Confront Early Mos Threshold Instability from Sodium Ion Contamination and Oxide Drift

# Confront Early MOS Threshold Instability: Sodium-Ion Contamination and Oxide Drift

## 1. Why Devices That Worked on the Bench Failed Under Sustained Bias

Devices that measured a stable, usable threshold voltage when first tested showed that threshold drifting measurably over hours to days of continuous operation under bias, and the root cause, traced through careful contamination analysis, was mobile sodium ions already present in the oxide (from furnace contamination, handling, or ambient sources) migrating toward the gate or the silicon interface under the sustained electric field — a distinct mechanism from the fixed-charge scatter Step 4 measured, and one that a one-time threshold measurement at fabrication could not catch at all. The drift mechanism, stated as a rate process:

Confront Early MOS Threshold Instability: Sodium-Ion Contami 1. Why Devices That Worked on the Bench Failed Under Sustained Bias MOBILE Na+ MIGRATING UNDER SUSTAINED GATE BIAS oxide bulk, Na+ ions initially scattered through the thickness under sustained positive bias, Na+ drifts toward the gate/oxide interface over hours to days a one-time post-fabrication test cannot catch a drift mechanism that only manifests under sustained bias

## 2. Real Diagram: Measured Threshold Shift Over Time, the Signature That Identified the Mechanism

The telltale signature distinguishing ion drift from simple fixed-charge scatter was its time dependence: threshold voltage measured at time zero, then again after hours of continuous bias, then again after a bias reversal — the reversible, bias-direction-dependent shift pattern that only a mobile charge species, not a frozen fixed charge, could produce.

Real Diagram 2. Real Diagram: Measured Threshold Shift Over Time, the Signature That Identified the Mec time under sustained bias bias reversed here threshold recovers on reversal reversible, bias-direction-dependent shift — the specific signature of a mobile ionic species, not a fixed defect

## 3. Measured Threshold Shift Over Time, the Signature That Identified the Mechanism

Identifying the mechanism precisely — mobile sodium, not some other unidentified defect — is what made the fix in the next step possible: a process solution aimed specifically at sodium contamination, not a general-purpose oxide-quality improvement that might or might not have addressed the real cause.

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