PHYSICAL PROPERTIES OF AU AND AL THIN FILMS MEASURED BY RESISTIVE HEATING

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1 Surface Review and Letters, Vol. 12, No. 1 (2005) c World Scientific Publishing Company PHYSICAL PROPERTIES OF AU AND AL THIN FILMS MEASURED BY RESISTIVE HEATING F. AVILÉS,O.CEHandA.I.OLIVA Centro de Investigación y de Estudios Avanzados del IPN, Unidad Mérida, Departamento de Física Aplicada, A. P. 73 Cordemex, 97310, Mérida, Yucatán México oliva@mda.cinvestav.mx Received 19 January 2005 The electrical resistivity (ρ), resistive thermal coefficient (α r), thermal expansion coefficient (α t ) and stress (σ) of Al and Au thin films deposited by thermal evaporation were measured while films were heated by Joule effect. Electrical resistivity measured by the four-probe technique was simultaneously measured with surface film temperature in real time. Au films show important variations in the α r and ρ properties when thickness ranged from 0.1 µm to1µm; contrarily, Al films show insignificant variations in the 0.1 µm to 2µm thickness range. The role of the surface aluminum oxide on these measurements and the intrinsic stress on films determined with grazing incidence X-rays are discussed. Keywords: Gold; aluminum; resistive thermal coefficients; metallic thin films. PACS Numbers: y, Gb, Jk, Bx, At 1. Introduction The new tendencies of miniaturization of electromechanical components are causing a revolution in the materials field. 1,2 Particularly, materials with thin film geometry deposited on thick substrates present physical properties that can be more relevant than for bulk. 3 There are several physical properties that change depending on the film thickness and kind of material. On this way, some thin film semiconductors present electrical and optical properties that depend on the thickness of the deposited films; 4 metals tend to increase their electrical resistivity when thickness decreases between the mean free path and certain coalescence limits. However, amazing results were recently observed for AuCu I alloy, showing that electrical resistivity maintains constant along nm thickness. 5 Moreover, the actual demand of microdevices used as microsensors to control or to measure certain physical parameters for specific applications is increasing. 6,7 These efforts follow two basic objectives: to increase the range of use and the capacity of the devices, as well as to reduce the device size and to increase the material quality. There are different physical properties that can be studied in a known material in order to obtain a possible new behavior. In this work, a study of thermal properties, related to the electrical properties of Au and Al thin films deposited by free evaporation on Corning glass and silicon (100) substrates is discussed. The importance and role of the metallic film-substrate geometry as well as the methodology used to obtain some thermal, mechanical and electrical properties are discussed. The obtained results may be of great Corresponding author. 101

2 102 F. Avilés, O. Ceh & A. I. Oliva interest due to the direct application in the microelectronic industry. 2. Theory 2.1. Electrical resistivity The collinear four-probe technique is a technique widely used to measure the electrical resistivity by superficial contact. This technique is mainly used in the semiconductor industry, research, and manufacturing field. 8 The electrical resistivity ρ of a thin film, whose length is much longer than its thickness d, can be obtained by: ρ = ( π ) ( ) ( ) V V d d, (1) ln 2 I I where V is the drop potential measured among the internal electrodes and I is the current applied through the external electrodes. In our case, resistivity measurements were carried out with a Jandel Universal Probe UNIV TC equipment, with 1 mm of separation among probes Resistive thermal coefficient It is known that electrical resistivity (i.e. the electrical resistance) of metal increases with the temperature. In the lineal range, the relationship between resistivity and temperature can be determined by means of the relationship. ρ ρ 0 = R R 0 = α r T, (2) where α r is the resistive thermal coefficient and R the electrical resistance at temperature T. Gradients, ρ = ρ ρ 0, R = R R 0,and T = T T 0 [with subscript 0 referring to the room temperature (used as reference)] can be determined between two different conditions. So, the coefficient α r can be determined if we know the change in the electrical resistance (or resistivity) when temperature changes. In our case, we measured T (t) by means of a smallmass thermocouple and its corresponding R(t) by the four-probe technique (being t, the time). By plotting T (t) and R(t) variables but eliminating the common variable t, we obtain the R(T ) function. In this way, the coefficient α r can be found as the slope of Eq. (2) Intrinsic stress A large number of methods for strain determination and related properties, such as the superficial texture and deformation, are based on data provided by the X-ray diffraction technique. This technique allows to determine the strain and thermal expansion by the small variations measured in the interplanar spacing 3 and the displacement of the main diffraction peaks around the standard position. 9 A displacement of the peaks position may assume that microcrystals are under uniform strain. 10 The unitary deformation (ε) in a given direction is determined by 11,12 : ( ) a a0 ε =, (3) a 0 where a 0 is the unstrained lattice parameter (standard value) without strain (standard value) and a is the measured lattice parameter. The origin of deformations can be mechanical or thermal and are related with the elastic and one-dimensional strain by, σ = Eε, (4) where E is the modulus of elasticity (Young s modulus) of the material Thermal expansion coefficient An important aspect in this work is the thermal behavior that the metallic film presents during heating. The film suffers a thermal expansion whose value depends on the temperature and the thermal expansion coefficient (α t ) of the material. This strain can be calculated by the following relationship: ε = α t T. (5) The deformation of the crystal is given by Eq. (3) and it is related to Bragg s diffraction angle by the following relationship 10 : ( ) sen θi sen θ f ε = (6) sen θ f Then, the α t parameter can be determined from the slope of the equation: ( ) sen θi sen θ f = α t T, (7) sen θ f

3 where θ i and θ f are respectively the initial and final positions of the diffraction peaks, and T is the change in temperature. Physical Properties of Au and Al Thin Films Measured by Resistive Heating Experimental System Gold and aluminum thin films (5 20 mm 2 )with different thickness were deposited on Corning glass 7059 and silicon (100) substrates by the free evaporation technique in a vacuum chamber ( 10 6 Torr). Molybdenum and tungsten crucibles were used to evaporate the high purity metals (99.999%). Substrates were cleaned according to a standard process. 13 Thin films thickness was controlled during deposition by a quartz crystal and verified after growth with a Dektak 3030 profile meter. The depositions rate in all cases was about 1 nm/s and the films thickness ranged between 0.1 and 2 µm. In order to apply an electrical current through the film (for thermal resistive coefficient determination), two electrodes of AWG 36 copper wire were attached to the ends of the film using conductive silver paint. A HP 6643A power supply provided the electrical current for the film. Two similar electrodes were fixed to the film to measure the corresponding potential drop. Temperature on metallic films surface were measured at high resolution by a small-mass Omega type K thermocouple. The microvoltages generated by the change of temperature were captured from adigitalthermometerviaagpibinterfacewitha programmable voltmeter HP-3458A and converted to temperature with the corresponding equation. 14 Homemade software in HP-VEE language was used to control and to acquire data for the PC. Changes on film crystallinity were measured by a diffractometer SIEMENS model D-5000 with grazing incidence technique by using a wavelength of λ = Å provided from a Cu(α) radiation. 4. Results 4.1. Electrical resistivity and resistive thermal coefficient Figures 1 to 3 show typical curves resulting from measurements of gold thin films deposited on silicon with a 0.11 µm thickness for electrical resistance measurements. Temporal changes of the electrical Fig. 1. Percentage change of electrical resistance versus time measured during heating of Au film (0.11 µm, thickness). Fig. 2. Change of temperature versus time measured during heating of Au film (0.11 µm, thickness). resistance shown in Fig. 1 are referred to the resistance of reference R 0, when 0.3 A of electrical current is applied. Figure 2 shows the temporal evolution of the metallic thin film temperature simultaneously measured, with the electrical resistance. Both curves show a similar behavior and a tendency to reach the steady state after 5 minutes. By eliminating the time as common factor from both figures, we determine the α r value through the slope of the plot (R R 0 )/R 0 versus (T T 0 ). The result is shown in Fig. 3. For this case, we obtain

4 104 F. Avilés, O. Ceh & A. I. Oliva Fig. 3. Resistive thermal coefficient (α r) calculated from Eq. (2) for a gold thin film. an α r = C 1 value for a range of temperatures between 25 and 41 C. This α r value obtained for the thin film differs from the reported bulk value, C 1. 1 This result provides a base to make a systematic study of this property in metallic thin films as a function of film thickness. Thus, the electrical resistivity values for gold and aluminum thin films were measured and compared with the bulk value. Table 1 shows the results of the electrical resistivity values (ρ) and of the resistive thermal coefficient (α r ) which were determined for different gold and aluminum thin film thicknesses. The experimental deviation calculated for the resistive thermal coefficient Table 1. Measured ρ and α r values for different thickness of Au and Al thin films. Material Thickness (µm) ρ (µohm-cm) α r ( C 1 ) Au Bulk Al Bulk of is basically due to the room temperature variations during measurements. Unlike bulk, the ρ in a thin film depends on several factors such as rate deposition, thickness, temperature and grain boundaries between others. 11 As thin film thickness decreases, the electron collisions with surfaces become important. Such confinement effect due to film thickness is clearly observed on gold thin films whose electrical resistivity values are higher than the reported for bulk. Besides, the resistive thermal coefficient shows a tendency to increase with thickness, until it eventually reaches its values in bulk. This is, again, a consequence of the gold film thickness. Regarding aluminum thin films results, their behavior cannot be interpreted in a simple way. Aluminum usually presents a native oxide film (Al 2 O 3 ) when exposed to atmospheric pressure, which changes substantially its surface properties. 15 As can be seen (Table 1), the measured ρ and α r values show slight variations with thickness. The analysis and behavior of the oxide layer in aluminum films require a more detailed study and will be the subject of another work Stress and thermal expansion coefficient The metallic films deformations due to a current flow were measured by grazing incidence X-ray diffraction technique (XRD). In order to obtain the strain, stress and thermal expansion coefficient of films, two gold thin films with 0.88 µm and 0.32 µm thickness were analyzed in situ while an electrical current was applied. Amorphous glass substrates were used to avoid the affectation of the main diffractive signals. The experimental procedure used was similar for both samples. First, the two main X-ray diffraction peaks of gold films were obtained with one degree as the grazing incidence angle; afterwards, different electrical currents (from 0.3 to 1.7 A) were applied through the films, monitoring in situ the two main diffraction peaks variations. Figure 4 shows the behavior of the (111) diffraction peak for a 0.88 µm gold film for the as-grown and for the different values of applied current. The initial position of the peak (as-grown) is far from the gold standard value, which indicates that the film has residual stress maybe generated during

5 Physical Properties of Au and Al Thin Films Measured by Resistive Heating 105 Fig. 4. Diffraction (111) peak displacements of gold thin films for different values of applied electrical current. preparation. According to Eqs. (4) and (6), residual stress quantify around 0.45 GPa. As the applied current increases (for heating) the peak intensity grows and tends to the standard value. Also, it was observed that when the electrical current is off, the peak position tends to return to its original value and shape, indicating that stresses are in the elastic limit and no recrystalization effects on the material, and no stress liberation occur. Table 2 shows the values obtained on the 2θ position of the main peak for the 0.88-µm-thickness gold film for different applied currents. The unitary deformation (ε) was calculated with Eq. (6), being θ i the angle for T =0. The residual stresses were determined by Eq. (4). The film temperature due to the different applied currents was measured using a small-mass type-k Table 2. Properties obtained for the peak (111) of Au film (0.88 µm). I(A) T ( C) 2θ ε (10 4 ) σ (GPa) Fig. 5. Thermal expansion coefficient (α t ) value calculated from Eq. (5) for a 0.88 µm thickness gold thin film. thermocouple, at steady state conditions. It is important to point out that temperature did not remain constant at steady state; contrarily, it ranged around 10% of its average value (this value is reported in Table 2). Because of its minor thickness the 0.32 µm film gives minor amplitude on the peak diffraction, showing a proportional behavior with the 0.88 µm film. Only the 0.88-µm-film-thickness results are shown. According to Eq. (7), plotting the unitary deformation (ε) as a function of the difference of temperature, it is possible to obtain the thermal expansion coefficient of the film. From the slope of Fig. 5, we obtained a α t = C 1 value for gold. The estimated experimental error is about C 1, mainly due to the variation in the measured superficial temperature. The α t value, reported by other authors for gold films is C The (200) peak behavior was similar to the (111) peak but with minor width and a slightly distorted peak. This behavior is interpreted as the film expansion follows a multidirectional way. Thus, stresses calculated with the unidirectional Hook s law represent only a simple estimation. A more realistic study of the stress and deformation needs to involve a biaxial stress state. 5. Conclusions We study the physical properties of Au and Al thin films with thickness between 0.1 and 2 µm deposited

6 106 F. Avilés, O. Ceh & A. I. Oliva on Corning glass and silicon (100) substrates. A methodology to characterize metallic thin films and to measure thermal and electrical properties as a function of thickness is proposed. The electrical resistivity measured on the thin films is larger than those measured in bulk. Such resistivity tends to decrease to the bulk as thickness increases. The relation between temperature and electrical resistivity present a lineal behavior in the range of temperatures measured between 25 and 80 C. The α r value measured on the gold thin films is smaller than for the bulk showing an increase with temperature, until the bulk value, when thickness increases. This result permits us to extend the study to other films thermal properties. Gold thin films deposited by free evaporation show intrinsic compression stress of about 0.45 GPa. The film heating by electrical current flow produces tension stress between 0.02 and 0.13 GPa (for heating around 50 C) and elastic deformations on gold thin films. Due to the small film thickness analyzed, the stress state is planar, as was deduced from XRD measurements. The α t measured for gold thin films between 0.1 and 0.8 µm thickness is 22.6 ± C 1.ForAlfilms,theobtainedα t values do not show any important changes due to the oxide layer formed when the film is exposed to the atmospheric pressure, standing as a protective layer. Acknowledgments Authors thank CONACYT (México) for the financial support given to project E. References 1. M. Ohring, Engineering Material Science (Academic Press, 1995). 2. N. Maluf, An Introduction to Microelectromechanical System Engineering (Artech House, 2000); M. Elwenspoek and R. Wiegerink, Mechanical Microsensor, Microtechnology and MEMS series (Springer, Berlin, 2001). 3. L. Eckertová, Physics of Thin Films (Plenum Press, New York, 1986). 4. A. I. Oliva, O. Solís-Canto, R. Castro-Rodríguez and P. Quintana, Thin Solid Films 391 (2001) W. Zhang, S. H. Brongersma, O. Richard, B. Brijs, R. Palmans, L. Froyen and K. Maex, J. Vac. Sci. Technol. B22 (2004) I. J. Busch-Vishniac, Electromechanical Sensors and Actuators (Springer-Verlag, New York, 1999). 7. M. Freygang, H. Glosch, H. Haffner and S. Messener, Tech. Digest 5th Int. Conf. New Actuators, (1996) Annual Book of ASTM Standards, West Conshohocken, P. A. Vol (1999), Standards F43, F84, F374, F A. I. Oliva, P. Quintana, O. Ceh, J. E. Corona and M. Aguilar, Thin Solid Films 353 (1999) B. D. Cullity and S. R. Stock, Elements of X-ray Diffraction (Prentice Hall, New Jersey, 2001). 11. M. Ohring, Materials Science of Thin Films (Academic Press, 2002). 12. S. Brennan, A. Munkholm, O. S. Leung and W. D. Nix, Physica B283 (2000) H. K. Pulker, Coatings on Glass (Elsevier, 1999). 14. The Temperature Handbook, Vol. 27 (Omega Press, 1999). 15. M. T. A. Saif, S. Zhang, A. Haque and K. J. Hsia, Acta Materialia 50 (2002) J. Lai, T. Perazzo, Z. Shi and A. Majundar, Sens. Actuators A58 (1997) 113. Appendix A. Bulk materials properties at room temperature used in this work. Material Crystal structure α t 10 6 E ρ α r a 0 (Å) (C 1 ) (GPa) (µω-cm) (C 1 ) Al FCC Au FCC Si Diamond Corning Amorphous Glass