[Al.sub.2][O.sub.3] films with Ni-based buffer layer prepared by plasma-ion assisted deposition on Cu substrate.

In this study, [Al.sub.2][O.sub.3] films with an Ni-based buffer layer were prepared on a Cu substrate by plasma-ion assisted deposition (PIAD). The main purpose of this study is to develop a novel electrical insulating film to be used at high temperature. X-ray diffraction (XRD) spectra show the [Al.sub.2][O.sub.3] films prepared by this method are amorphous. The results of atomic force microscopy (AFM), scanning electron microscopy (SEM), and auger electron spectroscopy (AES) analyses reveal that the [Al.sub.2][O.sub.3] films perfectly adhere to the substrate through the buffer layer, no visible defects were observed, and no impurity from Ni or Cu was detected. The diffusion of Cu into the [Al.sub.2][O.sub.3] film at high temperature is suppressed. [Al.sub.2][O.sub.3] films with an Ni-based buffer layer exhibit excellent resistivity (>[10.sup.10][ohm]xcm) even after experiencing a high temperature environment as high as 600[degrees]C 10 times.

Keywords: [Al.sub.2][O.sub.3] films, PIAD, Ni-based buffer layer, insulation properties, atomic force microscopy, ESCA, XPS, auger, SIMS, X-ray scattering crystallography

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The development of space science and technology has put forward new requests for electrical insulating film. In some special electric motors, insulating films must withstand temperatures as high as 600[degrees]C for very short times. Commonly used organic insulating materials can only withstand temperatures of 450[degrees]C at most. Inorganic oxide materials possess inherent advantages in this aspect. Thus, it is of practical significance to investigate such inorganic oxide film materials and preparation technologies.

[Al.sub.2][O.sub.3] is a promising film material because of its high resistivity, temperature stability, and chemical stability. [Al.sub.2][O.sub.3] films have been demonstrated to be applicable in DRAMs, (1) gate insulators, (2) film capacitors, (3) and so on. It is reasonable to believe that [Al.sub.2][O.sub.3] could be utilized to obtain good insulating films for use at high temperature.

[Al.sub.2][O.sub.3] films have been previously formed by a variety of methods, including sol-gel, (4) chemical vapor deposition, (5) and physical vapor deposition. (6-7) Plasma-ion assisted deposition (PIAD) is a technique that utilizes energetic ions to bombard the thin film during deposition, improving the packing density and thereby producing very stable layers. (8-10)

In this article, the primary work was carried out for the investigation of insulation film to be used at high temperature. [Al.sub.2][O.sub.3] films were deposited on a Cu substrate by PIAD. Considering the difference between Cu substrate and [Al.sub.2][O.sub.3] film, for example, thermal expansion, an Ni-based buffer layer was applied. The resistivity of the films was studied. Several characterization techniques including scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and auger electron microscopy (AES) were used for the physical characterization of [Al.sub.2][O.sub.3] films with an Ni-based buffer layer.

EXPERIMENTAL

A Cu substrate was immersed in an acid mixture of nitric acid, phosphoric acid, and acetic acid for 5 min after acetone cleaning. The PIAD system used in this study was an APS-904 type machine (Leybold, Germany). The Ni-based buffer layer was evaporated on a tungsten boat at 100[degrees]C. The [Al.sub.2][O.sub.3] pellets were of high purity (99.99%), 1-2.5 mm in diameter. High purity argon and oxygen were used as bombarded and reactive gases, respectively. [Al.sub.2][O.sub.3] pellets were evaporated at a temperature of 150[degrees]C with a base pressure of 2 x [10.sup.-3] Pa. The evaporation rate was controlled around 0.4 nm/sec.

The as-deposited [Al.sub.2][O.sub.3] films were rapidly heated to 600[degrees]C, held for 5 min, then cooled in atmosphere. The resistivity was determined by a high resistance meter. The crystal structure was confirmed by Rigaku B/Max-2550V XRD. The surface and cross-section morphology were observed through SPI3800 and SPA300HV AFM and JSM-6700F field emission SEM. AES depth profiles were obtained using a Microlab310F auger microprobe.

RESULTS AND DISCUSSIONS

XRD Patterns

The crystal structure of [Al.sub.2][O.sub.3] films investigated by XRD method is shown in Figure 1. XRD patterns indicate that all the peak positions and intensities correspond to Cu substrate. No peaks relative to the preferred crystal orientation of alumina could be detected; thus, it is concluded that the [Al.sub.2][O.sub.3] films prepared by PIAD were amorphous.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

AFM Investigation of the Surface Morphology

The AFM surface morphology of the NiAl/[Al.sub.2][O.sub.3] film is shown in Figure 2. [Al.sub.2][O.sub.3] films prepared by PIAD presented good surface features. No obvious defects such as holes could be seen on the film surface, which should be beneficial for the insulating properties of [Al.sub.2][O.sub.3] films. A highly undulating surface is observed. The average roughness of a pure Cu substrate is also given for comparison, as indicated in Figure 2b. This roughness is characteristic of the unpolished Cu substrate, which was only rinsed chemically before deposition.

SEM Image of Cross-Section Morphology

Figure 3 shows the cross-section image of [Al.sub.2][O.sub.3] film with an NiAl buffer layer after 600[degrees]C test. The thickness of the [Al.sub.2][O.sub.3] film is about 500 nm. Ni-based alloy was chosen as the buffer layer due to the close crystal structure and crystal constant compared with Cu substrate. Ni-based alloy accommodates the thermal expansion coefficient differences between [Al.sub.2][O.sub.3] films and Cu substrate well, and improves the binding strength between them. The interface of Ni-based alloy and Cu substrate is blurred because their atomic number is very close. The interface of Ni-based alloy and [Al.sub.2][O.sub.3] film can be clearly seen in Figure 3. There is no peeling off of [Al.sub.2][O.sub.3] film from the buffer layer. No obvious defects such as pores or cracks are seen. There is no trace of column structure of [Al.sub.2][O.sub.3] film in the depth profile. The [Al.sub.2][O.sub.3] film perfectly adheres to the substrate through the buffer layer.

AES Analysis

To determine the diffusion behavior of the interface at high temperature, AES depth profiling was examined. Auger electron spectra depth profile of [Al.sub.2][O.sub.3] film with and without NiAl buffer layer after 600[degrees]C test are given in Figure 4. The etching end point was about 20 nm away from the interface of substrate or buffer layer and the [Al.sub.2][O.sub.3] film. In Figure 4a, peaks at around 919 ev and 836 ev correspond to Cu, and those around 506 ev and 1379 ev correspond to [Al.sub.2][O.sub.3], which indicates that the Cu substrate diffused into the [Al.sub.2][O.sub.3] layer when heated to 600[degrees]C. However, in Figure 4b, the peaks all correspond to [Al.sub.2][O.sub.3], and no peaks of impurity from Ni or Cu are detected. This suggests that the diffusion of Cu into [Al.sub.2][O.sub.3] film at high temperature was effectively supressed. Thus it is beneficial for the insulation properties of [Al.sub.2][O.sub.3] film at high temperature.

With the above results, the function of the Ni-based buffer layer can be further discussed as follows. There is a tendency of Al and Ni to be oxidized when exposed to oxygen.

[FIGURE 4 OMITTED]

2Al + 3/2 [O.sub.2] [right arrow] [Al.sub.2][O.sub.3] (1)

and Ni + 1/2 [O.sub.2] [right arrow] NiO (2)

From the thermodynamic data, Gibbs-free energy [DELTA]G and reaction constant k of reaction (1) and (2) at different temperatures can be calculated. The results are given in Table 1.

At room temperature, [DELTA]G and k of reaction (1) is large enough to make the reaction proceed rightwards in a minute or so. That is why [Al.sub.2][O.sub.3] passivation film forms spontaneously. With increasing temperature, k decreases, yet still stays above [10.sup.70]. This value could make reaction (1) go along rightwards irreversibly. Meanwhile, compared with reaction (1), [DELTA]G and k of reaction (2) could be ignored. It has been reported that, for the bimetal layer consisting of Ni and Al, a series of differentiated Auger electron spectra, recorded during the oxidation in-situ, revealed that only the oxidation of Al occurred and that of Ni was restrained. (1) This result is in accordance with our thermodynamic calculation.

Actually, in our study, oxygen plasma with partially ionized oxygen was introduced during the deposition of [Al.sub.2][O.sub.3] film. Obviously, at the beginning, an [Al.sub.2][O.sub.3] passivation thin film formed on the surface of the NiAl buffer layer automatically. It is reasonable to believe that the buffer layer is favorable for the deposition of [Al.sub.2][O.sub.3] film, the interface microstructure, and the properties improvement of the [Al.sub.2][O.sub.3] films as well.

Insulation Properties

Samples were treated according to the procedure described in the Experimental Section. Table 2 shows the measured resistivity of samples under different conditions.

The results demonstrate that [Al.sub.2][O.sub.3] films with an Ni-based buffer layer exhibit excellent resistivity at temperatures as high as 600[degrees]C (>[10.sup.10][ohm]xcm). Even after being subjected to 10 thermal cycles, the [Al.sub.2][O.sub.3] films still show good resistivity (>[10.sup.11][ohm]xcm). The [Al.sub.2][O.sub.3] films with an Ni-based buffer layer prepared by plasma-ion assisted deposition do appear to ensure that the films possess good insulation properties at high temperature.

CONCLUSION

[Al.sub.2][O.sub.3] films with an Ni-based buffer layer have been prepared on Cu substrate by plasma-ion assisted deposition. No visible defects have been observed and no impurity from Ni or Cu has been detected in the [Al.sub.2][O.sub.3] films. An Ni-based buffer layer compensates well for the difference of thermal expansion between films and Cu substrate, and improves the adhesion of [Al.sub.2][O.sub.3] film to Cu substrate. At high temperature, the diffusion of Cu into [Al.sub.2][O.sub.3] films is supressed. The [Al.sub.2][O.sub.3] films exhibit excellent resistivity at temperatures as high as 600[degrees]C (>[10.sup.10][ohm]xcm), and still present good resistivity (>[10.sup.11][ohm]xcm) even after exposure to 10 thermal cycles. This suggests that the [Al.sub.2][O.sub.3] films with an Ni-based buffer layer prepared by plasma-ion assisted deposition may be suitable for high temperature space technology applications.

References

(1) Jeliazova, Y. and Franchy, R., Appl. Surf. Sci., 187, 51 (2002).

(2) Voigt, M. and Sokolowski, M., Mater. Sci. Eng., B, 109, 99 (2004).

(3) Yoshitake, M., Mebarki, B., and Lay, T.T., Surf. Sci., 511, L313 (2002).

(4) Bahlawane, N., Thin Solid Films, 396, 126 (2001).

(5) Nable, J.C., Suib, S.L., and Galasso, F.S., Surf. Coat. Technol., 186, 423 (2004).

(6) Zywitzki, O., Goedicke, K., and Morgner, H., Surf. Coat. Technol., 51-152, 14 (2002).

(7) Ide-Ektessabi, A., Uehara, H., and Kamitani, S., Thin Solid Films, 447-448, 388 (2004).

(8) Liu, W.J., Guo, X.J., and Chien, C.H., Surf. Coat. Technol., 196, 68 (2005).

(9) Thielsch, R., Gatto, A., Heber, J., and Kaiser, N., Thin Solid Films, 410, 86-93 (2002).

(10) Atanassov, G., Turlo, J., Fu, J.K., and Dai, Y.S., Thin Solid Films, 342, 83-92 (1999).

Lixin Song, ([dagger]) Lingnan Wu, Jiehua Wu, and Lili Zhao--Chinese Academy of Sciences*

* Shanghai Institute of Ceramics, Shanghai, 200050, People's Republic of China.

([dagger]) Author to whom correspondence should be addressed. Email: [e-mail].

Table 1 -- Gibbs-Free Energy and Reaction Constant k

       Free Energy [DELTA]G(kj/mol)   Reaction Constant k
       [Al.sub.2][O.sub.3]  NiO       [Al.sub.2][O.sub.3]  NiO

 293K  -1583.332            -209.958  [infinity]           7.37 x
                                                             [10.sup.37]
 873K  -1401.676            -159.476  4.60 x [10.sup.85]   5.58 x
                                                             [10.sup.9]
1023K  -1354.696            -146.937  4.49 x [10.sup.70]   4.61 x
                                                             [10.sup.7]

Table 2 -- Resistivity of [Al.sub.2][O.sub.3] Films Measured Under
Different Conditions

Resistivity                      Condition (1) (a)
([ohm]xcm)                Room Temperature   600[degrees]C

Ni/[Al.sub.2][O.sub.3]    1~2 x [10.sup.12]  2~3 x [10.sup.10]
NiAl/[Al.sub.2][O.sub.3]  2~3 x [10.sup.12]  1~2 x [10.sup.10]

Resistivity                       Condition (2) (a)
([ohm]xcm)                Room Temperature   600[degrees]C

Ni/[Al.sub.2][O.sub.3]    5~6 x [10.sup.11]  1~2 x [10.sup.10]
NiAl/[Al.sub.2][O.sub.3]  7~8 x [10.sup.11]  1~2 x [10.sup.10]

(a) Condition (1) refers to thermal cycle once, condition (2) refers to
thermal cycle 10 times.