Field emission of electrons into vacuum from suitable cathode materials is currently the most promising source of electrons in vacuum devices. These devices include flat panel displays, klystrons, traveling wave tubes, ion guns, electron beam lithographic apparatus, high energy accelerators, free electron lasers, electron microscopes and microprobes. The most promising application is the use of field emitters in thin matrix-addressed flat panel displays. See, for example, the December 1991 issue of Semiconductor International, p. 46; C. A. Spindt et at., IEEE Transactions on Electron Devices, vol. 38, p. 2355 (1991); I. Brodie and C. A. Spindt, Advances in Electronics and Electron Physics, edited by P. W. Hawkes, vol. 83, pp. 75-87 (1992); and J. A. Costellano, Handbook of Display Technology, Academic Press, New York, pp. 254 (1992), all of which are incorporated herein by reference.
A typical field emission device comprises a cathode including a plurality of field emitter tips and an anode spaced from the cathode. A voltage applied between the anode and cathode induces the emission of electrons towards the anode.
A conventional electron field emission flat panel display comprises a flat vacuum cell having a matrix array of microscopic field emitters formed on a cathode of the cell (the back plate) and a phosphor coated anode on a transparent front plate. Between cathode and anode is a conductive element called a grid or gate. The cathodes and gates are typically skewed strips (usually perpendicular) whose regions of overlap define pixels for the display. A given pixel is activated by applying voltage between the cathode conductor strip and the gate conductor. A more positive voltage is applied to the anode in order to impart a relatively high energy (400-3,000 eV) to the emitted electrons. See, for example, U.S. Pat. Nos. 4,940,916; 5,129,850; 5,138,237 and 5,283,500, each of which is incorporated herein by reference.
Ideally, the cathode materials useful for field emission devices should have the following characteristics:
(i) The emission current is advantageously voltage controllable, preferably with drive voltages in a range obtainable from off-the-shelf integrated circuits. For typical device dimensions (1 .mu.m gate-to-cathode spacing), a cathode that emits at fields of 25 V/.mu.m or less is suitable for typical CMOS circuitry. PA0 (ii) The emitting current density is advantageously in the range of 0.1-1 mA/mm.sup.2 for flat panel display applications. PA0 (iii) The emission characteristics are advantageously reproducible from one source to another, and advantageously stable over a long period of time (tens of thousands of hours). PA0 (iv) The emission fluctuation (noise) is advantageously small so as not to limit device performance. PA0 (v) The cathode is advantageously resistant to unwanted occurrences in the vacuum environment, such as ion bombardment, chemical reaction with residual gases, temperature extremes, and arcing; and PA0 (vi) The cathode is advantageously inexpensive to manufacture, without highly critical processes, and is adaptable to a wide variety of applications.
Previous electron emitters were typically made of metal (such as Mo) or semiconductor (such as Si) with sharp tips in nanometer sizes. Reasonable emission characteristics with stability and reproducibility necessary for practical applications have been demonstrated. However, the control voltage required for emission from these materials is relatively high (around 100 V) because of their high work functions. The high voltage operation aggravates damaging instabilities due to ion bombardment and surface diffusion on the emitter tips and necessitates high power densities to produce the required emission current density. The fabrication of uniform sharp tips is difficult, tedious and expensive, especially over a large area. In addition, the vulnerability of these materials to ion bombardment, chemically active species and temperature extremes is a serious concern.
Diamond is a desirable material for field emitters because of its negative electron affinity and its robust mechanical and chemical properties. Field emission devices employing diamond field emitters are disclosed, for example, in U.S. Pat. Nos. 5,129,850 and 5,138,237 and in Okano et al., Appl. Phys. Lett., vol. 64, p. 2742 (1994), all of which are incorporated herein by reference. Flat panel displays which can employ diamond emitters are disclosed in co-pending U.S. patent applications Ser. No. 08/220,077 filed by Eom et al on Mar. 30, 1994 (abandoned); Ser. No. 08/299,674 filed by Jin et al. on Aug. 31, 1994 (Issued as U.S. Pat. No. 5,504,385, on Apr. 2, 1996); Ser. No. 08/299,470 filed by Jin et at. on Aug. 31, 1994; Ser. No. 08/331458 filed by Jin et at. on Oct. 31, 1994; Ser. No. 08/332179 filed by Jin et at. on Oct. 31, 1994; and Ser. No. 08/361616 filed by Jin et al. Dec. 22, 1994. These six applications are incorporated herein by reference.
While diamond offers substantial advantages for field emitters, there is a need for diamond emitters capable of emission at yet lower voltages. For example, flat panel displays typically require current densities of at least 0.1 mA/mm.sup.2. If such densities can be achieved with an applied voltage below 25 V/.mu.m for the gap between the emitters and the gate, then low cost CMOS driver circuitry can be used in the display. Unfortunately, good quality, intrinsic diamond cannot emit electrons in a stable fashion because of its insulating nature. To effectively take advantage of the negative electron affinity of diamond to achieve low voltage emission, diamonds need to be doped into n-type semiconductivity. But the n-type doping process has not been reliably achieved for diamond. Although p-type semiconducting diamond is readily available, it is not helpful for low voltage emission because the energy levels filled with electrons are much below the vacuum level in p-type diamond. Typically, a field of more than 70 V/.mu.m is needed for p-type semiconducting diamond to generate an emission current density of 0.1 mA/mm.sup.2.
An alternative method to achieve low voltage field emission from diamond is to grow or treat diamond so that the densities of defects are increased in the diamond structure. This method is disclosed in pending U.S. patent application Ser. No. 08/331458 filed by Jin et al. on Oct. 31, 1994. Such defect-rich diamond typically exhibits a full width at half maximum (FWHM) of 7-11 cm.sup.-1 for the diamond peak at 1332 cm.sup.-1 in Raman spectroscopy. The electric field required to produce an electron emission current density of 0.1 mA/mm.sup.2 from these diamonds can reach as low as 12 V/.mu.m.
Another approach is to coat a flat device substrate with ultra-fine diamond particles and then to activate the particles into low-voltage electron emitters (&lt;12 V/.mu.m) by hydrogen plasma heat treatment. This method is disclosed in the aforementioned application Ser. No. 08/361616.
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