Chan, HLW

Cheng, KC

PolyU Institutional Repository

Characterization of Piezoelectric Ring Used for Wire Bonding Transducer Application Kei Chun Cheng and Helen Lai Wah Chan Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong. ABSTRACT Resonant vibration modes excited in a piezoelectric PZT ring which is closely related to its geometry and dimensions were studied. By a thinning test of the ring, the electromechanical properties of the ring were measured as a function of its thickness and shifts in resonance vibrations were noted. Thickness of the ring was also optimized in relation to the electromechanical performance of the ring used for wire bonding application. Index Terms - piezoelectric, ring, electromechanical, transducer I. INTRODUCTION Nowadays, ultrasonic wire bonding, used as the first-level packaging technique, is a crucial technique of microelectronic packaging [l]. With the rapid development of IC chips in recent years, not only the circuits inside the chips become complicated due to the multi-functional design, but also their size tend to be as small as possible for reducing the defective fault and for raising the operation speed. So, there is a need to develop a higher frequency bonder transducer in order to obtain higher yield. Increasing the transducer’s operation frequency would raise a problem of the reliability on its operation performance. The reason is that the operation resonance mode existing in the transducer would be easily coupled with other higher resonance modes such as radial, wall thickness modes and their harmonics exciting in the piezoelectric PZT rings which are clamped together in the transducer, thus causing difficulties in transducer control. Furthermore, PZT material has a high mechanical Q factor, locking the operating frequency in a designated range becomes increasingly difficult as the frequency goes up. Therefore, a study of vibration modes and electromechanical properties of the ring is extremely important for optimising the transducer’s performance operating at higher frequency. 11. FABRICATION OF A PZT RING The ring is made of modified lead zirconate titanate (PZT)-802 powder supplied by Piezo Kinetics Inc. This is a hard PZT material that can withstand high levels of electrical excitation and mechanical stress and it is commonly used in making high voltage high power generators and transducers [2]. There are three steps in the fabrication PZT rings [3]: or of 1. Ceramic powder was poured in a steel mould and formed into a ring by dry pressing. 2. The ring sample was calcinated at 750°C for 60 minutes to remove PVA binder. 3. Calcinated sample was sintered at 1325°C for 90 minutes so that combined reaction in the powder can be completed. In order to elicit piezoelectric performance of the ring, the sintered sample was then poled along the thickness direction by applying a d.c. field of 6 kV/mm for 30 minutes at 120°C. III. RESONANCE MODES IN A PZT RING Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on November 28, 2008 at 02:59 from IEEE Xplore.  Restrictions apply.There are three possible fundamental modes of vibrations in a piezoelectric ring when an electric field parallel to the poling direction (Fig. 1) was applied to the electroded face [41. Resonant frequency fr-rad fr-wdl fr-thk ode x2 'I; Measured Calculated Error (%) value ( W z )  value ( W z )  130.40 121.17 -7.12 478.25 444.10 -7.14 1170.85 1183.90 1.11 4 Poling XI direction Fig.1: Schematic diagram of a PZT ring. 1. The thickness mode vibration occurs when the applied frequency is close to the thickness resonant frequency f1-W which induces a change in thickness Tr k ATr. 2. The radial mode vibration occurs when the applied frequency is close to the radial resonant frequency fr-rad which induces a change in the mean diameter d,+ Ad,. The radial mode always appears as the lowest frequency as the diameter is the largest dimension. 3. The wall thickness mode fr-wd occurs when the frequency is close to the resonant frequency along the wall thickness direction, causing a change in the wall thickness Wr k AWr. The resonant frequencies of each mode [5] are given by Thickness resonant frequency frmthk 1 1 ................................... (1) fr-thk = 27, JZ' Radial resonant frequency fr-rad 1 (2) - ................................. - ndmr JZ Wall thickness resonant frequency fr-wall (3) 1 ................................. thickness, p is the density and 'I;r is the thickness of the ring. s: and SE are the elastic compliances at constant electric field and constant charge density, respectively. Dimensions of the PZT ring used in studying the characteristics of the vibration modes are: thickness Tr = 1.78 mm, outer diameter douer = 13.00 mm and inner diameter dimer = 5.20 mm. Three strong resonance vibrations can be identified from the electrical impedance and phase as a function of frequency as shown in Fig. 2. 360 300 240 180 D 120 2j 60 8 0 -60 -120 200 400 600 800 1000 1200 1400 Frequency (kHz) Fig. 2: Electrical impedance Z and phase 8 against frequency of a PZT (PIU 802) ring of thickness = 1.78 mm. Measured resonant frequencies of each mode can be compared with the calculated values and are shown in Table 1. Table 1. Measured and calculated results The optimal ring thickness of 1.78 mm has been selected from the thinning test and will be discussed in the next section. Error between the measured and calculated thickness mode is relatively small. Errors in the radial and wall thickness mode is larger due to the variation in the inner and outer diameter. where de(&uer+dher)/2 is the mean diameter, w~(&ukr-dhCr)/2 is the mean wall 65 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on November 28, 2008 at 02:59 from IEEE Xplore.  Restrictions apply.IV. THINNING TEST RESULTS OF A RING By thinning a PZT ring of 4.6 mm thick, it is possible to find how various material parameters such as k,, a, N3t and resonant frequencies of different vibration modes change with thickness. This knowledge is extremely important for optimizing the dimension required for specific ultrasonic applications. 132h - 1  13  " 3 1 2 8 ~ ~ ~ ~ " " ~ " " ' " " ~ ~ ' ~ ' ~ ~ ' " " ' " ' ~ ~  1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Thickness of PZT ring (mm) Fig. 3: fr-rad VS thickness of the PZT ring. fr-rad is slightly lower in thicker ring (> 4 mm thick) as shown in Fig. 3 which may be due to the presence of a strong thickness mode near to the radial mode. When the outer diameter d, is larger than 2.7 times the thickness q, the radial frequency is quite constant indicating that it is independent of the thickness. 5 0 0 ~  h 8 480 1 ,f 460 5' E 440 EJ $ 4 0 0  420 e! 380 B Fig. 4: fr-wd VS thickness of the PZT ring. Thickness resonance is the strongest vibration mode in a ring poled along the thickness direction. A strong and pure thickness mode is extremely important if the thickness mode is to be used in ultrasonic applications. Fig. 5 shows how the thickness mode resonance varies with the thickness of the ring. + lzo0l 1000 \ b 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Thickness of PZT ring (mm) Fig. 5:  fr-thk VS thickness of the PZT ring. The strongest and pure thickness resonance was found at a thickness of 1.78 mm and the material parameters of the PZT ring show good agreements compared with the data supplied by Morgan [2]. The ring with this thickness exhibits the highest electromechanical factor kt of 0.43 (Fig.6) and has a minimum electrical impedance of 2.7a at the thickness mode resonance (Fig. 7). Furthermore, high mechanical quality factor @ of 1213 (Fig. 8) and thickness frequency constant N3t of 2084 Hz-m (Fig. 9) has been obtained at the thickness of 1.78 111111. Another strong and pure resonance mode at a thickness of 2.5 mm was also found but its material parameters are a little bit lower than that of the PZT ring with a thickness of 1.78 mm. This can be another criteria for the choice of the ring thickness. When the thickness is over 3 mm, the material parameters measured are getting worse presumably due to the thickness resonance mode is approaching the wall thickness resonance causing strong mode coupling. It is also observed in Fig. 4 that the wall thickness resonant frequency decreases almost linearly with the increase in thickness. 66 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on November 28, 2008 at 02:59 from IEEE Xplore.  Restrictions apply.Z S O O l  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Thickness of PZT ring (mm) Fig. 6: kt vs thickness of the PZT ring. 80L . . . . ,  . . . . ,  . . . . ,  . . . . ,  . . . . ,  . . . . , ,  . . . . ,  1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Thickness of PZT ring (mm) Fig. 7: Z vs thickness of the PZT ring. 1x00 ~ . . . . , . . . . , . . . . , . . . . , . . . . , . . . . , . . . . ,  1600 1 I 4 0 0 ~ ~ ~ " ' ~ ~ ~ " ~ ~ ' " ' ~ ' ~ " ~ ' " ' ~ ~ " ~ ' ~ ~ ~  1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Thickness of PZT ring (mm) Fig. 8: QM vs thickness of the PZT ring. 8 2000 % C a 1800 h .O 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Thickness of PZT ring (mm) Fig. 9: N3t vs thickness of the PZT ring. V. CONCLUSION By carrying out thinning tests of the PZT ring, mode coupling and the electromechanical properties have been studied. Optimal thickness of the PZT ring can be selected for making a transducer for ultrasonic wire bonding application. ACKNOWLEDGEMENT Financial support from the Industrial Support Fund of the Hong Kong Special Administrative Region (project no. AF/147/98) is acknowledged. REFERENCES 1. Tummala, R.R. and Rymaszewski, E.J., Microelectronic Packaging Handbook, Van Nostrand Reinhold, 1989. 2. Piezoelectric ceramics data book for designers, Morgan Matroc Limited, England, 1987. part I", Ferroelectric materials and their applications, pp. 101-162, North-Holland , 1991. 4. Hueter, T.F. and Bolt R.H. Sonics, John Wiley & Sons, 1955. 5. "Guide to dynamic measurements of piezoelectric ceramics with high electromechanical coupling", IEC standard, 1976. 3. Xu, Y. "Perovskite-type ferroelectrics: 67 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on November 28, 2008 at 02:59 from IEEE Xplore.  Restrictions apply.

Characterization of piezoelectric ring used for wire bonding transducer application

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