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There is a great need in experimental biology for tools to study interactions between biological molecules and to profile expressure levels of large numbers of proteins. This paper describes the fabrication, packaging and testing of a resonant mass sensor for the detection of biomolecules in a microfluidic format. The transducer employs a suspended microchannel as the resonating element, thereby avoiding the problems of damping and viscous drag that normally degrade the sensitivity of resonant sensors in liquid. Our device differs from a vibrating tube densitometer in that the channel is very thin, which enables the detection of molecules that bind to the channel walls; this provides a path to specificity via molecular recognition by immobilized receptors. The fabrication is based on a sacrificial polysilicon process with low-stress low-pressure chemical-vapor deposited (LPCVD) silicon nitride as the structural material, and the resonator is vacuum packaged on the wafer scale using glass frit bonding. Packaged resonators exhibit a sensitivity of 0.8 ppm/(ng * [cm.sup.2]) and a mechanical quality factor of up to 700. To the best of our knowledge, this quality factor is among the highest so far reported for resonant sensors with comparable surface mass sensitivity in liquid. Index Terms--Biomedical transducers, chemical transducers, density measurement, Q factor, mass sensor, microbalance, vacuum packaging.

We have micromachined a mechanical sensor that uses interferometry to detect the differential and absolute deflections of two adjacent cantilevers. The overall geometry of the device allows simple fluidic delivery to each cantilever to immobilize molecules for biological and chemical detection. We show that differential sensing is 50 times less affected by ambient temperature changes than the absolute, thus enabling a more reliable differentiation between specific cantilever bending and background effects. We describe the fabrication process and show results related to the dynamic characterization of the device as a differential sensor. The root-mean-squared (rms) sensor noise in water and air is ~1 nm over the frequency range of 0.4-40 Hz. We also find that in air, the deflection resolution is limited only by the cantilever's thermomechanical noise level of 0.008 [Angstrom]/[Hz.sup.1/2] over the frequency range of 40-1000 Hz. [781] Index Terms--AFM technology, BioMEMS, biosensor, chemical-biological functionalization, diffraction, interdigitated, silicon nitride.

High-resolution atomic force microscopy (AFM) of soft or fragile samples requires a cantilever with a low spring constant and a sharp tip. We have developed a novel process for making such cantilevers from silicon nitride with oxidation-sharpened silicon tips. First, we made and sharpened silicon tips on a silicon wafer. Next, we deposited a thin film of silicon nitride over the tips and etched it to define nitride cantilevers and to remove it from the tips so that they protruded through the cantilevers. Finally, we etched from the back side to release the cantilevers by removing the silicon substrate. We characterized the resulting cantilevers by imaging them with a scanning electron microscope, by measuring their thermal noise spectra, and by using them to image a test sample in contact mode. A representative cantilever had a spring constant of ~0.06 N/m, and the tip had a radius of 9.2 nm and a cone angle of 36[degrees] over 3 [micro]m of tip length. These cantilevers are capable of higher resolution imaging than commercially available nitride cantilevers with oxidation-sharpened nitride tips, and they are especially useful for imaging large vertical features. [735] Index Terms--Atomic force microscopy (AFM), microfabricated cantilevers, microfabricated tips, micromachining, oxidation sharpening.

A high-resolution accelerometer with a bulk-micromachined silicon proof mass and an interferometric position sensor was developed for measuring vibratory accelerations. The interferometer consists of interdigitated fingers that are alternately attached to the proof mass and support substrate. Illuminating the fingers with coherent light generates a series of diffracted beams. The intensity of a given beam depends on the out-of-plane separation between the proof mass fingers and support fingers. Proof masses with mechanical resonances ranging from 80 Hz to 1 kHz were fabricated with a two mask process involving two deep reactive ion etches, an oxide etch stop, and a polyimide protective layer. The structures were packaged with a laser diode and photodiode into 8.6-[cm.sup.3] acrylic housings. The 80-Hz resonant proof mass has a noise equivalent acceleration of 40 ng/rt Hz and a dynamic range of 85 dB at 40 Hz. [730]

Examines the development of a high-resolution accelerometer with a bulk-micromachined silicon proof mass and interferometric position sensor for measuring vibratory accelerations. Structure of the interferometer; Intensity of the beams; Fabrication of the proof masses with mechanical resonances.