Method for polishing a wafer and method for manufacturing an integrated circuit

0001This invention relates to the use of chemical mechanical planarization (CMP) in the manufacture of semiconductor integrated circuits and more particularly to prediction of performance characteristics of polyurethane pads used for CMP of semiconductor wafers.

0002During fabrication of integrated circuits, it is often desirable to planarize and/or polish the surface of a semiconductor wafer. One method of performing these tasks is referred to as chemical mechanical planarization (CMP). In general, the CMP process involves rotation or random movement of a wafer on a polishing pad in the presence of a polishing slurry. The polishing pad is typically formed of a polyurethane material.

0003Downward pressure on the wafer against the pad, rotational speed of the wafer and the pad, slurry content and pad characteristics determine the rate at which material is removed the from the surface of the wafer, and the uniformity of the resulting wafer surface.

0004Determination of how long a wafer should be planarized or polished has proven to be a difficult task. An apparatus and method for in-situ measurement of the thickness of a material to be planarized for CMP end point determination is described in U.S. Patent No. Re. 34,425 to Schultz.

0005Methods of controlling the pressure exerted on the wafer against the pad, rotational speed or random movement of the wafer on the pad and slurry composition are well known in the art. Condition and performance characteristics of the polyurethane pad are more difficult to determine. The ability of a pad to planarize the surface of a wafer varies substantially from pad to pad and over the life of an individuals pad.

0006After a wafer has been through the CMP process the pad will be conditioned to prepare it for another wafer. The conditioning process comprises a controlled abrasion of the polishing pad surface for the purpose of returning the pad to a state where is can sustain polishing. The ability of the conditioning process to return the pad to a state where it can efficiently planarize an additional wafer is dependent upon the pad itself and the conditioning parameters. After planarizing several hundred wafers, the pad may no longer be useful for planarizing wafers despite the conditioning process.

0007A method of staining polishing pads using a Rhodamine dye to delineate pad structure and detect contamination particles is disclosed in IBM Technical Disclosure Bulletin 21(4), 1433 (1978), which also states that the concentrations of 0.01% of the Rhodamine dye will fluoresce under ultraviolet light to provide a further means of pad structure delineation.

0008In Mat.Res.Soc.Symp.Proc. 260, 53-64 (1992) some of the problems limiting broad manufacturing implementation of CMP as a planarization process for interlevel dielectrics are reviewed. The authors propose an explanation for the observed decrease in polish rates as the polish pad ages, based on slurry transport.

0009In Mat.Res.Soc.Symp.Proc. 337, 637-644 (1994) the authors describe a study which attempts to correlate the intrinsic polymer properties and cellular structure of the pad material to CMP process indices like polishing rate and planarity. For example, the drop off in removal rate as a function of time can be attributed to the mechanical response of polyurethanes under conditions of critical shear.

0010The ability to predict a performance characteristics of new and used polyurethane pads would be a great benefit to users and manufacturers of such pads.

0011The present invention provides a method for polishing a wafer, a method for manufacturing an integrated circuit as defined in claims 1 and 5. For example, a measurement of a change in pad size following soaking in an organic solvent of a polyurethane pad manufactured for chemical mechanical planarization (CMP) of semiconductor wafers is used to predict performance characteristics of the pad, and to adjust polishing time.

0012After manufacturing a lot, one pad or a portion of a pad from the manufacturing lot may be soaked in an organic solvent which causes the pad material to swell. It is believed that the relative increase in size is indicative of chemical bonding of polymer chains within the pad. The increase in pad size is indicative of the performance characteristics of the pad. Statistical Process Control methods may be used to optimise the pad manufacturing process. A manufacturing lot may consist of any number of pads which are deemed to have been manufactured under conditions which tend to cause all pads within the lot to have very similar performance characteristics. Measurements of a change in size following soaking of the pad in organic solvent allow predicted pad characteristics to be available for each pad. The predicted performance characteristics may be used as a measure of quality of the pad, and may also be provided to pad end users.

0013Pad characteristic measurements may be taken before any wafers are planarized. Measurements may also be taken after each wafer is planarized or at intervals throughout the life of the pad. Repeated use of the pad impacts the polishing/planarizing ability of the pad. During the CMP process, polyurethane pads are often exposed to high pH (9.0 to 13.0) and high temperature (0 to 90C) environments. A correlation between pad swelling characteristics and pad performance has been noted in pads that have been exposed to such conditions. The predicted effect of conditioning can then be used to predict performance characteristics of the pad. The measurement of pad fluorescence characteristics also allows for worn or substandard pads to be replaced prior to wafer processing.

0014The features of the invention as well as objects and advantages will be best understood by reference to the appended claims, detailed description of particular embodiments and accompanying drawings where: <ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 is a plot of pad swelling versus wafer material removal rate.</li></ul>

0015Fig. 1 is a plot of the swelling of a portion of a CMP pad soaked in N-Methyl-2-pyrrolidone (NMP) for twenty-four hours versus the rate of planarization of a semiconductor device wafer which is planarized by the pad. Increases in swelling beyond twenty-four hours are not very large; however, longer or shorter periods of time may be used. The swelling measurement shown is a measurement of increase in pad area. The increase in pad volume, or simply the increase in length of a strip of pad material may also be used. Greater swelling indicates that the planarization rate will be lower. It is believed that other organic solvents such as MEK, MIBK, THF, Xylene and MeC12 may be used with similar results.

0016The plot of Fig. 1 shows that measurement of polyurethane pad swelling can be used to predict the planarization characteristics of the pad. The predicted planarization characteristics allow for a determination of planarization time in a CMP process. Predicted planarization characteristics of a CMP pad can also be used for process control and quality control in the manufacture of CMP pads. This data may be sent with the pads to CMP pad customers in the form of predicted planarization characteristics for particular CMP processes. The inventive method of measuring pad swelling characteristics may be used to perform incoming inspection on the pads. Substandard pads can be rejected before they are ever used.