Bipolar multi-times-programming (MTP) RRAM

We found the WOx film also exhibits bistable resistance switching behavior by a bipolar operation. In our experiment, the resistance state can be switched to HRS by applying a positive pulse voltage. On the other hand, the HRS can also be reserved to LRS by using a negative pulse voltage. The suitable pulse width is about 80 ns and the positive and negative voltage is +5 V and -4 V, respectively. By using this bipolar operation, it is easy to define the resistance state to LRS and HRS. Moreover, these two resistance states exhibit good performance on the reliability test, which will be

Figure 4-39. The resistance switching character by 80 ns pulse bipolar operation.

88

Figure 4-39 shows the resistance variation after an applied pulse (80ns). This WOx

film shows a typical bipolar resistive switching characteristic. With a positive applied voltage, we can clearly observe that the resistance increases when voltage is above 2.5V.

The maximum resistance is about 10⁴ Ω when the positive voltage reaches about +5V.

Moreover, the resistance drops suddenly if the applied voltage is above the Vth (+5V), and the resistance becomes more unstable than before. In the meantime, the resistive switching character disappeared because the memory cell was destroyed when the applied voltage is above the Vth.

Oppositely, the resistance decreases when we apply a negative pulse voltage. When the applied negative bias reaches -4V, the resistance is about 20 Ω. This resistance is similar to the fail point when the applied positive bias is above Vth. In our experiment, this resistive memory exhibits good performance with these two operation conductions (+5V, -4V).

Figure 4-40 shows the cycle endurance test with this bipolar operation. It is clear to see that the on/off ratio keeps at about x10 during the cycle operation process for a thousand times. Both LRS and HRS exhibit small fluctuation in the cycle process and these two states never overlap in the cycle process.

Figure 4-40. Above thousand cycle operation in the endurance test.

Figure 4-41 shows the thermal stability experiment. In this figure, it is clear to see that both LRS and HRS keep their resistance state and the on/off ratio keeps at about 100X for long time baking. Not only 150℃ but also 250℃ baking experiments indicated this WOx film has excellent thermal stability.

89

Figure 4-41. The thermal stability test at 150 (left) and 250℃ (right).

Figure 4-42 shows the influence of stress on the LRS. In this experiment, the LRS also shows good performance in the stress test. The low resistance keeps its resistance state over several thousand seconds in 200 mV stress environment. For the higher stress about 400 mV, the resistance state reduces to roughly 40% after a thousand seconds on the stress test. Moreover, in 1000 mV stress environment above 1000 sec, the resistance state still keeps at the LRS without overlapping with HRS.

Figure 4-42. The stress test for LRS.

However, the width of applied pulse shot also plays an important role in the resistive switching character. Figure 4-43 shows the pulse width effect on the resistive switching character. We change pulse width from 9 ns to 80 ns to observe the influence on resistive switching behavior. Basically, the resistance switching behaviors are similar:

first, the resistance increases and then suddenly drops after the threshold voltage (Vth).

Next, the maximum resistance of those samples is in the range about 10⁴~10⁵ Ω and all

90

the breakdown resistance is below 100 Ω. In this figure, it is clear to observe that the Vth

increases when we reduce the pulse width of applied voltage. The relationship of Vth

and pulse width are also shown in figure 4-44.

0 2 4 6 8 10 12 14

Figure 4-43. The resistive variation by applied bias with different pulse width.

0 20 40 60 80

Figure 4-44. The relation between pulse width and threshold voltage.

The WOx RRAM also shows the possibility of the multi-bits programmable. Figure 4-45 shows above 100 cycle endurance of 2 bits/cell operation. By fixing the pulse width (80ns) and changing applied voltage (from -4 to 5 V), it is easy to form four different resistance states. These four resistance states remain after 100 cycle operation.

This result indicates that the WOx RRAM is promising for multi-level-cell (MLC) application.

91

1 10 100

10² 10³ 10⁴

Resistance ()

Cycle (times)

"00" state

"01" state

"10" state

"11" state

Figure 4-45. The endurance test of 2 bits/cell bipolar operation memory.

Figure 4-46 shows the electric character of LRS and HRS. This figure indicates the linear I-V curve of LRS. This metallic state is close to the minimum-metal-conductivity (MMC). However, the non-linear I-V curve of HRS follows the VRH conduction mechanism.

Figure 4-46. The I-V curve of LRS and HRS states.

Figure 4-47 shows the temperature effect on the HRS with current close to zero.

Moreover, the good hyperbolic-sine fitting of the I-V curve (the inset of figure 4-47) further strengthens the VRH conduction mechanism. With the calculation of hyperbolic-sine fitting, we got the hopping distance about 15Å .

92

Figure 4-47. The temperature dependent electrical character and VRH fitting curve (inner) for high resistance state.

Figure 4-48 shows the temperature effect on LRS. The measurement range of temperature effect changes from room-temperature to 4K. It is clear to see that the resistance decreases when the temperature decreases. The temperature effect shows the same behavior both in low and high temperatures. The inset shows the resistance behavior which change from room-temperature to high temperature. These results indicate that the LRS follows the metal conduction mechanism.

Figure 4-48. The temperature dependent electrical character for low resistance state.

Figure 4-49 shows the temperature effect on both HRS and LRS. The temperature dependent curve of HRS is different from LRS. It is clear to see that the resistance decreases when the temperature is increased. This result indicates the semiconductor behavior on HRS.

93

Figure 4-49. The temperature dependent electrical character of WOx film.