Overlay Metrology Correction for Process Variation
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Solution Overview
Problem
Diffraction-based overlay metrology techniques struggle to accurately measure overlay errors due to the interference of local process variations, which can cause asymmetry between alignment pads even in the absence of alignment errors, making it difficult to separate overlay shift and process variation contributions.
Innovation Solution
The method involves measuring the overlay pattern before and after forming the top diffraction gratings, generating differential spectra from both states, and using ratios of these spectra to calculate the overlay error independently of process variations, allowing for direct calculation or fitting techniques to determine the error.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If diffraction based overlay measurement is used to measure alignment error, then overlay measurement capability is enabled, but measurement precision deteriorates due to local process variations causing asymmetry between alignment pads
Solution Approach 1:
The measurement is segmented into two separate steps: first measuring the incomplete overlay pattern (without top diffraction gratings) to capture process variations, then measuring the completed overlay pattern (with top diffraction gratings) to capture both process variations and overlay errors. By separating these measurements, the contribution of local process variations can be isolated and removed from the final overlay error calculation.
Solution Approach 2:
The measurement of the incomplete overlay pattern is performed as a preliminary action before forming the top diffraction gratings. This preliminary measurement captures the baseline process variations that will also affect the final measurement, allowing these variations to be subtracted from the completed pattern measurement to obtain the true overlay error.
2Reliability
If alignment pads are used for overlay measurement, then overlay error detection is enabled, but measurement reliability deteriorates when process variations create false asymmetry
Solution Approach 1:
The measurement process is divided into two segments: a preliminary measurement of the incomplete pattern that captures only process variations, and a final measurement of the completed pattern that captures both process variations and overlay errors. This segmentation allows the information about process variations to be extracted and removed, leaving only the true overlay error information.
Solution Approach 2:
The contribution of local process variations is extracted from the measurement data by comparing the incomplete pattern measurement with the completed pattern measurement. By taking out the process variation component, the remaining signal represents the true overlay error, improving measurement reliability.
3Productivity
If submicrometer feature sizes are used to increase integration density, then device capability is improved, but overlay alignment tolerance is reduced making measurement more difficult
Solution Approach 1:
The invention transitions from measuring only the final completed pattern to measuring in two dimensional states: the incomplete pattern (before top grating formation) and the completed pattern (after top grating formation). This additional measurement dimension provides the extra information needed to separate process variations from overlay errors, enabling accurate measurement even at submicrometer feature sizes where tolerances are tight.
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 overlay error measurement by isolating and correcting for process variations, thereby improving the reliability of semiconductor device alignment at submicrometer feature sizes.
Implementation Method 1
diffraction based overlay metrology method and apparatus... The plurality of measurement locations are illuminated with incident radiation that reacts with the diffraction gratings and the resulting radiation is then detected
Data Source
AI summary
A diffraction based overlay metrology system produces the overlay error independent of effects caused by local process variations. Generally, overlay patterns include process variations that provide spectral contributions, along with the overlay shift, to the measured overlay error. The contributions from process variations are removed from the determined overlay error. In one embodiment, the local process variations are removed by measuring the overlay pattern before and after the top diffraction gratings are formed. A plurality of differential spectra from the measurement locations of the completed overlay pattern can then be used with a plurality of ratios of differential spectra from measurement locations of the incomplete overlay pattern can then be used to determine the overlay error by either direct calculation or by fitting techniques. In another embodiment, the local process variations are removed with no premeasurement but with careful construction of the overlay patterns.