Michelle D. Chabot

Novel fabrication of micromechanical oscillators with nanoscale sensitivity at room temperature

In this paper, we report on the design, fabrication, and implementation of ultrasensitive micromechanical oscillators. Our ultrathin single-crystal silicon cantilevers with integrated magnetic structures are the first of their kind: They are fabricated using a novel high-yield process in which magnetic film patterning and deposition are combined with cantilever fabrication. These novel devices have been developed for use as cantilever magnetometers and as force sensors in nuclear magnetic resonance force microscopy (MRFM). These two applications have achieved nanometer-scale resolution using the cantilevers described in this work. Current magnetic moment sensitivity achieved for the devices, when used as magnetometers, is 10/sup -15/ J/T at room temperature, which is more than a 1000-fold improvement in sensitivity, compared to conventional magnetometers. Current room temperature force sensitivity of MRFM cantilevers is /spl sim/10/sup -16/ N//spl radic/Hz, which is comparable to the room temperature sensitivities of similar devices of its type. Finite element modeling was used to improve design parameters, ensure that the devices meet experimental demands, and correlate mode shape with observed results. The photolithographic fabrication process was optimized, yielding an average of /spl sim/85% and alignment better than 1 /spl mu/m. Postfabrication-focused ion-beam milling was used to further pattern the integrated magnetic structures when nanometer scale dimensions were required.

A student’s guide to searching the literature using online databases

A method is described to empower students to efficiently perform general and specific literature searches using online resources. The method was tested on undergraduate and graduate students with varying backgrounds in scientific literature. Students involved in this study showed marked improvement in their awareness of how and where to find accurate scientific information.

Microfabrication of single-crystal silicon multiple torsional oscillators

Micro-oscillators of different designs and dimensions have been fabricated for use in a nuclear magnetic resonance force microscope. The various designs include double and triple torsional oscillators which have high Qs at room temperature (approximately equals 10,000) when operating at the upper cantilever and upper torsional resonances. Depending on design and dimensions, the resonance frequencies vary from tens to hundreds of kHz. Typical dimensions of the designs are (200 X 150) micrometers 2 X 200 nm thick. To fabricate these devices, microelectric fabrication techniques were employed. Si (100) wafers were patterned, etched, and boron-implanted at a dose of 4.2 X 1016 cm-2 and an energy of 134 keV. A post-implant anneal was then performed at 1000 degree(s)C, followed by a KOH wet-etch which leaves the free-standing boron-doped oscillators. Depending on the doping level, anneal, and etch parameters, the thickness of the oscillators varies from 100 - 400 nm. In order to optimize the design and fabrication process, resonance frequencies and Qs have been characterized using fiber-optic interferometry. For example, the upper cantilever resonance of one design has been found to have a minimum detectable force of 1.5 X 10-16 N/(root)Hz at room temperature.

Microcantilever Torque Magnetometery Study of Patterned Magnetic Films

In this work, we develop a new process for preparing patterned magnetic film on cantilever and show a primarily result of magnetic interactions in a paired magnetic bar measured by MTM. The process of patterning the magnetic film on the cantilever is following: (a) deposit a multilayer Au (200nm)/Cr (10nm) on cantilever, (b) patterning using a focused ion beam (FIB) milling, (c) magnetic film deposition through a mask, and (d) a lift-off process.

Single-crystal silicon triple-torsional micro-oscillators for use in magnetic resonance force microscopy

Single-crystal silicon triple-torsional micro-oscillators have been fabricated, characterized, and modeled primarily for use in a magnetic resonance force microscope. These structures exploit a high-Q triple-torsional mode of oscillation while providing added stability. Fabrication involves lithography, reactive ion etch, and a final KOH wet-etch, with the final oscillator material being single-crystal boron-doped silicon. Typical oscillators were 250 nm thick and 10 - 200 microns in lateral dimensions. Finite element modeling provided the sequence and structure of the ten lowest-frequency modes and indicated that the upper torsional mode best isolates the motion from losses to the base. The oscillators were excited piezoelectrically and the resulting frequency-dependent motion was detected with fiber-optic interferometry, with a 0.002 nm/Hz1/2 resolution. Phase-sensitive motion detection at various points on the oscillator facilitated the assignment of the principle modes. Magnetic excitation was also investigated in order to best excite the torsional resonances. Cobalt micromagnets with moments below 10-12 J/T were electron-beam deposited onto oscillators, and the magnetic forces were measured. MRFM, the primary intended application of these novel structures, is discussed; in particular, an overview is given of an experiment which uses a double-torsional micro-oscillator for the force detection of nuclear magnetic resonance. All topics discussed in this work are being combined in order to achieve a NMRFM single-sweep sensitivity as low as 10-16 N/Hz1/2 at room temperature.

Oscillator microfabrication, micromagnets, and magnetic resonance force microscopy

We report our advances in nuclear magnetic resonance force microscopy (NMRFM) in three areas: 1) MEMS microfabrication studies of single-crystal-silicon mechanical oscillators using double-sided processing; 2) micromagnetometry, anisotropy, and dissipation studies of individual permalloy micromagnets on oscillators; and 3) mechanical-oscillator detection of NMR in the magnet-on-oscillator scanning mode. In the first area, we report details of our back-etch microfabrication process, and characterize oscillator resonant frequency, quality factor, and spring constant by measuring the noise spectral density of oscillator motion. In the second studies, we report changes in the resonant frequency and quality factor for each of four modes of our oscillators for two shapes and sizes of permalloy thin-film (~30 and 180~nm) micromagnets; a simple, quantitative model is used to describe both low-field softening and high-field stiffening. Finally, we report scanning-mode NMR force detection of an ammonium-sulfate single-crystal interface and a polymethyl-methyl-acrylate thin film at room temperature. These latter studies use 2-μm-radius permalloy magnets on silicon oscillators to image the NMR response from resonant volumes as small as 3 μm3. These NMRFM studies are the first reported that attain sub-micron resonant-slice resolution at room temperature.