Silicon Pedestal Research Articles

Prototype Optical Bionic Microphone with a Dual-Channel Mach-Zehnder Interferometric Transducer.

A prototype optical bionic microphone with a dual-channel Mach-Zehnder interferometric (MZI) transducer was designed and prepared for the first time using a silicon diaphragm made by microelectromechanical system (MEMS) technology. The MEMS diaphragm mimicked the structure of the fly Ormia Ochracea's coupling eardrum, consisting of two square wings connected through a neck that is anchored via the two torsional beams to the silicon pedestal. The vibrational displacement of each wing at its distal edge relative to the silicon pedestal is detected with one channel of the dual-channel MZI transducer. The diaphragm at rest is coplanar with the silicon pedestal, resulting in an initial phase difference of zero for each channel of the dual-channel MZI transducer and consequently offering the microphone strong temperature robustness. The two channels of the prototype microphone show good consistency in their responses to incident sound signals; they have the rocking and bending resonance frequencies of 482 Hz and 1911 Hz, and their pressure sensitivities at a lower frequency exhibit an "8"-shaped directional dependence. The comparison indicates that the dual-channel MZI transducer-based bionic microphone proposed in this work is advantageous over the Fabry-Perot interferometric transducer-based counterparts extensively reported.

Radiation pressure and electrostriction induced enhancement for Kerr-like nonlinearities in a nanoscale silicon pedestal waveguide

We report an enhancement of effective nonlinear-index coefficient from the combination of radiation pressure and electrostriction in an air-cladding silicon pedestal waveguide. The effective nonlinear-index coefficient was experimentally measured by characterizing spectral broadening with incident pulses in the pedestal waveguide. Larger spectral broadening was observed in the pedestal waveguide compared to a conventional silicon waveguide. Theoretically, the primary enhancement of the effective nonlinear-index coefficient can be calculated by using Kramers–Kronig relations, which is associated with the change in absorption from stimulated Brillouin scattering in the pedestal waveguide. The calculations were consistent with experimental results. By tailoring the pillar width of the pedestal waveguide, about 27.6% enhancement in the effective nonlinear-index coefficient of the pedestal waveguide was possible at the center wavelength of 1555 nm.

Optomechanical microrheology of single adherent cancer cells.

There is a close relationship between the mechanical properties of cells and their physiological function. Non-invasive measurements of the physical properties of cells, especially of adherent cells, are challenging to perform. Through a non-contact optical interferometric technique, we measure and combine the phase, amplitude, and frequency of vibrating silicon pedestal micromechanical resonant sensors to quantify the “loss tangent” of individual adherent human colon cancer cells (HT-29). The loss tangent, a dimensionless ratio of viscoelastic energy loss and energy storage — a measure of the viscoelasticity of soft materials, obtained through an optical path length model, was found to be 1.88 0.08 for live cells and 4.32 0.13 for fixed cells, revealing significant changes (p < 0.001) in mechanical properties associated with estimated nanoscale cell membrane fluctuations of 3.86 0.2 nm for live cells and 2.87 0.1 nm for fixed cells. By combining these values with the corresponding two-degree-of-freedom Kelvin-Voigt model, we obtain the elastic stiffness and viscous loss associated with each individual cell rather than estimations from a population. The technique is unique as it decouples the heterogeneity of individual cells in our population and further refines the viscoelastic solution space.

Enabling silicon-on-silicon photonics with pedestalled Mie resonators.

High-refractive-index Mie resonators are regarded as promising building blocks for low-loss all-dielectric nanophotonic applications. To avoid the otherwise excessive damping and loss of symmetry such devices typically need to be implemented over a low-index substrate, which hampers their integration in many practical applications. In this paper we propose a new photonic structure consisting of silicon-on-silicon spheroidal-like resonators, each one supported by a slim silicon pedestal that makes the micro-cavities stand optically separated from the substrate while providing both mechanical stability and electrical contact with the substrate. These structures are produced in high-quality monocrystalline Si and their size and arrangement can be precisely controlled through standard lithography. We demonstrate that such structures present an optical performance similar to the one achieved with low-index substrates, opening new avenues for developing novel hybrid photonic/electronic devices.

Study of the pedestal process for reducing sidewall scattering in photonic waveguides.

In this work we investigate the principles of an alternative method for defining sidewall in optical waveguides fabricated using planar technology. The efficiency of this method is demonstrated through simulations and experimental results regarding propagation losses of a solid core ARROW waveguide fabricated on silicon substrate. It is well known that waveguides fabricated using sidewalls etched via Reactive Ion Etching (RIE) can present high sidewall roughness, especially if metallic hard-masks are used. This is largely responsible for the undesirable losses observed in these waveguides. The basic strategy of the proposed method is to do the etching step, in the fabrication of the waveguides, before the deposition of the core, so as to have the lower cladding layer and part of the silicon substrate etched away. Only after this, is the core of the waveguide deposited. This results in a waveguide sustained by a silicon pedestal. With this process, losses as low as 0.45 dB cm-1 for multimode and 0.84 dB cm-1 for single mode waveguides are obtained. The numerical simulations demonstrate that roughness in sidewalls implicates in propagation losses which are at least five times larger that those in the bulk of the material, thus corroborating the idea behind the proposed method.

Silica-based microcavity fabricated by wet etching**Project supported by the Postdoctoral Science Foundation of China (Grant No. 2015M582041) and the Special Project on the Integration of Industry, Education and Research of Aviation Industry Corporation of China.

Silica whispering gallery mode (WGM) microcavities were fabricated by the buffered oxide etcher and potassium hydroxide wet etching technique without any subsequent chemical or laser treatments. The silicon pedestal underneath was an octagonal pyramid, thus providing a pointed connection area with the top silica microdisk while weakly influencing the resonance modes. The sidewalls of our microdisks were wedge shaped, which was believed to be an advantage for the mode confinement. Efficient coupling from and to the 60 μm diameter microdisk structure was achieved using tapered optical fibres, exhibiting a quality factor of 1.5 near a wavelength of 1550 nm. Many resonance modes were observed, and double transverse electric modes were identified by theoretical calculations. The quality factor of the microdisks was also analysed to deduce the cavity roughness. The wet etching technique provides a more convenient avenue to fabricate WGM microdisks than conventional fabrication methods.

Dynamic mechanical measurement of the viscoelasticity of single adherent cells

Many recent studies on the viscoelasticity of individual cells link mechanics with cellular function and health. Here, we introduce a measurement of the viscoelastic properties of individual human colon cancer cells (HT-29) using silicon pedestal microelectromechanical systems (MEMS) resonant sensors. We demonstrate that the viscoelastic properties of single adherent cells can be extracted by measuring a difference in vibrational amplitude of our resonant sensor platform. The magnitude of vibration of the pedestal sensor is measured using a laser Doppler vibrometer (LDV). A change in amplitude of the sensor, compared with the driving amplitude (amplitude ratio), is influenced by the mechanical properties of the adhered cells. The amplitude ratio of the fixed cells was greater than the live cells, with a p-value &amp;lt;0.0001. By combining the amplitude shift with the resonant frequency shift measure, we determined the elastic modulus and viscosity values of 100 Pa and 0.0031 Pa s, respectively. Our method using the change in amplitude of resonant MEMS devices can enable the determination of a refined solution space and could improve measuring the stiffness of cells.

Novel fabrication method of microlens arrays with High OLED outcoupling efficiency

We presented a novel fabrication method of pyramidal and hemispherical polymethylmethacrylate (PMMA) microlens arrays to improve the outcoupling efficiency. Pyramidal microlens arrays were fabricated by replica molding processes using concave-pyramidal silicon molds prepared by the wet etching method. Concave-hemispherical PMMA thin film was used as a template for fabrication of the hemispherical microlens array. The concave-hemispherical PMMA template was prepared by blowing a N2 gas stream onto the thin PMMA film suspended on a silicon pedestal. A PMMA microlens arrays with hemispherical structure were fabricated by a replica molding process. The outcoupling efficiency of the hemispherical microlens array was greater than that of the pyramidal microlens array. The outcoupling efficiency of hemispherical microlens arrays with a higher contact angle was larger than that of those with lower contact angle. This indicates that, for the hemispherical microlens with larger contact angle, more light can be extracted from the OLEDs due to the decrease in the incident angle of the light at the interface between an air and a hemispherical microlens arrays. After attaching a hemispherical microlens array with contact angle of 50.4° onto the OLEDs, the luminance was enhanced by approximately 117%.

Air-clad silicon pedestal structures for broadband mid-infrared microphotonics

Toward mid-infrared (mid-IR) silicon microphotonic circuits, we demonstrate broadband on-chip silicon structures, such as: (i) straight and bent waveguides and (ii) beam splitters, utilizing an air-clad pedestal configuration which eliminates the need for typical mid-IR-lossy oxide cladding. We illustrate a sophisticated fabrication process that can create high-quality pedestal structures in crystalline silicon, while preserving its mid-IR transparency. A fundamental waveguide mode is observed between λ=2.5 μm and λ=3.7 μm, and an optical loss of 2.7 dB/cm is obtained at λ=3.7 μm. Our pedestal silicon structures show 50:50 mid-IR power splitting enabling the further development of mid-IR silicon microphotonics.

Chip-scale Mid-Infrared chemical sensors using air-clad pedestal silicon waveguides

Towards a future lab-on-a-chip spectrometer, we demonstrate a compact chip-scale air-clad silicon pedestal waveguide as a Mid-Infrared (Mid-IR) sensor capable of in situ monitoring of organic solvents. The sensor is a planar crystalline silicon waveguide, which is highly transparent, between λ = 1.3 and 6.5 μm, so that its operational spectral range covers most characteristic chemical absorption bands due to bonds such as C-H, N-H, O-H, C-C, N-O, C=O, and C≡N, as opposed to conventional UV, Vis, Near-IR sensors, which use weaker overtones of these fundamental bands. To extend light transmission beyond λ = 3.7 μm, a spectral region where a typical silicon dioxide under-clad is absorbing, we fabricate a unique air-clad silicon pedestal waveguide. The sensing mechanism of our Mid-IR waveguide sensor is based on evanescent wave absorption by functional groups of the surrounding chemical molecules, which selectively absorb specific wavelengths in the mid-IR, depending on the nature of their chemical bonds. From a measurement of the waveguide mode intensities, we demonstrate in situ identification of chemical compositions and concentrations of organic solvents. For instance, we show that when testing at λ = 3.55 μm, the Mid-IR sensor can distinguish hexane from the rest of the tested analytes (methanol, toluene, carbon tetrachloride, ethanol and acetone), since hexane has a strong absorption from the aliphatic C-H stretch at λ = 3.55 μm. Analogously, applying the same technique at λ = 3.3 μm, the Mid-IR sensor is able to determine the concentration of toluene dissolved in carbon tetrachloride, because toluene has a strong absorption at λ = 3.3 μm from the aromatic C-H stretch. With our demonstration of an air-clad silicon pedestal waveguide sensor, we move closer towards the ultimate goal of an ultra-compact portable spectrometer-on-a-chip.

Multitip atomic force microscope lithography system for high throughput nanopatterning

An atomic force microscope (AFM) system with multiple parallel lithography probes of equal heights on a single cantilever was created in order to improve the throughput of AFM lithography. The multitip probe was fabricated by electron-beam (e-beam) lithography and a dry silicon etching process. Several carbon islands were made on a single cantilever in a straight line by e-beam lithography and were used as an etch mask, whereas the silicon pedestal structure of the multitip probe was fabricated by reactive ion etching (RIE). Finally the carbon islands were sharpened by a RIE process using oxygen gas. The multitip probe was successfully applied to form multidot pattern arrays on a negative resist film coated on silicon by low electric field induced AFM lithography. A pedestal nanopillar structure was utilized as a convenient support feature that enabled better control of multiple nanotip arrays for AFM writing. The authors fabricated such a nanopedestal array with extremely sharp nanoneedle tips.

Far-field observation of the radial profile of visible whispering-gallery modes in a single microdisk based on Si-nanocrystal/SiO2 superlattices

The radial profile of visible whispering gallery modes (WGM) from a single microdisk based on silicon nanocrystals (Si-nc) and SiO2 superlattices was observed. Using thermal evaporation, the active layer, which consists of 30 pairs of ∼3nm thick Si-ncs and ∼4nm thick SiO2 layer, was fabricated on Si substrate. Si-ncs with diameters of 3nm, which are well defined by SiOx layer thickness, were formed by high temperature annealing at 1100°C for 60min under N2 environment. After standard photolithography and dry etching procedure, a microdisk with 8.8μm diameter on a silicon pedestal was successfully obtained. We calculated the expected radial profiles of the WGMs by solving the Maxwell equations using appropriate boundary conditions. Comparison with finite-difference time-domain (FDTD) simulation depicts similar radial profiles of the WGMs. Using a confocal microphotoluminescence setup, light emission from the top of a single disk was analyzed depending on the detection position. Thanks to the birefringence of nature of Si-nc/SiO2 superlattices, well-isolated sharp TE mode WGMs could be detected from the top, without using polarizers. Irrelative to detection position, broad Si-nc background luminescence is consistently found. And as the detection point was moved from the center to the outside of the disk, WGMs fields were strongly decreased especially for the detection at the outside. Taking experimental circumstances into account, the radial profile of WGMs field was estimated and was then compared to the experimental WGM profile. The expectation was consistent with experimental results confirming the confinement of WGMs fields within a disk. Although FDTD simulation reveals that the geometrical Q factor can reach &amp;gt;105, the maximum Q factor we observed was 2.5×103. This implies that the scattering and absorption losses must be suppressed in order to enhance the microdisk performance. Simulations show significant WGM field at the top/bottom surface of the disk and visible imperfections of the disk surface was confirmed by scanning electron microscopy, and are expected to produce considerable surface scattering loss limiting the overall disk cavity Q factor. However, such nondirectional surface scattering allowed us to successfully detect WGMs from the top of the disk.

Silicon Etching in Rapid Thermal Chemical Vapor Deposition of Tisi2

AbstractDownscaling of microelectronics devices into the deep submicron regime requires ultrashallow junctions with reliable, low-resistivity contacts. The conventional self-aligned TiSi2 technology exhibits a serious limitation in forming contacts to ultra-shallow junctions due to silicon substrate consumption. Selective chemical vapor deposition of TiSi2 is being investigated because of its potential for overcoming this difficulty. In this process Si and Ti are supplied from the gas phase. The standard source gas for Ti has been TiCl4 while several gases including SiH4, Si2H6 and SiH2Cl2 are available for Si. The reports on this process indicate that optimized process conditions can deliver TiSi2 films without substrate consumption. Although this promise is significant, the deposition has a complicated chemistry involving processes such as silicon etching, silicon consumption or silicon pedestal deposition taking place along with TiSi2 deposition. Although, suppression of Si-substrate etching by excess H2 has been reported previously, a broad quantitative analysis has been lacking up until this reporting. In this work, we have examined silicon etching trends as a function of temperature for different H2:TiCl4 flow ratios using thermodynamic equilibrium calculations. We have also performed experiments in a lamp heated rapid thermal chemical vapor deposition reactor to study substrate etching over the temperature range of 600°C to 800°C and for H2 flows from 0 to 1000 sccm. A silicon conversion efficiency is defined as a measure of the amount of Si converted to TiSi2 relative to total Si used from the substrate and it is evaluated via both thermodynamic calculations and experiments with good agreement between the two. Our calculations suggest that at high temperatures, etching occurs mainly via formation of SiCl2. Addition of H2 into the reaction chemistry encourages formation of HCl reducing the amount of Cl available for SiCl2 formation responsible for substrate etching. Our results show that by optimizing the H2 flow rate and the process temperature silicon substrate etching can be effectively suppressed.

Self-aligned silicon field emission cathode arrays formed by selective, lateral thermal oxidation of silicon

This article reports on the fabrication of single crystal silicon, field emission tip arrays. The tips are formed by exposing a ‘‘capped’’ silicon pedestal to a lateral high temperature thermal oxidation. Tips formed using this process have uniform height and profile, and have small radii of curvature—typically less than 20 nm. We have fabricated tip arrays with up to 2500 elements and with the tip-to-tip spacing ranging from 1.0 to 5.0 μm. The thermal SiO2 has a nominal thickness of 600 nm and serves as insulation between the extraction grid and the substrate. The extraction grid is a self-aligned, sputter deposited Ti0.1W0.9 film. Extraction grid apertures range from 900 nm to 1.7 μm. The process sequence is flexible allowing the fabrication of tips with different heights. The vertical position of the tip apex with respect to the extraction grid aperture, therefore, is a controllable parameter. The preliminary emission measurements are presented here for arrays containing 400 elements. All testing is done in ultrahigh vacuum (typically less than 5.0×10−10 Torr). The areas of the arrays fabricated range from 40 μm×40 μm to 100 μm×100 μm. The tip heights range from 1.0 to 1.4 μm above the substrate silicon. The maximum current produced from a 400 element array is only 0.45 μA at 140 Vdc. The low array current indicates that only a small number of tips are active at a given voltage.

Selective RTLPCVD of TiSi 2 without substrate consumption

Among the various materials likely to be used as substitutes for polysilicon, TiSi 2 appears to offer a greater number of advantages compared to other silicides used in VLSI devices. It has, indeed, the lowest resistivity (15μω· cm) and can be selectively deposited through chemical vapor deposition techniques. Whatever the starting gaseous phase, TiCl 4-H 2 or TiCl 4-SiH 4-H 2, selectivity never fails even in the case of submicronic lines. The problem, however, originates from an undesirable etching of the silicon substrate even through the use of the TiCl 4-SiH 4-H 2 gaseous phase. Different reaction mechanisms have been recently proposed by J.L. Regolini, D. Bensahel, G. Bomchil and J. Mercier [Appl. Surf. Sci. 38 (1989) 408], which involve: (i) silicon substrate etching by chlorinated species, (ii) silicon substrate consumption to form TiSi 2, (iii) TiSi 2 deposition from the gaseous species, (iv) silicon deposition from SiH 4 cracking, and which show that TiSi 2 selectively deposited layers can be achieved at 800°C without any silicon substrate consumption through a careful control of the various gas flow ratios. In particular, they pointed out that the TiCl 4 thermal diffusivity is about one order of magnitude lower than the SiH 4. Hydrogen is thus used in excess allowing a faster thermallization of the gas mixture [J. Mercier, J.L. Regolini and D. Bensahel, J. Electron. Mater. 19-3 (1990) 253]. As far as we are concerned, TiSi 2 selectively deposited films are obtained on 4 inch silicon wafer from the TiCl 4-SiH 4-H 2 gaseous phase by using a RTLPCVD reactor where gas mixture is injected all around the substrate. Thickness measurements show great differences between the edge and the center of the wafer. These differences can be interpreted as variations in the gas mixture composition: TiCl 4, a heavy molecule, is too slow to reach the wafer center, and mainly reacts at the edge of the substrate leading to the formation of a thick TiSi 2 layer with silicon consumption and silicon etching. On the contrary, at the center of the substrate, the gas mixture becomes impoverished in TiCl 4, and produces a silicon pedestal under a thin TiSi 2 film. Between the center and the edge of the wafer, the gas mixture composition achieves the right balance, allowing for TiSi 2 to deposit with neither silicon etching and consumption, or silicon pedestal deposition. Regarding our reactor, the solution allowing for deposition of TiSi 2 films with a proper thickness homogeneity on a 4 inch diameter silicon substrate should be obtained through a special geometry of gas mixture injectors. Mention is made of recent results achieved through different gas introduction systems.