U.S. patent application number 11/137673 was filed with the patent office on 2006-01-05 for plasma processing method and apparatus.
Invention is credited to Taichi Hirano, Jun Hirose, Hiroyuki Ishihara, Akira Koshiishi, Kohji Numata, Masahiro Ogasawara, Jun Ooyabu, Michishige Saito, Hiromitsu Sasaki, Tetsuo Yoshida.
Application Number | 20060000803 11/137673 |
Document ID | / |
Family ID | 32396266 |
Filed Date | 2006-01-05 |
United States Patent
Application |
20060000803 |
Kind Code |
A1 |
Koshiishi; Akira ; et
al. |
January 5, 2006 |
Plasma processing method and apparatus
Abstract
A plasma processing method is arranged to supply a predetermined
process gas into a plasma generation space in which a target
substrate is placed, and turn the process gas into plasma. The
substrate is subjected to a predetermined plasma process by this
plasma. The spatial distribution of density of the plasma and the
spatial distribution of density of radicals in the plasma are
controlled independently of each other relative to the substrate by
a facing portion opposite the substrate to form a predetermined
process state over the entire target surface of the substrate.
Inventors: |
Koshiishi; Akira;
(Nirasaki-shi, JP) ; Hirose; Jun; (Nirasaki-shi,
JP) ; Ogasawara; Masahiro; (Nirasaki-shi, JP)
; Hirano; Taichi; (Nirasaki-shi, JP) ; Sasaki;
Hiromitsu; (Kurihara-gun, JP) ; Yoshida; Tetsuo;
(Nirasaki-shi, JP) ; Saito; Michishige;
(Nirasaki-shi, JP) ; Ishihara; Hiroyuki;
(Nirasaki-shi, JP) ; Ooyabu; Jun; (Nirasaki-shi,
JP) ; Numata; Kohji; (Kyoto-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
32396266 |
Appl. No.: |
11/137673 |
Filed: |
May 26, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP03/15029 |
Nov 25, 2003 |
|
|
|
11137673 |
May 26, 2005 |
|
|
|
Current U.S.
Class: |
216/67 ;
156/345.47 |
Current CPC
Class: |
H01J 37/32082 20130101;
H01J 37/32422 20130101; H01J 37/32935 20130101 |
Class at
Publication: |
216/067 ;
156/345.47 |
International
Class: |
C23F 1/00 20060101
C23F001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 26, 2002 |
JP |
2002-341949 |
Oct 17, 2003 |
JP |
2003-358432 |
Claims
1. A plasma processing method comprising: supplying a predetermined
process gas into a plasma generation space in which a target
substrate is placed, and turning the process gas into plasma; and
subjecting the substrate to a predetermined plasma process by the
plasma, wherein spatial distribution of density of the plasma and
spatial distribution of density of radicals in the plasma relative
to the substrate are controlled independently of each other by a
set of first and second RF discharge regions and a set of first and
second process gas delivery regions, respectively, to form a
predetermined process state over an entire target surface of the
substrate, and wherein the set of first and second RF discharge
regions and the set of first and second process gas delivery
regions are disposed in layouts independently of each other.
2. The method according to claim 1, wherein a facing portion
opposite the substrate is divided into two regions as the first and
second RF discharge regions on a peripheral side and a central
side, respectively, in a radial direction relative to a center
through which a vertical line extending from a center of the
substrate passes; and the second RF discharge region on the facing
portion is divided into two regions as the first and second process
gas delivery regions on a peripheral side and a central side,
respectively, in the radial direction.
3. The method according to claim 2, wherein the first RF discharge
region is disposed radially outside an outer peripheral edge of the
substrate.
4. The method according to claim 1, wherein an RF output from a
single RF power supply is divided at a predetermined ratio, and
thereby discharged from the first RF discharge region and the
second RF discharge region.
5. The method according to claim 1, wherein a process gas supplied
from a single process gas supply source is divided at a
predetermined ratio, and thereby delivered from the first process
gas delivery region and the second process gas delivery region.
6. The method according to claim 5, wherein the process gas is
delivered from the first and second process gas delivery regions at
deferent flow rates per unit area.
7. The method according to claim 1, wherein the process gas is a
mixture gas of a plurality of gases, the plurality of gases are
delivered from the first process gas delivery region at a first gas
mixture ratio, and the plurality of gases are delivered from the
second process gas delivery region at a second gas mixture ratio
different from the first gas mixture ratio.
8. A plasma processing method comprising: exposing a target
substrate to plasma of a predetermined process gas; and subjecting
the substrate to a predetermined plasma process by the plasma,
wherein spatial distribution of density of the plasma and spatial
distribution of density of radicals in the plasma are controlled
independently of each other relative to the substrate to form a
predetermined process state over an entire target surface of the
substrate, and wherein processing rates at respective positions on
the target surface of the substrate are mainly controlled in
accordance with the plasma density spatial distribution, and one or
both of processing selectivity and processing shapes at respective
positions on the target surface are mainly controlled in accordance
with the radical density spatial distribution.
9. The method according to claim 1, wherein the first and second RF
discharge regions comprise an RF electrode to be supplied with an
RF.
10. A plasma processing apparatus arranged to turn a process gas
into plasma in a plasma generation space within a process container
configured to have a vacuum atmosphere therein, and subject a
target substrate placed within the plasma generation space to a
predetermined plasma process, the apparatus comprising: a plasma
density control section configured to control spatial distribution
of density of the plasma relative to the substrate; and a radical
density control section configured to control spatial distribution
of density of radicals in the plasma relative to the substrate
independently of the plasma density spatial distribution, wherein
the plasma density control section comprises a set of first and
second RF discharge regions configured to control the plasma
density spatial distribution, the radical density control section
comprises a set of first and second process gas delivery regions
configured to control the radical density spatial distribution, and
the set of first and second RF discharge regions and the set of
first and second process gas delivery regions are disposed in
layouts independently of each other.
11. The apparatus according to claim 10, wherein a facing portion
opposite the substrate and in contact with the plasma generation
space is divided into two regions as the first and second RF
discharge regions on a peripheral side and a central side,
respectively, in a radial direction relative to a center through
which a vertical line extending from a center of the substrate
passes; and the second RF discharge region on the facing portion is
divided into two regions as the first and second process gas
delivery regions on a peripheral side and a central side,
respectively, in the radial direction.
12. The apparatus according to claim 11, wherein the first RF
discharge region is disposed radially outside an outer peripheral
edge of the substrate.
13. The apparatus according to claim 10, wherein the process gas is
a mixture gas of a plurality of gases, the plurality of gases are
delivered from the first process gas delivery region at a first gas
mixture ratio, and the plurality of gases are delivered from the
second process gas delivery region at a second gas mixture ratio
different from the first gas mixture ratio.
14. The apparatus according to claim 10, wherein an RF power is
discharged toward the plasma space from the first and second RF
discharge regions at a predetermined ratio; and the process gas is
delivered toward the plasma space from the first and second process
gas delivery regions at a predetermined ratio.
15. The apparatus according to claim 14, wherein the plasma density
control section comprises an RF distributor configured to divide
and transmit an RF with a constant frequency, output from an RF
power supply, at a predetermined ratio to the first and second RF
discharge regions; and the radical density control section
comprises a process gas distributor configured to divide and supply
the process gas, output from a process gas supply source, at a
predetermined ratio to the first and second process gas delivery
regions.
16. The apparatus according to claim 15, wherein the RF distributor
includes an impedance control section configured to variably
control one or both of an impedance of a first feed circuit from
the RF power supply to the first RF discharge region, and an
impedance of a second feed circuit from the RF power supply to the
second RF discharge region.
17. The apparatus according to claim 10, wherein the first and
second RF discharge regions respectively comprise first and second
electrodes electrically insulated from each other.
18. The apparatus according to claim 17, wherein the first and
second process gas delivery regions include a number of process gas
delivery holes formed on the second electrode.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a Continuation Application of PCT Application No.
PCT/JP03/15029, filed Nov. 25, 2003, which was published under PCT
Article 21(2) in Japanese.
[0002] This application is based upon and claims the benefit of
priority from prior Japanese Patent Applications No. 2002-341949,
filed Nov. 26, 2002; and No. 2003-358432, filed Oct. 17, 2003, the
entire contents of both of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates to a technique for subjecting
a target substrate to a plasma process, and specifically to a
plasma processing technique for processing a substrate, using
radicals and ions derived from plasma. Particularly, the present
invention relates to a plasma processing technique utilized in a
semiconductor process for manufacturing semiconductor devices. The
term "semiconductor process" used herein includes various kinds of
processes which are performed to manufacture a semiconductor device
or a structure having wiring layers, electrodes, and the like to be
connected to a semiconductor device, on a target substrate, such as
a semiconductor wafer or a glass substrate used for an LCD (Liquid
Crystal Display) or FPD (Flat Panel Display), by forming
semiconductor layers, insulating layers, and conductive layers in
predetermined patterns on the target substrate.
[0005] 2. Description of the Related Art
[0006] In manufacturing semiconductor devices and FPDs, plasma is
often used for processes, such as etching, deposition, oxidation,
and sputtering, so that process gases can react well at a
relatively low temperature. Parallel-plate plasma processing
apparatuses of the capacitive coupling type are in the mainstream
of plasma processing apparatuses of the single substrate type.
[0007] In general, a parallel-plate plasma processing apparatus of
the capacitive coupling type includes a process container or
reaction chamber configured to reduce the pressure therein, and an
upper electrode and a lower electrode disposed therein in parallel
with each other. The lower electrode is grounded and configured to
support a target substrate (semiconductor wafer, glass substrate,
or the like) thereon. The upper electrode and/or lower electrode
are supplied with RF voltage through a matching unit. At the same
time, a process gas is delivered from a showerhead provided on the
upper electrode side. Electrons are accelerated by an electric
field formed between the upper electrode and lower electrode and
collide with the process gas, thereby ionizing the gas and
generating plasma. Neutral radicals and ions derived from the
plasma are used to perform a predetermined micro-fabrication on the
surface of the substrate. In the process described above, the two
electrodes function to form a capacitor.
[0008] The majority of ions in the plasma are positive ions, and
the number of positive ions is almost the same as that of
electrons. The density of the ions or electrons is far smaller than
the density of neutral particles or radicals. In general, plasma
etching is arranged to cause radicals and ions to act on the
substrate surface at the same time. Radicals perform isotropic
etching on the substrate surface by means of chemical reactions.
Ions are accelerated by an electric field and vertically incident
on the substrate surface, and perform vertical (anisotropic)
etching on the substrate surface by means of physical actions.
[0009] Conventional plasma processing apparatuses are arranged to
cause radicals and ions generated in plasma to act on the substrate
surface with the same density distribution. In other words, where
the radical density is higher at the substrate central portion than
at the substrate peripheral portion, the ion density (i.e.,
electron density or plasma density) is also higher at the substrate
central portion than at the substrate peripheral portion.
Particularly, in parallel-plate plasma processing apparatuses
described above, if the frequency of the RF applied to the upper
electrode is set higher, when the RF is supplied from an RF power
supply through a feed rod to the electrode backside, it is
transmitted through the electrode surface by means of the skin
effect and is concentrated at the central portion of the electrode
bottom surface (plasma contact surface). As a consequence, the
electric field intensity at the central portion of the electrode
bottom surface becomes higher than the electric field intensity at
the peripheral portion, so both the radical density and ion density
(electron density) become higher at the electrode central portion
than at the electrode peripheral portion. However, if radicals and
ions are always limited or restricted to such a relationship that
they have the same distribution in acting on the substrate surface,
it is difficult to perform a predetermined plasma process on the
substrate, and it is particularly difficult to improve the
uniformity in process state or process result.
BRIEF SUMMARY OF THE INVENTION
[0010] An object of the present invention is to provide a plasma
processing apparatus and method, which can optimize a plasma
process in which radicals and ions act on a target substrate at the
same time.
[0011] According to a first aspect of the present invention, there
is provided a plasma processing method comprising: [0012] exposing
a target substrate to plasma of a predetermined process gas; and
[0013] subjecting the substrate to a predetermined plasma process
by the plasma, [0014] wherein spatial distribution of density of
the plasma and spatial distribution of density of radicals in the
plasma are controlled independently of each other relative to the
substrate to form a predetermined process state over an entire
target surface of the substrate.
[0015] According to a second aspect of the present invention, there
is provided a plasma processing apparatus arranged to turn a
process gas into plasma in a plasma generation space within a
process container configured to have a vacuum atmosphere therein,
and subject a target substrate placed within the plasma generation
space to a predetermined plasma process, the apparatus comprising:
[0016] a plasma density control section configured to control
spatial distribution of density of the plasma relative to the
substrate; and [0017] a radical density control section configured
to control spatial distribution of density of radicals in the
plasma relative to the substrate independently of the plasma
density spatial distribution.
[0018] According to the first and second aspects, the spatial
distribution of plasma density (i.e., electron density or ion
density) and the spatial distribution of radical density are
controlled independently of each other relative to the target
substrate to optimize the balance or synergy between radical base
etching and ion base etching.
[0019] In order to achieve this, the facing portion opposite the
target substrate may comprise first and second RF discharge regions
configured to control the plasma density spatial distribution, and
first and second process gas delivery regions configured to control
the radical density spatial distribution, in layouts independently
of each other. In this case, by adjusting the balance (ratio) of
the RF electric field intensity or input power between the first
and second RF discharge regions, the spatial distribution of plasma
density (ion density) can be controlled. Further, by adjusting the
balance (ratio) of the gas flow rate between the first and second
process gas delivery regions, the spatial distribution of radical
density can be controlled. If the first and second RF discharge
regions respectively agree with or correspond to the first and
second process gas delivery regions, change in the input power
ratio affects the spatial distribution of radical density, while
change in the gas flow-rate ratio affects the spatial distribution
of plasma density (ion density). By contrast, where the division
layout of the RF discharge regions and the division layout of the
process gas delivery regions are independent of each other, such an
interlinking relationship is cut off, so that the plasma density
distribution and radical density distribution can be controlled
independently of each other.
[0020] In one design according to this independent type layout, the
facing portion may be divided into two regions as the first and
second RF discharge regions on a peripheral side and a central
side, respectively, in a radial direction relative to a center
through which a vertical line extending from a center of the target
substrate passes. Further, the second RF discharge region on the
facing portion may be divided into two regions as the first and
second process gas delivery regions on a peripheral side and a
central side, respectively, in the radial direction. More
preferably, the first RF discharge region is disposed radially
outside the outer peripheral edge of the target substrate.
[0021] With this layout, the control over the plasma density
spatial distribution performed by adjusting the ratio of electric
field intensity or input power between the first and second RF
discharge regions does not have a substantial influence on the
control over the radical density spatial distribution performed by
adjusting the ratio of process gas flow rate between the first and
second process gas delivery regions. Specifically, the process gas
delivered from the first and second process gas delivery regions is
dissociated within an area corresponding to the second RF discharge
region. Thus, where the balance of electric field intensity or
input power between the first and second RF discharge regions is
changed, the balance of radical generation amount or density
between the first and second process gas delivery regions is not
substantially affected. As a consequence, the plasma density
spatial distribution and radical density spatial distribution can
be controlled independently of each other.
[0022] In one design, an RF output from a single RF power supply
may be divided at a predetermined ratio, and thereby discharged
from the first RF discharge region and the second RF discharge
region. Further, a process gas supplied from a single process gas
supply source may be divided at a predetermined ratio, and thereby
delivered from the first process gas delivery region and the second
process gas delivery region. In this case, the process gas may be
delivered from the first and second process gas delivery regions at
substantially deferent flow rates per unit area. Where the process
gas is a mixture gas of a plurality of gases, the plurality of
gases may be delivered from the first process gas delivery region
at a first gas mixture ratio, and the plurality of gases may be
delivered from the second process gas delivery region at a second
gas mixture ratio different from the first gas mixture ratio.
[0023] In one design, processing rates at respective positions on
the target surface of the target substrate may be mainly controlled
in accordance with the plasma density spatial distribution.
Further, one or both of processing selectivity and processing
shapes at respective positions on the target surface of the target
substrate may be mainly controlled in accordance with the radical
density spatial distribution.
[0024] In the plasma processing apparatus according to the second
aspect, the plasma density control section may comprise an RF
distributor configured to divide and transmit an RF with a constant
frequency, output from an RF power supply, at a predetermined ratio
to the first and second RF discharge regions. The radical density
control section may comprise a process gas distributor configured
to divide and supply the process gas, output from a process gas
supply source, at a predetermined ratio to the first and second
process gas delivery regions. In this case, the RF distributor
preferably includes an impedance control section configured to
variably control one or both of an impedance of a first feed
circuit from the RF power supply to the first RF discharge region,
and an impedance of a second feed circuit from the RF power supply
to the second RF discharge region. The first and second RF
discharge regions may respectively comprise first and second
electrodes electrically insulated from each other. The first and
second process gas delivery regions preferably include a number of
process gas delivery holes formed on the second electrode.
[0025] According to the first and second aspects, it is possible to
optimize a plasma process arranged to cause radicals and ions to
act on a target substrate at the same time.
[0026] Additional objects and advantages of the invention will be
set forth in the description which follows, and in part will be
obvious from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out hereinafter.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0027] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate embodiments of
the invention, and together with the general description given
above and the detailed description of the embodiments given below,
serve to explain the principles of the invention.
[0028] FIG. 1 is a sectional side view showing a plasma etching
apparatus according to a first embodiment of the present
invention;
[0029] FIG. 2 is an enlarged partial side view showing a main part
of the plasma etching apparatus shown in FIG. 1;
[0030] FIG. 3 is a circuit diagram showing an equivalent circuit of
a main part of plasma generating means according to the first
embodiment;
[0031] FIG. 4 is a graph showing distribution characteristics of
electric field intensity (relative value), obtained by a function
of adjusting the balance of electric field intensity according to
the first embodiment;
[0032] FIG. 5 is a graph showing ratio characteristics of electric
field intensity, obtained by a function of adjusting the balance of
electric field intensity according to the first embodiment;
[0033] FIGS. 6A and 6B are graphs showing spatial distribution
characteristics of electron density according to the first
embodiment;
[0034] FIGS. 7A and 7B are graphs showing spatial distribution
characteristics of etching rate according to the first
embodiment;
[0035] FIG. 8 is a sectional side view showing a plasma etching
apparatus according to a second embodiment of the present
invention;
[0036] FIGS. 9A and 9B are graphs showing spatial distribution
characteristics of etching rate according to the second
embodiment;
[0037] FIGS. 10A and 10B are graphs showing spatial distribution
characteristics of etching rate according to the second
embodiment;
[0038] FIG. 11 is a graph showing characteristics of variable
capacitance vs. inner input power according to the second
embodiment;
[0039] FIG. 12 is a circuit diagram showing an equivalent circuit
of a plasma generation RF feed circuit according to the second
embodiment;
[0040] FIG. 13 is a view showing an effect of a conductive member
disposed around an upper feed rod according to the second
embodiment;
[0041] FIG. 14 is a graph showing characteristics of variable
capacitance vs. bottom self-bias voltage according to the second
embodiment;
[0042] FIGS. 15A and 15B are circuit diagrams each showing the
circuit structure of a low-pass filter according to the second
embodiment;
[0043] FIG. 16 is a diagram showing an effect of resistance
provided in a low-pass filter according to the second
embodiment;
[0044] FIG. 17 is a graph showing the optimum range of resistance
provided in a low-pass filter according to the second
embodiment;
[0045] FIG. 18 is a sectional side view showing a main part of the
plasma etching apparatus according to the second embodiment;
[0046] FIGS. 19A to 19E are graphs showing spatial distribution
characteristics of electron density, using as parameters the inner
diameter and protruded length of an upper electrode protrusion
according to the second embodiment;
[0047] FIGS. 20A and 20B are graphs showing characteristic lines of
electron density uniformity, using as two-dimensional parameters
the inner diameter and protruded length of an upper electrode
protrusion according to the second embodiment;
[0048] FIG. 21 is a sectional side view showing a plasma etching
apparatus according to a third embodiment of the present
invention;
[0049] FIGS. 22A and 22B are graphs showing spatial distribution
characteristics of electron density to demonstrate an effect of a
shield member according to the third embodiment;
[0050] FIG. 23 is a graph showing spatial distribution
characteristics of electron density, using inner/outer input power
ratio as a parameter, according to a fourth embodiment of the
present invention;
[0051] FIG. 24 is a graph showing spatial distribution
characteristics of polymer film deposition rate, using inner/outer
input power ratio as a parameter, according to the fourth
embodiment;
[0052] FIG. 25 is a graph showing spatial distribution
characteristics of etching depth, using inner/outer input power
ratio as a parameter, according to the fourth embodiment;
[0053] FIG. 26 is a graph showing spatial distribution
characteristics of CF.sub.2 radical density, using center/periphery
gas flow-rate ratio as a parameter, according to a fifth embodiment
of the present invention;
[0054] FIG. 27 is a graph showing spatial distribution
characteristics of Ar radical density, using center/periphery gas
flow-rate ratio as a parameter, according to the fifth
embodiment;
[0055] FIG. 28 is a graph showing spatial distribution
characteristics of N.sub.2 radical density, using center/periphery
gas flow-rate ratio as a parameter, according to the fifth
embodiment;
[0056] FIG. 29 is a graph showing spatial distribution
characteristics of SiF.sub.4 reaction products, using
center/periphery gas flow-rate ratio as a parameter, according to
the fifth embodiment;
[0057] FIG. 30 is a graph showing spatial distribution
characteristics of CO reaction products, using center/periphery gas
flow-rate ratio as a parameter, according to the fifth
embodiment;
[0058] FIG. 31 is a diagram showing a mechanism of radical
generation (dissociation) obtained by a simulation according to the
fifth embodiment;
[0059] FIGS. 32A to 32C are views showing an examination model for
BARC etching and sets of measurement data according to a sixth
embodiment of the present invention;
[0060] FIGS. 33A to 33C are views showing an examination model for
SiO.sub.2 etching and sets of measurement data according to a
seventh embodiment of the present invention; and
[0061] FIG. 34 is a diagram showing an application example of
independent control over two systems for plasma density
distribution and radical density distribution.
DETAILED DESCRIPTION OF THE INVENTION
[0062] Embodiments of the present invention will be described
hereinafter with reference to the accompanying drawings. In the
following description, the constituent elements having
substantially the same function and arrangement are denoted by the
same reference numerals, and a repetitive description will be made
only when necessary.
First Embodiment
[0063] FIG. 1 is a sectional side view showing a plasma etching
apparatus according to a first embodiment of the present invention.
This plasma etching apparatus is structured as a parallel-plate
plasma etching apparatus of the capacitive coupling type. The
apparatus includes a cylindrical chamber (process container) 10,
which is made of, e.g., aluminum with an alumite-processed
(anodized) surface. The chamber 10 is protectively grounded.
[0064] A columnar susceptor pedestal 14 is disposed on the bottom
of the chamber 10 through an insulating plate 12 made of, e.g., a
ceramic. A susceptor 16 made of, e.g., aluminum is disposed on the
susceptor pedestal 14. The susceptor 16 is used as a lower
electrode, on which a target substrate, such as a semiconductor
wafer W, is placed.
[0065] The susceptor 16 is provided with an electrostatic chuck 18
on the top, for holding the semiconductor wafer W by an
electrostatic attraction force. The electrostatic chuck 18
comprises an electrode 20 made of a conductive film, and a pair of
insulating layers or insulating sheets sandwiching the electrode
20. The electrode 20 is electrically connected to a direct-current
(DC) power supply 22. With a DC voltage applied from the DC power
supply 22, the semiconductor wafer W is attracted and held on the
electrostatic chuck 18 by the Coulomb force. A focus ring made of,
e.g., silicon is disposed on the top of the susceptor 16 to
surround the electrostatic chuck 18 to improve etching uniformity.
A cylindrical inner wall member 26 made of, e.g., quartz is
attached to the side of the susceptor 16 and susceptor pedestal
14.
[0066] The susceptor pedestal 14 is provided with a cooling medium
space 28 formed therein and annularly extending therethrough. A
cooling medium set at a predetermined temperature, such as cooling
water, is circulated within the cooling medium space 28 from a
chiller unit (not shown) through lines 30a and 30b. The temperature
of the cooling medium is set to control the process temperature of
the semiconductor wafer W placed on the susceptor 16. Further, a
heat transmission gas, such as He gas, is supplied from a heat
transmission gas supply unit (not shown), through a gas supply line
32, into the interstice between the top surface of the
electrostatic chuck 18 and the bottom surface of the semiconductor
wafer W.
[0067] An upper electrode 34 is disposed above the susceptor 16 in
parallel with the susceptor. The space between the electrodes 16
and 34 is used as a plasma generation space. The upper electrode 34
defines a surface facing the semiconductor wafer W placed on the
susceptor (lower electrode) 16, and thus this facing surface is in
contact with plasma generation space. The upper electrode 34
comprises an outer upper electrode 36 and an inner upper electrode
38. The outer upper electrode 36 has a ring shape or doughnut shape
and is disposed to face the susceptor 16 at a predetermined
distance. The inner upper electrode 38 has a circular plate shape
and is disposed radially inside the outer upper electrode 36 while
being insulated therefrom. In terms of plasma generation, the outer
upper electrode 36 mainly works for it, and the inner upper
electrode 38 assists it.
[0068] FIG. 2 is an enlarged partial side view showing a main part
of the plasma etching apparatus shown in FIG. 1. As shown in FIG.
2, the outer upper electrode 36 is separated from the inner upper
electrode 38 by an annular gap (slit) of, e.g., 0.25 to 2.0 mm, in
which a dielectric body 40 made of, e.g., quartz is disposed. A
ceramic body may be disposed in this gap. The two electrodes 36 and
38 with the dielectric body 40 sandwiched therebetween form a
capacitor. The capacitance C.sub.40 of this capacitor is set or
adjusted to be a predetermined value, on the basis of the size of
the gap and the dielectric constant of the dielectric body 40. An
insulating shield member 42 made of, e.g., alumina
(Al.sub.2O.sub.3) and having a ring shape is airtightly interposed
between the outer upper electrode 36 and the sidewall of the
chamber 10.
[0069] The outer upper electrode 36 is preferably made of a
conductor or semiconductor, such as silicon, having a low
resistivity to generate a small Joule heat. The outer upper
electrode 36 is electrically connected to a first RF power supply
52 through a matching unit 44, an upper feed rod 46, a connector
48, and a feed cylinder 50. The first RF power supply 52 outputs an
RF voltage with a frequency of 13.5 MHz or more, such as 60 MHz.
The matching unit 44 is arranged to match the load impedance with
the internal (or output) impedance of the RF power supply 52. When
plasma is generated within the chamber 10, the matching unit 44
performs control for the load impedance and the output impedance of
the RF power supply 52 to apparently agree with each other. The
output terminal of the matching unit 44 is connected to the top of
the upper feed rod 46.
[0070] The feed cylinder 50 has a cylindrical or conical shape, or
a shape close to it, and formed of a conductive plate, such as an
aluminum plate or copper plate. The bottom end of the feed cylinder
50 is connected to the outer upper electrode 36 continuously in an
annular direction. The top of the feed cylinder 50 is electrically
connected to the bottom of the upper feed rod 46 through the
connector 48. Outside the feed cylinder 50, the sidewall of the
chamber 10 extends upward above the height level of the upper
electrode 34 and forms a cylindrical grounded conductive body 10a.
The top of the cylindrical grounded conductive body 10a is
electrically insulated from the upper feed rod 46 by a tube-like
insulating member 54. According to this design, the load circuit
extending from the connector 48 comprises a coaxial path formed of
the feed cylinder 50 and outer upper electrode 36 and the
cylindrical grounded conductive body 10a, wherein the former
members (36 and 50) function as a waveguide.
[0071] Returning to FIG. 1, the inner upper electrode 38 includes
an electrode plate 56 having a number of gas through-holes 56a, and
an electrode support 58 detachably supports the electrode plate 56.
The electrode plate 56 is made of a semiconductor material, such as
Si or SiC, while the electrode support 58 is made of a conductor
material, such as aluminum with an alumite-processed surface. The
electrode support 58 has two gas supply cells, i.e., a central gas
supply cell 62 and a peripheral gas supply cell 64, formed therein
and separated by an annular partition member 60, such as an O-ring.
The central gas supply cell 62 is connected to some part of a
number of gas delivery holes 56a formed at the bottom surface, so
as to constitute a central showerhead. The peripheral gas supply
cell 64 is connected to other part of a number of gas delivery
holes 56a formed at the bottom surface, so as to constitute a
peripheral showerhead.
[0072] The gas supply cells 62 and 64 are supplied with a process
gas from a common process gas supply source 66 at a predetermined
flow-rate ratio. More specifically, a gas supply line 68 is
extended from the process gas supply source 66 and divided into two
lines 68a and 68b connected to the gas supply cells 62 and 64. The
branch lines 68a and 68b are provided with flow control valves 70a
and 70b disposed thereon, respectively. The conductance values of
the flow passages from the process gas supply source 66 to the gas
supply cells 62 and 64 are equal to each other. Accordingly, the
flow-rate ratio of the process gas supplied into the two gas supply
cells 62 and 64 is arbitrarily adjusted by adjusting the flow
control valves 70a and 70b. The gas supply line 68 is provided with
a mass-flow controller (MFC) 72 and a switching valve 74 disposed
thereon.
[0073] The flow-rate ratio of the process gas supplied into the
central gas supply cell 62 and peripheral gas supply cell 64 is
thus adjusted. As a consequence, the ratio (FC/FE) between the gas
flow rate FC from the central showerhead and the gas flow rate FE
from the peripheral showerhead is arbitrarily adjusted. As
described above, the central showerhead is defined by gas
through-holes 56a at the electrode central portion corresponding to
the central gas supply cell 62, while the peripheral showerhead is
defined by gas through-holes 56a at the electrode peripheral
portion corresponding to the peripheral gas supply cell 64.
Further, flow rates per unit area may be set different, for the
process gas delivered from the central showerhead and peripheral
showerhead. Furthermore, gas types or gas mixture ratios are
independently or respectively selected, for the process gas
delivered from the central showerhead and peripheral
showerhead.
[0074] The electrode support 58 of the inner upper electrode 38 is
electrically connected to the first RF power supply 52 through the
matching unit 44, upper feed rod 46, connector 48, and lower feed
cylinder 76. The lower feed cylinder 76 is provided with a variable
capacitor 78 disposed thereon, for variable adjusting
capacitance.
[0075] Although not shown, the outer upper electrode 36 and inner
upper electrode 38 may be provided with a suitable cooling medium
space or cooling jacket (not shown) formed therein. A cooling
medium is supplied into this cooling medium space or cooling jacket
from an external chiller unit to control the electrode
temperature.
[0076] An exhaust port 80 is formed at the bottom of the chamber
10, and is connected to an exhaust unit 84 through an exhaust line
82. The exhaust unit 84 includes a vacuum pump, such as a turbo
molecular pump, to reduce the pressure of the plasma process space
within the chamber 10 to a predetermined vacuum level. A transfer
port for a semiconductor wafer W is formed in the sidewall of the
chamber 10, and is opened/closed by a gate valve 86 attached
thereon.
[0077] In the plasma etching apparatus according to this
embodiment, the susceptor 16 used as a lower electrode is
electrically connected to a second RF power supply 90 through a
matching unit 88. The second RF power supply 90 outputs an RF
voltage with a frequency of from 2 to 27 MHz, such as 2 MHz. The
matching unit 88 is arranged to match the load impedance with the
internal (or output) impedance of the RF power supply 90. When
plasma is generated within the chamber 10, the matching unit 88
performs control for the load impedance and the internal impedance
of the RF power supply 90 to apparently agree with each other.
[0078] The inner upper electrode 38 is electrically connected to a
low-pass filter (LPF) 92, which prevents the RF (60 MHz) from the
first RF power supply 52 from passing through, while it allows the
RF (2 MHz) from the second RF power supply 98 to pass through to
ground. The low-pass filter (LPF) 92 is preferably formed of an LR
filter or LC filter. Alternatively, only a single conductive line
may be used for this, because it can give a sufficiently large
reactance to the RF (60 MHz) from the first RF power supply 52. On
the other hand, the susceptor 16 is electrically connected to a
high pass filter (HPF) 94, which allows the RF (60 MHz) from the
first RF power supply 52 to pass through to ground.
[0079] When etching is performed in the plasma etching apparatus,
the gate valve 86 is first opened, and a semiconductor wafer W to
be processed is transferred into the chamber 10 and placed on the
susceptor 16. Then, an etching gas (typically a mixture gas) is
supplied from the process gas supply source 66 into the gas supply
cells 62 and 64 at predetermined flow rates and flow-rate ratio. At
the same time, the exhaust unit 84 is used to control the pressure
inside the chamber 10, i.e., the etching pressure, to be a
predetermined value (for example, within a range of from several
mTorr to 1 Torr). Further, a plasma generation RF (60 MHz) is
applied from the first RF power supply 52 to the upper electrode 34
(36 and 38) at a predetermined power, while an RF (2 MHz) is
applied from the second RF power supply 90 to the susceptor 16 at a
predetermined power. Furthermore, a DC voltage is applied from the
DC power supply 22 to the electrode 20 of the electrostatic chuck
18 to fix the semiconductor wafer W on the susceptor 16. The
etching gas delivered from the gas through-holes 56a of the inner
upper electrode 38 is turned into plasma by glow discharge between
the upper electrode 34 (36 and 38) and susceptor 16. Radicals and
ions generated in this plasma are used to etch the target surface
of the semiconductor wafer W.
[0080] In this plasma etching apparatus, the upper electrode 34 is
supplied with an RF within a range covering higher frequencies
(form 5 to 10 MHz or more at which ions cannot follow). As a
consequence, the plasma density is increased with a preferable
dissociation state, so that high density plasma is formed even
under a low pressure condition.
[0081] In the upper electrode 34, the inner upper electrode 38 is
also used as a showerhead directly across the semiconductor wafer
W, such that the flow-rate ratio of the gas delivered from the
central showerhead (62 and 56a) and peripheral showerhead (64 and
56a) can be arbitrarily adjusted. As a consequence, the spatial
distribution of gas molecular or radical density can be controlled
in the radial direction, so as to arbitrarily control the spatial
distribution of an etching characteristic on the basis of
radicals.
[0082] Further, as described later, the upper electrode 34 is
operated as an RF electrode for plasma generation, such that the
outer upper electrode 36 mainly works for it, and the inner upper
electrode 38 assists it. The ratio of electric field intensity
applied to electrons below the RF electrodes 36 and 38 can be
adjusted by these electrodes. As a consequence, the spatial
distribution of plasma density can be controlled in the radial
direction, so as to arbitrarily and finely control the spatial
distribution of a reactive ion etching characteristic.
[0083] It should be noted here that the control over the spatial
distribution of plasma density has substantially no influence on
the control over the spatial distribution of radical density. The
control over the spatial distribution of plasma density is
performed by varying the ratio of electric field intensity or input
power between the outer upper electrode 36 and inner upper
electrode 38. On the other hand, the control over the spatial
distribution of radical density is performed by varying the ratio
of process gas flow rate, gas density, or gas mixture between the
central showerhead (62 and 56a) and peripheral showerhead (64 and
56a).
[0084] The process gas delivered from the central showerhead (62
and 56a) and peripheral showerhead (64 and 56a) is dissociated in
an area directly below the inner upper electrode 38. Accordingly,
even if the balance of electric field intensity between the inner
upper electrode 38 and outer upper electrode 36 is changed, it does
not have a large influence on the balance of radical generation
amount or density between the central showerhead (62 and 56a) and
peripheral showerhead (64 and 56a), because both showerheads belong
to the inner upper electrode 38 (within the same area). Thus, the
spatial distribution of plasma density and the spatial distribution
of radical density can be controlled substantially independently of
each other.
[0085] Further, the plasma etching apparatus is arranged such that
most or the majority of plasma is generated directly below the
outer upper electrode 36, and then diffuses to the position
directly below the inner upper electrode 38. According to this
arrangement, the showerhead or inner upper electrode 38 is less
attacked by ions from the plasma. This effectively prevents the gas
delivery ports 56a of the electrode plate 56 from being
progressively sputtered, thereby remarkably prolonging the service
life of the electrode plate 56, which is a replacement part. On the
other hand, the outer upper electrode 36 has no gas delivery ports
at which electric field concentration occurs. As a consequence, the
outer upper electrode 36 is less attacked by ions, and thus there
arises no such a problem in that the outer upper electrode 36
shortens the service life in place of the inner upper electrode
38.
[0086] As described above, FIG. 2 shows a main part of the plasma
etching apparatus (particularly, a main part of plasma generating
means). In FIG. 2, the structure of the showerheads (56a, 62, and
64) of the inner upper electrode 38 is not shown. FIG. 3 is a
circuit diagram showing an equivalent circuit of a main part of
plasma generating means according to the first embodiment. In this
equivalent circuit, the resistance of respective portions is not
shown.
[0087] In this embodiment, as described above, the load circuit
extending from the connector 48 comprises a coaxial path formed of
the outer upper electrode 36 and feed cylinder 50 and the
cylindrical grounded conductive body 10a, wherein the former
members (36 and 50) function as a waveguide J.sub.0. Where the
radius (outer radius) of the feed cylinder 50 is a.sub.0, and the
radius of the cylindrical grounded conductive body 10a is b, the
characteristic impedance or inductance L.sub.0 of this coaxial path
is approximated by the following formula (1).
L.sub.0=K.times.In(b/a.sub.0) (1)
[0088] In this formula, K is a constant determined by the mobility
and dielectric constant of a conductive path.
[0089] On the other hand, the load circuit extending from the
connector 48 also comprises a coaxial path formed of the lower feed
rod 76 and the cylindrical grounded conductive body 10a, wherein
the former member (76) functions as a waveguide J.sub.i. Although
the inner upper electrode 38 is present on the extension of the
lower feed rod 76, the impedance of lower feed rod 76 is dominant,
because the difference in diameters between them is very large.
Where the radius (outer radius) of the lower feed rod 76 is
a.sub.i, the characteristic impedance or inductance L.sub.i of this
coaxial path is approximated by the following formula (2).
L.sub.i=K.times.In(b/a.sub.i) (2)
[0090] As could be understood from the above formulas (1) and (2),
the inner waveguide J.sub.i for transmitting RF to the inner upper
electrode 38 provides an inductance L.sub.i in the same manner as a
conventional ordinary RF system. On the other hand, the outer
waveguide J.sub.0 for transmitting RF to the outer upper electrode
36 provides a very small inductance L.sub.0 because of a very large
radius. As a consequence, in the load circuit extending from the
connector 48 toward the side opposite to the matching unit 44, RF
is transmitted more easily through the outer waveguide J.sub.0
having a lower impedance (a smaller voltage drop). The outer upper
electrode 36 is thereby supplied with a larger RF power P.sub.0, so
the electric field intensity E.sub.0 obtained at the bottom surface
(plasma contact surface) of the outer upper electrode 36 becomes
higher. On the other hand, RF is transmitted less easily through
the inner waveguide J.sub.i having a higher impedance (a larger
voltage drop). The inner upper electrode 38 is thus supplied with
an RF power P.sub.i smaller than the RF power P.sub.0 supplied to
the inner upper electrode 38, so the electric field intensity
E.sub.i obtained at the bottom surface (plasma contact surface) of
the inner upper electrode 38 becomes lower than the electric field
intensity E.sub.0 on the outer upper electrode 36 side.
[0091] As described above, according to this upper electrode 34,
electrons are accelerated by a stronger electric field E.sub.0
directly below the outer upper electrode 36, while electrons are
accelerated by a weaker electric field E.sub.0 directly below the
inner upper electrode 38. In this case, most or the majority of
plasma P is generated directly below the outer upper electrode 36,
while a subsidiary part of the plasma P is generated directly below
the inner upper electrode 38. Then, the high density plasma
generated directly below the outer upper electrode 36 diffuses
radially inward and outward, so the plasma density becomes more
uniform in the radial direction within the plasma process space
between the upper electrode 34 and susceptor 16.
[0092] Incidentally, in the coaxial path formed of the outer upper
electrode 36 and feed cylinder 50 and the cylindrical grounded
conductive body 10a, the maximum transmission power Pmax depends on
the radius a.sub.0 of the feed cylinder 50 and the radius b of the
cylindrical grounded conductive body 10a, and is given by the
following formula (3). Pmax/E.sub.0.sup.2 max=a.sub.0.sup.2
[In(b/a.sub.0)].sup.2/2Z.sub.0 (3)
[0093] In the above formula, Z.sub.0 is the input impedance of this
coaxial path viewing from the matching unit 44, and E.sub.0max is
the maximum electric field intensity of the RF transmission
system.
[0094] In the formula (3), the maximum transmission power Pmax
takes on the maximum value when (b/a.sub.0).apprxeq.1.65. In other
words, when the ratio (b/a.sub.0) of the radius of the cylindrical
grounded conductive body 10a relative to the radius of the feed
cylinder 50 is about 1.65, the power transmission efficiency of the
outer waveguide J.sub.0 is best. Accordingly, in order to improve
the power transmission efficiency of the outer waveguide J.sub.0,
the radius of the feed cylinder 50 and/or the radius of the
cylindrical grounded conductive body 10a are selected so that the
ratio (b/a.sub.0) is preferably set to be at least within a range
of from 1.2 to 2.0, and more preferably within a range of from 1.5
to 1.7.
[0095] According to this embodiment, the lower feed rod 76 is
provided with the variable capacitor 78 disposed thereon as means
for adjusting the ratio or balance between the outer electric field
intensity E.sub.0 directly below the outer upper electrode 36 (or
the input power P.sub.0 into the outer upper electrode 36 side) and
the inner electric field intensity E.sub.i directly below the inner
upper electrode 38 (or the input power P.sub.i into the inner upper
electrode 38 side), in order to arbitrarily and finely control the
spatial distribution of plasma density. The capacitance C.sub.78 of
the variable capacitor 78 is adjusted to increase or decrease the
impedance or reactance of the inner waveguide J.sub.i, thereby
changing the relative ratio between the voltage drop through the
outer waveguide J.sub.0 and the voltage drop through the inner
waveguide J.sub.i. As a consequence, it is possible to adjust the
ratio between the outer electric field intensity E.sub.0 (outer
input power P.sub.0) and the inner electric field intensity E.sub.i
(inner input power P.sub.i).
[0096] In general, the ion sheath impedance that causes an electric
potential drop of plasma is capacitive. In the equivalent circuit
shown in FIG. 3, it is assumed (constructed) that the sheath
impedance capacitance directly below the outer upper electrode 36
is C.sub.po, and the sheath impedance capacitance directly below
the inner upper electrode 38 is C.sub.pi. Further, the capacitance
C.sub.40 of the capacitor formed between the outer upper electrode
36 and inner upper electrode 38 cooperates with the capacitance
C.sub.78 of the variable capacitor 78 in changing the balance
between the outer electric field intensity E.sub.0 (outer input
power P.sub.0) and inner electric field intensity E.sub.i (inner
input power P.sub.i). The capacitance C.sub.40 can be set or
adjusted to optimize the variable capacitor's 78 function of
adjusting the balance of electric field intensity (input
power).
[0097] FIGS. 4 and 5 show test examples (simulation data) in
relation to the variable capacitor's 78 function of adjusting the
balance of electric field intensity according to this embodiment.
FIG. 4 shows distribution characteristics of electric field
intensity (relative value) in the radial direction of the
electrode, using the capacitance C.sub.78 of the variable capacitor
78 as a parameter. FIG. 5 shows the relative ratio between the
outer electric field intensity E.sub.0 and inner electric field
intensity E.sub.i, using the capacitance C.sub.78 of the variable
capacitor 78 as a parameter.
[0098] In this simulation, the diameter of the semiconductor wafer
W was set at 200 mm, the radius of the inner upper electrode 38
(with a circular plate shape) at 100 mm, and the inner radius and
outer radius of the outer upper electrode 36 (with a ring shape) at
101 mm and 141 mm, respectively. In this case, the area of the
semiconductor wafer W was 314 cm.sup.2, the area of the inner upper
electrode 38 was 314 cm.sup.2 the same as that of the wafer W, and
the area of the outer upper electrode 36 was 304 cm.sup.2 slightly
smaller than that of the wafer W. Typically, on the face opposite
the wafer W, the planar area of the outer upper electrode 36 is
preferably set to be about 1/4 times to about 1 times the planar
area of the inner upper electrode 38.
[0099] As shown in FIG. 4, the outer electric field intensity
E.sub.0 directly below the outer upper electrode 36 is higher than
the inner electric field intensity E.sub.i directly below the inner
upper electrode 38, and a large step of electric field intensity is
thereby formed near the boundary between the electrodes 36 and 38.
Particularly, the outer electric field intensity E.sub.0 directly
below the outer upper electrode 36 is apt to take on the maximum
value near the boundary abutting the inner upper electrode 38, and
gradually decrease radially outward therefrom. In this example, as
shown in FIG. 5, where the capacitance C.sub.78 of the variable
capacitor 78 is changed within a range of from 180 to 350 pF, the
ratio E.sub.i/E.sub.0 between the electric field intensities
E.sub.i and E.sub.0 can be continuously controlled within a range
of from about 10 to 40%. Where C.sub.78 falls within a range of
from 125 to 180 pF, the load circuit produces resonance and thereby
becomes uncontrollable. In principle, within a stable domain, where
the capacitance C.sub.78 of the variable capacitor 78 is increased,
the reactance of the inner waveguide J.sub.i decreases. As a
consequence, the inner electric field intensity E.sub.i directly
below the inner upper electrode 38 is relatively increased, so that
the ratio E.sub.i/E.sub.0 between the outer electric field
intensity E.sub.0 and inner electric field intensity E.sub.i is set
to be higher.
[0100] According to this embodiment, the reactance of the outer
waveguide J.sub.0 provided by the feed cylinder 50 can be very
small, so the impedance reactance of the load circuit, viewing from
the output terminal of the matching unit 44, takes on a capacitive
negative value. This means that there is no resonance point at
which reactance causes polar inversion from an inductive positive
value to a negative value, on the waveguide extending from the
output terminal of the matching unit 44 to the capacitive ion
sheath. Since no resonance point is formed, no resonance electric
current is generated, thereby reducing the RF energy loss and
ensuring stable control of the plasma density distribution.
[0101] FIGS. 6A (bias in the ON-state), 6B (bias in the OFF-state),
7A (in the X direction), and 7B (in the Y direction) show examples
(experimental data) concerning electron density distribution
characteristics and etching rate distribution characteristics,
obtained by the plasma etching apparatus according to this
embodiment. In this experiment, as in the electric field intensity
distribution characteristics shown in FIGS. 4 and 5, the
capacitance C.sub.78 of the variable capacitor 78 was used as a
parameter. The electron density was measured at positions in the
radial direction, using a plasma absorption probe (PAP). Further, a
silicon oxide film disposed on a semiconductor wafer was etched,
and the etching rate was measured at wafer positions in the radial
direction. Also in this experiment, the radius of the inner upper
electrode 38 was set at 100 mm, and the inner radius and outer
radius of the outer upper electrode 36 (with a ring shape) at 101
mm and 141 mm, respectively. Principal etching conditions were as
follows: [0102] Wafer diameter=200 mm; [0103] Pressure inside the
chamber=15 mTorr; [0104] Temperature (upper electrode/chamber
sidewall/lower electrode)=60/50/20.degree. C.; [0105] Heat
transmission gas (He gas) supply pressure (central portion/edge
portion)=15/25 Torr; [0106] Distance between the upper and lower
electrodes=50 mm; [0107] Process gas (C.sub.5F.sub.8/Ar/O.sub.2)
flow rate.apprxeq.20/380/20 sccm; and [0108] RF power (60 MHz/2
MHz).apprxeq.2200 W/1500 W (C.sub.78=500 pF, 1000 pF), 1800 W
(C.sub.78=120 pF).
[0109] Referring to FIGS. 6A and 6B, the capacitance C.sub.78 of
the variable capacitor 78 was set at 120 pF so that the ratio
E.sub.i/E.sub.0 between the outer electric field intensity E.sub.0
and inner electric field intensity E.sub.i was set to be higher. In
this case, the distribution characteristic of electron density or
plasma density was formed such that the density took on the maximum
value near the electrode center, and monotonously decreased
radially outward therefrom. It is thought that, in this case, the
plasma diffusion rate overcame the difference between the plasma
generation rate directly below the outer upper electrode 36 or the
main plasma generation section, and the plasma generation rate
directly below the inner upper electrode 38 or sub plasma
generation section. As a consequence, the plasma gathered to the
central portion from all around, and the plasma density was thereby
higher at the central portion than at the peripheral portion.
[0110] On the other hand, the capacitance C.sub.78 of the variable
capacitor 78 was set at 1000 pF so that the ratio E.sub.i/E.sub.0
between the outer electric field intensity E.sub.0 and inner
electric field intensity E.sub.i was set to be lower. In this case,
the distribution characteristic of electron density was formed such
that the density took on the maximum value on the outer side (near
a position 140 mm distant from the center) of the wafer rather than
the inner side in the radial direction, and became almost uniform
on the inner side (0 to 100 mm) of the wafer. It is thought that,
this was so because the plasma generation rate directly below the
inner upper electrode 38 increased, and plasma diffusion radially
outward was thereby enhanced. In any case, the spatial distribution
characteristic of electron density or plasma density can be
flexibly and finely controlled by finely adjusting the capacitance
C.sub.78 of the variable capacitor 78 within a suitable range.
[0111] Further, the electron density at respective positions was
higher to some extent in the case (FIG. 6A) with an RF bias (2 MHz)
applied to the lower electrode 16, as compared to the case (FIG.
6B) without any RF bias applied to the lower electrode 16. However,
their distribution patterns were almost the same.
[0112] Referring to the experimental data shown in FIGS. 7A and 7B,
different patterns of the spatial distribution characteristic of
etching rate were formed by adjusting the capacitance C.sub.78 of
the variable capacitor 78, in accordance with the spatial
distribution characteristics of electron density shown in FIGS. 6A
and 6B. Accordingly, the spatial distribution characteristic of
etching rate on the wafer surface can be flexibly and finely
controlled by finely adjusting the capacitance C.sub.78 of the
variable capacitor 78 within a suitable range.
[0113] Further, in the plasma etching apparatus according to this
embodiment, the flow-rate ratio of the gas delivered from the
central portion and peripheral portion can be adjusted by the
showerhead mechanism of the inner upper electrode 36, as described
above. This function allows the spatial distribution characteristic
of etching rate to be controlled on the basis of radicals.
Second Embodiment
[0114] FIG. 8 is a sectional side view showing a plasma etching
apparatus according to a second embodiment of the present
invention. In FIG. 8, the constituent elements having substantially
the same arrangement and function as those of the apparatus
according to the first embodiment (FIG. 1) are denoted by the same
reference numerals.
[0115] One of the features of the second embodiment resides in that
the feed cylinder 50 or transmission path for transmitting the RF
from the RF power supply 52 to the outer upper electrode 36 is made
of a cast metal. This cast metal is preferably a metal having a
high conductivity and workability, such as aluminum. As one of the
advantages, cast metals can realize a low cost, and thus reduce the
cost for the member to 1/7 or less of that provided by a plate
material. As another advantage, cast metals can be easily
integrated, and thus can reduce the number of RF connection
surfaces in the member, thereby reducing the RF loss.
[0116] Further, even where the feed cylinder 50 is made of a cast
metal, the RF transmission efficiency is not lowered. Specifically,
referring to the experimental data shown in FIGS. 9A (cast metal),
9B (plate), 10A (cast metal), and 10B (plate), there was no
substantial difference in etching rate between the case where the
feed cylinder 50 was made of a plate material, and the case where
the feed cylinder 50 was made of a cast metal. FIGS. 9A and 9B show
spatial distribution characteristics of etching rate over a silicon
oxide film (SiO.sub.2). FIGS. 10A and 10B show spatial distribution
characteristics of etching rate over a photo-resist (PR) film. In
this test example, principal etching conditions were as follows:
[0117] Wafer diameter=300 mm; [0118] Pressure inside the chamber=25
mTorr; [0119] Temperature (upper electrode/chamber sidewall/lower
electrode)=60/60/20.degree. C.; [0120] Heat transmission gas (He
gas) supply pressure (central portion/edge portion)=15/40 Torr;
[0121] Distance between the upper and lower electrodes=45 mm;
[0122] Process gas (C.sub.5F.sub.8/Ar/O.sub.2) flow
rate.apprxeq.30/750/50 sccm; [0123] RF power (60 MHz/2
MHz).apprxeq.3300 W/3800 W; and [0124] Measurement time=120
seconds.
[0125] A second feature of the second embodiment resides in that a
conductive ring member 100 is disposed around the feed rod 76
inside the feed cylinder 50. The main role of the conductive member
100 is to reduce the inductance around the feed rod 76 so as to
improve the range of the variable capacitor's 78 function of
adjusting the balance between outer and inner input powers, as
described below.
[0126] In this plasma processing apparatus, as described above, the
ratio between the input power P.sub.0 into the outer upper
electrode 36 and the input power P.sub.i into the inner upper
electrode 38 can be arbitrarily adjusted by adjusting the
capacitance C.sub.78 of the variable capacitor 78. In general, the
capacitance C.sub.78 of the variable capacitor 78 is adjusted
stepwise, using a step motor or the like. For this capacitance
adjustment, it is necessary to avoid the uncontrollable resonance
domain described above (in FIG. 5, 125 pF<C.sub.78<180 pF).
For this reason, the experimental test examples described above
according to the first embodiment (FIGS. 6A, 6B, 7A, and 7B) mainly
used a stable domain (C.sub.78.gtoreq.=180 pF) on the right side of
the resonance domain. However, the stable domain on the right side
has a limit in increasing the ratio of the inner input power
P.sub.i, and also entails a large power loss. On the other hand, as
shown in FIGS. 4 and 5, a domain (C.sub.78.ltoreq.125 pF) on the
left side of the resonance domain has an advantage in increasing
the ratio of the inner input power P.sub.i, as well as a smaller
power loss. However, the domain on the left side of the resonance
domain becomes closer to the resonance domain, as the ratio of the
inner input power P.sub.i is increased. In the case of a
characteristic line with a large change rate (inclination), such as
a characteristic line A shown in FIG. 11, it becomes very difficult
to perform fine adjustment immediately before the resonance
domain.
[0127] In order to solve this problem, it is effective to modify
the characteristic line of capacitance vs. inner input power ratio,
as indicated with a characteristic line B shown in FIG. 11, i.e.,
the change rate (inclination) in the domain on the left side of the
resonance domain is set smaller so as to expand the adjusting
range. Then, in order to obtain a characteristic line with a gentle
and broader inclination as indicated with the characteristic line B
shown in FIG. 11, it is effective to reduce the inductance L.sub.i
around the feed rod 76, as described below.
[0128] FIG. 12 is a circuit diagram showing an equivalent circuit
of a plasma generation RF feed circuit according to the second
embodiment. The reactance .omega.L.sub.i around the feed rod 76
always takes on an absolute value larger than the reactance
1/.omega.C.sub.78 of the capacitor 78, and thus the composite
reactance X of the inner waveguide J.sub.i is always inductive and
can be expressed by X=.omega.L.sub.a. A parallel circuit formed of
this apparent inductance L.sub.a and the capacitance C.sub.40 falls
in a resonance state, when the susceptance 1/.omega.L.sub.a of the
inductance L.sub.a and the susceptance .omega.C.sub.40 of the
capacitance C.sub.40 cancel each other to be zero, i.e., when
1/.omega.L.sub.a=1/(.omega.L.sub.i-1/.omega.C.sub.78)=.omega.C.sub.40
is satisfied. In this formula, with decrease in L.sub.i, the value
of C.sub.78 to establish the resonance condition described above
increases, thereby approaching a characteristic line with a gentle
and broader inclination immediately before the resonance domain, as
indicated with the characteristic line B shown in FIG. 11. In the
equivalent circuit shown in FIG. 12, the inductance L.sub.0 of the
outer waveguide J.sub.0 is not shown, for the sake of simplicity of
explanation. Even if the inductance L.sub.0 is included in this
equivalent circuit, the principle is the same.
[0129] FIG. 13 shows an effect of the conductive member 100
according to this embodiment. When an electric current I varying
with time flows through the feed rod 76, a loop magnetic flux B is
generated around the feed rod 76, and an inductive electric current
i interlinked with the magnetic flux B flows through the conductive
member 100 due to electromagnetic induction. As a consequence, a
loop magnetic flux b is generated inside and outside the conductive
member 100 by the inductive electric current i, and the magnetic
flux B is cancelled by that much corresponding to the magnetic flux
b inside the conductive member 100. Thus, the conductive member 100
disposed around the feed rod 76 can reduce the net magnetic flux
generation amount around the feed rod 76, thereby reducing the
inductance L.sub.i.
[0130] As regards the appearance structure of the conductive member
100, a single ring shape continuous in an annular direction is
preferably used, but a plurality of conductive members disposed in
an annular direction may be used instead. As regards the inner
structure of the conductive member 100, a hollow ring body with a
cavity shown in FIG. 13 may be used, but a solid block structure,
as shown in FIG. 8, can provide a large inductance reduction
effect. The volume of the conductive member 100 is preferably set
larger, and is ideally or most preferably set to fill the space
inside the feed cylinder 50. In practice, however, it is preferable
for the conductive member 100 to occupy 1/10 to 1/3 of the space
surrounded by the feed cylinder 50 and outer upper electrode 36.
The conductive member 100 is made of an arbitrary conductor
material, such as aluminum or a cast metal. The conductive member
100 is disposed while being electrically insulated from conductive
bodies around it, such as the feed rod 76 and inner upper electrode
38.
[0131] FIG. 14 shows experimental data of demonstration examples,
in relation to the broadening effect described above of the
conductive member 100 according to this embodiment. Referring to
FIG. 14, a characteristic line B' was obtained by the apparatus
structure according to this embodiment, and a characteristic line
A' was obtained by the apparatus structure with no conductive
member 100. These characteristic lines A' and B' have upside-down
shapes of the characteristic lines A and B shown in FIG. 11,
respectively. Specifically, in a parallel-plate plasma apparatus of
this type, as the ratio of the input power (inner input power
P.sub.i) into the central portion of the upper electrode 34 is
increased, the plasma density becomes higher near the substrate W
on the susceptor 16, and the bias frequency Vpp on the susceptor 16
(in inverse proportion to the plasma density) decreases. According
to this relationship, measurement values of Vpp obtained by
respective step values of the variable capacitor 78 (a control
variable in proportion to the value of the capacitance C.sub.78)
were plotted to obtain the characteristic lines A' and B', which
thus have upside-down shapes of the characteristic lines A and B
shown in FIG. 11, respectively. According to this embodiment,
thanks to the conductive member 100 disposed around the feed rod
76, when the balance between the outer and inner input powers is
adjusted by the variable capacitor 78, the ratio of inner input
power P.sub.i can be stably and finely controlled to a value as
high as possible immediately before the resonance domain, as
demonstrated by the characteristic line B' shown in FIG. 14.
[0132] A third feature of the second embodiment relates to a
low-pass filter 92 connected between the inner upper electrode 38
and ground potential. As shown in FIG. 15A, the low-pass filter 92
according to this embodiment is formed of a variable resistor 93
and a coil 95 connected in series, and arranged not to allow the
plasma generation RF (60 MHz) to pass through, but to allow an
alternating frequency of the bias RF (2 MHz) or less and DC to pass
through. According to the low-pass filter 92, the DC potential or
self-bias voltage Vdc on the inner upper electrode 38 can be
adjusted by adjusting the resistance value R93 of variable resistor
93.
[0133] More specifically, as shown in FIG. 16, as the resistance
value R93 of the resistor 93 is set lower, the voltage drop through
the resistor 93 decreases, and the negative DC potential Vdc
thereby becomes higher (closer to ground potential). Conversely, as
the resistance value R93 of the resistor 93 is set higher, the
voltage drop through the resistor 93 increases, and the DC
potential Vdc becomes lower. If the DC potential Vdc is too high
(in general, it is higher than -150V), the plasma potential
increases, thereby causing abnormal discharge or arcing. On the
other hand, if the DC potential Vdc is too low (in general, it is
lower than -450V), the inner upper electrode 38 is intensely
attacked by ions, thereby hastening wear-out of the electrode.
[0134] In another perspective, as shown in FIG. 17, the DC
potential Vdc has an appropriate range (from -450V to -150V) to
prevent or suppress both of abnormal discharge and electrode
wear-out, and the resistance value R93 has a range (from Ra to Rb)
corresponding to this appropriate range. Accordingly, the DC
potential Vdc can be adjusted to be within the appropriate range
described above (from -450V to -150V) by setting or adjusting the
resistance value R93 of the resistor 93 to be within the range
described above (from Ra to Rb). Further, depending on the value of
an RF power applied to the entire upper electrode 34 (outer upper
electrode 36 and inner upper electrode 38), the appropriate range
(from Ra to Rb) of the resistance value R93 changes. For example,
in one experiment, the lower limit resistance value Ra=about 1
M.OMEGA. was provided by an RF power of 3000 W.
[0135] Further, as shown in FIG. 15B, the inner upper electrode 38
may be connected to ground through a variable DC power supply 97 to
directly control the DC potential Vdc by the voltage of the power
supply. The variable DC power supply 97 is preferably formed of a
bipolar power supply.
[0136] A fourth feature of the second embodiment resides in that,
in the upper electrode 34, the bottom surface of the outer upper
electrode 36 is protruded downward, i.e., toward the susceptor 16,
more than the bottom surface of the inner upper electrode 38. FIG.
18 is a sectional side view showing a main part of the plasma
etching apparatus according to the second embodiment. In this
example, the outer upper electrode 36 is formed of two parts
divided in the vertical direction, i.e., an upper first electrode
member 36A and a lower second electrode member 36B. The main body
or first electrode member 36A is made of, e.g., alumite-processed
aluminum, and is connected to the feed cylinder 50. The replacement
part or second electrode member 36B is made of, e.g., silicon, and
is detachably fixed to and in close contact with the first
electrode member 36A by bolts (not shown). The second electrode
member 36B is protruded by a predetermined value H, relative to the
bottom surface of the inner upper electrode 38. A member 102 for
enhancing thermal conductance, such as a silicone rubber sheet, is
interposed between two electrode members 36A and 36B. The contact
surfaces of the two electrode members 36A and 36B may be coated
with Teflon.TM. to reduce thermal resistance.
[0137] In the outer upper electrode 36, the protruded length H and
inner diameter .PHI. of the protrusion part 36B can define the
intensity or direction of an electric field provided from the outer
upper electrode 36 or upper electrode 34 to the plasma generation
space. Thus, they are important factors to thereby determine the
spatial distribution characteristic of plasma density.
[0138] FIGS. 19A to 19E show examples (experimental data)
concerning spatial distribution characteristics of electron
density, using as parameters the protruded length H and inner
diameter .PHI. of the protrusion part 36B. Also in this experiment,
the electron density was measured at positions in the radial
direction, using a plasma absorption probe (PAP). The diameter of a
semiconductor wafer was set at 300 mm. As regards the main
parameters .PHI. and H, the experimental examples shown in FIG. 19A
employed .PHI.=329 mm and H=15 mm, the experimental examples shown
in FIG. 19B employed .PHI.=329 mm and H=20 mm, the experimental
examples shown in FIG. 19C employed .PHI.=339 mm and H=20 mm, the
experimental examples shown in FIG. 19D employed .PHI.=349 mm and
H=20 mm, and the experimental examples shown in FIG. 19E employed
.PHI.=359 mm and H=25 mm. As a secondary parameter, the ratio
P.sub.i/P.sub.0 (RF power ratio) between the inner input power
P.sub.i and outer input power P.sub.0 was set at four different
values, i.e., (30/70), (27/73), (20/80), and (14/86).
[0139] In the experimental data shown in FIGS. 19A to 19E, an
inflection point F at which the electron density quickly drops was
moved radially outward with increase in the inner diameter .PHI. of
the protrusion part 36B of the outer upper electrode 36, and was
moved up with increase in the protruded length H of the protrusion
part 36B. The ideal distribution characteristic is a characteristic
in which an inflection point F is located directly above the wafer
edge (a position of 150 mm), and the density is flat at a high
value between the center and edge. In light of this, one
characteristic (.PHI.=349 mm and H=20 mm) shown in FIG. 19D
obtained by an RF power ratio P.sub.i/P.sub.0 of 30/70 is closest
to the ideal state.
[0140] FIG. 20A shows characteristics of total uniformity U.sub.T
and edge uniformity U.sub.E in the spatial distribution of electron
density, using .PHI. and H as two-dimensional parameters. The total
uniformity U.sub.T stands for planar uniformity over the entire
region in the radial direction from the wafer central position
(R.sub.0) to the wafer edge position (R.sub.150), as shown in FIG.
20B. The edge uniformity U.sub.E stands for planar uniformity over
a region near the wafer edge, such as a region from a position of
radius 130 mm (R.sub.130) to the wafer edge position
(R.sub.150).
[0141] As understood from the characteristics shown in FIG. 20A,
the protruded length H of the protrusion part 36B is remarkably
influential on the total uniformity U.sub.T, and also has a large
effect on the edge uniformity U.sub.E. On the other hand, the inner
diameter .PHI. of the protrusion part 36B is influential on the
edge uniformity U.sub.E, but is scarcely influential on the total
uniformity U.sub.T. In total, the protruded length H of the
protrusion part 36B is preferably set at 25 mm or less, and most
preferably set to be near 20 mm. The inner diameter .PHI. of the
protrusion part 36B is preferably set to be within a range of from
348 mm to 360 mm, and most preferably set to be near 349 mm. The
range of from 348 mm to 360 mm means that the protrusion part 36B
is disposed at a position 24 mm to 30 mm distant from the wafer
edge radially outward.
[0142] It should be noted that the protrusion part 36B of the outer
upper electrode 36 applies an electric field to the plasma
generation space radially inward from around, thereby providing an
effect of confining plasma. For this reason, the protrusion part
36B is disposed preferably or almost essentially at a position
outside the wafer edge in the radial direction, in order to improve
uniformity in the spatial distribution characteristic of plasma
density. On the other hand, the width of the protrusion part 36B in
the radial direction is not important, and thus can be arbitrarily
set.
Third Embodiment
[0143] FIG. 21 is a sectional side view showing a main part of a
plasma etching apparatus according to a third embodiment of the
present invention. The parts of this apparatus can be the same as
those of the second embodiment except for the featuring parts. The
third embodiment has a feature in that a shield member 104 is
disposed along the protrusion part 36B of the outer upper electrode
36 according to the second embodiment.
[0144] For example, the shield member 104 is formed of an aluminum
plate with an alumite-processed surface, and physically and
electrically coupled to the sidewall of the process container 10.
The shield member 104 extends essentially in the horizontal
direction from the container sidewall to the position below the
protrusion part 36B of the outer upper electrode 36 to cover the
bottom surfaces of the protrusion part 36B and the ring-shaped
shield member 42 in a non-contacting or insulated state. The second
electrode member 36B of the outer upper electrode 36 has an
L-shaped cross section with a peripheral portion extending downward
in the vertical direction to form a protrusion. The protruded
length H and inner diameter .PHI. of this protrusion can be defined
in accordance with the same numerical conditions as those of the
second embodiment.
[0145] A function of the shield member 104 is to shield or seal RF
discharge from the bottom surfaces of the protrusion part 36B of
the outer upper electrode 36 and ring-shaped shield member 42, so
as to suppress plasma generation directly below them. As a
consequence, it is possible to primarily enhance the plasma
confining effect directly above the wafer.
[0146] FIGS. 22A (with the shield member) and 22B (without the
shield member) show experimental data concerning the plasma
confining effect provided by the shield member 104. Where the
shield member 104 was not used, as shown in FIG. 22B, the plasma
electron density once dropped and then increased again to form a
peak outside the wafer edge position (150 mm) in the radial
direction. This was so because RF power was discharged vertically
downward from the bottom surfaces of the protrusion part 36B of the
outer upper electrode 36 and ring-shaped shield member 42, whereby
plasma was also generated directly below them and thus electrons
and ions were present there. Since a certain amount of plasma was
present in a space distant from the wafer edge position radially
outward, the plasma density directly above the wafer decreased by
that much.
[0147] On the other hand, where the shield member 104 was used
according to this embodiment, as shown in FIG. 22A, the electron
density (plasma density) essentially monotonously decreased
radially outward outside the wafer edge position (150 mm), while it
was higher directly above the wafer as a whole. This was so because
the bottom surfaces of the protrusion part 36B of the outer upper
electrode 36 and ring-shaped shield member 42 did not work as an RF
path any more due to the presence of the shield member 104, whereby
plasma generation directly below them was remarkably reduced. The
plasma confining effect or plasma diffusion-preventing effect of
the shield member 104 was enhanced, with increase in the RF power
of the RF power supply 52.
[0148] Further, as a secondary effect, where plasma generation is
remarkably reduced by the shield member 104 outside the wafer edge
position, as described above, etching species, such as radicals and
ions, are reduced. As a consequence, it is possible to effectively
prevent undesirable polymer films from being deposited on portions
inside the container (particularly near the shield member 104).
[0149] For example, conventionally, where a Low-k film (an
inter-level insulating film with a low dielectric constant) is
etched, a plasma etching is performed, and then ashing (resist
removal) is performed using O.sub.2 gas within the same chamber. At
this time, reactive species (such as CF and F), which have been
deposited as polymers inside the container during the previous
plasma etching, are activated by active oxygen atoms in plasma, and
cause damage to the Low-k film (Low-k damages), such that they etch
the via-holes of the film into a bowing shape or invade the film
and change its k value. According to this embodiment, however, the
shield member 104 can effectively prevent undesirable deposition of
reactive species during plasma etching, thereby solving problems
concerning Low-k damages described above. The shield member 104 may
be made of an arbitrary conductor or semiconductor (such as,
silicon), or a combination of different materials.
[0150] FIG. 21 also shows an arrangement of cooling medium passages
106 and 108 formed in the upper electrode 34 (36 and 38). A cooling
medium set at an adjusted temperature is circulated within each of
the cooling medium passages 106 and 108 from a chiller unit (not
shown) through lines 110 and 112. Specifically, the cooling medium
passages 106 are formed in the first electrode member 36A of the
outer upper electrode 36. Since the second electrode member 36B is
coupled with the first electrode member 36A through a coating or
sheet 102 for enhancing thermal conductance, it can also be
effectively cooled by the cooling mechanism.
[0151] The electrodes are supplied with a cooling medium even when
the RF power supplies 52 and 90 are in the OFF-state.
Conventionally, the plasma processing apparatus of this type
employs an insulative cooling medium, such as Galden.TM.. In this
case, when the cooling medium flows through a cooling medium
passage, it generates an electrostatic charge by friction, by which
the electrode enters an abnormally high voltage state. If an
operator's hand touches the electrode in this state during a
maintenance operation or the like in which the RF power supplies
are in the OFF-state, the operator may get an electric shock.
However, the plasma processing apparatus according to this
embodiment allows electrostatic charge generated in the inner upper
electrode 38 to be released to ground through the resistor 93 of
the low-pass filter 92 (see FIG. 8), whereby an operator is
prevented from getting an electric shock.
Fourth Embodiment
[0152] Using the plasma etching apparatus according to the third
embodiment (FIGS. 8 and 21), an experiment was conducted of etching
a silicon oxide film (SiO.sub.2) to form a hole having a diameter
(.PHI.) of 0.22 .mu.m. In this experiment, etching characteristics
(particularly etching rate) were examined, using as a parameter the
RF power input ratio (P.sub.i/P.sub.0) between the outer upper
electrode 36 and inner upper electrode 38. Other etching conditions
are shown below. FIGS. 23 to 25 show experimental result data.
[0153] Wafer diameter=300 mm; [0154] Pressure inside the chamber=20
mTorr; [0155] Temperature (upper electrode/chamber sidewall/lower
electrode)=20/60/60.degree. C.; [0156] Heat transmission gas (He
gas) supply pressure (central portion/edge portion)=20/35 Torr;
[0157] Distance between the upper and lower electrodes=45 mm;
[0158] Protruded length (H) of the outer upper electrode=15 mm;
[0159] Process gas
(C.sub.5F.sub.8/CH.sub.2F.sub.2/N.sub.2/Ar/O.sub.2).apprxeq.10/20/110/560-
/10 sccm; [0160] RF power (60 MHz/2 MHz).apprxeq.2300 W/3500 W; and
[0161] Etching time=120 seconds.
[0162] As shown in FIG. 23, as the ratio of the inner input power
P.sub.i increased, i.e., 14%, 18%, and 30%, the electron density or
plasma density became higher in proportion to the P.sub.i ratio
near the wafer central portion, while it did not change near wafer
edge portion. Accordingly, where the RF power input ratio
(P.sub.i/P.sub.0) is adjusted on the basis of this, the spatial
distribution characteristic of plasma density can be controlled in
the radial direction.
[0163] FIG. 24 shows the measurement result of deposition rate of a
polymer film, formed from reaction products and reactive species,
at respective positions in the radial direction, wherein the
deposition rate is in proportion to radical density. This
experiment was conducted to see the effect of change in the RF
power input ratio (P.sub.i/P.sub.0) on the radical density. A bare
silicon wafer was used as a sample substrate on which the polymer
film was deposited. As indicated by the experimental data shown in
FIG. 24, change in the RF power input ratio (P.sub.i/P.sub.0) only
had a very small influence on the polymer film deposition rate
i.e., the spatial distribution characteristic of radical
density.
[0164] FIG. 25 shows etching depth measured at respective positions
of the wafer in the radial direction, obtained by the SiO.sub.2
etching described above. As shown in FIG. 25, as the ratio of the
inner input power P.sub.i increased, i.e., 14%, 18%, and 30%, the
etching depth became larger in proportion to the P.sub.i ratio near
the wafer central portion, while it did not differ so much near
wafer edge portion. This was similar to that of the electron
density (FIG. 24).
[0165] As described above, judging from the experimental data shown
in FIGS. 23 to 25, the following matters have been confirmed.
Specifically, by adjusting the RF power input ratio
(P.sub.i/P.sub.0) between the outer upper electrode 36 and inner
upper electrode 38, the spatial distribution characteristic of
plasma density in the radial direction can be controlled without
substantially affecting the spatial distribution characteristic of
radical density, i.e., independently of the control over the
spatial distribution of radical density. Accordingly, the
uniformity of etching depth i.e., etching rate, can be improved by
adjusting the RF power input ratio (P.sub.i/P.sub.0). It should be
noted that, if the plasma etching apparatus according to the first
or second embodiment (FIGS. 1, 8, and 18) is used, the same
experimental result as described above is obtained.
Fifth Embodiment
[0166] Using the plasma etching apparatus according to the third
embodiment (FIGS. 8 and 21), a simulation was conducted of etching
a silicon oxide film (SiO.sub.2) with a CF family process gas. In
this experiment, the distributions of radicals or reaction products
were examined, using as a parameter the ratio (FC/FE) between the
flow rate FC of a process gas delivered from the central showerhead
(62 and 56a) and the flow rate FE of the process gas delivered from
the peripheral showerhead (64 and 56a). In this simulation, it was
assumed that neither reaction nor absorption of reaction products
or reactive species was caused on the wafer surface, but the
following reaction was simply caused on the blanket SiO.sub.2 film.
2CF.sub.2+SiO.sub.2.fwdarw.SiF.sub.4+2CO
[0167] Other principal etching conditions are shown below. FIGS. 26
to 30 show simulation result concerning radicals and reaction
products. FIG. 31 shows the type and generation rate (denoted by a
percentage value in brackets) of radicals generated by gradual
dissociation from molecules of the main etching gas
(C.sub.4F.sub.8). [0168] Wafer diameter=200 mm; [0169] Pressure
inside the chamber=50 mTorr; [0170] Temperature (upper
electrode/chamber sidewall/lower electrode)=20/60/60.degree. C.;
[0171] Heat transmission gas (He gas) supply pressure (central
portion/edge portion)=10/35 Torr; [0172] Distance between the upper
and lower electrodes=30 mm; [0173] Protruded length (H) of the
outer upper electrode=15 mm; [0174] Process gas
(C.sub.4F.sub.8/N.sub.2/Ar).apprxeq.5/120/1000 sccm; and [0175] RF
power (60 MHz/2 MHz).apprxeq.1200 W/1700 W.
[0176] As shown in FIG. 26, the distribution characteristic of
CF.sub.2 density, which is the main reactive species, was
remarkably influenced by the gas flow-rate ratio (FC/FE) between
the center and periphery. Specifically, as the ratio of the central
gas flow rate FC was increased, the CF.sub.2 density became higher
near the wafer central portion while it did not change so much near
the wafer edge portion. As shown in FIG. 28, the distribution
characteristic of CO radical density also showed a similar change
with change in the gas flow-rate ratio (FC/FE) between the center
and periphery. By contrast, as shown in FIG. 27, the distribution
characteristic of Ar radical density scarcely changed with change
in the gas flow-rate ratio (FC/FE) between the center and
periphery.
[0177] As regards reaction products, as shown in FIGS. 29 and 30,
either of SiF.sub.4 density and CO density was remarkably
influenced by the gas flow-rate ratio (FC/FE) between the center
and periphery. More specifically, as the ratio of the central gas
flow rate FC was reduced, each of the SiF.sub.4 density and CO
density became higher near the wafer central portion while it did
not change so much near the wafer edge portion. Where the central
gas flow rate FC was equal to the peripheral gas flow rate FE
(FC/FE=50/50), the density became higher near the wafer central
portion than near the wafer edge portion. As described above,
reaction products tended to gather to the central side, because
reaction products were less moved laterally by a fresh gas flow at
the central portion, as compared with the peripheral portion.
[0178] If reaction products have a non-uniform distribution on a
wafer, they not only affect the uniformity of process gas supply
rate or chemical reaction among respective positions, but also may
directly affect the etching shape or selectivity. According to this
embodiment, as shown in FIGS. 29 and 30, where the central gas flow
rate FC is set to be higher than the peripheral gas flow rate FE
(FC/FE.apprxeq.70/30 in this shown example), the space density
distribution of reaction products can become uniform. It should be
noted that, if the plasma etching apparatus according to the first
or second embodiment (FIGS. 1, 8, and 18) is used, the same
simulation result as described above is obtained.
Sixth Embodiment
[0179] Using the plasma etching apparatus according to the third
embodiment (FIGS. 8 and 21), an experiment was conducted of etching
a BARC (antireflective film). In this experiment, the etching shape
and selectivity were examined, using the gas flow-rate ratio
(FC/FE) between the center and periphery as a parameter. A test
sample shown in FIG. 32A was used. A mask having an opening
diameter (.PHI.) of 0.12 .mu.m, a photo-resist film having a
thickness of 350 nm, a BARC film having a thickness of 80 nm, and
an SiO.sub.2 film having a thickness of 700 nm were used. "Oxide
loss" and "resist remaining amount" were measured as examination
items for the selectivity, and "bottom CD" was measured as an
examination item for the etching shape or dimensional accuracy.
FIG. 32B shows measurement values of the respective examination
items at FC/FE=50/50. FIG. 32C shows measurement values of the
respective examination items at FC/FE=70/30. As regards the
measurement point, "center" denotes a position at the wafer center,
and "edge" denotes a position 5 mm distant from the wafer notch end
toward the center. Principal etching conditions were as follows:
[0180] Wafer diameter=300 mm; [0181] Pressure inside the
chamber=150 mTorr; [0182] Heat transmission gas (He gas) supply
pressure (central portion/edge portion)=10/25 Torr; [0183] Distance
between the upper and lower electrodes=30 mm; [0184] Protruded
length (H) of the outer upper electrode=15 mm; [0185] Process gas
(CF4).apprxeq.200 sccm; [0186] RF power (60 MHz/2 MHz).apprxeq.500
W/600 W; and [0187] Etching time=30 seconds.
[0188] As regards the examination items for the BARC etching, the
"oxide loss" is the etched depth of the underlying SiO.sub.2 film
provided by over etching of the BARC etching. For this value, a
smaller value is better, but a priority resides in that a smaller
difference in this value over the wafer (particularly the
difference between the center and edge) is better. The "resist
remaining amount" is the thickness of the photo-resist remaining
after the etching. For this value, a larger value is better, and a
smaller difference in this value is also better. The "bottom CD" is
the bottom diameter of a hole formed in the BARC. For this value, a
value closer to the mask opening diameter .PHI. is better, and a
smaller difference in this value is also better.
[0189] As shown in FIG. 32B, where the central gas flow rate FC was
equal to the peripheral gas flow rate FE (5:5), the differences
between the center and edge were large in all the examination
items, and particularly the difference was large in the "resist
remaining amount". By contrast, as shown in FIG. 32B, where the
central gas flow rate FC was larger than the peripheral gas flow
rate FE (7:3), all the examination items stably took on better
values and became uniform between the center and edge, i.e., the
selectivity and etching shape were remarkably improved.
[0190] As described above, according to this embodiment, within the
process container 10, particularly within the plasma generation
space defined between the upper electrode 34 and lower electrode
16, the inner upper electrode 38 of the upper electrode 34 is used
while the ratio (FC/FE) between the process gas flow rate FC
delivered from the central showerhead (62 and 56a) and the process
gas flow rate FE (64 and 56a) delivered from the peripheral
showerhead is adjusted. As a consequence, the spatial distribution
of radical density can be controlled to uniformize etching
characteristics (such as the selectivity and etching shape) on the
basis of radicals. It should be noted that, if the plasma etching
apparatus according to the first or second embodiment (FIGS. 1, 8,
and 18) is used, the same measurement result as described above is
obtained.
Seventh Embodiment
[0191] Using the plasma etching apparatus according to the third
embodiment (FIGS. 8 and 21), an experiment was conducted of etching
an SiO.sub.2 film. In this experiment, the etching shape was
examined, using the gas flow-rate ratio (FC/FE) between the center
and periphery as a parameter. A test sample shown in FIG. 33A was
used. A mask having an opening diameter (.PHI.) of 0.22 .mu.m, a
photo-resist film having a thickness of 500 nm, a BARC film having
a thickness of 100 nm, and an SiO.sub.2 film having a thickness of
1 .mu.m were used. "Etching depth", "top CD", and "bottom CD" were
measured as examination items for the etching shape. FIG. 33B shows
measurement values of the respective examination items at
FC/FE=50/50. FIG. 33C shows measurement values of the respective
examination items at FC/FE=10/90. Principal etching conditions were
as follows: [0192] Wafer diameter=300 mm; [0193] Pressure inside
the chamber=20 mTorr; [0194] Temperature (upper electrode/chamber
sidewall/lower electrode)=20/60/60.degree. C. [0195] Heat
transmission gas (He gas) supply pressure (central portion/edge
portion)=20/35 Torr; [0196] Distance between the upper and lower
electrodes=45 mm; [0197] Protruded length (H) of the outer upper
electrode=15 mm; [0198] Process gas
(C.sub.5F.sub.8/CH.sub.2F.sub.2/N.sub.2/Ar/O.sub.2).apprxeq.10/20/110/560-
/10 sccm; [0199] RF power (60 MHz/2 MHz).apprxeq.2300 W/3500 W;
[0200] RF power ratio (inner input power P.sub.i/outer input power
P.sub.0)=30:70; and [0201] Etching time=120 seconds.
[0202] As regards the examination items for the SiO.sub.2 etching,
the "etching depth" is the depth of a hole formed in the SiO.sub.2
film by the etching time (120 seconds), i.e., it corresponds to the
etching rate. The "top CD" and "bottom CD" are the top and bottom
diameters of the hole formed in the SiO.sub.2 film, and, as these
values are closer to each other, the vertical shape characteristic
of the hole (anisotropy) is better. As a matter of course, for each
of the examination items, it is more preferable that the difference
between the center and edge is smaller.
[0203] As shown in FIG. 33B, where the central gas flow rate FC was
equal to the peripheral gas flow rate FE (5:5), the difference was
large in the "etching depth", and values of the ratio of bottom
CD/top CD at respective positions were small, i.e., the holes were
more tapered. By contrast, as shown in FIG. 33B, where the central
gas flow rate FC was smaller than the peripheral gas flow rate FE
(1:9), the "etching depth" i.e., etching rate became more uniform,
and the vertical shape characteristic was improved and more
uniform.
[0204] As described above, also in this embodiment, by adjusting
the ratio (FC/FE) between the inner gas flow rate FC and the outer
gas flow rate FE, the spatial distribution of radical density can
be controlled to uniformize etching characteristics (particularly
the etching shape) on the basis of radicals. It should be noted
that, if the plasma etching apparatus according to the first or
second embodiment (FIGS. 1, 8, and 18) is used, the same
measurement result as described above is obtained.
[0205] According to the embodiments described above, the plasma
density distribution and the radical density distribution can be
controlled independently of each other within the plasma generation
space defined in the process container 10. This independent control
over two systems can be used to preferably deal with various plasma
process applications, such as those shown in the map of FIG.
34.
[0206] The embodiments described above may be modified in various
manners, in accordance with the technical ideas of the present
invention. For example, only the outer upper electrode 36 may be
supplied with an RF from the first RF power supply 52 through the
matching unit 44 and feed cylinder 50, while the inner upper
electrode 38 being supplied with no RF. Also in this case, the
inner upper electrode 38 can function as a showerhead or function
as an electrode for an RF output from the second RF power supply 90
to flow to ground. Alternatively, the inner upper electrode 38 may
be replaced with a single-purpose showerhead with no electrode
function. In the embodiments described above, the outer upper
electrode 36 is formed of one or single ring electrode, but it may
be formed of a plurality of electrodes combined to form a ring as a
whole. Further, the inner diameter of the outer upper electrode 36
may be set very small, or the outer upper electrode 36 may be
formed of a circular plate. Depending on the application, the
second RF power supply 90 may be omitted. The present invention can
be applied not only to plasma etching, but also to various plasma
processes, such as plasma CVD, plasma oxidation, plasma
nitridation, and sputtering. As regards a target substrate, the
present invention can be applied not only to a semiconductor wafer,
but also to various substrates for a flat panel display, photo
mask, CD substrate, and print substrate.
[0207] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
* * * * *