Plasma Processing and Gaseous Dielectrics

Customer Needs

To maintain its competitive world position, the U.S. semiconductor industry is continually developing microelectronic devices with smaller feature dimensions. This trend requires ever increasing control of the plasma discharges used in the fabrication processes, preferably in real time. Additionally, the development of new processes relies increasingly on predictive system modeling due to the increasing complexity of the fabrication tools and systems. The activities performed in this project assist the industry in developing complex plasma processes and provide the fundamental data required to develop and validate plasma models.

Technical Strategy

Standardized, reference discharges are used to develop and validate mass spectrometric and optical diagnostics for use as plasma monitors. The most intensively studied discharges are generated in Gaseous Electronics Conference (GEC) Radio Frequency Reference Cells, which are used by numerous research labs around the world. The performance of diagnostics can be determined by studying well-defined discharges, and the measurements can be used to validate various plasma models. Additionally, the assessment and determination of fundamental data describing collision processes in reactive plasmas allows for the development of accurate plasma models.

The determination of the identity and energy of ions generated in plasma discharges is critical for the understanding and modeling of reactive plasmas, particularly those containing complex gas mixtures.

DELIVERABLES: By 2002, develop improved laboratory diagnostic measurement techniques.

The density of reactive radicals produced in processing plasmas is an equally important parameter for determining the performance of an etching plasma. Advanced optical absorption methods are being developed to enable the accurate measurement of absolute plasma radical densities.

DELIVERABLES: By 2002, measure radical densities in reference reactive plasmas containing fluorocarbon gases, and correlate with measurements of ion flux densities.

Electron-interaction data are the most fundamental input parameters of plasma processing models. NIST provides the most reliable source of such data in the world for a small number of plasma processing gases. The assessment and derivation of available electron-interaction data continues for gases of interest to the plasma processing community as determined by interactions with semiconductor manufacturers and plasma tool companies.

DELIVERABLES: By 2002, complete an assessment of the available electron-interaction data for the most commonly used plasma processing etching gases.

Accomplishments

Detailed measurements of ion energy distributions in CF4 high-density plasmas with the lower electrode under various biasing conditions were performed. Energetic ions play a crucial role in plasma etching and other plasma processes. Ions exiting the plasma are accelerated to high energies by strong, radio-frequency electric fields in plasma sheaths, which are thin regions located at the boundaries of plasmas. The complicated ion dynamics in plasma sheaths are usually modeled using simplifying assumptions that have never been sufficiently validated. The tests, performed in CF₄ discharges, showed that ion energy distributions predicted by simple, commonly used, analytical sheath models did not agree with measurements. The data are currently being used to evaluate a more sophisicated model developed by researchers in the NIST Chemical Science and Technology Laboratory.

Ion energy distributions in a high-density CF4 plasma: measurements (top) and model predictions (bottom).

NIST hosted the Ninth International Symposium on Gaseous Dielectrics from May 21 to May 25, 2001, at the Turf Valley Resort & Conference Center, in Ellicott City, Maryland. The Symposium was organized by staff of the Electricity Division and co-chaired by Loucas Christophorou and Jim Olthoff. The symposium enjoyed sponsorship from the Air Force Research Laboratory, ABB, Hitachi, Ltd., Kansai Electric Power Company, Mitsubishi Electric Corporation, Tokyo Electric Power Company, and Toshiba Corporation. The Symposium had over 100 participants from 16 countries and covered an array of topics from the basic physics of gaseous dielectrics and understanding of fundamental gas discharge mechanisms to the very practical issues of recycling of used SF₆. The main theme of the meeting was SF₆ replacements and in particular SF₆/N₂ mixtures. The meeting was attended by some of the world's foremost authorities in the basic and applied aspects of gaseous dielectrics, as well as in the engineering and industrial aspects.

Absolute, mass-resolved ion fluxes and densities of selected radicals were simultaneously measured in the presence of a silicon wafer in capacitively coupled plasmas generated in processing gases including C₄F₈ and SF₆. The results indicate that the wafer significantly influences the ion and radical composition in each gas as evidence of the complex chemistries present in the etching plasmas. These results are useful to validate and refine reactor modeling codes used in the development of plasma processing methods.

Electron drift velocities and effective ionization coefficients were measured and analyzed for C₂F₄. C₂F₄ is a dominant fragment formed in highly dissociated C₄F₈ etching plasmas. These data were required as input parameters for newly developed process models describing C₄F₈ etching processes. The determination of these previously unavailable data enabled the development of a more accurate chemical code describing the primary reactions in C₄F₈ discharges.

FY Outputs

Standard Reference Data

Measurement techniques and data provided by NIST continue to assist our customers in industry to improve their plasma modeling and characterization efforts. Examples from this fiscal year include:

Mass-resolved ion fluxes and ion energy distributions measured in high density SF₆ plasmas used by Motorola for modeling deep trench etching processes.

The web-based NIST "Electron Interactions with Plasma Processing Gases" database (http://eeel.nist.gov/817/refdata/) that distributes fundamental data to plasma modelers throughout industry and academia. This web site has experienced thousands of hits in FY 2001 and tens of thousands of hits throughout its history.

Collaborations

Collaborations continued with researchers in the Physics Laboratory and Chemical Science and Technology Laboratory (NIST) related to the correlation of optical and electrical measurements with mass spectrometric diagnostic results.

Electron transport measurements in plasma processing gases are being made in collaboration with researchers from Centro de Cincias Fisicas, UNAM, Mexico.

Recent Publications

R. Champion, I. Dyakov, B. Peko, and Y. Wang, ìCollisional decomposition of SF₆-,î J. Chem. Phys. 115, 1765 (2001).

A. N. Goyette, Y. Wang, and J. K. Olthoff, ìComparison of the identities, fluxes, and energies of ions formed in high density fluorocarbon discharges,î in AIP Conference Proceedings 550, Proceedings of the 2000 International Conference on Characterization and Metrology for ULSI Technology, 238-242 (2001).

A. N. Goyette, Y. Wang, and J. K. Olthoff, ìIon compositions and energies in inductively coupled plasmas containing SF₆,î J. Vac. Sci. Technol. A 19, 1294 (2001).

A. N. Goyette, J. de Urquijo, Y. Wang, L.G. Christophorou, and J.K. Olthoff, ìElectron transport, ionization, and attachment coefficients in C₂F₄ and C₂F₄/Ar mixtures,î J. Chem. Phys. 114, 8932 (2001).

L. G. Christophorou and J. K. Olthoff, ìElectron Attachment Cross Sections and Negative Ion States of SF₆,î International Journal of Mass Spectrometry 205, 27-41 (2001).

L. G. Christophorou and J. K. Olthoff, ìElectron Interactions with CF₃I,î Journal of Physical and Chemical Reference Data 29, 553-569 (2000).

L. G. Christophorou and J. K. Olthoff, ìElectron Interactions with c-C₄F₈,î Journal of Physical and Chemical Reference Data 30, 449-473 (2001).

Y. Wang, M. Misakian, A. N. Goyette, and J. K. Olthoff, "Ion fluxes and energies in inductively coupled radio-frequency discharges containing CHF₃," J. Appl. Phys. 88, 5612-5617 (2000).

Martin Misakian, Eric C. Benck, and Yicheng Wang, ì Time evolution of ion energy distributions and optical emission in pulsed inductively coupled radio frequency plasmas.î J. Appl. Phys. 88, 4510 (2000).

A.N. Goyette, Yicheng Wang and J.K. Olthoff , ìInductively coupled plasmas in low global-warming-potential gases,î J. Phyis. D: Appl. Phys. 33, 2004 (2000).

A.N. Goyette, Yicheng Wang, M. Misakian, and J.K. Olthoff ìIon fluxes and energies in inductively coupled radio-frequency discharges containing C₂F₆ and c-C₄F₈,î J. Vac. Sci. Technol. A18 (2000).

Martin Misakian and Yicheng Wang, ìCalculation of ion energy distributions from radio frequency plasmas using a simplified kinetic approach,î J. Appl. Phys. 87, 3646 (2000).

Yicheng Wang, Eric C. Benck, Martin Misakian, and J.K. Olthoff , ìTime-resolved measurements of ion energy distributions and optical emissions in pulsed radio-frequency discharges,î J. Appl. Phys. 87, 2114 (2000).

J. K. Olthoff and Y. Wang, "Studies of Ion Bombardment in High Density Plasmas Containing CF₄," J. Vacuum Sci. and Technol. A, 17, No. 4, pp. 1552-1555 (Jul/Aug 1999).

Y. Wang, L. G. Christophorou, J. K. Olthoff, and J. K. Verbrugge, "Electron Drift and Attachment in CHF₃ and its Mixtures with Argon," Chem. Phys. Lett., 304, pp. 303-308 (May 1999).

Y. Wang and J. K. Olthoff, "Ion Energy Distributions in Inductively-Coupled Radio-Frequency Discharges in Argon, Nitrogen, Oxygen, Chlorine, and Their Mixtures," J. Appl. Phys., 85, No. 9, pp. 6358-6365 (May 1999).

M. A. Sobolewski, J. K. Olthoff, and Y. Wang, "Ion Energy Distributions and Sheath Voltages in Radio-Frequency-Biased, Inductively Coupled, High-Density Plasma Reactor," J. Appl. Phys. 85, No. 8, pp. 3966-3975 (Apr 1999).

L. G. Christophorou and J. K. Olthoff, "Electron Interactions with Cl₂," J. Phys. Chem. Ref. Data, 28, No. 1, pp. 131-169 (Apr 1999).

M.V.V.S Rao, R. J. Van Brunt, and J. K. Olthoff, "Kinetic-energy Distributions of Positive and Negative Ions in Townsend Discharges in Oxygen," Phys. Rev. E 59, No. 4, pp. 4565-4572 (Apr 1999).

G. D. Cooper, J. E. Sanabia, J. H. Moore, J. K. Olthoff, and L. G. Christophorou, ìTotal Electron Scattering Cross Section for Cl2,î J. Chem. Phys., 110, No. 1, pp. 682-683 (Jan 1999).

S. G. Walton, J. C. Tucek, R. L. Champion, and Y. Wang, "Low Energy, Ion-Induced Electron and Ion Emission from Stainless Steel: The Effect of Oxygen Coverage and the Implications for Discharge Modeling," J. Appl. Phys. 85, No. 3, pp. 1832-1837 (Feb 1999).