Paul Ronsheim

Rapid annealing and the anomalous diffusion of ion implanted boron into silicon

The anomalous diffusion of ion implanted boron into silicon is shown to be a transient effect with a decay time that decreases rapidly with increasing anneal temperature. The decay time is approximately 45 min at 800 °C and decreases to the order of a second at 1000 °C. The anomalous displacement in the low concentration region is greater at low temperatures but a larger fraction of the boron is redistributed at high temperature. Sheet resistance measurements agree with the idea that the moving fraction of the boron atoms is electrically active and limited to the intrinsic carrier concentration at the anneal temperature. The activation energy for the decay of the transient is greater than that for the diffusion coefficient, which makes an appropriate rapid thermal anneal cycle an important practical process in the fabrication of shallow p‐n junctions.

Ultrathin high-K gate stacks for advanced CMOS devices

Reviews recent progress in and outlines the issues for high-K high-temperature (/spl sim/1000/spl deg/C) poly-Si CMOS processes and devices and also demonstrate possible solutions. Specifically, we discuss device characteristics such as gate leakage currents, flatband voltage shifts, charge trapping, channel mobility, as well as integration and processing aspects. Results on a variety of high-K candidates including HfO/sub 2/, Al/sub 2/O/sub 3/, HfO/sub 2//Al/sub 2/O/sub 3/, ZrO/sub 2/, silicates, and AlN/sub y/(O/sub x/) deposited on silicon by different deposition techniques are shown to illustrate the complex issues for high-K dielectric integration into current Si technology.

Imaging of Arsenic Cottrell Atmospheres Around Silicon Defects by Three-Dimensional Atom Probe Tomography

Discrete control of individual dopant or impurity atoms is critical to the electrical characteristics and fabrication of silicon nanodevices. The unavoidable introduction of defects into silicon during the implantation process may prevent the uniform distribution of dopant atoms. Cottrell atmospheres are one such nonuniformity and occur when interstitial atoms interact with dislocations, pinning the dislocation and trapping the interstitial. Atom probe tomography has been used to quantify the location and elemental identity of the atoms proximate to defects in silicon. We found that Cottrell atmospheres of arsenic atoms form around defects after ion implantation and annealing. Furthermore, these atmospheres persist in surrounding dislocation loops even after considerable thermal treatment. If not properly accommodated, these atmospheres create dopant fluctuations that ultimately limit the scalability of silicon devices.

High temperature stability of Al2O3 dielectrics on Si: Interfacial metal diffusion and mobility degradation

We have examined the stability of Al2O3/Si heterostructures and show that significant Al diffusion occurs into the silicon for temperatures of 1000 °C and more. This may be caused by dissociation of small quantities of Al2O3 and subsequent dissolution of the Al into the silicon. Such diffusion may be reduced, though not eliminated via an interfacial silicon oxynitride diffusion barrier. Using long channel metal gate Al2O3/Si n field effect transistor data, we show that anneals at 1000 °C result in a degradation of the electron mobility by a factor of 2.

Implantation damage and the anomalous transient diffusion of ion‐implanted boron

The effect of the implantation of silicon ions on the anomalous transient diffusion of ion‐implanted boron is investigated. It is found that silicon ion fluences well below that necessary to amorphize the lattice substantially reduce the anomalous transient diffusion of subsequently implanted boron. The sheet resistance, however, is increased by the additional silicon implant. The implantation of silicon ions into activated boron layers causes additional anomalous diffusion at substantial distances beyond the range of the silicon ions. The anomalous motion is also reduced in regions where the damage is greater. The effects can be explained in terms of the generation of point defect clusters which dissolve during anneal and the sinking of point defects in the regions of high damage by the formation of interstitial type extended defects.

Threshold voltage control in NiSi-gated MOSFETs through silicidation induced impurity segregation (SIIS)

Silicidation-induced impurity segregation was found to be an excellent method for adjusting the workfunction of NiSi gates. Continuous workfunction control over 300 mV was obtained with P, As, and Sb used as gate impurities. Fully depleted silicon-on-insulator devices were fabricated with a tunable V/sub t/.

Universal tunneling behavior in technologically relevant P/N junction diodes

Band-to-band tunneling was studied in ion-implanted P/N junction diodes with profiles representative of present and future silicon complementary metal–oxide–silicon (CMOS) field effect transistors. Measurements were done over a wide range of temperatures and implant parameters. Profile parameters were derived from analysis of capacitance versus voltage characteristics, and compared to secondary-ion mass spectroscopy analysis. When the tunneling current was plotted against the effective tunneling distance (tunneling distance corrected for band curvature) a quasi-universal exponential reduction of tunneling current versus, tunneling distance was found with an attenuation length of 0.38 nm, corresponding to a tunneling effective mass of 0.29 times the free electron mass (m0), and an extrapolated tunneling current at zero tunnel distance of 5.3×107 A/cm2 at 300 K. These results are directly applicable for predicting drain to substrate currents in CMOS transistors on bulk silicon, and body currents in CMOS transistors in silicon-on-insulator.

Threshold voltage control in NiSi-gated MOSFETs through SIIS

Complete gate silicidation has recently been demonstrated as an excellent technique for the integration of metal gates into MOSFETs. From the various silicide gate materials NiSi has been shown to be the most scalable. In this paper, a versatile method for controlling the workfunction of an NiSi gate is presented. This method relies on doping the poly-Si with various impurities prior to silicidation. The effect of various impurities including B, P, As, Sb, In, and Al is described. The segregation of the impurities from the poly-Si to the silicide interface during the silicidation step is found to cause the NiSi workfunction shift. The effect of the segregated impurities on gate capacitance, mobility, local workfunction stability, and adhesion is studied.

Quantitative analysis of one-dimensional dopant profile by electron holography

The one-dimensional dopant profile of a silicon p–n junction was characterized using off-axis electron holography in a transmission electron microscope (TEM). Quantitative comparisons were made with simulated voltage profiles based on data obtained from secondary-ion mass spectroscopy. Close agreement was obtained over a range of sample thicknesses, and a spatial resolution of 5 nm and sensitivity of 0.1 V were established. By using a sample that had been wedge polished and briefly ion milled, depleted surface layers did not need to be taken into account, and beam-induced charging was removed by carbon coating one exposed surface of the TEM specimen.

Effect of plasma N2 and thermal NH3 nitridation in HfO2 for ultrathin equivalent oxide thickness

Two methods of HfO2 nitridation including plasma N2 nitridation and thermal NH3 anneal were studied for ultrathin HfO2 gate dielectrics with <1 nm equivalent oxide thickness (EOT). The detailed nitridation mechanism, nitrogen depth profile, and nitrogen behavior during the anneal process were thoroughly investigated by XPS and SIMS analysis for the two types of nitridation processes at different process conditions. Intermediate metastable nitrogen was observed and found to be important during the plasma nitridation process. For thermal NH3 nitridation, pressure was found to be most critical to control the nitrogen profile while process time and temperature produced second order effects. The physical analyses on the impacts of various process conditions are well correlated to the electrical properties of the films, such as leakage current, EOT, mobility, and transistor bias temperature instability.