Synchrotron X-ray Measurements

Our objective is to provide comprehensive descriptions of the structure of advanced materials and devices by performing synchrotron-based measurements to enable the development and optimization of such materials and devices. Our research will establish structure-property relationships for advanced materials, thereby accelerating the introduction of these materials into devices and systems with advanced functionality for a broad spectrum of high-technology applications.

Our approach is to apply state-of-the-art synchrotron measurement capabilities at the National Synchrotron Light Source (NSLS) and the Advanced Photon Source (APS) to generate structure data that cannot be measured by other methods. We collaborate with leading researchers from industry, other government agencies,and universities to address the nation's most pressing measurement needs. The methods that we employ include: (1) near-edge x-ray absorption fine structure(NEXAFS) spectroscopy; (2) extended x-ray absorption fine structure (EXAFS) spectroscopy; (3) variable kinetic energy x-ray photoelectron spectroscopy (VKEXPS); (4) grazing incidence x-ray diffraction; and (5) small angle x-ray scattering and reflectivity. Methods (1) - (3) are available at the NIST beamlines at the NSLS, while methods (4) and (5) are located at the APS.

Impact and Customers:

• Establishing structure-property relationships for advanced materials is critical to the development and optimization of products in many technology sectors. Synchrotron measurements provide structure data that cannot be attained by other methods.

• Synchrotron measurements of the depth dependence of the interfacial structure (e.g., chemistry and local bonding) in high dielectric constant gate stack devices have enabled SEMATECH to optimize processing of such devices.

• Synchrotron measurements have elucidated the surface chemistry of an environmentally friendly automotive oil additive produced by Afton Chemical.

• NIST researchers collaborate with more than 15 companies to provide synchrotronbased structure measurements in novel materials for next-generation device applications.

One of the semiconductor industry's "Grand Challenges" is to develop an alternative to the SiO2 gate dielectric. Integrated circuits exhibiting greater speed and smaller power consumption are no longer attainable with ultrathin (≤ 2 nm)SiO2 gate dielectrics due to their high direct tunneling leakage currents. Our collaboration with SEMATECH has led to extensive evaluation of Hf-based oxide thinfilms as promising high dielectric constant (high-k) replacement materials. We have measured core level binding energy spectra as a function of depth for nitrided HfO2/SiO2/Si gate stacks by VKE-XPS with tunable photon energy. An HfO2/SiO2"interface effect" is detected by the shift of the Si4+ 1s core peak to lower binding energy, indicating that the HfO2 film getters oxygen from the underlying SiO2, thereby rendering it oxygen deficient; this will result in the creation of interfacial carrier trap centers, that will degrade device reliability.

We have also studied HfO2 alternative gate dielectrics using grazing-incidence small-angle X-ray scattering and reflectivity to measure the internal structure and interfacial roughness of ultrathin (1 nm to 3 nm) HfO2 films. Another class of advanced materials are the perovskites (CaTiO3 structure), which display a wide range of useful electrical, magnetic, mechanical, and optical properties. Their integration with Si will lead to next-generation metal-ferroelectric-semiconductor field-effect transistors. We have demonstrated, using a combination of EXAFS and first-principles theoretical calculations, that when strain is imposed on SrTiO3, room temperature ferroelectricity can result, even though SrTiO3 is not normally ferroelectric at any temperature. The existence of the ferroelectric distortion allows for the possibility of strain-engineered ferroelectric devices on Si.

Finally, we have applied our methods to vehicular engine wear problems. Afton Chemical Corp. investigated a titanium additive chemistry as a replacement for traditional phosphorus-containing additives. Engine oils incorporating the Ti additive (at 10 ppm) have been found to be effective in reducing wear in automobile engine tests. We used NEXAFS to investigate the chemical bonding mechanism of the Ti additive with the metal surface of actual rocker arms from the engine tests. Our results enable us to postulate that Ti provides antiwear enhancement through the formation of FeTiO3 on the engine surface.