Effect of rapid thermal annealing of sputtered aluminium nitride film in an oxygen ambient

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Materials Science in Semiconductor Processing 9 (2006) 1137–1141

Effect of rapid thermal annealing of sputtered aluminium nitride film in an oxygen ambient Kyungsoo Jang, Kwangsoo Lee, Junsik Kim, Sunghyun Hwang, Jeongin Lee, Suresh Kumar Dhungel, Sungwook Jung, Junsin Yi School of Information and Communication Engineering, Sungkyunkwan University, 300 Chunchun-Dong, Jangan-Gu, Suwon 440-746, Republic of Korea Available online 20 December 2006

Abstract Aluminium nitride (AlN) thin films were deposited by radio frequency (RF) magnetron sputtering on p-type silicon (Si) substrate of (1 0 0) orientation using only argon (Ar) gas at substrate temperature of 300 1C. In order to achieve improved electrical properties, we performed post-deposition rapid thermal annealing (RTA). Sputtered AlN films were annealed in an oxygen ambient at temperatures of 600, 700, and 800 1C using RTA for 30 min. The orientation of the AlN crystal in the film was investigated using X-ray diffraction (XRD). The characteristic spectra by functional group were analyzed by Fourier transformation infrared (FTIR) spectroscopy. The electrical properties of the AlN thin films were studied through capacitance–voltage (C– V) characteristics in metal–insulator–semiconductor (MIS) device using the films as insulating layers. The flatband voltages (VFB) in C– V curves were found to depend on crystal orientations. Negative VFB was found in the case when AlN (1 0 0) peak was found. Also, when AlN (1 0 3) peak was observed upon increasing the annealing temperature, the value of VFB was positive and after annealing at 700 1C, AlN (1 0 3) peak intensity was found to be maximum and VFB was as high as+6.5 V. r 2006 Published by Elsevier Ltd. Keywords: Aluminium nitride; Sputtering; RTA; FTIR; XRD; Negative VFB

1. Introduction In recent years, aluminium nitride (AlN) films have found a widespread application stemming from their great technological advantages in the microelectronics industry owing to the wide band gap of AlN, low electrical but high thermal conductivity, high optical transmission, and high decomposition temperature [1–4]. Since an AlN film Corresponding author. Tel.: +82 31 290 7139; fax: +82 31 290 7159. E-mail address: [email protected] (J. Yi).

1369-8001/$ - see front matter r 2006 Published by Elsevier Ltd. doi:10.1016/j.mssp.2006.10.052

with a wurzite hexagonal structure has a piezoelectric property with a high acoustic velocity, AlN is well suited as a piezoelectric material for highfrequency surface acoustic wave (SAW) devices [5]. There is a current trend of using metal–insulator– semiconductor (MIS) structures in electronic devices [6,7]. Aluminium oxide (Al2O3)-based dielectric thin films [8] have been extensively studied as a promising gate insulator alternative to conventional silicon dioxide (SiO2) and silicon oxynitride (SiON) because of their high thermal stability against crystallization as compared with other candidates such as zirconium dioxide (ZrO2) and

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hafnium dioxide (HfO2). Until now only a few works have reported negatively charged MIS structure [9–12]. Employing a negatively charged insulator–semiconductor structure to the back surface of p-type silicon solar cell could replace the conventional back surface passivating dielectric layers like silicon nitride (Si3N4) and silicon dioxide (SiO2). Such conventional films have been mostly reported to have fixed a positive charge. The inevitable trend towards thinner bulk silicon solar cells makes the issue of the field-induced passivation by a fixed negative charge more and more significant. In this connection, the scientific community has for a long time been hunting for a proper dielectric film with an appreciable fixed negative charge. It has been found that thermally oxidized AlN films have excellent electrical and optical properties. AlN films are characterized by a high density of fixed negative charges at the interface which can be used for field-induced surface passivation creating a very effective back surface field (BSF) layer at the silicon interface. AlN films have been prepared by various methods including radio frequency (RF) magnetron sputtering [13,14], CVD [15,16], and reactive molecular beam epitaxy [17–19]. Among them, sputtering has advantages over conventionally used high temperature techniques for thin film deposition because of its simplicity, low cost, and the ability to obtain good quality films with the requested properties [20–22]. The properties of AlN films are not only influenced by the deposition method but also depend on the post-deposition heat treatment, such as annealing [23]. In this work, the influence of rapid thermal annealing (RTA) in oxygen ambient on the electrical properties of AlN is studied. A comparison to the case without oxidizing treatment is also done. 2. Experimental In this study, samples were prepared by RF magnetron sputtering system with a magnet assembly and a power of 100 W. Low RF power of 100 W was used in order to minimize the substrate heating and the lattice damage due to electron and ion bombardment. The substrates were thoroughly cleaned just prior to the deposition. The cleaning sequence of the silicon substrate was designed for the removal of organic impurities (trichloroethylene, acetone, methanol, sulphuric acid/hydrogen peroxide). As the target, AlN with 99.95% purity

Table 1 AlN film deposition parameter and condition using sputtering Parameter

Condition

Base pressure Working pressure Target (purity) Sputtering gas Substrate temperature RF power Annealing temperature (Reference: as deposited) Annealing gas (time)

6.6  IE-3 Pa 0.267 Pa AlN (99.95%) Argon 300 1C 100 W 600, 700, 800 1C Oxygen (30 min)

and a diameter of 4 in. was used. The sample temperature was determined with a thermocouple inserted through the side and screwed to the bottom of the substrate holder. The p-type silicon substrate was in thermal contact to the substrate holder. The nitride was sputtered on Si(1 0 0) substrate at 300 1C for 50 min at a base pressure of 6.6  103 Pa. The working pressure while using only argon gas was 0.267 Pa. An ionization gauge was applied to monitor the pressure. To reach the desired gas pressure and ratios, pre-sputtering was carried out for 10 min using 99.99% pure argon (Ar) gas. Various parameters, such as pressure, temperature, and RF power, were varied for the deposition of AlN thin films. The deposition conditions and parameters used in this work are shown in Table 1. AlN films were subjected to the post-deposition RTA at 600, 700, and 800 1C for 30 min in an oxygen ambient. To make the MIS (Al/AlN/po1004Si) structure, aluminium dots of different diameters such as 840, 550, 369, and 240 mm were deposited on AlN film coated on the same substrate by using a metal mask during thermal evaporation. The high frequency (1 MHz) C– V characteristics of the MIS structure were measured with an MDC 617 and Keithley LF 4192 analysis system. Crystalline phases and growth directions were determined by the X-ray diffraction (XRD) (Rigaku, 12 kW, Japan) with CuKa radiation operated at 40 kV and 100 mA. To analyze the characteristic spectrum by functional group, Fourier transformation infrared (FTIR) spectroscopy (Bruker, IFS-66/S, Germany) was also employed. 3. Results and discussion Fig. 1 shows the C– V curves of the MIS structures taken from accumulation to inversion and obtained

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at 1 MHz. A big change in the flatband voltage (VFB) is observed when one compares the oxidized and asdeposited AlN films, as shown in Fig. 1. Fig. 2 and Table 2 reveal that the flatband voltage (VFB) and the flatband voltage difference (DVFB) are closely related to the annealing temperature. In the case of as-deposited film, a very big negative VFB value of 18.5 V is obtained. In this case, there is no fixed negative charge at all. In order to investigate the stability of negative charge in the film, after deposition the samples were annealed in oxygen ambient at 600, 700, and 800 1C. This procedure generated VFB of at least +3.5 V. In the case of oxidation, the negative charge density was observed to be sufficiently high, such as in the

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case of Al2O3. This means that the property of the film is improved after annealing. When the annealing temperature was 700 1C, VFB was as high as +6.5 V. This size of this variation shows that the oxidation temperature plays a main role in the increment of flatband voltage. Table 3 shows the flatband voltage and the negative charge density obtained by different groups. The flatband voltage is given by V VB ¼ Fms 

QOX , C OX

where VFB is the flatband voltage, Fms is the work function of the gate, QOX is the fixed negative charge, and COX is the oxide capacitance. In this study, AlN films were deposited using RF magnetron sputtering and oxidized at 700 1C. At these conditions a flatband voltage of +6.5 V and a high number of density of negative charge of 1  1013 cm2 was obtained, which is significantly higher than in as-deposited AlN films. Fig. 3 shows the XRD spectrum of the AlN film deposited on Si substrate. From the XRD pattern, we establish the orientation of AlN crystal; in Table 2 Flatband voltage (VFB) and flatband voltage difference (DVFB) related to annealing temperature

Fig. 1. Capacitance–voltage curves of MIS (Al/AlN/Si) structure. The line labels are: (A) as deposited, (B) 600, (C) 700, (D) 800 1C.

Temperature (1C)

VFB (V)

DVFB (V)

As deposited 600 700 800

18.5 +4.5 +6.5 +3.5

0 +23 +25 +22

Fig. 2. Flatband voltage (VFB) and flatband voltage difference (DVFB) related to annealing temperature.

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Table 3 Comparison of material, VFB, QOX/q, deposition method Group

Material

VFB (v)

QOX/q (cm2)

Deposition method

IMEC (Belgium) Dirk Konig (University of Chemnitz) J. Kolodzey (University of Delaware) Hideki Murakami (University of Hiroshima) Our group

(Al2O3)  (TiO2) AlF3/SiO2 Al2O3 AlON

0.6 3.76 1.13 0.6

5  1011 2  1011 1  1011 4  1012

Atomic layer deposition CVD Sputtering CVD

AlN

6.5

1  1013

Sputtering

Fig. 3. XRD pattern analysis of deposited AlN film and annealing controlled AlN film. The peak and line labels are: (A) As deposited, (B) 600, (C) 700, (D) 800 1C, (I) AlN (1 0 0), (II) Al2O3 (1 2 2), (III) AlN (1 0 3), and (IV) Al2O3 (2 1 4).

addition, the influence of the annealing temperature on the peak intensities may be observed. Indeed, the XRD pattern exhibits different peak intensities after different annealing temperatures, namely 600, 700, and 800 1C. Fig. 3(a) overviews the complete XRD patterns associated with each annealing temperature. As seen in Fig. 3(b), the AlN(1 0 0) peak located at 2y ¼ 33:11 appears only in the asdeposited AlN film. The Al2O3(1 2 2) peak located

at 2y ¼ 61:61 is presented in Fig. 3(c), while the AlN(1 0 3) peak located at 2y ¼ 65:81 and the Al2O3(2 1 4) peak located at 2y ¼ 66:41 are shown in Fig. 3(d). From Fig. 3(d), it follows that the AlN(1 0 3) peak from the as-deposited film is negligible: its intensity is low as compared to that of the peaks arising from annealed AlN films. Finally, Fig. 4 displays the results of FTIR spectroscopy. The strong absorption peak at 668 cm1

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quently, we expect that AlN films could be applied to enhance efficiency of crystalline silicon solar cell by using them for back surface passivation.

References

Fig. 4. FTIR spectra of AlN films. The peak and line labels are: (A) as deposited, (B) 600, (C) 700, (D) 800 1C, (I) Al–O bond at 613 cm1, and (II) Al–N bond at 668 cm1.

is associated with the Al–N bond and is due to transverse optical phonon modes of AlN. The absorption peak associated with the Al–O bond (at 613 cm1) is visible as well. Also this figure shows that the physical properties of the AlN film depend on annealing temperature. 4. Conclusion We have investigated the effect of deposition conditions for the RF magnetron sputtering of AlN on 1–20 Ocm p-type (1 0 0) silicon substrates. The film quality has been presented as a function of gas pressure, sputtering power, substrate temperature, and annealing temperature. The optimal conditions for the best VFB at 100 W were identified to be: the gas pressure of 0.267 Pa, the substrate temperature of 300 1C, and the annealing temperature of 700 1C. The fact that the optimal negative VFB can be obtained through high-temperature annealing opens the possibility to control the electrical properties of AlN thin films to a significant degree. The total fixed interface charge of MIS structure consisting of (Al/AlN/P/1 0 0SSi) was estimated by C– V measurement to be equal to 1  1013 cm2. Conse-

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