Optical Focus Sensor Using Beam Splitter for Lithography

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Solution Overview

Problem

Current methods for determining whether a substrate is in focus in lithographic processes are complex and require intricate components, making them undesirable for efficient focus detection.

Innovation Solution

An optical focus sensor is developed, utilizing a splitter to create two sub-beams that pass through different apertures, allowing detectors to measure radiation intensity and determine the substrate's position relative to the focal plane of an objective lens, thereby generating a focus error signal.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If the knife edge method is used to determine focus, then focus detection capability is achieved, but device complexity increases

Engineering Contradiction:

Improvefocus detection capabilityVSAvoidcomplex parts

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The radiation beam is divided into two separate sub-beams that travel through different optical paths. Each sub-beam passes through a different aperture (first aperture in the focal plane, second aperture out of focus), allowing parallel measurement of focus conditions without requiring complex sequential operations or sophisticated components.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent creates a simplified optical copy of the focus detection function by using two separate detection paths instead of the complex knife-edge mechanism. The first detector receives light through the first aperture while the second detector receives light through the second aperture, providing focus information through a simpler replicated structure.

Inventive Principle:

Principle #26Copying

2Measurement precision

If complex parts are used for focus detection, then measurement capability is improved, but ease of operation deteriorates

Engineering Contradiction:

Improvefocus detection accuracyVSAvoidoperational simplicity

Core Design Contradiction:

Measurement precisionVSEase of operation

Solution Approach 1:

By segmenting the beam into two independent paths with simple apertures and detectors, the system achieves focus detection capability while maintaining operational simplicity. Each path is independent and straightforward, avoiding the operational complexity of knife-edge mechanisms.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent replaces the mechanical knife-edge method with an optical system using apertures and detectors. This substitution eliminates the need for mechanical movement and adjustment, thereby improving ease of operation while maintaining measurement precision.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Device complexity

If a simplified focus sensor is used, then device complexity is reduced, but measurement precision may deteriorate

Engineering Contradiction:

Improvesensor structureVSAvoidfocus position accuracy

Core Design Contradiction:

Device complexityVSMeasurement precision

Solution Approach 1:

The system generates a focus error signal by comparing the outputs from the two detectors. This feedback mechanism allows the system to accurately determine focus position and provide corrective information, maintaining measurement precision despite the simplified sensor structure.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The patent utilizes changes in light intensity parameters detected by the two detectors to determine focus position. By measuring intensity variations through the different apertures and processing the differential signal, accurate focus measurement is achieved with a simple sensor structure.

Inventive Principle:

Principle #35Parameter changes

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 solution simplifies focus detection by providing a clear indication of the substrate's position relative to the focal plane, enabling precise adjustments and improving the efficiency of lithographic processes.

Implementation Method 1

a splitter arranged and constructed to split a radiation beam into a first sub-beam and a second sub-beam

Methodology Applied

Scientific EffectOptical beam splitting:

Implementation Method 2

a back focal plane of an objective lens used to illuminate a substrate

Methodology Applied

Scientific EffectOptical focusing: Focusing

Implementation Method 3

direct the first sub-beam to a first detector via a first aperture, and to direct the second sub-beam to a second detector via a second aperture

Methodology Applied

Scientific EffectRadiation detection: Photoelectric Effect

Data Source

PatentUS8125615B2Optical focus sensor, an inspection apparatus and a lithographic apparatus

Publication Date: 2012.02.28 ASML NETHERLANDS BV

AI summary

To detect whether a substrate is in a focal plane of an inspection apparatus, an optical focus sensor is arranged to receive a radiation beam via an objective lens. The optical focus sensor includes a splitter configured to split the radiation beam into a first sub-beam and a second sub-beam. With an aperture and a detector in the light path of each of the sub-beams it is possible to detect whether the substrate is in focus by comparing the amount of radiation received by each of the detectors.