duv (deep ultraviolet)

DUV (Deep Ultraviolet) lithography uses short-wavelength ultraviolet light — primarily 193nm (ArF) and 248nm (KrF) — to pattern semiconductor wafers, and has been the workhorse lithography technology for the majority of semiconductor manufacturing history, enabling feature sizes from 250nm down to approximately 38nm through resolution enhancement techniques. DUV lithography operates on the principle of photochemical reactions: the short-wavelength UV light passes through a patterned photomask, is focused by a projection lens system onto the wafer coated with photoresist, and the exposed resist undergoes chemical changes that allow selective removal during development. The fundamental resolution limit is governed by the Rayleigh criterion: Resolution = k₁ × λ / NA, where λ is the wavelength, NA is the numerical aperture of the projection lens, and k₁ is a process-dependent factor (theoretical minimum 0.25, practical minimum ~0.28-0.35). For 193nm immersion (193i) with NA = 1.35, the single-exposure resolution limit is approximately 38nm — pushing below this requires multiple patterning techniques (LELF, SADP, SAQP) that use 2-4 exposure steps per layer. Resolution enhancement techniques that extended DUV capability far beyond its natural resolution include: optical proximity correction (OPC — modifying mask patterns to compensate for optical distortion), phase-shift masks (PSM — using phase differences to improve contrast), off-axis illumination (OAI — tilting the illumination to optimize the diffraction pattern for specific feature types), source-mask optimization (SMO — jointly optimizing the illumination source shape and mask pattern), and immersion lithography (using water between the lens and wafer to increase the effective NA from 0.93 to 1.35 by replacing air with a higher refractive index medium). DUV lithography remains extensively used even in advanced fabs alongside EUV — many non-critical layers at 5nm and 3nm nodes are still printed with 193i DUV because it is more mature, higher throughput, and lower cost than EUV. ```svg Immersion lithography (193i): water under the lens to print smallerA water film raises the 193 nm ArF numerical aperture from 0.93 to 1.35 — the DUV workhorse beside EUV1 · The water trick: raise NAfinal projection lensθultra-pure water · n = 1.44wafer + resistNA = n · sin θWater lets the lens collect a widercone → NA 1.35 (vs 0.93 in air).CD = k₁λ/NA: ~65 nm → ~38 nm2 · A fluid-control machinelenswaterscanwatermark23.000 ± 0.001 °C sets the index ndegassed — bubbles print as defects1–3 L/min flush · ultra-pure looptopcoat / hydrophobic resist stop leachreceding angle > 70° or watermarksAs much a fluid machine as an optic.3 · Where it sitsOne 193i exposure: ~38 nm half-pitch.Multi-patterning shrinks it:single38 nmSADP19 nmSAQP9.5 nmhalf-pitch · ArFi + multi-patterningArFi + MP vs EUVEUV prints the hardest layers, but mostlayers still use DUV/immersion — faster,cheaper, mature.A leading fab = EUV plus a largeinstalled base of immersion scanners.The water trickA 193 nm ArF beam through water(n=1.44) collects a wider cone —NA 0.93→1.35, ~35% smaller CD.Fluid-control machineDegassed, 23.000±0.001°C water;topcoat + high contact angle beatbubbles and watermark defects.Still the workhorseWith SADP/SAQP it reaches ~9.5 nmhalf-pitch; most layers in an EUV fabstill run on immersion scanners. ```

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