Lamella Endpointing Using Reference-Sample Distance Determination
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing methods for producing thin lamellas for TEM analysis face challenges in achieving accurate endpointing due to low yield and imprecision in thickness control, particularly with conventional metrology techniques being impractical for samples thinner than 10 nm.
Innovation Solution
Utilizing structural reproducibility between congruent reference and active structures, data from the reference structure is used as an index to accurately determine the etch position in the active structure by comparing image data and feature attributes, enabling precise control of etching processes such as ion beam milling, plasma etching, or laser etching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional metrology techniques (laser or ultrasonic interferometry) are used for thickness measurement, then measurement capability is available for macroscopic samples, but the techniques become impractical for lamellas thinner than 10 nm due to insufficient positional accuracy
Solution Approach 1:
The patent introduces a reference structure as an intermediary element that enables indirect measurement. Instead of directly measuring the thin lamella with impractical interferometry techniques, the system measures a thicker reference structure with known geometric relationship to the lamella, then calculates the lamella thickness from the reference measurement and predetermined offset distance.
Solution Approach 2:
The patent creates a reference structure that is a geometric copy or replica of the target lamella structure, but with modified dimensions (thicker) that make it suitable for conventional measurement techniques. The reference structure replicates the key geometric features and spatial relationships, allowing measurements to be transferred to the actual lamella through calculated transformation.
2Ease of manufacture
If open-loop depth control is used for milling, then the manufacturing process is simple, but the manufacturing precision is poor with accuracy of only 10-100 nm resulting in low yield
Solution Approach 1:
The patent implements a feedback mechanism where the thickness of the reference structure is measured after milling using conventional interferometry, and this measurement is used to determine the endpointing accuracy. The feedback loop allows verification and adjustment of the milling process to achieve the desired 1 nm precision endpointing while maintaining process simplicity.
3Manufacturing precision
If closed-loop control with accurate metrology is used for milling, then the manufacturing precision can be improved, but the device complexity increases due to requirements for accurate metrology systems
Solution Approach 1:
The reference structure serves as an intermediary that simplifies the metrology system. Instead of requiring complex in-situ metrology during the milling process, the system uses a simpler measurement approach on the reference structure, which then provides the necessary feedback for closed-loop control through calculated transformation of measurements.
Data Source
AI summary
Methods and apparatus are disclosed for determining a distance from a cut face of an active sample to a target plane, using data acquired from a reference sample. The active and reference samples have congruent structure, allowing reference data to be used as an index. An SEM image of the cut face is compared with the reference data to determine position within the active sample, and thereby the remaining distance to the target plane. The technique can be applied repeatedly between phases of ion beam milling until an endpoint at the target plane is reached. Consistent, accurate endpointing is achieved. The technique is suitable for preparing 5-100 nm thick lamella for TEM analysis of electronic circuits and can be used in a wide range of applications. Variations are disclosed.