A Comparative Study on the Microstructure and Properties of ITO Targets and Thin Films Prepared from Two Different Powders

doi:10.1007/s40195-020-01129-6

Abstract

Abstract:

With the rapid development of indium tin oxide (ITO) in the electronic display industry, choosing which raw powders to prepare high-quality ITO targets has always been a controversial topic. In the work, in order to clearly understand the effect of the raw powders on the microstructure and properties of ITO targets and thin films, tin-doped indium oxide (dITO) and In₂O₃-SnO₂ mixed (mITO) powders were chosen to prepare ITO targets for depositing the films and a comparative study on their microstructure and properties was conducted. It is found that, (1) dITO targets possess a higher solid solubility of tin in indium oxide and more uniform elemental distribution, while there are a higher density, a finer grain size and a higher mass ratio of In₂O₃ to SnO₂ for the mITO targets; (2) dITO films with more coarser columnar grains and a rougher surface prefer to grow along the [100] direction in an Ar atmosphere; (3) the conductive property of ITO films only depends on the doping amount of tin and is independent of the raw powders and the preparation process of the target source; (4) dITO films possess the superior optical property and narrower optical band gap; (5) the etching property of mITO films is superior to that of dITO films due to the lower solid solubility of tin in indium oxide.

Key words: Tin-doped indium oxide, In₂O₃-SnO₂ mixed powder, Solid solubility of tin in indium oxide, Photoelectric property, Etching property

Fangsheng Mei, Tiechui Yuan, Ruidi Li, Jingwei Huang. A Comparative Study on the Microstructure and Properties of ITO Targets and Thin Films Prepared from Two Different Powders[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(5): 675-693.

Figures/Tables 25

Table 1 Analyzed impurity compositions of the raw powders (ppm)

Powder	Fe	Al	Si	Ni	Cu	Pb	Cr	Tl	Cd	Total
Tin-doped indium oxide	4	5	10	5	2	2	1	2	2	< 100
In₂O₃	5	3	8	5	2	2	1	2	2	< 100
SnO₂	25	20	15	7	8	5	5	2	2	< 100

Table 2 Detailed preparation parameters of ITO targets

Process	Parameters
Ball milling	(1) High energy planetary ball milling: a solid loading of 25.00 vol.% (70.40 wt%), 250 r/min for 3 h with zirconia balls and deionized water
	(2) Binder: 1.0 wt% PVA (Polyvinyl alcohol)
	(3) Dispersant: 0.4 wt% NH₄PAA (Ammonium polyacrylate)
Molding	Hydraulic molding with a forming pressure of 50 MPa and cold isostatic pressing with a forming pressure of 250 MPa
Dewaxing	At 650 °C for 2 h in air
Sintering	At 1580 °C for 10 h in an oxygen flow of 10 L/min

Table 3 Detailed sputtering process parameters

Process	Parameters
Substrate temperature (°C)	25 (room temperature)
Sputtering gas	Ar
Sputtering pressure (Pa)	0.50
DC powder (W)	60 (2.12 W/cm²)
Sputtering time (min)	5, 10, 20
Target-to-substrate distance (mm)	153

Fig. 1 Phase compositions of the raw powders: a In2O3-10 wt% SnO2 mixed powders b 10 wt% tin-doped indium oxide powders

Fig. 2 Microtopography (a1-c1 SEM; (a2-c2 TEM) of the raw powders: a1, a2, a3 In2O3; b1, b2, b3 SnO2; c1, c2, c3 10 wt% tin-doped indium oxide. a3-c3 HRTEM images corresponding to a2-c2, respectively. a4-c4 Selected area electron diffraction (SAED) patterns corresponding to a2-c2, respectively

Table 4 Elemental content (wt%) measured by XRF in the targets

_Target	_In	_Sn	_O	_{Other elements}
_mITO	_78.560	_8.812	_12.521	_Balance
_dITO	_75.867	_8.679	_15.362	_Balance

Table 5 Analyzed impurity compositions (ppm) measured by ICP-AES in the targets

Target	Fe	Al	Si	Ni	Cu	Pb	Cr	Na	Mg	Zr	Total
mITO	30	18	29	10	30	5	5	34	20	60	< 1000
dITO	36	19	20	12	28	5	5	48	14	25	< 1000

Fig. 3 XRD diffraction patterns a of ITO specimens prepared from In2O3-10 wt% SnO2 mixed (mITO) and 10 wt% tin-doped indium oxide (dITO) powders and the enlarged partial drawings b: 30°-31°. Inset in a is the diffraction peak intensities of crystal planes

Fig. 4 SEM images a, c of the fracture surface and grain morphology b, d after corrosion of ITO specimens prepared from In2O3-10 wt% SnO2 mixed (mITO: a, b) and 10 wt% tin-doped indium oxide (dITO: c, d) powders

Fig. 5 a Relative density, the average grain size, resistivity, b oxide content of ITO targets prepared from In2O3-10 wt% SnO2 mixed (mITO) and 10 wt% tin-doped indium oxide (dITO) powders

Fig. 6 Elemental distributions of In a, b, Sn c, d, O e, f of ITO specimens prepared from In2O3-10 wt% SnO2 mixed (mITO: a, c, e) and 10 wt% tin-doped indium oxide (dITO: b, d, f) powders

Table 6 In and Sn exploration spots results (wt%) in Fig. 2(e, f) by EDX

Spots	In	Sn	In/Sn	Spots	In	Sn	In/Sn
m-pt1	81.92	3.38	24.24	d-pt1	75.78	7.37	10.28
m-pt2	83.67	2.31	36.22	d-pt2	87.06	4.17	20.88
m-pt3	50.93	33.06	1.54	d-pt3	59.40	30.68	1.94

Fig. 7 Microstructure and EDX maps of the coarse grain in mITO target characterized by TEM in the cross section: a grain structure in the target; b highly magnified TEM images of the coarse grain (red frame in a); c selected area electron diffraction (SAED) patterns of (b); d HRTEM image obtained from the dark particle (blue frame in b); e, f energy-dispersive X-ray (EDX) composition maps corresponding to b

Fig. 8 XPS images of ITO films deposited for different time

Table 7 Measured content of elements in ITO films (wt%)

Element	mITO			dITO
Element	5 min	10 min	20 min	5 min	10 min	20 min
In	73.46	75.52	80.40	70.05	72.97	81.94
Sn	7.30	7.49	6.61	8.28	7.83	5.97
O	18.44	16.26	12.80	20.84	18.68	12.09
C	0.80	0.73	0.19	0.83	0.52	0

Fig. 9 a X-ray diffraction patterns of ITO films deposited for different time and the partial enlarged drawings: b 28°-32°; c 34°-36°

Table 8 Diffraction angle positions of (222) and (400), I(222)/I(400) and the average grain size of ITO films deposited for different time

Items		mITO			dITO
Items		5 min	10 min	20 min	5 min	10 min	20 min
Diffraction angle position (°2Th.)	(222)	-	30.12	30.12	-	29.68	29.68
Diffraction angle position (°2Th.)	(400)	-	35.00	35.00	-	34.76	34.76
I₍₂₂₂₎/I₍₄₀₀₎		-	1.115	0.892	-	0.781	0.611
Average crystalline size (nm)		-	37	27	-	37	34

Fig. 10 SEM images of surface morphology of ITO films deposited for different time (a, b 5 min; c, d 10 min; e, f 20 min) using different targets a, c, e mITO; b, d, f dITO). Insets are the AFM images

Fig. 11 SEM images of cross-sectional morphology of ITO films deposited for different time (a, b 5 min; c, d 10 min; e, f 20 min) using different targets (a, c, e mITO; b, d, f dITO)

Fig. 12 a Growth rates of film thickness, b surface roughness of ITO films deposited for different time

Fig. 13 Surface morphology of ITO targets (a mITO; b dITO) after sputtering. Cracks, nodules and pores are marked by white dotted lines, red dotted lines and white arrows, respectivelyAs a whole, the targets with different microstructure and properties would affect the crystalline structure, grain growth and morphology of the films as well as the photoelectric and etching properties.

Fig. 14 a Sheet resistance, c carrier concentration and Hall mobility, d sheet carrier density of ITO films deposited for different time. b Schematic diagram of electron transmission paths in ITO films with different crystal growth directions of thin films

Fig. 15 a-c Transmittance at a wavelength of 300-800 nm and d the plots of (αhv)2 versus hv of ITO films deposited for different time (a 5 min; b 10 min; c 20 min). Inset in c is the schematic diagram of photon paths passing through ITO films with different crystal growth directions of thin films

Fig. 16 Surface morphology after etching of ITO films deposited for 5 min

Fig. 17 EDX spectra after etching of ITO films deposited for 5 min a, b mITO; c, d dITO. a, c Corresponding to point 1, and b, d corresponding to point 2 in Fig. 16a, c

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