Lower Specification Limit

LSL (Lower Specification Limit) is the minimum acceptable value for a measured parameter — the lower engineering boundary below which the product fails to meet performance, reliability, or quality requirements.

LSL in Practice

• CD Control: LSL for gate CD might be target - 2nm — below this causes leakage or reliability issues.
• Film Thickness: LSL for barrier layer thickness — below this allows metal diffusion.
• Adhesion Strength: LSL for film adhesion — below this causes delamination.
• Drive Current: LSL for transistor Idsat — below this means the transistor is too slow.

Why It Matters

• Pass/Fail: Measurements below LSL result in product rejection — the lower quality boundary.
• Cpk (Lower): $Cpk_{lower} = \frac{\bar{x} - LSL}{3\sigma}$ — measures capability relative to the lower limit.
• Asymmetric Risk: Upper and lower failures often have different consequences — LSL and USL may have different criticalities.

LSL is the minimum required — the lower engineering limit below which product performance or reliability is compromised.