Shielded-Elevated Coplanar Waveguides (SE-CPWs) with low loss have been successfully developed for the first time for RF GaN on low-resistivity silicon (LR-Si) substrates (σ < 40 Ω.cm). Transmission losses (S 21 ) of less than 0.4 dB/mm at X-band and better than 2 dB/mm at K-band with less than 20 dB return loss were exhibited by the developed SE-CPW, making them comparable in performance to those on traditional (semi-insulating) SI substrates. The developed waveguides use air-bridge technology to suspend CPW tracks above the HEMT GaN layer on LR-Si, directly above an additional thin layer of SiN and shielded ground planes. EM simulation was used to adjust structure parameters for performance optimization. In this work, we eliminated RF energy coupled into the substrate, paving the way for a cost-effective and higher integration GaN MMICs on LR-Si.This work was supported in part by the EPSRC III-V national
center pump-priming scheme

Eblabla, A

Elgaid, K

Guiney, I

Wallis, DJ

Apollo (Cambridge)

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS 1Novel Shielded Coplanar Waveguides onGaN-on-Low Resistivity Si Substratesfor MMIC ApplicationsA. Eblabla, D. J. Wallis, I. Guiney, and K. ElgaidAbstract—Shielded-Elevated Coplanar Waveguides (SE-CPWs)with low loss have been successfully developed for the ﬁrst timefor RF GaN on low-resistivity silicon (LR-Si) substrates. Transmission losses of less than 0.4 dB/mmat X-band and better than 2 dB/mm at K-band with less than20 dB return loss were exhibited by the developed SE-CPW,making them comparable in performance to those on traditional(semi-insulating) SI substrates. The developed waveguides useair-bridge technology to suspend CPW tracks above the HEMTGaN layer on LR-Si, directly above an additional thin layer of SiNand shielded ground planes. EM simulation was used to adjuststructure parameters for performance optimization. In this work,we eliminated RF energy coupled into the substrate, paving theway for a cost-effective and higher integration GaN MMICs onLR-Si.Index Terms—AlGaN/GaN HEMTs, Attenuation constant andconformal mapping, coplanar waveguides (CPWs), low resistivitysilicon substrates.I. INTRODUCTIONO VER a number of years, mm-wave systems using highpower, high frequency transistors have found broad ap-plication in the telecommunication and radar ﬁelds. In orderto become economy viable, such applications require low-costtransistors which can deliver signiﬁcant power at mm-wave fre-quencies. The wide energy gap , high break-down ﬁelds , high saturation electronvelocities and high channel electron den-sities found in GaN HEMTs enable the fabri-cation of devices which can deliver high power densities at bothmicrowave and mm-wave frequencies [1], [2]. Recent work hasextended the frequency performance of GaN HEMTs grown onsilicon, with cutoff frequencies as high as at-tained for 75 nm gate AlGaN/GaN HEMTs with high saturationcurrent density and a low[3].Realization of low-loss waveguides and capacitors and in-ductors with high-quality factor at mm-wave frequencies are ofManuscript received October 06, 2014; revised December 30, 2014; acceptedMarch 11, 2015. This work was supported in part by the EPSRC III-V nationalcenter pump-priming scheme.A. Eblabla and K. Elgaid are with the School of Engineering, The Universityof Glasgow, Glasgow G12 BQQ, U.K. (e-mail: khaled.elgaid@ glasgow.ac.uk).D. J. Wallis and I. Guiney are with the Cambridge Centre for GaN, The Uni-versity of Cambridge, Cambridge CB2 1TN, U.K.Color versions of one or more of the ﬁgures in this letter are available onlineat http://ieeexplore.ieee.org.Digital Object Identiﬁer 10.1109/LMWC.2015.2429120great importance for circuit application where interconnects andpassive components are needed. Therefore, low-loss transmis-sion media is necessarily required for cost-effective GaN-basedMMIC technology.Previously, a number of researchers have attempted to de-velop high quality CPWs on LR Si substrates. Some reportshave shown a reduction in losses using thick insulators suchas polyamide [4], benzocyclobutene (BCB) [5], AjinomotoBuild-up Film (ABF) and SU-8 [6] but these still have relativelyhigh attenuation at mm-wave frequencies. On the other hand,reports have shown that attaching silicon dioxide to thesurface of the Si substrate, induces a low-resistivity layer atthe interface between the and the Si surface [7]. Thesephenomena cause degradation of the attenuation constant. Sev-eral approaches have been suggested to avoid such undesirableeffects, such as introducing polysilicon at the Si- interface[8], removing the Si substrate from the backside substrate bymicromachining [9] and using the ﬂoating shield technique[10]. However, each of these methods requires complex andlengthy fabrication processes.In this letter, a Shielded-Elevated CPW (SE-CPW) is pro-posed to suppress the effect of the conductive substrate. In thisstructure, a ground plane is placed on top of the substrate toprovide near-complete isolation of the elevated CPW from thesubstrate. The structure provides transmission lines fabricatedon GaN-on-LR Si with much reduced dielectric loss and with avery low attenuation per unit length up to 67 GHz.II. EXPERMENTAL WORKThe study was carried out on AlGaN/GaN HEMTs suppliedby the Cambridge Centre for GaN in the University of Cam-bridge. The epitaxial layers were grown byMOCVD on 675LR P-type Si (111) substrates. The original layer stack, from thesubstrate up, consists of a 200 nmAlN nucleation layer followedby a 750 nm Fe-doped AlGaN graded buffer to accommodatethe lattice and thermal expansion miss-match, a 1.4 insu-lating GaN buffer and channel layer, a 20 nmbarrier and a 2 nm GaN cap, as shown in Fig. 1.The fabrication process was carried out at the James WattNanofabrication Center (JWNC) at the University of Glasgow.The transistor active region (the upper two layers) were etchedaway prior to transmission media fabrication. As in a standardMMIC process, the CPWs were fabricated on the isolationmesa ﬂoor on top of a 200 nm dielectric layer. The ﬁrststep of SE-CPW fabrication is to deﬁne the lower ground planeby e-beam exposure and development of a 1520 nm thicknessbi-layer PMMA, followed by electron beam evaporation of a1531-1309 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.2 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERSFig. 1. Oblique projection of the fabricated 50 SE-CPW line with, , and of length 1 mm.Fig. 2. Scanned Electron Microscopy (SEM) of the fabricated SE-CPW line.50 nm NiCr/400 nm Au layer and lift-off. Following this, theelevated traces' structures, including a number of supportingposts, were realized. These posts were deﬁned by spinningAZ4562 photoresist at 3000 rpm for 30 s (corresponding to5.5 elevation height). A 50 nm Ti/50 nm Au seed layer wasthen deposited prior to electroplating. Next, the elevated traceswere formed in 2 S1818 photoresist followed by exposure,development and ashing in a dry etch machine. The samplewas then metallized using 2 cold electroplating. Finally,different levels of exposure and development were performed.Fig. 2 shows SEM photos of the fabricated SE-CPW line.III. MEASUREMENTS AND MODELLINGOn-wafer S-parameter measurements were performed withan Agilent PNA network analyzer over the range 0.1–67 GHz.The system was calibrated using Line-Reﬂect-Reﬂect-Match(LRRM) calibration, and the lines were probed using Pico-probes with 100 pitch on the top of a double set of postslocated at either end. Then the samples were placed on a thickquartz spacer to eliminate any possible microstrip-like modecaused by the metal chuck.For modelling, the prefabricated SE-CPW lines were de-signed and simulated using a 3-D full-wave electromagneticsimulation tool, Anasoft HFSSTM, at the University ofGlasgow. The structure was precisely optimized during thesimulation for better performance and to minimize RF energydispersion introduced by the lossy substrate.IV. RESULTS AND DISCUSSIONWe initially investigated the performance of the SE-CPW bycomparing its measured and simulated data with that of a con-ventional CPW line between 0.1 and 67 GHz. Measured andFig. 3. Measured and simulated S-parameters for conventional CPW andSE-CPW lines. (a) Transmission coefﬁcient . (b) Reﬂection coefﬁcient.simulated S-parameters are in very close agreement, as shownin Fig. 3. It is clearly shown that the effect of the conductivesubstrate is dominant, even at low frequencies, for conventionalCPW lines. Since all the CPW traces are in direct physical con-tact with the substrate, most of the E-ﬁeld is coupled onto theconductive substrate even whilst employing a thin insu-lating layer. The losses are mainly due to two phenomena: oneis capacitive coupling which allows the ﬂow of conduction cur-rent not only through the metal strips but also through the lossysubstrate contact areas. The other reason is inductive couplingwhich induces current loops and associated losses by penetra-tion of the magnetic ﬁeld through the lossy substrate. Lossescontinuously increase over the whole frequency band, reachinga maximum of more than 14 dB at 67 GHz. On the other hand,by using an SE-CPW structure, improvements of the line perfor-mance in terms of dB/mm (up to 12 dB/mm) over a conventionalCPW line can be achieved. Losses also increase gradually overthe entire band for the SE-CPW line, showing a comparable per-formance to that of a CPW line fabricated on a semi-insulatingGaAs substrate [11], [12] as illustrated in Fig. 3(a).Not only does SE-CPW introduce improvements in termsof transmission losses , but it also provides acceptablematching, with as low as 22 dB/mm return loss, and more than20 dB/mm reduction over a conventional CPW line. Moreover,This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.EBLABLA et al.: NOVEL SHIELDED CPWS ON GaN-ON-LOW RESISTIVITY Si SUBSTRATES FOR MMIC APPLICATIONS 3Fig. 4. Measured and simulated of the attenuation constant, , of conventionalCPW and SE-CPW lines.the resonant frequency is shifted by nearly 5 GHz using the pro-posed transmission media. Consequently, SE-CPW lines miti-gate the unwanted in-band resonances, providing even better ef-ﬁciency than CPW-on-GaAs at K-band, as shown in Fig. 3(b).Another ﬁgure of merit in evaluating the performance of pas-sive components is the attenuation constant, , detailed in [13].Fig. 4 clearly shows that the attenuation constant rises steeplyto very high values for conventional CPW lines; 1 dB/mmat 76 GHz. However, a reduction of the attenuation constantof more than 0.6 dB/mm can be obtained using the SE-CPWstructure, owing to the electrical isolation of the signal tracefrom the bottom ground plane and the substrate. EM simulationwas carefully optimized to insure complete-isolation of theelevated signal and ground CPW traces by employing groundholes. These holes need to be as small as possible to reducethe effect of the conductive substrate. There are small areas inthe lower ground in which the line has physical contact withthe substrate and the supporting posts are in physical contactwith the substrate via exposed rectangular areas in the lowerground plane bond pad layer. For these reasons, SE-CPW linehas slightly larger attenuation constant (less than 0.2 dB/mm)compared to that of CPW-on-GaAs line at K-band.We believe that the SE-CPW could operate with similar per-formance without the CPW ground planes; although variationsin gap width (S) in proportion to the elevation height (h) maymake it behave as a conductor-backed CPW.V. CONCLUSIONWe have developed and characterized a high-performancenovel SE-CPW transmission media on AlGaN/GaN HEMTbuffer layers grown on LR-Si. The developed SE-CPW line haseffectively suppressed the effects of the low resistivity substrateresulting in excellent power conﬁnement. A transmission loss,, and attenuation constant, , of better than 2.4 dB/mm and0.4 dB/mm at 67 GHz were achieved respectively. These re-sults are comparable to those achieved for CPWs implementedon SI substrates such as GaAs and InP, demonstrating thefeasibility and viability of mm-interconnects on GaN-on-LRSi epitaxial layer stacks. The proposed SE-CPW transmissionmedia provides promising results of a mature GaN-on-LR Siprocess technology.REFERENCES[1] R. J. Trew, “SiC and GaN transistors—is there one winner formicrowave power applications?,” Proc. IEEE, vol. 90, no. 6, pp.1032–1047, Jun. 2002.[2] R. T. I. M. Kemerley, H. B. Wallace, and M. A. X. N. Yoder, “Impactof wide bandgap microwave devices on DoD systems,” Proc. IEEE,vol. 90, no. 6, pp. 1059–1064, 2002.[3] S. Ganguly, B. Song, W. S. Hwang, Z. Hu, M. Zhu, J. Verma, H. G.Xing, and D. Jena, “AlGaN/GaN HEMTs on Si by MBE with regrowncontacts and ,” Phys. Status Solidi, vol. 11, no. 3–4, pp.887–889, Feb. 2014.[4] L. P. B. K. George E. Ponchak and A. Margomenos, “Low-loss CPWon low-resistivity si substrates with a micromachined polyimide inter-face layer for rﬁc interconnects,” IEEE Trans. Microw. Theory Tech.,vol. 49, no. 5, pp. 866–870, 2001.[5] T. K. W. H. Haydl, J. Braunstein, L. F. E. M. Schlechtweg, and P.Tasker, “Low-Loss coplanar waveguides interconnects on low-resis-tivity silicon substrate,” IEEE Trans. Comp. Packag. Technol., vol. 27,no. 3, pp. 507–512, 2004.[6] D. Kim, D. H. Kim, J. M. Yook, J. I. Ryu, S. Park, J. C. Kim, andA. Cpws, “RF Passive Components on a Low Resistivity Si substratewith an ABF isolation layer,” in Proc. 5th Eur. Microw. Integr. CircuitsConf., Sep. 2010, pp. 317–320.[7] C. R. Neve, D. Lederer, G. Pailloncy, D. C. Kerr, J. M. Gering, T. G.Mckay, M. S. Carroll, and J. Raskin, “Impact of Si substrate resistivityon the non-linear behaviour of RF CPW transmission lines,” in Proc.3rd Eur. Microw. Integr. Circuits Conf., Oct. 2008, pp. 36–39.[8] T. H. Substrates, C. R. Neve, and J. Raskin, “RF harmonic distortionof CPW lines on,” IEEE Trans. Electron Devices, vol. 59, no. 4, pp.924–932, 2012.[9] H. C. Waveguide, S. T. Todd, X. T. Huang, J. E. Bowers, and N. C.Macdonald, “Fabrication, modeling, characterization of high-aspect-ratio coplanarwaveguide,” IEEE Trans. Microw. Theory Tech., vol. 58,no. 12, pp. 3790–3800, 2010.[10] T. Shun, D. Cheung, and J. R. Long, “Shielded passive devices forsilicon-based monolithic microwave and millimeter-wave integratedcircuits,” IEEE J. Solid-State Circuits, vol. 41, no. 5, pp. 1183–1200,2006.[11] K. I. Elgaid, “AKa-BandGaAsMESFETMonolithic Downconverter,”Ph.D. dissertation, Dept. Elect. Eng., Univ. Glasgow, Glasgow, U.K.,1998.[12] T. K. W. H. Haydl, J. Braunstein, L. F. E. M. Schlechtweg, and P.Tasker, “Attenuation of millimeterwave coplanar lines on galliumarsenide and indium phosphide over the range 1–60 GHz,” in IEEEMTT-S Int. Dig., 1992, no. l, pp. 3–6.[13] J. Zhang, S. Member, S. Alexandrou, T. Y. Hsiang, and S. Member,“Attenuation characteristics of coplanar waveguides at subterahertzfrequencies,” IEEE Trans. Microw. Theory Tech., vol. 53, no. 11, pp.3281–3287, 2005.

Novel Shielded Coplanar Waveguides on GaN-on-Low Resistivity Si Substrates for MMIC Applications

Shielded-Elevated Coplanar Waveguides (SE-CPWs) with low loss have been successfully developed for the first time for RF GaN on low-resistivity silicon (LR-Si) substrates. Transmission losses of less than 0.4 dB/mm at X-band and better than 2 dB/mm at K-band with less than 20 dB return loss were exhibited by the developed SE-CPW, making them comparable in performance to those on traditional (semi-insulating) SI substrates. The developed waveguides use air-bridge technology to suspend CPW tracks above the HEMT GaN layer on LR-Si, directly above an additional thin layer of SiN and shielded ground planes. EM simulation was used to adjust structure parameters for performance optimization. In this work, we eliminated RF energy coupled into the substrate, paving the way for a cost-effective and higher integration GaN MMICs on LR-Si

Eblabla, A.

Wallis, D.J.

Guiney, I.

Elgaid, K.

Enlighten

English

Novel shielded coplanar waveguides on GaN-on-low resistivity Si substrates for MMIC applications

A. Eblabla

D. J. Wallis

I. Guiney

K. Elgaid

Crossref

IEEE Microwave and Wireless Components Letters

Shielded-Elevated Coplanar Waveguides (SE-CPWs) with low loss have been successfully developed for the first time for RF GaN on low-resistivity silicon (LR-Si) substrates (σ<;40 Ω·cm). Transmission losses (S21) of less than 0.4 dB/mm at X-band and better than 2 dB/mm at K-band with less than 20 dB return loss were exhibited by the developed SE-CPW, making them comparable in performance to those on traditional (semi-insulating) SI substrates. The developed waveguides use air-bridge technology to suspend CPW tracks above the HEMT GaN layer on LR-Si, directly above an additional thin layer of SiN and shielded ground planes. EM simulation was used to adjust structure parameters for performance optimization. In this work, we eliminated RF energy coupled into the substrate, paving the way for a cost-effective and higher integration GaN MMICs on LR-Si

Eblabla, Abdalla

Wallis, David

Elgaid, Khaled

Online Research @ Cardiff

Novel shielded coplanar waveguides on GaN-on-Low resistivity Si substrates for MMIC applications

Enlighten: Publications

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS 1
Novel Shielded Coplanar Waveguides on
GaN-on-Low Resistivity Si Substrates
for MMIC Applications
A. Eblabla, D. J. Wallis, I. Guiney, and K. Elgaid
Abstract—Shielded-Elevated Coplanar Waveguides (SE-CPWs)
with low loss have been successfully developed for the first time
for RF GaN on low-resistivity silicon (LR-Si) substrates
. Transmission losses of less than 0.4 dB/mm
at X-band and better than 2 dB/mm at K-band with less than
20 dB return loss were exhibited by the developed SE-CPW,
making them comparable in performance to those on traditional
(semi-insulating) SI substrates. The developed waveguides use
air-bridge technology to suspend CPW tracks above the HEMT
GaN layer on LR-Si, directly above an additional thin layer of SiN
and shielded ground planes. EM simulation was used to adjust
structure parameters for performance optimization. In this work,
we eliminated RF energy coupled into the substrate, paving the
way for a cost-effective and higher integration GaN MMICs on
LR-Si.
Index Terms—AlGaN/GaN HEMTs, Attenuation constant and
conformal mapping, coplanar waveguides (CPWs), low resistivity
silicon substrates.
I. INTRODUCTION
O VER a number of years, mm-wave systems using highpower, high frequency transistors have found broad ap-
plication in the telecommunication and radar fields. In order
to become economy viable, such applications require low-cost
transistors which can deliver significant power at mm-wave fre-
quencies. The wide energy gap , high break-
down fields , high saturation electron
velocities and high channel electron den-
sities found in GaN HEMTs enable the fabri-
cation of devices which can deliver high power densities at both
microwave and mm-wave frequencies [1], [2]. Recent work has
extended the frequency performance of GaN HEMTs grown on
silicon, with cutoff frequencies as high as at-
tained for 75 nm gate AlGaN/GaN HEMTs with high saturation
current density and a low
[3].
Realization of low-loss waveguides and capacitors and in-
ductors with high-quality factor at mm-wave frequencies are of
Manuscript received October 06, 2014; revised December 30, 2014; accepted
March 11, 2015. This work was supported in part by the EPSRC III-V national
center pump-priming scheme.
A. Eblabla and K. Elgaid are with the School of Engineering, The University
of Glasgow, Glasgow G12 BQQ, U.K. (e-mail: khaled.elgaid@ glasgow.ac.uk).
D. J. Wallis and I. Guiney are with the Cambridge Centre for GaN, The Uni-
versity of Cambridge, Cambridge CB2 1TN, U.K.
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LMWC.2015.2429120
great importance for circuit application where interconnects and
passive components are needed. Therefore, low-loss transmis-
sion media is necessarily required for cost-effective GaN-based
MMIC technology.
Previously, a number of researchers have attempted to de-
velop high quality CPWs on LR Si substrates. Some reports
have shown a reduction in losses using thick insulators such
as polyamide [4], benzocyclobutene (BCB) [5], Ajinomoto
Build-up Film (ABF) and SU-8 [6] but these still have relatively
high attenuation at mm-wave frequencies. On the other hand,
reports have shown that attaching silicon dioxide to the
surface of the Si substrate, induces a low-resistivity layer at
the interface between the and the Si surface [7]. These
phenomena cause degradation of the attenuation constant. Sev-
eral approaches have been suggested to avoid such undesirable
effects, such as introducing polysilicon at the Si- interface
[8], removing the Si substrate from the backside substrate by
micromachining [9] and using the floating shield technique
[10]. However, each of these methods requires complex and
lengthy fabrication processes.
In this letter, a Shielded-Elevated CPW (SE-CPW) is pro-
posed to suppress the effect of the conductive substrate. In this
structure, a ground plane is placed on top of the substrate to
provide near-complete isolation of the elevated CPW from the
substrate. The structure provides transmission lines fabricated
on GaN-on-LR Si with much reduced dielectric loss and with a
very low attenuation per unit length up to 67 GHz.
II. EXPERMENTAL WORK
The study was carried out on AlGaN/GaN HEMTs supplied
by the Cambridge Centre for GaN in the University of Cam-
bridge. The epitaxial layers were grown byMOCVD on 675
LR P-type Si (111) substrates. The original layer stack, from the
substrate up, consists of a 200 nmAlN nucleation layer followed
by a 750 nm Fe-doped AlGaN graded buffer to accommodate
the lattice and thermal expansion miss-match, a 1.4 insu-
lating GaN buffer and channel layer, a 20 nm
barrier and a 2 nm GaN cap, as shown in Fig. 1.
The fabrication process was carried out at the James Watt
Nanofabrication Center (JWNC) at the University of Glasgow.
The transistor active region (the upper two layers) were etched
away prior to transmission media fabrication. As in a standard
MMIC process, the CPWs were fabricated on the isolation
mesa floor on top of a 200 nm dielectric layer. The first
step of SE-CPW fabrication is to define the lower ground plane
by e-beam exposure and development of a 1520 nm thickness
bi-layer PMMA, followed by electron beam evaporation of a
1531-1309 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
2 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS
Fig. 1. Oblique projection of the fabricated 50 SE-CPW line with
, , and of length 1 mm.
Fig. 2. Scanned Electron Microscopy (SEM) of the fabricated SE-CPW line.
50 nm NiCr/400 nm Au layer and lift-off. Following this, the
elevated traces' structures, including a number of supporting
posts, were realized. These posts were defined by spinning
AZ4562 photoresist at 3000 rpm for 30 s (corresponding to
5.5 elevation height). A 50 nm Ti/50 nm Au seed layer was
then deposited prior to electroplating. Next, the elevated traces
were formed in 2 S1818 photoresist followed by exposure,
development and ashing in a dry etch machine. The sample
was then metallized using 2 cold electroplating. Finally,
different levels of exposure and development were performed.
Fig. 2 shows SEM photos of the fabricated SE-CPW line.
III. MEASUREMENTS AND MODELLING
On-wafer S-parameter measurements were performed with
an Agilent PNA network analyzer over the range 0.1–67 GHz.
The system was calibrated using Line-Reflect-Reflect-Match
(LRRM) calibration, and the lines were probed using Pico-
probes with 100 pitch on the top of a double set of posts
located at either end. Then the samples were placed on a thick
quartz spacer to eliminate any possible microstrip-like mode
caused by the metal chuck.
For modelling, the prefabricated SE-CPW lines were de-
signed and simulated using a 3-D full-wave electromagnetic
simulation tool, Anasoft HFSSTM, at the University of
Glasgow. The structure was precisely optimized during the
simulation for better performance and to minimize RF energy
dispersion introduced by the lossy substrate.
IV. RESULTS AND DISCUSSION
We initially investigated the performance of the SE-CPW by
comparing its measured and simulated data with that of a con-
ventional CPW line between 0.1 and 67 GHz. Measured and
Fig. 3. Measured and simulated S-parameters for conventional CPW and
SE-CPW lines. (a) Transmission coefficient . (b) Reflection coefficient
.
simulated S-parameters are in very close agreement, as shown
in Fig. 3. It is clearly shown that the effect of the conductive
substrate is dominant, even at low frequencies, for conventional
CPW lines. Since all the CPW traces are in direct physical con-
tact with the substrate, most of the E-field is coupled onto the
conductive substrate even whilst employing a thin insu-
lating layer. The losses are mainly due to two phenomena: one
is capacitive coupling which allows the flow of conduction cur-
rent not only through the metal strips but also through the lossy
substrate contact areas. The other reason is inductive coupling
which induces current loops and associated losses by penetra-
tion of the magnetic field through the lossy substrate. Losses
continuously increase over the whole frequency band, reaching
a maximum of more than 14 dB at 67 GHz. On the other hand,
by using an SE-CPW structure, improvements of the line perfor-
mance in terms of dB/mm (up to 12 dB/mm) over a conventional
CPW line can be achieved. Losses also increase gradually over
the entire band for the SE-CPW line, showing a comparable per-
formance to that of a CPW line fabricated on a semi-insulating
GaAs substrate [11], [12] as illustrated in Fig. 3(a).
Not only does SE-CPW introduce improvements in terms
of transmission losses , but it also provides acceptable
matching, with as low as 22 dB/mm return loss, and more than
20 dB/mm reduction over a conventional CPW line. Moreover,
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
EBLABLA et al.: NOVEL SHIELDED CPWS ON GaN-ON-LOW RESISTIVITY Si SUBSTRATES FOR MMIC APPLICATIONS 3
Fig. 4. Measured and simulated of the attenuation constant, , of conventional
CPW and SE-CPW lines.
the resonant frequency is shifted by nearly 5 GHz using the pro-
posed transmission media. Consequently, SE-CPW lines miti-
gate the unwanted in-band resonances, providing even better ef-
ficiency than CPW-on-GaAs at K-band, as shown in Fig. 3(b).
Another figure of merit in evaluating the performance of pas-
sive components is the attenuation constant, , detailed in [13].
Fig. 4 clearly shows that the attenuation constant rises steeply
to very high values for conventional CPW lines; 1 dB/mm
at 76 GHz. However, a reduction of the attenuation constant
of more than 0.6 dB/mm can be obtained using the SE-CPW
structure, owing to the electrical isolation of the signal trace
from the bottom ground plane and the substrate. EM simulation
was carefully optimized to insure complete-isolation of the
elevated signal and ground CPW traces by employing ground
holes. These holes need to be as small as possible to reduce
the effect of the conductive substrate. There are small areas in
the lower ground in which the line has physical contact with
the substrate and the supporting posts are in physical contact
with the substrate via exposed rectangular areas in the lower
ground plane bond pad layer. For these reasons, SE-CPW line
has slightly larger attenuation constant (less than 0.2 dB/mm)
compared to that of CPW-on-GaAs line at K-band.
We believe that the SE-CPW could operate with similar per-
formance without the CPW ground planes; although variations
in gap width (S) in proportion to the elevation height (h) may
make it behave as a conductor-backed CPW.
V. CONCLUSION
We have developed and characterized a high-performance
novel SE-CPW transmission media on AlGaN/GaN HEMT
buffer layers grown on LR-Si. The developed SE-CPW line has
effectively suppressed the effects of the low resistivity substrate
resulting in excellent power confinement. A transmission loss,
, and attenuation constant, , of better than 2.4 dB/mm and
0.4 dB/mm at 67 GHz were achieved respectively. These re-
sults are comparable to those achieved for CPWs implemented
on SI substrates such as GaAs and InP, demonstrating the
feasibility and viability of mm-interconnects on GaN-on-LR
Si epitaxial layer stacks. The proposed SE-CPW transmission
media provides promising results of a mature GaN-on-LR Si
process technology.
REFERENCES
[1] R. J. Trew, “SiC and GaN transistors—is there one winner for
microwave power applications?,” Proc. IEEE, vol. 90, no. 6, pp.
1032–1047, Jun. 2002.
[2] R. T. I. M. Kemerley, H. B. Wallace, and M. A. X. N. Yoder, “Impact
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Novel Shielded Coplanar Waveguides on GaN-on-Low Resistivity Si Substrates for MMIC Applications

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