Tungsten Chemical Vapor Deposition Research Articles

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.

The novel precleaning treatment for selective tungsten chemical vapor deposition

The new solutions, hydroxylamine sulfate [(NH2OH)2H2SO4] combined with CuSO4, for cleaning Al via were investigated. It is found that the cleaning capability of hydroxylamine sulfate combined with CuSO4 is better than that of hydroxylamine sulfate. Low via resistance of electrical test structure is obtained if the via is cleaned by this new cleaning solution. The hydroxylamine sulfate can efficiently remove Al3O2 and leave the clean Al on the surface of via. Then, the Cu ion in this new solution will immediately react with clean Al and form a copper passivating layer on the surface of via. The copper is more stable than aluminum in the environment and hard to be oxidized. Therefore, hydroxylamine sulfate combined with CuSO4 can provide excellent cleaning capability for aluminum via holes. Also, the clean surface on the bottom of via is helpful for tungsten nucleation in via during CVD-W deposition. Therefore, a low via resistance and good selectivity of tungsten plug are obtained when the Al via is precleaned with this new solution.

Integration methodology of chemical vapor deposition TiN, chemical vapor deposition W and W chemical mechanical planarization for sub-quarter micron process application

Chemical vapor deposition (CVD) W plugs have been widely used for device metallization with excellent conformity in small contacts/vias with high aspect ratio [J. E. J. Schmitz, Chemical Vapor Deposition of Tungsten and Tungsten Silicides (Noyes, 1991)]. However, some unexpected plug fill such as plug loss and key hole exposing post tungsten chemical mechanical planarization (WCMP) still happened while going to smaller plug size and using metalorganic chemical vapor deposition (MOCVD) TiN barrier. In this study, MOCVD TiN and CVD W fill followed by WCMP for plug metallization were investigated. Extensive analysis had been conducted on various types of defective W plugs. Organic contaminants (hydrocarbon by-products) in MOCVD TiN deposition would prevent W deposition taking place inside the plug. For W wise, lower process temperature, carefully adjusted WF6/SiH4 and WF6/H2 partial pressure ratios had demonstrated better plug fill and electrical performance [T. E. Clark et al., J. Vac. Sci. Technol. B 9, 1478 (1991)]. In the WCMP approach, the effects of different oxidizer concentration in slurry were characterized. Eliminating the seam formation during the CVD W process can help avoid slurry attack in WCMP. The optimized integration scheme of MOCVD TiN barrier, CVD W, and WCMP was successfully achieved and is applied on 0.20 μm Logic production.

Suppression of Fluorine Penetration by Use of In Situ Stacked Chemical Vapor Deposited Tungsten Film

In this study, amorphous‐like tungsten films were deposited by a chemical vapor deposition (CVD) process. The deposited film has shown reduced resistance compared with , and it also blocked the fluorine atoms from diffusing into the gate oxide. Furthermore, when the amorphous‐like tungsten film was deposited prior to a typical CVD tungsten film with a columnar structure, it not only showed excellent barrier characteristics in impeding fluorine impurities, but its resistance was also substantially lower than a single layer of a‐W film. It is also found in our work that a bilayer film containing typical CVD tungsten/amorphous‐like CVD tungsten is a better wordline structure due to its extraordinarily low resistivity and low fluorine contamination. © 1999 The Electrochemical Society. All rights reserved.

Design of a 300 mm chemical vapor deposition tungsten reactor using computational fluid dynamics

Computer simulation is used to optimize the design of a 300 mm chemical vapor deposition tungsten reactor, in order to improve its concurrency of operations, relative to 200 mm designs. Typically, in tungsten sequential batch systems, some deposition stations sit idle while a newly loaded wafer is heated up and exposed to a protective silane flow. This idling is necessary to prevent the device attack and gas phase nucleation that can result from early introduction of WF6 to the chamber. In the 300 mm chamber design presented here, this problem is addressed by altering the chamber geometry to greatly improve the isolation of individual deposition stations. This allows incompatible processes to take place simultaneously on adjacent stations with minimal cross talk, providing a substantial improvement to the throughput of the system.

Influence of SiH2 CI 2 on the Kinetics of the Chemical Vapor Deposition of Tungsten by SiH4 Reduction of WF 6

The influence of adding dichlorosilane to the silane ‐based reduction reaction of tungsten hexafluoride has been investigated to enhance the properties of this chemical vapor deposition process, especially the control of the growth rate. The growth rate of tungsten by the reduction reaction based on and has been measured. It is shown that the kinetics of the silane‐dichlorosilane process can be characterized by a surface reaction limitation. The sole effect of the in the process is blocking of surface sites by preferential absorption, thereby reducing the growth rate of the tungsten film. © 1999 The Electrochemical Society. All rights reserved.

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.

Homogeneous Tungsten Chemical Vapor Deposition on Silane Pretreated Titanium Nitride

Homogeneous nucleation of tungsten chemical vapor deposition (CVD) films on metallorganic (MO) CVD substrates has been achieved by pretreating the substrate with at a wafer susceptor temperature of . Deposition of approximately a monolayer of silicon from the pretreatment results in continuous, uniform tungsten films less than thick. Tungsten nucleation films grown on MOCVD without a pretreatment were heterogeneous, with tungsten islands not coalescing into a continuous film until the thickness reached . Experimental results are presented along with implications for use of tungsten CVD for vertical interconnects in high aspect ratio vias. ©1999 The Electrochemical Society

Low‐Resistance Contacts with Chemical Vapor Deposited Tungsten on GaAs Grown by Molecular Layer Epitaxy

Approximately 100 Å thick, mirror‐like, ordered tungsten layers were fabricated on (100) GaAs with chemical vapor deposition, using a precursor. Native oxides on GaAs were reduced with pretreatment in a chemical vapor deposition chamber of tungsten. Secondary ion mass spectrometry and Rutherford backscattering spectroscopy analyses indicate tungsten‐GaAs mixed layer not thicker than 20 Å. The chemical vapor deposited tungsten was applied to fabrication of state of the art, low resistance contacts to GaAs layers grown with molecular layer epitaxy. Optimization of GaAs doping, pretreatment, and tungsten chemical vapor deposition conditions were necessary to obtain low contact resistance. Contact resistance, measured with the transmission line method, was to Te doped n‐type GaAs and below to C doped p‐type GaAs. Electrically active donor/acceptor concentrations in GaAs layers grown by molecular layer epitaxy were about one order of magnitude lower than expected from literature trend lines for these contact resistance. © 1999 The Electrochemical Society. All rights reserved.

Thermal fatigue tests on CVD-W/Cu divertor mock-ups

Tungsten is a promising candidate for plasma-facing materials to cover the surface of the divertor plate in the design of the International Thermonuclear Experimental Reactor (ITER). The Japan Atomic Energy Research Institute (JAERI) has developed divertor mock-ups coated with tungsten using the chemical vapor deposition (CVD) technique. Thermal fatigue and screening tests of ITER-relevant divertor mock-ups carried out in an electron beam gun facility are presented. The two mock-ups consist of a 2 mm thick CVD tungsten layer on heat sinks made of OFHC–Cu and 30 wt.% Cu-infiltrated tungsten, respectively. The cooling channel is equipped with a twisted tape as heat transfer enhancement. Both mock-ups successfully withstood a steady-state deposited heat flux of 5 MW m −2 for 1000 cycles. These experiments show that CVD W-coated divertor mock-ups meet ITER requirements for plasma-facing components. These tests also demonstrate the capability of these mock-ups to withstand a transient deposited heat flux of 22 MW m −2 for 10 s without failure.

Model Reduction for a Tungsten Chemical Vapor Deposition System

A model of a tungsten chemical vapor deposition (CVD) system is developed to study the CVD system thermal dynamics and wafer temperature nonuniformities during a processing cycle. We develop a model for heat transfer in the system’s wafer/susceptor/guard ring assembly and discretize the modeling equation with a multiple-grid, nonlinear collocation technique. This weighted residual method is based on the assumption that the system’s dynamics are governed by a small number of modes and that the remaining modes are slaved to these slow modes. Our numerical technique produces a model that is effectively reduced in its dynamical dimension, while retaining the resolution required for the wafer assembly model. The numerical technique is implemented with only moderately more effort than the traditional collocation or pseudospectral techniques. Furthermore, by formulating the technique in terms of a collocation procedure, the relationship between temperature measurements made on the wafer and the simulator results produced with the reduced-order model remain clear.

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.

Metallic point contacts formed by physical vapor deposition and chemical vapor deposition: microscopy study and point-contact spectroscopy

We have made an electron-microscopy study of nanoholes in membranes in successive stages of metal deposition using two different techniques: physical vapor deposition (PVD) and chemical vapor deposition (CVD). One-sided PVD (thermal evaporation) of gold and silver was used, as is relevant for heterocontacts. The key results in this case are: 1) the holes are not filled during deposition and 2) closing of the holes is accomplished by lateral growth of the film on the membrane. In the case of CVD of tungsten we found that nanoholes in membranes are filled at the beginning of the deposition, and that the process is capable of filling holes as small as 10 nm. Fabricated devices (α-tungsten) show good quality point-contact spectra which are characteristic of ballistic transport through the constriction. A very interesting stepwise current increase was observed for one amorphous tungsten point contact.

Suppression of Fluorine Impurity in Blanket Chemical Vapor Deposited Tungsten Film for Via Fills with A Novel Two-step Deposition Technique

Blanket chemical vapor deposited tungsten (CVD-W) offers the potential to fabricate reliable contacts for submicron multilevel metallization. In via filling application, various aluminum fluorides were formed by the reduction of WF6 with aluminum underlayer. These compounds will stay at CVD-W/Al interface and act as insulating layers which cause electrical degradation. In addition, impurities like fluorine or oxygen induce the formation of β-W lattices as well as high film resistivity. In this work, a two-step chemical vapor deposition of tungsten was developed to suppress the fluorine impurities in tungsten films and at CVD-W/Al interface for blanket CVD-W application. The first step involves a gas phase nucleation with high SiH4/WF6 flow ratio (i.e., 2.5) to deposit a thin tungsten film as the glue layer. It was found that the probability of the WF6 reduction with underlying aluminum was suppressed because the WF6 was completely consumed by SiH4 before arriving onto aluminum surface. Meanwhile, this gas phase nucleated tungsten exhibits blanket deposition capability and an amorphous structure. The second step includes typical CVD-W process (i.e., SiH4/WF6 flow ratio <1) to grow thick tungsten film. Secondary Ion Mass Spectroscopy (SIMS) measurements indicate that the fluorine impurities in tungsten film and at CVD-W/Al interface are drastically reduced. Also, the gas phase nucleated tungsten can be reproducibly deposited without attacking the aluminum underlayer. Moreover, a lower tungsten resistivity, lower via resistance and longer electromigration lifetime are achieved in the Al/W/Al Kelvin structures produced by two-step deposition technique than those of the typical CVD-W films deposited directly on aluminum.

SiH4 ‐ WF 6 Gas‐Phase Nucleated Tungsten as an Adhesion Layer in Blanket Chemical Vapor Deposition for Ultralarge Scale Integration

In this work, the thin tungsten film nucleated by an in situ gas‐phase reaction in the low pressure regime (100 mTorr) was investigated and used as an adhesion layer to replace TiN in a blanket chemical vapor deposition of tungsten (CVD‐W) process. The deposition rate, step coverage, and film resistivity were studied as a function of the process parameters. Deposition rates from 360 to over 3000 nm/min were observed and increased with increasing deposition pressure and temperature. Moreover, we found that the deposition rate fell to zero when the temperature was less than 150°C. Also, the gas‐phase reaction vanished when the flow ratio was smaller than 1.6. On the other hand, the step coverage decreased with increasing deposition rate. Finally, the tungsten film resistivity was 167 μΩ‐cm which was comparable to that of a sputtered TiN film (about 150 μΩ‐cm) and also exhibited good adhesion ability on oxide when the temperature was higher than 200°C. Overall, the results indicate that this in situ gas‐phase nucleated tungsten film is an attractive replacement for TiN film in the blanket CVD‐W technique because of reduced process complexity, excellent step coverage, and low resistivity.

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.

A New Tungsten Gate Metal Oxide Semiconductor Capacitor Using a Chemical Vapor Deposition Process

A new process for tungsten gate metal oxide semiconductor (MOS) capacitors has been developed using chemical vapor deposition (CVD) of tungsten on a thin poly‐Si layer of appropriate thickness. The poly‐Si acts as a sacrificial layer and is consumed during the CVD of tungsten (W). This process yields a nearly pure W metal gate after reduction of at 300°C. Compared with sputtered tungsten films, the CVD tungsten film has lower resistivity and lower intrinsic film stress. In addition, the CVD tungsten metal gate MOS capacitor has a lower interface state density (Dit) and a higher charge‐to‐breakdown (Qbd), than sputter‐deposited tungsten gate MOS capacitors.

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.

Reduction of Selectivity Loss Probability on Dielectric Surface during Chemical Vapor Deposition of Tungsten Using Fluorinated Oxide and Removing Silanol Units on Dielectric Surface

Dielectric materials and surface pretreatment are important for the control of selectivity loss during the chemical vapor deposition (CVD) of W in SiH4/ H2/ WF6 ambient. Water-related silanol (SiOH) units on an oxide surface, which come from a wet cleaning step and moisture in clean room air, are found to act as nucleation centers, resulting in selectivity loss. Removing these silanol units by in situ NF3 plasma pretreatment effectively inhibits the nucleation of W. Reduction of the dielectric constant, as well as doping of fluorine in oxide, was shown to suppress the selectivity loss. This is due to the high electronegativity of the Fx SiOy film surface, which repels the WF6 molecules. The resistance to selectivity loss of W on Fx SiOy is better than that on BPSG. From these results, nucleation of W is inferred to initiate from the adsorption of WF6 on the insulator surface in the WF6/ SiH4/ H2 ambient. On the other hand, a novel method that uses Fx SiOy as an intermetal dielectric (IMD) or that involves capping on a dielectric material ( e.g., BPSG), was proposed to reduce the selectivity loss probability during selective growth processes.

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.