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CMP: Where Does It End? Ron Allen, Charles Chen, Tom Trikas, Kurt Lehman, Robert Shinagawa, and Vijay Bhaskaran, KLA-Tencor Corporation Brian Stephenson and David Watts, Ebara Technologies Inc.
We describe the design, operation, and algorithms for an in-situ CMP endpoint detection and control system, with particular emphasis on copper polishing. The system’s eddy current-based sensor gives absolute surface metal thickness. Its multi-angle reflectometer gives eight optical reflectance measurements. The endpointer improves on existing sensors and techniques in several ways. It can process reflectance traces individually according to their endpoint sensitivity, which applies to dielectric polishing and copper barrier removal processes. Also, it merges reflectance signals for higher signal-to-noise ratios, which benefits copper CMP. Finally, the system can fuse the reflectance data with thickness readings for more robust endpoint detection.
Introduction
Chemical Mechanical Planarization (CMP) is a widely accepted polishing and patterning method in microelectronics fabrication. Though CMP is indeed crucial to some processes—copper (Cu), for instance, for which plasma etching remains problematic—process engineers must still cope with underpolish and overpolish problems. Stopping a metal polish step too soon (underpolishing) leaves metal or barrier material residues, which cause electrical shorts in the target layer. Underpolishing dielectric films causes open circuit defects. Polishing too long (overpolishing) results in metal dishing and dielectric erosion, ultimately leading to metal pooling and short circuits in higher metal layers. Simple time-based polishing is widespread in fabs. But Cu electroplating produces film thickness variations that thwart timed recipes, and the requisite rate monitor wafers are becoming prohibitively expensive. CMP tool conditions such as temperature, pad condition, and wafer pressure profile also affect the polish rate and uniformity. To facilitate CMP integration into largevolume production, process controllers must have cross-wafer information available and monitor each wafer’s polish profile to 54
Winter 2002
Yield Management Solutions
determine the process endpoint (the precise time at which the target material has been removed) or the remaining requisite layer thickness. KLA-Tencor’s Precision In-Situ CMP Endpoint (PRECICE™) system addresses these issues by providing real-time film thickness measurements, reflectance data, and endpoint detection for a variety of polishing processes, including Cu CMP. Design: sensors, controller, and communications links
The system contains an eddy current-based sensor; a single wavelength, multi-angle reflectometer; a data acquisition and control processor; and communication links to the CMP host computer. The sensors mount beneath the CMP tool’s rotary platen (Figure 1). The eddy probe has a drive coil that induces a current in the wafer, a sense coil to find in-phase and quadrature components of the induced voltage, and signal generation and data acquisition electronics1. The control computer’s calibration curves give absolute metal thickness values, independent of temperature and pad wear. The standard optical sensor wavelength is 808 nm. Two methods exist for creating an optical path to the wafer: a flexible polyurethane window inserted into the pad and a self-clearing objective (SCO) that uses a timed flow of deionized water (DIW). Thus, wafer incidence angles vary; with a SCO they range from 6.7° to 56.3°. A rotary union and slip ring on the table shaft bring fluid lines and electrical paths to table-mounted sensors.