Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2002-07-01
2003-10-14
Wells, Nikita (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492200, C250S251000
Reexamination Certificate
active
06633047
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to apparatus and method for introducing controlled amounts of selected impurity atoms into a surface of a target.
As the number of VLSI devices, integrated with a drastically reduced size on a single chip, has been increasing, the thickness of a gate insulating film for an MOS transistor has been tremendously reduced these days. As for a device with a design rule of 0.5 &mgr;m, the thickness of a gate insulating film was once ordinarily set at about 10 nm. In a device now available with a design rule of 0.25&mgr;m, however, the thickness of the film is almost halved, i.e., about 5 nm. A gate insulating film with such a very small thickness is extremely sensitive to charge buildup damage caused by implantation of dopant ions into the gate electrode of the device.
Recently, in order to process a wafer of a much greater size satisfactorily and to increase a throughput sufficiently, the maximum density of a beam current for an ion implanter has been increased. However, if the density of a beam current, created during ion implantation, is increased, then a positive charge buildup phenomenon, resulting from an impacting ion beam, gets even more remarkable. Accordingly, to suppress such a phenomenon, the ability of supplying electrons should be improved for an electron supply system.
FIG. 10
illustrates a cross-sectional structure for a conventional impurity introducing apparatus. As shown in
FIG. 10
, a guide tube
12
is provided to face a wafer
11
held by a wafer holder
10
. A tube bias is applied from a first voltage supply
13
to the guide tube
12
. An ion beam
14
, which has been generated by an ion beam generator (not shown), travels inside the guide tube
12
leftward to impinge on the surface of the wafer
11
.
An arc chamber
16
is provided beside the guide tube
12
and includes a filament
17
therein. A filament voltage is applied from a second voltage supply
18
to both terminals of the filament
17
. An arc voltage is applied from a third voltage supply
19
to between one of the terminals of the filament
17
and the arc chamber
16
. And arc current is supplied from a current source
20
into the arc chamber
16
.
Argon (Ar) gas, for example, is introduced from a gas feed system
21
into the arc chamber
16
. By supplying the Ar gas or the like into the arc chamber
16
and applying respective predetermined voltages to the arc chamber
16
and to the filament
17
, plasma is created inside the arc chamber
16
. And electrons, included in the plasma created, are supplied into the guide tube
12
to have a certain energy distribution.
The guide tube
12
, first, second and third voltage supplies
13
,
18
,
19
, arc chamber
16
, filament
17
, current source
20
and gas feed system
21
constitute an electron supply system
22
for supplying electrons to be introduced into the wafer
11
. The electrons
23
, which have been supplied from the arc chamber
16
into the guide tube
12
, are attracted to a positive ion beam
14
to be distributed around the beam
14
and introduced into the wafer
11
together with the beam
14
. The other electrons, which have not been attracted to the vicinity of the ion beam
14
, are also attracted and introduced into the wafer
11
by an electric field formed between the guide tube
12
and the wafer
11
.
However, the present inventors found that, if ions were implanted into a gate electrode on the wafer
11
using this conventional impurity introducing apparatus, the smaller the thickness of a gate oxide film, the higher the percentage of dielectric breakdown caused in the gate oxide film.
FIGS.
11
(
a
) and
11
(
b
) illustrate relationships between the thickness of a gate oxide film and the percentage of breakdown caused in the film, in which ions are implanted into the gate electrode of an MOS transistor, exhibiting antenna effect, using the conventional impurity introducing apparatus. Herein, the “antenna effect” is a phenomenon that if the area of a gate electrode is set larger than that of a gate electrode actually formed in a transistor, then a gate insulating film is affected by the charge of ions and electrons to a higher degree. FIGS.
11
(
a
) and
11
(
b
) illustrate results obtained by implanting the ions into p- and n-type semiconductor substrates, respectively. In this case, As
+
ions are implanted under the conditions that accelerating voltage is 20 keV, implant dose is 5×10
15
/cm
2
and a beam current is 10 mA. In the MOS transistor, the area of the gate insulating film is 1×10
−6
mm
2
, the area of the gate electrode is 1×10
−1
mm
2
, and the antenna ratio is 1×10
5
.
In FIGS.
11
(
a
) and
11
(
b
), the abscissas indicate thick-nesses of the gate oxide film, while the ordinates indicate percentages of breakdown caused in an MOS transistor exhibiting the antenna effect, where the breakdown voltage of the gate oxide film thereof is 8 MV/cm or less. FIGS.
11
(
a
) and
11
(
b
) also illustrate respective results obtained with the flux of electrons supplied from an electron supply system varied, where the respective fluxes of electrons are represented as 0.5, 1.0, 1.5 and 2.0 by normalizing the flux of electrons supplied under standard conditions at 1.0. As can be understood from FIGS.
11
(
a
) and
11
(
b
), the smaller the thickness of the gate oxide film, the higher the percentage of breakdown caused in the gate oxide film, even though the flux of electrons supplied from the electron supply system
22
remains the same.
SUMMARY OF THE INVENTION
A prime object of the present invention is preventing the percentage of breakdown caused in a gate insulating film from increasing even if the thickness of the film is reduced.
To achieve this object, an apparatus for introducing an impurity according to the present invention includes: means for introducing an impurity having charges into a target to be processed, which is a semiconductor substrate or a film formed on the substrate; means for supplying electrons into the target to cancel the charges of the impurity; and means for controlling the maximum energy of the electrons supplied by the electron supply means at a predetermined value or less.
The apparatus of the present invention includes the means for controlling the maximum energy of the electrons, supplied by the electron supply means, at a predetermined value or less. Accordingly, it is possible to prevent the target to be processed or the semiconductor substrate, on which the target is formed, from being negatively charged up.
In one embodiment of the present invention, the impurity introducing means is preferably means for implanting ions as the impurity.
In such an embodiment, it is possible to prevent a negative charge buildup phenomenon from being caused during the ion implantation.
In another embodiment of the present invention, if an insulating film with a thickness of t (nm) is formed on the semiconductor substrate, then the predetermined value is preferably 2 t (eV).
In such an embodiment, it is possible to prevent dielectric breakdown from being caused in the insulating film due to the negative charge buildup phenomenon.
In still another embodiment, the apparatus preferably further includes means for measuring the energy of the electrons supplied by the electron supply means.
In such an embodiment, the energy of the electrons supplied by the electron supply means can be known.
In such a case, the energy measuring means preferably includes means for measuring the maximum energy of the electrons supplied by the electron supply means.
Then, it is easier to control the maximum energy of the electrons, supplied by the electron supply means, at the predetermined value or less.
In an alternate embodiment, the energy measuring means preferably makes the control means control the maximum energy of the electrons, supplied by the electron supply means, at the predetermined value or less based on the measured energy of the electrons.
In such an embodiment, the maximum energy of the electrons, supp
Kubo Hiroko
Niwayama Masahiko
Yoneda Kenji
Matsushita Electric - Industrial Co., Ltd.
Nixon & Peabody LLP
Studebaker Donald R.
Wells Nikita
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