Polysilicon Gate Deposition and Patterning

Polysilicon Gate Deposition and Patterning is the CMOS process module that deposits and patterns the doped polysilicon (poly) layer that serves as the gate electrode in traditional gate-first integration or as a sacrificial mandrel in replacement metal gate (RMG) processes — with poly CD (critical dimension) directly setting the transistor gate length, making poly deposition uniformity, photoresist patterning, and etch profile control among the most critical process steps in CMOS manufacturing.

Polysilicon Deposition (LPCVD)

- Precursor: SiH₄ (silane) at 600–630°C, pressure 0.1–1 Torr → amorphous Si or poly-Si.
- Below 580°C: Amorphous silicon → annealed above 900°C → recrystallizes to poly.
- 580–630°C: Poly-Si directly → preferred for gate (established grain structure).
- Thickness: 100–150 nm for gate poly (must survive etch and silicidation without full consumption).
- Uniformity: ±1% thickness across 300mm wafer → critical for CD control via reflectometry endpoint.

In-Situ vs Ex-Situ Doping

- In-situ doped: PH₃ (n-type) or B₂H₆ (p-type) added during deposition → doped during growth.
- Advantage: Uniform doping, no additional implant step.
- Disadvantage: Changes deposition rate and grain structure; n/p poly cannot be different in same deposition run.
- Ex-situ (implant doped): Undoped poly → separate B or P implant → more control over doping level.
- Common for gate poly: Separate doping steps for n-poly (NMOS gate) and p-poly (PMOS gate) in CMOS.
- Doping level: 10²⁰ – 10²¹ atoms/cm³ → degenerate semiconductor → metal-like conductivity.

Hard Mask and ARC for Gate Patterning

- Gate patterning demands: Best CD control in entire process → dedicated hardmask + photoresist.
- Stack: Poly / SiO₂ hard mask / SiON or BARC / photoresist.
- Hard mask function: Etch resist during poly etch (photoresist can't survive long poly etch).
- ARC (Anti-Reflective Coating): Reduce standing wave and CD variation from reflection at poly/oxide interface.

Gate Poly Etch

- Chemistry: HBr/Cl₂ main etch → profile control; Cl₂ for lateral etch rate control.
- Selectivity requirements:
- Poly over gate oxide (SiO₂): > 50:1 selectivity → stop etch without consuming thin gate oxide (< 3 nm).
- Poly over STI (SiO₂): Same selectivity → avoid STI erosion.
- Profile: Near-vertical sidewall (89–90°) → precise CD transfer from resist to poly.
- Over-etch: 10–20% over-etch to clear residues → must not penetrate gate oxide.
- CD bias: Poly CD = resist CD - CD bias (from etch loading, plasma, etch profile) → calibrate in OPC.

Poly CD Uniformity

- Gate length variation → Vth variation → circuit speed spread.
- Within-wafer CDU (CD uniformity): Target < ±3% (3σ) at 45nm node → < ±1% at 7nm (EUV).
- Loading effects: Dense poly array etches differently than isolated poly → OPC correction.
- Poly line edge roughness (LER): Line edges not straight → LER → random Lg fluctuation → Vth variation.

Dummy Gates and Gate Density Rules

- Optical lithography: Best poly CD near target pitch → isolated poly prints at different CD than dense.
- Dummy gate fill: Fill open areas with non-functional poly gates → improve optical proximity consistency → better CDU.
- Design rules: Minimum gate density rule → ensures CDU within spec; maximum gate space rule → avoids OPC issues.

Poly in Replacement Metal Gate (RMG) Flow

- RMG: Poly gate is dummy → patterned and etched → source/drain epi and silicide formed → dielectric fill → CMP planarize → poly selectively removed → metal gate deposited in void.
- Advantage: Metal gate deposited last → avoids high-temperature degradation of metal work function.
- Poly removal: H₃PO₄ or TMAH (wet) or H₂/Cl₂ (dry) → high selectivity poly over SiO₂.

Polysilicon gate deposition and patterning are the pattern-definition steps that set the fundamental transistor gate length with sub-nanometer accuracy — because every 1nm variation in gate poly CD translates to a measurable Vth shift and drive current change, achieving ±0.5nm CD uniformity across a 300mm wafer using optimized LPCVD deposition followed by hard-mask-protected plasma etching with carefully calibrated OPC corrections represents one of the most precise manufacturing achievements in high-volume fabrication, one that enabled CMOS scaling from the 1µm through the 28nm planar node before replacement metal gate and EUV took over at finer dimensions.