⏸ Pause Refresh Failure Physics

(0.25µm-generation DRAM)

This document describes the physical origin, manifestation, and evolution of Pause Refresh failures observed in 0.25µm-generation DRAM.

Pause refresh failure was the primary refresh-related reliability limiter for this technology node, dominating both yield and high-temperature reliability.

🧪 1. Failure Detection

Fail Bit Map Acquisition under Pause Stress

Pause refresh failure was first identified through fail bitmap acquisition under refresh-pause stress conditions.

Fig. Fail bit distribution obtained under Pause Refresh stress in 0.25µm-generation DRAM

Observations

• Fail bits appeared spatially random
• No row-, column-, or block-level clustering
• No periphery or edge concentration

📌 Initial implication:
The failure was not driven by layout topology or array structure.

🔍 2. Failure Mode Identification

Single-Bit Dominant Behavior

Detailed bitmap analysis revealed:

• Failures occurred predominantly as isolated single-bit errors
• No word-line, bit-line, or block-level correlation
• No pattern-dependent aggregation

Mechanisms ruled out

📌 Conclusion:
The dominant mechanism was intrinsic cell-level degradation.

🌡 3. Temperature Dependence

Failure Population Evolution

Temperature sweep testing showed:

• Fail bit count increased exponentially at high temperature
• Some failing bits recovered at lower temperature
• Fail population evolved continuously, not discretely

📌 Key insight:
The failure mechanism was thermally activated and reversible, indicating leakage-driven behavior, not permanent breakdown.

⚛️ 4. Physical Root Cause

n⁺ / p⁻ Junction Leakage

All observed characteristics are consistent with:

• n⁺ / p⁻ junction leakage current
• Shockley–Read–Hall (SRH) generation via defect states
• Strong exponential temperature dependence

💡 Critical finding
Storage capacitance (C_cell) margin was sufficient.
Retention loss was leakage-dominated, not C-limited.

This shifted engineering focus away from capacitor scaling toward junction integrity.

🧱 5. Device Structure Origin

Localized Leakage Paths

Fig. Local leakage paths responsible for Pause Refresh failures in 0.25µm-generation DRAM

Leakage was localized at:

• Junction periphery regions
• STI-adjacent diffusion edges
• High electric-field corners

📌 These locations are highly sensitive to process-induced damage.

⚙️ 6. Process Dependency

Plasma Damage Sensitivity

Strong correlation was observed between Pause failures and:

• Plasma etching exposure
• Aggressive ashing / dry cleaning
• Increased interface state density (D_it)

📌 Key lesson:
Process-induced latent damage surfaced only under pause stress.

🛠 7. Countermeasure Direction

Damage Avoidance, Not Circuit Compensation

Effective mitigation focused on preventing damage, not masking it:

• Plasma condition optimization
• Soft-landing etch techniques
• Ashing damage reduction
• Interface recovery anneals

These measures resulted in a significant reduction of Pause Refresh fail population at wafer level.

🧠 Summary — Failure Causality Chain

Pause Refresh failure in 0.25µm DRAM followed a clear, traceable chain:

Fail bitmap acquisition
→ Single-bit dominant failure mode
→ Strong temperature dependence
→ n⁺ / p⁻ junction leakage
→ Plasma damage–driven defect generation

📘 Legacy insight:
Reliability was limited not by design intent, but by process-induced leakage physics.

This causal structure continues to reappear in modern memory technologies under different names.

🔗 Related Documents

• process_flow.md
• wafer_test_bin.md