Abstract:
A method of preventing the formation of cracks on the backside of a silicon (Si) semiconductor chip or wafer during the processing thereof. Also provided is a method for inhibiting the propagation of cracks, which have already formed in the backside of a silicon chip during the processing thereof and prior to the joining thereto of a substrate during the fabrication of an electronic package. The methods entail either treating the backside with a wet etch, or alternatively, applying a protective film layer thereon prior to forming an electronic package incorporating the chip or wafer.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method of preventing the formation of cracks on the backside of a silicon (Si) semiconductor chip or wafer during the processing thereof. Furthermore, the invention is also concerned with a method for inhibiting the propagation of cracks, which have already formed in the backside of a silicon chip during the processing thereof and prior to the joining thereto of a substrate during the fabrication of an electronic package. Moreover, the invention is also directed to the provision of an electronic package incorporating a substrate, particularly such as in a flip chip arrangement, wherein the substrate is to be joined to a silicon semiconductor chip or wafer, which may be subject to the formation of scratches and cracks, and wherein novel measures are taken to prevent the formation or propagation of the scratches and/or cracks in the backside of the chip. 
         [0003]    In essence, the backsides of silicon chips or wafers are occasionally subjected to the forming of scratches on the backside surfaces during fabrication or processing. These scratches may be caused by the polishing, milling or grinding of the backside of the respective silicon chip or wafer, such as for example, by means of a grinding process, wherein the surface finishing is accomplished by either numerically controlled milling machines, which may be followed by a manual polishing, various chemical and mechanical polishing compositions or slurries, or wet etching and the like procedures which are intended to remove the machine milling or fabrication marks and scratches. 
         [0004]    Moreover, there is also the possibility that scratches or cracks may be formed on the backside of the silicon chip or wafer during the process of joining the latter to a substrate at high temperatures, such as during reflow, whereby during subsequent of the joined cooling components, the silicon chip tends to shrink a lesser amount than the substrate, due to a higher coefficient of thermal expansion (CTE) being present for the substrate in comparison with that of the silicon chip. This differential in shrinkage generates high tensile stresses during thermal cycling in the backside of the silicon chip or wafer tending to cause the formation of microcracks in the silicon chip at the locations of existing minute scratches. Consequently, it is imperative that steps be taken to prevent the cracks propagating from the backside scratches on the silicon chip or wafer towards the front surface thereof, whereby such scratches can be nucleation sites for the cracks during package fabrication or thermal cycling, and which may cause package failure if propagating to the front side of the silicon chip or wafer, which represents the electronic device region of the electronic package incorporating the electrical contacts and circuitry, by a potential short-circuiting thereof. 
         [0005]    2. Discussion of the Prior Art 
         [0006]    Various methods and devices are currently known in the technology, which are adapted in diverse modes to inhibit or restrict the formation of scratches and propagation of microcracks in which the surfaces, and particularly the backside surfaces of semiconductor chips or wafers, which may be constituted of a silicon (Si) material. 
         [0007]    Chang, et al., U.S. Pat. No. 6,887,793 B2, disclose a method for plasma etching a wafer subsequent to the backside grinding thereof, and wherein this etching is adapted to remove a photoresist coating layer present on the wafer surface. The method disclosed in this patent, has nothing in common with the chip or wafer backside treatment analogous to the present invention in preventing the formation of scratches or propagation of cracks in the backside of the wafer or chip. 
         [0008]    Peterson, et al. U.S. Pat. No. 6,844,623 B1 disclose the application of a temporary coating for the protection of a microelectronic device wherein the coating is applied to a wafer surface and thereafter removed. This has no bearing on the method of treatment for protection of the backside of a semiconductor wafer or chip in a manner analogous to the present invention, which is adapted to either inhibit the formation of cracks or scratches or the propagation of cracks in the silicon wafer or chip. 
         [0009]    Hendrix, et al., U.S. Pat. No. 6,514,835 B1 provide for a stress control method for thin films due to mechanical deformation of wafer substrates. Again, this has nothing in common with the inventive methods of protecting the backside of a silicon semiconductor chip or wafer from the formation or propagation of scratches and cracks, which could potentially adversely influence the integrity of an electronic package utilizing the wafer or chip. 
         [0010]    Kane, et al., U.S. Pat. No. 6,790,125 B2 disclose an arrangement for finishing the backside of a wafer or chip by means of a milling machine, and wherein a subsequent polishing or wet etching method is employed to remove machine milling marks and scratches. The foregoing has nothing in common with the present inventive method of protecting the backside of a silicon wafer or chip from the deleterious effects of scratches and cracks which may be present therein, or which have a tendency to substantially propagate internally of the wafer or chip. 
         [0011]    Finally, Maurice, et al., U.S. Patent Publication No. 2004/0241461 A1 discloses a method of protecting the back surface of a wafer by the application thereon of a capping layer thereon during the handling of the wafer. Again, this type of application of the capping layer is different from and has nothing in common with the method of protecting the backside of the wafer or chip pursuant to the present invention. 
         [0012]    Accordingly, in order to prevent the formation of backside cracks or the propagation of cracks formed in the backside of a wafer or chip during processing thereof, pursuant to the invention, alternative methods may be utilized, which present significant advantages over the current state of the technology. 
       SUMMARY OF THE INVENTION 
       [0013]    Pursuant to one particular aspect or embodiment of the invention, the backside of a silicon wafer or chip is equipped with a protective film layer, which is constituted of a material possessing a high degree of hardness, which will prevent the formation of cracks in the backside of the chip and propagation thereof during the subsequent processing of the wafer, such as under high thermal cycling conditions, and protect any electronic components and devices located on the front side of the chip or wafer. 
         [0014]    In accordance with a further aspect of the invention, the backside of a silicon wafer or chip may be equipped with a protective film layer having a high fracture toughness, in effect, resistance to fracture, which will prevent the propagation of any scratches which are present in the backside surface of the wafer or chip, and which, during high temperature processing and thermal cycling when combining the wafer or chip with a substrate, may cause high tensile stresses encountered in the chip or wafer due to differences in the coefficients of thermal expansion with the substrate, to have the scratches from cracks propagating into the wafer or chip tending to adversely influence the integrity of an electronic package incorporating the silicon chip or wafer. 
         [0015]    As represented in a further aspect of the present invention, rather than equipping the backside of the chip or wafer with a film layer, as described hereinabove, the backside of the wafer or chip may be wet etched in order to blunt the internal tip or sharp leading point or apex of any micro crack, which may have already propagated into the silicon wafer or chip. This will reduce any stress concentrations and inhibit the crack from propagating further towards the front side of the chip or wafer where the crack could potentially adversely influence the integrity and functioning of electronic contacts or devices located on that surface. 
         [0016]    Accordingly, it is an object of the present invention to provide novel methods for inhibiting the formation or propagation of micro cracks in the backside surface of semiconductor chips or wafers. 
         [0017]    Another object of the present invention resides in an arrangement for inhibiting the formation or propagation of micro cracks in the backside surfaces of semiconductor chips or wafers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Accordingly, in order to clearly represent the inventive aspects, reference may now be made to the following detailed description, taken in conjunction with the accompanying drawings; in which: 
           [0019]      FIG. 1  illustrates generally diagrammatically a side sectional view of one embodiment of an electronic package incorporating a silicon semiconductor chip structure; 
           [0020]      FIG. 2  illustrates the arrangement of  FIG. 1 , showing the silicon chip with the presence of a crack in the backside surface; 
           [0021]      FIG. 3  illustrates, on an enlarged scale, the fragmentary encircled portion A of the silicon chip displaying a scratch and microcrack which has formed in the backside surface thereof; 
           [0022]      FIG. 4  illustrates, in a view similar to  FIG. 3 , a wet etch treatment of the silicon chip to blunt the microcrack in order to reduce generated stresses; 
           [0023]      FIG. 5  illustrates in a diagrammatic view similar to  FIG. 1 , the application of a crack preventive layer on the backside of the semiconductor chip; and 
           [0024]      FIG. 6  illustrates, on an enlarged scale, a fragmentary portion of the chip of  FIG. 5  showing the film layer for preventing the formation of cracks in the silicon chip. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Referring in detail to the drawings, wherein throughout the various embodiments, similar or identical components are designated with the same reference numerals,  FIG. 1  illustrates, generally diagrammatically, an electronic package  10  including a substrate layer  12 , on which there is arranged a silicon semiconductor chip  14  (or wafer) which has a backside surface  16  facing away from the substrate  12  and a front side  18 , which includes a device region  20 , in effect, electronic circuitry and connections (not shown) and to which there are adhesively fastened or soldered arrays of solder balls  22 , which are in contact with the facing surface  24  of the substrate  12 . All. of the foregoing components may be fabricated in the flip-chip technology, as is well known in the state of the art. Hereby, the solder balls  22  may be connected to the semiconductor chip  14  through C- 4  (controlled collapse chip connects), as widely employed. Communicating with the opposite side  26  of the substrate by means of electrical connections to further electrical components, such as bond pads or the like, are the solder balls through suitable electrically conductive vias  28 , extending through the substrate  12 . The silicon chip  10  may include an underfill  30  in the region facing the substrate  12  extending about the solder balls  22 , and is encompassed by a suitable dielectric encapsulant  32  so as to form a protective environment and which is also in contact with the facing surface  24  of the substrate about the periphery of the underfill  30 . 
         [0026]    As indicated in  FIG. 2  of the drawings, upon occasion, the backside surface  16  of the silicon chip  14  (or wafer), as represented on an enlarged scale in  FIG. 3  by the encircled portion A of  FIG. 1 , shows a scratch  34  having been previously formed in the silicon chip, possibly during grinding or the like surface finishing. This scratch may then, during packaging of the components and exposure to reflow and/or thermal cycling, nucleate into a sharp-pointed micro crack  38  due to a stress concentration at the sharp bottom tip or apex  36  of the scratch, and which micro crack  38  extends into the chip interior so as to end in a pointy tip  40 . 
         [0027]    As shown in  FIGS. 2 and 3  of the drawings, the micro crack  38  may extend downwardly, so as to cause it to propagate towards the front surface  18  of the chip  14 , which faces the region of the active devices and electronics. This microcrack  38 , when propagating through the entire thickness of the silicon chip, may then potentially result in a short-circuiting and create functional problems in the electronic package, and possibly render the latter subject to failure. 
         [0028]    In this instance, prior to the forming of the package, in the event that the backside surface  16  of the silicon chip  14  evidences the presence of a scratch  34  which has propagated into a sharp-pointed micro crack  38  extending at least partially through the thickness of the silicon chip towards the front surface  18  thereof, the chip is subjected to a wet etch process in order to widen and blunt the tip  40  of the micro crack  38 , as shown in  FIG. 4 , and thereby eliminate any stress concentration factor which would tend to cause the micro crack  38  to propagate further through thickness of the silicon chip  14 . This wet etch may be constituted of a solution of HF/HNO 3 , or other solutions which may exert such a blunting effect on the configuration of the microchip. 
         [0029]    Pursuant to a further method of the present invention, as shown in  FIGS. 5 and 6  of the drawings, wherein various structural components are identical or similar to those in  FIG. 1 , set forth that prior to the processing and joining of the silicon wafer or chip  14  to the substrate  12  and encapsulation thereof to form the electronic package  50 , the backside surface  16  of the silicon wafer or chip  14  is equipped with a protective film layer  52  of a crack prevention material, as described hereinbelow 
         [0030]    In one instance, this film material  52  possesses the property of high fracture toughness, such as an aluminum alloy. This film material, which is intended to prevent the scratching of the silicon wafer or chip  14 , after being contacted and located therebeneath may be subject to scratches  54 , which do not pass through so as not to penetrate into the silicon backside surface  16 ; i.e., thereby protecting the latter from forming micro scratches. Materials which also possess a high degree of fracture toughness and which would be suitable for the film layer when applied to the backside surface  16  may be WSi 2  having a fracture toughness of approximately 5.6 MPa m 1/2 ; SiO 2  with 0.85 to 1.15 MPa m 1/2 ; or aluminum alloys possessing a toughness of 33 to 54 MPa m 1/2 . Also employable is tungsten (W) with a toughness of 5.6 to 8.7 MPa m 1/2 , which is applied after a silicide process, or SiC with a fracture toughness of 3.1 MPa m 1/2 ; Si 3 N 4  with 8.3 MPa m 1/2  and wherein the silicon (Si) per se, has a toughness of 0.79 to 0.95 MPa m 1/2 . The aluminum is normally applied subsequent to a contact process, whereas the tungsten is ordinarily applied after effectuating a silicide process step. 
         [0031]    In the event that the backside surface  16  of the silicon wafer or chip  14  is provided with a film layer  52  possessing a high degree of hardness in order to prevent the formation of scratches in the chip backside surface  16 , as also shown in  FIG. 6  of the drawings, applied at preferably the beginning of the packaging process is a film layer constituted of Si 3 N 4 , SiC, DLC or WSi 2 ; whereby Si 3 N 4  has a hardness of about 14 GPa; SiC of 15-30 GPa; and a diamond-like carbon at 20 GPa; and with the silicon (Si) of approximately 12 GPa. 
         [0032]    This process of applying the crack prevention film material to the backside surface of the silicon chip or wafer prior to implementing the process of heat bonding with the substrate  14 , encompassing with the encapsulant  32  will prevent the formation or propagation of micro cracks in the silicon chip or wafer. 
         [0033]    It may also be advantageous that, under some circumstances, the backside surface of the chip or wafer is subjected to a wet etch treatment, as described hereinabove, and thereafter has a protective film layer  52  applied thereto. 
         [0034]    From the foregoing, it becomes readily apparent that by utilizing the foregoing methods of protecting the silicon chip or wafer from the propagation or formation of cracks tending to propagate towards the front side of the chip containing the electronic devices or connections, this will provide an assurance that subsequent to the adhesion to the substrate and encapsulation with the dielectric adhesive, the electronic package will be secure from potential failure during operation. 
         [0035]    While the present invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the present invention. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated, but to fall within the spirit and scope of the appended claims.