Introduction to random access memory

Random access memory (RAM) is a kind of memory that can store the data, the random means that any piece of data can be returned in a constant time, regardless of its physical location and whether or not it is related to the previous piece of data. It can be classified into two groups according to whether their data can keep when the power turn on or off. The first group is called the volatile memory, which is that the storage data will lose directly as soon as the power is turned off. The second group is called the non-volatile memory, which is that the data can retain for a long period of time without the power supply. Theses two groups are discussed as the following description.

1.1.1 Volatile memory

There are two main kinds of volatile memory. The dynamic random access memory (DRAM) and static random access memory (SRAM) separately. First, Fig.

1-1 show the basic cell of the DRAM, which is consisted of one access transistor and one capacitor (1T1R) per bit. Although its advantage is for the high density (6-12F²), but the leakage current from the capacitor makes the data lose within milliseconds, so it is needed to refresh the capacitor to retain the date. Next, the Fig. 1-2 shows the SRAM, it is not like the DRAM, it dose not need to periodically refresh the data for its combination is six transistor, and its merits is for the high speed write/erase than

the DRAM, but its disadvantage is for its bigger size (50-80F²).

1.1.2 Non-volatile memory

Due to the popularity of the 3C products, like cell phone, mp3 player, and pc parts, the requirement of the non-volatile memory is increased dramatically in recent years. The ideal specs for the non-volatile memory is consisted with low operation voltage, low power consumption, high operation speed, high endurance, long retention time, nondestructive readout, simple structure, small size, low cost, etc[ ].

But the ideal non-volatile memory that suit for all the properties above is still not in this word for the real commercial products.

The mainstream non-volatile memory nowadays is flash, included NOR flash and NAND flash. The NOR flash has high operation speed, and the NAND flash has higher density for the larger data storage. A primary structure of flash memory is MOSFET-like transistor with a floating gate, which was invented by Sze, which is shown in Fig. 1-3. The charge stored in the floating gate can affect the threshold voltage, so we can use the different Vth to operate the logic of high or low.

However, the flash has some issues like high operation voltage, low operation speed, poor retention time, and coupling interface effect during scaling down [83].

Hence, some modified flash memory, such as charge-trapping flash (SONOS) is going to replace traditional flash. But researcher still want to find the next-generation non-volatile memory, it can combined with the advantages of DRAM, SRAM, Flash with the high write/erase speed. There are four possible candidates, including ferroelectric random access memory (FeRAM) [84v], magnetroresistive random

access memory (MRAM) [80], phase change random access memory (PCRAM) or ovonic unified memory (OUM) [76], and resistive random access memory, which are discussed as following.

1.1.3 Next-generation non-volatile memory

1.1.3.1 FeRAM

Ferroelectric random access memory material can be applied with a electric field to change it polarization, and its structure is the ABO3 as shown in Fig. 1-3. The A, B, and O atoms are located at corner, body center, and face center of the cubic respectively. When a electric field is applied to the atom, the atom location will be decided by the polarity of the electrical field. The polarization hysteresis curve is shown in the Fig. 1-4. FeRAM can be subdivided into two groups; the first is called the metal-ferroelectric-semiconductor (MFSFET) as shown in Fig. 1-5. The structure is similar to the MOSFET except the oxide film is replaced by the ferroelectric film.

The polarization (+Pr or -Pr) of the ferroelectric film will affect the drain current, hence it is non-volatile with nondestructive readout property. The second type is the DRAM-like (1T1C) similar with the Fig. 1-1, where the dielectric of the capacitor is replaced by the ferroelectric film, and this kinds of DRAM-like is destructive readout and need to re-write process.

1.1.3.2 MRAM

The basic cell of magnetroresistive random access memory is the magnetic junction that consist of one thin tunneling layer in the middle of two magnetic material layers as in Fig. 1-6 The magnetization of one magnetic layer (reference

layer) is fixed and kept in a specific direction. The other layer can be switched to parallel or anti-parallel to the reference layer by applying a specific magnetic field..

The logic of high or low is determined by the resistance of the parallel or anti-parallel.

To read the state, a small current go through the tunneling layer and to detect the resistance. The issue of MRAM is that its scaling ability and it limit its further development.

1.1.3.3 PCRAM (OUM)

Phase change memory or called ovonic unified memory (OUM) is a promising technology for the ideal non-volatile memory. Its structure is shown in Fig. 1-7 [59], where the GeSbTe (GST) chalcogenide alloy material is an important material for PCRAM. The logic of high or low is determined by the phase of the amorphous or polycrystalline. In the off process, a high magnitude pulse with a short tailing edge is applied on the programmable phase change material. The temperature will exceed the melting point to eliminate the polycrystalline phase, and the device is cooled to freeze to the amorphous structure. In on process, a moderate magnitude pulse with sufficient duration is applied to ensure the phase has time for crystal growth. To read it, a low magnitude with long duration time is applied to read the amorphous (off-high) or polycrystalline (on-state). The issue of the PCRAM is that its consume high power during its switching for the high temperature it used.

1.1.3.4 RRAM

Resistive random access memory (RRAM) is another candidate for the next generation nonvolatile memory devices. In this thesis, the discussion is about RRAM,

so the complete introduction on RRAM is in the next section.

1.2 RRAM

Resistive random access memory can change its state by the electrical field or current effect, the conductive of the RRAM can be switched between on-state or off-state. As in Fig. 1-8 shows that on-state is the high conductive or low resistance;

and the off-state is the low conductive or high resistance. The strengths of RRAM are the high cell density array, high operation speed, low power consumption, high endurance, simple structure, lower scale limit, 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.

1.2.1 Structure

1.2.1.1 Basic structure

The basic structure of RRAM is made up of metal-insulator-metal (MIM) in Fig.

1-8 [28]. The top and bottom electrodes could be used by the metal or conducting transition metal oxides, the difference of the material is according to the crystalline, work functions and the ability of oxygen absorption. In the other hand, the adhesion layer should be also considered as well, and the main character of resistive switching is determined by the insulator layer that sandwiched between the electrodes.

1.2.1.2 1D1R and 1T1R

The advance structure is 1D1R (a diode and a resistor) in Fig. 1-9 [82] or 1T1R

(a transistor and a resistor) structures in Fig. 1-10 [86]. These structures must be used to prevent misreading as shown in Fig. 1-11. I. G. Baek et al. [28] shows that if a cell is at off-state and its neighboring cells are at on-state, it will be misread as on-state because of the leakage current path around its neighboring cells. So the reading value is not the correct logic we want to know. 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.

1.2.2 Fabrication method

The insulator layer is called “resistance switching layer” in the following sections, and the deposition methods are many kinds, 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 1-1. The RF magnetron 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 is suitable for high reactive metal like Ni, Ti or Cu to form metal oxides and the low process cost; 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 methods has related with the resistive switching characteristics for the different quality of the deposition layer.

1.2.3 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.

1.2.3.1 Binary material

The binary oxides adopted in RRAM application, such as TiO2 [1-16], , NiO [27-38], Al2O3 [4,65], CuxO [39-43], Fe2O3 [44], ZnO [45,46], HfO2 [47], SiO2 [48,49]

and MoOx [50]. In this thesis, the ZrO2 [17-26] is used a major material. These candidates have been widely used in other field of CMOS device, hence the compatibility with modern CMOS process would be suitable. Moreover, this material group of binary oxides has simpler element components for it is easier to control the proportion of metal and oxygen elements.

1.2.3.2 Other materials

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 [51]. Many BSZT in RRAM are doped with V [52], Cr [66,67,70], etc. Dopants are prone to occupy sites of intrinsic oxygen vacancies, and thus restrain the formation of them [52]. Because of the more complicated chemical components, the control of the materials is not as easy as binary oxides. Besides, there are still problems in the CMOS etching process [27]. Hence, it is not the optimistic for this material to be the future RRAM material.

The manganites discussed in RRAM is the carrier-doped manganites with

perovskite structure, R1-xAxMnO3, where R and A are rare-earth and alkaline-earth ions, respectively [53-58]. They are not classified in the above perovskite system here because of their unique characteristics of conducting ferromagnets below a Curie temperature [53]. The magnitude of the manganites with perovskite structures exhibit a magnetoresistive response that is many orders of larger than that found for other materials, beside the electrical resistive switching behaviors. It is the epitaxial sample that are generally prepared by PLD [55,57] or floating-zone melt-growth method [53]

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) [59], sulfides (e.g. Cd1-xZnxS [60]), and organic materials including Rose Bengal sodium salt (RB) [61], copperphthalocyanine (CuPc) [62], 2-amino-4,5-imidazole dicarbonitrile (AIDCN) [62] 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.