krf (krypton fluoride)

KrF (Krypton Fluoride) excimer lasers produce 248nm deep ultraviolet light and serve as the light source for DUV lithography systems used to pattern semiconductor features in the 250nm to 90nm range. The KrF excimer laser operates similarly to ArF — electrically exciting a krypton-fluorine gas mixture to form unstable KrF* excimer molecules that emit 248.327nm photons upon dissociation. KrF lithography was the industry workhorse from approximately 1996 to 2005, enabling the critical transition from the i-line (365nm mercury lamp) era to deep ultraviolet, and driving the 250nm, 180nm, 150nm, 130nm, and 110nm technology nodes. KrF laser characteristics include: pulse energy (10-40 mJ), repetition rate (up to 4 kHz), bandwidth (< 0.6 pm FWHM with line narrowing), and high reliability (billions of pulses between gas refills). KrF photoresists use chemically amplified resist (CAR) chemistry based on polyhydroxystyrene (PHS) platforms — the first generation of chemically amplified resists developed for manufacturing. The acid-catalyzed deprotection mechanism enables high photosensitivity, reducing exposure doses compared to non-amplified resists, which was essential given the lower brightness of early excimer sources. Resolution limits: with NA up to ~0.85 and k₁ ≥ 0.35, KrF achieves minimum features of approximately 100-110nm in single exposure. Resolution enhancement techniques (OPC, phase-shift masks, off-axis illumination) extended KrF capability to sub-100nm for select layers. While ArF (193nm) and EUV (13.5nm) have superseded KrF for leading-edge critical layers, KrF lithography remains in active production use for: non-critical layers (implant, contact, metal layers with relaxed pitch requirements), mature technology nodes (28nm and above — many foundries still run high-volume 28nm and 40nm production on KrF tools), MEMS and specialty devices, and compound semiconductor patterning. KrF scanners are significantly lower cost to purchase and operate than ArF or EUV systems, making them economically attractive for layers that don't require the finest resolution.