Optical Element Heating-State Estimation for EUV Mirror Preheating
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Solution Overview
Problem
Existing methods for determining the heating state of EUV mirrors in microlithographic projection exposure apparatuses are unreliable due to inhomogeneous temperature distributions, leading to incorrect temperature measurements and inadequate pre-heating adjustments, which cause thermally induced deformations and optical aberrations.
Innovation Solution
A method for estimating the average temperature at the incidence surface of an optical element using a calibration parameter that varies based on the illumination setting and other factors, such as the reticle used, to improve the accuracy of temperature sensor-based characterization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If temperature sensors are attached to the mirror's back side at a distance from the optical effective surface, then the mirror can be monitored for thermal effects, but the measured temperature values become sensitive to sensor position due to inhomogeneous temperature distributions
Solution Approach 1:
The patent introduces a calibration parameter as an intermediary factor that mediates between the temperature sensor readings and the actual average temperature at the incidence surface. This calibration parameter accounts for the inhomogeneous temperature distribution and the specific illumination setting, allowing accurate temperature estimation without requiring the sensor to be in direct contact with the optical surface.
Solution Approach 2:
The patent changes the approach from using a fixed temperature conversion factor to using a calibration parameter that varies with illumination settings. By adapting the calibration parameter to match the specific illumination conditions (e.g., dipole, quadrupole), the system accurately reflects the actual temperature distribution pattern, resolving the contradiction between sensor placement convenience and measurement precision.
2Ease of operation
If a fixed calibration parameter is used for temperature estimation, then the measurement process is simple, but the temperature estimation becomes inaccurate when illumination settings change
Solution Approach 1:
The patent transitions from a static, fixed calibration parameter to a dynamic calibration parameter that adapts to changing illumination settings. The calibration parameter is selected or calculated based on the current illumination mode (e.g., dipole, quadrupole), allowing the temperature estimation to remain accurate across different operational conditions while maintaining ease of use through automated selection.
Solution Approach 2:
The calibration parameter is made variable according to illumination settings. Different illumination modes produce different temperature distribution patterns, so the calibration parameter is adjusted to match the specific illumination condition. This ensures high measurement precision across diverse operational scenarios without complicating the user interface.
3Extent of automation
If pre-heating power is adjusted based on incorrect temperature measurements, then the control system responds to sensor data, but thermally induced deformations and optical aberrations occur
Solution Approach 1:
The patent implements a feedback mechanism where the temperature estimation (using the calibration parameter) is fed back to the pre-heating control system. This closed-loop control allows the pre-heating power to be adjusted based on accurate temperature estimates, compensating for thermal effects before they cause deformations or aberrations, thereby maintaining imaging precision while preserving automation.
Solution Approach 2:
The accurate temperature estimation enabled by the calibration parameter allows the control system to perform preliminary heating adjustments before thermal deformations occur. By anticipating and compensating for thermal effects based on real-time temperature trends, the system prevents imaging precision degradation rather than reacting after problems have occurred.
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
Enhances the reliability of temperature estimation, allowing for precise control of pre-heating mechanisms to mitigate thermal deformations and maintain optimal imaging properties.
Implementation Method 1
As a result of absorption of the radiation emitted by the EUV light source among other reasons, the EUV mirrors can heat up
Implementation Method 2
the EUV mirrors can heat up and can undergo an associated thermal expansion or deformation
Data Source
AI summary
A method and a device determine the heating state of an optical element in an optical system, for example in a microlithographic projection exposure system. Electromagnetic radiation hits an incidence surface of the optical element during operation of the optical system. Using a calibration parameter, an average temperature at the incidence surface is estimated on the basis of a temperature measurement carried out via at least one temperature sensor located a distance from the incidence surface. The calibration parameter is selected differently in accordance with the illumination setting which is set in the optical system.