In this paper, viable transmission media technology has been demonstrated for the first time on GaN on low-resistivity silicon) substrates (ρ < 40 $\Omega$·cm) at H-band frequencies (220-325 GHz). The shielded-elevated coplanar waveguide (CPW) lines employ a standard monolithic microwave integrated circuit compatible air bridge process to elevate the CPW traces above a 5-μm layer of benzocyclobutene on shielded metalized ground plates. An insertion loss of less than 2.3 dB/mm was achieved up to 325 GHz, compared with 27 dB/mm for CPW fabricated directly on the substrate. To prove the efficiency of the technology, a short-circuited stub filter with a resonant frequency of 244 GHz was used. The filter achieved an unloaded Q-factor of 28, along with an insertion loss of 0.35 dB and a return loss of-34 dB. To our knowledge, these results are the best reported to date for GaN-based technology.This work was supported by the EPSRC under Grant EP/N014820/1

Eblabla, AM

Elgaid, K

Guiney, I

Li, X

Wallis, DJ

IEEE Transactions on Terahertz Science and Technology

In this paper, viable transmission media technology has been demonstrated for the first time on GaN on low-resistivity silicon) substrates (ρ <; 40 Ω·cm) at H-band frequencies (220-325 GHz). The shielded-elevated coplanar waveguide (CPW) lines employ a standard monolithic microwave integrated circuit compatible air bridge process to elevate the CPW traces above a 5-μm layer of benzocyclobutene on shielded metalized ground plates. An insertion loss of less than 2.3 dB/mm was achieved up to 325 GHz, compared with 27 dB/mm for CPW fabricated directly on the substrate. To prove the efficiency of the technology, a short-circuited stub filter with a resonant frequency of 244 GHz was used. The filter achieved an unloaded Q-factor of 28, along with an insertion loss of 0.35 dB and a return loss of - 34 dB. To our knowledge, these results are the best reported to date for GaN-based technology

Eblabla, Abdalla Mohamed

Li, Xu

Wallis, David J

Guiney, Ivor

Elgaid, Khaled

Online Research @ Cardiff

GaN on low-resistivity silicon THz high-Q passive device technology

In this paper, viable transmission media technology has been demonstrated for the first time on GaN on low-resistivity silicon) substrates (ρ &lt; 40 Ω·cm) at H-band frequencies (220–325 GHz). The shielded-elevated coplanar waveguide (CPW) lines employ a standard monolithic microwave integrated circuit compatible air bridge process to elevate the CPW traces above a 5-μm layer of benzocyclobutene on shielded metalized ground plates. An insertion loss of less than 2.3 dB/mm was achieved up to 325 GHz, compared with 27 dB/mm for CPW fabricated directly on the substrate. To prove the efficiency of the technology, a short-circuited stub filter with a resonant frequency of 244 GHz was used. The filter achieved an unloaded Q-factor of 28, along with an insertion loss of 0.35 dB and a return loss of – 34 dB. To our knowledge, these results are the best reported to date for GaN-based technology

Wallis, David J.

Enlighten

English

GaN on low resistivity silicon THz high -Q passive device technology

Apollo (Cambridge)

1   Abstract In this paper, viable transmission media technology has been demonstrated for the first time on GaN-on-low resistivity silicon (LR- -band frequencies (220-325 GHz). The shielded-elevated (SE) CPW lines employ a standard MMIC compatible air bridge process to elevate the CPW traces above a 5 µm layer of benzocyclobutene (BCB) on shielded metalized ground plates. An insertion loss of less than 2.3 dB/mm was achieved up to 325 GHz, compared with 27 dB/mm for CPW fabricated directly on the substrate. To prove the efficiency of the technology, a short-circuited stub filter with a resonant frequency of 244 GHz was realised. The filter achieved an unloaded Q-factor of 28, along with an insertion loss of 0.35 dB and a return loss of -34 dB. To our knowledge, these results are the best reported to date for GaN-based technology.   Index Terms GaN-HEMTs, low resistivity silicon substrates, coplanar waveguides (CPWs), H-band, High-Q THz filters, TMICs.  I. INTRODUCTION  Hz technology has many applications in imaging and sensing, spectroscopy, astronomy and communications [1] [2]. The short wavelength of THz frequencies makes it a promising technology due to the unique interaction of its spectral regime with matter and the achievable high resolution imaging [3]. This interest in new emerging applications is motivated by the recent advances in high-speed semiconductor devices and nanotechnology; which have enabled the realization of TMIC (THz Monolithic Integrated Circuits) [4]. The advantage of using III-Nitride based material devices in TMIC such as higher power density and power added efficiency makes it more suitable than other material systems such as GaAs, InP or Si [5]. The utilization of TMIC technology in THz frequencies application is a critical component to suppress unwanted moding effects and hence reducing signal loss. In addition, TMIC offers the advantage of higher functionality, low system costs and smaller chip size. Currently, GaN high-electron-mobility-transistors (HEMTs) on semi-insulating (SI) SiC have achieved a cutoff frequency   Manuscript received December 17, 2015; revised June 24, 2016; accepted September 30, 2016. K. Elgaid, A. Eblabla and X. Li are with the School of Engineering, The University of Glasgow, (e-mail: Khaled.Elgaid@ glasgow.ac.uk).  D. J. Wallis and I. Guiney are with the Cambridge Centre for GaN, The University of Cambridge.   (fT) of 450 GHz through the intensive progress in GaN HEMTs scaling technologies toward THz operation; applying such devices in TMIC technology will yield systems operating in frequencies higher than their fT [6]. However, SiC substrates are expensive and have limited availability in large substrate diameters.     The potential use of GaN HEMTs grown on LR Si for MMIC circuits offers the advantage of cost-effective and large diameter wafers, which make manufacturing costs of GaN-on- LR Si potentially competitive with existing high-resistivity (HR) Si and SiC technologies. However, RF substrate coupling effects are the main cause of performance degradation when considering LR Si as a substrate [7]. Therefore, substrate loss suppression is a crucial step towards the industrialization of high-quality interconnects and passive elements using GaN-on-LR Si technology.  However, At frequencies above 220 GHz, designing high-Q passive components is very challenging as the wavelength becomes comparable with the substrate thicknesses [8]. To capitalize on the advantages in utilizing GaN on LR Si for millimeter-wave and THz applications, a technology with minimal substrate coupling is required. Insertion of a low-loss, low dielectric constant, k, layer of BCB as an insulator has proven to be a successful technique for substrate coupling reduction [9]. This approach, compared to other complicated techniques  [10] [11], has the advantage of accommodating active circuits underneath the passive components and interconnections with no degradation of active device performance [12].     In this work, substrate coupling effects were eliminated by the complete isolation of the substrate, where the CPW GaN on Low Resistivity Silicon THz High-Q Passive Device Technology Abdalla Eblabla, Xu Li, David J. Wallis, Ivor Guiney and Khaled Elgaid.  T   Fig. 1: Oblique projection of the fabricated 1 mm- -CPW on BCB with G = 21 µm, S = 18 µm and Wg = 100 µm. 2  interconnect have no direct contact to the lossy Si in addition to EM shielding. The developed SE-CPW transmission media use a reliable fabrication process to elevate all the CPW traces above a 5 µm layer of BCB on shielded metalized ground plates. Hence, the insertion loss was dramatically reduced to less than 2.3 dB/mm, compared to 27 dB/mm for that of CPW fabricated directly on the substrate over the whole H-band frequency range. For further investigation of the viability of the proposed technology, a band bass filter was simulated, fabricated and characterized. A sharp resonance with Q-factor of 28 was obtained at a center frequency of 244 GHz. This indicates the suitability of III-V-on-LR Si technology for millimeter-wave and THz applications.    I. MATERIAL AND DEVICE TECHNOLOGY     The study was performed on a GaN-based material structure that could be used to realize AlGaN/GaN HEMTs. The epitaxial layers were grown by Metal-Organic Chemical Vapor Deposition (MOCVD) on 675 µm thick -type Si (111) substrates. The layer stack, from the substrate up, consists of a 250 nm AlN nucleation layer followed by a 850 nm Fe-doped AlGaN graded buffer to accommodate the lattice and thermal expansion mismatch, a 1.4 µm insulating GaN buffer and channel layer, a 25 nm Al0.25Ga0.75N barrier and a 2 nm GaN cap, as shown in Fig. 1. Epitaxial layers and growth procedure are detailed in [13].      All levels of device definition were realized using optical lithography and all steps required to realize the transmission media are compatible with standard MMICs technology. As in a standard MMIC process, passive devices were fabricated on the mesa floor, where the transistor active region (the upper two layers) were etched away.     Fabrication process of the SE-CPW technology started with spinning a 5 µm-thick BCB film, and then fully cured at 250 0C in N2 atmosphere. Following this, Ti/Au (50/600 nm) was deposited to form the fabrication alignment markers and shielding plates. Next, air-bridge technology was utilized to elevate the CPW traces. Finally, the sample was metalized using 2 µm Au electroplating. Fig. 2 shows a micrograph of the fabricated devices.  II. MODELLING AND MEASUREMENTS     A 3-D full-wave electromagnetic simulation tool, Ansoft HFSS was employed to design the SE-CPW technology. Optimization of structure geometrics and highs was carried during the simulation for better performance and to ensure the suppression of RF energy dispersion introduced by the conductive Si substrate.      On-wafer measurements of small-signal S-parameter were performed using an Agilent PNA network analyzer over the range 220-325 GHz (H-band). The system was calibrated using short-open-load-thru (SOLT) calibration based on an off-chip ISS impedance standard. 50 µm-pitch WR-03 waveguide Picoprobes were used for probing; where a durable device input/output pads were realised by placing adjacent two air-bridges, as shown in Fig 2. The samples were placed on a thick quartz spacer to eliminate any possible parasitic substrate modes caused by the metal chuck.  III. RESULTS AND DISCUSSION A. Transmission media    RF signal coupling to the lossy substrate was investigated by the comparison of CPW layout fabricated directly on GaN-on-LR Si substrate with that using the SE-CPW with BCB technology (by shielding using metal layer, inserting low dielectric constant BCB and additional elevation using air-bridge support), as shown in Fig. 3. In addition, measured results of the elevated CPW using air-bridge technology and shielding metal (excluding the BCB insert) was included, as shown in Fig. 3.     From Fig. 3a, substrate losses were clearly dominant for the CPW structures realized directly on the substrate for the whole H-band frequency range, where an insertion loss of more than 23 dB/mm was observed. This means that most of the E-field was penetrating through to the lossy Si substrate. The insertion loss was reduced across the frequency band to less than 5.4 dB/mm by using an elevated CPW and shielding plates on the    (a)    (b)  Fig. 2: Scanned electron microscopy (SEM) image of the fabricated devices on BCB. (a) SE-CPW, and (b) Band-pass filter. 3  substrate (excluding the BCB insert). This reduction in substrate effects is mainly due to the electric field being mostly concentrated within the air-gap between the shielding plate and elevated CPW traces. However, as the SE-CPW structure requires the use of ground holes in the shielding plate, there are small areas in the shielding plate in which the CPW signal trace is in direct contact with the lossy substrate. Losses are very sensitive to these exposed areas (ground holes) especially at H-band frequency range, where the wavelength dimensions become comparable to the substrate exposed area at the base of the air-bridge support. To overcome this issue, the SE-CPW was fabricated on top of a 5 µm thick layer of BCB. This eliminated substrate coupling effects where the CPW traces are totally isolated from the lossy substrate. Hence, the return loss was improved to less than 2.3 dB/mm up to 325 GHz, the lowest reported to date for GaN based technology.      To further characterize the transmission media performance developed in this work, attenuation constant calculated [14]. As shown in Fig. 3b, using the proposed SE-CPW  was noticeably minimized especially at the higher frequency range of the H-d technology as low as of 0.3 Np/mm was obtained up to 325 GHz, and is the best reported to date at THz frequencies. This proves the complete isolation of the conductive substrate and THz signal confinement in the transmission media of this work. The measured results obtained are verified by the very close agreement between measured and simulated S-parameters, as shown in Fig. 3.      B. Short-circuited stub filters    The short-circuited shunt matching network is a key topology for a variety of circuits including band-pass filters, diode detectors and matching / DC return networks [4] [15]. However, in the H-band frequency range,  short-circuited matching stubs exhibit low quality factor even for SI GaAs substrates which makes them a non-viable component for TMIC technology [16].     Fig. 4 shows a 3-D plot of the fabricated SE-CPW on BCB dielectric short-circuited stub filter structure. The short stub characteristic impedance, Z0 dimensions are; W= 39 µm and S = 19 µm. While the feed line was designed to be Z0 W = 24 µm and S = 30 µm.     Fig. 5a shows the simulated and measured S-parameters of the SE-CPW short stubs on BCB dielectric insert. It is clear that insertion of an insulating layer of BCB between the GaN-on-LR Si substrate and shielding plate resulted in superior performance and a relatively sharp resonance, with a Q-factor of 28 and return loss of -34 dB and at 244 GHz. In addition, an insertion loss and 3dB-bandwidth of as low as 0.35 dB and 101 GHz were obtained respectively at a resonance frequency of 244 GHz. This improvement in performance is an indication of the complete isolation of the lossy substrate.     Previous work done by other researchers obtained a Q-factor of 21 and insertion loss of -4.1 dB/mm on RF CMOS technology operating at 60 GHz [10] . These results obtained by the newly developed technology are superior to those of the same structure using shielded elevated CPW on air-bridge supports without the BCB dielectric insert layer as shown in Fig. 5b. Higher insertion loss was observed in additional to the low quality factor.   -35-30-25-20-15-10-50220 240 260 280 300 320Frequency (GHz)SE-CPW on BCB [Measured]SE-CPW on BCB [Simulated]SE-CPW [Measured]CPW directly on the substrate [Measured]  (a)  00.511.522.533.54220 240 260 280 300 320Frequency (GHz)SE-CPW on BCB [Measured]SE-CPW on BCB [Simulated]SE-CPW [Measured]CPW directly on the substrate [Measured] (b)  Fig. 3.  S-parameters results for the fabricated 1 mm-  transmission media. (a) Insertion loss, and (b) attenuation constant.    Fig. 4: Oblique projection of the fabricated SE-CPW short-circuited stub filter with the fabricated dimensions. 4  IV. CONCLUSIONS In this paper, for the first time, high-Q transmission media on GaN-on-LR Si substrates ( 40 .cm) technology are simulated, fabricated and measured at H-band. The SE-CPW lines employ a standard MMIC compatible air bridge process to elevate the CPW traces above a 5 µm layer of BCB on shielded metalized ground plates. An insertion loss and attenuation constant of less than 2.3 dB/mm and 0.4 Np/mm were achieved, respectively up to 325 GHz. The viability of the proposed SE-CPW technology was investigated by the realization of a short-circuited stub filter. The filter achieved an unloaded Q-factor and insertion loss of 28 and 0.35 dB, respectively at 244 GHz. To our knowledge, these results are the best transmission media performance reported to date for mm-wave and THz GaN based technology. The proposed SE-CPW on BCB offers a promising technology for the integration of high RF performance active devices and low-losses passive elements for the realization of TMIC technology.  V. ACKNOWLEDGMENT     This work was supported by the EPSRC under grant EP/N014820/1.   VI. REFERENCES   [1] D. L. Woolard, et al.Proc. IEEE, vol. 93, no. 10, pp. 1722 1745, 2005. [2] V. Sanphuang, et al. Bandwidth reconfigurable THz filter employing phase-change m in Antennas and Propagation & USNC/URSI National Radio Science Meeting, 2015 IEEE International Symposium on, 2015, pp. 2289 2290. [3] M. Mikulla, et al. -speed technologies based on III-V  European al. Microwave Integrated Circuits Conference (EuMIC), 2013 European, 2013, pp. 169 171. [4] E. Feiginov, et al., Semiconductor Terahertz Technology: Devices and Systems at Room Temperature Operation John Wiley & Sons, Ltd, 2015. [5] A. Brown, et al. - in 2011 IEEE MTT-S International Microwave Symposium, 2011, pp. 1 1. [6] K. Shinohara, et al. schottky diodes for submillimeter-wave MMIC a IEEE Trans. Electron Devices, vol. 60, no. 10, pp. 2982 2996, 2013. [7] A. Eblabla, et al. shielded coplanar waveguides on GaN-on-low resistivity Si substrates for MMIC a  IEEE Microw. Wirel. Components Lett., vol. 25, no. 7, pp. 1012, 2015. [8] D. M. Pozar, Microwave engineering th ed, John Wiley & Sons, Inc. 2012. [9] prescription for sub-millimeter-wave transistor c IEEE Trans. Terahertz Sci. Technol., vol. 3, no. 4, pp. 433 439, 2013. [10] A. L. Franc, et al. -performance shielded coplanar waveguides for the design of CMOS 60-IEEE Trans. Electron Devices, vol. 59, no. 5, pp. 1219 1226, 2012. [11] J.-J. Lee and C.- A slow-wave microstrip line with a high-Q and a high dielectric constant for millimeter-wave CMOS a IEEE Microw. Wirel. Components Lett., vol. 20, no. 7, pp. 381 383, 2010. [12] M. J. Anand, et al. -dielectric benzocyclobutane encapsulated AlGaN/GaN HEMTs with Improved off-state Jpn. J. Appl. Phys., vol. 036504, 2015. [13] A. Eblabla, et al. performance GaN high electron mobility transistors on low resistivity silicon for X-band applicat IEEE Electron Device Lett., vol. 36, no. 9, pp. 899 901, 2015. [14] J. Zhang, et al. characteristics of coplanar waveguides at subterahertz f IEEE Trans. Microw. Theory Tech., vol. 53, no. 11, pp. 3281 3287, 2005. [15] F. A -band elevated CPW band-pass filters on GaAs s IEEE Microwave Conference (EuMC), Paris, pp. 9 12, 2010. [16] enhancement of millimetre-wave resonators using elevated IEEE Electron. Lett., vol. 45, no. 25, pp. 1326 1327, 2009.     -45-40-35-30-25-20-15-10-50220 240 260 280 300 320Frequency (GHz)Measured (S21)Measured (S11)Simulated (S21)Simulated (S11)  (a)   -45-40-35-30-25-20-15-10-50220 240 260 280 300 320Frequency (GHz)Measured (S21)Measured (S11)  (b)  Fig. 5.  S-parameters results of the fabricated SE-CPW short-circuited stub filter (a) with BCB insert, and (b) without BCB insert. 5  Abdalla Eblabla received the B.S. degree in communication engineering from Misurata University, Misurata, Libya, in 2008 and the M.S. degree in electronics and electrical engineering from the University of Glasgow, Glasgow, United Kingdom, in 2012, He is currently pursuing the Ph.D. degree in electronics and electrical engineering at the University of Glasgow, Glasgow, United Kingdom. His research interests include design, fabrication and characterization of microwave and millimeter-wave components and circuit design.    Xu Li received the B.S. degree in physical chemistry from Central South University, Changsha, China, in 1981, the M.S. degree in metallurgical chemistry from the Changsha Institute of Mining and Metallurgy, Changsha, in 1986, and the Ph.D. degree in applied chemistry from the University of Strathclyde, Glasgow, U.K., in 1997. He has been with the University of Glasgow, Glasgow, since 2004.     David J. Wallis has a first class honours degree in Physics and Computing and a PhD in Microstructural Physics from the University of Cambridge.  He has more than 25 years experience in the characterisation and growth of III-V semiconductor materials and has been involved in the growth of GaN by MOCVD since 2002. He was the technical lead for GaN characterisation and growth at QinetiQ (formerly RSRE and DERA) until 2011. Between 2012 and 2016 Dr Wallis worked for Plessey Semiconductors Leading the technical development of GaN based LEDs on large area Si substrates for commercial applications. In March 2016 he took up an EPSRC Manufacturing fellowship based at the University of Cambridge and is currently leading the development of GaN based device structures for a variety of optical and electronics applications.    Ivor Guiney is a MOCVD expert and is the growth specialist for GaN power and RF applications in the University of Cambridge. Prior to this role, he had various project management roles in industry which were focused on the growth of graphene, GaN, GaAs, InP and GaSb device structures by MOCVD and various dielectrics by ALD. The main areas of device development in this work were in GaN blue LEDs on sapphire, GaN on Si and SiC power electronics, InGaAsP-based telecomms modulators and IR-laser diodes. He has authored or co-authored over 30 publications and has given two invited talks at international conferences on the topics of GaN optoelectronics and power electronics. He currently sits on the Semiconductor Group Committee of the Institute of Physics as the treasurer.   Khaled Elgaid B.Sc. degree from the University of Evansville, Evansville, USA, M.Sc. degree from the University of Cincinnati, Cincinnati, USA, and the Ph.D. degree from the University of Glasgow, Glasgow, U.K.  He is a Senior Lecturer of High Frequency Compound Semiconductor Devices and Integrated Circuits at the University of Glasgow, Glasgow, U.K., where he is the Head of High-Frequency Integrated Technology research group in the School of Engineering. Currently his research interests in the design and realization of compound semiconductor microwave, millimeter-wave and Terahertz Monolithic integrated circuits using state of the art in-house nanotechnology facilities. These activities utilise different compound semiconductors platform including GaN and GaAs. The group enhances its research activities with further integration including on chip antennas at sub Millimeter-Wave and Terahertz Frequencies. His research activities are funded by government agencies and world leading industrial organizations in the area of high-speed wireless communications, radar, imaging and sensors applications.     

GaN on Low-Resistivity Silicon THz High-Q Passive Device Technology

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