Optical Metrology Focus Error Reduction via Dispersion Inversion

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Solution Overview

Problem

High sensitivity to focus errors and oblique illumination limitations hinder the performance of optical metrology systems, particularly in large numerical aperture systems, leading to reduced precision and increased measurement times.

Innovation Solution

The measurement spot is imaged onto the detector with the direction aligned with the plane of incidence oriented perpendicular to the direction of wavelength dispersion, reducing focus error sensitivity and allowing for faster focus times and higher throughput without compromising measurement accuracy.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If the measurement spot is imaged onto the detector with the direction aligned with the plane of incidence oriented parallel to the direction of wavelength dispersion (conventional configuration), then the measurement precision is maintained, but the focus error sensitivity is high and the measurement time is long

Engineering Contradiction:

Improvemeasurement precisionVSAvoidmeasurement time

Core Design Contradiction:

Measurement precisionVSLoss of time

Solution Approach 1:

The patent inverts the conventional orientation relationship between the plane of incidence and wavelength dispersion direction. Instead of aligning them parallel as in conventional systems, the patent orients them perpendicular to each other, fundamentally changing the geometric configuration to reduce focus error sensitivity and enable faster measurements while maintaining precision

Inventive Principle:

Principle #13The other way round (Inversion)

2Productivity

If the measurement spot is imaged onto the detector with the direction aligned with the plane of incidence oriented perpendicular to the direction of wavelength dispersion, then the throughput is enhanced and focus error sensitivity is reduced, but this represents a departure from conventional configuration

Engineering Contradiction:

ImprovethroughputVSAvoidsystem configuration complexity

Core Design Contradiction:

ProductivityVSDevice complexity

Solution Approach 1:

The patent transitions from a one-dimensional alignment (parallel orientation) to a two-dimensional perpendicular orientation between the plane of incidence and wavelength dispersion directions. This dimensional change in the geometric configuration enables reduced focus error sensitivity and improved throughput while managing system complexity through careful optical design

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

3Speed

If high numerical aperture is used in the optical metrology system, then the measurement speed and throughput are improved, but the sensitivity to focus errors increases significantly

Engineering Contradiction:

Improvemeasurement speedVSAvoidfocus accuracy

Core Design Contradiction:

SpeedVSMeasurement precision

Solution Approach 1:

The patent applies the inversion principle by changing the conventional parallel orientation to a perpendicular orientation between the plane of incidence and wavelength dispersion directions. This geometric inversion specifically addresses the focus error sensitivity issue in high NA systems, enabling faster measurement speeds without sacrificing focus accuracy

Inventive Principle:

Principle #13The other way round (Inversion)

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 configuration significantly reduces focus error sensitivity, enabling precise measurements with shorter measurement, acquisition, and measurement (MAM) times, and enhances the throughput of optical metrology systems, especially in large numerical aperture systems.

Implementation Method 1

The beam of collected light 17 is diffracted by diffraction grating 22 to spatially disperse the beam of collected light according to wavelength

Methodology Applied

Scientific EffectDiffraction: Diffraction

Implementation Method 2

The CCD detector 23 converts the collected light into electrical signals indicative of spectral intensity of the collected light

Methodology Applied

Scientific EffectPhotoelectric Effect: Photoelectric Effect

Data Source

PatentUS9970863B2Optical metrology with reduced focus error sensitivity

Publication Date: 2018.05.15 KLA CORP

AI summary

Methods and systems for performing broadband spectroscopic metrology with reduced sensitivity to focus errors are presented herein. Significant reductions in sensitivity to focus position error are achieved by imaging the measurement spot onto the detector such that the direction aligned with the plane of incidence on the wafer surface is oriented perpendicular to the direction of wavelength dispersion on the detector surface. This reduction in focus error sensitivity enables reduced focus accuracy and repeatability requirements, faster focus times, and reduced sensitivity to wavelength errors without compromising measurement accuracy. In a further aspect, the dimension of illumination field projected on the wafer plane in the direction perpendicular to the plane of incidence is adjusted to optimize the resulting measurement accuracy and speed based on the nature of target under measurement.