critical dimension (cd)

Critical Dimension (CD) is the smallest or most critical feature size that determines device performance, such as gate length. **Significance**: CD directly affects transistor performance - smaller gates = faster switching. **Definition**: The specific dimension that must be tightly controlled. Usually minimum linewidth. **Targets**: Specified in nanometers. Advanced nodes <10nm for gate CD. **Control**: CD uniformity across wafer and wafer-to-wafer is critical. Tight specifications. **Measurement**: CD-SEM (scanning electron microscope) measures actual dimensions. Scatterometry for grating structures. **CD uniformity**: Target is minimal variation. Specified as 3-sigma or range. **Process impact**: Lithography dose, focus, etch, CMP all affect final CD. **CD bias**: Difference between mask CD and wafer CD. May be intentional (OPC). **After-develop vs after-etch**: Measure CD at both stages. Final CD after etch is what matters. **Device impact**: CD variation causes variations in electrical performance. Tight CD = tight Vt, speed, power specifications.