Abrading – Abrading process – Side face of disk
Reexamination Certificate
1998-12-16
2001-07-17
Butler, Rodney A. (Department: 3725)
Abrading
Abrading process
Side face of disk
Reexamination Certificate
active
06261158
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to chemical-mechanical polishing (CMP) and, more particularly, a CMP system using two or more slurries during the CMP process.
BACKGROUND INFORMATION
CMP is often used in the fabrication of integrated circuits to planarize a surface of a wafer to facilitate subsequent photolithographic process steps or to globally remove portions of a layer formed on the wafer. In particular, CMP can be used in fabricating inter-layer interconnects (e.g., metal plugs for contacts or vias), or for forming intra-layer interconnects (e.g., copper interconnect lines in a damascene process). In addition, CMP can be used in dual damascene processes in which both inter-layer and intra-layer interconnects are formed using deposition of a single metal layer.
FIG. 1
is a diagram illustrative of a conventional CMP system
10
for polishing a wafer
11
. CMP system
10
includes a wafer carrier WC
1
to hold wafer
11
, a platen P
1
with a polishing pad
13
, and a slurry dispenser
15
with slurry S
1
. In conventional CMP system
10
, a down force is applied to wafer carrier WC
1
to achieve a polish pressure of F
D1
. To polish the surface of wafer
11
, wafer carrier WC
1
is also rotated at a rate of &ohgr;
WC1
, while platen P
1
is typically rotated in the opposite direction at a rate of &ohgr;
P1
. In this example, dispenser
15
dispenses slurry S
1
to the surface of polish pad
13
to facilitate the polishing process. In this example, slurry S
1
is a slurry designed for metal polishing. A metal slurry (i.e., a slurry for metal polishing) is typically water-based, having abrasive particles on the order of twenty to two hundred nanometers in diameter in colloidal suspension. The slurry density is about 1% to 5% by weight, with a pH typically ranging from 3 to 11.
FIGS. 2A and 2B
are diagrams illustrative of a cross-section of wafer
11
(
FIG. 1
) during a conventional single-step CWP process. To facilitate understanding of this description, the same reference numbers are used in several of the drawings to indicate elements having the same or similar function or structure. Referring to
FIG. 2A
, wafer
11
(
FIG. 1
) has a semiconductor substrate
21
with built in active device such as a transistor gate, upon which a dielectric layer
23
, a barrier layer
25
and a metal layer
27
are formed. It will be appreciated that barrier layer
25
can also serve as an adhesion or glue layer for metal layers that do not readily adhere to dielectric layer
23
. For example, metal layer
27
can be a Tungsten (W) layer, with CMP system
10
(
FIG. 1
) being used to remove the top portion of metal layer
27
to form W plugs in the contact holes in dielectric layer
23
. Barrier layer
25
would typically be formed from Titanium (Ti), Titanium Nitride (TiN) or a Ti/TiN stack for barrier and adhesion purposes.
FIG. 2B
shows the resulting structure after the conventional CMP process is performed.
More specifically, referring to
FIGS. 1
,
2
A and
2
B, wafer
11
is held in wafer carrier WC
1
, with metal layer
27
facing polishing pad
13
. Dispenser
15
dispenses metal slurry S
1
onto the polishing pad
13
at a flow rate of FR
1
. Slurry S
1
is typically chosen to more selective with respect to metal layer
27
relative to barrier layer
25
and dielectric layer
21
. Platen P
1
and wafer carrier WC
1
are rotated at rates &ohgr;
P1
, and &ohgr;
WC1
, respectively. In addition, a down force F
D1
is applied to wafer carrier WC
1
to perform this conventional metal polishing process.
In this conventional process, dielectric layer
23
is used as an polish stopping layer for the CMP process. The polishing may continue for a relatively short period after the endpoint is detected to ensure that all of barrier layer
25
is cleared from the surface of dielectric layer
23
. Because of the relatively high selectivity of slurry S
1
for metal layer
27
, the removal rate of metal layer
27
is greater than the removal rates for barrier layer
25
and dielectric layer
23
. As a result, at the end of barrier layer removal, a relatively large amount of metal recess (or dishing) occurs in the contact holes, as indicated by metal layer
27
, in
FIG. 2A
being below the level of the surrounding dielectric region. The amount of metal recess is indicated in
FIG. 2B
as &dgr; In addition, dielectric layer
23
is removed at a faster rate near the contact holes relative to the field regions. It is believed that the contact holes may cause this difference in removal rate by weakening the structural integrity of dielectric layer
23
in the region of the contact holes. This effect is referred to herein as dielectric erosion. The amount of dielectric erosion is indicated in
FIG. 2B
as a. It will be appreciated that in a typical CMP application, it is desirable to minimize both &agr; and &dgr; while maximizing the throughput of wafers being processed by CMP system
10
.
Another problem encountered in CMP is illustrated in FIG.
3
.
FIG. 3
shows a portion of dielectric layer
23
having microscratches
31
formed thereon, after being polished using metal slurry S
1
. Typically, a slurry that is effective in removing metal and barrier layers also causes microscratching of dielectric layers. Microscratching is generally undesirable in metal polishing because in severe cases, microscratching may result in short-circuiting of metal interconnects subsequently formed on the dielectric layer.
One conventional approach to solving the above dishing problem is illustrated in FIG.
4
.
FIG. 4
illustrates a two-platen two-slurry CMP system
40
. CMP system
40
includes CMP subsystems
41
and
42
. In this example, CMP subsystem
41
is implemented with CMP system
10
(FIG.
1
), and CMP subsystem
42
is essentially a duplicate of CMP subsystem
41
. In particular, CMP subsystem
42
includes a wafer carrier WC
2
, a platen P
2
with a polishing pad
43
, and a slurry dispenser
45
with slurry S
2
. A down force is applied to wafer carrier WC
2
to achieve a polish pressure of F
D2
. Wafer carrier WC
2
is rotated at a rate of &ohgr;
WC2
, while platen P
2
is rotated in the opposite direction at a rate of &ohgr;
P2
. Dispenser
45
dispenses slurry S
2
onto polishing pad
43
during this second polishing process. Subsystems
41
and
42
may be stations on a multi-station CMP machine, or separate CMP machines.
In this conventional approach, CMP subsystem
41
is used for metal polishing of wafer
11
using slurry S
1
, wafer carrier WC
1
and platen P
1
, as described above for CMP system
10
. Then, wafer
11
is cleaned (not shown) and transported to CMP subsystem
42
for dielectric polishing using a slurry S
2
. To reduce microscratching and/or dielectric erosion, slurry S
2
is optimized for polishing dielectric layer
23
(i.e., sometimes referred to as “second platen buffing”). To reduce dishing, the metal layer polishing is performed using barrier layer
25
(
FIG. 2A
) as the metal polish endpoint. Wafer
11
is then transported to CMP subsystem
42
for barrier layer polishing using slurry S
2
, which is optimized for barrier layer polishing. These conventional systems use two platens because slurries S
1
and S
2
are typically incompatible. For example, metal slurries typically have a low pH while dielectric slurries have a high pH. Mixing these slurries on the same platen generally causes the abrasive particles in the slurries to flocculate and fall out of suspension, which undesirably reduces uniformity of the metal removal. However, the need for transporting the wafer to a second platen undesirably reduces throughput of CMP system
40
. Therefore, there is a need for a CMP system that reduces dishing, dielectric erosion and microscratching without reducing wafer uniformity and throughput.
SUMMARY
In accordance with the present invention, a multi-step CMP system is provided that uses a single platen. This multi-step CMP system is advantageously used to form metal interconnects such as contact, vias and damascene lines. In this application, the w
Dai Fen
Gotkis Yehiel
Holland Karey
Mitchel Fred
Schey Dennis
Butler Rodney A.
Merchant & Gould P,C,
Speedfam-IPEC
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