FIELD OF THE INVENTION  
       [0001]     This invention relates to capacitor circuits, and more specifically to capacitor circuits that are operative across a range of power supply voltages.  
       BACKGROUND OF THE INVENTION  
       [0002]     Capacitors are widely used in semiconductor technology. They may be used, for example, for noise decoupling, for blocking direct current, and for charge storage in radio-frequency (RF) and analog applications. Semiconductor capacitors typically take the form of diffusion capacitors, trench capacitors, gate capacitors, and metal-insulator-metal (MIM) capacitors. Typically, these devices depend on an insulating layer, frequently silicon dioxide, to form the dielectric between the terminals of the capacitor. Like other layers in a semiconductor device, these dielectric layers are continuously scaled in order to make devices smaller, to increase performance, and to reduce cost of production. As dielectric layers are scaled, especially when scaled faster than the voltage or power supply level, premature punch-through, breakdown, and other dielectric damage is frequently encountered. As a result, obtaining highly-reliable, high-value capacitors becomes increasingly more challenging.  
         [0003]     Adding to this challenge is the fact that integrated circuits must frequently be designed to work within a wide range of operating voltages. For example, an integrated circuit may need to work within a power supply voltage range from 1.2 volts to 3.6 volts. This is especially true when a designer wishes that a device be compatible with both modern and older semiconductor technologies.  
         [0004]     One way to resolve the problem of voltage-induced dielectric damage in semiconductor capacitors is to simply connect two or more capacitors in series. With this configuration, the voltage drop across any one of the capacitors is reduced and, correspondingly, the reliability of that capacitor is maintained. Nevertheless, maximum total capacitance for a given array of capacitors is achieved by wiring the capacitors in parallel. As a result, it is most advantageous to wire an array of capacitors in parallel when the power supply voltage is low enough that capacitor reliability is not adversely affected. This allows the circuit to have the maximum total capacitance for a given area. Conversely, when the power supply voltage is high enough to adversely affect capacitor reliability, it is usually advantageous to wire the array of capacitors in series to such an extent that reliability is maintained at the cost of reduced total capacitance. Accordingly, in those integrated circuits that are used over a wide range of power supply voltages, it is desirable to be able to selectively and dynamically modify the wiring configuration of capacitors depending upon the level of power supply voltage.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention addresses the above-identified need by providing, in an illustrative embodiment, a novel capacitor circuit that allows its constituent capacitors to be dynamically configured as a function of a level of the voltage across the capacitors. When a supply voltage to the capacitor circuit is greater than a specified level, the circuit selectively connects the capacitors together in series, so as to distribute the supply voltage across the capacitors in such a manner as to improve reliability of the capacitors. When the supply voltage is less than the specified level, the circuit connects the capacitors together in parallel, so as to increase an overall capacitance of the circuit for a given area.  
         [0006]     In accordance with one embodiment of the invention, a capacitor circuit having improved reliability includes at least first and second capacitors, a first terminal of the first capacitor connecting to a first source providing a first voltage, a first terminal of the second capacitor connecting to a second source providing a second voltage, the first voltage being greater than the second voltage. The capacitor further includes a voltage comparator having a first input for receiving a voltage representative of the first voltage, a second input for receiving a third voltage provided by a third source, and an output for generating a control signal. The control signal is a function of a difference between the voltage representative of the first voltage and the third voltage. A switch is connected to second terminals of the first and second capacitors. The switch is selectively operable in one of at least a first mode and a second mode in response to the control signal, wherein in the first mode the switch is operative to connect the first and second capacitors together in parallel, and in the second mode the switch is operative to connect the first and second capacitors together in series. The first mode is indicative of the voltage representative of the first voltage being less than or about equal to the third voltage, and the second mode is indicative of the voltage representative of the first voltage being greater than the third voltage.  
         [0007]     In an illustrative embodiment, a capacitor circuit comprises two capacitors, a voltage comparator, and a switch. Three voltage levels are provided: a power supply voltage, Vcc, a ground level, Vss, and a reference source voltage, Vref. Vref is placed at a voltage approximately equal to the maximum sustainable voltage at which the capacitors can achieve a desired reliability lifetime. The voltage comparator is operative to cause the switch to connect the two capacitors in parallel when Vcc is less than Vref so that the full power supply voltage, Vcc, is dropped across each capacitor. Total circuit capacitance is thereby maximized. When Vcc exceeds Vref, the switch connects the two capacitors together in series such that Vcc is distributed across the combination of capacitors. In this way, the reliability of the capacitors in the circuit is maintained even when the power supply voltage exceeds the maximum voltage for sustained capacitor reliability. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a schematic diagram illustrating an exemplary capacitor circuit, formed in accordance with one embodiment of the invention.  
         [0009]      FIG. 2A  is a schematic diagram illustrating an exemplary switch comprising metal-oxide-semiconductor (MOS) devices suitable for use in the capacitor circuit of  FIG. 1 , in accordance with one embodiment of the invention.  
         [0010]      FIG. 2B  is a schematic diagram illustrating an exemplary switch comprising cantilever-type microelectromechanical system (MEMS) devices suitable for use in the capacitor circuit of  FIG. 1 , in accordance with a second embodiment of the invention.  
         [0011]      FIG. 2C  is a schematic diagram illustrating an exemplary switch comprising hinge-type MEMS devices suitable for use in the capacitor circuit of  FIG. 1 , in accordance with a third embodiment of the invention.  
         [0012]      FIG. 3  is a schematic diagram illustrating an exemplary capacitor circuit comprising two groups of capacitors, formed in accordance with an embodiment of the invention.  
         [0013]      FIG. 4  is a schematic diagram illustrating an exemplary capacitor circuit comprising three groups of capacitors, formed in accordance with another embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     The present invention will be illustrated below in conjunction with illustrative embodiments of capacitor circuits. It should be understood, however, that the invention is not limited to the particular circuitry arrangements of the illustrative embodiments. Moreover, in the illustrative embodiments, the capacitor circuit will be illustrated as a component of an integrated circuit. Nevertheless, the scope of this invention is intended to include a capacitor circuit formed of discrete device components mounted on, for example, a printed circuit board, substrate, etc. These and other possible modifications to the illustrative embodiments within the scope of this invention will be apparent to those skilled in the art.  
         [0015]      FIG. 1  is a schematic diagram depicting an exemplary capacitor circuit  100 , formed in accordance with one embodiment of the present invention. The circuit  100  comprises two capacitors  110 ,  120 , a switch  130 , and a voltage comparator  140 , or alternative voltage detection circuitry. Three voltage levels are provided to the circuit  100 : a power supply voltage, Vcc; a ground level, Vss; and a reference source voltage, Vref. It is to be understood that in an alternative embodiment of the invention, more than one power supply voltage level may be detected, and thus additional reference source voltages may be provided to the circuit  100 . A first terminal of capacitor  110  is connected to the power supply voltage Vcc, and a second terminal of capacitor  110  is connected to switch  130  at node  5 . A first terminal of capacitor  120  is connected to switch  130  at node  6 , and a second terminal of capacitor  120  is connected to ground level Vss. Switch  130  connects to Vcc at node  1  and connects to Vss at node  4 . At least a given one of capacitors  110  and  120  preferably comprises an MOS gate oxide capacitor, a trench capacitor, a diffusion capacitor, a metal-insulator-metal (MIM) capacitor and/or a discrete capacitor, although the present invention is not limited to only these particular types of capacitors.  
         [0016]     Vcc, or a voltage representative thereof, is connected to a first input of the voltage comparator  140  at node  8 . Vref is connected to a second input of the voltage comparator  140  at node  9 . Power is supplied to the voltage comparator  140  by connecting the comparator to Vcc at node  7  and to Vss at node  10 . In order to reduce glitches which may be generated by the voltage comparator  140 , particularly when Vcc is substantially close to Vref, hysteresis can be employed. Various methodologies for implementing hysteresis in a comparator are well known by those skilled in the art.  
         [0017]     Voltage comparators are frequently found in electronic circuits and will be familiar to those skilled in the art. Moreover, voltage comparators are described in various publications including, for example, P. Horowitz and W. Hill,  The Art of Electronics , Newnes Publishing 1991, which is incorporated by reference herein. Generally, the output(s) of a voltage comparator is (are) switched to a logical high or low state depending on the relative magnitudes of the voltages at the inputs thereof. In the embodiment shown in  FIG. 1 , the voltage comparator  140  compares the voltages on nodes  8  and  9  and generates an output which is a function of a difference between the two voltages. If the voltage on node  8  (Vcc) is higher than the voltage on node  9  (Vref), then the voltage comparator outputs at nodes  2  and  3  go high and low, respectively. If, on the other hand, the voltage on node  8  (Vcc) is lower than that on node  9  (Vref), the voltage comparator outputs at nodes  2  and  3  go low and high, respectively. As shown in  FIG. 1 , comparator  140  includes two outputs which generate corresponding output signals that are logical complements of one another. This is often referred to as a differential output comparator. It is to be appreciated, however, that a comparator having a differential output is not a requirement of the invention.  
         [0018]     In accordance with an aspect of the invention, the outputs of the voltage comparator  140  at nodes  2  and  3  are operative to cause switch  130  to electrically connect capacitors  110  and  120  together in parallel when Vcc is less than Vref, and to cause the switch  130  to connect the capacitors  110 ,  120  together in series when Vcc is greater than Vref. Numerous circuit configurations of switch  130  can be used to implement this function. By way of example, and without loss of generality,  FIGS. 2A through 2C  show three illustrative embodiments for implementing switch  130 , in accordance with the invention. It is noted that the reference designations depicted in  FIGS. 2A-2C  correspond directly to the reference designations shown in  FIG. 1 .  
         [0019]     The illustrative switch embodiment depicted in  FIG. 2A  comprises MOS transistor devices. More specifically, switch  130 - a  comprises two n-channel MOS (NMOS) transistor devices  210 ,  220 , and one p-channel MOS (PMOS) transistor device  230 . The NMOS transistor device  210  has a gate (G) connected to node  3 , a source (S) connected to node  6 , and a drain (D) connected to node  5 . The second NMOS transistor device  220  has a gate connected to node  2 , a source connected to node  4 , and a drain connected to node  5 . The PMOS transistor device  230  is configured such that its gate is connected to node  3 , its source is connected to node  6 , and its drain is connected to node  1 .  
         [0020]     The functioning of switch  130 - a  in  FIG. 2A  involves the operation of typical NMOS and PMOS transistor devices and will, as a result, be easily comprehended by one skilled in the art. As described above, when Vcc is greater than Vref, the outputs of the voltage comparator  140  at nodes  2  and  3  will be high and low, respectively. A high voltage on node  2  causes NMOS transistor device  220  to turn on such that a substantially low-resistance (e.g., less than about ten ohms) electrical path is created between nodes  4  and  5 . This is operative to electrically connect the second terminal of capacitor  110  in  FIG. 1  to Vss. At the same time, a low voltage on node  3  turns off NMOS transistor device  210  and turns on PMOS transistor device  230 . Nodes  1  and  6  are thereby electrically connected together through the PMOS transistor device  230 . This results in Vcc being applied to the first terminal of capacitor  120 . The end result of these connections is that the capacitors  110 ,  120  are wired together in parallel. Vcc is, thereby, applied across each capacitor  110 ,  120 .  
         [0021]     On the other hand, where Vcc is lower than Vref, the voltage comparator  140  of  FIG. 1  operates such that node  2  goes low and node  3  goes high. As a result, NMOS transistor device  220  and PMOS transistor device  230  are turned off while NMOS transistor device  210  is turned on. Nodes  5  and  6  are, as a result, connected together, resulting in capacitors  110 ,  120  being wired in series with one another between Vcc and Vss.  
         [0022]     Alternatively, in accordance with another aspect of the invention, switch element  130  may comprise MEMS devices. MEMS devices may integrate mechanical elements, sensors, actuators, electronics, etc., in a common silicon substrate through microfabrication technology. While the electronic components may be fabricated using, for example, a standard lithographic integrated circuit process (e.g., complementary MOS (CMOS), Bipolar, or BiCMOS processes), the micromechanical components may be fabricated using compatible micromachining processes that selectively etch away parts of the silicon wafer and/or add new structural layers to form mechanical and electromechanical devices. The operation and formation of MEMS devices is well described in readily-available publications, including, for example,  The MEMS Handbook , edited by M. Gad-El-Hak, CRC Press, 2001, which is incorporated herein by reference.  
         [0023]     MEMS devices for semiconductor applications have many advantages, including low insertion loss, low direct current (DC) power consumption, high linearity, and broad bandwidth performance.  FIG. 2B  shows a schematic diagram of an exemplary switch  130 - b  comprising cantilever-type MEMS devices for use in implementing the switch  130  depicted in  FIG. 1 , in accordance with a second illustrative embodiment of the invention. This switch  130 - b  comprises three cantilever MEMS devices  240 ,  250  and  260 . Referring to MEMS device  240 , when a high voltage is placed on a control terminal of the device at node  2  and a low voltage is placed on node  4 , such as when comparator  140  detects that Vcc is greater than Vref, the cantilever membrane is repelled such that it swings into a position where it electrically connects nodes  4  and  5  together. Similarly, in MEMS device  260 , the high voltage on a control terminal at node  2  causes nodes  1  and  6  to be connected together. In MEMS device  250 , a low voltage on a control terminal at node  3  cause the cantilever membrane in the device to be positioned such that nodes  5  and  6  are electrically disconnected from one another (e.g., open circuit). As before, this configuration results in capacitors  110 ,  120  being wired in parallel. Alternatively, where voltage comparator  140  places a low voltage on node  2  and a high voltage on node  3 , such as when the comparator  140  detects that Vcc is less than Vref, nodes  5  and  6  will be electrically connected together, while nodes  1  and  6 , and nodes  4  and  5 , will be open circuit. Capacitors  110 ,  120  are, thereby, wired in series with one another.  
         [0024]     In accordance with yet another aspect of the invention, the switch  130  shown in  FIG. 1  may comprise hinge-type MEMS devices.  FIG. 2C  is a schematic diagram illustrating an exemplary switch  130 - c  comprising hinge-type MEMS devices, in accordance with a third illustrative embodiment of the invention. Hinge-type MEMS devices are described, for example, in U.S. Pat. No. 6,143,997, which is incorporated herein by reference. Advantageously, hinge-type MEMS devices may overcome the need for high actuation voltages found in other types of MEMS switches. In switch  130 - c , hinge plates  270 ,  280  are attracted toward either a first control terminal at node  2  or a second control terminal at node  3  by a high voltage applied to either node  2  or node  3 , respectively. When node  2  is high and node  3  is low (e.g., when Vcc is less than Vref), hinge plates  270 ,  280  move toward node  2  and thereby electrically connect nodes  4  and  5  together, and nodes  1  and  6  together. Capacitors  110 ,  120  are connected together in parallel in this manner. When node  2  is low and node  3  is high (e.g., where Vcc is greater than Vref), hinge plates  270 ,  280  move toward node  3 , thereby connecting nodes  5  and  6  together. This operation causes the capacitors  110 ,  120  to be connected in series.  
         [0025]     Based on the preceding, it is clear that the level of the reference source voltage, Vref, is largely determinative of whether capacitors  110 ,  120  are wired in series or in parallel in the capacitor circuit embodiment of  FIG. 1 . In accordance with a preferred embodiment of the invention, Vref is selected to be approximately equal to a specified maximum sustainable voltage at which the capacitors  110 ,  120  can achieve a desired reliability lifetime. Such a maximum sustainable voltage is typically determined by stressing prototypes of integrated circuit capacitors prior to large scale production. Typically, one or more capacitors are exposed to a fixed electric field at a fixed temperature and a time-to-fail is measured. To meet certain lifetime requirements, a capacitor with a silicon dioxide thickness in a range of about 3-5 nanometers will typically have a maximum sustainable voltage of about 1.2 volts. As the dielectric layer becomes thicker, the maximum sustainable supply voltage will generally increase. However, the actual maximum sustainable voltage achievable by a given capacitor will likely depend on a combination of one or more characteristics of the capacitor and its dielectric. Beyond dielectric thickness and temperature, these characteristics may include, for example, dielectric impurity content, capacitor shape, method of dielectric growth, and others.  
         [0026]     It will be recognized by those that skilled in the art that the circuit depicted in  FIG. 1  would work in a manner consistent with that described above if a fraction of Vcc (e.g., Vcc/2) were applied to an input of the voltage comparator  140 , as opposed to the full power supply voltage, and if Vref were reduced by the same ratio. The scope of this invention is intended to cover such a configuration. For example, a voltage equal to one half of the supply voltage Vcc could be input into the voltage comparator  140  at node  8 . If Vref at node  9  were also reduced by one half, the illustrative capacitor circuit  100  of  FIG. 1  would work identically to that where the full Vcc and Vref are applied to the voltage comparator  140 . A voltage can be reduced by many methods including, but not limited to, implementation of a simple voltage divider circuit comprising two or more resistors (not shown). Such a voltage divider is described, for example, in  The Art of Electronics , cited earlier.  
         [0027]     The scope of the present invention is also not restricted to a capacitor circuit comprising only one pair of capacitors, as shown in  FIG. 1 . For example,  FIG. 3  shows a schematic diagram of an exemplary capacitor circuit  300 , formed in accordance with an alternative embodiment of the invention. The capacitor circuit  300  comprises n pairs of capacitors  310 - 1  and  320 - 1  through  310 -n and  320 -n, and n switches  330 - 1  through  330 -n, where n is an integer greater than one. As apparent from the figure, a single voltage comparator  340  may be used to generate one or more control signals for selectively controlling a connection of the capacitors as a function of the supply voltage Vcc, in a manner similar to that described above in connection with  FIG. 1 .  
         [0028]     An upper group of capacitors may be defined, comprising capacitors  310 - 1  through  310 -n, referred to collectively as  310 . Each of the capacitors  310  is preferably configured such that a first terminal is connected to Vcc and a second terminal is connected to node  11 . Similarly, a lower group of capacitors may be defined, comprising capacitors  320 - 1  through  320 -n, referred to collectively as  320 . Each of the capacitors  320  is preferably configured such that a first terminal is connected to node  12  and a second terminal is connected to Vss. Switches  330 - 1  through  330 -n are preferably connected between nodes  11  and  12 . Each switch may be configured in a manner consistent with switch  130  depicted in  FIG. 1 . Like the single capacitor pair arrangement of  FIG. 1 , the operation of switches  330 - 1  through  330 -n depends primarily on the outputs of the voltage comparator  340 . When the supply voltage Vcc exceeds Vref, the two capacitor groups  310 ,  320  are connected together in series, otherwise the capacitor groups are connected together in parallel.  
         [0029]     It should be noted with respect to the illustrative capacitor circuit  300  shown in  FIG. 3  that, although a single switch could be used to implement the selective connection of the upper and lower capacitor groups, using a plurality of switches is preferable. Utilizing a plurality of switches ensures a more uniform and effective switching action, thereby enhancing circuit performance.  
         [0030]     If the supply level is expected to be higher than twice Vref, then three or more groups of capacitors may be necessary in order to effectively distribute the voltage across the capacitors so that capacitor reliability is not adversely affected.  FIG. 4  shows a schematic diagram of an exemplary capacitor circuit  400  which may be employed when the supply voltage Vcc is too high to be distributed effectively across two capacitors (or two groups of capacitors), in accordance with another aspect of the invention. In this illustrative embodiment, three groups of capacitors are employed, namely, capacitors  410 - 1  through  410 -n, referred to collectively as  410 , capacitors  415 - 1  through  415 -n, referred to collectively as  415 , and capacitors  420 - 1  through  420 -n, referred to collectively as  420 , where n is an integer greater than one. The invention, however, is not limited to the particular circuit arrangement shown.  
         [0031]     In the exemplary capacitor circuit  400 , a first voltage comparator  440  is preferably used to compare Vcc and a first reference voltage, Vref. When Vcc exceeds Vref, switches  430 - 1  through  430 -n are preferably operative to connect capacitors  410  in series with capacitors  415 , in a manner consistent with that previously described in conjunction with  FIG. 1 . Simultaneously, a second voltage comparator  445  is preferably used to compare Vcc with a second reference voltage, which may be, for example, equal to twice Vref (2×Vref). When Vcc exceeds twice Vref, the third group of capacitors,  420  are added in series with capacitor groups  410  and  415 . In this manner, the supply voltage Vcc is distributed across two groups of capacitors connected in series when Vref&lt;Vcc&lt;2×Vref, and Vcc is distributed across three groups of capacitors connected in series when Vcc&gt;2×Vref. When Vcc is less than Vref, all of the capacitors are preferably connected in parallel, thereby maximizing the total capacitance for a given area.  
         [0032]     It should be noted that certain components and/or devices may be formed using standard semiconductor processing techniques. For example, the capacitors employed in the present invention may comprise trench capacitors, diffusion capacitors, gate capacitors, metal-insulator-metal capacitors, etc. Semiconductor processing techniques required to form such capacitors, as well as other devices in the capacitor circuit, will be familiar to those skilled in the art. Such IC processing techniques frequently include, but are not limited to, one or more of the following: deposition, etching, silicon growth, photolithography, ion implantation, cleaning, and polishing. Each of these IC processing techniques are further described in a number of publications, including, for example, S. M. Sze,  VLSI Technology , McGraw-Hill, 1988, which is incorporated herein by reference.  
         [0033]     At least a portion of the capacitor circuit of the present invention may be implemented in an integrated circuit, although the techniques of the present invention are similarly applicable to a circuit comprised of discrete components. In forming integrated circuits, a plurality of identical die are typically fabricated in a repeated pattern on a surface of a semiconductor wafer. Each die includes a device described herein, and may include other structures or circuits. The individual die are cut or diced from the wafer, then packaged as an integrated circuit. One skilled in the art would know how to dice wafers and package die to produce integrated circuits. Integrated circuits so manufactured are considered part of this invention.  
         [0034]     Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be made therein by one skilled in the art without departing from the scope of the appended claims.
A capacitor circuit having improved reliability includes at least first and second capacitors, a first terminal of the first capacitor connecting to a first source providing a first voltage, a first terminal of the second capacitor connecting to a second source providing a second voltage, the first voltage being greater than the second voltage. The capacitor further includes a voltage comparator having a first input for receiving a voltage representative of the first voltage, a second input for receiving a third voltage provided by a third source, and an output for generating a control signal. The control signal is a function of a difference between the voltage representative of the first voltage and the third voltage. A switch is connected to second terminals of the first and second capacitors. The switch is selectively operable in one of at least a first mode and a second mode in response to the control signal, wherein in the first mode the switch is operative to connect the first and second capacitors together in parallel, and in the second mode the switch is operative to connect the first and second capacitors together in series. The first mode is indicative of the voltage representative of the first voltage being less than or about equal to the third voltage, and the second mode is indicative of the voltage representative of the first voltage being greater than the third voltage.