Fabrication of Ultra-Low-Dielectric-Constant (ULK) films for silicon nanotechnology
To achieve the target dielectric value of 2.1 required for the 22 nm technology node, atomic and nanometer scale pores are introduced into this new class of material by using a porogen subtraction method. Current ULK films consist of 20-40% nano-pores by volume with diameters no greater than 5 nm. With careful control of the deposition and curing process conditions, it is now possible to manufacture ULK films with dielectric constant values as low as 2.5. We are developing processes to incorporate more nano-pores into advanced films, without collapsing the silicon matrix materials during post deposition curing.
Elastic constraint effects on small scale fracture
Tsui’s group focuses on the influence of substrate elastic constraint effects on the channel cracking behaviors of low-dielectric-constant thin film (low-k). Since ULK is a much more compliant material, as compared to the rigid silicon substrate, the effective substrate elastic modulus experienced by the low-k channel crack tip is reduced with an increasing ULK buffer layer thickness. This increases the crack tip energy release rate and the crack propagation velocity.
Mechanical reliability of nano-scale devices
Mechanical reliability of nano-scale devices, such as copper/ULK interconnect structures in integrated circuits, flexible electronics, and Micro-Electro Mechanical Systems (MEMS) devices, have stimulated significant interests in recent years. Since the structures of these devices are extremely small, crack tip stress fields can easily be affected by the plastic and elastic properties of the surrounding materials. Our group observes and characterizes channel cracks and their propagation in 45 nm technology node integrated circuits. For example, the elevated strain energy release rate produced by the thermal expansion coefficient mismatch between copper interconnect lines and the low-k material often leads to a critical crack. When this crack is filled with copper due to either mechanical or electrical stress, an electrical short between the two structures occurs and causes device failure. Studies like this have the potential to generate tens of millions of dollars in revenue at semiconductor companies.
Fracture of porous brittle materials used in advanced semi-conductor devices
Another important aspect of mechanical failure in the fields of advanced silicon or biological nanotechnology is the increasing application of brittle, porous materials. Chemical reactants, such as water molecules, can diffuse and easily absorb into these materials, thereby changing the local crack tip environment and resulting in degraded fracture resistances.
Biomechanics–nano-scale properties of human cells
Our research team applies nanoindentation techniques to measure mechanical properties of different human vertebral bone tissues and cell orientations. Our results demonstrated that some of the previously published elastic and plastic mechanical properties are several times lower than these actual values. We also revealed the highly elastic anisotropic nature of bone tissue.
Nano-wire and micro-machine fabrication
By using advanced integrated circuit processing techniques, we were able to produce nano-wires in mass, with uniform sizes and properties, on silicon substrates. These nano-wire materials can be composed of semiconductor, metal, or amorphous dielectrics. Because the wires are attached and protected by the silicon substrate during the manufacturing process, the final wire length is only limited by the wafer size and lithographic layout. In addition, the wires can compose of single or multiple layers of materials with different chemical, mechanical, and electrical properties. These novel composite nano-wires may provide new applications, such as, nano-scale mechanical manipulators and other sensor technologies.
Research interests
- Plasma-enhanced chemical vapor deposition
- Porous ultra-low-dielectric constant materials
- Nano-mechanics
- Thin film mechanical properties
- Nanoindentation
