Patent Grant
7622052
The present invention generally relates to chemical mechanical planarization, and more particularly relates to methods for chemical mechanical planarization and to detecting the endpoint of a chemical mechanical planarization operation.
The manufacture of many types of work pieces requires the substantial planarization of at least one surface of the work piece. Examples of such work pieces that require a planar surface include semiconductor wafers, optical blanks, memory disks, and the like. One commonly used technique for planarizing the surface of a work piece is the chemical mechanical planarization (CMP) process, a process commonly practiced in a multi-zonal processing apparatus. In the CMP process a work piece, held by a work piece carrier head, is pressed against a polishing pad and relative motion is initiated between the work piece and the polishing pad in the presence of a polishing slurry. The mechanical abrasion of the surface combined with the chemical interaction of the slurry with the material on the work piece surface ideally produces a surface of a desired shape, usually a planar surface. The terms “planarization” and “polishing,” or other forms of these words, although having different connotations, are often used interchangeably by those of skill in the art with the intended meaning conveyed by the context in which the term is used. For ease of description such common usage will be followed and the term “chemical mechanical planarization” will generally be used herein with that term and “CMP” conveying either “chemical mechanical planarization” or “chemical mechanical polishing.” The words “planarize” and “polish” will also be used interchangeably.
The construction of the carrier head of a CMP apparatus and the relative motion between the polishing pad and the carrier head as well as other process variables have been extensively engineered in an attempt to achieve a desired rate of removal of material across the surface of the work piece and hence to achieve the desired final surface shape. For example, the carrier head generally includes a flexible membrane that contacts the back or unpolished surface of the work piece and accommodates variations in that surface. A number of pressure chambers are provided behind the membrane so that different pressures can be applied to various zones on the back surface of the work piece to cause desired variations in polishing rate across the front surface of the work piece.
End point detection probes are often used to detect the completion of a polishing operation. The completion of the polishing operation is signaled or “called”, in accordance with a detection algorithm, as a function of the remaining material thickness. Upon detection of the end point signal, the CMP operation is either terminated immediately or after some prescribed delay denoted as an “over polish time.” The proper identification of endpoint is an important step in a CMP operation. Consider, for example, the removal of a copper layer from the surface of a semiconductor wafer as part of the process of forming a damascene pattern of interconnect metallization on that semiconductor wafer. If the endpoint is called too soon, an undesired layer of copper will remain on the semiconductor wafer causing an electrical short between unrelated metal conductors. If the endpoint is called too late, the polishing operation may cause damage to either the interconnect metal pattern or to the underlying insulator layers.
A number of different mechanisms and methods are available and commonly used for detecting the end point of a CMP operation. Such mechanisms and methods include optical end point detectors, eddy current monitors, measuring the drag experienced by the motors generating the relative motion between the work piece and the polishing pad, and the like. Each of these end point detection mechanisms and methods suffers from technical hurdles, especially when the layer being polished or removed is very thin. Specifically, calibration of the mechanism or method can be difficult at very thin layers, and noise sources can become a significant source of variation and error from work piece to work piece.
Accordingly, it is desirable to provide a chemical mechanical planarization method including an accurately determined end point. In addition, it is desirable to provide a method for accurate end point detection of a CMP operation. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.
The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein
The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, or the following detailed description.
CMP apparatus 20 is illustrative of apparatus such as the Xceda CMP system that is available from Novellus Systems, Inc. of San Jose, Calif. CMP apparatus 20 includes a carrier head 22 and a polishing pad 24. Carrier head 22 includes a flexible diaphragm 26 that presses against the upper surface of a semiconductor wafer 28, the lower surface of which is to be planarized. Preferably the diaphragm includes a plurality of concentric zones 29 that can be individually pressurized to predetermined pressures to exert different appropriate pressures against various zones of the wafer. A retaining ring or wear ring 30 serves to restrict the lateral movement of the wafer and to keep the wafer centered under the carrier head. Polishing pad 24 is supported on a platen 32. Mechanisms (not illustrated) are connected to the carrier head and to the platen that enable the wafer and/or the polishing pad to be placed in motion during a CMP operation so that relative motion is established between the wafer and the polishing pad. Preferably the carrier head, and hence the wafer, is rotated about an axis 34 that is perpendicular to the surface of the wafer and to the layer that is being polished on the lower surface of the wafer. Platen 32 and polishing pad 24 can be moved in a linear, rotational, orbital, or other pattern, but preferably is moved in an orbital pattern. A polishing slurry is supplied to the polishing pad, for example through a manifold (not illustrated) within and beneath platen 32. The slurry can then be injected through an array of holes in the top of the platen and through corresponding holes in the polishing pad. The composition of the polishing slurry is determined by the composition of the material being removed from the semiconductor wafer or other work piece. For example, if the layer to be polished from the work piece is a layer of copper, the polishing slurry might include alumina as an abrasive and hydrogen peroxide.
CMP apparatus 20 also includes an end point detector system. The end point is usually defined as the completion or near completion of the removal of a layer overlying the semiconductor wafer. A number of different systems can be used to detect the end point of the CMP operation. When the layer being removed is formed of an electrically conductive material such as copper or other metal, the preferred end point detector system, in accordance with an embodiment of the invention, is an eddy current end point detection system, although other systems such as an optical detector system can by used in accordance with some embodiments of the invention. An eddy current end point detection system, which is generally located in the platen, includes an eddy current generator and an eddy current detector. The eddy current generator includes a mechanism for generating an oscillating magnetic field. The oscillating magnetic field induces eddy currents in the electrically conductive layer material overlying the semiconductor wafer. The eddy current detector includes a tuned circuit that is electrically in parallel with the impedance represented by the layer of conductive material. As the thickness of the layer of conductive material changes during the CMP operation, the impedance of that layer changes, causing a change in impedance matching of the tuned circuit. This change can be correlated to the thickness of the conductive layer. As explained above, however, the measurement of thickness of the conductive layer, and hence the end point of the CMP operation, can be inaccurate, especially for thin layers, because of problems with calibration of the system and noise. Additional inaccuracies result from the nonuniform nature of both the layer being polished and the layer underlying the layer being polished. The underlying layer, for example, may have been patterned as part of the process of manufacturing the semiconductor device or other work piece.
In accordance with one embodiment of the invention, a preferred end point detection system includes four separate eddy current end point detection systems 50, each including an eddy current generators and an associated eddy current detector, arrayed along a radius of platen 32 as illustrated in
A method in accordance with one embodiment of the invention is illustrated in flow chart format in
In accordance with one embodiment of the invention, the method of end point detection continues by dividing the diameter scan into a plurality of zones as indicated at step 104. The number of zones can be chosen dependent upon the initial characteristics of the work piece that is to be polished, and can vary from as little as one up to several zones. When more than one zone is used, the zones typically represent concentric rings on the surface of the wafer. As indicated at step 106, an average thickness is calculated for each of the zones, with the average thickness calculated by averaging the thickness measurements reported for that zone. At step 108 those average thickness values are stored. In a preferred embodiment the value that is calculated and stored is the average value, however any thickness based metric can be utilized. For ease of discussion, any such value will hereinafter be referred to as an average value.
While proceeding down the flow chart, the method in accordance with this embodiment of the invention includes taking continuous measurements in a loop completed by path 109. As indicated by path 109, additional thickness measurements are continuously made, additional diameter scans are generated, one for each of the predetermined intervals of time, the diameter scan is divided into zones, an average thickness is calculated for each zone, and the calculated average thickness values are stored. In step 110 a difference is calculated for each sequential pair of stored values of average thickness for each of the zones. The calculated difference can be expressed as an incremental change in thickness of the layer being polished or as an incremental removal rate of that layer where the removal rate is the change in layer thickness per unit of time for each zone. The incremental change in thickness or incremental removal rate for each zone is stored at step 112. In a preferred method the average removal rate is used, but any removal rate based metric can be used. Again, for ease of discussion, any such value will hereinafter be referred to as the average removal rate.
At step 114 a moving average of the stored incremental data is calculated for each zone. If the diameter scan is generated every second, the moving average can be, for example, a five second moving average. At step 116 a predetermined threshold value is established. The threshold value can be expressed as a minimum incremental change in measured thickness or as a minimum removal rate, depending on how the incremental data for each zone is expressed.
Although individual measurements of thickness may be subject to error, it has been found that incremental changes in thickness or incremental changes in removal rate, especially when smoothed by taking a moving average, lead to an accurate end point detection. As the end point of the CMP operation is approached, the incremental changes approach zero. In step 118 the moving averages calculated in step 114 for each of the zones are compared to the threshold value established in step 116. If the moving average for a zone is less than the established threshold, the method in accordance with one embodiment of the invention takes path 120 and “calls” an end point for that zone as indicated in step 122. That is, when the incremental changes in thickness or incremental changes in removal rate decreases below some predetermined value, the end point is called. To “call” an end point is to ascertain that a predetermined point in the process has been reached. The same comparison is made for each of the zones. If all of the zones have called an end point (step 124), the end point of the CMP operation is called as indicated at step 126. If all of the zones have not called an end point, the method returns to step 100 and continues until all of the zones do have a moving average less than the threshold and an end point is called for all zones. If the comparison made at step 118 determines that the calculated moving average is not less than the established threshold, the method takes path 128 and returns to step 100.
At step 202 relative motion is initiated between the work piece and the polishing pad. In accordance with a preferred embodiment of the invention the work piece carrier and the work piece are set in rotational motion about an axis perpendicular to the layer that is to be planarized, and the polishing pad is set into orbital motion. Although various parameters can be used for the relative motion, typical parameters are a rotation of about 11 revolutions per minute (rpm) and an orbital radius of about 1.6 centimeters at an orbit speed of about 600 rpm. At step 204 the work piece is moved into contact with the polishing pad to initiate polishing or planarization of the layer on the work piece.
At step 206 measurement of thickness of the layer on the work piece by the plurality of end point detectors is initiated. The end point detectors make measurements periodically, such as every one millisecond. Measurements made by the plurality of end point detectors are reported to a computer or other end point controller. In accordance with a preferred embodiment of the invention at step 208 a diameter scan is generated based on the measurements reported to the computer. The diameter scan can be generated and updated at some predetermined interval of time such as once every second. At step 210 each of the diameter scans is divided into one or more zones representative of zones, preferably concentric annular zones, on the work piece.
At step 212 the end point is determined for each of the zones in the same manner as explained above with respect to the flow chart of
While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof. Although some of the various exemplary embodiments of the invention have made specific reference to semiconductor wafers as an example of work pieces and to copper or other metal layers on the semiconductor wafers, the invention is not limited to application to these specific work pieces. Additionally, although in preferred embodiments of the invention the end point detector of choice is an eddy current based end point detector, the invention is also applicable to thickness measurements made by other types of end point detectors.
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