Metal Films and Sheets - Sample Preparation for TEM

Application Note for Leica EM RES102 - Material Research

November 16, 2016

Most metal films already have a thickness that requires no further mechanical pre-preparation. Frequently, however, they are also domed, which can lead to undefined milling angles. this is a disadvantage, particularly for films that contain inclusions, and that therefore mostly require very flat milling angles.

Milling angles that are too high, and which tereby also lead to excessive energy transfer through the ions, often result in the destruction of the sample material. The latter should be proved by the use of various milling angles. To do this, thin Al and Ti films were prepared.

Metal films

The metal films can be prepared automatically with the Leica EM RES102. In addition, single milling steps or complete programs can be written. These will then be checked by the computer and, with the help of an optical automatic termination, will be stopped at the optimum point of time. The presence of the user during the milling process is not necessary. Time-consuming sample preparations can thereby be carried out overnight.

Preparation Conditions
Mechanical pre-preparation: Disc puncher to cut out 3 mm discs.

Ion milling

Sample holder:Quick-clamp-holder
Acceleration voltage:7 kV / 2 kV (final thinning)
Milling angle:16° and 6°
Sample movement:Rotation

 

Propgrammed milling sequence on the touch screen

Results
The metal films can be prepared to the electron-transparent over a wide range. The influence of the milling angle is clearly discernible with Al films. While the sample processed with a milling angle of 6° is free from visible defects (see Fig.1), the sample prepared with 16° shows a high defect density (Fig.2).

Fig.1: TEM image of an Al film that has been ion milled with a milling angle of 6°
Fig.2: TEM image of an Al film that has been ion milled with a milling angle of 16° with clearly visible defect structures (dislocation network)
Fig.3: TEM image of a Ti film that has been ion milled with a milling angle of 6°
Fig.4: TEM image of a Ti film that has been ion milled with a milling angle of 6°

Metal sheets

Metal sheets are thicker than 100 µm. Mechanical pre-preparation is necessary to obtain an acceptable initial thickness and a good surface quality for ion milling. An ultrasonic cutter can be used to get a 3 mm disc. It is also possible to use the core drill of the Leica EM TXP.

Preparation Conditions
Mechanical pre-preparation: TXP core drill to get a 3 mm disc. TXP for double sided polishing down to a final thickness of 50 µm. Diamond foils: 6 µm, 3 µm, 1 µm and 0.5 µm at 2000 rpm.

Ion milling
1. Step:

Acceleration voltage:7 kV
Current:2.6 mA
Milling angle:±6°
Sample movement:Rotation
Milling time:4h

 

2. Step:

Acceleration voltage:2 kV
Current:1.5 mA
Milling angle:±6°
Sample movement:Rotation

 

Results
Clean and large electron transparent area.
Some grains show lattice damages which may be due to the rolling process during manufacture.

Fig.5: TEM image of ion milled V2A steel sheet
Fig.6: TEM image of ion milled V2A steel sheet

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