Schematic diagram of concept of the developed polymer photomask for grayscale lithography (not to scale). Thickness variations in the patterns of the doped PDMS mask create corresponding changes in UV absorption.
Fabrication process of the developed photomask showing (a) mold with complementary patterns, (b) PDMS prepolymer mixed with dye dispensed over mold, followed by thermal curing, (c) cured PDMS layer with imprinted patterns, and (d) free PDMS layer with patterns may be used as is or with a glass support as grayscale photomask.
Absorption spectrum showing molar extinction coefficient data of Coumarin 314 dye in toluene and PDMS. The three wavelength components used in the experiments with the grayscale photomask are indicated at 365, 405, and , respectively.
Dependence of UV absorption on thickness of doped PDMS photomask and wavelength of incident UV light. Solid lines show the absorption measured with PDMS layers of different thicknesses. Dashed lines show absorption data extracted from molar extinction coefficient values in the absorption spectrum in Fig. 3. Undoped PDMS layer did not show any UV absorption regardless of wavelength.
Stereo microscope images showing (a) test pattern on PDMS photomask, (b) resulting 3D microstructure pattern in positive photoresist after lithography with doped PDMS mask, and (c) absence of patterns when undoped PDMS mask with similar patterns is used in photolithography.
3D microlens structures in AZP 4620 photoresist: (a) optical microscope image and (b) surface profile along the dashed line.
Photoresist profile depth as a function of (a) UV exposure dose and wavelength and (b) dye concentration in photomask at an exposure dose of at .
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