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Characterization of 0.18-μm CMOS MEMS Capacitive Ultrasonic Sensors for Fast Photoacoustic Imaging
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 168 (2016) 713 – 716
30th Eurosensors Conference, EUROSENSORS 2016
Characterization of 0.18-Pm CMOS MEMS Capacitive Ultrasonic Sensors for Fast Photoacoustic Imaging Yi-Chia Shih and Michael S.-C. Lu Department of Electrical Engineering, National Tsing Hua University, 101 Sec. 2 Kuang-Fu Rd., Hsinchu 30013, Taiwan, R.O.C.
Abstract This work presents a 4 u 4 capacitive ultrasonic sensor array fabricated in a 0.18-Pm CMOS process. Compared to prior CMOS MEMS ultrasonic sensors, the smaller electrode separation in the new platform significantly improves the capacitive sensitivity. The sensor has a measured center frequency at 3.8 MHz with a fractional bandwidth of 74%. The measured sensitivity is 499 mVpp/MPa/V. With the advance in technology, individual readout circuit can be placed directly beneath each sensor to allow unlimited scalability. Feasibility for fast 3D photoacoustic imaging is demonstrated with the sensing array presented in this work. © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: CMOS, Capacitive sensors, Ultrasound, Photoacoustic imaging.
1. Introduction Photoacoustic imaging is a promising noninvasive imaging modality to provide the structural and functional information for a wide spectrum of applications, such as imaging of blood vessels and detection of melanoma and breast cancer. It overcomes the penetration limit of pure optical imaging into scattering biological tissues to provide sub-millimeter ultrasonic resolution for an imaging depth up to centimeters. Non-ionizing laser pulses delivered into biological tissues are absorbed and converted to photoacoustic waves due to transient thermo-elastic expansion, which are representative of optical absorption distribution. Photoacoustic image reconstruction by 2-D scanning has been demonstrated in our prior work [1]; however, the technique requires a long time for data acquisition and post processing. The acoustic lens system [2] has been proposed to provide real-time 3-D photoacoustic imaging. In our proposed scheme as depicted in Fig. 1, a 4f acoustic lens system is used to image the produced photoacoustic pressure that represents the absorption distribution onto an image plane where a CMOS MEMS capacitive sensor array is placed to receive the transmitted pressure. The system provides uniform axial and lateral magnification of the imaged
1877-7058 © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.256
714
Yi-Chia Shih and Michael S.-C. Lu / Procedia Engineering 168 (2016) 713 – 716
pressure distribution. A small displacement in the object plane results in the same amount of displacement for the focused image plane. Due to the low speed of sound, the axial resolution is preserved by the propagation of transmitted acoustic pressure to facilitate 3-D imaging. In our previous work, a 4 u 4 sensor array implemented in a 0.35-Pm CMOS MEMS platform [3] was demonstrated; however, scalability is limited since individual readout cannot be placed underneath each sensor. In the new 0.18-Pm CMOS MEMS platform presented in this work, the readout size and performance can be improved to allow unlimited scalability; in addition, the air cavity between the capacitive electrodes is significantly reduced by using one of the MIM (metal-insulator-metal) capacitor layers as the sacrificial material, leading to increased sensitivity. The design, fabrication and characterization of the CMOS capacitive sensor array will be described in the following sections.
Fig. 1. The acoustic lens system is used to focus the emitted ultrasound to the CMOS capacitive sensors for fast photoacoustic image.
(a)
(b)
Fig. 2. (a) Cross-sectional view of the 0.18-Pm CMOS MEMS capacitive ultrasonic sensor. (b) SEM of the 4 u 4 capacitive sensor array. Each sensing unit as shown by the dash line consists of four membranes with a total capacitance of 540 fF.
2. Sensor Fabrication and Design The capacitive ultrasonic sensor array was fabricated in the TSMC 0.18-Pm one-polysilicon six-metal (1P6M) CMOS process based on the process reported in [1]. Sacrificial wet etch of stacked metal (aluminum) and via (tungsten) layers through the passivation openings was performed for structural release. As shown in Fig. 2(a), the thin metal (thickness = 0.19 Pm) typically used to form MIM capacitors (with metal-5) was removed to form air cavity, leading to enhanced capacitive sensitivity. The release holes were sealed by parylene-C deposition (thickness = 2.4 Pm). As shown in Fig. 2(a), the metal-5 and metal-6 layers were used as the capacitive electrodes. Inner diameter of the circular sensing membrane is 80 Pm, producing a capacitance of 135 fF. Four membranes are connected in parallel to form a single sensing element with a total size of 240 u 240 Pm2. A 4 u 4 sensor array is implemented on the CMOS chip as shown in Fig. 2(b). Upon photoacoustic imaging, the capacitively transduced
Yi-Chia Shih and Michael S.-C. Lu / Procedia Engineering 168 (2016) 713 – 716
signal is amplified, high-pass filtered, followed by peak detection to obtain the ultrasound intensity, as shown by the readout schematic in Fig. 3. A dc bias is applied to the pre-amp to produce the motional current, which is integrated by the feedback capacitance CF (CF = 100 fF). Contrast of the photoacoustic image depends on the accuracy of the peak detection circuit. Thanks to the migration from the 0.35-Pm to the 0.18-Pm CMOS process, signal routing becomes more convenient with more metallization layers and the whole readout can be placed beneath the sensing membranes to provide unlimited scalability.
Fig. 3. Schematic of the capacitive readout. Intensity of the ultrasonic signals is determined by the peak-detection circuit.
1 0.8
a.u.
0.6 0.4 0.2 0 0
2
4 6 Frequency (MHz)
8
10
Fig. 4. Spectrum of the measured capacitive ultrasonic signal. The center frequency is at 3.8 MHz with a fractional bandwidth of 74%.
3. Experimental Results For characterization, the CMOS sensor chip was first wirebonded to a printed circuit board and the bond pads were covered by epoxy to provide electrical isolation while the chip was immersed in water for test. Sinusoidal signals in the ultrasound range (1 to 10 MHz) with amplitudes from 0.1 to 1 V were applied to the peak detection circuit for test. The results showed that the percentage errors were mostly limited with r4% when the input amplitudes were between 0.1 to 0.5 V. As predicted by simulations, the result was better with larger inputs. The errors became much worse when the inputs were less than 100 mV. An ultrasonic transducer (V310-SM, Panametrics-NDT; diameter = 6 mm) was used to characterize capacitive transduction. Sensed capacitive signals were transformed to the corresponding spectra as depicted in Fig. 4, which shows a measured resonant frequency near 3.8 MHz with a fractional bandwidth of 74%. Measured output increases linearly with the bias voltage across the sensing capacitor to provide a sensitivity of 499 mVpp/MPa/V. Fig. 5 shows that the peak value of the pre-amp output produced by ultrasound transmission can be successfully captured by the peak detector. Fig. 5(a) and (b) show the measured results when the capacitor was biased at 8 V and 20 V, respectively. The acoustic pressure produced by using a visible light source is in the MPa range. By
715
716
Yi-Chia Shih and Michael S.-C. Lu / Procedia Engineering 168 (2016) 713 – 716
considering the attenuation of the acoustic lens in the system, the sensing capacitor can be biased with a suitable voltage (like 10 V) to generate enough signals.
2.5
Signal (V)
3
reset
2 1.5
sensed output from pre-amp
1 0.5 0 0
reset peak detector output
peak detector output
Signal (V)
3
2 1
sensed output from pre-amp
0 2
4 Time (Ps) (a)
6
8
0
2
4 Time (Ps)
6
8
(b)
Fig. 5. Peak values (red) of the capacitively sensed signals (blue) are successfully captured by the peak-detection circuit with the dc bias of the capacitive electrodes set at 8 V and 20 V [(a) and (b)], respectively .
4. Discussion and Conclusion This work uses a convenient micromachining process to develop a capacitive sensor array in a 0.18-Pm CMOS process for fast photoacoustic imaging. Monolithic integration effectively reduces the parasitic effect and enhances the signal-to-noise ratio. Compared to prior work developed in the 0.35-Pm CMOS process, scalability of the sensor array is not limited with the improved readout. Amplification and peak detection of the ultrasonic signals are successfully demonstrated by the capacitive readout. Our future work will be focused on the integrated test with the acoustic lens system. Acknowledgements This work is supported by the Ministry of Science and Technology, Taiwan, Republic of China. The authors would like to thank the National Chip Implementation Center for support of chip fabrication and the National Center for High-Performance Computing for support of simulation tools. References [1] M. L. Li, P. H. Wang, P. L. Liao, M. S.-C. Lu, Three dimensional photoacoustic imaging by a CMOS micromachined capacitive ultrasonic sensor, IEEE Elec. Dev. Lett. 32 (2011) 1149–1151. [2] J. J. Niederhauser, M. Jaeger M. Frenz,Real-time three-dimensional optoacoustic imaging using an acoustic lens system, Appl. Phys. Lett. 85 (2004) 846–848. [3] C. A. Kuo, M. S.-C. Lu, Characterization of CMOS MEMS capacitive ultrasonic sensors for fast photoacoustic imaging, Proc. Eurosensors (2014), Brescia, Italy.
ScienceDirect Procedia Engineering 168 (2016) 713 – 716
30th Eurosensors Conference, EUROSENSORS 2016
Characterization of 0.18-Pm CMOS MEMS Capacitive Ultrasonic Sensors for Fast Photoacoustic Imaging Yi-Chia Shih and Michael S.-C. Lu Department of Electrical Engineering, National Tsing Hua University, 101 Sec. 2 Kuang-Fu Rd., Hsinchu 30013, Taiwan, R.O.C.
Abstract This work presents a 4 u 4 capacitive ultrasonic sensor array fabricated in a 0.18-Pm CMOS process. Compared to prior CMOS MEMS ultrasonic sensors, the smaller electrode separation in the new platform significantly improves the capacitive sensitivity. The sensor has a measured center frequency at 3.8 MHz with a fractional bandwidth of 74%. The measured sensitivity is 499 mVpp/MPa/V. With the advance in technology, individual readout circuit can be placed directly beneath each sensor to allow unlimited scalability. Feasibility for fast 3D photoacoustic imaging is demonstrated with the sensing array presented in this work. © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: CMOS, Capacitive sensors, Ultrasound, Photoacoustic imaging.
1. Introduction Photoacoustic imaging is a promising noninvasive imaging modality to provide the structural and functional information for a wide spectrum of applications, such as imaging of blood vessels and detection of melanoma and breast cancer. It overcomes the penetration limit of pure optical imaging into scattering biological tissues to provide sub-millimeter ultrasonic resolution for an imaging depth up to centimeters. Non-ionizing laser pulses delivered into biological tissues are absorbed and converted to photoacoustic waves due to transient thermo-elastic expansion, which are representative of optical absorption distribution. Photoacoustic image reconstruction by 2-D scanning has been demonstrated in our prior work [1]; however, the technique requires a long time for data acquisition and post processing. The acoustic lens system [2] has been proposed to provide real-time 3-D photoacoustic imaging. In our proposed scheme as depicted in Fig. 1, a 4f acoustic lens system is used to image the produced photoacoustic pressure that represents the absorption distribution onto an image plane where a CMOS MEMS capacitive sensor array is placed to receive the transmitted pressure. The system provides uniform axial and lateral magnification of the imaged
1877-7058 © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.256
714
Yi-Chia Shih and Michael S.-C. Lu / Procedia Engineering 168 (2016) 713 – 716
pressure distribution. A small displacement in the object plane results in the same amount of displacement for the focused image plane. Due to the low speed of sound, the axial resolution is preserved by the propagation of transmitted acoustic pressure to facilitate 3-D imaging. In our previous work, a 4 u 4 sensor array implemented in a 0.35-Pm CMOS MEMS platform [3] was demonstrated; however, scalability is limited since individual readout cannot be placed underneath each sensor. In the new 0.18-Pm CMOS MEMS platform presented in this work, the readout size and performance can be improved to allow unlimited scalability; in addition, the air cavity between the capacitive electrodes is significantly reduced by using one of the MIM (metal-insulator-metal) capacitor layers as the sacrificial material, leading to increased sensitivity. The design, fabrication and characterization of the CMOS capacitive sensor array will be described in the following sections.
Fig. 1. The acoustic lens system is used to focus the emitted ultrasound to the CMOS capacitive sensors for fast photoacoustic image.
(a)
(b)
Fig. 2. (a) Cross-sectional view of the 0.18-Pm CMOS MEMS capacitive ultrasonic sensor. (b) SEM of the 4 u 4 capacitive sensor array. Each sensing unit as shown by the dash line consists of four membranes with a total capacitance of 540 fF.
2. Sensor Fabrication and Design The capacitive ultrasonic sensor array was fabricated in the TSMC 0.18-Pm one-polysilicon six-metal (1P6M) CMOS process based on the process reported in [1]. Sacrificial wet etch of stacked metal (aluminum) and via (tungsten) layers through the passivation openings was performed for structural release. As shown in Fig. 2(a), the thin metal (thickness = 0.19 Pm) typically used to form MIM capacitors (with metal-5) was removed to form air cavity, leading to enhanced capacitive sensitivity. The release holes were sealed by parylene-C deposition (thickness = 2.4 Pm). As shown in Fig. 2(a), the metal-5 and metal-6 layers were used as the capacitive electrodes. Inner diameter of the circular sensing membrane is 80 Pm, producing a capacitance of 135 fF. Four membranes are connected in parallel to form a single sensing element with a total size of 240 u 240 Pm2. A 4 u 4 sensor array is implemented on the CMOS chip as shown in Fig. 2(b). Upon photoacoustic imaging, the capacitively transduced
Yi-Chia Shih and Michael S.-C. Lu / Procedia Engineering 168 (2016) 713 – 716
signal is amplified, high-pass filtered, followed by peak detection to obtain the ultrasound intensity, as shown by the readout schematic in Fig. 3. A dc bias is applied to the pre-amp to produce the motional current, which is integrated by the feedback capacitance CF (CF = 100 fF). Contrast of the photoacoustic image depends on the accuracy of the peak detection circuit. Thanks to the migration from the 0.35-Pm to the 0.18-Pm CMOS process, signal routing becomes more convenient with more metallization layers and the whole readout can be placed beneath the sensing membranes to provide unlimited scalability.
Fig. 3. Schematic of the capacitive readout. Intensity of the ultrasonic signals is determined by the peak-detection circuit.
1 0.8
a.u.
0.6 0.4 0.2 0 0
2
4 6 Frequency (MHz)
8
10
Fig. 4. Spectrum of the measured capacitive ultrasonic signal. The center frequency is at 3.8 MHz with a fractional bandwidth of 74%.
3. Experimental Results For characterization, the CMOS sensor chip was first wirebonded to a printed circuit board and the bond pads were covered by epoxy to provide electrical isolation while the chip was immersed in water for test. Sinusoidal signals in the ultrasound range (1 to 10 MHz) with amplitudes from 0.1 to 1 V were applied to the peak detection circuit for test. The results showed that the percentage errors were mostly limited with r4% when the input amplitudes were between 0.1 to 0.5 V. As predicted by simulations, the result was better with larger inputs. The errors became much worse when the inputs were less than 100 mV. An ultrasonic transducer (V310-SM, Panametrics-NDT; diameter = 6 mm) was used to characterize capacitive transduction. Sensed capacitive signals were transformed to the corresponding spectra as depicted in Fig. 4, which shows a measured resonant frequency near 3.8 MHz with a fractional bandwidth of 74%. Measured output increases linearly with the bias voltage across the sensing capacitor to provide a sensitivity of 499 mVpp/MPa/V. Fig. 5 shows that the peak value of the pre-amp output produced by ultrasound transmission can be successfully captured by the peak detector. Fig. 5(a) and (b) show the measured results when the capacitor was biased at 8 V and 20 V, respectively. The acoustic pressure produced by using a visible light source is in the MPa range. By
715
716
Yi-Chia Shih and Michael S.-C. Lu / Procedia Engineering 168 (2016) 713 – 716
considering the attenuation of the acoustic lens in the system, the sensing capacitor can be biased with a suitable voltage (like 10 V) to generate enough signals.
2.5
Signal (V)
3
reset
2 1.5
sensed output from pre-amp
1 0.5 0 0
reset peak detector output
peak detector output
Signal (V)
3
2 1
sensed output from pre-amp
0 2
4 Time (Ps) (a)
6
8
0
2
4 Time (Ps)
6
8
(b)
Fig. 5. Peak values (red) of the capacitively sensed signals (blue) are successfully captured by the peak-detection circuit with the dc bias of the capacitive electrodes set at 8 V and 20 V [(a) and (b)], respectively .
4. Discussion and Conclusion This work uses a convenient micromachining process to develop a capacitive sensor array in a 0.18-Pm CMOS process for fast photoacoustic imaging. Monolithic integration effectively reduces the parasitic effect and enhances the signal-to-noise ratio. Compared to prior work developed in the 0.35-Pm CMOS process, scalability of the sensor array is not limited with the improved readout. Amplification and peak detection of the ultrasonic signals are successfully demonstrated by the capacitive readout. Our future work will be focused on the integrated test with the acoustic lens system. Acknowledgements This work is supported by the Ministry of Science and Technology, Taiwan, Republic of China. The authors would like to thank the National Chip Implementation Center for support of chip fabrication and the National Center for High-Performance Computing for support of simulation tools. References [1] M. L. Li, P. H. Wang, P. L. Liao, M. S.-C. Lu, Three dimensional photoacoustic imaging by a CMOS micromachined capacitive ultrasonic sensor, IEEE Elec. Dev. Lett. 32 (2011) 1149–1151. [2] J. J. Niederhauser, M. Jaeger M. Frenz,Real-time three-dimensional optoacoustic imaging using an acoustic lens system, Appl. Phys. Lett. 85 (2004) 846–848. [3] C. A. Kuo, M. S.-C. Lu, Characterization of CMOS MEMS capacitive ultrasonic sensors for fast photoacoustic imaging, Proc. Eurosensors (2014), Brescia, Italy.