Silicon micromachined waveguide components for terahertz systems

Doctoral Thesis in Electrical Engineering

Silicon micromachined waveguide

components for terahertz systems

BERNHARD BEUERLE

Silicon micromachined waveguide

components for terahertz systems

BERNHARD BEUERLE

Doctoral Thesis in Electrical Engineering KTH Royal Institute of Technology Stockholm, Sweden 2020

Academic Dissertation which, with due permission of the KTH Royal Institute of Technology, is submitted for public defence for the Degree of Doctor of Philosophy on Tuesday the 24th of November 2020, at 14:00 online.

© Bernhard Beuerle ISBN 978-91-7873-682-9 TRITA-EECS-AVL-2020:57

Sammanfattning

Denna avhandling presenterar mikrobearbetade vågledarkomponenter i kisel för användning inom sub-terahertz och terahertz (THz) system tillverka-de med djup reaktiv jonetsning (DRIE). Även om tillverka-den huvudsakliga drivkraften för utvecklingen av THz system ursprungligen har varit rymdbaserade instru-ment för astrofysik, Jord- och planetvetenskapliga uppdrag så har den senaste utvecklingen av aktiva och passiva THz komponenter även skett inom områ-den som avbildning, säkerhet, kommunikation och biologisk instrumentering. Traditionellt så har den primära teknologin för komponenter och sammanlän-kare runt THz området varit ihåliga vågledare i metall, tillverkade med hjälp av datornumerisk kontrollerad (CNC) fräsning. Dessa system är stora och hand-monterade och blir dyrare och svårare med ökande komplexitet av systemet. Under senare år har mikrobearbetat kisel dykt upp som ett möjligt alternativ för THz-komponenter och integrerade system och ser lovande ut för mer kom-pakta integrerade system.

Den första delen av avhandlingen rapporterar om ny teknologi för mikro-berarbetade vågledare i kisel, med låg förlust, som använder kisel-på-isolator (SOI) skivor. Flertalet vågledarkomponenter så som hybridkopplare, delare och matchade laster med låg-förlustteknologi för frekvensområdet 220 - 330 GHz har tillverkats och karaktäriserats. Vidare presenteras även en undersök-ning av tillverkundersök-ningsprecision och repeterbarhet för hög-Q filter i sub-THz fre-kvensområdet för samma vågledarteknologi.

Den andra delen introducerar en ny CPW-sond-till-mikroberabetade våg-ledare övergångskoncept for karaktärisering av vågvåg-ledare på skiva. Övergång-en är tillverkad simultant tillsammans med de testade Övergång-enheterna från dÖvergång-en förs-ta delen av avhandlingen. Den består av en CPW-sondsinterface och en pin ut-skjutande inuti vågledarhåligheten verkande som en E-fältsond för att excitera den dominerande moden av den rektangulära vågledaren. Övergången var ut-vecklad och karaktäriserad för frekvensområdet 220 – 330 GHz och användes med framgång för på-skivan karaktärisering av vågledarkomponenterna som tidigare presenterats. Möjligheten till att skala upp konceptet visas med hjälp av en modifierad övergång kapabel att karaktärisera komponenter upp till 500 GHz.

I den sista delen av avhandlingen utreds integrering av mikrobearbetade, monolitiska integrerade kretsar (MMIC) med mikrobearbetade vågledare av kisel. Ett nytt integrationskoncept för THz-system presenteras och en påföl-jande övergångsstruktur for integreringen av SiGe MMICs tillsammans med mikrobearbetade vågledare av kisel vid D-band frekvenser (110 – 170 GHz) har karaktäriserats. Vidare presenteras dessutom en gemensamt utvecklad övergång från InP MMICs till mikroberabetade, rektangulära vågledare av kisel bestående av en kompakt mikrostripp-till-vågledarövergång och en vertikal till horisontell vågledarböj i mikroberarbetad kiselvågledarteknologi. Konceptet har tillverkats och karaktäriserats i en påföljande konfiguration för frekvens-området 220 - 330 GHz. Båda koncepten är skalbara till högre frekvenser och metoden för montering är kompatibelt med kommersiella monteringsverktyg.

Abstract

This thesis presents silicon micromachined waveguide components for sub-terahertz and terahertz (THz) systems fabricated by deep reactive ion etching (DRIE). Historically the main driving force for the development of THz systems has been space-based scientific instruments for astrophysics, plane-tary and Earth science missions. Recent advances in active and passive com-ponents for the THz frequency range increased its usage in areas such as imaging, security, communications and biological instrumentation. Tradition-ally the primary technology for components and interconnections approach-ing THz frequencies has been hollow metal waveguides fabricated by com-puter numerical controlled (CNC) milling. Systems using this technology are bulky and hand-assembled, getting more expensive and complicated with an increasing complexity of the system. In recent years silicon micromachining has emerged as a viable alternative for THz components and integrated sys-tems promising more compact integrated syssys-tems.

The thesis reports on a new low-loss silicon micromachined waveguide technology using silion-on-insulator (SOI) wafers. Several low-loss waveguide components in the frequency range of 220–330 GHz have been fabricated and characterized, such as hybrid couplers, splitters and matched loads. Further-more, an investigation of fabrication accuracy and repeatability for high-Q fil-ters in the sub-THz frequency range using the same waveguide technology is presented.

For on-wafer waveguide characterization a novel CPW probe to micro-machined waveguide transition concept is introduced. The transition is co-fabricated together with the devices under test in the same waveguide tech-nology using SOI techtech-nology. It consists of a CPW probing interface and a pin protruding into the waveguide cavity acting as an E-field probe to excite the dominant mode of the rectangular waveguide. Designed and characterized for the frequency range of 220–330 GHz, the transition was successfully used for on-wafer characterization of the waveguide components previously sented. The scalability of the concept to higher frequencies is shown by pre-senting a modified transition capable of device characterization up to 500 GHz. The integration of monolithic micromachined integrated circuits (MMICs) with silicon micromachined waveguides is investigated, with a focus on scal-ability to higher frequencies and their compatibility with industrial assembly tools. A new integration concept for THz systems is presented and a back-to-back transition structure for the integration of SiGe MMICs with silicon micro-machined waveguides at D-band frequencies (110–170 GHz) has been char-acterized. Furthermore, a co-designed transition from InP MMIC to silicon micromachined rectangular waveguide is presented, consisting of a compact microstrip to waveguide transition and a vertical waveguide to in-plane wave-guide bend in the silicon micromachined wavewave-guide technology. The concept has been fabricated and characterized in a back-to-back configuration for the frequency range of 220–330 GHz.

We were past the point of debating the wisdom of this move; it was already done, and our only hope was to get to the other side.

Contents

Contents i

List of Publications iii

1 Introduction 1

1.1 Terahertz frequency range . . . 3 1.2 Monolithic microwave integrated circuits for the terahertz frequency

range . . . 4 1.3 Silicon micromachining . . . 4

2 Silicon micromachined waveguide components 7

2.1 Background . . . 7 2.2 Motivation and challenges . . . 10 2.3 Results and novelty . . . 18

3 On-wafer device characterization of silicon micromachined waveguide

components 21

3.1 Background . . . 21 3.2 Motivation and challenges . . . 24 3.3 Results and novelty . . . 27

4 Integrating MMICs and silicon micromachined waveguide components 29

4.1 Background . . . 29 4.2 Motivation and challenges . . . 31 4.3 Results and novelty . . . 32

5 Conclusions 35

6 Outlook 37

Acknowledgments 39

Bibliography 41

List of Publications

This thesis is based on the following papers in peer-reviewed international jour-nals:

I. ”A Very Low Loss 220–325 GHz Silicon Micromachined Waveguide Tech-nology,”

Bernhard Beuerle, James Campion, Umer Shah and Joachim

Oberham-mer, in IEEE Transactions on Terahertz Science and Technology, vol. 8, no. 2, pp. 248–250, 2018

https://doi.org/10.1109/tthz.2018.2791841

II. ”Investigation of Fabrication Accuracy and Repeatability of High-Q Silicon-Micromachined Narrowband Sub-THz Waveguide Filters,” Oleksandr Glubokov, Xinghai Zhao, James Campion, Bernhard Beuerle, Umer Shah and Joachim Oberhammer, in IEEE Transactions on

Mi-crowave Theory and Techniques, vol. 67, no. 9, pp. 3696–3706, 2019

https://doi.org/10.1109/tmtt.2019.2926244

III. ”A CPW Probe to Rectangular Waveguide Transition for On-wafer Micro-machined Waveguide Characterization,”

Bernhard Beuerle, James Campion, Oleksandr Glubokov, Umer Shah

and Joachim Oberhammer, in IEEE Transactions on Microwave Theory

and Techniques, under review, 2020

IV. ”On-wafer Micromachined Waveguide Characterization with a CPW Probe to Rectangular Waveguide Transition up to 500 GHz,”

Bernhard Beuerle, James Campion, Oleksandr Glubokov, Umer Shah

and Joachim Oberhammer, manuscript in preparation for journal submis-sion, 2020

V. ”Toward Industrial Exploitation of THz Frequencies: Integration of SiGe MMICs in Silicon-Micromachined Waveguide Systems,”

James Campion, Ahmed Hassona, Zhongxia Simon He, Bernhard

Beuerle, Adrian Gomez-Torrent, Umer Shah, Sandro Vecchiattini,

Richard Lindman, Torbjorn S. Dahl, Yinggang Li, Herbert Zirath and Joachim Oberhammer, in IEEE Transactions on Terahertz Science and

Technology, vol. 9, no. 6, pp. 624–636, 2019

https://doi.org/10.1109/tthz.2019.2943572 iii

iv LIST OF PUBLICATIONS

VI. ”Integrating InP MMICs and Silicon Micromachined Waveguides for sub-THz Systems,”

Bernhard Beuerle, Jan Svedin, Robert Malmqvist, Vessen Vassilev, Umer

Shah, Herbert Zirath and Joachim Oberhammer, in IEEE Electron Device

Letters, under review, 2020

The contribution of Bernhard Beuerle to each publication, major (•••), partial (••),

or minor (•):

Design Fabrication Experiments Analysis Writing I. ••• ••• ••• ••• ••• II. • • • • • III. ••• ••• ••• ••• ••• IV. ••• ••• ••• ••• ••• V. • •• • • • VI. ••• ••• ••• ••• •••

The work has also been presented at the following reviewed international confer-ences:

7. ”Micromachined multilayer bandpass filter at 270 GHz using dual-mode circular cavities,”

Oleksandr Glubokov, Xinghai Zhao, Bernhard Beuerle, James Campion, Umer Shah and Joachim Oberhammer, in IEEE MTT-S International

Mi-crowave Symposium (IMS), 2017

https://doi.org/10.1109/mwsym.2017.8058894

8. ”Integrated Micromachined Waveguide Absorbers at 220–325 GHz,”

Bernhard Beuerle, James Campion, Umer Shah and Joachim

Oberham-mer, in 47th European Microwave Conference (EuMC), 2017 https://doi.org/10.23919/eumc.2017.8230942

9. ”Micromachined cavity resonator sensors for on chip material character-isation in the 220–330 GHz band,”

Dragos Dancila, Bernhard Beuerle, Umer Shah, Anders Rydberg and Joachim Oberhammer, in 47th European Microwave Conference (EuMC), 2017

https://doi.org/10.23919/eumc.2017.8231000

10. ”A 230–300 GHz Low-loss Micromachined Waveguide Hybrid Coupler,” Jan Svedin, Robert Malmqvist, Bernhard Beuerle, Umer Shah and Joachim Oberhammer, in 47th European Microwave Conference (EuMC), 2017

v 11. ”A 220–325 GHz Low-Loss Micromachined Waveguide Power Divider,”

Robert Malmqvist, Andreas Gustafsson, Jan Svedin, Bernhard Beuerle, Umer Shah and Joachim Oberhammer, in IEEE Asia Pacific Microwave

Conference (APMC), 2017

https://doi.org/10.1109/apmc.2017.8251436

12. ”Micromachined Waveguides with Integrated Silicon Absorbers and At-tenuators at 220–325 GHz,”

Bernhard Beuerle, Umer Shah and Joachim Oberhammer, in IEEE/MTT-S International Microwave IEEE/MTT-Symposium - IMIEEE/MTT-S, 2018

https://doi.org/10.1109/mwsym.2018.8439364

13. ”Low-Loss Silicon Micromachined Waveguides Above 100 GHz Utilising Multiple H-Plane Splits,”

Bernhard Beuerle, James Campion, Umer Shah and Joachim

Oberham-mer, in 48th European Microwave Conference (EuMC), 2018 https://doi.org/10.23919/eumc.2018.8541605

14. ”Leaky Wave Antenna at 300 GHz in Silicon Micromachined Waveguide Technology,”

Dragos Dancila, Bernhard Beuerle, Umer Shah, Joachim Oberhammer and Anders Rydberg, in 44th International Conference on Infrared,

Mil-limeter, and Terahertz Waves (IRMMW-THz), 2019

Acronyms

AR aspect ratio.

ARDE aspect ratio dependent etching. BOX buried oxide layer.

CMOS complementary metal–oxide–semiconductor. CNC computer numerical control.

CPW coplanar waveguide.

CTE coefficient of thermal expansion. DL device layer.

DRIE deep reactive-ion etching. DUT device under test.

GSG ground-signal-ground.

HBT heterojunction bipolar transistor. HL handle layer.

IL insertion loss. InP indium phosphide.

LIGA Lithographie, Galvanoformung, Abformung. MEMS microelectromechanical systems.

MMIC monolithic microwave integrated circuit.

viii ACRONYMS OMT orthomode transducer.

PECVD plasma-enhanced chemical vapor deposition. RIE reactive-ion etching.

RL return loss.

SEM scanning electron microscope. Si silicon.

SiGe silicon-germanium.

SIMOX separation by implantation of oxygen. SiO₂ silicon dioxide.

SOI silicon on insulator. SOP system-on-package. TDM time-division multiplexing. THz terahertz.

THz-TDS terahertz time-domain spectroscopy. TRL thru-reflect-line.

Chapter 1

Introduction

This chapter familiarizes the the reader with the objective of this thesis. A brief in-troduction to the terahertz frequency range and monolithic microwave integrated circuits capable of operating in this frequency range is given. An overview of sili-con micromachining follows, including deep reactive-ion etching as the basis of the work performed in this thesis.

The terahertz (THz) frequency range is finding an ever-increasing usage in a va-riety of fields such as space and Earth observation sciences, security, communi-cations and biological instrumentation. Monolithic microwave integrated circuits (MMICs) have seen dramatic improvements in the last decades to extend their maximum frequency into the THz frequency range. Advances have been made in the millimeter wave frequency range to build system-on-package (SOP) systems and thus implementing more functionality on chip level [1]. However, those tech-niques are still not mature enough to be applied to frequencies above 200 GHz [2] and waveguide-based systems are the preferred choice.

Traditionally those waveguide systems have been fabricated by computer nu-merical control (CNC) metal machining. However, by design those systems are bulky, heavy and expensive, and with higher system complexity increasingly diffi-cult to fabricate [2]. Figure 1.1 shows a direct detection polarimeter system op-erating at 200 GHz implemented in a metal machined waveguide system [3] and illustrates its bulky nature. In order to further exploit the THz frequency range and open it up for new applications, high performance and very compact systems are needed. Micromachining is the prime contender and promises higher integration and packaging for THz systems.

To leverage the advantages of micromachined waveguide systems a number of challenges remain. For one advanced micromachining platforms are needed to

2 CHAPTER 1. INTRODUCTION

Figure 1.1: Image of a direct detection polarimeter system operating at 220 GHz (image from [3]).

not only fabricate single micromachined waveguide components, but also more complex components and systems. Furthermore, these platforms need to be suit-able for easy and efficient integration with active components.

This thesis addresses these points by introducing a novel silicon microma-chined waveguide technology based on silicon on insulator (SOI) wafers in Chap-ter 2. The technology has been successfully used to fabricate high performance low loss components, and an investigation into fabrication accuracies and repeata-bility is given. The arepeata-bility to perform rapid characterization of silicon microma-chined devices without the need of expensive and complex test fixtures, intro-ducing additional errors due to assembly, requires transitions from available test equipment to the micromachined waveguide platform. A novel coplanar wave-guide (CPW) to silicon micromachined wavewave-guide transition presented in Chap-ter 3 opens up the possibility to use widely available coplanar waveguide (CPW) ground-signal-ground (GSG) probes and techniques for on-wafer device charac-terization. Finally in Chapter 4 two novel concepts of interconnecting monolithic microwave integrated circuits (MMICs) with silicon micromachined waveguide sys-tems in the previously presented waveguide technology are introduced. Both con-cepts are designed to be able to be used with industrial assembly equipment and thus promise efficient and inexpensive fabrication of complete THz microsystems.

1.1. TERAHERTZ FREQUENCY RANGE 3 0 200 400 600 800 1000 10−2 10−1 100 101 102 103 104 H2O H2O O2 O2 H2O H2O Frequency (GHz) At tenuation (dB km − 1)

Figure 1.2: Specific attenuation due to atmospheric gases. Data from [11, 12].

1.1 Terahertz frequency range

THz, or submillimeter, radiation lies between millimeter waves and infrared light in the electromagnetic spectrum and comprises the frequencies between 100 GHz and 3 THz (subject to definition, the upper bound especially is ambigiuous). De-spite great scientific interest since the early part of the 20th century [4–8] this part of the electromagnetic spectrum has been fairly untapped up until recently. One of the reasons for this is the limited atmospheric propagation path hinder-ing commercial interests in THz systems [9]. For a long time the only applications have been high-resolution spectroscopy and remote sensing. A variety of light-weight molecules have their thermal emission lines in this frequency range, al-lowing astronomers, Earth, planetary, space scientists and chemists to measure and map them [9, 10]. Figure 1.2 shows the specific atmospheric attenuation up to 1000 GHz, indicating some molecular emission lines such as the water line at 557 GHz.

Enabled by advances in both active and passive components for the THz fre-quency range new applications are emerging [13]. Besides advances in system developments for the aforementioned scientific areas, applications include the detection of concealed objects [14] and THz imaging systems [15, 16]. Using THz frequencies for fixed wireless communication is actively explored [17] to satisfy the demand for higher data rate with continued growth indicated by industry [18].

4 CHAPTER 1. INTRODUCTION

1.2 Monolithic microwave integrated circuits for the terahertz

frequency range

“It is appropriate to view the submillimeter wave region as a transition region ly-ing between the millimeter wave and the infrared portions of the electromagnetic spectrum and posssessing as yet no hallmark of its own” [19]. This statement from the opening remark from the Proceedings of the Symposium on Submillime-ter Waves in 1970 describes the state of research in the THz frequency range at the outset of its exploitation. The reason for this was mainly due to the general unavailability of sources, sensors and instruments [13].

Efforts have been under way since the 1960s and 1970s to exploit the THz frequency range with quasi-optical methods. Approaches to generate and de-tect THz radiation for spectroscopy applications were based on, molecular va-por lasers, free electron lasers and synchrotrons. But it took up until the 1990s that one could speak of real-world applications based on these technologies. Ad-vances in the fabrication techniques of nonlinear optical materials and rapid im-provements in the reliability, stability, size and cost of lasers required sped up the development of these systems [20]. One new technique made possible through advances in those technologies is terahertz time-domain spectroscopy (THz-TDS), using femto-second lasers to obtain a rabid spectral measurement over a large bandwith [20].

Coming from the microwave part of the electromagnetic spectrum THz radi-ation emitters have been predominantly built using Schottky diode multipliers and mixers [21]. To achieve a fully electronic THz approach a transistor technol-ogy reaching the THz frequency range is needed. Since around a decade indium phosphide (InP) transistors are reaching fmax>1THz [22] and silicon-germanium

(SiGe) fmax>0.5THz [23] with further increases on the roadmap [24, 25]. Several

MMICs for various applications have been demonstrated for both InP [22, 26–30] and SiGe [31–33]. Traditional, cost-effective CMOS is also pushing into the THz frequency range for planetary missions [34] as well as communications [35–37]. Figure 1.3 gives an overview of the current state of affairs for SiGe, InP and com-plementary metal–oxide–semiconductor (CMOS) transistor technologies in rela-tion to their maximum frequency.

1.3 Silicon micromachining

Silicon is the second most common element on earth after oxygen. Besides its abundance it took until 1823 to characterize it in pure form due to its high chemi-cal affinity to oxygen. But only after the invention of the transistor in 1949 started

1.3. SILICON MICROMACHINING 5

millimeter waves terahertz radiation infrared

30 GHz 100 GHz 300 GHz 1 THz 3 THz

SiGe HBTs (2019) [38]

SiGe (Technology roadmap) [39] InP HEMT, HBT (2019) [22]

InP (Technology roadmap) [26] CMOS (2019) [40] GaN HEMT (2019) [41] GaAs mHEMT (2019) [42] 0.7 THz 2.5 THz 1.5 THz 2.0 THz <0.4THz >0.4THz >1.0THz

Figure 1.3: Current and projected fmax capabilities of different transistor

tech-nologies in relation to the terahertz frequency range [21].

silicon mask 1 (a) 1 (b) <100> <111> (c) <110> <111> (d)

Figure 1.4: Typical silicon wet etching profiles [43] for (a) isotropic etching with agitation, (b) isotropic etching without agitation, (c) anisotropic etching on <100> surfaces, 54.7° to plane angle [46]) and (d) anisotropic etching on <100> sur-faces.

the triumphant advance of silicon to revolutionize electronics. In the wake of the rapid development of the semiconductor industry the versatility of silicon gave rise to its use as a mechanical material [43]. Existing processing equipment and techniques were adapted to create miniature mechanical devices and compo-nents in silicon giving birth to the field of microelectromechanical systems (MEMS).

The fabrication of MEMS structures using etching techniques to remove part of a substrate or thin film is called micromachining [44]. The excellent mechanical properties of silicon make it a suitable choice as substrate. There are generally two modes of etching: isotropically and anisotropically as seen in Figure 1.4. In 1962 Honeywell introduced one of the first silicon sensors, fabricated by applying isotropic silicon wet etching [44, 45] (Figure 1.4(a) and (b)). Using orientation dependent etchants for anisotropic wet etching of silicon (Figure 1.4(c) and (d)) it was possible to fabricate more precise structures [46]. However, these etching techniques did not allow the fabrication of very precise structures with high aspect ratios (ARs).

6 CHAPTER 1. INTRODUCTION C₄F₈ + + ₊ (a) Deposition SF₆ + + ₊ (b) Breakthrough SF₆ + + ₊ (c) Etch

Figure 1.5: Three-step time-division multiplexing (TDM) BOSCH process [48–50]: (a) plasma deposition of C₄F₈passivation layer on surfaces; (b) physical plasma SF₆etching, removing passivation on horizontal surfaces; (c) isotropic chemical plasma SF₆etching.

Deep reactive-ion etching (DRIE) is a dry etching technique, using plasma to create structures in bulk silicon with high ARs [47]. Two techniques are applied: cryogenic etching and etching with the Bosch process. High aspect ratios struc-tures with near vertical sidewalls can be created with both. Notwithstanding the lower sidewall roughness of cryogenic etching, waveguide components are mostly fabricated with the more widely available Bosch process [47]. Further higher se-lectivity between mask and bulk silicon and a higher etch rate can be achieved.

The Bosch process was developed in the 1990s at Bosch in Germany [48–50] and uses the gases C₄F₈and SF₆in a time-division multiplexing (TDM) system. Modern DRIE etch systems are capable of switching between the different steps in1s or less. The basic process is illustrated in Figure 1.5. In the first step a passivation layer of C₄F₈is uniformly deposited on the wafer. This layer will act as an etch protection layer on non-horizontal surfaces in the subsequent steps. In the following breakthrough step SF₆ ions bombard the exposed wafer area, removing the passivation layer on the horizontal surface. The last etch step is a more isotropic etching. Chemical etching of silicon at the exposed areas takes place, while vertical sidewalls are protected. Sidewall scallops are a product of cycling through these processing steps, and their size depends on the duration of the steps. The interested reader can study the Bosch process in greater detail in [51, 52].

Chapter 2

Silicon micromachined waveguide

components

After an introduction to micromachined waveguides this chapter introduces the un-derlying silicon micromachined waveguide technology of this thesis, presented in Paper I. The fabrication process using silicon on insulator wafers is described and multiple components fabricated in this technology are presented. An investigation of fabrication and assembly tolerances on device performance for cavity resonator filters in this technology is given in Paper II.

2.1 Background

Provided a wavelength scaled processing technology is available, planar transmis-sion lines such as microstrip and CPW lines are viable to be used in MMICs for the THz frequency range [28]. Figure 2.1 shows that the losses increase only gradu-ally with frequency if all dimensions of the transmission line can be scaled with the wavelength [28]. However, with increasing frequency comes a greater effect of parasitics and mechanical limitations [2]. Approaches like system-on-package (SOP) [1] lack the maturity to be applied reliably above 200 GHz. As of today this keeps waveguide-based systems as the preferred option for systems in the THz frequency range.

Traditionally metal machining has been the dominating fabrication method for waveguides at microwave frequencies. When scaling waveguides to higher fre-quencies, achieving the tighter tolerances due to smaller features sizes and

8 CHAPTER 2. SILICON MICROMACHINED WAVEGUIDE COMPONENTS 0 0.1 IL (dB /λ ) _WM864 WM570 WM380 0 100 200 300 400 500 600 700 0 0.2 0.4 0.6 0.8 1.0 1.2 Frequency (GHz) IL (dB /λ ) microstrip CPW

Figure 2.1: Insertion loss per wavelength for different transmission lines up to 700 GHz. Note that the dimensions for the planar transmission lines (microstrip, CPW) were chosen using simple scaling [28].

mensions requires very high precision and accuracy. Advances in computer nu-merical control (CNC) milling equipment in recent decades deliver low loss wave-guide components by metal machining. Modern CNC mills are able to achieve tolerances of 10 µm, aspect ratios of 50:1 and channels as small as 25 µm can be milled [53]. Recently features with tolerances of±5/10µm were realized [54].

Commonly metal machined components were fabricated and systems assembled discretely, resulting in very expensive and bulky systems and introducing addi-tional losses. CNC equipment improvements in recent years enabled higher in-tegration of metal machined THz systems [55]. However, inherent fundamental limitations of this technology remain. For one, CNC is a serial process and cost rises with complexity of the system. Furthermore, vertical integration of CNC at frequencies above 500 GHz is not feasible [47]. Thin metal layers are mechan-ically unstable and tend to warp, whereas for thicker layers the CNC equipment sets a limit to the size of vertical interconnects [2].

Micromachining is an ideal candidate as the preferred fabrication technique for waveguides in the THz range, enabling very accurate features and high aspect ratio structures [47]. The first micromachined waveguide component for the ter-ahertz frequency range was reported, and the use of air-filled rectangular wave-guides was proposed in 1980 [56, 57]. Nonetheless it took more than a decade until the first micromachined rectangular waveguide was demonstrated [58]. Sev-eral different micromachining techniques have since been used to create wave-guide components for the THz frequency range and are introduced here in brief.

2.1. BACKGROUND 9

(a)

(b) (c)

Figure 2.2: Waveguide components using different microfabrication processes. (a) SEM images of a 600–750 GHz SIS mixer fabricated by metal electroplating with SU-8 [59]. (b) SEM image of a 90° waveguide twist at 500–750 GHz fabri-cated using UV LIGA [60]. (c) [61]

Thick resist electroforming is based on the Lithographie, Galvanoformung, Ab-formung (LIGA) process [62, 63] using X-ray lithography to achieve very high as-pect ratios up to 100:1. The structures are developed using a thick resist such as SU-8 or KMPR on a seed layer on top of a carrier substrate. Thickness for SU-8 can be up to 1 mm [64]. Through the openings in the resist pattern metal is elec-troplated. This enables the fabrication of components completely in metal with photolithographic precision [47]. A number of waveguide components for the THz have been reported [59, 60, 65–73] and examples are shown in Figure 2.2(a) and (b).

Thick resists used for electroforming can also be used to create waveguides directly [74]. In this case the resist forms a permanent part of the waveguide structure, in contrast to electroforming where the resist is removed at the end leaving an all-metal structure [47]. The major advantage of this technology is the

10 CHAPTER 2. SILICON MICROMACHINED WAVEGUIDE COMPONENTS

option to use standard lithography tools for fabrication. However, there are ma-jor disadvantages of permanent thick resist micromachining. With the coefficient of thermal expansion (CTE) being very high, delamination of the structures from the carrier can occur if the structures are not released [47]. Moreover, th ther-mal conductivity of the resist is very low, rendering the technology difficult to be used as an integration platform for active components (with efforts being inves-tigated to mitigate the problem but increasing fabrication complexity [75]). Nev-ertheless this technology is promising for very precise low-cost THz waveguide components, and a number of components at different frequencies have been demonstrated [61, 76–82]. Figure 2.2(c) shows a WR-3 band waveguide filter fabricated in this technology [61].

Silicon micromachining is the most promising technique for waveguide com-ponents in the THz frequency range [2, 47]. Using DRIE near vertical sidewalls and high aspect ratio structures can be etched, and a variety of different wave-guide components for the terahertz frequency have been reported [83–89].

Silicon on insulator technology

Silicon on insulator (SOI) technology dates back to the 1960s and has since seen widespread use in the semiconductor industry. SOI is an advanced version of a conventional silicon wafer and consists of a layer of silicon – device layer (DL) – separated by a layer of silicon dioxide – buried oxide layer (BOX)– on top of bulk silicon – handle layer (HL) [90]. SOI wafers can be fabricated using either sepa-ration by implantation of oxygen (SIMOX) [91], wafer bonding [92] or seed meth-ods [93]. The stacking of different layers makes SOI an interesting and widely used option in the MEMS field. A variety of sensors are fabricated using SOI and are commercially available [94].

Recently SOI substrates have seen wider use in micromachining for THz cir-cuits, namely for superconducting THz mixers, micromachined on-wafer probes and heterogeneous integration of THz devices [69, 95].

2.2 Motivation and challenges

A new silicon micromachined waveguide technology based on SOI technology is introduced. Using SOI wafers instead of conventional silicon wafers for microma-chined waveguides has a number of advantages:

2.2. MOTIVATION AND CHALLENGES 11

DRIE _{DRIE DRIE}

(a) (b) (c)

Figure 2.3: Silicon micromachined waveguide types. (a) E-plane split. Both guide halves are etched and joined together. (b) Single H-plane split. The wave-guide cavity is etched into a silicon wafer and sealed with a top wafer. (c) Double H-plane split presented in this thesis. The waveguide cavity is etched into the handle layer of an SOI wafer, where the buried oxide layer acts as an etch stop.

• lower insertion loss of waveguide components due to lower surface rough-ness of the waveguide walls without increasing fabrication complexity • better process control and higher etch accuracy across the wafer, enabling

the fabrication of more complex components and increasing the yield • possibility to integrate with active MEMS devices fabricated in the DL of the

SOI wafer

Given a good joint between the separate parts of a machined waveguide after assembly, the insertion loss of a waveguide depends on the type and thickness of the metallization and the surface roughness of the walls [96–100]. Metal wave-guides are usually made in an E-plane split configuration, so that the joint is situ-ated in the broad wall [101]. For silicon micromachined waveguides this configu-ration enables comparatively easy integconfigu-ration with active components by means of an E-field probe inserted into the waveguide in the E-plane [102]. However, all four walls of an E-plane split waveguide fabricated using DRIE are etched as can be seen in Figure 2.3(a), and experience high surface roughness. For H-plane split silicon micromachined waveguides the waveguide cavity is etched into one wafer and the waveguide is sealed by bonding a top wafer to the waveguide wafer (Fig-ure 2.3(b)). In this case the broad wall formed by the top waveguide is polished

12 CHAPTER 2. SILICON MICROMACHINED WAVEGUIDE COMPONENTS

and has a very low surface roughness, with the other three walls having higher roughness. By adding oxidation steps the surface roughness of the waveguide walls can be reduced [103–106] at the cost of increasing fabrication complexity. Using SOI wafers reduces the surface roughness of one waveguide wall with-out additional fabrication steps. The waveguide cavity is etched in the handle layer (HL) of the SOI wafer. The buried oxide layer (BOX) layer acts as an etch stop, and after metallization both broad walls of the waveguide are polished surfaces with very low surface roughness (Figure 2.3(c)).. In case of in-plane waveguide components, this constitutes a double H-plane split waveguide where both broad walls are polished with very low surface roughness.

Additional benefits besides the reduced surface roughness for waveguides al-low more accurate features. Two aspects of DRIE on conventional silicon wafers either increase fabrication complexity or decrease the yield.

• Aspect ratio dependent etching (ARDE), the dependence of the etch rate to the aspect ratio of the etched structure, causes different etch depths for waveguide cavities with different aspect ratios [107].

• DRIE tools have an inherent nonuniform etching characteristic further af-fecting the etch rate across the wafer. Modern tools employ complex tech-niques to remedy this problem, but those tools are not commonly available outside of MEMS fabs.

In the waveguide technology presented in this thesis the etch depth of the waveguide cavity is set by the thickness of the SOI handle layer. Thus the height only depends on the tolerance specified by the SOI manufacturer and not on the etch rate across the wafer and different aspect ratios.

SOI technology also enables creating complex multi-chip devices with very high fabrication precision [108]. Furthermore, integration of active MEMS struc-tures like waveguide switches for example for radiometer applications [109, 110] with waveguide components is explored [111].

Microfabrication process

All devices fabricated in the course of the work presented in this thesis were fab-ricated with the same core microfabrication process. Initially the technology was demonstrated in Paper I for the frequency band of 220–325 GHz. For components with features of different height in the waveguide cavity (all devices characterized with the on-wafer transition introduced in Chapter 3) a multistep etch process is applied [106, 111]. The fabrication process is outlined in Figure 2.4. This process

2.2. MOTIVATION AND CHALLENGES 13 t_H tB tD tox device layer handle layer (a)

mask Dburied oxide layer

thermal oxide layer

(b)

mask H1

(c)

PECVD oxide layer

(d) mask H2 (e) DRIE (f) DRIE (g) (h) DRIE (i) (j)

gold metallization layer

(k)

F 200 °C

(l)

Figure 2.4: Fabrication process flow for devices with a two mask etch process in the handle layer of the SOI wafer (mask H1 and H2). (a) SOI consisting of de-vice layer (DL), buried oxide layer (BOX), handle layer (HL) and thermal silicon dioxide (SiO₂) layers on both sides. (b) Patterning of SiO₂layer on top of DL with mask D. (c) Patterning of SiO₂layer on top of HL with mask H1. (d) Deposition of plasma-enhanced chemical vapor deposition (PECVD) layer on top of mask H2. (e) Patterning of PECVD SiO₂ layer with mask H2. (f) DRIE of DL. (g) DRIE of HL with mask H2, (h) removal of remaining mask H2 and (i) DRIE of HL with mask H1. (j) Reactive-ion etching (RIE) of exposed BOX layer. (k) Metallization by sputtering of gold from both sides. (l) Thermocompression bonding of the chip to a sealing bottom chip at 200 °C. This process flow includes the integration of an E-field probe for on-wafer characterization presented in Chapter 3.

14 CHAPTER 2. SILICON MICROMACHINED WAVEGUIDE COMPONENTS 5 µm Au Si 1.32 µm 463 nm (a) 500 µm

SOI device layer

SOI handle layer

(b)

Figure 2.5: Scanning electron microscope images of (a) gold coverage after gold deposition via sputtering (image from [111]). (b) Etched waveguide cavity for 220–330 GHz in an SOI handle layer, with the device layer forming one broad wall of the waveguide.

flow includes features inside the waveguide cavity in the handle layer of the SOI wafer with a reduced height (such as the E-field probe of the transition introduced in Chapter 3). Devices without those features, for example the cavity waveguide filters presented later in this chapter, have a simpler fabrication process using only one mask for the handle layer etch.

(a) Dimensions of the bare SOI wafer are chosen to fit the required dimensions of the waveguide components: device layer thickness t_D = 30µm, handle

layer thickness t_H = 275µm, buried oxide layer thickness tB = 3µm and

thermal oxide layer thickness t_ox = 2µm on both sides.

(b) Patterns in the device layer – mask D – are transferred into the thermal oxide layer on top of it.

(c) Etching of the first mask H1 for the handle layer into the thermal oxide layer. (d) Deposition of 3 µm of plasma-enhanced chemical vapor deposition (PECVD)

oxide on top of mask H1.

(e) Etching of the second mask H2 for the handle layer into the PECVD oxide layer.

(f) DRIE of the device layer with mask D.

(g) DRIE of the handle layer with mask H2, etching the waveguide cavity but masking features with different height within the waveguide. The buried ox-ide layer acts as an etch stop.

2.2. MOTIVATION AND CHALLENGES 15 (h) Removing the remaining oxide mask H2 by reactive-ion etching (RIE). (i) DRIE of the handle layer with mask H1, etching down features to the desired

height.

(j) RIE of the exposed buried oxide layer, with the handle layer facing up. (k) Sputtering of a layer of gold on both sides creates the metallization layer.

To ensure good sidewall coverage 2 µm (measured on a flat surface) of gold are deposited (Figure 2.5(a) depicts the different metallization thicknesses for flat surfaces and straight sidewalls in the used sputtering process). The final waveguide cavity after metallization but before bonding can be seen in Figure 2.5(b).

(l) As the last step the chip is joint together with its a corresponding chip in multi-chip assemblies or a flat chip. The chips are aligned and placed in a chuck for thermocompression bonding at 200 °C.

Two main challenges arise when fabricating waveguides using SOI technology. • Notching can occur during DRIE when the buried oxide layer acting as an etch stop is reached. The handle layer at the interface with the buried oxide layer is then horizontally etched due to charging effects [112]. For wave-guides this creates a gap between the broad and the narrow wall. After metallization no ohmic connection exists, increasing the insertion loss sig-nificantly [113, 114]. Depending on the DRIE tool this effect can be avoided by tuning the etching recipe, specifically adjusting the pulse behaviour of the plasma generation in the etch step [115].

• When etching big open areas to a depth of several hundred micrometers it is difficult to maintain a straight sidewall [116]. For waveguide components with sensitive dimensions sloped sidewalls can cause frequency shifts. The slope of the sidewall is thereby a function of the AR of the etched structure, and often behaves nonlinearly for deeper etches.

A stable DRIE process assures both no notching and a reliable sidewall profile for given apertures. This knowledge can be taken into account during the design phase for waveguide components.

Fabrication accuracy and repeatability

In [117] a dual-mode circular cavity filter at 270 GHz was presented (schematic in Figure 2.6(a)). The filter was designed by taking into account the sidewall angle

16 CHAPTER 2. SILICON MICROMACHINED WAVEGUIDE COMPONENTS cavity 1 cavity 2 WR-3.4 interfaces (a) 200 µm 275 µm 25.7 µm (b)

Figure 2.6: (a) Schematical three-dimensional view of the filter presented in [117]. (b) Cross-sectional view of the sidewall of an open cavity etched in the handle layer of an SOI wafer with thickness t_H = 275µm. The measured

under-etch of 25.7 µm gives a sidewall angle of α≈85°.

(a) _(b)

Figure 2.7: Measured (a) S-parameters and (b) transmission coefficient of the fab-ricated filter in [117], compared to simulation results (figures taken from [117]).

2.2. MOTIVATION AND CHALLENGES 17

1 mm

waveguide

cavity absorberwedge

(a) 2 mm waveguide cavity Si absorber wedge (b)

Figure 2.8: Matched loads in the silicon micromachined waveguide technology, realized with (a) semi-rigid absorber material [118] and (b) silicon wedge [119] placed inside a waveguide cavity.

for the cavity, derived for different apertures from a previous test fabrication run. Measurement results for the fabricated filter are shown in Figure 2.7, indicating no frequency shift in the passband. Critical to the performance of the multi-chip filter besides the sidewall slope are chip-to-chip and chip-to-flange alignment. Paper II gives an investigation into fabrication accuracy and repeatability for cav-ity filters.

Fabricated waveguide components

Figure 2.5(b) shows the cross-section of a waveguide cavity for the frequency band of 220–330 GHz, demonstrating the technology (Paper I). Both for device characterization as well as waveguide systems there is a need to terminate ports with matched loads. Semi-rigid absorber material or silicon wedges can be in-serted into etched waveguide cavities and integrated directly into waveguide com-ponents and systems (Figure 2.8). The silicon absorbers presented in [119] were used in the characterization of an orthomode transducer [108, 121]. Prior to that, the matched loads demonstrated in [118] were used to terminate ports to char-acterize a waveguide splitter [122] and coupler [120] fabricated in this waveguide technology. SEM images of the fabricated hybrid coupler presented in [120] are shown in Figure 2.9.

18 CHAPTER 2. SILICON MICROMACHINED WAVEGUIDE COMPONENTS 1 mm hybrid coupler transitions (a) 200 µm corrugation α (b)

Figure 2.9: Scanning electron microscope images of (a) the fabricated hybrid cou-pler at 220–330 GHz [120], and (b) detailed view showing a sidewall angle of α.

2.3 Results and novelty

The work described in this chapter and published in Paper I introduces a novel sil-icon micromachined waveguide technology based on silsil-icon on insulator wafers for components in the terahertz frequency range. Compared to micromachined waveguides etched in a conventional silicon wafer the bottom surface of the wave-guide is not etched, but polished. This gives a much lower surface roughness reducing the overall insertion loss of components fabricated in this technology. Moreover, there is no need to control the etch depth according to the waveguide dimensions as the buried oxide layer acts as an etch stop and the etch depth is set by the specification of the SOI wafer. This is especially important for com-ponents with structures of varying width, where aspect ratio dependent etching makes it nontrivial to control the etch depth for all areas. Additionally, the use of SOI wafers also allows to create precise features in the device layer, enabling for example the fabrication of high performance filter components and representing a core characteristic to create more complex waveguide components and systems. Waveguides fabricated in this technology at 220–330 GHz show losses compa-rable to special machined waveguides. Table 2.1 compares the insertion loss to other reported CNC and micromachined waveguides.

For components with tight design tolerances knowledge about process and assembly tolerances is crucial even in the design phase. Fabrication accuracy and repeatability was investigated and results are presented in Paper II.

2.3. RESULTS AND NOVELTY 19 Table 2.1: State of the art waveguide technologies fabricated by CNC or microma-chining (table from Paper I).

Ref. Technology Split h (nm) f (GHz) IL (dB/mm) [88] CNC, Al E – 325–360 0.20–0.25 [129] CNC, Au plated E – 220–330 0.03–0.06 [101] CNC, Au plated E – 210–280 0.014–0.018 [61] SU-8 E – 220–325 0.03–0.05 [102] DRIE H 75 500–700 0.10–0.20 [87] DRIE E 110 500–750 0.08–0.12 [87] DRIE H 20 / 110 500–750 0.06–0.12 [106] DRIE, ox. step E 43 500–750 0.05–0.07 This work DRIE double– H 2 / 1601 _{220–325 0.02–0.07}

Based on the work presented in this chapter a number of very low loss and high performance devices have been fabricated and successfully characterized:

• cavity filters at 270 GHz [117], 300 GHz [123] and 450 GHz [124] • waveguide hybrid coupler at 220–330 GHz [120]

• waveguide splitter at 220–330 GHz [122]

• integrated absorbers and attenuators at 220–330 GHz [118, 119] • waveguides, filters and multiplexers at D-band [125–127]. • cavity resonator sensors at 220–330 GHz [128]

The same technology was used for orthomode transducer 220–330 GHz [108], showing its capability to be used for very complex waveguide structures. The work presented in the subsequent chapters is likewise based upon this technology.

Chapter 3

On-wafer device characterization of

silicon micromachined waveguide

components

Here a novel concept for a coplanar waveguide to micromachined rectangular wave-guide transition for on-wafer device characterization is presented. A brief introduc-tion to on-wafer device characterizaintroduc-tion familiarizes the reader with the topic. Fully integrated in the previously introduced silicon micromachined waveguide technol-ogy, fabrication challenges of the transition are addressed. The transition was de-signed, fabricated and characterized for 220–330 GHz in Paper III. In Paper IV an improved design scalable to higher frequencies is presented.

3.1 Background

One of the challenges of terahertz system development is the measurement and characterization of individual components. Especially when new fabrication tech-niques are involved, it is essential to characterize the components before system integration [47]. With vector network analyzer (VNA) extender modules reaching frequencies over 1 THz [130–132] measuring S-parameters of devices at tera-hertz frequencies has become increasingly accessible. This allows direct char-acterization of micromachined waveguide components, and three approaches to waveguide measurements are used [47]:

• to a waveguide at the edge of the micromachined waveguide part (in-plane) 21

22 CHAPTER 3. ON-WAFER DEVICE CHARACTERIZATION OF SILICONMICROMACHINED WAVEGUIDE COMPONENTS flange alignment pin chip align. pin micromachined waveguide filter aluminium split-blocks WR-6 interface port 1 port 2 (a) (b)

Figure 3.1: Test fixtures and assembly for a silicon micromachined waveguide fil-ter for D-band [125]. (a) The DUT is placed between two aluminium split-blocks and (b) connected to waveguide flanges.

• to an out-of-plane waveguide • to an on-wafer transition

A micromachined device with waveguides at the edge of the part is shown in Figure 2.2(c). Such devices have to be placed between metal machined test fix-tures in order to connect them to waveguide flanges. These fixfix-tures require high precision and are expensive to fabricate. Aligning the device under test (DUT) to the test fixtures is challenging and risks introducing a gap when contact surfaces of the DUT are not flat after assembly. With increasing frequencies these assem-bly challenges are getting more critical for accurate scattering parameter mea-surements. Devices with out-of-plane waveguide interfaces are either placed be-tween test fixtures with improved alignment [125] (Figure 3.1), or can be attached directly to waveguide flanges [117, 121].

The ability to directly interface with micromachined waveguide components by either waveguide or coplanar waveguide probes removes the necessity for ex-ternal test fixtures. In [102] a concept to use special machined open-ended wave-guide probes at 500–750 GHz for characterization was introduced (setup shown in Figure 3.2(a)). Special open waveguide probes are fabricated, and alignment marks on the wafer help with accurate placement at the cost of additional wafer space. E-plane bend on-wafer transitions from horizontal silicon micromachined waveguide to out-of-plane waveguide for device characterization at 220–325 GHz were demonstrated in [104] and [105] (Figure 3.2(b)). Here both rotational and lateral misalignment was limited by a specially designed recess in the top layer

3.1. BACKGROUND 23

(a) (b) (c)

Figure 3.2: On-wafer probe measurement setups. Probing (a) an out-of-plane, vertical waveguide at 500–750 GHz [102], (b) a non-contact wafer probe [105] and (c) a CPW to micromachined waveguide transition, both implemented for 220–325 GHz [133]. frequency extenders VNA motorized stages microscope CPW probes DUT vacuum chuck

Figure 3.3: Terahertz measurement setup. Motorized stages for frequency exten-ders and the vacuum chuck for device under test (DUT) placement, allowing fully automated measurements.

of the transition. IL was 0.4–0.6 dB [105] over the whole band for back-to-back transition.

Coplanar waveguide (CPW) ground-signal-ground (GSG) probes are used for on-wafer calibration of active and passive devices [134] and are widely available from the microwave to the THz frequency range [135–139]. Cavity-backed CPW

24 CHAPTER 3. ON-WAFER DEVICE CHARACTERIZATION OF SILICONMICROMACHINED WAVEGUIDE COMPONENTS

CPW

slots E-field_probe backshort CPW probe micromachined waveguide z x y ref. plane Tier 2 calibration (A) CPW slots E-field probe CPWprobe micromachined waveguide z x y ref. plane Tier 2 calibration (B)

Figure 3.4: Three-dimensional sketches of the presented CPW to micromachined waveguide transition for 220–330 GHz, transition (A) with rectangularly shaped layout demonstrated in Paper III) and transition (B) with an improved diamond layout (Paper IV).

to silicon micromachined waveguide transitions were presented in [104, 133] en-abling rapid on-wafer device characterization. By implementing on-wafer calibra-tion standards for self-calibracalibra-tion techniques such as thru-reflect-line (TRL) the calibration reference plane can then be moved directly into the micromachined waveguide, enabling accurate de-embedded scattering parameters of the wave-guide device under test (DUT).

Figure 3.3 shows the measurement setup used for the work presented in this thesis. Motorized stages for both extenders with CPW probes as well as the chuck allow fully automated measurements.

3.2 Motivation and challenges

Paper III and IV report on the development of on-wafer coplanar waveguide to sil-icon micromachined waveguide transitions enabling on-wafer device characteri-zation. Co-fabricated with in-plane waveguide DUTs within the technology pre-sented in Chapter 2, no additional processing and assembly steps are required. An initial transition (A) was developed and is presented in Paper III. A modified and improved transition design (B) is presented in Paper IV and allows on-wafer device characterization up to 500 GHz.

3.2. MOTIVATION AND CHALLENGES 25

DRIE

(a)

E-field

probe guardstructure

(b) DRIE (c) (d) 50% HF (e)

gold metallization layer

(f)

Figure 3.5: Modified fabrication flow for devices incorporating guard structures for straight sidewalls. For initial steps refer to Figure 2.4. (a) DRIE of HL with mask H2, (b) removal of remaining mask H2 and (c) DRIE of HL with mask H1. (d) RIE of exposed BOX layer. (e) Wet etching with 50% HF removes the BOX layer below guard structures and releases them. (f) Metallization by sputtering of gold from both sides, before thermocompression bonding at 200 °C (Figure 2.4(l)).

Figure 3.4 shows a three-dimensional representation of both transition ge-ometries (A) and (B) demonstrated in this chapter. The concept of the novel micro-machined transition is based on the excitation of the dominant waveguide mode using an E-field probe [140]. In [141] a micromachined transition based on this concept at W-band was demonstrated, averaging 2.25 dB over the whole band. However, as the waveguide and the E-field probe are fabricated in two different chips, this approach requires assembly and is prone to misalignment especially at higher frequencies. By using SOI wafers for the waveguide technology, it is possi-ble to precisely etch features into the device layer of the wafer (with the waveguide cavities etched into the handle layer). A CPW probing interface is etched into the device layer, and an E-field probe fabricated in the handle layer below it protrudes into the waveguide cavity. The signal is coupled to the E-field probe through the two slots of the CPW interface.

To fabricate the probe inside the waveguide cavity the two-mask etching pro-cess of the handle layer of the SOI as described in Chapter 2 and shown in Fig-ure 2.4 was applied. This transition has been successfully fabricated and used for waveguide component characterization in a first revision. However, etching wide open areas while having small features like the probe structure exposed adds challenges to the etching process [116]. To normalize the AR for critical features both of the DUT as well as the transition, guarding structures were inserted [127]. This ensures a more uniform etching process and straight sidewalls. Figure 3.5

26 CHAPTER 3. ON-WAFER DEVICE CHARACTERIZATION OF SILICONMICROMACHINED WAVEGUIDE COMPONENTS 500 µm guard structure E-field probe (a) 500 µm CPW slots E-field probe (b) 1 mm E-field probe etch holes in DL (c)

Figure 3.6: SEM images of calibration standards during fabrication: (a) Reflect standard with transition (A) after first DRIE of the HL (step Figure 3.5(a)). (b) Thru standard with transition (A) after second DRIE of the HL, etching the E-field probe down to the designed height (step Figure 3.5(c)). (c) Offset short standard with transition (B) after guard structures were released in a 50% HF bath (step Fig-ure 3.5(e)). Visible are the etch holes in the device layer to etch the buried oxide layer below the guard structures.

illustrates the modified process flow, described in detail in Paper III. Scanning electron microscope (SEM) images of the transition during fabrication are shown in Figure 3.6.

(a) Reflect calibration standard with transition (A) after the first DRIE etch of the handle layer with guard structures around the E-field probe (Figure 3.5(a)). (b) Thru calibration standard with transition (A) after the second DRIE etch of the handle layer (Figure 3.5(b)). The top oxide mask was removed, and both the E-field probe and the guard structures are etched down to the desired height of the probe.

(c) Offset short calibration standard with transition (B) after releasing the guard structures in a 50% HF bath (Figure 3.5(e)). The etch holes can be seen in the device layer.

The transition was first designed for 220–330 GHz with a rectangular layout and transversal slots of the CPW probing interface (see Figure 3.4(A), presented in Paper III). The design proved to be not suitable to be scaled to higher frequencies. An improved design based on a diamond shaped layout was developed and the model for 220–330 GHz is shown in Figure 3.4(B). The transition is demonstrated in Paper IV for both 220–330 GHz and 330–500 GHz, and measurement results at 220–330 GHz are given.

To characterize the single-ended transition offset short calibration kits were fabricated. A two-tiered calibration was performed to de-embed the two-port

3.3. RESULTS AND NOVELTY 27 scattering parameters of the transition [136]. When used for on-wafer device characterization, calibration standards for thru-reflect-line (TRL) calibration [142] are placed next to the DUTs on the wafer.

3.3 Results and novelty

A novel fully micromachined coplanar waveguide to silicon micromachined wave-guide transition for on-wafer device characterization was presented. The design of the transition allows its fabrication in the same silicon micromachined wave-guide technology presented in Chapter 2. Paper III reports on the design, fabri-cation and characterization of the transition at 220–330 GHz. The insertion loss of the single-ended transition is 0.3–1.5 dB over the whole band, and return in excess of 8 dB. The feasibility of the transition for on-wafer device characteriza-tion was proven by successfully characterizing silicon micromachined waveguide components [118–120, 122, 126, 128, 143].

A modified design of the transition allows scaling to higher frequencies. Pa-per IV reports on the implementation, fabrication and characterization of this tran-sition at 220–330 GHz and shows simulation results for a scaled version up to 500 GHz. Insertion loss of the measured single-ended transition is 0.2–0.7 dB over the whole frequency band of 220–330 GHz and return loss better than 10 dB, showing good agreement with simulation results.

Chapter 4

Integrating MMICs and silicon

micromachined waveguide

components

The integration of active monolithic microwave integrated circuits with silicon mi-cromachined waveguide systems is discussed in the final chapter of this thesis. Two concepts are presented, integrating silicon-germanium (Paper V) and indium phos-phide MMICs with silicon micromachined waveguides (Paper VI), respectively. Both approaches are scalable to higher frequencies and compatible with commercially available die bonding equipment for ease of assembly.

4.1 Background

Conventional THz systems using metal machined waveguides are manufactured in a serial manner, making them extremely expensive and only available for scien-tific instrumentation or in research projects. Cheaper and more compact systems are needed in order to open up the terahertz frequency range for new applica-tions. Indium phosphide (InP) transistor technologies have maximum frequen-cies well into the THz frequency range and are used in a variety of systems. For some applications like wireless communications the cost of InP MMICs is pro-hibitively high. Recently cost-effective silicon-germanium (SiGe) transistor tech-nologies have made big advances and are pushing f_maxup to 720 GHz [38].

30CHAPTER 4. INTEGRATING MMICS AND SILICON MICROMACHINED WAVEGUIDECOMPONENTS 2 cm 1.5 cm IF/LO Routing (CPW) DC Routing Silicon-Micromachined Waveguide Diplexer Silicon-Micromachined Integration Platform Slot Radiator Micromachined Waveguide Open Waveguide Interface Bond Wires MMIC

Figure 4.1: Integration concept (I). THz microsystem concept presented in Pa-per V. SiGe MMICs are partly placed inside a silicon micromachined waveguide cavity and couple into the waveguide with an on-chip H-plane transition. The con-cept includes biasing and feeding tracks fabricated in the micromachined parts.

One approach of more compact and integrated systems is to realize all com-ponents of the system on the substrate itself. For InP complete front-ends and receivers have been reported up to 300 GHz [144–147], and advances for SiGe transistors led to transceivers demonstrated at 100–300 GHz [148, 149] How-ever, this approach is not mature enough for higher frequencies and integration and packaging concepts are needed [2]. As discussed in the previous chapters waveguides are the transmission medium of choice in THz systems. Silicon micro-machining promises higher integration and much lower costs compared to metal machined waveguides, driving the adaption of low-cost systems to exploit the THz frequency range.

Integration concepts of plane split waveguides with MMIC using on-chip E-field probes [102, 150] and transitions from coplanar waveguides to microma-chined waveguides suitable for packaging have been reported [104, 151]. How-ever, only few complete systems using micromachined packaging technologies have so far been demonstrated [47]. In [152] a power splitter/combiner module at 200–270 GHz using Nuvotronics’ micromachining technology integrated with Teledyne InP HBT MMICs, and NASA Jet Propulsion Laboratory demonstrated both a micromachined radiometer/spectrometer front-end at 520–600 GHz [153, 154] and an eight-pixel transceiver array for a 340 GHz imaging radar [88].

4.2. MOTIVATION AND CHALLENGES 31 micromachined vertical waveguide (top Si chip) micromachined horizontal waveguide (bottom Si chip) E-plane bend (bottom Si chip) InP MMIC back-to-back CPW probing interface

Figure 4.2: Integration concept (II). MMIC to silicon micromachined waveguide transition presented in Paper VI. The silicon micromachined part consists of a top and bottom chip, and the MMIC is fabricated in the Teledyne TSC250 InP DHBT process. A microstrip line couples into a vertical substrate waveguide connected to a vertical waveguide in the top silicon chip. In the bottom silicon an E-plane bend connects the vertical to a horizont in-plane waveguide.

4.2 Motivation and challenges

Any system integrating active devices needs to address three requirements [47]: • a suitable packaging environment

• transmission medium for RF interconnections • DC biasing and control signal connections

Two integration concepts for SiGe and InP MMICs with silicon micromachined waveguide systems presented in Chapter 2 were developed. Both promise highly compact and integrated THz systems, are designed to be compatible with indus-trial assembly tools and thus allows for rapid and precise alignment and assembly. (I) Paper V reports on a novel integration platform for SiGe MMICs. The concept is shown in Figure 4.1. No vertical stacking and interconnects are required as all features are in the H-plane. Integration of the MMIC into the wave-guide is provided by an in-line H-plane transition. The wavewave-guides are fab-ricated in the technology presented in Chapter 2. DC biasing and RF feeding

32CHAPTER 4. INTEGRATING MMICS AND SILICON MICROMACHINED WAVEGUIDECOMPONENTS

lines are directly implemented in the micromachined components. Initially designed, fabricated and characterized at 110–170 GHz all micromachined components are scalable to higher frequencies.

(II) In Paper VI a transition from MMIC to silicon micromachined waveguide sys-tems is shown (Figure 4.2). The transition is co-designed in an InP MMIC fabricated in Teledyne’s TSC250 InP DHBT process and a silicon microma-chined two-chip stack. A microstrip line on the MMIC couples to a vertical substrate waveguide sitting on a vertical ridge waveguide etched in the top silicon chip. Back-to-back transitions were designed, fabricated and charac-terized at 220–330 GHz. The two transitions were connected in the bottom silicon chip by E-plane bends and a horizontal in-plane waveguide. Similar to the concept for SiGe MMIC integration this approach using SOI wafers allows the fabrication of biasing and feeding traces directly in the silicon mi-cromachined waveguide parts.

4.3 Results and novelty

In this chapter two novel integration concepts for monolithic microwave integrated circuits and silicon micromachined waveguide components were introduced. The micromachined part of both concepts was fabricated in the silicon micromachined waveguide technology using SOI wafers presented in Chapter 2.

• A complete integration and packaging concept including biasing and RF feed-ing lines for the integration of SiGe and silicon micromachined waveguide systems was presented. Initially implemented at 110–170 GHz and pre-sented in Paper V, all micromachined components can be easily scaled to higher frequencies. Using this initial implementation D-band transceiver modules achieving a data rate of 2.66 Gbit/s with 16QAM over a 750 MHz channel were presented [155].

• The second concept introduces a co-designed transition from an InP MMIC to silicon micromachined waveguide. The transition sits on top of the mi-cromachined waveguide chip, enabling assembly with industrial assembly equipment. The micromachined part consists of a top chip with a verti-cal waveguide feeding the in-plane waveguide in the bottom chip. Imple-mented for 220–330 GHz in Paper VI, insertion loss is 3–6 dB and return loss in excess of 5 dB. The higher than simulated insertion loss is attributed to an additional post-processing step to open up the waveguide opening in the backside metallization of the MMIC. The waveguide openings for future design iterations are realized during the MMIC fabrication, and a lower in-sertion loss is expected. The concept is scalable to higher frequncies. The

4.3. RESULTS AND NOVELTY 33 feasibility of the transition for integration and packaging for THz systems has been shown, and the fabrication of MMICs and silicon micromachined waveguide components for more complex systems is ongoing.

Chapter 5

Conclusions

This thesis compiles the development of silicon micromachined waveguide com-ponents with the goal of creating highly compact and integrated systems for the further exploitation of the terahertz frequency range. The underlying waveguide technology is realized with silicon on insulator wafers. This reduces the surface roughness of the waveguide and thus decreases the insertion loss.

Using this technology several waveguide components have been successfully characterized for 220–330 GHz:

• silicon micromachined waveguides, achieving an insertion loss of 0.02–0.07 dB/mm

• a hybrid coupler with coupling coefficients of−3.2_±0.4 dB

• a power divider with transmission coefficients of−3.5±0.4 dB

• integrated absorbers for matched loads • very low-loss cavity resonator filters

An investigation into fabrication accuracy and repeatability for multi-chip fil-ters was given, showing the maturity of the fabrication process and the applied alignment and assembly techniques.

Essential during development of terahertz waveguide systems is the ability to measure and characterize individual components. A novel coplanar waveguide probe to micromachined waveguide transition was presented. The transition is co-fabricated with the devices under test in the same waveguide technology pre-viously introduced without further assembly, enabling rapid and accurate on-wafer device characterization. Initially demonstrated at 220–330 GHz with an insertion loss of better than 1.5 dB, the transition was successfully used for on-wafer de-vice characterization. An improved design of the transition achieved a measured