afm (atomic force microscopy)

AFM (Atomic Force Microscopy) measures surface topography at nanometer to sub-angstrom vertical resolution by scanning a sharp probe tip across the surface. **Principle**: Tip on flexible cantilever scans surface. Tip-surface forces (van der Waals, contact, electrostatic) deflect cantilever. Laser reflected from cantilever onto position-sensitive detector measures deflection. **Modes**: **Contact mode**: Tip touches surface. Measures deflection. Can damage soft surfaces. **Tapping mode**: Tip oscillates near surface. Amplitude change detects surface. Gentler, most common for semiconductors. **Non-contact**: Tip oscillates above surface. Detects force gradient. Minimal surface interaction. **Resolution**: Vertical resolution <0.1nm (sub-angstrom). Lateral resolution limited by tip radius (~2-10nm). **Applications in semiconductor**: Surface roughness measurement (RMS roughness), CMP surface quality, step height measurement, LER/LWR analysis, grain size characterization. **Scan area**: Typically 0.1 x 0.1 um to 100 x 100 um. Larger scans take longer. **Scan speed**: Slow compared to optical methods. Minutes per image. Not suitable for high-volume inline use. **CD-AFM**: Specialized tips (flared or tilted) can measure sidewall profiles and trench CDs. True 3D metrology. **Tip artifacts**: Tip shape convolves with surface features. Tip wear degrades resolution over time. Tip radius limits ability to image steep sidewalls. **Data**: Produces 3D height map. Statistical roughness parameters (Ra, RMS) calculated from data.