Patent Publication Number: US-6218214-B1

Title: Integrated circuit package for flip chip and method of forming same

Description:
This application is a Division of Ser. No. 09/059,080 filed Apr. 13, 1998. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an integrated circuit package for flip chips and, more particularly, to an integrated circuit package and method of forming the integrated circuit package using a ceramic substrate for receiving a flip chip. 
     BACKGROUND OF THE INVENTION 
     Large scale integrated circuit chips have many input and output connections. In order to accommodate the many connections, manufacturers have produced leadless chip carriers that use a wire bonding process. However, a wire bonding process can be expensive. In order to reduce the complexity and expense of a wire bonding process, manufacturers have increasingly used flip chip technology. 
     In a flip chip, an integrated circuit carries a pad arrangement on the top surface and is turned upside down (i.e., flipped), thus allowing direct coupling between the pads and matching contacts on the main circuit board or chip carrier. In a typical flip chip, solder or gold bumps are formed on the integrated circuit input/output terminals. The flip chip is directly bonded to a chip carrier or other structure by a solder connection. 
     U.S. Pat. No. 5,019,673 to Juskey, et al. discloses a flip chip package for integrated circuits that allows flip chip removal for ready testing and/or replacement. In the &#39;673 patent, a flip chip package includes an over-molding of an integrated circuit assembly. A flip chip is mounted to a thin chip carrier. The flip chip includes an array of bumped pads, which fill an array of matching conductive through holes on the chip carrier. The chip carrier includes an array of bumped contacts on its back surface, which corresponds to bumped pads of the flip chip. The transfer over-molding of the integrated circuit assembly provides a layer of epoxy around the exposed surfaces of the flip chip, thus allowing an environmentally protected and removable integrated circuit package. 
     Although the structure does provide a removable flip chip package, processing requirements, such as microwave systems, require even greater space saving structures and manufacturing techniques that are efficient and low cost. The requirements for increased miniaturization of low volume and proprietary systems has generated a need for increased density of active devices, such as flip chips. A process for implementing flip chip packaging on a die, such as a mimic die, is not available because of grounding difficulties. This is seen especially with Galium Arsenide devices having an air bridge. It is also difficult to under fill these types of flip chips because of the shorting problems. Additionally, these types of chips also require grounding on the back side. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide an integrated circuit package and method of forming same that allows flip chip packaging such that dies can be mounted in a relationship for same space packaging and increased density. 
     In accordance with the present invention, an integrated circuit package includes a ceramic substrate having a cut out configured to receive a flip chip. The cut out includes vias forming through holes. A flip chip is received within the cut out of the ceramic substrate and has conductive bumps formed thereon corresponding to the electrical contact pads of the flip chip. The conductive bumps are received within the through holes in the ceramic substrate. The through holes are also plated with a conductive ink. A controlled impedance line is secured to the conductive bumps by adhesive means, such as conductive epoxy, and acts as a simulated coax. 
     A second integrated circuit chip is mounted back-to-back on the flip chip received within the cut out. The flip chip and second integrated circuit chip are grounded together, such as by conductive epoxy. In accordance with one aspect of the present invention, the flip chip and second integrated circuit chip are stepped in configuration. Conductive bumps are formed as epoxy having a conductive material impregnated therein. 
     In still another aspect of the present invention, the integrated circuit package comprises a ceramic substrate having a cut out, which includes through holes. A flip chip is received within the cut out of the ceramic substrate and has conductive bumps formed thereon which are received through the through holes of the ceramic substrate. A heat sink is mounted on the flip chip. A second integrated circuit can be mounted on the heat sink and the second integrated circuit and can be electrically connected to conductive patterns formed on the ceramic substrate. The second integrated circuit is also grounded to the flip chip. 
     In a method aspect of the present invention for forming an integrated circuit package, the steps comprise forming a ceramic substrate having a cut out configured to receive a flip chip wherein the cut out includes through holes. The method further comprises forming conductive bumps onto a flip chip that correspond to the electrical input/output contacts of the flip chip, and receiving the flip chip within the cut out of the ceramic substrate so that the conductive bumps are received within the through holes of the ceramic substrate. The method also includes securing a controlled impedance line to the conductive bumps. In still another aspect of the present invention, the method comprises mounting a second integrated circuit chip on the flip chip in back-to-back relationship and grounding together the flip chip and second integrated circuit chip, such as by an adhesive epoxy. The method can further comprise mounting a heat sink on the flip chip followed by mounting a second integrated circuit chip on the heat sink. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become apparent from the detailed description of the invention which follows, when considered in light of the accompanying drawings in which: 
     FIG. 1 is a schematic sectional view of a flip chip and econd integrated circuit chip formed as a silicon die received within the ceramic substrate in back-to-back relationship, and showing a controlled impedance line that simulates coax when connected to conductive bumps of the flip chip. 
     FIG. 2 is another sectional view of a flip chip having a heat sink received on the flip chip, and received within a ceramic substrate. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     The present invention now provides an integrated circuit package, illustrated generally at 5, and a method of forming the integrated circuit package that allows for back side grounding of a flip chip and similar mimics and provides an integrated circuit package for “same space” packaging, if flip chips. This packaging feature of the present invention can be applied to a phased array component level assembly. It allows increased miniaturization of low volume and proprietary systems and allows increased density of active devices, such as flip chips. 
     This process for implementing flip chip packaging on a mimic die has not been currently available because of grounding difficulties between two chips. The present invention now allows back-to-back flip chip packaging with grounding. Excessive wire bonds are also eliminated to provide increased performance and repeatability. The integrated circuit package and method of forming the package is especially relevant for Galium Arsenide devices. These type of devices typically have air bridges, thus preventing flip chips from being placed on their face, which could cause shorts. These Galium Arsenide chips also must be grounded on the back side. Thus, the present invention allows not only back side grounding, but also allows for back-to-back grounding of two chips, thus allowing a space saving of 30% to 50%. 
     Referring now to FIG. 1, there is illustrated a ceramic substrate  10  that has been formed in a stepped configuration having a first cut out  12  and first ledge  14 , followed by larger cut out  16  and second ledge  18  to receive a Galium Arsenide chip configured as a flip chip  20 . The ceramic substrate  10  is typically formed in its green state by stacking together layers  10   a-e  of green tape, which in the green state can be easily formed before it is hardened by heating. Typically, the green tape is about three mil thickness. 
     As is well known to those skilled in the art, the green tape ceramic has metallized conductive patterns  22  formed thereon, forming signal paths, such as the grounding signal path on layer  10   d.  Such patterns  22  can be formed by silk screening or other techniques known to those skilled in the art. Once the metallized conductive patterns  22  are formed on the green tape ceramic sheets, the sheets are stacked together forming the desired ceramic substrate. The type of ceramic substrate used by the present invention is well known to those skilled in the art. 
     The ceramic substrate has a cut out  12  that allows a flip chip to be received within the cut out, and a second cut out  16  for a heat sink or second integrated circuit chip. The cut out includes vias formed as through holes  24 . The cut outs  12 , 16  can be formed by processing techniques known to those skilled in the art. The cut out  12  is dimensioned to receive the flip chip (also referred to as a mimic chip). 
     In accordance with the present invention, the flip chip  20  has conductive bumps  26  formed thereon corresponding to the electrical input/output contacts of the flip chip. The conductive bumps  26  can be formed, such as by bumping using gold (Au) wire. It is also possible to form the conductive bumps using an epoxy having a conductive material (such as silver powder) impregnated throughout the epoxy to form conductive bumps. 
     The through holes  24  are also plated with a conductive ink  28  to allow ready conduction. As shown in FIG. 1, the flip chip is received within the first cut out  12  and the conductive bumps are received in the through holes  24 . Control impedance lines  30 , which in effect simulate coax, are secured by conductive epoxy  32  to the conductive bumps  26 . 
     A second integrated circuit chip  34  (also a flip chip in the illustrated example) is mounted back-to-back on top of the flip chip  20 . This second integrated circuit chip  34  can be secured by a conductive epoxy  36  to allow back-to-back grounding of both chips, via conductive grounded patterns that had previously been formed on the green tape ceramic. Wire bonds  38  can connect the second integrated circuit chip to a conductive pattern  38   a  that has been formed on the ceramic substrate, such as by silk screening as noted before. 
     In the embodiment shown in FIG. 2, instead of a conductive epoxy situated between the second integrated circuit chip and the flip chip, a copper or copper/tungsten heat sink  40  (or other suitably formed heat sink) has been mounted on top of the flip chip  20 . The second integrated circuit chip  34  is mounted on the heat sink  40 . A conductive epoxy  41  can be used to secure the flip chip, heat sink, and second integrated circuit chip, and allow back-to-back grounding as explained before. It is also possible for the heat sink  40  to be grounded, such as by wire bonds. This is advantageous if a second integrated circuit chip is not mounted on top of the heat sink. 
     In accordance with a method aspect of the present invention, the integrated circuit package  5  is formed by techniques known to those skilled in the art. In the present invention, a green tape ceramic (formed as three mil sheets, which can be stacked) is cut out. Holes are drilled into the green tape ceramic forming the vias or through holes  24 . A laser or other drilling means known to those skilled in the art can be used. The through holes  24  are plated with conductive material, such as a conductive ink  28 . At the same time, a layer of conductive circuitry is formed on the green tape ceramic such as by screen printing on the green tape ceramic with techniques known to those skilled in the art. The green tape ceramic is also cut to later form the desired cut outs  12 , 16  to receive the flip chip  20  and other associated components, such as second integrated circuit chip  34 . Different layers of green tape ceramic are then pressed together to form the stepped configuration as illustrated in FIGS. 1 and 2. The applied circuit patterns can include ground layers in any desired position as desired by one skilled in the art. 
     The flip chip, such as the desired Galium Arsenide chip, has gold ball bumps formed on the die as conductive bumps  26 , by gold wire application, and flattened with a tamping tool. The gold gall bumps correspond to electrical input/output contacts and are positioned to the align with holes formed in the green tape ceramic. The flip chip die acts as a mimic die and is pressed into the green tape ceramic so that the gold ball bumps extend through the formed through holes. An adhesive, such as a conductive epoxy  32 , is then placed onto the hole and gold ball bump and a controlled impedance line secured by the conductive epoxy to the gold ball bumps. Thus, there is no lead line for the wire bonds that could cause noise and other problems. This controlled impedance line simulates a coax. As illustrated in FIG. 1, the cut out is stepped to allow another die to mount back-to-back. 
     A conductive epoxy  36  is placed on the back side and establishes the ground for the flip chip  20 , and in some instances, for the second integrated circuit chip. As shown in FIG. 1, the second integrated circuit chip  34  is placed back-to-back to the flip chip and can be wire bonded to the conductive patterns formed on the green tape ceramic. 
     FIG. 2 illustrates an embodiment where a heat sink  34 , such as a copper or copper/tungsten heat sink, is placed on top of the flip chip  20 . The second integrated circuit chip can be placed on top of the heat sink. As described before, the conductive epoxy can secure the flip chip, heat sink and the second integrated circuit chip. 
     The present invention now allows for increased miniaturization and eliminates a multiplicity of wire bonds, while also allowing for flip chip packaging on a mimic die. Thus, greater flip chip density can be realized, while saving costs. 
     Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed, and that the modifications and embodiments are intended to be included within the scope of the dependent claims.