Microwave Heating of Thin Au Film

Transcription

1 Materials Transactions, Vol. 48, No. 3 (27) pp. 531 to 537 #27 The Japan Institute of Metals Microwave Heating of Thin Au Film Hidekazu Sueyoshi and Shigeki Kakiuchi* Department of Nano Structure and Advanced Materials, Graduate School of Science and Engineering, Kagoshima University, Kagoshima 89-65, Japan Thin Au film was prepared by sputtering and evaporation methods with a quartz substrate, followed by microwave irradiation in air (frequency of microwave: 2.45 GHz, incident flux of microwave: 563 W, irradiation time: 6 s). As a result, it was confirmed that microwave heating of thin Au film is feasible. The growth of crystalline and particles due to microwave heating was confirmed from AFM observation and XRD analysis. Thin Au film is continuously heated during microwave irradiation, regardless of a preparation method of thin film. Microwave heating depends on the amount of microwave absorption on a thin Au film, which is related to the thickness and microstructure of the thin Au film. The rate of temperature rise depends on the ratio of a thickness to resistivity of thin Au film. [doi:1.232/matertrans ] (Received November 2, 26; Accepted December 15, 26; Published February 25, 27) Keywords: microwave irradiation, heating, thin gold film, film thickness, microstructure, sputtering, evaporation 1. Introduction Metal bulk is excellent reflector of microwave energy and in general is not heated significantly by microwave. Therefore, few application have been conducted so far on heating of metal bulk. 1) On the other hand, recently there has been much interest in microwave heating and sintering of metal powder. 2 7) This is because metal powder has a large specific surface area compared to metal bulk. Thin metal film is similar to metal powder from a view point of specific surface area. However, microwave characteristics of thin metal film may differ from those of metal powder. For example, as the thickness of the thin metal film is decreased to that of a few atomic layers conductivity drops below that of the metal bulk. The first mechanism on the scattering of the conduction electrons at the surface was described by Fuchs and Sondheimer (F-S theory). 8,9) Mayadas and Shatzkes 1) showed that a major portion of the total resistivity in polycrystalline films comes from electron scattering at grain boundaries (M-S theory). Furthermore, Steinhoegl et al. 11,12) have proposed a model that the surface scattering model (F-S theory) has been combined with the grain boundary scattering model (M-S theory). Thus, physical properties in thin metal film depend upon both the thickness and microstructure. Therefore, microwave heating of thin metal film may also differ from that of metal powder. Recently, novel microwave measurement technique involving the transmission of a microwave signal through a resistive film has been demonstrated in order to measure the sheet resistance of thin metal films. 13) The authors have reported microwave application for heating of a thin Au film which has been used in a surface plasmon resonance analysis. 14) However, there have been few reports regarding microwave heating of thin metal films and its mechanism has not been well understood to date. In the present study, microwave characteristics of thin Au film were examined. *Graduate Student, Kagoshima University PC Fig. 1 Microwave cavity Quartz Fiber Convertor 2. Experimental Procedure Microwave generator Sensor for pyrometer Specimen Antenna Quartz stand Schematic diagram of microwave irradiation test. Thin Au film was prepared using sputtering and evaporation methods. A substrate was quartz and its shape was mm. A 2.45 GHz, 1.8 kw microwave generator was used as microwave source. Figure 1 shows a schematic diagram of microwave irradiation test. Distribution of a microwave field was complex because of a multi-mode microwave cavity (section: mm, height: 335 mm). A glass beaker containing distilled water (2 g) was placed on a quartz stand setting in the center of the bottom, followed by microwave irradiation at 1 kw for 1 s. We obtained an incident flux of microwave using the following equation: P ¼ mct W =7 ð1þ where P is the incident flux of a microwave, m the mass of water, C the specific heat of water, and T W the temperature rise of water. We postulated that microwave-heat convert efficiency is 7%. Heat transfer between water and glass beaker was ignored because only a small temperature rise (1 K) of glass beaker was achieved when water-free glass beaker was irradiated under the same conditions.

2 532 H. Sueyoshi and S. Kakiuchi Irradiation Power, P/W Vertical Distance, H/mm Fig. 2 Relationship between P and vertical distance from the bottom, H. 7 Temperature, T/K Time, t/s Fig. 3 Change in temperature of sputtered thin Au film with a thickness of 34.6 nm during microwave irradiation. Figure 2 shows a relationship between P and vertical distance from the bottom, H. P was irregularly changed with vertical distance from the bottom. The thin Au film was placed at the position of 9 mm from the bottom where it received the maximum incident flux of microwave. Microwave irradiation was conducted at 1 kw for 6 s in air. As shown in Fig. 2, net incident flux of microwave was 563 W. The temperature of thin Au film was measured using a glass fiber type of radiation pyrometer (Chino Co. Ltd., IR-FL3). Microwave cavity was shielded to prevent the entrance of light. The surface morphology of thin Au film was observed using an atomic force microscopy (AFM, Digital Instruments Co. Ltd., Nano Scope III). The thickness of thin Au film was measured using AFM (scratch method). Structural analysis was performed with X-ray diffraction (XRD, Rigaku Co. Ltd., CN437A1). The resistivity of thin Au film was measured using four-point probe technique at room temperature. 3. Results Figure 3 shows the change in temperature of sputtered thin Au film with a thickness of 34.6 nm during microwave Fig. 4 AFM images of sputtered thin Au film with a thickness of 34.6 nm. as-sputtered, and after microwave irradiation. irradiation. The radiation pyrometer used in the present study was unable to measure the temperature lower than 373 K because of the detection limit of the sensor. No temperature measurement which appears immediately after an in put of the microwave power is due to this. As shown in Fig. 3, abrupt temperature rise was observed in the first 6 s. Then the rate of temperature rise decreased with irradiation time, and the temperature remained almost unchanged at a late stage of microwave irradiation. The temperature reached to 578 K after 6 s. Because a quartz substrate having low dielectric properties little heated by microwave irradiation, the temperature rise is thought to be due to heating of thin Au film. Unchanging of the temperature at the late stage of microwave irradiation may be caused by establishment of a heating/heat radiation equilibrium. Figure 4 shows AFM images of sputtered thin Au film with

3 Microwave Heating of Thin Au Film (2) (2) Fig. 5 XRD patterns of sputtered thin Au film with a thickness of 34.6 nm. as-sputtered, and after microwave irradiation. a thickness of 34.6 nm. After microwave irradiation the growth of Au particles due to microwave irradiation appeared as shown in Fig. 4. Figure 5 shows XRD patterns of this thin Au film. In assputtered thin Au film, a clear peak of the plane in which closest packing can be achieved appeared. But peak intensities of other planes were very low. This suggests that there is a preferential crystallographic orientation for sputtering deposition of Au. The full-width at half maximum of the plane was.544. On the assumption that the full-width at half maximum depends on only a crystalline size, we estimated the crystalline size, d, from the Scherrer formula. 15) The value of d in as-sputtered thin Au film was 15.7 nm. After microwave irradiation the heightened and sharpened peak of the plane was observed as shown in Fig. 5. The value of full-width at half maximum was.348. As a result, we obtained d ¼ 25:2 nm. A good correspondence between the change in crystalline size and morphological change (the change in particle size of Au) in AFM image (Fig. 4) was seen. It is well known that the growth of crystalline and particle is promoted by heating. In microwave irradiation of sputtered thin Au film with a thickness of 75.9 nm, no temperature measurement was Fig. 6 AFM images of sputtered thin Au film with a thickness of 75.9 nm. as-sputtered, and after microwave irradiation. observed at all irradiation times. Figure 6 shows AFM images of this thin Au film. The particle size of as-sputtered thin Au film was larger than that of as-sputtered thin Au film with a thickness of 34.6 nm (Fig. 4). Each sputtering was carried out under the same conditions, and the thickness of thin Au film was controlled by changing spattering time. The growth of Au particles may be due to prolonged sputtering time. As shown in Fig. 6, there was little morphological difference between as-sputtered and after microwave irradiation. Figure 7 shows XRD patterns of this thin Au film. In assputtered thin Au film, a clear peak of the plane appeared. Peak intensities of other planes were very low. The value of the full-width at half maximum of plane was.434. As a result, we obtained d ¼ 19:9 nm. The value of d was larger than that of as-sputtered thin Au film with a

4 534 H. Sueyoshi and S. Kakiuchi (2) (22) (311) (2) (22) (311) Fig. 7 XRD patterns of sputtered thin Au film with a thickness of 75.9 nm. as-sputtered, and after microwave irradiation. 7 Temperature, T/K Time, t/s Fig. 8 Change in temperature of evaporated thin Au film with a thickness of 29.5 nm during microwave irradiation. thickness of 34.6 nm. As mentioned above, this may be due to crystalline growth during long sputtering. As shown in Fig. 7, XRD pattern of thin Au film after microwave Fig. 9 AFM images of evaporated thin Au film with a thickness of 29.5 nm. as-evaporated, and after microwave irradiation. irradiation was similar to that of as-sputtered thin Au film. The full-width at half maximum of the plane was.414. The resultant d was nearly equal to the value for assputtered thin Au film, d ¼ 2:9 nm. These results suggest that sputtered thin Au film with a large thickness is more difficult to heat using microwave irradiation. Figure 8 shows the change in temperature of evaporated thin Au film with a thickness of 29.5 nm during microwave irradiation. Abrupt temperature rise was observed in the first 6 s. Then the rate of temperature rise decreased with increasing microwave irradiation time. The temperature reached to 575 K after 6 s. Figure 9 shows AFM images of evaporated thin Au film with a thickness of 29.5 nm. As shown in Fig. 2, the coalescence and growth of Au particles due to microwave irradiation appeared. From XRD patterns of this thin Au film,

5 Microwave Heating of Thin Au Film 535 ρ m / µωm we obtained d ¼ 16:4 nm for as-evaporated thin Au film and d ¼ 26:4 nm for thin Au film after microwave irradiation. These results suggest that evaporated thin Au film is heated by microwave irradiation, resulting in the growth of crystalline and particle. In evaporated thin Au film with a thickness of 92.6 nm, no temperature measurement was observed during microwave irradiation. AFM image of as-evaporated thin Au film showed that the particle size of Au was larger than that of thin Au film with a thickness of 29.5 nm (Fig. 9). Evaporation was carried out under the same conditions, and the thickness of thin Au film was controlled by changing running time. The growth of Au particles may be due to prolonged running time. After microwave irradiation, a slight growth of Au particles due to microwave irradiation was observed. From XRD patterns of this thin Au film, we obtained d ¼ 19:9 nm for as-evaporated thin Au film and d ¼ 28:7 nm for thin Au film after microwave irradiation. According to these results, the evaporated thin Au film with a thickness of 92.6 nm may not be attained the temperature higher than 373 K, though it was heated by microwave irradiation. Thus, microwave characteristic (film-thickness dependence) of evaporated thin Au film is similar to that of sputtered thin Au film. 4. Discussion R=.5 R=.7 R= / µωm ρ c Fig. 1 Comparison of measured resistivity, m, with calculated resistivity, c. Although metal bulk reflects most of incident microwaves, a part of incident microwaves penetrates into the metal from the surface. Depth of penetration, that is, skin depth (defined as the distance from the surface into the material at which the power drops to e 1 of the original value) is given by 16) ¼ð2=! Þ 1=2 where is the skin depth,! angular frequency (! ¼ 2f, where f is frequency of microwave), free space permeability, and conductivity. Substituting the value of for Au ð2þ in bulk ( ¼ 43: /m) 17) into eq. (2), we obtain ¼ 1:54 mm. This value is larger than the thickness of thin Au film used in the present study. This suggests that most of incident microwaves permeate through the thin Au film. Bosman et al. 16) have studied microwave absorption of contaminant (supposing as thin film disk) on a microwave window. According to the results, the temperature rise of the contaminant was expressed as: ¼ 2c=! ð3þ = ¼ s=2 ð4þ L=s ¼ L=2 2 ð5þ T ¼ 1 K a 1 mm L=s ð1 þ L=sÞ 2 A 1 8 W/m 2 1 =ð1 3 Wm 1 K 1 ð6þ Þ where T is the rate of temperature rise, free space wave length, c the speed of light in vacuum, a the radius of thin film disk, L the thickness of thin film disk, A the average incident flux of microwave, and the thermal conductivity of thin film disk. Replacing thin Au film square (1 13 mm) used in the present study by a disk with equivalent area, we obtain a ¼ 6:43 mm. From eq. (5), the value of L=s is 283 (using for Au in bulk). Dividing incident flux of microwave (563 W) by the area of thin Au film, we obtain A ¼ 4: W/m 2. Substituting these values and ¼ 293 J/(mK) 17) into eq. (6), we obtain T ¼ :333 K/s. The calculated value is considerably small compared to the rate of temperature rise in the first 6 s in Fig. 3, although no temperature measurement appears immediately after an in put of the microwave power. The difference between the calculated and experimentally obtained values may be caused by using for Au in bulk. According to the F-S theory, the conductivity of thin film is smaller than that of bulk because of surface scattering. On the other hand, according to the M-S theory, the conductivity of thin film depends mainly on microstructure because of grain boundary scattering. As a result, the rate of temperature rise is varied with changing thickness and microstructure of thin film. Bhat et al. 18) have reported a relationship between skin depth and frequency of microwave in thin Kanthal (Fe-22Cr- 5.8Al) film evaporated on alumina substrate, which has been discussed on the basis of the F-S theory. According to the results, the skin depth in thin Kanthal film was 134 nm at 3.5 GHz. The value was considerably small compared to the skin depth obtained in Kanthal bulk. And also Bhat et al. 18) showed the variation of microwave loss with thickness of the thin Kanthal film. These results suggest that the skin depth and microwave absorption on a thin film depend on thickness and microstructure of the thin film. When a, A and are constants in eq. (6), T depends on L=s. Substituting eqs. (2) and (3) and ¼ 1= into eq. (5), L=s ¼ 6L= ð7þ As a result, T depends on L= as the following equation:

6 536 H. Sueyoshi and S. Kakiuchi T ¼ 1 K a 1 mm 6L= ð1 þ 6L=Þ 2 A 1 8 W/m 2 1 =ð1 3 Wm 1 K 1 ð8þ Þ From a combination of the F-S and M-S theories, the resistivity of thin metal film is given by the following equation: 11) ( 1 1 ¼ þ 2 3 ln 1 þ 1 þ C ð1 PÞ U ) S l ð9þ ¼ l R ð1þ d 1 R where is the resistivity of the thin metal film, the resistivity of metal in bulk, l the mean free path within a grain, d the average distance of the grain boundaries, R the reflectivity coefficient at grain boundary, P the specularity parameter, C a constant (C ¼ 1:2), U the perimeter and S the area of the cross-section of the thin metal film. The first term on right side of eq. (9) shows the contribution of grain boundary scattering, and the second term shows the contribution of surface scattering. The values of R ¼ :9 for Au 19) and R ¼ :5 for Cu 11) have been reported. Figure 1 shows a comparison of the resistivity, m, which is measured using four-point probe technique, with calculated resistivity, c, which is obtained by substituting the value of R, l ¼ 4 nm, 11) P ¼ :5 (with respect to epitaxial single crystal), 2) ¼ :23 m m, 17) and the value of d (crystalline size) into eqs. (9) and (1). In the case of R ¼ :7, good agreement with experimental data was achieved. Figure 11 shows a relationship between and thickness of thin Au film for R ¼ :7. As shown in Fig. 11, increases significantly with decreasing thickness of thin Au film (the contribution of surface scattering). Also increases with decreasing d (the contribution of grain boundary scattering). Figure 12 shows a relationship between T which is calculated from eq. (8) and L= in thin Au film. As shown in Fig. 12, T depends on L= regardless of preparation method of thin Au film. As only the thickness of thin Au film is increased under the equivalent microstructure, L= increases considerably because of not only an increase in L but also a decrease in due to surface scattering, resulting in a decrease in T. On the other hand, when only d is increased under the same film thickness, L= increases with a decrease in due to grain boundary scattering, resulting in a decrease in T. The values of T for thin Au film with a small thickness (for example: 3.19 K/s for sputtered thin Au film with a thickness of 34.6 nm and 3.67 K/s for evaporated thin Au film with a thickness of 29.5 nm) nearly agree with the rates of temperature rise in the first 6 s in Figs. 3 and 8. The values of T for thin Au film with a large thickness (for example: 1.17 K/s for sputtered thin Au film with a thickness of 75.9 nm and 1.12 K/s for evaporated thin Au film with a thickness of 92.6 nm) are considerably small compared to that of thin Au film with a small thickness. Considering heat radiation from thin Au film, a practical T ρ / µωm d=15 nm d=2 nm Thickness of thin Au film, L/nm Fig. 11 Relationship between and thickness of thin Au film for R ¼ :7. T/ Ks Evaporation Sputtering Lρ 1 / Ω 1.6 Fig. 12 Relationship between T which is calculated from eq. (8) and L= in thin Au film. is thought to be very low. The result that no temperature measurement was observed at all irradiation times in thin Au film with a large thickness may be due to such low T. According to eqs. (8), (9) and (1), crystalline growth due to heating results in a decrease in T. On the other hand, it is well known that for metal increases with increasing temperature. This phenomenon contributes to an increase in T. As a result, T is given by a combination of the decrease in T due to crystalline growth, the increase in T due to temperature dependence of and the decreases in T due to heat radiation from thin Au film. As shown in Figs. 3 and 8, T decreased with microwave irradiation time when it exceeded 6 s. This may be caused by above mentioned behavior. Also the temperature was found to be constant at the late stage of microwave irradiation as shown in Figs. 3 and 8. This is because heating/heat radiation equilibrium was achieved. As mentioned above, it was confirmed that microwave heating of thin Au film is feasible. This suggests that microwave irradiation is able to use heating thin film of other metals, except for Au. The rate of temperature rise depends

7 Microwave Heating of Thin Au Film 537 on the amount of microwave absorption on a thin metal film, which is related to the thickness and microstructure of the thin metal film. 5. Conclusions Thin Au film was prepared by sputtering and evaporation methods with a quartz substrate, followed by microwave irradiation in air (frequency of microwave: 2.45 GHz, incident flux of microwave: 563 W, irradiation time: 6 s). The following conclusions were obtained: (1) Thin Au film is continuously heated during microwave irradiation, regardless of a preparation method of thin film. (2) Microwave heating depends on the amount of microwave absorption on a thin Au film, which is related to the thickness and microstructure of the thin Au film. (3) The rate of temperature rise depends on the ratio of a thickness to resistivity of thin Au film. REFERENCES 1) D. E. Clark and W. H. Sutton: Mater. Sci. 26 (1996) ) R. Roy, D. Agrawal, J. Cheng and S. Genevanishvilli: Nature 399 (1999) ) J. Cheng, R. Roy and D. Agrawal: J. Mater. Sci. Lett. 2 (21) ) T. Hayashi and Y. Asano: Report of Industrial Technology Center Gifu Prefectural Government, No. 6 (25) ) S. Takayama, Y. Saito, M. Sato, T. Nagasaki, T. Muroga and Y. Ninomiya: Proc. 9th Inter. Conf. on Microwave & RF Heating, (23) 64m. 6) S. Yanagita: New Application Technology of Microwave, (NTS, Tokyo, 23) pp ) N. Yoshikawa, E. Ishizuka and S. Taniguchi: Mater. Trans. 47 (26) ) K. Fuchs: Proc. Cambridge Phil. Soc. (1938) ) E. H. Sondheimer: Adv. Phys. 1 (1952) ) A. F. Mayadas and M. Shatzkes: Phys. Rev. B (197) ) W. Steinhoegl, G. Schindler, G. Steinlesberger, M. Traving and M. Engelhardt: IEEE Proc. of the 23 Inter. Conf. on Simulation of Semiconductor processes and Devices, (23) pp ) W. Steinhoegl, G. Schindler, G. Steinlesberger, M. Traving and M. Engelhardt: J. Appl. Phys. 97 (25) ) M. H. J. Lee and R. J. Collier: Microelectronic Enginerring (24) ) H. Sueyoshi and S. Kakiuchi: KINZOKU 76 (26) ) I. Nitta: X-Ray Crystallography, (Maruzen, Tokyo, 1959) p ) H. Bosman, Y. Y. Lau and R. M. Gilgenbach: Appl. Phys. Lett. 82 (22) ) Japan Inst. Metals: A Data Book on Metals, (Maruzen, Tokyo, 1974) p ) K. S. Bhat, S. K. Datta and C. Suresh: Thin Solid Films 332 (1998) ) C. Durken and M. E. Welland: Phys. Rev. B 61 (2) ) D. S. Campbell: The Use of Thin Films in Physical Investigations, (Academic Press Inc., New York, 1966) p. 315.