Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2002-11-25
2004-11-02
Cao, Phat X. (Department: 2814)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S639000, C438S677000
Reexamination Certificate
active
06812140
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a semiconductor fabrication method. More particularly, the present invention relates to a method for improving the contact profile.
2. Description of Related Art
Sub-quarter micron multi-level metallization is not only an important technology of very large scale integration (VLSI) but also one of the key technologies for the next generation of ultra large scale integration (ULSI). The multilevel interconnects that lie at the heart of this technology require reliable formation. Reliable formation of these interconnect features is very important to the success of ultra large scale integration and to the continued effort to increase circuit density and quality on individual substrates and dies.
Generally speaking, a contact window is an opening for forming a plug which connects the device on the substrate and conductive line. Typically the method for forming the contact window comprises depositing a silicon oxide layer on a substrate whereon the devices are formed. The material of the silicon oxide layer is formed by a low pressure chemical vapor deposition (LPCVD) process and the starting material for this process is tetraethyl orthosilicate and ozone. The thickness of the silicon oxide layer is about 1000 angstroms. Next, a borophosphosilicate glass layer is formed on the silicon oxide layer as an interlayer-dielectric layer. The thickness of the borophosphosilicate glass layer is about 5000 angstroms.
A chemical mechanical polishing (CMP) process is used to planarize the surface of the borophosphosilicate glass layer. At this stage, a thermal flow process also can be used to achieve the same effect. A photo-resist layer is then formed on the borophosphosilicate glass layer. The photo-resist layer is patterned and the position of the contact window is then defined. The patterned photo-resist layer is used as a mask, the borophosphosilicate glass layer and the silicon oxide layer are etched and the contact window is formed therein. The device on the substrate is exposed in the bottom of the contact window. Typically, the device is a gate or source/drain region.
Because the device on the substrate is exposed in the bottom of the contact window, the silicon material of the substrate reacts with oxygen-containing gas and a native oxide layer forms thereon. The native oxide layer must be removed before the tungsten plug is formed to decrease the sheet resistance between the tungsten plug and the device. The method of removing the native oxide comprises a physical dry etching process and a wet etching process.
Because the native oxide layer is very thin, typically several angstroms to tens of angstroms, the plasma used in physical dry etching easily damages the substrate for the reason of using the argon ion plasma. For this reason, wet etching is typically used to remove the native oxide layer. The method of the wet etching process comprises a substrate sunken in the aqueous solution of hydrogen fluoride and ammonia to remove the native oxide layer. Subsequently, a conformal titanium/titanium nitride (Ti/TiN) layer or a tantalum/tantalum nitride (Ta/TaN) layer is deposited to cover the bottom and the sidewall of the contact window as a glue layer or a barrier layer. A tungsten layer is deposited on the glue or barrier layer by a chemical vapor deposition process and fills the contact window. Finally, a tungsten etching back process is executed to further expose the surface of the borophosphosilicate glass layer and the tungsten plug is complete.
However, there is a serious problem in the method for fabricating a tungsten plug mentioned above. The etching rates of the silicon oxide layer and the borophosphosilicate glass layer in the wet etching process are different. As a result, the contact profile is discontinuous as shown in FIG.
1
.
FIG. 1
is a cross-sectional diagram showing a contact window disclosed in the prior art. There is a device
102
in a substrate
100
. The method for forming the contact window
108
comprises depositing a silicon oxide layer
104
on the substrate
100
wherein the devices
102
are formed. Then a borophosphosilicate glass layer
106
is formed on the silicon oxide layer
104
as an inter-layer-dielectric layer. A lithographic process and an etching process are performed and the contact window
108
is then defined in the silicon oxide layer
104
and the borophosphosilicate glass layer
106
. The device
102
in the substrate
100
is exposed in the bottom of the contact window
108
. The native oxide layer (not shown in the scheme) on the surface of the device
102
is removed by a wet etching process.
For the reason that the etching rate of the silicon oxide layer
104
is faster than the etching rate of the borophosphosilicate glass layer
106
in the aqueous solution of hydrogen fluoride and ammonia, when the substrate is submerged in the aqueous solution of hydrogen fluoride and ammonia to remove the native oxide layer (not shown in the scheme), the under cut of the silicon oxide layer
104
pointed out by arrow
110
occurs.
With further reference to
FIG. 1
, the profile of the contact window
108
is discontinuous because of the under cut of the silicon oxide layer
104
. Therefore, the discontinuity of the profiles of the subsequently deposited titanium layer
112
/titanium nitride layer
114
are discontinuous as well. Because the silicon oxide layer
104
and the borophosphosilicate glass layer
106
at the position pointed out by arrow
110
are out of the protection provided by titanium layer
112
/titanium nitride layer
114
, the precursors tungsten hexafluoride and hydrogen used for tungsten deposition process penetrate through the interface of the silicon oxide layer
104
and the borophosphosilicate glass layer
106
at the position pointed out by arrow
110
. The tungsten is deposited at the interface of the silicon oxide layer
104
and the borophosphosilicate glass layer
106
and the resultant bridge shorts the device.
Reference is made to
FIG. 2
, which is a transmitting electron microscope image of FIG.
1
. All devices and material in the figure are represented by an abbreviation. For example, “BPSG” is borophosphosilicate glass. “W” is tungsten for a plug. “TEOS” is silicon oxide made from tetraethyl orthosilicate. “Si” is silicon substrate. “WSi” is tungsten silicide and “poly-Si” is poly silicon. The WSi and poly-Si comprise the gate. The bridge shorts the gate and the tungsten plug pointed out by arrow
200
in
FIG. 2
makes the transistor fail.
The etching rate of the silicon oxide layer
104
is faster than the etching rate of the borophosphosilicate glass layer
106
in the aqueous solution of hydrogen fluoride and ammonia. A conventional method of solving this problem is to change the composition of the etching solution (such as, hydrogen fluoride: ammonia: deionized water=1:7:130). The etching selectivity between the silicon oxide layer
104
and the borophosphosilicate glass layer
106
is less than 1 in this etching solution. Therefore, the under cut of the silicon oxide layer
104
in the contact window
108
will be avoided and the profile of the contact window
108
is retained.
Although the etching solution provided by the prior art can theoretically solve the problem of different etching rates for oxide and borophosphosilicate glass, this problem cannot be completely solved by this solution. When the borophosphosilicate glass is heated to a high temperature, such as during a deposition process or re-flow process, the boron ions and the phosphorous ions diffuse from the borophosphosilicate glass layer into the silicon oxide layer. The concentration of the boron and phosphorous ions decreases from the top of the borophosphosilicate glass layer to the bottom of the borophosphosilicate glass layer in a gradient. The boron and phosphorous ions have the same distribution in the silicon oxide layer. In other words, the portion of the silicon oxide layer nearer the borophosphosilicate glass layer has a higher boron and pho
Chang Ching-Tsai
Huang Kite
Cao Phat X.
Dickinson Wright PLLC
Doan Theresa T.
Winbond Electronics Corporation
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