Sputtering targets preparation

Introduction Most of Aluminum doped Zinc Oxide (AZO) and Indium Tin Oxide (ITO) TCOs for industrial applications are made by sputtering of ceramic targets [1,2]. Therefore investigations that lead to the reduction of targets cost and improvements concerning both materials consumption efficiency and films characteristics are still of great interest. If the sputtering target is made of the same material as the thin film, the characteristics of the transparent electrodes are influenced by those of the target. On the other hand, the quality of the target is influenced by the started powder and its sintering conditions. AZO have many advantages over ITO as a TCO, owing to its low cost, abundance, high thermal stability and absence of toxicity [3] being an ideal replacement of ITO [4,5]. In this study, several AZO ceramic targets were prepared using a nanostructured Al2O3 doped ZnO (98:2 wt %) powder obtained by a new patented process of INNOVNANO for the synthesis of nanoparticles [6]. The effect of powder particle size distribution, green density, temperature and holding time on sintering were discussed. Also the final density, grain size and second phase (ZnAl2O4) distribution were investigated and correlated with targets electrical properties. Electrical resistivity values around 3.0-7.0x10-3 Ω.cm have been presented for the sintered targets, which are comparable to that results presented by other authors [7,8]. Due to the nanopowder characteristics the sputtering targets can be sintered at substantially lower temperatures (1150-1250°C), keeping a good homogeneity and dispersion of secondary phase (ZnAl2O4) even for fully dense targets. Based on these results a two inches ceramic target was prepared and used as sputtering target material for thin films deposition. The electro-optical properties of the films produced at room temperature with thicknesses around 360 nm, besides being highly uniform exhibit a resistivity of about 110-3 Ω.cm and a transmittance in the visible range above 90 %.

Targets densification / Electrical characterization

Structural analysis

Synthesis method

ALUMINUM-DOPED ZINC OXIDE - AZO SPUTTERING TARGETS OBTAINED FROM NANOPOWDERS: PROCESSING AND APPLICATION

Nuno Neves1,2*, Raquel Barros1,2*, Elsa Antunes1, Elvira Fortunato2, Rodrigo Martins2 , Isabel Ferreira2

1.Materiais Avançados, INNOVNANO, SA, 7600-095 Aljustrel, Portugal 2.CENIMAT-I3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal

*corresponding authors: Tel:+351 21 2948562, Fax: +351 21 2948558, email: nuno.neves@fct.unl.pt, rb@fct.unl.pt

AZO powder was heat treated at 600°C to remove part of the carbon inherent to the detonation synthesis process.

High energy milling in planetary mill before Cold Isostatic Pressing (CIP) to deagglomerate the nanoparticles.

Sintering and characterization of two inches ceramic targets (XRD, SEM, TEM, Hall effect measurements).

SEM image showing the general morphology of as-produced nanocrystalline Al-doped ZnO

powder.

Innovnano AZO powder – Homogeneous distribution of Al in each individual ZnO particle

AZO Powder

Specific surface

area

Particle size distribution

(µm)

(m2.g-1) D10 D50 D90

Before milling 6.09 0.022 0.251 1.305

After milling 12.11 0.020 0.169 0.394

INNOVNANO AZO powder (as-produced)

Heat treatment (600°C)

High energy milling (ethanol)

Drying (70°C)

AZO ceramic targets

Conclusions

Thin films properties

Schematic presentation of INNOVNANO synthesis process – Nanocrystalline ceramic powder formation

INNOVNANO nanocrystalline Al doped ZnO powder (AZO) with 99.9 % purity was used to produce AZO targets. The powder was obtained by a new patented process based on the detonation of an emulsion containing both Zn and Al in the final proportion (98:2 wt % respectively). In this synthesis method, high pressure (> 2.000 bars) and high temperature (range of 500°C to 3000°C) are combined with ultrafast quenching (up to 108 K.s-1 to 109 K.s-1) in one single reaction step, which has the great advantage of having Al more uniformly distributed in ZnO matrix comparing to traditional mechanical grinding method.

Particle size distribution (weight fraction) of INNOVNANO Al doped ZnO 98:2 wt%

Several targets were prepared in order to study the effect of sintering parameters on target properties. Highly dense targets were obtained at substantially lower temperatures (1150-1250 °C) compared to conventional ones (1400 °C) keeping a good homogeneity even for fully dense targets. Electrical resistivity of the targets between 3.0-7.0x10-3 Ω.cm were obtained. Microstructural analysis show a well dispersed spinel phase (ZnAl2O4) even for fully dense targets which is correlated with the good dispersion of Al in ZnO structure inherent to the synthesis process. High density targets with controlled grain size and second phase distribution promote electrical conductivity. High quality targets can be easily prepared and used to deposit Al-doped ZnO films by rf magnetron sputtering with low resistivity (1.0210-3 Ω.cm) and an average transmittance above 90% in visible range, as required for most optoelectronic applications where a transparent conductive oxide electrode is required and has to be processed at room temperature.

References

[1] E. Fortunato, P. Barquinha, A. Pimentel, et al, Adv. Mater., 17 [5] 590-4 (2005).

[2] E. Fortunato, H. Hosono, C. Granqvist, J. Wager, Thin Solid Films, 516 [7] 1313 (2008).

[3] S. H. Jeong, J. H. Boo, Thin Solid Films, 447-8 105-110 (2004).

[4] M. K. Jayaraj, A. Antony, M. Ramachandran, Bull. Mater.Sci., 25 [3] 227-30 (2002).

[5] C. P. Liu, G. R. Jeng, J. Alloys Compd., 468 [1-2] 343-9 (2009).

[6] J. M. Calado, E. M. Antunes, patent WO 2009144665 (2009).

[7] H. S. Huang et al, Thin Solid Films, 518 [21] 6071-5 (2010).

[8] T. Minami, J.-ichiOda, J.-ichiNomoto, T. Miyata, Thin Solid Films,519[1] 385-90 (2010).

Uniaxial cold pressing / CIP

Sintering (1000-1500°C) 0

10

20

30

40

50

60

70

80

90

100

0,01 0,1 1 10

Re

lati

ve

weig

ht

(%)

Particle diameter (µm)

AZO Before milling

AZO After milling

1000°C 2h

1200°C 2h

1300°C 2h

600 800 1000 1200 1400 1600-25

-20

-15

-10

-5

0

Line

ar S

hrin

kage

(%

)

Temperature (C)

Before milling

After milling

600 800 1000 1200 1400 1600

-0,7

-0,6

-0,5

-0,4

-0,3

-0,2

-0,1

0,0

0,1

Before milling

After milling

dY/d

t (%

.min

-1)

Temperature (C)

30 40 50 60 70

Before milling

After milling

Zincite (ZnO)

Inte

nsity

(a.

u.)

2 theta (degree)

Dilatometer tests Sintering

cicle Density

(%) Grain size

(μm)

1150°C/2h 96.2 1.54

1150°C/4h 96.8 1.67

1150°C/8h 97.6 1.78

1150°C/16h 99.1 2.12

Higher green densities lead to superior final densities;

The increase in holding time and

sintering temperature promotes the densification and grain growth of the targets;

Microstructural analysis show a well

dispersed spinel phase in ZnO structure at temperatures below 1400°C;

1000 1100 1200 1300 1400 150060

65

70

75

80

85

90

95

100

Re

lative

de

nsity (

%)

Temperature (C)

GD 46

GD 53

GD 62

1100 °C 1200 °C

1300 °C 1400 °C

Electrical conductivity of the sintered targets increase with densification between 1150 and 1300 °C;

At higher sintering temperatures (> 1400 °C the larger ZnAl2O4 grains tend to be located in ZnO grain boundaries and triple junction points causing a decrease in homogeneity and consequently in electrical conductivity.

AZO films morphology and cross-section structure deposited at room

temperature with rf power of 250 W and deposition pressure of 1.5 mTorr.