Abstract:
An encapsulant applicator comprising a flexor formed of a resilient material and a substantially rigid blade is described. The blade is attached to the flexor in a way that during a smoothing process, a force applied through the flexor is distributed across the second edge of the blade. Another aspect of the invention pertains to a system for forming a substantially uniform layer of material on a surface of a semiconductor wafer. The system of the present invention includes a stencil, an applicator and a conveyor device. The stencil is placed over the surface of the wafer so that an opening in the stencil exposes a portion of the surface of the wafer. The conveyor device is connected to the flexor so that during the smoothing process, the conveyor device moves the applicator across the opening of the stencil. Yet another aspect of the invention pertains to a method for applying a substantially uniform layer of flowable material to a surface of a semiconductor wafer using the applicator as described.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor integrated circuit manufacturing processes. More specifically, the present invention relates to applying encapsulant material onto the surfaces of semiconductor wafers. 
     BACKGROUND 
     As is well known in the art, integrated circuit devices are fabricated on semiconductor wafers. After the integrated circuit devices (or die) have been fabricated, they are separated from the wafer during what is referred to as a singulation process. One such singulation process, referred to as a saw operation, utilizes a saw having a circular (or other appropriate shape) blade to cut through the wafers along what are referred to as scribe lines (that delineate the individual die). One problem associated with this saw operation, however, is that when the saw blade cuts through the wafer, the surfaces of the wafer can be damaged due to stresses induced in the wafer that typically results in chipping, cracking, etc. 
     A conventional approach to protecting the surfaces of the wafer during the saw operation utilizes a protective layer of encapsulant material (such as epoxy). In addition to protecting the wafer surfaces, the protective layer of encapsulant provides a surface upon which identifying indicia (i.e., markings) can be placed. These markings are typically used to identify particular classes, or grades, of die (based upon, for example, speed, power, etc.). In order, therefore, to provide optimal protection as well as a surface suitable for clear marking, the protective layer of encapsulating material must be as uniform as possible since a non-uniform layer of encapsulant substantially reduces the protective properties of the encapsulant layer as well as reduces the ability to produce a clear and legible mark. 
     One approach to forming a uniform layer of encapsulant involves the use of encapsulant applicators that typically take the form of what is referred to as squeegees. A conventionally structured squeegee, however, has the unfortunate tendency to deflect under pressure during a smoothing operation resulting in a non-uniform layer of encapsulant. 
     In view of the foregoing, an apparatus and a method of forming a layer of encapsulant material having a uniform thickness onto semiconductor wafers would be desirable 
     SUMMARY 
     The present invention provides an encapsulant applicator and a method for using the applicator capable of forming a substantially uniform layer of encapsulant material on a surface of a semiconductor wafer. The applicator includes a flexor formed of a resilient material having a first end, and a substantially rigid blade. The blade has a first edge and a second edge, wherein the first edge is attached to the first end of the flexor in a way that during a smoothing process, a force applied through the flexor is distributed across the second edge of the blade. 
     Another aspect of the invention pertains to a system for forming a substantially uniform layer of material on a surface of a semiconductor wafer. The system of the present invention includes a stencil, an applicator and a conveyor device. The stencil is placed over the surface of the wafer so that an opening in the stencil exposes a portion of the surface of the wafer. The applicator includes a flexor formed of a resilient material having a first end and a second end and a substantially rigid blade having a first edge and a second edge. The first edge of the blade is attached to the first end of the flexor in a way that during a smoothing process, a force applied through the flexor is distributed across the second edge of the blade. The conveyor device is connected to the second end of the flexor so that during the smoothing process, the conveyor device moves the applicator across the opening of the stencil. 
     Yet another aspect of the invention pertains to a method for applying a substantially uniform layer of flowable material to a surface of a semiconductor wafer using the applicator as described. The method involves sweeping the applicator across an opening of a stencil such that the applicator is in contact with the stencil. 
    
    
     These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures, which illustrate by way of example the principles of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 illustrates a perspective view of one embodiment of a system for applying an encapsulating material onto the surface of a semiconductor wafer. 
     FIG. 2 illustrates a side plan, cross-sectional view of an alternative embodiment of a system for applying an encapsulating material onto the surface of a semiconductor wafer. 
     FIG. 3 illustrates a side plan, cross-sectional view of an applicator of the present invention according to one embodiment. 
     FIG. 4 illustrates a front plan view of the applicator of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described in detail with reference to a few preferred embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps have 7 not been described in detail in order to not unnecessarily obscure the present invention. 
     The present invention provides an encapsulant applicator capable of forming a substantially uniform layer of encapsulant material on a surface of a wafer. Generally, the encapsulant applicator includes a substantially rigid and elongated blade that is attached to a flexible strip of material (referred to as a flexor) that acts to regulate the pressure with which the blade is pressed upon the stencil during a smoothing operation. 
     In the described embodiment, during the smoothing operation, encapsulant material is deposited on a surface of the wafer defined by what is referred to as a stencil. As well known in the art, stencils are used to expose a selected portion of a surface of the wafer onto which the encapsulant is deposited. After the encapsulant has been deposited, in one embodiment, a rounded edge of the blade is swept across the stencil thereby removing excess deposited encapsulating material. The rigidity of the blade allows the blade to form a substantially uniform layer of encapsulating material upon the surface of the wafer so defined by the stencil. It is this uniform layer of encapsulating material that substantially protects the surface of the wafer from chipping during the singulation process as well as provides a substantially planar surface suitable for effective marking of the individual wafer. 
     The invention will now be described in terms of a semiconductor wafer and the associated integrated circuits formed thereon. It should be noted, however, that this invention can be used for any situation where the formation of a substantially uniform layer of viscous material is important. Such situations include, for example, applying encapsulating material during the described integrated circuit singulation procedure, applying solder paste onto wafers during processes for forming solder bumps, and applying under bump metallurgy to allow for better solder bump connections. 
     Turning now to FIG. 1, a side perspective view of an encapsulant application system  100  in accordance with the present invention is shown. The system  100  includes a semiconductor wafer  102  that is secured to a pallet  104  on top of which is placed a stencil  106 . In the described embodiment, the stencil  106  is formed of a sheet of stainless steel full hard having an opening  108 , that exposes a selected portion of a surface  105  of the wafer  102  Once the stencil  106  is in place and well secured, liquid or gel-like encapsulating material (not shown) such as epoxy is applied within the opening  108  of the stencil  106  such that the exposed surface  105  of the wafer  102  is completely covered by the encapsulant. 
     An encapsulant applicator  110  is secured to an applicator holder  112 , in such a way that a lengthwise portion of the applicator  110  lies flat on (or parallel with) the surface of the stencil  106 . The holder  112 , in turn, is secured to a conveyor device  114  that is positioned above the stencil  106  and which slides across the stencil opening  108  during a smoothing operation. In this way, during the smoothing operation, the conveyor device  114  moves the encapsulant applicator  110  across the opening  108  of the stencil  106  in such a way that any excess encapsulating material is removed and a substantially uniform layer of encapsulating material is left behind on the selected portion  105  of the wafer  102 . 
     In the described embodiment, the applicator  110  is formed of a flexor  116  onto which is attached a blade  118 . The flexor  116 , in turn, connects the blade  118  to the applicator holder  112 . The holder  112  positions the applicator  110  such that the length of the blade  118  is in contact with the stencil  106 . The holder  112  also presses the applicator  110  towards the stencil  106  such that the blade  118  maintains firm contact with the stencil  106 . The flexor  116  may bend under the pressure applied by the holder  112 , thereby regulating the amount of pressure that the blade  118  exerts upon the stencil  106 . The flexor  116  maintains at least a minimum amount of pressure on the blade  118  such that the blade  118  is able to maintain contact with the stencil  106  as the blade  118  encounters the excess encapsulating material on the surface  105  of the wafer  102 . At the same time, the flexor  116  prevents the application of excessive pressure that would cause the blade  118  to damage the stencil  106  during the smoothing operation. The flexibility of the flexor  116  also equalizes the pressure exerted by the holder  112  such that an equal amount of pressure is applied along the length of the blade  118 . The regulating and equalizing functions of the flexor  116  allow the applicator  110  to form a flat and uniformly thick layer of encapsulating material. 
     As the applicator  110  is swept over the stencil opening  108 , only the outer portions of the blade  116  are in contact with the stencil  106 . The applicator  110  is able to form an encapsulating layer of material having a uniform thickness due to the rigidity of the blade  118 . Specifically, the rigidity of the blade  118  prevents the pressure applied by the holder  112  from bending the middle portion of the blade  118  downwards into the stencil opening  108 . Such bending would cause the middle portion of the blade  118  to remove more encapsulating material than the outer portions of the blade, thereby forming an encapsulating layer with a concave profile and a non-uniform thickness. 
     Since integrated circuit devices may be formed on wafers of varying diameters, stencils are created to have proportionally sized openings. Consequently, in order to properly apply a uniformly thick layer of encapsulating material within the entire stencil opening, the length of the applicator must be at least as long as the diameter of the stencil opening. For example, applicators  110  of the present invention may be tailored for standard wafer sizes of 4, 5, 6 and 8 or more inches. 
     Depending upon various semiconductor fabrication parameters, either a single applicator or multiple applicators are attached to the applicator holder  112 . Such fabrication characteristics may include viscosity of the encapsulating material and the size of the wafer. Embodiments using a single applicator may be designed to remove encapsulating material while sweeping the applicator across the stencil opening in a single direction. However, in alternative embodiments, the holder positions the applicator  110  such that it removes excess encapsulating material as it sweeps over the stencil opening in both the back and forth directions. On the other hand, embodiments utilizing multiple applicators may be arranged such that each respective applicator is positioned to sweep away excess encapsulating material as it is moved in one specific direction over the surface of the stencil  106 . When two applicators are attached to the applicator holder  112 , for example, the first (or front) applicator is raised relative to the second (or rear) applicator so that only the first applicator sweeps across the stencil opening in one direction. Upon sweeping in the opposite direction, the first applicator is raised and the second applicator is lowered so that only the second applicator sweeps across the stencil opening. It should be noted that each of the multiple applicators may effectively remove excess encapsulating material in both the back and forth directions in a configuration wherein both the first and second applicators make contact with and sweep across the stencil opening in both directions. It should also be appreciated that the applicator  110  may be used to apply materials other than encapsulating material. For example, the applicator  110  may also be used to apply solder paste onto wafers through a printed screen in order to create solder bump contacts on the wafer. 
     FIG. 2 illustrates an enlarged, cross-sectional view of the applicator  110  while being swept across a layer of encapsulating material  200  within the opening  108  of the stencil  106  from left to right. FIG. 2 shows how the encapsulating material  200 , which has been swept by the blade  118  (to the left of the blade  118 ), has a flat and smooth surface  202  since the excess encapsulating material has been swept away by the blade  118 . To the right of the blade  118 , however, the surface of the encapsulating material  200  is still uneven and rises above the height of the stencil  106 . Directly in front of the blade  118  (to the right of the blade  118 ) the excess encapsulating material  200  is being pushed off of the layer of material  200  at or below the level of the stencil  106 . 
     As can be seen in FIG. 2, the applicator holder  112  clamps onto the upper length-wise portion of the flexor  116 . The upper length-wise portion of the blade  118  is attached to the bottom length-wise portion of the flexor  116 . The flexor  116  is shown as bending under the downward pressure applied by the applicator holder  112 . The flexibility of the flexor  116  regulates the amount of pressure transferred to the blade  118  and therefore the amount of pressure applied by the blade  118  on to the stencil  106 . Additionally, the flexor  116  equalizes the pressure applied by the holder  112  along the length of the blade  118 . 
     It is important that the blade  118  have a degree of rigidity sufficient to resist deformation while passing over the opening of the stencil  106 . Specifically, the blade  118 , which is pressed onto the stencil by the holder  112 , must be sufficiently rigid so not to bend into the stencil opening  108  and towards the wafer  102 . This allows the applicator  110  to provide the semiconductor wafer  102  with a layer of protective encapsulating material that has a substantially uniform thickness. The edge of the blade  118  that is in contact with the stencil is generally straight such that the resulting epoxy layer can be given a substantially flat surface. To ensure that the surface of the encapsulating material  200  receive a smooth surface, the surface of the blade  118  that sweeps away excess encapsulating material may be ground to have a smooth finish. A smooth encapsulant layer  200  allows for the application of identification marks upon the encapsulant layer  200  that are clearer and therefore more easily identified during manufacturing processes. The smooth surface of the blade  118  also provides an operational benefit of reducing the amount of encapsulating material that may adhere to the blade  118  itself. 
     To describe the structure of the applicator  110  in greater detail, the side and front views of one specific embodiment of the applicator  110  is illustrated in FIGS. 3 and 4. In this embodiment, the applicator  110  has a length, L, of approximately 8 inches. This applicator  110  generally is used to smooth a layer of encapsulating material applied to the surface of wafers having 6-inch diameters or less. It is possible to use this applicator with respect to wafers of up to 8-inch diameters. In alternative embodiments, the applicator  110  may have a different length so long as it has a length sufficient to cover the diameter of the opening of a stencil. For example, to apply encapsulating material to an 8-inch wafer, an applicator  110  having a length of at least 10 inches can be used. 
     In this embodiment of the flexor  116 , the flexor  116  has a height, H 1 , of 1 inch and a thickness, T 1 , of 8-10 mils. As may be appreciated, the height and thickness of the flexor  116  may vary depending upon specific design requirements. For example, the flexor  116  may be formed to have varying heights, H 1 , and thickness, T 1 , such that the flexor  116  exhibits a specific degree of elasticity. The flexor  116  is made of stainless steel however, it may also be made from other flexible materials. 
     The blade  118 , in this embodiment, has a thickness, T 2 , in the range of approximately 180-250 mils and a height, H 2 , of approximately 0.6 inches. Also, as may be appreciated, the thickness and the height of the blade  118  may vary depending upon the specific design requirements. For example, the blade  118  may be formed to have varying heights, H 2 , and thickness, T 2 , such that the blade  118  exhibits a specific degree of rigidity. The blade  118  is preferably made out of 440C stainless steel due to its resistance to corrosion. In alternative embodiments, the blade  118  may be composed of alternative materials that may also be given a smooth surface and which are also corrosion resistant. For example, the blade  118  could be composed of a composite material of plastic and epoxy. 
     As illustrated in FIG. 3, some embodiments of the applicator  110  have an edge that is finely ground to have a rounded surface  300 . By running the rounded edge  300  of the blade  118 , as opposed to a sharp edge, along the surface of the stencil, scraping-type damage that the blade  118  may cause to the stencil  106  is minimized. Preferably, the radius for the rounded edges, R, of the blade  118  is approximately 35-45 mils. However, the radius of the rounded edges may vary outside of this range depending upon the particular type of stencil that is used, the encapsulating material involved, or the pressure applied to the applicator  110 . 
     In this embodiment of the applicator  110 , the flexor  116  and the blade  118  are connected via screws  302 . Alternatively, it is possible to connect the flexor  116  and the blade  118  using adhesive or any other appropriate attaching mechanisms. 
     While this invention has been described in terms of several preferred embodiments, there are alteration, permutations, and equivalents, which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.