A New Solution for CVD

A new combustion chemical vapor deposition process is yielding high-quality thin films and coatings at a lower cost compared to conventional coating technologies



Coatings and nanopowders are used in a wide range of applications, from cutting tools to silicon chips, and have become a part of everyday life. Thin films (defined as <10 microns) function as resistors, dielectrics, conductors, catalysts, gas barriers, corrosion inhibitors and appearance enhancers. Nanopowders typically have an average particle size of less than 100 nm and function differently than larger sized powders. They are used in variety of different applications, including pigments, batteries, high-temperature catalysis, cosmetics, ink jet printing and chemical mechanical polishing.

Over the past few decades, several methods have been developed to deposit thin films. These techniques fall into four major categories: evaporative, glow discharge, gas phase chemical and liquid phase chemical.¹ Most thin-film techniques are vacuum-based, expensive and batch-process limited. Recent attempts to develop a non-vacuum technique have resulted in the invention of a new open atmosphere process called combustion chemical vapor deposition (CCVD).^2,3 This innovative technique overcomes many of the shortcomings of traditional thin-film vapor deposition methods, while yielding similar or better quality coatings at a lower cost.

Features and Benefits of the CCVD Process

The CCVD process relies on its ability to produce thin films in the open atmosphere using vapor precursors or inexpensive precursor chemicals in solution. Synthesis of coatings through the liquid solution CCVD technique involves four steps:

Solution Preparation. Starting precursors of materials, such as nitrates, acetylacetonates and ethylhexanoates, are dissolved in suitable aqueous or organic solvent(s).

Atomization. The solution is delivered by means of a pump to a proprietary atomizer,* where it is converted into submicron droplets.

Combustion. The metastable droplets are transported by a suitable gas to a flame, where they are vaporized and combusted.

Deposition. Coatings are conveyed onto a substrate by sweeping through the flame plasma. The heat from the flame supplies the energy required to evaporate the atomized droplets, which causes the precursors to react or decompose to form the desired material while flowing to vapor deposit onto the substrate.

The CCVD process has successfully deposited more than 100 materials, including oxides, metals, phosphates, carbonates and sulfates.^4-15 Additionally, when using a special spraying process** developed in conjunction with the proprietary atomizer, even polymers and polymer composite thin films can be deposited. The CCVD technique enables coatings to be "grown" onto substrates ranging from polymers to metals to ceramics. Amorphous, polycrystalline, preferred orientation and epitaxial coatings of materials have been synthesized using CCVD, with microstructures ranging from porous to very dense. CCVD is a true vapor process, since it is not line-of-sight restricted and produces epitaxial, conformal and nanostructured coatings.⁸

The ability to produce coatings in the open atmosphere makes this process amenable to continuous production. The CCVD technology is less expensive in terms of capital (equipment) and operating cost (elimination of costly precursors) compared to traditional CVD. CCVD also uses inexpensive starting materials, which do not need to have a high vapor pressure, and produces benign byproducts. A schematic representation of the CCVD process is shown in Figure 1.

Since the concentration and constituents in the precursor solution can be easily adjusted, the CCVD technique allows doping and tailoring of thin-film stoichiometry. This facilitates the straightforward production of complex oxides such as barium strontium titanate (BST) and lanthanum-doped lead zirconium titanate (PLZT) with good control of stoichiometry and composition. Composite coatings (for producing materials with unique properties) have also been used to produce electrically insulating coatings with improved thermal management properties.¹⁰ Additionally, a variation of CCVD known as combustion chemical vapor condensation (CCVC) is used to make nanopowders, wherein the vapor is allowed to condense homogenously and the substrate is replaced with a collection device. In short, the open atmosphere CCVD process offers the following capabilities:

• Deposits high-quality coatings onto a wide variety of substrates
• Produces conformal and non-line-of-sight coatings
• Facilitates tailored nanostructures
• Grows amorphous, crystalline or oriented coatings of metals, oxides and composites
• Permits continuous production of coatings
• Enables quick material development cycles
• Reduces capital and operating costs
• Results in environmentally friendly byproducts
• Allows a high degree of compositional control and ease of doping
• Produces multilayered structures

*The Nanomiser^®, **The NanoSpray^SM process

Targeted Applications and Materials

Frequency Agile Materials and Devices

Tunable microwave devices take advantage of the electric field dependence of the dielectric constant of ferroelectric materials.^16-18 Thin films of ferroelectrics with the required material and electronic properties are being developed for applications such as filters, antennas, voltage controlled oscillators and phase shifters.^9,11,18 BST is a leading ferroelectric candidate material for these applications. Voltage tunable devices are also being developed that will address the wireless industry's continuing needs, along with advanced tunable capacitors, resonators and phase shifters.

New radio frequency (RF) wireless products are resulting from the unique capability to deposit a BST dielectric as nanostructured coatings on commercially viable sapphire substrates. Such coatings lead to tunable components that reduce size, save power, and improve flexibility for cell phones and other RF mobile devices, as well as phase shifting components¹¹ that enable beam steering for wireless local area network (WLAN) systems for interference mitigation.

Sensors

The need for simple, accurate and inexpensive chemical and biological sensors exists in many important applications, including food inspection for bacteria, real-time feedback in chemical production and continuous monitoring of drinking water for contamination. This technology is in urgent demand for sensors to monitor against potential chemical and biological attacks.

A unique integrated optic chip technology (see Figures 2 and 3) has been developed that provides a simple solution. The detection circuits in the patented optical chip permit direct, real-time simultaneous measurement of chemicals and biomolecules in both liquids and gases. These integrated optic chips are relatively inexpensive to make with the CCVD technology.

Embedded Passives and Electronics

Currently, many printed wiring boards use electronic passive components (resistors, capacitors and inductors) that are picked, placed and then soldered onto the board surface. Embedded passives are a recent concept involving the integration of thin-film-based electronic components that are patterned and laminated within the multilayer printed circuit board structure.^9,19,20 Proprietary nanoengineered resistive and dielectric coatings enable embedded passive components to improve performance, decrease size and reduce the use of solder. These coatings have been licensed on a non-exclusive basis by Rohm and Haas Electronic Materials. It is expected that this technology will replace surface-mount passives with thin-film passives incorporated inside a multi-layer circuit board. Also under development are organic matrix materials for embedded capacitors that yield values from 10 to >100 nF/cm².

Other electronic applications currently being developed include low-K dielectrics, thermal management and power capacitor coatings.¹⁰

Nanopowders

Nanopowders continue to replace conventional powders in many applications because of their unique properties, such as high surface-area-to-volume ratios, easy formability and improved performance in end use products. They have utility in a variety of applications, including electronics, cosmetics, catalysts and polishing.^21,22 For example, because of improved catalytic performance from nanomaterials, current catalysts will be replaced with next-generation advanced catalysts that require little or no expensive precious metals such as platinum.^23,24 Nanomaterials will be more expensive than the larger grain materials per unit weight, but the performance will be superior on a cost basis. Using the CCVC modification of the CCVD technology discussed previously, nanopowders can be produced in either a dry or wet form.

The liquid-sourced CCVC process (see Figure 4) is a cost-effective method for producing a variety of mixed metal oxides and metal nanopowders. The benefits of this solution combustion technology include the simplicity of equipment, the flexibility to form complex compositions, and higher production rates at a lower cost.

The spraying process and proprietary atomizer discussed previously can produce aerosols with controllable droplet size distribution (Figure 5). In the CCVC production of nanopowders, the spraying process is used to convert a starting liquid solution containing chemical precursors into an ultra-fine aerosol that is efficiently combusted to produce nanopowders.

Both metal and oxide materials-including platinum, silver, gold, ceria, zinc oxide, alumina, silica, YSZ and BST-have been synthesized using this process.^23,24 Nanopowders produced through the CCVC process offer the potential benefits of low production costs, high yield and ease of scalability.

The resulting nanopowders allow manufacturers in a wide range of industries to fabricate nanodevices by building them from the bottom up, starting with vapor and nano-cluster sized building blocks. Nanopowders can be produced on a continuous basis, in almost unlimited compositions, faster and at a significantly lower cost through the use of the proprietary atomizer device and CCVC process. The process is currently being used to develop and produce specialized nanomaterials for applications in pigments, solid oxide fuel cell components, batteries, high-temperature catalysis, cosmetics, ink jet printing, medical implants and prosthetics, capacitors, chemical mechanical polishing, and other areas.

Barrier Coatings

In areas outside of the ceramic industry, the technology has specific capabilities in depositing polymer and oxide materials for barrier coatings used in food and beverage packaging, oxidation/moisture protection, and corrosion protection.

Creating Next-Generation Technologies

The CCVD process, as well as CCVC and other variations, offers novel ways to synthesize coatings and nanostructures of oxides, metals and polymer composites in the open atmosphere. As a result of its ability to deposit unique materials, the process lends greater flexibility in enabling next-generation products in the broadband, electronics and advanced energy sectors. Commercialization efforts are underway that capitalize on the benefits of the CCVD spraying and atomizing technologies-including substantial cost savings, versatility and tailor-made coatings.

Editor's note: All of the technologies discussed in this article, unless otherwise noted, have been developed or are under development by nGimat.

Acknowledgements

Funding for the technologies discussed in this article has been received through Small Business Innovative Research from the Ballistic Missile Defense Organization, Department of Energy, Department of Navy, Air Force, National Aeronautics and Space Administration, National Science Foundation, Defense Advanced Research Projects Agency, and the National Institute of Standards and Technology's (NIST) Advanced Technology Program.

About the Author

Andrew Hunt, Ph.D., is chief executive officer, chief technology officer and founder of nGimat, and inventor of the Nanomiser device and patented CCVD technology. He has published two book chapters and more than 15 scientific papers, and has several dozen patents issued or pending as a result of his work at nGimat. He can be reached at nGimat Co., 5315 Peachtree Industrial Blvd., Atlanta, GA 30341; (678) 287-2400; e-mail ahunt@ngimat.com ; or http://www.ngimat.com .

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