Patent Publication Number: US-10312212-B2

Title: Self-adhesive die

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional of and claims priority to U.S. application Ser. No. 14/034,078, filed on Sep. 23, 2013. Said application is incorporated herein by reference. 
    
    
     FIELD 
     The invention embodiments described herein relate generally to semiconductor device packaging and associated die attachment methods. In particular, the invention relates to cost effective packaging methods that provide high thermal performance at the interface of die to lead frame when implemented in molded packages. The principles herein are also applicable to other semiconductor packages and devices. 
     BACKGROUND 
     The embodiments of the invention relate generally to the packaging of integrated circuits (ICs); more particularly, a new die attach method suitable for use in packaging IC dice are described to eliminate die attach materials and reduce the cost, while providing good thermal performance and without the concern of resin bleed-out. 
     There are a number of conventional processes for packaging integrated circuit (IC) dice. By way of example, many IC packages utilize a metallic lead frame. The lead frame typically includes a plurality of leads or contacts, and optionally a die attach pad (paddle) upon which a die may be attached by means of a suitable adhesive material. The die is typically electrically connected to the lead frame leads by appropriate connectors such as bonding wires. In general, the die and portions of the lead frame are encapsulated with a molding material to protect the electrical connections and the delicate electrical components on the active side of the die. 
     The above-mentioned adhesive material can be in the format of paste (called die attach paste) or film (die attach film). Die attach paste, typically contains about 80 wt % silver filler and about 20 wt % polymer resin and additives. There are many limitations with die attach paste including resin bleed out (RBO), inconsistent die attach thickness, outgassing of organic components onto the bond pad on die surface causing non-stick on pad (NSOP) or causing the corrosion, and so on. Die attach film (DAF) has been developed to address some of the problems such as RBO and inconsistent die attach thickness. However, there are still some problems associated DAF such as limited thermal conductivity, sawing and die pickup issues. Moreover, the cost of DAF material is much higher than die attach paste. 
     On the other hand, much effort has been focused on gecko-inspired adhesives, which primarily rely on Van der Waals forces for adhesion by growing nanomaterials (such as carbon nanotubes) on the backside of the wafer or the chip. The growth temperature is about 750-850° C. However microelectronic chips cannot withstand so high temperature. 
     In view of the foregoing, there is continuing effort to produce cost effective packaging methods that provide high thermal performance at the interface of die to lead frame when implemented in molded packages. 
     SUMMARY 
     The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present some concepts of the invention in a simplified form as a prelude to a more detailed description that is presented later. 
     In accordance with an embodiment of the present application, an apparatus is provided. The apparatus comprises: a semiconductor die, having a top side and a backside; a plurality of silicon nanowires formed on the backside of the semiconductor die; a substrate, having a top side with a die attach pad and a backside; wherein the backside of integrated circuit die is positioned on the die attach pad while maintaining the top surface of the die in a parallel with the top surface of the substrate; and wherein the semiconductor die is mechanically attached to the substrate by the silicon nanowires using pressure between 0.01 Megapascal MPa) and 1 MPa. 
     In accordance with another embodiment of the application, a method of attaching a semiconductor die to a substrate is provided. The method comprising the steps; providing a semiconductor wafer containing integrated circuits, wherein the wafer has a top side and a backside; printing an ink containing metal nanoparticles of the back side of the wafer; etching a plurality of cavities on the backside of the wafers using metal assisted chemical etching to form a plurality of silicon nanowires; separating the semiconductor wafer into individual integrated circuit die; providing a substrate, having a top side with a die attach pad, and a backside; positioning the backside of integrated circuit die on the die attach pad while maintaining the top surface of the die in a parallel with the top surface of the substrate; and attaching the integrated circuit die to the substrate by the silicon nanowires using pressure between 0.01 and 1 MPa. 
    
    
     
       DESCRIPTION OF THE VIEWS OF THE DRAWING 
         FIG. 1  is sectional view, including an expanded view of area of interest of a die mounted integrated circuit in according with an embodiment of this invention. 
         FIG. 2  through  FIG. 4  are illustrations of steps in the fabrication of a wafer formed according to an embodiment of this invention. 
         FIG. 5A  is a scanning electron microscope (SEM) photo of Si nanowire arrays on Si. 
         FIG. 5B  is an illustration of the metal-assisted chemical etching of localized regions in a silicon wafer. 
         FIG. 6  is a flow diagram of the process used to mount a die according to an embodiment of this invention. 
     
    
    
     In the drawings, like reference numerals are sometimes used to designate like structural elements. It should also be appreciated that the depictions in the figures are diagrammatic and not to scale. 
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     The embodiments of the invention are described with reference to the attached figures. The figures are not drawn to scale and they are provided merely to illustrate the invention. Several aspects of the invention are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide an understanding of the invention. One skilled in the relevant art, however, will readily recognize that the invention can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the invention. The embodiments of the invention are not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the present invention. 
     The embodiments of the invention relate generally to the packaging of integrated circuits. As explained in the background section, the testing and operation of an integrated circuit (IC) package may subject the package to temperature extremes and other stresses. Such stresses may cause delamination and degrade the thermal performance of the package. The present invention provides a simple, cost-effective approach for die attached to lead frame surface. 
     Referring next to  FIG. 1 , an improved packaging structure design in accordance with one aspect of the invention will be described.  FIG. 1  illustrates a cross-sectional view of one embodiment of the present invention where a die  100  is secured to the die attach pad  101  with Silicon nanowires. The nanowires are between the 10 nm and 1 μm in diameter and 0.1-20 μm long and adhere the bottom of the die to the top surface of the die attach pad. The bottom surface of the die  100  includes a plurality of silicon nanowires. 
     The bond between the die pad  101  and the back of the die  100  is obtained by applying pressure to the die/die attach pad combination. Pressure is typically in the range of 0.01-1 Megapascal (MPa). In addition, the silicon nanowire can be modified with Amine-based, sulfur-based and phosphorous-based silane coupling agents to substantially enhance the adhesion between silicon nanowire and lead frame surface. 
     The die mount procedure mentioned above improves adhesion through increasing the bonding area and possibly forming mechanical interlocking using roughened lead frame, thus enabling enhanced package performance and reliability. 
     Embodiments of the present invention will now be described with reference to  FIGS. 2-6 . A particular embodiment of the regions pertaining to the die  100 , silicon nanowires, die attach pad  101  will be described. 
       FIG. 2  is an exemplary sample of a semiconductor wafer  200  having top  201  and backside  202  surfaces. 
       FIG. 3  illustrates application of metal nanoparticles to the backside of the wafer  200 . Application can be accomplished in a variety of methods. In this particular embodiment, the particles are applied by printing the backside of the wafer with an ink containing metal nanoparticles. The metal nanoparticles can be Platinum (Pt), Gold (Au) or Silver (Ag). The ink is then dried to evaporate the volatile suspension vehicle. 
       FIG. 4  shows the backside of the wafer after silicon nanowires, are formed utilizing Metal-assisted Chemical Etching to form the nanowires. 
       FIG. 5B  is a diagrammatic illustration of the Metal-Assisted Chemical Etching process. In this particular embodiment, the etching is accomplished using the chemical reaction:
 
Si+2H 2 O 2 +6HF→H 2 SiF 6 +2H 2 O+H 2  
 
     As shown in the illustration in  FIG. 5B , where a metal catalyst and a semiconductor interface form a cathode and anode, respectively. Through the metal catalyst, charge injection is sustained from a solution to the substrate and charge is balanced by the cathodic and anodic reactions. The presence of a local site for reaction the metal nanoparticle defines the selectivity of the etching mechanism. 
       FIG. 5A  shows a SEM photo of typical arrays of silicon nanowire arrays formed using the Metal-Assisted Chemical Etching process. 
     Referring next to the flow chart  600  presented in  FIG. 6 , one suitable approach for forming a package having good electrical and thermal performance and low delamination at the die-die attach pad interface will be described. Unless explicitly indicated otherwise, the operations described below may be performed concurrently or in any order. Operations may be added to or removed from flow chart  600 . 
     Step  601  involves providing a semiconductor wafer containing integrated circuit die, having a top side and a bottom side. 
     Step  602  involves depositing metal nanoparticles on the backside of the semiconductor wafer utilizing a printing process wherein the printing process prints ink containg metal nanoparticles on the wafer. The ink is then dried to allow the suspension vehicle in the ink to evaporate leaving the metal nanoparticles on the backside of the wafer. 
     Step  603  involves etching cavities in backside of wafer using metal assisted chemical etching to form nanowires in the backside of wafer. The cavities formed in the backside of the wafer are typically less than 20 μm deep. 
     Step  604  involves separating the integrated circuit die using a method selected from sawing, scribing and breaking or laser cutting. 
     Step  605  involves providing a substrate for mounting the die. The substrate can be a lead frame, a laminate or a ceramic package. 
     Step  606  involves adhering the backside of integrated circuit die to the top surface of the substrate with the nanowires therebetween. Adhering the backside of the integrated circuit die to the top surface of the substrate is accomplished by the physical phenomenon called the Van der Waals Force or chemical interaction if coupling agents are used. The Van der Waals Force is the sum of the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds, the hydrogen bonds, or the electrostatic interaction of ions with one another or with neutral molecules or charge molecules. Since the backside of the die has a plurality of silicon nanowires, intimate contact is achieved with the substrate, which leads to the Van der Waals interaction. The interaction with each silicon nanowire is small, however, since the total number of silicon nanowires is large, the total Van der Waals force is large enough to cause excellent self adhesion of the die to the substrate. Better adhesion can be further achieved with substrates through chemical modification of silicone nanowire surface with silane coupling agents or having a rough top surface contact area of a substrate such as roughened lead frame. Since there is no interface between die and silicon nanowires, the thermal performance will be effectively improved. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only and not limitation. Numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above described embodiments. Rather, the scope of the invention should be defined in accordance with the following claims and their equivalents.