Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2002-01-18
2003-10-21
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S42300F
Reexamination Certificate
active
06635889
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an ion implantation apparatus and a tuning method for an ion source system thereof.
In recent ion implantation apparatuses, with the shrinking of semiconductor devices, the energy level for implanting ions is being lowered to reduce the depth of ion implantation. In a lower energy range, however, the extraction voltage from an ion source is lower. This has been causing ion extraction efficiency to deteriorate and ion beams to repel one another due to the electric charges thereof with consequent divergence of ion beams, which is known as space charge effect. Hence, there has been a problem in that the degraded transporting efficiency prevents sufficient implantation ion beam current from being obtained.
The above problem will be described with reference to the accompanying drawings.
FIG. 1
shows the construction of a beam line from an ion source of an ion implantation apparatus to a mass analysis slit. In
FIG. 1
, an ion beam
5
extracted from an ion source
1
through the intermediary of an extraction electrode
2
is subjected to the mass analysis by a mass analysis magnet
3
and a mass analysis slit
4
located at downstream side of the mass analysis magnet
3
thereby to select only required ion species.
FIG. 2
shows the construction of the extraction unit of the ion source used with the ion implantation apparatus. The descriptions will be given of an ion source for taking out ions carrying positive electric charges. In
FIG. 2
, a positive voltage is being applied to an entire ion source
6
. The distal end portion of the ion source
6
has an arc chamber
7
for generating plasma
8
. The arc chamber
7
includes an opening
8
a
for extracting ions from the plasma
8
.
Although not shown in
FIG. 2
, a magnet or a source magnet that acts to generate a magnetic field for efficiently generating plasma in the arc chamber
7
is installed outside the arc chamber
7
.
The extraction electrode for extracting ions is generally constructed by a plurality of electrodes each having a slit. Of the plurality of electrodes, the last-or post-electrode as observed from the ion source
6
is usually referred to as a ground electrode
9
. As a whole, ions
13
are extracted from the plasma
8
in the arc chamber
7
by an extraction electric field directed from the positive potential of the ion source
6
to the ground electrode
9
. At the same time, the ions are accelerated to a desired level of extracting energy.
A suppression electrode
10
is provided on the upstream side of the ground electrode
9
. The suppression electrode
10
is subjected to a negative potential with respect to the ground electrode
9
so as to form a negative voltage barrier. The negative voltage barrier prevents the extraction electric field from inversely accelerating ions, that is, accelerating electrons from the ground electrode
9
toward the ion source
6
, while the extraction electric field should accelerate ions from the ion source
6
toward the ground electrode
9
. Thus, the suppression electrode
10
serves to form the negative voltage barrier for minimizing the chance of electrons from going out into the extraction electric field.
The extraction electrodes, namely, the suppression electrode
10
and the ground electrode
9
, are usually secured by a supporting member to a vacuum chamber or the like that accommodates the extraction electrodes. In
FIG. 2
, however, the extraction electrodes and the vacuum chamber are separately provided. More specifically, the extraction electrodes are supported by a supporting member
12
connected to a driving mechanism
11
. This allows the extraction electrodes to be moved in the longitudinal direction (in the direction of the gap axis), i.e., in the upstream or downstream direction of an ion beam stream, and also in the lateral direction (in the direction of the side axis), i.e., the direction orthogonal to the ion beam stream, thus permitting its positional relationship with the arc chamber
7
to be adjusted. In some cases, adjusting devices for tilt axis adjustment and vertical axis adjustment may be added. The tilt axis adjustment is performed to adjust the tilt angles of the extraction electrodes with respect to a central axis in the same direction as that of the gap axis. The vertical axis adjustment is performed to adjust the vertical movement in the direction orthogonal to the ion beam stream.
FIG. 3
schematically illustrates the electrical potential of the extracting system. In the zone from the ion source to the suppression electrode, an ion
14
is accelerated from an ion source potential
15
, which is a high potential, toward a suppression electrode potential
16
, which is a low potential. After the suppression electrode potential
16
, the ion
14
is decelerated to a ground electrode potential
17
. Hence, the ion energy (keV) upon completion of the extracting operation will take the value obtained by multiplying the voltage difference {extraction voltage (kV)} between the positive potential applied to the ion source and the potential of the ground electrode (generally a ground potential) by the valence of the ion.
The ion having the desired energy obtained by passing through the ground electrode is transported to a mass analysis magnet, which is the next destination.
The potential gradient from the ion source to the suppression electrode is known as an extraction electric field, and directly influences the extraction of ions from an arc chamber. As shown in
FIG. 2
, the zone wherein the extraction electric field acts, i.e., the zone extending from the ion source
6
or the arc chamber
7
to the suppression electrode
10
, is referred to as an “extraction gap” for convenience. When the size of the extraction gap is fixed, the gradient of potential becomes more gentle as the extraction voltage decreases. In other words, as the extraction voltage decreases, the extraction electric field becomes lower. On the other hand, when the extraction voltage is fixed, the gradient of potential grows steeper as the size of the extraction gap is reduced, making it possible to increase the extraction electric field.
Thus, the size of the extraction gap is an extremely important factor directly related to the efficiency of extracting ions from an ion source. For this reason, as shown in
FIG. 2
, the ion extracting system of a typical ion implantation apparatus has the driving mechanism
11
for moving the extraction electrode thereby to permit the adjustment of the size of the extraction gap. A gap axis is used primarily for adjusting the aforesaid extraction electric field. A side axis and a tilt axis are used to make fine adjustment for aligning the direction of an ion beam to be extracted with a design beam axis.
The energy of ions depends upon the voltage difference (extraction voltage) between the positive potential applied to the ion source and the potential of the ground electrode (generally the ground potential). Therefore, to take out low-energy ions, the voltage of the ion source has to be reduced. For example, to extract 80 (keV) ions by monovalent ions, a voltage of 80 (kV) is applied to the ion source. To extract 0.5 (keV) ions by monovalent ions, the voltage of only 0.5 (kV) can be applied to the ion source.
If the voltage applied to the ion source is decreased with the extraction gap size remaining unchanged, the extraction electric field applied to the extraction gap weakens. As a result, the ion extraction efficiency deteriorates with a consequent reduction in ion current that can be taken out. To avoid this, when low-energy ions are extracted, adjustment is performed by the driving mechanism to reduce the size of the extraction gap so as to bring the suppression electrode and the ground electrode closer to the ion source. In other words, the deterioration in the extraction efficiency is compensated for by controlling the weakening of the extraction electric field.
Ions are characterized by their tendency to repel each other because of their own positive electric
Kabasawa Mitsuaki
Sugitani Michiro
Takahashi Yuji
Tsukihara Mitsukuni
Arent Fox Kintner Plotkin & Kahn
Lee John R.
Leybourne James J
Sumitomo Eaton Nova Corporation
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