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. 2010;10(11):9541-63.
doi: 10.3390/s101109541. Epub 2010 Oct 28.

Novel Designs for Application Specific MEMS Pressure Sensors

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Free PMC article

Novel Designs for Application Specific MEMS Pressure Sensors

Giulio Fragiacomo et al. Sensors (Basel). .
Free PMC article

Abstract

In the framework of developing innovative microfabricated pressure sensors, we present here three designs based on different readout principles, each one tailored for a specific application. A touch mode capacitive pressure sensor with high sensitivity (14 pF/bar), low temperature dependence and high capacitive output signal (more than 100 pF) is depicted. An optical pressure sensor intrinsically immune to electromagnetic interference, with large pressure range (0-350 bar) and a sensitivity of 1 pm/bar is presented. Finally, a resonating wireless pressure sensor power source free with a sensitivity of 650 KHz/mmHg is described. These sensors will be related with their applications in harsh environment, distributed systems and medical environment, respectively. For many aspects, commercially available sensors, which in vast majority are piezoresistive, are not suited for the applications proposed.

Keywords: MEMS; MOEMS; capacitive; micromachined; optical; pressure; silicon; wireless.

Figures

Figure 1.
Figure 1.
Artistic view of a touch mode capacitive pressure sensors based on an array of capacitive elements. (a) Artistic view of the old design, the membrane (active area) has been made transparent in order to show the support structure. (b) Artistic view of the new design, part of the membrane has been removed in order to show the nanopillars structure.
Figure 2.
Figure 2.
Schematic of the touch mode regime. The plate comes into contact with the insulation layer deposited on the substrate. The touching area and the suspended area of the plate are defined by the pressure dependent variables ab(p) and av(p) which are linked by the radius of the plate a0. A polar coordinate system with the origin placed in the center of the unloaded plate has been used. The center deflection of the plate is measured along the negative z direction.
Figure 3.
Figure 3.
Capacitance as a function of the pressure. The analytical model (solid line) fits the experimental data (circles) and allows the extraction of important sensor parameters such as the parasitic capacitance, the insulating layer thickness and the flexural rigidity. As expected, this curve is highly nonlinear, especially in the transition region (see inlet) between normal mode and touch mode operations. The capacitance of the sensor is measured with a HP 4294A Precision Impedance Analyzer which accuracy is better than 0.1% in the range of interest. The pressure is varied with a Druck DPI 520 pressure controller which accuracy is 0.025% of the reading. Therefore the error bars cannot be seen given the high accuracy of the instrument used.
Figure 4.
Figure 4.
Process sequence for touch mode capacitive pressure sensors with low hysteresis. (a) Phosphorous doping of both the double side polished (DSP) silicon wafer and the top (SOI) wafer. (b) Growing of a 600 nm SiO2 layer and etching of the cavity. (c) Etching of the nanopillar structure on the bottom of the cavity and performing a second oxidation. (d) Etching of the insulation groove, fusion bonding and etching of the handle wafer. (e) Metalization of the contacts.
Figure 5.
Figure 5.
Profilometer measurement of the pillar structure etched on the bottom of the cavities. The 50 nanometer pillars are measured inside the walls of the honeycomb structure which height is roughly 600 nm.
Figure 6.
Figure 6.
A SEM image of a fabricated chip. Part of the top plate (A) has been removed in order to see the underlying pillar structure (B) which has been fabricated on the bottom of the cavities etched in the silicon wafer (C).
Figure 7.
Figure 7.
Measured capacitance as a function of the pressure at varying temperatures. From these curves the relative pressure coefficient in the different operating modes has been extracted. The capacitance of the sensor is measured with a HP 4294A Precision Impedance Analyzer which accuracy is better than 0.1% in the range of interest. The pressure is varied with a Druck DPI 520 pressure controller which accuracy is 0.025% of the reading. Therefore the error bars cannot be seen given the high accuracy of the instrument used.
Figure 8.
Figure 8.
Hysteresis, κ, comparison between two chips of the same batch, one of them fabricated with the pillar structure (black crosses), the other without (green triangles). Capacitances curves have been added to show qualitatively the hysteresis of the sensors.
Figure 9.
Figure 9.
The optical pressure sensor, which 3D sketch (left figure) and cross section (right figure) are shown, consists of a circular plate with a waveguide on top. A Bragg grating is integrated into the half-circle of the waveguide. When the plate deforms the tangential strain in the grating causes a uniform change in grating period.
Figure 10.
Figure 10.
The radial and tangential strain in a circular plate in arbitrary units.
Figure 11.
Figure 11.
The power through an half circumference shaped 2 μm wide waveguide as a function of its radius.
Figure 12.
Figure 12.
SEM image of circular plate pressure sensor and waveguide with integrated Bragg grating.
Figure 13.
Figure 13.
The figures shows the old and the new design of the passive bladder pressure sensor. The bottom plate and membrane are fabricated in silicon, 2, and the top plate is a glass substrate, 1. The inductor is electroplated copper, 3, and the opposite capacitor plate is made on the pressure sensitive membrane with a deposited gold film 4. (a) A 3D sketch of the old design. Notice that the copper coil, 3, continues to the center of the sensor. The top plate, 1, is a glass substrate. The bottom plate, 2, is a silicon substrate with a etched membrane covered with a gold layer, 4. (b) A 3D sketch of the new design. Notice the bossed membrane in the the center of the sensor, 5, and the electroplated copper coil, 3, that is solid in the center. The membrane deflects in the region, 4, that is situated between the bossed structure, 5, and the silicon substrate, 2. (c) A 2D sketch of the old design. (d) A 2D sketch of the new design.
Figure 14.
Figure 14.
Equivalent circuit diagram of a transformer circuit where the oscillation circuit on the primary side, made up by C1 and L1, can be tuned to that of the secondary side C2 and L2.
Figure 15.
Figure 15.
A structural mechanical simulation made in COMSOL for 1 bar pressure is shown. The black outline is the unstrained structure. Notice that the largest stress is located in the thin membrane, and the bossed structure is only moving vertically movement according to the center defection. Axial symmetry is employed in this simulation, the y-axis is considered the axis of rotation.
Figure 16.
Figure 16.
A static electrical simulation made in COMSOL is shown. The inductor is enclosed in an air domain to encapsulate all of the electromagnetic field. The cutout section in the red box shows a zoom of the inductor, that is approximated with concentric rings. Axial symmetry is employed in this simulation, the y-axis is considered the axis of rotation.
Figure 17.
Figure 17.
The figures shows simulations of a bossed membrane structure with parameters: gap distance at vacuum = 80 μm, radius of membrane = 3 mm, spiral width = spiral spacing = spiral thickness = 50 μm and thickness of bossed structure = 100 μm. The design parameters are not used because it would limit the range, and in these simulations the general behavior is investigated. The internal radius of the bossed structure is varied from 0 to 3 mm. (a) Sensitivity vs. boss radius. An optimal internal radius of the bossed membrane is around 1.2 mm. (b) Q-factor vs. boss radius. A clear peak is seen at approximately 2.4 mm.

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