multi-patterning

**Multi-Patterning (SADP/SAQP)** is **a set of lithographic patterning techniques that use self-aligned spacer deposition and mandrel removal cycles to multiply the spatial frequency of features beyond the resolution limit of a single lithographic exposure, enabling the fabrication of line/space patterns at pitches below what even EUV lithography can print in a single pass** — with self-aligned double patterning (SADP) halving the pitch and self-aligned quadruple patterning (SAQP) quartering it. - **SADP Process**: A mandrel pattern is printed at relaxed pitch using 193i or EUV lithography; conformal spacer material (typically SiO2 or SiN) is deposited over the mandrels by ALD or PECVD; anisotropic spacer etch removes the horizontal portions, leaving spacers on both mandrel sidewalls; the mandrel is selectively removed, and the remaining spacers serve as a hard mask at half the original pitch. - **SAQP Extension**: The SADP spacer pattern becomes the new mandrel for a second spacer deposition and etch cycle, producing features at one-quarter of the original lithographic pitch; SAQP is essential for metal and fin patterning at nodes of 7 nm and below where pitches of 24-30 nm are required but single-exposure EUV resolution is limited to approximately 30-36 nm pitch. - **Spacer Thickness Control**: The final feature width equals twice the spacer thickness, making ALD deposition uniformity (within plus or minus 0.3 nm across the wafer) the primary determinant of critical dimension (CD) uniformity; any spacer thickness variation directly maps to CD variation. - **Mandrel CD and Pitch Walking**: Variations in mandrel CD cause alternating wide and narrow spaces in the final pattern, a defect known as pitch walking; maintaining mandrel CD uniformity below 0.5 nm 3-sigma is essential to keep pitch walking within the electrical tolerance of the circuit. - **Line-Edge Roughness (LER)**: Each spacer transfer step can amplify or smooth LER depending on deposition conformality and etch anisotropy; SADP typically smooths LER on the spacer-defined edges while preserving roughness on the mandrel-defined edges, creating asymmetric roughness profiles. - **Cut and Block Patterning**: After spacer patterning creates a continuous grating, separate cut mask lithography and etch steps remove unwanted line segments to define the desired circuit layout; cut placement accuracy and etch selectivity are critical for avoiding shorts and opens. - **Design Rule Implications**: Multi-patterning imposes strict design rule constraints including unidirectional routing, fixed-pitch grids, and color-aware decomposition that limit layout flexibility; designers must work within these constraints to ensure manufacturability. Multi-patterning remains essential in the toolbox of advanced semiconductor manufacturing, complementing EUV lithography at the tightest pitches where even high-numerical-aperture EUV cannot achieve single-exposure resolution.