Junseok Chae

Noise analysis and characterization of a sigma-delta capacitive microaccelerometer

This paper reports a high-sensitivity low-noise capacitive accelerometer system with one micro-g//spl radic/Hz resolution. The accelerometer and interface electronics together operate as a second-order electromechanical sigma-delta modulator. A detailed noise analysis of electromechanical sigma-delta capacitive accelerometers with a final goal of achieving sub-/spl mu/g resolution is also presented. The analysis and test results have shown that amplifier thermal and sensor charging reference voltage noises are dominant in open-loop mode of operation. For closed-loop mode of operation, mass-residual motion is the dominant noise source at low sampling frequencies. By increasing the sampling frequency, both open-loop and closed-loop overall noise can be reduced significantly. The interface circuit has more than 120 dB dynamic range and can resolve better than 10 aF. The complete module operates from a single 5-V supply and has a measured sensitivity of 960 mV/g with a noise floor of 1.08 /spl mu/g//spl radic/Hz in open-loop. This system can resolve better than 10 /spl mu/g//spl radic/Hz in closed-loop.

A monolithic three-axis micro-g micromachined silicon capacitive accelerometer

A monolithic three-axis micro-g resolution silicon capacitive accelerometer system utilizing a combined surface and bulk micromachining technology is demonstrated. The accelerometer system consists of three individual single-axis accelerometers fabricated in a single substrate using a common fabrication process. All three devices have 475-/spl mu/m-thick silicon proof-mass, large area polysilicon sense/drive electrodes, and small sensing gap (<1.5 /spl mu/m) formed by a2004 sacrificial oxide layer. The fabricated accelerometer is 7/spl times/9 mm/sup 2/ in size, has 100 Hz bandwidth, >/spl sim/5 pF/g measured sensitivity and calculated sub-/spl mu/g//spl radic/Hz mechanical noise floor for all three axes. The total measured noise floor of the hybrid accelerometer assembled with a CMOS interface circuit is 1.60 /spl mu/g//spl radic/Hz (>1.5 kHz) and 1.08 /spl mu/g//spl radic/Hz (>600 Hz) for in-plane and out-of-plane devices, respectively.

A μL-scale micromachined microbial fuel cell having high power density

We report a MEMS (Micro-Electro-Mechanical Systems)-based microbial fuel cell (MFC) that produces a high power density. The MFC features 4.5-μL anode/cathode chambers defined by 20-μm-thick photo-definable polydimethylsiloxane (PDMS) films. The MFC uses a Geobacter-enriched mixed bacterial culture, anode-respiring bacteria (ARB) that produces a conductive biofilm matrix. The MEMS MFC generated a maximum current density of 16,000 μA cm(-3) (33 μA cm(-2)) and power density of 2300 μW cm(-3) (4.7 μW cm(-2)), both of which are substantially greater than achieved by previous MEMS MFCs. The coulombic efficiency of the MEMS MFC was at least 31%, by far the highest value among reported MEMS MFCs. The performance improvements came from using highly efficient ARB, minimizing the impact of oxygen intrusion to the anode chamber, having a large specific surface area that led to low internal resistance.

An in-plane high-sensitivity, low-noise micro-g silicon accelerometer with CMOS readout circuitry

A high-sensitivity, low-noise in-plane (lateral) capacitive silicon microaccelerometer utilizing a combined surface and bulk micromachining technology is reported. The accelerometer utilizes a 0.5-mm-thick, 2.4/spl times/1.0 mm/sup 2/ proof-mass and high aspect-ratio vertical polysilicon sensing electrodes fabricated using a trench refill process. The electrodes are separated from the proof-mass by a 1.1-/spl mu/m sensing gap formed using a sacrificial oxide layer. The measured device sensitivity is 5.6 pF/g. A CMOS readout circuit utilizing a switched-capacitor front-end /spl Sigma/-/spl Delta/ modulator operating at 1 MHz with chopper stabilization and correlated double sampling technique, can resolve a capacitance of 10 aF over a dynamic range of 120 dB in a 1 Hz BW. The measured input referred noise floor of the accelerometer-CMOS interface circuit is 1.6/spl mu/g//spl radic/Hz in atmosphere.

Fabrication and Characterization of a Wafer-Level MEMS Vacuum Package With Vertical Feedthroughs

This paper reports a MEMS vacuum package with vertical feedthroughs formed in a glass substrate all at the wafer level. This approach satisfies requirements for MEMS vacuum packages, including small size, vacuum/hermetic capability, sealed and low parasitic feedthroughs, wafer-level processing, compatibility with most MEMS processes, and low cost. It also enables flip-chip solder attachment to a PC board. The package has an integrated micro-Pirani gauge on a glass substrate for in situ monitoring, a silicon cap, and vertical feedthroughs through the glass. The integrated Pirani gauge has 0.6 milli-torr resolution at 0.1 torr and 0.2 torr resolution at 100 torr. Using the Pirani gauge, the fabricated vacuum package is characterized. The package has maintained ~33 torr base pressure with plusmn1.5 torr uncertainty for more than four months without a getter. The long-term measured pressure uncertainty is from the measurement setup and environment, and can be improved using a getter inside the package. [2007-0160]

Noise analysis and characterization of a sigma-delta capacitive silicon microaccelerometer

This paper reports a high-sensitivity low-noise capacitive accelerometer system with one micro-g//spl radic/Hz resolution. The system operates as a 2/sup nd/-order electromechanical /spl Sigma/-/spl Delta/ modulator together with the interface electronics. A detailed noise analysis of electromechanical /spl Sigma/-/spl Delta/ capacitive accelerometers with a final goal of achieving sub-/spl mu/g resolution is also presented. The circuit has more than 120 dB dynamic range and can resolve better than 10 aF. The complete module operates from a single 5V supply and has a measured sensitivity of 960 mV/g with a noise floor of 1.08 /spl mu/g//spl radic/Hz in open-loop. This system can resolve better than 10 /spl mu/g//spl radic/Hz in closed-loop.

Methods of reducing non-specific adsorption in microfluidic biosensors

Non-specific adsorption (NSA) of biomolecules is a persistent challenge in microfluidic biosensors. Microfluidic biosensors often have immobilized bioreceptors such as antibodies, enzymes, DNAs, etc, via linker molecules such as SAMs (self-assembled monolayers) to enhance immobilization. However, the linker molecules are very susceptible to NSA, causing false responses and decreasing sensitivity. In this paper, we present design methods to reduce the NSA of alkanethiol SAMs, which are popular linker molecules on microfluidic biosensors. Three design parameters were studied for two different chain-length SAMs (n = 2 and 10): (i) SAM incubation time, (ii) surface roughness [0.8 nm and 4.4 nm RMS (root mean square)] and (iii) gold crystal re-growth along (1?1?1) the target orientation. NSA was monitored by surface plasmon resonance (SPR). The results suggest that increased SAM incubation time reduces NSA, and that short-chain SAMs respond more favorably than the long-chain SAMs. Both SAMs were shown to be sensitive to surface roughness, and long-chain SAMs reduced NSA by 75%. Gold crystal re-growth along (1?1?1) the target orientation profoundly reduced NSA on the short-chain SAM. On a gold surface where surface roughness was 0.8 nm and there was strong directional alignment along the (1?1?1) gold crystal, final concentrations of nonspecifically bound proteins were 0.05 ng mm?2 (fibrinogen) and 0.075 ng mm?2 (lysozyme)?significantly lower than other known methods. The results show that optimizing three parameters (SAM incubation time, gold surface roughness and gold crystal orientation) improved SAM sensitivity for fibrinogen?anti-fibrinogen conjugates by a factor of 5 in 2.94 pM, suggesting that the methods are effective for reducing NSA in microfluidic biosensors.

Surface plasmon resonance protein sensor using Vroman effect

We report a new surface plasmon resonance (SPR) protein sensor using the Vroman effect for real-time, sensitive and selective detection of protein. The sensor relies on the competitive nature of protein adsorption onto the surface, directly depending upon proteins molecular weight. The sensor uses SPR for highly sensitive biomolecular interactions detection and the Vroman effect for highly selective detection. By using the Vroman effect we bypass having to rely on bio-receptors and their attachment to transducers, a process known to be complex and time-consuming. The protein sensor is microfabricated to perform real-time protein detection using four different proteins including aprotinin (0.65kDa), lysozyme (14.7kDa), streptavidine (53kDa), and isolectin (114kDa) on three different surfaces, namely a bare-gold surface and two others modified by OH- and COOH-terminated self-assembled monolayer (SAM). The real-time adsorption and displacement of the proteins are observed by SPR and evaluated using an atomic force microscope (AFM). The sensor can distinguish proteins of at least 14.05kDa in molecular weight and demonstrate a very low false positive rate. The protein detector can be integrated with microfluidic systems to provide extremely sensitive and selective analytical capability.

A micromachined Pirani gauge with dual heat sinks

This paper reports a micromachined Pirani gauge with dual heat sinks that can be integrated with microelectromechanical systems (MEMS) devices inside a vacuum package to monitor long-term pressure changes and stability inside the package. The Pirani gauge utilizes small gaps (<1 /spl mu/m) between its heater and two thermal heat sinks to obtain large dynamic range (20 mtorr to 2 torr) and high sensitivity (3.5/spl times/10/sup 5/ (K/W)/torr). The gauge is 2/spl times/2 mm/sup 2/ in size, is fabricated using the dissolved wafer process (DWP) on a glass substrate, and utilizes dielectric bridges for signal routing. Measurements show the low end of the dynamic range can be extended by reducing the gap distance between the heater and thermal sinks, which matches well with analytical modeling. This gauge shows an uncertainty of 50 /spl mu/torr and a detectable leak rate of 3.1/spl times/10/sup -16/ cm/sup 3//s, assuming a common micropackage volume of 1.6/spl times/10/sup -5/ cm/sup 3/, which represents at least four orders of magnitude improvement over traditional leak testing.

Transfer of metal MEMS packages using a wafer-level solder transfer technique

This paper presents a modular, low profile, wafer-level encapsulation technology for microelectromechanical systems (MEMS) packaging. Electroplated caps are formed on top of a solder transfer layer previously deposited on a carrier wafer, then simultaneously transferred and bonded to a device wafer by a novel solder transfer method and transient liquid phase (TLP) bonding technology. The solder transfer method is enabled by the dewetting of the solder transfer layer from the carrier wafer and TLP bonding of the cap to the device wafer during bonding. The bond and transfer cycle has a maximum temperature of 300/spl deg/C and lasts about 2.5 h. This approach has been demonstrated with nickel (Ni) caps as thin as 5 microns, with thicker caps certainly possible, ranging in size from 200 /spl mu/m to 1 mm. They were transferred with a lead-tin (Pb-Sn) solder layer and bonded with nickel-tin (Ni-Sn) TLP bonding with greater than 99% transfer yield across the wafer.