Production XPS Platform Turbocharges Thin-Film Metrology
Alexander E. Braun, Senior Editor -- Semiconductor International, 3/1/2003
Almost since the first semiconductors made their appearance, SiO2 has been the material of choice for the gate dielectric. As scaling has progressed to improve functionality and performance, transistor size has followed and, with it, so has the gate dielectric — diving from shrink to smaller shrink.
Unfortunately, as SiO2 gets thinner, the performance manufacturers expect of their leading-edge devices is degraded. For shrinks to continue, major problems that must be overcome involve an increase in leakage current and boron diffusion from the doped gate electrode.
For years, the semiconductor industry has unflaggingly pursued the Holy Grail of a dielectric material with high k, small equivalent oxide thickness (EOT) and uncompromised electrical characteristics. While this quest for a manufacturable high-k material continues, the use of nitrogen in the SiO2 matrix has supplied a suitable interim scaling step. This slightly more complex mix of SiO2 and nitrogen is rapidly replacing plain SiO2, as device manufacturers struggle to fulfill Moore's Law. While the addition of nitrogen to the gate dielectric has bridged a gap between device design and device manufacturing, the use of this element is not without its snags. One of these difficulties is the fact that, in the past, processing controls — in this case metrology — have been based on the industry's profound technical and empirical knowledge of SiO2, acquired over many years of use.
X-ray photoelectron spectroscopy (XPS) may help solve this problem. XPS is a conceptually simple, decades-old analytical technology created to measure the compositional attributes of thin films and thin surface-related properties over a wide spectrum of different conditions. XPS works by illuminating a spot on the wafer surface with low-energy X-rays, where they interact with the atoms in the material and knock off an electron from an atom of that material. The electron and its energy level are then detected. Because an electron's energy is characteristic to the element from which the electron came, an XPS system is capable of determining what type of atoms are present in a material by measuring the energy spectrum from all the electrons detected. It also can determine how many atoms there are of each of the elements present. Once this information is obtained, it becomes possible to determine thickness and other physical properties.
PHI-Evans (Sunnyvale, Calif.) is beta-testing its ReVera thin-film compositional metrology tool (Figure). This XPS-based system, packaged into a production-level platform, enables direct measurement of the physical composition and thickness of ultrathin films such as gate dielectrics. Until now, XPS system design has been based on analytical laboratory requirements. The new tool's advantage over traditional XPS systems is that it has been designed to incorporate a fast, production-level interpretation capability. This capability is driven by proprietary algorithms developed to convert the measurements taken by the tool into thickness and material information. The platform can provide full wafer maps of the dielectric composition and thickness in a few minutes, and its design should enable it to find use both in manufacturing and process development.
With this tool, the device manufacturer is able to observe variations of important properties of SiON within a wafer, wafer-to-wafer, or lot-to-lot. Because the system is based on XPS technology, it will directly apply to whatever material the industry chooses for the next generation of high-k, since those materials will be similar and also require direct measurement of compositional properties. The tool's sensitivity exceeds precision and sensitivity requirements even when used with the thinnest materials.
For additional information on inspection, measurement and test, go to www.semiconductor.net/imt.
