High-quality ultrathin chemical-vapor-deposited Ta2O5 capacitors prepared by high-density plasma annealing

Materials Science and Engineering B 106 (2004) 234–241

High-quality ultrathin chemical-vapor-deposited Ta2 O5 capacitors prepared by high-density plasma annealing C.H. Liu a , S.J. Chang b,∗ , J.F. Chen b , S.C. Chen c , J.S. Lee b , U.H. Liaw d b

a Department of Electronic Engineering, Nan-Jeon Institute of Technology, Yan-Hsui, Taiwan 737, ROC Department of Electrical Engineering, Institute of Microelectronics, National Cheng Kung University, Tainan 70101, Taiwan, ROC c Department of Electronic Engineering, Institute of Electronics and Information Engineering, National Yunlin University of Science and Technology, Touliu, Taiwan 640, ROC d Department of Electronic Engineering, Chin-Min College, To-Fen, Taiwan 351, ROC

Received 3 April 2003; accepted 4 September 2003

Abstract Highly reliable ultrathin low-pressure chemical-vapor-deposited (LPCVD) tantalum pentoxide (Ta2 O5 ) capacitors were fabricated by using high-density plasma (HDP) annealing in N2 O at 400 ◦ C after film deposition. It was found that HDP annealing in N2 O could significantly reduce the leakage current of Ta2 O5 capacitors so as to produce better time-dependent dielectric breakdown characteristics than either conventional oxygen-plasma annealing or high-density plasma annealing in O2 . It was also found that HDP annealing in N2 O will not significantly increase the thickness of interface layer between Ta2 O5 and the underlying silicon bottom electrode. Therefore, highly reliable ultrathin Ta2 O5 capacitors can be fabricated by using HDP annealing in N2 O. The leakage current mechanism was also investigated. It was found that Poole–Frenkel emission and Fowler Nordheim (FN) tunneling was the main leakage mechanism in low and high electric field, respectively, as for the deposited Ta2 O5 films. For the N2 O HDP-annealed Ta2 O5 films, it was found that, the main leakage mechanism was Schottky emission in low electric field and Poole–Frenkel emission in high electric field. © 2003 Elsevier B.V. All rights reserved. Keywords: Chemical-vapor-deposited; Capacitors; Annealing

1. Introduction In the development of integrated circuit, metal-oxidesemiconductor (MOS) device plays an important role. The characteristics of the MOS device are dominated by the characteristics of the insulator layer. The development of high-density MOS, dynamic random access memory (DRAM) has been accomplished by reducing the thickness of the SiO2 storage capacitors to maintain the required charge storage level. One of the most important features for a material to be used as the insulating dielectric in DRAM is the low leakage current. However, the reduction of the SiO2 thickness will result in a significant increase in leakage current. Therefore, various high dielectric constant (k) materials have been recommended to replace SiO2 . Under the same equivalent oxide thickness, it is required that the ∗ Corresponding author. Tel.: +886-62757575 x 62391; fax: +886-62761854. E-mail address: [email protected] (S.J. Chang).

0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.09.030

leakage current of the high dielectric constant material has to be lower than that of the conventional SiO2 . Tantalum pentoxide (Ta2 O5 ) has been regarded as one of the most promising element to replace SiO2 due to its high dielectric constant of about 20–60 [1–5]. The excellent properties of good step coverage [6,7] and good dielectric strength [8,9] after annealing also make Ta2 O5 suitable for high-density DRAM applications. However, the as-deposited Ta2 O5 exhibits a much larger leakage current than that of SiO2 due to the existence of organic impurities and/or oxygen vacancies in the as-deposited Ta2 O5 . These trap or defect states will induce a large leakage current through Frenkel–Poole emission or trap-assisted tunneling [10]. Various post-deposition annealing techniques such as plasma O2 annealing, plasma N2 O annealing [11], furnace O2 annealing [3,5], furnace N2 O annealing [8], rapid thermal O2 annealing [4], and rapid thermal N2 O annealing [12,13] were proposed to reduce the leakage current and to improve the quality of the as-deposited Ta2 O5 films. However, most of these annealing processes were operated at high

C.H. Liu et al. / Materials Science and Engineering B 106 (2004) 234–241

2. Experimental The Ta2 O5 films used in this study were all deposited on 200 mm p-type (1 0 0) silicon wafer by low-pressure chemical-vapor deposition (LPCVD) using tantalum ethylate (Ta(OC2 H5 )5 ) and oxygen gas (O2 ) as the Ta and O sources, respectively. During Ta2 O5 deposition, the gas flow rates of Ta(OC2 H5 )5 and O2 were kept at 0.18 sccm and 3.2 slm, respectively, the chamber temperature was kept at 445 ◦ C and the chamber pressure was kept at 9 Torr. The thickness of as-deposited Ta2 O5 films measured by ellipsometry was about 10 nm. The films were subsequently annealed by conventional plasma O2 (PO2 ), HDP O2 (HDPO2 ) and HDP N2 O (HDPN2 O). The HDP annealing processes were performed at 400 ◦ C for 60 s. During HDP annealing, the chamber pressure was kept at 5 mTorr. The high plasma density (1 × 1011 to 1 × 1012 ions/cm3 ) in the HDP chamber was generated by a 380 kHz rf inductive coupler operated at 2500 W. On the other hand, conventional plasma O2 annealing was performed in a plasma enhanced chemical-vapor deposition (PECVD) chamber using a 380 kHz rf generator at 400 ◦ C with different chamber pressures and plasma power densities. After annealing, titanium nitride (TiN) was deposited onto the Ta2 O5 surface by dc sputtering. Then, a 500 nm thick aluminum (Al) layer was thermal evaporated on top of the TiN to serve as the top electrodes of the capacitors. The top electrodes were subsequently patterned and etched to define the 7.85×10−5 cm2 capacitor area. Another 500 nm thick Al layer was deposited onto the backside of the Si wafer to serve as the bottom electrodes. Finally, all samples were sintered in N2 ambient at 400 ◦ C for 30 min to complete the fabrication of Ta2 O5 capacitors. The physical properties of the Ta2 O5 films were characterized by transmission electron microscopy (TEM) and secondary ion mass spectroscopy (SIMS). The capacitance–voltage (C–V) and current–voltage (I–V) characteristics of these fabricated Ta2 O5 capacitors were then measured by an HP4284B LCR meter operated at 1 MHz and an HP4156 semiconductor parameter analyzer, respectively. During I–V

measurements, the top gate electrodes of the Ta2 O5 capacitors were negatively biased with respect to the bottom electrodes.

3. Results and discussions Fig. 1 shows the I–V characteristics of the Ta2 O5 films annealed by O2 plasma with different rf power densities under a chamber pressure of 200 mTorr for 30 min. It can be seen that although the as-deposited Ta2 O5 film exhibited a large leakage current, O2 plasma annealing could effectively suppressed the leakage current and the leakage decreases as the rf power increases. Also, it was found that the effective oxide thickness of the as-deposited Ta2 O5 film was 2.18 nm while that of the O2 plasma-annealed Ta2 O5 film was 2.19, 2.22 and 2.24 nm when the rf power density equals to 0.308, 0.616 and 0.925 W/cm2 , respectively. The leakage current decreases and the small increment of effective oxide thickness could be attributed to the diffusion of atomic oxygen (O) into Ta2 O5 films [14]. During O2 plasma annealing, these oxygen species could eliminate organic impurities (i.e. hydrogen and carbon) in the Ta2 O5 films by oxidizing them to form oxidative products (CO, CO2 , and H2 O). These oxygen species could also fill the oxygen vacancies inside Ta2 O5 films [6]. A small amount of oxygen species could even reach Ta2 O5 /Si interface and react with silicon substrate to form additional interfacial oxide (SiOx ). As a result, the effective oxide thickness became slightly larger after O2 plasma annealing. Fig. 2 shows the I–V characteristics of the Ta2 O5 films annealed by O2 plasma under different chamber pressures with a 0.925 W/cm2 rf power density for 30 min. It can be seen that the leakage current is smaller when the chamber pressure is lower. Such an observation can be understood by the fact that when the chamber pressure is lower, the reactive oxygen species can gain more energy and thus we can achieve a smaller leakage

1x103

Leakage current density (A/cm2)

temperatures in oxygen containing ambiance and a thin SiOx layer could be easily formed at the Si/Ta2 O5 interface. The SiOx layer will significantly lower the effective capacitance per unit area [2]. On the other hand, leakage current will rapidly increase if the Ta2 O5 films were crystallized. Thus, low-temperature annealing processes, such as plasma annealing, are preferred since the formation of interfacial SiOx layer and the crystallization of Ta2 O5 film could both be minimized. In this paper, we adopted a new process technique of high-density plasma (HDP) N2 O annealing to obtain highly reliable Ta2 O5 films. HDP contains a large number of oxygen species, which can effectively decrease the concentration of organic impurities and repair the oxygen vacancies in the Ta2 O5 films. The detailed physical and electrical properties of HDP N2 O-annealed Ta2 O5 films will be reported.

235

O2 plasma

1x101 1x10-1 1x10-3 1x10-5 1x10-7

As-deposited 2 0.308 Watt/cm 2 0.616 Watt/cm 2 0.925 Watt/cm

1x10-9 1x10-11

0

1

2

3

4

5

6

7

8

9

Voltage (Volt) Fig. 1. Current density vs. the applied voltage for Ta2 O5 capacitors annealed by O2 plasma as a function of the rf power density.

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1x103

Leakage current density (A/cm2)

Leakage current density (A/cm2)

1x103 O2 plasma

1x101 1x10-1 1x10-3 1x10-5 1x10-7 As-deposited 500 mtorr 200 mtorr

1x10-9 1x10-11

0

1

2

3

4

5

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7

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9

Voltage (Volt)

current. Fig. 3 shows the I–V characteristics of the Ta2 O5 films annealed by O2 plasma under a chamber pressure of 200 mTorr with a 0.925 W/cm2 rf power density for different amount of annealing times. It can be seen that the leakage current decreases as the annealing time increases. Also, it was found that the effective oxide thickness was 2.20, 2.24, and 2.29 nm when the annealing time equals 10, 30, and 40 min, respectively. HDP annealing on the as-deposited Ta2 O5 films was also performed in both O2 (HDPO2 ) and N2 O (HDPN2 O) environments. For comparison, we call the conventional O2 plasma annealing process with a 0.925 W/cm2 rf power density under a chamber pressure of 200 mTorr for 40 min, which gives us the smallest O2 plasma leakage current, “PO2 annealing”. Fig. 4 shows the I–V characteristics of the as-deposited Ta2 O5 film and the Ta2 O5 films annealed by different methods (i.e. PO2 , HDPO2 and HDPN2 O). Compared with PO2 -annealing, it was found that HDPO2 was more effective in suppressing leakage current. Also, HDPN2 O annealing could further reduce the leakage current

Leakage current density (A/cm2)

1x103 O2 plasma

1x101 1x10-1 1x10-3 1x10-5 As-deposited 10 mins 30 mins 40 mins

1x10-9 1x10-11

0

1

2

3

4

5

6

1x10-1 1x10-3 1x10-5 As-deposited PO2 HDPO2 HDPN2O

1x10-7 1x10-9 1x10-11

0

1

2

3

4

5

6

7

8

9

Voltage (Volt)

Fig. 2. Current density vs. the applied voltage for Ta2 O5 capacitors annealed by O2 plasma as a function of the chamber pressure.

1x10-7

1x101

7

8

9

Voltage (Volt) Fig. 3. Current density vs. the applied voltage for Ta2 O5 capacitors annealed by O2 plasma as a function of annealing time.

Fig. 4. Current density vs. the applied voltage for Ta2 O5 capacitors annealed by high-density plasma annealing in O2 and N2 O as well as conventional plasma annealing in O2 .

down to 2 × 10−10 A/cm2 at 1.5 V, while the leakage current was 6 × 10−10 and 2 × 10−8 A/cm2 for the HDPO2 - and PO2 -annealed samples. It was also found that the effective oxide thickness was 2.29, 2.31, and 2.40 nm, respectively, for the PO2 -, HDPO2 - and HDPN2 O-annealed samples. Although the 60 s HDP annealing time was much shorter than the 40 min annealing time of the conventional O2 plasma annealing, much more active oxygen species were generated during HDP annealing. Thus, we can still achieve a much smaller leakage current by using HDP annealing. Previously, it has been shown that oxygen ions (O2 + ) will induce some plasma damage and atomic oxygen (O) is more effective in reducing leakage current [14]. However, it has been shown that the number of oxygen ion (O2 + ) is much larger than the number of atomic oxygen (O) in O2 plasma [14]. In contrast, the number of atomic oxygen (O) is much larger than the number of oxygen ion (O2 + ) in N2 O plasma. Thus, oxygen ions (O2 + ) plasma-induced damage is smaller in HDPN2 O annealing so that we can achieve an even smaller leakage current, as compared to HDPO2 annealing. The small HDPN2 O leakage current could also be partially attributed to the high-density reactive atomic oxygen (O) generated by the dissociation of N2 O during HDPN2 O annealing. Comparing with the oxygen ions (O2 + ), the excited oxygen atoms (O) can diffuse more effectively into the Ta2 O5 films. Furthermore, it has been reported that the energy needed to break nitrogen–oxygen bond in N2 O molecule is smaller than the energy needed to break oxygen–oxygen bond in O2 molecule [11]. Thus, HDPN2 O annealing can provide us more active oxygen atoms (O) and thus a smaller leakage current in Ta2 O5 films. Fig. 5(a)–(d) show the SIMS profiles of the as-deposited Ta2 O5 film and the Ta2 O5 films annealed by PO2 , HDPO2 , and HDPN2 O, respectively. It was found that the residual carbon and hydrogen concentrations in Ta2 O5 films both decreased after annealing. It was also found that the residual

C.H. Liu et al. / Materials Science and Engineering B 106 (2004) 234–241

22

10

10

6

10

237

22

6

10 PO2

As-deposited

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C 18

10

10

17

10

10

16

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18

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22

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10 HDPN2O

Concentration (atoms/cc)

S i( C oun t s ) - - >

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H

19

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3

C

10

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10

(c)

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10 300

21

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2

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18

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17

1

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16

0

Depth (angstroms)

5

10

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10

Secondary ion intensity (cts/sec)

21

10 300

6

10

HDPO2

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Depth (angstroms)

(b)

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C

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(a)

21

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(d)

0

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Concentration (atoms/cc)

20

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Secondary ion intensity (cts/sec)

5

Concentration (atoms/cc)

10 Si(Counts)-->

Secondary ion intensity (cts/sec)

21

10

0

50

100

150

200

250

10 300

Depth (angstroms)

Fig. 5. SIMS depth profiles of Ta2 O5 film on Si substrate: (a) as-deposited, (b) after PO2 annealing, (c) after HDPO2 annealing, and (d) after HDPN2 O annealing.

carbon and hydrogen concentrations in Ta2 O5 films were the lowest for samples annealed in HDPN2 O, followed by HDPO2 . These results agree well with those observed from I–V measurements shown in Fig. 4. Fig. 6(a)–(c) show the high resolution TEM micrographs of the as-deposited Ta2 O5 film and the Ta2 O5 films annealed by HDPO2 and HDPN2 O, respectively. It was found that the thickness of Ta2 O5 films is 8.8–8.9 nm for all three samples, and the thickness of interfacial oxide is 0.9, 10 and 1.1 nm for the as-deposited Ta2 O5 film and the Ta2 O5 films annealed by HDPO2 and HDPN2 O, respectively. Such an interlayer was thought to be the transition oxide layer (SiOx ) formed during the exposition of the silicon wafer in atmosphere and the subsequent deposition process at an oxygen-contained ambient.

Thus, the slight increase in interfacial oxide layer thickness for HDP-annealed samples is attributed to the oxidation of silicon substrates during HDP annealing. It should be noted that although the leakage current is significantly reduced as shown in Fig. 4, the increase in interfacial layer thickness is negligibly small for the HDP-annealed samples. Fig. 7 shows the time-dependent dielectric breakdown (TDDB) results for Ta2 O5 films annealed by PO2 , HDPO2 and HDPN2 O. During TDDB measurements, the top electrodes of the Ta2 O5 capacitors were negatively biased with an effective oxide field (Eeff ) equals 28 MV/cm. In other words, the stress voltage (Vg ) defined by Eeff Tox was applied onto the three annealed capacitors. Tox is the effective oxide thickness obtained from C–V measurements defined

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Fig. 6. Cross-sectional TEM micrographs of Ta2 O5 film on Si substrate: (a) as-deposited, (b) after HDPO2 annealing, and (c) after HDPN2 O annealing.

by Tox = ε0 εs A/C, where C is the capacitance measured at 1 MHz, ε0 the permittivity in vacuum, εs the dielectric constant of SiO2 (3.82), and A the area of the capacitors. During TDDB measurement, no random failure modes were observed, which suggests high-quality and good uniformity for the plasma-annealed CVD Ta2 O5 capacitors. It can be seen that the HDPN2 O-annealed sample has the longest breakdown time among the three annealed samples. The TDDB stress time of 50% cumulative failure for HDPN2 O capacitors is about 3.5 and 5.3 times longer than that of HDPO2 and PO2 capacitors. Fig. 8 shows the dependence

of TDDB lifetime on the electric field for the three annealed Ta2 O5 capacitors. Again, HDPN2 O capacitor shows the longest lifetime among the three capacitors. The extrapolated long-term lifetime indicates that HDPN2 O capacitors can survive 10 years at a stress field of about 16 MV/cm. Fig. 9(a)–(c) show the leakage currents measured under three different electrical fields as functions of temperature for the as-deposited Ta2 O5 film and the Ta2 O5 films annealed by HDPO2 and HDPN2 O, respectively. As shown in Fig. 9(a), it was found that the leakage current at low field (i.e. 1 MV/cm) increases with increasing temperature

C.H. Liu et al. / Materials Science and Engineering B 106 (2004) 234–241

239

1E-1

99

As-deposited 1E-2

Current density (A/cm2)

Cumulative failure (%)

Eeff = 28 MV/cm

90

70 50 30 PO2 HDPO2 HDPN2O

10

1 10 3

10 4

1E-4 Ea = 0.181 eV

1E-5

1E-6 1.5

3

3.5

HDPO2

10

10 years

Current density (A/cm2)

1E-6 Ea = 0.245 eV

1E-7 Ea = 0.391 eV

1E-8

1 MV/cm 2 MV/cm 3 MV/cm 2

Ea = 0.472 eV

2.5

3

3.5

1000/T (1/K)

(b) 1E-3

HDPN2O 1E-4

Ea = 1.086 eV Ea = 1.022 eV

1E-6 1E-7 1E-8

1E-10 1.5

(c)

Ea = 1.097 eV

1E-5

1E-9

8

10

Ea = 1.071 eV Ea = 1.067 eV

1E-10 1.5

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6

Ea = 1.019 eV

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1E-9

Current density (A/cm2)

Time to 50% Cumulative Failure (sec)

2.5

1E-3 1E-4

10

Ea = 0.304 eV

1 MV/cm 2 MV/cm 3 MV/cm 2

Ea = 0.502 eV Ea = 0.546 eV

2.5

3

3.5

1000/T (1/K)

Fig. 9. Arrhenius plots of the leakage current density of Ta2 O5 film E = 1, 2, and 3 MV/cm: (a) as-deposited, (b) after HDPO2 annealing, and (c) after HDPN2 O annealing.

4

10

PO2 HDPO2 HDPN2O

0

10

2

1000/T (1/K)

10 5

with a 0.18 eV activation energy, which suggests that the leakage current is not dominated by tunneling mechanism [15,16]. Under high electric field (i.e. 2 and 3 MV/cm), the leakage is less temperature dependent, which suggests that the leakage current is probably dominated by Fowler Nordheim (FN) tunneling [17–19]. As shown in Fig. 9(b) and (c), the temperature-dependent behaviors of leakage current for HDPO2 -annealed films and HDPN2 O-annealed films are similar. Regardless of applied electric field, the activation energies measured at high-temperature region were all in between 1.0 and 1.1 eV for both HDPO2 - and HDPN2 O-annealed samples. Such a result again suggests that the leakage current is not dominated by tunneling mechanism [15,16]. In order to clarify the dominant mechanism of leakage current in these samples, leakage current

10

1 MV/cm 2 MV/cm 3 MV/cm

(a)

Fig. 7. TDDB stress time dependence of cumulative failure of Ta2 O5 capacitors for three different annealed samples.

2

Ea = 0.055 eV

1E-3

Stress Time (sec)

10

Ea = 0.071 eV

15

20

25

30

35

40

Effective Electric Field (MV/cm) Fig. 8. Dependence of TDDB lifetime on the electric field for three different annealed Ta2 O5 capacitors.

density (J) and leakage current density divided by electric field (log10 (J/E)) measured at room temperature are plotted as functions of electric field (E1/2 ), as shown in Figs. 10 and 11. The dynamic dielectric constant is determined from the slop of the straight line in the foregoing plots. Only the self-consistent dynamic dielectric constant can ensure the conduction mechanism of the leakage current in Ta2 O5

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Table 1 The dynamic dielectric constant (εd ) and the conduction mechanism of the Ta2 O5 film before and after plasma annealing at room temperature √ √ Region d log(J)/d E d log(J/E)/d E εd Conduction mechanism As-deposited I III

6.39E−03

PO2 I II III IV

4.89E−03

1.0045548 6.8756476

– Poole–Frenkel

4.51E−04 7.20E−03

28.539862 0.7054957 808.45373 3.1644679

Schottky – – Poole–Frenkel

1.04E−03 6.67E−03

15.335416 0.832076 152.04888 3.6968705

Schottky – – Poole–Frenkel

1.05E−03 7.63E−03

11.11004 0.6291929 149.5315 2.8183352

Schottky – – Poole–Frenkel

1.20E−03 7.63E−03

HDPO2 I II III IV

1.64E−03 7.02E−03

HDPN2 O I II III IV

1.92E−03 8.08E−03

1x10

1x102

J ( A/cm2 )

1x100

¢¼

-8

1x10

¢»

-10

1x10

-12

1x10

-14

1x10

As-deposited PO2 HDPO2 HDPN2O

-16

1x10

-18

1x10

0

1

E

2 0.5

3

0.5

( MV/cm )

the HDP annealing process, the conduction mechanisms of the leakage current in Ta2 O5 film were changed. For the plasma-annealed Ta2 O5 film, the Schottky emission mechanism dominates the leakage current at low electric field and the Poole–Frenkel emission mechanism dominates the leakage current at high electric field.

¢º

-2

¢¹ 1x10-4 1x10-6 As-deposited PO2 HDPO2 HDPN2O

1x10-8 1x10-10 1x10-12

-6

1x10

Fig. 11. Poole–Frenkel plot of the Ta2 O5 film before and after high-density plasma annealing.

4

1x10

-4

1x10

J/E ( A/V.cm )

film [20]. It means that the dynamic dielectric constant (εd ) should be between the optical dielectric constant (εop ) (square of the refractive index obtained from ellipsometry, εop = n2 ) and the static dielectric constant (εs ) obtained from C–V measurement or very close to the optical dielectric constant. The optical dielectric constant is about 4 and the static dielectric constant is about 26. Figs. 10 and 11, respectively, show the Schottky plot and the Poole–Frenkel plot of the Ta2 O5 film before and after plasma annealing. The calculation of the dynamic dielectric constant in log10 (J) versus E1/2 plot and log10 (J/E) versus E1/2 plot is shown in Table 1. For the as-deposited Ta2 O5 capacitors, we identified the leakage current was dominated by the Poole– Frenkel emission mechanism at low electric field. After the conventional plasma annealing process or

0

1

E

2 0.5

3

0.5

( MV/cm )

Fig. 10. Schottky plot of the Ta2 O5 film before and after high-density plasma annealing.

4. Conclusion The HDPN2 O annealing reveals the best ability to improve the electric characteristics and TDDB reliability of the LPCVD Ta2 O5 film as compared with the HDPO2 annealing and conventional plasma O2 annealing. These results showed that the low-temperature HDPN2 O annealing

C.H. Liu et al. / Materials Science and Engineering B 106 (2004) 234–241

has the potential of fitting the manufacturing requirement of future generation of integrated circuits. Ta2 O5 , like most insulators, shows complicated current–voltage characteristics depending on the temperature and electric field regime. At room temperature, conduction mechanisms such as Poole–Frenkel conduction at high field and Schottky emission at low field have been identified in the plasma-annealed Ta2 O5 films on Si substrates with an interfacial SiO2 layer. Acknowledgements This work is partially supported by the National Science Council under contract number NSC-89-2215-E-006-005. References [1] K. Kishiro, N. Inoue, S.C. Chen, M. Yoshimaru, Jpn. J. Appl. Phys. 37 (1998) 1336. [2] S.W. Park, Y.K. Baek, J.Y. Lee, C.O. Park, H.B. Im, J. Electron. Mater. 21 (1992) 635. [3] S. Zaima, T. Furuta, Y. Yasuda, J. Electrochem. Soc. 137 (1990) 1297. [4] A. Pignolet, G.M. Rao, S.B. Krupanidhi, Thin Solid Films 258 (1995) 230.

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