U.S. patent application number 11/755385 was filed with the patent office on 2007-09-27 for etching method and apparatus.
This patent application is currently assigned to TOKYO ELECTRON LIMITED. Invention is credited to Susumu SAITO, Akitaka Shimizu.
Application Number | 20070221258 11/755385 |
Document ID | / |
Family ID | 36034595 |
Filed Date | 2007-09-27 |
United States Patent
Application |
20070221258 |
Kind Code |
A1 |
SAITO; Susumu ; et
al. |
September 27, 2007 |
ETCHING METHOD AND APPARATUS
Abstract
An etching method capable of controlling the film thickness of a
hard mask layer uniformly is provided. A plasma etching is
performed on a native oxide film by using an etching gas
containing, for example, CF.sub.4 and Ar while a thickness of a
silicon nitride film is being monitored and the etching is finished
when the thickness of the silicon nitride film reaches a
predetermined value. Then, a plasma etching is performed on a
silicon substrate by employing an etching gas containing, for
example, Cl.sub.2, HBr and Ar and using the silicon nitride film as
a mask while a depth of a trench is being monitored and the etching
is finished when the depth of the trench reaches a specified
value.
Inventors: |
SAITO; Susumu;
(Nirasaki-shi, JP) ; Shimizu; Akitaka;
(Nirasaki-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
TOKYO ELECTRON LIMITED
Tokyo
JP
|
Family ID: |
36034595 |
Appl. No.: |
11/755385 |
Filed: |
May 30, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11224949 |
Sep 14, 2005 |
|
|
|
11755385 |
May 30, 2007 |
|
|
|
60614042 |
Sep 30, 2004 |
|
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Current U.S.
Class: |
134/56R ;
257/E21.218; 257/E21.231; 257/E21.528 |
Current CPC
Class: |
H01L 21/308 20130101;
H01L 22/26 20130101; H01L 21/3065 20130101 |
Class at
Publication: |
134/056.00R |
International
Class: |
H01L 21/00 20060101
H01L021/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 14, 2004 |
JP |
2004-266538 |
Claims
1. An etching apparatus for performing an etching on a substrate
including a mask layer having a pattern; a first layer formed in a
groove portion of the pattern; and a second layer formed beneath
the mask layer and the first layer, comprising: a plasma source for
generating a plasma; a processing chamber for performing the
etching on the substrate by using the plasma; a substrate
supporting table, installed in the processing chamber, for mounting
the substrate thereon; a gas exhaust unit for depressurizing the
processing chamber; a gas supply unit for supplying a gas into the
processing chamber; a film thickness monitoring unit for monitoring
a thickness of the mask layer; and a controller for controlling an
etching process based on film thickness information provided from
the film thickness monitoring unit.
2. The etching apparatus of claim 1, wherein the film thickness
monitoring unit includes: a light source for irradiating a light
toward the substrate; a spectrometer unit for dispersing a
reflected light from the substrate into its spectrum; a light
detection unit for detecting the spectrum; and an operation unit
for processing detection results from the light detection unit
based on a calibration curve obtained in advance, wherein the film
thickness monitoring unit is configured to comprise the steps of
irradiating the light to the mask layer; detecting a reflected
light from a surface of the mask layer and a reflected light from
an interface between the mask layer and the second layer;
calculating a spectral reflectance; and measuring the thickness of
the mask layer based on the calibration curve obtained in advance
by employing a curve fitting method.
3. The etching apparatus of claim 1, wherein the controller
terminates an etching of the first layer and starts an etching of
the second layer when the thickness of the mask layer reaches a
specified value based on the film thickness information provided
from the film thickness monitoring unit.
4. The etching apparatus of claim 1, wherein the mask layer is a
silicon nitride film.
5. The etching apparatus of claim 1, wherein the second layer is a
silicon layer and the first layer is a native oxide film formed on
a surface of the silicon layer in the groove portion.
6. The etching apparatus of claim 5, wherein after etching the
native oxide film, an etching of the silicon layer is performed by
using the mask layer as a mask while an etched depth of the silicon
layer is being monitored and finished when the etched depth reaches
a specified value.
7. The etching apparatus of claim 6, which is applied to a trench
etching in shallow trench isolation.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS:
[0001] This document claims priority to Japanese Patent Application
Number 2004-266538, filed Sep. 14, 2004 and U.S. Provisional
Application No. 60/614,042, filed Sep. 30, 2004, the entire content
of which are hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to an etching method and
apparatus; and, more particularly, to an etching method and
apparatus applicable to a device isolation technique such as
shallow trench isolation (STI) in a manufacturing process of a
semiconductor device.
BACKGROUND OF THE INVENTION
[0003] An STI is known as a technique for electrically isolating a
device formed on a silicon substrate. The STI process involves the
steps of etching a silicon substrate through a mask of, for
example, a silicon nitride film to form a trench therein; filling
the trench with an oxide film such as SiO.sub.2; and finally
planarizing the substrate by chemical mechanical polishing (CMP) by
way of using the mask (silicon nitride film) as a stopper (see, for
example, U.S. Pat. No. 6,844,265).
[0004] The silicon trench etching process of the STI includes a
break-through (BT) step of performing an etching to remove a native
oxide film formed on the surface of a silicon layer after
patterning by using, for example, the silicon nitride film as a
mask and a main step of performing an etching on the silicon layer
from which the native oxide film is removed, to thereby form a
trench in the silicon layer (see, for example, Japanese Patent
Laid-open Publication No. H11-214356).
[0005] Moreover, as a technique related to the STI, there is
proposed a method for forming in advance an end point detection
layer in the mask to facilitate the detection of an end point of a
CMP process, to thereby control a thickness of a device isolation
film (see, for example, Japanese Patent Laid-open Publication No.
2003-45956).
[0006] Meanwhile, U.S. Patent Publication No. 2004/0191932
discloses a method for performing a plasma etching on a silicon
oxide film by using a resist film as a mask, while monitoring the
thickness of the resist film by employing an optical method, to
thereby prevent a reduction in the thickness of the resist
film.
[0007] Since the native oxide film formed on the surface of the
silicon substrate after the patterning can be removed in a
relatively short period of time ranging from about 5 to 10 seconds,
the end point of the etching in the BT step is conventionally set
based on a lapse of predetermined time. Though the BT step is
performed under a condition in which even a hard mask such as a
silicon nitride film can be easily etched, if the end point of the
BT step is controlled indiscriminately based only on time, the
etching will be terminated at a predetermined time point regardless
of the residual film thickness of the mask, resulting in
non-uniformity in the film thickness of the mask after the BT step.
That is, since in general there is variation in the film thickness
of the mask present even prior to performing the etching, the film
thickness of the mask may still remain non-uniform after the BT
step is performed. In the main step following the BT step, the
etching rate of silicon is high, whereas etching is performed very
slowly on an oxide film or a nitride film. Therefore, a surface
polishing is to be conducted during the subsequent CMP process
under a condition in which the film thickness of the mask is not
uniform.
[0008] In the CMP process, since polishing is typically set to be
finished upon the exposure of the mask layer such as the silicon
nitride film, the residual amount of the buried oxide film may
become irregular if the film thickness of the mask layer is
non-uniform. Moreover, if the film thickness of the mask is
non-uniform, some part of the mask may be left as a residue when
the mask is removed after the CMP process by, for example, wet
etching. Though it is technically possible to prepare an end point
detection layer in the mask to avoid these problems, as disclosed
in Japanese Patent Publication No. 2003-45956 cited above, it is
not considered practical since it will increase the number of
processes required.
SUMMARY OF THE INVENTION
[0009] It is, therefore, an object of the present invention to
provide an etching method and apparatus capable of controlling the
film thickness of a hard mask layer uniformly.
[0010] In order to achieve the object, in accordance with a first
aspect of the present invention, there is provided a an etching
method of a substrate to be processed including a mask layer having
a predetermined pattern; a layer to be etched, formed in a groove
portion of the pattern; and a base layer formed beneath the mask
layer and the layer to be etched, wherein an etching is performed
on the layer to be etched while a thickness of the mask layer is
being monitored and is finished when the thickness of the mask
layer reaches a predetermined value.
[0011] It is preferable that the mask layer is a silicon nitride
film. Further, preferably, the base layer is a silicon layer and
the layer to be etched is a native oxide film formed on a surface
of the silicon layer in the groove portion.
[0012] Additionally, after etching the native oxide film, an
etching may be performed on the silicon layer by using the mask
layer as a mask while an etched depth of the silicon layer is being
monitored and finished when the etched depth reaches a specified
value.
[0013] The etching method is preferably applied to a trench etching
in shallow trench isolation. In this case, the monitoring of the
thickness of the mask layer may involve the steps of irradiating a
light to the mask layer; detecting a reflected light from a surface
of the mask layer and a reflected light from an interface between
the mask layer and the silicon layer; calculating a spectral
reflectance; and measuring the thickness of the mask layer based on
a calibration curve obtained in advance by employing a curve
fitting method.
[0014] In accordance with a second aspect of the present invention,
there is provided an etching apparatus for performing an etching on
a substrate to be processed including a mask layer having a
predetermined pattern; a first layer to be etched, formed in a
groove portion of the pattern; and a second layer to be etched,
formed beneath the mask layer and the first layer to be etched,
including a plasma source for generating a plasma; a processing
chamber for performing an etching processing on the substrate by
using the plasma; a substrate supporting table, installed in the
processing chamber, for mounting the substrate thereon; a gas
exhaust unit for depressurizing the processing chamber; a gas
supply unit for supplying a gas into the processing chamber; a film
thickness monitoring unit for monitoring a film thickness of the
substrate by measuring the thickness optically; and a controller
for controlling an etching process based on film thickness
information provided from the film thickness monitoring unit,
wherein the controller terminates the etching of the first layer
and concurrently starts the etching of the second layer when the
thickness of the mask layer reaches a predetermined value based on
the film thickness information provided from the film thickness
monitoring unit.
[0015] The film thickness monitoring unit may include a light
source for irradiating a light toward the substrate to be
processed; a spectrometer unit for dispersing a reflected light
from the substrate to be processed into its spectrum; a light
detection unit for detecting the spectrum; and an operation unit
for processing detection results from the light detection unit
based on a calibration curve obtained in advance, wherein the film
thickness monitoring unit is configured to have the steps of
irradiating a light to the mask layer; detecting a reflected light
from a surface of the mask layer and a reflected light from an
interface between the mask layer and the second layer to be etched;
calculating a spectral reflectance; and measuring the thickness of
the mask layer based on a calibration curve obtained in advance by
employing a curve fitting method.
[0016] In accordance with a third aspect of the present invention,
there is provided a computer storage medium for storing therein a
control program operable on a computer, wherein the control program
is executed to control an etching apparatus for use in an etching
method of the first aspect.
[0017] In accordance with a fourth aspect of the present invention,
there is provided a computer storage medium for storing therein a
control program operable on a computer, wherein the control program
is executed to control an etching apparatus of the second
aspect.
[0018] In accordance with the present invention, etching is
performed while the thickness of the mask layer is being monitored,
whereby the thickness of the mask layer can be made uniform. For
instance, the present invention may be applied to etching of the
native oxide film in STI. Thus, the thickness of the buried oxide
film can be made uniform and generation of mask residues can be
prevented in a subsequent CMP process.
[0019] Further, the first layer to be etched, i.e., the native
oxide film, is etched and then the second layer to be etched, i.e.,
the silicon layer, is etched while the etched depth of the silicon
layer being monitored, whereby the residual film thickness of the
mask and the depth of the trench can be controlled at the same
time. Thus, the STI can be performed precisely.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The above and other objects and features of the present
invention will become apparent from the following description of
preferred embodiments given in conjunction with the accompanying
drawings, in which:
[0021] FIGS. 1A to 1C provide schematic cross sectional views of a
wafer to describe a preferred embodiment of the present
invention;
[0022] FIG. 2 sets forth a schematic view of a plasma etching
apparatus in accordance with the present invention; and
[0023] FIG. 3 illustrates a film thickness measuring unit.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] Hereinafter, a preferred embodiment of the present invention
will be described with reference to the accompanying drawings.
[0025] FIGS. 1A to 1C show schematic longitudinal cross sectional
views of a wafer W in a silicon trench etching process such as STI
in accordance with the preferred embodiment of the present
invention. As shown in FIG. 1A, a silicon nitride film 102 such as
a Si.sub.3N.sub.4, which serves as a hard mask, is formed on a
silicon substrate (silicon layer) 101 that forms the wafer W. The
silicon nitride film 102 is patterned after a predetermined shape,
and a native oxide film 103 of SiO.sub.2 is formed in a groove
portion 110 of the pattern on the silicon substrate 101.
[0026] In a BT step, an etching is performed to remove the native
oxide film 103 formed on the surface of the silicon substrate 101
after patterning. Specifically, a plasma etching is conducted by
using an etching gas containing, for example, CF.sub.4 and Ar in
the BT step. At this time, since the silicon nitride film 102 is
also etched, the etching is conducted while the thickness of the
silicon nitride film 102 is being monitored. Then, when the
thickness of the silicon nitride film 102 is reduced to a
predetermined value from its initial thickness (marked by a dashed
line), as shown in FIG. 1B, the etching of the native oxide film
103 is finished.
[0027] Thereafter, an etching is performed by using the residual
silicon nitride film 102 as a mask to form a trench in the silicon
substrate 101 (main step). That is, the silicon substrate 101
formed of single crystalline silicon is plasma-etched through a
hard mask formed of the silicon nitride film 102 by using an
etching gas containing, for example, Cl.sub.2, HBr and Ar, to
thereby form a trench 111 in the silicon substrate 101 as shown in
FIG. 1C. For instance, the trench etching in the main step is
performed while the depth of the trench 111 is being monitored and
the etching may be finished when the depth of the trench 111
reaches a preset value.
[0028] FIG. 2 shows a schematic configuration view of a plasma
etching apparatus in accordance with the preferred embodiment of
the present invention. A plasma etching apparatus 1 is configured
as a capacitively coupled parallel plate type etching apparatus in
which an upper and a lower electrode plate are disposed in parallel
to each other and high frequency power supplies are connected to
the electrode plates, respectively.
[0029] The plasma etching apparatus 1 includes a cylindrical
chamber 2 formed of aluminum whose surface is, for example, alumite
treated (anodic oxidized), and the chamber 2 is grounded. Installed
on a susceptor support 4 in the chamber 2 is a susceptor 5 for
horizontally mounting thereon an object to be processed, i.e., a
semiconductor wafer W (hereinafter, simply referred to as a
"wafer"), which is formed of, for example, silicon and has
specified films formed thereon. Further, the susceptor 5 also
serves as a lower electrode, and a high pass filter (HPF) 6 is
connected to the susceptor 5.
[0030] A temperature control medium space 7 is provided inside the
susceptor support 4, and a temperature control medium is introduced
into the temperature control medium space 7 through an inlet line 8
to be circulated therethrough, so that the susceptor 5 can be
maintained at a predetermined temperature.
[0031] The susceptor 5 has an upper central portion of disk shape,
which protrudes higher than its peripheral portion, and an
electrostatic chuck 11 that is shaped substantially identical to
the wafer W is mounted on the upper central portion of the
susceptor 5. The electrostatic chuck 11 includes an electrode 12
embedded in an insulating member. The electrostatic chuck 11
electrostatically attracts and holds the wafer W by Coulomb force
generated by a DC voltage of, for example, 1.5 kV applied to the
electrode 12 from a DC power supply 13 coupled to the electrode
12.
[0032] Further, formed through an insulating plate 3, the susceptor
support 4, the susceptor 5 and the electrostatic chuck 11 is a gas
channel 14 for supplying a heat transfer medium, for example, a He
gas, to the rear surface of the wafer W that is kept under a
predetermined back pressure. Thus, heat is transferred between the
susceptor 5 and the wafer W through the heat transfer medium, so
that the wafer W can be maintained at a predetermined
temperature.
[0033] Moreover, an annular focus ring 15 is disposed on the
peripheral portion of the susceptor 5 to surround the wafer W
loaded on the electrostatic chuck 11. The focus ring 15 is formed
of an insulating material such as ceramic or quartz and serves to
improve uniformity of etching.
[0034] An upper electrode 21 is disposed above the susceptor 5 to
face it in parallel and is supported at an upper portion of the
chamber 2 via an insulating member 22. The upper electrode 21
includes an electrode plate 24 that faces the susceptor 5; and an
electrode support 25 that serves to support the electrode plate 24
and is made of a conductive material, for example, aluminum whose
surface is alumite treated. The electrode plate 24 is formed of,
for example, quartz and is provided with a number of injection
openings 23. The distance between the susceptor 5 and the upper
electrode 21 is adjustable.
[0035] A gas inlet port 26 is formed at a center of the electrode
support 25 of the upper electrode 21 and coupled to a gas supply
line 27. Further, the gas supply line 27 is connected to a
processing gas supply source 30 via a valve 28 and a mass flow
controller 29, and an etching gas for plasma etching is supplied
from the processing gas supply source 30. Though there is shown in
FIG. 2 only one processing gas supply source 30, the plasma
processing apparatus is provided with a plurality of process gas
supply sources capable of supplying, for example, a CF.sub.4 gas,
an Ar gas, a Cl.sub.2 gas, an HBr gas, an O.sub.2 gas and the like
into the chamber 2 while the flow rates thereof being individually
controlled.
[0036] A gas exhaust pipe 31 is connected to a bottom portion of
the chamber 2 and coupled to a gas exhaust unit 35. The gas exhaust
unit 35 includes a vacuum pump such as a turbo molecular pump, and
serves to reduce the inner pressure of the chamber 2 down to a
predetermined vacuum level, e.g., 1 Pa or less. Further, a gate
valve 32 is installed on a sidewall of the chamber 2. The wafer W
is transferred between the chamber 2 and an adjacent load lock
chamber (not shown) while the gate valve 32 is opened.
[0037] A first high frequency power supply 40 is connected to the
upper electrode 21 via a matching unit 41. Further, a low pass
filter (LPF) 42 is coupled to the upper electrode 21. The first
high frequency power supply 40 has a frequency ranging from 50 to
150 MHz. By applying a high frequency power in such a range, a
high-density plasma in a desired dissociation state can be
generated within the chamber 2, which makes it possible to execute
a plasma processing under a low pressure. The frequency of the
first high frequency power supply 40 preferably ranges from 50 to
80 MHz. Typically, its frequency is chosen to be 60 MHz as
illustrated in FIG. 2 or thereabouts.
[0038] Further, a second high frequency power supply 50 is
connected to the susceptor 5 serving as the lower electrode via a
matching unit 51. The second high frequency power supply 50 has a
frequency ranging from several hundreds of kHz to less than twenty
MHz. By applying a power of a frequency in such a range, a proper
ionic action can be facilitated without causing any damage on the
wafer W. Typically, the frequency of the second high frequency
power supply 50 is chosen to be, for example, 13.56 MHz as shown in
FIG. 2 or 800 kHz.
[0039] The plasma etching apparatus 1 includes a film thickness
measuring unit 70 serving as a film thickness monitoring device.
The film thickness measuring unit 70 irradiates a light of multiple
wavelengths toward the wafer W, and measures a film thickness by
detecting a reflected light therefrom.
[0040] FIG. 3 illustrates the film thickness measuring unit 70
schematically. The film thickness measuring unit 70 includes a
light source 71 for irradiating a light toward the wafer W; a
spectrometer unit 72 having a polychrometer for dispersing a
reflected light from the wafer W into its spectrum; a light
detection unit 73 for detecting the spectrum; and an operation unit
74 for processing the detection results from the light detection
unit 73 based on a calibration curve obtained in advance.
[0041] When measuring the thickness of the silicon nitride film 102
during an etching process, a light from the light source 71 is
irradiated toward the surface of the wafer inside the chamber 2 of
the plasma etching apparatus 1 through an irradiation window (not
shown) via an optical fiber 75 and a lens 76. The irradiated light
is reflected at an interface of each layer on the wafer W. Then,
the reflected light is sent to the spectrometer unit 72 to be
spread its spectrum. The spectrum is detected by the light
detection unit 73 and a spectral reflectance ratio is calculated by
the operation unit 74.
[0042] The thickness of the silicon nitride film 102 serving as a
mask is measured as follows. The light reflected from the surface
of the silicon nitride film 102 and the light reflected from an
interface between the silicon nitride film 102 and the silicon
layer 101 interferes with each other. The intensity of the
interfered light varies depending on the thickness of the silicon
nitride film 102. Therefore, the thickness of the silicon nitride
film 102 can be measured by detecting the interfered light.
[0043] When measuring the thickness of the silicon nitride film
102, a spectral reflectance is calculated in advance for an
arbitrarily chosen thickness of the silicon nitride film 102 under
the condition that an incident light falls on the wafer W
vertically. Then, a ratio of the calculated spectral reflectance to
a spectral reflectance measured at a time when no silicon nitride
film 102 is formed (spectral reflectance ratio) is calculated to
thereby obtain a calibration curve to be stored in a memory of the
operation unit 74. It is preferable to obtain the calibration curve
in such a specified range that covers the variation of the
thickness of the silicon nitride film 102 to be etched during the
etching process.
[0044] Thereafter, a light of multiple wavelengths is irradiated to
the wafer W whose film thickness is to be measured. Then, the light
reflected from the wafer W is dispersed into its spectrum and a
spectral reflectance is calculated. Further, a ratio of the
calculated spectral reflectance to a spectral reflectance measured
at a time when no silicon nitride film 102 is formed on the wafer W
(spectral reflectance ratio) is calculated, and the obtained
spectral reflectance ratio is stored in the memory of the operation
unit 74. Afterward, the measured values are fitted to a curve by
employing a curve fitting method to compare it with the calibration
curve, whereby the thickness of the silicon nitride film 102 can be
found.
[0045] Each component of the plasma etching apparatus 1 is
connected to and controlled by a process controller 60 with a CPU.
A process manager can operate the plasma etching apparatus 1 by a
user interface 61 connected to the process controller 60, and the
user interface 61 includes a keyboard for inputting a command, a
display for showing an operational status of the plasma etching
apparatus 1 and the like.
[0046] Moreover, also connected to the processing controller 60 is
a memory unit 62 for storing therein a recipe including a control
program, processing condition data and the like to be used in
realizing various processings performed in the plasma etching
apparatus 1 under the control of the process controller 60.
[0047] Further, when receiving a command from the user interface 61
or film thickness information (control signal indicating an end
point of etching) from the operation unit 74 of the film thickness
measuring unit 70, a necessary recipe is retrieved from the memory
unit 62 to be executed on the process controller 60, whereby a
desired processing is performed in the plasma processing apparatus
1. Moreover, the necessary recipe to be used can be retrieved from
a readable storage medium such as a CD-ROM, a hard disk, a flexible
disk or the like, or retrieved through an on-line connected via,
for example, a dedicated line to another apparatus available all
the time.
[0048] Hereinafter, there will be explained a process for forming a
groove (trench) by performing an etching on the wafer W made of
single crystalline silicon by using the plasma processing apparatus
1 with the above-described configuration.
[0049] First, the gate valve 32 is opened and then the wafer W, on
which the native oxide film 103 of silicon dioxide and the silicon
nitride film 102 are formed, is carried into the chamber 2 from a
load lock chamber (not shown) to be mounted on the electrostatic
chuck 11. A DC voltage is then supplied from the DC power supply 13
to the electrostatic chuck 11, so that the wafer W is
electrostatically attracted by the electrostatic chuck 11 to be
adsorbed thereon.
[0050] Next, the gate valve 32 is closed and the chamber 2 is
evacuated to a predetermined vacuum level by the gas exhaust unit
35. Then, the valve 28 is opened, and an etching gas containing,
for example, CF.sub.4 and Ar is supplied into a hollow portion of
the upper electrode 21 from the processing gas supply source 30 via
the process gas supply line 27 and the gas inlet port 26 while its
flow rate is controlled to be, for example, CF.sub.4/Ar=100/200
mL/min by the mass flow controller 29. The etching gas is
discharged uniformly towards the wafer W through the injection
openings 23 of the electrode plate 24, as indicated by arrows in
FIG. 2.
[0051] In this BT step, while the inner pressure of the chamber 2
is maintained at a predetermined pressure level of, for example,
2.7 Pa (20 mTorr), a high frequency voltage of 600 W is applied to
the upper electrode 21 from the first high frequency power supply
40, and another high frequency voltage of 220 W is applied to the
susceptor (lower electrode) 5 from the second high frequency power
supply 50, whereby the etching gas is converted into a plasma so
that an etching can be performed on the native oxide film 103 on
the wafer W. Further, a back pressure may be 1333 Pa (10 Torr) at
both a central portion and an edge portion of the wafer W.
[0052] In the BT step, etching is performed while the thickness of
the silicon nitride film 102 is monitored by using the film
thickness measuring unit 70 on the basis of the reflected light of
multiple wavelengths ranging from, for example, 240 nm to 350 nm,
as described above. Then, when the thickness of the silicon nitride
film 102 is reduced to a predetermined value, the etching is
finished. Thus, the end point of etching is determined based on the
thickness of the silicon nitride film 102 during the etching of the
native oxide film 103, whereby the thickness of the silicon nitride
film 102 can be made uniform. Therefore, generation of mask
residues or variation in the residual amount of buried oxide film
can be prevented in a subsequent CMP process.
[0053] In a main step following the BT step, a trench is formed in
the silicon substrate 101. That is, the valve 28 is opened, and an
etching gas containing, for example, Cl.sub.2, HBr and O.sub.2
gases is supplied into a hollow portion of the upper electrode 21
from the processing gas supply source 30 via the process gas supply
line 27 and the gas inlet port 26 while the flow rates of the gases
are controlled to be, for example, Cl.sub.2/HBr/O.sub.2=55/55/6
mL/min, respectively. The etching gas is discharged uniformly
towards the wafer W through the injection openings 23 of the
electrode plate 24, as indicated by arrows in FIG. 2.
[0054] In this main step, while the inner pressure of the chamber 2
is maintained at a predetermined pressure level of, for example,
about 6.7 Pa (50 mTorr), a high frequency voltage of 250 W is
applied to the upper electrode 21 from the first high frequency
power supply 40, and another high frequency voltage of 350 W is
applied to the susceptor (lower electrode) 5 from the second high
frequency power supply 50, whereby the etching gas is converted
into a plasma such that an etching can be performed on the native
oxide film 103 on the wafer W. Further, a back pressure may be set
to be 1333 Pa (10 Torr) at both the central portion and the edge
portion of the wafer W.
[0055] In the main step, etching is further performed while the
depth of the trench 111 being formed in the silicon substrate 101
is being monitored by means of the film thickness measuring unit
70, and when the depth of the trench 111 reaches a predetermined
value, the etching is finished. Further, in case of measuring the
depth of the trench 111 in the main step, it is not necessary to
detect the light of multiple wavelengths, but it is possible to
measure the depth of the trench 111 by detecting only a reflected
light of an arbitrarily chosen wavelength of, for example, 261
nm.
[0056] After the main step is completed, a typical STI process,
that is, burying the oxide film and planarizing by CMP are
performed, to thereby carry out device isolation.
[0057] Hereinafter, results of experiments conducted to confirm the
effect of the present invention will be explained. An etching
process was performed on a first to a fifth wafer by using the
plasma etching apparatus 1 shown in FIG. 2. The first, the third
and the fifth wafer are sample wafers, while the second and the
fourth were dummy wafers. As for each sample wafer, the silicon
nitride film (SiN) 102 and the native oxide film 103 were formed in
the silicon substrate 101 as shown in FIG. 1A. On the other hand,
each dummy wafer was a bare silicon substrate formed of silicon
only.
[0058] The plasma etching was preferably conducted under the
following condition: CF.sub.4 and Ar were introduced into the
chamber 2 at flow rates of 100 and 200 mL/min, respectively while
the inner pressure of the chamber 2 was maintained at about 2.7 Pa;
high frequency voltages of 600 W and 220 W were applied to the
upper electrode 21 and the susceptor (lower electrode) 5 from the
first and the second high frequency power supply 40 and 50,
respectively; a back pressure was set to be 1333 Pa (10 Torr) at
both the central portion and the edge portion of the wafer W; and
the temperatures of the upper electrode 21, a sidewall and the
susceptor 5 (i.e., wafer W) in the chamber 2 were set to be
80.degree. C., 60.degree. C. and 30.degree. C., respectively.
[0059] As for each of the sample wafers (the first, the third and
the fifth one), the thickness of the silicon nitride film 102 was
monitored by the film thickness measuring unit 70, and the plasma
etching was finished when the thickness of the silicon nitride film
102 was reduced to 70 nm. Further, for confirmation, the thickness
of each sample wafer was actually measured before and after the
etching by using a film thickness measuring device ASET F5
(manufactured by KLA-Tencor Corporation). The results are provided
in Table 1, wherein a "measurement value" represents a film
thickness obtained by the film thickness measuring unit 70 while an
"actual measurement value" represents an actual film thickness
measured by the film thickness measuring device ASET F5.
TABLE-US-00001 TABLE 1 Thickness of SiN (nm) Before After Etching
processing processing time Measure- Actual Measure- Actual Sample
(sec- ment measurement ment measurement No. onds) value value value
value First 11.4 86.0 86.5 69.8 72.8 Third 11.3 84.6 85.2 69.8 71.4
Fifth 9.4 83.7 83.9 69.8 72.8 Variation -- -- 2.6 -- 1.4 between
wafers
[0060] From the Table 1, it is found that the variation in the
thickness of the silicon nitride film 102 among the first, the
third and the fifth sample wafer was reduced to 1.4 nm from 2.6 nm
after performing an etching of a BT step while the thickness of the
silicon nitride film 102 was being monitored. From the result, it
is proved that the film thickness of a hard mask can be controlled
to be uniform by performing an etching of a BT step while the film
thickness of the hard mask being monitored.
[0061] Accordingly, the native oxide film 103 is etched while the
thickness of the silicon nitride film 102 being monitored and then
the silicon layer 101 is etched while the depth of the trench 111
being monitored, whereby the thickness of a mask layer (silicon
nitride film 102) and the depth of the trench can be maintained
uniform after the etching. Moreover, in this case, since the etched
amount of silicon layer 101 is small in the BT step and the etched
amount of silicon nitride film 102 is small in the main step, the
STI can be performed precisely without errors.
[0062] While the invention has been shown and described with
respect to the preferred embodiment, it will be understood by those
skilled in the art that various changes and modifications may be
made without departing from the spirit and scope of the invention
as defined in the following claims.
[0063] For example, though a capacitively coupled parallel plate
type etching apparatus is used in the above preferred embodiment,
one of various plasma etching apparatuses, e.g., an inductively
coupled type etching apparatus, can be employed as long as it is
able to generate a plasma within a gas pressure range in accordance
with the present invention. Furthermore, though the preferred
embodiment has been described for the STI, the present invention
can be applied to measurement of a film thickness in another
etching processing.
* * * * *