Lithography Focus Measurement Using Asymmetry Ratios
Solution Overview
Problem
Current methods for measuring lithographic apparatus focus are limited by the dependence on process conditions, particularly resist parameters like BARC thickness, which can lead to errors in focus measurements using side wall angle asymmetry analysis.
Innovation Solution
The method involves forming two periodic structures with different focus-sensitive asymmetries on a substrate, measuring their spectra, and determining the focus by calculating the ratio of asymmetries, which is independent of processing conditions, using a scatterometer configured to analyze the radiation spectrum.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If side wall angle asymmetry analysis is used to measure focus, then focus measurement can be performed, but measurement precision deteriorates due to dependence on process conditions like BARC thickness
Solution Approach 1:
The patent changes the measurement parameter from absolute side wall angle asymmetry to the ratio of asymmetries between two periodic structures. This parameter transformation eliminates dependence on process conditions like BARC thickness, as the ratio remains constant even when absolute values change due to process variations. The method uses two periodic structures with different pitches, where the ratio of their asymmetries provides focus information independent of resist parameters.
Solution Approach 2:
The patent introduces a ratio calculation as an intermediary step between the raw asymmetry measurements and the final focus determination. By using the ratio of asymmetries from two different periodic structures as the intermediary parameter, the method decouples the focus measurement from process condition dependencies, achieving more reliable and consistent measurements.
2Reliability
If single periodic structure asymmetry measurement is used, then focus information can be obtained, but measurement reliability worsens due to sensitivity to process variations
Solution Approach 1:
The inspection target is segmented into two distinct periodic structures with different pitches, each contributing to the overall asymmetry measurement. By dividing the measurement into two separate structures rather than relying on a single structure, the method obtains two independent asymmetry values whose ratio provides focus information that is robust against process variations.
Solution Approach 2:
The two periodic structures serve multiple functions: they both provide asymmetry measurements for focus determination, and their ratio relationship provides process-condition-independent measurement. The dual-structure design makes the measurement system universal across different process conditions, as the ratio relationship holds regardless of specific resist parameters.
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 allows for accurate focus determination in lithographic processes, reducing errors caused by process variations such as BARC thickness and providing a high signal-to-noise ratio, thus enhancing focus monitoring in lithographic processing.
Implementation Method 1
One form of specialized inspection tool is a scatterometer, in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured
Implementation Method 2
properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate
Data Source
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AI summary
A method is used to determine focus of a lithographic apparatus used in a lithographic process on a substrate. The lithographic process is used to form at least two periodic structures on the substrate. Each structure has at least one feature that has an asymmetry between opposing side wall angles that varies as a different function of the focus of the lithographic apparatus on the substrate. A spectrum produced by directing a beam of radiation onto the at least two periodic structures is measured and ratios of the asymmetries are determined. The ratios and a relationship between the focus and the side wall asymmetry for each structure is used to determine the focus of the lithographic apparatus on the substrate.