2011 International Students and Young Scientists Workshop „Photonics and Microsystems”

978-1-4577-1652-2/11/$26.00 ©2011 IEEE

Analysis of substrate type and thickness influence on wettability of Nb2O5 thin films

Michał Mazur1, Shigeng Song2, Jarosław Domaradzki1, Danuta Kaczmarek1, Damian Wojcieszak1,

Karolina Sieradzka1, Frank Placido2, Pietro Gemmellaro3

1Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology, Janiszewskiego 11/17, 50-372 Wroclaw, Poland

2 Thin Film Centre, University of the West of Scotland, Paisley, Scotland PA1 2BE 3Dipartimento di Scienze Chimiche (Department of Chemical Science), University of Catania,

Viale A. Doria 6, 95125 Catania, Italy Email address of corresponding author: michal.mazur@pwr.wroc.pl

Abstract - For the purpose of this paper Nb2O5 thin films were manufactured by microwave-assisted reactive magnetron sputtering. Nb2O5 thin films with different thickness were deposited on standard microscope slides, SiO2 and silicon substrates. Wettability and optical properties of manufactured coatings were investigated. Knowledge of the macroscopic contact angle for materials allows to predict whether a liquid droplet will bead up on or spread out over a solid surface. In this paper contact angle, critical surface tension and surface free energy of Nb2O5 thin films with thickness of 565, 300, 200, 100 and 50 nm were investigated. Measurements of surface free energy and contact angle was carried out with a computer controlled goniometer system with water, ethylene glycol and diiodomethane. Contact angle measurements were performed according to the sessile drop method. The results have shown that all manufactured Nb2O5 thin films had the water contact angle of about 90 degrees. There was no difference in wettability for thin films with different thickness and deposited on different substrates. Also optical properties of deposited thin films were compared. Transmittance spectra of Nb2O5 thin films were measured using Hitachi U-3501 spectrophotometer. Based on transmittance and reflectance spectra, measured with the aid of Aquila nkd-8000 instrumentation, thickness, refractive index and extinction coefficient of Nb2O5 thin films were calculated using Drude – Lorentz model. Transparency in visible light range of deposited thin films varied from about 70 % up to 80 %. Refractive index and extinction coefficient were 2.32 and 4.55⋅⋅⋅⋅10-5, respectively.

I. INTRODUCTION

Thin oxide films based on different metals are increasingly applied for optical and microelectronic applications [1]. In optical applications, among others, niobia (Nb2O5) thin films are used as high index and low loss materials for optical waveguides [2], interference filters, anti-reflective coatings and electroluminescent devices [3]. Niobium oxide based materials

are very attractive for electronic applications, particularly the niobium pentoxide, the most stable of the niobium oxides. Niobia, along with other metals from V group, have also been investigated as possible candidate for corrosion barrier coatings and thin film catalysts [4,5]. More recently, there has been considerable interest in using them as high permittivity dielectric to replace the gate oxide in microelectronic devices [6–8]. These films are attractive candidates for the above mentioned technologies due to their superior thermal stability and mechanical resistance compared to traditional high permittivity materials [9]. Nb2O5 evidences a set of physical properties which are very important for its use in electronic passive components such as dielectric constant (�’ = 41) and a reported band gap of ~3.4–5.3 eV. Recently, this material has also been appointed as a substitute of tantalum pentoxide (�’ = 27) for application in solid state capacitors [10]. Nb2O5 is also one of the useful optical materials for its desirable properties, such as stability in air and water, resistance to acid and base, high refractive index (n = 2.4 at 550 nm), low absorption and high transparency in the UV–vis–NIR region [11]. It has been widely used in, for example, intelligent windows [12], solar cell [13], chemical sensor [14], etc. Niobium oxide film is also a promising counter electrode material in electrochromic devices, which can be used in batteries, solar cells, solar energy applications, sensors and display devices [15]. Niobia has excellent chemical stability and corrosion resistance in both acid and base media which makes it useful in a wide range of electrochromic devices [16].

One of the advantages of niobia coatings is the possibility to obtain different colors varying from brown for amorphous layers, grey for crystalline layers with small crystallite sizes (<25 nm) and blue for those with large crystallite sizes (>30 nm) [17].

In this paper wettability and optical properties of niobium pentaoxide with different thickness and deposited on different substrates were determined.

95II. EXPERIMENTAL DETAILS

Among others, reactive sputtering method is a well-established technique for industrial thin film preparation and research work. By parameter controlling, reactive sputtering allows to obtain crystalline or amorphous films with certain structure and morphology [18].

Homogenous and high optical quality niobium pentaoxide thin films were deposited on standard microscope slides, SiO2

and Si substrates by microwave assisted reactive magnetron sputtering. To improve plasma ionization microwave source was used. MicroDyn® system was equipped with high purity Nb target. The substrate holder was a large drum, rotated with the speed of 1 cycle per second during the deposition. Chamber was evacuated by turbomolecular pump backed by a rotary pump. After reaching operating pressure microwave plasma was turned on to pre-clean substrates surfaces from organic residues, dust and debris. The system was operated with optical monitoring, which allows on controlling of thin film thickness during deposition process. Five sets of Nb2O5 thin films have been prepared with different thickness. The thickness of Nb2O5

thin films varied to observe the dependence of optical properties, contact angle and surface free energy. The thickness of samples was 50, 100, 200, 300 and 565 nm. The deposition conditions were kept at the same level in case of every sample. Deposition parameters are shown in Table 1.

Thickness of Nb2O5 thin films was measured during the deposition processes with Coat Monitor software, which was based on optical monitoring of samples. Thickness was fitted to the known n and k value used by this software. After reaching the exact value of thickness the process was automatically stopped.

TABLE 1

DEPOSITION PARAMETERS OF NB2O5 THIN FILMS PREPARED BY

MICROWAVE ASSISTED MAGNETRON SPUTTERING

Parameter: Nb2O5

Voltage [V] 280 - 310

Current [A] 15.75

Power [kW] 4.4-4.9

Microwave power [kW] 2.71- 2.78

Reflected microwave power [kW] 0.02 - 0.05

Ar flow [sccm] 190

Ar partial pressure [Torr] 2.5·10-3

O2 flow [sccm] 79

O2 partial pressure [Torr] 1.0·10-3

Vacuum pressure [Torr] 1.0·10-5

Sputtering pressure [Torr] 4.3 - 5.0·10-3

Preclean time [s] 300 s.

The transmittance of the Nb2O5 thin films, deposited on microscope slides substrates, was measured using Hitachi U-3501 spectrophotometer. To calculate the thickness, refractive indexes and extinction coefficients of Nb2O5 thin films, the transmittance and reflectance spectra were measured using Aquila nkd8000 instrumentation and then the data were fitted

using a Drude-Lorentz model implemented in Pro-Optix software.

Contact angle measurement is a widely accepted method to characterize surfaces of thin films. Contact angle measurement

(θ) can be used to relate the key thermodynamic parameters of a surface through the Young equation given by [19]:

γlv cosθ = γsv − γsl,

where γlv, γsv and γsl are the liquid–vapour, solid–vapour and solid–liquid interface energies of a solid, respectively. Only

two of the terms contained in the Young equation, namely θand γlv, can be determined experimentally [19]. The contact angle of a liquid drop on a solid surface is defined by the mechanical equilibrium of the drop under the action of three interfacial tensions. Schematic of a sessile-drop contact angle system is shown in Fig. 1 [20].

Fig. 1. Schematic of a sessile-drop contact angle system [20]

The measurement of contact angle of three liquids i.e. distilled water, ethylene glycol and diiodomethane was carried out with computer control Theta Lite tensiometer system manufactured by Attension. Contact angle measurements were performed according to the sessile drop method. On the basis of contact angle results of different liquids, critical surface tension and surface free energy was determined using a special software provided with the equipment.

III. RESULTS

Transmittance spectra of Nb2O5 thin films with different thickness, deposited on microscope slides, are shown in Fig. 2.

400 500 600 700 800 9000.0

0.2

0.4

0.6

0.8

1.0

Tra

nsm

itta

nce

Wavelength [nm]

Transmittance ofNb2O

5 thin films

with different thickness:

50 nm

100 nm

200 nm

300 nm

565 nm

Fig. 2. Transmittance spectra of Nb2O5 thin films with different thickness deposited on microscope slides

(1)

96Transparency in the visible light range of deposited thin

films varied from about 70 % up to 80 %. The cut off wavelength was established on the basis of transmittance spectra and it was equal to about 320 nm. With the increase of thickness, more interference peaks at the transmittance spectra are visible.

Refractive index and extinction coefficient of Nb2O5 thin films are shown in Fig. 3 and Fig. 4, respectively.

400 500 600 700 800 900 10002,2

2,3

2,4

2,5

2,6

2,7

n

Wavelength [nm]

refractive index of Nb2O

5

Fig. 3. Refractive index of Nb2O5

400 500 600 700 800 900 100010

-5

10-4

10-3

10-2

k

Wavelegth [nm]

extinction coefficient of Nb2O

5

Fig. 4. Extinction coefficient of Nb2O5

The refractive index of Nb2O5 at the λ = 550 nm is 2.32 for all manufactured thin films. The extinction coefficient is very

low and at the λ = 550 nm is equal to 4.55⋅10-5. Such extinction coefficient testifies of very low absorbance in the visible spectral range.

Results of wettability investigation of Nb2O5 thin films with different thickness and deposited on different substrates, for distilled water, ethylene glycol and diiodomethane have shown that value of the contact angle is independent from the thin film thickness. Also substrate, on which thin film is deposited, does not influence the contact angle between liquid droplet and the surface of coating. The highest contact angle was obtained for distilled water and it was equal to about 90 degrees in case of every measurement. Contact angle for ethylene glycol and diiodomethane was about 65 and 53 degrees, respectively. Results of contact angle investigation of distilled water, ethylene glycol and diiodomethane have been shown in Fig. 5.

Droplet images of distilled water, ethylene glycol and diiodomethane on Nb2O5 thin films with different thickness, deposited on microscope slides are shown in Table 2.

0 100 200 300 400 500 6000

20

40

60

80

100

Co

nta

ct a

ng

le (

de

g)

Thickness of Nb2O

5 thin films (nm)

Nb2O

5 thin films deposited on:

microscope slide

SiO2

Si

distilled water

ethylene glycol

diiodomethane

Fig. 5. Contact angle measurements results of Nb2O5 thin film with different thickness, deposited on different substrates

TABLE 2

DROPLETS IMAGES OF DISTILLED WATER, ETHYLENE GLYCOL

AND DIIODOMETHANE ON NB2O5 THIN FILMS WITH DIFFERENT

THICKNESS

Droplet image Nb2O5

thickness Water Ethylene glycol Diiodomethane

565 nm/ microscope

slide

300 nm/ microscope

slide

200 nm/ microscope

slide

100 nm/ microscope

slide

50 nm/ microscope

slide

97In Table 3 droplets images of distilled water, ethylene

glycol and diiodmethane on 300 nm thick Nb2O5 thin films, deposited on different substrates have been shown.

TABLE 3 DROPLETS IMAGES OF DISTILLED WATER, ETHYLENE GLYCOL

AND DIIODOMETHANE ON 300 NM THICK NB2O5 THIN FILMS, DEPOSITED ON DIFFERENT SUBSTRATES

Droplet image Substrate of thin film Water Ethylene glycol Diiodomethane

Nb2O5/ microscope

slide

Nb2O5/ SiO2

Nb2O5/ Si

On the basis of contact angle investigation, critical surface tension of Nb2O5 thin films with different thickness and deposited on microscope slide substrates was calculated using Zisman approach [20]. Critical surface tension is often presented as the highest value of surface tension of a liquid which will completely wet solid surface. Results are shown in Fig. 6.

0 100 200 300 400 500 6000

10

20

30

40

Cri

tical su

rface t

en

sio

n (

mN

/m)

Thickness of Nb2O

5 thin films (nm)

critical surface tension of Nb2O

5 thin film

deposited on microscope slide

Fig. 6. Results of critical surface tension calculations of Nb2O5 thin films with different thickness deposited on microscope slide substrate

Results have shown that critical surface tension is almost independent of thickness of Nb2O5 thin films and it is equal to 23.7±1.7 (mN/m).

Influence of thickness of Nb2O5 thin films deposited on microscope slide substrates on surface free energy was determined according to geometric Fowkes approach [20]. Total value of surface energy was very similar for all measured samples and it was equal to 31.9±1 (mN/m). Total value was mainly dependent on dispersive component and there was almost no influence of polar component. Results of surface free energy are shown in Fig. 7.

0 100 200 300 400 500 6000

10

20

30

40

Su

rfa

ce f

ree

en

erg

y (

mN

/m)

Thickness of Nb2O

5 thin film (nm)

total surface free energy

dispersive component

polar component

Fig. 7. Results of surface free energy investigation for Nb2O5 thin films with different thickness deposited on microscope slide substrates

IV. CONCLUSIONS

Nb2O5 thin films with different thickness were deposited on microscope slides, silica and silicon substrates by microwave assisted reactive magnetron sputtering process. The thickness of samples was 50, 100, 200, 300 and 565 nm. Transparency level of deposited samples was about 70 – 80 % and the cut off wavelength was 320 nm according to transmittance measurements. Refractive index and extinction coefficient of

manufactured thin films were 2.32 and 4.55⋅10-5 at λ = 550 nm, respectively. Value of extinction coefficientt testifies of very low absorbance in the visible spectral range.

Wettability investigation have shown that the value of contact angle is independent from Nb2O5 thin films thickness and also from substrate on which the coating is deposited. The highest contact angle of about 90 degrees was obtained for distilled water, while for ethylene glycol and diiodomethane was about 65 and 53 degrees respectively. On the basis of contact angle of those three liquids, critical surface tension and surface free energy was determined and results have shown that their values are also independent from Nb2O5 thin films thickness.

ACKNOWLEDGMENT

This work was financed from fellowship co-financed by European Union within European Social Fund and from the sources given by the NCBiR in the years 2011-2013 as a development research project number N N515 4963 40.

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