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 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. 

With the development of terahertz (THz) technology, a variety of application demands are growing rapidly, such as high-rate communications, THz radars, environmental monitoring, medical imaging, and space exploration. However, the fabrication, integration, and packaging techniques for THz components and systems pose great challenges for a large-scale, cost-effective production. The current THz technology relies on the conventional and expensive serial fabrication and packaging techniques, such as computer numerical control (CNC) high-precision machining, which are suitable only for high-end research instrumentation or one-off prototypes. Nowadays, THz microelectromechanical system (MEMS) is a leading candidate to realize high-precision, low-cost, large-volume fabrication, and integration for miniaturized THz components and systems. In this thesis, several key components and technologies of THz MEMS are developed towards the progression of future THz microsystem front-ends.

In the following research, D-band filters and diplexers have been implemented by an advanced Si micromachining technology based on a releasable-filling-structure (RFS) approach which can achieve high-precision geometries for THz waveguide devices. Fabrication imperfection is a big issue which affects the performance of the devices, namely, insertion loss, bandwidth, and operation frequency. The RFS-based silicon-on-insulator (SOI) micromachining technology improves the deep reactive ion etching (DRIE) processing performance, especially sidewall verticality, by utilizing extra structures to fill the large areas, which in turn, can obtain uniform etching aspect ratios.

The state-of-the-art MEMS phase shifter based on a waveguide-integrated SOI micromachining technology has been successfully demonstrated at 220-330 GHz, with a full-band and low insertion-loss characterization. MEMS comb-drive actuators are integrated in the device layer of the SOI substrate, which move Si slabs in a rectangular waveguide of the handle layer for changing the propagation constant. Integrating tunability or reconfigurability into THz microsystems is a very crucial aspect for implementing signal modulation, frequency band selection, beam scanning, and calibration applications in THz MEMS front-ends. The demonstrated work paves the way towards a three-dimensional (3D)-micromachined, SOI integrated rectangular waveguide microsystem.

A series of high-Q multilayer filters have been achieved by a vertically stacked Si-chip micromachining technology. The frequencies cover 270 GHz, 300 GHz, 450 GHz, 687 GHz and 700 GHz. The fabrication accuracy and repeatability of this kind of THz waveguide filters based on this vertically stacked multilayer platform have been investigated by experiments. A versatile axial-port-integrated multilayer device concept has been proposed for enabling the direct on-flange characterization and integration for advanced THz waveguide components. H-plane waveguide filters with versatile axial-interfaces based on the vertically stacked multilayer platform has been successfully demonstrated. This vertically stacked multilayer Si micromachining technology shows a promising potential in implementing highly integrated, compact 3D THz microsystems.

I takt med utvecklingen av THz teknologi har behovet av en mängd applicationer växt explosionsartat, som t.ex höghastighetskommunikation, THz radars, miljömonitorering, medicinsk imaging och rymdutforskning. Tillverkningen, integreringen och förpackningstekniker kvarstår dock som stora utmaningar för THz-komponenter och system för att uppnå storskalig och kostnadseffektiv produktion. Nuvarande THz-teknologier förlitar sig på konventionella, dyra metoder för serietillverkning och förpackning, såsom computer numerical control (CNC) högprecisionsbearbetning, som enbart lämpar sig för avancerade forskningsinstrument eller engångsprototyper. Nu för tiden är THz MEMS en lovande kandidat för att realisera hög precision, låg kostnad och höga volymer vid tillverkning och integrering av miniatyriserade, passiva THz komponenter och system. I den här avhandlingen utvecklas flera nyckelkomponenter och teknologier inom THz MEMS till fördel för framtidens THz mikrosystem.

I ett av projekten har D-band filter och diplexer implementerats med hjälp av en avancerad mikrobearbetad vågledarteknologi som baseras på en frigörbar fyllnadsstruktur som uppnår geometrier med hög precision för THz vågledarenheter. Imperfektioner inom tillverkning är ett stort problem som påverkar prestationen hos enheter när det gäller inkopplingsförluster, bandbredd och operationsfrekvens. RFS-baserad SOI mikrobearbetningstekniker förbättrar deep reactive ion etching (DRIE) processens prestation och enhetlighet, särskilt inom sidväggs-vertikalitet, genom att utnyttja extra strukturer för att fylla de stora etsytorna och uppnå enhetliga aspect ratios av etsdiken.

En toppmodern MEMS fasväxlare baserad på en vågledar-integrerad SOI mikrobearbetningsteknik har framgångsrikt demonstrerats vid 220-330 GHz med karaktärisering av fullband med låg inkopplingsförlust. MEMS comb-drive aktuatorer är integrerade i device-lagret på SOI-skivan, vilka förflyttar kiselplattor i den rektangulära vågledaren för att förändra fortplantningskonstanten. Att integrera inställ- och konfigurerbarhet i THz-MEMS är en kritisk aspekt för att implementera signalmodulering, val av frekvensband, strålskanning och kalibrering i THz MEMS front-ends. Det demonstrerade arbetet banar väg för tredimensionella (3D) mikrobearbetade SOI-integrerade rektangulära vågledar-plattformar.

En serie multilager-filter med höga Q-värden har åstadkommits genom kiselbearbetningstekniker inom vertikala staplade kiselchip. Frekvenserna täcker 270, 300, 450, 687 och 700 GHz. Tillverkningsnoggrannheten och repeterbarheten av denna typ av THz-vågledarfilter baserade på  vertikala staplade multilager har undersökts. En mångsidig axiell portintegrerad multilagerenhet har föreslagits för att möjliggöra den direkta standardiserade fläns-koaxial-kopplingen. H-plan vågledarfilter med detta mångsidiga koaxialgränssnitt har framgångsrikt demonstrerats. Denna kiselbearbetningsteknik inom vertikala staplade multilager uppvisar potential för att implementera integrerade, kompakta 3D-mikrosystem inom THz applikationer.

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]

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).

A D-hand 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 degrees sidewall angles for the 400-mu m-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 lass of 6 and 10 dB for the lower and the upper hand, 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 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 mm2, 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.