Low Frequency Noise Behavior in Resistive Memory Devices with Hf/HfO2 Stack

Resistive-random-access-memory (RRAM) devices are considered to be one of the most potential candidates of next generation emerging memory technologies due to their excellent device properties, such as high density, low power, low voltage, and high speed. RRAM features simple metal–insulator–metal (MIM) structures and is fully compatible with traditional CMOS fabrication processes. Low-frequency noise (LFN) measurement is a technique to characterize electrically the trap-assisted conduction processes in dielectrics. In this work, we report the LFN characteristics of Hf/HfO2-based bipolar RRAM and investigate the current conduction mechanism and internal physics.


Introduction
Resistive-random-access-memory (RRAM) devices is a new type of non-volatile memory, which is stored by applying a pulsed voltage to a metal-oxide film to create a large difference in resistance [1][2] [3].The simple structure, in which two electrodes sandwich the metal oxide in the center, simplifies the manufacturing process while achieving excellent performance such as low power consumption and high-speed rewriting [4].The advantage of variable resistive memory is that it consumes less power.Resistive-random-access-memory (RRAM) devices will be a form of memristor.Like a resistor, a memristor generates and maintains a safe current through a device.But unlike a resistor, a memristor "remembers" the amount of charge that has previously passed through it even after the power is turned off.Two sets of memristors are more capable of performing the same function as a transistor, but are much smaller [5].The RRAMs have good low-frequency noise characteristics, and measuring the low-frequency noise characteristics is one of the effective techniques for obtaining the characteristics of body defects and interface state traps in the gate oxide layer, as well as for studying the conduction mechanisms and internal properties.

Device Components
The Hf/HfO2-based bipolar RRAM structure, consisting of 65-nm PVD (Physical Vapor Deposition) TiN/5-nm ALD (Atomic Layer Deposition) HfO2/10-nm PVD Hf/30-nm PVD TiN, was fabricated from IMEC with a back-end-compatible thermal budget, not exceeding 400 .The RRAM device was patterned in a cross-point shape with a 55 nm×55 nm drawn size.The TEM images are given in Figure 1.

1Resistive State of the RRAMs
The RRAMs can be in one of three resistance states: (1) virgin-state, (2) on-state which features low resistance (LRS), and (3) off-state, which features relatively large resistance (HRS).The virgin state is the as-grown state of the device.A forming process of applying a sufficiently high bias to a device in the virgin-state is used to irreversibly activate the switching between the on and off states of these devices.Agilent 4156C semiconductor parameter analyzer was used to measure the DC switching characteristics.Ten SET/RESET switching cycles after the forming step are shown in Figure 2. The cycle-to-cycle variation is relatively small.In this work, the forming voltage (Vforming) and the SET voltage (Vset) are taken at 100 μA current compliance.
The RESET voltage (Vreset) is recorded when the current is the maximum during the RESET transition.

Figure 2.
Ten SET/RESET switching cycles after forming step with good uniformity.

Low Frequency Noise Measurement System
HP4140A was used to provide the DC bias for noise measurement.The noise current was fed into a Stanford Research System SR560 low noise amplifier, and the output signals were analyzed by a Stanford Research System SR760 spectrum analyzer.The electrical measurement was done in a shielded environment.The bottom electrode was grounded and the bias was applied to the top electrode in all the measurements.Figure 3 shows the scheme of low frequency noise measurement system for RRAM device in this work.The LFN measurement was done in Institute of Microelectronics, Chinese Academy of Sciences..

Low Frequency Noise Characterization Test
Figure 4 gives the low frequency noise characteristics of Hf/HfO2 stack for LRS (resistance value of 1.5 kΩ) and HRS (resistance value of 36 kΩ ) states, sweeping the TE bias from 0.1 V to 0.5 V with 0.1 V step.In general, the low frequency noise power spectral densities increase with the BE bias.Furthermore, the low frequency noise power spectral densities during HRS was almost one order of magnitude higher than that of devices during LRS.

Linear Fitting Properties
Figure 5 shows the linear fitting for the slopes of low frequency noise characteristics of Hf/HfO2 stack in LRS (resistance value ranges from 1.2 to 1.8 kΩ) and HRS (resistance value ranges from 30 to 41 kΩ) states with 0.1 V BE bias.The slopes for LRS state range from -0.82 to -0.77 (absolute value less than 1), while the slopes for HRS state range from -1.54 to -1.35 (absolute value larger than 1).The low frequency noise power spectral densities in LRS and HRS states both follow the ଵ ೌ dependence.

Low-frequency Noise Power Spectral Density as a Function of DC Bias Voltage
Figure 6 gives low frequency noise power spectral densities in LRS (resistance value of 1.8 kΩ) and HRS (resistance value of 86 kΩ) states, sampled at different frequency points (30, 50 and 100 Hz), are as functions of the DC bias voltage.LFN power spectral densities in HRS state increase slowly with BE bias.But there is a turn point for LFN power spectral densities in LRS state, which is about 0.3 V.It seems that the higher BE bias is optimal for maximum signalto-noise ratio (SNR) for the read operation, but it may influence the resistance value of RRAM for the long time LFN measurement.

Physical Mechanisms Analysis
Figure 7 gives the physical mechanisms analysis.LFN types can be ascribed to defects fluctuating between a neutral and a charged state.These defects consist of an oxygen vacancy or an electron trap close to or inside the CF (conductive filament).The higher probability of observing individual defects in the LFN spectrum of HRS can be ascribed to the low number of defects associated with a very narrow constriction.In LRS, conductive filament is so thick that it is easier for current to flow across it.There are less carriers transported by the trap/defect insisted tunneling process.That's why low frequency noise is lower in LRS.In HRS, conductive filament is so narrow that it is harder for current to flow across it.So some carriers are transported by the trap/defect assisted tunneling process.That's why low frequency noise is higher in HRS.

Conclusion of the Experiment
In summary,

Figure 3 .
Figure 3. Low frequency noise measurement system for RRAM device.

Figure 4 .
Figure 4. Low frequency noise characteristics of (a) HRS RRAM and (b) LRS RRAM sweeping from 0.1 V to 0.5 V with 0.1 V step.

Figure 5 .
Figure 5. Slope fitting for low frequency noise characteristics of (a) HRS RRAM and (b) LRS RRAM with 0.1 V bias.

Figure 6 .
Figure 6.Low frequency noise power spectral densities LRS and HRS RRAMs, sampled at different frequency points.

Figure 7 .
Figure 7. Illustration of the conduction process in resistive switching memory: electrons in LRS RRAM are mainly transporting through the conductive filament, while more electrons in HRS RRAM prefer to transport by trap-assist-tunneling (TAT) process.

ଵೌ
behavior of low frequency noise in Hf/HfO2 stack has been investigated.The LRS and HRS states show different slopes and magnititudes from LFN characteristics.The LFN behavior difference is attributed to the distribution of electron flowing path and the traps around the CFs.