Abstract:
A method, apparatus, and computer program are provided for correcting the voltage across a thin oxide Complementary Metal-Oxide Semiconductor (CMOS) capacitor. Due to ever-decreasing thicknesses of capacitors in CMOS applications, leakage through the capacitor by electron tunneling and impurities has become a significant problem. For example, in Phased Lock Loops (PLLs), leaky capacitors can cause static phase errors. To combat the problem, a scaled capacitor and current mirrors are used to provide a correction current to a leaky capacitor to maintain a proper voltages.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates generally to the field of Complementary Metal-Oxide Semiconductor (CMOS) technology and, more particularly, to ameliorating device current leakage.  
       BACKGROUND  
       [0002]     The progress of electronic circuits was accomplished partially, as a result of the downsizing of components, such as vacuum tubes, for more than a century. The downsizing of active or passive components decreases their capacitance, resulting in an increase of the circuit operating speed and decrease of its power consumption. The size reduction increases the component density in the circuit, and enhances parallel operation capability, resulting in another increase in the circuit speed.  
         [0003]     Historically, Field Effect Transistor (FET) technology scaling trends seek to improve gate delay by about 30% and the reduction of transition-energy by approximately 30% to 65% per generation. Typically, this is accomplished by scaling supply voltages and/or shrinking the process technology. In developing semiconductors, maximum supply voltages are limited by gate oxide wear-out, and minimum supply voltage levels are typically set by practical noise-margin and performance considerations. The components, though, should maintain proper device behavior at smaller and smaller channel lengths and progressively thinner gate dielectrics, which is in turn dependent on maintaining an adequately large lateral-to-vertical aspect ratio for a device. Thus, the ability to scale semiconductor gate dielectrics can be limited by both the scalability of the supply voltage and the desire to preserve the device&#39;s aspect ratio.  
         [0004]     Static Random Access Memory (SRAM) circuits, for example in sub-0.3 μm CMOS technologies, exhibit profound read sensitivities to increased leakage current. Due to the limited scalability in supply voltages in high-performance applications, high electric fields may develop across the thin (˜1.5 nm) Silicon Dioxide (SiO 2 ) gate oxide. This field distorts the silicon band gap, such that electrons may more easily travel from the valence to the conduction band, from the gate to the channel and body. This problem is known as tunneling. This tunneling current along with sub-threshold leakage mechanisms, combine to affect the buildup of a voltage differential between the SRAM&#39;s bit lines such that the current-sinking behavior of the selected SRAM cell&#39;s wordline means Negative-Channel FETs (NFETs) must contend with significant leakage current from the non-selected devices.  
         [0005]     It can be difficult to predict the limit of the down-sizing, although the ultimate limit of the downsizing is the distance of atoms in silicon crystals and that is about 0.3 nm. Some signal moderation effect such as through a single atomic size gate electrode might be possible, but the moderated signal would be too weak to transfer to another node. In addition, there is no practical solution at this moment for interconnects to contact to such small atomic nodes. Thus, the limit of the downsizing is considered from the viewpoint of the integration of individual components into circuits.  
         [0006]     Therefore, there is a need for a device that addresses at least some of the issues related to integration limits, performance limits, power increases, reliability factors and design/production costs until integrated devices development expands beyond CMOS type devices.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention provides a method for current leakage correction for a leaky capacitor. A voltage across the leaky capacitor is measured. The measured voltage to a scaled capacitor is provided, wherein the scaled capacitor has an area reduced by a scaling factor in comparison to the leaky capacitor. Also, a sustaining charge to the leaky capacitor is provided. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following Detailed Description taken in conjunction with the accompanying drawings, in which:  
         [0009]      FIG. 1A  is a block diagram depicting a circuit containing a thin oxide leaky capacitor;  
         [0010]      FIG. 1B  is a graph depicting an ideal voltage across a capacitor relative to a current pulse;  
         [0011]      FIG. 2  is a timing diagram of Phased Lock Loop (PLL) filter capacitor voltage affected by a leakage current resulting in a static phase error; and  
         [0012]      FIG. 3  is a block diagram depicting a leakage correction circuit coupled to a thin oxide leaky capacitor. 
     
    
     DETAILED DESCRIPTION  
       [0013]     In the following discussion, numerous specific details are set forth to provide a thorough understanding of the present invention. However, those skilled in the art will appreciate that the present invention may be practiced without such specific details. In other instances, well-known elements have been illustrated in schematic or block diagram form in order not to obscure the present invention in unnecessary detail. Additionally, for the most part, details concerning network communications, electromagnetic signaling techniques, and the like, have been omitted inasmuch as such details are not considered necessary to obtain a complete understanding of the present invention, and are considered to be within the understanding of persons of ordinary skill in the relevant art.  
         [0014]     It is further noted that, unless indicated otherwise, all functions described herein may be performed in either hardware or software, or some combination thereof. In one embodiment, however, the functions can be performed by a processor, such as a computer or an electronic data processor, in accordance with code, such as computer program code, software, and/or integrated circuits that are coded to perform such functions, unless indicated otherwise. In the present specification, the same reference characters are used to refer to terminals, signal lines, and their corresponding signals.  
         [0015]     Referring to  FIGS. 1A and 1B  of the drawings, the reference numeral  100  generally designates a block diagram depicting a circuit containing a thin oxide leaky capacitor. Also, the reference numeral  150  generally designates is a graph depicting an ideal voltage across a capacitor relative to a current pulse. The circuit  100  comprises a current source  102 , a capacitor  104 , a first leakage current source  106 , and ground  108 .  
         [0016]     The circuit  100  is utilized in a variety of applications, such as PLLs. However, as a result of the thin film dielectric in the capacitor  104 , leakage current across the capacitor  104  can substantially affect the behavior of the capacitor. The intention in the circuit  100  is to generate a voltage V c  of  FIG. 1B  across the capacitor  104  of  FIG. 1A  that is proportional to the width of a current pulse Δt of  FIG. 1B .  
         [0017]     If a high impedance path (not shown) is provided at the first node  110  between the current source  102  and the capacitor  104 , then for hold states when the impedance is high, the voltage V c  of  FIG. 1B  across the capacitor  104  of  FIG. 1A  decreases due to leakage. In other words, when the current source  104  is effectively “shut off,” the V c  of  FIG. 1B  across the capacitor  104  of  FIG. 1A  decreases at a rate higher than the normal rate of capacitive discharge. Thus, the voltage V c  of  FIG. 1B  across the capacitor  104  of  FIG. 1A  is as follows:  
             Vc   =         1   C     ⁢     ∫     Iup   ·     ⅆ   t           -       1   C     ⁢     ∫     Ileak   ·     ⅆ   t           +     Vc   ⁡     (   0   )                 (   1   )             
 
 V c  is the output voltage across the capacitor. C is the electrical capacitance in Farads. I leak  is the leakage current in Amperes. I up  is the height of the rise of current during the leading edge of a clock signal, and dt is the change in time (also known as D (delta time). 
 
         [0018]     The circuit  100  operates by driving a current across a capacitor  104 . A current source or charge pump  102  provides a current to a first node  110 . Also, the current source  102  can be either a negative charge source or a positive charge source. A capacitor  104  is coupled to the first node  110  and to ground  108  at a second node  112 . The charge leakage is represented by the first leakage current source  106 . The first leakage current source  106  is coupled at first end to the first node  110  and at a second end to the second node  112 .  
         [0019]     Referring to  FIG. 2  of the drawings, the reference numeral  200  generally designates a timing diagram of PLL filter capacitor voltage affected by a leakage current resulting in a static phase error. In general, thin oxide capacitors do not have ideal electrical characteristics due to tunneling leakage.  
         [0020]     Although the tunneling leakage is exponentially related to the voltage V c2  across a capacitor (not shown),  FIG. 2  is shown as a lineal representation. Also, I UP  and I PUMP  are the height of the rise of current during the leading edge of a clock signal and a supply current, respectively. Since the capacitor voltage decreases due to leakage during the hold period of T minus T SPE , a phase error T SPE  develops which the phase lock loop tries to correct at the next reference clock cycle, according to the following formula:  
                 ∫   0   Tspe     ⁢     Iup   ·     ⅆ   t         =       ∫   Tspe   T     ⁢     Ileak   ·     ⅆ   t                 (   2   )             
 
 This static phase error (SPE) results in a continuous phase error between the reference clock and the PLL feedback clock, causing tracking and cycle-cycle jitter, a potentially unstable loop and system failure. 
 
         [0021]     Currently, several different methods are available to attempt to moderate or reduce the impact of SPE on a circuit. One method involves reducing leakage current across a capacitor (not shown) by reducing the capacitor area. However, this method can degrade loop performance since other loop parameters must be increased to compensate for a reduction in capacitance. Thicker oxides can be used, though these can introduce additional costs of manufacturing. Increasing a supply current I PUMP  may also result in difficulties maintaining optimal PLL characteristics. Lastly, adjustments to the reference clock frequency, though these suffer from an effective minimum in present systems of T≧˜2 nanoseconds.  
         [0022]     Referring to  FIG. 3  of the drawings, the reference numeral  300  generally designates a block diagram depicting a leakage correction circuit coupled to a thin oxide leaky capacitor. The circuit  300  comprises a charge pump (CP) circuit  350  and a correction circuit  352 . The CP  350  further comprises a current source  302 , a first capacitor  304 , ground  308 , and a first leakage current source  306 . The correction circuit  352  further comprises ground  308 , a second capacitor  316 , a second leakage current source  344 , a first Positive-Channel Field Effect Transistor (PFET)  312 , a second PFET  314 , a first Negative-Channel Field Effect Transistor (NFET)  318 , a second NFET  320 , a third NFET  338 , and a fourth NFET  322 .  
         [0023]     The CP  350  operates by driving a current across a capacitor  304 . A current source or charge pump  302  provides a current to a first node  310 . Also, the current source  302  can be either a negative charge source or a positive charge source. A capacitor  304  is coupled to the first node  310  and to ground  308  at a second node  334 . The charge leakage is represented by the first leakage current source  306 . The first leakage current source  306  is coupled at first end to the first node  310  and at a second end to the second node  334 .  
         [0024]     In comparison, the correction circuit  352  is more complicated than the CP  350 . The correction circuit  352  is coupled to the CP  350  at the first node  310 . The drain of the first PFET  312 , a first end of the second leakage current source  344 , and a first end of the second capacitor are coupled to the first node  310 . A second end of the second capacitor  316  and a second end of the second current leakage source  344  are coupled at a third node  332  to the drain of the first NFET  318 . Also, the gate of the first NFET  318  is coupled to the body of the first NFET  318  is coupled to the fourth node  330 . The body of the second NFET  320  is also coupled to the fourth node  330 . Also, the sources of the first NFET  318  and the second NFET  320  are coupled to ground  308 .  
         [0025]     In addition to the aforementioned connections, there are a variety of other connections that should be made for the current mirror  352  to operate. The drain of the second NFET  320 , the source of the third NFET  338 , and the source of the fourth NFET  322  are coupled to a fifth node  336 . The drain of the third NFET  338 , the source of the first PFET  312 , and the source of the second PFET  314  are coupled to a voltage source  346 . The drain of the fourth NFET  322 , the drain of the second PFET  314 , and the gate of the second PFET are coupled to a sixth node  328 . The gate of the second PFET  314  is also coupled to the gate of the first PFET  312  through a seventh node  324 .  
         [0026]     The gates of the third NFET  338  and the fourth NFET  322  are then coupled to the CP  350  (not shown). The voltages input into the gate of the fourth NFET  322  at an eighth node  342  and into the gate of the third NFET  338  at a ninth node  340  vary depending on the state of the current source  302 . If the current source is at a high impedance state, as described in  FIG. 1 , then an active high signal is input into the gate of the fourth NFET  322  at the eighth node  342 . If the current source is not at a high impedance state, as described in  FIG. 1 , then an active high signal is input into the gate of the third NFET  338  at a ninth node  340 .  
         [0027]     The circuit  300  further maintains the voltage on the first capacitor  304  of the CP  350  by using the characteristics of the correction circuit  352 . The first capacitor  304  has a first area (A) and a first capacitance (C) associated with it. The second capacitor  316  is a replica of the first capacitor  304  with a second area (A/N) and a second capacitance (C/N), where N is a scaling factor. The width (W) and length (L) of the current mirror  350  is varied such that the voltage across the second capacitor  316  is substantially equal to the voltage across the first capacitor  304 . Therefore, since the voltage across the second capacitor  316  is substantially equal to the voltage across the first capacitor  304  and since the area of the second capacitor  316  is decreased by a factor of N, then the charge leakage represented by the second leakage current source  316  is also decreased by a factor of N (I leak /N).  
         [0028]     The reduced current can then be multiplied by N+1 by using N+1 identical mirror devices in parallel. The identical mirrors comprise the second NFET  320 , the third NFET  338 , the fourth NFET  322 , and the second PFET  314 . Also, a device with a width ((N+1)W) to develop a tail current equal to a second reduced current ((N+1)I leak /N) for a the first PFET  312 .  
         [0029]     The first PFET  312  is configured such that a first reduced current ((N+1)I leak /N) is injected into first node  310  using additional current mirrors to exactly compensate the leakage of the first capacitors  304  and the second capacitor  316  during the hold state. In this manner the effective leakage current is reduced to zero. Additional area required by the circuit is negligible since N can be large and the mirror devices can be small.  
         [0030]     It will further be understood from the foregoing description that various modifications and changes may be made in the preferred embodiment of the present invention without departing from its true spirit. This description is intended for purposes of illustration only and should not be construed in a limiting sense. The scope of this invention should be limited only by the language of the following claims.