Out of Control (OOC) is the SPC designation indicating that a process has exceeded its statistical control limits or violated control chart rules, signaling that an assignable cause (a specific, identifiable source of variation) has affected the process. OOC triggers investigation and corrective action.

When a Process Is Out of Control

A process is declared OOC when its control chart shows any of these conditions:

• Point beyond 3σ: A single measurement exceeds the upper or lower control limit.
• Run rules violated: Patterns like 8 consecutive points on one side of the mean, 2 of 3 points beyond 2σ, or 4 of 5 points beyond 1σ (Western Electric rules).
• Trend: A consistent upward or downward pattern of 6+ consecutive points.
• EWMA/CUSUM alarm: The cumulative statistic exceeds its decision boundary.

The OOC Response Process

• Stop (if critical): For critical process steps, production on the affected tool may be halted until the cause is identified and corrected.
• Flag Wafers: Wafers processed since the last known-good measurement are flagged for additional inspection or disposition review.
• Investigate: Engineers identify the assignable cause — what specific change caused the process excursion?
• Correct: Fix the root cause — adjust the recipe, replace a consumable, repair hardware, etc.
• Verify: Run monitor wafers to confirm the process has returned to its in-control state.
• Disposition: Determine whether flagged wafers can continue processing, need rework, or must be scrapped.

Common Causes of OOC in Semiconductor Fabs

• Hardware Degradation: Worn chamber components, deteriorating electrodes, aging RF generators.
• Consumable End-of-Life: Gas filters, ESC surfaces, polishing pads nearing replacement.
• Contamination: Particles, metal contamination, or moisture in the process chamber.
• Recipe Drift: Unintended changes in gas flow, temperature, or power delivery.
• Maintenance Issues: Post-PM requalification problems, incorrect part installation.
• Environmental: Fab temperature/humidity excursions, utility (gas, water) quality changes.

OOC Severity Levels

• Warning (Soft OOC): Process is trending toward limits — increase monitoring frequency but continue production.
• Action (Hard OOC): Process has violated control limits — stop the tool, investigate, correct.
• Critical: Multiple parameters OOC simultaneously or extreme excursion — immediate tool shutdown and escalation.

OOC management is the core feedback loop of semiconductor process control — the speed and effectiveness of OOC response directly determines fab yield and productivity.