Abstract:
Embodiments of the invention include a semiconductor integrated circuit package that includes a substrate having an integrated circuit die attached thereto. The package includes a ESD shield attached to the substrate. The ESD shield configured to increase the ESD hardness of the package. The ESD shield can further serve to stiffen the package to prevent warping and operate as a heat spreader.

Description:
TECHNICAL FIELD  
       [0001]     The invention described herein relates generally to semiconductor device packaging. In particular, the invention relates to a method and package for a semiconductor device that is resistant to electrostatic discharge that can be caused by various factors.  
       BACKGROUND  
       [0002]     The semiconductor industry makes wide use of standard BGA (ball grid array) type semiconductor packages. Such packages generally include a BT (bismaleimide triazine) core having various metallization and solder mask layers to form the substrate. A semiconductor die is attached to the substrate and electrically connected to various electrical connections of the substrate using ball attach or wire-bonding techniques. The wire bonds and the die are typically encapsulated with a protective layer of encapsulant. Such packages and the methods of their construction are well known to persons having ordinary skill in the semiconductor packaging arts. Additionally, ball attach arrangements are provided with an underfill encapsulant to protect the bonding arrangements of the solder balls.  
         [0003]     Commonly, the packages are then provided with a stiffener and a heat spreader to complete the packages. These completed packages are then subject to a series of tests used to qualify the packages to insure they meet all the necessary specifications prior to shipping.  
         [0004]     One such test subjects the package to a series of electrostatic discharge (ESD) events to determine the packages robustness and resistance to these ESD events. In the present art, each of the ball pins of a ball grid array type package are tested for charge coupled electrostatic discharge failures. To alleviate this problem each die includes shunt circuitry associated with each external connection. One purpose of this shunt circuitry is to provide a level of resistance to ESD events.  
         [0005]     Commonly, testing is done using a device specifically constructed for administering such testing. One example of such a tester is an Orion CDM Tester produced by Oryx Instruments Corporation of Fremont California. Such equipment can be programmed to implement testing for compliance in accordance with a number of test standards. Typical examples being provided by the JEDEC, AEC, and ESDA standards as well as others. One particular standard being JEDEC No. JESD22-C101C.  
         [0006]      FIG. 1  is a simplified schematic cross-section view of a portion of a standard PBGA (plastic ball grid array) package  101  arranged on a tester. Commonly, such packages  101  include a substrate  102  or core. The core is typically sandwiched between two metallization layers which also include layers of solder mask. Most commonly, the core  102  is formed of fiber material suspended in a cured a BT resin material. This core  102  is then treated to form metallization layers. Commonly, copper materials or coated copper materials are used. Other conductive materials are also used. A solder mask layer is then formed over the metallization layers. Typically, the solder mask layer is photolithographically patterned to create a solder mask that can be used to define a corresponding pattern in the metallization layers. Such substrates are commonly very thin, for example, less than about 0.60 mm thick. The methods of accomplishing this are very well known to those having ordinary skill in the art  
         [0007]     With continued reference to  FIG. 1 , the substrate  102  forms part of a semiconductor package  101 . Ball attach pads are typically formed on a backside  105  surface of the substrate  102 . Solder balls  115  are typically formed on the ball attach pads. Additionally, a semiconductor integrated circuit die  110  is mounted to the front side  106  surface of the substrate  102 . In many implementations, solder balls electrically connect the die  110  to associated electrical contact points on the substrate  101 . Vias (not shown in this simplified view) formed in and through the substrate  102  enable the electrical communication between circuitry of the die  110  and the solder balls  115  mounted on the backside  105  of the package  101 . The die and electrical connections at the interface between the die  110  and substrate  102  are commonly encapsulated in a protective layer  112  of encapsulating underfill material.  
         [0008]     Additionally, many prior art devices include a metal stiffener  103  and a heat spreaders  104 . Such elements are known to persons having ordinary skill in the art.  
         [0009]     With continued reference to  FIG. 1 , the package  101  is typically placed upside down (solder ball side up) on a tester  120 . As shown here, the package is placed on an insulated tester board for testing. For example, the package  101  is placed on a tester chuck  12  which has a layer of insulating material  122  (e.g., FR-4or other such materials). The tester board ( 121 ,  122 ) is set at some predetermined electrical potential (e.g., 500 volts). Then a testing probe  123  descends to contact each of solder balls of the package  101 . The probe is commonly set at ground. If the package survives pre-selected test routine without damage the packaged is “qualified”.  
         [0010]     This commonly constructed package has provided satisfactory ESD protection until recently. Now, with increasing scaling of circuit elements formed on the die, smaller less ESD resistant devices and elements are becoming more common. These devices have increased vulnerability to ESD events. Thus, the traditional package format is increasingly lacking in the ability to protect these vulnerable elements from ESD events. Accordingly, the incidence of ESD induced package failure has been rising and is expected to continue to do so as circuitry and device sizes continue to shrink.  
         [0011]     Accordingly, what is needed is a packaging design and approach that provides increased resistance to ESD induced package failure.  
       SUMMARY OF THE INVENTION  
       [0012]     In accordance with the principles of the present invention, an improved semiconductor package and methods for its fabrication are disclosed.  
         [0013]     In general, the present invention is directed toward methods and packages that increase the package impedance of a semiconductor package thereby increasing the resistance of the overall package to ESD associated package failures.  
         [0014]     One embodiment of the invention comprises a semiconductor integrated circuit package having a substrate with a die attached to the front side thereof. The package includes a die electrically connected with the substrate. The package further including a discharge shield configured to protect the die from electrostatic discharge events. In still another related embodiment, the ESD shield can comprise a frame and cap elements.  
         [0015]     In a method embodiment, aspects of the invention involve a method for forming an ESD hardened semiconductor integrated circuit package. The method including providing a substrate having a semiconductor integrated circuit die mounted thereon. An electrostatic discharge shield is mounted on the substrate over the die. A thermal grease is placed between the top and die to facilitate heat flow from the die to the ESD shield.  
         [0016]     Other aspects and advantages of the invention will become apparent from the following detailed description and accompanying drawings which illustrate, by way of example, the principles of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The following detailed description will be more readily understood in conjunction with the accompanying drawings, in which:  
         [0018]      FIG. 1  is a simplified schematic views of a conventional substrate and package used to package semiconductor IC chips.  
         [0019]     FIGS.  2 ( a )- 2 ( b ) are simplified cross-sectional views of embodiments of the invention depicting various ESD shield embodiments formed in accordance with the principles of the invention . 
     
    
       [0020]     It is to be understood that in the drawings like reference numerals designate like structural elements. Also, it is understood that the depictions in the Figures are not necessarily to scale.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The present invention has been particularly shown and described with respect to certain embodiments and specific features thereof. The embodiments set forth hereinbelow are to be taken as illustrative rather than limiting. It should be readily apparent to those of ordinary skill in the art that various changes and modifications in form and detail may be made without departing from the spirit and scope of the invention.  
         [0022]     In general, the present invention encompasses semiconductor package designs that include an ESD shield formed over the die in order to reduce the incidence of ESD induced devices failures. In the following detailed description, semiconductor package embodiments will be disclosed. In particular, the depicted structures show package embodiments having various ESD shield embodiments suitable for increasing package resistance to ESD induced package and device failures.  
         [0023]      FIG. 2 ( a ) depicts one embodiment of a semiconductor package constructed in accordance with the principles of the invention. The depicted embodiment is view in plan view. In some ways the depicted embodiment is similar to a prior art package. However, there are several significant distinctions. The package  200  includes a substrate  202 . The substrate  202  in the depicted embodiment can be an ordinary BT substrate such a described herein above. The substrate  202  can be an ordinary two-layer PBGA substrate. Commonly, such substrates are on the order of 0.60 mm thick although substrates of any thickness can be employed. A die  210  is mounted to the substrate  202 . Such mounting is done using any of a number of different standard techniques.  
         [0024]     In the depicted embodiment, the die  210  is electrically connected to the substrate using a set solder balls  211  that contact via structures (not shown) to electrically connect the die  210  with the backside solder balls  215  of the package. Generally, a layer of encapsulant underfill  212  material is injected between the balls  211  between the die  210  and substrate  202 . Such underfill material is of a type generally known in the art.  
         [0025]     In order to pass the specification, a package must be able to withstand a certain level of electrostatic discharge to be properly “qualified”. Using, for example, a JEDEC standard Charged Device Model (CDM) Electrostatic Discharge specification a device constructed in accordance with the principles of the invention would be able to function properly after being subjected to a current of about 5 A for about 1 nanosecond (ns). Of course other standards exist (such as the previously referenced AEC and ESDA standards) and the inventors contemplate that a package constructed in accordance with the principles of the invention will be sufficiently robust to protect such packages from ESD related failures.  
         [0026]     Again referring to  FIG. 1 , the embodiment includes an electrostatic discharge shield  220  mounted on the substrate  202 . The shield  210  is configured to protect the die  210  from electrostatic discharge events. The electrostatic discharge shield  220  can be simply configured. In the depicted embodiment, the electrostatic discharge shield  220  is simply placed on the substrate  210  in place of the prior art stiffener  103  and heat spreader  104 . It can be affixed using a standard adhesive if desired. One example is a thermal setting epoxy. For example, MC-723 manufactured by Ablestik can be employed. Of course many other adhesive known to those having ordinary skill in the art can also be employed. Additionally, other means of affixing the shield can be employed. Mounting pegs, solder and numerous other approaches can be employed.  
         [0027]     In one particularly useful embodiment, the shield  220  is used to replace the stiffener and the heat spreader. In such an embodiment, a thermal grease  224  is spread between the die and shield to enhance the thermal transfer between the die  210  and the shield  220  which serves as the heatspreader.  
         [0028]     The inventors have discovered that AlSiC (aluminum silicon carbide) works well as an ESD shield  220 . This is because AlSiC has a sufficiently high resistivity enabling the shield to maximize impedance between the die and an external ESD generating source, thereby minimizing ESD current into the die. This also results in reduced capacitance between the die and the external ESD generating source. For example, AlSiC has a resistivity in the range of about 30-50 μΩ·cm (10 −6  ohm centimeters). AlSiC also provides a sufficiently high thermal conductivity to enable its effective use as a heat spreader. For example, AlSiC has a thermal conductivity in the range of about 130-200 W/mK. What makes AlSiC a surprising choice is the CTE (coefficient of thermal expansion) mismatch between AlSiC and ordinary substrate materials like BT. BT has a CTE of about 17 ppm/° C. whereas the CTE for AlSiC is in the range of about 6-9 ppm/° C. The inventors have found that under most operational conditions such a mismatch can still be functional in a package. The inventors also believe that shields comprising BeO and A 1   2 O 3  can also be employed to some good effect. The inventors further contemplate that materials having a resistivity greater than about 30 μΩ·cm and high thermal conductivity high enough to facilitate its use as a heat spreader can also be employed.  
         [0029]     It should be noted that although the invention is described here with respect to a two-layer BGA package, the principles and methodologies described here can readily be used to construct four and six (or more) layer packages, as well as, EPBGA (enhanced plastic ball grid array) packages, leaded packages (those with leads rather than solder balls), or chip scale packages (those that are 15×15 mm 2  or smaller). The support elements can be formed on the front side and/or the backside of the substrate, depending on the amount of stiffening desired.  
         [0030]     Another embodiment is depicted in simplified schematic sectional view in  FIG. 2 ( b ). Again, in many ways the depicted embodiment is similar to that shown, for example, in  FIG. 1 . The package  250  again includes a substrate  202 . Typically, the substrate  202  is an ordinary BT substrate such a described herein above. Again, the substrate  202  can be an ordinary two-layer PBGA substrate or alternatively a four and six (or more) layer package. Also the package can comprise EPBGA (enhanced plastic ball grid array) packages, leaded packages (those with leads rather than solder balls), or chip scale packages. The electrical connections include wire bonds  251  that connect the die  210  to the bottom solder balls  215 . A layer  253  of protective encapsulant material is used to protect and encapsulate the wires  251 . In this embodiment, the ESD shield comprises an ESD shield frame  253  that is mounted on the substrate  202 . The ESD frame  253  is formed of material having good electrical insulation properties. For example, AlSiC can be used as well as other electrically insulative materials. Such mounting can be conducted using a variety of mounting techniques known to those having ordinary skill in the art. In the depicted embodiment an adhesive layer  254  is used to adhere the frame  253  to the substrate  202 . For example, a thermal setting adhesive can be used. An ESD shield cap  255  is then attached to the top of the frame  253 . The ESD cap  255  is formed of material having good electrical insulation properties. Also, in some embodiments it is desirable that the cap  255  have a relatively high thermal conductivity enabling it to function as a heatspreader. Moreover, in embodiments where the frame  253  also demonstrates a high thermal conductivity the frame expands the capacity of the shield as a heat spreader. Therefore, AlSiC also provides an excellent cap  255  material. The cap  255  can be mounted to the frame  253  conducted using a variety of mounting techniques known to those having ordinary skill in the art. In the depicted embodiment an adhesive layer  256  is used to adhere the cap  255  to the frame  253 . Also, a thermal grease  256  can be positioned between the die and the shield to enhance the heat transfer capacity of the package  250 .  
         [0031]     In the depicted embodiments, the shield  220  is separated from the top of the die by about 75 microns and is about 1.2 mm tall. In one embodiment of the invention the package is configured to dissipate an ESD of at least 20 Watts (W).  
         [0032]     Some of the advantages of package embodiments described herein include manufacturability advantages related to the fact that these embodiments require no change in existing die design. Additionally, the inventors contemplate that the shunts currently employed for ESD purposes are no longer as important and in some cases may not be necessary at all. To that end, the removal of the shunts from the die, frees up more space on the die for operational circuitry thereby expanding the functionality of the die. Additionally, the introduction of the heat shield does not substantially affect package manufacturing process flow with the shield replacing existing stiffeners and heat spreaders.  
         [0033]     The present invention has been particularly shown and described with respect to certain preferred embodiments and specific features thereof. However, it should be noted that the above-described embodiments are intended to describe the principles of the invention, not limit its scope. Therefore, as is readily apparent to those of ordinary skill in the art, various changes and modifications in form and detail may be made without departing from the spirit and scope of the invention as set forth in the appended claims. Other embodiments and variations to the depicted embodiments will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims. In particular, it is contemplated by the inventors that support elements constructed for the purpose of increasing the rigidity of a semiconductor package can be formed on the package in any configuration. Although only two configurations are expressly disclosed herein, it should be appreciated by anyone having ordinary skill in the art that, using the teachings disclosed herein, many different package support configurations can be implemented and still fall within the scope of the claims. Further, reference in the claims to an element in the singular is not intended to mean “one and only one” unless explicitly stated, but rather, “one or more”. Furthermore, the embodiments illustratively disclosed herein can be practiced without any element which is not specifically disclosed herein.