Apparatus for separating wafers from polishing pads used in chemical-mechanical planarization of semiconductor wafers

The present invention is a planarizing machine for use in chemical-mechanical planarization of semiconductor wafers that has a moveable platen, a polishing pad, a wafer carrier, and a wafer separator. The polishing pad is positioned on the platen, and it has a planarizing surface with an operational zone upon which the wafer may be planarized. The wafer carrier holds a wafer and is positionable opposite the polishing pad to engage the wafer with the operational zone of the polishing pad. The wafer separator engages either the polishing pad, the wafer, or the wafer carrier to urge a portion of the wafer away from the pad.

TECHNICAL FIELD 
The present invention relates to chemical-mechanical planarization of 
semiconductor wafers, and more specifically to a planarizing machine with 
a separator for separating a planarized wafer from a polishing pad. 
BACKGROUND OF THE INVENTION 
Chemical-mechanical planarization ("CMP") processes are frequently used to 
planarize the surface layer of a wafer in the production of ultra-high 
density integrated circuits. In a typical CMP process, a planarizing 
surface on a polishing pad is covered with a slurry solution containing 
small, abrasive particles and reactive chemicals. A wafer is mounted in a 
wafer holder, and the wafer holder is positioned opposite the polishing 
pad. The wafer and/or the polishing pad are then moved relative to one 
another allowing the abrasive particles in the slurry to mechanically 
remove the surface of the wafer, and the reactive chemicals in the slurry 
to chemically remove the surface of the wafer. 
CMP processes must consistently and accurately planarize a uniform, planar 
surface on the wafer at a desired end-point. Many microelectronic devices 
are typically fabricated on a single wafer by depositing layers of various 
materials on the wafer, and manipulating the wafer and the other layers of 
material with photolithographic, etching, and doping processes. In order 
to manufacture ultra-high density integrated circuits, CMP processes must 
provide a highly planar surface so that the geometries of the component 
parts of the circuits may be accurately positioned across the full surface 
of the wafer. Integrated circuits are generally patterned on a wafer by 
optically or electromagnetically focusing a circuit pattern on the surface 
of the wafer. If the surface of the wafer is not highly planar, the 
circuit pattern may not be sufficiently focused in some areas, resulting 
in defective devices. Therefore, it is important to consistently and 
accurately create a uniformly planar surface on the wafer. 
Several factors influence the uniformity of a planarized surface of a 
wafer, one of which is the distribution of the slurry between the 
polishing pad and the wafer. A uniform distribution of slurry between the 
pad and the wafer results in a more uniform surface on the wafer because 
the abrasive particles and the chemicals in the slurry will react more 
evenly across the whole wafer. One type of polishing pad provides a number 
of wells in the pad substrate that are uniformly spaced apart from one 
another across the surface of the pad. Each well holds a volume of slurry, 
and as the pad passes across the surface of the wafer, the slurry is drawn 
out of the wells into the space between the wafer and the pad. As the 
slurry is drawn out of the wells, a vacuum is created in the wells that 
holds the wafer next to the planarizing surface of the pad. 
CMP processes must also provide a high throughput of finished devices to 
lower the unit cost of each device. The wafers, therefore, are generally 
between six inches and eight inches in diameter so that hundreds of 
microelectronic devices may be simultaneously fabricated on a single 
wafer. When six to eight inch diameter wafers are planarized in the 
presence of a slurry, however, a significant surface tension exists 
between the wafer, slurry, and polishing pad that holds the wafers next to 
the polishing pad. 
One problem with current CMP planarizers is that after the CMP process is 
finished, it is difficult to remove large wafers from conventional 
polishing pads, or any wafer from polishing pads with slurry wells. Wafers 
are attached to the wafer carrier by drawing a vacuum on the backside of 
the wafer that is low enough to prevent the wafer from being damaged. 
After planarizing, wafers are conventionally removed from polishing pads 
by simply lifting the wafer carrier. Such a low vacuum, however, generally 
does not provide enough force to overcome the surface bond between large 
wafers and the polishing pads. Similarly, such low vacuums are also 
insufficient to overcome the bond between wafers and polishing pads with 
slurry wells. Therefore, it would be desirable to develop a CMP machine 
that can separate virtually any type of wafer from any type of polishing 
pad. 
SUMMARY OF THE INVENTION 
The inventive machine is a planarizer for use in chemical-mechanical 
planarization of semiconductor wafers that has a moveable platen, a 
polishing pad, a wafer carrier, and a wafer separator. The polishing pad 
is positioned on the platen, and it has a planarizing surface with an 
operational zone upon which the wafer may be planarized. The wafer carrier 
holds a wafer, and it is positionable opposite the polishing pad to engage 
the wafer with the operational zone of the polishing pad. The wafer 
separator engages either the polishing pad, the wafer, or the wafer 
carrier to lift a portion of the wafer away from the pad. 
In an inventive method for chemical-mechanical planarization of a 
semiconductor wafer, the wafer is held by a wafer carrier and pressed 
against the polishing pad in the presence of a slurry. At least one of the 
wafer or the polishing pad is moved with respect to the other to remove 
material from the surface of the wafer. After a desired mount of material 
is removed from the surface of the wafer, a portion of the wafer is 
separated from the pad to break a surface bond between the wafer and the 
polishing pad.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a chemical-mechanical planarization machine 
that can separate virtually any type of wafer from any type of polishing 
pad after the wafer has been planarized. Conventional chemical-mechanical 
planarization machines typically cannot remove large wafers from polishing 
pads, or most any type of wafer from pads with slurry wells, because the 
vacuum on the backside of the wafer is insufficient to break the bond 
between such wafers and polishing pads. The present invention provides a 
wafer separator that acts against only a portion of the wafer, and 
preferably only a peripheral portion of the wafer. By acting against only 
a portion of the wafer instead of the whole surface area, a relatively 
small force can separate the wafer from the polishing pad. The present 
invention is described in detail in FIGS. 1-8, in which like reference 
numbers refer to like parts throughout the various figures. 
FIGS. 1 and 2 illustrate a chemical-mechanical planarization machine 10 
with a platen 20, a wafer carrier 30, a polishing pad 40, and a wafer 
separator 70. The platen 20 has a top surface 22 upon which the polishing 
pad 40 is positioned. A drive assembly 26 rotates the platen 20 as 
indicated by arrow A, and/or reciprocates the platen 20 back and forth as 
indicated by arrow B. The motion of the platen 20 is imparted to the pad 
40 because the polishing pad 40 is adhered to the top surface 22 of the 
platen 20. 
The wafer carrier 30 has a lower surface 32 to which a wafer 60 may be 
attached by drawing a vacuum on the backside of the wafer. A resilient pad 
34 may be positioned between the wafer 60 and the lower surface 32 to 
enhance the connection between the wafer 60 and the wafer carrier 30. The 
wafer carrier 30 may have an actuator assembly 36 attached to it for 
imparting axial and/or rotational motion as indicated by arrows C and D, 
respectively. The actuator assembly 36 is generally attached to the wafer 
carrier 30 by a gimbal joint that allows the wafer carrier 30 to pivot 
freely about the three orthogonal axes centered at the end of the actuator 
36. 
Several embodiments of a planarizer with a wafer separator are within the 
scope of the invention. In one series of embodiments, the wafer separator 
70 is positioned towards the perimeter of the pad, and it has a contact 
surface 72 that engages either the pad 40, the wafer 60, or the wafer 
carrier 30. The wafer separator 70 may be passive, in which a peripheral 
portion of the wafer 60 is urged away from the pad 40 by positioning the 
pad 40 on the wafer separator 70, or moving the wafer 60 and/or the wafer 
carrier 30 against the wafer separator 70. Alternatively, the wafer 
separator 70 may be active, in which the wafer separator 70 is moved 
against one of the pad 40, the wafer 60, or the wafer carrier 30 to 
separate the wafer 60 from the pad 40. The wafer separator 70 has many 
configurations, including a ring (shown in FIG. 2) that has an upper 
surface that defines the contact surface 72. The wafer separator 70 may 
alternatively be a number of tapered segments (not shown) positioned about 
the perimeter of the pad 40. The ring may have a wedge-shaped 
cross-section, a semi-circular shaped cross-section, a semi-elliptical 
cross-section, or any other suitable cross-section that provides an 
inclined contact surface that lifts a portion of the wafer 60 from the pad 
40. The wafer separator 70 may be positioned on the pad, the platen, or 
separately from the pad and platen. 
In the embodiment of the invention illustrated in FIGS. 1 and 2, the wafer 
separator 70 is a ring-like ridge positioned on the top surface 22 of the 
platen 20 towards the perimeter of the platen 20. The wafer separator 70 
has a wedge-shaped cross-section with an upper surface 72 that defines the 
contact surface. The perimeter of the pad 40 is positioned on the contact 
surface 72 to form a non-planar section 43 on the pad 40. 
FIG. 3 shows the operation of the embodiment of the wafer separator 70 
illustrated in FIGS. 1 and 2. The wafer 60 is substantially rigid and 
cannot conform to the non-planar section 43 of the pad 40. Thus, when the 
wafer 60 is brought over to the non-planar section 43, a peripheral 
portion of the bottom surface 62 of the wafer 60 is pried away from the 
upper surface 42 of the pad 40 to form a gap 80. Once the gap 80 is 
formed, the wafer 60 can be fully separated from the pad 40 by lifting the 
wafer carrier 30 upwardly in the direction of arrow C (shown in FIG. 1). 
FIG. 4 illustrates another embodiment of the invention, in which the wafer 
separator 70 is positioned on the upper surface 42 of the polishing pad 
40. The wafer separator 70 is positioned towards the perimeter of the 
polishing pad 40 so that it is outside of an operational zone on the pad 
where the wafer 60 is planarized. In operation, the wafer carrier 30 and 
wafer 60 are moved across the pad until at least one of them engages the 
wafer separator. Referring to FIG. 5, the contact surface 72 engages 
either a forward edge 31 of the wafer carrier 30 (shown by FIG. 5), or a 
peripheral portion of the wafer 60 itself (not shown). As the forward edge 
31 of the wafer carrier 30 rides up over the contact surface 72 of the 
wafer separator 70, the peripheral portion of the wafer 60 proximate to 
the forward edge 31 is lifted away from the pad 40. When the wafer 
separator 70 engages the wafer 60 (not shown), the peripheral portion of 
the wafer 60 proximate to the wafer separator 70 is pried from pad 40. 
Thus, the wafer separator 70 allows the wafer 60 to be easily removed from 
the pad 40. 
FIGS. 6 and 7 illustrate additional embodiments of the invention in which 
the wafer separator 70 is positioned radially outwardly from the perimeter 
of the platen 20. In FIG. 6, the wafer separator 70 is attached to the 
platen 20 by an arm 73. While in FIG. 7, the wafer separator 70 is 
attached to a wall 24 of the planarizer 10. As with the embodiments 
discussed above with respect to FIGS. 1-5, the wafer separators 70 
illustrated in FIGS. 6 and 7 operate by separating a peripheral portion of 
the wafer 60 from the pad 40. The wafer separators 70 shown in FIGS. 6 and 
7 are attached to the platen 20 and the wall 24, respectively, at an 
elevation that aligns the contact surface 72 with either the wafer 60 or 
the wafer carrier 30. 
FIGS. 1-7 illustrate a passive wafer separator 70 that operates by 
positioning the pad 40 on the contact surface 72 of the wafer separator 
70, or by moving the wafer 60 and the wafer carrier 30 to engage the 
contact surface 72. In related embodiments (not shown), the wafer 
separator 70 may be active such that it can be moved to engage the 
appropriate item on the planarizer. For instance, a wafer separator 70 may 
be attached to an actuator (not shown) that is connected to the wall 24 
(shown in FIG. 7) of the planarizer 10. The actuator may be extended 
radially inwardly towards the center of the platen 20 to engage the wafer 
separator 70 with either the pad 40, the wafer 60, or the wafer carrier 
30. The present invention, therefore, is not limited to passive wafer 
separators. 
FIG. 8A illustrates another type of active wafer separator 170. The active 
wafer separator 170 is a piston 171 with an extensible rod 172. The piston 
171 is positioned in a hole 23 towards the perimeter of the platen 20. In 
operation, the wafer carrier 30 and wafer 60 are translated across the 
surface of the pad 40 until the front edge 31 of the wafer carrier 30 is 
positioned over the rod 172. The rod 172 is then engaged with the wafer 
carrier 30, and the wafer carrier 30 and wafer 60 are lifted from the pad 
40. FIG. 8B shows another embodiment in which the active wafer separator 
170 is attached to the wall 24 of the planarizer 10. In this embodiment, 
the wafer carrier 30 and wafer 60 are translated across the surface of the 
pad 40 and over the peripheral edge of the platen 20. In still another 
embodiment (not shown), the hole 23 may be positioned at or near the 
center of the pad 40 so that a central portion of the pad may be deformed 
upwardly to separate any portion of the wafer from the pad. Thus, the 
present invention covers separating any portion of the wafer from the pad. 
One advantage of the present invention is that it provides a 
chemical-mechanical planarizer 10 with a wafer separator that separates 
virtually any type of wafer from any type of polishing pad. The present 
invention is particularly useful in connection with larger wafers having 
diameters between 6 and 8 inches, and polishing pads with slurry wells. 
The present invention, however, is not limited to such particular uses and 
may be useful for smaller wafers as well. 
While the detailed description above has been expressed in terms of 
specific examples, those skilled in the art will appreciate that many 
other structures could be used to accomplish the purpose of the disclosed 
procedure. Accordingly, it can be appreciated that various modifications 
of the above-described embodiment may be made without departing from the 
spirit and scope of the invention. Therefore, the spirit and scope of the 
present invention are to be limited only by the following claims.