* Electronics Engineering Laboratory,

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1 Fabrication of Dielectric-Filled Rectangular Waveguide using Thick-Film Processing M.S.Aftanasar', P.R.Young*, I.D.Robertson', S.Lucyszyn3 1 Microwave and Systems Research Group (MSRG), School of Electronics, Computing and Mathematics, University of Surrey, Guildford, GU2 7XH, United Kingdom ( [email protected]) * Electronics Engineering Laboratory, University of Kent at Canterbury, Canterbury, Kent, CT2 7NT, United Kingdom Department of Electrical and Electronic Engineering, Imperial College London, London, SW7 2BT, United Kingdom Abstract: This paper describes the use of photoimageable thick-film technology for the fabrication of rectangular waiveguides. Thick-film technology is attractive for volume manufacture and recent materials developments, such as photoimageable dielectric and conductor pastes have led to improved edge definition and smaller feature sizes, so that thick-film circuits can equal the performance of thinfilm processing. A dielectric-filled rectangular waveguide fabricated using photoimageable thick-film materials is presented. The waveguide incorporates a new transition from WOW probe to rectangular waveguide. Reds show low attenuation across the GHz frequency range. 1. INTRODUCTION The potential of the microwave and millimetric-wave frequency is currently being exploited in many fields, which include: broadband communication, radar, instrumentation and radiometry. Many advantages are gained from operating in this frequency range. One of which is the miniaturisation of system e.g. planar circuits, antenna etc. But the main problem is the cost of high manufacturing. Fortunately, due to the advancement in fabrication technology and material rheology, the problem is slowly removed with the introduction of new fabrication technique available today. Rectangular waveguide is well known for its ability to handle high-power. It is a single conductor transmission line able to propagate TE or TM modes. It also has low attenuation dire to its metal-walls enclosing the structure, providing shielding from outside interference. 'This makes it desirable for application in high frequency device. Furthermore, introducing dielectric into the hollow waveguide allows miniaturisation of the structure. In 1983, J.H.Hinken successhlly manufactured low cost Integrated Waveguide Technology (INWATE) [ 11. It utilises dielectric-filled rectangular waveguide as a transmission line. Later, this method was used in integration with other components such as filters, couplers and planar circuits [2-51. Other more recent advances utilise LTCC technique with Green tape dielectric [6]. Bulk micromachining method has also been successfully applied for hollow rectangular waveguide [7]. Other fabrication process includes plastic injection [SI, thick photoresist [9] and multilayer process using the photolithography technique [lo-121. But most methods require many processing techniques and, as with conventional manufacturing, integration with planar circuitry can be a problem. In this paper a new method has been proposed. The rectangular waveguide is constructed using low cost thick film technology. The materials used in the process are photoimageable conductor and dielectric paste. The detail of this manufacturing process is /0 1 /$ IEEE 82

2 included. The paper will also look at the feasibility of using photoimageable material in the construction process and overall performance of the manufactured rectangular waveguide. 2. FABRICATION PROCESS The waveguide was designed to operate between 60-90GHz (E-Band). The standard dimension for air filled (hollow) rectangular waveguide is 3.1 x 1.5 mm. But with the dielectric-filled rectangular waveguide, the dimension is scaled down according to the dielectric constant (er) of the material. For this frequency range, the dimension calculated is 1.22 x 0.6 mm. Figure 1 shows briefly the method of fabrication of the dielectric-filled rectangular waveguide. MI MI (a) 1 st Metal (MI) layer on Alumina (b) Dielectric Layer on MI MI (c) hosed and Developed Dielectric Layer (d) 2nd Metal (M2) layer filling the gap between dielectric to make vias and enclose the dielectric-filled rectangular waveguide Figure 1. Fabrication of the Thick Film Rectangular Waveguide The waveguide is designed on unpolished 99.6% Alumina. (a) The first layer of photoimageable gold paste is screen printed twice on the Alumina. This is to eliminate any pinholes and to deposit even layer surface on the Alumina. The MI layer is then baked and cured, producing a thickness of 7pm. This is sufficient for the skin depth of E-Band. Then, (b) a layer of photoimageable dielectric paste (E, = 7) is printed twice on the first gold layer. This layer is dried. (c) It is then exposed and developed in normal room temperature. This process produces the vias for the waveguide sidewall, and eliminates area not requiring any dielectric. (d) Finally, to enclose the dielectric structure and to fill the sidewall with metal, another layer of gold paste is screen-printed, exposed and developed. A complete structure is shown in Figure 2. The gap for the sidewall is = 1 OOpm wide /01/~ IEEE 83

3 Figure 2. SEM picture of top view of the waveguide sidewall The photoimageable dielectric and conductor paste used in this thick film process is very easy to :implement and able to produce result comparable to thin film performances. It follows conventional thick film processing procedure with an additional process for exposure and development. It can achieve thick film conductor line and space resolution of 20cun/30 pm with fired thickness of lopm on unpolished Alumina [13]. Several microstip lines with varying width was constructed and shown in Figure 3(a). The advancement in the material rheology allows the processing and fabrication to be carried out in normal laboratory room condition. The photoimageable dielectric processing can be caniad out in normal light and does not require any complicated procedure for processing. It is capable in producing very small via holes of =50p. A CPW probe pad is designed as shown in Figure 3(b) to show the size of the via holes interconnecting the top metal layer to the lower metal layer. The surface smoothness of the waveguide also plays an important aspect in the loss of the waveguide. Unlike the conventional spinning process or evaporation, thick film process requires the use of mesh wires, which restricts the size it is able to print and also the smoothness of its lines. Fortunately, there are improvements in the mesh wiring for mesh screens. For example, the new calendered mesh wiring is made with double rolling to smoothen the bumps made by the conventional wire overlaying and Figure 3. (a) 'Examples of different line width tested using photoimageable conductor paste. The top line is 20p, followed by 25p.m and 3 Example of 6 0p via hole for WOW probe /01~$ IEEE 84

4 3. MEASUREMENT The waveguide is designed to perform in E-Band and is tested using the HP8510XF Vector Network Analyser (VNA). This requires a specially design transition to launch power into the waveguide. A novel transition is used to couple the planar microstrip to waveguide. The SEM picture of the transition is shown in Figure 4. Other alternative transition was presented by Deslandes and Wu[14]. The difference in our transition is that it is closed on either end with a small slot opening for tapered microstrip line. This is likely to minimise the radiation loss at the waveguide ends. The end of microstrip is coupled to the RFOW probe station via CPW pads, which is then connected to the VNA. Figure 4. SEM picture showing the transition from microstrip to waveguide. Notice the tapering and the top and side walls of the waveguide. There is no specially designed test fixture for the waveguide. The use of CPW pads allows improve the reproducibility in testing, i.e. reinsertion of the test sample, and sets the reference plane for the de-embedding purposes. The arrangement for the measurement process is shown in Figure 5. For the measurement purposes, several lengths of waveguide were fabricated to calculate the loss in the waveguide by subtraction. These waveguides are shown in Figure 6. RFOW P - - Mrip to RWG transition - 1 DUT - - Mstrip to RWG transition - - RFOW Figure 5. Diagram showing the arrangement for the measurement /01/$ IEEE 85

5 2.490 mm Through Line mm Delay Line (Back-to-back transition) Two nun Off-Set Shorts mm Delay Line Figure 6. Different types of waveguide for measurement 4. RESULT One main problem with this method is the inability for the screen printing process to produce higher aspect ratio. With only 2 prints, the measured height of the waveguide is 1 8 ~ Several. layer can be printed but it might result in problems for precision multilayering and processing. According to [l5], this will result in higher attenuation due to the increase in the surface to cross-sectional area. Figure 7 shows the normalised attenuation result of the waveguide design. It seems to be very close to the calculated loss value of 0.5 db mm-'. More work is currently being carried out to improve the result and to produce thicker dielectric film Frequency (GHz) Figure 7. Measured attenuation of the Dielectric FiUed Rectangular Waveguide in db mm-' 5. CONCLliTSION A new dielectric-filled rectangular waveguide was fabricated and designed to operate in E-band. "lie process utilises mature thick film process and uses photoimageable dielectric and conductcr paste. The process follows an easy procedure without using any complicated process, mak:ing it low cost and also suitable for mass production. The new waveguide incorporates new transition fiom microstrip to waveguide transition /0:L/$ IEEE 86

6 Other than cost, there are many advantages seen in this process. One dominant advantage is that both planar and waveguide can be designed and integrated together on one substrate. Also, the size of the dielectric filled waveguide is comparable to the planar structure, making it suitable for integration. Hence, further work is underway to design some working devices utilising this fabrication process, and to improve the thickness of the layers. ACKNOWLEDGEMENT M.S.Aftanasar would like to thank USM (Universiti Sains Malaysia) for financial support. Also, to Dr. J. Minalgiene from Hibridas Enterprise for the fabrication process. This work is part of a joint ESPRC project entitled GHz Multi-Chip Module Technology, involving the Universities of Surrey, Kent and Glasgow. REFERENCES [ 11 Hinken, J.H.: Waveguides Become Integrated Circuits In New Space and Cost Saving Method, MSN, No.9, NOV. 1983, pp [2] Meier, U., Hinken, J.H. and Stenzl, W.: A broadband FET amplifier in Integrating waveguide Technology with an E-Plane microstrip insert, Conference Proceedings - European Microwave Proceedings 1987, pp [3] Hinken, J.H.: Band-pass Filters in integrating waveguide technology and adapters to standard waveguides, Conference Proceedings - European Microwave Conference 1984, pp [4] Heme, F., Modelski, J. and Hinken, J.H.: Investigation of the temperature sensitivity of dual-band dual-mode INWATE filters for satellite converters, Conference Proceedings - European Microwave Conference 1988, pp [5] Hinken, J.H.: Simple Design of Broadband monomode Tapers between Hollow and Dielectric Filled Rectangular Waveguides, AEU, Band 36, 1982 [6] Menzel, W. and Kassner, J.: Millimeter-Wave 3D Integration Techniques Using LTCC and related multilayer circuits, 3flh European Microwave conference, Paris 2000 [7] McGrath, W.R., Walker, C., Yap, M. and Tai, Y.: Silicon micromachined waveguides for millimetre-wave and submillimeter-wave frequencies, IEEE Microwave and guided was letters, vol. 3, pp.61-63, March 1993 [SI Wolfgang, M. and Jiirgen, K.: Novel Techniques for Packaging and Interconnects in MM-Wave Communication fiont-ends, IEE Electronics & Communication, Seminar on Packaging and Interconnects at microwave and mm-wave frequency, June 2000 [9] Digby, J. W., McIntosh, C. E., Parkhurst, G. M., Hadjiloucas, J. W., Chamberlain, J. M., Pollard, R. D., Miles, R. E., Steenson, D. P., Cronin, N. J. and Davies, S. R.: Fabrication and characterisation of micromachined rectangular waveguide components for use at millimetre-wave and terahertz frequencies, IEEE Trans. Microwave Theory Tech., Vol. 48, No. 8, pp , Aug [lo] Lucyszyn, S. Wang, Q. H. and Robertson, I. D.: 0.1 THz rectangular waveguide on GaAs semiinsulating substrate, Electron. Lett., vol. 31, no. 9, Apr. 1995, pp [ 111 Lucyszyn, S., Budimir, D.,Wang, Q. H. and Robertson, I. D.: Design of compact monolithic dielectric-filled metal-pipe rectangular waveguides for millimetre-wave applications, IEE Proc. Part H (Microwaves, Antennm and Propagation), vol. 143, no. 5, Oct. 1996, pp [12] Lucyszyn, S., Silva, S. R. P., Robertson, I. D., Collier, R. J., Jastrzebski, A. K., Thayne, I. G. and Beaumont, S. P.: Terahertz multi-chip module (T-MCM) technology for the 21 century, IEE Colloquium Digest on Multi-Chip Modules and RFICs, London, May 1998, pp [13] Muckett, S. and Minagiene, J.: Advances in Super Fine Thick Film Materials & Processing, 2000 International Symposium on Microelectronics and Packaging. [ 141 Deslandes, D. and Wu, K.: Integrated Microstrip and Rectangular Waveguide in Planar Form, IEEE Microwave and Wireless Components Letters, Vol. 11, No. 2, Feb. 2001, pp [15] Ramo, S., Whinnery, J.R and Duzer, T.V.: Field and Waves in Communication Electronics, 3rd edition, John Wiley & Sons Inc., /01/$ IEEE 81