Autofocus Beam Splitter for Refractive Index Compensation
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Solution Overview
Problem
Autofocus systems face instability due to environmental factors like refractive index changes and mechanical component fluctuations, affecting the reliability of substrate height measurements, especially in large substrates and immersion lithography systems.
Innovation Solution
The system splits an autofocus beam into a reference and measurement component, using a beam splitting optic and a reflector to create separate air spaces, allowing for detection and compensation of instabilities, enabling accurate substrate height determination by subtracting environmental and mechanical errors from the measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a high angle of incidence is used to achieve high sensitivity to height changes, then measurement precision is improved, but sensitivity to environmental changes increases
Solution Approach 1:
The patent divides the optical path into two separate segments: a measurement beam path that interacts with the substrate at high angle of incidence for height measurement, and a reference beam path that travels through a different optical route. This segmentation allows the measurement beam to maintain high sensitivity while the reference beam experiences environmental variations, enabling compensation through differential measurement.
Solution Approach 2:
The patent introduces a reference beam as an intermediary element that experiences the same environmental factors (temperature, pressure, humidity) as the measurement beam but does not interact with the substrate. By measuring the reference beam's optical path length changes, the system can compensate for environmental effects on the substrate height measurement.
2Area of stationary object
If the beam path length in air is increased for large substrates, then measurement coverage is improved, but sensitivity to refractive index changes increases
Solution Approach 1:
The patent segments the optical measurement into two independent paths: one that covers the large substrate area at high angle of incidence and another reference path that provides environmental compensation. This allows the measurement beam to traverse long air paths over large substrates while the reference beam independently monitors and compensates for refractive index changes in the air.
3Reliability
If environmental control is applied to reduce refractive index changes, then measurement stability is improved, but system complexity increases
Solution Approach 1:
The patent introduces a reference beam as an intermediary that experiences the same environmental conditions as the measurement beam. Instead of controlling the environment, the system uses the reference beam to measure and compensate for environmental effects, thereby achieving measurement stability without requiring complex environmental control systems.
Solution Approach 2:
The patent implements a feedback mechanism where the reference beam continuously monitors environmental variations and provides compensation signals to correct the measurement beam's optical path length changes. This feedback approach maintains measurement stability by dynamically compensating for environmental effects rather than attempting to control the environment itself.
4Ease of operation
If mechanical components like vibrating mirrors are used for autofocus, then focusing capability is improved, but mechanical instability increases
Solution Approach 1:
The patent introduces a reference beam path as an intermediary that does not pass through the vibrating mirror and other mechanical components. This reference beam serves as a stable reference that experiences the same mechanical vibrations and instabilities as the measurement beam, allowing for differential measurement and compensation of mechanical effects.
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 compensates for instabilities, improving the reliability of substrate height measurements by isolating and correcting for environmental and mechanical factors, thus enhancing the accuracy of autofocus systems, particularly in large-scale applications.
Implementation Method 1
an autofocus beam is split into a reference beam component and a measurement beam component, by a beam splitting optic
Implementation Method 2
the reference beam component is directed at the reflector, and a reflected reference beam component is directed from the reflector
Implementation Method 3
refractive index changes of air due to temperature, atmospheric pressure, or humidity changes or gradients
Data Source
AI summary
An autofocus system and method designed to account for instabilities in the system, e.g. due to instabilities of system components (e.g. vibrating mirrors, optics, etc) and/or environmental effects such as refractive index changes of air due to temperature, atmospheric pressure, or humidity gradients, is provided. An autofocus beam is split into a reference beam component (the split off reference channel) and a measurement beam component, by a beam splitting optic located a predetermined distance from (and in predetermined orientation relative to) the substrate, to create a first space between the beam splitting optic and the substrate. A reflector is provided that is spaced from the beam splitting optic by the predetermined distance, to create a second space between the reflector and the beam splitting optic. The measurement beam component is directed at the substrate and a reflected measurement beam component through the first space between the substrate and the beam splitting optic, while the reference beam component is directed at the reflector and a reflected reference beam component is directed from the reflector through the second space between the beam splitting optic and the reflector. The reflected reference and measurement beam components are returned to the beam splitting optic, and emerge substantially collinear from the beam splitting optic. The reference and measurement beam components are then detected, and provide information that enables compensation for changes in the z position of the substrate that are due to instabilities in the autofocus system components and/or environmental factors.