1996 Annual Report

TECHNICAL ACTIVITIES: Electronic Packaging and Interconnection

A major element of the U.S. microelectronics business group is the semiconductor and the electronic interconnection industries. Combined, these two industries deliver the power and functionality of semiconductor technology to the hands of the users. They also face changing consumer expectations, product evolutions, design cycles, and international competition with a pace and urgency never before seen in world commerce.

In response to the importance of electronic packaging to the microelectronics industry, this program is focusing on industry's most pressing challenges surrounding the utilization of advanced materials and material processes in semiconductor packaging, electronic interconnection, and assembly. Initiated in 1994, the program complements semiconductor fabrication activities supported by the NIST National Semiconductor Metrology Program (http://www.eeel.nist.gov/810.01/index.html ).

MSEL's program deals with industry's most pressing materials issues which are associated with the product and technology priorities contained within leading industry roadmaps. Roadmapping activities sponsored by major industry associations, namely The National Technology Roadmap for Semiconductors, The National Technology Roadmap for Electronic Interconnects, and more recently The National Electronics Manufacturing Initiative, have led to projects dealing in matters such as electrical, thermal, and mechanical characteristics of thin film materials; solders, solderability and solder joint design (http://www.ctcms.nist.gov/programs/solder); interfaces and adhesion; moisture measurement and control; and electromigration. These projects are conducted in conjunction with collaborators from numerous industrial consortia, individual companies, academia, and other government agencies.

The mission of MSEL's program is to develop and deliver to the U.S. electronics and electronic materials industries measurement tools and data for materials and processes used in semiconductor packaging and module interconnection. This program is based upon three primary needs:

Develop techniques and procedures for making in-situ, in-process and in-use measurements on materials and material assemblies having micrometer- and submicrometer-scale dimensions.

Record and quantify the divergence of material properties from their bulk values as dimensions are reduced and interfaces are approached.

Develop fundamental understanding of materials needed for future packaging, interconnection and assembly schemes.

More information about this program, and other NIST activities in electronic packaging, interconnection and assembly, is contained in Electronics Packaging, Interconnection and Assembly at NIST: Guide and Resources, NISTIR 5817. This publication presents a complete inventory of NIST's activities in this area and lists information on how to contact project leaders. Copies may be obtained by contacting Michael Schen at (301) 975-6741 or michael.schen@nist. gov.

Significant Accomplishments
Detailed analysis of transient charge response obtained from thermal pulse instrumentation allows evaluation of the barrier to heat transmission across the interface between a dielectric polymer film and a conducting substrate.

NMR and NIR spectroscopy have detected liquid-like water as well as molecularly dispersed water in silica-filled epoxy molding compound and a polyimide when equilibrated in liquid water.

An improved measurement of work of adhesion between solid surfaces using the Johnson-Kendall-Roberts (JKR) technique has been realized by detecting and allowing for the displacement which takes place between the two contacting surfaces.

X-ray reflectivity has been used to measure the hygroscopic expansion of thin films of polyimide on silicon for a variety of film thicknesses. The results imply the existence of an interfacial region which absorbs much more water than the bulk.

Improved Measurement Technique for Hydrothermal Expansion of Polymer Thin Films

F.I. Mopsik, M.A. Schen, C.K. Chiang, S.C. Roth, and G.T. Davis

Objectives

Determine accuracy and precision of NIST-designed capacitor cell technique for measuring out-of plane expansion of thin polymer films. Investigate the dimensional stability of electronics packaging materials subjected to temperature and humidity changes. Work with standards-setting bodies to introduce NIST metrology as a new standard test method.

Technical Description

Measurements will be made on a set of well-characterized materials to evaluate the use of a precision capacitance gauge for measuring small dimensional changes in thin films. Studies will be made using this gauge to measure thermal expansion properties of polymer materials used in microelectronics as well as their response to changes in humidity.

External Collaborations

IBM (Endicott), DEC, NRC, NSWC (Crane), Cornell, U. TX (Austin), U. MD, Cornell, Purdue, NASA (JPL), SRC

Planned Outcomes

• Improved standard test method for expansion of thin films.
  
• reliable data on the expansion behavior of electronic packaging materials with variations in temperature and humidity.
  
  
  

• Accomplishments: The NIST-designed capacitance cell for the measurement of thermal expansion of thin polymer films was subjected to tests designed to ascertain accuracy and precision using thin, polished pieces of fused quartz and single crystal silicon cut from a wafer. Prolonged measurements showed that it took too many temperature cycles to achieve elimination of interfacial layers of air between the electrodes and the polished specimens as indicated by gradual drifts in the measured sample thickness. Heating the assembly to moderate temperatures in a vacuum oven shows promise as a measurement protocol to eliminate air layers.
  
  The cell was constructed of Zerodur which has a thermal expansion coefficient of essentially zero. However, in calibration tests, a small temperature-dependent residual capacitance was found which is attributed to a small capacitance involving the Zerodur which has a large variation of permittivity with temperature. Although the residual was reduced by additional shielding, it could not be simply eliminated. A new cell has been designed using quartz electrodes and we are experimenting with metallic films that will be more robust than gold.
  
  An environmental chamber has been purchased and it has been interfaced to a computer. Control has been optimized so that it can run long enough unattended to allow for the slow equilibrations expected for changes associated with changes in moisture. Also, optimization has been made so that humidity and temperature control will be sufficiently tight to allow for the desired precision in the thickness measurements.
  
• Outputs
  
• Publications
  M.A. Schen, F.I. Mopsik, W. Wu, W.E. Wallace, N.C. Beck Tan, G.T. Davis and W. Guthrie, Advances in the Measurement of Polymer CTE: Micrometer- to Atomic Scale Measurements, Polymer Preprints 37 180 (March 1996).
  
• Presentations
  G.T. Davis, M.A. Schen, F.I. Mopsik, W.L. Wu, W.E. Wallace, N.C. Beck Tan and W. Guthrie, Advances in the Measurement of Polymer CTE: Micrometer- to Atomic Scale Measurements, ACS, New Orleans March 25, 1996.
  Thermal Properties Measurement
  A. S. DeReggi, P. Bloss

  
  Objectives
  Develop laser-pulse-based electrothermal method of measuring (i) the thermal diffusivity of thin dielectric films adhered to substrates, (ii) the thermal resistance of the dielectric-substrate interface or of an adhesive layer.
  Technical Description
  Heat from a laser pulse is used to generate a condenser-microphone-like electrical response from a voltage-biased dielectric specimen that first expands upon gaining thermal energy from the laser pulse and then contracts to its original thickness upon losing this thermal energy to the substrate after diffusive transport across the dielectric. The shape of the electrical response depends on the thermal properties of the dielectric, the thermal barrier properties of the interface, or adhesive layer, and the thermal sinking properties of the substrate.
  Curve-fitting of the measured response to the theoretical response derived from the heat diffusion equation is used to determine the thermal properties of the dielectric and the characteristics of the thermal contact with the substrate.
  External Collaborations
  We are collaborating with Prof. Tom Avedesian at Cornell University on his SRC-sponsored project on thermal diffusivity measurements in electronic packaging materials.

  
  Planned Outcomes
  

  • New thin film thermal metrology methods.
    
  • Reliable data of thermal properties of polymeric electronic packaging materials.
    Accomplishments
    The mathematical techniques for extracting the parameters of interest from the transient charge response following the thermal pulse have been proven reliable by using simulated data.
    Error analysis and establishment of minimum measurement accuracy has been performed.
    A new sample chamber has been designed and constructed.
    Experimental measurements on polyimide and FEP films glued to a conducting substrate yielded thermal diffusivities of the films which agree with literature values. The thermal barrier deduced for the glue layer of conducting epoxy was also consistent with the thickness and reported conductivity of the epoxy.
  • 
    Outputs
    Publications
    Aimé S. DeReggi and Sigfried Bauer, Adaptation of the thermal pulse method to the measurement of the thermal diffusivity of dielectric films on conducting substrates, 1995 Annual Report of Conf. on Electr. Insul. and Diel. Phenom., pp. 536-539. IEEE Pub. No. 95CH35842.
    Peter Bloss, Aimé S. DeReggi, and Hartmut Schafer, Adaptation of the thermal pulse method to the determination of thermal parameters of thin layers, Proc. 9th Int. Symp. on Elect., pp. 684-689 (1996). IEEE Public. No. 96 CH35808.
    Peter Bloss, Aimé S. DeReggi, and Hartmut Schafer, Application of the regularization technique to the deconvolution of thermal pulse data, Proc. 9th Int. Symp. on Elect., pp. 171-176. IEEE Public. No. 96 CH35808.
    Peter Bloss, Aimé S. DeReggi, and Hartmut Schafer, Thermal properties and electric field profile in metallized dielectric film adhesively bonded to substrate,1996 Annual Report of Conf. on Electr. Insul. and Diel. Phenom., pp. 721-725. IEEE Pub. No. 96CH35985.
  • 
    Presentations
    A.S. DeReggi and S. Bauer, Adaptation of the Thermal Pulse Method to the Measurement of the Thermal Diffusivity of Dielectric Films on Conducting Substrates, Conf. Elec. Insul. and Dielec. Phenom., Virginia Beach, VA October 23, 1995.
    A.S. DeReggi, Thermal Pulse Investigation of the Generation, Transport and Trapping of Space Charge in Microelectronics Buried Oxide Structures, Conf. Elec. Insul. and Dielec. Phenom., Virginia Beach, VA October 23, 1995.
    Industry Coordination and Workshops in Electronic Packaging
    Michael Schen
  • 
    Objectives
    Enhance NIST's role and effectiveness in metrology for electronics packaging, interconnection and assembly by coordinating and cooperating with U.S. industry and other government agencies in technology planning, infrastructure research and development, implementation of industry established technology roadmap research priorities, and technology benchmarking.
    Technical Description
    The NIST materials program in electronics packaging, interconnection and assembly has the role of helping the U.S. microelectronics and electronic materials industries identify, develop and use improved materials technology for the design, manufacture and reliability assessment of new electronics products and processes. As part of this, NIST takes an active role in working closely with companies, associations, consortia, universities and other government agencies in a number of different fashions; all aimed at enhancing the linkage between industry's measurements and standards needs and NIST's research. The methods being used to accomplish this and build NIST's contribution to this business arena include the following activities:
    participate in national technology planning activities,
    incorporate industry established technology roadmap priorities into NIST's research efforts,
    conduct research in measurement science to support the Nation's electronic materials and microelectronics infrastructure,
    apply NIST competencies to industry driven development exercises,
    cooperate in the evaluation of technical and manufacturing capabilities of industry,
    disseminate information related to its program and its capabilities to U.S. planners and practitioners, and
    conduct industry-led workshops to enhance the exchange of information between the diverse businesses and organizations supporting the microelectronics industry and goverment bodies.
  • 
    External Collaborations
    In FY 1996, significant coordination and workshop collaborations took place with the following organizations:
    Private Firms IBM Endicott
    Intel
    Motorola
    National Semiconductor
    Government Defense Advanced Research Projects Agency
    Department of Commerce Technology Administration
    Department of Energy
    National Aeronautics and Space Administration
    National Science Foundation
    Naval Research Laboratory
    Office of Science and Technology Policy
    United States Air Force, Rome Laboratory
    Associations Institute for Interconnecting and Packaging Electronic Circuits
    Optoelectronics Industry Development Association
    Semiconductor Industry Association
    Consortia Semiconductor Research Corporation (SRC)
    SEMATECH
    Interconnection Technology Research Institute (ITRI)
    National Center for Manufacturing Sciences (NCMS)
    National Electronics Manufacturing Initiative (NEMI)
  • 
    Accomplishments
    In April 1994, NIST published a new report entitled Electronics Packaging, Interconnection and Assembly at NIST: Guide and Resources, NISTIR 5817. The report presents a comprehensive inventory of NIST laboratory activities in electronic packaging and includes a summary of ongoing ATP awards in the area. Intended to assist researchers and manufacturers, the Guide presents a concise snapshot of NIST research activities, the measurement capabilities and publications associated with each of these activities, and the names, telephone numbers and Internet addresses of people to contact for further information. The Guide contains the contributions of over twenty-five different principal NIST researchers and managers who are working collaboratively with researchers and manufacturers from some 60 separate U.S. microelectronics and materials companies, consortia, trade associations, standards bodies, universities and other government agencies. The report is available free of charge and efforts are underway to make it available on NIST's web page.
    In FY 1996, NIST and the President's National Science and Technology Council's (NSTC) Electronic Materials Working Group (EMWG) also released a report, Beyond the Technology Roadmaps: An Assessment of Electronic Materials Research and Development, NISTIR 5777. This report assesses the R&D needs of U.S. industry in electronic materials. The report covers industry's short and long term materials issues in microelectronics, photonics, RF & microwave technologies, mass storage, module interconnects, materials characterization, and research opportunities. NIST participates along with other government agencies that have activities in electronic materials in the EMWG to enhance the efficient utilization of government resources and coordinate among agencies. The EMWG was established to support the activities of both the Materials Technologies and the Electronics Subcommittees of the NSTC's Civilian Industrial Technology Committee. It has the objective of pursuing methods to team industry groups and government agencies to create a national strategy in high leverage and/or critical materials and material processing technologies to significantly enhance U.S. competitiveness in electronics and supporting industries, and to provide a forum, knowledge base and recommendations for coordination among government agencies.
    In the areas of infrastructure research and development and implementation of technology roadmap research priorities, NIST is cooperating with the Semiconductor Research Corporation (SRC) and SEMATECH in a number of ways to better link NIST's activities to industry's needs and SRC and SEMATECH activities. In the case of the SRC, NIST is a participating member of the SRC Packaging Science's Technical Advisory Board and has become a mentor for a number of their individual research projects in packaging. Technical collaborations between SRC award recipients and NIST have also taken place. NIST has also built new links to SEMATECH's Assembly and Packaging efforts through its involvement in SEMATECH workshops and programs.
    NIST continues to work closely with the Institute for Interconnecting and Packaging Electronic Circuits (IPC), which represents the printed wiring board and assembly industries, in the roadmapping of their technology needs for the early part of the next century as well as their standards activities. NIST is a member of the IPC Roadmap Steering Committee and contributed substantially to their 1996 report, Technology Benchmarking: Worldwide Competitive Analysis of Electronic Interconnections. NIST is also a member of the IPC's Test Methods Subcommittee (7-11) and made major contributions to newly issued IPC test methods. At the IPC's 1996 Capital Hill Day event, NIST Director Arati Prabhaker was the invited banquet speaker and spoke about Government and NIST's continued support for this business segment.
    Workshops are an extremely important tool for NIST to not only understand industry's greatest needs but to also foster technical exchange between researchers and technologists within public and private sectors. In addition to sponsoring workshops, members of NIST's packaging team also participate in a large number of other industry and government sponsored workshops throughout the year. Since the inception of our program in electronic packaging materials, NIST has conducted the following industry workshops:
    6th International Workshop on Moisture in Microelectronics, sponsored by NIST and the U.S. Air Force Rome Laboratory, October 15-17, 1996, Gaithersburg, MD.
    Materials Metrology and Data for Commercial Electrical and Optical Packaging and Interconnection Technologies, sponsored by NIST, Institute for Interconnecting and Packaging Electronic Circuits, Optoelectronics Industry Development Association, Semiconductor Research Corporation, May 5-6, 1994, Gaithersburg, MD.
    Workshop on Moisture Measurement and Control for Microelectronics, sponsored by NIST and the U.S. Air Force Rome Laboratory, April 5-7, 1993, Gaithersburg, MD.
    Measurement of the Properties of Materials in Microelectronic Packaging, sponsored by NIST, Semiconductor Research Corporation, Microelectronics and Computer Technology Corporation, Microelectronics Center of North Carolina, U.S. Army Harry Diamond Laboratory, U.S. Air Force Wright Research and Development Center, and U.S. Naval Weapons Support Center, May 1-3, 1990, Gaithersburg, MD.
    Impacts
    Over 300 copies of NISTIR 5520, Metrology and Data for Microelectronic Packaging and Interconnection, have been distributed among companies, consortia, associations, universities and other government agencies. This report remains the only public document that builds upon technology roadmaps for the microelectronics industry to identify fundamental challenges in materials science and engineering for electronics packaging.
    To date, NIST has distributed nearly 3000 copies of it's report Electronics Packaging, Interconnection and Assembly at NIST: Guide and Resources to U.S. companies, universities, associations, and consortia. At the 1996 Semicon West Expo, this report, commonly referred to as the 'yellow book,' was the most sought after item at the NIST exhibition booth. In addition, as a consequence of the publication, NIST has hosted a number of visitors wishing to tap into the organization's competencies. It is our view this and future similar reports will be essential towards insuring NIST's technical contributions reach the right people at the right time.
    The Electronic Materials Working Group report, Beyond the Technology Roadmaps: An Assessment of Electronic Materials Research and Development, NISTIR 5777, has also seen wide distribution since it's publication this year. To date over 1000 copies have been requested. The report, the first of its kind to comprehensively assess materials R&D issues across today's electronic and optical technologies, is being used by planners and practitioners from industry, government and academia.
    NIST took a leadership role this year in reviewing and enhancing a number of new IPC test methods originating from the Deposited Dielectrics Task Group. Enhancement of the seven new test methods by NIST was recognized by the IPC and the Chairman of the IPC Test Methods Subcommittee as "a valuable contribution" by NIST and recommended that NIST's recommended changes be incorporated into the final standards.
  • 
    Outputs
    Publications
    Michael A. Schen, ed., NISTIR 5817, Electronics Packaging, Interconnection and Assembly at NIST: Guide and Resources, April 1996.
    Michael Schen, Thomas J. Russell, Robert F. Leheny, Henry Simon, Verne Hess, Gerald Borsuk, eds., NISTIR 5777, Beyond the Technology Roadmaps: An Assessment of Electronic Materials Research and Development, MatTech Electronic Materials Working Group report, March 1996.
    Michael A. Schen, ed., Government and Industry Relations, in "Technology Benchmarking: Worldwide Competitive Analysis of Electronic Interconnections," IPC June 1996.
    M.A. Schen, G.T. Davis, F.I. Mopsik, W.L. Wu, W.E. Wallace, J.R. Manning, C.A. Handwerker, D.T. Read, Electronics Packaging Materials Research at NIST, Mat. Res. Soc. Symp. Proc., 390, 19-31, 1995.
    M.A. Schen, R. Scace, T. Leedy, NIST Strategies, Activities and Collaborations in Electronics Packaging, Interconnection and Assembly, Proceedings of the Eleventh Biennial University, Government, Industry Microelectronics Symp., IEEE Catalog #95CH35779, Austin, TX, 1995
    M.A. Schen, G.T. Davis, F.I. Mopsik, W.L. Wu, W.E. Wallace, J.R. Manning, C.A. Handwerker, D.T. Read, Electronics Packaging Materials Research at NIST, in Electronic Packaging Materials Science VIII, R.C. Sundahl, K.A. Jackson, K-N. Tu, P. Børgesen, ed., Proceedings of the Materials Research Society (Materials Research Society, Pittsburgh, PA) 390, pgs. 19-32, 1995.
  • 
    Presentations
    M.A. Schen, NIST's Activities in Packaging and Interconnection, SEMATECH/ITRI Workshop, Moisture in Microelectronics, Austin, TX, March 1996.
    M.A. Schen, Competitive Analysis of World Products, IPC Printed Circuits Expo, San Jose, CA, March 1996.
    M.A. Schen, Technical Advances in Materials for Electronics Packaging and Interconnection: A Report on NIST Research, ISHM Capital Chapter, Columbia, MD, November 1995.
    M.A. Schen, Electronic Packaging Measurement Capabilities at NIST, Chesapeake Electronics Show, Greenbelt, MD, November 1995.
    M.A. Schen, Implications of the National Semiconductor Roadmap for Module Interconnection and Assembly, IPC Roadmap Steering Committee, Tempe, AZ, 1995.
    M.A. Schen, NIST Strategies, Activities and Collaborations in Electronics Packaging, Interconnection and Assembly, Eleventh Biennial University, Government, Industry Microelectronics Symposium, Austin, TX, 1995.
    Ultra-thin Polymer Films
    W. Wu, W.E. Wallace, E.K. Lin, G.T. Davis
    Objective
    Determine the role of interfaces on the properties of polymers by measuring the properties of ultra-thin polymer films adhered to well characterized substrates. Measure the thickness at which properties of thin films depart from those of the bulk material. Compare results with statistical mechanics calculations using a self-consistent mean-field procedure.
    Technical Description
    The physical properties of polymer thin films are measured using a variety of techniques including specular and off-specular x-ray and neutron reflectivity, energy-dispersive x-ray reflectivity, total external reflectance x-ray fluorescence, MeV ion scattering, atomic force microscopy, and ellipsometry. For single-component films, the physical properties measured include the coefficient of thermal expansion, glass transition temperature, and density. For liquid crystal materials, the density profiles were determined and correlated with an isotropic liquid crystal transition zone near the interface. For multicomponent films, interdiffusion dynamics and interfacial segregation are measured. Surface roughness is examined through a combination of off-specular reflectivity and atomic force microscopy. Typical materials considered include polystyrene, poly(methyl methacrylate), polyvinyl pyridine, polyimides, and epoxies. By varying the thickness of the film, the type of polymer and its molecular weight, the sample temperature and environment, and the film substrate surface, fundamental physical insight is gained into the behavior of polymer molecules at impenetrable interfaces. The experimental results are compared with self-consistent mean field calculations to build a theoretical framework for future predictions of polymer film properties. In the course of this work, new techniques have been developed to aid in measuring the subtle properties of polymer thin films. These include a new theory to interpret off-specular reflectivity data and a new method of x-ray reflectivity to determine thin film density.
  • 
    External Collaborations
    Gelest, Inc., University of Pennsylvania, U. Tennessee, U. Delaware, Army Research Laboratory, Sandia National Laboratory
    
    Accomplishments
    Developed a new theory to quantify surface roughness from off-specular reflectivity data.
    
  • Developed a method that improves the accuracy in the measurement of thin film density by at least a factor of five.
    
  • Demonstrated that the polymer thin film coefficient of thermal expansion depends sensitively on the substrate surface for film thicknesses up to three times the polymer radius of gyration.
    
  • Discovered that the polymer interdiffusion dynamics are significantly retarded for interfaces within several polymer radii of gyration of an impenetrable interface.
  • 
    Outputs
    Publications
    W.L. Wu, W.E. Wallace, J.H. van Zanten, B.J. Bauer, D.W. Liu, and A. Wong, Diffusion of Linear Polystyrene into Crosslinked Polystyrene, Polymer (in press)
    W.E. Wallace, W.L. Wu, and R.A. Carpio, Chemical-Mechanical Polishing of SiO₂ Thin Films Studied by X-Ray Reflectivity, Thin Solid Films 280 (1996) 37
    J.H. van Zanten, W.E. Wallace, and W.L. Wu, Substrate Interaction Induced Transitions in Ultrathin Polymer Films, Physical Review E 53 (1996) R2053
    W.E. Wallace and W.L. Wu, Determining Thin Film Density by Energy-dispersive X-ray Reflectivity: Application to a Spin-on-Glass Dielectric, Materials Research Society Proceedings 406 (in press)
    W.L. Wu, Application of Off-Specular X-ray Reflectivity to Surface Characterization, SPIE Proceedings, Denver, (1996) in press.
  • 
    Presentations
    W.E. Wallace, Analysis of Silica Thin Film Derived from Pyrolysis of Polysilsesquioxanes, MRS, San Francisco, CA April 10, 1996.
    W.E.Wallace, A Novel Method for Determining Thin Film Density by Energy Dispersive X-Ray Reflectivity, MRS, Boston, MA November 29, 1995.
    W.E. Wallace, Gas Absorption During Ion Irradiation of a Polymer Target, MRS, Boston, MA November 28, 1995.
    W.L. Wu, Molecular Structure of Bimodal Epoxy Networks, Workshop on Development and Characterization of Inhomogenieties during Network Formation, Czech Republic Academy of Science, Prague, Czech Republic, September 16, 1996.
    W.L. Wu, Molecular Structure of Bimodal Epoxy Networks, Gordon Research Conference on Thermosetting Resins, Plymouth State College, Plymouth, NH, July 9, 1996
    W.L. Wu, Application of Off-Specular X-Ray Reflectivity for Surface Characterization, SPIE-International Society of Optical Engineering, Denver, CO August 8, 1996
    W.L. Wu, N.C. Beck Tan, B.J. Bauer, J. Plestil and K. Dusek, Structure in Bimodal Epoxy Networks, APS, St. Louis, MO March 20, 1996.
    W.L. Wu, Diffusion of Linear Chains into Crosslinked Networks, Intersociety Polymer Conference, Baltimore, MD October 8, 1995.
    Dielectric Measurements of Thin Films from DC to 1 GHZ
    F. I. Mopsik, C.K. Chiang, S.C. Roth, and G.T. Davis
  • 
    Objectives
    Inductance-Capacitance-Resistance bridges (LCR meters) are designed for accurate measurements over a discrete range of frequency, often with constraints on the sample configuration. The objective of this project is to establish measurement protocols and sample configurations for the accurate determination of dielectric constant and loss at frequencies from DC to 1 GHZ of thin polymeric films important to the electronic packaging industry, many of which can be obtained only as these thin films.
  • 
    Technical Description
    Measurements will be made with known standards and films to establish the LCR meter limits. Measurements will include the use of other methods, such as the Time Domain Spectrometer to evaluate and extend the measurement range in a known way. Sample configurations and data reduction methods will be determined to optimize the film parameters.
  • 
    Planned Outcomes
    We expect to develop a sample configuration and measurement protocol for the accurate measurement of dielectric constant and loss of thin films for frequencies approaching 1 GHz. The method is expected to provide consistent results for both producers and users of polymeric resins used in electronic packaging. Current standards apply to lower frequencies and thicker materials.
  • 
    Accomplishments
    The LCR meters that operate to 30 MHZ were shown to be much more reproducible, uncertainties of less than 1%, if a coaxial admittance is directly connected to their terminals. No 4-wire configuration could achieve this level of precision. Calibration procedures showed, however, that the bridges still had significant systematic errors approaching 5%. If 100 ohm or 50 ohm resistance standards were used, it was found that measurements on a broad range of standard, high frequency capacitors from 10 to 1000 pF could be recomputed so that the errors, referred to the terminals of the bridge, could be made to much less than 1%, including loss uncertainty.
  • 
    For polymer samples made from low loss polypropylene, this accuracy still held if they were mounted directly at the bridge terminals. This was true even up to near 1 GHz using a Coaxial LCR meter. For temperature control, however, the samples will have to be mounted at the end of precision coaxial air-filled lines. For this case, good results were found if the admittance was measured at the bridge terminals using the proper calibration procedures. The sample is then evaluated at the other end of the line using values determined directly on the line by direct propagation using transmission line theory.
  • 
    The main remaining issues are the estimation of residual jig capacitances, the variation of the propagation constant of the air line with temperature and the best estimation of the samples geometric capacitance for a predetermined electrode configuration.
  • 
    Outputs
    Publications
    F.I. Mopsik, Dielectric Relaxation Phenomena in Composite Systems, MRS Proceedings Fall 1995.
    Presentations
    F.I. Mopsik, Dielectric Relaxation Phenomena in Composite Systems, MRS, Boston, MA November 29, 1996.
    Residual Stress in Polymer Films Adhered to Silicon Substrates
    A. S. DeReggi, C. K. Chiang
  • 
    Objectives
    Evaluate wafer bow technique for deducing the residual stress in polymer-coated silicon wafers resulting from differential contraction after cure. Investigate all factors affecting measurement and modeling of such stress.
  • 
    Technical Description
    Average wafer curvature is determined, using a commercial instrument and accompanying software, from measurements of the reflection angle of a near-normal-incidence laser beam, at 50 points sampled while the incident beam is scanned linearly along a diameter of a wafer. A temperature-regulated wafer holder allows varying the temperature in a user-determined program in the range from ambient to 500 C, allowing the monitoring of wafer bow while a polymer precursor solution spin-coated on the wafer is heated to a curing temperature and is then cooled back to room temperature. An enclosure allows controlling the atmospheric environment of the wafer-supported polymer film. Sources of errors are identified and measurement protocols are being developed.
  • 
    Accomplishments
    Showed that bow of polyimide-coated silicon wafer depends strongly on the relative humidity of its atmospheric environment. Proposed fast-responding humidity sensor based on measurement of capacitance of air gap between uncoated bowed wafer surface and a planar counter-electrode. Showed that stress measurement must take account of relative humidity.
  • 
    Moisture Effects in Electronic Packaging Polymers
    M.A. Schen, D.L. VanderHart, B. Dickens, G.T. Davis
  • 
    Objectives
    Develop measurement approaches that allow the determination of the physical nature and distribution of water within filled polymer systems and apply these approaches to the evaluation of industrial materials and processes used in the manufacture of semiconductor packages, electronic interconnects or assembled systems.
  • 
    Technical Description
    Two individual investigations have been underway in FY 1996 related to measuring the effects of moisture in polymeric dielectrics and filled polymer resins used in electronic packaging applications. One of the key reliability and performance issues associated with the manufacture and use of electronic systems that utilize either plastic semiconductor packages or multilayer polymer interconnects is the effects of moisture on the mechanical reliability of the polymer encapsulant, in the case of plastic packages, or on the electrical performance of the interlayer dielectric, in the case of multilayer interconnects. Consequently, metrology aimed at documenting the effect of moisture on the reliability performance characteristics of industrially important materials is vital as more widespread utilization of plastics in packaging occurs.
    In the first activity, NIST uses solid state proton NMR to: a) characterize the state of water in plastic encapsulant materials (mold compound); b) identify the presence of and the amount of void volume in these materials; and c) ascertain whether these voids contribute to package delamination and/or cracking, commonly referred to as 'popcorning' during solder reflow.
    In the second activity, NIST employs near infrared spectroscopy to yield information on the physical state of absorbed moisture in polyimide films. The frequency of the NIR absorbance of a water molecule depends on the hydrogen bonding of the molecule. Furthermore, since the absorbance is weak, samples much thicker than those required for the mid IR range can be examined (typically 25 m to 1 cm).
    External Collaborations
    National Semiconductor, DARPA's Plastic Packaging Consortium (this includes National Semiconductor, Amoco Electronic Materials Plaskon Division, IPAC, Sheldahl Corp., Olin Corp., Dexter Electronic Materials, Sandia National Laboratories, Delco Electronics, Leading Technology Corp.)
    Planned Outcomes
    Provide industry with convenient metrology tools to examine moisture ingress into mold compounds on a molecular scale beyond simple weight uptake.
    Provide experimental data which will validate or invalidate the appropriateness of commonly used conditioning treatments, for example steam ageing, for the accelerated testing of moisture sensitivity of mold compounds. NIST will work with standards bodies, such as IPC and JEDEC, and individual companies to refine current practices and standards.
    Accomplishments
    In the solid state NMR studies, a 'typical' cresol-novolac compounding material, supplied via a collaboration with National Semiconductor, has been examined. It was demonstrated that NMR is capable of distinguishing water distributed in the polymer matrix from water that is self associated within regions believed to be voids within the mold compound.
    Results from these studies led to the following conclusions:
    a) Of the total water picked up by the mold compound, about three fourths becomes associated with the matrix and one fourth associates as liquid water within voids after sample equilibration in liquid water. By measuring the amount of water in voids, the void content has been assessed to be about 0.2% of the overall volume.
    b) Voids do not contain liquid water after equilibration at 89% relative humidity (RH) and 29 C, but rather, fill with water at RH levels somewhere between 89% and 100%. This late-filling behavior for the voids is anticipated on the basis of thermodynamic arguments.
    c) It is unlikely that voids act as reservoirs for liquid water, even when electronic assembly is carried out in warm, humid climates.
    d) Voids, as formed, do not appear to provide a low-resistance, interconnected path to surfaces.
    e) Rapid heating of mold compound to simulate solder reflow conditions does not appear to change the rate at which water within the voids empty upon exposure to vacuum. Hence, rapid heating of encapsulant previously exposed to high humidity conditions does not generate persistent, new, low-resistance paths for water to escape.
    f) Simulation of solder reflow for mold compound previously exposed to high humidity conditions leaves a 400-micron thick specimen thoroughly dry. Consequently, moisture will migrate during reflow to encapsulant surfaces and interfaces where it can either escape or accumulate.
    The NIR spectra of water were examined as function of concentration in a variety of polar and non-polar organic solvents. It was established that a good correlation exists between the quantity of water present and the area under the NIR absorption peak from 5400 to 4000 cm¹,
    even though the shape of the peak changes significantly. Comparing the spectra of water absorbed by polyimide films equilibrated at a variety of relative humidities with those obtained from various solvents reveals that the water absorbed by polyimides is molecularly dispersed, as opposed to being in water clusters, even when equilibrated in liquid water. When filled with unsilanized silica and equilibrated at high humidities, polyimide films do show evidence for accumulation of liquid-like water. The NIR spectra of neat samples (unfilled) of a cresol-novolac epoxy encapsulation resin when soaked in water for two days did not differ from the same resin equilibrated with room air (nominally 25C and 50 % RH).
    Outputs
    Publications
    David L. VanderHart, Partitioning of Water Between Voids and the Polymer Matrix in a Molding Compound by Proton NMR, Proceedings of 6th International Workshop on Moisture in Microelectronics, sponsored by NIST and the U.S. Air Force Rome Laboratory, Gaithersburg, MD, Oct. 15-17, 1996.
    Presentations
    M.A. Schen, Moisture and Stress Measurements at NIST, Technology Reinvestment Program Plastic Packaging Consortium, Sheldahl, Longmont, CO September 10, 1996.
    N.C. Beck Tan, W.E. Wallace, W.L. Wu and G.T. Davis, Interface Effects on Moisture Absorption in Thin Polyimide Films on a Silicon Substrate, MRS, Boston, MA November 29, 1995.
    Interfacial Energy between Solids
    W. Wu, Pearl Chin and G.T. Davis
    Objective
    Improve the Johnson-Kendall-Roberts (JKR) technique for measuring the interfacial energy between solids by allowing for nonlinear elastic effects and finite size effects in the data analysis.
    Technical Description
    The interfacial energy between polymers and solid substrates is an important parameter controlling the structure and physical properties of polymers near the interface. The JKR technique has been demonstrated as a viable method for determining interfacial energies. To improve the accuracy of the measurement, the displacement as well as the contact area are measured simultaneously, this is in contrast to all the JKR work published so far where the only parameter measured is the contact area. In addition, a methodology has been established to identify and to discriminate regions where the nonlinear elastic effect and the finite size effect render the results inapplicable to the current JKR theory. Efforts have also been started to modify the JKR theory to accommodate these effects. To improve the precision of this measurement, work is now in progress to automate the data acquisition and analysis.
    External Collaboration
    Pearl Chin recently obtained her Ph D from the University of Delaware under the guidance of Prof. Roy McCullough in the Department of Chemical Engineering and performed her experimental work on the JKR technique at NIST under the guidance of Dr. Wen-li Wu.
    Accomplishments
    An instrument has been constructed which enables the simultaneous measurement of contact area and displacement.
    A methodology has been developed to identify the data domain over which the JKR theory in its current form is applicable.
    Outputs
    Publications
    P. Chin, Characterization of Polymer-Solid Adhesion, PhD Thesis, University of Delaware, (1996).
    P. Chin, R.L. McCullough and W. Wu, An Improved Procedure for Determining The Work of Adhesion Between Polymer-Solid Contact, J. Of Adhesion Science (submitted)
    Aging of Light Emitting Polymers
    G.T. Davis, E. Ettedgui, and C.K. Chiang
    Technical Objectives
    Determine the important parameters and underlying mechanisms responsible for the time-dependent decay of light intensity from electroluminescent polymer devices.
    Technical Description
    The examination of polymer devices using optical, scanning electron and atomic force microscopies reveals how device morphology evolves during operation as well as during storage. Absorption and fluorescence spectra of polymer devices give a global (not spatially resolved) indication of the optical properties of the electroluminescent polymer. These may indicate useful changes in the light emission properties of the polymer as a result of surface treatments. Imaging the polymer devices during operation using a CCD camera reveals the spatial distribution as well as time dependent evolution of the electroluminscence intensity. Electrical impedance spectroscopy measures changes in the electrical characteristics of polymer devices non-destructively during operation as well as during storage.
    External Collaborations
    We are collaborating with Prof. Frank Karasz and his group at the University of Massachusetts who are supplying polymer samples and light emitting devices.
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    Accomplishments
    Investigations have concentrated on two electroluminescent polymers, poly(p-phenylenevinylene) and a copolymer of derivatized phenylene vinylene and dioxyoctane.
    Optical and scanning electron microscopies reveal that the metal cathode overlying the polymer layer breaks down by forming holes which grow until they become adjacent to one another and merge. Eventually, the electrical continuity of the cathode is compromised and the device ceases to function.
    Optical and scanning electron microscopies also reveal that the polymer film itself may not be stable since its morphology gradually evolves in a manner reminiscent of dewetting during storage in ambient atmosphere. The resulting loss of polymer film integrity leads to catastrophic device failure after a few seconds of operation.
    Cathode formation by the deposition of metal on the polymer film affects the absorption and fluorescence properties of the device depending on the metal. A gold layer 40 Å thick attenuates the fluorescence intensity by 50% and results in the formation of a new broad absorption band centered at approximately 630 nm. Removing the gold restores the device to its original state. Aluminum deposition, on the other hand, does not appear to affect either the fluorescence intensity or the absorption spectrum of the device.
    Areas of inhomogeneity in the polymer film appear when imaging the electroluminescence intensity distribution across the device using a CCD camera. These areas correspond to some regions of non-uniform film thickness as found using optical and atomic force microscopies, suggesting a dependence of device failure on polymer film uniformity.
    Impedance spectroscopy reveals changes in the interface between the electrode and the polymer following device degradation.
  • 
    Outputs
  • 
    Publications
    E. Ettedgui, G.T. Davis, B. Hu and F.E. Karasz, Degradation of Polymer Based Light Emitting Diodes during Operation, Synthetic Metals (in press).
    Presentations
    E. Ettedgui, G.T. Davis, B. Hu and F.E. Karasz, Degradation of Polymer Based Light Emitting Diodes during Operation, International Conference on Science and Technology of Synthetic Metals, Salt Lake City, UT August 2, 1996.