About RRAM – 6 Filament Materials: OxRAM vs. CBRAM

So far, we've been talking about "when voltage is applied, a filament is formed." However, depending on what the filament is made of, RRAM can be broadly divided into two types.

1. OxRAM (Oxide RRAM): The way defects called oxygen vacancies gather to form filaments.
2. CBRAM (Conductive Bridge RAM): A method in which ‘real metals’ such as copper (Cu) or silver (Ag) move to build a bridge.

These two devices differ significantly, from the electrodes they use to their operating characteristics. Understanding where your desired device falls into is the first step in device design.

1. OxRAM: Movement of Oxygen Vacancies (VCM)

This is the most widely studied standard method in both academia and industry (TSMC, Infineon, etc.). Its official name is VCM (Valence Change Memory).

1.1. Operation Mechanism

Oxygen ions (O^2-) within the insulator (oxide) are pushed out by the voltage. The vacant space becomes an oxygen vacancy, a defect through which electrons can move. When these vacancies are connected in a line, they form a filament through which current flows.

• Main materials: HfO₂, Ta₂O₅, TiO₂ ... transition metal oxides.
• Electrodes (Important): Since only oxygen vacancies need to move, the electrode itself should not react. Therefore, inert electrodes such as TiN, Pt, W, and Ir are used.

1.2. Advantages and Disadvantages

• Merit: It uses a semiconductor standard material (HfO₂), providing excellent CMOS process compatibility. It also boasts excellent thermal stability and data Retention characteristics.
• disadvantage: The operating voltage is somewhat high, and the movement of oxygen ions may be slower than that of metal ions.

2. CBRAM: ECM

It is also called PMC (Programmable Metallization Cell) and is a technology commercialized by companies such as Adesto. Its official name is ECM (Electrochemical Metallization).

2.1. Operation Mechanism

This is like a microscopic Electroplating process.

1. Oxidation (Anode): At the active electrode (Ag, Cu), the metal is oxidized and dissolves into the insulator in the form of ions (Ag⁺, Cu²⁺).
2. Migration: Metal ions move through the electric field to the opposite electrode.
3. Reduction (Cathode): It meets electrons at the opposite electrode and is reduced back into metal and accumulated.
4. Growth: This lump of metal grows to form a bridge connecting the two electrodes.

• Main materials: GeSe, Ag₂S or porous oxide (SiO₂) acting as a solid electrolyte.
• Electrode (core): One electrode must be an active electrode, such as Ag or Cu, which is easily dissolved. The other electrode must be an inactive electrode, such as Pt or W.

2.2. Advantages and Disadvantages

• Merit: The operating voltage is very low (Low Power), and the switching speed is fast. The On/Off resistance difference (Ratio) is very large, making signal discrimination easy.
• disadvantage: The metal filaments (especially silver and copper) produced are too diffusion-prone. Over time, the filaments tend to melt away, resulting in poor retention.

3. OxRAM vs CBRAM

This is the table you must first decide on when writing a paper or designing a device.

Comparison items	OxRAM (Valence Change)	CBRAM (Electrochemical)
filament material	Oxygen Vacancy	metal atoms (Ag, Cu)
core electrode	Inert (TiN, Pt, W)	Active (Ag, Cu) + Inert
Switch layer	HfO2, Ta2O5, TiOx	GeSx, Ag2S, SiO2
Operating power	Medium to high	Very low
Retention	Stable	relatively unstable
Application	Embedded NVM, AI Synapse	Low Power IoT, Security Devices

4. RRAM Practical Tips: Where Does My Device Fit?

Sometimes, during research, the boundaries become blurred. For example, if a Cu electrode is placed on top of a HfO₂ layer, is this OxRAM or CBRAM?

• The correct answer is “Hybrid with strong CBRAM characteristics.”
• HfO₂is a material originally developed for OxRAM, utilizing oxygen vacancies. However, the moment a Cu electrodeis used, copper ions penetrate the HfO₂ . Because the mobility of copper ions is much faster than that of oxygen vacancies, switching occurs primarily through copper filaments (CBRAM).

⚠️ Caution: If you're researching OxRAM, never use electrodes like Ag, Cu, Au recklessly. These metals have strong penetrative properties and can contaminate the device's characteristics. The gold standard for OxRAM research is a MIM structure like TiN/HfO2/TiN .

5. Conclusion: Choose the right material for your purpose.

• Reliability (Retention, Endurance) is important and you want to apply it directly to the foundry process? -> Choose OxRAM (HfO₂/TiN).
• Need low power operation or very high on/off ratio? -> Choose CBRAM (Ag/GeS).

References: Nanoionics-based resistive switching memories