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Sang-Gook Kim

• Title: Professor
• Department: Department of Mechanical Engineering

Graduate Students

• K. Smyth, Research Assistant, ME
• R. Xu, Research Assistant, ME

Research engineers

• Jeffrey Chou, Postdoctoral fellow, ME
• Phillip Lee, Postdoctoral fellow, ME

Support Staff

• R. Hardin, Senior Administrative Assistant

Publications

F. Sammoura and S.G. Kim, “Theoretical Modeling and Equivalent Electric Circuit of a Bimorph Micromachined Ultrasonic Transducer,” IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, Vol. 59, No. 5, Page 990, 2012

De Volder M., Decoster J., Reynaerts D., Van Hoof C., Kim SG, High Damping Carbon Nanotube Hinged Micromirrors, Small, 2012

S.G. Kim, S. Priya, I. Kanno, “Piezoelectric MEMS for Energy Harvesting,” MRS Bulletin, V. 37, p.1039, 2012

S.G. Kim, S. Bathurst and F. Sammoura, “Design Framework for Micro and Nano-scale Products,” 22^nd CIRP Design Conference, Bangalore, India, 2012 (invited)

B. Commeau, R. Karnik, S.G. Kim, “Development and Growth of an Undergraduate Micro/Nano Engineering Laboratory Course,” ASEE Annual Conference, 2012

F. Sammoura and S.G. Kim, “Modeling of the Neutral Axes of a Circular Piezoelectric Micromachined Transducer in Transmit and Receive Mode,” Solid-State Sensor and Actuator Workshop, Hilton Head, SC, 2012

K. Smyth, F. Sammoura, S.G. Kim, “Modeling and Optimization of Electrode Configuration for Bimorph and Unimorph Piezoelectric Micro-Machined Ultrasonic Transducers (PMUTs), International Workshop on Acoustic Transduction Materials and Devices, PA, 2012

H. Lee, S. Bathurst and S.G. Kim, “Thermal Stability of Nano-Structured Selective Emitters for Thermo Photovoltaic,” TechConnect World, Santa Clara, CA, 2012

F. Sammoura, K. Smyth, S.G. Kim, “Working Equations of a Circular Multimorph Piezoelectric Micromachined Ultrasonic Transducer,” IEEE Industrial Electronics Society (IECON 2012), Montreal, Canada, 2012

F. Sammoura, K. Smyth, S.G. Kim, “An Analytical Analysis of the Sensitivity of Plate Deflection and Bandwidth of Circular Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) to Residual Stress,” IEEE International Ultrasonics Symposium (IUS), Dresden, Germany, 2012

H. Lee, S. Bathurst and S.G. Kim, “Thermally Stable Two-Dimensional Photonic Crystal for Selective Emitters”, MRS Fall Meeting, Boston, 2012

R. Xu and S.G. Kim, “Figures of Merit (FOMS) of Piezoelectric Materials in Energy Harvesters,” POWER MEMS 2012, Atlanta, GA, 2012 (Poster Presentation)

F. Sammoura and S.G. Kim, “Optimizing the Electrode Size of Circular Bimorph Plates with Different Boundary Conditions for Maximum Deflection of Piezoelectric Micromachined Ultrasonic Transducers,” Ultrasonics, v 53, P 328-334, 2013

S. Bathurst and S.G. Kim, “Printing of Uniform PZT Thin Films for MEMS Applications,” CIRP Annals -Manufacturing, Vol. 62, No. 1, 2013

K. Smyth, F. Sammoura, S. Bathurst and S.G. Kim, “Analytic Solution for N-electrode actuated Piezoelectric Disk with Application to Piezoelectric Micromachined Ultrasonic Transducers,” IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, in press

H. Lee, K. Smyth and S.G. Kim, “Nanostructures which can survive the high temperature solar energy applications,” Nanotechnology for Solar Energy Collection and Conversion – NNI Signature Initiative Symposium, Washington D.C., 2013

R. Xu and S.G. Kim, “Wide Bandwidth Piezoelectric MEMS Energy Harvesting,” MRS Spring Meeting, San Francisco, 2013

K. Smyth, F. Sammoura, and S. G. Kim, “An Accurate Equivalent Circuit for the Clamped Circular Multiple-Electrode PMUT with Residual Stress,” 2013 IEEE International Ultrasonics Symposium (IUS), Prague, Czech Republic, (Oral presentation)

H.J. Lee, K. Smyth, S. Bathurst, J. Chou, M. Ghebrebrhan, N. Saka and S.G. Kim, “Hafnia-plugged microcavities for thermal stability of selective emitters”, Applied Physics Letters, accepted for publication

F. Sammoura, K. Smyth and S.G. Kim, “An Equivalent Network Representation of a Clamped Bimorph Piezoelectric Micromachined Ultrasonic Transducer with Circular and Annular Electrodes using Matrix Manipulation Techniques,” IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, accepted for publication

Thermal Ink Jet Printing of CNT films

• Authors: P. Lee, S. G. Kim



Figure 1: The SEM images of carbon nanotubes percolation network with different pitch sizes.

Ink jet printing allows for rapid, scalable, and low-cost patterning process. It does not need a vacuum environment or toxic etching process, which facilitates the integration of ink jet printing into other micro-machining process. And ink jet printing can pattern on curved surface or 3D structure. The characteristics of this technique are advantageous in industry. Previously, we demonstrated highly repeatable and uniform PZT thin film with ink jet printing^[1],^[2]. With this know-how in PZT film printing, we apply carbon nanotubes to ink jet printing. Due to their mechanical, thermal conductive and electrical properties, carbon nanotubes find diverse applications. The percolation network of carbon nanotubes is a transparent and conductive material, and strong bonding among carbon nanotubes enables fabrication of a bendable, foldable, and stretchable electrode. Previous research from other groups fabricated an ink jet-printed carbon nanotubes conductor^[3], but the performance was not good enough to replace the current ITO transparent electrode. In this research, we controlled the amount of carbon nanotube deposition by changing the pitch of the printed line, which determines conductivity and transparency. Electrodes of 30% transparency can be fabricated using 10-um pitch, which has 36.6 S/cm conductivity and 247 ohm/sq sheet resistance. Increasing the pitch size up to 20 um gives the electrode properties of 74% transparency, 30.8 S/cm conductivity and 1.1 Kohm/sq sheet resistance. This result shows better performance compared with previous research. Figure 1 shows the SEM image of fabricated electrodes depending on the pitch size. Printed film shows uniformly deposited carbon nanotubes percolation network within 30- um pitch size. In the future, we plan to produce devices that utilize the full capabilities of this process to achieve better performance in both conductance and transparency, which will enable applications such as touch panels, organic light-emitting diodes, and solar cells.

1. S. P. Bathurst and S. G. Kim, “Designing direct printing process for improved piezoelectric micro-devices,” CIRP Annals – Manufacturing Technology, vol. 58, pp. 193-196, 2009. [↩]
2. S. P. Bathurst, H. W. Lee, and S. G. Kim, “Ink jet printing of PZT thin films for MEMS applications,” Proc. International Conference on Digital Fabrication, 2008, pp. 897-901. [↩]
3. W. R. Small, F. Masdarolomoor, G. G. Wallace, and M. in het Panhuis, “Inkjet deposition and characterization of transparent conducting electroactive polyaniline composite films with a high carbon nanotube loading fraction,” Journal of Materials Chemistry, vol. 17, no. 41, pp. 4359-4361, 2007. [↩]

Thin Film Piezoelectric Micro-machined Ultrasonic Transducer for Medical Imaging

• Authors: K. Smyth, F. Sammoura, S. Bathurst, S. G. Kim
• Sponsorship: MIT/Masdar Institute of Science and Technology Collaborative

Ultrasound is an attractive 3D medical imaging technique because it is relatively inexpensive, portable, compact, and non-invasive. However, for 3D real time imaging to be commercially realizable, scans must be consistent and high resolution and occur at a fast acquisition rate–all factors that are inhibited by the current bulk piezoelectric transducer technology^[1].  Highly manual manufacturing limits the size of current transducers to millimeter length scales and the high acoustic impedance of the bulk piezoelectric limits resolution reducing bandwidth and sensitivity^[2].  At high volume, micro-fabrication is high yield and less expensive, and it would enable element miniaturization for high resolution, small form factor ultrasound probes.

Our group has designed a piezoelectric micro-machined ultrasonic transducer (pMUT) that transmits acoustic signals via high frequency vibrations of a thin diaphragm.  These vibrations are actuated by applying a voltage across a thin film piezoelectric lead zirconate titanate (PZT) film deposited via a sol-gel technique.  For sensing, acoustic waves reflected from an imaging target strain the diaphragm generating a current signal.

Device fabrication begins with growth of thermal oxide on a silicon-on-insulator wafer.  The bottom electrode is then deposited via a lift-off process, and PZT is deposited and patterned with a wet etch.  The lift-off process is then repeated to create the top electrode.  Finally, diaphragms are released and the substrate is divided into chips (Figure 2) with a back-side deep reactive ion etch.  A schematic of the fabricated device is shown in Figure 1.

Through electrode size optimization, our pMUT is designed to maximize deflection, which is ideal for generating the high acoustic pressure necessary to overcome signal attenuation in deep penetration imaging^[3].  In the future, we hope to incorporate the optimized pMUT transducer design in pMUT arrays with a small form factor to enable 3D real time medical imaging.



Figure 1: Schematic of pMUT. Top left: Isometric view of pMUT device with line indicating cut for cross-section diagram. Bottom: Cross-section schematic of completed pMUT.



Figure 2: Fabricated pMUT chip. Columns form 1D device arrays. Inset: Top view microscope image of completed pMUT.




1. B. Khuri-Yakub, O. Oralkan, and M. Kupnik, “Next-Gen Ultrasound,” IEEE Spectrum, vol. 46, no. 5, pp. 44–54, May 2009. [↩]
2. T. L. Szabo, Diagnostic ultrasound imaging: inside out, ser. Biomedical Engineering. Elsevier Academic Press, 2004. [↩]
3. K. Smyth, S. Bathurst, F. Sammoura, and S. G. Kim, “Analytic Solution for N-electrode actuated Piezoelectric Disk with Application to Piezoelectric Micromachined Ultrasonic Transducers,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, to be published. [↩]

Wide-bandwidth, Low-frequency, Low-g Piezoelectric MEMS Energy Harvesters

• Authors: R. Xu, S.-G. Kim
• Sponsorship: MIT/International Iberian Nanotechnology Laboratory Collaboration Fund

Our group proposes the piezoelectric energy harvester based on a nonlinear resonator to address the narrow bandwidth issue of conventional energy harvesters. The wide bandwidth and significantly higher power than previously reported devices have been proved by our experiments (Fig. 1), which showed a ~20% bandwidth and a 2W/cm³ power density. Increasing number of researchers have been exploring different nonlinear resonance based designs, including beam stiffening, asymmetry structure, buckled structure, and magnetic restoring force, to name a few. The Duffing mode resonance has been observed in diverse designs, and the wide bandwidth is their shared property. However, the theoretical proof of a wide bandwidth and high-power density energy harvester is not available. By building the electromechanical model for the nonlinear system, we have found the analytical solution of the output power of a nonlinear resonance based piezoelectric energy harvester and have proved a much wider power bandwidth can be obtained by varying electrical load, and the maximum power can be achieved by matching the electrical damping to mechanical damping”. By understanding the advantages of the nonlinear design, we are making new designs to bring it closer to real applications. The two gap between the nonlinear piezoelectric energy harvesters at MEMS scale now are, the operating frequency (>1000Hz) is usually much higher than the frequencies of ambient vibrations (~100Hz), and the excitation level when testing in laboratory (4~5g) is also higher than the common vibration’s amplitude (<0.5g). By optimizing the beam composition, tuning the proof mass and redesigning the beam geometry, we anticipate that our new design will work in the frequency range between 100Hz to 200Hz at the excitation of 0.5g. The new design will be fabricated and tested soon.



Figure 1 Fabricated nonlinear resonance based energy harvester.