Spintronics Integrating magnetic materials with semiconductors
Bulk micromachining Wet Chemical etching: Masking layer Bulk Si Isotropic Bulk Si Anisotropic Surface micromachining http://www.darpa.mil/mto/mems Carving of layers put down sequentially on the substrate by using selective etching of sacrificial thin films to form freestanding/completely released thin-film microstructures HF can etch Silicon oxide but does not affect Silicon
MEMS: Foundry services SAMPLES: Sandia Agile MEMS Prototyping, Layout tools, Education and Services Current process: SUMMIT V) Sandias Ultra-planar Multi-level MEMS Technology) - 5 levels of poly-silicon - $10,000 / design MUMPS: Multi-user MEMS processes Derived from the BSAC processes at U.C. Berkeley 3-levels of poly-Si Process steps for fabricating a MEMS device MUMPS: Multi-user MEMS processes > Commercially operated, a repository of processing, design libraries
> Standard processing steps, can be custom-designed Poly-MUMPS: Three-layer polysilicon process Metal-MUMPS: Ni electroplating process SOI-MUMPS: Silicon-on-Insulator micromachining process The CRONOS process for a micromotor e.g., Synchronous motor Stator Rotor The CRONOS process for a micromotor Poly-silicon (POLY): Structural Material Silicon Oxide/PSG (OXIDE): Sacrificial material Silicon Nitride (NITRIDE): for isolation - 8 photo-masks: 8 levels of processing http:// mems.sandia.gov/
The cross-sections are depicted in MEMS processing Photoresist Photoresist (PR) Photoresist RIE removes POLY0 Photoresist washed away Oxide sacrificial layer deposited by LPCVD PR applied, dimples patterned, and PR washed away (PSG : OXIDE) Oxide patterned and etched, Poly1 deposited
proof of mass motion Metal can Electrostatic projection displays Surface micromach. + XeF2 release Electrostatic torsion of suspended tensile beams Glass bonded Catalytic combustible Surface micromach. gas sensor Resistance change due to heat of reaction
RF switches Cantilever actuation Surface micromach. DNA amplification Bonded etched glass with PCR Pressure driven flow across T-controlled zones Lab on a chip Electrophoresis & electrowetting Bulk & Surface micromachining
Custom mount Glass bonded Microcapillaries Microfluidics & Polymers A project on the frontier application areas of MEMS/NEMS Required: A written report + Presentation The project should address the following issues: (1) What is new or novel about this application? (2) Is there any new physical principle being used (3) Where is this headed? (commercial potential, offshoot into new areas of engineering ) (4) Most importantly, YOUR ideas for improvement. Presentations (15 minutes/team of two) A Piezoresistive Pressure Sensor Piezoresistance: the variation of electrical resistance with strain
- Origin in the deformation of semiconductor energy bands - NOT the same as piezo-electricity Transduction of stress into voltage Application: Manifold-Absolute-Pressure (MAP) sensor: Motorola One of the largest market segments of mechanical MEMS devices Piezoresistivity Piezoresistive effect is described by a fourth-rank tensor E = e [1 + J at small strains Electric field Resistivity tensor (2nd rank) Stress Current density Tensor notation
Stress ij Strain xx xy xz yx yy yz zx zy zz ij xx xy xz yx yy yz zx zy zz
4th rank tensor (81 elements) ij = Cijkl kl From symmetry (no net force in equilibrium) ij = ji 6 independent variables xx yy zz yz zx xy xx yy zz yz zx xy
C15 C25 C35 C45 C55 C56 C16 C26 C36 C46 C56 C66 xx yy zz yz zx xy (6 X 6) matrix, 21 independent elements (as, Cij = Cji) For cubic materials, e.g. single crystal Silicon, there are only 3 independent constants
Piezoresistive effect is described by a fourth-rank tensor E = e [1 + J Electric field Resistivity tensor (2nd rank) Stress Current density x 1, y2, z 3, [11, 22, 33, 23, 31, 12] [1, 2, 3, 4, 5, 6] E1 = [1+ ] J1 + J2J3 e E2 = [1+ ] J2 + J1J3 e E3 = [1+ ] J3 + J1J2 e Piezoresistive coefficients e
e e Measurement of Piezoresistance coefficients Practical Piezoresistance measurements Slide courtesy: M. Wu Longitudinal & transverse piezoresistance R = ll + tt l: longitudinal, t: transverse R Longitudinal & Transverse piezoresistance coefficients Longitudinal direction
l Transverse direction t (001) 11 (110) (110) 1/3 (110) 1/2 () (111) 1/3
(110) 1/2 () (001) (110) 1/2 1/2 Piezoresistive coefficients of Si - decrease as the doping level/temperature increases Type Resistivity Units
6.6 -1.1 138.1 C.S. Smith, Phys. Rev. B, vol. 94, pp.42-59, (1954). Concept of a piezoresistive sensing scheme Max. surface stress Proof Mass Substrate Flexure If piezo-resistor is along : n-type: l: -31.2 10-11 Pa-1, t: -17.6 10-11 Pa-1 p-type: l: 71.8 10-11 Pa-1, t: -66.3 10-11 Pa-1
Longitudinal - easier to align Transverse - more sensitive Principle of measurement Diaphragm Poisson ratio, R1 = ( + = (67.6 10-11) l t l l R1 CROSS-SECTION
R2 = - (61.7 10-11) l R2 R2 TOP VIEW WHEATSTONE BRIDGE R3 R1 R4 V - R1 = R3 = (1+ 1) Ro
R2 = R4 = (1 - ) Ro i = ii R3 R2 + Vo R1 R4 Resistance change due to stress Support Lc: cantilever length x: distance from support
t: thickness Cantilever x Piezoresistors 3 w Cantilever tip displacement (w) for a point load = max 2 Radius of curvature = 1/ = l = E [(t/2 Stress = E Strain d2w dx2 = 3 wmax (Lc - x)
Lc3 R = l l R x Lc 2 1- x 3Lc The Motorola MAP sensor S. Senturia, page 461, Microsystem design http://www.motorola.com/automotive/prod_sensors.html - MAP: Manifold Absolute Pressure
- Sensor measures mass airflow into the engine, to control air-fuel ratio - Uses piezoresistance to measure diaphragm bending with integrated signal-conditioning and calibration circuitry Process flow for MAP sensor -Bipolar (NPN) instead of MOS processing on (100) wafers - uses only one piezo-resistor: Xducer Al metallization n+ - collector n+ - Emitter p-base n+ - buried layer OXIDE n-epi <100> p-Si substrate
p-type piezoresistor Pressure sensor fabrication and packaging Piezoresistor element DIAPHRAGM Glass frit/Anodic bond
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