Sputtering target with a partially enclosed vault

Details Sputtering target with a partially enclosed vault Sputtering target with a partially enclosed vault

2002-03-12

2004-06-01

McDonald, Rodney G. (Department: 1753)

Chemistry: electrical and wave energy

Apparatus

Coating, forming or etching by sputtering

C204S298110, C204S298180, C204S298190, C204S298200, C204S298210

Reexamination Certificate

active

06743342

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to plasma sputter reactors. In particular, the invention relates to complexly shaped sputter targets.
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. Most commercial sputter reactors rely upon magnetron sputtering in a plasma reactor. The most common commercial sputter reactor is a magnetron sputter reactor in which a metal target of the material to be sputter deposited is placed in opposition to the wafer to be sputter coated. The vacuum chamber containing the wafer and target is filled with a few milliTorr of argon. The target is then electrically biased to a few hundred volts DC, which excites the argon into a plasma. The resulting positively charged argon ions are attracted to the negatively biased target and dislodge (sputter) metal atoms from the target. Some of the metal atoms fall on the wafer and coat a thin metal layer on it. Typically, a set of magnets, called a magnetron, is placed in back of the target to create magnetic field lines parallel to the front face of the target, thereby trapping electrons and increasing the plasma density adjacent the target and thus increasing the sputtering rate. In reactive sputtering, a reactive gas such as nitrogen is also admitted to the chamber, and the reactive gas reacts with the sputtered metal atoms to form a metal compound, such as a metal nitride, on the wafer surface.
The older, conventional magnetron sputter reactors produce a relatively low-density plasma of the argon ions and, as a result, the sputtered metal atoms are mostly neutral, only a few percent of them being ionized. It has become recognized in recent years that a higher fraction of metal ions would be very beneficial, particularly for coating the sides and bottoms of holes having high aspect ratios. Such holes may be via or contact holes or may be DRAM trenches. The mostly ballistic sputtering process described to this point is ill suited for reaching into holes having aspect ratios significantly larger than one at the same time that vias of modern integrated circuits often have aspect ratios of 5 and greater. However, it has been recognized that a negatively biased wafer can accelerate metal ions in the direction normal to the wafer surface, thereby draw the sputtered metal ions deep into the hole.
Generally, increasing the density of the argon plasma increases the ionization fraction of the sputtered atoms. Several approaches have been used to produce a high density plasma. In one approach, additional RF energy is inductively coupled into a plasma source region remote from the wafer. In a second approach, often called a hollow cathode reactor, a non-planar target surrounds the top and sides of a plasma region adjacent the target, thereby reducing the plasma loss and increasing the plasma density. In a third approach, often called self-ionized plasma (SIP) sputtering, a small intense magnetron concentrates the target power in a reduced area, thereby increasing the power density and hence increasing the plasma density adjacent to the magnetron. The small magnetron is scanned around the target to produce more uniform sputtering.
An advanced sputter reactor that advances on the second and third approach is the SIP
+
sputter reactor marketed by Applied Materials, Inc. of Santa Clara, Calif. and schematically illustrated in FIG.
1
. Reactors of this type have been described by Gopalraja et al. in U.S. Pat. No. 6,277,249 and U.S. patant application, Ser. No. 09/703,601, filed Nov. 1, 2000 and now issued as U.S. Pat. No. 6,451,177, both of which are incorporated by reference herein in their entireties. The lower part of the reactor
10
includes an electrically grounded chamber including sidewalls
12
generally symmetric about a central axis
14
. A vacuum pumping system
16
reduces the base pressure within the chamber to the neighborhood of 10
−8
Torr. However, working gas is supplied from an argon source
18
through a mass flow controller
20
to maintain the argon pressure in a range of 0.1 to 10 milliTorr. If a nitride film is being formed by reactive sputtering, nitrogen is additionally supplied.
A wafer
22
to be sputter coated is supported on a temperature controlled pedestal electrode
24
. The wafer
22
may be secured to the pedestal electrode
24
by a clamp ring
26
, but an electrostatic chuck may alternatively be used. A grounded shield
28
supported on the sidewalls
12
protects the chamber walls and sides of the pedestal
24
from being coated with sputtered material and further acts as a cathode for the diode sputtering process. The argon working gas is admitted into a processing space
30
over the wafer
22
through gaps between the pedestal
24
, the wafer clamp
26
, and the grounded shield
28
. The high density plasma being generated benefits from an electrically floating shield
32
supported on the grounded shield
28
through an isolator
34
.
The SIP
+
reactor
10
is most visibly distinguished by a target and magnetron assembly
40
including a vault-shaped target
42
supported on the chamber sidewalls
12
through a second isolator
43
. The target
42
is composed of the metal to be sputtered. Copper sputtering is the most prevalent initial use of the SIP
+
reactor
10
, but other metals can be used in the target
42
. The vault-shaped target
42
includes an annular vault
44
extending around the central axis
14
with its open end or throat facing the wafer
22
. The vault
44
includes an outer sidewall
46
, an inner sidewall
48
, both extending generally parallel to the central axis
14
, and a roof
50
extending generally perpendicular to the central axis
14
. A central well
52
is formed on the back of the target
42
inside the annular inner sidewall
48
. The target
42
is supported on the isolator
43
by an outwardly extending flange
54
. A projection
56
extending downwardly from the outer sidewall
46
forms a plasma dark space in opposition to the floating shield
32
.
A DC power source
58
electrically biases the target
42
to a negative voltage of about −600 VDC with respect to the grounded shield
28
. This voltage is sufficient to maintain an argon plasma within the processing space
30
. If a substantial fraction of the sputtered atoms are ionized, it is advantageous to induce a negative DC bias on the pedestal electrode
24
by biasing it with an RF power supply
60
connected to the pedestal electrode
24
through an unillustrated capacitive coupling circuit. A controller
62
controls the sputtering process and may be programmed for a multi-step process according to which it separately controls the chamber pressure, target power and wafer bias.
In magnetron sputtering, magnets are positioned in back of the target
42
to increase the plasma density adjacent to the face of the target
42
. The SIP
+
target and magnetron assembly
40
includes both stationary and rotating magnetic parts. The stationary part includes a large number of permanent magnets
70
of a first vertical polarity arranged around the outside of the outer vault sidewall
46
. A cylindrical magnet
72
of an opposite second vertical polarity is disposed within the vault well
52
behind and inside the vault inner sidewall
48
. Although the cylindrical magnet
72
is rotating for reasons relating to unillustrated target cooling, its magnetic field is essentially stationary. The two sets of magnets
70
,
72
create anti-parallel magnetic fields close to interior sides of the vault
44
adjacent the opposed sidewalls
46
,
48
. The rotating part includes a nested magnetron
74
positioned over the vault roof
50
and including an outer annular magnet
76
of the first magnetic polarity surrounding an inner cylindrical magnet
78
of the second magnetic polarity. The nested magnetron
74
is unbalanced in that the total (spatially integrated) magnetic flux produced by th

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