диафрагмированные волноводные фильтры / 434c9dc2-611a-4cbc-98fc-4522ea8f8bac

Abstract—In this letter, several bandpass filters operating in the WR-1.5 band (500 to 750 GHz) are presented. The deep reactive ion etching (DRIE) silicon micromachining process is used for the fabrication of the filters. Two canonical filter topologies based on E- and H-plane are implemented. The work presented here has two specific objectives: a) to get important fabrication process parameters, such as tolerances, vertical angles, surface roughness, and repeatability and b) to validate the proper working of the waveguide filters in the terahertz band. These filters do not have any tuning element. Experimental results show better than 10 dB return loss and approximately 1 and 2.5 dB insertion loss (for 6% fractional bandwidth) for the E- and H-plane topology, respectively. The obtained results are in agreement with fabrication tolerances of 2 and vertical angles deviations up to 3 .

Index Terms—Deep reactive ion etching (DRIE), filter, micromachining, terahertz, WR-15.

I. INTRODUCTION

P RECISION metal machining techniques using high-speed end-mills can achieve typical tolerances of approximately 5 . Even 3 tolerances are possible at the expense of very

high cost [1]. For the design of most of the wide band waveguide components in the WR-1.5 band (500 to 750 GHz) accuracy of at least 5 in the fabricated dimensions is needed. Moreover, for a high performance operation, tolerances as low as 2 are desired. This problem gets worse when dealing with even higher frequencies up to 2 THz or with strongly resonant structures, such as waveguide filters. In order to improve precision, silicon micromachining techniques have emerged as a very attractive alternative. Low-cost, high accuracy, and batch processing capability are the main advantages of this technique.

Manuscript received January 18, 2013; revised March 20, 2013; accepted April 08, 2013. Date of publication April 26, 2013; date of current version June 03, 2013. This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, under a contract with the National Aeronautincs and Space Administration. This work was supported in part by the Spanish government program TEC2010-17795, the CONSOLIDER CSD2008-00068 and a Ph.D. grant from Universidad Politécnica de Madrid.

C. A. Leal-Sevillano, J. R. Montejo-Garai, and J. M. Rebollar are with the Departamento de Electromagnetismo y Teoría de Circuitos, ETSI Telecomunicación, Universidad Politécnica de Madrid, Madrid 28040, Spain (e-mail: caleal@etc.upm.es).

T. J. Reck, C. Jung-Kubiak, and G. Chattopadhyay are with the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 USA.

J. A. Ruiz-Cruz is with the Escuela Politécnica Superior, Universidad Autónoma de Madrid, Madrid 28049, Spain.

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.2013.2258097

Fig. 1. (a) SEM image of a H-plane filter. (b) SEM image of an E-plane filter. The dimensions of the input rectangular waveguide are 400 200 . (c) Halves of some of the fabricated filters. (d) Detail of one of the pieces.

There are different micromachining techniques which can be used to fabricate these devices at terahertz frequencies. Amongst them are the photoresist materials based techniques such as SU-8 [2] and LIGA [3] and processes based on silicon etching such as DRIE [4], [5]. The first waveguide components implemented with these micromachining techniques were wide band structures, like couplers [6]. Recently, simple waveguide filters have been presented with LIGA process [3] up to 300 GHz and with DRIE [7] up to 570 GHz. Waveguide filters are resonant devices and thus very sensitive to fabrication tolerances. The narrower the bandwidth the higher the insertion loss and sensitivity to fabrication tolerances. These devices, therefore, are more suited to experimentally validate micromachining processes. Moreover, successful implementation would open the possibility of additional passive components at terahertz frequencies.

In this letter, we report our work on waveguide filters fabricated using the DRIE silicon micromachining process. Several designs with different filter topology, center frequency, and fractional bandwidths are presented in order to explore and validate the process at frequencies covering the entire WR-1.5 band.

II. FILTER DESIGN

Canonical bandpass filters are based on direct coupled synchronous resonant circuits [8]. Butterworth and Chebyshev

electric responses can be implemented with these topologies. Furthermore, the circuit values can be obtained by the well-known synthesis techniques presented in [8], [9]. Different physical realizations in rectangular waveguide technology are possible. We used two simple topologies for the filters, E-plane (with capacitive irises) and H-plane (with inductive irises), see Fig. 1(a) and (b). It also allowed us to explore E- and H-plane split-block design and their performances.

The initial dimensions of the filters are obtained from the circuit values by means of full-wave simulations. The equivalent circuit of the irises can be modeled as parallel capacitors for the E-plane irises and parallel inductances for the H-plane irises. From these equivalent circuits, the height and width of each of the irises are adjusted to implement the desired coupling between the cavities. The thickness of all the irises was set to 80 . An additional waveguide section was included in the equivalent circuit of the irises to account for their finite thickness. In the designed filters, the resonant mode is used, leading to an initial cavity length of a half wavelength at the center frequency of the filter. The length of each of the cavities are corrected by the values obtained from the irises simulations. Notice that the inductive irises correct for shorter cavity lengths and the capacitive irises to larger lengths. Finally, a full-wave optimization of the complete filter is carried out. In the case of H-plane filters, symmetric irises have been used, although asymmetric irises designs are also possible. For the E-plane case, asymmetric irises have been used in order to obtain the desired couplings with moderate waveguide heights. The different designed filter specifications are summarized in Table I.

For the full-wave simulations, an in-house computer aided design based on the Mode Matching Technique was used. For the E-plane filters only the modes (or alternative the modes) are generated in the structure. For the H-plane filters only the modes are generated. The Mode Matching Technique is especially well suited for E-plane and H-plane topologies, leading to a high convergence rate and accurate results. The full-wave responses of the different designed filters are shown in Figs. 2–5.

III. FABRICATION AND MEASUREMENT

The DRIE micromachining process described in [10] was used for the fabrication of the filters. The filters are split at the middle of the rectangular waveguide into two equal halves with the same depth. Each of the halves are patterned using UV lithography and etched using a DRIE process. Afterwards, a gold layer is sputtered over all the waveguides.

Once the pieces are metalized and released, the different filters are assembled. The two halves of each filter are aligned using special -shaped silicon compression pins (in Fig. 1(d)

Fig. 2. (a) Measurements and simulations of the filter 1 in Table I. (b) Detail of the transmission in the bandpass. An equivalent conductivity of was used in the simulations.

Fig. 3. (a) Measurements and simulations of the filter 2 in Table I. (b) Detail of the transmission in the bandpass. An equivalent conductivity of was used in the simulations.

the pockets for the -shaped silicon compression pins can be seen), then the two pieces are positioned between two metal blocks, as described in [11]. Using these -pins, a misalignment between both halves below 1 is obtained.

The measurements were carried out with Agilent PNA-X vector network analyzer (VNA) with VDI WR-1.5 frequency extenders and used TRL calibration.

IV. RESULTS AND DISCUSSION

Figs. 2 and 3 show the measured results of both E-plane filters, filter 1 and 2 in Table I. They show very good agreement between simulations and measurements. The degradation in the return loss to 10 dB is explained by the measured tolerances of 2 in the fabricated dimensions. The vertical walls were also measured with a deviation from 90 of approximately 2 . This angle is the main cause of the small frequency shifts presented in the measurements. From the insertion loss measurements an equivalent conductivity of is obtained. This value is close to the nominal conductivity of gold, which means that very good rms surface roughness was achieved in the fabrication. The insertion loss results are excellent and comparable with the best achievable metal machining process.

The measured results for the H-plane filters, filter 3 and 4 in Table I, are shown in Figs. 4 and 5. In this case, a deviation of

Fig. 4. (a) Measurements and simulations of the filter 3 in Table I. (b) Detail of the transmission in the bandpass. An equivalent conductivity of was used in the simulations.

In this letter, several waveguide filters in the 500 to 750 GHz frequency band have been presented. Both canonical geometries the E- and H-plane, based on capacitive and inductive irises, have been used. Different fractional bandwidths and center frequency designs have been presented with the aim to fully explore the entire WR-1.5 frequency band.

The filters showed high performance with low insertion loss. Moreover, tolerances of 2 and deviation in the vertical walls of 2 and 3 for the E-plane and H-plane filters, respectively, have been obtained. The inclination of the vertical walls has been shown to be a very important fabrication parameter, leading to frequency shifts in some of the filters. Nevertheless, this can be compensated by taking advantage of the batch processing capability of micromachining techniques and designing the same device for slightly different taper angles.

The presented results show that waveguide filters working in the WR-1.5 band can be fabricated using silicon micromachining. These new components in the terahertz band can be used to enhance the capabilities and performance of passive subsystems.

Fig. 5. (a) Measurements and simulations of the filter 4 in Table I. (b) Detail of the transmission in the bandpass. An equivalent conductivity of was used in the simulations.

3 was measured in the vertical walls. The simulations with this value explain the 10 GHz frequency shift observed in the measurements. These results also show that the H-plane topology is slightly more sensitivity to inclination in the vertical walls than the E-plane. This is due, in part, to the shorter cavities obtained in this case after the length correction due to the thick irises.

4, respectively. Since the metal coating process used was the same as the E-plane filters, the degradation in terms of equivalent conductivity can be attributed to interrupting the currents with the H-plane splitting. This is also verified by the lower conductivity obtained for the filter centered at a higher frequency. We believe that the issues due to air gaps in the H-plane might be avoided by using a thermo bonding technique.

In should be pointed out that the measurement presented in Figs. 2–5 have the reference planes at the input/output of the silicon pieces, see Fig. 1(d). Thus, an extra length of around 5 mm at the input and output of the filters are included in the measurements.

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