Refractory Silicides Research Articles

Reaction-formed silicon carbide

The reaction bonding of silicon carbide (SiC) typifies liquid-solid reaction processes for the synthesis of refractory ceramic composites. These processes have particular advantages over conventional sintering and hot-pressing techniques in their lower processing temperatures, shorter times and near-net shape fabrication capabilities. Two particular modifications that we have employed in order to improve the mechanical properties and the ultimate use temperature of reaction-bonded SiC are the use of microporous carbon pre-forms derived from polyfurfural alcohol for refinement of microstructure, and the use of alloyed melts in order to replace detrimental residual silicon with a refractory silicide. The control of reaction rate is always a key issue in reaction processing. We have studied the kinetics and mechanisms of the liquid SiC reaction. Experiments on carbon fibers and plates show that the principle mechanism is one of solution-reprecipitation. There is an increased solubility of carbon at very fine SiC particles formed by the spallation of the misfitting carbide from the carbon interface, leading to reprecipitation of SiC at defective seed crystals. Molybdenum and boron at low concentrations (3.2 mol.%) have little effect on reaction kinetics, whereas aluminum is able to impede the reaction through the formation of an interfacial carbide layer.

High reliability sputtered Schottky diodes on GaAs

We describe developments to improve reliability and power handling for high frequency applications in whisker-contacted Schottky barrier diode mixers, detectors and multipliers fabricated using sputtered refractory metals and silicides. The lift-off process has been used to fabricate diodes on GaAs with ideality factors of 1.18 and 1.06 for W/GaAs and TiSix/GaAs contacts respectively.

High Electron Mobility Transistors with Optically Processed Refractory Silicide Metallizations: Thermal and Microwave Analysis

ABSTRACTThe enhanced high temperature gate metallizations consisting of sputtered TiWSi or TiWN were investigated in order to attain high temperature stability at temperatures in excess of 250°C. The TiWN/Au system resulted in a sheet resistance of only 11.5 mΩ/□ while TiWSi/Au resulted in 75.0 mΩ/□. The HEMTs and FETs processed with additional stable ohmic contacts of epitaxial Ge/Pd structures exhibited a stable transconductance of 160 -180 mS/mm at temperatures of 300°C. Thermal analysis indicated the peak junction temperature increase with an input power of 200mW to be less than 18°C at substrate temperature of 60°C.

Gate-resistance-limited switching frequencies of power MOSFETs

High-frequency switching limitation of a power MOSFET resulting from large gate resistance is studied. It is shown that a maximum gate switching frequency can be identified to minimize resistive power dissipation in the gate. Power MOSFETs with refractory silicide gates are shown to result in more than a fivefold improvement in this frequency compared to conventional heavily POCl/sub 3/-doped polysilicon-gated MOSFETs with metal gate runners.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effect of gate resistance on high-frequency power switching efficiencies of advanced power MOSFETs

Accurate gate resistance values for large-area power MOSFETs have been obtained from calorimetric power loss measurements. It is shown that advanced power MOSFETs fabricated using refractory silicide gates exhibit a fivefold reduction in the gate resistance compared to conventional power MOSFETs with heavily POCl/sub 3/-doped polysilicon gates. Power MOSFETs with integral Schottky diodes are shown to further reduce the resistive-gate power dissipation. A simple analysis is used to evaluate the switching efficiency of power MOSFETs as a function of output current and switching frequency.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Refractory and silicide gate metallisations for GaAs MESFET's

Abstract Driven by the need for high device reliability in MESFET power transistors operating at elevated temperatures, and more recently the development of self-aligned gate structures, high stability contacts are being investigated in many laboratories. This paper will consider two classes of metallisations that have proved very promising. These include the refractory metals such as W, Ta and Ti together with the refractory silicides such as TaSi x TiSi x and WSi x . The work carried out in this area will be reviewed and appraised. The basic interface has been characterised by the use of RBS and SIMS studies. More recent studies have been concerned with the use of these results in the fabrication and evaluation of transistor structures.

Removal of End-of-Range Ion Implantation Defects in Silicon by Near Noble and Refractory Silicide Formation

AbstractComplete removal of end-of-range (EOR) defects in ion implanted silicon has been achieved by the formation and growth of near noble silicides (CoSi 2 and NiSi 2 ) and refractory silicides (MoSi 2 and WSi2). Continued generation of vacancies during the silicide growth was found to be essential to reduce EOR defects. The results are consistent with the suggestion of the presence of a vacancy diffusion barrier near the EOR defects

A study of the oxidation of selected metal silicides

I n situ ellipsometry was used to investigate the oxidation behavior of a group of silicides, CoSi2, CrSi2, Ir3Si5, Ru2Si3, and WSi2, on single-crystal silicon substrates. These observations were complemented by ex situ ellipsometric measurements on these silicides, as well as on Mn11Si19, MoSi2, NiSi, NiSi2, and ReSi2. Refractive index measurements necessary for the oxide thickness calculations were made using ellipsometry on the bare silicide surfaces. Three regimes of oxidation behavior were identified: one group of silicides oxidized much faster than Si itself, CoSi2, CrSi2, NiSi, and NiSi2; another group was intermediate in oxidation rate between the fast group and Si at low temperature, typically comprising the refractory silicides MoSi2 and WSi2; the third group oxidized at about the same rate as Si and included the semiconducting silicides Ir3Si5, ReSi2, and Ru2Si3. The oxidation of Mn11Si19 led to an oxide containing a small amount of Mn. The oxidation rates of the silicides scale with both the reported carrier concentrations and the measured absorption index. Thus, the electronic properties of the silicides are thought to be responsible for the oxidation behavior. Buffered HF etch rates of the oxides grown on silicides indicate that the oxides grown on the rapidly oxidizing silicides are less dense than the oxides on the intermediate and slowly oxidizing silicides.

Contact and metallization problems in GaAs integrated circuits

Despite the rapid maturing of GaAs as an integrated circuit technology, both for digital and analog applications, the rather empirical nature of our contact metallurgy betrays a lack of basic scientific understanding of the metal–semiconductor interface and its electrical properties. Schottky barrier heights to GaAs are considered to be fixed and determined by Fermi level pinning due to surface states. On the scale of the control of barrier height required for integrated circuits (tens of millivolts), this is not the case. We have no accepted model which allows us to predict barrier heights on ideal barriers, much less ones on processed real surfaces. Where film stress is present, such as with refractory silicides used for self‐aligned gate metal gate field‐effect transistors, run‐to‐run variations may amount to hundreds of millivolts. Schottky barrier heights of metal–AlGaAs barriers, used with modulation‐doped field‐effect transistors, are now known, but not to the desired accuracy. Ohmic contacts are required to both n‐ and p‐type material over a large doping range, including contact to the two‐dimensional gas in modulation‐doped devices. The widely used alloyed contact does not appear to be extendable to lower contact resistance, better area uniformity, or processing temperature sensitivity. Some novel alternatives will be discussed, and their potential accessed. The need for minimal contact resistance to p‐doped bases of heterojunction bipolar transistors has focused attention recently on contacts to p‐GaAs. There is a distinct trend toward the use of refractory metal contacts due to the elevated temperatures to which they are subjected during processing which cause severe degradation to the presently used gold‐based metallurgy.

Self-aligned silicides or metals for very large scale integrated circuit applications

The increases in the packing density and the resulting shrinkage of the silicon integrated circuit dimensions led to the investigation and successful application of the deposited refractory silicide layers as the gate and interconnection metallization. The continued shrinking of the device dimensions has now turned attention to further lowering of the resistance at the gate level and to finding a contact metallization for the shallow junctions. Although refractory metals are being considered for the former, self-aligned silicides of cobalt, titanium, platinum, and nickel offer the possibility of satisfying both the gate and interconnection and contact metallization requirements. This paper will review the present status of the refractory silicide and refractory metal technologies and compare them with the upcoming self-aligned silicide technology.

Codeposited silicides in very-large-scale integration

Recently, with a view to their application as gate and interconnection metallization in metal/oxide/semiconductor devices, silicides have been formed by codepositing a metal and silicon mixture in the desired composition followed by annealing at higher temperatures. In this paper the codeposition of silicide films is reviewed. Both refractory and group VIII metal silicides have been deposited in this manner. Co-sputtering from two elemental targets for research purposes, and from one sintered target for routine depositions, are the preferred methods. Unannealed as-deposited films are conducting, and they crystallize on annealing to form lower resistivity silicides. The resistivity depends on the silicon-to-metal ratio and the contaminants. Other properties and processing factors are also discussed. The possible existence of a range of homogeneity in the refractory silicides, but not in group VIII metal silicides, is suggested.

Interactions of amorphous alloys with Si substrates and Al overlayers

The reactions of Co-Mo, Co-Ta, and Ni-Ta amorphous films with Si substrates and Al overlayers were analyzed by a combination of backscattering spectrometry, transmission electron microscopy, and x-ray diffraction measurements. When the alloy is in contact with Si, the reaction temperature lies between 550 and 650 °C and phase separation is generally observed with near noble silicides formed next to the Si substrate and refractory silicides formed on the surface. When the alloy is in contact with Al, the reaction occurs at temperatures of ∼450 °C with a two-layer structure in the final product. The crystallization temperature of an amorphous alloy is generally higher than its reaction temperature and there is no correlation between the two parameters. The reaction takes place when the annealing temperature reaches the value at which compounds can be formed with both constituents of the alloy.

Thin Film Properties of Sputtered Niobium Silicide on SiO2, Si3 N 4, and N+ Poly‐Si

Thin film properties of niobium silicide sputtered from a slightly silicon‐rich , cold‐pressed alloy target onto , , and n+ doped poly‐Si have been investigated. The structural and compositional properties were examined with x‐ray diffraction, Rutherford backscattering spectrometry (RBS), and secondary ion mass spectrometry (SIMS). X‐ray diffraction revealed that was the predominant silicide phase present, unlike those films reported previously, which contained significant amounts of an intermediate silicide phase . These films had a Si/Nb ratio of 2.1 as determined from RBS and contained lower levels of common contaminants (such as , , and carbon). Isochronal and isothermal annealing showed that the major decrease in resistivity occurred in the first 5 min, and a resistivity value of ∼70 μΩ‐cm was obtained after annealing at 1000°C. During annealing, phosphorus was found to diffuse through rapidly, similar to other refractory silicides.

4507852 Method for making a reliable OHMIC contact between two layers of integrated circuit metallizations

A method of fabricating a semiconductor integrated circuit by providing a semiconductor body having a major surface; depositing a first layer of a conductive material on the major surface of the semiconductor body, and depositing a layer of a refractory silicide on the first layer of conductive material. Portions of said refractory silicide layer are marked to define a first pattern thereon; and the silicide layer is etched down to the first conductive layer in order to produce the pattern defined by the masking step. Portions of the body are masked again to define a second pattern thereon; etching said first conductive layer in said second masking pattern. The semiconductor body is sintered to stabilize the contact between first conductive layer and layer of refractory silicide. A layer of dielectric material is deposited on body. Portions of said dielectric material are masked to define a third pattern thereon; and the dielectric material is etched to silicide layer in accordance with the third pattern. A second layer of conductive material is deposited on body; and the body is sintered again in order to stabilize the contact between the second conductive layer and the silicide layer.

Epitaxial growth of FeSi2 in Fe thin films on Si with a thin interposing Ni layer

Thin interposing Ni layers between Fe thin films and substrate Si have been shown to be very effective in inducing the growth and improving the quality of epitaxial FeSi2 on silicon. The formation of a transition layer with graded concentration is conceived to facilitate the epitaxial growth of FeSi2 on silicon. The thin interposing layer scheme may be extended to promote the epitaxial growth of a number of refractory silicides on silicon.

Localized epitaxial growth of hexagonal and tetragonal MoSi2 on (111) Si

Both hexagonal and tetragonal forms of MoSi2 (h-MoSi2 and t-MoSi2) were grown epitaxially on (111)Si for the first time. The best epitaxy was obtained in samples annealed at 1050 °C for 1 h. The orientation relationships between epitaxial h-MoSi2 and Si were determined to be [0001]MoSi2//[111]Si and (202̄0)MoSi2//(202̄)Si whereas those between epitaxial t-MoSi2 and Si were analyzed to be [110]MoSi2//[111]Si and (004)MoSi2//(2̄02)Si. Regular interfacial dislocations, 100 Å in spacing, were identified to be of edge type with (1)/(6) 〈112〉 Burgers vectors. Preliminary investigations indicated that a number of other technologically important refractory silicides can all be grown epitaxially on silicon. The results present an exciting prospect to fabricate novel classes of devices with desirable characteristics.

Growth of thin films of refractory silicides on Si(100) in ultrahigh vacuum

Thin films (30–150 nm thick) of tungsten and molybdenum silicide were grown by evaporation under ultrahigh vacuum conditions on Si(100) substrates cleaned according to the classical procedures used in silicon molecular beam epitaxy. Two methods for silicide formation were studied: (1) metal deposition onto substrates at room temperature followed by in situ annealing; (2) metal deposition onto heated substrates at temperatures ranging between 600 and 800°C, i.e. higher than the threshold temperature for silicide formation (in this case the silicide grows during the evaporation of the metal). Excellent values of the resistivity were obtained in both cases despite the low annealing temperature and the thickness of the silicide layers. These results are discussed in terms of the crystal structure of the silicides observed using transmission electron microscopy. It was found that silicides produced by the second process grew epitaxially on Si(100).

Epitaxial growth of ZrSi2 on silicon with an ion beam mixing assisted scheme

Ion beam mixing has been demonstrated to play a critical role in growing refractory silicide epitaxially on silicon for the first time. Epitaxial ZrSi2 has been successfully grown on (111) and (001) Si. Best epitaxy was obtained in samples irradiated by As+ with an ion range close to the original Zr/Si interface to a dose of 1×1016/cm2 followed by 1100 °C annealing. The breaking of metal–oxygen bonds, dissolution of native silicon dioxide, as well as dispersion of impurities present at the interface by ion beam are thought to be beneficial in inducing the epitaxial growth of ZrSi2 on Si. The orientation relationships between epitaxial ZrSi2 and Si were found to be [010]ZrSi2//[001]Si, (002̄)ZrSi2//(22̄0)Si (with about 1° misorientation), [31̄0]ZrSi2//[112]Si, and (130)ZrSi2//(111̄)Si. Hexagonal and square networks of interfacial dislocations were observed. They were identified to be of edge type with (1)/(6) 〈112〉 or 1/2 〈110〉 Burgers vectors. The average spacings were measured to be 140 and 800 Å correspondingly. The results represent the first successful attempt to grow orthorhombic structure refractory silicide epitaxially on silicon.

Interactions in metallization systems for integrated circuits

Conductive films are required to provide interconnection between contacts on the devices and between devices and outside world. Aluminum has been the most popular metal. Conducting polycrystalline silicon (polysilicon) film has been the conductor for gate and interconnection. PtSi has been used as a Schottky barrier contact and also simply as a contact for deep junctions. Refractory silicides are now used as gate and interconnection metallizations. Titanium/palladium/gold or titanium/platinum/gold beam lead technology was successful in providing high reliability connection to the outside world. Several similar schemes have been suggested or used in the integrated circuits. A considerable amount of work is carried out in adopting such a metallization scheme in integrated circuits. These include a study of metallurgical and chemical interactions between various materials and effect of such interactions on the properties of the materials and devices. In this paper a review of these studies will be presented.

Stoichiometric shifts in cosputtered refractory silicide films during subsequent heat treatment

Stoichiometric shifts in the refractory metal silicides due to subsequent thermal processing have been an important consideration in VLSI processing. Optimal stoichiometry for interconnect application has been controversial. Shift in silicon-to-metal atomic ratio of the metal silicide films, due to heat treatment under inert and oxidizing ambients, have been studied. Existence of a thermodynamical equilibrium stoichiometry has been investigated by studying films on both silicon and silicon dioxide. The silicide films were deposited by planar magnetron cosputtering and Rutherford backscattering was the primary technique used for monitoring the stoichiometry.