High aspect ratio gratings fabricated by electrodeposition
A method is provided for making gratings of gold or other metal in silicon substrates. The disclosed method may achieve high aspect ratios. According to the disclosed method, a silicon wafer is through-etched. A seed layer of metal is vapor-deposited on one side of the wafer, and a layer of metal is electrodeposited on the seed layer. The electrodeposited metal plugs the trenches and provides a conductive surface for subsequent electrodeposition. The trenches are then filled by electrodeposition from within the trenches, so that the walls of the metal grating grow on the metal plugs.
1. A method for fabricating a metal grating in a silicon substrate having a thickness and first and second faces, comprising:
lithographically defining a grating pattern comprising parallel elongated etch regions of equal widths on the first face, wherein the etch region width is smaller than the substrate thickness by a factor of 50 or more;
etching the silicon substrate so as to create a multiplicity of through-etched trenches that extend all the way through the substrate;
at the first or second face of the silicon substrate, plugging the through-etched trenches with electrodeposited metal; and
filling the through-etched trenches with electrodeposited metal by growth from the plugged face.
2. The method of claim 1 , wherein the etching of the silicon substrate comprises:
etching the first face so as to create a multiplicity of parallel side- 1 trenches extending from the first face partway into the substrate; and
etching material away from the second face so as to expose bottom ends of the side- 1 trenches, thereby to create a multiplicity of through-etched trenches that extend all the way through the substrate.
3. The method of claim 1 , further comprising, before the through-etched trenches are plugged with electrodeposited metal, evaporatively depositing a metal layer as a seed layer for the plugging step.
4. The method of claim 3 , further comprising, before depositing the seed layer, coating the through-etched trenches on inner walls thereof with a dielectric material.
5. The method of claim 4 , wherein the dielectric material is aluminum dioxide, and wherein the coating is performed by atomic layer deposition.
6. The method of claim 1 , wherein the electrodeposited metal that fills the through-etched trenches is gold.
7. The method of claim 1 , wherein the through-etched trenches are at least 0.8 cm long, and wherein the grating pattern is at least 0.8 cm across.
8. The method of claim 1 , wherein the through-etched trenches are at least 550 μm deep.
9. The method of claim 1 , wherein the through-etched trenches have a pitch of 20 μm or less.
10. The method of claim 1 , wherein the through-etched trenches are separated by silicon walls with a thickness of 5 μm or less.