Gate electrode connection structure by in situ chemical...

Details Gate electrode connection structure by in situ chemical... Gate electrode connection structure by in situ chemical...

2000-09-08

2001-06-26

Mills, Gregory (Department: 1763)

Coating apparatus

Gas or vapor deposition

C118S697000, C156S345420, C438S592000

Reexamination Certificate

active

06251190

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the fabrication of integrated circuits (IC's). More particularly, the invention provides a technique, including a method and apparatus, for forming a gate stack structure having an improved gate electrode connection structure formed by chemical vapor deposition (CVD) of tungsten (W) and plasma enhanced chemical vapor deposition (PECVD) of tungsten nitride (W
x
N) films.
2. Description of the Background Art
Modern integrated IC's contain large numbers of transistors. These transistors are generally field effect transistors that contain a source region and a drain region with a gate electrode located in between the source and drain regions.
A typical gate structure contains a thin polysilicon electrode that lies on top of a thin layer of gate oxide such as silicon dioxide (SiO
2
). The gate electrode and gate oxide are formed between semiconducting source and drain regions, that define an underlying well of p-type or n-type silicon. The source and drain regions are doped opposite to the well to define the gate location, a layer of insulating material such as silicon oxide (SiO
x
) or silicon nitride (SiN
x
) is deposited on top of the source and drain regions and an aperture or via is formed in the insulating material between the source and drain regions. The gate structure within the via contains a thin oxide layer, a polysilicon layer and a metal plug. The metal plug is formed by vapor depositing a metal such as tungsten on top of the polysilicon gate electrode. To complete the connection, the silicon then is caused to diffuse into the tungsten during a thermal annealing process forming a layer of relatively uniform tungsten silicide (WSi
x
) as the connection to the gate electrode. Without annealing, the silicon will ultimately diffuse into the tungsten forming a non-uniform layer of tungsten silicide.
A gate electrode having an electrical connection made of pure tungsten would be more desirable than a tungsten silicide electrode since tungsten has a lower resistivity than tungsten silicide. Unfortunately, silicon diffuses into the tungsten forming tungsten silicide. The diffusion can be prevented by depositing a layer of tungsten nitride (W
x
N) as a diffusion barrier. W
x
N is a good conductor as well as an excellent diffusion barrier material. Such a barrier layer is formed by reducing tungsten hexafluoride (WF
6
) with ammonia (NH
3
) in a chemical vapor deposition (CVD) process.
Unfortunately, the above described process results in the formation of contaminant particles in the form of solid byproducts. These byproducts include ammonia adducts of tungsten hexafluoride ((NH
3
)
4
WF
6
), ammonium fluoride (NH
4
F) and other ammonium complexes. Many of these particles become attached to the deposition chamber's interior. During temperature fluctuations within the chamber, the deposits flake off the walls and contaminate the wafer. Further, the tungsten nitride that is deposited using the above described process has a polycrystalline structure in which there are many grain boundaries. As a result, the diffusion barrier properties of the tungsten nitride are compromised. In addition, tungsten nitride films deposited by the traditional method tend not to adhere very well to the substrate upon which they are deposited.
Therefore, a need exists for a gate structure having a low resistivity tungsten gate electrode connection with a compatible diffusion barrier to prevent diffusion of silicon into the tungsten and a concomitant method and apparatus for manufacturing same.
SUMMARY OF THE INVENTION
The disadvantages associated with the prior art are overcome by the present invention of a gate electrode connection structure having a diffusion barrier of tungsten nitride (W
x
N) deposited on top of a polysilicon gate electrode by a CVD process using two gaseous mixtures which do not have a gas phase reaction with each other until energy is applied to the gaseous mixtures. The gate connection structure further comprises a conductive layer of tungsten deposited using a plasma enhanced chemical vapor deposition (PECVD) process. According to a first embodiment of the invention, tungsten may be deposited on top of the tungsten nitride diffusion barrier using a PECVD method to form the gate electrode.
According to a second embodiment of the present invention, the tungsten is deposited using a multi-step CVD process in which diborane (B
2
H
6
) is added during a tungsten nucleation step.
The W
x
N deposition process of the present invention is performed by providing a gaseous mixture in a chamber that contains a wafer, and energizing the gaseous mixture to form a plasma. The gaseous mixture includes a first gaseous composition containing nitrogen and hydrogen and a second gaseous composition containing tungsten. The first gaseous composition is one that does not have a gas phase reaction with the second gaseous composition to form tungsten nitride, unless energy is provided to the gaseous mixture. The tungsten containing composition may be tungsten hexafluoride (WF
6
). The first gaseous composition may include a mixture of N
2
and H
2
. Additionally, the gaseous mixture may include an argon dilutant.
The gaseous mixture may be energized to form a plasma within a deposition zone. In the plasma, the N
2
nitrogen dissociates into nitrogen ions, and the tungsten separates from the fluorine. The nitrogen ions and tungsten then combine to form tungsten nitride (W
2
N). The tungsten nitride reacts with a heated wafer surface in the chamber, so that a layer (or film) of tungsten nitride grows on the wafer's upper surface.
The hydrogen and fluorine combine to form hydrogen fluoride (HF) as a gaseous reaction byproduct that is discarded, i.e., removed from the chamber. Fewer contaminant particles are generated by depositing tungsten nitride in accordance with the present invention. This reduction in contaminant particles is achieved by eliminating the ammonia reaction in the gas phase that forms ammonium containing contaminants.
To improve the adhesion of the tungsten nitride film to the substrate, and especially to a wafer having an insulating layer upon which the tungsten nitride is deposited, the wafer is pretreated with a plasma before depositing the tungsten nitride film. The pretreatment can be accomplished in the same chamber as the tungsten nitride deposition by providing only the first gaseous composition and energizing it to form a plasma. As such, the wafer is pre-treated with a plasma of, for example, hydrogen or a gaseous mixture containing hydrogen and, for example, nitrogen. Once the wafer is treated, the tungsten hexafluoride can be added to the existing plasma to begin tungsten nitride deposition.
Tungsten nitride that is deposited in accordance with the present invention is more amorphous than traditionally deposited tungsten nitride and therefore, acts as a better diffusion barrier.
Once the tungsten nitride is deposited, a bulk layer of tungsten is deposited as a metallization layer to complete the gate structure. The bulk tungsten deposition is accomplished using CVD deposition of tungsten by thermal reduction of tungsten hexafluoride.
The tungsten deposition process according to a second embodiment of the present invention comprises a nucleation step followed by a bulk deposition step. In the nucleation step, a process gas including a tungsten-containing source, a group III or V hydride, and a reduction agent is flowed into a deposition zone of a substrate processing chamber while the deposition zone is maintained at or below a first pressure level. During the nucleation step, other process variables are maintained at conditions suitable to deposit a first layer of the tungsten film over the substrate. Next, during the bulk deposition step, the flow of the group III or V hydride into the deposition zone is stopped, and afterwards, the pressure in the deposition zone is increased to a second pressure above the first pressure level and other process parameters are maintain

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