Method for Low Temperature Production of Nano-Structured Iron Oxide Coatings

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application Ser. No. 60/778,729 filed on Mar. 2, 2006, U.S. Provisional Patent Application Ser. No. 60/778,730 filed on Mar. 2, 2006 and U.S. Provisional Patent Application Ser. No. 60/811,403 filed on Jun. 5, 2006 the entire disclosures of which are incorporated by reference.

FIELD OF THE INVENTION

The present invention provides a method for forming nano-structured iron oxide coatings on a substrate.

BACKGROUND OF THE INVENTION

Several techniques are known for depositing iron oxide coatings onto a substrate. Most of the methods, however, are limited in that substrate temperatures greater than 400° C. are used. This is because the oxides are pyrolytically formed on the substrate surface. Such procedures inherently limit the types of substrates that may be used, since substrates melting at the high temperatures are prohibited.

It is accordingly an object of the present invention to provide a method of depositing iron oxide on a substrate at temperatures substantially below 400° C.

SUMMARY OF THE INVENTION

The present invention provides a method for forming nano-structured iron oxide coatings on a substrate. The method includes the steps of: (a) subjecting a chamber containing a plasma source to vacuum; (b) feeding iron pentacarbonyl and O₂ into a chamber containing a plasma source, wherein the O₂ is fed into the chamber at a rate greater than that of the iron pentacarbonyl; (c) subjecting the substrate to the chamber, wherein the substrate is at a temperature less than 250° C., thereby forming an iron oxide coating on the substrate, wherein the iron oxide is greater than 90 percent in the α-hematite form.

Iron Oxide Coating

The iron oxide coating formed by the method of the present invention, which is undoped, is typically greater than 90 percent a-hematite. Oftentimes, at least 95 percent or 97.5 percent of the material is a-hematite.

The iron oxide materials do not include a significant amount of either magnetite or maghemite forms of iron oxide. They typically contain less than 10 percent magnetite and/or maghemite, and oftentimes they contain less than 5 percent magnetite and/or maghemite.

The surface of the coatings of the iron oxide materials typically exhibit individual structures (e.g., disc-like structures, box-like structures, diamond-like structures, etc.) that lie in a non-parallel orientation (e.g., vertical) with respect to the substrate plane. Such structures typically have a ratio of long dimension to short dimension of at least 2:1. Oftentimes the ratio is at least 3:1 or 4:1. In certain cases, the ratio is at least 5:1 or 6:1.

The iron oxide coatings typically contain at least 10 individual structures on their surface within a 0.25 μm² area. Oftentimes, the coatings contain at least 25 or 50 disc-like structures on their surface within a 0.25 μm² area.

Method of Deposition

Iron pentacarbonyl and O₂ are fed into a chamber, containing a plasma source, through two separate feed lines. The O₂ is fed in at a rate at least 4 times greater than that of the iron pentacarbonyl. The chamber is subjected to vacuum prior to deposition and maintained under vacuum throughout the procedure. A substrate is subjected to the chamber, resulting in the production of an iron oxide coating on the substrate. During the deposition, the substrate is at a temperature less than 250° C.

The plasma source is typically a high density plasma source, and it is oftentimes an argon plasma source. In certain cases, O₂ is fed into the chamber at a rate at least 8 times greater than that of the iron pentacarbonyl, and oftentimes it is fed at a rate at least 12 times greater. The chamber is typically subjected to a vacuum of at least 0.10 torr, and, in some cases, to a vacuum of at least 0.01 torr or even 0.005 torr. Substrates may be of any suitable composition. Nonlimiting examples include a spectrally transparent cyclic-olefin copolymer, pure poly(norbomene), and a conducting glass plate having an F-doped SnO₂ overlayer. The substrate temperature during the deposition is usually less than 200° C. In certain cases it may be less than 175° C., 150° C., or 125° C.

Substrates are usually passed through the chamber during the coating process at a rate of at least 1 mm/s. Oftentimes, the substrates are passed through at a rate of at least 3 mm/s, 5 mm/s, or even 7 mm/s. Iron oxide coatings on the substrate are typically greater than 90 percent in the α-hematite form. In certain cases, the coatings are greater than 95 percent or even 97.5 percent in the α-hematite form. Coating thicknesses on the substrate usually exceed 500 Å, and can exceed 750 Å or even 1000 Å.

The following are non-limiting examples of the method of the present invention:

1. Plasma Source: High density.

• • O₂ Feed Rate: At least 50 sccm.
  • Iron Pentacarbonyl Feed Rate: At least 10 sccm.
  • Chamber Pressure: Less than 0.1 torr.
  • Substrate Composition: Spectrally transparent cyclic-olefin polymer.
  • Substrate Temperature: Less than 250° C.
  • Iron Oxide Form: Greater than 90 percent a-hematite.
  • Iron Oxide Coating Thickness: Greater than 500 Å.

2. Plasma Source: High density.

• • O₂ Feed Rate: At least 75 sccm.
  • Iron Pentacarbonyl Feed Rate: At least 15 sccm.
  • Chamber Pressure: Less than 0.1 torr.
  • Substrate Composition: Spectrally transparent cyclic-olefin polymer.
  • Substrate Temperature: Less than 250° C.
  • Iron Oxide Form: Greater than 90 percent α-hematite.
  • Iron Oxide Coating Thickness: Greater than 500 Å.

3. Plasma Source: High density.

• • O₂ Feed Rate: At least 75 sccm.
  • Iron Pentacarbonyl Feed Rate: At least 15 sccm.
  • Chamber Pressure: Less than 0.1 torr.
  • Substrate Composition: Spectrally transparent cyclic-olefin polymer.
  • Substrate Temperature: Less than 200° C.
  • Iron Oxide Form: Greater than 90 percent a-hematite.
  • Iron Oxide Coating Thickness: Greater than 500 Å.

4. Plasma Source: High density.

• • O₂ Feed Rate: At least 75 sccm.
  • Iron Pentacarbonyl Feed Rate: At least 15 sccm.
  • Chamber Pressure: Less than 0.1 torr.
  • Substrate Composition: Spectrally transparent cyclic-olefin polymer.
  • Substrate Temperature: Less than 175° C.
  • Iron Oxide Form: Greater than 90 percent α-hematite.
  • Iron Oxide Coating Thickness: Greater than 500 Å.

5. Plasma Source: High density argon.

• • O₂ Feed Rate: At least 100 sccm.
  • Iron Pentacarbonyl Feed Rate: At least 15 sccm.
  • Chamber Pressure: Less than 0.01 torr.
  • Substrate Composition: Spectrally transparent cyclic-olefin polymer.
  • Substrate Temperature: Less than 175° C.
  • Iron Oxide Form: Greater than 95 percent α-hematite.
  • Iron Oxide Coating Thickness: Greater than 500 Å.
  • Substrate Pass-Through Rate: At least 3 mm/s.

6. Plasma Source: High density argon.

• • O₂ Feed Rate: At least 150 sccm.
  • Iron Pentacarbonyl Feed Rate: At least 15 sccm.
  • Chamber Pressure: Less than 0.01 torr.
  • Substrate Composition: Spectrally transparent cyclic-olefin polymer.
  • Substrate Temperature: Less than 150° C.
  • Iron Oxide Form: Greater than 95 percent α-hematite.
  • Iron Oxide Coating Thickness: Greater than 750 Å.
  • Substrate Pass-Through Rate: At least 3 mm/s.

7. Plasma Source: High density argon.

• • O₂ Feed Rate: At least 150 sccm.
  • Iron Pentacarbonyl Feed Rate: At least 15 sccm.
  • Chamber Pressure: Less than 0.01 torr.
  • Substrate Composition: Spectrally transparent cyclic-olefin polymer.
  • Substrate Temperature: Less than 150° C.
  • Iron Oxide Form: Greater than 95 percent a-hematite.
  • Iron Oxide Coating Thickness: Greater than 1000 Å.
  • Substrate Pass-Through Rate: At least 3 mm/s.

8. Plasma Source: High density argon.

• • O₂ Feed Rate: At least 150 sccm.
  • Iron Pentacarbonyl Feed Rate: At least 15 sccm.
  • Chamber Pressure: Less than 0.01 torr.
  • Substrate Composition: Spectrally transparent cyclic-olefin polymer.
  • Substrate Temperature: Less than 150° C.
  • Iron Oxide Form: Greater than 95 percent a-hematite.
  • Iron Oxide Coating Thickness: Greater than 1000 Å.
  • Substrate Pass-Through Rate: At least 5 mm/s.

9. Plasma Source: High density argon.

• • O₂ Feed Rate: At least 150 sccm.
  • Iron Pentacarbonyl Feed Rate: At least 15 sccm.
  • Chamber Pressure: Less than 0.01 torr.
  • Substrate Composition: Poly(norbomene).
  • Substrate Temperature: Less than 150° C.
  • Iron Oxide Form: Greater than 95 percent a-hematite.
  • Iron Oxide Coating Thickness: Greater than 1000 Å.
  • Substrate Pass-Through Rate: At least 5 mm/s.

10. Plasma Source: High density argon.

• • O₂ Feed Rate: At least 150 sccm.
  • Iron Pentacarbonyl Feed Rate: At least 15 sccm.
  • Chamber Pressure: Less than 0.01 torr.
  • Substrate Composition: Conducting glass plate having an F-doped SnO₂ overlayer
  • Substrate Temperature Less than 150° C.
  • Iron Oxide Form: Greater than 95 percent a-hematite.
  • Iron Oxide Coating Thickness: Greater than 1000 Å.
  • Substrate Pass-Through Rate: At least 5 mm/s.

EXAMPLE Example 1 Deposition of Iron Oxide on Cyclic Olefin Copolymer

A sheet of Topas cyclic olefin copolymer was coated with iron oxide in the following manner. Iron pentacarbonyl and O₂ were fed into a chamber, containing a high density argon plasma source operating at 3000 W (Sencera, Charlotte, N.C.), at a rate of 20 sccm and 240 sccm respectively through two separate feed lines. The chamber was pumped down to 0.005 Torr prior to deposition and maintained at that pressure throughout the process. The sheet, which was at a temperature of 140° C., was passed over the feed outlets on a moving carriage at a speed of 5 mm/s to achieve an iron oxide deposit thickness of 1500 Å. An XRD pattern of the film showed it was an exact match for a-hematite iron oxide.