Advances in electronic devices generally include reducing the size of the components that form integrated circuits. As semiconductor devices have become more highly integrated in recent years, circuit interconnections have become finer and distances between those circuit interconnections have become smaller. For example, in the case of photolithography, which can form interconnections smaller than 0.5 μm wide, surfaces on which pattern images are to be focused should be as flat as possible. Further, with smaller circuit components the value of each unit area of a semiconductor wafer becomes higher because the ability to use all of the wafer area for integrated circuit components improves. To properly form an integrated circuit that employs a much higher percentage of usable wafer area, contaminant particle counts on the semiconductor wafer surface must be reduced to very low levels. In order to clean a semiconductor wafer and remove unwanted particles, a process known as chemical mechanical polishing or chemical mechanical planarizing (hereinafter “CMP”) has become popular.
The polishing is typically accomplished using a polishing pad attached to a platen surface of a CMP machine. A workpiece to be polished, such as a semiconductor wafer, is arranged in a manner such that a surface to be polished faces the polishing pad. The polishing pad and workpiece are independently rotated while an abrasive liquid or slurry is supplied onto the polishing pad. The workpiece is then pressed against the polishing pad at a predetermined pressure and the surface of the workpiece is polished to a flat mirror finish.
Polishing pads have a limited service life and become less effective over time. In turn, this results in inconsistent quality in the semiconductor wafers. As the polishing pad begins to lose its effectiveness, the semiconductor wafers polished later in the pad's life may be less smooth, possibly resulting in significant amounts of lost or unsaleable product. Thus, the polishing pads must be changed from time to time.
Current methods of determining the wear of a polishing pad, and thus whether the polishing pad is in need of replacement, are unsatisfactory. Analysis of the pad typically requires visual inspection. However, the pad usually cannot be visually inspected without removing the polishing pad from the CMP machine. Once removed, the polishing pad typically cannot be placed back on the machine, even if visual inspection determines some usable life remains. As a result, statistical analysis, based on experimental determination of pad wear over time under various operating conditions, is often used to make decisions about pad replacement. This is unsatisfactory for the reason, among others, that statistical analysis represents an average based on the wear of previous pads and is unable to account for any one particular pad or variations in quality or aberrant pads of inferior quality.
Furthermore, analysis of each polishing pad typically takes several hours or more and is often conducted by highly paid, highly skilled technicians whose time might be better used on other projects.
Another method of monitoring and analyzing polishing pad effectiveness is performing an analysis of the smoothness of the polished semiconductor wafer product. However, due to other steps required in wafer manufacture, this analysis is impractical until later in the manufacturing process. Thus, by the time defects in any one polishing pad are discovered and needed replacement is determined, the worn out pad may already have improperly polished many wafers remaining upstream of a suitable wafer analysis point, resulting in significant loss of product that must be discarded or recycled.
Conversely, using methods such as statistical analysis may also result in removing the polishing pad too soon, meaning that the full usable life of the polishing pad has not been obtained, resulting in the inefficient use of consumable materials. Thus, this also results in undesirable and uneconomic business practices.
What is needed is a system that can provide more efficient analysis of CMP polishing pads. What is also needed is a system that can analyze CMP polishing pads without removing the pad from the CMP machine.