Fabrication and characterization of wafer-bonded cMUT ... - IEEE Xplore

10.1109/ULTSYM.2014.0046

Fabrication and characterization of wafer-bonded cMUT arrays dedicated to ultrasound-image-guided FUS Dominique Gross, Marie Perroteau and Dominique Certon GREMAN UMR CNRS 7347 Université François Rabelais Tours, France Email: [email protected]

Caroline Coutier

Mathieu Legros

CEA LETI Laboratoire Composants Micro Capteurs Grenoble, France Email: [email protected]

Vermon S.A. Tours, France Email: [email protected]

Abstract—In this paper, the development of wafer-bonded cMUTs arrays, which will be implemented on a dual-mode probe dedicated to liposomes activation and high frequency imaging, is presented. The process flow is briefly described, and optical and electrical characterizations are reported. Then, experimental electro-acoustic tests confirming the expected performances of each array are presented and discussed.

I.

I NTRODUCTION

For many years, focused ultrasound are used in several kinds of medical applications: pathological tissue necrosis by hyperthermia, lithotripsy, etc. These kinds of noninvasive treatments are often greatly facilitated by the simultaneous imaging of the region of interest (ROI). This is why many devices with both treatment and imaging modalities have been developed [1] [2]. Unfortunately, FUS treatments often require relatively large acoustic powers, for a long time, which could induce strong self-heating into classical piezoelectric transducers [3]. To reduce this spurious side-effect, the cMUT technology (capacitive Micromachined Ultrasonic Transducer) appears as a great alternative, as the membrane is the only moving component. In this paper, we present the development of cMUT linear arrays dedicated to thermosensitive liposomal (TSL) anticancer drug release (at low frequency - LF) with simultaneous high frequency (HF) imaging for preclinical evaluation. The first part of this proceeding describes the design and the fabrication process, whereas the second and third ones are devoted to the optical and electrical characterizations, and the first electroacoustic tests on bare chips. The applications and modalities of use of these arrays, as well as the platform developed to drive them, is beyond the scope of this proceeding and are described in [4]. II.

D ESIGN

AND FABRICATION PROCESS

A. Probe and arrays specifications The simultaneous imaging and heating ability of the dualmode probe requires the use of two completely different cMUT arrays, each one been exclusively dedicated to one task: one

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TABLE I.

C HARACTERISTICS OF BOTH ARRAYS

membrane size [μm2 ] gap height [nm] membrane thickness [nm] elements columns per element elevation [mm] width [mm] pitch [μm]

LF

HF

40.5x40.5 500 800 8 44 5 16 2300

15x21 100 800 128 4 2.8 16 125

HF linear array is placed at the center of the probe, while four LF linear arrays dedicated to FUS surrounded it [4]. To efficiently release the liposomes in the organism, the LF part of the probe should deliver enough pressure amplitude in the ROI. Novell et al. showed that TSLs are optimally released from a local temperature of 42 ◦C, which corresponds to a minimal pressure amplitude of about 1.25 MPa at 1 MHz [5], with a 1 kHz PRF and 40% duty cycle. Other works from the same team [6] show that this temperature can be reached with different pressure amplitudes by tuning the PRF and the duty cycle. Each LF array is mechanically ordered so that it points to the investigation depth (set at 20 mm) thanks to a mechanical frame. Moreover, electric delays are used to perform focusing in the other direction. The membranes of the LF arrays have been designed to work in the range [1 MHz - 5 MHz], which match with the typical releasing frequency of TSLs. As numerous classical imaging transducers, the HF array is composed of 128 elements. The targeted central frequency is in the range [15 MHz - 20 MHz] . Some design specifications of both arrays are reported in Table I. B. Fabrication process The fabrication process of the cMUT devices was based on a wafer-bonding method using a SOI (Silicon On Insulator) wafer. 6 levels of masks were required to perform the fabrication. As the application of each kind of arrays is completely different, two gap heights were designed. One of the specificity of this process flow is its ability of preparing two depths of cavities on the same wafers that permits the realization of both HF and LF devices at the same time, in the same

2014 IEEE International Ultrasonics Symposium Proceedings

B. Electrical characterization

Fig. 1. Process flow of fabrication of the CMUT devices. (a) realization of trenches, (b) filling of trenches with oxide, (c) cavity etching for LF devices, (d) cavity etching for HF devices, (e) SiN and SiO2 thin layer deposition, (f) SOI direct bonding, (g) handle and BOX removal, (h) element etching, (i) passivation deposition and contacts opening and (j) metal pad deposition and patterning.

Electrical measurements on bare chips were carried out with an impedance analyzer to assess several electrical parameters describing the performances of the arrays, and this for different bias voltages. Figure 4 reports the real and imaginary parts of the impedance measured without fluidloading condition. One can recognize the well-known softening effect which shift the electrical resonance of the LF and HF devices from 6.3 MHz to 5 MHz and 31 MHz to 20.5 MHz respectively. The measured in-series resistance value is very small, in average lower than 10 Ω on the whole frequency range. Furthermore, several cycles of increase/decrease in bias voltage, first applied by the top electrode and after by the bottom one, was performed to point out potential charging effects. These latter are mainly weak on the majority of chips, but can also be significant on some of them, which reflects once again the existence of local heterogeneities. From these measurements, the HF and LF capacitance (CHF and CLF ) of both arrays were deduced, as well as the coupling coefficient kt with the following relation: kt = 1 −

CHF CLF

(1)

The kt value is between 0.5 and 0.6 on both arrays (Figure 5), which confirms the good homogeneity of cells on one element. IV.

After optical and electrical characterization, first acoustic performances of each kind of arrays were assessed thanks to laser interferometry and hydrophone measurements.

run of fabrication, and on the same chip. The membrane was highly doped, which avoid the use of an additional metallic top electrode. To ensure a good homogeneity of the electric potential on all the cells, a peripheral metallic trace connected to the bond pad has been deposed around each LF element. The fabrication was performed with the 200 mm MEMS CEA Leti platform and is described in Figure 1. Pictures of the fabricated devices are presented in Figure 2. III.

A RRAYS

CHARACTERIZATION

A. Optical characterization Once the fabrication step performed, the collapse voltage (Vc ) of each kind of array was measured thanks to a DHM (Digital Holographic Microscope, Lyncée Tec, Lausanne, Switzerland). The static collapse/snapback voltages were determined by measuring the displacement profile of several membranes with different bias voltages. The results are reported on Figure 3. The averaged collapse voltage is about 110 V for both arrays, with a very low standard deviation. This measurement shows a really good homogeneity from element-to-element on one single chip, but these values can change from one chip to another. Notice the strong hysteresis on the LF array, which is almost absent on the HF one. The difference between the collapse position and the initial position represents the maximum displacement sweepable by the membrane, taking into account the initial deflection due to the atmospheric pressure. The initial deflection is about 60 nm on the LF arrays, almost non-existent on the HF ones.

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A. LF array Acoustic simulations of the pressure field radiated by the LF part of the final probe were performed thanks to the DREAM Toolbox [7]. From these simulations, an acoustic focusing gain describing the ratio between the pressure radiated by the whole probe at the focal point and the one emitted by one single element of array at the same point was determined (see Figure 6). This allows us to evaluate in a first step the pressure amplitude emitted by the future probe at its focal point by measuring the pressure of one single element at the focal distance. For this paper, we focus on the results at 1 MHz, as

150 500

local membrane displacement [nm]

Pictures of the LF (a) and HF (b) arrays

local membrane displacement [nm]

Fig. 2.

E LECTRO - ACOUSTIC MEASUREMENTS

400

300

200

100

0 0

25

50 75 100 bias voltage [V]

125

150

125 100 75 50 25 0 0

20 40 60 80 100 120 140 160 180 bias voltage [V]

Fig. 3. Displacement of the center of the membrane for each kind of array as function of the bias voltage. Left: LF array membrane displacement of 25 cells, right: HF array membrane displacement of 40 cells.


Normalized real part

500

6

7

8 9 10 Frequency [MHz]

20

30

Peak−to−peak pressure [kPa]

5

40

Normalized imaginary part 4

300

200

100

0 0

5

6

7

8 9 10 20 Frequency [MHz]

30

40

Fig. 4. Normalized real and imaginary parts of the electrical impedance in air over bias voltage (from 10% to 100% of Vcollapse ). Solid line: LF array, dash line: HF array.

75 100 125 Peak−to−peak excitation voltage [V]

150

175

5

10

15

20

25

30

HF array: 0 to Vc

0.6

HF array: Vc to 0

100 80

−5

0.7

200

Time [µs] 00

60 −10

LF array: Vc to 0

Magnitude [dB]

t

k

50

Fig. 7. Peak-to-peak pressure amplitude at 1 MHz and 20 mm against peakto-peak excitation voltages, for different bias voltages (from 20% Vc (clear gray) to 90% Vc (black) with a step of 10% Vc .

0.8

0.5

25

LF array: 0 to Vc

0.4

40 20

−15

0 −20 −20 −25

−40

0.3

Pressure [kPa]

4

400

−60

−30

−80

0.2 −35

−100

0.1 −40 0.5

0 0

Fig. 5.

25

50 75 bias voltage [V]

100

Thus, one LF array was wire-bonded on a custom PCB and covered with a 500 μm silicone layer, whose acoustic impedance is close to that of water. Then, the device was immersed into a water bath and placed at 20 mm in front of a hydrophone (HGL-0085, Onda Corporation, Sunnyvale, USA). The pressure was recorded for different excitation and

Focusing gain

25 20 15 10 5 10

20 30 Depth [mm]

40

1.5

2 2.5 Frequency [MHz]

3

3.5

−120 4

Fig. 8. Example of LF signal emitted by one element. The first oscillations on the temporal signal are due to the electrical coupling affecting the hydrophone.Vpp =85%Vc , Vbias =80%Vc

Coupling coefficient kt of one element from each array.

it is the optimal frequency for TSLs release. The focusing gain at the ROI is close to 25, so it was deduced that a minimal peak-to-peak amplitude of 100 kPa is required for one element to reach the targeted pressure amplitude at the focal point of the probe.

0 0

1

50

Fig. 6. Focusing gain along the depth for an excitation with a central frequency of 1 MHz. Focusing depth = 20 mm. .

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bias voltages (Figure 7). These measurements were carried out for different frequencies in the range [1 MHz - 5 MHz]. Once more, only the results at 1 MHz are reported here. The operating conditions were: 1 kHz PRF, 10 cycles. These curves exhibit several operating conditions which fulfill the output pressure requirements given by the simulations. But, since the beamformer developed to drive the LF arrays will be limited to 100 V peak-to-peak, it will be necessary to polarize the cMUT arrays to at least 60% of Vc . High nonlinearities can be observed on the pressure spectrum (Figure 8), as the cMUT chip is working in driven mode, i.e. below its resonance frequency [8]. Nevertheless, the harmonic components should not been an issue for the final application, as they will contribute to the resulting acoustic power produced close to the focal point. To complete these measurements, aging tests were carried out on the same packaged device. The chip worked for 3 hours at 3 different frequencies and the signal was recorded every 2 minutes. Moreover, the operating conditions were, this time, in agreement with the ones required for heating applications, i.e. 1 kHz PRF and 50% duty cycle. The chip was biased at 80% of Vc and excited with a 80 V peak-to-peak signal. The results are reported on Figure9. No significant loss was observed on the signal amplitude. This latter seems to even slightly increase at the end of the measurement. This first aging test should be


Peak−to−peak pressure [kPa]

600 500

5 MHz

3 MHz

400 300 200 100

1 MHz

0 0

Magnitude [dB]

Local displacement [nm]

Fig. 9.

20

40

60

80 100 Time [min]

120

140

160

180

Aging tests on LF array at 3 different excitation frequencies

20 0 −20 1.9

2

2.1

2.2 2.3 Time [µs]

2.4

2.5 2.6 outer cell inner cell

ACKNOWLEDGMENT The Agence Nationale de la Recherche and the Fonds Européen de Développement Régional are acknowledged for their financial supports on the project (THERANOS ANR-10TECS-007/FEDER THERANOS 3431-35438).

0 −10 −20 −30 0

10

20 30 Frequency [MHz]

40

tation of two different gap heights is validated, as characterizations exhibit promising results, and a well functionality. The first electo-acoustic tests on LF array revealed several operating points to reach the required pressure amplitude on the final probe. Aging test did not point out significant malfunctions which could compromise the desired application of these arrays. Moreover, the displacement response of HF cells show good bandwidth and central frequency. Additional characterizations on the final probe, performed at higher levels of development, are presented in [4]. This first paper confirms the viability of manufacturing two completely different kinds of array on a same wafer, and demonstrated the proper functioning of the devices. Imaging testings on wire phantom are currently under progress. As a short-term goal, in vitro assessment of the heating capabilities of the LF arrays are planned. Finally, in vivo tests of TSLs releasing on living mouses will be performed.

50

R EFERENCES [1]

Fig. 10. Temporal and spectral displacement of one outer and one inner cell [2]

confirmed in future works to ensure stability over time of our device, and this with several operating conditions. [3]

B. HF array Electro-acoustic tests on HF array were performed on bare chip immersed in oil, under a heterodyne laser interferometer (described in [8]). This setup allows the measurement of the dynamic displacement at the center of one membrane, by numerical demodulation of the received beam. One element was excited with one single pulse centered at 25 MHz, with a bias voltage of 80% of Vc and a peak-to-peak excitation voltage of 60 V. The displacements of one inner and one outer cell were measured and are reported in Figure 10 with their corresponding spectra. Crosstalk via fluid coupling (baffle effect) between inner and outer cells can easily been identified on these curves: after the pulse duration, inner and outer cells vibrate freely in phase opposition. This phenomenon causes the disruptions that can been seen on their spectra around 11 MHz. Fortunately, this spurious mode is only strongly visible on a local displacement measurement, and its contribution in far field is low. The central frequency is 17.6 MHz with a fractional −6 dB bandwidth of 136%. One should notice that this result, even if it shows good functionality in a wide frequency range, does not represent the behavior of a whole element, where all the cells are taken into account. Additional results on a packaged array are presented in [4], and show that the bandwidth on pressure measurements is reduced. V.

[4]

[5]

[6]

[7] [8]

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C ONCLUSION

HF and LF cMUT arrays were designed and fabricated thanks to a SOI based wafer-bonding process. The implemen-

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