Focused ion beam (FIB) analysis techniques have been used to prepare samples like solar cells, integrated circuits, batteries and magnetic media on electrical and thermal E-chips. This sample preparation procedure is quite tedious.
However by paying special attention to preparation, it can be made routine. This article covers a few techniques for FIB sample preparation and offers recommendations for effective sample preparation.
Method 1 - In Situ Lift-Out Method
The In situ lift-out method involves the following:
Firstly using normal FIB sample preparation procedures, a thin section is extracted from the area of interest, welded and thinned to less than 100nm on a half-grid. Figure 1 shows the E-chip configuration.
Figure 1. E-chip configuration
It is important to mount the E-chip on the FIB stage for the in situ technique, within the span of the micromanipulator tip in order to enable transfer. The steps detailed here work with the Omniprobe micromanipulator system, however may or may not be compatible with micromanipulator systems from other manufacturers.
Secondly, as an alternative method, thinning of the sample can be performed when it is still secured in the substrate before lift out takes place and by skipping the transfer step to the half grid.
Next, the transfer of the thinned sample to the E-chip can be performed using one of the following three techniques. The chip must be oriented in a manner that enables transfer from the sample substrate or half grid is straightforward. Transfers are made easy when the E-chip is tilted at 45° angle.
- In case of the thinned sample being positioned to a half grid, the section from the half grid is kept free after securing the micromanipulator tip to the sample. The tip can be rotated by the user with the instrument software, in case the micromanipulator system has a rotation option installed. Rotate in such a way that the thin section’s flat side is parallel to the E-chip membrane. In case of the E-chip being tilted at a 45° angle, the micromanipulator must be rotated accordingly.
- While mounting the thinned sample to a half grid, before the tip is remounted to the thinned sample, the half grid is removed from the FIB chamber and laid flat. The sample is oriented such that the flat side and the E-chip membrane are in identical orientation. The half grid is then re-inserted with an E-chip into the FIB chamber, the tip is remounted and the thinned sample is released to transfer to the E-chip membrane.
- In certain cases, manual rotation of the tip is possible if the manipulator system does not have rotation. In case this can be done, then the tip is rotated such that the thinned sample’s flat portion is parallel to the E-chip membrane. Again, in case the E-chip is inclined, ensure that E-chip orientation and the thinned sample’s flat side are in alignment. Be careful while rotating the manipulator to avoid damaging the sample or internal microscope components. Now the thinned sample must be positioned on the membrane.
The sample must just contact the E-chip membrane, in case too much pressure is applied, fracture of the membrane may occur. While using a thermal E-chip, the sample is oriented such that the interest area is above a hole in the ceramic membrane.
For electrical e-chips, the sample must always be near the electrical leads so contact may be done easily and may be positioned above a hole in the membrane if applicable.
Once the sample is positioned on the membrane, welding can be done. In order to hold the sample in position, static forces are also strong enough. Next, the micromanipulator tip is released.
In case electrical E-chip and electrical contact to the sample is needed, Pt or W from an electrode is deposited on the E-chip to the area of interest with FIB induced deposition.
Method 2 - Ex Situ Lift-Out Method
The Ex Situ Lift-Out method involves the following steps:
- Firstly, a thinned area of the sample is milled, thinned and released while it still remains the substrate.
- Secondly, the thinned sample and the substrate are removed from the FIB chamber. The ex situ tool is used to lift the sample and position it on the E-chip membrane. In case a Thermal E-chip is used, one must make sure that the preferred area is above a hole in the ceramic membrane. In case an Electrical E-chip is used, make sure the section is close to the electrical leads in order to easily establish electrical contact. The area of interest can be laid above a hole in the membrane to increase electron transparency and resolution.
- Next, the thin section will adhere to the membrane through electrostatic forces. It is also possible to insert the E-chip into the FIB for additional processing, such as making electrical contacts or sample welding to the membrane. Note that at increased temperatures deposited metal forming an electrical contact or weld point may melt. This must be kept in mind while mounting samples.
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This information has been sourced, reviewed and adapted from materials provided by Protochips.
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