Predictive EUV Mirror Surface Temperature Estimation
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Solution Overview
Problem
In microlithographic projection exposure apparatuses, existing temperature sensors fail to accurately represent the surface temperature of EUV mirrors due to radiation absorption near the surface, leading to delayed regulation of pre-heaters and potential optical aberrations.
Innovation Solution
A method that estimates the surface temperature of an optical element by considering both the absolute temperature values measured at a distance and their temporal changes, using a predictive model that incorporates the time derivative of the sensor temperature to improve the accuracy of temperature characterization and regulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If temperature sensors are attached to the mirror's rear side at a distance from the optical effective surface, then the mirror structure remains intact and installation is simplified, but the temperature values measured are not representative of the surface temperature due to radiation absorption near the surface
Solution Approach 1:
The patent introduces a predictive model as an intermediary between the temperature sensor and the mirror surface. The sensor measures temperature at its location, and the predictive model estimates the surface temperature by considering the temporal changes in sensor temperature, effectively mediating the measurement gap between sensor position and surface position
Solution Approach 2:
The patent applies preliminary heating using infrared radiation before the lithographic exposure process. By pre-heating the mirror surface in advance, the system compensates for the temperature measurement delay and ensures the mirror reaches the desired operating temperature before actual use
2Measurement precision
If temperature sensors are placed close to the mirror surface to improve measurement accuracy, then temperature characterization improves, but the sensors interfere with the optical path and installation space is constrained
Solution Approach 1:
The predictive model serves as an intermediary that eliminates the need for physical proximity between sensor and surface. It translates distant sensor measurements into accurate surface temperature estimates without requiring the sensor to be positioned in the optical path or in contact with the mirror surface
3Stability of the object's composition
If pre-heating power is increased to compensate for delayed temperature regulation, then surface temperature stability improves, but energy consumption increases
Solution Approach 1:
The patent implements a feedback control system where the predictive model continuously monitors sensor temperature and its temporal changes, then adjusts the pre-heating power dynamically. This feedback mechanism ensures pre-heating is applied only when and where needed, maintaining surface temperature stability while minimizing energy consumption
Solution Approach 2:
The system transitions from static pre-heating to dynamic control by continuously adjusting pre-heating power based on real-time sensor measurements and predictive modeling. The pre-heating intensity varies over time according to the actual thermal state, optimizing energy efficiency
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 enables more reliable temperature-sensor-based characterization and regulation of the heating state, reducing thermal deformations and associated optical aberrations, thereby enhancing the stability and throughput of the microlithographic process.
Implementation Method 1
measuring values of a first temperature that the optical element has at a first position using a temperature sensor
Implementation Method 2
due to the radiation absorption taking place in the mirror substrate material predominantly already close to the surface at penetration depths of a few micrometers
Implementation Method 3
estimating a second temperature that the optical element has at a second position, which is located at a distance from the first position, on the basis of the values measured in step a), wherein estimating the second temperature in step b) is accomplished while taking into account a temporal change in the values measured in step a)
Data Source
AI summary
The disclosure provides a method and to an apparatus for determining the heating state of a mirror in an optical system, in particular in a microlithographic projection exposure apparatus. A method for determining the heating state of an optical element includes: measuring values of a first temperature that the optical element has at a first position using a temperature sensor; and estimating a second temperature that the optical element has at a second position, which is located at a distance from the first position, on the basis of the measured values, wherein estimating the second temperature is accomplished while taking into account a temporal change in the previously measured values.