Forming a transparent window in a polishing pad for a chemical mechanical polishing apparatus

#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).

#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 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.

#The accompanying drawings, which are incorporated and constitute a part of the specification, schematically illustrate an embodiment of the invention, and together with the general description given above and the detailed description given below, serve to explain the principles of the invention.

#FIG. 1 is a side view of a chemical mechanical polishing (CMP) apparatus typical of the prior art.

#FIG. 2 is a side view of a chemical mechanical polishing apparatus with endpoint detection constructed in accordance with the present invention.

#FIG. 2 depicts a portion of a CMP apparatus modified in accordance with one embodiment of the present invention. A hole 30 is formed in the platen 16 and the overlying platen pad 18. This hole 30 is positioned such that it has a view of the wafer 14 held by a polishing head 12 during a portion of the platen's rotation, regardless of the translational position of the head 12. A laser interferometer 32 is fixed below the platen 16 in a position enabling a laser beam 34 projected by the laser interferometer 32 to pass through the hole 30 in the platen 16 and strike the surface of the overlying wafer 14 during a time when the hole 30 is adjacent the wafer 14.

#A detailed view of the platen hole 30 and wafer 14 (at a time when it overlies the platen hole 30) are shown in FIGS. 3A-C. As can be seen in FIG. 3A, the platen hole 30 has a stepped diameter, thus forming a shoulder 36. The shoulder 36 is used to contain and hold a quartz insert 38 which functions as a window for the laser beam 34. The interface between the platen 16 and the insert 38 is sealed, so that the portion of the chemical slurry 40 finding its way between the wafer 14 and insert 38 cannot leak through to the bottom of the platen 16. The quartz insert 38 protrudes above the top surface of the platen 16 and partially into the platen pad 18. This protrusion of the insert 38 is intended to minimize the gap between the top surface of the insert 38 and the surface of the wafer 14. By minimizing this gap, the amount of slurry 40 trapped in the gap is minimized. This is advantageous because the slurry 40 tends to scatter light traveling through it, thus attenuating the laser beam emitted from the laser interferometer 32. The thinner the layer of slurry 40 between the insert 38 and the wafer 14, the less the laser beam 34 and light reflected from the wafer, is attenuated. It is believed a gap of approximately 1 mm would result in acceptable attenuation values during the CMP process. However, it is preferable to make this gap even smaller. The gap should be made as small as possible while still ensuring the insert 38 does not touch the wafer 14 at any time during the CMP process. In a tested embodiment of the present invention, the gap between the insert 38 and wafer 14 was set at 10 mils (250 μm) with satisfactory results.

#FIG. 3B shows an alternate embodiment of the platen 16 and pad 18. In this embodiment, the quartz insert has been eliminated and no through-hole exists in the pad 18. Instead, the backing layer 20 (if present) of the pad 18 has been removed in the area overlying the hole 30 in the platen 16. This leaves only the polyurethane covering layer 22 of the pad 18 between the wafer 14 and the bottom of the platen 16. It has been found that the polyurethane material used in the covering layer 22 will substantially transmit the laser beam 34 from the laser interferometer 32. Thus, the portion of the covering layer 22 which overlies the platen hole 30 functions as a window for the laser beam 34. This alternate arrangement has significant advantages. First, because the pad 18 itself is used as the window, there is no appreciable gap. Therefore, very little of the slurry 40 is present to cause the detrimental scattering of the laser beam. Another advantage of this alternate embodiment is that pad wear becomes irrelevant. In the first-described embodiment of FIG. 3A, the gap between the quartz insert 38 and the wafer 14 was made as small as possible. However, as the pad 18 wears, this gap tends to become even smaller. Eventually, the wear could become so great that the top surface of the insert 38 would touch the wafer 14 and damage it. Since the pad 18 is used as the window in the alternate embodiment of FIG. 3B, and is designed to be in contact with the wafer 14, there are no detrimental effects due to the wearing of the pad 18. It is noted that tests using both the opaque open-cell and transparent grooved surface types of pads have shown that the laser beam is less attenuated with the transparent grooved surface pad. Accordingly, it is preferable that this type of pad be employed.