Microlithography Illumination System Beam Pointing Stability
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Solution Overview
Problem
Microlithographic projection exposure apparatuses face challenges in maintaining a stable angular distribution of projection light at the mask level due to beam pointing fluctuations, especially when an optical integrator is not used, which can lead to significant displacements in the irradiance distribution and compromised imaging quality.
Innovation Solution
An illumination system with a spatial light modulator comprising an array of light deflecting elements, where the control unit adjusts the light deflecting elements to ensure that those at increasing and decreasing slopes of the irradiance distribution overlap in the pupil plane, maintaining a constant total irradiance and minimizing the impact of beam pointing fluctuations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If an optical integrator is not used to reduce system complexity and cost, then beam pointing fluctuations cause significant displacements in the irradiance distribution on the mirror array, but using an optical integrator increases system complexity and cost
Solution Approach 1:
The control unit continuously monitors the positions of light spots in the pupil plane and dynamically adjusts the tilt angles of individual micromirrors in response to beam pointing fluctuations. This closed-loop feedback mechanism compensates for irradiance distribution displacements without requiring an optical integrator, thereby maintaining stable angular light distribution while reducing system complexity
Solution Approach 2:
The system changes the tilt angles of micromirrors as adjustable parameters to compensate for beam pointing fluctuations. By dynamically modifying these angular parameters, the control unit ensures that light spots from different micromirrors maintain proper overlap in the pupil plane, achieving stability without additional optical components
2Reliability
If beam pointing fluctuations occur over long distances between light source and mirror array, then the irradiance distribution displaces significantly, but reducing the distance may compromise the optical design
Solution Approach 1:
The feedback control mechanism monitors light spot positions and adjusts micromirror tilt angles in real-time to compensate for displacements caused by beam pointing fluctuations over long distances, maintaining irradiance distribution stability without requiring a shorter optical path
Solution Approach 2:
The system preemptively compensates for beam pointing fluctuations by using the feedback control to adjust micromirror positions before significant image quality degradation occurs, cushioning against the effects of long-distance beam instability
3Adaptability or versatility
If the angular irradiance distribution is adapted for maximum flexibility, then different illumination settings can be produced, but beam pointing fluctuations cause unacceptable changes in the angular distribution
Solution Approach 1:
The feedback control unit continuously monitors light spot positions and dynamically adjusts micromirror tilt angles to maintain stable angular irradiance distribution despite beam pointing fluctuations, enabling flexible illumination settings while ensuring reliability
Solution Approach 2:
The system uses dynamically adjustable micromirror tilt angles to adapt to beam pointing fluctuations in real-time, allowing the angular irradiance distribution to be flexibly configured while maintaining stability through active control
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 control scheme stabilizes the angular light distribution at the mask level, reducing residual fluctuations to tolerable levels without the need for an optical integrator, thus ensuring consistent imaging quality and flexibility in producing various irradiance distributions.
Implementation Method 1
Each mirror can be tilted about two orthogonal tilt axes so that incident projection light is reflected along a direction which is determined by the tilt angles of the respective mirror
Implementation Method 2
A condenser lens arranged between the mirror array and a pupil plane translates the reflection angles produced by the mirrors into locations in the pupil plane
Implementation Method 3
The control unit is configured to control the light deflecting elements in such a way that a first light deflecting element, which is located at the increasing slope, and a second light deflecting element, which is located at the decreasing slope, deflect impinging projection light so that it at least partly overlaps in the pupil plane
Data Source
AI summary
An illumination system of a microlithographic projection exposure apparatus comprises a spatial light modulator which is arranged between a light source and a pupil plane. The spatial light modulator includes an array of micromirrors or other light deflecting elements each being capable of individually deflecting impinging projection into various directions. An irradiance distribution on the mirror array or its envelope has, along a direction X an increasing slope and a decreasing slope. The control unit controls the mirrors in such a way that a first mirror, which is located at the increasing slope, and a second mirror, which is located at the decreasing slope, deflect impinging projection light so that it at least partly overlaps in the pupil plane. This ensures that the angular irradiance distribution at mask level is substantially independent from beam pointing fluctuations.