passivation layer deposition

**Passivation Layer Deposition** is the **final protective thin-film coating applied over the completed integrated circuit — typically a bilayer of silicon nitride (SiN) over silicon dioxide (SiO2) or a polyimide-based organic film — that seals the chip against moisture, ionic contamination, mechanical damage, and environmental degradation for the entirety of its operational lifetime**. **Why Passivation Is Non-Negotiable** The aluminum or copper bond pads and top metal interconnects are reactive metals. Without passivation, atmospheric moisture penetrates the chip, mobile sodium and potassium ions drift under bias voltage and shift transistor thresholds, and copper corrodes into resistive oxides. An unpassivated chip can fail within hours of powered operation in a humid environment. **Passivation Materials** - **PECVD Silicon Nitride (SiN)**: The workhorse passivation film. SiN is an excellent moisture barrier (water vapor transmission rate <1e-3 g/m²/day at 300 nm thickness), mechanically hard (scratch resistant), and has good step coverage over the final metal topography. Deposited at 300-400°C, compatible with all BEOL metals. - **PECVD Silicon Dioxide (SiO2)**: Often deposited first as a stress-buffer layer between the compressive SiN and the metal underneath. The SiO2/SiN bilayer provides better adhesion and reduced stress-induced cracking compared to SiN alone. - **Polyimide / PBO (Polybenzoxazole)**: Organic passivation used in advanced packaging, redistributed layer (RDL) processes, and MEMS. Spin-coated and cured at 350°C, polyimide provides a thick (5-20 um), planarizing, and mechanically compliant passivation that absorbs thermal-mechanical stress during packaging and solder bump attachment. **Process Integration** 1. **Deposit Passivation Stack**: SiO2 (100-300 nm) + SiN (300-800 nm) by PECVD over the finished BEOL. 2. **Pad Opening Etch**: Litho and etch steps open windows in the passivation over the bond pads — exposing the aluminum or copper pad for wire bonding, flip-chip bumping, or probe testing. 3. **Post-Pad Etch Clean**: Remove etch polymer and native oxide from the pad surface to ensure low-resistance bonding. **Reliability Implications** - **HAST (Highly Accelerated Stress Test)**: Chips are exposed to 130°C, 85% relative humidity, and bias voltage for hundreds of hours. The passivation must prevent moisture ingress throughout this extreme test. - **Crack Resistance**: During dicing (sawing the wafer into individual dies), mechanical vibration can propagate cracks along the die edge. The passivation must be tough enough to arrest crack propagation before it reaches active circuitry. Passivation Layer Deposition is **the chip's suit of armor** — the last process step in fabrication and the first line of defense against the harsh physical world that will surround the chip for its entire operational lifetime.