Hyo-Jin Nam

Design, fabrication and RF performances of two different types of piezoelectrically actuated Ohmic MEMS switches

In this paper, we have proposed, fabricated and characterized piezoelectrically actuated RF MEMS (radio-frequency micro-electro-mechanical system) switches. They have been designed to operate at a low operation voltage for advanced mobile/wireless handset applications. The proposed switches are largely composed of piezoelectric cantilever actuators with an Au contact electrode and CPW (coplanar wave) transmission lines suspended over the substrate. Two different types of RF MEMS switches have been suggested to find the better geometry. One has the structure of one single piezoelectric cantilever and a contact electrode attached to its edge with three hinges (type-A), and the other contains four piezoelectric cantilevers that are symmetrically combined through each hinge to support a centered contact electrode (type-B). The two different fabricated (type-A and type-B) RF MEMS switches have insertion losses of −0.22 and −0.23 dB at an operation voltage of 2.5 V and a frequency of 2 GHz, respectively. Although the difference in insertion loss is trivial, there exist different dependences of insertion loss on applied voltage between them. The insertion losses of type-A switches are changed with varying operation voltage because the touching area between the contact electrode and the signal transmission lines is variable. Meanwhile, the type-B switches show nearly constant insertion losses regardless of operation voltage. The type-A and type-B switches have isolation values of −40.8 and −42.5 dB at a frequency of 2 GHz, respectively.

Ultrahigh-density phase-change data storage without the use of heating.

Non-volatile memories based on scanning probes offer very high data densities, but existing approaches require the probe to be heated, which increases the energy expenditure and complexity of fabrication. Here, we demonstrate the writing, reading and erasure of an ultrahigh-density array of nanoscopic indentations without heating either the scanning probe tip or the substrate. An atomic force microscope tip causes microphase transitions of the polystyrene-block-poly(n-pentyl methacrylate) of a block copolymer to occur at room temperature by application of pressure alone. We demonstrate a data storage density of 1 Tb in(-2), which is limited only by the size of the tip. This demonstration of a pressure-based phase-change memory at room temperature may expedite the development of next-generation ultrahigh-density data storage media.

A fully wafer-level packaged RF MEMS switch with low actuation voltage using a piezoelectric actuator

In this paper, a fully wafer-level packaged RF MEMS switch has been demonstrated, which has low operation voltage, using a piezoelectric actuator. The piezoelectric actuator was designed to operate at low actuation voltage for application to advanced mobile handsets. The dc contact type RF switch was packaged using the wafer-level bonding process. The CPW transmission lines and piezoelectric actuators have been fabricated on separate wafers and assembled together by the wafer-level eutectic bonding process. A gold and tin composite was used for eutectic bonding at a low temperature of 300 °C. Via holes interconnecting the electrical contact pads through the wafer were filled completely with electroplated copper. The fully wafer-level packaged RF MEMS switch showed an insertion loss of 0.63 dB and an isolation of 26.4 dB at 5 GHz. The actuation voltage of the switch was 5 V. The resonant frequency of the piezoelectric actuator was 38.4 kHz and the spring constant of the actuator was calculated to be 9.6 N m−1. The size of the packaged SPST (single-pole single-through) switch was 1.2 mm × 1.2 mm including the packaging sealing rim. The effect of the proposed package structure on the RF performance was characterized with a device having CPW through lines and vertical feed lines excluding the RF switches. The measured packaging loss was 0.2 dB and the return loss was 33.6 dB at 5 GHz.

Contact materials and reliability for high power RF-MEMS switches

This paper presents test and characterization of various thin film contact materials for reliable high power RF- MEMS switches. We selected Au, Pt, Ir, and AuPt alloys for contact materials and fundamentally studied on contact phenomena and reliability of similar or dissimilar contacts using a contact measurement apparatus at high current condition. We also investigated the electrical contact behavior of the MEMS switches. From these studies, Au-to-Pt, Pt-to-Pt and Au-to-Ir contact showed reliable characteristics for the high power RF-MEMS switches.

NANOSCALE INVESTIGATION OF DOMAIN RETENTION IN PREFERENTIALLY ORIENTED PBZR0.53TI0.47O3 THIN FILMS ON PT AND ON LANIO3

We report results on domain retention in preferentially oriented PbZr0.53Ti0.47O3 (PZT) thin films on Pt and on LaNiO3 (LNO) electrodes. Domain images are obtained by detecting an electrostatic force exerted on the biased conductive probe. We demonstrate that polarization loss of PZT domains on LNO electrodes occurs less under no external field rather than that of PZT on Pt. The time dependence of the remnant polarization is found to follow a stretched exponential decay.

Microcantilevers integrated with heaters and piezoelectric detectors for nano data-storage application

A thermomechanical writing system and a piezoelectric readback system have been demonstrated using silicon cantilevers integrated with heaters and piezoelectric sensors for a low-power scanning-probe-microscopy data-storage system. A thin polymethylmethacrylate film has been used as a media to record data bits of 50 nm in diameter and 25 nm in depth using the silicon cantilever. The sensitivity of 0.22 fC/nm was also obtained using the fabricated cantilever. Finally, to obtain readback signals using the piezoelectric cantilever, a patterned oxide wafer with 30 nm depth was scanned to show the distinctive charge signals.

Electromagnetic Two-Dimensional Scanner Using Radial Magnetic Field

In this paper, we present the design, fabrication, and measurement results of a two-dimensional electromagnetic scanning micromirror actuated by radial magnetic field. The scanner is realized by combining a gimbaled single-crystal-silicon micromirror with a single turn electroplated metal coil, with a concentric permanent magnet assembly composed of two concentric permanent magnets and an iron yoke. The proposed scanner utilizes the radial magnetic field rather than using a lateral magnetic field oriented 45deg to the horizontal and vertical scan axes to achieve a biaxial magnetic actuation. The single turn coil fabricated with electroplated copper achieves a nominal resistance of 1.2 Omega. A two-dimensional scanner with mirror size of 1.5 mm in diameter was fabricated. Maximum optical scan angle of 8.8deg in horizontal direction and 8.3deg in vertical direction were achieved. Forced actuation of the gimbal at 60 Hz and resonant actuation of the micromirror at 19.1-19.7 kHz provide slow vertical scan and fast horizontal scan, respectively. The proposed scanner can be used in raster scanning laser display systems and other scanner applications.

Investigation of the electrical contact behaviors in Au-to-Au thin-film contacts for RF MEMS switches

This paper presents the electrical contact behaviors of gold-to-gold thin-film contacts under high current conditions and describes the major factors that influence these contact behaviors for radio frequency microelectromechanical system switches. The fundamental phenomena in the contact resistance versus contact force curve were investigated with a contact measurement setup, which we devised. Based on the experimental results, the contact resistance behavior with increasing contact force can be divided into three regions: first, an unstable contact regime before the minimum contact force is reached; second, a stable contact regime where the plastic deformation of the asperities explains the behavior and, finally, a saturated regime where the thickness effect of the film on a hard substrate becomes evident. The minimum contact force required for a stable contact depended on the roughness of the surface and the current flow through the contact. The smooth surface of the Au film and the metal softening caused by a high current flow of 100 mA had the effect of lowering the minimum contact force. After the minimum contact force is exceeded, the contact resistance initially follows the asperity plastic deformation behavior, where the contact resistance is related to the resistivity and hardness of the contact material. In the higher contact force regime, the decrease in the contact resistance becomes almost negligible. With increasing contact force, the effective contact area is limited by the effect of the film thickness on a hard substrate. From these results, we found the principal factors influencing the contact behaviors of the Au-to-Au contacts to be the resistivity, hardness and softening temperature of the contact materials and the design factors such as the surface roughness, the film thickness and the substrate.

The Effects of the Preparation Conditions and Heat-Treatment Conditions of Pt/Ti/SiO2/Si Substrates on the Nucleation and Growth of Pb(Zr,Ti)O3 Films

Pb(Zr,Ti)O3 films were deposited using DC magnetron multi-target reactive sputtering on Pt/Ti/SiO2/Si substrates. The effect of the substrate on the structure of the deposited film was investigated because the nucleation of the perovskite PZT film would be largely affected by the substrate surface conditions. As the thickness of the Ti underlayer increases and that of the Pt layer decreases, Ti out-diffusion to the Pt substrate surface through the grain boundary of the Pt layer is enhanced so that the formation of the perovskite phase is promoted by facilitating the incorporation of Pb component. At the same time, the degree of (100) preferred orientation of the perovskite phase decreases. This change in the preferred orientation of the PZT film is also observed with decreasing the preparation temperature of the Pt/Ti layers. The structural phase of the PZT film and the resultant electrical properties have a sensitive dependence on the thermal history (such as processing time and environment) of the substrate prior to the deposition of the PZT film. The heat-treatment in Ar environment promotes the out-diffusion of Ti and the out-diffused Ti, which is expected to cover a large area of the PZT surface due to its good wettability, facilitates the formation of perovskite PZT film. The heat-treatment in oxygen environment, however, is scarcely effective in obtaining perovskite-phase PZT films because the out-diffusion of Ti to the Pt surface is suppressed by titanium oxide formed along the grain boundaries of the Pt layer and the surface titanium oxide will be localized only at the grain boundaries of the Pt surface due to poor wettability of titanium oxide on Pt.

A high-sag microlens array film with a full fill factor and its application to organic light emitting diodes

This paper reports a novel high-sag microlens array (MLA) film with a full fill factor and presents its application to organic light emitting diodes (OLEDs) for the purpose of improving their outcoupling efficiency. A gapless hexagonal MLA film having a high sag ratio is proposed and fabricated by a simple micromachining process including trench formation and the conformal vapor phase deposition of a polymer. By applying the MLA films to OLED panels, the outcoupling efficiency was increased by a maximum of 48% without any apparent color shift. A high-sag MLA film with a full fill factor is expected to impart remarkable optical efficiency to various display or lighting applications with flat panel light sources including OLEDs.