WO1994025976A1

WO1994025976A1 - Method of making field emission tips using physical vapor deposition of random nuclei as etch mask

Info

Publication number: WO1994025976A1
Application number: PCT/US1994/004568
Authority: WO
Inventors: Nalin Kumar
Original assignee: Microelectronics And Computer Technology Corporation; Si Diamond Technology, Incorporated
Priority date: 1993-04-23
Filing date: 1994-04-22
Publication date: 1994-11-10
Also published as: US5312514A; AU6668794A

Abstract

A method of making sub-micron low work function field emission tips (32, 66) without using photolithography. The method includes physical vapor deposition of randomly located discrete nuclei to form a discontinuous etch mask (20, 50). In one embodiment an etch is applied to low work function material (14) covered by randomly located nuclei to form emission tips (32) in the low work function material (14). In another embodiment an etch is applied to base material (44) covered by randomly located nuclei to form tips (58) in the base material (44) which are then coated with low work function material (60) to form emission tips (66). Diamond is the preferred low work function material (14, 60).

Description

DESCRIPTION

METHOD OF MAKING FIELD EMISSION TIPS USING PHYSICAL VAPOR DEPOSITION OF RANDOM NUCLEI AS ETCH MASK

Technical Field The invention relates to a method of making field emission tips using randomly located discrete nuclei deposited by physical vapor deposition as an etch mask.

Background Art Field emitters are widely used in ordinary and scanning electron microscopes since emission is affected by the adsorbed materials. Field emitters have also been found useful in flat panel displays and vacuum microelectronics applications. Cold cathode and field emission based flat panel displays have several advantages over other types of flat panel displays, including low power dissipation, high intensity and low projected cost. U.S. Patent No. 4,806,202 discloses a sputtered aluminum etch mask for forming a grass type oxide residue. Similarly, U.S. Patent No. 4,465,551 discloses an evaporated aluminum etch mask for forming a graded index layer for optical reflection reduction. A primary shortcoming and deficiency in the prior art is the inability to form fine conical or pyramid shaped emission tips without photolithography.

Disclosure of Invention Preferred embodiments of the invention provide a process for fabricating field emission tips with sharp sub-micron features without photolithography by physical vapor deposition of randomly located discrete nuclei to form a discontinuous etch mask. The nuclei are preferably non- polymerized with a melting point above 661 " C to assure an ion etch produces pyramid shaped tips with a suitable electric field enhancement factor. In a first embodiment an etch is applied to low work function material covered by randomly located nuclei to form emission tips in the low work function material. In a second embodiment an etch is applied to a base material covered by randomly located nuclei to form tips in the base material which are then coated with low work function material to form emission tips. Diamond is the preferred low work function material.

Brief Description of Drawings Figs. LA- ID show cross-sectional views of successive stages of fabricating a field emitter device in accordance with a first embodiment, Figs. 2A-2E show cross-sectional views of successive stages of fabricating a field emitter device in accordance with a second embodiment, Fig. 3 shows an elevational perspective view of a field emitter device fabricated in accordance with Figs. 1A-1D, and Fig. 4 shows an elevational perspective view of a field emitter device fabricated in accordance with Figs. 2A-2E.

Best Mode for Carrying Out the Invention

First Embodiment Referring now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views, and more particularly to Figs. 1A-1D, there are shown successive cross- sectional views of fabricating a field emitter device generally designated 10 according to a first embodiment. With reference now to Fig. 1A, a large area substrate 12 with a planar top surface is provided. Substrate 12 is preferably glass, silicon or a metal although other materials can be used that provide a base upon which a plurality of emission tips can be fabricated. A continuous layer of a low work function material 14 is disposed on substrate 12. A low work function material should be 4.5 electron-volts or less, preferably 2.5 electron-volts or less. Diamond (amorphic or nanocrystalline) is the preferred low work function material; see, for instance, U.S. Patent Nos. 5,199,918; 5,180,951; and 5,141,460. Other suitable low work function materials include tri- chromium mono-silicon (Cr Si), tantalum-nitride (TaN), low work function compounds, metals (cesium) and cermets (e.g., tri-chromium mono-silicon silicon-dioxide (CrgSi-S ^), tri-chromium mono-silicon magnesium-oxide (CrgSi-MgO), gold silicon-dioxide (Au-Siθ2), and gold magnesium-oxide (Au-MgO)). Low work function material 14 may be deposited on substrate 12, for instance, by sputtering, evaporation (including magnetically filtered cathode arc evaporation), laser deposition or chemical vapor deposition although the preferred technique depends on the particular material. The preferred deposition techniques for diamond films are disclosed in U.S. Patent Nos. 5,098,737 and 4,987,007. An optional adhesion layer 16 such as 500 angstroms titanium, chromium, tantalum, titanium-tungsten or nickel-chromium can be deposited between substrate 12 and material 14. Referring now to Fig. IB, an etch mask 20 of randomly dispersed nuclei is deposited by physical vapor deposition on material 14. Discontinuities between the nuclei are formed in situ as mask 20 is deposited, as opposed to depositing a blanket layer and then selectively patterning openings such as by photolithography. More particularly, etch mask 20 is formed by depositing in situ a discontinuous layer of randomly located, spaced, discrete nuclei with discontinuities therebetween so as to form etch mask 20 on low work function material 14 thereby exposing some portions of the low work function material 14 (beneath the discontinuities) while covering other portions of material 14 (beneath the nuclei). The random location of the nuclei ensures that the locations of the discontinuities, the exposed and covered portions of material 14, and the pattern of mask 20 are random as well. As may be seen, mask 20 serves to expose portions 22 of material 14 while covering portions 24 of material 14. Mask 20 is deposited by physical vapor deposition, such as by ionized metal cluster beams, liquid metal cluster beams, sputtering (including ion beam sputtering and magnetic ion beam sputtering) or evaporation. Those skilled in the art recognize that physical vapor deposition is distinct from chemical vapor deposition. In physical vapor deposition, as used herein, particles moving toward a substrate either fail to react or combine with a gas to form an oxide, nitride, carbide or the like. For instance, physical vapor deposition of a compound of a metal and a gas onto a surface is described in U.S. Patent No. 5,196,102. Physical vapor deposition does not require chemical reduction of a vapor in contact with a substrate. Thus, physical vapor deposition does not encompass photodecomposition of a gaseous precursor (whether by blanket illumination through a photomask or by direct-write of a laser beam) or electron beam decomposition of a gas phase material at a substrate. Moreover, physical vapor deposition avoids the need for highly uniform vapor composition and flow (which limits maximum substrate size) and toxic organo-metallic precursors, as typically required by chemical vapor deposition. Suitable mask materials include aluminum-oxide (A^Og), molybdenum, gold, and allotropic forms of carbon (including diamond and graphite). For example, diamond particles on the order of 50 angstroms diameter may be sprinkled on the substrate. It is noted that these mask materials are non-polymerized materials with relatively high melting points (i.e., above the 660.37 " C melting point of aluminum and preferably above 1000° C) to assure properly shaped emission tips. Aluminum-oxide has a very low sputtering yield (i.e., for an incoming atom how many atoms are etched off). A low sputtering yield in mask 20 relative to material 14 allows deep valleys to be etched in material 14 while using a relatively thin etch mask 20. Sputtering yields of various materials in argon are tabulated on p. 4-40 in Maissel and Glang, HANDBOOK OF THIN FILM TECHNOLOGY, 1983 Reissue, McGraw-Hill Book Company. The discontinuities extending through the thickness of etch mask 20 are formed in situ as physical vapor deposition occurs using techniques known to those having skill in the art. For instance, Maissel and Glang at p. 8-33 report a nucleation and growth model in which a thin film having a thickness of 100 angstroms or less is generally discontinuous. Furthermore, Maissel and Glang report experimental verification of this model at pp. 10-42 and 10-43. In Maissel and Glang the nucleation site is a random site where the first set of atoms/molecules reside during the initial formation of a thin film on a substrate. A majority (if not essentially all) of the nuclei grow as deposition occurs. Growth of the nuclei is three- dimensional although growth parallel to the substrate may be greater than growth normal to it. Moreover, the majority (if not essentially all) of growth of the nuclei does not result from coalescence of the nuclei. Even though the number and sizes of the nuclei which comprise the discontinuous thin film layer depend on substrate temperature, activation of nuclei adatoms, and duration of deposition, under proper conditions it is possible to accurately control the size of the nuclei. For instance, Fig. 21(a) on p. 10-42 of Maissel and Glang shows a large number of discrete three- dimensional nuclei in which the width of each nucleus is less than 1000 angstroms and each nucleus is spaced by a discontinuity of less than 1000 angstroms from the nearest adjacent nucleus. A thin film less than 10 angstroms thick may fail to provide a reliable etch mask 20. Therefore, a discontinuous layer of etch mask 20 between 10 and 100 angstroms thick is generally preferred. With reference now to Fig. 1C, an etch is applied to the top of device 10 to remove portions of material 14 exposed by mask 20. Ion etching (which includes dry etching and plasma etching) is preferred and is illustrated herein. Wet chemical etching, for example, is not normally preferred due to undercutting of material 14 beneath the mask. Likewise, a reactive etch (where a gas reacts with the surface to accelerate the etch) is normally unnecessary. It should be noted that mask 20 need not be erodible by the etch to form pyramid shaped emission tips since "shadowing" may occur whereby ions are deflected from the edge of the mask thereby accelerating the etch rate of exposed material spaced from the openings in the mask. Thus, under appropriate conditions the emission tips may be formed using a non-erodible etch mask. Returning to the example, material 14, mask 20 and the ion etch are selected so that as the ion etch is applied mask 20 erodes slowly and material 14 etches at a greater rate than mask 20. For practical purposes, material 14 should etch at a greater rate than mask 20. Preferably, material 14 etches at at least twice the rate of mask 20. As a result, vertical valleys 30 formed at the exposed portions 22 extend partially through material 14, preferably between 100 angstroms and 1 micron deep, although if desired valleys 30 could extend completely through material 14. As is seen, randomly located emission tips 32 are formed between valleys 30. Preferably, the distance between adjacent tips exceeds the height of the tips for field enhancement purposes. Referring now to Fig. ID, the ion etch is continued so as to completely remove mask 20 and deepen the valleys. Emission tips 32 eventually assume a conical-pyramid shape with pointed tops due to preferential etching of the grain boundaries as has been previously demonstrated by heavy ion bombardment of copper films. In the event the low work function material 14 contains different materials with different etch rates the ion etch should be carefully monitored to prevent completely removing one material thereby changing the work function of the material which remains on the substrate. For example, ion etching tantalum-nitride may remove the nitride leaving a tantalum layer with too high a work function. Alternatively, if desired, the ion etch can be halted before mask 20 is completely removed. The remainder of mask 20 may then be removed by a second etch, such as wet chemical etching or dry etching, without removing additional material 14. The resultant emission tips would assume a rectangular-pyramid shape with relatively flat tops, as seen in Fig. lC. This would not normally be preferred, however, since the conical-pyramid shape most enhances the electrical field distribution near the emission surface as field emission occurs. While the relative simplicity of the embodiment in Figs. 1A-1D is advantageous, certain difficulties may also arise. For instance, there may be only a small difference in the etch rates between the low work function material 14 and an aluminum-oxide mask 20. A limited thickness needed for a discontinuous mask 20 deposited in situ might severely limit the depth of valleys 30. Furthermore, certain low work function materials such as compounds or cermets may be destroyed by ion etching.

Second Embodiment With reference now to Figs. 2A-2E, there are shown successive cross- sectional views of fabricating a field emitter device 40 according a second embodiment which overcomes the previously described difficulties for the embodiment in Figs. 1A-1D at the cost of added complexity. Referring now to Fig. 2A, a relatively thick (1000 angstroms to 3 microns) continuous layer of base material 44 is sputter deposited on a substrate 42 (similar to substrate 12) with optional adhesion layer 46 (similar to adhesion layer 16) sandwiched therebetween. A preferred base material 44 is an electrical conductor such as 3 micron thick copper, although resistive or semiconductive base materials are also suitable. With reference now to Fig. 2B, a discontinuous layer of an etch mask 50 (similar to mask 20) is deposited on base material 44 such that the discontinuities are formed in situ between randomly scattered nuclei as deposition occurs, thereby exposing portions 52 of material 44 while covering portions 54 of material 44. The same materials, thicknesses, deposition techniques and so on described previously for mask 20 apply to mask 50. Referring now to Fig. 2C, an etch (illustrated as an ion etch) is applied to the top of device 40. Material 44, mask 50 and the ion etch are selected such that material 44 etches at a greater rate than mask 50, preferably at at least twice the etch rate. As a result, vertical valleys 56 formed at the exposed portions 52 extend partially through material 44, and tip bases 58 are formed between valleys 56. If desired, valleys 56 could extend completely through material 44. With 1000 electron-volt argon ions in the present illustration the large difference in sputtering yields of an aluminum-oxide mask 50 and copper base material 44 (0.04 and 3.2, respectively) produces very high aspect ratios for valleys 56. With reference now to Fig. 2D, the ion etch is continued so as to completely remove mask 50. In addition, base tips 58 assume a conical- pyramid shape. Referring now to Fig. 2E, a coating of low work function material 60 (similar to material 14) is deposited on base material 44, thereby forming randomly located conical-pyramid shaped emission tips 66 on base tips 58. An optional adhesion layer 68 (similar to adhesion layer 16) can be sandwiched between materials 44 and 60 if desired. Alternatively, as previously described, the ion etch could be halted before completely removing mask 50, and a wet chemical etch or dry etch could remove the rest of mask 50 without removing additional base material 44 prior to depositing low work function material 60 thereon. However, the resultant base tips 58 and emission tips 66 would assume the rectangular-pyramid shape seen in Fig. 2C which, as mentioned above, is not normally preferred. Furthermore, in the embodiment of Figs. 2A-2E it is not essential that the mask be completely removed since low work function material 60 is coated on the substrate after the mask is formed. Thus (although not shown), emission tips 66 may include mask 50 sandwiched between materials 44 and 60 if desired. With reference now to Figs. 3 and 4, elevational perspective views are shown of field emitter devices fabricated in accordance with Figs. 1A- ID and 2A-2E, respectively. Other such possibilities should readily suggest themselves to persons skilled in the art. For example, the emission tips could assume circular or irregular pyramid shapes. Furthermore, the emission tips may be used as cold cathodes in a wide variety of systems and devices such as flat panel displays. In addition, the method herein may suitably comprise, consist of, or consist essentially of the forementioned process steps. Various changes, substitutions and alterations can be made herein without departing from the scope of the appended claims.

Claims

1. A method of making field emission tips (32), including disposing a continuous layer of a low work function material (14) over a substrate (12), wherein the improvement comprises: depositing in situ by physical vapor deposition a non-polymerized randomly patterned etch mask (20) with a melting point above 661 ° C over the low work function material (14), said etch mask (20) comprising randomly located discrete nuclei with discontinuities therebetween thereby exposing portions (22) of the low work function material (14) beneath the discontinuities; etching the exposed portions (22) of the low work function material (14); and removing the etch mask (20) thereby forming exposed emission tips (32) of low work function material (14).

2. A method of making field emission tips (66), including disposing a continuous layer of a base material (44) over a substrate (42), wherein the improvement comprises: depositing in situ by physical vapor deposition a non-polymerized randomly patterned etch mask (50) with a melting point above 661 " C over the base material (44), said etch mask (50) comprising randomly located discrete nuclei with discontinuities therebetween thereby exposing portions (52) of the base material beneath the discontinuities; etching the exposed portions (52) of the base material (44) after the deposition of nuclei begins to form tips (58) in the base material (44); and depositing a low work function material (60) over the tips (58) in the base material (44) thereby forming exposed emission tips (66) of low work function material (60).

3. The method as recited in claims 1 or 2 wherein the low work function is less than 4.5 electron-volts.

4. The method as recited in claims 1 or 2 wherein the low work function material (14,60) is diamond.

5. The method as recited in claims 1 or 2 wherein the etch mask (20,50) is selected from the group consisting of aluminum-oxide, molybdenum, gold, carbon, graphite, and diamond.

6. The method as recited in claims 1 or 2 wherein the etch mask (20,50) consists of the nuclei.

7. The method as recited in claims 1 or 2 wherein the distance between adjacent emission tips (32,66) exceeds the height of the emission tips (32,66).

8. The method as recited in claims 1 or 2 wherein the physical vapor deposition is provided by one of an ionized metal cluster beam, a liquid metal cluster beam, evaporation or sputtering.

9. The method as recited in claims 1 or 2 wherein the etching is by an ion etch, the exposed portions (22,52) etch at a greater rate than the etch mask (20,50), and the emission tips (32,66) assume a pyramid shape.

10. The method as recited in claims 1 or 2 wherein the etching begins after the deposition of nuclei begins.