PET films deposited by dc magnetron sputtering

Journal of Non-Crystalline Solids 331 (2003) 41–47 www.elsevier.com/locate/jnoncrysol

Influence of negative metal ion bombardment on the properties of ITO/PET films deposited by dc magnetron sputtering Daeil Kim

*

SKION Corp., 50 Harrison St., Hoboken, NJ 07030, USA Received 1 May 2002; received in revised form 25 June 2003

Abstract Transparent conducting indium tin oxide (ITO) thin films were deposited on a polyethylene terephthalate (PET) substrate at a low substrate temperature by dc magnetron sputtering using a negative metal ion source and an ITO target. During separate deposition runs, the cesium partial pressure was varied from 1 · 10
3 to 2.2 · 10-3 Pa to investigate the effect of ion beam bombardment on the surface morphological, electrical and optical properties of the films. The resistivity of ITO films decreased with PCs and reached as low as 6.2 · 10
4 X cm at PCs of 1.7 · 10
3 Pa. Optical transmittance compared with bare PET substrate also varied significantly with PCs and the highest optical transmittance of 87% (at k ¼ 550 nm) was obtained at PCs of 1.7 · 10
3 Pa. According to a result obtained by AFM, surface roughness of the ITO/PET film showed remarkable change from 2.8 to 1.1 nm with PCs . However, above optimal PCs condition (>1.7 · 10
3 Pa), the electrical, optical property and surface morphological properties deteriorated. From SEM, AFM, and Hall measurements, relatively low resistivity and high transmittance of ITO film deposited at PCs ¼ 1:7  10
3 Pa is caused by an increase in charge carrier concentration and flat surface morphology with secondary negative metal ion beam bombardment.  2003 Elsevier B.V. All rights reserved. PACS: 41.75.C; 78.20

1. Introduction Indium tin oxide (ITO) is a degenerate, wide gap semiconductor. It is extensively used as transparent electrode for flat panel display devices,

* Present address: Samsung Advanced Institute of Technology, Process Engineering Center, P.O. Box 111, Suwon 440600, South Korea. Fax: +82-31 280 6879. E-mail address: [email protected] (D. Kim).

solar cells, sensors, and organic light emitting diodes (OLED) because of its high optical transmittance and low resistivity. To satisfy specific requirements of each optoelectronic application many deposition methods have been developed such as reactive sputtering [1], reactive evaporation [2], chemical vapor deposition [3], spray pyrolysis [4] and direct metal ion beam deposition [5]. Among the various deposition methods, reactive sputtering is the one of the most popular techniques to produce large area of ITO films and

0022-3093/$ - see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2003.09.024

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D. Kim / Journal of Non-Crystalline Solids 331 (2003) 41–47

in general, it needs a high substrate temperature (250–350 C) to obtain high optical transmittance (90% in visible region) and low resistivity (2 · 10
4 X cm). Thus, in conventional reactive sputtering process, most of ITO films were deposited on glass substrates which can withstand a high substrate temperature [1]. Currently, there is a strong commercial need for high quality transparent conducting oxide films deposited on polymeric substrates for color LCDs or flexible LCDs [6]. However, high quality ITO film deposition on flexible polymer substrates is currently difficult by conventional sputtering process because the substrate temperature should be less than 200 C due to low thermal stability of polymer substrate. In this study, the specific resistivity, optical transmittance, and surface morphology of as deposited ITO films on unheated polyethylene terephthalate (PET) substrates were investigated by dc magnetron sputtering using a negative ion source (SNIS) [7]. In our previous report, we showed that in SNIS, secondary negative metal ions are sputter generated from cesiated target surface by surface ionization and the ions transfer kinetic energy to the surface adatoms [8]. Thus, the effect of ion beam bombardment on the optoelectronic properties of ITO films depends on the secondary negative metal ion yield at cesiated target surface and in turn, the ion yield strongly depends on the Cs partial pressure (PCs ) on the sputtering atmosphere. In this study we investigated the optimal PCs on the structural, optical, and electrical properties of ITO film deposited on PET substrate by dc magnetron sputter type negative ion source (SNIS).

2. Experimental The experimental dc magnetron sputter type negative metal ion beam deposition system is shown schematically in Fig. 1. A commercial Cs vapor emitter (SKION Corp. Omnipotent series) was placed beside a rectangular ITO target (In2 O3 90 wt% SnO2 10 wt%, Size; 17 · 10 cm2 ). The ultrasonically cleaned PET thin film (80 lm thick, 7 · 7 cm2 size) was mounted below the target at a

DC power Ar Target

Cs reservoir

Cs vapor

Pump

Reactive gas inlet

Substrate Water

Fig. 1. Schematic diagram of the dc magnetron sputter type negative metal ion source.

distance of 15 cm. Substrate temperature was kept constant at about 30 C due to water cooled substrate and measured directly with K-type thermocouple. Prior to deposition, the chamber was evacuated to a back ground pressure of 5 · 10
5 Pa initially and then Cs vapor emitted from a heated reservoir to the target. The PCs varied with a reservoir temperature over the range 1 · 10
3 – 2.2 · 10
3 Pa. In order to consider the effect of the Cs content to the sputtering atmosphere on the properties of the ITO thin films, the ITO deposition occurred without oxygen gas flow. Ar gas was introduced as sputtering gas with the flow rate of 15 sccm. The typical deposition pressure with Ar and Cs vapor was 5 · 10
2 Pa. The amounts of the mean sputtering power and current were 250 W (2.1 W cm
2 ) and 0.7 A, respectively. Deposition rate was 14–16 nm/min. For surface roughness and optoelectronic properties characterizations, thickness was constant at about 100 nm, so the measured structural, optical, and electrical properties of ITO films are comparable. Thickness measurements were done with an alpha step surface profile measuring system.

D. Kim / Journal of Non-Crystalline Solids 331 (2003) 41–47

The optical transmittance was measured with a UV–visible spectrophotometer as a function of wavelength. Bare PET substrates were used as a transmission reference. The sheet resistance and the room temperature carrier density were measured using a four point probe and hall measurements, respectively. The surface morphology was measured by atomic force microscopy (AFM) in air and scanning electron microscopy (SEM).

3. Results Fig. 2 shows the variation of resistivity of ITO/ PET film as a function of PCs . The resistivity decreased with PCs and reached its minimum value of 6.2 · 10
4 X cm at PCs ¼ 1:7  10
3 Pa. Table 1 shows a property of ITO/glass film prepared as a function of Cs presence in the sputtering atmo-

43

sphere. The film prepared without Cs presence shows a resistivity of 9.4 · 10
4 X cm and the resistivity decreased with PCs and optimized at PCs ¼ 1:7  10
3 Pa. Fig. 3 shows the influence of PCs on the optical transmittance. The optical transmittance is always above 80% when the PCs ranges from 1 · 10
3 to 2.4 · 10
3 Pa and the highest optical transmittance of 87% was obtained at PCs ¼ 1:7  10
3 Pa. From the optical transmission and electrical resistivity measurements, it can be concluded that Cs incorporation in the sputtering atmosphere contributes to the generation of secondary negative metal ion and the transferred kinetic energy onto growing ITO film by ion beam bombardments leads higher carrier concentration and better crystallinity [8].

90

Average

120 Sheet resistance, Rs

Optical transmittance at 550nm (%)

130

110 100 90 80 70

86 84 82 80 78 76 0.8

60 0.8

Average 88

1.0

1.2 1.4 1.6 1.8 2.0 2.2 Cs Partial pressure, Pcs 10-3 Pa

2.4

1.0

Fig. 2. Variation of the sheet resistance as a function of the PCs .

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

Cs partial pressure, x10-3 Pa

2.6

Fig. 3. Variation of the optical transmittance at 550 nm wavelength as a function of the PCs .

Table 1 Variation of electrical resistivity, optical transmittance, and a figure of merit of ITO/glass films (100 nm thick) deposited on a glass substrate under different Cs partial pressure PCs Cesium partial pressure (·10
3 Pa)

Resistivity (·10
4 X cm)

Transmittance (%) (k ¼ 550 nm)

Figure of merit (T 10 =Rsh ) (10
4 X
1 )

1.0 1.2 1.4 1.7 2.0 2.2

9.0 7.3 5.9 4.3 4.5 5.1

72 75 80 81 80 80

4.2 7.7 18.1 28.3 23.9 21.0

Substrate temperature was kept constant at 70 C.

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D. Kim / Journal of Non-Crystalline Solids 331 (2003) 41–47

Fig. 4 shows the fundamental absorption edge shift for ITO films deposited under different PCs as a function of wavelength. As PCs is increased (1– 1.7 · 10
3 Pa), the fundamental absorption edge shifts to shorter wavelength. This shift is called a Moss–Burstein shift [9]. In a carrier density measurement, the carrier density increases with increasing PCs , taking a maximum carrier density of 3.0 · 1021 cm
3 at PCs ¼ 1:7  10
3 Pa, and then decreases with increasing PCs . Surface morphology is one of the most important properties of the ITO films and it is optimized accordingly to meet the requirements in the various optoelectronic applications that involving ITO thin film. For instance, ITO films used in liquid crystal displays favor a relatively rough surface due to good adhesion of subsequently coated polymeric layer on its surface, whereas ITO film with a rough surface in OLED application is very detrimental because the rough anode surface (ITO film) causes electrical short with high electric fields [10]. Fig. 5 shows the AFM images of bare PET substrate (a) and ITO films deposited under different PCs of (b) 1 · 10
3 , (c) 1.4 · 10
3 (d) 1.7 · 10
3 , respectively.

As the thickness of ITO films were maintained about 100 nm, surface roughness due to thickness variation can be negligible. Surface roughness of bare PET and ITO films in Fig. 5(a) and (b) are 1.33 and 2.0 nm respectively. As increasing PCs AFM images in Fig. 5(c) and (d) show the substantial decreased surface roughness compared with the images of the film shown in Fig. 5(b) due to increased ion beam bombardment. Fig. 6 shows the SEM images of the ITO films prepared on the PET films. ITO film (a) prepared without Cs incorporation in the sputtering atmosphere was amorphous, while the film (b) prepared at PCs ¼ 1:7  10
3 Pa shows grains on the surface. From Fig. 6 it can be concluded that the Cs incorporation in the sputtering atmosphere is a very effective way to prepare polycrystalline ITO thin film without intentional substrate heating treatment. Fig. 7 shows the variation of surface roughness as a function of PCs . Especially, the ITO film deposited at PCs ¼ 1:7  10
3 Pa shows the lower surface roughness (1.16 nm) than that of the bare PET substrate. From Figs. 5 and 6, it can be concluded that surface roughness also strongly depends on the PCs . However, at high PCs conditions, surface morphology is saturated at about 1.4 nm. In our previous report, it had been reported that for increasing Cs dose, negative metal ion yield increased significantly and then saturated beyond an optimized Cs flux [11]. When the PCs exceed optimal PCs , the work function of the target surface is changed as the same with Cs work function. Thus, the excessive Cs presence in the sputtering atmosphere is not favorable.

4. Discussion In order to investigate the performance of transparent conducting ITO thin films, a figure of merit (FH ) defined by Haacke [12] was considered. The FH is one of the important indices to judge the effectiveness of process parameter. The FH is defined by Fig. 4. Absorption edge shift of as deposited ITO films as a function of wavelength.

FH ðX
1 Þ ¼ T 10 =Rs ;

D. Kim / Journal of Non-Crystalline Solids 331 (2003) 41–47

45

Fig. 5. AFM image of ITO thin film prepared under different cesium partial pressure: (a) bare PET substrate, (b) 1.0 · 10
3 Pa, (c) 1.4 · 10
3 Pa and (d) 1.7 · 10
3 Pa.

where T is the optical transmittance at a specific wavelength (k ¼ 550 nm) and Rs is sheet resistance. It is known that the higher the FH , the better the quality of the transparent conducting thin film [13]. Thus, as shown in Fig. 8, the optoelectrical property of ITO film strongly depends on the PCs and the most effective PCs condition is 1.7 · 10
3 Pa, where FH is the highest. In order to consider the effect of substrate materials on the optoelectrical property of ITO film, an ITO film (100 nm thick) was deposited on a glass substrates under the same condition de-

scribed in the experimental procedures. As shown in Table 1, for the ITO/glass prepared at PCs ¼ 1:7  10
3 Pa the value of FH is higher than those for other low PCs conditions. This means that the best performance in ITO/glass was also achieved by a film deposited at PCs ¼ 1:7  10
3 Pa in the sputtering atmosphere with /TC ¼ 28:3 10
4 X
1 . In Table 1, since the films prepared on glass substrates show the higher optical transmittance and electrical conductivity than that of the ITO/PET film prepared with same PCs condition.

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D. Kim / Journal of Non-Crystalline Solids 331 (2003) 41–47

Surface roughness, Ra (nm)

2.2

Average

2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

Cs partial pressure, x10-3 Pa

Fig. 7. Surface average roughness (Ra) of the ITO films deposited under different PCs .

Figure of Merit, x10-3 ohm-1

3.5 Average 3.0 2.5 2.0 1.5 1.0 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

Cs partial pressure, x10-3Pa

Fig. 8. Variation of figure of merit as a function of PCs . Fig. 6. SEM image of ITO thin film prepared on the PET substrate (a) without Cs incorporation and (b) with Cs incorporation (PCs ¼ 1:7  10
3 Pa) in the sputtering atmosphere.

5. Conclusions Transparent conducting ITO thin films were deposited on water cooled PET substrates by dc magnetron sputter type negative metal ion beam deposition method. In order to investigate the effect of secondary negative metal ion beam bombardment on the properties of ITO films, Cs partial pressure (PCs ) in the sputtering atmosphere were varied from 1.0 · 10
3 to 2.2 · 10
3 Pa.

For increasing PCs , optical transmission, electrical resistivity and surface roughness varied significantly. At PCs ¼ 1:7  10
3 Pa, we obtained ITO films with 6.2 · 10
4 X cm in resistivity and 87% in optical transmittance at 550 and 1.16 nm in average surface roughness. From the AFM, SEM images and Hall effect measurements, it can be concluded that the higher optical transmittance and lower resistivity of ITO film prepared at PCs ¼ 1:7  10
3 Pa than that of the other PCs conditions is caused by smooth surface morphology, high carrier concentration of the film and poly crystallization.

D. Kim / Journal of Non-Crystalline Solids 331 (2003) 41–47

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[6] Y. Shigesato, R. Koshiishi, T. Kawashima, J. Ohsaako, Vacuum 59 (2000) 614. [7] D. Kim, S. Kim, US patent no. 6383345. [8] D. Kim, S. Kim, Thin Solid Films 408 (2002) 204. [9] M. Bender, W. Seelig, C. Daube, H. Frankenberger, B. Ocker, J. Stollenwerk, Thin Solid Films 326 (1998) 72. [10] F. Neusch, L. Rothberg, E. Forsyth, Q. Le, Y. Gao, Appl. Phys. Lett. 74 (1999) 880. [11] D. Kim, S. Kim, J. Vac. Sci. Technol. A 20 (4) (2002) 1314. [12] G. Haacke, J. Appl. Phys. 47 (1976) 4086. [13] S. Ray, R. Banerjee, N. Basu, A. Barua, J. Appl. Phys. 54 (1983) 3497.