Micromachined Tunable Dielectric Resonators Wanling Pan1, 2, Paolo Fiorini1, Orsola Di Monaco1, Kris Baert1, Bart Nauwelaers2, Robert Mertens1, 2 1
2
IMEC-MCP, Kapeldreef 75, B-3001, Leuven, Belgium K.U.Leuven, ESAT, Kasteelpark Arenberg 10, B-3001, Leuven, Belgium E-mail:
[email protected]
Abstract --- Micromachined dielectric resonators (DR) and cavity filters are used in microwave technology. For many applications the resonance frequency of the DR must have a small degree of tunablity, to cope with drifts of different origins. Tuning can be obtained electrically or mechanically. Mechanical tuning is based on a movable metal plate placed above the resonator puck. In this work we report about the implementation of mechanical tuning in a DR by means of MEMS technology. High resistivity silicon-based, cylindrical dielectric resonators have been fabricated using micromachining techniques. Depending on the dimensions and on the excited mode, resonance frequencies are in the range of 25~50GHz. In order to obtain frequency tuning, a membrane has been fabricated and coupled to the DR as the tuning element. Membranes are made of low stress SiN and are covered with a metal layer. Preliminary measurements applied to a high resistivity silicon DR indicate that by applying a pressure of 500Pa, a tuning range of 0.6% can be achieved. Keywords --- micromachining, dielectric resonator, tuning, membrane
I. INTRODUCTION In the last years the Micro Electro Mechanical Systems (MEMS) technology has been extensively used to fabricate small and high performance RF components. Most of the attention has been focused on switches, varactors and mechanical resonators. [1] Micromachined dielectric resonators (DR’s) and cavity filters, both made of high resistivity silicon, have also been implemented. A DR, operating at a frequency of 96 GHz and featuring a quality factor of nearly 10000 has been demonstrated. [2] A more compact cavity filter, with a much lower quality factor, has also been proposed. [3] This last device has the advantage of being more easily integrated with monolithic microwave integrated circuits (MMIC’s). In this paper, MEMS technology is used to fabricate a DR made of high resistivity silicon (HR-Si). Furthermore a small degree of frequency tuning (less than 1%) is
implemented by using a micromachined membrane. This level of tuning is necessary in many applications to cope with drifts of different origins. II. HIGH RESISTIVITY SILICON RESONATOR Basically, a DR is a cylindrical rod that acts like an airfilled metallic cavity resonator, with the advantage that it is much smaller in size. [4] Conventional dielectric resonators are working at TE01δ or TM01δ mode. In order to achieve higher quality factor, it is also possible to excite high order modes in the DR. Among them the Whispering Gallery Modes (WGM’s) are of particular importance, dielectric resonators operating in these modes exhibit high values of quality factor Q. [5] The WGM is characterized by a standing wave traveling along the resonator boundary (Fig. 1). In a first and rough approximation, the resonance is obtained when the circumference of the traveling circle is an integer multiple of the wavelength of the electromagnetic field in the resonator itself. This integer is the mode number, a higher mode means more concentrated electromagnetic field, and hence a higher quality factor. Surface-wave lines, such as microstrip or coplanar waveguide lines are used to excite the WGM. When the input and output lines are aligned to appropriate positions, WGM wave is excited and transmitted through electromagnetic coupling. (Fig.1) In general materials for fabricating DR’s must have a large dielectric constant (small size) and low dielectric losses (high Q). A silicon DR might be attractive in view of a simple, low cost integration with other electronic components. Because dielectric loss is proportional to the conductivity of the material [6], high resistivity silicon is preferred. The high resistivity silicon used in our fabrication has a dielectric constant of 11.5 and a resistivity of more than 5×103Ω⋅cm. Resonators have been fabricated by etching through a silicon wafer using standard ICP (inductively coupled plasma) etching method. The diameters of the resonators vary from 6 to 12mm, larger diameters giving lower resonance frequency. The resonance frequency ramps up very steeply when the thickness is reduced. To keep it
the other hand it is space consuming and more complicated to control electronically. The use of a micromachined membrane as top plate, whose position can be simply controlled by compact actuators, can eliminate the drawbacks of mechanical tuning. (See Fig.3)
IN RF OUT RF
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Figure 1. A schematic description of the whispering gallery mode.
within the range of our measurement instruments, three DR’s with thickness of about 600 µm each were bonded together to form a thicker one. In order to reduce radiation losses and electromagnetic noise from the outside world, a metallic cavity is used to make a shielded dielectric resonator (SDR). Preliminary measurements show that the working frequency ranges from 25 to 50GHz, depending on the excited mode and dimensions. One of these resonance peaks observed in a 12mm diameter DR is shown in Fig.2. It corresponds to mode 7. The quality factor is about 300 and is at the moment limited by the measurement set up and not by the intrinsic properties of the dielectric material. It should be noted that this measurement is not calibrated. This is at the moment not important, as we are interested mainly in the resonance frequency, and not in the insertion loss.
Figure 3. Schematic of tuning with a membrane.
By using approximate analytical models [8], we have calculated how the tuning plate distance influences the frequency of our HR-Si DR. Results are shown in Fig.4. It can be seen that when the tuning plate is not too far from the DR (less than 400µm), moving the plate 100µm could result in a frequency change of about 1.7%. This displacement can be achieved by a micromachined membrane. It must be noted that, because of symmetry reasons, the analytical model considers two simultaneously moving plates, one above and the other below the DR. The modeled tuning range is then an overestimation of the experimental one obtained by moving only one plate. 36
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Figure 2. Resonance peak of a high resistivity silicon DR (mode 7).
III. MEMS TUNING Tuning of a DR can be obtained electrically or mechanically [7]. Mechanical tuning is based on a movable metal plate placed above the resonance puck. The resonance frequency can be controlled by varying the distance between the metal plate and the top of the resonator. With respect to electrical tuning, this method offers the advantage of a wider range, and does not require a special design for each operating frequency. On
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Figure 4. Frequency vs displacement of the metal plate.
To fabricate the membrane, low-stress SiN layers of 4µm and 400nm are first deposited by PECVD on the front and back sides of a silicon wafer. The stress of the 4µm SiN layer, which is meant to form the membrane, is measured to be 6MPa, compressive. This value is within the error range of the measurement instrument, which means that the membrane can be regarded as stress-free. The 400nm layer on the backside is patterned and is used as a mask to etch through the wafer (standard KOH etching process). An aluminum layer is then deposited on the membrane and will act as a metal plate. In the first trial, flat membranes, with diameters ranging from 6mm to 20mm are fabricated.
moving the membrane positioned at about 400µm above the resonator with an equivalent pressure of about 500Pa. This range is in good agreement with the model.
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We have first verified the effectiveness of membrane tuning on a commercial ceramic DR (DI-E4030 from Temex SA). A membrane with a 10mm radius is set at a distance of about 200µm above the resonator. An external load is applied to the center of the membrane to deflect it, and resonance frequencies are measured for different loads. Results are reported in Fig.5. These results have been compared with those obtained by standard mechanical tuning. In this case frequencies are measured for a wide range of plate positions, and data are reported in Fig.6. We see that tuning is more efficient when the plate is closer to the DR.
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Figure 7. Frequency-load relationship for a HR-Si DR.
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Figure 5. Resonant frequency vs weight of the load for a commercial DR
From Fig.6, it can be seen that starting from 200µm above the DR, a metal plate displacement of 70µm could tune the frequency by 0.3%. From Fig.5, we see that such tuning amplitude requires a load weight of about 11g, equivalent to about 550Pa applied to the whole area of the membrane. From these figures we can estimate a membrane sensitivity of ~0.13µm/Pa, which fits with calculations based on standard formulas. [9] 37.1
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The preliminary results that we have shown confirm that a membrane can be used for efficient tuning of a DR. This calls for an analysis of the possible actuation mechanisms. One preliminary consideration is important: the deflection needed for an efficient tuning (50~100µm) requires large forces. They can be reduced by using corrugated membranes, with sensitivity up to 10 times larger than the one of flat membranes. The fabrication of the membranes is in progress. The following brief analysis of actuation mechanism is based on the availability of these membranes. The most straightforward solution would be electrostatic actuation. Unfortunately simple calculations show that a 100µm deflection requires several hundred volts, even using a corrugated, sensitive membrane. [10] A second attractive possibility is to use a piezoelectric actuation. In this case a piezoelectric layer (AlN or PZT) is deposited on the membrane and will deflect it upon bias application. Using CoventorWareTM software we have carried on simulations of these actuation mechanisms. Preliminary estimation shows that a 100µm deflection could be expected by applying a voltage of less than 50V. IV. SUMMARY
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Figure 6. Frequency – plate distance relationship for a commercial DR
The same membrane-controlled tuning set up has been applied on a high-resistivity-silicon DR. As shown in Fig.7, a tuning range of 0.6% has been realized by
High resistivity silicon dielectric resonators have been modeled and fabricated with standard micromachining techniques, with working frequencies ranging from 25GHz to 50GHz. Mechanical tuning has been realized by using the deflection of an Al coated SiN membrane. Work is in progress to improve the sensitivity of the membranes and to implement a piezoelectric actuation of the membrane.
ACKNOWLEDGEMENTS The authors would like to thank Dr. Cristina Rusu for her help in membrane fabrication. REFERENCES [1] J Jason Yao, “RF MEMS from a device perspective”, J. Micromech. Microeng. 10 (2000) R9-R38 [2] B. Guillon, K. Grenier, P. Pons, J. L. Cazaux, J. C. Lalaurie, D. Cros, R. Plana, “Silicon micromachining for millimeter-wave applications”, Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films - March 2000 -- Volume 18, Issue 2, pp. 743-745 [3] Karl M. Strohm, Franz Josef Schmückle, Bernd Schauwecker, Johann-Friedrich Luy, Wolfgang Heinrich, “Silicon Micromachined RF MEMS Resonators”, IMS2002 [4] Losee, Ferril. “RF systems, components, and circuits handbook”, ISBN 0-89006-933-6, pp.479-482, 1997 Artech House Norwood, MA [5] B.Guillon, D.Cros, P.Pons, K.Grenier, T.Parra, J.L.Cazaux, J.C.Lalaurie, J.Graffeuil, R.Plana, “Design and Realization of High Q Millimeter-wave Structures through Micromachining Techniques”, Microwave Symposium Digest, 1999 IEEE MTT-S International ,Vol.4,1999,pp 1519 –1522 [6] P. Blondy, D. Cros, F. Balleras, C. Masit, “W band silicon dielectric resonator for semiconductor substrate characterization”, International Microwave Symposium Digest, IEEE, New York, NY, USA; vol.3, 1998; pp.1349-1352 [7] Kajfez, Darko and Guillon, Pierre, “Dielectric Resonators”, ISBN 0-89006-201-3, 1986 Artech House Dedham, MA. [8] E.N. Ivanov, D.G.Blair, V.I.Kalinichev, “Approximate approach to the design of shielded dielectric disk resonators with whispering-gallery modes”, IEEE-Transactions-onMicrowave-Theory-and-Techniques. Vol.41, No.4; April 1993; pp.632-638. [9] Patrick R. Scheeper, Wouter Olthuis, Piet Bergveld, “The Design, Fabrication and Testing of Corrugated Silicon Nitride Diaphragms”, Journal of MEMS, Vol.3, No.1, March 1994, pp36-42. [10] E.H. Yang, S.S. Yang, S.W. Han and S.Y. Kim, “Fabrication and dynamic testing of electrostatic actuators with p+ silicon diaphragms”, Sensors and Actuators-A, Vol.A 50, No.1-2; Aug. 1995; pp.151-156.
