Preparation

2.2.4 Gas flow controlling system:

In general, the percentage of oxygen in the sputtering atmosphere played an important role in oxide ceramics. We used Gas MFC (mass flow meter) to control the flow rate and atmosphere contents during the sputtering process.

So we could find out that the dependence of the mass ratio for the device performance by tuning recipe

2.2.5 Plasma controlling system:

This system consists of a RF power generator, a network-matching box, and a 3-inch magnetron gun. The RF power generator has only one working frequency 13.56MHz, and the network-matching box could minimum reflection power by adjusting the capacitance of the whole circuit. We were able to gain the stable plasma by the controlling system.

2.2.6 Cooling system:

There was cooling water which flows in the pipe welded on the chamber and in the magnetron gun. During the sputtering process, the heating lamps and plasma always produced a lot of redundant heat energy in the chamber.

We needed cooling water to prevent from mechanical breakdown and maintain the sample uniformity.

2.3 Preparation of Devices

In the experiment, we fabricate our samples into a simple MIM

(Metal-Insulator-Metal) structure. After RCA clean, an oxide layer with thickness of 200nm was grown on (100) silicon wafer to prevent the leakage current from the substrate. The LNO bottom electrode with thickness of 150nm was deposited on the oxide by sputtering. Then, the doped SZO films, which have the properties of the resistive switching, were deposited on the LNO bottom electrode.

2.3.1 Preparation of Sputtering Targets

Because the LNO and SZO thin films were deposited by sputtering, we needed two kinds of disk-shaped sputter targets, including the LNO and the doped SZO powder targets.

(1) Synthesis of the LNO powder target

The LNO and SZO targets were prepared by the conventional solid-state powder-mixing method. There were six steps in the synthesis processes. First, two kinds of oxide powders, La2O3 and NiO, were mixed by the rule of stoichiometry. We were especially careful of the equivalent mole because 1 mole of LNO was composed of 0.5 of mole La2O3 and 1 mole of NiO. Second, the mixed powder was put into a jar with anhydrous alcohol and rolling glass balls, and then was mixed adequately by a grinder. Third, the mixture was dried by 80℃ oven. The fourth step was the sintering step. It is the most critical process, because the sintering temperature and the heating time would affect on the LNO qualities including the resistance and orientation of the LNO sputtered films. We put the dried mixture in a furnace to execute a sequence of sintering: 600℃ (4 hours)Æ1300℃ (10 hours).In the fifth step, the mixed powder was put in the beaker and baked it in the oven at 150℃ for 2 hours.

Finally, the mixed powder put in the disk-shaped target was squeezed by a high pressure of 2000 pounds for 60 seconds such that we could produce a

compact target for sputtering work. The preparation flow of the LNO target is showed in Fig. 2-5.

(2) Synthesis of the doped SZO powder

We synthesized the SZO powder from two kinds of oxide powder, SrCO3

and ZrO2.In order to substitute Zr atom, we must consider the suitable ionic radius compared with Zr atom. Considering all the conditions, transition metal oxide V2O5 was added to form the doped SZO powder. Because V has freaky oxidation number, it could show more effect on the electric properties of our memory thin films. For example, when we wanted to synthesize 0.3％V doped SZO powder, we should use 1 mole SrCO3, 0.997 mole ZrO2, and 0.0015 mole V2O5. After mixing above elements of the doped SZO powder, we followed the same steps as synthesis of LNO powder. The mixed powder was put into a furnace of a sequence of sintering process. In the last step, a disk-shaped target was made by a high pressure of 2000 pounds for 1min.

The manufacturing process is showed in Fig. 2-6

2.3.2 Thin Films Depositions

The LNO bottom electrode and the doped SZO films were deposited by RF magnetron sputter sequentially. To meet our demands for different process recipe, we could control several parameters to deposit the films based on the plasma theorem and the models of the thin film growth. There are many parameters including the chamber pressure, the RF power, the working temperature, the ambient conditions, and the deposition time. In general, chamber pressure affected the Mean Free Path (MFP) of plasma which is relative to the deposition rate. The lower pressure we choice, the larger MFP we create in the chamber, which leads to the higher deposition rate. Moreover, the deposition rate is dependent on the RF power as well. In the experiment,

when depositing both the LNO and the doped SZO thin films, we set the RF power 150W and the chamber pressure in 10 mtorr. In addition, the temperature and the ambient condition could have influence on the density of the defects, the crystallization, the conductivity, the stoichiometry, and the dielectric constant of thin films.

For the accuracy of the atmosphere, we need the base pressure about 3 x 10^-6 before sputtering. Next, to get the ambient condition, we control the flow rate of Ar and O2 by MFC, and the working pressure is kept by the valves among low pressure where the plasma is generated.

z The heat Treatment for Thin films after deposition

There are two purposes for our experiment to using Rapid Thermal Annealing systems. One was that we can get stronger crystallization orientation or better conductivity of the LNO bottom electrode. The other was that we could control the properties of our sample by changing RTA atmosphere (O2 or N2) or the heating profile of RTA temperature. The RTA model is FE-004A made by JETFIRST.

z Deposition of the Top Electrode

Before the Al top electrodes were on the doped SZO films, the sample had been adhered to a metal mask. The metal mask had different hole with three kinds of diameters that are 150μm, 250μm, and 350μm. So we could define different area for the top electrode, which are 1.767×10^-4 cm2, 4.908×10^-4 cm², 9.612×10^-4 cm².

Aluminum (Al) was used as the top electrode which was deposition by a thermal evaporation coater (EBX-6D) manufactured by ULVAC. We loaded our samples with metal masks on the spinning holder, which made the

deposition rate more uniform. Then, the rough pump and the turbo pump would work in term in order that the base pressure before deposition reached 5×10-6 torr.

2.4 Measurements and Analysis