Lingbo Zhu

Low temperature carbon nanotube film transfer via conductive polymer composites

A low temperature process for transferring carbon nanotube (CNT) film from a silicon wafer to a copper surface via conductive polymer composites is proposed. The morphologies and electrical properties of the transferred CNT films were studied. An ohmic contact was formed between a CNT film and a highly conductive polymer composite and the resistance of the as-transferred CNT film was 0.08 Ω, while a semiconductor joint was formed between a CNT film and a high resistivity polymer composite.

Aligned carbon nanotubes for electrical interconnect and thermal management

As IC performance increases, many technical challenges appear in the areas of power delivery, thermal management, I/O density, and thermal-mechanical reliability. To address these problems, the use of aligned carbon nanotubes (CNTs) is proposed in IC packaging as electrical interconnect and thermal interface materials. The superior electrical, thermal, and mechanical properties of CNTs promise to bring revolutionary improvement in reducing the interconnect pitch size, increasing thermal conductivity, and enhancing system reliability. Carbon nanotubes (CNTs) are the fascinating one-dimensional molecular structures that can be either metallic or semiconducting, depending on their diameter and helicity. In order to create interconnect structures comprised of CNTs units, it is necessary to control both the growth of CNTs in predefined orientations and configurations, and the interface with other materials such as metal electrodes. In this paper, the growth of aligned carbon nanotubes (ACNTs) by chemical vapor deposition (CVD) is discussed. It is a promising approach, since CVD growth is scalable and can be adapted to produce well-controlled large-area CNT arrays. Two approaches for CVD nanotube growth were explored. One needs pre-patterning catalysts. By this process, highly-aligned carbon nanotubes (CNTs) with high density were synthesized on Al/sub 2/O/sub 3//Fe coated silicon substrates of several square centimeter area. The other process is using vapor-phase mixture of xylene and ferrocene, which avoids pre-patterning the catalysts. The ferrocene was the nanotube nucleation initiator and xylene the carbon source. The as-grown CNTs were characterized by high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), and Raman spectroscopy. The feasibility of ACNT interconnect was investigated through the study of CNT/solder interface after a solder reflow process. Preliminary results indicated that molten Sn/Pb solder could wet the CNT surface and form good solder joints. The ACNT structures will be proposed to develop ultra-fine pitch electrical interconnections and high thermally conductive interface materials.

Assembling Carbon Nanotube Films as Thermal Interface Materials

As the feature size of integrated circuits (ICs) decreases and the IC clock frequency and transistor density per chip increase, the power density increases exponentially. As such, heat generation has become a critical issue in advanced high performance ICs in recent years. The current thermal solutions will soon be inadequate for heat dissipation. New thermal management strategies and high thermal conductivity materials are urgently needed for electronic thermal management within the chip and surrounding packaging. Carbon nanotubes (CNTs), which are reported to be the most thermally conductive material (>3000 W/K.m), are a promising candidate for thermal management in microelectronics. However, several existing technical barriers have restrained their application of CNTs in microelectronic devices. One of the main challenges is that the high temperature required to grow high quality CNTs (>600degC) which is too high to be compatible with back-end microelectronic fabrication processes. Another major obstacle is the poor adhesion between CNTs and substrates, which results in high interface thermal resistance and poor long term reliability. To address these challenges, we proposes to use a novel CNT transfer process, which features separated steps of an in-situ open-ended CNT synthesis and a low-temperature CNT assembly to engineer the well-aligned open-ended CNT architectures for microelectronic thermal management. In-situ growth of high density open-ended CNTs with vertical alignment was first developed in our laboratory. To successfully achieve CNT assembly with good thermal performance, the following issues should be addressed: (1) Appropriate metals/solders should be selected to form good thermal and electrical coupling with CNTs. (2) The wetting of solder metal alloys in CNT channels and on the CNT outer walls by capillary force should be further clarified. (3) An appropriate solder reflow process should be developed to guarantee good wetting on CNTs. Preliminary results show that the transferred open-ended CNT structures can have very strong adhesion to the substrate, which promise to improve the CNT-metal interface properties. Initial thermal measurements of the CNT assembly with CNT height of ~180 mum show that the thermal conductivity and total thermal resistance of the assembly were 81 W/m.K and 0.43 cm2 K/W, respectively. The work on the improvement of thermal performance of the CNT assembly is ongoing, including the interfacial solder layer thickness optimization and material selection, and CNT film quality.

Polymer‐Infiltrated Aligned Carbon Nanotube Fibers by in situ Polymerization

Carbon nanotube-polymer composite fibers are obtained by infiltration of a monomer liquid into aligned carbon nanotube aerogel fibers with subsequent in situ polymerization. The monomer, methyl methacrylate (MMA), was infiltrated into the aerogel fibers of multi-walled carbon nanotubes (MWNTs) at room temperature and subsequently polymerized at 50 °C into poly(methyl methacrylate) (PMMA). Cross-sections of the PMMA/MWNT composite fibers showed that the PMMA filled the spaces of the nanotube fibers and bound the nanotubes together. PMMA in the composite fibers exhibited local order. The resultant composite fibers with 15 wt.-% nanotube loading exhibited a 16-fold and a 49-fold increase in tensile strength and Youngs modulus, respectively, compared to the control PMMA.

Lotus effect surface for prevention of microelectromechanical system (MEMS) stiction

Due to the surface smoothness of micromachined structures, strong adhesion forces between these fabricated structures and the substrate can be developed. Once contact is made, the magnitude of these forces is sufficient to deform and attract these structures to the substrate, resulting in device failure. This type of failure is one of the dominant sources of yield loss in microelectromechanical system (MEMS) fabrications. The basic approaches to prevent stiction include increasing surface roughness and/or lowering solid surface energy by coating with low surface energy materials. By nature, the Lotus Effect surface is an excellent model surface of a combined effect of hydrophobicity and micro/nano scale structure topography. Such surfaces have water droplet contact angles of 150/spl deg/ or higher. The intrinsically superhydrophobic surfaces can avoid an attractive capillary force which pulls the MEMS microstructure to the substrate; as such they reduce van der Waals forces as well. To prepare a lotus effect surface, aligned carbon nanotubes (ACNTs) that are perpendicular to the substrate surface are created. The nanotubes were grown in a chemical vapor deposition (CVD) tube furnace system from a vapor-phase mixture of xylene and ferrocene. The ferrocene was the nucleation initiator and xylene as the carbon source. Multiwalled carbon nanotubes of 20-30 nm in diameter were fabricated onto SiO/sub 2/ surfaces that were deposited by the plasma enhanced chemical vapor deposition (PECVD) method. The average center-to-center spacing (pitch) between adjacent nanotubes was /spl sim/50 nm. The as-grown vertical nanotubes showed good adhesion to the substrate, which made the nano-scaled roughness possible. The initial water contact angle on the as-grown aligned CNT surface was 155/spl deg/. To improve the stability of the superhydrophobic surface, the aligned CNTs were modified by fluorinated polymers formed by PECVD. The as-grown CNTs were characterized using scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM).

Optimizing geometrical design of superhydrophobic surfaces for prevention of microelectromechanical system (MEMS) stiction

Due to the surface smoothness of micromachined structures, strong adhesion forces between these fabricated structures and the substrate can be developed. The major adhesion mechanisms include capillary forces, hydrogen bonding, electrostatic forces and van der Waals forces. Once contact is made, the magnitude of these forces is in some cases sufficient to deform and pin these structures to the substrate, resulting in device failure. This type of failure is one of the dominant sources of yield loss in MEMS. The basic approaches to prevent stiction are increasing surface roughness and/or lowering solid surface energy by coating with low surface energy materials. Combination of micro- and nano-meter scale roughness can dramatically increase the surface roughness. However, in fabrication process, how to optimally design surface geometry with micro-/nano-meter roughness is still not clear. The objectives of this paper are to experimentally study the wetting and hydrophobicity of water droplets on two-tier rough surfaces for comparison with theoretical analyses, and to optimize the surface geometrical design for fabricating stable superhydrophobic surfaces. Two model systems are fabricated: carbon nanotube arrays on silicon wafers and carbon nanotube arrays on carbon nanotube films, to compare wetting on micro-patterned silicon surfaces with wetting on nano-scale roughness surfaces. All surfaces are coated with 20 nm thick fluorocarbon films to obtain low surface energies and to improve the stability of the superhydrophobic surface, formed by plasma enhanced chemical vapor deposition (PECVD). The results show that the microstructural characteristics must be optimized to achieve stable superhydrophobicity on micro-scale rough surfaces. However, the presence of nano-scale roughness allows a much broader range of surface design criteria, decreases the contact angle hysteresis to less than 1deg and establishes stable and robust superhydrophobicity, although nano-scale roughness could not increase the apparent contact angle significantly if the micro-scale roughness dominates. The results of the research could guide the optimized designs of the surfaces for prevention of microelectromechanical (MEMS) stiction

A rapid growth of aligned carbon nanotube films and high-aspect-ratio arrays

The remarkable properties of carbon nanotubes (CNTs) make them attractive for microelectronic applications, especially for interconnects and nanoscale devices. In this paper, we report an efficient process to grow well-aligned CNT films and high-aspect-ratio CNT arrays with very high area distribution density (>1600 µm−2). Chemical vapor deposition (CVD) was invoked to deposit highly aligned CNTs on Al2O3/Fe coated silicon substrates of several square centimeter area using ethylene as the carbon source, and argon and hydrogen as carrier gases. The nanotubes grew at a high rate of ∼100 µm/min. for nanotube films at 800°C, while the nanotube arrays grew at ∼140 µm/min. even at 750°C, due to the base growth mode. The CNTs were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and x-ray photoelectron spectroscopy (XPS). The results demonstrated that the CNTs are of high purity and form densely aligned arrays with controllable size and height. The as-grown CNT structures have considerable potential for thermal management and electrical interconnects for microelectronic devices.

A Novel Nanocomposite With Photo-Polymerization for Wafer Level Application

A novel nanocomposite photo-curable material which can act both as a photoresist and a stress redistribution layer applied on the wafer level was synthesized and studied. In the experiments, 20-nm silica fillers were modified by a silane coupling agent through a hydrolysis and condensation reaction and then incorporated into the epoxy matrix. A photo-sensitive initiator was added into the formulation which can release cations after ultraviolet exposure and initiate the epoxy crosslinking reaction. The photo-crosslinking reaction of the epoxy made it a negative tone photoresist. The curing reaction of the nanocomposites was monitored by a differential scanning calorimeter with the photo-calorimetric accessory. The thermal mechanical properties of photo-cured nanocomposites thin film were also measured. It was found that the moduli change of the nanocomposites as the filler loading increasing did not follow the Mori-Tanaka model, which indicated that the nanocomposite was not a simple two-phase structure as the composite with micron size filler. The addition of nano-sized silica fillers reduced the thermal expansion and improved the stiffness of the epoxy, with only a minimal effect on the optical transparency of the epoxy, which facilitated the complete photo reaction in the epoxy.

Low Temperature Carbon Nanotube Film Transfer via Conductive Adhesives

A low temperature process for transferring carbon nanotube (CNT) film from a silicon wafer to a copper surface via conductive adhesives is proposed. Both the close-and open-ended CNT films were successfully transferred to a desirable substrate. The morphologies and electrical properties of the transferred CNT films were studied. An ohmic contact was formed between a CNT film and a highly conductive adhesive, while a semiconductor joint was formed between a CNT film and a high resistivity adhesive.

Surface treatment of MWCNT array and its polymer composites for TIM application

The multiwalled carbon nanotube (MWCNT) surface chemistry changes by microwave and UV/ozone treatments were studied and the treatment effects on the thermal conductivity of their polymer nanocomposites were investigated. Contact angle measurements, X-ray photoelectron spectroscopy (XPS) and Raman spectra were used for the CNT surface characterization. The surface functionalization and the CNT purification/annealing should be balanced for improving the thermal transport property of the MWCNT/polymer nanocomposites.