U.S. patent application number 10/675922 was filed with the patent office on 2004-08-05 for plasma doping method and plasma doping apparatus.
Invention is credited to Mizuno, Bunji, Nakayama, Ichiro, Okumura, Tomohiro, Sasaki, Yuichiro.
Application Number | 20040149219 10/675922 |
Document ID | / |
Family ID | 32776780 |
Filed Date | 2004-08-05 |
United States Patent
Application |
20040149219 |
Kind Code |
A1 |
Okumura, Tomohiro ; et
al. |
August 5, 2004 |
Plasma doping method and plasma doping apparatus
Abstract
In order to realize a plasma doping method capable of carrying
out a stable low-density doping, exhaustion is carried out with a
pump while introducing a predetermined gas into a vacuum chamber
from a gas supplying apparatus, the pressure of the vacuum chamber
is held at a predetermined pressure and a high frequency power is
supplied to a coil from a high frequency power source. After the
generation of plasma in the vacuum chamber, the pressure of the
vacuum chamber is lowered, and the low-density plasma doping is
performed to a substrate placed on a substrate electrode. Moreover,
the pressure of the vacuum chamber is gradually lowered, and the
high frequency power is gradually increased, thereby the
low-density plasma doping is carried out to the substrate placed on
the substrate electrode. Furthermore, a forward power Pf and a
reflected power Pr of the high frequency power supplied to the
substrate electrode are sampled at a high speed, and when a value
of which the power difference Pf-Pr is integrated with respect to
time reaches a predetermined value, the supply of the high
frequency power is suspended.
Inventors: |
Okumura, Tomohiro;
(Kadoma-shi, JP) ; Nakayama, Ichiro; (Kadoma-shi,
JP) ; Mizuno, Bunji; (Ikoma-shi, JP) ; Sasaki,
Yuichiro; (Kawasaki-shi, JP) |
Correspondence
Address: |
SHERIDAN ROSS PC
1560 BROADWAY
SUITE 1200
DENVER
CO
80202
|
Family ID: |
32776780 |
Appl. No.: |
10/675922 |
Filed: |
September 29, 2003 |
Current U.S.
Class: |
118/723I ;
438/513 |
Current CPC
Class: |
H01J 37/321 20130101;
H01L 21/2236 20130101; H01J 37/32412 20130101 |
Class at
Publication: |
118/723.00I ;
438/513 |
International
Class: |
C23C 016/00; H01L
021/04; H01L 021/26; H01L 021/42 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 2, 2002 |
JP |
PAT. 2002-290074 |
Oct 2, 2002 |
JP |
PAT. 2002-290075 |
Oct 2, 2002 |
JP |
PAT. 2002-290076 |
Claims
1. A plasma doping method for doping impurities into a sample or
into a film on the surface of the sample, comprising: a first step
of placing said sample on a sample electrode in a vacuum chamber; a
second step of evacuating said vacuum chamber with supplying a
doping material gas into said vacuum chamber, and supplying a high
frequency electric power to a plasma source with controlling the
pressure of said vacuum chamber at a first pressure so as to
generate plasma in said vacuum chamber; and a third step of
controlling the pressure of said vacuum chamber into a second
pressure lower than said first pressure with maintaining the
generation of the plasma.
2. A plasma doping method for doping impurities into a sample or
into a film on the surface of the sample, comprising: a first step
of placing said sample on a sample electrode in a vacuum chamber; a
second step of evacuating said vacuum chamber with supplying a gas
containing an inert gas other than helium into said vacuum chamber,
and supplying a high frequency electric power to a plasma source
with controlling the pressure of said vacuum chamber at a first
pressure so as to generate plasma in said vacuum chamber; and a
third step of evacuating said vacuum chamber with supplying a gas
containing helium into said vacuum chamber with maintaining the
generation of the plasma, so as to control the pressure of said
vacuum chamber into a second pressure.
3. A plasma doping method for doping impurities into a sample or
into a film on the surface of the sample, comprising: a first step
of placing said sample on a sample electrode in a vacuum chamber; a
second step of evacuating said vacuum chamber with supplying a gas
into said vacuum chamber, and supplying a high frequency electric
power to a plasma source with controlling the pressure of said
vacuum chamber at a first pressure so as to generate plasma in said
vacuum chamber; and a third step of controlling the pressure of
said vacuum chamber into a second pressure different from said
first pressure with maintaining the generation of the plasma, and
supplying a high frequency electric power larger than the high
frequency electric power in said second step to the plasma
source.
4. A plasma doping method for doping impurities into a sample or
into a film on the surface of the sample, comprising: a first step
of placing said sample on a sample electrode in a vacuum chamber; a
second step of evacuating said vacuum chamber with supplying a gas
not containing a doping material gas into said vacuum chamber, and
supplying a high frequency electric power to a plasma source with
controlling the pressure of said vacuum chamber at a first pressure
so as to generate plasma in said vacuum chamber; and a third step
of evacuating said vacuum chamber with supplying a gas containing a
doping material gas into said vacuum chamber with maintaining the
generation of the plasma, so as to control the pressure of said
vacuum chamber into a second pressure different from said first
pressure.
5. A plasma doping method, wherein a vacuum chamber comprising a
plasma generating apparatus is evacuated with supplying a gas into
said vacuum chamber, and a high frequency electric power is
supplied to the plasma generating apparatus via a matching circuit
of plasma generating apparatus comprising toroidal cores serving as
two variable impedance elements, whereby plasma is generated in
said vacuum chamber, and impurities are doped into a sample placed
on a sample electrode in said vacuum chamber or into a film on the
surface of the sample, characterized in that at least one of
control parameters, such as gas species, gas flow rate, pressure
and high frequency electric power, is changed with maintaining the
generation of plasma.
6. A plasma doping method, wherein a vacuum chamber comprising a
plasma generating apparatus is evacuated with supplying a gas into
said vacuum chamber, and a high frequency electric power is
supplied to the plasma generating apparatus via a matching circuit
of plasma generating apparatus, whereby plasma is generated in said
vacuum chamber, and impurities are doped into a sample placed on a
sample electrode in the vacuum chamber or-into a film on the
surface of the sample, characterized in that at least one of
control parameters of gas species, gas flow rate, pressure and high
frequency electric power is changed gradually in 1 second through 5
seconds with maintaining the generation of plasma.
7. A plasma doping method, wherein a vacuum chamber comprising a
plasma generating apparatus is evacuated with supplying a gas into
said vacuum chamber, a high frequency electric power is supplied to
said plasma generating apparatus so as to generate plasma in said
vacuum chamber, and a high frequency electric power is supplied to
a sample electrode on which a sample is placed in said vacuum
chamber, whereby impurities are doped into said sample placed on
said sample electrode in said vacuum chamber or into a film on the
surface of said sample, characterized in that when the forward
power of the high frequency electric power supplied to said plasma
generating apparatus or said sample electrode is denoted by Pf and
when the reflected power thereof is denoted by Pr, the power
difference Pf-Pr is sampled in every interval of 1 millisecond
through 100 milliseconds, and when the integration of the power
difference Pf-Pr with respect to time reaches a predetermined
value, the supply of said high frequency electric power is
stopped.
8. A plasma doping method in accordance with claim 7, wherein at
least one of control parameters, such as gas species, gas flow
rate, pressure and high frequency electric power, is changed during
the process of plasma doping with maintaining the generation of
plasma.
9. A plasma doping apparatus comprising: a vacuum chamber; a gas
supplying apparatus for supplying a gas into said vacuum chamber;
an evacuating apparatus for evacuating said vacuum chamber; a
regulating valve for controlling the pressure of said vacuum
chamber into a predetermined value; a sample electrode for placing
a sample in said vacuum chamber; a plasma generating apparatus; a
matching circuit for plasma generating apparatus comprising
toroidal cores serving as two variable impedance elements; and a
high frequency power supply for supplying a high frequency electric
power to said plasma generating apparatus via said matching circuit
for plasma generating apparatus.
10. A plasma doping apparatus comprising: a vacuum chamber; a gas
supplying apparatus for supplying a gas into said vacuum chamber;
an evacuating apparatus for evacuating said vacuum chamber; a
regulating valve for controlling the pressure of said vacuum
chamber into a predetermined value; a sample electrode for placing
a sample in said vacuum chamber; a plasma generating apparatus; a
high frequency power supply for supplying a high frequency electric
power to said plasma generating apparatus; a high frequency power
supply for supplying a high frequency electric power to said sample
electrode; a sampler which, when the forward power of the high
frequency electric power supplied to the plasma generating
apparatus or the sample electrode is denoted by Pf and when the
reflected power thereof is denoted by Pr, samples the power
difference Pf-Pr in every interval of 1 millisecond through 100
milliseconds; and a controlling apparatus which, when the
integration of the power difference Pf-Pr with respect to time
reaches a predetermined value, stops the supply of the high
frequency electric power.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a plasma doping method for
doping impurities into the surface of a solid sample such as a
semiconductor substrate, and to an apparatus for implementing this
method.
[0002] A technique for doping impurities into the surface of a
solid sample is disclosed, for example, in the prior art of the
U.S. Pat. No. 4,912,065, wherein a plasma doping method is
implemented such that impurities are ionized and then doped into a
solid material at a low energy.
[0003] A plasma doping method of the prior art method for doping
impurities is described below with reference to FIG. 14.
[0004] FIG. 14 shows a schematic configuration of a plasma doping
apparatus used in the prior art plasma doping method. In FIG. 14, a
vacuum chamber 100 accommodates a sample electrode 106 for placing
a sample (work) 109 such as a silicon substrate thereon. A gas
supplying apparatus 102 for supplying doping material gas such as
B.sub.2H.sub.6 containing a desired element and a pump 103 for
evacuating the vacuum chamber 100 are provided outside the vacuum
chamber 100. These apparatuses allow the vacuum chamber 100 to be
maintained at a predetermined pressure. A microwave is emitted from
a microwave waveguide 119 into the vacuum chamber 100 through a
quartz plate 107 as a dielectric window. By a mutual action of this
microwave and a DC magnetic field generated by an electric magnet
114, electron cyclotron resonance plasma is produced within a
region encompassed by a dash-dotted line 120 in the vacuum chamber
100. The sample electrode 106 is connected to a high frequency
power supply 110 through a capacitor 121, so that the electric
potential of the sample electrode 106 is controlled.
[0005] In the plasma doping apparatus having the above-mentioned
configuration, the doping material gas such as B.sub.2H.sub.6
introduced in the vacuum chamber 100 from the gas supplying
apparatus 102 is made into the plasma state by plasma generating
means comprising the microwave waveguide 119 and the electric
magnet 114, so that boron ions in the plasma 120 are doped into the
surface of the sample 109 by means of the electric potential
provided from the high frequency power supply 110.
[0006] On the sample 109 doped with the impurities as mentioned
above, a metallic wiring layer is formed in another process. Then,
a thin oxide film is formed on the metallic wiring layer in a
predetermined oxidizing atmosphere. After that, gate electrodes are
formed on the sample 109 by a CVD apparatus and the like, so that
MOS transistors, for example, are obtained.
[0007] The doping material gas such as B.sub.2H.sub.6 containing
impurities express electrical activity when doped into a sample
such as a silicon substrate, however there is a problem that such a
gas is generally hazardous for human body.
[0008] Further, in the plasma doping method, all of the substances
contained in the doping material gas are doped into the sample.
Description is made in the case of the doping material gas composed
of B.sub.2H.sub.6, for example. Only the boron works as effective
impurities in the doped state, however hydrogen is also doped into
the sample simultaneously. This doping of hydrogen into the sample
causes the problem that lattice defects are generated during the
subsequent heat treatment such as an epitaxial growth process.
[0009] For the purpose of resolving these problems, in another
prior art method disclosed in JP-A Hei 09-115851, an impurity solid
material containing the substance of impurities that express
electrical activity when doped into a sample is placed in a vacuum
chamber, while inert gas plasma is generated in the vacuum chamber.
The impurities are emitted from the impurity solid material and are
sputtered by the ions of the inert gas plasma. FIG. 15 shows the
configuration of a plasma doping apparatus used in this prior art
plasma doping method. In FIG. 15, a vacuum chamber 100 accommodates
a sample electrode 106 for placing a sample 109 composed of a
silicon substrate thereon. A gas supplying apparatus 102 for
supplying the inert gas and a pump 103 for evacuating the vacuum
chamber 100 are disposed outside the vacuum chamber 100. These
apparatuses allow the vacuum chamber 100 to be maintained at a
predetermined pressure. A microwave is emitted from a microwave
waveguide 119 into the vacuum chamber 100 through a quartz plate
107 as a dielectric window. Due to a mutual action of the microwave
and a DC magnetic field generated by an electric magnet 114, the
electron cyclotron resonance plasma is produced within a region
encompassed by a dash-dotted line 120 in the vacuum chamber 100.
The sample electrode 106 is connected through a capacitor 121 to a
high frequency power supply 110, so that the electric potential of
the sample electrode 106 is controlled. An impurity solid material
122 containing impurity element such as boron is placed on a solid
material holding bed 123. The electric potential of the solid
material holding bed 123 is controlled by a high frequency power
supply 125 connected thereto via a capacitor 124.
[0010] In the plasma doping apparatus having the above-mentioned
configuration, the inert gas such as argon (Ar) introduced from the
gas supplying apparatus 102 is ionized into the plasma state by
plasma generating means comprising the microwave waveguide 119 and
the electric magnet 114. A part of impurity atoms sputtered from
the impurity solid material 122 into the plasma 120 are ionized and
then doped into the surface of the sample 109.
[0011] Nevertheless, both of the above-mentioned prior art methods
shown in FIG. 14 and FIG. 15 still have the problem that low
density doping is not achieved stably and that the reproducibility
of the processing is poor. When low density doping is performed
using the doping material gas, it is required that the pressure of
the vacuum chamber is reduced, and the partial pressure of the
doping material gas is made low. For the purpose of the latter, in
general, the doping material gas is diluted with helium, which is
an inert gas. This is because helium ions have a lower sputtering
yield and hence have the advantage that ion irradiation damage to
the sample caused by the ions is suppressed. Nevertheless, helium
also has the disadvantage that the start of its discharge is
difficult at lower pressures. There is a difficulty in processing
in desired low doping conditions.
[0012] Similarly, even when low density doping is performed using
the impurity solid material in place of the doping material gas,
the pressure of the vacuum chamber is reduced. In the case that
argon is used as the inert gas, although the start of discharge at
lower pressures is easier than the case of helium, processing in
desired low density doping conditions is still difficult. This
difficulty is essentially the same as the case of the use of the
doping material gas.
[0013] On the other hand, JP-A 2000-309868 discloses a method in a
sputtering apparatus using argon gas, wherein the pressure of the
vacuum chamber is increased in a plasma generating step, so as to
ensure the generation of the plasma. Nevertheless, this method is
required to rise the pressure, and is not directly applicable to
the plasma doping process, which is extremely sensitive to
impurities.
[0014] An effective method for improving the generation-quality is
to change discharge conditions so as to increase the pressure of
the vacuum chamber in the generation step, and to decrease the
pressure in the doping step. Nevertheless, the change in the
pressure causes a substantial change in the discharge impedance.
This impedance change cannot be sufficiently rapidly tracked by a
matching circuit used for impedance matching of the high frequency
electric power. This causes a problem of the generation of a large
reflected power. More specifically, a typical matching circuit
comprises, as variable impedance elements, two variable capacitors
(or stubs in case of a microwave) having a mechanical section which
is driven by a motor. Thus, impedance matching adjustment typically
takes one second or longer because of the mechanical rotation by
the motor. The reflected power degrades the reproducibility of the
processing. The reflected power is liable to generate a noise and
hence erroneous operation of apparatuses occurs. In a worse case,
the rotation (movement) of the variable capacitors (stubs) overruns
the appropriate position, and causes the extinction of the
plasma.
[0015] Further, once a microwave or a high frequency electric power
is supplied to the plasma doping apparatus, the matching circuit
provided between the high frequency power supply and the plasma
generating apparatus or the sample electrode begins to operate. At
that time, it takes generally several 100 milliseconds through
several seconds from the start of operation of the matching circuit
to its achievement of the full suppression of reflected power.
Further, this necessary time varies in each of the repeated
processes, and hence degrades the controllability and
reproducibility. There is a difficulty in obtaining stably a
desired doping density.
[0016] In particular, in the case of low density doping at a doping
density of 1.times.10.sup.11 atm/cm.sup.2 through 1.times.10.sup.15
atm/cm.sup.2, the processing time is as short as several seconds
through ten and several seconds. Accordingly, the processing is
affected strongly by the variation in the reflected power.
[0017] Furthermore, when at least one of control parameters, such
as gas species, gas flow rate, pressure and high frequency electric
power is changed during the process of plasma doping with
maintaining the generation of plasma, a large reflected power is
liable to occur at the time of change. Variation in this reflected
power is large, and thus the controllability and reproducibility in
the doping density are degraded.
BRIEF SUMMARY OF THE INVENTION
[0018] An object of the invention is to provide a plasma doping
method permitting stable low density doping and an apparatus for
implementing the method.
[0019] An aspect of the invention is a plasma doping method for
doping impurities into a sample or into a film on the surface of
the sample, comprising: a first step of placing a sample on a
sample electrode in a vacuum chamber; a second step of evacuating
the vacuum chamber with supplying a doping material gas into the
vacuum chamber, and supplying high frequency electric power to a
plasma source with controlling the pressure of the vacuum chamber
at a first pressure so as to generate plasma in the vacuum chamber;
and a third step of controlling the pressure of the vacuum chamber
into a second pressure lower than the first pressure with
maintaining the generation of the plasma.
[0020] According to this plasma doping method of the invention, the
vacuum chamber is evacuated with supplying a doping material gas
into the vacuum chamber, and a high frequency electric power is
supplied to a plasma source with controlling the pressure of the
vacuum chamber at a first pressure so as to generate plasma in the
vacuum chamber. In this state, the pressure of the vacuum chamber
is controlled into a second pressure lower than the first pressure.
This provides a plasma doping method permitting stable low density
doping.
[0021] Another aspect of the invention is a plasma doping method
for doping impurities into a sample or into a film on the surface
of the sample, comprising: a first step of placing a sample on a
sample electrode in a vacuum chamber; a second step of evacuating
the vacuum chamber with supplying a gas containing an inert gas
other than helium into the vacuum chamber, and supplying high
frequency electric power to a plasma source with controlling the
pressure of the vacuum chamber at a first pressure so as to
generate plasma in the vacuum chamber; and a third step of
evacuating the vacuum chamber with supplying a gas containing
helium into the vacuum chamber with maintaining the generation of
the plasma, so as to control the pressure of the vacuum chamber
into a second pressure.
[0022] According to this plasma doping method of the invention, the
vacuum chamber is evacuated with supplying a gas containing an
inert gas other than helium into the vacuum chamber, and a high
frequency electric power is supplied to a plasma source with
controlling the pressure of the vacuum chamber at a first pressure
so as to generate plasma in the vacuum chamber. In this state, the
vacuum chamber is evacuated with supplying a gas containing helium
into the vacuum chamber so as to control the pressure of the vacuum
chamber into a second pressure. This provides a plasma doping
method permitting stable low density doping.
[0023] Another aspect of the invention is a plasma doping method
for doping impurities into a sample or into a film on the surface
of the sample, comprising: a first step of placing a sample on a
sample electrode in a vacuum chamber; a second step of evacuating
the vacuum chamber with supplying a gas into the vacuum chamber,
and supplying high frequency electric power to a plasma source with
controlling the pressure of the vacuum chamber at a first pressure
so as to generate plasma in the vacuum chamber; and a third step of
controlling the pressure of the vacuum chamber into a second
pressure with maintaining the generation of the plasma, and
supplying high frequency electric power larger than that of the
second step to the plasma source.
[0024] According to this plasma doping method of the invention, the
vacuum chamber is evacuated with supplying a gas into the vacuum
chamber, and a high frequency electric power is supplied to a
plasma source with controlling the pressure of the vacuum chamber
at a first pressure so as to generate plasma in the vacuum chamber.
In this state, the pressure of the vacuum chamber is controlled
into a second pressure and a high frequency electric power larger
than that at the generation of the plasma is supplied to the plasma
source. This provides a plasma doping method permitting stable low
density doping.
[0025] Another aspect of the invention is a plasma doping method
for doping impurities into a sample or into a film on the surface
of the sample, comprising: a first step of placing a sample on a
sample electrode in a vacuum chamber; a second step of evacuating
the vacuum chamber with supplying a gas not containing a doping
material gas into the vacuum chamber, and supplying high frequency
electric power to a plasma source with controlling the pressure of
the vacuum chamber at a first pressure so as to generate plasma in
the vacuum chamber; and a third step of evacuating the vacuum
chamber with supplying a gas containing a doping material gas into
the vacuum chamber with maintaining the generation of the plasma,
so as to control the pressure of the vacuum chamber into a second
pressure different from the first pressure.
[0026] According to this plasma doping method of the invention, the
vacuum chamber is evacuated with supplying a gas not containing a
doping material gas into the vacuum chamber, and a high frequency
electric power is supplied to a plasma source with controlling the
pressure of the vacuum chamber at a first pressure so as to
generate plasma in the vacuum chamber. In this state, the vacuum
chamber is evacuated with supplying a gas containing a doping
material gas into the vacuum chamber so as to control the pressure
of the vacuum chamber into a second pressure. This provides a
plasma doping method permitting stable low density doping.
[0027] Another aspect of the invention is a plasma doping method,
wherein a vacuum chamber comprising a plasma generating apparatus
is evacuated with supplying a gas into the vacuum chamber, and high
frequency electric power is supplied to the plasma generating
apparatus via a matching circuit for plasma generating apparatus
comprising toroidal cores serving as two variable impedance
elements without any mechanically moving section. Thereby plasma is
generated in the vacuum chamber, and impurities are doped into a
sample placed on a sample electrode in the vacuum chamber or into a
film on the surface of the sample. In the plasma doping method, at
least one of control parameters, such as gas species, gas flow
rate, pressure, and high frequency electric power is changed with
maintaining the generation of plasma.
[0028] According to this plasma doping method of the invention, a
high frequency electric power is supplied to the plasma generating
apparatus via a matching circuit for plasma generating apparatus
comprising toroidal cores serving as two variable impedance
elements. Thereby plasma is generated in the vacuum chamber. In
this state, at least one of control parameters, such as gas
species, gas flow rate, pressure, and high frequency electric power
is changed. This provides a plasma doping method permitting stable
low density doping with excellent reproducibility.
[0029] Another aspect of the invention is a plasma doping method,
wherein a vacuum chamber comprising a plasma generating apparatus
is evacuated with supplying a gas into the vacuum chamber, and a
high frequency electric power is supplied to the plasma generating
apparatus via a matching circuit for plasma generating apparatus.
Thereby plasma is generated in the vacuum chamber, and impurities
are doped into a sample placed on a sample electrode in the vacuum
chamber or into a film on the surface of the sample. In the plasma
doping method, at least one of control parameters, such as gas
species, gas flow rate, pressure, and high frequency electric power
is changed gradually in 1 seconds through 5 seconds with
maintaining the generation of plasma.
[0030] According to this plasma doping method of the invention, a
high frequency electric power is supplied to the plasma generating
apparatus via a matching circuit for plasma generating apparatus,
whereby plasma is generated in the vacuum chamber, and impurities
are doped into the surface of the sample. Then, at least one of
control parameters, such as gas species, gas flow rate, pressure,
and high frequency electric power is changed gradually in 1 second
through 5 seconds with maintaining the generation of plasma. This
provides a plasma doping method permitting stable low density
doping with excellent reproducibility.
[0031] The plasma doping methods according to the inventions are
useful especially in the case that at least one of control
parameters, such as gas species, gas flow rate, pressure, and high
frequency electric power is changed during the process of plasma
doping with maintaining the generation of plasma.
[0032] Another aspect of the invention is a plasma doping method,
wherein a vacuum chamber comprising a plasma generating apparatus
is evacuated with supplying a gas into the vacuum chamber. A high
frequency electric power is supplied to the plasma generating
apparatus so as to generate plasma in the vacuum chamber, and a
high frequency electric power is supplied to a sample electrode in
the vacuum chamber, whereby impurities are doped into the sample
placed on the sample electrode in the vacuum chamber or into a film
on the surface of the sample. When a forward power of the high
frequency electric power supplied to the plasma generating
apparatus or the sample electrode is denoted by Pf and the
reflected power thereof is denoted by Pr, the power difference
Pf-Pr is sampled in intervals of 1 millisecond through 100
milliseconds, and when the integration of the power difference
Pf-Pr with respect to time reaches a predetermined value, the
supply of the high frequency electric power is stopped.
[0033] According to this plasma doping method of the invention,
when a forward power of the high frequency electric power supplied
to the plasma generating apparatus or the sample electrode is
denoted by Pf and the reflected power thereof is denoted by Pr, the
power difference Pf-Pr is sampled in intervals of 1 millisecond
through 100 milliseconds. When the integration of the power
difference Pf-Pr with respect to time reaches a predetermined
value, the supply of the high frequency electric power is stopped.
This provides a plasma doping method excellent in the
controllability and reproducibility of the doping density.
[0034] A plasma doping apparatus of the present invention
comprising: a vacuum chamber; a gas supplying apparatus for
supplying a gas into the vacuum chamber; an evacuating apparatus
for evacuating the vacuum chamber; a regulating valve for
controlling the pressure of the vacuum chamber into a predetermined
value; a sample electrode for placing a sample in the vacuum
chamber; a plasma generating apparatus; a matching circuit for
plasma generating apparatus comprising toroidal cores serving as
two variable impedance elements without a mechanical moving-section
and a high frequency power supply for supplying high frequency
electric power to the plasma generating apparatus via the matching
circuit for plasma generating apparatus.
[0035] According to this plasma doping apparatus according to the
invention, a high frequency power supply is provided such as to
supply high frequency electric power to a plasma generating
apparatus via a matching circuit for plasma generating apparatus
comprising toroidal cores serving as two variable impedance
elements without any mechanically moving section. This permits
stable low density doping and excellently reproducible plasma
doping.
[0036] Another aspect of a plasma doping apparatus of the present
invention comprising: a vacuum chamber; a gas supplying apparatus
for supplying a gas into the vacuum chamber; an evacuating
apparatus for evacuating the vacuum chamber; a regulating valve for
controlling the pressure of the vacuum chamber into a predetermined
value; a sample electrode for placing a sample in the vacuum
chamber; a plasma generating apparatus; a high frequency power
supply for supplying high frequency electric power to the plasma
generating apparatus; a high frequency power supply for supplying
high frequency electric power to the sample electrode. This plasma
doping apparatus further comprises: a sampler, when the forward
power of the high frequency electric power supplied to the plasma
generating apparatus or the sample electrode is denoted by Pf and
when the reflected power thereof is denoted by Pr, samples the
power difference Pf-Pr in intervals of 1 millisecond through 100
milliseconds; and a controlling apparatus, when the integration of
the power difference Pf-Pr with respect to time reaches a
predetermined value, stops the supply of the high frequency
electric power.
[0037] According to the plasma doping apparatus of the invention,
the power difference Pf-Pr is sampled in every interval of 1
millisecond through 100 milliseconds, while when the integration of
the power difference Pf-Pr with respect to time reaches a
predetermined value, the supply of the high frequency electric
power is stopped. This provides a plasma doping apparatus excellent
in the controllability and reproducibility of the doping
density.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0038] FIG. 1 is a cross sectional view of a plasma doping
apparatus for implementing a plasma doping method according to
first through fourth embodiments of the invention;
[0039] FIG. 2 is a cross sectional view of another example of a
plasma doping apparatus for implementing a plasma doping method
according to first through fourth embodiments of the invention;
[0040] FIG. 3 is a cross sectional view of another example of a
plasma doping apparatus for implementing a plasma doping method
according to first through fourth embodiments of the invention;
[0041] FIG. 4 is a cross sectional view of another example of a
plasma doping apparatus for implementing a plasma doping method
according to first through fourth embodiments of the invention;
[0042] FIG. 5 is a cross sectional view of another example of a
plasma doping apparatus for implementing a plasma doping method
according to first through fourth embodiments of the invention;
[0043] FIG. 6 is a cross sectional view of a plasma doping
apparatus for implementing a plasma doping method according to a
fifth embodiment of the invention;
[0044] FIG. 7 is a circuit diagram of a matching circuit for plasma
generating apparatus used in the fifth embodiment of the
invention;
[0045] FIG. 8 is a timing chart showing the operation according to
the fifth embodiment of the invention;
[0046] FIG. 9 is a timing chart showing the operation according to
a sixth embodiment of the invention;
[0047] FIG. 10 is a cross sectional view of a plasma doping
apparatus for implementing a plasma doping method according to a
seventh embodiments of the invention;
[0048] FIG. 11 is a timing chart showing the operation according to
the seventh embodiment of the invention;
[0049] FIG. 12 is a graph showing the relation of sampling interval
with variation in doping density according to the seventh
embodiment of the invention;
[0050] FIG. 13 is a cross sectional view of another example of a
plasma doping apparatus for implementing a plasma doping method
according to the seventh embodiments of the invention;
[0051] FIG. 14 is the cross sectional view of the plasma doping
apparatus for implementing the prior art plasma doping method;
and
[0052] FIG. 15 is the cross sectional view of another example of
the plasma doping apparatus for implementing the prior art plasma
doping method.
DETAILED DESCRIPTION OF THE INVENTION
[0053] Preferred embodiments of the invention are described below
in detail with reference to FIG. 1 through FIG. 13.
[0054] [First Embodiment]
[0055] A plasma doping method according to a first embodiment of
the present invention is described below with reference to FIG.
1.
[0056] FIG. 1 is a cross sectional view of a plasma doping
apparatus for implementing a plasma doping method according to the
first embodiment of the invention. In FIG. 1, a predetermined gas
is continuously introduced from a gas supplying apparatus 2 into
the internal space 1a of a vacuum chamber 1, while the vacuum
chamber 1 is evacuated by a turbo molecular pump 3 serving as an
evacuating apparatus. In the state that the pressure of the
internal space 1a of the vacuum chamber 1 is maintained at a
predetermined value by means of a regulating valve 4, a high
frequency electric power of 13.56 MHz is supplied from a high
frequency power supply 5 to a spiral coil 8 provided in the
vicinity of a dielectric window 7 opposing to a sample electrode 6.
According to this configuration, plasma of an induction coupling
type is generated in the internal space 1a of the vacuum chamber 1,
so that plasma doping is performed on a silicon substrate 9 as a
sample (work to be processed) placed on the sample electrode 6. A
high frequency power supply 10 is provided for supplying high
frequency electric power to the sample electrode 6. Thus, the high
frequency power supply 10 can control the electric potential of the
sample electrode 6 such that the substrate 9 has a negative
potential relative to the plasma. The turbo molecular pump 3 and an
evacuation opening 11 are arranged immediately under the sample
electrode 6. The regulating valve 4 is arranged also immediately
under the sample electrode 6 but immediately over the turbo
molecular pump 3, and serves as a valve for increasing and reducing
the pressure. The sample electrode 6 is mounted in the vacuum
chamber 1 by four supporting members 12. The dielectric window 7 is
composed mainly of quartz glass.
[0057] A silicon substrate 9 for work is placed on the sample
electrode 6. In the state that the temperature of the sample
electrode 6 is maintained at 10.degree. C., helium gas is
introduced into the vacuum chamber 1 at the rate of 50 sccm
(standard cc/min), and diborane gas (B.sub.2H.sub.6) serving as a
doping material gas is introduced therein at the rate of 3 sccm. In
the state that the pressure of the vacuum chamber 1 is controlled
at the first pressure of 3 Pa (Pascal), the high frequency electric
power at 800 W of 13.56 MHz is supplied to the coil 8 serving as a
plasma source. Then, plasma is generated in the vacuum chamber 1.
At one second after the generation of the plasma, in the state that
the plasma is generated, the pressure of the vacuum chamber 1 is
controlled into a second pressure of 0.3 Pa lower than the first
pressure (3 Pa). Once the plasma becomes stable, a high frequency
electric power at 200 W (13.56 MHz) is supplied to the sample
electrode 6 for 7 seconds. As a result of this process, boron has
been doped in the vicinity of the surface of the substrate 9. The
doping of boron into a film prepared on the surface of a substrate
9 as the sample is processed similarly. A doping density of
2.5.times.10.sup.13 atm/cm.sup.2 has been obtained.
[0058] As described above, the generation of the plasma is
performed at a pressure (3 Pa) higher than the pressure (0.3 Pa) in
the doping process. Stable generation of the plasma can be
realized. Further, in the above-mentioned process, the plasma is
composed mainly of helium causing only a reduced amount of ion
irradiation damage to the sample. Therefore, stable low density
doping is carried out.
[0059] In the step of generating the plasma, an inert gas other
than helium may be used as another method. In an inert gas other
than helium, a low limit of pressure for generation is lower than
in helium. Thus, when an inert gas other than helium is used, the
plasma generation can be performed advantageously at a lower
pressure than in helium.
[0060] In the step of generating the plasma, the high frequency
electric power supplied to the plasma source may be reduced. In
this case, adverse influence which is given to the substrate 9
during the generation step can be reduced advantageously.
[0061] In the step of generating the plasma, the supply of the
doping material gas into the vacuum chamber may be stopped. This
reduces adverse influence to the substrate 9 during the generation
step.
[0062] [Second Embodiment]
[0063] A plasma doping method according to a second embodiment of
the present invention is described below with reference to FIG. 1,
which has bee used similarly in the description of the first
embodiment. The description made in the first embodiment as to FIG.
1 is applied to the second embodiment similarly.
[0064] The configuration and the basic operation of the plasma
doping apparatus of FIG. 1 have been described in the first
embodiment of the invention, and hence duplicated description is
omitted.
[0065] In FIG. 1, a substrate 9 is placed on the sample electrode
6. In the state that the temperature of the sample electrode 6 is
maintained at 10.degree. C., argon gas is introduced into the
vacuum chamber 1 at the rate of 50 sccm, and diborane gas
(B.sub.2H.sub.6) serving as a doping material gas is introduced at
the rate of 3 sccm. In the state that the pressure of the vacuum
chamber 1 is controlled at the first pressure of 0.8 Pa, high
frequency electric power at 800 W is supplied to the coil 8 serving
as a plasma source. Then, plasma is generated in the vacuum chamber
1. At one second after the generation of the plasma, in the state
that the plasma is generated, helium gas is introduced into the
vacuum chamber 1 at a rate of 50 sccm, whereas the supply of argon
gas is stopped. Once the plasma becomes stable, a high frequency
electric power at 200 W is supplied to the sample electrode 6 for 7
seconds. As a result of this process, boron has been doped in the
vicinity of the surface of the substrate 9. A doping density is
4.2.times.10.sup.13 atm/cm.sup.2.
[0066] As mentioned above, a stable generation of the plasma is
realizable by using an inert gas (argon, for example) other than
helium. After generation of the plasma, the plasma mainly composed
of helium is used, which gives a slight ion irradiation damage to
the sample. Thereby, a low density doping can be stably carried
out.
[0067] In the step of generating the plasma in the second
embodiment, the pressure of the vacuum chamber may be increased.
Thereby, there is an advantage that the plasma is generated
stably.
[0068] In the step of generating the plasma, the high frequency
electric power supplied to the plasma source may be reduced.
Thereby, adverse influence to the sample during the generation step
is reduced.
[0069] In the step of generating the plasma, the supply of the
doping material gas into the vacuum chamber may be stopped.
Thereby, adverse influence to the sample during the generation step
is reduced.
[0070] [Third Embodiment]
[0071] A plasma doping method according to a third embodiment of
the present invention is described below with reference to FIG. 1.
The description made in the first embodiment as to FIG. 1 is
applied to the third embodiment similarly.
[0072] The configuration and the basic operation of the plasma
doping apparatus of FIG. 1 have been described in the first
embodiment of the invention, and hence duplicated description is
omitted.
[0073] In FIG. 1, a substrate 9 is placed on the sample electrode
6. In the state that the temperature of the sample electrode 6 is
maintained at 10.degree. C., helium gas is introduced into the
vacuum chamber 1 at the rate of 50 sccm, and diborane gas
(B.sub.2H.sub.6) serving as a doping material gas is introduced at
the rate of 3 sccm. In the state that the pressure of the vacuum
chamber 1 is controlled at a first pressure of 3 Pa, high frequency
electric power at 100 W is supplied to the coil 8 serving as a
plasma source. Then, plasma is generated in the vacuum chamber 1.
At one second after the generation of the plasma, in the state that
the plasma is generated, the pressure of the vacuum chamber 1 is
controlled into the second pressure of 0.3 Pa lower than the first
pressure (3 Pa), while the high frequency electric power supplied
to the coil is increased into 800 W. Once the plasma becomes
stable, high frequency electric power at 200 W is supplied to the
sample electrode 6 for 7 seconds. As a result of this process,
boron has been doped in the vicinity of the surface of the
substrate 9. A doping density is 2.4.times.10.sup.13
atm/cm.sup.2.
[0074] As mentioned above, in the step of generating the plasma,
the electric power supplied to the plasma source is reduced.
Thereby, adverse influence to the sample during the generation step
can be reduced. The plasma mainly composed of helium is used, which
gives a slight ion irradiation damage to the sample. Thereby, a low
density doping can be stably carried out.
[0075] In the step of generating the plasma in the third
embodiment, the second pressure may be identical to the first
pressure. Even in this case, adverse influence to the sample is
reduced during the generation step.
[0076] In the step of generating the plasma, an inert gas other
than helium may be supplied. In an inert gas other than helium, a
low limit of pressure for plasma generation is lower than in
helium. Thus, the plasma generation can be performed advantageously
at a lower pressure.
[0077] In the step of generating the plasma, the supply of the
doping material gas into the vacuum chamber may be stopped. This
reduces adverse influence to the sample during the generation
step.
[0078] [Fourth Embodiment]
[0079] A plasma doping method according to a fourth embodiment of
the present invention is described below with reference to FIG. 1.
The description made in the first embodiment as to FIG. 1 is
applied to the fourth embodiment similarly.
[0080] The configuration and the basic operation of the plasma
doping apparatus of FIG. 1 have been described in the first
embodiment of the invention, and hence duplicated description is
omitted.
[0081] In FIG. 1, a substrate 9 is placed on the sample electrode
6. In the state that the temperature of the sample electrode 6 is
maintained at 10.degree. C., helium gas is introduced into the
vacuum chamber 1 at a rate of 50 sccm. In the state that the
pressure of the vacuum chamber 1 is controlled at a first pressure
of 3 Pa, high frequency electric power at 800 W is supplied to the
coil 8 serving as a plasma source. Then, plasma is generated in the
vacuum chamber 1. At one second after the generation of the plasma,
in the state that the plasma is generated and that the pressure of
the vacuum chamber 1 is controlled into a second pressure of 0.3 Pa
lower than the first pressure (3 Pa), diborane gas (B.sub.2H.sub.6)
serving as a doping material gas is introduced at a rate of 3 sccm.
Once the plasma becomes stable, high frequency electric power at
200 W is supplied to the sample electrode 6 for 7 seconds. As a
result of this process, boron has been doped in the vicinity of the
surface of the substrate 9. A doping density is 2.3.times.10.sup.13
atm/cm.sup.2.
[0082] As described above, in the step of generating the plasma, a
gas not containing the doping material gas is used. This reduces
adverse influence to the sample during the generation step. The
plasma mainly composed of helium is used, which gives a slight ion
irradiation damage to the sample. Thereby, a low density doping can
be stably carried out.
[0083] In the step of generating the plasma in the fourth
embodiment, the second pressure is preferably identical to the
first pressure. In this case, adverse influence to the sample is
reduced during the generation step.
[0084] In the step of generating the plasma, preferably, an inert
gas other than helium is supplied. An inert gas other than helium
has a low limit pressure for plasma generation is lower than in
helium. Thus, the plasma generation can be performed advantageously
at a lower pressure.
[0085] In the step of generating the plasma, it is preferable that
the high frequency electric power supplied to the plasma source is
reduced. Thereby, there is an advantage that adverse influence to
the sample during the generation step is reduced.
[0086] In the above-mentioned first to fourth embodiments of the
invention, the implementation of the invention is not limited to
the use of the apparatus shown in FIG. 1. FIG. 1 shows merely one
of variations in the shape of the vacuum chamber, the scheme of the
plasma source, and overall layout. The plasma doping method
according to the invention is obviously applicable to various
apparatuses other than that illustrated in FIG. 1.
[0087] For example, the plasma doping method according to the
present invention is applicable to the apparatuses of FIG. 2 where
the coil 8 has the shape of a planar spiral.
[0088] Further, the plasma doping method according to the invention
is applicable to the apparatuses of FIG. 3 where an antenna 13 is
used in place of the coil 8 and where an electric magnet 14 is used
as a magnetic-field generating apparatus. In this case, helicon
wave plasma is generated in the vacuum chamber 1. The helicon wave
plasma has a high density rather than the inductively coupled
plasma. It is preferable that a DC magnetic field or a low
frequency magnetic field of 1 kHz or lower is generated in the
vacuum chamber 1 by adjusting the electric current following
through the electric magnet 14.
[0089] The plasma doping method according to the present invention
is applicable to the apparatuses shown in FIG. 4 of which an
antenna 13 is used in place of the coil 8 and two electric magnets
14a and 14b are used as magnetic-field generating apparatuses. In
this case, by flowing electric currents in the opposite directions
to each other the two electric magnets 14a and 14b, magnetically
neutral loop discharge plasma can be generated in the vacuum
chamber 1. The magnetically neutral loop discharge plasma is
generated at a higher density than the inductively coupled plasma.
By adjusting the electric currents following through the electric
magnets 14a and 14b, a DC magnetic field or a low frequency
magnetic field of 1 kHz or lower may be applied into the vacuum
chamber 1.
[0090] FIG. 5 is a cross sectional view of another example of a
plasma doping apparatus. In FIG. 5, a predetermined gas is
continuously introduced from a gas supplying apparatus 2 into a
vacuum chamber 1, while the vacuum chamber 1 is evacuated by a
turbo molecular pump 3 serving as an evacuating apparatus. In the
state that the pressure of the vacuum chamber 1 is maintained at a
predetermined value by means of a regulating valve 4, high
frequency electric power of 13.56 MHz is supplied from a high
frequency power supply 5 to a coil 8 provided in the vicinity of a
dielectric window 7 opposing to a sample electrode 6. According to
this configuration, inductively coupled plasma is generated in the
vacuum chamber 1, so that plasma doping is performed on a silicon
substrate 9 serving as a sample placed on the sample electrode 6. A
high frequency power supply 10 is provided for supplying high
frequency electric power to the sample electrode 6. Thus, the high
frequency power supply 10 can control the electric potential of the
sample electrode 6 such that the substrate 9 has a negative
potential relative to the plasma. The turbo molecular pump 3 and an
evacuation opening 11 are arranged immediately under the sample
electrode 6. The regulating valve 4 is arranged also immediately
under the sample electrode 6 but immediately over the turbo
molecular pump 3, and serves as a valve for increasing and reducing
the pressure. The sample electrode 6 is mounted in the vacuum
chamber 1 by four supporting members 12. The dielectric window 7 is
composed mainly of quartz glass, and contains boron as
impurities.
[0091] A high frequency power supply 16 is provided to supply a
high frequency electric power of 500 kHz to a bias electrode 15
provided between the coil 8 and the dielectric window 7. The bias
electrode 15 is composed of a large number of strip-shaped
electrodes arranged in a radial manner as known in the art. The
longitudinal direction of each strip-shaped electrode is arranged
perpendicular to the conductor of the spiral coil 8. This
arrangement of the bias electrode 15 allows the high frequency
electromagnetic field emitted from the coil 8 to enter almost
completely into the vacuum chamber 1. The bias electrode 15 covers
almost entire area of the dielectric window 7, so as to control the
amount of boron sputtered from the boron-containing quartz glass of
the dielectric window 7 into the plasma. In the output of the high
frequency power supply 5, provided is a reflection wave detecting
circuit 20 comprising a band pass filter 17 and a reflection wave
meter 18. The band pass filter 17 is provided as a circuit to
eliminate the influence of which the modulation by the high
frequency electric power of 500 kHz from the high frequency power
supply 16 gives to the detection of the reflection wave of the high
frequency electric power of 13.56 MHz supplied from the high
frequency power supply 5. The band pass filter 17 also eliminates
the influence of which the plasma sheath thickness in the surface
of the dielectric window 7 varies at 500 KHz of frequency by
supplying the high frequency electric power of 500 kHz. The band
pass filter 17 extracts solely the 13.56-MHz component from the
reflection wave of the high frequency electric power of 13.56 MHz,
so as to transmit the component to the reflection wave meter 18. In
this configuration, the processing is performed in the state that
the reflection wave of the high frequency electric power of 13.56
MHz is monitored by the reflection wave meter 18. This permits
real-time detection of troubles in the matching state and in the
13.56-MHz high frequency power supply.
[0092] This configuration permits such processing that no doping
material gas is introduced into the vacuum chamber 1, and
impurities can be doped into a sample or into a film on the surface
of the sample by the doping material generated from the dielectric
window 7 containing necessary impurities in the state of solid.
[0093] In the above-mentioned embodiments of the invention, in
order that the generation and the low density doping are ensured
under a condition that the second pressure is lower than the first
pressure, the first pressure is preferably 1 Pa through 10 Pa,
while the second pressure is preferably 0.01 Pa through 1 Pa. More
preferably, the first pressure is 2 Pa through 5 Pa, while the
second pressure is 0.01 Pa through 0.5 Pa.
[0094] In the case that an inert gas other than helium is used,
preferable is the use of at least one selected from the group
consisting of neon, argon, krypton, and xenon. These inert gases
cause only smaller adverse influence to the sample than other kinds
of gases.
[0095] In the case that the high frequency electric power supplied
to the plasma source is reduced in the generation step, it is
preferred that the low density doping is achieved under the
condition that the generation is ensured and that adverse influence
to the sample is suppressed during the generation step. For this
purpose, the high frequency electric power supplied to the plasma
source in the generation step is preferably {fraction (1/100)}
through 1/2 of that in the doping step. More preferably, the high
frequency electric power supplied to the plasma source in the
generation step is {fraction (1/20)} through 1/5 of the high
frequency electric power supplied to the plasma source in the
doping step.
[0096] In the case that a doping material gas is introduced into
the vacuum chamber, for the purpose of achieving low density
doping, the partial pressure of the doping material gas is
preferably {fraction (1/1,000)} through 1/5 of the pressure of the
vacuum chamber in the doping step. Moreover, the partial pressure
of the doping material gas is preferably {fraction (1/100)} through
{fraction (1/10)} of the pressure of the vacuum chamber in the
doping step.
[0097] The above-mentioned embodiments have been described for the
case that the sample is a semiconductor substrate composed of
silicon. However, the plasma doping method according to the
invention is applicable also to the processing of samples composed
of other various kinds of substances.
[0098] The above-mentioned embodiments have been described for the
case that the impurities are composed of boron. However, in the
case that the sample is a semiconductor substrate composed of
silicon, the plasma doping method according to the invention is
effective especially for the case that the impurities are arsenic,
phosphorus, boron, aluminum and antimony. This is because these
impurities can form a shallow junction in the transistor
section.
[0099] The plasma doping method according to the invention is
effective especially for the case of a low doping density.
Specifically, the method is effective as a plasma doping method
aiming at a doping density of 1.times.10.sup.11 atm/cm.sup.2
through 1.times.10.sup.17 atm/cm.sup.2. The method is more
effective as a plasma doping method aiming at a doping density of
1.times.10.sup.11 atm/cm.sup.2 through 1.times.10.sup.14
atm/cm.sup.2.
[0100] The method according to the invention is effective also in
the case that electron cyclotron resonance (ECR) plasma is used.
However, the method is effective especially in the case of plasma
other than the ECR plasma. The ECR plasma has the advantage that
the plasma generation is easy even at low pressures. Nevertheless,
apparatuses using the ECR plasma have a strong DC magnetic field in
the vicinity of the sample. Therefore, charge separation of
electrons and ions is liable to occur, and hence it has a
disadvantage which is inferior in uniformity of a doping amount.
The low density doping which is excellent in uniformity can be
realized by applying the present invention to a plasma doping
method using other high density plasma source without using of the
ECR plasma.
[0101] [Fifth Embodiment]
[0102] A plasma doping method according to a fifth embodiment of
the present invention is described below with reference to FIG. 6
through FIG. 8.
[0103] FIG. 6 is a cross sectional view of a plasma doping
apparatus for implementing the plasma doping method according to
the fifth embodiments of the invention. In FIG. 6, a silicon
substrate 9 as a sample is placed on the sample electrode 6
provided in a vacuum chamber 1. A predetermined gas is continuously
introduced from a gas supplying apparatus 2 into the vacuum chamber
1, while the vacuum chamber 1 is evacuated by a turbo molecular
pump 3 serving as an evacuating apparatus. In the state that the
pressure of the vacuum chamber 1 is maintained at a predetermined
value by means of a regulating valve 4, a high frequency electric
power of 13.56 MHz is supplied from a high frequency power supply 5
to a coil 8 serving as a plasma generating apparatus provided in
the vicinity of a dielectric window 7 opposing to a sample
electrode 6. According to this configuration, inductively coupled
plasma is generated in the vacuum chamber 1, so that plasma doping
is performed on a silicon substrate 9 placed on the sample
electrode 6. A high frequency power supply 10 is provided for
supplying high frequency electric power to the sample electrode 6.
Thus, the high frequency power supply 10 can control the electric
potential of the sample electrode 6 such that the silicon substrate
9 has a negative potential relative to the plasma. The turbo
molecular pump 3 and an evacuation opening 11 are arranged
immediately under the sample electrode 6. The regulating valve 4 is
arranged also immediately under the sample electrode 6 but
immediately over the turbo molecular pump 3, and serves as a valve
for increasing and reducing the pressure. The sample electrode 6 is
mounted in the vacuum chamber 1 by four supporting members 12. The
dielectric window 7 is composed mainly of quartz glass. A matching
circuit 33 for plasma generating apparatus is provided between the
high frequency power supply 5 and the coil 8.
[0104] The matching circuit 33 for the plasma generating apparatus
is marked and is known in the art, and is described simply with
reference to the block diagram of FIG. 7. In FIG. 7, the output
terminal 33b is connected to the coil 8. When high frequency
electric power is supplied from the high frequency power supply 5
to the input terminal 33a of the matching circuit 33, in response
to a signal from a sensor 14, a computing circuit 15 outputs
control signals to toroidal cores 16a and 16b. Consequently, the
permeabilities of the toroidal cores 16a and 16b are changed, and
thus the high frequency inductances change. Thereby, the matching
circuit 33 is adjusted to a desired matching state. The matching
circuit 33 for plasma generating apparatus uses the toroidal cores
16a and 16b which have no mechanically moving section and which can
adjust the impedance in response merely to an electric signal.
Thus, the time necessary for the matching is 1 milliseconds or
shorter.
[0105] A silicon substrate 9 is placed on the sample electrode 6.
In the state that the temperature of the sample electrode 6 is
maintained at 10.degree. C., helium gas is introduced into the
vacuum chamber 1 at a rate of 50 sccm, and diborane gas
(B.sub.2H.sub.6) serving as a doping material gas is also
introduced at the rate of 3 sccm. In the state that the pressure of
the vacuum chamber 1 is controlled at a first pressure of 2 Pa,
high frequency electric power at 150 W is supplied to the coil 8
serving as a plasma source. Then, plasma is generated in the vacuum
chamber 1.
[0106] The process according to the present embodiment is described
below with reference to the timing chart of FIG. 8. The supply of
the high frequency electric power at 150 W to the coil 8 is started
at time t.sub.1. At 0.1 seconds after the start of the supply of
the high frequency electric power, in the state that the plasma is
generated, the pressure of the vacuum chamber 1 is controlled into
a second pressure of 1 Pa lower than the first pressure (2 Pa) by
adjusting the regulating valve 4.
[0107] Then, at 0.07 seconds after, the high frequency electric
power supplied to the coil 8 is increased into 800 W, while high
frequency electric power at 200 W is supplied to the sample
electrode 6. As a result of this process, boron has been doped in
the vicinity of the surface of the substrate 9. When this process
was repeated successively for 100 times, the mean of doping density
was 2.5.times.10.sup.13 atm/cm.sup.2, while the variation was
.+-.1.4%.
[0108] For comparison, similar process was performed using a prior
art matching circuit using variable capacitors. In this process, a
large reflection wave was produced at the timing of the change of
the control parameters, that is, pressure and high frequency
electric power. Further, the magnitude of this reflection wave
varied in each of the repeated processes. As a result, when the
process was repeated successively for 100 times, the mean of doping
density was 2.4.times.10.sup.13 atm/cm.sup.2, and the variation was
as large as .+-.2.8%. FIG. 8 shows the waveforms of the reflection
waves in the present embodiment and in the prior art.
[0109] As described above, by performing plasma generation at a
pressure higher than in the doping step, stable plasma generation
is realizable. Therefore, a low density doping can be stably
carried out by using of plasma composed mainly of helium causing
only a reduced amount of ion irradiation damage to the sample. This
permits stable low density doping. Further, in the step of
generating the plasma, the high frequency electric power supplied
to the plasma source is reduced. Thereby adverse influence to the
sample during the generation step can be reduced. Furthermore, in
the present embodiment, the matching circuit 33 for plasma
generating apparatus is used, which comprises the toroidal cores
16a and 16b serving as two variable impedance elements without any
mechanically moving section. Therefore, no large reflection wave
occurs even when the control parameters are changed. This permits
excellently reproducible processing.
[0110] In the fifth embodiment, in the step of generating the
plasma, an inert gas other than helium may be supplied. In an inert
gas other than helium, the low limit of pressure for generation is
lower than in helium. Thus, the plasma generation can be performed
advantageously at a lower pressure.
[0111] In the step of generating the plasma, the supply of the
doping material gas into the vacuum chamber 1 may be stopped. In
this case also, there is an advantage that adverse influence to the
sample during the generation step can be reduced.
[0112] [Sixth Embodiment]
[0113] A plasma doping method according to a sixth embodiment of
the present invention is described below with reference to FIG.
9.
[0114] A plasma doping apparatus for implementing the plasma doping
method according to the sixth embodiment have a configuration
similar to that shown in FIG. 6 and FIG. 7, and hence duplicated
description is omitted.
[0115] A substrate 9 is placed on the sample electrode 6. In the
state that the temperature of the sample electrode 6 is maintained
at 10.degree. C., helium gas is introduced into the vacuum chamber
1 at a rate of 50 sccm, and also diborane gas (B.sub.2H.sub.6)
serving as a doping material gas is introduced at the rate of 3
sccm. In the state that the pressure of the vacuum chamber 1 is
controlled at a first pressure of 2 Pa, high frequency electric
power at 200 W is supplied to the coil 8 serving as a plasma
source. Then, plasma is generated in the vacuum chamber 1.
[0116] The process according to the present embodiment is described
below with reference to the timing chart of FIG. 9. The supply of
the high frequency electric power at 200 W to the coil 8 is started
at time t.sub.1. At 0.5 seconds after the start of the supply of
the high frequency electric power, in the state that the plasma is
generated, the pressure of the vacuum chamber 1 is controlled into
a second pressure of 1 Pa lower than the first pressure (2 Pa) by
gradually increasing the openness of the regulating valve 4. At the
same time, the high frequency electric power supplied to the coil 8
was increased into 800 W gradually in 2.0 seconds. The pressure and
the high frequency electric power as control parameters are changed
gradually in 2.0 seconds. After that, high frequency electric power
at 200 W is supplied to the sample electrode 6. As a result of this
process, boron can be doped in the vicinity of the surface of the
substrate 9. When this process was repeated successively for 100
times, the mean of doping density was 2.5.times.10.sup.13
atm/cm.sup.2, and the variation was .+-.0.9%.
[0117] As described above, plasma generation is performed at a
pressure higher than in the doping step. Thereby stable plasma
generation is realizable. Furthermore, stable low density doping
can be carried out by using the plasma composed of mainly helium
causing only a reduced amount of ion irradiation damage to the
sample. Further, in the step of generating the plasma, the high
frequency electric power supplied to the plasma source is reduced.
Thereby, adverse influence to the sample during the generation step
is reduced. Furthermore, in the present embodiment, the matching
circuit 33 for plasma generating apparatus is used, which comprises
the toroidal cores 16a and 16b serving as two variable impedance
elements without any mechanically moving section. Therefore, no
large reflection wave occurs even when the control parameters are
changed. Excellently reproducible processing can be carried out.
Further, since the control parameters are changed gradually in 2.0
seconds, the impedance change is also gradual. This reduces the
reflection wave further, in comparison with the fifth embodiment
shown in FIG. 8. Thus, reproducibility is improved.
[0118] In the sixth embodiment, in the step of generating the
plasma, an inert gas other than helium may be supplied. In an inert
gas other than helium, the low limit of pressure for generation is
lower than in helium. Thus, the plasma generation can be performed
advantageously at a lower pressure.
[0119] In the step of generating the plasma, the supply of the
doping material gas into the vacuum chamber may be stopped. This
reduces adverse influence to the sample during the generation
step.
[0120] The present embodiment has been described for the case that
the control parameters are changed gradually in 2 seconds. However,
the control parameters are changed preferably in 1 second through 5
seconds. In the case that a matching circuit for plasma generating
apparatus comprising variable capacitors is used, when the control
parameters are changed in less than 1 second, a large reflection
wave is liable to occur. When the control parameters are changed in
greater than 5 seconds, the processing time increases so as to
reduce the productivity.
[0121] The fifth and sixth embodiments of the present invention
disclose merely examples of applicable forms of the plasma doping
method according to the invention. The plasma doping method
according to the invention is obviously applicable to apparatuses
of diverse variations with respect to the shape of the vacuum
chamber, the scheme of the plasma source, and overall layout.
[0122] For example, the coil 8 may have a planar shape as shown in
FIG. 2. Further, helicon wave plasma and magnetically neutral loop
plasma may be used.
[0123] It is possible that no doping material gas is introduced
into the vacuum chamber 1, and impurities are doped into a sample
or into a film on the surface of the sample by using doping
material generated from an impurity material in the state of
solid.
[0124] The plasma doping method according to the invention is
useful especially in the case that at least one of control
parameters, such as gas species, gas flow rate, pressure, and high
frequency electric power, is changed in the state that the plasma
is generated.
[0125] In the case that the pressure is changed from the first
pressure to the second pressure in the state that the plasma is
generated, it is preferable that the second pressure is lower than
the first pressure. In order that the generation and the low
density doping are ensured, the first pressure is preferably 1 Pa
through 10 Pa, and the second pressure is preferably 0.01 Pa
through 1 Pa. More preferably, the first pressure is 2 Pa through 5
Pa, and the second pressure is 0.01 Pa through 0.5 Pa.
[0126] In the case that an inert gas other than helium is used, it
is preferable to use at least one selected from the group
consisting of neon, argon, krypton, and xenon. These inert gases
cause only smaller adverse influence to the sample than other kinds
of gases.
[0127] In the case that the high frequency electric power supplied
to the plasma source is reduced in the generation step, it is
preferred that the low density doping is achieved under the
condition that the generation is ensured and that adverse influence
to the sample is suppressed during the generation step. For this
purpose, the high frequency electric power supplied to the plasma
source in the generation step is preferably {fraction (1/100)}
through 1/2 of the high frequency electric power supplied to the
plasma source in the doping step. More preferably, the high
frequency electric power supplied to the plasma source in the
generation step is {fraction (1/20)} through 1/5 of the high
frequency electric power supplied to the plasma source in the
doping step.
[0128] In the case that a doping material gas is introduced into
the vacuum chamber, for the purpose of achieving low density
doping, the partial pressure of the doping material gas is
preferably {fraction (1/1,000)} through 1/5 of the pressure of the
vacuum chamber in the doping step. More preferably, the partial
pressure of the doping material gas is preferably {fraction
(1/100)} through {fraction (1/10)} of the pressure of the vacuum
chamber in the doping step.
[0129] The above-mentioned embodiments of the invention have been
described for the case that the sample is a semiconductor substrate
composed of silicon. However, the plasma doping method according to
the invention is applicable also to the processing of samples
composed of other various kinds of substances.
[0130] The above-mentioned embodiments of the invention have been
described for the case that the impurities are composed of boron.
However, in the case that the sample is a semiconductor substrate
composed of silicon, the plasma doping method according to the
invention is effective especially for the case that the impurities
are composed of arsenic, phosphorus, boron, aluminum, and antimony.
This is because these impurities can form a shallow junction in the
transistor section.
[0131] The plasma doping method according to the invention is
effective especially for the case of a low doping density.
Specifically, the method is effective as a plasma doping method
aiming at a doping density of 1.times.10.sup.11 atm/cm.sup.2
through 1.times.10.sup.17 atm/cm.sup.2. The method is more
effective as a plasma doping method aiming at a doping density of
1.times.10.sup.11 atm/cm.sup.2 through 1.times.10.sup.14
atm/cm.sup.2.
[0132] The method according to the invention is effective also in
the case that electron cyclotron resonance (ECR) plasma is used.
However, the method is effective especially in the case of plasma
other than the ECR plasma. The ECR plasma has the advantage that
the plasma generation is easy even at low pressures. Nevertheless,
apparatuses using ECR plasma have a strong DC magnetic field in the
vicinity of the sample. Therefore, the charge separation of
electrons and ions is liable to occur, and hence it has a
disadvantage which is inferior in uniformity of a doping amount.
The low density doping which is excellent in uniformity can be
realized by applying the present invention to a plasma doping
method using other high density plasma source without using of the
ECR plasma.
[0133] [Seventh Embodiment]
[0134] A plasma doping method according to a seventh embodiment of
the present invention is described below with reference to FIG. 10
through FIG. 12.
[0135] FIG. 10 is a cross sectional view of a plasma doping
apparatus for implementing a plasma doping method according to the
seventh embodiments of the invention. In the plasma doping
apparatus of FIG. 10, the output of a high frequency power supply
10 is supplied through a sampler 37 to a sample electrode 6. The
sampler 37 provides a control signal to a control apparatus 44. The
output of the control apparatus 44 is provided to the high
frequency power supply 10 so as to control this high frequency
power supply. A predetermined gas is continuously introduced from a
gas supplying apparatus 2 into a vacuum chamber 1, while the vacuum
chamber 1 is evacuated by a turbo molecular pump 3 serving as an
evacuating apparatus. In the state that the pressure of the vacuum
chamber 1 is maintained at a predetermined value by means of a
regulating valve 4, a high frequency electric power of 13.56 MHz is
supplied from a high frequency power supply 5 to a coil 8 serving
as a plasma generating apparatus provided in the vicinity of a
dielectric window 7 opposing to a sample electrode 6. According to
this configuration, inductively coupled plasma is generated in the
vacuum chamber 1, so that plasma doping is performed on a silicon
substrate 9 serving as a sample placed on the sample electrode
6.
[0136] A high frequency power supply 10 is provided for supplying
high frequency electric power to the sample electrode 6. Thus, the
high frequency power supply 10 can control the electric potential
of the sample electrode 6 such that the silicon substrate 9 has a
negative potential relative to the plasma. The turbo molecular pump
3 and an evacuation opening 11 are arranged immediately under the
sample electrode 6. The regulating valve 4 is arranged also
immediately under the sample electrode 6 but immediately over the
turbo molecular pump 3, and serves as a valve for increasing and
reducing the pressure. The sample electrode 6 is mounted in the
vacuum chamber 1 by four supporting members 12. The dielectric
window 7 is composed mainly of quartz glass.
[0137] When the forward power of the high frequency electric power
supplied to the sample electrode 6 is denoted by Pf and the
reflected power thereof is denoted by Pr, the sampler 37 samples
the power difference Pf-Pr which is the difference of the forward
power Pf and the reflected power Pr in every interval of 1
millisecond through 100 milliseconds. When the integration of the
power difference Pf-Pr with respect to time reaches a predetermined
value, the controlling apparatus 44 stops the supply of the high
frequency electric power.
[0138] A substrate 9 is placed on the sample electrode 6. In the
state that the temperature of the sample electrode 6 is maintained
at 10.degree. C., helium gas is introduced into the vacuum chamber
1 at the rate of 50 sccm, and also diborane gas (B.sub.2H.sub.6)
serving as a doping material gas is introduced at the rate of 3
sccm. In the state that the pressure of the vacuum chamber 1 is
controlled at a first pressure of 2 Pa, high frequency electric
power at 150 W is supplied to the coil 8 serving as a plasma
source. Then, plasma is generated in the vacuum chamber 1. The
process according to the present embodiment is described below with
reference to the timing chart of FIG. 11.
[0139] The supply of the high frequency electric power to the coil
8 is started at time t.sub.1. At one second after the start of the
supply of the high frequency electric power, in the state that the
plasma is generated, the pressure of the vacuum chamber 1 is
controlled into a second pressure of 1 Pa lower than the first
pressure (2 Pa) by adjusting the regulating valve 4. Then, at 0.8
seconds after the start of control of the regulating valve 4, the
high frequency electric power supplied to the coil is increased
into 800 W, while high frequency electric power at 200 W is
supplied to the sample electrode 6. As a result of this process,
boron has been doped in the vicinity of the surface of the
substrate 9. When the forward power of the high frequency electric
power supplied to the sample electrode 6 is denoted by Pf and the
reflected power thereof is denoted by Pr, the sampler 37 samples
the power difference Pf-Pr in every interval of 80 milliseconds.
When the integration of the sampled power difference Pf-Pr with
respect to time reaches a predetermined value, the controlling
apparatus 44 stops the supply of the high frequency electric power.
That is, when the area of the shaded region in the graph in the
bottom of FIG. 11 reaches 1400 W second, the supply of the high
frequency electric power to the plasma generating apparatus 8 and
the sample electrode 6 is stopped.
[0140] When this process was repeated successively for 100 times,
the mean of doping density was 3.5.times.10.sup.13 atm/cm.sup.2,
while the variation was .+-.1.2%.
[0141] Further, the variation of doping density was measured with
changing the sampling interval for the power difference Pf-Pr. FIG.
12 is a graph showing the variation of doping density when the
process was repeated successively for 100 times for each value of
the sampling interval. For the sampling interval of 100
milliseconds or less, the variation decreases rapidly into a value
less than .+-.1.5%. For much smaller sampling interval of 10
milliseconds or less, the variation decreases as small as a value
less than .+-.0.5%.
[0142] The reason of this excellent reproducibility in the
processing is that the present method utilizes the fact that the
doping density is proportional to both the high frequency electric
power supplied to the sample electrode 6 and the processing time
under the condition of the same gas species, the same gas flow
rate, and the same gas pressure. When the integration of the power
difference Pf-Pr with respect to time reaches a predetermined
value, the doping density also reaches a predetermined value.
Accordingly, by stopping the supply of the high frequency electric
power at that time, the doping density is adjusted into the
predetermined value. Even in the case that variation occurs in the
generation of the reflection wave, the effective electric power
supplied to the sample electrode 6 is detected by sampling the
power difference Pf-Pr. Therefore, exact doping density is known
from the integration of the sampled power difference Pf-Pr with
respect to time.
[0143] The present embodiment has been described for the case that
when the forward power of the high frequency electric power
supplied to the sample electrode 6 is denoted by Pf and when the
reflected power thereof is denoted by Pr, the power difference
Pf-Pr is sampled. Another implementation is also possible such that
when the forward power of the high frequency electric power
supplied to the coil 8 serving as the plasma generating apparatus
is denoted by Pf and when the reflected power thereof is denoted by
Pr, the power difference Pf-Pr is sampled. An exemplary
configuration of a plasma doping apparatus in this case is shown in
FIG. 13.
[0144] In FIG. 13, a predetermined gas is continuously introduced
from a gas supplying apparatus 2 into a vacuum chamber 1, while the
vacuum chamber 1 is evacuated by a turbo molecular pump 3 serving
as an evacuating apparatus. In the state that the pressure of the
vacuum chamber 1 is maintained at a predetermined value by means of
a regulating valve 4, a high frequency electric power of 13.56 MHz
is supplied from a high frequency power supply 5 to a coil 8
serving as a plasma generating apparatus provided in the vicinity
of a dielectric window 7 opposing to a sample electrode 6.
According to this configuration, inductively coupled plasma is
generated in the vacuum chamber 1, so that plasma doping is
performed on a silicon substrate 9 serving as a sample (work to be
processed) placed on the sample electrode 6. A high frequency power
supply 10 is provided for supplying a high frequency electric power
to the sample electrode 6. Thus, the high frequency power supply 10
can control the electric potential of the sample electrode 6 such
that the silicon substrate 9 has a negative potential relative to
the plasma. The turbo molecular pump 3 and an evacuation opening 11
are arranged immediately under the sample electrode 6. The
regulating valve 4 is arranged also immediately under the sample
electrode 6 but immediately over the turbo molecular pump 3, and
serves as a valve for increasing and reducing the pressure. The
sample electrode 6 is mounted in the vacuum chamber 1 by four
supporting members 12. The dielectric window 7 is composed mainly
of quartz glass. When the forward power of the high frequency
electric power supplied to the coil 8 serving as a plasma
generating apparatus is denoted by Pf and when the reflected power
thereof is denoted by Pr, a sampler 37 samples the power difference
Pf-Pr in every interval of 1 millisecond through 100 milliseconds.
When the integration of the power difference Pf-Pr with respect to
time reaches a predetermined value, a controlling apparatus 44
stops the supply of the high frequency electric power. The present
method utilizes the fact that the doping density is proportional to
both the high frequency electric power supplied to the plasma
generating apparatus and the processing time under the condition of
the same gas species, the same gas flow rate, and the same gas
pressure. Thus, at the time when the integration of the power
difference Pf-Pr with respect to time reaches a predetermined
value, the supply of the high frequency electric power is stopped.
This provides more accurate doping density even in case that
variation occurs in the generation of the reflection wave.
[0145] The seventh embodiment of the invention discloses merely an
example of applicable forms of the plasma doping method according
to the present invention. The plasma doping method according to the
invention is obviously applicable to apparatuses of diverse
variations with respect to the shape of the vacuum chamber, the
scheme of the plasma source, and overall layout.
[0146] For example, the coil 8 may have a planar shape as shown in
FIG. 2. Further, helicon wave plasma and magnetically neutral loop
plasma may be used.
[0147] It is possible that no doping material gas is introduced
into the vacuum chamber 1, and doping material generated from an
impurity material of solid state may be used to dope impurities
into a sample or into a film on the surface of the sample.
[0148] The plasma doping method according to the invention is
useful especially in the case that at least one of control
parameters, such as gas species, gas flow rate, pressure, and high
frequency electric power is changed in the state that the plasma is
generated. This is because the reflection wave is liable to be
generated in the timing of the change of the control
parameters.
[0149] The above-mentioned embodiment of the invention has been
described for the case that the sample is a semiconductor substrate
composed of silicon. However, the plasma doping method according to
the invention is also applicable to the processing of samples
composed of other various kinds of substances.
[0150] The above-mentioned embodiment of the invention has been
described for the case that the impurities are composed of boron.
However, in the case that the sample is a semiconductor substrate
composed of silicon, the plasma doping method according to the
invention is effective especially for the case that the impurities
are composed of arsenic, phosphorus, boron, aluminum, and antimony.
This is because these impurities can form a shallow junction in the
transistor section.
[0151] The plasma doping method according to the invention is
effective especially for the case of a low doping density.
Specifically, the method is effective as a plasma doping method
aiming at a doping density of 1.times.10.sup.11 atm/cm.sup.2
through 1.times.10.sup.17 atm/cm.sup.2. The method is more
effective as a plasma doping method aiming at a doping density of
1.times.10.sup.11 atm/cm.sup.2 through 1.times.10.sup.14
atm/cm.sup.2. This is because in such low density doping, the
processing time is as short as several seconds through ten and
several seconds, and hence the influence due to variation of the
reflection wave becomes large.
[0152] When the forward power of the high frequency electric power
supplied to the plasma generating apparatus or the sample electrode
is denoted by Pf and when the reflected power thereof is denoted by
Pr, the sampling interval for the power difference Pf-Pr is
preferably 1 millisecond through 100 milliseconds. A sampling
interval less than 1 millisecond requires a sampler having
extremely high performance and a controlling apparatus having high
calculation capability, and hence causes an increase in the
apparatus cost. A sampling interval greater than 100 milliseconds
causes poor reproducibility.
[0153] When the forward power of the high frequency electric power
supplied to the plasma generating apparatus or the sample electrode
is denoted by Pf and when the reflected power thereof is denoted by
Pr, the sampling interval for the power difference Pf-Pr is more
preferably 1 millisecond through 10 milliseconds. A sampling
interval less than 10 milliseconds results in excellent
reproducibility.
[0154] In the sampling of the power difference Pf-Pr, each of Pf
and Pr may be separately sampled so that their difference is
calculated or alternatively obtained by means of operation in a
circuit. Alternatively, assuming that the forward power Pf is equal
to a setting value, the reflected power Pr alone may be sampled so
that the power difference Pf-Pr is calculated.
[0155] Further, in the integration of the power difference Pf-Pr
with respect to time, adjustment sampled values may be interpolated
with a straight line. Alternatively, the sampled values may be
treated such as to define a step-formed function.
* * * * *