G. J. M. Dormans

Measurement of piezoelectric coefficients of ferroelectric thin films

This article presents measurements of piezoelectric coefficients of lead zirconate titanate (PZT) thin films. The normal load method is used to measure the coefficients for PZT films with various compositions prepared by the sol‐gel technique or by organometallic chemical vapor deposition (OMCVD). The as‐deposited OMCVD films have a piezoelectric coefficient of 20–40×10−12 m/V, whereas the unpoled sol‐gel films are not piezoelectric. After poling the thin films having a composition near the morphotropic phase boundary; these values increase to 200×10−12 m/V for OMCVD films and 400×10−12 m/V for sol‐gel films. The difference may arise from an incomplete poling of the OMCVD films.

Ferroelectric properties and fatigue of PbZr0.51Ti0.49O3 thin films of varying thickness: Blocking layer model

Ferroelectric capacitors having Pt bottom and top electrodes and a ferroelectric film of composition PbZr0.51Ti0.49O3 (PZT) were fabricated and investigated. The PZT films of thicknesses varying from 0.12 to 0.69 μm were prepared by organometallic chemical‐vapor deposition. Annealed capacitors were investigated by capacitance, hysteresis, and pulse switching measurements. It is found that the thickness dependence of the reciprocal capacitance, the coercive voltage, and the polarization measured by pulse switching can all be explained by a blocking layer model, in which a dielectric layer of thickness dbl and relative permittivity ebl is situated between the PZT film and an electrode. It is shown that (i) the coercive field is independent of thickness having a value of 2.4 V/μm; (ii) the ratio ebl/dbl is in the range 20–28 nm−1; (iii) the voltage across the blocking layer is proportional to the polarization, Vbl=cP, where c=4.1±0.5 V m2/C; and (iv) the polarization depends on the electric field in the PZT ...

Stresses in Pt/Pb(Zr,Ti)O3/Pt thin‐film stacks for integrated ferroelectric capacitors

A study of the stresses in a ferroelectric capacitor stack deposited on an oxidized silicon substrate is presented. The capacitor stack was prepared with sputtered Pt bottom and top electrodes and a ferroelectric film of composition PbZrxTi1−xO3 (PZT) with x≊0.5 which was deposited using a modified sol‐gel technique. The stresses were determined by the changes in the radius of curvature of the wafer following the deposition steps, during and after annealing treatments, and after etching steps in which the top electrode, the PZT film, and the bottom electrode were successively removed. The largest stress effects are found in the Pt electrodes which are deposited under conditions giving an intrinsic compressive stress. An annealing treatment exceeding 500 °C changed the stress of the bottom electrode from ≊−750 MPa (compressive) to a large tensile stress (≊1 GPa). This stress is largely thermal and is caused by the differences in thermal‐expansion coefficients of the Pt film and the Si substrate. The stress...

OMCVD of cobalt and cobalt silicide

Cobalt and cobalt silicide layers were deposited by OMCVD using the Co precursors Co(C5H5)2, Co2(CO)8, Co(C5H5)(CO)2 and CoCF3(CO)4, and the Si precursors SiH4 and Si2H6. Strongly textured (111)-β Co layers were grown from Co(C5H5)2, Co(C5H5)(CO)2 and CoCF3(CO)4 at temperatures above 300°C in H2 at atmospheric pressure. Growth from Co(C5H5)2 is inhibited on Si substrates. For temperatures ≥600°C the Co layers deposited from Co(C5H5)(CO)2 react with the Si(100) substrate to form CoSi2(00l) aligned with the substrate orientation. Co2(CO)8 gives amorphous Co between 200 and 300°C. The upper temperature is set by the occurrence of homogeneous gas-phase reactions at atmospheric reactor pressure. Cobalt silicide layers can be grown from CO2(CO)8 and (di)silane at temperatures between 200 and 400°C. The Co/Si ratio in the layers decreases with increasing temperature and is independent of the gas-phase Co/Si ratio. Stoichiometric CoSi2 is obtained at ~ 300°C. Both Co(C5H5)2 and Co(C5H5)(CO)2 react with (di)silane, leading to the incorporation of carbon in the layer. The Co/Si ration and the carbon content in the layers are practically independent of the deposition conditions. With CoCF3(CO)4 no contamination-free silicide could be grown. The carbon incorporation with Co(C5H5)2 and Co(C5H5)(CO)2 can be avoided by a pulsed growth method in which the Co precursor and the Si precursor are introduced alternately into the reactor. With Co(C5H5)2 the growth is then inhibited on Si substrates.

Chemical Vapor Deposition of Electroceramic Thin Films

A nonconventional way of producing nonvolatile memories is to use ferroelectrics, a class of electroceramic materials. These materials have a remanent polarization. The direction of this polarization can be changed by an electric field. Ferroelectric materials possess a “natural memory,” so to speak. Ferroelectrics have been known for a long time, and the idea to use them for binary data storage originates in the 1950s. The basic element of this type of memory is formed by a ferroelectric capacitor—a ferroelectric layer sandwiched between electrodes. Early prototypes were unsuccessful because rather high voltages were needed to switch the ferroelectric capacitor (200–300 V) and the memories suffered from crosstalk. (Programming one particular cell influenced neighboring cells.) The revival of ferroelectric memories was driven by the development of thin-film deposition techniques that allowed the formation of capacitors with ferroelectric thin films of submicron thicknesses. These capacitors can be switched with normal intergrated-circuit (IC) voltages. The crosstalk problem is circumvented by isolating each memory cell by a transistor (similar to a dynamic random-access memory [DRAM]). Compared to “standard” nonvolatile memories, ferroelectric memories offer the advantage of very fast access times (both for reading and writing), low-voltage operation, and good write/read endurance. A ferroelectric material that is already being used in commercially available memories is lead zirconate titanate, PbZr x Ti 1− x O 3 . To combine a ferroelectric material with IC technology is a challenge, and many problems have been (and will be) encountered.

OMCVD of transition metals and their silicides using metallocenes and (di) silane or silicon tetra-bromide

In this work we investigated the feasibility of OMCVD of metals and silicides with the metallocenes of Fe, Co and Ni and various Si-precursors. Layers of pure Fe and Co can be deposited with H2 as the carrier gas at atmospheric pressure. This deposition is found to be completely inhibited on bare silicon substrates. Nickelocene cannot be used because of preliminary reduction by H2 at transport temperature. The Si-precursors silane and disilane are found to react with the metallocenes, leading to the incorporation of ∽ 30 at% carbon in the silicide, independent of deposition conditions. For cobalt silicide deposited on silicon substrates in H2, a thin layer of carbon-free epitaxial CoSi2 is found at the substrate interface, even on Si(100). The carbon incorporation can be reduced to ∽ 15 at% by using silicon tetra-bromide with an increase in the relative thickness of the carbon-free layer. The deposition of silicon from the non-conventional precursor silicon tetra-bromide was briefly studied. Carbon incorporation can be completely avoided by introducing the metallocene and the silicon precursor alternately in the reactor. The stoichiometry of the layer can then be controlled by adjusting the partial pressures of the precursors and/or the pulse durations. The silicide growth by this method is also inhibited on silicon substrates.

Composition-controlled growth of PbTiO3 on SrTiO3 by organometallic chemical vapour deposition

Abstract PbTiO 3 thin films have been grown on (001)SrTiO 3 by organometallic chemical vapour deposition (OMCVD) using the precursors tetra-ethyl-lead (TEL) and tetra-isopropoxy-titanium (TIP). The kinetics of the growth process have been studied as a function of the substrate temperature and the partial pressures of TEL, TIP and oxygen. At temperatures of 700°C and higher, the composition of the layers corresponds to stoichiometric PbTiO 3 , independent of the deposition temperature, oxygen partial pressure and the ratio of TEL and TIP partial pressures, which was varied between 0.2 and 1.7. This independency of growth condition can be explained by a mechanism involving the competition between the desorption of PbO and a fast formation of PbTiO 3 by the reaction of PbO with TiO 2 . At 700°C the growth rate of PbTiO 3 is proportional to the TIP partial pressure, but decreases with increasing TEL partial pressure, indicative of a competitive adsorption of Pb- and Ti-containing species. The growth rate is limited by the supply of TIP. Under optimal growth conditions, layers are obtained having a Rutherford backscattering channeling minimum yield as low as 3%.

Pulse switching characterization of organometallic chemical vapor deposited PbZrxTi1−xO3 thin films for high-density memory applications

Abstract This paper addresses the electrical characterization by fast pulse measurements of organometallic chemical vapor deposited PbZrxTi1−xO3 (PZT) thin films with platinum electrodes on oxidized silicon wafers. Investigations of the switched and non-switched polarization with the pulse amplitude, number of switched cycles (fatigue), pulse width, time (ageing), and the sensitivity to disturbing pulses were made. Different processing conditions (i.e., annealing in oxygen after the deposition and structuring of the top electrode), compositions, thicknesses and operating temperatures (0–100°C) can affect the thin films ferroelectric properties. Understanding these effects and trends aids in the optimization of PZT for high-density non-volatile random access memories and possibly other device applications. To supplement the understanding of the ferroelectric behavior in the films, transmission electron microscope investigations were made on two different compositions and unfatigued/fatigued capacitors.

High quality lead zirconate titanate films grown by organometallic chemical vapour deposition

Abstract Organometallic chemical vapour deposition is a suitable technique for the deposition of thin films of oxidic compounds such as lead zirconate titanate, PbZrxTi1−xO3. Above a deposition temperature of about 600°C stoichiometric PbZrxTi1−xO3 films can be grown on platinized silicon wafers within a large process window, independent of the precursor partial pressures and the deposition temperature. This is the result of a self-regulating mechanism. The PbZrxTi1−xO3 films have excellent ferroelectric properties exhibiting high values, up to 60μC/cm2, for the remanent polarisation. The value of the coercive field strength varies between 50 and 180 kV/cm, dependent on the composition. Layers with comparable properties can also be grown at lower temperatures, down to 500°C. In this case careful control of the gas-phase composition is required to obtain films with the correct stoichiometry.

Processing and performance of integrated ferroelectric and CMOS test structures for memory applications

Abstract The feasibility of integrating ferroelectric thin films with silicon CMOS technology was investigated by processing a ferroelectric process evaluation module which contains ferroelectric and CMOS test structures and some memory cells. The smallest cells have a ferroelectric capacitor (FECAP) of 25 μm2. The FECAPs were made with Pt/Ti electrodes and with Pb(Zr,Ti)O3 deposited by a modified sol-gel technique or by organometallic chemical vapour deposition. The back-end processing includes the insulation and interconnection of the FECAPs and the MOS transistors. The ferroelectric processing has only a slight influence on the CMOS properties. The properties of the FECAPs improve significantly by an additional anneal in oxygen. Both CMOS and FECAP properties allow a proper functioning of the memory cells. These can be reliably operated at supply voltages as low as 3 V and pulse widths down to 20 ns. The endurance of the memory cells exceeds 1013 read/write cycles.