Chang-Wook Baek

Direct nanomechanical machining of gold nanowires using a nanoindenter and an atomic force microscope

Nanomechanical characterization of gold nanowires with a height of 160 nm, width of 350 nm and length of 5 µm has been carried out. Hardness and elastic modulus of the unreleased wires were measured by nanoindentation techniques using a nanoindenter. Post-fabrication mechanical machining of the gold nanowires is demonstrated. An array of nanoscale indents was successfully made on the gold nanowires. Nanochannels, nanoslots and complex nanopatterns were fabricated on a single gold nanowire by directly scratching/sliding the wire surface with the atomic force microscope (AFM) tip. Bending tests were performed to generate nanogaps on the wires using the nanoindenter. Direct machining techniques using a nanoindenter and an AFM should find more applications in the integration and manufacturing of micro/nanodevices.

The SiOG-based single-crystalline silicon (SCS) RF MEMS switch with uniform characteristics

This paper details single-crystalline silicon (SCS) direct contact radio frequency microelectromechanical systems (RF MEMS) switch designed and fabricated using an SiOG (silicon-on-glass) substrate, so as to obtain higher fabrication and performance uniformity compared with a conventional metal switch. The mechanical and electrical performances of the fabricated silicon switch have been tested. In comparison with a conventional metallic MEMS switch, we can obtain higher productivity and uniformity by using SCS, because it has very low stresses and superior thermal characteristics as a structural material of the switch. Also, by using the SiOG substrate instead of an SOI substrate, fabrication cost can be significantly reduced. The proposed switch is fabricated on a coplanar waveguide (CPW) and actuated by electrostatic force. The designed chip size is 1.05 mm/spl times/0.72 mm. Measured pull-in voltage and actuation voltage were 19 V and 26 V, respectively. Eighteen identical switches taken randomly throughout the wafer showed average and standard deviation of the measured pull-in voltage of 19.1 and 1.5 V, respectively. The RF characteristics of the fabricated switch from dc to 30 GHz have been measured. The isolation and insertion loss measured on the four identical samples were -38 to -39 dB and -0.18 to -0.2 dB at 2 GHz, respectively. Forming damping holes on the upper electrode leads to a relatively fast switching speed. Measured ON and OFF time were 25 and 13 /spl mu/s, respectively. In the switch OFF state, self-actuation does not happen up to the input power of 34 dBm. The measured holding power of the fabricated switch was 31 dBm. Stiction problem was not observed after 10/sup 8/ cycles of repeated actuation, but the contact resistance varied about 0.5-1 /spl Omega/ from the initial value.

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.

A new micromachined overlay CPW structure with low attenuation over wide impedance ranges and its application to low-pass filters

In this paper, a new micromachined overlay-coplanar-waveguide (OCPW) structure has been developed and its characteristics are studied in detail as a function of the line parameters. In OCPW, the edges of the center conductors are lifted by micromachining techniques and partially overlapped with the ground plane to facilitate low-impedance lines. The elevated center conductors help to reduce the conductor loss by redistributing the current over a broad area. Comparative experiments on low-loss and lossy substrates also confirm the screening effect from the substrate losses by confining the electric field in the air between the overlapped conductor plates. Compared with the coplanar-waveguide (CPW) lines, the OCPW lines show wider impedance range (25-80 /spl Omega/) and lower loss (<0.95 dB/cm at 50 GHz). The advantages of OCPW for low-Z/sub 0/ lines are utilized to realize a high-performance stepped-impedance low-pass filter at X-band. The OCPW filter shows distinct advantages over the conventional CPW filter in terms of size, loss, skirt, and stopband characteristics.

Circular/Linear Polarization Reconfigurable Antenna on Simplified RF-MEMS Packaging Platform in K-Band

A linear-polarization (LP)/circular-polarization (CP) switchable reconfigurable antenna using a radio frequency micro-electro-mechanical-system (RF-MEMS) switch is proposed and a novel package platform is introduced. Since the package substrate of the novel package platform is used as a radiation aperture substrate of the antenna structure, the fabrication process can be simplified, and degradation of radiation performance can be prevented. As the radiation aperture exists on the top side of the package substrate, an aperture-coupling feed structure is employed. To implement the LP-CP-switchable antenna based on the package platform, a stub, acting as the other feed, is added to a slot ring and is connected to the RF-MEMS switch with an on/off state. The proposed antenna is simulated using the Finite-Element Method, after which it is fabricated and measured. The measured 10 dB impedance bandwidths (BWs) are 22.90% (LP state) and 28.43% (CP state). The measured 3 dB axial ratio BW is 13.07% in the CP state. The measured gains at 21 GHz are 2.63 dBi (LP state) and 3.90 dBi (CP state).

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.

Effect of mechanical process parameters on chemical mechanical polishing of al thin films

The effects of polishing pressure and abrasive on the chemical mechanical polishing of blanket and patterned aluminum thin films were investigated. The CMP process experiments were conducted using a soft pad and slurry mainly composed of acid solution and Al2O3 abrasive. The result of the blanket film showed that, as the concentration of abrasive in the slurry increased, surface roughness deteriorated but waviness improved. The planarity of the patterned Al films was slowly improved by the CMP when the widths of the gaps between the patterns were relatively small. An attempt was made to find the optimum CMP process conditions by which the patterned Al thin film could be planarized with a fine surface. The most satisfactory film surface was obtained when the applied pressure was low (10 kPa) and the abrasive concentration was relatively high (5 wt.%).

2-D mechanical beam steering antenna fabricated using MEMS technology

A mechanical beam steering antenna capable of beam steering with two degrees of freedom is proposed and fabricated using MEMS technology. The V-band antenna element is implemented on the polymer platform which is capable of rotation with two degrees of freedom so that the beam can be directed to any desired direction. Radiation pattern of the fabricated antenna was measured and was in good agreement with the simulation results.

Electrostatically driven low-voltage micromechanical RF switches using robust single-crystal silicon actuators

In this paper, we demonstrate an electrostatic RF MEMS switch with a low actuation voltage, which is obtained simply by optimizing the design and fabrication of the robust single-crystal silicon (SCS) actuator, as well as a uniform switch performance. Through the simple approach, the pull-in voltage of the proposed electrostatic switch could be reduced simply and efficiently to approximately 10 V without sacrificing the structural stabilities and degrading the switch performances. For a total of 68 switches on a wafer, the fabrication and measurement yields were obtained to be higher than 94 and 73%, respectively. The switch performances of 50 identical switches except for 4 initially broken and 14 non-actuated switches were successfully characterized. The measured pull-in voltage was 10.7 ± 1.5 V and that of 40 switches (80%) was as low as 12 V. Nevertheless, the 50 identical switches showed considerably good and uniform performances with little deviations in terms of the RF and pull-in voltage characteristics. In addition, self-actuating behavior of the switch was not observed up to the input power of 37 dBm although the actuation voltage was reduced considerably. Low insertion losses of 0.13 ± 0.06, 0.15 ± 0.05, 0.19 ± 0.06 and 0.2 ± 0.07 dB were acquired at 2, 5, 10 and 15 GHz, respectively. The isolation characteristics could be obtained to be 41.25 ± 0.78, 33.25 ± 0.98, 27.17 ± 0.82 and 23.57 ± 0.75 at 2, 5, 10 and 15 GHz, respectively. Numerical calculations and simulations based on the fabricated results of the switches clearly demonstrated that the performance deviations among the tested switches were mainly due to the fabrication errors and not structural deformations.