Patent Publication Number: US-11393865-B1

Title: Optical receiver systems and devices with detector array comprising a plurality of substrates aligned with an encapsulation layer comprising an alignment structure

Description:
CROSS-REFERENCE TO RELATED DISCLOSURES 
     This application is a continuation of and claims priority to U.S. patent application Ser. No. 15/294,335, titled “Receiver Array Packaging,” filed Oct. 14, 2016, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. 
     Photodetectors can detect light within a specific spectral band or at one or more wavelengths of light. An array of photodetectors may be used to image a field of view of a scene. Some photodetector arrays are one-dimensional (1×N) or two-dimensional (A×B). Some photodetector arrays are constrained due to substrate and package size limitations. 
     SUMMARY 
     The present disclosure generally relates to an optical receiver system with a detector array that spans multiple substrates. Each portion of the detector array may have a respective read-out integrated circuit and/or other circuitry. 
     In a first aspect, a system is provided. The system includes a plurality of substrates disposed in an edge-to-edge array along a primary axis. Each respective substrate of the plurality of substrates includes a plurality of detector elements. Each detector element of the plurality of detector elements generates a respective detector signal in response to light received by the detector element. The plurality of detector elements is arranged with a detector pitch between adjacent detector elements of the plurality of detector elements. Each respective substrate also includes a signal receiver circuit configured to receive the detector signals generated by the plurality of detector elements. The respective substrates of the plurality of substrates are disposed such that the detector pitch is maintained between adjacent detector elements on their respective substrates. 
     In a second aspect, an optical receiver device is provided. The optical receiver device includes a plurality of substrates disposed in an edge-to-edge array along a primary axis. Each respective substrate of the plurality of substrates includes a plurality of detector elements disposed along a first surface of the respective substrate. The plurality of detector elements is arranged with a detector pitch between adjacent detector elements of the plurality of detector elements. Each detector element of the plurality of detector elements generates a respective detector signal in response to light received by the detector element. Each respective substrate of the plurality of substrates also includes a ball grid array (BGA) or a land grid array (LGA) disposed along a second surface of the respective substrate. The second surface is opposite the first surface. Each respective substrate of the plurality of substrates additionally includes a signal receiver circuit configured to receive the detector signals generated by the plurality of detector elements. The respective substrates of the plurality of substrates are disposed such that the detector pitch is maintained between adjacent detector elements on their respective substrates. 
     Other aspects, embodiments, and implementations will become apparent to those of ordinary skill in the art by reading the following detailed description, with reference where appropriate to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1A  illustrates a system, according to an example embodiment. 
         FIG. 1B  illustrates a system, according to an example embodiment. 
         FIG. 1C  illustrates a system, according to an example embodiment. 
         FIG. 1D  illustrates a system, according to an example embodiment. 
         FIG. 1E  illustrates a system, according to an example embodiment. 
         FIG. 2A  illustrates an optical receiver device, according to an example embodiment. 
         FIG. 2B  illustrates an optical receiver device, according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Example methods, devices, and systems are described herein. It should be understood that the words “example” and “exemplary” are used herein to mean “serving as an example, instance, or illustration.” Any embodiment or feature described herein as being an “example” or “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or features. Other embodiments can be utilized, and other changes can be made, without departing from the scope of the subject matter presented herein. 
     Thus, the example embodiments described herein are not meant to be limiting. Aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are contemplated herein. 
     Further, unless context suggests otherwise, the features illustrated in each of the figures may be used in combination with one another. Thus, the figures should be generally viewed as component aspects of one or more overall embodiments, with the understanding that not all illustrated features are necessary for each embodiment. 
     I. Overview 
     An optical receiver system may include a plurality of substrates arranged in an array. Each substrate may include a plurality of detector elements coupled to a signal receiver circuit. In an example embodiment, the signal receiver circuit may include an application-specific integrated circuit (ASIC) and/or a field-programmable gate array (FPGA). For example, the signal receiver circuit may include an amplifier configured to receive photosignals from the detector elements. 
     In some embodiments, the plurality of detector elements may include 16 detector elements arranged along an axis parallel to a surface of the substrate. The detector elements may include avalanche photodiodes (APDs), such as single photon avalanche diodes (SPADs). In an example embodiment, the APDs may be operated in linear mode and/or in Geiger mode. In some cases, the axis may be perpendicular to at least one edge of the substrate. Furthermore, the plurality of substrates may be arranged in an end-to-end (or edge-to-edge) fashion such that a detector pitch (e.g., distance from detector centers) is constant, even between detector elements arranged on different substrates. As an example, 13 substrates with respective sets of detectors may be arranged in a row such that all 208 detectors are aligned in a single row. In other words, the substrates may be diced, etched, or otherwise formed such that each substrate and detector arrangement is essentially “borderless” at least along the detector axis. In such a scenario, the “long line” arrangement of detector elements may provide high resolution along at least one scanning axis of a light detection and ranging (LIDAR) system. 
     In an example embodiment, the detectors may be arranged along a first side of the substrate. A second side of the substrate may include a ball grid array (BGA). Accordingly, the substrate may be electrically coupled to a power supply and/or other electronic components. 
     In some embodiments, each detector is wire bonded to a respective signal receiver circuit. Furthermore, for each detector/signal receiver circuit wire bond (e.g., sixteen wire bonds per substrate), a return wire bond may be made from the bond pad on the signal receiver circuit to a ground plane on the substrate. The return wire bonds and the detector/signal receiver circuit wire bonds may be arranged such that the wire loop areas (e.g., cross-sectional inductive loop area) are similar or identical. Such an arrangement may eliminate or reduce some parasitic capacitive or inductive coupling due to the wire bonds. 
     The substrate, detectors, and wire bonds may be encapsulated with an encapsulation material. In some embodiments, the encapsulation material may be applied via a transfer mold process. However, other processes for applying the encapsulation material are contemplated (e.g., deposition, injection molding, reflow, backfill, etc.). The encapsulation material may be registered (e.g., via fiducial marks on the substrate) with the detector elements. In an example embodiment, the encapsulation material may include epoxy, silicone, or other polymer. Other encapsulation materials are possible. The encapsulation material may be applied or fabricated such that it has varying depth with respect to a surface of the substrate. For instance, the encapsulation may be thinner (e.g., may form a trench or depression) over the respective detectors. In a specific example, the encapsulation proximate to each detector may include an 800 micron wide trench with a 60-degree sidewall profile. 
     The local area of the encapsulation above the respective detectors may include one or more microlenses. That is, each detector may receive light via a respective microlens formed in the encapsulation material. The microlenses may be hemispherical, although other shapes are possible. In some embodiments, the microlenses may be formed with the encapsulation material. However, the microlenses may include other shapes and materials. In some embodiments, the microlenses may include pre-molded lens elements. In such scenarios, the pre-molded microlenses may be positioned via pick and place system or another precision placement method. 
     The microlenses may provide collection of light from a wide range of incidence angles. In some embodiments, the microlenses may be offset by 200 microns with respect to a surface of the detectors. Also, the microlenses may be recessed with respect to a primary surface of the encapsulation material (i.e., a surface where the encapsulation material is thickest). In a specific example, the microlenses may have a diameter greater than at least one physical dimension of the respective detector and/or the detector pitch. For instance, the microlens may have a diameter of 440 microns with a 400 micron pitch. As such, the microlenses may be “clipped” along at least one side. However, other geometries are possible. In some example embodiments, the shape of the microlens may be based on an optical path of incident light (e.g., f-number of other collection/focusing optics). 
     II. Example Systems 
       FIG. 1A  illustrates a system  100 , according to an example embodiment. System  100  includes a substrate  110   a  and a plurality of detector elements  120 . System  100  also includes a signal receiver circuit  140 , signal bond wires  142 , signal pads  144 , and ground pads  146 . 
     Substrate  110   a  may be a semiconductor material such as silicon or gallium arsenide. In some embodiments, the substrate  110   a  may include InGaAs grown on an InP substrate. It will be understood that other substrate materials are possible. Additionally or alternatively, substrate  110   a  may include a printed circuit board (PCB) or another substrate material. In some embodiments, substrate  110   a  may be a flexible substrate. 
     The detector elements  120  are configured to detect light within a desired spectral range and/or at one or more wavelengths. In an example embodiment, the detector elements  120  may be configured to detect light at 1550 nm, however, other wavelengths and/or spectral ranges are possible. In an example embodiment, at least one of the detector elements  120  could be an avalanche photodiode (APD) or a single photon avalanche diode (SPAD). For example, detector element  120  may include an InGaAs APD configured to detect light at wavelengths around 1550 nm. Other types of photodetectors are possible and contemplated herein. 
     Each detector element of the plurality of detector elements  120  may be die-bonded to a respective capacitor  124 . The capacitor device  124  may provide low-pass signal filtering. The capacitor device  124  may be die-bonded to the substrate  110   a . In an example embodiment, the capacitor device  124  may be a tantalum or aluminum capacitor. In some embodiments, a ground return bond  148  may be connected from the respective capacitor devices  124  to a respective ground pad  146 . 
     In an example embodiment, the signal receiver circuit  140  may include at least one of an application-specific integrated circuit (ASIC) or a field-programmable gate array (FPGA), such as a Xilinx XCVU3P Virtex UltraScale+FPGA. For instance, the signal receiver circuit  140  may represent an amplifier or another type of analog front end system configured to receive respective photosignals from the plurality of detector elements  120 . As an example, a detector element may provide a respective photosignal to the signal receiver circuit  140  via wire bond  122 . In an example embodiment, the ground return bonds  148  and the detector/signal receiver circuit wire bonds  122  may be arranged such that the wire loop areas (e.g., cross-sectional inductive loop area) are similar or identical. Such an arrangement may eliminate or reduce some parasitic capacitive or inductive coupling due to the wire bonds. 
     The signal receiver circuit  140  may receive and amplify the respective photosignals from the plurality of detector elements  120 . The signal receiver circuit  140  may carry out a variety of other functions including, but not limited to, signal routing/selection (e.g., switch, multiplexer, or demultiplexer), and signal processing (e.g., denoising, decoding, or encoding). The signal receiver circuit  140  may additionally or alternatively be configured to provide various image processing tasks based on the received photosignals (e.g., time averaging). 
     In an example embodiment, the signal receiver circuit  140  could include a transimpedance amplifier (TIA), such as a Maxim MAX  3658  low noise TIA. In other embodiments, the TIA may be embedded in a custom ASIC. 
     In an example embodiment, the plurality of detector elements  120  may include sixteen detector elements arranged in a single column (e.g., a linear array). For example, the detector elements of the plurality of detector elements  120  could be arranged along, or could be at least parallel to, a primary axis  130 . It will be understood that other arrangements of the respective detector elements are possible. For instance, the detector elements could be arranged in two columns that are parallel to primary axis  130 . While  FIG. 1A  illustrates sixteen detector elements, more or fewer detector elements are contemplated. 
     In an example embodiment, each detector element could be substantially square with a 350 micron side length. Furthermore, the detector pitch could be 400 microns along the primary axis  130 . That is, a center-to-center distance between neighboring detector elements could be 400 microns. Put another way, assuming a 350 micron detector side length, when arranged along the primary axis  130 , the detector elements may have 50 microns between them. It will be understood that other values for the detector pitch are possible and contemplated. For example, with smaller detector elements, detector pitches of less than 50 microns are possible. 
     In an example embodiment, during back end of line (BEOL) processing, the substrate  110   a  may be diced within 25 microns of an outermost detector element. In such a situation, another substrate similar to that of system  100  may be arranged right next to system  100  to maintain identical detector pitch across the different substrates. 
     As shown in the transverse cross sectional view of  FIG. 1A , in some embodiments, the system  100  may include an encapsulation  150  overlaying at least the plurality of detector elements  120 . The encapsulation  150  may include an epoxy or silicone. In some embodiments, the encapsulation  150  may include Sil-Poxy silicone adhesive, SolEpoxy OP7200, Nitto NT-324H, Nuva-Sil Epoxy resin, or silica. While a variety of application methods are contemplated, in an example embodiment, the encapsulation  150  may be provided using a one- or two-step transfer mold process. In an embodiment, the transfer mold may be registered to substrate with a fiducial mark on the substrate  110   a , or another type of alignment feature or landmark. 
     The encapsulation  150  may include a trench portion  154  that may be disposed above the plurality of detector elements  120 . In an example embodiment, the trench portion  154  could have sidewalls with a 60 degree sidewall angle. In some embodiments, the trench portion  154  may provide some measure of detector isolation from neighboring devices. For example, in some embodiments, optical crosstalk may be reduced below −30 dB. As an illustrative example, the trench portion  154  may be 800 microns in width (measured from an opening of the trench). It will be understood that other trench profiles (e.g., depth, width, sidewall angle) are contemplated. Specifically, trench profiles may be selected in an effort to reduce optical crosstalk between neighboring detectors. 
     In an example embodiment, the encapsulation  150  proximate to each detector element of the plurality of detector elements  120  may include a microlens  152 . In an example embodiment, the microlens  152  may have a hemispherical shape, although other shapes are contemplated. Further, the hemispherical microlens  152  may have a diameter that is larger than a size of the detector. For example, with a square detector having a 350 micron side length, the hemispherical microlens  152  could have a diameter of 440 microns. It will be understood that the dimensions and/or shape of the microlens  152  may be selected based on an incident optical beam (e.g., from a given field of view). For example, the microlens  152  may be adjusted based on a predetermined f-number of the optical system and/or other characteristics of the optical system. 
     The encapsulation  150  may magnify the active area and may also protect the system  100  from scratches and other damage. For example, the encapsulation  150  may provide protection for wirebonds and may be substantially optically clear within the wavelengths of interest. 
     In some embodiments, the microlens  152  is recessed with respect to a primary surface  151  of the encapsulation  150 . In this way, the microlens  152  may be better protected against physical damage. 
     In some embodiments, the plurality of detector elements  120  may be disposed along a first surface (e.g., the top) of the substrate  110   a  and a ball grid array  156  (BGA) may be disposed along a second, opposite surface of the substrate  110   a . Among other possibilities, the ball grid array  156  may provide one or more electrical interconnects to other electrical systems, devices, and/or elements. For example, the substrate  110   a  may be electrically coupled to a power supply and/or other electronic components (e.g., a read-out integrated circuit (ROIC)) via BGA  156 . 
     While a ball grid array is described and illustrated with regard to system  100 , other interconnect types are contemplated. For example, substrate  110   a  could alternatively or additionally include a pin grid array and/or be compatible with a land grid array. 
     While the system  100  could be used for many different applications, system  100  could represent at least a portion of a receiver of a light detection and ranging (LIDAR) system. That is, system  100  could be used to receive return laser pulses from a field of view of the LIDAR system. As such, system  100  may provide a “border-less” carrier substrate, which may provide more efficient returns from a vertically-scanning LIDAR beam. 
       FIG. 1B  illustrates the system  100 , according to an example embodiment. As illustrated in the Back View, substrate  110   a  of system  100  may include a ball grid array  156 . 
     Furthermore, as illustrated in the Axial Elevation View of  FIG. 1B , the encapsulation  150  may include a plurality of microlenses  152 . In an example embodiment, the microlenses  152  may be substantially centered over the respective detector elements. Furthermore, each of the microlenses  152  may have a diameter of 440 microns. As such, if the detector pitch is 400 microns, the microlenses  152  may be “clipped” along the primary axis  130 . 
     While microlenses  152  are illustrated and described herein as having a hemispherical shape, it will be understood that other shapes and lens types are possible and contemplated. For example, the microlenses  152  may be formed from micro-Fresnel lenses, which may focus light by refraction in a set of concentric curved surfaces. Yet further, microlenses  152  may be formed from binary optics. Such binary optical lenses may resemble a stepped arrangement. 
       FIG. 1C  illustrates a system  160 , according to an example embodiment. System  160  may include a plurality of substrates  110   a - 110   e , etc. that may be disposed in an edge-to-edge array along the primary axis  130 . As described elsewhere herein, the respective substrate  110   a - e  may be diced within 25 microns of an outermost detector element (e.g., detector  1  and detector  16 ). As such, the respective substrates  110   a - e  may be arranged next to one another while maintaining identical detector pitch (e.g., 400 microns) and identical detector spacing (e.g., 50 microns) across the different substrates. 
     In an example embodiment, system  160  may include thirteen substrates (e.g.,  110   a - m ) disposed in an edge-to-edge array along the primary axis  130 . System  160  may include more or fewer substrates. Furthermore, other geometries are contemplated. For example, a plurality of substrates could be disposed in an edge-to-edge array in two-dimensions (e.g., along an x-axis and a y-axis.) 
       FIG. 1D  illustrates a system  160 , according to an example embodiment. Namely,  FIG. 1D  provides a detail view of the interface between substrate  110   a  and substrate  110   b . In an example embodiment, substrate  110   a  and substrate  110   b  may be etched, wafer sawed or otherwise processed such that when abutted against one another, their respective detector arrays may maintain a constant detector spacing (e.g., detector pitch). 
     In other words, for a given detector pitch  164 , substrates  110   a  and  110   b  could be arranged along interface  163  such that an inter-substrate detector pitch  166  is substantially identical to the “on-chip” detector pitch  164 . In some embodiments, the inter-substrate detector pitch  166  could differ by less than 1% or less than 5% from the detector pitch  164 . As used herein, “substantially identical” may be a difference between the inter-substrate detector pitch  166  and the detector pitch  164  that still permits full operation of the plurality of detector elements as an extended optical detector array. 
       FIG. 1E  illustrates a system  160 , according to an example embodiment.  FIG. 1E  provides a detail axial elevation view of the interface between substrate  110   a  and  110   b . For a given on-substrate detector pitch  172  (e.g., 400 microns), substrates  110   a  and  110   b  may be arranged such that an inter-substrate detector pitch  174  is substantially identical to detector pitch  172 . In some embodiments, the inter-substrate detector pitch  174  could differ by less than 1% or less than 5% from the detector pitch  172 . 
     In an example embodiment, any of the systems described herein may form part of an optical receiver device. That is, systems  100  and  160  may make up at least a portion of an optical receiver device. In such a scenario, the optical receiver device may include a plurality of substrates disposed in an edge-to-edge array along a primary axis. Each respective substrate of the plurality of substrates may include a plurality of detector elements disposed along a first surface of the respective substrate. Each detector element of the plurality of detector elements is disposed according to a detector pitch. Each respective substrate may also include a ball grid array (BGA) or a land grid array (LGA) disposed along a second, opposite, surface of the respective substrate and a signal receiver circuit configured to receive respective photosignals from the plurality of detector elements. The respective substrates of the plurality of substrates are disposed such that the detector pitch is maintained between adjacent detector elements on their respective substrates. 
     In an example embodiment, the plurality of substrates may be aligned relative to one another via a variety of methods, as illustrated in  FIGS. 2A and 2B .  FIG. 2A  illustrates an optical receiver device  200 . Optical receiver device  200  may be similar or identical to systems  100  and  160 , as illustrated and described with regard to  FIGS. 1A-E . In such a scenario, each substrate ( 110   a  and  110   b ) may include one or more alignment structures  210 . As an example, the alignment structures  210  could be one or more slots and/or tabs configured to provide lateral alignment between the substrates  110   a  and  110   b  when fit together in an edge-to-edge fashion. In some embodiments, the alignment structures  210  may be incorporated into the encapsulation layer. For instance, one or more extruded, spherical shapes may be formed in the encapsulation layer of each of the plurality of substrates. During a mechanical alignment process, the plurality of substrates may be aligned to one another by fitting the spherical shapes of each encapsulation layer into a common mating surface that may have corresponding mating features (e.g., embossed spherical shapes) at appropriate intervals. 
       FIG. 2B  illustrates an optical receiver device  220 , according to an example embodiment. Optical receiver device  220  may be similar or identical to systems  100  and  160 , as illustrated and described with regard to  FIGS. 1A-E . Optical receiver device  220  may include a flexible or rigid base substrate  230 . As an example, the base substrate  230  may be a printed circuit board. In one embodiment, the printed circuit board could be a flexible printed circuit board. The base substrate  230  may include alignment structures, such as solder balls  232 , contact pads, or other types of physical and/or electrical coupling structures. That is, the base substrate  230  may provide an extended mating surface for the plurality of substrates (including substrates  110   a  and  110   b ). The plurality of substrates may be mated with the base substrate  230  via a bonding process (e.g., bump bonding), physical coupling, gluing, and pick-and-place methods, among others. When mated with the base substrate  230 , the plurality of substrates may be substantially aligned in one or more directions (e.g. height and/or lateral position). 
     While  FIG. 2B  illustrates the base substrate  230  as being substantially flat, it is understood that the base substrate  230  may include a concave or convex shape such that the plurality of substrates (e.g.,  110   a  and  110   b ) may be oriented along the concave or convex shape. In an example embodiment, the base substrate  230  may be a rigid or flexible substrate having a curved shape corresponding to a focal plane and/or a field curvature. That is, the base substrate  230  may be shaped according to a field curvature (e.g., a Petzval field curvature) of an optical system. 
     In an example embodiment, the optical receiver device may be configured to receive LIDAR laser pulses. Additionally or alternatively, the optical receiver device may include at least 13 substrates. In such a scenario, a total number of detector elements, N, may be at least 208. 
     As described herein, the detector elements may be disposed in a linear array. As an example, the linear array may include a 1×N array of detector elements oriented parallel to the primary axis. In some embodiments, the detector pitch of the linear detector array is less than 500 microns. While some embodiments herein are described as having a linear array format, it is understood that 2-dimensional arrays are also contemplated. In such scenarios, the 2-D array may include M×N detector elements. 
     The particular arrangements shown in the Figures should not be viewed as limiting. It should be understood that other embodiments may include more or less of each element shown in a given Figure. Further, some of the illustrated elements may be combined or omitted. Yet further, an illustrative embodiment may include elements that are not illustrated in the Figures. 
     A step or block that represents a processing of information can correspond to circuitry that can be configured to perform the specific logical functions of a herein-described method or technique. Alternatively or additionally, a step or block that represents a processing of information can correspond to a module, a segment, or a portion of program code (including related data). The program code can include one or more instructions executable by a processor for implementing specific logical functions or actions in the method or technique. The program code and/or related data can be stored on any type of computer readable medium such as a storage device including a disk, hard drive, or other storage medium. 
     The computer readable medium can also include non-transitory computer readable media such as computer-readable media that store data for short periods of time like register memory, processor cache, and random access memory (RAM). The computer readable media can also include non-transitory computer readable media that store program code and/or data for longer periods of time. Thus, the computer readable media may include secondary or persistent long term storage, like read only memory (ROM), optical or magnetic disks, compact-disc read only memory (CD-ROM), for example. The computer readable media can also be any other volatile or non-volatile storage systems. A computer readable medium can be considered a computer readable storage medium, for example, or a tangible storage device. 
     While various examples and embodiments have been disclosed, other examples and embodiments will be apparent to those skilled in the art. The various disclosed examples and embodiments are for purposes of illustration and are not intended to be limiting, with the true scope being indicated by the following claims.