C. Sudhama

Electrical and reliability properties of PZT thin films for ULSI DRAM applications

The electrical and reliability characteristics of ferroelectric capacitors fabricated using sol-gel derived 50/50 lead-zirconate-titanate (PZT) thin films have been examined for ULSI DRAM (dynamic random access memory) applications. Various electrode materials, film thicknesses (200 nm to 600 nm) and capacitor areas were used. A large stored-energy density (Q/sub c/) of 15 mu C/cm/sup 2/ (at 125 kV/cm) was measured using different methods. The results indicate that PZT thin films exhibit material properties which might satisfy the requirements of ULSI DRAMs.<<ETX>>

Effects of post‐deposition annealing ambient on the electrical characteristics and phase transformation kinetics of sputtered lead zirconate titanate (65/35) thin film capacitors

The fabrication of high dielectric constant lead zirconate titanate thin films for dynamic random access memory application involves post‐deposition annealing to convert the low dielectric constant pyrochlore phase to the desired high dielectric constant perovskite phase. The results of our experiments detailing the dielectric characteristics such as degradation field (the electric field at which a sudden increase in leakage takes place), leakage current density, time‐dependent dielectric breakdown characteristics, charge storage density, coercive field, and charge storage density are reported. The charge storage density was measured using both double‐bipolar pulses and quasistatic capacitance‐voltage techniques. The results of these two measurement techniques are discussed for oxygen and nitrogen annealed films. Additionally, a comparative study of the phase transformation kinetics when the post‐deposition annealing is carried out in a nonoxidizing ambient such as nitrogen, as against an oxidizing ambien...

Annealing of lead zirconate titanate (65/35) thin films for ultra large scale integration storage dielectric applications: phase transformation and electrical characteristics

Thin film capacitors of Lead Zirconate Titanate (PZT, 400 nm) of Zr/Ti ratio 65/35, deposited by reactive dc-magnetron sputtering, with low leakage current and high charge storge density(’Qc) for use as capacitor dielectrics in ultra-large scale integration dynamic random access memory (ULSI DRAM) cells have been fabricated and studied. The equivalent SiO2 thickness for the optimized film is 5.3å for the fresh film and 9.1å after 1010 unipolar stress cycles (0 to -3 V). The leakage current density is 1.32 × 10-7A/cm2 for 3 V operation which is equivalent to an effective SiO2 field of 55 MV/ cm. X-ray diffraction analysis reveals that as-deposited films (Tdep. = 200‡ C) contain no detectable perovskite phase. Post-deposition annealing is therefore essential, and critical to the fabrication of high quality capacitors for memory applications. The pyrochlore-to-perovskite phase transformation, the evolution of the microstructure, composition and the presence of different phases in the film have been studied as a function of annealing conditions. There is a purely outward radial growth of the interphase boundary, resulting in the increase in the curved surface area of the cylindrical perovskite aggregates throughout the film thickness with increasing thermal budget. The increase in perovskite phase content with annealing time at a constant annealing temperature indicates that a diffusional phase transformation from the pyrochlore to cubic perovskite phase above the Curie temperature occurs as a first step in the formation of the ferroelectric perovskite phase. The variation of two important dielectric properties, charge storage density and leakage current density is reported as a function of the annealing time and temperature. Furthermore, the variation of the charge storage density due to unipolar dynamic electrical stress is studied. The total area under the large-frequency C-V curve (which is the total reversible polarization) increases under unipolar dynamic stress (0 to -3 V) after 1010 stress cycles. The degradation in charge storage density is found to be primarily due to an increase in remanent polarization caused by the shift in the hysteresis loop as a result of the reduction in the internal bias field under the influence of the unipolar dynamic stress.

Impact of post processing damage on the performance of high dielectric constant PLZT thin film capacitors for ULSI DRAM applications

The exposure of PLZT thin films to post deposition processes may have a detrimental effect on the properties of the films. Hence for the proper integration of these films into CMOS technology, these effects need to be studied and improved upon. Fresh devices have been exposed to x-rays, plasma and forming gas anneals. Each of these processes is seen to cause a significant change in the DRAM polarization, leakage current and the reliability properties. An annealing technique has been proposed for a complete recovery in these properties.<<ETX>>

Comparative study of the perovskite phase microstructure evolution and electrical properties of lead zirconate titanate thin-film capacitors annealed in oxygen and nitrogen ambients

Lead zirconate titanate [PZT, Pb(ZrxTi1−x)O3, x=0.65, 400 nm] thin-film capacitors with very low leakage current and large stored charge densities (Q′c=Pmax–Pr) for ultralarge scale integration dynamic random access memory (ULSI DRAM) memory applications have been fabricated by reactive dc magnetron sputtering followed by an annealing step. In this letter, we present a comparative study of the significant influence of the use of an oxidizing (O2) as against a nonoxidizing annealing ambient (N2) on the evolution of the perovskite phase microstructure and the electrical properties of PZT capacitors. The phase transformation kinetics are faster in a N2 annealing ambient at low temperatures, yielding capacitors with a large Q′c at low annealing thermal budgets. However, N2 annealed films exhibit a lower breakdown field and time to breakdown which is in agreement with a degradation model based on the field-induced concentration polarization of oxygen vacancies.

Thickness-scaling of sputtered PZT films in the 200 nm range for memory applications

In this paper, we present electrical and material properties of thin films (100 to 400 nm) of magnetron-sputtered ferroelectric PZT for memory applications. The optimal lead-compensation power (and the resulting film composition) is independent of film-thickness. Reduction of film-thickness leads to a reduction in the crystallization temperature (from 700°C for 400 nm films to 575°C for 100 nm films), and yields evidence for a two-step growth of perovskite rosettes. An optimized 100 nm film yields 12 μC/cm2 for 1.5V operation and fatigues by 25% after 1010 unipolar stress cycles.

Effects of electrical stress parameters on polarization loss in ferroelectric P(L)ZT thin film capacitors

The effects of stress parameters (e,g,, stress magnitude, frequency, and pulse shape) on the loss of DRAM polarization during unipolar pulse stressing in sol-gel P(L)ZT thin film capacitors have been studied. The results indicate that there is a strong correlation between the fatigue rate and the DRAM polarization for the fresh device. It has been found that contrary to what one might expect, the fatigue rate does not increase indefinitely with increasing stress voltage but saturates at the same voltage at which the polarization-voltage loop saturates. Though the effect of the rise time of the stress pulses on the fatigue rate is negligible, the fatigue rate is strongly dependent on the pulse width of the trapezoidal stress pulses.<<ETX>>

La doped PZT films for gigabit DRAM technology

Very low leakage current density (5/spl times/10/sup -7/ A/cm/sup 2/ even at 125/spl deg/C) and high charge storage density (100 fCspl mu/m/sup 2/) under V/sub DD2=1 V conditions have been achieved using 5% La doped PZT films for gigabit DRAM capacitor dielectrics. In addition, the fatigue and TDDB measurements indicate good reliability of this capacitor.<<ETX>>

Effects of nonlinear storage capacitor on DRAM READ/WRITE

Important aspects of nonlinear storage capacitor switching and their impact on DRAM READ/WRITE operations are explained using a simple model and PSpice simulation. The voltage signal and charge-transfer rate are found to be dependent not only on the total charged stored, but also on the exact shape of the storage capacitor Q-V curve. Typical paraelectric capacitors are shown to deliver a smaller voltage signal than a linear capacitor that has the same stored charge at the operating voltage. Further, typical paraelectric capacitors have slower READ but faster WRITE compared to the linear capacitor.<<ETX>>

The effect of deposition temperature on the material and electrical properties of PZT thin films for ULSI DRAM applications

Abstract The effects of deposition temperature on the properties of thin films of sputtered lead-zirconate-titanate (PZT) have been studied for ULSI DRAM storage capacitor dielectric applications. The films were deposited by reactive dc magnetron sputtering from a multi-component target. The grain size for the films deposited at 400°C was found to be less than 1000 A, while it was ∼ 10–30 μm for films deposited at 200°C. Small grain-sized material is desirable since it leads to better cell-to-cell uniformity in terms of charge storage capacity and other electrical and reliability properties. The optimum lead compensation was found to increase as the deposition temperature (T dep) increased. Leakage current density stays fairly constant as T dep is varied. As-deposited films, with a deposition temperature of 500°C, were rich in the perovskite phase and showed a high charge storage density of 11.2 μC/cm2 and a low leakage current density of 5.1 × 10−7 A/cm2 (both at 1.5 V). This implies the possibility of e...