CMU MEMS Laboratory Publication Abstract


in M.S. Thesis, August 2003, Carnegie Mellon University, Pittsburgh, PA.
A Force-Detection NMR Sensor in CMOS-MEMS
K. M. Frederick
Nuclear Magnetic Resonance (NMR) on micro-liter samples using the Force Detection NMR (FDNMR) method will enable mobile and embedded detection of many elements without a multi-tesla superconducting magnet. This report presents a FDNMR sensor which has the ability to detect hydrogen of a 0.52 µL water sample in a 1 tesla magnetic field. The sensor is a micromachined cantilevered paddle within a CMOS chip measuring 2.5 mm x 2.5 mm x 540 µm with integrated amplification electronics. To begin the paddle fabrication, a 30 µm thick silicon membrane with a thin layer of CMOS interconnect on top, is made by Deep Reactive Ion Etching (DRIE) the backside of the chip. The backside area is patterned with photoresist and etched into many close-proximity, slightly undercut high-aspect-ratio trenches to achieve a uniform membrane thickness. The resulting membrane measures 1370 µm by 1850 µm, with center to edge thickness variations less than 5 µm. A 0.6 mm x 0.6 mm x 0.25 mm piece of pure nickel is glued to the etched membrane surface as a detector magnet. Freon plasma etches trenches into the silicon dioxide interconnect layer not masked by aluminum, and a final DRIE extends the trenches through the silicon membrane. The final etch releases the folded-mass cantilever with 1 mm long spring beams and a 1.1 mm2 paddle surrounded by a capacitive bridge sensor to measure vertical displacement. Integrated electronics drive the input and amplify the output of the balanced capacitive bridge which is made of lateral air gap capacitors between interdigitated micromechanical fingers. Force induced vertical cantilever displacement imbalances the bridge and the amplified output signal is measured by external test equipment. The bridge has the dynamic range to measure up to ± 0.5 µm displacement at the maximum sensitivity of 5.12 mV/nm and up to ± 2.0 µm with less sensitivity. The FDNMR sensor is tested in response to electrostatic, mechanical, acceleration, and magnetic forces. The cantilevered paddle at 760 Torr resonates at 3.6 kHz (Q = 20) compared to 3.0 kHz (Q = 430) by simulation. The bridge output amplitude is linear with oscillating magnetic force amplitude. Estimated noise bandwidth is extracted from time-domain averaging of experimental data. Assuming a 1 MHz system bandwidth, 512 measurements equal a 1.95 kHz noise bandwidth and a 2.4 Å displacement noise floor with an SNR of 1. This displacement corresponds to a 4.4 pN oscillating force at 2.5 mTorr. A 100 Hz noise bandwidth will detect the 1.0 pN FDNMR force from a 0.52 µL water sample in a 1 tesla magnetic field.
© 2003 Carnegie Mellon University, Department of Electrical and Computer Engineering.
Full paper (PDF) (opens in new window).

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