диафрагмированные волноводные фильтры / 743d6f4a-196c-49c4-a386-7d2736a2c2cc

Microfabrication technology for waveguide components at submillimetre wavelengths

James G. Partridge and Steven R. Davies

Department of Physics, University of Bath, Bath BA2 7AY, United Kingdom

ABSTRACT

For applications at submillimetre wavelengths, an increasing emphasis is being made on more integrated front-end circuits, in which semiconductor devices plus components of the embedding structures in which they are mounted are formed as part of the same fabrication process. The work reported here concerns the development of micromachined Schottky barrier diode devices, photolithographically formed waveguide cavities and their potential for integration. Micromachined millimetre wave rectangular waveguide components have been fabricated in the negative epoxy based photoresist SU–8. Inductive iris and E- plane septa band-pass filters (centre frequency 135GHz) with respective bandwidths of 5% and 10% were formed using this low-cost method. Using dual layer SU8 processing, accurate positioning of all micromachined waveguide components within standard two-port and three-port RF test fixtures was achieved. A 6dB branch-line coupler operating at 220GHz has also been realised and similar techniques are currently being applied to micromachined rectangular waveguide tuning structures. The methods employed are suitable for sub-millimetre wave application and waveguide components operating at frequencies approaching 1THz have been fabricated.

Keywords: micromachined, planar, Schottky diode, waveguides, SU8

1. INTRODUCTION

Applications such as satellite remote sensing, high resolution radar, plasma diagnostics and molecular line spectroscopy have resulted in considerable interest in the development of terahertz circuits and systems. In order for these systems to operate at terahertz frequencies the dimensions of the active devices must be reduced to a minimum and the parasitic elements associated with both the device and its embedding circuit need to be as low as possible. The effect of device dependent parasitics can be minimised mainly by controlling the device area and the epitaxial doping and layer profile; careful design of the device packaging is needed to reduce circuit parasitics.

For applications in astronomy, atmospheric sensing and communications systems, there is now an increasing demand for the development of low-noise instrumentation for operation further into the submillimetre part of the spectrum. The internet and mobile communications have become increasingly important as communications media in the modern world. There will continue to be an increasing demand for more services, faster access, combined with mobility and flexibility. In communications systems, higher frequencies offer greater bandwidth and speed (100s of megabits per second), higher antenna gain for a given antenna size, narrower antenna beam widths and smaller system size. The potential for submillimetre-wave wireless communication networks is very real. Certain applications can make use of the strong atmospheric absorption that occurs in parts of the millimetre and submillimetre spectrum. The high absorption and narrow beamwidths make these wavelengths well suited to applications such as covert, intra-building, high-speed, wireless communication links. To fully exploit these applications, inexpensive and reliable components and systems are needed, offering high levels of integration and the possibility of mass-production.

The development of instrumentation for millimetre-wave astronomy is now fairly mature. It is important that technologies suitable for terahertz astronomy are similarly developed. Low-noise instrumentation for such applications has generally been based upon single-ended mixers mounted in waveguide cavities or open-structure mounts such as corner-cube reflectors. The use of Schottky barrier diodes as the non-linear mixing element permits operation at ambient temperature and their characterisation as mixers is well understood, making them the preferred detector element for many applications. They can be operated at temperatures ranging from 300 K down to 10 K and have been used in spectral line receivers at frequencies from < 100 GHz to > 3 THz. Good performance at millimetre wavelengths has been achieved with planar air-bridged

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Schottky diodes [1]. Performance is improved by reducing the parasitic capacitance of the structure by removing high permittivity GaAs from around the contact pads. Air-bridged planar diodes have shown good performance up to around 600 GHz [2]. For operation above this frequency it is still the case that the best performance is achieved using whisker contacted devices. Whisker-contacted devices offer the attraction of small diode area, reducing device capacitance, but have poor repeatability, poor resistance to thermal and vibrational stress and are difficult to integrate with the mixer circuitry.

At millimetre and submillimetre wavelengths, waveguide mounts conventionally follow a split-block design, such as that shown in Figure 1. Sections of the mount are fabricated through electroforming onto a mandrel, milled in the shape of the desired waveguide cavity. The mandrel is then etched away and the sections of the block accurately machined to size. Although the technology used to fabricate such mounts is now mature, they are difficult and expensive to make, particularly at higher frequencies. This has tended to limit the applications to highly specialised ones such as astronomy and atmospheric remote sensing.

An increasing emphasis is being made on developing more integrated device structures, whereby some of the elements of front-end circuit construction are incorporated into the fabrication of the device itself. One can envisage the mixer diode circuit being integrated with not only passive components such as waveguide probes, transmission lines and impedance tuners, but also with active components, such as low-noise FET amplifier stages as part of diode-based MMICs.

The work reported here concerns the development of micromachining methods to fabricate both semiconductor detector devices plus their waveguide mounting structures for use at submillimetre wavelengths.

Figure 1. Conventionally machined crossed-guide waveguide mixer block, fabricated at the Rutherford Appleton Laboratory, U.K.

2. SEMICONDUCTOR DEVICES

One aspect of our work involves the integration of GaAs Schottky barrier diodes with the mounting structures in which they are embedded. Photolithographic patterning methods permit the fabrication and alignment of submillimetre-wave waveguide components. Two approaches to fabricating integrated devices are briefly described here.

2.1 Cantilever diodes

We have used micromachining methods to produce a sub-micron diameter device which combines the convenience of the planar diode with the low parasitics of a whisker contacted device [3]. Figure 2 is a schematic diagram showing the structure of the new device which we have termed the cantilever diode. It consists of an electroplated platinum whisker forming a point contact onto a Schottky barrier junction on epitaxial GaAs. The top of the electroplated pin is contacted by a metal cantilever arm. Away from the pin this cantilever is shaped to form an anode contact pad on the top surface of a SiO2 layer sputtered onto the surface of the GaAs. The cathode is formed by an alloyed ohmic contact adjacent to the anode.

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Devices have been fabricated on MBE grown epitaxial layers with semi-insulating GaAs substrates. These epitaxial layers consists of: GaAs(semi-insulating substrate)-AlGaAs undoped (1µ m)-GaAs n++doped 2× 1018 (5µ m) followed by the Schottky layer of GaAs n+doped 2× 1017 (0.12µ m). At the start of the process a 0.2 micron thick layer of SiO2 is sputtered onto the GaAs. A window is opened to accommodate the anode and ohmic contact. The ohmic contact is then formed by a standard evaporation and alloying process. The fabrication of the anode and cantilever contact is divided into two separate stages. The first stage forms the anode and the whisker structure and the second forms the cantilever arm. The whisker is formed by electroplating platinum through a small hole, pre-formed in a thick layer of photoresist. The photoresist is typically of thickness 10 - 20 microns.

Following the successful fabrication of the pin, the sample is baked in a conventional oven at a temperature of 90° C for 20 minutes. Using an appropriate mask a step is formed by UV exposing and developing part of the photoresist near to the whisker head as shown in Figure 2. A shadow mask is then used to evaporate a gold cantilever arm seed layer, starting from the substrate over the photoresist step and to the top of the pin structure. The arm is then copper plated using an acid based copper solution for extra strength and support. Figure 3 shows a device fabricated using the above procedure with a whisker pin having sub micron diameter at the base. Devices have been made with idealities of ~ 1.3; this will improve with better GaAs surface preparation immediately prior to anode formation. The junction capacitance of the cantilever diode has yet to be measured, but is expected to be < 1fF. The cantilever diode has (a) a sub-micron anode, (b) contact finger entirely in air, not dielectric and (c) a large gap between finger and substrate, minimising capacitance.

2.2 Window diodes

The second integrated device structure we are investigating is the window diode. In the window diode, a window is formed around the diode and its associated circuitry by completely etching away the GaAs from around the diode, leaving the diode area suspended on a thin membrane of material. A frame of GaAs remains intact around the device to support the membrane and to permit device handling and mounting [4]. The completed diode is mounted in a suitably formed waveguide block such that only the membrane and the thin layer of GaAs forming the device is present in the waveguide. The window diode readily lends itself to integration on a larger scale. The GaAs frame holding the diode plus membrane could easily be enlarged so as to incorporate active devices such as GaAs MMIC circuitry, including FET amplifiers. The window diode could readily be designed to accompany micromachined mounting structures or incorporated into non-waveguide based systems, perhaps coupled to planar antennas. Figure 4 shows a schematic diagram illustrating the concept of the window diode.

Air-bridged diode

GaAs frame

Window

Membrane (3-5 um)

Waveguide

Figure 4. Window diode concept.

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3. WAVEGUIDE COMPONENTS

Using photolithographic methods to pattern three dimensional structures means that accurate location and dimensioning of components is automatically achieved. We have used positive photoresists to fabricate structures of heights up to 200 m and the epoxy-based negative resist EPON SU–8 to fabricate structures over a millimetre in height. We have fabricated a variety of passive waveguide components using such techniques, including waveguide impedance transformers, sliding short circuit tuners, inductive iris bandpass filters and directional couplers.

Prior to the commercial availability of SU–8, our early work focused on using thick layers of positive photoresist. Hoescht resists AZ4562 and AZ4533 have been successfully used to fabricate structures up to 200 m thick. Such thick photoresist films are difficult to repeatably apply via low-speed spinning. We found that we could repeatably control the thickness of thick resist layers to ± 2 m across a 1 cm diameter by using standard sized substrates and controlling the volume of resist applied to the substrate. Successfully outgassing the resist solvents without bubbling and cracking of the resist requires a ramp baking method [5] in which the bake temperature is slowly raised (10° C/hour) to a maximum, where it is maintained for several hours. The thick resist films are exposed and developed to form waveguide formers. Following metallisation of the resist former, the resist is dissolved out to leave a waveguide cavity. These methods have been successfully employed to produce waveguide structures with operating frequencies ranging from 200 – 1600 GHz [6]. A waveguide structure with integral inductive iris for operation at 600 GHz is shown in Figure 5. Thick layers of conventional photoresist have poor transmission of UV light resulting from the dye content of the resist. This and the difficulty of baking thick resist films without cracking or bubbling limit the maximum thickness of resist that can be successfully patterned. Greater thicknesses may be achievable via multi-layer methods [7] and through the commercial development of reduced-dye photoresists for MEMS applications.

Figure 5. 600 GHz waveguide structure fabricated using thick positive photoresist.

More recently, our attention has switched to the negative acting, epoxy-based resist SU–8. This is a negative resist designed for high aspect ratio processing and resistance to harsh plating solutions. Achievable film thickness is up to 1000µm and beyond, and providing the bake and exposure are sufficient, the near transparent SU–8 allows successful imaging through these ultra-thick layers. The resist is based on epoxy resin technology, which possesses intrinsic adhesion characteristics superior to conventional thick resists. This is particularly beneficial on substrate coatings that may cause adhesion problems with normal resists, such as gold. SU–8 has sensitivity in the near-UV, deep UV and E-beam regions and low optical absorption allows near vertical sidewall profiles and reported aspect ratios exceeding 18:1. Once exposed, the imaged SU–8 must undergo a post-exposure bake in order to fully cross-link the polymer chains in the exposed regions.

In addition to its intended use as a mould material for electroplating, SU–8 possesses mechanical properties that allow it to be used for moving parts. Applications in MEMs include sensors and cogs, whilst in the microwave and sub-mm wave field, passive structures including waveguide filters and tuning elements are possible. The ability to produce multi-height structures is particularly advantageous in the context of millimetreand submillimetre-wave micromachined components.

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Following exposure and development, the SU–8 former is metallised via a combination of sputter deposition and electroplating to form a waveguide cavity. A number of different passive waveguide structures have been fabricated. These are described in the following sections.

3.1 Coupled resonator waveguide filters

Inductive iris coupled filters are regularly employed at microwave frequencies but for submillimetre wavelengths are prohibitively difficult to fabricate using conventional machining methods. Micromachining provides an inexpensive means to produce submillimetre-wave iris filters with very high accuracy. Figure 6 shows a micromachined bandpass waveguide filter with a centre frequency of 275 GHz. A planar lid completes the waveguide cavity. The iris fins are of width 60 m and height 400 m. Figure 7 shows a micromachined iris filter designed to have a bandpass with centre frequency of 137 GHz. The upper photograph shows the metallised, micromachined SU–8 waveguide structure (with lid removed). The lower photograph shows the filter mounted in a test fixture for characterisation. The test fixture includes standard WR–6 (WG–29) waveguide flanges coupled to both input and output of the filter. During the fabrication process, the test fixture itself is used as a photomask to pattern alignment features in the SU–8 structure; this is carried out in a specially modified mask aligner. Thus, the registration between the waveguides of the final filter structure and those of the fixture flanges is extremely good. RF tests on such structures have been carried out to verify their performance. Measurements were made using a simple test system with RF power supplied by a tunable 110 – 170 GHz backward wave oscillator and throughput power measured with a calibrated Anritsu power head (MP82B4) and meter (ML4803A). A 10dB directional coupler feeding a second power head was placed between the BWO and the test fixture to permit calibration of the throughput power. Measurements were initially made on both a straight through section of metallised SU–8 waveguide and a section of conventional WR–6 copper waveguide. In each case, the waveguide sections were of length 10 mm. The transmission loss of the SU–8 waveguide plus test fixture was measured to be ~ –3 dB over the range 110 – 160 GHz. This figure includes losses due to input and output mismatch losses introduced through coupling to WR–8 waveguides on both input and output sides of the test fixture (WR–6 not available). Correcting for estimated losses arising from mismatches and from the test fixture itself suggests that the SU–8 waveguides had losses of < 0.5 dB per wavelength.

Figure 8 shows the calculated response for the filter structure of Figure 7, modelled using Ansoft High Frequency Structure Simulator (HFSS), a commercial finite element analysis software package. Figure 9 shows the measured response of a fabricated structure such as the one shown in Figure 7.

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Figure 8. Filter response modelled using Ansoft HFSS

Micromachined Iris Bandpass Filter

Frequency (GHz)

Figure 9. Measured filter response

We believe the transmission losses seen in Figure 9 arise from two sources –– imperfect coupling of the waveguide lid to the top of the cavity and insufficient metallisation deposited onto the iris fins and into angled corners of the cavity. Good coupling of the lid is important for these filters due to the requirement for intimate contact over all the irises. Hence the loss is more pronounced than was experienced for the straight through waveguide. In addition, making the irises narrower will improve the shape of the passband. We are investigating ways to improve these aspects of our fabrication processes and anticipate improved results.

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3.2 Micromachined directional couplers

The photographs shown in Figure 10 illustrate a branch-line directional coupler (again with planar lid removed), designed to couple –6dB of throughput power to the branch line over a frequency range centred at 220 GHz. The coupler is made from metallised SU–8 and is mounted in a test fixture similar to the one used for the iris filters. Figure 11 is an SEM image of the tiny coupling channels linking the through and branch waveguides. The waveguides have a height of 600 m; the coupling channels have widths as small as 130 m. As with the iris filters described above, such structures would present a huge challenge to conventional machining methods. Completed directional couplers have been fabricated and RF tests of their performance will shortly be carried out.

Figure 10. Micromachined 220 GHz 6dB directional coupler, mounted in RF test fixture (waveguide lid removed)

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3.3 Other passive waveguide components

In addition to the filter and coupler structures described above, we have fabricated a variety of passive waveguide structures, including H-plane and tapered waveguide impedance transformers (Figures 3 and 12) and E-plane septa bandpass filters. We have also used sacrificial etch techniques to produce high tolerance, adjustable waveguide tuners of the type shown in Figure 12.

4. CONCLUSIONS

Improvements in planar device technology have extended their cut-off frequencies; future generations of front-end circuitry will require a more integrated approach to device/cavity design. The combination of micromachining techniques used to fabricate both semiconductor devices and the embedding structures in which they are mounted offer the potential for highly integrated submillimetre-wave front-end circuits.

ACKNOWLEDGEMENTS

This work has been carried out with the financial support of the EPSRC and PPARC UK Research Councils. We are grateful to Dr. D.N. Matheson of the CCLRC Rutherford Appleton Laboratories, UK for the use of the equipment used to perform RF characterisation of our micromachined structures.

REFERENCES

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2.Hessler J. L., Hall W. R., Crowe T. W., Weikle R. M., Beaver B. S., Bradly R. F., and Pann S. K., “Submillimetre Wavelength Waveguide Mixers with Whiskerless Diodes for Spaceborne Missions,” Seventh International Symposium on Space terahertz technology, Charlottesville, 1996.

3.H.Kazemi, S.R. Davies, S. T. G. Wootton and N. J. Cronin (in preparation).

4.C. M. Mann, “Integrated waveguides and mixers” in New Directions in Terahertz Technology, Ed. J. M. Chamberlain and R.E. Miles, Kluwer Academic Publishers, Dordrecht, pp. 183-191, 1997.

5.D.A. Brown, A.S. Treen and N.J. Cronin, “Micromachining of terahertz waveguide components with integrated active devices,” Proc. 19th Int. IR Millimeter Waves Conf., pp. 359-360, October 1994.

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7.B.E.J. Alderman, C.M. Mann, M.L. Oldfield and J.M. Chamberlain, “An embedded mask for applications in highfrequency integrated circuits using positive resist technology,” J. Micromech. Microeng. Vol. 10, pp. 334-336, 2000.

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