Integrated Laser Profiler Monitors Copper CMP

In chemical mechanical planarization (CMP), removal rate is affected by feature size, local metal density, preceding dielectric planarization, metal thickness uniformity, and CMP process parameters. A major problem in copper CMP is that the dielectric material polishes at a different rate than the copper. This occurs because the latter is soft compared with the dielectric material that surrounds it, which typically is silicon dioxide and is quite hard. Because copper polishes faster, its features can be easily affected by dishing and erosion.

Traditionally, chipmakers have used stand-alone, off-line metrology tools, which tend to be slower and may add wafer contamination risks. A fundamental need in the copper CMP process is the capability to accurately track and determine metal loss while simultaneously evaluating the height differential between a copper feature and the surrounding dielectric material.

An integrated-environment, non-destructive, real-time laser profiler system capable of directly measuring multiple metal layers across the wafer has been developed by Nanometrics (Milpitas, Calif.). The Nano CLP-9010 integrated metrology tool specifically targets copper processing's unique requirements in the 300 mm area (Fig. 1). It is capable of accurately determining metal loss associated with a copper feature relative to the surrounding dielectric material. Because it does not just use reflectance when there are multiple layers, there is no interference to complicate the measurement process, and the technique will work at virtually every metal layer in the process. The tool is capable of predicting the minimum copper thickness on the largest features, allowing for a direct calculation of thickness as well as the cross-sectional area of the line. The system was recently evaluated by a leading U.S. DRAM manufacturer on a polisher under production conditions.

1. The optical path difference between the measurement beam and reference beam results in a phase shift, which enables the step height to be accurately calculated. The phase shift from the dielectric/substrate interface is accounted for through an additional reflectometer measurement. (Source: Nanometrics)

2. The optical system has no moving parts, using a laser beam that is broken into two spots by a Wollaston prism, and projected onto the wafer surface. There, the surface topography causes a phase difference between the two spots, which is then measured by the detector circuit. (Source: Nanometrics)

The process control capabilities that are enabled by the system allow for the prediction of excessive metal loss and residuals. Because of its high sampling capabilities, misprocessed wafers can be detected before becoming a major problem. The tool, which will also work for tungsten CMP processes, can measure a wide range of feature sizes (1 µm and larger), monitor 100% of the metal layers, and measure in actual devices and scribe lines. It provides a 0.1 nm height resolution, dynamic repeatability of 1 nm (1s ), and 40 wph throughput at a nine-sites-per-wafer measurement protocol.

For additional information on inspection, measurement and test, go to www.semiconductor.net/imt.