Abstract:
Apparatus and methods for shielding a feature projecting from a first area on a substrate to a plasma while simultaneously removing extraneous material from a different area on the substrate with the plasma. The apparatus includes at least one concavity positioned and dimensioned to receive the feature such that the feature is shielded from the plasma. The apparatus further includes a window through which the plasma removes the extraneous material. The method generally includes removing the extraneous material while shielding the feature against plasma exposure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of application Ser. No. 11/003,062, filed Dec. 3, 2004, which is hereby incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates generally to plasma processing, and more particularly, to plasma processing methods and apparatus for selectively removing extraneous material from a substrate. 
       BACKGROUND OF THE INVENTION 
       [0003]    Plasma processing systems are routinely used to modify the surface properties of substrates used in applications relating to integrated circuits, electronic packages, and printed circuit boards. In particular, plasma processing systems are used to treat surfaces in electronics packaging, for example, to increase surface activation and/or surface cleanliness for eliminating delamination and bond failures, improving wire bond strength, ensuring void free underfilling of chips on circuit boards, removing oxides, enhancing die attach, and improving adhesion for die encapsulation. Typically, substrates are placed in the plasma processing system and at least one surface of each substrate is exposed to the plasma. The substrate&#39;s outermost atomic layers may be removed from the surface by physical sputtering, chemically-assisted sputtering, chemical reactions promoted by reactive plasma species, and combinations of these mechanisms. The physical or chemical action may also be used to condition the surface to improve properties such as adhesion or to clean undesired contaminants from the substrate surface. 
         [0004]    During semiconductor manufacture, semiconductor die are commonly electrically coupled by wire bonds with leads on a metal carrier, such as a lead frame. Lead frames generally include a number of pads each having exposed leads used to electrically couple a single semiconductor die with a circuit board. One semiconductor die is attached to each pad and external electrical contacts of the die are wire bonded with nearby portions of the leads. Each semiconductor die and its wire bonds are encapsulated inside a package consisting of a molded polymer body designed to protect the semiconductor die and wire bonds from the adverse environment encountered during handling, storage and manufacturing processes as well as to dissipate the heat generated from the semiconductor die during operation. The molded packages project as three-dimensional features from one side of the otherwise generally-planar lead frame. 
         [0005]    During the molding process, the lead frame and the multiple attached semiconductor die are positioned between two mold halves. One mold half includes numerous concavities each receiving one of the semiconductor die and mimicking the shape and arrangement of the packages. The mold halves are pressed together in an attempt to seal the entrance mouths to the concavities. The molding material, which is injected into the mold, fills the open space inside the concavities for encapsulating the semiconductor die and wire bonds. However, molding material can seep out of the concavities and flow between the mold halves to form thin layers or flash on the exposed portions of the leads. This thin flash has a thickness typically less than about 10 microns. Flash affects the ability to establish high quality electrical connections with the exposed portions of the leads and, hence, with the encapsulated semiconductor die. 
         [0006]    Various conventional approaches have been developed for alleviating the effects of flash. Flash may be prevented by covering the backside of the lead frame with tape during the molding process. However, adhesive may be transferred from the tape to the lead frame backside and remain as a residue after the tape is removed. In addition, tapes suitable for this application are relatively expensive, which needlessly contributes to the cost of manufacture. Flash may be removed after molding by mechanical and chemical techniques, or with a laser. These removal approaches also suffer from restrictions on their use. For example, the lead frame is susceptible to damage from mechanical flash removal techniques, such as chemical mechanical polishing. Chemical processes may be ineffective unless highly corrosive chemicals are used, which potentially raises issues of worker safety and waste disposal of the spent corrosive chemicals. Laser removal is expensive due to the equipment costs and leaves a residual carbon residue behind on the lead frame. 
         [0007]    There is thus a need for plasma processing systems and methods that can efficiently and effectively remove extraneous amounts of material, such as excess molding material, from an area on a substrate while shielding other areas on the substrate from the plasma. 
       SUMMARY OF THE INVENTION 
       [0008]    Embodiments of the present invention address these and other problems associated with removing extraneous material from an area on a substrate with a plasma without exposing features on other areas on the substrate to the plasma. To that end, the present invention provides a shielding assembly for holding a substrate during treatment with a plasma. The substrate has a first area, a feature projecting from the first area, and a second area covered by an extraneous material. The shielding assembly comprises a first member including a concavity positioned and dimensioned to receive the feature and to shield the feature from the plasma and a second member including a window for passing the plasma into contact with the extraneous material for removing the extraneous material from the second area with the plasma. 
         [0009]    One situation in which the shielding assembly of the present invention is particularly beneficial is in removing flash from a lead frame without exposing the molded packages, that project from the otherwise generally-planar lead frame, to the plasma. The semiconductor die inside the semiconductor packages are sensitive to plasma exposure and, therefore, it is desirable to shield the package from the plasma during a plasma deflashing process. 
         [0010]    The shielding assembly may be a component of a processing system further including a vacuum chamber enclosing a processing space capable of being evacuated to a partial vacuum, an electrode positioned in the processing space, and a gas port defined in the vacuum chamber for admitting a process gas into the processing space. The system further includes a power supply electrically coupled with the electrode, the power supply operative for converting the process gas to the plasma. The fixture is positioned in the processing space at a location appropriate for plasma treatment. 
         [0011]    In another aspect of the invention, a method is provided for plasma treating a substrate having a first area, a feature projecting from the first area, and a second area covered by an extraneous material. The method comprises placing the substrate in a processing space of a vacuum chamber and generating a plasma in the processing space. The first area of the substrate is covered with a shielding assembly having a concavity configured to receive and shield the feature from the plasma. The second area is exposed to reactive species from the plasma effective for removing the extraneous material. 
         [0012]    In yet another embodiment, a method is provided for plasma treating a substrate in the processing space of the vacuum chamber. The substrate has a first side, a plurality of features projecting from the first side, a second side opposite to the first side, and an extraneous material on the second side. The method comprises covering the first side of the substrate with a mask having a plurality of concavities each positioned and dimensioned to receive a respective one of the plurality of features projecting from the first side of the first substrate, and sealing about each of the plurality of concavities to prevent the ingress of a plasma into the plurality of concavities. A plasma is generated in the processing space and the second side of the substrate is exposed to the plasma to remove the extraneous material. 
         [0013]    These and other objects and advantages of the present invention shall become more apparent from the accompanying drawings and description thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0014]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the principles of the invention. 
           [0015]      FIG. 1  is a diagrammatic view of a plasma treatment system for plasma treating substrates in accordance with the principles of the present invention; 
           [0016]      FIG. 2  is an exploded view of a shielding assembly for use with the plasma treatment system of  FIG. 1 ; 
           [0017]      FIG. 3  is a perspective view of the assembled shielding assembly of  FIG. 2 ; 
           [0018]      FIG. 4  is a perspective view of the mask of  FIG. 2  illustrating the loading of the substrate into the mask; and 
           [0019]      FIG. 5  is a detailed view in partial cross-section of a portion of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    With reference to the  FIG. 1 , a plasma treatment system  10  includes a treatment chamber  12  constituted by walls that enclose a processing space  14 . During a plasma process, the treatment chamber  12  is sealed fluid-tight from the surrounding ambient environment, evacuated to a suitable partial vacuum, and supplied with a process gas appropriate for the intended plasma treatment. A vacuum pump  16  is used to evacuate the processing space  14  of treatment chamber  12  through a valved vacuum port  17 . Vacuum pump  16  may comprise one or more vacuum pumping devices with controllable pumping speeds as recognized by persons of ordinary skill in the art of vacuum technology. 
         [0021]    Process gas is admitted at a regulated flow rate to the processing space  14  from a process gas source  18  through an inlet gas port  21  defined in the treatment chamber  12 . The flow of process gas from the process gas source  18  to the processing space  14  is typically metered by a mass flow controller (not shown). The gas flow rate from the process gas source  18  and the pumping rate of vacuum pump  16  are adjusted, as needed, to create a processing pressure and environment suitable for plasma generation and suitable for the intended treatment process. Processing space  14  is continuously evacuated simultaneously as process gas is introduced from the process gas source  18  so that fresh gases are continuously exchanged within the processing space  14  when the plasma is present, and any spent process gas and volatile species removed from a substrate  20  are eliminated from the processing space  14 . 
         [0022]    A power supply  22  is electrically coupled with, and transfers electrical power to, an electrode  24  inside of the treatment chamber  12 . Power transferred from the power supply  22  is effective for forming a plasma  26  from the process gas confined within processing space  14  and also controls the direct current (DC) self-bias. Although the invention is not so limited, the power supply  22  may be a radio-frequency (RF) power supply operating at a frequency between about 40 kHz and about 13.56 MHz, preferably about 13.56 MHz, although other frequencies may be used, and a power level, for example, between about 4000 watts and about 8000 watts at 40 kHz or 300 watts to 2500 watts at 13.56 MHz. Those of ordinary skill in the art will appreciate that different treatment chamber designs may permit different bias powers. A controller (not shown) is coupled to the various components of the plasma treatment system  10  to facilitate control of the etch process. 
         [0023]    Plasma treatment system  10  may assume different configurations understood by those of ordinary skill in the art and, therefore, is not limited to the exemplary configuration described herein. For example, the plasma  26  may be generated remote of treatment chamber  12  and delivered to the processing space  14 . Plasma treatment system  10  is further understood to include components not shown in  FIG. 1  that are necessary for operation of system  10 , such as a gate valve disposed between the processing space  14  and the vacuum pump  16 . 
         [0024]    A shield or shielding assembly  30  holds one or more substrates  20  ( FIG. 2 ) in the exemplary treatment system  10  at a position in the processing space  14  of treatment chamber  12  suitable for performing the plasma treatment. Three-dimensional features  28  that project from one side  20   a  of the substrate  20  and the opposite side  20   b  of substrate  20  may be approximately planar. The three-dimensional features  28  are to be protected during the plasma treatment and, accordingly, are to be shielded from the plasma  26  during plasma treatment of substrate  20 . The invention contemplates that shielding assembly  30  may hold a single substrate  20  for plasma treatment. 
         [0025]    With reference to  FIGS. 2-4 , the shielding assembly  30  includes a plurality of first members or masks  34 , a second member or upper frame  36 , and a third member or lower plate  32  that may rest on the powered electrode  24 . Each of the masks  34  is adapted to mask side  20   a  of a corresponding one of substrates  20  so that the three-dimensional features  28  are shielded from the plasma in processing space  14 . The upper frame  36  secures the substrates  20  and masks  34  with the lower plate  32 . 
         [0026]    The lower plate  32  includes a projecting annular rim  38  and parallel, equally spaced ribs  40  each of which extends between opposite sides of the rim  38 . The rim  38  and ribs  40  cooperate to define recesses  42  below a plane defined by the rim  38 . Each recess  42  is dimensioned with a length, width, and depth appropriate to receive a single mask  34 . After the masks  34  are positioned in the recesses  42  and the substrates  20  are positioned in the shielding assembly  30 , an annular peripheral portion  44  of the upper frame  36  may physically contact the rim  38  of lower plate  32  for establishing good electrical and thermal contact. The ribs  40  are generally positioned between adjacent masks  34 . The lower plate  32  may be attached to the electrode  24  or, alternatively, may otherwise be positioned in the processing space  14  at a location suitable for plasma processing. 
         [0027]    Each mask  34  is constructed with multiple concavities  46  each of which is correlated with the three-dimensional features  28  carried on side  20   a  of one or more substrates  20 . Generally, the concavities  46  are arranged, dimensioned and positioned as the reverse image or complement of three-dimensional features  28  projecting from side  20   a.  The depth of the concavities  46  is preferably adjusted so that the rim  38  of lower plate  32  contacts the peripheral portion  44  of upper frame  36 . 
         [0028]    Each mask  34  is oriented spatially with the concavities  46  facing away from the powered electrode  24 . One or more substrates  20  are positioned inside each of mask  34  with the concavities  46  and three-dimensional features  28  coincident and registered. As a result, an exposed upper surface  20   b  of each substrate  20  faces away from the powered electrode  24  and the substrates  20  are oriented such that the three-dimensional features  28  face toward the powered electrode  24 . 
         [0029]    Each of the concavities  46  has dimensions (length, width, and depth) with sufficient clearance to receive one of the three-dimensional features  28 . The concavities  46  may be dimensioned equally or have individual dimensions tailored to accommodate three-dimensional features  28  of differing dimensions across the substrate  20 . As a result, each of the concavities  46  defines a seal with the substrate  20  about the perimeter of each three-dimensional feature  28  adequate to prevent the ingress of the plasma  26 . The invention contemplates that a single mask  34  may be sufficient to shield the substrates  20  and/or that a single concavity  46  may be effective for shielding the three-dimensional features  28  from the plasma  26 . For example, a single mask  34  having a single concavity  46  extending about the periphery of the mask  34  may be effective for shielding the lower surface  20   a  of the substrates  20  from the reactive species in the plasma  26 . 
         [0030]    With continued reference to  FIGS. 2-4 , the upper frame  36  is positioned on the substrates  20  held by the masks  34 . The mass of the upper frame  36  applies a downward force that secures the substrates  20  and masks  34  with the lower plate  32 . The upper frame  36  includes equidistant parallel ribs  48  extending between two opposite sides of the generally rectangular opening defined inside the annular peripheral portion  44 , which divide this space inside the peripheral portion  44  into a plurality of windows  50 . When the shielding assembly  30  is assembled, the ribs  48  are generally positioned between adjacent substrates  20 . Cross members  52  function to strengthen the upper frame  36  and only cover portions of the substrates  20  for which plasma treatment is not required or desired. The specific location of the cross members  52  will depend upon the arrangement of the three-dimensional features  28  on substrate  20  and will operate to divide the windows  50  into even smaller windows. The present invention contemplates that the upper frame  36  may be constructed to deliberately shield areas of the upper surface  20   b  of the substrate  20  from the plasma. Key pins  54  in the diagonal corners of the upper frame  36  are registered with corresponding key bores  56  in the lower plate  32 , of which only one key bore  56  is visible in  FIG. 2 , to ensure registration between these components during assembly of the shielding assembly  30 . 
         [0031]    The lower plate  32 , mask  34 , and upper frame  36  may be formed from any suitable material, like aluminum, characterized by an acceptable thermal and electrical conductivity. An exemplary mask  34  is formed from a five (5) mm thick sheet of aluminum and the concavities  46  are arranged and positioned at locations corresponding to the arrangement and positioning of the three-dimensional features  28  of the substrate  20 . 
         [0032]    In an alternative embodiment of the invention, the recesses  42  in the lower plate  32  may be directly formed into the electrode  24 . The recesses  42 , which serve to prevent lateral movement of the masks  34  and to locate the masks  34  at fixed positions relative to the windows  50  in upper frame  36 , may be replaced by any structure capable of preventing lateral movement. Alternatively, if lateral movement of the individual masks  34  relative to the upper frame  36  is not a concern, such as if masks  34  are all coupled together, the lower plate  32  may be omitted in its entirety. 
         [0033]    In an exemplary intended use of the plasma treatment system  10 , each of the substrates  20  may be a lead frame having semiconductor die encapsulating packages as three-dimensional features  28  and each mask  34  is configured with concavities  46  dimensioned and arranged for masking the packages of the lead frame. The lead frame is plasma treated to remove thin layers of molding material (i.e., flash) created by a molding process during a previous manufacturing stage. 
         [0034]    Embodiments of the present invention overcome the various deficiencies of conventional removal techniques as extraneous material is removed from a substrate  20  without resort to wet chemical etching techniques, mechanical techniques, or the use of a laser, and without damaging the substrate  20 . The process of the present invention is particularly applicable for removing unwanted thin layers of molding material or flash covering the electrical contacts of a lead frame. Flash results from the molding process encapsulating die carried by the lead frame inside respective packages constituted by the molding material. 
         [0035]    In use and with reference to  FIGS. 1-5 , the masks  34  are positioned in the recesses  42  defined in the lower plate  32 , which rests on the powered electrode  24 , and are oriented with the concavities  46  facing away from the electrode  24  and lower plate  32 . The substrates  20  are then associated with the masks  34  such that the three-dimensional features  28  carried by each substrate  20  are received in the corresponding set of concavities  46 . Finally, the upper frame  36  is positioned on the substrates  20  held by the masks  34 . The engagement between key pins  54  of the upper frame  36  and the corresponding key bores  56  defined in the lower plate  32  registers the lower plate  32  and upper frame  36  during assembly of the shielding assembly  30 . 
         [0036]    Adjacent to some or all of the three-dimensional features  28  are structures  58 , of which one structure  58  is shown in the detailed view of  FIG. 5 . The structures  58  may be, for example, the exposed electrically conductive leads of a lead frame. Areas  60  on the structure  58  may be covered by a thin layer of extraneous material, such as flash from a molding operation creating a package encapsulating a semiconductor die, that the plasma processing is intended to remove. 
         [0037]    After the shielding assembly  30  is assembled, the processing space  14  is then evacuated by vacuum pump  16 . A flow of process gas is introduced from process gas source  18  to raise the partial vacuum in the treatment chamber  12  to a suitable operating pressure, typically in the range of about 150 mTorr to about 1200 mTorr, while actively evacuating the processing space  14  with vacuum pump  16 . The power supply  22  is energized for supplying electrical power to the electrode  24 , which generates plasma  26  in the processing space  14  proximate to the substrate  20  and DC self-biases the electrode  24 . The substrate  20  is exposed to reactive species from the plasma  26  in a treatment process suitable for removing the thin layer of extraneous material from the covered areas  60  ( FIG. 5 ) on the substrate  20 . 
         [0038]    The plasma  26  contains reactive species, including atomic radicals and ions, that interact with material on the surface of the substrate  20  being modified. Extraneous material in covered areas  60  ( FIG. 5 ) of the substrate  20  is transformed by surface reactions with the atomic radicals and ions to a volatile gaseous reaction product that leaves the surface as a gas, which is evacuated from the treatment chamber  12  by the vacuum pump  16 . Flash constituted by a variety of different materials, such as different types of molding materials used to encapsulate semiconductor die, may be removed using different plasma compositions. Any surface reaction residue may be removed by providing a different plasma composition. 
         [0039]    While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicants&#39; general inventive concept.