Aluminum metallization by a barrier metal process

Details Aluminum metallization by a barrier metal process Aluminum metallization by a barrier metal process

1993-06-28

2001-03-06

Bowers, Charles (Department: 2813)

Semiconductor device manufacturing: process

Coating with electrically or thermally conductive material

To form ohmic contact to semiconductive material

C438S652000, C438S653000

Reexamination Certificate

active

06197686

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for metallization in the preparation of semiconductor devices. More particularly, it relates to a method for aluminum metallization for uniformly burying an aluminum-based material in a contact section having a barrier metal structure.
2. Description of Related Art
In keeping up with an increasing refinement of the design rule for semiconductor devices, as exemplified by VLSIs or ULSIs of recent origin, the contact hole formed in an interlayer insulating film for establishing electrical an connection between an upper metallization and a lower metallization is becoming increasingly smaller in diameter, such that the aspect ratio now exceeds unity. The upper metallization is usually formed by depositing an Al-based material by sputtering. However, it is difficult with this method to achieve sufficient step coverage to fill the contact hole having such a high aspect ratio so that a disconnection is frequently produced.
Recently, a high temperature sputtering has been proposed as a method for improving step coverage. This technology resides in carrying out sputtering with heating a wafer by a heater block or the like to several hundreds of degrees centigrade and applying an RF bias through the heater block, as introduced in, for example, Monthly Semiconductor World, December issue, 1989, pages 186 to 188, by Press Journal, or in IEEE/IRPS, 1989, pages 210 to 214. It is possible with this method to improve step coverage under Al reflow effects under elevated temperatures and ion impacts by the application of the bias voltage for forming the Al-based layer having a flattened surface. In these treatises, it is reported that, when a titanium (Ti) layer is provided as an underlying layer for the Al-based layer, the Ti layer contributes to surface migration of Al atoms to achieve excellent step coverage.
Meanwhile, the Ti layer provided as an underlying layer for the layer of the Al-based material is naturally expected to display the function as a barrier metal layer. However, the Ti layer, while being an excellent contact material from the viewpoint of achieving low-resistance ohmic contact, cannot achieve the function of the barrier metal, if used alone. The reason is that, even if the Ti layer is interposed by itself between the silicon (Si) substrate and the Al-based layer, the reaction between Si and Ti and the reaction between Ti and Al proceed simultaneously, so that Al spikes onto the Si substrate cannot be prevented from being produced. The conventional practice has been to adopt a two-layer barrier metal composed of, for example, a TiN layer superimposed on a Ti layer (Ti/TiN system). More recently, a two-layer barrier metal composed of a Ti layer and a TiON layer (Ti/TiON system), produced by introducing oxygen during deposition of the TiN layer, has been proposed with a view to improving the effects of preventing Al diffusion in the TiN grain boundary by oxygen segregation in the boundary.
However, when barrier metal such as Ti/TiON barrier metal, is previously formed in a contact section, an Al-based material cannot be filled uniformly in the contact hole if the Al-based material is to be deposited by high temperature bias sputtering. It is assumed that, in a wafer shown in
FIG. 1
, an interlayer insulating film
3
having a contact hole
4
is formed on a silicon substrate
1
, in which an impurity diffusion layer
2
is formed previously so that the contact hole
4
is contiguous to the impurity diffusion region
2
, and a Ti layer
5
and a TiON layer
6
are stacked step by step so as to overlie at least the contact hole
4
to provide a barrier metal layer
7
. If it is attempted to deposit a layer
8
of an Al-based material on the wafer by high temperature bias sputtering, the material cannot be deposited or buried uniformly in the contact hole, because voids
9
tend to be produced. It is because Al in the course of high temperature sputtering is in an intermediate state between the liquid state and the solid state and highly sensitive to the morphology of the surface of the underlying layer. That is, the TiON layer
6
has a columnar crystal structure in which the crystals are oriented with the longitudinal direction thereof extending substantially orthogonally with respect to the film surface, so that the layer exhibits rough surface morphology and is inferior in wettability and reactivity with respect to the Al-based material. Our experiments indicated that the buried state of the Al-based material could not be improved when the deposition rate was lowered to about half the usual deposition rate for the purpose of promoting the reaction at the boundary between TiON and Al.
OBJECT AND SUMMARY OF THE INVENTION
In view of difficulties met in the prior art in forming a contact hole satisfying high barrier properties and superior step coverage properties simultaneously, an object of the present invention is to provide a method for forming a metallization which will satisfy these two requirements simultaneously.
The present invention is based on two improvements in the metallization process. One improvement resides in providing a Ti/TiON/Ti three-layer barrier metal structure and forming a layer of the Al-based material by a high temperature process, and the other resides in executing a high temperature process in at least a portion of the deposition process of the Al-based material under conditions of a lower deposition rate.
The first point of improvement is aimed at improving wettability and reactivity between the Al-based material and the underlying layer for facilitated intrusion of the Al-based material in the contact hole by using the three-layer barrier metal structure in which a Ti layer is added to the conventional Ti/TiON structure so that one of the Ti layers is arranged at a contact surface with a layer of the Al-based material. According to the present invention, the three-layer barrier metal structure is used and the Al-based material is buried in the contact hole by a one-stage high temperature process or by a two-stage process. With the latter two-stage process, a first layer of an Al-based material is deposited in a first step to a thin thickness so as not to completely fill the contact hole and a second layer of the Al-based material is deposited to a thicker thickness in the second high-temperature step. For the high temperature process, high temperature sputtering, high temperature CVD or high temperature vapor deposition, may be employed.
The second point of improvement is optimizing the conditions of the high temperature process for improving process reliability and reproducibility. According to the present invention, it is the Ti layer that is in contact with the layer of the Al-based material, as explained hereinabove. However, this Ti layer partially reflects the rough surface morphology associated with the columnar crystal structure of the underlying TiON layer, so that it is inferior in smoothness to the Ti layer formed directly on the surface of e.g. a silicon substrate. For this reason, it is highly effective to optimize the conditions for the high temperature process.
It is noted that the substrate heating temperature is selected to be 450 to 550° C. in consideration of migration characteristics on deposition of the Al-based material on the substrate. It is accepted in general that, when forming a film of an eutectic-forming alloy by a vacuum thin film forming technique, the temperature, which is not higher than about 75% of the eutectic point, is insufficient for atoms etc. flying from a target or a source of evaporation to the substrate to undergo migration on the substrate. Since the eutectic point of an Al-1% Si alloy, typical of an Al-based material, is about 580° C., the contact hole can be difficult to fill at a temperature range lower than 450° C. because the migration is suppressed at such a temperature. Conversely, at a temperature higher than 550° C., migration is promoted excessively so that the film tends to be formed in separate island

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