contact hole

Contact holes are small vertical openings in dielectric that enable electrical connections between metal layers and transistors below. **Function**: Connect first metal layer down to transistor (source, drain, gate contacts). **Shape**: Ideally cylindrical. Round in layout, may print slightly elliptical. **Size**: Diameter typically 1-2X minimum CD. Aspect ratio (depth/width) up to 10:1 or more. **Lithography challenge**: Contacts are isolated features, harder to print than lines/spaces. Lower contrast. **Etch challenge**: High aspect ratio contact holes require specialized anisotropic etch. **Fill challenge**: Must fill narrow hole with metal. Barrier and seed layers consume space. **Resistance**: Smaller contacts have higher resistance. Multiple contacts per transistor for low resistance. **Overlay critical**: Contacts must land precisely on underlying features. Misalignment causes device failure. **Dual damascene**: Contact and first metal trench etched together, filled with copper simultaneously. **SAC (Self-Aligned Contact)**: Contact is self-aligned to gate structure, relaxing overlay requirements.