Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-07-31
2003-06-03
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492200, C250S492220, C250S294000, C250S296000, C250S298000, C250S3960ML
Reexamination Certificate
active
06573517
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an ion implantation apparatus and, in particular, to an ion implantation apparatus which is capable of producing an ion beam having no energy contamination.
In a recent ion implantation process for semiconductor device production, implantation energy has been getting lower and lower to make the depth of implanted ions shallower with scale reduction of micro patterning of the semiconductor device.
Extraction voltage of ion source is lowered to generate the low energy ions. However, the lower the extraction voltage is, the worse the extraction efficiency of ions from ion source is.
Further, the ions repel each other due to electric charge of themselves. This mutual repulsion causes a rapid increase of the ion beam diameter. This is what is called “Space charge effect”. This effect becomes stronger with the decrease in ion energy. Consequently, transport efficiency is also reduced in low energy region. As a result, enough ion beam current can not be obtained.
To compensate for this difficulty, so called, post-deceleration technology has been used conventionally. In this technology, the ions are extracted from the ion source at a relatively high extraction voltage, and are mass analyzed and transported near to the target wafers, then the ions are decelerated down to the desired energy by a reverse electric field.
In such a deceleration method, there is an advantage that relatively higher beam current of low energy ions can be obtained easily. However, if the ions are neutralized by the reaction with residual gas before the deceleration position, such neutralized particles can not be decelerated by the reverse electric field. Consequently, such neutralized particles are implanted into the target wafer with original energy that is different from desired energy. This phenomenon is what is called “Energy contamination”.
Similar phenomena occur also in post-acceleration method in which the ions are accelerated by a forward electric field after mass analyzing to obtain higher energy ion beam.
If the ions are neutralized by the reaction with residual gas before the acceleration position, such neutralized particles can not be accelerated by the forward electric field. Consequently, such neutralized particles are implanted into the target wafer with original energy that is different from desired energy.
On the contrary, if the electrons of the ions are stripped more by the reaction with residual gas before the acceleration position, ions become higher valence (multi-charged) ions. Such multi-charged ions are accelerated by the forward electric field the valence times more than the single charged ions and implanted into the target wafer with different energy that is higher than the desired energy.
Thus, the energy contamination often occurs in the apparatus equipped with post-acceleration as well.
Referring to
FIG. 1
, description will be made about a related ion implantation apparatus equipped with post-deceleration or post-acceleration.
In
FIG. 1
, an ion beam extracted from an ion source
41
is mass-analyzed by a mass analyzing magnet
42
and a mass analyzing slit
43
to select desired ion species.
Specifically, immediately after the ion beam passes the mass analyzing magnet
42
at a point A, only desired ions exist on a trajectory that can pass through the mass analyzing slit
43
.
In this case, energy of the desired ions at the Point A is determined in dependence upon the extraction voltage of the ion source
41
and the valence number of the ion. Therefore, the desired ions, which are on the trajectory towards the mass analyzing slit
43
, have no energy dispersion at the point A.
After the ions pass through the mass analyzing slit
43
, the ions are decelerated or accelerated in a post-stage electrode portion
44
. In this event, the ions are decelerated or accelerated in accordance with a direction of an electric field applied to the post-stage electrode portion
44
.
Namely, when a reverse electric field is applied, the ions are decelerated. On the other hand, when a forward electric filed is applied, the ions are accelerated.
The mass analyzing slit
43
is generally located nearby a downstream side of the post-stage electrode portion
44
, hence an electrode part of the post-stage electrode portion
44
may perform the function of the mass-analyzing slit
43
in many cases.
As a specific example, description will be made about such a case that boron ions (B+) having one valance are implanted into a silicon wafer
46
in a wafer-processing chamber
45
with the energy less than 1 keV by using the post-deceleration.
In this event, a mode is classified into a first mode (drift mode) and a second mode (deceleration mode).
In the first mode, the ions are extracted from the ion source
41
with an extraction voltage less than 1 kV, and are implanted without the post-deceleration.
In the second mode, the ions are extracted with a relatively higher voltage (for example, n kV), and a reverse electric field is applied to the post-deceleration electrode portion
44
to finally produce the ion having energy less than 1 keV.
In the first mode, the extraction efficiency is degraded because the extraction voltage is low. Further, the transport efficiency is not high because the beam diverges by the space charge effect. Consequently, the beam current becomes small.
In the first mode, the energy contamination does not occur, however the beam current becomes small. In consequence, implantation time becomes long to implant the predetermined implantation quantity.
In the second mode, the current is increased in comparison with the first mode because of relatively higher extraction voltage than the first mode.
For example, when the ions are extracted from the ion source
41
with the voltage of several to 10 kV, the ions are transported towards the post-stage electrode portion
44
with the initial energy of several to 10 keV.
Further, the reverse electric field is applied such that the ions are decelerated down to energy of ½-{fraction (1/10)} at the post-stage electrode portion
44
to finally produce the ion beam with the energy less than 1 keV.
However, a part of ions lose their charge by reaction with residual gas and become neutral particles in an area (an area B in
FIG. 1
) between the exit position (Point A) of the mass analyzing magnet
42
and a deceleration position.
In consequence, the part of the ions as the neutral particles are not affected by the reverse electric field for the deceleration. Thereby, the ions are implanted with the initial energy of several to 10 keV. As a result, not only the desired boron with the energy less than 1 keV but also the boron ions with the initial energy are inevitably implanted.
Thus, the beam current in the second mode of the deceleration is higher than the first mode, and the implantation time is advantageously short.
On the contrary, the particles not having the desired energy are inevitably mixed. This phenomenon is referred to as the energy contamination.
From the view point of getting the beam current as much as possible, deceleration ratio (namely, a ratio of energy before the deceleration to the energy after the deceleration) is to be desirably higher. However, as the deceleration ratio is higher, content or quantity of the energy contamination is generally higher.
FIG. 2
shows a typical example of implanted profile when the energy contamination takes place by the deceleration of the second mode in comparison with the profile of the first mode.
It is found out that the implanted profile of the second mode has higher energy component which is implanted to deeper position with the energy before deceleration.
To eliminate the components of energy contamination on the ion implantation apparatus with post-deceleration or post-acceleration, additional element for re-deflecting by electric field or magnetic filed has been conventionally used on the downstream trajectory of post-stage electrode.
In particular, this conventional method has been widely used on the apparatus equippe
Hidaka Yoshitomo
Inada Kouji
Kabasawa Mitsuaki
Sugitani Michiro
Tsukihara Mitsukuni
Lee John R.
Souw Bernard E.
Sumitomo Eaton Nova Corporation
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