Patent Publication Number: US-7719310-B2

Title: Semiconductor integrated circuit device and method for manufacturing the same

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese patent application JP 2008-089136 filed on Mar. 31, 2008, the content of which is hereby incorporated by reference into this application. 
     FIELD OF THE INVENTION 
     The invention relates to an on-chip decoupling capacitor mounted on a semiconductor integrated circuit. 
     BACKGROUND OF THE INVENTION 
     A transient variation (referred to as an AC noise on power supply line) in power supply voltage, attributable to a current flowing at the time of a circuit in operation, (a current flowing at the time of transition in circuit condition, called a switching current in a CMOS circuit) occurs to a semiconductor integrated circuit. If the AC noise on power supply line is large in magnitude, this will cause malfunction of a circuit in the worst case, and there is therefore the need for suppressing the AC noise on power supply line to fall within a normally operable range. 
     As a margin for the AC noise on power supply line has since decreased due to reduction in voltage of a semiconductor integrated circuit, as seen lately, in particular, so has increased importance attached to suppression of variation in power supply voltage. Further, there is the need for lowering the AC noise on power supply line occurring to the semiconductor integrated circuit in consideration of an increase in importance attached to EMC (Electro-Magnetic-Compatibility). 
     For example, in JP-A-2001-339288, an RC filter is added to a power supply line, attempting to decrease the noise. With the RC filter, however, there occurs drop in power supply, due to current flowing to the power supply line, and a resistor as a component of the filter. 
     If a capacitor called a decoupling capacitor is placed between power supplies (VDD and VSS) in order to suppress the AC noise on power supply line, this will provide effective means. JP-A-2004-327820 has disclosed an example of using the decoupling capacitor. In JP-A-2004-327820, there has been proposed a design idea whereby the decoupling capacitor is connected to power supply lines during a circuit operation period, and is electrically separated from the power supply lines during a non-operation period, thereby reducing a leak current in order to cope with an increase in gate leak current accompanying process miniaturization. 
       FIG. 1  shows an example of a decoupling capacitor. The decoupling capacitor has a configuration wherein an NMOS transistor  1  has a gate connected to VSS, having a source and a drain, connected to VDD, respectively. Further, the decoupling capacitor has a configuration wherein a PMOS transistor  2  has a gate connected to VDD, having a source and a drain, connected to VSS, respectively. Further, “a MOS transistor” referred to in description given hereinafter may be a common field effect transistor including a MIS (Metal-Insulator-Semiconductor) transistor. 
     Further, progress has since been made in reduction in film thickness of a gate oxide film, accompanying miniaturization in process. In IEEE Proceedings of 7th International Symposium on Quality Electronic Design 2006, a cross-coupled decoupling capacitor as shown in  FIG. 2  has been proposed for the purpose of enhancement in resistance to electrostatic breakdown occurring as a result of reduction in the film thickness. With the cross-coupled decoupling capacitor, if a gate is directly connected to VDD, VSS, respectively, there occurs deterioration in electrostatic breakdown resistance, so that with respect to MOS transistors  3 ,  4 , respectively, source-to-drain resistance is inserted between the gate and VDD, VSS, respectively, thereby implementing enhancement in the electrostatic breakdown resistance. 
     The AC noise on power supply line is influenced by not only the decoupling capacitor but also electrical characteristics of a package, a PCB (Printed-Circuit-Board), and so forth. For example, in IEEE Signal Propagation on Interconnects, 2004, Proceedings, pp. 45-48, it has been proposed to suppress a voltage bounce of a power supply line, due to resonance occurring to a power source, by insertion of a resistor of a suitable resistance (the resistance is called a damping resistance). 
     SUMMARY OF THE INVENTION 
     In  FIG. 3 , there is shown an equivalent circuit of the cross-coupled decoupling capacitor shown in  FIG. 2 . Resistors  5 ,  8  each represent the source-to-drain resistance of the MOS transistor, and capacitors  6 ,  7  each represent a gate capacitance. As is evident from the equivalent circuit, with the cross-coupled decoupling capacitor, the source-to-drain resistance is inserted so as to be equivalent to series-connection to the capacitor. Voltage drop (IR drop) occurring upon current flowing through the resistance between the source and the drain is the cause of occurrence of the AC noise on power supply line. 
     It is therefore a first object of the present invention to provide means for suppressing occurrence of the AC noise on power supply line when the cross-coupled decoupling capacitor is in use. 
     There is the case where addition of the damping resistance having an adequate resistance value is required in order to suppress resonance, and so forth, depending on circuit operation, a design of the package·PCB, when the decoupling capacitor is placed. Accordingly, it is a second object of the present invention to provide means for suppressing resonance caused by the decoupling capacitor. 
     The means for solving the first object are as follows. That is, a decoupling capacitor is made up of MOS transistors having a low threshold voltage Vth. By so doing, source-to-drain resistance of the MOS transistor, equivalently inserted, is reduced, so that the AC noise on power supply line can be reduced. 
     The means for solving the second object includes the following three means. A first means is to use a decoupling capacitor made up of MOS transistors having plural kinds of threshold voltages Vth, thereby using the resistance between the source and the drain as the damping resistance. In a second means, the threshold voltage Vth can be varied by controlling a substrate bias voltage. In so doing, the source-to-drain resistance of the MOS transistor, equivalently inserted, can be controlled, which will act as the damping resistance, so that resonance can be suppressed. In a third means, the source-to-drain resistance can be equivalently controlled by controlling a gate electrode voltage of the MOS transistor. As with the second means, the resonance can be suppressed. 
     The present invention is advantageous in that IR drop is firstly reduced. Furthermore, additional damping resistance is not required, and the noise accompanying the resonance phenomenon can be suppressed. In other words, the AC noise on power supply line can be reduced, thereby preventing a circuit from malfunctioning. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing an example of a decoupling capacitor; 
         FIG. 2  is a schematic view showing an example of a cross-coupled decoupling capacitor with enhanced resistance to electrostatic breakdown; 
         FIG. 3  is a schematic view showing an equivalent circuit using resistance and capacitance of the cross-coupled decoupling capacitor shown in  FIG. 2 ; 
         FIG. 4  is a general view showing a system LSI; 
         FIG. 5  is a circuit diagram of two kinds of decoupling capacitors having threshold voltages Vth differing from each other; 
         FIG. 6  is a schematic view showing an equivalent circuit using resistance and capacitance of each of the two kinds of the decoupling capacitors having the threshold voltages Vth differing from each other; 
         FIG. 7  is a view showing an equivalent circuit expressing the decoupling capacitor by use of resistance, and capacitance; 
         FIG. 8  is a view showing an example of a flow chart for determining capacitance of a plurality of kinds of decoupling capacitors having threshold voltages Vth differing from each other, and an application ratio of each kind of the threshold voltage Vth; 
         FIG. 9  is a view showing an example of a flow chart for placing the plurality of the kinds of the decoupling capacitors having the threshold voltages Vth differing from each other; 
         FIG. 10  is a view showing an example of circuit diagram for dynamically controlling the threshold voltage Vth; 
         FIG. 11  is a view showing an example where a signal shown in  FIG. 10  is controlled; 
         FIG. 12  is a view showing a control flow chart of the signal shown in  FIG. 10 ; 
         FIG. 13  is a circuit diagram of a variation of the circuit described with reference to  FIG. 10 ; 
         FIG. 14  is a view showing an example where a signal shown in  FIG. 13  is controlled; and 
         FIG. 15  is a view showing a control flow chart of the signal shown in  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the invention are described hereinafter with reference to a working example. 
       FIG. 4  is a view showing a general view of a system LSI. The system LSI comprises a decoupling capacitor  9 , and a logic circuitry  10 . The decoupling capacitor  9  and the logic circuitry  10  are connected between power supply lines VDD and VSS. Operating voltages from the supply lines VDD, and VSS, respectively, are fed to the logic circuitry  10 . 
     The logic circuitry  10  provides the internal logic of the system LSI, and is comprised of MOS transistors having plural kinds of threshold voltages Vth. A circuit  11  is comprised of a MOS transistor having a high threshold voltage, a circuit  12  is comprised of a MOS transistor having a standard threshold voltage, and a circuit  13  is comprised of a MOS transistor having a low threshold voltage. In this connection, the low threshold voltage, the standard threshold voltage, and the high threshold voltage each indicate a relative voltage, and doe not indicate a specific voltage. In  FIG. 4 , the logic circuitry  10  is comprised of MOS transistors having three kinds of threshold voltages Vth, however, the number of the kinds of threshold voltages Vth may be any suitable number not less than two. Those threshold voltages Vth are controlled by parameters of a process step such as an ion implantation process step, and so forth. 
     The decoupling capacitor  9  is the cross-coupled decoupling capacitor shown in  FIG. 2 , intended for reduction of the AC noise on power supply line occurring inside the system LSI. For the cross-coupled decoupling capacitor, use is made of one of the MOS transistors used in the logic circuitry, having the lowest threshold voltage. 
     The decoupling capacitor is generally provided in a cell library containing a decoupling capacitor cell. In this case, it is assumed that a cell library having a decoupling capacitor cell composed of MOS transistors having a low threshold voltage Vth is prepared. 
     By so doing, a source-to-drain resistance in the MOS transistors composing the cross-coupled decoupling capacitor is reduced. Then, it is possible to reduce the AC noise on power supply line due to a voltage drop (IR drop) occurring when current flows through the resistance between the source and the drain. 
     For reference purposes, there is described hereinafter relationship between a MOS transistor threshold voltage Vth, and a source-to-drain resistance Rch. The source-to-drain resistance (referred to as Rch), posing a problem in a decoupling capacitor with enhanced resistance to electrostatic breakdown, is dependent on the respective threshold voltages Vth of the MOS transistors composing the decoupling capacitor. The relationship between the MOS transistor threshold voltage Vth and the resistance Rch can be approximately expressed by expression (1) as follows:
 
1 /Rch =[(μ· Cox·Wg )/ Lg ]*( Vgs−Vth )  expression (1)
 
where μ indicates mobility, Cox a capacitance unit of a gate oxide film, Wg, L g  indicate a gate with, and a gate length, respectively, and Vgs indicates a gate-source voltage. It is evident from the expression (1) that Rch can be lowered by use of a MOS transistor having a low threshold voltage Vth. That is, with the decoupling capacitor with enhanced resistance to electrostatic breakdown, it is possible to decrease IR drop by use of the MOS transistor having the low threshold voltage Vth.
 
     Further, in  FIG. 4 , the cross-coupled decoupling capacitor is used in description; however, use can be made of the decoupling capacitor of a type shown in  FIG. 1 , as well. As shown in IEEE Journal of Solid-State Circuits, Vol. 32, No. 4, April, 1997, pp. 574-576, with the decoupling capacitor as well, a source-to-drain resistance Rch can act as a parasitic resistance. The source-to-drain resistance Rch is used as a resistance for suppressing resonance. 
       FIG. 5  shows the case where use is made of two kinds (a low threshold voltage, and a high threshold voltage) of decoupling capacitors having threshold voltages Vth differing from each other in place of the decoupling capacitor shown in  FIG. 2 , comprised of the MOS transistors having the lowest threshold voltage. MOS transistors  14 ,  15  each have the low threshold voltage, and MOS transistors  16 ,  17  each have the high threshold voltage. 
     In order to attain a resistance value as desired, use is made of the decoupling capacitors having threshold voltages Vth differing from each other, and control is implemented through a composite resistance of the decoupling capacitors. In this connection, it is to be pointed out that the low threshold voltage, and the high threshold voltage each indicate a relative voltage, and doe not indicate a specific voltage. 
     If the decoupling capacitor is made up of MOS transistors having a single threshold voltage, a capacitance value of the decoupling capacitor, and a damping resistance value thereof are dependent only on the number of decoupling capacitor cells, so that the capacitance value, and the damping resistance value cannot be independently determined. Accordingly, with the use of the decoupling capacitors having the threshold voltages differing from each other, it is possible to attain an optional capacitance value of the decoupling capacitor, and an optional damping resistance value. The attainment of the optional capacitance value, and resistance value is shown as follows: 
       FIG. 6  shows an equivalent circuit of the decoupling capacitors shown in  FIG. 5 . One decoupling capacitor is expressed by one parasitic resistance, and one parasitic capacitance, and respective parasitic resistance values, and parasitic capacitance values are designated R 1 , C 1  and R 2 , C 2 . In this case, if composite impedance is expressed by use of “s” of Laplace transformation, the composite impedance can be approximately expressed by expression 2 as follows: 
     
       
         
           
             
               
                 
                   
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     As described above, it is evident that the optional resistance value can be attained by use of the decoupling capacitors having the threshold voltages Vth differing from each other. Thus, the optional resistance value can be attained, and it is therefore possible to suppress resonance, and so forth, due to designing of the package PCB, without the addition of the damping resistance having an adequate resistance value. The damping resistance refers to a resistance of the resistor inserted for suppressing the resonance phenomenon of the power supply voltage, due to resonance occurring to the power source. 
     In  FIGS. 5 , and  6 , use is made of two kinds of the decoupling capacitors, however, there is no need for limiting the decoupling capacitors to two kinds, and use can be made of not less than three kinds of the decoupling capacitors having threshold voltages Vth differing from each other. 
     Further, with the decoupling capacitor shown in  FIG. 1  as well, a source-to-drain resistance Rch can act as the parasitic resistance, as shown in IEEE Journal of Solid-State Circuits, Vol. 32, No. 4, April, 1997, pp. 574-576. It is possible to use the source-to-drain resistance Rch as a resistance for suppressing resonance. In order to attain a resistance value as desired, use can be made of the decoupling capacitors having threshold voltages Vth differing from each other, and control is implemented with the composite resistance of the decoupling capacitors. 
       FIG. 8  shows a designing process for composing an integrated circuit by use of a plurality of decoupling capacitors having threshold voltages Vth differing from each other. In  FIG. 8 , indicated at  18  to  21 , respectively, is information necessary for designing while indicated at  22  through  24  is a flow chart for designing with the use of the information. There are prepared respective equivalent circuits of a PCB, a package, interconnects on a substrate, MOS transistors, and a decoupling capacitor on the basis of each kind of the threshold voltage Vth. 
     In step  22 , there are provisionally determined total capacitance of decoupling capacitors to be mounted, and an application ratio on the basis of each kind of the threshold voltage Vth. Thereafter, in step  23 , there is run a circuit simulation whereby operation of an integrated circuit apparatus is simulated. Then, in step  24 , checking is made on whether or not the AC noise on power supply line (in terms of magnitude, and waveform) meets a standard. If the AC noise on power supply line does not meet the standard, the process reverts to the step  22 , running the circuit simulation again after changing the total capacitance of the decoupling capacitors, and the application ratio on the basis of each kind of the threshold voltage Vth. 
     The process described as above is repeated until the AC noise on power supply line meets the standard. Herein, the standard refers to the magnitude of the AC noise on power supply line, and the waveform thereof, necessary for ensuring that the integrated circuit apparatus is prevented from malfunctioning. 
     In this case, determination on the total capacitance of the decoupling capacitors, and the application ratio on the basis of each kind of the threshold voltage Vth is made by making use of the circuit simulation, however, there is no need for limiting such determination to the circuit simulation, and use may be made of, for example, an analytic formula, and numerical calculation utilizing a computer. 
       FIG. 9  shows a procedure for placing the decoupling capacitors. The decoupling capacitors are generally provided in the cell library containing decoupling capacitor cell. In this case, it is assumed that use is made of a cell library having three kinds of decoupling capacitor cells having a high threshold voltage Vth, standard threshold voltage Vth, and low threshold voltage Vth, respectively. Obviously, there is no need for limiting the decoupling capacitor cells to three kinds, and the number of kinds of decoupling capacitor cells may be any suitable number not less than one kind. 
     First, in step  25 , the total capacitance of decoupling capacitors to be mounted, and the application ratio on the basis of each kind of the threshold voltage Vth are determined by a method shown with reference to working example  1 . Next, a decoupling capacitor having a low threshold voltage Vth is disposed in step  26 , a decoupling capacitor having a standard threshold voltage Vth is placed in step  27 , and a decoupling capacitor having a high threshold voltage Vth is placed in step  28  in that order. Now, those decoupling capacitors are placed in that order for the following reason, however, sequence in which those decoupling capacitors are placed is not necessarily limited to that order, and the decoupling capacitors may be simultaneously placed. Because a parasitic resistance value of the decoupling capacitor having the low threshold voltage Vth is low, IR drop thereof is small. The reason why those decoupling capacitors are placed in the sequence described is that it is desirable to convergently dispose the decoupling capacitor having the low threshold voltage Vth in the vicinity of, for example, a high-speed operating circuit, and so forth, sensitive to the AC noise on power supply line, while placing the decoupling capacitor having the standard threshold voltage Vth, and the decoupling capacitor having the high threshold voltage Vth at other locations, respectively. 
     In  FIG. 10 , there is shown an example of a circuit wherein the threshold voltage Vth is controlled by changing a substrate bias voltage of a MOS transistor. A control circuit  31  is a circuit for receiving a reset signal, thereby controlling the substrate bias voltage. MOS transistors  29 ,  30  make up a decoupling capacitor for suppressing the AC noise on power supply line. The respective substrate biases of the MOS transistors  29 ,  30  are controlled by the control circuit  31 , and the respective threshold voltages Vth of the MOS transistors  29 ,  30  are changed according to an operating state of the circuit to thereby control the respective threshold voltages Vth such that the AC noise on power supply line is reduced to the minimum. More specifically, upon detection of the reset signal being turned from OFF to ON, the substrate bias voltage is adjusted during a time period over which the AC noise on power supply line due to resonance occurs, thereby causing the threshold voltages Vth to turn High. That will cause a parasitic resistance of the decoupling capacitor to increase, which will act as the damping resistance, thereby enabling the AC noise on power supply line to be suppressed. 
     The AC noise on power supply line due to the resonance will increase in the case where a large change in power consumption occurs. In this connection, the case of occurrence of a large change in power consumption includes timing immediately after the power source is turned on. If the threshold voltages Vth are turned High upon the occurrence of the resonance, this will cause the resistance Rch between the source and the drain to turn high, which will act as the parasitic resistance. There is no necessity of fixing the threshold voltages Vth, and the threshold voltages Vth may be changed according to the operating state of the circuit, thereby enabling the respective threshold voltages Vth to be controlled such that the AC noise on power supply line is reduced. 
     Furthermore, the occurrence of a large change in power consumption is not limited to the timing immediately after the power source is turned on, and it is obviously effective to turn the threshold voltages Vth High in other cases as well  FIG. 11  shows a change in potential, at each of nodes shown in  FIG. 10 . In the initial state, a substrate bias potential of a PMOS transistor is set to the VDD potential, and a substrate bias potential of an NMOS transistor is set to the VSS potential. 
     At a point in time when the reset signal is turned effective (from Low to High), the substrate bias potential (VBB_P) of the PMOS transistor is changed to a potential higher than VDD while the substrate bias potential (VBB_N) of the NMOS transistor is changed to a potential lower than VSS. In the figure, there is shown a state where after the elapse of predetermined time, the VBB_P potential reverts to the VDD potential, and the VBB_N potential reverts to the VSS potential. 
       FIG. 12  shows a control flow chart of respective nodes shown in  FIG. 11 . In step  32 , the reset signal is detected, and in step  33 , the substrate bias potential of the PMOS transistor is set to the potential higher than the VDD potential while the substrate bias potential of the NMOS transistor is set to the potential lower than the VSS potential such that the threshold voltage Vth of the decoupling capacitor is turned High. In step  34 , checking is made on whether or not predetermined time for causing the AC noise on power supply line to be at the standard level or lower has elapsed. Thereafter, in step  35 , the respective substrate bias potentials are caused to revert to an original state. 
     In  FIGS. 10 to 12 , respectively, the reset signal is used, however, use may be made of a signal for detecting a large change in power consumption. For example, in the case of a design using a gated clock, a power change occurs by gating (enabling or disabling) of the clock. Use may be made of a signal (an enable signal, and so forth) for controlling the enabling the disabling, in place of the reset signal. 
       FIG. 13  is a circuit diagram of a circuit intended for fulfilling the same function as that for the circuit described with reference to  FIG. 9 . A control circuit  40  is a circuit for receiving a reset signal, thereby controlling respective gate voltages of MOS transistors  36 ,  39 . MOS transistors  37 ,  38  make up a decoupling capacitor for suppressing the AC noise on power supply line. Respective gate potentials of the MOS transistors  36 ,  39  are under control of the control circuit  40 , and are changed according to an operating state of a circuit, thereby causing a source-to-drain resistance Rch of each of the MOS transistors  36 ,  39  to be changed. 
       FIG. 14  shows a change in potential, at each of nodes shown in  FIG. 13 . In the initial state, the MOS transistors  36 ,  39  are in the ON-state, respectively, and are in such a state as is fixed to the VSS potential, and the VDD potential, respectively. The control circuit  40  detects that the reset signal is turned from OFF to ON to thereby turn a potential (CTLP) of the MOS transistor  36  higher than the VSS potential, and a potential (CTLN) of the MOS transistor  39  lower than the VDD potential. After the elapse of predetermined time, the CTLP potential is caused to revert to the VSS potential, and the CTLN potential is caused to revert to the VDD potential. 
       FIG. 15  shows a control flow chart of respective nodes shown in  FIG. 13 . In step  41 , the reset signal is detected. In step  42 , the CTLP potential, and the CTLN potential each are set to an intermediate potential between the VDD potential and the VSS potential such that the resistance Rch between the source and the drain of each of the MOS transistors  36 ,  39  is turned High. In step  43 , checking is made on whether or not predetermined time for causing the AC noise on power supply line to be at the standard level or lower has elapsed. In step  44 , the CTLP potential, and the CTLN potential each are caused to revert to an original potential. 
     By so doing, the AC noise on power supply line due to resonance can be suppressed as with the case shown in  FIG. 10 . More specifically, if resonance occurs, the source-to-drain resistance of the MOS transistor is temporarily increased, which will act as the damping resistance, so that the AC noise on power supply line can be suppressed. 
     In  FIGS. 12 to 15 , respectively, the reset signal is used; however, use may be made of a signal for detecting a large change in power consumption. For example, in the case of the design using the gated clock, power change occurs by gating (enabling or disabling) of the clock. Use may be made of a signal (the enable signal, and so forth) for controlling the enabling the disabling, in place of the reset signal. 
     Having specifically described the invention developed by the inventor, et al. with reference to the embodiment of the invention, it is to be understood that the invention be not limited thereto and it is obvious that various changes and modifications may be made in the invention without departing from the spirit and scope thereof.