E-Beam Technique Measures Product Wafer Composition, Thickness

The proliferation of CMOS materials is now a key trend. At the 130 nm node, about 20 materials were used. At the 65 nm node, the list may grow to 34 or more, with additions such as silicon-on-insulator (SOI) and SiGe substrates, ultralow-k dielectrics, HfO₂ and SiON gates, and atomic-layer deposition (ALD) barrier and seed layers. Additionally, alloyed films such as CoWP capping layers will be increasingly used in copper interconnects. Thinner multilayer films, such as TaN/Ta bilayer barriers, will also become prevalent.

This complexity and the tighter process margins associated with smaller design rules have introduced process integration problems that conventional metrology techniques cannot address. New parameters have emerged for film process control, including composition, density and porosity.

Film composition, especially, is a critical process control parameter. In the FEOL, nitrogen composition is important for reducing transistor leakage for heavily nitrided gate dielectrics at the 90 nm node. In the BEOL, variations in barrier/seed composition in the copper interconnect can reduce barrier adhesion, resulting in voids and leading to catastrophic loss of field functionality. Therefore, the composition of nitrogen in TaN/Ta barriers becomes critical in preventing electromigration failures and device leakage. With CoWP cap layers, composition is crucial for electromigration resistance.

The MetriX 100 platform’s e-beam column offers tunable landing energy for various elements and film thicknesses. Its fixed-wavelength X-ray detection spectrometers maximize X-ray collection efficiency, and each is customized for a single element. (Source: KLA-Tencor)

Conventional film monitoring techniques can be partially or fully destructive, or have difficulty in separating individual-layer thickness measurements on multilayer film stacks. Significantly, they cannot determine film composition in production. For example, sheet resistance metrology provides only a single combined-thickness measurement. Photo-acoustic metrology can be unreliable with <70 Å thick films. X-ray reflectivity metrology only measures multiple film layers' thickness. X-ray fluorescence, used for some thicker films, fails to offer a solution for product wafer line monitoring at the 90 nm node and below.

KLA-Tencor (San Jose) has introduced its MetriX 100 non-contact metal films metrology tool, expected to provide independent production line measurements of film composition and thickness. It works on ultrathin multilayer stacks, provides precision and accuracy necessary for production monitoring from the 90 nm node, and the sensitivity to measure ultrathin film layer changes down to <10 Å with production-level precision.

The system's e-beam technology stimulates X-rays from films being measured. The emissions are detected and counted by detectors situated around the e-beam. Film thickness and composition are measured by quantifying the X-rays. Enabling in-line product wafer monitoring required redesign of the e-beam column, X-ray detectors, and the modeling algorithms that calculate thickness and composition.

The system determines film thickness by measuring elemental X-ray lines' intensity, each with its characteristic energy. Similarly, the ratio of the X-ray lines of the elements of a compound film layer allows measurement of its composition. The system does this simultaneously and independently, providing thickness and composition data. Because the film's elements are known in advance, the tool does not scan a full X-ray spectrum; its spectrometers are designed to look at the X-ray energy lines of interest.

The X-ray detectors combine the best of high-sensitivity spectrometers. Up to six spectrometers around the e-beam overcome difficulties associated with collecting signals over a wide range of energy lines, enabling the simultaneous collection of multiple energy lines. The e-beam column provides a range of beam currents and landing energy voltages, producing the necessary quality and quantity of X-ray signals. The high beam current capability provides measurement precision, since precision is directly proportional to the X-ray photon intensity count. The landing energies range allows a variety of elements to be excited. Since X-rays penetrate the surface, depth resolution capability can be increased to obtain a signal from a buried layer. This permits the simultaneous measurement of multilayer film stacks and provides the future potential for detection of subsurface voids.

The system's primary focus is on copper applications — barrier seed, ECP, copper CMP, and the emerging CoWP cap layers. It is also effective for front-end applications, such as salicide and gate metals, gate dielectric composition, ultrashallow junctions, and MRAM film stacks.

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