• Document: Micro- and Nano-Technology... for Optics
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Micro- and Nano-Technology... ... for Optics 3.2 Lithography U.D. Zeitner Fraunhofer Institut für Angewandte Optik und Feinmechanik Jena “Printing on Stones” Map of Munich Stone Print Shadow Printing Photomask Wikipedia Mask Wafer Curtesy: R. Völkel, Suss Microoptics Contact Printing light mask resist substrate Mask Aligner Mask Aligner high pressure Mercury Emission Spectrum Hg-vapor lamp i h g e - line Proximity Printing light mask proximity gap resist substrate Pattern Generation by Photolithography Standard contact photolithography with a Mask Aligner: mask illumination photomask diffraction pattern z geometric shadow printing reduction of resolution with increasing z The inverse microscope microscope lithography light source image object microscope lens projection lens object image light source Projection Lithography light mask Resolution: 𝜆 𝑅 = 𝑘1 𝑁𝐴 projection R … minimum feature size optics k1 … optics dependent factor  … wavelength NA …numerical aperture of resist imaging system substrate microelectronic chips High-End Lithography Tool on Si-wafers DUV lithography stepper, =193nm EUV lithography stepper, =13.5nm (ASML) (ASML)  very low flexibility Photo Resist UV-exposure: Example: DNQ-based positive resist:  Photoinitiator creates reactive species  Chemical solubility in alkaline media changes Positive Resist: “hard“ resist D1 Dose resist thickness after Dth development x 1 resist D    log 10  2   D1  D2 exposure dose D Printing Result: Hard Resist Dose Dth x Resist pattern: resist  (almost) binary profile Gradation Curve “hard“ resist suitable for D1 binary pattern resist thickness after development “soft“ resist 1 D    log 10  2   D1  D2 exposure dose D dose range for variable dose writing of continuous surface reliefs Printing Result: Soft Resist Dose x Resist pattern: resist  continuous surface profile (typically nonlinear wrt. exposure dose) Technology for continuous profiles variable do

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