Park, Ik Keun

Park, Tae Sung

English

Exeley Inc.

Thickness Measurement of Aluminum Thin Film using Dispersion Characteristic  of Surface Acoustic Wave  Ik Keun Park,  Department of Mechanical &Automotive Engineering Seoul National University of Science and Technology Seoul, Korea ikpark@seoultech.ac.kr Tae Sung Park Department of Mechanical Engineering Hanyang University Seoul, Korea taesung78@seoultech.ac.kr  Abstract— In this study, we suggest a method to measure the thickness of thin films nondestructively using the dispersion characteristics of a surface acoustic wave propagating along the thin film surface. To measure the thickness of thin films, we deposited thin films with different thicknesses with range of 200~1000 nm on a Si(100) wafer by controlling the deposition time by DC sputtering technique. The thickness of the thin films was measured using a scanning electron microscope. Subsequently, the surface wave velocity of the thin films with different thickness was measured using V(z) curve method of scanning acoustic microscopy. The correlation between the measured thickness and surface acoustic wave velocity was verified. The wave velocity of the film decreased as the film thickness increased. Consequently, film thickness can be determined by measuring the dispersion characteristics of the surface acoustic wave velocity. Keywords-component; scanning acoustic microscope; V(z) curve; dipersion;surface wave velocity; thin film thickness  I.  INTRODUCTION   In recent years, nano-structured thin film systems are often applied in industries (e.g., MEMS/NEMS device, semiconductor, display, optical coating or the like). Thin film are used for many and varied purpose to provide resistance to abrasion, erosion, corrosion, galling, tarnish, wear, radiation damage, or high temperature oxidation and to reduce friction or electrical resistance, provide lubrication, prevent sticking, and also to provide special magnetic or dielectric properties[1-4].  Particularly, aluminum thin films are widely used for optical mirrors, fiber-optic probes, thin film transistor and flat panel displays[5-8]. The physical and elastic properties of the Al thin films depend strongly on their film thickness. Film thickness significantly affects coating quality and costs. Coating thickness plays an important role in product quality, process control and cost control and it can be measured with many different methods such as atomic force microscopy, alpha-step and scanning electron microscope and so on[9-10]. However, these methods do not maintain the functionality of films after testing and the thickness of the film might be changed when cutting or eliminating layers of the system because these all are destructive method. In this study, V(z) curve method was applied to measure the thickness of the thin film using scanning acoustic microscopy nondestructively.   II. THEORY  A. V(z) curve Figure 1 shows a schematic diagram of surface acoustic wave (SAW) propagation between the acoustic lens and the specimen via a coupling medium (e.g., distilled water).  In theory, the oscillations of the received signal can be explained by the interference between the specularly reflected acoustic wave (path #1) and the surface acoustic wave (path #2). A surface acoustic wave can be generated at a liquid-solid interface if the half aperture angle of the acoustic lens is greater than the second critical angle for the surface. A surface acoustic wave can propagate along the specimen surface and radiate energy into the liquid at the critical angle. The surface acoustic wave can interfere with the specularly reflected acoustic waves at the acoustic sensor and this interference will produce alternating maxima and minima in the acoustic output signal depending on the distance between the acoustic lens and the specimen.   Figure 1 Example of the V(z) curve The plot of the interference is the so-called V(z) curve[11-13]. Here, “V” represents the amplitude of the received signal and is a function of “z”, which represents the distance between the specimen and the acoustic lens. Figure 1 shows an example of the V(z) curve. The specimen is silica glass. The temperature of the coupling medium (i.e., de-ionized water) is 23. The operating frequency of the acoustic lens (Olympus; model: AL4M631) is 400 MHz. The velocity of the SAW is calculated by measuring the spacing of the minima zD    B. Calculation of surface wave velociry In this study, we use the ray model to calculate the surface wave velocity illustrated in Fig. 2. The ray model is based on  the ray interference theory [14-15]. According to the ray theory, when the specimen is displaced from z=0 toward the acoustic lens by z, the path #1 and #2 respectively experience the following phase change. Here, z=0 is set at the focal point of the acoustic lens and the positive z direction is away from the lens. When the specimen is at a distance z from the lens focal plane, the phase lags of #1d  (for path #1) and # 2d  (for path #2) are expressed by: #1#2( ) 2 2( )w ww Rz BO k z kz AOC k AC kdd= - × = - × ×= - × + ×   (1) Here #1d  and # 2d  are the phase changes for path #1 and path #2. The relative phase change between the two paths is given as follows: #2 #1( ) ( )12 1 tancosw R RRz z zz k kd dqqD = -é ùæ ö= × × - + ×ê úç ÷è øë û  (2) where wk  and Rk  stand for the wavenumber in the water between the lens and specimen and of the surface wave respectively. A phase change of 2p  in relative phase difference corresponds to a dip or peak interval in V(z) curve. Using Eq. (2) and Snell’s law, it follows that: 2 (1 cos )wRvzf qD =× × -     (3) Using Snell’s law again for the angle Rq  in the denominator, we can rewrite Eq. (3) in the form that explicitly express the surface acoustic wave velocity  Rv  as a function of  zD . 21 12wRwvVvf z=æ ö- -ç ÷× ×Dè ø    (4)   Figure 2 Schematic diagram showing the principle of the V(z) curve  where, wv   is the acoustic velocity in water and f  is the working frequency. This simple ray optics theory allows quantitative measurement of the phase velocity of a surface acoustic wave from V(z) curve, and characterization of the acoustic properties of materials. By changing the distance between the lens and specimens, (changing distance z), we can get a V(z) curve. The velocity of the SAW is calculated from the spacing of the   in Fig. 2 through Eq. (4).  III. EXPERIMENTAL DETAILS  Aluminum films, having thickness in the 200~1000 nm range, were deposited on silicon substrate by DC magnetron sputtering technique [10-11]. The deposition time was controlled to obtain the thin film specimens with different film thickness. The chamber pressure during sputtering was about 5ｘ10-6 Torr. The target used for film preparation was high-purity (99.99%) Al. The films were grown on silicon substrates with previous cleaning process. During film growth the distance substrate-source was 70 mm. The average deposition rate was 1.1 nm/sec approximately. Table 1 showed the deposition time for each specimen to obtain the expected film thickness.  TABLE 1 Deposition time for obtaining the expected film thickness Sample Number Film Thickness(nm) (expected) Deposition Time 1 200 3min 2s 2 400 6min 4s 3 600 9min 5s 4 800 12min 7s 5 1000 15min 9s  Figure 3 Schematic diagram of SAM for measuring the V(z) curve  The film thickness was measured by scanning electron microscope. The surface wave velocity was measured by V(z) curve method of scanning acoustic microscope(SAM). Fig. 3 is a schematic diagram of the scanning acoustic microscope (Model; Olympus UH3). The SAM was operated with frequency at 400 MHz The acoustic lens used was the model AL4M631, comprising a zinc oxide transducer (diameter 383 ㎛) for converting the electric signals generated by a RF tone-burst source, the sapphire buffer rod, and the spherical recessed portion of the acoustic lens. The lens has an aperture angle of 120°, and the focal distance of 577.52 ㎛. Deionized water was used as a coupling medium between the lens and the specimen.  The acoustic wave velocity in water is the important factor in V(z) measurement, it can directly affect the accuracy of the surface acoustic wave velocity according to Eq. (4) [18-19]. During experiments, the water temperature can be kept at around 22.5℃ so as to reduce the temperature error.  IV. RESULTS  Aluminum films with different thickness were deposited on glass substrate in order to verify a change of the surface wave velocity depending on the film thickness.  Table 2 shows the measured film thickness using scanning electron microscope. The film thickness increased as the deposition time was increased expectably. A V(z) curve was obtained by scanning acoustic microscope for each specimen.  The zD  was measured for each specimen and then the surface acoustic wave velocity was calculated by Eq. (4). A surface wave velocity was measured three times per specimen to reduce the measurement error. As mentioned above, a surface acoustic wave velocity in water was changed in accordance with water temperature. When a temperature of water the wave velocity is 1490m/s. Table 3 shows measured surface acoustic wave velocity. The correlation between the measured thickness and surface wave velocity was verified. The surface wave velocity tends to decreased as the thickness of aluminum thin film increased as shown Fig. 4.  TABLE 2 Measured film thickness using scanning electron microscope Sample Number Film Thickness(nm) (measured) 1 217 2 469 3 311 4 744 5 951    TABLE 3 Measured surface acoustic wave velocity  Unit :m/s Sample Number P1 P2 P3 Mean Value 1 3,066 3,069 3,081 3,072 2 3,044 3,077 3,077 3,066 3 3,044 3,041 3,051 3,045 4 3,009 3,016 3,044 3,023 5 3,006 3,009 3,006 3,007   Figure 4 A change of the surface wave velocity in accordance with film thickness  V. CONCLUSION  In this study we suggest a method that can measure the thickness of thin films nondestructively using dispersion characterization of surface acoustic wave propagating the thin film surface. A surface wave velocity was measured by V(z) curve method using scanning acoustic microscope.  To verify the correlation between the measured thickness and surface wave velocity, the film thickness was measured using scanning electron microscope. As film thickness increased, the surface wave velocity decreased. Consequently, thin film thickness can be determined by measuring the dispersion characteristic of surface acoustic wave velocity using V(z) curve method.  ACKNOWLEDGMENT   This work was supported by Radiation Technology R&D program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2013M2A2A9043274)  REFERENCES  [1] H. S. Ju and B. R. 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Thickness Measurement of Aluminum Thin Film using Dispersion Characteristic&nbsp; of Surface Acoustic Wave

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Thickness Measurement of Aluminum Thin Film using Dispersion Characteristic  of Surface Acoustic Wave

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Thickness Measurement of Aluminum Thin Film using Dispersion Characteristic&nbsp; of Surface Acoustic Wave

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Thickness Measurement of Aluminum Thin Film using Dispersion Characteristic of Surface Acoustic Wave