⚙️ 0.25µm 64M DRAM Process Flow (Reconstructed)

This document presents a reconstructed, end-to-end process flow for a 0.25µm-generation 64M DRAM, covering both cell array and peripheral CMOS regions.

⚠️ Important Note
This process flow is reconstructed from manufacturing experience and period-consistent knowledge.

It is intended solely for educational, analytical, and archival use
— not as a foundry-ready manufacturing recipe.

📌 Scope

This document emphasizes process–failure causality rather than numerical optimization.

🧭 High-Level Flow Overview

1. Isolation & well formation
2. Gate stack and word-line formation
3. LDD / spacer / deep source–drain formation
4. Bit-line formation
5. Capacitor formation (stacked, ONO dielectric)
6. Interlayer dielectric & contacts
7. Metal interconnects and passivation

📘 Each block below highlights why it mattered,
not merely what was done.

1️⃣ Isolation & Well Formation

• LOCOS-based isolation
• Pre-oxide and sacrificial oxidation steps
• Triple-well structure adopted in cell region

Engineering intent

• Substrate noise suppression
• Improved soft-error immunity
• Retention margin stabilization

🔎 Key sensitivity
Early surface condition directly affected later-stage
junction leakage variability.

2️⃣ Gate Stack & Word Line Formation

• Thin gate oxide (~80 Å class)
• Poly-Si + WSi word-line stack
• KrF lithography at critical CD limits

Engineering intent

• Aggressive CD control for WL pitch
• Sheet resistance reduction

🔎 Key sensitivity
Plasma exposure during gate etch propagated into
junction damage and retention failures observed much later.

3️⃣ Source / Drain Formation

• Independent optimization for:
  • Cell NMOS
  • Peripheral NMOS / PMOS
• LDD → spacer → deep S/D sequence

Engineering intent

• Balance drive current and leakage
• Protect short-channel behavior

🔎 Key sensitivity
Excessive plasma exposure or HF-based cleaning
amplified junction leakage dispersion.

4️⃣ Bit Line Formation

• Simultaneous bit-line and contact formation
• WSi-based low-resistance interconnect

Engineering intent

• Reduce RC delay
• Simplify integration sequence

🔎 Key sensitivity
Overlay margin and contact integrity directly affected
early functional yield.

5️⃣ Capacitor Formation

• Stacked capacitor structure
• Surface roughening for capacitance boost (≈1.5–1.8×)
• ONO dielectric

Engineering intent

• Secure sufficient storage capacitance
• Enable refresh margin

🔎 Key sensitivity
Capacitance itself was adequate;
retention failures were leakage-dominated, not C-limited.

6️⃣ Interlayer Dielectric & Contacts

• BPSG deposition and reflow
• Tungsten plug (etch-back, no CMP)

Engineering intent

• Planarity improvement
• Contact resistance stabilization

🔎 Key sensitivity
Contact leakage and interface damage mapped directly to
Pause Refresh failures at high temperature.

7️⃣ Metal Interconnect & Passivation

• Dual-layer Al-based interconnect
• Final hydrogen sinter

Engineering intent

• Interconnect reliability
• Leakage suppression and passivation stabilization

📐 Appendix A. Memory Cell Layout (Planar View)

Appendix A — Notes (Planar Layout)

This figure provides a simplified top-down (planar) layout view of a DRAM memory cell, illustrating the spatial relationship between:

• Active regions
• Word-line gate structures
• Bit-line contacts
• Isolation regions

🎯 Purpose of this appendix

• Not to define exact layer dimensions
• But to provide spatial context for process–failure interaction

Local process choices (plasma, cleaning, junction handling) act on
specific lateral regions, whose proximity strongly influences:

• Wafer test results
• Retention behavior
• Disturb sensitivity
• Yield and binning outcomes

🧱 Appendix B. Memory Cell Cross Section (Schematic)

Appendix B — Notes (Cross Section)

This schematic shows a reconstructed vertical cross-sectional view of a representative 0.25µm-generation DRAM memory cell.

It highlights the relative positioning of:

• Active regions (n⁺ diffusion in p-well)
• Word-line (WL) gate stack
• Bit-line (BL) contact
• Storage node (SN)
• LOCOS-based isolation

⚠️ This is not a foundry-exact device profile.
It is an abstracted physical model for understanding process–failure causality.

Mapping process sensitivities to structure

• Gate etch plasma exposure
  → Junction damage beneath WL edges
  → Increased leakage, retention degradation

• Contact etch & post-clean
  → Leakage paths at BL–diffusion interface

• LOCOS edge geometry
  → Local electric-field enhancement
  → Disturb-related failures

Together with the planar layout, this cross-section enables lateral + vertical analysis of memory cell behavior.

🧠 Summary

This reconstructed process flow demonstrates how seemingly local process decisions—

• Plasma conditions
• Cleaning strategies
• Junction handling

propagated upward into:

• Wafer test binning behavior
• Retention and disturb failures
• Yield excursions
• Final product and business decisions

📘 This is the essence of a Legacy Process Case.

🔗 Related Documents

• wafer_test_bin.md
• pause.md