short-term variation

**Short-term variation** is the **rapid random or cyclic fluctuation in process outputs over short windows such as wafer-to-wafer or lot-to-lot intervals** - it reflects immediate noise sources that affect repeatability. **What Is Short-term variation?** - **Definition**: High-frequency output spread occurring within stable operating periods. - **Typical Sources**: Sensor noise, flow jitter, control-loop response limits, and micro-environment disturbances. - **Statistical Form**: Appears as local dispersion around process center without long-term mean shift. - **Measurement Need**: Requires sufficient sampling resolution to separate noise from drift. **Why Short-term variation Matters** - **Repeatability Impact**: High short-term spread reduces within-lot consistency and margin to spec. - **Capability Penalty**: Increased sigma directly lowers Cp and Cpk even when mean is centered. - **Detection Challenge**: Excess noise can mask emerging special-cause signals. - **Tuning Priority**: Reducing short-term variation often improves quality with minimal process change. - **Customer Reliability**: Better repeatability supports tighter product performance distribution. **How It Is Used in Practice** - **Noise Characterization**: Quantify short-window variance by tool, chamber, and operating condition. - **Control Optimization**: Tune PID loops, replace unstable components, and harden sensor filtering. - **SPC Configuration**: Use chart selection and sampling plans appropriate for high-frequency behavior. Short-term variation is **a core determinant of process repeatability** - controlling fast noise sources is necessary to maintain strong capability and stable wafer-level performance.