Polysilicon Backside Seal (PBS) is the deposition of an undoped polycrystalline silicon layer (typically 0.5-1.5 microns thick) on the wafer backside to provide a thermally stable, particle-free extrinsic gettering layer — the dense network of grain boundaries in the polysilicon film creates an enormous density of trapping sites for metallic impurities, and unlike mechanical backside damage, polysilicon backside seal remains effective through all subsequent high-temperature processing steps, making it the premium extrinsic gettering solution for advanced CMOS logic and memory manufacturing.

What Is Polysilicon Backside Seal?

• Definition: A process step in which a thin polycrystalline silicon film is deposited by LPCVD on the non-active backside of the wafer before the start of front-end processing, creating a layer whose grain boundaries serve as permanent, thermally stable gettering sinks for transition metal impurities.
• Grain Boundary Density: Polysilicon deposited at typical temperatures (580-640 degrees C) has grain sizes of 20-100 nm, producing a grain boundary density of 10^10-10^11 cm of boundary per cm^3 — this enormous boundary area provides a vast number of trapping sites that far exceeds the capacity of mechanical damage or even most BMD distributions.
• Trapping Mechanism: Metals diffusing through the wafer to the backside encounter the polysilicon grain boundaries where they segregate preferentially (segregation coefficients of 10-1000 for transition metals at grain boundaries) and become trapped in electrically inactive configurations — once trapped, the metals remain immobilized through all subsequent processing.
• Thermal Stability: Unlike mechanical backside damage that anneals out above 1000 degrees C, polysilicon grain boundaries are thermodynamically stable and actually improve in gettering effectiveness after high-temperature processing through grain growth that drives boundary segregation to the fewer remaining boundaries — PBS provides gettering throughout the entire thermal budget.

Why Polysilicon Backside Seal Matters

• Advanced Node Standard: PBS is the default extrinsic gettering technique for 300mm wafers at advanced logic and memory nodes — its combination of thermal stability, no particle generation, and wafer stress symmetry makes it compatible with the stringent requirements of sub-10nm manufacturing.
• No Particle Generation: Unlike mechanical backside damage, polysilicon deposition is a clean CVD process that generates no particulates — this is critical for 300mm fab environments where particles on the backside can transfer to the frontside of adjacent wafers during cassette handling.
• Stress Symmetry: The polysilicon film on the backside creates a stress that partially balances the stress from frontside deposited films — this stress symmetry reduces wafer bow and improves lithography overlay compared to bare or mechanically damaged backsides.
• Wafer Vendor Integration: Most CZ silicon wafer vendors offer PBS as an available wafer specification option — the polysilicon is deposited at the wafer vendor facility before shipping to the fab, so the fab receives wafers with gettering already built in.
• Dual Protection with IG: PBS combined with intrinsic gettering provides two independent gettering defense layers — metals moving toward the bulk encounter BMD gettering sites, while metals moving toward the backside encounter the polysilicon getter, providing comprehensive protection regardless of the contamination flux direction.

How Polysilicon Backside Seal Is Implemented

• LPCVD Deposition: Low-pressure chemical vapor deposition using silane (SiH4) at 580-640 degrees C produces a uniform polysilicon film on the wafer backside — temperature and pressure control the grain size, which influences gettering capacity through the resulting grain boundary density.
• Film Thickness: Typical thickness of 0.5-1.5 microns provides sufficient grain boundary volume for effective gettering while minimizing stress and process time — thicker films provide more gettering capacity but increase deposition time and stress.
• Undoped Film: The polysilicon is intentionally left undoped to maximize the number of available trapping sites at grain boundaries — doping would partially passivate boundary dangling bonds and reduce gettering capacity.

Polysilicon Backside Seal is the premium extrinsic gettering solution for advanced semiconductor manufacturing — its thermally stable grain boundary network provides permanent, particle-free metallic impurity trapping that remains effective through all processing temperatures, making it the preferred backside gettering technique for the most demanding CMOS logic and memory products.