Abrading – Abrading process – Glass or stone abrading
Reexamination Certificate
2000-08-31
2003-04-01
Wilson, Lee (Department: 3723)
Abrading
Abrading process
Glass or stone abrading
Reexamination Certificate
active
06540590
ABSTRACT:
FIELD
This invention pertains generally to systems, devices, and methods for polishing and planarizing substrates, and more particularly to a Chemical Mechanical Planarization or Polishing (CMP) apparatus and method.
BACKGROUND
Chemical Mechanical Planarization or Polishing, commonly referred to as CMP, is a method of planarizing or polishing semiconductor and other types of substrates. Planarizing a surface of a semiconductor substrate or wafer between certain processing steps allows more circuit layers to be built vertically onto a device. As feature size decreases, density increases, and the size of the semiconductor wafer increase, CMP process requirements become more stringent. Wafer to wafer process uniformity as well as uniformity of planarization across the surface of a wafer are important issues from the standpoint of producing semiconductor products at a low cost. As the size of structures or features on the semiconductor wafer surface have been reduced to smaller and smaller sizes, now typically about 0.2 microns, the problems associated with non-uniform planarization have increased. This problem is sometimes referred to as a Within Wafer Non-Uniformity (WIWNU) problem.
Many reasons are known in the art to contribute to uniformity problems. These include the manner in which wafer backside pressure is applied to the wafer during planarization, edge effect non-uniformities arising from the typically different interaction between the polishing pad at the edge of the wafer as compared to at the central region, and non-uniform deposition of metal and/or oxide layers to might desirably be compensated for by planarizing or adjusting the material removal profile during polishing. Efforts to simultaneously solve these problems have not heretofore been completely successful.
With respect to the nature of the wafer backside polishing pressure, conventional machines typically use hard backed polishing heads to press the wafer against a polishing surface, that is heads having a hard receiving surface that presses directly against the backside of the semiconductor wafer. As a result any variation in the receiving surface of the head, or the presence of any material trapped between the wafer and the receiving surface results in a non-uniform application of pressure to the backside of the wafer. Thus, the front surface of the wafer typically does not conform to the polishing surface resulting in planarization non-uniformities. Moreover, such hard backed head designs often must utilize a relatively high polishing pressure (for example, pressure in the range between about 6 psi and about 8 psi) to provide any reasonable degree of conformity between the wafer and the polishing surface. Such relatively high pressures effectively deform the wafer causing too much material to be removed from some areas of the wafer will be removed and too little material from others resulting in bad planarization.
Attempts have been made to remedy the above problems with hard backed heads by providing an insert between the receiving surface and the wafer to be polished in an attempt to provide some softness in an otherwise hard backed system. This insert is frequently referred to as the wafer insert. These inserts are problematic because they frequently result in process variation leading to wafer-to-wafer variation. This variation is not constant or generally deterministic. One element of the variation is the absorption of water or other fluids such as slurry used in the polishing process. Because the amount of water absorbed by the insert tends to increase over its lifetime, there is frequently process variation from wafer-to-wafer. These process variations may be controlled to a limited extend by preconditioning the insert by soaking the insert in water prior to use and by replacing the insert before its characteristics change beyond acceptable limits. This tends to make the initial period of use more like the later period of use, however, this can increase equipment maintenance costs and decrease process throughput. Moreover, unacceptable process variations are still observed due to, for example, variations in the thickness of the insert, wrinkling of the insert and material being trapped between the hard backed head and the insert or the insert and the wafer.
Use of the insert has also required fine control of the entire surface to which the insert is adhered as any non-uniformity, imperfection, or deviation from planarity or parallelism of the head surface would typically be manifested as planarization variations across the wafer surface. For example, in conventional heads, an aluminum or ceramic plate is fabricated, then lapped and polished before installation in the head. Such fabrication increases the costs of the head and of the machine, particularly if multiple heads are provided.
On the other hand, when a soft backed head is used, the soft material of the insert does not distort the wafer as the wafer is pressed against the polishing pad. As a result, lower polishing pressures may be employed, and conformity of the wafer front surface to the polishing pad is achieved without distortion so that both polishing uniformity and good planarization may be achieved. Better planarization uniformity is achieved at least in part because the polishing rate on similar features from die to die on the wafer is the same.
In recent years, some attempts have been made to utilize soft backed heads, however, they have not been entirely satisfactory. One type of soft backed head is described in U.S. Pat. No. 6,019,671, to Shendon, hereby incorporated by reference. Shendon teaches a membrane or flexible member stretched across the lower surface of the head to form a chamber or cavity which is pressurized to press the substrate against the polishing surface. While a significant improvement over hard backed heads with or without inserts this approach is not wholly satisfactory for a number of reasons. One problem with this approach is that it does nothing to reduce or eliminate the non-uniformities due to material trapped between the membrane and the wafer. Another problem is the membrane prevents the use of vacuum to hold the wafer to the head during a load or unload operation. Moreover, the use of the membrane can actually increase non-uniformities by introducing new variables, such as variation in the thickness or flexibility of the membrane across its surface and possible wrinkling of an improperly installed membrane.
Other soft backed head designs use a seal between the edge of the wafer and the head to form a cavity which is then pressurized to directly press the wafer against the polishing surface during polishing and planarization. One approach is described in U.S. Pat. No. 5,635,083, to Breivogel, et al., hereby incorporated by reference. Breivogel teaches the use of a lip seal against the outer edge of the backside of said wafer to form a seal between the head and the wafer to which pressurized air is admitted. Unfortunately, while such an approach provides a soft backed head that eliminates some of the problems associated with hard backed heads and soft backed heads having membranes, it does not permit sufficient engagement between the wafer and the receiving surface to provide torque to the wafer in machines where the head rotates during the polishing operation. Another problem with this approach is that although vacuum can be used to hold the wafer to the head, because the wafer is supported only at the edge an unacceptable degree of bowing can occur resulting in damage to or loss of the wafer.
With respect to correction or compensation for edge polishing effects, attempts have been made to adjust the shape of the retaining ring and to modify a retaining ring pressure so that the amount of material removed from the wafer near the retaining ring is modified. Typically, more material is removed from the edge of the wafer, that is the wafer edge is over polished. In order to correct this over polishing, usually, the retaining ring pressure is adjusted to be somewhat higher than the wafer backside pressure
Kajiwara Jiro
Wang Huey-Ming
Dorsey & Whitney LLP
Multi-Planar Technologies, Inc.
Wilson Lee
LandOfFree
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