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.