Reactive Ion Etching Of Silicon Research Articles

Submicron-patterning of bulk titanium by nanoimprint lithography and reactive ion etching

Nanopatterns on titanium may enhance endosseous implant biofunctionality. To enable biological studies to prove this hypothesis, we developed a scalable method of fabricating nanogrooved titanium substrates. We defined nanogrooves by nanoimprint lithography (NIL) and a subsequent pattern transfer to the surface of ASTM grade 2 bulk titanium applying a soft-mask for chlorine-based reactive ion etching (RIE). With respect to direct write lithographic techniques the method introduced here is fast and capable of delivering uniformly patterned areas of at least 4 cm2. A dedicated silicon nanostamp process has been designed to generate the required thickness of the soft-mask for the NIL–RIE pattern transfer. Stamps with pitch sizes from 1000 nm down to 300 nm were fabricated using laser interference lithography (LIL) and deep cryogenic silicon RIE. Although silicon nanomachining was proven to produce smaller pitch sizes of 200 nm and 150 nm respectively, successful pattern transfer to titanium was only possible down to a pitch of 300 nm. Hence, the smallest nanogrooves have a width of 140 nm. An x-ray photoelectron spectroscopy study showed that only very few contaminations arise from the fabrication process and a cytotoxicity assay on the nanopatterned surfaces confirmed that the obtained nanogrooved titanium specimens are suitable for in vivo studies in implantology research.

High aspect ratio lead zirconate titanate tube structures: I. Template assisted fabrication - vacuum infiltration method

Polycrystalline Pb(Zr0.52Ti0.48 )O3 (PZT) microtubes are fabricated by a vacuum infiltration method. The method is based on repeated infiltration of precursor solution into macroporous silicon (Si) templates at a sub-atmospheric pressure. The pyrolyzed PZT tubes of a 2-?m outer diameter, extending to over 30 ?m in length were released from the template using a selective isotropic-pulsed XeF2 reactive ion etching of silicon. Free-standing microtubes, partially anchored at the bottom of the Si template, were then crystallized in pure oxygen atmosphere at 750 ?C for 2 min using a rapid thermal annealer. The perovskite phase of the final PZT tubes was confirmed by X-ray diffraction (XRD) analysis. The XRD spectrum also revealed a small amount of the pyrochlore phase in the structure and signs of possible fluoride contamination caused most likely by the XeF2 etching process. The surface morphology was examined using scanning electron microscopy. It was demonstrated that the whole surface of the pore walls was conformally coated during the repeated infiltration of templates, resulting in straight tubes with closed tips formed on the opposite ends as replicas of the pore bottoms. These high aspect ratio ferroelectric structures are suggested as building units for developing miniaturized electronic devices, such as memory storage (DRAM trenched) capacitors, piezoelectric scanners and actuators, and are of fundamental value for the theory of ferroelectricity in systems with low dimensionality.

Single mask, simple structure micro rotational motor driven by electrostatic comb-drive actuators

We report a design and fabrication of a new micro rotational motor (MRM) using silicon micromachining technology with the overall diameter of 2.4 mm. This motor utilizes four silicon electrostatic comb-drive actuators to drive the outer ring (or rotor) through ratchet teeth. The novel design of the anti-reverse structure helps us to overcome the gap problem after deep reactive ion etching of silicon. The MRM was fabricated by using silicon on insulator wafer with the thickness of the device layer being 30 µm and one mask only. The motor was successfully tested for performance. It was driven by periodic voltage with different frequencies ranging from 1 to 50 Hz. The angular velocity of the outer ratchet ring was proportional to the frequency. Moreover, when the driving frequency is lower than 30 Hz, the experiment results perfectly match the theoretical calculation.

Wafer Bevel Protection During Deep Reactive Ion Etching

During deep reactive ion etching of silicon used for through silicon via or deep trench isolation processing, the bevel of the wafer is also etched away. The etching of the bevel results in a deep step at the litho edge bead removal or in a degraded bevel shape, source of yield loss or processing issues. Two methods are proposed here to prevent the bevel degradation during deep reactive ion etching using an oxide hard mask. In one case this oxide mask is deposited in the second case the oxide hard mask is grown.

Hot roll embossing in thermoplastic foils using dry-etched silicon stamp and multiple passes

Hot roll embossing is a promising technique for manufacturing and patterning of micron and sub-micron features. It attracted attention due to its high volume production and large area processing. In this work, we describe a hot-roll-embossing process for manufacturing flexible devices in different commercially available thermoplastic polymer foils using a microstructured silicon wafer as a flat stamping tool. Larger features of a 100μm width were defined by photolithography and dry etching. A process combining deep reactive ion etching and reactive ion etching of silicon in fluorinated plasma was developed to achieve patterns in silicon with slightly positively tapered sidewalls to allow for easy demoulding. 100μm features were successfully replicated in cyclic-olefin-copolymer (COC) and poly-methyl-methacrylate (PMMA), using relatively low temperature and multiple passes. Smaller features with 1μm width were patterned by electron beam lithography and transferred by DRIE in silicon stamps, then replicated in COC foils.

Fabrication of AD/DA microfluidic converter using deep reactive ion etching of silicon and low temperature wafer bonding

The purpose of this work is to describe an original process that has been designed for the fabrication of a microfluidic converter. The fabrication is based on deep reactive ion etching of silicon and low temperature full wafer adhesive bonding. The technology development includes an improvement of the bonding process in order to produce an adaptive strength of SU-8 bond which not only ensures absence of debonding failures during the silicon deep etching procedure and the subsequent dicing procedure, but also avoids the potential SU-8 overflow leakage into channels due to the bonding step. Besides, the originality of the work is not only in the process but also in the design of the device. Common actuation method for microfluidic system is either based on closed-channel continuous-flow microfluidic (CMF) or droplet-based microfluidic (DMF). Both of them have advantages and disadvantages, and their integration on a single system is in dire need. In this paper, we briefly discuss the concept of microfluidic converter, integrating CMF with DMF, which can: (i) continuously preload reagents, (ii) independently manipulate several droplets, (iii) recombine and export samples into closed-channel continuous flow, making it ideal for interfacing to liquid-handling instruments and micro-analytical instruments.

Study of macroporous silicon electrochemical etching in 3D structured N type silicon substrates

AbstractIn this paper, the electrochemical etching of 3D n‐type substrates is investigated. These types of 3D structures are of interest to increase the active surface of some systems. Our aim is to improve a MEMS (Micro‐Electro Mechanical System) micro fuel cell power through total surface enhancement without any modification of the cell footprint. To reach this objective, we perform a gas diffusion layer using localized macro‐porous silicon. The porous area total surface is improved through trench formation (before porous silicon etching). This is supposed to allow a better power to surface ratio for manufactured fuel cell. The Figure on the right hand side introduces the aimed MEMS based micro fuel cell. The geometrical parameters of the designed structures and the manufacturing process influence are presented. To perform 3D structures, two types of anisotropic etching techniques have been used, alkaline etching of silicon and deep reactive ion etching (DRIE). Additional steps such as a doping layer have been used to improve the results obtained with the initial micro‐fabrication process. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ga + beam lithography for suspended lateral beams and nanowires

The authors demonstrate the fabrication of suspended nanowires and doubly clamped beams by using a focused ion beam implanted Ga etch mask followed by an inductively coupled plasma reactive ion etching of silicon. This method will demonstrate how a two-step, completely dry fabrication sequence can be tuned to generate nanomechanical structures on either silicon substrates or silicon on insulator (SOI). This method was used to generate lateral nanowires suspended between 2 μm scaled structures with lengths up to 16 μm and widths down to 40 nm on a silicon substrate. The authors also fabricate 10 μm long doubly clamped beams on SOIs that are 20 nm thick and a minimum of 150 nm wide. In situ electrical measurements of the beams demonstrate a reduction of resistivity from >37.5 Ω cm down to 0.25 Ω cm. Transmission electron microscopy for quantifying both surface roughness and crystallinity of the suspended nanowires was performed. Finally, a dose array for repeatable fabrication of a desired beam width was also experimentally determined.

Ga+ beam lithography for nanoscale silicon reactive ion etching

By using a dry etch chemistry which relies on the highly preferential etching of silicon, overthat of gallium (Ga), we show resist-free fabrication of precision, high aspect rationanostructures and microstructures in silicon using a focused ion beam (FIB) and aninductively coupled plasma reactive ion etcher (ICP-RIE). Silicon etch masks are patterned viaGa + ion implantation in a FIB and then anisotropically etched in an ICP-RIE using fluorinatedetch chemistries. We determine the critical areal density of the implanted Ga layerin silicon required to achieve a desired etch depth for both a Pseudo Bosch (SF6/C4F8) and cryogenicfluorine (SF6/O2) silicon etching. High fidelity nanoscale structures down to 30 nm and high aspect ratiostructures of 17:1 are demonstrated. Since etch masks may be patterned on unevensurfaces, we utilize this lithography to create multilayer structures in silicon. The linearselectivity versus implanted Ga density enables grayscale lithography. Limitson the ultimate resolution and selectivity of Ga lithography are also discussed.

Silicon Micro-Fabrication Technologies for Micro-Filters

Silicon micro-fabrication techniques allow for the development of microfluidic systems with very accurate control of size and uniformity of structures. In this paper we report on the silicon fabrication process of micro-filters for versatile application in fluidics systems. Micro-filters are composed of an ordered array of pillars and supply channels. Depending on pillar pitch, they can be used for, e.g., electrophoresis, chromatography and purification of biological mixtures. In this paper we focus on high performance liquid chromatography. The process that we have developed for micropillar fabrication consists of defining first 1μm diameter pillars with an inter-pillar distance of 1μm or less in an oxide hard mask with a DUV stepper, stitching is used to form few cm long patterns across the 200mm wafers. Second, the supply channels are defined with 1× alignment lithography. After definition of supply channels, deep reactive ion etching of silicon is performed with an optimised recipe to etch submicron pillars and supply channels of 100μm wide at the same time. The simultaneous etch of both structures avoids complex lithography steps otherwise necessary to protect the pillars while etching the supply channels or vice versa as would be done conventionally. Wafers are then anodically bonded to 200mm Pyrex wafers in order to seal the channels. Pyrex wafer also allows the use of optical detection system. Feed through holes for accessing the supply channels are etched on the backside of Si wafer. Filter characterization has been performed: a plate height of 1μm was measured and successful separation of 3 coumarin dyes is achieved.

Techniques of cryogenic reactive ion etching in silicon for fabrication of sensors

Cryogenic etching of silicon, using an inductively coupled plasma reactive ion etcher (ICP-RIE), has extraordinary properties which can lead to unique structures difficult to achieve using other etching methods. In this work, the authors demonstrate the application of ICP-RIE techniques which capitalize on the cryogenic properties to create different sensors geometries: optical, electrical, magnetic, and mechanical. The three techniques demonstrated are (1) single step deep etches with controllable sidewall profiles. Demonstrating this, silicon pillars with over 70μm depth and less than 250nm sidewall roughness were etched using only 1.6μm of photoresist for use as solar cells. (2) Using the cryogenic etch for thick metallization and liftoff with a thin photoresist mask. Demonstrating this second technique, a magnetic shim was created by deposition of 6.5μm of iron into 20μm deep etched trenches, using the remaining 1.5μm photoresist etch mask as the liftoff mask. Using the same technique, 15μm of copper was lifted off leaving a 20μm deep plasma enhanced chemical vapor deposition silicon oxide coated, silicon channel with copper. (3) Use of a two step cryogenic etch for deep etching with reduced sidewall undercutting. This was demonstrated by fabrication of deep and anisotropic microelectromechanical systems structures; a mechanical resonator was etched 183μm deep into silicon with less than 3μm of undercutting. This work also describes the etch parameters and etch controls for each of these sensors.

Arbitrary-shaped nanochannels fabricated by polymeric deformation to achieve single DNA stretching

This paper reports the development of a novel technique for the fabrication of nanochannels required for DNA molecule stretching. This nanochannel is fabricated by a combination of reactive ion etching of silicon, which controls the dimension of the final structure, and simple deformation of polydimethylsiloxane (PDMS) films onto the fabricated structure. The results of fluorescent dye injection and DNA stretching experiment carried out using the fabricated nanochannel are shown. In the case of 500 and 100 nm-scale nanochannels, the extended length of the DNA molecule in the 100 nm-scale showed different aspects, as suggested in the Odijk regime.

Silicon Nanotweezers With Subnanometer Resolution for the Micromanipulation of Biomolecules

The metabolism and toxicity of dapsone was compared in vitro and in vivo in rat, mouse and man. Metabolism was assessed by high-pressure liquid chromatography-mass spectrometry and methemoglobin formation has been used as a toxic endpoint. The greatest toxicity in vitro was seen in microsomes prepared from male Wistar rats (36.6 +/- 1.5% methemoglobin), although toxicity was also seen in microsomes from the female rat (8.2 +/- 1.3%), male CD1 (4.2 +/- 1.6%) and human (10. 9 +/- 1.1%). The rank order of toxicity agreed with the formation of the hydroxylamine metabolite in vitro. All microsomes were also capable of catalyzing the reverse reaction, i.e., reduction of the hydroxylamine to dapsone. However, in vivo administration of dapsone resulted in significant (P < 0.05) methemoglobinemia only in male rats and humans. This species difference in the susceptibility to dapsone toxicity could not be attributed solely to the sensitivity of the target erythrocytes, because the order of sensitivity to dapsone hydroxylamine was human > mouse > rat. Analysis of bile and urine revealed the formation of dapsone hydroxylamine and its glucuronide in male rats and humans, but not in female rats or mice. This species difference in the metabolism and toxicity of dapsone has important implications in the safety evaluation of related compounds for man.

Monolithic Integration Fabrication Process of Thermoelectric and Vibrational Devices for Microelectromechanical System Power Generator

This paper describes a fabrication process to integrate different types of energy-converting microelectromechanical system (MEMS) devices for a power generator. MEMS structures and a fabrication process for the monolithic integration are proposed for the purpose of accumulating small amounts of energy from various surrounding environments. For the MEMS power generator, thermocouples and movable plates are simultaneously fabricated in thick-film formation processes with deep reactive ion etching of silicon and gold electroplating. The fabricated thermoelectric devices of gold and silicon produced voltages in accordance with the temperature difference at the thermocouples. The vibrational devices with gold movable plates resonated with external vibrations. These results confirm that the monolithic integration process of MEMS devices can be established for harvesting thermal and vibrational energies from their surroundings.

Deep Vertical Etching of Silicon Wafers Using a Hydrogenation-Assisted Reactive Ion Etching

A novel hydrogenation-assisted deep reactive ion etching of silicon is reported. The process uses sequential hydrogen-assisted passivation and plasma etching at low-density plasma powers to stimulate the vertical removal of the exposed Si substrate. The main feature of this technique is the sequential alternation of the electrodes while switching between different gases. Three-dimensional structures with aspect ratios in excess of 40:1 and features as small as 0.7 mum have been realized. The net etch rate is about 0.25 mum/min, although higher rates are expected to be achievable.

Revisiting micro hot-embossing with moulds in non-conventional materials

Manufacturing moulds in non-metallic tooling materials using non-conventional structuring techniques is explored for rapid and/or alternative tooling for hot-embossing of polymers. Some challenges and advantages of producing embossing tools by deep reactive ion etching in silicon and/or replica-casting in poly(dimethylsiloxane) and utilizing them in the replication process are highlighted.

Characterizing and Predicting Spatial Nonuniformity in the Deep Reactive Ion Etching of Silicon

We present a way of predicting spatial nonuniformity in deep reactive ion etching (DRIE). In addition to well-known feature size or aspect-ratio-dependent etch (ARDE) nonuniformity, etch rate can vary between regions of the wafer, such that supposedly identical microelectromechanical systems (MEMS) fabricated on that wafer may vary in performance or fail in use. Wafer- and die-scale uniformity may be improved by changing conditions in the etching chamber, but this usually compromises etching speed and may accentuate ARDE. An alternative approach is to lay out the patterns being etched so that they compete for reactants in a controlled way. We demonstrate a way of precharacterizing any DRIE tool plus associated “recipes” of operating parameters. Several test wafers are etched and measured, and simple data-fitting algorithms are run. The model constructed captures etch nonuniformity occurring over the diameter of a wafer plus variations caused by the localization of patterns within the wafer. Our technique models etch rate variation across experimental wafers with fitting errors of between 0.8 and 4.5% root–mean–square per wafer. Our model can then predict etch rate on a lateral grid for any etched pattern. We envisage this method being integrated into computer-aided MEMS design systems.

Simulation of anisotropic etching of silicon in SF 6 + O 2 plasma

The two-dimensional reactive ion etching of silicon in SF 6 + O 2 plasma is considered. The profiles of etched trenches are calculated as a function of mask dimensions, concentrations of chemically active plasma components and ion bombardment parameters. The theoretically calculated profiles of etched trenches are compared with experimentally measured. The physical and chemical mechanisms underlying the processing are examined using numerical simulations. It is found that the decrease in temperature increases the etching anisotropy due to the decreased desorption of formed SiF 4 molecules from the sidewalls.

Maximum achievable aspect ratio in deep reactive ion etching of silicon due to aspect ratio dependent transport and the microloading effect

When etching high-aspect-ratio silicon features using deep reactive ion etching (DRIE), researchers find that there is a maximum achievable aspect ratio, which we define as the critical aspect ratio, of an etched silicon trench using a DRIE process. At this critical aspect ratio, the apparent etch rate (defined as the total depth etched divided by the total elapsed time) no longer monotonically decreases as the aspect ratio increases, but abruptly drops to zero. In this paper, we propose a theoretical model to predict the critical aspect ratio and reveal its causal mechanism. The model considers aspect ratio dependent transport mechanisms specific to each of the reactant species in the three subprocesses of a time-multiplexed etch cycle: deposition of a fluorocarbon passivation layer, etching of the fluorocarbon polymer at the bottom of the trench, and the subsequent etching of the underlying silicon. The model predicts that the critical aspect ratio is defined by the aspect ratio at which the polymer etch rate equals the product of the deposition rate and the set time ratio between the deposition and etching phases for the time-multiplexed process. Several DRIE experiments were performed to qualitatively validate the model. Both model simulations and experimental results demonstrate that the magnitude of the critical aspect ratio primarily depends on (i) the relative flux of neutral species at the trench opening, i.e., the microloading effect, and (ii) aspect ratio dependent transport of ions during the polymer etching subprocess of a DRIE cycle.

An interconnected 2D-TM EBG structure for millimeter and submillimeter waves

Due to their unique properties, electromagnetic bandgap (EBG) materials are of high interest for applications in communication technology for many frequency bands from microwave up to optical frequencies. We have investigated in both simulation and experiment a two dimensionally periodic EBG structure made by reactive ion etching of silicon with a bandgap for transverse magnetic waves in the millimeter wave range around 100 GHz. The structure comprises both a large bandgap and a high mechanical stability due to interconnecting dielectric bridges.