Polymer Nanocomposites, Printable and Flexible Technology for Electronic Packaging

Transcription

1 Polymer Nanocomposites, Printable and Flexible Technology for Electronic Packaging Rabindra N. Das, Frank D. Egitto, Bill Wilson, Mark D. Poliks, and Voya R. Markovich Endicott Interconnect Technologies, Inc., 193 Clark Street, Endicott, New York, Telephone No: (67) , Abstract: Printing technologies provide a simple solution to build electronic circuits on o low cost flexible substrates. Nanocomposites will play important role for developing advanced printable and flexile technology. Advanced printing is relatively new technology and need more characterization and optimization for practical applications. In the present paper, we examine the use of nanocomposites or materials in the area of printable and flexible technology. A variety of printable nanomaterials for electronic packaging have been developed. This includes nano capacitors and resistors. Nanocomposites can provide high capacitance densities, ranging from 5nf/inch 2 to 25 nf/inch 2, depending on composition, particle size and film thickness. A variety of printable discrete resistors with different sheet resistances, ranging from 1 ohm to 12 Mohm, processed on large panels (19.5 inches x 24 inches) have been fabricated. Low resistivity nanocomposites, with volume resistivity in the range of 1-4 ohm-cm to 1-6 ohm-cm depending on composition, particle size, and loading can be used as conductive joints for high frequency and high density interconnect applications. The paper also describes a flexible Technology for fine line structures. Several substrates including polyimide, LCP (liquid crystal polymer) were used in flexible technology for a laminate chip carrier and printed wiring board (PWB). The present process allows fabrication of traces having line width narrower thin the range of 14μm. 1. Introduction: There has been increasing interest in the development of electronic circuits on flexible substrates to meet the growing demand for low-cost, large-area, flexible and lightweight devices, such as roll-up displays, e-papers, connectors, and keyboards. Organic/polymer and nanocomposite materials [1, 2] have attracted a lot of attention for building large-area, mechanically flexible electronic devices. These materials are widely pursued since they offer numerous advantages in terms of ease of processing, good compatibility with a variety of substrates, and great opportunity for structural modifications. Organic light-emitting diodes for flat-panel displays appear ready for mass production [3,4], and significant progress has also been made in organic thin-film transistors [5] and solar cells [6,7]. There is also a strong desire to develop new, large-scale advanced materials that can meet the growing demand for miniaturization, high-speed performance, and flexibility for microelectronic products. An effort in this direction is presented in this paper. Figure 1. Overview of some of the potential applications of nanocomposites in microelectronics. This paper addresses the utilization of polymer nanocomposites as it relates to printable and flexible technology for electronic packaging. Printable technologies such as screen-printing, ink-jet printing, and microcontact printing provide a fully-additive, non-contacting deposition method that is suitable for flexible production. The electronic applications of printable, high-performance nanocomposite mateirals such as adhesives (both conductive and non-conductive), interlayer dielectrics (low-k, low loss dielectrics), embedded passives (capacitors, resistors), circuits, etc. are discussed. Also addressed are investigations of printable optically/magnetically active nanocomposite and polymeric materials for fabrication of devices such as inductors, embedded lasers, and optical interconnects. Here a polymer matrix and a range of metal/ceramic fillers with particle size ranging from 1 nm to 1 microns have been used. Addition of different fillers into the polymer matrix controls the overall electrical properties of the composites (Figure 1). For example, addition of zinc oxide nanoparticles into the polymer shows laser-like behavior upon optical pumping, and addition of barium titanate (BaTiO 3 ) nanoparticles results in high capacitance. 2. Results and Discussion: Printable nanocomposites have potential applications at all levels of microelectronics (see Figures 2A-I). Printed features with desired properties, thickness, and tolerance present significant challenges. Embedded resistors, capacitors, and conducting circuit lines can use ink-jet or screen printing for different applications. Figure 2 shows a series of printable and flexible materials. Ink-jet printing is best performed with inks having low viscosity, in the range of

2 7-1 cp. Low viscosity helps to generate nano to submicron thin structures. Screen printing is best performed with higher viscosity pastes (1,-3, cps), and generates 1-25 micron thick features. Figure 2A-D represents features made with screen printing. It can be seen that screen printing can produce line features in the range of 1 microns. Figures 2E-L represent features made with ink-jet printing, showing minimum line feature size in the range of 3 microns (Figures 2I,J), and ~5 microns dot patterns (Figure 2E). Space between two ink-jet printed lines can be reduced to 5 microns (Figure 2F). Figures 2K-L represent ink-jet printing on flexible substrates. A B C D E F G H I J K L Figure 2 : Various printable and flexible materials (A) screen printed capacitors with printed area ((19.5 inch X 24 inch)), (B) enlarged screen print, (C)-(D) screen printed ZnO nanocomposites, (E) ink-jet printed 5 µm dots, (F)-(H) ink-jet printed 75-1 µm lines, (I)-(J) ink-jet printed 25-3 µm lines, and (K)-(L) ink-jet printing on flexible plastics.

3 4 Capacitance (nf)/inch Figure3A: SEM image of BaTiO 3 epoxy nanocomposite Frequency (khz) Figure 3D: Capacitance density as a function of frequency for three different screen printable material compositions fabricated using various ratios of BaTiO 3 and polymer in the nanocomposite. Figure 3B: Schematic presentation for making screen printable thin film embedded capacitors and resistor. Capacitor Figure 3C: Cross-section view of screen printed embedded capacitors. A novel class of polymer nanocomposites which has shown a high dielectric constant, is a BaTiO 3 epoxy nanocomposite. A real challenge in the development of thin film nanocomposites is the incompatibility that exists between the typically hydrophilic nanoparticles and hydrophobic polymer matrix, which leads to nanoparticle agglomeration. As a result inferior coatings with poor performance are obtained. We have identified proper surface treatment that results in excellent dispersability of the nanoparticles and good quality, monolithic coatings when materials are subsequently deposited onto a metallized substrate. Figure 3A show the scanning electron micrographs of nanocomposite thin films. Nano particles formed uniform dispersion in the epoxy matrix. The particles in the epoxy matrix are so intimately compacted that analysis of individual particle is difficult. However, closer observation of the micrographs clearly reveals a uniform distribution of closely packed, well connected particles. These nanocomposites are used to fabricate thin film embedded capacitors. High temperature/pressure lamination was used to embed capacitors in multilayer printed circuit boards. The capacitor fabrication is based on a sequential build-up technology employing a first etched Cu electrode. After patterning of the electrode, the nanocomposite can be deposited and laminated within a printed wiring board (PCB). Nanocomposite can be directly deposited by printing. Figure 3B shows a flow chart for making screen printed discrete embedded capacitors and resistors. Capacitance values are defined by the feature size, thickness and dielectric constant of the polymer-ceramic compositions. Figure 3C shows a representative crosssectional view of screen printed embedded capacitors. Measurement of electrical properties of capacitors fabricated from nanocomposite prints and having areas of ~2-1 mm 2 showed high capacitance density ranging from 5 nf/inch 2 to 25 nf/inch 2, depending on composition, particle size, and thickness of the prints. Thin film capacitors fabricated from 4-6% v/v BaTiO 3 epoxy nanocomposites showed a stable capacitance density in the range of 5-2 nf/inch 2.

4 Measurement of electrical properties of capacitors fabricated from 7% v/v nanocomposite showed capacitance density of about 25 nf/inch 2. Figure 3D shows the room temperature capacitance profile measured at 1kHz - 1MHz for a BaTiO 3 epoxy nanocomposite screen printed film as a typical representative example. It was found that with increasing frequency (1-1 khz), the capacitance density decreased. 7 6 Resistance (ohm) ohm 51 ohm (S) 69 ohm 14 ohm 224 ohm Figure 5: Fine feature circuitization on flexible films having 14 μm lines and spaces. A Temperature (deg. C) Resistance (K-Ohm) k-ohm k-ohm k-ohm Temperature (deg. C) B Figure 4: Change in resistance with Temperature. Nanocomposites are also attractive for resistor applications because variable resistor materials can be formed simply by changing the metal insulator ratio. These compositions, however, have practical advantage only when they are capable of being printed in the internal layers of circuit boards. We have developed various discrete resistors with sheet resistance ranging from 1 ohm to 12 Mohm. Resistors in various ranges offer low temperature processing and resistor materials can be printed in the same internal layer. Representative examples of temperature profiles (25 o C -15 o C) of thin film resistors are shown in Figure 4. The electrical properties of resitors fabricated from epoxy nanocomposites showed a stable resistance over this temperature range. C Figure 6: Cross sections of thin substrates (A) LCP 4 metal layers, (B) Rigid LCP 1 metal layer structures and (C) LCP-PTFE based structures (dark color : PTFE and light color : LCP).

5 In addition, we are developing flexible packages for a variety of applications. Here we discuss several classes of flexible materials that can be used to form high-performance flexible packaging. A variety of materials including polyimide, PTFE, and liquid crystal polymer (LCP) has been used to develop flexible packages. In addition, copper thinner than 5 μm is routinely used, with copper layers as thin as.2 μm used for semi-additive approaches. The use of very thin copper facilitates manufacture of fine-line circuit features, and traces narrower than 14μm have been produced (Figure 5). A smooth copper-polymer interface is ideal for high speed applications and for fine line etching. Selection of an appropriate material provides good copper adhesion to the base film, even after exposure to harsh environmental and processing conditions. We are also investigating flexible packages with embedded passives. A key element of this flexible package is incorporation of integrated decoupling capacitance/resistance layers. One material that meets all of these requirements is a liquid crystal polymer (LCP) dielectric which has a unique combination of features and performance. Due to its design flexibility, lighter weight and especially hermeticity, LCPbased substrates have potential to be a favorable alternative to low temperature co-fired ceramic (LTCC) substrates. In addition, the lower dielectric constant of LCP can reduce crosstalk and noise coupling compared to LTCC. We are investigating multi metal layer flexible structures. Development of Z-interconnect structures with pure LCP or mixed dielectric composites using both LCP and PTFE will be favorable for high-performance hermetic packaging. Figure 6 shows representative cross-sectional views of LCP and LCP-PTFE based structures. LCP and PTFE-based substrates are flexible at lower metal layer count, but become rigid for high layer count structures. Addition of nanoparticles to electrically conductive adhesives (ECAs), used in the fabrication of novel z- interconnect structures [8] reduces sintering temperature without compromising electrical conductivity. Conducting adhesives cured at 2 o C for 2 hours showed low volume resistivity. All silver nano-micro composites showed a resistivity of about 1-4 to 1-6 ohm-cm. Resistance decreases with increasing curing temperature due to sintering of metal particles. Sintering temperature can be greatly reduced when the size of particles is decreased to 1-15 nanometers. Due to the decrease of size, diffusion and growth of the nanoparticles is much easier, and large grains (hundreds/thousand of nanometers) can be produced efficiently. These nano-micro particles diffuse with each other and are gradually sintered by neck-formation (Figure 7) between the adjacent particles during the thermal anneal process on the substrate. The grains of nano-micro particles convert into a continuous surface. Figure 7 shows nano-micro composites sintered within micro-vias of LCP substrates. The ratio of nano to micro particle loading is important for low-temperature sintering. An excess amount of microparticles will prevent low temperature diffusion/sintering. Figure 7: SEM micrographs for the nano-micro filled sintered conducting adhesives Figure 8: Low and high magnification SEM micrograph of sintered adhesive within LCP substrates. Addition of nanoparticles to electrically conductive adhesives (ECAs), used in the fabrication of novel z- interconnect structures [8] reduces sintering temperature without compromising electrical conductivity. Conducting adhesives cured at 2 o C for 2 hours showed low volume

6 resistivity. All silver nano-micro composites showed a resistivity of about 1-4 to 1-6 ohm-cm. Resistance decreases with increasing curing temperature due to sintering of metal particles. Sintering temperature can be greatly reduced when the size of particles is decreased to 1-15 nanometers. Due to the decrease of size, diffusion and growth of the nanoparticles is much easier, and large grains (hundreds/thousand of nanometers) can be produced efficiently. These nano-micro particles diffuse with each other and are gradually sintered by neck-formation (Figure 7) between the adjacent particles during the thermal anneal process on the substrate. The grains of nano-micro particles convert into a continuous surface. Figure 8 shows nano-micro composites sintered within micro-vias of LCP substrates. The ratio of nano to micro particle loading is important for low-temperature sintering. An excess amount of microparticles will prevent low temperature diffusion/sintering. 3. Conclusions In conclusion, polymer nanocomposites can be used to enhance the conductivity of ECAs, form printable integrated resistors with controlled sheet resistance, and form capacitors with high capacitance density. Mixing silver nanoparticles and microparticles has been shown to improve the sintering behavior, and hence the conductivity, of the ECAs. A variety of nanocomposites well suited to fabrication of printed embedded passive components has been developed. These materials enable fine-feature definition with excellent control of layer thickness. The nanocomposites can produce variable resistance, ranging from 1 ohm to 12 Mohm. Collectively, the results suggest that flexible and printable materials may be attractive for a range of applications, not only where flexibility is required, but also in large-area microelectronics such as radiofrequency structures, medical devices, etc. 2 Resistance (Ohm) DL-1 DL-2 LCP Points Figure 9B: Variation of resistance values for a number of resistors on LCP and other substrates (DL-1 and DL-2). The resistance varies from point to point due to differences of resistor area resulting from photoprocessing variation.. Lamination was performed at lower temperatures for DL-1 and DL-2 than for LCP. References Figure 9A : Resistor etch flow chart. Figure 9A shows a general flow chart to make embedded resistors fabricated from a LCP based substrate. The fabricated embedded resistor can also act as a subcomposite and can be laminated with other sub-composites for making high layer count boards with embedded resistors. Resistance values are defined by the feature size. Figure 9A shows a four-layer LCP laminate structure with the resistor foil at each layer. Resistance change for embedded resistors for different materials set is not significant. Resistance on LCP material was comparable when laminated with other low temperature dielectric (Figure 9B). 1. R. H. Friend., R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C.D. D. C.Taliani,, D. Bradley, A..Dos Santos, J. L. Brédas, M. Lögdlund, M. R. Salaneck, Nature, Vol.397,pp A. Lappas, A. Zorko, E. Wortham, R. N. Das, E. P. Giannelis, P. Cevc, and D. Arcon, Chem. Mater.,Vol. 17, pp , W. E. Howard, Sci. Am.Vol.29, pp.76-81, H. Sirringhuas, N. Tessler, R. H. Friend, Science Vol. 28,pp.1741, C. D. Dimitrakopoulos, D. J. Mascaro, IBM J. Res. Dev.Vol.45,pp.11-27, G. Yu, J. Gao, J. C. Hummelen,F. Wud, A. J. Heeger, Science Vol.27,pp.1789, C. J. Brabec, N. S. Sariciftci, J. C. Hummelen, Adv. Funct. Mater. Vol. 11, pp.15-26, R. N. Das., K. Papathomas, J. Laufer and F. D. Egitto, 57th Electronic Components and Technology Conference proceedings (May 29 June1, 27)pp