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The big and small CMUTs are designed to have a center frequency of 3 MHz and 30 MHz respectively, a bandwidth of 100 % and a coupling coefficient of 0.5. The CMUT array contains an infinite number of elements, each consisting of Digital Object Identifier: 10.1109/ULTSYM.2008.0520 Keywords; CMUT, receive amplifiers, neighbor coupling Figure 1. Illustration of the CMUT design. Membrane stack: 200 nm PECVD Si 3 N 4 30 nm Aluminum 100 nm LPCVD Si 3 N 4 60 nm cavity depth Silicon substrate μ μ m
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Co-optimization of CMUT and receive amplifiers to suppress effects of neighbor coupling between CMUT
elements
Sigrid Berg, Trond Ytterdal and Arne Rønnekleiv Department of Electronics and Telecommunications
Norwegian University of Science and Technology, NTNU N-7491 Trondheim, Norway
Email: [email protected]
Abstract—Capacitive Micromachined Ultrasound Transducers (CMUTs) promise high transducer performance for several ultrasound applications. When using the CMUT array for medical applications where a focused ultrasound image with a 90 degree image sector is needed, we need a large number of individual elements. In off-axis beam steering, neighbor elements operate at different phase. This leads to unwanted acoustic effects caused by the interaction with the fluid medium outside the array. We see high-Q resonances close to the center frequency of the array at off-axis angles, which we want to reduce. We propose to use Transimpedance Amplifiers (TIAs) and Charge Sampling Amplifiers (CSAs) where we can easily adjust the input impedance, which opens up the possibility to design amplifiers that are optimized for an ultrasound system with CMUTs. Simulations show that a low impedance path results in suppression of the effects of resonances for both CSAs and TIAs and that co-optimization is important since the frequency of the CMUT array affects the Q-factor of the unwanted resonances. Even though we introduce an impedance mismatch the noise figure is still at an acceptable level. We present simulations in water, blood plasma with estimated data, and olive oil and show that the viscosity of the medium greatly influences the presence of resonances. This indicates that effects that might be present in human tissue may be much reduced in olive oil or other vegetable oils.
Keywords; CMUT, receive amplifiers, neighbor coupling
I. INTRODUCTION
Neighbor coupling between CMUT elements at the CMUT-water interface is a problem that many research groups have addressed [1], [2]. In a CMUT array made for medical imaging each element consists of several CMUTs, and when we steer the ultrasonic beam to an off-axis direction, the various elements operate at different phase. This excites local high-Q resonances which have a negative effect on the overall performance of the array by shorting the radiating part of the array impedance. Simulations show that these unwanted resonances occur in a frequency range below the center frequency, but well within the 100 % bandwidth of the device.
Traditionally, the receive amplifiers used in ultrasound systems have mainly been voltage amplifiers based on bipolar transistors (BJTs) (see, for example, [3]) with high input
impedance. We propose to use Transimpedance Amplifiers (TIAs) and Charge Sampling Amplifiers (CSAs) where we can easily adjust the input impedance, which opens up the possibility to design amplifiers which are optimized for an ultrasound system with CMUTs.
II. METHOD
The simulations to calculate the output impedance from the CMUT array is performed in MATLAB (Mathworks Inc., version 7.6). We use a mathematical CMUT model developed by A. Rønnekleiv [4], which describes the motion of the membrane as a combination of free acoustic modes of an acoustically isolated CMUT, and the coupling of these modes to and through the fluid outside the CMUTs.
A critical parameter deciding the Q-factor of the resonances is the product of the frequency and the shear viscosity of the fluid outside the CMUT array. We therefore present simulation data for three different liquids, water, blood plasma with estimated data, and olive oil and of two different CMUT arrays with a difference in center frequency of a factor 10.
The small CMUT cells have a radius of 5.7 μm, and the center to center distance between two cells is 12.5 μm. The membranes of the CMUTs consist of a three layer stack as shown in Fig. 1. The vacuum cavity is 60 nm deep. The big CMUT cells are 10 times larger in every dimension.
Membrane stack:
200 nm PECVD Si3N4
30 nm Aluminum
100 nm LPCVD Si3N4
60 nm cavity depthSilicon substrate0.95 – 4 μm
5.7 μm
12.5 μm
Circular CMUTs
Figure 1. Illustration of the CMUT design.
The big and small CMUTs are designed to have a center frequency of 3 MHz and 30 MHz respectively, a bandwidth of 100 % and a coupling coefficient of 0.5. The CMUT array contains an infinite number of elements, each consisting of
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Digital Object Identifier: 10.1109/ULTSYM.2008.0520
Authorized licensed use limited to: Univ of Calif San Diego. Downloaded on August 17,2010 at 22:20:22 UTC from IEEE Xplore. Restrictions apply.
two by two circular CMUTs, making the element size 25x25 μm, as illustrated in Fig. 2, or 250x250 μm.
25 μm
Each element consists of four CMUTs
Figure 2. Illustration of the array configuration.
Compared to water, the attenuation in blood plasma is greater, and is given by dB/cm03.0 43.1f=α [5], giving an attenuation at 30 MHz of 3.88 dB/cm. We assume that it is only the shear part of the viscosity that contributes to the losses, resulting in an increase of the shear viscosity by a factor of 7.12 compared to water. This should be an upper limit to the shear viscosity, as losses may also come from the extensional part of the viscosity. In water this part dominates. Since a lot of measurements presented in literature are done in vegetable oil [6], we also present simulations of waves transmitted into olive oil. The shear viscosity of olive oil at 20oC is a factor 115 higher than water at 37oC [7], [8].
For the circuit simulations, Eldo (Mentor Graphics, version 2008.1) was used together with behavioral level models for the different circuit elements. Some of the circuits that were simulated are switched, and thus, the signal power for a single frequency input signal is non-zero for more than one frequency. This effect was taken into account by fitting an Eldo CMUT model to the calculated CMUT impedance at all important frequencies.
By providing a low impedance path through the input impedance of the receive amplifier, CMUT charge can be extracted even though unwanted modes have been excited. A simple transimpedance amplifier (TIA) can be used to provide a low impedance termination of the CMUT element. A schematic of a simple TIA is shown in Fig. 3. For large operational amplifier voltage gain Av, the input resistance, assuming otherwise ideal amplifier, is proportional to Rf /Av. Hence, the input resistance can be adjusted by changing Rf and/or Av.
Another front-end amplifier proposed by us is the charge sampling amplifier (CSA). Sampling at the front-end is beneficial in systems that require some amount of beam forming at the front-end since delay elements is simple to implement in discrete-time circuits.
Like the TIA, the CSA also offers adjustable input impedance that can be optimized for a given CMUT design. A schematic of such an amplifier including a single CMUT element is shown in Fig. 4. During the clock phase ϕ1, the charge from the CMUT flows onto the capacitor Cs and is subsequently sampled and held at the end of ϕ1. The stored charge is read out by the next stage before ϕ2 goes high and
resets the CSA, preparing it for the next clock period. The purpose of the amplifier is twofold. Firstly, it drives the input of the next stage and, secondly, saves area since the Miller-effect is exploited to lower the physical size of the sampling capacitance Cs. The input impedance can be adjusted by changing Cs and/or Av. The switch S1 is inserted to periodically restore the DC voltage across the transducer. The CSA has one advantage over TIAs with resistive feedback in that CSAs do not require large resistors which require large silicon area. Since the CSA employs switched capacitors that model resistors, the frequency response is similar to the TIA response at low frequency compared to the sampling frequency.
Figure 3. Schematic of a simple TIA.
CSAs employing both self-biased single ended and differential amplifiers have been implemented in 90nm CMOS [9], [10].
Figure 4. Simplified schematic of the CSA.
III. RESULTS AND DISCUSSION
When the CMUT array is connected to a high impedance amplifier, we see the effect of high-Q resonances at 18.5 MHz, well within the 100% bandwidth of the transducer. This is illustrated in Fig. 5.
The effect of reducing the input impedance of the amplifier is illustrated in Fig. 6 where we have plotted the normalized transfer function of the CMUT and the TIA. The transfer function is defined as output voltage over the square root of the incoming power. We clearly see how the effect of the
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unwanted resonance at 18.5 MHz is significantly reduced by decreasing the input resistance.
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Frequency [MHz]
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Figure 5. Normalized transfer function of combined CMUT and TIA for a high input impedance TIA. The resonances occur when the beam is steared into off-axis directions.
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Figure 6. Normalized transfer function of combined CMUT and TIA for two different values of input resistance. Off-axis steering angle was 30°.
In Fig. 7 we see that when using the CSA the high-Q resonances are reduced when the input resistance is reduced by a factor of 250.
The low amplifier input impedance means that the shorting due to unwanted resonances is strongly reduced, and hence their influence on the current from the CMUT is greatly reduced.
One of the main advantages of the CSA is that since sampling is employed the circuit provides good basis for implementing electronic focusing on the chip. Also, area is saved over the TIA since the area consuming feedback resistor is replaced by a small capacitor and a few small switches. The most important drawback of the CSA compared to the TIAs is increased complexity including the need for clock circuitry.
As mentioned the viscosity of the fluid outside the CMUT array affects the unwanted resonances. In Fig. 8 we present simulation data showing the difference between water, blood plasma and olive oil. A higher viscosity of the fluid leads to lower resonance peaks.
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Figure 7. Normalized transfer function of combined CMUT and CSA for two different values of input resistance. The blue curve shows the simulated transfer function using an amplifier with 250 times lower input resistance compared to the one used to produce the red curve. . Off-axis steering angle was 14°.
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Water
Blood plasma
Olive oil
Figure 8. Normalized transfer function of combined CMUT and TIA for three different media. Amplifier input resistance was 100kΩ and off-axis steering angle was 30°.
CMUT arrays for medical imaging will operate in contact with various human tissues, such as blood plasma. We see that the resonances are not as severe in blood plasma as in water, but the reduction of the Q-factor of the resonances when using the CMUT array in blood plasma instead of water could in reality be lower, as the shear viscosity may be lower than our estimate. We then would observe less reduction in unwanted resonances.
The unwanted resonances that might be present in blood plasma and other human tissue might not be possible to measure in olive oil due to its high viscosity. This should be taken into account when characterizing CMUT arrays designed for 2D and 3D imaging.
In Fig. 9 we compare the transfer function from two different CMUT arrays where the one consisting of big CMUTs is operating on frequencies ten times lower than the one consisting of small CMUTs. We see from the simulations that the low frequency transducers experience a higher Q-factor of the resonances than the high frequency transducers with the same receive amplifier input impedance.
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Normalized frequency
Big CMUTs
Small CMUTs
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Figure 9. Normalized transfer function in water of combined CMUT and TIA for two different sizes of CMUT area . The off-axis steering angle was 14° and the input resistance was 100kΩ.
This shows that to be able to reduce the resonances sufficiently in low frequency transducers, the input impedance of the receive amplifiers needs to be further reduced. Co-optimization of the CMUT and receive amplifier design is important to suppress the neighbor coupling sufficiently.
To show that the noise properties of the amplifier are not severely degraded when the input impedance is reduced we simulated the noise figure of the CMUT/CSA system. The results are shown in Fig. 10. We note that even though absorption of energy is small due to the mismatch of the source and load impedances, the circuit still manages to detect the signal without adding much noise. Notice that suppression of noise is not as efficient as suppression of ripple. The unwanted modes add noise to the front end, and hence increase the effective noise factor for the CMUT array in directions and frequency ranges where they are excited. The noise is very narrow banded. Hence, the noise figure will still be acceptable looking at the entire bandwidth of the transducer.
IV. CONCLUSIONS
We have shown that adjusting the input impedance of the receive amplifier is a powerful tool to suppress neighbor coupling between CMUT elements operating in liquid. This can easily be achieved by employing transimpedance amplifiers (TIAs) or charge sampling amplifiers (CSA). One added benefit of CSAs is that since this type of amplifier operates in discrete time it is trivial to implement delay for beam-steering.
Co-optimizing the receive amplifiers with the CMUT array design is important as we showed that the neighbor coupling was more pronounced in a low frequency CMUT array than a high frequency array with the same receive amplifier. We also showed that even though we introduce an impedance mismatch the noise figure is not considerably degraded.
ACKNOWLEDGMENT
Financial support from the Research Council of Norway through the project Smart Microsystems for Diagnostic
Imaging in Medicine (project number 159559/130) is gratefully acknowledged.
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Figure 10. Noise figure of CSA and CMUT in water for an off-axis steering angle of 14° and low input impedance.
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