Richard R. A. Syms

Surface tension-powered self-assembly of microstructures - the state-of-the-art

Because of the low dimensional power of its force scaling law, surface tension is appropriate for carrying out reshaping and assembly in the microstructure size domain. This paper reviews work on surface tension powered self-assembly of microstructures. The existing theoretical approaches for rotational assembly are unified. The demonstrated fabrication processes are compared. Mechanisms for accurately determining the assembled shape are discussed, and the limits on accuracy and structural distortion are considered. Applications in optics, electronics and mechanics are described. More complex operations (including the combination of self-assembly and self-organization) are also reviewed.

Demonstration of three-dimensional microstructure self-assembly

Self-assembly of three-dimensional microstructures using the surface tension force of molten solder to produce out-of-plane rotation is demonstrated. The generic nature of the technique is illustrated by reconfiguring structures formed in both Ni metal and single crystal Si. The structures do not have a hinge to constrain the rotation. This considerably simplifies fabrication and eliminates problems associated with the compatibility of a suitable hinge material. Details of the fabrication processes are given and results are presented for rotated structures.

Monolithic MEMS quadrupole mass spectrometers by deep silicon etching

A wafer-scale, batch fabrication process for constructing quadrupole mass spectrometers using microelectromechanical systems (MEMS) technology is described. The device is formed from two bonded silicon-on-insulator (BSOI) substrates, which are attached together to form a monolithic block. Deep etched features and springs formed in the outer silicon layers are used to locate cylindrical metal electrode rods, while similar features formed in the inner silicon layers are used to define integrated ion entrance and exit optics. The precision of the assembly is determined by lithography and deep etching, and by the mechanical definition of the bonded silicon layers. Mass filtering is demonstrated, with a mass range of /spl ap/ 400 a.m.u. and a mass resolution of 1 a.m.u. at 219 a.m.u., using quadrupoles with rods of 500 /spl mu/m diameter and 30 mm length, operating at 6 MHz RF frequency.

A theory of metamaterials based on periodically loaded transmission lines: Interaction between magnetoinductive and electromagnetic waves

The propagation of waves in a metamaterial consisting of split ring resonators (SRRs) and metallic rods is considered in several steps. The first involves the rods in isolation, the second the SRRs in isolation, and the third a combination of the two, which includes the coupling between neighboring SRRs and allows the propagation of magnetoinductive (MI) waves. The mathematical formulation is based on a conventional description of loaded transmission lines. A dispersion equation is derived to show the main features of known experimental results, including all the stop bands and passbands, the latter exhibiting both forward and backward waves. The interaction between electromagnetic and MI waves is presented in the form of a coupled dispersion equation. The applicability of the approaches based on negative material parameters is discussed.

Low-loss magneto-inductive waveguides

Radio-frequency experimental measurements of magnetically coupled chains of L-C resonators supporting magneto-inductive (MI) waves are presented. MI waveguides are constructed using printed circuit board (PCB) inductors and external capacitors. Methods for increasing the nearest-neighbour coupling and reducing the non-nearest neighbour coupling using double-sided PCBs are demonstrated. Conditions for low propagation loss are determined, and a method of approximate matching to 50 Ω transmission lines is presented. Propagation losses of 0.12 dB per element and coupling losses of 0.4 dB are achieved, using elements with Q-factors of 110 at ≈150 MHz frequency. Simple recursive Fabry–Perot filters are demonstrated by inserting reflectors into the waveguide.

Fabrication of buried channel waveguides on silicon substrates using spin-on glass.

A new process for the deposition of thick (≈10-µm) films of silica and titania-doped silica on silicon substrates is described. Films are built up by repetitive operation of a simple process cycle in which a layer of sol-gel material is deposited by spin coating, then densified by rapid thermal annealing. Stressfree layers are obtained through careful choice of the anneal temperature. Bilayer structures suitable for waveguide fabrication may also be constructed by performing two successive deposition runs using sol-gel precursors with different titania concentrations. These bilayers may be patterned topographically into ridges by using reactive ion etching, and the ridges may be planarized by applying additional layers of sol-gel material to form buried channel waveguides.

Electrothermal frequency tuning of folded and coupled vibrating micromechanical resonators

The use of constrained thermal expansion to tune the resonant frequency of vibrating micromechanical resonators is explored. A simple model is developed to predict the power sensitivity obtained with folded and unfolded geometries, including the effects of electrothermal heating, and conduction and convection cooling. It is shown that the sensitivity of folded structures can change sign as the ambient gas pressure is lowered in contrast to the behavior of unfolded structures. Tuning is then by tensile axial stress rather than compressive stress. Using folded laterally resonant bulk-micromachined comb-drive electrostatic actuators, tuning ranges of -25% and +50% are obtained (at atmospheric pressure and at 10 mTorr, respectively, with powers of 10 and 1.5 mW, respectively). A nested coupled resonator is then presented in which the frequency of one resonator may be tuned without effecting that of the other, thus allowing frequency matching to be obtained.

Fiber-device-fiber gain from a sol-gel erbium-doped waveguide amplifier

An erbium-doped silica-on-silicon planar waveguide optical amplifier is described. The active core is a topographic guide formed from aluminophosphosilicate glass doped with erbium and ytterbium. The buffer is formed from silica deposited by thermal oxidation and the cladding from borophosphosilicate glass obtained by plasma-enhanced chemical vapor deposition. The use of low process temperatures allows relatively heavy doping and careful control of the core etching allows low background insertion losses to be obtained. Spontaneous emission and gain measurements are given and 5.4-dB fiber-device-fiber gain is demonstrated using a 5-cm-long chip pumped using a 980-nm laser diode at 175-mW pump power.

Silicon based quadrupole mass spectrometry using microelectromechanical systems

The conventional quadrupole mass spectrometer (QMS) arrangement uses circular metallic rods as the mass filter excited electrically at voltages up to 1 kV depending upon the application. If the size and voltages can be reduced then the range of applications for QMS instruments would increase. The application of microelectromechanical systems (MEMS) technology allows the fabrication of submillimeter versions of such structures. In this article the development of a miniature QMS is reported in which the conventional rod arrangement has been replaced with a microengineered version. The structure is made in silicon with metallized specially drawn glass fibers of length 20–30 mm and diameter 0.5 mm to act as the quadrupole rods. This is about one order of magnitude smaller than most conventional QMS filters, with the potential for further reduction in size. The MEMS mass filter was mounted onto a commercial ion source, which was in turn attached to a vacuum flange and supplied by an electronic drive circuit at...

Equilibrium of hinged and hingeless structures rotated using surface tension forces

Two different geometries for the rotation of microstructures by the surface tension force of molten solder are investigated theoretically and experimentally. The geometries are based on structures with and without a hinge acting to constrain the motion of a flap to pure rotation. The equilibrium geometry of a hinged structure is first found analytically by considering the surface energy of the solder. An analysis of hingeless structures is then performed that shows a hinge to be unnecessary under certain conditions. Macroscopic experiments performed using printed circuit board parts are then described; the results show that the behaviors of hinged and hingeless structures are similar.