Patent Publication Number: US-2021175386-A1

Title: Integrated module assembly for optical integrated circuits

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 62/944,533, filed Dec. 6, 2019, and which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to the field of optical devices and assemblies. More specifically, embodiments of the present invention pertain to optical devices involving wafer level chip scale package (WLCSP) and printed-circuit board (PCB) devices. 
     BACKGROUND 
     Assembly of integrated circuits containing optical devices (e.g., transmitters and/or receivers) tends to be more expensive than assembly of other components. Particularly, it can be difficult for optical ICs to take advantage of WLCSP assembly, as the active surface of the device faces down toward a PCB that is typically opaque. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an example application using the module assembly, in accordance with embodiments of the present invention. 
         FIG. 2  is an example WLCSP device, in accordance with embodiments of the present invention. 
         FIG. 3  is an example individual lens device, in accordance with embodiments of the present invention. 
         FIG. 4  is an example multiple lens device, in accordance with embodiments of the present invention. 
         FIG. 5  is an example device with lenses using PCB holes for alignment, in accordance with embodiments of the present invention. 
         FIG. 6  is an example device with spherical ball lenses that fit in the PCB hole, in accordance with embodiments of the present invention. 
         FIG. 7  is a first example clear PCB device, in accordance with embodiments of the present invention. 
         FIG. 8  is a second example clear PCB device, in accordance with embodiments of the present invention. 
         FIG. 9  is a third example clear PCB device, in accordance with embodiments of the present invention. 
         FIG. 10  is a flow diagram of an example method of making an integrated module assembly, in accordance with embodiments of the present invention. 
         FIG. 11  is a fourth example clear PCB device, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Reference will now be made in detail to particular embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents that may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be readily apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, processes, components, structures, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention. 
     Some portions of the detailed descriptions which follow are presented in terms of processes, procedures, logic blocks, functional blocks, processing, schematic symbols, and/or other symbolic representations of operations on data streams, signals, or waveforms within a computer, processor, controller, device, and/or memory. These descriptions and representations are generally used by those skilled in the data processing arts to effectively convey the substance of their work to others skilled in the art. Usually, though not necessarily, quantities being manipulated take the form of electrical, magnetic, optical, or quantum signals capable of being stored, transferred, combined, compared, and otherwise manipulated in a computer or data processing system. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, waves, waveforms, streams, values, elements, symbols, characters, terms, numbers, or the like. 
     Referring now to  FIG. 1 , shown is an example application using the module assembly, in accordance with embodiments of the present invention. In example  100 , a redistribution layer (RDL) as used in wafer level chip scale packaging (WLCSP) is shown. WLCSP refers to the technology of packaging an integrated circuit at a wafer level, resulting in a device that is practically the same size as the die. While the name implies devices would be packaged, the bare die can actually be modified to add environmental protection layers and solder balls that are then used as the direct connection to the package carrier or substrate. WLCSP technology can allow devices to be integrated in the design by using the smallest possible form factor, and WLCSP devices may require no additional process steps on surface mount assembly lines. 
     In WLCSP, the bare die can be processed to have solder balls attached directly to the device, which may remove the need for external casing and wiring in some cases. In this particular example, silicon die  112  can be covered with a nitride passivation layer (e.g.,  110 ), except for pad openings (e.g.,  114 ) in some cases. For example, RDL  108  can be used to route between pad  114  and a region for external connectivity. A polymer dielectric (e.g.,  106 ) can then be added, followed by deposition of under bump metallization (UBM), such as  104 . A solder ball (e.g.,  102 ) can then be attached to each UBM pad. After processing, the device can be a die with an array pattern of solder balls, which may then be attached at a pitch that is comparable to traditional circuit board (e.g., printed-circuit board [PCB]) assembly processes. In this way, there may be no need for external packaging material in order to protect the chip. In particular embodiments, optical transceivers and/or optical receivers can be implemented in WLCSP technology, and then may be connected to a corresponding PCB to form a full integrated module assembly. 
     Referring now to  FIG. 2 , shown is an example WLCSP device (top view), in accordance with embodiments of the present invention. Example  200  is one particular example of an optical module in WLCSP device implementation. In addition to optical module implementation, such technology may be used in supporting analog and digital circuitry. In this particular example, optical transmitter  206 -T and optical receiver  206 -R can be on opposite sides at the edges of the WLCSP/die  204 . This arrangement allows multiple rows of blocking balls (e.g.,  102 ) and vias to be placed between the optical transmitter and receiver. Balls  102  can be near the edge of the die, but not exactly where the optical transmitter/receiver are located. Lenses  202  can be in alignment with the corresponding optical device  206 . 
     In one embodiment, an integrated module assembly can include: (i) an optical integrated circuit having first and second optical devices; (ii) a PCB having first and second holes therein, where the optical integrated circuit is coupled upside down to a first side of the PCB; and (iii) first and second lenses coupled to a second side of the PCB, where the first and second sides of the PCB are opposite thereto; and (iv) where the first lens is in alignment with the first hole and the first optical device, and the second lens is in alignment with the second hole and the second optical device. 
     In one embodiment, an integrated module assembly can include: (i) an optical integrated circuit comprising first and second optical devices; (ii) a PCB having first and second clear regions therein, where the optical integrated circuit is coupled upside down to a first side of the PCB; (iii) first and second lenses coupled to the first side of the PCB; and (iv) where the first lens is in alignment with the first clear region and the first optical device, and the second lens is in alignment with the second clear region and the second optical device. 
     In one embodiment, a method of making an integrated module assembly can include: (i) forming an optical integrated circuit comprising first and second optical devices; (ii) forming a PCB having first and second clear regions therein; (iii) arranging the optical integrated circuit upside down on the PCB; (iv) forming first and second lenses on the PCB; and (v) aligning the first lens with the first clear region and the first optical device, and aligning the second lens with the second clear region and the second optical device. 
     In particular embodiments, by using an optically clear PCB, the PCB may no longer block light to and from the active surface of the die that includes optical devices. In some cases, the PCB may be used directly as the optical sensor window in the housing of the final product. As an example of an application using this assembly technology, an optical distance detector can include an integrated circuit having an optical transmitter (e.g., light-emitting diode [LED], vertical-cavity surface-emitting laser [VCSEL], etc.) and one or more optical receivers (e.g., PIN diodes, etc.), in addition to analog and digital circuitry. 
     A VCSEL is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, as opposed to edge-emitting semiconductor lasers or in-plane lasers that emit from surfaces formed by cleaving the individual chip out of a wafer. VCSELs are used in various laser products, including computer mice, fiber optic communications, laser printers, Face ID, and smart glasses. A PIN diode is a diode with a wide, undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts. The wide intrinsic region makes this device particularly suitable for attenuators, fast switches, photodetectors, and high voltage power electronics applications. 
     The LED may alternatively be a separate die or “chiplet” made in a different process technology than the rest of the circuit, as LEDs are typically made in a technology that is difficult to integrate with standard CMOS devices. The LED transmitter may thus be a separate die from the rest of the circuit, or it may be a chiplet that is mounted of the surface of the main die. The optical receiver (e.g., phototransistor, PIN diode, etc.) may be integrated on the main die, or this may be a chiplet or separate die as well. An “optical device” as described herein can be an optical transmitter or an optical receiver. 
     WLCSP assembly can include layers of copper (e.g., RDL) and insulators. The materials used for insulation are transparent for most wavelengths, including infrared (IR). One of the materials that can be used for repassivation is a polyimide (e.g., Kapton, which is transparent in infrared). This material typically blocks blue and green light, but for WLCSP, the layers are so thin that the material may be partially transparent also for green and blue. In certain embodiments, one or more optical devices can be integrated with, or otherwise attached thereto, the WLCSP assembly. An “optical integrated circuit” as described herein can include, e.g., the WLCSP assembly/structure that includes one or more optical devices. 
     Referring now to  FIG. 3 , shown is an example individual lens device, in accordance with embodiments of the present invention. For a WLCSP device, the optical sensors (e.g., light receivers) can be mounted upside down toward the PCB. For example, “holes” or otherwise clear regions can be formed in the PCB for the optical sensor to see the outside world. In particular embodiments, actual holes can be formed in the PCB, and/or optically clear regions can be formed in the PCB. In addition, the light transmitters (e.g., LED, VCSEL, etc.) can also be mounted downward toward the PCB in alignment with holes/clear regions therein. Further, lenses can be included that are attached to the PCB, and/or are formed within the PCB itself in some cases. 
     In example  300 , the WLCSP can include silicon die  308  with optical receiver  306 -R and optical transmitter  306 -T, as well as repassivation layer(s) and polyimide (e.g., Kapton)  310 , UBM  318 , and solder balls  304 . The PCB can include PCB traces/pads  312 , PCB solder mask  314 , and PCB core  316 . In addition, via  322  can be included in PCB core  316  in order to form connections PCB traces and pads  312 . In example  300 , the lenses  302  may be individual lenses (e.g., one for the optical transmitter and one for the optical receiver), and light beams  320  can be as shown emanating from optical transmitter  306 -T, and coming into optical receiver  306 -R. 
     Depending on which particular WLCSP technology is used,  318  may be just the under bump metallization, or can be both the UBM and RDL, as this particular example does not differentiate between the two. Also, UBM may or may not be a separate layer, depending on the particular WLCSP technology used. In  1 , the UBM is shown as a separate layer. In some WLCSP technologies, bumps may be placed directly on die pads, but usually a UBM layer is still used. RIM, may be utilized when the bumps need to be in a different physical location than the pads (see, e.g.,  FIG. 2 ). RDL is typically a single layer on the die, but multi-layer RDL can also be used in some cases. In particular embodiments, any suitable WLCSP technology may be employed in order to implement the optical integrated circuit. In this particular case, a 2-layer PCB is shown; however, a multi-layer PCB or even a single-layer PCB can be utilized in certain embodiments. 
     Referring now to  FIG. 4 , shown is an example multiple lens device, in accordance with embodiments of the present invention. In example  400 , a cast module that includes multiple lenses can be utilized can be utilized instead of individual lenses for each optical device. In example  400 , the WLCSP can include silicon die  308  with optical receiver  306 -R and optical transmitter  306 -T, as well as repassivation layer(s)/polyimide  310 , UBM  318 , and solder balls  304 . The PCB can include PCB traces/pads  312 , PCB solder mask  314 , and via  322  in PCB core  316  to connect PCB traces and pads  312 . As shown, light beams  320  can be transmitted from optical transmitter  306 -T, and light beams can be received by optical receiver  306 -R. In example  400 , lenses  402  may be a cast module that includes multiple lenses with curved lens shapes corresponding to each optical device, as well as a common flat surface that mounts to the bottom of the PCB. 
     Referring now to  FIG. 5 , shown is an example device with lenses using PCB holes for alignment, in accordance with embodiments of the present invention. In example  500 , the WLCSP can similarly include silicon die  308  with optical receiver  306 -R and optical transmitter  306 -T, as well as repassivation layer(s)/polyimide  310 , UBM  318 , and solder balls  304 . The PCB can include PCB traces/pads  312 , PCB solder mask  314 , and PCB core  316 , and via  322  in PCB core  316  to connect PCB traces and pads  312 . As shown, light beams  320  can be transmitted from optical transmitter, and light beams can be received by optical receiver  306 -R. In example  500 , lenses  502  can use the holes in the PCB for alignment. As such, lenses  502  can be partially within the PCB holes and partially outside of the holes. Lenses  502  can have curved surfaces that fit within the corresponding holes on one side, and another curved surface on the other side. This arrangement may allow for improved alignment between the lens, hole, and corresponding optical device  306 . 
     Referring now to  FIG. 6 , shown is an example device with spherical ball lenses that fit in the PCB hole, in accordance with embodiments of the present invention. In example  600 , the WLCSP can similarly include silicon die  308  with optical receiver  306 -R and optical transmitter  306 -T, as well as repassivation layer(s)/polyimide  310 , UBM  318 , and solder balls  304 . The PCB can include PCB traces/pads  312 , PCB solder mask  314 , and PCB core  316 , and via  322  can be included in PCB core  316  to connect PCB traces and pads  312 . As shown, light beams  320  can be transmitted from/to optical transmitter  306 -T and optical receiver  306 -R. In example  600 , lenses  602  can be spherical ball lenses that fit in the PCB hole. As such, lenses  602  can be partially within the PCB holes and partially outside of the holes, while maintaining the spherical ball shape. Lenses  602  can be perfect spheres, or may be more elliptical in some cases. The particular type of lenses utilized in certain embodiments can depend on the optical requirements of the given application. 
     Referring now to  FIG. 7 , shown is a first example clear PCB device, in accordance with embodiments of the present invention. This particular example utilizes a clear and flat PCB. In example  700 , the WLCSP can include silicon die  308  with optical receiver  306 -R and optical transmitter  306 -T, as well as repassivation layer(s)/polyimide  310 , UBM  318 , and solder balls  304 . The PCB can include PCB traces/pads  312 , PCB solder mask  314 , and PCB core  704 , and via  322  can be included in PCB core  704  to connect between PCB traces and pads  312 . As shown, light beams  720  can be transmitted from optical transmitter  306 -T, and light beams may be received by optical receiver  306 -R. For example, PCB core  704  can be an optically clear material, and in particular may have clear regions corresponding to the optical devices and lenses, such as in place of holes in the PCB. 
     In particular embodiments, a clear PCB can be employed as shown, and such a clear PCB may also shield the surface of the die from the outside environment. Thus, the clear regions as described herein can include substantial portions of the PCB, but designated clear regions/holes may be aligned with optical transceiver  306 -T and optical receiver  306 -R. The clear PCB in this case can also serve as the external window of the optical structure, thereby reducing both cost and size of the final module. In example  700 , lenses  702  can be spherical ball lenses on one side and flat surfaces on the other, and that may at least partially fit in, or are otherwise aligned with, the PCB hole. Further, lenses  702  in this example can be arranged between the WLCSP and the PCB, as opposed to being located on the bottom of the PCB. 
     Referring now to  FIG. 8 , shown is a second example clear PCB device, in accordance with embodiments of the present invention. In example  800 , a clear PCB that is flat is shown. In example  800 , the WLCSP can similarly include silicon die  308  with optical receiver  306 -R and optical transmitter  306 -T, as well as repassivation layer(s)/polyimide  310 , UBM  318 , and solder balls  304 . The PCB can include PCB traces/pads  312 , PCB solder mask  314 , and PCB core  704 , and vias  322  formed in PCB core  704  to connect between PCB traces and pads  312 . As shown, light beams  720  can be transmitted from optical transmitter  306 -T. In addition, light beams can be received by optical receiver  306 -R. In example  800 , lenses  702  can be spherical ball lenses on one side and flat surfaces on the other, and that may at least partially fit in, or are otherwise aligned with, the PCB hole. In this example, the optical transmitter  306 -T and optical receiver  306 -R are placed father apart in order to substantially avoid optical interference. Multiple solder balls  304  and vias  322  can further block such interference between the optical devices  306 . 
     Referring now to  FIG. 9 , shown is a third example clear PCB device, in accordance with embodiments of the present invention. In example  900 , the clear PCB may be shaped to act as a lens itself (e.g.,  902 ) for the transmitted and received light beams  920 . In example  900 , the WLCSP can include silicon die  308  with optical receiver  306 -R and optical transmitter  306 -T, as well as repassivation layer(s)/polyimide  310 , UBM  318 , and solder balls  304 . The PCB can include PCB traces/pads  312 , PCB solder mask  314 , and PCB core  904 , and vias  322  formed in PCB core  704  to connect PCB traces and pads  312 . As shown, light beams  920  can be transmitted from optical transmitter  306 -T, and light beams can be received by optical receiver  306 -R. 
     In example  900 , additional lenses  702  can be shaped within the PCB hole itself between the WLCSP and the PCB, and/or lenses may be formed as a curved shape  902  within PCB core  904 . In some cases, curved lens  902  can be the only lens utilized for each optical device, while in other cases both of lenses  702  and  902  can be utilized in alignment with the corresponding optical device. Any suitable curved shape  902  for forming an optical lens can be employed in certain embodiments. In addition, the optical transmitter  306 -T and optical receiver  306 -R can similarly be placed father apart and with multiple solder balls and vias  322  therebetween in order to substantially avoid optical interference. 
     Referring now to  FIG. 10 , shown is a flow diagram of an example method of making an integrated module assembly, in accordance with embodiments of the present invention. In example  1000 , the method can include formation of an optical integrated circuit including first and second optical devices at  1002 . For example, this optical integrated circuit can be implemented using WLCSP technology, and may be fully integrated and/or may utilize one or more chiplets. At  1004 , a PCB having first and second clear regions (e.g., holes or optically transparent regions) therein can be formed. At  1006 , the optical integrated circuit may be arranged upside down on the PCB. 
     At  1008 , first and second lenses can be formed on the PCB. For example, the lenses can be separate lenses (see, e.g.,  FIG. 3 ), multiple lenses in a cast module (see, e.g.,  FIG. 4 , and/or the lenses may be integrated within a clear PCB (see, e.g.,  FIG. 9 ). At  1010 , the first lens can be aligned with the first clear region and the first optical device, and the second lens can be aligned with the second clear region and the second optical device. The alignment can be facilitated by utilizing the PCB holes themselves in some cases, such as in the examples of  FIGS. 5 and 6 . Of course, the ordering of the various steps shown in  FIG. 10  represent but one possible ordering, and any suitable ordering of these particular steps, as well as numbers of steps or sub-steps, can be supported in certain embodiments. 
     In particular embodiments, various alternatives can be utilized in order to reduce optical reflections between the optical transmitter and the optical receiver. For example, the clear PCB may only need to be clear at proximate locations or regions corresponding to and in alignment with the transmitter and receiver. In this case, the rest of the area can be covered by traces, (matte) black silk screen, or any other suitable material in order to block optical reflections. For example, in very low cost applications, printed traces and glue may be utilized instead of copper traces and solder. Alternatively, a glass/ITO process, as found in liquid crystal display (LCD) panels, can be utilized. In that case, the traces may be clear, and the silk screen can do the blocking of light reflections. Indium tin oxide (ITO) is a ternary composition of indium, tin, and oxygen in varying proportions. 
     As shown in various examples, one or more solder bumps can be placed directly between the optical transmitter and the optical receiver in order to block the direct light path therebetween. Even though this direct path may be blocked, solder bumps tend to be shiny, and the amount of light coming in through secondary light reflections may actually increase as a result. To help alleviate this potential problem, and/or if the PCB itself becomes an unwanted optical path between transmitter and receiver, one or more vias (e.g.,  322 ) can be placed within the PCB in order to break that unwanted path. 
     Referring now to  FIG. 11 , shown is a fourth example clear PCB device, in accordance with embodiments of the present invention. In example  1100 , if the air gap between the WLCSP/die and the PCB represents another potential problem, an optically clear underfill material  1102  (e.g., clear silicone, clear epoxy, etc.) can be utilized in that air gap. With an appropriate material choice, there may essentially be two fewer interfaces on each side where the refractive index changes. For example, by selectively only placing the underfill material  1102  at the locations of the transmitter  306 -T and receiver  306 -R (e.g., one small drop in each location), problems related to air bubbles in the underfill can substantially be avoided, and the optical coupling between the two may accordingly be reduced. Further, the solder balls  304  can generally provide suitable mechanical strength in pairing the WLCSP and the PCB with appropriate dimensions. As another example, in addition to the clear underfill  1102  that lies in the desired optical path, a regular opaque underfill can be added to fill that air gap for the rest of the die. Such an opaque underfill can block additional light, while also increasing mechanical strength, if required. 
     As shown in  FIGS. 8 and 9 , e.g., the optical transmitter and optical receiver can be on opposite sides at the edges of the WLCSP/die. This arrangement allows multiple rows of blocking balls (e.g.,  304 ) and vias (e.g.,  322 ) to be placed between the optical transmitter and receiver. Balls can still be near the edge of the die, but not exactly where the optical transmitter/receiver are located. Having the optical transmitter and receiver at the edge may also facilitate the dispensing of a small drop of clear underfill corresponding to each optical device, as discussed above. If lenses are to be added to a clear PCB, there are multiple options supported in certain embodiments. For a glass PCB, these may be separate lenses (see, e.g.,  FIGS. 7 and 8 ). For a plastic material, and in some cases for a glass material, the lenses may be molded as part of the PCB, e.g., as shown in  FIG. 9 . In addition, while only one side of the PCB is shown as molded in  FIG. 9 , one or both sides of the PCB may be so molded form lenses in certain embodiments. 
     Particular embodiments may be applicable to an optical distance measurement circuit using infrared light, but can also be used for any other optical circuit using wavelengths where the materials in the optical path are sufficiently transparent, such as (e.g., infrared) image sensors. Certain embodiments can also be extended to a wider optical wavelength range by using different (optically clear) materials (e.g., polyimide materials) as the insulator on top of the WLCSP, which is effectively between the WLCSP and the PCB. Particular embodiments may be especially suitable for manufacturers of systems that need robotic vision, proximity sensing, and/or distance measurement. 
     The drawings herein primarily show a single optical element (e.g., lens, molded transparent PCB, etc.) in the light path. However, an even more sophisticated optical system can be implemented in certain embodiments by having more than a single optical element, such as by combining the molded PCB shown in  FIG. 9  with the external lenses as shown in  FIG. 3 . This is also exemplified in  FIG. 9  with lenses  702  and molded curved lenses  902  in the optical path. In addition, any suitable optical devices can be used in certain embodiments. For example, the optical transmitter may be a LED for a low cost, short range applications, but a VCSEL may provide a more cost-effective in some cases. 
     While the above examples include circuit, operational, and structural implementations of certain memory cells and programmable impedance devices, one skilled in the art will recognize that other technologies and/or cell structures can be used in accordance with embodiments. Further, one skilled in the art will recognize that other device circuit arrangements, architectures, elements, and the like, may also be used in accordance with embodiments. Further, the resistance levels, operating conditions, and the like, may be dependent on the retention, endurance, switching speed, and variation requirements of a programmable impedance element. 
     The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.