Field Emission Devices With Improved Field Emission Surfaces

1. Field of the Invention

This disclosure is directed toward field emission surfaces, and is more particularly directed toward improvements in cold, low field, high current, low noise field emission devices and surfaces. Such devices are used in field emission display devices such as video displays and information displays.

2. Background of the Art

U.S. Pat. No. 4,663,559 and E.P.S. 0,228,616 B1, both to Alton O. Christensen (Christensen), disclose a field emission device which produces high current, low noise, low lateral energy, stochastic electron emission from a multiplicity of insulative particles subject to a field. The insulative particles are in and of a surface thickness comprised of a random mixture of insulative and conductive particles. Emission is achieved at applied potentials of about 5 volts which produce a field sufficient to emit electron currents of nanoamperes to milliamperes. Single devices or arrays of devices may be batch fabricated. Each device has an integral, implicitly self-aligned electron optic system comprising means for modulating, focusing and deflecting the formed current beam, and means for shielding the device from ambient magnetic fields.

The Institute of Electrical and Electronic Engineers (IEEE) sponsored annual International Vacuum Microelectronics Conference Proceedings, as well as many other publications in the art, are replete with many field emission materials, devices, and fabrication techniques. Worldwide field emitter development is recognized as having been fostered by the work of Spindt as disclosed in U.S. Pat. No. 3,755,704 as supported by Gray of NRL. Emphasis in the field, as reflected in recent conferences devoted to the technology, has been directed toward field emitters for excitation of phosphors to make information and video display products. Emission characteristics have been published for a wide variety of metals and metal compounds, such as borides, carbides, and nitrides. Field emission displays have been demonstrated, for example, by Coloray Inc., SRI, and PixTech Inc. using gated, pointy molybdenum metal emitters. Other developers have demonstrated field emission displays using pointy silicon emitters. SI Diamond, Inc. has demonstrated field emission displays using diamond-like carbon surfaces.

All prior art field emitters suffer from at least three or four significant deficiencies which affect the reliability and operating life when these devices are utilized in information or video displays. These deficiencies are summarized as follows:

(a) The emission surfaces oxidize and/or are poisoned by gasses within the display, or by gases generated by a phosphor used in the display thereby limiting the operating life of the emission device.

(b) The turn-on potentials and current modulating potentials are in the range of 25 to 300 volts. This range of potentials requires considerable power, dissipates considerable heat, and requires expensive high voltage control and address circuitry to operate the display device.

(c) The high fields in excess of 3.times.10.sup.9 volts/meter, required for higher current emission, stress and tend to modulate the emitting surface thereby producing mechanically and temporarily unstable emission sites. This results in bursts and deficits of current from the emission sites.

(d) The stochastic nature of field emission, and added burst noise, requires incorporation of high-valued resistors added in series typically with the cathode terminal of the emitter to limit noise. Such resistors increase the potential required to produce currents required to excite display phosphors. Such resistors are inadequate since they reduce the potential to the entire emission surface, and not just to the emission area affected. The parasitic capacitance of the resistors together with the high value of series resistance produces a time constant delay in action that may well limit efficacy.

An object of this invention is to provide a field emission device with an emitter surface which is insensitive to gases that oxidize or poison the emission surface. Such gases and other contaminants emanate from a phosphor when the emission device is used in phosphor display devices.

Another object of the invention is to provide a field emission device that can be operated at lower potentials thereby reducing power requirements and minimizing heat dissipation requirements.

Still another object of the invention is to provide a field emission device which is operated at lower field in order to reduce mechanically and temporarily unstable emission sites which result in current bursts and current deficits at these sites. Even at reduced operating fields, the device emits current sufficient to operate display devices.

A still further object of the invention is to provide a field emission device which incorporates internal resistors which provide series resistance to limit noise at affected emission areas thereby eliminating the need to limit noise by incorporating high-valued resistors in series typically in series with the cathode terminal of the emission device thereby reducing the potential to the entire emission surface and increasing potentials required to produce current sufficient to excite display phosphor.

There are other objects and advantages of the present invention that will become apparent in the following disclosure.

This disclosure teaches the improvement of operating efficiency and operating life of prior art field emitters. Field emission devices which utilize a cermet as an emitter are disclosed in the previously referenced U.S. Pat. No. 4,663,559 and E.P.S. 0,288,616 B1 to Christensen, which are assigned to the assignee of the present disclosure and which are incorporated into this disclosure by reference. With these devices, emission is obtained from a multiplicity of SiO.sub.2 and Cr.sub.3 Si sites. The average barrier to emission varies between 0.8 eV and 2.3 eV, depending upon the applied potential, the percentage of emission area occupied by SiO.sub.2, and the operative field factor for Cr.sub.3 Si sites between SiO.sub.2 sites.

This invention extends the prior art of Christensen cermet field emitter. Briefly, the invention provides:

(a) an emission device with an emitter surface of thickness which is about the electron ballistic transport length therein, thereby providing an emission surface which is insensitive to gases that oxidize or poison the emission surface and which is not detrimental to current emission;

(b) an alternate metal to Cr.sub.3 Si for the cermet of insulative particles (preferably SiO.sub.2) and conductive particles, which is preferably Al.sub.2 Li.sub.3 thereby lowering the operating potential required to operate the device such that sufficient current is emitted to activate a phosphor display; and

(c) other qualified materials of characteristics similar to SiO.sub.2 to form a co-deposited, graded cermet with Cr.sub.3 Si and Al.sub.2 Li.sub.3.

The emission cermet is comprised of an increasing percentage of SiO.sub.2 which is co-deposited with a decreasing percentage of the metal Cr.sub.3 Si or Al.sub.2 Li.sub.3. The bulk resistivity of both Cr.sub.3 Si and Al.sub.2 Li.sub.3 are many times greater that the resistivity of Al alone. Within the cermet, the graded co-deposition produces reduced cross sections of Cr.sub.3 Si or Al.sub.2 Li.sub.3 which form M paths of high resistance, low RC time constant connection to N possible sites of the emission surface. These paths form the internal resistors which provide series resistance to limit noise at affected emission areas thereby eliminating the need to limit noise by incorporating high-valued resistors in series typically in series with the cathode terminal of the emission device, thereby reducing the potential to the entire emission surface and increasing potentials required to produce current sufficient to excite display phosphor.

The emission surface is formed as a contiguous layer on the side of the deposited cermet in which the SiO.sub.2 concentration is increasing and the concentration of conductive particles is decreasing. The emission surface is deposited at the time of cermet deposition and is an integral part of the graded cermet. The surface is, in fact, an extension of the particle gradation within the cermet and is a later of 100 percent insulative particles which are preferably SiO.sub.2 (silica). For purposes of discussion, however, the emission surface will sometimes be referred to in the context of a separate layer. The emission surface is at least one atomic layer thick and is preferably about the thickness of the electron ballistic transport length therein. That electron ballistic transport length is greater when the cermet metal is Al.sub.2 Li.sub.3 since the work function is about 1.5 eV less than the 2.54 eV work function of Cr.sub.3 Si, and uses less of the electron temperature limit of ballistic transport. The use of Al.sub.2 Li.sub.3 thereby lowers the operating potential required to operate an emission device employing the cermet such that sufficient current is emitted to activate a phosphor display. The silica emission surface is insensitive to gases that oxidize or poison the emission surface. As an example, such gases and contaminants emanate from phosphors in field emission display devices.

In the preferred embodiment, as in the devices disclosed in the Christensen references, emission of microampere currents from N regions of the emission surface is obtained from the conduction band of SiO.sub.2 of emission barrier less that 1 eV at fields of the order of 5.times.10.sup.8 to 7.times.10.sup.8 volts/meter for applied gate potentials of 3.5 to 5 volts. Such low fields are not sufficient to mechanically or temporally modulate the emission surface and thereby create unwanted bursts or deficits in currents. As discussed previously, such emission current bursts and deficits are prevalent in prior art emission devices.

Deposited upon any organic or inorganic conductor, or upon n-type wide and medium band gap materials, the cermet reduces their respective emission barriers of 3 to 5 eV to less than 1 eV. The SiO.sub.2 --Cr.sub.3 Si cermet deposited upon n-silicon emitters reduces their emission barrier from about 4 eV to less than 1.5 eV, which is the sum of 0.55 eV Cr.sub.3 Si metal-silicon barrier and the less than 1 eV conduction band width of SiO.sub.2.

The noise reduction achieved by the previously described cermet resistances, together with the multiplicity of emission sites of the emission surface, increases the current-plus noise to noise ratio by at least 10 log N decibels over all prior art emitting surfaces.

Attention is briefly directed toward the preferred metals of the cermet. Al.sub.2 Li.sub.3 has a work function of 1.06 eV and, like Cr.sub.3 Si, makes ohmic contact to SiO.sub.2. Al.sub.2 Li.sub.3 also makes ohmic contact to all n-type medium and high band gap materials, organics, metal borides, carbides and nitrides. Al.sub.2 Li.sub.3 is highly reactive and must have its open surface passivated by SiO.sub.2, as is the case of the cermet with the 100% SiO.sub.2 emission surface of the disclosed invention. The reactivity of Al.sub.2 Li.sub.3 also serves to form an ohmic contact with diamond and diamond like carbon as alternate materials for the SiO.sub.2 insulative particles.

Attention is next briefly directed toward the insulative component of the cermet. Although SiO.sub.2 is the preferred material for the insulative component of the emission surface and graded portion of the cermet, any material having characteristics similar to SiO.sub.2 can be used. SiO.sub.2 passivates Al.sub.2 Li.sub.3, has an electron mobility of about 3.times.10.sup.5 meter.sup.2 per volt-second, and has a conduction band width of less than 1 eV. Organic compounds, or metal borides, carbides, or nitrides qualified for use in the cermet of the present invention, have resistance to oxidation and contamination, electron mobility, 1 eV or less conduction band width, and, if required, passivate Al.sub.2 Li.sub.3.

So that the manner in which the above recited features, advantages and objects of the present invention are obtained and can be understood in detail, more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings.

FIG. 1 is a partial section view of a prior art gated field emission device, with its series current limiting resistance, in the process of emitter deposition;

FIG. 2 is a sectional view of the cermet and emission surface of the present invention showing built-in current limiting resistances;

FIG. 3 is a sectional view of in-process addition of the cermet and emission surface to a typical prior art emitter; and

FIG. 4 is a sectional view of an improved emission surface on a field emission device.

Attention is first directed to FIG. 1 which is a partial section view of a prior art gated field emission device, with its series current limiting resistance, in the process of emitter deposition. The structure, generally denoted by the numeral 10, is in process of fabrication. The inventor makes no claims concerning this illustration of prior art. FIG. 1 illustrates a step in the fabrication process after, but not limited to, the previously cited U.S. Pat. No. 3,755,704 reference to Spindt wherein an emitter material 15 is molybdenum which is deposited at grazing incidence to form a conical shaped emitter 16.

Still referring to FIG. 1, the device 10 is formed on a substrate metal 11 which serves as the cathode for the device and which interconnects the device to the cathode of similar devices. A device surrounding gate is denoted by the numeral 13 and comprises metal. An oxide layer 12 contacts the substrate 11 and the gate 13 and electrically isolates the gate 13 from the substrate 11. A layer 14 is a layer of metallic or insulative material and serves a sacrificial parting layer in the manufacture of the device as will be illustrated in subsequent discussion. The layer 15 is excess emitter metal deposited in the formation of the emitter 16. A layer 19 functions as the previously discussed series resistor substrate-to-cathode of prior art devices required to control emitter current bursts and emitter current deficits. The layer 19 is, of course, formed of highly resistive material.

In the case of the device of FIG. 1 wherein 16 is a semiconductor, such as silicon, substrate 11 is also a semiconductor. In this case, the emitter 16 is formed by the etching of it semiconductor substrate 11, and layers 14 and 15 are absent.

The cermet device of the present invention is shown in FIG. 2 and is generally denoted by the numeral 20. The cermet is shown deposited on the emitter 16 and, as discussed previously, comprises a co-deposited, graded composition of metallic material 33 and insulative material 32, and a layer 34 of insulative material which forms the emission surface of the cermet. The metal is preferably particles of Cr.sub.3 Si or alternately Al.sub.2 Li.sub.3. The insulative material is preferably particles of SiO.sub.2. The concentration of SiO.sub.2 particles increases toward the layer 34 while the concentration of metal increases toward the emitter 16. The co-deposited, graded portion 18 of the cermet and emission surface layer 34 are actually deposited on the emitter with the same graded deposition operation, with the concentration of deposited SiO.sub.2 particles being increased as the cermet is built upon the emitter 16 until no metal 33 is being deposited and only SiO.sub.2 32 is being deposited thereby producing the oxide layer 34. This final oxide surface layer 34 has a thickness 34' of the electron ballistic transport length in SiO.sub.2 of about 5 nanometers (mm), such that the thickness 34' is several atomic layers of SiO.sub.2. Al.sub.2 Li.sub.3, used as metal 33 in the cermet portion 18 to produce ohmic contact to medium band-gap materials and diamond-like carbon, requires overlayer passivation such as provided by the layer 34 of SiO.sub.2. This layer forms the silica emission surface which is insensitive to gases that oxidize or poison the emission surface, such gases and contaminants emanate from phosphors in field emission display devices.

Deposition of the cermet is accomplished from two cooperating deposition sources, each with independently controlled energies and rates of deposition. With independent control of the deposition sources, the grading of the cermet portion 18 as to percentages of oxide and metal, and the size of the particles being deposited, can be precisely controlled. Furthermore, the thickness 34' of the oxide layer 34 can be precisely controlled. Dual ion beams have been used as deposition sources to deposit the cermet 20 upon the depositional surface 16. The co-deposition is better accomplished by atomic layer epitaxy, which allows the composition of the cermet to be varied in each successively formed atomic layer, starting with a layer of 100% metal 33 contacting the deposition surface 16 and terminating with a layer 34 of 100% oxide forming an emission surface.

Still referring to FIG. 2, the numeral 35 indicates a typical conducting, resistive channels of metal 33 which exhibit decreased cross section and length. As shown in FIG. 2, these channels extend from the emitter 16 to the layer 34. The resistivities of the preferred cermet metals 33, which are Cr.sub.3 Si and Al.sub.2 Li.sub.3, are many times greater than aluminum. These constricted conduction channels, typified by the numeral 35, function as current limiting resistors within the cermet to particular areas of emission on the surface of the layer 34. This results in the suppression of emission current bursts and emission current deficits. The resistors 19 required in prior art devices (see FIG. 1) reduces the potential to the entire emission surface. These prior art devices, when used as an example in screen display applications, require increased operating potentials to produce emission currents sufficient to excite display screen phosphors.

Focusing again on the resistor channels 35, the cermet device shown in FIG. 2 present invention exhibits a significant improvement over the prior art devices represented in FIG. 1 in that current limiting of the present invention tends to (a) stabilize the current of a particular area of the surface of the emission layer 34 and not of the entire emission surface as is the case of the prior art devices, and (b) decrease the noise component in the emitter current, acting with very short time constant, and may well eliminate the need for external resistors 19 used in emitters described in the prior art literature. The current limiting resistors' magnitude may be increased by increasing the initial percentage of the oxide in the cermet thereby further constricting the cross sections of the limiting resistor paths 35. In summary, emission devices employing the cermet and internal, site specific resistor channels 35 of the present invention can be operated at lower operating potentials therefore reducing power requirements and heat dissipation requirements.

Attention is now drawn to FIGS. 2 and 3. FIG. 3 shows a field emission device 30 in the process of fabrication. The layers of material 11, 12, 13, 14 and 15 have been discussed previously. The emitter 16 is formed in the shape of a pyramid or preferably a cone with the point of the cone truncated to receive the cermet material 20. The cermet includes the graded region 18 and the emission surface 34 as shown in FIG. 2. Co-deposition of the insulative and conductive particles of the cermet is initiated, and deposits of cermet mixture are formed on the truncated point of the emitter 16 as well as on the layer 15 as excess cermet material 17. The previously discussed deposition of the emitter 16 at grazing incidence produced the excess layer 15 of emitter material with an aperture 15' with conical shaped walls as shown in FIG. 3. Deposition of the cermet material upon the emitter 16 continues through the aperture 15' until the aperture is closed with cermet material 17. The cermet material deposited upon the emitter 16 is essentially conical in shape, with the graded region 18 of the cermet being passivated by the emission layer 34.

Referring now to both FIG. 3 and FIG. 4, the layers 17 and 15 are removed by dissolution of the sacrificial layer 14, thereby producing a truncated conical emitter 16 with and essentially conical cermet 20 as shown in FIG. 4.

If the emitter material 16 is etched silicon, then the layer 15 is not present. Using as an example a device of the type shown in FIG. 1 wherein 16 is a semiconductor, such as silicon, and substrate 11 is also a semiconductor. In this case, the emitter 16 is formed by the etching of it semiconductor substrate 11, and layers 14 and 15 are absent.

The function of the emitter 16 of the previously described field emission structure can be varied depending upon the type or classification of the structure. More specifically, the specification of the element 16 depends upon whether the element is to be operated:

(a) as a surface emitter itself, wherein the material of 16 is a low resistance interconnected metal such as copper or aluminum;

(b) as an improvement to a metal emitter 16, such as molybdenum, shown in FIGS. 2, 3, and 4;

(c) as an improvement to a silicon emitter as shown in FIGS. 2 and 4;

(d) as an improvement to emitter materials such as diamond or diamond-like carbon n-doped and contacted by Al.sub.2 Li.sub.3 as shown in FIGS. 2, 3, and 4; or

(e) as an improvement to emission of metal nitrides or metal carbides and the like as shown in FIGS. 2, 3 and 4.

FIG. 4 illustrates an example of an improved field emission device as taught by this disclosure. The device is indicated as a whole by the numeral 40. It comprises an essentially conical cermet 20 comprising a graded region 18 and an emission layer 34 (see FIG. 2). The cermet is deposited upon the truncation of the conical emitter 16 which is electrically connected to a metallic substrate 11. The element 13 is a gate with a preferably cylindrical aperture within which the emitter 16 and cermet 20 are centered. The layer 12 is an oxide layer which isolates the gate 13 from the substrate 11.

Still referring to FIG. 4, the element 22 represents a phosphor in the case where the field emission device is used in a display device. More specifically, a typical display device comprises a multiplicity of field emission devices sharing a common substrate 11 and each directed toward an assigned target area of the phosphor 22. Integrated circuit control means supported by the common substrate 11 are used to control the multiplicity of field emission devices as disclosed in the previously cited references of Christensen.

The field emission device 40 can be used in one of various types of r-f amplifiers. In this application, the element 22 in FIG. 4 represents the anode of the device 40.

The foregoing disclosure is directed toward the preferred embodiments of the invention, but the scope of the invention is defined by the claims which follow.