Abstract:
The specification describes a thin film Integrated Passive Device (IPD) design that achieves isolation between conductive runners by shielding the top and bottom regions of a noisy runner with metal shield plates. The shield plates are derived from metal interconnect layers. The invention can be implemented by merely modifying the mask pattern for the metal interconnect layers. No added elements or steps are needed to fabricate the IPDs. The invention is suitable for use in Multi-Chip Modules (MCMs) or other arrangements where digital circuits and RF circuits are in close proximity.

Description:
RELATED APPLICATION  
       [0001]     This application is a Division of application Ser. No. 11/127889, filed May 5, 2005. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to reducing noise due to electromagnetic interference (EMI) in thin film integrated passive devices (IPDs). More specifically it addresses reducing noise created by noisy conductive runners by shielding the offending runners with metal layers.  
       BACKGROUND OF THE INVENTION  
       [0003]     Detrimental EMI effects in electrical devices such as RF integrated circuits are well known. With advances in component density, and the high frequencies and diversity of elements characterized by state of the art thin film IPDs, detrimental noise problems in IPDs is a relatively new issue. However, for optimum performance, noise from conductive runners, typically digital lines such as clock lines, needs to be addressed. The need for addressing EMI in IPDs arises especially in thin film IPDs, and thin film IPDs in Multi-Chip Modules, or similar arrangements where runners carrying digital signals are located in close proximity with IPD components carrying RF signals.  
       BRIEF STATEMENT OF THE INVENTION  
       [0004]     According to one aspect of the invention, a thin film IPD design that achieves isolation between conductive runners is produced by shielding the top and bottom regions of a noisy runner with metal shielding plates. The shielding plates are derived from metal interconnect layers. The invention can be implemented by merely modifying the mask pattern for the metal interconnect layers. No added elements or steps are needed to fabricate the IPDs. The invention is suitable for use in Multi-Chip Modules (MCMs) where digital circuits and RF circuits are in close proximity. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0005]     The invention may be better understood when considered in conjunction with the drawing in which:  
         [0006]      FIG. 1  is a schematic view of an IPD incorporated in a Multi-Chip Module (MCM) arrangement;  
         [0007]      FIG. 2  is a section view showing a portion of the IPD to illustrate a typical multi-level metallization design;  
         [0008]      FIG. 3  shows one embodiment of the shielding plates of the invention;  
         [0009]      FIG. 4  is a plan view of the section through  4 - 4  of  FIG. 3 ;  
         [0010]      FIG. 5  is a plan view of the section through  5 - 5  of  FIG. 3 ;  
         [0011]      FIGS. 6 and 7  are schematic views illustrating process steps suitable for the manufacture of devices according to the invention;  
         [0012]      FIG. 8  is a schematic view of a larger portion of the metal layer of  FIG. 7  that forms part of the shielding; and  
         [0013]      FIGS. 9-11  are schematic views illustrating process steps following that represented by  FIG. 7 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]      FIG. 1  is a view of an IPD MCM, where IPD  11  is shown mounted on a printed circuit board (PCB)  12 . An IC chip  13  is shown mounted on the IPD. For simplicity, only one IC chip is shown. Typically there may be more than one. These IC chips can be an analog device, digital device, mixed signal device, RF device, and/or a micro-electro-mechanical-system (MEMS) based switch or oscillator. In most applications of the invention described here, at least one of the MCM chips will be a digital IC chip. Digital IC circuits contain noisy conductor runners, such as clock circuit runners. These are the conductors most likely to be addressed by the shielding arrangement of the invention. However, parasitic interactions occur between sensitive elements in the IPD and other external influences in the vicinity of the IPD. These adverse influences become more severe as the size, and thickness, of the IPD chip is reduced.  
         [0015]     A common choice for mounting the MCM to the PCB is solder. Referring again to  FIG. 1 , a solder bonding layer is shown at  14 , with PCB ground plane at  15 . The upper surface of the IPD is coated with a ground plane, and metal runners for interconnecting the IC chip, typically placed on two levels. These are represented as layer  16  in the figure, and connect to the IC chip through solder bumps  17 . The solder connection may be different from conventional flip chip connections in which the distance between the flip chip and substrate is kept at a maximum to maintain reliability, i.e. typically 70 to 120 um. It may have a small bump height, under 70 um, to accommodate the flatness variations between the IPD and the IC. Due to the similarity in thermal coefficient of expansion between the active IC and the IPD, there is less concern for solder joint reliability with a shortened distance. Furthermore, this reduction in distance will also contribute to the reduction of the overall thickness of the module. With this approach, that thickness, including the overmolding, made be made small, for example, not exceeding 1.0 mm overall thickness. This reduction in the distance also reduces the parasitic resistance and inductance, to further improve the performance of the RF circuits. Besides solder, other commonly known methods, for example, gold-to-gold, gold-to-aluminum, and conductive adhesives, are also within this scope of this invention.  
         [0016]     Wire bonds  18  connect the ground plane and runners to interconnections  19  on the PCB. Beside wire bonding, holes can be etched in the IPD, connecting the metal traces on the top and bottom surfaces of the IPD. This etching process is a commonly practiced MEMS manufacturing process. The IPD, with through holes, is attached to the substrate by conventional flip chip assembly process. The through hole connection further reduces the interconnect distance between the IPD and the substrate.  
         [0017]     It is understood by those in the art that the assembly shown in  FIG. 1  is encapsulated in a polymer housing. The plastic housing may be a plastic overmolded body, as in most conventional devices, or may be a plastic cavity package or other suitable protective package.  
         [0018]      FIG. 2  is a section view of a portion of the IPD  11  of  FIG. 1 . The figure illustrates the interconnections provided for two typical components, capacitor  22 , and resistor  23 . The bottom capacitor plate is indicated at  24 , the capacitor dielectric at  25 , and the top capacitor plate at  26 . The resistor body is shown at  27 . Runners  31 .  32  and  33  interconnect the bottom capacitor plate and the resistor body. Runner  35  interconnects the capacitor and resistor as shown. In this illustration runner  37 , on the same metal level as interconnection  35 , is a digital interconnection carrying a noisy signal. Under-bump-metallization (UBM) is shown at  36 , on the next metal level. The UBM is provided to accommodate a solder bump, such as that shown at  17  in  FIG. 1 .  
         [0019]     There are four levels of metallization shown in  FIG. 2 . The first level is lightly cross-hatched, and comprises capacitor plate  24 . The second level is more heavily cross-hatched, and comprises the upper capacitor plate  26  and the runners  31 ,  32 , and  33 . The third level is lightly shaded, and comprises capacitor-resistor interconnection  35  and noisy runner  37 . The fourth level is darkly shaded, and comprises UBM  36 . It will be understood by those skilled in the art that a thin film IPD may have fewer, or more, metal levels. It will also be understood that runner  37 , a noisy runner as will be seen, may reside at any position or level in the structure.  
         [0020]     It can be seen that if runner  37  carries a clock signal, or other noisy signal, the potential for EMI with capacitor  22 , or with other elements in the multi-level structure is significant.  
         [0021]     To overcome this problem, runner  37  is shielded in a metal structure like that shown in  FIG. 3 . In this layout, the noisy runner is located in an intermediate level, and metal shields are formed around it. The noisy runner is indicated by  47 . The interconnection arrangement of  FIG. 3  still has four levels, but runner  47  is part of level  3 , instead of level  4 . This is not a necessary feature of the invention, but simply an illustration of the variety of applications suitable for the invention. Also in this illustration, level  3  has other interconnect element(s) represented by  51 . In  FIG. 3 , the IPD substrate is shown at  41 , covered with dielectric layer  42 . Interlevel dielectric layers are  44 ,  46 , and  48 . Metal level  1  is shown at  43 , metal level  2  comprises runner  55  and shield plate  45 , metal level  3  comprises runners  51  and  47 , and metal level  4  comprises runner  56  and shield plate  49 . In the shielding region, the shield plate  45  of metal  2  and the shield plate  49  of metal  4  are connected with a series of vias represented by  52  and  53 . The shield plates  45  and  49  are connected to ground or another fixed potential. Electrically, they may not be part of the circuit interconnections, i.e., the circuits may function without these elements. However, a feature of the invention is that the plates  45  and  49  formed as parts of metal interconnect layers that do function as circuit interconnections. A circuit interconnection in metal level  2  is indicated by runner  55 , a circuit interconnection in metal  3  is indicated by runner  51 , and a circuit interconnection in metal level  4  is represented by runner  56 . It should be apparent that features  45  and  55  are formed simultaneously by patterning the level  2  metal layer, and features  49  and  56  are formed simultaneously by patterning the level  4  metal layer. While elements  55  and  56  are referred to as runners for circuit interconnection, they can as well be parts of circuit components for example, capacitor plates or inductor spirals. It is also within the scope of the invention to form shield plates  45  and  49  in metal levels that do not have other interconnection runners. Also, it should be understood that either or both of shield plates  45  and  49  may form parts of circuit interconnections as long as they are at the same circuit potential. Typically this will be ground.  
         [0022]      FIG. 4  shows a view through  4 - 4  of  FIG. 3 . The runner being shielded, i.e. the noisy runner, is shown at  47 , and the vias that form part of the shield are shown in  52  and  53 .  
         [0023]      FIG. 5  shows the section  5 - 5  of  FIG. 3 . This is a section through the upper shield plate, i.e.  49 . The runner  47  is indicated by the dashed line and is beneath the shield plate. The shield plate has a width, W S , that preferably is substantially larger that the width W R  of runner  47 . A typical width for runner  47  is 5-75 microns. A recommended width W S  for the shield  49  is: 
 W S &gt;1.5 W R    
 and preferably: 
 W S &gt;2 W R    
         [0024]     The overall shielding structure of  FIGS. 3-5  may be recognizable as the functional equivalent of a Faraday cage.  
         [0025]      FIGS. 2-5  illustrate but one interconnection arrangement for a typical IPD. A wide variety of similar arrangements will be found in typical IPDs. As mentioned above, an aspect of the preferred embodiments of the invention is that the shielding structure is formed as part of the interconnection layers. Features in the device can be identified as being formed as part of a common metal level because the distance separating the features from another level, or from the substrate, will be approximately the same.  
         [0026]      FIGS. 6-11  illustrate steps in the fabrication of the structure shown in  FIGS. 3-5 .  
         [0027]     In  FIG. 6  the IPD substrate is shown at  61 , covered with an insulating layer  62 . The IPD substrate may be polysilicon, and the insulating layer SiO 2 . More details on the fabrication of IPDs may be found in co-pending application Ser. No. 11/030,754, filed Jan. 6, 2005, which is incorporated herein by reference. To aid in understanding the relationship between the shield plates and other electrical features in the circuit(s)  FIG. 6  and subsequent figures show two portions of the substrate  61 , and  61 a, separated by an arbitrary distance.  
         [0028]      FIG. 7  illustrates the application of the first level metal. The metal portion  65  will form the lower part of the shield, and corresponds to element  45  in  FIG. 3 . The portion  66  of the first level metal, spatially removed from portion  65 , forms a circuit element, in this case the lower plate of a capacitor. The circuit element may comprise any feature of the electrical circuit of the device. For example, element  66  may correspond to a portion of plate  24  in  FIG. 2 . Another likely choice is a runner interconnecting another circuit component. The term circuit element means an element that is a part of an electrical circuit, and is connected to power, ground, or a digital or RF signal.  
         [0029]     The first level metal layout in this embodiment is shown in plan view in  FIG. 8 , where the lower shield plate is shown at  65 , and a plurality of lower capacitor plates at  66 .  
         [0030]     It will be understood by those skilled in the art that the metal levels are fabricated in a conventional manner, with a blanket deposition of metal on the surface of the device, the application of a suitable masking layer, typically a photolithograhic mask, and removal of the exposed metal by etching. The etching step may involve a liquid etchant, or may be a plasma etching step. These processing operations are well known and need no further exposition here.  
         [0031]      FIG. 9  shows a second level metal, comprising metal runner  68 . This is the runner that carries a noisy signal, and will accordingly be shielded.  
         [0032]     In  FIG. 10 , interlevel dielectric layer  71  is shown applied over the second level metal. Vias  72 , corresponding for example to elements  52  and  53  in  FIG. 4 , are formed through interlevel dielectric layers  67  and  71 .  
         [0033]      FIG. 11  shows a third metal level comprising the top shield  73 , and metal vias  74  connecting the top shield  73  to the bottom shield  65 . Elements  73 ,  74 , and  68  form a Faraday cage as described earlier. The third level metal also comprises a circuit element  76 . This element is part of the overall electrical circuit for the device. It may function, for example, as a runner forming an interconnection for circuit components, or it may comprise a portion of a circuit element itself, for example the top plate of a capacitor, or an inductor spiral. The point is that both shielding plates  65  and  73  are formed as part of a metallization layer that has other elements that perform an electrical function in the circuit.  
         [0034]     In an alternative embodiment of the invention, just one of the shielding layers comprises a portion of a metallization layer forming elements of the electrical circuit. The other shielding layer is part of a metallization layer formed only to provide a shield plate.  
         [0035]     For purposes of definition, the portions of the metallization that serve the shielding function but may not perform an active device interconnection are defined as shield metallization, or shield metallization portions. The parts of a metallization layer that serve as interconnections for electrical components in the device may be referred to as active metallization or active metallization portions. A metallization layer or metallization level means a pattern of metal deposited in a single deposition step, and generally wherein the metal resides on a common level. Alternatively, the shield metallization portions may be referred to as metal islands, to indicate that they may be electrically isolated from surrounding interconnect metal on a given level. A ground connection to the shield plates may be made through a via to another level.  
         [0036]     Electrical components in an IPD typically comprises capacitors, resistors and inductors. The capacitors will normally be formed using two metallization levels, i.e. one level for the bottom capacitor plate and one level for the top capacitor plate. Resistor contacts may be formed using one metallization level for both resistor contacts or two metallization levels for the resistor contacts. Inductors may be formed using one level for the inductor spiral and one or more levels for the inductor contacts. Thus, in the thin film implementation of an IPD (as described in the application referenced earlier) there are typically more than one metallization level that interconnects the IPD components. When reference is made to IPD active metallization it should be understood to mean any combination of the parts of the IPD components, including the contacts, and the metal runners interconnecting them.  
         [0037]     In the structure shown to illustrate the invention, the metal shields are formed at one level above and one level below the noisy runner. While that arrangement is most likely, a possible alternative in some cases may be to place one or both of the metal shields on a level removed from the noisy runner, for example, level one may include a metal shield, level two a noisy runner, and level four another metal shield. In this case the vias that interconnect the metal shields will extend through two levels.  
         [0038]     Reference made above, and in the appended claims, to “first”, “second” etc. in connection with metallization levels, is intended to convey a sequence, so the first metallization layer or level refers to the first in the recited sequence, and may or may not be the first layer or level in the device.  
         [0039]     Reference to an insulating substrate is intended to mean that the surface of the substrate comprises insulating material. The surface may be the surface of a bulk insulating substrate, or may be a layer of insulating material covering the bulk substrate. In high performance IPDs it is usually important that the bulk material as well as any surface layers be insulating. As should be evident from the thorough description of substrate materials in the application referenced above, un-doped, or very lightly doped, polysilicon qualifies from this standpoint, and is a preferred bulk substrate. The polysilicon may be covered with SiO 2 .  
         [0040]     The metallization layers may be formed by either substractive or additive processing. The term selective deposition, or selectively depositing, is intended to refer to both.  
         [0041]     Various additional modifications of this invention will occur to those skilled in the art. All deviations from the specific teachings of this specification that basically rely on the principles and their equivalents through which the art has been advanced are properly considered within the scope of the invention as described and claimed.