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Tungsten Plug Measurement for CMP Development and Production

by Anna Mathai, Technical Marketing Engineer; Paul Sullivan, Software Manager; and Jason Schneir, Product Marketing Manager

Chemical Mechanical Planarization (CMP) processes are widely used in the semiconductor industry to enable multilevel device processing and smaller device features. Process variations in CMP can lead to many failure modes. For example, most metal polishing processes lead to recessed metal features because of the differential polishing at the dielectric/liner/metal interface. In addition, changes in the process, such as the pH level of the slurry, polishing speed or conditioning of the pads will change the plug recess. Plug recess needs to be carefully monitored since if it is too large, it will degrade the electrical connections between the vias and metal interconnects. This can lead to increased electrical resistance and limit the performance of the integrated circuit.

CMP process sectors have commonly used profilers to measure the post-CMP planarization of the wafer. As the feature size of ultra large-scale integration technology decreases, this introduces stringent requirements on the spatial resolution of the CMP metrology tools. KLA-Tencor’s High-Resolution Profiler (HRP) is designed to monitor metal CMP processes in the fab, as is demonstrated here on measurements of tungsten plugs.

Measuring plug recess

In a production environment, it is critical for the plug recess measurements to be made automatically on an entire cassette of wafers without operator intervention.

The HRP loads each wafer automatically with a pre-assigned orientation and uses optical pattern recognition to locate an isolated plug to within a 10 x 10 µm area.

Since CMP provides low optical contrast, edge enhanced pattern recognition models are used.

Next, the feature-find algorithm scans the stylus in the x-direction with a preassigned scan length (see schematic in figure 1). If no plug is found that meets preset depth and width criteria, the stylus steps over a pre-assigned distance in the

y-direction and takes another x-scan (scans 1-3). In this way, the stylus quickly locates the plug and then takes a scan in the y-direction (scan 4) to determine the center of the plug. Then, a high-resolution image is acquired, positioning the isolated plug to within 0.5 µm of the center of the scan (shown by dotted lines). Figure 2 shows an HRP high-resolution image of an isolated tungsten plug.

Scan 1

Next, the data is analyzed by the Scan 2 HRP advanced cusScan 3 tomizable measurement software, Answer!™ (see figFinal highure 3). Answer! resolution scan centered seamlessly integrates Scan 4 on feature sophisticated macros written in a highlevel programming Figure 1. The HRP feature-find algorithm. language with the HRP software. This architecture enables KLA-Tencor to quickly develop custom macros for specific process measurement needs.

The Answer! plug recess algorithm thresholds the data to automatically find the plug and separate it from the background. Then, the algorithm extracts a representative line profile through the plug. Finally, it segments the data vertically, based on user-selectable parameters, in order to compute the plug recess and width.

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Results

excellent long-term repeatability and to be insensitive to the stylus shape.

Figure 4 summarizes ten dynamic measurements on a single isolated tungsten plug, with the wafer unloaded and re-loaded between measurements. We found the plug recess measurement to have excellent repeatability of 0.9 nm (1σ).

Figure 2. HRP high-resolution image of 0.25 µm geometr y isolated tungsten plug.

In process development, the HRP has proven to be an invaluable characterization tool for CMP applications, including post-CMP Shallow Trench Isolation (STI), tungsten and copper CMP, and inter-level dielectric CMP. The HRP is used by both semiconductor manufacturers and CMP equipment manufacturers routinely to qualify and monitor the performance of CMP polishers.

The measurement was repeated after 12 hours and again after 48 hours, using the same stylus. The measurement was also repeated using two other styli. Overall, we found the plug recess measurement to have

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Figure 3. The HRP Answer! plug recess macro

Plug recess (nm)

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automatically measures tungsten plug recess and width.

The First Production-Ready 300 mm Resistivity Measurement System The new OMNIMAP® RS-100 delivers the features customers have been asking for.

• Accurate edge measurement to 1 mm from the conductive film • Powerful and easy to use Windows NT® platform • Full GEM/SECS HSMS compatibility and more • Expanded sensitivity for highly conductive interconnects such as copper and thick aluminum • Automated probe conditioner • 3D uniformity maps • 200 mm and 300 mm configurations • Meets SEMI safety and ergonomic requirements

Visit our website at www.kla-tencor.com circle RS#003