Tilted Focal Plane Imaging for Extended Depth of Field
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Solution Overview
Problem
Current three-dimensional measurement techniques for semiconductor devices face challenges such as limited speed, sensitivity to surface contrast and texture, and complex mechanical designs, particularly in achieving high accuracy and speed while maintaining an effective depth of field.
Innovation Solution
An imaging system with a tilted focal plane is used, allowing only a small portion of the object plane to be in focus, enabling fast height measurement by scanning the system relative to the object and using focus stacking to combine images for an extended depth of field, thus avoiding the need for complex motorized focusing systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the aperture size of the lens is increased to improve optical resolution, then measurement precision is improved, but depth of field decreases
Solution Approach 1:
The patent divides the object plane into multiple height zones and captures images at different focal planes corresponding to each zone. By segmenting the measurement space vertically and combining images from multiple focal planes, the system achieves both high optical resolution (large aperture) and extended effective depth of field coverage.
2Manufacturing precision
If motorized focusing is implemented to enhance depth of field, then depth of field is improved, but device complexity increases
Solution Approach 1:
The patent replaces the motorized mechanical focusing system with a static optical system that uses multiple fixed focal planes. Instead of moving the lens to adjust focus, the system uses a tilted focal plane geometry combined with image processing to achieve extended depth of field measurement without any moving mechanical parts.
3Measurement precision
If interferometry is used for three-dimensional measurement, then measurement precision is improved, but productivity decreases
Solution Approach 1:
The patent performs preliminary capture of multiple images at different focal planes simultaneously (or in rapid succession) before any height calculation is performed. This preliminary imaging phase captures all necessary depth information in advance, allowing subsequent height calculation to proceed rapidly through image processing without requiring slow scanning or iterative measurements.
4Productivity
If on-the-fly grabbing technology is used to improve machine vision speed, then productivity is improved, but measurement precision decreases
Solution Approach 1:
The patent changes the optical parameter of focal plane orientation from parallel to tilted relative to the object plane. This geometric parameter change allows the system to maintain sharp focus across a wider depth range during rapid scanning, enabling high-speed on-the-fly imaging without sacrificing image clarity or requiring long exposure times.
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 faster and more accurate height measurement with improved depth of field and field of view, reducing sensitivity to surface characteristics and enabling high-resolution imaging without the need for complex mechanical systems.
Implementation Method 1
A straightforward way to improve machine vision accuracy is to increase the aperture size of the lens for higher optical resolution. However, increasing aperture size will lead to decreasing Depth of Field (DOF).
Implementation Method 2
Only in-focus light rays 114 which pass through the detector pinhole aperture 116 are collected by an image sensor 118, whereas out-of-focus light rays are mostly blocked.
Data Source
AI summary
An apparatus for measuring a height of an object plane or multiple points on an object is disclosed. The apparatus comprises an imaging system having a focal plane passing through a focal point of the imaging system, wherein the focal plane of the imaging system is tilted at an oblique angle with respect to the object plane such that only a small portion of the object is in focus. Alternatively, the focal plane is tilted at an oblique angle with respect to a scanning direction of the imaging system during relative movement between the imaging system and the object.