Albert M. Leung

Micromachined accelerometer based on convection heat transfer

A micromachined thermal accelerometer that is simple, reliable, and inexpensive to make has been developed at Simon Fraser University. The operating principle of this accelerometer is based on the free-convection heat transfer of a small hot air bubble in a sealed chamber. An experimental device that requires only four masking steps to fabricate has been built. This device has demonstrated a 0.6 milli-g sensitivity that can theoretically be extended to sub-micro-g level: A 2-axis accelerometer based on the same operating principle has also been fabricated and tested.

Laser, direct-write microlithography of soluble polythiophenes

Abstract Focused laser light of wavelength 442 nm was used to induce photochemical crosslinking in thin films of poly(3-hexylthiophene). Irradiated regions were insoluble and thus patterns of micron resolution could be achieved by dissolution of unexposed regions. The minimum gel dose required to leave an insoluble residue was 7.7 mJ cm −2 and the dose required for 50% gel formation was 61 mJ cm −2 . Patterns of the polymer were oxidized with nitrosonium tetrafluoroborate. The resistance of oxidized polymer channels having dimensions 40 μm wide, 90 nm thick and 200 μm long was 150 kΩ. The bulk conductivity of deposited and oxidized polymer was ∼ 6 Ω −1 cm −1 . Two dominant photochemical mechanisms are present during photolysis: photosensitization of singlet oxygen and Diels-Alder addition of singlet oxygen to thienyl residues; photo-oxidation of the alkyl side chain. The latter is believed to be responsible for crosslinking and photo-insolubilization.

Intracranial Pressure Telemetry System Using Semicustom Integrated Circuits

A new generation of implantable, telemetric transmitters for intracranial pressure (ICP) measurements have been developed. A unique technique used in packaging the silicon piezoresistive pt essure transducer provides excellent long-term stability. Pulse code modulation is used for data transmission over a radio frequency (RF) link. To minimize the component count, two semicustom, bipolar integrated circuits are used. The transmitter electronics are housed inside a 29 ×20 ×7 mm titanium package along with the pressure transducer and two lithium batteries. Even though the transmitter consumes less than 0.4 mW of power, it is turned on remotely via RF signal transduction only on demand in order to extend the lifetime of the batteries to years. The pressure input of the transmitter has a dynamic range of ¿100- +200 mmHg with a 0.3 mmHg resolution and a 1 mmHg accuracy. Long-term in vitro and in vivo pressure baseline stabilities of better than 1 and 2 mmHg per month, respectively, have been achieved.

The nanogap Pirani—a pressure sensor with superior linearity in an atmospheric pressure range

We have designed and fabricated a surface micromachined Pirani pressure sensor with an extremely narrow gap between its heater and heatsink (substrate) with superior output linearity in the atmospheric pressure range. The gap size of the device has been reduced to 50 nm by using a layer of PECVD amorphous silicon as a sacrificial layer and a xenon difluoride (XeF2) gas phase etching technique. Such a narrow gap pushes the transition from molecular to continuum heat conduction to pressures beyond 200 kPa. The higher transition pressure increases the measurement range and sensitivity of the gauge in atmospheric pressures. The gas phase etching of the sacrificial layer eliminates stiction problems related to a wet etching process. The active area of the sensor is only a 6 × 50 µm2 microbridge anchored to the substrate at both ends. An innovative fabrication technique was developed which resulted in a virtually flat microbridge with improved mechanical robustness. This process enabled us to have a very well-controlled gap between the microbridge and the substrate. The device was tested in a constant heater temperature mode with pressure ranges from 0.1 to 720 kPa. The heater power was only 3 mW at 101 kPa (atmospheric pressure), which increased to about 8 mW at 720 kPa. The output sensitivity and nonlinearity of the device were 0.55% per kPa at 101 kPa and ±13% of the output full scale, respectively.

Micromachined three-axis thermal accelerometer with a single composite heater

A novel three-axis thermal accelerometer is designed, fabricated, and characterized in this paper. The device includes two half sensor plates attached to buckled cantilevers to form out-of-plane structures. Cantilevers are assembled by a single push of a microprobe and preserve their shapes when they latch into stoppers anchored to the substrate. The fabrication process is based on surface micromachining on silicon substrates using polyimide as the structural layer and amorphous silicon as the sacrificial layer. The fabricated devices are individually packaged and characterized. Using a total heater power of 2.5 mW, the X, Y, and Z axes, respectively, showed sensitivities of 66, 64, and 25 µV g−1. Compared to the earlier versions of the same class accelerometers, the fabricated single heater accelerometer demonstrates more than fourfold sensitivity improvement.

Monolithically fabricated polymermems 3-axis thermal accelerometers designed for automated wirebonder assembly

This paper reports on two novel 3-axis thermal accelerometers based on different mechanical structures that are fabricated using polyimide PI-2611, and assembled using a standard wire-bonder. One accelerometer design has an un-amplified linear DC sensitivity of plusmn45 muV/g, plusmn60 muV/g, and plusmn35 muV/g on the X, Y, and Z axes respectively. The second design has a sensitivity of plusmn17 muV/g, plusmn8.5 muV/g, and plusmn14 muV/g respectively. Both accelerometers are assembled by applying a lateral push to each of the out-of-plane parts using an unmodified wire-bonder. This paper will detail the fabrication, design, assembly, and functional results of the two 3-axis thermal accelerometer designs.

Robust MEMS Gyroscope Based on Thermal Principles

Two variants of a novel single-axis thermal gyroscope without seismic mass are designed, fabricated, and characterized in this paper. The operating principle of the device is differential temperature detection due to the Coriolis effect on an oscillatory gas stream created by alternating two resistive microheaters. The fabrication process is based on a bulk micromachining technology on a silicon substrate using platinum as the only conductor layer. The device structure consists of two resistive temperature detectors equally spaced from the two microheaters. The 170-nm-thick platinum heater and detector microstructures are freely suspended over a cavity etched into the substrate, with minimal structural support. A computer-controlled precision rotary stage is constructed to accurately measure the device performance. The devices demonstrate excellent linearity within the tested ±3.5 revolution per second angular rate of rotation and show sensitivities of 0.947 and 1.287 mV/ °/s at 20 mW heater powers. The robustness of the devices has been validated by the drop shocks of 2,722 to 16,398g (9.81 m/s2).

The nanogap pirani - a pressure sensor with superior linearity in atmospheric pressure range

This paper reports a surface micromachined Pirani pressure sensor with superior linearity in atmospheric pressure range. The extremely narrow gap of the device (50 nm) increases the transition pressure of the device up to 300 kPa. When the heater power is kept constant, large change in the gap thermal conductivity at different pressures alters the heater temperature and introduces undesirable nonlinearity to the device output. We tested the device with constant heater temperature in order to eliminate the effect of the heater temperature variation. Testing the device with constant heater temperature, together with the high transition pressure of the device resulted in superior output linearity and significant sensitivity to absolute pressure from 101 kPa to 650 kPa.

Buckled cantilevers for out-of-plane platforms

In this paper, we show how surface-micromachined buckled cantilevers can be used to construct out-of-plane structures. We include the relevant theory necessary to predict the height and angle of plates attached to buckled cantilevers, as well as the mechanical stresses involved in assembly. These platforms can be assembled to any angle between 0° and 90° with respect to the substrate by changing the attachment point and the amount of deflection. Example devices were fabricated using PolyMUMPs™ and assembled. Using these devices, the deflection of the buckled cantilevers was verified, as well as the placement for raised platforms.

Sensitivity Improvement of Micromachined Convective Accelerometers

The sensitivity of a micromachined convective accelerometer filled with different gases at various conditions is theoretically and experimentally investigated. It is shown that the sensitivity of the accelerometer is proportional to the Rayleigh number for the contained gas. The Rayleigh numbers for nine gases are calculated, and the corresponding sensitivity improvements are predicted. To test our predictions, bulk silicon micromachining technology is used to fabricate and package several thermal accelerometers. A precision rotary stage is set up enclosing a packaged device into a gas chamber and rotating its sensitive axis against gravity. Data acquisition is done by a high-precision digital multimeter over full cycle rotations for seven candidate gases: nitrogen (N2), argon (Ar), carbon dioxide (CO2), sulfur hexafluoride (SF6), hexafluoroethane (C2F6), octafluoropropane ( C3F8), and octafluorocyclobutane ( C4F8). The measured sensitivities are in good agreement with the theoretical predictions. An upper limit to the sensitivity improvement by boosting the Rayleigh number for the contained gas is measured. Sensitivity decline beyond a critical Rayleigh number is reported for the first time since the advent of the convective accelerometers without proof mass.