  218 . The cavities  322 ,  324 ,  326 , and  328  have properties similar to the properties of the cavity  114  as described above, and thus these properties are not described again here. Each of the light filters  330 ,  332 ,  334 , and  336  may have physical properties similar to the properties of the light filter  120  as described above, and thus such properties are not described again here, with the exception that the specific material compositions of the light filters  330 ,  332 ,  334 , and  336  may vary so different wavelengths of light are rejected.  FIG. 3B  is a top-down view of the sensor package  300 , and  FIG. 3C  is a perspective view of the sensor package  300 . 
       FIGS. 4A-4F  are a process flow for manufacturing a sensor package having a wavelength-specific light filter, in accordance with various examples. The process flow is presented in the context of the example sensor package  100  described above, but the process flow may be adapted to manufacture any sensor package described herein, as well as variations of the sensor packages described herein.  FIG. 5  is a flow diagram of a method  500  for manufacturing a sensor package having a wavelength-specific light filter, in accordance with various examples. The method  500  is now described in parallel with the process flow of  FIGS. 4A-4F . The method  500  includes obtaining a die pad and a conductive terminal ( 502 ).  FIG. 4A  is a profile cross-sectional view of the die pad  102  and multiple conductive terminals  104 . The method  500  includes coupling a semiconductor die to the die pad, the semiconductor die having a light sensor ( 504 ).  FIG. 4B  is a profile cross-sectional view of the semiconductor die  106  having a light sensor  110  and coupled to the die pad  102 . For example, a die attach layer (not expressly shown) is useful to couple the semiconductor die  106  to the die pad  102 . The method  500  includes coupling a bond wire to the semiconductor die and to the conductive terminal ( 506 ).  FIG. 4C  is a profile cross-sectional view of the bond wires  108  coupled to the conductive terminals  104  and to the semiconductor die  106 . 
     The method  500  includes positioning the die pad, the conductive terminal, the semiconductor die, and the bond wire in a mold chase ( 508 ). The method  500  also includes lowering a mold chase member having a dam toward the semiconductor die such that the dam covers the light sensor ( 510 ).  FIG. 4D  is a profile cross-sectional view of the structure of  FIG. 4C  positioned inside a mold chase having a mold chase bottom member  400  and a mold chase top member  402 . The mold chase top member  402  includes a dam  404 , which, responsive to the mold chase being closed, extends to and contacts the light sensor  110  at the active surface  107  of the semiconductor die  106 . 
     The method  500  includes injecting a mold compound into the mold chase to cover the die pad, the conductive terminal, the semiconductor die, and the bond wire(s) with the mold compound ( 512 ). The dam is configured to prevent the mold compound from covering the light sensor ( 512 ).  FIG. 4E  is a profile cross-sectional view of a mold compound  112  covering some or all of the die pad  102 , the conductive terminals  104 , the semiconductor die  106 , and the bond wires  108 . The dam  404  prevented mold compound  112  from flowing over and covering the light sensor  110  and areas of the active surface  107  circumscribing the light sensor  110 . Consequently, the cavity  114  is formed. Cavity  114  has dimensions commensurate with the dimensions of the dam  404 . Thus, the dimensions of the cavity  114  may be manipulated by adjusting the dimensions of the dam  404 . The light sensor  110  is exposed to the cavity  114 . 
     The method  500  includes depositing a light filter solution including a combination of silicone, an epoxy base including metal particles, and an organic dye onto the light sensor ( 514 ). The combination is configured to reject light having a wavelength in a target wavelength range ( 514 ). The method  500  also includes curing the light filter solution to form a light filter ( 516 ).  FIG. 4F  is a profile cross-sectional view of the structure of  FIG. 4E  with the light filter  120  having been deposited in liquid form (e.g., by syringe deposition) in the cavity  114  and cured. In examples, the liquid light filter solution may be cured by applying heat in the range of 150-200 degrees Celsius for a time period ranging from 30-60 minutes. 
       FIG. 6  is a block diagram of an electronic device  600  including a sensor package having a wavelength-specific light filter, in accordance with various examples. The electronic device  600  may be a smartphone, a tablet, a laptop computer, a desktop computer, a camera, a webcam, or any other suitable device that may have use for ambient light data. The electronic device  600  includes a system board  602 . Circuitry  604  is mounted on the system board  602  and may include memory (e.g., including executable code), a processor, a microcontroller, a graphics processor, a sound card, connections (e.g., copper traces) between such components, etc. A sensor package  606  may be mounted on the system board  602  and coupled to the circuitry  604 . The sensor package  606  is representative of the example sensor packages  100 ,  200 , and  300  described herein. 
     In this description, the term “couple” may cover connections, communications, or signal paths that enable a functional relationship consistent with this description. For example, if device A generates a signal to control device B to perform an action, then: (a) in a first example, device A is directly coupled to device B; or (b) in a second example, device A is indirectly coupled to device B through intervening component C, if intervening component C does not substantially alter the functional relationship between device A and device B, so device B is controlled by device A via the control signal generated by device A. 
     A device that is “configured to” perform a task or function may be configured (e.g., programmed and/or hardwired) at a time of manufacturing by a manufacturer to perform the function and/or may be configurable (or reconfigurable) by a user after manufacturing to perform the function and/or other additional or alternative functions. The configuring may be through firmware and/or software programming of the device, through a construction and/or layout of hardware components and interconnections of the device, or a combination thereof. 
     A circuit or device that is described herein as including certain components may instead be adapted to be coupled to those components to form the described circuitry or device. For example, a structure described as including one or more semiconductor elements (such as transistors), one or more passive elements (such as resistors, capacitors, and/or inductors), and/or one or more sources (such as voltage and/or current sources) may instead include only the semiconductor elements within a single physical device (e.g., a semiconductor die and/or integrated circuit (IC) package) and may be adapted to be coupled to at least some of the passive elements and/or the sources to form the described structure either at a time of manufacture or after a time of manufacture by an end-user and/or a third-party. 
     While certain components may be described herein as being of a particular process technology, these components may be exchanged for components of other process technologies. Circuits described herein are reconfigurable to include the replaced components to provide functionality at least partially similar to functionality available prior to the component replacement. Components shown as resistors, unless otherwise stated, are generally representative of any one or more elements coupled in series and/or parallel to provide an amount of impedance represented by the shown resistor. For example, a resistor or capacitor shown and described herein as a single component may instead be multiple resistors or capacitor, respectively, coupled in parallel between the same nodes. For example, a resistor or capacitor shown and described herein as a single component may instead be multiple resistors or capacitor, respectively, coupled in series between the same two nodes as the single resistor or capacitor. Unless otherwise stated, “about,” “approximately,” or “substantially” preceding a value means+/−10 percent of the stated value. Modifications are possible in the described examples, and other examples are possible within the scope of the claims.