A 6.0 MHz 0.15mm Pitch Phased Array Ultrasonic Probe Using PMN-PT Single Crystal

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A 6.0 MHz 0.15mm Pitch Phased Array Ultrasonic Probe Using PMN-PT Single Crystal

Sung Min Rhim, Ho Jung, Ji Sub Jun*, and Jae Sub Hwnag*

Humanscan Co., Ltd., 320, Siwha APT Factory, Siwha Ind. Estate 5Ra 301 #672, Sunggok-dong, Ansan-si, Kyunggi-do, 425-110 Korea

*Medison Co., Ltd., 997-10, daechi-dong, Gangnam-gu, Seoul, 135-280 Korea E-mail address: [email protected]

Abstract - The performance of 64 channel 6.0 MHz phased array ultrasonic probe using 0.67Pb(Mg1/3 Nb2/3)O3- 0.33PbTiO3 single crystal was investigated. 2 way sensitivity of PMN-PT phased array is about 2 dB higher than that of PZT ceramic phased array. The -6dB bandwidth of PMN-PT phased array is about 100% and this is about 30% broader than that of PZT ceramic phased array. Especially this is very useful for harmonic imaging in Cardiac imaging. Sensitivity variation of PMN- PT phased array is similar and it is almost same with PZT ceramic phased array.

I . I NTRODUCTION

The low frequency phased array (2-3MHz) has been commercialized with high sensitivity and broad bandwidth comparing with conventional PZT ceramic probe [1]. And the possibility of multilayer single crystal probe was shown [2].

However, to make a high frequency phased array is more difficult because single crystal wafer thickness is too thin. The thickness of single crystal wafer for 5MHz phased array is about 0.15mm. The maximum frequency having reasonable yield for commercialization was about 5MHz. We have to reduce the thickness of single crystal wafer about 0.2-0.3mm in order to increasing the frequency about 1MHz. Because difficult process and low yield due to thin wafer thickness, the commercialization of high frequency probe using single crystal is very difficult. And the micro crack in thin wafer

happened during process is one of the reason for large variation between elements. Even though 7.5MHz linear array was presented [2], this is not for commercialized product but prototype for development and we have to overcome some problem for commercialization. This purpose of this study is possibility proof of single crystal for not only low frequency but also high frequency application (> 5MHz). In this paper, the fabrication of a 64 channel phased array with a center frequency of 6MHz MHz using PMN-PT single crystal is reported, and ultrasound images obtained using this high frequency PMN-PT phased array.

Fig. 1. Diced element surface having 0.15mm element pitch and 0.03mm kerf.

II . P ROPERTIES OF P MN- P T S INGLE

C RYSTAL AND F ABRICATION M ETHOD

OF P HASED A RRAY

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0.67Pb(Mg1/3 Nb2/3)O3-0.33PbTiO3 PMN-PT single crystal was used for high frequency phased array. The (001) direction PMN-PT wafer was lapped to about 0.15 mm in thickness.

After Cr/Cu/Au electroding with a thickness of 0.05/1.0/0.2 µm by DC sputtering, they were cut to 11 mm × 11 mm. The electroded wafer was then poled by applying a DC bias field of 3 kV/cm at room temperature for 30 min in silicone oil.

Table 1 is summary of PMN-PT single crystal properties comparing with conventional PZT ceramics. The clamped dielectric constant of PMN-PT single crystal is almost same with that of 3203HD PZT ceramics. However electromechanical coupling of PMN-PT single crystal is higher than that of ceramics. And low acoustic impedance is one of PMN-PT single crystal strengths for acoustic matching between probe to human body and it is possible to use comparable low acoustic impedance matching layer about 6- 7MRayls comparing with that of ceramic about 8-10MRayls.

Table I. Electrical and mechanical properties of 3203HD PZT ceramics and PMN-PT single crystal.

Properties unit 3203HD PMN-PT

Density kg/m³ 7800 8000

K3T 3800 5500

K3S 1250 1200

tanδ 2 <1

k33’ 0.68 0.85

vL’ m/s 4100 3300

Z MRayls 32 27

The specifications of the fabricated probe are a center frequency of 6 MHz, and 64 channels, each having dimensions of 10 mm × 0.12 mm. A backing material, whose acoustic impedance is 3.6 ×10⁶ kg/m²s, consisting of a mixture of several ceramic powders and epoxy resin was bonded to the multilayer PMN-PT wafer using an epoxy resin

cured at 35^oC for 24 hours. Then, signal and ground flexible printed circuits were attached along each edge of these transducers using silver epoxy cured at 60 ºC for 6 hours. Two acoustic matching layers were formed. A mixture of alumina powder and an epoxy resin with an acoustic impedance of 7.9

× 10⁶ kg/m²s was used for the first matching layer, and an epoxy resin sheet with an acoustic impedance of 3.0 × 10⁶ kg/m²s was used for the second matching layer. The acoustic lens is made of a RTV resin. The dicing pitch and kerf were 0.15 mm and 0.03 mm, respectively. The all elements were working after probe fabrication.

0.0 0.5 1.0 1.5 2.0 2.5

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

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-6dB PL = 0.189 usec -20dB PL = 0.462 usec Loop Senstivity = -60dB

Output voltage (V)

Time (usec)

0 3 6 9 12

-40 -35 -30 -25 -20 -15 -10 -5

0

(b)

-20dB -6dB

Center freq. = 6.1 MHz Bandwidht = 130 % Center freq. = 5.9 MHz Bandwidht = 98 %

Frequency (MHz)

Amplitude (dB)

Fig. 2. (a) waveform and (b) frequency spectrum of 6MHz 64channel PMN-PT phased array.

2005 IEEE Ultrasonics Symposium 220

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III . P ROPERTIES OF P MN- P T S INGLE

C RYSTAL P HASED A RRAY

Figure 1 is diced element surface having 0.15mm pitch and 0.03mm kerf. There is no any problem during dicing process such as chipping and de-bonding of element.

Pulse echo signal was measured at 75mm depth using the conventional pulse echo method in water at room temperature.

Figure 2 (a) and (b) show the waveform and frequency spectrum of 6MHz 64channel phased array using PMN-PT single crystal, respectively. Center frequency is about 6MHz and -6dB fractional bandwidth is almost 100%. The -20dB pulse length is about 0.19usec and this is only 2.7 cycle described by following: # of cycle = center frequency × pulse length.

0 2 4 6 8 10 12

-40 -30 -20 -10 0

Amplitude (dB)

Frequency (MHz) PZT probe PMN-PT probe

Fig. 3. Frequency spectrum PZT probe vs. PMN-PT probe.

Figure 3 is frequency spectrum of PZT and PMN-PT probes. The -6dB low frequency and high frequency edges of PZT probe are about 3.3MHz and 6.7MHz, respectively. The low edge of PMN-PT probe (3.0MHz) is similar with PZT probe but the high frequency edge is about 2MHz higher than that of PZT probe. This means that it is possible for PMN-PT probe to use lower Tx frequency for better penetration. This is very useful for harmonic imaging especially in cardiac

application. And if the Tx frequency is lower, the Rx frequency of harmonic component is higher and this means better resolution. Therefore, there is strength to both penetration and resolution in case of broad bandwidth.

Table II gives a numerical summary after averaging all of elements. The center frequencies of PMN-PT probe are slight higher than that of PZT probe. The echo amplitude of PMN- PT probe is about 2 dB higher than that of PZT probe. -6dB fractional bandwidth of PMN-PT probe is about 30% broader than that of PZT ceramic probe.

Table II. Numerical summaries of pulse echo characteristics of PZT, single layer PMN-PT, and multilayer PMN-PT probes.

Properties PZT PMN-PT

Relative sens. dB 0 +2.1

Center freq. MHz 5.0 5.9

-6dB bandwidth % 68 98

-20dB bandwidth % 94 130

-6dB pulse length usec 0.312 0.189 -20dB pulse length usec 0.736 0.462

0 10 20 30 40 50 60

-65 -64 -63 -62 -61 -60 -59 -58 -57 -56 -55

2 way sensitivity (dB)

Element #

PZT PMN-PT

Fig. 4. 2 way sensitivity variations of PZT probe and PMN-PT single crystal probe.

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(a)

(b)

Fig. 5. Comparison of harmonic B-mode images of (a) PZT probe and (b) PMN-PT probe.

Figure 4 shows 2 way sensitivity variations of PZT probe and PMN-PT single crystal phased array. The variation range of PMN-PT probe between maximum and minimum is only 1.96dB and standard deviation is 0.43dB. These values are almost same with PZT ceramic probe and it is enough for commercialization.

Figure 5 (a) and (b) shows B-mode harmonic image of PZT and PMN-PT single crystal probes using ACCUVIX XQ (Medison Co., Ltd.), respectively. The system settings of both cases are just same except Tx frequency. The Tx voltage of both is same and Tx frequency is a little different because bandwidth of two probes is different. The B-mode sensitivity of PMN-PT probe is higher than that of PZT ceramic probe and contrast and resolution of PMN-PT probe is also better

than those of PZT probe. These consist with 2 way pulse echo data in table II. If the system condition optimization will be done, the image quality of PMN-PT probe should be better.

IV . C ONCLUSIONS

A 64 channel 6.0 MHz phased array using PMN-PT single crystal was fabricated. -6dB fractional bandwidth of PMN-PT probe is about 100% and this is about 30% broader than that of PZT probe. Especially, the high frequency component of PMN-PT probe is larger with almost same as low frequency component. This means that it is possible to use lower Tx frequency for better penetration and to use higher Rx frequency of harmonic component for better resolution. The harmonic B-mode image of PMN-PT probe shows better penetration and resolution comparing with PZT probe. The element sensitivity variation of PMN-PT probe is almost same as PZT probe.

V. R EFERENCES

[1] Sung Min Rhim, Ho Jung, Seduk Kim, and Sang-Goo Lee, “A 2.6MHz Phased Array Ultrasonic Probe Using 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 Single Crystal Grown by the Bridgman Method,” Proc, IEEE Ultrason.

Symp., pp1114-1119, 2002.

[2] Sung Min Rhim, Ho Jung and Ki-Jong Lee, “Multilayer PMN-PT Single Crystal Transducer for Medical Application,” Proc, IEEE Ultrason. Symp., pp1021- 1024, 2004.

[3] Sung Min Rhim, Hyung Ham Kim, Ho Jung, Seduk Kim, and Sang-Goo Lee, “A 128 Channel 7.5MHz Linear Array Ultrasonic Probe Using PMN-PT Single Crystal,” Proc, IEEE Ultrason. Symp., pp782-785, 2003.