Research
 Overview of researchNanofabrication is a key enabling aspect of nanotechnology and it represents the focus of this proposal. Nanofabrication refers to the procedures to deposit, remove, modify and pattern substances on the 1-100 nm length scale in an externally controlled fashion. Typically a sequence of such steps is required to form devices and integrate them into a complete nano-system, similar to how microelectronic circuits are formed at a somewhat larger scale. Because the development of nano-materials and devices and their integration into functioning nano-systems is reliant on powerful nanofabrication techniques, developing and exploiting nanofabrication is a strategic focus of Dr. Cadien's research. ALDThere are three main ways of depositing thin films of metals, oxides, and nitrides. These are sputtering, evaporation, and chemical vapour deposition (CVD). Both sputtering and evaporation are line of sight deposition techniques which means that films deposited this way are not conformal on high aspect structures. For example, in trenches with depth to width ratios of > 2:1, sputtered and evaporated films show poor uniformity. CVD is a more conformal deposition method but requires high substrate temperatures and struggles with defects due to gas phase nucleation of particles. ALD on the other hand occurs at much lower temperatures than CVD, and since there is little possibility of gas phase nucleation there is no issue with particles and pin hole free films can be deposited. In addition, ALD is a surface reaction driven process and has been shown to uniformly coat features with aspects ratios > 100:1 . ALD is extremely scaleable and can uniformly coat the surface of nanotubes and nanoparticles as well as the surface of large planar and three dimensional objects. Finally, due to its ability to deposit a uniform monolayer, ALD is powerful technique to engineer interfaces. Atomic layer deposition (ALD) is a form of chemical vapor deposition (CVD). In CVD all of the reactant precursors are mixed in the reaction chamber and deposit simultaneously on a heated substrate. In ALD the precursors are pulsed into the reaction chamber alternately, with each pulse separated by a purge so that the reactants never mix in the gas phase. Enough precursor is used in each pulse to saturate the substrate surface. The precursors and process conditions are chosen so that the growth is self limiting. A good example of an ALD process is the deposition of zinc sulfide (ZnS) used by companies such as Planar Technology for making electroluminescent displays. The precursors for this process are zinc chloride and hydrogen sulfide. A schematic diagram of a typical viscous flow reactor used to deposit ZnS is shown below in Figure 1.
In this reactor design nitrogen purge gas is continuously flowed at several hundred sccm and the throttle valve on the pump is adjusted to maintain a pressure of about 1 mbar (1 Torr). The gas switching valves are alternately energized to deposit zinc chloride (ZnCl2) then hydrogen sulfide (H2S) on the surface. The chemical sequence is described in more detail in figure 2a and b.
In the first drawing in figure 2a the substrate is shown being exposed to zinc chloride in the vapor phase, and then a self-limiting monolayer of ZnCl2 attaching to surface, flowed by a purge sequence to remove excess reactant. When hydrogen sulfide is exposed to the surface it reacts with the zinc chloride surface to form zinc sulfide on the surface and hydrogen chloride as a byproduct. This process is repeated to build up thicker films of zinc sulfide. The simplified ALD deposition reaction is: ZnCl2 + H2S à ZnS + 2HCl. In figure 2b the timing sequence of the gas switching valves is shown schematically. The gap between pulses is to allow the prior reactant to be purged out of the system.
It should be noted that there are also high vacuum reactors where the reactant backfills the vacuum deposition chamber when the gas switching valve is turned and the vacuum pump valve is closed. The chamber is evacuated by closing the gas switching valve and opening the valve to the pump.
ALD reactions are self-limiting surface reactions. This property is what gives rise to outstanding step coverage, uniformity and scalability of ALD. ALD has been used to grow thin films of oxides, nitrides, metals, and semiconductor films at very low temperatures on a variety of substrates. This technology has the promise of have a major impact on silicon technology, micro electro mechanical systems (MEMS), nanotechnology, and other technologies. In order to achieve this, a fundamental understanding of film nucleation, precursors, film composition, film stress, and structure-property relationships is required.
Extensive research has been done in the application of ALD to integrated circuits. However there is little work going on in the application areas such as fuels cells and catalysts for oil refining identified in this program. There are two main university programs in the United States. The program at Harvard University is focused on precursor synthesis and the program at the University of Colorado is focused on deposition of different materials. In addition there is a research group at the University of Helsinki that has worked on synthesis and deposition.
Today, one of the biggest challenges in ALD is fundamental understanding of film nucleation. Typically, ALD oxides will nucleate well on oxide surfaces, and ALD nitride films on nitride surfaces, and ALD metal films on metal surfaces. However, it is possible to nucleate ALD oxides on silicon surfaces, but there is a delay of several growth cycles before deposition begins. Nucleation issues can also lead to the formation of islands at the film/substrate interface . It has also been observed that ALD films are typically 80% of bulk density. Understanding the origin of this deficiency is important to improving film properties. Researchers have also demonstrated that ALD films can improve solid oxide fuel cells , oil refining by tailoring catalysts, modify the hydrophobicity of surface with ALD layers , coat mesoporous membranes to tailor pore size, and improve MEMS performance .
APPROACHES TO RESEARCH In the following paragraphs the research approaches are described. A primary goal is improvement in understanding of the complex process/parameter space of ALD, followed by objectives pertaining to ALD applications: fuels cells, catalysts for oil refining, and improved micro fluidic and MEMS devices. Fundamental Understanding of ALD To expand the applications and improve the properties of ALD films a more complete understanding of nucleation and film growth is required. The impact of surface type and condition, temperature, and precursor exposure on absorption kinetics, multilayer adsorption, and island formation will be investigated. In order to accomplish this, an apparatus will be built to combine ALD with surface analytical techniques, and allow direct observation of nucleation on metal or semiconductor surfaces. Research activities will be focused on nucleation, growth, and properties of various thin film materials such as aluminum oxide (Al2O3), platinum (Pt), yttria stabilized zirconia (YSZ) and tantalum nitride (TaN). Nucleation activity will include the effects of substrate type, substrate preparation and surface condition, initiation and monitoring of growth using a quartz crystal microbalance, and in situ measurement of growth using surface analytical techniques. Film composition will be measured by Auger electron spectroscopy and X-ray photoelectron spectroscopy, and crystal structure by x-ray diffraction. As part of the initiation of growth studies, the interface between the films and the substrate will be examined by cross sectional transmission electron microscopy (TEM). Growth activities will include the effects of process conditions (temperature, pulse duration, etc) on growth rate and film properties such as crystal structure, density, adhesion, electrical and optical properties (as appropriate). It is envisioned that discoveries from the study of nucleation and growth may lead to innovations in reactor design that would improve in situ diagnostics and accelerate film growth. Applications of ALD Fuel cells Solid oxide fuel cells (SOFC) have a hard, non-porous ceramic compound as an electrolytic. They are expected to be 80-85% efficient with waste heat recovery. They also can tolerate high levels of sulfur and carbon monoxide. One of the major problems with SOFC is high operating temperatures of about 1000oC. In order to reduce the operating temperature of SOFC, thin films of YSZ are required. One objective of this research is to develop ALD growth of YSZ and other novel materials to enable lower temperature solid oxide fuel cells. The process must be developed and characterized to find the effects of composition, deposition conditions, and thermal stability on crystal structure and ionic conductivity. Techniques such as impedance spectroscopy, thermal gravimetric analysis, and differential scanning calorimetry will be used to measure these properties. Polymer electrolyte membrane (PEM) fuel cells operate at relatively low temperatures (~80oC) and require noble metal catalysts at both the anode and cathode. The catalyst is precipitated on carbon matrices that form the anode and cathode. One of the major costs in a fuel cell is the due to the noble metal catalyst. An objective of this aspect of this research is to study the ALD growth and nucleation of noble metal catalyst, such as Pt, on carbon surfaces with the goal of reducing cost as well as optimizing the surface area. Metals do not wet carbon, and two dimensional growth and adhesion are difficult to obtain. The research program will also focus on surface preparation to overcome these problems. Catalysts for oil refining ALD can be used to deposit catalysts on mesoporous catalytic membranes because of its unique capabilities. ALD can also be used to control the pore size and pore wall composition in these membranes. This research would focus on depositing Pt, Pd, V, V2O5, and Co on catalytic membranes. In these experiments, alumina would be used to modify the pore size, and a monolayer of catalyst would be deposited on the alumina. Pore size and structure would be determined by TEM and SEM, and pore surface area by nitrogen gas adsorption (BET isotherm). Improved micro fluidic and MEMS devices Micro fluidic devices are beginning to see real application in drug discovery and other areas where miniaturization of assays and sample handling can be beneficial . However, numerous practical obstacles still exist, including sample dispersion (i.e. spreading or broadening), and adsorption of molecules to the device walls. These and other problems lead to loss of valuable sample and less than optimal device operation. ALD films considered here will be used to modify the hydrophobicity of surfaces in micro fluidic devices and improve surface smoothness, surface tension, energy dissipation, and chemical reactivity. Films such as Al2O3 will be measured for roughness atomic force microscopy, surface tension by measuring the contact angle of a water droplet on the film surface, and stability in water, acidic, and basic environments as a function of film properties. Applications of these films to micro fluidic devices will be carried out in collaboration with Chris Backhouse at the University of Alberta. Micro-electromechanical systems (MEMS) are subject to several issues including mechanical wear, static friction failure, and electrical breakdown that can potentially be alleviated using nanometer thick films that impart protection without negatively impacting device performance. In this research, we will develop ALD films and processes to improve MEMS performance. Initially this project will focus on developing a low temperature Al2O3 process to improve electrical isolation and ZrO2 films to improve wear resistance and reduce the coefficient of friction. Improved diffusion barriers for integrated circuits As interconnect linewidths and films thickness have continued to shrink, copper effective resistivity has started to increase exponentially leading to increased RC signal delay. One potential solution to this problem is to shrink the diffusion barrier layer thickness so that the thickness copper in the damascene structure can be increased. This research will focus on developing nanometer thick copper barrier films that can survive IC fabrication condition using ternary and quaternary ALD alloys and compounds.
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