Frank D. Egitto

Plasma modification of polymer surfaces for adhesion improvement

Polymers have wide-ranging applications in food packaging and decorative products, and as insulation for electronic devices. For these applications, the adhesion of materials deposited onto polymer substrates is of primary importance. Not all polymer surfaces possess the required physical and/or chemical properties for good adhesion. Plasma treatment is one means of modifying polymer surfaces to improve adhesion while maintaining the desirable properties of the bulk material. This paper addresses the interaction of organic surfaces with the various components of a plasma, with examples taken from a review of the pertinent literature.

Plasma etching and modification of organic polymers

Etching and modification of polymers by plasmas is discussed in terms of the roles played by atomic and molecular oxygen, atomic fluorine, CFx radicals, ions, high energy metastable species, and photons. Addition of fluorine-containing gases to oxygen can increase both 0 atom densities in the plasma and polymer etching rates. The etching rate be- havior generally exhibits a maximum at a specific concentration of this additive. Process parameters which alter the concentrations of 0 and F atoms in the plasma or affect the rate of delivery of these species to the polymer surface shift the position of this maximum with respect to feed gas composition. However, the gas composition which yields maximum rates exhibits a strong dependence on polymer structure, specifically, its degree of unsaturation. This is explained on the basis of molecular orbital (MO) arguments which predict that the surfaces of unsaturated polymers have a higher affinity than saturated polymer surfaces for atomic fluorine. Favored reaction pathways leading to volatile etching products are pro- posed based on MO calculations of relative bond strengths for various oxygenated and fluorinated organic model compounds. Although fluorine abstraction of hydrogen plays a major role in generating radical sites on saturated polymer surfaces, it is likely that etching of unsaturated moieties proceeds through a saturated radical intermediate resulting from addition reactions of fluorine atoms. Excessive amounts of fluorine in the plasma result in reduced etching rates and incorporation of fluorine and/or CFx radicals into the polymer. Polymer film surfaces are also modified by high energy metastables and ultraviolet radiation generated from noble gas plasmas, The effect of vacuum ultraviolet radiation from helium microwave plasmas on films of polytetrafluoroethylene and polyethylene is addressed.

Oxygen plasma modification of polyimide webs: effect of ion bombardment on metal adhesion

Webs of Kapton 200-H and Upilex-S polyimide films were treated using oxygen plasma prior to sequential sputter deposition of chromium and copper in a roll metallization system. Two plasma system configurations were employed for treatment. In one configuration, the sample traveled downstream from a microwave plasma; in the other, the web moved through a DC-generated glow discharge. For the DC-glow treatment, the potential difference between the plasma and the web, Φf, and relative ion densities, n+, were measured at various values of chamber pressure and DC power using a Langmuir probe. Although samples treated downstream from the microwave plasma were not subjected to bombardment by energetic ions, Φf for the DC-glow operating conditions was between 5 and 13 eV. For both films, advancing DI water contact angles of less than 20° were achieved using both modes of treatment. Contact angles for untreated films were greater than 60°. However, 90° peel tests yielded values of 15 to 20 g/mm for microwave plasma ...

Surface composition and distribution of fluorine in plasma‐fluorinated polyimide

Surface composition, fluorine distribution, and morphology were determined for polyimide films modified downstream from microwave plasmas containing CF4/O2. Complementary analytical techniques including x‐ray photoelectron spectroscopy, Rutherford backscattering spectroscopy, and scanning electron microscopy yielded a more complete understanding of polyimide fluorination and subsequent etching of the modified film. Depth of fluorination increased nonlinearly with treatment time for films exposed downstream from a CF4‐rich plasma. Exposure downstream from an O2‐rich plasma resulted in a reduction of thickness in both the fluorinated layer and the unmodified polyimide during etching. Finally, a model for fluorination of polyimide and subsequent removal is proposed.

Nano‐ and micro‐filled conducting adhesives for z‐axis interconnections: new direction for high‐speed, high‐density, organic microelectronics packaging

Purpose – The purpose of this paper is to discuss the use of epoxy‐based conducting adhesives in z‐axis interconnections.Design/methodology/approach – A variety of conductive adhesives with particle sizes ranging from 80 nm to 15 μm were laminated into printed wiring board substrates. SEM and optical microscopy were used to investigate the micro‐structures, conducting mechanism and path. The mechanical strength of the various adhesives was characterized by 90° peel test and measurement of tensile strength. Reliability of the adhesives was ascertained by IR‐reflow, thermal cycling, pressure cooker test (PCT), and solder shock. Change in tensile strength of adhesives was within 10 percent after 1,000 cycles of deep thermal cycling (DTC) between −55 and 125°C.Findings – The volume resistivity of copper, silver and low‐melting point (LMP) alloy based paste were 5 × 10−4, 5 × 10−5 and 2 × 10−5 Ω cm, respectively. Volume resistivity decreased with increasing curing temperature. Adhesives exhibited peel strength...

Z-Axis Interconnection for Enhanced Wiring in Organic Laminate Electronic Packages

Greater I/O density at the die level, coupled with more demanding performance requirements, is driving the need for improved wiring density and a concomitant reduction in feature sizes for electronic packages. Traditionally, greater wiring densities are achieved by reducing the dimensions of vias, lines, and spaces, increasing the number of wiring layers, and utilizing blind and buried vias. However, each of these approaches possesses inherent limitations, for example those related to drilling and plating of high aspect ratio vias, reduced conductance of narrow circuit lines, and increased cost of fabrication related to additional wiring layers. One method of extending wiring density beyond the limits imposed by these approaches is a strategy that allows for metal-to-metal z-axis interconnection of subcomposites during lamination to form a composite structure. Conductive joints can be formed during lamination using an electrically conductive paste. As a result, one is able to fabricate structures with vertically-terminated vias of arbitrary depth. Replacement of conventional plated through holes with vertically-terminated vias opens up additional wiring channels on layers above and below the terminated vias, enables die shrink, and eliminates via stubs which cause reflective signal loss. In addition, parallel lamination of testable subcomposites offers yield improvement, shorter cycle times, and ease of incorporating features conducive to high speed data rates. As a case study, an example of a z-axis interconnect construction for a flip-chip plastic ball grid array package with a 150 m die pad pitch is given. The processes and materials used to achieve smaller feature dimensions, satisfy stringent registration requirements, and achieve robust electrical interconnections are discussed

Removal of Poly(Dimethylsiloxane) Contamination from Silicon Surfaces with UV/Ozone Treatment

UV/ozone cleaning is known to be effective for removing thin organic contaminants, but removal of silicon containing contaminants is questionable. Organo-silicon contaminants, e.g., silicones, can result from a variety of integrated circuit chip and electronic packaging fabrication processes. In this investigation, films of poly(dimethylsiloxane) (PDMS) on silicon substrates, with and without a gold coating, have been used to simulate such contamination up to a thickness of 50 nm. Although treatment consistently reduced the advancing DI water contact angle, in some cases from a value greater than 100° to a value less than 5°, the hydrophilic nature of the treated surfaces was not due to complete contaminant removal, i.e., a significant amount of modified contaminant remained on the surface. High resolution x-ray photoelectron spectroscopy (XPS) in the Si 2 p region suggest that O-Si-C bonds in the siloxane, observed prior to treatment, are converted to SiO x , where x is between 1.6 and 2. The time required to reduce the contact angle to a minimum value was greater for the thicker PDMS film samples. Deflection testing was used to evaluate the adhesion of an epoxybased adhesive to intentionally-contaminated silicon chips, before and after UV/ozone treatment. Although PDMS contamination induced loss of adhesion between the chip and the adhesive, complete conversion to silicon oxides by UV/ozone treatment of contaminants having a thickness of 5.0 nm has been demonstrated to restore adhesion to a value equivalent to that of uncontaminated silicon chip surfaces.

Influence of Nanoparticles, Low Melting Point (LMP) Fillers, and Conducting Polymers on Electrical, Mechanical, and Reliability Performance of Micro-Filled Conducting Adhesives for Z-Axis Interconnections

This paper discusses micro-filled epoxy-based conducting adhesives modified with nanoparticles, conducting polymers, and low melting point (LMP) fillers for z-axis interconnections, especially as they relate to package level fabrication, integration, and reliability. A variety of conducting adhesives with particle sizes ranging from 80 nm to 15 mum were incorporated as interconnects in printed wiring board (PWB) or laminated chip carrier (LCC) substrates. SEM and optical microscopy were used to investigate the micro-structure, and conducting and sintering mechanisms. Volume resistivity of modified adhesives is in the range of 10-5 to 10-6 ohm-cm. Adhesives formulated with a conducting polymer exhibited tensile strength with Goulds JTC-treated Cu ges 3800 PSI, and as low as 1800 PSI for a conducting polymer-LMP based system. There was no delamination of conductive joints after 3X IR-reflow, pressure cooker test (PCT), and solder shock. Among all, the conducting polymer modified micro-filled adhesives showed the highest mechanical strength. The paper also describes a combinatorial approach to the synthesis of LMP coated particles. Several conductive adhesives were used in a z-axis interconnect construction for a laminate chip carrier and printed wiring board (PWB). The present process allows fabrication of z-interconnect conductive joints having diameters in the range of 55-300 microns. The processes and materials used to achieve smaller feature dimensions, satisfy stringent registration requirements, and achieve robust electrical interconnections are discussed.

Electrical conductivity and reliability of nano- and micro-filled conducting adhesives for z-axis interconnections

This paper discusses epoxy-based conducting adhesives for z-axis interconnections. Recent work on adhesives formulated using controlled-sized particles to fill small diameter holes is highlighted, particularly with respect to their integration in laminate chip carrier substrates, and the reliability of the electrically conductive joints formed between the adhesive and metal surfaces. A variety of conductive adhesives with particle sizes ranging from 80 nm to 15 mum were laminated into printed wiring board substrates. SEM and optical microscopy were used to investigate the micro-structures, conducting mechanism and path. Volume resistivity of Cu, Ag and low melting point (LMP) alloy based paste were 5 times 10-4 ohm-cm, 5 times 10-5 ohm-cm, and 2 times 10-5 ohm-cm, respectively. Volume resistivity decreased with increasing curing temperature. The mechanical strength of the various adhesives was characterized by 90 degree peel test and measurement of tensile strength. Adhesives exhibited peel strength with Goulds JTC-treated Cu as high as 2.75 lbs/inch for silver, and as low as 1.00 lb/inch for LMP alloy. Similarly, tensile strength for silver, Cu and LMP alloy was 3370, 2056 and 600 psi, respectively. Reliability of the adhesives was ascertained by IR-reflow, thermal cycling, pressure cooker test (PCT), and solder shock. Change in tensile strength of adhesives was within 10 % after 1000 cycles of deep thermal cycling (DTC) between -55 degC and 125 degC. There was no delamination for silver, copper and LMP alloy samples after 3X IR-reflow, PCT, and solder shock. Among all, silver-based adhesives showed the lowest volume resistivity and highest mechanical strength. It was found that with increasing curing temperature, the volume resistivity of the silver-tilled paste decreased due to sintering of metal particles. Sinterability of silver adhesive was further evaluated using high temperature/pressure lamination, and shows a continuous metallic network when laminated at 365 degC. As a case study, an example of silver-filled conductive adhesives as a z-axis interconnect construction for a flip-chip plastic ball grid array package with a 150 mum die pad pitch is given. This effort is an integrated approach centering on three interrelated fronts: (1) materials development and characterization; (2) fabrication, and (3) integration at the device level

Laser Micromachining of Barium Titanate

This paper discusses laser micromachining of barium titanate (BaTiO3)-polymer nanocomposite thin films. In particular, recent developments on high-capacitance, large-area, thin, flexible, embedded capacitors are highlighted. A variety of barium titanate (BaTiO3)-epoxy polymer nanocomposite-based flexible/rollable thin films ranging from 2 to 25 mum thick were processed on large-area substrates (330 mm times 470 mm, or 495 mm times 607 mm) by liquid coating processes. The electrical performance of composites was characterized by dielectric constant (Dk), capacitance, and dissipation factor (loss) measurements. Nanocomposites provided high capacitance density (10-100 nF/in2) and low loss (0.02-0.04) at 1 MHz. Scanning electron microscopy (SEM) micrographs showed uniform particle distribution in the coatings. Uniform mixing of nanoparticles in the epoxy matrix results in high dielectric (> 3 times 107 V/m) and mechanical strengths (> 3700 PSI). Reliability of the capacitor was ascertained by thermal cycling. Capacitance change was less than 5% after baking at 140degC for 4 h, and 1100 cycles from -55degC to 125degC (deep thermal cycle). A frequency-tripled Nd:YAG laser operating at a wavelength of 355 nm was used for the micromachining study. The micromachining was used to generate arrays of variable-thickness capacitors from the nanocomposites. The resultant thickness of the capacitors depends on the number of laser pulses applied.