Carbon Film for Field Emission Devices

The present invention relates in general to field emission devices, and in particular, to a carbon emission film exhibiting certain characteristics.

Field emission devices show promise for enabling simpler and more efficient field emission devices, such as flat panel displays and other types of display systems where simplicity of manufacture is desired. Field emission devices that utilize flat cathodes are preferred over cathodes requiring micro-tip type structures. Therefore, there is a need in the art for better and more efficient field emission materials, which are practical for using in such flat cathodes.

The present invention provides a field emission film for use in manufacturing a flat cathode, which exhibits certain physical characteristics. With 244 nanometers ("nm") and a 2-7 milliwatts ("mW") excitation source, and within the wave number from 1100 to 1850 cm.sup.-1, the carbon film has a distinct ultraviolet ("UV") Raman band in the range from 1578 to 1620 cm.sup.-1 with a fill width at half maximum ("FWHM") from 25 to 165 cm.sup.-1. There can be another less significant line in the range from 1318 to 1340 cm.sup.-1 with a FWHM at least 18 cm.sup.-1. Additionally, there is sometimes a wide band (FWHM>180 cm.sup.-1) between 1360 and 1420 cm.sup.-1. This wide band can appear as either a separate band or as a shoulder of the excitation line near 1580 cm.sup.-1. In the visible Raman, these films exhibit the carbon DIG pair bands around 1350 cm.sup.-1 and 1580 cm.sup.-1, respectively.

The carbon film may be thinner than 300 nanometers. The substrate on which the film is deposited can be conductive or non-conductive. In the case of a non-conductive substrate, the substrate can be coated either with a continuous conductive layer or a fine network of conductive material.

The carbon layer can be deposited by chemical vapor deposition, physical vapor deposition, electrolysis, printing or painting. The film can be continuous or non-continuous, with a dense array of particles, each having a size less than 300 nanometers.

The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a UV Raman spectrum of a sample cathode of the present invention;

FIG. 2 illustrates a UV Raman spectrum of another sample cathode of the present invention;

FIG. 3 illustrates another UV Raman spectrum of a sample cathode in accordance with the present invention;

FIG. 4 illustrates a phosphor screen image of the emitting carbon film whose UV Raman spectrum is illustrated in FIG. 1;

FIG. 5 illustrates a phosphor screen image of the emitting carbon film whose UV Raman spectrum is illustrated in FIG. 2;

FIG. 6 illustrates a phosphor screen image of the emitting carbon film whose UV Raman spectrum is illustrated in FIG. 3;

FIG. 7 illustrates a field emission device incorporating the present invention;

FIG. 8 illustrates an apparatus for depositing the carbon film of the present invention;

FIG. 9 illustrates a graph of a Raman spectra of a prior art DLC film; and

FIGS. 10-12 illustrate visible Raman spectra of the sample cathodes illustrated in FIGS. 1-3, respectively.

In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known circuits have been shown in block diagram form in order not to obscure the present invention in unnecessary detail. For the most part, details concerning timing considerations and the like have been omitted inasmuch as such details are not necessary to obtain a complete understanding of the present invention and are within the skills of persons of ordinary skill in the relevant art.

Referring to FIG. 7, there is illustrated a field emission device manufactured with a carbon film 703 in accordance with the present invention. On substrate 701, conductive layer 702 is deposited and carbon layer 703 is deposited on top thereof. The anode includes substrate 704, which may be a glass substrate, conductive layer 705, which may be indium-tin oxide, and a phosphor layer 706 for receiving electrons emitted from carbon layer 703. Electrons are emitted from layer 703 in response to an appropriate electric field between the anode and the cathode. The apparatus in FIG. 7 is illustrated as a diode structure, but one or more grids may be included. The apparatus may be used to produce a field emission lamp or a matrix-addressable color display. For further discussion of such configurations, please refer to U.S. Pat. Nos. 5,449,970 and 5,548,185, which are hereby incorporated by reference herein.

Referring to FIG. 8, the carbon layer 703 may be deposited using a hot filament assisted chemical vapor deposition ("CVD") process. The substrate 803 (optionally with the conductive layer 702 deposited thereon) is placed on a holder 801 in a CVD reactor 802. A hydrogen gas 805 is flowed into the reactor 802 for approximately less than 10 minutes. Then a hydrogen 805 and methane 806 mixture with the methane percentage less than 50% is flowed into the reactor 802 for less than one hour. Another hydrogen 805 and methane 806 mixture with the methane percentage lower than in the above step is flowed into the reactor 802 for less than two hours. Then, a hydrogen 805 flow for less than 15 minutes is performed within the CVD reactor 802.

A small amount of oxygen, nitrogen, or boron dopant can also be included in the above gas flow.

The filament 804 temperature is set in the range from 1600.degree. C. to 2400.degree. C. and the substrate 803 temperature is between 600.degree. C. and 1000.degree. C. Deposition pressure is between 5-300 torr.

The carbon film that results from this process exhibits superior emission qualities to those of prior art diamond-like carbon or CVD diamond films. A UV Raman spectrum was measured on three samples of carbon films deposited in the manner set forth above. With a 244 nm and 2-7 mW excitation source, and within the frequency from 1100 cm.sup.-1 to 1850 cm.sup.-1, the carbon film produced in the manner above has a distinct UV Raman band in the range from 1578 cm.sup.-1 to 1620 cm.sup.-1 with a FWHM from 25 to 165 cm.sup.-1. There may also be another less significant line in the range from 1318 to 1340 cm.sup.-1 with a FWHM greater than 18 cm.sup.-1. Between 1360 to 1410 cm.sup.-1, there is, sometimes, also a band with a FWHM greater than 180 cm.sup.-1.

Though the carbon layer described above was deposited using a hot filament assisted CVD process, the carbon layer may also be deposited using physical vapor deposition, electrolysis, printing, or painting. The carbon film can be continuous or noncontinuous, and may have a dense array of particles each having a size less than 300 nanometers.

As noted above, three samples of the carbon film were deposited in the manner described above. A Raman spectrum on the first sample is shown in FIG. 1 having a distinct UV Raman band at 1580.8 cm.sup.-1 with an 89.7 cm.sup.-1 FWHM. There is a much less significant line at 1329.7 cm.sup.-1 with a 24.6 cm.sup.-1 FWHM.

A second sample of carbon layer was tested for its UV Raman spectra, which is illustrated in FIG. 2. This sample exhibited a UV Raman band at 1583.4 cm.sup.-1 with a 45.3 cm.sup.-1 FWHM.

A third carbon layer sample was measured for its UV Raman spectra as illustrated in FIG. 3. This carbon layer had a distinct UV Raman band at 1612.2 cm.sup.-1 with a 77 cm.sup.-1 FWHM. This film also exhibited a shoulder at 1408 cm.sup.-1.

FIGS. 4-6 illustrate photographs of phosphor screen images of the three carbon film samples illustrated in FIGS. 1-3, respectively. These screen images were produced by a field emission device similar to that illustrated in FIG. 7. The phosphor screen image in FIG. 4, pertaining to the Raman spectra illustrated in FIG. 1, was produced with a 5.3 volts per micrometer electric field.

The phosphor screen image illustrated in FIG. 5, pertaining to the UV Raman spectra illustrated in FIG. 2, was produced with a 6.3 volts per micrometer electric field.

The phosphor screen image illustrated in FIG. 6, pertaining to the UV Raman spectra illustrated in FIG. 3, was produced with a 5.7 volts per micrometer electric field.

The carbon film of the present invention is thin (less than 300 nanometers) and consists of a unique range of combinations of amorphous, highly disordered graphitic carbon and some randomed and ordered sp.sup.3 bonded carbon. The content, or the amount, of the ordered sp.sup.3 bonded carbon (i.e., diamond structure) in these films is so small that in a UV Raman spectra, with a typical instrumental operating condition, its signature Raman excitation line around 1332 cm.sup.-1 does not appear in most cases or is less significant compared to those from sp.sup.2 bonded carbon, despite the fact that the UV Raman is 25 times more sensitive to diamond/graphitic carbon ratios than conventional visible Raman spectrum (see FIGS. 1-3).

The domain size of the ordered sp.sup.3 carbon is likely less than 60 angstroms judging from the FWHM and frequency shift of its Raman line (see FIG. 1). The existence of sp.sup.3 bonding, particularly the amorphous sp.sup.3 in these films, is more often inferred from the up frequency shift and/or a strong shoulder on the lower frequency side of the typical sp.sup.2 carbon excitation line around 1580 cm.sup.-1 as shown in FIG. 3. This shoulder can sometimes appear as a broad band between 1360 to 1410 cm.sup.-1 with a FWHM larger than 180 cm.sup.-1, when the 1580 cm.sup.-1 line is not as strong. In some films, an almost typical graphitic carbon line with a slight up frequency shift and broadening appeared, indicative of little content of sp.sup.3 bonded carbon and more graphitic carbon structure in the film (see FIG. 2).

FIGS. 10-12 illustrate visible Raman spectra on the three samples illustrated with respect to FIGS. 1-3, respectively. The visible Raman spectra were produced with a 514.5 nm and 10 mW excitation source. These three Raman spectra clearly show the D/G pair of peaks at around 1350 cm.sup.-1 (D peak) and 1580 cm.sup.-1 (G peak). Typically, the D peak of the carbon film of the present invention will be between 1340 cm.sup.-1 and 1380 cm.sup.-1, while the G peak will be between 1578 cm.sup.-1 and 1620 cm.sup.-1.

Prior carbon films that had been reported suitable for field emission application include CVD or defect enriched CVD diamond films, such as nitrogen or boron doped or ion implanted diamond films and diamond-like carbon (DLC) films with predominantly sp.sup.3 bonding. Consisting mostly of diamond bonded carbon and its domain size being much larger than 60 angstroms, CVD or defective CVD diamond films typically exhibit a dominant diamond excitation line near 1332 cm.sup.-1 with a FWHM less than 18 cm.sup.-1, and a broad and often much less significant band of a FWHM larger than 170 cm.sup.-1 near 1580 cm.sup.-1 in the UV Raman spectrum. It should be noted that although there have been reports showing that fine grained CVD diamond films sometimes have a weak or almost invisible shift around 1332 cm.sup.-1 in the visible Raman, their UV Raman shows a strong and sharp diamond line because of the ultrasensitivity of UV Raman to the diamond/graphitic carbon ratio. In other words, the film claimed within the present invention contains less diamond structure. In most cases, its presence is completely undetectable even by UV Raman.

DLC films are amorphous carbon with mixed sp.sup.2 and sp.sup.3 bonding. Depending on the method of making it, the sp.sup.3 /sp.sup.2 bonding ratio in the film can change widely from mostly sp.sup.2 to mostly sp.sup.3. There has been a widely accepted assumption that there are sp.sup.3 clusters in these films and that these clusters resemble diamond so that they have a negative electron affinity. Based on the above assumption, DLC films with predominantly sp.sup.3 bonding have been claimed suitable for field emission application. Although the UV Raman spectra of these DLC films also exhibit an excitation line from 1580 to 1620 cm.sup.-1, their visible Raman does not exhibit the 1580 cm.sup.-1 line (G band) nor the 1350 cm.sup.-1 line (D band). Instead, they have a distinct asymmetric and broad band located between 1510 to 1600 cm.sup.-1. In addition, even in UV Raman, these predominantly sp.sup.3 bonding DLC films always have another wide band or shoulder peaking at approximately 1150 cm.sup.-1, and the FWHM of their 1580 cm.sup.-1 lines is often larger than 165 cm.sup.-1. This second band at 1150 cm.sup.-1 provides another distinction between the prior art DLC films and the carbon films of the present invention. The Raman spectra of a DLC film is illustrated in FIG. 9 showing the asymmetric band located between 1510 to 1600 cm.sup.-1 in the visible Raman, with this peak shifting to 1600 cm.sup.-1 as the photon energy is increased to the UV Raman spectrum. Further discussion may be found within Resonant Raman Scattering of Amorphous Carbon and Polycrystalline Diamond Films, J. Wagner, M. Ramsteiner, Ch. Wild and P. Koidl, Physical Review B, Jul. 15, 1989, Vol. 40, pp. 1817; Ultraviolet Raman Spectroscopy Characterizes Chemical Vapor Deposition Diamond Film Growth and Oxidation, Richard W. Bornett, et al., Journal of Applied Physics, 77 (11), June 1995, pp. 5996; and UV Studies of Tetrahedral Bonding in Diamond-Like Amorphous Carbon, V. L. Merkuluv, et al., Physical Review Letters, June 1997, pp. 4869, which are hereby incorporated by reference herein.

As discussed above, pertaining to the UV Raman spectrum, the peak position, line width, line shape, and their intensity ratios are characteristic of the bonding structure of the carbon film. The frequency and FWHM range, as well as the line shape and intensity ratio given in this invention represent a group of carbon film of ordered and disordered sp.sup.2 and sp.sup.3 bonding structure that has not been utilized in the prior art as a field emitter. FIGS. 1-6 show three examples of this group of carbon films that have exhibited excellent field emission properties.

The advantages of a carbon film of the present invention with the particular Raman spectra disclosed include that the film can emit at an extraction electric field lower than 2 volts per micrometer, the carbon film has an emission site density of at least 10.sup.4 sites/centimeter.sup.2 at 10 volts per micrometer, and the carbon film can provided a current density of 100 milliamps per centimeter.sup.2 or higher. The emission properties make these carbon films particularly desirable for highly uniform, high performance field emission electron devices. It should be emphasized that although there might have been reports that some prior art diamond or DLC films can emit at a comparable electrical field or deliver a current density that is comparable, there has never been a report on the emission site density of those films. In fact, often only a few emission sites have been shown and a reported emission current density is inferred from a current gathered from those few sites. The carbon film disclosed herein emits all over the surface uniformaly.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.