IEEE MICROWAVE AND GUIDED WAVE LETTERS, VOL. 9, NO. 2, FEBRUARY 1999 63

A New Micro-Machined Millimeter-Wave andTerahertz Snap-Together Rectangular

Waveguide TechnologyC. E. Collins,Member, IEEE, R. E. Miles,Member, IEEE, J. W. Digby,Student Member, IEEE, G. M. Parkhurst,R. D. Pollard,Fellow, IEEE, J. M. Chamberlain, D. P. Steenson, N. J. Cronin, S. R. Davies, and J. W. Bowen

Abstract—A novel technique for micro-machining millimeterand submillimeter-wave rectangular waveguide components isreported. These are fabricated in two halves which simply snaptogether, utilizing locating pins and holes, and are physicallyrobust, and cheap, and easy to manufacture. In addition,SSS-parameter measurements on these structures are reported forthe first time and display lower loss than previously reportedmicro-machined rectangular waveguides.

Index Terms— Micro-machining, millimeter-wave, passivestructures, photolithographic techniques, rectangular waveguide.

I. INTRODUCTION

COMMERCIAL interest in the millimeter-wave frequencyrange is increasing for a variety of applications includ-

ing automotive radar, point-to-point communications, passiveimaging, and remote sensing [1], [2]. There is also a greatpotential in exploiting the large bandwidths available in thesubmillimeter-wave range as soon as a suitable cost-effectivetechnology becomes available [3]. Research is ongoing intotransmission line technologies for use above 100 GHz [4]where standard planar transmission media (such as microstripand coplanar waveguide) become very lossy. Rectangularwaveguide remains a popular choice at these frequenciesowing to its low loss, although its major drawback is thatit becomes difficult and expensive to machine conventionallyowing to its small size and consequently tight manufacturingtolerances.

Manuscript received September 16, 1998. This work was supported by theUK Engineering and Physical Sciences Research Council (EPSRC) as part ofthe TINTIN program.

C. E. Collins was with the Institute of Microwaves and Photonics, Uni-versity of Leeds, Leeds, LS2 9JT, U.K., and the School of Physics andAstronomy, University of Nottingham, Nottingham, NG7 2 Rd., U.K. Sheis now with a commercial MMIC facility in the United States.

R. E. Miles, R. D. Pollard, and D. P. Steenson are with the Institute ofMicrowaves and Photonics, School of Electronic and Electrical Engineering,University of Leeds, Leeds, LS2 9JT, U.K.

J. W. Digby was with the School of Physics and Astronomy, University ofNottingham, Nottingham, NG7 2Rd., U.K. He is now with the Radar Division,Alenia Marconi Systems, Frimley, Surrey, U.K.

G. M. Parkhurst and J. M. Chamberlain are with the School of Physics andAstronomy, University of Nottingham, Nottingham, NG7 2 Rd., U.K.

N. J. Cronin and S. R. Davies are with the School of Physics, Universityof Bath, Claverton Down, Bath, BA2 7AY, U.K.

J. W. Bowen is with the Department of Cybernetics, University of Reading,Whiteknights, Reading, RG6 6AY, U.K.

Publisher Item Identifier S 1051-8207(99)02659-8.

Previously reported techniques for micro-machining low-cost millimeter and submillimeter-wave rectangular waveg-uides suffer from some limitations. Conventional microwavemonolithic techniques have been used to fabricate a 4-m-high rectangular waveguide structure [5] which exhibited veryhigh loss ( 40 dB/wavelength at 100 GHz). Micro-machinedrectangular waveguides using standard UV photoresist formershave also been reported [6] which suffer from a difficultyin coupling into standard waveguides owing to a maximumheight limitation of 100 m (1/13th of full height at 100 GHz),and therefore can also be lossy. The LIGA X-ray lithographicprocess has been used in the micro-machining of taller high-aspect-ratio structures [7], but this is both time consumingand expensive to implement.

A recently introduced negative photoresist (known as EPONSU-8TM) [8]) has been successfully used to micro-machine700- m-high rectangular waveguides in a single UV exposure[9]. This Letter reports a new technique for the fabricationof micro-machined rectangular waveguides in two halvesusing multiple layers of SU-8 photoresist and presents somemeasured -parameter results. The multiple resist layers fa-cilitate the integration of locating holes and pins enabling thewaveguide halves to be simply snapped together, preciselyand repeatably. Also, significantly greater waveguide heightshave been achieved than previously reported using micro-machining techniques [5], [6], thus reducing attenuation andmismatch loss. The waveguides are physically more robust,thus simplifying the measurement process and increasing theease-of-handling; they are cheap and easy to make, and willlend themselves to highly accurate manufacture of all kindsof waveguide components at millimeter-wave and terahertzfrequencies.

II. WAVEGUIDE FABRICATION

The new waveguide fabrication procedure is detailed inFig. 1. A sacrificial layer of aluminum is first evaporated ontoa substrate, and then a thick layer of SU-8 photoresist is spunon top and baked. This SU-8 layer will eventually form thebottom and top walls of the waveguide. This first layer ofSU-8 is exposed, a second thick layer is applied, and thewhole sample baked again. The second layer is then exposed,carefully lining up the alignment marks on the mask with thosealready exposed on the first layer of resist, and the sample is

1051–8207/99$10.00 1999 IEEE

64 IEEE MICROWAVE AND GUIDED WAVE LETTERS, VOL. 9, NO. 2, FEBRUARY 1999

Fig. 1. New technique for micro-machining rectangular waveguide usingEPON SU-8TM photoresist.

given a post-exposure bake and developed. The second resistlayer forms the side walls of the waveguide; one waveguidepiece also contains locating pins and the other matching holesto enable the halves to be snapped together with the edgesexactly aligned. Subsequent immersion in potassium hydroxidesolution removes the sacrificial aluminum layer and releasesthe SU8 waveguide halves; these are sputtered with gold toa thickness of at least 600 nm (2 skin depths at-band) tocoat the inside walls. The locating pins are designed slightlysmaller ( 10 m) than the holes to allow for this thickness of

(a)

(b)

Fig. 2. Photographs of T-guide structures (a) before and (b) after assembly.

sputtered gold and facilitate a tight but repeatable connection.The final stage in the procedure is to simply snap the twowaveguide halves together. Waveguides fabricated using thisnew technique will be referred to as terahertz guides (or “T-guides”) throughout the remainder of this Letter. Photographsof a straight 700-m-high -band (75–110 GHz) T-guide,before and after connecting the two halves together, are shownin Fig. 2.

III. M EASUREMENT TECHNIQUE AND RESULTS

The -band frequency range was chosen for the design ofsome preliminary waveguide structures due to the availabilityof standard measurement equipment at these frequencies. Theresults presented here are for straight lengths of waveguide,cross-section 2.54 mm by 700m ( half height). A stan-dard TRL calibration was performed at the waveguide testports of a -band vector network analyzer and the T-guideswere then connected between the test ports and their-parameters measured. The guides were clamped between metaltest blocks containing -band flange detail to enable themto be connected directly to the standard waveguide flangesof the network analyzer. The lower test block contained ashallow groove into which the SU8 pieces could simplyslide to achieve alignment. The measurements displayed aperiodic ripple caused by multiple reflections between the fullto reduced height waveguide junctions. The effect of thesereflections was removed by renormalizing the measured-parameters to the characteristic impedance of the T-guiderather than the full height test port impedance [9]. In orderto achieve this, both of these characteristic impedances needto be known; three-dimensional (3-D) electromagnetic fieldsimulations were performed of both the T-guide and full-

COLLINS et al.: NEW MILLIMETER-WAVE RECTANGULAR WAVEGUIDE TECHNOLOGY 65

Fig. 3. Measured loss of T-guide sample atW -band.

Fig. 4. Measured loss per guide wavelength (�g) of T-guide atW -band.

height -band waveguide in order to determine these. The-parameters were then renormalized at each measurement

point, taking account of the change in waveguide characteristicimpedance with frequency.

The renormalized measurement results obtained are shownin Fig. 3. The loss demonstrated is mainly due to the joinbetween the waveguide halves occurring in the narrow wall,nevertheless, the loss is significantly lower than for previouslyreported micro-machined waveguides [5], [6] and comparableto that reported for the single layer of SU-8 technique [9]. Thetwo waveguide halves were disconnected and reconnected, andthe measurements repeated; very little difference was observedin the measured -parameters which demonstrates the repeata-bility of this procedure. The measured sample was severalwavelengths long, and the results actually correspond to aninsertion loss of between 0.4–0.8 dB per guide wavelengthover most of the measurement frequency range, as shown inFig. 4. While this loss may seem high, it is anticipated that,at 200 GHz and above, these waveguides could be fabricatedin the opposite orientation with a “trough” in both waveguidehalves. This would make waveguides with the join in the centerof the broad wall possible, with widths up to 1.4 mm (GHz), thus greatly reducing the loss.

IV. DISCUSSION AND CONCLUSIONS

A new technique for micro-machining millimeter-wave andterahertz rectangular waveguides has been demonstrated andmeasured results reported for the first time. The waveguides

produced using this technique have been named terahertzguides (or “T-guides”) by the authors and can achieve highdimensional precision owing to the use of photolithographictechniques. The photoresist used requires only a single UVexposure to form features up to 1 mm in height, and multiplelayers of this resist have been used to fabricate waveguidesin two halves incorporating locating holes and pins to enablethem to be precisely snapped together. The new techniqueis simple, cost-effective, highly accurate, and achieves goodwaveguide wall quality. The measured results presented showthat the micro-machined waveguides achieve much lower lossthan previously reported on-chip waveguides [5], [6] dueto their additional height, as well as being physically morerobust. They do not suffer from the difficulty of coupling intoconventionally machined waveguides as they can be bolteddirectly onto standard flanges. This technique will also be veryuseful at submillimeter-wave frequencies. The majority of theloss demonstrated by the T-guides is due to the connectionbetween the two waveguide halves occurring in the narrowwall. Above 200 GHz, where standard machining becomeseven more difficult, the waveguides could be fabricated sothat the join occurs in the center of the broad wall to greatlyreduce loss.

ACKNOWLEDGMENT

The authors wish to thank T. Moseley, University of Leeds,U.K., for machining the sample holders, HP-EEsof for thedonation of their high frequency structure simulator software(HFSS), and J. W. Digby wishes to thank British Telecom(BT) for a CASE research studentship.

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