Carrier head for chemical mechanical polishing a substrate

A carrier head for a chemical mechanical polishing apparatus includes a flexible membrane with a lip portion to engage a substrate to form a seal for improved vacuum-chucking.

BACKGROUND 
The present invention relates generally to chemical mechanical polishing of 
substrates, and more particularly to a carrier head for chemical 
mechanical polishing a substrate. 
Integrated circuits are typically formed on substrates, particularly 
silicon wafers, by the sequential deposition of conductive, semiconductive 
or insulative layers. After each layer is deposited, it is etched to 
create circuitry features. As a series of layers are sequentially 
deposited and etched, the outer or uppermost surface of the substrate, 
i.e., the exposed surface of the substrate, becomes increasingly 
nonplanar. This nonplanar surface presents problems in the 
photolithographic steps of the integrated circuit fabrication process. 
Therefore, there is a need to periodically planarize the substrate 
surface. 
Chemical mechanical polishing (CAP) is one accepted method of 
planarization. This planarization method typically requires that the 
substrate be mounted on a carrier or polishing head. The exposed surface 
of the substrate is placed against a rotating polishing pad. The polishing 
pad may be either a "standard" or a fixed-abrasive pad. A standard 
polishing pad has durable roughened surface, whereas a fixed-abrasive pad 
has abrasive particles held in a containment media. The carrier head 
provides a controllable load, i.e., pressure, on the substrate to push it 
against the polishing pad. A polishing slurry, including at least one 
chemically-reactive agent, and abrasive particles, if a standard pad is 
used, is supplied to the surface of the polishing pad. 
The effectiveness of a CMP process may be measured by its polishing rate, 
and by the resulting finish (absence of small-scale roughness) and 
flatness (absence of large-scale topography) of the substrate surface. The 
polishing rate, finish and flatness are determined by the pad and slurry 
combination, the relative speed between the substrate and pad, and the 
force pressing the substrate against the pad. 
One problem encountered in CMP is that a central portion of the substrate 
is often underpcolished. This problem, which may be termed the "center 
slow effect", may occur even if pressure is uniformly applied to the 
backside of the substrate. 
Another problem is the difficulty in removing the substrate from the 
polishing pad surface once polishing has been completed. As mentioned, a 
layer of slurry is supplied to the surface of the polishing pad. When the 
substrate is placed in contact with the polishing pad, the surface tension 
of the slurry generates an adhesive force which binds the substrate to the 
polishing pad. The adhesive force may make it difficult to remove the 
substrate from the pad. 
Typically, the substrate is vacuum-chucked to the underside of the carrier 
head, and the carrier head is used to remove the substrate from the 
polishing pad. When the carrier head is retracted from the polishing pad, 
the substrate is lifted off the pad. However, if the surface tension 
holding the substrate on the polishing pad is greater than the 
vacuum-chucking force holding the substrate on the carrier head, then the 
substrate will remain on the polishing pad when the carrier head retracts. 
This may cause the substrate to fracture or chip. In addition, failure to 
remove the substrate can cause a machine fault requiring manual 
intervention. This requires shutting down the polishing apparatus, 
decreasing throughput. To achieve reliable operation from the polishing 
apparatus, the substrate removal process should be essentially flawless. 
Several techniques have been employed to reduce the surface tension between 
the substrate to the polishing pad. Once such technique is to slide the 
substrate horizontally off the polishing pad to break the surface tension 
before vertically retracting the carrier head. This technique may, 
however, scratch or otherwise damage the substrate as it may detach from 
the carrier head as it slides off the edge of the polishing pad. The 
mechanical configuration of the CMP apparatus may also prohibit use of 
this technique. 
Another technique is to treat the surface of the polishing pad to reduce 
the surface tension. However, this technique is not always successful, and 
such treatment of the pad surface may adversely affect the finish and 
flatness of the substrate and reduce the polishing rate. 
Another technique is to apply a downward pressure to the edge of the 
substrate to create a seal that prevents ambient atmosphere from 
interfering with the vacuum-chucking process. However, this technique may 
require complex pneumatic controls for the carrier head. In addition, the 
structure of the carrier head may prevent the application of pressure to 
the edge of the substrate. 
SUMMARY 
In one aspect, the invention is directed to a carrier head for chemical 
mechanical polishing of a substrate. The carrier head has a base and a 
flexible membrane extending beneath the base to define a pressurizable 
chamber. A lower surface of the flexible membrane provides a mounting 
surface for applying a load to a substrate. The flexible membrane includes 
an inner portion and a lip portion surrounding the inner portion, the lip 
portion positioned and arranged such that, when a substrate is positioned 
against the mounting surface and the chamber is evacuated to pull the 
inner portion of the flexible membrane away from the substrate, the lip 
portion will be pulled against the substrate to form a seal therebetween. 
Implementations of the invention may include one or more of the following. 
The flexible membrane may include a juncture formed between the lip 
portion and the inner portion. The juncture may be twice as thick as the 
inner portion. The inner portion may be about 29 and 33 mils thick and the 
juncture may be about 60 and 66 mils thick. The lip portion may be thicker 
adjacent the juncture than at an outer rim portion thereof, and may taper 
from a thickness about equal to the thickness of the juncture to a 
thickness about equal to the thickness of the inner portion. An edge 
portion of the flexible membrane may connect the inner portion and lip 
portion to the base. At least part of the edge portion might fold over the 
lip portion, or the edge portion might not extend over the lip portion. 
The lip portion may contact a perimeter portion of the substrate. A 
retaining ring may surround the mounting surface to maintain the substrate 
beneath the carrier head. The flexible membrane may be connected to a 
support structure, and the support structure may be movably connected to 
the base. An edge portion of the flexible membrane may extend between an 
outer surface of the support structure and an inner surface of a retaining 
ring. An edge portion of the flexible membrane may extend around an outer 
surface of the support structure and across a portion of a top surface of 
the support structure. The support structure may include a support plate 
and a clamp, and the flexible membrane may be clamped between the support 
plate and the clamp. A projection may extend downwardly from a lower 
surface of the support structure. The projection may be formed integrally 
with the support structure, or it may comprise a layer of compressible 
material disposed on the lower surface of the support structure. The lip 
portion may project downwardly from the flexible membrane to extend past 
the projection from the support structure. 
In another aspect, the invention is directed to a method of chemical 
mechanical polishing. A substrate is positioned on a mounting surface of a 
carrier head that includes a base and a flexible membrane extending 
beneath the base to define a pressurizable chamber, a lower surface of the 
flexible membrane providing the mounting surface. The chamber is 
pressurized to urge the substrate into contact with a moving polishing 
surface, and the chamber is evacuated to pull an inner portion of the 
flexible membrane away from the substrate and pull a lip portion of the 
membrane against the substrate to form a seal therebetween. 
Implementation of the invention may include pressurizing the chamber to 
force the inner portion of the flexible membrane outwardly and urge the 
lip portion of the flexible membrane away from the substrate to break the 
seal. 
Advantages of the invention may include the following. The substrate can be 
reliably removed from the polishing pad. Underpolishing of the center of 
the substrate is reduced, and the resulting flatness of the substrate is 
improved. 
Other advantages and features of the invention will be apparent from the 
following description, including the drawings and claims.

Like reference numbers are designated in the various drawings to indicate 
like elements. A primed reference number indicates that an element has a 
modified function, operation or structure. 
Detailed Description 
Referring to FIG. 1, one or more substrates 10 will be polished by a 
chemical mechanical polishing (CMP) apparatus 20. A description of a 
similar CMP apparatus may be found in U.S. Pat. No. 5,738,574, the entire 
disclosure of which is incorporated herein by reference. 
The CMP apparatus 20 includes a lower machine base 22 with a table top 23 
mounted thereon and a removable upper outer cover (not shown). Table top 
23 supports a series of polishing stations 25, and a transfer station 27 
for loading and unloading the substrates. The transfer station may form a 
generally square arrangement with the three polishing stations. 
Each polishing station includes a rotatable platen 30 on which is placed a 
polishing pad 32. If substrate 10 is an eight-inch (200 millimeter) or 
twelve-inch (300 millimeter) diameter disk, then platen 30 and polishing 
pad 32 will be about twenty or thirty inches in diameter, respectively. 
Platen 30 may be connected to a platen drive motor (not shown) located 
inside machine base 22. For most polishing processes, the platen drive 
motor rotates platen 30 at thirty to two-hundred revolutions per minute, 
although lower or higher rotational speeds may be used. Each polishing 
station may further include an associated pad conditioner apparatus 40 to 
maintain the abrasive condition of the polishing pad. 
A slurry 50 containing a reactive agent (e.g., deionized water for oxide 
polishing) and a chemically-reactive catalyzer (e.g., potassium hydroxide 
for oxide polishing) may be supplied to the surface of polishing pad 32 by 
a combined slurry/rinse arm 52. If polishing pad 32 is a standard pad, 
slurry 50 may also include abrasive particles (e.g., silicon dioxide for 
oxide polishing). Typically, sufficient slurry is provided to cover and 
wet the entire polishing pad 32. Slurry/rinse arm 52 includes several 
spray nozzles (not shown) which provide a high pressure rinse of polishing 
pad 32 at the end of each polishing and conditioning cycle. 
A rotatable multi-head carousel 60, including a carousel support plate 66 
and a cover 68, is positioned above lower machine base 22. Carousel 
support plate 66 is supported by a center post 62 and rotated thereon 
about a carousel axis 64 by a carousel motor assembly located within 
machine base 22. Multi-head carousel 60 includes four carrier head systems 
70 mounted on carousel support plate 66 at equal angular intervals about 
carousel axis 64. Three of the carrier head systems receive and hold 
substrates and polish them by pressing them against the polishing pads of 
the polishing stations. One of the carrier head systems receives a 
substrate from and delivers the substrate to transfer station 27. The 
carousel motor may orbit the carrier head systems, and the substrates 
attached thereto, about carousel axis 64 between the polishing stations 
and the transfer station. 
Each carrier head system includes a polishing or carrier head 100. Each 
carrier head 100 independently rotates about its own axis, and 
independently laterally oscillates in a radial slot 72 formed in carousel 
support plate 66. A carrier drive shaft 74 extends through slot 72 to 
connect a carrier head rotation motor 76 (shown by the removal of 
one-quarter of cover 68) to carrier head 100. There is one carrier drive 
shaft and motor for each head. Each motor and drive shaft may be supported 
on a slider (not shown) which can be linearly driven along the slot by a 
radial drive motor to laterally oscillate the carrier head. 
During actual polishing, three of the carrier heads are positioned at and 
above the three polishing stations. Each carrier head 100 lowers a 
substrate into contact with a polishing pad 32. Generally, carrier head 
100 holds the substrate in position against the polishing pad and 
distributes a force across the back surface of the substrate. The carrier 
head also transfers torque from the drive shaft to the substrate. 
Referring to FIGS. 2 and 3, carrier head 100 includes a housing 102, a base 
104, a gimbal mechanism 106, a loading chamber 108, a retaining ring 110, 
and a substrate backing assembly 112. A description of a similar carrier 
head may be found in U.S. application Ser. No. 08/745,670 by Zuniga, et 
al., filed Nov. 8, 1996, entitled A CARRIER HEAD WITH A FLEXIBLE MEMBRANE 
FOR A CHEMICAL MECHANICAL POLISHING SYSTEM, and assigned to the assignee 
of the present invention, the entire disclosure of which is incorporated 
herein by reference. 
Housing 102 can be connected to drive shaft 74 to rotate therewith during 
polishing about an axis of rotation 107 which is substantially 
perpendicular to the surface of the polishing pad during polishing. 
Loading chamber 108 is located between housing 102 and base 104 to apply a 
load, i.e., a downward pressure, to base 104. The vertical position of 
base 104 relative to polishing pad 32 is also controlled by loading 
chamber 108. 
Substrate backing assembly 112 includes a support structure 114, a flexure 
diaphragm 116 connecting support structure 114 to base 104, and a flexible 
member or membrane 118 connected to support structure 114. Flexible 
membrane 118 extends below support structure 114 to provide a mounting 
surface 192 for the substrate. The sealed volume between flexible membrane 
118, support structure 114, flexure diaphragm 116, base 104, and gimbal 
mechanism 106 defines a pressurizable chamber 190. Pressurization of 
chamber 190 forces flexible membrane 118 downwardly to press the substrate 
against the polishing pad. A first pump (not shown) may be fluidly 
connected to chamber 190 to control the pressure in the chamber and thus 
the downward force of the flexible membrane on the substrate. 
Housing 102 may be generally circular in shape to correspond to the 
circular configuration of the substrate to be polished. A cylindrical 
bushing 122 may fit into a vertical bore 124 through the housing, and two 
passages 126 and 128 may extend through the housing for pneumatic control 
of the carrier head. 
Base 104 is a generally ring-shaped body formed of a rigid material and 
located beneath housing 102. A passage 130 may extend through the base, 
and two fixtures 132 and 134 may provide attachment points to connect a 
flexible tube between housing 102 and base 104 to fluidly couple passage 
128 to passage 130. 
An elastic and flexible membrane 140 may be attached to the lower surface 
of base 104 by a clamp ring 142 to define a bladder 144. Clamp ring 142 
may be secured to base 104 by screws or bolts (not shown). A second (not 
shown) may be connected to bladder 144 to direct a fluid, e.g., a gas, 
such as air, into or out of the bladder and thereby control a downward 
pressure on support structure 114. Specifically, bladder 144 may be used 
to cause a projection 179 from a support plate 170 of support structure 
114 to press a central area of flexible membrane 118 against substrate 10, 
thereby applying additional pressure to the central portion of the 
substrate. 
Gimbal mechanism 106 permits base 104 to pivot with respect to housing 102 
so that the base may remain substantially parallel with the surface of the 
polishing pad. Gimbal mechanism 106 includes a gimbal rod 150 which fits 
into a passage 154 through cylindrical bushing 122 and a flexure ring 152 
which is secured to base 104. Gimbal rod 150 may slide vertically along 
passage 154 to provide vertical motion of base 104, but it prevents any 
lateral motion of base 104 with respect to housing 102. 
An inner edge of a generally ring-shaped rolling diaphragm 160 may be 
clamped to housing 102 by an inner clamp ring 162. An outer clamp ring 164 
may clamp an outer edge of rolling diaphragm 160 to base 104. Thus, 
rolling diaphragm 160 seals the space between housing 102 and base 104 to 
define loading chamber 108. A third pump (not shown) may be fluidly 
connected to loading chamber 108 to control the pressure in the loading 
chamber and the load applied to base 104. 
Retaining ring 110 may be a generally annular ring secured at the outer 
edge of base 104, e.g., by bolts (not shown). When fluid is pumped into 
loading chamber 108 and base 104 is pushed downwardly, retaining ring 110 
is also pushed downwardly to apply a load to polishing pad 32. A bottom 
surface 194 of retaining ring 110 may be substantially flat, or it may 
have a plurality of channels to facilitate transport of slurry from 
outside the retaining ring to the substrate. An inner surface 196 of 
retaining ring 110 engages the substrate to prevent it from escaping from 
beneath the carrier head. 
Support structure 114 of substrate backing assembly 112 includes support 
plate 170, an annular lower clamp 172, and an annular upper clamp 174. 
Support plate 170 may be a generally disk-shaped rigid member having a 
plurality of apertures 176 formed therethrough. The outer surface of 
support plate 170 may be separated from inner surface 196 of retaining 
ring 110 by a gap having a width of about 3 mm. An annular recess 178 
having a width W1 of about 2-4 mm, e.g., 3 mm, may be formed in the outer 
edge of support plate 170. In addition, projection 179 (see FIG. 3) may 
extend downwardly from a central region of the bottom surface of the 
support plate. The projection may be formed by attaching a carrier film to 
the bottom of the support plate, or it may be formed integrally with the 
support plate. Support plate 170 may not include apertures through the 
area above projection 179. Alternately, the apertures may extend through 
both the support plate and the projection. 
Flexure diaphragm 116 of substrate backing assembly 112 is a generally 
planar annular ring. An inner edge of flexure diaphragm 116 is clamped 
between base 104 and retaining ring 110, and an outer edge of flexure 
diaphragm 116 is clamped between lower clamp 172 and upper clamp 174. 
Flexure diaphragm 116 is flexible and elastic, although it could be rigid 
in the radial and tangential directions. Flexure diaphragm 116 may formed 
of rubber, such as neoprene, an elastomeric-coated fabric, such as 
NYLON.TM. or NOMEX.TM., plastic, or a composite material, such as 
fiberglass. 
Flexible membrane 118 is a generally circular sheet formed of a flexible 
and elastic material, such as chloroprene or ethylene propylene rubber. 
Flexible membrane 118 includes an inner portion 180, an annular edge 
portion 182 which extends around the edges of support plate 170 to be 
clamped between the support plate and lower clamp 172, and a flexible lip 
portion 186 which extends outwardly from a juncture 184 between inner 
portion 180 and edge portion 182 to contact a perimeter portion of a 
substrate loaded in the carrier head. The juncture 184 is located 
generally beneath recess 178 in support plate 170, and is thicker, e.g., 
about twice as thick, than inner portion 180 or edge portion 182. 
The lip portion 186 may be wedge-shaped and taper from a thickness about 
equal to that of the juncture to a thickness at its outer rim 188 about 
equal to that of inner portion 180 of flexible membrane 118. Outer rim 188 
of lip portion 186 may be angled toward the substrate. Specifically, the 
lip portion should extend sufficiently downwardly so that, if chamber 190 
is evacuated and flexible membrane 118 is pulled upwardly, rim 188 of lip 
portion 180 still extends below projection 179 on support plate 170. This 
ensures that a seal can be formed between the substrate and flexible 
membrane even if projection 179 prevents the application of pressure to 
the edge of the substrate. As discussed in greater detail below, lip 
portion 186 assists in the removal of the substrate from the polishing 
pad. 
In one implementation, the inner and edge portions of flexible membrane 118 
may be about 29-33 mils thick, whereas the juncture section may be about 
60-66 mils thick and may extend inwardly from the edge portion about 1-5 
mm, e.g., 3.5 mm. The lip portion may extend downwardly at an angle of 
about 0-30.degree., e.g., 15.degree., from inner portion 180, and may 
extend about 1-5 mm, e.g., 3.5 mm, beyond edge portion 182. 
As previously discussed, one reoccurring problem in CMP is underpolishing 
of the substrate center. Carrier head 100 may be used to reduce or 
minimize the center slow effect. Specifically, by providing support plate 
170 with a projection 179 which contacts the upper surface of the flexible 
membrane in a generally circular contact area near the center of the 
substrate-receiving surface, additional pressure may be applied by bladder 
144 to the potentially underpolished region at the center of the 
substrate. This additional pressure increases the polishing rate at the 
center of the substrate, improving polishing uniformity and reducing the 
center slow effect. 
When polishing is completed, fluid is pumped out of chamber 190 to vacuum 
chuck the substrate to flexible membrane 118. Then loading chamber 108 is 
evacuated to lift base 104 and backing structure 112 off the polishing 
pad. 
As mentioned above, another reoccurring problem in CMP is the difficulty in 
removing the substrate from the polishing pad. However, carrier head 100 
substantially eliminates this problem. Referring to FIG. 4A (for 
simplicity, only the elements involved in chucking and dechucking the 
substrate are illustrated in FIGS. 4A and 4B), when chamber 190 is 
evacuated, inner portion 180 of flexible membrane 118 is pulled inwardly. 
This causes a decrease in pressure in the volume between the backside of 
the substrate and the mounting surface of the flexible membrane. The 
decrease in pressure causes lip portion 186 to be drawn against a 
perimeter portion of the substrate to form a seal therebetween. This 
provides an effective vacuum-chuck of the substrate to the flexible 
membrane. Thus, when loading chamber 108 is evacuated, substrate 10 will 
be securely held to the carrier head. In addition, the seal is 
sufficiently fluid-tight that it may not be necessary to apply an 
additional downward force to the portion of the flexible membrane over the 
perimeter of the substrate to form the seal. Consequently, the seal may be 
implemented without requiring additional pneumatic controls in the carrier 
head. 
Referring to FIG. 4B, to remove the substrate from the carrier head, fluid 
is pumped into chamber 190. This causes inner portion 180 to bulge 
outwardly, causing juncture 184 to pivot downwardly. Consequently, lip 
portion 186 pivots upwardly so that it lifts away from the substrate. This 
breaks the seal between the flexible membrane and substrate, and the 
downward pressure from the inner portion of the flexible membrane dechucks 
the substrate from the carrier head. The thickness of juncture 184 should 
be selected to provide sufficient rigidity to ensure that the lip portion 
pivots upwardly when the inner portion of flexible membrane 118 is urged 
downwardly. 
Referring to FIG. 5, a carrier head 100' may include a flexible membrane 
118' that folds over lip portion 186'. An advantage of this implementation 
is that the gap between the outer cylindrical surface of support plate 
170' and the inner surface of retaining ring 110 is smaller. The edge 
portion 182' of flexible membrane 118' includes a folded portion 198 which 
extends over lip portion 186' to connect to juncture 184'. The folded 
portion 198 may fit into recess 178' in support plate 170'. Support plate 
170' may also include a projection 179' that is formed integrally with the 
support plate. 
The present invention has been described in terms of a number of 
embodiments. The invention, however, is not limited to the embodiments 
depicted and described. Rather, the scope of the invention is defined by 
the appended claims.