TABLE 49.3 Material Properties and Applications Areas Coercive Electro. Electro- Ferroelectric Epsilon Polarization Aging Optical Mechanical NV RAM DRAM X kxxxx X Optical Modulator A piezoelectric component often has a very simple geometric shape, especially when it is prepared for neasurement purposes. There will be mechanical resonances associated with the major dimensions of a sample piece. The resonance spectrum will be more or less complicated, depending on the shape of a sample piece. If the object has a simple shape, then some of the resonances will be well separated from each other and can b associated with specific vibrations and dimensions(modes). Each of these resonances has an electrical alent, and inspection of the equivalent circuit shows that there will be a resonance(minimum impedance an antiresonance(maximum impedance). Thus an impedance plot can be used to determine the frequencies and also the coupling constants and mechanical parameters for the various modes. 49.4 Ferroelectric and High Epsilon Thin Films While PZT and other ferroelectric(FE) bulk materials have had major commercial importance, thin films prepared from these materials have only recently been the focus of significant research efforts. In this section the material properties and process issues will be discussed. Because of the potentially large payoff, major efforts ive been directed at developing the technologies for depositing thin films of ferroelectric and non-ferroelectric but high epsilon(high dielectric constant)thin films A recent trend has been the ever increasing density of dynamic random access memory(DRAM). The storage capacitor in these devices is becoming a major limiting factor because the dielectric has to be very thin in order to achieve the desired capacitance values to yield, in turn, a sufficient signal for the storage cell. It is often also desirable to have nonvolatile operation(no data loss on power loss). These two desires have, probably more than anything else, driven the development of high epsilon and FE thin films. Of course, these are not the only applications of FE films. Table 49.3 lists the applications of FE (nonvolatile, NV)and high epsilon films(volatile) and highlights which of the properties are important for their use. It is seen that the memory application is very demanding. Satisfying all these requirements simultaneously has produced significant challenges in the manufacture of these films Perhaps the least understood and to some extent still unsolved problem is that of fatigue. In nonvolatile memory applications the polarization represents the memory state of the material (up =bit 1; down bit 0) In use the device can be switched at the clock rate, say 100 MHz. Thus for a lifetime of 5 years the material must withstand =106 polarization reversals or large field reversals. Typical materials for ferroelectric applica tions are PZTs with the ratio of zirconium to titanium adjusted to yield the maximum dielectric constant and polarization. This maximum will be near the morphotropic phase boundary for PZT. Small quantities of other naterials can be added, such as lanthanum or niobium to modify optical or switching characteristics. The Sol-Gel method discussed below is particularly suitable for incorporating these additives. Devices made from materials at the current state of the art loose a significant portion of their polarization after 100 to 10 2 cycles rendering them useless for their intended memory use because of the associated signal loss. This is a topic of intensive investigation and only one proprietary material has emerged which might be suitable for memory use(Symetric Corporation). High epsilon nonferroelectric materials are of great interest for DRAM applica tions. As an example, major efforts are extant to produce thin films of mixtures of barium and strontium titanate(BST). Dielectric constants of 600 and above have been achieved(compared to 4-6 for silicon oxides and nitrides) In applications for FE films, significant opportunities also exist for electro-optical modulators for fiber-optic devices and light valves for displays. Another large scale application is actuators and sensors. For the latter the c 2000 by CRC Press LLC© 2000 by CRC Press LLC A piezoelectric component often has a very simple geometric shape, especially when it is prepared for measurement purposes. There will be mechanical resonances associated with the major dimensions of a sample piece. The resonance spectrum will be more or less complicated, depending on the shape of a sample piece. If the object has a simple shape, then some of the resonances will be well separated from each other and can be associated with specific vibrations and dimensions (modes). Each of these resonances has an electrical equiv￾alent, and inspection of the equivalent circuit shows that there will be a resonance (minimum impedance) and an antiresonance (maximum impedance). Thus an impedance plot can be used to determine the frequencies and also the coupling constants and mechanical parameters for the various modes. 49.4 Ferroelectric and High Epsilon Thin Films While PZT and other ferroelectric (FE) bulk materials have had major commercial importance, thin films prepared from these materials have only recently been the focus of significant research efforts. In this section the material properties and process issues will be discussed. Because of the potentially large payoff, major efforts have been directed at developing the technologies for depositing thin films of ferroelectric and non-ferroelectric but high epsilon (high dielectric constant) thin films. A recent trend has been the ever increasing density of dynamic random access memory (DRAM). The storage capacitor in these devices is becoming a major limiting factor because the dielectric has to be very thin in order to achieve the desired capacitance values to yield, in turn, a sufficient signal for the storage cell. It is often also desirable to have nonvolatile operation (no data loss on power loss). These two desires have, probably more than anything else, driven the development of high epsilon and FE thin films. Of course, these are not the only applications of FE films. Table 49.3 lists the applications of FE (nonvolatile, NV) and high epsilon films (volatile) and highlights which of the properties are important for their use. It is seen that the memory application is very demanding. Satisfying all these requirements simultaneously has produced significant challenges in the manufacture of these films. Perhaps the least understood and to some extent still unsolved problem is that of fatigue. In nonvolatile memory applications the polarization represents the memory state of the material (up º bit 1; down º bit 0). In use the device can be switched at the clock rate, say 100 MHz. Thus for a lifetime of 5 years the material must withstand .1016 polarization reversals or large field reversals. Typical materials for ferroelectric applica￾tions are PZTs with the ratio of zirconium to titanium adjusted to yield the maximum dielectric constant and polarization. This maximum will be near the morphotropic phase boundary for PZT. Small quantities of other materials can be added, such as lanthanum or niobium to modify optical or switching characteristics. The Sol-Gel method discussed below is particularly suitable for incorporating these additives. Devices made from materials at the current state of the art loose a significant portion of their polarization after 1010 to 1012 cycles, rendering them useless for their intended memory use because of the associated signal loss. This is a topic of intensive investigation and only one proprietary material has emerged which might be suitable for memory use (Symetric Corporation). High epsilon nonferroelectric materials are of great interest for DRAM applica￾tions. As an example, major efforts are extant to produce thin films of mixtures of barium and strontium titanate (BST). Dielectric constants of 600 and above have been achieved (compared to 4–6 for silicon oxides and nitrides). In applications for FE films, significant opportunities also exist for electro-optical modulators for fiber-optic devices and light valves for displays. Another large scale application is actuators and sensors. For the latter the TABLE 49.3 Material Properties and Applications Areas Ferroelectric Epsilon Polarization Coercive Field Leakage Aging Electro￾Optical Electro￾Mechanical NV RAM X X X X X DRAM X X X Actuator X X Display X X X X Optical Modulator X X X X