Bi-layer Stack Diffractive Optical Element Fabrication

The patent segments the fabrication process into two distinct parts: a single lithography step that creates a mask pattern, and a subsequent etching step that transfers the pattern. This segmentation eliminates the need for multiple iterative lithography-etching cycles, thereby preventing accumulation of alignment errors while maintaining the ability to create multilevel structures through the etching process alone.

The patent applies preliminary action by performing the lithography step once to create a complete mask pattern before the etching process begins. This preliminary mask creation allows the entire multilevel structure to be defined in advance, eliminating the need for subsequent realignments and preventing error accumulation that would occur with iterative approaches.

The patent replaces mechanical反复 etching processes with a chemical etching approach using selective etchants. The bi-layer stack structure enables selective chemical etching that removes material layer-by-layer without the mechanical damage and roughness associated with repeated plasma or mechanical etching cycles, thereby maintaining surface quality while achieving high step counts.

The patent uses composite materials in the form of a bi-layer stack where each layer has different etch selectivity. This composite structure allows selective removal of alternating layers through chemical etching, creating smooth stepped surfaces without the roughness that would result from multiple conventional etching steps on a single material layer.

The patent segments the continuous photoresist profile into discrete etchable layers through the bi-layer stack structure. Instead of relying on the limited gray-scale resolution of photoresist (around 16 levels), the segmentation approach uses the alternating etch-selective layers to create precisely defined discrete steps, thereby increasing the number of resolvable levels beyond the limitations of photoresist exposure nonlinearity.

The patent introduces an intermediary structure—the bi-layer stack with alternating etch-selective and non-etch-selective layers—that mediates between the simple binary photoresist pattern and the desired multilevel structure. This intermediary enables the transformation of a single-layer mask pattern into multiple precisely controlled etching steps, overcoming the resolution limits of conventional gray-scale lithography.