Method of forming a contact

Details Method of forming a contact Method of forming a contact

1997-03-12

2001-10-30

Everhart, Caridad (Department: 2825)

Semiconductor device manufacturing: process

Coating with electrically or thermally conductive material

To form ohmic contact to semiconductive material

C438S656000, C438S664000, C438S680000, C438S661000

Reexamination Certificate

active

06309967

ABSTRACT:

BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates to the formation of high aspect ratio submicron VLSI contacts. More specifically, the present invention is directed to depositing germanium from germane gas using an LPCVD process into a contact opening in order to remove native silicon dioxide from the contact opening. The layer of germanium in the bottom of the contact opening is consumed during annealing to form a low resistance contact.
2. The Relevant Technology
The movement toward progressive miniaturization of semiconductor devices has resulted in increasingly compact and efficient semiconductor structures. This movement has been accompanied by an increase in the complexity and number of such structures aggregated on a single semiconductor integrated chip. As feature sizes are reduced, new problems arise which must be solved in order to economically and reliably produce the semiconductor devices. The submicron features which must be reduced include, for instance, the width and spacing of metal conducting lines as well as the size of various geometric features of active semiconductor devices.
As an example, the requirement of submicron features in semiconductor manufacturing has necessitated the development of improved means of making contact with the various structures. The smaller and more complex devices are achieved, in part, by reducing device sizes and spacing and by reducing the junction depth of regions formed in the semiconductor substrate. Among the feature sizes which are reduced in size are the contact openings through which electrical contact is made to active regions in the semiconductor devices. As both the contact size and junction depth are reduced, new device metallization processes are required to overcome the problems which have been encountered.
Historically, device interconnections have been made with aluminum or aluminum alloy metallization. Aluminum, however, presents problems with junction spiking. Junction spiking results in the dissolution of silicon into the aluminum metallization and aluminum into the silicon. Typically, when aluminum contacts with the silicon substrate directly, the aluminum eutectically alloys with the silicon substrate at temperatures lower than 450° C. When such a reaction occurs, silicon is dissolved into the aluminum electrode, and there is a tendency for silicon thus dissolved into the electrode to be precipitated at a boundary between the electrode and the substrate as an epitaxial phase. This increases the resistivity across the contact. Furthermore, aluminum in the electrode is diffused into the silicon substrate from the electrode and forms an alloy spike structure in the substance.
The resulting alloy spike structure is a sharp, pointed region enriched in aluminum. The alloy spikes can extend into the interior of the substrate from the boundary between the electrode and the substrate to cause unwanted short circuit conduction at the junction of the semiconductor in the substrate, particularly when the junction is formed in an extremely shallow region of the substrate. When such an unwanted conduction occurs, the semiconductor device no longer operates properly. This problem is exacerbated with smaller device sizes, because the more shallow junctions are easily shorted, and because the silicon available to alloy with the aluminum metallization is only accessed through the small contact area, increasing the resultant depth of the spike.
Contact openings have also been metallized with chemical vapor deposited tungsten. This process has also proven problematic. The tungsten is typically deposited in an atmosphere of fluorine, which attacks the silicon, creating “wormholes” into the active region. Wormholes can extend completely through the active region, thereby shorting it out and causing the device to fail. Tungsten also presents a problem in that it does not adhere well directly to silicon.
3. Prior State of the Art
In order to eliminate the problems associated with the reaction between the silicon substrate and the metallization material, prior art solutions have typically used a diffusion barrier structure in which the reaction between the silicon substrate and the electrode is blocked by a barrier layer provided between the electrode and the substrate. Such a barrier layer prevents the diffusion of silicon and aluninum. It also provides a surface to which the tungsten will adhere and which will prevent tungsten and fluorine from diffusing into the active region. Prior art
FIGS. 1 through 4
of the accompanying illustrations depict one conventional method known in the art of forming contacts having a diffusion barrier. In
FIG. 1
, a contact opening
18
is etched through an insulative layer
16
overlying an active region
14
on a silicon substrate
12
. Insulating layer
16
typically comprises a passivation layer of intentionally formed silicon dioxide in the form of borophosphosilicate glass (BPSG). Contact opening
18
provides access to active region
14
by which an electrical contact is made. Layer
20
is a thin native oxide layer which forms on the active region from exposure to ambient. As shown in
FIG. 2
, a titanium metal layer
22
is then sputtered over contact opening
18
so that the exposed surface of active region
14
is coated.
A high temperature anneal step is then conducted in an atmosphere of predominantly nitrogen gas (N
2
). Native oxide layer
20
is dissolved and titanium metal layer
22
is allowed to react with active region
14
and change titanium metal layer
22
into a dual layer. As shown in
FIG. 3
, a layer of titanium silicide (TiSi
x
)
26
is formed by the anneal step, and provides a conductive interface at the surface of active region
14
. A layer of titanium nitride (TiN
x
)
24
is also formed, and acts as a diffusion barrier to the interdiffusion of tungsten and silicon or aluminum and silicon, as mentioned above. Under such conditions, the lower portion of titanium metal layer
22
overlying active region
14
, after dissolving layer
20
, reacts with a portion of the silicon in active region
14
to form titanium silicide region
26
. Concurrently, the upper portion of titanium metal layer
22
reacts with the nitrogen gas of the atmosphere to form titanium nitride layer
24
.
The next step, shown in
FIG. 4
, is metallization. This is typically achieved by chemical vapor deposition (CVD) of tungsten, or by the deposition of aluminum using any of the various known methods. These include aluminum reflow sputtering, and chemical vapor deposition. In the case of tungsten, the titanium nitride helps improve the adhesion between the walls of the opening and the tungsten metal. In the case of both tungsten and aluminum, the titanium nitride acts as a barrier against the diffusion of the metallization layer into the diffusion region and vice-versa.
Spiking and wormholes can still occur, even with the use of a deposition barrier, particularly when the diffusion barrier is too thin. This frequently occurs at the comers of the contact opening, where it is difficult to form a thick layer, particularly if the aspect ratio of the contact is high. Contact opening
18
of
FIG. 3
is filled by a layer of aluminum
32
in
FIG. 4
which depicts the effects of spiking, with a spike
34
extending through active region
14
, the effect of which is to short active region
14
out.
The compound titanium nitride (TiN) is well suited to forming a diffusion barrier, as it is extremely hard, chemically inert, an excellent conductor, and has a high melting point. It also makes excellent contact with other conductive layers. Titanium nitride is typically formed by the reaction of sputtered titanium during annealing in nitrogen, or can be deposited directly on the substrate by reactive sputtering, evaporation, chemical vapor deposition and the like before the deposition of the metallization.
As device dimensions continue to shrink and the contact openings become deeper and narrower, contact walls become vertical and most of the metal deposition techniqu

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