MetrologyF E A T U R E S

Tungsten Plug Measurement for CMPDevelopment 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 multileveldevice processing and smaller device features. Process variations in CMP can lead to many failure modes. For example, mostmetal polishing processes lead to recessed metal features because of the differential polishing at the dielectric/liner/metal inter-face. In addition, changes in the process, such as the pH level of the slurry, polishing speed or conditioning of the pads willchange the plug recess. Plug recess needs to be carefully monitored since if it is too large, it will degrade the electrical con-nections between the vias and metal interconnects. This can lead to increased electrical resistance and limit the performanceof the integrated circuit.

CMP process sectors have commonly usedprofilers to measure the post-CMP pla-narization of the wafer. As the feature sizeof ultra large-scale integration technologydecreases, this introduces stringentrequirements on the spatial resolution ofthe CMP metrology tools. KLA-Tencor’sHigh-Resolution Profiler (HRP) isdesigned to monitor metal CMP processesin the fab, as is demonstrated here onmeasurements of tungsten plugs.

Measuring plug recessIn a production environment, it is criticalfor the plug recess measurements to bemade automatically on an entire cassetteof wafers without operator intervention.The HRP loads each wafer automaticallywith a pre-assigned orientation and usesoptical pattern recognition to locate anisolated plug to within a 10 x 10 µm area.Since CMP provides low optical contrast,edge enhanced pattern recognition modelsare used.

Next, the feature-find algorithm scans thestylus in the x-direction with a pre-assigned scan length (see schematic in fig-ure 1). If no plug is found that meets pre-set depth and width criteria, the stylussteps over a pre-assigned distance in the

A

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

Next, the data isanalyzed by theHRP advanced cus-tomizable measure-ment software,Answer!™ (see fig-ure 3). Answer!seamlessly integratessophisticated macroswritten in a high-level programminglanguage with theHRP software. This architecture enables KLA-Tencor toquickly develop custom macros for specific process mea-surement needs.

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

Scan 4

Scan 1

Scan 2

Scan 3

Final high-resolution scan centered on feature

Figure 1. The HRP feature-find algorithm.

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F E A T U R E S

Figure 3. The HRP Answer! plug recess macro

automatically measures tungsten plug recess

and width.

Figure 2. HRP high-resolution image of 0.25

µm geometry isolated tungsten plug.

198 Angstroms

The new OMNIMAP® RS-100 delivers the feahave been asking for.

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The First Production300 mm ResistivityMeasurement System

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Automated mea-

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ResultsFigure 4 summarizes ten dynamicmeasurements on a single isolatedtungsten plug, with the waferunloaded and re-loaded betweenmeasurements. We found the plugrecess measurement to have excel-lent repeatability of 0.9 nm (1σ).

The measurement was repeated after12 hours and again after 48 hours,using the same stylus. The measure-ment was also repeated using twoother styli. Overall, we found theplug recess measurement to have

tures customers

measurement to 1 mm the conductive filmPowerful and easy to use Windows NT® platform

Full GEM/SECS HSMSompatibility and morexpanded sensitivity for highly onductive interconnects such as

copper and thick aluminum• Automated probe conditioner

• 3D uniformity maps• 200 mm and 300 mm configu• Meets SEMI safety and ergono

-Ready

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excellent long-term repeatabilityand to be insensitive to the stylusshape.

In process development, the HRPhas proven to be an invaluable char-acterization tool for CMP applica-tions, including post-CMP ShallowTrench Isolation (STI), tungsten andcopper CMP, and inter-level dielec-tric CMP. The HRP is used by bothsemiconductor manufacturers andCMP equipment manufacturers rou-tinely to qualify and monitor theperformance of CMP polishers.

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