Electrical particle gun

An electrical particle gun with applications in electronic and ionic microlithography for the integrated circuits industry comprises an emission chamber. In the emission chamber are a source adapted to emit the particles and devices for accelerating and focussing the particles. An exciter circuit is electrically connected to a decelerator electrode adjacent the focussing device. This enables pulsed emission of particles with an energy level lying within a predetermined band and filtering of the particles according to their energy level.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The present invention relates to an electrical particle gun enabling pulsed 
beams of particles to be generated and filtered according to their energy 
level. 
2. Description of the prior art 
Current electronic or ionic microlithographic techniques use pulsed 
electron or ion beams to etch circuits according to a reference design or 
diagram. For testing the circuits obtained in this way the particle beam 
has to be a pulsed beam with a high repetition frequency. Such pulsed 
beams are obtained by means of systems enabling transmission of the 
particle beam to be blocked or authorized. Currently used systems employ 
capacitor plates, resonant cavities or unipotential electrostatic lenses. 
Systems involving the use of unipotential electrostatic lenses are covered 
by publications including U.S. Pat. No. 4,439,685. In this system the 
unipotential lens is disposed after a first focussing lens of the 
focussing optical column held at the reference potential, so that the 
unipotential lens is totally separated from the source of electrical 
particles. 
An object of the present invention is to provide a multifunction electrical 
particle gun enabling pulsed beams of electrical particles to be generated 
with a very high switching frequency. 
Another object of the present invention is to provide an electrical 
particle gun enabling these electrical particles to be filtered according 
to their energy level to obtain a beam with reduced energy spread and/or 
pulsed emission at high switching frequencies. 
A further object of the present invention is to provide an electrical 
particle gun that can be integrated into multistage accelerator systems. 
SUMMARY OF THE INVENTION 
The present invention consists in an electrical particle gun comprising an 
emission chamber and, in said emission chamber, a source adapted to emit 
said particles, devices for accelerating and focussing said particles, a 
decelerator electrode adjacent said focussing device, an exciter circuit 
and an electrical connection between said decelerator electrode and said 
exciter circuit, enabling pulsed emission of said particles with an energy 
level lying within a predetermined band and filtering of said particles 
according to their energy level. 
The invention finds an application in electron or ion microlithography in 
the integrated circuit industry. 
It will be better understood from the following description given by way of 
non-limiting example only with reference to the appended drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1a, the electrical particle gun in accordance with the 
invention comprises, within an emission chamber that is not shown in the 
figure, a source 1 emitting electrical particles and devices 2 for 
accelerating and focussing these particles. 
In the vicinity of the focussing device 2 is a decelerator electrode 3, 
electrically connected to an exciter circuit 4 and enabling the pulsed 
emission of particles with energy levels within a predetermined energy 
band and filtering of the latter according to their energy level. 
In accordance with the invention, the circuit 4 serves to apply to the 
decelerator electrode 3 an excitation signal consisting of a DC voltage of 
predetermined level, denoted u, on which is superimposed a pulse or a 
stream of pulses of amplitude .delta.V serving to block the particles. 
The term electrical particle gun means an electron gun in the case where 
the electrical particles emitted are electrons or an ion gun in the case 
where the electrical particles emitted are positively or negatively 
charged ions. In the case of an electron gun the emission chamber is an 
electronic vacuum chamber, in the known way, whereas in the case of an ion 
gun the emission chamber may comprise a vacuum chamber in the case where 
the ion gun emits ions by virtue of field emission from a metal deposited 
on the cathode of the source or a chamber into which a gas to be ionized 
is introduced. A more detailled description of electron or ion gun 
structures that may be used will be given later in the present 
description. 
In the first embodiment as shown in FIG. 1a, the excitation circuit 4 
comprises, connected in cascade, a variable DC voltage generator 40 
producing a bias voltage u relative to the reference voltage and a pulse 
generator 41 delivering one or more pulses of amplitude .delta.V. 
In a second embodiment of the excitation circuit 4, as shown in FIG. 1b, 
the circuit comprises a generator 40 producing a variable DC voltage u 
connected to the decelerator electrode 3 and a pulse generator delivering 
pulses of amplitude .delta.V connected to the decelerator electrode 3 by a 
capacitive device 42. 
In the case where the electrical particles emitted are negatively charged, 
as in the case of emission of electrons, the emission source 1 is 
negatively biased to a value U relative to the reference voltage. The 
source 1 is positively biased to a value U relative to the reference 
voltage in the case where the particles emitted are positively charged. 
The voltage U is produced by a conventional type DC generator and typical 
values of the bias voltage U can be as high as several tens of kilovolts 
relative to the reference voltage. 
The particle accelerator and focussing devices 2 consisting of an 
accelerator lens 2 are arranged in such a way that the accelerator lens 2 
is biased positively by means of a DC generator producing a voltage U1 
relative to the reference voltage in the case of emission of negatively 
charged electrical particles. The accelerator lens 2 is negatively biased 
by the aforementioned DC generator 6 to the value U1 relative to the 
reference voltage in the case of emission of positively charged electrical 
particles. 
In accordance with the present invention, the decelerator electrode 3 is 
biased relative to the emission source 1 to a low DC voltage of between 
zero and a few volts, negative or positive according to whether the 
particles emitted are negatively or positively charged. 
Operation of the electrical particle gun in accordance with the invention 
will now be described with reference to FIG. 2. 
FIG. 2 is a graph showing the distribution of the number of electrical 
particles emitted, or of the derivative of this number, as a function of 
the acceleration energy of the electrical particles in the gun. The curve 
shown in FIG. 2 is representative of the energy spectrum of the electrical 
particles contained in the particle beam shown at 7 in FIG. 1a and 1b for 
an acceleration voltage U delivered by the DC generator 5 relative to one 
or more accelerator anodes schematically represented at 8 in FIGS. 1a and 
1b. 
In the case where the decelerator electrode 3 is biased to a potential U, 
the corresponding energy level being denoted eU (at 1 on the graph), it is 
assumed that all the electrical particles emitted by the emission source 1 
pass through the decelerator electrode 3 and reach the anode or anodes 8. 
In this case all the particles which have an energy E greater (in absolute 
value) than eU can pass through the decelerator electrode. It will be 
understood that in the case of negatively charged particles or electrons 
the energy E is lower in value and in sign than the value eU where e 
designates the unit charge of the electron in value and in sign, whereas 
in the case of positive electrical particles the energy E is greater than 
the value eU. 
In the case where a voltage u is superimposed by means of the DC voltage 
generator 40, for example, the relative energy level of the decelerator 
electrode 3 relative to the energy levels of the particles in the beam is 
held at a value e(U-u) corresponding to the point marked 2 in FIG. 2. In 
this case the relative energy level of the decelerator electrode 3 is 
greater (in absolute value) than the energy levels of the particles 
contained in the particle beam 7, the latter being blocked by the 
decelerator electrode 3. 
In the case where a pulse of amplitude .delta.V is superimposed on the 
voltage u delivered by the DC voltage generator 40, the relative energy 
level of the decelerator electrode 3 when the applied pulse is present 
becomes e(U-u+V) and correponds to point 3 in FIG. 2 conditioned by the 
amplitude .delta.V of the pulses. In this latter case all the electrical 
particles of higher energy (in absolute value) than the relative energy of 
the decelerator electrode 3 corresponding to point 3 in FIG. 2 will be 
transmitted by the decelerator electrode 3 for the duration of the pulse 
whereas all other electrical particles, the energy of which is lower (in 
absolute value) than the relative energy level of the decelerator 
electrode 3 when pulses are present, that is the level e(U-u+V), will be 
blocked by the decelerator electrode 3. In FIG. 2 the electrical particles 
blocked are shown by the shaded part in the case where the electrical 
particles emitted are electrons and where the voltage u generated by the 
DC voltage generator 40 is a negative voltage. 
For optimum operation of the electrical particle gun in accordance with the 
invention, in particular with regard to chromatic aberration, the various 
component parts of the particle gun as shown in FIGS. 1a and 1b may be 
arranged in such a way that the particle beam 7 enters the decelerator 
electrode 3 with a focussing half-angle .alpha. in a ratio of less than 
1/5 to the emission half-angle .alpha.0 of the particles at the point of 
entry to the accelerator lens 2. The source 1 and the decelerator 
electrode 3 will accordingly be positioned allowing for the operating 
parameters of the accelerator lens 2 in such a way as to achieve this 
result. 
The particle beam 7 may advantageously be focussed by the accelerator lens 
2 at the object nodal point 30 of the decelerator electrode 3 so that the 
beam of particles emerging from the latter appears to issue from the image 
nodal point 31 of the decelerator electrode 3. The angle of emergence of 
the beam is then equal to the angle of entry at the decelerator electrode 
3. The object nodal point of the decelerator electrode 3 is substantially 
at the center thereof. 
The conditions covering focussing of the particle beam as previously 
described make it possible to minimize chromatic aberration. It is to be 
understood that these conditions are not limiting and that it is possible 
to depart slightly from the aforementioned operating conditions, although 
a particle beam of reduced quality is then obtained. 
It is to be understood that to obtain minimum chromatic aberration the 
accelerator lens 2 must be itself constructed so as to feature low 
spherical and chromatic aberration. This condition will be met in the case 
where the lens has a short focal length, which implies that the particle 
source 1 must normally be situated in the vicinity of the optical center 
of the accelerator lens 2. 
In accordance with the invention the decelerator electrode 3 must make it 
possible to establish on the optical axis near the center of the 
decelerator electrode a potential differing by at most a few volts from 
the potential at which the decelerator electrode 3 is held by the exciter 
circuit 4 according to the amplitudes u of the DC voltage and .delta.V of 
the pulses to which the decelerator electrode 3 is subjected. 
The decelerator electrode 3 may advantageously be a pierced electrode, the 
orifice 300 being substantially symmetrical relative to the optical axis. 
The orifice 300 advantageously has a diameter .phi. when the electrode has 
a thickness a such that the following relation is satisfied: 
.phi..ltoreq./a.ltoreq.3/5. The previously stated condition relating to 
the dimensions of the orifice and the electrode makes it possible to 
obtain the previously defined potential conditions substantially at the 
center thereof. 
To enable use of the electrical particle gun in accordance with the 
invention with control pulses having very short switching times, below one 
nanosecond, in order to enable use of the electron gun in accordance with 
the invention in switching mode at very high switching frequencies, that 
is several hundred megahertz, it is advantageous to arrange the 
decelerator electrode and the other electrodes of the electron gun, and in 
particular the accelerator electrodes 2, in such a way that the stray or 
electrostatic capacitance between the decelerator electrode 3 and the 
other electrodes of the electron gun is minimized. This makes it possible 
to use switching pulses of very short duration and even pulse streams with 
a very high repetition frequency. 
To obtain a decelerator electrode 3 of low electrostatic capacitance the 
aforementioned electrode may advantageously have a reduced cross-section 
as compared with the other electrodes of the gun. Also, the distance 
between the decelerator electrode 3 and the other electrodes may be 
increased, the only constraint relative to increasing this distance, which 
implies consequential focussing of the particle beam 7, being that it 
increases the overall size of the electrical particle gun. The distance 
between the decelerator electrode 3 and the other electrodes is typically 
one centimeter, for example. 
Various embodiments of an electrical particle gun in the case where the 
particles emitted are negatively charged, the gun constituting an electron 
gun, will now be described with reference to FIGS. 3, 4 and 5. 
In FIGS. 3 and 4 the various components of the electron gun in accordance 
with the present invention from figures 1a and 1b, with the exception of 
the decelerator electrode 3, are very similar to the components of the 
electron gun described in published European Patent Application No. 0 095 
969 of Dec. 7, 1983. For a description of the arrangement of the various 
components constituting the electron gun shown in FIGS. 3 and 4 in 
particular reference may be had to the text of the aforementioned European 
patent application. A parts list of the various components referenced in 
FIGS. 3 and 4 appears at the end of this description. 
Referring to FIG. 3, the electron gun in accordance with the invention 
comprises a magnetic lens 50 constituting the accelerator lens 2. With 
regard to the operating conditions of the electromagnetic lens 50, these 
correspond to the operating conditions described in the aforementioned 
European patent application, the magnetic lens 50 constituting a lens of 
short focal length. 
As seen in FIG. 3, the decelerator electrode may consist of an electrode 
denoted 71 comprising a hollow electrode body substantially enclosing the 
magnetic lens 50. The decelerator electrode 71 has an outlet face 710 
facing the magnetic lens 50 in which is an outlet orifice 711 disposed 
substantially on the optical axis. It is to be understood that the 
decelerator electrode 71 meets (in terms of its geometrical construction 
parameters relative to its thickness a and the diameter .phi. of the 
orifice 711) the condition whereby it is possible to obtain on the optical 
axis a potential within a few volts of the potential of the source 1, as 
previously defined. It is seen in FIG. 3 that the orifice 711 is flared at 
its ends. In this, optional case the thickness a of the electrode is to be 
understood as including these flared portions, this thickness having to be 
great enough to meet the previously defined conditions in respect of the 
potential at the center of the orifice 711 on the optical axis. 
In accordance with the invention, the decelerator electrode 71 is connected 
to the excitation generator 4 through the intermediary of the high-tension 
lead-through 25 and a supplementary electrical contact 250. 
An alternative embodiment in which the decelerator electrode has a low 
electrostatic capacitance relative to the magnetic lens 51 will now be 
described with reference to FIG. 4. 
In this figure the decelerator electrode 71 may advantageously be of 
reduced cross-section relative to the magnetic lens 50. It may 
advantageously be mounted on insulative spacers or rods 75 bearing against 
the corresponding surface 510 of the magnetic lens. The insulative spacers 
74 may advantageously be made from a material such as alumina. 
The electrical connection between the decelerator electrode 71 and the 
excitation generator 4 may advantageously be provided, as shown in FIG. 4, 
by a rigid conductor 74 connecting the electrode to the ring or shoulder 
33, 33a. 
The FIG. 4 embodiment makes it possible to obtain a decelerator electrode 
whose stray electrostatic capacitance relative to the magnetic lens 51 is 
in the order of one picofarad. 
The connection of the ring 33 to the excitation generator 4 may 
advantageously be made by a cable entering the high-tension lead-through 
25 and connected to the part 41 which is in turn connected to the 
decelerator electrode 71. 
Another embodiment of the electrical particle gun in accordance with the 
invention will now be described with reference to FIG. 5. 
In the embodiment shown in this figure the electrical particle gun may 
advantageously provide for the emission of electrons or even of ions, 
depending on the emission mode chosen. In FIG. 5 the magnetic lens 51 
previously employed is replaced by an electrostatic lens consisting of the 
parts denoted 76, 77, 78 and 79. The electrostatic lens constitutes a 
unipotential electrostatic lens of which the first electrode 76 and third 
electrode 78 are held at the potential required to extract the particles 
from the emission source while, the second electrode 77 is held at a 
different potential serving to vary the focussing properties of the lens. 
By virtue of an advantageous characteristic of the FIG. 5 embodiment, and 
with a view to obtaining a high performance particle gun from the point of 
view of spherical and chromatic aberration, the lens may be an assymetric 
type lens. In this case the orifice of the second electrode 79 would be 
nearer the first electrode 76 than the third electrode 78. Moreover, for 
this lens to function with a short focal distance the first electrode 76 
would be placed near the source, preferably at a distance of a few 
millimeters. Given these conditions, the spherical and chromatic 
aberration coefficients of the lens would be in the order of a few 
centimeters. 
The FIG. 5 embodiment is not limited to the implementation of an electron 
gun, of course. To the contrary, this embodiment advantageously lends 
itself to the implementation of a field emission ion gun. For a more 
detailed description of the operating conditions of a gaseous ion gun such 
as that previously mentioned reference may usefully be had to a paper by 
W. H. ESCOVITZ, T. R. FOX and R. LEVI-SETTI entitled Scanning Transmission 
Ion Microscope with a Field Ion Source, published in Pro. Nat. Acad. Sci. 
USA, Vol. 72, No. 5, pp-1826, 1828, May 1985. 
There has been described an electrical particle gun enabling pulsed 
emission of particles having energy levels within a predetermined energy 
band. 
The electrical particle gun in accordance with the invention is 
particularly remarkable in that it is designed and adapted through the use 
of a particularly simple design of decelerator electrode to control or 
switch the particle beam "at source", that is to say in the vicinity of 
the source, where the particles of the beam still have a low acceleration 
energy level. It is advantageously distinguished from the prior art 
devices in that it makes it possible, especially where the particle gun is 
a field emission gun and the particles emitted are electrons, to eliminate 
one focussing lens from the focussing column. 
The electrical particle gun in accordance with the invention is not limited 
to field emission guns, of course; specifically, it encompasses 
conventional electron and ion guns. The aforementioned elimination of the 
focussing lens consequently makes it possible to reduce the length of the 
particle trajectory and thus to attenuate effects which tend to increase 
the dimension of the probe or the section of the beam in the scanning 
plane. An increased scanning spatial resolution is thus obtained. 
Parts list of the most important parts shown in FIGS. 3, 4 and 5: 
24 polmethylmethacrylate sleeve 
25 alumina insulative sheath - high-tension leadthrough 
26 homologous disk 
27 base support 
28 electrical lead-through 
30 annular shape device 
31 ring 
33 ring 
33A outside shoulder 
34 alumina cylinder 
35 screw 
37 magnetic focussing device support member 
38 retaining screw 
39 metal plate 
40 electrical lead-through 
41 inner ring 
42 screw 
43 lead-through members 
44 lead-through members 
45 insulative disk 
50 magnetic lens 
51 coil 
52 sealed carcasse 
53 magnetic circuit 
60 polepiece 
61 polepiece first part 
62 polepiece second part 
63 spacer 
72 part of intermediate electrode 
250 electrical contact 
610 upper hole 
620 lower hole