Oxidation and wet etching behavior of sputtered Mo-Ti-Al filmsTanja Jörg, Anna M. Hofer, Harald Köstenbauer, Jörg Winkler, and Christian Mitterer

Citation: Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 36, 021513 (2018);View online: https://doi.org/10.1116/1.5009289View Table of Contents: http://avs.scitation.org/toc/jva/36/2Published by the American Vacuum Society

Oxidation and wet etching behavior of sputtered Mo-Ti-Al films

Tanja J€orga) and Anna M. HoferDepartment of Physical Metallurgy and Materials Testing, Montanuniversit€at Leoben, Franz-Josef-Straße 18,8700 Leoben, Austria

Harald K€ostenbauer and J€org WinklerBusiness Unit Coating, PLANSEE SE, Metallwerk-Plansee-Straße 71, 6600 Reutte, Austria

Christian MittererDepartment of Physical Metallurgy and Materials Testing, Montanuniversit€at Leoben, Franz-Josef-Straße 18,8700 Leoben, Austria

(Received 14 October 2017; accepted 9 January 2018; published 24 January 2018)

Exposure of Mo thin films above 300 �C in ambient atmosphere results in surface oxidation,

leading to the formation of colored oxide films deteriorating their performance in thin film

transistor liquid crystal displays. In this study, the influence of the alloying elements Ti and Al

on microstructure, electrical properties, oxidation and wet etching behavior of Mo thin films

was investigated. Mo1�x�yTixAly films (with x� 0.08 and 0� y� 0.24) were deposited by direct

current magnetron sputtering and annealed in ambient air at 330 �C for 1 h. The oxidation

resistance of Mo-Ti-Al films was enhanced with increasing Al content. A minimum Al content of

y¼ 0.16 was necessary to form a thin protective Al2O3 surface layer and to prevent the formation

of colored molybdenum oxides. The wet etching rate of the Mo-Ti-Al films in a commonly used

mixture of phosphoric-, acetic-, and nitric acid decreased with increasing Al content, but was still

acceptable for thin film transistor liquid crystal displays applications. Published by the AVS.https://doi.org/10.1116/1.5009289

I. INTRODUCTION

Sputtered Mo thin films are used in thin film transistor

liquid crystal displays (TFT-LCD) as conducting materials.

Their low electrical resistance and easy chemical patterning

make them an excellent material for gate, source/drain elec-

trodes and signal and data bus-lines.1,2 In addition, Mo thin

films serve as bottom and capping layers in the gate metalli-

zation of TFTs, due to their good adhesion on glass and low

contact resistance to silicon and indium tin oxide.3,4 They

are also utilized as diffusion barrier layer between Cu and Si

in the Cu process, based on their high melting point and very

low diffusion coefficients.5,6 Since annealing treatments are

used in TFT-LCD processing,7 these films have to withstand

thermal exposure without deterioration of their properties.

However, Mo is known to exhibit poor oxidation resistance

in air and is prone to oxidation above 300 �C.8,9 Previous

studies on binary Mo alloys indicated that alloying can be

used to improve the oxidation resistance.10 For example,

Park et al. have reported that Ti is corrosion resistant in oxi-

dizing atmospheres, and that alloying of Ti to Mo leads to

the formation of a passivation layer.11 However, Ti cannot

be dissolved in phosphoric, acetic, nitric acid solutions

(PAN),12 which is a key process step in the production of

TFT materials. In contrast to Ti, Al is known for its superior

oxidation resistance and at the same time its good wet etch-

ing behavior in PAN.12 Thus, the aim of the present work is

to synthesize ternary Mo-Ti-Al thin film alloys by magne-

tron sputter deposition and to study their oxidation resistance

and wet etching ability for varying Al contents.

II. EXPERIMENTAL DETAILS

The films were grown in a laboratory-scale unbalanced

direct current (dc) magnetron sputter system, using one

Mo0.9Ti0.1 (atomic fractions) and one Al target (150.8 mm� 6 mm), which were mounted on two unbalanced

magnetrons (AJA International, Inc.). The substrate material,

boron-silicate glass (Corning EAGLE2000TM AMLCD,

size 50� 50 mm2), and Si (100) were ultrasonically cleaned

in a commercial detergent from Borer Chemie AG or etha-

nol, respectively, and fixed on a rotatable sample holder

positioned about 55 mm above the magnetrons. After reach-

ing a base pressure of less than 2� 10�3 Pa, plasma etching

of the substrates was performed by applying a pulsed dc dis-

charge of �500 V and 250 kHz at an Ar gas pressure of 1 Pa

for 2 min. Depositions were carried out at an Ar pressure of

0.4 Pa, while no intentional substrate heating was used.

Different Al contents within the films were obtained by vary-

ing the dc current on the Al target between 0 and 0.15 A,

whereas the current on the Mo0.9Ti0.1 target was held con-

stant at 0.35 A. Depending on the Al current, the growth rate

varied from 31 to 42 nm/min; hence, the deposition time was

adjusted to obtain a film thickness of �250 nm for all films.

Film thickness measurements were performed by investi-

gating the fracture cross-sections of the films grown on Si

with a scanning electron microscope (SEM, Zeiss Evo-50).

The chemical composition of the films on Si was evaluated

using energy-dispersive X-ray spectroscopy (EDX) with an

Oxford Instruments INCA extension attached to the SEM.

All further investigations were conducted on the films grown

on boron-silicate glass. The crystallographic structure was

characterized by X-ray diffraction (XRD) in h/2h geometrya)Electronic mail: tanja.joerg@stud.unileoben.ac.at

021513-1 J. Vac. Sci. Technol. A 36(2), Mar/Apr 2018 0734-2101/2018/36(2)/021513/5/$30.00 Published by the AVS. 021513-1

using a Bruker-AXS D8 Advance diffractometer equipped

with Cu-Ka radiation and parallel beam optics. The wet etch-

ing rate of the as-deposited films was determined by the

weight difference of the films before and after dipping into

the etchant. The etching solution (PAN) was composed of

66% phosphoric acid, 10% acetic acid and 5% nitric acid by

weight. All wet etching tests were carried out at a stirring

rate of 300 rpm at 40 �C. To evaluate the oxidation behavior

of the films, the as-deposited samples were annealed in a

chamber furnace in air at 330 �C for 1 h. The electrical sheet

resistivity and reflectance UV-Vis spectra of the films were

determined prior and after the annealing step with a Jandel

RM2 four-point probe and a Perkin Elmer Lambda 950

photo-spectrometer, respectively. The Raman spectra of the

annealed films were obtained using a Jobin Yvon LabRam

confocal Raman spectrometer employing a Nd:YAG laser

(k¼ 532 nm, 10 mW). The oxidation states of the films after

annealing were studied by X-ray photoelectron spectroscopy

(XPS) using an Omicron Multiprobe system with a mono-

chromized Al Ka beam (1486.6 eV) and a resolution <0.5 eV.

In order to remove the volatile surface contaminants, the

samples were heated for 20 min at 300 �C before XPS

measurements.

III. RESULTS AND DISCUSSION

Figure 1 illustrates the X-ray diffractograms of all

Mo1�x�yTixAly films in the as-deposited state (x and y in at.

% determined by EDX). It should be noted that the ratio of

Ti:Al in the films almost linearly increases from 1:1 via 1:2

to 1:3 with a corresponding decrease of the Mo content. The

diffraction patterns reveal the formation of a single-phase

body-centered cubic Mo-based solid solution (space group

Im-3m, ICDD 03-065-7442) for all films synthesized. While

(110)-oriented crystallites dominate for all compositions

studied, diffraction of (100)-oriented crystallites enhances

with increasing Al content. The results are consistent with a

previous study by Lorenz et al.,13 who investigated Mo-Al

thin films containing up to 32 at. % Al.

The evolution of the wet etching rate in PAN of the unal-

loyed Mo and Mo1�x�yTixAly films with increasing Al con-

tent is displayed in Fig. 2. It can be seen that for adding

approximately 8 at. % Ti to Mo only slightly decreases the

dissolution rate by 7.4%. For comparison, the wet etching

rate of a Mo-Ti film containing 18 at. % Ti was determined

208 6 2 nm/min. This substantial reduction of the wet etch-

ing rate is attributed to the insolubility of Ti in PAN.12 In

contrast, Al has a higher solubility than Ti, but the incorpora-

tion of Al in Mo still gradually decreases the wet etching

rate with increasing Al content. This is in good agreement to

Kim et al.,14 who reported that Mo exhibits a higher wet

etching rate than Al in PAN and that the contact of the less

noble Al decreases the dissolution rate of the nobler Mo.

Annealing tests in air at 330 �C were performed to investi-

gate the oxidation resistance of the alloyed films compared to

the pure Mo film. It is evident, that the Al content significantly

influences the oxidation behavior of the films. The pure Mo

film [see Fig. 3(a)] exhibits poor resistance to oxidation at

330 �C, leading to the formation of a colored oxide film. A

similar behavior can be observed for the Mo0.92Ti0.08 and

Mo0.92Ti0.08Al0.08 films. However, the ternary Mo1�x�yTixAlyfilms with y> 0.08 [Figs. 3(d)–3(e)] remain colorless after the

heat treatment, retaining their initial mirrorlike appearance,

indicating the formation of a very thin transparent protective

oxide film.

The influence of the annealing treatment on the electrical

resistivity is plotted in Fig. 4 as a function of the Al content.

A pure Mo film (300 nm thick), as reported in our earlier

work,15 has a resistivity of about 10 lX cm. Alloying 8 at.%

Al to the Mo0.92Ti0.08 solid solution causes a sharp rise of

electrical resistivity to approximately 100 lX cm. However,

further Al additions result in a linear increase of the resistiv-

ity. This behavior is in good agreement with the develop-

ment of resistivity against alloying content of solid solutions

and can be explained by the different atom size of the alloy-

ing elements, which lead to an increase of electron scatter-

ing. Furthermore, alloying elements introduce local charge

differences, due to their different valence electrons, which

FIG. 1. (Color online) X-ray diffractograms of as-deposited Mo1�x�yTixAlyfilms.

FIG. 2. (Color online) Wet etching rate in PAN for Mo1�x�yTixAly films

(with x� 0.08 and 0� y� 0.24) plotted over the Al content. Open symbol

denotes the etching rate of the unalloyed Mo film.

021513-2 J€org et al.: Oxidation and wet etching behavior of sputtered Mo-Ti-Al films 021513-2

J. Vac. Sci. Technol. A, Vol. 36, No. 2, Mar/Apr 2018

results in a higher electron scattering probability.16 The

obtained resistivity values are comparable to those found for

Mo-Al alloys13 and are considered acceptable for TFT-LCD

applications.17 No significant difference between the electri-

cal resistivity before and after annealing is observed. While

annealing could be expected to slightly reduce the electrical

resistivity due to annihilation of point defects,18 the forma-

tion of oxide films—assuming that the oxide films are not

destroyed during four-point probe measurement—might

compensated for the defect annihilation.

The reflectance spectra of the Mo0.84Ti0.08Al0.08 and

Mo0.76Ti0.08Al0.16 films in the as-deposited and annealed

state are presented in Fig. 5. Both films show in the as-

deposited state an average reflectance of above 50%, con-

firming the metallic appearance. Moreover, the Al fraction

in the films has a strong influence on the optical properties

of the annealed samples. The reflectance of the Mo film con-

taining 8 at. % Al decreases significantly after annealing and

shows an absorption edge below 400 nm. The strong absorp-

tion in the violet region of the spectrum is responsible for

the yellowish color of the film [see Fig. 3(c)] and indicates

the formation of an oxide layer. However, by increasing

the Al content to 16 at. %, the decrease in reflectivity after

annealing is reduced and the formation of the absorption

edge is prevented. As a result, the optical appearance

remains metallic [Fig. 3(d)].

The oxide films formed during annealing are too thin to

be detected by XRD and in SEM cross-sections. To obtain

more information about the surface oxides, which are

responsible for the thermal coloration of the films, Raman

spectroscopy was performed. Molybdenum oxides are well

suited for characterization by Raman scattering.19 In contrast,

annealing of bulk Al below 350 �C leads to the formation of

an amorphous Al2O3 film,20 which cannot be detected by

Raman spectroscopy.21,22 Figure 6 shows the Raman spectra

of the annealed samples, which are in good agreement with

the results of visual inspection and spectroscopy. The Raman

spectrum of the reference Mo film is characteristic for poly-

crystalline MoO3 bands (995, 818 and 665 cm�1). Thus, the

annealing treatment causes surface oxidation of the unalloyed

Mo film, leading to the formation of MoO3. The Raman spec-

tra of the Mo0.84Ti0.08 and Mo0.84Ti0.08Al0.08 film are nearly

identical. Both indicate broadening of the peaks, suggesting

the presence of mixed oxides.23 In contrast, no Raman peaks

are observable for the ternary Mo1�x�yTixAly films with

y> 0.08, indicating the absence of the Raman-active MoO3

and TiO2. Therefore, it can be concluded that an increased Al

FIG. 3. (Color online) Photographs of the investigated films after annealing at 330 �C for 1 h in air.

FIG. 4. (Color online) Electrical resistivity of the Mo1�x�yTixAly films (with

x� 0.08 and 0� y� 0.24) vs the Al content in the as-deposited and

annealed state. The electrical resistivity of a 300 nm thick Mo film reported

in our earlier work (Ref. 15) is given as a reference.

FIG. 5. (Color online) Optical reflectance spectra before and after annealing

of (a) Mo0.84Ti0.08Al0.08 and (b) Mo0.76Ti0.08Al0.16.

021513-3 J€org et al.: Oxidation and wet etching behavior of sputtered Mo-Ti-Al films 021513-3

JVST A - Vacuum, Surfaces, and Films

content prevents the formation of MoO3 and results in growth

of a protective Al2O3 film.

In order to study the influence of Al on the oxidation

states of Mo and to verify the presence of the assumed

Al2O3 film, XPS analyses were conducted. As a result of

spin-orbit (j-j) coupling, the Mo3d spectrum consists of the

characteristic Mo3d5/2-Mo3d3/2 doublet.24 Figure 7 repre-

sents the Mo3d XPS spectra of the Mo-Ti-Al films with dif-

ferent Al contents. Again, the spectra of the Mo0.84Ti0.08 and

Mo0.84Ti0.08Al0.08 films exhibit substantial similarity. Curve

fitting of the Mo3d5/2 peak indicates two spectral lines at

231.8 and 232.7 eV, which are assigned to Mo5þ and Mo6þ

states. These peak positions agree well with earlier reports.25

Clearly, the intensity of the Mo6þ oxidation state exceeds

the one of the Mo5þ oxidation state and is the major contribu-

tion to the Mo3d5/2 signal of the MoTi8 and MoTi8Al8 films.

Hence, annealing of the low-alloyed films in air is enhancing

the Mo6þ state, evidencing the formation of MoO3. However,

the XPS spectra of the ternary Mo1�x�yTixAly films with

y> 0.08 alter with increasing Al content. For y¼ 0.16 several

oxidation states can be identified, with binding energies of the

Mo3d5/2 corresponding to metallic Mo0 (227.9 eV), Mo4þ

(229.1 eV), Mo5þ (232.0 eV) and Mo6þ (233.1 eV). Similar

findings have also been reported by Kim et al.26 As the Al

content is further increased (y¼ 0.24), the same oxidation

states and one additional species of Mo is observed, with a

binding energy of 228.3 eV. Since the binding energy of this

oxidation state is between Mo4þ and the metallic Mo0, it is

referred to as Modþ. Choi and Thompson assigned this Mo

species to Mo3þ, Xiang et al. to Mo2þ.24,27 Increasing the Al

content decreases the intensity of the Mo6þ peaks and favors

reduced oxidation states of Mo. Accordingly, the intensity

of the metallic Mo0 peak is the highest observed in the

Mo0.69Ti0.07Al0.24 film (see Fig. 7).

The results of peak area analysis of the Ti2p and Al2p

bands are summarized in Table I. It shows that an increasing

Al content leads to enhanced formation of Al2O3. As a conse-

quence, it can be presumed that the formation of Al2O3 is

responsible for the reduced oxidation states of Mo. Therefore,

the oxidation behavior of Mo-Ti-Al films is primarily con-

trolled by the Al content in the films. This conclusion is in

agreement to a study by Kai et al.28 on the oxidation of a Zr-

Cu-Al-Ni amorphous alloy in air at 300–425 �C.

IV. CONCLUSIONS

The influence of different Al contents on phase evolution,

wet etching behavior and oxidation resistance of Mo-Ti-Al

films deposited by dc magnetron sputtering was investigated.

All films show a single-phase body-centered cubic structure

based on the Mo phase. An increasing Al content results in a

decline of the wet etching behavior of the Mo alloy films in a

66 wt. % phosphoric acid, 10 wt. % acetic acid and 5 wt. %

nitric acid (PAN), but is still in an acceptable range for thin

film transistor liquid crystal display applications. The forma-

tion of MoO3 during the annealing at 330 �C for 1 h is respon-

sible for the thermal coloration of the unalloyed Mo and the

low-alloyed Mo0.84Ti0.08 and Mo0.84Ti0.08Al0.08 films. The

further increasing Al content promotes the formation of an

amorphous Al2O3 layer, which reduces the oxidation states of

Mo. Thus, the oxidation behavior of Mo1�x�yTixAly films is

primarily controlled by the Al content, where a minimum

alloying element content of x¼ 0.08 and y� 0.16 was found

to be necessary to enhance the oxidation resistance at still

acceptable wet etching rates.

ACKNOWLEDGMENTS

Funding for this research has been provided by the€Osterreichische Forschungsf€orderungsgesellschaft mbH within

the framework of the Headquarters project E2SPUTTERTECH

(Project No. 841482). The authors are grateful to Alexander

FIG. 6. (Color online) Raman spectra of the Mo1�x�yTixAly films compared

to a Mo reference after annealing at 330 �C for 1 h in air.

FIG. 7. (Color online) Mo3d XPS spectra of the Mo1�x�yTixAly films after

annealing at 330 �C for 1 h in air.

TABLE I. Results of quantitative peak area analysis of the Ti2p and Al2p

XPS bands.

Atomic fraction (%)

Ti

(453.9 eV)

TiO2

(458.9 eV)

Al

(72.3 eV)

Al2O3

(74 eV)

Mo0.92Ti0.08 — 2.51 — —

Mo0.84Ti0.08Al0.08 — 2.61 — 3.58

Mo0.76Ti0.08Al0.16 — 2.59 0.18 7.18

Mo0.69Ti0.07Al0.24 0.15 2.45 1.59 11.29

021513-4 J€org et al.: Oxidation and wet etching behavior of sputtered Mo-Ti-Al films 021513-4

J. Vac. Sci. Technol. A, Vol. 36, No. 2, Mar/Apr 2018

Fian (Joanneum Research, Weiz, Austria) for XPS

measurements and to Ronald Bakker (Montanuniversit€at

Leoben) for the assistance with Raman spectroscopy. Gerhard

Hawranek (Montanuniversit€at Leoben) is thanked for SEM

characterization.

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