Abstract:
A sacrificial material applied to a thin die prior to die attach provides stability to the thin die and inhibits warpage of the thin die as heat is applied to the die and substrate during die attach. The sacrificial material may be a material that sublimates at a temperature near the reflow temperature of interconnects on the thin die. A die attach process deposits the sacrificial material on the die, attaches the die to a substrate, and applies a first temperature to reflow the interconnects. At the first temperature, the sacrificial material maintains substantially the same thickness. A second temperature is applied to sublimate the sacrificial material leaving a clean surface for the later packaging processes. Examples of the sacrificial material include polypropylene carbonate and polyethylene carbonate.

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to integrated circuits. More specifically, the present disclosure relates to packaging integrated circuits. 
     BACKGROUND 
     Electronic devices are continually shrinking in size to improve portability of the electronic devices. For example, cellular telephones have recently decreased in size to fit in shirt pockets, and are continuing to decrease in size. As the devices shrink, the components inside the device, including the integrated circuits, also shrink. In integrated circuits, a significant amount of the overall thickness is the die rather than circuitry on the die. One method of decreasing the integrated circuit thickness uses thinner dies for the integrated circuits. 
     Thin dies are fragile and difficult to handle during manufacturing processes. For example, when heating a thin die during reflow, unbalanced stresses in the die cause the die to warp. Warpage results in poor contact between interconnects (e.g., non-wets) leading to yield and reliability problems at die thicknesses less than 100 micrometers. A conventional die attach process having wafer warpage is illustrated in  FIGS. 1A-1B . 
       FIG. 1A  is a cross-sectional view illustrating a conventional packaged integrated circuit before heating. A die  120  with interconnects  122  is coupled to a substrate  102  having interconnects  110 . The interconnects  122  are attached to the interconnects  110  through a flux material  112 . The die  120  and the substrate  102  are heated to reflow the interconnects  122  and the interconnects  110  and bond the interconnects  122  with the interconnects  110 . 
       FIG. 1B  is a cross-sectional view illustrating a conventional packaged integrated circuit after heating. During heating, the die  120  may warp due to unbalanced stresses. Warpage at edges of the die  120  is larger than the center of the die  120 . As a result, connections  130  are created between the interconnects  122  and the interconnects  110  in the center of the die  120 . However, non-wets  132  occur between the interconnects  122  and the interconnects  110  at the edges of the die  120 . 
     The non-wets  132  reduce yield and reliability of integrated circuits manufactured that include the die  120 . Thus, there is a need for an improved method of attaching thin dies during manufacturing of integrated circuits. 
     BRIEF SUMMARY 
     According to one aspect of the disclosure, a method of packaging includes depositing a sacrificial material on a die. The method also includes attaching a first group of interconnects of the die to a second group of interconnects of a substrate after depositing the sacrificial material on the die. The method further includes heating the die to a first temperature after depositing the sacrificial material. The first temperature causing the first group of interconnects of the die to connect to the second group of interconnects of the substrate. The method also includes heating the die to a second temperature after heating the die to the first temperature. The second temperature causes the sacrificial material to sublime. 
     According to another aspect of the disclosure, a method of packaging includes the step of depositing a sacrificial material on a die. The method also includes the step of attaching a first group of interconnects of the die to a second group of interconnects of a substrate after depositing the sacrificial material on the die. The method further includes the step of heating the die to a first temperature after depositing the sacrificial material. The first temperature causing the first group of interconnects on the die to connect to the second group of interconnects on the substrate. The method also includes the step of heating the die to a second temperature after heating the die to the first temperature. The second temperature causing the sacrificial material to sublimate. 
     According to a further aspect of the disclosure, an apparatus includes a substrate having a first group of interconnects. The apparatus also includes a die having a second group of interconnects attached to the first group of interconnects. The apparatus further includes a sacrificial layer on a side of the die opposite the second group of interconnects. The sacrificial layer has a sublimation temperature above the liquidus temperature of the second group of interconnects. 
     According to another aspect of the disclosure, an apparatus includes a substrate having a first group of interconnects. The apparatus also includes a die having a second group of interconnects attached to the first group of interconnects. The apparatus further includes means for reducing warpage on a side of the die opposite the second group of interconnects. The warpage reducing means has a sublimation temperature above the liquidus temperature of the second group of interconnects. 
     The foregoing has outlined rather broadly the features and technical advantages of the present disclosure in order that the detailed description that follows may be better understood. Additional features and advantages will be described hereinafter which form the subject of the claims of the disclosure. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the technology of the disclosure as set forth in the appended claims. The novel features which are believed to be characteristic of the disclosure, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure, reference is now made to the following description taken in conjunction with the accompanying drawings. 
         FIG. 1A  is a cross-sectional view illustrating a conventional packaged integrated circuit before heating for die attach. 
         FIG. 1B  is a cross-sectional view illustrating a conventional packaged integrated circuit after heating for die attach. 
         FIG. 2  is a flow chart illustrating an exemplary process flow for die attach with thin dies according to one embodiment. 
         FIG. 3A  is a cross-sectional view illustrating an exemplary die after deposition of a sacrificial material according to one embodiment. 
         FIG. 3B  is a cross-sectional view illustrating an exemplary die after heating to a first temperature according to one embodiment. 
         FIG. 3C  is a cross-sectional view illustrating an exemplary die after heating to a second temperature according to one embodiment. 
         FIG. 3D  is a cross-sectional view illustrating an exemplary die after removal of the sacrificial material according to one embodiment. 
         FIG. 4A  is a graph illustrating a temperature applied during die attach according to one embodiment. 
         FIG. 4B  is a graph illustrating a thickness of a sacrificial material during die attach according to one embodiment. 
         FIG. 5  is a flow chart illustrating an exemplary process flow for die attach with stacked thin dies according to one embodiment. 
         FIG. 6  is a block diagram showing an exemplary wireless communication system in which an embodiment of the disclosure may be advantageously employed. 
         FIG. 7  is a block diagram illustrating a design workstation used for circuit, layout, and logic design of a semiconductor component according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Depositing a sacrificial material during packaging of an integrated circuit (IC) with a thin die temporarily increases the thickness of the thin die to provide additional support for the die. For example, during solder reflow high temperatures applied to a die may cause warpage of a die having a thickness below 100 micrometers. A sacrificial material may be deposited on the thin die before stacking on a substrate and before application of high temperatures in order to inhibit warpage of the die during reflow. The sacrificial material may be used in packaging techniques such as face-to-face die bonding or other technologies in which contacts of the die are facing the substrate or printed circuit board. 
       FIG. 2  is a flow chart illustrating an exemplary process flow for die attach with thin dies according to one embodiment. An exemplary process for die attach begins at block  210  with depositing sacrificial material.  FIG. 3A  is a cross-sectional view illustrating an exemplary packaged integrated circuit after deposition of a sacrificial material according to one embodiment. A sacrificial material  330  is deposited on a die  320 . The die  320  may be, for example, silicon, glass, or sapphire. The sacrificial material  330  may be, for example, polyethylene carbonate (PEC) or polypropylene carbonate (PPC). According to one embodiment, the sacrificial material  330  is spun on to the die  320  at a thickness of between 10 and 100 micrometers. In another embodiment, the sacrificial material  330  may be deposited by chemical vapor deposition (CVD). The die  320  having interconnects  322  is placed on a substrate  302  having interconnects  304 . According to one embodiment, the interconnects  322  are microbumps for flip chip packaging. According to another embodiment, through silicon stacking with or without through vias may be used for stacking. A flux material  306  between the interconnects  304  and the interconnects  322  holds the die  320  in place before heating. According to one embodiment, the flux material may be rosin-based. 
     In one embodiment, the sacrificial material  330  has a sublimation temperature above the liquidus temperature of the interconnects  304  and the interconnects  322 . For example, the liquidus temperature of eutectic SnPb is approximately 183 degrees Celsius and the liquidus temperature of SAC 305  is approximately 221 degrees Celsius. 
     The die attach process continues to block  220  and heats the die  320  to a first temperature.  FIG. 3B  is a cross-sectional view illustrating an exemplary die after heating to a first temperature according to one embodiment. The first temperature may be selected to significantly bond the interconnects  322  with the interconnects  304 . During bonding of the interconnects  322 , the flux material  306  activates and cleans oxide from the solder surface. At the first temperature, thickness of the sacrificial material  330  is substantially constant and provides support for the die  320  to inhibit warpage. 
     After heating the die to the first temperature at block  220 , the die is heated to a second temperature at block  230 .  FIG. 3C  is a cross-sectional view illustrating an exemplary die after heating to a second temperature according to one embodiment. At the second temperature, the sacrificial material  330  sublimes resulting in removal of substantially all the sacrificial material  330 . According to one embodiment, the second temperature is applied for approximately 45-90 seconds to cause sublimation of the sacrificial material  330 . According to another embodiment, the sacrificial material  330  may be partially etched or ground away and a heating process applied to remove remaining residue of the sacrificial material  330 . 
       FIG. 3D  is a cross-sectional view illustrating an exemplary packaged integrated circuit after removal of the sacrificial material according to one embodiment. Removal of substantially all of the sacrificial material  330  on the die  320  facilitates good adhesion to subsequent backside die attach material or overmold materials during packaging. Sacrificial material  330  remaining on the die  320  may inhibit bonding of additional materials to the die  320 . 
     According to one embodiment, a heating process for die attach of the die  320  to the substrate  302  is described with respect to  FIGS. 4A-4B .  FIG. 4A  is a graph illustrating a temperature applied during die attach according to one embodiment. A line  400  represents the temperature applied to a die during a die attach process.  FIG. 4B  is a graph illustrating a thickness of a sacrificial material during die attach according to one embodiment. A line  420  represents thickness of a sacrificial material on a die during a die attach process. 
     At block  220 , the die is heated to the first temperature, T 1 , at a first time, t 1 , as illustrated by point  402  on the line  400 . A thickness of the sacrificial material at the first time, t 1 , is illustrated as point  422  on the line  420 . At the first temperature, T 1 , interconnects of the die bond to interconnects of the substrate. The first temperature, T 1 , may be, for example, a liquidus temperature of the interconnects. At the first temperature, the thickness of the sacrificial material is substantially constant as indicated by the point  422  on the line  420 . 
     At block  230 , the die is heated to a peak temperature of the process, a second temperature, T 2 , at a second time, t 2 , as illustrated by point  404  on the line  400 . A thickness of the sacrificial material at the second time, t 2 , is illustrated as point  424  on the line  420 . At the second temperature, T 2 , the sacrificial material thins until substantially no sacrificial material remains on the die. The second temperature, T 2 , may be, for example, a decomposition temperature of the sacrificial material. 
     Although the line  400  is shown as one set of temperature, the line  400  may take on different profiles. For example, the line  400  may be a continuous ramp without local maximums. In one embodiment, a continuous ramp may be used in tape automated bonding (TAB) to sublimate the sacrificial material  330 . 
     A sacrificial material applied to a thin die during die attach provides additional support for the thin die and inhibits warpage of the thin die. After die attach using the sacrificial material, the thin die may be incorporated into an integrated circuit. The sacrificial material may be selected such that the sacrificial material remains substantially the same thickness at temperatures used for bonding of the interconnects, such as solder liquidus temperatures and sublimates at peak temperatures of the die attach process. 
     A die attach process using the sacrificial material may apply a first temperature for reflow during which the sacrificial material remains substantially the same thickness. The die attach process may apply a second temperature during which the sacrificial material decomposes resulting in removal of substantially all of the sacrificial material. The sacrificial material allows manufacturing using thin dies, such as those below 100 micrometers in thickness, and production of thin electronic devices. 
     The die attach process and sacrificial material may also be applied during stacking of dies as illustrated in the flow chart of  FIG. 5 , which continues from the flow shown in  FIG. 2 . According to one embodiment, at block  540  a second tier die may be attached to a first tier die with a flux material through a interconnects on the first tier die and the second tier die. At block  550 , a second sacrificial material is deposited on the second tier die to inhibit warpage of the second tier die. At block  560 , the second tier die is heated to a third temperature causing the interconnects on the second tier die to connect to the interconnects of the first tier die. At block  570 , the second tier die is then heated to a fourth temperature causing the sacrificial material to sublimate. According to one embodiment, the third and fourth temperature are equal to the first and second temperature, respectively. 
       FIG. 6  shows an exemplary wireless communication system  600  in which an embodiment of the disclosure may be advantageously employed. For purposes of illustration,  FIG. 6  shows three remote units  620 ,  630 , and  650  and two base stations  640 . It will be recognized that wireless communication systems may have many more remote units and base stations. Remote units  620 ,  630 , and  650  include improved packaged ICs  625 A,  625 C, and  625 B, respectively, which are embodiments as discussed above.  FIG. 6  shows forward link signals  680  from the base stations  640  and the remote units  620 ,  630 , and  650  and reverse link signals  690  from the remote units  620 ,  630 , and  650  to base stations  640 . 
     In  FIG. 6 , the remote unit  620  is shown as a mobile telephone, the remote unit  630  is shown as a portable computer, and the remote unit  650  is shown as a computer in a wireless local loop system. For example, the remote units may be cell phones, hand-held personal communication systems (PCS) units, portable data units such as personal data assistants, fixed location data units such as meter reading equipment, set top boxes, music players, video players, entertainment units, navigation devices, or computers. Although  FIG. 6  illustrates remote units according to the teachings of the disclosure, the disclosure is not limited to these exemplary illustrated units. The disclosure may be suitably employed in any device which includes packaged ICs. 
       FIG. 7  is a block diagram illustrating a design workstation used for circuit, layout, and logic design of a die or a circuit implemented on a die as disclosed below. A design workstation  700  includes a hard disk  701  containing operating system software, support files, and design software such as Cadence or OrCAD. The design workstation  700  also includes a display to facilitate design of a circuit  710  or a semiconductor component  712  such as a wafer or die. A storage medium  704  is provided for tangibly storing the circuit design  710  or the semiconductor component  712 . The circuit design  710  or the semiconductor component  712  may be stored on the storage medium  704  in a file format such as GDSII or GERBER. The storage medium  704  may be a CD-ROM, DVD, hard disk, flash memory, or other appropriate device. Furthermore, the design workstation  700  includes a drive apparatus  703  for accepting input from or writing output to the storage medium  704 . 
     Data recorded on the storage medium  704  may specify logic circuit configurations, pattern data for photolithography masks, or mask pattern data for serial write tools such as electron beam lithography. The data may further include logic verification data such as timing diagrams or net circuits associated with logic simulations. Providing data on the storage medium  704  facilitates the design of the circuit design  710  or the semiconductor component  712  by decreasing the number of processes for designing semiconductor wafers. 
     The methodologies described herein may be implemented by various components depending upon the application. For example, these methodologies may be implemented in hardware, firmware, software, or any combination thereof. For a hardware implementation, the processing units may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, electronic devices, other electronic units designed to perform the functions described herein, or a combination thereof. 
     For a firmware and/or software implementation, the methodologies may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. Any machine-readable medium tangibly embodying instructions may be used in implementing the methodologies described herein. For example, software codes may be stored in a memory and executed by a processor unit. Memory may be implemented within the processor unit or external to the processor unit. As used herein the term “memory” refers to any type of long term, short term, volatile, nonvolatile, or other memory and is not to be limited to any particular type of memory or number of memories, or type of media upon which memory is stored. 
     If implemented in firmware and/or software, the functions may be stored as one or more instructions or code on a computer-readable medium. Examples include computer-readable media encoded with a data structure and computer-readable media encoded with a computer program. Computer-readable media includes physical computer storage media. A storage medium may be any available medium that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer; disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. 
     In addition to storage on computer readable medium, instructions and/or data may be provided as signals on transmission media included in a communication apparatus. For example, a communication apparatus may include a transceiver having signals indicative of instructions and data. The instructions and data are configured to cause one or more processors to implement the functions outlined in the claims. 
     Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the technology of the disclosure as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.