Capsule Ultrasound Device

Farah Memon, Gerard Touma, Junyi Wang, Spyridon Baltsavias, Azadeh Moini, Chienliu Chang, Morten Fischer

Rasmussen, Amin Nikoozadeh, Jung Woo Choe, Amin Arbabian, R. Brooke Jeffrey, Eric Olcott,

Butrus T. Khuri-Yakub

Stanford University, USA

Corresponding email: farahm@stanford.edu

Abstract—We are developing a capsule ultrasound (CUS)

device to serve as a wireless, portable, and ultrasonic pill for

investigating the multiple layers of the complete gastrointestinal

(GI) tract, in particular, the small intestine. This capsule will

acquire ultrasound images with 360 degrees field-of-view (FOV)

and a penetration depth of 5 cm using a 128-element and

cylindrically-shaped capacitive micromachined ultrasonic

transducer (CMUT) array, wrapped around the center of its

body. Simulation results indicate that linear array imaging with a

fixed focus of F#4 and 16 active elements produces valuable

images. We have designed a CMUT for this application and the

fabrication process to create cylindrical CMUT arrays has been

established. We report our fabrication progress and show test

devices that we successfully made and bent around a glass tube.

In addition, the design of the application-specific integrated

circuit (ASIC) and the wireless transmitter, responsible for the

acquisition and wireless transmission of ultrasonic data

respectively, is described.

Keywords—CMUT; capsule endoscope; wireless; medical

devices; medical imaging; ultrasound

I. INTRODUCTION

To examine the gastrointestinal (GI) tract, in particular the small intestine, various medical diagnostic tools, such as capsule endoscopes, and techniques, including endoscopy and computed tomography, are utilized [1][2][3]. The main problem with these stated diagnostic devices and methods is that they are inadequate in providing a clear picture of the different layers of the small intestine, such as the mucosa and the submucosa. As a result, properly diagnosing lesions and tumors in the small intestine is a significant challenge. To solve this problem, we are in the process of developing the capsule ultrasound (CUS) device. This device is designed to have a diameter of 1 cm and a height of approximately 2.5 cm. Secured around the body of the device will be a cylindrical capacitive micromachined ultrasonic transducer (CMUT) array, illustrated in Fig. 1. The CUS device is to be swallowed by the patient so that it can scan the entire digestive system with a 360o field-of-view (FOV) and a penetration depth of 5 cm by collecting ultrasonic data for B-mode images with a frame rate of 2-4 images per second and wirelessly transmitting the data to an external unit. In this paper, we report our progress towards the development of the CUS device.

Fig. 1. The cylindrical CMUT array for the CUS device with 128 total elements, 16 active elements (shown in pink), and a diameter of 1 cm.

For image acquisition, linear array imaging with a fixed and identical focus on transmit and receive, will be implemented using the cylindrical CMUT array. The transducer will have 128 total and 16 active elements with a center frequency of 5 MHz. Apart from the CMUT array, an application-specific integrated circuit (ASIC) will be situated in the CUS device. It will have multiple components including ASIC TX, ASIC RX, Power Management Unit, and Control Unit. These units will be responsible for transmitting ultrasound pulses, receiving and performing beamforming with the RF signals, providing DC bias to different components, and controlling the entire operation of the device, respectively. The beamformed data from the ASIC will be relayed to memory from which the wireless transmitter will read and transmit the data to an external processing unit. Lastly, the CUS device will also house batteries and a clock for operation. The entire block diagram of the CUS device is presented in Fig. 2.

Fig. 2. A block diagram of the CUS device indicating the CMUT array, wireless transmitter, memory, clock, batteries, and the ASIC.

978-1-4799-8182-3/15/$31.00 ©2015 IEEE 2015 IEEE International Ultrasonics Symposium Proceedings

10.1109/ULTSYM.2015.0168

We will first discuss simulations that demonstrate the ability of the CUS device to produce valuable medical images and transition towards the design of the CMUT array. Next, we will focus on the fabrication process required to make cylindrical CMUT arrays for the CUS device. We will conclude with an update on the ASIC and wireless transmitter design.

II. CUS DEVICE SIMULATIONS

To evaluate the performance of the cylindrical CMUT array, we used Field II [4][5]. Using Field II, we defined a cylindrical array with a 5 MHz operating frequency, set up a phantom consisting of point targets, cysts, and anechoic regions, and gathered beamformed data by simulating linear array imaging with the focus set using an F#4 for both transmit and receive. As seen from Fig. 3, a fixed focus using F#4 produces a meaningful medical image.

III. CMUT ARRAY DESIGN

Based on the simulation results, the CMUT was designed for a 5 MHz center frequency. In addition, we desire a low collapse voltage, an important consideration for a portable CUS device. Table I lists the CMUT array parameters for our design.

Fig. 3. Simulated image of a phantom with a fixed focus using F#4. Image shown with a dynamic range of 50 dB.

TABLE I. CMUT ARRAY PARAMETERS

CMUT Array Parameter Value

Cell diameter 57 μm

Cell pitch 61 μm

Plate thickness 2 μm

Gap height 110 nm

Insulation layer thickness 110 nm

Silicon nitride (deposited on top electrode) thickness

110 nm

Fig. 4. Arrangement of cells (shown in purple) and the separation between elements (shown in blue) in the CMUT array.

Fig. 5. The peak-to-peak output pressure is 342 kPa with the CMUT array biased at 24 V and excited using a 15 V unipolar pulse.

Fig. 4 displays the tight arrangement of the cells for the CMUT design, which gives a fill-factor of 77%. This high fill-factor will result in a large output pressure. In addition, the CMUT array is designed to have a large transmit efficiency and receive sensitivity in low voltage regime. As a result, the collapse voltage has been designed to be between 35 V – 40 V. Fig. 5 shows simulated output pressure of the CMUT array. With the CMUT biased using 24 V DC and pulsed with 15 V unipolar pulse, the peak-to-peak surface pressure is higher than 300 kPa.

IV. FABRICATION OF CMUT ARRAYS

In order to create CMUT arrays for the CUS device, we derived a fabrication plan and are currently working to complete all of the steps of the process. We use a wafer-bonding process for CMUT fabrication, and etch trenches between elements to enable bending of the CMUT array around the CUS device [6]. A small silicon bridge, connecting the CMUT element to a frame, is preserved during the etching process, as displayed in Fig. 6. We plan to integrate the CMUT array with a flexible PCB (flex), detach the frame from of the CMUT array, and bend the CMUT-flex assembly in a cylindrical fashion.

Fig. 6. (a) A bottom view of the CMUT array and (b) a side view of a CMUT element.

(a)

(b)

Fig. 8. (a) (b) Test devices wrapped around a 10 mm diameter glass tube.

(a) (b)

Fig. 9. (a) Static cell deflection of CMUT cells. (b) Multiple CMUT cells separated by zig-zag trenches.

(a) (b)

(c) (d)

(e) (f)

(g)

Fig. 7. Fabrication Process for cylindrical CMUT Arrays.

A. Fabrication Process

The fabrication process starts with the oxidation of a low-resistivity SOI wafer. The oxide on the SOI wafer is patterned and etched to create the cell cavities [Fig. 7(a)]. Next, the patterned SOI wafer is fusion bonded to an oxidized low-resistivity prime wafer and the bonded pair is oxidized at 1050oC for 4 hours. This process results in an oxide-to-oxide bond between the two wafers. The handle and the BOX layer of the SOI wafer are removed using wet-etching techniques, leaving behind the silicon plate [Fig. 7(b)]. Next, aluminum is evaporated on the silicon plate, followed by the etching of the aluminum and the silicon plate to separate the CMUT elements [Fig. 7(c)].

Next steps include removing the oxide covering the gap height and the insulation layer using dry-etching techniques, and etching silicon on the prime wafer using deep-reactive ion etching (DRIE) to create front-side trenches. Silicon nitride is deposited on the front-side using plasma-enhanced chemical vapor deposition (PECVD) and patterned to access the front-side electrode [Fig. 7(d)]. On the back-side of the wafer, the oxide is etched and under bump metallization (UBM) pads are placed using a lift-off technique [Fig 7(e)]. The wafer is then bonded to a carrier wafer using a wafer bonding agent and back-side trenches are etched and merged with the front-side trenches. The silicon bridge between the elements and the frame is accomplished by two lithography steps to etch the back-side trenches and using the oxide on the bottom of the wafer to control the bridge height. Lastly, the trenches are filled with polydimethylsiloxane (PDMS) and the carrier wafer is de-bonded [Fig. 7(g)].

B. Fabrication Progress

Prior to making cylindrical CMUT arrays, we made test devices to validate our fabrication process. These test devices are silicon elements separated via trenches and connected to a common frame. After device fabrication, we attached these test

devices to a piece of tape, removed the frame, and bent the test devices around a 10 mm diameter glass tube. Fig. 8 shows some cylindrical test devices, demonstrating that we can incorporate trenches within CMUT elements to make cylindrically-shaped arrays.

Currently, we are working towards fabricating cylindrical CMUT arrays. We have completed the wafer-bonding and the front-side trench etching and are working towards completing the latter steps and testing the devices. Fig. 9 shows images of the fabrication process acquired using an optical 3D profiler. Shown in the figure is cell-deflection of the CMUT cells after the wafer-bonding process and multiple CMUT elements separated by zig-zag trenches after depositing aluminum and separating the elements.

V. ASIC AND WIRELESS TRANMITTER

The ASIC and wireless transmitter are two integral components of the CUS device that we are in the process of designing.

A. ASIC

As shown in Fig. 2, the ASIC is divided into multiple blocks. . Both the transmit (ASIC TX) and receive (ASIC RX) blocks consist of 128 channels. Fig. 10 display the transmit and receive channels for an active element. In the transmit phase, in order to pulse with a fixed focus, each CMUT element has an associated delay embedded in a shift register. The delay is compared to the value of a counter; when the delay equals the counter, the high-voltage (HV) pulser triggers the CMUT array to emit an ultrasound pulse. The one shot

circuit ensures proper pulse-width for the emitted ultrasound pulses. In receive mode, the CMUT is connected to a trans-impedance amplifier (TIA). The TIA is designed for a bandwidth greater than 10 MHz to properly capture the CMUT broadband signal. The TIA is followed by a time-gain compensation (TGC) amplifier, with a variable gain ranging from 0 dB to 40 dB, a 10-bit analog-to-digital converter (ADC), and a digital delay line and adder to perform beamforming. In order to perform linear array imaging, delays for each element are connected serially and shifted by one element after the acquisition of every beamformed signal. In order to have only 16 active elements at a time, inactive elements are associated with a delay that does not trigger the comparator to turn on during the transmit phase. In the receive phase, the inactive elements are not connected to an ADC and hence do not contribute to the beamformed signal.

The Power Management Unit will mostly comprise of charge pumps that amplify voltage from the batteries to provide the DC bias for the CMUT and the pulser. Lastly, the Control Unit will have digital circuitry to regulate the activity of the rest of the units in the ASIC.

We have designed most of the components of the ASIC and are working on the layout. After the tape-out of the ASIC, we will test its performance.

B. Wireless Transmitter

This module aims to transmit 2-4 frames/sec data to an external receiver, through highly attenuating and thick human tissue layers, while consuming low power and adhering to FCC regulations. Taking into account the frequency dependency of tissue attenuation [7] and the bandwidth and output power limit of available bands [8], the 902-928MHz ISM band was chosen for transmission. Finally, for a robust, low complexity design for both the transmitter and receiver and high energy efficiency [9], Frequency Shift Keying (FSK) was chosen as the modulation scheme. The specifications for the wireless transmitter are shown in Table II.

ASIC – TX Channel

ASIC – RX Channel

Fig. 10. (a) TX Channel and (b) RX Channel Block Diagram.

TABLE II. WIRELESS TRANSMITTER PARAMETERS

Specification Value

Data rate 6-10Mbit/s

Average power dissipation < 6mW

Bit error-rate < 10

-5

Output power > 3dBm

VI. CONCLUSION

In this paper, we report our progress towards making a prototype of the CUS device. We have created a design and a fabrication process for the CMUT arrays and are working to complete the CMUT array fabrication. In addition, design of the ASIC and the wireless transmitter is in progress. The future work involves integrating all the three components: CMUT array, ASIC, and wireless transmitter, for imaging experiments.

ACKNOWLEDGMENT

This research is supported by General Electric Medical Systems. Farah Memon is supported by National Science Foundation Graduate Research Fellowship Program. Fabrication of CMUT arrays is performed at Stanford Nanofabrication Facility, which is a member of National Nanotechnology Infrastructure Network.

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