Selective Tungsten Deposition Research Articles

Selective formation of tungsten nanowires

We report on a process for fabricating self-aligned tungsten (W) nanowires with polycrystalline silicon core. Tungsten nanowires as thin as 10 nm were formed by utilizing polysilicon sidewall transfer technology followed by selective deposition of tungsten by chemical vapor deposition (CVD) using WF6 as the precursor. With selective CVD, the process is self-limiting whereby the tungsten formation is confined to the polysilicon regions; hence, the nanowires are formed without the need for lithography or for additional processing. The fabricated tungsten nanowires were observed to be perfectly aligned, showing 100% selectivity to polysilicon and can be made to be electrically isolated from one another. The electrical conductivity of the nanowires was characterized to determine the effect of its physical dimensions. The conductivity for the tungsten nanowires were found to be 40% higher when compared to doped polysilicon nanowires of similar dimensions.

Fabrication of Self-Aligned Sub-100 nm Iron Wires by Selective Chemical Vapor Deposition

We have demonstrated the ability to fabricate and self-align sub-100 nm iron wires using a combination of silicon nitride spacer technology and selective deposition of iron and tungsten by chemical vapor deposition (CVD). The discovery of selective deposition of CVD iron, from pentacarbonyl [Fe(CO) 5 ] precursor, on silicon nitride surfaces over tungsten surfaces is the key factor that allows the self-alignment of iron wires. The density and conductivity of the CVD iron layers improved as the deposition temperature increased. Deposition time of 1 min was sufficient to deposit a perfectly aligned, continuous iron wire. The deposited iron layer shows 100% selectivity.

Selective tungsten deposition into ordered nanohole arrays of anodic porous alumina by electron-beam-induced deposition

The selective deposition of a metal (tungsten) into ordered nanohole arrays of an anodic porous alumina membrane was performed using an electron-beam-induced deposition process. After deposition, the membrane was observed and analyzed using electron microscopy and energy-dispersive X-ray spectroscopy. It is shown that the deposition was preferentially conducted in the holes in the irradiated area of the electron beam. A calculation of the electron-beam intensity explains the reason for the preferential deposition in the holes.

Si Consumption in Selective Chemical Vapor Deposition of Tungsten Using SiH4 Reduction of WF 6

Silicon consumption in a selective tungsten chemical vapor deposition (SWCVD) process using reduction of was investigated. In the SWCVD process, the Si surface was consumed through the reduction of the until a continuous W cladding layer was formed. The dependence of Si consumption on growth temperature and impurity concentration was investigated by measuring the decrease in silicon on insulator (SOI) thickness using an ellipsometer. The consumption decreased with a decrease of the growth temperature dependent on an activation energy of 0.104 eV; the consumption increased in proportion to the phosphorus concentration, in the region where the concentration was above about It was demonstrated that SWCVD could be applied to fabricate a W cladding layer on the gate/source/drain regions of the fully depleted 20 nm SOI complementary metal oxide semiconductor field effect transistors (CMOSFETs). Moreover, optimizing the fabrication process to avoid excess impurity concentration in the regions of the n-MOSFETs will make this technology suitable for use with thinner SOI devices. © 2001 The Electrochemical Society. All rights reserved.

Chemical Vapor Deposition Coating for Micromachines

AbstractTwo major problems associated with Si-based MEMS devices are stiction and wear. Surface modifications are needed to reduce both adhesion and friction in micromechanical structures to solve these problems. In this paper, we will present a process used to selectively coat MEMS devices with tungsten using a CVD (Chemical Vapor Deposition) process. The selective W deposition process results in a very conformal coating and can potentially solve both stiction and wear problems confronting MEMS processing. The selective deposition of tungsten is accomplished through silicon reduction of WF6, which results in a self-limiting reaction. The selective deposition of W only on polysilicon surfaces prevents electrical shorts. Further, the self-limiting nature of this selective W deposition process ensures the consistency necessary for process control. Selective tungsten is deposited after the removal of the sacrificial oxides to minimize process integration problems. This tungsten coating adheres well and is hard and conducting, requirements for device performance. Furthermore, since the deposited tungsten infiltrates under adhered silicon parts and the volume of W deposited is less than the amount of Si consumed, it appears to be possible to release stuck parts that are contacted over small areas such as dimples. Results from tungsten deposition on MEMS structures with dimples will be presented. The effect of wet and vapor phase cleans prior to the deposition will be discussed along with other process details. The W coating improved wear by orders of magnitude compared to uncoated parts. Tungsten CVD is used in the integrated-circuit industry, which makes this approach manufacturable.

Selective W for Coating and Releasing MEMS Devices

AbstractTwo major problems associated with Si-based MEMS (MicroElectroMechanical Systems) devices are stiction and wear. Surface modifications are needed to reduce both adhesion and friction in micromechanical structures to solve these problems. In this paper, we will present a CVD (Chemical Vapor Deposition) process that selectively coats MEMS devices with tungsten and significantly enhances device durability. Tungsten CVD is used in the integrated-circuit industry, which makes this approach manufacturable. This selective deposition process results in a very conformal coating and can potentially address both stiction and wear problems confronting MEMS processing. The selective deposition of tungsten is accomplished through the silicon reduction of WF6. The self-limiting nature of this selective W deposition process ensures the consistency necessary for process control. The tungsten is deposited after the removal of the sacrificial oxides to minimize stress and process integration problems. Tungsten coating adheres well and is hard and conducting, requirements for device performance. Furthermore, since the deposited tungsten infiltrates under adhered silicon parts and the volume of W deposited is less than the amount of Si consumed, it appears to be possible to release stuck parts that are contacted over small areas such as dimples. The wear resistance of selectively coated W parts has been shown to be significantly improved on microengine test structures.

Secondary ion coincidence in highly charged ion based secondary ion mass spectroscopy for process characterization

Coincidence counting in highly charged ion based secondary ion mass spectroscopy has been applied to the characterization of selective tungsten deposition via disilane reduction of tungsten hexafluoride on a patterned SiO2/Si wafer. The high secondary ion yield and the secondary ion emission from a small area produced by highly charged ions make the coincidence technique very powerful.

Improving the Characteristics of Ultra-Thin-Film Fully-Depleted Metal-Oxide-Semiconductor Field Effect Transistors on SIMOX (Separation by IMplanted OXygen) by Selective Tungsten Deposition on Source and Drain Region

Selective tungsten (W) chemical-vapor-deposition (CVD) with hydrogenation and hydrogen-termination (HHT) is applied to ultra-thin-film fully-depleted (FD) metal-oxide-semiconductor field effect transistors (MOSFETs) on SIMOX (Separation by IMplanted OXygen) for reducing the sheet resistance of source and drain regions. 0.25-µm-gate MOSFETs on SIMOX (MOSFETs/SIMOX) with a top Si layer with a thickness of 50 nm are fabricated using selective W-CVD, and their characteristics, including hot-carrier effects and latch-onset voltage, are systematically investigated. It is found that selective W-CVD with HHT can reduce the source/drain (S/D) sheet resistance in 50-nm-thick ultra-thin-film SIMOX to 10 Ω/sq. or less. This ensures that W deposition increases the drain saturation current. It is also found that W deposition largely suppresses the parasitic bipolar effects. Consequently, the anomalous subthreshold slope diminishes, hot carrier reliability improves, and the latch-onset voltage rises to over 2.5 V. Moreover, it is clarified that the parasitic bipolar effects are largely suppressed when the W layer is within 0.3 µm from the source/body junction. This is because W effectively extracts the holes generated by impact ionization, and thus suppresses the accumulation of holes which would otherwise induce an increase in the body potential.

Simulation of selective tungsten chemical vapour deposition

A detailed numerical model is presented for the simulation of hydrodynamics and chemistry in selective tungsten low pressure chemical vapour deposition. It combines models for multicomponent transport phenomena in the gas phase and ab initio elementary reactions at the wafer surface. A mechanism and model are proposed to relate non-selective tungsten nucleation on oxide surfaces to the gaseous by-product WF 5. Model predictions of gas temperatures and composition as well as deposition rates have been validated quantitatively with ( in situ laser diagnostic) experiments. Predictions for the dependence of selectivity loss on operating conditions show excellent qualitative agreement with experimentally observed trends. The use of the model in the optimization of selective tungsten deposition processes is illustrated.

Chemical mechanical polishing for selective CVD-W

This work investigates chemical mechanical polishing (CMP) for W-filled contact holes, vias, and trenches by selective chemical vapor deposition. A novel process that combines the CMP technique with selective chemical vapor deposition of tungsten (CVD-W) was employed to remove nail heads due to overgrowth and W-paiticles on the surface of dielectric due to selectivity loss. The overfilled nail heads and the selectivity loss can be completely removed with very low down-pressure (3 psi) in a very short polishing time (30 s). This indicates that the novel process is very promising for ULSI multilevel interconnection application. The removal rate selectivities of W to thermal oxide, PECVD-TEOS, and BPSG were found to be 47:1, 30:1 and 15:1, respectively, while the selectivities of W to the barrier metals of TiW, Ti and Ta were determined to be 0.6:1, 6:1 and 28:1, respectively.

Use of oxygen-stabilized C60 films for selective chemical vapor deposition

Thin C60 films exposed to ultraviolet/visible light in the presence of oxygen were used as a selective mask for tungsten chemical vapor deposition on silicon substrates. An uptake of oxygen in the fullerene films as well as a significant increase in their thermal stability resulted from the simultaneous exposure to the radiation and oxygen. The thermal stability and inertness of these films to tungsten hexaflouride, which is readily reduced by silicon to form the metal at 350 °C, allowed selective deposition of tungsten in both ultrahigh vacuum and low pressure environments. X-ray photoelectron spectroscopy and x-ray fluorescence spectroscopy were used to characterize the tungsten deposited on the C60 mask and the unmasked silicon substrate.

Characteristics of Selective Chemical Vapor Deposition of Tungsten on Aluminum with a Vapor Phase Precleaning Technology

A simple and efficient precleaning of an aluminum surface with hydrochloric acid vapor prior to selective chemical vapor deposition of tungsten was investigated. The vapor phase precleaning was shown to remove the aluminum native oxide as well as reduce the aluminum fluorides formed at the W/Al interface during initial reaction of with the aluminum underlayer. It was found that the precleaning process is spontaneous and the removal rate of aluminum is related to the concentration of reactants used. After precleaning, the aluminum surface was free of oxide and covered with a large amount of Cl species that were examined by x‐ray photoelectron spectroscopy. These Cl species occupy active sites on the wafer surface and prevent further adsorption of on the aluminum during the initial stage in selective deposition of tungsten. Therefore, the probability of reaction between and the underlying aluminum is greatly suppressed. By using a hydrochloric acid vapor to pretreat the aluminum trench and via hole patterned substrates, a smooth and dense tungsten film and low selectivity loss (<50 pcs/cm2) were obtained. A low concentration of aluminum fluorides accumulated at the W/Al interface that were involved in low via resistance was also observed.

Modeling of selective tungsten low-pressure chemical vapor deposition

A comprehensive numerical model for selective low-pressure chemical vapor deposition (LPCVD) of tungsten from hydrogen and WF 6 in a single-wafer reactor has been developed. The computational fluid dynamics (CFD) code PHOENICS-CVD is used for transient solutions of the two-dimensional transport equations of heat, momentum and chemical species in the reactor. A detailed model is used for the kinetics of tungsten deposition at the surface. A model for the nucleation of intermediates, which are formed at the surfaces where tungsten is deposited and are transported through the gas phase, is used to describe the loss of selectivity. This opens the way for qualitative study of the influence of process conditions and reactor configurations on the selectivity of this process. The selectivity is found to improve at high flow rates, low total pressures and low partial pressures of WF 6.

Influences of damage and contamination from reactive ion etching on selective tungsten deposition in a low-pressure chemical-vapor-deposition reactor

Reactive ion etching (RIE) used in contact hole formation can result in damage and contamination of the underlying silicon substrate. In this work, influences of these phenomena on selective tungsten deposition in a low-pressure chemical-vapor-deposition reactor have been studied. The damage was generated because of ion bombardment and radiation-induced bonding changes in silicon lattices. It causes large Si consumption, rough W/Si interface during tungsten deposition, and large leakage current of W/Si Schottky structure. Simultaneously, contamination occurred with two forms of residual layers and impurity permeation layers in fluorocarbon-based RIE chemistries. The CF4/CHF3/O2 RIE of oxide produces the SiFxCyOz complex layers deposited on the sidewall and on the Si surface as well as the embedding of impurities such as F and C in the Si substrate. The creep-up, selectivity loss, lateral encroachment, high W film resistivity, and rough W/Si interface have been observed in the contaminated samples. Accordingly, a post-RIE etching technique used to remove the damage and contaminants before tungsten growth has been developed. A CF4/O2 plasma etching followed with an O2 plasma ashing step exhibits the capability of efficient surface cleaning. The excellent characteristics of W films such as elimination of encroachment and creep-up, low selectivity loss (i.e., 0.25 pcs/cm2), low resistivity of W films, smooth W/Si interface, and very low leakage current of W/Si contacts are thus obtained.

Deposition properties of selective tungsten chemical vapor deposition

This work investigates the basic deposition properties of selective tungsten chemical vapor deposition (W-CVD) using the process of silane reduction of WF6 with the SiH4/WF6 flow rate ratio less than 0.6 over the temperature range 280–350 °C. Selective W-CVD was performed on a contact hole patterned silicon substrate with in situ NF3 plasma etching of the silicon substrate prior to the selective tungsten deposition. The W deposition rate, deposition selectivity and W film resistivity were investigated with respect to the SiH4 partial pressure, WF6 partial pressure, total (SiH4 + WF6) deposition pressure, as well as the flow rate of the hydrogen carrier gas. It was found that the deposition rate is proportional to the SiH4 partial pressure. With a constant SiH4/WF6 flow rate ratio, the deposition rate increases with total pressure. At given flow rates of SiH4 and WF6, the deposition rate and film resistivity are dependent on the flow rate of the carrier gas. The experimental results show that the reverse bias junction leakage for the wet etching pretreated W/p+-n junction diode is smaller than that of the plasma pretreated diode. On the other hand, the contact resistance of the Al/W/n+p diode is smaller than that of the Al/W/p+n diode.

An Efficient Preclean of Aluminized Silicon Substrate for Chemical Vapor Deposition of Submicron Tungsten Plugs

Preclean of aluminum trench and via patterned substrates is vital for successful selective chemical vapor deposition of tungsten (CVD‐W). A convenient preclean method uses in situ plasma etching to remove the native metal oxide prior to conducting the CVD‐W. During the plasma etching, however, the outsputtered aluminum oxide and aluminum can be redeposited on the sidewall of the trench and via hole and on the surface of the dielectric layer, where W nucleation is induced, resulting in creep‐up and selectivity loss during tungsten deposition. By using a solution of hydroxylamine sulfate to pretreat the aluminum trench and via hole patterned substrates, we successfully avoid the creep‐up and selectivity loss of W deposition.

Selective deposition of tungsten on ITM-CoSi 2

In the present work a silane reduced selective tungsten deposition on ITM processed SALICIDE CoSi 2 was evaluated. The contact resistance of W/CoSi 2/Si contacts are lower compared to W Si contacts. CoSi 2 was formed by a selective silicide formation process using a single RTA step. During the ion implantation the interface between cobalt and silicon was mixed up and a possible native oxide has been cracked. This promotes a better metal-silicon reaction. Contact resistance and sheet resistance measurements were performed using Kelvin structures (CBKR) and contact chains or van der Pauw structures, respectively. A processing window was determined with respect to a good selectivity and low contact resistances. SIMS measurements were made in order to investigate the influence of the flow ratio of SiH 4 WF 6 on the sheet resistance.

Nucleation on SiO2 during the Selective Chemical Vapor Deposition of Tungsten by the Hydrogen Reduction of Tungsten Hexafluoride

A horizontal hot‐wall chemical vapor deposition (CVD) quartz reactor with rectangular cross section was used to study the effect of different process conditions on the nucleation of tungsten on during selective WCVD by the reduction of . The experimental procedure included placing a metallic surface at the center of the reactor, and small samples of at different positions both upstream and downstream with respect to the metallic surface. Digitized scanning electron microscopy micrographs were used to determine the particle size distributions of nuclei on the surfaces. We found that the amount of nucleation on decreases when smaller metallic surfaces are present and for lower temperatures and shorter process times. Although nucleation was always greatest on samples closest to the metal sample, the effect of flow rate depended on the position of the . A statistical nearest neighbor analysis indicated a clustering of W nuclei on the . A simplified mathematical model was developed to predict concentration profiles of a gaseous intermediate generated at the metal surface during the thermal decomposition of the source gas. This intermediate has been proposed as being the reactive species that causes nucleation on surface. Qualitative agreement between experimental and theoretical results reinforce the proposed role of the intermediate with this species being characterized by a short lifetime.

A Simple and Efficient Pretreatment Technology for Selective Tungsten Deposition in Low-Pressure Chemical Vapor Deposition Reactor

In this paper we develop a two-step pretreatment method which is a simple but effective cleaning process that can be used to achieve a high deposition rate and excellent interface characteristics, as well as improve the selectivity. In the first step, we use CF4/O2 mixing gases as a plasma etching system without substrate bias. The removal of damage and contaminants by reactive ion etching (RIE) during contact hole opening can thus be obtained. We consider that the role of oxygen is first to oxidize the polymer residues (i.e., fluorocarbons like Cx Fy ) and the RIE-damaged regions on the silicon surface, and it is then followed with CF4 to remove these oxides. In experiments, the plasma modification rate can be controlled at 120 Å/min. Results of the high deposition rate, smooth interface, low selectivity loss and the other improvements are observed. These are superior to those of cases without pretreatment. The second step is that anhydrous HF cleaning was used to remove surface oxide which was formed during the first step. Obviously, this new cleaning technology is very efficient not only for contact hole pretreatment but also for polysilicon materials. This also implies that this technology may have potential for polycide gate applications.

Selective deposition of tungsten by chemical vapour deposition from SiH 4 reduction of WF 6

Tungsten films were deposited on tungsten-patterned wafers by the selective chemical vapour deposition (CVD) process from SiH 4-reduction of WF 6 in a cold-wall reduced pressure CVD reactor. The experimental conditions such as reactive gas flow ratio, temperature, total pressure etc. providing good selective deposition were investigated (in terms of voids and homogeneity of the coatings in each plug) and particularly the pretreatment of wafers by different sorts of plasma. The tungsten deposition rate and dependence of the selectivity loss on the SiO 2 film were checked using a scanning electron microscopy technique, vs. the different parameters. These results show the importance of the plasma pretreatment and of the first reactive gas introduced for the selectivity. A temperature window for optimum selectivity was established and an apparent activation energy was measured. From all these results, an interesting correlation could be made between the selectivity loss and the rate-controlling mechanism. Finally the flexibility of this process is demonstrated by the feasibility of selective tungsten deposition on aluminium-patterned wafers. The conditions for tungsten and for aluminium are very different; in particular, the electrical results obtained on aluminium show a high sensitivity to the pretreatment carried out before deposition.