How To: TEM Lamella Preparation Using FIB-SEM

Table of Contents

Background #

A TEM lamella is an electron transparent sample analyzed in a transmission electron microscope (TEM). TEM lamella can range in thickness depending on the accelerating voltage of the TEM. A good rule of thumb is for sample thickness is 0.8-1.5 nm per kV of accelerating voltage. This means that a 200 kV microscope can image samples ranging from 160-300 nm thick.

There are a multitude of ways to prepare TEM lamellas. For the sake of this website, we will focus on preparing TEM lamella using a FIB-SEM (Focused Ion Beam-Scanning Electron Microscope). FIB-SEMs are sometimes also referred to as DualBeams since they have both and electron and ion columns.

Overview #

FIB-SEMs make it convenient to prepare TEM samples at precisely selected areas. This is not to be mistaken to mean that preparing TEM lamellas is easy. TEM lamella preparation requires the operation of many accessories simultaneously like: micromanipulator, Gas Injection System (GIS), and TEM Grids. Successful TEM Lamella preparation is facilitated by deep knowledge of the system’s geometry.

Figure 1. Schematic of the TEM lamella preparation process: FIB milling of the bulk sample into a thin foil, followed by transfer into a TEM, and finally imaging of the sample within the TEM.

The example illustrated above (Figure 1) shows a typical TEM lamella prep from a bone-titanium implant sample. The images at the bottom exhibit some TEM measurements of one of the prepared TEM lamellas. The lattice fringes (HRTEM image) as well as the electron diffraction pattern indicate that the sample is polycrystalline. The example experiment allowed researchers to study the mineralization of bone at precisely selected distances from the titanium implant, something which can be difficult to do with conventional preparation techniques.

General Lamella Preparation Process #

The process can, in general, be separated into the following 6 individual steps:

  1. Deposition of a protective layer with beam induced deposition
  2. Prepare lamella via cross-sectioning
  3. J-cut
  4. Lift-Out
  5. Lamella Thinning
  6. Final Polishing

Depending on the material, there are many nuances in each step and it is worth while to look up recipes for specific samples. 

Figure 2. Micrographs of a sample after the cross-sectioning step (a), the J-cut step (b), and the in-situ lift-out step with a micromanipulator (c) of a TEM lift-out preparation.

Lamella Cross-sectioning #

Cross-sectioning for lamella preparation follows the general procedure used for preparation of a normal cross-section, but material is removed from both the front and back faces of the target area (Figure 2a). The thickness of the cross-sectioned area can vary depending on the target end application, but is typically around 1-2 μm.

J-cut #

After the cross-sectioning of the sample is complete, a J-cut is made to disconnect the bottom and sides of the cross-section from the bulk of the sample (Figure 2b). The J-cut is typically made from three rectangular cuts milled in a parallel pattern. The depth of the cuts will depend on the thickness of the cross-section slice.

Lift-Out #

Once a sample is mostly free from the bulk material after a J-cut, the cross-sectioned material can be removed from the bulk with either in-situ lift-out with a micromanipulator setup (Figure 2c), or with an ex-situ lift-out setup. After a section is removed, it is attached to TEM grid for final thinning. The choice of TEM grid will depend on the lift-out setup and the final geometry of a sample.

Thinning #

Once a lamella is mounted onto a TEM grid, the sample is further thinned until the point of electron transparency. The thinning steps are done with a lamella oriented at a shallow angle versus the ion beam (Figure 3a). Material from a lamella is typically removed from both the front and backside, tilting a sample ±1 to 5° relative to the ion beam. The exact tilt used will vary across samples and instruments.

Figure 3. TEM lamella that is electron transparent at 5 kV (a), but not electron transparent at 2 kV (b).

While thinning, a lamella is also typically imaged with the electron beam of a FIB-SEM to monitor the thickness of a lamella. Using low electron accelerating voltages, one can determine the approximate thickness of a section by when the sample reaches electron transparency.

Final Polishing #

The last step in TEM lamella preparation is a final polishing of the sample to minimize sample contamination by ion implantation and remove amorphous material that is generated by ion beam damage. There are many different methods that are used for final polishing: lower accelerating voltage glancing angle milling, low accelerating voltage perpendicular milling, milling with an alternative ion beam with lower implantation depths, and likely many more. The method used for final polishing will be material and application dependent.

Technique
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