DRAM 1T1C Memory Cell Fabrication Process: 10-Step Industrial Engineering Breakdown
This interactive digital twin details the complete industrial fabrication sequence of modern 6F² DRAM 1T1C (1-Transistor 1-Capacitor) memory cells, featuring buried wordlines (b-WL), bitline direct contacts, and extreme aspect ratio (>40:1) MIM cylindrical capacitors.
10-Step DRAM 1T1C Semiconductor Fabrication Sequence
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Step 1: Active Area STI Isolation & 6F² Diagonal Island Patterning
In single-crystal silicon, high-resolution lithography and anisotropic silicon dry etch form 6F² diagonal honeycombed active silicon islands, filled with flowable CVD oxide and polished flat by CMP.
Equipment: ArFi Immersion Scanner (ASML Twinscan) + High-Aspect-Ratio RIE (Lam Research Kiyo). Materials: P-Type Single-Crystal Si + SiO2 STI Oxide (Depth: ~150nm).
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Step 2: Buried Wordline (b-WL / BCAT) Saddle-Fin Trench Etch
Anisotropic plasma dry etching cuts deep trenches across both silicon active islands and STI, creating saddle-fin conduction profiles for the buried access transistor.
Equipment: High-Density Silicon RIE System (TEL Tactras). Materials: Recessed Silicon Cavities (Depth: ~120nm, Width: ~25nm).
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Step 3: Gate Dielectric, Tungsten b-WL Metallization & SiN Capping
Thermal oxidation forms high-reliability gate oxide, followed by conformal ALD TiN and CVD Tungsten fill. Recess etch-back and CMP planarize with SiN capping to seal the wordline beneath the surface.
Equipment: Radical Gate Oxidation Furnace + W-CVD System + CMP Polisher. Materials: SiO2/SiON Gate Dielectric + TiN Barrier + Tungsten (W) + Si3N4 Cap (~40nm).
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Step 4: Bitline Direct Contact (DC) Self-Aligned Etch & Poly Plug
Self-aligned contact (SAC) etching opens direct contact holes to the shared central active silicon node, filled with heavily doped N+ polysilicon.
Equipment: Self-Aligned Contact RIE Etcher (Applied Materials Centura). Materials: N+ Doped Poly-Si (Doping: 2e20 cm⁻³).
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Step 5: Bitline Poly-Si/W Stack Patterning & Sidewall Spacers
Depositing a composite stack of doped poly-Si, TiN, low-resistance Tungsten, and SiN hardmask. Anisotropic etching patterns bitlines running orthogonal to buried wordlines, encapsulated by SiBCN spacers.
Equipment: Metal RIE Etcher + ALD Spacer Reactor. Materials: Poly-Si / TiN / W (~40nm) / Si3N4 Cap + Low-k SiBCN Spacers.
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Step 6: Storage Node Contact (SNC) Etch & Hexagonal Landing Pads
Deep contact etching between bitlines down to silicon active source regions. Filled with selective tungsten and CMP polished into hexagonal landing pads to provide a broad base for tall capacitors.
Equipment: High Aspect Ratio Contact Etcher + Tungsten CMP System. Materials: Ti/TiN Liner + Tungsten (W) Hexagonal Landing Pads.
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Step 7: Thick Sacrificial Mold Oxide & Dual Si3N4 Supporter Stack
PECVD depositing a thick 1.2-1.5μm sacrificial SiO2 mold oxide, intercalated with two thin Si3N4 supporter membranes to mechanically stabilize capacitors.
Equipment: Ultra-Thick Film PECVD System (Applied Materials Producer GT). Materials: Sacrificial SiO2 Mold (~1400nm) + Dual Si3N4 Supporter Layers (~35nm each).
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Step 8: Extreme High Aspect Ratio (>40:1) Capacitor Hole Etch
Using thick boron-doped amorphous carbon hardmask (B-ACL) and cryogenic fluorocarbon plasma RIE to punch vertical cylindrical holes through the entire mold stack down to each landing pad.
Equipment: Extreme HAR Cryogenic Dielectric Etcher (Tokyo Electron TEL Tactras / Lam Vantex). Materials: Deep Cylindrical Storage Node Voids (Depth: ~1400nm, Diameter: ~35nm).
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Step 9: Mold Oxide Selective Wet Strip & Freestanding TiN Cylinder Release
ALD deposits thin TiN bottom electrode, followed by selective buffered oxide etch (BOE) stripping all sacrificial SiO2 mold while preserving Si3N4 supporter lattices to prevent cylinder collapse.
Equipment: Single-Wafer Wet Clean Station (SCREEN SU-3200). Materials: TiN Bottom Electrode (~8nm) + Dilute BOE Chemistry, selectivity > 200:1 over Si3N4.
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Step 10: ALD MIM ZAZ (ZrO2/Al2O3/ZrO2) High-k Dielectric & Top Plate Fill
Atomic Layer Deposition coats nanometer ZAZ high-k dielectric stack and TiN/W top plate electrode held at VDD/2, completing the MIM storage capacitor.
Equipment: Atomic Layer Deposition System (TEL Trias ALD / ASM Pulsar). Materials: ZrO2 (~2.5nm) / Al2O3 (~1nm) / ZrO2 (~2.5nm) ZAZ High-k Stack (~6nm, k~38) + TiN/W Top Plate.
Frequently Asked Questions (FAQ) on DRAM 1T1C Fabrication
- Why are Buried Wordlines (b-WL) essential in modern DRAM?
- Buried wordlines sink the gate electrode entirely beneath the silicon wafer surface. This eliminates parasitic capacitive coupling between wordlines and bitlines, reduces bitline capacitance by over 30%, provides a saddle-fin conduction channel with increased effective channel length (Leff), and drastically suppresses off-state subthreshold leakage.
- What is the 6F² cell layout in DRAM manufacturing?
- 6F² is an ultra-dense layout geometry where each memory cell occupies only 6 times the minimum lithographic feature area squared (F²). By slanting active silicon islands at an angle, two access transistors share a single central bitline contact, maximizing bit density per silicon wafer.
- What is the ZAZ High-k dielectric stack in DRAM MIM capacitors?
- ZAZ stands for Zirconia-Alumina-Zirconia (ZrO2 / Al2O3 / ZrO2). ZrO2 provides a very high dielectric constant (k ~ 35-45 in tetragonal phase), while an ultra-thin middle layer of Al2O3 (k ~ 9) disrupts grain boundaries and increases the bandgap offset, suppressing quantum mechanical leakage currents below 10^-7 A/cm².