Nano-Structures for CD and Interconnect Metrology

Research Activities

This project, in collaboration the NIST Precision Engineering and Statistical Engineering Divisions, has recently designed and implemented a screening experiment to identify which combinations of six pattern-transfer process factors drive down SCCDRM (Single-Crystal CD Reference-Material) reference-feature CDs and their uncertainties.  Initial results include CD reductions to 25 nm with superior uniformities. 

SCCDRM chip layouts have been designed for three new fabrication ventures featuring 200 mm wafer monolithic implementations. One of these is for a new hybrid optical/e-beam-direct-write process designed to reduce CDs to below 20 nm.  Another is to take advantage of an offer by SEMATECH to collaborate on the fabrication of a new generation of SCCDRMs using state-of-the-art 193 nm lithography. The third is to respond to an interest expressed by other NIST operating units to sponsor a joint SRM venture. 

Our SCCDRM implementation is well suited to the fabrication of calibrated reference materials for the emerging metrology known as optical-CD (OCD).  We have now accomplished the first-ever fabrication and extensive inspection by SEM of gratings suitable for this purpose.  

A paper that describes a new simulation program and its use to study the effects of surface and grain-boundary scattering on the effective resistivity of copper in thin planar films and small cross-section lines was completed. The paper was published by the Microelectronics Reliability journal in July 2006. 

Project staff completed for publication a new JEDEC standard for electro-migration stress testing with constant current and temperature.

Project staff prepared a comprehensive manuscript on second-generation SCCDRM reference-materials for publication as the lead article in the May-June 2006 issue of the NIST Journal of Research.

As a result of close extended collaboration with the University of Edinburgh , the project has been able to develop and report a unique copper damascene process for the fabrication of a scaled electrical-CD test structure having copper-only features. The purpose is to facilitate studies of electron transport in pure copper without having to correct for the complexities of the interaction of copper with barrier-metal films. This implementation enables the separation of the effects of surface and grain-boundary scattering, as a function of the cross-section dimensions of the conductor by electrical testing. Our joint paper describing the accomplishment was presented at the ICMTS 2006 conference in Austin , Texas , on March 8, 2006 . 

In close collaboration with the Laboratory for Interconnect and Packaging at the University of Texas at Austin , test structures for the investigation of the effect of linewidth scaling on electron transport in nickel mono-silicide features have been designed and fabricated. The features were patterned on (110) silicon-on-insulator wafers with i-line lithography that replicated test structures from which voltage/current (V/I) measurements could be extracted. Subsequently, the patterning of single-crystal features with direct-write electron-beam lithography has been developed in order to facilitate future reduction of the linewidths of NiSi features, which have a highly controlled surface microstructure, to linewidths below 40 nm. Our joint paper illustrating these results was also presented at the ICMTS 2006 conference in Austin , Texas , on March 8, 2006 . 

Work has continued on a new CD metrology based on coplanar waveguide test structures. Extensive e-m field modeling of characteristic impedance and distributed capacitance, which we have conducted in collaboration with the Department of Electrical and Computer Engineering at George Washington University , indicates that the extraction of these values from S-parameter measurements can be applied to sampling CDs of test-structure features that are printed on photomasks. At this time, the design of a set of thru and de-embedding structures has been designed, and a supplier of the substrate has been identified. Our joint abstract on the results has been accepted for presentation at the SPIE Bay Area Chrome-Users Symposium in September 2006.  It now seems possible that coplanar waveguide test structures may provide a means of extracting electrical and dimensional information of copper interconnect features fabricated with barrier-metal coatings.

A new JEDEC standard, JESD202, was completed and published in March 2006. The standard describes an accelerated stress test method for determining sample estimates and their confidence limits of the median-time-to-failure, sigma, and early percentile of a log-normal distribution, which are used to characterize the electromigration failure-time distribution of equivalent metal lines subjected to a constant current-density and temperature stress. Procedures are provided to analyze complete and singly, right-censored failure-time data. 

The computer-aided design (CAD) of a new test chip for the fabrication of test structures that can be electrically calibrated to serve as an overlay reference material for high-density interconnect fabrication has been completed. A program to simulate and validate the calibration of the standard has been applied to the design, and the fabrication of a wafer-lot has been initiated. 

In collaboration with NIST operations in Boulder , Colorado , we applied Maxwell software to the calculation of RLGC parameters of cylindrical copper conductors with diameters ranging from 50 nm to 5000 nm, which were coated with TaN barrier metal of thicknesses typical of state-of-the-art IC-interconnect applications. At frequencies above 10 GHz, significant increases in resistance were observed as a consequence of the skin-effect induced amplification of transmission losses caused by barrier-metal coatings. A selection of these results have now been compared with those obtained for copper-only wires that were obtained from exact solutions of Maxwell-based Bessel equations. Initial results indicate close comparisons for the cases that have so far been compared.

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Date created: 2/2/2006
Last updated: 8/14/2007