Focus Control for Multi-Layer Metrology Targets
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Solution Overview
Problem
The increasing numerical aperture of optical systems in inspection apparatuses decreases the depth of focus, leading to inaccurate metrology measurements, especially in multi-layered integrated circuits where the distance between target structures exceeds the depth of focus, resulting in only one target being in focus at a time.
Innovation Solution
The method involves controlling a characteristic of the radiation or collected scattered radiation to adjust the focus point of the optical system, allowing both target structures of a metrology target to be kept in focus simultaneously by using a radiation beam delivery system, a radiation collection system, and a radiation control system to manage the focus point based on received scattered radiation values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the numerical aperture of the optical system is increased to improve measurement capability, then the resolution is improved, but the depth of focus decreases
Solution Approach 1:
The patent implements dynamic focus control by varying the focus setting during the measurement process. The focus is adjusted based on the vertical position of target structures within the depth of focus range, allowing the system to adaptively maintain measurement accuracy across multiple layers despite the reduced depth of focus caused by high numerical aperture.
Solution Approach 2:
The patent changes the focus parameter dynamically during measurement. By adjusting the focus setting according to the known vertical positions of target structures (obtained from process data), the system compensates for the limited depth of focus, enabling accurate measurements of targets at different heights within the same measurement cycle.
2Measurement precision
If the numerical aperture is increased to enhance inspection capability, then the imaging quality is improved, but the focus control precision requirement increases
Solution Approach 1:
The patent performs preliminary actions by obtaining process data about target structure positions before conducting the actual measurement. This advance knowledge of the vertical positions of target structures allows the system to pre-calculate the appropriate focus settings needed during measurement, thereby reducing the risk of focusing errors and improving measurement reliability.
Solution Approach 2:
The patent incorporates feedback mechanisms where measurement results are compared against expected values, and focus control is adjusted accordingly. The system uses information about target structure positions to feedback-adjust the focus settings, ensuring that measurements remain accurate even with the reduced depth of focus associated with high numerical aperture.
3Manufacturing precision
If the depth of focus is increased to measure multi-layer structures, then the measurement range is improved, but the numerical aperture must be reduced
Solution Approach 1:
Rather than statically increasing the depth of focus by reducing numerical aperture, the patent dynamically adjusts the focus setting during measurement. This allows the system to maintain high numerical aperture for resolution while sweeping through different focus positions to capture targets at various heights, effectively achieving multi-layer measurement capability without sacrificing resolution.
Solution Approach 2:
The patent employs periodic focus adjustment during the measurement process. The focus is varied systematically to scan through the vertical range of target structures, allowing the system to periodically capture in-focus images of targets at different heights. This periodic focus variation enables measurement of multi-layer structures while maintaining high numerical aperture.
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 increases the depth of focus, enhancing the accuracy of metrology measurements by ensuring both target structures remain in focus, thereby improving the precision of measurements in complex integrated circuit structures.
Implementation Method 1
collecting scattered radiation that has been scattered by the target using a radiation collection system
Data Source
AI summary
An inspection apparatus includes an optical system, which has a radiation beam delivery system for delivering radiation to a target, and a radiation beam collection system for collecting radiation after scattering from the target. Both the delivery system and the collection system comprise optical components that control the characteristics of the radiation and the collected radiation. By controlling the characteristics of one or both of the radiation and collected radiation, the depth of focus of the optical system may be increased.