Pressure measurements typically are made as absolute, gauge, or differential (or relative) measurements. Absolute pressure sensors measure a pressure relative to a vacuum, gauge sensors measure a pressure relative to atmospheric pressure, and differential sensors measure a pressure difference between two inputs. Absolute pressure sensors are normally built with the vacuum inside a sensor's cavity, and have a sensed pressure applied to their top side. This side can be identified as the one that contains things for interconnection. A pressure gauge consists in principle of a spring element (measuring element) and a measuring or sensor organ. Common commercially available pressure gauges based on silicon technology may use membranes as the spring element, provided with piezoresistive resistors as the sensor organs. Absolute pressure sensors and sealed gauge pressure sensors both need a hermetic sealing of a relatively small cavity at the active diaphragm to get a reference pressure, preferably a vacuum enclosure. This can be accomplished on a wafer basis using e.g. silicon wafer bonding under vacuum conditions. A differential pressure sensor measures the difference of two pressures acting on opposite surfaces of the diaphragm. The differential pressure sensors include at least first and second pressure inlet ports. The first (low) pressure inlet port may be attached to a low pressure inlet line and the second (high) pressure port may be attached to a higher pressure inlet line. The differential pressure transmitter returns an electrical signal indicative of the difference in pressure between the higher pressure line and the low pressure line. The differential pressure transmitter is intended to measure a difference in pressure with substantially no flow between the pressure lines. To measure a difference between two pressures (differential pressure), two sensing chambers are commonly used, one for each pressure, the sensing chambers being spatially and mechanically connected with one another and being provided with one sensing capacitor each. In this manner it is possible to produce an electric signal which corresponds to the difference between a pressure acting on one of the sensing chambers and a pressure acting on the other sensing chamber.

Pressure sensors generally contain movable or deformable bodies, most often a deflectable diaphragm, and they can be either of the main types absolute pressure sensors and differential or relative pressure sensors. Pressures are measured by bringing the medium to be measured up to the sensor, or by transmitting the pressure prevailing in the medium to the sensor. The pressure sensors are often subjected to high loads that depend on the current state of the medium in which the measurement is performed. Frequently, the pressures acting on the pressure sensor vary considerably. A pressure sensor must therefore withstand high loads, and it must deliver exact measuring results. Pressure sensors are commonly used to monitor and/or control the pressure and flow of process fluids such as, for example, oil, water, gases, etc. In many cases, the pressure sensors are integral to a fluid flow regulator that is serially interposed in the process fluid flow path. Some pressure sensors are integral to a monitoring device that does not perform a regulation function and that is appended to or serially interposed in the process fluid flow path. Generally, a pressure sensor for sensing a fluid pressure has a diaphragm that acts as a pressure sensing element and is configured such that deflection (pressure deformation) of a diaphragm under a fluid pressure applied through a pressure port is converted to an electrical signal, thereby enabling the fluid pressure to be measured. A diaphragm pressure sensor is constructed in such a way as to sense pressure by converting the displacement of a diaphragm generated by pressure applied to the diaphragm into an electronic signal. The movement or deformation of the diaphragm can be sensed in different ways such by measuring the change of the capacitance of a suitable adapted capacitor, measuring the change of electric characteristics of a piezoresistive body or the change of the resistance of an electrical conductor coupled to the movement of the diaphragm and thereby being in varying strained states. Ceramic pressure-measuring cells are advantageously used in pressure measurement technology, since ceramic pressure-measuring cells have a measuring accuracy which is stable over a very long time. However, in comparison with metal, ceramic pressure sensors have a rougher surface and are often restrained by means of a generally nonreplaceble seal made of an organic material, for example an elastomer, in a pressure-tight manner in a housing which can then fastened at a measuring location by means of a process connection.

Semiconductor pressure sensors are getting attention for the development of a semiconductor technology and a micro-machining technology. Since creep rarely occurs in the semiconductor pressure sensor, a superior linearity can be obtained. The semiconductor pressure sensor is small, lightweight, and very strong against vibration. Compared with a mechanical sensor, the semiconductor pressure sensor is more sensitive, more reliable, and presents a higher production yield. A semiconductor pressure sensor generally includes a semiconductor substrate, a diaphragm and diffusion gauge resistors. The diaphragm is formed in the main surface of the substrate. The diffusion gauge resistors are formed by ion implantation and diffusion. Then, a detection signal corresponding to pressure applied to the diaphragm can be generated based on a resistance value change of the diffusion gauge resistors. In a typical semiconductor pressure sensor, a pressure stimulus causes distortion or strain of a diaphragm, which is usually formed of a monocrystalline silicon. The semiconductor pressure sensor device detects a pressure employing an effect converting a pressure added to a semiconductor into an electronic signal. A device which includes a semiconductor sensor substrate having a diaphragm of appropriate thickness for generating a predetermined electronic signal corresponding to a predetermined pressure and a glass base as a support member to fix the semiconductor sensor substrate is known as such a semiconductor pressure sensor device. The typical semiconductor pressure sensor is generally classified into a capacitive type and a piezoresistive type. Piezoresistive sensors are generally considered to be more robust than capacitive sensors. Another advantage is that they give an output signal proportional to the input with good linearity. Capacitive sensors, on the other hand, have the advantage over the piezoresistive type in that they consume less power, but have a non-linear direct output signal and are more sensitive to electromagnetic interference.

The piezoresistive pressure sensor device is one of the pressure-sensing devices for the semiconductors, and the principle of sensing is to sense the pressure from the outward environment by means of the piezoresistance effect of the silicon. The piezoresistive effect is one in which the resistance of an element changes as a result of length changes for that element generally as a result of stress. Resistance changes can behave anisotropically in directions parallel to and perpendicular to the direction of the stress. The piezoresistive pressure sensor is formed by diffusing impurities onto a semiconductor and has advantages such as easy fabrication and superior linearity. Though a simplified processing circuit can be applied in the piezoresistive pressure sensor, a correction circuit is usually added thereto to overcome a poor temperature characteristic thereof. In a piezoresistive pressure sensor, a strain gauge with a piezoresistive effect is formed on a semiconductor diaphragm. The strain gauge is deformed by a pressure applied to the diaphragm, and a change in resistance of the strain gauge caused by the piezoresistive effect is detected, thereby measuring the pressure. The diaphragm is formed by engraving one surface of a semiconductor wafer by etching. The thickness of the diaphragm largely influences the characteristics of the semiconductor pressure sensor. Accordingly, the thickness of the diaphragm must be controlled precisely. Piezoresistive pressure sensor chip includes several diffusion resistors. The diffusion resistors are disposed on a diaphragm made of a material (for instance, single crystal silicon) capable of providing the piezoresistance effect, and are connected into a bridge circuit. A pressure signal is taken out from the bridge circuit in accordance with changes in resistance values of the diffusion resistors which are caused by displacement of the diaphragm. Piezoresistive pressure sensors are distinguished by high long-term stability, a wide operating temperature range and a large measuring range in conjunction with low temperature dependence and high measurement dynamics. Piezoelectric pressure sensors have many applications such as underwater hydrophones, strain sensors and vibration sensors. Many types of piezoelectric sensors have been proposed regarding different piezoelectric materials, methods of construction and various other features. For instance, pressure sensors for measuring the engine intake pressure in automobiles generally use semiconductor pressure sensor chips utilizing piezoresistance effect as the pressure detecting device. For the purposes of detection of combustion conditions or knocking, improvement of fuel economy, and cleaning of exhaust gas in an internal combustion engine, the pressure in a combustion chamber of the internal combustion engine is detected by a pressure sensor built in a spark plug.

A majority of the currently employed pressure sensors are piezoresistive devices. However, capacitive pressure sensor devices are becoming increasingly more of the focus in the industry because of their higher pressure sensitivity, lower temperature sensitivity, and reduced power consumption. Capacitive pressure sensors typically measure pressure by the capacitive changes resulting from variations in the distance between a movable diaphragm and a substrate that occur because of pressure changes. In the capacitive pressure sensor, an exterior pressure or stress, causes a change in a gap interposed between opposing electrodes, so that capacitance therebetween is changed. The amount of the changed capacitance is then converted into an electrical signal, which involves with the magnitude of the stress or the pressure. A capacitive pressure sensor has a substrate on which is provided a diaphragm, which changes its shape in accordance with pressure, and a substrate which an electrode is provided on; these substrates are bonded together so as to face each other with a gap therebetween. Pressure is detected based on change in the capacitance between the diaphragm and the electrode. Capacitive pressure sensors typically include a fixed element having a rigid, planar conductive surface forming one plate of a substantially parallel plate capacitor. A displaceable conductive member, such as a metal diaphragm, or a plated non-conductive member, such as a metalized ceramic diaphragm, forms the other plate of the capacitor. Generally, the diaphragm is edge-supported so that a central portion is substantially parallel to and opposite the fixed plate. Capacitive sensors can be made highly accurate and repeatable. In gage transmitters that electronically calculate a pressure difference based on two absolute pressure sensor outputs, accuracy and repeatability of the sensors are particularly important to avoid introducing errors in the subtraction process. Capacitive silicon sensors can be made to be small in size and can easily be made by surface micromachining. However, they are not very robust and their pressure sensitive diaphragm needs to be protected against the pressure media by a gel or other flexible material in most applications.