Retention test and switching cycles of RTA in O2 ambient at 400oC for 60 seconds under 0.1mA compliance current are shown in Fig. 3-18. The retention test was directly given 0.08V (=Vread) voltage bias and logarithmicly detected off state current (Ioff) from 0 to 104 seconds. Off state conductance as shown in Fig. 3-18 was Ioff divided by Vread. After retention test of off state, the cell was set to on state under 0.1mA compliance current. Similarly to off state retention test, on state retention was conducted again. On state conductance was also shown in Fig. 3-18. From Fig. 3-18, the retention test is agreeable to I-V sweep method. Also, this RRAM cell exhibits non-destructive readout and good data retention.
Fig. 3-19 shows average of maximal reset current versus compliance current under various process conditions. From Fig. 3-19, we can conclude that Ar RTA increases maximal reset current, temperature of Ar RTA especially. However, O2 RTA can decrease maximal reset current to about compliance current even under different compliance currents. The effect that low compliance current led to low reset current has been mentioned in Gao et al.’s study [38].
Table 3-5 also lists the performance of the control sample and O2 RTA at 400oC for 60 seconds under 0.1mA compliance current. We used low cost nickel (Ni) as top electrode and applied O2 RTA to diminished compliance current to 0.1mA, maximal reset current to 0.18mA, set power to 7.46μW, reset power to 69.1μW, conductance of off state to 1.09μS, conductance of on state to 0.39mS.
50 100 150 200 Compliance Current : 0.1 mA
RTA: O2, 400oC, 60sec
Fig. 3-18 Retention test and switching cycles of RTA in O2 ambient at 400oC for 60 seconds under 0.1mA compliance current
0.0 0.2 0.4 0.6 0.8 1.0
Ave. of Max reset current (mA)
Compliance Current (mA)
Fig. 3-19 Average of maximal reset current versus compliance current under various process conditions
Table 3-5 Summary of measured performance and comparison with the other works
* The definition of set power and reset power is not clear.
** Measurement was conducted by pulse I-V measurement.
Control O2 400oC 60sec 2007 JJAP [23] 2008 IEDM [16]
Structure Ni/HfOx/TiN Ni/HfOx/TiN Pt/HfOx/TiN Pt/TaOx/Pt
Compliance current (mA) 0.5 0.1 0.1 -
Operation mode uni-polar uni-polar uni-polar bi-polar
Vread (V) 0.08 0.08 0.5 -
Goff (S) 2.41×10-5 1.09×10-6 1×10-6 4×10-4
Gon (S) 4.33×10-3 3.89×10-4 9.09×10-5 6.67×10-3
Max Ireset (mA) 1.39 0.18 < 0.1 0.15
Vset (V) 1.41 1.33 2.4 -1.5 **
Vreset (V) 0.43 0.67 1.5 2 **
Set Power (W) 6.68×10-5 7.46×10-6 2.5×10-4 * - Reset Power (W) 4.15×10-4 6.91×10-5 1.5×10-4 * -
CHAPTER 4 CONCLUSION
The resistive switching characteristics of a Ni/HfOx/TiN structure have been demonstrated. Although the exact mechanism of RRAM is not yet clear, the filament model is plausible. Proper annealing can slightly elevate endurance. Ar RTA leads to large current of on state and reset process. Proper O2 annealing can reduce compliance current, operation power, and conductance of on and off state while maintaining sufficient ratio. If time and temperature of O2 annealing are too high, RRAM cell exhibits nonlinear I-V curves of on state or no resistive switching. In addition, non-destructive readout and data retention were tested in this thesis. Base on above reason, RRAM is suitable for the use in low power and nonvolatile memory portable device applications.
CHAPTER 5 FUTURE WORK
I am presenting preliminary results of a pilot experiment that will be further analyzed, expanded and replicated. The switching operation of set and reset under negative bias is the needful due to asymmetrical MOM structure. Chemical analysis is used to explore oxygen concentration of HfOx films. Pulse I-V measurement can provide not only an extra degree of freedom of measurement, time but also more precise reliability tests, endurance and data retention. Future research is necessary to determine with certainty the relationship between optimal condition of O2 RTA and thickness or oxygen content of HfOx. It may be of interest for future research that the effect of thermal treatment on RRAM which is constructed by the other metal top electrode. We are hopeful that future research will provide more detailed results which may support these views.
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