Analyzer Offers Fast, Non-Destructive Characterization
Alexander E. Braun, Senior Editor -- Semiconductor International, 1/1/2001
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Traditionally, shallow trench characterization has been done using slower and localized techniques such as atomic force microscopy (AFM) or destructive methods such as scanning electron microscopy (SEM). These techniques yield information, but generally do not accommodate high-throughput sampling. Under certain circumstances, they may not lend themselves to wafer uniformity studies and can be difficult to apply in manufacturing environments. Etch depth control across the wafer is critical in trench etch. Because it is a blind etch — and uniformity of fill and CMP depend on trench uniformity — trench depth control is crucial. In situ metrology capable of providing rapid feedback on trench depth and uniformity could be useful, minimizing test wafers and scrap.
n&k Technology Inc. (Santa Clara, Calif.) has developed a fast, non-destructive characterization method. It is non-localized, returning average results from an illuminated area of 50 µm, and provides considerable information about these structures and their surroundings. Besides measuring standard quantities such as trench depth and width, the TMS Analyzer also provides data about the thicknesses and properties of films deposited inside and outside trench regions.
The system uses broadband spectrophotometry — covering a wavelength range of 190 to 1000 nm — and incorporates all-reflective optics and a microspot for measuring patterned wafers. Actual analysis is based on the Forouhi-Bloomer dispersion equations for refractive index (n) and extinction coefficient (k), plus other algorithms.
In a demonstration, the system characterized four test wafers comprised of trench structures. Apart from intentional trench depth changes, the structures were nominally identical. The samples were divided into halves; one half was measured using a SEM, the other using the analyzer (Figure). Average trench depth was measured at 59 locations across the diameter of each half wafer and a SEM used to determine trench depth at a location at the center of each half wafer.
The data show that average trench depths measured by the TMS closely agree with SEM measurements. Multiple measurements provide more information about these structures and their surroundings. The distinct center-to-edge variation is shown in the Figure: more pronounced with deeper trenches in wafers 3 and 4. Within minutes, the analyzer can distinguish geometry process variations on the order of 100 Å.
The technique's detail capability can be considerable. When a more detailed examination of trench depth data presented in the graph's top curve was made by re-plotting data on an expanded y axis, several topological features became apparent, including a 0.25 µm total variation in average trench depth, a cyclic repetition every seventh die, and an asymmetry of average depths about the wafer centerline.
This multiple-point measurement and analysis technique can provide detailed depth and width data, and map process distributions of entire 300 mm wafers in minutes. Critical CMP process limitations such as dishing and erosion are also addressed. The technology can characterize fine geometries required to meet dual-damascene process ground rules, providing new levels of visibility. Ultimately, this non-destructive, non-contact characterization system may find its greatest utility in the manufacturing environment to improve process control and throughput, the true goals of in situ metrology. •
For additional information on inspection, measurement & test, go to www.semiconductor.net/imt
