Electromechanical Lifting Actuation of a MEMS Cantilever and Nano-Scale Analysis of Diffusion in Semiconductor Device Dielectrics

This dissertation presents experimental and theoretical studies of physical phenomena in micro- and nano-electronic devices. Firstly, a novel and unproven means of electromechanical actuation in a micro-electro-mechanical system (MEMS) cantilever was investigated. In nearly all MEMS devices, electric forces cause suspended components to move toward the substrate. I demonstrated a design with the unusual and potentially very useful property of having a suspended MEMS cantilever lift away from the substrate. The effect was observed by optical micro-videography, by electrical sensing, and it was quantified by optical interferometry. The results agree with predictions of analytic and numerical calculations. One potential application is infrared sensing in which absorbed radiation changes the temperature of the cantilever, changing the duty cycle of an electrically-driven, repetitively closing micro-relay. Secondly, ultra-thin high-k gate dielectric layers in two 22 nm technology node semiconductor devices were studied. The purpose of the investigation was to characterize the morphology and composition of these layers as a means to verify whether the transmission electron microscope (TEM) with energy dispersive spectroscopy (EDS) could sufficiently resolve the atomic diffusion at such small length scales. Results of analytic and Monte-Carlo numerical calculations were compared to empirical data to validate the ongoing viability of TEM EDS as a tool for nanoscale characterization of semiconductor devices in an era where transistor dimensions will soon be less than 10 nm

Stress Analysis of Free-Standing Silicon Oxide Films Using Optical Interference

ABSTRACT We report a method for stress measurement and analysis in silicon oxide thin films using optical interference. Effects of design and fabrication on stress have been studied by fabricating submicron-thick slabs of oxide anchored at one end and extending over a reflective surface. Optical interference occurs between reflections from the surface and the oxide slab, giving rise to light and dark fringes that may be imaged with a microscope. Analysis of the interference pattern at different wavelengths gives the radius of curvature and means of stress mapping. The accuracy exceeds non-interferometric profilometry using optical or confocal microscopes, and it can be more quantitative than scanning electron microscopy. This nondestructive profilometry method can aid the stress optimization of silicon oxide or other transparent thin films to achieve specific mechanical characteristics in MEMS devices

Far-Infrared Absorber Based On Standing-Wave Resonances In Metal-Dielectric-Metal Cavity

Thin-film resonant absorbers for the far-IR spectral range were fabricated, characterized, and modeled. The 3-μm-thick structure comprises a periodic surface array of metal squares, a dielectric spacer and a metallic ground plane. Up to 95% absorption for the fundamental band at ∼53.5μm wavelength (5.6 THz) is achieved experimentally. Absorption bands are independent of the structure period and only weakly dependent on polarization and incident angle. The results are well explained in terms of standing-wave resonances within individual metal-dielectric-metal cavities. The structure has application as a wavelength selective coating for far-IR bolometers

Mems Clocking-Cantilever Thermal Detector

We present performance calculations for a MEMS cantilever device for sensing heat input from convection or radiation. The cantilever deflects upwards under an electrostatic repulsive force from an applied periodic saw-tooth bias voltage, and returns to a null position as the bias decreases. Heat absorbed during the cycle causes the cantilever to deflect downwards, thus decreasing the time to return to the null position. In these calculations, the total deflection with respect to absorbed heat is determined and is described as a function of time. We present estimates of responsivity and noise. © 2013 SPIE

Release of MEMS devices with hard-baked polyimide sacrificial layer

Removal of polyimides used as sacrificial layer in fabricating MEMS devices can be challenging after hardbaking, which may easily result by the end of multiple-step processing. We consider the specific commercial co-developable polyimide ProLift 100 (Brewer Science). Excessive heat hardens this material, so that during wet release in TMAH based solvents, intact sheets break free from the substrate, move around in the solution, and break delicate structures. On the other hand, dry reactive-ion etching of hard-baked ProLift is so slow, that MEMS structures are damaged from undesirably-prolonged physical bombardment by plasma ions. We found that blanket exposure to ultraviolet light allows rapid dry etch of the ProLift surrounding the desired structures without damaging them. Subsequent removal of ProLift from under the devices can then be safely performed using wet or dry etch. We demonstrate the approach on PECVD-grown silicon-oxide cantilevers of 100 micron × 100 micron area supported 2 microns above the substrate by ∼100-micron-long 8-micron-wide oxide arms. © 2013 SPIE

Release Of Mems Devices With Hard-Baked Polyimide Sacrificial Layer

Removal of polyimides used as sacrificial layer in fabricating MEMS devices can be challenging after hardbaking, which may easily result by the end of multiple-step processing. We consider the specific commercial co-developable polyimide ProLift 100 (Brewer Science). Excessive heat hardens this material, so that during wet release in TMAH based solvents, intact sheets break free from the substrate, move around in the solution, and break delicate structures. On the other hand, dry reactive-ion etching of hard-baked ProLift is so slow, that MEMS structures are damaged from undesirably-prolonged physical bombardment by plasma ions. We found that blanket exposure to ultraviolet light allows rapid dry etch of the ProLift surrounding the desired structures without damaging them. Subsequent removal of ProLift from under the devices can then be safely performed using wet or dry etch. We demonstrate the approach on PECVD-grown silicon-oxide cantilevers of 100 micron × 100 micron area supported 2 microns above the substrate by ∼100-micron-long 8-micron-wide oxide arms. © 2013 SPIE

Thermomechancial Characterization in a Radiant Energy Imager Using Null Switching

Abstract Thermomechanical noise for a MEMs-based infrared detector using null switching (US patent 7977635) depends on vibrational amplitude, since IR radiation is transduced to a change in the duty cycle of a repetitively closing switch. Equipartition theorem determines the maximum rms vibrational amplitude for the fabricated cantilever switch at its natural frequency. This determines the timing uncertainty, NEP, and NETD. We estimate NETD to fall in the range 0.13 -1.5 K for a device with ~20 μm pitch

Stress Analysis Of Free-Standing Silicon Oxide Films Using Optical Interference

We report a method for stress measurement and analysis in silicon oxide thin films using optical interference. Effects of design and fabrication on stress have been studied by fabricating submicron-thick slabs of oxide anchored at one end and extending over a reflective surface. Optical interference occurs between reflections from the surface and the oxide slab, giving rise to light and dark fringes that may be imaged with a microscope. Analysis of the interference pattern at different wavelengths gives the radius of curvature and means of stress mapping. The accuracy exceeds non-interferometric profilometry using optical or confocal microscopes, and it can be more quantitative than scanning electron microscopy. This nondestructive profilometry method can aid the stress optimization of silicon oxide or other transparent thin films to achieve specific mechanical characteristics in MEMS devices. © 2013 Materials Research Society

Patterning Of Oxide-Hardened Gold Black By Photolithography And Metal Lift-Off

A method to pattern infrared-absorbing gold black by conventional photolithography and lift-off is described. A photo-resist pattern is developed on a substrate by standard photolithography. Gold black is deposited over the whole by thermal evaporation in an inert gas at ∼1 Torr. SiO2 is then deposited as a protection layer by electron beam evaporation. Lift-off proceeds by dissolving the photoresist in acetone. The resulting sub-millimeter size gold black patterns that remain on the substrate retain high infrared absorption out to ∼5 μm wavelength and exhibit good mechanical stability. This technique allows selective application of gold black coatings to the pixels of thermal infrared imaging array detectors. © 2013 Elsevier B.V. All rights reserved

Patterning And Hardening Of Gold Black Infrared Absorber By Shadow Mask Deposition With Ethyl Cyanoacrylate

Patterning of gold-black infrared absorbing films by stencil lithography and hardening by polymer infusion is reported. Gold black nano-structured films are deposited through a thin metal shadow mask in a thermal evaporator in ~400 mTorr pressure of inert gas, followed by ethyl cyanoacrylate fuming through the same mask to produce rugged IR absorptive patterns of ~100 micron scale dimensions. Infrared absorptivity is determined by transmission and reflectivity measurements using a Fourier spectrometer and infrared microscope. Results indicate that the optimized hardening process reduces the usual degradation of the absorptivity with age. This work has potential application to infrared array bolometers. © 2013 SPIE