Overlay Metrology for Thick Stacks Using Dark Field Scatterometry
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Solution Overview
Problem
Conventional metrology techniques face challenges in accurately determining overlay for thick stacks due to unreliable intensity asymmetry measurements, especially when the distance between layers is significant, as they lack a region of stable intensity for averaging.
Innovation Solution
The method involves determining intensity asymmetry measurements from pairs of complementary pixels in first and second images of the target, obtained from complementary non-zero diffraction orders, to characterize the target and perform overlay metrology using a metrology apparatus with an illumination system and detection system capable of dark field methods.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional intensity asymmetry measurement methods are used on thick stacks, then overlay measurement can be performed, but the measurement reliability deteriorates due to lack of stable intensity region
Solution Approach 1:
The patent segments the target into multiple regions with different depths, measuring each region separately and combining results. This allows reliable overlay measurement on thick stacks by processing shallower and deeper regions independently rather than attempting a single measurement across the entire thick target.
Solution Approach 2:
The patent introduces a depth dimension by focusing at different depths within the target stack. By performing measurements at multiple focal planes and combining them, the method overcomes the limitation of thick stacks where single-plane measurements become unreliable.
2Reliability
If larger targets are used for measurement, then measurement reliability improves, but device complexity increases due to separate acquisitions for each target area
Solution Approach 1:
The patent merges multiple focal plane measurements into a single comprehensive overlay determination process. By combining results from measurements at different depths within one unified workflow, it achieves reliable overlay measurement without requiring separate acquisition processes for different target areas.
Solution Approach 2:
The patent creates a universal measurement method that works for both thin and thick stacks using the same dark field scatterometry approach with focal plane adjustments. This multi-functional method eliminates the need for different measurement procedures for different stack thicknesses.
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 enables accurate overlay metrology on thick stacks by providing stable and reliable intensity asymmetry measurements, improving the precision of overlay determination and reducing errors associated with thick stack sensitivity variations.
Implementation Method 1
These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation
Implementation Method 2
Diffraction-based overlay using dark-field detection of the diffraction orders enables overlay measurements on smaller targets
Data Source
AI summary
Disclosed is a method of determining a characteristic of a target on a substrate and corresponding metrology apparatus and computer program. The method comprises determining a plurality of intensity asymmetry measurements from pairs of complementary pixels comprising a first image pixel in a first image of the target and a second image pixel in a second image of the target. The first image is obtained from first radiation scattered by the target and the second image is obtained from second radiation scattered by the target, the first radiation and second radiation comprising complementary non-zero diffraction orders. The characteristic of the target is then determined from said plurality of intensity asymmetry measurements.