Fabrication method of cylindrical gratings
A phase mask method to geometrically transform and to optically transfer a standard planar radial grating pattern into a cylindrical photoresist pattern at the circularly cylindrical wall of a given element. The planar radial grating pattern can be written with an integer number of lines having strictly constant period without any stitching problem. The photolithographic transfer is made by an illumination device providing a normal incident beam on the phase mask. The annular radial grating diffracts this normal incident beam, formed by plane waves, into two cylindrical waves of the first diffraction order (T +1 and Tr −1 ) which impinge on the circularly cylindrical wall and interfere in a photoresist layer deposited on the circularly cylindrical wall to give rise to an interferogram.
1. A fabrication method of a cylindrical grating at a circularly cylindrical surface of an element, characterized by the following steps:
providing a annular phase mask formed by a plane radial grating;
coating said circularly cylindrical surface with a photoresist layer;
arranging said circularly cylindrical surface relative to said annular phase mask so that the central axis of this circularly cylindrical surface comprises the center of the annular phase mask, so that said central axis is substantially perpendicular to the plane defined by said annular phase mask, and so that a medium or media between this annular phase mask and the photoresist layer on the circularly cylindrical surface is transparent at least for a given wavelength;
exposing said annular phase mask with a light wave at said given wavelength and under substantially normal incidence, said plane radial grating having a period selected so as to have the +1 st and −1 st diffraction orders (Tr +1 and Tr −1 ) propagating in said medium or media between the annular phase mask and the photoresist layer and giving rise to an interferogram in said photoresist layer;
developing the photoresist layer resulting in a circularly cylindrical photoresist corrugation grating ( 9 ) having lines parallel to said central axis.
2. The fabrication method of a cylindrical grating according to claim 1 , wherein said annular phase mask is a transmission phase mask directly arranged on a end face of said element ( 3 ) which is transparent for said given wavelength.
3. The fabrication method of a cylindrical grating according to claim 2 , wherein a layer is deposited on the area outside said annular phase mask, this layer being absorptive or reflective for said given wavelength.
4. The fabrication method of a cylindrical grating according to claim 1 , wherein said annular phase mask is a transmission phase mask arranged on, under or into a stand alone disk or wafer which is arranged relative to said element so that said interferogram occurs in a given section of said circularly cylindrical surface of this element, this disk or wafer being transparent for said given wavelength.
5. The fabrication method of a cylindrical grating according to claim 1 , wherein said annular phase mask is a reflection phase mask arranged on, under or into a standalone disk or wafer which is arranged relative to said element so that said interferogram occurs in a given section of said circularly cylindrical surface of this element.
6. The fabrication method of a cylindrical grating according to claim 5 , wherein a layer is deposited on the area outside said annular phase mask, this layer being absorptive or transmissive for said given wavelength.
7. The fabrication method of a cylindrical grating according to claim 1 , wherein said circularly cylindrical surface is convex and defines an external wall of said element.
8. The fabrication method of a cylindrical grating according to claim 1 , wherein said circularly cylindrical surface is concave and defines an internal wall of said element.
9. The fabrication method of a cylindrical grating according to claim 1 , wherein it comprises a final step consisting in forming said cylindrical grating by physically transferring or embossing or injection molding or extruding said circularly cylindrical photoresist corrugation grating.