Silicide Thickness Research Articles

Hot-carrier scattering in a metal: A ballistic-electron-emission microscopy investigation on PtSi

Ballistic-electron-emission microscopy (BEEM) has been used to study the PtSi/n-type Si(100) interface. Hot-electron transport has been investigated by measuring the ballistic-electron transmission as a function of position (BEEM imaging), energy (BEEM spectroscopy), and PtSi thickness (BEEM attenuation length). Hot-hole transport and electron-electron scattering have also been investigated as a function of these parameters. This study has been conducted on in situ fabricated PtSi/n-type Si(100) Schottky diodes in UHV with the silicide thickness ranging from 20 to 500 \AA{}. The PtSi films were granular and the BEEM transmissivity was found to be homogeneous on individual grains but strongly varying from one grain to another. We attempt to compare the absolute BEEM current level with existing models and find its limiting value for small silicide thicknesses to be close to 1 depending on the model considered, indicating efficient transmission due to strong forward focusing of the electrons in the base or enhancement due to multiple scattering within the grains. BEEM current as a function of silicide thickness shows a change from a rapid to a slower decrease at about 150 \AA{}. Reverse BEEM (RBEEM) attenuation lengths also show this qualitative behavior, but the attenuation with increasing PtSi thickness is distinctly weaker, indicating that BEEM and RBEEM intensities are differently influenced by the elastic and inelastic hot-carrier mean free paths. RBEEM spectra are found to be anomalous, indicating an anomalous distribution of the tunnel-injected hot holes in the base.

The process limitation for forming Ti silicided shallow junction by BF2+ implantation into thin polycrystalline Si films and subsequent Ti silicidation

Excellent shallow p+n junctions have been formed by implanting BF2+ ions into thin polycrystalline Si films and subsequent annealing; these junctions which show a leakage of 1 nA/cm2 and a junction depth of about 0.05 μm. An anomalous boron diffusion occurs when subsequent silicidation is carried out. Silicidation using 300-Å Ti just slightly affected the junction profile. However, the junction is considerably deepened for 600-Å Ti silicidation, yielding a resulting depth of about 0.11 μm. The large boron redistribution is attributed to the point defects induced by silicidation. Hence, proper silicide thickness should be chosen to retain the junction profile as well as to reduce the parasitic source/drain resistance.

Stress-induced precipitation of dopants diffused into Si and TiSi2 and CoSi2 implanted layers

As, P and B ions were implanted into TiSi2 and CoSi2 thin layers and subsequently diffused into the underlying silicon substrate either by a rapid thermal process or by a long annealing in a furnace. For all the species, shallow junctions (20-400 nm depth) were obtained with a high dopant concentration (1019-1020 cm-3) at the silicide/silicon interface. In the case of As, the comparison between the amount of diffused substitutional arsenic and the carrier profile indicates the presence of inactive substitutional atoms at the silicide/silicon interface. These As atoms form coherent precipitates at the silicide/silicon interface as evidenced by transmission electron microscopy cross sections. Precipitates were also found for P and B dopants, but for this latter species they were located into the silicon substrates at 30 nm from the interface. The precipitate formation has been associated with the high tensile stress induced by the silicide layer on the surface silicon region and to its influence on the solid solubility of the dopant. The precipitate density decreases if the silicide thickness is reduced, i.e. if the tensile stress produced by silicide layer becomes lower. These precipitates are easily dissolved during high-temperature annealing after removal of the silicide layer.

A photoemission study of erbium silicide ultra-thin films epitaxially grown on Si(111)

We have investigated the growth and the electronic structure of ultra-thin (0.2 to 6 ML) Er films deposited at room temperature on a Si(111)7 × 7 surface and subsequently annealed to 600°C using LEED, ARUPS, AES and XPS. Our aim was to study the early stages of the (epitaxial) erbium silicide/Si(111) interface formation. We have found two different regimes: 1. - In the submonolayer range, our data indicate the growth of two-dimensional islands having a (1 X 1) symmetry. The band structure of this phase has been mapped. - Above 1 ML, the film surface shows an asymmetric 3 × 3 R30° superstructure. The ARUPS spectra rapidly evolve towards those of thick silicide layers, although some differences remain clearly observable close to the Γ point. These results - complemented with AES and XPS data - will be discussed in relation with our present knowledge of the erbium silicide structure.

Kinetics of diffusion growth of silicides in silicon–thin-metal-film systems

A kinetic model for diffusional growth of silicides in thin-metal-film–silicon systems is proposed. The time dependence of the growth has been shown to be a function of the morphology of the growing silicide and the controlling diffusion process (diffusion in the film, interface diffusion). If the phase grows only in depth the parabolic dependence of silicide thickness h on time t in most cases follows the relation h≂t0.5. If silicide grows only in width w, then w≂t. In the case of simultaneous change of thickness and width when h/w=const the growth is proportional to t0.33.

Effects of BF2+ implants on titanium silicide formation by rapid thermal annealing

The formation of titanium silicides on Si implanted with different BF2+ dosages has been studied by secondary ion mass spectrometry and transmission electron microscopy measurements. The thickness of the silicide layer formed in the temperature ranging from 600 to 800 °C has been investigated as a function of the implanted BF2+ dosage up to 1×1016 cm−2. Annealing at 700 °C results in conversion of the titanium film into predominantly C49 TiSi2, and most of it is transformed into the C54 phase at 800 °C or higher, resulting in a lower sheet resistance (16 μΩ cm). The titanium silicide thickness formed after the rapid thermal annealing (RTA) treatment depends on the implanted BF2+ dosage, caused by the native oxide enhanced by increased damage. Boron is redistributed into the silicide layer up to the solid solubility limit during annealing, leading to an accumulation at the silicide/silicon interface. The lowest contact resistance (with a size of 0.7 μm×0.7 μm) of 35 Ω is obtained at the annealing temperature of 700 °C.

Highly selective sputtering of silicon from TiSi2 at elevated temperature

We demonstrate almost 100% selective sputtering of silicon from TiSi2 using a combination of low energy ion bombardment and elevated temperature. TiSi2 was prepared by annealing 1000-Å-thick Ti on (100)Si in He at 635 °C for 30 min to produce 2300-Å thick TiSi2. Ion beam etching was carried out using 300 eV argon with a flux of 0.27 mA/cm2 at temperatures from 33 to 700 °C. In situ sheet resistance measurements were used to monitor the decrease in silicide thickness as a function of time. Near room temperature, ion etching causes normal sputtering of the silicide. However, at temperatures of 500–700 °C, the sheet resistance remains almost unchanged during ion beam etching. Analysis by Rutherford backscattering, with and without Xe markers, shows that Si atoms sputtered from the surface of these high-temperature samples are continuously replaced by Si diffusing from beneath the silicide layer. The thickness and composition of the silicide remain almost unchanged, but the dramatic change in sputtering behavior shows that the surface is enriched in Si. Compared with the room-temperature values, the absolute sputtering yield of Ti at 500–700 °C is decreased by a factor of 5, and the yield of Si is increased by a factor of 2. The net result is almost 100% selective sputtering of silicon.

Ultra-shallow junction formation using silicide as a diffusion source and low thermal budget

Ultra-shallow p/sup +//n and n/sup +//p junctions were fabricated using SADS (silicide-as-diffusion-source) and ITS (ion-implantation-through-silicide processing) of 45-nm CoSi/sub 2/ films (3.5 Omega / Square Operator ) using a low thermal budget. The best junctions of either type were made by moderate 10-s RTA (rapid thermal annealing) at 800 degrees C, where the total junction depth, counting the silicide thickness, is believed to be under 60 nm. Diffusion-limited current predominated down to 50 degrees C in junctions made under these conditions. The initial implantation energy had only a minor effect on the junction leakage, where shallower implants required slightly higher temperatures to form low leakage diodes, resulting in diodes which were somewhat more susceptible to shorting during silicide agglomeration at high temperatures. The ITS scheme, where dopant is implanted slightly beyond the silicide, gives an equally low leakage current. Nevertheless, the ITS scheme gives deeper junctions than the SADS process, and it is difficult to control the position of the ITS junction due to silicide/silicon interface fluctuations. >

Incorporation of metal silicides and refractory metals in VLSI technology

The advantages and issues associated with the incorporation of metal silicides and the selective deposition of refractory metals into VLSI device technology are illustrated using examples from 1 to 0.25 μm CMOS technology where the silicide or metal are formed over a pre-existing junction. While the drive current characteristics, latch-up resistance, and series resistance of junction-clad devices ae generally improved, other characteristics, such as hot electron stability, threshold voltage control, and short channel effect may be adversely effected. Reducing the metal (silicide) thickness to reduce silicon consumption and thereby allow scaling the junction depth results in films having considerably higher resistivity and poorer thermal stability. The use of silicide as a diffusion source is shown to be one possible way to scale the technology to smaller dimensions while minimizing the scaling of the silicide thickness. Here we report low leakage (<10 nA/cm 2)n + and p + junctions where the junction motion beyond the silicide is believed to be less than 1 nm.

Thermal spike mixing in cobalt silicide formation

Ion beam mixing in cobalt silicide has been studied using silicon, argon and helium ions. Both Co-Si and Co-SiO2-Si systems have been studied, the latter in the form of coated silicon wafers which have not had their native oxide removed. Dose and temperature dependence and the effects of post-bombardment annealing have been studied. Oxide free samples show both temperature dependent and temperature independent mixing regimes. Mixing is retarded in samples with an oxide layer and this is reflected in the mixing activation energies which for Si+ ions are 0.90+or-0.28 eV and 2.17+or-0.79 eV for the two types of sample. There is evidence that implanted Si dilutes the interfacial barrier and enhances migration. The results indicate that the chemical species of the ion is important in determining the degree of mixing. Argon ion dose dependence experiments indicate a minimum dose of 1*1016 ions cm-2 for the onset of mixing (at 350 degrees C) in both types of sample. Temperature dependence studies have yielded mixing activation energies for Ar+ ions of 0.46+or-0.10 eV from the Co-Si samples and 0.64+or-0.18 eV for the samples with oxide present. These values are intermediate between those obtained for Si and Xe bombarded samples. He+ ion bombardment studies indicate that mixing is not associated with ionization spikes. Considerations of ion concentration, bombardment flux, silicide thickness and activation energies favour thermal spike mixing rather than models based on radiation enhanced diffusion.

Thermal stability of TiSi2 films on single crystal and polycrystalline silicon

The stability of selectively formed TiSi2 films on single crystal and polycrystalline silicon layers at elevated process temperatures is reported. Extensive electrical and analytical studies were performed to understand the high-temperature stability of TiSi2 films as a function of (i) substrate dopant concentration, (ii) titanium silicide thickness, (iii) silicide formation sequence, and (iv) silicide post-processing steps. It is shown that all four process variations have a profound influence on the thermal stability of TiSi2 films. It is observed that titanium silicide films formed on single crystal silicon are stable at higher processing temperatures compared to those formed on polysilicon substrates under similar conditions. The degradation of high-temperature stability of TiSi2 films on polycrystalline silicon can be related to increased number of defects and grain boundaries. It is shown that TiSi2 films can be successfully used in silicon integrated circuit applications where the post-silicide processing temperatures do not exceed 1000 °C.

Conduction mechanism in PtSi/Si Schottky diodes.

The conduction mechanism in PtSi/Si Schottky diodes has been studied in the temperature range of 80 to 300 K. Above 100 K the forward current-voltage (I-V) characteristic of the diodes is highly ideal and obeys the thermionic-emission theory including image-force lowering of the barrier. Certain diodes show a deviation from this behavior, which is due to carrier recombination in the depletion region. Factors that contribute to carrier recombination are a small area-to-periphery ratio and a thick silicide layer. Carrier recombination is also responsible for the soft behavior of the reverse I-V characteristic. The temperature dependence of the barrier height and the bias-independent position of the quasi-Fermi-level at the interface are shown to be associated with a high density of interface states. The activation energy found for the current transport at low temperatures suggests that these states are due to substitutional Pt atoms on the Si lattice.

Optical and electrical characterization of β-FeSi2 epitaxial thin films on silicon substrates

Electrical measurements have been carried out on epitaxial FeSi2 layers on silicon substrates, the silicide thickness being either 180 or 350 Å. A direct gap of 0.85 eV was measured by optical absorption. Current-voltage characteristics of mesa-structures Cr/Fe/FeSi2/Si show a p-type semiconductor behavior. Capacitance-voltage and capacitance-temperature data at different frequencies indicate a large response of deep levels or interface states. Admittance spectroscopy yields the activation energy and capture cross section of two levels. Finally an energy-band diagram is proposed.

Application of MBE-grown epitaxial Si/CoSi/sub 2//Si heterostructures for overgrown silicon permeable-base transistors

The fabrication of overgrown silicon permeable-base transistors (PBTs) is described starting from MBE-grown epitaxial Si/CoSi/sub 2//Si heterostructures. Gate fingers are patterned by Ar sputtering followed by low-pressure vapour phase epitaxial overgrowth at 800 degrees C and 0.2 mbar. Results on surface morphology, Schottky-diode characteristics, and PBT performance are presented. It has been recognized that a serious problem of Si PBTs is the field crowding effect at the rim of the thin silicide gate. It cases breakdown at low voltages and limits PBT performance. A diagram indicating the possible operating areas depending on silicide thickness, channel doping, and PBT geometry is presented. >

Influence of Structure on the Soft X-Ray Optical Properties of Metallic Multilayers

AbstractMultilayer thin film structures for reflecting soft x-rays are now being fabricated in a number of laboratories. However, understanding of. the optical properties of these structures is presently limited by lack of knowledge of the microstructure of the layers, as well as of the details of the interfaces. In this paper we present results from our studies of multilayers grown by molecular beam epitaxy (MBE), characterized in situ by reflection high energy electron diffraction (RHEED), low energy electron diffraction (LEED), Auger, and x-ray photoelectron spectroscopy (XPS), and characterized ex situ by scanning tunneling microscopy (STM), transmission electron microscopy (TEM), x-ray diffraction, and Rutherford back scattering (RBS). In the case of Mo/Si multilayers, we observe the formation of an amorphous interfacial silicide, which can have a positive effect on the performance of these evaporated multilayer mirrors. We observe a contraction in the period of these multilayers as the deposition temperature is raised from 50 °C to 250 °C, corresponding to an increase in the thickness of the interfacial silicide. This contraction indicates that the silicide is more dense than the average atomic density of its components. We also discuss Ag/B and Pd/B multilayers, which have very similar theoretical performance. However, due to differences in the multilayer structures formed, the actual performance of multilayers made from these materials is radically different. The structural differences originate from different growth modes for Ag and Pd on B.

Electronic and IR-optical properties of silicide/silicon interfaces

Responsivity measurements of silicide/silicon IR detectors, i.e. PtSi/Si and IrSi/Si are interpreted in the light of physical models for photoemission. The quantum yield for photoemission is composed of the optical absorption and the electrical emission yield which impose partially conflicting requirements for the optimization of the silicide thickness. For thin (2–6 nm) silicide films, the IR responsivity is enhanced by diffuse wall scattering of charge carriers at the interfaces. The enhancement is high near the long wavelength cut-off of the detector and decreases to shorter wavelengths due to inelastic scattering of mobile carriers for increasing photoexcitation energy. The electronic properties of the interface are also characterized by tunneling spectroscopy. The results indicate a disordered interface which is created by interdiffusion of the components at elevated temperatures for silicide formation. Interdiffusion increases the emission barrier and reduces the responsivity.

Comparison of transformation to low-resistivity phase and agglomeration of TiSi/sub 2/ and CoSi/sub 2/

The phase transformation and stability of TiSi/sub 2/ on n/sup +/ diffusions are investigated. Narrower n/sup +/ diffusions require higher anneal temperatures, or longer anneal times, than wider diffusions for complete transitions from the high-resistivity C49 phase to the low-resistivity C54 phase. A model is presented which explains this in terms of the probability of forming C54 nuclei on narrow diffusions and the influence of diffusion width on C54 grain size. The results are that more C49 and C54 nucleation events are required to completely transform narrow lines. For thin TiSi/sub 2/ (40 nm), there is a narrow process window for achieving complete transformation without causing agglomeration of the TiSi/sub 2/. The process window decreases with decreasing silicide thickness. A significantly larger process window is achieved with short-time rapid annealing. Similar studies are performed for CoSi/sub 2/ on n/sup +/ and p/sup +/ diffusions. No linewidth dependence is observed for the transformation from CoSi/sub x/ to CoSi/sub 2/. There is a broad process window from 575 degrees C to 850 degrees C using furnace annealing, for which the low-resistivity phase is obtained without causing agglomeration.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Infrared response from metallic particles embedded in a single-crystal Si matrix: The layered internal photoemission sensor

Infrared radiation at wavelengths of 1–2 μm has been detected in a new device labeled the layered internal photoemission sensor. The device structure, which is grown by molecular beam epitaxy, incorporates epitaxial CoSi2 particles with dimensions of 10–50 nm. Radiation absorbed by these particles photoexcites carriers into a surrounding single-crystal silicon matrix. A peak quantum efficiency of 1.3% is measured, which is approximately six times higher than in planar CoSi2 Schottky diodes with 5 nm silicide thickness.

Effects of stress in TiSi2 gate metal-oxide-silicon structures

Metal-oxide-silicon capacitors with TiSi2 gate were studied to determine the relation between gate-induced stress and the electrical characteristics of the system. TiSi2 is found to exert a compressive stress on the oxide film. With increasing silicide thickness, i.e., larger stress, the rate of trapping of injected electrons, which is a measure of the defect (trap) density, was found to increase. Moreover, the rate of generation of Si-SiO2 interface states upon electron injection shows a similar correlation with the stress.

Synthesis of chromium silicide with laser pulses

Thin chromium films, 60 nm thick, were deposited onto single-crystal silicon wafers. The samples were irradiated with 30 ns single pulses from a Nd: glass laser at fluences ranging from 0.4 to 2.25 J/cm2. Rutherford backscattering spectrometry, transmission electron microscopy and electron diffraction measurements evidence the formation of CrSi2 layers at the Cr/Si interface. The silicide thickness depends on the laser fluence.