Ion source

An ion beam for use in fabrication and processing of semiconductors, thin films or the like. For making uniform the radial distribution of an ion beam extracted from the ion source, a plasma chamber is formed by extending a plasma producing chamber in the direction in which microwave energy is introduced. The plasma chamber thus formed is provided with second magnetic means for generating a magnetic field of multicusp geometry.

BACKGROUND OF THE INVENTION 
The present invention generally relates to an ion source. More 
particularly, the invention concerns an ion source which can be 
advantageously employed in fine processing of semiconductors, thin films 
or the like by making use of an ion beam. 
In recent years, in accompaniment to tendency of dry semiconductor 
processes being increasingly adopted, there arises a great demand for an 
etching apparatus in which an ion beam is made use of. Implementation of 
semiconductor devices with high fineness has made a remarkable progress, 
as demonstrated by the fact that a minimum line width of the submicron 
order is being realized in the fabrication of LSI devices. In light of the 
present state of the art, the dry etching apparatus destined to perform 
the process with high fineness of the submicron order is imposed in 
respect to the performance capability thereof with such requirements that 
the divergence angle of the ion beam is constricted to a very small value 
for satisfying the desired high processing precision and that the ion beam 
should have a very uniform density. As exemplary structure of the ion beam 
etching apparatus of this sort, there may be mentioned the structure 
disclosed in "Extended Abstracts for The 32nd Spring Meeting, Mar. 29-Apr. 
1, 1985 of The Japan Society of Applied Physics and Related Societies", p. 
316. The ion beam etching apparatus disclosed in the above publication is 
composed of a cylindrical vacuum container serving as a vacuum chamber, a 
microwave waveguide connected to one end of the vacuum container, and a 
coil disposed around the outer periphery of the vacuum container for 
generating a magnetic field of the direction which is in parallel with the 
direction the microwave energy is introduced. A microwave power is 
supplied from a microwave oscillator (not shown). When the angular 
frequency of the microwave is represented by .omega. dimensions are so 
selected that the following relation can be satisfied at least 
approximately: 
##EQU1## 
where m.sub.e represents mass of electron, e represents electric charge of 
electron, and B represents density of magnetic flux. When the microwave of 
2.45 MHz is used, the density of magnetic flux (B) is set approximately 
equal to 875 Gauss. 
When the above conditions are met, electrons are accelerated due to 
cyclotron resonance of electrons taking place within the vacuum chamber, 
resulting in that ionization is promoted intensively by the accelerated 
electrons, whereby a plasma is produced. From this plasma, ions are 
extacted through ion extracting means including an electrode biased with a 
positive voltage, an electrode biased with a negative voltage and an 
electrode applied with the ground potential. This known ion source suffers 
however a shortcoming that the uniformity of the plasma density 
distribution is poor with an axial or center portion of the plasma 
exhibiting a higher density. As a consequence, the density of the 
extracted ion beam is high only at the axial center portion thereof. In 
other words, the ion beam as extracted is of nonuniformity in the ion 
density distribution in the radial direction thereof. Further, since the 
magnetic lines of force generated by the coil tend to leak toward the ion 
extracting electrode array, the paths of ions tend to be bent, giving rise 
to a problem that the divergence angle of the ion beam is increased. 
SUMMARY OF THE INVENTION 
In light of the state of the art described above, it is a primary object of 
the present invention to provide an ion source in which the extracted ion 
beam exhibits a uniform distribution of ion density in the radial 
direction thereof. 
Another object of the present invention is to provide an ion source in 
which the extracted ion beam exhibits a high uniformity in the ion density 
distribution and an extremely small divergence angle so that the ion 
source can be advantageously employed in the processing requiring an 
extremely high fineness or precision. 
In view of the first mentioned object, it is proposed according to the 
present invention that a plasma producing chamber for generating a plasma 
within the interior thereof is extended in the direction in which the 
microwave energy is introduced or injected, to thereby form a plasma 
chamber which is provided with second magnetic means for generating a 
magnetic field of multicusp-like geometry. 
Further, in view of the second mentioned object, it is taught by the 
present invention that in the structure mentioned above, a magnetic shield 
member is provided between the second magnetic means and the first 
magnetic means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now, the present invention will be described in detail in conjunction of 
the preferred embodiments shown in the drawings, exemplary of the 
invention. 
FIG. 1 shows an embodiment of the ion source According to the present 
invention. Referring to the figure, a coil 3 for generating a magnetic 
field 7 (.phi. denotes the flux density thereof) in the axial direction is 
disposed around the outer periphery of a cylindrical vacuum container 1. A 
top end plate 14 of the vacuum container 1 has formed therein a dielectric 
window 12 which transmits therethrough microwave energy and is coupled to 
a waveguide 2 for feeding microwave energy from a microwave oscillator 
(not shown). In the structure according to the instant embodiment, the 
vacuum chamber 1 is provided with an extension extending in the axial 
direction to form a plasma chamber 10 having an outer periphery around 
which a large number of permanent magnets 11 are disposed for generating 
magnetic fields of multicusp geometry in both the circumferential and 
longitudinal directions. The plasma chamber 10 has an aperture 15 at which 
an ion extracting electrode array 9 including electrodes 4 and 5 biased to 
a positive potential and a negative potential, respectively, and a 
grounded electrode 6 is provided. 8 denotes a vacuum chamber having a 
substrate (not shown) provided therein which is subjected to matching such 
as etching, milling or the like with the ion beam extracted through the 
ion extracting electrode array 9. When microwave energy is introduced 
through the dielectric window 12, electrons are accelerated due to 
electron cyclotron resonance within the interior space of the vacuum 
container 1 surrounded by the magnetic field generating coil 3, whereby 
molecules of a neutral gas within the vacuum container 1 is ionized to 
produce a plasma 16. This plasma 16 diffuses in the axial direction and is 
confined within an inner space of the plasma chamber 10. 17 denotes the 
plasma confined within the plasma chamber 10. As a consequence, the plasma 
can present an extremely uniform density distribution in the radial 
direction due to the diffusion. Further, since the ion extracting 
electrode array 9 is disposed remotely from the coil 3 for generating the 
magnetic field of the axial direction, the intensity of the magnetic field 
generated by the coil 3 is very feeble at the location of the ion 
extracting electrode array 9 and scarcely exert influence to the paths of 
the ions. In particular, when a magnetic shield member 13 is provided 
between the coil 3 and the permanent magnet assembly 11 for generating the 
magnetic field of multicusp geometry, influence of the magnetic field 
generated by the coil 3 to the paths of ions extracted through the ion 
extracting electrode array 9 can be reduced substantially to zero. 
Consequently, the ion beam extracted through the ion extracting electrode 
array 9 has an extremely uniform distribution of ion density in the radial 
direction with the divergence angle of the ion beam being correspondingly 
set to a minimum value, whereby an ion beam can be obtained which is 
advantageously suited for the very fine processing. 
FIG. 2 shows an ion source according to a second embodiment of the present 
invention. Referring to the figure, a partition plate 18 having a 
plurality of openings is disposed at the boundary between the vacuum 
container 1 and the plasma chamber 10. With this structure, there can be 
obtained similar effects to those of the first described embodiment of the 
invention. 
FIG. 3 shows a third embodiment of the present invention. In the case of 
this embodiment, the inner diameter of the plasma chamber 10 is selected 
smaller than that of the vacuum container 1. In contrast, in the case of a 
fourth embodiment shown in FIG. 4, the inner diameter of the plasma 
chamber 10 is selected greater than that of the vacuum chamber 1. In both 
of the embodiments shown in FIGS. 3 and 4, there can be obtained ions of 
low energy as compared with those produced in the ion source shown in FIG. 
1 due to additional effect of adiabatic expansion. 
In any of the embodiments of the invention described so far, the coil 3 and 
the permanent magnets 11 are both disposed externally of the vacuum 
chamber 1 and the plasma chamber 10. It should however be appreciated that 
effectiveness of the ion sources according to the invention can remain 
unchanged even when these magnetic means are provided internally. 
As will be appreciated from the foregoing description, with the ion source 
arranged such that a plasma chamber is formed by extending the vacuum 
chamber for producing therein a plasma in the direction in which the 
microwave is introduced and that second magnetic means for producing a 
magnetic field of multicusp geometry is provided in association with the 
plasma chamber, ion density distribution of the ion beam as extracted from 
the plasma can be made very uniform in the radial direction of the beam. 
Further, by additionally providing the magnetic shield member between the 
first and second magnetic means, the divergence angle of the ion beam can 
be retained at an extremely small value. In this way, the ion source 
advantageously suited for the fine processing of semiconductors or the 
like can be realized, to great advantages.