U.S. patent application number 10/531245 was filed with the patent office on 2006-05-04 for atomic layer deposition of noble metals.
Invention is credited to Toh-Ming Lu, John Joseph Senkevich.
Application Number | 20060093848 10/531245 |
Document ID | / |
Family ID | 32107939 |
Filed Date | 2006-05-04 |
United States Patent
Application |
20060093848 |
Kind Code |
A1 |
Senkevich; John Joseph ; et
al. |
May 4, 2006 |
Atomic layer deposition of noble metals
Abstract
The present invention relates to ALD processes for deposition of
a metal selected from Pd, Rh, Ru, Pt and Ir wherein a layer
including the metal is formed on a surface composed of a material
selected from W, Ta, Cu, Ni, Co, Fe, Mn, Cr, V Nb, tungsten
nitride, tantalum nitride, titanium nitride, dielectrics and
activated dielectrics at a temperature ranging from >60.degree.
C. to <260.degree. C. The layer is formed by sequentially
pulsing into a chamber containing the surface a precursor for the
metal and a reducing gas selected from hydrogen, glyoxylic acid,
oxalic acid, formaldehyde, 2-propanol, imidazole and
plasma-activated hydrogen.
Inventors: |
Senkevich; John Joseph;
(Rolla, MO) ; Lu; Toh-Ming; (Loudonville,
NY) |
Correspondence
Address: |
HESLIN ROTHENBERG FARLEY & MESITI PC
5 COLUMBIA CIRCLE
ALBANY
NY
12203
US
|
Family ID: |
32107939 |
Appl. No.: |
10/531245 |
Filed: |
October 15, 2003 |
PCT Filed: |
October 15, 2003 |
PCT NO: |
PCT/US03/32644 |
371 Date: |
April 14, 2005 |
Current U.S.
Class: |
428/585 |
Current CPC
Class: |
Y10T 428/12285 20150115;
B82Y 30/00 20130101; C23C 16/18 20130101; C23C 16/45553 20130101;
C23C 16/0272 20130101 |
Class at
Publication: |
428/585 |
International
Class: |
B22D 7/06 20060101
B22D007/06 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 15, 2002 |
US |
60418519 |
Claims
1. An ALD process for deposition of a metal selected from Pd, Rh,
Ru, Pt and Ir comprising forming a layer comprising the metal on a
surface comprising a material selected from W, Ta, Cu, Ni, Co, Fe,
Mn, Cr, V Nb, tungsten nitride, tantalum nitride, titanium nitride,
dielectrics and activated dielectrics at a temperature ranging from
>60.degree. C. to <260.degree. C.
2. An ALD process according to claim 1, wherein forming a layer
comprises sequentially pulsing into a chamber containing the
surface a precursor for the metal and a reducing gas selected from
hydrogen, glyoxylic acid, oxalic acid, formaldehyde, 2-propanol,
imidazole and plasma-activated hydrogen.
3. An ALD process for deposition of a metal selected from Pd, Rh,
Ru, Pt and Ir comprising providing a surface comprising a material
selected from noble metals, W, Ta, TaN, tungsten nitride, tantalum
nitride, titanium nitride, Cu, Ni, Co, Fe, Mn, Cr, V and Nb in a
reaction chamber; pulsing a precursor for the metal into the
chamber at a temperature ranging from >60.degree. C. to
<260.degree. C.; and pulsing hydrogen gas into the chamber.
4. An ALD process according to claim 3 wherein the surface is a
noble metal.
5. An ALD process according to claim 3 wherein the surface is a
pretreated metallic surface selected from W, Ta, tungsten nitride,
tantalum nitride, and titanium nitride.
6. An ALD process according to claim 3 wherein the surface is a
metal selected from Cu, Ni, Co, Fe, Mn, Cr, V and Nb.
7. An ALD process for deposition of a metal selected from Pd, Rh,
Ru, Pt and Ir comprising providing a surface comprising a material
selected from noble metals, W, Ta, Cu, Ni, Co, Fe, Mn, Cr, V Nb,
tungsten nitride, tantalum nitride, titanium nitride, dielectrics
and activated dielectrics in a reaction chamber at a temperature
ranging from >60.degree. C. to <260.degree. C.; pulsing a
precursor for the metal into the chamber; and pulsing into the
chamber a reducing gas selected from glyoxylic acid, oxalic acid,
formaldehyde, 2-propanol, and imidazole.
8. An ALD process according to claim 7 wherein the reducing gas is
glyoxylic acid.
9. An ALD process according to claim 7 wherein the activated
dielectric surface comprises at least one of thiol, sulfide,
tetrasulfide, phosphine, phosphide or amine groups.
10. An ALD process according to claim 7 wherein the activated
dielectric surface comprises thiol groups.
11. An ALD process according to claim 7 wherein the dielectric is
selected from CVD polymers, organic-inorganic hybrids, and silicon
or metals having an oxide-terminated surface.
12. An ALD process for deposition of a metal selected from Pd, Rh,
Ru, Pt and Ir comprising providing a substrate in a reaction
chamber; pulsing a precursor for the metal into the chamber at a
temperature ranging from >60.degree. C. to <260.degree. C.;
and pulsing plasma-activated hydrogen gas into the chamber.
13. An ALD process according to claim 1, wherein the precursor is a
metal .beta.-diketonate compound.
14. An ALD process according to claim 1, wherein the precursor is a
metal-hfac compound.
15. An ALD process according to claim 1, wherein the precursor is
selected from Pd(hfac).sub.2, Ru(hfac).sub.2, Rh(hfac).sub.2,
Pt(hfac).sub.2, Ir(hfac).sub.2, Ir(acac).sub.2, Pd(tmhd).sub.2,
Ru(tmhd).sub.2, Rh(tmhd).sub.2, Pt(tmhd).sub.2, and
Ir(tmhd).sub.2.
16. An ALD process according to claim 1, wherein the metal is
Pd.
17. An ALD process according to claim 1, wherein the precursor is
Pd(hfac).sub.2.
Description
RELATED APPLICATION
[0001] This application claims the benefit of U.S. Patent
Application Ser. No. 60/418,519, filed Oct. 15, 2002.
BACKGROUND OF THE INVENTION
[0002] The use of self-limiting chemical reactions to sequentially
grow monolayers of transition metals will positively impact many
and diverse applications. For example, among the applications for
metal atomic layer deposition (ALD) are noble metal catalysts on
rough electrode and mesoporous bulk materials, Cu seed layers for
the electrochemical deposition and chemical vapor deposition of Cu,
conformal adhesion layers to Cu metallic overlayers and
alkylthiolate self-assembled monolayers. Palladium is useful as a
catalyst in fuel cells and for hydrogenation reactions, gas
sensors, and hydrogen permselective membranes. Palladium, like the
other noble metals, is rather costly; therefore, a driving force
exists to reduce the quantity used as a function of its activity.
Atomic layer deposition is an ideal method for extracting the
highest performance out of noble metal catalysts with the least
cost, since its growth is highly conformal and well controlled. As
a result, monolayer-by-monolayer control of growth can take place
over complicated geometries and bulk mesoporous supports.
[0003] In an ALD process, a substrate is exposed sequentially and
alternately to at least two mutually reactive reactants. The
substrate is heated to a temperature that is high enough to prevent
condensation of the reactants but low enough to prevent thermal
decomposition of each of the reactants. The substrate is exposed to
the first reactant, which is a coordination compound and is
chemisorbed onto the surface of the substrate. Ideally this is one
monolayer but often less due to precursor desorption (after the
pulse is stopped) and steric effects due to bulky ligands as part
of the coordination compound used as an ALD precursor. Excess
reactant is then evacuated from the deposition chamber so parasitic
CVD does not result. Then the substrate is exposed to the second
reactant which reacts with the chemisorbed first reactant. The
purpose of this pulse is to remove the organic ligand as part of
the coordination compound (reactant 1) and reduce the metal ion to
its elemental state. When this reaction is complete a submonolayer
of the metal results at the substrate surface. This cycle is
repeated until a sufficiently thick film has been deposited onto
the substrate. More than two reactants can be used, in particular
for the deposition of ternary or more complicated compounds or
multilayers. The general technique has been known since 1980, see
the review article of Suntola, "Atomic Layer Epitaxy" in: "Handbook
of Christal Growth 3, Thin Films and Epitaxy, part B. Growth
Mechanisms and Dynamics", by D. T. J. Hurle, Ed. Elsevier, 1994,
Chapter 14, p 601-663. Because only a monolayer of the material is
deposited per cycle, a sufficient number of cycles need to be
executed to achieve the required film thickness. In order to
minimize the cycle time without compromising the effectiveness of
the purging of the reactor, the volume of the reactor should be
minimized and the flow dynamics of the reactor should be
optimized.
[0004] It should be noted that ALD differs from CVD and therefore
has different precursor requirements than CVD. ALD is performed in
a cyclic fashion with sequential alternating pulses of precursor,
reactant and purge gas. The ALD precursor must exhibit
self-limiting behavior, such that the precursor is adsorbed on the
substrate only up to a monolayer. Because of this self-limiting
requirement, only one monolayer or sub-monolayer of precursor is
deposited per operation cycle, and additional precursor will not be
deposited on the grown layer even when excess precursor is
supplied. In CVD, precursor and reactants arrive at the substrate
simultaneously with film growth resulting from continuous chemical
reactions of precursors on the substrate surface. Uniform and
reproducible growth of the film is dependent on maintenance of the
correct precursor and reactant flux at the substrate. The growth
rate is proportional to the precursor flux at the substrate and to
the substrate temperature. Because of the different growth
mechanisms, the precursor requirements differ for ALD and CVD. In
ALD, the precursor must readily adsorb at bonding sites on the
growth surface in a self-limiting mode, and once adsorbed must
readily react with co-reactant to form the desired film. In CVD,
the precursor and the co-reactants must react appropriately at the
substrate surface simultaneously to form the desired film. Thus,
many useful CVD precursors are not viable as ALD precursors, and it
is not trivial or obvious to select a precursor for the ALD
method.
[0005] Metal ALD to date has had limited success. This is primarily
due to the lack of precursor chemisorption on oxide-terminated
surfaces and the lack of appropriate reducing agents. The lack of
chemisorption with palladium (II) hexafluoroacetylacetonate
[PdII(hfac).sub.2] on oxidized Ta, Cu, and P surfaces has been
reported by the inventors (Senkevich, et al., Chem. Vapor Dep.
2002, 8, 189). The inefficacy of hydrogen as a reducing agent on
non-metallic surfaces is well established and is attributed to
molecular hydrogens lack of polarizability and high bond strength
of 435.99 kJ/mol. Alcohols and aldehydes were unsuccessfully used
as reducing agents for Cu ALD with CuII(hfac).sub.2 as a precursor
(Solanki et al, Electrochem. Solid-State Lett. 2000, 3, 479).
Blackburn et al. (Chem. Mater. 2000, 12, 2625) also found no growth
of Pd via chemical fluid deposition on oxide-terminated surfaces
below 230.degree. C. As a result, there is still great interest in
developing an ALD technique where metallorganics can be used and
deposited on non-noble metal surfaces, for example, polymers,
oxidized metals and oxidized barrier layers.
SUMMARY OF THE INVENTION
[0006] It has been unexpectedly discovered that noble metals
including Pd, Pt, Rh, Ru and Ir may be deposited by an ALD process
on a variety of substrates. Even more surprisingly, the technique
is substrate independent since any surface can be sulfide or thiol
terminated via self-assembled chemistry, or plasma-enhanced surface
modification.
[0007] Accordingly, in one aspect, the present invention relates to
an ALD process for deposition of a metal selected from Pd, Rh, Ru,
Pt and Ir comprising forming a layer comprising the metal on a
surface comprising a material selected from W, Ta, Cu, Ni, Co, Fe,
Mn, Cr, V Nb, tungsten nitride, tantalum nitride, titanium nitride,
dielectrics and activated dielectrics at a temperature ranging from
>60.degree. C. to <260.degree. C. Forming a layer may include
sequentially pulsing into a chamber containing the surface a
precursor for the metal and a reducing gas selected from hydrogen,
glyoxylic acid, oxalic acid, formaldehyde, 2-propanol, imidazole
and plasma-activated hydrogen.
[0008] In another aspect, the present invention relates to an ALD
process for deposition of a metal selected from Pd, Rh, Rut Pt and
Ir including providing a surface comprising a material selected
from noble metals, W, Ta, TaN, tungsten nitride, tantalum nitride,
titanium nitride, Cu, Ni, Co, Fe, Mn, Cr, V and Nb in a reaction
chamber; pulsing a precursor for the metal into the chamber at a
temperature ranging from >60.degree. C. to <960.degree. C.;
and pulsing hydrogen gas into the chamber.
[0009] In yet another aspect, the present invention relates to an
ALD process for deposition of a metal selected from Pd, Rh, Ru, Pt
and Ir including providing a surface comprising a material selected
from noble metals, W, Ta, Cu, Ni, Co, Fe, Mn, Cr, V Nb, tungsten
nitride, tantalum nitride, titanium nitride, dielectrics and
activated dielectrics in a reaction chamber at a temperature
ranging from >60.degree. C. to <260.degree. C.; pulsing a
precursor for the metal into the chamber; and pulsing into the
chamber a reducing gas selected from glyoxylic acid, oxalic acid,
formaldehyde, 2-propanol, and imidazole. The activated dielectric
surface may include thiol, sulfide, tetrasulfide, phosphine,
phosphide or amine groups.
[0010] In yet another aspect, the present invention relates to an
ALD process for deposition of a metal selected from Pd, Rh, Ru, Pt
and Ir including providing a substrate in a reaction chamber;
pulsing a precursor for the metal into the chamber at a temperature
ranging from >60.degree. C. to <260.degree. C.; and pulsing
plasma-activated hydrogen gas into the chamber.
[0011] For any of these processes, the precursor is preferably a
metal .beta.-diketonate compound, more preferably, a metal-hfac
compound. Specific examples of preferred precursors for all of the
processes are Pd(hfac).sub.2, Ru(hfac).sub.2, Rh(hfac).sub.2,
Pt(hfac).sub.2, Ir(hfac).sub.2, Ir(acac).sub.2, Pd(tmhd).sub.2,
Ru(tmhd).sub.2, Rh(tmhd).sub.2, Pt(tmhd).sub.2, and
Ir(tmhd).sub.2.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1--The chemical structures of the Pd ALD precursor and
novel reducing agent used in this study.
[0013] FIG. 2--The structure of the tetrasulfide SAM and Ir
substrates used in this study.
[0014] FIG. 3--The sequential growth of Pd ALD films has a function
of the number of cycles for Pd.sup.II(hfac).sub.2 pulsed separately
with H.sub.2. The films at 130.degree. C. show less growth with
more cycles due to the desorption of Pd.sup.II(hfac).sub.2 on Pd
versus Ir. The ALD process. Pd is not deposited monolayer by
monolayer due to the presence of bulky organic ligands.
[0015] FIG. 4--The Pd 3d XPS spectra for 60 .ANG. of Pd deposited
on the tetrasulfide SAM and for 55 .ANG. of Pd deposited on Ir. The
peak position of the Pd 3d.sub.5/2 peak is slightly shifted from
elemental Pd, which might be attributed to charge referencing or
slight chemical contamination from the dissociation of the hfac
ligand.
[0016] FIG. 5--The C 1s XPS spectra for the Pd ALD films deposited
on the tetrasulfide SAM and Ir surfaces. The high binding energy
shoulder on the peak associated with the C--C or adventitious
bonding is associated with --CF.sub.X, C.dbd.O, or C--O bonding due
to the dissociation of the hfac ligand.
[0017] FIG. 6--The F 1s XPS spectra for the Pd ALD films deposited
on the tetrasulfide SAM and Ir surfaces. The peak position of the
spectra is correlated with the fluorine being associated with
carbon.
[0018] FIG. 7--S 2p XPS spectrum showing the Pd--S interfacial
reaction between Pd.sup.II(hfac).sub.2 and the tetrasulfide
SAM.
[0019] FIG. 8--The Ir 4f XPS spectra showing the Pd--Ir interfacial
reaction between Pd.sup.II(hfac).sub.2 and the Ir surface. The
binding energy shift is only obvious for the 240 cycle Pd film
because only the interfacial chemistry contributes to the Ir
signal.
[0020] FIG. 9--Reflection high energy electron diffraction (RHEED)
images and spectra for 55 .ANG. of Pd on Ir (top), 60 .ANG. of Pd
on tetrasulfide SAM (middle), and the 150 .ANG. Ir substrate
(bottom). The Pd on Ir exhibits a higher quality texture than the
Ir substrate itself However, the Pd on the tetrasulfide SAM is
nearly random, which is attributed to the higher temperatures
needed to achieve the Pd seed layer.
[0021] FIG. 10--Proton Nuclear Magnetic Resonance Spectroscopy of
the glyoxylic acid reducing agent before (bottom spectrum) and
after (top spectrum) dehydration in vacuum (10 sccm Ar carrier and
10.sup.-4 Torr base pressure) at 60.degree. C. for 60 min. The
broad peaks at 7.2 and 8.5 ppm are attributed to a proton exchange
mechanism between glyoxylic acid and water. When the peak is at
higher chemical shifts it is more associated with the acid than
free water.
DETAILED DESCRIPTION OF THE INVENTION
[0022] Metals that may be deposited by the ALD processes of the
present invention include Pd, Pt, Ru, Rh and Ir, and particularly
Pd. Suitable precursors for the metals sublime without
decomposition, exhibit self-limiting chemistry and are stable at
deposition temperatures. Further, they should be susceptible to
decomposition on the substrate and fragments of the ligand(s)
should be removable there from. Precursors are typically
coordination compounds based on bidentate ligands such as
.beta.-diketonates or carboxylates. The .beta.-diketonates are
particularly useful. Examples of .beta.-diketonate ligands include
hfac, acetylacetonate (acac), tfac, fod, and
tetramethylheptanedionate (tmhd). Adducts of the coordination
compounds with Lewis bases may be also be utilized. Examples of
Lewis bases that may be suitable include pyridine, 4,4' bipyridyl,
imidazole, ammonia, n-propanol, n-butanol, and methanol. Examples
of coordination compounds that are typically suitable as precursors
include Pd(hfac).sub.2, Pt(hfac).sub.2, Ru(hfac).sub.2,
Rh(hfac).sub.2, Ir(hfac).sub.2, and Ir(acac).sub.2. Precursors for
palladium include palladium (II) hexafluoroacetylacetonate,
palladium (II) tetramethylheptanedionate, palladium (I)
hexafluoroacetylacetonate with 2-methylallyl ligand, palladium (I)
acetylacetonate with 2-methylallyl ligand and palladium (II)
hexafluoroacetylacetonate with Lewis base adducts and palladium
(II) carboxylate complexes
[0023] Deposition temperatures range from >60.degree. C. to
<260.degree. C., and preferably, from about 80.degree. C. to
about 200.degree. C. The temperature is typically chosen with a
particular precursor in mind, especially since, as noted above, the
precursor must be stable at the deposition temperature. The
reducing agent used also determines the deposition temperature. For
catalytic processes using hydrogen or activated hydrogen as the
reducing agent, temperatures typically range from about 60.degree.
C. to about 130.degree. C., preferably from about 80.degree. C. to
about 100.degree. C. For organic reducing agents, deposition
temperatures are much higher, typically >150.degree. C., and in
some cases, >200.degree. C.
[0024] Suitable reducing agent in gas form, or reducing gases, for
use in the processes of the present invention are able to remove
the organic ligand that is a part of the coordination compound and
reduce the metal ion to its elemental state. These include, for
appropriate substrates or surfaces, hydrogen, organic reducing
agents and plasma-activated hydrogen.
[0025] Hydrogen is capable of reducing the metal ions on surfaces
including noble metals (Ru, Rh, Pd, Os, Pt), oxide-terminated
metallic surfaces where the oxide can be removed with a
pretreatment (W, Ta, tungsten nitride, tantalum nitride, titanium
nitride) and metallic surfaces where a transmetallation reaction
can occur (Cu, Ni, Co, Fe, Nm, Cr, V, Nb). For such a reaction to
occur, the metal being deposited must be more noble than the
substrate, the substrate can not be terminated with an oxide, and
the coordination compound that is a product of the reaction (e.g.,
Cu(hfac).sub.2 from Pd(hfac).sub.2 on Cu), must be stable.
[0026] Where it is desired to deposit a metal layer on a surface or
substrate other than the above, organic reducing agents or gases or
plasma-activated hydrogen may be used. Activated hydrogen is
substrate-independent because the plasma creates atomic hydrogen
and hydrogen ions that are energetic and can reduce metal ions on
essentially any surface. In addition, the metal layer produced is
of high chemical quality. Examples of means for activating hydrogen
include radiofrequency (RF) or microwave plasmas, either
near-surface or remote, such as capacitively coupled plasmas,
inductively coupled plasmas, microwave cavity plasmas, and electron
cyclotron resonance plasmas.
[0027] Organic reducing agent such as glyoxylic acid, oxalic acid,
formaldehyde, 2-propanol and imidazole may be used for noble
metals, W, Ta, tungsten nitride, tantalum nitride, titanium nitride
Cu, Ni, Co, Fe, Nm, Cr, V, and Nb surfaces, and also for activated
dielectric surfaces. Dielectric substrates which can be activated
toward ALD at the surface include metal and silicon oxides, CVD
polymers such as the parylenes, hybrid organic-inorganic composites
and oxide-terminated metals, although almost any surface may be
activated using an appropriate technique. Such materials may be low
.kappa. dielectrics, high .kappa. dielectrics, metal oxides,
silicon oxides, hybrid materials composed of organic and inorganic
constituents, polymeric materials, and diffusion barrier materials.
The dielectric surface may be activated by attaching functional
groups containing sulfur, phosphorous or nitrogen using solution or
vapor deposition techniques. Essentially any form of sulfur that
may be attached to the surface of the substrate may be used, and
particularly sulfur in the form of thiol (--SH), thio ether
(--S--), sulfide (--SS--), tetrasulfide (--SSSS--) groups may be
used. Phosporous is typically in the form of phosphine or phosphide
groups. Nitrogen is typically in the form of primary, secondary,
tertiary and quaternary amine functionality. For example, a
self-assembled monolayer or multilayer (SAM) may be formed at the
surface of the substrate using multifunctional silanes. These
techniques are well known and have been described in the
literature. Self-assembled monolayer/multilayers (SAM's) grow in an
ordered structure due to the chemical anisotropy that exists within
the molecules. Three types of SAM's are common: alkyl thiolate
SAM's (on Ag, Au, and Cu) (Laibinis et al., J. Am. Chem. Soc.
113:7152-7167 (1991)), trichlorosilyl SAM's (on hydroxylated
surfaces) (Vuillaume et al., Appl. Phys. Lett. 69(11):1646-1648
(1996)), and trialkoxysilyl SAMs (on hydroxylated surfaces)
(Dressick et al., J. Electrochem. Soc. 141(1):210-220 (1994)).
Namely, the alkylthiolate, trichlorosilyl, and the trialkoxysilane
groups have significantly different reactivity than terminal groups
on the other side of the SAM molecule, e.g. pyridine
(--C.sub.5H.sub.4N), methyl (--CH.sub.3), phenyl
(--C.sub.6H.sub.5), and mercaptan (--SH), and thus the SAM molecule
will not react with itself SAMs anchored by trichlorosilyl or
trialkoxysilyl groups may be used with surfaces having surface
hydroxy groups. For example, the surface of the native oxide of
Si(100) is reactive with chloro- or alkoxy-silanes. For materials
that do not possess surface hydoxy functionality, such as barrier
layers, the surface may be hydroxylated in a separate step prior to
forming the SAM. These techniques are also described in
US-2002-0182385.
[0028] In some cases, where the deposition temperature with the
organic reducing agents is higher than desired, e.g.
.about.200.degree. C., then only a seed layer of Pd is deposited
using the organic reducing agent since the deposition rate of noble
metal may be be prohibitively slow. Once the seed layer is
deposited, the temperature may be lowered to, for example,
80.degree. C. and the ALD growth of the metal layer resumes at a
much faster rate. This rate difference is due to significant
desorption of the metallorganic that takes place at higher
temperatures due to low interaction energy between the precursor
and the freshly created metal surface.
[0029] The purge time, that is, the time between pulse A
(precursor) and pulse B (the reducing agent), should be large
enough that no parasitic CVD occurs. This typically depends on the
conductance of the vacuum system and the pumping speed of the
vacuum pump stack. A minimum of 10 s is frequently needed but this
can be reduced to .about.1 s. The reducing agent (pulse B) pulse
duration is the same as pulse A (precursor).
EXPERIMENTAL
Palladium ALD on Ir:
[0030] The use of Ir as an ALD substrate accomplishes two things.
First, it has been established that Pd.sup.II(hfac).sub.2
preferentially chemisorbs on Ir. Second, Ir is known to dissociate
molecular hydrogen, which acts as a reducing agent and removes the
hfac ligands from the Pd.sup.II(hfac).sub.2 metallorganic. Further,
it is important to establish two experimental findings to make the
ALD process viable. First, the precursor (or metallorganic) should
be able to be sublimed without decomposing. This can be undertaken
with thermogravimetric analysis. Fortunately, Pd.sup.II(hfac).sub.2
has already been shown to sublime without decomposing. Second, the
metallorganic should not decompose at the substrate's surface until
temperatures that exceed the deposition temperature of the metal.
This has been shown previously with Pd.sup.II(hfac).sub.2 to be
230.degree. C. on Ir and on a unspecified surface.
[0031] As a result of the above work the processing window for the
ALD of Pd is between 60.degree. C. and 260.degree. C. The lower
limit is due to the condensation of the precursor and the upper
limit is due to its thermal decomposition (non-self-limiting
growth). The substrate structure of Ir and the tetrasulfide SAM is
shown in FIG. 2 and explained in more detail in the experimental
section. FIG. 3 shows the thickness of the ALD Pd thin films as a
function of the number of cycles. The areal density of the Pd films
was obtained by RBS and then converted to a thickness value by: 1
.ANG.=6.80.times.10.sup.14 atoms/cm.sup.2 obtained from palladium's
density and molecular weight.
[0032] As can be seen from FIG. 3, the Pd films grown at
130.degree. C. do not exhibit a linear relationship between film
thickness and the number of cycles. This paralinear growth is
attributed to the difference in bond energies between
Ir--Pd.sup.II(hfac).sub.2 and Pd--Pd.sup.II(hfac).sub.2. The atomic
polarizabilities of Ir and Pd are 7.6.+-.1.9 .ANG..sup.3 and
4.8.+-.1.2 .ANG..sup.3 respectively. The larger polarizability
helps to stabilize the van der Waals forces between the Ir surface
and the Pd.sup.II(hfac).sub.2 metallorganic. The change between the
initial growth (Ir--Pd.sup.II(hfac).sub.2) and the subsequent
growth (Pd--Pd.sup.II(hfac).sub.2) does not occur until nearly 10
.ANG. due to the growth mechanism of ALD Pd via
Pd.sup.II(hfac).sub.2. It should be realized that
monolayer-by-monolayer deposition of metallic atoms does not occur
but instead the monolayer deposition of, for example,
Pd.sup.II(hfac).sub.2 occurs assuming no desorption. The primary
reason for the lack of monolayer-by-monolayer growth is due to the
large size of the organic ligands associated with the metallorganic
precursor.
[0033] At 80.degree. C. the vapor pressure of Pd.sup.II(hfac).sub.2
is much lower and the Pd--Pd.sup.II(hfac).sub.2 bond is strong
enough that little desorption occurs. However, when Pd is grown on
Ir via Pd.sup.II(hfac).sub.2/H.sub.2 an incubation period is
consistently observed. It is known that H.sub.2 has a very high
reactivity with Pd.sup.II(hfac).sub.2 and therefore this
observation is likely attributed to carbon residing at the Ir
surface. After the prerequisite number of cycles of H.sub.2, the
carbon can be adequately cleaned and Pd ALD can commence. The
adventitious carbon on the Ir surface was observed with XPS (not
shown) and was significantly greater then on, for example, a
SiO.sub.2 surface. Further, as the temperature of the substrate is
raised, the ability of hydrogen to clean the Ir surface should
increase.
Palladium ALD on Ir and the Tetrasulfide SAM (XPS):
[0034] Ex situ X-ray photoelectron spectroscopy is used to
investigate the chemical quality of the Pd ALD films. In
particular, comparisons are made for Pd deposited on Ir versus the
tetrasulfide SAM. The primary difference between the two types of
depositions is that a Pd `seed` layer is deposited via ALD on the
tetrasulfide SAM at temperatures >200.degree. C., where
glyoxylic acid is active, and then Pd is deposited by the
Pd.sup.II(hfac).sub.2/H.sub.2 process at 80.degree. C. used with
Ir. The process is outlined in FIG. 2 and explained in the
Experimental section. Glyoxylic acid was chosen as an effective
metallorganic reducing agent due to its use as an electroless Cu
reducing agent and its thermal dissociation to form CO.sub.2 and
H.sub.2CO at temperatures greater than 200.degree. C.
[0035] FIG. 4 shows the Pd 3d XPS spectra for Pd deposited on Ir
and on the tetrasulfide SAM. The Pd 3d.sub.5/2 peak for the
tetrasulfide SAM surface is referenced to carbon (C 1s peak) at
285.0 eV from previous work. The Pd 3d.sub.5/2 peak for the Ir
surface is referenced to adventitious carbon at 284.6 eV for the C
1s peak. The value for adventitious carbon on Ir was determined
with a separate experiment. This measurement must be done with care
since the tail of the Ir 4d.sub.5/2 peak at 297 eV can interfere
with the C 1s peak at 284.6 eV. As a result, quantitative results
are hard to come by, but the peak position is easily
obtainable.
[0036] The peak positions of both the Pd 3d.sub.5/2 peaks in FIG. 4
at 335.7 eV are close to what has been previously established in
the literature for elemental Pd. Elemental Pd has been shown to
have a Pd 3d.sub.5/2 binding energy of 334.8, 335.3, and 335.4 eV.
The slight shift to larger binding energies may be attributed to
charge referencing or slight contamination from the dissociation of
the hfac ligand. However, the relationship between the chemical
contamination and the binding energy shift may be less important
since the Pd deposited on the tetrasulfide SAM has higher
contamination levels (C and F) in contrast to the Pd deposited on
Ir but still has the same Pd 3d binding energy.
[0037] The C 1s and F 1s XPS spectra of Pd deposited on the
tetrasulfide SAM and Ir surfaces are shown in FIGS. 5 and 6. The Pd
deposited on the tetrasulfide SAM has significantly higher levels
of carbon and fluorine compared to the Ir substrate. This is likely
caused by the high temperatures (>200.degree. C.) needed to
deposit the Pd seed layer with glyoxylic acid. Higher temperatures
favor a greater degree of ligand dissociation, which is problematic
with the .beta.-diketonate metallorganics. Both C 1s spectra show
evidence of CF.sub.X, C.dbd.O, or C--O bonding due to dissociation
from the original hfac ligand. Much of the carbon that exists in
FIG. 6 is attributed to adventitious sources existing at the top of
the Pd deposit due to ex situ analysis. Proper chemical analysis of
the Pd ALD films would be better undertaken in situ; however,
experimental difficulties preclude this set up at this time.
[0038] Golub et al. have shown that the peak position of the F 1s
peak is correlated to atomic percent fluorine that exists in
polymer repeat units. Poly(tetrafluoroethylene) (PTFE) has a atomic
percent fluorine of 67% and a corresponding binding energy of 689.2
eV. The same --CF.sub.2-- group has a binding energy in the C is
spectra of 290.0 eV to 291.4 eV. Lin et al. showed that physisorbed
Pd.sup.II(hfac).sub.2 on a clean Cu surface at 120 K had a F 1s
binding energy of 688.2 eV. After annealing the physisorbed
Pd.sup.II(hfac).sub.2 at 473 K they saw the formation of a new peak
at 682.9 eV. However, the fluorine associated with the Pd ALD films
in FIG. 6 are at 688.9 eV (on tetrasulfide SAM) and 689.5 eV (on
Ir), which may indicate a loss of carbonyl groups from the hfac
ligand. Also, it should be noted that carbon bonded fluorine
typically has larger F 1s binding energies as compared to metal
fluorides and the binding energies of the F 1s spectra in FIG. 6 is
consistent with the former.
Interfacial Interactions Between Pd.sup.II(hfac).sub.2 and the
Tetrasulfide SAM and Ir Surfaces:
[0039] It was established previously that Pd.sup.II(hfac).sub.2
interacts weakly or not at all with hydroxyl or oxygen terminated
surfaces. Further, weak van der Waals interactions are not strong
enough to make ALD possible. As a result, chemisorption needs to
exist between Pd.sup.II(hfac).sub.2 and the tetrasulfide SAM and Ir
surfaces. That chemisorption exists means that interfacial Pd--S
and Pd--Ir bonding should be evident. In order to see such bonding
in the XPS spectra, for example, the Pd--S signal should be large
relative to the C--S or S--S bonding, if the S 2p spectrum is used
for analysis.
[0040] FIG. 7 shows the S 2p spectrum for a monolayer of
Pd.sup.II(hfac).sub.2 deposited on the tetrasulfide SAM surface
(FIG. 2). The tetrasulfide SAM has a peak at 164.1 eV in the S 2p
XPS spectrum, which has previously been established. After pulsing
Pd.sup.II(hfac).sub.2 over the surface, chemisorption is evident
and a significant binding energy shift occurred in the S 2p
spectrum as shown in FIG. 8. The composite S 2p peak exists at
163.1 eV, whereas the deconvoluted spectra has two peaks, one
associated with the S 2p.sub.3/2 peak at 162.7 eV and one at 163.9
eV associated with the S 2p.sub.1/2 peak. The spin orbital
splitting is 1.18 eV between the S 2p peaks. The shift in the Pd 3d
spectrum for one monolayer versus a 60 .ANG. film is not
significant.
[0041] The Pd ALD films deposited on Ir are slightly different than
the ones deposited on the tetrasulfide SAM because the Ir film is
150 .ANG. versus on just a monolayer for the sulfur terminated SAM.
As a result, interfacial bonding is only evident in FIG. 8 for the
31 .ANG. Pd film that was deposited with 240 cycles at 130.degree.
C. Before that point the Ir--Ir bonding overwhelms the Pd--Ir
bonding and this peak is convoluted in the Ir 4f spectra. The Ir
4f.sub.7/2 peak is centered at 60.91 eV, which is literature value.
The Pd--Ir interfacial bond is centered at 60.72 eV. No literature
value exists for this bonding but IrSi bonding has been reported at
60.70 eV, which may be similar to Pd--Ir.
Reflection High Energy Electron Diffraction (RHEED):
[0042] The structure of the Pd ALD films is most easily obtained by
using a RHEED camera under ultra high vacuum (UHV) conditions.
RHEED is sensitive to the structure of angstrom thick metallic
films, whereas conventional X-ray diffraction is not appropriate
due to the small X-ray cross-section.
[0043] FIG. 9 shows the RHEED images and the corresponding spectra
of Pd on Ir, Pd on the tetrasulfide SAM, and the Ir substrate
itself obtained from digitizing the images. The .about.150 .ANG. Ir
thin film thermally evaporated on hydrogen-terminated Si(100)
exhibits some slight (111) orientation but nearly exhibits random
texture. The calculated lattice constant is 3.81.+-.0.06 .ANG.
based on a d-spacing of 2.86.+-.0.04 .ANG. for the (111) plane,
which is near the literature value of 3.839 .ANG..
[0044] Despite the nearly random texture of the Ir substrate, the
55 .ANG. Pd ALD film deposited on it has much preferred (111)
texture. The lattice constant is 3.8.+-.0.1 .ANG. based on the
(111) d-spacing. The grain size is 60 .ANG., which is very similar
to the thickness of the Pd film. Also, it has a texture spread of
.about.70.degree. at the surface and .about.40.degree. in the bulk
of the film. The high quality Pd ALD film would be expected for the
deposition of a metal on a higher surface free energy metal that is
relatively smooth, .about.10 .ANG. RMS (from AFM measurements).
[0045] The Pd ALD film grown on the tetrasulfide SAM possesses very
small grains and has nearly random texture. This is
uncharacteristic of a metal film that exhibits monolayer wetting as
shown previously. However, the difficulty that arises with the use
of Pd.sup.II(hfac).sub.2 is the high volatility it possesses at low
temperatures, e.g. 1 Torr at 60.degree. C. Further, the hfac ligand
shows some instabilities even at room temperature and as the dosing
or substrate temperature is raised the dissociation of the hfac
ligand becomes worse. As previously explained, glyoxylic acid is
only effective as a reducing agent at temperatures greater than
200.degree. C. Therefore, the processing method developed here is
more suitable for lower vapor pressure metallorganics that do not
dissociate at the deposition temperatures needed to undertake ALD
for their respective metal.
Chemical Analysis of Glyoxylic Acid:
[0046] Glyoxylic acid was used here to deposit a Pd seed layer via
ALD on top of the tetrasulfide SAM. After the seed layer is
deposited, Pd can be deposited via H.sub.2 at low temperatures
where the growth rate per cycle is higher. The preliminary
experiments showed that glyoxylic acid is only effective as a
reducing agent above 200.degree. C., which is the temperature where
it thermally decomposes. At lower temperatures, .about.200.degree.
C., a 5:1 ratio exists between CO.sub.2 and formaldehyde for the
decomposition products. As the temperature is raised a lower ratio
is evident but CO.sub.2 is always the dominant species.
Nevertheless, glyoxylic acid is an effective reducing agent since
it decomposes cleanly to yield oxidation products and electrons
available for reduction reactions with the metallorganic.
[0047] The as-received glyoxylic acid is a hydrate and water is
known to dissociate .beta.-diketonate metallorganics. Proton
Nuclear Magnetic Resonance Spectroscopy (.sup.1H-NMR) was used to
analyze the glyoxylic acid as-received and after dehydration to
ensure that glyoxylic acid was utilized as a reducing agent rather
than effects caused by the presence of water. FIG. 10 shows the
.sup.1H-NMR spectra for the glyoxylic acid before and after
dehydration under vacuum heating to 60.degree. C. for 60 min. There
are four peaks in the proton spectra for glyoxylic acid dissolved
in d.sub.6-acetone: The tetramethylsilane reference peak at 0 ppm,
2.1 ppm acetone (impurity in the d.sub.6-acetone), 5.3-6.1 ppm
glyoxylic acid, and a broad peak attributed to proton exchange
between the glyoxylic acid and water. Two items are apparent from
the .sup.1H-NMR spectra The integrated area between the broad peak
at 7.2 ppm for the as-received sample and the group of peaks at
5.3-6.1 ppm associated with glyoxylic acid is 2.9:1. In contrast,
the ratio for the dehydrated sample is 0.8:1. Second, the peak
associated with proton exchange shifts from 7.2 ppm (FWHM=17.1 Hz)
to 8.5 ppm (FWHM=113 Hz). The conclusions to be made from these
observations are that much less water exists in glyoxylic acid
sample without taking into consideration the water in the
d.sub.6-acetone and the water adsorbed after the sample was
dehydrated. Further, the protons are much more associated with the
acid than `free` water after dehydration. This last conclusion
comes from the larger chemical shift for the dehydrated sample. As
a result, glyoxylic acid is effective as a reducing agent, there is
little associated water, and therefore water plays a very little
role in the removal of the hfac ligands from the
Pd.sup.II(hfac).sub.2 metallorganic.
[0048] The thicknesses of the Pd ALD films were measured as a
function of the number of ALD cycles via Rutherford Backscattering
Spectrometry. At a deposition temperature of 130.degree. C. the
relationship was not linear due to the high volatility of
Pd.sup.II(hfac).sub.2 or in other words its high desorption rate
from the freshly created Pd surface. However, at lower temperatures
(80.degree. C.) the desorption rate is not significant and linear
growth is observed. The Pd ALD films were also characterized by
X-ray Photoelectron Spectroscopy and Reflection High Energy
Electron Diffraction. The films grown on the tetrasulfide SAM had a
higher levels of carbon and fluorine contamination attributed to
the high temperature need to deposit the Pd seed layer. This was
related to the dissociation of the hfac ligand. When the films were
grown on Ir the contamination levels were lower due to the
temperature dependence of the hfac dissociation. Further, the films
grown on Ir exhibited (111) texture with a grain size of .about.60
.ANG. for a 55 .ANG. thick film. In contrast, the Pd films grown on
the tetrasulfide SAM showed very small grains with nearly random
texture. This relative poor structure, as compared to the Pd film
grown on Ir, was attributed to the dissociation of the hfac ligand
thus allowing fluorine and carbon contamination at the Pd seeded
surface.
EXPERIMENTAL
[0049] Two surfaces were used for the atomic layer deposition of
palladium as shown in FIG. 2. 150 .ANG. of Ir (99.95% Alfa Aesar,
Ward Hill, Mass.) was electron beam deposited onto a Si (100)
wafer. The other surface was a tetrasulfide terminated
self-assembled monolayer solution grown on the native oxide of Si
(100) via bis[3-(triethoxysilyl)propyl]-tetrasulfide (>97%
Gelest Inc., Morrisville, Pa.). The details of the growth and
characterization of this monolayer has been published
previously.
[0050] A custom-built vacuum chamber with computer controlled gas
flow was used for the atomic layer deposition of palladium via
Pd.sup.II(hfac).sub.2. The base pressure of the vacuum system is
5.0.times.10.sup.-4 Torr with the use of a roots blower/direct
drive roughing pump (Leybold RUVAC WS/WSU 151 and TRIVAC D25 BCS
(hydrocarbon prepped)) and a 3.7 L foreline trap (bronze gauze) to
prevent oil backstreaming. The vacuum chamber is a warm walled to
prevent condensation of the Pd.sup.II(hfac).sub.2 precursor.
Further, pneumatic valves can turn off or turn on the gas flow and
mass flow meters can control the flow of gases into the system
under steady state conditions.
[0051] Two types of depositions were undertaken, one on a noble
metal surface (Ir) and one on a tetrasulfide SAM. The depositions
(outlined in FIG. 2) on Ir were undertaken at a deposition
temperature of 80.+-.4.degree. C. or 130.+-.4.degree. C. and a
Pd.sup.II(hfac).sub.2 sublimation temperature of
49.8.+-.0.2.degree. C. The deposition chamber walls were kept at
60-65.degree. C. and the lines between the sublimation tube and the
deposition chamber at 90-95.degree. C. to ensure no cold spots
existed to condense Pd.sup.II(hfac).sub.2. During each experiment,
30 sccm Ar (99.999% Air Products) was flowed as a purge gas, 10
sccm Ar as a carrier gas for Pd.sup.II(hfac).sub.2, and 8.0 sccm
H.sub.2 (99.999% Air Products) as a reducing gas. The pulse
sequence was 5 s of Pd.sup.II(hfac).sub.2 followed by 10 s of `dead
time` with just 30 sccm Ar flowing as a purge gas, and then 20 s of
H.sub.2. The number of cycles per deposition varied from 8 to 240
for Pd ALD on Ir.
[0052] The depositions on the tetrasulfide SAM were slightly
different due to the inefficacy of glyoxylic acid (Alfa Aesar,
Mass.) as a reducing agent below 200.degree. C. and the high rate
of desorption of Pd.sup.II(hfac).sub.2 on Pd at temperatures above
200.degree. C. due to its high vapor pressure and low interaction
energy with Pd. This behavior may be followed in terms of Langmuir
kinetics. As a result, these depositions consisted of two parts.
The first part consisted of depositing a `seed` layer of Pd, where
Pd.sup.II(hfac).sub.2 was pulsed for 5 s alternatively with 60 s
pulses of glyoxylic acid (sublimation temperature 60.+-.2.degree.
C. and 15 sccm Ar carrier). As with all of the depositions a 10 s
dead time was used. It took 120 cycles to deposit the `seed` layer
of Pd, after that point the Pd.sup.II(hfac).sub.2/H.sub.2 process
was used at 80.degree. C. for 120 cycles more.
[0053] The Reflection High Energy Electron Diffraction Images
(RHEED) images were taken with a 15 keV electron beam source that
produces a 100 .mu.m diameter spot size. The pressure of the
stainless steel chamber was .about.1.times.10.sup.-9 Torr. The
high-energy electron beam was incident at 1.degree. from the sample
plane. The diffracted electrons pass through two metal retarding
grids before striking the phosphor-coated screen, which is imaged
using a 16-bit Princeton Instruments CCD camera. The Nuclear
Magnetic Resonance Spectroscopy (FT-NMR) work on the analysis of
the water in the glyoxylic acid was undertaken on a Varian Unity
INOVA 300 (300 MHz). .sup.1H-NMR spectra were undertaken on
dehydrated and as-received glyoxylic acid with 8 scans with a pulse
width of 8 is at 90.degree. in d.sub.6-acetone with 1 v/v %
tetramethylsilane (TMS) as a proton standard.
* * * * *