EUV Mirror Thermal Control via Active Cooling
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Solution Overview
Problem
In microlithography, EUV mirrors experience thermal deformation due to radiation absorption, leading to imaging quality issues, especially with increasing light source power, and existing pre-heating solutions are inadequate in managing spatial and temporal heat variations.
Innovation Solution
An optical system with a mirror having a cooling channel for heat dissipation, where the temperature and flow rate of a cooling fluid are adjusted based on measured or estimated thermal load to maintain the mirror's average temperature near the zero crossing temperature, effectively reducing thermal deformations and optical aberrations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the power of the light source is increased to improve productivity and reduce photoresist sensitivity, then the imaging performance deteriorates due to thermal deformation of the EUV mirror
Solution Approach 1:
The patent applies preliminary action by pre-heating the EUV mirror before the lithographic exposure process begins. This pre-heating brings the mirror to its operating temperature in advance, preventing thermal deformation during actual operation when high-power EUV radiation is applied. The mirror is heated to a temperature close to its zero-crossing temperature where thermal expansion effects are minimized.
Solution Approach 2:
The patent employs parameter changes by dynamically adjusting the temperature of the EUV mirror during operation. A temperature control unit modifies heating power based on detected mirror temperature and thermal load conditions, maintaining the mirror at optimal operating parameters. The system also adjusts cooling fluid flow rate and temperature to manage thermal effects while preserving imaging performance at high light source powers.
2Reliability
If pre-heaters are used to mitigate thermal deformation, then temporal heat variations are addressed, but spatial variations in heat introduction remain unmanaged
Solution Approach 1:
The patent applies local quality by using multiple independent heating zones on the EUV mirror surface, each可控 by separate heating elements. This allows different regions of the mirror to be heated to different temperatures according to their specific thermal requirements. The system can apply localized heating or cooling to compensate for spatial variations in heat introduction from the light source, ensuring uniform thermal distribution across the mirror surface.
Solution Approach 2:
The patent implements feedback control through temperature sensors that continuously monitor the thermal state of the EUV mirror. The detected temperature information is fed back to the temperature control unit, which adjusts heating and cooling parameters in real-time. This closed-loop feedback mechanism enables the system to respond to both temporal and spatial thermal variations, maintaining optimal mirror temperature distribution despite changing operational conditions.
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
This approach effectively mitigates thermal-induced mirror deformations and maintains high imaging performance even at higher light source powers by actively controlling the cooling fluid's temperature and flow rate, aligning with the zero crossing temperature to minimize material and illumination-related distortions.
Implementation Method 1
at least one cooling channel in which a cooling fluid is capable to flow is arranged in the mirror substrate in order to dissipate heat that is generated in the mirror substrate due to absorption of electromagnetic radiation
Implementation Method 2
heat that is generated in the mirror substrate due to absorption of electromagnetic radiation incident from a light source
Data Source
AI summary
Disclosed are an optical system, in particular for microlithography, and a method for operating an optical system. According to one disclosed aspect, the optical system includes at least one mirror (100, 500, 600) having an optical effective surface (101, 501, 601) and a mirror substrate (110, 510, 610), wherein at least one cooling channel (115, 515, 615) in which a cooling fluid is configured to flow is arranged in the mirror substrate, for dissipating heat that is generated in the mirror substrate due to absorption of electromagnetic radiation incident from a light source on the optical effective surface, and a unit (135, 535, 635) to adjust the temperature and/or the flow rate of the cooling fluid either dependent on a measured quantity that characterizes the thermal load in the mirror substrate or dependent on an estimated/expected thermal load in the mirror substrate for a given power of the light source.