POLYCRYSTALLINE SILICON || SILICON PEIZO RESISTORS || MODULE 4 || EC 465 || KTU MEMS || LECTURE 26

Hi all, Today we are going to discuss two topics in this lecture, polycrystalline silicon and silicon pisor resistase. First, let us look at polycrystalline silicon. Silicon in polycrystalline form can be deposited onto silicon substrate by chemical vapour deposition. If you have not studied in VLSI, then you must have studied CVD process or chemical vapour deposition. Polycrystalline silicon in polycrystalline form can be deposited onto silicon substrate by chemical vapour deposition process. It becomes a principal material in surface micromachining. Low pressure CVD process is frequently used for depositing polycrystalline silicon onto silicon substrate. Temperature involved in this process is about 600 to 650 degree celsius. Polycrystalline silicon is widely used in IC industry for resistase, gates, transistors, thin film transistors etc. Highly doped polycrystalline silicon and phosphorus for n-type and boron for p-type can drastically reduce resistivity of polycrystalline silicon to produce conductors and control switches. Piso resistance is defined as a change in electrical resistance of solids when subjected to stress field. Highly doped polycrystalline silicon and phosphorus for n-type and boron for p-type can drastically reduce resistivity of polycrystalline silicon to produce conductors and control switches. Both p- and n-type silicon exhibit excellent piso resistive effect. Delta R is equal to pi into sigma. Delta R is equal to delta Rxx, Ryy, Rzz, Rxy, Rxz, Ryz, the whole transpose, which represents change of resistance in infinitesimally small cubical piso-resistive crystal element with coordinates corresponding to Delta R is equal to delta Rxx, Ryy, Rzz, Rxy, Rxx, Ryz, the whole transpose, which represents change of resistance in infinitesimally small cubical piso-resistive crystal element with coordinates corresponding to π is referred to Piezo resistive coefficient matrices δ Riz equal to π x σ delta R is equal to pi into sigma delta R is the change in resistance sigma is the change in resistance corresponding to stress component sigma is the stress component we have learnt that there are two types of stress components normal stress and shear stress sigma xx, sigma yy, sigma zz is the normal stress component sigma xy, sigma xz, sigma yz is the shear stress component what is pi? pi is the change in resistance corresponding to stress component we have written the equation and the corresponding change in resistance values in the resistive coefficient matrix, pi 11 and pi 12 are associated with normal stress component pi 44 is related to shear stress component the actual values of these coefficients depend on the angles of piezo resistors with respect to silicon crystal lattice we have used the equation 7.6 to get the figure sigma yy, sigma xx, sigma zz is the normal stress component sigma xy, sigma yz, sigma xz is the shear stress component we have drawn the diagram corresponding to the equation silicon piezo resistors are used in the form of strips in most of the applications of MEMS and micro resistors this is a figure of that in such cases, only the in-plane stresses in the x and y directions need to be accounted for the change of electrical resistor in a silicon piezo resistor gauge can be expressed as delta R divided by R is equal to pi L, sigma L plus pi T, sigma T delta R and R are change in resistance and R is the original resistance the value of original resistance R in the equation can be given either by R is equal to Rho L by A L and A are the length and cross sectional area of the piezo resistor the stress components in the longitudinal and tangential direction are denoted as sigma L and sigma T one major drawback of silicon piezo resistor is strong temperature dependence of their piezo resistivity the sensitivity of piezo resistivity to the applied stress deteriorates rapidly with increase of temperature the doping concentration of piezo resistivity should be kept below 10 rise to 19 per cm3 because the piezo resistivity coefficient drops considerably above this dose and error based breakdown become an issue the doping concentration of piezo resistivity should be kept below 10 rise to 19 per cm3 because the piezo resistivity coefficient drops considerably above this dose and error based breakdown become an issue the doping concentration of piezo resistivity should be kept below 10 rise to 19 per cm3 today we learnt about polycrystalline silicon and silicon piezo resistivity I hope you understood these two topics see you in the next lecture thank you