Patent Publication Number: US-8987885-B2

Title: Packaged microdevices and methods for manufacturing packaged microdevices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/987,890 filed Jan. 10, 2011, now U.S. Pat. No. 8,354,301, which is a divisional of U.S. application Ser. No. 11/509,990 filed Aug. 25, 2006, now U.S. Pat. No. 7,868,440, each of which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention is related to microdevices and methods for packaging microdevices using gold-to-gold interconnects or other types of conductive elements in high-density contact arrays. 
     BACKGROUND 
     Microelectronic devices generally have a die (i.e., a chip) that includes integrated circuitry having a high density of very small components. In a typical process, a large number of dies are manufactured on a single wafer using many different processes that may be repeated at various stages (e.g., implanting, doping, photolithography, chemical vapor deposition, plasma vapor deposition, plating, planarizing, etching, etc.). The dies typically include an array of very small bond-pads electrically coupled to the integrated circuitry. The bond-pads are the external electrical contacts on the die through which the supply voltage, signals, etc., are transmitted to and from the integrated circuitry. The dies are then separated from one another (i.e., singulated) by dicing the wafer and backgrinding the individual dies. After the dies have been singulated, they are typically “packaged” to couple the bond-pads to a larger array of electrical terminals that can be more easily coupled to the various power supply lines, signal lines, and ground lines. 
     Electronic products require packaged microelectronic devices to have an extremely high density of components in very limited space. For example, the space available for memory devices, processors, displays, and other microelectronic components is quite limited in cellphones, PDAs, portable computers, storage devices, media players and many other products. As such, there is a strong drive to reduce the surface area or “footprint” of a microelectronic device on a printed circuit board, lead frame, or other type of substrate. Reducing the size of a microelectronic device is difficult because high performance microelectronic dies generally have more bond-pads that result in larger ball-grid arrays and thus larger footprints. 
     “Flip-chip” packages are attractive for such high performance, small microelectronic devices. These devices are referred to as “flip-chips” because they are typically manufactured on a wafer and have an active side with bond-pads that initially face upward. After completing the die, it is singulated and inverted or “flipped” such that the active side bearing the bond-pads faces downward for attachment to a substrate. The bond-pads are usually coupled to terminals, such as conductive “bumps,” that electrically and mechanically connect the die to the substrate. The bumps on the flip-chip are generally formed from solder, conductive polymers, or other materials. When the bumps are made from solder, they are reflowed to form discrete solder joints between the flip-chip component and the substrate. This leaves a small gap between the die and the substrate. To enhance the integrity of the joint between the die and the substrate, an underfill material is generally introduced into the gap. The underfill material bears some of the stress placed on the components and protects the components from moisture, chemicals, and other contaminants. The underfill material can include filler particles to increase the rigidity of the material and modify the coefficient of thermal expansion of the material. 
     Most flip-chip devices use a lead-tin solder that requires flux to remove oxide during assembly. Although lead-tin solders provide high yields and reliable connections, soldering generally involves potentially hazardous materials and presents other challenges. First, it is generally costly and inefficient to handle hazardous materials. Second, the temperatures of reflow processes may be above the upper limits for some of the materials used in the packages. Third, solder interconnects are relatively large compared to gold-to-gold interconnects. Many solder-based flip-chip packages accordingly require a redistribution layer on the die that redistributes the very fine pitch of the bond-pads to an array having a larger pitch to accommodate the solder interconnects. 
     Gold-to-gold interconnects are one alternative to solder interconnects. Gold-to-gold interconnects generally have gold stud bumps placed on the die bond-pads through a modification of the “ball bonding” process used in conventional wire-bonding. In ball bonding, the tip of the gold bond wire is melted to form a sphere, and the bonding tool presses this sphere against a bond-pad while applying mechanical force, heat and ultrasonic energy to create a metallic connection. The gold is broken just above the ball to form a gold ball or “gold stud bump” on the bond-pad. After placing the gold stud bumps on a chip, they may be flattened by mechanical pressure to provide a flat-top surface and uniform bump height (i.e., co-planarity). Gold stud bumps are relatively easy to form with conventional wire-bonding equipment, and they do not use hazardous materials that require expensive and sophisticated handling processes. Gold stud bumps can also be quite small, and are thus very useful for fine pitch arrays with a large number of very small bond-pads. 
     Gold stud bumps, however, have only been used in limited applications because it is challenging to use them in many types of packaged devices. For example, because gold stud bumps are quite small, they must have good co-planarity and there must be good parallelism between the die and substrate to obtain good diffusion bonding. Misalignment between the die and the substrate, or non-uniform bump heights, may cause openings in the interconnects. As a result, gold stud bumps are not used in high density arrays on laminate substrates formed from organic dielectric materials. Additionally, large arrays require more ultrasonic power and greater down forces to attach the gold stud bumps to the bond-pads, and this can damage the dies. Therefore, it would be desirable to develop a packaged device and a method for packaging devices in which gold stud bumps can be formed in large arrays on organic substrates or other types of substrates to enable gold interconnects to be used in a wider range of applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side cross-sectional view of a packaged microdevice in accordance with an embodiment of the invention. 
         FIGS. 2A-2C  illustrate stages in a method for packaging a microdevice in accordance with an embodiment of the invention. 
         FIG. 2A  is a schematic side cross-sectional view of a substrate with first conductive elements and a die with second conductive elements at one stage of a method for packaging microdevices in accordance with the invention. 
         FIG. 2B  is a schematic side cross-sectional view in which the die is inverted such that the first conductive elements are aligned with the second conductive elements. 
         FIG. 2C  is a schematic side cross-sectional view of the substrate and the die after connecting the first conductive elements to the second conductive elements. 
         FIG. 3  is a schematic side cross-sectional view illustrating a stage in another embodiment of a method for packaging microdevices in accordance with the invention. 
         FIG. 4  is a schematic side cross-sectional view illustrating a stage in still another embodiment of a method for packaging microdevices in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION 
     A. Overview 
     The following disclosure describes several embodiments of microdevices and methods for packaging microdevices. One embodiment of a packaged microdevice comprises a substrate having a mounting area, contacts in the mounting area, and external connectors electrically coupled to corresponding contacts. The microdevice also includes a die located across from the mounting area and spaced apart from the substrate by a gap. The die has an integrated circuit and pads electrically coupled to the integrated circuit. The microdevice further includes first and second conductive elements in the gap that form interconnects between the contacts of the substrate and corresponding pads of the die. The first conductive elements are electrically connected to contacts on the substrate, and the second conductive elements are electrically coupled to corresponding pads of the die. The first conductive elements are attached to the second conductive elements at corresponding interfaces such that the interconnects connect the contacts of the substrate directly to corresponding pads on the die within the gap. 
     In several embodiments, the first conductive elements comprise first gold bumps deposited onto the contacts of the substrate, and the second conductive elements comprise second gold bumps deposited onto the pads of the die. The interfaces can comprise ultrasonic joints or other fixed joints (e.g., welds) between corresponding first and second gold bumps. In additional embodiments, the substrate can further comprise an organic dielectric material and conductive traces electrically coupling the contacts to corresponding external connectors. In many embodiments, the pads on the die are juxtaposed to corresponding contacts on the substrate and the first gold bumps are superimposed with corresponding second gold bumps such that the interconnects couple the pads to the contacts without a redistribution structure on the die between the pads and the contacts. 
     In another embodiment, a packaged microelectronic device comprises a substrate having a mounting area, contacts in the mounting area, and external connectors electrically coupled to the contacts. The packaged device can further include a die located across from the mounting area and spaced apart from the substrate by a gap. The die has pads and an integrated circuit electrically coupled to the pads. The packaged device further includes gold interconnects in the gap between the substrate and the die. The individual interconnects have a first cross-sectional dimension at the contacts and a length between the contacts and the die that is greater than the cross-sectional dimension. 
     Another aspect of the invention is directed towards methods for packaging a microelectronic device. One embodiment of such a method includes arranging a plurality of first gold elements in a pattern corresponding to a pattern of contacts on a substrate, and attaching the first gold elements to corresponding second gold elements. The individual pairs of first and second gold elements form individual interconnects. Additionally, the first gold elements are attached to contacts of the substrate, and the second gold elements are attached to corresponding pads of a microelectronic die. 
     In one specific example, the first gold elements are arranged in the pattern corresponding to the pattern of contacts on the substrate by depositing first gold bumps onto the contacts of the substrate. In this embodiment, the second gold elements comprise second gold bumps, and the method further comprises depositing the second gold bumps onto the pads of the dies. The method can further comprise attaching the first gold bumps to corresponding second gold bumps after the second gold bumps have been deposited onto the pads. 
     In a different embodiment, the first gold elements are arranged in the pattern corresponding to the pattern of contacts on the substrate by forming first gold bumps on the contacts of the substrate. The first gold elements can be attached to corresponding second gold elements by depositing second gold bumps onto the first gold bumps, and then the method can further comprise attaching the second gold bumps to corresponding pads of the die after attaching the first gold bumps to the second gold bumps. 
     In an alternative embodiment, the second gold bumps are deposited onto the pads of the die before attaching the first gold bumps to the second gold bumps, and then the first gold bumps are then attached to the second gold bumps. The contacts of the substrate are attached to the first gold bumps after the first gold bumps have been attached to the second gold bumps. 
     Another embodiment of a method of packaging a microelectronic device comprises forming a plurality of first gold elements on contacts of a substrate, and forming a plurality of second gold elements in electrical connection with pads of a microelectronic die. The method further includes aligning the first gold elements with corresponding second gold elements, and attaching the first gold elements to the second gold elements to form electrical interconnects between the substrate and the die. 
     Still another embodiment of a method of packaging a microelectronic device comprises providing a substrate having an organic dielectric material, contacts arranged in an array in a mounting area, and external connectors electrically coupled to the contacts. The method further includes providing a microelectronic die having pads arranged in an array corresponding to the contact array and an integrated circuit electrically coupled to the pads. The method further includes attaching a plurality of the first gold elements to a plurality of second gold elements, attaching the first gold elements to corresponding contacts, and attaching the second gold elements to corresponding pads. The individual pairs of first and second gold elements comprise individual interconnects between the substrate and the die. 
     Specific details of several embodiments of the invention are described below with reference microelectronic devices with a single microelectronic die attached to a substrate. However, in other embodiments, the microelectronic devices can have two or more stacked microelectronic dies electrically coupled to a substrate. The microelectronic devices can be processors, memory devices (DRAM, SDRAM, flash, etc.), imagers, sensors, filters (SAW filters) or other types of devices that require an electrical connection between the dies and a substrate. Several details describing well-known structures or processes often associated with fabricating microelectronic dies and devices are not described herein for purposes of brevity. Also, several of the embodiments of the invention can have different configurations, components, or procedures than those described in this section. A person of ordinary skill in the art, therefore, will accordingly understand that the invention may have other embodiments with additional elements, or that the invention may have other embodiments without several of the elements and features shown and described below with reference to  FIGS. 1-4 . 
     B. Embodiments of Packaged Microdevices 
       FIG. 1  is a schematic side cross-sectional view of a packaged microelectronic device  100  in accordance with an embodiment of the invention. In this embodiment, the packaged device  100  includes a substrate  110 , a die  120  spaced apart from the substrate  110  by a gap  130 , and a plurality of interconnects  140  electrically coupling the die  120  to the substrate  110 . The individual interconnects  140  can include a first conductive element  142  electrically coupled to the substrate  110  and a second conductive element  144  electrically coupled to the die  120 . As explained in more detail below, the first and second conductive elements  142  and  144  are separate components that are attached to each other at interfaces  146 . The first and second conductive elements  142  and  144  can be gold bumps or other small conductive elements that can fit in the gap  130  between the substrate  110  and the die  120  to electrically connect the die  120  directly to the substrate  110  without a redistribution structure on the die  120 . The first and second conductive elements  142  and  144  also combine to have a standoff height or length that mitigates the need to have (a) highly uniform bump heights and (b) parallelism between the substrate  110  and the die  120 . Therefore, as explained in more detail below, the features and advantages of the interconnects  140  enable gold-to-gold interconnects or other small interconnects to be used in a wide range of applications that are currently limited to solder-based interconnects. 
     The substrate  110  includes a dielectric material  111  having a die surface  112  with a mounting area  113  and an external surface  114 . The substrate  110  further includes a plurality of contacts  115  in the mounting area  113 , external connectors  116  at the external surface  114 , and electrical traces  117  between the contacts  115  and corresponding external connectors  116 . The dielectric material  111  can be an organic material, a ceramic material, or another suitable dielectric material. In many applications, the traces  117  are copper lines on layers of an organic dielectric material  111  that are laminated together. 
     The die  120  can be a semiconductor die or other type of microelectronic die. In many applications, the die  120  has an integrated circuit  122  and a plurality of pads  124  electrically coupled to the integrated circuit  122 . The pads  124  can be external bond-pads as shown in  FIG. 1 , or the pads  120  can be embedded in the die  120  and connected to the interconnects  140  by through-wafer interconnects. The die  120  can be a processor, a memory device, an imager, a sensor, a filter, or other type of microelectronic device. Suitable memory devices, for example, include DRAM and flash memory devices. 
     In the embodiment of the packaged microdevice  100  shown in  FIG. 1 , the contacts  115  are arranged in a contact array, and the pads  124  are arranged in a pad array that mirrors the contact array. The die  120  is positioned relative to the substrate  110  such that individual pads  124  are aligned with corresponding individual contacts  115  across the gap  130 . The pads  124  are accordingly juxtaposed or otherwise superimposed relative to corresponding contacts  115 . In other embodiments, the contact array may be a universal array with a large number of contacts such that the array of pads  124  is juxtaposed to only a subset of the contacts  115  on the substrate  110 . 
     The first conductive elements  142  are electrically connected to the contacts  115  of the substrate  110 , and the second conductive elements  144  are electrically coupled to corresponding pads  124  of the die  120 . Individual first conductive elements  142  are attached to corresponding individual second conductive elements  144  at interfaces  146  such that individual pairs of first and second conductive elements  142  and  144  form the direct interconnects  140  in the gap  130  between the contacts  115  and corresponding pads  124 . As a result, in many embodiments the multi-bump interconnects  140  electrically couple the pads  124  to the contacts  115  without constructing a redistribution layer on the die  120 . The first conductive elements  142  can be gold bumps, and the second conductive elements  144  can be gold bumps formed separately from the first gold bumps  144 . The first conductive elements  142  and second elements  144 , moreover, can be attached at the interfaces  146  such that the interconnects  140  have a length “L” between the contacts  115  and corresponding pads  124  that is greater than a cross-sectional dimension “W” at the contacts  115  and/or the pads  124 . 
     The packaged microdevice  100  can further include an optional resist layer  150  (shown in broken lines) on the die surface  112  of the substrate  110  with an opening (not shown) in which the contacts are located. The packaged device  100  can further include an optional casing  160  molded around the die  120  to protect the die  120  from external hazards. In several embodiments, the packaged device  100  can further include an under fill material in the gap  130  to protect the interconnects  140  when the casing  160  is molded around the die  120 . 
       FIGS. 2A-2C  are schematic side cross-sectional views illustrating different stages of a method for fabricating the packaged microdevice  100  in accordance with an embodiment of the invention. Like references numbers refer to like components in  FIGS. 1-2C . Referring to  FIG. 2A , this embodiment of the method includes depositing the first conductive elements  142  onto the contacts  115  and depositing the second conductive elements  144  onto the pads  124 . The first and second conductive elements  142  and  144  are accordingly separate components in this embodiment such that the first conductive elements  142  are attached to the contacts  115  and the second conductive elements  144  are attached to the pads  124  before the first and second conductive elements  142  and  144  are connected to each other. The first conductive elements  142  can be first gold stud bumps and the second conductive elements  144  can be second gold stud bumps. In one embodiment, the first and second gold stud bumps are formed using a wire bonding machine by forming a gold sphere at the tip of the wire, attaching the sphere to the contact  115  or pad  124 , and severing the sphere from the wire. This stage of the method further includes flipping the die  120  (arrow F) to invert the die  120  over the substrate  110  for further processing. In an alternative embodiment, the substrate  110  can be flipped to be inverted over the die  120 . 
       FIG. 2B  illustrates a stage after which the die  120  has been flipped to be inverted over the substrate  110 . At this stage, the method includes aligning the pads  124  with corresponding contacts  115  such that the first conductive elements  142  face corresponding second conductive elements  144 . The method continues by moving die  120  and/or the substrate  110  toward each other until the first conductive elements  142  contact corresponding second conductive elements  144 . 
       FIG. 2C  illustrates a stage at which the first conductive elements  142  contact corresponding second conductive elements  144 . At this stage, the method continues by applying a down force (arrow D) to the die  120  while transmitting an energy (arrow E) to the first and second conductive elements  142  and  144 . The down force and energy form a fixed joint at the interfaces  146  between the first conductive elements  142  and corresponding second conductive elements  144 . The energy can be ultrasonic energy or heat such that the interface  146  is a diffusion joint between the first conductive elements  142  and corresponding second conductive elements  144 . Individual pairs of first and second conductive elements  142  and  144  accordingly form direct interconnects between the contacts  115  and the pads  124 . After connecting the first elements  142  to the second elements  144 , an under fill material may be placed in the gap  130  and a casing may be molded over the die  120  to further protect the die  120 . 
     One advantage of several embodiments of the packaged device  100  is that the multi-bump interconnects  140  with two or more stacked bumps provide a standoff height between the substrate  110  and the die  120 . This can compensate for non-uniformities in the height/lengths of the interconnects and/or a lack of parallelism between the die  120  and the substrate  110 . The larger standoff height accordingly enables laminates and other types of non-ceramic substrates to be used with gold-to-gold interconnects or other types of small interconnects in flip-chip applications. As a result, the advantages of using a gold interconnect or other type of small, lead-free interconnect for environmental purposes can be realized in applications that use laminated substrates and/or high density arrays with high pin counts. 
     Another advantage of several embodiments of the packaged device  100  illustrated in  FIGS. 1-2C  is that the first and second conductive elements  142  and  144  can be formed of gold balls or other small, conductive stud bumps that are much smaller than solder balls. This enables flip-chip interconnection between very fine pitched bond-pad arrays on a die and corresponding contact arrays on a substrate without having to use a redistribution layer. It will be appreciated that redistribution layers have dielectric layers, conductive traces and additional pads to effectively increase the spacing between the pads to a larger array on the die. The direct connection provided by the interconnects  140 , however, eliminates the need to effectively space the pads apart by a greater distance. As such, several embodiments of the packaged device  100  eliminate the costs and time for manufacturing redistribution layers on wafers. 
     Still another advantage of several embodiments of the packaged device  100  illustrated in  FIG. 1  is that the design of the dies is not restricted to bond-pad arrangements that can be wire bonded or need a redistribution layer to be coupled to the substrate. This enables higher pin counts and/or smaller packages. As a result, high performance devices can have relatively small packages for use in cell phones, PDAs, portable computers and other products where space is a premium. 
     The specific embodiment in which the first conductive elements  142  are attached to the second conductive elements  144  only after the first conductive elements have been deposited onto the contacts  115  and the second conductive elements  144  have been deposited onto the pads  124  is expected to provide better diffusion bonding between the first and second conductive elements  142  and  144 . More specifically, the down force used to connect the conductive elements together (see  FIG. 2C ) can be less because the slip interface between the first and second conductive elements  142  and  144  forms a stronger diffusion joint when the first and second conductive elements  142  and  144  are first attached to the substrate  110  and the die  120 , respectively. This is expected to reduce or otherwise mitigate damage caused to the die  120  during the attaching stage illustrated in  FIG. 2C . 
     C. Additional embodiments of Packaged Microdevices 
       FIG. 3  is a schematic side cross-sectional view illustrating a die  120  being attached to a substrate  110  to form a packaged device in accordance with another method of the invention. Like reference numbers refer to like components in  FIGS. 1-3 . In the embodiment illustrated in  FIG. 3 , the second conductive elements  144  are deposited onto the pads  124  of the die  120 , and then the first conductive elements  142  are attached to the second conductive elements  144  to form multi-bump interconnects  140  on the die  120 . The die  120  is then inverted as shown in  FIG. 3  to align the interconnects  140  with corresponding contacts  115  of the substrate. The first conductive elements  142  are then attached to corresponding contacts  115  of the substrate  110  by applying a down force and an appropriate energy to the interconnects  140 . 
       FIG. 4  is a schematic side cross-sectional view illustrating a stage in forming a packaged device in accordance with another method of the invention. In this embodiment, the first conductive elements  142  are deposited onto the contacts  115  and then the second conductive elements  144  are attached to the first conductive elements  142  to form multi-bump interconnects  140  on the substrate  110 . The die  120  is then positioned such that the pads  124  are aligned with corresponding multi-bump interconnects  140 . The substrate  110  and/or the die  120  are then moved toward each other until the second conductive elements  144  contact corresponding pads  124  of the die  120 . The appropriate down force and energy are then applied to the interconnects to attach the second conductive elements  144  to corresponding pads  124 . 
     From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. For example, the illustrated interconnects have two separate conductive elements, but in other embodiments, the individual interconnects can have three or more conductive elements. Additionally, the flip-chip interconnects can be used between stacked dies in a stacked die arrangement. Accordingly, the invention is not limited except as by the appended claims.