This study aims to establish a quick and sustainable approach for optimising the growth of millimetre-tall vertically aligned carbon nanotube (VACNT) arrays through simple catalyst tuning. To achieve this swiftly, a rapid and handy characterisation protocol is essential. Herein, we propose and validate such a method for the rapid characterisation of VACNTs, which concurrently improves upon the traditional weight-gain density calculation for VACNT arrays. We demonstrate the efficacy of this combined strategy by systematically investigating the influence of iron (Fe) catalyst film thickness (0.8-2.0 nm) on the microstructure and density of VACNT arrays synthesised via thermal chemical vapour deposition (TCVD). Catalyst morphology was characterised by atomic force microscopy (AFM), while transmission electron microscopy (TEM) provided nanotube diameter and wall-number distributions. Scanning electron microscopy (SEM) measured array height for areal density estimation, with Fourier component analysis of SEM cross-sections applied to correct height underestimations from tube tortuosity. It was observed that increasing Fe thickness enlarges and broadens catalyst particle distributions, enabling modest tuning of VACNT dimensions without compromising packing density. An optimum film thickness of similar to 1.1 nm was identified, maximising areal density at similar to 2 x 10(10) CNTs cm(-2). The integrated imaging and analysis approach presented here offers a practical and efficient pathway for rapid VACNT microstructure control and density assessment, facilitating future optimisation cycles. Furthermore, a transition in growth mode was observed, which further confirms the decisive role of the catalyst film thickness.

This paper presents a wafer-scale silicon microfabrication technology for the sub-terahertz (sub-THz) waveguide device mass production. Based on the effective scheme, a WR-5 (140-220 GHz) straight rectangular waveguide and a WR-2.8 (260-400 GHz) rectangular waveguide bandpass filter are implemented as demonstrated examples. The silicon deep reactive ion etching (DRIE) process is employed to etch through the total thickness of the silicon wafer and form the main waveguide channels. Then, a low-temperature thermal compression process is used to bond the trough-etched wafer with the top and bottom metallised silicon wafers to form the closed waveguide structures without any precise alignment process. The fabricated waveguide has the benefit of low transmission loss (0.03-0.05 dB mm-1) at the whole G band. Besides, to measure the fabricated WR-2.8 waveguide filter and solve the measuring equipment standard waveguide difference, silicon micromachined waveguide transitions are explored and fabricated to match two different frequency-band modules for measuring the waveguide filters in the desired full frequency band, which also has a potential application for the different size waveguide conversion. The measured results agree well with the simulated ones. The measured 3 dB bandwidth is 9.3%, with a central frequency of 343 GHz; the average insertion loss (IL) is about 1.6 dB in the pass band, including two extra straight waveguides of 8 mm length on input/output ends and two external waveguide-to-waveguide transitions. The proposed method provides a feasible and cost-effective solution for the mass production of high-performance waveguide devices and integrated systems in sub-THz frequency bands and beyond.

In this paper, we present a fifth-order sub-THz direct-coupled E-plane waveguide bandpass filter fabricated through micromachining. The all-pole filter operates at the center frequency of 400 GHz with fractional bandwidth of 5% and occupies an area of only 1 mm2. The filter can be directly inserted between two standard WR-2.2 waveguide flanges due to axially arranged interfaces. The compactness and axial interfaces are simultaneously achieved by bending of the E-plane filter. The structure is realized using an E-plane split design where waveguides are etched in the handle layer of a silicon-on-insulator (SOI) wafer and couplings are realized through E-plane septa fabricated in the SOI device layer. External couplings are facilitated by means of slots in the device layer. The proposed design is highly resistant to the underetching effect, as E-plane septa are not influenced by the underetching, thus enabling accurate coupling control. The measured return loss is better than 20 dB in the most of the passband, with the worst-case return loss of 11 dB, and the insertion loss of only 1.1 dB is measured. A very good agreement between measured and simulated data is obtained.

This paper explores the impact of etch holes on sub-terahertz (THz) waveguide transmission performance and presents a silicon micromachined WR-03 waveguide with etch holes. The influence of etch-hole size and grid size is investigated through simulations. The waveguide prototype is realized through deep reactive ion etching (DRIE) and low-temperature thermal-compression bonding. Measurements reveal a low transmission loss (average 0.04 dB/mm) for the etched waveguide, with negligible influence on transmission loss observed for etch hole sizes below a specific threshold, which agrees with the simulations. The findings guide the design and optimization of micromachined THz waveguide components with etch holes applied in some specific situations.

This paper investigates the use of vertically aligned carbon nanotube (VACNT) arrays as building blocks for mm-wave waveguide devices. The fabrication, characterization, and performance evaluation of VACNT-based waveguides are presented. The waveguides are fabricated using a low-temperature 3D packaging technology, and their transmission properties are assessed through S-parameter measurements. The results demonstrate the potentiality of the VACNT-based waveguide topologies within the mm-wave frequency range. Simulations are conducted to understand the loss mechanisms in VACNT-based waveguides and compared with experimental measurements. The low conductivity of the CNT array introduces additional loss compared to metal-based waveguides. Bonding misalignment is also found to impact the overall loss.

This article presents a new, very compact silicon-micromachined two-port waveguide platform that features axial ports but allows to integrate waveguide devices, which is enabled by integrated broadband, back-to-back, stepped E- and H-plane bends. This approach of rotating the waveguide plane in two axes enables devices of complex geometry to be directly mounted between two waveguide flanges. A prototype transition operating at 220–320 GHz has experimentally demonstrated an insertion loss of less than 0.2 dB and a return loss of better than 18.5 dB throughout the entire WM-864 waveguide band, characterized by an integrated on-chip waveguide calibration kit. Two 5th-order waveguide filters have been designed as complex demonstrator devices and co-fabricated with the proposed axial integration platform, demonstrating a direct-coupled filter at 270 GHz and a cross-coupled filter at 300 GHz with three transmission zeros. The filters with 1.85% and 2% fractional bandwidths demonstrate a measured insertion loss of 1.92 and 1.50 dB, respectively, to which the two transitions combined add a total of 0.3 dB, and a return loss of 19 and 15 dB in the passbands, respectively. The unloaded $Q$ -factors for the resonator cavities of the two filters were extracted to 750 and 900, respectively. These results are so far unparalleled for filters of similar complexity in this frequency range. The sensitivity to fabrication tolerances is analyzed.

In this paper, we present two bandpass filters at 687.5 and 700 GHz with fractional bandwidths (FBW) of 3.64% and 1% respectively. Both 4th-order all-pole filters utilize a pair of dual-mode cavities: the first filter uses elliptic cavities with quasi-TMno degenerate modes, while the second one uses rectangular cavities with TM410-TM140 modes. In the latter, the coupling slots between the cavities are arranged to enhance the stopband performance by suppressing spurious resonances in the stopband. The filters are fabricated using silicon micromachining with gold metallization. The measured average insertion loss in the passband of the 3.64% FBW filter is 1.45 dB, and 2.5 dB for the 1% FBW filter. The experimentally extracted unloaded quality-factors are 450 for the elliptic cavities and 950 for the rectangular cavities. The measured filter performance of the first prototypes agrees very well with the simulation results, exhibiting a frequency shift of less than 0.7%. These are the best insertion loss and quality factors ever published in this frequency range for narrow-band filters, and the first time that a 1%-FBW microwave filter is demonstrated above 500 GHz.

In this work, we propose a new concept of phase shifting by tuning the coupling distance between two parallel silicon slabs that support the propagation of an in-phase supermode. A prototype device was implemented in the 220-330-GHz band by integrating the micro-electro-mechanical system (MEMS)-actuated phase-shifting mechanism, based on two moveable parallel-coupled high-resistivity silicon slabs, inside a metallized hollow rectangular waveguide in silicon-on-insulator (SOI) micromachining technology. The prototype device has been characterized to a maximum continuous phase tunability of 550 degrees with a maximum insertion loss of 1.87 dB achieved at 330 GHz. A maximum figure-of-merit (FOM) of 375 degrees/dB is achieved at around 320 GHz, which presently is the highest FOM value reported for any phase shifter in the subterahertz (sub-THz) frequency range. The bandwidth covers the whole waveguide band with an average and worst case insertion loss of 1.94 and 2.5 dB, respectively, a worst case insertion-loss variation of 0.7 dB and a phase error better than 4 degrees for all phase states. A large displacement of MEMS electrostatic comb-drive actuators that are co-fabricated in the micromachined waveguide platform is achieved by a driving voltage of 50 V. The prototype footprint is $7.3 \times 5.3$ mm(2).

This letter presents an under-etching prevention method for an aqueous hydrofluoric acid (HF) releasing process by adding a single-side low-pressure chemical vapor deposition (LPCVD) silicon protection layer. The proposed method enables advanced silicon-on-insulator (SOI) based millimeter-wave/terahertz (mmW/THz) MEMS waveguide devices, which require for RF performance a complete metallization film over the SOI buried-oxide (BOX) layer, and simultaneously need a locally under-etched BOX layer for implementing MEMS actuators reconfiguring the devices. A comparison between the WR-3.4 waveguide (220-330 GHz) using the proposed under-etching prevention method and the one without using it proves the effectiveness and feasibility during HF releasing processes. A MEMS tunable phase shifter driven by comb-drive actuators has been successfully implemented by applying this method in a micromachined waveguide. [2021-0036]

A D-band waveguide diplexer, implemented by silicon micromachining using releasable filling structure (RFS) technique to obtain high-precision geometries, is presented here for the first time. Prototype devices using this RFS technique are compared with devices using the conventional microfabrication process. The RFS technique allows etching large waveguide structures with nearly 90° sidewall angles for the 400-μm-tall waveguides. The diplexer consists of two direct-coupled cavity six-pole bandpass filters, with the lower and the upper band at 130-134 and 141-148.5 GHz, respectively. The measured insertion loss of the two bands is 1.2 and 0.8 dB, respectively, and the measured return loss is 20 and 18 dB, respectively, across 85% of the passbands. The worst case adjacent channel rejection is better than 59 dB. The unloaded quality factors of a single cavity resonator are estimated from the measurements to reach 1400. Furthermore, for the RFS-based micromachined diplexer, an excellent agreement between measured and simulated data was observed, with a center frequency shift of only 0.8% and a bandwidth deviation of only 8%. In contrast to that, for the conventionally micromachined diplexer of this high complexity, the filter poles are not well controllable, resulting in a large center frequency shift of 3.5%, a huge bandwidth expanding of over 60%, a poor return loss of 6 and 10 dB for the lower and the upper band, respectively, and an adjacent channel rejection of only 22 dB.

The microfabrication technology based on Silicon-on-Insulator (SOI) for waveguide devices at sub-terahertz frequencies is investigated in this work. The relationship between the critical micromachining parameters and the performance of waveguide devices is discussed for achieving a better understanding of the SOI-based waveguide micromachining processes. Some typical sub-THz waveguide structures are demonstrated to verify the analysis to the proposed micromachining processes accordingly. SOI-based micromachining technology is one of the most promising methods to realize sub-THz components and their integration.

This letter presents two silicon-micromachined narrowband fourth-order waveguide filter concepts with center frequency of 450 GHz, which are the first narrowband submillimeter-wave filters implemented in any technology with a fractional bandwidth as low as 1%. Both filters designs are highly compact and have axial port arrangements, so that they can be mounted directly between two standard waveguide flanges without needing any split-block interposers. The first filter concept contains two TM110 dual-mode cavities of circular shape with coupling slots and perturbations arranged in two vertically stacked layers, while the second filter concept is composed of four TE101 series resonators arranged in a folded, two-level topology without cross-couplings. Prototype devices are fabricated in a multilayer chip platform by high-precision, low-surface roughness deep-silicon etching on silicon-on-insulator wafers. The measured passband insertion loss of two prototype devices of the dual-mode circular-cavity filters is 2.3 dB, and 2.6 dB for three prototypes of the folded filter design. The corresponding extracted unloaded quality factors of the resonators are 786 +/- 7 and 703 +/- 13, respectively, which are the best so far reported for submillimeter-wave filters in any technology. The presented filters are extremely compact in terms of size; their footprints have areas of only 0.53 and 0.55 mm(2), respectively, and the thickness between the waveguide flanges is 0.9 mm.

This paper presents a narrowband silicon-micro machined 4th-order waveguide filter concept at 300 GHz with 2% fractional bandwidth, which is highly compact and has axial port arrangements, so that it can be mounted directly between two standard waveguide flanges without needing any split-block interposers. This filter is composed of four TE101 series resonators arranged in one plane which leads to a very small size in real application direction. Underetching effects on the response of the filter are investigated. Prototype devices are fabricated by deep-silicon etching on silicon-on-insulator wafers with a standard process. The measured passband insertion loss is 1.65 dB. The corresponding extracted unloaded quality factor of the resonators is about 600. The filters demonstrate excellent repeatability of the measured S-parameters on a single chip. The presented filters are extremely compact in terms of size; their footprints have areas of only 2 mm 2 , and the length reaches 0.6 mm in the practical application direction.

This paper investigates the fabrication accuracy and repeatability of micromachined quadruplet filters designed at a center frequency of 270 GHz with a 5-GHz bandwidth using a versatile multilayer chip platform which allows for axially arranged waveguide ports. A large number of narrowband silicon-micromachined filters arranged on multiple chips are investigated for fabrication imperfections, assembly misalignment, and fabrication yield, employing fabrication-prediction and different chip-to-chip self-alignment feature strategies. A numerical technique for characterization of the entire fabrication process of the filters through extracting the error statistics for coupling coefficients of a large number of different samples from separately assembled chips is proposed. A total of 47 test filters in effectively 15 different design variants have been fabricated in two fabrication runs, evaluated, and analyzed. The most critical sources of errors are determined. The expected accuracy of the entire filters fabrication process is demonstrated through the yield analysis based on the collected error statistics.

A WR-3 dual-band bandpass filter based on micromachining fabrication process is presented in this paper. The topological structure of the dual-band filter is formed by a pair of parallel coupled single-band bandpass filter and two T-shaped matching stubs. While each single-band filter is designed by Chebyshev 6-stage inductive obstacle structures. Then the dual-band filter is fabricated and measured. The results show that the dual passbands have about 7 GHz downwards shift than the designing. The rectangle coefficient of the dual passbands is 1.33 and 1.42, respectively. The return loss of the proposed filter is better than 14 and 12 dB while the insertion loss is around 3.6 and 7.1 dB. The whole size of the fabricated filter is about 21 mm × 11 mm × 1.48 mm. It demonstrates a practical way to design, fabricate and measure the dual-band filter working in terahertz range with good rectangle coefficient.

A new method for preparing the Spindt field emission cathode array was proposed by combining the polystyrene nanosphere self-assembly with micro-mechanical manufacturing technologies.The fabricated hole array based on the present method possessed a high density and highly uniform periodic sub-micrometer size,where the diameter of the gate hole was in the range of 300-500 nm,the depth of the hole was 500 nm,and the periodic of the structural unit was 750 nm.The high micro-hole density could reach 108/cm2,being more than ten times the density of ordinary lithography.The field emission characteristics of the Spindt cathode were simulated with the particle-in-cell solver of CST particle studio.Simulation results showed that the radius of the emission tip,diameter of the gate hole as well as the relative height difference between the emitter tip and the gate are key factors affecting the field emission current.

This paper gives an overview on recent achievements in micromachined technology for millimeter and submillimeter-wave applications, from 130 to 750 GHz. The micromachined components presented inclu ...

In this paper, we present a microfabricated fourth-order sub-THz WR-3.4 bandpass waveguide filter based on TM110 dual-mode circular-shaped cavity resonators. The filter operates at the center frequency of 270 GHz with fractional bandwidth of 1.85% and two transmission zeros are introduced in the upper and in the lower stopband using a virtual negative coupling. The microchip filter is significantly more compact than any previous dual-mode designs at comparable frequencies, occupying less than 1.5 mm 2 . Furthermore, in contrast to any previous micromachined filter work, due to its axially arranged interfaces it can be directly inserted between two standard WR-3.4 rectangular-waveguide flanges, which vastly improves system integration as compared to previous micromachined filters; in particular no custom-made split-block design is required. The cavities are etched in the handle layer of a silicon-on-insulator (SOI) wafer, and coupling is realized through rectangular slots fabricated in the SOI device layer. Couplings of the degenerate modes in one cavity are facilitated by means of small perturbations in the circular cavity shapes. The measured average return loss in the passband is −18 dB and worst-case return loss is −15 dB, and an insertion loss of only 1.5 dB was measured. The excellent agreement between measured and simulated data is facilitated by fabrication accuracy, design robustness and micromachined self-alignment geometries.

We investigated a dielectric transmission line consisting of three dielectric substrate layers for millimeter-wave (mmW) applications. This line is referred to as a narrowed dielectric microstrip line (N-DML). The effective dielectric constant method is used to analyze the propagation characteristics of the N-DML. The result of the analysis of the normalized phase constant is consistent with the simulation. For demonstration, two J-band N-DML prototypes of different lengths with transitions to rectangular waveguides were designed, fabricated, and measured. Good agreement between the simulated and measured results is observed. In particular, due to its nonmetal structure, the N-DML shows a low average attenuation constant of only 0.52 dB/cm over 220-280 GHz in the experiment. Due to its good performance, ease of fabrication, low loss, and low cost, the N-DML is a suitable candidate for various mmW applications.