U.S. patent application number 12/220233 was filed with the patent office on 2008-11-20 for process to open carbon based hardmask overlying a dielectric layer.
Invention is credited to Shawming Ma, Bryan Pu, Shing-Li Sung, Judy Wang.
Application Number | 20080286977 12/220233 |
Document ID | / |
Family ID | 37902459 |
Filed Date | 2008-11-20 |
United States Patent
Application |
20080286977 |
Kind Code |
A1 |
Wang; Judy ; et al. |
November 20, 2008 |
Process to open carbon based hardmask overlying a dielectric
layer
Abstract
A method of opening a carbon-based hardmask layer composed of
amorphous carbon containing preferably at least 60% carbon and
between 10 and 40% hydrogen. The hardmask is opened by plasma
etching using an etching gas composed of H.sub.2, N.sub.2, and CO.
The etching is preferably performed in a plasma etch reactor having
an HF biased pedestal electrode and a capacitively VHF biased
showerhead.
Inventors: |
Wang; Judy; (Cupertino,
CA) ; Sung; Shing-Li; (Hsin-Chu, TW) ; Ma;
Shawming; (Sunnyvale, CA) ; Pu; Bryan; (San
Jose, CA) |
Correspondence
Address: |
Applied Materials, Inc.;Patent/Legal Dept., M/S 2061
P.O. Box 450A
Santa Clara
CA
95052
US
|
Family ID: |
37902459 |
Appl. No.: |
12/220233 |
Filed: |
July 22, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11244422 |
Oct 5, 2005 |
7432210 |
|
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12220233 |
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Current U.S.
Class: |
438/710 ;
257/E21.214; 257/E21.253; 257/E21.257 |
Current CPC
Class: |
H01L 21/31144 20130101;
H01J 37/3266 20130101; H01L 21/31122 20130101; H01J 37/32165
20130101; H01J 37/32082 20130101 |
Class at
Publication: |
438/710 ;
257/E21.214 |
International
Class: |
H01L 21/302 20060101
H01L021/302 |
Claims
1. A method of etching a carbon-based layer formed over a substrate
and comprising at least 40 at % carbon, the method comprising
exposing the carbon-based layer to a plasma of an etching gas
having active components consisting essentially of hydrogen,
nitrogen, and carbon monoxide.
2. The method of claim 1, wherein the carbon-based layer comprises
at least 60 at % carbon.
3. The method of claim 2, wherein the carbon-based layer comprises
between 10 and 40 at % hydrogen.
4. The method of claim 1, wherein the actives components of the
etching gas consist of hydrogen, nitrogen, and carbon monoxide.
5. The method of claim 1, wherein a flow of the hydrogen is between
50% and 300% of a flow of the nitrogen.
6. The method of claim 1, further comprising optically patterning a
photoresist layer formed over the carbon-based layer.
7. The method of claim 6, further comprising etching a dielectric
layer underlying the carbon-based layer using the etched
carbon-based layer as a hardmask.
8. The method of claim 1, further comprising placing the substrate
into a plasma etch chamber and maintaining a pressure of the
chamber at more than 50 milliTorr during the etching.
9. The method of claim 8, wherein the pressure of the chamber is no
more than 20 milliTorr during the etching.
10. A method of etching a carbon-based layer overlying a dielectric
layer and comprising at least 40 at % carbon, the method comprising
exposing the carbon-based layer to a plasma of an etching gas
having active components comprising hydrogen, nitrogen, and carbon
monoxide and including no effective amount of fluorine.
11. The method of claim 10, wherein the carbon-based layer
comprises at least 60 at % carbon.
12. The method of claim 11, wherein the carbon-based layer
comprises between 10 and 40 at % hydrogen.
13. The method of claim 10, wherein the active components of the
etching gas consist essentially of hydrogen, nitrogen, and carbon
monoxide.
14. The method of claim 13, wherein the active components of the
etching gas consist of hydrogen, nitrogen, and carbon monoxide
15. The method of claim 10, wherein a flow of the hydrogen is
between 50% and 300% of a flow of the nitrogen.
16. The method of claim 1, further comprising optically patterning
a photoresist layer formed over the carbon-based layer.
17. The method of claim 16, further comprising etching the
dielectric layer using the etched carbon-based layer as a
hardmask.
18. The method of claim 17, wherein the dielectric layer comprises
an oxide and wherein the step of etching the dielectric layer
includes a fluorine-based etch.
19. The method of claim 10, further comprising placing the
substrate into a plasma etch chamber and maintaining a pressure of
the chamber at more than 50 milliTorr during the etching.
20. The method of claim 19, wherein the pressure of the chamber is
no more than 20 milliTorr during the etching.
Description
RELATED APPLICATION
[0001] This application is a continuation of Ser. No. 11/244,422,
filed Oct. 5, 2005.
FIELD OF THE INVENTION
[0002] The invention relates generally to etching of semiconductor
integrated circuits. In particular, the invention relates to
etching of masks containing a high carbon fraction.
BACKGROUND ART
[0003] Plasma etching is one process used in the definition of the
structure of a silicon integrated circuit. One example involves the
etching of via holes through a dielectric layer to form a vertical
metallic interconnect structure, which in some advanced designs may
simultaneously form the horizontal interconnect structure. The
dielectric layer is conventionally formed of a material based on
silicon dioxide, also called oxide. More advanced dielectrics have
included fluorine or other dopants to reduce the dielectric
constant. Yet other dielectric compositions may be used. The long
established photolithographic process deposits a generally planar
layer of photoresist material onto the unpatterned oxide with
perhaps an anti-reflective coating (ARC) therebetween. The
photoresist is optically patterned according to a desired patterned
and then developed to remove the unexposed photoresist in positive
lithography or exposed photoresist in negative lithography. The
patterned photoresist then serves as a mask for a further step of
etching the exposed oxide and intermediate ARC if present.
Dielectric etch processes have been developed which provide a
reasonable etch selectivity between the oxide and photoresist.
[0004] The advance of integrated circuit technology has depended in
large part on the continuing shrinkage of the horizontal features
such as the via holes through the oxide layer. Via widths are now
decreasing to below 100 nm. However, the oxide thickness has held
steady at around 1 .mu.m and there are many structures in which
oxide thicknesses of 3 .mu.m or more are desired. Such high aspect
ratios of the holes to be etched in the oxide layer have presented
several problems between the photolithography and the etching. To
maintain depth of field in the optical patterning, the thickness of
the photoresist should not be much greater than the size of the
feature being defined in the oxide layer, e.g., 100 nm in the above
example. As a result, the etch selectivity, that is, the ratio of
the oxide etch rate to the photoresist etch rate must be 10 or
greater if the mask is to remain until the via hole has been etched
to its bottom. However, photoresists are typically based on soft
organic materials. Obtaining such high selectivity of photoresist
has been difficult to achieve while simultaneously achieving other
requirements such as vertical profiles in the narrow via holes.
[0005] It is desired to transition the lithography from 248 nm
radiation for exposing the photoresist from a KrF layer to 193 nm
radiation from an ArF laser. However, the 193 nm radiation presents
problems. The photoresist sensitive to the shorter wavelengths is
generally softer and the maximum thickness of the photoresist is
generally reduced to less than 400 nm to accommodate the shallower
depth of field at the shorter wavelength.
SUMMARY OF THE INVENTION
[0006] A method of etching a carbon-based layer, particularly for
opening a hardmask of such material through a photoresist mask, by
plasma etching in an etching gas comprising and preferably
consisting of hydrogen, nitrogen, and carbon monoxide. The hardmask
may be used as a mask for etching an underlying layer, such as a
dielectric inter-metal layer requiring a high aspect-ratio via
hole. The method is especially useful when the dielectric
inter-metal layer is composed of a low-k dielectric.
[0007] The hydrogen and nitrogen are preferably supplied in a ratio
of between 1:2 and 2:1. The chamber pressure is preferably
maintained above 50 milliTorr.
[0008] The plasma etch chamber preferably includes a pedestal
electrode biased by an HF power supply operating in the range of 1
to 143 MHz and a showerhead electrode operating in the range of 150
to 350 MHz.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a cross-sectional view of an exemplary structure
that can be formed according to the invention.
[0010] FIG. 2 is a schematic cross-sectional view of a plasma etch
reactor in which the invention is advantageously practiced.
[0011] FIG. 3 is a chart illustrating the uniformity and profile
characteristics for two different ratios of two of the etching
gases.
[0012] FIG. 4 is a chart illustrating the uniformity and profile
characteristics for different ratios of flow of the etching gases
to the center and edge of the wafer.
[0013] FIG. 5 is a chart illustrating the uniformity and profile
characteristics as a function of chamber pressure.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0014] Etching selectivity can be increased by using a photoresist
mask to pattern a hardmask intermediate the dielectric and
photoresist layers. The generally thinner hardmask is more easily
etched using the available photoresist and the tougher hardmask is
used to pattern the thicker underlying dielectric layer. Hardmasks
are needed in dielectric etching as the feature size decreases to
less than 100 nm and using 193 nm photoresist patterning radiation
available from an ArF laser. Hardmasks have been proposed in the
past, typically composed of silicon nitride or silicon oxynitride.
However, a particularly advantageous hardmask material is a
carbon-based material such as Advanced Patterning Film (APF)
available from Applied Materials, Inc. of Santa Clara, Calif. Its
deposition by plasma enhanced chemical vapor deposition (PECVD) has
been described by Fairbairn et al. in U.S. Pat. No. 6,573,030 using
a hydrocarbon, for example, propylene C.sub.3H.sub.6, as a
precursor. Wang et al. in U.S. Published Application 2005/0199585
and Lei et al. in U.S. Published Application 2005/0167394 have
described its use as a hardmask. These three documents are
incorporated herein by reference. Fairbairn has characterized this
material as being composed of at least 40 at % of carbon and
between 10 and 60 at % of hydrogen and therefore of at least 40 at
% of carbon. A tighter compositional range is, however, preferred
of at least 60 at % of carbon and between 10 and 40 at % of
hydrogen. Dopants have been proposed to control the dielectric
constant and refractive index, but an APF inter-layer dielectric
patterned through an effective anti-reflective coating does not
seem to require substantial components other than carbon and
hydrogen. It is believed to form as an amorphous material although
its growth condition and precursors may change the crystallography.
APF grown at 400.degree. C. has been observed to have a density of
1.1 g/cm.sup.2, a hardness of 2.2 MPa, a strength of 2.2 MPa, and
an optimized C/H atomic ratio of 63/37. The ratio of single
hydrocarbon bonds (C--H) to double hydrocarbon bonds (C.dbd.H) is
observed to be 5. Recently developed APF grown at higher
temperatures may show somewhat different characteristics.
[0015] An example of a use of a carbon-based hardmask is
illustrated in the cross sectional view of FIG. 1 for a contact
via. However, it must be emphasized that the invention is not
limited to this structure and may be applied to opening
carbon-based layers other than APF over other materials. Over a
silicon substrate 12 are deposited typically by chemical vapor
deposition (CVD) an etch stop layer 14, for example, of silicon
nitride, a dielectric layer 16, for example, of a low-k oxide doped
with a halogen, and a carbon-based hardmask layer 18 to facilitate
the photographic patterning of the after deposited photoresist
layer 22, typically spun on in wet form and then dried. The ARC
layer 20 includes a bottom DARC layer 24 of, for example, silicon
oxynitride, and an upper BARC layer 26 of organic material. The
DARC layer 24 is used in part to promote adhesion of the BARC layer
26 to the carbon-based hardmask layer.
[0016] The photoresist layer 22 is then photographically exposed
through a reticle in a step and repeat exposer tool and developed
to leave an aperture over the desired contact via 28. The BARC and
DARC layers 26, 24 are then removed using conventional plasma
etching, for example, using CF.sub.4. Then the exposed areas of the
carbon-based hardmask layer 18 are etched through or opened using
the remaining portions of the patterned photoresist layer 26 or the
ARC layer 18 as the etch defining mask. Thereafter a fluorine-based
chemistry, for example, of C.sub.4F.sub.6 and Ar or other
hydrogen-free or hydrogen-containing fluorocarbon, etches through
the dielectric layer 16 using the hardmask layer 18 as the mask for
at least the final portions of the etch and stopping at the etch
stop layer [[16]] 14 because of the selective nature of the
properly chosen fluorine-based oxide etch. The thin etch stop layer
14 is then removed by a sputter etch or relatively non-selective
chemical plasma etch step, for example, using CF.sub.4 or O.sub.2
for the DARC and BARC, to expose the underlying silicon wafer 12 to
allow its doping by ion implantation and contacting through a
contact layer such as a refractory silicide and a metallization
metal filled into the via hole. The remaining portion of the
carbon-based hardmask layer 18 may be removed in an oxygen plasma.
It is understood that the process can be easily extended to a dual
damascene structure having a lower-level via hole and an
upper-level trench for horizontal connections.
[0017] The opening of the hardmask layer should satisfy several
different criteria. It should produce a vertical profile in the
hardmask layer to maintain the critical dimension (CD) established
by the photoresist patterning. For 100 nm features, the variation
of the CD at the bottom of the opened hardmask should be less than
10 nm. For commercial production, the etch rate should be
relatively high, for example, greater than 500 nm. Production of
particles should be relatively low and not significantly increase
during repeated cycling of the equipment. Because the hardmask
layer typically has thickness of greater than 400 nm or 600 nm and
via widths of less than greater than 200 nm are required, the etch
should be highly anisotropic and produce vertical profiles.
Particles may become a concern because of the fluorine-based oxide
etching preferably performed in the same chamber as the opening of
the hardmask.
[0018] An advantageous process for opening the carbon-based
hardmask is based on the etching gas mixture of H.sub.2/N.sub.2/CO.
The hydrogen and nitrogen are the primary etching species. The
carbon monoxide provides sidewall passivation, which is important
for maintaining the vertical profile.
[0019] The hardmask open process together with other steps in
defining the via contact of FIG. 1 may be performed in a
capacitively coupled plasma etch chamber 30 schematically
illustrated in the cross-sectional view of FIG. 2 and described by
Hoffman et al. in U.S. Pat. Nos. 6,853,141 and 6,894,245, both
incorporated herein by reference. Hoffman et al. expands upon
features of the former in U.S. patent application Ser. No.
11/046,538, filed Jan. 28, 2005 and now published as U.S. patent
publication 2005/0178745. The Enabler chamber available from
Applied Materials incorporates parts of the disclosed chamber. The
etch chamber 30 separately biases the showerhead and the pedestal
supporting the wafer and attempts to decouple the VHF source power
applied to the showerhead and producing the plasma from the HF
source power applied to the pedestal and producing a DC self bias
which affects the energy of an etching ion. The chamber 30 also
includes careful selection of source frequency and careful coupling
of the VHF power into the chamber, features best described in the
cited application. Other features to be briefly described greatly
improve the uniformity of etching.
[0020] The etch chamber 30 includes a main chamber body 32
including a baffled annular pumping port 34 to a vacuum pump 36
allowing the chamber to be pumped to 100 milliTorr and below. A
pedestal electrode 38 supports a wafer 40 be etch processed in
opposition to a showerhead 42 supplying etching gas into a
processing space 44 above the wafer 40. A wafer port 48 with an
associated slit valve allows the wafer 40 to be inserted into the
chamber 30. An HF power supply 50 RF biases the pedestal electrode
40 through a capacitive matching circuit 52 to produce the DC self
bias on the wafer 40. The frequency of the HF power supply 50 may
be in the low megahertz range. In some applications not
specifically discussed here, two HF power supplies operating
respectively at 1.8 MHz and 2.0 MHz may both input to the matching
circuit 52. A broader preferred range for the HF frequency is
between 1 and 14 MHz.
[0021] The RF biasing of the showerhead 42 is carefully controlled
through a coaxial stub 56 including an inner conductor 58 and an
outer conductor 60 separated by a insulator 62 and terminated by a
short 64. The stub 56 has a predetermined length, for example, a
quarter wavelength of a VHF frequency that provides both high
coupling and a wide output impedance. A VHF power supply 68, for
example, operating at 162 MHz, is connected through a coaxial cable
70 to the inner conductor 58 at a distance from the short 64 which
provides high power coupling. At the other end of the stub 56, the
outer conductor 60 is grounded to the chamber body 32 and the inner
conductor 58 is connected to a flared conductor 72, which is
capacitively coupled to the showerhead 42 through a insulating ring
74 of carefully controlled thickness, which effectively isolates
the VHF biasing of the showerhead 42 from the DC self-biasing of
the pedestal electrode 38. A broader preferred range for the VHF is
between 150 and 325 MHz.
[0022] The showerhead 74 contains a large number of apertures 78 to
evenly supply processing gas into the processing space 44. However,
the apertures are divided into an annular outer zone 80 and an
annular inner zone 82 connected via respective foam-filled
manifolds 84, 86 and gas supply lines 88, 90 through a bore 92
within the stub 56 to inner and outer gas supplies 94, 96. Thereby,
the process gas may be differentially supplied to inner and outer
portions of the wafer 40. A typical diameter of the inner zone 82
is 8.1 inches (206 mm) for a 300 mm wafer. Heating or cooling fluid
is supplied to the back of the showerhead 44 and returned therefrom
through fluid lines 104 passing through the stub bore 92 and
connected to a thermal fluid source 106.
[0023] The dual zone showerhead provides a mean of tuning the
radial distribution of all species of the process gas including
neutral atoms or molecules. The ionized species can be separately
tuned by two coaxial coils 112, 114 placed in back of the
showerhead and supplied with separately controllable amounts of DC
current by a plasma steering controller 116 to produce magnetic
fields in the processing space 44. The first coil 112 is placed in
a radially outer position outside of the showerhead 42 and a short
distance above a level of the showerhead 42. The second coil 114 is
placed in a radially inner position and a longer distance above the
showerhead, preferably adjacent the outside of the top of the
flared conductor 72. When approximately equal currents of the same
polarity pass through the two coils 112, 114, a cusp-shaped
magnetic field is produced having significant radial components in
the processing space 44 between the showerhead 42 and the wafer 40,
which can steer the ionized components of the process gas.
[0024] According to one aspect of the invention, the carbon-based
layer is removed by a plasma etch with an etching gas including
nitrogen, hydrogen, and carbon monoxide. Preferably, the etching is
performed in an etch chamber, such as that of FIG. 1, including
magnetic enhancement and a showerhead capacitively biased at a VHF
frequency.
Example 1
[0025] A hardmask open process has been optimized for an APF layer
grown at 400.degree. C. to a thickness of 900 nm on a 300 mm
silicon wafer and thereafter covered with the ARC layer and
patterned photoresist. The wafer is placed into the etch chamber of
FIG. 2 configured for 300 mm wafers. The ARC layer is removed by a
conventional etch in which 200 sccm of CF.sub.4 is supplied at a
pressure of 150 milliTorr while the pedestal electrode is biased
with 600 W of 13.56 MHz RF power.
[0026] After a transition step, the APF layer is then etched with
an etching gas flow of 150 sccm of N.sub.2, 450 sccm of H.sub.2,
and 50 sccm of CO. The different process gases are metered by
respective mass flow controllers. The component fractions delivered
to the inner and outer zones are the same and, in this example,
equal amounts of process gas are delivered to the two zones. The
chamber pressure is maintained at 100 milliTorr and the pedestal
electrode is held at 40.degree. C. The showerhead electrode is
supplied with 1500 W of 162 MHz source power and the pedestal
electrode is supplied with 900 W of 13.56 MHz bias power. Five amps
of current of the same polarity are passed through each of the
coils to produce the cusp-shaped magnetic field. The etch rate for
180 s of etching averaged over the wafer is observed to be about
431 nm/min with a non-uniformity of 7.7%.
Example 2
[0027] In a second example, the current supplied to the inner coil
is reduced to 2 A while the current to the outer coil remains at 5
A. Other conditions remain the same as in the first example. The
average etch rate is observed to increase to 469 nm/min but the
non-uniformity increases to 11.5%.
Example 3
[0028] In a third example, the 162 MHz source power is increased to
2000 W while other conditions remain the same as in the first
example. The average etch rate is observed to be about 549 nm/min
with a non-uniformity reduced to 3.1%.
Example 4
[0029] In a fourth example, the 162 MHz source power is set to 2000
W and the supply of N.sub.2 is increased to 300 sccm and the supply
of H.sub.2 is decreased to 300 sccm. Otherwise the conditions of
the first example are used. The average etch rate is observed to be
516.5 nm/min with a non-uniformity of 2.1%. There is some
non-uniformity in the hole profile across the wafer.
Example 5
[0030] In a fifth example, two process conditions are compared to
determine the effect of varying the ratio of nitrogen to hydrogen.
In one set of tests, the hydrogen flow is 450 sccm and the nitrogen
flow is 150 sccm. In the other set, the hydrogen flow is 300 sccm
and the nitrogen flow is 300 sccm. That is, the total flow of the
two gases believed primarily responsible for the etching is
maintained the same. The carbon monoxide is believed to be
primarily useful in passivating the sidewall and thus improving the
profile.
[0031] The results are summarized in the chart of FIG. 3, which
shows the bottom critical dimension (BCD), that is, the width at
the bottom at different locations on the wafer for the two
H.sub.2/N.sub.2 flow ratios. The chart also displays the variance
of the BCD. Generally the lower H.sub.2/N.sub.2 ratio below 2:1
provides smaller BCD with better BCD uniformity. Also, more ARC
remains at the end of the APF etch with the lower ratio. However,
at the lower ratio there is more undercutting at the DARC/APF
interface, which contributes to a less vertical profile. For other
chamber optimizations, it is believed that the preferred
H.sub.2/N.sub.2 flow ratio falls within the range of 3:1 to
1:2.
[0032] Similar results are observed when the source power is
increased to 2000 W.
Example 7
[0033] In a seventh example, the ratio of the flow of process gases
between the inner and outer zones is varied while otherwise
maintaining the process conditions of the first example. The
results in the chart of FIG. 4 show the BCD at six points on the
wafer for no process gas in the outer zone and for the ratios for
outer to inner flow rates of 1/4, 1, and 4. It is seen that more
edge flow increases the BCD particularly at the center while
solving the polymer residue at the top of the profile. The effects
arise from less gas being supplied to the center and the increased
pumping. Also, the gas residence time is increased. The higher edge
flow also improves the uniformity. Generally flow ratios of between
1/2 to 2 produce better results although that APF at the unity flow
ratio is about 10% lower.
Example 8
[0034] In an eighth example, the chamber pressure is varied while
otherwise the conditions of the first example are followed. The
results of FIG. 5 show that at pressures above 50 milliTorr, the
vertical profile is improved and the uniformity of BCD is greatly
improved. It is believed 5 that for differently optimized recipes,
the minimum of the optimum pressure range may be reduced to 20
milliTorr.
[0035] All the described recipes include an etching gas having
active constituents of which at least 90% are hydrogen and
nitrogen. It is possible to add an inert gas such as argon, but it
is expected to have little effect on the APF etching chemistry.
[0036] APF is being developed by others to be grown at 550.degree.
C. and is believed to be harder than the 400.degree. C. APF.
However, the same etching chemistry of H.sub.2/N.sub.2/CO should
provide similarly good results although with varying optimization.
The etching chemistry is applicable also to other carbon-based
materials having similar compositions.
[0037] It is possible to use intermediate hardmask layers between
the carbon-based layer and the photoresist. The ARC layer in fact
acts partially as a hardmask layer once the ARC layer has been
etched through and the chemistry changed for etching carbon-based
material. The carbon-based hardmask greatly simplifies the etching
of the underlying layer, particular of low-k dielectric or very
thick oxide layers desired for some applications. The carbon-based
material is relatively impervious to fluorine-based plasma etching,
which is particularly effective for oxide etching. That is, very
high selectivity is available for etching oxide over the defining
carbon-hard mask.
[0038] The hard mask can be defined with a relatively thin
photoresist layer, thus allowing very narrow mask features, but the
carbon-based hardmask can be deeply and vertically etched using the
chemistry of the invention. The hardmask can also be etched in the
same plasma etch reactor used for the overlying ARC layer and the
underlying oxide or other layer for which it defines the etch
patterning.
* * * * *