Self ionized plasma sputtering

Details Self ionized plasma sputtering Self ionized plasma sputtering

2001-07-30

2002-12-24

Ryan, Patrick (Department: 1745)

Chemistry: electrical and wave energy

Apparatus

Coating, forming or etching by sputtering

C204S298160

Reexamination Certificate

active

06497802

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to sputtering of materials. In particular, the invention relates to the magnet and sputtering conditions used to enhance sputtering.
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. A conventional PVD reactor
10
is illustrated schematically in cross section in
FIG. 1
, and the illustration is based upon the Endura PVD Reactor available from Applied Materials, Inc. of Santa Clara, Calif. The reactor
10
includes a vacuum chamber
12
sealed to a PVD target
14
composed of the material, usually a metal, to be sputter deposited on a wafer
16
held on a heater pedestal
18
. A shield
20
held within the chamber protects the chamber wall
12
from the sputtered material and provides the anode grounding plane. A selectable DC power supply
22
negatively biases the target
14
to about −600 VDC with respect to the shield
20
. Conventionally, the pedestal
18
and hence the wafer
16
are left electrically floating.
A gas source
24
supplies a sputtering working gas, typically the chemically inactive gas argon, to the chamber
12
through a mass flow controller
26
. In reactive metallic nitride sputtering, for example, of titanium nitride, nitrogen is supplied from another gas source
27
through its own mass flow controller
26
. Oxygen can also be supplied to produce oxides such as Al
2
O
3
. The gases can be admitted to the top of the chamber, as illustrated, or at its bottom, either with one or more inlet pipes penetrating the bottom of the shield or through the gap between the shield
20
and the pedestal
18
. A vacuum system
28
maintains the chamber at a low pressure. Although the base pressure can be held to about 10
−7
Torr or even lower, the pressure of the working gas is typically maintained at between about 1 and 1000 mTorr. A computer-based controller
30
controls the reactor including the DC power supply
22
and the mass flow controllers
26
.
When the argon is admitted into the chamber, the DC voltage between the target
14
and the shield
20
ignites the argon into a plasma, and the positively charged argon ions are attracted to the negatively charged target
14
. The ions strike the target
14
at a substantial energy and cause target atoms or atomic clusters to be sputtered from the target
14
. Some of the target particles strike the wafer
16
and are thereby deposited on it, thereby forming a film of the target material. In reactive sputtering of a metallic nitride, nitrogen is additionally admitted into the chamber
12
and it reacts with the sputtered metallic atoms to form a metallic nitride on the wafer
16
.
To provide efficient sputtering, a magnetron
32
is positioned in back of the target
14
. It has opposed magnets
34
,
36
creating a magnetic field within the chamber in the neighborhood of the magnets
34
,
36
. The magnetic field traps electrons and, for charge neutrality, the ion density also increases to form a high-density plasma region
38
within the chamber adjacent to the magnetron
32
. The magnetron
32
is usually rotated about the center of the target
14
to achieve full coverage in sputtering of the target
14
. The form of the magnetron is a subject of this patent application, and the illustrated form is intended to be only suggestive.
The advancing level of integration in semiconductor integrated circuits has placed increasing demands upon sputtering equipment and processes. Many of the problems are associated with contact and via holes. As illustrated in the cross-sectional view of
FIG. 2
, via or contact holes
40
are etched through an interlevel dielectric layer
42
to reach a conductive feature
44
in the underlying layer or substrate
46
. Sputtering is then used to fill metal into the hole
40
to provide inter-level electrical connections. If the underlying layer
46
is the semiconductor substrate, the filled hole
40
is called a contact; if the underlying layer is a lower-level metallization level, the filled hole
40
is called a via. For simplicity, we will refer hereafter only to vias. The widths of inter-level vias have decreased to the neighborhood of 0.25 &mgr;m and below while the thickness of the inter-level dielectric has remained nearly constant at around 0.7 &mgr;m. That is, the via holes have increased aspect ratios of three and greater. For some advanced technologies, aspect ratios of six and even greater are required.
Such high aspect ratios present a problem for sputtering because most forms of sputtering are not strongly anisotropic so that the initially sputtered material preferentially deposits at the top of the hole and may bridge it, thus preventing the filling of the bottom of the hole and creating a void in the via metal.
It has become known, however, that deep hole filling can be facilitated by causing a significant fraction of the sputtered particles to be ionized in the plasma between the target
14
and the pedestal
18
. The pedestal
18
of
FIG. 1
, even if it is left electrically floating, develops a DC self-bias, which attracts ionized sputtered particles from the plasma across the plasma sheath adjacent to the pedestal
18
and deep into the hole. Two associated measures of the effective of hole filling are bottom coverage and side coverage. As illustrated schematically in
FIG. 2
, the initial phase of sputtering deposits a layer
50
, which has a surface or blanket thickness of s
1
, a bottom thickness of s
2
, and a sidewall thickness of s
3
. The bottom coverage is equal to s
2
/s
1
, and the sidewall coverage is equal to s
3
/s
1
. The model is overly simplified but in many situations is adequate.
One method of increasing the ionization fraction is to create a high-density plasma (HDP), such as by adding an RF coil around the sides of the chamber
12
of FIG.
1
. An HDP reactor not only creates a high-density argon plasma but also increases the ionization fraction of the sputtered atoms. However, HDP PVD reactors are new and relatively expensive. It is desired to continue using the principally DC sputtering of the PVD reactor of FIG.
1
.
Another method for increasing the ionization ratio is to use a hollow-cathode magnetron in which the target has the shape of a top hat. This type of reactor, though, runs very hot and the complexly shaped targets are very expensive.
It has been observed that copper sputtered with either an inductively coupled HDP sputter reactor or a hollow-cathode reactor tends to form an undulatory film on the via sidewall and the deposited metal tends to dewet. This is particularly serious when the sputtered copper layer is being used as a seed layer for a subsequent deposition process such as electroplating to complete the copper hole filling.
A further problem in the prior art is that the sidewall coverage tends to be asymmetric with the side facing the center of the target being more heavily coated than the more shielded side. Not only does this require excessive deposition to achieve a seed layer of predetermined thickness, it causes cross-shaped trenches used as alignment indicia in the photolithography to appear to move as the trenches are asymmetrically narrowed.
Another operational control that promotes deep hole filling is low chamber pressure. At higher pressures, there is a higher probability that sputtered particles, whether neutral or ionized, will collide with atoms of the argon carrier gas. Collisions tend to neutralize ions and to randomize velocities, both effects degrading hole filling. However, as described before, the sputtering relies upon the existence of a plasma at least adjacent to the target. If the pressure is reduced too much, the plasma collapses, although the minimum pressure is dependent upon several factors.
The extreme of low-pressure plasma sputtering is sustained self-sputtering (SSS), as disclosed by Fu et al. in U.S. patent application Ser. No. 08/854,008, filed May 8, 1997.

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