Coating processes – Coating by vapor – gas – or smoke
Reexamination Certificate
2001-06-21
2003-09-23
Meeks, Timothy (Department: 1762)
Coating processes
Coating by vapor, gas, or smoke
C427S255180, C427S255392, C427S255393, C438S680000, C438S682000, C438S683000
Reexamination Certificate
active
06623798
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an improved chemical vapor deposition process, and particularly to a process for the deposition of tungsten silicide from dichlorosilane (DCS), and an apparatus for performing the same.
2. Description of the Related Art
As the packing density of semiconductor devices in integrated circuits (IC) increases, feature sizes of patterns formed on the IC and the space between the patterns are becoming smaller and smaller. Conventionally, polysilicon has been used for making electrical connections, such as between a gate electrode and a bitline. However, as the pattern size and spacing becomes smaller, the RC time delay and the IR voltage drop continue to increase because of the relatively high resistivity of polysilicon. Accordingly, a polycide having similar characteristics to those of polysilicon and a resistivity less than that of polysilicon, has been used as a lower resistance alternative to polysilicon. Often, polycide is a multi-layer structure including a refractory metal silicide (silicide) over a layer of doped silicon.
Refractory metal silicides may be made from tungsten, molybdenum, titanium or tantalum, all of which have a relatively high melting point and are useful in the manufacture of VLSI circuits. A silicide may be combined with a highly doped polysilicon to form a polycide gate electrode. A known method for depositing the refractory metal silicide is by low-pressure chemical vapor deposition (LPCVD). Particularly, in case of using a tungsten suicide combined with a polysilicon, it is known that characteristics such as self-passivation, stability to wet chemicals, surface roughness, adhesion, oxidation and reproducibility are excellent.
Tungsten silicide (WSi
x
) thin films have been deposited by a low-pressure chemical vapor deposition (LPCVD) method onto semiconductor substrates using silane (SiH
4
) and tungsten hexafluoride (WF
6
) as precursor gases. One problem with this process is that the deposited film does not have a conformal shape over stepped topographies as desired. Another problem with this process is that films thus deposited may have a high residual fluorine content that can adversely affect device performance. For example, when the semiconductor wafer is exposed to an elevated temperature (e.g., about 800° C. or higher), as during annealing, the excess fluorine ions can migrate through the underlying polysilicon layer into an underlying silicon oxide layer (the gate oxide). This fluorine migration can adversely impact the electrical properties of the silicon oxide, which in turn can lead to an adverse change in electrical properties of semiconductor devices including such layers.
When using a multi-chamber vacuum processing system according to a known method, a substrate to be coated with tungsten silicide first is cleaned using a fluorine plasma scrub to remove native oxide from the polysilicon layer. The cleaned substrate is then transferred into a substrate transfer chamber. This transfer chamber has a nitrogen or argon atmosphere to assist in preventing re-oxidation of the substrate, and contains a robot to transfer the substrate into a processing chamber (e.g., a tungsten deposition chamber) through a slit valve having an O-ring seal. This CVD process has become widely used for deposition of tungsten suicide from SiH
4
and WF
6
. However, as substrates become larger and feature sizes for devices become smaller, the above problems of the step coverage and the residual fluorine content may be more pronounced.
An improved process for depositing WSi
x
films using dichlorosilane (DCS) instead of SiH
4
has been proposed. The resultant WSi
x
films have a reduced fluorine content and are more conformal than WSi
x
films deposited using SiH
4
as the precursor gas, thereby providing a solution to the SiH
4
-based deposition process limitations.
FIG. 1
is a schematic view of a conventional deposition apparatus for depositing a tungsten silicide from dichlorosilane (DCS). A substrate such as a silicon wafer is introduced into the load-lock chamber-A
50
or load-lock chamber-B
52
of a chemical vapor deposition (CVD) apparatus. For example, after loading the wafer into the load-lock chamber-A
50
, the pressure of load-lock chamber-A
50
is reduced to about 200 mTorr. Thus, the load-lock chamber-A
50
is maintained substantially at vacuum. Then, a slit valve (not shown) between a transfer module
54
and the load-lock chamber-A
50
is opened and the wafer in the load-lock chamber-A
50
is transferred into a processing chamber-C
56
or a processing chamber-D
58
by the robot arm of the transfer module
54
.
When transferring the wafer into the processing chamber-D
58
, the DCS deposition process progresses while introducing reaction gases into the processing chamber-D
58
. After the DCS deposition is completed, the processing chamber-D
58
is pumped down to a pressure of about 20 mTorr. After completing this pumping step, a slit valve (not shown) between the transfer module
54
and the processing chamber-D
58
is opened and the wafer is transferred into a cooling stage
60
by the robot arm of the transfer module
54
. After cooling the wafer in the cooling stage
60
, the slit valve between the transfer module
54
and the load-lock chamber-B
52
is opened and the wafer is transferred into the load-lock chamber-B
52
.
After all wafers are transferred into a cassette in the load-lock chamber-B
52
via the above-described steps, a vent gas such as nitrogen (N2) or argon (Ar) is supplied through a vent line connected with the load-lock chamber
52
until the pressure of the load-lock chamber-B
52
reaches a pressure of about 760 Torr. As a result, the load-lock chamber-B
52
is vented and the wafers are removed from the CVD system.
The reaction gases introduced into the processing chamber during the above described DCS deposition process are WF
6
and SiH
2
Cl
2
gases. The reaction used to produce WSi
x
is:
WF
6
+SiH
2
Cl
2
+P→WSi
x
+by-product gases
where, phosphorous (P) is contained in the silicon wafer (e.g., the polysilicon layer under the tungsten suicide film).
When unloading the wafer from the load-lock chamber after completion of the DCS deposition process, fumes are generated. When the temperature of the DCS deposition process is 600° C. or more, a substantial amount of fumes are generated. Further, as the concentration of P in the underlying polysilicon layer increases, these fumes are generated even more. That is, the generation of such fumes is more severe when the DCS deposition process is carried out on the bitline polysilicon layer of which the doping concentration is higher than that of the gate polysilicon layer.
As shown in the above reaction, such fumes are generated due to the by-product gases absorbed on a surface of the wafer after completion of the DCS deposition process. The components of such fumes may be phosphorus-based and/or chlorine-based gases. As a result of analyzing the components of the fumes, the phosphorous-based gas of about 56 parts-per-billion was detected by the phosphine (PH
3
) measuring system, and thus, the by-product gases generating the fumes proved to be the P-based gases. Since the P-based and/or Cl-based fumes are noxious to the human body, the semiconductor process may not be further proceeded without removing such fumes in consideration of the safety. Accordingly, it is necessary to remove the residual gases absorbed on the wafer surface after the DCS deposition process is completed.
What is needed, therefore is a CVD method and apparatus which substantially eliminate noxious gases which plague conventional methods
SUMMARY OF THE INVENTION
In accordance with an illustrative embodiment of the present invention, there is provided a chemical vapor deposition method that includes depositing a silicide on a substrate and purging residual gases remaining from the depositing step by flowing air including gaseous H
2
O (H
2
O(g)).
According to another illustrative embodim
Kim Young-Cheon
Lee Hyeon-Deok
Park In-Sun
Shin Ju-Cheol
Whang-Bo Chul
Meeks Timothy
Samsung Electronics Co,. Ltd.
Volentine & Francos, PLLC
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