Y. T. Cheng

Localized silicon fusion and eutectic bonding for MEMS fabrication and packaging

Silicon fusion and eutectic bonding processes based on the technique of localized heating have been successfully demonstrated. Phosphorus-doped polysilicon and gold films are applied separately in the silicon-to-glass fusion bonding and silicon-to-gold eutectic bonding experiments. These films are patterned as line-shape resistive heaters with widths of 5 or 7 /spl mu/m for the purpose of heating and bonding. In the experiments, silicon-to-glass fusion bonding and silicon to gold eutectic bonding are successfully achieved at temperatures above 1000/spl deg/C and 800/spl deg/C, respectively, by applying 1-MPa contact pressure. Both bonding processes can achieve bonding strength comparable to the fracture toughness of bulk silicon in less than 5 min. Without using global heating furnaces, localized bonding process is conducted in the common environment of room temperature and atmospheric pressure. Although these processes are accomplished within a confined bonding region and under high temperature, the substrate temperature remains low. This new class of bonding scheme has potential applications for microelectromechanical systems fabrication and packaging that require low-temperature processing at the wafer level, excellent bonding strength, and hermetic sealing characteristics.

Localized bonding with PSG or indium solder as intermediate layer

The feasibility of localized bonding with PSG (phosphosilicate glass) or indium solder as the intermediate layer have been demonstrated. Both localized PSG-to-glass and indium-to-glass bonds are accomplished on a square bonding area (/spl sim/500/spl times/500 /spl mu/m/sup 2/) encompassed by 5 /spl mu/m wide microheaters made of phosphorus doped polysilicon. Either PSG or indium solder are deposited on top of the microheater and function as the intermediate bonding material. The separation of heating and bonding materials by the intermediate layer greatly improved the controllability of the bonding process. Moreover, the whole bonding process can be achieved in less than 2 minutes at atmospheric pressure and room temperature environment. This new bonding scheme has potential application for MEMS device packaging that requires low temperature processing at the wafer-level, but high temperature at the bonding interface.

Fabrication and hermeticity testing of a glass-silicon package formed using localized aluminum/silicon-to-glass bonding

A hermetic package based on localized aluminum/silicon-to-glass bonding has been successfully demonstrated. Less than 0.2 MPa contact pressure with 46 mA current input for two parallel 3.5 m wide polysilicon microheaters can achieve a strong and reliable bond in 7 minutes. Accelerated testing in an autoclave shows some packages survive more than 200 hours at 3 atm, 100%RH and 128/spl deg/C. Premature failure has been attributed to some unbonded regions on the failed samples. The bonding yield and reliability has been improved by increasing bonding time and applied pressure. Devices using this process have lasted for more than 320 hours under accelerated conditions. The packaging technology is being applied to vacuum encapsulation of resonant devices.

MEMS fabrication based on nickel-nanocomposite : film déposition and characterization

Electrosynthesized metal-nanocomposites represent a unique class of microelectromechanical systems (MEMS) structural material due to their compatibility with MEMS and CMOS fabrication technologies via a one-step, selective on-chip deposition process at low temperatures (∼50–90 ◦ C). In this paper, particle-reinforced, electrosynthesized nickel-nanocomposites of various compositions have been successfully demonstrated as potential structural materials for MEMS. We have achieved low-temperature, stress-free, wafer-level fabrication of nano-composite MEMS via both an electroless nickel (EN) and an electrolytic nickel (EL) deposition process, with the addition of uniformly dispersed micro- and nanoparticles of either cordierite (diameter ≈ 100 nm–5 µm) or diamond (diameter ∼ 4 nm). The as-deposited nickel-cordierite films exhibit better thermal compatibility with silicon, compared to nickel. The measured coefficients of thermal expansion (CTE) of EN-cordierite and EN are 17.34 ppm K −1 and 26.69 ppm K −1 , respectively. Stress–temperature measurement of EN-cordierite composite also confirms that residual stress decreases with the incorporation of cordierite. Finally, by adding various concentrations of nanodiamond particles into an EL matrix, it is found that higher diamond concentration renders these films more compressively stressed.

Vacuum packaging technology using localized aluminum/silicon-to-glass bonding

A vacuum package based on localized aluminum/silicon-to-glass bonding has been successfully demonstrated. With 3.4 watts heating power, /spl sim/0.2 MPa applied contact pressure, and 90 minutes wait time before bonding, vacuum encapsulation at 25 mtorr can be achieved. Folded-beam comb drive /spl mu/-resonators are encapsulated and used as pressure monitors. Long-term testing of un-annealed vacuum-packaged /spl mu/-resonators with a Q of 2500 has demonstrated stable operation after 20 weeks. A /spl mu/-resonator with Q of /spl sim/9600 has been vacuum encapsulated and shown to be stable after 7 weeks.

A low-power milliwatt electromagnetic microspeaker using a PDMS membrane for hearing aids application

This paper demonstrates a low-power electromagnetic microspeaker with a PDMS membrane for hearing aids application. For a 1.76mW power input, the speaker with a 3.5mm in diameter and 3.3µm thick membrane can generate a sound with the sound pressure level (SPL) of 106dB @1kHz in a 2c.c. coupler.

Design and fabrication of a microplatform for the proximity effect study of localized ELF-EMF on the growth of in vitro HeLa and PC-12 cells

This paper presents a platform technology with experimental results that show the scientists and biologists a way to rapidly investigate and analyze the biological effects of localized extremely low frequency (ELF) electromagnetic field (EMF) on living cells. The proximity effect of the localized ELF-EMF on living cells is revealed using the bio-compatible microplatform on which an on-glass inductive coil array, the source of the localized ELF-EMF in micro scale, is designed, fabricated and operated with a field strength of 1.2 ± 0.1 mT at 60 Hz for cell culturing study. After a 72 h ELF-EMF exposure, HeLa (human cervical cancer) and PC-12 (rat pheochromocytoma) cells exhibit about 18.4% and 12.9% cell proliferation rate reduction, respectively. Furthermore, according to the presented dynamic model, the reduction of the proliferation can be attributed to the interference of signal transduction processes due to the tangential currents induced around the cells.

A flexible, non-intrusive power sensor tag for the electricity monitoring of two-wire household appliances

This paper presents a flexible non-intrusive power sensor tag with good proximity for accurate electric current and voltage sensing on a typical SPT-2 18AWG zip-cord power line of household appliances. The current and voltage sensors are simultaneously fabricated on a 100μm-thick flexible PET substrate. For the design of a 50-turns coil combing with two sensing electrodes in an area of 1.3×1 cm2, the sensor tag exhibits a sensitivity of 31.1μV/A and 98.9 mV/115 V for detecting 60Hz electric current and voltage, respectively.

Nickel nano-composite film for MEMS applications

We have successfully demonstrated low-temperature, stress-free, wafer-level fabrication of nano-composite MEMS via both an electroless nickel (EN) and an electrolytic nickel (EL) deposition process, with the addition of uniformly dispersed nanoparticles of either cordierite (diameter /spl ap/100 nm/spl sim/5 /spl mu/m) or diamond (diameter /spl sim/4 nm). The as-deposited nickel-cordierite films exhibit better thermal compatibility with Si, compared to nickel. The measured coefficient of thermal expansion (CTE) of EN-cordierite and EN is 17.34 ppm/K and 26.69 ppm/K, respectively. Stress-temperature measurement of EN-cordierite composite also confirms that residual stress decreases with the incorporations of cordierite. Finally, by adding various concentrations of nanodiamond particles into an EL matrix, it is found that higher diamond concentrations render the film to be more compressively stressed.

SU-8 serial MEMS switch for flexible RF applications

This paper presents a novel SU-8 micromachining process for MEMS series switch fabrication. The switch is designed with a clamped–clamped SU-8 (5 µm)/Cu (2 µm)/SU-8 (3 µm) beam structure driven by electrostatic force, which is fabricated on a silicon substrate with a resistivity of ~5 Ω cm. Experimental results show that the switch can exhibit better than −4.48 dB insertion loss and −28.2 dB isolation up to 12 GHz. Such a large insertion loss is mainly caused by substrate loss which can be further reduced down to −0.75 dB as long as the substrate resistivity is increased up to 100 Ω cm.