Chemical and Biomolecular Engineering

Top 20 Doctoral Program — National Research Council

Faculty

Demetre J. Economou
Dr. Demetre J. Economou

Professor of Chemical and Biomolecular Engineering
Hugh Roy and Lillie Cranz Cullen Distinguished University Chair

Office Location: S239, Engineering Building 1
Phone: 713-743-4320   |   Fax: 713-743-4323
Email: economou [at] uh [dot] edu
economou's research

Education: 

Diploma, Chemical Engineering, National Technical University of Athens, Greece (1981)
M.S., Chemical Engineering, University of Illinois at Urbana-Champaign (1983)
Ph.D., Chemical Engineering, University of Illinois at Urbana-Champaign (1986)

Research Interests: 

Dr. Economou’s research focuses on plasma science and engineering as applied to etching and deposition of thin solid films for microelectronic device fabrication, nanotechnology, plasma medicine, and surface modification of materials. The projects described below are in collaboration with Prof. V. M. Donnelly.

Electron and ion energy distributions in low-temperature plasma reactors

The electron energy distribution function (EEDF), as well as the energy of ions bombarding the substrate (ion energy distribution or IED) are crucial for controlling etching rate and selectivity in advanced plasma processes used in the fabrication of devices with features down to 10 nm. Particle-in-Cell simulations with Monte Carlo Collisions (PIC-MCC) are employed to simulate the spatiotemporal evolution of the EEDF and IED in capacitively- and inductively-coupled plasmas. Of special interest is the application of tailored voltage waveforms to control the profile of the IED. Simulations are complemented with experimental measurements using Langmuir probes as well as non-intrusive optical diagnostics (EEDF), and retarding field energy analysis (IED). Recently we developed a methodology to obtain nearly monoenergetic (tight energy spread) ions bombarding the substrate. This was achieved by pulsing the plasma power and applying a synchronous DC bias voltage during the afterglow.

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In-Plasma Photo-Assisted Etching

While studying ion-assisted etching of p-type silicon in chlorine-containing plasmas near the threshold energy a new, important phenomenon was discovered: in-plasma photo-assisted etching. This mechanism was first discovered in mostly Ar plasmas with a few percent added Cl2, but was found to be even more important in pure Cl2 plasmas. Nearly monoenergetic ion energy distributions (IEDs) were obtained by applying a synchronous DC bias on a “boundary electrode” during the afterglow of a pulsed, inductively-coupled, Faraday-shielded plasma.  Such precisely controlled IEDs allowed the study of silicon etching as a function of ion energy, at near-threshold energies. Etching rates increased with the square root of the ion energy above the observed threshold of 16 eV, in agreement with published data (see figure). Surprisingly, a substantial etching rate was observed, independent of ion energy, when the ion energy was below the ion-assisted etching threshold. Experiments ruled out chemical etching by Cl atoms, etching assisted by Ar metastables, and etching mediated by holes and/or low energy electrons generated by Auger neutralization of low-energy ions, leaving photo-assisted etching as the only plausible explanation. Experiments were carried out with light and ions from the plasma either reaching the surface or being blocked, showing conclusively that the “sub-threshold” etching was due to photons, predominately at wavelengths <1700 Å. The photo-assisted etching (PAE) rate was equal to the ion-assisted etching rate at 36 eV, causing substantial complications for processes that require low ion energies to achieve high selectivity and low damage, such as atomic layer etching. Under these conditions, PAE likely plays an important role in profile evolution of features etched in Si with chlorine-containing plasmas, causing the commonly observed sloped sidewalls and undesired microtrenching. On the other hand, PAE can be beneficial by promoting extremely high selectivity in plasma etching of nanopatterns where, under certain conditions, plasmon resonance (plasmonics) may also play a role.

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Etching rate of blanket p-type Si as a function of E1/2 (E=ion energy), in different continuous wave argon/halogen plasmas. Pulsed DC bias.

Atomic Layer Etching (ALE)

Etching with atomic layer precision is a critical requirement for advancing nanoscience and nanotechnology. Current plasma etching techniques do not have the level of control or damage-free nature that is needed for patterning delicate sub-10 nm structures. In addition, ALE methods proposed in the past, based on pulsed gases with long reactant adsorption and purging steps, are very slow, even for etching extremely thin films. In this project, principles and techniques are developed for a practical method of etching surfaces, one atomic layer at a time, using a combination of pulsed plasma and monoenergetic ion bombardment. With this novel methodology it should be possible to obtain ALE at a substantially higher rate (~30X), compared to other methods. Plasma experiments and simulations are performed to understand the complex interaction between the pulsed plasma and the resulting ion energy distributions. Measurements of time-resolved ion bombardment energy and angular distributions are coupled with etching experiments including the effect of noble gas ion mass, and reactant (Cl, Br, I) mass and electronegativity on sub-surface lattice damage and etching with monolayer precision. Plasma and surface diagnostics are employed to measure product removal rate as a function of chemisorbed layer surface coverage and substrate damage.

Low Temperature Atmospheric Pressure Plasmas

Interest in low-temperature atmospheric-pressure plasmas is fueled to a large extent by realized and potential biomedical applications. For selected area exposure, so-called atmospheric pressure plasma jets (APPJ) are most common. The plasma generated by this source extends up to several cm from the end of the tube where it mixes with open air, making it ideal for treating specimens, including bacteria-covered surfaces, or living tissue. Although the jet appears to be continuous, it consists of periodic streamers or “bullets” that propagate at speeds of 10 km/s or more. This project is a combined experimental-simulation study of APPJs interacting with surfaces. A schematic of the experimental setup is shown in the figure below. Optical emission spectroscopy (OES), in a wide range of wavelengths (UV to near IR), is the main plasma diagnostic. We are developing a new OES technique to be able to probe the last 100 nm of gas near a surface. At the same time, we are employing a plasma transport and reaction fluid model to predict the spatiotemporal profiles of plasma species and electric field. The physics of bullet interaction with specimens is of particular interest. The fluxes of important species (e.g., O atoms and ozone in the case of He plasma gas in an O2 ambient) on the surface of the specimen are predicted for both insulating and conducting surfaces, and compared to data.

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An atmospheric pressure plasma jet impinging on a quartz substrate.

Nanopantography

In nanopantography, standard photolithography, thin film deposition, and etching are used to fabricate arrays of ion-focusing micro-lenses (e.g., small round holes through a metal/insulator structure) on a substrate such as a silicon wafer.  The substrate is then placed in a vacuum chamber, a broad area collimated beam of ions is directed at the substrate, and electric potentials are applied to the lens arrays such that the ions focus at the bottoms of the holes (e.g., on the wafer surface).  When the wafer is tilted off normal (with respect to the ion beam axis), the focal points in each hole are laterally displaced, allowing the focused beamlets to be rastered across the hole bottoms. In nanopantography, the desired pattern is replicated simultaneously in many closely spaced holes over an area limited only by the size of the broad-area ion beam.  With the proper choice of ions and downstream gaseous ambient, the method can be used to deposit or etch materials.  Data show that simultaneous impingement of an Ar+ beam and a Cl2 effusive beam on an array of 950 nm dia. lenses can be used to etch 10 nm dia. features into a Si substrate, a reduction of 95X. Simulations indicate that the focused “beamlet” diameter scale directly with lens diameter, thus a minimum feature size of ~1 nm should be possible with 90 nm dia. lenses. Thus far we have been able to write holes with diameter as small as 3 nm using a 230 nm diameter lens (see figure below). Transfer of patterns defined by nanopantography using highly selective plasma etching of Si, with the native silicon oxide as hard mask, can improve patterning speed and etch profile. With this method, arrays of high aspect ratio (>5) nanofeatures were fabricated in silicon with no mask undercut. The ability to fabricate complex patterns using nanopantography, followed by highly selective plasma etching, was also demonstrated. We expect nanopantography to become a viable method for overcoming one of the main obstacles in practical nanoscale fabrication – rapid, large-scale fabrication of virtually any shape and material nanostructure. Unlike all other focused ion or electron beam writing techniques, this self-aligned method is virtually unaffected by vibrations, thermal expansion, and other alignment problems that usually plague standard nanofabrication methods. This is because the ion focusing optics are built on the wafer.

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economou-hole.jpg

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Interlocking "UH" logo developed using amplification by plasma etching of latent pattern produced by nanopantography. 80 of the 7.5 million lenses are shown. Thinnest line is ~ 13 nm.

Awards & Honors: 

2011-2014: Editorial Board, Journal of Applied Physics & Applied Physics Letters.
2011: W. T. Kittinger Teaching Excellence Award (Highest teaching honor of the College of Engineering)
2010-present: Hugh Roy and Lillie Cranz Cullen Distinguished University Chair
2009: Esther Farfel Award (Highest honor accorded to a UH faculty member)
2008: Plasma Prize, Plasma Science and Technology Division, American Vacuum Society
2008: Fluor Daniel Faculty Excellence Award, College of Engineering, University of Houston (Highest Award of the College of Engineering)
2008: Senior Faculty Award for Excellence in Research and Scholarship, University of Houston (Highest research award of the University of Houston)
2003: Fellow, American Vacuum Society
2003: Outstanding Teaching Award, Cullen College of Engineering, University of Houston
2002: Sigma Xi Faculty Research Award
Guest Editor: IEEE Trans. Plasma Science, Special Issues, August 1995, October 1999, August 2003 and October 2007
Guest Editor: Thin Solid Films, Special Issues, 2000, 2007
1998-present: International Editorial Board, Materials Science in Semiconductor Processing
1999: Senior Faculty Research Excellence Award, College of Engineering, University of Houston
1996-2010: John and Rebecca Moores Professor
1995: Excellence in Research and Scholarship Award, University of Houston
1992: Best Young Author Paper Award of the Journal of Electrochemical Society
1991: Young Faculty Research Excellence Award, Cullen College of Engineering, University of Houston

Journal Papers / Refereed Journal Publications

  1. B. Bruneau et al.,

    “Effect of gas properties on the dynamics of the electrical slope asymmetry effect in capacitive plasmas: comparison of Ar, H2 and CF4,” Plasma Sources Sci. Technol. 25, 01LT02

    , 2016
  2. Lei Liu, Shyam Sridhar, Vincent M. Donnelly and Demetre J. Economou,

    “Ignition Delay in Pulsed Inductively Coupled Plasma (ICP) in Tandem with Auxiliary ICP,” J. Phys. D: Appl. Phys. 48, 485201

    , 2015
  3. Lei Liu, Shyam Sridhar, Weiye Zhu, Vincent M. Donnelly, Demetre J. Economou, Michael D. Logue and Mark J. Kushner,

    “External Control of Electron Energy Distributions in a Dual Tandem Inductively Coupled Plasmas,” J. Appl. Phys. 118, 083303

    , 2015
  4. Qiaowei Lou, Sanbir Kaler, Vincent M. Donnelly and Demetre J. Economou,

    “Optical Emission Spectroscopic Studies and Comparisons of CH3F/CO2 and CH3F/O2 Inductively Coupled Plasmas,” J. Vac. Sci. Technol. A33, 021305

    , 2015
  5. S. Tian, V. M. Donnelly, D. J. Economou, and P. Ruchhoeft,

    “Sub-10 nm nanopantography,”Appl. Phys. Lett. 107, 193109

    , 2015
  6. Siyuan Tian, Vincent M. Donnelly and Demetre J. Economou,

    "Transfer of Nanopantography-Defined Patterns Using Highly Selective Plasma Etching," J. Vac. Sci. Technol. B, 33, 030602

    , 2015
  7. Vladimir Samara, Jean-Francois de Marneffe, and Demetre J. Economou,

    “In-situ monitoring of etch uniformity using plasma emission interferometry,” J. Vac. Sci. Technol. B,  33, 031206

    , 2015
  8. Demetre J. Economou,

    “Pulsed Plasma Etching for Semiconductor Manufacturing,” J. Phys. D: Appl. Phys.47, 303001, [Invited Topical Review]

    , 2014
  9. Erdinc Karakas, Sanbir Kaler, Qiaowei Lou, Vincent. M. Donnelly, and Demetre J. Economou,

    “Measurements of Absolute CO Number Densities in CH3F/O2 Inductively-Coupled Plasmas by Optical Emission Self-actinometry,” J. Phys. D: Appl. Phys.47, 085203

    , 2014
  10. P. Diomede, Demetre J. Economou, T. Lafleur, J.-P. Booth, and S. Longo,

    “Radio-frequency capacitively coupled plasmas in hydrogen excited by tailored voltage waveforms: comparison of simulations with experiments,” Plasma Sources Sci. Technol.23, 065049 {This paper was selected by the Editors of PSST as a 2014 Highlight, see http://iopscience.iop.org/0963-0252/page/Highlights-of-2014}

    , 2014
  11. Paola Diomede and Demetre J. Economou,

    “Kinetic simulation of capacitively coupled plasmas driven by trapezoidal asymmetric voltage pulses,” J. Appl. Phys.115, 233302

    , 2014
  12. Paola Diomede, Demetre J. Economou and Vincent M. Donnelly,

    “Instabilities in Capacitively Coupled Plasmas Driven by Asymmetric Trapezoidal Voltage Pulses,” IEEE Trans. Plasma Sci.42, 2822

    , 2014
  13. Weiye Zhu, Shyam Sridhar, Lei Liu, Eduardo Hernandez, Vincent M. Donnelly, and Demetre J. Economou,

    “Photo-Assisted Etching of Silicon in Chlorine- and Bromine-Containing Plasmas,” J. Appl. Phys.115, 203303

    , 2014
  14. Demetre J. Economou,

    “Tailored ion energy distributions on plasma electrodes,” J. Vac. Sci. Technol. A31, 050823 [Invited paper commemorating the 60th Anniversary of the American Vacuum Society]

    , 2013
  15. E. Karakas, V. M. Donnelly, and D. J. Economou,

    “Langmuir Probe and Optical Emission Spectroscopy of CH3F/O2 Inductively Coupled Plasmas,” J. Appl. Phys.113, 213301

    , 2013
  16. Erdinc Karakas, Vincent M. Donnelly and Demetre J. Economou,

    “Abrupt transitions in species number densities and plasma parameters in a CH3F/O2 inductively coupled plasma,” Appl. Phys. Lett.102, 034107

    , 2013
  17. H. Shin, W. Zhou, L. Liu, S. Sridhar, V. M. Donnelly, D. J. Economou, C. Lenox, and T. Lii,

    “Selective Etching of TiN over TaN and vice-versa in Chlorine-Containing Plasmas,” J. Vac. Sci. Technol. A31, 031305

    , 2013
  18. Paola Diomede, Doosik Kim and Demetre J. Economou,

    “Particle-in-cell simulation of electron and ion energy distributions in dc/rf hybrid capacitively-coupled plasmas,” AIChE J., 59(9), 3214. Special Issue of AIChE Journal in memory of Neal Amundson

    , 2013
  19. Zhuo Chen, John A. Mucha, Vincent M. Donnelly, and Demetre J. Economou,

    “Plasma Enhanced Layer-by-Layer Deposition and Nano-crystallization of Hydrogenated Amorphous Silicon Films,” J. Vac. Sci. Technol. B31, 061209

    , 2013
  20. H. Shin, W. Zhu, D. J. Economou and V. M. Donnelly,

    “Ion Energy Distributions, Electron Temperatures and Electron Densities in Ar, Kr and Xe Pulsed Discharges,” J. Vac. Sci. Technol. A30, 031304 [5 pages]

    , 2012
  21. H. Shin, W. Zhu, D. J. Economou and V. M. Donnelly,

    “The Surprising Importance of Photo-Assisted Etching of Silicon in Chlorine-Containing Plasmas,” J. Vac. Sci. Technol. A30, 021306 [10 pages]

    , 2012
  22. Michael D. Logue, Hyungjoo Shin, Weiye Zhu, Lin Xu, Vincent M. Donnelly, Demetre J. Economou, and Mark J. Kushner,

    “Ion Energy Distributions in Inductively Coupled Plasmas Having a Biased Boundary Electrode,” Plasma Sources Sci. & Technol.21, 065009

    , 2012
  23. P. Diomede, D. J. Economou and V. M. Donnelly,

    “Rapid Calculation of the Ion Energy Distribution on a Plasma Electrode,” J. Appl Phys,111, 123306.

    , 2012
  24. P. Diomede, S. Longo, D. J. Economou and M. Capitelli,

    “Hybrid Simulation of a DC-Enhanced Radio Frequency Capacitive Discharge in Hydrogen,” J. Phys. D: Appl. Phys., 45, 175204 [14 pages]

    , 2012
  25. H. Shin, W. Zhu, L. Xu, T. Ouk, D. J. Economou and V, M. Donnelly,

    “Control of ion energy distributions using a pulsed plasma with synchronous bias on a boundary electrode,”Plasma Sources Sci. Technol.20, 055001 [9 pages]  

    , 2011
  26. P. Diomede, M. Nikolaou and D. J. Economou,

    “Voltage Waveform to Achieve a Desired Ion Energy Distribution on a Substrate in Contact with Plasma,” Plasma Sources Sci. Technol.,20, 045011 [9 pages]

    , 2011
  27. P. Diomede, V. M. Donnelly and D. J. Economou,

    “Particle-in-Cell Simulation of Ion Energy Distributions on an Electrode by Applying Tailored Bias Waveforms in the Afterglow of a Pulsed Plasma,” J. Appl. Phys.109, 083302 [7 pages]

    , 2011
  28. Q. Li, Y.-K. Pu, M. A. Lieberman and D. J. Economou,

    “A Dynamic Model of Streamer Coupling for the Homogeneity of Glow-Like Dielectric Barrier Discharges at Near-Atmospheric Pressure,” Phys. Rev. E.83, 046405

    , 2011
  29. S. G. Belostotskiy, O. Tola, V. M. Donnelly, D. J. Economou, and N. Sadeghi,

    “Time and Space Resolved Measurements of Ar(1s5) Metastable Density in a Microplasma Using Diode Laser Absorption Spectroscopy,” J. Phys. D: Appl. Phys. 44, 145202

    , 2011
  30. G. Belostotskiy, O. Tola, V. M. Donnelly, D. J. Economou, and N. Sadeghi,

    “Gas Temperature and Electron Density Profiles in an Argon DC Microdischarge Measured by Optical Emission Spectroscopy,” J. Appl. Phys.107, 053305, 7 pages

    , 2010
  31. D. J. Economou,

    “Modeling and Simulation of Fast Neutral Beam Sources for Materials Processing,” Plasma Processes and Polymers6, 308-319

    , 2009
  32. S. G. Belostotskiy, V. M. Donnelly, D. J. Economou, and N. Sadeghi,

    “Spatially Resolved Measurements of Argon Metastable (1s5) Density in a High Pressure Microdischarge Using Diode Laser Absorption Spectroscopy,” IEEE Trans. Plasma Sci.37, 852-858

    , 2009
  33. Z. Chen, V. M. Donnelly, D. J. Economou, L. Chen, M. Funk, and R. Sundararajan,

    “Measurement of electron temperatures and electron energy distribution functions in dual frequency capacitively-coupled CF4/O2 plasmas using trace rare gases-optical emission spectroscopy (TRG-OES),” J. Vac. Sci. Technol. A.27, 1159

    , 2009
  34. D. J. Economou,

    “Fast (10s -100s eV) Neutral Beams for Materials Processing,” J. Phys. D: Appl. Phys.41, 024001 [11 pages]

    , 2008
  35. Lin Xu, Azeem Nasrullah, Zhiying Chen, Manish Jain, Demetre J. Economou, Paul Ruchhoeft, and Vincent M. Donnelly,

    “Etching of nanopatterns in silicon using nanopantography,” Appl Phys. Lett.92, 013124

    , 2008
  36. Lin Xu, Lee Chen, Merritt Funk, Alok Ranjan, Mike Hummel, Ron Bravenec, Radha Sundararajan, Demetre J. Economou, and Vincent M. Donnelly,

    “Diagnostics of ballistic electrons in a dc/rf hybrid capacitively coupled discharge,” Appl. Phys. Lett.93, 261502

    , 2008
  37. Sergey Belostotskiy, Rahul Khandelwal, Qiang Wang, Vincent M. Donnelly, Demetre J. Economou, and Nader Sadeghi,

    “Measurement of Electron Temperature and Density in an Argon Microdischarge by Laser Thomson Scattering,” Appl. Phys. Lett.92, 221507

    , 2008
  38. Sergey Belostotskiy, Vincent M. Donnelly, and Demetre J. Economou,

    “Influence of Gas Heating on High Pressure DC Microdischarge I-V Characteristics,” Plasma Sources Sci. Technol.17, 045018

    , 2008
  39. A. Ranjan, C. Helmbrecht, V. M. Donnelly, D. J. Economou, and G. Franz,

    “Effect of Surface Roughness on the Energy Distribution of Fast Neutrals and Residual Ions Extracted from a Neutral Beam Source,” J. Vac. Sci. Technol. B.25, 258-263

    , 2007
  40. D. Economou,

    “Fundamentals and Applications of Ion-Ion Plasmas,” Appl. Surf. Science,253, 6672-6680

    , 2007
  41. L. Xu, N. Sadeghi, V. M. Donnelly, and D. J. Economou,

    “Nickel Atom and Ion Density in an Inductively Coupled Plasma with an Internal Coil,” J. Appl. Phys.101, 013304

    , 2007
  42. Q. Wang, F. Doll, V. M. Donnelly, D. J. Economou, N. Sadeghi, and G. Franz,

    “Experimental and Theoretical Study of the Effect of Gas Flow on Gas Temperature in an Atmospheric Pressure Microplasma,” J. Phys. D: Appl. Phys.40, 4202-4211

    , 2007
  43. S. Belostotskyi, Q. Wang, V. M. Donnelly, D. J. Economou, and N. Sadeghi,

    “Three Dimensional Gas Temperature Measurements in Atmospheric Pressure Microdischarges Using Raman Scattering,” Appl. Phys. Lett.89, 251503

    , 2007
  44. S. K. Nam, D. J. Economou, and V. M. Donnelly,

    “Particle-in-Cell Simulation of Ion Beam Extraction from a Pulsed Plasma Through a Grid,” Plasma Sources Sci. Technol.16, 90-96

    , 2007
  45. Sang Ki Nam, Demetre J. Economou and Vincent M. Donnelly,

    “Generation of Fast Neutral Beams by Ion Neutralization in High Aspect Ratio Holes: A Particle-in-Cell Simulation Study,” IEEE Trans. Plasma Sci.35, 1370-1378

    , 2007
  46. A. Ranjan, V. M. Donnelly, and D. J. Economou,

    “Energy Distribution and Flux of fast Neutrals and Residual Ions Extracted from a neutral beam Source,” J. Vac Sci. Technol. A,24, 1839-1846

    , 2006
  47. O Polomarov, C. Theodosiou, I. Kaganovich, B. Ramamurthi, and D. J. Economou,

    “Self-Consistent Modeling of Non-Local Inductively Coupled Plasmas,” IEEE Trans. Plasma Science34, 767-785

    , 2006
  48. Q. Wang, D. J. Economou, and V. M. Donnelly,

    “Simulation of Direct Current Micro-Plasma Discharge in Helium at Atmospheric Pressure,” J. Appl. Phys.100, 023301

    , 2006
  49. S. K. Nam, D. J. Economou, and V. M. Donnelly,

    “Particle-in-Cell Simulation of Beam Extraction Through a Hole in Contact with Plasma,” J. Phys. D: Appl. Phys.39, 3994-4000

    , 2006
  50. L. Xu, D. J. Economou, V. M. Donnelly and P. Ruchhoeft,

    “Extraction of a Nearly Monoenergetic Ion Beam from a Pulsed Plasma,” Appl. Phys. Lett.87, 041502

    , 2005

 


Patents

  1. “Atomic Layer Etching with Pulsed Plasmas,” Patent Applied for, December 15, 2010, with V. M. Donnelly.
    Date: 12/15/2010
  2. U.S. Patent #4,859,277 “Method for Measuring Plasma Properties in Semiconductor Processing,” with G. Barna.
  3. U.S. Patent #7,358,484 “Hyperthermal Neutral Beam Source and Method for Operating,” with L. Chen and V. Donnelly.
  4. U.S. Patent #7,883,839 “Method and Apparatus for Nanopantography,” with V. Donnelly, P. Ruchhoeft, L. Xu; S. C. Vemula; and M. Jain.