directed self assembly

**Directed Self-Assembly (DSA)** is a **patterning technology that uses the thermodynamic self-organization of block copolymers (BCP) to create sub-10nm features** — guided ("directed") by a pre-pattern to produce regular arrays with feature sizes beyond conventional lithography limits. **Block Copolymer Self-Assembly** - BCP: Two chemically distinct polymer blocks (A-B) covalently bonded. - Immiscible blocks phase-separate into periodic nanoscale domains. - PS-b-PMMA (polystyrene-block-polymethyl methacrylate): Standard DSA polymer. - Period $L_0$: 20–50nm for typical BCPs (can reach < 10nm with high-χ BCPs). - Morphology: Lamellae (lines), cylinders, spheres — tuned by volume fraction. **Guiding Strategies** **Graphoepitaxy**: - BCP fills lithographically-defined trenches or wells. - BCP period determined by trench width (must be multiple of $L_0$). - No need for surface chemistry control inside trench. **Chemical Epitaxy**: - Lithographically define alternating surface chemistry stripes. - BCP domains align to surface chemistry pattern. - Can multiply original pattern frequency: 1 guide stripe → 4 BCP stripes. - Critical for cutting metal tracks in EUV or LELE patterning. **DSA in HVM Integration** - **Contact hole shrink**: BCP fills hole, one block etched, smaller hole remains → < 20nm contacts. - **Line/space patterning**: BCP lamellae create < 15nm half-pitch lines. - **Frequency doubling**: One litho step + DSA = 2x the pattern density. **Challenges** - Defect density: BCP domains can have dislocations, disclinations → yield risk. - Process window: Temperature, time, surface energy control are tight. - Long-range order: BCP natural period is only ~20–30nm; guided order over mm² needed. - Pattern rectification only: DSA adds resolution but can't create arbitrary patterns. DSA is **a promising multi-patterning complement at sub-5nm nodes** — particularly for contact hole patterning and line multiplication where its natural periodicity matches device requirements.