One-time-programming (OTP) RRAM

For the first tuning of the electrical character of our WOx device, we tried several pulse conductions such as the combinations of pulse width and pulse voltage. We found the WOx can be used for the one time programming (OTP) application. In our experiment, we found that there are two ways to be used in this OTP application. One is to use several pulse shots with constant voltage (5 V, 70 ns) to form the different resistance states. The resistance state is dependent on the numbers of pulse shot. Figure 4-31 shows the relation between the resistance state and the numbers of applied pulse shots. For example, the initial resistance state of our fresh sample is about 1 kΩ and this resistance state is defined as “01” state. However, the resistance state is reversed from this “01” state by one pulse shot and this reversed resistance state (“00” state ~ 20 kΩ) keeps about the same one if the number of shots is below Nc1 (~60). When the number of shots is above Nc1, the resistance decreases from 20 kΩ to 100 Ω (“10” state) and it keeps at the same one if the number of shots is below Nc2 (~120). Moreover, the lowest resistance of this WOx film is about 30 Ω (“11” state) when the number of shots is above Nc2. With this kind of operation, we can easily program this two bits per cell (2 bits/cell) OTP resistance switching memory device.

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Figure 4-31. The resistance state verse the number of pulse shots.

However, these resistance states are very sensitive with the pulse width. Figure 4-32 shows the influence of the different pulse width on resistance switching character.

For 100 ns pulse, the HRS (“00” state) was formed by only one shot. And the total number of shots at other two LRS (“10” and “11” states) is smaller than the number of shots by 70 ns pulse width. This result indicates the pulse width dependence relationship on the WOx film.

Figure 4-32. The resistance switching by 70 ns and 100 ns pulse width.

Another way to be used for the OTP operation is to change the pulse width with a fixed applied voltage. Figure 4-33 shows the relation between resistance state and the pulse width, with fixed the voltage at about 1.5V. The HRS (“00” state) can be programmed by a 60 ns pulse and the second HRS (“01” state) can be programmed by a 45 ns pulse. Moreover, the LRS (“11” state) can also be programmed by a 200 ns pulse applied. This method is more efficient for the OTP application because all the resistance state can be formed by only one shot. This one shot OTP operation not only increases the programming speed but also reduces the programming cost.

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Figure 4-33. The one shot OTP operation character.

Figure 4-34 shows several shot programming operation with different pulse width.

For 1.5V applied voltage, the initial resistance can’t be reversed by applying a 10 ns pulse bias. It can be attributed to the fact that lower energy (too shorter pulse and low voltage) can’t revise the resistance state. With the pulse width increasing (also means more power), the resistance state is reversed by the applied pulse shot. In this figure, the resistance state can be switched to HRS by 50 ns and 80 ns pulse shot. By the way, with the addition of pulse shots, the resistance state reverses slowly to the LRS. However, long pulse shot can switch the resistance state to the low resistance state. In this figure, the resistance switching behavior shows no difference between 1 us and 10 ms pulse shot. These LRS are created by the high energy pulse. Moreover, these LRS decreased slowly with more applied pulse.

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Figure 4-34. The influence of the pulse width with applied voltage about 1.5 V.

According to our previous experiments, it is easy to program these resistance states in the bi-stable 2 bits/cell device. Moreover, these states exhibit excellent read disturb performance. Figure 4-35 shows the read disturb character of our 2 bits/cell memory device. The applied reading voltage is about 10 mV and these resistance states still remain their resistance state after one thousand times of reading process.

Figure 4-35. The 10mV read disturb of 2bits/cell WOx film

These 2bits/cell resistance states also exhibited excellent data retention and thermal stability. The left of figure 4-36 shows data retention performance for more than 10⁴ sec at room temperature, and the right of figure 4-36 shows thermal stability at 150℃ for more than several hundred hours. These results indicated that our OTP WOx film is a suitable material for the OTP non-volatile memory application.

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Figure 4-36. The data retention (left) and thermal stability (right) of 2bits/cell WOx film

Figure 4-37. The electrical character of four resistance states.

Figure 4-37 shows the I-V curve of these four resistance states. These resistance states exhibit both linear and non-linear electrical behavior. The electrical character of LRS (“11” and “10” states) shows linear I-V curve and they follow the Ohm’s law. It indicates the metal dominant behavior in these two low resistance states. However, the HRS (“01” and “00” states) shows non-linear I-V curve and these curves are well fitted by T^-1/4. It indicates that these electrical characters follow the VRH behavior.

Figure 4-38. Temperature effect on “00” state. The dashed line is optimized fitting by power law.

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Figure 4-38 shows the temperature effect on the current “00” state with applied voltage near zero. This figure indicates the VRH conduction mechanism as described by Eq. (3-4). If the voltage is close to zero, kT x sinh(CV/kT) becomes a temperature independent constant and Eq. 3-4 depends only on exp(T^-1/4). We can simplify Eq. (3-4) to Eq. (4-1) and the important parameters are as followed:

CV

The average hopping distance for electrons is 15 Å .