Tungsten Silicide Films Research Articles

Superconducting nanowire single-photon detector made of ultrathin γ-Nb4N3 film for mid-infrared wavelengths

Superconducting nanowire single-photon detectors (SNSPDs) made of ultrathin δ-NbN films have been widely applied in both visible and infrared wavelengths. For mid-infrared (MIR) wavelengths, SNSPDs made of tungsten silicide films with a lower critical temperature were reported up to 9.9 μm wavelength. In this study, we demonstrate the potential of NbN-SNSPDs for use in MIR applications. SNSPDs made of γ-Nb4N3 films (critical temperature of 5.1 K for 6.5 nm thick films) instead of δ-NbN films were fabricated. The dependence of the normalized detection efficiency on the bias current indicated a clear trend toward a saturated plateau for wavelengths up to 2145 nm. The calculated cut-off wavelengths indicated the possibility of using γ-Nb4N3 SNSPDs for longer MIR wavelengths.

Promising Superconducting Materials for Highly Sensitive Detectors of the Infrared and Terahertz Ranges

Modern technologies of photonics, astrophysics, medicine and security systems have a demand for development of new types of sensitive detectors and/or optimization of existing ones. As an example, a strong demand exists for improvement of the characteristics of highly sensitive detectors based on superconducting materials. One way to optimize the performance of such detectors is to select a suitable superconducting material. This is due to the fact that the technical characteristics of devices are determined by relaxation mechanisms of nonequilibrium processes that occur in the material upon absorption of electromagnetic radiation. In this paper, we focused on the study of the relaxation of nonequilibrium processes in superconducting materials such as highly boron-doped polycrystalline diamond films, highly disordered titanium nitride (TiN) films and ultrathin amorphous tungsten silicide films (WSi). The experimental data allowed us to determine the temperature dependence of the inelastic relaxation time in the studied materials. These results can help us to evaluate the applicability of these materials for the different types of superconducting detectors.

Nonbolometric bottleneck in electron-phonon relaxation in ultrathin WSi films

We developed the model of the internal phonon bottleneck to describe the energy exchange between the acoustically soft ultrathin metal film and acoustically rigid substrate. Discriminating phonons in the film into two groups, escaping and nonescaping, we show that electrons and nonescaping phonons may form a unified subsystem, which is cooled down only due to interactions with escaping phonons, either due to direct phonon conversion or indirect sequential interaction with an electronic system. Using an amplitude-modulated absorption of the sub-THz radiation technique, we studied electron-phonon relaxation in ultrathin disordered films of tungsten silicide. We found an experimental proof of the internal phonon bottleneck. The experiment and simulation based on the proposed model agree well, resulting in tau_{e-ph} = 140-190 ps at T_C = 3.4 K, supporting the results of earlier measurements by independent techniques.

Tungsten Silicide Superconducting Nanowire Single-Photon Test Structures Fabricated Using Optical Lithography

Single-pixel fiber-coupled superconducting nanowire single-photon detectors (SNSPDs) operating at 1550 nm and utilizing amorphous superconducting tungsten silicide (WSi) films have proven ability to detect photons with: high system-detection efficiency (SDE) of up to 93%, low-jitter on the order of ∼150 ps, dark count rates of ∼1 kcps, and fast reset times on the order of tens of nanoseconds. Additionally, WSi SNSPD devices with 12-pixels have recently demonstrated downlink data rates of 79 Mbps between a terminal in orbit around the moon and a terminal on earth, as part of the Lunar Laser Communication Demonstration (LLCD) at the Lunar Lasercomm OCTL Terminal (LLOT). To further extend the performance of SNSPD devices for optical and quantum communication for terrestrial and space-based applications, the next generation of devices will need to incorporate hundreds to thousands of SNSPD pixels and to be free-space coupled. The wire widths necessary for optimal performance of WSi (∼120–220 nm) devices have to date been achieved using electron-beam lithography (EBL) to pattern photoresists for etch-back fabrication methods. The high cost and time to fabricate kilo-pixel arrays of SNSPDs using EBL will become prohibitive in producing such devices. Here, we report fabrication of a WSi SNSPD test structure with 64 pixels using optical lithography instead of EBL. Specifically, we used Canon EX3 and EX6 deep-UV (DUV) steppers with KrF excimer lasers <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$(\lambda=248\ \mbox{nm} ) $</tex-math></inline-formula> in the Micro Devices Laboratory at the Jet Propulsion Laboratory to fabricate the array. Dies with 8 × 8 pixels with 166-nm-wide wires were produced, with pixels having a 100 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu\mbox{m} $</tex-math></inline-formula> pitch in the vertical and horizontal directions. Two improvements were observed: 1) the time to pattern the 8 × 8 SNSPD pixels on 3.5 mm × 3.5 mm dies filling a 4-in Si wafer required ∼24 hours using EBL while optical lithography wrote the same dies in approximately 15 minutes; and 2) the cost to write one 4-in wafer using EBL was comparable to the cost for one optical mask for use in the stepper to write many 4-in wafers. While fabrication times and costs will vary from facility to facility, the improvements in speed and cost for optical lithography versus EBL are apparent, and this technological advance should scale and enable fast and rapid production of kilo-pixel arrays in the future.

Tungsten Silicide Alloys for Microwave Kinetic Inductance Detectors

Microwave kinetic inductance detectors are used to detect photons over a large range of wavelengths from submillimeter to X-ray. The common material requirements for this application are: high internal quality factor (Qi), high kinetic inductance fraction, long quasiparticle lifetime, and in the case of X-ray photons stopping power (i.e., dense, high atomic number materials). Superconducting tungsten silicide alloys have a tunable TC with silicon content, a high normal state resistivity, and a high density. In this work, we investigate the properties of thin films of tungsten silicide made of two different stoichiometry: WSi2 and W5Si3 with particular attention to their application to microwave kinetic inductance detectors. We present a study of the structural and transport properties of films deposited under different conditions for both stoichiometry. Quarter wavelength microwave coplanar waveguide resonators have been fabricated from films of both stoichiometry and we present measurements of the microwave properties of these films as well as quasiparticle lifetimes using X-ray photons.

Low Temperature Chemical Vapor Deposition of Silicon-rich Tungsten Silicide Films from Tungsten Hexafluoride - Disilane Pre-activated Mixtures

Abstract Kinetics of chemical vapor deposition (CVD) of silicon-rich tungsten silicide films from WF6/Si2H6 mixtures was investigated as a function of the configuration of a hot-wall tubular reactor. The step coverage within micron-sized trenches was analyzed to determine the overall sticking probability, which is the ratio between the film-forming species deposited on a surface and the flux of that species to the surface. The overall sticking probability was found in the range 0.01 - 0.30 as the deposition temperature was varied from 120 to 360°C. The activation energy was 25 kJ/mol. These sticking probabilities were lower than those for CVD of tungsten silicide film from WF6/SiH4 mixtures, which yielded better step-coverage profiles. The extinction temperature, Tex, below which no deposition was observed, ranged from 70 ‹ Tex ‹ 80°C. Tex = 70°C corresponded to an inner reactor diameter, d, of 22 mm, and Tex = 80°C corresponded to d = 11 mm. The dependence of Tex on d strongly suggests that radical chain reactions are involved in the CVD process. The reaction producing active intermediate species occurs in the gas phase and these active species are consumed on the surface of the reactor wall. Therefore, the volume to surface ratio of the reactor may be the key factor to control Tex, and an increase in d leads to a decrease in Tex. For Tex = 80°C, pre-heating of the gas phase can be used to induce the chain reaction and increase the silicon content in the tungsten silicide films.

Evaluation and Analysis of Physical Properties of Nanomaterials for Highly Sensitive Mechanical Sensing Devices

This technical note is a review of the contract research at Ritsumeikan University for the “Highly Integrated, Complex MEMS Production Technology Development” Project (“Fine MEMS” Project) sponsored by New Energy and Industrial Technology Development Organization (NEDO). Our research focuses on characterization of physical properties of nanoscale microelectronic materials and carbon nanotube (CNT) to elucidate new effects for potential applications in advanced micro/nano electromechanical devices, and consists of two main parts. The first part deals with experimental evaluation of mechanical, thermal, and electrical properties of tungsten silicide (WSi) and CNT thin films. We have clarified that the WSi film will be promising as a structural material for high-performance capacitive micro accelerometer. In the second part, piezoresistive effect of single crystal silicon nanostructures has been theoretically and experimentally investigated. The p-type ultra-thin silicon nanowire (SiNW), in particular <001> oriented SiNW, will be a good piezoresistor with a giant longitudinal piezoresistance coefficient. We have carried out the research in cooperation with Hitachi, Ltd. and National Institute of Advanced Industrial Science and Technology (AIST), and these cooperative relationships have greatly contributed to the advancement of the research and development. Achievements in the contract research are very promising and significant for mechanical sensing applications.

Kinetic study of chemical vapor deposition of WSi x films from WF 6 and SiH 2Cl 2: Determination of molecular size and reactivity of gas species

A kinetic study of chemical vapor deposition of tungsten silicide (WSi x ) films from WF 6 and SiH 2Cl 2 was done, focusing on the deposition rate profile, atomic composition ratio (ratio of Si/W), and the reactivity of film precursors. The deposition mode was changed from blanket WSi x deposition to selective W deposition by decreasing the ratio of SiH 2Cl 2 to WF 6 and also by decreasing the substrate temperature, T sub. The rate-determining step for film deposition was determined to be diffusional mass transport of the film-forming species. The molecular diameter of the film precursor was calculated from measured diffusion coefficients to be 6–7 Å, which is close to the diameter of WF 6 molecules. The temperature at which film formation was extinguished, T ex, was measured as a function of the inner diameter of the reactor. T ex was found to depend on the inner diameter of the reactor, which indicates that the first step of the WF 6/SiH 2Cl 2 process is controlled by radical chain reactions in the gas phase to form active intermediate species. The sticking probability of the film precursor was also calculated by measuring the step coverage in micron-sized trenches. For T sub > 600 °C, the sticking probability was about 0.35 with an activation energy of 19 kJ/mol. However, for T sub = 500 °C, the sticking probability decreased to 0.15, which corresponds to an activation energy of 61 kJ/mol.

Application of In-Situ Nitrogen Plasma Treated Amorphous-like Tungsten Silicide Film as Barrier Material in the Application of Copper Gate Electrode

An amorphous-like tungsten silicide film, deposited by chemical vapor deposition at a higher flow ratio of 2.5 of WF6/SiH4, was used as the barrier layer in the copper gate electrode. This film processes superior barrier characteristics to those of the typical tungsten film at a lower flow ratio of 0.4 of WF6/SiH4 because it lacks the fast diffusion paths of columnar grain boundaries. We proved that the film successfully suppressed copper atom diffusion toward the gate oxide during annealing at 600°C for 30 min. We also found that the amorphous-like tungsten silicide film exhibits better barrier characteristics after in-situ nitrogen plasma treatment at 300°C for 5 min.

Stress changes in chemical vapor deposition tungsten silicide (polycide) film measured by x-ray diffraction

Stresses in chemical vapor deposited polycide tungsten silicide (poly-Si/WSi2) were evaluated at each stage of the fabrication. The individual layers of the Si/SiO2/poly-Si/WSi2/poly-Si multilayered structure were deposited sequentially on separate wafers and subjected to x-ray diffraction analysis in the as deposited and annealed conditions to determine the changes in strain occurring in WSi2. Samples cut from wafers containing all the layers were either heat treated at 1193 K for 30 min or capped with a 25 nm thermal oxide and the strain in the WSi2 film was also analyzed by x-ray diffraction. The change in strain of the WSi2 layer, following each step of the fabrication process, was evaluated by the lattice parameter variation of the c axis. The stress is affected by the layers of the multilayered film. An as deposited poly-Si layer on top of WSi2 reduces its stress, since it introduces a compressive component. The stress in WSi2 is further reduced upon annealing. Poly-Si also maintains a Si supply at the poly-Si/SiO2 interface thus eliminating Si out-diffusion during heat treatment in an oxygen containing ambient. Capping the system by a thin oxide layer modifies the stress pattern of the WSi2 which becomes compressive.

Tungsten silicide for the alternate gate metal in metal-oxide-semiconductor devices

We proposed Si-rich tungsten silicide (WSix) films for the alternate gate electrode of deep-submicron metal–oxide–semiconductor (MOS) field effect transistors. The investigation of WSix films deposited directly on SiO2 indicated that the annealing of as-deposited films using a rapid thermal processor (RTP) results in low resistivity, as well as negligible fluorine (F) diffusion. Specifically, the resistivity of RTP-annealed samples at 800 °C for 3 min in vacuum was ∼150 μΩ cm, and the irregular growth of an extra SiO2 layer due to F diffusion during annealing has not been observed. In addition, the analysis of the WSix–SiO2–Si (MOS) capacitors exhibits excellent electrical characteristics.

Formation of tungsten silicide films by ion beam synthesis

In the present study, a MEVVA ion implanter was employed to implant tungsten ions into silicon wafers at an elevated temperature of 100°C. The acceleration voltage was 40 kV and the charge states of the implanted tungsten ions were 1 + (8%), 2 + (34%), 3 + (36%), 4 + (19%), and 5 + (3%). The ion fluences were 1×10 17 and 3×10 17 ions/cm 2. The as-implanted specimens were furnace annealed in Ar under various temperatures for 30 min. The tungsten silicide film was analyzed by secondary ion mass spectroscopy, X-ray diffraction, cross-sectional transmission electron microscopy, Rutherford backscattering spectroscopy, and a four-point probe. The results indicate that the tungsten silicide film thickness is approximately 30–40 nm, depending on annealing temperature as well as ion fluence. The sheet resistance of the tungsten silicide film closely correlates to the depth profile of ion-implanted tungsten, the Si/W ratio, the crystallographic structure, and the microstructure of the tungsten silicide film. The maximum sheet resistance was obtained at annealing temperatures of 400°C and 550°C for ion fluences of 1×10 17 and 3×10 17 ions/cm 2, respectively, while the minimum sheet resistance was obtained at annealing temperatures above 800°C for both ion fluences. The hexagonal crystallization phase of tungsten silicide, existing at annealing temperatures between 400 and 550°C and leading to smaller mean depth of the synthesized layer, can be clearly observed only at 3×10 17 ions/cm 2. The tetragonal crystallization phase of tungsten silicide, starting to form at an annealing temperature of 550°C and resulting in larger mean depth of the synthesized layer, is clearly observed at annealing temperatures above 800°C for both ion fluences. The microstructure of the as-implanted tungsten silicide film possesses amorphous and non-continuous properties as well as a rough surface. However, surface roughness can be markedly improved and a continuous and epitaxial layer of tungsten silicide can be obtained when the annealing temperature is increased to 800°C or higher.

Modification of Stress in CVD Tungsten Silicide (Polycide) Film by SiO2Capping

ABSTRACTStresses in chemical vapor deposited polycide tungsten silicide (poly-Si/WSi2) wereevaluated at each stage of fabrication. The individual layers of the Si/SiO2/Poly-Si/WSi2/Poly-Si multilayer structure were deposited sequentially on separate wafers and subjected to x-ray diffraction analysis in the as deposited and annealed conditions to determine the changes in strain occurring in WSi2. Samples cut from wafers containing all the layers were capped with a 25nm thermal oxide and the strain in the WSi2film was also analyzed by XRD. The change in strain of the WSi2layer, following each step of the fabrication process, was evaluated by the lattice parameter variation of the c axis. The layers of the multilayered film affect the stress in the WSi2. A poly-Si layer on top of WSi2reduces its stress, since it introduces a compressive component, which further decreases upon annealing. It also maintains a Si supply at the poly- Si/SiO2interface, thus, eliminating Si outdiffusion during heat treatment in an oxygen containingambient. Capping the system by a thin oxide layer modifies the stress pattern of the WSi2, which becomes compressive.

Stability of boron and phosphorus implanted tungsten silicide structures at high temperatures

We report on the stability of sputter-deposited tungsten silicide (WSi2.6) films that were implanted with boron or phosphorus and annealed at high temperatures using rapid thermal annealing. Depending on process conditions, some films were found to be unstable, resulting in peeling and lifting from the substrate. The regime of stability was experimentally determined and found to be a function of the implant dose, species, and anneal temperature. Films implanted with boron were found to be more stable than those implanted with phosphorus. With the appropriate choice of dopant dose, anneal temperature, and capping layer type, the film stacks were found to be stable upon subsequent furnace annealing steps at 900 °C.

Barrier Characteristics of Chemical Vapor Deposited Amorphous‐like Tungsten Silicide with In Situ Nitrogen Plasma Treatment

Our team investigated the characteristics of inserting an 80 nm amorphous‐like barrier layer between aluminum film and a shallow diode to retard aluminum and silicon interdiffusion. In one chamber without breaking vacuum, we used a nitrogen plasma treatment to stuff nitrogen atoms into the grain boundaries of amorphous‐like tungsten silicide film. The nitrogen atoms eliminated the fast diffusion paths of film, thus giving the amorphous‐like tungsten silicide film a smaller diffusion coefficient. We then examined the failure of diodes with amorphous‐like barriers which were annealed at 575°C for 30 min and which had leakage currents of and . These diodes failed due to the diffusion of aluminum along the sidewalls of the barriers and the field oxide interface. In the search for a solution to this problem we investigated the use of tetraethylorthosilicate, which is known for its thermal stability as a stress buffer. In this experiment we used tetraethylorthosilicate to form a “contact array structure” which in turn prevented diode failure at 575°C annealing for 30 min, and furthermore, the diodes in the contact array structure only began to show evidence of degradation at 600°C. © 1999 The Electrochemical Society. All rights reserved.

Preparation of WSI 2 by rapid thermal annealing of CVD thin films of tungsten

Tungsten silicide (WSi i ) films were prepared by rapid thermal annealing (RTA) of W films obtained by chemical vapor deposition (CVD) from carbonyl precursor—W(CO) 6. The RTA process proceeds at 800–1400°C in different gas environments—argon, nitrogen, vacuum, etc. Investigations of the crystal phase structure were performed by Reflection High Energy Electron Diffraction (RHEED) method. Difference in the phase composition was observed for thin and thick WSi 2. Close to the surface of W films, a pure metal rich phase is formed, and at the interface W\\Si, a phase rich of Si is found. Diffusion was considered to be the controlling process in the kinetics formation of WSi 2. The influence of the gas environment on WSi 2 was also studied. The comparison made shows that nitrogen favours WSi 2 formation. Some of the samples have shown a certain degree of texturing.

Formation of High Conductivity WSix Layer and its Characterization as a Gate Electrode

A novel integrated clustered electrode for gate electrode application has been developed. This method includes a sequential deposition of in situ phosphorus doped polycrystalline silicon and dichlorosilane‐based tungsten silicide films using a clustered platform without a vacuum break. The dopant atom, phosphorus, in the polycrystalline silicon substrate enhances the reduction of , resulting in the formation of an amorphous layer with a uniform thickness and a lower resistivity of 234 μΩ cm. Upon thermal annealing, the composition of the silicide was converted into with an accompanying thickness increase of the silicide. At the interface of /polycrystalline silicon, no defects due to the thickness change were observed at all, while the grain size and the resistivity of the silicide were measured to be 100–400 nm and about 36 μΩ cm, respectively. As a result of its application to a complementary metal oxide semiconductor device having a 0.25 μm minimum linewidth, the line resistance was three times lower than with the conventional . In addition, characteristics such as the dopant depletion of the gate electrode and the reliability of the gate oxide were superior to those of the conventional . This systematic study has also reviewed the correlation between the silicide's structural properties and device performance.

Tungsten silicide composition analysis by Rutherford backscattering spectroscopy, Auger electron spectroscopy, and x-ray photoelectron spectroscopy

While Rutherford backscattering spectroscopy (RBS) is the accepted method for determining the composition of tungsten silicide films, the systems are expensive and not widely available. Furthermore, the depth resolution of RBS is of the order of 150 Å, which does not allow for the compositional analysis of thinner films. Also, chemical information is not available from RBS. These are critical issues as the geometries of ultralarge-scale integration devices become smaller. However, with special care applied to data acquisition and reduction, both Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) provide composition agreement with RBS. Additionally, AES and in particular XPS provide chemical information about thin films that is unavailable from RBS analysis. To demonstrate that AES and XPS provide composition analysis of thin films comparable to RBS, post-anneal tungsten silicide films deposited on silicon with both chemical and vapor deposition processes were analyzed by AES, XPS, and RBS. With care taken in data acquisition and reduction, silicon-to-tungsten ratios were comparable among the three techniques. Moreover, using AES and XPS, surface oxide thickness and interfacial layer composition can be determined.

Electrically conductive tungsten silicide coatings for EMI/RFI shielding of optically transparent windows

Conductive metal mesh coatings on external surfaces of infrared (IR) windows reduce the electromagnetic and radio frequency interference (EMI/RFI) but are mechanically soft and easily damaged. Surface-doped semiconductors, such as gallium arsenide, have optical absorption and emission problems, while semiconducting carbide coatings, such as germanium carbide, suffer performance loss at the low end of the required temperature range. Electrically conductive tungsten silicide have been investigated for use in erosion-resistant EMI/RFI protective coatings for IR windows and radomes. Tungsten silicide films were sputtered on zinc sulphide substrates, electrical properties were evaluated, and shielding effectiveness was measured over 400 MHz to 18 GHz. The relevant results are presented and discussed.

Stress Analysis of Transistor Structures Considering the Internal Stress of Thin Films

Stress fields in transistor structures are analyzed with consideration of the internal stresses of thin films. Internal stresses of amorphous silicon and tungsten silicide films are measured by detecting changes in the surface curvature of film-covered substrates as a function of temperature. Internal stresses of both films change upon annealing due to phase transitions, and reach about 1000 MPa. The stress predicted for transistor structures without considering the internal stress of the films differs markedly from results obtained using microscopic Raman spectroscopy. On the other hand, the stress predicted with consideration of film internal stress agrees very well with measured data. Stress design is performed for an actual transistor structure by adjusting the annealing temperature depending on the internal stress of an amorphous silicon thin film to eliminate the generation of dislocations. It is confirmed that stress design is effective in eliminating dislocations in transistor structures, thus improving device reliability.