Tungsten Silicide Films Research Articles

Effective elastic constants of thin-film tungsten-silicide from surface acoustic wave analysis

The dispersion of surface acoustic waves propagating on single-crystal (100) silicon substrates covered by thin films of tungsten-silicide has been investigated using an acoustic material signature technique at a frequency of 339 MHz. The tungsten-silicide layers (500–2500 Å) were deposited using a co-sputtering technique, isochronally annealed at temperatures of 700 and 850 °C for 45 min in an argon atmosphere and examined using Rutherford backscattering spectrometry. Dispersion curves were calculated using a Tiersten model [J. Appl. Phys. 40, 2 (1969)] and fitted to the experimental dispersion data. The effective elastic constants for annealed and ‘‘as-deposited’’ layers were determined as best-fit parameters.

Raman Scattering from Rapid Thermally Annealed Tungsten Silicide

ABSTRACTRaman Scattering as a technique for studying the formation of tungsten silicide is presented. The tungsten silicide films were formed by rapid thermally annealing tungsten films that were sputter deposited on silicon substrates. The Raman scattering data is correlated with data from resistivity measurements, Auger and Rutherford Backscattering measurements, and scanning electron microscopy.

Growth of tungsten silicide films by low pressure CVD method

Growth of tungsten silicide films on (111)-oriented Si substrates through the reactions via WF 6 and SiH 4 gases in a vertical type low pressure chemical vapour deposition (LPCVD) system has been investigated. It is found that the properties of the silicide film are highly dependent upon the flow rates of the reacting gases and the substrate temperature. If the substrate temperature is in the range 350–450°C and the ratio of flow rates for WF 6 to SiH 4 (26% diluted in H 2) is 1:1.35 then a smooth, pinhole free, WSi 2 film can be obtained at growth rates between 250 and 350 Å min −1. Changes in film properties after annealing at temperatures igher than 700°C have also been found. After annealing, the films exhibit well-defined crystallinity and grain size can be as large as 600 Å. In addition, the sheet resistance decreases due to a change of structure from hexagonal WSi 2 to tetragonal WSi 2 which is accompanied by an improvement of the crystallinity and the growth of the WSi 2 grain size.

Characterization of tungsten silicide films from a cold-pressed, vacuum-sintered composite target

Thin tungsten silicide films were deposited onto Si(100) wafers from a cold-pressed, vacuum- sintered composite target in a Varian 3180 sputtering system. The films were annealed in a Varian IA-200 rapid isothermal annealer, resulting in high-quality films of ±5% thickness uniformity and a low resistivity of 45 μΩcm. Variation of the film composition is observed when depositing the films with heated or voltage-biased substrates. We conclude that the historical problems associated with composite target preparation of sputtered silicides, i.e., low purity and poor mechanical integrity of the target, have been overcome.

Changes in Resistivity and Composition of Chemical Vapor Deposited Tungsten Silicide Films by Annealing

Chemical vapor deposited (CVD) tungsten silicide films were formed by a cold wall reactor. These films were annealed in to investigate changes in resistivity, composition, thickness, and impurity. The change in resistivity after 1000°C annealing becomes larger as the film reaches the stoichiometric value. A composition change occurs in a film whose composition Si/W is more than 2.6. Excess Si in the films is segregated in the boundary between and poly‐Si. A thickness change of about 15% occurs after 1000°C annealing at on ; this value is smaller than the calculated value. F and H, which are impurities in films decrease gradually and diffuse into gate after 1000°C annealing.

Analysis of the effects of annealing on resistivity of chemical vapor deposition tungsten–silicide films

Chemical vapor deposition WSix films were formed with varying compositions and film thicknesses. The were films annealed in N2 for 30 min and their resistivities were measured. The maximum resistivity was obtained for annealing temperatures between 500 and 600 °C if the film was comparatively richer in tungsten and was thick. The film was analyzed by x-ray diffraction and secondary ion mass spectrometry. The maximum resistivity is considered to be related to crystallization, which changes amorphous WSix into a mainly hexagonal structure, and to the difference in the conduction mechanism of electrons between amorphous, hexagonal, and tetragonal WSi2. It is clear that impurities in the film do not contribute to the maximum resistivity. There is also no relationship between resistivity and the number of grains of tetragonal WSi2 per unit length for annealing below 600 °C.

Low-temperature annealing characteristics of chemical vapor deposited WSi2 film

Resistivity annealing characteristics of tungsten silicide films made by the chemical vapor deposition technique at 0.3 Torr and 275–425 °C has been investigated paying particular attention to their low temperature annealing behavior. The Si-to-W atomic ratio was controlled in the range of 2.1–4.1 by means of a substrate temperature. Resistivity of the annealed sample with the lower atomic ratio is shown to increase initially in the temperature range below about 500 °C. It was concluded that this increase is explained neither by impurity effect nor by compositional change of Si and W as detected by Rutherford backscattering spectroscopy and secondary ion mass spectroscopy. X-ray diffraction analysis shows that this phenomenon is rather well correlated with the formation of hexagonal phase of WSi2.

Characterization of W/WSIx/Si Contact Structures under Thermal Annealing for Some X Values

ABSTRACTThermal stability of W/WSix/Si (X=1.4, 2.1, 2.5) contact structures has been characterized at annealing temperature up to 900°C. Thermal reaction in these contact structures was strongly affected by the X values of deposited WSix films. The X values were presicely controlled by a novel co-sputter deposition apparatus. After annealing at 900°C, in the samples of X=1.4 and 2.1, the W films were stably maintained. On the other hand, in the sample of X=2.5, the structure changed into the WSi2. 2/Si resulting in the disappearance of the W film. The silicidation of the W film in the sample of X=2.5 was likely due to a diffusion of Si into the W film through the precipitated region of the excess Si in the tungsten silicide film. The W/WSi2.2/Si structure can provide the electrically stable W-Si contact at annealing temperature up to 800°C.

Analysis of stress in chemical vapor deposition tungsten silicide film

Stress in chemical vapor deposition (CVD) tungsten silicide film was studied by changing the flow ratio of SiH4 and WF6, deposition temperature, and annealing temperature. It was revealed that the stress is tensile on the order of 109 dyn/cm2. Stress is not significantly influenced by the flow rate of SiH4, but it is greatly influenced by the composition ratio of Si and W. As the film becomes Si rich, the stress is reduced remarkably, and after annealing its value peaks at 500 °C. This behavior was analyzed using an x-ray diffractometer and secondary ion mass spectrometry (SIMS). Annealing behavior of the stress can not be explained solely by the Nakjima–Kinoshita model which indicates that increase of tensile stress is caused by grain size increase, but it can be fully explained by adding to the model an expression which takes the grain structure and orientation into account. A tensile stress increase accompanies reaction between the tungsten silicide film and Si substrate at a high temperature of 1100 °C.

Effects of biased cosputtering on resistivity and step coverage in tungsten silicide films

Electrical resistivity and step coverage on thick oxide steps of various slopes were examined in tungsten silicide films sputter deposited by multilayering of W and Si utilizing magnetron targets. The desired composition was obtained by adjusting power levels to the targets and bias voltage. A broad minimum in electrical resistivity was obtained with bias voltage ranging from −40 to −80 V. Typical resistivity in films of 2500 Å thickness after annealing at 1000 °C was 52–66 μΩ cm when deposited on poly-Si substrates. The effect of bias was found to produce films with much lower levels of impurity such as N, O, C, and H and the sputtering gas Ar. Proper bias voltage (−50 V) was necessary to achieve good step coverage on thick oxide step with various slopes. Zero-bias sputtering provided minimum step coverage, while too high bias (−100 V) led to poor step coverage and formation of rounded corners and faceting.

Capacitance–voltage characterization of silicide–GaAs Schottky contacts

Capacitance–voltage carrier concentration profiling has been used to investigate the high temperature stability of refractory metal silicide films on GaAs. This technique is more sensitive to silicide–semiconductor interactions than is forward I–V characterization since tenacious surface Fermi level pinning of GaAs can yield stable diode barrier height and ideality factor measurements even for some cases of gross silicide–semiconductor interaction. Using C–V characterization we have found tungsten silicide film compositions that exhibit excellent high temperature stability on GaAs and have suggested failure mechanisms for other less stable film compositions.

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).

Electrical transport properties of tungsten silicide thin films

The electrical transport properties of cosputtered tungsten silicide films were investigated. The microstructure of the annealed film was determined by x-ray diffraction. Both resistivity and Hall coefficients for the WSi2 films were measured in the temperature range 80–300 K. The current carriers in tungsten disilicide were found to be positive holes. The carrier concentration determined from this experiment is ∼1×1022 cm−3 which does not change with either annealing process or measuring temperature. The resistivity of the WSi2 film in the measured temperature range increases approximately linearly with temperature. The carrier mobility and its temperature dependence were also studied.

Formation and characterization of tungsten silicide layers

Tungsten silicide films formed via furnace annealing were studied. The tungsten layers were deposited either by evaporation or by r.f. sputtering onto Si(100) substrates as well as onto silicon layers deposited in situ. Tungsten deposited at room temperature yields poor silicides owing to the lack of permeability at the interface with silicon. This as well as the formation of voids in the substrate are discussed. Deposition onto substrates heated to 500 °C, however, always allows the formation of a silicide during subsequent annealing.

A critical comparison of silicide film deposition techniques

Three different film deposition techniques are compared. WSi 2 was chosen as a test sample. Fabrication of films with the desired composition and lowest electrical resistivities are emphasized. Rutherford backscattering analysis was found to be a very useful tool to perform rapid non-destructive examination of in-depth composition variation and determination of relative thickness. Co-evaporation of tungsten and silicon was found to provide films with the lowest resistivity but was the most difficult method of film deposition. Co-sputtering is slightly simpler than co-evaporation but it is preferable to sputter from a compound or composite target of high purity if available. Low pressure chemical vapor deposition is the simplest method for tungsten silicide film deposition with uniformity and high throughput. However, these films contain more impurities, and thus the resulting resistivities are higher than those of purer films deposited by other techniques.

Conductivity changes in tungsten silicide films due to rapid thermal processing

Several limitations of the polysilicon gate in VLSI have led to the development of a silicide/polysilicon material as all alternative to polysilicon. Recently, rapid thermal processing has been investigated for annealing such polycide films. We report here the electrical-conductivity changes during the process of rapid thermal annealing in CVD tungsten silicide films. It is shown that electrical resistivity initially increases due to changes in the silicon to tungsten ratio and then drops to about one-tenth of the initial value, thus suggesting a minimum time and power required for achieving low-resistivity tungsten silicide films in VLSI interconnections.

Early stages of silicide formation on W, Ni, and Pt surfaces, an atom probe, and field ion microscope study

The atomic structures and composition of silicide layers grown on W, Ni, and Pt surfaces have been studied in the atom probe and the field ion microscope. Four stages of silicide growth on W surfaces have been identified. On the W{112} plane, the atomic image structure of the top surface layer in the first stage, and the first three atomic layers in the second stage of silicide formation do not correlate well with the atomic image structures of WSi2. The W{110} plane does not react with Si atoms during the first two stages. In the third stage, the growth of thin WSi2 films proceeds nearly epitaxially from the W{001} planes to cover the entire W substrate, forming mismatches of silicide lattices along the [110] zone lines of the W. In the final stage, thick layers of polycrystalline silicides are formed. Their orientations are no longer well correlated to the W substrate. Atom-probe analyses show the stoichiometry of these atomically perfect polycrystallines to be WSi2. Identifications of crystal planes of WSi2 can be best done by comparing the image structures with the atomic arrangements of W in the silicide surface planes since the majority of Si atoms are not imaged in the field ion microscope. A pure Si atomic layer on top of the WSi2 (112) plane gives a faint image of ∼1% the regular image intensity of a W layer. FIM images show the W–WSi2 phase boundary to be very sharp. Thick layers of silicide of platinum and nickel have also been analyzed in the atom probe. The platinum silicide layers near the Pt interface have the composition of Pt2Si. Beyond the relatively sharp interface, a few Si atoms can still be found tens of atomic layers inside the platinum matrix. The composition of nickel silicide varies from one region to another. No sharp boundaries seem to exist between different phases. We also find the thick tungsten silicide films to be resistive to oxidation.

プラズマＣＶＤ法により形成したタングステンシリサイド膜の評価

Refractory metals and their silicides are the major candidates for low resistivity interconnect material for replacing polysilicon. Their films are commonly deposited by co-sputtering or evaporation method. In this paper, tungsten-silicide films are prepared by plasma CVD and their properties are studied by means of X-ray diffraction, Auger electron spectroscopy (AES), and Rutherford back-scattering (RBS). The film properties depend on souce gas ratio (SiH4 to WF6), discharge frequency and annealing temperature.When the source gas ratio is small (SiH4/WF6=1), pure tungsten films are obtained, whereas tungsten-silicide films are obtained, when the ratio is large (SiH4/WF6=4). By means of RBS, the ratio of Si/W is found to increases from 0.1 to 12 with the source gas ratio. When the ratio of Si/W=0.1, the resistivity decreases to 1×10-5 ohm-cm after annealing. The decreasing rate depends on deposit frequency and it is found the difference is due to the structural difference of the film.It is found by X-ray diffraction analysis that the films deposited with small gas ratio had peaks reflecting surface orientations, most intensive of which are W (200) and W (110) for those of 50 kHz and 13.56 MHz, respectively. When the source gas ratio is large, however, it shows the peaks typical of bulk tunsten at as-deposited. After annealing, only films deposited at 50 kHz show tungsten-silicide peaks.

Electrical Properties of Composite Evaporated Silicide/Polysilicon Electrodes

MOS‐type capacitor structures were fabricated employing a composite polycrystalline silicon/refractory metal silicide electrode. Electrical characterization of oxidized structures has revealed that their behavior, compared with that of polysilicon devices, can remain unaltered by the addition of the conductive tungsten or molybdenum silicide film (other than an approximately 10–15× reduction in net resistivity of the electrode material). This compatibility, however, requires a minimum polysilicon thickness, or else an abnormally high incidence of shorting occurs following oxidation. The required thickness of the underlying polysilicon is larger than that required only to provide sufficient silicon for the oxidation process.

Resistivity and oxidation of tungsten silicide thin films

The resistivity and oxidation properties of sputtered tungsten silicide films were examined for nominally stoichiometric films and for films containing excess silicon. The films containing excess silicon had a lower resistivity and a larger grain size after annealing and better oxidation characteristics than did the nominally stoichiometric films.