Via Bottom Profiling Using Scattered Light in Interferometry
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Solution Overview
Problem
Conventional optical metrology methods fail to accurately profile the bottom surface of vias in printed circuit boards reinforced with oriented fibers due to obstruction by protruding fibers, leading to incorrect measurements as the bottom surface is obscured and considered noise.
Innovation Solution
Utilizing a scanning interferometer with a numerical aperture of at least 0.4 and leveraging the surface roughness of the via to diffuse light back to the objective, allowing for the processing of previously discarded fringe data to achieve precise measurements of the bottom surface, even when obscured by fibers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional optical metrology methods are used to profile vias, then the measurement process is simple, but the measurement precision deteriorates because the bottom surface is obscured by protruding fibers
Solution Approach 1:
The patent converts the harmful effect of fiber obstruction into a beneficial measurement approach by utilizing the scattered light from the rough bottom surface. Instead of discarding the obscured regions as noise, the method processes the fringe data from these regions by leveraging the surface roughness to diffuse light back to the objective, thereby obtaining accurate measurements of the previously inaccessible bottom surface.
Solution Approach 2:
The patent changes the measurement parameters by using a scanning interferometer with a specific numerical aperture (at least 0.4) and processing fringe data differently. The method segments the via profile into regions and processes fringe data from obscured regions by adjusting the optical parameters and signal processing techniques to accommodate the scattered light from the rough surface.
2Measurement precision
If the numerical aperture is increased to see through the fiber shelf, then the measurement precision improves, but the device complexity increases
Solution Approach 1:
The patent specifies using a scanning interferometer with a numerical aperture of at least 0.4, which is a parameter change that enables sufficient light to reach the bottom surface and return to the objective. This parameter specification provides a clear guideline for instrument selection and configuration, balancing measurement capability with practical device requirements.
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 enables more accurate and precise profiling of the bottom surface of vias, resulting in reliable and repeatable measurements that match mechanical measurements, overcoming the limitations of conventional methods by incorporating scattered light from the rough surface.
Implementation Method 1
The signal captured in low-coherence interferometry (including structured light metrology) is fringes
Implementation Method 2
leveraging the surface roughness of the via to diffuse light back to the objective
Data Source
AI summary
The rough bottom surface of a recessed feature partially obscured by an overlying structure is profiled interferometrically with acceptable precision using an objective with sufficiently large numerical aperture to illuminate the bottom under the obscuring structure. The light scattering produced by the roughness of the surface causes diffused light to return to the objective and yield reliable data fringes. Under such appropriate numerical-aperture and surface roughness conditions, the bottom surface of such recessed features can be profiled correctly simply by segmenting the correlograms produced by the scan and processing all fringes that correspond to the bottom surface elevation.