Charged particle beam apparatus with charge-up compensation

A charged particle beam apparatus comprising a particle source for generating a particle beam to irradiate a specimen located in a field space of a particle lens of the apparatus is provided with an auxiliary particle source for generating a particle beam of low energy particle to be injected into the lens field space and to be directed to the surface of the specimen by the same lens field.

BACKGROUND OF THE INVENTION 
The invention relates to a charged particle apparatus comprising a particle 
source for generating a particle beam irradiating a specimen to be located 
in the lens field space of a lens which forms part of a charged particle 
lens system of the apparatus. 
Such an apparatus in the shape of an electron beam apparatus is known from 
U.S. Pat. No. 4,306,149. This document discloses an electron microscope in 
which an object is located in a lens field space of an objective lens. 
This lens is provided with an auxiliary lens to enable easy switching 
between different modes of operation. In U.S. Pat. No. 4,820,898 an ion 
beam apparatus for which the invention is applicable is disclosed. 
If in such an apparatus electrically non-conductive specimens are examined 
the specimen may be charged up. This charging-up of the specimen may 
result in additional electric fields in the volume around the specimen 
which may have adverse effects on the imaging properties of the objective 
lens and hence on the image quality. This drawback has been neglected or 
accepted in the past because due to the moderate vacuum in the apparatus 
it rapidly vanished as a consequence of contamination on the surface of 
the specimen. The surface becomes electrically conductive, and the 
charging-up vanishes. No possibility of avoiding contamination apart from 
heating up the specimen is known. Unfortunately carbon deposits on the 
specimen surface are also liable to impair the image quality oz to 
detoriate right landing positions in a beam writing system such an 
electron beam pattern generator or ion beam implantating apparatus. 
In more modern instruments in which a relatively high vacuum can be 
realized which is necessity for studies of biological specimens etc., no 
substantial contamination occurs, and the phenomena of charging-up occur 
with all their drawbacks. 
One known method of neutralizing the specimen charge consists in producing 
slow charged particles of opposite sign from an auxiliary electron or ion 
source The slow charged particles will then be attracted to the specimen 
by the electrostatic field of the specimen charge and thus neutralize it. 
This method is, however, not applicable if the specimen is located in a 
region with a strong magnetic field because, according to Busch theorem, 
slow particles cannot enter a strong magnetic field unless they start from 
the auxiliary source with specific initial conditions giving them 
corresponding initial values of the angular momentum about the optical 
axis. If the value of the angular momentum of a charged particle is 
outside a certain limited range of values, then the particle is rejected 
by a strong magnetic field ("magnetic mirror"). This restriction of the 
range of initial values of the angular momentum of the charged particles 
emitted from an auxiliary source is of great practical importance if the 
charged particles are electrons. On the other hand, if the charged 
particles are ions, then they can, due to their large mass, reach the 
specimen with almost any practically realizable value of their initial 
angular momentum. A neutralization of the specimen charge by ions would, 
however, have the drawback that their presence would change the 
composition of the specimen to be examined. 
SUMMARY OF THE INVENTION 
The invention aims at neutralizing the specimen charge using an auxiliary 
charged particle placed close to the lens field space in which the 
specimen is located. According to the invention, the electrical and 
geometrical parameters of the auxiliary particle source are chosen to give 
the relatively slow particles initial velocities and directions enabling 
them to be directed to the specimen surface across the magnetic lens 
field. 
Since the charging-up phenomena are mainly generated by secondary electron 
emission from the specimen upon impinging of the relatievely highly 
energetic primary electrons from the main electron beam, the specimen 
charge has preferentially positive sign. Thus the invention provides means 
for the generation of electrons to be directed to the specimen surface in 
order to compensate for the loss of electrons due to secondary emission. 
Using an auxiliary electron source which injects electron into the local 
magnetic lens field in such a manner that they are guided to the specimen 
and stay there, the charge-up can he fully neutralized. It would also be 
possible to use secondary electrons from an objective diaphragm from which 
secondary electrons are released by a part of the main electron beam 
impinging thereupon. This method, however, has the drawback that the 
geometrical shape of the apparatus in a vital region would have to be 
adapted thereto. The necessity of such an adaptation should be avoided 
because it might strongly limit the field of view on the specimen in most 
modes of examination. 
In a preferred embodiment the objective lens field itself is used to guide 
an electron beam from the auxiliary electron source onto the specimen. In 
order to achieve this, the values of the initial velocities and initial 
directions of the slow electrons from the auxiliary source must lie within 
specific intervals in order to prevent the magnetic mirror effect. If the 
initial conditions are correctly chosen, then the electrons will move 
along spiral trajectories around the magnetic field lines, and the flood 
beam formed by such electrons may cover the whole area of the specimen 
under examination. The operational parameters of the auxiliary electron 
source can also be adapted, using a control system, to relevant properties 
of the main electron beam such as the current value thereof and to 
secondary properties of the specimen if known. 
In a further embodiment the secondary electron source is an electron 
emitter of any kind for example a p-n electron emitter such as disclosed 
in U.S. Pat. No. 4,303,930 provided with additional electron-optical means 
to control the initial directions and initial energies of the electrons in 
the auxiliary electron beam, and with control means for its current. 
In an embodiment for local field neutralization the auxiliary electron gun 
can he provided with a scanning system coupled with the scanning system 
for the main beam in order to inject the neutralizing auxiliary particle 
beam which is now focused into a crossover on the momentary impinging 
point of the main beam.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An electron microscope as shown in the Figure comprises an electron source 
1 with an electron emitter 2 and an anode 3, a condensor system 4 with a 
condensor aperture 5, a pre-specimen beam deflection system 6, an 
objective lens system 7 with a first pole piece 8 and a second pole piece 
9, a post-specimen deflection system 10, a magnifying lens system 11, a 
differential pump aperture 12 and a detection system for operation as a 
scanning transmission electron microscope (STEM). All elements are 
arranged around an electron beam 15 to be generated by the electron gun, 
and being indicated as the main electron beam. The first pole piece 8 of 
the objective lens system 7 comprises a coil 20, a yoke 21, scanning coils 
22, a vacuum tube 23 and an auxiliary lens system 24 for which a coil 25 
as well as a gap 26 in the ferromagnetic yoke are indicated. 
The second pole piece 9 is normally substantially similar to pole piece 8 
apart from the deflection coil system and the auxiliary lens which may, 
however, also be introduced in the second pole piece. In front of the 
second pole piece an objective aperture 28 is positioned which aperture 
normally is removable and interchangeable in size. Between the two pole 
pieces a specimen 29 to be examined is positioned. 
As already indicated in some modes of operation the possibility exists to 
generate secondary electrons from the aperture 28 by the main beam Under 
certain conditions these secondary electrons can be collected by the 
specimen in order to neutralize the charging-up. The apparatus further 
comprises an auxiliary electron source 30 in the shape of an electron gun 
with an electron emitter 32, an anode 34, and an electron-optical beam 
focussing system 36. Such an auxiliary gun is described in U.S. Pat. No. 
3,731,094 for example and is used there to neutralize space-charge 
phenomena. Relevant elements of the auxiliary gun are connected to a 
control unit 38 for controlling an auxiliary electron beam 40 to be 
injected into an objective lens field area 42. The control unit 38 for 
controlling an auxiliary electron beam 40 to be injected into an objective 
lens field area 42. The control unit 38 is linked to a control unit 44 
which controls the main electron beam such that parameters for the main 
beam can be introduced into the control system for the auxiliary beam and 
if appropriate vice versa. The auxiliary electron beam in general is 
adapted to the lens field of the objective lens such that it will be 
guided thereby to the specimen, irradiating a whole relevant area of the 
specimen with electrons which have a velocity small enough to cause no 
substantial secondary emission but will be absorbed in the specimen as 
long as this is positively charged up. Due to local fields near to the 
specimen as a consequence of local charging-up phenomena the electrons of 
the auxiliary beam will move to locations with the highest potential. 
These locations will thus become neutralized. If appropriate the electron 
source can be incorporated in part of the yoke of the objective lens.