Heating Light Deflection for EUV Mirror Thermal Control
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Solution Overview
Problem
Microlithographic projection exposure apparatuses face challenges in efficiently and gently heating optical elements due to low reflectivity and thermal issues with EUV light, leading to heat absorption and potential deformation of mirrors, which affects imaging quality.
Innovation Solution
A microlithographic projection exposure apparatus using a heating light source with a diffractive or refractive freeform deflection element to distribute heating light efficiently across the optical elements, minimizing losses and avoiding high radiation intensities that could damage sensitive coatings, while allowing for variable intensity distributions and flexible design.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If mirrors are used for EUV projection exposure apparatus, then the apparatus can operate at short wavelengths (13.5 nm), but the low reflectivity of mirrors causes light loss and reduces throughput
Solution Approach 1:
A heating light source operating at a different wavelength (infrared or visible range) is introduced as an intermediary to preheat the mirror substrate before EUV exposure. This mediator enables thermal management without interfering with the primary EUV light path, allowing the mirror to reach optimal operating temperature while maintaining EUV reflectivity
Solution Approach 2:
The temperature of the mirror substrate is actively changed and controlled to optimize its optical properties for EUV reflection. By adjusting the thermal state of the mirror through separate heating, the coefficient of thermal expansion is compensated and reflectivity is maximized, thereby improving light throughput without changing the mirror's physical structure
2Loss of energy
If mirrors with complex multi-layer coatings are used to improve reflectivity, then more light is reflected, but thermal problems arise due to absorption of EUV light leading to temperature increase
Solution Approach 1:
The mirror substrate is preheated to its optimal operating temperature using a heating light source before the actual EUV exposure begins. This preliminary thermal conditioning ensures that the mirror is already at the correct temperature to minimize thermal expansion and maintain coating integrity, preventing excessive temperature increases during exposure
Solution Approach 2:
A dedicated heating light source serves as an intermediary mechanism that transfers thermal energy to the mirror substrate independently of the EUV exposure process. This mediator allows precise thermal control without adding to the EUV light load, separating the thermal management function from the imaging function
3Temperature
If heating light is concentrated on specific regions of the mirror, then localized heating is achieved, but high radiation intensities may damage sensitive coatings
Solution Approach 1:
The heating light beam is divided into multiple segments or zones using optical elements such as diffractive optical elements or microlens arrays. This segmentation distributes the heating function across multiple lower-intensity beams that collectively cover the required area, preventing any single point from receiving damaging radiation intensity while still achieving the desired localized heating effect
Solution Approach 2:
Instead of using a single high-intensity heating beam, multiple partial heating beams are employed that each deliver a lower, non-damaging intensity. The cumulative effect of these partial actions achieves the required heating without exceeding the damage threshold of the sensitive EUV coatings
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 solution enables efficient and gentle heating of optical elements with low heat loss, reducing the risk of damage and maintaining imaging quality by distributing heating power effectively and adaptively adjusting intensity distributions.
Implementation Method 1
The illumination optical unit includes a deflection element formed by a diffractive optical element or as a refractive freeform element, wherein the deflection element directs the heating light impinging thereon simultaneously in different directions
Implementation Method 2
a heating light source, which differs from the projection light source and which is configured to generate heating light, to which the layer is not light-sensitive and which is at least partly absorbed by the optical element
Implementation Method 3
The illumination optical unit includes a deflection element formed by a diffractive optical element or as a refractive freeform element
Data Source
AI summary
A microlithographic projection exposure apparatus includes a projection light source. The apparatus also includes a heating light source for generating heating light which is at least partly absorbed by an optical element. An illumination optical unit directs the heating light onto the optical element such that the heating light has a predefined intensity distribution on an optical surface of the optical element. The illumination optical unit includes a deflection element which is a diffractive optical element or a refractive freeform element. The deflection element simultaneously directs the heating light impinging thereon in different directions.