Microstructured Silicon Research Articles

Controllable Si micro-structuring by temporally modulated single-shot femtosecond pulse lithography

Micro-structured silicon surface plays a significant role in the electronics industry. Crystalline and amorphous phases of silicon, which have significantly different physical properties, can be transformed into each other using femtosecond laser, and assisted chemical etching enables the easy fabrication of various micro-structures. However, efficient and controllable fabrication of micro-cylinders, micro-rings, and micro-ring grooves on silicon surfaces still remains a challenge. Here, we propose a temporally modulated single-shot femtosecond pulse lithography technology, combining laser-induced silicon modification and chemical etching. In this technology, the occurrence of recrystallization allows for the flexible manipulation of the shape of the modified area, transitioning from circular to annular shapes by adjusting the laser fluence. This corresponds to the formation of micro-cylinders and micro-rings after etching. In the case of temporally shaped pulses, with an increasing pulse delay, the micro-rings formed after etching gradually transform into micro-ring grooves, as the recrystallization area created by the first sub-pulse is reacted by the second sub-pulse. Due to the characteristics of the single shot, the technology, when combined with the flying punch method, can be used for the high-efficiency fabrication of large-area silicon surface micro-structure arrays.

Detachable artificial micro-structured silicon chip for reconfigurable acoustofluidic particle manipulation

The utilization of reconfigurable acoustofluidic manipulation facilitates diverse and complex modes of particle patterning. Traditional approaches employ a variety of advanced acoustic transducers and equipment configurations to modify the acoustic field, which is challenging for disposable, portable, and cost-effective biomedical applications. For the first time, we demonstrate the feasibility of using an artificial micro-structured waveguide silicon chip to generate a variety of unique modes of the acoustic field for reconfigurable and versatile particle manipulation. The distribution of acoustic field for eleven different designs of artificial micro-hole structures and bare surfaces was simulated and subsequently fabricated using laser-cut techniques for acoustofluidic demonstration. The impact of using Polydimethylsiloxane (PDMS) film and glass sheet as the sealing layer on the particle pattern shape and moving velocity was characterized. It was observed that by adjusting the excitation frequency and microchamber size, the modes of the particle pattern were changed, thereby demonstrating the remarkable flexibility of particle reconfiguration. The investigation of thermal effect quantification validates the possibility of long-term operation while maintaining a high level of biocompatibility. Additionally, the plug-and-play assembly mode enables the elimination of microfluidic components to prevent the occurrence of cross-contamination. Such reconfigurable acoustofluidic manipulation platforms have the potential to serve as sample treatment tools in chemical and biomedical instruments.

Accurate Prediction of Antimicrobial Susceptibility for Point-of-Care Testing of Urine in Less than 90 Minutes via iPRISM Cassettes.

The extensive and improper use of antibiotics has led to a dramatic increase in the frequency of antibiotic resistance among human pathogens, complicating infectious disease treatments. In this work, a method for rapid antimicrobial susceptibility testing (AST) is presented using microstructured silicon diffraction gratings integrated into prototype devices, which enhance bacteria-surface interactions and promote bacterial colonization. The silicon microstructures act also as optical sensors for monitoring bacterial growth upon exposure to antibiotics in a real-time and label-free manner via intensity-based phase-shift reflectometric interference spectroscopic measurements (iPRISM). Rapid AST using clinical isolates of Escherichia coli (E. coli) from urine is established and the assay is applied directly on unprocessed urine samples from urinary tract infection patients. When coupled with a machine learning algorithm trained on clinical samples, the iPRISM AST is able to predict the resistanceor susceptibility of a new clinical sample with an Area Under the Receiver Operating Characteristic curve (AUC) of ∼ 0.85 in 1 h, and AUC > 0.9 in 90 min, when compared to state-of-the-art automated AST methods used in the clinic while being an order of magnitude faster.

Acoustic Signal Monitoring in Laser Ablation of Anti-Reflective Microstructured Silicon Surface

Acoustic Signal Monitoring in Laser Ablation of Anti-Reflective Microstructured Silicon Surface

Self‐Aligned Polymer Film Patterning on Microstructured Silicon Surfaces

AbstractBased on a recently developed selective etching process for silicon samples with buried implants, this work presents a method for utilizing the generated material contrast between silicon and silicon oxide to selectively deposit polymers on the patterned surface. Besides depositing polystyrene selectively on the etched parts via drop‐casting and dewetting, polymer chains are selectively grafted to the oxidic surface parts. To this end, the oxide surfaces are selectively modified via functional alkoxysilanes, whereas etched silicon surfaces are unreactive. Using either a two‐step or a straight‐forward one‐step process employing commercially available chemicals, atom transfer radical polymerization (ATRP) initiator functions are attached selectively to the non‐etched surface areas, and subsequently used to polymerize either methyl methacrylate (MMA) or N‐isopropyl acrylamide (NIPAM) in a controlled surface‐initiated activator regenerated by electron transfer (ARGET)‐ATRP process. This procedure allows for the further self‐aligned functionalization of the micropatterned samples, enabling the production of highly functional, patterned surfaces, potentially suitable for applications in areas such as cell adsorption, microfluidics, or functional microsystems, e.g., MEMS.

Inverted pyramid structure on monocrystalline silicon processed by wet etching after femtosecond laser machining in air and deionized water

The micro-structured silicon has been preliminarily applied to improve the surface and optical properties of micro-solar cells, micro-optical systems, and aerospace engineering. However, the morphological changes and optical properties of monocrystalline silicon during femtosecond laser machining assisted with chemical etching are not clear, especially in a liquid environment. In this paper, monocrystalline silicon was ablated by femtosecond laser in air and deionized water respectively and then etched by alkaline potassium hydroxide (KOH). The reflectivity, absorptivity, and transmissivity of monocrystalline silicon were tested. Experimental results showed that inverted pyramid (IP) structure could be processed on a monocrystalline silicon wafer by ablating the surface with a high-power laser under the air environment and a low-power laser under the deionized water environment, followed by KOH etching. Meanwhile, the optical properties of the monocrystalline silicon with IP structure were effectively improved.

Negative‐Index Acoustic Metamaterial Operating above 100 kHz in Water Using Microstructured Silicon Chips as Unit Cells

AbstractA major challenge for negative‐index acoustic metamaterials is increasing their operational frequency to the MHz range in water for applications such as biomedical ultrasound. Herein, a novel technology to realize acoustic metamaterials in water using microstructured silicon chips as unit cells that incorporate silicon nitride membranes and Helmholtz resonators with dimensions below 100 μm fabricated using clean‐room microfabrication technology is presented. The silicon chip unit‐cells are then assembled to form periodic structures that result in a negative‐index metamaterial. Finite‐element method (FEM) simulations of the metamaterial show a negative‐index branch in the dispersion relation in the 0.25–0.35 MHz range. The metamaterial is characterized experimentally using laser‐doppler vibrometry, showing opposite phase and group velocities, a signature of negative‐index materials, and is in close agreement with FEM simulations. The experimental measurements also show that the magnitude of phase and group velocities increase as the frequency increases within the negative‐index band, confirming the negative‐index behavior of the material. Acoustic indices from –1 to –5 are reached with respect to water in the 0.25–0.35 MHz range. The use of silicon technology microfabrication to produce acoustic metamaterials for operation in water opens a new road to reach frequencies relevant for biomedical ultrasound applications.

Publisher's Note: “Preliminary benchmarks and analysis of boundary conditions in a trenched microstructured silicon radiation detector” [J. Appl. Phys. 131, 134503 (2022)

Publisher's Note: “Preliminary benchmarks and analysis of boundary conditions in a trenched microstructured silicon radiation detector” [J. Appl. Phys. 131, 134503 (2022)

Preliminary benchmarks and analysis of boundary conditions in a trenched microstructured silicon radiation detector

Microstructured neutron detectors have the benefit of enhanced neutron detection efficiency as compared to planar devices, achieved by etching 6LiF-filled trenches on the top surface of a silicon PIN diode. This sensor geometry results in a complex electric field distribution and depletion characteristics within the diode under reverse bias. For the first time on record, the effects of a fixed oxide charge on the microstructured device depletion characteristics and mobile carrier transport is investigated. Prototype detectors were fabricated with non-conformal surface doping. Capacitance voltage and current voltage measurements were performed for these prototypes and compared with COMSOL Multiphysics simulations. A spectral response from an 241Am alpha particle source was acquired and analyzed. It was found that monoenergetic alpha particles produce three prominent peaks in the pulse height spectrum output by the device. The peaks were confirmed by simulations to correlate with dead layers and incident trajectories into the microstructure. It was also found that significant differences in pulse rise time result, corresponding with events arriving in a low-field region in the fins and a high-field region in the bulk. Geant4 was utilized for radiation transport, interaction modeling, and benchmarking the spectral data. The results of this simulation work provide confidence in the ability to attain and benchmark electrical characteristics and spectral data for semiconductor radiation detectors employing complex microstructures.

The nature of column boundaries in micro-structured silicon oxide nanolayers

Columnar microstructures are critical for obtaining good resistance switching properties in SiOx resistive random access memory (ReRAM) devices. In this work, the formation and structure of columnar boundaries are studied in sputtered SiOx layers. Using TEM measurements, we analyze SiOx layers in Me–SiOx–Mo heterostructures, where Me = Ti or Au/Ti. We show that the SiOx layers are templated by the Mo surface roughness, leading to the formation of columnar boundaries protruding from troughs at the SiOx/Mo interface. Electron energy-loss spectroscopy measurements show that these boundaries are best characterized as voids, which in turn facilitate Ti, Mo, and Au incorporation from the electrodes into SiOx. Density functional theory calculations of a simple model of the SiO2 grain boundary and column boundary show that O interstitials preferentially reside at the boundaries rather than in the SiO2 bulk. The results elucidate the nature of the SiOx microstructure and the complex interactions between the metal electrodes and the switching oxide, each of which is critically important for further materials engineering and the optimization of ReRAM devices.

Fluid driven self-assembly of woven Ag nanowire grid for ultra-flexible transparent electrodes

Highly oriented woven Ag-NW grids has great potential for enhancing the mechanical stabilities of flexible transparent electrodes (TEs) but few researchers reported. This work presents a flow driven self-assembly approach for preparing the highly oriented woven Ag-NW grids. Due to the coupling effect of micro-structured silicon mold and thermocapillary flow induced by surface tension, two flow states of Ag-NWs ink are formed in sequence, which leads to the enrichment of Ag-NWs. Meanwhile, the selective deposition enables the highly oriented woven feature of Ag-NWs. Using elastomer casting and transferring processes, TEs of Ag-NWs embedded in PDMS are successfully obtained. Such TEs show remarkable mechanical performances with a low sheet resistance of ∼20 Ω/□. The change of resistance is less than 0.8% even under the extreme bending deformation. A resistance increase of less than 2.9% is observed when the sample undergoes 14,000 cycles. The stretching results also demonstrate excellent mechanical performance. The uniform electroluminescence of electroluminescent device under the flat and bending states verified that a defect-free TE have been fabricated. These results imply that the as-fabricated electrode is a promising candidate for flexible optoelectronic applications.

Piezoresistive Electronic-Skin Sensors Produced With Self-Channeling Laser Microstructured Silicon Molds

Highly sensitive and cost-effective flexible piezoresistive sensors are pursued as essential components of electronic skin (e-skin) for a variety of applications such as soft robotics and body prosthesis. A common strategy for achieving high sensitivity of these sensors is to fabricate their electrode material with complex 3-D microstructures, but manufacturing cost-effective micro-structured sensors with high reliability and reproducibility is a challenge. Herein, a robust approach for producing high-reproductive microstructured polydimethylsiloxane (PDMS) films composing the sensors is proposed, based on silicon molds with a honeycomb-like architecture fabricated rapidly and cost-effectively at a standoff distance by femtosecond laser pulses in the self-channeling regime. Single-walled carbon nanotubes (SWNTs) are embedded in the micro-structured PDMS films composing sandwich-structural flexible piezoresistive sensors. The sensing devices exhibit good performance with high sensitivity, fast response time, and excellent stability and are used with success for the detection of hand motion pressure and blood pressure at the wrist, thus showing a great potential as components of e-skin for detection of various human motions.

Piezoresistive electronic skin based on diverse bionic microstructure

With the introduction of micro nano structure, the performance of electronic skin (E-skin), as a flexible pressure sensor, has been significantly improved. However, most of the pre templates for constructing micro nano structure are fabricated by photolithography, which endows them with the limitation of surface microstructure and the difficulty of forming complex multi-level structures. Distinguished from the artificial micro-structured silicon template, there are abundant microstructure distributed on the surface of natural plant leaves for us to choose. The diversity of plant leaves provides more possibility for us to explore the internal relationship between the microstructure distributed on electrode surface and the performance of E-skin device. Herein, four kinds of plant leaves are selected as templates to fabricate E-skins with different bionic microstructures. The various sensing performances of four kinds of devices are evaluated through the real-time monitoring of current signal, and the device based on nerium oleander template exhibits the most outstanding performance (sensing range: 0–0.8 kPa, sensitivity: 8.5 kPa−1, detection limit: 0.001 kPa). Furthermore, quantitative indicators (radius/height and density) for the surface microstructure of E-skin are proposed to explain the performance differences between these devices. The device with fine (small values of radius/height) and high-density microstructure trends to exhibit high sensitivity and low detection limit as well as good durability. Additionally, the activating phenomenon of the E-skin during repeated compression was observed and the underlying mechanism was discussed. Finally, the E-skin based on nerium oleander template has been successfully employed to detect the subtle distinction of physiological signals in a series of fitness actions.

Ultrafast pulse laser inscription and surface quality characterization of micro-structured silicon wafer

• Established an efficient methodology based on Ultrafast laser inscription principle for fabricating higher quality microchannels. • The proposed methodology will be an alternative technique for the usual industrial practice to attain higher ablation depth. • For the first time, ALF over the crystalline coating of Si3N4 on silicon was analysed and reported to be predominant on structural integrity. • The occurrence of the bulging phenomenon on coated silicon surface at low repetition rates were reported. • The surface temperature was predicted which justified the enhancement in surface integrity processed at a specific range of lasing parameters. We report the applicability of the ultrafast pulse laser inscription technique to achieve high ablation depth on uncoated silicon wafer despite its higher surface reflectivity. The proposed methodology of this research work can be an alternative approach for the usual industrial practice of coating silicon surface with highly reflective materials for increasing the absorption phenomenon. To unveil the potential of the proposed methodology, a comparative study was carried out by fabricating microchannels of higher depth on uncoated and coated silicon wafer by varying repetition rate from 10 kHz to 500 kHz at a constant pulse energy of 18 μJ. The formation of ablation depth, ablation width and amorphous layer thickness was taken as the standard for evaluating the effectiveness of the proposed methodology. The experimental results revealed the formation of a higher ablation depth of 6.6 μm and an amorphous layer thickness of 0.039 μm for uncoated silicon material. Whereas, in the case of coated silicon material the ablation depth was found to be 3.199 μm with an amorphous layer thickness of 0.101 μm. This justified the applicability of the ultrafast pulse laser inscription technique for achieving quality microchannels having higher depth on silicon material without any surface coating. The underlying mechanism for the improved performance is due to the low temporal separation (μs) property of ultrafast lasers which results in negligible heat diffusion into the bulk material, thereby minimizing the collateral thermal damages. This was further proved based on an analytical model by evaluating the surface temperature at various repetition rates. The experimental and analytical results from the present work will be highly beneficial for the electronic industry, where the laser micro structuring of MEMS components made of silicon material is highly challenging due to its reflective property.

Wet chemical etching of single-bore microstructured silicon dioxide fibers

We model the process of wet chemical etching of the external surface of a single-bore microstructured silicon dioxide fiber in hydrofluoric acid (HFA) while water is pumped through the internal channel to prevent etching of it. The model uses the Stokes flow for the velocity throughout the system and the advection–diffusion equation for the concentration of HFA. We determine the etch rate as a function of HFA concentration using data from experiments designed for this purpose, from which we calculate the change in the fiber surface. We solve our equations using a time-stepping finite-element method and verify our model by comparing to results found experimentally. We investigate the effects of different water flow rates, diffusivity, buoyancy, and bore radius. We find the water being pumped through the bore does not fully protect it and there is some etching of the internal channel, which is difficult to see in experimental images. We also obtain an estimate of the diffusivity of high-concentration HFA in water.

Grinding performance improvement of laser micro-structured silicon nitride ceramics by laser macro-structured diamond wheels

Silicon nitride ceramics are widely used in various industrial fields because of their excellent characteristics: high hardness, high elastic modulus, abrasion resistance, and high heat resistance. Diamond wheel grinding is the predominant and most productive method to machine silicon nitride ceramics. However, a lot of heat is generated due to high friction between a diamond grinding wheel and extremely rigid silicon nitride during grinding. This causes surface/subsurface damage, wheel wear, etc., which impairs the surface quality of silicon nitride. This impairment can restrict the use of silicon nitride ceramic components. To improve the surface quality and service life of grinding wheels, a laser macro-micro combination structured grinding (LMMCSG) method was presented. The results indicated that the grinding force ratio and surface roughness when using LMMCSG were respectively 31% and 40% lower than the grinding force ratio and surface roughness when using conventional grinding. Moreover, the LMMCSG method effectively reduced the wheel wear and workpiece subsurface damage.

New insights on boiling carbon dioxide flow in mini- and micro-channels for optimal silicon detector cooling

Whilst the thermal management needs of future silicon detectors are increasing, the required mass and volume minimization of all detector ancillaries gets more demanding. This requires highly effective active cooling in very small channels. In the context of the AIDA-2020 project, a new test stand has been developed to characterize, with unprecedented level of accuracy, boiling flows of CO2 in mini- and micro-channels with hydraulic diameter ranging from 2 down to 0.1 mm. The heat transfer coefficient and pressure drop behaviour in stainless steel tubular evaporators for saturation temperatures from ＋20 to −25 °C, mass fluxes from 1200 to 100 kg m−2 s−1 and heat fluxes from 0.5 to 3.5 W/cm2 are discussed for one diameter. In addition, high speed camera observations of CO2 flow patterns recorded on micro-structured silicon cold plates are used to help with the interpretation of the heat transfer coefficient and pressure drop trends reported.

Molecular and Material Engineering of Photocathodes Derivatized with Polyoxometalate-Supported {Mo3S4} HER Catalysts.

Molecular engineering of efficient HER catalysts is an attractive approach for controlling the spatial environment of specific building units selected for their intrinsic functionality required within the multistep HER process. As the {Mo3S4} core derived as various coordination complexes has been identified as one as the most promising MoSx-based HER electrocatalysts, we demonstrate that the covalent association between the {Mo3S4} core and the redox-active macrocyclic {P8W48} polyoxometalate (POM) produces a striking synergistic effect featured by high HER performance. Various experiments carried out in homogeneous conditions showed that this synergistic effect arises from the direct connection between the {Mo3S4} cluster and the toroidal {P8W48} units featured by a stoichiometry that can be tuned from two to four {Mo3S4} cores per {P8W48} unit. In addition, we report that this effect is preserved within heterogeneous photoelectrochemical devices where the {Mo3S4}-{P8W48} (thio-POM) assembly was used as cocatalyst (cocat) onto a microstructured p-type silicon. Using a drop-casting procedure to immobilize cocat onto the silicon interface led to high initial HER performance under simulated sunlight, achieving a photocurrent density of 10 mA cm-2 at +0.13 V vs RHE. Furthermore, electrostatic incorporation of the thio-POM anion cocat into a poly(3,4-ethylenedioxythiophene) (PEDOT) film is demonstrated to be efficient and straightforward to durably retain the cocat at the interface of a micropyramidal silicon (SimPy) photocathode. The thio-POM/PEDOT-modified photocathode is able to produce H2 under 1 Sun illumination at a rate of ca. 100 μmol cm-2 h-1 at 0 V vs RHE, highlighting the excellent performance of this photoelectrochemical system.

Topological optimization of patterned silicon anode by finite element analysis

A silicon-based anode in lithium-ion battery exhibits several times higher gravimetric energy storage capacity compared to an established carbon-based anode. However, the cycling performance of the silicon anode is poor due to the extremely large volume variation during the intercalation of lithium ions. The micro-structuring of silicon facilitates cycling performance. In particular, patterned microstructures are discussed as a possible solution. The large volumetric change can be adopted in such structures by bending walls and rotation around fixed vertexes. Nevertheless, the cycling performance of known patterned anodes remains poor due to plastic deformations. In this paper, a new square-based-patterned silicon anode is proposed and analyzed using the finite element method. The maximal stress in the topologically optimized structure is below the yield strength of lithiated silicon. In contrast to known structures, the deformed pattern of the new structure is explicitly defined by its initial geometry. A similar modification of the honeycomb-based-patterned anode leads to a slightly larger bending stress, but still below the yield stress of lithiated silicon. The related pure elastic deformation behavior is favorable to a prolonged cycling life of the micro-structured silicon anode. The developed approach can be applied for analysis of other severely swelling metamaterials.

Formation of micro structured doped and undoped hydrogenated silicon thin films

The microcrystalline hydrogenated-silicon (μc-Si: H) (also called polymorphous silicon) consisting a two-phase mixture of amorphous and structured silicon is being used for electronic or optoelectronic based thin-film devices. The μc-Si: H thin films are deposited using radio frequency (13.56 MHz) Plasma Enhanced Chemical Vapour Deposition (RF-PECVD) by varying doping gases (diborane (B2H6) and phosphine (PH3)) flow and hydrogen-silane dilution ratio (R = H2/SiH4) to optimize the crystalline fraction and electrical conductivity. Micro-Raman spectroscopy is used to investigate these effect on the transition fraction regime from amorphous into micro-structured silicon. Qualitative and quantitative properties have been studied by deconvolution of the micro Raman spectra which allows to determine the crystalline fraction in the film and also some investigation regarding the correlation between electrical and structural properties are presented for different annealing temperature (from 300 to 550 °C) and various film thickness ranges (10–100 nm). In this work, we present the characterization of thin films (both doped and undoped) deposited at the temperature of 250 °C on quartz substrate after annealed at 550 °C in N2-ambient, as a result crystallinity percentage up to 90% for p-type, 96% for n-type and 80% for undoped films are achieved. A detailed characterization of the microcrystalline silicon (μc-Si: H) has been demonstrated in this paper: structural properties through Raman spectroscopy, electrical properties through Four-point probe station and optical properties using Ellipsometer.