Endpoint the Thinning of a TEM Sample Face Following Sample Liftout Preparation Steps and Prior to Final Sample Mounting
Publication Date: 2008-Apr-06
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Thin one of the two lamella faces prior to lifting it and mounting it to the TEM grid. After performing the bulk mill steps needed to prepare a sample for lift out, use secondary electron imaging to endpoint thinning on one face of the sample. Then, it may be possible to transfer the sample to a TEM grid with no damage or an acceptably small amount of damage to the thinned face. The thinned face should be oriented away from the ion beam during the lift out and mount to the TEM grid.
Endpoint the Thinning of a TEM Sample Face Following Sample Liftout Preparation Steps and Prior to
Final Sample Mounting
Transmission Electron Microscope (TEM) samples are cut from a substrate using a focused ion beam, mounted on a TEM grid, and thinned by the ion beam. A sample can be either a cross-sectional sample or a plan view sample depending on the orientation of the sample in the substrate before extraction. The orientation of the sample in the substrate before extraction affects the orientation at which the samples are mounted on the TEM grid and presented to the ion beam for thinning. Some systems, such as the Expida™ System from FEI Company can include a TEM Sample Unloader ("TSU™"), in which the TEM grid is mounted in a capsule that can be readily removed from the system without breaking vacuum. The TSU cartridge holds the TEM grid tilted at a 38° angle to allow the plane of the grid (and samples mounted on it) to be positioned perpendicular to the electron beam for imaging with the in-system STEM detector.
When thinning a sample using the ion beam, it is necessary to stop thinning when the appropriate thickness is achieved and before the feature to be examined is removed. When thinning TEM lamella mounted parallel to the grid in one of these 'tilted' cartridges it is only possible to view a single face of the lamella using the secondary electron detectors, which are mounted off the optical axis. There is no physical orientation that will allow secondary electron imaging of the other lamella face while imaging. There may also be other cases where neither the electron transmission imaging of the lamella nor secondary electron imaging of the visible features are sufficient to endpoint the thin on the obscured face.
To view the sample to determine whether thinning is complete, it has been necessary to either: (1) physically removing the TEM grid from the vacuum chamber and rotating it in the cartridge or loading it into a second tool; or (2) use a combination of second electron imaging and transmission electron imaging to endpoint the thin of the face obscured from the secondary electron detector.
Below is presented a method of determining the end point of a thinning operation. The present invention is useful for any TEM preparation system where the final mounting of the TEM sample obscures one face of the final lamella from the secondary electron detector. The invention provides increased reliability in endpointing material removal on the obscured face. F...