EUV Projection Mirror Rib Structure for Stable High-Bandwidth Control
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Solution Overview
Problem
The challenge in EUV lithography is achieving high control bandwidth in projection exposure systems with larger mirrors, as the natural frequency of the mirrors decreases with increasing size, leading to unstable closed-loop control and increased material costs.
Innovation Solution
An optical element design featuring a mirror body with a stiff base portion and thinner mirror portion, supported by a rib structure, which allows for actuator connectors and measurement targets, enabling a lightweight and stable mirror configuration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If the mirror size is increased to achieve high numerical aperture, then the optical performance is improved, but the natural frequency decreases and control stability deteriorates
Solution Approach 1:
The mirror body is divided into a mirror portion and a base portion with different thicknesses and functions. The mirror portion has thinner walls for weight reduction, while the base portion has greater thickness for providing a stable support structure for actuators and sensors, thereby maintaining control stability despite large mirror area.
Solution Approach 2:
Different regions of the mirror body are given different structural properties: the mirror portion is made thinner for weight reduction while the base portion is made thicker for stability. This local differentiation allows the large mirror to maintain both optical performance and control stability.
2Area of moving object
If the mirror size is increased to achieve high numerical aperture, then the optical performance is improved, but the material cost increases
Solution Approach 1:
The mirror body is segmented into a thin-walled mirror portion and a thicker base portion. This segmentation allows the majority of the mirror area to use less material, significantly reducing material costs while the base portion provides necessary structural support.
Solution Approach 2:
The mirror portion is designed with thin walls, utilizing the principle of thin-shell structures that can provide sufficient optical performance with minimal material, thereby reducing material costs for large-area mirrors.
3Weight of moving object
If the mirror walls are made thinner to reduce weight, then the control bandwidth is improved, but the structural strength decreases
Solution Approach 1:
The mirror body is divided into a thin-walled mirror portion for weight reduction and a thicker base portion for structural strength. The base portion provides a rigid foundation that compensates for the reduced strength of the thin mirror portion, enabling weight reduction without compromising overall structural integrity.
Solution Approach 2:
The wall thickness is locally optimized: thin in the mirror portion for weight reduction and control bandwidth improvement, and thick in the base portion for maintaining structural strength and supporting actuators.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The design achieves a significant weight reduction and higher control bandwidth, reducing production costs while maintaining optical performance and stability.
Implementation Method 1
The optically active surface is suitable for reflecting illumination radiation, such as EUV radiation
Data Source
AI summary
An optical element for a projection exposure system. The optical element comprises a mirror body having a mirror section with an optically active surface and a base section provided on the rear side of the mirror section. The base section has a greater stiffness than the mirror section. The optical element also comprises multiple actuator connectors for connecting actuators to the optical element. The actuator connectors are provided on the base section. The optical element further comprises a stiffening rib structure attached on the rear side of the mirror section.