Resistive random-access memory
Encyclopedia
Resistive random-access memory (RRAM) is a new non-volatile memory type being developed by many companies. The technology bears some similarities to CBRAM
and phase change memory.
Different forms of RRAM have been disclosed, based on different dielectric materials, spanning from perovskite
s to transition metal oxides
to chalcogenide
s. Even silicon dioxide has been shown to exhibit resistive switching as early as 1967, and has recently been revisited.
, which is normally insulating, can be made to conduct through a filament or conduction path formed after application of a sufficiently high voltage. The conduction path formation can arise from different mechanisms, including defects, metal migration, etc. Once the filament is formed, it may be reset (broken, resulting in high resistance) or set (re-formed, resulting in lower resistance) by an appropriately applied voltage. Recent data suggest that many current paths, rather than a single filament, are probably involved.
A memory cell can be deduced from the basic memory cell in three different ways. In the simplest approach, the pure memory element can be used as a basic memory cell, resulting in a configuration where parallel bitlines are crossed by perpendicular wordlines with the switching material placed between wordline and bitline at every cross-point. This configuration is called a cross-point cell. Since this architecture will lead to a large parasitic current flowing through non selected memory cells, the cross-point array has a very slow read access. A selection element can be added to improve the situation. A series connection of a diode in every cross-point allows to reverse bias all non selected cells. This can be arranged in a similar compact manner as the basic cross-point cell. Finally a transistor device (ideally a MOS Transistor) can be added which makes the selection of a cell very easy and therefore gives the best random access time, but comes at the price of increased area consumption.
For random access type memories, a transistor type architecture is preferred while the cross-point architecture and the diode architecture open the path toward stacking memory layers on top of each other and therefore are ideally suited for mass storage devices. The switching mechanism itself can be classified in different dimensions. First there are effects where the polarity between switching from the low to the high resistance level (reset operation) is reversed compared to the switching between the high and the low resistance level (set operation). These effects are called bipolar switching effects. On the contrary, there are also unipolar switching effects where both, set and reset operation, require the same polarity, but different voltage magnitude.
Another way to distinguish switching effects is based on the localization of the low resistive path. Many resistive switching effects show a filamentary behavior, where only one or a few very narrow low resistive paths exist in the low resistive state. In contrast, also homogenous switching of the whole area can be observed. Both effects can occur either throughout the entire distance between the electrodes or happen only in close proximity to one of the electrodes. Filamentary and homogenous switching effects can be distinguished by measuring the area dependence of the low resistance state.
1. phase change chalcogenides like Ge2Sb2Te5 or AgInSbTe
2. binary transition metal oxides like NiO or TiO2
3. perovskites like Sr(Zr)TiO3 or PCMO
4. solid-state electrolytes like GeS, GeSe, or Cu2S
5. organic charge transfer complexes like CuTCNQ
6. organic donor–acceptor systems like Al AIDCN
7. various molecular systems
or MRAM
without sacrificing programming performance, retention or endurance. On April 30, 2008 HP announced a memristor
, a new circuit element that is another possible demonstration of RRAM, and on July 8 they announced they would begin prototyping RRAM using their memristors. At IEDM 2008, the highest performance RRAM technology to date was demonstrated by ITRI
, showing switching times less than 10 ns and currents less than 30 microamps. At IEDM 2010, ITRI also broke the speed record, showing <0.3 ns switching time, while also showing process and operation improvements to allow yield up to 100%.
ITRI has recently shown that RRAM is scalable below 30 nm. The motion of oxygen atoms is a key phenomenon for oxide-based RRAM; one study has indicated that oxygen motion may take place in regions as small as 2 nm. It is believed that if a filament is responsible, it would not exhibit direct scaling with cell size. Instead, the current compliance limit (set by an outside resistor, for example) could define the current-carrying capacity of the filament.
A significant hurdle to realizing the potential of RRAM is the sneak path problem which occurs in larger passive arrays. In 2010, complementary resistive switching (CRS) was introduced as a possible solution to the interference from sneak-path currents. In the CRS approach, the information storing states are pairs of high and low resistance states (HRS/LRS and LRS/HRS) so that the overall resistance is always high, allowing for larger passive crossbar arrays.
A drawback to the initial CRS solution is the high requirement for switching endurance caused by conventional destructive readout based on current measurements. A new approach for a nondestructive readout based on capacity measurement potentially lowers the requirements for both material endurance and power consumption.
Programmable metallization cell
The programmable metallization cell, or PMC, is a new form of non-volatile computer memory being developed at Arizona State University and its spinoff, Axon Technologies....
and phase change memory.
Different forms of RRAM have been disclosed, based on different dielectric materials, spanning from perovskite
Perovskite
A perovskite structure is any material with the same type of crystal structure as calcium titanium oxide , known as the perovskite structure, or XIIA2+VIB4+X2−3 with the oxygen in the face centers. Perovskites take their name from this compound, which was first discovered in the Ural mountains of...
s to transition metal oxides
Transition metal oxides
Transition metal oxides comprise a class of materials that contain transition elements and oxygen. They include insulators as well as metals. Often the same material may display both types of transport properties, hence a Metal-Insulator transition, obtained by varying either temperature or...
to chalcogenide
Chalcogenide
A chalcogenide is a chemical compound consisting of at least one chalcogen ion and at least one more electropositive element. Although all group 16 elements of the periodic table are defined as chalcogens, the term is more commonly reserved for sulfides, selenides, and tellurides, rather than...
s. Even silicon dioxide has been shown to exhibit resistive switching as early as 1967, and has recently been revisited.
Mechanism
The basic idea is that a dielectricDielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
, which is normally insulating, can be made to conduct through a filament or conduction path formed after application of a sufficiently high voltage. The conduction path formation can arise from different mechanisms, including defects, metal migration, etc. Once the filament is formed, it may be reset (broken, resulting in high resistance) or set (re-formed, resulting in lower resistance) by an appropriately applied voltage. Recent data suggest that many current paths, rather than a single filament, are probably involved.
A memory cell can be deduced from the basic memory cell in three different ways. In the simplest approach, the pure memory element can be used as a basic memory cell, resulting in a configuration where parallel bitlines are crossed by perpendicular wordlines with the switching material placed between wordline and bitline at every cross-point. This configuration is called a cross-point cell. Since this architecture will lead to a large parasitic current flowing through non selected memory cells, the cross-point array has a very slow read access. A selection element can be added to improve the situation. A series connection of a diode in every cross-point allows to reverse bias all non selected cells. This can be arranged in a similar compact manner as the basic cross-point cell. Finally a transistor device (ideally a MOS Transistor) can be added which makes the selection of a cell very easy and therefore gives the best random access time, but comes at the price of increased area consumption.
For random access type memories, a transistor type architecture is preferred while the cross-point architecture and the diode architecture open the path toward stacking memory layers on top of each other and therefore are ideally suited for mass storage devices. The switching mechanism itself can be classified in different dimensions. First there are effects where the polarity between switching from the low to the high resistance level (reset operation) is reversed compared to the switching between the high and the low resistance level (set operation). These effects are called bipolar switching effects. On the contrary, there are also unipolar switching effects where both, set and reset operation, require the same polarity, but different voltage magnitude.
Another way to distinguish switching effects is based on the localization of the low resistive path. Many resistive switching effects show a filamentary behavior, where only one or a few very narrow low resistive paths exist in the low resistive state. In contrast, also homogenous switching of the whole area can be observed. Both effects can occur either throughout the entire distance between the electrodes or happen only in close proximity to one of the electrodes. Filamentary and homogenous switching effects can be distinguished by measuring the area dependence of the low resistance state.
Material systems for resistive memory cells
A large number of inorganic and organic material systems showing thermal or ionic resistive switching effects have been demonstrated in the literature. These can be grouped into the following categories:1. phase change chalcogenides like Ge2Sb2Te5 or AgInSbTe
2. binary transition metal oxides like NiO or TiO2
3. perovskites like Sr(Zr)TiO3 or PCMO
4. solid-state electrolytes like GeS, GeSe, or Cu2S
5. organic charge transfer complexes like CuTCNQ
6. organic donor–acceptor systems like Al AIDCN
7. various molecular systems
Demonstrations
Papers at the IEDM Conference in 2007 suggested for the first time that RRAM exhibits lower programming currents than PRAMPhase-change memory
Phase-change memory is a type of non-volatile computer memory. PRAMs exploit the unique behavior of chalcogenide glass. Heat produced by the passage of an electric current switches this material between two states, crystalline and amorphous...
or MRAM
MRAM
Magnetoresistive Random-Access Memory is a non-volatile computer memory technology that has been under development since the 1990s. Continued increases in density of existing memory technologies – notably flash RAM and DRAM – kept it in a niche role in the market, but its proponents...
without sacrificing programming performance, retention or endurance. On April 30, 2008 HP announced a memristor
Memristor
Memristor is a passive two-terminal electrical component envisioned by Leon Chua as a fundamental non-linear circuit element relating charge and magnetic flux linkage...
, a new circuit element that is another possible demonstration of RRAM, and on July 8 they announced they would begin prototyping RRAM using their memristors. At IEDM 2008, the highest performance RRAM technology to date was demonstrated by ITRI
Itri
Itri is a small city and comune in the central Italian region of Latium and the Province of Latina.Itri is an agricultural centre divided in two parts by a small river, the Pontone. It lies in a valley between the Monti Aurunci and the sea, not far from the Gulf of Gaeta...
, showing switching times less than 10 ns and currents less than 30 microamps. At IEDM 2010, ITRI also broke the speed record, showing <0.3 ns switching time, while also showing process and operation improvements to allow yield up to 100%.
Future applications
RRAM has the potential to become the front runner among other non-volatile memories. Compared to PRAM, RRAM operates at a faster timescale (switching time can be less than 10 ns), while compared to MRAM, it has a simpler, smaller cell structure (less than 8F² MIM stack). Compared to flash memory and racetrack memory, a lower voltage is sufficient and hence it can be used in low power applications.ITRI has recently shown that RRAM is scalable below 30 nm. The motion of oxygen atoms is a key phenomenon for oxide-based RRAM; one study has indicated that oxygen motion may take place in regions as small as 2 nm. It is believed that if a filament is responsible, it would not exhibit direct scaling with cell size. Instead, the current compliance limit (set by an outside resistor, for example) could define the current-carrying capacity of the filament.
A significant hurdle to realizing the potential of RRAM is the sneak path problem which occurs in larger passive arrays. In 2010, complementary resistive switching (CRS) was introduced as a possible solution to the interference from sneak-path currents. In the CRS approach, the information storing states are pairs of high and low resistance states (HRS/LRS and LRS/HRS) so that the overall resistance is always high, allowing for larger passive crossbar arrays.
A drawback to the initial CRS solution is the high requirement for switching endurance caused by conventional destructive readout based on current measurements. A new approach for a nondestructive readout based on capacity measurement potentially lowers the requirements for both material endurance and power consumption.