Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02690697 2010-01-21
SELECTIVE ETCHING
AND
FORMATION OF XENON DIFLUORIDE
BACKGROUND OF THE INVENTION
[0002] In the electronics industry various deposition techniques have been
developed
wherein selected materials are deposited on a target substrate to produce
electronic
components such as semiconductors. One type of deposition process is chemical
vapor deposition (CVD), wherein gaseous reactants are introduced into a heated
processing chamber resulting in films being deposited on the desired
substrate. One
subtype of CVD is referred to a plasma enhanced CVD (PECVD) wherein a plasma
is
established in the CVD processing chamber.
[0003] Generally, all methods of deposition result in the accumulation of
films and
particulate materials on surfaces other than the target substrate, that is,
the deposition
materials also collect on the walls, tool surfaces, susceptors, and on other
equipment
used in the deposition process. Any material, film and the like that builds up
on the
walls, tool surfaces, susceptors and other equipment is considered a
contaminant and
may lead to defects in the electronic product component.
[0004] It is well accepted that deposition chambers, tools, and equipment must
be
periodically cleaned to remove unwanted contaminating deposition materials. A
generally preferred method of cleaning deposition chambers, tools and
equipment
involves the use of perfluorinated compounds (PFC's), e.g., C2F6, CF4, C3F8,
SF6, and
NF3 as etchant cleaning agents. In these cfeaning_ operations a chemically
active
fluorine species, which is normally carried in a process gas, converts the
unwanted and
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contaminating residue to volatile products. Then, the volatile products are
swept with
the process gas from the reactor.
[0005] Ion implantation is used in integrated circuit fabrication to
accurately introduce
controlled amounts of dopant impurities into semiconductor wafers and is a
crucial
process in microelectronic/semiconductor manufacturing. In the ideal case, all
feedstock molecules would be ionized and extracted, but in reality a certain
amount of
feedstock decomposition occurs, which results in the deposition on and
contamination
on the surfaces in the ion source region, or parts of the ion implanter tool,
such as, low
voltage insulators and the high voltage components. The known contamination
residues
are silicon, boron, phosphorus, germanium or arsenic. It would be a
significant advance
in the art of ion implantation to provide an in situ cleaning process for the
effective,
selective removal of unwanted residues deposited throughout the implanter,
particularly
in the ion source region, during implantation. Such in situ cleaning would
enhance
personnel safety and contribute to stable, uninterrupted operation of the
implantation
equipment. A gas-phase reactive halide composition, e.g., XeF2 , NF3 , F2 ,
XeF6,
SF6 , C2F6 , IFs or IF7 , is introduced to the contaminated parts for
sufficient time and
under sufficient conditions to at least partially remove the residue from the
components,
and to do so in such a manner that residue is removed selectively with respect
to the
materials from which the components of the ion implanter are constructed.
[0006] In a micro electro mechanical system (MEMS), a mixture of sacrificial
layers
(usually with amorphous silicon) and protective layers, thus device structure
layers, are
formed. Selectively removing the sacrificial materials is a critical step for
the structure
release etching process, where several microns of sacrificial material need to
be
removed isotropically without damaging other structures. It has been
understood that the
etching process is a selective etching process that does not etch the
protective layers.
Typical sacrificial materials used in MEMS are: silicon, molybdenum, tungsten,
titanium,
zirconium, hafnium, vanadium, tantalum, niobium. Typical protective materials
are nickel,
aluminum, photoresist, silicon oxide, silicon nitride.
[0007] In order to efficiently remove the sacrificial material, the release
etching utilizes
an etchant gas capable of spontaneous chemical etching of the sacrificial
layers,
preferably isotropic etching that removes the sacrificial layers. Because the
isotropic
etching effect of xenon difluoride is great, xenon difluoride (XeF2) is used
as the etchant
for lateral etching process.
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[0008] However, xenon difluoride is expensive, and is a material difficult to
deal with.
Xenon difluoride is unstable on contact with air, light or water vapor
(moisture). All
xenon fluorides must be protected from moisture, light and air to avoid
formation of
xenon trioxide and hydrogen fluoride. Xenon trioxide is a colorless,
nonvolatile solid
that's dangerously explosive. Hydrogen fluoride is not only dangerous, but
also reduces
efficiency of etching.
[0009] Additionally, xenon difluoride is a solid having a low vapor pressure
which
makes deliver of xenon difluoride to the process chamber difficult.
[0010] The following references are illustrative of processes for the
deposition of films
in semiconductor manufacture and the cleaning of deposition chambers, tools
and
equipment and the etching of substrates, the etching of sacrificial layers in
a MEMS,
and for the cleaning of the ion source region in an ion implantation system
used in the
fabrication of a microelectronic device:
[0011] US 5,421,957 discloses a process for the low temperature cleaning of
cold-
wall CVD chambers. The process is carried out, in situ, under moisture free
conditions.
Cleaning of films of various materials such as epitaxial silicon, polysilicon,
silicon nitride,
silicon oxide, and refractory metals, titanium, tungsten and their silicides
is effected
using an etchant gas, e.g., nitrogen trifluoride, chlorine trifluoride, sulfur
hexafluoride,
and carbon tetrafluoride.
[0012] US 6,051,052 discloses the anisotropic etching of a conduct material
using
fluorine compounds, e.g., NF3 and C2F6 as etchants in an ion-enhanced plasma.
The
etchants consist of a fluorine containing chemical and a noble gas selected
from the
group consisting of He, Ar, Xe and Kr. The substrates tested include
integrated circuitry
associated with a substrate. In one embodiment a titanium layer is formed over
an
insulative layer and in contact with the tungsten plug. Then, an aluminum-
copper alloy
layer is formed above the titanium layer and a titanium nitride layer formed
above that.
[0013] US 2003/0047691 discloses the use of electron beam processing to etch
or
deposit materials or repair defects in lithography masks. In one embodiment
xenon
difluoride is activated by electron beam to etch tungsten and tantalum
nitride.
[0014] GB 2,183,204 A discloses the use of NF3 for the in situ cleaning of CVD
deposition hardware, boats, tubes, and quartz ware as well as semiconductor
wafers.
NF3 is introduced to a heated reactor in excess of 350 C for a time
sufficient to remove
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silicon nitride, polycrystalline silicon, titanium silicide, tungsten
silicide, refractory metals
and silicides.
[0015] Holt, J. R., et al, Comparison of the Interactions of XeF2 and F2 with
Si(100)(2X1), J. Phys. Chem. B 2002, 106, 8399-8406 discloses the interaction
of XeF2
with Si(100)(2X1) at 250 K and provides a comparison with FZ. XeF2 was found
to react
rapidly and isotropically with Si at room temperature.
[0016] Chang, F. I., Gas-Phase Silicon Micromachining With Xenon Difluoride,
SPIE
Vol. 2641/117-127 discloses the use of XeF2 as a gas phase, room temperature,
isotropic, silicon etchant and noted that it has a high selectivity for many
materials used
in microelectromechanical systems such as aluminum, photoresist and silicon
dioxide.
At page 119 it is also noted that XeF2 has a selectivity of greater that
1000:1 to silicon
dioxide as a well as copper, gold, titanium-nickel alloy and acrylic when
patterned on a
silicon substrate.
[0017] Isaac, W.C. et al, Gas Phase Pulse Etching of Silicon For MEMS With
Xenon
Difluoride, 1999 IEEE, 1637-1642 discloses the use of XeF2 as an isotropic gas-
phase
etchant for silicon. It is reported that XeF2 has high selectivity to many
metals,
dielectrics and polymers in integrated circuit fabrication. The authors also
note at page
1637 that XeF2 did not etch aluminum, chromium, titanium nitride, tungsten,
silicon
dioxide, and silicon carbide. Significant etching also had been observed for
molybdenum:silicon; and titanium:silicon, respectively.
[0018] Winters, et al, The Etching of Silicon With XeF2 Vapor, Appl. Phys.
Left. 34(1)
1 January 1979, 70-73 discloses the use of F atoms and CF3 radicals generated
in
fluorocarbon plasma induced dissociation of CF4 in etching solid silicon to
produce
volatile SiF4 species. The paper is directed to the use of XeF2 to etch
silicon at 300 K at
1.4 X 10-2 Torr. Other experiments showed that XeF2 also rapidly etches
molybdenum,
titanium and probably tungsten. Etching of Si02, Si3N4 and SiC was not
effective with
XeF2 but etching was effective in the presence of electron or ion bombardment.
The
authors concluded that etching of these material required not only F atoms but
also
radiation or high temperature.
[0019] US 6870654 and US 7078293 both disclose a structure release etching
process by using an etchant having a fluorine group or a chroleine group to
replace
xenon difluoride, avoiding the difficulties resulting from using the xenon
difluoride.
However, the etching effect is not as efficient as the use of xenon
difluoride. Therefore,
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US 6870654 and US 7078293 disclose a special structure for facilitate the
structure
release etching process such that the processing time etc is commensurate with
that of
xenon difluoride.
[0020] US20060086376 discloses the use of XeFZfor cleaning the residues
(silicon,
boron, phosphorus, germanium or arsenic) from the components of the ion
implanter,
in the fabrication of a microelectronic device.
[0021] Specifically, US20060086376 relates to the in situ removal of residue
from the
vacuum chamber and components contained therein by contacting the vacuum
chamber and/or components with a gas-phase reactive halide composition, e.g.,
XeF2
for sufficient time and under sufficient conditions to at least partially
remove the residue
from the components, and to do so in such a manner that residue is removed
selectively with respect to the materials from which the components of the ion
implanter
are constructed.
[0022] One of industry objectives is to find new etchants that can be used to
remove
difficult to remove titanium nitride (TiN) films from silicon dioxide (Si02)
and silicon nitride
(SiN) coated surfaces. Theses surfaces are found in the walls of semiconductor
deposition chambers, particularly quartz chambers and quartz ware,
semiconductor tools
and equipment. Many of the conventional fluorine based etchants that attack
TiN films
also attack SiO2 and SiN surfaces and, therefore, unacceptable for removing
TiN
deposition products from semiconductor deposition chambers and equipment.
[0023] Another industry objective is to provide a method for selective removal
of
silicon, from silicon dioxide (quartz) surfaces such as those commonly found
in
semiconductor deposition chambers and semiconductor tools, as well as devices
in
MEMS.
[0024] There is yet another industry objective for providing a method for
generating or
forming xenon difluoride on site as needed for lower cost of ownership.
BRIEF SUMMARY OF THE INVENTION
[0025] This invention relates to an improved process for the selective removal
of
titanium nitride (TiN) films and deposition products from silicon dioxide
(quartz) surfaces
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such as those commonly found in semiconductor deposition chambers and
semiconductor tools as well as silicon nitride (SiN) surfaces commonly found
in
semiconductor tool parts and the like. In a basic process for removing
undesired
components contaminating a surface an etchant is contacted with the undesired
component in a contact zone and the undesired component converted to a
volatile
species. The volatile species then is removed from the contact zone. The
improvement
in the basic process for removing undesired TiN deposition materials from a
surface
selected from the group consisting of Si02 and SiN in a contact zone resides
in
employing xenon difluoride (XeF2) as the etchant. Conditions are controlled so
that said
surface selected from the group consisting of Si02 and SiN is not converted to
a volatile
component.
[0026] Significant advantages in terms of selective etching of TiN films and
deposition
materials which are very difficult to remove from semiconductor deposition
chambers
(sometimes referred to as reaction chambers), tool parts, equipment and the
like include:
an ability to selectivity remove TiN films from quartz, i.e., Si02, and SiN
coated surfaces found in the cleaning of deposition chambers;
an ability to remove TiN films from quartz surfaces at modest
temperatures; and,
an ability to activate perfluoro etching agents in remote plasma to remove
TiN films from Si02 and SiN surfaces without adverse effects normally caused
by
fluorine atoms attacking in the remote plasma.
[0027] The present invention also discloses a process for the selective
etching of
a first material to a second material, comprising:
providing a structure containing the first material and the second material in
a
chamber;
providing an etchant gas comprised of xenon(Xe), an inert gas and a fluorine
containing chemical to the chamber;
contacting the structure with the etchant gas and selectively converting the
first
material to a volatile species; and
removing the volatile species from the chamber;
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wherein the first material is selected from the group consisting of silicon,
molybdenum,
tungsten, titanium, zirconium, hafnium, vanadium, tantalum, niobium, boron,
phosphorus,
germanium, arsenic, and mixtures thereof; and the second material is selected
from the
group consisting of silicon dioxide, silicon nitride, nickel, aluminum, TiNi
alloy,
photoresist, phosphosilicate glass, boron phosphosilicate glass, polyimides,
gold,
copper, platinum, chromium, aluminum oxide, silicon carbide , and mixtures
thereof.
[0028] The present invention also discloses a process of forming xenon
difluoride in a
chamber, comprising:
providing a fluorine containing chemical selected from the group consisting of
NF3, C2F6, CF4, C3F8i SF6, a plasma containing F atoms generated from an
upstream
plasma generator, and mixtures thereof to the chamber; and
forming the xenon difluoride by reacting xenon with the fluorine containing
chemical in the chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] Fig. 1 is a plot of the etch rate of a silicon substrate as a function
of the level of
Xe vis-a-vis Ar in an NF3 remote plasma.
[0030] Fig. 2 is a plot of the etch rate of Si02 as a function of the level of
Xe vis-a-vis
Ar in an NF3 remote plasma.
[0031] Fig. 3 is a plot comparing the etch selectivity of silicon to silicon
dioxide as a
function of the level of Xe vis-a-vis Ar in an NF3 remote plasma.
[0032] Fig. 4 is a plot of Fourier Transform Infrared spectrum (FTIR) spectra
from
Ar/NF3 and Xe/NF3 in an NF3 remote plasma.
[0033] Fig. 5 is a plot of Fourier Transform Infrared spectrum (FTIR) spectra
from
Xe/NF3 in an NF3 remote plasma.
[0034] Fig. 6 is a plot of XeFZand XeF4 Fourier Transform Infrared spectrum
(FTIR)
peak heights as a function of Xe/(Xe+Ar) in an NF3 remote plasma.
[0035] Fig. 7 is a plot of XeF2 and XeF4 Fourier Transform Infrared spectrum
(FTIR)
peak heights as a function of Xe/NF3 flow ratio in an NF3 remote plasma.
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[0036] Fig. 8 is a plot of XeF2 Fourier Transform Infrared spectrum (FTIR)
peak height
and etch selectivity of silicon to silicon dioxide as a function of Xe/(Xe+Ar)
in an NF3
remote plasma.
[0037] Fig. 9 is a plot of the etch rate of TiN as a function of temperature
and the level
of Xe vis-a-vis Ar in an NF3 remote plasma.
[0038] Fig. 10 is a plot of the etch rate of silicon dioxide as a function of
temperature
and the level of Xe vis-a-vis Ar in an NF3 remote plasma.
[0039] Fig. 11 is a plot comparing the etch selectivity of TiN to silicon
dioxide as a
function of the level of Xe vis-a-vis Ar in an NF3 remote plasma.
DETAILED DESCRIPTION OF THE INVENTION
[0040] The deposition of titanium nitride (TiN) is commonly practiced in the
electronics
industry in the fabrication of integrated circuits, electrical components and
the like. In
the deposition process some of the TiN is deposited on surfaces other than the
surface
of the target substrate, e.g., walls and surfaces within the deposition
chamber. It has
been found that XeF2 is effective as a selective etchant for TiN contaminating
silicon
dioxide (SiO2) and silicon nitride (SiN) surfaces. With this finding one can
use xenon
difluoride (XeF2) as an etchant for removing unwanted TiN films and deposition
materials contaminating surfaces such as those found in semiconductor reactor
or
deposition chambers, tools, equipment, parts, and chips coated or lined with
silicon
dioxide (quartz) or silicon nitride.
[0041] In the removal of unwanted TiN residues from SiO2 and SiN surfaces,
such as
those in a deposition chamber, XeF2 is contacted with the surface in a contact
zone
under conditions for converting TiN to volatile TiF4, and then removing the
volatile
species from the contact zone. Often, the XeF2 is added along with an inert
gas, e.g.,
N2, Ar, He, and the like.
[0042] In carrying out the process for removing TiN deposition materials from
SiN and
SiO2 surfaces, XeF2 may be preformed prior to introduction to the contact
zone, or for
purposes of this invention, and by definition herein, XeF2 may be formed by
two
methods.
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[0043] In one embodiment of the in situ formation of XeF2, xenon (Xe) is added
to a
fluorine containing chemical and charged to a remote plasma generator. There
Xe
reacts with F atoms present in the resulting remote plasma to form XeF2.
[0044] In the other embodiment, a variation of the in situ embodiment, the
fluorine
containing chemical is added to the remote plasma generator and then Xe and
the
remote plasma containing F atoms are added to a chamber downstream of the
remote
plasma generator. There Xe reacts with F atoms to form XeF2 in the chamber.
The
chamber can be any type of a chamber, such as, but not limited to a processing
chamber, a deposition chamber, a cleaning chamber, a reactor, and a plasma
generator.
[0045] Illustrative of this fluorine containing chemical for forming XeF2
includes F2,
NF3, perfluorocarbons as C2F6, CF4, C3F8, sulfur derivatives such as SF6, and
remote
plasma containing F atoms generated from an upstream plasma generator. In the
preferred embodiment NF3 is used as the fluorine containing chemical for the
formation
of XeF2.
[0046] The fluorine containing chemical can be generated onsite. For example,
F2 is
generated onsite using a halogen generator and then introducing the F2 to the
process.
This would be a possible means to mitigate the hazards associated with
handling
fluorine.
[0047] A wide range of Xe to fluorine containing chemical can be used in the
in situ
process of forming XeF2. The mole ratio of Xe to fluorine containing chemical
is
dependant upon the amount of XeF2 formed vis-a-vis the level of. F atoms in
the remote
plasma.
[0048] Not being bound by theory it is believed that the remote p,asma acts as
a
source for excitation and dissociation of the fluorine containing chemical
that are
introduced as the fluorine source. The fluorine radicals subsequently react
with the Xe
present in the reg;on just after the plasma generation zone. The path length
of this zone
in addition to the energy used to excite the fluorine containing species as
well as Xe,
are believed to be critical parameters in balancing a preference for XeF2 and
a
mitigation of XeF4.
[0049] Furthermore, it is believed that if the Xe is introduced in the space
just after
the plasma excitation zone can lead to even further reductions in XeF4
formation
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because the Xe has not been excited. It is well known that Xenon has a very
low
metastable energy state. The formation of this metastable state can lead to
additional
collisional reactions between XeF2 molecules formed within this zone. These
collisions
could result in dissociation of XeF2 to XeF and F radicals. These species
could then
lead to further reactions with other XeF2 molecules to form XeF4. Thus, by
introducing
the Xe post plasma excitation, the Xe metastables are not formed. Thus, the
formation
of XeF4 will be decreased. This is disclosed in the second embodiment, i.e.
the variation
of the in situ embodiment, wherein Xe is added to remote plasma containing F
atoms
generated upstream in a plasma generator.
[0050] Preferred mole ratios are from 1:10 to 10:1 Xe to fluorine containing
chemical.
Optionally an inert gas, e.g., argon can be included in the remote plasma
generation of
XeF2 as a means of adjusting the selectivity etching of TiN to Si02 and SiN or
Si to Si02
and SiN.
[0051] Pressures suited for the removal of TiN from Si02 and SiN surfaces
range from
0.5 to 50 Torr, preferably from 1 to 10 Torr. Temperatures for effecting
selective etching
of TiN films from silicon dioxide surfaces (quartz) and SiN surfaces depend
primarily on
which method the process is carried out. By that it is meant that if XeF2 is
preformed
and added directly to the contact zone, temperatures should be elevated to at
least 100
C, e.g., 100 to 800 C, preferably from 150 to 500 C. Pressures for XeF2
should be at
least 0.1 Torr, e.g., 0.1 to 20 Torr, preferably from 0.2 to 10 Torr. In
contrast to prior art
processes where the rate of etching (Si etching) decreases as the temperature
is
increased, here the rate of etching increases with an increase in temperature.
It is
believed the increase in temperature increases the rate of TiN etching because
TiF4 is
volatile under these conditions and is easily removed from the Si02 and SiN
surface.
Lower temperatures leave TiF4 species near the Si02 and SiN surfaces blocking
the
attack of XeF2.
[0052] In the in situ process of forming XeF2 cleaning or etching is done in
the
presence of a remote plasma. Temperatures when a remote plasma is present may
range from ambient to 500 C, preferably from ambient to 300 C.
[0053] The disclosed processes of forming XeF2 provide significant advance for
the in
situ cleaning process. In that, they not only provide a process to produce
XeF2 at a
lower cost, they also provides an effective, selective removal of unwanted
residues
without requiring a major shut down, further lower the cost of maintenance. In
addition,
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the disclosed processes use high vapor pressure gases rather than a low vapor
pressure solid. This improves productivity since higher gas flows, and hence
higher
etch rates, are possible.
[0054] A further benefit from using the disclosed processes of forming XeF2,
is that in
addition to XeF2 some free fluorine radicals will also be provided which might
help
facilitate the removal of residues that may not be reactive when in contact
with just
XeF2. This is beneficial to all selective cleaning/etching applications, such
as cleaning
parts and semiconductor tools coated with Si02 having some residues deposited
unwanted thereon; the etching of sacrificial layers in a MEMS, and the residue
cleaning
in the ion source region of an ion implantation system used in the fabrication
of a
microelectronic device.
[0055] The following examples are provided to illustrate various embodiments
of the
invention and are not intended to restrict the scope thereof.
Example 1
Effectiveness of XeF2 In Etching Of Deposition Materials
At Various Temperatures and Pressures
[0056] In this example, the etch rates for TiN, Si02, and SiN were determined
using
XeF2 as the etchant at various temperatures and pressures. Experimental
samples
were prepared from Si wafers coated with thin films of TiN, Si02, and SiN.
Etch rates
were calculated by the thin film thickness change between the initial film
thickness and
that thickness after a timed exposure to the etching or processing conditions.
[0057] To effect etching bulk XeF2 gas was introduced from a cylinder into the
reactor
chamber through an unused remote plasma generator. The XeF2 gas pressure in
the
reactor chamber was held constant by turning off the flow from the cylinder
once the
desired pressure was reached.
[0058] The test coupons were placed on the surface of a pedestal heater which
was
used to maintain different substrate temperatures. The results are shown in
Table I
below.
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Table I
ETCH RATES FOR VARIOUS MATERIALS USING XeF2
Material Temperature Pressure Etch
( C) (Torr) Rate
nm/min
Ti N 25 1 0
TiN 100 1 0
TiN 150 1 8
TiN 200 1 13
TiN 300 0.5 20
Si02 300 0.5 0
SiN 100 1 0
SiN 150 1 0
SiN 300 1 0
[0059] The above results show that at a pressure of 0.5 to 1 Torr, XeF2 was
effective
in etching TiN films at elevated temperatures of from 150 to 300 C and not
effective at
25 C room temperature. Surprisingly XeF2 did not etch an SiO2 or an SiN
surface at
any of the temperatures and pressures employed but did etch TiN films at such
temperatures. Because of the inability of XeF2 to etch Si02 and SiN surfaces
at these
elevated temperatures, but did etch TiN films, it was concluded that XeF2
could be used
as a selective etching agent for TiN films and particles from Si02 and SiN
surfaces.
Example 2
Selective etching of Si to SiO2
[0060] In this example, an MKS Astron remote plasma generator was mounted on
top
of a reactor chamber. The distance between the exit of the Astron generator
and the
sample coupon was about six inches. The remote plasma generator was turned on
but
the pedestal heater in the reactor chamber was turned off. The chamber was
kept at
room temperature. The etch rate of both Si and Si02 substrates using remote
plasma
was measured.
[0061] The process gas to the remote plasma was NF3 and it was mixed with a
second gas stream in various amounts. The second gas stream was comprised of
either Xe, argon (Ar), or a combination thereof. The total gas flowrate to the
reactor
chamber was fixed at 400 sccm and the NF3 flowrate was fixed at 80 sccm. While
keeping the total flowrate of the second gas stream at 320 sccm, the ratio of
the
flowrate of Xe to the total flowrate of the second gas stream (Xe/(Ar+Xe)) was
varied
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between 0 (only Ar as the additional process gas) and 1 (only Xe as the
additional
process gas). The results of Si substrate etching are shown in Figure 1 and
the results
of Si02 substrate etching are shown in Figure 2.
[0062] As Figure 1 shows, addition of Xe to the process gas, NF3, enhanced the
Si
etch rate. What was unexpected is that the addition of Xe to a remote plasma
generator along with NF3 would generate a plasma that enhanced Si etching.
[0063] Figure 2 shows that the addition of Xe to an NF3/argon plasma inhibited
the
Si02 substrate etch rate and this was unexpected. F atoms present in a remote
plasma
usually attack Si02 based substrates.
[0064] Along with the analysis of Figure 1, it was surmised that the addition
of Xe to
the plasma resulted in enhancing Si substrate etching, but reducing or
inhibiting Si02
substrate etching as noted in Example 1.
[0065] Fig. 3 is provided to compare the effect of the addition of Xe to the
NF3
process gas on the etch selectivity for Si vis-a-vis Si02. As can be seen by
comparing
the results in Figures 1 and 2, Fig. 3 shows that the etch selectivity for Si
relative to
Si02 increased as the amount of Xe in the process gas was increased.
Specifically, the
selectivity increased from 30 to 250 (> 8 times) as the percentage of Xe in
the gas
stream was increased from 0% to 100%.
[0066] Typical sacrificial materials in MEMS are: silicon, molybdenum,
tungsten,
titanium, zirconium, hafnium, vanadium, tantalum, niobium. Typical protective
materials
are nickel, aluminum, photoresist, silicon oxide, silicon nitride.
Example 3
Selective etching of molybdenum(Mo) to Si02
[0067] A large 2.5 m length 25 cm diameter cylindrical SS etch chamber was
used to
determine etch rate of another common sacrificial material in MEMS
applications:
molybdenum (Mo). The remote plasma was generated using a water-cooled MKS
Astron
AX7670 6 slpm unit. The plasma source was connected to the chamber by a 10 cm
long
transport tube with a 4 cm inner diameter. The sample was placed 2 feet from
the
loading/unloading end of the tube.
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[0068] At 2.75 torr, 275 sccm NF3 flow and 600 sccm Xe or Ar flow, etch rate
of Mo =
1.1 microns/min. Etch rate of Si02was 82 nm/min for NF3/Ar gas mixture and was
26
nm/min for NF3/Xe mixture. Thus, selectivity of Xe/NF3 mixture was at least 3
times that
of Ar/NF3 mixture. Note that Mo etch rate is limited by the surface oxide.
With a surface
preparation treatment to break its native oxide, the etch rate of Mo can be
enhanced to >
2.7 micron/min.
Example 4
In Situ Formation of XeF2 Via Reaction Of Xe and NF3
[0069] In this example the procedure of Example 2 was followed. An Applied
Materials
P5000 DxZ2 PECVD chamber fitted with a 6slpm MKS Astron eX remote plasma
generator was used for Fourier Transform Infrared spectrum (FTIR) studies. The
FTIR
measurements were made downstream of the chamber pump at ambient pressure. A
5.6
m path length cell at 150 C was used. Instrument resolution was 2 cm-'.
[0070] Figure 4 shows the FTIR spectra collected under same conditions as in
Example 2: pressure of 4 torr, total gas flow of 400 sccm, NF3 flow of 80
sccm, total flow
of Xe and Ar of 320 sccm. Clear dominant peaks were seen in the 500-600 cm-'
range in
the Xe/NF3 spectra whereas Ar/NF3 spectra showed no peaks in that region. The
two
major peaks at 551.5 cm-' and 570.3 cm-' were identified as XeF2 peaks.
Reference
spectra from XeF3 manufacturers showed peak positions at 550.8 and 566.4 cm-'.
[0071] Figure 5 shows that where Xe and NF3 were present, 3 dominant peaks
were
observed at 551, 570 and 590 cm-1. XeF2 was identified by peaks at 551, 567 cm-
'
whereas XeF4 is detected at 580, 590 cm"'. The peak at 567 cm-' was therefore
a
combination of the 567 and 580 cm-' peaks. Thus, both XeF2 and XeF4 were
formed in
the Xe/NF3 mixture. No evidence of XeF6 or XeOF4 formation was found from FTIR
spectra.
[0072] Table II shows several conditions where pressure was varied from 0.5 to
5 torr,
Xe flow rate was varied from 200-1000 sccm, and NF3 flow rate was varied from
50 to
500 sccm. In all cases, XeF2 peaks were detected. The peak values were
recorded here.
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Table II
Pressure (torr) 0.5 4 5 2.75 5 2
NF3 (sccm) 50 80 200 275 50 500
Xe (sccm) 200 320 500 600 1000 1000
Peak Value (530.1 cm-1) 0.06 0.07 0.16 0.22 0.09 0.33
Peak value (570.3 cm-1) 1.01 1.18 1.35 1.35 1.36 1.42
Peak value (590 cm-1) 0.43 0.43 1.63 1.42 0.18 1.58
Peak Value (603.1 cm-1) 0.07 0.07 0.44 0.27 0.04 0.31
[0073] The peaks tended to saturate under some conditions, hence the leading
edge of
the XeF2 peak at 520.1 cm-' and the trailing edge of the XeF4 peak at 603.1 cm-
' were
analyzed in addition. The XeF2 /XeF4 ratio was defined as the ratio of peak
height values
at 530 cm-' and 603 cm-'.
[0074] The experimental results using response surface regression were
summarized
below in Table Ill.
Table Ill
?eak HeigF,t 530.1 551.5 592 603.1 Ratio 603.1/530.1
Sigr,3ficance Leading XeF2 signal XeF2 niax XeF4max Trai4ing XeF4 signal
XeF2/XeF4
Xe Weak Moderate Weak Weak Strong up
NF3 Strong Strong Strong Strong Strong down
p Weak Moderate Strong Strong Strong do+vn
[0075] Note that the flow of Xe is > the flow of NF3 under all conditions
here, so NF3
was a stronger factor. Higher NF3 flow increased both XeF2 and XeF4 peaks, and
Xe had
a weak influence on the peaks (due to excess Xe being present). Pressure had a
weak
effect on XeF2 peaks but strong on XeF4 peaks. Astron operating pressure is
typically 1-
10 torr.
[0076] Therefore, pressure is a key parameter to control the formation of
XeF4. XeF4
can hydrolyze to produce Xe03 - which is an explosive and shock sensitive
compound.
Under the current experimental conditions, the XeF2/XeF4 ratio can be
maximized under
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CA 02690697 2010-01-21
high Xe, low NF3 and low pressure conditions. For example, the flow rate of Xe
was 1000
sccm, the flow rate of NF3 was 50 sccm, the pressure was at 0.5 torr.
[0077] Figure 6 shows the XeF2 FTIR peak height and XeF4 FTIR peak height as a
function of Xe/(Xe+Ar). The unit for the peak height was arbitrary. As Xe flow
fraction
increased, XeF2 produced increased and XeF4 fraction decreased. High Xe flow
desired
to maximize the formation of XeF2 relative to XeF4. Figure 7 shows the ratio
of XeF2 / XeF4
FTIR peak heights as a function of the ratio of Xe/NF3 flow rates. Clearly,
high ratios of
Xe/NF3 were desired to maximize the formation of XeF2 relative to XeF4
[0078] Figure 8 shows the XeF2 FTIR peak height (right Y-axis) and the etch
selectivity
of Si/SiOZ(left Y-axis) as a function of Xe/(Xe+Ar). The etch selectivity of
Si/Si02 was
clearly correlated with the in situ formation of XeF2.
[0079] The use of plasma excitation to produce XeF2 can also be used to
manufacture
XeF2 for use as an etchant in processes not directly associated with where it
was made.
The data shows that there are conditions that clearly benefit the production
of XeF2 and
minimize the production of XeF4. It is very desirable to minimize the
production of XeF4
because of its explosivity if it subsequently reacts to form Xe03. As XeF2 is
formed in the
reaction zone after the plasma generator it can be removed from the zone by
the use of
cryogenic trapping to condense out the material onto a cold surface. The solid
XeF2 can
then be extracted from the process chamber and reloaded into delivery
cylinders for use
in etching processes. Because of the excess xenon is introduced to reduce XeF4
formation, it is very beneficial to utilize xenon recovery or recycling of the
xenon back
into the process to assure productive use of all the xenon required for the
manufacture of
XeF2.
Example 5
Effect of Remote Plasma and Temperature
On Etch Rate Of TiN and Si02
[0080] In this example the procedure of Example 2 was followed except both the
remote plasma generator and the pedestal heater were turned on to allow for
determination of the etch rate of both TiN and Si02 using remote plasma at
various
substrate temperatures.
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[0081] In a first set of experiments the etch rate of TiN and Si02 was
measured using
a mixture of NF3 and Xe as the process gas. The flowrate of Xe was fixed at
320 sccm.
The temperature was varied from 100 C to 150 C. The results of these
experiments
are shown as the square points in Figures 9 and 10 for TiN and Si02,
respectively.
[0082] In a second set of experiments the etch rate of TiN and Si02 was
measured
using a mixture of NF3 and argon (Ar) as the process gas. The flowrate of Ar
was fixed
at 320 sccm. The temperature was varied from 100 C to 150 C. The results of
these
experiments are shown as the diamond points in Figures 4 and 5 for TiN and
Si02,
respectively.
[0083] As Figure 9 shows, the addition of Xe to the process gas enhanced the
TiN
etch rate at temperatures generally above 130 C. Figure 10 shows that the
addition of
Xe to NF3 inhibited the Si02 etch rate for all temperatures studied vis-a-vis
the addition
of Ar to NF3. The effect of the addition of Xe to the process gas on the etch
selectivity
can be seen by comparing the results in Figures 9 and 10.
Figure 11 shows, the etch selectivity for TiN relative to Si02 and the graph
shows that
the TiN selectivity begins to increase at temperatures above about 110 C, and
rapidly
above 120 C, with the addition of Xe to the NF3 process gas relative to Ar.
[0084] Summarizing, Example 1 shows that XeF2 is a selective etchant for TiN
films in
relation to silicon dioxide and silicon nitride substrates when such etching
is performed at
elevated temperatures.
[0085] Examples 2 and 3 shows that the addition of Xe to an NF3 process gas in
a
remote plasma generator (or a reactor or a chamber) can increase the etch
selectivity of
Si or Mo relative to Si02 as compared to the etch selectivity when only NF3 is
used as
the process gas.
[0086] Example 4 shows that when Xenon and a fluorine containing gas such as
NF3
were introduced to a plasma generator(or a reactor or a chamber), in situ
formation of
XeF2 was observed. There is an economic advantage (i.e., lower cost of
ownership) of
combining xenon with a fluorine containing gas such as NF3 rather than
directly
employing XeF2 for a cleaning process. A further benefit from using the
disclosed
processes of forming XeF2 is that they provide in addition to XeF2 some free
fluorine
radicals which help facilitate the removal of residues that may not be
reactive when in
contact with just XeF2.
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CA 02690697 2010-01-21
[0087] Example 5 shows that the addition of Xe to an NF3 process gas in a
remote
plasma can increase the etch selectivity of TiN relative to Si02 at high
(elevated)
temperatures as compared to the etch selectivity when only NF3 is used as the
process
gas. The increased selectivity of TiN relative to Si02 is important in quartz
tube furnace
applications and to parts and semiconductor tools coated with Si02 having TiN
deposits
thereon. This methodology may facilitate the cleaning of deposition reactors
in between
deposition cycles by interfacing a remote downstream plasma unit onto the
process
reactor and admitting the process gases. There may be economic advantages
(i.e.,
lower cost of ownership) of combining xenon with a fluorine containing gas
such as NF3
rather than employing XeF2 for such a cleaning process.
[0088] The cleaning processes described in the examples could also be employed
in
an off-line process reactor whose sole purpose is to clean process reactor
parts prior to
their re-use. Here, a remote downstream plasma reactor would be interfaced
onto an
off-line process reactor into which parts (components from the deposition
reactor) are
placed. Xenon and a fluorine containing gas such as NF3 would then be
introduced to
the remote downstream unit prior to the admission of the process gases into
the
chamber containing the parts to be cleaned.
[0089] The increased selectivity of Si, Mo, or TiN relative to Si02 , and the
disclosed
processes of forming XeF2 are important in many applications: such as,
cleaning parts
and semiconductor tools coated with Si02 having Si, Mo, or TiN deposits
unwanted
thereon; the etching of sacrificial layers in a MEMS; and the residue cleaning
in the ion
source region of an ion implantation system used in the fabrication of a
microelectronic
device.
[0090] The applications can be extended to clean other unwanted materials such
as:
tungsten, titanium, zirconium, hafnium, vanadium, tantalum, niobium, boron,
phosphorus,
germanium, arsenic, and mixtures; from Si3N4, Al, AI203, Au, Ga, Ni, Pt, Cu,
Cr, TiNi
alloy, SiC, photoresist, phosphosilicate glass, boron phosphosilicate glass,
polyimides,
gold, copper, platinum, chromium, aluminum oxide, silicon carbide and the
combinations
thereof.
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