Quan Tran

Metal contact reliability of RF MEMS switches

It is well-recognized that MEMS switches, compared to their more traditional solid state counterparts, have several important advantages for wireless communications. These include superior linearity, low insertion loss and high isolation. Indeed, many potential applications have been investigated such as Tx/Rx antenna switching, frequency band selection, tunable matching networks for PA and antenna, tunable filters, and antenna reconfiguration. However, none of these applications have been materialized in high volume products to a large extent because of reliability concerns, particularly those related to the metal contacts. The subject of the metal contact in a switch was studied extensively in the history of developing miniaturized switches, such as the reed switches for telecommunication applications. While such studies are highly relevant, they do not address the issues encountered in the sub 100μN, low contact force regime in which most MEMS switches operate. At such low forces, the contact resistance is extremely sensitive to even a trace amount of contamination on the contact surfaces. Significant work was done to develop wafer cleaning processes and storage techniques for maintaining the cleanliness. To preserve contact cleanliness over the switch service lifetime, several hermetic packaging technologies were developed and their effectiveness in protecting the contacts from contamination was examined. The contact reliability is also very much influenced by the contact metal selection. When pure Au, a relatively soft metal, was used as the contact material, significant stiction problems occurred when clean switches were cycled in an N2 environment. In addition, various mechanical damages occurred after extended switching cycling tests. Harder metals, while more resistant to deformation and stiction, are more sensitive to chemical reactions, particularly oxidation. They also lead to higher contact resistance because of their lower electrical conductivity and smaller real contact areas at a given contact force. Contact reliability issues could also be tackled by improving mechanical designs. A novel collapsing switch capable of generating large contact forces (>300μN) was shown to be less vulnerable to contamination and stiction.

An ultraclean tip-wear reduction scheme for ultrahigh density scanning probe-based data storage.

Probe-based memory devices using ferroelectric media have the potential to achieve ultrahigh data-storage densities under high write-read speeds. However, the high-speed scanning operations over a device lifetime of 5-10 years, which corresponds to a probe tip sliding distance of 5-10 km, can cause the probe tip to mechanically wear, critically affecting its write-read resolution. Here, we show that the long distance tip-wear endurance issue can be resolved by introducing a thin water layer at the tip-media interface-thin enough to form a liquid crystal. By modulating the force at the tip-surface contact, this water crystal layer can act as a viscoelastic material which reduces the stress level on atomic bonds taking part in the wear process. Under our optimized environment, a platinum-iridium probe tip can retain its write-read resolution over 5 km of sliding at a 5 mm/s velocity on a smooth ferroelectric film. We also demonstrate a 3.6 Tbit/inch(2) storage density over a 1 × 1 μm(2) area, which is the highest density ever written on ferroelectric films over such a large area.

Scanning probe charge reading of ferroelectric domains

A scanning probe charge-detection technique based on direct piezoelectric effect is demonstrated to read alternating bit polarizations in a ferroelectric media The bit signal is generated by spatially modulating charges interacting with a probe tip scanning in contact with the media. A periodicity of the bits is used with an appropriate scan speed to modulate the signal frequency. A signal-to-noise ratio of 10 dB has been achieved for a contact force of 100 nN. The modulation of the bit signal frequency into the 2 kHz data rate is achieved by coupling 0.4 μm spacing between alternating polarizations with scanning speed of 1.6 mm/s.

Tuning the Built-in Electric Field in Ferroelectric Pb(Zr0.2Ti0.8)O3 Films for Long-Term Stability of Single-Digit Nanometer Inverted Domains

The emergence of new technologies, such as whole genome sequencing systems, which generate a large amount of data, is requiring ultrahigh storage capacities. Due to their compactness and low power consumption, probe-based memory devices using Pb(Zr(0.2)Ti(0.8))O(3) (PZT) ferroelectric films are the ideal candidate for such applications where portability is desired. To achieve ultrahigh (>1 Tbit/in(2)) storage densities, sub-10 nm inverted domains are required. However, such domains remain unstable and can invert back to their original polarization due to the effects of an antiparallel built-in electric field in the PZT film, domain-wall, and depolarization energies. Here, we show that the built-in electric-field can be tuned and suppressed by repetitive hydrogen and oxygen plasma treatments. Such treatments trigger reversible Pb reduction/oxidation activity, which alters the electrochemistry of the Pb overlayer and compensates for charges induced by the Pb vacancies. This tuning mechanism is used to demonstrate the writing of stable and equal size sub-4 nm domains in both up- and down-polarized PZT films, corresponding to eight inverted unit-cells. The bit sizes recorded here are the smallest ever achieved, which correspond to potential 60 Tbit/in(2) data storage densities.

A Stamp-Sealed Microshell Package for RF MEMS Switches

This paper describes a unique method of encapsulating MEMS switches at the wafer level using a thin-film “microshell” lid and a novel micro-embossing, or “stamping” technique to seal the lid. After fabrication of the MEMS switch and subsequent formation of the microshell, the switches are released through gold tunnels that allow the penetration of a chemical etchant. In a controlled ambient, a “stamp” wafer is aligned to the device wafer, and the wafers are thermally compressed together. This process applies pressure across each tunnel to fuse the gold, thereby sealing the microshell packages. By sealing and passivating the switches at the wafer level, the wafers can be exposed to backend processing, packaging, and assembly steps such as dicing without damaging the sensitive MEMS devices. Furthermore, the size, cost, and complexity of the packaged system are significantly reduced compared to standard wafer bonding processes.Copyright