Abrading – Precision device or process - or with condition responsive... – By optical sensor
Reexamination Certificate
2000-09-14
2002-09-24
Nguyen, George (Department: 3723)
Abrading
Precision device or process - or with condition responsive...
By optical sensor
C451S041000, C451S287000, C451S526000
Reexamination Certificate
active
06454630
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to semiconductor manufacture, and more particularly to a method for forming a transparent window in a polishing pad for use in chemical mechanical polishing (CMP).
BACKGROUND OF THE INVENTION
In the process of fabricating modem 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 inprocess wafer highly irregular, with bumps, areas of unequal elevation, troughs, trenches, and/or other surface irregularities. These 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 other parts will not. In fact, surface variations on the order of less than 1000 Å over a 25×25 mm die would be preferable. In addition, 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 effect some type of planarization, or leveling, of the IC structures. In fact, most high density IC fabrication techniques make use of some method to form a planarized wafer surface at critical points in the manufacturing process.
One method for achieving semiconductor wafer planarization or topography removal is the chemical mechanical polishing (CMP) process. In general, the chemical mechanical polishing (CMP) process involves holding and/or rotating the wafer against a rotating polishing platen under a controlled pressure. As shown in
FIG. 1
, a typical CMP apparatus
10
includes a polishing head
12
for holding the semiconductor wafer
14
against the polishing platen
16
. The polishing platen
16
is covered with a pad
18
. This pad
18
typically has a backing layer
20
which interfaces with the surface of the platen and a covering layer
22
which is used in conjunction with a chemical polishing slurry to polish the wafer
14
. However, some pads have only a covering layer and no backing layer. The covering layer
22
is usually either an open cell foamed polyurethane (e.g. Rodel IC1000) or a sheet of polyurethane with a grooved surface (e.g. Rodel EX2000). The pad material is wetted with the chemical polishing slurry containing both an abrasive and chemicals. One typical chemical slurry includes KOH (Potassium Hydroxide) and fumed-silica particles. The platen is usually rotated about its central axis
24
. In addition, the polishing head is usually rotated about its central axis
26
, and translated across the surface of the platen
16
via a translation arm
28
. Although just one polishing head is shown in
FIG. 1
, CMP devices typically have more than one of these heads spaced circumferentially around the polishing platen.
A particular problem encountered during a CMP process is in the determination that a part has been planarized to a desired flatness or relative thickness. In general, there is a need to detect when the desired surface characteristics or planar condition has been reached. This has been accomplished in a variety of ways. Early on, it was not possible to monitor the characteristics of the wafer during the CMP process. Typically, the wafer was removed from the CMP apparatus and examined elsewhere. If the wafer did not meet the desired specifications, it had to be reloaded into the CMP apparatus and reprocessed. This was a time consuming and labor-intensive procedure. Alternatively, the examination might have revealed that an excess amount of material had been removed, rendering the part unusable. There was, therefore, a need in the art for a device which could detect when the desired surface characteristics or thickness had been achieved, in-situ, during the CMP process.
Several devices and methods have been developed for the in-situ detection of endpoints during the CMP process. For instance, devices and methods that are associated with the use of ultrasonic sound waves, and with the detection of changes in mechanical resistance, electrical impedance, or wafer surface temperature, have been employed. These devices and methods rely on determining the thickness of the wafer or a layer thereof, and establishing a process endpoint, by monitoring the change in thickness. In the case where the surface layer of the wafer is being thinned, the change in thickness is used to determine when the surface layer has the desired depth. And, in the case of planarizing a patterned wafer with an irregular surface, the endpoint is determined by monitoring the change in thickness and knowing the approximate depth of the surface irregularities. When the change in thickness equals the depth of the irregularities, the CMP process is terminated. Although these devices and methods work reasonably well for the applications for which they were intended, there is still a need for systems which provide a more accurate determination of the endpoint.
One such system employs a CMP apparatus in which a hole is formed in a platen and the overlying platen pad. The hole is positioned so that it has a view of the wafer held by a polishing head during a portion of the platen's rotation. A laser interferometer is fixed below the platen in a position enabling the laser beam projected by the laser interferometer to pass through the hole in the platen and strike the surface of the overlying wafer during the time when the hole is adjacent to the wafer. Various polishing pad embodiments include a transparent window in the pad. One of the concerns with the disclosed polishing pad arrangements is the leakage of slurry into the hole below the window of the polishing pad. This is a serious concern because any more than a trace amount of slurry will tend to scatter the light traveling through it, thus attenuating the laser beam emitted from the laser interferometer. The slurry leakage will thus cause inaccurate measurements with a laser interferometer, or even inoperability of the device.
In one method for detecting the end point in an in-situ polishing process, a platen is provided with a hole, or aperture, through which a laser interferometer is able to transmit laser light to the surface of the wafer being polished. The pad is configured with a transmissive portion that is positioned over the aperture and the rotatable platen. Thus, a relatively clear path to the wafer surface is provided by the combination of the platen and the pad. In one embodiment, the platen hole is formed with a stepped diameter to form a shoulder. A quartz insert is contained within the shoulder and functions as a window for the laser beam. The interface between the platen and the insert is sealed. The quartz insert protrudes above the top surface of the platen and partially into the platen pad in order to minimize the gap between the top surface of the insert and the surface of the wafer. This minimizes the amount of slurry trapped in the gap, thus reducing the attenuation of the laser beam emitted from the laser interferometer. It is desirable to make the gap as small as possible to reduce the amount of slurry in the gap. The fixing of the quartz insert within the platen is a concern, however since the wear of the pad could become so great that the top surface of the insert would touch the wafer and damage the wafer. In order to overcome this problem, another embodiment of the arrangement provides a polishing pad that has an integral window. For example, the window may be made of a polyurethane material that will not scratch the wafer and is co-planar with the top surface of the polishing pad. One of the disadvantages
Applied Materials Inc.
Moser Patterson & Sheridan
Nguyen George
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