Abrasive tool making process – material – or composition – Pore forming
Reexamination Certificate
1998-02-24
2004-04-13
Singh, Arti R. (Department: 1771)
Abrasive tool making process, material, or composition
Pore forming
C051S298000, C428S131000, C428S133000, C428S315500, C451S041000, C451S527000, C451S530000, C451S537000
Reexamination Certificate
active
06719818
ABSTRACT:
BACKGROUND
1. Technical Field
This invention relates to semiconductor manufacture, and more particularly, to an apparatus and method for chemical mechanical polishing (CMP) and in-situ endpoint detection during the CMP process.
2. Background Art
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. 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 26×25 mm die would be preferable. In addition, if the aforementioned 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 aforementioned surface irregularities can lead to poor yield and device performance. Consequently, it is desirable to effect some type of planarizatlon, 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 the aforementioned 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 aforementioned polishing platen
16
is typically 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
. Although some pads
18
have only the covering layer
22
, 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 aforementioned 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. Alternately, 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.
SUMMARY
The present invention is directed to a novel apparatus and method for endpoint detection which can provide this improved accuracy, The apparatus and method of the present invention employ interferometric techniques for the in-situ determination of the thickness of material removed or planarity of a wafer surface, during the CMP process.
Specifically, the foregoing objective is attained by an apparatus and method of chemical mechanical polishing (CMP) employing a rotatable polishing platen with an overlying polishing pad, a rotatable polishing head for holding the wafer against the polishing pad, and an endpoint detector. The polishing pad has a backing layer which interfaces with the platen and a covering layer which is wetted with a chemical slurry and interfaces with the wafer. The wafer is constructed of a semiconductor substrate underlying an oxide layer. And, the endpoint detector includes a laser interferometer capable of generating a laser beam directed towards the wafer and detecting light reflected therefrom, and a window disposed adjacent to a hole formed through the platen. This window provides a pathway for the laser beam to impinge on the wafer, at feast during the time that the wafer overlies the window.
The window can take several forms. Among these are an insert mounted within the platen hole. This insert is made of a material which is highly transmissive to the laser beam, such as quartz. In this configuration of the window, an upper surface of the insert protrudes above a surface of the platen and extends away from the platen a distance such that a gap is formed between the upper surface of the insert and the wafer, whenever the wafer is held against the pad. This gap is preferably made as small as possible, but without allowing the insert to touch the wafer, Alternately, the window can take the form of a portion of the polishing pad from which the adjacent backing layer has been removed. This is possible because the polyurethane covering layer is at least partially transmissive to the laser beam. Finally, the window can take the form of a plug formed in the covering layer of the pad and having no backing layer. This plug is preferably made of a polyurethane material which is highly transmissive to the laser beam.
In one embodiment of the present invention, the hole through the platen, and the window, are circular in shape. In another, the hole and window are arc-shaped. The arc-shaped window has a radius with an origin coincident to the center of rotation of the platen. Some embodiments of the invention also have
Birang Manoocher
Gleason Allan
Johansson Nils
Applied Materials Inc.
Fish & Richardson P.C.
Singh Arti R.
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