Tilted Slit Confocal System for Automated Focus Tracking
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Solution Overview
Problem
Conventional automated focus methods for semiconductor wafers, such as 3-D NAND wafers, face challenges with process variation and wafer patterns, leading to inconsistent focusing and reduced defect detection sensitivity, especially in thick wafer stacks where diffractive patterns cause loss of focus and spherical aberrations.
Innovation Solution
An optical characterization system with tilted confocal slit devices in both illumination and collection paths provides continuous through-focus information without moving parts, allowing for precise tracking of the best focal plane and maintaining sample focus using a closed-loop control system, enabling accurate focus tracking across varying wafer surfaces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional automated focus methods are used for thick wafer stacks, then focus detection is performed, but process variation and wafer patterns cause inconsistent focusing and reduced defect detection sensitivity
Solution Approach 1:
The patent segments the detection function by using separate illumination and collection paths with slit devices at different conjugate focal planes. The first slit device at the first conjugate focal plane and the second slit device at the second conjugate focal plane divide the detection process into distinct optical channels, allowing independent optimization of each path for reduced cross-talk and improved focus precision.
Solution Approach 2:
The patent introduces tilted confocal slit devices as intermediary optical elements between the illumination source and the sample. These slit devices act as spatial filters that selectively transmit light from specific focal planes, enabling precise focus detection while blocking out-of-focus light that would otherwise cause cross-talk and reduce measurement reliability.
2Ease of operation
If conventional focus methods are used, then focus tracking is attempted, but diffractive patterns cause loss of focus and spherical aberrations
Solution Approach 1:
The patent replaces conventional mechanical focus adjustment mechanisms with an optical characterization system that uses tilted confocal slit devices. Instead of mechanically moving the sample or objectives to find focus, the system uses optical filtering through the slit devices to detect and track focus position, eliminating mechanical errors and diffractive pattern interference.
Solution Approach 2:
The patent changes the optical parameters of the detection system by introducing tilted slit devices at specific conjugate focal planes with defined tilt angles. This parameter change transforms the detection method from conventional wide-field imaging to confocal line scanning, which provides superior depth discrimination and eliminates spherical aberrations by restricting the acceptance angle of collected light.
3Productivity
If tilted confocal slit devices are used in both illumination and collection paths, then continuous through-focus information is obtained without moving parts, but system complexity increases
Solution Approach 1:
The patent merges the focus detection and focus tracking functions into a single integrated optical characterization system. The tilted confocal slit devices in both illumination and collection paths work together as a unified confocal arrangement, simultaneously providing through-focus information and enabling closed-loop focus tracking without requiring separate detection and actuation systems.
Solution Approach 2:
The tilted confocal slit device configuration serves multiple functions: it acts as a spatial filter for out-of-focus light rejection, provides through-focus information through the confocal arrangement, enables focus position detection via the tilted geometry, and facilitates closed-loop control. This multi-functionality reduces the need for additional specialized components, offsetting the initial complexity increase.
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 axial resolution, reduces cross-talk from lower surfaces, and maintains consistent defect detection sensitivity across the wafer, even in regions with process variation, by differentiating top and lower surface reflections and compensating for variable reflectivity.
Implementation Method 1
An optical characterization system with tilted confocal slit devices in both illumination and collection paths provides continuous through-focus information
Implementation Method 2
by differentiating top and lower surface reflections
Data Source
AI summary
A system for automated focus tracking of a sample is disclosed. The system comprises an illumination source, a set of illumination optics in an illumination path, a set of collection optics in a collection path, a first slit device in the illumination path, a second slit device in the collection path, at least one detector configured to generate an image of the sample, and a controller configured to receive through-focus information from the image, and provide corrective motion to a stage holding the sample to maintain a position of the sample at a selected focus. A method for automated focus tracking of a sample is also disclosed.