U.S. patent application number 11/730244 was filed with the patent office on 2007-08-02 for plasma doping method and plasma doping apparatus.
This patent application is currently assigned to MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.. Invention is credited to Hiroyuki Ito, Bunji Mizuno, Katsumi Okashita, Tomohiro Okumura, Yuichiro Sasaki.
Application Number | 20070176124 11/730244 |
Document ID | / |
Family ID | 37396640 |
Filed Date | 2007-08-02 |
United States Patent
Application |
20070176124 |
Kind Code |
A1 |
Sasaki; Yuichiro ; et
al. |
August 2, 2007 |
Plasma doping method and plasma doping apparatus
Abstract
Disclosed is a plasma doping method that, even though a plasma
doping treatment is repeated, can make a dose from a film to a
silicon substrate uniform for each time. According to an embodiment
of the invention, there is provided a plasma doping method that
places a sample on a sample electrode in a vacuum chamber,
generates plasma in the vacuum chamber, and causes impurity ions in
the plasma to collide against a surface of the sample so as to form
an impurity doped layer in the surface of the sample. The plasma
doping method includes a maintenance step of preparing the vacuum
chamber having a film containing an impurity formed on an inner
wall thereof such that, when the film containing the impurity fixed
to the inner wall of the vacuum chamber is attacked by ions in the
plasma, the amount of an impurity to be doped into the surface of
the sample by sputtering is not changed even though the plasma
containing the impurity ions is repeatedly generated in the vacuum
chamber, a step of placing the sample on the sample electrode, and
a step of irradiating the plasma containing the impurity ions so as
to implant the impurity ions into the sample, and doping the
impurity into the sample by sputtering from the film containing the
impurity fixed to the inner wall of the vacuum chamber.
Inventors: |
Sasaki; Yuichiro; (Tokyo,
JP) ; Okashita; Katsumi; (Osaka, JP) ; Ito;
Hiroyuki; (Chiba, JP) ; Mizuno; Bunji; (Nara,
JP) ; Okumura; Tomohiro; (Osaka, JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, NW
WASHINGTON
DC
20005-3096
US
|
Assignee: |
MATSUSHITA ELECTRIC INDUSTRIAL CO.,
LTD.
Osaka
JP
|
Family ID: |
37396640 |
Appl. No.: |
11/730244 |
Filed: |
March 30, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11647235 |
Dec 29, 2006 |
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11730244 |
Mar 30, 2007 |
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PCT/JP06/09509 |
May 11, 2006 |
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11647235 |
Dec 29, 2006 |
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Current U.S.
Class: |
250/492.21 ;
257/E21.143 |
Current CPC
Class: |
H01J 37/32412 20130101;
C23C 14/48 20130101; H01L 21/2236 20130101 |
Class at
Publication: |
250/492.21 |
International
Class: |
H01J 37/08 20060101
H01J037/08 |
Foreign Application Data
Date |
Code |
Application Number |
May 12, 2005 |
JP |
2005-140405 |
Claims
1-22. (canceled)
23. A silicon substrate with a diameter of 300 mm in which boron is
introduced to a surface thereof by a plasma doping process using
successive plasma containing the boron, wherein the introduced
boron has such a profile that a depth at which a boron
concentration is 5.times.10 .sup.18cm.sup.-3 is in the range of 7
to 15.5 nm, wherein the abruptness of the depth profile of boron is
in the range of 1.5 to 3 nm/decade in a distance in which the boron
concentration is lowered from 1.times.10.sup.19 cm.sup.-3 to
1.times.10.sup.18 cm.sup.-3; and wherein a standard deviation of a
boron dose on surfaces of the silicon substrate other than a
portion in 3 mm from the edge thereof is 2% or less.
24. A silicon substrate with a diameter of 300 mm in which a boron
introduced layer is formed in a surface thereof by a plasma doping
process using successive plasma containing boron, wherein the boron
introducing layer is formed by generating plasma containing boron
in a vacuum vessel having an inner wall on which a film containing
boron is formed, wherein the introduced boron has such a profile
that a depth at which a boron concentration is 5.times.10.sup.18
cm.sup.-3 is in the range of 7 to 15.5 nm, wherein a abruptness of
the boron depth is in the range of 1.5 to 3 nm/decade in a distance
in which the boron concentration is lowered from 1.times.10.sup.19
cm.sup.-3 to 1.times.10.sup.18 cm.sup.-3; and wherein a standard
deviation of a boron dose on surfaces of the silicon substrate
other than a portion is 3 mm from the edge thereof is 2% or
less.
25. The silicon substrate according to claim 23, wherein the
abruptness of the depth profile of boron is in the range of 1.7 to
2.7 nm/decade.
26. The silicon substrate according to claim 24, wherein the
abruptness of the depth profile of boron is in the range of 1.7 to
2.7 nm/decade.
27. A plasma doping apparatus for forming an impurity introduced
layer on a processing substrate, the plasma doping apparatus
comprising: a vacuum vessel having an inner wall on which a film
containing a first impurity is formed; a sample support which is
surrounded by the inner wall of the vacuum vessel and on which the
processing substrate is placed; a coil and antenna provided
opposite to the sample support and serving as a plasma source; a
first high-frequency power supply supplying a high frequency power
to the coil and antenna; and a second high-frequency power supply
providing a high-frequency power to the sample support, wherein a
gas containing the first impurity are introduced into the vacuum
vessel to generate successive plasma by the coil and antenna, the
first high-frequency power supply, and the second high-frequency
power supply.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a plasma doping method, a
plasma doping apparatus used therefor, and a silicon substrate
formed using the same. In particular, the invention relates to a
method of performing plasma doping that dopes an impurity into a
surface of a solid sample, such as a semiconductor substrate or the
like.
[0003] 2. Description of the related art
[0004] As a technology for doping an impurity into a surface of a
solid sample, there is known a plasma doping (PD) method that
ionizes the impurity and dopes the ionized impurity into a solid at
low energy (for example, see Patent Document 1).
[0005] Meanwhile, among methods of doping an impurity, an ion
implantation method is most widely used at present. As will be
apparent from Non-Patent Document 1, the plasma doping method is
also described in ITRS2003 as a next-generation technology of ion
implantation. The plasma doping method is different from the ion
implantation method.
[0006] A technical difference between ion implantation and plasma
doping will now be described in detail.
[0007] In the ion implantation method, an apparatus having the
following configuration is used. The apparatus includes an ion
source that generates plasma from gas, an analysis magnet that
performs mass separation in order to select desired ions among ions
extracted from the ion source, an electrode that accelerates the
desired ions, and a process chamber that implants the accelerated
desired ions into a silicon substrate. In the ion implantation, in
order to implant the impurity shallow, it is preferable to set
energy extracting ions from the ion source and acceleration energy
small.
[0008] However, when the extraction energy is set small, the number
of ions to be extracted is decreased. In addition, when the
acceleration energy is set small, while an ion beam is transported
from the ion source to a wafer, a beam diameter is widened due to a
repulsive force by charges between the ions. Accordingly, the ion
beam may collide against the inner wall of a beam line, and thus a
large number of ions may be lost. For this reason, throughput of an
implantation processing may be lowered. For example, when B+ ions
are implanted, if the acceleration energy becomes 2 keV or less,
the throughput starts to be lowered. Then, if the acceleration
energy becomes 0.5 keV or less, the beam transportation itself may
be difficult. Further, even though the acceleration energy is
lowered to 0.5 keV, the B ions may be implanted at a depth of
approximately 20 nm. That is, in case of forming an extension
electrode having a thinner thickness, productivity may be lowered
drastically.
[0009] In contrast, in the plasma doping method, an apparatus
having the following configuration is used. The apparatus includes
a plasma generation source that induces plasma into a cylindrical
vacuum chamber, in which a silicon substrate can be disposed, a
bias electrode, on which the silicon substrate is disposed, and a
bias power supply that adjusts a potential of the bias electrode.
That is, the apparatus has the configuration, in which the analysis
magnet and the acceleration electrode are not provided, different
from the apparatus used in the ion implantation. The bias electrode
that serves as a plasma source and a wafer holder is provided in
the vacuum chamber. Then, the ions are accelerated and introduced
by a potential to be generated between the plasma and the wafer.
With this configuration, since low-energy plasma can be directly
used, a large amount of low-energy ions can be irradiated onto the
wafer, compared with the ion implantation. That is, a dose rate is
considerably high. For this reason, in the low-energy B-ion
implantation, high throughput can be kept.
[0010] With the application of the plasma doping method, the
inventors have developed a process technology that forms a
source-to-drain extension electrode having a very small thickness
and low resistance. This new process technology is known as a
process technology that has particular effects (Non-Patent Document
2).
[0011] In this method, doping material gas, such as B.sub.2H.sub.6,
that is introduced from a gas introduction port is plasmized by a
plasma generation unit having a microwave waveguide and an electric
magnet. Then, boron ions in plasma are supplied to a surface of a
sample by a high-frequency power supply.
[0012] With the reduction in size and high integration of a
semiconductor device, characteristics in an impurity doped region
are very important. Among these, a dose (impurity doping amount)
determines low resistance that is one of important elements in
determining element characteristics. Accordingly, the control of
the dose is very important.
[0013] If the plasma doping method is used, it can be seen that the
source-to-drain extension electrode having a very small thickness
and low resistance can be formed. However, a control method of the
dose for controlling the element characteristics has not been
developed yet. Up to now, a method that changes the dose by
changing a plasma doping time has been tested, but this method does
not obtain sufficient control precision and thus is
unpractical.
[0014] In this situation, as a method that can increase safety by
diluting toxic B.sub.2H.sub.6 having a serious risk to the human
body as large as possible, can stably generate and keep plasma
without degrading doping efficiency, and can easily perform the
control of the dopant dose, the inventors has suggested the
following method. In this method, B.sub.2H.sub.6 gas as a material
containing an impurity to be doped is diluted with He gas having
small ionization energy, then He plasma is generated earlier, and
subsequently B.sub.2H.sub.6 is discharged (Patent Document 2). In
this method, there has been suggested that the concentration of
B.sub.2H.sub.6 gas is preferably less than 0.05%.
[0015] When the concentration is low, for example, approximately
0.05%, although it is not reported that the dose is easily
controlled, the dose is changed by changing the time while the gas
concentration is kept constant. That is, when the B.sub.2H.sub.6
gas concentration is low, the change in the dose is small with
respect to the change in time, and thus the dose is easily
controlled. Here, there is a progress in that the control precision
of the dose is increased. However, in a plasma doping method that
forms an impurity doped layer in the surface of the sample by
generating plasma in the vacuum chamber and causing impurity ions
in plasma to collide against the surface of the sample, the dose
may be changed each time plasma is irradiated onto the silicon
substrate, regardless of the same plasma condition, and
reproducibility may be degraded. This is because, even though
plasma is generated in the vacuum chamber in order to implant ions
into the silicon substrate, the state in the vacuum chamber is
changed for each time accordingly. Accordingly, it is difficult to
adjust the dose with good reproducibility. In addition, since the
state in the vacuum chamber is changed for each time, it is
difficult to keep an in-plane dose of the silicon substrate
uniformly. The dose can be made uniform by adjusting possible
parameters or the shape of the apparatus, but the uniform dose
cannot be repeatedly reproduced.
[0016] Patent Document 1: U.S. Pat. No. 4,912,065
(Specification)
[0017] Patent Document 2: JP-A-2004-179592
[0018] Patent Document 3: Japanese Patent No. 3340318
[0019] Non-Patent Document 1: Column of Shallow Junction Ion Doping
of FIG. 30 of Front End Process in International Technology Roadmap
for Semiconductors 2001 Edition (ITRS2001)
[0020] Non-Patent Document 2: Y. Sasaki, et al., Symp. on VLSI
Tech. p 180 (2004)
[0021] Non-Patent Document 3: B. Mizuno et al., Plasma Doping into
the side-wall of a sub-0.5 .mu.m width Trench, Ext. Abs. of
International Conference on SSDM, p. 317 (1987)
[0022] Non-Patent Document 4: B. Mizuno, et al., Plasma Doping for
silicon, Surface Coating tech., 85, 51 (1996)
[0023] Non-Patent Document 5: B. Mizuno, et al., Plasma Doping of
Boron for Fabricating the Surface Channel Sub-quarter micron
PMOSFET, symp. VLSI Tech, p. 66 (1996)
[0024] As described above, it is known that the state in the vacuum
chamber is changed for each time, but why the change occurs is not
clear. As the result of various studies, the inventors have focused
that a film is formed on an inner wall of a vacuum chamber as a
plasma chamber and the state of the film is changed. Specifically,
if a plasma doping treatment is repeated using mixture gas plasma
of B.sub.2H.sub.6 gas and helium gas, the color concentration of
the film and the formation area of the film are changed. That is,
the inventors have focused that the thickness of the film becomes
larger and the formation area of the film is increased. The
invention has been finalized from this viewpoint. The inventors
have supposed that the plasma concentration of the surface of the
sample is changed when the film containing the impurity fixed to
the inner wall of the vacuum chamber is attacked (sputtered) by
ions in plasma or the variation is changed by the thickness of the
film and the formation area of the film. In addition, the inventors
have supposed that the change depends on the density of the
impurity contained in a unit volume of the film.
[0025] According to the experiment result of the inventors, the
dose of the impurity that is doped into the surface of the silicon
substrate from a film containing the impurity fixed to the inner
wall of the vacuum chamber is changed for each time.
[0026] The invention has been finalized in consideration of the
above problems, and it is an object of the invention to provide a
plasma doping method that can make a dose from a film to a silicon
substrate for each time uniform even though a plasma processing is
repeated.
[0027] The invention performs dose control and in-plane uniformity
on the basis of a technical spirit that reverse the common
knowledge of plasma doping up to now. According to the common
knowledge of known plasma doping, the impurity is doped from ions,
gas, radicals in plasma, and the material is supplied from a gas
pipe connected to the vacuum chamber as gas. That is, the amount of
the impurity contained in gas, the gas concentration or pressure, a
mixture ratio of gas, and the like, determines the amount of the
impurity doped into the surface of the semiconductor substrate.
Accordingly, it is designed such that a plasma density or gas flow
and a pressure distribution are made uniform on the surface of the
semiconductor substrate. In addition, the adjustment of the dose is
performed by adjusting the concentration of the impurity contained
in gas to be supplied so as to adjust the concentration of the
impurity contained in plasma or by adjusting an irradiation time of
plasma.
[0028] In contrast, as the premise of the invention, it has been
focused that a ratio of an impurity that is supplied from the gas
pipe as gas, then plasmized, and subsequently doped into the
surface of the semiconductor substrate is merely 15% to 30% of the
total amount of the impurity to be doped by plasma doping. This is
a numeric value that reverses the known common knowledge. In the
related art, design of a process and the entire apparatus is
performed on the basis of the spirit that the dose from gas plasma
is a principal factor. In addition, in the invention, as the
principal factor corresponding to remaining 85% to 70%, there is
apparently the following phenomenon. That is, the film containing
the impurity formed to be fixed to the inner wall of the vacuum
chamber is exposed to plasma and sputtered while plasma doping is
repeatedly performed. Then, the impurity doped into the film once
is discharged in plasma again, and the discharged impurity is doped
into the surface of the semiconductor substrate. The inventors have
thought that a more accurate determination of the ratio of the
factors relative to the dose requires future studies, and depends
on the condition of plasma doping, but it is important to reduce
the ratio of the dose from plasma that is considered as the
principal factor in the related art. As considered in the related
art, even though the parameters of plasma are adjusted, it can be
seen that it is impossible to control the dose, stably keep
repetitive uniformity, and perform a process with good
reproducibility. That is, in order to control the dose, stably keep
repetitive uniformity, and perform a process with good
reproducibility, it is necessary to control a dose from the film
containing the impurity as the principal factor and to secure
stability. That is, it is necessary to adjust the film containing
the impurity fixed to the inner wall of the vacuum chamber.
[0029] According to an aspect of the invention, there is provided a
plasma doping method that places a sample on a sample electrode in
a vacuum chamber, generates plasma in the vacuum chamber, and
causes impurity ions in the plasma to collide against a surface of
the sample so as to form an impurity doped layer in the surface of
the sample. The plasma doping method includes a maintenance step of
preparing the vacuum chamber having a film containing an impurity
formed on an inner wall thereof such that, when the film containing
the impurity fixed to the inner wall of the vacuum chamber is
attacked by ions in the plasma, the amount of an impurity to be
doped into the surface of the sample by sputtering is not changed
even though the plasma containing the impurity ions is repeatedly
generated in the vacuum chamber, a step of placing the sample on
the sample electrode, and a step of irradiating the plasma
containing the impurity ions so as to implant the impurity ions
into the sample, and doping the impurity into the sample by
sputtering from the film containing the impurity fixed to the inner
wall of the vacuum chamber.
[0030] According to this configuration, the film is formed on the
inner wall of the vacuum chamber such that the amount of the
impurity to be doped into the sample by sputtering from the film
containing the impurity of the inner wall of the vacuum chamber is
not changed and then plasma doping is performed. Therefore,
impurity doping can be stably performed with good
reproducibility.
[0031] In the plasma doping method according to the aspect of the
invention, the maintenance step may include, before forming the
film containing the impurity, a substep of removing the film
containing the impurity fixed to the inner wall of the vacuum
chamber.
[0032] With this configuration, after the impurity stuck to the
inner wall of the vacuum chamber is removed once, the film
containing the impurity is formed again according to the
conditions. Therefore, reliability can be improved.
[0033] In the plasma doping method according to the aspect of the
invention, the sample may be a silicon substrate, and the film
containing the impurity fixed to the inner wall of the vacuum
chamber may be formed such that, when a dose of the impurity is the
same level with a tolerance of .+-.10% even though the plasma
containing the impurity ions is repeatedly generated in the vacuum
chamber, the dose is made uniform in a surface of the silicon
substrate.
[0034] With this configuration, the control can be performed with
high accuracy.
[0035] The plasma doping method according to the aspect of the
invention may further include a step of adjusting the shape of the
inner wall of the vacuum chamber such that the amount of an
impurity to be stuck to the inner wall of the vacuum chamber has a
desired value.
[0036] For example, the shape of the inner wall of the vacuum
chamber is adjusted such that, when the formation of the film is
completed, the total distribution of the distribution of the
impurity to be doped from the plasma containing the impurity ions
and the distribution of the impurity to be doped by sputtering from
the film containing the impurity fixed to the inner wall of the
vacuum chamber is made uniform in the surface of the silicon
substrate. It is important and more preferable to perform the
adjustment such that the concentration is made uniform when the
formation of the film is completed. In such a manner, uniform
plasma doping can be realized with good repeatability. When the
shape of the inner wall of the vacuum chamber is adjusted such that
the distribution of the dose of the impurity is made uniform before
the film is formed or when the film is being formed, since the
state of the inner wall of the vacuum chamber, that is, the state
of the film, is changed while plasma doping is repeated, it is
difficult to reproduce uniformity.
[0037] The plasma doping method according to the aspect of the
invention may further include a step of adjusting a gas supply
method such that the amount of an impurity to be stuck to the inner
wall of the vacuum chamber has a desired value.
[0038] For example, a film containing an impurity having a dark
color, that is, a large thickness is likely to be formed in the
vicinity of a gas jetting port. For this reason, the dose becomes
large in a portion near the gas jetting port and becomes small in a
portion distant from the gas jetting port. Accordingly, in-plane
uniformity of the dose can be improved by adjusting the gas jetting
port and the semiconductor substrate. For example, in-plane
uniformity can be improved by moving, for example, rotating the
semiconductor substrate with respect to the gas jetting port.
[0039] In the plasma doping method according to the aspect of the
invention, the maintenance step may include a substep of providing
the vacuum chamber, from which the film containing the impurity is
removed, in a plasma doping apparatus and then generating the
plasma containing the impurity ions in the vacuum chamber so as to
form the film containing the impurity ions.
[0040] According to this configuration, a high-accurate impurity
profile can be obtained with good controllability without using a
special device.
[0041] In the plasma doping method according to the aspect of the
invention, the step of forming the film containing the impurity
ions may provide the vacuum chamber, from which the film containing
the impurity is removed in the maintenance step, in a plasma doping
apparatus separately provided in order to form the film and may
generate the plasma containing the impurity ions in the vacuum
chamber so as to form the film containing the impurity ions.
[0042] According to this configuration, desired control is
performed using an additional device, and thus a high-accurate
impurity profile can be obtained with good controllability.
[0043] The plasma doping method according to the aspect of the
invention may further include a step of doping the impurity into
the sample by sputtering from the film containing the impurity
fixed to the inner wall of the vacuum chamber while measuring and
managing a temperature of the inner wall of the vacuum chamber.
[0044] With this configuration, it has been found that the amount
of the impurity to be doped from the film containing the impurity
into the semiconductor substrate is changed by the temperature of
the inner wall of the vacuum chamber. Accordingly, in order to keep
the amount of the impurity constant, it is preferable to keep the
temperature of the inner wall of the vacuum chamber constant.
Further, in order to set the amount of the impurity to be doped
from the film to a desired value, it is preferable to adjust the
temperature of the inner wall of the vacuum chamber to a desired
temperature.
[0045] Moreover, in the invention, a dummy chamber may be disposed
in the vacuum chamber to cover the inner wall, and a film may be
formed on the inner wall of the vacuum chamber. In vacuum
equipment, there are many cases where the dummy chamber is called
an inner chamber. In the invention, the formation of the film on
the inner wall of the vacuum chamber, the studies of the shape of
the inner wall, or the management of the temperature has been
described. However, as for the inner chamber, the same effects can
be obtained through the same studies. Therefore, the inner chamber
still falls within the scope of the invention. In addition, the
inner chamber does not have a function of holding the vacuum state,
but can be simply detached, easily cleaned, and used as consumption
goods. Accordingly, when the inner chamber is provided, it is
desirable in that, instead of detaching and cleaning an expensive
vacuum chamber, only the inner chamber can be detached and
cleaned.
[0046] In the plasma doping method according to the aspect of the
invention, the plasma may be plasma of gas containing boron.
[0047] According to this configuration, the boron film can be
formed on the inner wall of the vacuum chamber. In addition, it is
configured such that, when the film containing the impurity fixed
to the inner wall of the vacuum chamber is attacked by the ions in
the plasma, the amount of the impurity to be doped into the surface
of the sample by sputtering is not changed even though the plasma
containing the impurity ions are repeatedly generated in the vacuum
chamber. Therefore, a high-accurate impurity profile can be
obtained with good controllability, together with the impurity by
sputtering.
[0048] In the plasma doping method according to the aspect of the
invention, the gas containing boron may be gas of molecules having
boron and hydrogen.
[0049] As the gas, BF.sub.3 or the like may be used, but, since F
has a high sputter rate, it is difficult form a stable film. In
order to form the stable film, gas having an atom having a low
sputter rate is preferably used. In addition, if the sputter rate
is high, the surface of the silicon substrate may be chipped off
during the plasma doping treatment, a device cannot be manufactured
according to design. Further, since the surface of the silicon
substrate doped with the impurity is chipped off, impurity doping
itself may not be performed with good controllability. Since
hydrogen has a sputter rate less than F, if the gas of molecules
having boron and hydrogen is used, a high-accurate impurity profile
can be obtained with good controllability.
[0050] In the plasma doping method according to the aspect of the
invention, the gas containing boron may be diborane
(B.sub.2H.sub.6).
[0051] According to this configuration, B.sub.2H.sub.6 is
industrially cheap, and is filled in a gas tank to be then
transported and preserved in a gas state, which results in ease of
handling. In addition, since only boron and hydrogen are contained,
a sputter rate is low, and thus a high-accurate impurity profile
can be obtained with good controllability.
[0052] In the plasma doping method according to the aspect of the
invention, the plasma may be plasma of gas that is obtained by
diluting gas of molecules having boron and hydrogen with rare
gas.
[0053] According to this configuration, if the concentration of the
gas containing boron is excessively high, the film may be easily
separated. If the film is separated, particles may be generated to
cause degradation of yield in manufacturing a semiconductor, which
causes an inconvenience. Accordingly, if the gas concentration is
lowered through the dilution with a different gas, a film that is
rarely separated can be formed. As the dilution gas, rare gas
having chemical stability is preferably used.
[0054] In the plasma doping method according to the aspect of the
invention, the rare gas may be an atom having an atomic weight
equal to or less than neon.
[0055] Among the rare gases, rare gas having a large atomic weight
has a high sputter rate, and thus it is difficult to form a stable
film. Further, it may chip off the surface of the silicon
substrate. Therefore, the rare gas having an atomic weight smaller
than neon is preferably used.
[0056] In the plasma doping method according to the aspect of the
invention, the rare gas may be helium. In particular, helium has
the smallest atomic weight and the lowest sputter rate among the
rare gases. Therefore, a stable film is easily formed, and chipping
of the silicon substrate can be suppressed to the minimum.
[0057] In the plasma doping method according to the aspect of the
invention, the plasma may be plasma of gas that is obtained by
diluting diborane (B.sub.2H.sub.6) with helium.
[0058] It is most preferable to use the gas diluted with helium
such that the gas concentration of B.sub.2H.sub.6 becomes low.
[0059] In the plasma doping method according to the aspect of the
invention, an implantation depth of boron may be in a range of 7.5
mm to 15.5 mm.
[0060] From the experiment result, if implantation energy
corresponding to the implantation depth of boron ranging from 7.5
nm to 15.5 nm is used, it can be seen that a film containing boron
is formed on the inner wall of the vacuum chamber such that sheet
resistance is saturated. In addition, it can be seen that, when the
formation of the film is completed, good in-plane uniformity is
obtained.
[0061] In the plasma doping method according to the aspect of the
invention, an implantation depth of boron may be equal to or less
than 10 nm.
[0062] Under a low energy condition corresponding to the
implantation depth of boron equal to or less than 10 nm, it is very
difficult to obtain uniformity. However, from the experiment
result, it can be seen that, according to the method of-the
invention, uniformity of 1.5% or less can be realized by adjusting
the PD time.
[0063] In the plasma doping method according to the aspect of the
invention, the plasma may use continuous plasma.
[0064] According to this configuration, uniformity of 1.5% or less
can be realized by adjusting the PD time using the continuous
plasma. In general, in plasma doping, there are developed a
technology using continuous plasma and a technology using pulse
plasma. When the pulse plasma is used, it has been reported that,
in an implantation technology for a deep region more than
approximately 20 nm, not the implantation to a shallow region as
intended in the invention, uniformity and reproducibility are
secured by plasma doping. However, as for the implantation into the
shallow region, uniformity and reproducibility are insufficient. In
contrast, in the invention, from various experiment results, in
case of the implantation into the shallow region by the continuous
plasma, uniformity and reproducibility can be secured.
[0065] According to another aspect of the invention, a plasma
doping apparatus that performs the above-described plasma doping
method includes a vacuum chamber, a sample electrode, a gas supply
device that supplies gas into the vacuum chamber, an exhaust device
that exhausts the vacuum chamber, a pressure control device that
controls a pressure in the vacuum chamber, and a power supply for a
sample electrode that supplies power to the sample electrode.
[0066] With this configuration, reproducibility of the dose of
boron doped by plasma doping can be secured through the pressure
control using the pressure control device.
[0067] The plasma doping apparatus according to another aspect of
the invention may further include a plasma generation device that
forms the film containing the impurity.
[0068] With this configuration, the state of the inner wall of the
vacuum chamber can be easily controlled.
[0069] The plasma doping apparatus according to another aspect of
the invention may further include a mechanism that adjusts a flow
distribution of the gas to be supplied to the vacuum chamber such
that the flow distribution of the gas can be adjusted after the
film containing the impurity is formed, without exposing the inner
wall of the vacuum chamber to atmosphere.
[0070] With this configuration, a desired internal state can be
easily obtained in a short time without using an additional device
and separately providing a preparation time forming the vacuum.
[0071] The plasma doping apparatus according to another aspect of
the invention may further include a mechanism that adjusts a
temperature of the inner wall of the vacuum chamber to a desired
temperature.
[0072] The temperature control of the inner wall of the vacuum
chamber can be realized by measuring the temperature using a
temperature sensor and heating the inner wall using a heater.
According to the experiment of the inventors, if the experiment is
performed with no temperature control, the temperature of the inner
wall of the vacuum chamber is initially at a room temperature, but
it is increased to 40.degree. C. to 90.degree. C. when the plasma
doping treatment is repeated. The increased temperature depends on
the number of processing times or the conditions. Then, if the
plasma doping treatment ends, the temperature is gradually
decreased to the room temperature. That is, the temperature varies
when the plasma doping treatment starts and when the plasma doping
treatment is repeated. In addition, the temperature of the inner
wall of the vacuum chamber is affected by a difference from an
external temperature. Accordingly, it is preferable to adjust the
temperature of the inner wall to a temperature that the inner wall
naturally reaches when the plasma doping treatment is repeated, for
example, a desired temperature of 40.degree. C. to 90.degree. C.
Therefore, the amount of the impurity to be doped from the film can
be adjusted to a desired value. Further, it is more preferable to
adjust the temperature of the inner wall to a desired temperature
50.degree. C. to 70.degree. C. As a result, since the adjustment to
a temperature that the inner wall naturally reaches under more
plasma doping conditions can be performed, good repeatability can
be obtained.
[0073] According to still another aspect of the invention, there is
provided a silicon substrate that has a diameter 300 mm and into a
surface of which boron is doped by plasma doping using continuous
plasma containing boron. In this case, a profile of doped boron has
a depth ranging 7 nm to 15.5 nm with a boron concentration of
5.times.10.sup.18 cm.sup.-3, steepness of the depth profile of
boron is in a range of 1.5 nm/dec 3 nm/dec upon evaluation at a
distance where the boron concentration is lowered from
1.times.10.sup.19 cm.sup.-3 to 1.times.10.sup.18 cm.sup.-3, and a
dose of boron has a standard deviation of 2% or less at a surface
excluding an end 3 mm of the silicon substrate. Among many products
that can be manufactured using the method according to the
invention, when the above-described substrate is manufactured, the
following marked effects can be obtained. If boron is doped at the
depth of the above range, a very fine source-to-drain extension
electrode of a MOSFET of a 65 nm node to 22 nm node can be formed.
Further, when boron is doped at the steepness of the above range, a
very fin drain current of the MOSFET can be increased. When boron
is doped by plasma doping, the very fine electrode of the MOSFET
can be produced with good productivity. In addition, since in-plane
uniformity of the dose can be improved by the 300 mm substrate,
productivity and yield can be improved. Moreover, this is verified
by way of the example using a silicon substrate as the
semiconductor substrate. A germanium substrate or a strained
silicon substrate may be used since an atom used in the germanium
substrate or the strained silicon substrate has an atomic weight
not different only negligibly from the silicon atom and thus it can
be supposed that the same effects are obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0074] FIG. 1 is a diagram showing the relationship between the
number of substrates subjected to plasma doping and sheet
resistance;
[0075] FIG. 2 is a diagram showing repetitive reproducibility of
sheet resistance (a ratio of sheet resistance ranges from 0.5 to
4.0);
[0076] FIG. 3 is a diagram showing repetitive reproducibility of
sheet resistance (a ratio of sheet resistance ranges from 0.8 to
1.2);
[0077] FIG. 4 is a diagram showing repetitive reproducibility of
in-plane uniformity;
[0078] FIG. 5 is a diagram showing repetitive reproducibility of
in-plane uniformity;
[0079] FIG. 6 is a diagram showing the relationship between the
number of substrates subjected to plasma doping and sheet
resistance;
[0080] FIG. 7 is a diagram showing repetitive reproducibility of
sheet resistance;
[0081] FIG. 8 is a diagram showing repetitive reproducibility of
in-plane uniformity;
[0082] FIG. 9 is a diagram showing the relationship between a
plasma doping time, and dose and in-plane uniformity;
[0083] FIG. 10 is a diagram showing a SIMS profile of boron
immediately after plasma doping;
[0084] FIG. 11 is a diagram showing the comparison result between
uniformity of a plasma doping region obtained by an example of the
invention and uniformity of a plasma doping region obtained by a
comparative example;
[0085] FIG. 12 is a diagram showing the comparison result between
uniformity of a plasma doping region obtained by an example of the
invention and uniformity of a plasma doping region obtained by a
comparative example;
[0086] FIG. 13 is a diagram showing the comparison result between
uniformity of a plasma doping region obtained by an example of the
invention and uniformity of a plasma doping region obtained by a
comparative example;
[0087] FIG. 14 is a diagram showing a plasma doping apparatus
according to a first embodiment of the invention;
[0088] FIG. 15 is a diagram showing a chamber provided in a vacuum
chamber and a cover closing an opening for a transfer arm according
to the invention; and
[0089] FIG. 16 is a diagram showing a plasma doping apparatus used
for comparison of uniformity in the first embodiment of the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0090] Hereinafter, embodiments of the invention will be described
with reference to the drawings.
First Embodiment
[0091] Hereinafter, a first embodiment of the invention will be
described in detail with reference to the drawings.
[0092] FIG. 14 is a cross-sectional view of a plasma doping
apparatus that is used in the embodiment of the invention.
[0093] The plasma doping apparatus includes a vacuum chamber 15
that has a film containing an impurity formed on an inner wall
thereof, a turbo molecular pump 6 that serves as an exhaust device
for exhausting the vacuum chamber 15, a pressure regulating valve 7
that serves as a pressure control device for controlling a pressure
in the vacuum chamber 15, a coil and antenna 3 that serves as a
plasma source provided in the vicinity of a dielectric window
facing a lower electrode 14, a high-frequency power supply 12 that
supplies high-frequency power of 13.56 MHz to the coil and antenna
3, and a high-frequency power supply 1 that serves as a voltage
source for supplying a voltage to the lower electrode 14. In the
plasma doping apparatus, a substrate to be processed (substrate) 13
is placed on the lower electrode 14 serving as a sample table, and
plasma irradiation is performed onto the substrate 13.
[0094] Here, a high frequency is supplied from the coil and antenna
3 through the high-frequency power supply 1 for generating plasma
and the matching box 2 for adjusting the discharge. Required gas is
supplied through the mass flow controllers (MFC) 4 and 5. A degree
of vacuum in the vacuum chamber 15 is controlled by the mass flow
controllers 4 and 5, the turbo molecular pump 6, the pressure
regulating valve 7, and the dry pump 8. Power is supplied to the
vacuum chamber 15 from the high-frequency power supply 12 through
the matching box 11. The substrate 13 to be processed that is
provided in the vacuum chamber 15 is placed on the sample table 14,
and then the power is supplied.
[0095] Next, a plasma doping process will be described.
[0096] A predetermined gas is introduced from the gas supply device
into the vacuum chamber 15 of the process chamber through the mass
flow controllers 4 and 5, while gas exhaust is performed by the
turbo molecular pump 6 as an exhaust device. Further, the vacuum
chamber 15 is kept at a predetermined pressure by the pressure
regulating valve 7 as a pressure control device. Then,
high-frequency power of 13.56 MHz is supplied from the
high-frequency power supply 1 to the coil 3 as a plasma source,
such that inductively coupled plasma is generated in the vacuum
chamber 15. In this state, the silicon substrate 13 as a sample is
placed on the lower electrode 14. Further, high-frequency power is
supplied by the high-frequency power supply 12, and then the
potential of the lower electrode 14 can be controlled such that the
silicon substrate (the substrate to be processed) 13 as a sample
has a negative potential with respect to plasma.
[0097] First, plasma of gas containing an impurity is generated in
the vacuum chamber so as to form a film. For example, plasma doping
may be repeatedly performed on a dummy substrate under a constant
plasma doping condition. As the film is formed, the film is
attacked by ions in the plasma, and the amount of an impurity to be
doped into the surface of the silicon substrate by sputtering is
increased.
[0098] The increase reaches the saturation before long, and, under
the constant plasma doping condition, a dose of the impurity to be
doped by the plasma doping treatment one time is made uniform even
when plasma doping is repeated. For example, if doping is performed
on the silicon substrate multiple times under the constant plasma
doping condition, and the dose into the silicon substrate is made
uniform, it can be seen that the formation of the film is
completed. If the first dose to the substrate tends to be smaller
the final dose to the substrate, the formation of the film is not
completed, and thus the formation of the film is continued.
[0099] The shape of the inner wall of the vacuum chamber is
adjusted such that, when the formation of the film is completed,
the total distribution of the distribution of the impurity to be
doped from the plasma containing the impurity ions and the
distribution of the impurity to be doped by sputtering from the
film containing the impurity fixed to the inner wall of the vacuum
chamber is made uniform in the surface of the silicon
substrate.
[0100] It is important and more preferable to perform the
adjustment such that the distribution is made uniform when the
formation of the film is completed. In such a manner, uniform
plasma doping can be realized with good repeatability. When the
shape of the inner wall of the vacuum chamber is adjusted such that
the distribution of the dose of the impurity is made uniform before
the film is formed or when the film is being formed, since the
state of the inner wall of the vacuum chamber, that is, the state
of the film, is changed while the plasma doping treatment is
repeated, it is difficult to reproduce uniformity.
[0101] After the silicon substrate 13 is placed on the sample table
14 as the lower electrode, while the vacuum chamber 15 is
exhausted, helium gas is supplied into the vacuum chamber 15 by the
mass flow controller 4 and diborane (B.sub.2H.sub.6) gas as doping
material gas is supplied into the vacuum chamber 15 by the mass
flow controller 5. At this time, the pressure regulating valve 7 is
controlled such that the pressure of the vacuum chamber 15 is kept
at 0.9 Pa. Next, high-frequency power 1500 W is supplied to the
coil 3 as a plasma source so as to generate plasma in the vacuum
chamber 15. Further, high-frequency power 200 W is supplied to the
lower electrode 14 such that boron is implanted in the vicinity of
the surface of the silicon substrate 13. Here, plasma that is
exposed to the silicon substrate 13 is mixture gas plasma of
B.sub.2H.sub.6 and He (B.sub.2H.sub.6/He plasma). Moreover, the
mixture ratio of B.sub.2H.sub.6 and He can be changed by changing a
ratio of flow rates of He gas and B.sub.2H.sub.6 gas flowing in the
mass flow controllers 4 and 5.
[0102] If a bias is applied by irradiating the mixture gas plasma
of B.sub.2H.sub.6 and He (B.sub.2H.sub.6/He plasma) onto the
silicon substrate, there is a time when doping and sputtering of
boron to the silicon substrate are saturated (balanced). That is,
if plasma irradiation starts, the dose is initially increased, and
thereafter, a time when the dose is substantially uniform without
depending on the time variation is continued. Accordingly, the dose
can be accurately controlled through a process window of the time
when the dose is made substantially uniform without depending on
the time variation. Further, in-plane uniformity can be obtained by
previously measuring, in the surface of the silicon substrate, the
time when the dose is made uniform and setting the doping time
according to the latest start time.
[0103] The film containing the impurity may be formed after the
vacuum chamber is mounted after the maintenance of the plasma
doping apparatus. After the film is formed, the impurity is doped
under an actual plasma doping condition. In such a manner, a
process can be performed while the formed film is not exposed to
atmosphere. In case of the boron film, boron tends to act with
moisture. Accordingly, it is preferable to use the above-described
configuration since, according to the above-described
configuration, the film can be formed without acting with moisture
and then used in the process.
[0104] The film containing the impurity may be formed by providing
a vacuum chamber, from which the film containing the impurity is
removed through a maintenance step, in a plasma generation device
separately prepared in order to form the film, and generating the
plasma containing the impurity ions in the vacuum chamber. If a
plurality of vacuum chambers are prepared, the vacuum chamber to
which the film is attached in advance may be provided in the plasma
doping apparatus, thereby performing the plasma doping treatment,
while the film is being attached by the plasma generation device
separately prepared. Accordingly, while the film is being attached
to the inner wall of the vacuum chamber, the plasma doping
treatment can be performed on the silicon substrate, thereby
improving productivity.
EXAMPLE 1
[0105] Example 1 of the invention will be described. Moreover, when
there is not particular description, the following experiment
method is common to the individual examples. First, using the
plasma doping apparatus shown in FIG. 14, a boron film is formed in
the vacuum chamber using mixture gas plasma of B.sub.2H.sub.6 and
He. The plasma doping apparatus used herein is an apparatus that is
typically used. As the PD conditions, the gas mixture ratios of
B.sub.2H.sub.6 and He are 0.05% and 99.95%, source power is 1500 W,
bias power is 135 W, and a pressure is 0.9 Pa. A plasma doping time
when a bias is applied is 60 seconds. Under the constant
conditions, plasma doping is performed on a substrate having a
diameter 300 mm in the vacuum chamber immediately after the
maintenance. Then, a heat treatment is performed on the first,
25-th, 75-th, 125-th, 250-th, and 500-th processed silicon
substrates at 1075.degree. C. for 20 seconds. Thereafter, sheet
resistance at 121 places excluding an end 3 mm of the substrate are
measured by a four-probe method, and a standard deviation with
respect to the average value of sheet resistance is calculated.
In-plane uniformity is represented by the standard deviation
(1.sigma.) of sheet resistance. At this time, the reason why the
heat treatment is not performed on the second to 24-th substrates
is that the treatment is performed using a dummy silicon substrate.
The dummy silicon substrate means that, an actual silicon substrate
is used, but a new substrate is not used for each PD treatment and
the PD treatment is repeatedly performed on the same silicon
substrate approximately one hundred times. This is a measure that
saves consumption of the silicon substrate without having an affect
on the experiment for forming the film containing boron on the
inner wall of the vacuum chamber.
[0106] FIG. 1 shows the relationship between the number of
substrates subjected to the plasma doping treatment and sheet
resistance. Even though the treatment is performed under the same
condition, sheet resistance is initially high and then gradually
decreased as the number of processed substrates increases. As the
analysis result of the film formed on the inner wall of the vacuum
chamber after the experiment, it can be seen that boron is
contained in the film. It is considered that the film is formed
while the PD treatment is repeated and then sheet resistance is
decreased.
[0107] The average value of sheet resistance of the 75-th, 125-th,
250-th, and 500-th processed substrates is 236.1 ohm/sq. FIGS. 2
and 3 are diagrams having a ratio of sheet resistance when 236.1
ohm/sq is substituted with `1` as a vertical axis and the number of
substrates subjected to the PD treatment as a horizontal axis. FIG.
3 shows the ratio of sheet resistance ranging from 0.8 to 1.2.
Referring to FIG. 3, the decrease of sheet resistance is not
saturated, that is, the formation of the film is not saturated, in
case of the 25-th processed substrate. However, in case of the
75-th substrate or later, sheet resistance is converged on a
variation within a tolerance of .+-.5%, and the decrease of sheet
resistance is saturated. That is, it can be seen that the formation
of the film is saturated at the time of the PD treatment on the
75-th substrate. After the formation of the film is completed,
while sheet resistance varies around the average value within a
small range, the PD treatment can be performed continuously many
times.
[0108] Referring to FIG. 2, sheet resistance of the first substrate
subjected to the PD treatment is approximately 3.2 times as much as
average sheet resistance (236.1 ohm/sq) of the 75-th substrate and
later. It is possible to consider that the dose is substantially in
proportion to sheet resistance. Accordingly, it means that the dose
to the first substrate is only approximately 30% of the dose to the
75-th substrate or later. In case of the first substrate, a factor
that boron is doped entirely depends on B.sub.2H.sub.6 gas plasma
introduced in forms of gas, ions, and radicals through the
plarmization of boron contained in B.sub.2H.sub.6 gas introduced
from the gas introduction port. At this time, since the film
containing boron is not formed, the dose from the film is zero.
Meanwhile, after the 75-th substrate or later, a factor depends on
the film containing boron formed on the inner wall of the vacuum
chamber, in addition to the B.sub.2H.sub.6 gas plasma. Since the PD
condition is unchanged for the first substrate or the 75-th
substrate, the dose from the B.sub.2H.sub.6 gas plasma is not
changed. The changed is the dose from the film. Table 1 shows the
dose of the first substrate and the 75-th substrate and later by
factors. It can be seen that, after the 75-th substrate and later,
the dose from the film containing boron becomes a principal factor
at approximately 70%. The dose from the B.sub.2H.sub.6 gas plasma
occupies only approximately 30%. This is a new fact that reverses
the concept of plasma doping in the related art. From this, it has
been found that, in order to obtain in-plane uniformity of the dose
by plasma doping and repeatability, it is important to focus the
dose from the film containing boron. In addition, it has been found
that it is important and preferable to make the dose from the film
uniform in the surface of the semiconductor substrate, to improve
repetitive reproducibility, and to perform the adjustment to a
desired dose. TABLE-US-00001 TABLE 1 Dose from B.sub.2H.sub.6 Gas
Plasma Dose from Film (Normalization Containing Boron Sheet Ratio
with Dose of (Normalization Resistance of First with Dose of
(ohm/sq) Dose Substrate) First Substrate) First Substrate 756 1 1 0
(Comparative Example) 75-th Substrate 236.1 3.2 1 2.2 and Later
(Example)
[0109] FIG. 4 shows the relationship between the number of
substrates subjected to the plasma doping treatment and in-plane
uniformity of sheet resistance. It can be seen that the in-plane
uniformity is improved from 5.28% in the first substrate to a range
of 2% to 3% in the 25-th substrate and later. This is because the
shape of the inner wall of the vacuum chamber is adjusted such that
the in-plane uniformity is improved when the formation of the film
is completed. In the related art, the shape of the vacuum chamber
or the position of the coil is adjusted so as to make the plasma or
gas distribution uniform. This corresponds to a case where the
maximum in-plane uniformity is obtained in the first substrate.
However, in this example, the in-plane uniformity is neglected
since the film is not completely formed, and the shape of the inner
wall of the vacuum chamber is adjusted while focusing on the state
when the formation of the film is completed. As a result, the 25-th
substrate and later when the film is being formed has in-plane
uniformity superior to the first substrate. Further, in the 75-th
substrate and later when the formation of the film is completed,
markedly improved in-plane uniformity with respect to the first
substrate can be continuously realized with good repetitive
reproducibility.
[0110] On the basis of the experiment result, the plasma doping
apparatus, in which the shape of the inner wall of the vacuum
chamber after the maintenance is adjusted in advance such that
in-plane uniformity is improved when the formation of the film is
completed, is prepared, and plasma doping is performed. In
particular, it is preferable to adjust the shape of a side surface
and a top surface in the inner wall of the vacuum chamber.
Specifically, a chamber serving as an inner wall is provided in the
vacuum chamber. This chamber cannot be kept in a vacuum state. This
chamber is used to form a film on its surface. With this chamber,
it is unnecessary to clean the entire vacuum chamber upon the
maintenance, and only the above-described chamber may be removed
and cleaned. If a plurality of chambers are prepared, while one
chamber is cleaned, production can be continued using another
chamber, and thus efficiency can be improved. In addition, the
above-described chamber does not have a vacuum keeping capability
compared with the vacuum chamber, and thus the structure can be
simplified, and cleaning can be easily performed.
[0111] The shape of the chamber has been studied as follows. The
chamber is substantially symmetric as viewed from the center of the
silicon substrate. However, it is necessary to form an opening,
through which a transfer arm for transferring the silicon substrate
into the vacuum chamber enters and leaves the vacuum chamber. At
this portion, symmetry is drastically lacking. Accordingly, in
order to keep symmetry as much as possible, the inventors have
studied for making the area of the opening as small as possible to
an extent such that the silicon substrate and the transfer arm get
therethrough. As a result, as shown in FIG. 5, in the PD treatment
of the 100-th substrate and later when the formation of the film is
completed, very good uniformity of 2% or less is obtained.
[0112] Although the area of the opening is made small herein, when
plasma doping is performed, a mechanism that covers the opening may
be provided to close the opening, and the film may be formed after
the opening is covered. In this case, symmetry is further
increased. As a mechanism 20 that covers the opening 16, a
plate-shaped cover 17 may be fixed from the outside of the chamber
15, as shown in FIG. 15. As a simple structure, a driving unit may
be provided outside the chamber, and thus the opening may be
covered without causing particles. FIG. 15(a) is a diagram showing
when a semiconductor substrate is transferred. FIG. 15(b) is a
diagram showing a state where plasma doping is performed.
[0113] On the basis of the experiment result, in order to improve
the in-plane uniformity when the formation of the film is
completed, a plasma doping apparatus, in which a gas supply method
is adjusted in advance immediately after the maintenance, is
prepared, and plasma doping is performed. As the gas supply method,
a head plate is disposed to the silicon substrate, 18 holes are
opened in the head plate like a shower head, and gas is supplied
from the holes into the vacuum chamber. Then, the positions of the
holes are adjusted such that the in-plane uniformity is improved
when the formation of the film is completed. The plasma doping
apparatus used herein is shown in FIG. 16.
[0114] In FIG. 16, a predetermined gas is introduced from a gas
supply device 102 into a vacuum chamber 101 while gas exhaust is
performed by a turbo molecular pump 103 as an exhaust device. The
vacuum chamber 101 can be kept at a predetermined pressure by the
pressure regulating valve 4. Then, high-frequency power of 13.56
MHz is supplied to a coil 108 provided in the vicinity of a
dielectric window 107 facing a sample electrode 106 by a
high-frequency power supply 105, such that inductively coupled
plasma can be generated in the vacuum chamber 101. A silicon
substrate 109 as a sample is placed on the sample electrode 106.
Further, a high-frequency power supply 110 that supplies
high-frequency power to the sample electrode 106 is provided. The
high-frequency power supply 110 functions as a voltage source for
controlling the potential of the sample electrode 106 such that the
substrate 109 as the sample has a negative potential with respect
to the plasma. In such a manner, ions in the plasma are accelerated
and collide against the surface of the sample, such that the
impurity can be doped into the surface of the sample. Moreover, the
gas supplied from the gas supply device 102 is exhausted from an
exhaust port 111 to the pump 103.
[0115] A flow rate of gas containing impurity material gas is
controlled to a desired value by a flow rate control device (mass
flow controller) provided in the gas supply device 102. In general,
gas obtained by diluting the impurity material gas with helium, for
example, gas obtained by diluting diborane (B.sub.2H.sub.6) with
helium (He) at 0.5% is used as impurity material gas, and the flow
rate of the impurity material gas is controlled by a first mass
flow controller. Further, the flow rate of helium is controlled by
a second mass flow controller. The gases whose flow rates are
controlled by the first and second mass flow controllers are mixed
in the gas supply device 102, and then introduced to a gas main
path 114 through a pipe (gas introduction path) 113. Subsequently,
the mixture gas is introduced into the vacuum chamber 101 by gas
outlet ports 115 through a plurality of holes connected to the gas
main path. The plurality of gas outlet ports 115 are configured to
blow off the gas from a surface facing the sample 109 toward the
sample 109.
[0116] The gas outlet ports 115 are substantially provided
symmetrically with respect to the center of the dielectric window
107. The gas outlet ports 115 have a structure to substantially
isotropically blow off the gas toward the sample. That is, 24 gas
outlet ports 115 are substantially isotropically arranged.
Reference numeral 116 denotes a matching box, and reference numeral
117 denotes a VDC monitor.
[0117] As a result, as shown in FIG. 6, in the PD treatment of the
375-th substrate and the later, sheet resistant is stabilized. The
average value of sheet resistance of the 375-th substrate and later
is 220 ohm/sq. In the first substrate and the 375-th substrate and
later, the PD condition is not changed, and thus the dose from
B.sub.2H.sub.6 gas plasma is not changed. The change occurs in the
dose from the film. Table 2 shows the dose by factors. In the
375-th substrate and the later, the dose from the film occupies
approximately 85% and the B.sub.2H.sub.6 gas plasma occupies
approximately 15%. It can be seen that the dose from the film
becomes a principal factor. TABLE-US-00002 TABLE 2 Dose from
B.sub.2H.sub.6 Gas Plasma Dose from Film (Normalization Containing
Boron Sheet Ratio with Dose of (Normalization Resistance of First
with Dose of (ohm/sq) Dose Substrate) First Substrate) First 1493 1
1 0 Substrate (Comparative Example) 375-th 220 6.8 1 5.8 Substrate
and Later (Example)
[0118] FIG. 7 is a diagram showing repetitive reproducibility of
sheet resistance. Here, a ratio of sheet resistance ranging from
0.8 to 1.2 is shown. In the 375-th substrate and later, sheet
resistance is converged on a variation within a tolerance .+-.5%
from the average value, and a decrease of sheet resistance is
saturated. That is, it can be understood that the formation of the
film is saturated by the PD treatment around the 375-th substrate.
After the formation of the film is completed, while sheet
resistance varies around the average value within a small range,
the PD treatment can be performed continuously many times.
[0119] FIG. 8 is a diagram showing repetitive reproducibility of
in-plane uniformity. In view of the in-plane uniformity when the
formation of the film is completed, the gas supply method is
adjusted immediately after the maintenance. As a result, in the PD
treatment of the 375-th substrate and later when the formation of
the film is completed, very good uniformity of 1.85% or less is
obtained.
[0120] Next, in the PD treatment from the 1350-th substrate to the
1353-th substrate, a change in in-plane uniformity when the PD time
is changed is tested. The PD time means a time when a bias is
applied by irradiating plasma. The PD time is changed to 14
seconds, 45 seconds, and 100 seconds. Moreover, in a continuous
treatment before the 1350-th substrate and after the 1354-th
substrate, the PD time is 60 seconds. Then, as data of the
treatment for 60 seconds, data for the treatment of the 1375-th
substrate closest to the 1350-th substrate is referred to. FIG. 9
shows the relationship between the PD time and the dose. FIG. 9
also shows a change in in-plane uniformity. If plasma irradiation
starts, the dose is initially increased, and then the dose is
increased such that 1.7E15 cm.sup.-2 becomes an asymptotic line. It
is recognized that there is a time when the change of the dose is
very small without depending on the time variation. Through a
process window of the time, the dose can be accurately controlled.
It is more preferable to set a time when the dose occupies 70% or
more of the asymptotic line. Accordingly, the in-plane uniformity
can be further improved. It is still more preferable to set a time
when the dose is closer to the asymptotic line. Therefore, the
optimum in-plane uniformity can be obtained. Actually, when the PD
time is 100 seconds, the in-plane uniformity of 1.34% at 1.sigma.
can be realized, regardless of the same condition, excluding the
shape of the chamber, the gas supply method, and the PD time.
[0121] Among the results, the distribution results of sheet
resistance at 121 places excluding an end 3 mm of the 300 mm
substrate are shown in FIGS. 11 to 14. FIGS. 11(a) to 11(c) are
diagrams showing in-plane uniformity of the samples of the first
substrate in the comparative example, the 1000-th substrate in the
example, and the 1375-th substrate in the example, respectively.
FIGS. 12(a) to 12(b) are diagrams showing in-plane uniformity of
the samples of the first substrate and the 125-th substrate in the
comparative example, respectively. FIGS. 13(a) to 13(c) are
diagrams showing the results after 14 seconds in the comparative
example, after 60 seconds in the example, and after 100 seconds in
the example.
[0122] As the SIMS analysis result of the silicon substrate
immediately after the PD treatment, an implantation depth of boron
is 9.4 nm (see FIG. 10). This means that implantation energy of
boron is very low. Low energy corresponding to the implantation
depth of 10 nm or less is widely industrially used in the related
art. However, even in the ion implantation method capable of
realizing excellent uniformity, it is not easy to realize
uniformity of 2% or less over the entire surface excluding an end 3
mm of the 300 mm substrate. Further, in a plasma doping method that
is known to have a difficulty in realizing uniformity, a degree of
difficulty is still more severe. In contrast, according to the
invention, when the implantation depth is 10 nm or less, uniformity
of 2% or less can be realized over the entire surface excluding the
end 3 mm of the 300 mm substrate at the implantation depth of 10 nm
or less. In addition, when the PD time is adjusted, uniformity of
1.5% or less can be realized.
[0123] FIG. 10 shows a SIMS profile when a bias voltage in the
plasma doping apparatus used in the invention is changed. The
implantation depth of boron can be changed in a range of 7.5 nm to
15.5 nm. If an implantation energy range corresponds to at least
the above-described implantation depth, the film containing boron
can be formed on the inner wall of the vacuum chamber so as to
saturate sheet resistance. Further, the shape of the inner wall of
the vacuum chamber can be adjusted in advance such that the
in-plane uniformity is improved when the formation of the film is
completed. That is, through the same experiment, the fact that the
method according to the invention can be used similarly and
validity are confirmed.
[0124] Moreover, in the example, in order to form the film, the PD
condition that is actually used after the film formation is
repeated. However, the film may be formed a different condition
from the PD condition to be actually used. Specifically, although
the gas mixture ratios of B.sub.2H.sub.6 and He are 0.05% and 0.95%
in the example, the gas mixture ratios may be 0.1% and 99.9%. It is
preferable in that, if the B.sub.2H.sub.6 concentration is higher,
the film can be formed in a short time. However, if the
B.sub.2H.sub.6 concentration is increased to 5%, it is known that
the film is not stabilized, and sheet resistance and in-plane
uniformity are also unstable. It is preferable in that, if the
B.sub.2H.sub.6 concentration is increased within a range in which a
stable film can be formed, the film can be formed in a short time.
After the film is formed in such a manner, from a practical
viewpoint, it is preferable to adjust the B.sub.2H.sub.6
concentration such that the dose has a desired value, and
immediately perform actual plasma doping. The optimum
B.sub.2H.sub.6 concentration upon the film formation will be
studied in future, but it belongs to a typical design item that
still falls within the scope of the invention. In addition, while
the film is formed, the dummy silicon substrate may be disposed on
the sample electrode. After the formation of the film is completed,
preferably, the dummy substrate is removed from the sample
electrode, and then a substrate to be processed is placed and an
actual treatment starts. Accordingly, an extra substrate for the
formation of the film is not used. From this viewpoint, it is
efficient.
Comparative Example 1
[0125] Comparative Example 1 will be described with reference to
FIG. 4.
[0126] After the maintenance of the vacuum chamber, that is, after
the film is removed, the plasma condition, the gas supply method,
and the shape of the inner wall of the vacuum chamber are adjusted
such that in-plane uniformity of the dose is improved. Here, a
concept of the dose from the film is unconscious. This approach
does not have the maintenance step of preparing the vacuum chamber
having the film containing the impurity on the inner wall thereof
such that, when the film containing the impurity fixed to the inner
wall of the vacuum chamber is attacked by ions in plasma, the
amount of the impurity to be doped into the surface of the sample
by sputtering is not changed even though plasma containing the
impurity ions is repeatedly generated in the vacuum chamber. A
known approach that adopts in order to improve repeatability and
in-plane uniformity with plasma doping almost corresponds to the
above-described case.
[0127] As the result of trial and error under the same plasma
condition and other different conditions, in-plane uniformity of
1.5% can be realized by the treatment of the first substrate. Sheet
resistance is 455 ohm/sq. These are the results according to the
known method. However, in a state where the in-plane uniformity of
1.5% is realized on the first substrate, the plasma doping
treatment is repeatedly performed on 150 substrates under the same
PD condition, and then the silicon substrate is examined. As the
examination result, uniformity is 6.0%, and sheet resistance is 165
ohm/sq. It can be understood that repeatability of sheet resistance
is not obtained. In general, the level of uniformity that is to be
demanded for a practical use is 2% or less, and preferably, 1.5% or
less. From this, in Comparative Example 1, even though the state
where uniformity of 1.5% is obtained through the maintenance and
condition adjustment is arranged, the maintenance is required again
upon the treatment after 150 substrates.
[0128] In Comparative Example 1, in a state before the film is
formed immediately after the maintenance, the shape of the inner
wall of the vacuum chamber or the gas supply method is adjusted
such that the in-plane uniformity is improved. However, in this
method, the film containing boron is formed on the inner wall of
the vacuum chamber by plasma doping, and the state is changed.
Accordingly, good in-plane uniformity cannot be repeatedly and
stably obtained, and the same sheet resistance cannot be repeatedly
obtained. In contrast, in the example, there is provided the
maintenance step that prepares the vacuum chamber having the film
containing the impurity on the inner wall such that the dose from
the film is not changed even though the plasma is repeatedly
generated. Then, in a state after the film containing boron is
stably formed on the inner wall of the vacuum chamber, the shape of
the inner wall of the vacuum chamber or the gas supply method is
adjusted. With this difference, in the example, the following
marked effects can be obtained. That is, even though production is
repeated for a long time, in-plane uniformity can be kept to a
stable and good level. Further, sheet resistance is stabilized and
thus the same sheet resistance can be obtained.
Comparative Example 2
[0129] As Comparative Example 2, not immediately after the
maintenance, when the film is being formed, which is not intended
for convenience of the experiment, the gas supply method and the
shape of the inner wall of the vacuum chamber may be adjusted such
that the in-plane uniformity is improved. An experiment of
repeatability of sheet resistance may start. Typically, when gas
plasma containing an impurity is used for plasma doping, for
example, when mixture gas plasma of B.sub.2H.sub.6 and He is used,
a film is naturally formed even though an experimenter does not
intend to form the film. Comparative Example 2 corresponds to a
case where an experimenter who makes an experiment on a depth
control of an impurity upon plasma doping starts an experiment on
in-plane uniformity or repeatability without performing the
maintenance of the vacuum chamber for convenience for the
experiment. However, even though plasma is repeatedly generated in
the vacuum chamber, the formation of the film is incomplete.
Further, the maintenance step that prepares the vacuum chamber
having the film containing the impurity on the inner wall such that
the dose from the film is not changed is not provided. This is
because the formation of the film is incomplete. In this case,
repeatability of sheet resistance is not obtained because of the
same reason as Comparative Example 1.
Comparative Example 3
[0130] In Comparative Example 2, there may be a case where the
formation of the film is unintentionally completed. However, it is
difficult to increase repetitive stability of sheet resistance and
in-plane uniformity to a reliable level. When it is against the
intention of the invention, in particular, it is very difficult to
increase repeatability of in-plane uniformity.
[0131] The reason will be described. In a general plasma doping
apparatus, after the film is formed, it is difficult to adjust the
shape of the inner wall of the vacuum chamber and the gas supply
method. As for the adjustment of the gas supply method,
specifically, a position where the gas is supplied to the vacuum
chamber may be adjusted. In order to change the shape of the inner
wall of the vacuum chamber, a part of the inner wall needs to have
mechanical drivability.
[0132] However, if a complex mechanical movement to change the
shape of the inner wall is performed in the vacuum chamber,
particles are likely to be generated. The mechanical movement needs
to be as small as possible. If the inner wall corresponding to a
portion where the film is formed is moved, a part of the film is
separated, which results in particles. When the position where the
gas is supplied to the vacuum chamber is changed, a mechanical
movement for changing the position of the gas outlet port occurs.
For this reason, the particles are generated for the same
reason.
[0133] Meanwhile, it may be considered that the shape of the inner
wall of the vacuum chamber and the gas supply method are adjusted
by exposing the vacuum chamber to atmosphere, with no mechanical
movement. However, in this case, it is not preferable in that the
generation frequency of the particles clearly becomes high compared
with a case where production can be performed with no exposure to
atmosphere after the formation of the film. In addition, in the
case of the film containing boron, it is particularly difficult.
From the experiment result, it is known that the film containing
boron is likely to act with moisture. For this reason, upon the
exposure to atmosphere, the film acts with moisture in atmosphere,
the quality of the film is changed. Accordingly, even though the
vacuum chamber is vacuumized and the adjustment is performed for
plasma doping, performance of the film before the exposure to
atmosphere is not obtained. Accordingly, in the case of the film
containing boron, it is impossible to adjust the shape of the inner
wall of the vacuum chamber and the gas supply method by exposing
the vacuum chamber to atmosphere after the film is formed.
[0134] In Comparative Example 2, the shape of the inner wall of the
vacuum chamber or the gas supply method is adjusted such that
repeatability and in-plane uniformity are improved after the film
is unintentionally formed. However, in this method, when the film
is being formed, repeatability is not obtained. In addition, when
the formation of the film is completed, it is difficult to improve
in-plane uniformity. In contrast, in the example, in a state where
the film containing boron is stably formed on the inner wall of the
vacuum chamber, the shape of the inner wall of the vacuum chamber
or the gas supply method is adjusted in advance such that in-plane
uniformity is improved. With this difference, in the example, the
following marked effects can be obtained. That is, in-plane
uniformity and repeatability can be stably kept to a good level
without causing the particles.
[0135] When the film contains boron, the vacuum chamber cannot be
exposed to atmosphere. Accordingly, in the comparative example, it
is more difficult to adjust in-plane uniformity. Therefore, the
effects of the example more markedly exhibit. As such, like the
example, it is preferable to adjust the shape of the inner wall of
the vacuum chamber and the gas supply method in advance before the
film is formed such that in-plane uniformity is improved after the
film is formed.
Comparative Example 4
[0136] There is disclosed a technology that causes sputtering gas
to collide against a solid target containing an impurity in a
plasma state so as to fly the impurity out of the target, and dopes
the flown impurity into the surface of the sample (Patent Document
3). In the technology disclosed in Patent Document 3, a microwave
of 1 GHz or more is introduced into a vacuum chamber. In a known
sputtering apparatus, a material forming the target is a metal.
Accordingly, in a parallel flat-plate type plasma generation device
not having a plasma generation unit, such as ECR or the like,
plasma is also generated.
[0137] However, when a target containing boron is used, since boron
has high insulation, a generated electric field is diffused, and
thus plasma is rarely generated. Then, ECR18 is provided so as to
introduce a microwave of 1 GHz or more into the vacuum chamber. As
such, if the microwave of 1 GHz or more is introduced into the
vacuum chamber, high-density plasma of 1000 times as much as plasma
in the parallel flat-plate type plasma generation device is
generated. Accordingly, the impurity, such as boron or the like,
can be implanted into the surface of the silicon substrate in a
short time. For this reason, the temperature of the silicon
substrate is not increased to 300.degree. C. or more, and thus it
is possible to prevent a resist pattern formed on the silicon
substrate from being burned.
[0138] In this method, the solid target containing the impurity is
prepared and placed in the vacuum chamber. However, in this
comparative example, the maintenance that forms the film containing
the impurity such that the amount of an impurity to be doped into
the surface of the sample by sputtering is not changed even though
the plasma containing the impurity ions is repeatedly generated in
the vacuum chamber is not performed. Accordingly, it is difficult
to obtain in-plane uniformity or repeatability. The reason will be
described. Even though the solid target is disposed in the vacuum
chamber, if the plasma containing the impurity used in the example
(boron in the example) is repeatedly excited in the vacuum chamber,
a film containing boron is being formed in the surface of the solid
target. Further, the film containing boron is formed on the surface
of the inner wall at a portion of the inner wall of the vacuum
chamber that is not covered with the solid target. Then, repetitive
excitation is performed, and the formation of the film is saturated
and completed after the excitation for a certain time. Accordingly,
only by disposing the solid target in the vacuum chamber, the
maintenance step that prepares the vacuum chamber having the film
containing the impurity on the inner wall such that dose from the
film is not changed even though the plasma is repeatedly generated
in the vacuum chamber is not provided. As a result, it is difficult
to obtain repeatability of sheet resistance.
[0139] Further, although a method of creating a target is not
clearly described, in general, it is considered that the target is
not created in the vacuum chamber, and the externally created
target is provided in the vacuum chamber. However, in this method,
a solid containing an impurity is necessarily exposed to atmosphere
one time. Accordingly, the solid may act with oxygen in atmosphere,
and an oxide film may be formed in the surface of the film.
Further, the solid may act with moisture in atmosphere, and the
quality of the film may be changed. The oxidization of the film or
the action with moisture in atmosphere has a large affect on the
dose of the impurity after plasma doping and uniformity. Therefore,
even though the shape of the solid target containing the impurity
or the installment place thereof is designed such that in-plane
uniformity is temporarily improved, uniform plasma doping cannot be
performed as design due to a change in atmospheric humidity or
temperature. For the same reason, repetitive reproducibility of the
dose is not obtained. Moreover, in a clean room in which humidity
or temperature is managed, humidity and temperature change a little
every day, and thus it is impossible to suppress oxidization or
action with moisture.
[0140] In contrast, in the invention, the film containing the
impurity is formed on the inner wall such that the dose from the
film containing the impurity fixed to the inner wall of the vacuum
chamber is not changed even though the plasma is repeatedly
generated. Accordingly, the marked effects, for example, repetitive
reproduction of the dose, can be obtained. Further, in the
invention, the film containing the impurity fixed to the inner wall
of the vacuum chamber is formed such that, when a dose of the
impurity is the same level with a tolerance of .+-.10% even though
the plasma containing the impurity ions is repeatedly generated in
the vacuum chamber, the dose is made uniform in a surface of the
silicon substrate. Accordingly, while good in-plane uniformity of
the dose is kept, repetitive reproducibility can be obtained. In
addition, in the invention, the step of forming the film containing
the impurity is performed by providing the vacuum chamber, from
which the film containing the impurity is removed in the
maintenance step, in the plasma doping apparatus and generating the
plasma containing the impurity ions in the vacuum chamber.
Accordingly, the film containing the impurity can be used for
plasma doping with no exposure to atmosphere. Therefore, there is
no affect of atmospheric humidity or temperature, and repetitive
reproducibility can be secured over the entire surface of the 300
mm substrate excluding the end 3 mm. In addition, very uniform
plasma doping of the impurity can be performed with the standard
deviation of 1.7%, and, if the PD time is adjusted, 1.5% or less.
TABLE-US-00003 TABLE 3 Repetitive Exposure to Stability of
Atmosphere of Repetitive In-Plane Film (solid) Stability of
Uniformity Containing Dose When PD When PD is Impurity is Repeated
Repeated Example No Comparative No x x Example 1 Comparative No
.DELTA. x Example 2 Comparative Exposure x x Example 4
[0141] (Example) A case where, before the film is formed, the shape
of the inner wall of the vacuum chamber and the gas supply method
are adjusted in advance such that in-plane uniformity is improved
after the film is formed
[0142] (Comparative Example 1) A case where, before the film is
formed, the plasma condition, the gas supply method, and the shape
of the inner wall of the vacuum chamber are adjusted such that
in-plane uniformity of the dose is improved
[0143] (Comparative Example 2 (3)) A case where, when the film is
unintentionally being formed, the gas supply method and the shape
of the inner wall of the vacuum chamber are adjusted such that
in-plane uniformity of the dose is improved
[0144] (Comparative Example 4) A technology that causes the
sputtering gas to collide against the solid target containing the
impurity in a plasma state so as to fly the impurity out of the
target, and dopes the flown impurity into the surface of the
sample
[0145] Plasma doping has attracted attention for 20 years since a
group including one of the inventors had stated an impurity
implantation method to a silicon trench in 1986 to 1987 (Non-Patent
Document 3). Thereafter, in a technical field for forming a shallow
junction, a MOS device fabricated using plasma doping had stated in
1996 (Non Patent Documents 4 and 5). For 10 years since then,
plasma doping has attracted attention and would be expected to be
put to practical use. However, plasma doping has been yet far from
practical use. One of the problems as the obstruction to practical
use was repetitive stability of the dose and in-plane uniformity.
The invention is to solve this problem.
[0146] In the case of Comparative Examples 1, 2, and 3, even though
the design items are arranged through the combination thereof,
sufficient repetitive stability of the dose and in-plane uniformity
when plasma doping is repeated is not obtained. Meanwhile, in the
example, repetitive stability of the dose and in-plane uniformity
is obtained. A resolution method of repetitive stability of the
dose and in-plane uniformity described herein is not easily made
through the combination of the comparative examples and also not
the design items of the apparatus technology. This method is an
unprecedented method and has marked effects.
[0147] According to the plasma doping method of the invention, it
is possible to realize a plasma doping method that can control an
impurity concentration profile with high accuracy and can form a
shallow impurity diffusion region. In addition, the plasma doping
method of the invention can be applied to the use, such as an
impurity doping process of a semiconductor or manufacturing of a
thin film transistor used in liquid crystal or the like.
* * * * *