Tubular magnet as center pole in unbalanced sputtering...

Chemistry: electrical and wave energy – Apparatus – Coating – forming or etching by sputtering

Reexamination Certificate

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C204S298200

Reexamination Certificate

active

06663754

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to sputtering of materials. In particular, the invention relates to the magnetron creating a magnetic field to enhance sputtering.
2. Background Art
Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of semiconductor integrated circuits. The semiconductor industry typically uses DC magnetron sputtering in which a wafer to be sputter deposited is placed in opposition to a metal target across a plasma reactor chamber filled with an argon working gas. The target is biased sufficiently negatively with respect to the chamber that the argon is excited into a plasma. The positively charged argon ions are strongly accelerated toward the target and sputter metal atoms from the target. The metal atoms dislodged from the target fall at least in part on the wafer and are deposited in a layer thereon.
In metal sputtering, the target or its least its inner surface has substantially the same metallic composition as that desired for the sputter deposited layer, for example, aluminum, copper, titanium, tantalum, tungsten, etc. In reactive sputtering, a chemically reactive gas such as nitrogen is additionally supplied into the chamber and the reactive gas reacts with sputtered metal atoms near the wafer surface to deposit a metal compound on the wafer, such as the refractory metal nitrides TiN, TaN, WN. The refractory nitrides are particularly useful as barrier layers between a dielectric and a later sputtered metal layer, and the associated refractory metal is often used as a glue layer promoting adhesion of the metal to the dielectric. Accordingly, it is often advantageous to use the same sputter reactor to deposit a bilayer liner of, for example, Ti/TiN, Ta/TaN, or W/WN. Sputtering is also used to coat the sides of a via hole with a thin copper seed layer that nucleates and provides an electrode for subsequent filling of copper into the hole by electrochemical plating (ECP).
However, for advanced integrated circuits, sputtering suffers from the fundamental problem that sputter deposition, as described above, is primarily a ballistic process between the target and wafer in which the sputtered atoms are emitted in a broad pattern about the normal to the target. Such a distribution is ill suited to filling narrow holes, such as via holes extending through an inter-level dielectric layer separating two layers of metallization. Such via holes in advanced devices have aspect ratios of 3:1 and greater. A broad sputtering pattern causes the top of the hole to close before the bottom is filled. That is, voids are created in the sputtered via metallization. Similarly, sputtered liner layers tend to be much thicker at the top of the via hole than at the bottom.
One method of adapting sputtering to deep hole filling, as well as other applications, is self-ionized plasma (SIP) sputtering, as disclosed by Fu in U.S. patent application Ser. No. 09/249,468, filed Feb. 12, 1999 and now issued as U.S. Pat. No. 6,290,825. and by Chiang et al. in U.S. patent application Ser. No. 09/414,014, filed Oct. 8, 1999 and now issued as U.S. Pat. No. 6,398,929, both incorporated herein by reference in their entities. SIP sputtering allows a significant fraction of the sputtered atoms to be ionized using a somewhat conventional sputtering reactor. The sputtered metal ions can be electrically attracted into narrow via holes in the wafer. Furthermore, the sputtered metal ions can in part be attracted back to the target to further sputter the target, thereby allowing the pressure of the argon working gas to be significantly decreased. In the case of copper, it is possible to eliminate the need for the argon working gas after the plasma has been ignited in a process called sustained self-sputtering (SSS).
An example of a SIP sputter reactor
10
is schematically illustrated in cross section in FIG.
1
. It includes chamber wall
12
supporting a biased metal target
14
through a dielectric isolator
16
. A wafer
18
is held on a pedestal electrode
20
by, for example, a clamping ring
22
although an electrostatic chuck may alternatively be used. The chamber walls
12
are protected from sputter deposition by an electrically grounded shield
24
, which also acts as an anode to the target cathode. An electrically floating shield
26
supported on a second dielectric isolator
28
is arranged about a central chamber axis
30
between the grounded shield
24
and the target
14
. A negative charge inherently builds up on the floating shield
26
during sputtering and repels plasma electrons, thereby reducing electron leakage and extending the plasma closer to the wafer
18
.
Argon working gas is supplied into the chamber
12
from a gas supply
32
and is metered by a mass flow controller
34
. The working gas flows into the processing region through a gap
35
between the pedestal
20
, the grounded shield
24
, and the wafer clamp
22
. A vacuum pumping system
36
connected to a pumping port
38
maintains the interior of the chamber
12
at a low but controllable pressure. A negative DC power supply
40
biases the target
14
to about −600 VDC, which after ignition excites the argon working gas into a plasma. The negative bias attracts the ions to the target
14
, where they sputter target atoms, which are thereafter deposited on the wafer
18
to form a layer of sputtered material. An RF power supply
42
applies RF power to the pedestal electrode
20
, which causes it to develop a negative DC self-bias in the presence of a plasma. A computerized controller
44
controls the power supplies
40
,
42
, the mass flow controller
32
, and the pumping system
36
, thereby controlling the sputtering conditions.
A magnetron
50
is located in back of the target
14
to generate a magnetic field adjacent to the front (bottom) of the target
14
. The magnetic field traps electrons, which raises the plasma density in a high-density plasma region
52
, thereby increasing the sputtering rate. An argon chamber pressure of about 6 to 10 milliTorr is typically required to ignite the plasma. However, if the density of metal ions in the high-density plasma region
52
is sufficiently high, the supply of argon can be reduced and sometimes eliminated so that a significant portion if not all of the target sputtering is effected by metal ions in the SIP process. Chamber pressure for SIP sputtering can be reduced to well below 1 milliTorr. The very low sputtering pressures are advantageous in reducing scattering of the sputtered atoms as they move towards the wafer and in reducing the temperature of the wafer since energetic argon ions are no longer bombarding it.
SIP sputtering is promoted by high target power and a small-area intense magnetic field produced by the magnetron
50
, as well as designing the magnetron to minimize plasma leakage to the shields and target. Such a magnetron
50
includes an inner magnet pole
53
of one magnetic polarity surrounded by an outer magnet pole
54
of the other magnetic polarity in a nested configuration. One or both magnet poles
53
,
54
may be composed of multiple magnets with perhaps a pole face linking the magnets within the pole. The illustrated magnetic polarities are the polarities at one end of the magnets with the other ends having the unillustrated opposite polarity. The inner and outer magnet poles
53
,
54
are magnetically coupled by a magnetic yoke
56
on their sides away from the target
30
. The magnetron
50
is an unbalanced magnetron in which the total magnetic flux, that is, flux density integrated over the surface of the pole face, produced by the outer pole
54
is significantly larger than the total magnetic flux produced by the inner pole
53
, for example, by a factor of at least 1.5. The integrated magnetic flux may be referred to as the total magnetic intensity. The unbalanced magnetron
50
produces a magnetic field distribution which has components extending from

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