Electrostatic-magnetic lens for particle beam apparatus

An electrostatic-magnetic lens is provided having either a symmetrical or asymmetrical magnetic lens which is overlaid with an electrostatic immersion lens. One electrode of the immersion lens is formed as a hollow cylinder, which is within an upper pole piece of the magnetic lens concentrically relative to the axis of symmetry thereof and extending into the region of the pole piece gap. The lower pole piece of the magnetic lens is preferably at a ground potential and clad with the beam guiding tube for protection against contamination and forming the lower electrode of the electrostatic immersion lens.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to an electrostatic-magnetic lens 
for a particle beam apparatus. 
2. Description of the Related Art 
Particle beam devices are currently being used in all areas of development 
and manufacture of microelectronic components. The various manufacturing 
stages of integrated circuits are monitored, masks and wafers are 
inspected, and micro-structures are generated by electron beam 
lithography, all with particle beam devices. 
Modified scanning electron microscopes that are equipped with retarding 
field spectrometers and fast beam blanking systems have obtained a 
particular significance in detecting logic and design errors in large 
scale integrated circuits, especially during the development phase. For 
example, the modified scanning electron microscopes are used to mesure the 
time dependence at selected nodes in the circuit. To avoid charging and/or 
damaging the radiation sensitive specimens, the electron optical devices 
are predominantly operated at low accerlating voltages of between 0.5 and 
5 kV, so that it is no longer possible to produce high resolution 
investigations as with conventional scanning electron microscopes. 
Therefore, all areas of the semiconductors industry have an increasing need 
for a high performance, low voltage scanning electron microscope which 
provides fast and high resolution investigations of microelectronic 
components. 
At low acceleration voltages, the resolution of a scanning electron 
microscope is determined by the beam diameter d on the specimen, which in 
turn is essentially defined by the Coulomb repulsion of the electrons, 
also known as the Boersch effect, which opposes focusing of the beam. 
Resolution is also hindered by the axial chromatic aberrations of the 
imaging lenses which increases with the chromatic aberration coefficient 
C.sub.F and, for a constant width of the energy distribution for the 
electrons, increases with decreasing primary energy. The beam diameter d 
is in accordance with the following equation 
EQU d=(d.sub.O.sup.2 +d.sub.F.sup.2).sup.1/2 ( 1), 
where the probe diameter d defines the resolution and d.sub.O denotes the 
geometrical optical probe diameter expanded by the Coulomb repulsion of 
the electrons between the beam source and the specimen, in other words, 
the lateral Boersch effect, and d.sub.F denotes the diameter of the 
chromatic aberration disk produced by the chromatic aberration, which by 
the relationship 
EQU d.sub.F =C.sub.F .multidot..alpha..multidot..DELTA.U/U (2) 
is dependent on the beam aperture, on the chromatic aberration coefficient 
C.sub.F of the lens, on the primary energy eU, and on the width of the 
energy distribution e.DELTA.U of the electrons. Therefore, it is only 
possible to improve the resolution by reducing the negative influences of 
the electron-electron interaction (the energetic Boersch effect which 
influences the energy width e.DELTA.U and the lateral Boersch effect which 
influences the probe diameter (d) and the chromatic aberration constant 
C.sub.F of the lenses used. 
In the publication by R. F. W. Pease "Low Voltage Scanning Electron 
Microscopy", Record of the IEEE 9th Annual Symposium on Electron, Ion and 
Laser Beam Technology, Berkeley, 9-11 May 1967, pp. 176-187, a scanning 
electron microscope is disclosed in which primary electrons are initially 
accelerated to high kinetic energies and are subsequently decelerated to 
the desired low final energy in an retarding field established immediately 
above the specimen. By measuring the beam cross section on the specimen, 
it can be shown that the objective lens of the disclosed arrangement in 
the retarding mode exhibits significantly smaller chromatic and spherical 
aberration constants than a magnetic single lens given conventional 
operation without a retarding field. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a lens for particle beam 
devices for decelerating high energy particles to a desired final energy 
which exhibits smaller aberrations than conventional magnetic lenses. This 
and other objects are inventively achieved in an electrostatic-magnetic 
lens having a magnetic lens generating a nearly rotational symmetric 
magnetic field and an electrostatic immersion lens generating a nearly 
rotational symmetric electrical field and including two electrodes at 
different potentials which are disposed within the magnetic lens. 
An advantage obtainable by the present invention is that the particle 
probes having small beam cross sections can be generated even for high 
beam currents and low accelerating voltages. 
Preferred developments and improvements of the invention include one pole 
piece of the magnetic lens forming an electrode of the electrostatic 
immersion lens, at least one of the electrodes of the electrostatic 
immersion lens in the shape of a hollow cylinder concentrically disposed 
within the upper pole piece of the magnetic lens, a second electrode in 
the form of a hollow cylinder provided within a hollow cylindrical 
electrode, a diaphragm within one of the electrodes that is conductively 
connected to the electrode, the diaphragm being an aperture diaphragm and 
being at the end of one of the electrodes, a beam guiding tube lining the 
lower pole piece of the magnetic lens and the magnetic lens being either 
symmetrical, asymmetrical, or conical.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An electrostatic-magnetic lens is shown in FIG. 1 for use in a particle 
beam device, such as a scanning electron microscope. A scanning electron 
microscope is shown in the above-identified publication from the Record of 
the IEEE 9th Annual Symposium on Electron, Ion and Laser Beam Technology, 
incorporated herein by reference. 
The electrostatic-magnetic lens shown in FIG. 1 is formed of a symmetrical 
or an asymmetrical magnetic lens ML which is overlaid by an electrostatic 
immersion lens. To achieve the smallest possible focal lengths, the 
magnetic flux generated with the assistance of an excitation coil SP is 
conducted through pole pieces UP and OP and is concentrated into a small 
spatial region around an axis of symmetry OA of the system. The magnetic 
field is rotational symmetric around the symmetric axis OA and achieves 
its maximum strength in a pole piece gap PS. 
In the illustrated exemplary embodiment, one electrode of the electrostatic 
immersion lens is in the form of a hollow cylinder RE which, together with 
a cylindrical insulator IS, is disposed in the upper pole piece OP of the 
magnetic lens ML. The cylindrical electrode RE is concentrically disposed 
relative to the axis of symmetry OA and extends into the region of the 
pole piece gap PS. A second electrode is formed by the lower pole piece UP 
and is preferably, although not necessarily, connected to ground. 
In FIG. 2, a second embodiment of an electrostatic-magnetic lens is shown 
including the lower pole piece UP of the magnetic lens ML being clad with 
a beam guiding tube SF of either magnetic or nonmagnetic material for 
protection against contamination. The lower pole piece UP is preferably 
connected to ground potential and forms the lower electrode of the 
electrostatic immersion lens in accordance with the invention. A 
rotational symmetric electrical retarding field is built up within the 
magnetic lens ML whenever the cylindrical electrode RE is connected to a 
positive potential in comparison to that of the lower pole piece UP, and 
more particularly, in comparison to the potential of an anode of the 
particle beam generator (not shown). 
The imaging properties of the particle-optical unit composed of the 
electrostatic immersion lens and the magnetic lens ML are essentially 
defined by voltages applied to the electrodes as well as the dimensions of 
the electrodes and by the magnetic field strength of the pole piece gap 
PS. It is therefore definitely not necessary that the cylindrical 
electrode RE and the opening in the lower pole piece UP have the same 
diameter. 
On the contrary, the position of the cylindrical electrode RE within the 
magnetic lens ML, its diameter and its spacing from the lower pole piece 
UP, is adapted to the required particle-optical properties of the overall 
system. Thus, the focal length of the electrostatic immersion lens is 
varied with the assistance of a circular aperture diaphragm BL which is 
disposed within the cylindrical electrode RE, as shown in FIG. 3, or which 
terminates in the region of the pole piece gap PS, as shown in FIG. 2. In 
FIG. 3, a continuous variation of the focal length is possible when two 
coaxially disposed cylindrical electrodes RE' and SE of different focal 
lengths are disposed within an upper pole piece OP' and the outer 
cylindrical electrode SE acts as a control electrode by being charged with 
a suitable voltage. Each of the cylindrical electrodes RE' and SE, in one 
embodiment, are terminated by a circular apertured diaphragm BL'. 
Due to the electrical retarding field superimposed on the focusing magnetic 
field, the electrostatic-magnetic lens of the invention exhibits 
noticeably smaller spherical and chromatic aberration constants than do 
known magnetic lenses. Thus, the aberration constants of the composite 
system essentially defined by the difference in potential between the 
cylindrical electrode RE and the lower pole piece UP are reduced by about 
a factor of ten in comparison to the aberration constants of a magnetic 
lens when the particles are decelerated in the retarding field of the 
immersion lens to one-tenth of their primary energy. The lenses of the 
invention also exhibits an advantage in that their particle-optical 
properties are easily calculated and are readily realized in practice due 
to the excellent centerability of the electrical and magnetic lens. 
Referring now to FIG. 4, an electrostatic-magnetic lens including a conical 
magnetic lens ML" is shown in which primed reference characters indicate 
substantially similar elements relative to FIGS. 1 through 3. Conical 
objective lenses are used, for example, in scanning electron microscopes 
so that it is possible to image and investigate large specimen at small 
working distances. As a consequence of the form of the pole pieces OP" and 
UP", the conical magnetic lens has a large pole piece gap PS" and, thus, a 
comparatively long focal length. This leads to large aberrations which 
increase with the focal length. As a result of the inventive arrangement 
of a cylindrical electrode RE" at positive potential within the upper pole 
piece OP", the imaging properties of the particle-optical unit having a 
conical magnetic lens ML" and an electrostatic immersion lens are 
noticeably improved in comparison to those using a conical single lens. 
This improvement in the imaging properties is achieved by the 
afore-described reduction of aberration constants and by the shift of the 
principal planes of the conical magnetic lenses ML" in the direction of 
the specimen. The shortening of the focal length associated with the 
magnetic lens ML" (C.sub.F .about.focal length) causes lower aberrations. 
In the exemplary embodiment, the lower pole piece UP" is preferably 
connected to ground potential to form one electrode of the electrostatic 
immersion lens. 
The electrostatic-magnetic lenses of the present invention are 
advantageously used in scanning particle microscopes and particularly in 
scanning electron microscopes. The Boersch effect in such microscopes 
limits the resolution, particularly at low accelerating voltages and the 
conventional lens systems used therein generate excessively high 
aberrations. Since the influence of the lateral Boersch effect on the 
probe diameter decreases at high kinetic energies, but the width of the 
energy distribution of the primary electrons, particularly in the beam 
generator, noticeably increases as a consequence of the energetic Boersch 
effect, the electrons should advantageously pass a first beam crossover, 
or source crossover (not shown), at low energies of, for example, 2 keV. 
The electrons are subsequently accelerated to high energies, of for 
example, 10 keV and are decelerated to the desired low final energy of, 
for example, 1 keV, shortly before they reach the specimen. The 
deceleration and focusing of the primary electrons advantageously occurs 
with the use of an electrostatic-magnetic lens of the present invention 
which is in place of a conventional condenser lens or objective lens of an 
electron optical column. The cylindrical electrode RE lies at anode 
potential, such as 9 kV, given an assumed cathode potential of -1 kV, an 
acceleration of the primary electrons to 10 keV and a desired final energy 
of the primary electrons of 1 keV. Typical dimensions of the 
electrostatic-magnetic lens are: 
A diameter of 6 mm and a gap of 3 mm of the cylindrical electrodes and 
a diameter of 10 mm of the upper and 6 mm of the lower pole piece and a gap 
of 4 mm of the magnetic lens. 
Although other modifications and changes may be suggested by those skilled 
in the art, it is the intention of the inventors to embody within the 
patent warranted hereon all changes and modifications as reasonably and 
properly come within the scope of their contribution to the art.