Ph.D.: University of California at Berkeley, 1983
Postdoctoral Fellow: California Institute of Technology, 1983-84
The research group is working on a number of topics including surface chemistry, thin film growth, nanostructure properties and various technological applications.
1. Fabrication and Properties of Nanocomposites
Nanocomposites are a new frontier in materials science because composites can have very different properties than their constituents. We are focusing on nanolaminate composite materials. Nanolaminates show unique physical properties when the nanolayer thickness is less than the characteristic length scale that defines the physical property. For example, thermal conductivity is reduced when the nanolayer thickness is less than the mean free path of the phonon that transfers the heat. Likewise, hardness is increased when the nanolayer thickness is less than the dislocation length for the slip plane motion that characterizes the response of the material to stress.
Our nanolaminate work has recently focused on Al2O3/ZnO [7,8] and Al2O3/ W nanolaminates. These are interesting nanolaminates for a variety of reasons. Electrically, Al2O3 is an insulator and ZnO and W are both conductors. Structurally, Al2O3 is amorphous and ZnO and W are both nanocrystalline. By depositing these nanolaminates with various compositional ratios and different nanolayer thicknesses, the electrical and structural properties of the film can be tuned over a wide range. The Al2O3/W nanolaminate may also be useful as a high temperature thermal barrier coating to protect turbine engine blades. The Al2O3/W nanolaminates may also be very useful as X-Ray Bragg mirrors in the hard X-Ray region.
2. Depositing Ultrathin, Conformal Coatings on Particles
Particles have many technological applications and are used to make a variety of composite materials. Coatings on particles can be used to change the surface properties while retaining the bulk properties. Coatings can also be used to functionalize, protect, strengthen or modify the properties of the particle. Despite the importance of coating particles, very few methods exist to deposit conformal thin films on particles.
Our previous work has developed ALD techniques to coat particles with ultrathin and conformal films. Recent investigations have studied Al2O3  and SiO2  deposition on BN particles, BN deposition on ZrO2 particles , and SiO2 on BaTiO3 particles. We have also demonstrated Al2O3 deposition on low-density polyethylene particles. Many of these studies are conducted using Fourier Transform Infrared (FTIR) spectroscopy because the particles have a very high surface area that facilitates these FTIR investigations.
3. Optimizing and Understanding Metal Oxide SemiconductorGas Sensors
An interesting class of gas sensors is based on the resistivity of metal oxide semiconductor thin films. Gases adsorb onto the surface of these metal oxide semiconductor thin films and alter the film resistivity. The surface adsorption perturbs the underlying charge carriers in the film or the electron conduction between individual crystalline grains in the film. However, the exact mechanism of the effect of surface adsorption is not clearly known. Optimization of these metal oxide semiconductor gas sensors must await additional understanding.
Our gas sensor work has focused on ZnO films. We have grown ZnO films using ALD techniques and simultaneously measured their resistivity . Sensitivity is extremely high for ultrathin ZnO films. The ZnO film resistivity also varies dramatically with adsorbed surface species. ZnO films terminated with Zn-CH3* species have a much higher sensitivity that ZnO films terminated with Zn-OH* species. Future research will explore the effect gas adsorption and attempt to understand and optimize the sensor sensitivity.
4. Metal Atomic Layer Deposition using Hydrogen Radicals
Many binary oxide and nitride materials can be deposited using ALD techniques. In contrast, the deposition of single-element metals is less compatible with the binary reaction sequence chemistry that defines the ALD process. To develop metal ALD procedures, a binary sequence can be defined using hydrogen radicals as one of the two reactants. The hydrogen radicals serve to strip the ligand from the metal precursor according to the general overall reaction scheme: MXn + nH --> M + nHX.
Our research is developing hydrogen radical plasma sources and optimizing hydrogen radical transport for efficient metal ALD. Hydrogen recombination to form H2 is a significant hydrogen radical loss mechanism. Based on these results, a reactor for metal ALD has been constructed to minimize hydrogen radical recombination. This reactor has been employed recently to deposit various metals such as Ti using TiCl4 + 4H --> Ti + 4HCl.
5. Growth of High-k Dielectrics to Replace SiO2 in Capacitors and Gates
SiO2 has been the primary insulator in silicon microelectronics for both capacitors and gate oxides. Miniaturization has progressively reduced the SiO2 thicknesses to the limit where electron tunneling through the insulator is a significant problem. Higher dielectric constant materials are needed to increase capacitance density and reduce electron tunneling. These high-k dielectrics are preferably amorphous to avoid grain boundary current leakage.
HfO2 and ZrO2 and their associated silicates and aluminates have emerged as leading high-k dielectric candidates. Our recent research has focused on Al2O3 ALD, HfO2 ALD and HfO2-silicate ALD. ALD processing yields excellent pin-hole free insulating films. However, a primary concern is the interfacial oxide that forms between the high-k dielectric and silicon. This interfacial oxide must be minimized to benefit from the high-k dielectric.
6. Deposition of Inorganic Films on Polymers
Polymers are pervasive because of their flexibility, light weight and low cost. Many polymer properties could be enhanced by the addition of an inorganic coating on their surface. This inorganic film could serve as a gas diffusion barrier for various packaging applications. The inorganic layer could also serve to protect the underlying polymer and give the polymer higher strength. Unfortunately, inorganic layers are difficult to deposit on polymers because the deposition of inorganics is usually performed at temperatures above the melting temperature of the polymer.
Our recent work has shown that Al2O3 ALD using Al(CH3)3 and H2O can deposit an Al2O3 film on polymers at fairly low temperatures below the polymer melting temperature. This Al2O3 coating serves as a gas diffusion barrier for the polymer. The Al2O3 film may also act as an adhesion layer for the deposition of other materials. These Al2O3 adhesion layers may be useful to facilitate the deposition of diffusion barriers on low-k dielectrics for backend interconnects.
7. Reliability Enhancement for Micro Electromechanical Systems (MEMS) Devices
Micro electromechanical systems (MEMS) devices are developing rapidly for a variety of applications. MEMS devices are fabricated using silicon micro-machining. The resultant MEMS devices suffer reliability problems because of mechanical wear between the moving parts and because of stiction from attractive surface forces and static charge buildup. Many of these reliability problems could be solved by applying the appropriate thin film coating to the MEMS device.
ALD is ideal for the deposition of conformal films on the high aspect ratio structures encountered in MEMS devices. Our research has demonstrated that Al2O3 ALD yields a hard and protective coating that reduces abrasive wear in MEMS devices. Nanolaminates can also provide films with enhanced hardness to minimize further mechanical wear. In addition, Al2O3/ZnO alloys have been shown to provide a variable resistivity film that can dissipate static charge and minimize stiction.
8. Fabrication of Superlattices for Extended Ultraviolet (EUV) and X-Ray Mirrors
Mirrors in the extended ultraviolet (EUV) and X-Ray regions are superlattices that reflect by Bragg diffraction. High reflectivities of ~70% in the EUV at ~12 nm can be obtained using Mo/Si superlattices. These EUV mirrors are important for next generation EUV lithography. Current Mo/Si superlattices fabricated using ion beam spattering suffer from high defect densities. Lower defect density superlattices will be necessary for EUV lithography.
Superlattices can be fabricated using ALD techniques. These superlattices may produce well-defined multilayers that challenge the ion beam spattering fabrication methods. We are currently setting up to deposit Mo/Si superlattices using ALD. After demonstrating the fabrication of these Mo/Si superlattices, the structural properties will be evaluated prior to EUV reflectivity measurements.