Abrading – Work holder – Vacuum
Reexamination Certificate
2000-06-30
2002-05-14
Banks, Derris H. (Department: 3723)
Abrading
Work holder
Vacuum
C451S289000
Reexamination Certificate
active
06386962
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to chemical mechanical planarization (CMP), and more particularly to wafer carriers for reducing edge effects during wafer processing by CMP.
2. Description of the Related Art
Fabrication of semiconductor devices from semiconductor wafers generally requires, among others, chemical mechanical planarization (CMP), buffing, and cleaning of the wafers. Modem integrated circuit devices typically are formed in multi-level structures. At the substrate level, for example, transistor devices are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive features are insulated from each other by dielectric material, such as silicon dioxide, for example. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove excessive metallization.
FIG. 1
shows a schematic diagram of a chemical mechanical planarization (CMP) process
100
performed on a semiconductor wafer
102
. In this process
100
, the wafer
102
undergoes a CMP process in a CMP system
104
. Then, the semiconductor wafer
102
is cleaned in a wafer cleaning system
106
. The semiconductor wafer
102
then proceeds to a post-CMP processing
108
, where the wafer
102
undergoes different subsequent fabrication operations, including deposition of additional layers, sputtering, photolithography, and associated etching.
The CMP system
104
typically includes system components for handling and planarizing the surface topography of the wafer
102
. Such components can be, for example, an orbital or rotational polishing pad, or a linear belt-polishing pad. The pad itself is typically made of an elastic polymeric material. For planarizing the surface topography of the wafer
102
, the pad is put in motion and a slurry material is applied and spread over the surface of the pad. Once the pad with the slurry is moving at a desired rate, the wafer
102
, which is mounted on a wafer carrier, is lowered onto the surface of the pad for planarizing the topography of the wafer surface.
In rotational or orbital CMP systems, a polishing pad is located on a rotating planar surface, and the slurry is introduced onto the polishing pad. In orbital tools the velocity is introduced via pad orbital motion and wafer carrier rotation and the slurry is introduced from underneath the wafer through multiple holes in the polishing pad. Through these processes, a desired wafer surface is polished to provide a smooth planar surface. The wafer is then provided to the wafer cleaning system
106
to be cleaned.
One of the main goals of CMP systems is to ensure the uniform removal rate distribution across the wafer surface. As is well known, the removal rate is defined by Preston's equation: Removal Rate=KpPV, where the removal rate of material is a function of loading pressure P and relative velocity V. The term, Kp, is Preston Coefficient, which is a constant determined by the composition of the slurry, the process temperature, and the pad surface.
Unfortunately, conventional CMP systems often suffer from edge effects that redistribute the removal rate and thus the uniformity across the wafer surface. The edge effects typically result from boundary conditions between a wafer edge and a polishing pad during CMP processing.
FIG. 2A
shows a cross-sectional view of a static model of conventional edge effect between a section of the wafer
102
and a polishing pad
204
. In this static model, a uniform pressure is exerted on the wafer
102
in the form of a downforce as indicated by vectors
206
. This down force
206
, however, causes a deformation, which is indicated by vectors
112
, of the pad
204
that is essentially transversal (i.e., normal) but with a substantial longitudinal-transversal perturbation zone near the edge
208
of the wafer
102
. Thus, this deformation results in a lower pressure zone
110
near the edge
208
. The edge
208
of the wafer
102
causes high pressure as indicated by vectors
111
, thereby producing non-uniform pressure areas near the edge
208
.
The creation of alternating pressure zones leads to non-uniform removal rate across the wafer.
FIG. 2B
illustrates a cross-sectional view of a dynamic model of the edge effect between a section of the wafer
102
and the polishing pad
204
. A section of a retaining ring
116
retains the wafer
102
in place to retain the wafer
102
in a wafer carrier (not shown) that controls the movement of the wafer
102
. In this configuration, the wafer
102
is in motion relative to the polishing pad
204
as indicated by vector V
rel
. The pad
204
is generally elastic. As the wafer
102
moves with the relative velocity V
rel
over the pad
204
, it thus causes elastic perturbation on the surface of the pad
204
.
The translational motion of the wafer
102
and the elastic perturbation produce a longitudinal-transversal pad deformation wave on the surface
114
of the polishing pad
204
according to conventional wave generation theory. The deformation wave is typically a fast relaxing wave due to suppressive action of the extended wafer surface and the high viscosity of the pad material. This causes local redistribution of the loading and pressures near the edge
208
of the wafer
102
. For example, low pressure zones
120
,
122
, and
124
are formed on the surface
114
of the pad
204
with progressively higher pressures relative to the distance from the edge
208
of the wafer
102
.
Each of the low pressure zones
120
,
122
, and
124
is defined by local minimum and maximum pressure regions that cause uneven planarization of the surface topography. For example, the local minimum pressure region
126
of the low pressure zone
120
causes lower removal rates, resulting in local under-planarization of the surface topography. Conversely, the local maximum pressure region
128
of the low pressure zone
120
causes higher removal rates, resulting in local over-planarization of the surface topography. Thus, the overall planarization efficiency of the wafer
102
is substantially degraded.
Furthermore, in conventional CMP systems the frontal wave maximum produces sealing effect at the edge of a wafer that substantially reduces entry of slurry under the wafer.
FIG. 2C
shows a cross-sectional view of a sealing effect between a section of the wafer
102
and the polishing pad
204
. The slurry is initially provided over the surface
114
of the polishing pad
204
. As the wafer
102
moves with velocity Vrel relative to the polishing pad
204
, the edge
208
of the wafer causes a high pressure as indicated by vector
152
. This high pressure causes loading concentration of the slurry
150
at the edge
208
of the wafer
102
, thereby restricting slurry transport underneath the wafer
102
. In addition, high loading at the edge
208
may squeeze out the slurry out of pores and grooves of the polishing pad
204
, creating slurry starvation conditions. As a result, internal sections of the wafer surface may not be provided with adequate amount of slurry for effective CMP processing.
Additionally, low pressure zones stimulate redeposition processes that can cause increased surface defectivity. Specifically, conventional CMP systems utilize dissolution and surface modification reactions, which are typically reducing volume type reactions stimulated by high pressure. In these reactions, pressure drops reverse the reaction, causing redeposition of dissolved by-products back to the wafer surface. Re-deposited material typically has uncontrollable composition and glues other p
Gotkis Yehiel
Owczarz Aleksander A.
Banks Derris H.
Lam Research Corporation
Martine & Penilla LLP
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