Multi-Depth Fiducial Alignment for Deep Feature Imaging
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Solution Overview
Problem
Charged particle microscopy systems face challenges in accurately measuring the location of features deep within a sample due to displacement issues such as sample shift and beam drift, particularly when the feature is out-of-focus in SEM images, making it difficult to determine the distance between the feature and the surface fiducial.
Innovation Solution
The method involves forming multiple fiducials at different sample depths, allowing for accurate alignment and imaging of features by ensuring that both the fiducials and the region of interest (ROI) are in-focus in the SEM images, achieved by milling the sample in a staircase fashion and using the relative locations of the fiducials to determine the ROI's position.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single surface fiducial is used for deep features, then the imaging process is simple, but the measurement precision deteriorates due to out-of-focus conditions
Solution Approach 1:
The single fiducial reference system is segmented into multiple fiducials positioned at different depths within the sample. Each fiducial serves as a depth-specific reference point, allowing features at different depths to be measured with appropriate focus conditions. This segmentation resolves the contradiction by maintaining measurement precision for deep features while keeping the overall process manageable through systematic depth stratification.
Solution Approach 2:
The reference system transitions from a two-dimensional surface fiducial to a three-dimensional multi-depth fiducial system. By adding the depth dimension with fiducials positioned at multiple z-locations, the system can accurately reference features throughout the sample volume, not just at the surface level. This dimensional expansion resolves the out-of-focus measurement problem for deep features.
2Measurement precision
If multiple fiducials at different depths are used, then the measurement precision for deep features improves, but the device complexity increases
Solution Approach 1:
Multiple fiducials are created and positioned at predetermined depths before the actual feature measurement process begins. This preliminary setup establishes a ready-to-use depth-referenced coordinate system, eliminating the need for complex real-time calculations during measurement. The fiducials are positioned in advance using the FIB, creating a prepared reference framework that simplifies subsequent imaging and measurement operations.
Solution Approach 2:
The multi-depth fiducials act as intermediary reference points between the imaging system and deep features. Rather than directly measuring deep features with potentially out-of-focus conditions, the system uses intermediate fiducial markers at known depths as mediators for accurate positioning and measurement. These intermediaries provide stable reference points that bridge the gap between surface imaging and deep feature location.
3Loss of information
If the feature is imaged at deep sample depth, then the feature location information is obtained, but the image quality deteriorates due to beam drift and sample shift
Solution Approach 1:
The multi-depth fiducial system provides feedback reference points at known positions throughout the sample depth. By periodically imaging these fiducial markers at different depths, the system can detect and compensate for beam drift and sample shift occurrences. The fiducials serve as feedback checkpoints that allow real-time correction of positioning errors, maintaining image quality reliability even when imaging deep features.
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 precise location and imaging of features deep within the sample, minimizing post-processing requirements and ensuring high-resolution imaging by maintaining the fiducials and ROI in-focus throughout the imaging process.
Implementation Method 1
A focused ion beam (FIB) and an electron beam can analyze defects and failures during micro-fabrication
Implementation Method 2
images of the exposed sample surfaces are taken using scanning electron microscopy (SEM)
Data Source
AI summary
Methods and systems for milling and imaging a sample based on multiple fiducials at different sample depths include forming a first fiducial on a first sample surface at a first sample depth; milling at least a portion of the sample surface to expose a second sample surface at a second sample depth; forming a second fiducial on the second sample surface; and milling at least a portion of the second sample surface to expose a third sample surface including a region of interest (ROI) at a third sample depth. The location of the ROI at the third sample depth relative to the first fiducial may be calculated based on an image of the ROI and the second fiducial as well as relative position between the first fiducial and the second fiducial.