On-chip Integrated Antenna Structures 1n CMOS for 60 GHz WPAN Systems

Transcription

1 IEEE Global Teleommuniations Conferene (Giobeom), Honolulu, HI, Nov. 30-De. 4, 2009, pp On-hip Integrated Antenna Strutures 1n CMOS for 60 GHz WPAN Systems Felix Gutierrez, Jr., Kristen Parrish, Theodore S. Rappaport Wireless Networking and Communiations Group Department of Eletrial and Computer Engineering The University of Texas at Austin Austin, TX Abstrat-We present several on-hip antenna strutures that may be fabriated with standard CMOS tehnology for use at millimeter wave frequenies. On-hip antennas for wireless personal area networks (WPANs) promise to redue interonnetion losses and greatly redue wireless transeiver osts, while providing unpreedented flexibility for devie manufaturers. We present the urrent state of researh in on-hip integrated antennas, highlight several pitfalls and hallenges for on-hip design, modeling, and measurement, and propose several antenna strutures that derive from the mirowave and HF ommuniation fields. We also desribe an experimental test apparatus for performing measurements on RFIC systems with on-hip antennas at The University of Texas at Austin. Index Terms-WPAN, 60 GHz, RFIC, on-hip antenna, millimeter wave, mm Wave ommuniations, passive radiating elements. I. INTRODUCTION Within the past deade, the wireless ommunity has beome inreasingly interested in the worldwide 60 Gigahertz (GHz) radio frequeny (RF) band [1-8]. In 2001, the United States Federal Communiations Commission (FCC) released 7 GHz of bandwidth (57-64 GHz) for unliensed use, while other governments have similarly allowed portions of the 60 GHz band to be used without a liense. While the preise frequeny alloation is different in eah ountry, all bands share a ommon 5 GHz of ontinuous unliensed bandwidth entered at 60 GHz[5, 6]. With suh large RF bandwidths available at 60 GHz, data rates of several gigabits per seond (Gbps) are feasible within loal areas, greatly surpassing ul1'ent wireless transmission apabilities. Wireless personal area networks (WPANs) at 60 GHz will enable a vast array of appliations suh as wireless memory and unompressed high definition video. However, for 60 GHz tehnology to be adopted rapidly, devie and system osts and power onsumption must be kept as small as possible. Fully integrated CMOS iruitry offers the greatest ost and power savings, espeially when onsidering pakaging, integration, and interonnet issues. Already there has been signifiant work in RF iruits and systems operating at 60 GHz using CMOS tehnology [9-16]. A key fator for the implementation of low ost 60 GHz systems is integrating an antenna on-hip with CMOS iruits so that an entire wireless ommuniation system an be manufatured in one step with foundry fabriation. Even if only passive antennas are fabriated on a hip, easy integration with other iruitry on a form fator suh as a printed iruit board (PCB) is possible with no power drain by the antennas. Essentially, on-hip antennas, whether passive or ative with basi RF amplifiation stages, would provide many degrees of freedom for the design and implementation of onsumer eletroni devies, while eliminating or greatly reduing material osts assoiated with RF antennas. The large antenna gains and pattern flexibility at 60 GHz will support reliable low power, high bandwidth ommuniations over short distanes with minimal power drain, as the antenna gains are traded for transmitted power onsumption to overome RF propagation losses and obstrutions in the hannel [5, 7, 8]. Integrating antennas with RF iruitry has been pioneered in [ 17, 18], and several papers have doumented work fabriating standalone integrated iruits at 60 GHz [9-11]. Combining RF iruits with integrated antennas at 60 GHz poses several major hallenges. Standard CMOS tehnology is not optimized for millimeter wave strutures, and losses in the IC metal layers and substrate greatly redue antenna performane. Another hallenge is the small distane (typially several mirons) between the radiating elements, the suitounding metal layers, and low-resistane silion that exists underneath the metal layers of the IC substrate, whih at to distort the normal radiation pattems that antenna engineers are austomed to in free spae. Also, aurate high frequeny modeling of devies and passive omponents at millimeter wave ti equenies is diffiult and aurate measurement of 3-D far field patterns for integrated antennas at 60 GHz has yet to be ahieved. Setion II presents several novel on-hip antenna onepts, with extensive modeling and simulation results. These antenna onepts indiate promising approahes for on-hip antennas. Setion III desribes an experimental measurement system being developed at The University of Texas at Austin for testing and measuring on-hip antenna radiation patterns and wireless ommuniation systems up to 67 GHz. The paper onludes with a synopsis of open researh issues.

2 II. NEW CONCEPTS FOR 60 GHz INTEGRATED ANTENNAS For on-hip antennas, interations with the ground plane and the lossy silion substrate auses near field energy to dissipate rapidly around the antenna, whih greatly redues radiation effiieny and gain. Researhers and amateur (ham) radio operators have long known of the issues of transmitting over ground planes and the benefits of phasing and swithing with passive high gain antennas. Antenna gain towards the horizon has usually been the figure of merit for hams. They have ommuniated over great distanes throughout the world by staking and phasing these diretional antennas in an array. It is oneivable to reate high gain antennas at millimeter-wave frequenies on ICs by properly saling antenna dimensions. High gain antennas suh as the rhombi [19], the Yagi, the long wire [20], and even low-profile [21] and loop antennas [22] have been ommonly used throughout history, and are now onsidered for the novel appliation of passive integrated millimeter wave antennas. This setion onsiders the design of popular HF antennas for use at millimeter wave frequenies on a CMOS substrate. A. Overview of Simulation Methods We onsidered a substrate model for standard CMOS 0.18p,m IC tehnology as shown in Figure 1. We model a hip with ommon dimensions of 5 mm x 5 mm with 6 metal layers above bulk silion. Sine the modeled ground plane is onfined to the dimensions of the hip, diffration may introdue radiation on the lower hemisphere (below the die). The dieletri between metal layers (silion dioxide) is modeled as lossless, while the losses in silion bulk substrate are modeled with ondutivity (a) of 10 S/m, as in [23] and in [24]. All simulations use a standard 50 D. soure impedane in series with the exitation. Anterma m.eta:l layer, 1 ~n:n.tbik - Meu!:l ~ m. thik Silion Dioxide,- via layer Port entry/feed system- Metal l....,., Via. hoe for. 'eed sy~ten1 the top IC layer (silion dioxide, r = 3.9), where 1 A freespae = ~j v Eof-to 1 1 Aeff = -- = ---- A freespae y'fjij )oerf-tof Fr where Afreespae is 5 mm for 60 GHz, and Aeffetive is alulated by inluding the effet of the dieletri onstant. Thus, Aeffetive = 2.53 mm. We explored on-hip antennas based on the basi halfwave dipole, the multi-element Yagi, and the rhombi antenna. These simple antennas have historially been very reliable and provide reasonable gain, high front-to-hak and front-to-side ratios, and radiation near the horizon. We identified the following key issues: a) how performane is a funtion of on-hip plaement of radiating elements; b) how design guidelines should be applied to on-hip antenna design; ) the toleranes that exist between the widths, lengths, and spaings for various radiating and parasiti elements; and d) the maximum and diretion of ahievable gain for a wide range of new integrated passive on-hip antennas. B. Feed Systems In order to aurately simulate antennas, the transmission lines and feed systems that arry the signal from the on-hip transmitter outputs to the antenna inputs must be onsidered. Two feed systems were examined in simulation and yielded very omparable results. The feedlines extended from Metal 6 down to Metal 1, to transfer the signal from a lower layer to the top layer, as shown in Figure 1. Two representative feedlines are shown in Figures 2 and 3. (1) (2) Fig. 1. Illustration of IC model inluding silion dioxide layers for a typial CMOS integrated iruit. Fig. 2. Feed system for dipole antenna. Realisti millimeter wave antennas were modeled with Ansoft HFSS (High Frequeny Strutural Simulator), a leading ommerial finite element method field solver whih simulates three-dimensional strutures and produes S-parameters and radiation patterns. HFSS is used in both [25] and [26], as well as numerous other works. The effetive wavelength at 60 GHz for an IC substrate is estimated by using the frequeny and material properties of C. On-hip Dipole Antenna at 60 GHz To understand on-hip antenna properties, we first onsidered half wave dipoles using a wide range of line widths and orientations. The best performane with respet to gain was obtained by plaing a enter fed dipole on the edge of the hip (See Figure 4).

3 Fig. 3. Feedline of Yagi antenna. Using equation (2), an on-hip dipole with length A/4 = p,m was simulated using a 30 p,m antenna width. Eah dipole )../4 element was shortened to 570 p,m to shift minimum S 11 up to 60 GHz as seen in Figure 5. When the width of the dipole elements is doubled to 60 p,m while keeping the lengths the same, we observed that S 11 is minimized at 56.5 GHz (as opposed to 59.5 GHz). If the dipole is mounted on the enter of the hip, as opposed to the edge, then the gain 'is a very poor dbi, due to the lossy substrate and the lose proximity of the radiation angle to the horizon. As shown in Figure 4, the edge mounted antenna (width of 30 p,m) exhibits maximum gain of -7.3 dbi at an elevation angle of 0 o above the substrate horizon (8 = 90 ), and has a typial broadside pattern. This diretionality provides the onhip iruitry some radiation protetion. Radiation effiieny is still poor, at approximately 9%. With element widths of 60 J..Lm, the maximum gain inreases to -6.7 dbi, approximately 9 db less than that of a free spae dipole antenna. These signifiant hanges highlight the importane of understanding on-hip antenna performane as it is relatively unknown today. Fig. 4. Top view of on-hip integrated dipole antenna (not to sale) and radiation pattern for this antenna. Maximum gain is -7.3 dbi (radial power units are in dbi). () is the elevation angle where the Z-axis is 0, and the XY plane is 90, is the azimuthal angle in the XY plane, where the X-axis is 0 and the Y-axis is 90. D. On-hip Yagi Antenna at 60 GHz The Yagi antenna uses parasiti metals around the dipole to reflet and diret energy to ahieve inreased gain. There are several degrees of freedom when designing a Yagi antenna, suh as the spaing, lengths, and widths of eah parasiti element. We examined how spaing and widths affet Yagi performane on silion, as well as the optimal antenna orientation to ahieve maximum gain. Simulations of a 2-element Yagi, onsisting of a driven element and a parasiti refletor, showed highest gain when a Yagi antenna was ornered along the hip edge as shown in Figure 6. Using a thinner dipole width w = 15 p,m, the Yagi dipole (driven element) length was adjusted to maintain a minimum S 11 at approximately 60 GHz. Optimal performane was found when eah leg of the driven element was inreased to e = 597 J..Lm (approximately 5% longer than the single dipole). The enter feed spaing between eah )../4 element was 7 t-tm e 30um um ; ' Fig. 5. widths. Sll harateristis of two dipole antennas with different element Fig. 6. Top view of three individually simulated two-element on-hip Yagi antennas (not to sale). The Yagi antenna was simulated at the enter, lower right orner, and right edge of the Smm x 5mm hip.

4 ~ ~ -~ --- ~ - ~- --~ -5~~--~--~--~~ o-n.;.t-7'k: a:r -15~~._~~ f71"--+-~ ~\ ~~o~~~~~~~ ~ &J--f<b'------T t... u; -30.::... 9.:.?.?..?:...,. -35-t---l--r-t H.:~.. ~~~-~e~.~ I--H''-t H = ~?~~i:~:.~.~-~~ ' ffi Freq [G-tz] Fig. 7. S ll (db) as the refletor is moved away from the driven element. Spaing is measured from the edge of the driven element to the edge of the refletor. Also shown is the entered 597 J.Lm dipole quarter-wave element without any refletor and the saled 574 J.Lm quarter-wave element dipole with the 0.11-X-spaed refletor. -4~--,---,--~----~~~ -6 ~-1~ ~ -12 _pipol~ - U) ~~fje ~ "t-----~ r ~--~-~~ Y.~~~ =~~ T... --~ r enter Ya Freq [GHz] ~~r;eyagi Fig. 9. S ll (db) parameter as the Y~tgi antenna is moved aross the hip. "Edge" denotes right edge, and "Edge Edge" denotes lower right orner. "Dipole" is the 597 J.Lm dipole plaed at the enter Fig Antenna gains as the Yagi antenna is moved aross the hip. The left plot is elevation ( = 90 ) and the right is azimuth (8 = 90 ), Radial power units are in dbi. The radiation plots from Figure 8 are reprodued here. Fig. 8. Gain of a dipole vs. a.25>.-spaed refletor Yagi. The left plot is elevation ( = 90 ) and the right is azimuth (8 = 90 ). Radial power units are in dbi. We first investigated performane as a funtion of the distane between the reflelor and the dtiven element of a Yagi antenna. The Yagi dipole was plaed at the enter of the 5mm x 5mm hip. Aording to [22], Yagi refletors in free spae should be spaed between 0.15,\ and 0.20,\, Given that the Yagi quarter~wave driven element resonated at I! = 597 p,m, we estimated the wavelength to be mm, whih is omparable (within 4%) to our result from (2), and denoted this length as,\. A refletor was iteratively positioned from 0.01,\ to 0.25,\ in inrements of 0.01,\ away from the dipole. We observed S 11 and gain. The refletor width wy was idential to the dipole (15 p,m) and the length was set to f!y = 0.55,\, as suggested by [22]. Figure 7 shows a omparison of S 11 of the Yagi antenna as the refletor was moved away from the dipole. Note the S 11 resonane is shifted to a lower frequeny when a refletor is added and the minimum Sll ourred when the Yagi refletor was 0.11,\ away from the dipole. To bring the resonane bak up to 60 GHz using a 0.11,\ spaed refletor, eah driven element was saled to 574 p,m in order to resonate at the appropriate frequeny, denoted in Figure 7. Gains inrease slightly with the addition of the refletor, whih is onsistent with antenna theory. Gains inreased as the refletor was moved away from the dipole. At spaings between 0.20,\ and 0.25,\, the maximum gains were very similar, with a refletor distane of 0.25,\ having the highest gain at the horizon, shown in Figure 8. At distanes larger than 0.25,\, gain was further diminished. Thus, it appears Yagi antennas with a 0.25,\ refletor spaing exhibit both a reasonable S 11 to a 500 soure impedane and maximum horizon gain. With a 0.25,\ spaing, we investigated the role of refletor width in Yagi performane. The spaing was held onstant at 0.25,\ from the edge of the driven element to the edge of the refletor. S 11 dereased slightly by 1 db overall as the refletor width was inreased from 15 J..Lm to 160 p,m. We noted inremental gain inreases as width was inreased, differing only by 0.5 db over the entire range of widths. This shows that the width of the refletor ondutor does not appreiably impat gain or impedane mathing of the on-hip Yagi. As we will show subsequently, this does not hold true for rhombi antennas. Finally, we investigated how Yagi performane hanges as the antenna is moved aross the hip. We used the 0.25,\ spaed refletor with width 160 p,m. We plaed the antenna at the enter, right edge, and lower right omer of the hip as seen in Figure 6. Figures 9 and 10 show the Sll and gain hanges as the antenna is moved, respetively. The performane is ompared to that of a dipole at the enter of the hip. The 2-element Yagi along the right orner of the hip ahieved the highest gain of dbi, a front-to-hak ratio of 10.4 db, and maximum radiated intensity at 20 o above the horizon. The effiieny was 15.8%, whih is muh greater than the numbers

5 reported in [23, 25, 26]. By moving the Yagi antenna to an edge, maximum gains improved by 7.65 db ompared to the Yagi loated at the enter of the hip and by db over a dipole at the enter of the hip. E. On-hip Rhombi Antenna at 60 GHz The rhombi antenna is a diamond shaped broadband diretional antenna ommonly used in the 3-30 MHz frequeny range by shortwave broadast stations and ham radio operators. Large rhombi strutures are onstruted several meters above ground. Here we onsider saling them to onhip solutions. To implement a true rhombi antenna, eah leg of the antenna must be longer than two wavelengths [27]. This poses a problem for on-hip antenna designers: if>. in the material is approximately 2.5 mm, we are barely able to fit a leg of length 2>. on the edge of our 5 mm hip, although larger hips ould easily be fabriated for more diretive passive antennas. We onsidered a rhombi antenna along the perimeter of the hip where eah leg length is approximately 4.9 mm (nearly 2>.) as shown in Figure 11. The rhombi antenna was fed with a 60 GHz soure and a 50 n soure impedane. Two primary design parameters of a rhombi antenna in free spae are the angle 1 (indiated in Figure 11) and the height above ground. The ground height is analogous to the substrate thikness of the hip and an be optimized using wafer thinning tehniques. A third rhombi design parameter, exlusive to small sale antennas suh as an on-hip antenna, is trae width (indiated in Figure 11. These three design parameters were varied to asertain the performane of on-hip rhombi antennas operating at 60 GHz. -1 o -~_- _------H~~~~~-8~~-~--~ ffi -12 -~--M-~~-~G~~~ ~ -14 -;... ;... _..,... ;.,.../ ~-16~...,... m-- --J I.. ~--; ~ ~ <{ Rhombi Angle (gamma) [deg] (a) Gain vs. angle 'Y Maximum gain observed at 39. ~ : -11 'ffi C) ro -13 $ ~ -15 -e- MaxGain Substrate Thikness [urn] (b) Gain vs. substrate thikness. Maximum gain observed at 110 f-tm...., ~---.., ,----, (: "--i'""'""'""'"'-;. ;.4*-"' ~ -3 -;j-~ j ~ ] -;..;r;:{i)fd.~~. -4-;...,..... <B -5-j...,... r;;t;~ , j m ~ _..,.., --e- Max Gain Trae Width [um) () Gain vs. trae width. Maximum gain observed at 1445 f-tlll. Fig. 11. Top view of a simulated 60 GHz on-hip rhombi antenna. The rhombi angle 1 and trae width are indiated. The point where the antenna is onneted to the 60 GHz soure is denoted "Feed Points." The first design parameter tested was the rhombi angle, 'Y The substrate thikness was held onstant at 750 f.tm, feed points were separated by 3 f.tm, and trae width was 10 f.tm. The angle 1, was inremented from 5o to 45 o. Antenna gain at the horizon ( = 90 o, fj = 90 o) as well as maximum gain were observed. The results in Figure 12(a) show that the angle of largest horizon gain is 1 = 39 o. Using')'= 39 o, the substrate thikness was dereased from Fig. 12. Gain of the 60 GHz on-hip antenna as a funtion of (a) angle/, (b) substrate thikness, and () trae width. Maximum gain omted at an angle of 39, substrate thikness of 110 J.Lm, and trae width of 1445 f-t111. "Max Gain" denotes the maximum antenna gain while "Horizon Gain" denotes the gain at =90, B =90 ). 750 fl,lti to 50 f.1iti. The results shown in Figure 12(b) demonstrate that horizon gain inreases as the substrate thikness is dereased. An optimal value for horizon gain is observed at m or approximately,\/25. A thinner substrate allows higher gains and effiienies sine the degrading ettels of the lossy silion substrate are redued. Using both a 39 o rhombi angle, and a m substrate thikness, the trae width of the on-hip rhombi was in-

6 reased from 10 J-Lm to 1900 J-Lm. The results are shown in Figure 12(). The maximum horizon gain ours with a trae width of 1445 J-Lm (about 57%>.) with a gain nearly -3 dbi. The angle of maximum gain begins to shift from the horizon as trae width inreases. Generally, free spae rhombi antennas are onstruted of eletrially thin opper wires, so the thikness of these wires is not a design parameter. However, repeated tests have shown that on-hip rhombi antenna performane is enhaned with thiker trae widths. Figure 13 shows a omparison between a onventional rhombi antenna with thin trae widths and a higher gain rhombi antenna with thiker trae widths. Azimuth and elevation radiation plots are presented in Figure 13. The rhombi antenna with trae width 200 J-Lm ahieves a horizon gain of -8 dbi while the rhombi with trae width = 1445 J-Lm an ahieve a greater horizon gain lose to 0 dbi....q- Azimuth 90 maximum gain was -0.2 dbi. The radiation effiieny was 85%, due primarily to a thinned lossy substrate. These gains ourred with 'Y = 39 o, 110 p,m substrate thikness, 1445 J-Lm trae width, and a feed width of 660 J-Lm. Although the simulation results are promising for the rhombi, the size of the antenna and the area used by it on the hip may ause eletromagneti interferene with iruit omponents, thus suggesting several rhombis may best be pakaged as a standalone passive hip. III. FACILITIES FOR ON-CHIP ANTENNA WORK AT THE UNIVERSITY OF TEXAS AT AUSTIN The Wireless Networking and Communiations Group (WNCG) at The University of Texas has invested approximately $1,000,000 to build a start-of-the-art measurement faility for RF iruitry and on-hip antennas up to 67 GHz (see Figure 14). The faility inludes a probe station onsisting of several devies for making in-situ measurements on iruit substrates, and allows for thorough haraterization of all S Parameters (e.g. VSWR, gain, oupling losses), as well as other related equipment operating up to 67 GHz. By using the probe station and vetor network analyzer, it is possible to reate a 3-D antenna pattem range. We have alulated the minimum far-field distane of the on-hip antenna, given by [28], to be only 10 mm. VNA ( GHz) 90 Monitor -180 Spetrum Analyzer \ External Computer Fig. 13. Comparison of two 60 GHz on-hip rhombi antennas. Inreasing trae widths inreases horizon gains, as seen in the radiation plots ( = 90 ' e = 90, radial units in dbi). The wide rhombi antenna showed gains inreased by 6-8 dbi when trae widths were inreased from 200 f-lid to 1445 f.tlll. Fig. 14. Measurement failities in the WNCG lab at UT Austin The rhombi antenna has been optimized with respet to horizon gains; however, to transfer maximum power to the antenna, the S 11 should be minimized at 60 GHz. If a mathing network is not available to minimize S 11, the width of the feed points an be adjusted. The rhombi feed width was adjusted to 660 J-Lm (26% of)..) to obtain an aeptable impedane math to the standard 50 n soure impedane. This aused the S 11 to derease to a fiat -13 db and thus allow most of the available power to be aepted by the antenna when transmitting. Additionally, the horizon gain of the antenna slightly inreased from -3 dbi to dbi with the inreased feed width. The S 11 of -13 db for the on-hip rhombi antenna was flat aross the entire GHz band whih shows wide bandwidth. The optimized rhombi antenna with radiation pattem is shown in Figure 13. Horizon gain was dbi and overall Fig. 15. Proposed far-field radiation pattern measurement system. In order to onfirm fabriated on-hip antennas, we are

7 onstruting an automated antenna measurement system that makes use of a rotating onial hom antenna. The system in Figure 15 is based on the system desribed in [29]. In our system, the reeiving antenna rotates on an axis at a onstant distane from the probe-fed IC antenna, aquiring field measurements for one ut of the radiation pattern (where the (azimuth) angle is fixed, while the (} (elevation) is inrementally stepped). This rotation is automated by a omputerontrolled motor that aurately reords position data while reeived power is measured. When one rotational "ut" is ompleted, the base of the rotation is moved as indiated along the plate edge, hanging the angle and repeating the (} angle sweep. This system allows us to onstrut a far field radiation pattern for a large range of elevation and azimuth angles. It is ruial that all refletive surfaes and other possible interferers, espeially the probe station, be overed with radioabsorbing material. Pre-test and alibration will aount for losses from ables, probe tips, and other elements in the measurement system. Wideband hannel sounding methods may be used here. IV. CONCLUSION We have presented various 60 GHz passive antennas simulated for implementation on an integrated iruit. Typial CMOS metals, dieletris, and substrates were used. Antennas were designed to fit on an IC die size of 5mm x 5mm. The antennas implemented were the dipole, Yagi, and rhombi antennas. We found key relationships between the dipole and the Yagi element spaing and found antenna performane is maximized when the antennas are plaed at the edge of the die. Based on Sll, the Yagi antenna resonated best with a 0.11).. refletor spaing. When moving from the enter of the hip to the right edge, the maximum gains of both the dipole and Yagi antenna inreased by nearly 7 db. At the orner of the hip, the dipole and the 2-element Yagi had maximum gains of -6.7 dbi and -3.5 dbi, respetively. A novel appliation of the rhombi antenna to IC tehnology is also explored. Here, we find the optimal angle, substrate thikness and trae width for implementing rhombi antennas in CMOS tehnology. Using these optimal parameters, simulations show that it is possible to ahieve an antenna gain of -0.2 dbi, the highest reported for any on-hip antenna. While our results show higher gains than previously reported, on-hip antennas tend to have very low effiienies and pose a unique hallenge for antenna designers. However, integrating antennas with iruitry will greatly advane ommuniations tehnology by improving performane and lowering osts for WPAN systems. REFERENCES [1] T. Manabe, Y. Miura, and T. Ihara, "Effets of antenna diretivity and polarization on indoor multipalhpropagalion harateristis at 60 GHz," IEEE JSAC, vol. 14, no. 3, pp , [2] P. 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