Abstract:
A semiconductor storage device floats the gate of a conventional transistor between two capacitors to store a logic state which can be utilized to store the condition of a circuit such as a latching type circuit such as a flip-flop or register prior to a power down operation to save power. The gate and first terminals of the two capacitors preferably share the same conductive line such as a polysilicon segment. A second transistor and a second set of capacitors store the complementary state of the logic state so that complementary signals are provided for detecting the stored logic state. After the time for power down has ended, the state of the semiconductor storage device made up of the two transistors and four capacitors is sensed, and the detected logic state is loaded back into the latching type circuit.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to semiconductors, and more specifically, to semiconductor devices having information storage capability. 
     BACKGROUND OF THE INVENTION 
     Storage devices are used in the vast majority of semiconductor integrated circuits. In many of these integrated circuits a need exists for the storage devices to be non-volatile meaning that stored data remains even when electrical power is removed from the integrated circuit. Typically, non-volatile storage devices require additional processing complexity to form non-volatile storage devices. For example, multiple layers of material such as polysilicon are required to implement a floating gate transistor which is a common transistor used to implement a nonvolatile storage device. Additionally, the programming and erasing of these devices is complex wherein multiple voltages at possibly different polarities are required. Another characteristic of many non-volatile storage devices is an undesirably large power consumption which is required to store the data in a non-volatile element. Further, a complex reference circuit is required to reliably sense the state of a non-volatile storage device. These characteristics of existing non-volatile storage devices create circuits which are either complex and therefore more costly to manufacture, which have significant power consumption, and/or which require significant circuit area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example and not limited to the accompanying figures, in which like references indicate similar elements. 
         FIG. 1  illustrates in schematic form a non-volatile latch in accordance with the present invention; and 
         FIG. 2  illustrates in topographical form a layout of the storage cell of  FIG. 1 . 
     
    
    
     Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. 
     DETAILED DESCRIPTION 
     Illustrated in  FIG. 1  is a schematic of a nonvolatile latch  10 . The nonvolatile latch  10  has a storage cell  11  and is coupled to a sense amplifier  16  and a latch  30 . The latch  30  is a conventional flip-flop and has a data input for receiving input data, a clock input for receiving a clock signal, and complementary data outputs for providing complementary signals Q and QB (Q Bar). The latch  30  has a sense enable input for receiving a sense enable signal, SE and a control input for receiving a signal labeled “Stored Q”. 
     The storage cell  11  has a transistor  12 , a transistor  14  and capacitors  18 ,  20 ,  22  and  24 . Each of transistor  12  and transistor  14  is an N-channel MOS (metal oxide semiconductor) transistor. Capacitor  18  has a first electrode connected to a node  32 . A level shifting circuit  26  has an input for receiving the signal Q from latch  30 . An output of level shifting circuit  26  is connected to node  32  and provides a program voltage labeled V PROG  which has a voltage magnitude that is shifted or increased from the input voltage of Q. A second electrode of capacitor  18  is connected to a gate of transistor  12  that is actually at a floating potential as will be discussed below. A first electrode of capacitor  20  is connected to a node  34 . A level shifting circuit  28  has an input for receiving the complementary signal QB from latch  30 . An output of level shifting circuit  28  is connected to node  34  and provides a complementary program voltage labeled V PROGB  which has a voltage magnitude that is shifted or increased from the input voltage of QB. A second electrode of capacitor  20  is connected to the gate of transistor  12 . A first electrode of a capacitor  22  is connected to node  32 . A second electrode of capacitor  22  is connected to a gate of transistor  14 . A first electrode of a capacitor  24  is connected to node  34 . A second electrode of capacitor  24  is connected to a gate of transistor  14 . Transistor  12  and transistor  14  each has a source connected to a power supply terminal for receiving a voltage labeled V SS . Transistor  12  has a drain connected to a first sensing input of sense amplifier  16 , and transistor  14  has a drain connected to a second sensing input of sense amplifier  16 . As will be further illustrated in connection with  FIG. 2 , the second electrode of capacitor  18  is formed by a same first portion of conductive material, such as polysilicon, as the gate or control electrode of transistor  12 . Similarly, the second electrode of capacitor  20  is formed by a same second portion of conductive material as the gate of transistor  12 . Additionally, the second electrode of capacitor  22  is formed by a same first portion of conductive material as the gate of transistor  14 . Further, the second electrode of capacitor  24  is formed by a same second portion of conductive material as the gate of transistor  14 . 
     In a program operation, input data is latched into the latch  30  under control of the Clock signal. When the Clock signal transitions to a high logic state, the Q output of latch  30  becomes the same value as what is presently at the input for the Input Data. The complementary signals Q and QB are level shifted and coupled to nodes  32  and  34 , respectively. As will be described below in connection with  FIG. 2 , the nodes  32  and  34  are an N-conductivity well region of the semiconductor device. The capacitance of capacitor  18  is dimensioned relative to the capacitance of capacitor  20  so that capacitor  18  has significantly more capacitance than capacitor  20 . Similarly, the capacitance of capacitor  22  is dimensioned relative to the capacitance of capacitor  24  so that capacitor  22  has significantly less capacitance than capacitor  24 . In one form, the ratios that are used vary within a range of two-to-one to ten-to-one wherein any ratio within that range may be used, although higher ratios may be implemented. The dimensioning may be implemented by either specifying the physical area of the capacitors or by sizing the thickness of the capacitive dielectric of each capacitor, or by specifying both of these parameters. 
     As a result of the capacitive size ratios, when a positive program voltage V PROG  is applied to node  32  and a smaller complementary program voltage is applied to node  34  the V PROG  voltage will be coupled in almost its entirety to the gate of transistor  12 . As a result, a high electric field will develop across capacitor  20  and lead to electron tunneling from node  34  onto the gate of transistor  12 . Reciprocally, the V PROGB  voltage will be coupled in almost its entirety to the gate of transistor  14 . As a result, a high electric field will develop across capacitor  22  and lead to electron tunneling from the gate of transistor  14  to node  32 . The electron tunnel mechanism is conventional Fowler-Nordheim or direct tunneling or a combination of both and will therefore not be described in any greater detail. As a result of the charge which has been established in storage cell  11 , the gate of transistor  14  will predominantly be positively charged whereas the gate of transistor  12  will predominantly be negatively charged. 
     Conversely, when a positive program voltage V PROG  is applied to node  32  and a larger complementary program voltage is applied to node  34  the V PROG  voltage will be coupled in almost its entirety to the gate of transistor  12 . As a result, a high electric field will develop across capacitor  20  and lead to electron tunneling to node  34  from the gate of transistor  12 . Reciprocally, the V PROGB  voltage will be coupled in almost its entirety to the gate of transistor  14 . As a result, a high electric field will develop across capacitor  22  and lead to electron tunneling from node  32  to the gate of transistor  14 . The electron tunnel mechanism is conventional Fowler-Nordheim or direct tunneling or a combination of both and will therefore not be described in any greater detail. As a result of the charge which has been established in storage cell  11 , the gate of transistor  12  will predominantly be positively charged whereas the gate of transistor  14  will predominantly be negatively charged. It should be understood that transistors  12  and  14  operate in a complementary manner to obtain a first charge state for a first complementary relationship between Q and QB. Conversely, transistors  12  and  14  operate in a complementary manner to obtain an opposite second charge state for a second complementary relationship between Q and QB. 
     In a read operation, the program voltage V PROG  and V PROGB  are forced to a substantially identical voltage which is small in magnitude relative to V PP . Due to the charge differential present on the gates of transistors  12  and  14 , the transistors will exhibit different drain currents. For example, if the gate of transistor  12  is positively charged relative to the gate of transistor  14 , transistor  12  will exhibit a higher drain current than transistor  14 . Conversely, if the gate of transistor  14  is positively charged relative to the gate of transistor  12 , transistor  14  will exhibit a higher drain current than transistor  12 . The sense amplifier  16  will detect the difference in the drain currents of transistors  12  and  14  and amplify the difference to provide an output signal in the form of the Stored Q signal. This signal represents the previous Q output of latch  30  that was stored by storage cell  11  in the last program operation performed by storage cell  11 . 
     It should be noted in connection with the schematic of  FIG. 1 , the gate of each of transistors  12  and  14  does not have a resistive path to ground and therefore is allowed to electrically float. Therefore, the gate of each of transistors  12  and  14  may be considered to be a floating gate. As will be made clear in connection with  FIG. 2 , portions of the same conductive material that is used for the gate of transistor  12  are shared to also function as one of the electrodes of each of capacitor  18  and capacitor  20 . Additionally, an N-conductivity well is used as the remaining electrode for capacitor  18  and a second N-conductivity well is used as the remaining electrode for capacitor  20 . 
     Illustrated in  FIG. 2  is one form of a layout of the storage cell  11  of  FIG. 1 . Elements in common with  FIG. 1  and  FIG. 2  are given the same reference number for purposes of being able to correlate the two figures. Node  32  in  FIG. 1  is implemented in  FIG. 2  as an active well region. It should be noted that the node  32  is an active region of silicon so that electrical contact may be made and is also an N-conductivity well at the same time. The N-well or node  32  encloses the active region. Electrical contact to node  32  may be made by a contact  52  positioned on an exposed surface of node  32 . Similarly, the node  34  is an active region of silicon so that electrical contact may be made and is also an N-conductivity well at the same time. The N-well or node  34  encloses the active region. Electrical contact to node  34  may be made by a contact  54  positioned on an exposed surface of node  34 . 
     Also illustrated in  FIG. 2  is an active region  36  which is formed, for example, as a rectangular area of silicon. Electrical contact to active region  36  is made via a plurality of contacts such as a contact  46 , a contact  48  and a contact  50 . The region  36  is termed “active” because exposed surfaces exist to the region for the purpose of making electrical contact to the region. The contact  46  functions as a source electrode contact for each of transistors  12  and  14 . The potential V SS  is connected to contact  46 . Contact  48  functions as a drain electrode contact for transistor  12 , and contact  50  functions as a drain electrode contact for transistor  14 . Each of the contacts  48  and  50  is connected to sense amplifier  16 . Overlying and overlapping a portion of the nodes  32  and  34  and active region  36  are continuous polysilicon layer  40  and continuous polysilicon layer  42 . The continuous polysilicon layer  40  overlies a portion of node  32  to form the first electrode of capacitor  18  having a size that is denoted by cross-hatching from upper-left to lower-right. The continuous polysilicon layer  40  also extends over a portion of active region  36  to form the floating gate electrode of transistor  12 . The continuous polysilicon layer  40  further extends and overlies a portion of the node  34  as denoted by cross-hatching from lower-left to upper-right. The portion overlapping node  34  forms the second electrode of capacitor  20 . Because node  32  is an active region, electrical contact is made to node  32  by a contact  52  which provides electrical contact to the first electrode of capacitors  18  and  22 . 
     Additionally, the continuous polysilicon layer  42  overlies a portion of node  32  to form the second electrode of capacitor  22  having a size that is denoted by cross-hatching from lower-left to upper-right. The continuous polysilicon layer  42  also extends over a portion of active region  36  to form the floating gate electrode of transistor  14 . The continuous polysilicon layer  42  further extends and overlies a portion of the node  34  as denoted by cross-hatching from upper-left to lower-right. The portion overlapping node  34  forms the second electrode of capacitor  24 . Electrical contact to the node  34  is made by a contact  54  which provides electrical contact to the first electrodes of capacitors  20  and  24 . 
     It should be noted that the layout of storage cell  11  is implemented with a conventional CMOS process and no additional specialized processing. For example, numerous nonvolatile storage cells require specialized process steps and structures so that a conventional MOS manufacturing flow must be supplemented or modified. As illustrated in  FIG. 2  the node  32  and the node  34  are overlapped by two monolithic areas of polysilicon, or other conductive material, in such a way that the overlap area of one polysilicon area is small over one node but large over the other node. When one of nodes  32  and  34  is biased, a high electric field develops across capacitors  20  and  22  which results in a tunneling current to boost the electrical potential of the gate of one of the transistors. The direction of tunneling current is determined by the relative amount of capacitive coupling between each gate and the nodes  32  and  34 . As described above, the biasing provided by V PROG  and V PROGB  causes the charging of one gate of transistors  12  and  14  and the discharging of the other through Fowler-Nordheim or direct tunneling. As illustrated in  FIG. 2  a monolithic floating gate is formed with a same material that is used to form a connected electrode of each of two adjacent capacitors. As a result of the gate of transistor  12  and transistor  14  being capacitively coupled and not grounded, the gates of each of transistors  12  and  14  is electrically floating. Charging of transistor  12  and discharging of transistor  14 , or the inverse operation, depending on the state of Q and QB, can also be achieved by applying a short voltage pulse to the V PP  supply, instead of a continuous V PP  signal. This pulse typically can have a duration of from approximately ten nanoseconds to several hundred milliseconds. When the power is removed, the charge on the gates of transistors  12  and  14  remains for a significant amount of time and therefore the storage cell  11  is nonvolatile. When power is subsequently restored, adequate charge is present to continue operation in a manner as if no interruption in the source of power occurred. Specifically, after restoring power, a sense operation can be performed that will provide an output, “STORED Q”, at the output of sense amplifier  16 . This value can subsequently be stored back to latch  30 . After this operation, the latch  30  will be in the same state as it was when the power was shut down. 
     As illustrated in  FIG. 1 , the dielectrics which separate the electrodes of capacitors  18 ,  20 ,  22  and  24  are the same thickness. In one form the dielectrics are sized to have a value that is within a range of fifteen Angstroms to one hundred Angstroms. In one form the dielectric thickness of capacitor  18  and capacitor  24  are the same and have a first thickness. In this form the dielectric thickness of capacitor  22  and capacitor  20  are the same and have a second thickness. The first thickness may or may not be the same as the second thickness. In this manner, the layout of  FIG. 2  exhibits structural balance which results in symmetry of charge. 
     By now it should be appreciated that there has been provided a nonvolatile storage cell for use in a standard MOS process. The layout of the storage cell  11  is symmetrical. In one form there is provided a semiconductor storage device in which a first transistor has a gate. A source is coupled to a first power supply terminal and a drain as a first output of the semiconductor storage device. A first capacitor has a first terminal coupled to the gate of the first transistor, a second terminal coupled to a first programming terminal, and a first capacitance. A second capacitor has a first terminal coupled to the gate of the first transistor, a second terminal coupled to a second programming terminal, and a second capacitance, wherein the first capacitance is greater than the second capacitance. A second transistor has a gate, a source coupled to the first power supply terminal and coupled to the source of the first transistor. A drain is a second output of the semiconductor storage device. A third capacitor has a first terminal coupled to the gate of the second transistor, a second terminal coupled to the first programming terminal, and a third capacitance. A fourth capacitor has a first terminal coupled to the gate of the second transistor, a second terminal coupled to the second programming terminal, and a fourth capacitance, wherein the fourth capacitance is greater than the third capacitance. In another form the first capacitance is at least two times greater than the second capacitance. In another form the first capacitance is at least five times greater than the second capacitance. In yet another form the gate of the first transistor, the first terminal of the first capacitor, and the first terminal of the second capacitor are portions of a first continuous conductive line. In yet another form the first continuous conductive line comprises a first continuous polysilicon segment. In another form the sources and drains of the first and second transistors are in a first active region and the second terminals of the first and the third capacitors are in a second active region. Additionally, the second terminals of the second and the fourth capacitors are in a third active region. In yet another form the first terminals of the first and the third capacitors are over the second active region. The first terminals of the second and the fourth capacitors are over the third active region. In another form the gate of the first transistor, the first terminal of the first capacitor, and the first terminal of the second capacitor are portions of a first continuous polysilicon segment. In another form the first terminal of the first capacitor is separated from the second active region by a first dielectric layer having a first thickness, and the first terminal of the fourth capacitor is separated from the third active region by a second dielectric layer having the first thickness. In yet another form the first programming terminal is for being at a higher voltage than the second programming terminal to establish a first logic state of the semiconductor storage device. The second programming terminal is for being at a higher voltage than the first programming terminal to establish a second logic state of the semiconductor storage device. 
     In another form a semiconductor storage device has a first transistor having a gate, a source coupled to a first power supply terminal, and a drain. A first capacitor has a first terminal coupled to the gate of the first transistor, a second terminal coupled to a first programming terminal, and a first capacitance. A second capacitor has a first terminal coupled to the gate of the first transistor, a second terminal coupled to a second programming terminal, and a second capacitance. The first capacitance is greater than the second capacitance. A second transistor has a gate, a source coupled to the first power supply terminal and coupled to the source of the first transistor, and a drain. A third capacitor has a first terminal coupled to the gate of the second transistor, a second terminal coupled to the first programming terminal, and a third capacitance. A fourth capacitor has a first terminal coupled to the gate of the second transistor, a second terminal coupled to the second programming terminal, and a fourth capacitance. the fourth capacitance is greater than the third capacitance. A sense amplifier has a first input coupled to the drain of the first transistor, a second input coupled to the drain of the second transistor, and an output. A latch has a first input for receiving a data signal, a second input coupled to the output of the sense amplifier, and an output. A coupling device is responsive to the output of the latch that selectively couples a programming voltage to one of the first and second programming terminals. In another form the latch is further characterized as responsive to a sense enable signal. The sense amplifier is further characterized as responsive to the sense enable signal. In another form the output of the latch provides a true signal and a complementary signal. In one form the coupling device is a first coupler responsive to the true signal and a second coupler responsive to the complementary signal. In another form the first capacitance is at least five times greater than the second capacitance. The gate of the first transistor, the first terminal of the first capacitor, and the first terminal of the second capacitor are portions of a first continuous polysilicon segment. In yet another form the sources and drains of the first and second transistors are in a first active region. The second terminals of the first and the third capacitors are in a second active region. The second terminals of the second and the fourth capacitors are in a third active region. The first terminals of the first and the third capacitors are over the second active region, and the first terminals of the second and the fourth capacitors are over the third active region. 
     In yet another form a semiconductor storage device has a first transistor having a gate comprised of a first polysilicon segment over a first active region, a source formed in the first active region, and a drain formed in the first active region. A first capacitor has a first terminal formed of the first polysilicon segment over a second active region, a second terminal in the second active region, and a first capacitance. A second capacitor has a first terminal formed of the first polysilicon segment over a third active region, a second terminal in the third active region, and a second capacitance, wherein the first capacitance is greater than the second capacitance. A second transistor has a gate formed of a second polysilicon segment over the first active region, a source formed in the first active region and coupled to the source of the first transistor, and a drain formed in the first active region. A third capacitor has a first terminal formed of the second polysilicon segment over the second active region, a second terminal in the second active region, and a third capacitance. A fourth capacitor has a first terminal formed of the second polysilicon segment over the third active region, a second terminal in the third active region, and a fourth capacitance, wherein the fourth capacitance is greater than the third capacitance. In another form the first capacitance is at least two times greater than the second capacitance. In yet another form the first capacitance is at least five times greater than the second capacitance. The second active region and the third active region are further characterized as well regions. In yet another form the first terminal of the first capacitor is separated from the second active region by a first dielectric layer having a first thickness. The first terminal of the fourth capacitor is separated from the third active region by a second dielectric layer having the first thickness. In another form the first terminal of the second capacitor is separated from the second active region by a first dielectric layer having a first thickness. The first terminal of the third capacitor is separated from the third active region by a second dielectric layer having the first thickness. 
     In the foregoing specification, the invention has been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. For example, the polysilicon layer  40  and polysilicon layer  42  may be implemented with conductive materials other than polysilicon. A conductive material such as any of numerous metals or metal alloys used in semiconductor manufacturing may be used. The nonvolatile storage cell  11  described herein may be used with circuitry other than sense amplifier  16  and latch  30 . While a differential bitcell is herein described, it should be understood that an alternate embodiment that does not utilize a differential output may be implemented wherein a single transistor and two capacitors are used to store data and that data is sensed using a reference signal. 
     Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. 
     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. The terms a or an, as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically.