Silicon-doped titanium wetting layer for aluminum plug

Details Silicon-doped titanium wetting layer for aluminum plug Silicon-doped titanium wetting layer for aluminum plug

1999-06-08

2001-05-15

Potter, Roy (Department: 2822)

Active solid-state devices (e.g., transistors, solid-state diode

Combined with electrical contact or lead

Of specified material other than unalloyed aluminum

Reexamination Certificate

active

06232665

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to sputter deposition processes for fabricating aluminum plugs which electrically connect different layers of a semiconductor integrated circuit. The invention also relates to the aluminum plugs created by such processes.
BACKGROUND OF THE INVENTION
Semiconductor integrated circuits generally have successive layers of semiconductor devices and metal conductors, each layer being separated by an intermediate dielectric layer. Electrical connections between semiconductors and/or conductors on different layers are accomplished by metal plugs which extend through the dielectric. A plug generally is fabricated by etching a hole which extends completely through a dielectric layer, and then depositing a metal so as to fill the hole.
Aluminum (optionally doped with copper) is a desirable material for plugs because of its high electrical conductivity. However, there are a number of difficulties in fabricating aluminum plugs.
One difficulty is completely filling each hole with aluminum so as to not leave any unfilled “void” within the hole, because voids increase the electrical resistance of the plug and create mechanical instability which can lead to broken connections. To maximize the density of an integrated circuit, the holes must be as narrow as possible and must have a high aspect ratio, that is, a high ratio of length to width. It is difficult to fill a narrow, high aspect ratio hole without voids.
One potential source of voids is poor adhesion between aluminum and silicon dioxide, the most commonly used dielectric, which can cause the aluminum plug to separate or “de-wet” from the side wall of the hole, thereby creating a void. A conventional solution to this problem is to deposit a wetting layer of titanium on the wall of the hole before filling the hole with aluminum. However, when depositing aluminum over titanium, the titanium and aluminum interdiffuse and react to form titanium trialuminide (TiAl
3
), which causes a number of problems. One problem caused by the formation of TiAl
3
is an increase in the resistance of the aluminum plug, because TiAl
3
has much higher resistivity than aluminum. (P. R. Besser et al., “Effect of Si on TiAl
3
Formation in TiAl Alloy Bilayers,” Mat. Res. Soc. Symp. Proc., vol. 355, pp. 631-636, 1995.) Another problem caused by the formation of TiAl
3
is the creation of voids within the plug, because TiAl
3
occupies less volume than the elemental titanium and aluminum consumed to produce the TiAl
3
. (R. K. Nahar et al., “Effect of Si on the reaction kinetics of Ti/AlSi bilayer structures,” Appl. Phys. Lett., vol. 50, no. 3, pp. 130-131, 1987.) A third problem is that the formation of TiAl
3
on the side wall of a hole narrows the aperture of the hole, and thereby impedes filling the remainder of the hole with aluminum.
One method that has been proposed for inhibiting interdiffusion between aluminum and an underlying titanium layer is to dope the aluminum with a small amount of silicon. (P. R. Besser et al., supra; and R. K. Nahar et al., supra.) However, incorporating any silicon in an aluminum plug is undesirable because conventional chemistries for etching aluminum will not react significantly with silicon, hence will leave behind unreacted silicon in the form of solid particles which can contaminate the semiconductor workpiece.
SUMMARY OF THE INVENTION
One aspect of the invention is a process for fabricating metal plugs, such as aluminum plugs, in a semiconductor workpiece. The invention is suitable for filling narrow, high aspect ratio holes, and the invention minimizes the formation of TiAl
3
or other reaction products arising from interdiffusion between the metal plug and the wetting layer. First, a wetting layer is created by covering the side walls of a hole with a titanium film doped with silicon. Second, the hole is filled by depositing a material consisting primarily of aluminum.
Advantageously, the silicon atoms in the wetting layer inhibit the titanium from reacting with the aluminum, and the wetting layer facilitates filling the hole with the aluminum material without leaving unfilled voids.
Preferably, the wetting layer is created by sputter deposition using a titanium sputtering target containing 0.1% to 20% wt silicon, most preferably 5% to 10% wt silicon. The hole preferably is filled by sputter deposition using an aluminum sputtering target, optionally containing dopants such as copper. To facilitate filling the hole without voids, the aluminum sputter deposition preferably is performed “hot”, specifically, with the workpiece at a temperature below the melting point of aluminum but high enough to promote reflow of the deposited aluminum.


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