Unipolar multi-times-programming (MTP) RRAM

The graded WOx RRAM also shows the potential of unipolar operation. It is clear to observe the unipolar operation when the positive voltage, is applied on top electrode.

The HRS can be formed by a short pulse (~100 ns) voltage and LRS can be formed by a longer pulse (1000 ns) voltage. However, we can’t observe the unipolar operation with applied negative voltage on top of electrode. Moreover, the oxidation time influences the WOx thickness and it also influences the maximum value of HRS. For the unipolar WOx RRAM, it also exhibited good performance of reliability test: cycle endurance above 1000, over 3000 hrs at 150℃ baking, 0.4 V stress above 1000 sec and 10x on/off ratio. Both HRS and LRS show the non-linear I-V curve and they follow the VRH conduction mechanism. With the calculation of VRH equation, we got the hopping distance about 24 and 16Å of HRS and LRS, respectively.

Figure 4-50 shows the resistive switching character by unipolar operation. The resistance state increases with the applied short pulse below 100 ns. When the applied pulse is above 100 ns, the resistance state switches from HRS to LRS. This unipolar operation of our system uses different pulse width (100 ns and 1000 ns for HRS and LRS programming, respectively) and fixes the positive applied voltage about 4V. With this kind of operation, it is easy to switch the resistance state.

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Figure 4-50. The pulse width influence of resistance state.

In our experiment, the oxidation layer is formed by down-stream-plasma process. The thickness of WOx film is dependent on the oxidation time. Figure 4-51 shows the relationship between sample thickness and oxidation time. The thickness seems not linear dependent with the oxidation time. The thickness of WOx film is about 90 and 140 Å by 200 and 1600 sec oxidation process, respectively.

Figure 4-51. The WOx thickness relationship with different oxidation time.

However, the oxidation time affects only the HRS. Figure 4-52 shows the unipolar resistive switching character with different oxidation time (OT). It is clear to see that a sample with longer oxidation time exhibits higher resistance state. This HRS can be

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formed by a short pulse about 50~300 ns. Moreover, all the LRS keeps about 300 Ω by 1000 ns pulse applied. However, the resistive switching character will be damaged when the applied pulse width is over 1000ns.

Figure 4-52. The unipolar operation character with different oxidation time.

The unipolar operation can’t work out with a negative pulse applied. Figure 4-53 shows the resistance change by using a negative pulse. We can observe that the resistance state decreases when the applied negative voltage is above -3 V. Both short and long pulse can only reduce the resistance state. This result indicates the direction of our WOx unipolar RRAM.

Figure 4-53. The resistance change by using negative voltage with varies pulse width

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The unipolar operation RRAM also exhibits good cycle endurance performance.

Figure 4-54 shows above 1000 times cycle operation. The on/off ratio is about 170x in the first cycle operation and it keeps about 3x after 1000 times of cycle operation. This result indicates that this unipolar RRAM is promising for non-volatile memory application.

Figure 4-54. The cycle endurance test of unipolar operation

In the thermal stability experiment, the resistance states also exhibited good thermal stability by means of this unipolar operation shown in figure 4-55. Both LRS and HRS kept their resistance state above 3000 hrs at 150℃ baking environment. At higher baking environment about 250℃, those resistance states kept the resistance state above 500 hrs. Moreover, after the high temperature baking experiment at both 150℃

and 250℃, the on/off ratio kept about x10 for a long time.

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Figure 4-55. The thermal stability of unipolar operation at 150 and 250℃.

Figure 4-56 shows the stress influence on both LRS and HRS. It is clear to see that the HRS always keeps its resistance state when the stress voltage is below 0.6 V. For the LRS, it is clear to see that the resistance increases when the applied stress is above 0.6 V and the increase of resistance is linearly dependent on the stress time. This figure also indicates good stress performance at 0.2 V.

Figure 4-56. The stress influence of unipolar RRAM.

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However, the higher stress above 0.6 V affected the HRS. Figure 4-57 shows the influence of HRS with stress voltage above 0.6 V. It is clear to see that the resistance of HRS decreases when the applied stress is above 1 V. Moreover, the resistance state switches to LRS immediately when the applied voltage is about 2 V. This figure also indicates that the HRS kept its memory state over 30 min in stress voltage about 1 V.

Figure 4-57. The stress test of high resistance state

In the unipolar operation, both LRS and HRS show non-linear I-V curve. Figure 4-58 shows the electric character of those states and the inset shows both I-V curves at the same current scale. Both LRS and HRS follows the VRH conduction mechanism in the unipolar operation. Figure 4-59 shows the temperature dependent on the resistance state. With the calculation of the VRH fitting curve, the hopping distance of LRS and HRS is about 16 and 24Å , respectively.

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Figure 4-58. The electric character of LRS and HRS.

Figure 4-59. The VRH fitting curve of unipolar operation RRAM.

Figure 4-60 shows the change of the hopping distance in the cycle operation. The hopping distance of both states almost overlap after 3000 cycling operation. However, figure 4-54 doesn’t show the overlap of both resistance states after 3000 times of cycling operation.

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Figure 4-60. The hopping distance varies in the cycle endurance test.