Pulsed Laser Plasma Light Source for Metrology
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Solution Overview
Problem
Current semiconductor metrology systems face limitations in defect sensitivity due to the wavelength-dependent radiance of existing light sources, self-absorption in laser-sustained plasmas, and the inability to generate high brightness, broadband illumination required for advanced metrology applications.
Innovation Solution
A pulsed laser is used to repeatedly ignite and extinguish a plasma in a cold gas, achieving high brightness and high color temperature illumination by focusing the laser beam with a high numerical aperture to induce dielectric breakdown, thereby generating broadband light with a modulation depth greater than 90%.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If laser sustained plasma is used to generate illumination light, then brightness is improved, but self-absorption occurs causing color temperature to be limited to approximately 12,000-15,000 degrees Kelvin
Solution Approach 1:
The patent employs periodic pulsed laser excitation to repeatedly ignite and extinguish the plasma. By controlling the pulse duration to be shorter than the plasma lifetime, the plasma is ignited during each pulse but extinguishes before the next pulse arrives. This periodic operation prevents the formation of a stable, hot plasma core that would cause self-absorption, while still generating sufficient brightness during each ignition event. The modulation depth greater than 90% confirms the effectiveness of this periodic action in controlling plasma temperature and eliminating self-absorption effects.
2Duration of action of stationary object
If electrode-based discharge arc lamp is used, then light emission is sustained, but radiance and power are limited due to electrostatic constraints on current density
Solution Approach 1:
The patent replaces the electrode-based mechanical discharge system with a laser-based plasma ignition system. Instead of using physical electrodes to generate and sustain plasma through electrical discharge, the invention uses pulsed laser radiation to ignite the plasma and then allows it to extinguish naturally between pulses. This substitution eliminates the electrostatic constraints on current density that limit radiance in electrode-based systems, while the periodic pulsed operation sustains light emission over time through repeated ignition events.
3Illumination intensity
If high power laser is used to increase plasma temperature, then brightness is improved, but plasma size increases causing diminishing impact on color temperature
Solution Approach 1:
The patent resolves this contradiction by using periodic pulsed laser excitation with pulse durations shorter than the plasma lifetime. This approach allows the plasma to be ignited at high temperature during each pulse, achieving high brightness, but then extinguishes before the plasma can expand and cool significantly. The key insight is that by operating in a pulsed regime rather than continuous, the system achieves high color temperature during the brief ignition phase without the plasma size increase that would occur with continuous high power laser heating.
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
The solution provides high radiance illumination with a color temperature exceeding 30,000 degrees Kelvin, enhancing the performance of metrology systems like Spectroscopic Ellipsometry and Beam-Profile Reflectometry by improving defect detection sensitivity and brightness.
Implementation Method 1
focusing the laser beam with a high numerical aperture to induce dielectric breakdown, thereby generating broadband light
Implementation Method 2
repeatedly ignite and extinguish a plasma in a cold gas, achieving high brightness and high color temperature illumination
Implementation Method 3
The solution provides high radiance illumination with a color temperature exceeding 30,000 degrees Kelvin
Data Source
AI summary
Methods and systems are described herein for producing high radiance illumination light suitable for semiconductor metrology. A cold gas is repeatedly ignited by a pulsed laser to periodically generate accessible, high brightness illumination light generated during each break-down event. The pulse duration and repetition period are set to ignite, but not sustain fully formed plasma. The central plasma core emits high color temperature light before a cooler plasma region forms around the central core. Thus, after ignition, the plasma is extinguished before the arrival of the next laser pulse. The repeated plasma ignition/extinction cycle generates illumination light at high color temperature that is accessible for illumination purposes in a metrology application. In one embodiment, a bulb filled with Xenon gas at 10 atmospheres is repeatedly ignited with a pulsed laser having pulse duration of 10 nanoseconds to generate illumination light with a color temperature of approximately 60,000 Kelvin.