About RRAM – 4 Forming and Compliance Current

What's the most common mistake RRAM researchers make? Mishandling the process? No. It's incorrectly setting up the measurement equipment, which can result in a complete device being burned out.

The resistance change (switching) of RRAM is a process in which tiny conductive paths called filaments are created and broken. However, if this filament formation is not controlled, the device will experience permanent insulation breakdown (hard breakdown), becoming a mere "wire."

In this article, we will explain the electroforming process , which is essential for reviving RRAM, and how to set the Compliance Current, which is the safety belt of the device.

1. RRAM's First Operation: Electroforming

Freshly processed RRAM devices (Pristine State, Virgin State) do not exhibit resistance change characteristics because the space between the upper and lower electrodes is sealed by a perfect insulator.

To make it work as RRAM, a process is needed to open up an initial filament pathin this insulator, called Electroforming, or Formingfor short.

Forming Features

1. High Voltage: A higher voltage (e.g., 5-7 V) is required than the typical SET operating voltage (e.g., 1-2 V). This is because the initial breakthrough is the most difficult.
2. Soft Breakdown: Rather than completely destroying the insulator, a "soft breakdown" must be induced, creating only a tiny leakage path.
3. One-time: This typically only needs to be performed once, when the device is first powered up.

2. Seat belt of the device: Compliance Current

When voltage is applied during the forming or SET process, filaments grow inside the insulator, and current begins to flow rapidly. This is the most dangerous moment.

Why Burn It? (Joule Heating Runaway)

1. The moment the filament is connected, the resistance (R) drops dramatically.
2. While the voltage (V) remains constant, the resistance decreases, resulting in an explosive increase in current (I)according to Ohm's Law (I = V/R).
3. This increase in current causes Joule heating, heating up the component.
4. The heat causes the filament to grow abnormally thick, ultimately melting or permanently bonding the component (Hard Breakdown).

The Role of Compliance Current

To prevent this situation, the Compliance Current (I_cc).

• Example settings: Even if the forming voltage is swept up to 5V, if I_ccis set to 1µA, the current stops at exactly 1µA when the device is turned on.
• Effect: Prevents the filament from becoming too thick (overshoot), making it ready for re-cutting (reset) the next time.

3. Practical Measurement Recipe (Step-by-Step)

This is standard procedure when using equipment such as the Keithley 4200 or Keysight B1500.

Step 1. Forming (Initial Path Breaking)

• Mode: DC Voltage Sweep
• Voltage: 0V → +5V (adjusted according to device characteristics)
• Compliance Current (I_cc): Very important! It is usually set low, between 10nA ~ 1µA .
• Result: Success occurs when the current reaches the I_cc level at a specific voltage (Forming Voltage).

Step 2. RESET (Cut the path)

• Mode: DC Voltage Sweep (If it is a bipolar device, the polarity is opposite)
• Voltage: 0V → -3V
• Compliance Current: Do not set or leave it high (e.g. 100µA).
  • Tip: RESET requires a large current to burn out the filament. If I_ccis set too low, RESET will not work.

Step 3. SET (Reconnect)

• Mode: DC Voltage Sweep
• Voltage: 0V → +3V
• Compliance Current: Set similar to or slightly higher than when forming (e.g. 10µA ~ 100µA).
  • Tip: The size of I_ccset at this time determines the resistance value of LRS (Low Resistance State). The higher the I_cc, the lower the resistance (the thicker the filament).

4. Advanced: Why You Die Even With I_cc(Overshoot)

“I clearly put in compliance current, but the device died!”

This is due to equipment limitations. There's a slight delay before the equipment responds, "The current is too high? Reduce it."

At this moment, a current overshoot phenomenon occurs, in which the charges stored in parasitic capacitors hidden in the cable or probe station all at once pour into the device.

Advanced Tip

1. Transistor connection (1T1R): Instead of the I_cc of the device, it is most likely to limit the current by the Gate voltage of the transistor (physical limitation).
2. Using built-in resistors: Connect a resistor in series with the device (Series Resistor) to prevent excessive current.

5. Conclusion

The most important thing in RRAM research is creating a filament of the "appropriate thickness." If it's too thin, it will break quickly (poor retention), and if it's too thick, it will never break again (non-resettable). Maintaining this appropriate balance is called compliance current.

When starting your first measurements in the lab, it is always a good idea to start with a low current (~1µA) and gradually increase it.

References: Investigation of compliance current effect on resistive switching properties