BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to non-volatile memory circuits. In particular, the present invention relates to non-volatile memory circuits that use ferroelectric material for persistent storage.

2. Discussion of the Related Art

Memory circuits using ferroelectric materials (e.g., lead zirconate Titanate (PZT)) have been proposed. One type of such memory circuit is simply a capacitor, such as that ferroelectric capacitor 100 of Figure 1(c). As shown in Figure 1(c), ferroelectric capacitor 100 includes a layer of ferroelectric material provided between two capacitor plates represented by electrodes formed between a "plate line" (PL) and a "bit line" (BL), respectively.

Ferroelectric capacitor 100 of Figure 1(c) may be used in one of two ways: (i) as non- volatile memory cell, or as (ii) a volatile memory cell. Figure 1(a) illustrates the programmed states of ferroelectric capacitor 100 when used as a non-volatile memory cell. As shown in Figure 1(a), when used as a non-volatile cell, a high programming voltage (e.g., VPP, where VPP may be 5v volts, for example) is applied across ferroelectric capacitor 100 to program ferroelectric capacitor 100 into a first phase or programmed state, representing the stored data bit "0". After the imposed voltage across ferroelectric capacitor 100 is removed, the ferroelectric material in ferroelectric capacitor 100 maintains the programmed "0" state for a long time (e.g., tens of years). Alternatively, ferroelectric capacitor 100 may be programmed into a second phase or programmed state by applying a high negative voltage (e.g., -VPP) across storage capacitor 100, representing the erased data bit "1". When the imposed voltage across capacitor 100 is removed, the ferroelectric material in storage capacitor 100 maintains the programmed "1" state for a long time (e.g., tens of years).

Figure 1(b) illustrates the programmed states of ferroelectric capacitor 100 when used as a volatile memory cell. As shown in Figure 1(b), when used as a volatile cell, a programming voltage (e.g., Vcc e.g., lv) that is much lower than the programming voltage for the non-volatile state is applied across ferroelectric capacitor 100 to program ferroelectric capacitor 100 into a first phase or programmed state, representing the volatile data bit "0". When the imposed voltage across capacitor 100 is removed, the ferroelectric material in storage capacitor 100 maintains the programmed "0" state for a relatively much shorter time period (e.g., seconds) than those of the non-volatile states. Alternatively, ferroelectric capacitor 100 may be programmed into a second phase or programmed state by applying a corresponding negative voltage (- Vcc) across storage capacitor 100, representing the volatile data bit "1". When the imposed voltage across ferroelectric capacitor 100 is removed, the ferroelectric material in storage capacitor 100 maintains the programmed "1" sate for a relatively shorter period of time (e.g., seconds) than those of the non-volatile states.

The higher programming voltage in non-volatile memory cells operation may reduce the endurance of ferroelectric capacitor 100 and provides relatively slow read and write speeds. The low programming voltage for volatile operations allows greater endurance and higher read and write speeds (e.g., comparable to conventional dynamic random access memory (DRAM) speeds). However, in many applications, as in DRAM, ferroelectric capacitor 100 is required to be refreshed periodically to prevent data loss, albeit at a lesser frequency than conventional

DRAMs.

Figure 2(a) shows a ferroelectric static random access memory (FeSRAM) cell 200, which operate as a non-volatile memory cell. As shown in Figure 2(a), FeSRAM cell 200 includes a conventional 6-transistor static random access memory (SRAM) cell formed by select transistors 201a and 201b, and cross-coupled inverters formed by transistors 202a, 202b, 203a and 203b. FeSRAM 200 also includes ferroelectric capacitors CO and CI respectively connected input terminals 204a and 204b of the cross-coupled inverters and the plate line carrying voltage signal PL. During operation, before FeSRAM cell is powered down, the complementary stored data bits of the SRAM cell held at terminals 204a and 204b are written into ferroelectric capacitors CO and CI, respectively. When power is restored, the complementary data bits in ferroelectric storage capacitors CO and CI are written back into the SRAM cell (i.e., to be held at terminals 204a and 204b again). Figure 2(b) shows, when power is restored, voltage signal VPW at the power supply line of the SRAM cell, voltage signal PL on the plate line and voltage signals BT and BC at the input terminals 204a and 204b of the cross-coupled inverters of the SRAM cell. Figure 2(c) shows, before FeSRAM cell 200 is powered down, voltage signal VPW at the power supply line of the SRAM cell, voltage signal PL on the plate line and voltage signals BT and BC at the input terminals of the cross-coupled inverters of the SRAM cell.

Initially, as shown in Figure 2(c), voltage signal VPW is at Vcc, plate line signal PL is at 0.5 Vcc, and voltage signals BT and BC are at Vcc and 0 volts, respectively. (Transistors 203 a and 202b are conducting, and transistors 203b and 202a are non-conducting). To write these states into ferroelectric capacitors CO and CI, both voltage signals VPW and PL are brought to programming voltage VPP, thereby bringing the voltage across ferroelectric storage capacitor CI to VPP, thus writing bit '0' into capacitor CI. After ferroelectric storage capacitor CI has been written, voltage PL on the plate line is brought to 0 volts, such that the voltage across ferroelectric storage capacitor CO is brought to -VPP, thereby writing bit Ί ' into capacitor CO. After ferroelectric storage capacitors CO and CI are both written, voltage VPW is also brought to 0 volts (i.e., powered down). When power is restored, as shown in Figure 2(b), voltage signal VPW is ramped up to

Vcc, plate line signal PL is held at 0 volts. The stored voltages in ferroelectric storage capacitors CO (at bit ' 1 ') and CI (at bit 'Ο') are sufficient to render transistors 203a and 202b conducting, and transistors 203b and 202a non-conducting. Accordingly, the voltage signals BT and BC at terminals 204a and 204b are brought to Vcc and 0 volts, respectively. Thus, the SRAM cell is returned to its state prior to the power down. At this point, voltage PL is returned to 0.5 VPP, which is its quiescent state. The subsequent operations of FeSRAM cell 200 are essentially those of a conventional SRAM cell.

As can be seen from the discussion above with respect to Figures 1(a) and 1(b), during normal operations, signal transitions at the input terminals of the cross-coupled inverters of the SRAM cells place ferroelectric storage capacitors CO and CI into their volatile states, and thus the resulting stored data are not preserved.

As voltage PL has signaling requirements that are different from either the power supply voltage VPW or the ground reference, it requires a separate decoder. The routing requirement also increases the size of FeSRAM cell 200 relative to the size of a

corresponding conventional SRAM cell.

SUMMARY

According to one embodiment of the present invention, a ferroelectric static random access memory (FeSRAM) cell includes (a) first and second cross-coupled inverters connected between a power supply voltage signal and a ground reference voltage signal and holding a data signal represented in a complementary manner in first and second common data terminals; (b) first and second select transistors coupled respectively to the first and second common data terminals of the cross-coupled inverters; and (c) first, second, third and fourth ferroelectric capacitors, wherein the first and second ferroelectric capacitors couple the first common data terminal to the power supply voltage signal and the ground reference voltage signal, respectively, and wherein the third and the fourth ferroelectric capacitors couple the second common data terminal to the power supply voltage signal and the ground reference voltage signal, respectively.

In one embodiment, the first select transistor is formed at the surface of a

semiconductor substrate, and the first and second ferroelectric capacitors are implemented between conductor layers above the select transistor within the silicon footprint of the first select transistor. The power supply voltage signal selectably provides a first voltage ("operating voltage") and a second voltage ("programming voltage") and wherein the programming voltage selectably programs one or more of the ferroelectric capacitors to non- volatile states. The operating voltage selectably programs one or more of the ferroelectric capacitors to volatile states.

The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1(a) illustrates the programmed states of storage operations of storage capacitor 100 when used as a non-volatile memory circuit.

Figure 1(b) illustrates the programmed states of storage operations of storage capacitor 100 when used as a volatile memory circuit

Figure 1(c) shows storage capacitor 100, which is a simple model of a ferroelectric memory circuit.

Figure 2(a) shows a ferroelectric static random access memory (FeSRAM) cell 200, which operate as a non-volatile memory cell.

Figure 2(b) shows, when power is restored, voltage signal VPW at the power supply line of the SRAM cell, voltage signal PL on the plate line and voltage signals BT and BC at the input terminals of the cross-coupled inverters of the SRAM cell.

Figure 2(c) shows, before FeSRAM cell 200 is powered down, voltage signal VPW at the power supply line of the SRAM cell, voltage signal PL on the plate line and voltage signals BT and BC at the input terminals of the cross-coupled inverters of the SRAM cell.

Figure 3(a) shows FeSRAM cell 300, in accordance with one embodiment of the present invention.

Figure 3(b) shows the operations to write the signal states at terminals 204a and 204b of the cross-coupled inverters into some of the ferroelectric capacitors COU, ClU, COD and C1D.

Figure 3(c) shows, upon restoring power, the operations to restore the states of signals BT and BC at terminals 204a and 204b, respectively, according to one embodiment of the present invention. Figure 4(a) summarizes storing non-volatile states in ferroelectric capacitors C1D and CIU of FeSRAM 300, when terminal 204a is at the VPP and terminal 204b is at ground voltage at the time of writing into the ferroelectric capacitors in preparation of powering down. Figure 4(b) summarizes storing non-volatile states in ferroelectric capacitors COD and

C0U of FeSRAM 300, when terminal 204b is at the VPP and terminal 204a is at ground voltage at the time of writing into the ferroelectric capacitors in preparation of powering down.

Figures 5(a) and 5(b) illustrate how select transistor 201b and ferroelectric capacitors CIU and COD may be implemented on the same silicon footprint of select transistor 201b.

To facilitate cross-referencing and to simplify the detailed description below, like elements in the figures are assigned like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides for the programming needs of an FeSRAM cell without requiring a plate line. Figure 3(a) shows FeSRAM cell 300, in accordance with one embodiment of the present invention. As shown in Figure 3, like FeSRAM cell 200 of Figure 2, FeSRAM cell 300 includes a conventional 6-transistor SRAM cell. To simplify this detailed description, the transistors of the conventional SRAM cells in FeSRAM cells 200 and 300 are assigned like reference numerals. Unlike FeSRSAM cell 200, having two ferroelectric storage capacitors CO and CI connected to a plate line, FeSRAM cell 300 includes four ferroelectric storage capacitors C0U, CIU, C1D and COD, without a plate line. Instead, ferroelectric storage capacitors C0U and CIU are coupled across power supply voltage VPW and cross-coupled inverter terminals 204a and 204b, respectively. Ferroelectric capacitors C1D and COD are connected across cross-coupled inverter terminals 204a and 204b and the ground reference voltage (Vss),

Figure 3(b) shows the operations to write the signal states at terminals 204a and 204b of the cross-coupled inverters into some of the ferroelectric capacitors C0U, CIU, COD and C1D. In Figure 3(b), initially, supply voltage VPW is at voltage Vcc, voltage BT is at voltage Vcc, while voltage BC is at 0 volts, suggesting that transistors 203 a and 202b are conducting while transistors 203b and 202a are not conducting. As shown in Figure 3(b), voltage VPW is brought to programming voltage VPP, which in turn, brings terminal 204a to programming voltage VPP as well, while the voltage at terminal 204b remains at 0 volts. As a result, programming voltage VPP is imposed across ferroelectric capacitors C1D and CIU, thus writing ferroelectric capacitor C1D to the non-volatile "1" state and ferroelectric capacitor ClU to the non-volatile '0' state. As full programming voltage VPP does not appear across ferroelectric capacitors COU and COD in these operations, the states in ferroelectric capacitors COU and COD are volatile states and are lost upon powering down. The preceding description therefore corresponds to the case when terminal 204a is at the VPP and terminal 204b is at ground voltage at the time of writing into capacitor C1D and ClU. Figure 4(a) summarizes this condition, pointing out that the programmed states at ferroelectric capacitors COU and COD are volatile and not preserved by the power down.

Figure 3(c) shows, upon restoring power, the operations to restore the states of signals BT and BC at terminals 204a and 204b, respectively, according to one embodiment of the present invention. As shown in Figure 3(c), when power is restored, the non-volatile states of ferroelectric capacitors ClU and C1D restore terminals 204a and 204b to Vcc and 0 volts, respectively, thus restoring them to their previous states prior to powering down.

In the event that, at the time of programming non-volatile states into the ferroelectric capacitors, terminal 204b is at the VPP and terminal 204a is at ground voltage, non-volatile states would be written into ferroelectric capacitor COU and COD, and volatile states (that are not preserved by the power down) would be written into ferroelectric capacitors C1D and ClU. This condition is summarized in Figure 4(b).

By eliminating the plate line, FeSRAM cell 300 may be provided at the same size as the conventional 6-transistor SRAM cell. This is achieved by providing the ferroelectric capacitors between conductor layers above the conventional 6-transistor SRAM cell. For example, Figures 5(a) and 5(b) illustrate how select transistor 201b and ferroelectric capacitors ClU and COD (i.e., the combination shown in the dashed oval of Figure 5(a)) may be implemented on the same silicon footprint of select transistor 201b. Figure 5(b) shows select transistor 201b being implemented by gate electrode WL and source and drain regions BC and BLC separated by a channel region. Contact CONT connects source region BC to the common capacitor plate M0 in a first conductor layer. The other capacitor plates of capacitor ClU and COD are implemented on a second conductor layer separated by ferroelectric material. As shown in Figure 5(c), power supply voltage VPW is routed to the capacitor plate of ferroelectric capacitor ClU at the second conductor level, while ground reference voltage Vss is routed to the capacitor plate of ferroelectric capacitor COD at the second conductor level.

In addition to its compact size, a FeSRAM of the present invention also has the advantage of not having to provide a decoder for signal PL of the plate line, which is eliminated.

The above detailed description is provided to illustrate specific embodiments of the present invention and is not to be taken as limiting. Numerous variations and modifications within the scope of the present invention are possible. The present invention is set forth in the accompanying claims.