Chien-Min Liu

Unidirectional growth of microbumps on (111)-oriented and nanotwinned copper.

Tiny Tinny Bumps One challenge in moving to three-dimensional integrated circuit architectures is the need for aligned interconnects to join neighboring layers. Hsiao et al. (p. 1007) applied rapid stirring to the direct current electroplating of copper to produce films with oriented copper grains that have a high density of nanotwin defects. The resulting material was an excellent platform for the growth of copper-tin intermetallic compounds in the form of arrays of microbumps potentially suitable for the soldering of electronic components. Oriented copper grains grown using direct-current electroplating serve as a template for intermetallic microbumps. Highly oriented [111] Cu grains with densely packed nanotwins have been fabricated by direct-current electroplating with a high stirring rate. The [111]-oriented and nanotwinned Cu (nt-Cu) allow for the unidirectional growth of Cu6Sn5 intermetallics in the microbumps of three-dimensional integrated-circuit packaging; a uniform microstructure in a large number of microbumps of controlled orientation can be obtained. The high-density twin boundaries in the nt-Cu serve as vacancy sinks during the solid-state reaction between Pb-free solder and Cu and greatly reduce the formation of Kirkendall (or Frenkel) voids.

Low-temperature direct copper-to-copper bonding enabled by creep on (111) surfaces of nanotwinned Cu

Direct Cu-to-Cu bonding was achieved at temperatures of 150–250 °C using a compressive stress of 100 psi (0.69 MPa) held for 10–60 min at 10−3 torr. The key controlling parameter for direct bonding is rapid surface diffusion on (111) surface of Cu. Instead of using (111) oriented single crystal of Cu, oriented (111) texture of extremely high degree, exceeding 90%, was fabricated using the oriented nano-twin Cu. The bonded interface between two (111) surfaces forms a twist-type grain boundary. If the grain boundary has a low angle, it has a hexagonal network of screw dislocations. Such network image was obtained by plan-view transmission electron microscopy. A simple kinetic model of surface creep is presented; and the calculated and measured time of bonding is in reasonable agreement.

Superparamagnetic and ferromagnetic Ni nanorod arrays fabricated on Si substrates using electroless deposition.

The microstructures and magnetic properties of nickel nanorods fabricated using an anodic alumina oxide template and electroless deposition were investigated. The as-deposited nanorods were found to contain nanocrystalline grains with an average size of approximately 2-3 nm. The temperature-dependent magnetic hysteresis curves indicated superparamagnetic behavior of the as-deposited rods as a result of the reduction of ferromagnetic crystallites. The superparamagnetic (SM) Ni nanorods transformed into ferromagnetic (FM) ones when annealed at 400 degrees C. Results from dark-field transmission electron microscopy reveal that the microstructure of the rods tends to form a laminar structure with grain growth parallel to the long axis of the rods, together with the enhancement of ferromagnetic ordering along the same direction. The results suggest that the SM-FM phase transition obtained is microstructure driven. The Ni nanorods manufactured by the electroless deposition also have the potential to serve as magnetic building blocks in nanoscale devices, such as high-frequency inductors. On-chip magnetic spiral inductors were fabricated using these nanorods, and it was demonstrated that the nanorods can enhance inductance up to 6 GHz.

Growth Mechanism of TiO2 Nanotube Arrays in Nanopores of Anodic Aluminum Oxide on Si Substrates by Atomic Layer Deposition

In this study, we combined atomic layer deposition and anodic aluminum oxide (AAO) on a silicon substrate and developed self-aligned Ti0 2 nanotube arrays. We studied the growth mechanism of TiO 2 nanotubes on the inner wall of AAO at 100 and 400°C. We found that at 100°C, TiO 2 grew in a layer-by-layer manner. Therefore, it can be grown into Ti0 2 nanotube arrays with very thin walls. However, at 400°C, Ti0 2 needs to first form a 2.5-nm amorphous layer, before becoming crystalline Ti0 2 via a phase transformation, and growing into crystalline TiO 2 nanotube arrays along the preferred plane {101} by means of a space-limited growth mechanism.

Fabrication and Characteristics of Self-Aligned ZnO Nanotube and Nanorod Arrays on Si Substrates by Atomic Layer Deposition

Vertically self-aligned ZnO nanorods and nanotubes are fabricated on Si substrates by atomic layer deposition with the assistance of anodic aluminum oxide at 250°C. These nanostructures are equal in height, isolated, and vertical to the Si substrate. With 550 deposition cycles, we can fabricate regular arrays of ZnO nanorods with an average diameter of 70 nm and with a height of 470 nm. In particular, the wall thickness of the nanotubes can be controlled precisely by using the atomic layer deposition approach. The measured wall thickness is 18.5 ± 1 nm after 250 deposition cycles, which yields a growth rate of 0.075 nm/cycle. A polycrystalline structure for both ZnO nanorods and nanotubes was confirmed by a transmission electron microscope and selected area diffraction pattern. Compared with the ZnO films and nanorods, the fabricated ZnO nanotubes exhibit an excellent performance on photoluminescence characteristics due to their larger surface area.

The heterojunction effects of TiO2 nanotubes fabricated by atomic layer deposition on photocarrier transportation direction

The heterojunction effects of TiO2 nanotubes on photoconductive characteristics were investigated. For ITO/TiO2/Si diodes, the photocurrent is controlled either by the TiO2/Si heterojunction (p-n junction) or the ITO-TiO2 heterojunction (Schottky contact). In the short circuit (approximately 0 V) condition, the TiO2-Si heterojunction dominates the photocarrier transportation direction due to its larger space-charge region and potential gradient. The detailed transition process of the photocarrier direction was investigated with a time-dependent photoresponse study. The results showed that the diode transitioned from TiO2-Si heterojunction-controlled to ITO-TiO2 heterojunction-controlled as we applied biases from approximately 0 to -1 V on the ITO electrode.

Formation of nearly void-free Cu3Sn intermetallic joints using nanotwinned Cu metallization

Cu3Sn intermetallic compounds (IMCs) are more resistant to fracture than solders. In addition, the Cu3Sn IMCs are more conductive than the solders. In this study, we manufactured Cu3Sn IMCs to serve as a joint using electroplated nanotwinned Cu as a metallization layer to react with pure Sn at 260 °C and 340 °C. The results show that there were almost no Kirkendall voids generated inside the Cu3Sn layer. In addition, the kinetics of the Cu3Sn growth was analyzed to predict the time needed to form the Cu3Sn joint.Cu3Sn intermetallic compounds (IMCs) are more resistant to fracture than solders. In addition, the Cu3Sn IMCs are more conductive than the solders. In this study, we manufactured Cu3Sn IMCs to serve as a joint using electroplated nanotwinned Cu as a metallization layer to react with pure Sn at 260 °C and 340 °C. The results show that there were almost no Kirkendall voids generated inside the Cu3Sn layer. In addition, the kinetics of the Cu3Sn growth was analyzed to predict the time needed to form the Cu3Sn joint.

Effect of Geometric Nanostructures on the Absorption Edges of 1-D and 2-D TiO2 Fabricated by Atomic Layer Deposition

2-Dimensional (2-D) TiO2 thin films and 1-dimensional (1-D) TiO2 nanotube arrays were fabricated on Si and quartz substrates using atomic layer deposition (ALD) with an anodic aluminum oxide (AAO) template at 400 °C. The film thickness and the tube wall thickness can be precisely controlled using the ALD approach. The intensities of the absorption spectra were enhanced by an increase in the thickness of the TiO2 thin film and tube walls. A blue-shift was observed for a decrease in the 1-D and 2-D TiO2 nanostructure thicknesses, indicating a change in the energy band gap with the change in the size of the TiO2 nanostructures. Indirect and direct interband transitions were used to investigate the change in the energy band gap. The results indicate that both quantum confinement and interband transitions should be considered when the sizes of 1-D and 2-D TiO2 nanostructures are less than 10 nm.

Ultraviolet Photoresponse of TiO2 Nanotube Arrays Fabricated by Atomic Layer Deposition

We used atomic layer deposition with anodic aluminum oxide, to fabricate self-aligned TiO2 nanotube arrays, on a Si substrate, at 400 C. Numerous p-n nanojunctions can be fabricated using this approach. In the absence of the external application of a bias voltage, these crystalline nanotubes make very sensitive ultraviolet sensors. The sensitivity is a result of the built-in voltage, generated by the nanojunction formed between the TiO2 nanotubes and the Si substrate, which separates electron-hole pairs and produces photocurrents. In addition, the one-dimensional structure of the TiO2 nanotubes provides a well-defined path for the transportation of the photo-generated carriers. VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3576076] All rights reserved.

Effect of grain orientations of Cu seed layers on the growth of -oriented nanotwinned Cu

We investigate the growth of Cu films on two different Cu seed layers: one with regular <111>-oriented grains and the other with very strong <111>-preferred orientation. It is found that densely-packed nanotwinned Cu (nt-Cu) can be grown by pulsed electroplating on the strong <111>-oriented Cu seed layer without a randomly-oriented transition layer between the nt-Cu and the Cu seed layer. The electroplated nt-Cu grow almost epitaxially on the seed layer and formed <111>-oriented columnar structures. However, with the regular <111>-oriented Cu seed, there is a randomly-oriented transition layer between the nt-Cu and the regular <111>-oriented Cu seed. The results indicate that the seed layer plays a crucial role on the regularity of <111>-oriented nanotwinned Cu.