Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
1999-07-08
2001-12-11
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S627000, C438S629000, C438S635000, C438S643000, C438S648000, C438S653000, C438S656000, C438S658000, C438S672000, C438S685000, C438S688000
Reexamination Certificate
active
06329282
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of improving the texture of aluminum metallization for tungsten etch back processing.
2. Brief Description of the Prior Art
In the fabrication of semiconductor devices, an often required step is that of chemical vapor deposited (CVD) tungsten plug processing wherein a titanium/titanium nitride stack is deposited prior to the CVD tungsten deposition. Etch back is then performed to remove the unwanted tungsten on the field oxide in order to reduce the lead resistance. Aluminum alloy films are then deposited for the leads. Typically, the tungsten etch back is stopped by the titanium nitride layer and a titanium nitride/aluminum/titanium nitride (TiN/Al/TiN) stack is deposited for the lead because better electromigration resistance is observed for this type of stack structure.
It has, however, been observed that there is texture or crystallographic orientation degradation of the TiN/Al/TiN stack when that stack is disposed over an underlying Ti/TiN stack which has gone through the tungsten etch back processing. It is desired that the (111) crystallographic orientation of the aluminum be, to the greatest possible extent, normal to the surface of the aluminum layer. For example, without etch back, the TiN/Al/TiN stack can have the texture of aluminum with (111) crystallographic orientation (Al(111)) texture of about 1.4 degrees for x-ray rocking curve full-width-at half-maximum (FEWM. However, after etch back, the FWHM dropped to about 3.5 degrees. This change is attributed to the damage to the TiN surface due to the tungsten etch back processing prior to the deposition of the aluminum.
Aluminum with (111) texture in a direction normal to the surface of the aluminum layer is most beneficial for electromigration improvements. The texture is controlled by the deposition conditions and, most profoundly, by the substrates (e.g. aluminum films can develop strong (111) texture when titanium is used beneath the aluminum). However, to prevent interaction between the aluminum and the titanium, it is necessary to form a layer of titanium nitride or other barrier material between the aluminum and the titanium. Also, the titanium nitride layer can prevent the reaction between tungsten hexafluoride (WF
6
) (generally used for tungsten plug filling) and titanium. Fortunately, titanium nitride has an atomic arrangement similar to that of aluminum (111) and titanium (0001). Therefore, by controlling the orientation of the titanium, the texture of the titanium can be transferred to the titanium nitride and then to the aluminum.
In accordance with standard fabrication techniques there is provided a substrate having a metal layer and an oxide layer thereon with the oxide layer extending over the metal layer and having a via extending therethrough to the metal layer, such as, for example, silicon dioxide on silicon. A layer of titanium is deposited over the oxide layer including the interior walls of the via and a titanium nitride layer is deposited over the entire layer of titanium including the interior walls of the via. Tungsten is then deposited over entire surface to cover the titanium nitride layer, fill the via. The tungsten layer is thereby separated from the oxide layer by the titanium and the layer of titanium nitride. The tungsten layer is then etched back using an appropriate etchant, preferably SF
6
as the etchant, with the titanium nitride layer acting as an etch stop so that the tungsten remains only within the via. As a result of this etch back of tungsten step, a degree of contamination remains on the surface of the titanium nitride layer and exposed tungsten within the via in the form of elemental sulfur, fluorine and possibly other contaminants. Accordingly, when an aluminum interconnect layer is then deposited over the exposed titanium nitride and tungsten within the via, due to the contamination, very little of the aluminum will have the desired (111) crystallographic orientation normal to the surface of the aluminum. As described above, it is highly desirable to have as much of the aluminum as possible having the (111) crystallographic orientation normal to the surface of the aluminum. Accordingly, a solution to this problem is highly desirable.
SUMMARY OF THE INVENTION
In accordance with the present invention, the texture of the titanium nitride layer and the aluminum is improved relative to the prior art, thereby providing a much high degree of aluminum with (111) crystallographic orientation relative to the surface of the aluminum layer than was obtained in the prior art.
Briefly, the CVD tungsten is deposited to fill the via or contact and the etch back of the tungsten, preferably using SF
6
, to remove the tungsten on the field oxide and over and external to the via proceeds as in the prior art with tungsten remaining in the vias with the etch back process controlled with end point detection such that the etch is stopped at the titanium nitride layer. A slight over etch is required to remove unetched tungsten nodules on the field.
At this point, in accordance with the present invention, an in-situ plasma, preferably an argon
itrogen plasma (though other plasmas can be used such as, for example, nitrogen, nitrogen/hydrogen, nitrogen/ammonia and combinations thereof), is used to remove the residual contaminants, such as, for example, sulfur and fluorine and any other contaminants on the titanium nitride surface when an SF
6
etchant is used. The addition of nitrogen to the plasma also assists in passivation of the tungsten surface with the formation of tungsten nitride which prevents reaction between the aluminum and the tungsten. A dramatic improvement was shown with this passivation processing to prevent the reaction between the aluminum and the tungsten even after annealing at 500° C. for 2.5 hours. Annealing generally takes place in a multilevel metallization process after completion of all other processing steps and generally at a temperature of from about 400 to about 500 degrees C.
A TiN/Al/TiN stack is then deposited on the undamaged and passivated TiN surface, it being understood that the first TiN layer can but need not be eliminated in view of the formation of the tungsten nitride layer over the exposed tungsten as described above. Hyper-textured aluminum was obtained with this cleaning and passivation process. It should be understood that in all cases wherein aluminum is used, an appropriate aluminum based alloy can be substituted therefore.
REFERENCES:
patent: 5318924 (1994-06-01), Lin et al.
patent: 5780908 (1998-07-01), Sekiguchi et al.
patent: 6069072 (2000-05-01), Konecni et al.
patent: 6159851 (2000-12-01), Chen et al.
Hong Qi-Zhong
Hsu Wei-Yung
Brady W. James
Gurley Lynne
Hoel Carlton H.
Niebling John F.
Telecky , Jr. Frederick J.
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