Lithographic Scatterometer Asymmetry Calibration
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Solution Overview
Problem
Lithographic scatterometers face challenges in achieving high accuracy for asymmetry measurements due to stray radiation and 'ghost' reflections, which are difficult to eliminate with existing anti-reflection coatings and surface cleaning methods, especially when measuring across a wide range of wavelengths.
Innovation Solution
A method and apparatus that include calibration measurements to correct for stray radiation by capturing images of periodic structures in multiple orientations and illumination modes, allowing for the calculation of corrected asymmetry measurements using specific calibration values to reduce the influence of stray radiation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If anti-reflection coatings and surface cleaning methods are used to reduce stray radiation, then measurement accuracy is improved, but stray radiation and ghost reflections persist especially when measuring across a wide range of wavelengths
Solution Approach 1:
The patent applies preliminary action by performing calibration measurements before actual asymmetry measurements to characterize and store stray radiation patterns. These calibration data are then used to correct subsequent measurements, eliminating the need to physically remove stray radiation during the measurement process itself.
Solution Approach 2:
The patent creates a digital copy or model of the stray radiation characteristics through calibration measurements. This mathematical model is then subtracted from actual measurements to remove the harmful effects of stray radiation, achieving correction without physical intervention in the optical path.
2Measurement precision
If substrate rotation mode is used to measure asymmetry by capturing images in multiple orientations, then measurement accuracy is improved, but measurement time and process complexity increase
Solution Approach 1:
The patent performs calibration measurements in multiple orientations as a preliminary step to establish stray radiation characteristics. Once calibrated, subsequent asymmetry measurements can be corrected using these pre-established calibration data, reducing the time needed for each actual measurement while maintaining accuracy.
Solution Approach 2:
The system uses the periodic structure itself to generate the calibration data needed for correction. By measuring the known periodic structure in multiple orientations, the system self-calibrates to characterize its own stray radiation patterns, eliminating the need for external calibration targets or complex additional equipment.
3Measurement precision
If multiple calibration measurements are performed to correct for stray radiation, then measurement accuracy is improved, but device complexity and measurement process complexity increase
Solution Approach 1:
The patent creates mathematical models (copies) of stray radiation characteristics through calibration measurements. These digital models are then processed computationally to correct actual measurements, transferring complexity from the physical measurement process to data processing, which simplifies the hardware requirements.
Solution Approach 2:
The patent changes the approach from physically modifying the optical system to mathematically adjusting measurement parameters. By transforming the problem from a hardware correction task to a data processing task, the patent reduces device complexity while maintaining or improving measurement accuracy through computational methods.
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 enhances the accuracy of asymmetry measurements without the need for new hardware, improving the precision of overlay, focus, and dose measurements in lithographic processes.
Implementation Method 1
measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a diffraction 'spectrum' from which a property of interest of the target can be determined
Implementation Method 2
these methods are susceptible to asymmetry in the optical paths used in the different modes, which will result in errors when measuring the asymmetry of the target. Accordingly, although various calibrations and corrections can be applied to reduce these errors
Data Source
AI summary
A scatterometer is used in a dark-field imaging mode to measure asymmetry-related parameters such as overlay. Measurements of small grating targets are made using identical optical paths, with the target in two orientations to obtain separate measurements of +1 and −1 diffraction orders. In this way, intensity scaling differences (tool asymmetry) are avoided. However, additive intensity defects due to stray radiation (ghosts) in the optical system cannot be avoided. Additive intensity issues strongly depend on the ratio between 0th and 1st order diffraction and are therefore strongly substrate (process) dependent. Calibration measurements are made on a few representative target gratings having biases. The calibration measurements are made, using not only different substrate rotations but also complementary apertures. Corrections are calculated and applied to correct asymmetry, to reduce error caused by stray radiation.