Phase Characterization of Targets Using Short Coherence Interferometry
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Solution Overview
Problem
Target imperfections pose a limiting factor in wafer metrology as overlay scatterometry methods become more accurate, while target inspection methods lag behind or rely on extensive modeling or slow measurement procedures.
Innovation Solution
A system comprising an illumination source with a coherence length of less than 30 microns, a phase controlled interferometer, and an imaging unit that generates interferometry images at the pupil and field planes to derive target characteristics by introducing controlled phase differences between radiation directed to the target and a reference optical element, allowing for accurate phase characterization and metrology.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If overlay scatterometry methods are used to improve measurement accuracy, then measurement precision is improved, but target imperfections become a limiting factor that worsens measurement reliability
Solution Approach 1:
A reference optical element is introduced as an intermediary to create a reference beam that serves as a mediator between the target and the detector. This reference beam provides a stable reference signal that compensates for target imperfections, allowing accurate measurement despite variations in target quality. The interferometer combines the target beam and reference beam to generate interference patterns that enable precise overlay measurement while compensating for target-related errors.
Solution Approach 2:
The system changes the coherence length parameter of the illumination source to less than 30 microns. This parameter change enables the generation of interference patterns that are sensitive to overlay variations while being less sensitive to target imperfections. The specific coherence length range optimizes the interference signal for overlay measurement and reduces the impact of target-related errors on measurement reliability.
2Measurement precision
If target inspection methods are used to characterize targets, then target characterization capability is improved, but the methods either lag behind metrology accuracy or depend on extensive modeling or slow measurement procedures
Solution Approach 1:
The system merges overlay metrology and target characterization into a single interferometric measurement process. By using the same optical path and detector for both functions, the system eliminates the need for separate inspection steps, reducing measurement time while maintaining high accuracy for both overlay and target characterization.
Solution Approach 2:
The reference optical element serves as a mediator that enables simultaneous extraction of multiple parameters from a single interferometric measurement. By analyzing the interference pattern between the target beam and reference beam, the system can determine both overlay information and target characteristics (such as side wall angles and film thickness) without requiring separate measurements or extensive modeling.
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 enhances measurement sensitivity and accuracy, improving current target-related inaccuracies by up to an order of magnitude, enabling precise characterization of target topography, side wall angles, and film thickness, and reducing spectral dependencies, thus aligning with overlay tool accuracies.
Implementation Method 1
an illumination source arranged to produce electromagnetic radiation having a coherence length of less than 30 microns
Implementation Method 2
a phase controlled interferometer arranged to receive the radiation from the illumination source, split the radiation between a target and the reference optical element
Implementation Method 3
the interferometer is arranged to controllably introduce a phase between radiation directed to the target and to the reference optical element
Implementation Method 4
generate, from reflected radiation from the target and from the reference optical element, an interference beam
Implementation Method 5
an imaging unit arranged to receive the interference beam and generate a plurality of interferometry images corresponding to different introduced phases
Data Source
AI summary
Systems and methods are provided which derive target characteristics from interferometry images taken at multiple phase differences between target beams and reference beams yielding the interferometry images. The illumination of the target and the reference has a coherence length of less than 30 microns to enable scanning the phase through the coherence length of the illumination. The interferometry images are taken at the pupil plane and/or in the field plane to combine angular and spectroscopic scatterometry data that characterize and correct target topography and enhance the performance of metrology systems.