Accuracy Improvements in LPC Measurements for CMP Slurries
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1157-E04-07
Accuracy Improvements in LPC Measurements for CMP Slurries Bruno Tolla and David Boldridge Cabot Microelectronics Corporation, 870 Commons Drive, Aurora, IL 60504, USA ABSTRACT We have examined the Large Particle Count (LPC) analytical method to see whether there are opportunities to improve both the accuracy and precision in hope of improving the utility of the LPC measurement. We have identified weaknesses in the current method that limit both its accuracy and its precision, and which can introduce count errors in excess of a factor of 10. We propose modifications to the current method which result in both accuracy and precision improvements. We recommend these improvements as absolutely necessary for any experiments designed to test the correlation between LPC and defectivity. INTRODUCTION Chemical Mechanical Planarization depends on specially designed slurries to produce a flat, defect free surface on a semiconductor wafer in preparation for subsequent processing steps.[1,2] These slurries consist of abrasive particles and active chemistry in a liquid carrier, typically water. To assure effective polishing, the abrasive particle size distribution is tightly controlled. The practitioners of CMP are understandably worried that small quantities of undesirably large abrasive or impurity particles could damage the wafers and lead to yield loss. The quality metric of Large Particle Count (LPC) was implemented to help guard against abnormally high levels of these large particles. The LPC is presumed to indicate the number of particles larger than a given size, usually stated as 0.56 µm or 1.01 µm. These particles represent only ppm of the mass of the slurry and ppb of the number of abrasive particles, necessitating the use of a highly selective measurement technology. The task of quantifying such a small fraction of the total particle size distribution is quite challenging, and the industry has implemented Single Particle Optical Sizing (SPOS) as a routine analytical technique.[3-8] This technique is nominally capable of detecting only the largest particles while ignoring the smaller, necessary abrasive component. While there is ample evidence that the SPOS technique can provide a warning signal in extreme cases, continued improvements in abrasive and slurry production have dramatically reduced the typical LPC levels. As a result, the correlation between LPC and defectivity has become much less clear.[3,9-13] Weak correlation between LPC and defectivity can be the result of either inadequacy of the LPC measurement protocol or the appearance of non-LPC related defect mechanisms. We have found compelling evidence that inappropriate calibration and operating practices can severely compromise both the accuracy and the precision of the technique. Bringing the calibration and operating procedures into conformance with best known practice are required if the test is have any hope of recovering some of its former utility.
EXPERIMENTAL The slurries used in this study are commercial dielectric slurries produced by
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