Single Crystal Piezoelectric Composites for Advanced NDT Ultrasound

Transcription

1 Single Crystal Piezoelectric Composites for Advanced NDT Ultrasound Xiaoning Jiang *a, Kevin Snook a, Wesley S. Hackenberger a, and Xuecang Geng b a TRS Technologies, Inc., 282 East College Avenue, Suite J., State College, PA 1681 b Blatek, Inc., 282 East College Avenue, Suite F., State College, PA 1681 ABSTRACT In this paper, the design, fabrication and characterization of PMN-PT single crystal/epoxy composites are reported for NDT ultrasound transducers. Specifically, 1-3 PMN-PT/epoxy composites with center frequencies of 5 MHz-4 MHz were designed and fabricated using either the dice-and-fill method or a photolithography based micromachining process. The measured electromechanical coefficients for composites with frequency of 5 MHz 15 MHz were about , and the coupling coefficients for composites with frequencies of 25 MHz- 4 MHz were about The dielectric loss remains low (<.5). These properties hold promise for advanced NDT ultrasound applications. Keywords: single crystal piezoelectrics, piezoelectric composites, NDE, NDT, ultrasound transducers. 1. INTRODUCTION Piezoelectric materials are broadly used in ultrasound transducers for medical imaging and therapy, non-destructive testing (NDT) and underwater sonar applications. Recently discovered single crystal piezoelectrics based on Pb(Zn 1/3 Nb 2/3 ) 1-x Ti x O 3 (PZN-PT) or Pb(Mg 1/3 Nb 2/3 ) 1-x Ti x O 3 (PMN-PT) exhibit large increases in piezoelectric coefficients and electromechanical coupling coefficients over conventional piezoelectric ceramics [1]. Due to a unique ferroelectric domain configuration, the crystals piezoelectric strain remains nearly hysteresis free up to levels of ~.5% to.6% depending on the crystal composition (Figure 1) [1]. The much higher figure of merit (d 33 *g 33 ) of single crystal piezoelectrics suggests that single crystal piezoelectrics are promising for both transmission and receiving ultrasound transducers [2]. Furthermore, the high electromechanical coupling coefficient (k 33 ) of PMN-PT single crystal will contribute to transducers with high sensitivity and broad bandwidth (Figure 2) [1,2]. Table 1 shows the properties comparison between major PZT ceramics and PMN-PT single crystal. 1$ C 12 $ Field (ky/cm) Field (ky/cm) Figure 1. Piezoelectric strain response from PZN-PT single crystal material compared to piezoelectric and electrostrictive ceramics. The crystal strain remains nearly hysteresis free up to ~.6%. At high fields very large strains > 1% can be achieved but with an increase in hysteresis. * xiaoning@trstechnologies.com; phone: ext 23; fax: Nondestructive Characterization for Composite Materials, Aerospace Engineering, Civil Infrastructure, and Homeland Security 27, edited by H. Felix Wu, Aaron A. Diaz, Peter J. Shull Proc. of SPIE Vol. 6531, 6531F, (27) X/7/$18 doi: / Proc. of SPIE Vol F-1

2 I'J 6 8 KR *,< 87.5e6 HAX luo.t START CO.O. s/div 2. d? S OO. T_ HoOO.Qo Figure 2. Impedance and phase spectrum of a PMN-PT single crystal sample (k 33 bar, 3mmx3mmx9mm) indicating a large electromechanical coupling coefficient. Table 1.Properties of <1> Oriented PMN-PT Compared to Several Standard Piezoelectric Ceramics Property Type III PZT Type II PZT Type VI PZT PMN-33%PT Single Xtal (TRS3) (TRS2) (TRS61) T K S K Tan δ <.8 T c ( C) ~ ρ (g/cm 3 ) E c (kv/cm) ~ d 33 (pc/n) d 31 (pc/n) k k k p N/A k t ~.5-.6 N 33 (Hz-m) ~ E s 33 (x1-11 m 2 /N) E s 11 (x1-11 m 2 /N) Q m Primary Applications Power Ultrasonics Sensors & Actuators Medical & NDT Ultrasoun d Useable in All Applications Piezoelectric composites, especially 2-2 and 1-3 composites, have been extensively studied for ultrasound applications because of their benefits such as high coupling coefficients, low acoustic impedance, short pulses, easy conformability, etc. [3-8]. Single crystal piezoelectric composites developed recently for medical and sonar transducers have shown that unprecedented bandwidth and sensitivity could be achieved by substituting this material for the ceramics. In this paper, single crystal piezoelectric composites fabrication and testing is presented for NDT ultrasound. Both the dice-and-fill technique and micromachining technology were used for single crystal composite preparation. Impedance and phase spectrum of composites were recorded using an impedance analyzer. Proc. of SPIE Vol F-2

3 2. EXPERIMENTAL DESIGN 2.1 Composite design Composite piezoelectric materials have high electromechanical coupling factors, low acoustic impedance, and are relatively easy to conform. The 2-2 and 1-3 composites are the two major composite formations. Main factors to be considered in composite design include acoustic impedance, electrical impedance, center frequency, electromechanical coupling coefficient, etc. The rule in frequency design is that the center frequency is not higher than the half of the 1 st lateral mode resonant frequency in the kerf material, which could ensure the elimination of the lateral mode interference. The frequency of the first lateral mode in a 1-3 composite kerf can be empirically expressed as VT f l = 2 2d p (1) where f l is the frequency of the first lateral mode, V T is the shear wave velocity and d p is the kerf width. For example, if the kerf width is 1µm, which is the present state of the art for dicing, the frequency of the first lateral mode is about 39MHz, limiting the operating frequency of this composite to about 2MHz. For a 2-2 composite, the frequency of the first lateral mode can be empirically expressed as VT f l = (2) 2d p In this instance, for a 1µm kerf, the first lateral mode resonance is at 55MHz allowing operation up to 28MHz. According to the above analysis it is very difficult to make single crystal 1-3 or 2-2 composites that operate in the 2 to 5 MHz range with present dice-and-fill techniques, however, the proposed photolithography based micromachining of single crystal could be used to fabricate composites with kerfs of 1 µm or less and the crystal width less than 5 µm. Another rule of thumb for composite thickness design is that the post height/width aspect ratio is usually designed to be > 2 so that the transducer resonates in a nearly pure mode, yielding the maximum electromechanical coupling coefficient. A high aspect ratio also dampens lateral modes. Both effects are needed to achieve a broad bandwidth and high sensitivity. In this report, 1-3 single crystal piezoelectric composites with center frequencies of 5 MHz, 1 MHz, 15 MHz, 25 MHz, and 4 MHz were designed, prototyped and characterized. 2.2 Composites Fabrication Many different processes for ceramic-polymer 2-2 composite and 1-3 composite transducers have been studied including dice-and-fill (Figure 3), molding [9], and combined tape casting and multilayer ceramic techniques [1]. Micromachining has also been used for fine scale composite transducer fabrication. 1-3 PZT/polymer composites have been fabricated by injection micromolding, where the mold inserts were made using X-Ray deep exposure or deep RIE of silicon. The demonstrated transducer had a frequency around 2 MHz [11,12]. For single crystal-polymer composite transducers, the only process used so far is dice-and-fill. Dicing speeds less than 2 mm/s are required to reduce breakage and chipping. For 2-2 composite single crystal transducers, a stacking technique may be used to fabricate transducers with frequencies up to 2 MHz, but not above 2 MHz since the post or beam widths would be smaller than 5 µm, which is not practical using mechanical machining. A laser micromachining and laminating method could potentially be used for single crystal piezoelectric composite fabrications, however, the process precision and low speed may limit its capability for this application. Recently, a photolithography based micromachining process was developed at TRS, which is a piezoelectric composite based micromachined ultrasound transducer (PC-MUT) technique, and this PC-MUT has been demonstrated successfully for high frequency medical imaging applications [13]. PC-MUT has several advantages including submicron machining precision, batch fabrication, and a low-stress mechanical environment for fragile, fine structures. Figure 4 shows schematically the process flow for the PC-MUT technique. In this paper, the dice-and-fill process was used for 5 MHz, 1 MHz and 15 MHz PMN-PT/epoxy 1-3 composites fabrication, and the 2 MHz and 4 MHz composites was fabricated using the PC-MUT technique. Proc. of SPIE Vol F-3

4 Poled PZT Ceramic Epoxy Fill of Saw Kerf Dicing Lapping to Remove Ceramic Back Plate or "Web" Diced Posts for 1-3 Composite Figure 3: Conventional PZT-Polymer Composite Fabrication. A 1-3 composite is shown. For 2-2 composites the ceramic is only diced in one direction to form beams instead of posts. PMN-PT Epoxy fill Photolithography PMN-PT Lapping Electroplating PMN-PT Electroding Etching PMN-PT Cr/Au Photo resist Ni epoxy Figure 4. Process flow for the PC-MUT technique. Proc. of SPIE Vol F-4

5 2.3 Composites Characterization Composites with Cr/Au electrodes were poled at an electric field of 5-1 kv/cm at room temperature, followed by characterization. A capacitance meter was used to record capacitance and dielectric loss at 1 KHz. The dielectric loss (free) was then calculated using the composite dimensions and the recorded capacitance. The clamped dielectric constant was obtained by measuring the capacitance at twice the anti-resonance frequency. The impedance and phase spectrum was recorded using an impedance analyzer, and the effective electromechanical coupling coefficients (k t ) were calculated using the following equation: 2 πf r π f a f r kt = tan( ), (3) 2 f a 2 f a where f r is the resonance frequency and f a is the anti-resonance frequency. The mechanical Q of the composites can also be calculated according to IEEE standard [14]. 3. EXPERIMENTAL RESULTS 3.1 Composites fabrications Dicing of single crystals is much more difficult than ceramics, since the lack of domain boundaries permits cracks to propagate through the material freely. Since traditional dicing essentially cracks and chips away at materials, cracks along certain orientations can quickly spread due to the stresses from the saw. Vibrations (or chatter) within the spindle can also create post breakage as the saw passes through the cut material. Dicing was performed using 1-18 µm wide blades from Disco Corp., and the entire 1-3 composite was diced at one time, to mimic a more production level process. The spindle speed and feed rate were varied to evaluate the conditions for the least macroscopic damage (i.e. chipping and cracking). Feed rates of 1 mm/sec were adequate to eliminate chipping, and ideal spindle speed varied with the thickness of blade. This was because the combination of reduced bond material and higher speeds resulted in more diamond grit pullout as the blade exited the crystal, resulting in chipping on the backside of the blade. A representative 1 MHz diced structure is shown in Figure 5 (a). The dicing depth was greater than 25 µm, and the maximum kerf width was approximately 22 µm for the widest blade. The design is adequate for 1 MHz, though lateral modes between 2-25 MHz limit the performance above 15 MHz. Epo-Tek 31 epoxy (Epoxy Tech., Billerica, MA) was used as the inactive filler material. The epoxy was back-filled onto the composite structure and degassed, before curing overnight at room temperature to minimize expansion during curing. Both sides of the composite were then lapped, and Cr-Au electrodes (5Å/15Å) were applied to the faces (Figure 5(b)). 2 µm 'c' )..t 1 t ii4.e'.nv.et t. '.. ii '4 j! 3 a2 ';4'f I ft ;::;. rr.. C r tzfl- I L.; L4j w ttk. 'J*s&j PJdF.. t,ç t&tac '.,Sj 4 4fltPtç.i ' U, c1.i,....aa'l trr p',' (a) (b) Figure 5. Pictures of PMN-PT posts and composites.fabricated using the dice-and-fill technique. (a) PMN-PT posts directly after dicing. (b) PMN-PT composites with Cr/Au electrode. Proc. of SPIE Vol F-5

6 For > 25 MHz PMN-PT single crystal 1-3 composite fabrication, PMN-PT single crystal wafers with Ni coating were used for photolithography processing. PMN-PT wafers were coated with photoresist using a spin coater. Photoresist was baked at an elevated temperature for several minutes and then readied for UV exposure. After UV exposure, the wafers were developed using photoresist developer and then a patterned photoresist structure was formed. A through-wafer Ni electro-plating process was used to form a Ni etching mask out of the photoresist pattern. The PMN-PT wafer with Ni etching mask was then put into a dry etching chamber for deep etching. Figure 6(a) shows an SEM picture of etched PMN-PT pillar arrays for 1-3 composite. The kerfs of etched PMN-PT single crystal post arrays were next filled with epoxy. Epoxy was cured at 6 C overnight. The wafer was then lapped on one side until the PMN-PT posts were exposed. The wafer was then flipped over for the second side lapping until the final thickness was achieved. The 1-3 single crystal/epoxy composite was then formed, and both sides of the composites were coated with 5 Å Cr and 2 Å Au as electrodes. Figure 6 (b) shows micromachined piezoelectric composites with electrodes. (a) (b) Figure 6. SEM picture of an etched PMN-PT single crystal and composites. (a) etched PMN-PT pillar arrays. (b) a micromachined PMN-PT/epoxy 1-3 composite with Cr/Au electrodes. 3.2 Composites properties The impedance and phase spectra of 5MHz, 1 MHz and 15 MHz PMN-PT/epoxy 1-3 composites were recorded using impedance analyzer and are shown in Figure 7. The impedance and phase Lc spectra of 25 MHz and 4 MHz composites fabricated using PC-MUT are shown in Figure 8. The selected properties of the composites prepared using the dice-andfill process as well as PC-MUT process are presented in Table 2. I Fh I Fh 1 krn k II\ fl!!!!!! - 13 HHHH23.33 lll tit 3 \U I 1 oom c1 13 c OM. 5.OM 1.OM 15.OM 2.OM 25.OM 3.OM 35DM 2.OM 4.OM.OM 8.OM l.om (a) (b) Proc. of SPIE Vol F-6

7 p;p;d]i D;b;]d];A;Dd \ H-.OOL rho 1 9 N 7 ODD x 9 7 If HDDD DD.DD!! HDOD HOOD DOD /l / 7!!!!!!]?_ D /1-7- \ Figure 7. Impedance and phase spectrum of PMN-PT single crystal 1-3 composites fabricated using the dice-and-fill process. (a) 5 MHz composite with kerf width of 25 µm, post width of 75 µm and composite thickness of.245 mm. (b) 1 MHz composite with kerf width of 2 µm, post width of 6 µm and composite thickness of.151 mm. (c) 15 MHz composite with kerf width of 13 µm, post width of 47 µm and composite thickness of.95 mm. (c) Impedance (Ohm) Frequency (MHz) (a) Phase (Degree) Impedance (Ohm) Frequency (MHz) (b) Phase (Degree) Figure 8. Impedance-phase spectrum of PMN-PT single crystal composites fabricated using the PC-MUT technique. (a) 25 MHz composite with kerf width of 5 µm, post width of 15 µm, and composite thickness of 6 µm. (b) 4 MHz composite with kerf width of ~ 4 µm, post width of ~ 17 µm and composite thickness of 4 µm. Table 2. PMN-PT/epoxy 1-3 composite properties. Composite center f r f a tan δ k t frequency (MHz) (MHz) 5 MHz MHz MHz MHz MHz It is noticed that effective coupling coefficients of PMN-PT single crystal composites decrease with increasing frequency, which is partially attributed to the fact that post aspect ratio decreases in the high frequency designs. Dielectric loss also increases with frequency for the piezoelectric materials. Proc. of SPIE Vol F-7

8 4. CONCLUSIONS PMN-PT single crystal/epoxy 1-3 composites with frequency ranging from 5 MHz to 4 MHz were successfully fabricated using the dice-and-fill and PC-MUT techniques. PMN-PT single crystal composites showed electromechanical coupling coefficients of ~ , and the loss remains low (<.5), which is promising for NDE/NDT ultrasound applications. ACKNOWLEDGMENTS Authors would like to acknowledge the processing helps from Matt Corbin, Hua Lei, Dr. Seongtae Kwon and Dr. Jun Luo at TRS. REFERENCES [1] S.E. Park and T.R. Shrout, Relaxor based ferroelectric single crystals for electromechanical actuators, Mat. Res. Innovat., 1, pp.2-25, [2] P. Marin-Franch, I. Pettigrew, M. Parker, K.J. Kirk, and S. Cochran, Piezocrystal-polymer composites: new materials for transducers for ultrasonic NDT, Insight, Vol. 46, No.11, 24. pp [3] R.E. Newnham, D.P. Skinner, and L.E. Cross, Connectivity and Piezoelectric-Pyroelectric Composites, Materials Res. Bull., Vol. 13, pp , [4] W. A. Smith and B.A. Auld, Modeling 1-3 Composite Piezoelectrics: Thickness-Mode Oscillation, IEEE Trans. Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 38, No.1, 1991, pp [5] H.L.W. Chan and J. Unsworth, Simple Model for Piezoelectric Ceramic/Polymer 1-3 Composites Used in Ultrasound Transducer Applications, IEEE Trans. Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 36, No. 4, 1989, pp [6] R. Lockwood, D.H. Turnbull, D.A. Christopher, and F.S. Foster, Beyond 3MHz: Applications of High Frequency Ultrasound, IEEE Engineering in Medicine and Biology, Nov/Dec, pp (1996). [7] T.R. Gururaja, Piezoelectrics for Medical Ultrasound Imaging, American Ceramic Society Bulletin, Vol. 73, No. 5, pp (1994). [8] T. Ritter, T. Shrout, R. Tutwiler and K. Shung, A 3-MHz Piezo-Composite Ultrasound Array for Medical Imaging Applications, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, V 49, n2, February, 22, pp [9] A. Safari, Development of Piezoelectric Composite for Transducers, J. Phys. III France 4 (1994), pp [1] W. Hackenberger, M.J. Pan, D. Kuban, T. Ritter and T. Shrout, Novel Method for Producing High Frequency 2-2 Composite from PZT Ceramic, Proc. IEEE Iltrasonics Symposium, pp , 2. [11] Y. Hirata, H. Okuyama, S. Ogino, T. Numazawa, and H. Tatakada, Piezoelectric Composites for Micro-ultrasonic Transducers Realized with Deep-etch X-ray Lithography, Proc. IEEE MEMS 95, pp , [12] S. Wang, J.F. Li, R. Watanabe, and M. Esashi, Fabrication of Lead Zirconate Titanate Microrods for 1-3 Piezocomposites Using Hot Isostatic Pressing with Silicon Molds, J. Am. Ceram. Soc., 82(1) pp , [13] X. Jiang, J.R. Yuan, et al., Microfabrication of Piezoelectric Composite Ultrasound Transducers (PC-MUT), Proc. IEEE Ultrasonic Symposium, 26. [14] IEEE Standard on Piezoelectricity, ANSI/IEEE Standard Proc. of SPIE Vol F-8