Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2001-01-24
2003-04-08
Meeks, Timothy (Department: 1762)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S685000, C427S099300, C427S253000
Reexamination Certificate
active
06544889
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method for chemical vapor deposition of a layer of tungsten (W) on a semiconductor substrate.
The chemical vapor deposition of tungsten on a semiconductor substrate, such as a silicon oxide wafer which may have portions of an integrated circuit structure already formed therein, such as, for example one or more transistors, is an integral part of most semiconductor fabrication processes.
An insulating layer, mostly a silicon oxide layer has usually been formed over this substrate and has been previously patterned to provide openings or vias to underlying portions of the integrated circuit structure.
Chemical vapor deposited W has been used as a conducting material to fill contact holes or via holes. The tungsten layer covers the complete substrate surface and is then etched or polished away, except from the holes.
Since a tungsten layer cannot be deposited by chemical vapor deposition directly on a silicon oxide layer, an intermediate layer with a good adhesion for both the insulating layer and tungsten, for instance a titanium nitride (TiN) layer on top of titanium is deposited.
The tungsten is usually deposited through the reduction of tungsten hexafluoride (WF
6
) in a two-steps process. The steps are different in pressure set points and used reductor, being in the first step mainly silane (SiH
4
) and then hydrogen (H
2
) only. The largest part of the film is deposited by H
2
reduction.
U.S. Pat. No. 5,028,565 of APPLIED MATERIALS, Inc., Santa Clara, Calif., LISA, discloses such method wherein tungsten is deposited on a wafer heated from about 350 to about 525° C. in a vacuum chamber wherein the pressure is maintained from 2.67 to 101.32 kPa (from about 20 to about 760 Torr). A combination is used of WF
6
gas, an inert carrier gas such as Ar, nitrogen and hydrogen. The flow rate of WF
6
is from about 20 to about 200 standard cubic centimeters per minute (hereafter abbreviated as sccm). The flow rate of the inert carrier Ar is from about 100 to about 5000 sccm, and the flow rate of nitrogen is from about 10 to about 300 sccm. The hydrogen flow rate is from about 300 to about 3000 sccm.
The N
2
in the gas mixture has been found to increase the reflectivity of the deposited layer which facilitates the use of photolitography in a subsequent patterning step, and to decrease the surface roughness.
U.S. Pat. No. 5,028,565 discloses however also that, especially when the intermediate layer is titanium nitride, it is important to form first a nucleation layer with from about 5 to about 50 sccm of WF
6
, from about 5 to about 50 sccm silane (SiH
4
), from about 500 to about 3000 sccm of Ar and from about 20 to about 300 sccm of N
2
.
It has been found that, without the nucleation layer, the tungsten layer was not uniform in thickness and resistivity.
Literature unanimously confirms the impossibility to obtain a tungsten film with good qualities, especially a good step coverage, a good layer uniformity and a low via resistance, without these two steps. The step coverage is the ratio of the thickness of the tungsten film at the side wall at half depth of the trench or contact hole and the nominal tungsten film thickness or the thickness of top layer.
EUI SONG KIM et al. for instance mention in their article “Studies on the nucleation and growth of chemical-vapor-deposited W on TiN substrates”, published in MATERIALS SCIENCE AND ENGINEERING, B 17 (1993) 137-142, that since it is not easy to nucleate W on TiN by H
2
reduction of WF
6
, it is now common to initiate nucleation of W by SiH
4
reduction first and then grow W film to the required thickness by H
2
reduction.
CAROL M. McCONICA et al., also mention in their article “Step coverage prediction during blanket LPCVD tungsten deposition from hydrogen, silane and tungsten hexafluoride”, published in the Proceedings of the V-Mic Conference of Jun. 13-14, 1988, pages 268-276, Session VII: “VSSI Multilevel Interconnection Dielectric Systems”, that the reduction with SiH
4
or a mixture of SiH
4
and H
2
offers many advantages over the reduction by H
2
alone, such as smaller temperature dependency in the growth rate, more uniform films and a larger growth rate, but that the major disadvantage of SiH
4
is the limited step coverage, in comparison to the hydrogen reduction.
A. HASPER et al. In “W-LPCVD step coverage and modeling in trenches and contact holes”, Proceedings of the workshop on tungsten and other refractory metals for VLSI/USII applications V, 127 (1990) S. S. WONG and S. FURUKAWA ed., Materials Research Society, Pittsburg Pa., USA, mention also that the reduction with SiH
4
offers many advantages like a high and temperature independent growth rate, a smaller grain size and has less interaction with silicon, but also that, when SiH
4
is added to a WF
6
/H
2
mixture, the step coverage drops.
In general, the hydrogen reduction gives better step coverage than the silane reduction, but the deposition rate of the hydrogen reduction method is significantly lower. Consequently, the second step in the tungsten deposition is therefore without SiH
4
as in the actual method recommended by the above mentioned company APPLIED MATERIALS, INC.
This method comprises a soak step with SiH
4
, to saturate and passivate the underlying layer, a nucleation step at a pressure of 4.00 kPa (30 Torr), wherein 30 sccm WF
6
is reduced by means of a mixture of 1000 sccm H2 and SiH
4
in a flow ratio WF
6
/SiH
4
of 2, and a bulk deposition step at a second pressure of 12.00 kPa (90 Torr) wherein sccm WF
6
is reduced by means of 700 sccm H
2
alone. The wafer is heated to 475° C. during the tungsten deposition. An extra pressurizing step is necessary between both steps as there is a difference in pressure.
A similar method, but with both steps under the same pressure, is disclosed in U.S. Pat. No. 5,795,824 of NOVELLUS SYSTEMS, INC., San Jose, USA. After an initiation step by providing 15 to 75 sccm SiH
4
and 1000 sccm Ar, the tungsten deposition is carried out under a pressure from 5.33 to 10.67 kPa (40-80 Torr) during successively two deposition steps: a nucleation deposition by providing from 1000 to 15000 sccm H
2
, from 50 to 800 sccm WF
6
and from 15 to 75 sccm SiH
4
and, in a different station, a bulk deposition by providing WF
6
, H
2
and Ar gases, possibly in successive layers until the final thickness of tungsten.
All the above mentioned known methods with a reduction of tungsten hexafluoride in two steps are rather complicated and relatively slow, while a relatively complicated deposition system is required.
SUMMARY OF THE INVENTION
An object of the invention is to provide a method for tungsten chemical vapor deposition which is more simple and cheaper and has a higher deposition rate than the above mentioned prior art methods while a more simple deposition system may be used, and whereby the characteristics of the tungsten film such as the step coverage, the via resistance, the reflectivity etc. are at least equal to or better than these of a film obtained via the prior art methods.
According to the invention, this object is accomplished in a method for tungsten chemical vapor deposition on a semiconductor substrate, comprising positioning said substrate within a deposition chamber, heating said substrate and depositing under low pressure the tungsten on the substrate by contacting the latter with a mixture of gases flowing through the deposition chamber comprising tungsten hexafluoride (WF
6
), hydrogen (H
2
) and at least one carrier gas, characterized in that the mixture of gases comprises also silane (SiH
4
) with such a flow rate that the flow ratio WF
6
/SiH
4
is from 2.5 to 6, the flow rate of WF
6
being from 30 to 60 sccm, while the pressure in the deposition chamber is maintained from 0.13 to 5.33 kPa (I and 40 Torr).
It is amazing that by adjusting the flow ratio of WF
6
/SiH
4
, within the indicated pressure window, a 100% step coverage can be obtained.
Therefore, the tungsten deposition may be carried out in a single step.
Reaction efficiency is high, what re
Baele Joris
Vercammen Hans
AMI Semiconductor Belgium BVBA
Meeks Timothy
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