EUV Laser Focus Control via Return Beam Diagnostics
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Solution Overview
Problem
In EUV lithography, maintaining the focus of the laser source on the target in a Laser Produced Plasma (LPP) EUV system is challenging due to thermal heating and expansion of optical components, affecting EUV light production efficiency and reliability.
Innovation Solution
A system and method utilizing a return beam diagnostics (RBD) module that splits reflected light into two optical paths of different lengths to determine target image sizes, generating focus correction signals to adjust the laser source focus, ensuring accurate alignment and maintaining optimal EUV production.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If thermal heating and expansion of optical components occurs during operation, then the focus of the laser source on the target deteriorates, but continuing operation is necessary for EUV production
Solution Approach 1:
The system performs preliminary focus measurements using the sensing device before EUV production begins, establishing a baseline focus position. During operation, continuous monitoring compares current focus conditions against this baseline, enabling proactive correction before focus degradation significantly impacts EUV production
Solution Approach 2:
The patent implements a feedback control loop where the sensing device continuously monitors focus conditions by detecting reflected light from the target. The controller receives this feedback and automatically adjusts the focusing element to maintain optimal focus, compensating for thermal effects in real-time during EUV operation
2Reliability
If focus correction is implemented continuously, then focus stability improves, but system complexity increases due to additional sensing and control components
Solution Approach 1:
The sensing device serves multiple functions: it monitors focus conditions, measures target position, and detects optical path changes. This multi-functionality reduces the need for separate dedicated sensors for each parameter, thereby limiting the increase in system complexity while maintaining reliable focus control
Solution Approach 2:
The patent uses reflected EUV light from the target as an intermediary carrier to convey focus information. Instead of requiring direct measurement of laser focus or complex interferometric setups, the system measures properties of the reflected light that indirectly indicate focus conditions, simplifying the overall measurement system
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 corrects the focus of the laser source, enhancing the reliability and efficiency of EUV light production by continuously monitoring and adjusting the focus, thereby improving the quality and consistency of sub-32 nm feature production in semiconductor manufacturing.
Implementation Method 1
a focusing element configured to receive light reflected from a target when the target is illuminated by a laser source, the focusing element focusing the received reflected light along an optical path
Implementation Method 2
a beam splitter located on the optical path configured to receive the focused reflected light and to split the focused reflected light into a first split beam and a second split beam
Data Source
AI summary
Focus of a laser beam on a target in a Laser Produced Plasma (LPP) Extreme Ultraviolet (EUV) light source is maintained by focusing reflected light from the target illuminated by a laser source, sampling the focused reflected light in a plurality of planes at different optical path lengths, and comparing the image sizes of the focused reflected light at the plurality of planes to determine a correction signal to correct the focus of the laser source. In an embodiment, the focused reflected light is split into a two optical paths of differing optical path lengths, with each optical path directed to a sensing device at an imaging plane. Since each of the optical paths is of a different length, the images of the target taken by the sensing device at the imaging plane will be of different sizes. By comparing the relative sizes of the target images from the optical paths, a correction signal is produced to correct the focus of the laser source.