Contrast Enhancement of Polished Cross Sections of Semiconductor Structures - Sample Preparation for SEM

Application Note for Leica EM RES102 - Industrial Manufacturing

October 28, 2016

The surfaces of polished cross sections often show fine scratches and residues of the removed material or of the abrasive material. The artefacts are strongly material-dependent, and are mostly only detectable at higher resolutions in the scanning electron microscope.

A further problem arises from the fact that the ground section mostly only has low contrast, i.e., in the structures of the semiconductor materials are very difficult to discern. Conventionally, the surfaces of the ground sections are wet-chemically etched to improve the contrast. As a result, specific acids are required for each material system.

With the use of ion beam milling, the ground sections of semiconductor structures can be "contrasted".

Preparation Conditions
The ground sections were cleaned and selectively milled with the help of ion beam milling process. In doing this, various milling angles and times were tested.

Acceleration voltage:6 kV
Milling angle:15° and 90°
Milling time:variable
Sample movement:rotation

 

Results
The surface of the sample was cleaned and lightly selectively milled with a milling angle of 15°. To obtain efficient contrasting, however, the sample has to be milled at a steeper angle. A significant increase in the contrast of the structure was, however, only achieved after an additional milling at 90°. The milling time required is dependent on the material combination and the task in hand, and so must be determined for each different preparation problem.

Fig.1: SEM image of a cross-section of a semiconductor structure with scratches and contamination form the grinding
Fig.2: SEM image of the cross-section after cleaning (15°) and contrast enhancement (90°)
Fig.3: SEM image of the cross-section after cleaning (15°) and contrast enhancement (90°)
Fig.4: SEM image of the cross-section after (15°) and selective milling (90°) with clearly visible grain structure of the W Plug.

Au wire bond (45 min polishing, 3 min contrast enhancement)

The gold wire bond looks perfect after ion polishing. The grain structure of gold and the structure of the bond area are visible (Fig.5). Nevertheless, an additional contrast enhancement step can expose the fine structure like intermediate layers.

1. Polishing

Acceleration voltage:6 kV
Gun current:2.2 mA
Sample movement:Oscillation ±60°
Milling angle:
Milling time:45 min

 

2. Contrast enhancement

Acceleration voltage:3 kV
Gun current:1.6 mA
Sample movement:Oscillation ±60°
Milling angle:35°
Milling time:6 min.

 

Results
Fig.6 shows the result of the short contrast enhancement step. Now the inner structure of the bond area is clearly visible.

Fig.5: Au wire bond after ion polishing
Fig.6: Au wire bond after polishing and contrast enhancement

Cu wire bond (1h 45 min polishing, 5 min contrast enhancement)

1. Polishing

Acceleration Voltage:6 kV
Gun current:2.2 mA
Sample movement:Oscillation ±60°
Milling angle:
Milling time:1 h 45 min

 

2. Contrast enhancement

Acceleration voltage:3 kV
Gun current:1.8 mA
Sample movement:Oscillation ±60°
Milling angle:35°
Milling time:5 min
Fig.7: Cu wire bond (1 h 45 min polishing, 5 min contrast enhancement)
Fig.8: Cu wire bond (1 h 45 min polishing, 5 min contrast enhancement)
Fig.9: Cu wire bond (1 h 45 min polishing, 5 min contrast enhancement)

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