ELECTRONIC PACKAGING, INTERCONNECTION AND ASSEMBLY PROGRAM
The objective of this program is to assist the U.S. microelectronics industry by addressing their most pressing materials measurement and standards issues. These issues are central to the development and utilization of advanced materials and material processes required by new product technologies, as outlined within leading industry roadmaps1. This program is part of a broad effort in the NIST Materials Science and Engineering Laboratory with the goal of serving as a key resource within the Federal Government for materials metrology development for commercial microelectronics manufacturing. More specifically the objectives can be categorized in the following four areas:
With these objectives in mind, the program currently consists of some eleven separate projects dealing in matters such as the electrical, thermal, and mechanical characteristics of polymer thin films; characteristics of interfaces and adhesion; and the nature of the built-up stress and moisture in plastic packages. From beginning to end, these projects are conducted in concert with partners from industrial consortia, individual companies, academia, and other government agencies. The program is strongly coupled with other microelectronics programs within government and industry, including the National Semiconductor Metrology Program (NSMP)2. The NSMP is a national resource responsible for the development and dissemination of new semiconductor measurement technology.
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. Copies may be obtained by contacting Wen-li Wu at (301) 975-6839 or wenli@nist.gov.
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1National Technology Roadmap for Semiconductors, Semiconductor Industry Association, San Jose, CA, 1994, 1997 (in draft); National Technology Roadmap for Electronic Interconnects, Institute for Interconnecting and Packaging Electronic Circuits, Lincolnwood, IL, 1995, 1997 (in draft); National Electronics Manufacturing Technology Roadmap, National Electronics Manufacturing Initiative, Inc., Herndon, VA, 1996.
2 http://www.eeel.nist.gov/810.01/index.html
Significant Accomplishments
Measurements of Hygrothermal Expansion of Polymer Thin Films
F. I. Mopsik, C.R. Snyder
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 with 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
Dr. E.O. Shaffer II of Dow Chemical Company provided materials for CTE measurements.
Planned Outcomes
Improved test method for measuring hygrothermal expansion of thin films providing reliable data on the expansion behavior of electronic packaging materials with variations in temperature and humidity.
Accomplishments
Measurements were performed on<0001>oriented Al2O3 single crystal samples to demonstrate the ability of the technique to provide accurate values for the thermal expansion coefficient of materials. Results are in excellent agreement with literature values. Additionally, the calculated expansion coefficient of the nichrome-plated fused quartz electrode matches the literature value within the experimental uncertainty.
The uncertainties from the humidity and PVT properties of air and water have been assessed. A relative uncertainty of 1x10-6 of the total thickness has been determined for dry conditions by measurements on single crystal Al2O3 and 4x10-5 for humid conditions (at 60 °C) by theoretical computations.
Validity of the data reduction technique, based upon first principles, was demonstrated by determining the thickness of Zerodur spacers as a function of relative humidity. The determined thicknesses were identical within the experimental uncertainty at 60 °C and 0 %, 20 %, 40 %, 60 %, and 80 % relative humidity.
Outputs
Publications
C.R. Snyder and F.I. Mopsik, A Precision Capacitance Cell for Measurement of Thin Film Out-of-Plane Expansion. I. Thermal Expansion, Rev. Sci. Instrum., 69(1998)3889.
C.R. Snyder and F.I. Mopsik, A Precision Capacitance Cell for Measurement of Thin Film Out-of-Plane Expansion. II. Hygrothermal Expansion, J. Appl. Phys., submitted.
C.R. Snyder and F.I. Mopsik, High Sensitivity Technique for Measurement of Thin Film Out-of-Plane Expansion, Proceedings of the 1998 International Conference on Characterization and Metrology of ULSI Technology, in press.
E.K. Lin, C.R. Snyder, F.I. Mopsik, W.E. Wallace, W.L. Wu, C.X. Zhang and R.M. Laine, Characterization of Epoxy-Functionalized Silsequioxanes as Potential Underfill Encapsulants, Proceedings of the Materials Research Society 1998 Spring Meeting, in press.
Presentations
C.R. Snyder and F.I. Mopsik, A Precision Capacitance Cell for Measurement of Thin Film Out-of-Plane Expansion: Thermal & Hygrothermal Expansion, 1998 International Conference on Characterization and Metrology of ULSI Technology, Gaithersburg, MD, March 25, 1998.
C.R. Snyder and F.I. Mopsik, A Precision Capacitance Cell for Measurement of Thin Film Out-of-Plane Expansion: Thermal & Hygrothermal Expansion, American Chemical Society National Meeting, Boston, MA, August 22, 1998.
Characterization of Polymer Films and Composites by Dielectric Techniques
Jan Obrzut
Objectives
Develop test procedure for dielectric characterization of thin films at microwave frequencies (up to 20 GHz). Evaluate dielectric properties of polymer composites filled with ferroelectric materials for electronic applications.
Technical Description
Dielectric measurement techniques in the microwave range are of primary importance to industry, and can provide substantial insight into interfacial interactions and molecular dynamics in thin polymer films.
There are two currently available standard testing methods for dielectric permitivity of polymer substrates at microwave frequencies. One is ASTM 3380 - 90/95 and the other is IPC-TM-650, No.: 2.5.5.5, revised in 1998. These test methods employ a strip line resonator circuit covering frequency range from 8 GHz to 12 GHz. The required sample thickness is about 1.6 mm. Substrates thinner than 1.27 mm with dielectric constant greater than 10.5 are not covered by the above testing methods. Many of the polymers in applications are available only in the form of thin films, typically a few micrometers in thickness. As the thickness of the dielectric being measured is reduced, the other dimensions of the test pattern are reduced as well. Such test patterns can only be made using very high-resolution photolithographic techniques. Moreover, the coupling efficiency between the feeding microstrip launchers and the resonating strip rapidly approaches zero, as the test pattern becomes smaller. This makes the dielectric measurements on the thin films in the microwave range rather difficult and unreliable, if performed in accordance with the available testing standards. Therefore, a new testing procedure specifically designed for thin dielectric films and laminates is needed.
To address this issue the applicability of the IPC-TM-650, No.:2.5.5.5 test method to thin films is being extended by designing a new test specimen and by developing a new coupling procedure for microstrip resonators. The first application of this new test method is the characterization of polymer composites for high-dielectric films.
New, chip scale packaging utilizes high-dielectric constant films to form embedded RC cells, which are integrated within the interlevel dielectric structure. Polymer composites filled with ferroelectric powders are candidates for such applications. However, the structure-dielectric property relationship for the high-dielectric constant composites is poorly understood. This situation is mostly due to lack of reliable experimental permittivity data in the frequency range from 0.5 GHz to 20 GHz. Currently, there is no satisfactory test procedure for evaluating dielectric dispersion and relaxation behavior of candidate materials at the above mentioned frequencies. Most of the problems in testing can be attributed to interfacial polarization, to the wave compression effect and to the cutoff frequency effect of distributed circuits.
External Collaborations
Philip Bowles of Delphi-Delco Electronics provided assistance in accelerated stress test of high-k films for their reliability.
Yueh-Ling Lee of DuPont provided copper clad laminates of high dielectric constant films.
John Lauffer of IBM constructed an artwork (mask) of the microwave test specimen.
Dale Murry of Litton Advanced Circuitry manufactured the test boards with embedded a dielectric test pattern.
Dr J. Dougherty of Penn State University provided detailed analysis of molecular dynamics of high-k films.
Dylan Williams of NIST Boulder provided test specimen design.
Planned Outcome
Metrology for Dielectric Permittivity of Thin Polymer Films in the Microwave Range.
A better understanding of coupling mechanism between ferroelectric filler and polymer matrix. There is a question whether and to what extent polarizability of the polymer matrix may control the apparent dielectric permittivity of the composite. The search will focus on model polymer matrices of different molecular structure, such as dielectric polymer matrix (polyimide), poled polymer matrix (PVDF) and an oriented chiral polymer matrix (modified cellulose).
Accomplishments
In collaboration with the industrial partners a test specimen and a test procedure has been developed for ferroelectric films. The method utilizes a modified, airline coaxial probe for frequency range from 0.5 GHz to 5 GHz. The uncertainty is currently about 0.5% for the dielectric constant. This is significant improvement over the existing methods utilizing four terminal fixtures, which have an upper frequency limit of about 800 MHz and uncertainty of about 10%. The test specimen (photo mask) has been accepted by the National Center for Manufacturing Sciences participants (DuPont, IBM, Delco, Raytheon, 3M) as a common dielectric test vehicle for the Embedded Capacitive Materials Project.
In collaboration with MSEL and EEEL staff, a new test specimen has been designed for thin dielectric films. The specimen utilizes triplate, microstrip resonating structures and co-planar waveguides. The microstrip resonators can be used at frequencies up to 20 GHz. The co-planar waveguiding structure can be used to test thin dielectric films at higher frequencies, above 20 GHz (Boulder). A new measurement protocol has been developed for resonators, which are end-fire coupled to microwave launchers. The implemented alignment procedure is analogous to those used in aligning fiber-optics devices. It allows precise controlling of coupling conditions, which eliminates a major source of experimental errors and improves overall accuracy.
An experimental set-up has been developed for dielectric relaxation spectroscopy and DC conductivity of composite materials. Currently, the experiment can be conducted in the frequency range of 1 mHz to 1 MHz and temperatures from -150 °C to 200 °C. The temperature stability is about ±1 °C. The set-up has been used for dielectric evaluation of DuPont High Dielectric Constant films. The dielectric permittivity and the DC conductivity of the DuPont 3579 high-k material have been evaluated and a summary report (6 pages) has been transmitted to DuPont and IBM. IBM plans to use the DuPont materials in new chip carrier applications.
Outputs
Publications
J. Obrzut, and Miguel Jimarez, Plastic Flip-Chip-BGA Carrier with Microvias for Chip Scale Packaging, SMTA, International Conference on Electronic Assembly, Atlanta, GA, June 16-18, 1998, pp. 31 - 38.
J. Obrzut, Interconnection Continuity Test for Packaged, Functional Modules, Proceedings of the 1998 International Conference on Characterization and Metrology of ULSI Technology, in press.
M. A. Jimarez, L. Li and J. Obrzut, Elastic Thermal Compensation of Chip Carriers, IBM Tech. Discl. Bull., 40(1997)85.
Patents
"Compliant Surface Layer for Flip-Chip Electronic Packages" IBM Docket #EN997150
"UV Absorbing Glass Cloth and Use Thereof" IBM Docket #EN996136
Presentations
J. Obrzut, and M. Jimarez, Plastic Flip-Chip-BGA Carrier with Microvias for Chip Scale Packaging, SMTA, International Conference on Electronic Assembly, Atlanta, GA, June 16, 1998.
M. Jimarez, C. Tytran, C. Loveland, and J. Obrzut, Technical Evaluation of CPS-BGA Carrier, 48th Electronic Components & Technology Conference, CPMT/IEEE, Seattle, WA, May 25, 1998.
J. Obrzut, Interconnection Continuity Test for Packaged, Functional Modules, 1998 International Conference on Characterization and Metrology of ULSI Technology, NIST Gaithersburg, MD, March 25, 1998.
J. Obrzut, Prototyping Electrooptic Devices using Polymer Waveguides, MRS Fall Meeting, Boston, MA, Dec. 3, 1997.
Miscellaneous
Membership on the IPC High Density Interconnect Task Force Committee.
Contributions to the following, new standard test methods:
Dielectric Measurements of Thin Films from DC to 1 GHZ
F. I. Mopsik
Objective
To establish measurement protocols and sample configurations for the accurate determination of dielectric constant and loss for frequencies from DC to 1 GHz suitable for thin polymeric films (> 5 µm) of importance to the electronic packaging industry.
Technical Description
Inductance-Capacitance-Resistance Bridges (LCR meters) are designed for accurate measurements over discrete ranges of frequencies with constraints on the sample configuration. 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 developed to optimize the measurements.
Planned Outcomes
We expect to develop a sample configuration and measurement protocol for the accurate measurement of dielectric constant and loss of thin films(> 5 µm) for frequencies approaching 1 GHz. The method should provide consistent results for both producers and users of polymeric resins used in electronic packaging. Current standards are primarily in the lower frequency ranges.
Accomplishments
A coaxial holder was constructed, allowing a single sample to be measured with accuracy better than 1 % from 1 kHz to 30 MHz, 1 % up to 500 MHz and less than 5 % to 1 GHz., as measured on a 25 (m thick polypropylene film. The film was metallized on one side and on a central 5 mm spot on the other, using a simple punch and mask. Reproducibility from sample to sample is within 1 %. This includes all sources, including sample variation, variations in metallization geometry and sample insertion.
The residuals of the holder have been determined with the main contribution being 3 pF from the coaxial adapter and line. The fringing fields for the sample and holder were evaluated and shown to be small. The uncertainty in this evaluation was shown to be less than the 1 % goal for the total measurement uncertainty. When this is complete, a test method will be proposed that will be capable of good reproducibility, limited mainly by determination of sample thickness at the 10 µm level.
Commercial capacitor-grade polypropylene film was acquired and initial measurements were made in the sample holder and the thin film two-fluid cell. Also, thickness determinations were made both by the thin film capacitance cell and a precision micrometer assembly. There was some discrepancy that has to be evaluated so that reliable thickness data can be used for data reduction.
Outputs
Presentations F. I. Mopsik, Extended Frequency Range Dielectric Measurements of Thin Films, 1998 International on Characterization and Metrology of ULSI Technology, Gaithersburg, MD, March 25, 1998.
Measurement of the Thermal Properties of Polymer Thin Films
A. S. DeReggi, Peter Bloss, Chad Snyder
Objective
Improve the methodology for measuring the thermal properties of substrate-supported and free-standing polymer thin films (>5 µm ), important for in-situ characterization of the thermal performance of materials used in microelectronics.
Technical Description
Laser pulse heating of the surface of an electroded and voltage-biased polymer film generates a transient pyroelectric-like response as the polymer first expands inhomogeneously and then contracts to its original thickness as it loses heat to the substrate (or to the surrounding air for the free-standing film) . The response depends on the bias voltage, temperature profile (which depends on the thermal properties of the polymer and time), and the electric field profile produced by the bias voltage (for nonpolar polymers) or by pre-existing charges and dipoles (for general or specially treated polymers). Theoretical modeling of the problem and deconvolution of the measured response using advanced numerical techniques yield the thermal properties and the electric field profile.
External Collaborations
Dr. Mel Zussman of Dupont provided test materials and material data. Dr. Thomas Avedesian of Cornell/SRC collaborated in developing alternative test methods and reference samples.
Dr. Eberhard Hartman of Institute of Surface Modification at Leipzig provided test samples and conducted complementary measurements that resulted in one joint publication.
Dr. Gerhard Sessler of Technical University at Darmstadt collaborated in developing alternative test methods and this work has resulted in one joint publication.
Planned Outcomes
New metrology for measuring thin film thermal and electrical properties.
Development of superior reference materials in the form of thin films for thermal and electrical properties.
Reliable thermal and collateral electrical data on polymeric microelectronics packaging materials.
Accomplishments Noise-subtraction techniques have allowed signal acquisition to begin at 10-7 s, the nominal end of the laser pulse, and consequently, resolution of electrical features within 0.1 micrometers of the incident surface.
A surface-bound negative charge layers in polyimide residing within 0.1 µm of the surface was discovered. These charges are present in polyimide specimens not previously subjected to an applied voltage. The electric field associated with these charges is not switchable by a normal bias voltage of either polarity. They are stable against heating of the specimen up to about 200 °C. These charge layers are significant because the electrical properties of thin films will become increasingly surface-sensitive as their thicknesses are decreased to the point where they can no longer be considered as large compared to the thickness of the charge layers.
The electric field associated with these charge layers may affect the operation of field-effect transistors and charged coupled devices and may raise or lower the dielectric breakdown voltage depending on the direction of the local field due to designed operating voltages between conductors in close proximity.
Preliminary surface-sensitive thermal pulse measurements were conducted in polyimide with thicknesses as small as 1.7 micrometers and found a decrease in thermal conductivity as compared to the value for bulk samples.
A steady state heat flow measurement apparatus to determine the thermal conductivity of thin films averaged over their thickness was designed and is under construction. Measurements obtained on the same samples with this apparatus and with the thermal pulse method will allow estimates to be made of the variation of thermal properties near the surface.
Charge distribution measurements were made on electron-beam-irradiated fluoroethylene propylene by using thermal pulse and acoustic pulse methods. This material is being considered as a reference material for the measurement of long-term (> 10 y) trapped-charge stability.
Poling effects in electron-beam-cured nanocomposites of powdered ferroelectric barium titanate dispersed in the polar binder tripropylene glycol diacrylate were observed. These nanocomposites are materials with a high dielectric permittivity with microelectronics application as the dielectric of integrated capacitors. The thermal pulse methods provides detailed information about the polarization profile and thus indirectly about the homogeneity of the dispersion.
Outputs
Publications
Aime S. DeReggi, Space charge characterization for the 21st century: from high voltage cables to microelectronics, Proc. 3rd Int. Conf. on Electric Charge in Solid Insulators, published in Le Vide: Science, Technique, et Applications, 287 Supp., 165-174 (1998).
Peter Bloss, Aime S. DeReggi, and Hartmut Schafer, Electric field profile and thermal properties in substrate-supported dielectric films, Phys. Rev. B, submitted.
Peter Bloss, and Aime S. DeReggi, Electrode thermal mass effects in thin film thermal pulse measurements, IEEE Trans. on Dielectrics. and Electr. Insul., submitted.
Peter Bloss, and Aime S. DeReggi, Electric field profiles in 1.7 micrometwer polyimide using improved thermal pulse method: Influence of water uptake, IEEE Trans. on Dielectrics and Electr. Insul., submitted.
Peter Bloss, Aime S. DeReggi, Hans-Jurgen Glasel, and Eberhard Hartmann, Thermal pulse investigation of polarization distribution in ceramic-polymer nanocomposites, Proc. World Ceramics Congress and Forum on New Materials (CIMTEC '98), in press.
P. Bloss, A. S. DeReggi, G.-M. Yang, G.M. Sessler, and H. Schafer, Thermal and acoustic pulse studies of space charge profiles in electron-irradiated fluoroethylene propylene, 1998 Annual Report on Conf. on Electr. Insul. and Diel. Phenom., in press.
Surface Roughness as Measured by Total-reflection X-ray Fluorescence
Wen-li Wu, William E. Wallace
Objective Develop metrology for measuring thin film surface roughness and planarity, which are important parameters for interlevel dielectric process control in microelectronics manufacture.
Technical Description Total-reflection x-ray fluorescence is employed. This is a technique which has been used in the semiconductor industry to measure wafer metal contamination before processing, (a so-called front-end of fabrication line measurement). Application of this technique to the determination of low-k dielectric (e.g. polymer) thin film roughness (e.g. planarity over metal topography) would extend the use of the technique to the back-end of the fabrication line. Industry would gain a new use for an existing tool. At present time there is no suitable non-contact technique for measuring dielectric film roughness when the film materials are transparent, as is the case in most instances.
Accomplishments Experiments have been performed using the new Philips x-ray reflectometer on phase-separated polystyrene/poly(vinyl methyl ether) thin films. These results were compared to atomic force microscope measurements. Indications are that that TXRF is sensitive to low amplitude (~10 nm RMS) but long wavelength (~0.1 µm) roughness. Most scanning techniques, such as AFM, have difficulty measuring long wavelength roughness especially if the wavelength is on the order of the maximum scan length. Thus, TXRF, a technique already familiar to industry, obtains a new application as a technique for roughness measurement.
Output
Publications
W. L. Wu, and W. E. Wallace, Microroughness of Thin Polymer Coatings Studied by Total External Reflection X-ray Fluorescence, Journal of Vacuum Science and Technology B 16(1998)1958.
W. L. Wu and W. E. Wallace, Characterization of Planarity of Polymer Thin Films on Rough Surfaces, Proceedings of the SPIE 3426(1998)222.
Polymer Precursors for Inorganic Low-k Dielectrics for Integrated Circuitry
William E. Wallace
Objective
Provide thin film measurement support to an industrially-led initiative to develop a novel polysilsesquioxane spin-on-glass.
Technical Description
Polysilsesquioxanes are a type of silicon-oxygen polymer that can readily be converted to silica, or organically-modified silica, by the application of heat or ultraviolet radiation. The polysilsesquioxane precursor must be stable enough to allow for room temperature processing (e.g. spin coating) but reactive enough to form an insoluble monolith with a modest heating or radiation schedule. For this reason many types of polysilsesquioxanes are being studied by a host of industrial concerns. Two of the major properties of the final monolith that affect it performance in service are density and composition, chiefly residual carbon content. For example, these two properties go a long way toward determining the dielectric constant for the final spin-on-glass thin film. Dielectric constant of the insulating material is an important design parameter in very large scale integration circuitry and of great concern to the microelectronics industry. NIST provides accurate thin film density measurements, a key metric in determining film performance.
External Collaborations
Barry Arkles of Gelest, Inc. has been collaborating on the preparation of monomers and the synthesis of novel polysilsesquioxanes.
Accomplishments
The density and composition of polysilsesquioxane-derived silica thin films processed with UV light were measured by energy-dispersive x-ray reflectivity and ion scattering respectively. Exposure to 12 min of intense UV light was found to produce the same level of conversion to organically-modified silica of essentially the same composition as heating at 350 °C for 4 h. Such a drastic reduction in processing time is of great interest to industry. Conversion by UV light also allows for the possibility of patterning the dielectric, a necessary step for damascene processing the new lithographic technique for ultra-large scale integration processing.
Work also continued on improved methods to more accurately measure thin film density because, generally speaking, density is the underlying parameter that controls many thin film properties of interest for glassy materials, for example, dielectric constant. Polystyrene continued to be used as the example material.
Outputs
Publications
Q. Pan, G. B. Gonzales, R. J. Composto, W. E. Wallace, B. Arkles, L. K. Figge, and D. H. Berry, Spin-on-Glass Thin Films Prepared form a Novel Polysilsesquioxane by Thermal and Ultraviolet-Irradiation Methods, Thin Solid Films, acccepted.
E. K. Lin, C. R. Snyder, F. I. Mopsik, W. E. Wallace, W. L. Wu, C. X. Zhang, and R. M. Laine, Characterization of Epoxy-Functionalized Silsesquioxanes as Potential Underfill Encapsulants, Materials Research Society Proceedings, in press.
W. E. Wallace, C. K. Chiang, and W. L. Wu, Energy-Dispersive X-ray Reflectivity and the Measurement of Thin Film Density for Interlevel Dielectric Optimization, Proceedings of the 1998 International Conference on Characterization and Metrology for ULSI Technology, in press.
W. E. Wallace, N. C. Beck Tan, S. Satija, and W. L. Wu, Mass Density of Polystyrene Thin Films Measured by Twin Neutron Reflectivity, Journal of Chemical Physics 108, 3798 (1998).
N. C. Beck Tan, W. L. Wu, W. E. Wallace, and G. T. Davis, Interface Effects on Moisture Absorption in Ultrathin Polyimide Films, Journal of Polymer Science B: Polymer Physics 36, 155 (1998).
Presentations
W. E. Wallace, Mass Density of Polystyrene Thin Films, American Physical Society March Meeting, Los Angeles, CA, March 1998.
W. E. Wallace, Measurement of Thin Film Thermomechanical Properties for Polymer Interlevel Dielectric Optimization, 1998 International Conference on Characterization and Metrology for ULSI Technology, Gaithersburg, MD, March 25, 1998.
Confinement Effects on Polymer Thin Film Tg and Morphology
Darrin Pochan, Eric Lin, Wen-li Wu
Objective
To determine the effects of polymer/solid substrate interaction and the presence of a polymer free surface on thin film properties such as the coefficient of thermal expansion (CTE), glass transition temperatures Tg and crystalline structure using x-ray and neutron reflectivity and diffraction. Both semicrystalline and amorphous polymers were included in this work.
Technical Description
Thin film thermal expansion/glass transition: The thermal expansion as a function of film thickness for thin deuterated polystyrene films is measured via neutron reflectometry. In addition, measurements are performed on samples with a free surface and subsequently on the same samples without free surfaces by capping them with a high Tg polymer confinement layer. Two confinement materials are used, bisphenol-A polycarbonate (Tg=150 °C) and a unique, soluble polyimide (Tg=300 °C) synthesized from 2,2'-bis(3,4-dicarboxyphenylhexafluoropropane dianhydride and 2,2'-(trifluoromethyl 4,4'-biphenyldiamine). The polymer layer to be studied, deuterated polystyrene, has a repulsive segmental interaction energy with both of the above materials thereby focusing on confinement as the root cause of any unique thermal behavior. Above film thicknesses (d) of 80.0 nm the thin films were found to exhibit bulk thermal expansion behavior and a transition from the bulk glass CTE to bulk rubber CTE at approximately 100 °C. In films with 40.0 nm. > d > 70.0 nm a transition was found to occur between capped and uncapped samples. Uncapped d-PS layers were again found to exhibit bulk thermal expansion/Tg. However, after capping the glassy CTE persisted 20-40 °C above the bulk Tg of 100 °C while the rubber CTE was found to be lower than the bulk rubber CTE. Film thicknesses > 35.0 nm were found to have glassy thermal expansion throughout the temperature range studied in both the uncapped and capped geometries. Therefore, only in the transition region of thickness (40.0 nm. > d > 70.0 nm and 3-5 Rg) is the effect of the free surface noticeable (a transition from bulk-like behavior with the free surface to a glassy state without the free surface). Below this transition region, the material expands as a glass throughout the temperature range studied (<180 °c)<>
Thin film crystallization: Initial neutron and x-ray reflectivity and diffraction experiments are carried out in an attempt to discern the effects of confinement in a thin-film geometry on the morphology and phase behavior of semi-crystalline polymers. Deuterated polypropylene was chosen as the model material. Neutron diffraction in transmission mode and synchrotron diffraction in reflection mode revealed strong orientation of crystallites with a monoclinic unit cell within the plane of the film. Neutron Reflectivity performed through the silicon substrate revealed a homogeneous polymer sublayer of constant density next to the substrate. Similar levels of crystallinity are observed in both as-cast and melt/slow cooled films despite gross differences in superstructure as determined via AFM measurements.
External Collaborations
Rainer Kolb of Exxon provided synchrotron beam time and performed experiments, including WAXS and SAXS, on thin semi-crystalline films.
Steven Z.D. Cheng of University of Akron provided high Tg soluble polyimide for confinement material.
Ron Trolard of Cambridge Isotope Laboratories provided detailed molecular analysis of deuterated semi-crystalline polymers, and custom synthesis of other deuterated polymers.
Planned Outcome
The thermomechanical behavior of thin, amorphous polymer films in the film thickness region of 40 nm > d > 70 nm will be focussed on to confirm the transition from bulk-like to completely glassy thermal expansion. Also, the interfacial energy of the substrate and capping materials will be altered in order to monitor its effect on the entire film's thermomechanical properties.
The unit cell, crystalline orientation and melting point of semi-crystalline polymers with a depth resolution of few nanometers will be determined. The emphasis will be placed in the region next to the substrates.
Accomplishments
Polystyrene: The effect of a free surface on thin film properties was determined using polystyrene as a model material. Thickness limits were determined. Above one limit the films exhibit bulk thermal expansion (90 nm) while below a second limit below glassy behavior persists throughout temperature range studied (30nm).
Transition region was mapped out in which free surface results in bulk-like thermal behavior. After capping the film behaves like a glass to temperatures ~20-40 °C above the bulk Tg and exhibits a CTE less than the bulk rubbery CTE.
Polypropylene: Unit cell of deuterated polypropylene ultra thin films was determined to be monoclinic alpha type. Diffraction studies suggest strong orientation of the c axis within the plane of the films.
A sublayer adjacent to the silicon substrate was observed via neutron reflectivity; this sublayer was found to have a density different from the rest of the polypropylene thin film.
Outputs
Presentations
D. J. Pochan, E. K. Lin, R. Kolb, W-L Wu, and S. Satija, Neutron Reflectometry for Interfacial Materials Characterization, 1998 International Conference on Characterization and Metrology for ULSI Technology, NIST, Gaithersburg, MD., March 23, 1998.
D. J. Pochan, E. K. Lin, W-L Wu, and S. Satija, Confinement Effects on the Thermal Expansion and Glass Transition Behavior of Thin Polymer Films Sandwiched between Polymer Layers, American Physical Society Meeting, Los Angeles, CA., March 16,1998.
D. J. Pochan, E. K. Lin, W-L Wu, and S. Satija, Confinement Effects on the Thermal Expansion and Glass Transition Behavior of Thin Polymer Films Sandwiched between Polymer Layers, American Chemical Society Meeting, Boston, MA., August 24, 1998.
Polymer Ultra-Thin Films - CTE and Viscoelastic Properties
Chris White, William E. Wallace, Wen-li Wu
Objective
Develop a dual quartz resonator technique for measuring the viscoelastic properties of polymer films as thin as several hundred Angstroms. Thin film viscoelastic properties and their temperature dependence will further our understanding of the fundamentals of polymer at interface. Determining the anomalous thin film thermomechanical properties is necessary to the design and implementation of novel thin film organic layers in modern microelectronics.
Technical Description
Development of a instrumental design for quantitative measurement of the viscoelastic properties of thin polymer films has not been possible in the past due to the lack of relevant working equations for this condition. The two widely used simplifying limits of the force balance equation, surface loading and gap loading are not applicable for ultra-thin polymer films. New working equations have been developed that do not rely on either of these simplifying limits. With this development, an instrument based on resonating quartz crystals has been designed. With an ultra-thin polymer layer attached, the resonance condition of the quartz crystal is shifted slightly and the width of the frequency peak is also broadened. At temperatures below Tg , the rigid glassy polymer film, moves in phase and with similar amplitude to the quartz driving surface. As the temperature is raised through the Tg of the polymer, the polymer film becomes more lossy and begins to move with a motion that is out of phase and with a different amplitude than that of the quartz driving surface. These changes in properties are measured by determining the resonance frequency of the quartz crystal and broadening of the resonance frequency peak.
External Collaboration
Barry Lucas, Nanoprobe Corporation, will conduct joint measurement and analysis of thin polymer films using nano-indentation techniques.
Planned Outcome
A state-of-the-art and high precision technique for measuring the viscoelastic properties of polymer films (< 1 omega (cap)m).
Accomplishments
A dual quartz crystal resonator has been designed, constructed, and tested. It currently has a dynamic range of 12 kHz-500kHz. It has temperature control of better than 0.1°C within a range of 25-200 °C. The measurement precision has been determined to be better than 1 part in 108 for the resonance frequency measurement and slightly lower precision for the resonance peak width measurement.
Films of 570 nm and 370 nm thick have been measured. They both show significant temperature dependence in resonance frequency shift and peak width. These are the first measurements of the viscoelastic properties of ultra-thin polymer films.
Output
Publications
C. C. White, and W. L. Wu, A Novel Method to Determine the Mechanical Properties of Ultra-Thin Films, Proceedings of the Materials Research Society Spring 1998 meeting, San Francisco, Ca., in press.
Presentations
C. C. White, and W. L. Wu, A Novel Method to Determine the Mechanical Properties of Ultra-Thin Films, Materials Research Society Meeting, San Francisco, CA., April 15, 1998.
C. C. White, and W. L. Wu, Determining The Viscoelastic Properties Of Ultra-Thin Polymer Films, 1998 Gordon Research Conference on Thin Film Mechanical Behavior, Plymouth, New Hampshire, June 24, 1998
C. C. White, and W. L. Wu, Determining The Viscoelastic Properties Of Ultra-Thin Polymer Films, American Chemical Society Meeting, Boston, MA, August 23, 1998.
Measuring Polymer Mobility near Solid Surfaces
Eric K. Lin, Rainer Kolb, Wen-li Wu
Objective
To gain a fundamental understanding of the effect of molecular size, surface interaction energy, and mechanical deformation on the mobility of polymer chains near the polymer/solid interface. This information is critical for understanding important physical properties such as the glass transition temperature of polymer chains in confined geometries, the dynamics of adhesion between polymer layers, and the effect of flow processing on entangled polymers at the polymer/solid interface.
Technical Description
The mobility of polymer chains over distances comparable to the size of the polymer chain is measured near the polymer/solid interface using neutron reflectometry which monitors the rate of interdiffusion between layers of deuterated poly(methyl methacrylate) (d-PMMA) and hydrogenated PMMA supported on polished silicon wafers. The thickness of the d-PMMA layer located next to the solid substrate is varied to investigate changes in polymer mobility as a function of distance from the substrate. Molecular size effects were investigated using different molecular weight pairs of d-PMMA and h-PMMA. The surface interaction energy between d-PMMA and the silicon surface was varied chemically through the use of different silane coupling agents. The effect of a shear deformation on the surface adsorption/desorption kinetics of an entangled polymer melt was measured in a newly constructed shearing device.
Accomplishments
Polymer mobility from different molecular weight pairs near an attractive solid substrate is found to dramatically decrease for polymer chains located less than 3 Rg (radius-of-gyration) from the surface. No changes in polymer mobility are observed at distances more than 5 Rg from the solid substrate.
Decreases in the polymer chain mobility are observed near the substrate surface for various substrate surface energies ranging from hydrophilic to hydrophobic.
Shear-induced polymer desorption from an attractive solid substrate is observed for the first time in a highly entangled polymer melt. The rate of desorption increases with increasing shear rate.
Output
Publications
E. K. Lin, R. Kolb, W. L. Wu, and and S. K. Satija, Shear Induced Polymer Desorption at the Melt/Solid Interface, Macromolecules, submitted.
E. K. Lin, W. L. Wu, and S. K. Satija, Polymer Mobility near the Melt/Solid Interface, Macromolecules, submitted.
E. K. Lin, d. J. Pochan, R. Kolb, W. L. Wu, and S. K. Satija, Neutron Reflectometry for Interfacial Materials Characterization, Proc. 1998 International Conference on Characterization and Metrology for ULSI Technology, 1998, in press.
Presentations
E. K. Lin, R. Kolb, W. L. Wu, and S. K. Satija, Shear-induced Polymer Melt Desorption, Gordon Conference: Polymer Physics, Newport, RI, August 17, 1998.
W. L. Wu, E. K. Lin, R. Kolb, S. K. Satija, Polymer Desorption at the Melt/Solid Interface after a Shear Deformation, American Physical Society, Los Angeles, CA, March 19, 1998.
R. Kolb, E. K. Lin, W. L. Wu, and S. K. Satija, Polymer Interdiffusion near Varying Solid Surfaces, American Physical Society, Los Angeles, CA, March 18, 1998.
W. L. Wu, E. K. Lin, R. Kolb, and S. K. Satija, Polymer Interdiffusion near Attractive Walls, Materials Research Society, San Francisco, CA, December 7, 1997.
E. K. Lin, R. Kolb, W. L. Wu, and S. K. Satija, Polymer Interdiffusion at the Melt/Solid Interface, American Institute of Chemical Engineers, Los Angeles, CA, November 18, 1997.
Novel Underfill Materials and Underfill Flow Modeling
Eric K. Lin, William E. Wallace, Wen-li Wu
Objective
To evaluate physical properties of novel epoxide functionalized silsesquioxane cubes critical to their application as potential underfill materials in flip-chip packaging. To develop the Lattice Boltzmann method as a new tool to model the complex multiphase underfill flow process.
Technical Description
Underfill materials are used to improve device reliability in flip-chip electronic packages by reducing stresses due to differences in the coefficients of thermal expansion between the silicon die and organic substrates. Current formulations are multiphase fluids consisting of low viscosity epoxy resins highly filled with silica particles. The underfill material is flowed under capillary action into the narrow gap between the chip and the substrate and in between the solder bump interconnects. The underfill materials exhibit complicated rheology and complex formulations due to the requirement of a dense suspension of silica particles. Inorganic/organic hybrid materials such as epoxy functionalized silsesquioxane cubes may provide a single phase fluid alternative to current underfill formulations. The critical physical properties of the hybrid material to be evaluated include the coefficient of thermal expansion of the cured material, viscosity, and glass transition temperature. These properties are measured with a variety of methods including X-ray reflectometry, differential scanning calorimetry, and rheometry.
Since the rheology and composition of current underfill materials are complex, computer modeling of underfill flow has been difficult to achieve because of the high filler content, the presence of a free surface, and the complex geometry of the flow volume with surfaces with different wetting characteristics. A working flow model is helpful in determining metrics to apply to new underfill formulations and to evaluate innovative underfill dispension technologies. The Lattice Boltzmann method is a new flow modeling technique which can model complex flow problems by solving a discretized version of the Boltzmann kinetic equations. The advantages of the technique include a straightforward inclusion of multiphase flow in complex geometries and easily parallelizable coding and has been successfully applied to many other technologically important problems such as multiphase flow through porous media. Here, the Lattice Boltzmann method is developed for application to the underfill flow process.
External Collaborations
Professor Richard M. Laine and Chunxin Zhang from the University of Michigan-Ann Arbor collaborated by preparing both tetra- and octa-epoxide functionalized silsesquioxane cube materials.
Dr. Michael A. A. Spaid from Corning, Inc. collaborated in code development of the Lattice Boltzmann method.
Accomplishments
Thin films of the tetra- and octa-epoxide functionalized silsesquioxane cubes cured with structurally different diamines were successfully prepared. The glass transition temperature of the cured epoxy-functionalized cube material is found to be more than 200 °C, a necessary condition for a useful underfill material.
The coefficient of thermal expansion is found to strongly depend upon the structure of the curing agent. Tailoring the thermal expansion is critical for a successful underfill material.
A working three dimensional multi-phase code using the Lattice Boltzmann method has been successfully developed and applied to model test problems.
Outputs
Publications
E. K. Lin, C. R. Snyder, F. I. Mopsik, W. E. Wallace, W. L., Wu, C. W. Zhang, and R. M. Laine, Characterization of Epoxy-Functionalized Silsesquioxanes as Potential Underfill Encapsulants, in Organic/Inorganic Hybrid Materials, ed. by R. M. Laine, C. Sanchez, E. Giannelis, and C. J. Brinker, Mat. Res. Soc. Proc., San Francisco, CA, in press.
Presentations
E. K. Lin, C. R. Snyder, F. I. Mopsik, W. E. Wallace, W. L., Wu, C. X. Zhang, and R. M. Laine, Characterization of Epoxy-functionalized Silsesquioxanes as Potential Underfill Encapsulants, Materials Research Society, San Francisco, CA, April 13, 1998.
Residual Stress in Polymer Films Adhered to Silicon Substrates
C. K. Chiang and A. S. DeReggi
Objective
To develop a test method based on a residual stress measurement for determining coefficient of thermal expansion and in-plane elastic modulus of thin polymer coatings.
Technical Description
Differences in coefficient of thermal expansion, CTE, between polymer coating and substrate lead to residual stress during a temperature excursion. For example, there always exists a tensile residual stress in polymer coatings prepared at elevated temperatures onto a low CTE substrate such as silicon wafer. Provided the elastic constants of the wafer are known quantities the bending or the warpage of the wafer can be used to determine residual stress. Depending on the substrate CTE and elastic modulus the residual stress of a polymer coating can be markedly different. By measuring the residual stress of a pair of identically prepared thin coatings on two different substrates with known properties, the CTE and elastic modulus of the coating can be deduced. By conducting this measurement at different temperatures one can measure the temperature dependence of CTE and elastic modulus. The above mentioned procedure was first used for investigating the properties of refractory films by Retajczyk and Sinha, ( Appl. Phys. Lett. 36, 162 (1980)). In our work silicon wafer and GaAs wafer were chosen as the substrates because of their wide usage in electronic industries.
External Collaborators
Dr. David Babb of Dow Chemical Company provided data on the thermal and mechanical properties of spin-on polymer films.
Dr. David Feiler and Dr. Wei Xia of Rockwell Semiconductor System provided test samples and their properties of spin-on low-k dielectric materials.
Accomplishments
A modification of a Tencor residual stress analyzer, an instrument commonly used in most microelectronic companies, resulted in a significant reduction of thermal noise. For example, our data collected at 180 °C showed that the random noise level was reduced by a factor of four with our modification. A reduction in thermal noise significantly improves the accuracy of stress measurement, and hence the accuracy of CTE and modulus values of the polymer coatings or films.
Output
Presentation
C. K. Chiang, and Aime S. DeReggi, Measurement of In-Plane CTE and Elastic Modulus of Polymer Thin-Films Using Bending Plate Technique, 1998 International Conference on Characterization and Metrology for ULSI Technology, Gaithersburg, MD, March 25, 1998.
