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Charging effect on non conducting sample and its elimination
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Objective:

The main motive of this experiment is to show, how the non conducting samples are charged during SEM analysis and how to minimize the charging.:

Instrument:

Field emission gun environmental scanning electron microscopy equipped with ETD and LFD with high vacuum and low vacuum mode.

Specimen:

Sintered pellet of Sr2- and Mg2- doped LaGaO3

Theory:

 During SEM analysis, the specimen is irradiated with an electron beam and in case of non conducting samples, it causes accumulation/build up of static electric charges on the specimen surface and hence charging effect arose. This static charge influences the electron signals and hence deteriorates the image information. To eliminate/reduce this charging and to achieve an informative image, different ways are adopted during the sample preparation or during the analysis, such as;

(a) with the use of less accelerating voltage,

(b) coating the non conducting samples with a thin conductive film,

(c) applying some biasing voltage to the sample surface,

(d) using low vacuum (in some SEM), and

(e) additionally, the samples should be mounted with a conductive bridge connecting the top of the sample surface to the sample holder by using some conductive coating or carbon tape.

How coating is helpful to reduce charging?

 Let's concentrate on Fig.1. It schematically illustrates the emission of secondary electron during SEM analysis of non-conductive specimens depending on the coating conditions. In Fig. 1(a), sample is coated fully and hence the developed charges are passed to the ground via the conducting layer for which negative charges are not accumulated over the specimen surface.  But in case of Fig.1 (b), the number of the secondary electrons emitted from the specimen may decrease due to the decrease of incident electrons landing to the specimen and hence image quality is deteriorated.

 

 

 

Fig.1: Schematic illustration of secondary electron emission during SEM analysis of non-conductive specimens depending on the coating conditions.

Some experiments to reduce the charging effect:

 In case of Fig.2 (a), the sample was analyzed at 20 KV without any coating and surface was fully charged reducing the information. However reducing the beam energy to 10 KV, charging was reduced (Fig. 2(b,c)). However apply some biasing voltage (500V), the charging effect was again reduced (Fig.2 (d)). Fig.3 (a) and Fig 3(b) shows the biasing effect. With the  various technique used here, charging effect was reduced. However for this sample to eliminate the charging effect completely  conductive coating is needed.

 

 

Fig.2: FESEM image of sintered pellet of doped LaGaO3 (without any coating); (a) at 20 KV, (b,c) at 10 KV, (d) at 10 KV applying biasing voltage of 500 V.

 

Fig.3: FESEM image of sintered pellet at 15 KV; (a) without any biasing, (b) with biasing of 500 V.

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