Georgia Institute of TechnologySchool of Chemical & Biomolecular Engineering

Nanostructures, Patterning, and Thin Film Deposition


Physiochemical properties and behavior of confined and nanoscale polymer films

Faculty: Clifford L. Henderson
Students: Lovejeet Singh (PhD)

The physiochemical properties and behavior of polymer thin and ultra-thin films is an important issue in a wide variety of applications including membrane based separations, coatings, microelectronics, and many other fields. For example, the diffusion of photoacid within chemically amplified photoresist (CAR) polymer thin films is of critical importance in determining the ultimate resolution of CAR materials used for nanopatterning in manufacturing devices such as modern computer microprocessors. As feature sizes in microelectronic devices decrease below 100 nanometers, the thickness of resist films used to fabricate such features is also decreasing rapidly. It is clear that for certain future patterning technologies, such as advanced 193 nm lithography and Extreme Ultraviolet Lithography (EUVL), that resist film thicknesses well below 200 nm may be required. The influence of thin film confinement on the behavior of such materials is an important issue and such effects must be understood to enable rational materials and process design.

Figure 1. Water diffusion coefficient versus initial polymer film thickness for poly(hydroxystyrene). Error bars represent 90% confidence limits.

The focus of our work has been on the experimental investigation of the influence of thin film confinement on a variety of physiochemical properties of polymer films including: glass transition temperature (Tg), coefficient of thermal expansion (CTE), diffusion coefficients of small molecules in these films, and thin film dissolution rates. Analytical methods such as variable angle spectroscopic ellipsometry (VASE), quartz crystal microbalance sorption experiments, and Fourier Transform Infrared Spectroscopy (FTIR) are being used to characterize the behavior of a variety of polymer thin films. We have found that thin film confinement can affect the polymer properties in dramatic ways over a range of different length scales. Our current focus is on expanding these studies to include new physiochemical properties and providing insights on the fundamental mechanisms that underly such behavior.

Selected Recent Papers:

  • Singh, Lovejeet; Ludovice, Peter J.; Henderson, Clifford L. "Effect of nanoscale confinement on the diffusion behavior of photoresist polymer thin films," Proceedings of SPIE-The International Society for Optical Engineering, 5376 (Pt. 1, Advances in Resist Technology and Processing XXI), pp. 369-378, (2004).

  • Singh, Lovejeet; Ludovice, Peter J.; Henderson, Clifford L. "Effect of thin film confinement on the transport properties of ultra-thin polymer films," Materials Research Society Symposium Proceedings, 790 (Dynamics in Small Confining Systems--2003), pp. 203-208, (2004).

  • Singh, Lovejeet; Ludovice, Peter J.; Henderson, Clifford L. "Influence of molecular weight and film thickness on the glass transition temperature and coefficient of thermal expansion of supported ultrathin polymer films," Thin Solid Films, 449(1,2), pp. 231-241, (2004).

Development of novel photoresist materials for sub-100 nm electron beam lithography and optical lithography

Faculty: Clifford Henderson
Students: Augustin Jeyakumar (PhD), Michael Romeo (MS), Cody Berger (PhD)

Microlithography, or often simply referred to as "lithography", refers to the process used to print the patterns that are used to fabricate devices at length scales ranging from the hundreds of microns to only a few nanometers in size. For example, microlithography is responsible for printing the circuit elements on each level of a semiconductor device and is considered "the key technology driver for the semiconductor industry." Indeed, it has been improvements in optical lithography that have enabled the extraordinary advances made in semiconductor performance over the past 40 years. Current microlithographic methods involve the use of patterns of light produced by optical exposure tools to form the desired relief patterns into photosensitive polymeric materials called photoresists (see Figure 1). The patterned photoresist acts as a protective masking layer for subsequent pattern transfer processes such as removal of substrate materials using reactive etching in oxygen or halogen plasmas. There is a rapidly approaching roadblock of critical importance in semiconductor processing technology, namely the development of advanced tools and photoresist materials for the patterning of sub-90 nm transistor features in advanced logic and memory devices. Likewise, there is a critical need for mass fabrication methods for nanometer scale features and devices in a variety of other fields. Developing microlithography processes that can be used to print smaller features can be addressed in three ways: (1) develop higher resolution exposure tools, (2) develop higher resolution photoresist materials, and (3) develop processes that produce higher resolution patterns for a given exposure tool and photoresist.

Figure 1. Schematic view of microlithography process: (1) photoresist film is first coated onto substrate by spin coating from organic solvent solution, (2) projection optical system is used to expose resist film to light in the shape of desired patterns, (3) this exposure creates a chemical change in the resist film which depending on the tone of the resist makes the exposed resist region more or less soluble in a developing solvent, (4) resist film is developed to produce desired protective relief pattern, (5) resist pattern is used in subsequent processes such as plasma etching, and (6) resist film is finally removed and process is repeated.

Our work focuses on several aspects of this problem: (1) the design, synthesis, & characterization of advanced organic and inorganic photoresist materials, (2) modeling and fundamental studies of the mechanisms which control photoresist performance, (3) development of high resolution lithographic processes using novel materials and methods, and (4) application of these methods to design, build, and characterize new nanoscale devices. The goal of this work is to provide a suite of materials and methods that can enable the production of sub-100 nm patterned devices for nanoelectronic, nanoelectromechanical (NEMS), and biological applications.

Selected Recent Papers:

  • Jeyakumar, Augustin; Henderson, Clifford L. "Enhancing the electron beam sensitivity of hydrogen silsesquioxane (HSQ)," Proceedings of SPIE-The International Society for Optical Engineering, 5376 (Pt. 1, Advances in Resist Technology and Processing XXI), pp. 490-501, (2004).

  • Hoskins, Trevor; Chung, Won Jae; Agrawal, Ankur; Ludovice, Peter J.; Henderson, Clifford L.; Seger, Larry D.; Rhodes, Larry F.; Shick, Robert A. "Bis(trifluoromethyl)carbinol-Substituted Polynorbornenes: Dissolution Behavior," Macromolecules, 37(12), pp. 4512-4518, (2004).

  • Jeyakumar, Augustin; Henderson, Clifford L.; Roman, Paul, Jr.; Suh, Seigi. "Electron beam lithography process using radiation sensitive carboxylate metalorganic precursors," Journal of Vacuum Science & Technology, B: Microelectronics and Nanometer Structures--Processing, Measurement, and Phenomena, 21(6), pp. 3157-3161, (2003).

Combined lithography and embossing methods for manufacturing complex nanoscale devices

Faculty: Clifford Henderson (ChBE), William King (ME)
Students: Yueming Hua (ChBE), Celesta White (ChBE), Harry Rowland (ME)

Imprint lithography (IL) is already one of the most rapidly growing areas of research in the microelectronics industry and IL is also becoming increasingly more important in the fabrication of microfluidic and nanofluidic devices and sensors. To imprint a surface, three basic components are needed: (1) a mechanical "stamp" or mold with relief patterns of the desired

Figure 1. General schematic of the combined optical lithography and imprint methods described in this work using photodefinable sacrificial materials. Photopatterning of the sacrificial material is performed to produce large scale and relatively isolated features while embossing is used for printing very fine structures that may be below the resolution limits of the sacrificial material.

features, (2) the material to be imprinted, usually a layer of polymer with suitable glass transition temperature (Tg) and molecular weight on an appropriate substrate, and (3) equipment for printing with adequate control of temperature, pressure, and control of parallelism of the stamp and substrate. In short, the process consists of pressing the stamp into the polymer film using pressures in the range of 5-40 MPa. The polymer film is sometimes heated to aid in the flow of the polymer into the small features of the stamp. The stamp is then detached from the printed substrate after cooling both the stamp and substrate.

There has been a tremendous amount of research directed at investigating imprint lithography in recent years. The versatility and flexibility of the imprinting techniques and materials provide a distinct technology advantage for many different applications and can easily be combined with existing technologies. Recently, we have investigated the combination of sacrificial polymeric materials and imprinting for use in manufacturing nanofluidic channels in biological applications and in the development of fully-released microporous polymer membranes. Our current work is focused on the imprinting of directly photodefinable sacrificial materials. The goal of this work is to develop a novel microfabrication method that combines the processing benefits and extremely high resolution of imprint lithography with the design versatility and large area patterning ability of traditional microlithography using photodefinable polymers such as photoresists. The general method being pursued in this work is shown in Figure 1. The outcome of this work will be a suite of tools, materials, and methods for the cost effective manufacturing of devices that contain features at a range of length scales down to the nanometer scale.

Selected Recent Papers:

  • White, Celesta E.; Anderson, Travis; Henderson, Clifford L.; Rowland, Harry D.; King, William P. "Microsystems manufacturing via embossing of photodefinable thermally sacrificial materials," Proceedings of SPIE-The International Society for Optical Engineering, 5374(Pt. 1, Emerging Lithographic Technologies VIII), pp. 361-370, (2004).

  • Henderson, Clifford L.; King, William P.; White, Celesta E.; Rowland, Harry R. "Microsystems manufacturing via embossing of thermally sacrificial polymers," Materials Research Society Symposium Proceeding, EXS-2(Nontraditional Approaches to Patterning), pp. 17-19, (2004).

Changes of chemical bonding and chain orientation in polymer films due to chemical mechanical polishing

Faculty: Dennis Hess
Students: Jie Diao

This research is focused on developing techniques to quantify chemical and mechanical changes near a polymer film surface in a process, e.g., chemical mechanical polishing (CMP), where the surface is modified due to chemical reaction and/or mechanical strain.

To quantify the chemical changes, a new iterative algorithm has been proposed to extract concentration depth profiles based on Fick's second law of diffusion in a multi-element system from angle resolved x-ray photoelectron spectroscopy (ARXPS) data. Parameters related to the concentration profiles are obtained by fitting the experimental angle-dependent relative photoelectron intensities to predictions from the algorithm. The use of relative instead of absolute photoelectron intensities eliminates error due to changes in absolute photoelectron intensities resulting from the change in system geometry in angle-resolved experiments. Simulations using an infinite source diffusion model have been conducted to study the influence of errors in the raw data and to demonstrate the robustness of the algorithm. The algorithm is tested based on preliminary experimental ARXPS data from chemically treated poly (biphenyl

dianhydride-p-phenylenediamine) (BPDA-PDA) films. The method can detect changes in chemical composition profile in a region less than 10 nm below the surface due to chemical treatments (see figure).

 

The mechanical changes will be studied by quantifying polymer chain reorientation with polarized infrared (IR) spectroscopy. Accurate optical constants of anisotropic BPDA-PDA films are important for successful quantification of chain orientation from polarized IR spectra. A new method has been proposed to determine the anisotropic optical constants of films based on matrix method. It has been demonstrated that this new method can generate n and k spectra in a wavelength range without assuming an arbitrary relationship between n, k and wavelength.

Selected Recent Papers/Patents/Theses/Reports:

  • J. Diao and D. W. Hess, Use of Angle Resolved XPS to Determine Depth Profiles Based on Fick's Law of Diffusion: Description of Method and Simulation Study, Journal of Electron Spectroscopy and Related Phenomena, 2004, 135, p. 87-104.

  • J. Diao and D. W. Hess, Through-plane Uniformity of Optical Anisotropy in Spin-coated BPDA-PDA Films, submitted to Thin Solid Films

Deposition of high dielectric constant insulators by metalorganic CVD

Faculty: Dennis Hess (ChBE), William Rees (Chemistry)
Students: (current) Qian Luo; (former) Ebony Mays

The SiO2 gate dielectric of complementary metal-oxide semiconductor (CMOS) devices is approaching its fundamental limit as the chip feature size is scaling down to the 65 nm node according to the International Technology Roadmap for Semiconductors (ITRS). As the SiO2 gate dielectric thickness decreases below 1 nm, the leakage current from direct tunneling increases exponentially. In order to continue the increased speed of succeeding device generations, new gate materials with high dielectric constants are required. Our research involves the deposition of different high dielectric constant (high k) insulators such as zirconium-tin-titanate and hafnium oxide by metalorganic CVD. The focus relates to the relationships between the effect of precursor structure and CVD variables (e.g., pressure, substrate temperature, oxidant) on the electrical properties of the deposited high k films. In order to maintain device performance with the equivalent oxide thickness (EOT) of the high k gate dielectric less than 1 nm, we are investigating the thickness dependence and intrinsic defects that create oxide charge near the interface between the high-k gate dielectric and the silicon surface. The thermal stability and diffusion barrier properties of the high k insulators are also of interest.

Selected Recent Papers/Patents/Theses/Reports:

  • E. L. Mays, D. W. Hess, and W. S. Rees, Jr., Chemical vapor deposition of zirconium tin titanate: A dielectric material for potential microelectronic", Proceedings - Electrochemical Society, Proceedings, Vol 8, 2003, p. 855-862.

  • E. L. Mays, D. W. Hess, and W. S. Rees, Jr., Deposition and characterization of zirconium tin titanate thin films as a potential high-k material for electronic devices. Journal of Crystal Growth, 2004, 261, p. 309-315.

Olefin polymerization catalysts via the nanoscopic design of porous silica to create well-defined immobilized organometallic species on surfaces.

Faculty: Christopher W. Jones
Students: (current) Mike McKittrick, Jason Hicks

In catalytic olefin polymerization, solid-supported transition metal complexes have been the dominant type of catalyst used in industry for a number of years. Despite this dominance, a fundamental understanding of active-site structure and polymerization mechanism are still not well-established in most cases. Though metallocenes have been supported on solids in numerous studies, no proven, well-characterized example of a single-site, supported metallocene olefin polymerization catalyst has been reported due to the difficulty of controlling nanoscale features on the surface.

Using a patterned aminosilica, with amine functionalities spaced at least 8 Angstroms apart, a well-defined immobilized organometallic species can be created on the surface.

Constrained Geometry Catalysts (CGCs), which are related to metallocenes, are among the most important single-site catalysts for olefin polymerization. CGCs have a reactive center that is among the least sterically hindered of all the single-site catalysts. In homogeneous CGCs, small changes in ligand structure and coordination environment can have a large affect on catalyst activity. Therefore, differences in supported CGC's molecular architecture may also substantially impact the catalysts' performance. By developing a new molecular patterning protocol that allows for control of the synthesis at the nanoscale, site-isolated, single-site CGC catalysts are prepared. Using these new immobilized molecular catalysts, we will be able to probe structure-activity relationships for this supported system.

Our new synthetic method has resulted in the preparation of supported, well-defined, isolated single-site CGCs. The protocol allows for characterization by multiple techniques at each stage of the synthesis. By performing a detailed characterization of the material, including the use of molecular probes at each stage, we are able to more clearly understand the development of the surface functionalities. Polymerization studies will be combined with molecular level modifications to develop structure-activity relationships for these novel materials.

Selected Recent Papers/Patents/Theses/Reports:

  • "Effect of Site-Isolation on the Preparation and Performance of Silica-Immobilized Ti CGC-Inspired Ethylene Polymerization Catalysts." M. W. McKittrick and C. W. Jones, Journal of Catalysis, 2004, in press.

  • "Role of Amine Structure and Site-Isolation on the Performance of Aminosilica-Immobilized Zr CGC-Inspired Ethylene Polymerization Catalysts" K. Yu, M. W. McKittrick, and C. W. Jones, Organometallics, 2004, in press.

  • "Towards Single-Site, Immobilized Molecular Catalysts: Site-Isolated Ti Ethylene Polymerization Catalysts Supported on Porous Silica" M. W. McKittrick and C. W. Jones, Journal of the American Chemical Society, 2004, 126, 3052-3053.

  • "Towards Single-Site Functional Materials - Preparation of Amine-Functionalized Surfaces Exhibiting Site-Isolated Behavior." M. W. McKittrick and C. W. Jones, Chemistry of Materials 2003, 15, 1132-1139.

Formation and characterization of nanoporous molecular sieving carbon materials

Faculty: William J. Koros
Students: (current) Jason Williams (former) Keisha Steel

Carbon molecular sieve (CMS) membranes are nanoporous materials formed from the high temperature pyrolysis of polymer precursors. The pore structure of CMS materials contains a bimodal distribution of sizes. Large pore (> 2 nm) act as sorption sites for gas penetrants. These pores are connected by ultramicropores (0.3 - 0.6 nm) which act as molecular sieves. Both of these pore sizes are characterized by distributions. An idealized model of the pore size

distribution in CMS membranes can be found in the Figure. The gas separation performance of CMS materials far exceeds polymeric membranes, the current industry standard. Steel et al. have shown that the microstructure of CMS materials can be tailored by altering the pyrolysis temperature, thermal soak time, and precursor composition. The current focus of CMS research in our group is to determine underlying factors which play a role in the development of the CMS nanostructure. Two key factors have been identified and include the polymer free volume and the composition of decomposition products which evolve during CMS nanostructure development. The goal of this project is to systematically determine the importance of each of these factors and investigate the CMS nanostructure using gas sorption and permeation as molecular probes.

Selected Recent Papers/Patents/Theses/Reports:

  • Steel, K.M., Koros, W.J., Investigation of porosity of carbon materials and related effects on gas separation properties. Carbon, 2003. 41: p. 253-266.

  • Steel, K., Carbon membranes for challenging gas separations. Ph.D. Dissertation, The University of Texas at Austin, 2000.

Synthesis and characterization of inorganic single-walled nanotubes and fabrication of nanotubular single-molecular sensors

Faculty: Sankar Nair
Students: (current) Sanjoy Mukherjee

The goal of this research is to design single-molecule sensing devices based on a 'solid-state' sensing element inserted in a thin membrane. The sensing element is a single molecule of designed properties and it senses other single molecules. We are inspired by living cell membranes, which are lipid bilayers incorporating functional biomolecules like ion channel proteins. Nature's designs however, are often not conducive to technological sensing applications. We wish to use the lipid bilayer as a chemically flexible platform for high-resolution solid-state sensor elements. This involves the fabrication of a short (< 100 Å) single-walled nanotube molecule of pore diameter ~ 15 Å (see Figure) with a hydrophilic inner wall

and with an organic-functionalized hydrophobic outer wall; and its insertion in an artificial lipid bilayer to study the transport of chain biomolecules like single-stranded (ss) DNA through a single nanotube by electrophysiological techniques. These sensors would enable the high-speed detection, sizing and sequencing of chain-like biomolecules (particularly ssDNA) which are of high importance to biotechnology. The reliability of currently pursued 'nanopore sensors' based on self-assembled protein channels is low because of a complex pore structure and flexibility of the seven protein segments assembled by non-covalent bonding. Our aim is to design and fabricate reliable nanopore sensors by replacing the protein with a solid-state cylindrical nanotubular molecule to obtain higher signal reproducibility and reliability. The fabrication of such devices raises many questions of fundamental engineering importance - for example, the growth of nanotubes with controlled length and diameter, their functionalization, and the mechanisms of biomolecule transport through nanopores. These issues are being studied in relation to the desired technological applications mentioned above.

Water-based coatings via miniemulsion polymerization

Faculty: F. Joseph Schork
Students: James Russum, Gengeng Qi
Postdoc: Willfred Smulders

Nanocomposites of acrylic polymers with the alkyds traditionally used in oil-based coatings can be made via miniemulsion polymerization. In these systems, the nanoparticles contain domains of alkyd, which give highly-cured, durable coatings, and domains of acrylic, which give film

Photograph of a miniemulsion. Note the total colloidal stability of the miniemulsion after 3 hrs of standing as compared with a conventional emulsion.

forming properties. The accurate placement (core-shell, etc.) of the domains within the nanoparticle determine its coatings properties. Through this technology it is possible to develop various coatings (architectural, industrial, etc.) which have the durability properties of solvent-based (oil) coatings and the environmental, health and convenience properties of water-based (latex) coatings.

Selected Recent Papers/Patents/Theses/Reports:

  • Tsavalas, John G. Yingwu Luo, Laila Hudda, and F. Joseph Schork, "Limiting Conversion Phenomenon in Hybrid Miniemulsion Polymerization," Polymer Reaction Engineering , 11(3), 277-304 (2003).

  • Landfester, Katharina, F. Joseph Schork and Victor A. Kusuma, "Particle Size Distribution in Miniemulsion Polymerization", Compte Rendus Chimie, 6(11-12), 1337-1342 (2003).

  • Water-Borne Polyurethane Coatings by Miniemulsion Polymerization," U.S. Patent Number 6,384,110, May 7, 2002.