A Built-In Self-Test Structure (BIST) for Resistive RAMs Characterization: Application to Bipolar OxRRAM

TLDR: In this paper, a built-in structure allows collecting statistical data related to the OxRRAM memory array (ON/OFF resistance distributions) for reliability assessment of the technology, which is a key parameter to determine the overall performance.

Abstract: Resistive Random Access Memory (RRAM) is a form of nonvolatile storage that operates by changing the resistance of a specially formulated solid dielectric material [1] . Among RRAMs, oxide-based Resistive RAMs (so-called OxRRAMs) are promising candidates due their compatibility with CMOS processes and high ON/OFF resistance ratio. Common problems with OxRRAM are related to high variability in operating conditions and low yield. OxRRAM variability mainly impact ON/OFF resistance ratio. This ratio is a key parameter to determine the overall performance of an OxRRAM memory. In this context, the presented built-in structure allows collecting statistical data related to the OxRRAM memory array (ON/OFF resistance distributions) for reliability assessment of the technology.

Figures

Figure 4. Programmable voltage source (a) and shift register (b)

Figure 3. OxRRAM sensing circuit

Figure 1. Typical I-V characteristic of a bipolar O xRRAM cell

Figure 2. 3×3 OxRRAM memory array

Figure 8. V IN SET and V IN RESET distributions versus cell variability [ βRedOx±5% and βHRS ±1.5%] after 100 Monte Carlo runs@Vread = [0V, 1V]

Figure 8. V IN SET and V IN RESET distributions versus cell variability [ βRedOx±5% and βHRS ±1.5%] after 100 Monte Carlo runs@Vread = [0V, 1V]

Frequently asked questions

Q1. What are the contributions mentioned in the paper "A built-in self-test structure (bist) for resistive rams characterization: application to bipolar oxrram" ?

In this paper, the authors proposed a built-in self-test structure ( BIST ) to evaluate OxRRAM variability at a circuit level. 

Q2. What is the definition of a conductive state?

Electroforming stage corresponds to a voltage-induced resistance switching from an initial very high resistance state (virgin state) to a conductive state. 

Q3. What are the main features of OxRRAM?

OxRRAM presents a lot of interesting features like high integration density, high-speed operations (write/erase/read) and satisfactory reliability performances in terms of retention and cycling. 

Q4. What is the proposed approach to OxRRAM?

The proposed OxRRAM modeling approach relies on electric field-induced creation/destruction of oxygen vacancies within the switching layer. 

Q5. What is the advantage of the BIST structure?

As variability in OxRRAMs presents a major challenge for fabrication process and design engineers, the BIST structure can be used to quickly evaluate cell variability impact on the ON/OFF resistance ratio of a whole memory array. 

Q6. What is the trend for the use of OxRRAMs?

For instance, for mobile applications containing OxRRAMs, the trend would be to reduce the cell consumption without compromising reliability. 

Q7. How many Monte Carlo simulations are performed to extract the sense amplifier VIN distributions?

For each VREAD value, 100 Monte Carlo simulations are performed after a SET and a RESET operation to extract the sense amplifier VIN distributions. 

Q8. What is the purpose of the study?

In this study, a large number of Monte Carlo simulations are performed to provide the statistic needed to characterize variability and its impact on the circuit. 

Q9. What is the redox rate of the cell?

Ox ererr dtdr ⋅ ⋅⋅+− ⋅ ⋅⋅−− ⋅⋅−⋅⋅−= α β αβ ReRe 1010max (1)where βRedOx is the nominal oxide reduction rate, Ea is the activation energy, αred and αox are the transfer coefficients (ranging between 0 and 1), kb is the Boltzmann constant, T is the temperature and Vcell the voltage across the cell. 

Q10. What is the definition of resistive switching in an OxRRAM element?

Resistive switching in an OxRRAM element corresponds to an abrupt change between a High Resistance State (HRS or OFF state) and a Low Resistance State (LRS or ON state). 

Q11. What is the IOX trap assisted current?

To take into account IOX trap assisted current (PooleFrenkel, Schottky emission, Space Charge Limited Current (SCLC)), a power law between the cell current and the applied bias is considered with two parameters AHRS and βHRS. 

Q12. What is the basic concept of a resistive memory?

In its simplest form, resistive memory element relies on a Metal/Insulator/Metal (MIM) stack that can be easily integrated into the Back-End Of Line (BEOL), paving the way for 3D technology. 

Q13. What is the control of the transmission gates of the voltage source?

The transmission gates of the voltage source are controlled by the shift register presented in Fig. 4b, which shifts a “1” and has all its other bits set to “0”. 

Q14. What is the case of bipolar switching?

In the case of bipolar switching, addressed in this paper, bipolar voltage sweeps are required to switchthe memory element (Fig. 1). 

Q15. What is the correlation curve in Fig. 8?

8. Therefore, the modified sense amplifier structure can be used as a powerful tool to track any resistance variations but also to characterize the memory array variability. 

Q16. What is the main contributor to LRS current?

the total current flowing through the cell is:OXCFCell III += (4)ICF is the main contributor to LRS current (ILRS) and IOX is the main contributor to HRS current (IHRS). 

Q17. What mechanisms may explain the resistance change?

Various mechanisms may explain the resistance change (oxygen vacancy migration [4], oxidation-reduction processes, thermal diffusion…). 

Q18. What is the total current in the cell?

the total current inthe OxRRAM includes two components, i.e. one is related to the conductivespecies (IFC) and the other to the conduction through the oxide (IOX).( )( )OXCFOXCFCF x Cell CF rrL V The authorσπσσπ ⋅⋅+−⋅⋅⋅= 2 max 2 (2)HRSxCell CellHRSOX LV SAIβ ⋅= (3)where Lx is the oxide thickness, SCell is the total area of the device, σOx the oxidationrate and σCF the reduction rate.