Projection-Mask Autofocus for Configurable Specimen Focus Offset
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Solution Overview
Problem
Existing auto-focusing systems struggle to maintain the best focal plane on a specimen surface with configurable focus offset, especially in semiconductor fabrication, leading to inconsistencies in defect detection sensitivity due to process variations and diffractive specimen patterns, which cause spherical aberrations and loss of focus.
Innovation Solution
An automated focusing system with a configurable focus offset, incorporating a projection mask, detector assembly, and a controller with processors to dynamically adjust the stage assembly based on projection mask image quality and normalized s-curves, ensuring optimal focal positioning.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Extent of automation
If existing auto-focusing systems are used to track specimen surface, then focus adjustment is automated, but focus consistency is lost due to process variations and diffractive patterns causing spherical aberrations
Solution Approach 1:
The patent introduces a projection mask as an intermediary element that projects a known test pattern onto the specimen surface. This mask serves as a reference that allows the system to measure and compensate for focus deviations caused by process variations and diffractive patterns, thereby maintaining focus consistency while preserving automation.
Solution Approach 2:
The system implements a feedback mechanism where the imaging system captures images of the projected pattern, and the controller analyzes these images to determine focus quality. Based on this analysis, the controller automatically adjusts the stage assembly to optimize focus, creating a closed-loop system that maintains focus consistency despite process variations.
2Manufacturing precision
If focus is adjusted to compensate for process variations, then image quality improves, but system complexity increases due to additional components and control mechanisms
Solution Approach 1:
The projection mask serves multiple functions: it provides a test pattern for focus measurement, acts as a reference for compensating process variations, and enables the feedback control mechanism. This multi-functionality reduces the need for separate components, thereby limiting the increase in system complexity while achieving improved image quality.
3Measurement precision
If projection mask image quality is used for focus determination, then focus accuracy improves, but measurement precision requirements increase
Solution Approach 1:
The projection mask introduces a specific test pattern with known local features at the specimen location. By analyzing the image quality of this localized pattern rather than the entire specimen, the system achieves accurate focus determination while reducing the overall measurement precision requirements, as only the local pattern features need to be measured with high precision.
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
The system maintains consistent focus on the specimen surface, enhancing image quality and defect detection sensitivity by dynamically adjusting the focus to compensate for process variations and specimen thickness, thereby improving imaging system performance.
Implementation Method 1
relay system configured to optically couple illumination transmitted through the projection mask to an imaging system, the relay system being configured to project one or more patterns from the projection mask onto a specimen disposed on a stage assembly of the imaging system and transmit an image of the projection mask from the specimen to the detector assembly
Data Source
AI summary
An auto-focusing system is disclosed. The system includes an illumination source. The system includes an aperture. The system includes a projection mask. The system includes a detector assembly. The system includes a relay system, the relay system being configured to optically couple illumination transmitted through the projection mask to an imaging system. The relay system also being configured to project one or more patterns from the projection mask onto a specimen and transmit an image of the projection mask from the specimen to the detector assembly. The system includes a controller including one or more processors configured to execute a set of program instructions. The program instructions being configured to cause the one or more processors to: receive one or more images of the projection mask from the detector assembly and determine quality of the one or more images of the projection mask.