Resistive random access memory

RRAMs make good use of the resistance changes as different memory states. By electrical field or current effects, the conductivity of the memory layer can be switched between high and low resistance reproducibly. The different resistance states stands for different digital states as a memory device. The strengths of RRAM are the high cell density array, high operation speed, low power consumption, high endurance and lower scale limit. Furthermore, RRAMs have the features of nonvolatility, long retention time, and non-destructive readout. In this section, the properties are discussed in view of the structure, fabrication, material classification, operation and circuit realization. By Table 1.1, RRAM have great potential for replacing the flash memory and will become mainstream memory in the future.

1.2.1 Structure and fabrication

The basic structure for RRAM is made up of only metal-insulator-metal, MIM, which can be further integrated into 1D1R (a diode and a resistor), 1T1R (a transistor and a resistor) structures (discussed later), or 1S1R (a switching and a resistor). The top and bottom electrodes could be metals or conducting transition metal oxides, the choice of which has impacts on the resistive switching properties because of their different crystallinities, work functions and the ability of Gibbs free energy. The adhesion and among layers should be considered as well. If high temperature process is needed, the thermal stress problem should be considered. The main character of resistive switching is the insulator layer sandwiched between the electrodes. The insulator for the MIM structure actually may be not really insulating, but also semiconducting, depending on the constitution and stoichiometry. As a result, the

insulator would be called “resistance switching layer” in the following sections. Usual deposition method of resistance switching layer are many and various, including radio-frequency (RF) magnetron sputtering, reactive sputtering, e-beam evaporation, spin coating (sol-gel), thermal oxidation, metal-organic chemical vapor deposition (MOCVD), pulsed laser deposition (PLD), atomic layer deposition (ALD), plasma-enhanced atomic layer deposition (PEALD), and melt-grown by FZ method, as listed in Table 1.2. Among them, the sputtering has lower cost and wide application but poor film uniformity; e-beam evaporation and spin coating has low process cost but poor film quality as well; thermal oxidation are suitable for high reactive metal like Ni, Ti or Cu to form metal oxides and inexpensive; MOCVD, PLD, ALD, and PEALD are able to produce high quality film with good step coverage but expensive;

the FZ method is able to fabricate perfect crystals with exact component proportion but not practical in semiconductor fabrication process. The different quality deposited by different methods has connection with the resistive switching characteristics.

1.2.2 Material classification

The resistive switching phenomena have been found in many materials. The research mainstream is focused on several groups, including binary oxides, perovskite oxides, manganites, and other alloy or polymers.

The binary oxides adopted in RRAM application, such as CuxO [1-16], TiO2

[17-32], NiO [23-53], ZrO2 [17-26] Al2O3 [66-68], HfO2 [69,70], Fe2O3 [71], ZnO [72,73] and MoOx [74], are candidates or have been widely used in other field of CMOS devices. Thus the compatibility with modern CMOS process would not be a problem. Moreover, this material group of binary oxides has simpler element

Another extensively studied material group is (Ba,Sr)(Zr,Ti)O3, BSZT. It has been studied as a role of the high-k dielectric for a long time [75]. Many BSZT in RRAM are doped with V [76], Cr [77,78], etc. Dopants are prone to occupy sites of intrinsic oxygen vacancies, and thus restrain the formation of them [76]. Because of the more components and the more complicated chemical environment, the control of the component proportion is not as easy as that of binary oxides.

The manganites discussed in RRAM usually represent the carrier-doped manganites with perovskite structure, R1-xAxMnO3, where R and A are rare-earth and alkaline-earth ions, respectively [79-84]. They are not classified in the above perovskite system here because of their unique characteristics of conducting ferromagnets below a Curie temperature [79]. Manganites with perovskite structures exhibit a magnetoresistive response that is many orders of magnitude larger than that found for other materials, beside the electrical resistive switching behaviors. It is the epitaxial samples that are generally prepared by PLD [81,83] or floating-zone melt-growth method [79] to obtain the precise element proportion and physical properties. For the same reason of perovskite oxides, the future for manganites in RRAM is not so promising.

The other materials such as chalcogenide (GeSbTe) [85], sulfides (e.g. Cd1-xZnxS [86]), and organic materials including Rose Bengal sodium salt (RB) [87], copperphthalocyanine (CuPc) [88], 2-amino-4,5-imidazole dicarbonitrile (AIDCN) [88] and so on, have been investigated for RRAM application. The chalcogenide material has been drawing many attentions recently due to Intel’s support, while the others are newly introduced to semiconductor processes. Besides, many organic polymers tend to degrade easily. Chalcogenide seems a more practical candidate in

this group of materials.

1.2.3 Operation and circuit realization

Basic operation of bistable resistive switching in a single cell can be achieved by DC sweep or pulse switching methods. Fig. 1.2 shows a typical I-V plot under DC sweep operation. Assuming the resistance state is first held in off state (high resistance state), the current suddenly increases as the DC bias sweeps toward positive direction and on state (low resistance state) is reached, which is defined as a process of “switch on” or “set” as indicated in the figure. The voltage where the current suddenly increase is the switch-on voltage. Then a negative voltage bias is applied to switch back to off state with a substantially current drop at the switch-off voltage, as indicated by the “reset” or “switch off” in the figure. It should be noted that this operation requiring different voltage polarities to switch on and off, whether positive on/negative off or positive off/negative on, is called bipolar operation. As for the unipolar operation, either polarity can be applied to switch on or off depending on the present memory state. On the basis of I–V characteristics, the switching behaviors can be classified into two types: unipolar (nonpolar) and bipolar, for which typical I–V curves are shown in Figs. 1.3a and 1.3b, respectively For the data reading operation, the bias should not exceed the range indicated as “read” in the figure to prevent memory state modification. Fig. 1.2 does not show the forming process required to initiate the resistive switching properties of as-deposited oxide films. The forming process is similar to soft oxide breakdown, leading to the conducting paths (filaments) composed of clusters of point defects. To unify and clarify the terms of operation parameters in the following text, the “switch on” and “switch off” would be used to describe the switching operation instead of “set” and “reset”; “Vset”, “Vreset”, “Ron”

and “off-state resistance”.

In the real circuits, it is the pulse switching that is the practical operation method for its fast operation speed and lower power consumption. The waveforms of switching on and off are shown in Fig. 1.4(a) and (b) respectively. The pulse heights and widths for switch on and off must strike a balance, in which the larger the pulse heights are, the shorter the pulse widths are needed. The reading pulses with small pulse heights are designed not to modify the memory states. This non-destructive readout property can be examined by the stress test, in which memory device samples are stressed by a small voltage bias for a long period and the details are described in chapter 2.

For the memory cell array, 1D1R, 1S1R, 1T1R structure as show in Fig. 1.5-9 must be used to prevent misreading as shown in Fig. 1.7. I. G. Baek et al. [33]

reported that if a cell is in off state and its neighboring cells are in on state, it will be misread as on state because of the leakage current path around its neighboring cells.

Therefore a rectifying element is required for each cell in an array to confine the current paths. The minimum sizes for 1D1R and 1T1R structures are 4F² and 6F² respectively, which meet the requirement for high density arrays.