# Pressure and Temperature Effects on Stoichiometry and Microstructure of Nitrogen-Rich TiN Thin Films Synthesized  via Reactive Magnetron DC-Sputtering

Authors: E. Penilla; J.  Wang
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Nitrogen-rich titanium nitride (TiN) thin films containing excess nitrogen up to 87.0 at.% were produced on (100) Si substrates via the reactive magnetron DC-sputtering of a commercially available 99.995 at.% pure Ti target within an argon-nitrogen (Ar-N2) atmosphere with a 20-to-1 gas ratio. The process pressure (PP) and substrate temperature (TS) at which deposition occurred were varied systematically between  0.26 Pa–1.60 Pa and between  15.0∘C–600∘C, respectively, and their effects on the chemical composition, surface morphology, and preferred orientation were characterized by energy dispersive X-ray spectroscopy (EDS), field emission scanning electron microscopy (FE-SEM), and X-ray diffraction (XRD). The EDS analysis confirms increasing nitrogen content with increasing PP and  TS. The SEM images reveal a uniform and crystallized surface morphology as well as a closely packed cross-sectional morphology for all crystalline films and a loosely packed cross-sectional morphology for  amorphous films. Films produced at lower PP and  TS have a pyramidal surface morphology which transitions to a columnar and stratified structure as PP and  TS increase.  The XRD analysis confirms the existence of only the δ-TiN phase and the absence of other nitrides, oxides, and/or sillicides in all cases. It also indicates that at lower PP and TS, the preferred orientation relative to the substrate is along the (111) planes, and that it transitions to a random orientation along the (200), (220), and (311) planes as PP and  TS increase and these results correlate with and qualify those observed by SEM.


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## Body

## 1. Introduction

Various
transition metal nitrides such as TiN thin films have attracted considerable
attention from researchers spanning the spectrum of the physical sciences.  This is a testament to the myriad of potential
applications that the TiN
system
affords because of its unique properties. They have been used as wear-resistant
coatings on high-speed cutting tools [1–3], as infrared filtering coatings on windows
[4], and as inexpensive decorative coatings on pens, watch dials, and so on [5]
because of their golden luster. TiN thin films have also been used as
microelectronic contacts [6], resistors [7], and, because of their high-chemical
stability, as diffusion barriers [8, 9] between interconnects of microchips. A
most recent application makes use of their biocompatibility and
hemocompatability [10, 11], as they have been integrated as surface layers
within orthopedic prostheses and cardiac valves [12, 13].TiN
thin films have been produced by various techniques, all derivatives of those used
in semiconductor processing, such as electroplating [14], laser ablation [15, 16], chemical vapor deposition (CVD) [17], as well as the physical vapor
deposition (PVD) techniques of evaporation and sputtering, reactive or
otherwise [18–22]. Much of the problem associated with their production has
been the need for high voltages and temperatures, which limit their integration
within CMOS process flows. As of late, however, the commercial availability of
TiN sputtering  targets has facilitated their use within certain scenarios, where
sputtering thin films is an option.Although many practical applications,
synthesis techniques, and deposition methods have been developed for TiN thin
films, the literature seems to indicate that the basic metallurgy and physics
of the TiN system
are not completely understood [1, 23, 24].
The literature indicates that the first attempts to derive the TiN phase diagram were
conducted by Wriedt and Murray as recently as 1987, but the complete phase diagram
covering the full compositional spectrum was not reported herein, namely, the
data pertaining to overstoichiometric TiN was not reported [24].
Additional studies by Ristolainen et al. report the stability of TiN films with
excess nitrogen contents up to 62 at.% via triode ion plating techniques [23]. Nonetheless, the limit of nitrogen solubility in
TiN still appears uncertain as do the properties of such nitrogen-rich films as
well as the effects of processing parameters on the basic metallurgy and
physics.All of the TiN films synthesized for this work, whose
phase existence is confirmed by EDS and XRD analyses, have an excess nitrogen
content of at least 70.4 at.% and up to 87 at.% and it is believed that this is
the first report of such produced films. This belief in tandem with the wide
range of potential applicability within industry and academia prompted the
authors’ interest in characterizing some of the films’ fundamental physics.
Scanning electron microscopy was used to elucidate the surface and
cross-sectional characteristics, and SEM in concert with EDS and XRD was instrumental
in determining the chemistry and crystallography.


## 2. Experimental Procedure

### 2.1.  Preparation of TiN Films

TiN
films with high nitrogen content were deposited onto polished (100) Si
substrates with a native oxide layer and overall thicknesses of 480μm by
sputtering a commercially developed 99.995 at.% Ti target (50.8 mm diameter ×
6.35 mm thick) within an Ar-N2 atmosphere. The target was
mechanically clamped to a water-cooled direct-current magnetron cathode of an ultrahigh
vacuum sputtering system (Orion 5 UHV sputtering system, AJA International, Inc., North Scituate, Mass, USA).
The target must remain at a constant temperature throughout because the sputter
yield and, as a consequence, the deposition rate are functions of target
temperature [25]. Target poisoning also
becomes an issue with the elevation of target temperature [25].
A DC-potential of 466 V was coupled to the cathode and this resulted in a driving
current of 536 mA and a power dissipation of 250 W.  All depositions occurred at a target to
substrate distance of 178 mm and lasted 6300 seconds.The purity of the gases was 99.9995% and the Ar-N2 ratio was 20 to 1 in all cases. Gas flow was controlled by 2 independently
calibrated mass flow controllers (MFC) capable of delivering up to 20 SCCM of
gas. The MFCs are integrated within a computer-controlled feedback loop to
ensure constant flow and gas mixture throughout the entire deposition process.
At full gas flow, PP was controlled by adjusting a three-position
throttle valve, which is placed in series between the preparation chamber and a
500 L turbomechanical pump, allowing for the adjustment of conductance between
the two. Films were synthesized with PP set points of 0.26 Pa, 0.53 Pa,
1.07 Pa, and 1.60 Pa.Before sputtering, the substrates were subjected to a
series of ultrasonicated chemical baths including acetone, isopropanol (IPA) and
deionized (DI) water. They were then blown dry and inserted into a prevacuum
load-lock chamber until reaching a pressure of 10−4 Pa.
This is followed by
insertion into the main preparation chamber where the substrates were pumped on
until the preparation chamber reached a base pressure of 10−6–10−7 Pa.
Further
cleaning was achieved by subjecting the substrates to an RF-etching bias (13.52 MHz) of 50 W for 10 minutes in a pure Ar plasma. The process pressure during
etching was 0.53 Pa.
This also removes the native oxide layer, ensuring a maximum absence of oxygen
within the preparation chamber whose presence could have become an issue, especially during
those depositions at elevated PP and TS.Following chamber and substrate conditioning, the
substrates were heated to the desired set points via a pair of 1 kW quartz
heating elements that are coupled to a proportional with integral and derivative
(PID) controller capable of maintaining set point ±1C°. TS is
monitored throughout by a k-type thermocouple which is integrated within a
feedback loop with the heating elements and the PID controller. Depositions
were performed with TS held constant during the entire processes at
set points of 15C°, 200C°, 400C°, and 600C°  for each of the PP set points
denoted above. Therefore, a total of 16 samples were synthesized by this
technique for characterization.


### 2.2. Film Characterization

All films were characterized ex situ
and at room temperature. Film thicknesses,tf, were experimentally
confirmed by scanning probe profilometry (SPP) and SEM  of their
cross-sections to be ~500 nm with no significant variation from film to film.
It is important to make this conclusion because it is well known that process
pressure and substrate temperature have an effect on the sputter yield and the
eventual film thickness [25] and that it has also been shown that the physical
properties of TiN films are dependent on film thickness [26].The surface and cross-sectional
morphology were qualitatively ascertained by the implementation of FE-SEM. For
cross-sectional characterization, the samples were cleaved in order to expose
their underlying microstructure. The instrument used was a Phillips XL30. All
samples were imaged with a 15 kV accelerating potential, 50 nm spot size, 30μm aperture, at a working distance of 10 mm, and all images were gathered by the secondary
electron (SE) detector.EDS of all samples was conducted within
the Phillips XL30 using the same parameters as above, which produced a projected interaction volume ~500 nm3 allowing for full
cross-sectional analysis of the synthesized TiN films. Counts were taken for 90
seconds and the spectra were analyzed with Phillips EDXi software, which allows
for background signal removal, peak identification, and peak quantification.Nondestructive XRD was performed on a
Bruker AXS D8 advanced X-ray diffractometer which implements a Cu Kα1 (λ = 1.54058 Å) radiation operated at 40 kV and 40 mA. All scans were taken in
continuous mode for a 2θ range of 33°–83°, a step size of .02°, and time step
of 2 seconds. All peaks derived beyond this range are secondary and therefore
scans beyond this range are not strictly necessary for crystallographic and/or
chemical identification.


## 2.1.  Preparation of TiN Films

TiN
films with high nitrogen content were deposited onto polished (100) Si
substrates with a native oxide layer and overall thicknesses of 480μm by
sputtering a commercially developed 99.995 at.% Ti target (50.8 mm diameter ×
6.35 mm thick) within an Ar-N2 atmosphere. The target was
mechanically clamped to a water-cooled direct-current magnetron cathode of an ultrahigh
vacuum sputtering system (Orion 5 UHV sputtering system, AJA International, Inc., North Scituate, Mass, USA).
The target must remain at a constant temperature throughout because the sputter
yield and, as a consequence, the deposition rate are functions of target
temperature [25]. Target poisoning also
becomes an issue with the elevation of target temperature [25].
A DC-potential of 466 V was coupled to the cathode and this resulted in a driving
current of 536 mA and a power dissipation of 250 W.  All depositions occurred at a target to
substrate distance of 178 mm and lasted 6300 seconds.The purity of the gases was 99.9995% and the Ar-N2 ratio was 20 to 1 in all cases. Gas flow was controlled by 2 independently
calibrated mass flow controllers (MFC) capable of delivering up to 20 SCCM of
gas. The MFCs are integrated within a computer-controlled feedback loop to
ensure constant flow and gas mixture throughout the entire deposition process.
At full gas flow, PP was controlled by adjusting a three-position
throttle valve, which is placed in series between the preparation chamber and a
500 L turbomechanical pump, allowing for the adjustment of conductance between
the two. Films were synthesized with PP set points of 0.26 Pa, 0.53 Pa,
1.07 Pa, and 1.60 Pa.Before sputtering, the substrates were subjected to a
series of ultrasonicated chemical baths including acetone, isopropanol (IPA) and
deionized (DI) water. They were then blown dry and inserted into a prevacuum
load-lock chamber until reaching a pressure of 10−4 Pa.
This is followed by
insertion into the main preparation chamber where the substrates were pumped on
until the preparation chamber reached a base pressure of 10−6–10−7 Pa.
Further
cleaning was achieved by subjecting the substrates to an RF-etching bias (13.52 MHz) of 50 W for 10 minutes in a pure Ar plasma. The process pressure during
etching was 0.53 Pa.
This also removes the native oxide layer, ensuring a maximum absence of oxygen
within the preparation chamber whose presence could have become an issue, especially during
those depositions at elevated PP and TS.Following chamber and substrate conditioning, the
substrates were heated to the desired set points via a pair of 1 kW quartz
heating elements that are coupled to a proportional with integral and derivative
(PID) controller capable of maintaining set point ±1C°. TS is
monitored throughout by a k-type thermocouple which is integrated within a
feedback loop with the heating elements and the PID controller. Depositions
were performed with TS held constant during the entire processes at
set points of 15C°, 200C°, 400C°, and 600C°  for each of the PP set points
denoted above. Therefore, a total of 16 samples were synthesized by this
technique for characterization.


## 2.2. Film Characterization

All films were characterized ex situ
and at room temperature. Film thicknesses,tf, were experimentally
confirmed by scanning probe profilometry (SPP) and SEM  of their
cross-sections to be ~500 nm with no significant variation from film to film.
It is important to make this conclusion because it is well known that process
pressure and substrate temperature have an effect on the sputter yield and the
eventual film thickness [25] and that it has also been shown that the physical
properties of TiN films are dependent on film thickness [26].The surface and cross-sectional
morphology were qualitatively ascertained by the implementation of FE-SEM. For
cross-sectional characterization, the samples were cleaved in order to expose
their underlying microstructure. The instrument used was a Phillips XL30. All
samples were imaged with a 15 kV accelerating potential, 50 nm spot size, 30μm aperture, at a working distance of 10 mm, and all images were gathered by the secondary
electron (SE) detector.EDS of all samples was conducted within
the Phillips XL30 using the same parameters as above, which produced a projected interaction volume ~500 nm3 allowing for full
cross-sectional analysis of the synthesized TiN films. Counts were taken for 90
seconds and the spectra were analyzed with Phillips EDXi software, which allows
for background signal removal, peak identification, and peak quantification.Nondestructive XRD was performed on a
Bruker AXS D8 advanced X-ray diffractometer which implements a Cu Kα1 (λ = 1.54058 Å) radiation operated at 40 kV and 40 mA. All scans were taken in
continuous mode for a 2θ range of 33°–83°, a step size of .02°, and time step
of 2 seconds. All peaks derived beyond this range are secondary and therefore
scans beyond this range are not strictly necessary for crystallographic and/or
chemical identification.


## 3. Results and Discussion

### 3.1. Chemical Composition

EDS analysis  was implemented to
elucidate the stoichiometric composition of the reactively sputtered TiN films and
it reveals an increasing nitrogen content within the films with increasing
process pressure and substrate temperature as is shown in Figure1. It is important to note that surface
roughness has an effect on EDS results with decreased reliability as the root
mean square surface roughness approaches the micron scale. As will be disclosed
in Section 3.2, the feature size of the produced films is nanometric and,
therefore, localized EDS analysis is minimally affected by the film surface
roughness [27]. The film produced at PP = 0.26 Pa and TS =15.0C°
contained 70.4 at.% nitrogen and the film produced at PP = 1.60 Pa and
TS = 600C°  had a nitrogen content of 87.0 at.%, and these were the
minimum and maximum nitrogen contents of all of the produced films. This
indicates that the combined effects of increasing PP and TS to the upper limit resulted in a 16.6 at.% increase in the nitrogen content of the TiN
films.Figure 1
Effect ofPP and TS variations on nitrogen solubility.The increase inPP from 0.26 Pa to 1.60 Pa
at a constant TS resulted in a maximum increase in nitrogen content
of 10.7 at.%. This value is obtained by taking the difference of the maximum
and minimum nitrogen contents for the denoted process pressure range at each
substrate temperature. The average increase in nitrogen content as a function
of PP was 9.16 at.% over the entire temperature range.Similarly, increasingTS from 15C° to 600C°  with PP held constant at 0.26 Pa caused the nitrogen content to
increase from 70.4 at.%   to 81.3 at.%, a
maximum increase of 10.9 at.%. On average, the increased substrate temperature
was responsible for a 7.46 at.% increase in nitrogen content. There appears to
be no sign of asymptotic behavior
with respect to nitrogen solubility as a function of either PP or TS;
nonetheless logic indicates that there must exist an upper limit to nitrogen
saturation.


### 3.2. Surface Morphology and Preferred Orientation: Effects of
IncreasingPP and TS

The XRD analysis of the four samples deposited withTS = 15C°
at each of the defined PP set points indicates that the synthesized
material is indeed titanium nitride, namely, the δ-TiN phase (cubic NaCl structure,
a = 4.244 Å) [28], and that the
preferred orientation of the films is along the (111) planes (2θ = 36.9°)
relative to the (100) Si substrates in all cases (Figure 2). The smaller peaks
at 2θ = 78.9°  correspond to the secondary (222) planes. Nominally, these peaks
are not necessary for crystallographic and/or chemical phase identification,
but in this case there is only the emergence of a single primary peak, making
it difficult to qualify the data. Hence the presence of the secondary peak in
its expected location along with the presence of the primary peak allows for
reliable spectrum qualification.Figure 2
XRD spectra of increasingPP for TS = 15C°.It is also important to notice that the spectra
indicate that no other nitrides, oxides, or sillicides of titanium or
permutations thereof were synthesized even at increased process pressures and
therefore that the films produced are nitrogen-rich TiN indeed.The sharpness of the XRD peaks indicates an ordered crystalline
structure. This can be qualitatively seen in the SEM images in Figure3, which indicates
that the nanostructured morphology is densely and uniformly packed throughout
with an approximate feature size of 75 nm. The intensity of the (111) peaks
decreases with increasing pressure and is almost indiscernible for the spectrum
corresponding to the 1.60 Pa film. This indicates that increasing PP at this temperature results in a transition from the (111) orientations to a quasiamorphous
phase.SEM images of increasingPP at TS = 15C° with a) 0.26 Pa, b) 0.53 Pa, c)1.07 Pa and d) 1.60 Pa.
(a)(b)(c)(d)The SEM images in Figure3 of these produced films
display the variation in their surface morphology. It quickly becomes evident
that the films deposited with 0.26 Pa ≤ PP ≤ 1.07 Pa have a
pyramidal surface structure with uniformity in the long range and that the
surface morphology becomes less distinctive when PP = 1.60 Pa. Based
on the information collected by XRD, it is apparent that these pyramids are
single crystals of TiN growing with their (111) planes parallel to the
substrate. In effect, we are seeing the 〈100〉 family planes as the sides
of these pyramids. The image of the 1.60 Pa film supports the conclusion drawn from the XRD spectrum that this film is an amorphous structure with
intermittent pyramidal, that is, isolated (111) single crystals.The overlain spectra of the films produced at a
substrate temperature of 200C°, (Figure 4), indicate that the δ-TiN phase
transitions from the (111) orientations to a combination of (111) and (200) orientations (2θ = 42.8°)
as the process pressure is taken to the upper limit. The scanning electron
micrographs in Figure 5 for the 0.26 Pa and 0.53 Pa films display almost no
difference between those films produced at similar pressures but at 15C°.  Again, we see a highly uniform and pyramidal
surface structure with no real discerning attributes between one and another. The transition
in surface morphology and preferred orientation does not occur at TS = 200C°  until PP≥ 1.07
Pa.  In this case, the elevated pressures do not cause amorphicivity, instead we see the
formation of large (~200 nm) δ-TiN (200) single crystals coexisting with the
familiar (111) crystals, especially in the film produced at PP =
1.07 Pa. The (200) and (220) planes
are referenced and analyzed because the (100) and (110) planes are necessarily
forbidden in the δ-TiN XRD spectrum even though they are of the same family
because δ-TiN has a cubic NaCl structure and these planes have mixed Miller indices.
Refer to [28] for a complete discussion.
As the process pressure is taken to the
upper limit, the intensity of the (200) peaks is dominant over the (111) peaks. This is
qualitatively confirmed by SEM imaging, whereby the majority of the crystals
seen are not the pyramidal (111) crystals but the larger (200) crystals,
however, this film is yet completely homogenous.Figure 4
XRD spectra of increasingPP for
TS = 200C°.SEM images of increasingPP at
TS = 200C° with a) 0.26 Pa, b) 0.53 Pa, c) 1.07 Pa and d) 1.60 Pa.
(a)(b)(c)(d)The films produced at a
substrate temperature of 400C°  exhibit further transition in their preferred
orientation and surface morphology.  With
this set of films, we again see the prominence of the (111) peaks decreasing as
the process pressure is elevated. Again, the emergence of the (200) peaks
results in these spectra, yet the interesting result here is that the increase
in substrate temperature has allowed this transition to occur at a PP of 0.26 Pa, moreover peaks at 2θ = 61.7°  and 74.4°, corresponding to the (220) and (311) planes, respectively, emerge and increase in
intensity with the elevation in the process pressure as shown in Figure 6. Interestingly
enough, the SEM images shown in Figure 7 again parallel the XRD results. The
surface morphology of the 0.26 Pa film is unmistakably comprised of pyramidal
(111) crystals intermixed with rectangular (200), and parallelogram (220) and
(311) crystals.  And as the process pressure is increased to
1.60 Pa, the films become less columnar and more stratified with the crystalline
planes growing parallel to each other, indicating the increased prevalence of the
(200), (220), and (311) orientations over a (111) alignment. The stratified
crystals are the (200) crystals growing atop one another.Figure 6
XRD spectra of increasingPP for TS = 400C°.SEM images of increasingPP at TS = 400C°. with a) 0.26 Pa,  b) 0.53 Pa, c) 1.07 Pa and d) 1.60 Pa
(a)(b)(c)(d)Again, the feature size
is also affected with those films synthesized at elevated set points exhibiting
features that are 200–500 nm in size as opposed to those produced at lower
pressures which still contain (111) oriented crystals of approximately 75–100 nm.Based
on the observed trend, it would seem likely that further increasing the
substrate temperature would completely eliminate the (111) orientations regardless
of the applied process pressure set point, but surprisingly, the films produced atTs=600°C
with PP = 0.26 Pa are still dominantly oriented
along the (111) planes, as can be seen in the XRD and SEM results displayed in Figures   8 and 9,  respectively. As expected,
however, grain coarsening and stratification are activated by increased
process pressures as in previous cases. The ultimate conclusion is that the
change in surface morphology and preferred orientation are a result of the
coupled parametric variations in PP and TS.Figure 8
XRD spectra of increasingPP for TS = 600C°.SEM images of increasingPP at
TS = 600C° with a) 0.26 Pa, b) 0.53 Pa, c) 1.07 Pa and d) 1.60 Pa.
(a)(b)(c)(d)


### 3.3. Cross-Sectional Morphology

As has been discussed, films produced
at lowerPP and TS were preferably oriented along the
(111) planes and that an increased PP at low TS results
in the amorphization of the films as shown in Figures 2 and 3. Moreover, a
combined increase in PP and TS results in a transition
from the pyramidal (111) orientations
to  stratified and columnar orientations.
Field emission scanning electron microscopy of the as-cleaved cross-sections
was implemented to qualify and correlate the cross-sectional microstructure of
each typical film type.The scanning electron micrographs reveal a closely packed and
columnar cross-section for those films with a preferred orientation along the
(111) planes as shown in Figure10(a). In essence, the columnar cross-sectional
microstructure supports the conclusion that the pyramidal structures are in
fact the 〈100〉 family of planes and that the film is growing with the
(111) planes parallel to the substrate.SEM images of typical film cross-sections:
(a) pyramidal surface withPP=0.53Pa  and Ts=200 °C ,  (b) amorphous surface with PP=1.06Pa and Ts=15 °C,  (c) transitional surface with PP=0.53Pa and  Ts= 600 °C, and (d) columnar/stratified
surface with PP=1.07Pa  and   Ts= 600 °C.
(a)(b)(c)(d)Interestingly, the scanning electron micrographs of the
cross-sections of the amorphous film reveal a quasiloosely packed underlying
microstructure (Figure10(b)), again correlating as expected with the results
derived from XRD and surface SEM.SEM images of the cross-sections of those films that are
transitioning from the (111) preferred orientation to the stratified and columnar
orientations as well as those which are fully stratified and columnar are shown
in Figures10(c) and 10(d). The micrographs again reveal a closely packed and columnar
cross-section with the absence of pockets or voids. This is in fact the expectation
since all of these films are crystalline and are being produced via a PVD
route, where films grow from the substrate layer by layer.


## 3.1. Chemical Composition

EDS analysis  was implemented to
elucidate the stoichiometric composition of the reactively sputtered TiN films and
it reveals an increasing nitrogen content within the films with increasing
process pressure and substrate temperature as is shown in Figure1. It is important to note that surface
roughness has an effect on EDS results with decreased reliability as the root
mean square surface roughness approaches the micron scale. As will be disclosed
in Section 3.2, the feature size of the produced films is nanometric and,
therefore, localized EDS analysis is minimally affected by the film surface
roughness [27]. The film produced at PP = 0.26 Pa and TS =15.0C°
contained 70.4 at.% nitrogen and the film produced at PP = 1.60 Pa and
TS = 600C°  had a nitrogen content of 87.0 at.%, and these were the
minimum and maximum nitrogen contents of all of the produced films. This
indicates that the combined effects of increasing PP and TS to the upper limit resulted in a 16.6 at.% increase in the nitrogen content of the TiN
films.Figure 1
Effect ofPP and TS variations on nitrogen solubility.The increase inPP from 0.26 Pa to 1.60 Pa
at a constant TS resulted in a maximum increase in nitrogen content
of 10.7 at.%. This value is obtained by taking the difference of the maximum
and minimum nitrogen contents for the denoted process pressure range at each
substrate temperature. The average increase in nitrogen content as a function
of PP was 9.16 at.% over the entire temperature range.Similarly, increasingTS from 15C° to 600C°  with PP held constant at 0.26 Pa caused the nitrogen content to
increase from 70.4 at.%   to 81.3 at.%, a
maximum increase of 10.9 at.%. On average, the increased substrate temperature
was responsible for a 7.46 at.% increase in nitrogen content. There appears to
be no sign of asymptotic behavior
with respect to nitrogen solubility as a function of either PP or TS;
nonetheless logic indicates that there must exist an upper limit to nitrogen
saturation.


## 3.2. Surface Morphology and Preferred Orientation: Effects of
IncreasingPP and TS

The XRD analysis of the four samples deposited withTS = 15C°
at each of the defined PP set points indicates that the synthesized
material is indeed titanium nitride, namely, the δ-TiN phase (cubic NaCl structure,
a = 4.244 Å) [28], and that the
preferred orientation of the films is along the (111) planes (2θ = 36.9°)
relative to the (100) Si substrates in all cases (Figure 2). The smaller peaks
at 2θ = 78.9°  correspond to the secondary (222) planes. Nominally, these peaks
are not necessary for crystallographic and/or chemical phase identification,
but in this case there is only the emergence of a single primary peak, making
it difficult to qualify the data. Hence the presence of the secondary peak in
its expected location along with the presence of the primary peak allows for
reliable spectrum qualification.Figure 2
XRD spectra of increasingPP for TS = 15C°.It is also important to notice that the spectra
indicate that no other nitrides, oxides, or sillicides of titanium or
permutations thereof were synthesized even at increased process pressures and
therefore that the films produced are nitrogen-rich TiN indeed.The sharpness of the XRD peaks indicates an ordered crystalline
structure. This can be qualitatively seen in the SEM images in Figure3, which indicates
that the nanostructured morphology is densely and uniformly packed throughout
with an approximate feature size of 75 nm. The intensity of the (111) peaks
decreases with increasing pressure and is almost indiscernible for the spectrum
corresponding to the 1.60 Pa film. This indicates that increasing PP at this temperature results in a transition from the (111) orientations to a quasiamorphous
phase.SEM images of increasingPP at TS = 15C° with a) 0.26 Pa, b) 0.53 Pa, c)1.07 Pa and d) 1.60 Pa.
(a)(b)(c)(d)The SEM images in Figure3 of these produced films
display the variation in their surface morphology. It quickly becomes evident
that the films deposited with 0.26 Pa ≤ PP ≤ 1.07 Pa have a
pyramidal surface structure with uniformity in the long range and that the
surface morphology becomes less distinctive when PP = 1.60 Pa. Based
on the information collected by XRD, it is apparent that these pyramids are
single crystals of TiN growing with their (111) planes parallel to the
substrate. In effect, we are seeing the 〈100〉 family planes as the sides
of these pyramids. The image of the 1.60 Pa film supports the conclusion drawn from the XRD spectrum that this film is an amorphous structure with
intermittent pyramidal, that is, isolated (111) single crystals.The overlain spectra of the films produced at a
substrate temperature of 200C°, (Figure 4), indicate that the δ-TiN phase
transitions from the (111) orientations to a combination of (111) and (200) orientations (2θ = 42.8°)
as the process pressure is taken to the upper limit. The scanning electron
micrographs in Figure 5 for the 0.26 Pa and 0.53 Pa films display almost no
difference between those films produced at similar pressures but at 15C°.  Again, we see a highly uniform and pyramidal
surface structure with no real discerning attributes between one and another. The transition
in surface morphology and preferred orientation does not occur at TS = 200C°  until PP≥ 1.07
Pa.  In this case, the elevated pressures do not cause amorphicivity, instead we see the
formation of large (~200 nm) δ-TiN (200) single crystals coexisting with the
familiar (111) crystals, especially in the film produced at PP =
1.07 Pa. The (200) and (220) planes
are referenced and analyzed because the (100) and (110) planes are necessarily
forbidden in the δ-TiN XRD spectrum even though they are of the same family
because δ-TiN has a cubic NaCl structure and these planes have mixed Miller indices.
Refer to [28] for a complete discussion.
As the process pressure is taken to the
upper limit, the intensity of the (200) peaks is dominant over the (111) peaks. This is
qualitatively confirmed by SEM imaging, whereby the majority of the crystals
seen are not the pyramidal (111) crystals but the larger (200) crystals,
however, this film is yet completely homogenous.Figure 4
XRD spectra of increasingPP for
TS = 200C°.SEM images of increasingPP at
TS = 200C° with a) 0.26 Pa, b) 0.53 Pa, c) 1.07 Pa and d) 1.60 Pa.
(a)(b)(c)(d)The films produced at a
substrate temperature of 400C°  exhibit further transition in their preferred
orientation and surface morphology.  With
this set of films, we again see the prominence of the (111) peaks decreasing as
the process pressure is elevated. Again, the emergence of the (200) peaks
results in these spectra, yet the interesting result here is that the increase
in substrate temperature has allowed this transition to occur at a PP of 0.26 Pa, moreover peaks at 2θ = 61.7°  and 74.4°, corresponding to the (220) and (311) planes, respectively, emerge and increase in
intensity with the elevation in the process pressure as shown in Figure 6. Interestingly
enough, the SEM images shown in Figure 7 again parallel the XRD results. The
surface morphology of the 0.26 Pa film is unmistakably comprised of pyramidal
(111) crystals intermixed with rectangular (200), and parallelogram (220) and
(311) crystals.  And as the process pressure is increased to
1.60 Pa, the films become less columnar and more stratified with the crystalline
planes growing parallel to each other, indicating the increased prevalence of the
(200), (220), and (311) orientations over a (111) alignment. The stratified
crystals are the (200) crystals growing atop one another.Figure 6
XRD spectra of increasingPP for TS = 400C°.SEM images of increasingPP at TS = 400C°. with a) 0.26 Pa,  b) 0.53 Pa, c) 1.07 Pa and d) 1.60 Pa
(a)(b)(c)(d)Again, the feature size
is also affected with those films synthesized at elevated set points exhibiting
features that are 200–500 nm in size as opposed to those produced at lower
pressures which still contain (111) oriented crystals of approximately 75–100 nm.Based
on the observed trend, it would seem likely that further increasing the
substrate temperature would completely eliminate the (111) orientations regardless
of the applied process pressure set point, but surprisingly, the films produced atTs=600°C
with PP = 0.26 Pa are still dominantly oriented
along the (111) planes, as can be seen in the XRD and SEM results displayed in Figures   8 and 9,  respectively. As expected,
however, grain coarsening and stratification are activated by increased
process pressures as in previous cases. The ultimate conclusion is that the
change in surface morphology and preferred orientation are a result of the
coupled parametric variations in PP and TS.Figure 8
XRD spectra of increasingPP for TS = 600C°.SEM images of increasingPP at
TS = 600C° with a) 0.26 Pa, b) 0.53 Pa, c) 1.07 Pa and d) 1.60 Pa.
(a)(b)(c)(d)


## 3.3. Cross-Sectional Morphology

As has been discussed, films produced
at lowerPP and TS were preferably oriented along the
(111) planes and that an increased PP at low TS results
in the amorphization of the films as shown in Figures 2 and 3. Moreover, a
combined increase in PP and TS results in a transition
from the pyramidal (111) orientations
to  stratified and columnar orientations.
Field emission scanning electron microscopy of the as-cleaved cross-sections
was implemented to qualify and correlate the cross-sectional microstructure of
each typical film type.The scanning electron micrographs reveal a closely packed and
columnar cross-section for those films with a preferred orientation along the
(111) planes as shown in Figure10(a). In essence, the columnar cross-sectional
microstructure supports the conclusion that the pyramidal structures are in
fact the 〈100〉 family of planes and that the film is growing with the
(111) planes parallel to the substrate.SEM images of typical film cross-sections:
(a) pyramidal surface withPP=0.53Pa  and Ts=200 °C ,  (b) amorphous surface with PP=1.06Pa and Ts=15 °C,  (c) transitional surface with PP=0.53Pa and  Ts= 600 °C, and (d) columnar/stratified
surface with PP=1.07Pa  and   Ts= 600 °C.
(a)(b)(c)(d)Interestingly, the scanning electron micrographs of the
cross-sections of the amorphous film reveal a quasiloosely packed underlying
microstructure (Figure10(b)), again correlating as expected with the results
derived from XRD and surface SEM.SEM images of the cross-sections of those films that are
transitioning from the (111) preferred orientation to the stratified and columnar
orientations as well as those which are fully stratified and columnar are shown
in Figures10(c) and 10(d). The micrographs again reveal a closely packed and columnar
cross-section with the absence of pockets or voids. This is in fact the expectation
since all of these films are crystalline and are being produced via a PVD
route, where films grow from the substrate layer by layer.


## 4. Conclusions

Reactive magnetron DC-sputtering of titanium was
implemented for the synthesis of TiN thin films. EDS analysis of the as-synthesized
films confirmed an overstoichiometric chemical composition with excess nitrogen
contents ranging between 70.4 at.% and 87.0 at.%. XRD analysis indicates the
independent existence of theδ-TiN phase, and that indeed the films produced
were a solid solution of the TiN
constituents. The effects of increasing PP and TS set points
during deposition affected nitrogen solubility within titanium. Increasing PP from 0.26 Pa to 1.60 Pa was responsible for a maximum nitrogen content increase
of 10.7 at.% and on average the variation in the PP increased the
nitrogen content by 9.16 at.%. Varying TS between 15.0C°–600C°
resulted in an increased nitrogen content of up to 10.9 at.% and an average
increase of 7.46 at.%.  The nitrogen
content in the films increased by 16.6 at.%, when both PP and TS were taken to their maximum set points.The surface morphology and preferred orientation were
also affected by these parametric variations.   Scanning electron microscopy
reveals nanometric surface roughness, and that increasedPP and TS resulted in a columnar and stratified surface morphology with long-range
uniformity. This columnation and stratification are explained by XRD analysis which indicates that
the preferred orientation of these films relative to the substrate is along the
(200), (220), and (311) planes. Those films produced at lower set points had a
pyramidal surface morphology, that is, the preferred orientation is along the
(111) planes.   Scanning electron microscopy of the as-cleaved
cross-sections reveals a closely packed and columnar underlying microstructure
for all crystalline films and a loosely packed cross-section for amorphous films.



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*Source: 267161-2008-08-06.xml*