Novel fabrication method of metallic glass thin films using carousel ...

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We propose a novel method of fabricating metallic glass thin films using a carousel type sputtering system. In conventional methods of fabricating metallic glass ...
Novel fabrication method of metallic glass thin films using carousel type sputtering system J. Sakurai*, S. Hata and A. Shimokohbe Precision and Intelligence Laboratory, Tokyo Tech., 4259 Nagatuta, Yokohama, Japan 226-8503 ABSTRACT We propose a novel method of fabricating metallic glass thin films using a carousel type sputtering system. In conventional methods of fabricating metallic glass thin films using alloy targets, control of the alloy composition is difficult. However, since r.f. power for each target can be controlled independently in the proposed system, it is easy to control the alloy composition. Thin films of various alloy compositions are fabricated. In this work, near-equiatomic CuZr thin films are fabricated by the sputtering system with rotational speeds of the substrate holder ranging from 10 to 50 rpm. Small-angle XRD revealed that the specimen fabricated with rotation at 10 rpm had a multilayer structure. The specimen fabricated with rotation at 50 rpm exhibited a glass transition temperature of 672 K, a crystallization temperature of 715 K, and a supercooled liquid region of 43 K. However, although the XRD results indicated that the specimen fabricated with rotation at 30 rpm was in an amorphous state, it exhibited solid-state amorphization rather than glass transition before the crystallization in DSC measurement. Thus, the specimen did not become a metallic glass. Clearly, sputtering rate is a very important parameter in the fabrication of metallic glass thin films by the proposed sputtering system. These results have shown the proposed method to be effective in fabricating metallic glass thin films. Keywords: MEMS, metallic glass thin film, carousel type sputtering system, glass transition, super liquid region

1. INTRODUCTION Amorphous alloys are expected to find great application in microelectromechanical systems (MEMS) such as micromachines and microprobes due to their desirable mechanical characteristics including high values for strength, elastic limit and electrical conductivity [1-3]. Such characteristics of amorphous alloys are superior to those of conventional materials used for MEMS such as poly-silicon [4] and polymers [5]. In addition to amorphous alloys being homogeneous and isotropic, because the size effect is almost negligible [6], they can be easily incorporated in MEMS design. Metallic glasses are a kind of amorphous alloy known to exhibit viscous flow at a certain temperature range known as the “supercooled liquid region”. In the supercooled liquid region, metallic glasses can be easily formed using the same techniques as glasswork. Moreover, because metallic glass thin films are easily prepared on Si substrates by sputtering, they can be formed by MEMS processes such as photolithography, dry or wet etching, and lift off processing. As a result, unique MEMS with 3D microstructures that differed from those obtained from LIGA processing [7], and which involved self-assembly of the structure [8], were fabricated using metallic glass thin films [9-12]. Conventional sputter-deposited thin films are prepared by parallel-plate sputtering using an alloy target. It is difficult to control the alloy composition in such a system because only one alloy composition is obtained for each alloy target, and this differs from the target composition [2]. Some researchers have tried to control the alloy composition of thin films by changing the apparent alloy composition of the target through the placement of small pure metal plates on the target [13,14]. However, this method only provides discontinuous alloy compositions. * [email protected]; phone +81-45-981-8437; fax +81-45-924-5046; www.nano.pi.titech.ac.jp

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Micro- and Nanotechnology: Materials, Processes, Packaging, and Systems II, Jung-Chih Chiao, David N. Jamieson, Lorenzo Faraone, Andrew S. Dzurak, ed., Proc. of SPIE Vol. 5650 (SPIE, Bellingham, WA, 2005) 0277-786X/05/$15 · doi: 10.1117/12.581811

In this work, preparation of Cu-Zr metallic glass thin films is attempted using a carousel type sputtering system with pure Cu and Zr metal targets. Cu-Zr metallic glass is a binary alloy system that exhibits a glass transition and is known to have a high glass forming ability and excellent mechanical properties [15,16]. Since r.f. power for each target can be controlled independently in this system, it is easy to control the composition of the thin films over a continuous range [17,18]. However, important problems must be resolved in the fabrication of metallic glass thin films using this system. Since each element is sputter-deposited independently on rotated substrates, the fabricated thin films tend to have a laminated structure, a so-called “multilayer”. It is not evident whether multilayer thin films exhibit glass transitions. The purposes of this work are to fabricate the Cu-Zr metallic glass thin films using the carousel type sputtering system and to optimize the fabrication conditions in order to obtain materials with glass transitions.

2.EXPERIMENTAL METHODS CuZr thin films were prepared using a carousel type r.f. magnetron sputtering system as shown in Fig.1. A hexagonal substrate holder was placed in the vacuum chamber. Six substrates were placed around the substrate holder, one on each side. The substrate holder was rotated at various speeds and was cooled at 283 K by a water-cooling device. Five targets were placed around the vacuum chamber. In order to prepare specimens easily for the experiments, the six glass substrates were covered with Al film, because CuZr thin films are easily peeled off Al film substrates rather than glass. Sputter parameters for the system were base pressure P0 of less than 5.0 u10-4 Pa, Ar pressure PAr of 1.0 Pa, pure Cu (99.99%) and Zr (99% up) targets and a sputtering deposition time of 6 h. The rotational speeds of the substrate holder Rs were 10, 30 and 50 rpm. The r.f. powers used for the Cu and Zr targets were 113 and 400 W, respectively. The alloy composition of the as-sputtered CuZr thin films was determined by an energy dispersion X-ray spectrometry (EDS). Table 1 shows that all specimens were about 50.5 at% Cu. The thickness of the as-sputtered thin films was measured by 3-D surface profilers and was found to be about 5-6 Pm. The phase and structure were identified by a large-angle X-ray diffractometer (XRD, 2T : 20-120o) and a small-angle XRD (2T : 1-10o), using Cu-KDradiation. Thermal properties were measured by a differential scanning calorimeter (DSC) with a heating rate of 0.33 K/s. Viscous flow behavior and mechanical properties were examined using thermo mechanical analysis (TMA). Viscous flow behavior of the thin film was examined by thermal expansion measurement of the strain-temperature curve under constant load at a heating rate of 0.33 K/s. Mechanical properties were examined by tensile

Fig.1

Carousel type sputtering system.

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tests measuring stress-strain curves at a strain rate of 0.0017 /s. The size of specimens used in the thermal expansion tests and tensile tests were 3 mm u 5 mm and 1 mm u 5 mm (gauge portion), respectively. Table 1 Alloy compositions of Cu50Zr50 thin films fabricated at various rotational speeds of substrate holder. (at%)

3.RESULTS AND DISCUSSION 3.1 Microstructure Fig. 2 shows large-angle XRD profiles of the as-sputtered Cu50Zr50 thin films fabricated at various rotational speeds of the substrate holder. A hollow pattern can be seen and crystalline Cu and Zr patterns cannot be seen for every specimen in Fig. 2. The results of large-angle XRD suggest the thin films might not be in a multilayer state but in an amorphous state. In order to investigate the exact microstructure of the as-sputtered Cu50Zr50 thin films, small-angle XRD measurement was carried out. Fig. 3 shows small-angle XRD profiles of the as-sputtered Cu50Zr50 thin film fabricated at various rotational speeds. There are no satellite peaks in the specimens fabricated at Rs=30 and 50 rpm indicating the thin films are in an amorphous state. Whereas, for the specimen fabricated at Rs=10 rpm, a broad satellite peak can be seen at around 4 o. This peak corresponds to Bragg reflection of the individual Cu and Zr layers. As a result, this specimen is not in an amorphous state but is like a multilayer thin film. The period of the Cu and Zr layers is 2.17 nm. It is considered that this structure consists of several Cu and Zr atom planes. However, since it was not an exact multilayer structure due to sputtering onto a rotating substrate using a carousel type sputtering system, the satellite peaks are broad as shown in Fig. 3. Multilayer thin films consisting of bilayer of several atom layers exhibit features similar to an amorphous state in large-angle XRD observation [19]. 3.2 Thermal properties Fig.4 (a) shows the DSC curves for crystallization in the as-sputtered Cu50Zr50 thin films fabricated at various rotational speeds For the specimen fabricated at Rs=50 rpm, an endothermic peak corresponding to a glass transition can be seen.

Fig. 2 Large-angle X-ray diffraction profiles of as-sputtered Cu50Zr50 thin films fabricated at various rotational speeds.

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Fig. 3 Small-angle X-ray diffraction profiles of as-sputtered Cu50Zr50 thin films fabricated at various rotational speeds.

With increasing temperature, two exothermic peaks occur. The primary large peak corresponds to crystallization of the amorphous thin film. The secondary small peak corresponds to transformation of the first crystallized phase to a second phase [20]. The glass transition temperature Tg is 672 K and the crystallization temperature Tx is 715 K, giving a supercooled liquid region of width 'T = 43 K. Thus, the proposed method is successful in fabricating metallic glass thin films. For the specimen fabricated at Rs=10 rpm, three exothermic peaks can be seen and the specimen did not exhibit a glass transition as shown in Fig.4. The start temperature of the second peak is 715 K, which is almost the same as Tx of the specimen fabricated at Rs=50 rpm. In addition, the heat flow of the specimen fabricated at Rs=10 rpm is almost the same as that fabricated at Rs =50 rpm. Thus, the second peak corresponds to crystallization with a Tx of 715 K. Likewise the third small peak corresponds to a phase transformation. The initial broad peak is considered to correspond to a solid-state amorphization (SSA) with a start temperature, TSSA, of 644 K. A binary multilayer system undergoing SSA satisfies the following conditions. (1) The two metals exhibit a large negative heat of mixing. (2) One of the metals exhibits anomalously fast diffusion in the crystal compared to the other metal [21,22]. The Cu-Zr system satisfies the first condition, because it is one of three empirical properties that metallic glasses exhibit [23]. Moreover, anomalously fast diffusion of Cu has been reported in D-Zr [24]. SSA has also been reported in a Zr-poor (Zr composition ranging from 3 to 9 at%) Cu/Zr multilayer thin film [20]. For the specimen fabricated at 30 rpm in the present research, an exothermic peak corresponding to crystallization is observed with a Tx of 715 K. Tx is constant for all specimens. Fig. 4 (b) shows an enlarged DSC curve of the reaction before crystallization in the specimen fabricated with Rs=30 rpm. It is considered that, for this intermediate rotational speed, an exothermic peak corresponding to SSA and an endothermic peak corresponding to the glass transition would overlap. It is difficult to evaluate whether this sample has formed into a metallic glass. Accordingly, the glass transition is evaluated by another method proposed in the next section. 3.3 Viscous flow behavior The formation of bulk metallic glasses is evaluated by measuring the glass transition, using DSC or differential thermal analysis (DTA) [15,16]. DSC and DTA measurements need a sufficient weight of sample to measure temperature and heat flux. In the case of sputter-deposited thin films, a lot of pieces need to be packed into the closed cell in order to prepare a sample of sufficient weight for detecting heat flow. However, it is difficult to prepare sputter-deposited thin films because they often develop a curled-up shape due to the residual stresses introduced during sputtering. In this section, we propose a new method of evaluating metallic glasses based on measuring thermo-mechanical properties, such as viscous flow, instead of thermal properties. The viscous flow behavior of the obtained thin films was

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Fig. 4 (a) DSC curves for crystallization in the Cu50Zr50 thin films fabricated at various rotational speeds. (b) Enlarge curve in the Cu50Zr50 thin films fabricated at Rs=30rpm.

examined by a thermal expansion test under constant load using TMA. The initial applied stress was caused by the weight of the chuck that gripped the end of the specimen and was less than 1 MPa. In the supercooled liquid region, the metallic glass thin film softened and then became elongated due to the initial stress. Fig.5 shows the strain-temperature curves of the Cu50Zr50 thin films fabricated at Rs=30 and 50 rpm. The two dashed lines correspond to Tg and Tx, respectively, of the specimen fabricated at Rs=50 rpm. For this specimen at temperatures under Tg, the strain increased slightly with increasing temperature because of thermal expansion. When the temperature

Fig. 5

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Strain-temperature curves in the Cu50Zr50 thin films fabricated at Rs=30 and 50 rpm.

exceeds Tg, the strain of the sample increases rapidly with increasing temperature. Because the metallic glass exhibits viscous flow in the supercooled liquid region, it is easily deformed by a small load. In the temperature range from Tx to 750 K, corresponding to the end temperature of crystallization, the strain scarcely increased with increasing temperature, because the volume of the sample decreased due to crystallization. When the specimen had crystallized completely, it could only then expand. However, the sample fabricated at Rs=30 rpm was not considered to exhibit viscous flow in the supercooled liquid region because a rapid increase of strain was not observed. Thus, TMA provides a measure of whether a metallic glass is formed. The strain is constant during crystallization but Tx and the end temperature of crystallization measured by TMA differed from results measured by DSC. It is consider that this difference was caused by the heterogeneous chemical composition of the sputter-deposited thin film. 3.4 Mechanical properties Fig.6 shows the stress-strain curve of a Cu50Zr50 thin film fabricated at Rs= 50 rpm. The specimen exhibited only elastic deformation and did not exhibit the plastic elongation that is the characteristic mechanical feature of Cu-based bulk metallic glasses [15,16]. The film finally fractured with a fracture strain Hf of 2.2%. Considering only the elastic limit strain Hl, the thin film is superior to Cu-based bulk metallic glasses. The Young’s modulus E and fracture stress Vf were 57 GPa and 1.2 GPa, respectively. The Young’s modulus of the thin film was lower than Cu-based bulk metallic glasses. Sputter-deposited thin films are coarser than the bulk material because large voids and Ar gas are introduced into the thin film during sputtering. Thus, thin films are softer than bulk materials. The fracture stress of thin films is also lower than Cu-based bulk metallic glasses. It is consider that the density also had an effect on the strength of the specimen.

Fig. 6

Stress-Strain curve in the Cu50Zr50 thin films fabricated at Rs=50 rpm.

3.5 Sputtering Conditions for fabrication of metallic glass In order to fabricate metallic glass thin films using the proposed sputtering system, sputtering conditions are important. The sputter rates of Cu and Zr were investigated. Each element was sputtered under the sputter conditions used in the previous experiments. Table 2 shows the results. We must consider the influence of interference between each element during sputtering. The actual sputter rates are considered to be smaller than these results. In estimation the atomic diameters of Cu and Zr are 0.27 nm and 0.31 nm, respectively [25]. In the case of the Cu50Zr50 thin film sputtered at Rs= 50 rpm, it is considered that Cu atoms were deposited sparsely onto the substrate while the substrate revolved around, because the rate obtained in the sputtering of Cu only is smaller than the atomic diameter of Cu. Although the sputter

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rate of Zr is similar to its atomic diameter, Zr atoms were deposited randomly onto the substrate while the substrate revolved around, because of the relatively sparse distribution of Cu atoms on the substrate. Thus this specimen did not have a multilayer structure but an amorphous structure. While in the case of the Cu50Zr50 thin film sputtered at Rs= 30 rpm, the sputter rate of each element is larger than its atomic diameter. In this case, a few atom layers of each element were deposited densely onto the substrate while the substrate revolved around. Accordingly, with heating, this specimen exhibited SSA. In fabrication of metallic glass thin films using the sputtering system proposed herein, it was found that the sputtering rate of each comprising element is controlled to be at least as small as its atomic diameter. Table 2

Sputtering rate of Cu and Zr elements under the used conditions.

CONCLUSIONS We have attempted the fabrication of metallic glass thin films by a novel carousel type sputtering system. In this work, Cu50Zr50 thin films are fabricated at various rotational speeds of the substrate holder. A glass transition can be seen by DSC and thermal expansion measurements in the specimen fabricated at Rs=50 rpm. Tg, Tx and 'T are 672, 715 and 43 K, respectively. When Rs was less than 50 rpm, the fabricated samples exhibited solid-state amorphization instead of glass transition before crystallization. Their TSSA and Tx values were about 640 K and 715 K, respectively. The Cu50Zr50 metallic glass thin film fabricated at Rs=50 rpm achieved a fracture strain of 2.2%, a fracture stress of 1.2 GPa and a Young’s modulus of 56 GPa. When the sputtering rate of each comprising element is controlled to be at least smaller than its atomic diameter, the proposed method is found to be effective in fabricating metallic glass thin films.

ACKNOWLEDGMENTS This research was supported by a grant from the Japan Society for the Promotion of Science (JSPS) as a grant-in-aid for Scientific Research (A) (2), 15206015, 2004, and for Young Scientists (A), 16686010, 2004, and by the Industrial Technology Research Grant Program of 2003 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The experiments were carried out at the Mechano-Micro Process Laboratory of the Tokyo Institute of Technology.

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