A Better Way to Monitor Your Copper Process

Since the inception of copper dual-damascene processing, there has been a critical need for a non-destructive monitoring system that can track non-uniformity and pattern-loading effects of copper CMP on product wafers. Now, with the scaling of multilevel copper interconnects and the shift to 300 mm wafers, the requirement has become even more imperative.

The BX-30 advanced interconnect measurement system from Boxer Cross (Menlo Park, Calif.) is an in-line optical metrology tool that measures metal resistance on product wafers. It measures resistance per unit length in fine-line arrays or resistance per unit area on copper bond pads or blanket films, which directly correlate to copper profile and film thickness. The tool targets the needs of fabs for CMP process control, and, by allowing straightforward characterization of copper features, it also aids in new tool and process qualification.

"Customers were having issues with pattern microloading on product wafers, which were not being detected by test wafers. When they transferred their processes to production, they were experiencing yield loss of die on the wafer periphery — clearly better characterization methods were needed," explained Yuval Wasserman, senior vice president at Boxer Cross.

With copper CMP, dishing and erosion issues can appear both within-wafer and within-die. "You see substantial differences between the test chip area and the actual active region of the device, especially at Metal 1 and Metal 2 levels, which contain a great variety of pattern density, from 0.12 µm isolated lines to high-density arrays," added Peter Borden, the company's chief technology officer. "You can't capture that whole variety in a small set of test structures, especially in the limited space available in scribe lines." Alternatively, engineers can select device structures that represent the most critical elements of the CMP process and measure those sites (five or nine per wafer) in process control mode. With the capability of measuring fine-line array and isolated line features, the tool is especially adept at detecting metal shorts, a common result of CMP under-polish.

IC manufacturers traditionally use stylus profilometry or FIB-SEM methods to analyze copper line profiles and film thicknesses. Unfortunately, neither method meets the needs for non-destructive, in-line monitoring and high throughput. Profilometry gives the profile change, but not film thickness; focused ion beam (FIB) milling and SEM viewing allow high-resolution analysis, but not real-time process feedback.

The BX-30 signal of 0.16 µm copper lines is converted to thickness, clearly showing erosion patterns in copper arrays. (Source: Boxer Cross)

The tool's throughput in process control mode is ³ 35 wph, and full wafer characterization (~100 sites) takes ~10 min. Copper line thickness directly correlates with FIB-SEM, profiler and electrical probing results. Boxer Cross's roadmap also includes integration of the system with copper CMP tools, and exploring other critical metal applications.

For additional information on yield management, click here.