U.S. patent application number 10/959585 was filed with the patent office on 2005-02-24 for plasma etching method and plasma etching unit.
Invention is credited to Hayashi, Hisataka, Honda, Masanobu, Matsuyama, Shoichiro, Nagaseki, Kazuya.
Application Number | 20050039854 10/959585 |
Document ID | / |
Family ID | 28786373 |
Filed Date | 2005-02-24 |
United States Patent
Application |
20050039854 |
Kind Code |
A1 |
Matsuyama, Shoichiro ; et
al. |
February 24, 2005 |
Plasma etching method and plasma etching unit
Abstract
The present invention is a plasma etching method including: an
arranging step of arranging a pair of electrodes oppositely in a
chamber and making one of the electrodes support a substrate to be
processed in such a manner that the substrate is arranged between
the electrodes, the substrate having a silicon film and an
inorganic-material film adjacent to the silicon film; and an
etching step of applying a high-frequency electric power to at
least one of the electrodes to form a high-frequency electric field
between the pair of the electrodes, supplying a process gas into
the chamber to form a plasma of the process gas by means of the
electric field, and selectively plasma-etching the silicon film of
the substrate by means of the plasma; wherein a frequency of the
high-frequency electric power applied to the at least one of the
electrodes is 50 to 150 MHz in the etching step.
Inventors: |
Matsuyama, Shoichiro;
(Nirasaki-Shi, JP) ; Honda, Masanobu;
(Nirasaki-Shi, JP) ; Nagaseki, Kazuya;
(Nirasaki-Shi, JP) ; Hayashi, Hisataka;
(Yokohama-Shi, JP) |
Correspondence
Address: |
SMITH, GAMBRELL & RUSSELL, LLP
1850 M STREET, N.W., SUITE 800
WASHINGTON
DC
20036
US
|
Family ID: |
28786373 |
Appl. No.: |
10/959585 |
Filed: |
October 7, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10959585 |
Oct 7, 2004 |
|
|
|
PCT/JP03/04410 |
Apr 7, 2003 |
|
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Current U.S.
Class: |
156/345.44 ;
156/345.43; 257/E21.218 |
Current CPC
Class: |
H01L 21/3065 20130101;
H01J 37/32082 20130101; H01J 37/3266 20130101 |
Class at
Publication: |
156/345.44 ;
156/345.43 |
International
Class: |
C23F 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 8, 2002 |
JP |
2002-105249 |
Claims
1. A plasma etching method comprising: an arranging step of
arranging a pair of electrodes oppositely in a chamber and making
one of the electrodes support a substrate to be processed in such a
manner that the substrate is arranged between the electrodes, the
substrate having a silicon film and an inorganic-material film
adjacent to the silicon film, and an etching step of applying a
high-frequency electric power to at least one of the electrodes to
form a high-frequency electric field between the pair of the
electrodes, supplying a process gas into the chamber to form a
plasma of the process gas by means of the electric field, and
selectively plasma-etching the silicon film of the substrate by
means of the plasma, wherein, in the etching step, a frequency of
the high-frequency electric power applied to the at least one of
the electrodes is 50 to 150 MHz and a pressure in the chamber is
not higher than 13.3 Pa.
2. A plasma etching method according to claim 1, wherein the
frequency of the high-frequency electric power applied to the at
least one of the electrodes is 100 MHz in the etching step.
3. A plasma etching method according to claim 1, wherein in the
etching step, power density of the high-frequency electric power is
0.15 to 5 W/cm.sup.2.
4. A plasma etching method according to claim 1, wherein in the
etching step, plasma density in the chamber is 5.times.10.sup.9 to
2.times.10.sup.10 cm.sup.-3.
5. A plasma etching method according to claim 1, wherein the
inorganic-material film comprises at least one of a silicon oxide,
a silicon nitride, a silicon oxinitride, and a silicon carbide.
6. A plasma etching method according to claim 1, wherein in the
etching step, the high-frequency electric power is applied to an
electrode supporting the substrate to be processed.
7. A plasma etching method according to claim 6, wherein in the
etching step, a second high-frequency electric power of 3.2 MHz to
13.56 MHz is applied to the electrode supporting the substrate to
be processed, the second high-frequency electric power being
overlapped with the high-frequency electric power.
8. A plasma etching method comprising: an arranging step of
arranging a pair of electrodes oppositely in a chamber and making
one of the electrodes support a substrate to be processed in such a
manner that the substrate is arranged between the electrodes, the
substrate having a silicon film and an inorganic-material film
adjacent to the silicon film, and an etching step of applying a
high-frequency electric power to at least one of the electrodes to
form a high-frequency electric field between the pair of the
electrodes, supplying a process gas into the chamber to form a
plasma of the process gas by means of the electric field, and
selectively plasma-etching the silicon film of the substrate by
means of the plasma, wherein in the etching step, a frequency of
the high-frequency electric power applied to the at least one of
the electrodes is 50 to 150 MHz, in the etching step, the
high-frequency electric power is applied to an electrode supporting
the substrate to be processed, in the etching step, a second
high-frequency electric power is applied to the electrode
supporting the substrate to be processed, the second high-frequency
electric power being overlapped with the high-frequency electric
power, and a frequency of the second high-frequency electric power
is 13.56 MHz.
9. A plasma etching method according to claim 8, wherein power
density of the second high-frequency electric power is not higher
than 0.64 W/cm.sup.2.
10. A plasma etching method according to claim 7, wherein in the
etching step, a self-bias electric voltage of the electrode
supporting the substrate to be processed is not higher than 200
V.
11. A plasma etching method according to claim 1, wherein a
distance between the pair of electrodes is shorter than 50 mm.
12. A plasma etching method comprising: an arranging step of
arranging a pair of electrodes oppositely in a chamber and making
one of the electrodes support a substrate to be processed in such a
manner that the substrate is arranged between the electrodes, the
substrate having a silicon film and an inorganic-material film
adjacent to the silicon film, and an etching step of applying a
high-frequency electric power to at least one of the electrodes to
form a high-frequency electric field between the pair of the
electrodes, supplying a process gas into the chamber to form a
plasma of the process gas by means of the electric field, and
selectively plasma-etching the silicon film of the substrate by
means of the plasma, wherein in the etching step, a frequency of
the high-frequency electric power applied to the at least one of
the electrodes is 50 to 150 MHz, and in the etching step, a
magnetic field is formed around a plasma region between the pair of
electrodes to achieve a plasma confining effect.
13. A plasma etching method according to claim 12, wherein strength
of the magnetic field formed around the plasma region between the
pair of electrodes is 0.03 to 0.045 T (300 to 450 Gauss).
14. A plasma etching method according to claim 13, wherein when the
magnetic field is formed around the plasma region between the pair
of electrodes, strength of the magnetic field on a focus ring
provided around the substrate to be processed is not lower than
0.001 T (10 Gauss) and strength of the magnetic field on the
substrate to be processed is not higher than 0.001 T.
15. A plasma etching method according to claim 1, wherein the
silicon film comprises poly-silicon.
16. A plasma etching method comprising: an arranging step of
arranging a pair of electrodes oppositely in a chamber and making
one of the electrodes support a substrate to be processed in such a
manner that the substrate is arranged between the electrodes, the
substrate having a silicon film and an inorganic-material film
adjacent to the silicon film, and an etching step of applying a
high-frequency electric power to at least one of the electrodes to
form a high-frequency electric field between the pair of the
electrodes, supplying a process gas into the chamber to form a
plasma of the process gas by means of the electric field, and
selectively plasma-etching the silicon film of the substrate by
means of the plasma, wherein in the etching step, the process gas
includes at least one of an HBr gas and a Cl.sub.2 gas, plasma
density in the chamber is 5.times.10.sup.9 to 2.times.10.sup.10
cm.sup.-3, a self-bias electric voltage of the electrode supporting
the substrate to be processed is not higher than 200 V, and a
pressure in the chamber is not higher than 13.3 Pa.
17. A plasma-etching-condition confirming method comprising: an
arranging step of arranging a pair of electrodes oppositely in a
chamber and making one of the electrodes support a substrate to be
processed in such a manner that the substrate is arranged between
the electrodes, the substrate having a silicon film and an
inorganic-material film adjacent to the silicon film, and a
plasma-forming step of applying a high-frequency electric power to
at least one of the electrodes to form a high-frequency electric
field between the pair of the electrodes, and supplying an Ar gas
into the chamber to form a plasma of the Ar gas by means of the
electric field, wherein in the plasma-forming step, a confirming
step is carried out to confirm that plasma density in the chamber
is not lower than 1.times.10.sup.10 cm.sup.-3 and that a self-bias
electric voltage of the electrode supporting the substrate to be
processed is not higher than 100 V.
18. A plasma etching unit comprising: a chamber configured to
contain a substrate to be processed having a silicon film and an
inorganic-material film adjacent to the silicon film, a pair of
electrodes arranged in the chamber, one of the pair of electrodes
being configured to support the substrate to be processed, a
process-gas supplying system configured to supply a process gas
into the chamber, a gas-discharging system configured to discharge
a gas in the chamber, and a high-frequency electric power source
configured to supply a high-frequency electric power for forming a
plasma to at least one of the electrodes, wherein a frequency of
the high-frequency electric power generated from the high-frequency
electric power source is 50 to 150 MHz, and a pressure in the
chamber is not higher than 13.3 Pa.
19. A plasma etching unit according to claim 18, wherein the
frequency of high-frequency electric power generated from the
high-frequency electric power source is 100 MHz.
20. A plasma etching unit according to claim 18, wherein power
density of the high-frequency electric power is 0.15 to 5
W/cm.sup.2.
21. A plasma etching unit according to claim 18, wherein the
high-frequency electric power is applied to an electrode supporting
the substrate to be processed.
22. A plasma etching unit according to claim 21, further comprising
a second high-frequency electric power source configured to apply a
second high-frequency electric power of 3.2 MHz to 13.56 MHz to the
electrode supporting the substrate to be processed, the second
high-frequency electric power being overlapped with the
high-frequency electric power.
23. A plasma etching unit comprising: a chamber configured to
contain a substrate to be processed having a silicon film and an
inorganic-material film adjacent to the silicon film, a pair of
electrodes arranged in the chamber, one of the pair of electrodes
being configured to support the substrate to be processed, a
process-gas supplying system configured to supply a process gas
into the chamber, a gas-discharging system configured to discharge
a gas in the chamber, and a high-frequency electric power source
configured to supply a high-frequency electric power for forming a
plasma to at least one of the electrodes, wherein a frequency of
the high-frequency electric power generated from the high-frequency
electric power source is 50 to 150 MHz, the high-frequency electric
power is applied to an electrode supporting the substrate to be
processed, a second high-frequency electric power source configured
to apply a second high-frequency electric power to the electrode
supporting the substrate to be processed is provided, the second
high-frequency electric power being overlapped with the
high-frequency electric power, and a frequency of the second
high-frequency electric power is 13.56 MHz.
24. A plasma etching unit according to claim 23, wherein power
density of the second high-frequency electric power is not higher
than 0.64 W/cm.sup.2.
25. A plasma etching unit according to claim 18, wherein a distance
between the pair of electrodes is shorter than 50 mm.
26. A plasma etching unit comprising: a chamber configured to
contain a substrate to be processed having a silicon film and an
inorganic-material film adjacent to the silicon film, a pair of
electrodes arranged in the chamber, one of the pair of electrodes
being configured to support the substrate to be processed, a
process-gas supplying system configured to supply a process gas
into the chamber, a gas-discharging system configured to discharge
a gas in the chamber, and a high-frequency electric power source
configured to supply a high-frequency electric power for forming a
plasma to at least one of the electrodes, wherein a frequency of
the high-frequency electric power generated from the high-frequency
electric power source is 50 to 150 MHz, and a magnetic-field
forming unit configured to form a magnetic field around a plasma
region between the pair of electrodes is provided, the magnetic
field achieving a plasma confining effect.
27. A plasma etching unit according to claim 26, wherein strength
of the magnetic field formed around the plasma region between the
pair of electrodes by the magnetic-field forming unit is 0.03 to
0.045 T (300 to 450 Gauss).
28. A plasma etching unit according to claim 27, wherein a focus
ring is provided around the substrate to be processed, and when the
magnetic-field forming unit forms the magnetic field around the
plasma region between the pair of electrodes, strength of the
magnetic field on the focus ring is not lower than 0.001 T (10
Gauss) and strength of the magnetic field on the substrate to be
processed is not higher than 0.001 T.
29. A plasma etching unit comprising: a chamber configured to
contain a substrate to be processed having a silicon film and an
inorganic-material film adjacent to the silicon film, a pair of
electrodes arranged in the chamber, one of the pair of electrodes
being configured to support the substrate to be processed, a
process-gas supplying system configured to supply a process gas
into the chamber, a gas-discharging system configured to discharge
a gas in the chamber, and a high-frequency electric power source
configured to supply a high-frequency electric power for forming a
plasma to at least one of the electrodes, wherein when a gas
including at least one of an HBr gas and a Cl.sub.2 gas is used as
the process gas, plasma density in the chamber is 5.times.10.sup.9
to 2.times.10.sup.10 cm.sup.-3, a self-bias electric voltage of the
electrode supporting the substrate to be processed is not higher
than 200 V, and a pressure in the chamber is not higher than 13.3
Pa.
30. A plasma etching unit comprising: a chamber configured to
contain a substrate to be processed having a silicon film and an
inorganic-material film adjacent to the silicon film, a pair of
electrodes arranged in the chamber, one of the pair of electrodes
being configured to support the substrate to be processed, a
process-gas supplying system configured to supply a process gas
into the chamber, a gas-discharging system configured to discharge
a gas in the chamber, and a high-frequency electric power source
configured to supply a high-frequency electric power for forming a
plasma to at least one of the electrodes, wherein when an Ar gas is
used as the process gas, plasma density in the chamber is not lower
than 1.times.10.sup.10 cm.sup.-3, a self-bias electric voltage of
the electrode supporting the substrate to be processed is not
higher than 100 V, and a pressure in the chamber is not higher than
13.3 Pa.
31. A plasma etching method according to claim 1, wherein in the
etching step, the pressure in the chamber is not higher than 4
Pa.
32. A plasma etching method according to claim 1, wherein in the
etching step, the pressure in the chamber is not higher than 1.33
Pa.
33. A plasma etching unit according to claim 18, wherein the
pressure in the chamber is not higher than 4 Pa.
34. A plasma etching unit according to claim 18, wherein the
pressure in the chamber is not higher than 1.33 Pa.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a plasma etching method and
a plasma etching unit of plasma-etching a silicon film formed on a
substrate to be processed such as a semiconductor wafer, which has
the silicon film and an inorganic-material film adjacent to the
silicon film.
DESCRIPTION OF THE RELATED ART
[0002] In a manufacturing step of a semiconductor device, a
multilayer film including a silicon film, such as a poly-silicon
film, and insulating films is formed on a semiconductor wafer, and
then a plasma-etching process is conducted in order to form a
predetermined wiring pattern.
[0003] In order to conduct the plasma-etching process, various
kinds of units are used. Among them, a capacitive-coupling type of
parallel-plate plasma etching unit is used mainly. In the
capacitive-coupling type of parallel-plate plasma etching unit, a
pair of parallel-plate electrodes (upper electrode and lower
electrode) are arranged in a chamber, a process gas is introduced
into the chamber, and a high-frequency electric power is applied to
at least one of the electrodes to form a high-frequency electric
field between the electrodes. By means of the high-frequency
electric field, plasma of the process gas is generated so that a
plasma-etching process is conducted to a substrate to be
processed.
[0004] In such a plasma processing unit, a high-frequency electric
power of 13.56 to 40 MHz is applied to the lower electrode in order
to conduct the etching process.
[0005] In such conditions, for example, when a silicon film such as
a poly-silicon film is etched with a mask of an inorganic-material
film such as an SiO.sub.2, the etching process is conducted under a
relatively high pressure in order to enhance an etching selectivity
with respect to the inorganic-material film.
[0006] However, when the etching process is conducted under the
conventional relatively high pressure, although the etching
selectivity of the silicon film with respect to the
inorganic-material film is enhanced, an etching geometric control
performance is not good. This problem is arisen not only in the
case using the mask of the inorganic-material film, but also in
another case wherein an inorganic-material film is formed as a base
of the silicon film.
SUMMARY OF THE INVENTION
[0007] This invention is developed by focusing the aforementioned
problems in order to resolve them effectively. An object of the
present invention is to provide a plasma etching method and a
plasma etching unit that can etch a silicon film adjacent to an
inorganic-material film with a high etching selective ratio and a
good etching geometric control performance.
[0008] According to a result of study by the inventors, in the
etching process of the silicon film such as a poly-silicon film,
plasma density is dominant, and ion energy contributes only a
little. On the other hand, in the etching process of the
inorganic-material film such as a SiO.sub.2 film or a SiN film,
both the plasma density and the ion energy are necessary. Thus, if
the plasma density is high and the ion energy is low to some
extent, an etching selective ratio of the silicon film with respect
to the inorganic-material film can be enhanced. In the case, the
ion energy of the plasma indirectly corresponds to a self-bias
electric voltage of an electrode at the etching process. Thus, in
order to raise the etching selective ratio of the silicon film with
respect to the inorganic-material film, finally, it is necessary to
etch the silicon film under a condition of high plasma density and
low bias.
[0009] On the other hand, in order to improve the etching geometric
control performance, it is necessary to conduct the process under a
low pressure. However, under the above conditions, a process under
a lower pressure can achieve a high etching selective ratio. That
is, if a high plasma density and a low self-bias electric voltage
are achieved, the etching selective ratio of the silicon film with
respect to the inorganic-material film can be enhanced under a
lower pressure. Thus, a high etching selective ratio and a good
etching geometric control performance may not be in conflict with
each other.
[0010] According to a further result of study by the inventors,
when the frequency of the high-frequency electric power applied to
the electrode is high, a condition wherein the plasma density is
high and the self-bias electric voltage is low can be
generated.
[0011] The present invention is a plasma etching method comprising:
an arranging step of arranging a pair of electrodes oppositely in a
chamber and making one of the electrodes support a substrate to be
processed in such a manner that the substrate is arranged between
the electrodes, the substrate having a silicon film and an
inorganic-material film adjacent to the silicon film; and an
etching step of applying a high-frequency electric power to at
least one of the electrodes to form a high-frequency electric field
between the pair of the electrodes, supplying a process gas into
the chamber to form a plasma of the process gas by means of the
electric field, and selectively plasma-etching the silicon film of
the substrate by means of the plasma; wherein a frequency of the
high-frequency electric power applied to the at least one of the
electrodes is 50 to 150 MHz in the etching step.
[0012] According to the present invention, since the frequency of
the high-frequency electric power applied to the electrode is 50 to
150 MHz, which is higher than prior art, even under a condition of
a lower pressure, a high plasma density and a low self-bias
electric voltage can be achieved. Thus, the silicon film can be
etched with a high etching selective ratio with respect to the
inorganic-material film and with a good geometric control
performance.
[0013] It is preferable that the frequency of the high-frequency
electric power applied to the electrode is 70 to 100 MHz, in
particular 100 MHz.
[0014] In addition, in the etching step, it is preferable that
power density of the high-frequency electric power is 0.15 to 5
W/cm.sup.2.
[0015] In addition, in the etching step, it is preferable that
plasma density in the chamber is 5.times.10.sup.9 to
2.times.10.sup.10 cm.sup.-3.
[0016] In addition, in the etching step, it is preferable that a
pressure in the chamber is not higher than 13.3 Pa.
[0017] In addition, the present invention is a plasma etching
method comprising: an arranging step of arranging a pair of
electrodes oppositely in a chamber and making one of the electrodes
support a substrate to be processed in such a manner that the
substrate is arranged between the electrodes, the substrate having
a silicon film and an inorganic-material film adjacent to the
silicon film; and an etching step of applying a high-frequency
electric power to at least one of the electrodes to form a
high-frequency electric field between the pair of the electrodes,
supplying a process gas into the chamber to form a plasma of the
process gas by means of the electric field, and selectively
plasma-etching the silicon film of the substrate by means of the
plasma; wherein in the etching step, the process gas includes at
least one of an HBr gas and a Cl2 gas, plasma density in the
chamber is 5.times.10.sup.9 to 2.times.10.sup.10 cm.sup.-3, and a
self-bias electric voltage of the electrode supporting the
substrate to be processed is not higher than 200 V.
[0018] According to the present invention, under a condition
wherein the plasma density in the chamber is 5.times.10.sup.9 to
2.times.10.sup.10 cm.sup.-3 and the self-bias electric voltage of
the electrode supporting the substrate to be processed is not
higher than 200 V, plasma of the gas including at least one of an
HBr gas and a Cl2 gas is generated, so that the silicon film can be
etched with a high etching selective ratio with respect to the
inorganic-material film and with a good geometric control
performance.
[0019] In the above, the inorganic-material film may comprises at
least one of a silicon oxide, a silicon nitride, a silicon
oxinitride, and a silicon carbide.
[0020] In addition, it is preferable that the high-frequency
electric power is applied to an electrode supporting the substrate
to be processed. In the case, a second high-frequency electric
power of 3.2 to 13.56 MHz may be applied to the electrode
supporting the substrate to be processed, the second high-frequency
electric power being overlapped with the high-frequency electric
power. By overlapping the second high-frequency electric power of a
lower frequency with the high-frequency electric power, plasma
density and ion drawing effect can be adjusted so that an etching
rate of the silicon film can be raised more while a high etching
selective ratio with respect to the inorganic-material film can be
assured.
[0021] It is preferable that a frequency of the second
high-frequency electric power is 13.56 MHz. If the frequency of the
second high-frequency electric power is 13.56 MHz, it is preferable
that power density of the second high-frequency electric power is
not higher than 0.64 W/cm.sup.2. In addition, if the second
high-frequency electric power of 3.2 to 13.56 MHz is applied, it is
preferable that a self-bias electric voltage of the electrode
supporting the substrate to be processed is not higher than 200
V.
[0022] In addition, the present invention is a plasma etching unit
comprising: a chamber configured to contain a substrate to be
processed having a silicon film and an inorganic-material film
adjacent to the silicon film; a pair of electrodes arranged in the
chamber, one of the pair of electrodes being configured to support
the substrate to be processed; a process-gas supplying system
configured to supply a process gas into the chamber; a
gas-discharging system configured to discharge a gas in the
chamber; and a high-frequency electric power source configured to
supply a high-frequency electric power for forming a plasma to at
least one of the electrodes; wherein a frequency of the
high-frequency electric power generated from the high-frequency
electric power source is 50 to 150 MHz.
[0023] It is preferable that the frequency of high-frequency
electric power generated from the high-frequency electric power
source is 70 to 100 MHz, in particular 100 MHz.
[0024] Preferably, power density of the high-frequency electric
power is 0.15 to 5 W/cm.sup.2.
[0025] In addition, it is preferable that a pressure in the chamber
is not higher than 13.3 Pa.
[0026] In addition, preferably, the high-frequency electric power
is applied to an electrode supporting the substrate to be
processed.
[0027] In addition, preferably, the plasma etching unit further
comprises: a second high-frequency electric power source configured
to apply a second high-frequency electric power of 3.2 MHz to 13.56
MHz to the electrode supporting the substrate to be processed, the
second high-frequency electric power being overlapped with the
high-frequency electric power. In the case, preferably, a frequency
of the second high-frequency electric power is 13.56 MHz. In
addition, preferably, power density of the second high-frequency
electric power is not higher than 0.64 W/cm.sup.2.
[0028] Herein, because of the Paschen's law, an electric-discharge
starting voltage Vs takes a local minimum value (Paschen's minimum
value) when a product pd of a gas pressure p and a distance d
between the electrodes takes a certain value. The certain value of
the product pd that corresponds to the Paschen's minimum value is
smaller when the frequency of the high-frequency electric power is
higher. Thus, when the frequency of the high-frequency electric
power is high, in order to decrease the electric-discharge starting
voltage Vs to facilitate and stabilize the electric-discharge
effect, the distance d between the electrodes has to be reduced, if
the gas pressure p is constant. Thus, in the present invention, it
is preferable that the distance between the electrodes is shorter
than 50 mm. In addition, when the distance between the electrodes
is shorter than 50 mm, residence time of the gas in the chamber can
be shortened. Thus, reaction products can be efficiently
discharged, and etching stop can be reduced.
[0029] In addition, it is preferable that the plasma etching unit
further comprises a magnetic-field forming unit configured to form
a magnetic field around a plasma region between the pair of
electrodes.
[0030] When the frequency of the applied high-frequency electric
power is high, the etching rate may be higher in a central portion
as a feeding position compared with in a peripheral portion.
However, if a magnetic field is formed around a plasma region
between the pair of electrodes, plasma confining effect can be
achieved so that the etching rate on the substrate to be processed
arranged in a processing space can be made substantially the same
between in an edge portion (peripheral portion) of the substrate to
be processed and in a central portion thereof. That is, the etching
rate can be made uniform.
[0031] It is preferable that strength of the magnetic field formed
around a plasma region between the pair of electrodes by the
magnetic-field forming unit is 0.03 to 0.045 T (300 to 450
Gauss).
[0032] In addition, it is preferable that a focus ring is provided
around the electrode supporting the substrate to be processed, and
that when the magnetic-field forming unit forms a magnetic field
around a plasma region between the pair of electrodes, strength of
the magnetic field on the focus ring is not lower than 0.001 T (10
Gauss) and strength of the magnetic field on the substrate to be
processed is not higher than 0.001 T.
[0033] When the strength of the magnetic field on the focus ring is
not lower than 0.001 T, drift movement of electrons may be
generated on the focus ring, so that the plasma density around the
focus ring is raised to make the plasma density uniform. On the
other hand, when the strength of the magnetic field on the
substrate to be processed is not higher than 0.001 T, which
substantially has no effect on the substrate to be processed,
charge-up damage can be prevented.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] FIG. 1 is a schematic vertical sectional view showing a
plasma etching unit of an embodiment according to the present
invention;
[0035] FIG. 2 is a horizontal sectional view schematically showing
a magnetic annular unit arranged around a chamber of the plasma
etching unit of FIG. 1;
[0036] FIG. 3 is a sectional views showing a structural example of
semiconductor wafer to which a plasma etching process according to
the present invention is applied;
[0037] FIG. 4 is a sectional views showing another structural
example of semiconductor wafer to which a plasma etching process
according to the present invention is applied;
[0038] FIG. 5 is a schematic sectional view partly showing a plasma
processing unit comprising a high-frequency electric power source
for generating plasma and a high-frequency electric power source
for drawing ions;
[0039] FIG. 6 is a graph showing relationships between the absolute
value of a self-bias electric voltage .vertline.Vdc.vertline. and
plasma density Ne, in respective cases wherein the frequency of the
high-frequency electric power is 40 MHz or 100 MHz, when the plasma
consists of argon gas;
[0040] FIG. 7A is a graph showing etching rates of a poly-silicon
film at a wafer position, in respective cases wherein the
high-frequency electric power is 500 W, 1000 W or 1500 W, when the
frequency of the high-frequency electric power is 100 MHz;
[0041] FIG. 7B is a graph showing etching rates of a poly-silicon
film at a wafer position, in respective cases wherein the
high-frequency electric power is 500 W or 1000 W, when the
frequency of the high-frequency electric power is 40 MHz;
[0042] FIG. 8 is a graph showing relationships between a
high-frequency electric power and an etching rate of the
poly-silicon film, in respective cases wherein the frequency of the
high-frequency electric power is 40 MHz or 100 MHz;
[0043] FIG. 9 is a graph showing relationships between a
high-frequency electric power and an etching rate of the SiO.sub.2
film, in respective cases wherein the frequency of the
high-frequency electric power is 40 MHz or 100 MHz;
[0044] FIG. 10 is a graph showing relationships between a
high-frequency electric power and an etching rate of the
poly-silicon film and relationships between a high-frequency
electric power and a ratio (an etching rate of the poly-silicon
film/an etching rate of the SiO.sub.2 film) corresponding to an
etching selective ratio, in respective cases wherein the frequency
of the high-frequency electric power is 40 MHz or 100 MHz;
[0045] FIG. 11 is a graph showing relationships between an etching
rate of the poly-silicon film and a ratio (an etching rate of the
poly-silicon film/an etching rate of the SiO.sub.2 film)
corresponding to an etching selective ratio, in respective cases
wherein the frequency of the high-frequency electric power is 40
MHz or 100 MHz;
[0046] FIG. 12A is a graph showing relationships between a pressure
in the chamber at the etching process and an etching rate of the
poly-silicon film, in respective cases wherein the frequency of the
high-frequency electric power is 40 MHz or 100 MHz;
[0047] FIG. 12B is a graph showing relationships between a pressure
in the chamber at the etching process and an etching rate of the
SiO.sub.2 film, in respective cases wherein the frequency of the
high-frequency electric power is 40 MHz or 100 MHz;
[0048] FIG. 13 is a graph showing relationships between a pressure
in the chamber and a ratio (an etching rate of the poly-silicon
film/an etching rate of the SiO.sub.2 film) corresponding to an
etching selective ratio, in respective cases wherein the frequency
of the high-frequency electric power is 40 MHz or 100 MHz;
[0049] FIG. 14 is a graph showing relationships between a pressure
in the chamber and an etching rate of the poly-silicon film and
relationships between a high-frequency electric power and a ratio
(an etching rate of the poly-silicon film/an etching rate of the
SiO.sub.2 film) corresponding to an etching selective ratio, in
respective cases wherein the frequency of the high-frequency
electric power is 40 MHz or 100 MHz;
[0050] FIG. 15 is a graph showing relationships between an etching
rate of the poly-silicon film and a ratio (an etching rate of the
poly-silicon film/an etching rate of the SiO.sub.2 film)
corresponding to an etching selective ratio, in respective cases
wherein the frequency of the high-frequency electric power is 40
MHz or 100 MHz;
[0051] FIG. 16 is a graph showing relationships between the
absolute value of a self-bias electric voltage
.vertline.Vdc.vertline. and plasma density Ne, when the plasma
consists of a HBr gas, in respective cases wherein the
high-frequency electric power is 500 W, 1000 W, 1500 W or 2000 W,
the frequency of the high-frequency electric power being 100 MHz,
and the second high-frequency electric power is 0 W, 200 W or 600
W, the frequency of the second high-frequency electric power being
13.56 MHz;
[0052] FIG. 17 is a graph showing relationships between a
high-frequency electric power and an etching rate of the
poly-silicon film and relationships between a high-frequency
electric power and a ratio (an etching rate of the poly-silicon
film/an etching rate of the SiO.sub.2 film) corresponding to an
etching selective ratio;
[0053] FIG. 18 is a graph showing relationships between a second
high-frequency electric power and an etching rate of the
poly-silicon film and relationships between a second high-frequency
electric power and a ratio (an etching rate of the poly-silicon
film/an etching rate of the SiO.sub.2 film) corresponding to an
etching selective ratio; and
[0054] FIG. 19 is a graph comparatively showing relationships
between an Ar-gas flow rate and a pressure difference .DELTA.P of a
central portion of the wafer and a peripheral portion thereof, in
respective cases wherein an electrode gap is 25 mm or 40 mm,
wherein the Ar gas is used as a plasma gas.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0055] An embodiment of the invention will now be described with
reference to the attached drawings.
[0056] FIG. 1 is a schematic sectional view showing a plasma
etching unit used for carrying out the present invention. The
etching unit of the embodiment includes a two-stage cylindrical
chamber vessel 1, which has an upper portion 1a having a small
diameter and an lower portion 1b having a large diameter. The
chamber vessel 1 may be hermetically made of any material, for
example aluminum.
[0057] A supporting table 2 is arranged in the chamber vessel 1 for
horizontally supporting a wafer W as a substrate to be processed.
The supporting table 2 may be made of any material, for example
aluminum. The supporting table 2 is placed on a conductive
supporting stage 4 via an insulation plate 3. A focus ring 5 is
arranged on a peripheral area of the supporting table 2. The focus
ring 5 may be made of any conductive material or any insulating
material. When the diameter of the wafer W is 200 mm.phi., it is
preferable that the focus ring 5 is 240 to 280 mm.phi.. The
supporting table 2, the insulation plate 3, the supporting stage 4
and the focus ring 5 can be elevated by a ball-screw mechanism
including a ball-screw 7. A driving portion for the elevation is
arranged below the supporting stage 4 and is covered by a bellows
8. The bellows 8 may be made of any material, for example stainless
steel (SUS). The chamber vessel 1 is earthed. A coolant passage
(not shown) is formed in the supporting table 2 in order to cool
the supporting table 2. A bellows cover 9 is provided around the
bellows 8.
[0058] A feeding cable 12 for supplying a high-frequency electric
power is connected to a substantially central portion of the
supporting table 2. The feeding cable 12 is connected to a
high-frequency electric power source 10 via a matching box 11. A
high-frequency electric power of a predetermined frequency is
adapted to be supplied from the high-frequency electric power
source 10 to the supporting table 2. A showerhead 16 is provided
above the supporting table 2 and oppositely in parallel with the
supporting table 2. The showerhead 16 is also earthed. Thus, the
supporting table 2 functions as a lower electrode, and the
showerhead 16 functions as an upper electrode. That is, the
supporting table 2 and the showerhead 16 form a pair of plate
electrodes.
[0059] Herein, it is preferable that the distance between the
electrodes is set to be shorter than 50 mm. The reason is as
follows.
[0060] Because of the Paschen's law, an electric-discharge starting
voltage Vs takes a local minimum value (Paschen's minimum value)
when a product pd of a gas pressure p and a distance d between the
electrodes takes a certain value. The certain value of the product
pd that corresponds to the Paschen's minimum value is smaller when
the frequency of the high-frequency electric power is higher. Thus,
when the frequency of the high-frequency electric power is high
like the present embodiment, in order to decrease the
electric-discharge starting voltage Vs to facilitate and stabilize
the electric-discharge effect, the distance d between the
electrodes has to be reduced, if the gas pressure p is constant.
Thus, it is preferable that the distance between the electrodes is
shorter than 50 mm. In addition, when the distance between the
electrodes is shorter than 50 mm, residence time of the gas in the
chamber can be shortened. Thus, reaction products can be
efficiently discharged, and etching stop can be reduced.
[0061] However, if the distance between the electrodes is too
short, pressure distribution on the surface of the wafer W as a
substrate to be processed (pressure difference between in a central
portion and in a peripheral portion) may become large. In the case,
problems such as deterioration of etching uniformity may be
generated. Independently on gas flow rate, in order to make the
pressure difference smaller than 0.27 Pa (2 mTorr), it is
preferable that the distance between the electrodes is not shorter
than 35 mm.
[0062] An electrostatic chuck 6 is provided on an upper surface of
the supporting table 2 in order to electrostaticly stick to the
wafer W. The electrostatic chuck 6 consists of an insulation plate
6b and an electrode 6a inserted in the insulation plate 6b. The
electrode 6a is connected to a direct-current power source 13.
Thus, when the direct-current power source 13 supplies an electric
power to the electrode 6a, the semiconductor wafer W may be stuck
to the electrostatic chuck 6 by coulomb force, for example.
[0063] The coolant passage not shown is formed in the supporting
table 2. The wafer W can be controlled at a predetermined
temperature by circulating a suitable coolant in the coolant
passage. In order to efficiently transmit heat of cooling from the
suitable coolant to the wafer W, a gas-introducing mechanism (not
shown) for supplying a He gas onto a reverse surface of the wafer W
is provided. In addition, a baffle plate 14 is provided at an
outside area of the focus ring 5. The baffle plate 14 is
electrically connected to the chamber vessel 1 via the supporting
stage 4 and the bellows 8.
[0064] The showerhead 16 facing the supporting table 2 is provided
in a ceiling of the chamber vessel 1. The showerhead 16 has a large
number of gas jetting holes 18 at a lower surface thereof and a gas
introducing portion 16a at an upper portion thereof. Then, an
inside space 17 is formed between the gas introducing portion 16a
and the large number of gas jetting holes 18. The gas introducing
portion 16a is connected to a gas supplying pipe 15a. The gas
supplying pipe 15a is connected to a process-gas supplying system
15, which can supply a process gas for etching that consists of a
reaction gas and a diluent gas.
[0065] As the reaction gas, any halogen gas may be used. As the
diluent gas, an Ar gas, a He gas, or any other gas generally used
in this field may be used.
[0066] The process gas is supplied from the process-gas supplying
system 15 into the space 17 of the showerhead 16 through the gas
supplying pipe 15a and the gas introducing portion 16a. Then, the
process gas is jetted from the gas jetting holes 18 in order to
etch a film formed on the wafer W.
[0067] A discharging port 19 is formed at a part of a side wall of
the lower portion 1b of the chamber 1. The discharging port 19 is
connected to a gas-discharging system 20 including a vacuum pump. A
pressure of an inside of the chamber vessel 1 may be reduced to a
predetermined vacuum level by operating the vacuum pump. A
transferring port for the wafer W and a gate valve 24 for opening
and closing the transferring port are arranged at an other upper
part of the side wall of the lower portion 1b of the chamber vessel
1.
[0068] A magnetic annular unit 21 is concentrically arranged around
the upper portion 1a of the chamber vessel 1. Thus, a magnetic
field may be formed around a processing space between the
supporting table 2 and the showerhead 16. The magnetic annular unit
21 may be caused to revolve around a center axis thereof (along an
annular peripheral edge thereof) by a revolving mechanism 25.
[0069] The magnetic annular unit 21 has a plurality of segment
magnets 22 which are supported by a holder not shown and which are
arranged annularly. Each of the plurality of segment magnets 22
consists of a permanent magnet. In the embodiment, 16 segment
magnets 22 are arranged annularly (concentrically) in a multi-pole
state. That is, in the magnetic annular unit 21, adjacent two
segment magnets 22 are arranged in such a manner that their
magnetic-pole directions are opposite. Thus, a magnetic line of
force is formed between the adjacent two segment magnets 22 as
shown in FIG. 2, so that a magnetic field of 0.02 to 0.2 T (200 to
2000 Gauss), preferably 0.03 to 0.045 T (300 to 450 Gauss), is
generated only around the processing space. On the other hand, in a
region wherein the wafer is placed, a substantially non-magnetic
field state is generated. The above strength of the magnetic field
is determined because of the following reasons: if the magnetic
field is too strong, a fringing field may be caused; and if the
magnetic field is too weak, plasma confining effect can not be
achieved. Of course, the suitable strength of the magnetic field
also depends on the unit structure or the like. That is, the range
of the suitable strength of the magnetic field may be different for
respective units.
[0070] When the above magnetic field is formed around the
processing space, strength of the magnetic field on the focus ring
5 is desirably not lower than 0.001 T (10 Gauss). In the case,
drift movement of electrons (E.times.B drift) is generated on the
focus ring, so that the plasma density around the wafer is
increased, and hence the plasma density is made uniform. On the
other hand, in view of preventing charge-up damage of the wafer W,
strength of the magnetic field in a portion where the wafer W is
positioned is desirably not higher than 0.001 T (10 Gauss).
[0071] Herein, the substantially non-magnetic state in a region
occupied by the wafer means a state that there is not a magnetic
field affecting the etching process in the area occupied by the
wafer. That is, the substantially non-magnetic state includes a
state that there is a magnetic field not substantially affecting
the wafer process.
[0072] In the state shown in FIG. 2, a magnetic field whose density
is not more than 0.42 mT (4.2 Gauss) is applied to a peripheral
area of the wafer. Thus, plasma confining function can be
achieved.
[0073] When a magnetic field is formed by the magnetic annular unit
of the multi-pole state, wall portions of the chamber 1
corresponding to the magnetic poles (for example, portions shown by
P in FIG. 2) may be locally whittled. Thus, the magnetic annular
unit 21 may be caused to revolve along the peripheral direction of
the chamber by the above revolving mechanism 25. Thus, it is
avoided that the magnetic poles are locally abutted (located)
against the chamber wall, so that it is prevented that the chamber
wall is locally whittled.
[0074] Each segment magnet 22 is configured to freely revolve
around a perpendicular axis thereof by a segment-magnet revolving
mechanism not shown. Then, when the segment magnets 22 are caused
to revolve, the state wherein the multi-pole magnetic field is
substantially formed and the state wherein the multi-pole magnetic
field is not formed can be switched. Depending on a process
condition, the multi-pole magnetic field may be effective or not.
Thus, when the state wherein the multi-pole magnetic field is
formed and the state wherein the multi-pole magnetic field is not
formed can be switched, a suitable state can be selected
correspondingly to the process condition.
[0075] As the state of the magnetic field is changed depending on
the arrangement of the segment magnets, when the arrangement of the
segment magnets is changed variously, various profiles of magnetic
field can be formed. Thus, it is preferable to arrange the segment
magnets so as to obtain a required profile of magnetic field.
[0076] The number of the segment magnets is not limited to the
above examples. The section of each segment magnet is not limited
to the rectangle, but may have any shape such as a circle, a
square, a trapezoid or the like. A magnetic material forming the
segment magnets 22 is also not limited, but may be any known
magnetic material such as a rare-earth magnetic material, ferrite
magnetic material, an Arnico magnetic material, or the like.
[0077] The above plasma etching unit can be used for an etching
process to a poly-silicon film adjacent to an inorganic-material
film such as an SiO.sub.2 film or an SiN film. An operation for the
etching process by means of the plasma etching unit is
explained.
[0078] For example, as shown in FIG. 3, a wafer W to be etched has
a structure wherein a poly-silicon film 32 is formed on a silicon
substrate 31 and wherein an inorganic-material film 33 having a
predetermined pattern as a hard mask is formed on the poly-silicon
film 32. Alternatively, as shown in FIG. 4, a wafer W has another
structure wherein an inorganic-material film 42 consisting of
SiO.sub.2 as a gate oxide film is formed on a silicon film 41,
wherein a poly-silicon film 43 as a gate is formed on the
inorganic-material film 42, and wherein a resist film 44 having a
predetermined pattern as a mask is formed on the poly-silicon film
43.
[0079] The inorganic-material film 33 consists of a material
generally used as a hard mask. As a suitable example, it may be a
silicon oxide, a silicon nitride, a silicon oxinitride, a silicon
carbide, or the like. That is, it is preferable that the
inorganic-material film 43 consists of at least one of the above
materials.
[0080] In each of the above wafers W, the poly-silicon film 32 or
43 is etched. At first, the gate valve 24 is opened, the wafer W is
conveyed into the chamber 1 by means of a conveying arm, and placed
on the supporting table 2. After that, the conveying arm is
evacuated, the gate valve 24 is closed, and the supporting table 2
is moved up to a position shown in FIG. 1. The vacuum pump of the
gas-discharging system 20 creates a predetermined vacuum in the
chamber 1 through the discharging port 19.
[0081] Then, a predetermined process gas, for example an HBr gas,
is introduced into the chamber 1 through the process-gas supplying
system 15, for example at a flow rate of 0.02 to 0.4 L/min (20 to
400 sccm). Thus, a pressure in the chamber 1 is maintained at a
predetermined pressure. In this state, a high-frequency electric
power whose frequency is 50 to 150 MHz, preferably 70 to 100 MHz,
is supplied from the high-frequency electric power source 10 to the
supporting table 2. In this case, power per unit area i.e. power
density is preferably within a range of about 0.15 to about 5.0
W/cm.sup.2. Then, a predetermined electric voltage is applied from
the direct current power source 13 to the electrode 6a of the
electrostatic chuck 6, so that the wafer W sticks to the
electrostatic chuck 6 by means of Coulomb force, for example.
[0082] When the high-frequency electric power is applied to the
supporting table 2 as the lower electrode as described above, a
high-frequency electric field is formed in the processing space
between the showerhead 16 as the upper electrode and the supporting
table 2 as the lower electrode. Thus, the process gas supplied into
the processing space is made plasma, which etches the poly-silicon
film on the wafer W.
[0083] During the etching step, by means of the annular magnetic
unit 21 of a multi-pole state, a magnetic field as shown in FIG. 2
can be formed around the processing space. In the case, plasma
confining effect is achieved, so that an etching rate of the wafer
W may be made uniform, even in a case of a high frequency like this
embodiment wherein the plasma tends to be not uniform. However,
depending on the process condition, it is preferable that the
magnetic field is not formed. In the case, the segment magnets 22
may be caused to revolve in order to conduct the etching process
under a condition wherein a magnetic field is substantially not
formed around the processing space.
[0084] When the above magnetic field is formed, by means of the
electrically conductive or insulating focus ring 5 provided around
the wafer W on the supporting table 2, the effect of making the
plasma process uniform can be more enhanced. That is, if a plasma
density at a peripheral portion of the wafer is high and an etching
rate at the peripheral portion of the wafer is larger than that at
a central portion of the wafer, by using a focus ring made of an
electrically conductive material such as silicon or SiC, even a
focus-ring region functions as the lower electrode. Thus, a
plasma-forming region is expanded over the focus ring 5, the plasma
process around the wafer W is promoted, so that uniformity of the
etching rate is improved. In addition, if a plasma density at the
peripheral portion of the wafer is low and an etching rate at the
peripheral portion of the wafer is smaller than that at the central
portion of the wafer, by using a focus ring made of an electrically
insulating material such as quartz, electric charges can not be
transferred between the focus ring 5 and electrons and ions in the
plasma. Thus, the plasma confining effect may be increased so that
uniformity of the etching rate is improved.
[0085] In order to adjust plasma density and ion-drawing effect,
the high-frequency electric power for generating plasma and a
second high-frequency electric power for drawing ions may be
overlapped with each other. Specifically, as shown in FIG. 5, in
addition to the high-frequency electric power source 10 for
generating plasma, a second high-frequency electric power source 26
for drawing ions is connected to the matching box 11, so that they
are overlapped. In the case, the frequency of the second
high-frequency electric power source 26 for drawing ions is
preferably 3.2 to 13.56 MHz, in particular 13.56 MHz. Thus, the
number of parameters for controlling ion energy is increased so
that an optimum processing condition can be easily set wherein an
etching rate of the poly-silicon film is raised more while a
necessary and sufficient etching selective ratio with respect to
the inorganic-material film is assured.
[0086] Herein, according to a result of study by the inventors, in
the etching process of the poly-silicon film, the plasma density is
dominant, and the ion energy contributes only a little. On the
other hand, in the etching process of the inorganic-material film,
both the plasma density and the ion energy are necessary. Thus, as
shown in FIGS. 3 and 4, in the etching process of the poly-silicon
film adjacent to the inorganic-material film, in order to etch the
poly-silicon film with a high etching selective ratio with respect
to the inorganic-material film, the plasma density has to be high
and the ion energy has to be low. That is, if the ion energy
necessary for etching the inorganic-material film is low and the
plasma density dominant for etching the poly-silicon film is high,
only the poly-silicon film can be selectively etched. Herein, the
ion energy of the plasma indirectly corresponds to a self-bias
electric voltage of an electrode at the etching process. Thus, in
order to etch the poly-silicon film with a high etching selective
ratio, finally, it is necessary to etch the organic-material film
under a condition of high plasma density and low self-bias electric
voltage.
[0087] On the other hand, in order to improve the etching geometric
control performance, it is necessary to conduct the etching process
under a low pressure. However, when the above condition is
satisfied, a process under a lower pressure can achieve a high
etching selective ratio. That is, if a high plasma density and a
low self-bias electric voltage are achieved, the etching selective
ratio of the poly-silicon film with respect to the
inorganic-material film can be enhanced even under a lower
pressure. Thus, a high etching selective ratio and a good etching
geometric control performance can not be in conflict with each
other. For that purpose, it was found that the frequency of the
high-frequency electric power to be applied to the electrode is 50
to 150 MHz, which is higher than prior art.
[0088] This is explained with reference to FIG. 6 as follows. FIG.
6 is a graph showing relationships between the absolute value of a
self-bias electric voltage .vertline.Vdc.vertline. and plasma
density Ne, in respective cases wherein the frequency of the
high-frequency electric power is 40 MHz or 100 MHz. The transverse
axis represents the absolute value of a self-bias electric voltage
.vertline.Vdc.vertline., and the ordinate axis represents the
plasma density Ne. In the case, as the plasma gas, Ar was used for
evaluation, instead of real etching gas. For each frequency,
applied high-frequency electric power was changed, so that values
of the plasma density Ne and the absolute value of a self-bias
electric voltage .vertline.Vdc.vertline. were changed. That is, in
the respective frequencies, if the applied high-frequency electric
power is large, both the plasma density Ne and the absolute value
of a self-bias electric voltage .vertline.Vdc.vertline. are large.
Herein, the plasma density was measured by means of a microwave
interferometer.
[0089] As shown in FIG. 6, in the case wherein the frequency of the
high-frequency electric power is conventionally 40 MHz, when the
plasma density is increased to enhance the etching rate of the
poly-silicon film, the absolute value of a self-bias electric
voltage .vertline.Vdc.vertline. is greatly increased. On the other
hand, in the case wherein the frequency of the high-frequency
electric power is 100 MHz that is higher than prior art, even when
the plasma density is increased, the absolute value of a self-bias
electric voltage .vertline.Vdc.vertline. is not so increased and
controlled substantially not higher than 100 V. That is, it was
found that a condition of high plasma density and low self-bias
electric voltage can be achieved. That is, if the frequency is
relatively low like a conventional art, when the etching rate of
the poly-silicon film is increased in a real etching process under
a low pressure, the inorganic-material film is also etched to the
same extent and good selective-etching performance is not achieved.
On the other hand, if the frequency is as high as 100 MHz, it was
found that the poly-silicon film can be etched with a high etching
selective ratio with respect to the inorganic-material film.
[0090] In addition, as seen from FIG. 6, in order to etch the
poly-silicon film under a low pressure with a higher etching
selective ratio by higher plasma density and lower self-bias
electric voltage than prior art, when the plasma of Ar gas is
formed, it may be thought preferable to form the plasma under a
condition wherein the plasma density is not less than
1.times.10.sup.10 cm.sup.-3 and the self-bias electric voltage of
the electrode is not higher than 100 V. Alternatively, the plasma
density is not less than 5.times.10.sup.10 cm.sup.-3 and the
self-bias electric voltage of the electrode is not higher than 200
V. Then, in order to satisfy such a plasma condition, it may be
estimated that the frequency of the high-frequency electric power
has to be 50 MHz or higher.
[0091] Thus, the frequency of the high-frequency electric power for
generating plasma is set not less than 50 MHz, as described above.
However, if the frequency of the high-frequency electric power for
generating plasma is higher than 150 MHz, the uniformity of the
plasma may be deteriorated. Thus, it is preferable that the
frequency of the high-frequency electric power for generating
plasma is not higher than 150 MHz. In particular, in order to
effectively achieve the above effect, it is preferable that the
frequency of the high-frequency electric power for generating
plasma is 70 to 100 MHz.
[0092] It is preferable that a pressure in the chamber at the
etching process is not higher than 13.3 Pa (100 mT). From a view of
preventing any conflict between the etching selective ratio of the
poly-silicon film with respect to the inorganic-material film and
the etching geometric control performance, it is more preferable
that a pressure in the chamber is not higher than 4 Pa (30 mT). If
the etching geometric control performance is thought to be more
important, it is further more preferable that a pressure in the
chamber is not higher than 1.33 Pa (10 mT).
[0093] Next, in order to obtain a real etching rate of an
poly-silicon film and an etching selective ratio with respect to an
inorganic-material film, etching experiments for whole-surface
formed films of an poly-silicon film and an inorganic-material film
(SiO.sub.2) were conducted. The result is explained.
[0094] Herein, a 200 mm wafer was used as the wafer W, an HBr gas:
0.2 L/min (0.02 L/min only when the pressure is 0.133 Pa) was
supplied as an etching gas, the gap between the electrodes was 27
mm, and the pressure in the chamber was 4 Pa.
[0095] FIG. 7A is a graph showing etching rates of a poly-silicon
film at a wafer position, in respective cases wherein the
high-frequency electric power is 500 W (1.59 W/cm.sup.2), 1000 W
(3.18 W/cm.sup.2) or 1500 W (4.77 W/cm.sup.2), when the frequency
of the high-frequency electric power is 100 MHz. FIG. 7B is a graph
showing etching rates of a poly-silicon film at a wafer position,
in respective cases wherein the high-frequency electric power is
500 W (1.59 W/cm.sup.2), 1000 W (3.18 W/cm.sup.2) or 1500 W (4.77
W/cm.sup.2), when the frequency of the high-frequency electric
power is 40 MHz. FIG. 8 is a graph showing relationships between a
high-frequency electric power and an etching rate of the
poly-silicon film, in respective cases wherein the frequency of the
high-frequency electric power is 40 MHz or 100 MHz. FIG. 9 is a
graph showing relationships between a high-frequency electric power
and an etching rate of the SiO.sub.2 film, in respective cases
wherein the frequency of the high-frequency electric power is 40
MHz or 100 MHz. FIG. 10 is a graph showing relationships between a
high-frequency electric power and an etching rate of the
poly-silicon film and relationships between a high-frequency
electric power and a ratio (an etching rate of the poly-silicon
film/an etching rate of the SiO.sub.2 film) corresponding to an
etching selective ratio, in respective cases wherein the frequency
of the high-frequency electric power is 40 MHz or 100 MHz. FIG. 11
is a graph showing relationships between an etching rate of the
poly-silicon film and a ratio (an etching rate of the poly-silicon
film/an etching rate of the SiO.sub.2 film) corresponding to an
etching selective ratio, in respective cases wherein the frequency
of the high-frequency electric power is 40 MHz or 100 MHz.
[0096] As seen from these drawings, the etching rate of the
poly-silicon film tends to be increased when the high-frequency
electric power is increased. However, there is no great difference
between those in the cases of 40 MHz and 100 MHz. In addition, at
the same gas pressure and the same power, the etching rates of the
poly-silicon film in the cases of 40 MHz and 100 MHz are at the
same level, but the etching rate of the SiO.sub.2 film in the case
of 40 MHz is higher than that in the case of 100 MHz. Thus, it was
confirmed that the ratio corresponding to an etching selective
ratio (an etching rate of the poly-silicon film/an etching rate of
the SiO.sub.2 film) is higher in the case of 100 MHz than in the
case of 40 MHz. That is, from the experimental result of the
samples for estimation, at the pressure of 4 Pa, it was confirmed
that the possibility of etching the poly-silicon film with a high
etching selective ratio is higher in the case of 100 MHz than in
the case of 40 MHz. If the power of the high-frequency electric
power is increased too much, the etching rate of the poly-silicon
film is increased but the etching selective ratio is decreased,
because the etching rate and the etching selective ratio of the
poly-silicon film are in a tradeoff relationship. Thus, it is
preferable that the power density of the high-frequency electric
power of 100 MHz is not higher than 5 W/cm.sup.2 (about 1500
W).
[0097] On the other hand, in the case of 100 MHz, when the power
density is decreased, the etching rate of the poly-silicon film is
decreased and the etching selective ratio with respect to the
SiO.sub.2 film is improved. If a base film of a film to be etched
is a gate oxide film such as a SiO.sub.2 film, since the base film
has usually a thickness of several nm, the etching rate of the
SiO.sub.2 film has to be decreased to an order of 0.1 nm/min. For
example, when the pressure condition is 1.33 Pa (10 mT) and the
power density is 1.5 W/cm.sup.2 (about 500 W), the etching rate of
the poly-silicon film is 100 nm/min, the etching selective ratio is
70, and the etching rate of the SiO.sub.2 film is 1.43 nm/min.
Thus, in order to decrease the etching rate of the SiO.sub.2 film
to the order of 0.1 nm/min, it is estimated that the power density
has to be decreased to about 0.15 to 0.3 W/cm.sup.2 (about 50 to
100 W). Taking into account the above point, it is preferable that
the minimum high-frequency electric power is not lower than 0.3
W/cm.sup.2, in particular not lower than 0.15 W/cm.sup.2 (about 50
W). In view of only the etching selectivity, it is preferable that
the high-frequency electric power is not higher than 1.5 W/cm.sup.2
(about 500 W).
[0098] Next, other etching processes were conducted while the flow
rate of the HBr gas was changed within a range of 0.02 to 0.2
L/min, the pressure in the chamber was changed within a range of
0.133 to 13.3 Pa, the high-frequency electric power was fixed to
500 W, and the other conditions were the same as the above.
[0099] FIG. 12A is a graph showing relationships between a pressure
in the chamber at the etching process and an etching rate of the
poly-silicon film, in respective cases wherein the frequency of the
high-frequency electric power is 40 MHz or 100 MHz. FIG. 12B is a
graph showing relationships between a pressure in the chamber at
the etching process and an etching rate of the SiO.sub.2 film, in
respective cases wherein the frequency of the high-frequency
electric power is 40 MHz or 100 MHz. FIG. 13 is a graph showing
relationships between a pressure in the chamber and a ratio (an
etching rate of the poly-silicon film/an etching rate of the
SiO.sub.2 film) corresponding to an etching selective ratio, in
respective cases wherein the frequency of the high-frequency
electric power is 40 MHz or 100 MHz. FIG. 14 is a graph showing
relationships between a pressure in the chamber and an etching rate
of the poly-silicon film and relationships between a high-frequency
electric power and a ratio (an etching rate of the poly-silicon
film/an etching rate of the SiO.sub.2 film) corresponding to an
etching selective ratio, in respective cases wherein the frequency
of the high-frequency electric power is 40 MHz or 100 MHz. FIG. 15
is a graph showing relationships between an etching rate of the
poly-silicon film and a ratio (an etching rate of the poly-silicon
film/an etching rate of the SiO.sub.2 film)-corresponding to an
etching selective ratio, in respective cases wherein the frequency
of the high-frequency electric power is 40 MHz or 100 MHz.
[0100] As seen from these drawings, at the same high-frequency
electric power and the same pressure in the chamber, the etching
rate of the poly-silicon film is a little higher and the etching
selective ratio is also higher in the case of 100 MHz than in the
case of 40 MHz. In addition, as the same high-frequency electric
power, a high etching selective ratio can be achieved at a lower
pressure in the case of 100 MHz than in the case of 40 MHz. In
addition, as shown in FIG. 15, at the same high-frequency electric
power and the same etching rate, the etching selective ratio is
higher in the case of 100 MHz than in the case of 40 MHz. That is,
in the case of 100 MHz, a high etching selective ratio can be
achieved under a condition of a lower pressure, which is
advantageous in the etching geometric control performance, so that
both the high etching selectivity and the good etching geometric
control performance can be achieved.
[0101] Regarding the effect of the pressure, in the both cases of
40 MHz and 100 MHz, when the pressure is higher, the etching rate
and the etching selective ratio of the poly-silicon film are
better. However, in view of the etching geometric control
performance of the poly-silicon film, it was confirmed that a lower
pressure is preferable, specifically not higher than 13.3 Pa.
[0102] Next, a real etching gas (HBr) was used and a high-frequency
electric power of 100 MHz was applied to measure the absolute value
of a self-bias electric voltage .vertline.Vdc.vertline. and plasma
density Ne. The measurement results are explained.
[0103] FIG. 16 is a graph showing relationships between the
absolute value of a self-bias electric voltage
.vertline.Vdc.vertline. and plasma density Ne, when the plasma
consists of a HBr gas and the frequency of the high-frequency
electric power is 100 MHz. The transverse axis represents the
absolute value of a self-bias electric voltage
.vertline.Vdc.vertline., and the ordinate axis represents the
plasma density Ne. The plasma density was measured by means of a
microwave interferometer.
[0104] Herein, the pressure in the chamber was 2.7 Pa (20 mTorr).
In addition, the high-frequency electric power of 100 MHz was
changed within a range of 500 to 2000 W so that the plasma density
Ne and the absolute value of a self-bias electric voltage
.vertline.Vdc.vertline. were changed. In addition, when the
high-frequency electric power of 100 MHz was 500 W, a second
high-frequency electric power of 0 W, 200 W or 600 W, whose
frequency was 13.56 MHz, was overlapped with the high-frequency
electric power.
[0105] As seen from FIG. 16, in the respective frequencies, when
the applied high-frequency electric power is larger, both the
plasma density Ne and the absolute value of a self-bias electric
voltage .vertline.Vdc.vertline. are larger.
[0106] As shown in FIG. 16, in the case of the plasma of the real
etching gas, compared with the plasma of the Ar gas (see FIG. 6),
the plasma density tends to be a little lower. In addition, when
the second high-frequency electric power of the lower frequency (13
MHz) is overlapped to increase the power, the self-bias electric
voltage tends to be increased.
[0107] In addition, as seen from FIG. 16, when the second
high-frequency electric power is not overlapped and the plasma
density is increased, the absolute value of a self-bias electric
voltage .vertline.Vdc.vertline. is not increased so much, but
maintained at about 100 V or less. That is, it was found that the
high plasma density and the low self-bias electric voltage can be
achieved.
[0108] FIG. 17 is a graph showing relationships between a
high-frequency electric power and an etching rate of the
poly-silicon film and relationships between a high-frequency
electric power and a ratio (an etching rate of the poly-silicon
film/an etching rate of the SiO.sub.2 film) corresponding to an
etching selective ratio, wherein the second high-frequency electric
power is not overlapped.
[0109] When the high-frequency electric power is increased, the
etching rate of the poly-silicon film is increased but the
selective ratio is decreased. Thus, it is preferable that the
high-frequency electric power is not higher than about 1500 W
(about 4.77 W/cm.sup.2). On the other hand, when the high-frequency
electric power is decreased, the etching rate is decreased but the
selective ratio is increased. Thus, it is preferable that the
high-frequency electric power is not lower than about 500 W (about
1.5 W/cm.sup.2).
[0110] As seen from FIGS. 16 and 17, it was confirmed that a
necessary etching rate of the poly-silicon film can be achieved
while the poly-silicon film can be etched with a high etching
selective ratio with respect to the inorganic-material film, by
means of the high frequency of 100 MHz.
[0111] In addition, as seen from FIGS. 16 and 17, it is thought
preferable that the plasma density is 5.times.10.sup.9 to
2.times.10.sup.10 cm.sup.-3 and the self-bias electric voltage of
an electrode is not higher than 200 V, in order to etch the
poly-silicon film with a high selective ratio and a required
etching rate at a low pressure.
[0112] Herein, as a process gas, instead of the gas including an
HBr, a gas including a Cl.sub.2 gas may be used. In the latter
case, it was confirmed that a suitable range of the plasma density
is the same as the above.
[0113] FIG. 18 is a graph showing relationships between a second
high-frequency electric power and an etching rate of the
poly-silicon film and relationships between a second high-frequency
electric power and a ratio (an etching rate of the poly-silicon
film/an etching rate of the SiO.sub.2 film) corresponding to an
etching selective ratio, wherein the high-frequency electric power
is fixed to 500 W and the second high-frequency electric power is
overlapped with the high-frequency electric power.
[0114] As seen from FIGS. 16 and 18, when the second high-frequency
electric power of 13 MHz is overlapped to increase the electric
power, the etching rate is increased and the self-bias electric
voltage of an electrode is also increased. When the self-bias
electric voltage is increased, the etching selective ratio tends to
be decreased. However, until the self-bias electric voltage reaches
200 V, that is, the second high-frequency electric power reaches
about 200 W (about 0.64 W/cm.sup.2), the etching selective ratio
can be maintained within an allowable range.
[0115] Thus, by increasing the overlapped second high-frequency
electric power (bias electric power), the etching rate can be
enhanced while the etching selective ratio can be maintained at 10
or more.
[0116] In the above experiments, the gap between the electrodes was
27 mm. As described above, if the distance between the electrodes
is too small, pressure distribution (pressure difference between at
a central portion and at a peripheral portion) on the surface of
the wafer W, which is a substrate to be processed, becomes so large
that deterioration of the etching uniformity or the like may be
generated. Thus, in practice, the distance between the electrodes
is preferably 35 to 50 mm. This is explained with reference to FIG.
19.
[0117] FIG. 19 is a graph comparatively showing relationships
between an Ar-gas flow rate and a pressure difference .DELTA.P of a
central portion of the wafer and a peripheral portion thereof, in
respective cases wherein the electrode gap is 25 mm or 40 mm,
wherein the Ar gas is used as a plasma gas. As shown in FIG. 20,
the pressure difference .DELTA.P is smaller when the gap is 40 mm
rather than 25 mm. In addition, in the case of the gap of 25 mm,
when the Ar-gas flow rate is increased, the pressure difference
.DELTA.P tends to be sharply increased. When the gas flow rate is
higher than about 0.3 L/min, it exceeds 0.27 Pa (2 mTorr) as an
allowable maximum pressure difference .DELTA.P, at which
deterioration of the etching uniformity or the like may not be
generated. On the other hand, in the case of the gap of 40 mm,
independently on the gas flow rate, the pressure difference is
smaller than 0.27 Pa (2 mTorr). Thus, it can be expected that the
allowable maximum pressure difference .DELTA.P at which
deterioration of the etching uniformity or the like may not be
generated is ensured, independently on the gas flow rate, if the
electrode gap is not less than about 35 mm.
[0118] The present invention is not limited to the above embodiment
but may be variously modified. For example, in the above
embodiment, the silicon film is the poly-silicon film. However, the
silicon film may be a mono-crystal silicon film, an amorphous
silicon film, or any other silicon film.
[0119] In addition, in the above embodiment, as the magnetic-field
generating means, the annular magnetic unit in the multi-pole state
is used wherein the plurality of segment magnets consisting of
permanent magnets are arranged annularly around the chamber.
However, the present invention is not limited to this manner if a
magnetic-field can be formed around the processing space to confine
the plasma. In addition, the peripheral magnetic field for
confining the plasma may be unnecessary. That is, the etching
process can be conducted under a condition wherein there is no
magnetic field. In addition, the present invention can be applied
to a plasma etching process conducted in a crossed electromagnetic
field wherein a horizontal magnetic field is applied to the
processing space.
[0120] In addition, in the above embodiment, the high-frequency
electric power for generating plasma is applied to the lower
electrode, but may be applied to the upper electrode. The layer
structure of the substrate to be processed is not limited to those
shown in FIGS. 3 and 4. In addition, the semiconductor wafer is
taken as an example of the substrate to be processed. However, this
invention is not limited thereto, but applicable to an etching
process for a silicon film in another type of substrate to be
processed.
* * * * *