electrostatic/electromagnetic field superposed objective lens

An objective lens where an electrostatic lens is superposed on an ordinary magnetic lens, so as to obtain high spatial resolution even at low accelerating voltage by decreasing the focal length.
For high spatial resolution observation, the electron probe diameter on a specimen needs to be made small. To achieve this, the accelerating voltage is set high so that the aberration of the objective lens becomes small. However, the observation of various specimens, such as the top surface of a specimen, a specimen (high polymer material, etc.) susceptible to thermal damage due to irradiation of a highly-accelerated electron beam and a specimen susceptible to electric charging, requires a low accelerating voltage (0.5 to 1 kV). For improving the spatial resolution while compensating the increased probe size due to the aberration at low accelerating voltage, it is necessary to shorten the focal length. When the focal length is shortened, the specimen position is close to the bottom of the objective lens and thus, the working distance (WD) becomes too short. It is noted that the ordinal WD requires several mm. To overcome this problem, superposing an electrostatic lens on the ordinary magnetic lens makes it possible to shorten the focal length without a very small WD. This feature achieves a smaller probe diameter than that provided by the objective lens which uses only a magnetic field. Thus, the specimen can be observed with high spatial resolution even at low accelerating voltage.
The structure of the electrostatic/electromagnetic field superposed objective lens is schematically explained in Fig. 1. A metal pipe, called "inner electrostatic electrode," is incorporated in the magnetic objective lens. In this electrode, a positive voltage (several kV) is applied to the magnetic objective lens. An electric field, which decelerates the electron beam, is generated between the bottom of the inner electrostatic electrode and the lower pole of the magnetic objective lens. Then, this generated electric field acts as an electrostatic lens. This electrostatic lens is created closer to the specimen than the magnetic lens. Since the bottom of the inner electrostatic electrode is placed above the lower pole of the magnetic objective lens, a small probe diameter is obtained without changing the WD (Fig. 2).
The major advantage of the electrostatic/electromagnetic field superposed objective lens is that high spatial resolution is achieved while maintaining the general versatility of the out-lens objective lens. The use of this unique lens enables high spatial resolution observation in various cases. That is, this superposed lens makes it possible to perform tilt observation of a large specimen, EDS and EBSD analyses, and observation with an added special stage (for heating and cooling, stretching, etc.) while the specimen does not interfere with the objective lens. Furthermore, a magnetic specimen can be observed with no influence from the magnetic field of the objective lens.

Fig. 1 Electrostatic/electromagnetic field superposed objective lens.
An inner electrostatic electrode is placed inside the magnetic lens. An electrostatic lens is created beneath the magnetic lens.

Fig. 2 Difference of the electron probe on the specimen for the magnetic objective lens and for the electrostatic/electromagnetic field superposed objective lens.
(a) Lens action only for the magnetic lens. The electron beam is focused onto the specimen only by the action of the magnetic lens.
(b) Lens action for the electrostatic/electromagnetic field superposed lens. An electrostatic field is generated at the bottom of the magnetic lens and thus, the electron beam is further focused. As a result, the focal length is shortened and a small probe is produced while the same WD is maintained.

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