Method to fabricate a tilted logpile photonic crystal
A method to fabricate a tilted logpile photonic crystal requires only two lithographic exposures and does not require mask repositioning between exposures. The mask and photoresist-coated substrate are spaced a fixed and constant distance apart using a spacer and the stack is clamped together. The stack is then tilted at a crystallographic symmetry angle (e.g., 45 degrees) relative to the X-ray beam and rotated about the surface normal until the mask is aligned with the X-ray beam. The stack is then rotated in plane by a small stitching angle and exposed to the X-ray beam to pattern the first half of the structure. The stack is then rotated by 180° about the normal and a second exposure patterns the remaining half of the structure. The method can use commercially available DXRL scanner technology and LIGA processes to fabricate large-area, high-quality tilted logpile photonic crystals.
1. A method to fabricate a tilted logpile photonic crystal, comprising:
providing a substrate coated with a photoresist;
providing a lithographic mask comprising a pattern of mask openings to pattern a tilted logpile structure in the photoresist with an incident beam from a collimated X-ray source;
mounting the mask spaced a distance from the surface of the photoresist;
tilting the mounted mask and substrate at a crystallographic symmetry angle toward the incident beam and aligning the incident beam with the mask openings;
rotating the mounted mask and substrate through a stitching angle about the mask normal;
exposing the photoresist to the incident beam through the mask openings to pattern a first half of the tilted logpile structure in the photoresist;
rotating the mounted mask and substrate by 180 degrees about the mask normal; and
exposing the photoresist to the incident beam through the mask openings to pattern a second half of the tilted logpile structure in the photoresist, thereby forming a tilted logpile structure patterned in the photoresist.
2. The method of claim 1 , wherein the tilted logpile structure comprises a cubic or rhombic structure.
3. The method of claim 2 , wherein the structure comprises a face-centered-cubic structure.
4. The method of claim 1 , wherein the stitching angle is less than 10 degrees.
5. The method of claim 1 , wherein the stitching angle θ is selected according to tan θ=0.5 w/s, where w is the width of the mask opening and s is distance spaced.
6. The method of claim 1 , wherein the crystallographic symmetry angle is 45 degrees.
7. The method of claim 1 , wherein the photoresist comprises a negative photoresist.
8. The method of claim 1 , wherein the photoresist comprises a positive photoresist.
9. The method of claim 1 , further comprising developing the patterned photoresist.
10. The method of claim 9 , further comprising depositing a dielectric material into the holes of the developed photoresist.
11. The method of claim 10 , wherein the depositing comprises electroplating or atomic layer deposition.
12. The method of claim 10 , wherein the dielectric material comprises plastic, metal, ceramic, or glass.
13. The method of claim 12 , wherein the metal comprises tungsten, gold, silver, nickel, or copper.
14. The method of claim 12 , wherein the metal comprises NiFe, NiFeCo, NiCr, or NiCo alloy.
15. The method of claim 10 , wherein the developed photoresist comprises an inverse tilted logpile structure.
16. The method of claim 15 , wherein the remaining photoresist is removed to provide a tilted logpile structure comprising rods of the dielectric material in air.