Chin Chong Yap

Flip Chip Based on Carbon Nanotube–Carbon Nanotube Interconnected Bumps for High-Frequency Applications

This paper presents a flip-chip structure based on carbon nanotube (CNT) interconnected bumps for high-frequency applications. The CNT bumps are grown directly on gold coplanar lines using the plasma-enhanced chemical vapor deposition approach, and the CNT bumps are interconnected using a flip-chip bonder. DC and high-frequency measurements from flip-chip input to output are characterized and compared against electromagnetic simulation of CNT bumps and gold bumps. S-parameter transmission of -2.5 dB up to 40 GHz was obtained using CNT bumps in this experiment. Experimental transmission across the CNT bumps demonstrates the feasibility of using CNT bundles for future interconnects at smaller scale (few micrometers) and at even higher frequencies. This is the first work using CNT bumps for flip-chip structures and serves as a platform for future studies of CNT interconnects above 40 GHz.

Impact of the CNT growth process on gold metallization dedicated to RF interconnect applications

Carbon nanotubes (CNTs) are a unique group of materials with high aspect ratio, mechanical and electrical properties, which are of great interests in the field of interconnects, and radio frequency applications. In order to incorporate CNTs into any of these applications successfully, one important issue that has to be resolved is the critical parameters (temperature and reactant gases) associated with the growth of the CNTs. As such, the effect of these growth requirements on the adjacent components should be studied. In this work, we examined specifically the effect of carbon nanotubes growth on the underlying metallization, in particular gold, dedicated for radio-frequency-based applications. The gold coplanar lines were annealed at 8008C in a plasma-enhanced chemical vapor deposition (PECVD) system to simulate the worst-case condition. The reflection and transmission parameters were analyzed using a probe station connected to a vector network analyzer. Carbon nanotubes grown on different barrier layers were also characterized using a scanning electron microscope and Raman spectroscopy to identify a suitable barrier layer for gold. Our results showed that it is promising to integrate carbon nanotubes grown using PECVD onto Au coplanar waveguide without degrading the S-parameters measurements up to 20 GHz.

Measurement and modeling of carbon nanotubes-based flip-chip RF device

A flip chip device based on carbon nanotubes (CNTs) interconnections characterized up to 40 GHz is presented. An analytical model using an RLC transmission line is fitted from measurements of the fabricated structure. CNTs have the potential to make smaller interconnections than metal-based ones with the same degree of performance. A value of 324 fcΩ is found for the contact resistance between CNTs. A CNT-bundle-based flip chip with -2.5 dB insertion loss and return loss below -13 dB is presented.

Carbon-nanotube-based RF components with multiple applications

Owing to their extraordinary electronic properties, carbon nanotubes (CNTs), as a building block, promise superior performance and novel functionalities for microwave electronic components. This review presents some of the last results obtained in three different fields of this fast moving area - antennas, interconnects and sensors. Electromagnetic and analytical models are used to evaluate and optimize component performance. Synergy between fabrication and simulation techniques allows the practical design of bundled-CNTs-based components. Opportunities are discussed from the fabrication of CNT materials (PECVD, CVD) to their implementation in components and systems by specific technological processes (Photolithography, Flip-Chip, Inkjet printing, etc.). Actual performance of fabricated prototypes and expected improvement based on the CNTs implementation are presented.

Characterization of CNT interconnection bumps implemented for 1st level flip chip packaging

The fabrication procedures to contact the carbon nanotubes (CNT) to metal surface determine the contact resistance of the CNT bump. The large contact resistance is seen as a limiting factor to realize practical application using CNT bumps. An alternative bonding solution using CNT in physical contact with CNT using CNT interconnection bump is proposed. A study was conducted to determine the relationship between the bonding loads and bump resistances. It was concluded that the bump resistance decreases exponentially with the bonding load due to the change in inter-tube spacing. The lowest achieved CNT interconnection bump resistivity value was 0.05 Ω.cm.

Designing Carbon Nanotube Interconnects for Radio Frequency Applications

With the blooming demand for wireless and mobile applications, the needs to develop existing RF technologies are increasing. For example, there is a need for RF technologies that can operate at much higher frequencies, which can be potentially used for satellite or tetra-hertz imaging applications. Thus, there is a strong interest for novel materials, which can have more reliable and stable high-frequency performance. Flip chip is one of the technology offering lower insertion loss, compact packages, and low-cost fabrication. The use of carbon nanotube (CNT) bumps to replace existing materials or applications is not unheard of. In this chapter, demonstration of a successfully CNT flip chip device at high frequency will be done. A parametric study using hybrid EM/analytical modeling of the device will be conducted.

The influence of titanium nitride barrier layer on the properties of CNT bundles

The use of carbon nanotubes (CNTs) for electrical interconnections is hinder by the possibility of growing CNT directly onto metallization. The introduction of barrier layer between catalyst and metallization is thus essential to permit the direct growth of vertically aligned CNT bundles using CVD approaches. As a result, the resultant CNT bundle resistivity is not only a function of the densities and quality of CNT growth, but is also affected by the thickness and resistivity of the barrier layer. CNT is growth on different thickness of TiN with 2 different underlying layers (Au and SiO2). It was observed that the length of CNT grown on Au is independent of TiN thickness, whereas the height decreases for Au underlayers case. In both scenarios, both have well aligned CNTs growth when the thickness of TiN is <; 90nm.

Carbon based multi-functional materials towards 3D system integration. Application to thermal and interconnect management

In order to meet the demands of increasing package density and miniaturization of devices without compromising performance, the most challenging issues to tackle are thermal and interconnect management. In this paper, we will first understand the interfacial transport between Si and Carbon for better system integration and discuss how novel carbon films can be used for thermal extraction. Second, we will show how carbon nanotubes can be used as interconnects using a flip chip approach as well as potential radio frequency applications.

Electrical transport in carbon nanotube intermolecular p-n junctions

Thermionic emission and tunneling are found to be the dominant transport mechanisms under the forward and reverse bias conditions, respectively, in a single-walled carbon nanotube based intermolecular p-n junction. At a reverse bias, the kink point on the plot of ln(I/V2) vs 1/V indicates that the transport mechanism experiences a transition from direct tunneling to the Fowler-Nordheim tunneling through the junction barrier. In contrast, the Arrhenius plot of the I-V curve at forward biases shows that tunneling dominates over the thermionic emission below 50 K.

Understanding the electrical transport properties of carbon nanotubes and its metal under-layers

Carbon nanotubes have been widely studied due to its excellent electrical, thermal and mechanical properties, showing promising applications in both active and passive components in electronic devices. However, for such devices to be developed, compatibility issues for obtaining quality carbon nanotubes with suitable underlying substrates have to be understood. In this work, a technique to study the impact of each under layer on carbon nanotubes would be discussed. Vertically aligned multiwalled carbon nanotubes are grown on patterned metal trace with suitable barrier layers. In situ electrical measurement of the nanotubes using manipulators in a field emission scanning electron microscope was performed to understand the electrical transport at each interface.