U.S. patent application number 10/813012 was filed with the patent office on 2004-11-11 for plasma processing method.
This patent application is currently assigned to TOKYO ELECTRON LIMITED. Invention is credited to Horiguchi, Katsumi, Ito, Kiyohito, Kanno, Keiichi, Yamamoto, Kenji.
Application Number | 20040222190 10/813012 |
Document ID | / |
Family ID | 33408627 |
Filed Date | 2004-11-11 |
United States Patent
Application |
20040222190 |
Kind Code |
A1 |
Horiguchi, Katsumi ; et
al. |
November 11, 2004 |
Plasma processing method
Abstract
In a plasma processing method, a silicon layer of an object to
be processed is etched by using a plasma of a processing gas
introduced into an airtight processing chamber through a patterned
mask. The processing gas contains a gaseous mixture of HBr, O.sub.2
and SiF.sub.4 and, additionally, one or both of SF.sub.6 gas and
NF.sub.3 gas; and a gas containing C and F is further added to the
processing gas.
Inventors: |
Horiguchi, Katsumi;
(Nirasaki-shi, JP) ; Yamamoto, Kenji;
(Kitakoma-gun, JP) ; Ito, Kiyohito; (Nirasaki-shi,
JP) ; Kanno, Keiichi; (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: |
33408627 |
Appl. No.: |
10/813012 |
Filed: |
March 31, 2004 |
Current U.S.
Class: |
216/79 ; 216/67;
257/E21.218 |
Current CPC
Class: |
H01J 37/32568 20130101;
H01L 21/3065 20130101 |
Class at
Publication: |
216/079 ;
216/067 |
International
Class: |
C23F 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2003 |
JP |
2003-096631 |
Claims
What is claimed is:
1. A plasma processing method comprising the step of: etching a
silicon layer of an object to be processed by employing a patterned
mask and by using a plasma of a processing gas introduced into an
airtight processing chamber, containing a gaseous mixture of HBr,
O.sub.2 and SiF.sub.4 and, additionally, one or both of SF.sub.6
and NF.sub.3, wherein a gas containing C and F is further added to
the processing gas.
2. The plasma processing method of claim 1, wherein the gas
containing C and F is one or more gases selected from the group
consisting of CF.sub.4, C.sub.4F.sub.8, C.sub.5F.sub.8,
C.sub.4F.sub.6, CHF.sub.3 and CH.sub.2F.sub.2.
3. The plasma processing method of claim 1, wherein the gas
containing C and F is added to the processing gas in a middle of
the etching step.
4. The plasma processing method of claim 3, wherein the gas
containing C and F is continuously added to the processing gas
until the end of the etching step.
5. The plasma processing method of claim 1, wherein the gas
containing C and F is added to the processing gas for a period of
time during the etching step.
6. The plasma processing method of claim 1, wherein the timing of
starting to add the gas containing C and F to the processing gas is
determined according to the opening diameter of holes or the
opening width of grooves formed by the etching step.
7. The plasma processing method of claim 1, wherein the opening
diameter of holes or the opening width of grooves formed by the
etching step is smaller than or equal to about 0.2 .mu.m.
8. The plasma processing method of claim 1, wherein the patterned
mask includes at least an oxide layer containing silicon.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a plasma processing
method.
BACKGROUND OF THE INVENTION
[0002] Recently, along with the trend for high density and high
integration of semiconductor devices, there has been increased
demand for forming holes (or grooves) having a high aspect ratio.
Moreover, it is preferable that such a hole has a smooth sidewall
approximately perpendicular to a surface of an opening thereof.
[0003] A plasma etching is conventionally used in forming holes
having a high aspect ratio in a silicon layer, by adopting certain
etching conditions exemplified below.
[0004] Conventionally, a plasma etching process is carried out by
setting the temperature of a lower electrode on which an object to
be processed is mounted in a hermetically sealed processing chamber
to be not greater than, e.g., 60.degree. C., and the inner pressure
thereof to be lower than or equal to 150 mTorr, wherein a gaseous
mixture of HBr, NF.sub.3 and O.sub.2 gases or HBr, SF.sub.6 and
O.sub.2 gases is used as a processing gas.
[0005] U.S. Pat. No. 5,423,941 also discloses a method for
performing an etching by using a gaseous mixture of HBr gas,
SiF.sub.4 gas, SF.sub.6 gas, and O.sub.2 gas containing He gas as a
processing gas in an airtightly sealed processing chamber, wherein
the pressure in the processing chamber is set to be between 50 and.
150 mTorr and a magnetic field of a magnitude smaller than or equal
to 100 Gauss is applied perpendicular to an electric field.
[0006] In the above etching methods, since HBr gas and/or SiF.sub.4
gas is included in the processing gas, a protective layer is formed
on an inner sidewall of a hole. Accordingly, it is possible to
vertically form a hole having an opening diameter smaller than or
equal to 1 .mu.m according to the size of a mask.
[0007] In the aforementioned conventional methods, however,
deposits (e.g., SiBr.sub.xO.sub.y x and y being the combination
ratios) may be accumulated at the openings of the mask. If an
opening diameter of the mask is large, such deposits do not affect
the etching process. However, when an opening diameter of the mask
becomes smaller than or equal to about 0.2 .mu.m, the deposits
accumulated at the openings of the mask may present a hampering
effect on a fine patterning of the hole.
[0008] In case the deposits are accumulated at the opening of the
mask, the opening diameter becomes narrower and, consequently, an
apparent aspect ratio of the hole increases. An increase in the
aspect ratio results in a decrement of an etching rate and,
further, a deterioration in the throughput. In addition, if the
aspect ratio is greater than or equal to, e.g., 50, it becomes
difficult to etch a bottom portion of the hole, so that the hole
may not attain a designed depth.
SUMMARY OF THE INVENTION
[0009] It is, therefore, a primary object of the present invention
to provide a new and improved plasma processing method capable of
forming fine holes (or grooves) of a high aspect ratio in a silicon
layer with a high etching rate.
[0010] In accordance with the present invention, there is provided
a plasma processing method including the step of: etching a silicon
layer of an object to be processed by employing a patterned mask
and by using a plasma of a processing gas introduced into an
airtight processing chamber, the processing gas containing a
gaseous mixture in which one or both of SF.sub.6 gas and NF.sub.3
gas are added to HBr gas, O.sub.2 gas and SiF.sub.4 gas, wherein a
gas containing C and F is further added to the processing gas. Due
to an effect of the gas containing C and F, deposits are prevented
from being accumulated at an opening of the mask and, further,
accumulated deposits are removed. Accordingly, it is possible to
form a deep hole in a silicon layer even in case a diameter of a
opening of the mask is very small.
[0011] The gas containing C and F may be a gaseous mixture having a
combination of one or more gases selected from the group consisting
of CF.sub.4 gas, C.sub.4F.sub.8 gas, C.sub.5F.sub.8 gas,
C.sub.4F.sub.6 gas, CHF.sub.3 gas and CH.sub.2F.sub.2 gas.
[0012] In order to prevent deposits from being accumulated at the
opening of the mask or remove accumulated deposits, it is
preferable to set the flow rate of the gas containing C and F added
to the processing gas to be smaller than or equal to, e.g., 10
sccm. Further, the added amount of the gas containing C and F is
preferably controlled according to the material or thickness of the
mask or the etched amount of the silicon layer. Furthermore, the
patterned mask preferably includes at least an oxide layer
containing silicon.
[0013] In accordance with the present invention, the addition of
the gas containing C and F to the processing gas may be initiated
either from the beginning or at a certain stage of the etching
process of the silicon layer of the object to be processed and then
terminated before or continued until the end of the etching
process. The time period of supplying the gas containing C and F is
preferably set based on, e.g., the accumulating state of deposits
at the opening of the mask and an opening diameter of a hole or an
opening width of a groove formed by the etching. Accordingly,
deposits are prevented from being accumulated at the opening of the
mask. Further, even if the deposits are excessively accumulated
thereat, it is possible to appropriately remove the deposits.
[0014] Especially, in case the opening diameter of the hole or the
opening width of the groove formed by the etching is smaller than
or equal to about 0.2 .mu.m, the accumulation of deposits at the
opening thereof has been problematic. Since, however, the
accumulation of deposits at the opening of the mask is suppressed
in accordance with the present invention, it is possible to form
deep holes or deep grooves in a silicon layer even in case the
opening diameter of the holes or the opening width of the grooves
is smaller than or equal to about 0.2 .mu.m.
[0015] Further, as used in this specification, 1 mTorr and 1 sccm
correspond to (1.times.10.sup.-3.times.101325/760) Pa and
(1.times.10.sup.-6/60) m.sup.3/sec, respectively.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] 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:
[0017] FIG. 1 shows a schematic cross sectional view illustrating a
plasma processing apparatus in accordance with a preferred
embodiment of the present invention;
[0018] FIG. 2 describes a schematic cross sectional view of an
object to be processed before a mask for use in etching a silicon
layer is formed;
[0019] FIG. 3 provides a schematic cross sectional view of the
object to be processed after the mask for use in etching the
silicon layer is formed;
[0020] FIGS. 4A to 4C represent schematic cross sectional views of
the object to be processed, showing accumulating states of deposits
at openings of the mask;
[0021] FIGS. 5A to 5C offer schematic cross sectional views of the
object to be processed, showing states of holes formed on a silicon
layer; and
[0022] FIG. 6 presents a graph indicating a relationship between an
opening diameter of the mask and an etching time.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0023] A preferred embodiment of the plasma processing method in
accordance with the present invention will now be described in
detail with reference to the accompanying drawings. In the
specification and the accompanying drawings, like reference
numerals will be given to like parts having substantially the same
functions, and redundant description thereof will be omitted.
[0024] FIG. 1 is a schematic cross sectional view showing the
structure of a plasma processing apparatus 100 in accordance with a
preferred embodiment of the present invention. A processing chamber
102 of the plasma processing apparatus 100 is made of, e.g.,
aluminum having an alumite processed (anodizing processed) surface
covered by a plate member (not shown) made of quartz. Further, the
processing chamber 102 is grounded as illustrated in FIG. 1.
[0025] Disposed inside the processing chamber 102 is a lower
electrode 104 also serving as a susceptor for mounting thereon an
object to be processed, e.g., a semiconductor wafer W. The lower
electrode 104 can be vertically moved by an elevating shaft (not
shown).
[0026] Formed under a bottom surface of the lower electrode 104 are
a quartz member 105 to be an insulating material and a conductive
member 107 being in contact with a bellows 109. The bellows 109
made of, e.g., stainless steel, is in contact with the processing
chamber 102. Accordingly, the conductive member 107 is grounded via
the bellows 109 and the processing chamber 102. Further, a bellows
cover 111 is installed in such a way that it surrounds the quartz
member 105, the conductive member 107 and the bellows 109.
[0027] An electrostatic chuck 110 connected with a high voltage DC
power supply 108 is installed to a mounting surface of the lower
electrode 104 on which the object to be processed is mounted.
Further, a focus ring 112 is provided around the electrostatic
chuck 110.
[0028] Connected with the lower electrode 104 via a matching unit
116 is a dual system of a high frequency power supply, i.e., a
first high frequency power supply 118 and a second high frequency
power supply 138. The frequency of a power outputted from the first
high frequency power supply 118 (hereinafter, referred to as a
first frequency) is set to be higher than that from the second high
frequency power supply 138 (hereinafter, referred to as a second
frequency). Accordingly, in the plasma processing apparatus 100,
dual high frequency power is applied to the lower electrode 104 and
each frequency power is controlled independently. Consequently, it
is possible to avoid a phenomenon in which sidewalls of the holes
formed on the object to be processed are eroded into a curved
shape, i.e., the so-called bowing phenomenon, thereby providing
holes having comparatively appropriate shapes.
[0029] It is preferable to set the first frequency to be greater
than or equal to, e.g., 27.12 MHz. Especially, in case a magnetic
field is not generated in a processing space, it is preferable to
set the first frequency to be greater than or equal to 27.12 MHz.
On the other hand, in case a magnet 130 and the like are provided
to generate a magnetic field in the processing space, the first
frequency can be set at 13.56 MHz, as will be further described
later. This is because the density of a plasma can be increased to
augment an etching rate of silicon by the magnetic field. In this
case, the second frequency is preferably set at, e.g., 3.2 MHz.
[0030] Provided at a top portion of the processing chamber 102 is
an upper electrode 124 grounded therethrough. Further, formed at
the upper electrode 124 is a plurality of gas inlet holes 126 for
introducing a processing gas. Each of the gas inlet holes 126 is
connected with a gas supply source (not shown), and the processing
gas is introduced into a processing space 122 via the gas inlet
holes 126.
[0031] Provided outside the processing chamber 102 is a magnet 130
for generating a horizontal magnetic field to the processing space
122. Due to the magnet 130, a magnetic field of a magnitude of 170
Gauss is generated at a central portion of the object to be
processed in the processing space 122. In case the magnitude of the
magnetic field generated by the magnet 130 is greater than or equal
to 170 Gauss, a high frequency power supply may be configured to
provide a single high frequency of, e.g., 13.56 MHz.
[0032] Installed at a lower portion of the processing chamber 102
is an exhaust opening 128 connected with an exhaust system (not
shown) such as a vacuum pump and the like. Due to such
configuration, an inner space of the processing container 102 can
be maintained at a certain vacuum level.
[0033] Hereinafter, operation of the plasma processing apparatus
100 will be described with reference to FIGS. 1 and 2. FIG. 2 shows
a schematic cross sectional view of an object to be processed 200
before being etched.
[0034] As the object to be processed 200, a semiconductor wafer W
having a diameter of, e.g., 200 mm, is used. As shown in FIG. 2,
formed on a surface of the semiconductor wafer W is a resist layer
202 on which holes having a diameter of 150 nm are patterned by a
photolithography process. Further, a silicon dioxide film layer
(SiO.sub.2 film) 204, e.g., a CVD oxide film, having a thickness
ranging from about 700 to 2200 nm is formed beneath the resist
layer 202. Furthermore, a silicon nitride film layer (SiN film) 206
having a thickness of about 200 nm is formed beneath the silicon
dioxide film layer 204. In some cases, a silicon dioxide film layer
(SiO.sub.2 film) having a thickness smaller than or equal to a few
nm may be formed as a gate insulating film between the silicon
nitride film layer 206 and a silicon (Si) layer 210.
[0035] An etching process is performed on the above-described
object to be processed 200 by using the resist layer 202 as a mask,
to thereby provide patterning of the silicon dioxide film layer 204
and the silicon nitride film layer 206. Thereafter, the resist
layer 202 is removed, as illustrated in FIG. 3. The silicon dioxide
film layer 204 and the silicon nitride film layer 206 serve as a
mask for etching the silicon layer 210.
[0036] Next, the object to be processed 300 having as the mask the
silicon dioxide film layer 204 and the silicon nitride film layer
206 that are patterned in a predetermined shape is loaded into the
processing chamber 102 via a loading/unloading opening (not shown)
and then mounted on the lower electrode 104. After the processing
chamber 102 is evacuated through the exhaust opening 128 by using
the vacuum pump (not shown), a processing gas is introduced from
the gas supply source (not shown) into the processing chamber 102
via the gas inlet holes 126.
[0037] As for the processing gas, a gaseous mixture in which
CF.sub.4 gas is added to HBr gas, NF.sub.3 gas, SiF.sub.4 gas and
O.sub.2 gas is used. Further, it is possible to use SF.sub.6 gas
instead of NF.sub.3 gas. The flow rate of each gas in the
processing gas is controlled as follows: HBr gas ranging from 100
to 600 sccm; O.sub.2 gas, from 1 to 60 sccm; SiF.sub.4 gas, from 2
to 50 sccm; SF.sub.6 gas, from 1 to 60 sccm; NF.sub.3 gas, from 2
to 80 sccm; and CF.sub.4 gas, from 5 to 50 sccm.
[0038] By controlling the flow rate of the processing gas at a
predetermined value, the pressure in the processing chamber 102 can
be set under a predetermined level, e.g., 200 mTorr, while setting
the temperature of each unit at a predetermined temperature.
Moreover, the first high frequency power having the first frequency
is applied from the first high frequency power supply 118 to the
lower electrode 104 via the matching unit 116 and, at the same
time, the second high frequency power having the second high
frequency is applied from the second frequency power supply 138
thereto via the matching unit 116.
[0039] As described above, the first frequency is preferably chosen
to be greater than or equal to 27.12 MHz. Therefore, the first
frequency involved herein is set to be 40.0 MHz. In the meantime,
the second frequency is set to be 3.2 MHz. In addition, the output
power of the first high frequency power supply 118 is controlled at
a value ranging, e.g., from 400 to 600 W, and that of the second
high frequency power supply 138 is controlled at a value ranging,
e.g., from 100 to 800 W.
[0040] By supplying the high frequency powers having different
frequencies of the dual system to the lower electrode 104, the
dissociation of the processing gas is facilitated and, therefore,
the silicon layer 210 can be etched more efficiently.
[0041] When the plasma processing apparatus 100 is ready as
described above, the etching process is performed on an object to
be processed 300. The object to be processed 300 is etched by using
the silicon dioxide film layer 204 and the silicon nitride film
layer 206 (hereinafter, referred to as `mask material` together) as
the mask material, thereby forming holes on the silicon layer 210,
as illustrated in FIG. 3.
[0042] Hereinafter, plasma etching conditions in accordance with
this embodiment will be described with reference to Table 1. Table
1 lists the etching conditions applied to a case where holes having
a diameter of 0.15 .mu.m (design value) are formed in the silicon
layer 210. Temperatures of the upper electrode 124, an inner wall
of the processing chamber 102 and the lower electrode 104 are
commonly set at 80.degree. C., 60.degree. C. and 120.degree. C.,
respectively.
1 TABLE 1 Pressure at backside Etch- Power (W) Processing gas of
substrate (Torr) ing Pressure 40.0 3.2 flow rate (sccm) Central
Peripheral time Step (mTorr) HHz MHZ HBr NF.sub.3 CF.sub.4
SiF.sub.4 O.sub.2 portion portion (sec) BT 50 400 100 150 2.5 0 0 1
13 15 10 S1 125 700 500 220 32 0 0 14 13 15 60 S2 200 600 500 270
35 0 0 3 10 15 20 S3 200 600 800 270 35 0 10 18 10 15 240 S4 200
600 800 270 35 10 10 18 10 15 240
[0043] The plasma etching method (plasma processing method) in
accordance with this embodiment includes a plurality of steps, as
shown in Table 1.
[0044] First of all, a breakthrough (BT) step is performed to
remove a silicon dioxide film layer generated due to a natural
oxidation on a surface of the silicon layer 210 to be etched (see
FIG. 3).
[0045] Next, a first step S1 is performed to form holes having,
e.g., a funnel shape, wherein a large inner diameter is provided at
an upper portion and a small inner diameter is provided at a lower
portion. In order to form such shaped holes during the first step,
the silicon layer 210 is etched such that the diameter of the holes
becomes smaller as the etching progresses in the depth direction.
At this time, the depth of the hole is, e.g., 1.5 .mu.m. The first
step S1 can be divided into a number of steps and different etching
conditions can be applied to each step. In such a case, it is
possible to form holes having more conformal shapes to design
values.
[0046] Thereafter, a second step S2, a third step S3 and a fourth
step S4 are performed to further etch the silicon layer 210. After
going through these steps, the holes formed in the silicon layer
210 can have a depth determined by the design value. As can be seen
from Table 1, different etching conditions are applied to second,
third and fourth steps S2, S3 and S4. By forming the holes while
varying the etching conditions as described above, it is possible
to obtain holes having a final shape corresponding to the design
value.
[0047] As a result of performing the aforementioned steps, the
holes each having a predetermined diameter and depth are formed in
the object to be processed 300.
[0048] In case of forming fine holes in the silicon layer 210, the
etching rate of silicon is deteriorated as the holes grow toward a
lower portion of the silicon layer 210. Therefore, in the plasma
etching method in accordance with this embodiment, the output of
the high frequency power supply 138 is increased during the latter
steps of the etching process, i.e., the third and the fourth steps
S3 and S4. Accordingly, the ion energy in the plasma increases such
that the etching rate at a lower deep portion of the silicon layer
210 can be prevented from being deteriorated. Further, during the
third and the fourth steps S3 and S4, the flow rate of O.sub.2 gas
is increased to accelerate an accumulation of a protective layer at
a top portion of the mask material. As a result, a high etching
selectivity can be obtained. Furthermore, it is preferable that the
timing of increasing the output power of the high frequency power
supply 138 be substantially equal to that of increasing the flow
rate of O.sub.2 gas.
[0049] Hereinafter, deposits accumulated at the openings of the
mask material during the etching process of the silicon layer 210
will be described with reference to FIGS. 4A to 4C.
[0050] In the plasma etching method in accordance with this
embodiment, a gaseous mixture in which CF.sub.4 gas is added to HBr
gas, NF.sub.3 gas (or SF.sub.6 gas), SiF.sub.4 gas and O.sub.2 gas
is used as the processing gas. Among them, HBr gas or SiF.sub.4 gas
in particular contributes to the forming of a protective layer on
inner walls of the holes. Since the protective layer is formed on
the inner walls of the holes, it is possible in this embodiment to
vertically and highly precisely form fine holes having an opening
diameter of 0.15 .mu.m (design value) in the silicon layer 210, as
defined in the mask.
[0051] However, as the etching for forming holes in the silicon
layer 210 progresses, deposits (e.g., SiBr.sub.xO.sub.y, x and y
being combination ratios) become excessively accumulated on the
opening portions of the mask.
[0052] FIG. 4A shows a schematic cross sectional view of the object
to be processed (a lower portion of the silicon layer 210 is
omitted), when the third step S3 is completed, i.e., 330 seconds
after the start of the etching process. Formed in the silicon layer
210 are holes 210a and 210b at this point not reaching a final
depth thereof. Further, since the etching has been performed for
330 seconds, the silicon dioxide film layer 204 that used to have a
thickness of DO before the etching has a reduced thickness of Da
(the remaining amount of silicon dioxide film layer mask), as
measured with respect to shoulder portions at the entrance of the
holes.
[0053] Additionally, as illustrated in FIG. 4A, deposits 310 are
accumulated at the openings of the mask. Accordingly, the opening
diameter Ra1 103 nm of the mask is narrower than the uppermost
opening diameter Ra2 133 nm of the holes 210a and 210b.
[0054] FIG. 4B represents a schematic cross sectional view of the
object to be processed (a lower portion of the silicon layer 210 is
omitted) when the entire etching process has been completed by
finishing the fourth step S4, i.e., 570 seconds after the start of
the etching process. The etching process has been continuously
performed on the silicon layer 210 for 240 seconds since the object
to be processed reached the state illustrated in FIG. 4A and,
accordingly, the thickness of the shoulder portion of the silicon
dioxide film layer 204 formed at the entrance of the holes is
reduced from Da to Db.
[0055] As shown in FIG. 4B, the deposits 310 are accumulated at the
openings of the mask even after the entire etching process has been
completed. However, the amount of the accumulated deposits 310 is
small, and the opening diameter Rb1 127 nm of the mask that is
measured when the entire etching process has been completed is
wider than the opening diameter Ra1 103 nm of the mask that is
measured when 330 seconds has passed since the start of the etching
process.
[0056] Under the etching conditions in accordance with this
embodiment, during the fourth step S4, CF.sub.4 gas is introduced
into the processing chamber 102. Due to the CF.sub.4 gas,
accumulation of deposits at the openings of the mask is suppressed.
Hereinafter, effects of the CF.sub.4 gas will be described in
detail.
[0057] FIG. 4C shows as in FIG. 4B, a schematic cross sectional
view of the object to be processed obtained after the entire
etching process has been completed by finishing the fourth step S4,
i.e., 570 seconds after the start of the etching process. However,
the object to be processed illustrated in FIG. 4C is different from
that shown in FIG. 4B in that the etching process of FIG. 4C is
performed thereon by using a processing gas that does not contain
CF.sub.4 gas. Specifically, during the etching process of the
object to be processed shown in FIG. 4b, CF.sub.4 gas having a flow
rate of 10 sccm was introduced into the processing chamber 102 in
the fourth step S4 (see Table 1). On the other hand, while the
object to be processed illustrated in FIG. 4C is etched, CF.sub.4
gas is not introduced into the processing chamber 102 during any
steps of the etching process.
[0058] In other words, the difference between the etching
conditions applied for obtaining the object to be processed
illustrated in FIG. 4B and those for the object to be processed
shown in FIG. 4C is whether or not the CF.sub.4 gas is introduced
into the processing chamber 102. As clearly can be seen by
comparing FIG. 4B to FIG. 4C, there is a big difference in the
amount of accumulated deposits between the case where the CF.sub.4
gas is added to the processing gas and the case where the CF.sub.4
gas is not added thereto.
[0059] In case the CF.sub.4 gas is not added to the processing gas,
the amount of the accumulated deposits 310 increases during a
latter part of the etching process in which the holes are deeply
formed. As a result, when the etching process is completed, the
opening diameter Rc1 of the mask becomes very narrow, i.e., 108 nm,
as shown in FIG. 4C.
[0060] On the other hand, in case the CF.sub.4 gas is added to the
processing gas, the amount of the deposits 310 accumulated at the
openings of the mask is small, as illustrated in FIG. 4B when the
entire etching process is completed. Moreover, the opening diameter
Rb1 127 nm of the mask is wider than the opening diameter Rc1 108
nm of the mask that is measured in case the processing gas does not
contain the CF.sub.4 gas. This means that in case the CF.sub.4 gas
is added to the processing gas, while the silicon layer 210 is
being etched during the latter part of the etching process in which
the holes are deeply formed, the opening diameter of the mask is
large.
[0061] On the other hand, the uppermost opening diameter Rb2 140 nm
of the holes 210a and 210b which is measured in case the CF.sub.4
gas is added to the processing gas, is not considerably different
from the uppermost opening diameter Rc2 139 nm of the holes 210a
and 210b which is measured in case the CF.sub.4 gas is not added to
the processing gas. That is, even if the CF.sub.4 gas is added to
the processing gas, the inner diameter of the holes formed in the
silicon layer 210 is not enlarged to thereby form appropriately
shaped holes.
[0062] However, the thicknesses Db and Dc of the silicon dioxide
film layer 204 formed at the entrance of the holes become different
between the two cases where the CF.sub.4 gas is added and not added
to the processing gas. By comparing FIG. 4B with FIG. 4C, it can be
seen that the etching rate of the silicon dioxide film layer 204
included in the mask material increases in case the CF.sub.4 gas is
added to the processing gas. Therefore, it is preferable to control
the timing of introducing the CF.sub.4 gas into the processing
chamber 102 and the flow rate thereof so that the silicon dioxide
film layer 204 can remain at least until the etching process for
forming holes on the silicon layer 210 is completed. The timing of
introducing the CF.sub.4 gas into the processing chamber 102 and
the flow rate thereof will be described later.
[0063] Hereinafter, the shapes of the holes formed on the silicon
layer 210 by etching the silicon layer 210 under the etching
conditions described in Table 1 will be described with reference to
FIGS. 5A to 5C.
[0064] FIG. 5A corresponding to FIG. 4A depicts the shapes of the
holes 210a and 210b formed in the depth direction when 330 seconds
has passed since the beginning of the etching process. Further,
FIG. 5B corresponding to FIG. 4B describes the shapes of the holes
210a and 210b formed in the depth direction when the entire etching
process has been completed by finishing the fourth step S4 where
the CF.sub.4 gas is introduced into the processing chamber 102,
i.e., when 570 seconds has passed since the beginning, of the
etching process. Furthermore, FIG. 5C corresponding to FIG. 4C
indicates the shapes of the holes 210a and 210b formed in the depth
direction when the entire etching process has been completed
without introducing the CF.sub.4 gas into the processing chamber
102, i.e., 570 seconds after the beginning of the etching process.
Additionally, in order to show the accurate shapes of the holes,
FIGS. 5A to 5C represent the schematic cross sectional views of the
objects to be processed that were cleaned by using a liquid
chemical, e.g., hydrofluoric acid. Accordingly, protective layers
formed on inner walls of the holes during the etching process or
deposits accumulated at the openings of the mask material are not
illustrated therein.
[0065] As shown in FIG. 5A, when 330 seconds has passed since the
beginning of the etching process, the depth Dsa of the holes 210a
and 210b reaches 5.37 .mu.m. Besides, at this point, the uppermost
opening diameter Rat of the holes 210a and 210b is 162 nm, and the
diameter Rab of a bottom portion of the holes 210a and 210b is 127
nm.
[0066] When the etching process (the fourth step S4) has been
performed on the silicon layer 210 for 240 seconds after reaching
the state shown in FIG. 5A obtained 330 seconds after the beginning
of the etching process, the depth Dsb of the holes 210a and 210b
reaches 7.81 .mu.m, as shown in FIG. 5B. During the fourth step S4,
the CF.sub.4 gas is introduced into the processing chamber 102.
[0067] In case the CF.sub.4 gas is not introduced thereinto during
the fourth step S4, the depth Dsc of the holes 210a and 210b merely
reaches 7.65 .mu.m, as illustrated in FIG. 5C.
[0068] As clearly can be seen by comparing FIG. 5B with FIG. 5C,
the depth of the holes formed in the silicon layer 210 becomes
different between the cases where the CF.sub.4 gas is added or not
added to the processing gas. Such difference originates from the
difference in the amounts of the deposits 310 accumulated at the
openings of the mask.
[0069] As illustrated in FIG. 4C, in case the CF.sub.4 gas is not
added to the processing gas, the deposits 310 are excessively
accumulated at the openings of the mask, thereby narrowing the
opening diameter Rc1 of the mask. As a result, an apparent aspect
ratio of the holes 210a and 210b being formed increases. Herein,
the "apparent aspect ratio" is defined as "a depth of the holes/the
opening diameter Rc1 of the mask".
[0070] If the apparent aspect ratio increases, the etching is rate
near a bottom portion of the holes 210a and 210b is remarkably
deteriorated during the latter part of the etching process where
the holes are deeply formed. Thus, it may be difficult to obtain a
designed depth.
[0071] As shown in FIG. 4B, by adding the CF.sub.4 gas to the
processing gas, accumulation of deposits 310 at the openings of the
mask is suppressed, thereby enlarging the opening diameter Rb1 of
the mask. Accordingly, it is possible to avoid the deterioration of
the etching rate near the bottom portion of the holes 210a and 210b
during the latter part of the etching process where the holes are
deeply formed, so that the desired depth of the holes can be
obtained. In case the CF.sub.4 gas is added to the processing gas
during the fourth step S4, the etching rate of the silicon layer
210 during the fourth step S4 is 610 nm/min, which is a favorable
value in comparison with the etching rate of the silicon layer 210,
i.e., 570 mm/min, that is measured in case the CF.sub.4 gas is not
added to the processing gas.
[0072] Further, in case the CF.sub.4 gas is added to the processing
gas (FIG. 5B), the uppermost opening diameter Rbt of the holes 210a
and 210b is 171 nm, and the diameter Rbb at the bottom of the holes
210a and 210b is 137 nm. On the other hand, in case the CF.sub.4
gas is not added to the processing gas (FIG. 5C), the uppermost
opening diameter Rct of the holes 210a and 210b is 168 nm, and the
diameter Rcb at the bottom of the holes 210a and 210b is 135 nm. In
other words, even if the CF.sub.4 gas is added to the processing
gas, the inner diameter of the holes formed on the silicon layer
210 is not enlarged.
[0073] Hereinafter, the timing of introducing CF.sub.4 gas into the
processing chamber 102 and the flow rate thereof will be
described.
[0074] Table 2 indicates relationships between the flow rate of
CF.sub.4 gas introduced into the processing chamber 102 and the
etching rate and the in-surface uniformity (uniformity of the
etching rate in surface of a semiconductor wafer) of the silicon
dioxide layer 204 of the mask material.
2TABLE 2 CF.sub.4 gas flow rate Etching rate In-surface (sccm)
(nm/min) uniformity (%) 0 55.5 .+-.8.2 5 70.4 .+-.15.9 10 79.3
.+-.15.9 30 117.6 .+-.9.6 50 153.8 .+-.16.7
[0075] If the flow rate of CF.sub.4 gas introduced into the
processing chamber increases, the etching rate of the silicon
dioxide layer 204 also increases. Meanwhile, values of the
in-surface uniformity are nearly constant regardless of variations
in the flow rates of CF.sub.4 gas. Therefore, it is preferable to
set the flow rate of CF.sub.4 gas mainly considering the etching
rate. Specifically, the flow rate thereof is controlled such that
the silicon dioxide film layer 204 in the mask material is not
completely consumed before the etching process for forming holes is
completed. For example, it is preferable to control the flow rate
of CF.sub.4 gas within the range from about 5 to about 50 sccm.
Under the etching conditions in accordance with the preferred
embodiment, the flow rate of CF.sub.4 gas is controlled to be
maintained at about 10 sccm, as illustrated in Table 1.
[0076] Hereinafter, the timing of introducing the CF.sub.4 gas into
the processing chamber 102 and the period thereof in accordance
with the preferred embodiment will be described with reference to
FIG. 6. The period of introducing CF.sub.4 gas into the processing
chamber 102 (the period of adding CF.sub.4 gas to the processing
gas) can be set from the beginning to the end of the etching
process, from the beginning to an intermediate stage of the etching
process, or from a certain stage to the end of the etching process.
Moreover, the CF.sub.4 gas can be introduced into the processing
container 102 during a predetermined time period in the etching
process. The timing of introducing CF.sub.4 gas into the processing
container 102 and the period thereof are preferably determined
based on the opening diameter or the depth of the holes 210a and
210b formed by the etching, the thickness of the silicon dioxide
film layer 204 in the mask material, the types of the processing
gas and the like.
[0077] The timing of introducing the CF.sub.4 gas into the
processing chamber 102 and the period thereof in the preferred
embodiment will now be described. FIG. 6 shows a relationship
between an etching time for forming the holes 210a and 210b in the
silicon layer 210 and an opening diameter of the mask.
[0078] Referring to FIG. 6, there are illustrated a circle
".largecircle." indicating an opening diameter of the mask, which
is measured when the CF.sub.4 gas is not added to the processing
gas; a square ".quadrature." representing an opening diameter of
the mask, which is measured when the CF.sub.4 gas is added to the
processing gas from the beginning of the fourth step S4, i.e., 330
seconds after the beginning of the etching process; and a triangle
".DELTA." presenting an opening diameter of the mask, which is
measured in case the CF.sub.4 gas is added to the processing gas
from 90 seconds after the beginning of the etching process, i.e.,
in the middle of the third step S3. The introducing timing of
CF.sub.4 gas represented by the square ".quadrature." in FIG. 6
corresponds to the etching conditions illustrated in Table 1.
[0079] According to the result represented by the circles
".largecircle." in FIG. 6, a large amount of deposits is
accumulated at the openings of the mask during a period spanning
from 90 to 330 seconds of the etching time. After 330 seconds has
passed since the beginning of the etching process, the amount of
newly accumulated deposits is small. During the period spanning
from 90 to 330 seconds of the etching time, the depth of the holes
210a and 210b is shallow yet whereas the etching rate of the
silicon layer 210 is high, so that a large amount of deposits is
considered to be accumulated at the openings of the mask.
[0080] Therefore, CF.sub.4 gas can be added to the processing gas
from 90 seconds after the beginning of the etching process (or from
the beginning of the etching process) till 330 seconds after the
beginning of the etching process (or to the end of the etching
process), as plotted by the triangle ".DELTA." instead of the
square ".quadrature." representing the timing of introducing
CF.sub.4 gas corresponding to the etching conditions of Table 1. In
this case, it is possible to suppress the accumulation of deposits
itself at the openings of the mask.
[0081] Further, as plotted by the squares ".quadrature.", the
deposits accumulated at the openings of the mask are etched and
removed by the CF.sub.4 gas that is added to the processing gas
since 330 seconds after the beginning of the etching process when
the amount of newly accumulated deposits at the openings of the
mask becomes negligible.
[0082] By adding the CF.sub.4 gas to the processing gas, the
opening diameter of the mask is enlarged by about 20 nm in
comparison with the case where the CF.sub.4 gas is not added
thereto. Moreover, by controlling the timing of adding the CF.sub.4
gas to the processing gas, the effect of suppressing the
accumulation of deposits at the openings of the mask and/or an
effect of removing the deposits accumulated thereat can be
achieved.
[0083] The timing of adding the CF.sub.4 gas to the processing gas
is preferably set when the opening diameter of the mask becomes
narrow (e.g., smaller than or equal to about 110 nm) so as to
deteriorate the etching rate of the silicon layer 210 or when
deposits are excessively accumulated at the openings of the mask.
Furthermore, it is preferable to set the timing of stopping the
addition of CF.sub.4 gas to the processing gas to coincide with the
time when the accumulation of the deposits at the openings of the
mask is stopped substantially (e.g., 330 seconds after the
beginning of the etching process). Moreover, it is also possible to
continuously add the CF.sub.4 gas until the end of the etching
process. In this case, however, the flow rate of CF.sub.4 gas
should be controlled at an appropriate value such that, especially,
the silicon dioxide film layer 204 is not completely consumed.
[0084] As described above, in accordance with the plasma etching
method of the preferred embodiment, an appropriate amount of
CF.sub.4 gas is added to the processing gas having a mixture of HBr
gas, NF.sub.3 gas (or SF.sub.6 gas), SiF.sub.4 gas and O.sub.2 gas
at an appropriate timing. Due to an effect of the CF.sub.4 gas, the
accumulation of deposits at the openings of the mask is suppressed
and, further, accumulated deposits can be removed. Accordingly, it
is possible to form deep holes in the silicon layer 210 even in
case the diameter of the openings of the mask is very small.
[0085] Although the present invention has been described with
respect to the preferred embodiments where holes are formed in a
silicon layer by the etching, the present invention can be applied
to a case where grooves are formed in a silicon layer. Similar
effects can be obtained when forming either grooves or holes in a
wafer, e.g., a silicon layer. Further, in case the grooves are
formed in the wafer, an opening diameter of the aforementioned
holes corresponds to an opening width of the grooves.
[0086] The preferred embodiments of the present invention have been
described where the CF.sub.4 gas is added to the processing gas.
However, in lieu of the CF.sub.4 gas, a CF-based gas such as
C.sub.4F.sub.8 gas, C.sub.5F.sub.8 gas and C.sub.4F.sub.6 gas and
the like or CHF-based gas such as CHF.sub.3 gas, CH.sub.2F.sub.2
gas and the like can be added to the processing gas. Similar
effects of preventing deposits from being accumulated at the
openings of the mask material can be obtained by employing any of
the aforementioned gases.
[0087] In addition, although the plasma etching method in
accordance with the preferred embodiments of the invention uses a
processing gas containing either the SF.sub.6 gas or the NF.sub.3
gas, the present invention is not limited thereto. Even if a
processing gas containing both the SF.sub.6 gas and the NF.sub.3
gas is used, similar effects can be obtained.
[0088] While the invention has been shown and described with
respect to the preferred embodiments, it will be understood by
those skilled in the art that various changes and modification may
be made without departing from the spirit and scope of the
invention as defined in the following claims.
* * * * *