Dose-Based End-Pointing for Low-kV FIB TEM Sample Preparation
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Solution Overview
Problem
Current methods for preparing transmission electron microscopy (TEM) samples using focused ion beam (FIB) milling face challenges in achieving precise thickness and position due to high-energy beam-induced damage and degraded resolution at lower energies, leading to uncertainty in lamella thickness and position.
Innovation Solution
A method involving high-energy FIB milling followed by low-energy FIB milling with dose-based end-pointing to remove damage layers, where the high-energy beam mills the sample to a slightly thicker initial state, and the low-energy beam precisely removes the damage layer based on characterized material removal rates, ensuring accurate control of the final lamella thickness and position.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high energy FIB (30 keV) is used for milling, then productivity and material removal rate are improved, but a substantial damage layer is produced in the final lamella
Solution Approach 1:
The milling process is divided into multiple stages: initial high-energy milling for bulk material removal, intermediate milling for damage layer reduction, and final low-energy milling for damage layer removal. This segmentation allows each stage to optimize for its specific function, resolving the contradiction between productivity and damage prevention.
Solution Approach 2:
The high-energy milling is performed first to remove the bulk of unwanted material and create the initial lamella structure. This preliminary action prepares the sample for subsequent low-energy damage removal steps, ensuring that the majority of material removal occurs before damage becomes a concern.
2Object-affected harmful factors
If low energy FIB (2-8 keV) is used for damage removal, then the damage layer is reduced, but FIB resolution and probe characteristics are substantially degraded
Solution Approach 1:
The process separates damage removal from final precision milling. Low-energy beams are used exclusively for damage layer removal where high resolution is not critical, while high-energy beams handle precision positioning. This segmentation allows each beam energy to optimize for its specific function without compromise.
Solution Approach 2:
The patent introduces an intermediate milling step that bridges high-energy and low-energy operations. This intermediate step performs rough damage layer reduction, allowing the final low-energy step to focus solely on complete damage removal without needing to achieve high resolution, thus mediating between the conflicting requirements.
3Object-affected harmful factors
If low energy FIB is used for imaging and placement, then damage is reduced, but control of placement accuracy and thickness precision is much worse
Solution Approach 1:
The process segments imaging and placement functions from damage removal functions. High-energy FIB is used for all imaging and placement operations where precision is critical, while low-energy FIB is restricted to damage layer removal where precision requirements are lower. This segmentation resolves the contradiction by assigning each function to the appropriate beam energy.
Solution Approach 2:
Instead of using low-energy beams for precision work (which would reduce damage but lose resolution), the patent inverts the approach: high-energy beams perform precision imaging and placement, while low-energy beams handle damage removal. This inversion allows each beam type to operate in its optimal performance regime.
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 precise control of lamella thickness and position with reduced damage, improving the accuracy and quality of TEM samples by minimizing errors to less than 2% in material removal, thereby enhancing structural characterization.
Implementation Method 1
a focused ion beam (FIB) system... to mill away the unwanted volume of material
Implementation Method 2
directing the low energy focused ion beam toward one or more of the exposed faces of the unfinished sample lamella... to remove at least a portion of the damage layer
Data Source
AI summary
A method, system, and computer-readable medium for forming transmission electron microscopy sample lamellae using a focused ion beam including directing a high energy focused ion beam toward a bulk volume of material; milling away the unwanted volume of material to produce an unfinished sample lamella with one or more exposed faces having a damage layer; characterizing the removal rate of the focused ion beam; subsequent to characterizing the removal rate, directing a low energy focused ion beam toward the unfinished sample lamella for a predetermined milling time to deliver a specified dose of ions per area from the low energy focused ion beam; and milling the unfinished sample lamella with the low energy focused ion beam to remove at least a portion of the damage layer to produce the finished sample lamella including at least a portion of the feature of interest.