Apparatus for polishing a substrate at radially varying polish rates

A polishing apparatus and method is disclosed, whereby fluid is delivered at dissimilar flow rates and pressures across a wafer. The fluid is delivered either directly to the wafer or through a polishing pad. Changing the fluid delivery allows the removal properties of the fluid to polish material from the wafer surface based on the location of that material relative to the center of the wafer. The fluid delivery system and the polishing pad oscillate relative to a rotating wafer. The radius of oscillation is relatively small compared to the size of the wafer to allow removal along one or more concentric rings and/or circles across the wafer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to polishing in general and, more particularly, to a 
polishing apparatus for removing, with or without a polishing pad, 
material from a substrate (or wafer) surface at dissimilar rates depending 
on the polish position relative to the center of the wafer. 
2. Description of Related Art 
The concept of polish is generally well known. A substrate surface can be 
polished using a fluid combined with an abrasive pad to present both 
chemical and mechanical removal. If the pad is rotated with enough 
abrasive force, surface material can be dislodged or ablated from across 
the substrate which, according to one example, can be a semiconductor 
wafer. The fluid and/or pad extending across the wafer help dislodge, for 
example, surface contaminants, and/or raised topological features from the 
surface. The abrasive pad is preferably used containing materials which 
roughen the wafer surface so as to enhance effectiveness of the chemical 
etchant within the fluid. The pad can rotate relative to the wafer 
surface. Combined with the pad may be a chemically reactive solution, 
commonly referred to as "slurry". A combination of polishing pad with 
chemical slurry is recognized in the industry as a chemical mechanical 
polish ("CMP"). 
A typical CMP process involves placing the wafer face-down on the polishing 
pad which is fixedly attached to a rotatable table. Elevationally raised 
portions of a thin film existing on the wafer surface is placed in direct 
contact with the rotating pad. A carrier may be used to apply downward 
pressure against the backside surface of the wafer possibly concurrent 
with upward pressure against the backside surface of the pad. 
The slurry may be introduced though a nozzle, whose distal opening may be 
placed proximate the pad, laterally offset from the wafer. The slurry 
initiates the polishing process by chemically reacting with the film being 
polished. The polishing process is facilitated by the rotational movement 
of the pad relative to the wafer (or vice versa) and slurry is provided to 
the wafer/pad interface. 
CMP is popular in most modern semiconductor wafer fabrication processes. 
For example, CMP is used to planarize tungsten-based interconnect plugs 
commensurate with the upper surface of the interlevel dielectric. As 
another example, CMP may be used to planarized fill dielectric placed in 
shallow trenches, the planarize fill dielectric thereby used as a field 
dielectric. Accordingly, CMP is principally popular as a planarization 
tool. 
It is oftentimes difficult to control polish rate across the wafer surface. 
If CMP is used, the polish rate is generally targeted to be equal across 
raised areas of the wafer surface. However, as the pad conforms to the 
wafer, or as it bows in an arcuate pattern in response to force applied 
thereto, the pad will unfortunately remove some portions of the wafer 
while leaving others. For example, if the pad contacts with more force at 
the center of the wafer rather than at the wafer perimeter, then too much 
of the film will be removed at the center relative to the perimeter. A 
need therefore exists to possibly compensate for dissimilar contact 
pressures between the pad and the wafer surface. As another example, it 
might be desirable to remove one or more films at dissimilar rates based 
on their radial distance from the center of the wafer. A need for this 
form of removal may stem from misapplication of the thin film, such as 
would be the case if the thin film accumulated near the perimeter of the 
wafer surface rather than at the surface. Removing thin film from the 
outer radial distance relative to the inner radial distance would be 
beneficial in this case. 
An apparatus and process is therefore needed to compensate for disparities 
at which an abrasive pad contacts and therefore removes material from a 
wafer surface. A need further exists for removing film material at 
dissimilar rates to offset films which have previously accumulated at 
dissimilar thicknesses across the wafer. The desired apparatus and method 
must therefore selectively remove material and/or film on the wafer 
surface depending on where that material and/or film resides. 
SUMMARY OF THE INVENTION 
The problems outlined above are in large part solved by an improved 
polishing apparatus and method. The present polishing technique 
purposefully removes materials and/or film thicknesses at dissimilar rates 
across a substrate. According to one embodiment, the substrate can be a 
semiconductor wafer. However, it is recognized that the present polishing 
technique can be applied to a substrate not limited to a semiconductor 
wafer. Accordingly, whenever "wafer" is referenced hereinbelow, it applies 
to any material composition which can be polished and is configured as a 
disk. This includes any device manufactured having a defined thickness and 
diameter used, for example, in manufacturing or in trade, a suitable 
disk-shaped wafer includes, for example, a CD-ROM, etc. 
More specifically, material may be removed at a faster rate as the distance 
from the center of the substrate increases, or vice versa. Alternatively, 
material may be removed at dissimilar rates along one or more concentric 
rings extending from the center of the substrate, or wafer. 
The removal rate can be steadily increased as the radial distance 
increases, or increased several defined distances from the wafer center. 
Yet further, the removal rate can be increased, decreased and thereafter 
increased again as the radial distance increases. It is therefore 
appreciated that the removal rate can be varied according to numerous 
permutations to achieve a limitless removal gradient across of two or more 
concentric ring boundaries along the wafer surface. 
Removal rate differential is accomplished by (i) controlling the position 
of the rotating wafer relative to an oscillating pad, and (ii) delivering 
fluid to pad locations based on the location's position relative to the 
center of the pad. Oscillation radius of the pad is limited relative to 
the overall radius of the wafer. For example, the pad may only oscillate 
(or orbit) approximately one half inch about a central axis. The central 
axis is shared by the wafer which rotates about that axis. By limiting the 
oscillation radius, points of removal on the wafer is dictated primarily 
by corresponding points on the pad. An "abrasive" point on the pad will 
abrade an area of the wafer directly above the point and extending 
approximately a half inch therefrom. Relative to an overall radius of 
modern wafer sizes, a half inch oscillation is quite small. Coupled with a 
disparate fluid delivery system, the pad oscillation mated with wafer 
rotation provides close control of abrasion along one or more concentric 
rings. The fluid delivery system operates to channel dissimilar fluid flow 
rates and pressures across the pad backside surface. The pad frontside 
surface contacts and/or interfaces with the frontside surface of the 
wafer. The fluid preferably comprises a slurry mixture containing solid 
particles. 
In lieu of the polishing pad, the fluid delivery system can forward 
pressurized fluid to the frontside surface of the wafer absent a polishing 
pad therebetween. The fluid is preferably pressured (but not necessarily 
pressurized) to ablate the wafer surface according to disparate pressure 
levels across the wafer. Absent a pad, the process is deemed a chemical 
polish ("CP") and not CMP. In either instance of CP or CMP, surface 
elevational disparities can be removed at a non-uniform rate depending on 
the magnitude of those disparities. 
Broadly speaking, the present invention contemplates a semiconductor wafer 
polishing apparatus. The polishing apparatus includes a plurality of 
apertures spaced from each other through a manifold. Having an inlet port 
at one end and an opening at an opposing end, the housing is configured to 
receive the manifold within the opening at the end opposite the inlet 
port. A conduit is coupled to channel dissimilar fluid flow rates and 
pressures from the inlet port through the apertures according to the 
position of those apertures within the manifold. 
The polishing apparatus further, or alternatively, contemplates a polishing 
pad configured within a housing. The housing is adapted for oscillating 
about a central axis. The semiconductor wafer is brought to contact or 
interface with the upper surface of the polishing pad. This semiconductor 
wafer is therefore adapted for rotation about the central axis. A fluid 
delivery system is connected to a lower surface of the polishing pad to 
vary fluid across the interface between the semiconductor wafer and the 
polishing pad. Fluid quantity (i.e., flow rate) and/or pressure is varied 
relative to changes in the radial distance from the central axis.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Turning now to the drawings, FIG. 1 provides a perspective view of a CMP 
apparatus 10. Apparatus 10 includes a wetted polishing surface which can 
be adapted to bear against a semiconductor wafer 12. Wafer 12 comprises 
any semiconductive material comprising a plurality of integrated circuits 
extending across the wafer as "die". CMP apparatus 10 is employed at one 
or more stages in the fabrication of the integrated circuits. CMP can be 
used, for example, to remove elevationally raised areas, surface defects, 
scratches, roughness, contaminants, or embedded particles of dust or dirt. 
CMP is often referred to as mechanical planarization, but is also utilized 
to clean the wafer surface to improve the quality and reliability of the 
ensuing circuit. 
In general, CMP involves rotating wafer 12 about an axis 14 while forcing 
the wetted surface against wafer 12. CMP apparatus 10 includes a polishing 
pad 16. Polishing pad 16 is made from a relatively porous, soft material, 
a suitable material being polyurethane. Alternatively, the polishing pad 
can be made from any hard material which does not conform as much as a 
polyurethane pad. In either instance, a pad having the various desired 
composition may be obtained, for example, from Rodel Corporation, as the 
IC-1000 pad or the Politex pad. The amount of hardness is dictated based 
on the material being removed and the chemical slurry being used. 
Pad 16 is preferably porous and may contain apertures therethrough to allow 
a slurry mixture to be pumped directly through pad 16 according to arrows 
18. The direction of fluid flow 18 is chosen such that it readily extends 
through pad 16 and impinges on wafer 12 at substantially perpendicular 
angles absent scattering as it traverses the pad. Pad 16 preferably moves 
in an orbital direction along a two-dimensional plane. The orbital 
direction is one which can be deemed as oscillation. Specifically, orbital 
direction vector is maintained in the two-dimensional plane but changes it 
back-and-forth movement at incrementally changing vectors. For example, 
the initial vector may be purely in the positive and negative x 
directions. Thereafter, the direction vector changes to gradually increase 
in the y direction relative to the x until it eventually is entirely in 
the y direction. Thereafter, the vector will continue to increment until 
again the vector oscillates entirely in the x direction, and so on. The 
various oscillation vectors are shown in FIG. 1 as reference numerals 20. 
The amount of movement along the oscillation vectors 20 is substantially 
limited. Preferably, the back-and-forth movement relative to axis 14 
occupies a radial movement from axis 14 less than one inch, and preferably 
less than one half inch. 
Arranged on the bottom surface of pad 16 is a manifold 22. Manifold 22 
contains a plurality of apertures which permit passage of fluid (i.e., 
slurry) through the apertures denoted as reference numerals 24. Apertures 
24 receive the polishing fluid, and pass that fluid through pad 16 to the 
region between pad 16 and wafer 12. 
FIG. 2 illustrates in more detail along a cross-section of CMP apparatus 
10. Apparatus 10 carefully and controllably places wafer 12 against pad 16 
using a carrier 26 to retain wafer 12 and a housing 28 to retain pad 16. 
Carrier 26 is used to rotate wafer 12 against pad 16 which is directed 
upward against the wafer during polishing. An upward force is applied from 
the pad to the rotating wafer 12. The upward force may be buffered, as 
desired. For example, the buffered force may comprise air pressure 30 
delivered through an inlet port 32 and into a chamber partially encircled 
by carrier 26. A plate 34 is responsive to the air pressure within chamber 
36 by forcing pad 16 in a downward direction when air is present. Air 
pressure within chamber 36 advantageously serves to buffer or filter 
transient variations in interface force between pad 16 and wafer 12. In 
many instances, air pressure within chamber 36 will offset or add to the 
upward force applied upon pad 16. Wafer 12, regardless of the pressure 
applied thereto, is retained within carrier 26. The inner surface of 
carrier 26 retains the outer perimeter surface of wafer 12 to prevent it 
from slipping laterally during polishing. Thus, polish pressure can be 
thought of as being applied both through the pad and through the carrier. 
Housing 28 serves somewhat the same purpose as carrier 26 in that it 
retains pad 16. Pad 16 and housing 28 form a chamber which can receive air 
pressure 40 through an inlet port 42. The air pressure within chamber 44 
serves to buffer the upward pressure applied on housing 28 against 
substrate 12. The combination of air within chambers 36 and 44 help 
modulate and maintain relatively constant pressure across the entire 
interface between wafer 12 and pad 16. Placed between pad 16 and chamber 
44 is manifold 22. Manifold 22 can be thought of as a relatively thin 
member, suitably made of aluminum having a plurality of apertures 24 
extending entirely through the cross-sectional thickness of manifold 22. 
The air pressure and/or fluid extending through inlet port 42 causes 
manifold 22 and pad 16 to extend upward. In so doing, manifold 22 may flex 
in an arcuate pattern as shown. Uneven pressure may result in a relatively 
severe, circular polishing pattern near the center of wafer 12. The 
circular polishing pattern at or near the center is dictated by the length 
of oscillation vectors 20. Abrasion primarily at the center region will 
not produce a desired uniformity across the entire wafer surface. 
Alternatively, polish only at the center may not remove thicker films 
which may not exist at the perimeter of the wafer, due to uneven chemical 
vapor deposition (CVD) or sputter deposition techniques. 
To offset the uneven nature by which pad 16 might abrade wafer 12 surface, 
uneven delivery of slurry may be desired. The uneven fluid delivery is 
shown as reference numeral 48, where the length of arrows indicate a 
greater channeling of fluid flow and pressure to the outer perimeter of 
pad 16 relative to the center of pad 16. The result of uneven fluid 
delivery is to accumulate more fluid (or slurry) at the perimeter of the 
wafer rather than at the center to offset possibly greater abrasive force 
of an arcuate pad applied at the center as shown. 
It is believed that by directing slurry with sufficient force at the out 
perimeter of the wafer, more wafer will be removed at those perimeter 
positions and relatively little slurry forwarded at the center of the 
wafer. The fluid delivery non-uniformity is shown in FIG. 2 to offset the 
abrasive pad-wafer contact nonuniformity. 
FIG. 3 illustrates instances where pad 16 may not necessarily bow upward if 
minimal polish pressure is applied through pad 16. This is contrary to 
that shown in FIG. 2. Instead, pad 16 maintains a relatively planar upper 
surface when brought to bear against a wafer. It might be desirable in 
many instances to apply more fluid to the center of the wafer then at the 
perimeter. Vectors 50 illustrate fluid flow and pressure differentials. 
The fluid flow and pressure differentials may be selected to remove more 
surface material at the center of the wafer, with gradual decrease as 
radial position extends to the perimeter of the wafer. 
FIG. 4 illustrates yet another embodiment in which fluid flow and pressure 
vectors 52 change according to their radial position to form removal rate 
differentials across concentric rings of the wafer. Removal rate vectors 
48, 50 and 52 (shown in FIGS. 2, 3 and 4) indicate greater removal along 
larger arrows than smaller arrows. Vector 52 indicates a removal rate at 
the center to be relatively high, decreasing towards the perimeter and the 
increasing again at the perimeter. 
FIG. 5 illustrates multiple concentric rings 54 of removal rate 
differentials formed across the surface of wafer 12. Removal ring 54a may, 
for example, indicate substantial removal within that area. The ring 
indicated by numeral 54b, outside area 54a, may indicate a lessened 
removal rate, relative to area 54a. The number of permutations at which 
removal rate differences can occur in radial directions is almost 
limitless based on the number of rings and dissimilar removal rates 
amongst those rings. 
FIG. 6 is a detailed cross-sectional view of a partial pad and manifold 
region. According to one embodiment, manifold 22 contains apertures 24 
which are of dissimilar size. Apertures 24 have larger or smaller openings 
depending on whether a greater or lesser amount of fluid, respectively, is 
to pass. As shown, aperture 24a is larger than aperture 24b, and aperture 
24b is larger than aperture 24c. This allows for a greater flow rate and 
pressure of fluid passing through aperture 24a than aperture 24c. The 
fluid flow and pressure rate differential is shown with dissimilar arrow 
lengths indicative of that differential as reference numerals 58a through 
58c. A larger aperture 24a allows greater fluid amounts and pressures to 
extend through pad 16 directly above the aperture. This forces the fluid 
to locally etch the wafer surface near the perimeter (i.e., above aperture 
24a) relative to the wafer surface near the center (i.e., above apertures 
24b and 24c). 
FIG. 7 illustrates and alternative embodiment in which apertures 24 are of 
the same size, however, tubes or ports 60 are connected to the aperture, 
each bearing fluid which passes therethrough at dissimilar flow rates 
and/or pressures. A greater pressure/flow 58a is within tube 60a than the 
pressure/flow 58b and 58c within tubes 60b and 60c, respectively. By 
affixing tubes of the same diameter to apertures of the same diameter, yet 
changing the flow/pressure within those tubes allows the same differential 
to occur across pad 16 and ultimately across the wafer surface, similar to 
the embodiment shown in FIG. 6. Thus, tubes 60 extend through the chamber 
between manifold 22 and housing 28 such that the fluid delivery is 
external to air delivery at the backside surface of manifold 22. 
Accordingly, FIG. 7 proposes separation of fluid and air delivery, whereas 
FIG. 6 may, if needed, combine the two. 
FIG. 8 illustrates yet another embodiment in which the polishing pad is 
removed. Instead of having a polishing pad, the flow rate and pressure of 
fluid being delivered directly removes wafer surface material. When using 
a polishing pad, both the mechanical abrasion of the pad in combination 
with fluid delivery etches the wafer surface at select regions. Manifold 
22 may be securely mounted to housing 28 since need for its upward 
movement is eliminated absent the pad. Varying fluid delivery rates 
through manifold 28 is adjusted by changing the opening size of apertures 
24 within manifold 22. Shown in FIG. 8 is a greater flow and pressure 
amount forwarded through apertures 24 near the perimeter of manifold 22 
rather than at the center. The larger arrows are indicative of the greater 
flow and pressure amounts relative to the smaller arrows, denoted as 
reference numeral 64. 
FIG. 8 generally depicts a chemical polish (or CP) technique. The fluid 
itself, when impinging on the wafer surface dislodges the outer surface 
materials being impinged. The fluid can be a slurry material and can 
contain various etch components. For example, the slurry can comprise 
silica particles and deionized water, along with possibly potassium 
hydroxide as the active element. The slurry can be suitably obtained from, 
for example, Cabot Corporation. The active agent can be, in lieu of for 
example potassium hydroxide, potassium dichromate, potassium iodate, 
potassium ferricyanide, potassium bromate, and/or vandium trioxide. The 
fluid may, in some instances, not contain silica particles or an active 
agent. Instead, the fluid may simply be deionized water pressure delivered 
upon the wafer. 
FIG. 9 illustrates a CP process where, in lieu of changing aperture sizes, 
the apertures remain the same in diameter. Yet, tubes 66 are affixed to 
the inner surfaces of apertures 24. Tubes 66 contain fluid delivered 
through apertures 24 at dissimilar flow rates and/or pressures, denoted as 
reference numeral 68. Tubes 66 extend from respective apertures 24 through 
inlet port 42. Accordingly, inlet ports 42 may require enlargement to 
accommodate numerous tubes 66. 
It will be appreciated to those skilled in the art having the benefit of 
this disclosure that this invention is believed to be capable of removing 
material and/or film from an upper surface of a semiconductor wafer. The 
fluid delivery system can be adapted to be placed with or without an 
abrasive pad. Fluid is delivered at dissimilar pressures and flow rates as 
the radial distance from the center of the wafer increases across the 
wafer. It is intended that the following claims be interpreted to embrace 
all such modifications and changes and, accordingly, the specification and 
drawings are to be regarded in an illustrative rather than a restrictive 
sense.