Single Etch Step Research Articles

A novel hardmask-to-substrate pattern transfer method for creating 3D, multi-level, hierarchical, high aspect-ratio structures for applications in microfluidics and cooling technologies

This letter solves a major hurdle that mars photolithography-based fabrication of micro-mesoscale structures in silicon. Conventional photolithography is usually performed on smooth, flat wafer surfaces to lay a 2D design and subsequently etch it to create single-level features. It is, however, unable to process non-flat surfaces or already etched wafers and create more than one level in the structure. In this study, we have described a novel cleanroom-based process flow that allows for easy creation of such multi-level, hierarchical 3D structures in a substrate. This is achieved by introducing an ultra-thin sacrificial silicon dioxide hardmask layer on the substrate which is first 3D patterned via multiple rounds of lithography. This 3D pattern is then scaled vertically by a factor of 200–300 and transferred to the substrate underneath via a single shot deep etching step. The proposed method is also easily characterizable—using features of different topographies and dimensions, the etch rates and selectivities were quantified; this characterization information was later used while fabricating specific target structures. Furthermore, this study comprehensively compares the novel pattern transfer technique to already existing methods of creating multi-level structures, like grayscale lithography and chip stacking. The proposed process was found to be cheaper, faster, and easier to standardize compared to other methods—this made the overall process more reliable and repeatable. We hope it will encourage more research into hybrid structures that hold the key to dramatic performance improvements in several micro-mesoscale devices.

Compact and broadband silicon mode-order converter using bricked subwavelength gratings

A compact and broadband silicon mode-order converter (MOC) scheme by employing reciprocal mode evolution between asymmetric input/output taper and bricked subwavelength gratings (BSWG) is proposed. In the proposed MOC, a quasi-TE0 mode is generated in the BSWG region, which can be regarded as an effective bridge between the two TE modes to be converted. Flexible mode conversion can be realized by only choosing appropriate structure parameters for specific mode transitions between input/output modes and the quasi-TE0 mode. By combing 3D finite difference time domain (FDTD) and particle swarm optimization (PSO) method, TE0-TE1 and TE0-TE2 MOCs are optimal designed, which can efficiently convert TE0 mode to TE1 and TE2 modes with lengths of 9.39 µm and 11.27 µm, respectively. Results show that the insertion losses of <1 dB and crosstalk of <-15 dB are achieved for both TE0-TE1 and TE0-TE2 MOCs, the corresponding working bandwidth are 128 nm (1511∼1639 nm) and 126 nm (1527∼1653 nm), respectively. Additionally, the MOCs can be fabricated with only single etch step with minimum feature size of 145 nm.

Palladium-intercalated MXene membrane for efficient separation of H2/CO2: Combined experimental and modeling work

Research on gas separation membrane has targeted upon maximizing the selectivity and permeance. The development of 2D materials (e.g., MXene) gives a novel avenue to surpass the “trade-off” among the membrane selectivity and permeability. However, randomly stacked nanosheets and the regulation of interlayer spacing for MXene nanosheets hinder the development of MXene membrane. To this end, we designed a Pd-intercalated lamellar MXene membrane to modify the surface chemical properties (H-spillover effect) of MXene (Ti3C2Tx) and to expand the interlayer spacing (from 2.29 to 3.26 Å) due to the strong interaction between Pd2+ and MXene nanosheets with negative charge. Here, we performed a single-step etching approach to prepare 2D Pd-MXene membrane and demonstrated its excellent H2 permeability of 2.66 × 10−7 mol m−2 s−1·Pa−1 and H2/CO2 selectivity of 242 at 25 °C, which greatly exceeds the current benchmark membranes. We also developed single gas transport mathematical mechanism models to interpret and support the experimental results. The combined experimental and modeling results show that interlayer spacing and H-spillover effect of Pd-MXene membrane benefit H2/CO2 separation.

Polarization-independent multimode interference coupler with anisotropy-engineered bricked metamaterial

Many applications, including optical multiplexing, switching, and detection, call for low-cost and broadband photonic devices with polarization-independent operation. While the silicon-on-insulator platform is well positioned to fulfill most of these requirements, its strong birefringence hinders the development of polarization-agnostic devices. Here we leverage the recently proposed bricked metamaterial topology to design, for the first time, to our knowledge, a polarization-independent 2 × 2 multimode interference coupler using standard 220 nm silicon thickness. Our device can be fabricated with a single etch step and is optimized for the O-band, covering a wavelength range of 160 nm with excess loss, polarization-dependent loss, and imbalance below 1 dB and phase errors of less than 5°, as demonstrated with full three-dimensional finite-difference time-domain simulations.

High-Performance Polarization Splitter-Rotator Based on Lithium Niobate-on-Insulator Platform

We propose and experimentally demonstrate a high-performance polarization splitter-rotator based on the lithium niobate-on-insulator platform, consisting of an adiabatic taper, an asymmetric directional coupler, and a multimode interference mode splitter. Two injecting modes with orthogonal polarization states were separated, and the transverse magnetic (TM) polarization state was rotated to the transverse electric (TE) polarization state simultaneously. The designed device is fabricated in a single-step etching process, providing an extinction ratio of 20 dB over 80 nm wavelength range and insertion loss of less than 0.65 dB for both TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> modes launched. Lithium niobate-on-insulator, polarization splitter-rotator, extinction ratio.

Neural optoelectrodes merging semiconductor scalability with polymeric-like bendability for low damage acute in vivo neuron readout and stimulation

Neural optoelectrodes can read and manipulate large numbers of neurons in vivo. However, state-of-the-art devices rely on either standard microfabrication materials (i.e., silicon and silicon nitride), which result in high scalability and throughput but cause severe brain damage due to implant stiffness, or polymeric devices, which are more compliant but whose scalability and implantation in the brain are challenging. Here, we merge the gap between silicon-based fabrication scalability and low (polymeric-like) stiffness by fabricating a nitride and oxide-based optoelectrode with a high density of sensing microelectrodes, passive photonic circuits, and a very small tip thickness (5 μm). We achieve this by removing all the silicon supporting material underneath the probe’s tip—while leaving only the nitride and glass optical ultrathin layers—through a single isotropic etch step. Our optoelectrode integrates 64 electrodes and multiple passive optical outputs, resulting in a cross-sectional area coefficient (the cross section divided by the number of sensors and light emitters) of 3.1—smaller than other optoelectrodes. It also combines a low bending stiffness (∼4.4 × 10−11 N m2), comparable or approaching several state-of-the-art polymeric optoelectrodes. We tested several mechanical insertions of our devices in vivo in rats and demonstrated that we can pierce the pia without using additional temporary supports.

Broadband adiabatic polarization rotator-splitter based on a lithium niobate on insulator platform

Polarization rotator-splitters (PRSs) are crucial components for controlling the polarization states of light in classical and quantum communication systems. We design and experimentally demonstrate a broadband PRS based on the lithium niobate on insulator (LNOI) platform. Both the rotator and splitter sections are based on adiabatically tapered waveguide structures, and the whole device only requires a single etching step. We show efficient PRS operation over an experimentally measured bandwidth of 130 nm at telecom wavelengths, potentially as wide as 500 nm according to simulation prediction, with relatively low polarization crosstalks of ∼ − 10 dB . Our PRS is highly compatible with the design constraints and fabrication processes of common LNOI photonic devices, and it could become an important element in future LNOI photonic integrated circuits.

Wafer-level testing of inverse-designed and adjoint-inspired vertical grating coupler designs compatible with DUV lithography.

Perfectly vertical grating couplers have various applications in optical I/O such as connector design, coupling to multicore optical fibers and multilayer silicon photonics. However, it is challenging to achieve perfectly vertical coupling without simultaneously increasing reflection. In this paper, we use the adjoint method as well as an adjoint-inspired methodology to design devices that can be fabricated using only a single-etch step in a c-Si 193 nm DUV immersion lithography process, while maintaining good coupling and low reflection. Wafer-level testing of devices fabricated by a pilot line foundry confirms that both design paradigms result in state-of-the-art experimental insertion loss (<2 dB) and bandwidths (∼20 nm) while having only moderate in-band reflection (<-10 dB). Our best design has a (median) 1.82 dB insertion loss and 21.3 nm 1 dB-bandwidth.

Design, fabrication, and characterisation of a silicon microneedle array for transdermal therapeutic delivery using a single step wet etch process

The fabrication of silicon in-plane microneedle arrays from a simple single wet etch step is presented. The characteristic 54.7° sidewall etch angle obtained via KOH etching of (100) orientation silicon wafers has been used to create a novel microneedle design. The KOH simultaneously etches both the front and back sides of the wafer to produce V shaped grooves, that intersect to form a sharp pyramidal six-sided microneedle tip. This method allows fabrication of solid microneedles with different geometries to determine the optimal microneedle length and width for effective penetration and minimally invasive drug delivery. A modified grooved microneedle design can also be used to create a hollow microneedle, via bonding of two grooved microneedles together, creating an enclosed hollow channel. The microneedle arrays developed, effectively penetrate the skin without significant indentation, thereby enabling effective delivery of active ingredients via either a poke and patch application using solid microneedles or direct injection using hollow microneedles. This simple, scalable and cost effective method utilises KOH to etch the silicon wafer in-plane, allowing microneedles with variable length of several mm to be fabricated, as opposed to out-of-plane MNs, which are geometrically restricted to dimensions less than the thickness of the wafer. These microneedle arrays have been used to demonstrate effective delivery of insulin and hyaluronic acid into the skin.

Polyimide moth-eye nanostructures formed by oxygen ion beam etching for anti-reflection layers

Here, we report the use of a simple oxygen ion-beam etching process to fabricate polyimide moth-eye-like nanostructures as anti-reflection layers. The etching process formed moth-eye nanostructures directly onto polyimide layer surfaces that covered both flat and curved surfaces. Notably, this approach removed the need for a masking process, which reduced the number of separate fabrication steps compared with conventional methods that require repeated moth-eye etching on flat and hard materials, such as ceramics and curve surfaces. The proposed method formed moth-eyes through a single step etching process on both flat and curved substrates. The moth-eyes had an average height and pitch of approximately 700 and 100 nm, respectively, on a synthesized polyimide layer on both flat and curved substrates. The reflectivity of the fabricated polyimide moth-eye was as low as 0.2%–0.48%. The utility of the method to form anti-reflection layers in practical optical devices is also confirmed.

SOI multimode waveguide bend with radially bridged SWGs for broadband (de)multiplexing -INVITED

A novel broadband multimode waveguide bend is proposed that supports the propagation of multiple TE modes on a silicon-on-insulator platform. The gradient curvature bend utilizes trapezoidal subwavelength grating (SWG) segments, connected by adiabatically tapered radial strips to achieve efficient mode (de)multiplexing. The inclusion of the radial strips offers an extra degree of design freedom, allowing the realization of a multimode bend with only one single full etch step. The access waveguide has a width of 2.075 μm with an effective radius of 10 μm. Propagation loss for all modes remains below 2.96 dB, and intermodal crosstalk has a maximum of -19 dB across a broad bandwidth of 100 nm, centred at 1550 nm. This work presents an excellent design choice for broadband mode-division multiplexing operations.

К 125-летию со дня рождения лауреата Нобелевской премии академика Николая Николаевича Семенова Архитектура мезы и эффективность InGaP/Ga(In)As/Ge солнечных элементов

It has been shown that the mesa architecture and achieved quality of the mesa sidewall of concentrator multijunction solar cells ensured increasing their efficiency up to 36.7% at the sunlight concentration before 100 (AMO; 0.136W/cm²). Creation of the mesa structure architecture with following separation of epitaxial plates of monolithic InGaP/GaAs/Ge nanoheterostructure into chips was carried out by single step chemical wet etching in the HBr:H2O2:H2O (8:1:100) through a photoresist mask to the depth of 12-18µm. Conditions of single step etching, which ensure formation of a smooth and even side surface of the InGaP/Ga(In)As/Ge nanoheterostructure mesa containing different in composition and thickness layers have been determined. Determination of the activation energy has shown that etching occurs in the diffusion region of the heterogeneous process. In raising etchant temperature from 2 to 36˚C, a change of the tilt angle in the Ge substrate region from 4.5 to 25 angular degrees is observed, what allows optimizing the amount of concentrator solar cells and their quality at final mechanical separation of the epitaxial plate into chips.

Silicon subwavelength modal Bragg grating filters with narrow bandwidth and high optical rejection.

Waveguide Bragg grating filters with narrow bandwidths and high optical rejections are key functions for several advanced silicon photonics circuits. Here, we propose and demonstrate a new, to the best of our knowledge, Bragg grating geometry that provides a narrowband and high rejection response. It combines the advantages of subwavelength and modal engineering. As a proof-of-concept demonstration, we implement the proposed Bragg filters in 220-nm-thick Si technology with a single etch step. We experimentally show flexible control of the filter selectivity, with measured null-to-null bandwidths below 2 nm, and strength of 60 dB rejection with a null-to-null bandwidth of 1.8 nm.

Inverse design of a single-step-etched ultracompact silicon polarization rotator.

We propose and experimentally demonstrate a novel ultracompact silicon polarization rotator based on equivalent asymmetric waveguide cross section in only single-step etching procedure for densely integrated on-chip mode-division multiplexing system. In the conventional mode hybridization scheme, the asymmetric waveguide cross section is employed to excite the hybridized modes to realize high performance polarization rotator with compact footprint and high polarization extinction ratio. However, the fabrication complexity severely restricts the potential application of asymmetric waveguide cross section. We use inverse-designed photonic-crystal-like subwavelength structure to realize an equivalent asymmetric waveguide cross section, which can be fabricated in only single-step etching process. Besides, a theory-assisted inverse design method based on a manually-set initial pattern is employed to optimize the device to improve design efficiency and device perform. The fabricated device exhibited high performance with a compact footprint of only 1.2 × 7.2 µm2, high extinction ratio (> 19 dB) and low insertion loss (< 0.7 dB) from 1530 to 1590 nm.

Flow control in fully enclosed microfluidics paper based analytical devices using plasma processes

Microfluidic paper based analytical devices (μ-PADs) have shown promising results for disease diagnostics in resource limited settings but they often lack effective fluid flow control in the hydrophilic channels. Flow control in μ-PADs is necessary to conduct multiple step assays with minimum user interference. Using fluorocarbon (pentafluoroethane) plasma deposition followed by O2 plasma etching with a metal mask, flow control functionality in fully enclosed channels was achieved. Channels with up to a 10-fold difference in wicking rate were fabricated by changing the O2 plasma etch time. Metal meshes were used to locally vary the O2 plasma etch rate and therefore fabricate channels with different wettability inside a single paper substrate in a single etch step. The channels in fully enclosed devices are isolated, thereby preventing contamination due to handling and environmental exposure. To gain additional reliability, channels were covered with non-porous adhesive tape to minimize the effect of evaporation on fluid flow to eliminate variation in device operation under different humidity conditions. Proof of concept was demonstrated by fabricating and operating a device with three channels that converged to a single reaction zone. Solutions of different pH values were sequentially transported to and reacted with an indicator deposited at the reaction zone. μ-PADs fabricated by this method are suitable for field use when conducting complex low-cost diagnostic tests.

Design and simulation of a compact and ultra-wideband polarization beam splitter based on sub-wavelength grating multimode interference coupler

A compact and ultra-wideband multimode interferometer (MMI)-based polarization beam splitter (PBS) is designed in a silicon-on-insulator (SOI) platform. A sub-wavelength grating (SWG) structure is employed in the multimode region to reduce the overall length of the structure and also increase its operating bandwidth. Instead of using the beat-length difference to separate the transverse electric (TE) and transverse magnetic (TM) polarized waves, the TM waves are directly coupled to the output bar port through an interconnecting waveguide, while the TE polarized waves are transmitted to the cross port by the self-imaging process. This substantially reduces the length of the PBS and the overall footprint can be as small as 4.8 × 21 µm2. The proposed device exhibits the extinction ratio of more than 15 dB in a 250 nm wavelength range (1.38–1.63 µm) and more than 12 dB in the 350 nm wavelength range (1.3–1.65 µm) for the TE and TM polarizations, respectively. In addition, the insertion loss remains below 1.2 dB for both the TE and TM polarization in a broad wavelength range from 1.38 to 1.54 µm. The proposed PBS can be fabricated by a single-step etching process on an SOI wafer. Moreover, our simulations show the robustness of the PBS performance to the fabrication errors’.

Simulation-based optimization of out-of-plane, variable-height, convoluted quartz micro needle arrays via single-step anisotropic wet etching

Variable-height, out-of-plane, convoluted microneedle arrays with high aspect ratio, tapered shafts, sharp tips, and smooth surfaces are required for applications where the microneedle heights must conform to a specific target geometry. Here, we show that it is possible to achieve such convoluted microneedle arrays on a Z-cut quartz substrate by a single wet etching step in saturated NH4HF2 at 80 °C. Due to the low etch rate of quartz, the procedure is based on carefully controlling the dimensions of every needle-like structure by properly dimensioning the masked area on top. In particular, we show that the mask dimensions for all the microneedles can be optimized via a combination of Level Set (LS) simulations of wet etching and an Evolutionary Algorithm (EA) to search the optimal dimensions. As an example, we fabricate two different microneedle arrays, where the needle heights follow two different spherical surfaces. Our experiments confirm that (i) the LS method correctly predicts the evolution of the etch front for microneedles etched on AT, BT and Z quartz cuts; and (ii) the LS + EA approach provides a suitable procedure to determine an optimal collection of mask sizes for the fabrication of convoluted microneedle arrays on the Z cut.

Experimental demonstration of metamaterial anisotropy engineering for broadband on-chip polarization beam splitting.

Subwavelength metamaterials exhibit a strong anisotropy that can be leveraged to implement high-performance polarization handling devices in silicon-on-insulator. Whereas these devices benefit from single-etch step fabrication, many of them require small feature sizes or specialized cladding materials. The anisotropic response of subwavelength metamaterials can be further engineered by tilting its constituent elements away from the optical axis, providing an additional degree of freedom in the design. In this work, we demonstrate this feature through the design, fabrication and experimental characterization of a robust multimode interference polarization beam splitter based on tilted subwavelength gratings. A 110-nm minimum feature size and a standard silicon dioxide cladding are maintained. The resulting device exhibits insertion loss as low as 1 dB, an extinction ratio better than 13 dB in a 120-nm bandwidth, and robust tolerances to fabrication deviations.

Mid-infrared widely tunable single-mode interband cascade lasers based on V-coupled cavities.

We demonstrate widely tunable single-mode V-coupled-cavity lasers emitting at wavelengths near 3 µm based on a type-II interband cascade (IC) structure. The mode selection is achieved using a half-wave V-coupler designed for the IC structure in the mid-infrared range. The laser waveguides and cavity structure are deeply etched in a single etching step, without any grating. By changing the injection current at a fixed heat-sink temperature, a tuning range over 35 nm can be achieved with a side-mode suppression-ratio up to 28 dB. The tuning range can be extended to 60 nm when combined with the adjustments of the heat-sink temperature.

Investigation and optimization of reactive ion etching of Si3N4 and polyphthalaldehyde for two-step gray scale fabrication of diffractive optics

Nanofabrication of x-ray diffractive optics using electron beam lithography requires a complex process of electron exposure optimization and resist development. Thermal scanning probe lithography (TSPL) offers a high resolution, maskless, gray scale patterning method with reduced complexity. Thin diffractive optics with high efficiency for the extreme ultraviolet (EUV) and soft x-ray (SXR) photon range could be fabricated by combining TSPL with a single etching step if the TSPL resist, polyphthalaldehyde (PPA), can be used as an etch mask to direct-etch the pattern into a substrate using reactive ion etching. This condition critically depends on high etch selectivity between the substrate and the PPA, because TSPL resolution deteriorates as the PPA patterning depth increases beyond tens of nanometers. In this work, the authors have evaluated the etch selectivity for PPA and Si3N4 using SF6/C4F8 gases and the influence of process parameters, including gas flow rate, vacuum pressure, radio frequency bias power, and inductively coupled plasma power. The experimental results indicate that an etch selectivity of 7 (Si3N4:PPA) is achievable, and the authors demonstrate that diffractive optics for EUV/SXR can be fabricated in only two steps.