The advent of integrated circuits, which became possible because of the tremendous progress in semiconductor technology, resulted in the low cost microprocessor. Thus if it is possible to design a low cost sensor which is silicon based then the overall cost of the control system can be reduced .We can have integrated sensors which has electronics and the transduction element together on one silicon chip. This complete system can be called as system-on-chip .The main aim of integrating the electronics and the sensor is to make an intelligent sensor, which can be called as smart sensor. Smart sensors then have the ability to make some decision. Physically a smart sensor consists of transduction element, signal conditioning electronic and controller/processor that support some intelligence in a single package. In this report the usefulness of silicon technology as a smart sensor, physical phenomena of conversion to electrical output using silicon sensors, characteristics of smart sensors. A general architecture of smart sensor is presented.
Smart sensors are sensors with integrated electronics that can perform one or more of the following function logic functions, two-way communication, make decisions.
3. Usefulness of Silicon Technology in Smart Sensor
There are very convincing advantages of using silicon technology in the construction of smart sensor. All integrated circuits employ silicon technology. A smart sensor is made with the same technology as integrated circuits. A smart sensor utilizes the transduction properties of one class of materials and electronic
properties of silicon (GaAs). A transduction element either includes thin metal films, zinc oxide and polymeric films. Integrating electronics circuits on the sensor chip makes it possible to have single chip solution. Integrated sensors provide significant advantages in terms of overall size and the ability to use small signals from the transduction element. The IC industry will get involved in smart sensor if a very large market can be captured and the production of smart sensor does not require non-standard processing steps.
3.1 Signal conversion effects
We know that silicon shows a suitable physical signal conversion effect. Many of the physical effects of silicon can be used in making sensors. Based on these effects, different types of sensors can be constructed which can be used for measuring different physical and chemical measurand.
Table1 below shows how different non electrical signal in which we can classify different measurand and Table 2 shows the physical effects for sensors in silicon.
One problem with silicon is that its sensitivities to strain, light and magnetic field show a large crosssensitivity to temperature. When it is not possible to have silicon with proper effect, it is possible to deposit layers of materials with desired sensitivity on the top of a silicon substrate. Thus we can have a magnetic field sensor by depositing Ni-Fe layer on the top of a silicon substrate.
3.2 Different Silicon Sensors Employing Above Effects
Radiant Signal Domain
Silicon can be used to construct a sensor for sensing wide range of radiant signal from gamma rays to infrared. Silicon can be used for the fabrication of photoconductors, photodiode, and phototransistor or to detect nuclear radiation.
Mechanical Signal Domain
Silicon can be used for measuring force and pressure because of the piezo resistance effect. This effect is large because the average mobility of electrons and holes in silicon is strongly affected by the application of strain. Silicon can also be used for the measurement of air or gas velocities. If we slightly heat a silicon structure having two temperature measuring devices, and is brought into airflow then the resulting a temperature difference is proportional to the square root of the flow velocity. Combining a piezo resistor, diffused in a cantilevered beam or a piezoelectric layer with silicon can make a miniature accelerometer. By photoelectric principle one can find angular position by employing two photodiodes (i.e. one for X co-ordinate and other for Y).
Thermal Signal Domain
We know that all electron devices in silicon show temperature dependence, this property of silicon can be used for the measurement of temperature. This can be achieved by using two bipolar transistors with a constant ratio of emitter current. Another way of measuring temperature is to integrate thermocouples consisting of evaporated aluminium films and diffused p-type and n-type layers. This is possible because Seebeck in silicon is very large.
Magnetic Signal Domain
Silicon is a non â€œmagnetic material but it can be used for the construction of Hall plates and transistor structures that are sensitive to magnetic fields. These sensors are constructed by depositing a thin magnetic Ni-Fe film on top of silicon chip that also contains electronic circuits.
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