Precision fabrication of patterned shapes on silicon wafers with ultra-absorbent black silicon
A method of fabricating a patterned mask on a silicon wafer includes grind-polishing a pre-cut silicon wafer; ion beam figuring the grind-polished silicon wafer, transferring a coronagraphic pattern to a surface of the ion beam figured silicon wafer; and in a cryogenic deep reactive ion etching process, applying black silicon to exposed silicon regions of the ion beam figured silicon wafer. The cryogenic etching process is used to fabricate black silicon to achieve a high aspect ratio structure with higher etch rate than conventional reactive ion etching.
1. A method of fabricating a patterned mask on a silicon wafer, comprising:
grind-polishing a pre-cut silicon wafer;
ion beam configuring the grind-polished silicon wafer using an infinite 2D array selected from of cones and pyramid structures with a one-unit cell with Floquet periodic boundary conditions on four sides whereby the one-unit cell's geometry consists of one of a selected pyramid and cone structure with matched layers (PML) above the structure, with remaining space between the structure and the PMLs filled with air configured as an absorber made of conductive material whereby at an interface of the conductive material and air, with the incident field partially reflected and partially transmitted into the structure;
transferring a coronagraphic pattern to a surface of the ion beam figured silicon wafer by photolithography; and
in a cryogenic deep reactive ion etching process, applying black silicon to exposed silicon regions of the ion beam figured silicon wafer.
2. The method of claim 1 , further comprising grind-polishing the pre-cut silicon wafer to a diffraction-limit of lambda/4.
3. The method of claim 1 , further comprising using the IBF process to improve the diffraction limit to lambda/20.
4. The method of claim 1 , further comprising measuring a wavefront error of the silicon wafer after the grind-polishing and before and after the IBF to ensure a flatness of a surface of the silicon wafer.
5. The method according to claim 1 , wherein after the ion beam figuring, the method further comprises:
depositing a highly reflective metal on a surface of the silicon wafer using vacuum deposition;
photolithographically transferring the coronagraphic pattern to the surface of the silicon wafer;
etching the coronagraphic pattern into the highly reflective metal, wherein regions of bare silicon are left exposed to form a patterned wafer; and
subjecting the patterned wafer to the cryogenic deep reactive ion etching to form absorbing black silicon in the exposed silicon regions.