Tungsten deposition process with dual-step nucleation

Details Tungsten deposition process with dual-step nucleation Tungsten deposition process with dual-step nucleation

2000-06-01

2002-03-19

Dang, Trung (Department: 2823)

Semiconductor device manufacturing: process

Coating with electrically or thermally conductive material

To form ohmic contact to semiconductive material

C438S655000, C438S656000, C438S675000, C438S683000

Reexamination Certificate

active

06358844

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to a tungsten deposition process that incorporates a nucleation step and more particularly, relates to a tungsten deposition process for filling via openings in a semiconductor substrate incorporating a dual-step nucleation process for depositing first a WSi
x
layer and then a W layer on top.
BACKGROUND OF THE INVENTION
In the fabrication of semiconductor devices, metal contacts and vias are frequently formed in contact holes and via openings on silicon wafers that have been pre-processed with insulating layers on top. Devices are then fabricated by connecting the components with metal contacts and vias to form the integrated circuit. In particular, aluminum, aluminum alloys, tungsten and tungsten alloys are used for depositing into contact holes and via openings on silicon substrates. The deposition process can be carried out either in a physical vapor deposition (PVD) chamber or in a chemical vapor deposition (CVD) chamber.
As the dimensions of semiconductor devices continuously to shrink in the miniaturization of modern semiconductor devices to the sub-half-micron range, via openings and contact holes must also shrink. Consequently, the openings and holes to be filled have larger aspect ratios, i.e., the ratios between the depth of the opening or hole and the diameter.
Difficulties have been encountering in depositing conductive metals into via openings and contact holes that have high aspect ratios by the conventional sputtering process. As the openings or holes become smaller and deeper, the bottom and sides of an opening or hole receive fewer deposited metal particles than the top surface of the device. The end result of such a phenomenon, sometimes called a shadowing effect, is that metal layers formed by the particles hang over the opening forming an overhang. The overhand closes before the opening is completely filled as the deposition process progresses and thus creating a void in the opening or hole.
One technique used to remedy the shadowing effect of the sputtering process is to use a tungsten chemical vapor deposition (W CVD) technique for filling openings and holes that have large aspect ratios. The W CVD process solves the difficult problems in metalization to ensure enough metal continuity in contact windows and vias. The step coverage of deep openings or holes by the W CVD particles is greatly improved over that possible by any other deposition techniques. In a W CVD process conducted on a silicon substrate for filling a contact hole, the basic chemistry may be a reaction such as
2WF
6
+3SiH
4
→2W+3S
i
F
4
.
During a W CVD deposition process, a wafer is usually held on a vacuum chuck that is heated to a temperature between about 400° C. and about 500° C. A shower head is positioned opposite to the wafer where WF
6
/SiH
4
gases are injected. Normally, a two-or three-step process is involved where SiH
4
is first introduced without any flow of WF
6
to initiate a deposition of a very thin seed layer of amorphous silicon as a prenucleation layer. The prenucleation process is then followed by a SiH
4
+WF
6
silane reduction nucleation process for depositing a thin W nucleation layer, and then the faster rate H
2
+WF
6
hydrogen reduction process for bulk W deposition. During the nucleation stage, less than 100 nm of tungsten is deposited, while the bulk of the tungsten deposition is by the hydrogen reduction process. The multi-stage deposition process is designed such that during the initial nucleation stage, the silicon from the source/drain area is not consumed in the reaction since WF
6
would react readily with Si. When WF
6
reacts with Si from the source/drain region, a defect known as junction leakage may occur. The introduction of SiH
4
first into the reaction avoids the consumption of Si from the substrate. The initial introduction of SiH
4
into the reaction without WF
6
for the deposition of the prenucleation layer of Si is known as a silane soak step.
In the W CVD process, a W CVD is frequently blanket-deposited onto a wafer surface and into the contact holes after a metal nucleation layer is first deposited on the entire wafer. The W deposited on the glue/barrier layer, i.e., a Ti/TiN layer, is then etched off in an etchback process by a process of reactive ion etching or by CMP process. After the etchback process, only the thicker W in the contact holes are left. Since the process relies on the removal of all W CVD except in the contact holes, the uniformity of the W deposition and the RIE etchback process is critical for the successful formation of W contact plugs. When the process is not accurately controlled, such as with the pre-disposition of a nucleation layer of Ti/TiN, the W contact plugs may be substantially recessed after the etchback process and thus results in poor step coverage on the device.
In the conventional single-step nucleation process conducted after a diffusion barrier layer deposition, the ratio of WF
6
/SiH
4
in the reactant gas mixture must be kept low in order to avoid chemical attack of WF
6
or defect formation known as volcano effect. At a low flow rate of WF
6
, the step coverage of the subsequent tungsten deposition for forming the plug is poor. The step coverage problem becomes more severe with decreasing feature size at the contact or via level which renders a perfect plug forming impossible. The step coverage in filling a contact of via is extremely important in a tungsten CVD process in order to achieve desirable resistance values and device reliability. The conventional single step nucleation process is therefore inadequate for achieving high reliability plug filling and for improving step coverage.
It is therefore an object of the present invention to provide a tungsten plug deposition process that does not have the drawbacks or shortcomings of the conventional tungsten deposition process incorporating single-step nucleation.
It is another object of the present invention to provide a tungsten plug deposition process incorporating a dual-step nucleation method.
It is a further object of the present invention to provide a tungsten plug deposition process incorporating a dual-step nucleation method such that two separate nucleation layers are deposited into via openings before the bulk tungsten deposition.
It is another further object of the present invention to provide a tungsten plug deposition process incorporating a dual-step nucleation method by using a reactant gas mixture of WF
6
/SiH
4
at different mix ratios for the two nucleation layers.
It is still another object of the present invention to provide a tungsten plug deposition process incorporating a dual-step nucleation method in which a first nucleation step is carried out by flowing a reactant gas mixture that is silicon rich followed by a second nucleation step by flowing a reactant gas mixture that is tungsten rich.
It is yet another object of the present invention to provide a tungsten plug deposition process incorporation a dual-step nucleation method by forming two separate nucleation layers consisting of a first layer of WSi
x
and a second layer of W.
It is still another further object of the present invention to provide a semiconductor structure that has tungsten plugs formed in a top surface including, in each plug, a first nucleation layer contacting the via openings of substantially WSi
x
and a second nucleation layer on top of the first nucleation layer of substantially W.
It is yet another further object of the present invention to provide a method for tungsten plug deposition incorporating a dual-step nucleation process in which a first nucleation layer is deposited by a reactant gas mixture of WF
6
/SiH
4
having a mix ratio smaller than 1:1 followed by the deposition of a second nucleation layer by flowing a reactant gas mixture of WF
6
/SiH
4
having a mix ratio larger than 2:1.
SUMMARY OF THE INVENTION
In accordance with the present invention, a tungsten plug deposition process incorporating a dual-step nucleation method and the semiconductor

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