Process capability in SPC measures a process's ability to produce output within specification limits — it quantifies how well the natural variation of the process fits within the required tolerance window. High capability means the process consistently produces results well within spec; low capability means the process frequently approaches or exceeds the limits.

Key Capability Metrics

• Cp (Process Capability Index):

Compares the specification width to the process spread. Does not consider process centering — it measures potential capability if the process were perfectly centered.

• Cpk (Process Capability Index, Centered):

Accounts for how close the process mean is to the nearer spec limit. Always ≤ Cp. Cpk = Cp only when the process is perfectly centered.

Interpreting Capability Values

Why Capability Matters in Semiconductors

• A CD process with Cpk < 1.33 produces too many out-of-spec features — directly causing yield loss.
• Critical steps (gate CD, overlay, film thickness for thin films) often require Cpk ≥ 1.67.
• Non-critical steps may accept Cpk ≥ 1.0, but improvement is expected.
• Cp vs. Cpk Gap: If Cp is high but Cpk is low, the process has adequate precision but is off-center — a simple target adjustment can improve Cpk.

Improving Process Capability

• Reduce σ: Tighten the process spread through equipment improvements, recipe optimization, or better raw materials. This improves both Cp and Cpk.
• Center the Process: Adjust the process mean to the midpoint of the specification range. This improves Cpk without changing Cp.
• Widen Specifications: If the specs are unnecessarily tight, relaxing them improves capability — but this requires design validation.

Process capability is the ultimate measure of process quality — it directly connects manufacturing variation to product specifications and defect rates.