Collaborators

• H. Asada, MIT, ME
• G. Barbastathis, MIT, ME
• V. Bulovic, MIT, EECS
• A. Chandrakasan, MIT, EECS
• G. Chen, MIT, ME
• H. Lee, MIT, EECS
• G. Rutledge, MIT, ChE
• M. Schmidt, MIT, EECS
• C. Sodini, MIT, EECS
• F. Sammoura, MASDAR

Graduate Students

• S. Bathurst, MechE
• A. Hajati, EECS
• H. Lee, ME
• S. Wang, EECS
• A. Charpentier, Ecole de Mines

Support Staff

• R. Hardin, Administrative Assistant

Publications

H. J. In, H. W. Lee, S-G Kim, G. Barbastathis, “Nanomanufacturing of Carbon Nanotubes on Titanium Nitride,” Int. J. of Nanomanufacturing, Vol. 6, No. 1-4, Page 46-54, 2010

Schlipf M., Stephen B., Kippenbrock K., Kim S.-G., Lanza G.,“On the structured Approach to Design and Integrate MEMS and Precision Engineering Systems” Journal of Manufacturing Science and Technology, v.3, p.236, 2010

J. Peck, S-G. Kim, “Improving Patient Flow through Axiomatic Design of Hospital Emergency Departments,” Journal of Manufacturing Science and Technology, accepted, 2010

J. Peck, D. Nightingale, S-G. Kim, “Axiomatic approach for efficient healthcare system design and optimization,” CIRP Annals -Manufacturing Technology, V.59, 2010

A. Hajati and S.G. Kim, “Design and Fabrication of a Nonlinear Resonator for Ultra Wide Bandwidth Energy Harvesting Applications,” IEEE MEMS 2011, Cancun, Mexico, 2011

A. Hajati and S.G. Kim, “Nonlinear Resonator for Ultra Wide Bandwidth Energy Harvesting,” Mater. Res. Soc. Symp. Proc., San Francisco, 2011 (invited)

To overcome the limitations of piezoelectric energy harvesters such as narrow bandwidth and low power density, our group has recently demonstrated a broadband harvester, which is based on amplitude-stiffened Duffing mode resonance. This nonlinear resonance greatly increases the bandwidth by keeping the harvester resonant until jumping down to a low energy state. Furthermore, the stretching strain of the nonlinear beam produces much higher maximum extractable electrical energy than that of a linear bending-based harvester. This design has been fabricated into a compact MEMS device, which is about the size of a US quarter coin.  The test results show more than one order of magnitude improvements in both bandwidth (~20% of the peak frequency) and power density (up to 2W/cm³) in comparison to the devices previously reported. To make the energy harvester better scavenge energy from ambient vibrations, which typically have low frequency spectra and low-g excitation, we are exploring new designs based on the nonlinear resonance. We have found that it is possible to bring the working frequency down to the range of 100 Hz to several hundred Hz, and lower the excitation level to ~0.5 g, by tuning the design parameters such as the dimensions of the resonator and external proof mass. The new low frequency, low-g designs will be implemented and tested soon. We anticipate that the broadband, low frequency, low-g piezoelectric energy harvesters will be used to power a wide range of devices including portable electronic devices and self-powered wireless sensors.

In this project, a piezoelectric 2-D array of ultrasound transducers will be developed for compact, portable 3-D ultrasound imaging systems. Piezoelectric materials have been used for macro-scale ultrasound systems due to their high polarization density. However, making tiny 2-D array of transducers with conventional piezoelectric materials (all ceramic or polymeric composite) has been extremely difficult. Dicing and bonding of crystallized piezoelectric ceramic bulk and subsequent delicate assembly operations require a lot of manual effort, which limits production yield, rate, and quality. In addition, piezo-ceramics inherently have high acoustic impedance, which is difficult to match in liquid or air medium. Capacitive Micromachined Ultrasonic Transducers (CMUTs) have been developed to leverage the MEMS fabrication techniques for small form factor transducer fabrication and to mitigate the acoustic impedance mismatch ^[1] . A CMUT consists of metallized silicon nitride membranes suspended above highly doped silicon bulk. These membranes vibrate when an electrostatic charge is generated under each membrane. Each membrane can also detect the reflected sound wave by measuring the capacitance change at the gap under each membrane. CMUTs offer greater bandwidth than piezoelectrics and are tunable ^[2] . Moreover, many of the available MEMS processing technologies could be used to make micro-scale arrays of CMUT elements effectively. However, CMUTs still have some technical issues such as high voltage requirement, which makes them not suitable for in vivo operations, result in insulator breakdown, and cause static charge accumulation at the membrane surface.

This research project will focus on developing PZT micromachined ultrasound transducers (PMUTs) and designing novel 2-D array PMUTs with a reliable PZT process technique of PZT. The initial goal of this project is to study the PZT structure appropriate for a 64×64 array and actuation voltage less than 10 volts. A prototype PZT structure will be fabricated and characterized to demonstrate the feasibility of the technology. In addition, the low voltage limits, potential efficiency, and sensitivity will be determined and optimized. Fabricating an array of PZT pillars with size less than 50 mm is one of the major challenges of this project. A new and flexible on-demand deposition process for high quality PZT thin films developed by Bathurst et al. will be used to solve this problem ^[3] .

In addition to the advanced medical applications, the core technology developed in this project will be applied to further improve the ultra-wide bandwidth of energy harvesters. This will lead energy harvesters to be deployable in real world applications including sensors for energy efficient buildings, structural monitoring devices of crude oil pipelines, and leak detectors in water supply networks.

1. I. Landebaum et al., “Surface micromachined capacitive ultrasonic transducers,” IEEE Trans. Ultrasonics Ferroelectrics and Frequency Control, vol. 45, no. 3, pp. 678-690, 1998.
2. B. Khuri-Yakub, “Next-generation ultrasound,” IEEE Spectrum, p. 45, May 2009.
3. S. Bathurst, J. Jeon, H. W. Lee, and S. G. Kim, ”PZT MEMS by thermal ink jet printing,” presented at Solid-State Sensor and Actuator Workshop, Hilton Head, SC, 2008.

We recently demonstrated a process of thermal ink jet printing of PZT thin films for MEMS applications ^{[1] [2]} .  Previous methods for deposition of solution-based PZT were painstaking and low-yield; they also imposed significant processing and design constraints. Thermal ink jet printing allows for rapid, low-cost deposition of patterned PZT films over a wide range of geometries and provides for greater flexibility in process sequencing. With this technique, PZT may be easily integrated into devices with large out-of-plane features after the micro-machining process, which enables the formation of more complex device structures. In 2010-2011 the printing process was modeled in detail, including the dynamics of droplet formation as well as the internal flows that occurring during film drying. Models proposed by others were extended to include printed PZT films ^[3] .  Experiments were carried out to confirm the modeling. Specifically, high-speed camera images (Figure 1) were taken to visualize the droplet formation, and the effects of surface tension and temperature were investigated through droplet drying tests. As a result the conditions required for highly repeatable and uniform printed films were determined. Further development work focused on the integration of printed PZT into a range of micro-machined structures including cantilevers and bridges with energy harvester applications as well as resonators for ultrasonic transduction (Figure 2). These devices provide a proof of concept for a fully integrated PZT device fabrication process. In the future we plan to produce devices that utilize the full capabilities of this process to reach energy densities and acoustic coupling greater than those of devices based on current deposition techniques.

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. H. Hu and R. G. Larson, “Marangoni effect reverses coffee-ring depositions,” The Journal of Physical Chemistry B, vol. 110, , pp. 7090-7094, Apr. 2006.