Christine Dehm

Ferroelectric strontium bismuth tantalate thin films deposited by metalorganic chemical vapour deposition (MOCVD)

Thin films of Sr 1-x Bi 2+x Ta 2 O 9 (SBT) have been deposited by metalorganic chemical vapor deposition (MOCVD) on 150 mm Si wafers with Pt/Ti electrodes. The choice of Bi precursor significantly affects the process; film homogeneity is significantly improved when using a β-diketonate Bi precursor in combination with compatible Sr and Ta precursors. A highly repeatable process has been developed, with good run-to-run composition and thickness control. Effects of Bi volatility have been investigated in annealing experiments that show the onset of Bi loss at ∼570°C at reduced pressure (1-10 Torr). Film properties relevant to integrated ferroelectric random access (Fe RAMS) memories have also been characterized. Remenant polarizations (2P r ) up to 24 μC cm -2 have been obtained at 5 V, with 90% saturation of 2P r at 1.5 V and a coercive voltage of 0.52 V for a 140 mn film. Electrical leakage current density values were < 2×10 -8 A cm -2 at 1.5 V. Fatigue endurance has been measured to 10 11 cycles with < 10% degradation in switched charge.

Properties of SrBi2Ta2O9 thin films grown by MOCVD for high density FeRAM applications

Abstract A novel low temperature MOCVD process for SrBi2Ta2O9 (SBT) thin films is described. The process, which uses Bi(thd)3 as the Bi source, allows deposition temperatures down to 300°C enabling to maintain the integrity of the Pt bottom electrode. Excellent run-to-run repeatability and a step coverage > 90% on a 0.5 μm structure have been demonstrated. After annealing at 800°C, 140 nm thick films showed remanent polarizations of 2Pr = 25 μC/cm2 @ 5V and leakage currents between 10-8-10-9 A/cm2 @ 3 V. Endurance after 2x1011 cycles was 90% using a 1.8 V, 1MHz square pulse signal. This manufacturable CVD process removes a further obstacle for production of high density ferroelectric memories (16M and above).

Imprint in ferroelectric SrBi2Ta2O9 capacitors for non-volatile memory applications

Abstract Imprint is known as a failure mechanism in ferroelectric capacitors due to a voltage shift in the hysteresis curve. A detailed study to investigate the time, temperature and bias voltage dependence of the voltage shift was performed on MOD SBT thin films. Lifetime extrapolation under operating conditions (125 °C) reveal values for the lifetime of well over ten years.

Origin of Imprint in Ferroelectric CSD SrBi 2 Ta 2 O 9 Thin Films

The imprint behavior of CSD processed SrBi 2 Ta 2 O 9 (SBT) thin films has been investigated as a function of time, applied bias, illumination with band gap light and post anneal under different oxygen partial pressures. Applying a bias in the direction of the polarization enhances the tendency of the capacitor to exhibit a voltage shift as well as illuminating the poled capacitor with band gap light. Post anneal after top electrode deposition and patterning under slightly reducing atmospheres does not affect the imprint rate. From these experimental results, a model is presented which explains the imprint behavior of SBT films by transport of electronic charges from the electrodes into the film and subsequent trapping of these charges near the interface.

MOCVD of SrBi 2 Ta 2 O 9 for Integrated Ferroelectric Capacitors

SrBi 2 Ta 2 O 9 (SBT) is a promising material for ferroelectric random access memories (FERAMs) because it has high resistance to fatigue and imprint combined with low coercive field. Metalorganic chemical vapor deposition (MOCVD) offers the ability to produce high quality, conformai SBT films for both high and low density memory applications. An MOCVD process based on liquid delivery and flash vaporization has been developed which allows precise delivery of low vapor pressure precursors to the process. Precursor decomposition has been examined over a wide temperature range and the effects of process pressure have been examined. It is shown that Bi(thdb is superior to Bi(Ph) 3 as a source of Bi, offering a wide decomposition window with compatible Sr and Ta precursors so that a simple, well-controlled, and repeatable process is achieved at low temperatures. Films with 90% conformallity have been grown on 0.6 μm structures with a 1:1 aspect ratio. The MOCVD process yields the fluorite phase, which is transformed to the ferroelectric layered perovskite phase upon annealing in oxygen. Dielectric constants (e) of 200 and remanent polarization (2P r )up to 16 μC/cm 2 have been achieved on 150 mm wafers.

Influence of dry etching using argon on structural and electrical properties of crystalline and non-crystalline SrBi2Ta2O9 thin films

Abstract After patterning the Platinum/crystalline SrBi2Ta2O9 bilayer by Argon based Reactive Ion Etching (RIE), a degradation of the remanent polarization and leakage current of the capacitors for smaller feature sizes is observed. To simulate the study of the side wall of the capacitors, etching of blanket SBT is used as a model experiment. It is shown that etching of crystalline SBT is damaging the SBT material, resulting in the formation of small crystallites (SEM), the appearance of an unknown peak (XRD) and reduction of the Bismuth content on the SBT surface (AES). Using non-crystalline SBT, neither a degradation of electrical properties for smaller feature sizes nor a structural damage of blanket SBT is found after etching and recrystallization annealing although after etching of non-crystalline SBT also a loss of Bi is seen as indicated by AES. Therefore the following model is proposed: Patterning the Pt/crystalline SBT capacitor leads to a Bi deficient edge of the dielectric. Due to the crystalli...

Role of recovery anneals for chemical solution deposition (CSD) based SrBi2Ta2O9 (SBT) thin films

Abstract Using a recovery anneal after deposition of the Pt top electrode and patterning the Platinum / SrBi2Ta2O9 bilayer has been established to obtain well shaped hysteresis curves with low leakage currents. Electrical properties of SBT test capacitors in dependence of temperature and time for the recovery anneal are discussed. Evidence for degradation of the electrical properties of SBT capacitors after patterning due to the appearance of a new unknown peak in X-ray diffraction (XRD) is presented.

Technology challenges and solutions for 1Gbit and beyond

Abstract The need for higher DRAM densities, for cost effective manufacturing and the price pressure puts the DRAM development on a highly innovative path. The fast pace with which DRAM cell sizes are reduced results in many technology issues. This talk discusses the deep trench cell architecture, its advantages and the main technology innovations that have made the aggressive scaling of the DRAM cell possible. The issues related to Gbit DRAMs, the new challenges and potential innovations will be presented.

Phase Transformation and Microstructural Properties in Sputtered Vs. CVD WSi, Films

CVD WSi, films produced by dichlorosilane reduction at 570°C and WSi, films sputter deposited at 50°C were characterized by in situ x-ray diffraction (IS-XRD), in situ resistivity (ISRes), in situ stress (IS-stress), ex situ/in situ transmission electron microscopy (EX/IS-TEM) and ex situ Auger electron spectrometry (EX-AES) over the temperature range 25–1100°C. The CVD films were crystalline after deposition, with columnar grains in the hexagonal phase and a Si:W atomic ratio of 2.6:1. The CVD films exhibited a sharp hexagonal to tetragonal phase transformation near 750°C. The final grain size was greater than the film thickness, with no evidence of voiding. Avrami analyses gave traditional curves with n values of 2 for the phase transition in the CVD films. In comparison, the sputtered films were amorphous as deposited (Si:W atomic ratio of 2.8:1 ) and crystallized to a different hexagonal phase microstructure than did the CVD films. The sputtered films showed a broad hexagonal to tetragonal phase transformation near 800°C, and a final grain size that was less than the fihn thickness with much voiding. A low Avrami exponent of 0.2 to 0.4 was obtained for the transformation of the sputtered films.

Properties of SrBi/sub 2/Ta/sub 2/O/sub 9/ thin films grown by MOCVD for high density FeRAM applications

Summary form only given. A new, low temperature SrBi/sub 2/Ta/sub 2/O/sub 9/ (SBT) MOCVD process exhibiting excellent run-to-run repeatability has been developed for high density FeRAM applications. 90% step coverage and good adhesion were achieved on feature sizes down to 0.5 /spl mu/m on both Pt or SiO/sub 2/ surfaces. Postanneal process optimization resulted in an increase of remanent polarization of about 30%. Electrical characterization showed dielectric constants (/spl epsiv/) of /spl ges/200 and remanent polarization values (2Pr) of up to 23 /spl mu/C/cm/sup 2/. Endurance has been demonstrated up to 10/sup 11/ cycles.