Effects and Interactions of Wafer Shape and Stepper Chucks on Wafer Flatness Control

Abstract

As submicron lithography is advancing and device manufacturers are producing geometries of half micron and below to fabricate 16 and 64Mbit DRAMS, imperfections in any stepper component become increasingly critical to acceptable yield and must be controlled. Additionally the effects of wafer shape can become a significant lithographic detractor. This paper describes a metrology for extracting information about the shape and performance of the stepper chuck. A preliminary characterization of the effect of wafer shape on chucked flatness is presented. During the exposure process, the wafer is clamped by vacuum against the stepper chuck. If the chuck deviates from perfect planarity, this deviation is transmitted to the top exposure surface of the wafer resulting in larger focal plane deviations. Although chucks are manufactured to tight tolerances, even 0. lum deviations are significant for today’s geometries. The vacuum clamping action of the stepper chuck can deviate from an ideal state of 100% surface-to-surface contact depending on the wafer shape. Bows of tens of microns have been shown to cause marked effects. Thickness-based flatness metrology systems are widely used to qualify wafers for lithography. These systems do not use mechanical references, and therefore, derive the true, undistorted wafer flatness. Steppers themselves can, in special operational modes, also measure wafer flatness via their focus systems. These measurements will, of course, include any stepper chuck induced flatness. Results cover the work conducted on 100” and 150” wafers using a variety of stepper tools. The wafers were measured on a thickness based flatness system and high-density flatness data sets were obtained (16,000 points per wafer). Data sets for each wafer were also obtained from several commercially available steppers operating in “wafer flatness mode” (1,000-2,000 points per wafer). The data were processed to create point-commensurate data sets for all wafers for both flatness methods. The metrology tool flatness data and the stepper flatness data were differenced on a point-by-point basis to extract a “stepper chuck signature” for each wafer. The individual signatures were ensemble averaged to reveal the statistically significant signature for each stepper chuck. Statistical signatures are presented including point-by-point variances. The relationship of these variances to wafer shape are discussed. From these experiments a new metrology process has been derived which measures the performance of the wafer flatness and chuck flatness interaction. Furthermore, this metrology process can measure and monitor the effects of wafer shape as a lithographic detractor. These techniques will promote better understanding of real world photolithography. This in turn will promote higher yields.