T. D. Stowe

Quality factors in micron- and submicron-thick cantilevers

Micromechanical cantilevers are commonly used for detection of small forces in microelectromechanical sensors (e.g., accelerometers) and in scientific instruments (e.g., atomic force microscopes). A fundamental limit to the detection of small forces is imposed by thermomechanical noise, the mechanical analog of Johnson noise, which is governed by dissipation of mechanical energy. This paper reports on measurements of the mechanical quality factor Q for arrays of silicon-nitride, polysilicon, and single-crystal silicon cantilevers. By studying the dependence of Q on cantilever material, geometry, and surface treatments, significant insight into dissipation mechanisms has been obtained. For submicron-thick cantilevers, Q is found to decrease with decreasing cantilever thickness, indicating surface loss mechanisms. For single-crystal silicon cantilevers, significant increase in room temperature Q is obtained after 700/spl deg/C heat treatment in either N/sub 2/ Or forming gas. At low temperatures, silicon cantilevers exhibit a minimum in Q at approximately 135 K, possibly due to a surface-related relaxation process. Thermoelastic dissipation is not a factor for submicron-thick cantilevers, but is shown to be significant for silicon-nitride cantilevers as thin as 2.3 /spl mu/m.

Attonewton force detection using ultrathin silicon cantilevers

A measured force resolution of 5.6×10−18 N/Hz at 4.8 K in vacuum using a single-crystal silicon cantilever only 600 A thick is demonstrated. The spring constant of this cantilever was 6.5×10−6 N/m, or more than 1000 times smaller than that of typical atomic force microscope cantilevers. The cantilever fabrication includes the integration of in-line tips so that the cantilever can be oriented perpendicular to a sample surface. This orientation helps suppress cantilever snap-in so that high force sensitivity can be realized for tip-sample distances less than 100 A.

Low‐stiffness silicon cantilevers for thermal writing and piezoresistive readback with the atomic force microscope

Low‐stiffness silicon cantilevers have been developed for proposed data storage devices based on the atomic force microscope, in particular thermomechanical recording. The cantilevers combine a sharp tip with an integrated piezoresistive sensor for data readback from a rotating polycarbonate disk. A novel process was developed to make shallow piezoresistors in cantilevers 1 μm thick, significantly thinner and therefore softer than previously possible. Readback was demonstrated at linear velocities up to 120 mm/s. Separate cantilevers with resistively heated tips were fabricated for writing data marks on polycarbonate, with measured thermal time constants of 30 μs.

Silicon dopant imaging by dissipation force microscopy

Noncontact damping of a cantilever vibrating near a silicon surface was used to measure localized electrical dissipation. The dependence of the damping on tip-sample distance, applied voltage, carrier mobility, and dopant density was studied for n- and p-type silicon samples with dopant densities of 1014–1018 cm−3. Dopant imaging with 150 nm spatial resolution was demonstrated.

Torsional force probes optimized for higher order mode suppression

We describe the design of a micromechanical torsional cantilever made from silicon nitride which is optimized for high force sensitivity and higher order mode suppression. The unique geometry and wide mode modal spacing of the torsional cantilever allow simplified feedback control for the purpose of damping thermomechanical noise.

Fabricating subcollimating grids for an x-ray solar imaging spectrometer using LIGA techniques

We are fabricating sub-collimating X-ray grids that are to be used in an instrument for the High Energy Solar Spectroscopic Imager (HESSI), a proposed NASA mission. The HESSI instrument consists of twelve rotating pairs of high aspect ratio, high Z grids, each pair of which is separated by 1.7 meters and backed by a single Ge detector. The pitch for these grid pairs ranges from 34 micrometers to 317 micrometers with the grid slit openings being 60% of the pitch. For maximum grid X-ray absorbing with minimum loss of the solar image, the grid thickness-to-grid-slit ratio must be approximately 50:1, resulting in grid thicknesses of 1 to 10 millimeters. For our proof-of-concept grids we are implementing a design in which a 34 micrometers pitch, free-standing PMMA grid is fabricated with 20 micrometers wide slits and an 800 micrometers thickness. Stiffeners that run perpendicular to the grid are placed every 500 micrometers . After exposure and developing, metal, ideally gold, is electrodeposited into the free-standing PMMA grid slits. The PMMA is not removed and the metal in the slits acts as the X-ray absorber grid while the PMMA holds the individual metal pieces in place, the PMMA being nearly transparent to the X-rays coming from the sun. For optimum imaging performance, the root-mean-square pitch of the two grids of each pair must match to within 1 part in 10000 and simultaneous exposures of stacked sheets of PMMA have insured that this requirement is met.