3-1 Characteristics of Ni/HfOx/TiN RRAM Cells
3-1.1 Basic 1R switching characteristics of Ni/HfO2/TiN cells
It has been reported that the HfOx based RRAM cell with Ni as the electrode shows good endurance in unipolar switching [49], and along with its good CMOS compatibility, it is attractive and worth exploring.
Fig. 3-1 shows the switching characteristics of a fresh Ni/HfOx(50nm)/TiN cell with compliance current of 1 mA. The stop voltage during the reset sweeping is 1 volt.
Successive 100 switching cycles are achieved in this device. Both the set and reset operations were done with positive voltage. Abruptly decreased current is observed in the reset switching, consistent with what has been reported [49][50].
Resistances of high-resistance state/low-resistance state (LRS/HRS) during the successive 100 cycles are shown in Fig. 3-2. The on/off ratio is around 400 in the Ni/HfOx(50nm)/TiN cell with the operation condition mentioned above. As a matter of fact, the on/off ratio is actually tunable with appropriate operation condition, and will be discussed later.
3-1.2 Effects of the compliance current
The compliance current is one of the most important operation parameters that influence the LRS resistance directly. Fig. 3-3 shows the relationship between the LRS resistance and the compliance current obtained from the characterization of the Ni/HfOx(40nm)/TiN cells. Also shown in the figure is the standard deviation of the LRS resistance. It can be clearly seen that the average LRS resistance decreases with increasing compliance current. It is also observed that the cycle-to-cycle variation of
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LRS resistances can be ignored when the compliance current is sufficiently large. The variation becomes obvious when the compliance current is smaller than 1 mA. It is probably due to the uncontrollable filament size in the presence of overshoot current during the set operation. When the compliance current is small, the overshoot current caused by the charges released from the parasitic capacitance [45] could be larger than the compliance current. The uncontrollable filament size makes it difficult for low power operation.
Another indicator of the filament size is the reset current at which the resistance starts to increase during the reset operation. Fig. 3-4 shows the relationship between the compliance current and the reset current. The reset current decreases with increasing compliance current when it is approximately larger than 0.1 mA, but almost no influence related to the compliance current when it is smaller than 0.1 mA.
As the LRS resistance, the cycle-to-cycle variation of reset current is obvious when the compliance current is smaller than 1 mA, but negligible when larger than 1 mA.
Fig. 3-5 shows the cumulative probability of HRS resistances of Ni/HfOx(40nm)/TiN RRAM cells with various compliance current. It seems that the compliance current has no significant influence on HRS resistance.
3-1.3 Effects of reset stop voltage
There are few works discussing the influence of the stop voltage during the reset operation Some of the papers described the parameters such as set voltage and HRS resistance of their devices with fixed stop voltage [25][51]. However, it seems not as easy to describe these parameters as they did. Instead, these parameters should be clarified more precisely with the measurement conditions including compliance current and stop voltage during reset (denoted as Vrs in the following discussion).
Fig. 3-6(a) shows the cumulative probability of HRS resistances with various Vrs.
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The HRS resistances also tend to increase with the Vrs, as the case in set voltage. Fig.
3-6(b) shows the cumulative probability of HRS resistances with different HfOx thickness and Vrs of 1 volt. We can find out that the influence of oxide thickness seems not as obvious as the Vrs. The importance of the Vrs can also be observed form Fig. 3-7, in which Fig. 3-7 (a) shows the relation of forming voltage with the oxide thickness, and Fig. 3-7 (b) shows the relation of set voltage with oxide thickness, both with Vrs of 1 volt. It can be obviously seen that the forming voltage increases with increasing HfOx thickness. Because the forming process needs a critical electric field in the HfOx layer to form conductive filaments, the thicker cells need a larger voltage to achieve it. But the thickness of HfOx seems to have no influence on the set voltage when the Vrs remains unchanged,. In contrast, Fig. 3-7(c) shows the relation of set voltage with Vrs with HfOx of 40 nm. It is observed that the set voltage increases with increasing Vrs.
3-2 Characteristics of TiN/Ti/HfOx/TiN RRAM Cells
3-2.1 Basic switching characteristics of TiN/Ti/HfO2/TiN cells
The HfOx-based RRAM cell with Ti electrode has been reported to have excellent bipolar performance, with good endurance and retention. More significantly, low power switching has been achieved in such cells, which is important for high density integration [27].
Fig. 3-8 shows the bipolar switching characteristics of a TiN/Ti/HfOx/TiN cell with compliance current of 0.1 mA. Unlike the Ni/HfOx/TiN cells, the resistance of TiN/ Ti/HfOx/TiN RRAM cell increases gradually during reset operation, which is in agreement with the results reported previously [27].
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3-2.2 Effects of compliance current
Fig. 3-9 shows the switching behaviors of a TiN/Ti/HfOx(20nm)/TiN RRAM cell with different compliance currents. The on/off ratio degrades when the compliance current is increased. With the same Vrs, the cell with larger compliance current has lower HRS resistance. It is probably caused by the formation of multiple conductive filaments or filaments with a larger cross section that makes the filaments more difficult to rupture during the reset operation. It seems not to be consistent with the results of Ni/HfOx/TiN RRAM cells, in which the compliance current has no influence on HRS resistances, as has been discussed in previous section.
Although the compliance current indeed affects the resistive switching characteristics, the 1R measurement configuration with overshoot current makes it difficult to show stable switching behavior with a given compliance current. As shown in Fig. 3-10, the TiN/Ti(20nm)/HfOx(20nm)/TiN RRAM cell successfully accomplishes 100 cycles only if we modify the compliance current to an appropriate value when the on/off ratio degrades cycle by cycle.
3-2.3 The dependence of forming voltage on reset current
Fig. 3-11 shows the relationship between the forming voltage and the reset current during the first reset process. We can observe that the reset current increases with increasing forming voltage. It is consistent with the result of a previous study [52], in which they claimed that the overshoot current during the forming operation
depends on the forming voltage. This conception can be easily understood from Fig.
3-12. With a larger forming voltage, more charges are stored in the parasitic capacitance before the cell reaches the forming voltage, resulting in larger overshoot current when the cell reaches the forming voltage.
Larger overshoot current leads to a larger reset current and thus more
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uncontrollable switching behavior. Fig. 3-13(a) shows a TiN/Ti/HfOx/TiN RRAM cell with forming voltage of approximately 4 V, the reset current in the following reset operation is approximate 0.6 mA. In the first set operation, the current reaches the compliance current before the cell reaches the set voltage, resulting in failed set operation. Fig. 3-13(b) shows a TiN/Ti/HfOx/TiN RRAM cell with forming voltage of approximately 1.5 V, and the reset current in the following reset operation is approximate 0.2 mA. Unlike the case mentioned above, the following set operation is successfully completed because of the smaller reset current during the first reset operation. The two cases reveal the importance for lowering the forming voltage in the 1R configuration because of the inevitable overshoot current.
3-2.4 The influence of Ti buffer layer thickness
It has been reported that the Ti buffer layer is able to degrade the dielectric strength of the dielectric layer [53]. It is because of the interfacial layer formed between the Ti buffer layer and the dielectric layer. The thickness of the Ti buffer layer and the dielectric layer must be optimized for steady switching behavior [53].
TiN/Ti/HfOx/TiN RRAM cells with different thicknesses of Ti buffer layer and HfOx was fabricated and characterized in this work. Major properties including the breakdown field, initial resistance, and the switching ability are summarized in Table 3-1.
Fig. 3-14(a) shows the influence of Ti buffer layer thickness on TiN/Ti/HfOx(10nm)/TiN RRAM cell. With 20 nm Ti buffer layer, the fresh cell is too leaky to form local conductive filament, and thus no switching behavior can be observed. With 10 nm Ti buffer layer, the leakage current of the fresh cell is reduced and the cell shows switching behavior, but failed in switching within 5 cycles.
Without Ti buffer layer, the leakage current of the fresh cell is further reduced but
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failed in reset operation. Fig. 3-14(b) shows the characteristics of TiN/Ti/HfOx(20nm)/TiN RRAM cells with various thickness of Ti buffer layer. The fresh cell with 20 nm Ti buffer layer also shows largest leakage current and the lowest forming voltage, and it can switch more than 5 cycles. The cell with 10 nm Ti buffer layer shows lower leakage current and higher forming voltage, and its switching ability is less than 5 cycles. Without Ti buffer layer, the cell fails in forming operation because it has the best dielectric strength. Among them, the RRAM cell with 20 nm Ti buffer layer and 20 nm HfOx dielectric layer shows the most stable switching behavior.
3-3 Switching Characteristics of Ni/HfOx/TiN Cells with External 1T1R Configuration
In order to eliminate the overshoot current, we have tried to use external 1T1R configuration in this work. In forming/set process, Vg of 4 volt is applied to the transistor, and the compliance current for RRAM cell is approximately 0.2 mA (see Chap. 2). In reset process, the voltage is applied to the RRAM cell directly without involving the transistor. Note that in the 1T1R configuration reported in the literature, the two devices were integrated together and the RRAM cell was connected to the drain of the transistor, Vg larger than that in forming/set process must be applied to the transistor in reset process. In our configuration, the RRAM cell and the transistor were fabricated independently, thus we can choose not to connect the transistor with the RRAM cell during reset operation so as to simplify the analysis.
Fig. 3-15 shows the switching characteristics of a Ni/HfOx(40nm)/TiN RRAM cell in our external 1T1R configuration. It can be observed that the reset current is still much larger than the compliance current as in 1R configuration. This indicates that
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the overshoot current still exists in the external 1T1R configuration. However, we can point out that the overshoot current is mainly caused by the existence of parasitic
Despite many efforts put into studies in these years, the mechanism of resistive switching is still in debate. The analyses of switching mechanism were mostly based on qualitative descriptions. In addition, there is a lack of quantitative analyses describing the switching procedure of a RRAM cell for it to achieve real application in IC industry.
Most of reports described the resistive switching mechanism of Ti/HfOx based RRAM cell using VCM model that has been mentioned in Chapter 1. The resistive switching is attributed to the formation/rupture of conductive filaments that are composed of oxygen vacancies in HfOx [28][54]. However, some reports related the resistive switching behavior to the charge trapping/detrapping process, which influenced the magnitude of space-charge-limited current [27].
There were also several models that have been reported to describe the switching behavior of the Ni/HfOx-based RRAM cell. For example, the switching behavior is suspected to be related to the filament composed of Ni ions [13][49]. Under the forming operation, the Ni ions migrate from the top electrode in the presence of electric field and form conductive paths that connect the top and bottom electrodes, so that the cell is switched to LRS. The size of the filaments is controlled by the