Oxygen vacancies migration

The oxygen deficient region (filaments) of a film is much more conducting than the stoichiometric region irrespective of whatever the defect type is. Generally the mobility of oxygen ions in binary oxides is much higher than metal ions, and the formation of the filaments could be induced by the rejection of O^2- ions [21,31].

During the filament formation process, it is the electrical field effect that takes an essential part, because the insulating parts among the filaments are stressed by the highest electrical field accompanied with point discharge effect. As for the filament rupture, it is the current-induced effect that rules the process in which Joule heating effect resulting from the current flow through the tiny filaments generates heat and raises temperature up to hundreds of degree Celsius at the local spot [68,69], providing enough energy for oxygen ions to reflow and making the conductive regions of non-stoichiometry return to insulator to rupture the filaments. Due to the

(Von, Voff, Ron and Roff) are inevitable. Well-designed structures may improve these variations, as proposed by I. G. Baek et al. [28] and D. C. Kim et al. [30], that the size shrinking of cell area and the improvement at the interface between the electrodes and oxide film help confine the numbers and location of the filaments respectively, as shown in Fig. 1.8 and 1.9, and hence reduce the variation of switching on/off parameters.

Recently, the concept of tree-shaped filament structure has been proposed [20,23]. The breakdown paths (filaments) are formed in a dielectric when the carriers are injected from a local point at the electrode interface. The initial cross-sectional area of the path for the carrier conduction is quite narrow, but with the propagation of the filaments the cross section largely increases by an increasing number of secondary formed paths. Therefore the region near the cathode might contain filaments with a larger conductivity compared to the region near the anode [20]. To be more specific, it is very likely that the filament is thick at the cathode and thin at the anode if merely considering a single filament, which matters in terms of conductivity and has impact on Joule heating effect. It is at the thinnest part of a filament near the anode that most thermal energy accumulates and the filament formation and rupture occur.

Experiments reported by K. M. Kim et al. show that only 3 ~ 10 nm of the filaments near the anode dominates the switching [23], and the schematics are shown in Fig.

1.20. Other experiment using infrared thermal microscopy gives further evidence for this model [78]. The infrared thermal micrograph of a Cr:SrTiO3 single crytal is shown in Fig. 1.21, which depicts the confinement of the current path during 5-mA current load and a hot spot near the anode electrode, confirming that in the anode region the heat energy accumulates and the filament is probably thinner compared to that in the cathode region.

Table 1.Comparison of various memories

Table 1.2. List of preparation methods of resistance switching layer.

Figure 1.1. Typical polarization vs. voltage hysteresis of a ferroelectric material. [82]

Figure 1.2. Typical I-V curves of RRAM switching in voltage sweeping mode.

Figure 1.3. (a) Nonpolar (unipolar) switching (b) Bipolar switching. [98]

Figure 1.4. Switching dynamics monitored with programming and reading pulses.

Pulse waveforms and transition for (a) switch on and (b) switch off. [33]

Figure 1.5. Conceptual schematic of 1D1R structure. [40]

Figure 1.6. Cross-sectional schematic and basic circuit diagram of 1T1R structure.

[24]

Figure 1.7. (a) Generalized cross-point structure with memory and switching elements. (b) Reading interference without switch elements. (c) Rectified reading

operation with switch elements. [40]

Figure 1.8. (a) Threshold switching of a Pt/VO2/Pt switch element and (b) Bistable resistance switching demonstrated for a Pt/NiO/Pt memory element. [91]

Figure 1.9. Programming characteristics of combined oxide switch and oxide memory elements. [91]

Figure 1.10. (a)The Schottky effect. (b)The Frenkel-Poole effect. [30]

Figure 1.12. Band diagram with the conditions of interface states for on state (LRS) and off state (HRS), respectively. [63]

Figure 1.13. Band diagram with the condition of bulk traps for on state (LRS) and off state (HRS), respectively. [63]

Figure 1.14. I-V characteristics of a Ag/La0.7Ca0.3MnO3 /Pt heterostructure (a) The set process (b) The reset process. [84]

Figure 1.15. Metal-insulator phase diagram based on the Hubbard model in the plane of U/t and filling n. Two routes for the MIT (metal-insulator transition) are shown: the

FC-MIT (filling-control MIT) and the BC-MIT (bandwidth-control MIT). [92]

Figure 1.16. Schematic illustration of energy levels for (a) a Mott-Hubbard insulator and (b) a charge-transfer insulator generated by the d-site interaction effect. [92]

Figure 1.17.The interface Mott transition, Schematic steps of the unipolar resistive switching process. [95]

Figure 1.18. The set and reset process of the Solid-state electrolyte (SSE). The green ball represent Ag⁺ ion, the grey ball represent Ag atom. The red arrow represent the direct of the ion.

Figure 1.19. Schematic pictures of high- and low-resistive state of the specimen. [28]

Figure 1.20. Schematic diagram for switching from on state to off state. (a) On state formed by positive bias on TE. [(b)-(e)] Nucleation and propagation of a filament

when negative bias is applied on TE. (f) Off state is attained. [23]

Figure 1.21. Infrared thermal micrograph of the memory cell with a current of 5 mA at a voltage of ~30 V applied. In the color scale, blue and red represent room and

elevated temperatures, respectively. [78]

Chapter 2

Experiment details

2.1 Sample fabrication

The structure of the samples studied in this study is depicted in Fig. 2.1. After standard RCA clean procedure, a 100-nm-thick SiO2 was grown on 4-inch Si. The bottom electrode, a 20-nm Ti followed by an 80-nm Pt, was deposited by e-beam evaporation on SiO2 at room temperature. Then, the resistance switching layer was prepared with 2 steps. First a 300nm thick Cu layer was deposited on Pt/TiSiO2/Si substrates by DC sputtering. Subsequently, the CuO film was grown by thermal oxidation of Cu layer. Finally, top electrodes were deposited also by e-beam evaporation and patterned by shadow masks. The process flow is depicted in Fig. 2.2, and all the details in each process are stated below.

2.1.1 Standard RCA clean

The bare Si wafers must be cleaned before further process. The RCA clean is the industry standard for removing contaminants from wafers, and the main steps are narrated as the following. The wafers were submerged in Caro’s acid (also called SPM), a solution of 3:1 H2SO4:H2O2, for 10 min at around 80^oC to remove organic contaminants from the surface of wafer, then in 1:100 HF:H2O (diluted HF, DHF) to etch chemical oxide produced in the previous step. The following steps were standard clean 1 (SC1) and standard clean 2 (SC2), in which the wafers were soaked in a solution with 1:4:20 NH4OH:H2O2:H2O and 1:1:6 HCl:H2O2:H2O, respectively, both

for 10 min at around 80^oC to eliminate particles and metallic contamination. Finally, the wafers were dipped in DHF again to remove the chemical oxide grown in the previous standard clean steps. It should be noted that each step was separated by DI water rinse for 5 min.

The mechanism for Caro’s acid to remove organic contaminants comes from its strong capability of dehydrating and oxidizing organic compound containing C-H bonding. As for SC1, the surface of Si wafer is oxidized by H2O2 into a thin SiO2

layer, the footing layer for particles and in the meantime etched by NH4OH. The particles attached on the surface would be removed as SiO2 layer on the surface finally vanishes. The following SC2 step further takes IA and IIA metal ions away, in which chlorides resulting from the combination of IA, IIA and Cl^- would dissolve in water and thus the metal ions are eliminated. On the other hand, IIIA metal ions such as Al would be reactive with NH4OH in SC1 and generates AlOH3 which later forms chlorides in SC2. Hence, to remove particles and metal ions effectively, the sequential order of SC1 and SC2 cannot be reversed.

2.1.2 Growth of SiO2

After RCA clean, 4-in-boron-doped Si wafers were sent into a furnace immediately for thermal oxidation, in which SiO2 was grown at 950^oC for 30 min in O2 and H2 atmosphere. The SiO2 layer playing a role of insulating layer avoiding current leakage from the Si substrate was expected to be 200 nm in thickness.

According to different mechanisms, there are two types of thermal oxidation, dry and wet oxidation. The former is oxidized in O2 ambient, in which oxygen ions are combined with Si atoms to form SiO2. The diffusion barrier for oxygen ions moving

through the existed SiO2 toward Si is getting larger as SiO2 is becoming thicker, and thus dry oxidation is not suitable for the growth of thick SiO2. For the 200-nm SiO2

required in this study wet oxidation was adopted to serving an insulated layer in order to avoid current leakage from the substrate.

2.1.3 Deposition of bottom electrode

The bottom electrode was deposited by e-beam dual gun evaporator (ULVAC EBX-10C). First, a 20-nm Ti was deposited on SiO2 to serve as an adhesion layer between SiO2 and the upper 80-nm Pt layer subsequently in situ deposited to ensure excellent conductivity along Pt/Ti dual layer.

E-beam evaporator uses an electron beam to heat a metal crucible and transform solid metal pellets into the vapor of metal atoms which finally reaches the wafers and forms continuous metal film on the surface. The deposition thickness is in situ monitored by the crystal sensor embedded in the evaporator system. During deposition, metal film is also deposited on the crystal sensor at the same time. Thicker film reduces the crystal oscillating frequency which can be detected by the system and the thickness of the deposited film can be calculated in real time.

2.1.4 Preparation of resistive switching layer

Two steps of resistive switching layer were fabricated in this thesis. The first step was a deposition of Cu film by sputtering a metal layer, and the second step was thermal oxidation Cu film form CuO insulator film.

In the first step, 300nm thick Cu film was deposited by DC physical vapor sputter system on Pt/Ti/SiO2/Si. The purity of the Cu metal target is 99.99%. During

film deposition, the respective parameters of the working pressure, substrate temperature, RF power, and gas flow, were 7.6*10^-3Torr, 25^oC, 190 W (corresponding deposition rates: 0.1nm/s), and Ar with a total flow rate of 24 sccm. To avoid oxidation with air form more the native oxide of the Cu, the device be reserved at vacuum box.

In the second step, the purpose of the step is oxidation Cu film, which is first step deposition process. Different thermal oxidation temperature and oxidation time were evaluated to oxidize the Cu films. Several process parameters such as oxidation time (15, 30, 60 min), oxidation temperature (300, 400 and 500 °C) and constant oxygen partial pressure (60sccm) were tested. In oxidation processing, we put device into thermal furnace at temperature, then the oxygen gas flow was controlled a flow meter at 60cc/min steady. The detail oxidation processing can be distinguished into three processing. In processing 1, this processing is rising temperature processing. The device was heated to oxidation temperature by 10^oC per minute. In processing 2, this processing is maintenance temperature processing. There are three oxidation temperatures (300, 400 and 500 °C) and three oxidation time, totally have nine oxidation condition had be tested. In processing 3, this processing is lowing temperature processing. We adopt nature cool down to avoid thermal stress, cause damage at device. The device was toke out from furnace at room temperature.

2.1.5 Deposition of top electrodes

After the fabrication of resistance switching layer, the top electrodes were prepared to form the structure of metal/resistive switching layer/metal. For the devices, the electrodes of Ti or Pt were deposited by the same e-beam evaporator, in which the

2.1.5.1 Shadow mask

The top electrodes mentioned above are patterned by the shadow mask having the dot-shaped holes with diameters of 350, 250 and 150 μm, i.e. areas of 9.26x10^-4, 4.91x10^-4 and 1.77x10^-4 cm², respectively.

2.1.5.2 Lithography

For the purpose of minus the size of the top electrode, it is used photo resistance to be the mask. The characteristic of this sample is used 60mins and 200^oC. First, the positive photo resistance AZ4620 is used on the Photo Resist Spinner, the 1200rps for 5 seconds and 4000rps for 25 seconds, After that, the sample is put on the hot plate at 90^oC and 5mins for soft bake. And then the mask that having the 10 μm x 10μm square is exposed using the intense light for 45 seconds, this step is that the pattern can be print on the ZrO2 layer. Finally, the sample is put in the AZ300 for 200 seconds to develop the pattern, and then rinse into the DI-water for 1 min.

The sample that has pattern is also deposited by Ti using the e-beam evaporation for 150nm, and then put it in the acetone for 1 min and rinse in the DI-water for 1 min.

The purpose of this step is to lift-off for removing the photo resistance, so the top electrode is Ti can be sustained on the ZrO2 layer, and the top electrode is 10 μm x 10μm square.

2.2 Resistive switching layer growing equipments

The fabrication of resistive switching layer used two main equipments. One is

DC sputter systems are widely adopted in film deposition due to the simple equipment structure, low cost and the capability of depositing all kinds of films;

another is thermal oxidation furnace, which is heating device to oxidation. The DC sputter system and thermal oxidation furnace are detailed introduced following.

2.2.1 DC sputter system

The DC sputter mainly used to deposit metal thin film. We used Cu metal target (99.99%). The sputter system configuration is shown in Fig. 2.3, and can be class to vacuum system, pressure monitor system, gas flow control system, plasma control system. The follow simple describe the function of the each system.

For vacuum system, this system can become high vacuum in chamber. The impurities and particles will be draw out of the chamber.

For gas flow control system, Mass flow controller (MFC) is used to control the gas flow delivered into the chamber and accurately controls the ratio among different gas ingredients.

For plasma control system, this system is used to generate plasma and control the plasma power. The plasma system is of capacitor structure. This is the simplest configuration that under low pressure condition the plasma density would be too low and the number of ions (Ar⁺) to bombard the target is not enough for film deposition.

It is the magnetron sputter that induces higher plasma density in low pressure situation which is better for deposited film quality. The magnetic field near the target would make electrons to move in a gyro motion, which have a longer path and therefore greater chance for the electrons to collide with other species and induce the plasma of

2.2.2 Thermal oxidation furnace

Thermal oxidation is accomplished using an oxidation furnace (or diffusion furnace, since oxidation is basically a diffusion process involving oxidant species), which provides the heat needed to elevate the oxidizing ambient temperature. A furnace typically consists of: 1) a cabinet; 2) a heating system; 3) a temperature measurement and control system; 4) fused quartz process tubes where the wafers undergo oxidation; 5) a system for moving process gases into and out of the process tubes; and 6) a loading station used for loading (or unloading) wafers into (or from) the process tubes.

The heating system usually consists of several heating coils that control the temperature around the furnace tubes. The devices are placed in quartz glassware known as boats, which are supported by fused silica paddles inside the process tube. A boat can contain two devices. The oxidizing agent (oxygen or steam) then enters the process tube through its source end, subsequently diffusing to the device where the oxidation occurs.

2.3 Analyses and measurements

In this thesis, several material analyses and electrical measurements were carried out to study the relationship between material and electrical characteristics. The details are stated as the following.

2.3.1 X-ray diffraction (XRD)

X-ray crystallography is the science of determining the arrangement of atoms within a crystal from the manner in which a beam of X-rays is scattered from the

electrons within the crystal. The key step in X-ray crystallography is the diffraction of X-rays from a crystalline material. It is the elastically scattered x-ray photons that are measured in diffraction measurement, as the scattered x-rays without losing any energy carry information about the electron distribution in materials.

Generally, thin films are classified according to the crystallization. There are three types of crystallization, such as amorphous, polycrystalline, and single crystalline. X-Ray diffraction analyses are used to investigate the crystal structure and the orientation of our sample. Furthermore, the relations between the crystallization and the heat treatment can be characterized from XRD results. In the experiment, the thin films show either amorphous or poly. Follow Scherrer’s formula, we could calculate the average grain size from XRD illustration:

D λcos

β θ

= ×

Where D is the apparent crystallite size (in the present case, the local strain effect was not taken into account), β is the full width at half maximum (FWHM) of the XRD peak and θ is the diffraction angle. In these analyses, X-ray is made with 0.02 degree beam divergence and operation configuration at 30KV, 20mA.

Thin film diffraction methods are used as important process development and control tools, as hard x-rays can penetrate through the epitaxial layers and measure the properties of both the film and the substrate. There are several special considerations for using XRD to characterize thin film samples. First, reflection geometry is used for these measurements as the substrates are generally too thick for transmission. Second, high angular resolution is required because the peaks from semiconductor materials

2.3.2 Transmission electron microscopy (TEM)

Transmission electron microscopy (TEM) is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through it. An image is formed from the electrons transmitted through the specimen, magnified and focused by an objective lens and appears on an imaging screen, a fluorescent screen in most TEMs, plus a monitor, or on a layer of photographic film, or to be detected by a sensor such as a CCD camera.

In material science/metallurgy the specimens tend to be naturally resistant to vacuum, but must be prepared as a thin foil, or etched so some portion of the specimen is thin enough for the beam to penetrate. Preparation techniques to obtain an electron transparent region include ion beam milling and wedge polishing. The focused ion beam (FIB) is a relatively new technique to prepare thin samples for TEM examination from larger specimens. Because the FIB can be used to micro-machine samples very precisely, it is possible to mill very thin membranes from a specific area of a sample, such as a semiconductor or metal.

There are a number of drawbacks to the TEM technique. Many materials require extensive sample preparation to produce a sample thin enough to be electron transparent, which makes TEM analysis a relatively time consuming process with a low throughput of samples. The structure of the sample may also be changed during the preparation process. Also the field of view is relatively small, raising the possibility that the region analyzed may not be characteristic of the whole sample.

There is potential that the sample may be damaged by the electron beam, particularly in the case of biological materials.

2.3.3 Secondary ion mass spectroscopy (SIMS)

Secondary ion mass spectrometry, SIMS, is the mass spectrometry of ionized particles which are emitted when a surface, usually a solid, is bombarded by energetic primary particles which may be electrons, ions, neutrals or photons. The emitted of secondary particles will be electrons, neutral species atoms or molecules or atomic

Secondary ion mass spectrometry, SIMS, is the mass spectrometry of ionized particles which are emitted when a surface, usually a solid, is bombarded by energetic primary particles which may be electrons, ions, neutrals or photons. The emitted of secondary particles will be electrons, neutral species atoms or molecules or atomic