SiO2 Interface by

Subscriber access provided by Nottingham Trent University

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Inhibition of Oxygen Scavenging by TiN at the TiN/SiO2 Interface by Atomic-Layer Deposited Al2O3 Protective Interlayer Elena O. Filatova, Sergei S. Sakhonenkov, Aleksei S. Konashuk, Sergey A. Kasatikov, and Valeri Vasilievich Afanas'ev J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05800 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Inhibition of Oxygen Scavenging by TiN at the TiN/SiO2 Interface by Atomic-Layer Deposited Al2O3 Protective Interlayer Elena O. Filatova1*, Sergei S. Sakhonenkov1, Aleksei S. Konashuk1, Sergey A. Kasatikov1 and Valeri V. Afanas’ev2 1Institute

of Physics, St-Petersburg State University, Ulyanovskaya Str. 1, Peterhof 198504, St.

Petersburg, Russia 2Department

of Physics, University of Leuven, Celestijnenlaan 200D, 3001, Leuven, Belgium.

Abstract

Chemical composition of interfaces between physical-vapor deposited TiN and SiO2 as affected by introduction of a thin (0.5 – 3 nm) alumina interlayer was studied using photoelectron spectroscopy with high kinetic energies of photoelectrons (HAXPES) and near edge X-ray absorption fine structure (NEXAFS). Our results reveal formation of TiO2 and titanium oxynitride phases both at the bottom interface of the TiN film and at its surface due to oxygen scavenging from the SiO2 and oxidation in air, respectively. Insertion of alumina layer as thin as units of nm prevents the TiO2 growth at the bottom TiN/SiO2 interface but leads to formation of aluminosilicate layer. The thickness of this silicate layer practically independent on the thickness

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 53

of Al2O3. Presumably, the observed formation of SiOx (x0.8).24 The oxynitride peaks at 457.3 eV and 456.2 eV were introduced according to ref.

33

The photoelectron peak at 458.8 eV is assigned to Ti4+

chemical state in TiOx phase.34 We did not introduced separately the lowest oxidation states of titanium in TiOx phase since their energy positions coincide with those of Ti oxynitride peaks.33,35 In the framework of this decomposition, the peaks originating from the Ti oxynitride provide information about all the intermediate chemical states of titanium atoms: both oxynitride states and lower oxidation states in TiOx phase. In order to estimate the thickness of the chemically different layers in the studied TiN/SiO2/Si stack, we used the approach described in more details in our previous reports.35,36 Using the decomposition analysis of Ti 2p spectra, a complex multiphase structure of the 10 nm TiN electrode related to its oxidation at both top and bottom interfaces can be represented as TiO2/TiNxOy/TiN/TiNxOy/TiO2/SiO2 multi-layer entity. In this model we did not take into account the formation of SiOx layer at the TiN/SiO2 interface due to absence of the corresponding Si2p


11


Page 12 of 53

signal components in the spectra measured at electron emission angles larger than 30⁰. All contributions stemming from silicon states were considered as a substrate signal: SiOx+SiO2+Si. Each layer was associated with its own photoelectron “fingerprint”: for TiN - peak at 455.1 eV and its shake-up satellite, for SiO2 - Si02p, Si3+2p and Si4+2p, for titanium oxynitride - peaks at 457.3 eV and 456.2 eV, for TiO2 - peak at 458.8 eV. The intensity of each line was normalized to the photoionization cross section,37 atomic concentration,38 inelastic mean free path21 and transmittance function of analyzer. It is worth of adding that in framework of our model the atomic composition of each layer is assumed to be constant across the layer, i.e. the model does not consider the inter-diffusion and the roughness factors assuming an abrupt interface between the layers. Then intensity of a particular line was divided by the sum of intensities of all considered lines that allowed us to estimate its relative contribution to the total intensity.

Figure 3. Experimental (dots) and simulated (solid lines) HAXPES peak intensities (3010 eV excitation) for different electron emission angles using the five-layer model for TiN/SiO2/Si sample shown in the inset. Size of dots corresponds to experimental uncertainty.


12

Page 13 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Using a recurrent formula (1) and the model discussed above the relative contributions of each line were estimated in order to determine the thickness of different constituting layers. The results of the calculations are summarized in Figure 3. As follows from the calculations, TiO2 and titanium oxynitride phases are formed both on the top of TiN electrode with total thickness of 2.8±0.1 nm and at the bottom TiN/SiO2 interface with total thickness of 2.2±0.3 nm. The total thickness of electrode turned out equals to 10.6 nm, which agrees fairly well with the target thickness of 10 nm. From the obtained results one can conclude that both TiO2 and titanium oxynitride were formed at interfaces of the PVD TiN layer.

― 𝑑𝑛

𝐹𝑛(𝜃) = (1 ― 𝑒𝜆𝑛cos (𝜃))

𝑛―1

∏𝑒

― 𝑑𝑖 𝜆𝑖cos (𝜃)

𝑖=0

(1)

Also the TiN/SiO2/Si sample was studied by traditional photoelectron spectroscopy in combination with Ar+ ion sputtering that allows to study separately the oxidation of surface from the atmosphere and interface between TiN and SiO2. Such approach allows to compare the results obtained in the framework of the developed by us model in HAXPES analysis with the results of direct measurements.


13


Page 14 of 53

Figure 4. Experimental and fitted photoelectron spectra collected from TiN/SiO2/Si stack before and after different Ar+ ion sputtering steps measured at the excitation energy of 1486.6 eV (Al Kα). Panels (a) and (c) show Ti2p photoelectron spectra; panels (b) and (d) show the O1s photoelectron spectra. Panels (c) and (d) show the Ti2p and O1s photoelectron spectra after 1080 s of Ar+ ion sputtering. Figure 4 shows the photoelectron spectra obtained at the excitation energy of 1486.6 eV (Al Kα) eV from the pristine TiN/SiO2/Si stack and those observed after different sputtering steps. Before sputtering, analysis of the Ti2p spectrum reveals the presence of a double peak structure with spin–


14

Page 15 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


orbit splitting of 5.6 eV and the intensity ratio of 2p3/2 to 2p1/2 components corresponding to the expected 2:1 that is a characteristic of TiO2 phase. Also a shoulder with complex form including two inflection points, located at the lower binding energies is revealed in the spectrum. It should be noted that the energy position of the inflection points coincides with the energy position of the lines belonging to the TiN and titanium oxynitride phases. During sputtering the TiO2 Ti2p doublet peak becomes weaker, ultimately disappears after 480 s of sputtering and reappears after 1080s. At the same time the peak originated from TiN phase becomes detectable in the spectrum. The intensity of this peak increases significantly as etched. Moreover, after 180s of etching the redistribution between intensity of TiN and titanium oxynitride lines is traced: the intensity assigned to titanium oxynitride phase is greatly increased. The presence of the TiN phase is traced in the form of asymmetry of the line from the higher binding energies. The deconvolution of the experimental Ti2p spectrum obtained after 1080 s of sputtering reveals a presence of both phases in the spectrum: an intense line originated from TiN phase and significantly lower intensity line associated with titanium oxynitride phase. Analysis of the O1s photoelectron spectra confirms the all discerned regularities. One should pay a special attention to O1s photoelectron spectrum measured after 1080 s etching. This spectrum consists of main line assigned to the SiO2 layer and small contribution from TiO2 and titanium oxynitride phases. This observation suggests the existence of interlayer at the interface between TiN and SiO2 layer composed of titanium oxynitride and TiO2 phase. Finally we can conclude that the developed by us approach, which is based on the analysis of HAXPES spectra, allows one to properly separate information about the oxidation of the surface from atmosphere and the hidden interface, and thus, to establish the intrinsic composition and extension of the interface.


15


Page 16 of 53

TiN/Al2O3/SiO2/Si stacks Next, to progress towards the interface control, we studied the influence of the ALD-grown Al2O3 IL between the TiN electrode and SiO2 underlayer in the TiN/Al2O3/SiO2/Si stack as a function of the Al2O3 thickness. The main goal was to reveal the effect of the oxygen bonding strength in the IL of different thickness on the TiN oxidation at the bottom interface because it might be expected that the Al2O3 IL will limit oxygen re-distribution and formation of vacancies.39

Figure 5 compares Al 2s and Si 2p photoelectron spectra measured using the excitation photon energy of 3010 eV at electron emission angle of 5⁰ from the TiN/Al2O3/SiO2/Si stacks with Al2O3 IL of different thickness inserted between the TiN and SiO2 layers. Also decomposition of the Al 2s and Si 2p photoelectron spectra is shown. Analysis of the energy of Al 2s photoelectron lines from different samples reveals that they are positioned at ≈119.8±0.3 eV corresponding to amorphous Al2O3 phase.40–42 The insignificant change of binding energy for Al 2s line observed in some stacks can be explained by variation of density of alumina.40 As can be seen from the Figure 5, the increasing the Al2O3 layer thickness appears to enhance the Al 2s peak intensity. Noteworthy that FWHM of Al 2s line of pure Al2O3 is about 1.8 eV42 thus a fairly large width of Al 2s photoelectron lines of studied samples (≈2.3 eV) can be explained by interaction of alumina and silicon dioxide layers and/or interface charges.


16

Page 17 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Experimental Al 2s and Si 2p photoelectron spectra collected from TiN/Al2O3/SiO2/Si stacks with different Al2O3 layer thickness at the excitation energy of 3010 eV and electron emission angle of 5° and their spectral decomposition. Spectral decomposition of the Al 2s emission indicates the presence of two components with energy splitting of ≈ 0.6 eV which is unaffected by the thickness of Al2O3 IL. According to refs.40,43 the component at higher binding energy can be assigned to amorphous Al2O3, whereas the peak at lower binding energy originates from the alumosilicate (Al-O-Si) layer formed as a result of interaction between Al2O3 and SiO2 layers. As already discussed above, the Si 2s spectrum from the reference TiN/SiO2/Si stack (cf. Figure 1a) indicates the presence of Si4+2s, Si3+2s and Si02s components stemming from SiO2, SiOx and


17


Page 18 of 53

the Si substrate, respectively. The insertion of Al2O3 IL (regardless of its thickness) results in appearance in the Si 2p spectrum of an additional component associated with silicate layer Al-OSi. The thickness of aluminosilicate layer practically independent on the thickness of Al2O3 layer. Using recurrence formula (1), the thicknesses of layers constituting the TiN/Al2O3(X nm)/SiO2/Si (X = 0, 0.5, 2, 3 nm) stacks were estimated as listed in Table 1. Analysis of the obtained results points to the absence (within the experimental sensitivity limit) of TiO2 phase at the bottom TiN/SiO2 interface in the samples with the inserted alumina layers of 2 and 3 nm thickness. Also one can see the same surface oxidation of all the studied samples from the atmosphere. This result correlates with results of XPS measurements at 1486.6 eV excitation photon energy. In this analysis Ti2p, N1s and O1s photoelectron spectra were almost indistinguishable for all the studied samples.

TiO2 (nm±0.1)

1,6

1,8

1,7

1,7

TiNxOy (nm±0.1)

1,2

1,2

1,2

1,2

TiN (nm±0.3)

5,6

5,6

5,9

5,9

TiNxOy (nm±0.3)

0,6

1

1,5

1,6

TiO2 (nm±0.4)

1,6

1,1

-

-

Al2O3 (nm)

0

0,5

2

3

Table 1. The inferred constituting layer thickness in samples with different thickness of Al2O3 ILs Therefore, one can assert with high degree of reliability that the oxidation of TiN is inhibited, the oxidized TiN interlayer thickness significantly decreases and its chemical composition turn to be close to titanium oxynitride. The presented results indicate that insertion of a thin Al2O3 IL between TiN and SiO2 inhibits the TiN bottom interface oxidation, i.e., prevents formation of the TiO2 layer. At the same time an aluminosilicate layer is formed at the interface Al2O3/SiO2.


18

Page 19 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Apparently then, a thin aluminosilicate film formed at the interface appears to be more thermodynamically stable than SiO2 and inhibits the oxygen scavenging effect responsible for the oxidation of TiN at the bottom interface.

NEXAFS studies The probing depth in the total electron emission yield method for titanium nitride is approximately 10 nm,44,45 i.e., it is comparable to the thickness of the TiN layer. This allows us to reliably detect signals from the whole electrode and to probe the interface between TiN and SiO2 layers applying the NEXAFS spectroscopy. Figure 6 shows NK, TiL2,3, OK absorption spectra of the TiN/SiO2 stack recorded using the total electron emission yield quantified by measuring electrical current from the sample.


19


Page 20 of 53

Figure 6. X-ray absorption spectra of the TiN/SiO2/Si stack measured in the vicinity of (a) Kabsorption edge of nitrogen; (b) L2,3- absorption edge of titanium; (c) K- absorption edge of oxygen. The red curve at panel (a) shows the model NK-absorption spectrum without contribution of molecular nitrogen peak (i.e., without peak Q). The NK - absorption spectrum of the TiN/SiO2 stack is presented in the Figure 6a. Taking into account that each N atom in TiN is six-fold coordinated, the DOS calculations based on the localized spherical wave (LSW) method with an extended basis set predict that the NK -absorption spectrum of TiN consists of the doublet structure (features a-b) at absorption threshold followed by a broad structure c-d above it. The first region a-b is attributed to the unoccupied N 2p states, which mix with the Ti 3d bands split into the t2g and eg sub-bands by the crystal field.46,47 The wider region c-d is attributed to unoccupied N 2p states, which are mixed with Ti4sp bands. This region, in particular the shoulder c, is sensitive to the long range order.48 Analysis of the measured N K-spectrum shown in Figure 6a indicates that the energy splitting between features a and b related to the ligand field equals to ≈2.6 eV, which is different from the known value for the stoichiometric titanium nitride (Δa-b ≈ 2.3eV).49 There is a sharp a peak Q (at around 401.5 eV) in the region of the eg subband. Similar narrow peak has been found in the earlier works27,50 and was associated with formation of unbonded nitrogen dissolved in the TiN matrix caused the (partial) substitution of nitrogen by oxygen upon TiN oxidation. This conclusion is supported by HAXPES studies discussed in the previous section pointing towards formation of unbonded nitrogen inside the titanium oxynitride layers. From these observations, we can conclude that formation of unbonded nitrogen is promoted by: i) the oxygen scavenging from the SiO2 film by chemically active metal Ti; and ii) oxidation of the outer TiN surface in air. It is plausible to assume that some nitrogen atoms in the octahedral surrounding of the Ti atom have been replaced by oxygen atoms.


20

Page 21 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The TiL2,3 absorption edge spectra of the analyzed TiN/SiO2 system are shown in Figure 6b. According to the classical concept, the NEXAFS excitation at the Ti 2p threshold in TiN should reflect the energies of the unoccupied Ti 3d states because it is dominated by the 2p → 3d dipole transitions in the Ti atoms.14 The measured Ti L2,3 absorption spectrum clearly reflects the spinorbit splitting of the initial Ti 2p state and the corresponding Ti 2p1/2 structures are marked by asterisks in Figure 6b. It is well known that the L2 absorption spectra is misrepresented by an additional damping channel caused by the L2L3V- Coster-Kronig transition,51 which leads to a shorter life time of the L2-holes. Due to this reason, it makes sense to discuss only the L3-absorption spectrum. The features a and b in Ti L3 spectrum of TiN stem from the allowed dipole transitions of Ti 2p3/2 electrons to the unoccupied 3d states split into 3dt2g (peak a) and 3deg (peak b) components in the octahedral surrounding. According to ref.,50 the ligand field splitting effect is weakly expressed (almost absent) in materials with metallic character such as, for example, metallic Ti or c-TiN. According to52,53 in the spectrum of pure TiO2 the splitting of a and b features is about 1.7 eV – 2.2 eV, in dependence on crystal structure and ratio of x and y in TixOy. In the Ti L2,3 spectrum of TiN/SiO2 system splitting Δa-b is about 1.7 eV. This observation suggests that some nitrogen atoms in the octahedral surrounding of the Ti cations have been replaced by oxygen atoms. The OK absorption spectrum shown in the Figure 6c further confirms oxidation of the TiN electrode. The spectrum consists of structured band a-b and broad band c-d. Analysis of the shape and the energy position of main features of the spectrum allows one to notice its qualitative similarity to the corresponding spectrum of TiO2.52 Then it is plausible to conclude that the features a and b are related to the Ti 3d (t2g - eg) states mixed with the O 2p states while the wide band c-d can be attributed to the O2p states mixed with the Ti 4sp bands.52 Therefore, NK-, TiL2,3 and OK-


21


Page 22 of 53

absorption spectra of TiN/SiO2 consistently point towards the oxidation of the titanium nitride electrode.

Figure 7. OK-absorption spectra of TiN/SiO2/Si, TiN/Al2O3(3nm)/SiO2/Si stacks and reference spectra of TiNxOy measured in ref.,54 amorphous TiO2 and amorphous Al2O3. The spectra were measured in total electron yield mode and are normalized to continuum edge jump. In order to understand the role of the Al2O3 layer in the process of the interface formation let us consider in more detail the OK-absorption spectra of the TiN/SiO2/Si and TiN/Al2O3(3 nm)/SiO2/Si stacks (Figure 7). Also spectra of reference materials are shown in Figure 7. Comparing the spectra of TiN/SiO2/Si and TiN/Al2O3(3 nm)/SiO2/Si stacks with that of TiO2 one can trace gradual merger of features a and b and a reduction of distance between a and d features for studied stacks (1,5 eV smaller for TiN/SiO2/Si stack relative that of TiO2). According to refs.54– 56

decrease of the distance between a and d features is related to lowering of oxidation state of

titanium. TiO2 has a maximal a-d distance, which is equal to 13 eV52,53,55,56 that is also the case for our reference TiO2. Consideration of the shape of c-d band and the value of a-d distance allows one to assume that main oxidation states for TiN/SiO2/Si and TiN/Al2O3(3 nm)/SiO2/Si stacks are


22

Page 23 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ti2+/Ti3+ with negligible contribution of Ti4+. Analysis of intensities of a-b and c-d bands further confirms this assumption: the spectra of both stacks are characterized by noticeably smaller a-b band relative c-d one in comparison with the spectrum of reference TiO2. As it follows from refs.,55,57 relative intensity of a-b band reflects an occupation of 3d valence states of transition metal. In case of titanium compounds lowering of Ti oxidation state corresponds to filling the Ti3d states and accordingly to reduction of a-b band relative intensity. One can see from the Figure 7 that the spectra of TiN/SiO2/Si stack and reference titanium oxynitride54 has a close similarity in the relative intensities of a-b and c-d bands and the a-d distance. In view of this and above mentioned we can say that oxidation of TiN layer proceeds mainly through formation of titanium oxynitride but one can’t exclude some lesser contribution of oxide with Ti4+ state regarding peaks a and b emerging at positions specific for TiO2. Let’s compare a shape of the spectra of TiN/SiO2/Si and TiN/Al2O3(3 nm)/SiO2/Si stacks, which is clearly different. Notice that accordingly to analysis of surface sensitive XPS, a surface oxidation of all the studied samples is nearly the same. This means that the difference in the shape of NEXAFS spectra of TiN/SiO2/Si and TiN/Al2O3(3 nm)/SiO2/Si stacks should reflect exactly difference in the oxidation process at the interface of the TiN with underlying oxide. Regarding the spectrum of TiN/Al2O3(3 nm)/SiO2/Si stack we can say that insertion of 3 nm Al2O3 interlayer results in lesser oxidation of TiN at the interface. Indeed the intensity of a-b band, which doesn’t overlap with absorption bands of underlying Al2O3 and SiO2, is noticeably reduced relative continuum edge jump. Moreover the contribution of the absorption spectrum of underlying Al2O3 is increased against the TiN/SiO2/Si spectrum as one can see from increased intensity of shoulder c. Also distinction of peaks a and b


23


Page 24 of 53

becomes almost imperceptible that could point out reduction of contribution of titanium oxide phase at the TiN/Al2O3 interface.

Figure 8. OK-absorption spectra of TiN/Al2O3(3nm)/SiO2/Si stack measured in total electron emission yield (TEY) and fluorescence (FY) modes and reference spectra of amorphous SiO2 and Al2O3 (a). Comparison of OK-absorption spectra of TiN/Al2O3(3nm)/SiO2/Si stack measured in FY mode and simulated one by linear combination of that measured in TEY and reference spectra of am-SiO2 and am-Al2O3 (b). OK-absorption spectra of TiN/Al2O3(3nm)/SiO2/Si sample were measured both in total electron emission yield and fluorescence yield modes, which are presented in Figure 8a. The probing depth of FY mode spectrum several times exceeds the thickness of top TiN layer that means TiN/Al2O3 interface oxidation contribution is undoubtedly well resolved in FY spectrum. Due to this reason a linear combination simulation of FY spectrum was performed: the TEY spectrum of the same sample was summed with the spectra of am-SiO2 and am-Al2O3 with different weights (all the spectra were normalized to continuum edge jump). Result of simulation is shown in Figure 8b. Weight coefficients are summarized in table 2. One can see that simulated curve rather well mimics


24

Page 25 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the spectrum measured in FY mode. This means that TiN/Al2O3 interface oxidation contribution is well resolved also in TEY mode spectrum, not only in FY mode one. The same linear combination procedure was performed for TiN/SiO2/Si sample. The simulated curve also well mimics corresponding spectrum measured in FY mode. It is indispensable to compare weight coefficients obtained for TiN/SiO2/Si and TiN/Al2O3(3nm)/SiO2/Si stack. As follows from the Table 2 the contribution of TEY mode spectrum of oxidized TiN is much smaller for TiN/Al2O3(3nm)/SiO2/Si stack that additionally confirms preventing oxidation of TiN by insertion of Al2O3 layer.

TiN/SiO2/Si

TEY mode spectrum am-Al2O3

am-SiO2

0.23

-

0.77

0.21

0.64

TiN/Al2O3(3nm)/SiO2/Si 0.15

Table 2. Weight coefficients of OK-absorption spectra (normalized to continuum edge jump) of reference am-Al2O3, am-SiO2 and TEY mode spectra of corresponding TiN/SiO2/Si and TiN/Al2O3(3nm)/SiO2/Si stacks in a linear combination simulation of FY mode OK-absorption spectra of TiN/SiO2/Si and TiN/Al2O3(3nm)/SiO2/Si stacks. Influence of the TiN synthesis method on interface structure Though the above results leave little doubt that the bottom TiN interface with oxide insulators appears to be oxidized due to the oxygen scavenging effect and the extent of this oxidation can be influenced by the ALD-grown Al2O3 IL, it is also worth of addressing the impact of the TiN deposition route on the degree of this chemical interaction. To understand this impact we studied additionally TiN/SiO2 stack with TiN layer synthesized on top of SiO2 by ALD from TiCl4 and NH3 at 300º C.


25


Page 26 of 53

Figure 9. Comparison of Ti2p photoelectron spectra of ALD-TiN/SiO2 (red curve) and PVDTiN/SiO2 stacks. Figure 9 compares the Ti2p photoelectron spectra of 10-nm thick TiN electrodes synthesized by ALD or PVD methods as measured at the same excitation photon energy of 3010 eV and electron emission angle of 5°. In the ALD-grown sample, the analysis of the line shapes and intensity suggests the significant increase the titanium oxynitride contribution (shoulder a) while the TiO2 peak loses intensity and shifts towards lower binding energies as compared to the PVD-grown TiN. This decrease in the binding energy of the Ti 2p component in TiO2 points towards formation of TiOx phase with x less than 2. The layer thicknesses evaluation using multi-layer model yields the following structure of the ALD-TiN/SiO2 stack: TiOx(1.4 nm)/TiNxOy(1.3 nm)/TiN(7.2 nm)/TiNxOy(0.8 nm), i. e., it contains no sign of the TiO2 phase found in the PVD-TiN/SiO2 stack. Therefore, we may conclude that ALD allows one to avoid formation of TiO2 layer at the interface between TiN and SiO2 while resulting in a more extended titanium oxynitride layer as compared to the PVD-TiN/SiO2 system. We believe that this kind of differences in the interface chemical


26

Page 27 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


structure may account for the reported sensitivity of the TiN EWF to the metal synthesis technology.10 Conclusions The presented results indicate that the TiN layer in the PVD-grown TiN/SiO2/Si stack represents a multilayered system, in which the formation of both the TiO2 and titanium oxynitride layers occurs at both sides of the TiN layer due to different processes: The oxygen scavenging from the underlying SiO2 film at bottom TiN/SiO2 interface and oxidation of the outer TiN surface in air. Insertion of a thin ALD-grown Al2O3 IL between TiN and SiO2 allows one to partially inhibit O scavenging and prevents formation of TiO2 layer at the TiN/SiO2 interface which can be explained by formation of aluminosilicate layer acting as a diffusion barrier. The thickness of this silicate layer is practically independent on the IL thickness (2 nm and 3 nm). Analysis of the ALD-grown TiN on top of SiO2 reveals that ALD method prevents the formation of TiO2 at the interface between TiN and SiO2 while resulting in a more extended titanium oxynitride IL as compared to the PVD-TiN/SiO2 system.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] Phone: +7 (812) 428 43 52 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


27


Page 28 of 53

ACKNOWLEDGMENT The work was partially supported by RSF grant 19-72-20125. We gratefully acknowledge the financial support by Helmholtz Zentrum Berlin (HZB) and also thank HZB and Elettra for the allocation of synchrotron radiation beamtimes. The authors acknowledge the Resource Centers of the Research Park of St. Petersburg State University “Physical methods of surface investigation” and "Interdisciplinary Resource Centre for Nanotechnology". REFERENCES (1)

IEEE. The International Roadmap for Devices and Systems 2017 Edition Executive Summary. IRDS 2017.

(2)

Patra, B.; Incandela, R. M.; Van Dijk, J. P. G.; Homulle, H. A. R.; Song, L.; Shahmohammadi, M.; Staszewski, R. B.; Vladimirescu, A.; Babaie, M.; Sebastiano, F.; et al. Cryo-CMOS Circuits and Systems for Quantum Computing Applications. IEEE J. SolidState Circuits 2018, 53, 309–321.

(3)

Kothari, S.; Vaidya, D.; Nejad, H.; Variam, N.; Ganguly, S.; Lodha, S. Plasma-Assisted As Implants for Effective Work Function Modulation of TiN/HfO2 Gate Stacks on Germanium. Appl. Phys. Lett. 2018, 112, 203503.

(4)

Narasimha, S.; Jagannathan, B.; Ogino, A.; Jaeger, D.; Greene, B.; Sheraw, C.; Zhao, K.; Haran, B.; Kwon, U.; Mahalingam, A. K. M.; et al. A 7 nm CMOS Technology Platform for Mobile and High Performance Compute Application. In 2017 IEEE International Electron Devices Meeting (IEDM); IEEE, 2017; pp 29.5.1-29.5.4.

(5)

Ragnarsson, L.-A.; Dekkers, H.; Matagne, P.; Schram, T.; Conard, T.; Horiguchi, N.;


28

Page 29 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Thean, A. V.-Y. Zero-Thickness Multi Work Function Solutions for N7 Bulk FinFETs. In 2016 IEEE Symposium on VLSI Technology; IEEE, 2016; pp 1–2. (6)

Hussain, M. M.; Smith, C. E.; Harris, H. R.; Young, C. D.; Tseng, H. H.; Jammy, R. GateFirst Integration of Tunable Work Function Metal Gates of Different Thicknesses into HighK Metal Gates CMOS FinFETs for Multi- VTh Engineering. IEEE Trans. Electron Devices 2010, 57, 626–631.

(7)

Collaert, N.; De Keersgieter, A.; Dixit, A.; Ferain, I.; Lai, L.-S.; Lenoble, D.; Mercha, A.; Nackaerts, A.; Pawlak, B. J.; Rooyackers, R.; et al. Multi-Gate Devices for the 32nm Technology Node and Beyond. In ESSDERC 2007 - 37th European Solid State Device Research Conference; IEEE, 2007; pp 143–146.

(8)

Seo, K.-I.; Haran, B.; Gupta, D.; Guo, D.; Standaert, T.; Xie, R.; Shang, H.; Alptekin, E.; Bae, D.-I.; Bae, G.; et al. A 10 nm Platform Technology for Low Power and High Performance Application Featuring FINFET Devices with Multi Workfunction Gate Stack on Bulk and SOI. In 2014 Symposium on VLSI Technology (VLSI-Technology): Digest of Technical Papers; IEEE, 2014; pp 1–2.

(9)

Lima, L. P. B.; Dekkers, H. F. W.; Lisoni, J. G.; Diniz, J. A.; Van Elshocht, S.; De Gendt, S. Metal Gate Work Function Tuning by Al Incorporation in TiN. J. Appl. Phys. 2014, 115, 074504.

(10)

Afanas’ev, V.; Kolomiiets, N.; Houssa, M.; Stesmans, A. Internal Photoemission Metrology of Inhomogeneous Interface Barriers. Phys. Status Solidi Appl. Mater. Sci. 2018, 215, 1– 12.


29


(11)

Page 30 of 53

Filatova, E. O.; Sakhonenkov, S. S.; Konashuk, A. S.; Afanas’ev, V. V. Control of TiN Oxidation upon Atomic Layer Deposition of Oxides. Phys. Chem. Chem. Phys. 2018, 20, 27975–27982.

(12)

Filatova, E. O.; Konashuk, A. S.; Sakhonenkov, S. S.; Sokolov, A. A.; Afanas’ev, V. V. ReDistribution of Oxygen at the Interface Between γ-Al2O3 and TiN. Sci. Rep. 2017, 7, 4541.

(13)

Schäfers, F.; Sokolov, A. The At-Wavelength Metrology Facility at BESSY-II. J. LargeScale Res. Facil. 2016, 2, A50.

(14)

Fano, U.; Cooper, J. W. Spectral Distribution of Atomic Oscillator Strengths. Rev. Mod. Phys. 1968, 40, 441–507.

(15)

Konashuk, A. S.; Filatova, E. O. Redistribution of Valence and Conduction Band States Depending on the Method of Modification of SiO2 Structure. Phys. Chem. Chem. Phys. 2017, 19, 26201-26209.

(16)

Gorgoi, M.; Svensson, S.; Schäfers, F.; Öhrwall, G.; Mertin, M.; Bressler, P.; Karis, O.; Siegbahn, H.; Sandell, A.; Rensmo, H.; et al. The High Kinetic Energy Photoelectron Spectroscopy Facility at BESSY Progress and First Results. Nucl. Instruments Methods Phys. Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip. 2009, 601, 48–53.

(17)

Schaefers, F.; Mertin, M.; Gorgoi, M. KMC-1: A High Resolution and High Flux Soft XRay Beamline at BESSY. Rev. Sci. Instrum. 2007, 78, 123102.

(18)

J. F. Moulder, J. Chastain, and R. C. K. Handbook of X-Ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data; Physical Electronics: Eden Prarie, United States of America, 1995.


30

Page 31 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19)

F. J. Himpsel, F. R. McFeely, A. Taleb-Ibrahimi, J. A. Y. Microscopic Structure of the SiO2/Si Interface. Phys. Rev. B 1988, 38, 6084-6096.

(20)

Gries, W. H. A Universal Predictive Equation for the Inelastic Mean Free Path Lengths of X-Ray Photoelectrons and Auger Electrons. Surf. Interface Anal. 1996, 24, 38–50.

(21)

Tanuma, S.; Powell, C. J.; Penn, D. R. Calculation of Electron Inelastic Mean Free Paths (IMFPs) VII. Reliability of the TPP-2M IMFP Predictive Equation. Surf. Interface Anal. 2003, 35, 268–275.

(22)

Seah, M.; Dench, W. Quantitative Electron Spectroscopy of Surfaces: A Standard Data Base for Electron Inelastic Mean Free Paths in Solids. Surf. Interface Anal. 1979, 1, 2-11.

(23)

Ghita, R.; Logofatu, C.; Negrila, C.-C.; Ungureanu, F.; Cotirlan, C.; Manea, A.-S.; Lazarescu, M.-F.; Ghic, C. Study of SiO2/Si Interface by Surface Techniques. In Crystalline Silicon - Properties and Uses; 2011.

(24)

Jaeger, D.; Patscheider, J. A Complete and Self-Consistent Evaluation of XPS Spectra of TiN. J. Electron Spectros. Relat. Phenomena 2012, 185, 523–534.

(25)

Milošv, I.; Strehblow, H. ‐H; Navinšek, B.; Metikoš‐Huković, M. Electrochemical and Thermal Oxidation of TiN Coatings Studied by XPS. Surf. Interface Anal. 1995, 23, 529– 539.

(26)

Glaser, A.; Surnev, S.; Netzer, F. P.; Fateh, N.; Fontalvo, G. A.; Mitterer, C. Oxidation of Vanadium Nitride and Titanium Nitride Coatings. Surf. Sci. 2007, 601, 1153–1159.

(27)

Esaka, F.; Furuya, K.; Shimada, H.; Imamura, M.; Matsubayashi, N.; Sato, H.; Nishijima, A.; Kawana, A.; Ichimura, H.; Kikuchi, T. Comparison of Surface Oxidation of Titanium


31


Page 32 of 53

Nitride and Chromium Nitride Films Studied by X-Ray Absorption and Photoelectron Spectroscopy. J. Vac. Sci. Technol. A Vacuum, Surfaces, Film. 1997, 15, 2521–2528. (28)

Bertóti, I.; Mohai, M.; Sullivan, J. L.; Saied, S. O. Surface Characterisation of PlasmaNitrided Titanium: An XPS Study. Appl. Surf. Sci. 1995, 84, 357–371.

(29)

Bruninx, E.; Van Eenbergen, A. F. P. M.; Van Der Werf, P.; Haisma, J. X-Ray Photoelectron Spectroscopy of Hafnium Nitride. J. Mater. Sci. 1986, 21, 541–546.

(30)

Tougaard, S. Universality Classes of Inelastic Electron Scattering Cross-Sections. Surf. Interface Anal. 1997, 25, 137–154.

(31)

Guha, S.; Narayanan, V. Oxygen Vacancies in High Dielectric Constant OxideSemiconductor Films. Phys. Rev. Lett. 2007, 98, 196101–196104.

(32)

Porte, L.; Roux, L.; Hanus, J. Vacancy Effects in the X-Ray Photoelectron Spectra of TiNx. Phys. Rev. B 1983, 28, 3214–3224.

(33)

Pulsipher, D. J. V.; Martin, I. T.; Fisher, E. R. Controlled Nitrogen Doping and Film Colorimetrics in Porous TiO2 Materials Using Plasma Processing. ACS Appl. Mater. Interfaces 2010, 2, 1743–1753.

(34)

Saha, N. C.; Tompkins, H. G. Titanium Nitride Oxidation Chemistry: An X-Ray Photoelectron Spectroscopy Study. J. Appl. Phys. 1992, 72, 3072–3079.

(35)

Filatova, E. O.; Kozhevnikov, I. V.; Sokolov, A. A.; Ubyivovk, E. V.; Yulin, S.; Gorgoi, M.; Schäfers, F. Soft X-Ray Reflectometry, Hard X-Ray Photoelectron Spectroscopy and Transmission Electron Microscopy Investigations of the Internal Structure of TiO2(Ti)/SiO2/Si Stacks. Sci. Technol. Adv. Mater. 2012, 13, 15001.


32

Page 33 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(36)

Filatova, E. O.; Sokolov, A. A.; Kozhevnikov, I. V. Characterization of High-K Dielectric Internal Structure by X-Ray Spectroscopy and Reflectometry: New Approaches to Interlayer Identification and Analysis. In High-k Gate Dielectrics for CMOS Technology; 2012; pp 225–271.

(37)

Yeh, J. J.; Lindau, I. Atomic Subshell Photoionization Cross Sections and Asymmetry Parameters: 1

SiO2 Interface by

Recommend Documents