Wet Etching Step Research Articles

Trimming method for a high-yield manufacturing of high-efficiency diffraction gratings.

The tolerances in manufacturing of fully dielectric diffraction gratings based on the leaky-mode resonance in the -1st diffraction order can be challenging, especially if the grating design exhibits high diffraction efficiency (DE) only within a comparatively narrow spectral bandwidth. To gain improved control on the spectral bandwidth exhibiting high DE, we implemented a two-step etching approach within the fabrication process of the grating. First, a dry and anisotropic etching step was used for pre-shaping the grating, followed by iterative isotropic wet-etching steps using an alkaline solution (KOH) at a temperature of 90°C to adjust the maximum efficiency around the desired wavelength with high precision. This straightforward method gave us very good control on tailoring the DE as a function of the wavelength and led to the demonstration of DEs as high as 99.7±0.2% in the -1st diffraction order at a wavelength of 1030nm and of >99% in the wavelength range between 1020 and 1070nm. The gratings were used as cavity end mirrors of an Yb:YAG thin-disk laser, generating an output power of 145W in fundamental mode operation (M2<1.1).

Scalable long-term extraction of photosynthetic electrons by simple sandwiching of nanoelectrode array with densely-packed algal cell film

Direct extraction of photosynthetic electrons from the whole photosynthetic cells such as plant cells or algal cells can be highly efficient and sustainable compared to other approaches based on isolated photosynthetic apparatus such as photosystems I, II, and thylakoid membranes. However, insertion of nanoelectrodes (NEs) into individual cells are time-consuming and unsuitable for scale-up processes. We propose simple and efficient insertion of massively-populated NEs into cell films in which algal cells are densely packed in a monolayer. After stacking the cell film over an NE array, gentle pressing of the stack allows a large number of NEs to be inserted into the cells in the cell film. The NE array was fabricated by metal-assisted chemical etching (MAC-etching) followed by additional steps of wet oxidation and oxide etching. The cell film was prepared by mixing highly concentrated algal cells with alginate hydrogel. Photosynthetic currents of up to 106 nA/cm2 was achieved without aid of mediators, and the photosynthetic function was maintained for 6 days after NE array insertion into algal cells.

Photonic Properties of Structurally-Engineered Nanoporous Anodic Alumina and Application to Biosensing

Nanoporous anodic alumina (NAA) is a material with growing interest in nanotechnology because of its high versatility and ease of preparation[1]. By electrochemical etching of aluminum in acidic electrolytes and under the adequate conditions of composition, anodization voltage or current and temperature, the NAA pores grow perpendicular to the aluminum surface in a self-arranged matrix. Pore radius and interpore distances can be modulated in the range of a few tens of nanometers and up to a few hundreds. A wide range of applications has been demonstrated in sensing[2], drug delivery[3], cell culture[4] or photovoltaics[5]. Technologies for NAA have evolved to tailor material properties to the desired applications, specifically photonic properties with a view to sensing[6]. These structures are applied to analytes such as glucose, proteins or antibodies, usually accomplished by reflection interference spectroscopy. This technique permits to monitor real-time changes in the optical properties of the NAA as they are infiltrated by solutions in a flow cell. Photonic properties are obtained by engineering the pore shape. For instance, by combining sequentially anodization steps with wet etching steps, two-layer systems can be obtained that provide self-calibration in sensing[7]. On the other hand, periodic modulation of the applied anodization voltage between two different values leads to periodic in-depth variations of pore morphology, and consequently to the formation of successive layers with different effective refractive index giving rise to distributed-Bragg reflectors (DBRs)[8]. Precise control of the anodization times for each voltage level and temperature permits to obtain high refractive index contrasts between the different layers in the DBR and consequently, wide photonic stop bands at the desired wavelengths in the spectrum. In this communication we report the more recent results in biosensing with NAA-engineered structures, focusing on NAA-based rugate filters (NAA-RF). These are obtained by the application of a periodic sinusoidal anodization current, composed of a constant offset current together with a sinusoidal component, which produce an in-depth continuous periodic variation in pore diameter. The consequent continuous variation in refractive index results in a 1-D photonic crystal structure with a stop band tunable within the UV-VIS-IR range. Figure 1 shows a SEM cross-section of a NAA-RF, where the pore modulation can be recognized. It has been shown that different sinusoidal components can be overlapped in the anodization current profile[9]. With this, photonic structures with several stop bands can be produced. Furthermore, such stop bands can be tailored at any position in the spectrum what permits to propose new applications such as information encoding. Nevertheless, this overlap of sinusoidal components has the drawback that the refractive index contrasts that can be achieved have to be shared among the different sinusoidal components, what results in stop bands with low reflectance. In this work, we study the preparation of NAA-RF based on the successive application of the different sinusoidal components. We call these structures stacked NAA-RF, since the different photonic crystals grow separately in the NAA. Figure 2 shows three selected sections of the applied sinusoidal current and of the measured electrochemical cell voltage. Figure 3 shows the spectrum resulting from such applied anodization current. Three clear stop bands with reflectances as high as 40% can be observed. In the communication we will report our results on the tunability of the stop bands and its sensitivity in biosensing applications. ACKNOWLEDGMENT This work was supported by the Spanish MINECO (TEC2015-71324-R), the Catalan authority AGAUR (2014SGR1344), and ICREA under the ICREA Academia Award. REFRENCES [1] J. Ferré-Borrull, J. Pallarès, G. Macias and L.F. Marsal, Materials 7, 2014, p. 5225. [2] G. Macias, J. Ferré-Borrull, J. Pallarès and L.F. Marsal, Analyst 140, 2015, p. 4848. [3] M. Alba, B. Delalat, P. Formentin, M.L. Rogers, L.F. Marsal and N. Voelcker, Small 11, 2015, p. 4626. [4] P. Formentin, U. Catalan, M. Alba, S. Fernandez-Castillejo, R. Sola, J. Pallares and L.F. Marsal, New Biotechnology 8, 2017, p. 675. [5] V.S. Balderrama, J. Albero, P. Granero, J. Ferre-Borrull, J. Pallares, E. Palomares and L.F. Marsal, Nanoscale 7, 2015, p. 13848. [6] J. Ferré-Borrull, E. Xifré-Pérez, J. Pallarès and L.F. Marsal in Nanoporous Alumina: Fabrication, Structure, Properties and Applications; Santos, A., Losic, D., Eds.; Springer-Verlag Berlin, 2015, p. 185. [7] G. Macias, L.P. Hernandez-Eguia, J. Ferré-Borrull, J. Pallarès and L.F. Marsal, ACS Applied Materials and Interfaces 5, 2013, p. 8093. [8] M. M. Rahman, L.F. Marsal, J. Pallares and J. Ferré-Borrull, ACS Applied Materials and Interfaces 5, 2013, p. 13375. [9] A. Santos, C.S. Law, T. Pereira and D. Losic, Nanoscale 8, 2016, p. 8091. Figure 1

Fluid-Imbibition Coupled Interferometry Study of Surface Modifications in Nanoporous Anodic Alumina for Biosensing

Nanoporous anodic alumina (NAA) is a material with great perspectives in nanotechnology due to its cost-effective fabrication, its chemical stability and its versatility in applications such as biosensing or drug delivery[1]. Optical sensing with NAA has gained much attention in recent years[2,3]. Nano-optofluidic methods[4] based on a solid carrier matrix, a liquid carrying analytes and on the physico-chemical characteristics at their interfaces have been proven sensitive for substances as small as nanoparticles, viruses[5] and even ions[6]. Fluid imbibition-coupled laser interferometry (FICLI)[7] is an optofluidic technique that permits the investigation of the surface and geometrical properties of nanoporous structures by non-invasive optical methods. It consists of the measurement of interferometric light intensity fluctuations in a beam reflected in a nanoporous thin film membrane as it is being filled by a liquid. In FICLI, time-resolved interferograms are measured and then analyzed on the basis of mathematical models of the filling kinetics. With this the surface properties and the pore internal geometries can be estimated. Recently, we established for FICLI an accuracy better than 2 nm in determining changes in NAA pore radius after different modifications involving radius increase (by chemical wet etching) or decrease (by the deposition of layer-by-layer coatings). Such accuracy leads to consider NAA and FICLI as a platform to develop biosensing systems. Such application depends critically on the functionalization of the pore surface, as it is necessary to enable the selective immobilization of the desired biomolecules. In this work, we investigate the influence of three different NAA functionalization methods on the FICLI pore radius estimate: i) electrostatic binding of bovine serum albumin (BSA), ii) covalent binding for the attachment of streptavidin and biotin and iii) specific immune complexation of antibody-antigen pairs. The study was carried on NAA membranes produced in both oxalic and sulfuric acid electrolytes, and together a chemical wet etching step for a given amount of time (between 0 min. and 24 min.) permits to use a wide range of pore dimensions (between 15 nm and 44 nm). NAA inner pore surface was modified in different steps and the FICLI technique was applied to estimate the changes in filling dynamics and the consequent pore radius change after each step. For the electrostatic attachment of BSA, NAA membranes with different pore radius were used as-produced and measured before and after incubation in a BSA solution. For the covalent attachment of streptavidin, NAA samples with 40 nm initial pore radius were investigated after two steps: i) silanization with (3-aminopropyl)-triethoxysilane (APTES) with cross-linking activation with glutaraldehyde (GTA) and ii) incubation with streptavidin. Finally, immune complexation within NAA pores was tested on NAA samples with 40 nm initial pore radius attaching different kinds of immunoglobulin (IgG) to the inner pore surface. Several experiments were conducted, in all of them the first step was to attach electrostatically protein A, which has the ability to link with antibody or antigen portions. Then, samples were incubated following one of three possibilities: i) human IgG, ii) anti-human IgG and iii) anti-goat IgG from rabbit followed by incubation with goat IgG. Results show that for the case of the electrostatic attachment, FICLI is able to detect the formation of a BSA layer on the NAA pores, independently of the initial pore radius with slight differences observed for different electrolyte used in the preparation. For covalent immobilization of streptavidin, FICLI estimates a much bigger radius reduction than expected from the molecule size, what can be explained by a multi-layer deposition of streptavidin. Finally, FICLI is able to demonstrate that different IgG attach differently to the NAA pores with a determinant influence of the pore size. ACKNOWLEDGMENT This work was supported by the Spanish MINECO (TEC2015-71324-R), the Catalan authority AGAUR (2014SGR1344), and ICREA under the ICREA Academia Award. REFERENCES [1] J. Ferré-Borrull, E. Xifré-Pérez, J. Pallarès and L.F. Marsal in “Nanoporous Alumina: Fabrication, Structure, Properties and Applications”; Santos, A., Losic, D., Eds.; Springer-Verlag Berlin, 2015, p. 185. [2] G. Macias, J. Ferré-Borrull, J. Pallarès and L.F. Marsal, Analyst 140, 2015, p. 4848. [3] T. Kumeria, A. Santos, M.M. Rahman, J. Ferré-Borrull, L.F. Marsal, D. Losic, ACS Photonics 1, 2014, p. 1298. [4] D. Psaltis, S.R. Quake, C.H. Yang, Nature 442, 2006, p. 381. [5] A. Mitra, B. Deutsch, F. Ignatovich, C. Dykes, L. Novotny, ACS Nano 23, 2010, p. 1. [6] T. Kumeria, M.M. Rahman, A. Santos, J. Ferré-Borrull, L.F. Marsal, D. Losic, ACS applied materials & interfaces 6, 2014, p. 12971. [7] C. Eckstein, E. Xifré-Pérez, M. Porta-i-Batalla, J. Ferré-Borrull, L.F. Marsal, Langmuir 32, 2016, p. 10467.

Wet-Etched Three-Level Silicon Interposer for 3-D Embedding and Connecting of Optoelectronic Dies and CMOS ICs

Ultracompact optical submodules for parallel optical interconnects are demonstrated based on a three-level silicon interposer, which is fabricated through a low-cost wet etching process. Using three steps of wet etching of silicon, a multilevel cavity is formed for embedding and flip-chipping optical and electrical dies, and opening optical through-silicon vias. In order to reduce thermal coupling between CMOS and GaAs dies, a 50-μm thermal isolation air gap is formed between dies as a part of the assembly concept, and thermal simulations and experiments are carried to validate its effectiveness. Based on this 3-D packaging concept, compact 4 mm × 6 mm, 10-Gb/s 12-channel transmitter and receiver submodules are fully assembled and tested. Clear and uniform eye patterns for both modules are captured at 10 and 15 Gb/s for every channel. Bit error rate (BER) testing is also performed. Both transmitter and receiver submodules show uniform BER curves, with receiver sensitivity spreading less than 1 dB at a BER lower than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> . Also, crosstalk for both modules is tested, yielding only a 0.1and 0.8-dB additional penalty for transmitter and receiver, respectively.

Fabrication and characterization of SU-8-based capacitive micromachined ultrasonic transducer for airborne applications

We present a successful fabrication and characterization of a capacitive micromachined ultrasonic transducer (CMUT) with SU-8 as the membrane material. The goal of this research is to develop a post-CMOS compatible CMUT that can be monolithically integrated with the CMOS circuitry. The fabrication is based on a simple, three mask process, with all wet etching steps involved so that the device can be realized with minimal laboratory conditions. The maximum temperature involved in the whole process flow was 140°C, and hence, it is post-CMOS compatible. The fabricated device exhibited a resonant frequency of 835 kHz with bandwidth 62 kHz, when characterized in air. The pull-in and snapback characteristics of the device were analyzed. The influence of membrane radius on the center frequency and bandwidth was also experimentally evaluated by fabricating CMUTs with membrane radius varying from 30 to 54 μm with an interval of 4 μm. These devices were vibrating at frequencies from 5.2 to 1.8 MHz with an average Q-factor of 23.41. Acoustic characterization of the fabricated devices was performed in air, demonstrating the applicability of SU-8 CMUTs in airborne applications.

Dry Etching with Nanoparticles: Formation of High Aspect-Ratio Pores and Channels Using Magnetic Gold Nanoclusters.

Methods for generating nanopores in substrates typically involve one or more wet-etching steps. Here a fundamentally different approach to produce nanopores in sheet substrates under dry, ambient conditions, using nanosecond-pulsed laser irradiation and magnetic gold nanoclusters (MGNCs) as the etching agents is described. Thermoplastic films (50-75 µm thickness) are coated with MGNCs then exposed to laser pulses with a coaxial magnetic field gradient, resulting in high-aspect ratio channels with tapered cross sections as characterized by confocal fluorescence tomography. The dry-etching process is applicable to a wide variety of substrates ranging from fluoropolymers to borosilicate glass, with etch rates in excess of 1 µm s-1 . Finite-element modeling suggests that the absorption of laser pulses by MGNCs can produce temperature spikes of nearly 1000 °C, which is sufficient for generating photoacoustic responses that can drive particles into the medium, guided by magnetomotive force.

Oblique Colloidal Lithography for the Fabrication of Nonconcentric Features.

Herein, we describe the development of oblique colloidal lithography (OCL) and establish a systematic patterning strategy for creating libraries of nanosized nonconcentric plasmonic structures. This strategy combines OCL, capillary force lithography, and several wet and ion etching steps. Hexagonal arrays of nonconcentric gold features were created on glass substrates with highly controllable geometric parameters. The size, geometry, and eccentricity of the gold features could be independently tuned by controlling the experimental conditions. Gaps within surface elements could be shrunk to as small as 30 nm, while the total patterned area was about l cm2. The goal was to devise a method that offers a high degree of control over the resolution and morphology of asymmetric structures without the need to resort to electron beam lithography. This technique also enabled the development of numerous surface patterns through the stepwise fabrication of separate elements. Complex features, including dots-surrounded nonconcentric targets, nonconcentric hexagram-disks, and nonconcentric annular aperture arrays, were demonstrated, and their optical properties were characterized. Indeed, spectroscopic studies and FDTD simulations demonstrated that Fano resonances could readily be generated by the nonconcentric gold features. Consequently, our patterning strategy should enable the high-throughput investigation of plasmonic coupling and Fano resonances as a function of the physical parameters of the elements within the nanopattern array.

Highly sensitive and scalable AAO-based nano-fibre SERS substrate for sensing application

Well-ordered periodic nanostructures are excellent substrates for many surface-enhanced Raman spectroscopy (SERS) applications. Conventional fabrication approaches such as high precision electron beam lithography or focused ion beam produce high resolution nano-features with great reproducibility at the expense of low throughput. In this work, a highly sensitive and scalable AAO-nano-fibre (ANF) SERS substrate is demonstrated by optimising the second anodisation time of the standard two-step anodisation of aluminium and performing an additional wet etching step on the resulting AAO substrate. The optimised ANF substrate exhibits SERS sensitivity that surpasses the AAO nanoholes and the metal-film-on-nanoparticles substrates. A detection limit of 0.1 nM is achieved with a signal-to-noise ratio of 2.6–3 using a low excitation power of 0.1 mW. The ANF substrate exhibits an enhancement factor of 9.28 × 106 and a standard deviation of no more than 8%. The results indicate that the highly sensitive and scalable ANF substrate is a promising substrate for commercial SERS application.

Process simplification for 15.6×15.6 cm2 interdigitated back contact silicon solar cells by laser doping

In this paper we propose a possible process simplification for pseudosquare 15.6×15.6cm2 Interdigitated Back Contact (IBC) CZ-Si solar cells. By using laser doping to create a selective BSF, one out of three diffusions and a wet etching step can be avoided, compared to a completely diffusion-based process flow. We show numerical optimization of this simplified cell structure in terms of: i) rear optics with additional SiNx capping layer, ii) contact fraction of the selective BSF. The results feature a top cell efficiency of 21.7% measured over the area of 239cm2. Further characterization implies that cells were limited mainly by: i) FF losses attributed to lateral majority carrier transport in the BSF region and increased J02 losses, ii) current loss which can be related to the BSF region.

Annealing-free copper source-drain electrodes based on copper–calcium diffusion barrier for amorphous silicon thin film transistor

The copper (Cu) source-drain electrodes based on copper–calcium (CuCa) diffusion barrier were fabricated without annealing process and in one wet etching step in order to develop the applications of Cu in hydrogenated amorphous silicon thin film transistor (α-Si:H TFT). The results show that oxygen flux and substrate temperature in depositing CuCa buffer layer affect greatly the adhesion of source-drain electrodes, and a perfect adhesion was obtained by an increasing oxygen flux to 4sccm or an increasing substrate temperature to 150°C, despite no annealing process. The specific resistance of source-drain electrodes has a slight increase with the increasing oxygen flux or substrate temperature or CuCa thickness. Auger electron spectroscopy (AES) show that the CuCa alloy barrier layer has perfect anti-diffusion between Cu film and α-Si:H. A much-desired taper angle of 43.4° and a little critical dimension (CD) bias of 0.91μm for the Cu/CuCa electrodes were obtained in one wet etching step. The α-Si:H TFT with the Cu/CuCa source-drain electrodes demonstrated the field-effect mobility of 0.73cm2/Vs, the subthreshold slope of 0.73V/dec, the threshold voltage of 0.45V, and the Ion/Ioff ratio of 106 due to the superior performances of the source-drain electrodes with the desired adhesion, specific resistance and taper angle despite no annealing process.

The properties of Cu metallization based on CuMgAl alloy buffer layer

The properties of Cu metallization based on CuMgAl alloy thin film as buffer layer were investigated in view of adhesion, diffusion and electronic properties in order to develop the applications of Cu metallization. The perfect adhesion of Cu film was obtained for the deposited CuMgAl buffer layer at 2.5sccm oxygen flux and 250°C substrate temperature. The resistivity of Cu film decreases with the increased substrate temperature and has a maximum for 2.5sccm oxygen flux in the deposition of CuMgAl film. A much-desired taper angle and a little critical dimension bias for the Cu/CuMgAl interconnect line were obtained in one wet etching step. Auger electron spectroscopy (AES) shows that the CuMgAl alloy barrier layer deposited at 2.5sccm oxygen flux and 250°C substrate temperature has good anti-diffusion between Cu film and substrate due to the formation of Mg and Al oxides in the interface of CuMgAl /substrate.

Symmetry-breaking nanostructures on crystalline silicon for enhanced light trapping in thin film solar cells.

We introduce a new approach to systematically break the symmetry in periodic nanostructures on a crystalline silicon surface. Our focus is inverted nanopyramid arrays with a prescribed symmetry. The arrangement and symmetry of nanopyramids are determined by etch mask design and its rotation with respect to the [110] orientation of the Si(001) substrate. This approach eliminates the need for using expensive off-cut silicon wafers. We also make use of low-cost, manufacturable, wet etching steps to fabricate the nanopyramids. Our experiment and computational modeling demonstrate that the symmetry breaking can increase the photovoltaic efficiency in thin-film silicon solar cells. For a 10-micron-thick active layer, the efficiency improves from 27.0 to 27.9% by enhanced light trapping over the broad sunlight spectrum. Our computation further reveals that this improvement would increase from 28.1 to 30.0% in the case of a 20-micron-thick active layer, when the unetched area between nanopyramids is minimized with over-etching. In addition to the immediate benefit to solar photovoltaics, our method of symmetry breaking provides a useful experimental platform to broadly study the effect of symmetry breaking on spectrally tuned light absorption and emission.

Optical Monitoring of the Capillary Filling Dynamics Variation in Nanoporous Anodic Alumina toward Sensing Applications.

Fluid imbibition-coupled laser interferometry (FICLI) is a technique in which the kinetics of a fluid infiltrating a nanoporous anodic alumina (NAA) membrane is monitored by the interference of a laser beam at the membrane top and bottom surfaces. Further processing of the measured data results in an estimate of the pore radius. In this work, we study the accuracy of FICLI in the detection of small changes in pore radius, and we evaluate the possibility of using such detection as a sensing paradigm. The accuracy is estimated by measuring samples with increasing pore radius, obtained by successive wet etching steps, and repeatability is evaluated by using different liquids. For decreasing pore radius, samples obtained by the successive deposition of polyelectrolyte double layers are used. With the aim of evaluating the possibility of the FICLI method to sense biological binding events, BSA attachment detection is demonstrated by applying FICLI to samples before and after immobilization of the protein. Results show that the technique permits an accurate estimation of the pore radius, the pore-etching rate (with a radius variation of retch,DI = 1.05 nm/min ± 0.11 nm/min), and the polyelectrolyte double layer thickness (with a radius variation of rPAH/PSS = 3.2 nm ± 0.2 nm per polyelectrolyte double layer). Furthermore, the pore radius reduction measured after BSA immobilization (dBSA = 4.9 nm ± 1.1 nm) is in good agreement with the protein size, as reported in the literature. With these results, we provide a sound basis for the applicability of FICLI as a sensitive technique for the characterization of NAA pore radius modifications.

Investigation of surface passivation schemes for p-type monocrystalline silicon solar cell

This paper represents an experiment to analyze the dark saturation current densities of passivated surfaces for monocrystalline silicon solar cells. The samples are diffused at peak temperatures of 800–950 °C. Basically, symmetrical lifetime samples with different doping profiles are prepared with alkaline textured and saw damage etched (planar) surfaces. After POCl3 diffusion, the phosphorous silicate glass layers are removed in a wet chemical etching step. Several designs are chosen for the determination of the sheet resistance (Rsh), the concentration profile for excess charge carrier and the minority carrier effective lifetime of the diffused surfaces. The dark saturation current densities (Jo) and the doping profiles are determined accordingly via quasi-steady state photoconductance decay measurement and electrochemical capacitance–voltage measurement. Three different passivation schemes are investigated as follows: silicon nitride (SiNx) deposited by plasma-enhanced chemical vapor deposition (PECVD) technique, silicon-rich oxynitride (SiriOxNy) capped with a PECVD SiNx layer, and thin thermally grown oxide, capped with a PECVD SiNx layer.

Effect of the duration of a wet KCN etching step and post deposition annealing on the efficiency of Cu2ZnSnSe4 solar cells

The influence of the duration of the KCN etching step on the efficiency of Cu2ZnSnSe4 (CZTSe) solar cells and Post deposition annealing (PDA) has been explored. CZTSe thin film absorbers prepared by selenization at 450°C were etched by 5wt% KCN/KOH from 30s up to 360s before solar cell processing. KCN etching times above 120s resulted in poor efficiencies. The fill factor (FF) and short circuit current density (Jsc) of these devices were affected severely. After annealing the solar cells at 200°C in N2 atmosphere the best devices degraded and poor devices improved. Combined physical and optoelectronic characterization of the solar cells showed that PDA modifies the bulk defect density and also surface composition which reflects in the solar cell performance.

Reliability of Highly Stable Amorphous-Silicon Thin-Film Transistors Under Low Gate-Field Stress—Part II: Optimization of Fabrication Conditions and Gate Voltage Dependence

Part I of this paper introduced a two-stage model for reliability characterization and lifetime prediction of amorphous-silicon thin-film transistors (a-Si TFTs) under low gate-field stress that includes both charge trapping in the silicon nitride (SiNx) gate dielectric and defect generation in the a-Si channel. In part II, the model is used to experimentally reduce the drain current instability under room temperature operation of a-Si TFTs under a prolonged gate bias of 5 V. Deposition conditions for the SiNx gate insulator and the a-Si channel layer were varied, and TFTs were fabricated with all reactive-ion-etch steps, or with all wet-etch steps. The stability of the a-Si channel also depends on the deposition conditions for the underlying SiNx gate insulator, and TFTs made with wet etching are more stable than TFTs made with reactive ion etching. Combining the various improvements raised the extrapolated 50% lifetime of the drain current of back-channel-passivated dry-etched TFTs under continuous operation in saturation at 20 °C with $V_{\mathrm{GS}}=\text{5} \text{V}$ from $\text{3}\times \text{10}^{4} \text{s}$ (9.2 h) to $\text{4}\times \text{10}^{7} \text{s}$ (1.4 years). We also extend the model, so that parameters from the degradation at one gate voltage can be used to estimate the degradation at other low gate voltages.

A novel technique for die-level post-processing of released optical MEMS

This work presents a novel die-level post-processing technique for dies including released movable structures. The procedure was applied to microelectromechanical systems (MEMS) chips that were fabricated in a commercial process, SOIMUMPs from MEMSCAP. It allows the performance of a clean DRIE etch of sidewalls on the diced chips enabling the optical testing of the pre-released MEMS mirrors through the chip edges. The etched patterns are defined by photolithography using photoresist spray coating. The photoresist thickness is tuned to create photoresist bridges over the pre-released gaps, protecting the released structures during subsequent wet processing steps. Then, the chips are subject to a sequence of wet and dry etching steps prior to dry photoresist removal in O2 plasma. Processed micromirrors were tested and found to rotate similarly to devices without processing, demonstrating that the post-processing procedure does not affect the mechanical performance of the devices significantly.

Dry-wet digital etching of Ge1−xSnx

The development of a precise micromachining process for Ge1–xSnx has the potential to enable both the fabrication and optimization of Ge1−xSnx-based devices in photonics and microelectromechanical systems. We demonstrate a digital etching scheme for Ge0.922Sn0.078 based on a two-stage, highly selective CF4 plasma dry etch and HCl wet etch. Using X-Ray Reflectivity, we show consistent etch control as low as 1.5 nm per cycle, which is defined as one dry etch step followed by one wet etch step. The etch rate increases to 3.2 nm per cycle for a longer dry etch time due to physical sputtering contributions, accompanied by an increase in RMS surface roughness. By operating within a regime with minimal sputtering, we demonstrate that good digital etch depth control and surface quality can be achieved using this technique.

Fabrication of narrow-gap nanostructures using electron-beam induced deposition etch masks

We present an optimized procedure to create narrow-gap nanostructures using electron-beam induced deposition etch masks. Incorporation of a Cr sacrificial layer in between Au metal layer and EBID mask improves the etch mask lithography in terms of dimension control and surface flatness. EBID mask writing is performed with a single-pass strategy varying the dose during writing. This patterning strategy allows us to create relatively thick EBID masks with steep, narrow gaps. An argon sputter etch is used to transfer the mask into the underlying material where the low etching rate of sacrificial layer compared to the EBID mask improves transfer of the mask pattern into the metal nanostructure. A Cr wet etch step is then used to remove sacrificial layer and EBID mask material. We show fabrication of Au dimer nanostructures with gap spacing well below 10nm, down to about 5nm, on Si substrates as well as ITO-coated glass. This method may enable the fabrication of nanostructures with small gap spacing and high degree of material purity and flatness on arbitrarily shaped surfaces.