middle-of-line process development

**MOL** (Middle-of-Line) is the **process module between the front-end transistor (FEOL) and the back-end interconnect (BEOL)** — encompassing the local contacts to transistor source, drain, and gate terminals that connect individual devices to the first metal interconnect layer. **Key MOL Process Steps** - **Contact Etch**: High-aspect-ratio contact holes through the ILD to reach S/D and gate. - **Silicide/Contact**: Form low-resistance contact at the S/D surface (Ti/TiN liner + silicide). - **Metal Fill**: Fill contacts with tungsten (W), cobalt (Co), or ruthenium (Ru). - **Local Interconnect (LI)**: Short-range wiring that connects closely spaced transistors locally. **Why It Matters** - **Contact Resistance**: MOL is the bottleneck for contact resistance — the largest contributor to parasitic resistance at advanced nodes. - **New Materials**: Transition from W to Co to Ru for contact fill to reduce resistance at smaller dimensions. - **Scaling**: MOL dimensions are the smallest in the chip — pushing the limits of etch, fill, and CMP. **MOL** is **the bridge between transistors and wires** — connecting the atomic-scale transistor terminals to the nanoscale interconnect network.