Abstract:
A rigid encapsulation package for semiconductor sensors, actuators, and devices is described. In one embodiment, a semiconductor pressure sensor includes a sensor element having a deformable diaphragm for measurement of pressure, and a cap that includes a recess. The cap is attached to the sensor element to form a cavity therebetween. The pressure sensor further includes a leadframe, interconnecting bond wires, a pressure port that is coupled to the sensor element, and a nominally rigid material formed over the sensor element, cap, leadframe, and bond wires. The material may include one or more of the following: epoxy, RTV, resins, and gel. The sensor element may include a built-in stress isolation flexible region. A second pressure port may optionally be attached to the housing for providing differential or gage pressure measurements.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the packaging of semiconductor sensors, actuators, and devices, and specifically, to packaging of semiconductor pressure sensors. 
     2. Background Information 
     Conventional semiconductor devices are packaged using a variety of methods including low cost, injection molding of plastic around the semiconductor die and leadframe. Certain types of semiconductor devices such as pressure sensors, actuators, and stress-sensitive electronic circuits typically cannot be injection molded in the conventional approach without degrading or impairing device operation. Semiconductor pressure sensors based on deformable diaphragms, for example, will typically shift in output signal, sensitivity, non-linearity, and hysteresis if not adequately isolated from contact with rigid materials. 
     Due to high stress sensitivity of the sensors, one method involves attaching a sensor die to a back plate having a closely matched thermal coefficient of expansion (e.g., Pyrex, silicon, or glass), which is then attached to a substrate. Typically, the substrate used is ceramic, PCB, plastic, or a metal header. Depending on the application and degree of accuracy required, the sensor die is attached to the substrate with soft epoxies or RTVs. The soft die attach material acts as a stress isolation mechanism, dampening the transfer of external stresses to the sensor die. The sensor package is housed in a protective enclosure in an effort to eliminate any physical contact with the sensor die itself (other than the measured media). This has typically resulted in a more complicated packaging and manufacturing process for the sensor. Although silicon sensors are similar to integrated circuits, sensor manufacturers have not been as successful, from a cost and reliability standpoint, in utilizing the advances in semiconductor packaging and manufacturing. 
     FIG. 1 illustrates a cross-sectional view of a prior-art semiconductor pressure sensor  1  housed in a plastic package  3 . The pressure sensor  1  includes a semiconductor sensor die  2  having a thin, pressure-sensitive diaphragm  9  that is bonded to the bottom of the plastic package  3  using silicone RTV, as shown by numeral  4 . The sensor die  2  typically contains a series of stress-sensitive resistors connected in a Wheatstone bridge configuration to provide a voltage output which varies with pressure applied across the diaphragm  9 . Bond wires  13  are used to connect the stress-sensitive resistors to a leadframe  5 , which is supported by the plastic package  3 . A lid  7  is attached over the plastic package  3  and contains a hole  8  for the application of external pressure from a top side  11  of the pressure sensor  1 . A second pressure port  10  may be optionally provided from a bottom side  12  of the sensor  1  for gage or differential pressure measurements. 
     The sensor die  2  is typically coated with a gel  6  to prevent direct environmental exposure and corrosion. The gel  6  is a compliant material, capable of transferring pressure to the diaphragm  9 . Although at first, the semiconductor package seems like an elegant solution to the environmental exposure issue, the gel is not impervious to gases and liquids. That is, gases and liquids tend to penetrate the gel, corroding the underlying metal traces, bond pads, and wire bonds. Conventional integrated circuit packaging materials such as thermosetting plastics offer improved environmental protection, though they have historically been unsuitable for pressure sensors since they are rigid, non-compliant, thermally mismatched, and hysteretic, preventing cost-competitive packaging techniques to be utilized. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a rigid encapsulation package for semiconductor sensors, actuators, and devices. The package includes a protective cap that is attached to a semiconductor die in which the cap and die are encapsulated with an injection-molded plastic. In one embodiment, the package is a semiconductor pressure sensor where the semiconductor die includes a deformable diaphragm, and the cap includes a recess. The cap is attached to the semiconductor die to form a cavity therebetween. The pressure sensor further includes a leadframe, one or more interconnecting bond wires, a pressure port that is coupled to the semiconductor die, and a nominally rigid material that is formed over the semiconductor die, cap, leadframe, and bond wires. The material may include one or more of the following: epoxy, RTV, resins, and gel. The semiconductor die may include a built-in stress isolation flexible region. A second pressure port may optionally be attached to the housing for providing differential or gage pressure measurements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a cross-sectional view of a prior-art semiconductor pressure sensor. 
     FIGS. 2 a  and  2   b  illustrate cut-away top and bottom views of a silicon pressure sensor according to one embodiment of the present invention. 
     FIG. 3 a  illustrates a cross-sectional view of the silicon pressure sensor of FIGS. 2 a  and  2   b  where the pressure sensor capsule is also over-molded on the wire bonding side. 
     FIG. 3 b  illustrates a cross-sectional view of a silicon pressure sensor according to another embodiment of the present invention. 
     FIG. 4 illustrates a schematic cross section of the sensor die bonded to the silicon cap according to a preferred embodiment of the present invention. 
     FIG. 5 a  illustrates a metal leadframe strip having sensor elements attached directly thereto according to one embodiment of the present invention. 
     FIGS. 5 b  and  5   c  illustrate different configurations of the pressure sensor after transfer molding. 
     FIGS. 6 a  and  6   b  illustrate cross-sectional views of an over-molded pressure sensor, according to one embodiment of the present invention. 
     FIGS. 7 a  and  7   b  illustrate perspective views of a pre-molded leadframe package having a pressure port, according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention comprises a method and apparatus for packaging semiconductor sensors, actuators, and devices, allowing a semiconductor device to be attached directly to a metal leadframe (via e.g., epoxy, RTV, or solder). The semiconductor devices, such as pressure sensors, are then placed in standard transfer molding equipment used for integrated circuits (“ICs”) to create a new generation of sensor packages that are fully encapsulated in a similar manner to ICs. This method of packaging allows for high volume, low-cost production of products (e.g., pressure sensors) that can serve many existing and emerging applications. 
     In one embodiment, the pressure sensor comprises a sensor die bonded to a cap, a stress-isolation mechanism, and at least one pressure port pathway for the communication of pressure from external sources. In addition, various over-molding methods are performed to produce high volume, low-cost packaging without undue degradation of the sensor performance. 
     FIGS. 2 a  and  2   b  illustrate cut-away top and bottom views of a silicon pressure sensor  100  according to one embodiment of the present invention. FIG. 3 a  illustrates a cross-sectional view of the silicon pressure sensor  100 . Referring to FIGS. 2 a ,  2   b , and  3   a , the silicon pressure sensor  100  has a top side  102  and a bottom side  104  which is exposed to pressure media (e.g., gas or fluid). The pressure sensor  100  comprises a molded plastic housing  106  which embodies a metal insert  108  (hereinafter referred to as a “pressure port”). 
     In one embodiment, the housing  106  is made from diallylphthalate thermoset or liquid crystal polymer (“LCP”) materials and the pressure port  108  is made from Invar  36 , nickel plated kovar, brass or a combination of such materials, providing corrosion resistance and solderability. The metal pressure port  108  may be attached to connecting tubing using epoxy, solders, brazing, welding, or press-fitting. In another embodiment, the pressure port  108  is made out of plastic for attaching the pressure port to connecting tubing using epoxy, RTVs, adhesive, or press-fitting. The pressure port  108  may optionally be snapped into the plastic housing  106 , in which case the pressure port may include an O-ring seal to prevent leakage (not shown). This allows the pressure port  108  to be configured into different shapes and sizes to maintain compatibility with different pressure interfaces. 
     The silicon pressure sensor  100  further comprises a sensor die  110  that includes a diaphragm  112 . A plurality of piezoresistive elements is disposed on the diaphragm  112  and connected in a Wheatstone bridge configuration for sensing external pressure. The pressure port  108  provides a hermetic path for fluids to a backside  115  of the sensor die  110 . A preform solder washer  114 , is provided for soldering and hermetically attaching the sensor die  110  directly to the pressure port  108 . In one embodiment, the preform solder washer  114  contains a combination of tin and silver, and preferably 96.5% tin and 3.5% silver having a eutectic melting point of 210° C. In another embodiment, the preform solder washer  114  contains a combination of gold and tin, and preferably 80% gold and 20% tin having a eutectic reflow temperature of 280° C. The backside  115  of the sensor die  110  may optionally be metallized with chromium and platinum for enhancing solderability and providing protection against exposure to fluids. The molded plastic housing  106  also embodies a metal leadframe  116  for externally interconnecting the circuitry contained within the silicon pressure sensor  100 . In one embodiment, the metal leadframe  116  is made from Alloy  42  having electroless nickel and gold plating. 
     The sensor die  110  is bonded to a silicon or glass cap wafer  118  having a recess which forms a cavity  120  therebetween for allowing the diaphragm  112  to deflect responsive to applied stress, and for providing a vacuum pressure reference. In particular, the sensor die  110  is bonded to the cap  118  in a region around the diaphragm  112  to prevent penetration of the cavity by over-molding or injection molding material. The cap  118  may be optionally bonded to active electronic circuitry  122  (e.g., an application specific integrated circuit “ASIC”) which provides amplification and signal correction. An exemplary active electronic circuitry includes electronically trimmable circuitry which negates the need for laser trimming. The active electronic circuitry  122  includes one or more pads  126  (FIG. 2 a ) for connecting the active electronic circuitry to the leadframe  116 . A conformal epoxy coating  124  (e.g., RTV material) is applied to the topside  102  of the sensor for completing the sensor and protecting the electronics contained therein. 
     FIG. 3 b  illustrates a cross-sectional view of a silicon pressure sensor according to another embodiment of the present invention. As shown therein, a second pressure port  109  is attached to the housing  106  (e.g., with RTV, epoxy, etc.) as shown by numeral  125 . In this embodiment, a passageway  119  is provided from the cavity  120 , through the cap  118  and encapsulant  124 , and to the second pressure port  109  for providing differential or gage pressure measurements. Prior to applying the encapsulant  124 , a thin film coating  123  such as Teflon, silicon nitride, silicon carbide, diamond-like films, gold, nickel, parylene, or gel may be optionally deposited on the internal surfaces exposed to the pressure media for additional protection. The coating  123  may also be deposited on the inside of the second pressure port  109 . The pressure ports  108  and  109  may be configured with various fittings, such as screw threads or barbed ends for connecting tubing. 
     FIG. 4 illustrates a schematic cross section of a stress isolated sensor die  110  bonded to the cap  118  according to a preferred embodiment of the present invention. As shown in FIG. 4, the sensor die  110  includes the diaphragm  112  and an integral stress isolation flexible region  130 . The stress isolation region  130  protects and isolates the diaphragm  112  from thermal, molding, and mounting stresses in the peripheral (or frame) region  132  of the sensor die  110 . The flexible diaphragm  112  includes piezoresistors for sensing bending stresses from pressure applied to the diaphragm  112 . A bond ring  134  is used to hermetically attach the cap  118  to the sensor die  110  and form a vacuum cavity  120  therebetween for providing an absolute pressure reference. By way of illustration, the bond ring  134 , comprising aluminum, gold, or polysilicon, may be formed on the sensor die  110 , while the cap  118  may be coated with germanium, uncoated, or gold coated, respectively. The bond is formed at the wafer level by placing the two wafers in physical contact under elevated temperature and pressure. 
     In addition to or in lieu of the active electronic circuitry  122  shown in FIG. 3 a , active electronic circuitry may be fabricated inside the bond ring  134  of the sensor die  110  and the cap  118 , as shown by dashed lines  128 . Fabricating the electronic circuitry within the bond ring  134  environmentally protects the same from gaseous and/or fluid media. The cap  118  may contain a series of through-the-wafer etched holes for electrically connecting pads (not shown) on top of the cap  118  to circuitry on the sensor die  110 . Active electronic circuitry may also be fabricated outside of the bond ring  134  (e.g., top side of cap  118 ). 
     FIG. 5 a  illustrates a metal leadframe strip  210  having one or more sensor capsules  212  attached directly thereto, according to one embodiment of the present invention. Referring to FIG. 5 a , the sensor capsule  212  comprises the semiconductor die  110  bonded to the cap  118  (FIG. 3 a ). Additionally, pressure ports  108  may be attached to the leadframe strip  210 , by an adhesive or may be welded thereto. The pressure port  108  is also attached to the sensor die  110  of the sensor capsule  212  (e.g., adhesive, solder, etc.). The ASIC  122  may also be attached to the cap  118  of the sensor capsule  212 . The sensor die  110  and ASIC  122  are wire bonded (e.g., using gold ball bonding wires) to the leadframe strip  210  prior to performing a transfer molding operation. 
     The leadframe strip  210  is then placed in a transfer molding equipment where a thermoset compound is molded over the sensor capsule  212 , ASIC  122 , leadframe, and pressure port  108 . The leads are trimmed to separate them electrically, allowing for on-strip testing and compensation. Finally, all tested and compensated units are separated from the leadframe strip  210  into individual units. Additionally, in order to provide further stress isolation from the molded compounds, a layer of silicone gel may be dispensed over the sensor capsule  212 , and ASIC  122  prior to the transfer molding. 
     FIGS. 5 b  and  5   c  illustrate different configurations of the pressure sensor after transfer molding. In these embodiments, the pressure port  108 , sensor capsule  212 , compensation ASIC  122 , and leadframe  116  (FIG. 3 a ) are all molded at the same time. The leads are configured in a dual-in-line package (“DIP”) format (FIG. 5 b ) or as a surface mount technology (“SMT”) format (FIG. 5 c ), though other configurations are possible. FIGS. 6 a  and  6   b  illustrate cross-sectional views of an over-molded pressure sensor, according to one embodiment of the present invention. In this embodiment, the packages incorporate mechanical snapping features  220  on two sides of the package to ease the attachment of the pressure port accessories and field installations (e.g., for attaching to various interfaces). 
     FIGS. 7 a  and  7   b  illustrate perspective views of a pre-molded leadframe package  310  having a pressure port, according to another embodiment of the present invention. In this embodiment, the sensor capsule  212  can be attached to a pre-molded package  310  before being encapsulated. The pre-molded leadframe package  310  comprises a metal leadframe  116  and a pressure port  108  which are molded together optionally with thermoset material or high temperature polymers such as LCP. This type of package typically has a cavity to allow for attachment of the sensor capsule  212  and bonding of the leadframe pads for electrical lead-outs. After completion of the assembly, the top cavity  315  with the sensor capsule  212  and optional ASIC  122  can be encapsulated with a conformal material for full environmental and handling protections. This approach is a more robust solution than today&#39;s state-of-the-art solutions due to elimination of any need for silicone gel and the ability to apply pressure from the bottom. 
     While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art.