Next-generation NVM

FRAM is a NVM based on the ferroelectric effect of the charge storage mechanism, and is quite different from the floating-gate-based NVM. Typical perovskite ferroelectric materials are BaTiO3, PbTiO3, PZT (PbZr1-xTixO3), PLZT (Pb1-xLxZrO3), PMN (PbMg1-xNbxO3), SBT (SrBi2Ta2O9), SBN (SrBi2Nb2O9), and among others. The ferroelectric effect is the ability of a material to store an electric polarization in the absence of an applied electric field. A FRAM device cell is fabricated by depositing a ferroelectric film in crystal form between two electrode plates to form a capacitor. The perovskite crystals of a ferroelectric material maintain two stable polarization states resulting from the alignment of internal dipoles, corresponding to states of logical ―1‖ and ―0,‖ which in turn reverses the alignment of these internal dipoles. A simplified model of a ferroelectric crystal is shown in Fig.

1-4(a) [6]. A mobile atom is in the center of the crystal, and application of an electric

field across the face of the crystal causes this atom to move in the direction of the field. A reversal of the field causes the atom to move in the opposite direction, resulting in a typical hysteresis curve, as shown in Fig. 1-4(b). The positions of the atom at top and bottom of the crystal are stable, and the atom remains in these states when the external field is removed. No external electric field or current is required for the ferroelectric material to remain polarized in either state; hence, a memory device can be built for storing digital (binary) data that will not require power to retain information stored within it. The critical temperature for ferroelectric materials is known as the Curie temperature; the perovskite structure assumes a cubic symmetry and is paraelectric when the Curie temperature is exceeded, that is, the films will lose the ferroelectric behavior. Therefore, the Curie temperature for a ferroelectric material must be high enough to maintain a wide margin of the application in semiconductor technology. The polarization versus temperature curve is shown in Fig. 1-4(c) [7,8].

To read a FRAM, detecting the position of the atoms within the petrovskite crystals is necessary. However, it cannot be directly sensed. The read operation involves applying an electric field across the capacitor, causing the mobile atoms to move across the crystals in the direction of the field if they are not already in the appropriated positions. The circuit dumps charge resulting from the applied field from the capacitor and compares it to the charge from a reference. A capacitor with atoms that switch states will emit a larger charge than a capacitor with atoms that do not switch. The memory sensing circuitry must determine which capacitors switched. The

―switched charge‖ allows the circuit to determine the state of the memory cell. Given that a memory read operation involves a change of state, the FRAM circuitry must restore the original memory state. Therefore, each read access is accompanied by a pre-charge operation that restores the memory states. To write a FRAM, no system overhead is required, and the operation is very similar to a read operation. The circuit

applies write data to the ferroelectric capacitors, and the new data switches (if necessary) the state of the ferroelectric crystals.

(b) MRAM

MRAM is solid state, nonvolatile magnetic storage device in which the stored data are represented by the magnetization direction and the readout is done by resistance measurements. According to the proponents of MRAM, this technology scales better to low voltages than DRAM, SRAM, and flash memories, and it requires fewer mask steps than the DRAM process. Early MRAMs were based on the anisotropic magnetoresistance (AMR) effect. Only less than 5% of the amplitude of the AMR effect in thin films limits the application only to military and space application. In 1988, the discovery of the giant magnetoresistive (GMR) effect [9]

changed this situation. This GMR technology (5-15%) has been applied in commercial products like hard disk drive (HDD) read heads and magnetic sensors. A breakthrough in the field of magnetic tunnel junctions was achieved sometime in 1995, when the large tunnel magnetoresistive (TMR) effect [10] was demonstrated at room temperature, further contributing to the development of MRAM. The TMR has the potential of reaching MR ratios of 30-40%. However, a tougher challenge lies in finding the right process recipe that would enable the production of reliable MRAMs.

In general, both GMR and TMR result in low resistance if the magnetization directions in the multilayer are parallel, and in high resistance when the magnetizations are anti-parallel. For the representation of storage bits, different possibilities exist. The pseudo-spin valves use two ferromagnetic layers that switch their magnetization direction at different magnetic fields, which can be accomplished by utilizing layers of different magnetic materials, or layers of the same material but of different thicknesses. A typical structure of MRAM cell is shown in Fig. 1-5.

The resistance of two thin ferromagnetic layers separated by a thin nonmagnetic conducting layer can be altered based on whether the moments of ferromagnetic layers are parallel or anti-arallel. The layers with parallel magnetic moments will have less scattering at the interface, longer mean free paths, and lower resistance. On the other hand, the layers with antiparallel magnetic moments will have more scattering at the interfaces, shorter mean free paths, and higher resistance. In order for the spin-dependent scattering to be a significant part of total resistance, the layers must be thinner than the mean free path of electrons in the bulk material. For many ferromagnetic materials, the mean free path is tens of nanometers, so the layers themselves must each be typically less than 10 nm.

The addressing of the MRAM is done using an array of crossing lines. Writing a certain cell is equivalent to setting a magnetization in the desired direction. By applying a current pulse to a bit line and a word line, lines experience the maximum magnetic field, thereby reversing its magnetization. All other MRAM cells below the bit or word line are exposed to the significantly lower field that is caused by a single current pulse and, therefore, will not change their magnetization directions.

(c) PCRAM

The phase-change memory technology stores information using structural phase changes in certain thin-film alloys. The semiconducting properties of a range of crystalline and amorphous chalcogenide alloys were investigated in the early 1950s.

Up to date, the chalcogenide is a proven phase-change material used in the rewritable CDs and DVDs [11]. The operation of PCRAM devices is based on the principle that the films can be designed to be highly resistive semiconductors in the amorphous phase and highly conductive semimetals in the crystalline phase [12]. These alloys should be stabilized in the crystalline state (in Fig. 1-6(a)) in which the atoms are

arranged in a regular periodic structure, as well as in the amorphous state (in Fig.

1-6(b)) with irregular atomic state [13,14]. The phase-change alloys are referred to as the chalcogenide materials. These two structural states have different optical and electrical properties, and two states by application of electric pulses of energy sufficient enough to overcome the energy barrier separating the two states.

The phase-change conversion is accomplished by appropriate heating and cooling of the material. A laser beam pulse energy is used to detect these structural phase changes. On melting, the material loses its crystalline structure; and rapidly cooling it below its glass transition temperature results in the amorphous phase. To keep the material from recrystallizing during cooling, the cooling rate must be faster than the crystal nucleation and growth rate. To switch the memory cells back to the conductive state, the material is heated to a temperature between the glass transitio n temperature and the melting temperature, causing the nucleation and crystal growth to occur rapidly, as shown in Fig. 1-6(c).

(d) RRAM

RRAM devices like PCRAM, based on different resistance state to distinguish the high or low state; these devices have emerged recently and are considered to be promising candidates of NVM. Unlike PCRAM, which exhibits homogeneous change on the resistance values inside the thin film, the change in resistance values in RRAM devices only occur at some certain localized positions among the insulator. Only a several nm order of the filamentary path in diameter is needed to perform a success ful resistance switching. Therefore, RRAM devices can offer great advantages of small cell size, excellent scalability and high density array. A simple fabrication process for RRAM devices is compatible with the standard semiconductor technology.

Furthermore, the voltage values required to switch RRAM devices are quite low,

sometimes in the range of approximately 1-3 V to accomplish a sufficiently large resistance difference, which exhibits low operation voltage and high-to-low resistance ratio. At present, a RRAM array with high speed operation, long endurance and retention time has been performed, verifying the high potential of the RRAM applications. Several oxide materials, such as binary metal oxides, perovskite-based oxide, chalogenide materials, solid-state electrolytes, and organic molecular materials, have been proposed to exhibit resistive switching (RS) properties, thereby showcasing the flexibility of RRAM devices.