Abstract:
A SIMM (single in-line memory module) board is provided with a plurality of integrated semiconductor memory or other integrated semiconductor circuit devices which include, as a part of each integrated circuit device, a current spike leveling capacitor. The capacitor is on the die side of circuitry connecting the device to the board. By connecting the on-chip capacitors of the integrated circuit devices in parallel, sufficient capacitance is provided to stabilize current to all of the integrated circuit devices.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of U.S. patent application Ser. No. 08/965,741, filed Nov. 7, 1997, pending, which is a continuation of U.S. patent application Ser. No. 08/671,248, filed Jun. 27, 1996, now U.S. Pat. No. 5,687,109, which is a continuation of U.S. patent application Ser. No. 08/178,716, filed Jan. 10, 1994, now abandoned, which is a continuation of U.S. patent application Ser. No. 08/034,001, filed Mar. 19, 1993, which is now U.S. Pat. No. 5,307,309, which is a continuation of U.S. patent application Ser. No. 07/774,121, filed Oct. 8, 1991, abandoned, which is a continuation of U.S. patent application Ser. No. 07/291,294, filed Dec. 27, 1988, now abandoned, which is a continuation-in-part of U.S. patent application Ser. No. 07/200,673, filed May 31, 1988, now abandoned. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to arrays of semiconductor circuit devices, in which a plurality of integrated circuit chips is mounted to a printed circuit board, or the like, for connection to a main circuit board (mother board). The invention is directed to power supply filtering of SIMM (single in-line memory module) arrays and similar arrays.  
           [0004]    This invention further relates to semiconductor devices and, more specifically, to circuit connections on semiconductor devices and to the reduction of voltage transients on the semiconductor devices.  
           [0005]    2. State of the Art  
           [0006]    Integrated semiconductor devices are typically constructed en masse on a wafer of silicon or gallium arsenide. Each device generally takes the form of an integrated circuit (IC) die, which is attached to a lead frame with gold wires. As shown in FIG. 1, the die and lead frame are then encapsulated in a plastic or ceramic package, which is then recognizable as an IC “chip”. IC chips come in a variety of forms, such as dynamic random access memory (DRAM) chips, static random access memory (SRAM) chips, read only memory. (ROM) chips, gate arrays, and so forth. The chips are interconnected in myriad combinations on printed circuit boards by a number of techniques, such as socketing and soldering.  
           [0007]    Interconnections among chips arrayed on printed circuit boards are typically made by conductive traces formed by photolithography and etching processes. Semiconductor circuit devices, including DRAMs, SRAMs and gate arrays, are essentially switching devices. As the output drivers within those chips create intermittent current flow on associated conductive traces, the traces behave as inductors, creating voltage surges which have the potential for creating logic errors. Other logic-damaging transient voltages, caused by voltage fluctuations at the power line and the interaction of other circuit components in the system, may also be present.  
           [0008]    Semiconductor devices typically take the form of a semiconductor die. The semiconductor die  11  is generally attached to a lead frame  13  within a package, by means of fine gold wires  15 , as shown in FIG. 1. these fine gold wires function as lead frame connection wires. the lead frame and die assembly is then encapsulated, in the form of the familiar integrated circuit “chip”. The packaged chip is then able to be installed on a circuit board by any number of techniques, such as socketing and soldering.  
           [0009]    In order to render innocuous the transient voltages which regularly appear in logic circuits, decoupling capacitors are commonly used as low-frequency bypass filters.  
           [0010]    The gold connection wires, because of their length relative to their diameter, function as inductors. As current through the gold connection wires is alternately switched on and off, voltage spikes occur. In order to reduce the effects of voltage transients, external capacitors have been installed either within the semiconductor package or on a circuit board onto which the semiconductor packages are installed. In either case, the capacitor is on an opposite side of the lead frame connection wire from the semiconductor die. This establishes the circuit shown in FIG. 2. This equivalent circuit represents an inappropriate arrangement for filtering voltage transients which would affect active circuit  11 .  
           [0011]    One circuit-board-mounted semiconductor chip array that is of particular interest is the SIMM (single in-line memory module). SIMM boards are typically constructed with such capacitors, which are usually located beneath or adjacent memory array circuit chips on the SIMM.  
           [0012]    SIMM (single in-line memory module) boards are circuit arrays which consist of byte multiples of memory chips arranged on a printed circuit board or comparable mounting arrangement. The SIMM board is connected to a circuit control board by an edge connector.  
           [0013]    The SIMM is a highly space-efficient memory board having no on board address circuitry and which is designed to plug directly into the address, data and power-supply buses of a computer so that the randomly-addressable memory cells of the SIMM can be addressed directly by the computer&#39;s CPU rather than by a bank-switching technique commonly used in larger memory expansion boards. Memory cells on the SIMM are perceived by the computer&#39;s CPU as being no different than memory cells found on the computer&#39;s mother board. Since SIMMs are typically populated with byte multiples of DRAMs, for any eight bit byte or sixteen bit byte or word of information stored within a SIMM, each of the component bits will be found on a separate chip and will be individually addressable by column and row. One edge of a SIMM module is a card-edge connector which plugs into a socket on the computer which is directly connected to the computer buses required for powering and addressing the memory on the SIMM.  
           [0014]    The control board may be any of a number of circuits which address memory arrays. Examples include computer mother boards, daughter boards which plug into a mother board, wherein the daughter board functions as a mother board for the SIMM module, peripheral devices with a capability of using add-on memory, and special purpose equipment which uses memory. It is also possible to use small modules of arrays of similar circuits for purposes other than memory applications.  
           [0015]    The capacitor on the SIMM, mounted external to the memory chips, establishes an inappropriate arrangement for filtering voltage transients. Therefore, it is desirable to provide capacitance on the other side of the inductor, i.e., the side of the inductor that the device is connected to.  
           [0016]    Present SIMM boards are provided with surface-mounted decoupling capacitors, which cannot be seen in plan view. In the usual case, one decoupling capacitor is mounted beneath each DRAM chip, between bus voltage (V CC ) input and the connection to ground. The V CC  bus and the ground-plane bus on the circuit board are not visible in a plan view since those particular bus traces are located between two of the board&#39;s six layers.  
           [0017]    In most cases, each of the module&#39;s decoupling capacitors are connected in parallel between the V CC  bus and the ground plane bus. As long as the dielectric of each of the eight capacitors is intact, the module is functional. However, a short in any one of the eight capacitors will result in the V CC  buss becoming shorted to the ground-plane bus, whereupon the module will begin to draw an inordinate amount of current which will invariably result in its destruction.  
           [0018]    Decoupling capacitors of the surface-mount type are particularly susceptible to shorting, since they have no leads to thermally isolate them as they are soldered to a circuit board with infrared energy at temperatures of up to 400° C. (700° F.). Even if a surface-mount capacitor survives the mechanical shock generated by the soldering process, it is still vulnerable to other types of mechanical stress. For example, by simply bending a SIMM having surface-mounted capacitors, the capacitors may be compromised. And, even if a SIMM passes testing (an indication that the decoupling capacitors are at least not shorted), it may have a relatively high failure rate when placed in use. SIMMs of the type shown in FIG. 9 have an unacceptable average failure rate traceable to shorted decoupling capacitors during the first 90 days of use of roughly 3-10 per 100,000.  
           [0019]    Single in-line packages (SIPs) are similar in design to SIMMs, except that instead of having a card edge-type connector, SIPs have pins which are either socketed or soldered for connection to a bus. The problems associated with the decoupling capacitor system of SIMMs also apply to SIPs.  
           [0020]    Most semiconductors, including all DRAMS, include capacitors. For example, a 4 megabit DRAM includes over 4 million capacitors each. For the purpose of storing individual bits of information, these capacitors are accessed by connections through access transistors and sense amplifiers, connected through a peripheral circuit. The present invention concerns adding filter capacitance to such devices in order to provide protection from voltage transients which may not be afforded by what may be millions of other capacitors on the semiconductor device.  
           [0021]    Semiconductor circuit devices are designed with an architecture which places their functional circuitry within a confined area, usually rectangularly shaped. At the perimeter (either outside or inside) of the rectangularly shaped area are a series of contact pads and a substantial amount of chip area which is occupied by conductor buses, but is unoccupied by active circuit devices. Unlike many of the circuit elements on a semiconductor circuit device, filter capacitors need not be built to precise specifications. It is, therefore, possible to utilize perimeter areas and portions of semiconductor chip areas which form major border areas between active portions of the semiconductor circuit device.  
           [0022]    There is a significant advantage in providing that any added circuit elements be on the same side of a chip wafer as other circuit elements, because of manufacturing techniques and tolerances. Conventionally, semiconductor circuit devices are arrayed on one side of a die wafer. It would, therefore, be advantageous to design a filtering element which would not significantly expand the die area (chip area) required for each die.  
           [0023]    There is a certain portion of the die area which is not particularly suitable for active circuitry. This includes chip area occupied by bus lines, which are normally metallization which overlays most or all of the patterned layers which make up the active circuitry on the die.  
         BRIEF SUMMARY OF THE INVENTION  
         [0024]    In accordance with the present invention, capacitance filtering is provided for a circuit having an array of similar semiconductor circuit devices, such as a SIMM (single in-line memory module) array of semiconductor circuit devices. The semiconductor circuit devices are formed with capacitors located primarily in border areas, including perimeter border areas and intermediate border areas. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0025]    [0025]FIG. 1 shows a top view of a semiconductor device attached by pads to the lead frame;  
         [0026]    [0026]FIG. 2 shows an equivalent circuit of a semiconductor device connected to a lead wire and an off-chip capacitor;  
         [0027]    [0027]FIG. 3 shows an equivalent circuit of a semiconductor circuit device connected through a lead frame connection wire and having an on-chip decoupling capacitor;  
         [0028]    [0028]FIG. 4 shows a top view of a semiconductor device which incorporates a decoupling capacitor;  
         [0029]    [0029]FIG. 5 shows a top view of a semiconductor device, in which a decoupling capacitor is placed along an intermediate boundary area of the chip architecture;  
         [0030]    [0030]FIGS. 6 and 7 show connection arrangements for N-channel and P-channel capacitors, respectively;  
         [0031]    [0031]FIG. 8 shows a cross-sectional view of an arrangement in which two capacitors are connected in series in order to increase breakdown voltage;  
         [0032]    [0032]FIG. 9 shows a SIMM module constructed in accordance with the present invention;  
         [0033]    [0033]FIG. 10 shows a parallel arrangement of capacitors on a memory array;  
         [0034]    [0034]FIG. 11 shows the use of a SIP board; and  
         [0035]    [0035]FIG. 12 shows a schematic block diagram representation of semiconductor circuit device having an on-chip regulator. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]    Referring to FIG. 1, a semiconductor device includes a die  11  which is connected to a lead frame  13  by a plurality of lead wires  15 . The lead wires  15  are attached to the die  11  at pads or contact points  17 .  
         [0037]    The lead wires  15  function as inductors  15 ′ and  15 ″, as schematically shown in FIGS. 2 and 3. While an external capacitor  21  is often provided, an appropriate filter capacitance would be located on the die side of the lead wire  15 ″, as schematically shown in FIG. 3, at  23 .  
         [0038]    [0038]FIG. 4 shows details of one end of the die  11  constructed with the present invention. A pair of capacitors is defined by an active area of the substrate  30  and a polysilicon (poly) layer which is formed into strips  25 ,  26 . The active area of the substrate  30  is in electrical communication with a first bus line V SS . The poly strips  25 ,  26  are in electrical communication with a second bus line V CC . Oxide is used to separate the strips  25 ,  26  from the active area of the substrate  30 .  
         [0039]    The capacitors defined by the strips  25 ,  26  are on a location of the die  11  which underlies VX and V CC , as well as other buses  31 . The buses  31  (including VX and V CC ) are typically metallization layers, and real estate occupied by the buses  31  cannot be used for most types of active circuitry. This is because active circuitry requires utilization of layers as outputs, which, in this case, is prevented by the buses  31  which are used for routing signals from the left end to the right end of the chip.  
         [0040]    [0040]FIG. 5 shows a configuration in which a pair of capacitors is defined by an active substrate area  55  along an intermediate portion of a semiconductor die  57 . A plurality of poly strips  25 - 26 , superimposed over the active poly area, defines a plurality of capacitors. Circuit buses  71  are superimposed over the capacitors formed by the poly strips so that the capacitors do not occupy real estate that could be used for most active circuit devices.  
         [0041]    The invention has been described in terms of connection to circuit buses which have external connections. It is possible that an additional circuit may be placed between the bus and an external connection. A likely example of such an additional circuit would be a voltage regulating circuit. It is possible to connect the capacitor to a bus which extends between such an additional circuit and a main portion of the integrated circuit device.  
         [0042]    The present embodiment contemplates the use of N channel capacitors, with V SS  connected to the active area of the substrate  30 ′ x  and V CC  connected to poly  75 . This is shown in FIG. 6. It is possible to construct P channel capacitors with V CC  connected to the active area of the substrate  30 ″ and V SS  connected to poly  85 . This is shown in FIG. 7. Each of these is an enhancement mode capacitor, which has a preferential voltage polarity. It is also possible to form these capacitors as depletion mode capacitors.  
         [0043]    It may also be the case that two capacitors may be connected in series in order to increase the total breakdown voltage of the combined capacitors. Enhancement mode capacitors require adjustment for their preferential voltage polarity. This can be accomplished through interconnects or similar means. Depletion mode capacitors, on the other hand, have less preferential voltage polarity. If the capacitors are not polarization sensitive, then the capacitors can have a common poly plate  91  or a common active area  95 , as schematically shown in FIG. 8.  
         [0044]    [0044]FIG. 9 shows a SIMM (single in-line memory module) board  101 , which consists of a printed circuit board  103 , on which are mounted a plurality of semiconductor memory devices such as DRAMs  105 . The printed circuit board  103  includes an edge connector  107 , which extends from the printed circuit board  103  in order to permit the SIMM board  101  to be plugged into a computer bus (not shown) on a computer. The computer bus has a capability of addressing the DRAMs  105  on the board in predetermined sequence, as defined by the SIMM protocol. Typically, an entire row of DRAMs  105  is simultaneously addressed to obtain a byte of information. Other addressing schemes are, of course, possible.  
         [0045]    [0045]FIGS. 10 and 11 show the use of capacitors on DRAMS  105 . A SIP (single in-line package) board  111 , as shown in FIG. 11 and similar boards, which use a connector to connect an array of similar components with parallel address circuitry through a connector may also be used with the invention.  
         [0046]    By providing a capacitor as a part of the individual DRAMs  105 , the SIMM board  101  does not have to be constructed with discrete capacitors mounted to the board. As mentioned, having a capacitor on the die serves to filter the effects of lead wire inductance. Further, since the SIMM board includes byte multiples of DRAMs, the capacitors on board each chip are connected in parallel. While this does not increase the capacitance on the die side of the lead wires, the total capacitance is thereby increased, with the added benefit that some capacitance is on the die side of the lead wire of each chip.  
         [0047]    The elimination of discrete capacitors further eliminates a failure mode. It has been found that as many as one in 10,000 discrete capacitors has failed subsequent to burn-in. This has resulted in an added field failure rate of close to one in 1000 for an 8 or 9 device part. Eliminating the discrete capacitors is believed to significantly reduce this failure rate.  
         [0048]    There is a possibility that the impedance of the multiple rows of the DRAMs  105  results in a mismatch (of impedance) of multiplexed RAS and CAS signal lines if the signals are intended for use with a single row of DRAMs. In order to cause the impedance to match that of inputs, termination capacitors (not shown) may be are used to compensate for the shifted impedance load of the DRAMs caused by the multiple rows of DRAMs  105 . The termination capacitors may be discrete elements, even if load capacitors are incorporated onto the DRAMs  105 .  
         [0049]    Referring to FIG. 12, a regulator  98  is on-chip with capacitor  23  and die  11 .  
         [0050]    What has been described is a specific embodiment of the invention. The invention is expected to be able to work with other memory devices, such as SRAMS and VRAMS. It is anticipated that variations can be made on the preferred embodiment and, therefore, the invention should be read as limited only by the claims.