Abstract:
The process of the invention starts with a metal panel, overlying the metal panel is created an interconnect substrate making use of BUM and thin film processing technology while the process of the invention enables the use of stacked vias and merged vias for the connection of the flip chip bumps. The process of the invention creates, for instance, two patterned layers on the surface of the metal panel whereby the metal panel is used as the ground terminal of the power supply. The first layer that is created on the surface of the metal panel can be the power supply layer (this layer can also be used for some fan-out interconnect lines), the second layer that is created on the surface of the metal panel is primarily used for (fan-out) interconnect lines. The flip chip bumps are, under the process of the invention, connected to the second layer of the interconnect substrate. Where the BGA balls also reside on the same surface as the flip chip bumps, the process of the invention does not require any additional structures such as a dam for the containment of insulating encapsulation material (underfill) that at times is provided around a perimeter of a well into which a flip chip is inserted, making the process of the invention most cost effective.

Description:
This application is related to Ser. No. 09/332,427 filed on 06/14/99 assigned to a common assignee. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method for creating a cavity down flip chip package. 
     (2) Description of the Prior Art 
     The semiconductor industry is known to be very competitive and is therefore constantly driven to improve semiconductor device performance at competitive prices. The objective of improving device performance can realistically only be achieved by reducing device dimensions, which leads to increased device densities. Devices of increased densities must further be combined to form multi-chip packages that contain not only the high-density semiconductor devices but also contain relatively complex means for the interconnection of the devices that are part of the package. 
     In many of the complex, multi-device packages a substrate, that is typically ceramic or plastic based, is used for the mounting of devices on the surface thereof and for the formation of the interconnect-interface between the devices and the surrounding circuitry. Many different approaches are used for the purpose of interconnecting multiple semiconductor devices, such as Dual-In-Line packages (DIP&#39;s), Pin Grid Arrays (PGA&#39;s), Plastic Leaded Chip Carriers (PLCC&#39;s) and Quad Flat Packages (QFP&#39;s). Multi layer structures have further been used to connect physically closely spaced integrated circuits with each other. Using this technique, a single substrate serves as an interconnect medium, multiple chips are connected to the interconnect medium forming a device package with high packaging density and dense chip wiring. The chip wiring contains layers of interconnect metal that are interconnected with interconnect vias, layers of dielectric (such as polyimide) or insulating layers separate metal layers that make up the interconnect network and the via and contact points that establish connections between the interconnect networks. The design of overlying and closely spaced interconnect lines are subject to strict rules of design that are aimed at improving package performance despite the high density packaging that is used. For instance, electrical interference between adjacent lines is minimized or avoided by creating interconnect lines for primary signals that intersect under 90 degree angles. Surface planarity must be maintained throughout the construction of multi-layer chip packages due to requirements of photolithography and package reliability. Many of the patterned layers within a layered structure form the base for overlying layers, lack of planarity can therefore have a multiplying effect on overlying layers. 
     The Quad Flat Package (QFP) has been created to achieve high pin count integrated packages with various pin configurations. The pin I/O connections for these packages are typically established by closely spaced leads distributed along the four edges of the flat package. This limits the I/O count of the packages and therefore the usefulness of the QFP. The Ball Grid Array (BGA) package has been created whereby the I/O connects for the package are distributed around the periphery of the package and over the complete bottom of the package. The BGA package can therefore support more I/O points and provides a more desirable package for high circuit density with high I/O count. The BGA contact points are solder balls that in addition facilitate the process of flow soldering of the package onto a printed circuit board. The solder balls can be mounted in an array configuration and can use 40, 50 and 60 mil spacings in a regular or staggered pattern. 
     Where circuit density keeps increasing and device feature size continues to be reduced, the effect of the interconnect metal within the package becomes relatively more important to the package performance. Factors that have a negative impact on circuit performance, such as interconnect line resistance, parasitic capacitance, RC-delay constants, crosstalk and contact resistance have a significant impact on the package design and its limitations. A significant power drop may for instance be introduced along the power and ground buses where the reduction of the interconnect metal does not match the reduction in device features. Low resistance metals (such as copper) are therefore finding wider application in the design of dense semiconductor packages. 
     Increased I/O combined with increased high requirements for high performance IC&#39;s has led to the development of Flip Chip packages. Flip chip technology fabricates bumps (typically Pb/Sn solder) on Al pads and interconnects the bumps directly to the package media, which are usually ceramic or plastic based. The flip-chip is bonded face down to the package through the shortest paths. These technologies can be applied not only to single-chip packaging, but also to higher or integrated levels of packaging in which the packages are larger, and to more sophisticated package media that accommodate several chips to form larger functional units. 
     The flip-chip technique, using an area array, has the advantage of achieving the highest density of interconnection to the device and a very low inductance interconnection to the package. However, pre-testability, post-bonding visual inspection, and Temperature Coefficient of Expansion (TCE) matching to avoid solder bump fatigue are still challenges. In mounting several packages together, such as surface mounting a ceramic package to a plastic board, the TCE mismatch can cause a large thermal stress on the solder lead joints that can lead to joint breakage caused by solder fatigue from temperature cycling operations. 
     For devices that have high power dissipation, cavity down BGA packages are frequently used. The BGA cavity down package has a structure such that the semiconductor die and the BGA balls both reside on the bottom side of the BGA substrate, that is the side of the BGA substrate that faces the printed wiring board. The structure thereby allows the top-side of the BGA substrate to be available for heat removal purposes. 
     As an illustration of a Prior Art cavity down package, a method of packaging a BGA is shown in FIG.  1 . The features of this package can be identified as follows: 
       10 ′ is the heatsink of the package, heatsink  10 ′ has a surface that is electrically conductive; mounted in the heatsink is 
       12 ′, the semiconductor device; contact points for the die  10 ′ are closely spaced around the periphery of the die; the chip  12 ′ is interconnected to surrounding circuitry be means of the interface 
       14 ′ which contains one or more layers of interconnect wiring; layer  14 ′ can contain a stiffener that provides rigidity to the substrate; contact points that have been provided in the surface of chip  12 ′ are connected to substrate  14 ′ by means of 
       16 ′, the wire bond connections; the wire bond connections provide a wire bonded connection between a contact points on the IC die  12 ′ and the copper traces  19 ′ contained in layer 
       18 ′, additional points of electrical contact are provided in the surface of substrate  14 ′ by means of points  18 ′ which are contact pads to which, typically, contact balls are connected for further mounting of the indicated package; device  12 ′ is mounted inside a cavity that has been provided in the surface of the heatsink  14 ′ and contacts the heatsink via 
     layer  20 ′, which is a thermally conductive adhesive layer, typically containing epoxy; the device  12 ′ is further protected from the environment by being encapsulated in layer 
       22 ′, which forms an epoxy based protective enclosure for device  12 ′; the layer 
       24 ′ is the adhesive interface between the substrate  14  and the heatsink  10 ′. 
     The Prior Art package that is shown in FIG. 1 contains a heat sink in which a cavity is provided for the insertion of a semiconductor die, a substrate that contains one or more layers of interconnect lines and methods of encapsulating the mounted semiconductor die. Other, simpler methods can be used for mounting a semiconductor whereby the die is mounted directly on the surface of a Printed Circuit Board (PCB) while layers of metal interconnect within the PCB are used to provide the I/O connections of the mounted die to surrounding circuitry. In most applications of this kind, the die is still provided with contact balls, these contact balls rest directly on the surface of the PCB and are connected to electrical points of contact that are opened in the surface of the PCB. 
     U.S. Pat. No. 5,583,378 (Marrs et al.) (cited by the inventor) shows a metal panel  204  used as a thermal conductor. The chip  202  is attached to the thermal conductor  204  by metal epoxies  206 . See FIG. 2A, also see FIGS. 4A through 4L. 
     U.S. Pat. No. 5,777,386 (Higashi et al.) (cited by the inventor) shows a flip chip package having a heat conductor but ceramic substrate. See col. 3, line 65. The package has heat conducting pattern 12 that conducts heat to the substrate (e.g. PCB) by solder balls. 
     U.S. Pat. No. 5,874,321 (Templeton, Jr. et al.) teaches a cavity up package with a conductive lid. 
     U.S. Pat. No. 6,020,637 (Karnezos) shows a heat spreader for a package. 
     U.S. Pat. No. 5,578,869 (Hoffman et al.) shows a metal base/panel for a package. 
     U.S. Pat. No. 5,289,337 (aghazadeh et al.) shows a related package. 
     SUMMARY OF THE INVENTION 
     A principle objective of the invention is to provide a method for the fabrication of high-density substrates that is used for the packaging of cavity-down flip chip semiconductor devices. 
     The process of the invention starts with a metal panel, overlying the metal panel is created an interconnect substrate making use of BUM and thin film processing technology while the process of the invention enables the use of stacked vias for the A connection of the flip chip bumps. The process of the invention creates, for instance, two patterned layers on the surface of the metal panel whereby the metal panel is used as the ground terminal of the power supply. The first layer that is created on the surface of the metal panel can be the power supply layer (this layer can also be used for some fan-out interconnect lines), selected points within the first layer are in direct contact with the metal panel for purposes of improved heat exchange to the metal panel and of improved ground power supply. The second layer that is created on the surface of the metal panel is primarily used for (fan-out) interconnect lines. The flip chip bumps are, under the process of the invention, connected to the second layer of the interconnect substrate. The process of the invention does not require any additional structures such as a dam for the containment of insulating encapsulation material (underfill) that at times is provided around a perimeter of a well into which a flip chip is inserted, making the process of the invention most cost effective. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a Prior Art cavity down package for mounting a wire-bonded chip. 
     FIG. 2 shows a cross section of the above referenced related application of a single chip flip chip with three interconnect layers. 
     FIGS. 3 a  though  3   f  shows the Prior Art BUM processing sequence. 
     FIGS. 4 a  through  4   f  shows a Prior Art thin film processing sequence. 
     FIG. 5 shows a cross section of the mounting of a cavity down flip chip on the surface of a metal panel whereby two conductive layers have been created overlying the metal panel. 
     FIG. 6 shows a cross section of the mounting of a cavity down flip chip on the surface of a metal panel whereby two conductive layers have been created overlying the metal panel, a stacked thermal via is used for this method. 
     FIG. 7 shows a cross section of another arrangement of a stacked metal via. 
     FIG. 8 shows a cross section of the application of a solder ball in combination with a stacked thermal via. 
     FIG. 9 shows a cross section of a multi-chip cavity down flip chip package of the invention. 
     FIGS. 10 a  through  10   i  show the processing flow for the creation of stacked vias. 
     FIGS. 11 a  through  11   f  show the processing flow for the creation of merged vias. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The above referenced related application that is assigned to a common assignee uses, for the creation of high-density BGA packages, a metal substrate on the surface of which is created a multi-layer interconnect substrate that is plastic based. The process of this related application starts with a metal panel, typically copper, but it can be other metals like aluminum or stainless steel, with a size of 18×24 inches, other sizes can obviously be used. The process of this related application takes advantage of what is known in the printed wiring board industry as the Build Up Multilayer (BUM) processing in combination with the use of thin film deposition equipment and processes that are used in the flat panel display industry to build high density packaging substrate devices. 
     To create this package, a first surface of the metal substrate is cleaned and used for the creation of the multi-layer interconnect substrate. This process of creation of the multi-layer interconnect substrate starts with the deposition of a layer of dielectric over the first surface (of the metal substrate) a set of vias is created through the layer of dielectric, over the layer of dielectric a layer of interconnect metal is deposited. The layer of interconnect metal is patterned to form the interconnect lines, this process can then be repeated with more depositions of layers of dielectric over which patterns of interconnect lines are formed. The last pattern of interconnect lines is covered with a solder mask and subsequently patterned to open an array of metal pads in the layer of dielectric, these metal pads are used for the connection of the BGA ball contact attachment. 
     The second surface of the metal substrate is next patterned to create openings in this second surface, these openings expose the first layer of dielectric that has been deposited on the first surface of the metal substrate. Openings are created in the exposed portions of the first layer of dielectric (typically using laser technology). These openings serve as pads for flip chip bump to connect the flip chips, which are mounted in the openings of the second surface of the metal substrate, to the interconnect substrate (that has been created as highlighted above on the first surface of a metal substrate). 
     To further illustrate the processing steps that are part of the above referenced related application that is assigned to a common assignee, FIG. 2 of this related application will be referred to as part of the present application. FIG. 2 shows a cross section of a single chip flip-chip package with three interconnect layers. 
     The interconnect substrate  12 ″ contains the totality of the sequence of layers that are created within the scope of the invention for making a high density packaging substrate. 
     The interconnect substrate  12 ″ has two surfaces. The surface of the interconnect that is in contact with the metal substrate  14 ″ is the-second surface of the interconnect substrate the surface of the interconnect substrate into which the contact balls  10 ″ are mounted is the first surface of the interconnect substrate. 
     The metal layer within the interconnect substrate  12 ″ that is closest to the first surface  24 ″ of the metal substrate  14 ″ is referred to as the bottom layer, the metal layer within the interconnect substrate  12 ″ that is furthest removed from the first surface  24 ″ of the metal substrate  14 ″ is referred to as the top layer. 
     The metal substrate  14 ″ has two surfaces, the first surface  24 ″ of the metal substrate is the surface on which the interconnect substrate  12 ″ is created. The second surface  26 ″ of the metal substrate is the surface into which openings are etched for the insertion of BGA chips. 
     The three interconnect layers within the interconnect substrate  12 ″ are highlighted as layer  18 ″ (which can be a ground layer), layer  20 ″ (which can be a power layer) and layer  22 ″ (which can be a signal layer). Metal substrate  14 ″, typically copper, is about 30 mils thick. The metal used for substrate  14 ″ is not limited to copper but can be another metal such as aluminum or stainless steel. The size of the metal substrate  14 ″ is typically 18×24 inches but the size of the metal substrate  14 ″ is not limited to these dimensions. The invention uses the Build Up Multilayer (BUM) technology (a printed circuit board technology) in combination with thin film deposition technology (a technology used for the creation of flat panels). 
     It must be noted from FIG. 2 that the metal substrate  14 ″ and the contact balls  10 ″ are mounted on opposite sites of the interconnect substrate  12 ″ while the metal substrate  14 ″ and the contact balls  10 ″ are also aligned with each other (the metal substrate  14 ″ is located above the contact balls  10 ″). The IC  16 ″ is mounted in an opening  28 ″ created in the metal substrate  14 ″. The opening  28 ″ into which the flip chip  16 ″ is mounted is a cavity that is created by masking and etching of the second side  26 ″ of the metal substrate  14 ″. 
     A dielectric layer (not shown) is first deposited over the metal substrate  14 ″ on the first side  24 ″ of the metal substrate  14 ″ as a first step in the creation of the interconnect substrate  12 ″. This deposition of the dielectric can be done by either lamination or by a coating and curing process. The layer of dielectric typically has a thickness of between about 10 and 40 μm. It a required that the dielectric has a CTE that is higher than the CTE of the metal substrate. This is to assure that, after the metal substrate with the deposited layer of dielectric are cooled down to room temperature, the dielectric film is under tension. The dielectric layer can be epoxy with or without a thin glass reinforcement, a polyimide film or any other build-up dielectric material. 
     The first step in the creation of the interconnect substrate  12 ″ is the creation of a thin film interconnect metal layer  22 ″ on top of the layer of dielectric. The thin film deposition technique contains the following steps of depositing an interconnect plating base by consecutive sputtering of Cr, Au and Cr, masking and patterning for semi-additive plating of the interconnect pattern, wet etching the thin Cr layer to expose the Au, depositing semi-additive plating of the interconnect pattern by depositing Au, Ni and Cu, removing of the mask for the semi-additive plating of the interconnect pattern, wet etching to remove the sputtered plating base from between the interconnect pattern and coating the created interconnect pattern with a layer of dielectric. For applications where solder connections need to be made to the interconnect pattern, the above dielectric forms a solder mask and vias are created in the dielectric for the solder connections. 
     The state of the art BUM technology provides the technology to add layers  20 ″ and  18 ″ on top of the created thin film interconnect layer  22 ″, these added layers  20 ″ and  18 ″ typically can be for power and ground interconnects but can also be used for fan-out interconnections. The last layer created in this manner, that is the layer  18 ″ in FIG. 2 or the layer furthest removed from the first side  24 ″ of the metal substrate  14 ″, must provide the interconnects with the contact balls of the BGA structure and must therefore be coated as a solder mask. 
     The second side  26 ″ of the metal substrate must next be prepared for the mounting of the flip chip;,an opening or cavity  28 ″ must therefore be created in the metal substrate  14 ″ that can accommodate the flip chip. The second side  26 ″ of the metal substrate  14 ″ is therefore masked and etched to create the opening  28 ″ in the metal substrate  14 ″. The size of this opening can vary and depends on the number of flip chips that are to be mounted on the interconnect substrate  12 ″. The wet etch of the second side  26 ″ of the metal substrate exposes the dielectric layer that has previously been deposited (on the first side  24 ″ of the metal substrate  14 ″, see above). Openings  30 ″ must be created in this layer of dielectric through which the flip chip  16 ″ can be connected to the contact points in the first layer  22 ″. A laser is used to create these openings. 
     The openings  32 ″ for the BGA solder ball connections are created to expose the top metal pads (the pads in the interconnect layer  12 ″ that are furthest away from the metal substrate  14 ″). The flip chip  16 ″ is inserted into the interconnect layer  12 ″ within the cavity  28 ″, the interconnect layer  12 ″ is brought into contact with the contact balls  10 ″, electrical contact has then been established between the bump pads  30 ″ of the flip chip  16 ″ and the contact balls  10 ″. 
     In summary, in the above referenced related application the minimum metal lines and spaces used are in the range of 12-40 μm and the dielectric insulation is 10-25 μm. Printed wiring board BUM is used in this related application to build the power supply layer as well as the interconnect line metal layer. High density interconnect layers are fabricated with the thin film interconnect layers. Typical-steps of BUM processing are used for metal depositions while thin film sputter equipment (typically used for a flat panel creation process) is used to sputter metal in depositing a plating base for the semi-additive plating to make the thin film lines. 
     While the above FIG. 2 shows a cross section of a single chip flip chip with three interconnect layers, other applications of the same above referenced related application can be used to create for instance a single chip package with two interconnect layers, multi-chip structures that contains three interconnect layers in the interconnect substrate or multi chip packages with four interconnect layers contained within the interconnect substrate. The number of chips that are mounted using this method and the number of interconnect layers that are created in the substrate of the package are not limited by the method of the above referenced related application. 
     The processing sequence that is used to create interconnect lines in the BUM technology is highlighted in FIGS. 3 a  through  3   f , as follows: 
     1) FIG. 3 a , the starting substrate  80  can be a cleaned metal panel (substrate) without any interconnect layers; 
     2) FIG. 3 b , coating of the substrate  80  with a layer  82  of dielectric; 
     3) FIG. 3 c , creating of vias  84  in the dielectric  82  for electrical connections to the substrate  80 ; 
     4) FIG. 3 d , etching and swelling of the dielectric  82  to roughen the surface and thereby promote adhesion for the subsequent electroless copper deposition; 
     5) FIG. 3 d , electroless seeding of the dielectric; 
     6) FIG. 3 e , plating of the panel with a layer  88  of copper; and 
     7) FIG. 3 f , masking and etching the deposited layer  88  of copper to create the metal pattern  90  in the BUM layer. 
     Thin film deposition technique contains the following steps, see FIGS. 4 a  through  4   f : 
     1) FIG. 4 a , depositing an interconnect plating base  50  by consecutive sputtering of Cr and Cu over a layer of dielectric  51  that has been deposited on the surface  49  of a substrate surface; conventional processing uses sputter and evaporator equipment to deposit the thin layer  50  of metal that serves as the plating base; a via  59  has been created in the layer  51  of dielectric; alternatively, the plating base can be made by etching and swelling the dielectric layer, followed by an electroless step to deposit a thin layer of copper on which may or may not be (electrolytically) plated another layer of copper, resulting in a plating base of about 1 to 2 μm thickness; 
     2) FIG. 4 b , a layer  52  of photoresist is deposited over the surface of the interconnect plating base  50 ; this layer  52  of photoresist is masked and patterned creating the pattern  53  of the interconnect lines; 
     3) FIG. 4 c , semi-additive plating  54  of the interconnect pattern is performed by depositing Cu in the openings that have been created in the layer  52  of photoresist; this plating  54  plates the surface of the (copper) lines that are to be created for the interconnect pattern; 
     4) FIG. 4 d , removing of the mask  52  (FIG. 4 b ) of photoresist that has been used as a mask for the semi-additive plating of the interconnect pattern; areas  56  are the regions in the plating base layer  50  that must be removed to create the interconnect pattern; 
     5) FIG. 4 e , wet etching to remove the plating base metal layer  56 , FIG. 4 d , from between the interconnect pattern  54 ; 
     6) FIG. 4 f , coating the created interconnect pattern with a layer  58  of dielectric; vias  63  and  65  have been created in the layer  58  of dielectric for points of electrical contact by either using the photolithographic approach of exposing and developing or by using a laser 
     7) for applications where wire bond connections need to be made to the interconnect pattern the top dielectric forms a solder mask and vias are created in the dielectric for the solder connections. 
     In the present invention, the minimum metal lines and spaces used are in the range of 12-40 μm and the dielectric insulation is 10-25 μm. Printed wiring board BUM technology is used in the present invention to build the power supply layer as well as the (relatively low density) interconnect line metal layer. High density interconnect layers are fabricated using thin film interconnect technology. The dielectric thickness that is obtained using thin film technology is much thinner than the dielectric thickness that is obtained using BUM technology, which is in the range of 50-75 μm. In using the BUM technology therefore a modified coating approach as practiced by the build up industry needs to be used. For example, in the case of the curtain coating or screen-printing, the process parameters are adjusted to achieve a thin dielectric coating of 12-25 μm required for this design. Alternatively, a spinning or a spinning-extrusion combination practiced in the display industry is used to accomplish the same purpose. To create high-density, thin film interconnect lines, the conventional exposure station of a hard contact on a corresponding photoresist layer is not capable to provide the resolution and the required yield. For the processes of the invention, a high resolution photo resist, with either projection printing or proximity printing, combined with laser beam scanning technology must be used to obtain a high yield. 
     FIG. 5 shows a cross section of a High Density BGA package where two patterned metal layers  40  and  41  are created on top of the metal substrate  14 , the metal panel  14  can be used as the ground terminal for the power supply. The two layers  40  and  41  combined form the interconnect substrate  42 . The first layer  40 , that is the layer closest to the metal panel  14 , essentially serves as the power supply layer. This layer  40  however can also be used for some fan out interconnections. The second layer  41  is the layer to which the flip chip bumps  30  are connected, this layer  41  is primarily used for the fan out interconnect wiring. Contact balls  10  are provided to further interconnect the package of the invention that is shown in cross section in FIG. 5 to surrounding circuitry and components. 
     It must be noted in FIG. 5 that a number of the wiring connects that from part of layer  40 , such as  31 ,  32 ,  33 , and  34 , make direct contact with the metal panel  14 . This is part of the method of the invention and provides significantly improved heat exchange between the interconnect substrate  42  and the metal panel  14 . 
     FIG. 6 shows how, to conduct heat from chip  16  to the metal panel  14 , a stacked via structure  43 / 44  is used where via  43  is metal  1  and via  44  is metal  2 . The stacked via  43 / 44  is made from a small via  43  in layer  40  that is immediately adjacent to the metal panel  14  and a larger via  44  that is made in the overlying layer  41 . A flip chip  16 , containing a matrix of metal flip chip bumps  30 , is joined to the metal pad  44  of metal  2 . The flip chip bumps  30  are made of solder or gold. The heat is therefore transferred from the flip chip  16  to the flip chip bump  30 , through the stacked via  43 / 44  to the metal panel  14 . 
     FIG. 7 shows how, in a variation of the stacked via of FIG. 6, the flip chip bump  30  can be placed centered to and on top of the stacked via  43 / 44 . The stacked via  43 / 44  provides a direct path of thermal heat transfer between the flip chip  16  and the metal panel  14 , forming an efficient method and path of heat exchange. 
     FIG. 8 shows how, in a manner that is similar to the previously highlighted heat conduction paths of FIG.  6  and FIG. 7, a heat conductivity path exists between the Printed Circuit Board  45 , on the surface of which a metal contact point  21  is provided, to metal panel  14  through the solder ball  46  that is in direct contact with the stacked vias  43 / 44 . The diameter of solder ball  46  is larger than the diameter of a flip chip bump  30  (FIGS. 5,  6 , and  7 ). For example, the diameter of solder ball  46  is 29 mil for a solder ball pitch of 1.27 mm, this can be compared with a flip chip bump of 5 mil for flip chips with a 0.25 mm bump pitch. 
     The basic design that has been highlighted up to this point can be used to package more than one flip chip. This is shown in FIG. 9 where two flip chips  17  and  18  are mounted on interface substrate  22 , which is attached to the metal substrate  14 . The number of flip chips that can be mounted in this manner is not limited to two flip chips under the method of the invention. FIG. 9 shows a total of three patterned metal layers, that is layers  26 ,  27 , and  28  where the three layers combined form the interconnect substrate  22 . In the cross section that is shown in FIG. 9 the metal panel  14  can still be used as the ground power supply. Layer  26  which is closest to the metal panel  14  and serves as the power layer, the top two layers  27  and  28  are used for interconnect. In general, the power supply layer  26  is fabricated using BUM technology while the high density interconnect layers  27  and  28  are created using thin film deposition methods. 
     The cross section of the design of the invention that is shown in FIG. 9 shows that the invention provides for the creation of multiple, overlying vias that form the electrical interface between the contact balls  10  and the interconnect substrate  22  and the ground plane that is provided by the metal panel  14 . FIG. 9 shows an example of a contact ball  23  that penetrates the interconnect substrate to the surface of layer  28 , a contact ball  24  that penetrates the interconnect substrate through layer  28  to the surface of layer  27  and a contact ball  25  that penetrates layers  28  and  27  of the interconnect substrate  22  to layer  26  where this&#39;contact ball is grounded to the metal panel  14 . Additional variations of this interconnect scheme can readily be identified from FIG. 9, providing further evidence of the flexibility of the process of the invention. 
     Additional detail will be provided below relating to the creation of interconnect vias. Interconnect vias can be either stacked vias of merged vias, the processes that are applied for the creation of these two types of vias are described following. 
     First will be listed in sequence of execution the processing steps that are required to create stacked vias, after this the processing steps of creating stacked vias will be explained in further detail using FIGS. 10 a  through  10   i.    
     The processing steps required to create stacked vias are: 
     provide a metal panel 
     coat the surface of the metal panel with a first layer of dielectric 
     deposit a first layer of copper foil on the surface of the first layer of dielectric, using methods of laminating or plating 
     create first via openings in the first copper foil using methods of masking and etching 
     drill first holes through the first layer of dielectric that align with the first via openings that have been created in the first copper foil using laser drilling 
     plate a first layer of metal over the surface of the first copper coil and the inside surfaces of the first openings that have been created through the first layer of dielectric 
     mask and etch the first layer of metal overlying the first layer of dielectric, creating first interconnect lines and first via pads on the surface of the first layer of dielectric 
     deposit a second layer of dielectric over the surface of the first layer of dielectric, including the surface of the first interconnect lines and the first via pads that have been created on the surface of the first layer of dielectric 
     laminate or plate a second layer of copper foil over the surface of the second layer of dielectric 
     create second via openings in the second layer of copper foil, these second via openings to align with the first via openings created in the first layer of copper foil 
     drill second holes through the second layer of dielectric that align with the first via openings that have been created in the first copper foil using laser drilling, removing the second layer of dielectric from the first vias that have been created in the first layer of dielectric 
     plate a second layer of metal over the surface of the second copper foil and the inside surfaces of the second openings that have been created through the second layer of dielectric, including the inside surfaces of the first vias, and 
     mask and etch the second layer of layer of metal overlying the second layer of dielectric, creating second interconnect lines and second via pads on the surface of the second layer of dielectric that overlay and interconnect with the first vias and first interconnect lines that have been created in the first layer of dielectric. 
     The above sequence of processing steps for the creation of stacked vias will now be described using FIGS. 10 a  through  10   i.    
     The process starts, FIG. 10 a , with a metal panel  60  over the surface of which is coated a layer  61  of dielectric. A copper foil  62  is created on the surface of the layer  61  of dielectric by process of laminating or panel plating. 
     Openings  72 , FIG. 10 b , are created in the copper foil  62  using conventional methods of masking and etching the layer  62  of copper foil. These openings  72  are further propagated into the underlying layer  61  of dielectric, the openings  72  penetrate the layer  61  of dielectric and therefore partially expose the surface of the metal panel  60 . The propagation of the openings  72  through the layer  61  of dielectric uses laser technology to drill the openings. 
     FIG. 10 c  shows a cross section of the metal panel after a metal layer  64  has been deposited inside the vias  72  and over the surface of the metal foil  62 . Masking and etching the metal layer  64 , using conventional methods of photolithographic exposure and development, creates (FIG. 10 d ) a via pad  73  and a line structure  74  overlying the layer  61  of dielectric and overlying the inside surfaces of openings  72 . 
     The process is now repeated, a second layer  66 , FIG. 10 e , of dielectric is deposited over the structure of FIG. 10 d  followed by creating a second copper foil  67  (by laminating or plating) on the surface of the second layer  66  of dielectric. For simplicity of presentation, patterned layers  62  of copper foil that have been shown in cross section in FIG. 10 d  are no longer shown in FIGS. 10 e  through  10   i  since these layers can, from a functional point of view, be considered as being part of and integrated with the overlying layers  73 / 74 . 
     The layer  67  of copper foil is masked and etched, creating openings  68 , FIG. 10 f , in the layer of copper foil. These openings  68  align with the underlying metal via pad  73  and the via that is connected to the underlying interconnect line  74 . Using laser technology, holes  68 , FIG. 10 g , are created through the second layer  66  of dielectric, partially exposing the surface of the via pad  73  and the via that is connected to the interconnect line  74 . 
     A layer  69  of metal, FIG. 10 h , is electroplated over the structure that is shown in cross section in FIG. 10 g , that is over the surface of the patterned layer  67  of metal foil and over the inside surfaces of the openings  68  that have been created in the second layer  66  of dielectric, including the inside surfaces of the via pad  73  and the via that is connected to interconnect line  74  that have been created in the (first) layer  61  of dielectric. 
     A final masking and etching of the deposited layer  69  results in the cross section that is shown in FIG. 10 i , showing a stacked via  70  that makes contact with the metal panel  60  by means of the via  73 , further showing a stacked via  71  which contacts the interconnect line  74  and also contacts the metal panel  60 . From the cross sections that are shown in FIG. 10 i , it is clear why these vias are called stacked vias: the layers of metal that form the vias overlay each other or are, in other words, stacked one on top of the other. 
     For simplicity of presentation, patterned layers  67  of copper foil that have been shown in cross section in FIG. 10 h  are not shown in FIG. 10 i  since these layers can, from a functional point of view, be considered as being part of and integrated with the overlying layers  70 / 71 . 
     The processing steps that are required to create merged vias are as follows: 
     provide a metal panel 
     coat the surface of the metal panel with a first layer of dielectric 
     deposit a first layer of copper foil on the surface of the first layer of dielectric, using methods of laminating or plating 
     create first via openings in the first copper foil using methods of masking and etching, the interconnection lines and via pads are also created in the same step 
     coat a second layer of dielectric of the surface of the first layer of dielectric and the first metal layer 
     deposit a second layer of copper foil on the surface of the second layer of dielectric, using methods of laminating or plating 
     create second via openings in the second copper foil-using methods of masking and etching, these second via openings align with the first via openings created in the first copper foil 
     use a laser drill to drill holes through the second or through the second and the first layer of dielectric, these holes are aligned with the first and the second via holes, exposing the first vias, further partially exposing the surface of the metal panel where holes are drilled that penetrate both layers of dielectric 
     plate metal over the inside surfaces of the holes created in the second or in the second and the first layer of dielectric, and 
     patterning and etching the plated metal, creating interconnect lines and contact vias on the surface of the second layer of dielectric. 
     FIGS. 11 a  through  11   f  describe the processing steps that are performed for the creation of merged vias. 
     The process starts, FIG. 11 a , with a metal panel  60  over the surface of which is coated a first layer  61  of dielectric. A copper foil  62  is created on the surface of the layer  61  of dielectric by process of laminating or panel plating. 
     Openings, FIG. 11 b , are next created in the layer  62 . These openings are surrounded by via pads  79  and  80 ,  77  represents interconnect lines that are created in the same processing step. Via pad  90  is also created in layer  62 . 
     A second layer  92  of dielectric, FIG. 11 c , is coated over the surface of the first layer of dielectric, including the surface of the metal pattern  77 ,  79 ,  80  and  90  that has been created on the surface of the fist layer  61  of dielectric. A layer  93  of copper foil is deposited over the surface of the second layer  92  of dielectric, FIG. 11 c , by methods of lamination or plating. 
     FIG. 11 d  shows how via holes  94  and  95  are created in the copper foil  93 , it must be noted that hole  94  aligns with the space over the surface of layer  61  between via pads  79  and  80 , FIG. 11 b , while hole  95  aligns with pad  90 , FIG. 11 b . The layer  93  remains in place surrounding the perimeter of openings  94  and  95 , layers  79 ′,  80 ′ and  91 ′. FIG. 11 d  further shows the extension of the holes  94  and  95  through the second layer  92  of dielectric, partially exposing the surface of the pattern  79 ,  80  and  90  that has been created in the first metal foil  62  of FIG. 11 a . It must be noted in FIG. 11 d  that the opening  94  penetrates both the second layer  92  and the first layer  61  of dielectric, further that hole  95  penetrates only the second layer  92  of dielectric. 
     A layer  96  of metal is next plated over the surface of the structure that is shown in FIG. 11 d , see FIG. 11 e , including the inside surfaces of the openings  94  and  95 . This layer  96  of metal is in contact with via pads  79 ,  80  and the via pad  90 . 
     As a final step in the creation of merged vias, layer  96  of metal is etched, leaving in place the merged via  97 , FIG. 11 f , that extends through the two layers  61  and  92  of dielectric and the via  98  that connects via pad  90  (FIG. 11 d ) with metal on the surface of layer  92  of dielectric. Metal interconnect lines  99  are also created at the same time. 
     It is clear from the cross section that is shown in FIG. 11 f  why the vias that are created are called merged vias, that is no two layers are deposited over each other to create the walls of the via (as was the case for the previously highlighted stacked vias) while at the same time a via can be created that penetrates both layers of dielectric and a via that penetrates only the upper layer of dielectric. 
     Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the spirit of the invention. It is therefore intended to include within the invention all such variations and modifications which fall within the scope of the appended claims and equivalents thereof.