Abrading – Abrading process – Glass or stone abrading
Reexamination Certificate
2000-08-17
2001-08-14
Eley, Timothy V. (Department: 3723)
Abrading
Abrading process
Glass or stone abrading
C451S287000, C451S388000
Reexamination Certificate
active
06273794
ABSTRACT:
BACKGROUND
The present invention relates generally to an apparatus and method for grinding a semiconductor wafer surface and, in particular, to grinding techniques that can be used to planarize a semiconductor surface during the fabrication of an integrated circuit.
In the process of fabricating modern semiconductor integrated circuits (ICs), it is necessary to form various material layers and structures over previously-formed layers and structures. However, the prior formations often leave the top surface topography of an in-process wafer highly irregular, with bumps, areas of unequal elevation, troughs, trenches and/or other surface irregularities. Such irregularities cause problems when forming the next layer. For example, when printing a photolithographic pattern having small geometries over previously-formed layers, a very shallow depth of focus is required. Accordingly, it becomes essential to have a flat and planar surface. Otherwise, some parts of the pattern will be in focus and others will not. Surface variations on the order of less than 1,000 angstroms (Å) over a 25×25 millimeter (mm) exposure area are preferred. Additionally, if the irregularities are not leveled at each major processing step, the surface topography of the wafer can become even more irregular, causing further problems as the layers stack up during further processing. Depending on the die type and the size of the geometries involved, the surface irregularities can lead to poor yield and device performance. Consequently, it is desirable to planarize, or level, the IC structures.
One technique for planarizing the surface of a wafer is chemical mechanical polishing (CMP). In general, CMP planarization involves holding a thin flat semiconductor wafer against a rotating wetted polishing surface, such as a compliant polishing pad, under a controlled downward pressure. During the CMP process, a slurry is provided to remove and flush away unwanted film material. In one exemplary implementation, a CMP process is used to remove an oxide coating to the level of previously-formed IC structures. In such processes, it is important to remove a sufficient amount of material to provide a smooth surface without removing an excessive amount of underlying materials.
Although CMP processes have proved useful in the fabrication of semiconductor ICs, they suffer from several drawbacks. First, CMP processes are relatively slow, with a removal rate on the order of about 1 micron per minute (&mgr;/min), and, therefore, limit the overall throughput of the fabrication process. Second, polishing pads typically used in CMP processes tend to have relatively short lifetimes and must be replaced frequently. Third, the use of slurry and other chemicals during the CMP process increases the overall cost of fabrication and results in the need for additional waste removal.
Grinding processes, in which a grinding wheel is pressed against the wafer surface to grind away semiconductor material, are sometimes used by manufacturers of semiconductor wafers to planarize the wafer surface or provide a smooth wafer edge. Grinding processes can avoid some of the foregoing problems associated with CMP processes. However, as explained below, such grinding processes have not generally been used during the fabrication of semiconductor ICs.
The topography of the front surface of a wafer may vary by as much as 1-2 microns (&mgr;) as a result of the natural distortions or warpage of the wafer as well as variations in the thickness of the wafer across its surface. In contrast to CMP processes in which the wafer is supported by a compliant pad, grinding processes use a hard grinding surface to remove from the wafer surface all materials in substantially an absolute geometrical reference plane. Therefore, because of the wafer's front surface topography, it is difficult to use a grinding process to planarize a wafer having one or more previously-formed layers without removing an excessive amount of underlying materials on at least some parts of the wafer.
Despite the apparent difficulties in using grinding processes to planarize the wafer during the fabrication of ICs, it would be beneficial to provide a planarization technique based on a grinding process that can provide a substantially flat surface across the entire wafer and that can overcome some of the drawbacks associated with current CMP processes.
SUMMARY
In general, according to one aspect, a semiconductor wafer fabrication apparatus includes a carrier head for holding a wafer and distributing a downward pressure across a back surface of the wafer. The apparatus also includes a wafer processing station disposed below the carrier head. The station includes a grinding wheel and a fluid bearing. The fluid bearing provides an upward pressure against a front surface of the wafer so as to substantially flatten the front surface of the wafer. The grinding wheel can be raised into contact with the front surface of the wafer and rotated to grind the front surface while the fluid bearing provides the upward pressure and the carrier head distributes the downward pressure.
According to another aspect, a semiconductor wafer fabrication apparatus includes a carrier head for holding a wafer and distributing a downward pressure across a back surface of the wafer. The apparatus further includes fluid bearing surface areas separated by a gap. The fluid bearing surface areas have openings through which a fluid can flow to provide an upward pressure against a front surface of the wafer when positioned over the bearing surface. The apparatus also includes a grinding wheel at least partially disposed within the gap. The carrier head can be moved to position the wafer over the bearing surface areas and the gap. The grinding wheel can be brought into contact with the front surface of the wafer to grind the front surface when the wafer is positioned over the bearing surface and the gap.
In another aspect, a method of grinding a semiconductor wafer includes positioning the wafer over fluid bearing surface areas separated by a gap, wherein the fluid bearing surface areas have openings through which a fluid flows to provide an upward pressure against the front surface of the wafer. A substantially uniform pressure is provided against the back surface of the wafer. A grinding wheel at least partially disposed in the gap is moved into contact with the front surface of the wafer. The grinding wheel is then rotated against the front surface of the wafer.
Various implementations include one or more of the following features. The grinding wheel can include an annular-shaped grinding surface and can encircle part of the bearing surface. In other embodiments, the grinding wheel can be disc-shaped. Alternatively, an abrasive drum can be used as the grinding wheel.
The grinding wheel can be rotated to grind the front surface of the wafer. Similarly, the carrier head can be moved about a plane substantially parallel to the front of the wafer so that substantially the entire front surface of the wafer comes into contact with the grinding wheel during grinding.
The carrier head can include a wafer backing assembly having a compliant material to provide a mounting surface for the wafer. The carrier head also can include a chamber that is pressurized to generate a downward pressure on the wafer backing assembly and press the wafer toward the bearing surface. By controlling the downward pressure from the carrier head and the upward pressure from the fluid bearing, the front surface of the wafer can be substantially flattened and maintained at a substantially uniform height when positioned for grinding by the grinding wheel. Additionally, closed-loop feedback can be used to adjust the amount of fluid flowing through the openings in the fluid bearing to control the upward pressure against the front surface of the wafer.
A cavity can be formed around the gap so that a pressure in the cavity is maintained at substantially the same pressure as a pressure at the fluid bearing surface opposite the front surface of the wafer.
Vari
Tietz James V.
White John M.
Applied Materials Inc.
Eley Timothy V.
Fish & Richardson
Nguyen Dung Van
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