Coherence Control in Photolithography Light Beams
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Solution Overview
Problem
In photolithography, high coherence of pulsed light beams leads to speckle noise, causing uneven exposure and defects in microelectronic features, as existing techniques to reduce coherence often increase the light beam's bandwidth.
Innovation Solution
A method to control the optical system by measuring the coherence of a pulsed light beam, comparing it to a target value, and adjusting the optical system to reduce coherence by modifying optical elements such as changing the shape, position, or curvature of optical surfaces, thereby reducing speckle without increasing bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If existing techniques are used to reduce coherence in the light beam, then speckle noise is reduced, but the bandwidth of the light beam increases
Solution Approach 1:
The patent implements a feedback control system that measures the coherence of each pulse of light beam and uses this measurement to generate a control signal. The control signal adjusts an optical element (such as a deformable mirror or spatial light modulator) to modify the wavefront of the subsequent pulse, thereby reducing coherence and speckle noise while maintaining precise control over the bandwidth parameter
Solution Approach 2:
The patent employs dynamic adjustment of optical elements between pulses. The optical element (e.g., deformable mirror) is modified based on real-time coherence measurements, allowing the system to adaptively reduce speckle noise for each subsequent pulse while maintaining the desired bandwidth characteristics through controlled temporal variations in the optical path
2Manufacturing precision
If coherence of the light beam is reduced, then uniform exposure is achieved, but control complexity increases
Solution Approach 1:
The control system measures the coherence property of each pulse and uses this feedback to automatically adjust the optical element. This closed-loop approach simplifies the overall control strategy by relying on real-time measurements rather than complex pre-programmed sequences, achieving uniform exposure through adaptive rather than predetermined adjustments
Solution Approach 2:
The system uses the measured coherence information from each pulse to automatically control the subsequent pulse without external intervention. The optical element self-adjusts based on the feedback signal, reducing the need for complex external control mechanisms while maintaining precise control over coherence and achieving uniform exposure
Data Source
AI summary
Techniques for controlling an optical system include accessing a measured value of a property of a particular pulse of a pulsed light beam emitted from the optical system, the property being related to an amount of coherence of the light beam; comparing the measured value of the property of the light beam to a target value of the property; determining whether to generate a control signal based on the comparison; and if a control signal is generated based on the comparison, adjusting the amount of coherence in the light beam by modifying an aspect of the optical system based on the control signal to reduce an amount of coherence of a pulse that is subsequent to the particular pulse.