Alignment Mark Sensing via Self-Referencing Interferometer
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Solution Overview
Problem
Existing alignment systems in lithographic apparatuses face challenges in accurately differentiating between signals from alignment marks and stray input from the substrate, leading to imperfect measurements due to product crosstalk, and are limited by the size of alignment marks, which restricts the ability to correct intrafield alignment errors and utilize valuable wafer real estate effectively.
Innovation Solution
A system that combines detector types to split the signal from alignment marks, allowing a detector to 'feel' the mark and a camera to 'see' it, enabling the identification and discard of spurious substrate signals, using a self-referencing interferometer and optical devices like beam splitters or flip mirrors to convey the signal to both components simultaneously or alternately, and performing image manipulation such as summing or differencing standing output images to enhance signal differentiation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single detector is used to sense alignment marks, then the system is simple and cost-effective, but it cannot differentiate between signals from alignment marks and stray input from the substrate, leading to measurement errors
Solution Approach 1:
The patent divides the sensing function into two separate detector types: a first detector for capturing phase information and a second detector for capturing intensity information. This segmentation allows each detector to specialize in one aspect of the alignment mark signal, enabling accurate differentiation between mark signals and substrate stray light without requiring a single complex detector system.
Solution Approach 2:
The patent combines the outputs from two different detector types through optical interference. The phase information from the first detector and intensity information from the second detector are merged to create a composite alignment signal that eliminates the ambiguity present in single-detector systems, achieving high measurement precision through synergistic combination of multiple sensing modalities.
2Reliability
If larger alignment marks are used to improve signal detection, then measurement reliability improves, but valuable wafer real estate is wasted and the number of correctable intrafield alignment errors is limited
Solution Approach 1:
The patent employs different detector types with different sensing characteristics at different locations in the optical path. The first detector is optimized for phase detection while the second detector is optimized for intensity detection, allowing each detector to extract specific information from the alignment mark signal. This local specialization enables reliable detection from smaller marks by optimizing each detector's response to specific signal characteristics rather than requiring large marks for general-purpose detection.
Solution Approach 2:
The patent changes the detection parameters by using two different detector types that respond to different physical properties of the alignment mark signal (phase and intensity). This parameter differentiation allows the system to detect alignment marks with higher signal-to-noise ratio even when the marks are smaller, because each detector type can be optimized for its specific detection mode rather than relying on the mark size to provide sufficient signal strength for a single detector.
3Measurement precision
If multiple detector types are combined to differentiate mark and substrate signals, then measurement precision improves, but the device complexity and cost increase
Solution Approach 1:
The patent introduces an optical device as an intermediary that directs the alignment mark signal to two different detector types. This intermediary optical system manages the signal distribution and coordination between detectors, simplifying the overall system architecture while enabling the use of multiple detector types. The optical intermediary handles the complexity of coordinating multiple sensors, allowing the detection system to achieve high precision without requiring complex electronic or mechanical coordination systems.
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 allows for more accurate alignment by distinguishing between mark and substrate signals, enabling the use of smaller alignment marks and improving overlay precision, thereby enhancing the correction of intrafield alignment errors and increasing the number of marks that can be used on the wafer.
Implementation Method 1
a sensor arranged to receive light diffracted by the alignment pattern and adapted to generate an output comprising at least one standing output image of the alignment pattern
Implementation Method 2
a self-referencing interferometer
Implementation Method 3
an optical device arranged to receive the output and adapted to convey the output to the detector and the camera
Data Source
AI summary
An apparatus for and method of sensing alignment marks in which a self-referencing interferometer based sensor outputs standing images of the alignment marks and camera device is used to capture the images as output by the sensor and a detector is used to obtain phase information about the alignment marks from the images as output by the sensor.