A variable capacitor made from single crystal silicon fracture surface pairs

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005.Includes bibliographical references (p. 263-279).Complementary and nano-smooth single-crystal-silicon surfaces have been fabricated by deliberately fracturing a weakened portion of a larger structure whose flexural mechanism refines and concentrates an externally applied load to a notched specimen region pre-.fracture and acts as a precision bearing post-fracture. When material is not ejected, crystalline silicon's extreme brittleness at room temperature results in complementary fracture surfaces; "closed" gaps of 20-30 nanometers are typical. Lithographic, focused ion beam, and anisotropically etched notches as well as crystal orientation and specimen thickness were studied, and a method was developed for fabricating smooth surfaces perpendicular to the wafer plane: 10 micrometer square specimens oriented with the (110) plane are fully notched by anisotropic etching (KOH) and fractured within a structure optimized to apply pure tension. Prototype MEMS variable capacitors employing fracture surfaces as separable parallel plates have been developed. The advantage of fracture surfaces over etched surfaces is their exceptional flatness or complementarity, which allow them to very closely approach each other prior to contact.(cont.) For a parallel-plate capacitor, a small minimum separation disproportionately improves dynamic range because capacitance scales with the inverse of separation. To take utmost advantage of the surfaces' qualities, unstable pull-in must be avoided. Modelling of the three-way force balance between the structure's compliance, the capacitive force between the fracture surfaces, and the zipping electrostatic actuator demonstrates the system is stable when the actuator is working against the spring force and not the capacitive force. The structure and the capacitor are tuned to ensure system stability to nanometer scale separations. During fabrication of the variable capacitor, it was discovered that high temperature processing associated with the thermal oxidation necessary for the actuators blunted the anisotropically etched notches. This blunting occurred even when the notches were covered with a nitride diffusion barrier, and was likely due to diffusional smoothing. Blunt notches produced material ejecting fractures with non-functional surfaces, but since material ejection is characteristic of higher stress fractures, it may be useful for toughening anisotropically etched structures.(cont.) The blunting problem was overcome by masking the notch etch with thermal oxide after all high temperature processing was complete.by Alexander D. Sprunt.Ph.D

A machine for tribological experimentation on indexing continuous media with specific application to semiconductor testing

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2001.Includes bibliographical references (p. 103).Technology was developed to facilitate electrical contact tribology experiments on continuous media, dramatically reducing the difficulty of employing virgin material for each test. Specifically, a tester was designed to accurately reproduce semiconductor contactor operating environments while measuring contact resistance in-situ, thereby effecting the study of operating temperature, test current, cleaning method, and cleaning interval on contactor life. To manipulate the continuous media while preserving exact constraint, novel web handling machine elements were devised. Universal joints and beam type flexible couplings were employed for gimballing and castering axes, both at standard caster radii and at roller center. A kinematic edge constraint was designed. The torque transmission properties of clamped connections were alloyed to the favorable kinematics of typical pinned type connections by compliantly mounting a spherical roller bearing as a pinch roller.by Alexander D. Sprunt.S.M

Second harmonic light scattering induced by defects in the twist-bend nematic phase of liquid crystal dimers

The nematic twist-bend (NTB) phase, exhibited by certain thermotropic liquid crystalline (LC) dimers, represents a new orientationally ordered mesophase -- the first distinct nematic variant discovered in many years. The NTB phase is distinguished by a heliconical winding of the average molecular long axis (director) with a remarkably short (nanoscale) pitch and, in systems of achiral dimers, with an equal probability to form right- and left-handed domains. The NTB structure thus provides another fascinating example of spontaneous chiral symmetry breaking in nature. The order parameter driving the formation of the heliconical state has been theoretically conjectured to be a polarization field, deriving from the bent conformation of the dimers, that rotates helically with the same nanoscale pitch as the director field. It therefore presents a significant challenge for experimental detection. Here we report a second harmonic light scattering (SHLS) study on two achiral, NTB-forming LCs, which is sensitive to the polarization field due to micron-scale distortion of the helical structure associated with naturally-occurring textural defects. These defects are parabolic focal conics of smectic-like ``pseudo-layers", defined by planes of equivalent phase in a coarse-grained description of the NTB state. Our SHLS data are explained by a coarse-grained free energy density that combines a Landau-deGennes expansion of the polarization field, the elastic energy of a nematic, and a linear coupling between the two

Light scattering study of the “pseudo-layer” compression elastic constant in a twist-bend nematic liquid crystal

The nematic twist-bend (TB) phase, exhibited by certain achiral thermotropic liquid crystalline (LC) dimers, features a nanometer-scale, heliconical rotation of the average molecular long axis (director) with equally probable left- and right-handed domains. On meso to macroscopic scales, the TB phase may be considered as a stack of equivalent slabs or “pseudo-layers”, each one helical pitch in thickness. The long wavelength fluctuation modes should then be analogous to those of a smectic-A phase, and in particular the hydrodynamic mode combining “layer” compression and bending ought to be characterized by an effective layer compression elastic constant Beff and average director splay constant Keff1. The magnitude of Keff1 is expected to be similar to the splay constant of an ordinary nematic LC, but due to the absence of a true mass density wave, Beff could differ substantially from the typical value of ∼10⁶ Pa in a conventional smectic-A. Here we report the results of a dynamic light scattering study, which confirms the “pseudo-layer” structure of the TB phase with Beff in the range 10³–10⁴ Pa. We show additionally that the temperature dependence of Beff at the TB to nematic transition is accurately described by a coarse-grained free energy density, which is based on a Landau-deGennes expansion in terms of a heli-polar order parameter that characterizes the TB state and is linearly coupled to bend distortion of the director