Abstract:
Consistent with an example embodiment, there is a method for assembling a wafer level chip scale processed (WLCSP) device from a wafer substrate, the method comprises grinding the back-side of the wafer substrate to a prescribed thickness. A plurality of trenches is sawed along a plurality of device die boundaries on a back-side surface of the wafer, the trenches having a bevel profile. The plurality of trenches is etched until the bevel profile of the plurality of trenches is rounded.

Description:
FIELD 
     The embodiments of the present invention relate to semiconductor device packaging and, more particularly, to modifying WLCSP device die that enhance the manufacturability and quality of products. 
     BACKGROUND 
     The electronics industry continues to rely upon advances in semiconductor technology to realize higher-function devices in more compact areas. For many applications, realizing higher-functioning devices requires integrating a large number of electronic devices into a single silicon wafer. As the number of electronic devices per given area of the silicon wafer increases, the manufacturing process becomes more difficult. 
     The packaging of an IC device is increasingly playing a role in its ultimate performance. For example, in mobile devices (i.e., mobile phones, tablet computers, laptop computers, remote controls, etc), bare WLCSP die may be used in their assembly. WLCSP components save valuable space in the mobile device. After assembly, in some example processes, customers encapsulate these WLCSP devices by injection molding or casing. This manual post-processing of the WLCSP may result in the device damage. 
     To accommodate the lack of space in the mobile devices, the WLCSP wafer is ground on its backside to a prescribed thickness. The grind process induces stress into the substrate, which may weaken and embrittle it. Further, the back-side edges of the bare die have sharp edges which are susceptible to damage from contact with assembly tooling. 
     There is a need for a WLCSP assembly process which can address the challenges raised by the needs of mobile applications. 
     SUMMARY 
     The present disclosure has been found useful in the packaging of semi-conductor devices which find their way into portable electronic devices. In particular, WLCSP products which are furnished as unpackaged die to manufacturers of mobile devices, who in turn encapsulate these devices directly onto a printed circuit board (in an effort to conserve valuable space in the mobile device) may subject these unpackaged die to rough handling. The handling may result in cracking or other latent damage which may not show up until the mobile device reaches the end user. Rounding of the edge profile of back-side of the unpackaged device die reduces the device&#39;s susceptibility to handling damage during assembly by the end user. 
     In example embodiment, there is a method for assembling a wafer level chip scale processed (WLCSP) device from a wafer substrate the method comprises grinding the back-side of the wafer substrate to a prescribed thickness. A plurality of trenches is sawed along a plurality of device die boundaries on a back-side surface of the wafer, the trenches having a bevel profile. A protective film is applied to the front-side surface of the wafer substrate. The plurality of trenches is etched until the bevel profile of the plurality of trenches is rounded. 
     In another example embodiment, an integrated circuit wafer comprises a silicon substrate. There is a plurality of active device die having boundaries on a front-side of the silicon substrate. Trenches having a rounded profile are formed on a back-side of the silicon substrate; the trenches located along device boundaries of the plurality of active device die. 
     In another embodiment, an integrated circuit device is made according to the described method. 
     The above summaries of the present disclosure are not intended to represent each disclosed embodiment, or every aspect, of the present invention. Other aspects and example embodiments are provided in the figures and the detailed description that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIGS. 1A-1B  are examples of damage to WLCSP device die that may occur during assembly; 
         FIG. 2  is a flow diagram of an assembly process according to example embodiments; 
         FIGS. 3A-3B  illustrate a device die before and after the rounding of the edges of the back-side according to an example embodiment; and 
         FIGS. 4A-4E  show cross-sectional views, the preparation of a wafer substrate yielding device die with a rounded edge profile according to an example embodiment. 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     The disclosed embodiments have been found useful in preventing damage to the Wafer Level Chip-Scale Product (WLCSP) devices during their assembly. Such a process may be integrated into the customary back-end assembly. In an example assembly process, bare WLSCP device die, when separated out from the completed wafer substrate, have a very sharp edge profile which makes the device die susceptible to cracking during handling and assembly into the end user&#39;s system. Refer to  FIGS. 1A and 1B . For example, chips  10  and  20  may occur during the system assembly device die  5  and  15 . From a side-view, a crack  25  may propagate from the chips  10  and  15  to active device die regions (see enlarged view  30 ). 
     In an example embodiment, the user takes a wafer substrate having devices. The wafer undergoes electrical tests (e.g., E-sort) to sort out non-functioning devices; other processes may forego E-sort and cull out non-functioning devices after packaging. The tested wafers are mounted on an adhesive tape. A trench is cut into the wafer back-side (in areas corresponding to the scribe lanes) with a beveled dicing blade to obtain a kerf having a beveled side wall. Through an etching process, the edges of the kerf are rounded; the rounding of the edges relieves any stress and micro-damage due to the back grinding and beveled sawing. The die strength on the backside of the device will increase and make the device less susceptible for mechanical impacts. 
     In one example process, a wet etch may be used. The silicon etchant, an example process may use, is a mixture of sulfuric acid, nitric acid, phosphorous acid, hydrofluoric acid and water. The specific ratios of acids and water used are determined by specific process requirements of etch rate, degree of anisotropy, the composition of the silicon substrate, degree of corner rounding, etc. During the etch process, the front side of the wafer is protected with a grinding tape. The wet etching may be a dip-etch or a spin etch. In another example process, a plasma etch may be used (e.g., SF 6 ). In another process, a vapor gas etch may be applied (e.g., XeF 2 ). Before an etching process, the front side of the wafer is covered with a protection layer; the protection layer may be a foil (e.g., a grinding foil, or a temporary acid-resistant coating). 
     The wafer is remounted on an FFC (Film Frame Carrier) also referred to as a “dicing frame.” The wafer is sawed or diced with appropriate blades. The FFC tape is stretched to separate the device die. 
     To further reduce the likelihood of edge damage to the die, in another example process, a “stealth dicing” process may be used for separating such a wafer into individual ICs. Channels are formed in the one or more metallization layers on a front-side of the wafer along respective lanes along which the ICs are to be separated. These (separation) lanes are located between ICs and extend between a front-side of the wafer at the metallization layer(s), and a backside of the wafer at the silicon substrate. After forming the channels, the backside of the silicon substrate is thinned, and laser pulses are applied via the backside of the silicon substrate to change the crystalline structure of the silicon substrate along the lanes. This change in the silicon structure weakens the silicon in the lanes. The changed portions in the silicon substrate and the channels facilitate the propagation of cracks in the silicon substrate along the lanes during expansion of the wafer, while mitigating propagation of cracks outside of the lanes. With this approach, wafer separation can be achieved while mitigating issues that can arise from the formation of cracks. 
     More details of “stealth dicing” may be found in U.S. patent application (Ser. No. 13/687,110) of Sascha Moeller and Martin Lapke titled, “Wafer Separation” filed on Nov. 28, 2012 and is incorporated by reference in its entirety. 
     Refer to  FIG. 2 . In an example embodiment according to the present disclosure, a wafer having undergone electrical testing is mounted on a sawing tape  110 . As required for a given end user, the wafer back-side may undergo a back-grind to reduce the wafer thickness  120 . Into the wafer back-side, trenches are cut between active device die with a blade having a beveled kerf  130 . 
     For example, a pre-grinding thickness, of an eight-inch wafer (20.32 cm) is about 725 μm, for a six-inch wafer (15.24 cm) is about 675 μm. Note that this technique may be applied to wafer substrates of any size and may be useful for twelve-inch (30.48 cm) substrates. Further, devices using balls, bumps, pads, etc. benefit from the protective material. In an example process, a WLCSP is ground to a thickness of about 400 μm with a ball bond side of about 200 μm. It is desirable to achieve a minimum wafer thickness; however, it is limited by the technical ability to thin down wafers with 200 μm bumps. Thickness, in an example process may be in the range of about 150 μm to about 250 μm. 
     The positioning of the trenches may be determined by infrared, x-ray, etc. or other suitable imaging techniques. With an etching process, the kerf edges are rounded  140 . In some end-user applications, the back-sides of the active device die may be laser-marked for product identification  145 . With the device die (active side up), the wafer is remounted onto another flexible foil tape  160 . The wafer is diced or sawed with a blade at the corresponding locations of the beveled trenches  170 . The flexible foil tape is stretched so that devices may be separated  180 . If required, a final testing of devices is done; the separated device die are packed and shipped to the end user  190 . 
     In another example, embodiment, a “stealth” dicing process may be used. After the kerf edges are etched  140 , a laser is applied to the backside of the silicon substrate so as to change the crystalline structure of portions of the silicon along the trenches  155 . Support tape is placed on the back-side of the wafer  157 . The wafer is oriented front-side up and the support tape is removed from the front-side  159 . As mentioned earlier, the front-side of the wafer is sawed  170 , the support tape stretched  180 , and the final testing, packing, and shipping of devices is completed 190. 
     Refer to  FIGS. 3A-3B . After undergoing the process outlined in  FIG. 2 , the WLSCP device die  310  with ball bonds  320  on the active side, has the sharp-edge profile of  3 A transformed to the rounded profile  330  of  FIG. 3B . 
     Refer to  FIGS. 4A-4E . In an example embodiment according to the disclosure, it is instructive to view the process in cross-section. A wafer  410  having ball bonds  420  defining each device. A bevel blade  405  defines the trench having the desired kerf. These trenches  430  are cut between active devices in X and Y directions. With an etchant  425 , the sharp bevel-edged trenches  430  are rounded. In some processes, to facilitate handling and manufacturing, an optional laser  425  may define product marking. The wafer  410  with the rounded trenches  435  is attached to an FFC material  440 . From the ball bond  420  side, the wafer is sliced with a blade  445 . Device die  410   a ,  410   b , and  410   c , for example are separated. The process may be applied to device die of a few square millimeters to those of in the square-centimeter range, that is, from tiny three-pin SCR type devices to very large scale systems on a chip. 
     Numerous other embodiments of the invention will be apparent to persons skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.