Design and Fabrication by Thermal Imprint Lithography and Mechanical Characterization of a Ring-Based PDMS Soft Probe fo...Published: 05 March 2019 by MDPI in Polymers
In this paper, the design, fabrication and mechanical characterization of a novel polydimethylsiloxane (PDMS) soft probe for delivering and sensing forces in biological systems is proposed. On the basis of preliminary finite element (FEM) analysis, the design takes advantage of a suitable core geometry, characterized by a variable spring-like ring. The compliance of probes can be finely set in a wide range to measure forces in the micronewton to nanonewton range. In particular, this is accomplished by properly resizing the ring geometry and/or exploiting the mixing ratio-based elastic properties of PDMS. Fabrication by the thermal imprint lithography method allows fast and accurate tuning of ring sizes and tailoring of the contact section to their targets. By only varying geometrical parameters, the stiffness ranges from 1080 mNm-1 to 50 mNm-1, but by changing the base-curing agent proportion of the elastomer from 10:1 to 30:1, the stiffness drops to 37 mNm-1. With these compliances, the proposed device will provide a new experimental tool for investigating force-dependent biological functions in sensory systems.
This work aims at developing a new and unconventional Sacrificial Stencil Mask (SSM) technology by exploiting Two-Photon Polymerization (2PP) in an IP-L/SU-8 double layer resist system. The process consists of the sequential deposition of two different resists, such as SU-8 and IPL, onto the same glass substrate, followed by 2PP lithography and distinct development processes. The 2PP writing process was used to polymerize structures inside the top and bottom resist layers to form, in one single exposure process, both SSM and a permanent polymeric structure, in our case a plain pedestal. The top IPL resist was developed using Isopropyl Alcohol (IPA), which does not affect either exposed or un-exposed SU-8 regions. In this way, structures written into the bottom layer remained latent, while exposed areas of the top IPL resist, including the stencil mask, were developed. The realization of 3D stencil masks, designed to be anchored inside the un-exposed bottom layer, was combined with metal evaporation to demonstrate the deposition of a plain metal line through the stencil mask. The final development of the bottom layer led to the lift off of the sacrificial stencil mask, uncovering the underlying, permanent polymer-metal structure. The combination of sacrificial polymer structures with permanent ones opens new possibilities in 3D MEMS design, enabling the integration of distributed electronic transducers in flexible polymeric structures.
Background: Flow stimuli in the natural world are varied and contain a wide variety of directional information. Nature has developed morphological polarity and bidirectional arrangements for flow sensing to filter the incoming stimuli. Inspired by the neuromasts found in the lateral line of fish, we present a novel flow sensor design based on two curved cantilevers with bending orientation antiparallel to each other. Antiparallel cantilever pairs were designed, fabricated and compared to a single cantilever based hair sensor in terms of sensitivity to temperature changes and their response to changes in relative air flow direction. Results: In bidirectional air flow, antiparallel cantilever pairs exhibit an axially symmetrical sensitivity between 40 μV/(m s-1) for the lower air flow velocity range (between ±10-20 m s-1) and 80 μV/(m s-1) for a higher air flow velocity range (between ±20-32 m s-1). The antiparallel cantilever design improves directional sensitivity and provides a sinusoidal response to flow angle. In forward flow, the single sensor reaches its saturation limitation, flattening at 67% of the ideal sinusoidal curve which is earlier than the antiparallel cantilevers at 75%. The antiparallel artificial hair sensor better compensates for temperature changes than the single sensor. Conclusion: This work demonstrated the successive improvement of the bidirectional sensitivity, that is, improved temperature compensation, decreased noise generation and symmetrical response behaviour. In the antiparallel configuration, one of the two cantilevers always extends out into the free stream flow, remaining sensitive to directional flow and preserving a sensitivity to further flow stimuli.
In this work we report the design, the fabrication and the characterization of an innovative soft tactile micro-actuator, also called TAXEL (TActile piXEL), which is developed to be integrated in a portable tactile display for providing text content and graphical information to visually impaired people through the sense of touch. It exploits a thermo-active approach, by taking inspiration from common thermometers: the actuator is activated by the thermal deformation of an active material, namely the metallic alloy Galinstan©, determined by heating the alloy through an underlying metallic resistor designed to work as a heater. The microfabrication of TAXELs is achieved in several steps consisting in heater fabrication, in SU8 micro chambers fabrication, in the deposition of Galinstan® inside and sealing by a PDMS membrane. Measurements of the TAXEL deformation have been accomplished by measuring the displacement of the PDMS sealing membrane, which is promoted by the expansion of the heated Galinstan® drop. These measurements have been achieved by using the Laser Doppler Vibrometer in the “topography mode “and revealed a total displacement of 50 μm when a tension of 2.4 V is applied at taxel terminals and, according to the Joule's law, a power converted from electrical energy to thermal energy of 7,2 W.
Including Liquid Metal into Porous Elastomeric Films for Flexible and Enzyme-Free Glucose Fuel Cells: A Preliminary Eval...Published: 22 November 2018 by MDPI in Journal of Low Power Electronics and Applications
This communication introduces a new flexible elastomeric composite film, which can directly convert the chemical energy of glucose into electricity. The fabrication process is simple, and no specific equipment is required. Notably, the liquid metal Galinstan is exploited with a two-fold objective: (i) Galinstan particles are mixed with polydimethylsiloxane to obtain a highly conductive porous thick film scaffold; (ii) the presence of Galinstan in the composite film enables the direct growth of highly catalytic gold structures. As a first proof of concept, we demonstrate that when immersed in a 20 mM glucose solution, a 5 mm-long, 5 mm-wide and 2 mm-thick sample can generate a volumetric power density up to 3.6 mW·cm−3 at 7 mA·cm−3 and 0.51 V without using any enzymes.
Microfabrication of pH-responsive 3D hydrogel structures via two-photon polymerization of high-molecular-weight poly(eth...Published: 01 September 2018 by Elsevier BV in Sensors and Actuators B: Chemical
3D pH-responsive microstructures by two-photon lithography (2 PL) in poly(ethylene glycol) diacrylates (PEG-DAs) hydrogels are particularly suitable for biosensing as structural and functional components. So far, 2 PL patterning of hydrogels have been successfully achieved only for low molecular-weight (≤ 700 Da MMw) PEG-DAs, which is unfortunately not mechanically compliant with single cell and tissues stiffness. We report an optimised protocol to setup a 2 PL fabrication of high MMw (10 kDa) PEG-DA-based formulations, suitable for pH sensing in soft biological tissues. Two different shapes (pyramids and domes) were obtained and tested for mechanical characterization and pH responsiveness at the microscale. Fast pH-induced swelling (<15 min) in microstructures allows for envisioning high MMw PEG-DA-based micro and nanostructures via 2 PL as a tunable pH responsive tool for biosensing applications in cell and tissue.
Determination of absorption and structural properties of cellulose-based hydrogel via ultrasonic pulse-echo time-of-flig...Published: 02 June 2018 by Springer Nature in Cellulose
Biodegradable cellulose-based hydrogels are attracting increasing interest in the academic and industrial fields thanks to their high swelling capacity and reproducibility, which allow many novel applications. These properties are enabled by amplification effect of their sensitiveness on a molecular level, translated into macroscopic effects such as a change in swelling degree. The monitoring of the hydrogel state is a crucial step for understanding the response of the hydrogel to external environment. Accordingly, the major aim of this study is to exploit ultrasound to characterize the swelling and degradation of cellulose-based hydrogel with different blend of molecular weight and degree of substitutions. The ultrasonic sensor used herein relies on the determination of a Pulse-echo time of flight. This technique provides dimensional information, thanks to its capability of monitoring the thickness of the swollen/unswollen hydrogel during sorption mechanism. Furthermore, by combining these data with a rheological characterization, the degree of crosslink and its modification during multiple swelling/deswelling cycles (due to ion strength variation) has been monitored. This technique could be an effective, alternative, fast and non-destructive method for real-time hydrogel characterization.
Tumor Cell Dynamics: Microenvironmental Stiffness of 3D Polymeric Structures to Study Invasive Rates of Cancer Cells (Ad...Published: 22 November 2017 by Wiley in Advanced Healthcare Materials
Three-dimensional cage-like microscaffolds with tailored Young's modulus are used to induce different invasive behaviours of cancer cells according to structural stiffness by Enrico Domenico Lemma, Ferruccio Pisanello, and co-workers in article number 1700888. Tumor cells invasion is boosted by softer architectures and by the presence of stiffness “weak spots” within overall rigid scaffolds, enabling more detailed analysis on mechanical interactions between tumor cells and the extracellular matrix.
Cells are highly dynamic elements, continuously interacting with the extracellular environment. Mechanical forces sensed and applied by cells are responsible for cellular adhesion, motility, and deformation, and are heavily involved in determining cancer spreading and metastasis formation. Cell/extracellular matrix interactions are commonly analyzed with the use of hydrogels and 3D microfabricated scaffolds. However, currently available techniques have a limited control over the stiffness of microscaffolds and do not allow for separating environmental properties from biological processes in driving cell mechanical behavior, including nuclear deformability and cell invasiveness. Herein, a new approach is presented to study tumor cell invasiveness by exploiting an innovative class of polymeric scaffolds based on two-photon lithography to control the stiffness of deterministic microenvironments in 3D. This is obtained by fine-tuning of the laser power during the lithography, thus locally modifying both structural and mechanical properties in the same fabrication process. Cage-like structures and cylindric stent-like microscaffolds are fabricated with different Young's modulus and stiffness gradients, allowing obtaining new insights on the mechanical interplay between tumor cells and the surrounding environments. In particular, cell invasion is mostly driven by softer architectures, and the introduction of 3D stiffness “weak spots” is shown to boost the rate at which cancer cells invade the scaffolds. The possibility to modulate structural compliance also allowed estimating the force distribution exerted by a single cell on the scaffold, revealing that both pushing and pulling forces are involved in the cell–structure interaction. Overall, exploiting this method to obtain a wide range of 3D architectures with locally engineered stiffness can pave the way for unique applications to study tumor cell dynamics.
The response to different force load ranges and actuation at low energies is of considerable interest for applications of compliant and flexible devices undergoing large deformations. We present a review of technological platforms based on nitride materials (aluminum nitride and silicon nitride) for the microfabrication of a class of flexible micro-electro-mechanical systems. The approach exploits the material stress differences among the constituent layers of nitride-based (AlN/Mo, SixNy/Si and AlN/polyimide) mechanical elements in order to create microstructures, such as upwardly-bent cantilever beams and bowed circular membranes. Piezoresistive properties of nichrome strain gauges and direct piezoelectric properties of aluminum nitride can be exploited for mechanical strain/stress detection. Applications in flow and tactile sensing for robotics are described.
Parylene C-based 2D STC fluidics, where pure water and water-based solutions can flow strictly confined by differences in surface energy. In the present study a new, facile and cheap method to obtain a 2D surface-tension-confined fluidic system on substrates conformally coated by parylene C is presented. It is based on the use of poly(dimethylsiloxane) (PDMS)-based soft masks obtained by molds produced by a 3D-printer. These masks, applied alternatively onto a parylene C-coated silicon substrate together with appropriate plasma treatments permit to obtain a superhydrophilic pattern on a superhydrophobic background, in which pure water, water-based solutions and polar solvents can flow. The flow of these liquids is strictly confined and is driven into the superhydrophilic pattern only by the differences in surface energy between it and the background, without any confinement effect provided by walls or capillary-driven channel, that are completely missing. According to the proposed fabrication method, all the desired fluidic systems can be fabricated in an easy and cheap way. The developed method for 2D surface-tension-confined fluidics on parylene C permits to obtain a highly versatile platform which can be applied on all desired substrates, without the need to etch the polymer surface in order to obtain channel walls, paving the way to employ green, easily available and cheaper substrates, such as cellulose for paper-based fluidics applications, improving, at the same time, the biopolymer surface properties.
To enhance today's artificial flow sensing capabilities in aerial and underwater robotics, future robots could be equipped with a large number of miniaturized sensors distributed over the surface to provide high resolution measurement of the surrounding fluid flow. In this work we show a linear array of closely separated bio-inspired micro-electro-mechanical flow sensors whose sensing mechanism is based on a piezoresistive strain-gauge along a stress-driven cantilever beam, mimicking the biological superficial neuromasts found in the lateral line organ of fishes. Aiming to improve state-of-the-art flow sensing capability in autonomously flying and swimming robots, our artificial lateral line system was designed and developed to feature multi-parameter freestream flow measurements which provide information about (1) local flow velocities as measured by the signal amplitudes from the individual cantilevers as well as (2) propagation velocity, (3) linear forward/backward direction along the cantilever beam orientation and (4) periodicity of pulses or pulse trains determined by cross-correlating sensor signals. A real-time capable cross-correlation procedure was developed which makes it possible to extract freestream flow direction and velocity information from flow fluctuations. The computed flow velocities deviate from a commercial system by 0.09 m s−1 at 0.5 m s−1 and 0.15 m s−1 at 1.0 m s−1 flow velocity for a sampling rate of 240 Hz and a sensor distance of 38 mm. Although experiments were performed in air, the presented flow sensing system can be applied to underwater vehicles as well, once the sensors are embedded in a waterproof micro-electro-mechanical systems package.
This paper presents the design and analysis of an innovative PDMS probe for investigating mechanotransduction and force generation by mechanosensory cells and organs. The low Young’s modulus of PDMS together with a novel ring-spring probe design allow an easily tunable stiffness of 1-100 mN/m or larger. The probe design limits the fluid drag and allows settling times of 200 μs or less depending on the compliance of the probe. Moreover, the custom-tailored tip geometry of this device allows good coupling to a variety of sensory structures. The probe can be used as a force sensor or a force actuator using force-displacement calibration curves computed by the Finite-Element Method (FEM). Finally, a FEM modal analysis for computing resonant frequencies confirmed that probe resonances are in the high kHz range, allowing use across the frequency range of most biological sensors.
Mechanical properties tunability of three-dimensional polymeric structures in two-photon lithographyPublished: 01 January 2016 by Institute of Electrical and Electronics Engineers (IEEE) in IEEE Transactions On Nanotechnology
Two-photon (2P) lithography shows great potential for the fabrication of three-dimensional (3-D) micro- and nanomechanical elements, for applications ranging from microelectromechanical systems to tissue engineering, by virtue of its high resolution (<;100 nm) and biocompatibility of the photosensitive resists. However, there is a considerable lack of quantitative data on mechanical properties of materials for 2P lithography and of structures obtained through this technique. In this paper, we combined static and dynamic mechanical analysis on purpose-designed microstructures (microbending of pillar-like structures and picometer-sensitive laser Doppler vibrometry of drum-like structures) to viably and nondestructively estimate Young's modulus, Poisson's ratio, and density of materials for 2P lithography. This allowed us to analyze several polymeric photoresists, including acrylates and epoxy-based materials. The experiments reveal that the 2P exposure power is a key parameter to define the stiffness of the realized structures, with hyperelasticity clearly observable for high-power polymerization. In the linear elastic regime, some of the investigated materials are characterized by a quasi-linear dependence of Young's modulus on the used exposure power, a yet unknown behavior that adds a new degree of freedom to engineer complex 3-D micro- and nanomechanical elements.
Two-Photon Polymerization Lithography and Laser Doppler Vibrometry of a SU-8-Based Suspended Microchannel ResonatorPublished: 01 August 2015 by Institute of Electrical and Electronics Engineers (IEEE) in Journal of Microelectromechanical Systems
We present the optical realization and characterization of complex suspended microchannel resonator (SMR) for biomechanical sensing applications. We exploit the flexibility of two-photon direct laser writing to optimize a highly versatile fabrication strategy based on a shell-writing procedure with the aim to reduce fabrication time of big inlet/outlet sections compatible with most microfluidic systems for lab-on-chip. Compared with standard microfabrication techniques, requiring several technological steps to obtain suspended hollow structures, this method allows to fabricate complex SMR sensors in only one fabrication step by virtue of its intrinsically 3-D nature. The realized resonant structure was characterized by laser doppler vibrometry, showing good agreement with finite-element methods simulations and an experimental quality factor of the fundamental mode of ~60.
We present an innovative design for a flexible probe to study mechanisms of biological force sensing and force generation in the piconewton to micronewton range. Made of polydimethylsiloxane (PDMS) and employing a novel ring-spring section with adjustable size, the device works both as a force sensor and force actuator by precise calibration of its tunable stiffness and optical measurement of ring deformation. In addition, the tip geometry of the probe can be properly shaped to fit the anatomical profile of the sensory receptor of interest and to reproduce the in vivo stimulation. Finally, use of Finite Element Method (FEM) modal analysis confirms that the resonance frequencies of probes are outside the frequency range of interest for many sensory systems.
Wind and fluid flow represent some of the most attractive renewable energy sources for addressing climate change, pollution and energy insecurity issues. Wind harvesting technologies, in particular, are the fastest-growing electric technologies in the world because of their efficiency and lower environmental impact with respect to traditional energy sources, despite exhibiting major drawbacks such as big infrastructure investment and environment invasiveness, producing high levels of noise and requiring the need of large areas for their installation. A single wind turbine can produce megawatts of power and they have the potential to cover the entire world’s energy demand in the next few years, but they have a technological limit in a cut-in wind speed of about 4 m s−1, below which the turbines do not operate, excluding them as an energy source for slow air flows. At the same time most of the wind available in the environment is below the turbines’ threshold speed. In this paper we show that small flags, made by piezoelectric thin film on flexible polymers and whose shape resembles the dry leaves of trees, can efficiently act as harvesters of energy from wind at extremely low speed, such as from a gentle blow or breath. We demonstrate that piezoelectricity on flexible polymers is achievable by depositing a thin film of piezoelectric aluminium nitride (AlN), sandwiched between metal electrodes with columnar grains coherent through the polycrystalline layers, on Kapton substrates. The prototype flags have a natural curling due to the release of the residual stress of the layers. While the curling is essential for the activation of the maximum flag oscillation, this system is so elastic and light that oscillations start at a cut-in flow speed of 0.4 m s−1, the lowest reported so far, with an open circuit peak to peak voltage of 40 mV. The voltage increases to 1.2 V when the flag is flattened and parallel to the fluid flow lines, with a generated power of 0.257 mW cm−3.
Parylene C is a polymer well-known for its inertness and chemical resistance, thus ideal for covering and sealing 3D substrates and structures by conformal coating. In the present study, the Parylene C surface is modified by functionalization with pH-responsive poly(methacrylic acid) microgels either over the whole surface, or in a pattern through a poly(dimethylsiloxane) stamp. The surface functionalization consists of two phases: first, an oxygen plasma treatment is used to make the surface superhydrophilic, inducing the formation of polar functional groups and surface topography modifications; then, the plasma-treated samples are functionalized by drop casting a solution of pH-responsive microgels, or in a pattern via microcontact printing of the same solution. While both techniques, namely, drop casting and microcontact printing, are easy to use, fast, and cheap, the microcontact printing was found to provide a more homogeneous functionalization and to be applicable to any shape of substrate. The functionalization effectiveness was tested by the repeated uptake and release of a fluorescent labeled monoclonal CD4 antibody at different pH values, thus suggesting a new sensing approach.
The trend of biomimetic underwater robots has emerged as a search for an alternative to traditional propeller-driven underwater vehicles. The drive of this trend, as in any other areas of bioinspired and biomimetic robotics, is the belief that exploiting solutions that evolution has already optimized leads to more advanced technologies and devices. In underwater robotics, bioinspired design is expected to offer more energy-efficient, highly maneuverable, agile, robust, and stable underwater robots. The 30,000 fish species have inspired roboticists to mimic tuna , rays , boxfish , eels , and others. The development of the first commercialized fish robot Ghostswimmer by Boston Engineering and the development of fish robots for field trials with specific applications in mind (http://www.roboshoal. com) mark a new degree of maturity of this engineering discipline after decades of laboratory trials.
Human perception of the environment relies on senses for transducing light, inertial, mechanical, and biochemical stimuli in electrical signals through a great variety of receptors: photoreceptors for sight, mechanoreceptors for touch and hearing, and chemoreceptors for taste and smell, in addition to thermoreceptors for temperature and nocioreceptors for pain/damage sensing. Artificial and biomimetic approaches to mimic these sensorial systems for prosthetic and robotic applications require new-concept and frontier technologies.
Bio-inspiration from natural structures and systems can be used to design innovative engineered solutions. Here natural sensor architectures inspire the design of micro-electronic-mechanical systems (MEMS) for flow sensing. In this chapter, we introduce an innovative approach to artificial flow sensing based on mimicking stereocilia and their mechanical properties. This method exploits the intrinsic differences in material properties of multilayered thin films such as thermal expansion properties, crystalline lattice order and interatomic distances. If a cantilever beam is multilayered, these properties create a stress gradient along the cantilever cross section, allowing an upwards bending, defined as ‘stress-driven geometry’. When inserted in a superficial fluid stream, the cantilever beam is deformed by the flow and acts as a fluid flow velocity sensor. It is shown that a Parylene post-processing conformal coating not only waterproofs the device, but also sets the flexural stiffness of the beam, thus tuning the dynamic range for flow measurements optimisation.
In this work, the theoretical analysis of an innovative polymeric strain sensor is proposed. In this device, composed of chalcogenide glass (As2S3) stripes, periodically repeated on a substrate of polydimethylsiloxane (PDMS), an applied strain stretches the soft substrate, altering the periodic arrangement of stripes and introducing a variation of the resonant peak of the optical response of the structure. The amount of strain is measured by analyzing the entity of the resonance displacement or intensity in the reflection spectra. In order to realize strain-sensing application, two regimes have been investigated, in dependence of the aperture between two adjacent stripes: large and small aperture. Numerical computations reveal that, for large aperture, this device is characterized by an excellent sensitivity equal to 1.4 nm/nm and by a linear intensity reflection-deformation calibration curve at a fixed wavelength equal to 1.5 μm. Similarly, the device with smaller aperture exhibits a resonance displacement-deformation calibration curve with a sensitivity of approximately 1.33 nm/nm and 1 nm/nm for first and second resonance, respectively.
Parylene conformal coating encapsulation as a method for advanced tuning of mechanical properties of an artificial hair ...Published: 01 January 2013 by Royal Society of Chemistry (RSC) in Soft Matter
A soft Parylene conformal coating encapsulation is demonstrated to be an efficient method to control the mechanical and sensory properties of a bioinspired artificial hair cell, tuning the mechanoreceptive responsivity from a sub-linear to a super-linear behaviour such as hair cells adapt to a natural environment.
In this work, we report on the fabrication and characterization of stress-driven aluminum nitride (AlN) cantilevers to be applied as flow sensor for fish lateral line system. The fabricated structures exploit a multilayered cantilever AlN/molybdenum (Mo) and a Nichrome 80/20 alloy as piezoresistor. Cantilever arrays are realized by using conventional micromachining techniques involving optical lithography and etching processes. The fabrication of the piezoresistive cantilevers is reported and the operation of the cantilever as flow sensor has been investigated by electrical measurement under nitrogen flowing condition showing a sensitivity to directionality and to low value applied forces.
Directional enhancement of refractive index and tunable wettability of polymeric coatings due to preferential dispersion...Published: 01 May 2010 by Elsevier BV in Thin Solid Films
A novel epitaxial lift-off process for III-nitrides, involving selective removal of a sacrificial (Al, In)N layer in a hot nitric acid etchant, is reported. This was applied to the fabrication of 1−λ GaN planar microcavities bounded by two dielectric DBRs, starting from epitaxial GaN–(Al, In)N–GaN trilayers grown on free-standing GaN or high-quality GaN template material. An optically smooth surface was retained on the GaN (0001̄) surface exposed to the nitric acid etch, with root mean square roughness values as low as 2 nm over 8 μm×8 μm areas. Photoluminescence and reflectivity spectra were recorded from completed microcavities, and the latter showed clear dips in the region of 3.5 eV.
Processing of GaN-AlInN-GaN epitaxial trilayers into 3-dimensional microstructures, using a combination of vertical dry etching and lateral wet etching, is discussed. The AlInN layers were grown so as to have an InN mole fraction close to the value of 17% required for lattice matching with GaN. Inductively coupled plasma etching with chlorine-argon gas mixtures was used to define mesa features with near-vertical sidewalls. Refluxing aqueous solutions of nitric acid of 2 molar concentration allowed highly selective lateral etching of the AlInN interlayers exposed on the mesa sidewalls, providing a novel sacrificial layer technology for the III-nitride materials. Lateral etch rates of 0.14-0.21 μm/hr were observed for 100-nm AlInN interlayers. Two distinct applications are discussed. In one example, lateral etching of an AlInN layer was used to expose the underside of epitaxial GaN disks for fabrication of planar microcavities. Here, retention of an optically smooth GaN (0001) surface on the underside of the disks is critical. Microbridges with potential for development as sensors were also demonstrated, and the deformation of these structures provides a sensitive probe of the local strain state of the undercut GaN layer.
We describe the fabrication and optical properties of a 3λ/2 InGaN/GaN-based microcavity using “upper” and “lower” silica/zirconia mirrors. The fabrication of this structure involved selective removal of an AlInN layer following multistep thinning of a free-standing GaN substrate. Photoluminescence spectra show a narrowing of the excitonic emission from InGaN/GaN quantum wells in the microcavity, giving a cavity quality factor Q exceeding 400.
(In,Ga)N∕GaN microcavities with double dielectric mirrors fabricated by selective removal of an (Al,In)N sacrificial lay...Published: 12 March 2007 by AIP Publishing in Applied Physics Letters
Comparable microcavities with 3λ∕2 (∼240nm) active regions containing distributed (In,Ga)N quantum wells,grown on GaN substrates and bounded by two dielectricmirrors, have been fabricated by two different routes: one using laser lift-off to process structures grown on GaN-on-sapphire templates and the second using freestanding GaN substrates, which are initially processed by mechanical thinning. Both exploit the properties of an Al0.83In0.17N layer, lattice matched to the GaN substrate and spacer layers. In both cases cavity quality factors >400 are demonstrated by measurements of the cavity-filtered room-temperature excitonic emission near 410nm.
Wet etching of AlInN–GaN epitaxial heterostructures, containing AlInN layers with InN mole fractions close to 0.17 has been studied. One molar aqueous solution of the chelating amine 1,2-diaminoethane (DAE) proved to selectively etch the AlInN layers, without the need for heating above room temperature, or photo-assistance. In experiments with a (0 0 0 1)-oriented AlInN-on-GaN bilayer, the mode of removal of the AlInN layer was predominantly lateral etching, initiated from the sidewalls of pit defects in the AlInN layer. The lateral etch rate was estimated at ∼60 nm/h. The GaN buffer layer surface was roughened concurrently with etching of the AlInN, although the DAE solution has no effect on as-grown GaN (0 0 0 1) surfaces. The roughening of the GaN surface is tentatively attributed to the charge accumulation layer expected at the AlInN–GaN heterointerface. The DAE etchant also proved effective at removing buried AlInN layers from trilayer and more complex multilayer structures, leading to the prospect of epitaxial lift-off processes, and the fabrication of three-dimensional engineered microstructures. These capabilities were demonstrated by the production of suspended microdisk structures from a GaN–AlInN–GaN trilayer, using a combination of dry and wet etching.
Thinning of N-face GaN (0001 ¯) samples by inductively coupled plasma etching and chemomechanical polishingPublished: 01 March 2007 by American Vacuum Society in Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films
The processing of N-polar GaN(0001 ¯) samples has been studied, motivated by applications in which extensive back side thinning of freestanding GaN (FS-GaN) substrates is required. Experiments were conducted on FS-GaN from two commercial sources, in addition to epitaxialGaN with the N-face exposed by a laser lift-off process. The different types of samples produced equivalent results. Surface morphologies were examined over relatively large areas, using scanning electron microscopy and stylus profiling. The main focus of this study was on inductively coupled plasma(ICP)etch processes, employing Cl2∕Ar or Cl2∕BCl3Ar gas mixtures. Application of a standard etch recipe, optimized for feature etching of Ga-polar GaN (0001) surfaces, caused severe roughening of N-polar samples and confirmed the necessity for specific optimization of etch conditions for N-face material. A series of recipes with a reduced physical (sputter-based) contribution to etching allowed average surface roughness values to be consistently reduced to below 3nm. Maximum N-face etch rates of 370–390nm∕min have been obtained in recipes examined to date. These are typically faster than etch rates obtained on Ga-face samples under the same conditions and adequate for the process flows of interest. Mechanistic aspects of the ICPetch process and possible factors contributing to residual surface roughness are discussed. This study also included work on chemomechanical polishing (CMP). The optimized CMP process had stock removal rates of ∼500nm∕h on the GaN N face. This was much slower than the ICPetching but showed the important capability of recovering smooth surfaces on samples roughened in previous processing. In one example, a surface roughened by nonoptimized ICPetching was smoothed to give an average surface roughness of ∼2nm.
This paper describes processing of GaN on the on the (000$ \bar 1 $) N‐face surface, using two different high‐density plasma etch techniques, inductively coupled plasma (ICP) etch, and electron cyclotron resonance (ECR) etching. ICP experiments used several different conditions employing Cl2–Ar–BCl3 or Cl2–Ar plasmas. The resulting maximum etch rates of 370–390 nm/min are approximately twice as high as etch rates for Ga‐face (0001) GaN with the same recipes. ECR etching employed a Cl2–CH4–Ar recipe, which produced an average etch rate of 55 nm/min in a 20‐minute etch process on N‐face GaN. Both etch techniques increased the roughness of N‐face GaN, but could produce surfaces with average roughness values below 3 nm. Selection of conditions with a dominant chemical etch contribution is important to maintain smooth surfaces. The use of both ICP and ECR etching in sequence is advantageous in situations where a GaN substrate several tens of microns in thickness must be thinned from the backside, stopping the etch in a suitable marker layer. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Wavelength dispersive X-ray analysis and cathodoluminescence techniques for monitoring the chemical removal of AlInN on ...Published: 01 October 2006 by Elsevier BV in Superlattices and Microstructures
This paper shows the selective etching process of an AlInN sacrificial layer, lattice-matched to GaN, on N-face (0001̄) GaN by an aqueous solution of 1,2-diaminoethane. Using the wavelength dispersive X-ray (WDX) spectrometers on an electron probe micro-analyser, together with an optical spectrometer and silicon CCD array added to the light microscope, and sharing the same focus as the electron microscope, cathodoluminescence spectra are collected from exactly the same spot as sampled by the WDX spectrometers. This technique allows the compositional properties of the etched AlInN layer and the optical properties of the semiconductor layers underlying the sacrificial layer to be scrutinised, verifying the etching selectivity and the efficiency of the process.
Nonequilibrium phonon generation in coupled Wannier-Stark ladders from a semiconductor superlattice in a three-terminal ...Published: 15 June 2006 by AIP Publishing in Journal of Applied Physics
Quantum cascade laser-based photoacoustic spectroscopy of volatile chemicals: Application to hexamethyldisilazanePublished: 01 May 2006 by Elsevier BV in Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
The use of photoacoustic spectroscopy and mid-infrared quantum-cascade lasers (QCLs) for the detection of hexamethyldisilazane (HMDS) is reported. A detection limit of 200 parts in 10(9) is found using a Fabry-Perot QCL operated at 8.4 microm in pulsed mode and a photoacoustic cell equipped with four electret microphones. The laser multimode spectrum matches the range of the N-H bending absorption band of HMDS. Further improvements to reach lower detection limits are discussed.
AlInN alloys achieve an in‐plane lattice match to hexagonal GaN at an indium nitride mole fraction of ∼18%. Meanwhile Al0.82In0.18N displays a refractive index contrast of ∼7% with GaN at visible wavelengths. We illustrate the use of Al0.82In0.18N insertion layers to control layer thicknesses during homoepitaxial growth of GaN‐based microcavities, using in situ optical reflectometry. The structures discussed are 3λ /2 microcavities incorporating distributed InGaN quantum wells tailored for emission at ∼400 nm. As‐grown samples have been characterised by techniques including cathodoluminescence spectroscopy. In addition to their role in growth monitoring, there are several post‐growth processing steps in which Al0.82In0.18N insertion layers can assist microcavity fabrication. We focus here on a demonstration of the ∼1:5 etch rate selectivity obtainable between Al0.82In0.18N and GaN in reactive ion etching. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Form only given. Cavity solitons are similar to spatial solitons but arise in dissipative systems, which bestows them special properties. They are generated by shining short and narrow laser pulses into resonant cavities filled with nonlinear samples of large section, and driven by a cw coherent holding beam. The cavity soliton, which appears as a bright spot in the transverse intensity profile, persists after the passage of the pulse, until it is switched off by another pulse. This talk will present the recent progress in the theoretical numerical studies of cavity solitons in semiconductor microresonators, following the development of more refined models to adequately describe the complex physics of broad-area semiconductor microresonators. In particular, a microscopic model is discussed to describe the nonlinear response (via the complex susceptibility) of a multiple quantum well sample.
Three-terminal mid-IR tunable emitters based on Wannier–Stark ladder transitions in semiconductor superlatticesPublished: 01 March 2004 by IOP Publishing in Semiconductor Science and Technology
We report on the narrow‐band photoluminescence associated with Fe2+internal transitions in Fe‐doped ZnSe thin epilayers. The growth procedure exploits, for the first time, the combination of 2D — molecular beam epitaxy of thin ZnSe layers, doping with Fe, diffusion and thermal annealing.
Monte Carlo simulation of tunable mid-infrared emission from coupled Wannier–Stark ladders in semiconductor superlattice...Published: 09 June 2003 by AIP Publishing in Applied Physics Letters
We present a theoretical and experimental study on the mid-infrared electroluminescence associated with transitions between electric-field-induced conduction states, forming the Wannier–Stark ladder, in strongly coupled GaAs/AlAs superlattices. The interwell and intrawell radiative transitions in the whole range of electric fields from the moderate localization to the resonance-induced delocalization regimes have been experimentally investigated. Monte Carlo simulations show a very good agreement between experimental and theoretical electroluminescence spectra. Results show that the application of an electric field in the range from 100 to 250 kV/cm shifts the emission peak, related with interwell diagonal transitions between Stark-localized ground states of two adjacent wells, up to the limit corresponding to the merging of this electroluminescence peak with the intersubband emission between excited and ground state of the same well. The theoretical investigation indicates that interwell scattering via LO phonons is responsible for the population of the excited state of the ladder.
Widely tunable mid-infrared emission from coupled Wannier–Stark ladders in semiconductor superlatticesPublished: 01 March 2002 by Elsevier BV in Physica B: Condensed Matter
Cavity solitons appear as stationary, isolated peaks of light superimposed onto a homogeneous background field in the transverse profile of the coherent field transmitted or reflected by a non-linear resonator. These self-organized structures are theoretically predicted and simulated in a broad area multi-quantum-well vertical microresonator. We develop models suited to describe the macroscopic properties of the medium and the nonlinear interaction with the coherent field. Parametric domains and operational regimes for stable solitons are investigated along with some quantitative appreciation of their characteristics. Intrinsic stability properties of solitons are investigated by means of semi-analytical techniques and this allows to describe the destabilizing mechanisms for solitons, mutual interaction properties, their response to perturbations and some of their dynamical features.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.