Abrading – Precision device or process - or with condition responsive... – Computer controlled
Reexamination Certificate
2001-05-17
2002-12-17
Nguyen, George (Department: 3723)
Abrading
Precision device or process - or with condition responsive...
Computer controlled
C451S008000, C451S287000
Reexamination Certificate
active
06494765
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of polishing, in particular to the field of chemical mechanical polishing. More particularly, the invention relates to methods and apparatus for controlling and optimizing chemical mechanical polishing processes and materials for polishing substrates used in the manufacture of semiconductor wafers and integrated circuits.
BACKGROUND OF THE INVENTION
Polishing processes play significant role in modern technologies, in particular in semiconductor fabrication. For example, at certain stages in the fabrication of devices on a substrate, it may become necessary to polish the surface of the substrate before further processing may be performed. In a polishing process, known as mechanical polishing, a polishing pad with abrasive particles repetitively passes over the surface of the substrate. Polishing may also be performed with a chemically active abrasive slurry. A polishing system that uses a chemical slurry is commonly known as a chemical mechanical polishing (CMP) system. In contrast with mechanical polishing, the slurry in a CMP system provides an increased removal rate of a substrate material. Additionally, by selecting particular chemicals, chemical slurry can be used to selectively polish certain films on a semiconductor substrate.
Chemical mechanical planarization, commonly referred to as CMP, may be used as a preparation step in the fabrication of substrates or semiconductor wafers to provide substantially planar front and back sides thereon. CMP is also used to remove high elevation features, or other discontinuities, which are created on the outermost surface of a wafer during the fabrication of microelectronic circuitry on the substrate.
The planarization method typically requires that the substrate be mounted in a wafer head or carrier, with the surface of the substrate to be polished exposed. The substrate supported by the head is then placed against a moving polishing pad mounted on a platen. The head holding the substrate may also rotate, to provide additional motion between the substrate and the polishing pad surface. Further, a polishing slurry (typically including an abrasive and at least one chemically reactive agent therein, which are selected to enhance the polishing of the topmost film layer of the substrate) is supplied to the pad to provide an abrasive chemical solution at the interface between the pad and the substrate. Pressure may be applied between the carrier and the platen to effectuate polishing. In some CMP machines the wafer rotates while the polishing pad is stationary, in others the pad moves while the wafer carrier is stationary, and in yet another type both the wafer carrier and the pad move simultaneously. The polishing pad may be pre-soaked and continually re-wet with a slurry that may have a variety of abrasive particles suspended in a solution of chemicals.
DESCRIPTION OF THE RELATED ART
U.S. Pat. No. 5,597,341 issued on Jan. 28,1997 to Kodera, et al, U.S. Pat. No. 5,234,867 issued on Aug. 10, 1993 to Schultz, et al., and U.S. Pat. No. 5,232,875 issued on Aug. 3, 1993 to Tuttle, et al illustrate several techniques and corresponding types of CMP systems for chemical mechanical planarization of the semiconductor wafer surfaces.
One type of CMP systems is shown schematically in FIG.
1
a. In this system a polishing pad
10
a
is mounted on a platen
12
a,
which rotates by means of a first motor
14
a
through a transmission
16
a.
A wafer
20
a
with a front surface
22
a
to be polished is held on a head
24
a.
In the illustrated apparatus, the polishing pad
10
a
has a diameter significantly larger than that of the wafer
20
a
(
FIG. 1
a
). The polishing head
24
a
is rotated by means of a second motor
26
a
through a transmission
28
a
and comprises a retaining ring
30
a,
which prevents the wafer from slipping out of the head during polishing. A slurry feeding system
32
a
pours a slurry on the top working surface of the pad
10
a.
FIG. 1
b
illustrates another embodiment of the aforementioned known CMP system. In this embodiment, a polishing pad
10
b
is mounted on a platen
12
b,
which is rotated by means of a first motor
14
b
through a transmission
16
b.
A wafer
20
b
with a front surface
22
b
to be polished is held on a head
24
b.
In the illustrated apparatus, the polishing pad
10
b
has a diameter significantly smaller than that of the wafer
20
b
(
FIG. 1
b
). The polishing head
24
b
is rotated by means of a second motor
26
b
through a transmission
28
b
and comprises a retaining ring
30
b,
which prevents the wafer from slipping out of the head during polishing. A slurry feeding system
32
b
pours a slurry on the front surface of the wafer
22
b.
In order to provide uniformity of polishing, in the CMP systems of the types shown in
FIGS. 1
a
and
1
b,
the distance between the polishing pad rotational axis and the wafer rotational axis is typically varied in an oscillatory manner. For this purpose, the substrate is repeatedly moved back and forth relative to the polishing pad. In
FIGS. 1
a
and
1
b
the oscillatory movement is shown by arrows
25
a
and
25
b,
respectively.
Another type of the CMP system, shown schematically in
FIG. 2
, is disclosed, e.g., in U.S. Pat. No. 5,899,800, issued on May 4,1999 to Shendon and in U.S. Pat. No. 6,184,139, issued on Feb. 6, 2001 to Adams et al. In the CMP apparatuses of these patents, the lower head comprising a polishing pad
10
c
mounted on a platen
12
c
is driven into orbital movements by means of an orbital drive
34
with a motor
36
, while the carrier
24
c
holding the wafer
20
c
rotates about the central axis O
1
—O
1
by a motor
26
c
via a transmission
28
c.
The pad diameter is slightly larger than the diameter of the wafer
20
c.
A polishing fluid (slurry) is introduced to the wafer directly through the openings
38
a,
38
b,
. . .
38
n
in the polishing pad
10
c
with point-of-use mix, which results in better wafer uniformity and reduced slurry consumption.
The efficiency of polishing greatly depends on the pad surface conditions and may reduce with time as polishing pad is worn out. Therefore in the course of polishing, the pad surface should be refreshed or “conditioned” after a period of use to provide for a more uniform polishing rate, from wafer to wafer, and to provide for better planarization uniformity across a single wafer. During the pad conditioning process, a pad conditioner arm with an abrasive lower surface is forced to come in contact with the pad upper surface while the pad rotates or oscillates and the conditioner arm moves back and forth on the polishing pad. While the operation of conditioning is an effective way of deterring the wear of the polishing pad, the pad requires replacement if its surface conditions are not recovered by conditioning.
Important characteristics of a planarization process in semiconductor wafer fabrication are a removal rate, uniformity, and end point detection (EPD). Removal rate is the rate of material removal from the surface of semiconductor wafer being polished. Preferably, the rate of removal should be such that any surface peaks are preferentially flattened and the resultant surface is as near perfectly planar as possible. Uniformity of the material removal over the entire wafer surface is critical in order to provide required flatness and to avoid over- or under-polished areas on the wafer. Detection of the end point (i.e. the moment when predefined degree of surface flatness or non-uniformity is attained and polishing process has to be terminated) is usually associated with polishing of wafers with multiple layers on the surface, when the uppermost layer has to be completely or partially removed to expose the next layer. It requires very accurate determination of transition from one layer to another.
There are several factors that may affect all the above-mentioned parameters.
Since various materials of the upper layer on a wafer, polishing pad, slurry, and retaining ring interact in a course of polishing, the comb
Gitis Norm
Vinogradov Michael
Center for Tribology, Inc.
Nguyen George
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