Effect of Bottom Electrode Roughness on Electrical Properties of Metal/Insulator/Metal (MIM) Devices

Mohamed A. Mohamed, Professor Qiangfei Xia, Peng Lin (PhD)University of Massachusetts Amherst

1. Motivation

• Metal/Insulator/Metal is a common structure for modern thin film devices.

• Previous research has looked into the effect of the shape of the bottom electrode on the electrical properties on the entire device.

• Recent research has compared structure of a flat electrode with a pyramid-shaped electrode and have shown interesting results with improvements in set/reset voltages and endurance of the device.

• The flat electrode had a set/reset voltage of -1.5/0.8 V with 400 cycles while the pyramid-shaped electrode had a set/reset voltage of -1/0.6 V with 2400 cycles.

2. Method

Evaporation angles of 0°, 35°, and 75° were used to deposit the bottom electrode using E-beam evaporation. Table 1 shows the parameters used and Figure 2 shows the conditions of evaporation.

Angles Base Pressure(Torr)

Distance to Source (m)

Thickness (Pt/Cr nm)

Deposition Rate (Å/s)

0° 2.5*10-6 0.5 22.86 0.4

35° 2.6*10-6 0.5 19.65 0.5

75° 2.5*10-60.5 47.32 1

Table 1: Parameters and thickness measurements after evaporation

0°35°

75°

Fig. 2. Evaporation of three different angles.

Substrate

Vapor Flow

Source

Pt (20 nm)Cr (2nm)

Si/SiO2

HfO2(5nm)

Ta (50nm)

Pd (40 nm)

Fig. 3. Device structure of the MIM device consisting of the following layers: Pd/Ta/PMMA/HfO2/Pt/Cr/SiO2/Si.

4. Analysis

The surface roughness of the bottom electrode was calculated using the atomic force microscope. The surface becomes rougher as the angle increases as shown in figure 4. Figure 5 shows the switching behavior for the three different angles, where we can observe a decrease in switching current and a decrease in set/reset voltage as the angle of evaporation is increased.

(a) (b) (c)

Fig. 4. AFM roughness profiles using atomic force microscope for (a) 0°, (b) 35°, and (c) 75°

3. Device Structure

The device structure consists of Platinum as the top electrode, HfO2as the switching layer using Atomic Layer Deposition, and Tantalum as the top electrode using DC sputtering.

(a) (b) (c)

PMMA (200nm)

Fig. 5. Switching behavior IV curves for (a) 0°, (b) 35°, and (c) 75°

5. Conclusion

By looking at table 2, we can see a decrease in set/reset voltages and a decrease in switch current as well. However, many of the 35° and 75° devices were unstable and had a low yield; but, 35° devices performed slightly better. Findings so far do not necessarily state a direct relationship between roughness and electrical properties and more testing has to be done.

Angle Set/Reset Voltage (V) Switch Current

0° 1.2/-1.5 10mA

35° 1.0/-1.2 2mA

75° 0.8/-1.4 0.1 mA

6. Acknowledgements

This project was funded by the National Science Foundation. Award number 1253073

7. References

[1] Xia, Qiangfei, J. Joshua Yang, Wei Wu, Xuema Li, and R. Stanley Williams. "Self-Aligned Memristor Cross-Point Arrays Fabricated with One Nanoimprint Lithography Step." Nano Letters Nano Lett. 10.8 (2010)

[2] Huang, Yu-Chih, Wan-Lin Tsai, Chia-Hsin Chou, Chung-Yun Wan, Ching Hsiao, and Huang-Chung Cheng. "High-Performance Programmable Metallization Cell Memory With the Pyramid-Structured Electrode." IEEE Electron Device Lett. IEEE Electron Device Letters 34.10 (2013)

Table 2: Set/Reset Voltages and switch current values for the three angles showing a steady decrease.

UMASS REU Poster Session