Pressure Sensors

The first high-volume production of a pressure sensor began in 1974 at National Semiconductor Corp. of Santa Clara, California. Pressure sensing has since grown to a large market with an estimated 60 million silicon micromachined pressure sensors manufactured in 2001. Nearly all units use bulk micromachining technology. Manifold-absolute-pressure (MAP) [14] and disposable blood pressure [15] sensing are the two single largest applications. The vast majority use piezoresistive sense elements to detect stress in a thin silicon diaphragm in response to a pressure load. A few designs use capacitive methods to sense the displacement of a thin diaphragm.

The basic structure of a piezoresistive pressure sensor consists of four sense elements in a Wheatstone bridge configuration that measure stress within a thin crystalline silicon membrane (see Figure 4.8). The stress is a direct consequence of the membrane deflecting in response to an applied pressure differential across the front and back sides of the sensor. The stress is, to a first order approximation, linearly proportional to the applied pressure differential. The membrane deflection is typically less than one micrometer. The output at full-scale applied pressure is a few millivolts per volt of bridge excitation (the supply voltage to the bridge). The output normalized to input applied pressure is known as sensitivity [(mV/V)/Pa] and is directly related to the piezoresistive coefficients, ? _// and ? _? (see Chapter 2). The thickness and geometrical dimensions of the membrane affect the sensitivity and, consequently, the pressure range...