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From micro to nano thermal effects in micromachined devices
8-Jun-2007 14:08 14:08
Age: 13 yrs


PR. JOSEPH J. TALGHADER



From micro to nano thermal effects in micromachined devices


Séminaire Scientifique

Joseph J. Talghader

University of Minnesota, email: joey@umn.edu

VENDREDI 08 JUIN 2007, 15h 00, Amphi 160 à l’ESIEE

 

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From Micro to Nano: Thermal Effects in Micromachined Devices


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Résumé:


Moving from the macro- to the micro- and nanoscale causes a great shift in the relative importance of various types of heat transfer. In our everyday experience, we are used to fluids of different temperatures moving past one another with ease, as can be attested by anyone who has ever watched a pot of boiling water. However, as size decreases, the thermal “pockets” that form to carry away a heated mass actually become larger than the volume of the structure in question. When this occurs, convection essentially becomes negligible and heat transfer is dominated by conduction. Another aspect of heat transfer that changes on small scales is interface thermal contact conductance (TCC). For interfaces with nanometer smoothness, trapped fluid tends to force asperities apart and reduces the thermal contact conductance.

Micromachined structures are very susceptible to thermal deformation because they are fabricated from films that are very thin. Any thermal expansion mismatch between free layers will cause shape changes, buckling, unwanted contact, or other problems. Multilayer thermal design can correct this but is made much simpler if one has thin films with a variety of thermal expansion coefficients. We recently have discovered that a family of oxides that have a negative thermal expansion in bulk form can be induced to take on negative thermal expansion in thin film form. Given the potential technical impact of thin film negative expansion materials, this is an area ripe for future investigation.

Unlike many devices where thermal effects are undesired, uncooled infrared detectors are designed to have extremely high thermal isolation so they are maximally sensitive to the heat generated by absorbed photons. Active thermal control of the microsystem can eliminate this problem. If a small portion of the microbolometer support is brought into contact with the substrate, then the thermal conductance can be increased so that the pixel does not heat excessively. The voltage applied to the support controls the TCC at the interface through the contact pressure and area. Using electrostatic actuation, one can also change the radiation absorption of a thermal detector. In reference, an optical cavity is designed into a microbolometer pixel such that light on resonance is coupled into the detector plate. By adjusting the gap spacing, the spectral sensitivity can be tuned across most of the LWIR, while retaining a wide-band detection mode.

Thermal and thermomechanical noise is often negligible even in microsystems. Exceptions to this are usually sensor elements, such as accelerometers, high finesse optical cavities, and thermal detectors, where the devices are actively designed to minimize other noise sources. As size is reduced from micro- to nano-, however, thermal noise becomes important for almost all devices. For example, the rms thermal fluctuations of an isolated polysilicon cube, 10nm on a side, will be on the order of 1K. With such large fluctuations, it becomes difficult to even use the traditional concept of temperature. This effect and others like it place strict limits on device performance that often have not been considered in the rush to nanotechnology.


 

 

Joseph Talghader obtained his B.S. in electrical engineering from Rice University in 1988. He was awarded an NSF Graduate Fellowship and attended the University of California at Berkeley where he received his M.S. in 1993 and Ph.D. in 1995. From 1992 to 1993 he worked at Texas Instruments as a Process Development Engineer. After graduating from Berkeley in 1995, he joined Waferscale Integration. In 1997 Dr. Talghader joined the faculty at the University of Minnesota, where he is now an Associate Professor. His group is particularly active in the areas of: uncooled infrared detectors, quantum-coupled nanostructures, negative thermal expansion and optical coating materials, and micro and nano- sensors for extreme environments. Dr. Talghader has received 3M Nontenured Faculty Awards on three occasions. He has published many journal and conference papers and patents. He has served on various program committees and reviews, including service as Program chair of the 2003 IEEE/LEOS Optical MEMS Conference and as Guest Editor of the IEEE Journal of Selected Topics in Quantum Electronics 2004 Special Issue on Optical Microsystems, Conference Chair of the 2006 IEEE/LEOS Optical MEMS and Nanophotonics Conference and Guest Editor of the IEEE Journal of Selected Topics in Quantum Electronics Special Issue on Optical Micro- and Nano- Systems.

 

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