Surface properties of refractory ceramic transition metal nitride thin films grown by magnetron sputtering are essential for resistance towards oxidation necessary in all modern applications. Here, typically neglected factors, including exposure to residual process gases following the growth and the venting temperature Tv, each affecting the surface chemistry, are addressed. It is demonstrated for the TiN model materials system that Tv has a substantial effect on the composition and thickness-evolution of the reacted surface layer and should therefore be reported. The phenomena are also shown to have impact on the reliable surface characterization by x-ray photoelectron spectroscopy.

Transition metal (TM) nitride-based thin films grown by physical vapor deposition (PVD) are widely used as protective layers on high-speed cutting tools,1,2 engine parts,3,4 as well as diffusion barriers in electronics.5–7 Coating performance does not only depend on the bulk mechanical properties such as hardness or elastic modulus but also on the resistance towards corrosion and/or oxidation during operation in hostile environments, which are to a large extent determined by the surface properties. Thus, ambient-coating interactions modifying the surface chemistry are essential for understanding the coating performance and the failure mechanisms. Numerous studies have been devoted to investigate oxidation mechanisms of the TM-based nitrides.8–10 More recently, the interaction between TiAlN surfaces and residual as well as environmental gas has been studied theoretically11 and experimentally.12 

Since the ambient-coating interactions define the impurity incorporation during thin film growth,13,14 it is reasonable to assume that they also affect the as grown film surfaces during the exposure to residual gases in the vacuum chamber and subsequently throughout the venting sequence and the following storage. Each of these steps affects surface chemistry, especially that films are grown at elevated temperatures (400–500 °C) to ensure high adatom surface mobility needed to form dense layers,15 which also increases the reaction rates. Unfortunately, the effects of or conditions for residual gas exposure, venting strategy, and the long-term storage are not explicitly addressed in the literature.

In this work, we seek to change the status quo by pursuing the role of venting temperature Tv, a “hidden” experimental variable often not considered and reported, but, as we show here, defining the surface chemistry of the TiN layers. We employ the previously developed Al-cap technique16 to separate the effects of residual gas exposure in the high-vacuum (HV) environment during the post-deposition phase from those introduced by the following venting sequence and air exposure. With the help of x-ray photoelectron spectroscopy (XPS) analyses performed on a series of TiN samples as a function of Tv, we find that the majority of surface reaction products, including TiO2, TiOxNy, and N2 previously detected after prolonged annealing experiments, form shortly after vent, provided that Tv is sufficiently high. This has implications for all sorts of practical studies where the surface composition of TM layers is assumed to be fixed once the same growth protocol is used. We show both, that this is definitely not the case for the TiN model materials system, and that the venting temperature is a key experimental variable. The implications are paramount even for a reliable surface characterization by XPS since (i) the reference core level spectra obtained from “as-received” films exhibit large dependence on Tv, and (ii) the ability to obtain clean oxide-free surfaces by in situ sputter etching decreases with the increasing venting temperature.

Polycrystalline TiN thin films are grown on Si(001) substrates biased at −60 V by reactive dc magnetron sputtering (DCMS) in a CC800/9 CemeCon AG system using a rectangular 8.8 × 50 cm2 target and a Ar:N2 (4:1) gas mixture. The target-to-substrate distance is 6 cm. The total pressure during deposition is 3 mTorr (0.4 Pa), while the system base pressure pb before and after the film growth is 2.3 × 10−6 Torr (0.3 mPa) and 1.5 × 10−7 Torr (0.02 mPa), respectively. The average target power during 25-min-long deposition is 4 kW, resulting in film thickness of 1.2 μm. The substrate temperature Ts is 430 °C as determined with the thermocouple placed next to the sample holder.

θ–2θ x-ray diffraction scans reveal that TiN films are single-phase NaCl-crystal structure with 111 preferred orientation. Rutherford backscattering spectroscopy (RBS) yields N/Ti = 1 ± 0.01, while time-of-flight energy elastic recoil detection analyses (ToF–E ERDA)17 give C and O bulk concentrations of 0.4 and 0.2 at. %, respectively.

Following TiN film growth, different venting scenarios with N2 (99.999% pure) are investigated (in all cases, Tv is measured next to the sample holder and corresponds to the sample temperature): (1) an immediate vent at Tv = Ts = 430 °C, (2) vent at Tv = 330 °C after 30 min exposure to residual gases, (3) vent at Tv = 213 °C after 100 min exposure, and (4) vent at Tv = 29 °C after 840 min exposure. Films are subsequently exposed to laboratory air (40% relative humidity) for 10 min necessary to transfer them to the load-lock chamber of the ultra-high-vacuum (UHV) XPS system. To quantify the effect of residual gas exposure in HV two additional TiN films are deposited and in situ capped with 15-Å-thick Al layers in the deposition system prior to air-exposure and loading into the XPS instrument.16 In the first case, Al capping is done immediately (<10 s) after TiN film growth at Ts = 430 °C, while the second sample is capped after 840 min exposure to residual gas in HV. This time is necessary for cooling the sample from 430 to 29 °C at pb = 1.5 × 10−7 Torr (0.02 mPa).

XPS spectra are acquired from TiN and Al/TiN films in a Kratos Analytical instrument, with a base pressure of 1.1 × 10−9 Torr (1.5 × 10−7 Pa), using monochromatic Al Kα radiation (hν = 1486.6 eV). All spectra are referenced to the adventitious C-C/C-H carbon contamination C 1s peak at 284.5 eV. Spectra deconvolution and quantification is performed using CasaXPS software employing Shirley-type background,18 Voigt-type line shapes, and manufacturer's sensitivity factors.19 The exception is the Ti 2p spectra that require asymmetric Voigt-type functions.20 

We showed recently that 15-Å-thick Al capping layers provide effective barriers to TiN sample oxidation and contamination during air exposure and allow subsequent quantitative XPS analyses in which destructive ion etching is avoided.16 The Ti 2p and N 1s spectra from Al/TiN samples were identical to those obtained from single-crystal TiN/MgO(001) films grown and analyzed in situ in a UHV XPS system.21 XPS-determined N/Ti concentrations acquired from Al/TiN/Si(001) samples were in excellent agreement with RBS and ToF-E ERDA analyses.

Here, we employ the Al-capping technique to separate the effects of exposure to residual gases in the HV environment from those that result from venting at different temperatures. Figures 1(a) and 1(b) show the Ti 2p and N 1s spectra from Al/TiN films capped either immediately after TiN film growth at 430 °C (marked as “Ref.1”) or following the 840-min-long exposure to residual gases in HV necessary to cool down to Tv = 29 °C for cap deposition (marked as “Ref.2”). For both samples, the venting temperature was 29 °C. There is only a very subtle change in the appearance of the Ti 2p and N 1s core level spectra indicating that prolonged exposure to residual gases has a minor effect on the chemistry of the surface region probed by XPS.22 

FIG. 1.

XPS (a) Ti 2p, (b) N 1s, (c) O 1s, and (d) C 1s core-level spectra acquired from a series of polycrystalline TiN/Si(001) films as a function of venting temperature. In addition, results for Al/TiN/Si(001) films capped either immediately after the TiN film growth at 430 °C (Ref.1) or following the 840 min-long exposure in high vacuum necessary to cool down to 29 °C for cap deposition (Ref.2) are also shown.

FIG. 1.

XPS (a) Ti 2p, (b) N 1s, (c) O 1s, and (d) C 1s core-level spectra acquired from a series of polycrystalline TiN/Si(001) films as a function of venting temperature. In addition, results for Al/TiN/Si(001) films capped either immediately after the TiN film growth at 430 °C (Ref.1) or following the 840 min-long exposure in high vacuum necessary to cool down to 29 °C for cap deposition (Ref.2) are also shown.

Close modal

Figures 1(a)–1(d) show four sets of Ti 2p, N 1s, O 1s, and C 1s spectra from TiN films as a function of venting temperature, while the XPS-derived surface elemental compositions are summarized in Figure 2. All core-level signals exhibit pronounced changes as a function of Tv indicating large influence of the latter parameter on the surface chemistry. In order to interpret these results in chemical terms, all spectra are deconvoluted taking particular care that not only a qualitative, but also a quantitative self-consistency across all core level signals is achieved.23 In the next step, using information about the chemical composition of the surface region, the thickness of the reacted layers is estimated from the relative intensities of contributing signals.

FIG. 2.

XPS-derived elemental composition in the surface region of TiN/Si(001) films plotted as a function of venting temperature.

FIG. 2.

XPS-derived elemental composition in the surface region of TiN/Si(001) films plotted as a function of venting temperature.

Close modal

The Ti 2p core-level spectra from the Al-capped TiN sample, thus representative of a passivated (native oxide-free) surface (Fig. 1(a)), consist of a spin-orbit split doublet with Ti 2p3/2 and Ti 2p1/2 peaks at 455.3 and 461.2 eV, respectively. Both Ti 2p components exhibit satellite features on the high binding-energy (BE) side, shifted ∼2.7 eV above the primary peaks, in agreement with previous XPS analyses of polycrystalline TiN layers grown in situ.24,25 The origin of the satellite peaks is under debate, and the primary interpretations are a decrease in the screening probability of the core-hole created during photoionization by Ti 3d electrons,24,26–28 and t1g→2t2g intraband transitions between occupied and unoccupied electron states near the Fermi level (shake-up events).29,30 The N 1s spectra obtained from the Al/TiN sample, Fig. 1(b), consists of a main peak at 397.5 eV and the low-intensity satellite feature at ∼400.2 eV.

With increasing Tv, two new features appear in the Ti 2p spectra at the expense of the component assigned to TiN bond, which decreases in intensity. We focus on the stronger spin-split component Ti 2p3/2. First, with Tv = 29 °C, a new Ti 2p3/2 peak appears at 458.5 eV, which is assigned to the TiO2 formation.31,32 This contribution coincides with the TiN satellite peaks and increases in intensity with increasing Tv to completely dominate the Ti 2p spectrum of the Tv = 430 °C sample. An additional peak at 456.9 eV, best visible for samples vented at 213 and 330 °C, indicates the formation of TiOxNy.33,34 The latter assignment is confirmed by the evolution of the corresponding N 1s spectra (Fig. 1(b)), which shows a continuous decrease of the original TiN peak at 397.5 eV with increasing Tv, accompanied by a simultaneous increase in the new TiOxNy feature at lower BE (396.4 eV).33,34 There is a good correspondence between the relative intensities of the Ti 2p3/2 and N 1s components due to oxynitride, which supports peak assignment.35 The N 1s spectrum indicates dramatic loss of N in the case of the Tv = 430 °C sample, with the domination of the TiOxNy component, and the presence of a new broad peak at 402.4 eV. The latter feature has been observed previously following 4 h anneals at 450 °C in O2 atmosphere36 or 1 h anneal at 400 °C in dry air,34 and is attributed to the interstitial molecular nitrogen formed during TiN oxidation according to TiN + O2 → TiO2 + 1/2N2. The evolution of the O 1s spectra (Fig. 1(c)) is fully consistent with the peak assignments made for Ti 2p and N 1s core-levels. O 1s signal is dominated by a peak at 529.9 eV assigned to TiO232 that increases with increasing Tv, in accordance with the corresponding component in the Ti 2p spectra. There is also a low-intensity feature at the higher BE side of the main O 1s peak at ∼531.5 eV, which is composed of several smaller contributions from CO, CO2, and TiNxOy. The C 1s spectra shown in Fig. 1(d) reveal that C is present on the surface in four chemical states: C–C/C–H (284.5 eV), C–O (286.1 eV), O–C=O (289.1 eV), and C–Ti (282.3 eV).37,38 Three first components which dominate the C 1s spectra are associated with the adventitious carbon contamination. TiC is detected in films with 29 ≤ Tv ≤ 330 °C.

Once the chemical species present in the surface region are identified, the thickness d of the surface layer chemically modified upon air exposure at different Tv can be estimated from the relative signal intensities in the Ti 2p spectra using39,40

d=λBln(NAλAIBNBλBIA+1),

in which λ is the inelastic electron mean free path, N is the volume density of Ti atoms, and I is the measured XPS peak intensity. Indices A and B refer to the original TiN film and the reacted TiO2 + TiOxNy surface layer, respectively. The rutile crystal structure is assumed for TiO2 and TiOxNy species, and for simplicity, we use λA=λB= 18 Å for the Ti 2p electrons excited with Al Kα x-rays.41 

Figure 3 is the schematic illustration, emerging from the above analyses, of the TiN film surface air-exposed at different Tv. Chemical modification of the native surface is lowest for the Tv = 29 °C sample. Both TiO2 and the oxynitride with the stoichiometry TiO0.3N0.7 are found at the surface, with the domination of the later species (62% vs. 38% TiO2). The combined thickness of the TiO2/TiO0.3N0.7 surface layer is 11 Å. The angle-resolved XPS shows an increasing TiO2/TiOxNy peak ratio with decreasing photoelectron take-off angle which indicates that the TiO2 component is located closer to the surface; however, no evidence for the layer-over-layer structure can be provided (hence a dashed line separating TiO2 and TiOxNy is used in Fig. 3). The amount of physisorbed adventitious carbon is highest of all samples (see Fig. 2) and dominated by C–C/C–H species, which likely results from the 840-min-long exposure in HV prior to venting. Small amounts of TiC are detected. With increasing Tv to 213 °C, the thickness of the modified layers increases to 16 Å, while the fraction changes to 50% TiO0.5N and 50% TiO2. The amount of C contamination is reduced with respect to the 29 °C sample predominantly due to the loss of the C–C/C–H component. The Tv = 330 °C sample possess 20 Å thick reacted layers of equal fraction TiO0.4N1.6 and TiO2, while the amount of adventitious C and TiC species is slightly lower than for the TiN film vented at 213 °C. Drastic changes take place once Tv is increased to 430 °C. The surface layer gets significantly thicker, 48 Å, and is predominantly composed of TiO2 (84%) with a minor oxynitride component TiO1.2N0.8 (16%). The carbon contamination increases with respect to the 330 °C sample, while the signal from TiC species is not detected.

FIG. 3.

Schematic representation of the effects of venting temperature on the surface composition of TiN/Si(001) films.

FIG. 3.

Schematic representation of the effects of venting temperature on the surface composition of TiN/Si(001) films.

Close modal

Clearly, the spatial elemental composition distribution, the phases formed, as well as the reaction layer architecture all strongly depend on the venting temperature. The thickness of the chemically stable TiO2 layer increases with Tv and eventually for Tv ≳ 400 °C dominates the surface. Thus, the commonly used concept of as-deposited film state is flawed, and the venting temperature has to be reported as it defines the surface chemistry.

These findings have also practical consequences for XPS analyses. In Figure 4, oxygen concentration depth profiles CO(h) obtained from TiN films vented at 29 °C and 430 °C are shown as a function of sputter time. Interestingly, CO(h) profile of the latter sample saturates at ∼14 at. %, which is 3× higher concentration than that obtained for the TiN film vented at 29 °C. Clearly, a relatively thick TiO2 layer formed during the high temperature vent prevents effective in situ cleaning presumably due to forward O implantation and re-deposition with negative effects on Ti 2p and N 1s spectra quality (as observed for these samples, but not shown here).

FIG. 4.

A comparison of XPS oxygen concentration depth profiles CO(h) obtained from air-exposed polycrystalline TiN/Si(001) films following the vent at 29 °C or 430 °C.

FIG. 4.

A comparison of XPS oxygen concentration depth profiles CO(h) obtained from air-exposed polycrystalline TiN/Si(001) films following the vent at 29 °C or 430 °C.

Close modal

In conclusion, by employing the Al-cap technique for the TiN model materials system, we were able to separate the surface chemistry effects of residual gas exposure in the high-vacuum environment during the post-deposition phase from those introduced by the following venting sequence. XPS analyses performed as a function of venting temperature Tv reveal that the majority of surface reaction products, including TiO2, TiOxNy, and N2 previously detected after prolonged anneal experiments, form shortly after vent, provided Tv is high enough. These results unequivocally demonstrate that Tv defines the surface chemistry of TiN layers and should therefore routinely be recorded and reported together with other processing conditions for such experiments and production. It is reasonable to assume that findings reported here are also relevant for other transition metal nitrides, as well as for compounds that form reaction products with gases contained in the atmosphere. The surface characterization by XPS is also affected since the reference core level spectra obtained from as-received films exhibit large dependence on Tv. In addition, the ability to obtain clean oxide-free surfaces by in situ sputter etching decreases with the increasing venting temperature.

The authors most gratefully acknowledge the financial support of the German Research Foundation (DFG) within SFB-TR 87, the VINN Excellence Center Functional Nanoscale Materials (FunMat) Grant No. 2005-02666, the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant No. SFO-Mat-LiU 2009-00971), and the Knut and Alice Wallenberg Foundation Grant No. 2011.0143. We thank Dr. Jens Jensen for help with RBS and ToF–E ERDA measurements.

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