Jung-Mu Kim

V-band 2-b and 4-b low-loss and low-voltage distributed MEMS digital phase shifter using metal-air-metal capacitors

Low-loss digital distributed phase shifters have been developed using micromachined capacitive shunt switches for V-band applications. Instead or conventional metal-insulator-metal capacitors, high-Q metal-air-metal capacitors were used in series with the microelectromechanical system (MEMS) shunt capacitive switches to minimize the dielectric loss. The operation voltage for the phase shifters was also reduced by applying the bias directly to the MEMS shunt switches through choke spiral inductors. Fabricated 2-b (270/spl deg/) and 4-b (337.5/spl deg/) distributed phase shifters showed low average insertion losses of 2.2 dB at 60 GHz and 2.8 dB at 65 GHz, respectively. The average phase errors for 2-b and 4-b phase shifters were 6.5% and 1.3%, respectively. The return losses are better than 10 dB over a wide frequency range from 40 to 70 GHz. Most of the circuits operated at 15-35-V bias voltages. These phase shifters present promising solution to low-loss integrated phase shifting devices at the V-band and above.

A V-band micromachined 2-D beam-steering antenna driven by magnetic force with polymer-based hinges

This paper presents a new type of antenna fabricated by micromachining technology for mechanical beam steering with two degrees of freedom of motion. A V-band two-dimensional mechanical beam-steering antenna was designed and fabricated on a single high-resistivity silicon substrate using microelectromechanical systems technologies. A fabricated antenna is driven by magnetic force to overcome the limit of electrostatic actuation, and a polymer-based hinge structure is used to increase the maximum scanning angle to as much as 40/spl deg/. Simulation result for validating the mechanical beam-steering concept is presented. In addition, mechanical properties such as static actuation angles are investigated together with microwave properties such as the return loss and radiation pattern at the V-band.

Packaging for RF MEMS devices using LTCC substrate and BCB adhesive layer

In this paper, a packaging method utilizing an LTCC (low temperature co-fired ceramic) substrate and a BCB (benzocyclobutene) adhesive layer has been developed for RF MEMS devices, and the RF performance and characteristic parameters of the package have been evaluated. LTCC substrates have good RF characteristics in high-frequency applications, and via feedthroughs can be easily incorporated during the manufacturing process. In this paper, an LTCC substrate is used as a capping wafer to reduce the complex processes for vertical interconnections. A layer of BCB, in the form of sealing rims, is used as an adhesive to bond the MEMS substrate with the LTCC cap due to the excellent properties of BCB as a packaging material. A CPW (coplanar waveguide) line has been fabricated on a quartz substrate and packaged to demonstrate the performance of the proposed packaging method. After forming the CPW lines, a 28 µm thick BCB layer is patterned by double-coating photolithography for an adhesive bonding. On the backside of the LTCC cap, a 150 µm deep cavity is formed to improve the RF characteristics. The CPW and the contact pad are connected electrically through the silver via-post in the LTCC substrate by screen-printed silver epoxy. The RF characteristics of the CPW line have been measured before and after packaging. The insertion loss of a bare CPW is 0.047 dB at 2 GHz and 0.092 dB at 20 GHz. After packaging, the insertion loss of the packaged CPW is 0.091 dB at 2 GHz and 0.312 dB at 20 GHz. A leak test has been performed using both IPA (isopropyl alcohol) soaking and the He leak tester. Most of the samples show no leakage for the IPA test, and a measured leak rate of 10−8 atm cc s−1 for the He leak test. In addition, the shear strength of the package was measured to be 25–35 MPa. From the experimental results, we showed the feasibility of a low-loss RF MEMS package from dc to 20 GHz with acceptable package performances.

Millimeter-wave MEMS tunable low pass filter with reconfigurable series inductors and capacitive shunt switches

In this paper, a new type of reconfigurable inductor is proposed for mm-waves based on the reconfiguration of slow-wave coplanar waveguide (CPW) lines using multiple-contact switches. It is then employed together with micro-electro-mechanical systems (MEMS) capacitive shunt switches to realize a reconfigurable low-pass filter, where the values of both inductors and capacitors are changed independently to achieve optimum filter characteristics. The measured 3-dB cut-off frequency of the reconfigurable low-pass filter changes from 53 GHz to 20 GHz while maintaining Tchebycheff low-pass prototype characteristics. The out-of-band rejection characteristics are also improved in this way. To our knowledge, this is among the first demonstrations that both types of MEMS contact switches and capacitive switches are used simultaneously to construct a monolithic tunable filter with independent L/C controlling capability.

Permittivity measurements up to 30 GHz using micromachined probe

We implemented a micromachined probe for the measurement of biological properties using MEMS technology, and experimentally showed the suitability of the micromachined probe in biological applications. The micromachined probe was fabricated on a silicon substrate, and to remove wave transmission through the silicon substrate, we etched the silicon substrate from beneath a lower ground and made the etched silicon surface conducting by using thermal evaporation of Cr/Au and a coating of conductive epoxy. The micromachined probe consists of a CPW and strip line between benzo cyclo butene (BCB) layers, which is known to be a material with high resistivity, low loss tangent, and low permittivity at high frequency. We measured the permittivity of a number of well-known liquids—0.5%, 0.9% and 1.3% saline, acetone, ethanol, and muscle and fat of pork—as biological samples using the micromachined probe after liquid calibration. The measured permittivity of 0.9% saline agreed well with the expected value of the Cole–Cole equation. In this paper, we first demonstrate that the micromachined probe can provide broadband measurement of measurable solid materials, such as biological samples, and also of well-known liquids at microwave frequencies. The size of the micromachined probe is 2000 µm (width) × 580 µm (thickness) × 30 000 µm (length), and the aperture size of the micromachined probe is only 650 µm × 70 µm. Therefore, we can extract the biological information from very small biological tissues and reduce radiation effects. Thus we show the feasibility of low-cost, small and portable permittivity measurement systems using a micromachined open-ended coaxial RF MEMS probe.

Low-loss analog and digital reflection-type MEMS phase shifters with 1:3 bandwidth

Broad-band analog and digital reflection-type phase shifters (RTPSs), showing constant phase shift over 1:3 bandwidth, have been developed using microelectromechanical systems (MEMS) technology. Two-unit RTPSs centered at two different frequencies have been cascaded to achieve flat phase responses over a wide frequency band. Each phase shifter uses air-gap overlay coplanar-waveguide couplers for low-loss 3-dB coupling and bridge-type MEMS capacitors for reflective terminations. The fabricated analog phase shifter shows the average insertion loss of 3.5 dB, and maximum phase error of /spl plusmn/4.9/spl deg/ from 15 to 45 GHz. The 2-bit digital RTPS shows reduced maximum phase error of /spl plusmn/2.8/spl deg/ together with a comparable insertion loss of 3.1 dB from 5 to 15 GHz. The details of the circuit design, analysis, and fabrication procedures are presented together with the measurement results. Compared with the similar constant phase shifters using semiconductor diodes and transistors, the micromachined RTPS of this study shows superior performance in terms of phase errors, insertion losses, and bandwidth.

Continuous anti-stiction coatings using self-assembled monolayers for gold microstructures

We have experimented with using continuous self-assembled monolayer (SAM) coatings on different surfaces, such as gold and silicon nitride, as anti-stiction coatings for radio-frequency microelectromechanical systems (RF MEMS). SAMs are coated on gold and silicon nitride surfaces in order to fabricate stiction-free gold structures after the wet release process. SAM coatings on gold and silicon nitride surfaces change the contact angle of a water droplet from 70.6° to 112.6° and from 36.7° to 115.8°, respectively. From atomic force microscopy, we can see that there is little change in the roughness of the gold and silicon nitride before and after the SAM coating. Using the electroplating technique and the wet release process, we have fabricated 4 μm thick gold cantilevers and bridges on a gold surface with a 6.5 μm gap from the substrate. Cantilevers and bridges are fabricated with different lengths in the range of 100–1000 μm with separations of 100 μm. It is observed that cantilevers and bridges with a length of 1000 μm on a gold substrate are released and standing free. A 1000 μm long gold cantilever has only 16.5 μm tip end deflection after the wet release process and SAM coating, much smaller compared with the 120 μm tip end deflection with the dry release process. Gold cantilevers, 100–500 μm long, on silicon nitride are released free and gold bridges on silicon nitride are released with a length of 1000 μm with continuous SAMs. The SAMs are coated on gold and silicon nitride surface in order. This method enables the gold cantilever and bridge on the gold or silicon nitride surface to be released with low deflection.

In vitro and in vivo measurement for biological applications using micromachined probe

We developed a small-sized micromachined probe for the measurement of biological properties using microelectromechanical systems (MEMS) technology. We also experimentally showed the suitability of the micromachined probe for biological applications through in vivo, as well as in vitro measurements of various types of tissue. We measured the permittivities of 0.9% saline and the muscle and fat of pork using the micromachined probe after liquid calibration. The measured permittivities of 0.9% saline and pork agreed well with both the expected values of the Cole-Cole equation along with the measured values obtained through the use of a 1-mm-diameter open-ended coaxial probe. We also performed in vivo measurements of breast cancer tissue implanted in an athymic nude mouse to show the suitability of the small-sized micromachined probe for practical biological applications. Through the obtained data, the capability of the micromachined probe of distinguishing different tissue types from one another was shown. The actual aperture size of the micromachined probe is only 240 /spl mu/m /spl times/ 70 /spl mu/m and, therefore, we can extract the biological information from very small biological tissues and drastically decrease the invasiveness of this method through the implementation of the small probe created through the use of MEMS technology.

Fabrication and Characterization of RF MEMS Package Based on LTCC Lid Substrate and Gold-Tin Eutectic Bonding

This paper reports on an RF MEMS package based on LTCC technology and gold-tin eutectic bonding, and also evaluates physical and RF characteristics of the proposed structure. The package consists of an LTCC lid substrate and a bottom substrate, and assembled by gold-tin layer (80 wt. % gold 20 wt. % tin) for hermetic sealing and low temperature bonding. The helium leak rate of 17 packages were 1.38 X 10-8 atm-cc/sec, and the shear strength of the bonded surface showed in the range of 36 - 50 MPa. The RF characteristics were measured up to 10 GHz, and the packaging loss was calculated to be 0.1349 dB at 2 GHz.

A compact low-loss reconfigurable monolithic low-pass filter using multiple-contact MEMS switches

A high-performance reconfigurable low-pass filter based on the quasifractal self-similar structures has been developed at millimeter-waves using multiple-contact MEMS switches. Three-cell cascaded low-pass structure was reconfigured to one-cell low-pass counterpart by activating the micromachined switches to achieve 3:1 frequency scaling. Two types of special multiple-contact MEMS switches were developed to reduce the number of switching elements as well as to reduce the insertion loss and, thus, extend the operating frequencies to mm-waves. In addition, bias-decoupling circuits were eliminated, resulting in a small chip size of 1.2 mm /spl times/ 1.5 mm. The measured 3-dB cut-off frequency of the reconfigurable low-pass filter changed from 67 GHz to 28 GHz with minor change in the insertion loss from 0.32 dB to 0.27 dB. This work demonstrates the possibility of high-performance compact-size reconfigurable filters at mm-waves using multiple-contact micromachined switches.