Single-crystal Silicon Films Research Articles

RF MEMS Oscillator with Integrated Resistive Transduction

A method to integrate micromechanical frequency-determining elements along with the corresponding electromechanical transducers into a poly or single-crystal silicon film layer is demonstrated. A resistor dissipating several microwatt of power induces high-frequency resonant mechanical motion in a shallow-shell membrane. Transduction from the mechanical to the electrical domain is performed using implanted piezoresistors, which are sensitive to strain produced by resonant motion. Self-sustained oscillations at 10 MHz are demonstrated when the device is directly coupled to a high-impedance operational amplifier and a positive feedback loop. Finally, this letter discuss how the resonator and transducers may be incorporated into standard integrated-circuit technology

Mechanical properties of a micron-sized SCS film in a high-temperature environment

The effects of varying temperature on the mechanical properties of a micron-sized single-crystal silicon (SCS) film were investigated using an ‘on-chip’ tensile test. Young's modulus slightly decreased as the test temperature increased from room temperature (RT) to 500 °C. Furthermore, the fracture stress at 500 °C decreased to 1.84 GPa, which is about 32% of that at RT. The stress–strain relationship of the SCS film at 400 °C and 500 °C showed elastic–plastic deformation, and SEM images showed crossed slip lines on the side surface of the fractured specimen. From the measured stress–strain relationship, plastic strain in micron-sized silicon seems to have occurred at temperatures lower than the brittle-to-ductile transition temperature in bulk silicon.

Five-nanometer thick silicon on insulator layer

Silicon on insulator (SOI) has been achieved using epitaxially grown Si∕Y2O3∕Si structure. Silicon film as thin as 5nm was achieved. Pulsed laser deposition technique was used for the epitaxial deposition of both the Y2O3 and silicon. The growth conditions were adjusted to achieve two-dimensional growths of single crystal silicon films on Y2O3. No dislocations were observed in these silicon epitaxial films. This approach will allow the independent thickness control of both silicon and the oxide in the nanometer range. Si∕CeO2∕Si SOI structure was not as successful due to the formation of an amorphous oxide film at the interfaces.

Cross comparison of thin-film tensile-testing methods examined using single-crystal silicon, polysilicon, nickel, and titanium films

This paper reports on the results of comparing different types of tensile testing methods that are used to evaluate thin-film properties. We tested the same material fabricated on a single wafer using different testing techniques at five different research institutions. The testing methods were different in the way the specimen was gripped. Materials tested were single-crystal silicon (SCS), polysilicon, nickel, and titanium films. Specimens with three different shapes were processed through the same fabrication steps. The tensile strength, fracture strain, and Young's modulus of the films were measured and compared. The measured values of the mechanical properties of all the testing methods were in good agreement with each other, thus demonstrating their accuracy.

A Three-Dimensional Stacked Fin-CMOS Technology for High-Density ULSI Circuits

In this paper, a three-dimensional CMOS technology is proposed and implemented using stacked Fin-CMOS (SF-CMOS) architecture. The technology is based on a double layer silicon-on-insulator wafer formed by two oxygen implants to create two single-crystal silicon films with an oxide isolation layer in between. The proposed approach achieves a 50% area reduction and significant shortening of the wiring distance between active devices through vertical connection when compared with conventional planar CMOS technology. The SF-CMOS technology also inherits the scalability and two-dimensional processing compatibility of the FinFET structure. SF-CMOS devices and simple circuits were fabricated and characterized.

TEM Characterization of Single Crystal Silicon Film on Glass Substrate

TEM Characterization of Single Crystal Silicon Film on Glass Substrate

Stacked 3-D Fin-CMOS technology

A stacked three-dimensional Fin-CMOS (SF-CMOS) technology has been proposed and implemented. The technology is based on a double-layer SOI wafer formed by performing two oxygen implants to form two single-crystal silicon films with isolation layer in between. The proposed approach achieves a 50% area reduction and significant shortening of wiring distance between the active devices when compared with existing planar CMOS technology. The SF-CMOS technology also inherits the scalability and two-dimensional processing compatibility of the FinFET architecture.

Mechanical characterization of single crystal silicon and UV-LIGA nickel thin films using tensile tester operated in AFM

This paper describes the mechanical characteristics of microscale single crystal silicon (SCS) and UV-LIGA nickel (Ni) films used for microelectromechanical systems (MEMS). A compact tensile tester, operated in an atomic force microscope (AFM), was developed for accurate evaluation of Young's modulus, tensile strain and tensile strength of microscale SCS and UV-LIGA Ni specimens. SCS specimens with nominal dimensions of 20 μm in thickness, 50 μm in width and 600 μm in length were prepared by a conventional photolithography and etching process. UV-LIGA Ni specimens, with a thickness of 15 μm, a width of 50 μm and a length of 600 μm in nominal dimensions, were also fabricated by electroplating using a UV thick photoresist mould. All specimens have line patterns on their specimen gauge section to measure axial elongation under tensile loading. The SCS specimens showed a linear stress–strain response and fractured in a brittle manner, whereas the UV-LIGA Ni specimens showed elastic–inelastic deformation behaviour. Young's modulus of SCS and UV-LIGA Ni specimens obtained from tensile tests averaged 169.2 GPa and 183.6 GPa, respectively, close to those of bulk materials. However, the tensile strength of both materials showed a larger value than the bulk materials: 1.47 GPa for the SCS and 0.98 GPa for the Ni specimens. Yield stress and breaking elongation of UV-LIGA Ni specimens were also quite different from those of the bulk Ni because of the specimen size effect on inelastic properties.

A Systematic Study of the Formation of Nano-Tips on Silicon Thin Films by Excimer Laser Irradiation

AbstractRecently, we reported conditions for controllable, direct laser fabrication of sharp conical tips with heights of about one micrometer and apical radii of curvature of several tens of nanometers. An individual cone is formed when a single-crystal silicon film on an insulator substrate is irradiated in air environment with a single pulse from a KrF excimer laser, homogenized and shaped to a circular spot several microns in diameter. In this work, we present a study of the formation of such tips as a function of the laser fluence, the film thickness, and the diameter of the irradiated spot. Atomic force microscopy and scanning electron microscopy were used to study the topography of the structures. A simple mechanism of formation based on movement of melted material is proposed. We have also studied structures (nano-ridges) that resulted from irradiation with narrow lines (width of several microns) instead of circular spots.

THERMAL CONDUCTION IN SILICON MICRO- AND NANOSTRUCTURES

Silicon micro- and nanostructures are found in integrated circuits, sensors, and actuators, and the performance and reliability of many of these devices are affected by thermal conduction. Conduction processes in micro- and nanostructures are complicated by phonon interactions with grain and material boundaries. Phonon confinement in nanostructures and highly nonequilibrium rates of phonon generation by electrons also influence heat conduction. This article reviews experimental and theoretical studies of the thermal conductivity of single-crystal and polycrystalline silicon films, with a focus on data relevant for modeling heat transport in modern devices. Data for single-crystal films indicate a room-temperature thermal conductivity reduction of up to 50% compared to bulk silicon due to phonon boundary scattering, while grain boundary scattering in polysilicon films results in even lower values. Simple analytical expressions for approximating the room-temperature thermal conductivity of silicon films are provided. Doping modifies heat conduction mainly due to impurity scattering, and thermal conductivity data for boron-, phosphorus-, and arsenic-doped silicon films demonstrate the extent of this effect. Hotspots smaller than the phonon mean free path cause intense localized heating in silicon and severe departure from equilibrium within the phonon system, which can substantially increase the effective thermal resistance for conduction. This review provides an overview of the nanoscale conduction effects that are becoming important for the design of silicon nanostructures.

Anisotropy of Fracture Strength and Fracture Toughness of Micro-Sized Single-Crystal Silicon

This paper presents the dependence of fracture toughness, fracture strength, and fracture behavior, such as crack propagation, on the crystal orientation of single-crystal silicon. We conducted on-chip tensile testing to measure fracture strength and fracture toughness of single-crystal silicon films with (100) and (110) surface in the and loading direction. The loading direction had a significant effect on fracture toughness, which was 2.17MPa√m in the direction and 1.27MPa√m in . However, the fracture strength varies with both loading direction and surface orientation. We observed a fracture specimen on which a (111) cleavage plane eventually appeared on any crystal types of the specimen.

Anisotropy in fracture of single crystal silicon film characterized under uniaxial tensile condition

In this work, we studied the effect of crystallographic orientation on fracture toughness and fracture paths of single crystal silicon film by a uniaxial tensile test. A focused ion beam (FIB) process was used to introduce a notch in the middle of silicon (1 1 0) and (0 0 1) film specimens that have different tensile orientations. The fracture toughness value varied from 1 to 2 MPa m 1/2, depending on the tensile orientation. The scanning electron microscope (SEM) observations suggested that the inclination angle of the low index planes, {1 1 1} and {1 1 0}, relative to the specimen surface affects the fracture path significantly. The {1 1 1} and {1 1 0} planes were the preferred cleavage planes.

Measurement for fracture toughness of single crystal silicon film with tensile test

We developed a method of measuring the fracture toughness, which is a material constant in the macroscopic domain, of micro-machined single crystal silicon film on the (1 0 0) plane. We notched the thin film specimen on a single edge and then conducted a uniaxial tensile test to specimen failure. The average value of measured fracture toughness was 1.58 MPa m 1/2, with scatter. This is slightly higher than, but comparable to, the value for bulk silicon. Scanning electron microscope (SEM) observation of the failed specimens revealed that the fracture developed mainly along the (1 1 0) cleavage plane.

Amorphization of silicon by femtosecond laser pulses

We have used femtosecond laser pulses to drill submicron holes in single crystal silicon films in silicon-on-insulator structures. Cross-sectional transmission electron microscopy and energy dispersive x-ray analysis of material adjacent to the ablated holes indicates the formation of a layer of amorphous Si. This demonstrates that even when material is ablated using femtosecond pulses near the single pulse ablation threshold, sufficient heating of the surrounding material occurs to create a molten zone which solidifies so rapidly that crystallization is bypassed.

Nanoscale thin single-crystal silicon and its application to electronics

We demonstrate a method for achieving thin (tens of nanometers) single-crystal silicon films with electronic-quality bulk and surface properties, atomic-scale surface roughness, and suitable for use in complex multilayered structures. The thin silicon film is achieved by defining thickness through the use of oxidation depth as a reference, transfer of film structure by exfoliation, and chemical-mechanical polishing for selective thin film definition. Such structures are useful in a variety of directions: in electronic devices such as field-effect transistors, in characterization studies such as those utilizing vertical transport using thicknesses of the order of mean free path length, etc. We demonstrate its use in a silicon metal-oxide-semiconductor transistor with two gates, one on each side of the thin silicon film on a planar substrate.

Contactless Measurement of Young's Modulus Using Laser Beam Excitation and Detection of Vibration of Thin-Film Microresonators

Young's modulus of a single-crystal thin silicon film was estimated by a noncontact method. The resonance frequencies of a microresonator fabricated using the film were measured by excitation and detection using multiple laser beams. A special configuration was adopted for the resonator to suppress the heating effect of the excite beam. Clear resonances were observed in the frequency characteristics of vibration. Young's modulus was determined from an elastic modification analysis of a spring-mass system using the resonance frequency together with the geometrical dimensions of the resonator. The deduced value of Young's modulus was 152±17 MPa, which agrees with reported values within experimental errors. The cantilever beams of the resonator had trapezoidal cross sections caused by the reactive ion etching (RIE) used in the fabrication, and their widths showed nonuniformity. The error mainly comes from the nonuniformity of the beam width along the cantilever.

Epitaxial silicon and germanium on buried insulator heterostructures and devices

Future microelectronics will be based upon silicon or germanium-on-insulator technologies and will require an ultrathin (<10 nm), flat silicon or germanium device layer to reside upon an insulating oxide grown on a silicon wafer. The most convenient means of accomplishing this is by epitaxially growing the entire structure on a silicon substrate. This requires a high quality crystalline oxide and the ability to epitaxially grow two dimensional, single crystal films of silicon or germanium on top of this oxide. We describe a method based upon molecular beam epitaxy and solid-phase epitaxy to make such structures and demonstrate working field-effect transistors on germanium-on-insulator layers.

Wavy surface nanostructures formed in amorphous silicon films by sputtering with nitrogen ions

Wavy nanostructures are formed on the ion-bombarded surface of single-crystal and amorphous silicon films prepared by various methods. The period and depth of this structure have been determined as functions of the angle of incidence and the energy of nitrogen ions used for the surface sputtering. A comparative analysis of SEM images of the wavy surface nanostructures on various silicon films has been performed using a two-dimensional Fourier transform. The films of amorphous silicon obtained by electron beam deposition and magnetron sputtering techniques are closest to single-crystal silicon films with respect to the wavy surface nanostructure formation.

Raman microscopy investigations of structural phase transformations in crystalline and amorphous silicon due to indentation with a Vickers diamond at room temperature and at 77 K

Raman microscopy has been used to investigate phase transitions within Vickers diamond residual indentations, made at room temperature and at 77 K, in single crystal Si(100) and 0.3 μm thick amorphous silicon film deposited on a sapphire single crystal. It has been found that for room temperature residual indentations in the single crystal silicon (Si-I) and amorphous silicon film phase transition to the Si-III phase occurs, as indicated by the existence of Raman peaks corresponding to this phase. On the other hand, in the case of the indentations made at 77 K, only the Si-I crystalline phase and amorphous silicon were found within the residual indentations. It has been suggested that when Vickers diamond indentations are made at 77 K, there is no phase transition to the Si-II (i.e. the β-Sn phase) during indenter loading, which would give rise to the Si-III phase on the removal of indenter load. Moreover, the existence of a very strong peak due to the Si-I phase gives further support to the suggestion that during Vickers indentations in single crystal silicon and in amorphous silicon at 77 K there is no structural phase transition. It has also been suggested that during room temperature indentations there is a substantial temperature rise, which may help to cause structural phase transitions.

Mechanical characterization of micro/nanoscale structures for MEMS/NEMS applications using nanoindentation techniques

Mechanical properties of micro/nanoscale structures are needed to design reliable micro/nanoelectromechanical systems (MEMS/NEMS). Micro/nanomechanical characterization of bulk materials of undoped single-crystal silicon and thin films of undoped polysilicon, SiO 2, SiC, Ni–P, and Au have been carried out. Hardness, elastic modulus and scratch resistance of these materials were measured by nanoindentation and microscratching using a nanoindenter. Fracture toughness was measured by indentation using a Vickers indenter. Bending tests were performed on the nanoscale silicon beams, microscale Ni–P and Au beams using a depth-sensing nanoindenter. It is found that the SiC film exhibits higher hardness, elastic modulus and scratch resistance as compared to other materials. In the bending tests, the nanoscale Si beams failed in a brittle manner with a flat fracture surface. The notched Ni–P beam showed linear deformation behavior followed by abrupt failure. The Au beam showed elastic–plastic deformation behavior. FEM simulation can well predict the stress distribution in the beams studied. The nanoindentation, scratch and bending tests used in this study can be satisfactorily used to evaluate the mechanical properties of micro/nanoscale structures for use in MEMS/NEMS.