Through-Substrate Imaging of High-Aspect-Ratio Structures for Bottom CD
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Solution Overview
Problem
Existing methods struggle to accurately characterize high aspect ratio structures, such as Through-Silicon Vias (TSVs), by measuring their critical dimension at the bottom individually and efficiently, due to difficulties in modeling measurement signals and the need for precise imaging through thick substrates.
Innovation Solution
A method involving illumination from a light source with wavelengths suitable for transmission through the substrate, combined with imaging from the bottom side to capture images of the structure's bottom, and using image processing techniques to measure lateral dimensions, along with optional low-coherence interferometry for depth measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If spectroscopic reflectometry is used to measure depth profiles of TSV arrays, then depth information can be obtained, but individual characterization of TSVs is not possible and accurate modeling of measurement signals is difficult
Solution Approach 1:
The patent divides the measurement approach into two independent parts: (1) using spectroscopic reflectometry for depth measurement of individual TSVs, and (2) using optical sectioning with confocal microscopy for lateral dimension measurement. This segmentation allows each technique to be optimized for its specific purpose, resolving the contradiction between depth measurement accuracy and individual characterization capability.
Solution Approach 2:
The patent introduces optical sectioning as an intermediary technique that enables lateral dimension measurement of individual TSVs without requiring direct contact or complex modeling. The confocal microscopy system acts as a mediator to capture images at different depths, allowing individual TSV characterization while maintaining the simplicity of the overall measurement process.
2Productivity
If a light spot covering multiple TSVs is used for spectroscopic reflectometry, then measurement speed is improved, but individual TSV characterization is lost
Solution Approach 1:
The patent segments the measurement process into two independent characterization steps: depth measurement using spectroscopic reflectometry (which can measure multiple TSVs simultaneously) and lateral dimension measurement using optical sectioning (which provides individual TSV data). This allows both high productivity and individual characterization to coexist.
Solution Approach 2:
The patent transitions from measuring only depth (one dimension) to measuring both depth and lateral dimensions (two dimensions) by combining spectroscopic reflectometry with optical sectioning. This dimensional expansion enables complete individual TSV characterization while maintaining the ability to measure multiple structures efficiently.
3Measurement precision
If imaging is performed through thick substrates, then bottom CD measurement becomes possible, but optical losses increase and image quality deteriorates
Solution Approach 1:
The patent changes the optical parameters by selecting specific wavelengths that are transparent to silicon (e.g., infrared wavelengths) and by adjusting the numerical aperture and focusing conditions of the imaging system. These parameter optimizations reduce optical losses and improve image quality when measuring through thick substrates, enabling accurate bottom CD measurement.
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
Enables accurate and efficient characterization of high aspect ratio structures by providing precise lateral and depth data, optimizing interferometric measurements to compensate for high losses and ensuring individual characterization of structures.
Implementation Method 1
a light source emitting light with a wavelength adapted to be transmitted through the substrate
Implementation Method 2
acquiring, with an imaging device positioned on the bottom side of said substrate, at least one image of a bottom of said at least one structure through the substrate
Data Source
AI summary
A method for characterizing structures etched in a substrate, such as a wafer is disclosed. A bottom of the structure is embedded in the substrate, the substrate having a top side in which the structures are etched and a bottom side opposite to the top side. The method includes the following steps: illuminating the bottom of at least one structure with an illumination beam issued from a light source emitting light with a wavelength adapted to be transmitted through the substrate, acquiring, with an imaging device positioned on the bottom side of said substrate, at least one image of a bottom of the at least one structure through the substrate, and measuring at least one data, called lateral data, relating to a lateral dimension of the bottom of the at least one HAR structure from the at least one acquired image. A system implementing such a method is also disclosed.