Thin pressure sensors

Doping of the siHcon can have a large effect on the etch rate, and layers of different materials such as Si02 and Si N can have different etch rates. Eor pressure sensors, thin diaphragms of Si or related materials are etched into the wafer (see Pressure measurements). 

Many of the variations developed to make pressure sensors and accelerometers for a wide variety of appHcations have been reviewed (5). These sensors can be made in very large batches using photoHthographic techniques that keep unit manufacturing costs low and ensure part-to-part uniformity. A pressure differential across these thin diaphragms causes mechanical deformation that can be monitored in several ways piezoresistors implanted on the diaphragm are one way changes in electrical capacitance are another. 

Silicon-based pressure sensors are amongst the most common devices making use of this process. A thin low-n-doped epitaxial layer on the wafer determines an etch stop depth and thus the thickness of e.g. the pressure sensor membrane. 

In addition to absolute pressure measurements, pressure sensors can be used to determine flow rates when combined with a well-defined pressure drop over a microfluidic channel. Integration of optical waveguide structures provides opportunities for monitoring of segmented gas-liquid or liquid-liquid flows in multichannel microreactors for multiphase reactions, including channels inside the device not accessible by conventional microscopy imaging (Fig. 2c) (de Mas et al. 2005). Temperature sensors are readily incorporated in the form of thin film resistors or simply by attaching thin thermocouples (Losey et al. 2001). 

Small leakage currents or a transistor-like action of the junction are sufficient to generate a small current that may cause undesired passivation. This can be circumvented by application of an additional potential to the etching layer, shown by the broken line in Fig. 4.16 a. This electrochemical etch-stop technique is favorable compared to the conventional chemical p+ etch stop in alkaline solutions, because it does not require high doping densities. This etch stop has mainly been apphed for manufacturing thin silicon membranes [Ge5, Pa7, Kll] used for example in pressure sensors [Hil]. 

Fig. 6.15. Basis of strain gauge pressure transducer (a) thin-film strain-gauge used as pressure sensor (b) deformation of conductor under tensile stress...

The aim is to eliminate entrance effects as much as possible and any influence on the flow of the pressure tap holes into the channels. This was achieved by integrating on the same silicon chip the microchannel, the pressure taps and the pressure sensors. The fabrication process and the operating mode are described in [28]. The pressure sensors are constituted cf a membrane which is deformed under the fluid pressure and on which is deposited a thin film strain gauge. This strain gauge forms a Wheatstone bridge whose the membrane deformation modifies the electrical resistances. 

The analysis of thin supported films by N2 adsorption is often difficult due to the very small percentage of pore volume contributed by the thin layer relative to that of the support. Usually it is necessary to scrape off most of the bulk support layer to increase the pore volume percentage of the thin film. Recent technical improvements in pressure sensors on commercial apparatus (reaching now a sensitivity of 5.10 mmHg) or new sophisticated detection techniques using surface acoustic waves may in some cases solve this problem. 

Fig. 4.1.12 Thin-film metal structures on the steel membrane of Bosch s high-pressure sensor and the equivalent circuit of a Wheatstone bridge [27]...

Here we show an example of applying the EFS method to a non-silicon-based pressure sensor to operate at high pressure ranges. The membrane of many high-pressure sensors (Bosch, WIKA) is manufactured of steel, with thin-film metal resistors as a measurement signal pickup (Fig. 4.1.12). 

Fig. 5.4.11 View of thin film system in high-pressure sensor elements of Nagano Keiki Co. (left) and Robert Bosch GmbH (right)...

Thin film technology on steel is well suited for high precision sensors in automotive applications. Well established applications include high pressure sensors in the range of 140 to 200 MPa. For these applications, a manufacturing volume of several million sensors per year has been reached. 

Bulk-micromachined membranes are usually formed from dielectric materials like silicon oxide or silicon nitride combined with additional materials for example, in pressure sensors, silicon is used to increase the membrane thickness to the required values and in thermal sensors, platinum or other metals are needed for the sensing elements. The overall stress state of the membranes has to be controlled well to prevent buckhng (under high compressive stress) or fracture (under high tensile stress). With proper processing control, silicon oxide and silicon nitride thin films meet this requirement, making them ideal candidates for membrane-type devices. 

To improve the fuel vapor-pressure sensor accuracy, we reduced the mechanical strain conveyed from the package to the sensor device. Fig. 7.3.14 shows the structure of the fuel vapor-pressure sensor for 5 kPa. We chose a relatively thin silicon diaphragm, 14 pm, to achieve the sensitivity required for 5 kPa detection. That makes the sensor device more susceptible to the mechanical strain conveyed from the resin package. To solve that problem, we analyzed the effect of the mechanical strain from the package by FEM. 

The material of the sensing layer strongly influences the characteristics of the pressure sensor and the difficulty of the calibration process. Fig. 7.4.6 shows the change in sensitivity of two different sensing layer materials (poly-Si and thin-film NiCr metal) against temperature. Poly-Si shows a nonlinear dependence on temperature, whereas NiCr is quite linear. Also the variation of the sensitivity is much lower for thin-film NiCr. Both high linearity and low variation of sensitivity contribute to a simple and therefore cost-effective calibration concept for high-pres-... 

Fig. 7.4.8 shows the results of endurance testing of a metal thin-film high-pressure sensor with the design shown in Fig. 7.4.7. Typical deviations from the ideal characteristic can be seen. The sensor has been tested in a gasoline direct injection car for 162000 km. The deviation is shown for pressures up to 140 bar and temperatures between —40 and 140 °C. Hysteresis can be seen for increasing and decreasing pressures at each measuring temperature. The maximum deviation of about 0.3% FSD demonstrates the long-term stability of the sensor package design and the thin-film technology.

Figure 2.23 (a) A schematic diagram of an intravascular fiber optic pressure sensor. Pressure causes deflection in a thin metal membrane that modulates the coupling between the source and detector fibers, (b) A characteristic curve for the fiber optic pressure sensor. (From Peura R A and Webster J G Bask sensors and principles in Medical Instrumentation Application and Design, 2nd edition, edited by J G Webster, copyright 1992 Houghton Mifflin Co., Boston.)... 

Figure 12.1 An illustration of the procedures leading up to the formation of a thin membrane used in a pressure sensor, (a) The spin-on of a positive photoresist (b) the exposed resist can then be removed by the developer leaving a window open to the Si02 (c) the wafer is immersed in a buffered HF solution, the Si02 is removed down to the silicon (d) the region of exposed silicon can now be etched to form a cavity. 

Recently, Tu and Zemel constructed a fiber-optic-based pressure sensor that employed an etched channel for guiding a single-mode optical fiber to a thin membrane perpendicular to the wafer surface [61]. This is depicted in figure... 

Figure 1 illustrates the operational principle of hydrogel-based sensors. Pressure sensor chips with a flexible thin silicon bending plate and with an integrated piezoresistive Wheatstone bridge inside this plate have been employed as... 

Piezoresistive sensors automatically normalized to ambient pressure are used as clinical pressure sensors today. They have to be protected against water by use of a filter in the sensing line. The installation is set up in the sidestream by means of a thin hose as the sensing line in which only a negligible flow occurs. Electronic ventilators utilize additional pressure sensors for device control. 

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