My Research Activities

For additional information, publications, etc., see my project research page on the MSAL website


My research activities are currently focused on creating cantilever microsystems that can detect various gasses, vapors, aerosols, or any other analyte of interest. The primary mechanism that is employed to measure these changes is a resonant frequency shift due to a increase of mass on a resonant cantilever. Some of the most sensitive readout schemes to measure these cantilevers employ free-space optics which are used to measure the deflection of cantilevers. This generally means the use of an external laser which needs to be focused onto the cantilever, and then reflected to a position sensitive photodetector. For cantilevers in the 10's of microns in size, this can be quite a challenge; however, this is a well established and very sensitive method to measure the resonant frequency changes of a cantilever.

The approach that my project is using is similar in that it involves a resonant cantilever. The main catch is that our design utilizes this optical readout scheme, but without the external free-space optical equipment that is usually present. Our system uses guided optics in the form of optical fiber and optical waveguides in Indium Phosphide (a direct bandgap semiconductor). Our waveguides form the core of the system, where the waveguide itself is utilized as the cantilever MEMS structure. This waveguide is doped slightly to allows the waveguide to be actuated electrostatically. The optical overlap between the cantilever waveguide and the output waveguide is very sensitive to this displacement and thus works well to detect motion. Using a detector off chip, the optical power is measured and a signal is processed.

The benefits of this approach are twofold: It allows for sensitive displacement measurement without external free-space optics, and it presents a monolithic fabrication scheme in III-V direct bandgap semiconductors. Why do we care about direct bandgap semiconductors? Because these materials can be grown in a controlled manner which allow for the generation, guiding, and detection of light all simultaneously within one material system. This means that the external equipment needed for the system such as lasers and photodetectors can be made on the same substrate and fabricated simultaneously on one chip. This ability makes the devices attractive to batch fabrication that industry normally requires for a device to actually be profitable.

Current Work

My current research is centered on both the integration of a laser into my waveguide cantilever detector to reduce the need for off-chip optical sources. This will be the first demonstration of such full optical component integration using this particular InP cantilever platform. My current challenges are involving the molecular beam epitaxy of a relatively complex layer structure including a laser waveguide and protodetector. Stay tuned for my future updates with exciting results in these areas.

Publications List:


N. P. Siwak, "Indium Phosphide MEMS Cantilever Waveguides for Chemical Sensing with Integrated Optical Readout," Master's Thesis, University of Maryland, College Park, MD, October 2007.

Journal Papers:

Nathan Siwak, Xiao Zhu Fan, Dan Hines, S. Kanakaraju, Neil Goldsman, and Reza Ghodssi, "Indium Phosphide MEMS Cantilever Resonator Sensors Utilizing a Pentacene Absorption Layer," Journal of Microelectromechanical Systems, vol. 18, no. 1, pp. 103-110, February 2009.

M. W. Pruessner, N. Siwak, K. Amarnath, S. Kanakaraju, W.-H. Chuang and R. Ghodssi, "End-coupled optical waveguide MEMS devices in the indium phosphide material system," Journal of Micromechanics and Microengineering (JMM), vol. 16, pp. 832-842, April 2006.

Conference Papers:

D. Hines, E. Williams, N. Siwak, and R. Ghodssi, "The Application of Nanoimprint Lithography and Transfer Printing in the Fabrication of Electronic and Mechanical Systems onto Flexible Substrates," 2009 Nanoelectronic Devices for Defense & Security (NANO-DDS), Fort Lauderdale, FL, September 28-October 2, 2009.

X. Z. Fan, N. Siwak, S. Kanakaraju, C. Richardson, and R. Ghodssi, "A Chemical Sensing Microsystem Utilizing an Adaptive Feedback Circuit," The 15th International Conference on Solid-State Sensors, Actuators, and Microsystems (Transducers '09), Denver, CO, June 21 - 25, 2009.

N. Siwak, X. Z. Fan, and R. Ghodssi, "Towards an Integrated Chemical Sensor Microsystem Utilizing Indium Phosphide Cantilevers and a Novel Feedback Circuit," EUROPTROBE IX, Dublin, Ireland, March 30 - April 2, 2008.

X. Z. Fan, N. Siwak, and R. Ghodssi, "Towards a Smart Adaptive Feedback Circuit for Microsensors," International Semiconductor Device Research Symposium (ISRS 2007), College Park, MD, December 11 - 14, 2007.

N. Siwak, X. Z. Fan, and R. Ghodssi, "Indium Phosphide Resonant Chemical Sensor with a Monolithically Integrated Optical Readout Scheme," Proceedings of The 6th Annual IEEE Conference on Sensors (IEEE Sensors 2007), Atlanta, Georgia, USA, October 28 - 31, 2007.

X. Z. Fan, N. Siwak, and R. Ghodssi, "An Adaptive Feedback Control Circuit for Resonator Sensors," American Vacuum Society 54th International Symposium, Seattle, WA, October 14 - 19, 2007. P. Dykstra, X. Z. Fan, M. Mischiati, L. Mosher, N. Siwak, and R. Ghodssi, "Bi-Directional Transport of Ultra-Low Volume Droplets Using Capacitive Sensing," American Vacuum Society 54th International Symposium, Seattle, WA, October 14 - 19, 2007.

N. Siwak, X. Z. Fan, D. Hines, E. Williams, N. Goldsman, and R. Ghodssi, "Chemical Sensor Utilizing Indium Phosphide Cantilevers and Pentacene as a Functionalization Layer," Proceedings of the 20th IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2007), pp. 489-492, Kobe, Japan, January 21 - 25, 2007.

N. Siwak, J. McGee, and R. Ghodssi, "Indium Phosphide Optical MEMS for Chemical and Biological Sensing," Proceedings of the IEEE/NLM Life Science Systems and Applications Workshop, pp. 86-87, Bethesda, MD, July 13-14, 2006.

N. Siwak, J. McGee, and R. Ghodssi, "InP Optical MEMS for Integrated Sensing and Photonics Applications," 2006 Solid-State Sensor, Actuator and Microsystems Workshop (Hilton Head 2006), Open Poster Session, Hilton Head, SC, June 4-8, 2006.

N. Siwak, M. W. Pruessner, J. McGee, and R. Ghodssi, "Indium Phosphide MEMS for Integrated Bio-Sensing," International Semiconductor Device Research Symposium 2005 (ISDRS 2005), Bethesda, MD, December 7-9, 2005.

J. McGee, N. Siwak, B. Morgan, and R. Ghodssi, "In-Plane Indium Phosphide Tunable Optical Filter Using Ridge Waveguides," International Semiconductor Device Research Symposium 2005 (ISDRS 2005), Bethesda, MD, December 7-9, 2005.

J. McGee, N. Siwak, and R. Ghodssi, "Monolithic In-Plane Tunable Optical Filter," American Vacuum Society 52nd International Symposium, Boston, MA, October 30 - November 4, 2005.