Lecture 31

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EEE 352: Lecture 31 http://www.fulton.asu.edu/~ferry/EEE 352.htm Dielectric Materials Polarizations – electronic and molecular Dielectric function and Debye equation Piezoelectric effect Scanning tunneling microscope Transducers Microelectromechanical systems Peter Debye Nobel prize in chemistry, 1936 “for his contributions to our knowledge of molecular structure through his investigations on dipole moments and on the diffraction of X-rays and electrons in gases”

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Dielectric Effects Metal plates Dielectric What makes e different from e0? POLARIZATION In electrostatics, the CONSTITUITIVE RELATION is Polarization Susceptibility

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Dielectric Effects POLARIZATION arises from charge shifts in the material—there is a macroscopic separation of positive charge (e.g., the ions) and negative charge (e.g., the BONDING ELECTRONS). Induced DIPOLE MOMENT POLARIZATION is then There are many sources of dipoles. Amount of charge shift

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Dielectric Effects ln(w) 10-15 eV 30-50 meV visible infrared Major source of POLARIZATION is distortion of the bonding electrons around atoms. This leads to the normal semiconductor dielectric constant. In POLAR materials, like GaAs and SiC, the different charge on the A and B atoms can be polarized as well, leading to a difference between the optical and the static dielectric constants. In Appendix C, the two values for GaAs are reversed!

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Molecular Polarization—Debye Equation Quite generally, we can write We assert that the polarization decays as Since the polarization must be related to f(w), we say

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Molecular Polarization—Debye Equation Longitudinal vibration frequency of the A-B molecule—the POLAR OPTICAL PHONON frequency

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Piezoelectric Effect In materials with NO REFLECTION SYMMETRY (like GaAs or many molecular species) the applied electric field produces a DISTORTION OF THE LATTICE (size change) and vice versa. FORCE ELECTRIC FIELD A common piezoelectric is Poly-Vinylidene Flouride, which is used in a variety of stereo headsets. The most common is crystalline quartz used as frequency control crystals—pressure applied to the quartz has a resonance which can be used in a feedback loop to create a highly-stable oscillator—the quartz crystal oscillator.

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Scanning Tunneling Microscope Piezoelectric transducers are used to give displacement in each of the 3 directions Typical size of the active parts http://www.zurich.ibm.com/st/nanoscience/#

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Scanning Tunneling Microscope Raster scanning of the tunneling head is used to generate a 3D map of the surface. Then the power of the computer is used to create the impressive graphics displays that result. Gerd Binnig Nobel prize in physics, 1986 Heinrich Rohrer Nobel prize in physics, 1986 “for their design of the scanning tunneling microscope”

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Scanning Tunneling Microscope Cu (111) surface steps Cs atoms on GaAs (110) surface Fe atoms on a Cu (111) surface

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Oscillating electric field Piezoelectric material Piezoelectric Effect surface acoustic wave device Surface acoustic wave Propagation of the surface wave can be used by a second TRANSDUCER to create a signal processing chip.

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Piezoelectric Effect surface acoustic wave device RF Resonator

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Piezoelectric Effect surface acoustic wave device Wafer with sets of SAW devices Tire pressure sensor using a SAW device--changes in pressure move the resonant frequency of the resonator

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Micro-Electro-Mechanical Systems (MEMs) * Micromachining—lithography and etching * Piezoelectric Cantilevers and sensors * Resonators * Other neat things MEMs accelerometer

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Microfabrication Photolithography, deposition, and etching are the standard tools of the semiconductor VLSI industry. They can also be used to create novel micro-electro-mechanical systems as well. To a large extent, these system used the microfabrication and piezoelectric properties of materials to achieve new functionality. semiconductor substrate New material grown/deposited upon the substrate Processing begins with the growth of an “active” layer on the substrate. There may well be other “active” layers within the substrate (as will be seen below). However, this active layer is one of the key layers.

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Microfabrication Photoresist is deposited and exposed by an appropriate light source. The exposed area is then etched by an appropriate liquid or gaseous etchant.

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A New Approach to Lithography Nanoimprint Lithography http://www.mems-exchange.org/services/embossing

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Microfabrication A common approach is to first PREFERENTIALLY etch the TOP layer, then PREFERENTIALLY etch the second layer to a prescribed depth. Etching can be stopped by timing the etch or by the use of an ETCH STOP LAYER. Silicon nitride layer on silicon (the bending arises from the stress in the deposited layer—once the underlying layer is removed, the stress induces the bending. Small gears etched in the top layer.

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MEMs Transducers Transducers work by translating force or acceleration into an electrical signal—such as a voltage—through the piezoelectric effect. The field can be detected by various transistors. Force sensors make good accelerometers for use in inertial navigation systems or for e.g. triggering an air bag.

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MEMs Transducers The system can also be used to create a mechanical resonator for use in an electrical system. Prof. Navid Yazdi worked on this system as part of his doctoral thesis at Michigan.

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MEMs Actuators The piezoelectric effect can be used to reverse the transducer property to make an ACTUATOR—an electrical signal creates a displacement. An electrostatic relay.

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Making Radio Frequency Circuits One of the major problems in analog microelectronics is the need for inductors. Semiconductor devices make good resistors and capacitors, but NOT inductors. MEMs can solve this problem.

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System on a Chip

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System on a Chip Microelectronics and MEMs can be used to create fluidic systems as well. This approach is being pursued today to make integrated chemical analysis systems, to analyze blood automatically, to make sensors and a variety of other applications. http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=011269yvHrz8vY1818#bulk http://touch.caltech.edu/research/research.htm http://www-bsac.eecs.berkeley.edu/groups/bmad/mems_reps/home.htm http://mmadou.eng.uci.edu/ http://www.zurich.ibm.com/st/mems/