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IBM Journal of Research and Development

Issue 6 • Date Nov. 1987

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Displaying Results 1 - 12 of 12
  • Contact metallurgy development for VLSI logic

    Page(s): 608 - 616
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (757 KB)  

    The criteria involved with the choice of an ohmic contact material for VLSI logic are discussed. The problems of aluminum penetration encountered with Al metallization and solid-phase epitaxy associated with Al-Si metallization make these interconnect materials incompatible with VLSI technology. The contact resistance characteristics of palladium and platinum silicides were compared to the contact resistance obtained using a titanium contact layer. The contact resistance of palladium silicide increased with extended annealing at 400°C, while the PtSi and Ti contact materials exhibited stable contact resistance under these conditions. A Ti/Al-Cu/Si process which is compatible with a lift-off patterning technique and partial coverage of contacts is described. Rutherford backscattering results indicate that copper and silicon additions to the aluminum metallization retard the Ti-Al reaction. SIMS data show that silicon in Ti/Al-Cu/Si films redistributes during heat treatment, accumulating at the Ti/Al-Cu interface. View full abstract»

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  • Electrical and microstructural investigation of polysilicon emitter contacts for high-performance bipolar VLSI

    Page(s): 617 - 626
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (948 KB)  

    Key electrical characteristics of polysilicon emitter contacts in bipolar transistors, such as contact resistance and recombination velocity, are extremely sensitive to the microstructure of the polysilicon/single-crystal silicon interface. In this study, we correlated the microstructural and electrical characteristics of this interface by performing cross-sectional transmission electron microscopy (XTEM) on actual transistors on the same chip where ring-oscillator speeds were measured. The base current and emitter resistance of the fastest devices approached values typical of single-crystal silicon emitters. Interpretation of these electrical data and of the SIMS impurity profile indicates that significant restructuring of the polysilicon/single-crystal interface had taken place. This conclusion was indeed confirmed by the XTEM results. Although the low-current performance was degraded because of higher junction capacitances, the high-current switching speed was improved because of the minimal emitter contact resistance. Since the current gain was sufficiently high and very uniform, it is concluded from this work that minimization of both junction depth and contact resistance is the most important design consideration for high-performance submicron transistors, rather than maximization of the gain enhancement of the polysilicon/single-crystal interface. View full abstract»

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  • Study of contact and shallow junction characteristics in submicron CMOS with self-aligned titanium silicide

    Page(s): 627 - 633
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (568 KB)  

    The contact resistance between TiSi2 and shallow n+/p+ source-drains in CMOS is studied for a variety of junction depths and silicide thicknesses. The contact contribution to the total device series resistance can be significant if excessive silicon and dopants are consumed during silicide formation. Low contact resistances are obtained for 0.15-µm n+ and 0.20-µm p+ junctions when the titanium thickness is reduced to keep a high doping concentration at the TiSi2/Si interface. Alternatively, a nonstandard process can be employed to implant additional dopants into the titanium. A thin layer of dopants then out-diffuses into the silicon after the silicide reaction and anneal to help reduce contact resistance and leakage currents. The latter technique is more extendable to CMOS devices which require thicker titanium films and/or shallower junctions. View full abstract»

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  • Oxidation of Si-rich chemical-vapor-deposited films of tungsten silicide

    Page(s): 634 - 640
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (687 KB)  

    We have studied dry oxidation characteristics of Si-rich WSix thin films prepared by LPCVD directly on SiO2, with x = 2.7 for as-deposited films. It has been reported previously that thin (less than 100 nm) CVD tungsten silicide adheres well to SiO2. Using Auger depth profiling and Rutherford backscattering spectroscopies, we find that silicon in excess of stoichiometric WSi2 diffuses through the silicide toward the surface to form a SiO2 passivating overlayer. The extracted activation energy for this oxidation process is Ea = 1.2 eV, consistent with oxygen diffusion in SiO2. A similar value of Ea is found for WSix deposited on polysilicon. During the anneal, the stoichiometry x of WSix decreases monotonically with the annealing temperature, reaching x = 2 after 30 min at 900°C or 20 min at 950°C. Longer times or higher temperatures result in silicon depletion, with x = 1.7 after 30 min at 1000°C. At the same time, the resistivity of WSix also decreases from ≃90 Ω/□ for 1500 Å as-deposited film to 5 Ω/□ for the films annealed at 1000°C, the value obtained in a standard homogenization anneal. A scanning electron micrograph (SEM) of 0.5-µm fine lines patterned using e-beam lithography reveals that the integrity of fine line structures, their adhesion to SiO2, and their vertical profiles remain unchanged after the oxidation process. We suggest that such Si-rich tungsten silicide can be useful as a gate electrode without the polysilicon underlayer, since no extra passivation is necessary and reoxidation and homogenization steps in the FET processing sequence can be accomplished simultaneously. View full abstract»

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  • Correlation analysis of particle clusters on integrated circuit wafers

    Page(s): 641 - 650
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (770 KB)  

    Defect clustering results in correlations between the numbers of defects or faults that occur on integrated circuit chips located adjacent to one another on semiconductor wafers. Until now, it has been believed that correlations of this type were not accounted for in existing yield models. It is shown in this paper that such correlations are present in yield models based on mixed or compound Poisson statistics. A quadrat analysis of particle distributions on semiconductor wafers is used to compare data and theory. The results show that the theoretical correlation coefficients are in agreement with the experimental ones. It was also determined from the particle data how these correlation coefficients vary as the distance between quadrats is increased. This variation provides a convenient method for determining the cluster dimensions. View full abstract»

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  • Modeling and analysis of computer system availability

    Page(s): 651 - 664
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1123 KB)  

    The quantitative evaluation of computer-system availability is becoming increasingly important in the design and configuration of commercial computer systems. This paper deals with methods for constructing and solving large Markov-chain models of computer-system availability. A set of powerful high-level modeling constructs is discussed that can be used to represent the failure and repair behavior of the components that comprise a system, including important component interactions, and the repair actions that are taken when components fail. If time-independent failure and repair rates are assumed, then a time-homogeneous continuous-time Markov chain can be constructed automatically from the modeling constructs used to describe the system. Markov chains having tens of thousands of states can be readily constructed in this manner. Therefore, techniques that are particularly suitable for numerically solving such large Markov chains are also discussed, including techniques for computing the sensitivities of availability measures with respect to model parameters. A computer system modeling example is presented to illustrate the use of these modeling and analysis techniques. The modeling constructs, automatic Markov-chain construction, and model-solution methods have been implemented in a program package called the System Availability Estimator (SAVE). View full abstract»

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  • Ferroresonance

    Page(s): 665 - 678
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (930 KB)  

    This paper describes a mathematical model for ferroresonant circuits that addresses some of the deficiencies of earlier analyses of ferroresonant regulators. Derived using piecewise-linear, normalized differential equations, the model accommodates nonlinear behavior and predicts circuit performance in terms of parameters such as line voltage, frequency, and load. A phase-plane analysis is used to simplify the determination of linear regions of operation between nonlinear events. Numerical solutions of the resulting equations are used to generate time-domain and parametric performance curves. The results compare well with experiments and suggest potential applications in the design of high-frequency voltage regulators. View full abstract»

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  • Font design for high-speed impact line printers

    Page(s): 679 - 684
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (475 KB)  

    In impact line printers that use print-band (or similar) technology, the higher speed required of the type band for higher print throughput results in wider printed strokes with increased slur. Ordinarily, font designers compensate for the increased printed strokewidth by narrowing the width of the engraved characters on the type band. While this approach corrects the total printed character stroke, the print quality is degraded because of increased slur. This paper presents an alternative design approach in which an examination of the essential parameters of print dynamics suggests a font design that incorporates wider strokewidths. View full abstract»

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  • Recent publications by IBM authors

    Page(s): 685 - 691
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (572 KB)  

    The informationl isted here is supplied by the Institute for Scientific Informarion and orher outside sources. Reprints ofthe papers may be obtained by writing directly to the first author cited. Information on books may be obtained by writing the publisher. Papers and books are listed alphabetically by author. View full abstract»

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  • Recent IBM patents

    Page(s): 692 - 693
    Save to Project icon | PDF file iconPDF (175 KB)  
    Freely Available from IEEE
  • Author index for Volume 31

    Page(s): 694 - 696
    Save to Project icon | PDF file iconPDF (193 KB)  
    Freely Available from IEEE
  • Subject index for Volume 31

    Page(s): 697 - 700
    Save to Project icon | PDF file iconPDF (280 KB)  
    Freely Available from IEEE

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The IBM Journal of Research and Development is a peer-reviewed technical journal, published bimonthly, which features the work of authors in the science, technology and engineering of information systems.

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Clifford A. Pickover
IBM T. J. Watson Research Center