solve mos threshold drift with phosphosilicate glass psg passivation

# Solve Threshold Drift With Phosphosilicate Glass Passivation

## 1. Why a Phosphorus-Doped Glass Layer Specifically Stops Sodium Migration

Once sodium contamination was identified as the drift mechanism, process engineers found that depositing a phosphosilicate glass (PSG) layer — silicon dioxide doped with phosphorus — either as a cap over the gate oxide or incorporated into the oxide-growth process itself, measurably trapped mobile sodium ions before they could reach the gate or the silicon interface, because phosphorus-doped glass has a specific chemical affinity for binding alkali ions that undoped oxide lacks. This is a targeted fix, not a general cleanliness improvement:

## 2. Real Diagram: Measured Drift, Before and After the PSG Fix, Same Test Protocol

Repeating Step 13's exact sustained-bias drift test on PSG-protected devices shows the threshold holding essentially flat over the same time window where unprotected devices had drifted substantially — the direct, measured confirmation that the identified mechanism and the targeted fix actually matched.

## 3. Measured Drift, Before and After the PSG Fix, Same Test Protocol

This specific fix — PSG passivation against mobile ionic contamination — became a standard step in MOS process flows well beyond this experimental era, carried forward into the commercial PMOS and later CMOS generations this project's own device-architecture series document; it is one of the few process elements from this experimental period that survived essentially unchanged into production.